The Belomorian eclogite province: sequence of events and age of the igneous and metamorphic rocks of the Gridino association

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Russian Geology and Geophysics 53 (2012) 1023–1054 www.elsevier.com/locate/rgg

The Belomorian eclogite province: sequence of events and age of the igneous and metamorphic rocks of the Gridino association K.A. Dokukina a,*, T.B. Bayanova b, T.V. Kaulina b, A.V. Travin c, M.V. Mints a, A.N. Konilov a, P.A. Serov b a

b

Geological Institute, Russian Academy of Sciences, Pyzhevskii per. 7, Moscow, 119017, Russia Geological Institute of the Kola Science Center, Russian Academy of Sciences, ul. Fersmana 14, Apatity, 184209, Russia c V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia Received 18 February 2011; received in revised form 29 November 2011; accepted 16 February 2012

Abstract Within the Belomorian eclogite province, near Gridino Village, rocks of different compositions (tonalite-trondhjemite-granodioritic gneisses, granites, mafic and ultramafic rocks) were metamorphosed. The metamorphism included subsidence with increasing pressure and temperature, an eclogite stage, decompression in the granulitic facies, and a retrograde stage in the amphibolitic facies. We attempted to characterize the succession and to date igneous and metamorphic events in the evolution of the Gridino eclogite association. For this purpose, we conducted the following studies: U–Pb isotope dating of zircon (conventional and SHRIMP II methods) from gneisses, a mafic dike, and a high-pressure granitic leucosome; U–Pb dating of rutile from mafic dikes; 40Ar/39Ar dating of amphibole and mica; and Sm–Nd studies of rocks and minerals. The Sm–Nd model ages of felsic (2.9–3.1 Ga) and mafic (3.0–3.4 Ga) rocks from the Gridino eclogite association and individual magmatic zircon grains with an age of ca. 3.0 Ga indicate the Mesoarchean age of the metamorphic-rock protoliths. The most reliable result is the upper age bound of eclogitic metamorphism (2.71 Ga), which reflects the time of the posteclogitic decompression melting of eclogitized rocks under high-pressure retrograde granulitic metamorphism. The mafic dikes formed from 2.82 Ga to 2.72 Ga, most probably, at 2.82 Ga, in accordance with the crystallization age of magmatic zircon from metagabbro. Superimposed amphibolitic metamorphism and the “final” exhumation of metamorphic complexes at 2.0–1.9 Ga are associated with the later Svecofennian tectonometamorphic stage. Successive cooling of the metamorphic associations to 300 ºC at 1.9–1.7 Ga is shown by U–Pb rutile dating and 40Ar/39Ar mica dating. © 2012, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. Keywords: eclogites; Archean; Paleoproterozoic; mafic dikes; U–Pb zircon dating; province

Introduction Manifestations of Early Precambrian eclogitic metamorphism have long been known in the eastern Fennoscandian Shield: on the southern Kola Peninsula and in northeastern Karelia, in the coastland and on the isles of the Kandalaksha Gulf (Batieva, 1958; Smirnova and Baboshin, 1967; Sudovikov, 1935). However, they have been dated reliably only in recent years, when Neoarchean and Meso-Neoarchean eclogites have been identified (Mints et al., 2010c; Volodichev et al., 2004). Recent studies have revealed the Meso-Neoarchean Belomorian eclogite province in the eastern Fennoscandian Shield (Mints et al., 2010a,b). * Corresponding author. E-mail address: [email protected] (K.A. Dokukina)

40 39 Ar/ Ar dating of mica and amphibole; Sm–Nd dating; Belomorian

Archean eclogites were previously known only as xenoliths in kimberlites (Pearson et al., 2003). The crustal eclogites known were no older than 2.0–1.8 Ga (Baldwin et al., 2004; Collins et al., 2004; Kröner et al., 2006; Möller et al., 1995), that is, the second half of the Paleoproterozoic. Archean crustal eclogites remained unknown for a long time, and this was partly why eclogitic metamorphism was considered impossible in the Archean crust. The absence of crustal eclogites was regarded as evidence supporting the model ideas that the geothermal gradient was high in the Earth’s early history (Baer, 1977; Green, 1975). The Gridino eclogitization affected the felsic and mafic rocks of the continental crust and mafic-dike swarms. We attempt to determine the sequence and age of igneous and metamorphic events in the evolution of the Gridino association. The paper presents new geochronological data, which

1068-7971/$ - see front matter D 201 2, V . S. S o bolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.rgg.2012.08.006 +

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include conventional and local U–Pb dating of zircon from metamorphosed mafic dikes, leucosome cutting dikes and host gneisses; that of rutile from the metamorphosed mafic dikes; Sm–Nd analysis of whole-rock samples and minerals from the mafic dikes and gneisses; 40Ar/39Ar dating of amphibole in dikes and that of mica in a felsic leucosome.

Regional geology The known eclogite bodies in the Belomorian province are confined to a NW-trending strip >500 km long and 50–60 km wide (Fig. 1). They are localized within the migmatized tonalite-trondhjemite-granodioritic gneisses of the Keret’ rock unit, which is usually regarded as the upper part of the Belomorian series. The Keret’ unit dips northeastward and is underlain by mafic and ultramafic rocks of protoophiolite association (Slabunov et al., 2006a) of the Central Belomorian greenstone belt. The composition of the greenstone association and the tectonic position of the belt suggest that it is a Meso-Neoarchean suture zone (Mints et al., 2010b,c). Correspondingly, the ortho- and paragneisses of the Keret’ unit, which overlies the suture and includes Meso-Neoarchean subduction-related eclogite bodies and eclogitized dikes, can be regarded as an active margin of the Kola paleocontinent (South Kola active margin, Fig. 1) (Mints et al., 2010a–c). The Central Belomorian suture helped to solve the old problem of demarcating the Kola megablock (province) from the Belomorian megablock (province) (Glebovitskii, 2005). These areas of Archean crust were divided for the first time by an Archean not Paleoproterozoic boundary. This boundary was previously associated with Paleoproterozoic structures: the Pechenga–Imandra–Varzuga volcanosedimentary belt or the Lapland–Kolvitsa granulite-gneiss belt (overview in (Mints et al., 2010b)). The Belomorian eclogite province comprises two rock associations, which contain mafic eclogite bodies differing in morphology and protolith origin. The Salma Meso-Neoarchean association consists of eclogitized oceanic rocks (Konilov et al., 2011; Mints et al., 2010b,c), whereas the Gridino association consists of eclogitized dikes and fragments of mafic rocks integrated into a continental substratum (Dokukina and Konilov, 2011; Volodichev, 1990; Volodichev et al., 2004). Starting from the works of V.S. Stepanov (1981), it has been customary to correlate the Gridino dikes with the complex of smallish intrusions (locally called “drusites”) which occur throughout the Belomorian accretionary-collisional orogen (Fig. 1) (Mints et al., 2010b), or megablock, in the conventional view, or mobile belt (Slabunov et al., 2005, 2006a,b). The term “drusites” (E.S. Fedorov, 1905) emphasizes the widespread occurrence of late- and postmagmatic coronary structures in the intrusive rocks of this complex. The drusite area (at least 700 km along strike and up to 100 km across) extends northwestward, right southwest of the eclogite area, with only a slight overlap. Stepanov (1981) divided the drusites into three age and compositional complexes: Archean gabbro-anorthosites, Early

Paleoproterozoic lherzolites-gabbronorites, and garnet gabbro. According to the subsequent isotope-geochronological studies, the age of the first two complexes fits into a narrow interval from 2.46 to 2.43 Ga, and the age of the garnet-gabbro dikes varies from 2.4 to 1.9 Ga, as distinct from the drusite complex (Glebovitskii, 2005). Unlike the drusites, the Gridino maficdike swarm underwent eclogitic and granulitic metamorphism (Dokukina and Konilov, 2011; Volodichev et al., 2005). Thus, the eclogitized dikes of the Gridino association differ from the compositionally similar drusites in metamorphic grade and spatial and structural assignment. The age relations between the drusites and eclogitized dikes remain disputable. The paper presents new geochronological data, which constrain intrusive and metamorphic events in the formation of the Gridino structural-compositional complexes (Fig. 1).

The Gridino eclogite association The Gridino metamorphic complex is regarded as an Archean eclogite-containing tectonic mélange (Volodichev et al., 2004), as illustrated in detail in (Sibelev et al., 2004; Slabunov et al., 2007). The complex is traced from northwest to southeast for ~50 km, within a strip up to 10 km wide. The mélange matrix consists of gneisses corresponding to tonalite-trondhjemites, amphibole granites, and biotite-amphibole gneisses. The clastic component consists of amphibolites, retrograde eclogites, zoisitites, metapyroxenites, metamorphosed gabbro, kyanite-garnet-biotitic gneisses, calciphyres, and marbles (Sibelev et al., 2004; Volodichev et al., 2004). The fragments are lens-like or isometric, with subdued edges, and vary in size from several centimeters to the first tens of meters. The mélange structures are intersected by numerous eclogitized mafic dikes, which is recognized as the Gridino dike swarm. The mafic dikes on the Karelian coast and on the isles of the Kandalaksha Gulf were first studied in detail by V.S. Stepanov (1981); the subsequent studies are associated with V.S. Stepanov and A.V. Stepanova (Stepanov, 1990; Stepanov and Stepanova, 2005, 2006; Stepanova and Stepanov, 2005, 2010; Stepanova et al., 2003). The dikes are most often typical fissure intrusions breaking through the host rocks. However, the deformed dikes often follow the structural pattern of the mélange: They undergo folding, conformable to the host gneisses, boudinage, or metamorphic banding with attendant migmatization (Dokukina and Konilov, 2011). The extreme degree of dike transformation is their transformation into amphibolite interbeds in zones of intense shearing. Several groups of dikes belonging to different complexes are recognized on the basis of intersections, mineral composition, and geochemistry (Stepanov, 1990; Stepanov and Stepanova, 2005, 2006; Stepanova and Stepanov, 2005; Stepanova et al., 2003). Note that the deformation style and intensity for individual dikes are age-unrelated. The eclogites and eclogite-like (plagioclase-bearing) rocks near Gridino Village were first studied by O.I. Volodichev (1977, 1990). Several hypotheses about the formation of

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Fig. 1. Tectonic position of the Salma and Gridino eclogite associations, which form the Archean Belomorian eclogite province in the eastern Fennoscandian Shield, modified after (Mints et al., 2010a). Inset shows the study localities in the Gridino zone (in dark gray): 1, subduction-related eclogites; 2, eclogitized dikes.

eclogitic parageneses in the Gridino dikes have been put forward by now: from an autoclave effect within magma chambers (Volodichev et al., 2005) to local pressures corresponding to eclogitic metamorphism within the amphibolitic metamorphic field (Travin and Kozlova, 2005). The model of V.M. Kozlovskii and L.Ya. Aranovich is based on a relationship of eclogitic and eclogite-like parageneses with tectonic strain and attendant metasomatism under high-pressure amphibolitic conditions. A special role in the formation of the eclogitic and eclogite-like parageneses belongs to the deformation intensity and deformation-related fluid permeability of the rocks as well as their silica saturation (Kozlovskii and Aranovich, 2008, 2010). We showed that the formation of high-pressure parageneses is due to the subsidence of the Gridino rock association to the subduction zone to the eclogitic level (Dokukina and Konilov, 2011).

Existing absolute-age estimates The absolute-age estimates available for the mélange zone vary widely. Detrital zircon grains with irregular cores and narrow metamorphic rims were dated in a lens of kyanite-garnet-biotitic gneisses from Cape Kirbei (Bibikova et al., 2003). The age of the cores is 2.97–2.87 Ga. One metamorphic grain was dated at 2.70 Ga. The U–Pb age of zircon from the

subduction-related eclogites on Stolbikha Island is estimated at 2.72 ± 0.01 and 1.92 Ga, and zircon from the trondhjemite vein cutting the mélange structure was dated by the U–Pb method at 2.70 ± 0.01 Ga (Volodichev et al., 2004). The dating of a gabbro intrusion on the Suprotivnye Isles yielded 2.71 ± 0.03 Ga (Slabunov et al., 2006b, 2008). Gabbro also includes zircon grains with ages of 2.82–2.79 and 2.74 Ga. These authors compare them with zircon from the granitoids and tonalite-trondhjemite-granodioritic gneisses and presume that these crystals might have been inherited by mafic magma during host-rock assimilation. From two eclogite samples (eclogitized dike of olivine gabbronorites in northwestern Gridino Village and a boudin of retrograde kyanite-bearing eclogites on Vysokii Island), zircon was extracted with a wide spectrum of Archean and Paleoproterozoic ages: Archean, captured from the country rocks, as hypothesized in the corresponding publications; Paleoproterozoic (ca. 2.4 Ga), which are considered coeval with the intrusion of a mafic melt, quickly followed by eclogitization (Slabunov et al., 2003; Volodichev et al., 2008, 2009); and Paleoproterozoic, with an age of ca. 1.9 Ga (Volodichev et al., 2009), which reflect later (Svecofennian) metamorphism. The ages obtained for zircon from the metamorphic and igneous rocks near Gridino Village fit into several groups, interpreted as follows: 2.97–2.87 Ga, sedimentation (Bibikova

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et al., 2003); ca. 3.0–2.74 Ga, formation of tonalitetrondhjemitic granitoids; 2.73–2.72 Ga, metamorphism, including eclogitic metamorphism (Slabunov et al., 2006c; Volodichev et al., 2004); 2.71–2.69 Ga, granitic collisional and gabbro late-collisional magmatism (Bibikova et al., 2003; Slabunov et al., 2006a, 2008; Volodichev et al., 2004); 2.47–2.39 and 2.12 Ga, intrusion and autonomous eclogitization of mafic dikes (Slabunov et al., 2011; Volodichev et al., 2008, 2009); 1.9–1.8 Ga, exhumation (Svecofennian tectonometamorphic stage). In a recent series of publications, S.G. Skublov et al. (2010a,b, 2011a,b) made persistent attempts at “the final dating of eclogites from the Belomorian mobile belt” (Skublov et al., 2011c), that is, the substantiation of the Late Paleoproterozoic Svecofennian (ca. 1.9 Ga) age of eclogitic metamorphism in the Belomorian province. Most of the isotope-geochronological data of Skublov et al. replicate what is known (Bibikova et al., 2003; Dokukina and Konilov, 2011; Dokukina et al., 2009, 2010; Konilov et al., 2011; Mints et al., 2010a–c; Volodichev et al., 2004); however, these authors make radically different conclusions. The latter are based only on the isotopic, geochemical, and geochronological characteristics of zircon, and, partly, the rock-forming minerals, without regard to the geologic position, geochemistry, or petrology of the eclogites themselves and their host rocks.

