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Metamorphic PT path and zircon U–Pb dating of Archean eclogite association in Gridino complex, Belomorian province, Russia
Accepted Manuscript Title: Metamorphic PT path and zircon U-Pb dating of Archaean eclogite association in Gridino complex, Belomorian province, Russia Author: Xiaoli Li Lifei Zhang Chunjing Wei I. Alexander Slabunov PII: DOI: Reference:
S0301-9268(15)00240-5 http://dx.doi.org/doi:10.1016/j.precamres.2015.07.009 PRECAM 4316
To appear in:
Precambrian Research
Received date: Revised date: Accepted date:
11-4-2015 9-7-2015 17-7-2015
Please cite this article as: Li, X., Zhang, L., Wei, C., Slabunov, I.A.,Metamorphic PT path and zircon U-Pb dating of Archaean eclogite association in Gridino complex, Belomorian province, Russia, Precambrian Research (2015), http://dx.doi.org/10.1016/j.precamres.2015.07.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Highlights
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We obtain peak metamorphic condition of Belomorian eclogite;
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We confirm the clock-wise metamorphic PT path of Belomorian eclogite assemblages;
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Neoarchean age of Belomorian eclogite has been determined;
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Subsequent high-temperature reworking during the exhumation of Belomorian eclogite
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assemblages has been suggested at Neoarchean;
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Metamorphic PT path and zircon U-Pb dating of Archaean eclogite association in Gridino
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complex, Belomorian province, Russia
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*Li, Xiaoli1, Zhang, Lifei1, Wei, Chunjing1, Slabunov, Alexander I.2
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1. School of Earth and Space Sciences, Peking University, Beijing 100871, China
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2. Institute of Geology, Karelian Research Center RAS, Petrozavodsk, 185910, Russia,
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*Corresponding author: Li, Xiaoli, Yiheyuan Road No.5, Haidian District, Beijing, 100871,
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China. Tel.: +86-010-62751168, Email: [email protected]
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Abstract
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The eclogites from the Gridino eclogite-bearing mélange complex in the Belomorian province,
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Fennoscandian Shield may represent one of the oldest terrain in the world. The Belomorian
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province is a superposition of Archaean and Palaeoproterozoic collisional orogens, which
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experienced repeated high-grade metamorphic reworking and intense polyphase tectonic
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deformation in the early Precambrian. Main purpose of this study is to determine the metamorphic
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age(s) and evolutionary PT path of eclogite as boudin-like body within the Gridino tectonic
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mélange. Representative rock materials of well-preserved, plagioclase-free eclogite and associated
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retrogressed eclogitic rock were sampled from Stolbikha Island in the Gridino area for our research.
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The eclogite sample mainly comprises omphacite, garnet, amphibole, rare quartz, and some
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accessory minerals (rutile, magnetite), whereas the retrogressed eclogite sample is made of diopside
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– Na-rich plagioclase symplectites, amphibole, quartz, and accessory minerals. Thermodynamic
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modeling points to a clockwise metamorphic PT path, with peak conditions of P = 18.5kbar, T =
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710 °C, and a “warm” subduction is inferred thus. LA-ICP-MS zircon U-Pb dating yields diverse
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metamorphic episodes from the Neoarchaean to the Palaeoproterozoic. While metamorphic zircon
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formed at eclogite-facies condition is identified by studying the trace element distribution and
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partitioning in and between zircon and co-existing eclogite-facies minerals (omphacite, garnet);
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consequently, Neoarchaean eclogite-facies metamorphic event of ca. 2.70 Ga can be distinguished
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for the Gridino eclogite complex, and a regional-scale thermal event may have occurred in the
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period of 2.70 Ga – 2.61 Ga.
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Keywords: Archaean eclogite, Belomorian province, phase equilibria modeling, zircon dating
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It was commonly considered that eclogite, a product of high-pressure (HP) metamorphism
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usually formed during subduction, could hardly occur in Archaean because of the distinct
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geodynamic regime in the Archaean eon (Baer, 1977; Green, 1975; Stern, 2005; Brown, 2014). The
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likely recognized oldest (2.0 Ga) eclogite occurrence was reported in the Usagaran belt of Tanzania
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(Möller et al., 1995). Also the eclogite occurrence in the Snowbird zone, Canada, was before
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speculated to form in Archaean (Percival, 1994), although it was eventually proved to be of
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Palaeoproterozoic (ca. 1.90 Ga) age (Baldwin, et al., 2004). However, about a decade ago, Archaean
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eclogite associations were firstly recognized in the Belomorian province of Fennoscandian shield
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(Volodichev et al., 2004; Konilov et al., 2004; Mints et al., 2010a) that could fundamentally update
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our understanding of the early geodynamics of our planet.
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Eclogite occurrence in the Belomorian province was already known long time ago (Eskola,
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1921). The eclogite association on the Stolbikha Island (Gridino complex) – the study area of
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current paper, were firstly reported in the 90s of last century (Volodichev, 1990; Mitrofanov, 1996),
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and since then debates on the petrology and metamorphic age of the eclogite were never stopped.
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Generally there are two major types of eclogite occurrence here such as boudin-like mafic eclogite
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bodies embedded in a polymetamorphic TTG matrix and eclogitized dykes (of gabbro-norite
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compositions) occurring in different generations. The first type was usually considered to be of
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Archaean age (Stolbikha Island) (Bibikova et al., 2003; Volodichev et al., 2004, 2005; Slabunov et 3 Page 3 of 70
al., 2006) and the second type usually occurring on adjacent archipelagos (e.g. Vargas Island,
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Izbunaya Luda Island) was suggested to be of Palaeoproterozoic age (Travin and Kozlova, 2005,
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2009; Kozlovsky and Aranovich, 2008; Dokukina et al., 2009, 2010). Later another eclogite
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associations of older ages (Meso-, Neoarchaean, Mints et al., 2010a, b; Shchipansky et al., 2012a, b)
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in the northern proximity named after the Salma and Kuru-Vaara complexes were discovered, along
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with some other eclogite-bearing outcrops of different scales continuously being reported in this
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region (for instance, in the Chupa and Khetolambino terrains), which could reach several of
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hundreds (personal communications with Shchipansky A.A.). The concept “Belomorian eclogite
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province” was then introduced by local Russian geologists, which comprises the whole Belomorian
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province and the southern part of Kola continent (Mints et al., 2014 and references therein). Debates on the age of the Belomorian eclogites and possible geodynamic models have never
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stopped ever since their first discover. There are in all four eclogite-facies metamorphic episodes
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were proposed by different researchers – Mesoarchaean twice (2.88 Ga and 2.82Ga, Salma complex)
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(Mints et al., 2010a, b; Konilov et al., 2011), Neoarchaean (2.72 Ga, Salma and Gridino complexes)
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(Bibikova et al., 2003; Volodichev et al., 2004; Slabunov et al., 2006, 2008; Shchipansky et al.,
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2012a, b) and Palaeoproterozoic (1.90 Ga, Gridino and Kuru-Vaara complexes), which was usually
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considered to be related to the Svecofennian orogeny (Travin and Kozlova, 2009; Skublov et al.,
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2010, 2011; Herwartz et al., 2012; Mel’nik et al., 2013). The genesis of Belomorian eclogite
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occurrence was another subject of discussion, as most researchers prefer to the subduction-collision
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model (e.g. Slabunov et al., 2006; Mints et al., 2014; Dokukina et al., 2014), although alternative
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minor opinions may also emerge (for instance, “autonomous” eclogite-facies metamorphism (e.g.
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Travin and Kozlova, 2005, 2009). In the pioneer works, estimation of metamorphic PT conditions
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were based on a series of conventional geothermo-, barometric calculations on certain mineral
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paragenesis that peak conditions of 14.2-17.5 kbar, 740-865 °C was suggested and, consequently,
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subduction with a geothermal gradient of ~ 12-13 °C/km was inferred that could answer a warm
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subduction regime (Volodichev et al., 2004; Mints et al., 2010a).
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In this paper, we will focus on the boudin-like mafic eclogite bodies within the TTG tectonic
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mélange (the first type eclogite), and to avoid misunderstanding, in the following text, the term
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Gridino eclogite will only refer to this type of eclogite. Pioneer researchers (e.g. Volodichev et al,
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2004; Slabunov, 2011) have discussed the age(s) and peak metamorphic conditions of similar
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eclogite rocks from this area. Here we will present new petrological, geochronological and
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geochemical data of the analogous boudin-like eclogite outcrops from the adjacent area, to conclude
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the metamorphic evolution PT path and the time(s) of eclogite-facies metamorphic event(s),
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involving complex analytical approaches. Research materials are eclogite and retrogressed eclogite
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rocks (apoeclogite as in Russian literature), which were collected during the geological excursion of
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GEC-2011 on Stolbikha Island near the village of Gridino addressed as Stop 1 in the fieldtrip
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guidebook (Volodichev et al., 2011). Phase equilibria modeling in the NCFMASH system combined
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with conventional geothermobarometry are utilized to build the metamorphic PT path of the
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Gridino eclogite. Age of eclogite-facies metamorphism is determined by in-situ LA-ICP-MS zircon
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U-Pb dating. By analyzing trace element compositions in zircons and co-existing eclogite minerals
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(omphacite, garnet), we are able to distinguish the metamorphic zircons formed at eclogite-facies
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condition, who were in equilibrium with omphacite-garnet assemblage, and, thus, the corresponding
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age(s). Besides, high-grade metamorphic zircons formed in the early Precambrian period may
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possess individual geochemical fingerprints that differ from the “modern” ones.
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2. Geological background
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The Belomorian province, along with the Karelian, Murmansk, Norrbotten and Kola
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provinces comprise the Archaean core of Fennoscandian (Baltic) shield (Fig.1a). The Svecofennian
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orogen (1.8-1.9 Ga) accreted to the old core in the west and the Kola-Lapland orogen (1.9-2.0 Ga)
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formed inside this core, while the Belomorian and Kola provinces are forlands of Kola-Lapland
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orogen (Fig.1a). The Karelian province (craton) is built up mainly by an ancient granite-greenstone 5 Page 5 of 70
complex (ca. 2.6-3.5 Ga) including tonalite-trondjemite-granodiorite (TTG) granitoids and gneisses,
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greenstone and paragneisses belts with mafic, komatiitic and intermediate-felsic metavolcanics,
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metasediments, banded iron formation (BIF), metamorphosed in greenschist to low-pressure
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amphibolite (up to granulite) facies with either andalusite or sillimanite formation (Gaál and
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Gorbatschev, 1987; Sorjonen-Ward and Luukkonen, 2005; Slabunov et al., 2006; Hölttä et al., 2014).
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The Kola province, or Inari-Kola microcontinent, consists of three types of terrains: the first
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predominate one includes Archaean greenstone, paragneiss, TTG-gneiss and granulite-gneiss
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complexes, the second is mixer of Neoarchaean and juvenile Palaeoproterozoic rocks, and the third
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– Palaeoproterozoic Lapland and Umba granulite (Balagansky, 2002; Glebovitskii, 2005; Daly et
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al., 2006; Balagansky et al., 2011). The Belomorian (tectonic) province mainly is composed of
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Archaean (2.7-2.9 Ga) TTG granitoid and gneiss, greenstone (metakomatiite, metabasalt, island-arc
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type felsic volcanic rocks, sediments of BIF, quartzite and conglomerate) and paragneiss
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(metagreywacke) complexes (Bibikova et al., 1999, 2004; Thurston and Kozhevnikov, 2000;
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Glebovitskii, 2005; Slabunov et al., 2006; Slabunov, 2008; Hölttä et al., 2014). Meanwhile,
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Palaeoproterozoic coronitic gabbroid (Stepanova and Stepanov, 2010), granitoid and pegmatite are
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also very common (Volodichev, 1990).
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Archaean associations in the Belomorian province are rather similar to those in the Karelian
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craton generally but distinguished in the metamorphic grades: there are two stages (Neoarchaean
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and Palaeoproterozoic) of amphibolite-facies metamorphism with kyanite (high- pressure)
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occurrence in the Belomorian province (Volodichev, 1990). Geological (Miller and Mil'kevich,
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1995) and seismic profiling data (Mints et al., 2009; Sharov et al., 2010) show that the internal
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structure of Belomorian province is composed of large scale intensely folded nappe, which form in
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the Archaean and Paleoproterozoic time. The Belomorian province was thrusted on the Karelian
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craton and, in turn, was thrusted by rocks of the Kola province during Lapland-Kola orogeny.
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There are two eclogite-bearing mélange complexes can be distinguished in the Belomorian
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province: the Salma complex in the northern part and the Gridino – in the eastern (Fig. 1a). These 6 Page 6 of 70
complexes form tectonic slices (Fig. 1b) (Slabunov, 2008; Balagansky et al., 2015). The matrix of
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the mélange is mainly built up of intensely migmatized and deformed TTG-gneiss (Sibelev et al,
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2004; Shchipansky et al., 2012a). Major types of eclogite occurrences are recognized as boudin-like
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bodies of several meters in diameter (Shirokaya Salma, Kuru-Vaara, Gridino localities) up to
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hundreds meters (Shirokaya Salma localities) (Volodichev et al., 2004, 2005, 2011; Mints et al.,
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2010a, b, c; Dokukina and Konilov, 2011; Shchipansky et al., 2012a). The Salma (Shirokaya Salma,
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Uzkaya Salma, Kuru-Vaara localities) and Gridino eclogites are considered to be of oceanic
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lithosphere subduction-related type, which formed in the intervals of 2.89-2.82 (for Salma) and 2.72
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Ga (for Salma and Gridino) (Volodichev et al., 2004; Mints et al., 2010a, b, c; Dokukina and
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Konilov, 2011; Shchipansky et al., 2012a). The second type of eclogites are eclogitized and partly
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boudinated mafic dyke swarms (Volodichev, 1990; Travin and Kozlova, 2005; Kozlovsky and
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Aranovich, 2008; Volodichev et al., 2004, 2012; Dokukina and Konilov, 2011; Mel’nik, 2015),
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which can be also noted in the Keret archipelago (Kozlovsky and Aranovich, 2008). In the Gridino
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area most eclogitized gabbro outcrops are systematically located in an archipelago of the White Sea
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(e.g., Vorotnaya Luda Island, Izbunaya Luda Island, Vargas Island), and a few occur at the coast
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near the village of Gridino (Volodichev et al, 2012, Dokukina and Konilov, 2011). The dyke-type
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eclogites have been well discussed in details in other reports (Dokukina et al., 2009, 2010, 2012,
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2014; Dokukina and Konilov, 2011; Stepanova and Stepanov, 2010; Babarina and Sibelev, 2015).
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The boudin-type eclogite, together with amphibolite and zoisitite rocks, constitute the primary
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components of the TTG gneiss matrix that form a polygenetic mélange complex; they are
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distributed irregularly in the northwest spread, a structure that is possibly related to the
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disintegration of certain multiple-built complexes in the subduction zone and collisional suture
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(Slabunov et al., 2007, 2015). The boudin-type eclogite was firstly dated at ca. 2.72 Ga by
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Volodichev et al. (2004), and it was considered to be of subduction-type genesis as the case in the
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Salma complex. However, later Skublov et al. (2011) proposed a late Palaeoproterozoic age of ca.
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1.90 Ga for the eclogite-facies metamorphism and considered the early obtained Archaean age was 7 Page 7 of 70
actually the time of protolith formation. Petrologic studies on both mafic and felsic rocks from the
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Gridino complex indicate a minimum pressure of 16-17.5 kbar for the eclogite-facies
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metamorphism, and subsequent retrogression via high-pressure granulite (14-10 kbar, 800-750 oC)
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then amphibolite (7.9-9.6 kbar, 530-700 oC) facies metamorphism can be expected (Dokukina et al.,
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2009; Konilov and Dokukina, 2011). Besides, according to the report of Perchuk and Morgunova
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(2014), an ultra-high-pressure metamorphism (coesite stability field) in the Gridino eclogitized
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gabbro was even proposed.
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Nowadays there are different models of crustal evolution for the Belomorian province, and all
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of them are in consensus on the subduction, accretion, plume and collision events. For instance, one
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group of geologists consider that the eclogite and other early continental crust with mafic dykes
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formed during the subduction-accretion event at ca. 2.82-2.90 Ga, and the eclogitized mafic dykes
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of Gridino type and the continent occurred during the collision at ca. 2.78-2.82 Ga. Subsequently,
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superplume influences on the continent uplifted to higher crustal level with granulite-facies event(s)
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started at ca. 2.70-2.72 Ga, while later new superplume events with mafic dyke and granitoid
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intrusion took place at ca. 2.4 Ga. At last, a new collisional event with exhumed middle crust
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occurred at ca. 1.77-1.90 Ga. (e.g. Mints et al., 2010b, 2015). Another few geologists however put
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forward a different model. They consider that the Earth crust of the Belomorian province was
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produced during the Mesoarchaean-Neoarchaean (Belomorian) collisional orogeny at ca. 2.65-2.90
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Ga (Slabunov, 2008; Slabunov et al., 2006; Hölttä et al, 2014), and it was greatly influenced by the
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Palaeoproterozoic (ca. 2.4 Ga) plume (Stepanova and Stepanov, 2010). During the late
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Palaeoproterozoic Lapland-Kola collisional orogeny it was somehow reworked and reshaped
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(Balagansky, 2002; Daly at al., 2006; Balagansky et al., 2015).
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In this work, we will accentuate on the boudin-shaped eclogite bodies within the TTG tectonic
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mélange of the Gridino complex (on Stolbikha Island) that include well-preserved eclogite with
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minimal retrogression (sample GS-1) and associated retrogressed eclogitic rock (samples GS-2 &
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GS-3) (Fig. 1c). Most exposed eclogite bodies have either been transformed into symplectite-rich 8 Page 8 of 70
retrogressed eclogite or garnet-bearing amphibolite; in the bodies’ central parts, however,
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sometimes pristine omphacite-garnet mineral assemblages mainly occur, although minor secondary
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plagioclase rims around garnet may also be observed. In the outcrops, several tonalitic-
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trondhjemitic and pegmatitic veins (~ 50 cm in width) extend dozens of meters and, occasionally,
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they transect some eclogite and amphibolite bodies and/or cross-cut the gneissose foliation. The
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country rocks are intensely deformed amphibole-biotite-bearing orthogneiss and granitoid of the
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TTG series – the main component of the tectonic mélange (Li et al., 2013a; Babarina and Sibelev,
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2015).
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Mineral chemistry analysis was conducted with the electron microprobe analyzer JEOL JXA-
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8100 at Peking University, with 15 kV accelerating voltage, 10 nA beam current, and 10-15 s
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counting time. SPI standards were utilized and the PRZ correction was applied. Mineral formulae
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were calculated with the MINPET software (ver. 2.02). Zircons were extracted from ~ 8-10 kg rock
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samples, mounted together with TEM standards in epoxy resin, and well-polished to expose their
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cores. The zircons were first investigated in the optical microscope and inclusions were identified
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by Raman spectroscopy. Back-scatter electron (BSE) and cathodoluminescence (CL) images were
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obtained with a scanning electron microscope, QUANTA-650FEG, equipped with INCA-Synergy
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and CHROMA-D detectors at Peking University. Qualitative and quantitative WDS/EDS analyses
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of mineral inclusions in zircons were performed with the afore mentioned microprobe operated at
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the same settings. In-situ analysis of trace elements and U-Pb isotopes was performed with an ICP-
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MS Agilent 7500 Ce equipped with a Complex Pro102 laser ablation system (LA-ICP-MS) at
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Peking University. Helium was used to enhance the transport efficiency, and nitrogen was added to
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the argon plasma to decrease the detection limit and improve the analytical precision. Each analyse
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incorporated approximately 15 s background acquisition (gas blank) followed by 30 s data
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acquisition from the sample. The laser spot was adjusted to ~ 30-35 μm. For zircons, U, Th and Pb
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concentrations were calibrated using
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analytic signals, time-drift correction, and quantitative calibration for trace element analysis and
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U-Pb dating were done with the ICPMSDataCal software. The Glitter 4.0 software (by CSIRO) was
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used for further calculation. PLE and TEM were used as external standards for U-Pb isotopes in a
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separate operation and standards #610, #612, and #614 were references for the trace elements.
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Common Pb correction was undertaken with the LAM-ICPMS Common Lead Correction (ver.3.15)
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algorithm from T. Andessen in MS Excel 2010, and concordia diagrams and weighted means were
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built and computed with Isoplot/Ex (ver.3.57). For garnet and omphacite, the average 29Si contents
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of 38 wt.% and 54 wt.%, respectively (basing on the minerals’ microprobe data), were references
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for the trace elements calculation and standards #610, #612, #614 were applied for calibration
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during analysis.
Si. Off-line selection and integration of background and
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4. Petrography and mineral chemistry
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Eclogite sample GS-1 is a nearly bimineralic rock mainly consisting of porphyroblastic garnet
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and omphacite, with subordinate amphibole and scarce quartz. Omphacite usually forms fine-
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grained crystals of 0.5-1.0 mm length and contains in part some mineral inclusions (rutile,
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accessory minerals); garnet occurs as euhedral crystals of 0.5-1.0 mm diameter and usually contains
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rutile, amphibole and quartz inclusions. Amphibole develops mainly interstitially, but may also
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occur as epitaxial replacement of omphacite (clinopyroxene); in addition, little amphibole is
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associated with plagioclase forming kelyphitic coronas around garnet, a texture reflecting
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retrogression (Fig. 2).
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Clinopyroxene-plagioclase symplectites along with a lot of quartz aggregates, some of which
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may have experienced recrystallization, and aggregates of plagioclase, which often possesses
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twinning, characterize retrogressed eclogitic rock samples GS-2 & GS-3. In both rocks, garnet 10 Page 10 of 70
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usually occurs as semi-euhedral porphyroblasts with multiple fractures or irregular shapes, the size
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of which ranges from 1.0 to 1.5 mm. Amphiboles develop widespread, replacing symplectitic and/or
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relic clinopyroxene (Fig. 2). Judging by the field correlation, retrogressed eclogite samples GS-2
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and GS-3 are likely retrogressed analogues of eclogite sample GS-1. Shown in Table 1, representative microprobe analyses of rock-forming minerals
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(clinopyroxene, garnet, plagioclase, and amphibole) can be generally characterized as follows
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(mineral abbreviations are after Whitney and Evans, 2010): Clinopyroxene is mostly omphacite in
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the eclogite sample GS-1 with Na2O = 3.31-4.80 wt.% and ∑FeO = 4.71-7.08 wt.%. The jadeite (Jd)
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component ranges from 16 to 30 mol.%, and the aegirine (Ae) component may reach 10 mol.% (Fig.
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3a). A Ca-Eskola (CaEs) component could hardly be estimated, whereas the Ca-Tschermak (CaTs)
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component ranges from 2 to 9 mol.%. The retrogressed eclogite samples GS-2 & GS-3 exclusively
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contain diopside with a little higher ∑FeO content – 7.94-9.33 wt.%, but barely a Jd component (<
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3 mol.%; Fig. 3a). No CaEs component could be recalculated, while a relatively high CaTs
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component – 8-14 mol.% – could be obtained. Garnet is a almandine-pyrope-grossular solid
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solution (Alm-Prp-Grs) with a predominant Alm component. For garnets from the eclogite sample
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(GS-1), the Prp component is a little higher than the Grs in general, while garnets from the
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retrogressed eclogite samples (GS-2 & GS-3) show the opposite. Most garnets in both eclogite and
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retrogressed eclogite samples do not show clear compositional zonation, only a few, however,
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present slight prograde zoning with Prp increasing (24 mol.% → 27 mol.%) and Grs decreasing (26
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mol.% → 23 mol.%) from core to rim (eclogite sample GS-1), or, conversely, show retrograde
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zoning (retrogressed eclogite sample GS-3; Fig. 3b). Amphiboles all are calcic types, pargasite (Prg)
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and/or edenite (Ed) (Fig. 3c). Plagioclase from the eclogite sample, forming kelyphitic coronas
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together with amphibole, is oligoclase (Ab 72-84 mol.%), while in the retrogressed eclogite samples,
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plagioclase mostly is andesine (Ab 51-67 mol.%; Fig. 3d). Zoisite (epidote) casually occurs in the
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retrogressed eclogite sample along with plagioclase and/or quartz, with an estimated Fe3+ = 0.40
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p.f.u.
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1 2
5. Metamorphic PT conditions
3 Phase equilibria modeling was undertaken in the NCFMASH model system (Na2O-CaO-FeO-
5
MgO-Al2O3-SiO2) for the eclogite sample GS-1. Titanium is omitted because its presence is limited
6
to the accessory minerals rutile/ilmenite in a trivial amount; potassium and manganese are also
7
excluded from the modal system for their negligible contents in feldspar and garnet, respectively.
8
Pure H2O is assumed to be in excess, considering the widespread presence of amphibole and
9
absence of carbonates. For the modeling, the rock’s bulk composition obtained by XRF analysis is:
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SiO2 = 48.81, Al2O3 = 12.38, FeO = 9.15, CaO = 13.38, MgO = 9.48, Na2O = 2.90 (in wt.%). THERMOCALC (ver. 3.33) by R. Powell was applied to build the PT pseudosection. Datasets
12
tc-ds55.txt & tc-ds55s.txt (updated 26/10/09) and the metabasite a-x file tc-NCFMASHm.txt were
13
used, with solution models of amphibole (Diener et al., 2007), clinopyroxene (Green et al., 2007),
14
chlorite (Holland et al., 1998), garnet (White et al., 2007), and plagioclase (Holland and Powell,
15
2003). In the PT pseudosection (Fig. 4), the eclogite-facies mineral assemblage garnet-
16
omphacite(±quartz) is predicted to be stable at 16-25 kbar at reference temperatures of 800-620 °C.
17
On garnet isopleth diagrams, Xprp (= Mg/(Mg+Ca+Fe2+) and Xgrs (= Ca/(Mg+Ca+Fe2+) increase and
18
decrease, respectively, with the pressure. The observed prograde zonation in garnet (core → rim)
19
implies a metamorphic evolutionary path within the stability field of the assemblage omphacite-
20
garnet-amphibole-quartz and, consequently, peak metamorphic conditions of 18.5 kbar, 710 °C for
21
((amphibole-) eclogite-facies) can be inferred (stage M1). If the peak metamorphic mineral
22
assemblage was amphibole-free (only comprising omphacite-garnet-quartz), following the garnet’s
23
zoning trend, pressures may actually have been little higher, 19 kbar at least, within a temperature
24
range of 720-730 °C (Fig. 4a). The occurrence of plagioclase along with amphibole (in trivial
25
amount) in kelyphitic coronas around garnet reflects uplift-retrogression, here named after stage M2,
26
with the respective stability field clinopyroxene-garnet-amphibole-plagioclase-quartz (Fig. 4b).
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Additionally, the conventional garnet-clinopyroxene thermometer (Krogh, 1988) evaluates the
2
PT range for the formation of the mineral assemblage omphacite-garnet-amphibole-quartz, which
3
yields maximum and minimum limits (as in solid lines) with Kd (distribution coefficient) values
4
varying from 7.80 to 8.55 – by omphacite rim composition with garnet core composition pair (Fig.
5
4b). The garnet-amphibole thermometer (Graham and Powell, 1984) was also applied for
6
appropriate mineral assemblages, where the garnet rim composition was used, that yields a higher
7
temperature range of 750-770 °C (reference pressures 15-17 kbar), values within the stability field
8
of clinopyroxene-garnet-amphibole-quartz±plagioclase. Thus, re-heating is suggested during
9
decompression (exhumation). Isopleths of omphacite’s Jd component (Xjd, Xjd = Na/(Na+Ca+Fe2+))
10
show that it decreases with the pressure. Altogether, a feasible decompressing stage (M2), in which
11
clinopyroxene, garnet, amphibole, plagioclase, and quartz are stable, can be concluded, considering
12
both garnet-amphibole thermometry and the composition of omphacite (Fig. 4b). Further
13
retrogression may be easily expected by the associated occurrence of retrogressed eclogites.
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To summarize, a pressure of about 18.5 kbar and a temperature of approximately 710 °C
15
represents the peak conditions of eclogite-facies metamorphism in the Gridino eclogite complex,
16
Belomorian province. Moreover, if we assume that there is no amphibole at all, the pressure might
17
be a little higher (19.5 kbar) and so does the peak temperature (720-730 oC). Consequently, a
18
subduction depth of nearly 61-64 km and a corresponding geothermal gradient of ~ 11-12 °C/km
19
can be inferred, which may be a littler “warmer” than the Phanerozoic (cold) subduction cases.
20
During subsequent decompression (exhumation), heating to > 750 °C occurred, when the pressure
21
dropped to about 15-16 kbar (stage M2). Further retrogression can be implied by the associated
22
symplectic eclogitic rock (samples GS-2, GS-3) that amphibolite-facies and lower grade
23
metamorphism is expected. Thus, in general, a clockwise metamorphic PT trajectory is concluded,
24
in accord with most orogen-type eclogites worldwide.
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6. Zirconology and the U-Pb dating 13 Page 13 of 70
1 2
6.1 Zircon morphology
3 The zircons extracted from the well-preserved eclogite sample GS-1 are mainly transparent,
5
euhedral to subhedral, and measure 100 to 200 µm in length. They contain several mineral
6
inclusions (garnet, rutile, diopside, plagioclase, amphibole, calcite, quartz and others) in limited
7
amounts. A few zircons occur as fragmental crystals or, rarely, as aggregates. On the basis of
8
cathodoluminescence (CL) imaging, four major textural types can be discriminated (Fig. 5): type A,
9
the most common one, possesses a clear core (domain 1, A1) – rim (domain 2, A2) zonal texture,
10
which could either be dark (low-CL) or bright (high-CL); type B, the second most one, is
11
characterized by fir-tree or patchy textures, often combined with type A zonal textures; type C
12
combines homogeneous zircons, being either completely dark or bright; and type D summarizes
13
zircons with compromised texture (fragmental sometimes), containing an obvious fluid-retreated
14
overprint as injection trails. In addition, for almost all zircon types, a thin bright “fringe” (shell) less
15
than 10μm thick can be widely observed. The zircons extracted from the retrogressed eclogitic
16
rocks (samples GS-2 & GS-3) show the same textures, yet they are smaller (50-150 µm), with type
17
A and type B textures equally distributed. Beside, almost all GS-2 and GS-3 zircons also possess a
18
thin bright fringe, mostly 10-15 µm thick with very few reaching 30-40 µm thickness.
