Ultra-high temperature overprinting of high pressure pelitic granulites in the Huai'an complex, North China Craton: Evidence from thermodynamic modeling and isotope geochronology

Accepted Manuscript Ultra-high temperature overprinting of high pressure pelitic granulites in the Huai'an complex, North China Craton: Evidence from thermodynamic modeling and isotope geochronology

Hao Liu, Xu-Ping Li, Fan-Mei Kong, M. Santosh, Han Wang PII: DOI: Reference:

S1342-937X(19)30069-3 https://doi.org/10.1016/j.gr.2019.02.003 GR 2111

To appear in:

Gondwana Research

Received date: Revised date: Accepted date:

29 September 2018 16 December 2018 2 February 2019

Please cite this article as: H. Liu, X.-P. Li, F.-M. Kong, et al., Ultra-high temperature overprinting of high pressure pelitic granulites in the Huai'an complex, North China Craton: Evidence from thermodynamic modeling and isotope geochronology, Gondwana Research, https://doi.org/10.1016/j.gr.2019.02.003

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Ultra-high temperature overprinting of high pressure pelitic granulites in the Huai’an complex, North China Craton: evidence from thermodynamic

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modeling and isotope geochronology

Shandong Provincial Key Laboratory of Depositional Mineralization &

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a

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Hao Liua, Xu-Ping Lia,*, Fan-Mei Konga, M. Santoshb,c, Han Wanga

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Sedimentary Minerals, Shandong University of Science and Technology, Qingdao 266590, China.

School of Earth Sciences and Resources, China University of Geosciences

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b

Beijing, 29 Xueyuan Road, Beijing 100083, China

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Department of Earth Sciences, University of Adelaide, Adelaide SA 5005,

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c

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Australia

*Corresponding author: [email protected]

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Abstract Paleoproterozoic granulite facies rocks are widely distributed in the North China Craton (NCC). The Huai’ an terrane, located within the northern segment of the Trans-North China Orogen (TNCO), a major Paleoproterozoic

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collisional belt in the central NCC expose mafic and pelitic granulites as well as

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TTG (tonalite-trondhjemite-granodiorite) gneisses. Here we investigate the

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pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages. The prograde

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metamorphism (M1) is recorded by relict biotite and the compositional profile

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of Xca (grt) isopleths. The Pmax (M2) is distinguished by the Xca (grt) isopleths, which corresponds to the kyanite stable area with an inclusion mineral

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assemblage of Grt-c – (Ky) - Qz - Rt - Kfs - liq suggesting that the pressures

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were higher than 12kbar with a temperature below 900°C. However, kyanite is absent in thin sections suggesting its consumption during later stages. The

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Tmax metamorphism (M3) is characterized by the assemblage: Grt-m - Qz - Pl -

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Rt - Kfs - Sil - liq in the garnet mantle and also reflected in the compositional profile. Two-feldspar geothermometry yields a P-T range of 940°C – 950°C and 9.5 – 10.5 kbar, indicating ultra-high temperature (UHT) metamorphic overprinting. The subsequent retrograde metamorphic stage (M4) is characterized by Grt-r - Bt - Sil - Kfs - Pl - Qz ± Rt ± Ilm with symplectites of Bt-Sil-Qz in the garnet rim suggesting garnet breakdown with P-T conditions estimated as 770°C – 840 °C and 6.5 – 8 Kbar. The pelitic granulites show a 2

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clockwise path, with P-T estimates higher than those in estimated in previous studies using conventional techniques. LA-ICP-MS U–Pb analysis of metamorphic zircon grains yield two groups of ages at 1972.9 ± 8.1Ma and 1873.3 ± 9.9Ma. We suggest that the protoliths of

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the Manjinggou HP-UHT granulites were deep subducted where they

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experienced HP metamorphism associated with the collision of the Ordos and

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Yinshan blocks at ca. 1.97 Ga. Subsequently, the UHT metamorphic overprint

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NCC along the TNCO at ca. 1.87 Ga.

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occurred during the assembly of the unified Western and Eastern Blocks of the

Key words:

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Pelitic granulite; High-pressure and ultra-high temperature metamorphism;

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1. Introduction

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Phase equilibrium modeling; Zircon U-Pb geochronology; North China Craton.

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The North China Craton (NCC) is one of the oldest cratons on the earth preserving the remnants of early crust as old as 3.8 Ga (Liu et al., 1992; Wang et al., 2015). The NCC is composed of a number of Archean microblocks which were incorporated into the Western and Eastern Blocks (Zhai et al., 1992, 1995, 2001, 2009; Zhai and Liu, 2001; Santosh, 2010; Santosh and Kusky, 2010; Zhai and Santosh, 2011; Yang and Santosh, 2017; Tang and Santosh, 2018) with final suturing along the Trans-North China Orogen (TNCO) 3

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during late Paleoproterozoic (Zhao et al., 1999, 2005, 2010). The Huai’an complex is located in the north-western margin of the TNCO (Zhao et al., 2005, 2009, 2010) with basement dominantly composed of pelitic and mafic granulites together with TTG gneisses (Zhao et al., 2005) (Fig. 1).

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The relationship between pelitic and mafic granulites of this complex and their

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metamorphic history remain debated: Guo et al. (1993) interpreted that the

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banded alternating structure of the HP granulite facies mélange belt in the Huai'an Manjinggou area resulted from tectonic uplift of mafic high pressure

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granulites. Zhang et al. (1994) suggested that the khondalite series rocks and

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the mafic HP granulite-bearing gray gneisses represent cover and basement, respectively, with a detachment fault between them. Zhao et al. (2005, 2010)

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considered the pelitic granulites in TNCO as allochthonous parts that

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experienced medium pressure granulite facies metamorphism, and were derived from the Western Block during the collision of the Western and Eastern

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Blocks at ~1.85Ga. Based on the recent finding of kyanite-garnet-K-feldspar

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assemblages in the pelitic granulites from the Manjinggou are of this complex, it was suggested that the pelitic granulites shared the same metamorphic history with associated mafic HP granulites from peak to retrograde stages with analogous HP granulite facies metamorphism (Wu et al., 2016). The pelitic granulites from NCC, previously considered as normal granulites, have in recent studies been re-interpreted as UHT granulites (Li and Wei, 2016). In this study, we present results from petrography, mineral chemistry, P-T 4

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pseudosection modelling and zircon U-Pb geochronology of pelitic granulites from the Manjinggou area of Huai’an complex. Our results show that these rocks experienced initial HP metamorphism, followed by UHT overprinting associated with the two major subduction-collision events in the NCC of

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amalgamation between the Yinshan and Ordos Blocks into the Western Block

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and the final assembly of the Western and Eastern Blocks.

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2. Geological setting

The NCC is a collage of crustal blocks which were amalgamated along three

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major Paleoproterozoic collisional belts namely the Inner Mongolia Suture

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Zone (also known as the Khondalite Belt), the Trans-North China Orogen, and the Jiao-Liao-Ji belt (Fig.1; Zhao et al.,2005, 2010; Santosh and Kusky, 2010;

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Zhao and Zhai, 2013). The Western Block formed through the collision of

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Yinshan block and Ordos block at ca. 1.95Ga, followed by the assembly of this block with the Eastern Block along the TNCO at ca. 1.85 Ga (Zhao et al., 2005).

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Alternate models suggest three Paleoproterozoic mobile belts: the Jiaoliao Belt,

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Jinyu Belt and Fengzhen Belt in the eastern, central and north-western NCC, respectively (Fig.1; Zhai et al., 2005; Li et al., 2018). The Huai’an complex occurs in the northern segment of the TNCO (Fig.1; Zhao et al., 2005) and exposes the high-grade metamorphic basement, considered

to

be

the

continuity

of

the

khondalite

series

in

the

Jining-Liangcheng terrane (Zhai et al., 1995). The TTG gneisses together with pelitic granulites and mafic granulites constitute more than 80% of the rocks in 5

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this complex. Previous studies reported magmatic zircon ages mostly around 2.5 Ga and metamorphic zircon grains that show a spread of ages between 1.9 and 1.8 Ga (Zhao et al., 2008; Guo et al., 2005; Liu et al., 2009, 2012). Our study area is located in the southern part of the Huai’an complex around

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Manjinggou where TTG gneiss together with pelitic and mafic granulites are

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the dominant rocks (Fig. 2), and these can be subdivided into six lithological

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units: the Shuigoukou TTG gray gneisses, banded gneisses, mafic granulites, khondalite series rocks, the Dongjiagou granitic gneiss and the Dapinggou

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garnet-bearing granite, from north to south (Fig.3; Guo et al., 1993; Wu et al.,

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2016; Zhao et al., 2008, 2010).

The Shuigoukou gray gneisses occur as thick banded sequences containing

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tonalitic gneisses with minor diorite gneisses, the protoliths of which formed at

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2.5-2.55Ga (Zhao et al., 2008). The banded gneisses exposed along the riverbed in Mangjinggou show intercalated granulite facies trondhjemites and

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mafic granulites (Zhai et al., 2009). The mafic granulites are exposed as

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prolate lenses or bands within the northern TTG gneisses and the southern metasediments, suggesting that the protoliths of the mafic granulites may be gabbroic dykes (Guo et al., 2002; Zhao et al 2005, 2009; Wu et al 2016). The Dongjiagou granitic gneiss and the Dapinggou garnet-bearing granite are exposed in the southern part of the area. The granitoids are mainly charnockites and S-type granites (Guo et al., 1993; Wu et al., 2016). Grayish khondalite series granulite facies metapelitic rocks

include 6

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sillimanite garnet gneisses and garnet-bearing quartzo-feldspathic gneisses with minor calc-silicate rocks (Guo et al., 1993, Li et al., 2011; Wang et al., 2011a) The mineral assemblage of kyanite- garnet- K-feldspar was also reported from this area, suggesting that the pelitic granulites experienced

HP

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metamorphism (Wu et al., 2016).

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3. Analytical methods

The compositions of minerals were analyzed at the Tongji University,

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Shanghai using a JXA-8230 electron microprobe with conditions of 15 kV

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accelerating voltage, 10 nA probe current, and a 1–3 µm diameter beam except for biotite (5μm). Four samples of pelitic granulites from the Huai’an

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terrene were analyzed for thermodynamic P-T pseudosection modelling and

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geochronological studies. The analytical spots for garnet profiles and representative mineral analyses are presented in Appendix Table 1 and Table

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1－2.