Problem formulation The current view on the age of the mafic dikes and eclogitic metamorphism are based on the fundamentally different ages of two eclogite types near Gridino Village (Slabunov et al., 2006c; Volodichev et al., 2004, 2009): Type 1 eclogites form small bodies, which are part of the mélange, whereas type 2 eclogites form in mafic dikes (Volodichev et al., 2008), which mostly cut the structural-compositional associations of the mélange but are partly deformed and integrated into the tectonic-mélange zone. The small bodies of type 1 Archean eclogites are dated at 2.72 Ga. These bodies, which are part of the mélange, are associated with the subduction of oceanic complexes (Volodichev et al., 2004) and subsidence to the eclogitic level (P = 14.0–17.5 kbar; T = 740–865 °C). The successive exhumation of the eclogites is illustrated by a trend of subisothermal decompression from 14.0 to 6.5 kbar. During the geochronological studies of the dikes, the researchers had difficulty extracting enough zircon and interpreting the ambiguous results. To date, geochronological studies of eclogitized dikes have been conducted only on a limited scale (Slabunov et al., 2011; Volodichev et al., 2004); new data are presented and analyzed below. To explain the essence of the discussion which we will have to touch upon, we emphasize that the eclogitized dikes of the Gridino swarm are assigned to the Paleoproterozoic drusite complex in all the publications of the research group (V.S. Stepanov, A.V. Stepanova, A.I. Slabunov, O.I. Volodi-

chev, and others), regardless of the obvious differences in the metamorphic grade and the spatial and structural isolation of the eclogitized dikes from the drusite field. Finds of 2.38– 2.4 Ga zircon are presumed to evidence that the dikes belong to the drusite complex (Slabunov et al., 2003; Volodichev et al., 2009). Note that the original conclusion about the dike age relied exclusively on geological data (area of occurrence, body appearance and morphology, relations with the host environment, bulk composition of the rocks). The dike metamorphism (Dokukina and Konilov, 2011) involves a prograde stage (an increase starting from 5 kbar and 600 °C); an eclogite stage (minimum estimated peak pressure, 15–17 kbar), a decompression stage under high-pressure granulitic conditions (P = 10–13.5 kbar; T = 800–850 °C), and a retrograde stage under amphibolitic conditions (P = 7.9–9.6 kbar; T = 530–700 °C). Indirect petrological evidence (including thermobarometry) indicates that the pressure of eclogitic metamorphism in the Belomorian province might have considerably exceeded the above estimates and reached ultrahigh-pressure values (Dokukina and Konilov, 2011). These hypotheses have recently been confirmed in (Morgunova and Perchuk, 2012); these authors have shown that the PT-conditions surrounding the metamorphism of a metagabbro dike in the same area reached the coesite stability field. We think that the above idea of repeated eclogitic metamorphism in the Gridino rocks (owing to oceanic-crust subduction in the Neoarchean and repeated intrusion of mafic dikes in the Paleoproterozoic) are not substantiated enough and further studies are required. (1) The age of the eclogitized dikes estimated through comparison with the drusites is not substantiated enough; (2) A whole series of so old eclogitic metamorphic events is questionable: If their reality is proven, each generation will be unique by itself; (3) According to field observations, the dike deformations and boudinage are accompanied by the formation of fragments morphologically and structurally indistinguishable from the Archean subduction-related eclogites; (4) Archean subduction-related eclogites and eclogitized dikes coexist in the same outcrops. The dikes intrude boudins and lenses of the subduction-related eclogites; (5) The dikes and boudins show the same PT-trend, at least in the retrograde area (Dokukina and Konilov, 2011): from the eclogitic facies (minimum pressure, 15–17 kbar) to amphibolitic retrograde metamorphism, with a decompression trend through the high-pressure granulitic region. The hypothesis about the geochemical likeness and, consequently, geologic similarity and coevality of the Gridino dike swarm and the mafic–ultramafic intrusions of the drusite complex has not been proven as yet. To test it, we compared the compositions of the Paleoproterozoic drusites using data from (Lobach-Zhuchenko et al., 1998; Sharkov et al., 2004) and mafic dikes from the Gridino association. We used compositional data on Neoarchean gabbro (Slabunov et al., 2008), subduction-related eclogites (Slabunov et al., 2005), Paleoproterozoic metagabbro dikes (Dokukina and Konilov, 2011; Stepanov and Stepanova, 2006; Volodichev et al., 2005;

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Fig. 2. Petrochemical rock types in the Gridino eclogite association on an A–F–M diagram (tholeiitic–calc-alkaline boundary after (Kuno, 1968)). 1, granitic leucosome; 2, tonalite-gneisses; 3, subduction-related (?) eclogites (Slabunov et al., 2005); 4, Neoarchean gabbro (Slabunov et al., 2008); 5, early metagabbro, according to (Stepanov and Stepanova, 2006) and our data; 6, early gabbronorites; 7, late gabbronorites; 8, late metagabbro; 9, Paleoproterozoic drusites, 2.40–2.45 Ga (Lobach-Zhuchenko et al., 1998). Shaded area shows the compositions of the Gridino dikes.

our data), and Paleoproterozoic gabbronorite dikes (Dokukina and Konilov, 2011; Volodichev et al., 2005, 2008; our data). On the A–F–M diagram, the dike compositions plotted between two Belomorian reference drusite complexes (Fig. 2). Note that only 14% of the Gridino dikes plot as drusites. Thus, the presumed petrochemical similarity between the drusites and Gridino dikes was not confirmed. We drew the same comparison for trace elements using different diagrams (Mints et al., 2010b). Showing a slight overlap, the drusites always form distinct groups, whereas the Gridino dikes show wide compositional variation and form individual trends. In general, the petro- and geochemical comparison between the Paleoproterozoic drusites and Gridino dikes does not confirm their similarity. Thus, an unbiased age estimate for the eclogitized dikes of the Gridino swarm requires detailed geochronological studies. Undoubtedly, geochronological studies of only the mafic dikes (time of magmatic crystallization and successive metamorphic events) or eclogites are not enough to reconstruct the age and sequence of the most significant events in the geologic history of the Gridino association. A realistic view on its evolution also requires age estimates for the major events in the evolution of the rocks enclosing and cutting the eclogitized dikes and framing eclogites. Objects of study The geochronological studies were conducted for Izbnaya Luda Island, Cape Vargas, and northeastern Gridino Village.

Izbnaya Luda Island Izbnaya Luda Island is located 4 km southeast of Gridino Village, elongated in the near-E–W direction, and ~900 × (50– 400) m in size. The isle consists of metamorphosed igneous complexes. Its structure is dominated by tonalite-gneisses, which vary in chemical composition from granodiorites and tonalites to plagiogranites and serve as a matrix for the other metamorphic rocks. Enclaves in granite-gneisses consist of symplectic eclogites; garnet, diopside-garnet, and orthopyroxene amphibolites (interbeds, blocks); metaperidotites (smal blocks); metaorthopyroxenites (deformed dikes or interbeds); and K-feldspar granites (small bodies, migmatites). The rocks are cut by dikes of early and late ferriferous metagabbro, quartz gabbronorites, and olivine gabbronorites. Mapping of the geologic structure of the isle revealed three domains (Central, Eastern, Western), which differ in structure, deformation style, predominant orientation, and succession in which the structural elements formed (Fig. 3). The Central domain (Fig. 3) consists of finely banded plagiogranite-gneisses, which serve as a matrix for small (10 cm–5 m) fragments of garnet ( ± clinopyroxene) amphibolites, tonalites, plagiogranites, and microclinic granites. The rocks show predominantly northwestward and near-E–W banding and are cut by differently oriented short faults (no more than the first meters). The eastern part of the domain contains eclogite inclusions, which are presumably Neoarchean (Volodichev et al., 2004). Displacements along the faults create breccia-like structures.

Fig. 3. Geological sketch map of Izbnaya Luda Island, with sampling sites and geochronological data, modified after (Dokukina and Konilov, 2011; Dokukina et al., 2010). Inset shows the part of the Central–Western domain boundary shown on the map by a rectangle. 1, Quaternary sediments; 2, mafic dikes (a, early metagabbro; b, metagabbronorites; c, late metagabbro); 3, banded gneissic granites of the Western and Eastern domains; 4, brecciated gneissic granites of the Central domain; 5, strips of mélange fragments (a, amphibolites; b, amphibolites, orthopyroxenites); 6, banding dips and strikes.

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The brecciated rocks are cut by mafic dikes (Fig. 3): two gabbronorite generations and late metagabbro. All the dikes evidence high-pressure metamorphism and are amphibolized in narrow zones at the contacts with the gneisses. They have undeformed straight intrusive bounds and steep westward dips; they contain granite-gneiss xenoliths and show thin apophyses. The dikes are thin, from several centimeters in the apophyses to several tens of centimeters and the first meters. The largest one (~20 m thick) breaks through the gneissic granites of the eastern part of Central domain (Fig. 3). Judging by the dike–host rock relations, the dikes within the Central domain intruded rigid consolidated rocks, which had experienced metamorphism and tectonic reworking. The Western domain is a zone of northwestward foliation affecting tectonic breccias and mafic dikes (Fig. 3). The Central and Western domains are demarcated reliably on the basis of dike turns and truncation: Their strike is bounded by a zone of intense tectonic reworking (Fig. 3, inset). Toward the domain boundary, the dike rocks of the Central domain grade into medium-grained amphibolites. Amphibolized dikes turn in the direction of foliation and then are truncated. The dike turns indicate dextral displacement. Dike fragments are preserved as flattened boudins of garnet and clinopyroxenegarnet amphibolites in the granite-gneisses within the Western domain. The Eastern domain (Fig. 3) is characterized by the near-N–S strike of the rock texture and the gentle dip of lineation. The Central–Eastern domain boundary is a N–Strending strike-slip zone up to 5 m wide. The Eastern domain is dominated by tonalite-gneisses, showing bands saturated with mafic-rock fragments. For more than 100 m, a strip is traced with numerous small angular or rounded fragments of massive coarse-grained orthopyroxenites and amphibolites in a gneiss matrix (up to 0.2–0.3 or, rarely, 0.5 m across). Smaller accumulations of orthopyroxenites and amphibolites are observed in other parts of the Eastern domain. The Eastern domain contains several large metamorphosed dikes. The thickest one (up to 25 m) features relics of magmatic textures and minerals. In the northern, coastal, part of the isle, the eastern contact of the dike cuts the banding of the host granite-gneisses for a length of ~30 m and the dike contains angular xenoliths of the latter. The other contacts of the dike are undulating, and the banding of the host granitegneisses is conformable to the contacts near the dikes. Granite pegmatite and tourmaline–carbonate veins are associated with the dike pinches, and the dike rocks are completely amphibolized. The dike swells retain mineral parageneses from the magmatic stage and the preamphibolitic stages of the metamorphic evolution. The pinch-and-swell structure reflects the dike deformation and early boudinage, which ended under amphibolitic conditions. The other dikes in the Eastern domain (Fig. 3) are completely metamorphosed. In general, the dike boundaries are conformable to the banding of the host granite-gneisses. However, relics of the contacts cutting the gneisses indicate that the mafic bodies are intrusive. For example, the metadiorite body in the southern Eastern domain is seen

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clearly to cut a gneiss strip saturated with orthopyroxenite fragments. Cape Vargas Cape Vargas is located 3 km north of Gridino Village. It comprises tonalitic granite-gneisses with amphibolic-gneiss interbeds containing numerous bodies of amphibolites, eclogite-like rocks, and eclogites of different sizes (from the first centimeters to the first meters) and of isometric to greatly flattened shape as well as metagabbro dikes, strongly deformed and migmatized together with the enclosing granitegneisses (Fig. 4). The granite-gneisses and metagabbro dikes form a folded complex with gentle bends and near-vertical axial planes. The banding of the granite-gneisses is conformable to the dikes. However, one metagabbro dike at the southern extremity of the cape retains cross-cutting contacts with the host granite-gneisses, saturated with amphibolite inclusions, for 30 m. Northward, this dike is deformed, folded, and boudinaged and has a pinch-and-swell structure. The host gneisses and dike rocks in the deformed areas underwent partial melting with the formation of a phengite-bearing felsic leucosome. Breccia structures similar to those studied on Izbnaya Luda Island were observed in a small area within the gneisses of the cape. Cape Vargas is cut by potassic granites, zircon from which was dated by laser ablation at Macquarie University (GEMOC ARC National Key Centre, Macquarie University, Sydney, Australia; analysts L.M. Natapov, E.A. Belousova) at 2721 ± 19 Ma (unpublished data) (Fig. 4, sample D57-1). Northeastern Gridino Village Gridino Village ends in the northeast with a cape consisting of tonalitic granite-gneisses, which contain boudins of retrograde eclogites and are cut by eclogitized gabbro and gabbronorite dikes (Fig. 5). The cape structure and the constituent rocks are described in detail in (Volodichev et al., 2008).

Techniques for isotope-geochemical studies The major-element contents (wt.%) were determined at the Center of Isotopic Research (VSEGEI, St. Petersburg) by X-ray phase analysis, and the trace-element contents (ppm) were determined by ICP MS at the same center on an ELAN 6100 DRC in the standard mode. Mineral monofractions were extracted at the Geological Institute of the Kola Science Center (Apatity) and at the Geological Institute of the Russian Academy of Sciences (Moscow). The rocks and minerals underwent Sm–Nd dating on a Finnigan MAT-262 (Apatity). Zircon underwent local isotope dating on a SHRIMP II SIMS (ion microprobe) at the Center of Isotopic Research, analyst A.N. Larionov. Concordia diagrams were plotted with Isoplot/Ex software (Ludwig, 1999). Correction for common

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K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

Fig. 5. Geological sketch map of northeastern Gridino Village, after (Volodichev et al., 2005). The numbers of geochronological samples V-16-65 and V-16-66 are from (Slabunov et al., 2011), and 1111-08 is from this paper. 1, olivine metagabbronorite; 2, metagabbro; 3, plagiogneisses; 4, banding dips and strikes.