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19
Altogether, morphologically, the zircons from the eclogite and eclogitic rocks are very alike
20
and do well meet the criteria of high-grade metamorphic genesis (Corfu et al., 2003; Liati et al.,
21
2009). The zonal type zircons are usually “mixed” by fir-tree/patchy texture, forming sort of hybrid
22
type, where the zonation might be compromised. The inner part of the zonal type zircon (usually of
23
low-CL) is often considered as a protolithic core, however, in the studied scenario, it should be
24
rechecked very carefully. Fluid-injection trails are common in all zircon types, with some silica- and
25
calcium-rich mineral inclusions (quartz and/or calcium-carbonate) usually concentrated around
26
them. Most injected fluid trails apparently are externally initiated (i.e. external fluid invasion),
14 Page 14 of 70
1
although a few may occur spontaneously somehow. When the fluid reworking has greatly
2
compromised the zircon, we sorted it into the type D.
3 4
6.2 Zircon U-Pb ages and geochemistry
5
7
Analytical data of U-Pb isotopes and trace element contents are given in Table 2 and 3, and
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the obtained concordia plots and REE distribution patterns are shown in Fig. 6 and 7 accordingly. For all analyzed zircons, obvious U-Pb age differences do not exist among textural types or
9
domains, and there are in all several age episodes can be concluded. For the well-preserved eclogite
10
sample GS-1, two major episodes can be discriminated (Table 2, Fig. 6): one (age group I), an upper
11
intercept age of 2698±28 Ma (n = 16, MSWD = 0.59), which, interpreted alternatively, corresponds
12
to a concordant age of 2689±5 Ma (n = 9, MSWD = 0.0026), and, two (age group IV), an upper
13
intercept age of 2607±44 Ma (n = 14, MSWD = 0.47). In addition, a single concordant
14
age of 1914±24 Ma is obtained for the A2 textural domain (outer part of zoned zircons), the
15
youngest age of all. Geochemically all dated zircons are characterized by low ∑REE contents, with
16
obvious HREE predominance, relatively medium-low absolute Y, Th, and U contents, and medium-
17
low Th/U ratios (Table 3). All chondrite-normalized REE pattern exhibit negative Eu and positive
18
Ce anomalies (one exception, a spot with positive Eu anomaly of Eu/Eu* = 1.44, age group IV
19
zircon) and relative flat HREE distribution (Fig. 7). Conventional Ti-in-zircon thermometry
20
(Watson et al., 2006) is considered to estimate the crystallization temperature of zircons; a value of
21
about 670 °C was obtained for both age groups.
206
Pb/238U
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For the retrogressed eclogitic rock GS-2, two episodes can be discriminated too (Table 2,
23
Fig.6), the age group II – 2649±31 Ma (n = 12, MSWD = 0.20) and the age group X – 2761±73 Ma
24
(n = 9, MSWD = 0.099). Spots from the fir-tree/patchy zircon domains (i.e. type B) determine the
25
age group II, and spots exclusively from inner zircon cores (type A1) the age group X.
26
Geochemically, zircons of both age groups II and X show similarities: low ∑REE, Y, Th and U 15 Page 15 of 70
1
contents, medium-low Th/U ratios, and negative Eu- and positive Ce-anomalies (Table 3). In the
2
chondrite-normalized pattern, the HREE distributions exhibit duality – either flat pattern or uneven
3
pattern with steep Lu-enrichment characterize zircons of one same age group (Fig. 7). For zircons from the retrogressed eclogitic rock GS-3, three episodes can be discriminated
5
including the afore mentioned age groups I and II, plus the age group III (Table 2, Fig. 6). The age
6
group I is determined by an upper intercept of 2707±31 Ma (n = 10, MSWD = 0.13), and,
7
alternatively, a concordant age of 2694±7.5 Ma (n = 6, MSWD = 0.13) obtained from several spots
8
from the A1 and A2 zircon domains (internal part of type A); the age group II – an upper intercept at
9
2655±46 Ma (n = 9, MSWD = 0.29) – is obtained from spots from the inner domains of type B
10
zircons (being mixed with fir-tree/patchy textures). The age group III – an upper intercept at
11
2671±51 Ma (n = 8, MSWD = 0.54) – is obtained for the remaining spots in the inner part of type B
12
zircons. Geochemically, zircons of all age groups (I, II, III) share general characteristics – low
13
∑REE, Th, U contents, medium Th/U ratios, and negative Eu- and positive Ce-anomalies. For age
14
groups II and III zircons, the HREE distributions can either be flat or steep, with Lu-enrichment in
15
all but age group I zircons, which feature flat HREE distributions only. Ti-in-zircon thermometry
16
does not yield obvious differences among all age group zircons (Table 3).
19
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6.3 Zircon trace element correlation
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Generally, the geochemical features of all analyzed Gridino zircons are quite consistent with
21
the literature data of high-grade metamorphic genesis (Rubatto, 2002; Corfu et al., 2003). All are
22
characterized by low ∑REE contents (less than 100 ppm) with obvious HREE predominance, low Y
23
contents (less than 200 ppm), medium-low Th and U contents (less than 100 ppm), and medium-
24
low Th/U ratios – Th/U = 0.2-0.8 (one exception of Th/U = 0.01 for the 1.91 Ga zircon; Fig. 8). The
25
Th/U ratio is commonly considered as indicator for HP eclogite-facies metamorphism, if it is lower
26
than 0.1; however, it could only be an artifact instead, because, under fluid participation, the 16 Page 16 of 70
1
mobility/solubility of Th and U can be very distinctive and, consequently, their concentrations and
2
the Th/U ratio might very unpredictably (Möller et al., 2002; Hoskin and Schaltegger, 2003).Thus,
3
if fluid is involved, as in the studied circumstance given presence of retrograde amphibole, zircon’s
4
Th/U ratio is not a robust indicator. For magmatic zircons, particular trace element features (correlations) have been well
6
discussed for provenance investigation (Grimes, et al., 2007, 2009); for high-grade metamorphic
7
zircons, similar correlations may apply. For instance, the zircons of age groups I & IV (from
8
eclogite sample GS-1) do share certain common characteristics and somehow differ from other
9
classical eclogite-/granulite-facies metamorphic zircons worldwide (Table 4; Fig. 9), where the
10
Th/U ratio indicator is mainly pronounced. On the Th/Yb vs. Y diagram, the zircons from the
11
Gridino eclogite (this work) usually plot at the same area as the Bohemian eclogite-granulite zircons
12
(Bröcker et al., 2010); besides, on the Th/U vs. Y diagram, they also fall into the similar field,
13
where the Bohemian eclogite-granulite zircons are spread. On the other side, the Triassic HP-UHP
14
eclogite zircons from the Dabie-Sulu orogen, China, (Chen, 2009; Liu et al., 2011) are clearly
15
distinctive on the Th/U vs. Y diagram, as are the Palaeoproterozoic HP granulite zircons from the
16
Hengshan terrane, North China Craton (Zhang, 2013). Apparently, the zircons from the Bohemian
17
eclogite-granulite complex are somehow comparable with the Gridino eclogite zircons, both subject
18
to high-pressure and high-temperature metamorphism.
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In the chondrite-normalized pattern, most zircons from the Belomorian eclogite (sample GS-1)
20
present quite flatt HREE distributions with negative Eu- and positive Ce-anomalies; however, the
21
zircons from the retrogressed eclogite samples (GS-2 & GS-3) possess sort of dual HREE
22
distribution with both flat patterns and steep Lu-enrichment in the same age group. Exception only
23
occurs for age group I zircons, where flat HREE pattern occurs, as in the eclogite sample. Positive
24
Ce-anomaly occurs with medium-low Ce/Ce* values (Ce/Ce* = 1-20) along with negative Eu-
25
anomaly in most cases (Eu/Eu* = 0.4-0.8), although, a very few may show small positive Eu-
26
anomaly with Eu/Eu* = 1.1-2.0 – mainly for the zircons from the retrogressed eclogite samples (age 17 Page 17 of 70
groups II, III and X). A negative Eu-anomaly in zircons can be related to many factors, such as the
2
co-existence with feldspar – a rival for the Eu2+ ion, the geochemical inheritance from the protolith,
3
the whole rock geochemistry (Eu-depletion), and the redox state of rock or melt (Schaltegger et al.,
4
1999; Hoskin and Ireland, 2000; Hoskin and Black, 2000; Rubatto, 2002; Hoskin and Schaltegger,
5
2003; Trail et al., 2012). In our case, there is only a trivial amount of plagioclase intergrowth with
6
amphibole – coronas around garnet – that could hardly have significant impact. The geochemistry
7
of the whole rock, however, does show a negative Eu anomaly (Eu/Eu* = 0.46-0.86), possibly an
8
important factor. Besides, the redox state of the whole rock under eclogite-facies metamorphism
9
may also play a role somehow. To estimate the oxygen fugacity in zircon formation, an empirical
10
algorithm using the Ce content as a proxy is considered (Trail et al., 2012), and an oxygen-fugacity
11
vs. U-Pb age diagram was obtained (Fig. 10). It shows that zircons from the studied Belomorian
12
eclogite and from the Hengshan HP granulite, both formed in Precambrian times, plot in a close
13
range of oxygen fugacities – -20 to -30, whereas the Paleozoic-Mesozoic high-grade metamorphic
14
zircons yield higher oxygen fugacities instead. Whether or not this phenomenon reflects a
15
significant change in Earth’s history, requires a more detailed investigation of a much larger dataset.
18
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6.4 Trace and rare earth element partitioning of zircons, garnet and omphacite
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In the eclogite, omphacite and garnet are the main rock forming minerals, and their co-
20
existence with zircon of geochronological significance can be used to decode dated metamorphic
21
ages. Garnet is an important container for trace elements, especially heavy REE and Y, rivals zircon,
22
which contains significant amounts of middle- to heavy-REEs, Y, U, and Th. In the Gridino eclogite
23
sample GS-1, garnet is mostly unzoned in the major elements (Ca-Mg-Fe), so it is in the REEs.
24
Compared with zircons, the garnets contain slightly less ∑REE and Y in ~ 10-20 ppm differences:
25
∑REEgrt = 16-43 ppm, av. 26 ppm; Ygrt = 28-79 ppm, av. 45 ppm. The chondrite-normalized REE (+
26
Y) patterns of the garnets show unanimous flatten HREE distributions with negative Eu anomaly 18 Page 18 of 70
(av. Eu/Eu* = 0.70; Table 6, Fig. 11). Omphacite hardly contains REEs and Y – ∑REE = 5-8 ppm
2
(av. 6 ppm), Y = 1-3 ppm (av. 2 ppm), and exhibit steep HREE-depleted patterns (Table 6, Fig. 11).
3
Both analyses of the omphacites and the garnets have near-homogenous chemical compositions
4
with little variation, which allows estimation of the trace elements (TE) partitioning coefficient
5
DTEomp/grt (DTEomp/grt = CTEomp/CTEgrt) using their average compositions. In results, the obtained
6
partitioning coefficient trend of the garnet and the omphacite, two key eclogite-facies minerals, is
7
comparable to the literature data of classical eclogites as from Western Alps and others (Rubatto and
8
Hermann, 2003, references therein). Therefore, in the geochemical scope, the analyzed garnets and
9
omphacites can be ascertained as authentic eclogite minerals.
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The trace elements partitioning between garnet and zircon from eclogites have been well
11
discussed in many studies in order to interpret zircon U-Pb ages (e.g. Schaltegger et al., 1999;
12
Hermann, 2002; Rubatto, 2002; Rubatto and Hermann, 2003, 2007). The trace elements distribution
13
coefficient of zircon and garnet (DTEzrn/grt = Czrn/Cgrt, C – concentration) can be calculated for the
14
zircons (age groups I, IV) and garnets from the eclogite sample GS-1, using their average
15
compositions. The obtained distribution coefficients between the zircon and the garnet show a near
16
“balanced” partition for Sm, Eu, Gd, Tb and Dy elements (DTEzrn/grt = ca. 1.0), although most trace
17
elements are preferentially incorporated into zircon (especially Ce), and the value trend is quite
18
comparable with the data of other classical granulite-facies and eclogite-facies rocks on Earth (Fig.
19
12, Rubatto, 2002; Rubatto and Hermann, 2003; Buick et al., 2006; Zhou et al., 2011). The obtained
20
trend of the distribution coefficient zircon to garnet for the Gridino eclogite is quite comparable
21
with data from the Western Alps eclogites, where equilibrium between zircon and garnet is
22
suggested (Rubatto and Hermann, 2003). Consequently, the zircons and garnets from the studied
23
eclogite sample are in equilibrium, corresponding to the eclogite-facies zircon formation (Fig. 12A).
24
Trace element partitioning coefficients between zircon and garnet were determined
25
experimentally at 20 kbar, 800-1000 °C (Rubatto and Hermann, 2007). The DREEzrn/grt trend obtained
26
here plots between the trends derived from the 900 and 950 °C experiments, which might suggest
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an intermediate value temperature that is actually far away from the estimated peak temperature (~
2
700-730 °C) discussed above. Likewise case was also met for the UHP eclogites of Central Dabie
3
(Zhou et al., 2011; Fig. 12B). The reason of the noneffective referencing may concern the host rock,
4
as being mentioned in the experimental work, where starting materials of granitic composition were
5
used, the temperature reference may only be applicable in the case of intermediate-felsic rocks
6
though.
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Mineral inclusions in zircons can provide very important implications on the formation
11
environment of a dated zircon domain. In zircons from the Gridino eclogite and retrogressed
12
eclogite rocks, diverse inclusions have been found in different textural domains (Table 5). They
13
were thoroughly analyzed by IR-Raman spectrometry and electron microprobe analysis – the
14
representative microprobe data are shown in Table 7.
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Garnet inclusions found in the internal zones (A1, B, C domains) of zircons from the eclogite
16
sample GS-1 usually contain 14-15 mol.% Prp and 33-34 mol.% Grs, which may be similar to few
17
matrix garnets (Prp ~ 15-20 mol.%), whereas most of the matrix garnets contain more Prp and less
18
Grs (Fig. 3). On the built PT pseudosection, however, none could be illustrated by isopleths
19
intersection. We consider these garnet inclusions may form in the eclogite-facies metamorphism.
20
Other garnet inclusions found in the fluid-reworked domain D* often contain much less Prp (8-10
21
mol.%, with Grs 30-31 mol.%) and are likely related to post-peak metamorphism (retrogression).
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A phase associated with quartz – likely biotite – was found in zircon’s internal domain A2
23
(zonal type A), where multiple fractures occur. Inheritance from the magmatic protolith for this
24
domain is possible. This phase contains nearly 7 wt.% K2O with FeO, MgO in majority (13.65 wt.%
25
and 10.90 wt.% correspondingly) and perceptible TiO2 (ca. 4 wt.%), although, due to its small size,
26
no good microprobe analyses could be obtained. A sulfide phase comprising Cu-Fe-S with 20 Page 20 of 70
hexagonal shape was found in the internal domain A2 (zonal type A), likely chalcopyrite. In the
2
association, there are also some fine trails of quartz + Ca-carbonate – they could be related to fluid
3
activity (hydrothermal reworking). Rutile, a common eclogite-facies Ti-bearing mineral, was found
4
in the internal parts of zircons (A2 domain). In the zircon domain D, which experienced an obvious
5
overprint by an external fluid, calcite and quartz are very common. Sometimes, garnet, amphibole,
6
biotite (XMg = 0.54), plagioclase (andesine, An = 41-44 mol.%), K-feldspar (Or = 81 mol.%, Ab =
7
19 mol.%), and rutile inclusions also occur in this domain. Inclusions of quartz – plagioclase – K-
8
feldspar assemblages found in the zircons are interpreted as entrapped melt and thus they may
9
indicate that partial melting during high-temperature reworking could be involved in the
10
retrogression, an inference in agreement with the eclogite’s PT path. In a highly luminescent zircon
11
fringe thought to represent a post-eclogite-facies event (retrogression), andesine (An = 42 mol.%)
12
was found.
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The zircons from the retrogressed eclogite samples GS-2 & GS-3 contain more mineral
14
inclusions. The garnet inclusions in the zircons’ inner and compromised (fluid-reworked) zones are
15
compositionally similar with the ones in the bulk rock (XMg = 0.10-0.14, XCa = 0.30-0.34).
16
Clinopyroxene of low jadeite component (diopside) was found mainly in the internal zones of
17
zircons (domains A1, A2, B, C), along with garnet and/or ± plagioclase (andesine, An = 43-46
18
mol.%); all are chemically identical to the rock forming minerals of the retrogressed eclogite. In the
19
internal domain Al of one zircon, where a diopside inclusion was found, a concordant
20
age of 2646±37 Ma was obtained (age group II). A Fe-oxide phase with ∑FeO = 84 wt.% – maybe
21
magnetite – was found in one zircon’s internal domain B. Titanite is a quite common mineral
22
inclusion in all zircon domains and might be produced due to decomposition of rutile under
23
influence of a carbonate-saturated (fluid). In the high-CL fringe classified as domain F, garnet,
24
titanite and rutile inclusions were also found and are related to the latest metamorphic event. In the
25
fluid-reworked D* domain, plagioclase (andesine), quartz, Ca-carbonate, rutile, and titanite
26
inclusions occur; all are thought to be of secondary genesis. Zoisite with low Fe content (∑FeO =
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3.7-3.8 wt.%) was found in association with quartz in one zircon’s inner domain A1 (from sample
2
GS-2) and may suggest a lower grade metamorphism. Combined biotite-muscovite-diopside
3
inclusions (Fig. 13) were found in one zircon’s inner domain A. It appears that biotite was generated
4
from muscovite, which is commonly believed to take place during greenschist-facies metamorphism
5
at 400-450 °C; the assemblage may thus be an early relict. Another biotite-quartz(-chlorite-alike
6
phase) and biotite-diopside (Jd = 6 mol.%) assemblage (Fig. 13) may also occur in the zircons inner
7
domains (A1 and B, sample GS-3), where the biotite is featured by relatively higher TiO2 content
8
(3.1-4.6 wt.%) and magnesium (XMg = 0.54-0.58), which may suggest a higher temperature
9
environment.
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10 7. Discussion
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Representative, well-preserved eclogite and retrogressed eclogitic rock samples from the
14
Stolbikha Island, Gridino eclogite complex, Belomorian province have been studied. The eclogite
15
GS-1 is mainly composed of omphacite and garnet, with subordinate amphibole and quartz. The
16
eclogitic rock samples GS-2 and GS-3, retrogressed equivalents of the pristine eclogite, are mainly
17
made of diopside-plagioclase-symplectites, garnet, amphibole, and quartz. Thermodynamic
18
modeling was performed on the well-preserved eclogite GS-1 to reconstruct the metamorphic PT
19
path. According to the geochemical studies (Slabunov, 2008; Volodichev et al., 2011), trace
20
elements in the Gridino eclogite samples are comparable with the ones of MORB, thus, we may
21
infer that the eclogite was derived from oceanic crust. For the Gridino eclogite and retrogressed
22
eclogite samples, zircon U-Pb dating yielded diverse age episodes, which require thorough
23
interpretation. Geochemical studies of zircons are a very powerful, commonly used instrument for
24
decoding their genesis (e.g. Hoskin and Schaltegger, 2003; Grimes et al., 2007; Shchipansky and
25
Slabunov, 2015); meanwhile, the study of the trace elements partitioning among zircons and other
26
relevant minerals is considered to be a more elaborate approach (e.g. Schaltegger et al., 1999;
27
Rubatto, 2002). The mineralogy of inclusion minerals in zircons can yield extremely significant
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1
clues sometimes (e.g. Hermann et al., 2001) and, besides, the thermodynamic modeling in a ZrO2-
2
bearing system is also proposed in concrete cases (e.g. Zhang, 2013). In what follows, we
3
scrupulously discuss the petrological, geochemical and geochronological data and explain our
4
points of view on the debated Archaean Belomorian eclogite association.
5 7.1 The metamorphic PT path of the eclogite
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The well-preserved eclogite GS-1 contains omphacite with high Jd content and a weak
9
compositional zonation from the core to the rim (Jd, 30 mol.% → 21 mol.%), and garnet with a
10
high Prp content, which is generally compositional homogenous. Most likely, re-equilibration
11
(higher temperature overprint) is – at least partially – responsible. Nevertheless, in individual cases,
12
(weak) prograde zonation of garnet is observed and permits the reconstruction of a prograde section
13
of the eclogite’s metamorphic PT path. Calcic amphibole (edenite, pargasite) is associated with the
14
omphacite-garnet assemblage (interstitial occurrence and epitaxial replacement of omphacite) and
15
either product of (amphibole-)eclogite-facies metamorphism or subsequent retrogression.
16
Thermodynamic modeling does, within errors of the method, coincide with both scenarios.
17
Plagioclase, found exclusively in the plagioclase-amphibole kelyphitic coronas around garnet in a
18
trivial amount, reflects uplift and provides hints on the eclogite’s retrograde evolution. The
19
formation of diopside-plagioclase-symplectites along with garnet, amphibole and quartz
20
characterizes the retrogressed eclogitic rocks. Garnets are pristine, yet they do not show
21
compositional zonation at all – re-equilibration during a high-temperature reworking can account
22
for. Basing on the field correlation, the retrogressed eclogite samples should be the retrogressed
23
equivalents of the well-preserved eclogite.
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A clockwise section of the metamorphic PT path of the studied Gridino eclogite was
25
reconstructed by combining thermodynamic modeling (NCFMASH system) with conventional
26
geothermometry. The result agrees well with many modern-time orogen-type eclogites worldwide. 23 Page 23 of 70
The peak eclogite-facies metamorphism occurs in the pressure-temperature range of 18.5-19.5 kbar,
2
700-730 °C, which corresponds to about 61-65 km subduction-exhumation depth. Consequently, a
3
relatively warm geothermal gradient of 11-12 °C/km is inferred. Retrogression of the eclogite was
4
possibly the result of a heat impact at 15-16 kbar at T > 750 °C; subsequently, both decompression
5
and cooling proceeded through the lower amphibole-facies represented by retrogressed eclogite. In
6
the early report (Volodichev et al., 2004), basing on massive conventional thermometric calculation,
7
a prograde metamorphic path with PT condition intervals of 14.0 – 17.5 kbar and 740 – 865 oC was
8
concluded. In comparison, our estimation is in accord with those data generally but with several
9
adjustments. For the peak conditions, higher pressure could be reached with appropriate
10
temperature ranges; in the retrogression, we suggest a high-temperature reworking process during
11
the decompression that a high-pressure granulite-facies metamorphism could be accounted for;
12
further retrogression (decompression and cooling) can lead to the formation of symplectic
13
retrogressed eclogitic rock in association, that (high-pressure) amphibolite-facies metamorphism
14
can be expected. In addition, our PT condition estimation is quite consistent with the data obtained
15
from other mafic and felsic rocks in the Gridino complex (Dokukina et al., 2009; Konilov and
16
Dokukina, 2011), that may fulfill a complete metamorphic picture of current area. As having been
17
well discussed by Mints and coauthors (2014), the Belomorian eclogite is quite comparable with
18
those of other high-pressure rocks on Earth and present no significant differences in the PT
19
trajectory for Precambrian and Phanerozoic eclogite-bearing complexes. Here we would like to
20
compare the PT paths of the eclogite and blueschist from the Pam Peninsula, NE New Caledonia
21
(Clarke et al., 1997) and the Eastern Blue Ridge, North Carolina, USA (Page et al., 2003) with our
22
data in this work as well (Fig. 14).
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7.2 The time of metamorphism and its geological implication
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Zircon U-Pb dating determines the metamorphic age of the studied Gridino eclogite
2
association. According to the elaborate studies on zircon morphology and geochemistry, we are able
3
to ascertain some dated zircons with particular geological significance. Altogether, six major
4
episodes can be summarized (and grouped) from all dated zircons (Fig. 15). The age groups I and
5
IV – ca. 2.72 Ga and ca. 2.62 Ga correspondingly – were obtained from zircons with high-grade
6
metamorphic morphology from the well-preserved eclogite sample. Geochemically, these zircons
7
are featured by low ΣREE (+Y) contents, flat HREE distribution pattern, and positive Ce- and
8
negative Eu-anomalies, all typical for eclogite-facies genesis. In addition, the trace element
9
correlation studies may show that the dated zircons from Gridino eclogite complex could share a
10
similarity with those from other eclogite- and/or granulite-bearing complexes. The REEs
11
partitioning between zircons and omphacite/garnet implies equilibrium and, consequently, age
12
groups I and IV zircons should both form during eclogite-facies metamorphism. Therefore, we may
13
conclude that both obtained ages – ca. 2.72 and ca. 2.61 – could represent (an) Archaean eclogite-
14
facies metamorphic event(s). Besides, the discriminated algorithm on oxygen fugacity estimation by
15
zircon’s Ce content could also denote a possible likeness of zircons formed in Precambrian. In the
16
earlier work (Volodichev et al., 2004), an age of ca. 2.72 Ga was suggested for the formation of the
17
Gridino eclogite that conforms to our result, while the later age might suggest a repeated eclogite-
18
facies metamorphism (subduction?). According to the known regional geology, the Belomorian
19
orogen was in an early collision stage of evolution (Bibikova et al., 1999; Slabunov, 2005, 2008;
20
Slabunov et al., 2006). During this period, the eclogite-bearing mélange was subject of repeated
21
“renovation” with intense tectonic deformation (Glebovitskii, et al. 2000). In such a scenario, two
22
consecutive (about 100 Ma in-between) eclogite-facies events could possibly have proceeded in
23
principle, besides, a repeated subduction is generally possible (Herwartz et al., 2011). However, no
24
more petrologic-mineralogical nor geochemical evidence could be found (metamorphic generations)
25
that we prefer to hold cautious evaluation on this scenario.
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25 Page 25 of 70
The oldest age of ca. 2.75-2.76 Ga (age group X) was obtained by a small amount of zircons
2
(in their internal textural parts) from the retrogressed eclogite sample GS-2 (Fig. 15), whose genesis
3
cannot be unequivocally determined, although we may propose an inherited magmatic age of the
4
protolith. In the another Belomorian eclogite-bearing complex named after Salma, a similar age of
5
ca. 2.77-2.78 was reported to be the magmatic age of associated granitic rocks (Kaulina et al., 2010;
6
Mints et al., 2010; Li et al. 2013b), thus, at this time, a heat-triggered event (magmatism or high-
7
temperature metamorphism) indeed occurred.
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The other ages, 2.65 Ga (group II) and 2.67 Ga (group III), were obtained from zircons from
9
the retrogressed eclogite samples, which are very alike to the zircons of age group IV. The gap of
10
about 10-20 Ma between these two age groups is insignificant and may be attributed to analytical
11
precision. Besides, some similar ages in the range of ca. 2.61-2.67 Ca were also obtained from
12
individual zircons. The scattering age points, on the other side, may be related to a long residence
13
time of high-grade metamorphic rocks with very slow cooling process (e.g. Ashwal et al., 1999)
14
during exhumation after the collision event, thus, altogether, we suggest that at this time, (a) long-
15
lived (~ 60 Ma) metamorphic event(s) occurred. The mineral inclusion studies revealed associations
16
of diopside, garnet and plagioclase – minerals characteristic for granulite-facies metamorphism – in
17
the zircons’ internal domains. In this period, continental collision proceeded and can be accounted
18
for the high-temperature event(s). As other individually obtained ages in the range of ca. 2.45 Ga,
19
they can be related to the superplume thermal event, which is accounted for the formation of mafic
20
dyke swarms in the Belomorian province during ca. 2.4-2.5 Ga (Stepanova and Stepanov, 2010).
21
Singular age point of ca. 1.91 Ga was obtained in a zircon’s outer fringe domain from the eclogite
22
sample, which is connected to the Lapland-Kola orogeny (Balagansky et al., 2015).
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23
In the early report by Skublov et al. (2011), similar eclogite samples were dated. The author
24
concluded that the eclogite-facies metamorphism occurred at late-Palaeoproterozoic age of ca. 1.90
25
Ga, and magmatic protolith of eclogite was formed at ca. 2.70 Ga. However, in that paper, the dated
26
zircons (sample 102) were clearly zonal with dark core and bright rim texture in the CL imaging, 26 Page 26 of 70
which is not so widely observed in our samples (as mentioned above, we are dealing with zircons of
2
none zonal textures). Thus, it is possible that we are dealing with different eclogite samples (of
3
retrogression?). Furthermore, that conclusion was mainly made on traditional geochemistry of
4
zircons (such as Th/U, HREE distribution indicators, etc), which cannot be depended solely without
5
solid mineralogical-petrologic observations – besides, as being discussed above by us and
6
Shchipansky and Slabunov (2015), trace elements partitioning studies among rock-forming
7
minerals and zircon can be more reliable. Besides, the justification and criticism on subsequently
8
inferred Palaeoproterozoic model (Skublov 2010, 2011), which appears to be invalidate, have been
9
well discussed by Mints et al. (2014). On the other side, as being speculated by Slabunov et al.,
10
(2011), one of the specifics of Gridnio eclogite could be a quick subduction after the protolith
11
formation, i.e. the eclogite-facies metamorphism could occur short after the magmatic event (~ ±
12
20-30 Ma). It may be the reason that both protolithic and metamorphic ages are quite close.
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Another debated on the Archaean age of the Belomorian eclogite arose from alternative
14
geochronological dating (Sm-Nd, Lu-Hf methods) of other eclogite-like outcrops (eclogitized dykes
15
in Gridino area, eclogite rocks in Salma complex) (e.g. Skublov et al., 2010; Berezin et al., 2012;
16
Herwartz et al., 2012). In those works, the dated rock samples were intensely retrogressed eclogitic
17
rock and it remains unclear, how strongly the isotopic systems have been compromised by later
18
“supraposition”. Widely discussed, the closure temperature (Tc) of the Sm-Nd isotopic system
19
depends on a lot of parameters such as the grain size of garnet, the whole rock major element
20
composition, the diffusion rate, the initial temperature and cooling rate. (Burton et al., 1995;
21
Granguly and Tirone, 1998, 1999). Altogether, a wide closure temperature range of 500-850 °C has
22
been suggested (grain sizes below 10 mm, cooling rate less than 5 °C/Ma; e.g. Thöni and Jagoutz,
23
1992; Hensen and Zhou, 1995; Li et al., 2000; Cherniak and Watson, 2001). The situation for the
24
Lu-Hf isotopic system is comparable given that its closure temperature can vary greatly with
25
garnet’s size (Scherer et al., 2000). In contrast, the closure temperature of the U-Pb isotopic system
26
in zircon is usually much higher, > 800-900 °C, and cannot be easily reset in most geological
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27 Page 27 of 70
circumstances – including high-grade metamorphism or anatexis (Burton et al., 1995; Lee et al.,
2
1997; Mezger and Krogstad, 1997). For instance, Sm/Nd ages of the Dabie UHP eclogite are
3
considered to reflect cooling reworking rather than eclogite-facies metamorphism (Liu et al., 2011).