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Zircon separation and CL imaging were carried out at the Mineral Separation Laboratory of the Hebei Institute of Regional Geological and Mineral Survey, Langfang, China. The zircon grains were separated from 3-5kg of each sample using standard heavy-liquid and magnetic techniques, with subsequent handpicking under a binocular microscope. All grains were examined using transmitted and reflected light photomicrographs under microscope so that the mineral inclusions inside could be avoided, The CL images were carefully 7

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analysed for internal structures prior to U-Pb dating. The zircon U–Pb dating and in situ trace element analyses were performed by LA-ICP-MS at the Nanjing FocuMS Technology Co. Ltd. Australian Scientific Instruments RESOlution LR laser-ablation system (Canberra, Australian) and Agilent

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Technologies 7700x quadrupole ICP-MS (Hachioji, Tokyo, Japan) were

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combined for the experiments. The 193 nm ArF excimer laser, homogenized

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by a set of beam delivery systems, was focused on zircon surface with fluence of 3.5J/cm2. Ablation protocol employed a spot diameter of 33 um at 6 Hz

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repetition rate for 40 seconds (equating to 240 pulses). Helium was applied as

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carrier gas to efficiently transport aerosol to ICP-MS. Zircon 91500 was used as external standard to correct instrumental mass discrimination and elemental

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fractionation during the ablation. Zircon GJ-1 was treated as quality control for

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geochronology. Lead abundance of zircon was external calibrated against NIST SRM 610 with Si as internal standard, while Zr as internal standard for

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other trace elements (Liu et al, 2010; Hu et al., 2011). Raw data reduction was

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performed off-line by ICPMSDataCal software (Liu et al., 2010). The calculation of ages, weighted mean and concordia diagrams were conducted by the ISOPLOT 3.0 (Ludwig, 2003) shown in Appendix Table 2. The REE compositions of zircons were normalized by chondrite values (Sun and McDonough, 1989) shown in Appendix Table 3. Mineral abbreviations used are after Whitney and Evans (2010).

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4. Petrography The samples selected for this study are all sillimanite-garnet-K-feldspar gneisses, which have experienced granulite facies metamorphism and were collected from outcrops of pelitic granulites about 300 meters north of

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Manjinggou (Fig.3, Fig. 4a–b). The mineral assemblages are garnet (20-30%),

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sillimanite (10-15%), K-feldspar (15-25%), plagioclase (10-15%), biotite

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(5-10%), quartz (20-30%), and minor accessory minerals including rutile, ilmenite and pyrite (Fig.4). The rocks are well foliated and carry oriented

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columnar sillimanite and elongated quartz, garnet and K-feldspar (Fig.4b, f).

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The garnet occurs generally as coarse porphyroblastic grains with a size of 1.8-3.5mm and commonly contain inclusions of quartz, biotite, sillimanite,

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K-feldspar, plagioclase and rutile (Fig.4e, Fig. 5) except for some peritectic

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grains (~0.4mm) which lack inclusions (Fig.4d). Garnet shows six different textural associations: (1) a clouded garnet core with inclusions of quartz,

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K-feldspar, biotite and rutile, representing the product of prograde

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metamorphism (M1); (2) a clear garnet mantle with rarely developed solitary inclusions of sillimanite, quartz, and plagioclase (Fig. 5a, b), representing a Pmax (see phase equilibria modeling part below) metamorphism (M2); (3) relatively thin garnet rim, which also carries mineral inclusions of quartz, K-feldspar, sillimanite and rutile (Fig. 4e, g; Fig. 5a, b), representing decompression stage during exhumation of the granulite (M3); (4) peritectic garnet with typical melting structure representing Tmax (see

phase equilibria 9

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modeling part below) metamorphism (M3) (Fig. 4d, 5a); (5) embayed porphyroblastic garnet rim in contact with symplectitic minerals like biotite, plagioclase, sillimanite, quartz and K-feldspar, which suggests temperature decrease (M4) (Fig.4c, e; Fig.5a, b), and (6) recrystallized small grains at

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porphyroblastic garnet rim associated with symplectite Bt + Sil + Pl or in the

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matrix of the rock (M4) (Fig. 4c, e, Fig. 5b).

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Sillimanite shows three different morphologies with two generations: (1) needle-like inclusions in garnet rim (M3) (Fig. 4g); (2) idiomorphic columnar

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crystals in the matrix with corroded edges intergrown with biotite, K-feldspar

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and quartz (M3) (Fig. 4f); (3) symplectite with quartz, biotite and plagioclase around garnet rim (M4) (Fig.4c, e, Fig. 5a, b).

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Biotite in the samples show three distinct associations: (1) small flaky

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inclusions in garnet core (M1) (Fig. 5a); (2) accompanied with sillimanite plagioclase and quartz in the embayed porphyroblastic garnet rim (M4); and (3)

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small laths in the matrix (after M4) (Fig.4h). Biotite is mostly a retrogressive

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mineral, occurring only at and after M4 metamorphic stage. Plagioclase is relatively low in content, which may be related to the low CaO and Na2O bulk composition. It occurs as inclusions in garnet rim, and represents Tmax metamorphism (M3) (Fig. 5b), or as acicular solid solutions within perthite, formed during decompression of pelitic granulite at M3 stage (Fig. 4i). The mineral is also present in the matrix of the rock, suggesting that it was formed as retrograde assemblage at/after metamorphic stage M4 (Fig. 4c, 10

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d, 5b). K-feldspar occurs in all the stages from garnet core to rim and in the matrix of the rock (Fig. 4 and Fig. 5). Quartz occurs as inclusions in the garnet core, mantle, rim and also as

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anhedral or elongated crystals in the matrix, and is present in all the

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metamorphic stages (Fig. 4e–l).

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The symplectite corona of sillimanite, plagioclase and biotite around the rim of the garnet may suggest a decompression processes (Fig. 5a, b), following

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the reaction: Grt + melt + Kfs = Bt +Pl+ Sill± Qz (Fig. 4c). Similar

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microstructures in pelitic granulites have been interpreted to be consistent with back reactions involving melt crystallization (Fig. 4e; Waters, 1988; Kriegsman

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and Alvarez-Valero, 2010; Zhang et al., 2017).

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Accessory minerals include rutile, zircon, monazite, xenotime, and apatite (Fig. 4–5). The presence of pyrite and pyrrhotite suggest a relatively low

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oxygen fugacity (Fig.4j). The rutile grains occur both within garnet and in the

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matrix, in all the stages. It is partly replaced by ilmenite during retrograde metamorphism after stage M3 in the matrix (Fig.4k; Fig.5). Melt is widely distributed and the microstructures suggest as follows: (1) the presence of elongated or rounded quartz in garnet suggest the involvement of melt in the growth of garnet during the M2 stage (Fig.4e, Fig.5; Groppo et al., 2012); (2) melt film and pocket in the boundary of quartz, feldspar (albite + K-feldspar) and peritectic garnet in the matrix during the M3 stage (Fig. 4d; 11

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Sawyer, 1999); (3) the textures of symplectite corona of sillimanite, plagioclase and biotite around garnet rim during the M4 stage (Fig.5a, b). Based on the textural observations and reaction relationships four metamorphic stages can de distinguished: the prograde metamorphic stage

metamorphic stage

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of the porphyroblastic garnet (M1); the peak

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represented by Grtc + Bt + Pl + Kfs + Qz + Rt, which were included in the core

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characterized by Grtm + Kfs + Qz + Rt + L (melt) as inclusions in the mantle of porphyroblastic garnet (M2); the third stage comprising garnet rim and its

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inclusions of Grtr + Sill + Kfs + Pl + Qz + Rt + L (M3); fourth metamorphic stage

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represented by recrystallized small grains, small symplectite assemblage and its melt corona in the porphyroblastic garnet rim as Grt + Sill + Kfs + Bt + Pl +

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Qz + Rt + Ilm + L (M4).

5. Mineral chemistry

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Samples HA5-1, HA5-2, HA5-3 and HA7-1 were selected for detailed

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mineral composition analysis, and representative mineral compositions are given in tables 1－3. Two porphyroblastic garnet profiles were analyzed to evaluate the compositional variation corresponding to textural zoning (Fig. 5a, b, Appendix Table 1). Garnet profile “a” contains Alm (XFe) ~57.6~63.0 mol%, Pyr (XMg) ~32.1–37.3 mol% , Grs (XCa) ~4.16–5.74 mol% and Sps (XMn) 0.3–0.6 mol%; and garnet profile “b” contains Alm ~57.8–62.9mol%, Pyr ~33.1–37.4mol%, 12

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Grs ~3.4–4.9mol% and Sps ~0.30–0.61mol%. The analyzed end member compositions of two garnet profiles are shown in table. 1 (Fig. 5a, b) which display distinct variations. The XMg, XFe and XMn contents at grain level are nearly same from core to rim without any obvious zoning which is consistent

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with the diffusional re-equilibrium during a HT metamorphic stage, followed by

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re-equilibration during retrograde metamorphism (Chakraborty and Ganguly,

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1991; Guilmette et al., 2011; Sorcar et al., 2014; Spear et al., 1990; Cooke et al., 2000) in both big garnet grains. On the contrary, the XCa contents display

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distinct zoning from garnet core, and mantle to rim in profiles “a” and “b”. For

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profile “a”, the XCa shows higher contents than in profile “b”. Profile “a” shows a slight increase from center outward in the garnet core, with the 5.21–5.74mol%

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representing the prograde metamorphism (shown as garnet core Grt1 for M1);

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decreasing outward to the garnet mantle to the range 5.52 – 4.63mol%, corresponding to the peak-stage (shown as garnet mantle Grt2 for M2). The

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values show further decrease at the rim to around 4.16–4.35mol%,

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corresponding to M3 stage (shown as garnet rim Grt3 for M3) (Appendix Table 1, Fig. 5a). The garnet core of profile “b” with a higher XCa content of 4.50–4.94mol% is considered to represent prograde metamorphism (Grt1 for M1), and the decrease XCa might have been affected by late high grade metamorphism; decrease of XCa in the mantle (Grt2 for M2) from ~3.91–4.43mol% at the peak-stage to 3.43–3.58mol% in the rim might correlate with decompression stage (Grt3 for M3) (Fig. 4c, e). The X(Ca) content 13

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in garnet is taken to represent growth zoning due to the slow diffusivity in garnet and have been used to constrain P-T conditions of migmatites from HP-HT terranes in phase equilibrium simulations (Wu et al 2016, 2017; Wang et al 2011a, b; Zhang et al., 2017).