Fig. 4. Geological sketch map of Cape Vargas, modified after (Dokukina and Konilov, 2011; Dokukina et al., 2009). 1, Quaternary sediments; 2, pegmatite veins; 3, potassic granites; 4, metamorphosed mafic rocks; 5, areas of partially molten mafic and felsic rocks, in which the phengite-bearing felsic leucosome is formed; 6, plagiogneisses; 7, tectonic breccias; 8, dips and strikes; 9, sampling site.

Pb was made in accordance with the measured 204Pb/206Pb ratio, after the model from (Stacey and Kramers, 1975). Conventional U–Pb dating was conducted at the Geological Institute of the Karelian Science Center. Chemical digestion of the minerals and U and Pb extraction were done by Krogh’s method (1973). Uranium and lead contents were determined by isotopic dilution with a 208Pb/235U tracer on MI-1201T and Finnigan MAT 262 mass spectrometers. The site coordinates and isotope ages were calculated in K. Ludwig’s programs (1999, 2000). The reproducibility error for the U/Pb ratios was 0.5%. Some zircon grains underwent U–Pb dating at the Geological Institute of the Karelian Science Center with a 205Pb/235U tracer on a Finnigan MAT 262 in the static mode with collectors. The technique is described briefly in (Bayanova et al., 2007). For the 40Ar/39Ar dating, weight samples of amphibole and mica mineral fractions, together with those of MSA-11 biotite

(OSO no. 129-88), used as a monitor, were packed in Al foil and sealed in an evacuated quartz capsule. Biotite sample MSA-11, prepared at the Fedorovsky All-Russian ScientificResearch Institute of Mineral Resources in 1988 as a standard K–Ar sample for K–Ar dating, was certified as an 40Ar/39Ar monitor with the help of international standard samples: Bern 4m muscovite and LP-6 biotite (Baksi et al., 1996). The average calibrated value (311.0 ± 1.5 Ma) was taken to be the integrated age of MSA-11 biotite. Afterward, the grains were irradiated in a Cd-plated channel of a VVR-K scientific reactor (Institute of Nuclear Physics of Tomsk Polytechnic University, Tomsk). The neutron-flux gradient did not exceed 0.5% of the sample size. Experiments on incremental heating were performed in a quartz reactor with an external furnace. Blank experiment for 40Ar (10 min at 1200 °C) did not exceed 5 × 10–10 ncm3. Argon was purified with Ti and ZrAl SAES getters. The Ar isotope composition was determined on a Noble Gas 5400 (Micromass, United Kingdom). The measurement error in the text, tables, and figures is within ± 1σ. The trace-element content of zircon (at sites of local U–Pb analysis) was determined on a Cameca IMS-4f at the Yaroslavl Branch of the Institute of Physics and Technology (analyst S.G. Simakin).

Geochronological data Studies of the geologic structure of Izbnaya Luda Island and Cape Vargas restored the sequence of tectonic, igneous, and metamorphic events (Dokukina and Konilov, 2011). The primary rock complexes formed as a result of sedimentary and igneous processes. Afterward, a metamorphic event took place, which transformed the primary rocks into metamorphic ones.

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

The subsequent events took place in the following sequence: The granite-gneiss complexes were intruded by several maficdike generations, which cut the previously formed folds, breccias, and mélange structures; high-pressure metamorphism was superimposed onto all the rock complexes (dikes, host rocks); the dikes were deformed, and new metamorphic banding formed in the gneisses, while felsic pegmatite and carbonate veins cut the granite-gneisses and dikes (last stage).

U–Pb zircon dating The mafic dikes on Izbnaya Luda Island did not lend themselves to U–Pb zircon dating because of the scarcity of the zircon weight samples. Gneisses, Izbnaya Luda Island, samples D26, D27, D28. The study was aimed at dating the brecciated gneisses of the Central domain and superimposed shearing. Gneiss was sampled from selected sites in the central and boundary parts of the Central domain (Fig. 3). Four geochronological samples underwent conventional U–Pb dating: three from the gneisses and one from a leucosome crossing a metagabbronorite dike. The gneisses have a homogeneous mineral composition: migmatized biotitic, garnet-biotitic, or garnet-amphibolic, sometimes with scapolite. The PT-conditions are 10–12 kbar and 750–800 °C (Dokukina and Konilov, 2011). The average composition of the rocks (Table 1) is tonalitic (SiO2 = 61–69 wt.%; Na dominates over K: Na2O + K2O = 5.12–6.35; Na2O = 3.46–4.57; K2O = 1.19–1.93 wt.%; relatively high bulk contents of REE, 74–162 ppm; a slight or considerable positive Eu anomaly, Eu/Eu* = 1.05–2.46; LREE saturation relative to HREE). The zircon grains from the oldest (according to the structural relations) gneisses in the Central domain (Fig. 6, b, Table 2, sample D27) turned out to be the youngest ones (2689 ± 14 Ma). Those from the rocks of the Western and Eastern domains (Fig. 6, a, c, samples D26, D28) were older (2816 ± 8 and 2771 ± 18 Ma, respectively). We have to acknowledge that the results do not show the older age of the rocks in the Central domain as compared with the boundary areas, which must have experienced later recrystallization. The zircon grains of different morphologies from the garnet-amphibole-biotitic gneisses at the Central–Western domain boundary (Fig. 6, a, Table 2, sample D26) show a distinct oscillatory zoning, typical of magmatic zircon (Corfu et al., 2003). Thus, the discordant age 2.82 Ga might correspond to the age of the tonalitic protolith. The prismatic zircon grains from the migmatized amphibole-biotitic gneisses at the Central–Eastern domain boundary (Fig. 6, c, sample D28) show no oscillatory zoning but quite similar internal structure. They might correspond to a metamorphic stage. However, the same sample contains semitransparent long-prismatic zircon grains with an oscillatory zoning (Fig. 6, c, Table 2), whose age corresponds to a concordant age of 2000 ± 10 Ma (2.00 ± 0.01 Ga). They are probably coeval with Paleoproterozoic migmatites, which could not be cleared enough as the sample was taken and treated.

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Zircon from the tonalite-gneisses of the Central domain (Fig, 6, b, Table 2, sample D27) has oscillatory cores and fractured rims, which characterize different stages in the rock formation. Local studies of similar zircon grains (see below) showed that the formation of such rims usually matches the Paleoproterozoic (ca. 1.9 Ga) stage in the rock transformation. Apparently, conventional zircon dating does not permit differentiating between the Archean and Paleoproterozoic stages; therefore, the estimated age 2.69 Ga can be regarded as the age of the tonalitic protolith, somewhat underestimated. For greater accuracy in the subsequent studies, zircon fractions should be dated separately by local methods. Hopefully, this will permit identifying separate events in the gneiss formation. Granitic leucosome, Izbnaya Luda Island, sample D1415. Sample D14-15 was taken from the granitic leucosome crosscutting a dike of quartz-bearing metagabbronorites in the Central domain (Fig. 3). The dike evidences eclogitic, granulitic, and amphibolitic metamorphism (Dokukina and Konilov, 2011; Konilov and Dokukina, 2011). Chemically, it almost does not differ from the gneisses dated (Table 1). The zircon grains are long-prismatic and prismatic, with dark cores in cathodoluminescence and fine light rims. The cores show a clear oscillatory zoning. The rims, which replicate the core outlines, like in Cape Vargas zircon (see below), are most probably young Paleoproterozoic overgrowths (ca. 1.9 Ga). Four crystals underwent single zircon dating (Fig. 7, Table 2) and showed an age of 2651 ± 2.4 Ma from the upper intercept. Metagabbro, Cape Vargas, sample D17-1. Twelve zircon grains from a metagabbro dike on Cape Vargas underwent local SHRIMP dating. A geochemical study was conducted to establish relationships between the dike, which retains crosscutting relations with the host gneisses, deformed by flat mafic bodies, and amphibolite boudins, which might be dike fragments (Fig. 4, Table 1) (Dokukina et al., 2009). These rocks are of similar compositions and plot as subalkalic gabbro (Na2O + K2O = 3.57–3.88 wt.%). They have low contents (wt.%) of MgO (6–6.5) and elevated contents of TiO2 (1–1.5) and FeO* (10.8–11.9). The content of Al2O3 varies from 14.8 to 15.8, and that of CaO varies from 9.0 to 11.3 wt.%. The REE pattern shows slight LREE enrichment with respect to chondrite ((La/Lu)N = 1.5–6.5) and a slight negative Eu anomaly (Eu/Eu* = 0.85–0.93). The rock-forming magmatic minerals have not been preserved in the mafic rocks. The rocks mainly consist of an equilibrium metamorphic garnet-clinopyroxene-plagioclase assemblage of the granulite stage with quartz. Differently amphibolized varieties are observed. Garnet contains rare omphacite (Jd up to 43 mol.%, Table 3, Fig. 8, a) with a high content of the Ca–Ts end member (for example, Ca–Ts = 6.5 mol.% at Jd = 32 mol.%, Table 3), which is a relic mineral from the previous, eclogite, stage. Zircon was extracted from the dike part in which it retains intrusive contacts with the enclosing migmatized gneisses (Figs. 4, 8, b, sample D17-1) (Dokukina et al., 2009). Here, the rock is represented by a garnet-clinopyroxene-plagioclase

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K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

Fig. 6. U–Pb concordia diagrams and optical photos of zircon from gneisses, Izbnaya Luda Island. a–c, See description in the text. The zircon size is from Table 2. Numbers near the ellipses are the numbers of the analyses in Table 2.

assemblage with superimposed brown-green amphibole and biotite (Fig. 8, c). The crystallization conditions of the Grt–Cpx–Pl assemblage are estimated from direct contacts (representative mineral analyses, Table 3) at 838–867 °C (Grt–Cpx geothermometer (Powell, 1985)) and 13.4–13.6 kbar (TPF-consistent Grt–Cpx–Pl–Qtz geobarometer (Fonarev et al., 1991)) or 12.7–15.5 kbar at a given temperature of 800 °C on a Cpx–Pl–Qtz geobarometer (McCarthy and Patiño Douce, 1998). The cores of large clinopyroxene grains, unlike their rims, have an elevated content of the Ca–Ts end member (for example, 14 mol.% Ca–Ts at 18 mol.% Jd), suggesting an

earlier, higher-pressure, metamorphic stage. Amphibolization, which ended the metamorphism of the metagabbro dike, took place at 600–650 °C and 8–9 kbar (geothermometer from (Holland and Blundy, 1994), geobarometer from (Kohn and Spear, 1990)). The zircon grains are very small (~100 µm), with different morphologies and internal structures, observed in cathodoluminescence (Fig. 9). Only ~30 grains and crystal fragments were extracted from 30 kg rock. The age of 12 representative grains varied widely from 1.0 to 3.0 Ga (Table 4, Fig. 9). Despite the morphologic and age diversity, we managed to

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K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054 Table 1. Representative chemical analyses of the mafic dikes and felsic rocks dated Component