4
Therefore, we believe that for the Belomorian eclogites, zircon U-Pb geochronology should be the
5
appropriate dating method considering their metamorphic PT evolution, only extreme cautious
6
interpretation on obtained ages should be applied.
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7 8. Conclusions
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Complex petrological and geochronological studies of the Gridino boudin-like eclogite
11
associations enable us to draw the following conclusions, which should provide new perspectives
12
on the knowledge of early Precambrian geology.
an
10
(1) The boudin-like eclogite from the Gridino mélange complex is mainly made of an
14
omphacite-garnet (± amphibole, quartz) assemblage, although most eclogite bodies have
15
been subjected to retrogression with symplectite-rich formation, a few of minimal
16
retrogression managed to preserve provide very valuable geological information.
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(2) Thermodynamic modeling and petrological observation permit us to reconstruct the
18
metamorphic evolution of the Gridino boudin-like eclogite that proceeded along a
19
clockwise PT trajectory, which is principally consistent with the previous estimation. Peak
20
pressure-temperature conditions of 18.5 kbar, 710 °C for the (amphibole-)eclogite-facies
21
metamorphism are estimated. Besides, further retrogression was characterized by re-
22
heating that may have reached a temperature above 750 °C;
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(3) The metamorphic PT path of the Gridino boudin-like eclogite is well consistent with other
24
estimations for the Gridino complex in all, and it is quite comparable with the one of most
25
modern high-pressure rocks formed in subduction zones, although a warmer subduction
26
can be speculated for the early Precambrian; 28 Page 28 of 70
(4) Trace elements partitioning studies among zircons and co-existing mineral assemblages
2
confirms their eclogite-facies metamorphism condition equilibrium (paragenesis), and the
3
geochronology of zircon shows that the eclogite-facies metamorphic event in the Gridino
4
complex should occur in Neoarchaean at ca. 2.70 Ga. While the subsequent discrete
5
period of ca. 2.61 – 2.67 Ga may represent a high-temperature overprint when the
6
continental collision was in progress.
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7 Acknowledgements
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We would like to thank Professor Volodichev O.I. from the Geological Institute of the
11
Karelian Research Center, Russian Academy of Sciences (Petrozavodsk) for his assistance in
12
organizing the fieldwork and providing first-hand regional geological data. We also want to thank
13
Professors Mints M.V. and Shchipanskiy A.A. from the Geological Institute of the Russian
14
Academy of Sciences (Moscow) for their consulting on the regional geology. We would like to
15
express our deep gratitude to senior engineers Dr. Su G. and Dr. Ma F. (Peking University) for their
16
assistance in microprobe and LA-ICP-MS analyzing operations. We are also very thankful to our
17
dear colleague Dr. Thomas Bader from Peking University for his assistance in correcting current
18
paper. This study was financially supported by the National Natural Science Foundation of China
19
(grants 41202032, 412111062 and 41311120071).
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9 Figure Captions:
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Fig. 1. (a) Main tectonic units of the Fennoscandian Shield; BP - Belomorian Province; KP - Kola
13
Province (after Balagansky, 2002, 2015; Daly et al., 2006; Slabunov et al., 2006); (b) Schematic
14
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15
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16
the Stolbikha Island in the Gridino eclogite-bearing mélange, Belomorian province (modified after
17
Sibelev et al., 2004). Samples were taken within the dashed square (Li et al., 2013a).
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M
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19
Fig. 2. Petrography of eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino
20
eclogite complex, Belomorian province (plane polarized light). The eclogite is mainly composed of
21
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22
(replacement) amphibole. The retrogressed eclogite is mainly composed of cracked garnet,
23
clinopyroxene-plagioclase-symplectites and retrograde amphibole (replacement). (Grt – garnet,
24
Omp – omphacite, Cpx – clinopyroxene, Amp – amphibole, Pl – plagioclase, Qz – quartz, Ttn –
25
titanite, Rt - rutile).
26 43 Page 43 of 70
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Fig. 3. Mineral classification diagrams showing the molecular compositions (microprobe data) of (a)
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3
(GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino eclogite complex, Belomorian
4
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8
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9
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the amphibole eclogite-facies (grt-omp-amp-qz) or the eclogite-facies (grt-omp-qz; inferred dashed
12
line; path I); (b) thermometric calculations on appropriate garnet-clinopyroxene (Kd = 7.80-8.55)
13
and garnet-amphibole (Kd = 3.24-3.96) assemblages are shown with corresponding max.-min. Kd
14
values, which, along with the isopleths of omphacite (clinopyroxene) were used to constrain the
15
plausible PT ranges (hatched circles). Heating during decompression is suggested (stage M2; two
16
possible retrograde paths II can be inferred). The further retrogression path can be concluded from
17
the associated retrogressed eclogite in the field grt-cpx-amp-pl-qz (path III) (grt-garnet, omp-
18
omphacite, cpx-clinopyroxene, amp-amphibole, pl-plagioclase, opx-orthopyroxene, chl-chlorite, zo-
19
zoisite, qz-quartz).
us
an
M
ed
pt
Ac ce
20
cr
7
21
Fig. 5. Typical CL images of representative zircons from eclogite (GS-1) and retrogressed eclogite
22
(GS-2, GS-3) from the Gridino eclogite complex, Belomorian province. Several textural types are
23
distinguished: type A – zoned zircons with clear inner (A1) and outer (A2) domains (low- and high-
24
CL); type B – fir-tree and/or patchy zircons; type C – homogenous zircons of low-CL or high-CL;
25
type D – variant zircons with fluid-compromised or fragmented appearance. Zircons of “mixed”
44 Page 44 of 70
1
textures can be very common for A, B, and C types. A thin, high-CL outer fringe occurs on almost
2
all zircons. Concordant LA-ICP-MS 206Pb/238U ages are shown with the laser spot circles.
3 4
Fig. 6. Zircon U-Pb concordia diagrams for eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3)
5
from the Gridino eclogite complex, Belomorian province. Representative CL images of each age
6
group’s zircons with laser spots and corresponding
7
sample GS-1, two age groups, I and IV, were determined; for the retrogressed eclogite sample GS-2,
8
two other age groups, II and X, were obtained; and for the retrogressed eclogite GS-3, three age
9
groups, including the age groups I, II and the new obtained age group III were determined.
Pb/238U ages are shown too. For the eclogite
us
cr
ip t
206
10
Fig. 7. Chondrite-normalized REE patterns for zircons of each age group from eclogite (GS-1) and
12
retrogressed eclogite (GS-2, GS-3) from the Gridino eclogite complex, Belomorian province. Age
13
group I and IV zircons are featured by flat HREE distribution, while the remaining age group
14
zircons show both flat (a) and steep Lu-enriched (b) pattern in co-existence.
ed
M
an
11
15
Fig. 8. The Th vs. U diagram reveals a moderate Th/U ratio in the range of 1 and 0.1 for all but one
17
dated zircons from eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino
18
eclogite complex, Belomorian province; the exception (Th/U = 0.01) is the analysis of the outer
19
fringe domain.
Ac ce
20
pt
16
21
Fig. 9. U/Yb vs. Y, Th/U vs. Y, Th vs. Yb and Th/Yb vs. Y discrimination diagrams for zircons from
22
UHP eclogite from the Sulu CCSD MH core (44 analyses, Chen, 2009) and North Dabie
23
granulitized HP-UHP eclogite (18 analyses, Liu et al., 2011), the Hengshan HP granulite (66
24
analyses, Zhang, 2013), Bohemian eclogite-granulite (16 analyses, Bröcker et al., 2010) and
25
Gridino eclogite (of age groups I, IV, this work) (54 analyses). Apparently, the zircons from the
45 Page 45 of 70
1
Bohemian eclogite-granulite complex are somehow comparable with the Gridino eclogite zircons,
2
which are both subject to complex high-pressure and high-temperature metamorphism.
3 Fig. 10. Correlation of zircon U-Pb age and oxygen fugacity. Zircons from early Precambrian high-
5
grade metamorphic complexes (Gridino, this work, and Hengshan, Zhang, 2013) may have similar
6
low oxygen fugacity, compared to other Paleozoic-Mesozoic zircons from the North Dabie
7
granulitized eclogite (Liu et al., 2011), Sulu CCSD MH core eclogite (Chen, 2009) and Bohemian
8
eclogite-granulite (Bröcker et al., 2010).
cr
ip t
4
us
9
Fig. 11. Chondrite-normalized REE + Y patterns of omphacite and garnet form eclogite of the
11
Gridino complex show that omphacite is featured by extreme low REE concentration with HREE
12
depletion, while garnet is enriched in REE with LREE depletion but flat-enriched HREE (+ Y)
13
pattern. The omphacite – garnet trace element partitioning coefficients (DTEomp/grt = CTEomp/CTEgrt)
14
for the Gridino eclogite (in circles) are consistent with reference data for Western Alps eclogites (in
15
squares; Rubatto and Hermann, 2003) and Dora-Maria, Central Dabie, and Liguria, NW Italy
16
(shaded zone; references in Rubatto and Hermann, 2003).
pt
ed
M
an
10
17
Fig. 12. (A) Trace element distribution coefficients (average contents) of age group I (zrn-I) and IV
19
zircon (zrn-IV) to garnet for the Gridino eclogite exhibit a similar trend as Zrn and Grt from the
20
Western Alps eclogite known to be in equilibrium (Rubatto, 2002), an indication for equilibrium of
21
zircon and garnet in the Gridino eclogite; (B) Trace element distribution coefficient of zircon to
22
garnet from granulite-facies rocks from the Limpopo belt (South Africa; Buick et al., 2006),
23
eclogite-facies rocks form the Dabie orogen (China; Zhou et al., 2011), eclogites from the Gridino
24
complex (this work, t.w.) and an experimental dataset (800 °C, 900 °C, 1000 °C; Rubatto and
25
Hermann, 2007).
Ac ce
18
26 46 Page 46 of 70
1
Fig. 13. BSE images of zircons from retrogressed eclogitic rock (sample GS-3), Gridino complex,
2
Belomorian province containing biotite-association inclusions. Their microprobe analyses are
3
shown in Table 7.
4 Fig. 14. Reconstructed metamorphic PT paths of the Gridino boudin-like eclogite (this work), the
6
eclogite-blueschist of the Pam Peninsula, NE New Caledonia (after Clarke et al., 1997), and the
7
Eastern Blue Ridge, North Carolina, USA (after Page et al., 2003) (modified after Mints et al.,
8
2010). (PP - prehnite-pumpellyite; LB - lawsonite blue-schist; GS - green-schist; EB - epidote-blue-
9
schist; EA - epidote-amphibolite; A - amphibolite; GA - garnet amphibolite; PG - pyroxene
cr
us
10
ip t
5
granulite; GG - garnet granulite).
238
an
11
U-206Pb age relative probability for (a) eclogite (sample GS-1) and (b)
Fig. 15. Zircon
13
retrogressed eclogite (sample GS-2) from the Gridino complex of Belomorian province.
M
12
17 18 19 20 21
pt
16
Ac ce
15
ed
14
22 23
47 Page 47 of 70
ip t
Ac c
ep te
d
M
an
us
cr
Table 1. Representative microprobe analyses of rock forming minerals (clinopyroxene, garnet, amphibole, plagioclase) from eclogite (sample GS-1) and retrogressed eclogite (samples GS-2 & GS-3) from the Gridino complex, Belomorian province. Clinopyroxene Garnet GS-1 GS-2 GS-3 Sample GS-1 GS-2 GS-3 Sample 1 2 3 4 1 2 1 2 1 2 3 4 1 2 1 2 N N 55.22 54.41 54.81 54.27 51.21 50.29 51.16 51.50 SiO2 37.57 38.10 38.42 38.22 38.69 38.12 37.48 38.45 SiO2 0.00 0.03 0.22 0.19 0.06 0.27 0.25 0.06 TiO2 0.07 0.01 0.05 0.04 0.00 0.01 0.00 0.00 TiO2 7.19 8.50 7.21 8.09 4.05 5.67 4.75 3.91 21.97 21.24 22.08 21.94 21.71 21.97 21.43 21.76 Al2O3 Al2O3 0.03 0.00 0.03 0.11 0.02 0.03 0.05 0.02 Cr2O3 0.03 0.21 0.00 0.09 0.06 0.06 0.12 0.07 Cr2O3 4.78 5.42 5.25 5.17 9.33 9.04 9.04 8.87 FeO 23.80 24.38 23.28 22.60 22.75 22.35 22.56 22.43 FeO 11.69 10.58 10.92 10.62 12.24 11.45 11.78 12.10 MgO 6.39 6.35 6.40 6.49 4.91 5.66 4.33 4.67 MgO 0.00 0.02 0.04 0.03 0.04 0.06 0.11 0.05 MnO 0.90 0.94 0.66 0.57 0.67 0.57 0.69 0.60 MnO 17.59 16.99 17.26 16.68 22.59 21.93 22.29 22.64 CaO 9.25 9.06 9.95 9.99 11.82 11.32 12.59 12.39 CaO 4.34 4.46 4.09 4.67 0.79 0.79 0.79 0.67 Na2O 0.00 0.03 0.00 0.00 0.00 0.02 0.00 0.00 Na2O 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 K2O K2 O 100.86 100.39 99.82 99.84 100.33 99.53 100.21 99.82 Total 99.98 100.32 100.84 99.95 100.61 100.08 99.19 100.36 Total p.f.u. 1.97 1.95 1.98 1.95 1.90 1.88 1.90 1.92 Si 2.90 2.94 2.94 2.94 2.98 2.94 2.93 2.97 Si 0.00 0.00 0.01 0.01 0.00 0.01 0.01 0.00 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ti 0.30 0.36 0.31 0.34 0.18 0.25 0.21 0.17 Al 2.00 1.93 1.99 1.99 1.97 1.99 1.98 1.98 Al 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.01 0.00 0.01 0.00 0.00 0.01 0.00 Cr 3+ 3+ 0.07 0.05 0.00 0.06 0.08 0.03 0.03 0.03 Fe 0.19 0.18 0.13 0.11 0.06 0.12 0.14 0.08 Fe 0.08 0.12 0.16 0.09 0.21 0.26 0.25 0.25 Fe2+ 1.35 1.40 1.36 1.34 1.40 1.32 1.33 1.37 Fe2+ 0.62 0.57 0.59 0.57 0.68 0.64 0.65 0.67 Mg 0.74 0.73 0.73 0.75 0.56 0.65 0.51 0.54 Mg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mn 0.06 0.06 0.04 0.04 0.04 0.04 0.05 0.04 Mn 0.67 0.65 0.67 0.64 0.90 0.88 0.89 0.91 Ca 0.77 0.75 0.82 0.82 0.98 0.94 1.06 1.03 Ca 0.30 0.31 0.29 0.33 0.06 0.06 0.06 0.05 Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 K K endmembers 70 68 71 67 94 94 94 95 46 47 46 46 47 45 45 46 WEF Alm 24 27 29 28 3 5 5 4 9 9 6 5 3 6 7 4 Jd And 6 4 0 6 3 1 1 1 17 16 21 22 29 26 29 31 Ae Grs 25 25 25 25 19 22 17 18 Prp 2 2 1 1 1 1 2 1 Sps 0 1 0 0 0 0 0 0 Uv 48 Page 48 of 70
ip t cr 4 42.94 0.36 15.28 0.00 12.45 12.65 0.09 10.93 3.47 0.19 98.37
GS-2 1 2 42.84 41.84 1.19 1.46 13.50 14.02 0.06 0.25 15.29 16.05 10.67 9.79 0.10 0.06 11.04 11.73 1.62 1.63 0.93 1.07 97.23 97.89
GS-3 1 2 42.61 43.14 1.17 0.96 13.98 13.53 0.15 0.09 15.60 15.99 10.33 10.23 0.07 0.11 11.90 11.84 1.58 1.47 1.12 1.02 98.51 98.39
6.61 0.08 2.08 0.02 0.00 1.49 3.08 0.02 1.64 0.84 0.07
6.30 0.04 2.64 0.00 0.00 1.53 2.77 0.01 1.72 0.99 0.04
6.41 0.13 2.38 0.01 0.00 1.91 2.38 0.01 1.77 0.47 0.18
6.31 0.13 2.44 0.02 0.00 1.93 2.28 0.01 1.89 0.45 0.21
us
GS-1 2 3 45.26 43.18 0.71 0.89 12.07 14.28 0.13 0.15 12.19 12.33 14.14 12.99 0.14 0.08 10.50 10.74 2.97 3.36 0.38 0.23 98.49 98.22
2.80 0.00 1.18 0.00 0.01 0.00 0.00 0.20 0.84 0.01
Ab An Or
79 21 0
80 19 1
2.63 0.00 1.36 0.00 0.00 0.00 0.00 0.37 0.65 0.01
2.55 0.00 1.45 0.00 0.00 0.00 0.00 0.45 0.57 0.01
2.59 0.00 1.40 0.00 0.00 0.00 0.00 0.42 0.57 0.01
2.51 0.00 1.48 0.00 0.00 0.00 0.00 0.48 0.52 0.01
63 36 1
56 44 1
57 42 1
51 48 1
Si Ti Al Cr Fe3+ Fe2+ Mg Mn Ca Na K
ep te
2.76 0.00 1.21 0.00 0.01 0.00 0.00 0.22 0.85 0.00
Ac c
Si Ti Al Cr Fe3+ Fe2+ Mg Mn Ca Na K
d
M
an
Plagioclase Amphibole GS-1 GS-2 GS-3 Sample Sample 1 2 1 2 1 2 1 N N 62.33 62.84 59.03 57.00 57.45 55.75 SiO2 42.83 SiO2 0.01 0.01 0.00 0.01 0.00 0.00 TiO2 1.68 TiO2 23.10 22.56 26.01 27.57 26.41 27.97 Al2O3 13.05 Al2O3 0.21 Cr2O3 Cr2O3 0.35 0.21 0.02 0.02 0.02 0.05 FeO 13.02 FeO 0.01 0.00 0.00 0.00 0.01 0.00 MgO 12.89 MgO 0.02 0.02 0.01 0.01 0.02 0.03 MnO 0.07 MnO 4.64 4.17 7.78 9.31 8.71 10.02 CaO 10.98 CaO 9.90 9.70 7.50 6.54 6.47 5.94 Na2O 3.48 Na2O 0.05 0.08 0.17 0.10 0.17 0.19 K2O 0.20 K2O 100.42 99.60 100.52 100.55 99.25 99.95 Total 98.41 Total 6.33 0.19 2.27 0.03 0.00 1.61 2.84 0.01 1.74 1.00 0.04
6.35 0.10 2.47 0.02 0.00 1.52 2.85 0.01 1.69 0.96 0.04
6.26 0.16 2.47 0.03 0.00 2.01 2.18 0.01 1.88 0.47 0.20
6.39 0.11 2.36 0.01 0.02 1.96 2.26 0.01 1.88 0.42 0.19
49 Page 49 of 70
ip t cr
Th/U
Isotopic ratios
13 11 75 112 101 48 22
40 2 88 2 28 10 19 10 60 58 9 8 12 36 60 3
3 4 33 14 31 10 9
0.31 0.20 0.45 0.25 0.47 0.33 0.47 0.44 0.36 0.35 0.35 0.34 0.26 0.47 0.20 0.29
Pb/ Pb 0.16349 0.18389 0.17603 0.18448 0.17975 0.15077 0.17965 0.18274 0.18305 0.17519 0.18549 0.18521 0.18271 0.18379 0.18361 0.18729
±1σ 0.00224 0.005 0.00225 0.0045 0.00265 0.00306 0.00283 0.00329 0.0023 0.00228 0.00281 0.00334 0.00289 0.00256 0.00224 0.00495
207
235
Pb/ U 9.44321 12.7892 11.06214 13.0977 12.34394 8.06743 11.83314 12.04471 13.01616 11.67632 13.35987 13.35851 12.93523 13.09326 13.05265 13.50255
±1σ 0.1267 0.36226 0.13684 0.33189 0.18045 0.1637 0.18628 0.22032 0.15931 0.14848 0.20346 0.24719 0.20644 0.1807 0.15427 0.37426
0.26 0.35 0.44 0.13 0.30 0.20 0.40
0.16765 0.16674 0.16961 0.17083 0.17039 0.17281 0.17257
0.00355 0.00601 0.00247 0.00234 0.00283 0.00386 0.00275
11.03 10.95561 11.23367 11.49334 11.4043 11.53554 11.73451
0.23884 0.41017 0.16293 0.15441 0.1905 0.20752 0.18755
an
130 11 194 10 58 32 41 23 166 167 27 24 47 77 296 10
206
d
207
M
Th(ppm)
ep te
Age group I GS-1-030 GS-1-037 GS-1-035 GS-1-048 GS-1-039 GS-1-028 GS-1-036 GS-1-012 GS-1-016 GS-1-022 GS-1-002 GS-1-004 GS-1-020 GS-1-021 GS-1-014 GS-1-023 Age group IV GS-1-005 GS-1-029 GS-1-007 GS-1-044 GS-1-046 GS-1-008 GS-1-018
U(ppm)
Ac c
Sample spot
us
Table 2. U-Pb isotope data of zircons from eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) of the Gridino complex, Belomorian province (LA-ICP-MS data): Age (Ma) 206
238
Pb/ U 0.41905 0.5046 0.45594 0.5151 0.49823 0.3882 0.47789 0.47819 0.5159 0.48356 0.52255 0.52328 0.51365 0.51687 0.51577 0.52305
±1σ 0.00455 0.01057 0.00467 0.00975 0.0058 0.00565 0.00592 0.00679 0.00534 0.00511 0.00646 0.00763 0.00654 0.00585 0.00521 0.01089
t207/235 2382 2664 2528 2687 2631 2239 2591 2608 2681 2579 2705 2705 2675 2686 2683 2715
±1σ 12 27 12 24 14 18 15 17 12 12 14 17 15 13 11 26
t206/238 2256 2634 2422 2678 2606 2114 2518 2519 2682 2543 2710 2713 2672 2686 2681 2712
±1σ 21 45 21 41 25 26 26 30 23 22 27 32 28 25 22 46
0.47733 0.47669 0.48052 0.48814 0.4856 0.48412 0.49334
0.00762 0.01249 0.00563 0.00528 0.00626 0.0064 0.00619
2526 2519 2543 2564 2557 2567 2583
20 35 14 13 16 17 15
2516 2513 2530 2563 2552 2545 2585
33 55 25 23 27 28 27
50 Page 50 of 70
0.00227 0.00269 0.00187 0.00224 0.00248 0.0052 0.0027
84 90 79 14 26 1 6 19 18 16 4 4
50 52 40 3 11 0 3 4 5 4 1 1
0.60 0.58 0.51 0.24 0.43 0.51 0.49 0.20 0.26 0.26 0.32 0.32
0.1789 0.18251 0.1788 0.18179 0.18161 0.18928 0.18017 0.17999 0.18337 0.17946 0.17925 0.18101
0.00271 0.00272 0.0035 0.006 0.00635 0.02091 0.00549 0.00504 0.00635 0.00418 0.00525 0.00739
27 17 6 6 30 14 66 79 103
9 5 2 3 10 3 33 40 69
0.32 0.30 0.37 0.49 0.34 0.24 0.50 0.51 0.67
0.15809 0.19165 0.17402 0.18017 0.19236 0.18179 0.19041 0.1788 0.13988
3
0.46
0.18455
9.89797 5.64893 7.6553 11.29756 12.13787 9.52468 12.12883
ip t
0.16306 0.11853 0.14342 0.17091 0.17629 0.15609 0.1762
6
cr
0.57 0.01 0.46 0.33 0.76 0.60 0.55
0.13564 0.12733 0.09685 0.14492 0.16772 0.32419 0.18526
0.4404 0.34576 0.38727 0.47958 0.49955 0.44272 0.49943
0.00492 0.00509 0.00405 0.00515 0.00552 0.01002 0.006
2425 1924 2191 2548 2615 2390 2614
13 19 11 12 13 31 14
2352 1914 2110 2525 2612 2363 2611
22 24 19 22 24 45 26
11.46454 11.92726 11.92561 12.47377 11.86134 8.56169 11.87969 12.34649 11.13565 12.55411 12.48617 12.74803
0.17002 0.17365 0.23654 0.42916 0.35398 0.94025 0.37348 0.35907 0.3345 0.30026 0.37771 0.54698
0.46495 0.47416 0.48393 0.49783 0.47368 0.32817 0.47839 0.49768 0.44044 0.50753 0.50539 0.51098
0.00514 0.00517 0.00709 0.0123 0.00865 0.02252 0.01051 0.01052 0.00759 0.00874 0.01066 0.01557
2562 2599 2599 2641 2594 2292 2595 2631 2535 2647 2642 2661
14 14 19 32 28 100 29 27 28 22 28 40
2461 2502 2544 2604 2500 1830 2520 2604 2353 2646 2637 2661
23 23 31 53 38 109 46 45 34 37 46 66
0.00428 0.00444 0.00582 0.00549 0.00327 0.006 0.00281 0.0035 0.00227
9.39537 14.11785 11.40238 11.87969 14.16082 12.47377 13.96238 11.92561 7.52064
0.25872 0.33932 0.39477 0.37348 0.24216 0.42916 0.20207 0.23654 0.11948
0.4312 0.53447 0.47539 0.47839 0.53412 0.49783 0.53202 0.48393 0.39008
0.00819 0.0096 0.0116 0.01051 0.00689 0.0123 0.00588 0.00709 0.00443
2377 2758 2557 2595 2761 2641 2747 2599 2175
25 23 32 29 16 32 14 19 14
2311 2760 2507 2520 2759 2604 2750 2544 2123
37 40 51 46 29 53 25 31 21
0.00527
13.19168
0.39319
0.51862
0.01128
2693
28
2693
48
us
33 0 81 56 71 3 29
ep te
d
M
an
59 33 175 170 93 5 52
Ac c
GS-1-006 GS-1-033 GS-1-011 GS-1-003 GS-1-047 GS-1-010 GS-1-045 Age group II GS-2-018 GS-2-016 GS-2-009 GS-2-015 GS-2-004 GS-2-001 GS-2-017 GS-2-019 GS-2-012 GS-2-007 GS-2-010 GS-2-025 Age group X GS-2-027 GS-2-003 GS-2-020 GS-2-017 GS-2-011 GS-2-015 GS-2-008 GS-2-009 GS-2-024 Age group I GS-3-012
51 Page 51 of 70
0.18617 0.18605 0.18525 0.18212 0.18116 0.1837 0.18483 0.18596 0.18527
0.00419 0.00408 0.00297 0.00276 0.00288 0.00318 0.00264 0.00299 0.00342
13.3619 13.28523 13.23547 12.05354 11.53068 12.01233 12.17345 12.96234 13.22893
9.75 17.44 8.69 7.81 28.85 78.96 14.59 6.97 40.65
2.74 8.31 2.36 2.7 8.32 36.38 5.73 2.82 15.33
0.28 0.48 0.27 0.35 0.29 0.46 0.39 0.40 0.38
0.18118 0.17469 0.17718 0.17987 0.17623 0.17806 0.17353 0.18026 0.17958
0.00378 0.00432 0.00741 0.00625 0.00379 0.00286 0.00341 0.00653 0.00318