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Biotite shows TiO2 contents of 5.31–5.66 wt% in retrograde M4 stage, and

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6.37 wt% in relict M1 stage (Table 1). The XMg content of biotite in contact with

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garnet rim or occurring as symplectitic corona ranges from 65.53 to 76.02mol%, and is regarded to be the product of retrograde metamorphism

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(shown as Bt4 for M4, Figs. 4e, 5a). Biotite in the matrix shows similar XMg of ca.

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63.96 mol%. The relict biotite inclusions in garnet core of profile “a” with an XMg content of 71.62mol% belongs to the pre-peak stage of metamorphism (Bt1 for

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M1 in table 1, Fig. 5a). Considering the relatively fast Mg-Fe diffusion in

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minerals like garnet and biotite in granulites (Spear et al., 1992, 1999), the higher XMg as well as TiO2 contents of biotite in stage M1 might have been

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modified during pre-peak and peak metamorphism.

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Plagioclase occurring as tiny inclusions in garnet rim is considered to have formed in the post-peak metamorphic stage (Pl3 for M3, Fig. 5b) with an XAn ~36.1–38.5 mol% which is distinct from the ca. 6.6mol% in the symplectite coronas around garnet rim, related to the M4 stage represented by Pl4. Plagioclase in perthite, as Pl3 is related to stage M3, and displays variable composition from XAn 4.8 mol% to 33.2mol% (Table 1; Fig. 4i and Fig. 8). K-feldspar is widely present in all metamorphic generations of the samples. 14

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The host K-feldspar composition of perthite at stage M3, XOr varies from 55.7 to 92.8 mol%, and in the matrix varies from 72.1 mol% to 93.8 mol%. Based on the mineral chemistry data and petrographic observations, four different stages can be distinguished: the pre-peak stage (M1) inferred by the

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garnet core of profile “a” with inclusions of relict biotite showing XCa increase

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from core to mantle. The XCa contents (5.21–5.74 mol%) are much higher than

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those from garnet profile b (Fig.5a, Grt1, Bt1). The peak stage (M2) indicated by the garnet mantle and rim of profile “a” shows the highest XCa content

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decrease from inner to outer domains, from 5.56 mol% to 4.21 mol%

the

garnet

mantle

of

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(Appendix Table 1, Fig.5a, Grt2). The post-peak stage (M3) is represented by profile

“b”

with

an

assemblage

of

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garnet-plagioclase-quartz-rutile-K-feldspar-sillimanite (Fig.5b, Grt3, Pl3, Sil3).

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The retrograde stage (M4) is characterized by garnet rim and the symplectite corona

surrounding

it,

consistent

with

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plagioclase-biotite-sillimanite-quartz-K-feldspar-rutile with or without ilmenite

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(Fig.4e, Sil4, Pl4, Bt4).

6. Phase equilibria modeling Phase equilibria is modeled with THERMOCALC v340 (Powell and Holland, 1988)

using

the

(Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3)

NCKFMASHTO system

which

provides realistic estimates for metapelites (White et al., 2007). The internally 15

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consistent thermodynamic data set used is ds62 with the re-parameterized a-x models (Powell et al., 2014; White et al., 2014). The sample HA5-1 was selected for further phase equilibria modeling due to following reasons: (1) it preserves well the microstructures and mineral assemblages from prograde to

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retrograde stages; (2) the garnet profiles preserve growth zoning that can be

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used to derive better constraints in the phase-equilibrium modeling; and (3) it

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is better to use bulk rock and mineral compositions from the same sample in phase equilibria modeling.

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The whole-rock chemical compositions were determined by X-ray

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fluorescence spectrometry (XRF, Axiosmax) analysis combined with standard wet chemical methods at the Hebei Institute of Regional Geological and

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Mineral Survey, China. The bulk-rock composition in wt% shows SiO2 = 61.95,

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Al2O3 = 19.49, TiO2 = 0.69, MgO = 2.52, CaO = 0.67, TFe2O3 =1.19, K2O = 4.49, Na2O = 1.22, MnO = 0.06, P2O5 = 0.05 and LOI = 1.12. The minor MnO

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and P2O5 contents were ignored in constructing the pseudosection, and other

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compositions are normalized to 100%, to construct the phase equilibria within the NCKFMASHTO system. The microstructural evidence indicates the involvement of melt. Therefore, so we defined the H2O contents with T-X(H2O) diagram to ensure that the final stage assemblage (stage M4) is stable above the solidus (Korhonen et al., 2013). In order to constrain an appropriate H2O content, a T-X(H2O) diagram was constructed at 8kbar, which is within the range of previous estimates of 16

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the final stage (an assemblage Grt - Sil - Pl - Bt - Kfs - Qz - Rt - Ilm) closest to the solidus pressure (Wu et al.,2016, 2017) (Fig.6). From texture and mineral assemblages, the pelitic granulite show the presence of rutile in all metamorphic stages (Fig. 4 and 5). The computed H2O content ranges from a H2O (X(H2O)=1). At 8 kbar,

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near-anhydrous composition (X(H2O)=0) to excess

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the subsolidus mineral assemblage is stable at X(H2O) > 0.01 (Fig. 6). At higher

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X(H2O) content (X(H2O) > 0.6), rutile is predicted to be absent from the investigated assemblages (Fig. 6). The mineral assemblages in the regions (3)

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and (4) of Fig. 6, and also the temperature of the solidus (< 800°C) between

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them are much lower than is appropriate for such a residual granulite facies assemblage. Based on the constraints among two dashed red lines, a X(H2O)

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value of 0.16 (~0.48 mol %) (as T = 820°C, taken from Wu et al., 2016)

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indicated by solid red line was selected for subsequent modeling of this bulk composition. Furthermore, the existence of pyrite and pyrrhotite indicates a

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low oxygen fugacity, representing a reducing environment; thus we choose the

AC

minimum X (Fe2O3) min =0.01 for calculation. Melt loss is considered to be an essential part for the preservation of granulite facies mineral assemblages (Powell, 1983; Waters, 1988; Powell and Downes, 1990; White and Powell, 2002; Wu et al., 2016; Zhang et al., 2017). However, melt loss was assumed to have occurred before granulite facies rocks reached their peak temperature metamorphic conditions (White and Powell, 2002; Kelsey et al., 2003). In addition, there is no obvious change of 17

ACCEPTED MANUSCRIPT 18

the mineral assemblages and compositions above solidus fields compared with the measured bulk composition of melt re-integrated bulk composition controlled P-T pseudosections (Indares et al., 2008; Groppo et al., 2012; Wu et al., 2016, 2017).

PT

The P-T pseudosection with measured bulk composition was constructed in

RI

the P-T range of 5-16 Kbar and 700 -1000 °C, a X(H2O) = 0.16, XFe2O3 = 0.01

SC

(Fig.7a, b). Quartz and K-feldspar are calculated in excess in the whole P-T region. Rutile is stable above 6 kbar with a decrease in consumption during

NU

replacement of ilmenite below 8 kbar. Biotite is modelled to occur below 840°C

MA

which is much lower than plagioclase stability (>1000°C) in the medium pressure region. Melt appears above 800°C in this study.

D

The isopleths of XCa [=Ca2+/(Ca2++Mg2++Fe2++Mn2+)] (Fig.7b) in garnet are

PT E

rather horizontal in most of the regions that can be used as an effective indicator of pressure. The isopleths of XAn in plagioclase show medium slope in

CE

the plagioclase-bearing assemblages above solidus that are useful to

AC

constrain temperature. The XMg isopleths of biotite show opposite changes below and above solidus. In the subsolidus area, the XMg content increases with the decrease of temperature but it rises with the cooling process above the solidus. The preserved mineral assemblage is not stable to constrain the precise prograde-metamorphic stage in the high-pressure region due to the absence of kyanite in our thin sections. However, mineral assemblages and 18

ACCEPTED MANUSCRIPT 19

compositions of HP-HT/UHT granulite facies are seldom completely preserved and undergo changes during later stages (Zhang et al., 2006; White and Powell, 2002; Brown, 2002; Wu et al., 2017). Wu et al. (2016) reported kyanite-garnet-K-feldspar from the khondalite series in this area that confirm

PT

the existence of HP stage.

RI

As for the pre-peak stage (M1), XCa of garnet profile “a” shows a slight

SC

increase from inner to outer domains of the core (from XCa = 0.052 to 0.057) which might represent the pre-peak prograde stage. The XCa = 0.0 45-0.057

NU

obtained from both garnet core of profile “a” and garnet profile “b” are plotted in

MA

the kyanite-bearing HP metamorphic regions as dashed arrow (1) below 790°C (Fig. 7b). Although Bt1 was found in the garnet core (Fig. 5b), due to the

D

lack of compositional data on biotite and plagioclase in the garnet core, the

PT E

exact PT conditions of stage M1 cannot be fixed. In the peak metamorphic assemblage (M2), we were not able to find

CE

preserved kyanite in the garnet core and mantle, and therefore the Pmax

AC

conditions, marked as yellow stars, were taken from previous studies that identified kyanite-bearing HP pelitic granulite from this study area (Wu et al., 2016) and define ~830 °C / 15 kbar. Nevertheless, garnet mantle compositions from garnet profile “a” and “b” record decrease (Grt2 from inner towards outside, XCa = 0.055 to XCa = 0.037) (Appendix Table 1, Fig. 5), based on which arrow (2) is marked in the figure 7b correlating to M2. With regard to the metamorphic stage M3 (table 1, Fig. 7b), the observed 19

ACCEPTED MANUSCRIPT 20

plagioclase-bearing

mineral

assemblage

is

stable

in

the

Grt-Sil

-Pl-Rt-Kfs-Qz-Liq domain. The XCa from garnet rim can be plotted in this area and show decompression from XCa = 0.0435 to XCa = 0.0343. The intersections of these XCa isopleths from the rim of Grt3 and the isopleths of

XAn=0.361 –

PT

0.385 from Pl3 inclusions in the garnet rim yield P-T conditions of ~860°C –

RI

930°C and 10.1 – 11.1 kbar.