Metagabbro

Leucosome

Gneisses

Pegmatite

Metagabbronorite Metagabbro

1

2

3

4

5

6

7

8

9

10

D17-1

D17-2

D17-3

D17-4

D26

D27

D28

D14-15

D14-2

D12-2

SiO2, wt.%

50.80

48.90

49.90

75.70

66.90

70.90

66.00

68.5

48.10

47.50

TiO2

1.46

0.98

1.01

0.05

0.48

0.30

0.49

0.37

0.61

2.20

Al2O3

15.80

14.80

15.40

13.80

15.60

15.30

16.30

15.8

13.40

13.60

FeO*

10.80

11.88

11.70

0.66

3.555

2.016

3.456

5.92

11.61

15.66

Fe2O3

–

–

–

–

3.780

2.16

3.18

–

–

–

FeO

–

–

–

–

<.25

<.25

0.60

–

–

–

MnO

0.16

0.22

0.17

0.02

0.069

0.032

0.054

0.039

0.20

0.24

MgO

6.35

6.50

6.01

0.32

2.11

1.04

2.13

1.96

12.10

5.72

CaO

8.99

11.30

9.73

1.86

4.13

3.10

4.17

3.34

9.98

9.87

Na2O

2.79

3.01

2.70

3.08

4.34

4.42

4.57

4.57

1.84

2.31

K2O

0.86

0.70

0.87

3.75

1.68

1.93

1.58

1.7

0.49

0.76

P2O5

0.30

0.07

0.11

0.05

0.25

0.096

0.11

0.24

0.06

0.44

Total

98.31

98.36

97.60

99.29

99.90

99.80

99.80

103

98.39

98.30

V, ppm

181

281

197

7.64

60

–

–

–

189

330

Cr

238

185

142

14.90

–

–

–

31

939

109

Co

45.6

51.1

52

2.76

11.7

6.310

11.5

10.8

69.6

56.6

Ni

94.1

73.6

94.3

7.50

13.2

9.2

17.2

20

246

68.6

Rb

14.9

8.77

30.8

67.40

44.9

56.2

45.8

69.1

10.8

18.6

Sr

256

65

142

259

562

391

489

353

107

178

Y

23.1

22.3

19.8

2.47

11

2.4

7.89

5.26

14.4

38.8

Zr

135

52.5

81.6

31

165

186

105

157

41.4

205

Nb

7.29

3.93

5.87

0.36

7.86

3.98

5.2

7.56

2.47

12

Ba

315

63.6

212

1930

380

490

420

430

302

321

La

20.7

5.18

11

1.83

30.8

24.1

17.6

26.5

5.07

22.2 52.6

Ce

46.6

13.5

24.7

2.98

66.7

41.2

36.6

53.20

11

Pr

5.62

1.76

2.95

0.24

8.41

3.98

4.27

5.78

1.40

6.76

Nd

24.1

7.99

12.70

0.91

30.1

12

16

19.60

6.34

28.6

Sm

5.05

2.62

3.03

0.16

5.12

1.41

2.76

3.09

1.71

6.82

Eu

1.36

0.82

1.00

0.76

1.22

0.71

0.8

0.80

0.66

1.96

Gd

4.77

3.16

3.56

0.22

3.82

1.06

2.44

2.22

2.14

7.67

Tb

0.75

0.58

0.58

0.05

0.49

0.11

0.32

0.28

0.39

1.20

Dy

4.45

3.87

3.38

0.39

2.14

0.52

1.47

1.08

2.61

7.00

Ho

0.84

0.80

0.71

0.09

0.38

0.074

0.26

0.19

0.55

1.48

Er

2.48

2.60

2.20

0.27

0.91

0.18

0.7

0.40

1.65

4.36

Tm

0.34

0.39

0.34

0.05

0.16

0.022

0.077

0.06

0.25

0.63

Yb

2.17

2.29

2.00

0.34

0.84

0.19

0.65

0.36

1.46

3.96

Lu

0.33

0.36

0.31

0.06

0.13

0.033

0.1

0.047

0.21

0.56

Hf

3.43

1.72

2.26

1.02

–

–

–

–

1.08

5.06

Ta

0.42

0.29

0.24

–

0.29

0.11

0.19

0.22

0.15

0.63

Th

1.24

0.38

1.65

0.13

2.23

3.24

1.44

4.68

1.26

1.95

U

0.23

0.45

0.16

0.19

0.12

0.15

<0.1

0.14

0.14

0.37

Note. 1–3, early metagabbro, Cape Vargas: 1, amphibolized Grt–Cpx–Pl rock of the dike; 2, boudin along the strike of dike D17-1 (detached and completely amphibolized dike fragment); 3, flattened folded amphibolite body; 4, phengite leucosome in a split zone of dike D17-1; 5–7, gneisses, Izbnaya Luda Island: 5, garnet–amphibole–biotitic gneisses, Western domain; 6, garnet–biotitic gneisses, Central domain; 7, amphibole–biotitic gneisses with scapolite, Eastern domain; 8, felsic pegmatite, formed in a rupture of metagabbronorite dike D14, Izbnaya Luda Island; 9, Grt–2Px rock from a granulitized zone of dike D14, analog of D14-13, Izbnaya Luda Island; 10, Grt–Cpx–Pl rock of dike D12, analog of D12-4, Izbnaya Luda Island. * All Fe is present as FeO.

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K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

Table 2. U-Pb isotope dating of zircon from gneisses and a granite leucosome, Izbnaya Luda Island Sample no. Weight, mg Size, length × width, mm

Content, ppm

Pb isotope composition*

Pb

206

206

206

Pb

207

206

204Pb

207Pb

208Pb

235

238

U

Pb

Pb

Isotopic ratios

Pb U

Age, Ma** Rho

Pb U

207

Pb

206

Pb

Garnet-biotite-amphibolic migmatized gneiss (sample D-26) 1

0.3

0.14 × 0.07

91.5

135.6

658

4.6275

4.0749

14.3322

0.525025

2810

0.98

2

0.2

0.25 × 0.05

87.5

132.4

603

4.2319

3.8902

13.3107

0.490183

2801

0.91

3

0.2

0.14 × 0.09

46.6

77.4

656

4.6179

4.6503

12.9377

0.478172

2813

0.98

Garnet-biotitic migmatized gneiss (sample D-27) 1

0.1

0.07 × 0.05

47.6

82.6

495.5

4.8198

5.1160

11.6855

0.463461

2679

0.96

2

0.2

0.09 × 0.05

27.5

49.1

288.1

4.5498

4.4519

10.7585

0.428102

2656

0.9

3

0.3

0.16 × 0.04

29.6

61.3

783.5

5.0741

6.3475

10.1754

0.406629

2667

0.9

Amphibole-biotitic gneiss with scapolite (sample D-28) 1

0.3

0.15 × 0.07

19.3

31.6

1247

4.9705

6.8339

13.7169

0.519397

2755

0.94

2

0.5

0.20 × 0.15

38.5

61.1

772

4.5404

4.4021

12.5827

0.486193

2722

0.92

3

0.2

0.20 × 0.15

29.8

68.5

13848

7.6070

3.9549

6.16999

0.365057

2104

0.79

4

0.5

0.20 × 0.07

7.6

19.9

585

4.7677

5.4055

12.0007

0.468713

2766

0.92

4.1177

2.7243

12.50120

0.494045

2685

0.98

Felsic pegmatite in a rupture of a metagabbronorite dike (sample D14-15) 1

0.2

0.18 × 0.11

2

0.2

0.18 × 0.11

24.5

40.3

153

3.8973

1.5984

8.42931

0.339214

2615

0.66

3

0.2

0.24 × 0.08

43.1

77.6

38

1.9748

0.9236

3.87586

0.155783

2640

0.74

4

0.1

0.09 × 0.05

54.8

719.2

187

3.9444

1.2456

1.03369

0.039688

2733

0.67

207.6

281.9

205

* All the ratios were corrected for blank Pb (0.08 ng) and U (0.04 ng) and mass discrimination (0.12 ± 0.04%). ** Correction for common Pb was determined according to the model from (Stacey and Kramers, 1975).

identify several main clusters among the zircon grains under study. The oldest zircon, whose age is 3.0 Ga (Table 4, Fig. 9, grain 4.1), is a crystal fragment with a coarse oscillatory zoning and a Th/U ratio of 0.63, typical of magmatic zircon. A five-grain cluster (Table 4, Fig. 9, grains 1.1, 2.1, 6.1, 9.1, 12.1) comprises elongated crystals showing a magmatic oscillatory zoning, variations in U (58–407 ppm) and Th (24–270 ppm) contents, large Th/U ratios (0.21–0.89), considerable HREE enrichment, and well-defined negative Eu (Eu/Eu* = 0.55) and positive Ce (Ce/Ce* = 13) anomalies; all these features are typical of magmatic zircon (Fig. 10, a, Table 5) (Hoskin and Schaltegger, 2003). The upper intercept for this zircon corresponds to 2822 ± 39 Ma. The magmatic appearance of the grains suggests two hypotheses about their formation: They crystallized from mafic magma or were captured by it during the assimilation of the host granitegneisses. Cluster 2 (Table 4, Fig. 9, grains 5.1, 7.1) differs from the other grains in small Th/U ratios (0.06–0.12), with 154– 273 ppm U and 14–108 ppm Th (Table 4). The grains of cluster 2 are rounded and show an oscillatory and sectorial internal zoning, which is typical of magmatic zircon (Corfu et al., 2003), but the small Th/U ratios are typical of metamorphic zircon (Rubatto, 2002). The upper intercept

corresponds to 2715 ± 15 Ma. The REE pattern for cluster 2, which shows positive Ce (1.97–2.10) and negative Eu (Eu/Eu* = 0.40–0.55) anomalies and slight HREE enrichment ((Lu/Sm)N = 5–9; (Lu/La)N = 33–67) (Fig. 10, b, Table 4), considerably differs from the REE pattern for cluster 1. Also, two zircon crystal fragments (Table 4, Fig. 9, nos. 10-1, 11-1), with low Th and U contents and elevated (magmatic) Th/U ratios (0.63–0.64), were dated at 2.38 and 2.44 Ga. They could have been correlated with the dike emplacement but for the U–Pb age of zircon from the granitic leucosome splitting the metagabbro dike (see below). Granitic leucosome, Cape Vargas, samples D17-4, D17-7. Geochronological samples were taken from a phengitebearing granitic leucosome, along the metagabbro dike (Fig. 4), in the area of its deformation and migmatization (Fig. 8, d, e). The leucosome (Table 1) belongs to potassic subalkalic granites (Na2O + K2O = 6.83; 3.75 wt.% K2O), characterized by a high SiO2 content (75.7 wt.%), an abnormally high Ba content (1930 ppm), low contents of all the other trace elements, a well-defined positive Eu anomaly (Eu/Eu* = 17.16), and low bulk contents of REE (11 ppm). All these features indicate the eutectic origin of the leucosome (Skjerlie and Johnston, 1996). According to petrological studies, the phengite-bearing leucosome formed at falling pressure, during the eclogitic-

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

granulitic transition (Dokukina et al., 2010). The magmatic parageneses comprise Ti- and Ba-bearing phengites (3.15–3.20 Si cations per 11 oxygen atoms), equilibrated with rounded clinozoisite-quartz symplectites, K- and K–Ba-feldspars, and felsic plagioclase. Reactions are observed in thin sections: New very fine-grained phengite intergrown with quartz forms after phengite (Fig. 8, f); also, biotite forms and, in turn, is surrounded by K-feldspar rims. Garnet and clinopyroxene develop after the clinozoisite-quartz symplectites (Table 3). The pressure for the phengites estimated with a geobarometer (Caddick and Thompson, 2008) corresponds to eclogitic conditions and equals 16–25 kbar for 650–800 °C. The temperature and pressure for the other minerals (Bt–Grt geothermometer, Grt–Cpx–Pl–Qtz geobarometer, TPF-consistent system (Fonarev et al., 1991), Grt–Cpx geothermometer (Powell, 1985), Cpx–Pl–Qtz geobarometer (McCarthy and Patiño Douce, 1998)) were estimated at 750–800 °C and 10.9–12.3 kbar (Dokukina et al., 2010; Dokukina and Konilov, 2011); this matches the high-pressure granulitic region. Sample D17-4 was taken from the phengite leucosome (Fig. 8, c) splitting the metagabbro dike (Fig. 8, e). The zircon grains are long-prismatic, brownish, and U-rich (461– 1836 ppm), with a morphology and U content corresponding to those of migmatitic zircon (Bibikova et al., 2004; Sedova et al., 2009). Five zircon samples underwent conventional U–Pb dating, and the upper intercept corresponds to 2634 ± 5 Ma (Dokukina et al., 2009). Sample D17-7 was taken from the same leucosome not far from sample D17-4 but outside the dike, in an area of superimposed amphibolization. Zircon from this sample, dated by the SHRIMP II method (Table 6), is divided into several morphologic groups. The first and largest zircon group consists of long-prismatic brown U-rich crystals with fine colorless rims (Fig. 11, a). This cluster is most similar to conventionally dated zircon. The zircon grains, dark and spotted in cathodoluminescence, rarely show relics of oscillatory zoning, violated most probably by metamictization. Only one U-rich (3319–3475 ppm) zircon grain (Table 6, site 4.1) was dated, with a concordant age of 2713 ± 6 Ma (2.71 ± 0.01 Ga) (Table 6, Fig. 11, e). Grain 4.1 is characterized by high contents of U, Y, Nb, and Ca, great REE enrichment as compared with the other zircon grains under study (ΣREE = 1205 ppm), and a small Th/U ratio (0.05). Note that the low Th/U ratios are determined by abnormally high U contents, while Th content is quite ordinary (Table 6, 158–169 ppm); here, this does not permit an unambiguous interpretation of the small Th/U ratios as evidence for the metamorphic crystallization of zircon. The REE pattern shows a slight negative Eu (Eu/Eu* = 0.57) and a positive Ce (Ce/Ce* = 1.62) anomaly, while the trend is relatively flat ((Lu/Sm)N = 21; (Lu/Sm)N = 131) (Fig. 12, a, Table 5). This trend is similar to those for zircon from the metagabbro dike, D17 (Fig. 11, grains 5.1, 7.1). Their age, within error limit, does not differ from that of grain 4.1 from the leucosome. Group 2 consists of brownish gray short-prismatic grains 100 × 150 µm in size, with fine colorless rims (Fig. 11, b).

1035

Fig. 7. U–Pb concordia diagram, optical photos, and cathodoluminescence images of zircon from felsic pegmatite, Izbnaya Luda Island (sample D14-15).

The cores are dark in cathodoluminescence, unzoned, with a high U content (309–935 ppm) and a small Th/U ratio (0.11–0.12, except one value, 0.61). Some grains have a “fir-tree” texture, typical of metamorphic zircon (Fig. 11, b, grain 7.1) (Corfu et al., 2003). The age of zircon from this group (cores) is 2707 ± 20 Ma (n = 4; MSWD = 2.4; Fig. 11, e); this does not differ, within error limit, from that of group 1 and cluster 2 from metagabbro. The REE pattern shows a slight positive Ce (Ce/Ce* = 6.6) and a negative Eu (Eu/Eu* = 0.33) anomaly, slightly elevated LREE contents, and slight HREE enrichment ((Lu/Sm) = 13; (Lu/La)N = 135); this determines the moderate slope of the REE pattern (Table 5, Fig. 12, b). Like in cluster 1, the small Th/U ratio in this zircon is determined by high U contents. Thus, the cluster might correspond to both metamorphic zircon (small Th/U ratio), which grows in the presence of a melt (in the restite part, synchronously with partial melting and the formation of a phengite leucosome) (Rubatto, 2002) and migmatitic zircon, which crystallizes in situ from a felsic melt (high U content). Sometimes the zircon grains have an intricate structure (Fig. 11, c): The core, with low U (115–129 ppm) and Th (59–71 ppm) contents and a distinct oscillatory zoning, is surrounded by a structureless rim, which is dark in cathodoluminescence and has the same properties as the cores of group 2 zircon. Only one oscillatory core (Fig. 11, c, grain 1.1) yielded a discordant 207Pb/206Pb age of 2793 ± 14 Ma, whereas the other oscillatory cores are younger and coeval with zircon from the previous groups (ca. 2.71 Ga) (Fig. 11, e), within error limit. The concordant age of the light-colored rims is 1938 ± 35 Ma (Fig. 11, d, e). Young rims are poor in all the elements except Hf (Table 5). The rims are characterized by a negligible Th content (0–1 ppm) and, correspondingly, a

1036

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

Table 3. Representative mineral analyses for the rocks of Cape Vargas Component Eclogite

Grt-Cpx-Pl rock from a metagabbro dike

Phengite-bearing granite leucosome

D25 (Fig. 8, a)