56.97 22.01 80.82 59.58 83.81 11.51 32.36 18.48
21.46 6.67 46.29 23.05 40.99 3.86 9.84 4.44
0.38 0.30 0.57 0.39 0.49 0.34 0.30 0.24
0.18188 0.181 0.18303 0.1371 0.16436 0.16006 0.16532 0.1694
0.00294 0.00351 0.00342 0.00224 0.00234 0.00354 0.00257 0.00325
0.3104 0.30156 0.21335 0.18122 0.1822 0.20907 0.16967 0.20886 0.24902
0.52073 0.51808 0.51835 0.48018 0.4618 0.47443 0.47785 0.50573 0.51807
0.00894 0.00894 0.00652 0.00567 0.0056 0.00633 0.00519 0.00634 0.00744
2706 2700 2697 2609 2567 2605 2618 2677 2696
22 21 15 14 15 16 13 15 18
2702 2691 2692 2528 2448 2503 2518 2638 2691
38 38 28 25 25 28 23 27 32
11.40496 11.01598 11.8614 12.57931 10.913 11.69588 10.32085 12.61512 12.56464
0.24149 0.27823 0.51755 0.45709 0.19199 0.18648 0.20423 0.47904 0.22434
0.45672 0.45751 0.48572 0.5074 0.44913 0.47657 0.43151 0.50775 0.50764
0.00716 0.00832 0.0149 0.01313 0.00554 0.00579 0.00629 0.01385 0.00682
2557 2524 2594 2649 2516 2580 2464 2651 2648
20 24 41 34 16 15 18 36 17
2425 2429 2552 2646 2391 2512 2313 2647 2647
32 37 65 56 25 25 28 59 29
12.82978 12.7302 12.94742 6.58223 9.5389 9.61945 10.01354 10.66537
0.20762 0.25273 0.24591 0.10505 0.13268 0.21442 0.15391 0.20554
0.51178 0.51029 0.51324 0.34834 0.42107 0.43605 0.43947 0.4568
0.0064 0.0077 0.00736 0.00406 0.00455 0.00679 0.0052 0.00644
2667 2660 2676 2057 2391 2399 2436 2494
15 19 18 14 13 21 14 18
2664 2658 2670 1927 2265 2333 2348 2425
27 33 31 19 21 30 23 28
an
M
d
ep te
ip t
0.20 0.30 0.31 0.32 0.32 0.35 0.55 0.38 0.30
cr
2 6 11 18 15 14 55 12 15
us
11 21 34 55 49 41 101 31 51
Ac c
GS-3-033 GS-3-001 GS-3-008 GS-3-005 GS-3-020 GS-3-013 GS-3-025 GS-3-010 GS-3-017 Age group II GS-3-003 GS-3-022 GS-3-023 GS-3-027 GS-3-002 GS-3-026 GS-3-007 GS-3-021 GS-3-032 Age group III GS-3-015 GS-3-006 GS-3-031 GS-3-019 GS-3-018 GS-3-029 GS-3-004 GS-3-028
52 Page 52 of 70
ip t cr
Dy
Ho
Er
Tm
Yb
Lu
Ti
δEu
δCe
Ti-in-zrn, oC
0.38 0.42 1.18 0.82 0.95 1.91 1.88 1.42 0.67 0.51 0.93 1.34 0.39 1.79 1.59 0.64
3.82 4.11 11.91 7.61 8.67 18.48 17.37 14.02 6.09 3.78 8.14 12.06 3.66 15.85 13.69 4.76
1.20 1.32 3.46 2.26 2.56 5.29 4.75 4.11 1.60 1.05 2.27 3.43 0.98 4.14 3.62 1.11
4.54 4.35 11.36 7.26 8.54 17.98 15.11 13.10 5.11 3.59 7.10 10.66 3.04 12.91 11.47 3.27
0.87 0.66 1.87 1.26 1.49 2.89 2.33 2.03 0.82 0.54 1.08 1.73 0.44 1.92 1.72 0.43
7.26 6.34 14.59 9.71 12.05 22.73 17.75 14.94 6.24 4.18 8.81 13.56 3.45 13.82 13.91 3.61
1.38 0.97 2.36 1.60 1.96 3.65 2.80 2.23 1.02 0.67 1.38 2.17 0.64 2.26 2.25 0.57
2.79 3.74 39.23 2.77 4.45 5.64 3.37 3.26 3.99 2.47 4.29 4.80 3.94 3.28 5.68 3.21
0.67 0.51 0.54 0.45 0.50 0.40 0.50 0.51 0.34 0.88 0.39 0.53 0.39 0.45 0.51 0.66
9.69 4.60 11.20 7.46 6.67 5.99 5.91 6.62 10.01 10.78 13.89 7.52 17.47 9.14 6.67 4.69
639 660 876 638 673 692 652 650 665 630 671 679 664 650 692 649
0.39 0.69 1.21 0.42 0.46 0.71 1.06 1.35 0.23 1.30 0.39 2.05
4.25 8.16 12.44 4.16 4.69 6.50 9.54 11.80 1.83 12.04 3.53 18.44
1.27 2.64 3.86 1.23 1.28 1.84 2.55 3.31 0.41 3.07 0.97 4.97
4.70 9.08 12.69 4.41 4.64 6.67 8.04 10.47 1.33 9.64 2.69 14.63
0.99 1.50 2.12 0.77 0.80 1.10 1.25 1.74 0.23 1.52 0.42 2.33
9.03 11.58 16.98 6.20 6.42 8.19 10.06 13.74 1.85 11.20 3.37 16.23
1.54 2.01 2.72 1.03 1.11 1.46 1.64 2.31 0.40 1.71 0.55 2.54
4.28 3.72 3.85 5.22 4.10 4.38 2.55 7.02 1.89 5.67 3.64 5.78
1.44 0.47 0.48 0.45 0.81 0.54 0.64 0.86 0.99 0.52 0.44 0.46
6.98 2.79 6.59 6.30 6.28 7.88 3.75 8.18 2.42 15.32 13.05 9.02
670 660 662 686 667 672 632 710 612 692 658 694
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
36.94 38.96 104.14 66.93 75.19 157.15 141.73 124.80 49.62 31.93 68.33 100.09 28.31 118.66 104.81 34.35
0.03 0.04 0.04 0.02 0.03 0.04 0.03 0.02 0.03 0.02 0.00 0.02 0.00 0.03 0.03 0.03
0.81 0.50 1.31 0.61 0.72 0.78 0.75 0.70 1.01 1.22 0.79 1.00 0.70 0.87 1.23 0.69
0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.02 0.02 0.03 0.02 0.03 0.01 0.02 0.04 0.03
0.20 0.13 0.26 0.15 0.12 0.38 0.23 0.35 0.12 0.25 0.17 0.42 0.12 0.24 0.53 0.47
0.15 0.17 0.57 0.35 0.52 1.10 0.82 0.92 0.25 0.31 0.38 0.95 0.29 0.80 1.34 0.72
0.13 0.10 0.32 0.19 0.26 0.44 0.47 0.42 0.12 0.23 0.19 0.47 0.11 0.46 0.58 0.39
1.21 1.22 3.45 2.48 3.06 6.22 5.55 4.59 2.18 1.47 3.03 4.98 1.49 6.17 6.26 3.22
40.67 79.94 118.72 39.29 41.77 58.11 80.42 95.00 13.23 96.26 28.15 138.24
0.03 0.03 0.14 0.04 0.03 0.05 0.04 0.06 0.04 0.04 0.02 0.03
0.87 0.41 1.76 0.71 0.92 1.29 0.88 1.96 0.37 1.94 1.18 1.03
0.03 0.03 0.03 0.02 0.04 0.03 0.06 0.05 0.03 0.02 0.02 0.02
0.30 0.17 0.19 0.18 0.31 0.29 0.58 0.55 0.38 0.32 0.19 0.28
0.45 0.28 0.49 0.28 0.32 0.40 0.85 1.00 0.20 0.73 0.30 1.10
M
d
ep te 0.34 0.15 0.28 0.12 0.25 0.25 0.41 0.76 0.16 0.38 0.11 0.56
an
Y
Ac c
Sample spot Age group I GS-1-030 GS-1-037 GS-1-035 GS-1-048 GS-1-039 GS-1-028 GS-1-036 GS-1-012 GS-1-016 GS-1-022 GS-1-002 GS-1-004 GS-1-020 GS-1-021 GS-1-014 GS-1-023 Age group IV GS-1-005 GS-1-029 GS-1-007 GS-1-044 GS-1-046 GS-1-008 GS-1-018 GS-1-006 GS-1-033 GS-1-011 GS-1-003 GS-1-047
1.07 1.86 3.38 1.54 1.71 2.72 3.38 4.88 0.89 4.16 1.35 7.19
us
Table 3. Geochemical features of zircons of different age groups of eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino complex, Belomorian province (LA-ICP-MS data):
53 Page 53 of 70
ip t
0.03 0.02
0.86 1.35
0.02 0.02
0.34 0.78
0.38 2.08
0.17 0.85
1.81 9.68
0.44 2.11
3.09 12.85
0.67 2.44
98.40 78.70 73.10 64.53 135.04 24.63 35.14 39.92 57.83 60.05 62.27 99.99
0.03 0.02 0.03 0.03 0.11 0.04 0.03 0.03 0.14 0.04 0.03 0.02
0.83 0.80 0.93 0.65 0.83 0.08 0.23 0.46 0.87 0.82 0.23 0.26
0.03 0.02 0.02 0.02 0.08 0.03 0.02 0.02 0.05 0.05 0.02 0.00
0.21 0.32 0.32 0.15 0.76 0.19 0.13 0.13 0.43 0.21 0.14 0.05
0.72 0.65 0.63 0.28 1.35 0.17 0.29 0.17 0.42 0.30 0.25 0.27
0.27 0.40 0.33 0.15 0.44 0.08 0.12 0.10 0.30 0.30 0.12 0.09
4.70 3.98 3.51 2.26 5.41 0.96 1.29 1.06 1.72 1.48 1.16 1.79
1.36 1.16 1.01 0.75 1.61 0.34 0.36 0.40 0.54 0.51 0.42 0.69
12.74 9.47 9.07 7.01 15.60 3.11 3.92 4.18 4.84 5.96 5.24 8.46
3.18 2.61 2.38 2.17 4.15 0.69 1.10 1.27 1.72 1.80 1.98 3.05
10.02 7.88 7.46 6.52 14.64 2.43 4.41 4.28 6.93 7.34 7.61 13.18
110.56 120.66 54.63 35.14 79.16 64.53 67.48 73.10 88.29
0.02 0.01 0.02 0.03 0.02 0.03 0.06 0.03 0.01
0.49 0.39 0.24 0.23 0.72 0.65 1.19 0.93 0.80
0.00 0.02 0.02 0.02 0.02 0.02 0.07 0.02 0.01
0.00 0.18 0.10 0.13 0.37 0.15 0.83 0.32 0.22
0.29 0.34 0.17 0.29 0.54 0.28 1.04 0.63 0.96
0.18 0.17 0.18 0.12 0.17 0.15 0.36 0.33 0.54
1.63 1.93 1.52 1.29 2.42 2.26 3.36 3.51 5.15
0.63 0.75 0.44 0.36 0.78 0.75 0.90 1.01 1.40
8.10 9.43 5.29 3.92 8.03 7.01 7.98 9.07 11.96
3.52 3.71 1.71 1.10 2.46 2.17 2.13 2.38 3.03
84.91 41.11 103.40 87.95 38.64 32.98 101.61 95.22 189.92 33.83
0.03 0.03 0.02 0.03 0.03 0.04 0.08 0.03 0.03 0.13
0.28 0.50 0.65 0.65 0.79 0.58 1.47 0.99 0.99 1.35
0.01 0.03 0.02 0.02 0.03 0.03 0.05 0.02 0.05 0.10
0.16 0.24 0.16 0.15 0.15 0.19 0.97 0.22 1.02 0.94
0.27 0.48 0.46 0.37 0.33 0.20 0.97 0.86 2.22 0.97
2.21 1.13 3.05 2.47 1.60 1.17 4.12 5.33 9.76 2.82
0.74 0.43 0.99 0.87 0.44 0.42 1.20 1.42 2.80 0.60
8.15 4.04 10.74 9.25 4.29 3.70 11.08 12.17 25.69 4.49
2.66 1.29 3.42 2.84 1.24 1.03 3.19 3.19 6.67 1.07
1.62 4.75
0.25 0.53
4.60 4.73
0.51 0.48
8.59 12.66
676 678
1.49 1.27 1.19 0.98 2.39 0.43 0.74 0.72 1.27 1.44 1.51 2.63
11.32 9.32 9.25 7.76 20.29 3.69 6.56 5.95 11.46 11.84 13.07 24.58
1.66 1.54 1.46 1.22 3.21 0.65 1.19 0.99 2.32 2.37 2.37 4.41
4.13 4.04 3.47 4.01 6.84 1.78 2.40 3.72 156.53 5.09 4.16 3.37
0.34 0.59 0.54 0.39 0.43 0.50 0.49 0.54 0.94 1.14 0.54 0.31
6.34 7.55 9.41 7.22 2.07 0.58 2.27 4.99 2.62 4.23 2.62 10.16
668 666 655 665 708 608 628 660 684 668 652
17.06 17.39 6.45 4.41 9.06 6.52 6.86 7.46 8.95
4.05 3.72 1.11 0.74 1.65 0.98 1.10 1.19 1.33
45.63 36.95 9.73 6.56 13.22 7.76 8.89 9.25 10.62
9.99 7.66 1.84 1.19 2.18 1.22 1.49 1.46 1.67
4.20 3.93 2.80 2.40 4.57 4.01 13.62 3.47 6.82
0.64 0.49 0.72 0.49 0.38 0.39 0.54 0.54 0.59
17.60 5.64 2.93 2.27 7.25 7.22 4.01 9.41 21.41
669 664 639 628 675 665 768 655 707
10.11 4.64 12.56 10.05 4.14 3.39 10.13 9.87 18.78 2.65
1.83 0.78 2.06 1.67 0.75 0.59 1.59 1.44 2.60 0.43
16.32 6.08 17.97 14.07 7.28 5.10 12.15 10.69 17.95 2.73
2.87 1.05 2.74 2.17 1.11 0.73 1.88 1.76 2.39 0.43
3.13 5.01 4.36 4.87 5.72 3.45 5.75 3.41 4.71 10.94
0.34 0.57 0.61 0.53 0.56 0.68 0.54 0.44 0.39 0.57
3.72 3.82 8.21 6.74 6.36 4.17 5.68 9.33 5.21 2.65
647 683 672 680 693 654 693 653 678 748
us
an
M
d
ep te 0.12 0.14 0.32 0.22 0.16 0.14 0.41 0.40 0.70 0.33
1.63 5.63
0.24 0.71
cr
20.11 73.13
Ac c
GS-1-010 GS-1-045 Age group II GS-2-018 GS-2-016 GS-2-009 GS-2-015 GS-2-004 GS-2-001 GS-2-017 GS-2-019 GS-2-012 GS-2-007 GS-2-010 GS-2-025 Age group X GS-2-027 GS-2-003 GS-2-020 GS-2-017 GS-2-011 GS-2-015 GS-2-008 GS-2-009 GS-2-024 Age group I GS-3-012 GS-3-033 GS-3-001 GS-3-008 GS-3-005 GS-3-020 GS-3-013 GS-3-025 GS-3-010 GS-3-017 Age group II
54 Page 54 of 70
0.19 1.13 0.48 0.47 0.32 0.44 0.85 0.35 0.78
0.07 0.50 0.31 0.26 0.18 0.15 0.92 0.14 0.34
1.24 4.52 1.95 2.49 2.40 3.30 2.07 2.90 3.44
70.28 105.33 78.08 41.04 81.71 29.75 85.80 65.30
0.03 0.03 0.02 0.06 0.04 0.06 0.03 0.02
1.12 1.05 1.12 0.88 1.31 0.43 0.81 0.32
0.07 0.03 0.03 0.07 0.12 0.03 0.02 0.02
0.45 0.49 0.53 0.75 0.70 0.15 0.29 0.06
0.70 1.19 0.83 0.68 1.25 0.13 0.38 0.17
0.34 0.50 0.35 0.21 0.42 0.08 0.29 0.09
2.53 5.47 4.52 1.88 4.10 0.78 2.24 0.83
0.57 1.53 0.52 0.80 0.93 1.24 0.53 0.87 0.87 0.80 1.68 1.18 0.53 1.12 0.28 0.79 0.35
3.29 6.94 2.77 3.61 3.20 4.90 1.60 2.68 2.38
15.99 30.98 12.52 14.16 11.89 17.04 5.91 9.52 8.53
3.58 6.56 2.66 2.68 2.19 2.88 1.17 1.54 1.32
36.70 62.65 24.94 23.66 18.27 22.46 10.72 12.43 11.44
7.14 12.92 4.99 4.25 2.92 3.69 1.95 2.12 1.82
3.21 3.27 4.55 3.13 5.59 3.50 99.60 3.48 11.96
0.34 0.58 0.83 0.59 0.45 0.27 2.02 0.29 0.53
4.65 2.19 3.39 5.61 6.86 7.11 4.12 5.14 4.47
649 650 675 647 691 655 655 756
7.90 14.05 9.77 4.58 9.29 2.54 8.67 4.58
2.23 3.54 2.45 1.24 2.49 0.82 2.88 1.99
7.21 10.69 7.79 3.91 8.04 3.58 10.79 9.97
1.08 1.65 1.24 0.67 1.34 0.80 1.97 2.46
8.38 12.88 9.99 5.09 9.56 8.09 16.79 24.49
1.23 1.85 1.52 0.92 1.61 1.76 2.84 5.41
3.52 4.48 4.65 6.55 6.75 3.79 5.71 2.73
0.69 0.50 0.44 0.53 0.52 0.56 0.76 0.58
4.49 7.47 9.77 3.11 3.15 2.42 7.29 3.55
656 674 677 704 706 661 693 637
ep te
d
7.42 17.89 7.30 9.87 9.31 14.72 5.18 8.64 8.19
ip t
0.15 1.23 0.24 0.27 0.19 0.12 0.89 0.18 0.65
cr
0.02 0.10 0.04 0.02 0.02 0.03 0.11 0.02 0.05
us
0.38 0.86 0.60 0.66 0.60 0.83 1.89 0.39 0.94
an
0.02 0.07 0.04 0.04 0.03 0.03 0.08 0.02 0.05
M
101.65 214.11 87.22 104.62 95.13 143.35 51.58 82.65 75.95
Ac c
GS-3-003 GS-3-022 GS-3-023 GS-3-027 GS-3-002 GS-3-026 GS-3-007 GS-3-021 GS-3-032 Age group III GS-3-015 GS-3-006 GS-3-031 GS-3-019 GS-3-018 GS-3-029 GS-3-004 GS-3-028
55 Page 55 of 70
ip t
an
us
cr
Table 4. Trace element content and U-Pb ages of zircons from the Sulu UHP eclogite, the Dabie HP/UHP eclogite, the Hengshan HP granulite, the Bohemian eclogite/granulite and the Grinio eclogite (age groups I, and IV) of different PT conditions ([1] Chen, 2009; [2] Liu et al., 2011; [3] Zhang, 2013; [4] Bröcker et al., 2010). Dabie eclogite[2] Hengshan[3] Bohemian[4] Gridino (this work) Sulu eclogite[1] HP UHP HP granulite eclogite granulite I age group IV age group ∑REE,ppm 9-39 5-38 2-40 13-934 11-74 57-110 15-82 8-71 Avg. 16 21 24 64 36 81 54 35 14-58 27
9-60 34
7-66 38
20-1101 103
23-119 62
25-176 96
28-157 80
13-138 66
Th, ppm Avg.
1-16 4
1-6 4
1-6 4
1-99 6
0-11 4
9-326 122
2-88 28
b.d.-81 27
U, ppm Avg. Age, Ma P, GPa T, °C
29-277 78 226-235 3-4.4 600-910
10-296 78 2698
5-175 69 2609
M
Y, ppm Avg.
ep te
d
39-380 14-527 228 271 210-215 222-227 2 4 940-990 910-980
38-824 155 1862 1.4-1.5 780-850
42-297 114-1040 136 535 330-350 > 2.7 1.8-2.8 690-820 800-1000
1.7-1.9 650-710
Ac c
Table 5. List of all mineral inclusions identified in different textural domains (A, B, C, D, F) of zircons from eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino complex of the Belomorian province. A question mark in the bracket indicates a possible mineral identified with EDS analysis yet without microprobe data. A – zonal texture of inner (A1) and outer (A2) domains; B – fir-tree/patchy textureal domains; C – homogeneous domains of low-CL or high-CL image; D* – domains compromised by fluid injection trails; F – outer fringe of high-CL. (Grt – garnet, Di – diopside, Amp – amphibole, Bt – biotite, Ms – muscovite, Pl – plagioclase, Kfs – K-feldspar, Zo – zoisite, Ap – apatite, Ttn – titanite, Rt – rutile, Qz – quartz, Cal – Ca-carbonate) A
A1
GS-1
Grt, Ap
GS-2 GS-3
Qz, Zo Di, Ms, Bt, Ap,
A2 Bt(?), Rt, Qz, Cal, Chalcopyrite(?) Di, Qz Grt, Pl, Ap, Ttn
B
C
Grt,
Grt, Qz, Cal,
Grt, Di, Pl, Amp, Qz, Di, Amp(?), Bt, Ttn, 56
Qz Grt, Di, Ttn
D* Grt, Bt, Pl, Kfs, Amp(?), Rt, Qz, Cal, Pl, Ttn, Qz, Cal Pl, Rt, Qz, Cal
F Pl – Grt, Ttn, Rt
Page 56 of 70
ip t
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
36.89 35.47 34.58 41.74 32.14 45.01 28.48 42.23 62.64 57.64 78.56 35.01 50.84
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.07 0.01 0.02 0.02 0.01 0.01
0.02 0.01 0.02 0.01 0.07 0.01 0.01 0.11 0.01 0.06 0.02 0.03 0.06
0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.03 0.02 0.01 0.02
0.16 0.10 0.15 0.06 0.40 0.08 0.06 0.19 0.07 0.33 0.09 0.20 0.42
0.39 0.50 0.50 0.40 0.63 0.32 0.19 0.48 0.29 0.86 0.38 0.66 0.89
0.21 0.38 0.36 0.17 0.40 0.24 0.16 0.39 0.21 0.51 0.33 0.44 0.59
1.68 2.70 3.04 2.39 2.81 1.90 1.66 2.52 2.53 4.33 3.65 3.04 3.95
0.63 5.83 1.50 4.57 0.70 0.78 5.96 1.33 3.60 0.52 0.83 6.04 1.25 3.55 0.47 0.81 7.54 1.66 4.03 0.51 0.78 5.47 1.15 3.41 0.52 0.64 6.37 1.82 6.14 0.97 0.65 5.02 1.11 3.11 0.44 0.73 6.71 1.62 5.04 0.75 0.99 9.39 2.40 7.30 1.05 1.33 10.36 2.35 6.22 0.95 1.10 12.49 3.12 9.45 1.44 0.84 6.00 1.27 3.78 0.54 1.12 8.97 2.01 5.33 0.78
5.19 3.64 3.66 3.62 3.63 8.03 3.58 5.98 8.06 6.47 9.61 3.98 5.26
0.80 0.48 0.48 0.40 0.51 1.31 0.50 0.91 1.09 0.92 1.30 0.62 0.71
2.24 3.09 2.03 1.86 1.66 0.74 1.43 1.61 0.90
0.41 0.54 0.30 0.21 0.28 0.13 0.09 0.11 0.08
1.54 1.81 0.90 0.89 0.90 0.63 0.44 0.51 0.51
0.25 0.31 0.16 0.17 0.16 0.18 0.12 0.12 0.14
1.67 2.03 1.16 1.26 1.12 1.40 1.01 1.04 1.27
0.99 1.13 0.74 0.90 0.72 0.80 0.81 0.77 0.97
0.39 0.40 0.28 0.29 0.27 0.23 0.25 0.24 0.27
1.09 1.49 0.97 1.01 0.97 0.74 1.04 0.82 0.99
0.17 0.21 0.14 0.11 0.12 0.08 0.14 0.10 0.09
0.06 0.12 0.08 0.08 0.09 0.04 0.06 0.09 0.05
0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01
ep te
d
M
an
us
Y
Ac c
Sample spot garnet GS-1-7 GS-1-22 GS-1-17 GS-1-23 GS-1-18 GS-1-21 GS-1-5 GS-1-24 GS-1-30 GS-1-10 GS-1-29 GS-1-19 GS-1-9 omphacite GS-1-11 GS-1-12 GS-1-13 GS-1-14 GS-1-15 GS-1-16 GS-1-20 GS-1-25 GS-1-26
cr
Ttn, Qz, Qz, Cal, Fe-oxide Table 6. Rare earth element (+Y) contents of garnet and omphacite in eclogite sample GS-1 from the Gridino complex, Belomorian province (LA-ICP-MS data):
0.72 0.93 0.69 0.58 0.50 0.26 0.47 0.47 0.40
0.11 0.13 0.10 0.08 0.08 0.03 0.07 0.06 0.04
0.18 0.30 0.16 0.14 0.15 0.04 0.13 0.15 0.05
0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01
57 Page 57 of 70
ip t
cr
Table 7. Representative microprobe analyses of mineral inclusions in zircon from eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino complex of the Belomorian province (Cpx – clinopyroxene, Grt – garnet, Pl – plagiocalse, Zo – zoisite, Mus – muscovite, Bi - biotite).
GS-3 251 49.64 0.27
GS-3 231 52.17 0.13
Sample N SiO2 TiO2
Garnet GS-1 208 38.82 0.02
GS-1 209 38.66 0.04
GS-3 233 38.54 0.01
Sample N SiO2 TiO2
Plagioclase GS-1 GS-1 215 222 56.68 56.92 0.01 0.04
GS-2 261 55.68 0.07
GS-3 237 56.42 0.00
Sample N SiO2 TiO2
Zoisite GS-2 258 38.41 0.13
Sample N SiO2 TiO2
Muscovite GS-3 243 48.09 1.28
Sample N SiO2 TiO2
Biotite GS-3 246 35.92 4.61
5.75 0.12 8.94 11.03 0.12 21.94 1.04 0.00 0.04 98.55
3.06 0.15 8.71 12.32 0.09 22.33 0.85 0.00 0.02 99.10
6.30 0.03 8.81 10.50 0.09 22.32 0.89 0.02 0.07 98.93
1.83 0.14 8.01 12.36 0.11 23.03 0.47 0.01 0.00 98.26
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
21.68 0.05 22.52 3.84 1.29 12.33 0.11 0.00 0.00 100.66
21.53 0.22 23.70 3.50 0.89 12.13 0.05 0.00 0.00 100.72
21.55 0.07 24.12 2.50 1.48 10.45 0.00 0.04 0.00 98.76
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
27.08 0.00 0.03 0.00 0.01 8.95 6.21 0.14 0.02 99.12
26.72 0.05 0.02 0.00 0.00 8.41 6.35 0.07 0.04 98.62
27.94 0.03 0.01 0.00 0.00 9.51 6.14 0.16 0.00 99.54
27.88 0.04 0.05 0.00 0.02 9.17 6.24 0.16 0.05 100.03
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
30.30 0.00 3.81 0.04 0.04 23.23 0.01 0.00 0.00 95.97
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
32.73 0.01 2.23 1.74 0.00 0.01 0.17 8.91 0.00 95.17
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
15.75 0.54 17.26 10.99 0.05 0.01 0.34 9.56 0.07 95.11
1.86 0.01 0.26 0.00 0.20
1.93 0.01 0.14 0.00 0.22
1.87 0.01 0.28 0.00 0.25
1.98 0.00 0.08 0.00 0.26
Si Ti Al Cr Fe2+
3.00 0.00 1.97 0.00 1.43
3.00 0.00 1.97 0.01 1.52
3.07 0.00 2.02 0.00 1.61
Si Ti Al Cr Fe2+
2.56 0.00 1.44 0.00
2.58 0.00 1.43 0.00
2.52 0.00 1.49 0.00
2.53 0.00 1.47 0.00
Si Ti Al Cr Fe2+
2.99 0.01 2.79 0.00 0.00
Si Ti Al Cr Fe2+
3.18 0.06 2.55 0.00 0.12
Si Ti Al Cr Fe2+
2.73 0.26 1.41 0.03 1.10
0.08 Fe3+ 0.62 Mg 0.00 Mn 0.89 Ca 0.08 Na 0.00 K 0.00 Ni 4.00 Sum endmembers 92 WEF 5 Jd 3 Ae
0.06 0.69 0.00 0.90 0.06 0.00 0.00 4.00
0.03 0.59 0.00 0.90 0.07 0.00 0.00 4.00
0.00 0.70 0.00 0.94 0.04 0.00 0.00 4.00
Fe3+ Mg Mn Ca Na K Ni Sum
0.03 0.44 0.08 1.02 0.02 8.00
0.02 0.41 0.06 1.01 0.01 8.00
0.00 0.30 0.10 0.89 0.00 8.00
Fe3+ Mg Mn Ca Na K Ni Sum
0.00 0.00 0.00 0.43 0.54 0.01 4.98
0.00 0.00 0.00 0.41 0.56 0.00 4.98
0.00 0.00 0.00 0.46 0.54 0.01 5.00
0.00 0.00 0.00 0.44 0.54 0.01 4.99
Fe3+ Mg Mn Ca Na K Ni Sum
0.25 0.01 0.00 1.94 0.00 0.00
Fe3+ Mg Mn Ca Na K Ni
0.00 0.17 0.00 0.00 0.02 0.75 -
Fe3+ Mg Mn Ca Na K Ni
0.00 1.25 0.00 0.00 0.05 0.93 -
94 3 3
93 6 1
96 4 0
Alm And Grs
48 1 33
51 1 32
55 0 31
Ab An Or
55 44 1
58 42 0
53 46 1
55 44 1
an
M
d
ep te
Ac c
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total p.f.u. Si Ti Al Cr Fe2+
us
Clinopyroxene GS-2 GS-2 260 262 49.25 51.26 0.32 0.31
Sample N SiO2 TiO2
7.99
58 Page 58 of 70
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Fig.5
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Fig.4
Fig.6 63 Page 63 of 70
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Fig.7
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Fig.8
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ip t cr us an M d ep te Ac c
Fig.9
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ip t cr us an M d ep te Ac c
Fig.10
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Fig.11
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S0301-9268(15)00240-5 http://dx.doi.org/doi:10.1016/j.precamres.2015.07.009 PRECAM 4316
To appear in:
Precambrian Research
Received date: Revised date: Accepted date:
11-4-2015 9-7-2015 17-7-2015
Please cite this article as: Li, X., Zhang, L., Wei, C., Slabunov, I.A.,Metamorphic PT path and zircon U-Pb dating of Archaean eclogite association in Gridino complex, Belomorian province, Russia, Precambrian Research (2015), http://dx.doi.org/10.1016/j.precamres.2015.07.009 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
Highlights
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We obtain peak metamorphic condition of Belomorian eclogite;
3
We confirm the clock-wise metamorphic PT path of Belomorian eclogite assemblages;
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Neoarchean age of Belomorian eclogite has been determined;
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Subsequent high-temperature reworking during the exhumation of Belomorian eclogite
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assemblages has been suggested at Neoarchean;
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Metamorphic PT path and zircon U-Pb dating of Archaean eclogite association in Gridino
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complex, Belomorian province, Russia
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*Li, Xiaoli1, Zhang, Lifei1, Wei, Chunjing1, Slabunov, Alexander I.2
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1. School of Earth and Space Sciences, Peking University, Beijing 100871, China
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2. Institute of Geology, Karelian Research Center RAS, Petrozavodsk, 185910, Russia,
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*Corresponding author: Li, Xiaoli, Yiheyuan Road No.5, Haidian District, Beijing, 100871,
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China. Tel.: +86-010-62751168, Email: [email protected]
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Abstract
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The eclogites from the Gridino eclogite-bearing mélange complex in the Belomorian province,
12
Fennoscandian Shield may represent one of the oldest terrain in the world. The Belomorian
13
province is a superposition of Archaean and Palaeoproterozoic collisional orogens, which
14
experienced repeated high-grade metamorphic reworking and intense polyphase tectonic
15
deformation in the early Precambrian. Main purpose of this study is to determine the metamorphic
16
age(s) and evolutionary PT path of eclogite as boudin-like body within the Gridino tectonic
17
mélange. Representative rock materials of well-preserved, plagioclase-free eclogite and associated
18
retrogressed eclogitic rock were sampled from Stolbikha Island in the Gridino area for our research.