SC

The retrograde stage M4 is characterized by the symplectite corona of Bt4 - Sil4 - Pl4 around garnet rim (Fig. 4c) together with Rt4 + Ilm4 and melt (Fig. 5b),

NU

which defines a P-T range of 820°C – 840°C and 6.5 – 8.5kbar in the mineral

MA

assemblage marked ⑧, passing through reaction Grt + Kfs + melt = Bt + Sil + Pl + Qz as temperature decreases (Fig. 7b). The measured XMg (bi) of 0.66

D

and 0.64 from garnet rim and matrix are modeled to yield temperature <840°C

PT E

/ 8.4 kbar. Moreover, the modes of biotite, plagioclase, sillimanite, quartz and ilmenite are predicted to increase at the expense of garnet, K-feldspar and

CE

rutile during the retrograde stage below the solidus. For example, as

AC

calculated P-T conditions decrease from 800°C / 7.2 kbar (point A) to 750°C / 6.6 kbar (point B) during retrogression, contents of garnet, K-feldspar and rutile decrease from 0.204 to 0.199, 0.326 to 0.317 and 0.002 to 0 respectively, whereas the modes of biotite, plagioclase, sillimanite, quartz and ilmenite increase from 0.0430 to 0.0434, 0.046 to 0.056, 0.114 to 0.115, 0.261 to 0.262 and 0.004 to 0.007 respectively (Fig. 7b, Table 4). These are consistent with the petrographic observations. 20

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Thus, four stages can be summarized: the pre-peak stage (M1), the peak stage (Pmax for M2), the post-peak stage (Tmax for M3) and the post-peak retrograde stage (M4). A corresponding clockwise P-T path is traced with HP-UHT metamorphic P-T history as shown by dashed lines with yellow stars

RI

PT

representing the Pmax stage (Fig. 7b).

SC

7. Two-feldspar geothermometer

Recent studies suggest that dissolution of apatite in the anatectic melt coupled

NU

with Ca garnet in apatite bearing aluminous granulites place restrictions on the

MA

accuracy of modeling metamorphic P-T conditions in NCKFMASHTO system using Xca in garnet (Indares and Kendrick, 2018). This would particularly affect

D

the P-T estimate of metamorphic stage M3 in our study, where melt

PT E

participates in the reaction and is also associated with apatite (Fig. 5b). In addition, it is difficult to constrain the peak temperature of metamorphism (M3)

CE

in the biotite-free P-T field at mid to high pressure stages. Two-feldspar

AC

geothermometer is widely used for P-T calculation in granulite facies rocks, and has been successfully applied in different cases (e.g., Hokada, 2001; Prakash et al., 2006; Jiao et al., 2011). Several groups of perthite/mesoperthite compositions (including Pl lamellae and Kfs host) were obtained from the four samples from Manjinggou pelitic granulite using electron microprobe analyses, and the data are listed in Table 2. Microscope and BSE images show that perthite grains mainly occur in the 21

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absence of coexisting plagioclase in these samples and the modal contents show ca. 15 vol%, 15 vol%, 40 vol% and 40 vol% in sample HA5-1, HA5-2, HA5-3 and HA7-1, respectively. The relative percentages of the calculated plagioclase lamellae and host

PT

K-feldspar were obtained by image analysis of the backscattered electron

RI

images. About two to four domains (204800-409600 pixels) were calculated in

SC

one feldspar profile and the average number of the pixels from the lamellae and host. The values thus measured are considered to represent the volume

NU

proportions for analysis on thin section surfaces. The weighted percentages of

MA

lamellae and host were converted by multiplying the volume proportions and densities of plagioclase lamellae and alkali feldspar, respectively (2.67g/cm 3

D

for plagioclase and 2.57 g/cm3 for alkali feldspar; Smith, 1974). The

PT E

re-integrated molar compositions of feldspar profiles were acquired by the calculation of the weight percentages and the chemical analysis from electron

CE

probe. The highest Ca and K contents of plagioclase lamellae and host

AC

K-feldspar were selected in order to reduce the error effect in the EPMA analysis between the host and lamellae domains (Hokada, 2001). The different calculation models of two-feldspar geothermometers yield slightly different results (Fuhrman and Lindsley, 1988; Lindsley and Nekvasil, 1989; Elkins and Grove, 1990; Benisek et al., 2004). In this study, we choose the model of Fuhrman and Lindsley (1988) for further analysis, which is one of the robust calibrations for temperature estimation of granulite facies rocks due 22

ACCEPTED MANUSCRIPT 23

to the following reasons.

(1) It is more accurate to take the Al-Si substitution

in to consideration by Al avoidance principle instead of counting the feldspar solution as an idealized single-site mixing owing to the existence of anorthite in feldspars. (2) The model of Fuhrman and Lindsley (1988) yielded lower

PT

temperature for antiperthite than perthite and mesoperthite, although the

RI

Lindsley and Nekvasi (1989) and Elkins and Grove (1990) methods yielded

SC

opposite results. Moreover, the lamella-free plagioclase exhibits temperatures lower than those from the coexisting perthite grains as confirmed by recent

NU

studies (Hokada, 2001; Jiao et al., 2011). Thus, the Al-avoidance models of

MA

Fuhrman and Lindsley (1988) and Benisek et al. (2004) are considered as the optimal choices.

D

As stated previously, the estimated P-T conditions of metamorphic M3 stage

PT E

range are ~860°C – 930°C / 10.1 – 11.1 kbar. Apatite occurs with garnet rim (Grt3) (Fig. 5b); however, the XCa of garnet controlled the pressure and not the

CE

temperature (Fig. 7b M3). If we remove the additional Ca due to the effect of

AC

apatite, both the XCa and pressure would decrease, and the corresponding temperature would show a slight increase. The pressure, therefore, is chosen as P = 9.5 kbar and P = 11.0 kbar, as estimated in our study for the M3 stage (Fig. 7b shown in coarse blue solid line with circled triangle). The summary of the analytical results is listed in Table 3 and shown in Fig 10. In the ternary feldspar diagram, the re-integrated compositions of feldspars in pelitic granulite from the Manjinggou area yield temperature of 826 – 946 °C 23

ACCEPTED MANUSCRIPT 24

/ 9.5 kbar and 836 – 950 °C / 11 kbar, marking the P-T conditions for M3 in Fig. 10 (yellow framed area). These temperatures mark the minimum temperature of peak metamorphic conditions owing to the exchange of CaAl-(Na, K) Si of intracrystalline closure instantly after the peak metamorphic conditions

PT

(Hokada, 2001; Jiao et al., 2011). Combining the above results, the P-T

RI

conditions of M3 stage can be revised up to 940 – 950 °C / 9.5 – 10.5 kbar, and

SC

the P-T path can be further improved as shown in Fig. 10.

The Ca2+ in apatite might thus influence the Xca content in garnet but not

NU

significantly to effect any changes in the P-T estimates of UHT metamorphism

MA

during Tmax stage except for a slightly high pressure and a lower temperature conditions as compared to results calculated by using two-feldspar

PT E

D

geothermometer.

8. Zircon U–Pb dating and REE

CE

The zircon LA-ICPMS U-Pb results are listed in Appendix Table 2. Zircon

AC

grains in the sample are stubby or round in shape, transparent or translucent in color, and generally about 100–200µm in size. The CL images show most grains are structureless or possess blurred zoning (Fig. 11), typical of zircon grains in high grade metamorphic rocks (Corfu et al., 2003; Li and Wei, 2016; Wu et al., 2017). The zircon grains have low Th contents (7–63 ppm) and moderate U contents (134–1067 ppm), yielding low Th/U ratios of 0.01–0.18 (Appendix Table 2), which are also consistent with their metamorphic origin 24

ACCEPTED MANUSCRIPT 25

(Rubatto, 2002; Hoskin and Schaltegger, 2003; Wu and Zheng, 2004, Chen et al., 2018). In chondrite-normalized REE diagrams the data show nearly flat HREE patterns (~YbN / GdN = 0.11–1.38 for group 1 zircons, 0.11–2.54 for group 2

PT

zircons), weakly positive Ce anomalies and mostly moderate Eu negative

RI

anomalies for group I zircons (~0.05–0.) and group 2 zircons (~0.05–1.85) (Fig.

SC

12a, b, Appendix Table 3). These features suggest that the zircons formed together with garnet and feldspar (Rubatto, 2002; Hoskin and Schaltegger,

NU

2003; Wu et al., 2008). Several plots of group 1 zircons, however, lack any

MA

obvious negative Eu anomalies (~0.45–0.94) and two spots from group 2 even show positive Eu anomalies (~1.25–1.85), which might have resulted from the

D

breakdown and consumption of feldspars or melting process (e.g. Rubatto,

PT E

2002).

A total of fifty-four analyses on sample HA5-1 show two groups of 207

Pb/206Pb weighted mean ages of 1972.9 ±

CE

metamorphic ages, and yielding

AC

8.1 Ma and 1873.3±9.9 Ma (Fig.13).

9. Discussion

9.1 Metamorphic P-T evolution of the pelitic granulites Previous studies on the pelitic granulites in Manjinggou area reported medium pressure and high pressure granulites, although the temperature estimates were all below 850°C (Guo et al., 2002; Wu et al., 2016 and 25

ACCEPTED MANUSCRIPT 26

reference therein). The distinction between the MP and HP metamorphism in this case was the absence or presence of kyanite. The MP granulites facies conditions were reported as ca. 800 ± 50°C / 8–10 kbar based on conventional geothermometry (Lu and Jin, 1993; Wu et al., 2016, and reference therein).

PT

The recent HP granulite model is based on the finding of kyanite in the

RI

Manjinggou pelitic granulite, revising the peak metamorphic conditions to

SC

810–860°C / 11.5–15 kbar on the basis of pseudosection approaches (Wu et al., 2016).