D17-2 (Fig. 8, c)

d17/5b

Omp7

Omp49

Grt28

Grt27

Cpx24

Inclusion in Grt

Inclusion in Grt

Center

Rim

SiO2, wt.% 52.81

53.02

39.55

37.83

TiO2

0.14

0

0.19

Al2O3

8.7

9.20

22.05

Cpx25

Pl26

GRT

d17/4

d17/6

Cpx2

Bt13

Pl1

Ph20

Ph3

Corona Corona Corona n=4 around Grt around Grt around Grt

Center

Matrix

Center

Center

Ph + Qtz symplectite

50.58

51.25

60.86

50.30

35.35

62.10

46.87

45.72

0

0.60

0.27

0

0.05

0.31

2.71

0

1.18

1.08

21.80

6.39

5.13

22.71

20.82

3.81

16.27

23.86

30.25

31.13

38.15

Cr2O3

0

0.24

0.01

0.06

0.10

0.13

0.36

0.06

0.11

0.03

0

0.01

0.08

FeO*

6.05

7.28

23.95

24.48

10.00

11.38

0.37

21.83

9.49

21.09

0

3.33

3.14

MnO

0.09

0

1.19

1.06

0

0.19

0

1.92

0.41

0.24

0.16

0

0

MgO

8.25

9.76

5.14

4.67

10.41

10.88

0

3.27

10.86

9.48

0

1.80

1.52

CaO

14.12

15.66

10.52

10.46

21.93

21.17

4.76

13.39

21.95

0.05

5.08

0.07

0

Na2O

5.9

4.5

0

0

1.37

1.66

8.37

0

0.91

0.20

8.18

0.33

0.24

K2O

0

0

0.08

0

0

0.10

0.34

0

0.04

10.93

0.24

10.55

10.40

BaO

–

–

–

–

–

–

–

0

0

0

0

0.61

1.68

Total

96.06

99.66

102.68

100.36

101.38

102.16

97.77

99.48

98.19

96.35

99.62

95

95

Si, ppm

1.987

1.935

2.996

2.951

1.871

1.894

2.766

3.005

1.924

2.724

2.759

3.186

3.131

IV

0.013

0.065

0.004

0.049

0.129

0.106

1.216

0

0.076

1.276

1.250

0.814

0.869

AlVI

0.372

0.331

1.965

1.956

0.150

0.117

0

1.933

0.096

0.201

0

1.609

1.644

Ti

0.004

0

0.011

0

0.017

0.008

0

0.003

0.009

0.157

0

0.061

0.055

Cr

0

0.007

0.001

0.004

0.003

0.004

0.013

0.004

0.003

0.002

0

0.001

0.005

Fe″

0.190

0.222

1.517

1.597

0.309

0.352

0.014

1.438

0.304

1.359

0

0.189

0.180

Mn

0.003

0

0.076

0.070

0

0.006

0

0.128

0.013

0.016

0.006

0

0

Mg

0.463

0.531

0.580

0.543

0.574

0.599

0

0.384

0.619

1.089

0

0.182

0.156

Ca

0.569

0.612

0.854

0.874

0.869

0.838

0.232

1.130

0.900

0.004

0.242

0.005

0

Na

0.430

0.318

0

0

0.098

0.119

0.738

0

0.068

0.030

0.705

0.044

0.032

K

0

0

0.008

0

0

0.005

0.020

0

0.002

1.074

0.014

0.915

0.909

Ba

–

–

–

–

–

–

–

0

0

0

0

0.016

0.045

Total

–

–

8.012

8.044

4.020

4.047

4.998

8.024

4.014

7.932

4.975

7.021

7.025

O

6

6

12

12

6

6

8

12

6

11

8

11

11

X′

0.291

0.295

0.723

0.746

0.350

0.370

0.234

0.789

0.329

0.555

0.252

0.510

0.536

Al

Note. Numbers near the mineral abbreviation show the number of the microprobe analysis in thin section. X′ = Fe/(Fe + Mg) for dark-colored minerals, or X′ = Ca/(Ca + Na + K) in plagioclase; n, number of analyses. Coexisting minerals underwent microprobe analysis in polished thin sections after microscopy. Mineral compositions were determined under a CamScan MV2300 microscope, equipped with an Oxford INCA EDS, at the Institute of Experimental Mineralogy (Chernogolovka). * All Fe is present as FeO.

very small Th/U ratio (0–0.01), gentle slope of the REE patterns, positive Ce (Ce/Ce* = 1.58–1.69) and negative Eu (Eu/Eu* = 0.41–0.51) anomalies, and slight HREE enrichment ((Lu/Sm)N = 17–79; (Lu/La) N = 89–392) (Table 5, Fig. 12, c). Metasomatic veinlets in a dike of eclogitized olivine gabbronorites (northeastern Gridino Village), sample 1111-08. We conducted special petrological and geochronological studies in the dike of eclogitized olivine gabbronorites (Fig. 5) dated previously (Slabunov et al., 2011), in which U- and Th-rich zircon with large Th/U ratios (0.67–2.69) yielded an age of 2393 ± 13 Ma, interpreted by these authors as that of magmatism. The estimates for this

zircon comprised both discordant (discordance extent 3–6) and inversely discordant ages (discordance from –1 to –3) (Slabunov et al., 2011, Table 1). The samples also contained oscillatory zircon with 206Pb/207Pb ages of 2.62–2.84 Ga, interpreted as xenogenic inclusions (Slabunov et al., 2011), and ca. 1.9 Ga zircon (Fig. 13). Geochronological sample 1111-08 was taken from metasomatic veinlets associated with the superimposed-deformation zone crosscutting the dike, in which olivine gabbronorite was completely transformed into quartz-free orthopyroxene eclogite under eclogitic conditions (Fig. 14, a, b) (Dokukina and Konilov, 2011). The veinlets consist of quartz-bearing sym-

1037

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054 Table 4. U-Th-Pb SHRIMP II dating of zircon from a metagabbro dike, sample D17-1 (Cape Vargas) Analysis no.

206

Pbc,

U

Th

232

Th/238U

206

Pb*, ppm

207

%

Pb*/235U(1)

±%

206

Pb*/238U(1)

±%

Rho

Age, Ma

ppm D-17-1.1.1

0.32

159

33

0.21

71.1

14.150

1.70

0.5179

0.80

0.46

2811 ± 25

D-17-1.2.1

0.65

313

270

0.89

62.0

5.816

1.40

0.2287

0.69

0.50

2693 ± 19

D-17-1.3.1

0.53

94

28

0.31

14.5

1.750

4.70

0.1785

1.30

0.27

960 ± 92

D-17-1.4.1

0.08

131

79

0.63

69.2

18.930

1.30

0.6159

0.85

0.66

3002 ± 15

D-17-1.5.1

0.18

273

31

0.12

126.0

13.870

1.40

0.5350

0.65

0.48

2725 ± 20

D-17-1.6.1

0.05

347

182

0.54

154.0

14.060

0.86

0.5168

0.61

0.71

2804.2 ± 10

D-17-1.7.1

0.09

229

14

0.06

98.5

12.710

1.20

0.5000

0.72

0.62

2692 ± 15

D-17-1.8.1

–

154

108

0.72

68.5

13.540

1.20

0.5192

0.84

0.68

2735 ± 15

D-17-1.9.1

–

407

247

0.63

185.0

15.290

0.77

0.5304

0.56

0.73

2898.2 ± 8.6

D-17-1.10.1

0.68

131

81

0.64

51.3

9.560

2.00

0.4526

0.97

0.49

2382 ± 29

D-17-1.11.1

0.39

82

58

0.72

32.4

10.000

2.20

0.4571

1.20

0.54

2441 ± 31

D-17-1.12.1

0.07

58

24

0.44

26.9

15.000

1.90

0.5407

1.30

0.69

2836 ± 23

Note. Errors are 1σ; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Temora Standard calibration was 0.37–0.91%. Pb corrected using measured 204Pb.

(1)

Common

Table 5. Trace and rare-earth element contents of zircon which underwent SHRIMP-II dating (ppm), Cape Vargas Analysis no.*

Ca

Ti

Li

Sr

Y

Nb

Ba

La

Ce

Pr

Metagabbro, sample D17-1 5.1

53.92

12.23

N.m.

2.35

789.26

20.96

5.04

9.30

79.88

9.38

7.1.

14.96

5.51

N.m.

0.84

237.32

25.57

1.53

1.98

13.22

1.35

12.1

8.59

4.87

N.m.

0.52

1003.39

26.88

0.72

0.11

4.56

0.07

N.m.

5.67

1933.07

26.98

4.69

5.68

39.33

6.24

Phengite leucosome, sample D17-7 4.1

116.83

40.11

8.1

11.33

11.09

N.m.

0.90

767.73

22.78

2.78

1.98

32.60

1.68

8.2

7.53

12.48

63.84

0.76

586.54

19.28

2.50

0.87

18.72

0.55

9.1

66.36

187.16

28.20

0.99

180.74

7.79

4.56

1.76

10.62

1.54

11.1

2.51

2.67

12.27

0.53

136.81

18.36

1.08

0.22

1.23

0.14

Analysis no.*

Nd

Sm

Eu

Gd

Dy

Er

Yb

Lu

Hf

Th

U

Metagabbro, sample D17-1 5.1

53.10

39.86

8.90

62.17

101.74

112.57

195.37

32.32

9063.32

39.91

294.40

7.1.

10.69

9.08

1.67

18.28

26.13

34.62

71.95

13.92

9909.28

21.70

248.60

12.1

1.11

2.53

0.59

16.36

72.32

163.38

273.84

46.55

6688.89

19.27

65.68

Phengite leucosome, sample D17-7 4.1

44.54

22.86

6.97

60.47

160.40

284.18

496.11

78.64

9698.55

133.76

3765.73

8.1

13.80

10.03

2.97

28.31

65.83

121.39

213.63

35.17

6594.62

60.23

155.13

8.2

4.65

5.76

1.30

25.76

63.40

75.07

84.80

12.31

8920.42

200.63

846.19

9.1

8.78

5.75

1.12

7.77

12.46

32.05

92.90

16.57

10195.75

2.04

206.87

11.1

0.79

0.71

0.14

1.49

6.83

24.56

53.25

9.21

11729.05

0.36

93.09

* Analysis and crystal numbers are given in accordance with SHRIMP II dating. N.m., Not measured.

1038

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

1039

Fig. 8. Photos of field exposures and thin sections of Cape Vargas rocks. a, Photo of thin section and BSE image (inset) of metamorphosed mafic rock, in which garnet retained omphacite; indices near Omp and Cpx show the Jd content at the site of clinopyroxene analysis (mol.%); b, undeformed part of the metagabbro dike from which geochronological sample D17-1 was taken; c, photo of a thin section of a Grt–Cpx–Pl rock at the boundary of a phengite-bearing leucosome (sampling site is shown in panel e; compositions of coexisting minerals are shown in Table 3); numbers near the minerals correspond to the number of the microprobe analysis in thin section, Table 3; d, photo of a deformed and migmatized metagabbro dike; e, part of the metagabbro dike penetrated by veinlets of the phengite-bearing leucosome from which geochronological sample D17-4 was taken; f, BSE image of the phengite-bearing leucosome; indices near Ph and Bt show Fe/(Fe + Mg) at the site of phengite or biotite analysis. Note the recrystallization of primary phengite with the formation of a quartz–phengite aggregate.

plectic eclogite with linear stringers of quartz-biotite-plagioclase (Fig. 14, b, c) or pure quartz composition. The quartz crosscuts are lined with garnet and rutile. Orthopyroxene-plagioclase reaction rims formed at the quartz–garnet boundary. Orthopyroxene-clinopyroxene-plagioclase symplectites contain omphacite relics (Jd up to 30%), and garnet contains kyanite and omphacite inclusions. Note that chlorine-rich apatite grains with up to 6.37 wt.% Cl, rutile, Ti-rich biotite (up to 7 wt.% TiO2), and linear trails of zircon grains occur throughout the symplectites (Fig. 14, d). The veinlet formation was probably due to the percolation of a Cl-saturated acid fluid, whose origin deserves individual discussion, into eclogite. Sample 1111-08 (430 g) yielded 50 zircon grains, which belonged to one population. The grains are of peculiar lumpy

shape with hollows, which are traces of gas-liquid inclusions (Fig. 15, a). They do not luminesce and, therefore, show no internal zoning in cathodoluminescence. They look in crosssection like split columnar, rounded, or ameboid grains growing through hard rock under stunted conditions. Zircon contains numerous inclusions of orthopyroxene, clinopyroxene, Ti-rich biotite, rutile, quartz, and chlorine-rich apatite— symplectic minerals and the minerals crystallized in the presence of a fluid (Fig. 15, b). The zircon grains feature homogeneous Th-rich areas and areas of superimposed recrystallization. The homogeneous ones have extremely high contents (ppm) of Th (2338–17,700), U (2355–8500), Y (5027–30,000), Hf (7704–9750), and REE (up to 14,500) and abnormally large Th/U ratios (1.0–2.8). The SHRIMP II dating of such zircon (Center of Isotopic Research) yielded both

Fig. 9. U–Pb concordia diagram and cathodoluminescence images of zircon (sample D17-1) from metagabbro, Cape Vargas. Age estimates: 1, oscillatory zircon with large Th/U ratios (0.21–0.89); 2, zircon with small Th/U ratios (0.06–0.12); 3, uninterpreted zircon. Figures near the ellipses and zircon grains are the numbers of the analyses in Table 4.

1040

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

concordant (Table 7, discordance 0–1) and inversely discordant age estimates (Table 7, discordance –1 to –9). The concordant U–Pb age of the zircon grains is 2394 ± 6 Ma. Altered areas in the grains are characterized by small and very small Th/U ratios (0.91–0.04) and low contents of (ppm) Th (18–3810), U (188–3263), Y (874–4864), and REE (1307– 4234); they contain up to 10 wt.% H2O and numerous thorite segregations and have elevated Na, K, Ca, and Fe contents. The age of zircon hydration and recrystallization is 1886 ± 10 Ma.

Sm–Nd dating of whole-rock samples and minerals

Fig. 10. REE pattern for zircon from a metagabbro dike, sample D17-1, Cape Vargas. a, Magmatic zircon; b, granulitic zircon. Chondrite is after (Sun and McDonough, 1989).