19
The eclogite sample mainly comprises omphacite, garnet, amphibole, rare quartz, and some
20
accessory minerals (rutile, magnetite), whereas the retrogressed eclogite sample is made of diopside
21
– Na-rich plagioclase symplectites, amphibole, quartz, and accessory minerals. Thermodynamic
22
modeling points to a clockwise metamorphic PT path, with peak conditions of P = 18.5kbar, T =
23
710 °C, and a “warm” subduction is inferred thus. LA-ICP-MS zircon U-Pb dating yields diverse
24
metamorphic episodes from the Neoarchaean to the Palaeoproterozoic. While metamorphic zircon
25
formed at eclogite-facies condition is identified by studying the trace element distribution and
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partitioning in and between zircon and co-existing eclogite-facies minerals (omphacite, garnet);
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consequently, Neoarchaean eclogite-facies metamorphic event of ca. 2.70 Ga can be distinguished
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for the Gridino eclogite complex, and a regional-scale thermal event may have occurred in the
3
period of 2.70 Ga – 2.61 Ga.
4 5
Keywords: Archaean eclogite, Belomorian province, phase equilibria modeling, zircon dating
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6 1. Introduction
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It was commonly considered that eclogite, a product of high-pressure (HP) metamorphism
10
usually formed during subduction, could hardly occur in Archaean because of the distinct
11
geodynamic regime in the Archaean eon (Baer, 1977; Green, 1975; Stern, 2005; Brown, 2014). The
12
likely recognized oldest (2.0 Ga) eclogite occurrence was reported in the Usagaran belt of Tanzania
13
(Möller et al., 1995). Also the eclogite occurrence in the Snowbird zone, Canada, was before
14
speculated to form in Archaean (Percival, 1994), although it was eventually proved to be of
15
Palaeoproterozoic (ca. 1.90 Ga) age (Baldwin, et al., 2004). However, about a decade ago, Archaean
16
eclogite associations were firstly recognized in the Belomorian province of Fennoscandian shield
17
(Volodichev et al., 2004; Konilov et al., 2004; Mints et al., 2010a) that could fundamentally update
18
our understanding of the early geodynamics of our planet.
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Eclogite occurrence in the Belomorian province was already known long time ago (Eskola,
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1921). The eclogite association on the Stolbikha Island (Gridino complex) – the study area of
21
current paper, were firstly reported in the 90s of last century (Volodichev, 1990; Mitrofanov, 1996),
22
and since then debates on the petrology and metamorphic age of the eclogite were never stopped.
23
Generally there are two major types of eclogite occurrence here such as boudin-like mafic eclogite
24
bodies embedded in a polymetamorphic TTG matrix and eclogitized dykes (of gabbro-norite
25
compositions) occurring in different generations. The first type was usually considered to be of
26
Archaean age (Stolbikha Island) (Bibikova et al., 2003; Volodichev et al., 2004, 2005; Slabunov et 3 Page 3 of 70
al., 2006) and the second type usually occurring on adjacent archipelagos (e.g. Vargas Island,
2
Izbunaya Luda Island) was suggested to be of Palaeoproterozoic age (Travin and Kozlova, 2005,
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2009; Kozlovsky and Aranovich, 2008; Dokukina et al., 2009, 2010). Later another eclogite
4
associations of older ages (Meso-, Neoarchaean, Mints et al., 2010a, b; Shchipansky et al., 2012a, b)
5
in the northern proximity named after the Salma and Kuru-Vaara complexes were discovered, along
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with some other eclogite-bearing outcrops of different scales continuously being reported in this
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region (for instance, in the Chupa and Khetolambino terrains), which could reach several of
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hundreds (personal communications with Shchipansky A.A.). The concept “Belomorian eclogite
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province” was then introduced by local Russian geologists, which comprises the whole Belomorian
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province and the southern part of Kola continent (Mints et al., 2014 and references therein). Debates on the age of the Belomorian eclogites and possible geodynamic models have never
12
stopped ever since their first discover. There are in all four eclogite-facies metamorphic episodes
13
were proposed by different researchers – Mesoarchaean twice (2.88 Ga and 2.82Ga, Salma complex)
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(Mints et al., 2010a, b; Konilov et al., 2011), Neoarchaean (2.72 Ga, Salma and Gridino complexes)
15
(Bibikova et al., 2003; Volodichev et al., 2004; Slabunov et al., 2006, 2008; Shchipansky et al.,
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2012a, b) and Palaeoproterozoic (1.90 Ga, Gridino and Kuru-Vaara complexes), which was usually
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considered to be related to the Svecofennian orogeny (Travin and Kozlova, 2009; Skublov et al.,
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2010, 2011; Herwartz et al., 2012; Mel’nik et al., 2013). The genesis of Belomorian eclogite
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occurrence was another subject of discussion, as most researchers prefer to the subduction-collision
20
model (e.g. Slabunov et al., 2006; Mints et al., 2014; Dokukina et al., 2014), although alternative
21
minor opinions may also emerge (for instance, “autonomous” eclogite-facies metamorphism (e.g.
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Travin and Kozlova, 2005, 2009). In the pioneer works, estimation of metamorphic PT conditions
23
were based on a series of conventional geothermo-, barometric calculations on certain mineral
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paragenesis that peak conditions of 14.2-17.5 kbar, 740-865 °C was suggested and, consequently,
25
subduction with a geothermal gradient of ~ 12-13 °C/km was inferred that could answer a warm
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subduction regime (Volodichev et al., 2004; Mints et al., 2010a).
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In this paper, we will focus on the boudin-like mafic eclogite bodies within the TTG tectonic
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mélange (the first type eclogite), and to avoid misunderstanding, in the following text, the term
3
Gridino eclogite will only refer to this type of eclogite. Pioneer researchers (e.g. Volodichev et al,
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2004; Slabunov, 2011) have discussed the age(s) and peak metamorphic conditions of similar
5
eclogite rocks from this area. Here we will present new petrological, geochronological and
6
geochemical data of the analogous boudin-like eclogite outcrops from the adjacent area, to conclude
7
the metamorphic evolution PT path and the time(s) of eclogite-facies metamorphic event(s),
8
involving complex analytical approaches. Research materials are eclogite and retrogressed eclogite
9
rocks (apoeclogite as in Russian literature), which were collected during the geological excursion of
10
GEC-2011 on Stolbikha Island near the village of Gridino addressed as Stop 1 in the fieldtrip
11
guidebook (Volodichev et al., 2011). Phase equilibria modeling in the NCFMASH system combined
12
with conventional geothermobarometry are utilized to build the metamorphic PT path of the
13
Gridino eclogite. Age of eclogite-facies metamorphism is determined by in-situ LA-ICP-MS zircon
14
U-Pb dating. By analyzing trace element compositions in zircons and co-existing eclogite minerals
15
(omphacite, garnet), we are able to distinguish the metamorphic zircons formed at eclogite-facies
16
condition, who were in equilibrium with omphacite-garnet assemblage, and, thus, the corresponding
17
age(s). Besides, high-grade metamorphic zircons formed in the early Precambrian period may
18
possess individual geochemical fingerprints that differ from the “modern” ones.
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2. Geological background
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The Belomorian province, along with the Karelian, Murmansk, Norrbotten and Kola
23
provinces comprise the Archaean core of Fennoscandian (Baltic) shield (Fig.1a). The Svecofennian
24
orogen (1.8-1.9 Ga) accreted to the old core in the west and the Kola-Lapland orogen (1.9-2.0 Ga)
25
formed inside this core, while the Belomorian and Kola provinces are forlands of Kola-Lapland
26
orogen (Fig.1a). The Karelian province (craton) is built up mainly by an ancient granite-greenstone 5 Page 5 of 70
complex (ca. 2.6-3.5 Ga) including tonalite-trondjemite-granodiorite (TTG) granitoids and gneisses,
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greenstone and paragneisses belts with mafic, komatiitic and intermediate-felsic metavolcanics,
3
metasediments, banded iron formation (BIF), metamorphosed in greenschist to low-pressure
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amphibolite (up to granulite) facies with either andalusite or sillimanite formation (Gaál and
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Gorbatschev, 1987; Sorjonen-Ward and Luukkonen, 2005; Slabunov et al., 2006; Hölttä et al., 2014).
6
The Kola province, or Inari-Kola microcontinent, consists of three types of terrains: the first
7
predominate one includes Archaean greenstone, paragneiss, TTG-gneiss and granulite-gneiss
8
complexes, the second is mixer of Neoarchaean and juvenile Palaeoproterozoic rocks, and the third
9
– Palaeoproterozoic Lapland and Umba granulite (Balagansky, 2002; Glebovitskii, 2005; Daly et
10
al., 2006; Balagansky et al., 2011). The Belomorian (tectonic) province mainly is composed of
11
Archaean (2.7-2.9 Ga) TTG granitoid and gneiss, greenstone (metakomatiite, metabasalt, island-arc
12
type felsic volcanic rocks, sediments of BIF, quartzite and conglomerate) and paragneiss
13
(metagreywacke) complexes (Bibikova et al., 1999, 2004; Thurston and Kozhevnikov, 2000;
14
Glebovitskii, 2005; Slabunov et al., 2006; Slabunov, 2008; Hölttä et al., 2014). Meanwhile,
15
Palaeoproterozoic coronitic gabbroid (Stepanova and Stepanov, 2010), granitoid and pegmatite are
16
also very common (Volodichev, 1990).
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Archaean associations in the Belomorian province are rather similar to those in the Karelian
18
craton generally but distinguished in the metamorphic grades: there are two stages (Neoarchaean
19
and Palaeoproterozoic) of amphibolite-facies metamorphism with kyanite (high- pressure)
20
occurrence in the Belomorian province (Volodichev, 1990). Geological (Miller and Mil'kevich,
21
1995) and seismic profiling data (Mints et al., 2009; Sharov et al., 2010) show that the internal
22
structure of Belomorian province is composed of large scale intensely folded nappe, which form in
23
the Archaean and Paleoproterozoic time. The Belomorian province was thrusted on the Karelian
24
craton and, in turn, was thrusted by rocks of the Kola province during Lapland-Kola orogeny.
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There are two eclogite-bearing mélange complexes can be distinguished in the Belomorian
26
province: the Salma complex in the northern part and the Gridino – in the eastern (Fig. 1a). These 6 Page 6 of 70
complexes form tectonic slices (Fig. 1b) (Slabunov, 2008; Balagansky et al., 2015). The matrix of
2
the mélange is mainly built up of intensely migmatized and deformed TTG-gneiss (Sibelev et al,
3
2004; Shchipansky et al., 2012a). Major types of eclogite occurrences are recognized as boudin-like
4
bodies of several meters in diameter (Shirokaya Salma, Kuru-Vaara, Gridino localities) up to
5
hundreds meters (Shirokaya Salma localities) (Volodichev et al., 2004, 2005, 2011; Mints et al.,
6
2010a, b, c; Dokukina and Konilov, 2011; Shchipansky et al., 2012a). The Salma (Shirokaya Salma,
7
Uzkaya Salma, Kuru-Vaara localities) and Gridino eclogites are considered to be of oceanic
8
lithosphere subduction-related type, which formed in the intervals of 2.89-2.82 (for Salma) and 2.72
9
Ga (for Salma and Gridino) (Volodichev et al., 2004; Mints et al., 2010a, b, c; Dokukina and
10
Konilov, 2011; Shchipansky et al., 2012a). The second type of eclogites are eclogitized and partly
11
boudinated mafic dyke swarms (Volodichev, 1990; Travin and Kozlova, 2005; Kozlovsky and
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Aranovich, 2008; Volodichev et al., 2004, 2012; Dokukina and Konilov, 2011; Mel’nik, 2015),
13
which can be also noted in the Keret archipelago (Kozlovsky and Aranovich, 2008). In the Gridino
14
area most eclogitized gabbro outcrops are systematically located in an archipelago of the White Sea
15
(e.g., Vorotnaya Luda Island, Izbunaya Luda Island, Vargas Island), and a few occur at the coast
16
near the village of Gridino (Volodichev et al, 2012, Dokukina and Konilov, 2011). The dyke-type
17
eclogites have been well discussed in details in other reports (Dokukina et al., 2009, 2010, 2012,
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2014; Dokukina and Konilov, 2011; Stepanova and Stepanov, 2010; Babarina and Sibelev, 2015).
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The boudin-type eclogite, together with amphibolite and zoisitite rocks, constitute the primary
20
components of the TTG gneiss matrix that form a polygenetic mélange complex; they are
21
distributed irregularly in the northwest spread, a structure that is possibly related to the
22
disintegration of certain multiple-built complexes in the subduction zone and collisional suture
23
(Slabunov et al., 2007, 2015). The boudin-type eclogite was firstly dated at ca. 2.72 Ga by
24
Volodichev et al. (2004), and it was considered to be of subduction-type genesis as the case in the
25
Salma complex. However, later Skublov et al. (2011) proposed a late Palaeoproterozoic age of ca.
26
1.90 Ga for the eclogite-facies metamorphism and considered the early obtained Archaean age was 7 Page 7 of 70
actually the time of protolith formation. Petrologic studies on both mafic and felsic rocks from the
2
Gridino complex indicate a minimum pressure of 16-17.5 kbar for the eclogite-facies
3
metamorphism, and subsequent retrogression via high-pressure granulite (14-10 kbar, 800-750 oC)
4
then amphibolite (7.9-9.6 kbar, 530-700 oC) facies metamorphism can be expected (Dokukina et al.,
5
2009; Konilov and Dokukina, 2011). Besides, according to the report of Perchuk and Morgunova
6
(2014), an ultra-high-pressure metamorphism (coesite stability field) in the Gridino eclogitized
7
gabbro was even proposed.
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Nowadays there are different models of crustal evolution for the Belomorian province, and all
9
of them are in consensus on the subduction, accretion, plume and collision events. For instance, one
10
group of geologists consider that the eclogite and other early continental crust with mafic dykes
11
formed during the subduction-accretion event at ca. 2.82-2.90 Ga, and the eclogitized mafic dykes
12
of Gridino type and the continent occurred during the collision at ca. 2.78-2.82 Ga. Subsequently,
13
superplume influences on the continent uplifted to higher crustal level with granulite-facies event(s)
14
started at ca. 2.70-2.72 Ga, while later new superplume events with mafic dyke and granitoid
15
intrusion took place at ca. 2.4 Ga. At last, a new collisional event with exhumed middle crust
16
occurred at ca. 1.77-1.90 Ga. (e.g. Mints et al., 2010b, 2015). Another few geologists however put
17
forward a different model. They consider that the Earth crust of the Belomorian province was
18
produced during the Mesoarchaean-Neoarchaean (Belomorian) collisional orogeny at ca. 2.65-2.90
19
Ga (Slabunov, 2008; Slabunov et al., 2006; Hölttä et al, 2014), and it was greatly influenced by the
20
Palaeoproterozoic (ca. 2.4 Ga) plume (Stepanova and Stepanov, 2010). During the late
21
Palaeoproterozoic Lapland-Kola collisional orogeny it was somehow reworked and reshaped
22
(Balagansky, 2002; Daly at al., 2006; Balagansky et al., 2015).
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23
In this work, we will accentuate on the boudin-shaped eclogite bodies within the TTG tectonic
24
mélange of the Gridino complex (on Stolbikha Island) that include well-preserved eclogite with
25
minimal retrogression (sample GS-1) and associated retrogressed eclogitic rock (samples GS-2 &
26
GS-3) (Fig. 1c). Most exposed eclogite bodies have either been transformed into symplectite-rich 8 Page 8 of 70
retrogressed eclogite or garnet-bearing amphibolite; in the bodies’ central parts, however,
2
sometimes pristine omphacite-garnet mineral assemblages mainly occur, although minor secondary
3
plagioclase rims around garnet may also be observed. In the outcrops, several tonalitic-
4
trondhjemitic and pegmatitic veins (~ 50 cm in width) extend dozens of meters and, occasionally,
5
they transect some eclogite and amphibolite bodies and/or cross-cut the gneissose foliation. The
6
country rocks are intensely deformed amphibole-biotite-bearing orthogneiss and granitoid of the
7
TTG series – the main component of the tectonic mélange (Li et al., 2013a; Babarina and Sibelev,
8
2015).
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Mineral chemistry analysis was conducted with the electron microprobe analyzer JEOL JXA-
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8100 at Peking University, with 15 kV accelerating voltage, 10 nA beam current, and 10-15 s
14
counting time. SPI standards were utilized and the PRZ correction was applied. Mineral formulae
15
were calculated with the MINPET software (ver. 2.02). Zircons were extracted from ~ 8-10 kg rock
16
samples, mounted together with TEM standards in epoxy resin, and well-polished to expose their
17
cores. The zircons were first investigated in the optical microscope and inclusions were identified
18
by Raman spectroscopy. Back-scatter electron (BSE) and cathodoluminescence (CL) images were
19
obtained with a scanning electron microscope, QUANTA-650FEG, equipped with INCA-Synergy
20
and CHROMA-D detectors at Peking University. Qualitative and quantitative WDS/EDS analyses
21
of mineral inclusions in zircons were performed with the afore mentioned microprobe operated at
22
the same settings. In-situ analysis of trace elements and U-Pb isotopes was performed with an ICP-
23
MS Agilent 7500 Ce equipped with a Complex Pro102 laser ablation system (LA-ICP-MS) at
24
Peking University. Helium was used to enhance the transport efficiency, and nitrogen was added to
25
the argon plasma to decrease the detection limit and improve the analytical precision. Each analyse
26
incorporated approximately 15 s background acquisition (gas blank) followed by 30 s data
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acquisition from the sample. The laser spot was adjusted to ~ 30-35 μm. For zircons, U, Th and Pb
2
concentrations were calibrated using
3
analytic signals, time-drift correction, and quantitative calibration for trace element analysis and
4
U-Pb dating were done with the ICPMSDataCal software. The Glitter 4.0 software (by CSIRO) was
5
used for further calculation. PLE and TEM were used as external standards for U-Pb isotopes in a
6
separate operation and standards #610, #612, and #614 were references for the trace elements.
7
Common Pb correction was undertaken with the LAM-ICPMS Common Lead Correction (ver.3.15)
8
algorithm from T. Andessen in MS Excel 2010, and concordia diagrams and weighted means were
9
built and computed with Isoplot/Ex (ver.3.57). For garnet and omphacite, the average 29Si contents
10
of 38 wt.% and 54 wt.%, respectively (basing on the minerals’ microprobe data), were references
11
for the trace elements calculation and standards #610, #612, #614 were applied for calibration
12
during analysis.
Si. Off-line selection and integration of background and
13 14
4. Petrography and mineral chemistry
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Eclogite sample GS-1 is a nearly bimineralic rock mainly consisting of porphyroblastic garnet
17
and omphacite, with subordinate amphibole and scarce quartz. Omphacite usually forms fine-
18
grained crystals of 0.5-1.0 mm length and contains in part some mineral inclusions (rutile,
19
accessory minerals); garnet occurs as euhedral crystals of 0.5-1.0 mm diameter and usually contains
20
rutile, amphibole and quartz inclusions. Amphibole develops mainly interstitially, but may also
21
occur as epitaxial replacement of omphacite (clinopyroxene); in addition, little amphibole is
22
associated with plagioclase forming kelyphitic coronas around garnet, a texture reflecting
23
retrogression (Fig. 2).
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Clinopyroxene-plagioclase symplectites along with a lot of quartz aggregates, some of which
25
may have experienced recrystallization, and aggregates of plagioclase, which often possesses
26
twinning, characterize retrogressed eclogitic rock samples GS-2 & GS-3. In both rocks, garnet 10 Page 10 of 70
1
usually occurs as semi-euhedral porphyroblasts with multiple fractures or irregular shapes, the size
2
of which ranges from 1.0 to 1.5 mm. Amphiboles develop widespread, replacing symplectitic and/or
3
relic clinopyroxene (Fig. 2). Judging by the field correlation, retrogressed eclogite samples GS-2
4
and GS-3 are likely retrogressed analogues of eclogite sample GS-1. Shown in Table 1, representative microprobe analyses of rock-forming minerals
6
(clinopyroxene, garnet, plagioclase, and amphibole) can be generally characterized as follows
7
(mineral abbreviations are after Whitney and Evans, 2010): Clinopyroxene is mostly omphacite in
8
the eclogite sample GS-1 with Na2O = 3.31-4.80 wt.% and ∑FeO = 4.71-7.08 wt.%. The jadeite (Jd)
9
component ranges from 16 to 30 mol.%, and the aegirine (Ae) component may reach 10 mol.% (Fig.
10
3a). A Ca-Eskola (CaEs) component could hardly be estimated, whereas the Ca-Tschermak (CaTs)
11
component ranges from 2 to 9 mol.%. The retrogressed eclogite samples GS-2 & GS-3 exclusively
12
contain diopside with a little higher ∑FeO content – 7.94-9.33 wt.%, but barely a Jd component (<
13
3 mol.%; Fig. 3a). No CaEs component could be recalculated, while a relatively high CaTs
14
component – 8-14 mol.% – could be obtained. Garnet is a almandine-pyrope-grossular solid
15
solution (Alm-Prp-Grs) with a predominant Alm component. For garnets from the eclogite sample
16
(GS-1), the Prp component is a little higher than the Grs in general, while garnets from the
17
retrogressed eclogite samples (GS-2 & GS-3) show the opposite. Most garnets in both eclogite and
18
retrogressed eclogite samples do not show clear compositional zonation, only a few, however,
19
present slight prograde zoning with Prp increasing (24 mol.% → 27 mol.%) and Grs decreasing (26
20
mol.% → 23 mol.%) from core to rim (eclogite sample GS-1), or, conversely, show retrograde
21
zoning (retrogressed eclogite sample GS-3; Fig. 3b). Amphiboles all are calcic types, pargasite (Prg)
22
and/or edenite (Ed) (Fig. 3c). Plagioclase from the eclogite sample, forming kelyphitic coronas
23
together with amphibole, is oligoclase (Ab 72-84 mol.%), while in the retrogressed eclogite samples,
24
plagioclase mostly is andesine (Ab 51-67 mol.%; Fig. 3d). Zoisite (epidote) casually occurs in the
25
retrogressed eclogite sample along with plagioclase and/or quartz, with an estimated Fe3+ = 0.40
26
p.f.u.
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5. Metamorphic PT conditions
3 Phase equilibria modeling was undertaken in the NCFMASH model system (Na2O-CaO-FeO-
5
MgO-Al2O3-SiO2) for the eclogite sample GS-1. Titanium is omitted because its presence is limited
6
to the accessory minerals rutile/ilmenite in a trivial amount; potassium and manganese are also
7
excluded from the modal system for their negligible contents in feldspar and garnet, respectively.
8
Pure H2O is assumed to be in excess, considering the widespread presence of amphibole and
9
absence of carbonates. For the modeling, the rock’s bulk composition obtained by XRF analysis is:
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SiO2 = 48.81, Al2O3 = 12.38, FeO = 9.15, CaO = 13.38, MgO = 9.48, Na2O = 2.90 (in wt.%). THERMOCALC (ver. 3.33) by R. Powell was applied to build the PT pseudosection. Datasets
12
tc-ds55.txt & tc-ds55s.txt (updated 26/10/09) and the metabasite a-x file tc-NCFMASHm.txt were
13
used, with solution models of amphibole (Diener et al., 2007), clinopyroxene (Green et al., 2007),
14
chlorite (Holland et al., 1998), garnet (White et al., 2007), and plagioclase (Holland and Powell,
15
2003). In the PT pseudosection (Fig. 4), the eclogite-facies mineral assemblage garnet-
16
omphacite(±quartz) is predicted to be stable at 16-25 kbar at reference temperatures of 800-620 °C.
17
On garnet isopleth diagrams, Xprp (= Mg/(Mg+Ca+Fe2+) and Xgrs (= Ca/(Mg+Ca+Fe2+) increase and
18
decrease, respectively, with the pressure. The observed prograde zonation in garnet (core → rim)
19
implies a metamorphic evolutionary path within the stability field of the assemblage omphacite-
20
garnet-amphibole-quartz and, consequently, peak metamorphic conditions of 18.5 kbar, 710 °C for
21
((amphibole-) eclogite-facies) can be inferred (stage M1). If the peak metamorphic mineral
22
assemblage was amphibole-free (only comprising omphacite-garnet-quartz), following the garnet’s
23
zoning trend, pressures may actually have been little higher, 19 kbar at least, within a temperature
24
range of 720-730 °C (Fig. 4a). The occurrence of plagioclase along with amphibole (in trivial
25
amount) in kelyphitic coronas around garnet reflects uplift-retrogression, here named after stage M2,
26
with the respective stability field clinopyroxene-garnet-amphibole-plagioclase-quartz (Fig. 4b).
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Additionally, the conventional garnet-clinopyroxene thermometer (Krogh, 1988) evaluates the
2
PT range for the formation of the mineral assemblage omphacite-garnet-amphibole-quartz, which
3
yields maximum and minimum limits (as in solid lines) with Kd (distribution coefficient) values
4
varying from 7.80 to 8.55 – by omphacite rim composition with garnet core composition pair (Fig.
5
4b). The garnet-amphibole thermometer (Graham and Powell, 1984) was also applied for
6
appropriate mineral assemblages, where the garnet rim composition was used, that yields a higher
7
temperature range of 750-770 °C (reference pressures 15-17 kbar), values within the stability field
8
of clinopyroxene-garnet-amphibole-quartz±plagioclase. Thus, re-heating is suggested during
9
decompression (exhumation). Isopleths of omphacite’s Jd component (Xjd, Xjd = Na/(Na+Ca+Fe2+))
10
show that it decreases with the pressure. Altogether, a feasible decompressing stage (M2), in which
11
clinopyroxene, garnet, amphibole, plagioclase, and quartz are stable, can be concluded, considering
12
both garnet-amphibole thermometry and the composition of omphacite (Fig. 4b). Further
13
retrogression may be easily expected by the associated occurrence of retrogressed eclogites.
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To summarize, a pressure of about 18.5 kbar and a temperature of approximately 710 °C
15
represents the peak conditions of eclogite-facies metamorphism in the Gridino eclogite complex,
16
Belomorian province. Moreover, if we assume that there is no amphibole at all, the pressure might
17
be a little higher (19.5 kbar) and so does the peak temperature (720-730 oC). Consequently, a
18
subduction depth of nearly 61-64 km and a corresponding geothermal gradient of ~ 11-12 °C/km
19
can be inferred, which may be a littler “warmer” than the Phanerozoic (cold) subduction cases.
20
During subsequent decompression (exhumation), heating to > 750 °C occurred, when the pressure
21
dropped to about 15-16 kbar (stage M2). Further retrogression can be implied by the associated
22
symplectic eclogitic rock (samples GS-2, GS-3) that amphibolite-facies and lower grade
23
metamorphism is expected. Thus, in general, a clockwise metamorphic PT trajectory is concluded,
24
in accord with most orogen-type eclogites worldwide.
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6. Zirconology and the U-Pb dating 13 Page 13 of 70
1 2
6.1 Zircon morphology
3 The zircons extracted from the well-preserved eclogite sample GS-1 are mainly transparent,
5
euhedral to subhedral, and measure 100 to 200 µm in length. They contain several mineral
6
inclusions (garnet, rutile, diopside, plagioclase, amphibole, calcite, quartz and others) in limited
7
amounts. A few zircons occur as fragmental crystals or, rarely, as aggregates. On the basis of
8
cathodoluminescence (CL) imaging, four major textural types can be discriminated (Fig. 5): type A,
9
the most common one, possesses a clear core (domain 1, A1) – rim (domain 2, A2) zonal texture,
10
which could either be dark (low-CL) or bright (high-CL); type B, the second most one, is
11
characterized by fir-tree or patchy textures, often combined with type A zonal textures; type C
12
combines homogeneous zircons, being either completely dark or bright; and type D summarizes
13
zircons with compromised texture (fragmental sometimes), containing an obvious fluid-retreated
14
overprint as injection trails. In addition, for almost all zircon types, a thin bright “fringe” (shell) less
15
than 10μm thick can be widely observed. The zircons extracted from the retrogressed eclogitic
16
rocks (samples GS-2 & GS-3) show the same textures, yet they are smaller (50-150 µm), with type
17
A and type B textures equally distributed. Beside, almost all GS-2 and GS-3 zircons also possess a
18
thin bright fringe, mostly 10-15 µm thick with very few reaching 30-40 µm thickness.
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Altogether, morphologically, the zircons from the eclogite and eclogitic rocks are very alike
20
and do well meet the criteria of high-grade metamorphic genesis (Corfu et al., 2003; Liati et al.,
21
2009). The zonal type zircons are usually “mixed” by fir-tree/patchy texture, forming sort of hybrid
22
type, where the zonation might be compromised. The inner part of the zonal type zircon (usually of
23
low-CL) is often considered as a protolithic core, however, in the studied scenario, it should be
24
rechecked very carefully. Fluid-injection trails are common in all zircon types, with some silica- and
25
calcium-rich mineral inclusions (quartz and/or calcium-carbonate) usually concentrated around
26
them. Most injected fluid trails apparently are externally initiated (i.e. external fluid invasion),
14 Page 14 of 70
1
although a few may occur spontaneously somehow. When the fluid reworking has greatly
2
compromised the zircon, we sorted it into the type D.
3 4
6.2 Zircon U-Pb ages and geochemistry
5
7
Analytical data of U-Pb isotopes and trace element contents are given in Table 2 and 3, and
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the obtained concordia plots and REE distribution patterns are shown in Fig. 6 and 7 accordingly. For all analyzed zircons, obvious U-Pb age differences do not exist among textural types or
9
domains, and there are in all several age episodes can be concluded. For the well-preserved eclogite
10
sample GS-1, two major episodes can be discriminated (Table 2, Fig. 6): one (age group I), an upper
11
intercept age of 2698±28 Ma (n = 16, MSWD = 0.59), which, interpreted alternatively, corresponds
12
to a concordant age of 2689±5 Ma (n = 9, MSWD = 0.0026), and, two (age group IV), an upper
13
intercept age of 2607±44 Ma (n = 14, MSWD = 0.47). In addition, a single concordant
14
age of 1914±24 Ma is obtained for the A2 textural domain (outer part of zoned zircons), the
15
youngest age of all. Geochemically all dated zircons are characterized by low ∑REE contents, with
16
obvious HREE predominance, relatively medium-low absolute Y, Th, and U contents, and medium-
17
low Th/U ratios (Table 3). All chondrite-normalized REE pattern exhibit negative Eu and positive
18
Ce anomalies (one exception, a spot with positive Eu anomaly of Eu/Eu* = 1.44, age group IV
19
zircon) and relative flat HREE distribution (Fig. 7). Conventional Ti-in-zircon thermometry
20
(Watson et al., 2006) is considered to estimate the crystallization temperature of zircons; a value of
21
about 670 °C was obtained for both age groups.