NU

In previous P-T pseudosection modeling of the pelitic granulite from

MA

Manjinggou area, the peak T conditions are too close to the solidus (e.g. Wu et al., 2016) that is incompatible with the observation under microscope that the

D

majority of the garnet grains grew in presence of melt as observed in this study

PT E

and previous studies (e.g. Wu et al., 2016; Zhang et al., 2017). Thus, the peak T condition should be higher than that inferred before, owing to diffusional

CE

homogenization of garnet or overestimate of the stability of biotite. Considering

AC

that there is no clear evidence that biotite participated in the peak stage, and in combination with the petrographic observations from this study, the peak stage is redefined as Grt – Pl – Sil – Kfs – Qz – Rt - liq (Fig. 7a and b). In accordance with the petrographic characteristics and phase equilibria modelling, the metamorphic evolution of the pelitic granulites in Manjinggou area can be subdivided into three different stages during the exhumation process: the Pmax, the Tmax and subsequent retrogressive cooling stages. 26

ACCEPTED MANUSCRIPT 27

Garnet is considered to be a robust mineral to diffusion equilibrium during most metamorphic stages (e.g. Spear et al., 1990; Florence and Spear, 1991). However,

it

may

undergo

homogenization,

especially

under

UHT

metamorphism and may not preserve the peak metamorphic stage (e.g. Wang

PT

et al., 2011b; Wu et al., 2016, 2017; Li & Wei., 2016; Zhang et al., 2017). The

RI

Fe and Mg in garnet may not therefore be suitable to constrain the peak

SC

temperature conditions of granulite facies rocks. There are examples of Ca2+ growth zoning in garnet, which are supposed to have preserved the early

NU

phases through slow diffusivity and have been used to constrain P-T

MA

conditions of migmatites in HP-HT terranes (e.g. Wang et al 2011a, b; Hollis et al., 2006; Wu et al 2016, 2017; Zhang et al,. 2017). As shown in Table. 1 and

D

Fig.5, the Xca of garnet displays regular changes as compared to other

PT E

compositions of the garnet profile from core, mantle to rim. Therefore, in here we used the different Xca in garnet core, mantle and rim and mineral

CE

assemblages to demarcate the four different stages.

AC

The pre-peak stage (M1) is defined by the compositional zoning of XCa in garnet from garnet profile “a” and “b”, which shows increasing pressure and temperature in the P-T pseudosection. It represents a prograde stage trajectory as expressed by dashed red line and define ca.< 790 °C / 13 kbar (Fig. 7b, Fig. 10). The P-T conditions of Pmax stage (M2) is not well constrained in this study due to the lack of petrographic evidence for kyanite. It is common in many 27

ACCEPTED MANUSCRIPT 28

HP-HT granulites that kyanite is not preserved during the MP-HT overprint (Zhang et al., 2006). We therefore used inferred trajectories from kyanite bearing assemblages reported in previous studies from this area (Wu et al., 2016, 2017; Yin et al., 2014; Zhao et al., 2010). Moreover, the

PT

Xca=0.043–0.049 mol/% obtained from the garnet mantle is also plotted in the

RI

kyanite-bearing assemblages of the HP metamorphic region, reaching a

SC

pressure similar to that reported as Wu et al. (2016) at ca. 830 °C / 15 kbar. (Fig. 7a, b).

NU

The PT metamorphic conditions of Tmax stage (M3) were obtained from Grt

MA

– Sil – Pl – Kfs – Qz – Rt – liq assemblage by using isopleths of Xca in garnet rim and XAn in associated Pl3 inclusions in the garnet rim, a P-T range of 900°C

D

– 940°C, and the estimate of 9.8-10.6 kbar was obtained in the thermodynamic

PT E

modelling diagram (Table 1, Fig. 7b). In order to remove the compositional effect of coexisting apatite on the associated garnet rim XCa value, Tmax was

CE

recalculated by using two-feldspar geothermometer, and the revised estimate

AC

shows 940 – 950 °C / 9.5 – 10.5 kbar, reaching UHT metamorphic conditions. The retrograde stage M4 stage is characterized by the symplectite corona of Bt4 - Sil4 - Pl4 - Rt4 - Ilm4 - melt around garnet rim (Fig. 4c, Fig. 5b), which defines a P-T range of 820°C – 840°C and 6.5 – 8.5kbar. The measured XMg(bi) of 0.66 and 0.64 from garnet rim and matrix are modeled to yield temperature <840°C / 8.4 kbar. The four metamorphic stages in the pelitic granulite from Manjinggou, define 28

ACCEPTED MANUSCRIPT 29

a clockwise P-T path, exhibiting an HP to UHT evolutionary characteristics (Fig. 10). Ultrahigh-temperature metamorphism is the most thermally extreme type of deep crustal metamorphism, with temperatures exceeding 900–1000 °C

PT

(Kelsey, 2008; Brown 2007, 2009; Harley, 2008; Santosh et al., 2012; Kelsey

RI

and Hand, 2015). In the recent years, several UHT granulite outcrops were

SC

discovered in the khondalite belt of the Western Block of the NNC, such as the sapphirine granulite in the Dongpo, Heling’er and Shaerqin of the Daqingshan

NU

terrae (e.g., Guo et al., 2006; Tsunogae et al., 2011; Guo et al., 2012; Liu et al.,

MA

2012; Jiao et al., 2015); sapphirine-bearing Mg–Al granulites in the Tuguiwula of Jining complex (Santosh et al., 2007a,b, 2009, 2012; Jiao and Guo, 2011);

D

and Spl - Qz bearing UHT granulite in the Xumayao, Xinghe area (Zhang et al.,

PT E

2012, Luo et al., 2012). There are several reports of UHT pelitic granulites without diagnostic UHT metamorphic minerals such as Spr + Qz, Spl + Qz,

CE

Osm, and Hyp – Sil + Qz (e.g. Kelsey et al., 2003; Jiao and Guo, 2011; Li and

AC

Wei, 2016). Thus, we infer that the metapelites in Manjinggou area of this study underwent an UHT metamorphism although they lack diagnostic UHT mineral assemblages due to the bulk composition constrains.

9.2. P-T path for the mafic and pelitic granulites in the Huai’an complex The early studies on metamorphic processes of HP mafic granulite by Zhai et al. (1992) and Guo et al. (1993) in the area reported peak HP 29

ACCEPTED MANUSCRIPT 30

metamorphism at ~800°C / >1.4 GPa (Fig. 14). The Sm-Nd mineral isochron and zircon U-Pb concordia ages suggested peak metamorphic ages of ca. 1.82–1.83 Ga from high pressure mafic granulites and was correlated to large scale overthrust of the crust in a collisional background, followed by

PT

retrogression (Guo et al., 1993).

RI

The P–T estimates of the pelitic granulites from the khondalite series were

SC

initially assumed as MP granulite facies at ~ 750–850 °C/ 8–10 kbar (Lu and Jin, 1993; Liu, 1997). The pelitic granulites from Manjinggou area were

NU

previously considered to be metamorphosed at the sillimanite stability field

MA

(Guo et al., 2002; Zhao et al., 2005, 2010). Recent finding of kyanit in the metepelites from Manjigngou with the mineral assemblage of Grt – Ky – Kfs –

D

Bt – Rt – Qz – Liq ± Ms (Wu et al., 2016) yielded prograde and retrograde PT

PT E

conditions of ~810–860°C / 11.5–15 kbar and ~850 °C / 9.5 kbar (Fig. 14). Their P-T path, represents exhumation as shown in Fig. 14, with a slight

CE

increase in temperature at the high presure region.

AC

Our study is the first record of UHT metamorphic overprint in Manjinggou terrane at

ca. 940 – 950 °C / 9.5 – 10.5 kbar following a HP pelitic granulite of

Pmax stage at ~830 °C / 15 kbar. In addition, a prograde compositional variation of XCa in the porphyroblastic garnet core was identified (Fig. 5a) based on which a clockwise P-T path is traced. The mafic HP granulites from Huangtuyao in the Huai’an complex also show a similar clockwise P-T path (Fig. 14) with Pmax at ~ 880–890 °C/11–13 kbar and subsequent 30

ACCEPTED MANUSCRIPT 31

near-isothermal decompression (Wang et al., 2016).

9.3 P-T path, geochronology and tectonic implications Previous studies indicate that the HP and MP granulites in the Khondalite

PT

Belt are characterized by clockwise P-T paths involving near-isothermal

RI

decompression (i.e. Lu and Jin, 1993; Liu et al., 1993; Zhao et al., 1999, 2002,

SC

2005). Examples include the pelitic granulite of khondalite series rock in the Jining complex, Khondalite Belt (Wang et al., 2011a,b), mafic HP granulite

NU

from Huangtuyao, Huai.an complex (Wang et al., 2016), and mafic HP

MA

granulite from the Huai’an Complex (Wu et al., 2018). As discussed above, the peak temperature conditions of metapelites in the Huai’an complex may have

D

been underestimated in the early studies (e.g. Lu and Jin, 1993; Wu et al.,

PT E

2016; Wang et al., 2016). Subsequently, the pelitic granulites in Manjinggou area were considered as HP-HT granulites based on the finding of kyanite (Wu

CE

et al., 2016). In this paper, the decompression from the Pmax mineral

AC

assemblages of UHT pelitic granulite stable fields at a thermal gradient ~15 °C /km (Fig.15) suggests an orogenic process involving crustal thickening followed by tectonic extension during exhumation to crustal level of ~30 km with temperature rising to UHT conditions. This scenario is comparable with that in the adjacent regions from Huai’an complex and those from the Khondalite Belt in the Western Block of the NCC. Similar P-T path with temperature increased during decompression were also reported from other 31

ACCEPTED MANUSCRIPT 32

localities of the Khondalite Belt and the TNCO (Qian et al., 2013; Yang et al., 2014; Wei et al., 2014; Wu et al., 2017; Li and Wei, 2016, 2018, Fig. 15). The two types of clockwise P-T paths with or without temperature increase during decompression might share the same geodynamic process (Fig. 14, 15).

PT

The published metamorphic ages from pelitic granulites terranes of both the

RI

Huai’an complex and Khondalite Belt can be divided into three groups: (1) > 2.