Gneisses, Izbnaya Luda Island, samples D26, D27, D28. The Sm–Nd model ages of the gneisses under study with respect to depleted mantle are the following: D26, 3089 Ma; D27, 2854 Ma; D28, 2973 Ma (Table 8, Fig. 3). Comparison between the U–Pb and model Sm–Nd age estimates for these samples (Table 8) shows their correlation, suggesting the Paleo-Mesoarchean (3.1–2.9 Ga) age of the protoliths and the Meso-Neoarchean (2.82–2.69 Ga) age of the most significant metamorphic recrystallization of the gneisses. The εNd estimates for the tonalite-gneisses (0.6–1.5) indicate the juvenile nature of the corresponding magmas. Mafic dikes. The Sm–Nd mineral isochrons (Grt, Cpx, Pl, WR) for dike D14-13 of quartz-bearing metagabbronorite and dike D12-4 of metagabbro (Fig. 16) yielded an age of 1.9 ± 0.1 Ga. Most of the minerals used for the isochrons have low closure temperatures of the Sm–Nd system. Therefore, the

Table 6. U-Th-Pb SHRIMP II dating of zircon from a phengite leucosome, sample D17-7, Cape Vargas Analysis no.

206

Pbc,

U

Th

206

232

Pb*

Th/238U D, %

Pb/235U(1) ±%

207

Pb*/238U(1) ±%

206

Rho

206

%

Pb*/238U age, Ma(1)

207 Pb/206Pb* age, Ma(1)

ppm D17-7-1.1

0.04

129

59

47.8

0.47

+20

11.7

1.9

0.433

1.7

0.9

2318

±33

2793

±14

D17-7-1.2

0.43

63

62

21.3

1.01

+4

7.5

2.6

0.392

2.0

0.8

2133

±36

2205

±29

D17-7-2.1

–

309

184

118

0.61

+11

10.8

1.6

0.446

1.5

0.9

2377

±30

2619

±10

D17-7-3.1

–

476

55

206

0.12

+4

13.0

1.6

0.504

1.5

1.0

2630

±32

2716

±8

D17-7-4.1

0.00

3319

158

1490

0.05

+0

13.4

1.5

0.522

1.4

1.0

2708

±32

2713

±6

D17-7-4.1a

–

3575

169

1560

0.05

+3

13.1

1.5

0.508

1.4

1.0

2647

±31

2713

±6

D17-7-5.1

0.18

115

63

40.6

0.56

+21

10.5

2.0

0.409

1.8

0.9

2212

±34

2700

±15

D17-7-5.2

–

38

1

11.4

0.02

+12

6.3

3.2

0.346

2.4

0.7

1916

±39

2131

±39

D17-7-6.1

–

935

99

393

0.11

+2

11.8

1.5

0.489

1.4

1.0

2566

±30

2600

±7

D17-7-7.1

0.07

271

32

108

0.12

+11

11.8

1.7

0.463

1.6

0.9

2455

±32

2700

±10

D17-7-8.1

–

120

71

51.9

0.61

+2

12.8

2.0

0.505

1.8

0.9

2634

±38

2683

±14

D17-7-8.2

–

522

158

223

0.31

+4

12.6

1.6

0.497

1.5

0.9

2599

±33

2689

±9

D17-7-9.1

0.13

155

1

44.6

0.01

+4

5.5

2.1

0.335

1.7

0.8

1863

±28

1928

±20

D17-7-10.1

0.01

672

78

291

0.12

+4

13.0

1.6

0.504

1.5

1.0

2631

33

2713

±7

D17-7-11.1

0.00

66

0

19.9

0.00

+3

5.8

3.0

0.349

2.1

0.7

1929

±35

1975

±39

Note. Errors are 1σ; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Temora Standard calibration was 0.42%. corrected using measured 204Pb. D, Discordance.

(1) Common

Pb

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

1041

Fig. 11. Cathodoluminescence and optical images of zircon from a phengite leucosome, sample D17-7, Cape Vargas. a, U-rich migmatitic zircon with destroyed cores; b, zircon with U-rich unzoned cores and small Th/U ratios; c, zircon with magmatic oscillatory cores; d, examples of Paleoproterozoic rims (1.9 Ga); e, U–Pb concordia diagram. Age estimates: 1, zircon 4.1; 2, U-rich unzoned cores with small Th/U ratios; 3, oscillatory cores; 4, U-poor rims with negligible Th/U ratios.

1042

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

The Sm–Nd model ages of the whole-rock samples from the dikes (3037, 3207, 3443 Ma) (Figs. 3, 4, Table 7) turned out to be even older than the model ages of the host gneisses. The age interval obtained (3.4–3.0 Ga) can be regarded as evidence for the Archean age of the juvenile mafic magmas and their relationship with a Paleoarchean source.

40

Ar/39Ar amphibole dating

Fig. 12. REE pattern in zircon from a phengite leucosome (sample D17-7, Cape Vargas). a, Migmatitic zircon; b, metamorphic zircon; c, Paleoproterozoic rims. Chondrite is after (Sun and McDonough, 1989).

estimates can be interpreted as the time the rocks cooled or the Sm–Nd mineral system was re-equilibrated during heating and subsequent cooling.

The 40Ar/39Ar isotope system in amphibole has a closure temperature varying within 550 ± 50 °C depending on the mineral composition, grain size, and system cooling rate (Harrison, 1981). The 40Ar/39Ar dating was aimed at determining the time of amphibolitic metamorphism, when both eclogitic and granulitic metamorphism have already ended. Samples were taken from dikes of several generations in the areas in which the dikes had been completely transformed under amphibolitic conditions. Four samples were taken within Izbnaya Luda Island (Fig. 3): three from dikes at the Central–Western domain boundary (samples 991-101, 991-3a, 991-26) and one from a rupture in a gabbronorite dike (sample 996-8). One more sample was taken from an amphibolized metagabbro dike on Cape Vargas (Fig. 4, sample D17-3). Amphibolite after olivine metagabbro, Izbnaya Luda Island, sample 991-101. The Ca/K ratios on the sample spectrum are consistent for three high-temperature steps, varying from 46 to 47 (Table 9). Argon from the last three steps might correspond to that released from the amphibole lattice. On the sample spectrum, age decreases (downward steps) with increasing temperature, suggesting an excess of radiogenic Ar. The points on the isochron diagram plot along the X-axis but it seems impossible to establish an isochron dependence (Fig. 17, a). An average age of 2029 ± 15 Ma was calculated for two high-temperature steps with consistent ages, which correspond to 33% of 39Ar released. Since we cannot exclude that this age is overestimated because of excessive

Fig. 13. Concordia diagram for zircon from a dike of eclogitized olivine gabbronorites, northeastern Gridino Village. a, After (Slabunov et al., 2011); b, this work. See sampling sites in Fig. 5.

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K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

Table 7. U–Th–Pb SHRIMP II dating of zircon from metasomatic veinlets in an olivine gabbronorite dike, sample 1111-08 (northeastern outskirts of Gridino Village) Site

206

Pbc, U

Th

206

Pb*

Th/U

238

U

206

%

±%

∗

206

Pb

ppm

Pb∗

207

±%

Pb∗

207

∗

±%

235

Pb

Pb∗

206

±%

Rho

Age, Ma

D, %

238

U

U

206

207

Pb

Pb

238

206

U

Pb

1.1

–

2455 6757

959.0

2.84

2.2

1.1

0.1541

0.3

9.66

1.2

0.455

1.1

1.0

2417

±23

2392 ±6

–1

2.1

0.35

739

217.0

0.13

2.9

2.5

0.1159

0.8

5.47

2.6

0.342

2.5

0.9

1898

±40

1893 ±15

0

3.1

–

2599 3717

1063.7

1.48

2.1

1.0

0.1545

0.4

10.15

1.1

0.476

1.0

0.9

2512

±22

2396 ±6

–6

4.1

–

6881 17,715

2902.4

2.66

2.0

1.0

0.1550

0.3

10.50

1.0

0.491

1.0

1.0

2575

±21

2402 ±5

–9

5.1

0.16

1960 2338

753.3

1.23

2.2

1.0

0.1545

0.4

9.53

1.1

0.448

1.0

0.9

2384

±20

2397 ±6

+1

6.1RE

–

1320 1311

523.7

1.03

2.2

1.0

0.1543

0.4

9.83

1.1

0.462

1.0

0.9

2448

±21

2394 ±7

–3

8.1

0.08

1759 508

509.1

0.30

3.0

1.0

0.1150

0.5

5.34

1.1

0.337

1.0

0.9

1871

±17

1880 ±8

+1

9.1

–

2342 4892

912.8

2.16

2.2

1.0

0.1537

0.3

9.61

1.1

0.454

1.0

1.0

2411

±21

2387 ±5

–1

10.1

–

429

10.1RE 0.26

93

18

2667 2346

126.0

0.04

2.9

0.5

0.1130

1.5

5.33

1.6

0.342

0.5

0.3

1896

±8

1848 ±28

–3

894.0

0.91

2.6

1.1

0.1338

0.4

7.20

1.2

0.390

1.1

0.9

2124

±20

2149 ±7

+1

Note. Errors are 1σ; Pbc and Pb* indicate the common and radiogenic portions, respectively. Error in Temora Standard calibration was 0.42%. D, Discordance. 40

Ar, a realistic estimate of the closure age of the amphibole isotope system will be no older than 2 Ga (Fig. 17, a). Amphibolite after metagabbronorite, Izbnaya Luda Island, sample 991-3a. The sample shows a spectrum with decreasing age (downward steps), which suggests an excess of radiogenic Ar. The last high-temperature step, which corresponds to 96% 39Ar released and Ca/K = 46 (Table 9), yielded an age of 2046 ± 16 Ma. That said, the closure age of the amphibole isotope system is no older than 2.05 ± 0.02 Ga (Fig. 17, b). The excess of radiogenic Ar is also confirmed by the fact that the points for the high-temperature steps on the isochron diagram form a linear regression (MSWD = 1.6), with an age of 1962 ± 33 Ma and a large initial ratio (40Ar/36Ar)0 = 3602 ± 444 (Fig. 17, b). Thus, the closure of the amphibole isotope system and the final cooling of the rock below 500 °C took place at 1.93–1.99 Ga. Amphibolite after metagabbro, Izbnaya Luda Island, amphibole, sample 991-26. The spectrum of sample 991-26 features a plateau of three successive steps, which corresponds to 98% 39Ar released (Table 9) and an age of 1948 ± 14 Ma. The Ca/K ratios for the plateau steps are consistent within error limit. On the isochron diagram, the points of all the temperature steps, except 800 °C, form a regression with an initial ratio (40Ar/36Ar)0 = 457 ± 27, MSWD = 2.6, and an age of 1948 ± 14 Ma, which is consistent, within error limit, with the plateau age. Thus, the latter might be coeval with the closure of the amphibole isotope system (Fig. 17, c) as a result of cooling below 500 °C. Amphibolite after metagabbronorite, Izbnaya Luda Island, sample 996-8. The spectrum of sample 996-8 features a three-step plateau, which corresponds to 94.5% of 39Ar released (Table 9) and an age of 1917 ± 14 Ma. The Ca/K ratio is small for the low-temperature steps, suggesting the contribution of altered crystal rims. At the same time, the steps show consistent values varying from 13.5 to 14.6. The points on the isochron diagram, except two low-temperature ones,

form a regression with an initial ratio (40Ar/36Ar)0 = 1875 ± 260, MSWD = 2.3, and an age of 1895 ± 20 Ma. The isochron age is consistent with the plateau age within error limit. Thus, the plateau age, as the more accurate one, might correspond to that of the closure of the amphibole isotope system (Fig. 17, d). Amphibolite after metagabbro, Cape Vargas, sample D17-3. The Ca/K ratios are consistent for the high-temperature steps, varying from 14 to 16 (Table 9). Argon from these steps might correspond to that released from the amphibole lattice. The spectrum for sample D17-3 shows decreasing age (downward steps), suggesting an excess of radiogenic Ar. The last high-temperature step yielded an age of 2008 ± 14 Ma. The excess of radiogenic Ar is also confirmed by the fact that the points for the high-temperature steps on the isochron diagram form a linear regression (MSWD = 2.1), with an age of 1947 ± 75 Ma and a large initial ratio (40Ar/36Ar)0 = 28340 ± 3660 (Fig. 18). Thus, the closure of the amphibole isotope system and the final cooling below 500 °C took place at 1.95 ± 0.08 Ga. 40

Ar/39Ar data on muscovite and biotite

Depending on the grain size and system cooling rate, the closure temperature of the 40Ar/39Ar isotope system in mica is 370 ± 40 °C in muscovite (Harrison et al., 2009) and 345–280 °C in biotite (Harrison et al., 1985). Phengite and biotite were dated in a sample of a phengite-bearing granitic leucosome on Cape Vargas (sample D17-4, Fig. 4). As was said, phengite is of magmatic and metamorphic origin, whereas biotite after phengite as a result of metamorphism. Spectra with well-defined plateaus were obtained (Fig. 19). Phengite and biotite yielded ages of 1689 ± 12 and 1793 ± 13 Ma, respectively. The ages contradict the experimental sequence of closure temperatures in the mica grains studied.

1044

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

Fig. 14. Photos of outcrops and thin sections of metasomatic veinlets in an eclogitized dike of olivine gabbronorites, northeastern Gridino Village. a, Photo of an outcrop of metasomatic veinlets crossing quartz-free orthopyroxene eclogites, after olivine gabbronorites, in northeastern Gridino Village; b, BSE image of the boundary (white dashed line) between the orthopyroxene eclogites (area 1) and metasomatic veinlets (area 2), which occur as linear zones consisting of two-pyroxene-plagioclase symplectites with linear quartz veinlets and quartz-biotite-plagioclase stringers; indices near Omp show Jd content (mol.%); c, quartz-biotite-plagioclase stringers penetrating orthopyroxene eclogite in a zone of deformation and symplectization; d, zircon trails associated with metasomatic veinlets of symplectization and matter supply.