206
Pb/238U
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For the retrogressed eclogitic rock GS-2, two episodes can be discriminated too (Table 2,
23
Fig.6), the age group II – 2649±31 Ma (n = 12, MSWD = 0.20) and the age group X – 2761±73 Ma
24
(n = 9, MSWD = 0.099). Spots from the fir-tree/patchy zircon domains (i.e. type B) determine the
25
age group II, and spots exclusively from inner zircon cores (type A1) the age group X.
26
Geochemically, zircons of both age groups II and X show similarities: low ∑REE, Y, Th and U 15 Page 15 of 70
1
contents, medium-low Th/U ratios, and negative Eu- and positive Ce-anomalies (Table 3). In the
2
chondrite-normalized pattern, the HREE distributions exhibit duality – either flat pattern or uneven
3
pattern with steep Lu-enrichment characterize zircons of one same age group (Fig. 7). For zircons from the retrogressed eclogitic rock GS-3, three episodes can be discriminated
5
including the afore mentioned age groups I and II, plus the age group III (Table 2, Fig. 6). The age
6
group I is determined by an upper intercept of 2707±31 Ma (n = 10, MSWD = 0.13), and,
7
alternatively, a concordant age of 2694±7.5 Ma (n = 6, MSWD = 0.13) obtained from several spots
8
from the A1 and A2 zircon domains (internal part of type A); the age group II – an upper intercept at
9
2655±46 Ma (n = 9, MSWD = 0.29) – is obtained from spots from the inner domains of type B
10
zircons (being mixed with fir-tree/patchy textures). The age group III – an upper intercept at
11
2671±51 Ma (n = 8, MSWD = 0.54) – is obtained for the remaining spots in the inner part of type B
12
zircons. Geochemically, zircons of all age groups (I, II, III) share general characteristics – low
13
∑REE, Th, U contents, medium Th/U ratios, and negative Eu- and positive Ce-anomalies. For age
14
groups II and III zircons, the HREE distributions can either be flat or steep, with Lu-enrichment in
15
all but age group I zircons, which feature flat HREE distributions only. Ti-in-zircon thermometry
16
does not yield obvious differences among all age group zircons (Table 3).
19
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6.3 Zircon trace element correlation
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Generally, the geochemical features of all analyzed Gridino zircons are quite consistent with
21
the literature data of high-grade metamorphic genesis (Rubatto, 2002; Corfu et al., 2003). All are
22
characterized by low ∑REE contents (less than 100 ppm) with obvious HREE predominance, low Y
23
contents (less than 200 ppm), medium-low Th and U contents (less than 100 ppm), and medium-
24
low Th/U ratios – Th/U = 0.2-0.8 (one exception of Th/U = 0.01 for the 1.91 Ga zircon; Fig. 8). The
25
Th/U ratio is commonly considered as indicator for HP eclogite-facies metamorphism, if it is lower
26
than 0.1; however, it could only be an artifact instead, because, under fluid participation, the 16 Page 16 of 70
1
mobility/solubility of Th and U can be very distinctive and, consequently, their concentrations and
2
the Th/U ratio might very unpredictably (Möller et al., 2002; Hoskin and Schaltegger, 2003).Thus,
3
if fluid is involved, as in the studied circumstance given presence of retrograde amphibole, zircon’s
4
Th/U ratio is not a robust indicator. For magmatic zircons, particular trace element features (correlations) have been well
6
discussed for provenance investigation (Grimes, et al., 2007, 2009); for high-grade metamorphic
7
zircons, similar correlations may apply. For instance, the zircons of age groups I & IV (from
8
eclogite sample GS-1) do share certain common characteristics and somehow differ from other
9
classical eclogite-/granulite-facies metamorphic zircons worldwide (Table 4; Fig. 9), where the
10
Th/U ratio indicator is mainly pronounced. On the Th/Yb vs. Y diagram, the zircons from the
11
Gridino eclogite (this work) usually plot at the same area as the Bohemian eclogite-granulite zircons
12
(Bröcker et al., 2010); besides, on the Th/U vs. Y diagram, they also fall into the similar field,
13
where the Bohemian eclogite-granulite zircons are spread. On the other side, the Triassic HP-UHP
14
eclogite zircons from the Dabie-Sulu orogen, China, (Chen, 2009; Liu et al., 2011) are clearly
15
distinctive on the Th/U vs. Y diagram, as are the Palaeoproterozoic HP granulite zircons from the
16
Hengshan terrane, North China Craton (Zhang, 2013). Apparently, the zircons from the Bohemian
17
eclogite-granulite complex are somehow comparable with the Gridino eclogite zircons, both subject
18
to high-pressure and high-temperature metamorphism.
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In the chondrite-normalized pattern, most zircons from the Belomorian eclogite (sample GS-1)
20
present quite flatt HREE distributions with negative Eu- and positive Ce-anomalies; however, the
21
zircons from the retrogressed eclogite samples (GS-2 & GS-3) possess sort of dual HREE
22
distribution with both flat patterns and steep Lu-enrichment in the same age group. Exception only
23
occurs for age group I zircons, where flat HREE pattern occurs, as in the eclogite sample. Positive
24
Ce-anomaly occurs with medium-low Ce/Ce* values (Ce/Ce* = 1-20) along with negative Eu-
25
anomaly in most cases (Eu/Eu* = 0.4-0.8), although, a very few may show small positive Eu-
26
anomaly with Eu/Eu* = 1.1-2.0 – mainly for the zircons from the retrogressed eclogite samples (age 17 Page 17 of 70
groups II, III and X). A negative Eu-anomaly in zircons can be related to many factors, such as the
2
co-existence with feldspar – a rival for the Eu2+ ion, the geochemical inheritance from the protolith,
3
the whole rock geochemistry (Eu-depletion), and the redox state of rock or melt (Schaltegger et al.,
4
1999; Hoskin and Ireland, 2000; Hoskin and Black, 2000; Rubatto, 2002; Hoskin and Schaltegger,
5
2003; Trail et al., 2012). In our case, there is only a trivial amount of plagioclase intergrowth with
6
amphibole – coronas around garnet – that could hardly have significant impact. The geochemistry
7
of the whole rock, however, does show a negative Eu anomaly (Eu/Eu* = 0.46-0.86), possibly an
8
important factor. Besides, the redox state of the whole rock under eclogite-facies metamorphism
9
may also play a role somehow. To estimate the oxygen fugacity in zircon formation, an empirical
10
algorithm using the Ce content as a proxy is considered (Trail et al., 2012), and an oxygen-fugacity
11
vs. U-Pb age diagram was obtained (Fig. 10). It shows that zircons from the studied Belomorian
12
eclogite and from the Hengshan HP granulite, both formed in Precambrian times, plot in a close
13
range of oxygen fugacities – -20 to -30, whereas the Paleozoic-Mesozoic high-grade metamorphic
14
zircons yield higher oxygen fugacities instead. Whether or not this phenomenon reflects a
15
significant change in Earth’s history, requires a more detailed investigation of a much larger dataset.
18
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6.4 Trace and rare earth element partitioning of zircons, garnet and omphacite
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In the eclogite, omphacite and garnet are the main rock forming minerals, and their co-
20
existence with zircon of geochronological significance can be used to decode dated metamorphic
21
ages. Garnet is an important container for trace elements, especially heavy REE and Y, rivals zircon,
22
which contains significant amounts of middle- to heavy-REEs, Y, U, and Th. In the Gridino eclogite
23
sample GS-1, garnet is mostly unzoned in the major elements (Ca-Mg-Fe), so it is in the REEs.
24
Compared with zircons, the garnets contain slightly less ∑REE and Y in ~ 10-20 ppm differences:
25
∑REEgrt = 16-43 ppm, av. 26 ppm; Ygrt = 28-79 ppm, av. 45 ppm. The chondrite-normalized REE (+
26
Y) patterns of the garnets show unanimous flatten HREE distributions with negative Eu anomaly 18 Page 18 of 70
(av. Eu/Eu* = 0.70; Table 6, Fig. 11). Omphacite hardly contains REEs and Y – ∑REE = 5-8 ppm
2
(av. 6 ppm), Y = 1-3 ppm (av. 2 ppm), and exhibit steep HREE-depleted patterns (Table 6, Fig. 11).
3
Both analyses of the omphacites and the garnets have near-homogenous chemical compositions
4
with little variation, which allows estimation of the trace elements (TE) partitioning coefficient
5
DTEomp/grt (DTEomp/grt = CTEomp/CTEgrt) using their average compositions. In results, the obtained
6
partitioning coefficient trend of the garnet and the omphacite, two key eclogite-facies minerals, is
7
comparable to the literature data of classical eclogites as from Western Alps and others (Rubatto and
8
Hermann, 2003, references therein). Therefore, in the geochemical scope, the analyzed garnets and
9
omphacites can be ascertained as authentic eclogite minerals.
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The trace elements partitioning between garnet and zircon from eclogites have been well
11
discussed in many studies in order to interpret zircon U-Pb ages (e.g. Schaltegger et al., 1999;
12
Hermann, 2002; Rubatto, 2002; Rubatto and Hermann, 2003, 2007). The trace elements distribution
13
coefficient of zircon and garnet (DTEzrn/grt = Czrn/Cgrt, C – concentration) can be calculated for the
14
zircons (age groups I, IV) and garnets from the eclogite sample GS-1, using their average
15
compositions. The obtained distribution coefficients between the zircon and the garnet show a near
16
“balanced” partition for Sm, Eu, Gd, Tb and Dy elements (DTEzrn/grt = ca. 1.0), although most trace
17
elements are preferentially incorporated into zircon (especially Ce), and the value trend is quite
18
comparable with the data of other classical granulite-facies and eclogite-facies rocks on Earth (Fig.
19
12, Rubatto, 2002; Rubatto and Hermann, 2003; Buick et al., 2006; Zhou et al., 2011). The obtained
20
trend of the distribution coefficient zircon to garnet for the Gridino eclogite is quite comparable
21
with data from the Western Alps eclogites, where equilibrium between zircon and garnet is
22
suggested (Rubatto and Hermann, 2003). Consequently, the zircons and garnets from the studied
23
eclogite sample are in equilibrium, corresponding to the eclogite-facies zircon formation (Fig. 12A).
24
Trace element partitioning coefficients between zircon and garnet were determined
25
experimentally at 20 kbar, 800-1000 °C (Rubatto and Hermann, 2007). The DREEzrn/grt trend obtained
26
here plots between the trends derived from the 900 and 950 °C experiments, which might suggest
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an intermediate value temperature that is actually far away from the estimated peak temperature (~
2
700-730 °C) discussed above. Likewise case was also met for the UHP eclogites of Central Dabie
3
(Zhou et al., 2011; Fig. 12B). The reason of the noneffective referencing may concern the host rock,
4
as being mentioned in the experimental work, where starting materials of granitic composition were
5
used, the temperature reference may only be applicable in the case of intermediate-felsic rocks
6
though.
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Mineral inclusions in zircons can provide very important implications on the formation
11
environment of a dated zircon domain. In zircons from the Gridino eclogite and retrogressed
12
eclogite rocks, diverse inclusions have been found in different textural domains (Table 5). They
13
were thoroughly analyzed by IR-Raman spectrometry and electron microprobe analysis – the
14
representative microprobe data are shown in Table 7.
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Garnet inclusions found in the internal zones (A1, B, C domains) of zircons from the eclogite
16
sample GS-1 usually contain 14-15 mol.% Prp and 33-34 mol.% Grs, which may be similar to few
17
matrix garnets (Prp ~ 15-20 mol.%), whereas most of the matrix garnets contain more Prp and less
18
Grs (Fig. 3). On the built PT pseudosection, however, none could be illustrated by isopleths
19
intersection. We consider these garnet inclusions may form in the eclogite-facies metamorphism.
20
Other garnet inclusions found in the fluid-reworked domain D* often contain much less Prp (8-10
21
mol.%, with Grs 30-31 mol.%) and are likely related to post-peak metamorphism (retrogression).
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A phase associated with quartz – likely biotite – was found in zircon’s internal domain A2
23
(zonal type A), where multiple fractures occur. Inheritance from the magmatic protolith for this
24
domain is possible. This phase contains nearly 7 wt.% K2O with FeO, MgO in majority (13.65 wt.%
25
and 10.90 wt.% correspondingly) and perceptible TiO2 (ca. 4 wt.%), although, due to its small size,
26
no good microprobe analyses could be obtained. A sulfide phase comprising Cu-Fe-S with 20 Page 20 of 70
hexagonal shape was found in the internal domain A2 (zonal type A), likely chalcopyrite. In the
2
association, there are also some fine trails of quartz + Ca-carbonate – they could be related to fluid
3
activity (hydrothermal reworking). Rutile, a common eclogite-facies Ti-bearing mineral, was found
4
in the internal parts of zircons (A2 domain). In the zircon domain D, which experienced an obvious
5
overprint by an external fluid, calcite and quartz are very common. Sometimes, garnet, amphibole,
6
biotite (XMg = 0.54), plagioclase (andesine, An = 41-44 mol.%), K-feldspar (Or = 81 mol.%, Ab =
7
19 mol.%), and rutile inclusions also occur in this domain. Inclusions of quartz – plagioclase – K-
8
feldspar assemblages found in the zircons are interpreted as entrapped melt and thus they may
9
indicate that partial melting during high-temperature reworking could be involved in the
10
retrogression, an inference in agreement with the eclogite’s PT path. In a highly luminescent zircon
11
fringe thought to represent a post-eclogite-facies event (retrogression), andesine (An = 42 mol.%)
12
was found.
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The zircons from the retrogressed eclogite samples GS-2 & GS-3 contain more mineral
14
inclusions. The garnet inclusions in the zircons’ inner and compromised (fluid-reworked) zones are
15
compositionally similar with the ones in the bulk rock (XMg = 0.10-0.14, XCa = 0.30-0.34).
16
Clinopyroxene of low jadeite component (diopside) was found mainly in the internal zones of
17
zircons (domains A1, A2, B, C), along with garnet and/or ± plagioclase (andesine, An = 43-46
18
mol.%); all are chemically identical to the rock forming minerals of the retrogressed eclogite. In the
19
internal domain Al of one zircon, where a diopside inclusion was found, a concordant
20
age of 2646±37 Ma was obtained (age group II). A Fe-oxide phase with ∑FeO = 84 wt.% – maybe
21
magnetite – was found in one zircon’s internal domain B. Titanite is a quite common mineral
22
inclusion in all zircon domains and might be produced due to decomposition of rutile under
23
influence of a carbonate-saturated (fluid). In the high-CL fringe classified as domain F, garnet,
24
titanite and rutile inclusions were also found and are related to the latest metamorphic event. In the
25
fluid-reworked D* domain, plagioclase (andesine), quartz, Ca-carbonate, rutile, and titanite
26
inclusions occur; all are thought to be of secondary genesis. Zoisite with low Fe content (∑FeO =
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3.7-3.8 wt.%) was found in association with quartz in one zircon’s inner domain A1 (from sample
2
GS-2) and may suggest a lower grade metamorphism. Combined biotite-muscovite-diopside
3
inclusions (Fig. 13) were found in one zircon’s inner domain A. It appears that biotite was generated
4
from muscovite, which is commonly believed to take place during greenschist-facies metamorphism
5
at 400-450 °C; the assemblage may thus be an early relict. Another biotite-quartz(-chlorite-alike
6
phase) and biotite-diopside (Jd = 6 mol.%) assemblage (Fig. 13) may also occur in the zircons inner
7
domains (A1 and B, sample GS-3), where the biotite is featured by relatively higher TiO2 content
8
(3.1-4.6 wt.%) and magnesium (XMg = 0.54-0.58), which may suggest a higher temperature
9
environment.
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10 7. Discussion
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Representative, well-preserved eclogite and retrogressed eclogitic rock samples from the
14
Stolbikha Island, Gridino eclogite complex, Belomorian province have been studied. The eclogite
15
GS-1 is mainly composed of omphacite and garnet, with subordinate amphibole and quartz. The
16
eclogitic rock samples GS-2 and GS-3, retrogressed equivalents of the pristine eclogite, are mainly
17
made of diopside-plagioclase-symplectites, garnet, amphibole, and quartz. Thermodynamic
18
modeling was performed on the well-preserved eclogite GS-1 to reconstruct the metamorphic PT
19
path. According to the geochemical studies (Slabunov, 2008; Volodichev et al., 2011), trace
20
elements in the Gridino eclogite samples are comparable with the ones of MORB, thus, we may
21
infer that the eclogite was derived from oceanic crust. For the Gridino eclogite and retrogressed
22
eclogite samples, zircon U-Pb dating yielded diverse age episodes, which require thorough
23
interpretation. Geochemical studies of zircons are a very powerful, commonly used instrument for
24
decoding their genesis (e.g. Hoskin and Schaltegger, 2003; Grimes et al., 2007; Shchipansky and
25
Slabunov, 2015); meanwhile, the study of the trace elements partitioning among zircons and other
26
relevant minerals is considered to be a more elaborate approach (e.g. Schaltegger et al., 1999;
27
Rubatto, 2002). The mineralogy of inclusion minerals in zircons can yield extremely significant
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1
clues sometimes (e.g. Hermann et al., 2001) and, besides, the thermodynamic modeling in a ZrO2-
2
bearing system is also proposed in concrete cases (e.g. Zhang, 2013). In what follows, we
3
scrupulously discuss the petrological, geochemical and geochronological data and explain our
4
points of view on the debated Archaean Belomorian eclogite association.
5 7.1 The metamorphic PT path of the eclogite
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The well-preserved eclogite GS-1 contains omphacite with high Jd content and a weak
9
compositional zonation from the core to the rim (Jd, 30 mol.% → 21 mol.%), and garnet with a
10
high Prp content, which is generally compositional homogenous. Most likely, re-equilibration
11
(higher temperature overprint) is – at least partially – responsible. Nevertheless, in individual cases,
12
(weak) prograde zonation of garnet is observed and permits the reconstruction of a prograde section
13
of the eclogite’s metamorphic PT path. Calcic amphibole (edenite, pargasite) is associated with the
14
omphacite-garnet assemblage (interstitial occurrence and epitaxial replacement of omphacite) and
15
either product of (amphibole-)eclogite-facies metamorphism or subsequent retrogression.
16
Thermodynamic modeling does, within errors of the method, coincide with both scenarios.
17
Plagioclase, found exclusively in the plagioclase-amphibole kelyphitic coronas around garnet in a
18
trivial amount, reflects uplift and provides hints on the eclogite’s retrograde evolution. The
19
formation of diopside-plagioclase-symplectites along with garnet, amphibole and quartz
20
characterizes the retrogressed eclogitic rocks. Garnets are pristine, yet they do not show
21
compositional zonation at all – re-equilibration during a high-temperature reworking can account
22
for. Basing on the field correlation, the retrogressed eclogite samples should be the retrogressed
23
equivalents of the well-preserved eclogite.
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A clockwise section of the metamorphic PT path of the studied Gridino eclogite was
25
reconstructed by combining thermodynamic modeling (NCFMASH system) with conventional
26
geothermometry. The result agrees well with many modern-time orogen-type eclogites worldwide. 23 Page 23 of 70
The peak eclogite-facies metamorphism occurs in the pressure-temperature range of 18.5-19.5 kbar,
2
700-730 °C, which corresponds to about 61-65 km subduction-exhumation depth. Consequently, a
3
relatively warm geothermal gradient of 11-12 °C/km is inferred. Retrogression of the eclogite was
4
possibly the result of a heat impact at 15-16 kbar at T > 750 °C; subsequently, both decompression
5
and cooling proceeded through the lower amphibole-facies represented by retrogressed eclogite. In
6
the early report (Volodichev et al., 2004), basing on massive conventional thermometric calculation,
7
a prograde metamorphic path with PT condition intervals of 14.0 – 17.5 kbar and 740 – 865 oC was
8
concluded. In comparison, our estimation is in accord with those data generally but with several
9
adjustments. For the peak conditions, higher pressure could be reached with appropriate
10
temperature ranges; in the retrogression, we suggest a high-temperature reworking process during
11
the decompression that a high-pressure granulite-facies metamorphism could be accounted for;
12
further retrogression (decompression and cooling) can lead to the formation of symplectic
13
retrogressed eclogitic rock in association, that (high-pressure) amphibolite-facies metamorphism
14
can be expected. In addition, our PT condition estimation is quite consistent with the data obtained
15
from other mafic and felsic rocks in the Gridino complex (Dokukina et al., 2009; Konilov and
16
Dokukina, 2011), that may fulfill a complete metamorphic picture of current area. As having been
17
well discussed by Mints and coauthors (2014), the Belomorian eclogite is quite comparable with
18
those of other high-pressure rocks on Earth and present no significant differences in the PT
19
trajectory for Precambrian and Phanerozoic eclogite-bearing complexes. Here we would like to
20
compare the PT paths of the eclogite and blueschist from the Pam Peninsula, NE New Caledonia
21
(Clarke et al., 1997) and the Eastern Blue Ridge, North Carolina, USA (Page et al., 2003) with our
22
data in this work as well (Fig. 14).
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23 24
7.2 The time of metamorphism and its geological implication
25
24 Page 24 of 70
Zircon U-Pb dating determines the metamorphic age of the studied Gridino eclogite
2
association. According to the elaborate studies on zircon morphology and geochemistry, we are able
3
to ascertain some dated zircons with particular geological significance. Altogether, six major
4
episodes can be summarized (and grouped) from all dated zircons (Fig. 15). The age groups I and
5
IV – ca. 2.72 Ga and ca. 2.62 Ga correspondingly – were obtained from zircons with high-grade
6
metamorphic morphology from the well-preserved eclogite sample. Geochemically, these zircons
7
are featured by low ΣREE (+Y) contents, flat HREE distribution pattern, and positive Ce- and
8
negative Eu-anomalies, all typical for eclogite-facies genesis. In addition, the trace element
9
correlation studies may show that the dated zircons from Gridino eclogite complex could share a
10
similarity with those from other eclogite- and/or granulite-bearing complexes. The REEs
11
partitioning between zircons and omphacite/garnet implies equilibrium and, consequently, age
12
groups I and IV zircons should both form during eclogite-facies metamorphism. Therefore, we may
13
conclude that both obtained ages – ca. 2.72 and ca. 2.61 – could represent (an) Archaean eclogite-
14
facies metamorphic event(s). Besides, the discriminated algorithm on oxygen fugacity estimation by
15
zircon’s Ce content could also denote a possible likeness of zircons formed in Precambrian. In the
16
earlier work (Volodichev et al., 2004), an age of ca. 2.72 Ga was suggested for the formation of the
17
Gridino eclogite that conforms to our result, while the later age might suggest a repeated eclogite-
18
facies metamorphism (subduction?). According to the known regional geology, the Belomorian
19
orogen was in an early collision stage of evolution (Bibikova et al., 1999; Slabunov, 2005, 2008;
20
Slabunov et al., 2006). During this period, the eclogite-bearing mélange was subject of repeated
21
“renovation” with intense tectonic deformation (Glebovitskii, et al. 2000). In such a scenario, two
22
consecutive (about 100 Ma in-between) eclogite-facies events could possibly have proceeded in
23
principle, besides, a repeated subduction is generally possible (Herwartz et al., 2011). However, no
24
more petrologic-mineralogical nor geochemical evidence could be found (metamorphic generations)
25
that we prefer to hold cautious evaluation on this scenario.
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The oldest age of ca. 2.75-2.76 Ga (age group X) was obtained by a small amount of zircons
2
(in their internal textural parts) from the retrogressed eclogite sample GS-2 (Fig. 15), whose genesis
3
cannot be unequivocally determined, although we may propose an inherited magmatic age of the
4
protolith. In the another Belomorian eclogite-bearing complex named after Salma, a similar age of
5
ca. 2.77-2.78 was reported to be the magmatic age of associated granitic rocks (Kaulina et al., 2010;
6
Mints et al., 2010; Li et al. 2013b), thus, at this time, a heat-triggered event (magmatism or high-
7
temperature metamorphism) indeed occurred.
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The other ages, 2.65 Ga (group II) and 2.67 Ga (group III), were obtained from zircons from
9
the retrogressed eclogite samples, which are very alike to the zircons of age group IV. The gap of
10
about 10-20 Ma between these two age groups is insignificant and may be attributed to analytical
11
precision. Besides, some similar ages in the range of ca. 2.61-2.67 Ca were also obtained from
12
individual zircons. The scattering age points, on the other side, may be related to a long residence
13
time of high-grade metamorphic rocks with very slow cooling process (e.g. Ashwal et al., 1999)
14
during exhumation after the collision event, thus, altogether, we suggest that at this time, (a) long-
15
lived (~ 60 Ma) metamorphic event(s) occurred. The mineral inclusion studies revealed associations
16
of diopside, garnet and plagioclase – minerals characteristic for granulite-facies metamorphism – in
17
the zircons’ internal domains. In this period, continental collision proceeded and can be accounted
18
for the high-temperature event(s). As other individually obtained ages in the range of ca. 2.45 Ga,
19
they can be related to the superplume thermal event, which is accounted for the formation of mafic
20
dyke swarms in the Belomorian province during ca. 2.4-2.5 Ga (Stepanova and Stepanov, 2010).
21
Singular age point of ca. 1.91 Ga was obtained in a zircon’s outer fringe domain from the eclogite
22
sample, which is connected to the Lapland-Kola orogeny (Balagansky et al., 2015).
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23
In the early report by Skublov et al. (2011), similar eclogite samples were dated. The author
24
concluded that the eclogite-facies metamorphism occurred at late-Palaeoproterozoic age of ca. 1.90
25
Ga, and magmatic protolith of eclogite was formed at ca. 2.70 Ga. However, in that paper, the dated
26
zircons (sample 102) were clearly zonal with dark core and bright rim texture in the CL imaging, 26 Page 26 of 70
which is not so widely observed in our samples (as mentioned above, we are dealing with zircons of
2
none zonal textures). Thus, it is possible that we are dealing with different eclogite samples (of
3
retrogression?). Furthermore, that conclusion was mainly made on traditional geochemistry of
4
zircons (such as Th/U, HREE distribution indicators, etc), which cannot be depended solely without
5
solid mineralogical-petrologic observations – besides, as being discussed above by us and
6
Shchipansky and Slabunov (2015), trace elements partitioning studies among rock-forming
7
minerals and zircon can be more reliable. Besides, the justification and criticism on subsequently
8
inferred Palaeoproterozoic model (Skublov 2010, 2011), which appears to be invalidate, have been
9
well discussed by Mints et al. (2014). On the other side, as being speculated by Slabunov et al.,
10
(2011), one of the specifics of Gridnio eclogite could be a quick subduction after the protolith
11
formation, i.e. the eclogite-facies metamorphism could occur short after the magmatic event (~ ±
12
20-30 Ma). It may be the reason that both protolithic and metamorphic ages are quite close.
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Another debated on the Archaean age of the Belomorian eclogite arose from alternative
14
geochronological dating (Sm-Nd, Lu-Hf methods) of other eclogite-like outcrops (eclogitized dykes
15
in Gridino area, eclogite rocks in Salma complex) (e.g. Skublov et al., 2010; Berezin et al., 2012;
16
Herwartz et al., 2012). In those works, the dated rock samples were intensely retrogressed eclogitic
17
rock and it remains unclear, how strongly the isotopic systems have been compromised by later
18
“supraposition”. Widely discussed, the closure temperature (Tc) of the Sm-Nd isotopic system
19
depends on a lot of parameters such as the grain size of garnet, the whole rock major element
20
composition, the diffusion rate, the initial temperature and cooling rate. (Burton et al., 1995;
21
Granguly and Tirone, 1998, 1999). Altogether, a wide closure temperature range of 500-850 °C has
22
been suggested (grain sizes below 10 mm, cooling rate less than 5 °C/Ma; e.g. Thöni and Jagoutz,
23
1992; Hensen and Zhou, 1995; Li et al., 2000; Cherniak and Watson, 2001). The situation for the
24
Lu-Hf isotopic system is comparable given that its closure temperature can vary greatly with
25
garnet’s size (Scherer et al., 2000). In contrast, the closure temperature of the U-Pb isotopic system
26
in zircon is usually much higher, > 800-900 °C, and cannot be easily reset in most geological
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27 Page 27 of 70
circumstances – including high-grade metamorphism or anatexis (Burton et al., 1995; Lee et al.,
2
1997; Mezger and Krogstad, 1997). For instance, Sm/Nd ages of the Dabie UHP eclogite are
3
considered to reflect cooling reworking rather than eclogite-facies metamorphism (Liu et al., 2011).
4
Therefore, we believe that for the Belomorian eclogites, zircon U-Pb geochronology should be the
5
appropriate dating method considering their metamorphic PT evolution, only extreme cautious
6
interpretation on obtained ages should be applied.
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7 8. Conclusions
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Complex petrological and geochronological studies of the Gridino boudin-like eclogite
11
associations enable us to draw the following conclusions, which should provide new perspectives
12
on the knowledge of early Precambrian geology.
an
10
(1) The boudin-like eclogite from the Gridino mélange complex is mainly made of an
14
omphacite-garnet (± amphibole, quartz) assemblage, although most eclogite bodies have
15
been subjected to retrogression with symplectite-rich formation, a few of minimal
16
retrogression managed to preserve provide very valuable geological information.
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(2) Thermodynamic modeling and petrological observation permit us to reconstruct the
18
metamorphic evolution of the Gridino boudin-like eclogite that proceeded along a
19
clockwise PT trajectory, which is principally consistent with the previous estimation. Peak
20
pressure-temperature conditions of 18.5 kbar, 710 °C for the (amphibole-)eclogite-facies
21
metamorphism are estimated. Besides, further retrogression was characterized by re-
22
heating that may have reached a temperature above 750 °C;
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(3) The metamorphic PT path of the Gridino boudin-like eclogite is well consistent with other
24
estimations for the Gridino complex in all, and it is quite comparable with the one of most
25
modern high-pressure rocks formed in subduction zones, although a warmer subduction
26
can be speculated for the early Precambrian; 28 Page 28 of 70
(4) Trace elements partitioning studies among zircons and co-existing mineral assemblages
2
confirms their eclogite-facies metamorphism condition equilibrium (paragenesis), and the
3
geochronology of zircon shows that the eclogite-facies metamorphic event in the Gridino
4
complex should occur in Neoarchaean at ca. 2.70 Ga. While the subsequent discrete
5
period of ca. 2.61 – 2.67 Ga may represent a high-temperature overprint when the
6
continental collision was in progress.