SC

0Ga (Wang et al., 2016; Zhao et al., 2010) (2)1.98 to 1.90 Ga (Santosh et al., 2017b; Zhao et al., 2010; Wu JL et al., 2016, 2017) (3) 1.88 to 1.82 Ga (Guo et

NU

al., 2005). The 2.3－2.0 Ga age was suggested to be related to A-type granites,

MA

which were generated by the partial melting of Archean tonalitic protoliths through heat input from mantle upwelling in a rift or intraplate geodynamic

D

setting (Zhai and Santosh, 2011, Zhang et al., 2014), which are considered to

PT E

be close to primary ages of detrital zircon (i.e. Santosh et al. 2007b; Li et al., 2011). The ages of 1.98－1.90 Ga and 1.88－1.82 Ga were controversially

CE

interpreted to represent two separate orogenic events (i. e. Zhao et al., 2005,

AC

2010; Guo et al., 2005, 2012; Li et al., 2011) or just one protracted orogenic process with slow cooling and uplift associated with the assembly of the Columbia supercontinent (Rogers and Santosh 2002; Santosh, 2010; Tang et al., 2016, 2017; Zhai and Santosh, 2011). The two separate orogenic events correspond to the timing of metamorphic events that affected the Khondalite Belt, the age ~1.95 Ga collisional event occurred between the Yinshan and Ordos blocks and the age ~ 1.85 Ga collisional event was considered to 32

ACCEPTED MANUSCRIPT 33

represent the timing of formation of the Trans-North China Orogen (Zhao et al., 2005, 2008, 2010; Tang and Santosh, 2018). On the basis of metamorphic evolution of the rocks, the metamorphic stages of M2 Pmax and M3 Tmax occurred above the solidus (Fig. 7 and 10), which

PT

indicate a melt-involved process. The textural features from petrographic

RI

studies also attest to these processed during M2 and M3 stages (Fig. 4c–e, Fig.

SC

5). The CL images of zircons from both group 1 and group 2 show dark luminescence and rounded embayed texture, indicating the effect of

NU

high-grade metamorphism and melting (Fig. 11, Santosh et al., 2007b; Jiao et

MA

al., 2013, 2017; Yang et al., 2014; Li and Wei, 2016; Wu et al., 2017). On the other hand, both populations of the zircon grains reveal steep positive slopes

D

of LREE patterns from La to Sm, and slight negative slopes HREE patterns

PT E

from Gd to Lu (~YbN / GdN = 0.11–1.38 for group 1 zircons, 0.11–2.54 for group 2 zircons; Fig. 12). These progressive changes in HREE fractionation are

CE

consistent with increased uptake of HREE by garnet between ca. 1.97 and

AC

1.87 Ga, and are typical of HT-UHT granulite facies metamorphic zircons (Jiao et al., 2013, 2017; Li and Wei, 2016, 2018; Wu et al., 2017). The pelitic granulites from this study, therefore, suggest that the collision event between the Yinshan and Ordos blocks occurred at ca. 1973

Ma as presented by the

post Pmax decompression P–T conditions, and the younger age of ca. 1873 Ma is interpreted to represent a slow cooling and exhumation process corresponding to the post-Tmax cooling P–T paths recorded by the pelitic 33

ACCEPTED MANUSCRIPT 34

granulites. In addition, the P-T paths from Khondalite Belt show both clockwise and counterclockwise P-T paths (Fig. 15). Counterclockwise P–T paths were reported mostly from the Tuguiwula and also from Xumayao UHT granulites

PT

(e.g. Santosh et al., 2009, 2012; Zhang et al. 2012; Fig. 15). Yang et al. (2014)

RI

also report a Spl-Qz bearing pelitic UHT granulite from Hongsigou of ~20km

SC

south of Tuguiwula, which present a clockwise P–T path. Similar clockwise P–T paths of UHT granulites were also demonstrated from Zhaijiayao, Gushan

NU

(Wang et al., 2011b; Li and Wei, 2016; Wu et al., 2017). These two type P-T

MA

paths of UHT metamorphic evolution, however, seemly show different timings, at ~1.92 Ga (Santosh et al., 2009, 2012) and younger than ~1.88 Ga (Zhang et

D

al. 2012; Li and Wei 2016; Wu et al., 2017), respectively. Our study from

PT E

Manjinggou in the Huan’an terrane also show UHT metamorphic evolution along clockwise P–T path, with the UHT event dated as 1.87 Ga. Similar UHT

CE

age of 1.87 Ga was also reported from the Khondalite Belt of North China

AC

Craton, for example, from the Hongsigou UHT granulite (Yang et al., 2014), the Daqingshan UHT metapelites (Jiao et al., 2015, 2017) and the Zhaojiayao and Tuguiwula UHT granulites from the Jining complex (Li and Wei, 2016). Jiao et al. (2018) suggested that there were two episodes of Paleoproterozoic UHT metamorphic events in the Khondalite Belt, separated at least by 50 million years at ca. 1.92 Ga and 1.87 Ga, both likely related to syn-extensional mafic magmatism during amalgamation of the North China Craton. Since it is also 34

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possible that a single and protracted UHT event can result in the above features, further studies are needed to address this debate. Both metamorphic evolution and geochronological characteristics of mafic and pelitic granulites are comparable between the western Khondalite Belt and

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the Huan’an complex as discussed above. Conventional tectonic models in

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previous investigations show there is an obvious cover and basement

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relationship between the metapelites and mafic HP granulites-bearing gray gneisses with a decollement between them (Zhang 1994, Fig. 16). Westward

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subduction of the Eastern Block underneath West Block of the NCC along the

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TNCO followed by exhumation / denudation bringing the relicts of the pelitic gneisses of Manjinggou, Huangtuyao and Sifangdun in direct contact with the

10. Conclusions

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Huai’an gneissic basement (Zhang et al., 1994).

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1. The relict mineral assemblages and compositions of the pelitic granulites

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in Manjinggou area, Huai’an complex, record a Tmax metamorphic stage at 940 – 950 °C / 9.5 – 10.5 kbar. The P-T data are thus revised to as UHT metamorphic conditions, which overprinted the HP metamorphism. 2. Zircon U-Pb studies analyses record two groups of metamorphic ages at ~1972.9 ± 8.1 Ma and ~1873.3 ± 9.9 Ma, which possibly correspond to the collision between the Ordos block and the Yinshan block, and amalgamation of the Western and Eastern Blocks of NCC along the 35

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TNCO.

Acknowledgements We thank Dr. Lingmin Zhang for her help during operation of the electron

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microprobe and processing the analytical results. We thank Dr. Li Tang and an

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anonymous reviewer for their comments and suggestions that greatly

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improved the manuscript. This research was supported by funds from the

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Research Fund (2015TDJH101).

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NSFC/NRF Research Cooperation Programme (41761144061) and SDUST

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Zhai, M.G., Liu, W.J., 2003. Palaeoproterozoic tectonic history of the North

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China Craton: a review. Precambrian Research 122, 183–199.

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Zhai, M.G., Guo, J.H., Liu, W.J., 2005. Neoarchean to Paleoproterozoic continental evolution and tectonic history of the North China Craton.

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Journal of Asian Earth Science, 24 (5) 547–561.

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Zhai, M.G., 2009. Two kinds of granulites (HT–HP and HT–UHT) in North China Craton:their genetic relation and geotectonic implications. Acta Petrologica Sinica 25, 1753–1771 (in Chinese with English abstract). Zhai, M.G., Santosh, M., 2011. Early Precambrian odyssey of the North China Craton: a synoptic overview. Gondwana Research 20, 6–25 Zhang, J.S., Dirks, P.H.G.M., Passchier, C.W., 1994. Extensional collapse and uplift of a poly-metamorphic granulite terrain in the Archean of north China. 52

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Precambrian Research, 67, 37–57. Zhang, H.F., Zhai, M.G., Peng, P., 2006. Zircon SHRIMP U-Pb age of the Paleoproterozoic high-pressure granulites from the Sanggan area, North China Craton and its geologic implications. Earth Science Frontiers 13,

PT

190–199 (in Chinese with English abstract).

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Zhang, H.T., Li, J.H., Liu, S.J., Li, W.S., Santosh, M. & Wang, H.H., 2012.

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Spinel + quartz-bearing ultrahigh-temperature granulites from Xumayao,

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Inner Mongolia Suture Zone, North China Craton: petrology, phase equilibria and counterclockwise p–T path. Geoscience Frontiers 3,

MA

603–611.

Zhang, Z.M., 2017. Oligocene HP metamorphism and anatexis of the Higher

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Himalayan Crystalline Sequence in Yadong region, east-central Himalaya. Gondwana Research 41, 173–187.

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Zhao, G.C., Wilde, S.A., Cawood, P.A., Lu, L.Z., 1999. Tectonothermal history of the basement rocks in the western zone of the North China Craton and

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its tectonic implications. Tectonophysics 310 37–53. Zhao, G.C., Sun, M., Wilde, S.A., 2002. Major tectonic units of the North China Craton and their Paleoproterozoic assembly. Science in China Series D: Earth Sciences 32, 538–549 (in Chinese). Zhao, G.C., Sun, M., Wilde, S.A., Li, S.Z., 2005. Late Archean to Paleoproterozoic evolution of the North China Craton: key issues revisited. 53

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Precambrian Research 136, 177–202. Zhao, G.C., Wilde, S.A., Sun, M., Guo, J.H., Kröner, A., Li, S.Z., Li, X.P., Wu, C.M., 2008. SHRIMP U-Pb zircon geochronology of the Huai’an Complex: constraints on Late Archean to Paleoproterozoic crustal accretion and

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collision of the Trans-North China Orogen. Amrican Journal of Science 308,

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270–303.

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Zhao GC, 2009. Metamorphic evolution of major tectonic units in basement of

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the North China Craton: Key issues and discussion. Acta Petrology Sinica 25(8), 1772–1792.

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Zhao, G.C., Wilde, S.A., Guo, J.H., Cawood, P.A., Sun, M., Li, X.P., 2010. Single zircon grains record two continental collisional events in the North

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China craton. Precambrian Research 177, 266–276.

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Figure and table captions

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Table 1 Representative electron microprobe analyses of other minerals for sample HA5-1 from the pelitic granulite in Manjinggou, Huai’an complex

Table 2 Representative electron microprobe analyses of perthite from pelitic granulite in the Manjinggou complex. (data select from the mineral pair of the maximum XAn in plagioclase lamellae and XOr in host K-feldspar) 54

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Table 3 Re-integrated compositions of feldspar with areal proportion and chemical compositions of lamellae and host domains Table 4 The calculated variations of major mineral contents at the chosen

RI

PT

P-T conditions during retrograde stage as shown in Fig. 7b

SC

Figure captions

Fig.1 Tectonic subdivision of the North China Craton. Lower yellow layer

NU

based on Zhai et al., (2005) and the overlay blue layer based on Zhao

MA

et al., (2005).