This might be due to the smaller size of the restitic phengite grains observed in thin sections.

rutile from dike D14-13 of quartz-bearing gabbronorites is 1755 ± 5 Ma (Fig. 20, b, Table 10). Rutile from dike D12-4 of metagabbro was dated at 1786 ± 26 Ma (Fig. 20, b, Table 10).

U–Pb rutile dating The closure temperature of the U–Pb system in rutile is 400–450 °C (Mezger et al., 1991) or 650–670 °C (Cherniak, 2000) depending on the grain size and system cooling rate. We dated rutile from metamorphosed dikes on Izbnaya Luda Island (Fig. 3). On the basis of the upper intercept, the age of

Discussion The geochronological studies yielded a long series of dates. We will try to group the figures in a logical way, obtain a good correlation with the igneous and metamorphic events

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

1045

Fig. 15. Zircon from metasomatic veinlets in eclogitized olivine gabbronorites, sample 1111-08, northeastern Gridino Village. a, SE images; b, BSE images of zircon dated. White circles show the site of SHRIMP II analysis (Table 7); numbers show the 206Pb/207Pb age (Ma); the Th/U ratio is in parentheses.

Fig. 16. Sm–Nd mineral isochrons for metamorphosed mafic dikes, Izbnaya Luda Island. a, Metagabbronorite, Grt–Opx–Cpx granulite, sample D14-13; b, metagabbro, Grt–Cpx–Pl rock, sample D12-4. WR, Whole-rock analysis. Parenthetical figure shows the analysis number from Table 8.

revealed by the geological and petrological studies, and restore the igneous and metamorphic evolution of the Gridino eclogite association with regard to the data published by other researchers.

Archean events The age estimate for zircon from the phengite-bearing granitic leucosome (ca. 2.71 Ga) is an important reference for dating the intrusion and eclogitic metamorphism of the metagabbro dike. The age 2.71 Ga, within error limit, does

not differ from the age 2721 ± 8 Ma, obtained for zircon from an eclogite boudin on Stolbikha Island, which might be coeval with eclogitic metamorphism, according to (Volodichev et al., 2004). Other researchers interpret the age ca. 2.7 Ga as that of granulitic metamorphism in the rocks of the Belomorian province (Lobach-Zhuchenko and Chekulaev, 2007; Mints, 2007). In the eclogites of the Salma association, this age is associated with granulitic metamorphism, superimposed onto earlier eclogite associations (Mints et al., 2010a,c). The ages obtained for different zircon fractions from the leucosome are very close, and their differences are within the analytical error. Uranium-rich metamict brown elongated

1046

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

Table 8. Sm-Nd isotope dating of felsic and mafic rocks from Izbnaya Luda Island Sample

Rock or mineral

Sm

Nd

147

Sm/144Nd

143

Nd/144Nd

T(DM)*

ppm D26

Grt-Hbl-Bt gneiss

5.545

35.276

0.095021

0.51073

±18

3089

D27

Grt-Bt gneiss

2.351

19.637

0.072361

0.510491

±18

2852

D28

Hbl-Bt gneiss

2.888

16.18

0.107907

0.511075

±17

2973

D12-4

Metagabbro

7.252

31.922

0.137334

0.51162

±11

3037

D12-4(1)

Clinopyroxene

2.152

16.012

0.081234

0.51089

±11

–

D12-4(2)

Garnet

7.446

23.059

0.195209

0.512378

±19

–

D12-4(3)

Plagioclase

3.838

7.193

0.322573

0.513925

±18

–

D14-13

Gabbronorite

1.741

6.781

0.155238

0.511913

±21

3207

D14-13(1)

Clinopyroxene

2.956

11.308

0.158041

0.511933

±16

–

D14-13(2)

Orthopyroxene

2.958

11.359

0.157425

0.511954

±24

–

D14-13(3)

Garnet

1.178

2.22

0.320737

0.513966

±23

–

D14-13(4)

Garnet

1.123

1.804

0.376241

0.514772

±62

–

D14-13(5)

Plagioclase

0.554

6.052

0.055354

0.510752

±13

–

D-17-1

Metagabbro

5.793

25.19

0.139

0.511451

±9

3443

Note. Whole-rock samples are in bold; the mineral monofractions analyzed in these rocks are in plain font. Analysis number is in parentheses. * Model age (Ma) with respect to depleted mantle after (Goldstein and Jacobsen, 1988).

Table 9. T, ºC

40

Ar/39Ar dating of amphibole and mica

40 Ar, 10–9 ncm3

40

Ar/39Ar ±1σ

38

Ar/39Ar ±1σ

37

Ar/39Ar ±1σ

36

Ar/39Ar ±1σ

Ca/K

Σ39Ar, % Age, Ma ±1σ

Amphibole from a metagabbro dike, sample D17-3, J = 0.004415 ± 0.000051, weight 82.5 mg 500

171.8

7088.3

499.5

0.1393

0.0843

3.8930

2.5086

0.6272

0.0831

14.0

0.3

6222.8

124.8

600

68.6

700

424.4

4227.0

368.1

0.0950

0.0955

9.5829

6.2878

0.9430

0.1196

34.5

0.5

5257.3

150.3

3582.5

54.5

0.0526

0.0099

5.9744

0.4592

0.2410

0.0155

21.5

1.8

5057.8

32.5

800

472.0

900

278.1

4405.8

35.2

0.0477

0.0171

1.7799

0.4445

0.1194

0.0080

6.4

3.0

5431.1

24.1

1743.3

17.1

0.0532

0.0108

2.3323

0.2845

0.0639

0.0098

8.4

4.8

3884.8

24.3

1000

685.4

841.1

2.1

0.0203

0.0022

3.9656

0.0376

0.0118

0.0024

14.3

14.0

2791.1

16.8

1075

3091.1

527.3

0.6

0.0198

0.0014

4.4655

0.0220

0.0013

0.0011

16.1

80.0

2168.3

14.7

1125

819.6

462.8

0.5

0.0198

0.0007

4.0315

0.0272

0.0023

0.0010

14.5

100.0

2006.1

14.1

Amphibole from a first-generation gabbronorite dike, sample 991-101, J = 0.004542 ± 0.000053, weight 71.4 mg 700

319.3

11164.0

281.0

0.1927

0.0424

4.4877

0.8608

0.3794

0.0269

16.2

0.9

7100.4

49.1

850

108.4

5378.3

308.7

0.0505

0.0506

11.6561

0.9408

0.2713

0.0595

42.0

1.6

5811.8

101.6

950

199.4

1218.6

8.8

0.0560

0.0102

14.2380

0.1776

0.0698

0.0073

51.3

6.9

3360.5

21.1

1000

909.6

487.5

0.9

0.0444

0.0011

12.9800

0.0287

0.0095

0.0017

46.7

67.3

2099.3

14.7

1050

200.6

461.1

2.2

0.0461

0.0028

12.8048

0.0889

0.0143

0.0047

46.1

81.4

2026.8

15.7

1150

266.3

464.8

1.6

0.0440

0.0021

13.0954

0.0594

0.0203

0.0034

47.1

100.0

2031.8

15.0

Amphibole from a second-generation gabbronorite dike, sample 991-3a, J = 0.004493 ± 0.000053, weight 49.9 mg 500

11.4

1553.2

311.3

0.0432

0.0442

0.3585

0.3659

0.5803

0.2318

1.3

0.4

3563.5

319.0

600

15.6

2027.9

260.7

0.0646

0.0653

12.2754

6.7358

0.6935

0.1564

44.2

0.9

4001.8

210.3

700

23.7

4555.5

1129.6

0.0954

0.0985

6.3147

6.5112

0.9194

0.3368

22.7

1.2

5426.7

426.8

800

18.3

2317.9

544.0

0.5684

0.2780

10.2030

9.6148

2.0704

0.5396

36.7

1.7

3895.7

380.6

950

41.8

1117.2

49.2

0.0057

0.0057

43.6778

14.0244

0.1408

0.0435

157.2

3.9

3181.5

70.4

1150

767.7

471.5

0.7

0.0640

0.0009

13.1066

0.0318

0.0094

0.0013

47.2

100.0

2044.7

14.6

(continued on next page)

1047

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054 Table 9 (continued) T, ºC

40 Ar, 10–9 ncm3

Ar/39Ar ±1σ

40

38

Ar/39Ar ±1σ

37

Ar/39Ar ±1σ

36

Ar/39Ar ±1σ

Ca/K

Σ39Ar, % Age, Ma ±1σ

Amphibole from a second-generation gabbronorite dike, sample 996-8, J = 0.004588 ± 0.000055, weight 36.7 mg 500

3.9

131.7

6.4

0.1097

0.0365

0.4548

0.4557

0.2379

0.0499

1.6

0.6

446.9

94.6

700

38.4

464.2

11.1

0.0799

0.0163

0.6467

0.2947

0.1590

0.0241

2.3

2.3

1927.5

37.4

900

82.0

538.5

5.6

0.0369

0.0083

1.6892

0.1579

0.0681

0.0104

6.1

5.5

2194.0

21.5

1025

874.7

416.3

0.4

0.0226

0.0005

4.0582

0.0098

0.0050

0.0009

14.6

49.2

1920.9

14.3

1075

818.2

413.5

0.4

0.0228

0.0010

3.7525

0.0126

0.0046

0.0010

13.5

90.4

1913.1

14.2

1150

194.2

419.1

1.3

0.0215

0.0044

3.8698

0.0392

0.0174

0.0031

13.9

100.0

1918.3

14.9

Amphibole from a second-generation metagabbro dike, sample 991-26, J = 0.004468 ± 0.000052, weight 33.0 mg 500

16.2

884.0

70.6

0.4211

0.0938

5.9997

2.0037

0.9512

0.1103

21.6

0.5

2357.3

118.1

600

4.6

1255.1

719.3

0.2542

0.2934

11.8963

11.4728

3.2237

1.9343

42.8

0.5

1542.6

830.8

800

75.5

2739.1

191.8

0.1457

0.0544

3.8878

1.2404

0.4666

0.0772

14.0

1.2

4574.3

118.6

900

32.0

1107.8

93.9

0.3859

0.0802

7.9498

1.4970

1.1307

0.1280

28.6

1.9

2696.1

128.0

1000

1263.1

439.1

0.6

0.0282

0.0014

5.6910

0.0297

0.0124

0.0013

20.5

72.8

1948.9

14.0

1050

14.2

708.3

49.4

0.2061

0.0985

9.8668

1.9938

0.9637

0.0969

35.5

73.2

1916.0

101.4

1150

477.0

438.9

0.7

0.0289

0.0010

5.7620

0.0244

0.0128

0.0014

20.7

100.0

1948.1

14.0

Biotite from a phengite leucosome, sample D17-4, J = 0.004384 ± 0.000050, weight 3.9 mg 500

11.2

189.0

4.7

0.0356

0.0157

–

–

0.0680

0.0256

–

1.0

999.7

40.3

650

388.1

384.1

0.2

0.0232

0.0010

–

–

0.0135

0.0014

–

17.6

1769.5

12.9

700

463.5

394.1

0.1

0.0221

0.0012

–

–

0.0044

0.0012

–

36.9

1806.7

13.1

800

336.2

393.1

0.6

0.0224

0.0016

–

–

0.0058

0.0014

–

50.9

1802.5

13.2

900

194.6

387.8

0.4

0.0268

0.0031

–

–

0.0131

0.0027

–

59.2

1780.5

13.2

1000

484.9

397.0

0.3

0.0192

0.0007

–

–

0.0061

0.0012

–

79.2

1813.6

13.1

1065

265.8

393.7

1.1

0.0144

0.0010

–

–

0.0116

0.0022

–

90.3

1799.2

13.5

1130

228.7

387.1

0.8

0.0213

0.0020

–

–

0.0081

0.0027

–

100.0

1783.0

13.3

Muscovite from a phengite leucosome, sample D17-4, J = 0.004383 ± 0.000050, weight 3.3 mg 600

30.7

348.1

4.8

0.0462

0.0171

–

–

0.0715

0.0141

–

1.7

1604.2

23.3

675

44.3

337.6

1.2

0.0167

0.0117

–

–

0.0355

0.0110

–

4.2

1604.5

16.5

725

64.0

352.8

2.6

0.0290

0.0059

–

–

0.0295

0.0047

–

7.6

1658.8

15.3

775

82.3

344.3

1.7

0.0299

0.0036

–

–

0.0346

0.0043

–

12.2

1627.1

14.0

825

349.0

357.4

0.5

0.0216

0.0004

–

–

0.0112

0.0015

–

30.7

1690.2

12.7

900

332.8

356.1

0.6

0.0197

0.0015

–

–

0.0051

0.0013

–

48.5

1691.6

12.7

975

234.0

356.3

0.5

0.0224

0.0019

–

–

0.0170

0.0016

–

61.0

1681.4

12.7

1065

377.0

357.3

0.3

0.0215

0.0010

–

–

0.0138

0.0006

–

81.0

1687.5

12.5

1130

357.0

357.0

0.4

0.0194

0.0014

–

–

0.0061

0.0006

–

100.0

1693.6

12.6

Note. J, Neutron flux intensity.

zircon grains with relics of a magmatic oscillatory zoning most probably crystallized from a granitic melt. The magmatic appearance, rock microstructures, and estimated PT-parameters of phengite crystallization suggest that anatectic melting of the rocks took place under eclogitic conditions. On the other hand, the zircon dated underwent metamictization and recrystallization, so that the age obtained might conceal earlier igneous and, probably, metamorphic events. The REE pattern, small Th/U ratio, and internal structure of zircon from group 2 suggest metamorphic crystallization

conditions. However, the high U content of zircon from this group is most probably due to its growth in the presence of a melt in the crystalline part of the granitic melt. This process is close to the formation of U-poor oscillatory cores, probably from the U-depleted residual felsic melt. The age 2.72 Ga, obtained for two zircon grains from a granulitized metagabbro dike (sample D17-1, Cape Vargas), which combine the properties of magmatic and metamorphic zircon, coincides with the above estimated age of the granitic

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K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

Fig. 17. 40Ar/39Ar data on amphibole from the amphibolized zones of mafic dikes from different generations, Izbnaya Luda Island. a, b, d, Gabbronorite: a, early, sample 991-101; b, late, sample 991-3a; d, late, sample 996-8; c, late metagabbro, sample 991-26.