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7 Acknowledgements
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We would like to thank Professor Volodichev O.I. from the Geological Institute of the
11
Karelian Research Center, Russian Academy of Sciences (Petrozavodsk) for his assistance in
12
organizing the fieldwork and providing first-hand regional geological data. We also want to thank
13
Professors Mints M.V. and Shchipanskiy A.A. from the Geological Institute of the Russian
14
Academy of Sciences (Moscow) for their consulting on the regional geology. We would like to
15
express our deep gratitude to senior engineers Dr. Su G. and Dr. Ma F. (Peking University) for their
16
assistance in microprobe and LA-ICP-MS analyzing operations. We are also very thankful to our
17
dear colleague Dr. Thomas Bader from Peking University for his assistance in correcting current
18
paper. This study was financially supported by the National Natural Science Foundation of China
19
(grants 41202032, 412111062 and 41311120071).
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Precambrian Research 101: 313-340.
5 6 7
Volodichev, O.I., 1990. Belomorian complex of Karelia: Geology and Petrology. Nauka, Lenigrad, pp. 245. [In Russian] Volodichev, O.I., Slabunov, A.I., Bibikova, E.V., Konilov, A.N. and Kuzenko, T.I., 2004. Archean
ip t
4
eclogites in the Belomorian Mobile Belt, Baltic Shield. Petrology 12: 540-560.
Volodichev, O.I., Slabunov, A.I., Stepanov, V.S., Sibelev, O.S., Travin, V.V., Stepanova, A.V. and
9
Babarina, I.I., 2005. The Archean and Paleoproterozoic eclogites and Paleoproterozoic
10
druzites in the Gridino area (White Sea). In: Shiptsov, V.V. (Eds.), The Proceedings:
11
Belomorian Mobile Belt and its analogues: Geology, geochronology, geodynamics,
12
minerageny, Karelia Research Center RAS, Petrozavodsk, Russia, p. 60-74. [in Russian]
13
Volodichev, O.I., Slabunov, A.I., Stepanova, A.V., Stepanov, V.S. and Sibelev, O.S., 2011.
14
Archean eclogites and Paleoproterozoic elogitized gabbroids, Gridino area, White Sea. In:
15
Shiptsov, V.V. (Eds.), The Proceedings: Granulite and Eclogite Complexes in the Earth's
16
History, Extended Abstracts and Field Guide, Karelia Research Center RAS, Petrozavodsk,
17
Russia, p. 37-46.
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an
us
cr
8
Volodichev, O. I., Slabunov, A. I., Sibelev, O. S., Skublov, S. G., and Kuzenko, T. I., 2012.
19
Geochronology, Mineral Inclusions, and Geochemistry of Zircons in Eclogitized
20
Gabbronorites in the Gridino Area, Belomorian Province. Geochemistry International 50:
21
657–670.
22 23 24 25
Ac ce
18
Watson, E.B., Wark, D.A. and Thomas, J.B., 2006. Crystallization thermometers for zircon and rutile. Contributions to Mineralogy and Petrology 151: 413-433. White, R.W., Powell, R. and Holland, T.J.B., 2007. Progress relating to calculation of partial melting equilibria for metapelites. Journal of Metamorphic Geology 25: 511-527.
42 Page 42 of 70
1 2
Whitney, D.L., Evans, B.W., 2010, Abbreviations for names of rock-forming minerals. American Mineralogist, 95: 185-187.
3
Zhang, Y., 2013. Phase equilibria modelling for metamorphic evolution of high-pressure granulites
4
and anatexis of gneisses in the Hengshan Complex, Shanxi Province. PhD dissertation, Peking
5
University, Beijing, pp. 117. [In Chinese] Zhou, L., Xia, Q., Zheng, Y. and Chen, R., 2011. Multistage growth of garnet in ultrahigh-pressure
7
eclogite during continental collision in the Dabie orogen: Constrained by trace elements and
8
U-Pb ages. Lithos 127: 101-127.
cr
ip t
6
10
us
9 Figure Captions:
an
11
Fig. 1. (a) Main tectonic units of the Fennoscandian Shield; BP - Belomorian Province; KP - Kola
13
Province (after Balagansky, 2002, 2015; Daly et al., 2006; Slabunov et al., 2006); (b) Schematic
14
geological map of Gridino area with location of eclogite-bearing mélage complex (modified after
15
Slabunov, 2008; Babarina et al., 2014; Babarina, Sibelev, 2015); (c) Schematic geological map of
16
the Stolbikha Island in the Gridino eclogite-bearing mélange, Belomorian province (modified after
17
Sibelev et al., 2004). Samples were taken within the dashed square (Li et al., 2013a).
ed
pt
Ac ce
18
M
12
19
Fig. 2. Petrography of eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino
20
eclogite complex, Belomorian province (plane polarized light). The eclogite is mainly composed of
21
a fine-grained omphacite-garnet assemblage, with subordinate interstitial and/or epitaxial
22
(replacement) amphibole. The retrogressed eclogite is mainly composed of cracked garnet,
23
clinopyroxene-plagioclase-symplectites and retrograde amphibole (replacement). (Grt – garnet,
24
Omp – omphacite, Cpx – clinopyroxene, Amp – amphibole, Pl – plagioclase, Qz – quartz, Ttn –
25
titanite, Rt - rutile).
26 43 Page 43 of 70
1
Fig. 3. Mineral classification diagrams showing the molecular compositions (microprobe data) of (a)
2
clinopyroxene, (b) garnet with zoning profiles, (c) amphibole and (d) plagioclase from eclogite
3
(GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino eclogite complex, Belomorian
4
province. Casual prograde and retrograde compositional zonation were found in garnet from
5
eclogite (GS-1) and retrogressed eclogite (GS-3), respectively.
ip t
6
Fig. 4. PT pseudosection of the Gridino eclogite sample GS-1 calculated with the Thermocalc
8
software in the NCFMASH modal system (H2O in excess). The bulk composition (in mol.%) is
9
shown on top of the diagrams. (a) Intersections of garnet isopleths (Xprp and Xgrs) reveal a section of
10
the prograde PT path (solid line) and, consequently, peak metamorphism (stage M1) proceeded in
11
the amphibole eclogite-facies (grt-omp-amp-qz) or the eclogite-facies (grt-omp-qz; inferred dashed
12
line; path I); (b) thermometric calculations on appropriate garnet-clinopyroxene (Kd = 7.80-8.55)
13
and garnet-amphibole (Kd = 3.24-3.96) assemblages are shown with corresponding max.-min. Kd
14
values, which, along with the isopleths of omphacite (clinopyroxene) were used to constrain the
15
plausible PT ranges (hatched circles). Heating during decompression is suggested (stage M2; two
16
possible retrograde paths II can be inferred). The further retrogression path can be concluded from
17
the associated retrogressed eclogite in the field grt-cpx-amp-pl-qz (path III) (grt-garnet, omp-
18
omphacite, cpx-clinopyroxene, amp-amphibole, pl-plagioclase, opx-orthopyroxene, chl-chlorite, zo-
19
zoisite, qz-quartz).
us
an
M
ed
pt
Ac ce
20
cr
7
21
Fig. 5. Typical CL images of representative zircons from eclogite (GS-1) and retrogressed eclogite
22
(GS-2, GS-3) from the Gridino eclogite complex, Belomorian province. Several textural types are
23
distinguished: type A – zoned zircons with clear inner (A1) and outer (A2) domains (low- and high-
24
CL); type B – fir-tree and/or patchy zircons; type C – homogenous zircons of low-CL or high-CL;
25
type D – variant zircons with fluid-compromised or fragmented appearance. Zircons of “mixed”
44 Page 44 of 70
1
textures can be very common for A, B, and C types. A thin, high-CL outer fringe occurs on almost
2
all zircons. Concordant LA-ICP-MS 206Pb/238U ages are shown with the laser spot circles.
3 4
Fig. 6. Zircon U-Pb concordia diagrams for eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3)
5
from the Gridino eclogite complex, Belomorian province. Representative CL images of each age
6
group’s zircons with laser spots and corresponding
7
sample GS-1, two age groups, I and IV, were determined; for the retrogressed eclogite sample GS-2,
8
two other age groups, II and X, were obtained; and for the retrogressed eclogite GS-3, three age
9
groups, including the age groups I, II and the new obtained age group III were determined.
Pb/238U ages are shown too. For the eclogite
us
cr
ip t
206
10
Fig. 7. Chondrite-normalized REE patterns for zircons of each age group from eclogite (GS-1) and
12
retrogressed eclogite (GS-2, GS-3) from the Gridino eclogite complex, Belomorian province. Age
13
group I and IV zircons are featured by flat HREE distribution, while the remaining age group
14
zircons show both flat (a) and steep Lu-enriched (b) pattern in co-existence.
ed
M
an
11
15
Fig. 8. The Th vs. U diagram reveals a moderate Th/U ratio in the range of 1 and 0.1 for all but one
17
dated zircons from eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino
18
eclogite complex, Belomorian province; the exception (Th/U = 0.01) is the analysis of the outer
19
fringe domain.
Ac ce
20
pt
16
21
Fig. 9. U/Yb vs. Y, Th/U vs. Y, Th vs. Yb and Th/Yb vs. Y discrimination diagrams for zircons from
22
UHP eclogite from the Sulu CCSD MH core (44 analyses, Chen, 2009) and North Dabie
23
granulitized HP-UHP eclogite (18 analyses, Liu et al., 2011), the Hengshan HP granulite (66
24
analyses, Zhang, 2013), Bohemian eclogite-granulite (16 analyses, Bröcker et al., 2010) and
25
Gridino eclogite (of age groups I, IV, this work) (54 analyses). Apparently, the zircons from the
45 Page 45 of 70
1
Bohemian eclogite-granulite complex are somehow comparable with the Gridino eclogite zircons,
2
which are both subject to complex high-pressure and high-temperature metamorphism.
3 Fig. 10. Correlation of zircon U-Pb age and oxygen fugacity. Zircons from early Precambrian high-
5
grade metamorphic complexes (Gridino, this work, and Hengshan, Zhang, 2013) may have similar
6
low oxygen fugacity, compared to other Paleozoic-Mesozoic zircons from the North Dabie
7
granulitized eclogite (Liu et al., 2011), Sulu CCSD MH core eclogite (Chen, 2009) and Bohemian
8
eclogite-granulite (Bröcker et al., 2010).
cr
ip t
4
us
9
Fig. 11. Chondrite-normalized REE + Y patterns of omphacite and garnet form eclogite of the
11
Gridino complex show that omphacite is featured by extreme low REE concentration with HREE
12
depletion, while garnet is enriched in REE with LREE depletion but flat-enriched HREE (+ Y)
13
pattern. The omphacite – garnet trace element partitioning coefficients (DTEomp/grt = CTEomp/CTEgrt)
14
for the Gridino eclogite (in circles) are consistent with reference data for Western Alps eclogites (in
15
squares; Rubatto and Hermann, 2003) and Dora-Maria, Central Dabie, and Liguria, NW Italy
16
(shaded zone; references in Rubatto and Hermann, 2003).
pt
ed
M
an
10
17
Fig. 12. (A) Trace element distribution coefficients (average contents) of age group I (zrn-I) and IV
19
zircon (zrn-IV) to garnet for the Gridino eclogite exhibit a similar trend as Zrn and Grt from the
20
Western Alps eclogite known to be in equilibrium (Rubatto, 2002), an indication for equilibrium of
21
zircon and garnet in the Gridino eclogite; (B) Trace element distribution coefficient of zircon to
22
garnet from granulite-facies rocks from the Limpopo belt (South Africa; Buick et al., 2006),
23
eclogite-facies rocks form the Dabie orogen (China; Zhou et al., 2011), eclogites from the Gridino
24
complex (this work, t.w.) and an experimental dataset (800 °C, 900 °C, 1000 °C; Rubatto and
25
Hermann, 2007).
Ac ce
18
26 46 Page 46 of 70
1
Fig. 13. BSE images of zircons from retrogressed eclogitic rock (sample GS-3), Gridino complex,
2
Belomorian province containing biotite-association inclusions. Their microprobe analyses are
3
shown in Table 7.
4 Fig. 14. Reconstructed metamorphic PT paths of the Gridino boudin-like eclogite (this work), the
6
eclogite-blueschist of the Pam Peninsula, NE New Caledonia (after Clarke et al., 1997), and the
7
Eastern Blue Ridge, North Carolina, USA (after Page et al., 2003) (modified after Mints et al.,
8
2010). (PP - prehnite-pumpellyite; LB - lawsonite blue-schist; GS - green-schist; EB - epidote-blue-
9
schist; EA - epidote-amphibolite; A - amphibolite; GA - garnet amphibolite; PG - pyroxene
cr
us
10
ip t
5
granulite; GG - garnet granulite).
238
an
11
U-206Pb age relative probability for (a) eclogite (sample GS-1) and (b)
Fig. 15. Zircon
13
retrogressed eclogite (sample GS-2) from the Gridino complex of Belomorian province.
M
12
17 18 19 20 21
pt
16
Ac ce
15
ed
14
22 23
47 Page 47 of 70
ip t
Ac c
ep te
d
M
an
us
cr
Table 1. Representative microprobe analyses of rock forming minerals (clinopyroxene, garnet, amphibole, plagioclase) from eclogite (sample GS-1) and retrogressed eclogite (samples GS-2 & GS-3) from the Gridino complex, Belomorian province. Clinopyroxene Garnet GS-1 GS-2 GS-3 Sample GS-1 GS-2 GS-3 Sample 1 2 3 4 1 2 1 2 1 2 3 4 1 2 1 2 N N 55.22 54.41 54.81 54.27 51.21 50.29 51.16 51.50 SiO2 37.57 38.10 38.42 38.22 38.69 38.12 37.48 38.45 SiO2 0.00 0.03 0.22 0.19 0.06 0.27 0.25 0.06 TiO2 0.07 0.01 0.05 0.04 0.00 0.01 0.00 0.00 TiO2 7.19 8.50 7.21 8.09 4.05 5.67 4.75 3.91 21.97 21.24 22.08 21.94 21.71 21.97 21.43 21.76 Al2O3 Al2O3 0.03 0.00 0.03 0.11 0.02 0.03 0.05 0.02 Cr2O3 0.03 0.21 0.00 0.09 0.06 0.06 0.12 0.07 Cr2O3 4.78 5.42 5.25 5.17 9.33 9.04 9.04 8.87 FeO 23.80 24.38 23.28 22.60 22.75 22.35 22.56 22.43 FeO 11.69 10.58 10.92 10.62 12.24 11.45 11.78 12.10 MgO 6.39 6.35 6.40 6.49 4.91 5.66 4.33 4.67 MgO 0.00 0.02 0.04 0.03 0.04 0.06 0.11 0.05 MnO 0.90 0.94 0.66 0.57 0.67 0.57 0.69 0.60 MnO 17.59 16.99 17.26 16.68 22.59 21.93 22.29 22.64 CaO 9.25 9.06 9.95 9.99 11.82 11.32 12.59 12.39 CaO 4.34 4.46 4.09 4.67 0.79 0.79 0.79 0.67 Na2O 0.00 0.03 0.00 0.00 0.00 0.02 0.00 0.00 Na2O 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 K2O K2 O 100.86 100.39 99.82 99.84 100.33 99.53 100.21 99.82 Total 99.98 100.32 100.84 99.95 100.61 100.08 99.19 100.36 Total p.f.u. 1.97 1.95 1.98 1.95 1.90 1.88 1.90 1.92 Si 2.90 2.94 2.94 2.94 2.98 2.94 2.93 2.97 Si 0.00 0.00 0.01 0.01 0.00 0.01 0.01 0.00 Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Ti 0.30 0.36 0.31 0.34 0.18 0.25 0.21 0.17 Al 2.00 1.93 1.99 1.99 1.97 1.99 1.98 1.98 Al 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Cr 0.00 0.01 0.00 0.01 0.00 0.00 0.01 0.00 Cr 3+ 3+ 0.07 0.05 0.00 0.06 0.08 0.03 0.03 0.03 Fe 0.19 0.18 0.13 0.11 0.06 0.12 0.14 0.08 Fe 0.08 0.12 0.16 0.09 0.21 0.26 0.25 0.25 Fe2+ 1.35 1.40 1.36 1.34 1.40 1.32 1.33 1.37 Fe2+ 0.62 0.57 0.59 0.57 0.68 0.64 0.65 0.67 Mg 0.74 0.73 0.73 0.75 0.56 0.65 0.51 0.54 Mg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Mn 0.06 0.06 0.04 0.04 0.04 0.04 0.05 0.04 Mn 0.67 0.65 0.67 0.64 0.90 0.88 0.89 0.91 Ca 0.77 0.75 0.82 0.82 0.98 0.94 1.06 1.03 Ca 0.30 0.31 0.29 0.33 0.06 0.06 0.06 0.05 Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Na 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 K K endmembers 70 68 71 67 94 94 94 95 46 47 46 46 47 45 45 46 WEF Alm 24 27 29 28 3 5 5 4 9 9 6 5 3 6 7 4 Jd And 6 4 0 6 3 1 1 1 17 16 21 22 29 26 29 31 Ae Grs 25 25 25 25 19 22 17 18 Prp 2 2 1 1 1 1 2 1 Sps 0 1 0 0 0 0 0 0 Uv 48 Page 48 of 70
ip t cr 4 42.94 0.36 15.28 0.00 12.45 12.65 0.09 10.93 3.47 0.19 98.37
GS-2 1 2 42.84 41.84 1.19 1.46 13.50 14.02 0.06 0.25 15.29 16.05 10.67 9.79 0.10 0.06 11.04 11.73 1.62 1.63 0.93 1.07 97.23 97.89
GS-3 1 2 42.61 43.14 1.17 0.96 13.98 13.53 0.15 0.09 15.60 15.99 10.33 10.23 0.07 0.11 11.90 11.84 1.58 1.47 1.12 1.02 98.51 98.39
6.61 0.08 2.08 0.02 0.00 1.49 3.08 0.02 1.64 0.84 0.07
6.30 0.04 2.64 0.00 0.00 1.53 2.77 0.01 1.72 0.99 0.04
6.41 0.13 2.38 0.01 0.00 1.91 2.38 0.01 1.77 0.47 0.18
6.31 0.13 2.44 0.02 0.00 1.93 2.28 0.01 1.89 0.45 0.21
us
GS-1 2 3 45.26 43.18 0.71 0.89 12.07 14.28 0.13 0.15 12.19 12.33 14.14 12.99 0.14 0.08 10.50 10.74 2.97 3.36 0.38 0.23 98.49 98.22
2.80 0.00 1.18 0.00 0.01 0.00 0.00 0.20 0.84 0.01
Ab An Or
79 21 0
80 19 1
2.63 0.00 1.36 0.00 0.00 0.00 0.00 0.37 0.65 0.01
2.55 0.00 1.45 0.00 0.00 0.00 0.00 0.45 0.57 0.01
2.59 0.00 1.40 0.00 0.00 0.00 0.00 0.42 0.57 0.01
2.51 0.00 1.48 0.00 0.00 0.00 0.00 0.48 0.52 0.01
63 36 1
56 44 1
57 42 1
51 48 1
Si Ti Al Cr Fe3+ Fe2+ Mg Mn Ca Na K
ep te
2.76 0.00 1.21 0.00 0.01 0.00 0.00 0.22 0.85 0.00
Ac c
Si Ti Al Cr Fe3+ Fe2+ Mg Mn Ca Na K
d
M
an
Plagioclase Amphibole GS-1 GS-2 GS-3 Sample Sample 1 2 1 2 1 2 1 N N 62.33 62.84 59.03 57.00 57.45 55.75 SiO2 42.83 SiO2 0.01 0.01 0.00 0.01 0.00 0.00 TiO2 1.68 TiO2 23.10 22.56 26.01 27.57 26.41 27.97 Al2O3 13.05 Al2O3 0.21 Cr2O3 Cr2O3 0.35 0.21 0.02 0.02 0.02 0.05 FeO 13.02 FeO 0.01 0.00 0.00 0.00 0.01 0.00 MgO 12.89 MgO 0.02 0.02 0.01 0.01 0.02 0.03 MnO 0.07 MnO 4.64 4.17 7.78 9.31 8.71 10.02 CaO 10.98 CaO 9.90 9.70 7.50 6.54 6.47 5.94 Na2O 3.48 Na2O 0.05 0.08 0.17 0.10 0.17 0.19 K2O 0.20 K2O 100.42 99.60 100.52 100.55 99.25 99.95 Total 98.41 Total 6.33 0.19 2.27 0.03 0.00 1.61 2.84 0.01 1.74 1.00 0.04
6.35 0.10 2.47 0.02 0.00 1.52 2.85 0.01 1.69 0.96 0.04
6.26 0.16 2.47 0.03 0.00 2.01 2.18 0.01 1.88 0.47 0.20
6.39 0.11 2.36 0.01 0.02 1.96 2.26 0.01 1.88 0.42 0.19
49 Page 49 of 70
ip t cr
Th/U
Isotopic ratios
13 11 75 112 101 48 22
40 2 88 2 28 10 19 10 60 58 9 8 12 36 60 3
3 4 33 14 31 10 9
0.31 0.20 0.45 0.25 0.47 0.33 0.47 0.44 0.36 0.35 0.35 0.34 0.26 0.47 0.20 0.29
Pb/ Pb 0.16349 0.18389 0.17603 0.18448 0.17975 0.15077 0.17965 0.18274 0.18305 0.17519 0.18549 0.18521 0.18271 0.18379 0.18361 0.18729
±1σ 0.00224 0.005 0.00225 0.0045 0.00265 0.00306 0.00283 0.00329 0.0023 0.00228 0.00281 0.00334 0.00289 0.00256 0.00224 0.00495
207
235
Pb/ U 9.44321 12.7892 11.06214 13.0977 12.34394 8.06743 11.83314 12.04471 13.01616 11.67632 13.35987 13.35851 12.93523 13.09326 13.05265 13.50255
±1σ 0.1267 0.36226 0.13684 0.33189 0.18045 0.1637 0.18628 0.22032 0.15931 0.14848 0.20346 0.24719 0.20644 0.1807 0.15427 0.37426
0.26 0.35 0.44 0.13 0.30 0.20 0.40
0.16765 0.16674 0.16961 0.17083 0.17039 0.17281 0.17257
0.00355 0.00601 0.00247 0.00234 0.00283 0.00386 0.00275
11.03 10.95561 11.23367 11.49334 11.4043 11.53554 11.73451
0.23884 0.41017 0.16293 0.15441 0.1905 0.20752 0.18755
an
130 11 194 10 58 32 41 23 166 167 27 24 47 77 296 10
206
d
207
M
Th(ppm)
ep te
Age group I GS-1-030 GS-1-037 GS-1-035 GS-1-048 GS-1-039 GS-1-028 GS-1-036 GS-1-012 GS-1-016 GS-1-022 GS-1-002 GS-1-004 GS-1-020 GS-1-021 GS-1-014 GS-1-023 Age group IV GS-1-005 GS-1-029 GS-1-007 GS-1-044 GS-1-046 GS-1-008 GS-1-018
U(ppm)
Ac c
Sample spot
us
Table 2. U-Pb isotope data of zircons from eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) of the Gridino complex, Belomorian province (LA-ICP-MS data): Age (Ma) 206
238
Pb/ U 0.41905 0.5046 0.45594 0.5151 0.49823 0.3882 0.47789 0.47819 0.5159 0.48356 0.52255 0.52328 0.51365 0.51687 0.51577 0.52305
±1σ 0.00455 0.01057 0.00467 0.00975 0.0058 0.00565 0.00592 0.00679 0.00534 0.00511 0.00646 0.00763 0.00654 0.00585 0.00521 0.01089
t207/235 2382 2664 2528 2687 2631 2239 2591 2608 2681 2579 2705 2705 2675 2686 2683 2715
±1σ 12 27 12 24 14 18 15 17 12 12 14 17 15 13 11 26
t206/238 2256 2634 2422 2678 2606 2114 2518 2519 2682 2543 2710 2713 2672 2686 2681 2712
±1σ 21 45 21 41 25 26 26 30 23 22 27 32 28 25 22 46
0.47733 0.47669 0.48052 0.48814 0.4856 0.48412 0.49334
0.00762 0.01249 0.00563 0.00528 0.00626 0.0064 0.00619
2526 2519 2543 2564 2557 2567 2583
20 35 14 13 16 17 15
2516 2513 2530 2563 2552 2545 2585
33 55 25 23 27 28 27
50 Page 50 of 70
0.00227 0.00269 0.00187 0.00224 0.00248 0.0052 0.0027
84 90 79 14 26 1 6 19 18 16 4 4
50 52 40 3 11 0 3 4 5 4 1 1
0.60 0.58 0.51 0.24 0.43 0.51 0.49 0.20 0.26 0.26 0.32 0.32
0.1789 0.18251 0.1788 0.18179 0.18161 0.18928 0.18017 0.17999 0.18337 0.17946 0.17925 0.18101
0.00271 0.00272 0.0035 0.006 0.00635 0.02091 0.00549 0.00504 0.00635 0.00418 0.00525 0.00739
27 17 6 6 30 14 66 79 103
9 5 2 3 10 3 33 40 69
0.32 0.30 0.37 0.49 0.34 0.24 0.50 0.51 0.67
0.15809 0.19165 0.17402 0.18017 0.19236 0.18179 0.19041 0.1788 0.13988
3
0.46
0.18455
9.89797 5.64893 7.6553 11.29756 12.13787 9.52468 12.12883
ip t
0.16306 0.11853 0.14342 0.17091 0.17629 0.15609 0.1762
6
cr
0.57 0.01 0.46 0.33 0.76 0.60 0.55
0.13564 0.12733 0.09685 0.14492 0.16772 0.32419 0.18526
0.4404 0.34576 0.38727 0.47958 0.49955 0.44272 0.49943
0.00492 0.00509 0.00405 0.00515 0.00552 0.01002 0.006
2425 1924 2191 2548 2615 2390 2614
13 19 11 12 13 31 14
2352 1914 2110 2525 2612 2363 2611
22 24 19 22 24 45 26
11.46454 11.92726 11.92561 12.47377 11.86134 8.56169 11.87969 12.34649 11.13565 12.55411 12.48617 12.74803
0.17002 0.17365 0.23654 0.42916 0.35398 0.94025 0.37348 0.35907 0.3345 0.30026 0.37771 0.54698
0.46495 0.47416 0.48393 0.49783 0.47368 0.32817 0.47839 0.49768 0.44044 0.50753 0.50539 0.51098
0.00514 0.00517 0.00709 0.0123 0.00865 0.02252 0.01051 0.01052 0.00759 0.00874 0.01066 0.01557
2562 2599 2599 2641 2594 2292 2595 2631 2535 2647 2642 2661
14 14 19 32 28 100 29 27 28 22 28 40
2461 2502 2544 2604 2500 1830 2520 2604 2353 2646 2637 2661
23 23 31 53 38 109 46 45 34 37 46 66