Fig.2 Regional geological sketch map of the Huai’an Complex in the northern

PT E

D

segment of the TNCO of the NCC (modified after Zhai et al., 2003). Fig.3 Geological lithologic units of Manjinggou area, Huai’an terrane (after

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Guo et al., 1993; Wu et al., 2016) Fig.4 Photomicrographs of sillimanite–garnet gneisses from Manjinggou of

AC

the Huai’an Complex. (a) field photograph of outcropped pelitic granulite in Manjinggou; (b) hand specimen of pelitic granulite; (c) garnet porphyroblast showing zoned texture; (d) peritectic garnet encircled by melt film; (e) porphyroblastic garnet with symplectite corona of sillimatie, plagioclase and biotite; (f) sillimanite with corroded edges intergrowth with K-feldspar and quartz matrix; (g)

55

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needle-like sillimanite inclusions in garnet rim; (h) late biotite in the matrix; (i) plagioclase as exsolution lamellate in perthite; (j) pyrite in fracture; (k) rutile equilibria with ilmenite in the late stage M4; (l) melt film in the boundary of quartz, microcline and K-feldspar. Mineral

PT

abbreviations after Whitney & Evans (2010), Per for perthite, Hol for

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hollow. Photos d, e, f and h were taken under plane-polarized light; l

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under cross-polarized light; j and k were taken under reflected light; c, g and i are BSE images. Photo e was taken from sample HA5-2; I

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from HA7-1 and all other photos from HA5-1.

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Fig.5 Representative profiles and chemical analyses of garnets from sample HA5-1, “a” and “b”, recording progressive and retrogressive

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Grtr for garnet rim.

D

processes respectively. Grtc for garnet core, Grtm for garnet mantle,

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Fig.6 T–X(H2O) diagram at 8 kbar for sample HA5-1. X(H2O) = 0 represents a near-anhydrous composition and X

(H2O)

=1 represents a composition

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with a solidus temperature below 800°C, yellow coarse dash line is solidus.

Fig.7 (a) P–T pseudosections calculated from measured bulk composition of sample HA5-1. (b) The pseudosections are presented with isopleths of XCa in garnet, XAn in plagioclase and XMg in biotite for relevant assemblages. Compositions used for determining P-T path are listed in

56

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Appendix Table 1 and Table 1. Fig.8 Photomicrographs of perthite texture in the pelitic granulite from Manjinggou of the Huai’an Complex. Photos a, c and e are BSE

PT

images; b, d, and f were taken under cross-polarized light. Fig. 9 Ternary plots of re-integrated feldspar compositions for all analyzed

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samples with the solvus calculated at 0.8 GPa using the model of

SC

Fuhrman and Lindsey (1988) as solid line and Benisek et al (2004) as

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dash line.

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Fig. 10 Integrated P-T path of pseudosections and are presented with isopleths of XCa in garnet, XAn in plagioclase and XMg in biotite for

D

relevant assemblages. Two-Fsp GT for two-feldspar geothermometer.

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Compositions used for determining P-T path are listed in Appendix Table 1, Table 1 and Table 3.

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Fig.11 Cathodoluminescence images of representative zircon grains from

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the Manjinggou meta-pelite (sample HA5-1), North China Craton. The circles mark the areas of the LA-ICP-MS analytical sites for

207

U -

206

Pb age; numbers refer to analysis in Appendix Table 2.

Fig.12 Zircon REE patterns of pelitic granulite (sample HA5-1) from Manjinggou. Chondrite values are from Sun and McDonough (1989). Fig.13 Concordia diagrams together with weighted means diagrams showing LA-ICP-MS U–Pb data of pelitic granulite (sample HA5-1) from 57

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Manjinggou. Fig.14 The summary P-T paths of mafic HP granulite and pelitic granulite from the Huai’an Complex. Blue solid-thin lines for HP mafic granulites, blue solid-coarse line for pelitic granulite from the

PT

Manjinggou terrane; dark grey solid-thin lines for mafic granulites from

SC

line shows speculative P-T segment.

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the Huangtuyao terrane; red coarse line represents this study, dashed

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Fig.15 The summary P-T path of granulites from the Western khondalite belt of Western Block, NCC. (1) Li & Wei, 2016 for pelitic HP granulite of

MA

Zhaojiayao, Jining complex; (2) Wang et al 2011 for the Jining pelitic granulite of khondalite series rock; (3) Li & Wei, 2018 for the pelitic

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UHT granulites in the Tuguiwula area, Khondalite Belt; (4) Santosh et al., 2012a for the UHT granulites in the Tuguiwula area; (5) Zhang et

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al. 2012 for the pelitic UHT granulite of the Xumayao, east most segment of the Khondalite Belt in the NCC; (6) Yang et al., 2014 for

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pelitic UHT granulite from Hongsigou of south of Tuguiwula; (7) Santosh et al., 2007a for UHT granulites from the Tuguiwula area; (8) Wu et al., 2017 for the pelitic HP-(U)HT granulite from Gushan, the eastern end of the Khondalite Belt; (9) the P-T path in the Manjinggou, Huai’an complex, this study. Abbreviations: NG – ‘normal’ granulite; UHTG

–

ultrahigh-temperature

granulite;

E-HPG

–

eclogite-high-pressure granulite. The effective sub-aluminous pelite 58

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solidus and areas of NG, UHTG and E-HPG are cited from Brown (2007).

The

transition

lines

of

Al2SiO5

are

calculated

by

THERMOCALC. Fig.16 (a) Sketch lithological map of Tuguiwula to Huai’An gniess terrane；(b)

PT

E-W directed cross-section between Tuguiwula and Waykou,

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illustrating the decollement structure between khondalite series and

AC

CE

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D

MA

NU

SC

TTG gneisses in Huai'an Complex (from Zhang, 1994).

59

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Table 1 Representative EMP analyses of other minerals for sample HA5-1 from the pelitic granulite in Manjinggou, Huai’an complex mineral

garnet profile Fig. 5a + 5b (corresponding to Appendix Table 1)

position

Grt1 in core

Grt2 in mantle

Grt3 in rim

stage

M1

M2

M3

biotite Bt1 in Bt4 in embayed in matrix position Grt rim Grt core M4

M1

(wt%)

Min

Max

Min

Max

Min

Max

plagioclase

Min

M4

T P

Pl3 inclusions in Grt rim of profile (table 1, Fig. 5)

Pl3 in Perthite

M3

M3

stage

Max

K-feldspar

(wt%)

1

2

3

I R

SC

Min

Max

Pl4 in the Pl4 around matrix Grt rim M4

Kfs3 in Perthite

in the matrix

M3

M3–M4

M4 Min

Max

Min

Max

65.26 64.77 0.00 0.04

65.82

63.69

0.04

0.05

18.83

19.42

0.00 0.56 0.00 0.01 0.19 3.04

0.00 0.49 0.00 0.00 0.00 0.65

12.39 14.94 100.87 99.24

SiO2

39.02

39.65

39.09

39.43

39.71

39.26

36.74

38.12

37.44

36.89

SiO2

58.77

58.25

56.98

69.51

64.78

66.66

66.61

TiO2

0.01

0.02

0.00

0.00

0.00

0.04

6.37

5.50

5.31

5.66

TiO2

0.00

0.07

0.00

0.05

0.07

0.02

0.00

Al2O3

21.50 0.00 27.68 0.27 9.62 1.64

22.07 0.02 27.13 0.20 9.38 2.09

21.37

22.05

21.80

21.50

14.03

15.04

16.32

15.28

Al2O3

25.79

26.74

26.36

19.06

18.39

20.77

20.72

0.00

0.04

0.02 27.83 0.20 9.53 1.34 0.01

0.05 27.38 0.21 9.11 2.00 0.08

0.03 27.91 0.17 9.78 1.26 0.02

0.00 28.86 0.24 8.24 1.55 0.03

0.05 10.44 0.00 14.79 0.03 0.21

0.10 12.76 0.02 13.62 0.00 0.19

0.07 9.14 0.00 16.28 0.01 0.15

0.12 13.29 0.01 13.24 0.03 0.16

Cr2O3 FeO MnO MgO CaO Na2O

0.00 0.11 0.00 0.00 7.32 7.08

0.03 0.50 0.03 0.01 7.78 7.04

0.04 0.47 0.00 0.01 8.00 7.02

0.00 0.26 0.04 0.07 0.87 9.11

0.00 0.01 0.01 0.01 0.11 0.77

0.01 0.04 0.02 0.01 1.88 9.80

0.00 0.21 0.04 0.00 1.39 10.83

18.48 18.75 0.00 0.00 0.00 0.05 0.01 0.02 0.00 0.00 0.22 0.10 5.29 0.74

0.03 99.76

0.00 99.38

0.00

0.00

100.60

100.31

100.69

0.01 99.73

9.71 92.35

9.55 94.91

9.70 94.41

9.69 94.38

0.11 0.04 0.06 99.17 100.47 98.92

0.62 99.59

15.12 99.26

10.20 15.51 99.46 99.98

3.02 0.00 1.98 0.00 0.00

3.02 0.00 1.95 0.00 0.00

3.02 0.00 1.99 0.00 0.00

3.03 0.00 1.96 0.00 0.00

3.04 0.00 1.96 0.00 0.00

2.79 0.36 1.25 0.00 0.00

2.82 0.31 1.31 0.01 0.00

2.64 0.00 1.37 0.00 0.00

2.59 0.00 1.40 0.00 0.02

2.58 0.00 1.41 0.00 0.02

3.03 0.00 0.98 0.00 0.01

2.67 0.00 1.33 0.00 0.00

0.06 99.27 8 2.92 0.00 1.07 0.00

0.08 99.88

3.01 0.00 1.95 0.00 0.04

K2O Total O Si Ti Al Cr

2.98 0.00 1.00 0.00

0.00

2.93 0.00 1.07 0.00 0.01

Fe Mn Mg Ca Na K XM g

1.74 0.02 1.10 0.14 0.00 0.00

1.73 0.01 1.07 0.17 0.01 0.00

1.80 0.01 1.10 0.11 0.00 0.00

1.75 0.01 1.04 0.16 0.01 0.00

1.78 0.01 1.11 0.10 0.00 0.00

1.87 0.02 0.95 0.13 0.01 0.00

0.66 0.00 1.67 0.00 0.03 0.94 0.72

0.79 0.00 1.50 0.00 0.03 0.90 0.66

Fe2+ Mn Mg Ca Na K X(An)