leucosome and might be associated with the formation of the melt in mafic rock. It might be argued that zircon from the phengite-bearing leucosome crosscutting the metagabbro dike (samples D17-4, D17-7) did not crystallize from a granitic melt but are xenogenic and were captured from the host gneisses. Studies of zircon from the gneisses showed that it is poorer in U (32–136 ppm) (Table 2) than that from the leucosome (520– 3000 ppm) (Table 6). As pointed out above, the host gneisses

and the leucosome show geochemical differences, which indicate their independent origin. Along with the crystal composition and morphology, these data suggest that zircon crystallized in situ from an anatectic melt. Thus, the leucosome age ca. 2.71 Ga is the upper bound of both the dike intrusion and the eclogitic event for two reasons. First, the leucosome intrudes a metagabbro dike which has already formed; second, judging by relics of high-pressure magmatic and superimposed granulitic mineral

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

associations, it formed during retrograde decompression as the eclogites rose up the crust under high-pressure granulitic conditions. The estimated Sm–Nd age of the Gridino dikes (3.4– 3.0 Ga) characterizes the time the mafic melts separated from the mantle source. In this case 2.82 Ga magmatic zircon from metagabbro might be coeval with magmatic crystallization. Thus, the intrusion of the Gridino mafic dikes might have taken place as early as the Mesoarchean. Note also a ca. 3.0 Ga zircon grain (Table 4, Fig. 9, 1.4.1) from a metagabbro dike, which is morphologically and geochemically similar to magmatic zircon. The Sm–Nd age of the metagabbro whole-rock sample, calculated with respect to depleted mantle, is T(DM) = 3.3 Ga; εNd(2800) = –2.4; εNd(2400) = –5.4. These estimates suggest that the juvenile mafic melt was contaminated with ancient continental crust. However, since the lower bound of the possible dike age has not been constrained reliably, this estimate suggests the relatively old age of the dike or the age of the gneissic protolith. The gneisses from Izbnaya Luda Island (2.82–2.77 Ga) are coeval, within age variation and analytical error, with metagabbro dike D17-1 (2.82 Ga). This has two possible explanations: Either zircon in the mafic dike was really captured from the 2.82 Ga country rocks and the mafic intrusion formed at 2.82–2.72 Ga or mafic magmatism was subsynchronous with the formation of tonalitic magmas. The temporal proximity of eclogitic and granulitic metamorphism remains a hypothesis for lack of reliable evidence constraining eclogitic metamorphism. Now we can speak only of the interval which included the eclogitic event (2.82– 2.71 Ga).

1049

Fig. 18. 40Ar/39Ar dating of amphibole from the amphibolized zone of a metagabbro dike, sample D17-3, Cape Vargas.

Problem of the age 2.4 Ga The Gridino mafic dikes, which contain Archean magmatic and metamorphic zircon, evidence a 2.4 Ga thermal event. Also, U–Pb ages of ca. 2.4 Ga were also obtained for the Gridino mafic rocks in the previous studies (Slabunov et al., 2003, 2011). Slabunov, Volodichev, and their coauthors think that exactly this zircon yields the dike crystallization age, whereas the other groups consist of inherited xenogenic crystals. This conclusion is inconsistent with an Archean U–Pb age of ca. 2.71 Ga, coeval with the formation of the phengitebearing veinlet cutting the dike. We think that comparison with the above geochronological studies of zircon from the same dike (Slabunov et al., 2011) gives an insight into the origin of 2.39 Ga zircon grains (gabbro ones, according to Slabunov et al.). The U and Th contents and Th/U ratio in Early Paleoproterozoic zircon permit identifying two clear subgroups in group II from (Slabunov et al., 2011, Table 1): first, sites 66.10.1 and 66.10.3 (2.45–2.44 Ga) show very low (metamorphic?) U and Th contents (56–60 and 36–41 ppm, respectively) and moderate Th/U ratios (0.67–0.71); second, sites 65.7.1, 65.7.2, 66.5.1, 66.6.1, and 66.4.1 (2.40–2.35 Ga) show the following values

Fig. 19. 40Ar/39Ar data on phengite (a) and biotite (b) from a phengite-bearing granitic leucosome (sample D17-4), Cape Vargas.

decreasing with age: Th, from 4392 to 404 ppm; U, from 1686 to 349 ppm; and Th/U, from 1.19 to 2.69. Both zircon fractions, dated at 2.39 Ga (zircon extracted in situ from the eclogitized dike (Slabunov et al., 2011) and obtained from metasomatic veinlets in the area in which the dike is strongly deformed and reworked by fluids), show

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K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

Fig. 20. U–Pb concordia diagrams for rutile from metamorphosed mafic dikes, Izbnaya Luda Island. a, Metagabbronorite, sample D14-13; b, metagabbro, sample D12-4. Figures near the ellipses are the numbers of the analyses in Table 10.

2.3–2.5 Ga for some crystal grains, zones, or areas. We think that the age ca. 2.4 Ga in the Gridino mafic rocks corresponds to a Paleoproterozoic thermal (or, more broadly, metamorphic) event, by analogy with the Salma eclogites. Local dating methods are expected to detect the same zircon in the host granite-gneisses. The 2.5–2.4 Ga thermal event is now associated by most researchers with the formation and development of a superplume in the mantle region beneath the Neoarchean continent, which predated the East European craton. This period is marked by general crustal stretching and downwarping and superplume-induced LIP formation: gabbro-anorthosite underplating and intraplating in the lower crust, intrusions of layered mafic-ultramafic rocks in the upper crust, minor gabbro-anorthosite and mafic-ultramafic (drusite) intrusions in the middle crust, and contrasting mafic-rhyolitic volcanism as well as

clearly identical morphologic and geochemical characteristics. The sample from the metasomatic veinlets is rich in zircon crystals, with more well-defined geochemical properties, but older zircon (including 2.45–2.44 Ga) is absent. These features are interpreted reliably with the assumption that the zircon grains described crystallized at 2.39 Ga in a dike previously affected by eclogitic metamorphism, in deformed zones, under the effect of a high-temperature, acid, probably magmatic, fluid. The date 2.39 Ga is unrelated to the age and eclogitization time of the mafic intrusion, because the zircon grains contain inclusions of minerals and symplectic intergrowths of the posteclogite decompression stage. According to our experience and ample data derived from site analysis of zircon from the Archean Salma eclogites (Konilov et al., 2011; Mints et al., 2010b,c), the predominant Archean values coexist more or less regularly with ages of

Table 10. U-Pb isotope dating of rutile from metamorphosed mafic dikes, Izbnaya Luda Island Sample no.

Weight, mg

Content, ppm

Pb isotope composition*

Pb

206

U

Pb/

204

Pb

206

207

Pb/

Pb

Isotopic ratios 206

Pb/

208

Pb

207

235

Pb/

U

Age, Ma** 206

Pb/

238

U

207

Rho

206

Pb/

Pb

Gabbronorite, Grt-2Px granulite (sample D14-13) 1

2.9

6.5

2

3.1

7.2

3

2.9

296.6

20.3

1191

8.4603

41.14

4.71047

0.320014

1745

0.90

24.4

1745

8.7155

59.087

4.48961

0.300442

1748

0.98

497.9

72

3.3315

1.4129

4.07739

0.267320

1810

0.98

Late metagabbro, Grt-Cpx-Pl rock (sample D12-4) 1

2.00

4.4

56.0

936

8.0594

21.91802

4.61910

0.306120

1743

0.67

2

1.65

17.2

56.5

1245

8.4351

38.76800

4.59990

0.304608

1760

0.36

3

2.00

10.4

34.8

1098

8.1931

27.62800

4.46438

0.295192

1784

0.95

* All the ratios were corrected for blank Pb (0.08 ng) and U (0.04 ng) and mass discrimination (0.12 ± 0.04%). ** Correction for common Pb was determined according to the model from (Stacey and Kramers, 1975).

K.A. Dokukina et al. / Russian Geology and Geophysics 53 (2012) 1023–1054

inland-basin formation, sedimentation, granulitic metamorphism, and partial melting of the Archean basement and basin sediments. This period in the evolution of the Kola–Karelian region is described in detail in (Glebovitskii, 2005; Mints et al., 2010b), with extensive reference lists.

Tectonometamorphic stage, 1.8–2.0 Ga Mapping of the key constituents of the Gridino association showed Late Paleoproterozoic (1.8–2.0 Ga) ages in zones of superimposed deformation and amphibolization. Here, eclogitic and granulitic mineral parageneses disappear or occur as relics. Uranium-poor colorless zircon rims with ages of 1.94 Ga around Archean magmatic and metamorphic zircon are due to thermal and deformation effects in the rocks of the Belomorian orogen as a whole, which accompanied the late Paleoproterozoic collisional events (Bibikova et al., 2004; Mints et al., 2010b,c; Slabunov et al., 2006b). The Sm–Nd isochron dates obtained for the gabbronorite and metagabbro dikes (1.92–1.91 Ga) correspond to the same age. Late Paleoproterozoic metamorphism led to isotope re-equilibration of the Sm–Nd system at the mineral level. The closure temperature of the Sm–Nd system in garnet is estimated at 650–670 °C (Ganguly et al., 1998), and this corresponds to the amphibolitic facies. Rock cooling below 550–500 °C is detected by 40Ar/39Ar analysis of amphibole (Harrison, 1981). Therefore, the end of Svecofennian metamorphism near Gridino Village is dated at 1.96 ± 0.01 Ga (mean weighted value for four dates). The presence of much excessive Ar suggests that the formation of the amphibole parageneses has a longer history from Archean high-pressure metamorphism to the late Paleoproterozoic. Note that the U–Pb age of the Paleoproterozoic zircon rims (1.94 ± 0.04 Ga) coincides, within error limit, with the amphibole age; that is, the new rims constrain amphibolitic. The U–Pb rutile ages (1.79–1.76 Ga) are considerably younger than those shown by the U–Pb or Sm–Nd isotope systems of other minerals and the 40Ar/39Ar system in amphibole. They reflect the time of metamorphic rock cooling to 400–450 °C (Mezger et al., 1991). These values are confirmed by 40Ar/39Ar dating of muscovite and biotite in a leucosome (sample D17-4), which constrains the time of the system cooling to 425–280 °C (Harrison et al., 1985, 2009) at 1.79–1.69 Ga.

Conclusions The studies of the Gridino eclogite association permitted dating the major igneous and metamorphic events, which form the following sequence. (1) The oldest magmatic zircon (3.00 Ga, U–Pb dating) in the metagabbro dike is coeval with the igneous protolith, dike, or the metamorphic rocks intruded by the dike. The Sm–Nd model ages of the felsic (3.1–2.9 Ga) and mafic (3.4–3.0 Ga)

1051

rocks from the Gridino association also indicate that the protoliths are Paleo-Mesoarchean; (2) The age 2.82 Ga (U–Pb dating), obtained for magmatic zircon from the metagabbro dike, might be that of mafic-melt crystallization or that of the igneous protolith of the metamorphic host rocks. In the second case, the dike age is 2.82– 2.71 Ga; (3) The entire Gridino complex experienced subsidence and eclogitic metamorphism between 2.82 and 2.71 Ga; (4) The U–Pb age (ca. 2.71 Ga) of magmatic and metamorphic zircon in the Gridino felsic and mafic rocks is coeval with retrograde granulitic metamorphism and is a possible upper age bound of eclogitic metamorphism. Zircon formed under decompression conditions simultaneously with partial rock melting; (5) The U–Pb age ca. 2.4 Ga reflects the deformations and thermal metamorphism superimposed on the eclogitized rocks, which are related to an Early Paleoproterozoic superplume in the mantle region beneath the Neoarchean continent, which predates the East European craton; (6) Superimposed amphibolitic metamorphism and the final exhumation of metamorphic complexes at 2.0–1.9 Ga are associated with late Paleoproterozoic collisional events, attributed to the appearance of the Svecofennian accretionary orogen immediately west of the Belomorian province. Successive cooling of the metamorphic associations to 300 °C at 1.9–1.7 Ga is shown by U–Pb zircon and rutile dating and 40 Ar/39Ar mica dating. We thank K.V. Van (Institute of Experimental Mineralogy, Chernogolovka) for participation in field studies and help with microprobe analyses; S.G. Simakin and E.V. Potapov (Yaroslavl Branch of the Institute of Physics and Technology) for geochemical studies of zircon; A.N. Larionov (VSEGEI), L.M. Natapov, and E.A. Belousova (Macquarie University) for geochronological studies of zircon. Special thanks go to A.L. Perchuk, whose comments helped greatly improve the manuscript. The study was supported by the Russian Foundation for Basic Research (grants no. 08-05-00350, 09-05-01006, 11-0500492, 12-05-00856).

References Baer, A.J., 1977. Speculation on the evolution of the lithosphere. Precambrian Res. 5 (3), 249–260. Baksi, A.K., Archibald, D.A., Farrar, E., 1996. Intercalibration of 40Ar/39Ar dating standards. Chem. Geol. 129 (3), 307–324. Baldwin, J.A., Bowring, S.A., Williams, M.L., Williams, I.S., 2004. Eclogites of the Snowbird tectonic zone: petrological and U–Pb geochronological evidence for Paleoproterozoic high-pressure metamorphism in the western Canadian Shield. Contrib. Mineral. Petrol. 147 (5), 528–548. Batieva, I.D., 1958. Alkali granites of Lakes Kanozero and Kolvitsa, in: Vorob’eva, O.A. (Ed.), Alkali Granites of the Kola Peninsula [in Russian]. Izd. AN SSSR, Moscow–Leningrad, pp. 135–145. Bayanova, T.B., Corfu, F., Todt, W., Pohler, W., Apanasevich, E.A., Levkovich, N.V., Zhavkov, V.A., 2007. Heterogeneity of 91,500 and TEMORA-1 standards for U–Pb single zircon dating, in: Proc. XVIII Symp. on Isotope Geochemistry (Moscow, 14–16 November 2007) [in Russian]. GEOKhI RAN, Moscow, pp. 42–43.

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Editorial responsibility: V.V. Reverdatto