0.00428 0.00444 0.00582 0.00549 0.00327 0.006 0.00281 0.0035 0.00227
9.39537 14.11785 11.40238 11.87969 14.16082 12.47377 13.96238 11.92561 7.52064
0.25872 0.33932 0.39477 0.37348 0.24216 0.42916 0.20207 0.23654 0.11948
0.4312 0.53447 0.47539 0.47839 0.53412 0.49783 0.53202 0.48393 0.39008
0.00819 0.0096 0.0116 0.01051 0.00689 0.0123 0.00588 0.00709 0.00443
2377 2758 2557 2595 2761 2641 2747 2599 2175
25 23 32 29 16 32 14 19 14
2311 2760 2507 2520 2759 2604 2750 2544 2123
37 40 51 46 29 53 25 31 21
0.00527
13.19168
0.39319
0.51862
0.01128
2693
28
2693
48
us
33 0 81 56 71 3 29
ep te
d
M
an
59 33 175 170 93 5 52
Ac c
GS-1-006 GS-1-033 GS-1-011 GS-1-003 GS-1-047 GS-1-010 GS-1-045 Age group II GS-2-018 GS-2-016 GS-2-009 GS-2-015 GS-2-004 GS-2-001 GS-2-017 GS-2-019 GS-2-012 GS-2-007 GS-2-010 GS-2-025 Age group X GS-2-027 GS-2-003 GS-2-020 GS-2-017 GS-2-011 GS-2-015 GS-2-008 GS-2-009 GS-2-024 Age group I GS-3-012
51 Page 51 of 70
0.18617 0.18605 0.18525 0.18212 0.18116 0.1837 0.18483 0.18596 0.18527
0.00419 0.00408 0.00297 0.00276 0.00288 0.00318 0.00264 0.00299 0.00342
13.3619 13.28523 13.23547 12.05354 11.53068 12.01233 12.17345 12.96234 13.22893
9.75 17.44 8.69 7.81 28.85 78.96 14.59 6.97 40.65
2.74 8.31 2.36 2.7 8.32 36.38 5.73 2.82 15.33
0.28 0.48 0.27 0.35 0.29 0.46 0.39 0.40 0.38
0.18118 0.17469 0.17718 0.17987 0.17623 0.17806 0.17353 0.18026 0.17958
0.00378 0.00432 0.00741 0.00625 0.00379 0.00286 0.00341 0.00653 0.00318
56.97 22.01 80.82 59.58 83.81 11.51 32.36 18.48
21.46 6.67 46.29 23.05 40.99 3.86 9.84 4.44
0.38 0.30 0.57 0.39 0.49 0.34 0.30 0.24
0.18188 0.181 0.18303 0.1371 0.16436 0.16006 0.16532 0.1694
0.00294 0.00351 0.00342 0.00224 0.00234 0.00354 0.00257 0.00325
0.3104 0.30156 0.21335 0.18122 0.1822 0.20907 0.16967 0.20886 0.24902
0.52073 0.51808 0.51835 0.48018 0.4618 0.47443 0.47785 0.50573 0.51807
0.00894 0.00894 0.00652 0.00567 0.0056 0.00633 0.00519 0.00634 0.00744
2706 2700 2697 2609 2567 2605 2618 2677 2696
22 21 15 14 15 16 13 15 18
2702 2691 2692 2528 2448 2503 2518 2638 2691
38 38 28 25 25 28 23 27 32
11.40496 11.01598 11.8614 12.57931 10.913 11.69588 10.32085 12.61512 12.56464
0.24149 0.27823 0.51755 0.45709 0.19199 0.18648 0.20423 0.47904 0.22434
0.45672 0.45751 0.48572 0.5074 0.44913 0.47657 0.43151 0.50775 0.50764
0.00716 0.00832 0.0149 0.01313 0.00554 0.00579 0.00629 0.01385 0.00682
2557 2524 2594 2649 2516 2580 2464 2651 2648
20 24 41 34 16 15 18 36 17
2425 2429 2552 2646 2391 2512 2313 2647 2647
32 37 65 56 25 25 28 59 29
12.82978 12.7302 12.94742 6.58223 9.5389 9.61945 10.01354 10.66537
0.20762 0.25273 0.24591 0.10505 0.13268 0.21442 0.15391 0.20554
0.51178 0.51029 0.51324 0.34834 0.42107 0.43605 0.43947 0.4568
0.0064 0.0077 0.00736 0.00406 0.00455 0.00679 0.0052 0.00644
2667 2660 2676 2057 2391 2399 2436 2494
15 19 18 14 13 21 14 18
2664 2658 2670 1927 2265 2333 2348 2425
27 33 31 19 21 30 23 28
an
M
d
ep te
ip t
0.20 0.30 0.31 0.32 0.32 0.35 0.55 0.38 0.30
cr
2 6 11 18 15 14 55 12 15
us
11 21 34 55 49 41 101 31 51
Ac c
GS-3-033 GS-3-001 GS-3-008 GS-3-005 GS-3-020 GS-3-013 GS-3-025 GS-3-010 GS-3-017 Age group II GS-3-003 GS-3-022 GS-3-023 GS-3-027 GS-3-002 GS-3-026 GS-3-007 GS-3-021 GS-3-032 Age group III GS-3-015 GS-3-006 GS-3-031 GS-3-019 GS-3-018 GS-3-029 GS-3-004 GS-3-028
52 Page 52 of 70
ip t cr
Dy
Ho
Er
Tm
Yb
Lu
Ti
δEu
δCe
Ti-in-zrn, oC
0.38 0.42 1.18 0.82 0.95 1.91 1.88 1.42 0.67 0.51 0.93 1.34 0.39 1.79 1.59 0.64
3.82 4.11 11.91 7.61 8.67 18.48 17.37 14.02 6.09 3.78 8.14 12.06 3.66 15.85 13.69 4.76
1.20 1.32 3.46 2.26 2.56 5.29 4.75 4.11 1.60 1.05 2.27 3.43 0.98 4.14 3.62 1.11
4.54 4.35 11.36 7.26 8.54 17.98 15.11 13.10 5.11 3.59 7.10 10.66 3.04 12.91 11.47 3.27
0.87 0.66 1.87 1.26 1.49 2.89 2.33 2.03 0.82 0.54 1.08 1.73 0.44 1.92 1.72 0.43
7.26 6.34 14.59 9.71 12.05 22.73 17.75 14.94 6.24 4.18 8.81 13.56 3.45 13.82 13.91 3.61
1.38 0.97 2.36 1.60 1.96 3.65 2.80 2.23 1.02 0.67 1.38 2.17 0.64 2.26 2.25 0.57
2.79 3.74 39.23 2.77 4.45 5.64 3.37 3.26 3.99 2.47 4.29 4.80 3.94 3.28 5.68 3.21
0.67 0.51 0.54 0.45 0.50 0.40 0.50 0.51 0.34 0.88 0.39 0.53 0.39 0.45 0.51 0.66
9.69 4.60 11.20 7.46 6.67 5.99 5.91 6.62 10.01 10.78 13.89 7.52 17.47 9.14 6.67 4.69
639 660 876 638 673 692 652 650 665 630 671 679 664 650 692 649
0.39 0.69 1.21 0.42 0.46 0.71 1.06 1.35 0.23 1.30 0.39 2.05
4.25 8.16 12.44 4.16 4.69 6.50 9.54 11.80 1.83 12.04 3.53 18.44
1.27 2.64 3.86 1.23 1.28 1.84 2.55 3.31 0.41 3.07 0.97 4.97
4.70 9.08 12.69 4.41 4.64 6.67 8.04 10.47 1.33 9.64 2.69 14.63
0.99 1.50 2.12 0.77 0.80 1.10 1.25 1.74 0.23 1.52 0.42 2.33
9.03 11.58 16.98 6.20 6.42 8.19 10.06 13.74 1.85 11.20 3.37 16.23
1.54 2.01 2.72 1.03 1.11 1.46 1.64 2.31 0.40 1.71 0.55 2.54
4.28 3.72 3.85 5.22 4.10 4.38 2.55 7.02 1.89 5.67 3.64 5.78
1.44 0.47 0.48 0.45 0.81 0.54 0.64 0.86 0.99 0.52 0.44 0.46
6.98 2.79 6.59 6.30 6.28 7.88 3.75 8.18 2.42 15.32 13.05 9.02
670 660 662 686 667 672 632 710 612 692 658 694
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
36.94 38.96 104.14 66.93 75.19 157.15 141.73 124.80 49.62 31.93 68.33 100.09 28.31 118.66 104.81 34.35
0.03 0.04 0.04 0.02 0.03 0.04 0.03 0.02 0.03 0.02 0.00 0.02 0.00 0.03 0.03 0.03
0.81 0.50 1.31 0.61 0.72 0.78 0.75 0.70 1.01 1.22 0.79 1.00 0.70 0.87 1.23 0.69
0.02 0.02 0.02 0.02 0.02 0.03 0.03 0.02 0.02 0.03 0.02 0.03 0.01 0.02 0.04 0.03
0.20 0.13 0.26 0.15 0.12 0.38 0.23 0.35 0.12 0.25 0.17 0.42 0.12 0.24 0.53 0.47
0.15 0.17 0.57 0.35 0.52 1.10 0.82 0.92 0.25 0.31 0.38 0.95 0.29 0.80 1.34 0.72
0.13 0.10 0.32 0.19 0.26 0.44 0.47 0.42 0.12 0.23 0.19 0.47 0.11 0.46 0.58 0.39
1.21 1.22 3.45 2.48 3.06 6.22 5.55 4.59 2.18 1.47 3.03 4.98 1.49 6.17 6.26 3.22
40.67 79.94 118.72 39.29 41.77 58.11 80.42 95.00 13.23 96.26 28.15 138.24
0.03 0.03 0.14 0.04 0.03 0.05 0.04 0.06 0.04 0.04 0.02 0.03
0.87 0.41 1.76 0.71 0.92 1.29 0.88 1.96 0.37 1.94 1.18 1.03
0.03 0.03 0.03 0.02 0.04 0.03 0.06 0.05 0.03 0.02 0.02 0.02
0.30 0.17 0.19 0.18 0.31 0.29 0.58 0.55 0.38 0.32 0.19 0.28
0.45 0.28 0.49 0.28 0.32 0.40 0.85 1.00 0.20 0.73 0.30 1.10
M
d
ep te 0.34 0.15 0.28 0.12 0.25 0.25 0.41 0.76 0.16 0.38 0.11 0.56
an
Y
Ac c
Sample spot Age group I GS-1-030 GS-1-037 GS-1-035 GS-1-048 GS-1-039 GS-1-028 GS-1-036 GS-1-012 GS-1-016 GS-1-022 GS-1-002 GS-1-004 GS-1-020 GS-1-021 GS-1-014 GS-1-023 Age group IV GS-1-005 GS-1-029 GS-1-007 GS-1-044 GS-1-046 GS-1-008 GS-1-018 GS-1-006 GS-1-033 GS-1-011 GS-1-003 GS-1-047
1.07 1.86 3.38 1.54 1.71 2.72 3.38 4.88 0.89 4.16 1.35 7.19
us
Table 3. Geochemical features of zircons of different age groups of eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino complex, Belomorian province (LA-ICP-MS data):
53 Page 53 of 70
ip t
0.03 0.02
0.86 1.35
0.02 0.02
0.34 0.78
0.38 2.08
0.17 0.85
1.81 9.68
0.44 2.11
3.09 12.85
0.67 2.44
98.40 78.70 73.10 64.53 135.04 24.63 35.14 39.92 57.83 60.05 62.27 99.99
0.03 0.02 0.03 0.03 0.11 0.04 0.03 0.03 0.14 0.04 0.03 0.02
0.83 0.80 0.93 0.65 0.83 0.08 0.23 0.46 0.87 0.82 0.23 0.26
0.03 0.02 0.02 0.02 0.08 0.03 0.02 0.02 0.05 0.05 0.02 0.00
0.21 0.32 0.32 0.15 0.76 0.19 0.13 0.13 0.43 0.21 0.14 0.05
0.72 0.65 0.63 0.28 1.35 0.17 0.29 0.17 0.42 0.30 0.25 0.27
0.27 0.40 0.33 0.15 0.44 0.08 0.12 0.10 0.30 0.30 0.12 0.09
4.70 3.98 3.51 2.26 5.41 0.96 1.29 1.06 1.72 1.48 1.16 1.79
1.36 1.16 1.01 0.75 1.61 0.34 0.36 0.40 0.54 0.51 0.42 0.69
12.74 9.47 9.07 7.01 15.60 3.11 3.92 4.18 4.84 5.96 5.24 8.46
3.18 2.61 2.38 2.17 4.15 0.69 1.10 1.27 1.72 1.80 1.98 3.05
10.02 7.88 7.46 6.52 14.64 2.43 4.41 4.28 6.93 7.34 7.61 13.18
110.56 120.66 54.63 35.14 79.16 64.53 67.48 73.10 88.29
0.02 0.01 0.02 0.03 0.02 0.03 0.06 0.03 0.01
0.49 0.39 0.24 0.23 0.72 0.65 1.19 0.93 0.80
0.00 0.02 0.02 0.02 0.02 0.02 0.07 0.02 0.01
0.00 0.18 0.10 0.13 0.37 0.15 0.83 0.32 0.22
0.29 0.34 0.17 0.29 0.54 0.28 1.04 0.63 0.96
0.18 0.17 0.18 0.12 0.17 0.15 0.36 0.33 0.54
1.63 1.93 1.52 1.29 2.42 2.26 3.36 3.51 5.15
0.63 0.75 0.44 0.36 0.78 0.75 0.90 1.01 1.40
8.10 9.43 5.29 3.92 8.03 7.01 7.98 9.07 11.96
3.52 3.71 1.71 1.10 2.46 2.17 2.13 2.38 3.03
84.91 41.11 103.40 87.95 38.64 32.98 101.61 95.22 189.92 33.83
0.03 0.03 0.02 0.03 0.03 0.04 0.08 0.03 0.03 0.13
0.28 0.50 0.65 0.65 0.79 0.58 1.47 0.99 0.99 1.35
0.01 0.03 0.02 0.02 0.03 0.03 0.05 0.02 0.05 0.10
0.16 0.24 0.16 0.15 0.15 0.19 0.97 0.22 1.02 0.94
0.27 0.48 0.46 0.37 0.33 0.20 0.97 0.86 2.22 0.97
2.21 1.13 3.05 2.47 1.60 1.17 4.12 5.33 9.76 2.82
0.74 0.43 0.99 0.87 0.44 0.42 1.20 1.42 2.80 0.60
8.15 4.04 10.74 9.25 4.29 3.70 11.08 12.17 25.69 4.49
2.66 1.29 3.42 2.84 1.24 1.03 3.19 3.19 6.67 1.07
1.62 4.75
0.25 0.53
4.60 4.73
0.51 0.48
8.59 12.66
676 678
1.49 1.27 1.19 0.98 2.39 0.43 0.74 0.72 1.27 1.44 1.51 2.63
11.32 9.32 9.25 7.76 20.29 3.69 6.56 5.95 11.46 11.84 13.07 24.58
1.66 1.54 1.46 1.22 3.21 0.65 1.19 0.99 2.32 2.37 2.37 4.41
4.13 4.04 3.47 4.01 6.84 1.78 2.40 3.72 156.53 5.09 4.16 3.37
0.34 0.59 0.54 0.39 0.43 0.50 0.49 0.54 0.94 1.14 0.54 0.31
6.34 7.55 9.41 7.22 2.07 0.58 2.27 4.99 2.62 4.23 2.62 10.16
668 666 655 665 708 608 628 660 684 668 652
17.06 17.39 6.45 4.41 9.06 6.52 6.86 7.46 8.95
4.05 3.72 1.11 0.74 1.65 0.98 1.10 1.19 1.33
45.63 36.95 9.73 6.56 13.22 7.76 8.89 9.25 10.62
9.99 7.66 1.84 1.19 2.18 1.22 1.49 1.46 1.67
4.20 3.93 2.80 2.40 4.57 4.01 13.62 3.47 6.82
0.64 0.49 0.72 0.49 0.38 0.39 0.54 0.54 0.59
17.60 5.64 2.93 2.27 7.25 7.22 4.01 9.41 21.41
669 664 639 628 675 665 768 655 707
10.11 4.64 12.56 10.05 4.14 3.39 10.13 9.87 18.78 2.65
1.83 0.78 2.06 1.67 0.75 0.59 1.59 1.44 2.60 0.43
16.32 6.08 17.97 14.07 7.28 5.10 12.15 10.69 17.95 2.73
2.87 1.05 2.74 2.17 1.11 0.73 1.88 1.76 2.39 0.43
3.13 5.01 4.36 4.87 5.72 3.45 5.75 3.41 4.71 10.94
0.34 0.57 0.61 0.53 0.56 0.68 0.54 0.44 0.39 0.57
3.72 3.82 8.21 6.74 6.36 4.17 5.68 9.33 5.21 2.65
647 683 672 680 693 654 693 653 678 748
us
an
M
d
ep te 0.12 0.14 0.32 0.22 0.16 0.14 0.41 0.40 0.70 0.33
1.63 5.63
0.24 0.71
cr
20.11 73.13
Ac c
GS-1-010 GS-1-045 Age group II GS-2-018 GS-2-016 GS-2-009 GS-2-015 GS-2-004 GS-2-001 GS-2-017 GS-2-019 GS-2-012 GS-2-007 GS-2-010 GS-2-025 Age group X GS-2-027 GS-2-003 GS-2-020 GS-2-017 GS-2-011 GS-2-015 GS-2-008 GS-2-009 GS-2-024 Age group I GS-3-012 GS-3-033 GS-3-001 GS-3-008 GS-3-005 GS-3-020 GS-3-013 GS-3-025 GS-3-010 GS-3-017 Age group II
54 Page 54 of 70
0.19 1.13 0.48 0.47 0.32 0.44 0.85 0.35 0.78
0.07 0.50 0.31 0.26 0.18 0.15 0.92 0.14 0.34
1.24 4.52 1.95 2.49 2.40 3.30 2.07 2.90 3.44
70.28 105.33 78.08 41.04 81.71 29.75 85.80 65.30
0.03 0.03 0.02 0.06 0.04 0.06 0.03 0.02
1.12 1.05 1.12 0.88 1.31 0.43 0.81 0.32
0.07 0.03 0.03 0.07 0.12 0.03 0.02 0.02
0.45 0.49 0.53 0.75 0.70 0.15 0.29 0.06
0.70 1.19 0.83 0.68 1.25 0.13 0.38 0.17
0.34 0.50 0.35 0.21 0.42 0.08 0.29 0.09
2.53 5.47 4.52 1.88 4.10 0.78 2.24 0.83
0.57 1.53 0.52 0.80 0.93 1.24 0.53 0.87 0.87 0.80 1.68 1.18 0.53 1.12 0.28 0.79 0.35
3.29 6.94 2.77 3.61 3.20 4.90 1.60 2.68 2.38
15.99 30.98 12.52 14.16 11.89 17.04 5.91 9.52 8.53
3.58 6.56 2.66 2.68 2.19 2.88 1.17 1.54 1.32
36.70 62.65 24.94 23.66 18.27 22.46 10.72 12.43 11.44
7.14 12.92 4.99 4.25 2.92 3.69 1.95 2.12 1.82
3.21 3.27 4.55 3.13 5.59 3.50 99.60 3.48 11.96
0.34 0.58 0.83 0.59 0.45 0.27 2.02 0.29 0.53
4.65 2.19 3.39 5.61 6.86 7.11 4.12 5.14 4.47
649 650 675 647 691 655 655 756
7.90 14.05 9.77 4.58 9.29 2.54 8.67 4.58
2.23 3.54 2.45 1.24 2.49 0.82 2.88 1.99
7.21 10.69 7.79 3.91 8.04 3.58 10.79 9.97
1.08 1.65 1.24 0.67 1.34 0.80 1.97 2.46
8.38 12.88 9.99 5.09 9.56 8.09 16.79 24.49
1.23 1.85 1.52 0.92 1.61 1.76 2.84 5.41
3.52 4.48 4.65 6.55 6.75 3.79 5.71 2.73
0.69 0.50 0.44 0.53 0.52 0.56 0.76 0.58
4.49 7.47 9.77 3.11 3.15 2.42 7.29 3.55
656 674 677 704 706 661 693 637
ep te
d
7.42 17.89 7.30 9.87 9.31 14.72 5.18 8.64 8.19
ip t
0.15 1.23 0.24 0.27 0.19 0.12 0.89 0.18 0.65
cr
0.02 0.10 0.04 0.02 0.02 0.03 0.11 0.02 0.05
us
0.38 0.86 0.60 0.66 0.60 0.83 1.89 0.39 0.94
an
0.02 0.07 0.04 0.04 0.03 0.03 0.08 0.02 0.05
M
101.65 214.11 87.22 104.62 95.13 143.35 51.58 82.65 75.95
Ac c
GS-3-003 GS-3-022 GS-3-023 GS-3-027 GS-3-002 GS-3-026 GS-3-007 GS-3-021 GS-3-032 Age group III GS-3-015 GS-3-006 GS-3-031 GS-3-019 GS-3-018 GS-3-029 GS-3-004 GS-3-028
55 Page 55 of 70
ip t
an
us
cr
Table 4. Trace element content and U-Pb ages of zircons from the Sulu UHP eclogite, the Dabie HP/UHP eclogite, the Hengshan HP granulite, the Bohemian eclogite/granulite and the Grinio eclogite (age groups I, and IV) of different PT conditions ([1] Chen, 2009; [2] Liu et al., 2011; [3] Zhang, 2013; [4] Bröcker et al., 2010). Dabie eclogite[2] Hengshan[3] Bohemian[4] Gridino (this work) Sulu eclogite[1] HP UHP HP granulite eclogite granulite I age group IV age group ∑REE,ppm 9-39 5-38 2-40 13-934 11-74 57-110 15-82 8-71 Avg. 16 21 24 64 36 81 54 35 14-58 27
9-60 34
7-66 38
20-1101 103
23-119 62
25-176 96
28-157 80
13-138 66
Th, ppm Avg.
1-16 4
1-6 4
1-6 4
1-99 6
0-11 4
9-326 122
2-88 28
b.d.-81 27
U, ppm Avg. Age, Ma P, GPa T, °C
29-277 78 226-235 3-4.4 600-910
10-296 78 2698
5-175 69 2609
M
Y, ppm Avg.
ep te
d
39-380 14-527 228 271 210-215 222-227 2 4 940-990 910-980
38-824 155 1862 1.4-1.5 780-850
42-297 114-1040 136 535 330-350 > 2.7 1.8-2.8 690-820 800-1000
1.7-1.9 650-710
Ac c
Table 5. List of all mineral inclusions identified in different textural domains (A, B, C, D, F) of zircons from eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino complex of the Belomorian province. A question mark in the bracket indicates a possible mineral identified with EDS analysis yet without microprobe data. A – zonal texture of inner (A1) and outer (A2) domains; B – fir-tree/patchy textureal domains; C – homogeneous domains of low-CL or high-CL image; D* – domains compromised by fluid injection trails; F – outer fringe of high-CL. (Grt – garnet, Di – diopside, Amp – amphibole, Bt – biotite, Ms – muscovite, Pl – plagioclase, Kfs – K-feldspar, Zo – zoisite, Ap – apatite, Ttn – titanite, Rt – rutile, Qz – quartz, Cal – Ca-carbonate) A
A1
GS-1
Grt, Ap
GS-2 GS-3
Qz, Zo Di, Ms, Bt, Ap,
A2 Bt(?), Rt, Qz, Cal, Chalcopyrite(?) Di, Qz Grt, Pl, Ap, Ttn
B
C
Grt,
Grt, Qz, Cal,
Grt, Di, Pl, Amp, Qz, Di, Amp(?), Bt, Ttn, 56
Qz Grt, Di, Ttn
D* Grt, Bt, Pl, Kfs, Amp(?), Rt, Qz, Cal, Pl, Ttn, Qz, Cal Pl, Rt, Qz, Cal
F Pl – Grt, Ttn, Rt
Page 56 of 70
ip t
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
36.89 35.47 34.58 41.74 32.14 45.01 28.48 42.23 62.64 57.64 78.56 35.01 50.84
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.07 0.01 0.02 0.02 0.01 0.01
0.02 0.01 0.02 0.01 0.07 0.01 0.01 0.11 0.01 0.06 0.02 0.03 0.06
0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.03 0.02 0.01 0.02
0.16 0.10 0.15 0.06 0.40 0.08 0.06 0.19 0.07 0.33 0.09 0.20 0.42
0.39 0.50 0.50 0.40 0.63 0.32 0.19 0.48 0.29 0.86 0.38 0.66 0.89
0.21 0.38 0.36 0.17 0.40 0.24 0.16 0.39 0.21 0.51 0.33 0.44 0.59
1.68 2.70 3.04 2.39 2.81 1.90 1.66 2.52 2.53 4.33 3.65 3.04 3.95
0.63 5.83 1.50 4.57 0.70 0.78 5.96 1.33 3.60 0.52 0.83 6.04 1.25 3.55 0.47 0.81 7.54 1.66 4.03 0.51 0.78 5.47 1.15 3.41 0.52 0.64 6.37 1.82 6.14 0.97 0.65 5.02 1.11 3.11 0.44 0.73 6.71 1.62 5.04 0.75 0.99 9.39 2.40 7.30 1.05 1.33 10.36 2.35 6.22 0.95 1.10 12.49 3.12 9.45 1.44 0.84 6.00 1.27 3.78 0.54 1.12 8.97 2.01 5.33 0.78
5.19 3.64 3.66 3.62 3.63 8.03 3.58 5.98 8.06 6.47 9.61 3.98 5.26
0.80 0.48 0.48 0.40 0.51 1.31 0.50 0.91 1.09 0.92 1.30 0.62 0.71
2.24 3.09 2.03 1.86 1.66 0.74 1.43 1.61 0.90
0.41 0.54 0.30 0.21 0.28 0.13 0.09 0.11 0.08
1.54 1.81 0.90 0.89 0.90 0.63 0.44 0.51 0.51
0.25 0.31 0.16 0.17 0.16 0.18 0.12 0.12 0.14
1.67 2.03 1.16 1.26 1.12 1.40 1.01 1.04 1.27
0.99 1.13 0.74 0.90 0.72 0.80 0.81 0.77 0.97
0.39 0.40 0.28 0.29 0.27 0.23 0.25 0.24 0.27
1.09 1.49 0.97 1.01 0.97 0.74 1.04 0.82 0.99
0.17 0.21 0.14 0.11 0.12 0.08 0.14 0.10 0.09
0.06 0.12 0.08 0.08 0.09 0.04 0.06 0.09 0.05
0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01
ep te
d
M
an
us
Y
Ac c
Sample spot garnet GS-1-7 GS-1-22 GS-1-17 GS-1-23 GS-1-18 GS-1-21 GS-1-5 GS-1-24 GS-1-30 GS-1-10 GS-1-29 GS-1-19 GS-1-9 omphacite GS-1-11 GS-1-12 GS-1-13 GS-1-14 GS-1-15 GS-1-16 GS-1-20 GS-1-25 GS-1-26
cr
Ttn, Qz, Qz, Cal, Fe-oxide Table 6. Rare earth element (+Y) contents of garnet and omphacite in eclogite sample GS-1 from the Gridino complex, Belomorian province (LA-ICP-MS data):
0.72 0.93 0.69 0.58 0.50 0.26 0.47 0.47 0.40
0.11 0.13 0.10 0.08 0.08 0.03 0.07 0.06 0.04
0.18 0.30 0.16 0.14 0.15 0.04 0.13 0.15 0.05
0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01
57 Page 57 of 70
ip t
cr
Table 7. Representative microprobe analyses of mineral inclusions in zircon from eclogite (GS-1) and retrogressed eclogite (GS-2, GS-3) from the Gridino complex of the Belomorian province (Cpx – clinopyroxene, Grt – garnet, Pl – plagiocalse, Zo – zoisite, Mus – muscovite, Bi - biotite).
GS-3 251 49.64 0.27
GS-3 231 52.17 0.13
Sample N SiO2 TiO2
Garnet GS-1 208 38.82 0.02
GS-1 209 38.66 0.04
GS-3 233 38.54 0.01
Sample N SiO2 TiO2
Plagioclase GS-1 GS-1 215 222 56.68 56.92 0.01 0.04
GS-2 261 55.68 0.07
GS-3 237 56.42 0.00
Sample N SiO2 TiO2
Zoisite GS-2 258 38.41 0.13
Sample N SiO2 TiO2
Muscovite GS-3 243 48.09 1.28
Sample N SiO2 TiO2
Biotite GS-3 246 35.92 4.61
5.75 0.12 8.94 11.03 0.12 21.94 1.04 0.00 0.04 98.55
3.06 0.15 8.71 12.32 0.09 22.33 0.85 0.00 0.02 99.10
6.30 0.03 8.81 10.50 0.09 22.32 0.89 0.02 0.07 98.93
1.83 0.14 8.01 12.36 0.11 23.03 0.47 0.01 0.00 98.26
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
21.68 0.05 22.52 3.84 1.29 12.33 0.11 0.00 0.00 100.66
21.53 0.22 23.70 3.50 0.89 12.13 0.05 0.00 0.00 100.72
21.55 0.07 24.12 2.50 1.48 10.45 0.00 0.04 0.00 98.76
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
27.08 0.00 0.03 0.00 0.01 8.95 6.21 0.14 0.02 99.12
26.72 0.05 0.02 0.00 0.00 8.41 6.35 0.07 0.04 98.62
27.94 0.03 0.01 0.00 0.00 9.51 6.14 0.16 0.00 99.54
27.88 0.04 0.05 0.00 0.02 9.17 6.24 0.16 0.05 100.03
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
30.30 0.00 3.81 0.04 0.04 23.23 0.01 0.00 0.00 95.97
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
32.73 0.01 2.23 1.74 0.00 0.01 0.17 8.91 0.00 95.17
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total
15.75 0.54 17.26 10.99 0.05 0.01 0.34 9.56 0.07 95.11
1.86 0.01 0.26 0.00 0.20
1.93 0.01 0.14 0.00 0.22
1.87 0.01 0.28 0.00 0.25
1.98 0.00 0.08 0.00 0.26
Si Ti Al Cr Fe2+
3.00 0.00 1.97 0.00 1.43
3.00 0.00 1.97 0.01 1.52
3.07 0.00 2.02 0.00 1.61
Si Ti Al Cr Fe2+
2.56 0.00 1.44 0.00
2.58 0.00 1.43 0.00
2.52 0.00 1.49 0.00
2.53 0.00 1.47 0.00
Si Ti Al Cr Fe2+
2.99 0.01 2.79 0.00 0.00
Si Ti Al Cr Fe2+
3.18 0.06 2.55 0.00 0.12
Si Ti Al Cr Fe2+
2.73 0.26 1.41 0.03 1.10
0.08 Fe3+ 0.62 Mg 0.00 Mn 0.89 Ca 0.08 Na 0.00 K 0.00 Ni 4.00 Sum endmembers 92 WEF 5 Jd 3 Ae
0.06 0.69 0.00 0.90 0.06 0.00 0.00 4.00
0.03 0.59 0.00 0.90 0.07 0.00 0.00 4.00
0.00 0.70 0.00 0.94 0.04 0.00 0.00 4.00
Fe3+ Mg Mn Ca Na K Ni Sum
0.03 0.44 0.08 1.02 0.02 8.00
0.02 0.41 0.06 1.01 0.01 8.00
0.00 0.30 0.10 0.89 0.00 8.00
Fe3+ Mg Mn Ca Na K Ni Sum
0.00 0.00 0.00 0.43 0.54 0.01 4.98
0.00 0.00 0.00 0.41 0.56 0.00 4.98
0.00 0.00 0.00 0.46 0.54 0.01 5.00
0.00 0.00 0.00 0.44 0.54 0.01 4.99
Fe3+ Mg Mn Ca Na K Ni Sum
0.25 0.01 0.00 1.94 0.00 0.00
Fe3+ Mg Mn Ca Na K Ni
0.00 0.17 0.00 0.00 0.02 0.75 -
Fe3+ Mg Mn Ca Na K Ni
0.00 1.25 0.00 0.00 0.05 0.93 -
94 3 3
93 6 1
96 4 0
Alm And Grs
48 1 33
51 1 32
55 0 31
Ab An Or
55 44 1
58 42 0
53 46 1
55 44 1
an
M
d
ep te
Ac c
Al2O3 Cr2O3 FeO MgO MnO CaO Na2O K2O NiO Total p.f.u. Si Ti Al Cr Fe2+
us
Clinopyroxene GS-2 GS-2 260 262 49.25 51.26 0.32 0.31
Sample N SiO2 TiO2
7.99
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Fig.1
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Ac c M
an
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us
ip t
ip t Ac c
ep te
d
M
an
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cr
Fig.2
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ep te
d
M
an
Fig.3
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Fig.5
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d
M
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Fig.4
Fig.6 63 Page 63 of 70
ip t cr us an M d ep te Ac c
Fig.7
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ip t cr us an M d ep te Ac c
Fig.8
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ip t cr us an M d ep te Ac c
Fig.9
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ip t cr us an M d ep te Ac c
Fig.10
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ip t cr us an M d ep te Ac c
Fig.11
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M
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cr
Fig.12
Fig.14
Ac c
ep te
Fig.13
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ip t cr us an M d ep te Ac c
Fig.15
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