0.00 0.00 0.00 0.35 0.62 0.01 0.36

0.00 0.00 0.00 0.37 0.61 0.00 0.38

0.00 0.00 0.00 0.39 0.62 0.00 0.39

0.00 0.00 0.00 0.04 0.77 0.03 0.048

0.00 0.00 0.00 0.33 0.65 0.01 0.33

0.00 0.00 0.00 0.10 0.87 0.00 0.11

0.00 0.00 0.00 0.07 0.92 0.00 0.07

Xca

0.045

0.057

0.037

0.055

0.034

0.043

X(Or)

0.01 0.63

0.00 0.62

0.00 0.61

0.04 0.918

0.01 0.65

0.00 0.89

0.00 0.91

Cr2O3 FeO MnO MgO CaO Na2O K2 O Total O Si Ti Al Cr Fe

3+ 2+

0.00

PT

D E

12

11

E C

C A

2.75 0.29 1.41 0.00 0.00

2.77 0.32 1.35 0.01 0.00

0.56 0.00 1.78 0.00 0.02 0.91 0.76

0.83 0.00 1.48 0.00 0.02 0.93 0.64

A M

U N

Fe

3+

X(Ab)

0.00

2.986 0.001 1.019 0.000 0.002

2.98 0.00 1.01 0.00 0.02

2.95 0.00 1.06 0.00 0.02

0.00 0.00 0.00 0.01 0.47 0.59 0.010

0.000 0.001 0.000 0.005 0.066 0.912 0.005

0.00 0.00 0.00 0.01 0.27 0.72 0.009

0.00 0.00 0.00 0.00 0.06 0.88 0.000

0.554 0.928

0.721

0.938

0.436 0.067

0.269

0.062

60

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XCa = Ca2+/(Ca2++Mg2++Fe2+); XMg= Mg2+/(Fe2++Mg2+); XAn= Ca2+/(Ca2++Na++K+); XOr= K+/(Ca2++Na++K+); XAb= Na+/(Ca2++Na++K+); Min & Max are depending on the XCa content for Grt, on the XAn for Pl and XOr for K-peldspar.

T P

I R

C S U

N A

D E

M

T P E

C C

A

61

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Table 2 Representative EMP analyses of perthite from pelitic granulite in the Manjinggou complex. (data select from the mineral pair of the maxium XAn in plagioclase lamellae and XOr in host K-feldspar) Sample

HA5-1-1

HA5-1-2

HA5-3-1

HA5-3-3

HA7-1-1

Texture

Perthitic feldspar

Perthitic feldspar

Perthitic feldspar

Perthitic feldspar

Perthitic feldspar

Domin

HA7-1-2

T P

Perthitic feldspar

HA7-1-4 Perthitic feldspar

Kfs (Host)

Pl (lamellae)

Kfs (Host)

Pl (lamellae)

Kfs (Host)

Pl (lamellae)

Kfs (Host)

Pl (lamellae)

Kfs (Host)

Pl (lamellae)

Kfs (Host)

Pl (lamellae)

Kfs (Host)

Pl (lamellae)

65.50 0.08 18.14 0.03 0.00 0.00 0.01 0.06 0.99 15.02 99.83

61.52 0.00 24.53 0.00 0.00 0.00 0.00 6.19 7.64 0.80 100.69

66.62 0.00 20.33 0.04 0.10 0.01 0.01 1.01 11.13 0.32 99.57

65.26 0.00 18.48 0.00 0.00 0.01 0.00 0.22 5.29 10.20 99.46

59.82 0.00 25.38 0.00 0.03 0.00 0.00 6.96 7.57 0.24 100.00

64.78 0.07 18.39 0.00 0.01 0.01 0.01 0.11 0.77 15.12 99.26

64.71 0.06 18.82 0.00 0.00 0.00 0.00 0.05 0.87 15.44 99.95

61.93 0.04 23.27 0.00 0.00 0.00 0.00 4.20 7.39 3.15 99.98

64.77 0.04 18.75 0.00 0.05 0.02 0.00 0.10 0.74 15.51 99.98

62.33 0.01 23.72 0.05 0.04 0.01 0.02 4.66 8.72 0.24 99.80

64.70 0.06 18.74 0.01 0.02 0.01 0.00 0.05 0.87 15.21 99.68

62.62 0.02 23.47 0.00 0.02 0.03 0.00 4.57 8.84 0.35 99.91

64.55 0.00 19.05 0.01 0.00 0.02 0.00 0.07 1.08 15.27 100.05

63.74 0.02 22.72 0.03 0.05 0.00 0.01 3.64 9.32 0.43 99.96

Mn Mg Ca Na K

3.01 0.00 0.98 0.00 0.00 0.00 0.00 0.00 0.00 0.09 0.88

2.72 0.00 1.28 0.00 0.00 0.00 0.00 0.00 0.29 0.66 0.05

2.94 0.00 1.06 0.00 0.00 0.00 0.00 0.00 0.05 0.95 0.02

2.98 0.00 1.00 0.00 0.00 0.00 0.00 0.00 0.01 0.47 0.59

3.00 0.00 1.00 0.00 0.00 0.00 0.00 0.00 0.01 0.07 0.89

2.67 0.00 1.33 0.00 0.00 0.00 0.00 0.00 0.33 0.65 0.01

2.98 0.00 1.02 0.00 0.00 0.00 0.00 0.00 0.00 0.08 0.91

2.77 0.00 1.23 0.00 0.00 0.00 0.00 0.00 0.20 0.64 0.18

2.99 0.00 1.02 0.00 0.00 0.00 0.00 0.00 0.01 0.07 0.91

2.77 0.00 1.24 0.00 0.00 0.00 0.00 0.00 0.22 0.75 0.01

2.99 0.00 1.02 0.00 0.00 0.00 0.00 0.00 0.00 0.08 0.90

2.78 0.00 1.23 0.00 0.00 0.00 0.00 0.00 0.22 0.76 0.02

2.97 0.00 1.04 0.00 0.00 0.00 0.00 0.00 0.00 0.10 0.90

2.82 0.00 1.18 0.00 0.00 0.00 0.00 0.00 0.17 0.80 0.02

Ab Or An

0.09 0.91 0.00

0.66 0.05 0.30

0.94 0.02 0.05

0.07 0.92 0.01

0.65 0.01 0.33

0.08 0.92 0.00

0.63 0.18 0.20

0.07 0.93 0.01

0.76 0.01 0.23

0.08 0.92 0.00

0.76 0.02 0.22

0.10 0.90 0.00

0.80 0.02 0.17

(wt%) SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O

Total Si Ti Al Cr Fe

3+

Fe

2+

T P E

C C

A

0.44 0.55 0.01

D E

C S U

N A

M

I R

formula calculation on the basis of O = 8

62

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Table 3 Re-intergrated compositions of feldspar with areal proportion and chemical compositions of lamellae and host domains

Areal（%） sample

Pl Kfs （lamellae） （host HA5-1 13.8 86.2 HA5-2 11.3 88.7 HA5-3-1 14.5 85.5 HA5-3-3 10.5 89.5 HA7-1 25.0 75.0 HA7-2 27.2 72.8 HA7-4 17.4 82.6

Re-integrated composition (mol%)

EPMA data（mol%） Ab 0.660 0.935 0.655 0.628 0.761 0.762 0.802

Pl domin Or 0.045 0.018 0.013 0.176 0.014 0.020 0.024

An 0.295 0.047 0.332 0.196 0.225 0.218 0.174

Ab 0.090 0.436 0.071 0.079 0.067 0.080 0.096

Kfs domin Or 0.906 0.554 0.923 0.919 0.928 0.917 0.900

An 0.003 0.010 0.005 0.002 0.005 0.003 0.004

Temperature (°C) / 9.5 Kbar

I R

T P

Temperature (°C) / 1.1 Gpa

Ab

Or

An

T(FL)

T(LN)

T(EG)

T(BN)

T(FL)

T(LN)

T(EG)

T(BN)

0.172 0.495 0.159 0.139 0.246 0.270 0.223

0.784 0.491 0.787 0.838 0.693 0.667 0.742

0.045 0.015 0.054 0.023 0.062 0.063 0.035

885 826 913 833 942 946 871

858 770 889 798 911 914 833

955 816 997 873 995 996 901

887 838 911 839 942 946 876

889 836 916 838 947 950 877

858 770 888 800 913 916 838

960 823 1001 878 1000 1002 908

888 851 911 843 946 951 882

N A

C S U

M

T(FL) from Fuhrman & Lindsley, 1988; T(LN) from Lindslry & Nekvasil, 1989; T(EG) from Elkins & Grove, 1990 and T(BN) from Benisek et al., 2004

D E

T P E

C C

A

63

ACCEPTED MANUSCRIPT 64

Table 4 The calculated variations of major mineral contents at the chosen P-T conditions during retrograde stage as shown in Fig. 7b. P(Kbar)

T(°C)

Grt

Ksp

Rt

Pl

Bt

Sil

Qz

Ilm

A

7.2

800

0.204

0.326

0.002

0.046

0.0430

0.114

0.261

0.004

B

6.6

750

0.199

0.317

0.000

0.056

0.0434

0.115

0.262

T P

I R 0.007

C S U

N A

D E

M

T P E

C C

A

64

ACCEPTED MANUSCRIPT 65

Highlights

 Ultra-high temperature metamorphic overprint on high

SC

RI

PT

pressure pelitic granulites

 The UHT pelitic granulites record a Tmax metamorphic stage

MA

NU

at 940 – 950 °C / 9.5 – 10.5 kbar.

 Two groups of zircon U-Pb ages at 1973 and 1873 Ma

PT E

stages.

D

correspond to the HP decompression and UHT cooling

CE

 HP metamorphism associated with collision of the Ordos and

AC

Yinshan blocks and UHT metamorphism with the assembly of Western and Eastern Blocks

65

Figure 1

Figure 2

Figure 3

Figure 4

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