P–T–t evolution of the high-pressure mafic granulites from northern Hengshan, North China Craton: Insights from phase equilibria and geochronology

Accepted Manuscript P–T–t evolution of the high-pressure mafic granulites from northern Hengshan, North China Craton: insights from phase equilibria and geochronology Yinghui Zhang, Chunjing Wei, Minjie Lu, Xiwen Zhou PII: DOI: Reference:

S0301-9268(18)30089-5 https://doi.org/10.1016/j.precamres.2018.04.022 PRECAM 5072

To appear in:

Precambrian Research

Received Date: Revised Date: Accepted Date:

7 February 2018 23 April 2018 27 April 2018

Please cite this article as: Y. Zhang, C. Wei, M. Lu, X. Zhou, P–T–t evolution of the high-pressure mafic granulites from northern Hengshan, North China Craton: insights from phase equilibria and geochronology, Precambrian Research (2018), doi: https://doi.org/10.1016/j.precamres.2018.04.022

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P–T–t evolution of the high-pressure mafic granulites from northern Hengshan, North China Craton: insights from phase equilibria and geochronology Yinghui Zhanga,b, Chunjing Weib*, Minjie Lua, Xiwen Zhoua

a

Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China.

b

MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space

Sciences, Peking University, Beijing, 100871, China;



Corresponding author: Chunjing Wei Tel: +86 13651355549 E-mail address:

[email protected]

Abstract The Hengshan Complex has been widely concerned for the occurrence of typical high-pressure mafic granulites, representing the Paleoproterozoic collisional orogeny in the North China Craton. Mafic granulites occur as boudins of various scales in tonalite–trondhjemite–granodiorite (TTG) gneisses. They are composed of garnet, clinopyroxene, plagioclase, orthopyroxene, hornblende, biotite, quartz, rutile and ilmenite. Garnet porphyroblasts are usually surrounded by plagioclase coronas, and early jadeite-rich clinopyroxene is totally decomposed to the symplectite of clinopyroxene and plagioclase. Two high-pressure granulites and one two-pyroxene granulite were studied by the pseudosection approach, and clockwise P–T paths with four metamorphic stages were recognized. The pre-peak prograde stage was identified from garnet core to mantle compositions, with increasing P–T from 630 to 710 °C at 11–13 kbar. The peak pressure stage hasn’t been well preserved but its mineral assemblages inferred from petrography to be garnet + jadeite-rich clinopyroxene + quartz + rutile ± plagioclase, with P–T condition of 760–820 °C at 15 kbar. The post-peak decompression with slight heating is characterized by symplectite, plagioclase coronas, and the formation of orthopyroxene. Locally, two-pyroxene granulite assemblages develop, defining P–T condition of 6–8 kbar and 840–860 °C for the temperature peak (Tmax). The post-Tmax cooling is represented by the occurrence of amphibolite facies assemblages especially in

boudin margins with P–T condition of 6–8 kbar and 720–760 °C, suggesting an isobaric cooling process with fluid infiltration. U–Pb dating of metamorphic zircon records main ages ranging from 1897–1830 Ma with weighted mean age 1862 ± 11 Ma for a high-pressure granulite, and 1837 ± 7 Ma for a two-pyroxene granulite. These metamorphic ages were interpreted to record the post-peak cooling stage especially the time close to solidus, which is also supported by the result of Ti-in-zircon thermometer. A summary of P–T–t evolution for metamorphic terrains corresponding to different crust levels in Hengshan–Wutai

area

indicates

that

the

main

episode

of

crust

thickening-dominated collisional orogeny is ~1.95 Ga or earlier and finished at ~1.92 Ga. This was followed by post-orogenic cooling-uplifting or the overprinting of another thermal-tectonic event during the period of 1.92–1.80 Ga. Keywords: high-pressure granulite; phase equilibria; P–T–t evolution; zircon dating; Hengshan Complex; North China Craton

1. Introduction The Paleoproterozoic high-pressure (HP) granulites from North China Craton (NCC) have been extensively investigated in recent decades for their potential particular tectonic significance and well preservation of mineral assemblages and

textural evidence suitable for estimating the metamorphic reactions and P–T conditions (Zhai et al., 1993; Guo et al., 1999; Zhao et al., 2001a; Guo et al., 2002; O'brien et al., 2005; Tam et al., 2012; Wu et al., 2012; Liu et al., 2013; Duan et al., 2015; Zhang et al., 2016). Thus, understanding the distribution, metamorphism, and geochronology of these HP granulites is essential to elucidate the tectonic attributes and evolution of the NCC in the Paleoproterozoic. A popular view suggests that the NCC experienced a series of oceanic subduction and collision between blocks (Fig. 1a): a collision between the Yinshan and Ordos Blocks forming the Khondalite Belt and Western Block at ~1.95 Ga, a collision between the Longgang and Nangrim blocks forming the Jiao–Liao–Ji Belt and Eastern Block at ~1.90 Ga, and finally, a collision between the Western and Eastern blocks at ~1.85 Ga forming the Trans-North China Orogen (TNCO) and the final amalgamation of the NCC (Zhao et al., 2001b, 2005, 2012). Another view argues that the NCC was cratonized by amalgamation of micro-blocks and island arcs at ~2.5 Ga, followed by a Paleoproterozoic

imprint

involving

a

series

of

tectonic

events

of

rifting–subduction–accretion–collision from an extensional regime (2.3–2.0 Ga) to a compressional

setting

(2.01–1.97

Ga)

and

HP

and

high-temperature–ultrahigh-temperature (HT–UHT) granulite facies metamorphism during 1.95–1.82 Ga (Zhai et al., 2000, 2005; Zhai, 2011; Zhai and Santosh, 2011). This granulite facies metamorphism was further divided into three major stages, including the peak HP–HT granulite facies at 1.98–1.90 Ga, moderate-pressure (MP)

granulite facies at 1.89–1.82 Ga and amphibolite facies at ~1.80 Ga (Zhou et al., 2017). The uplifting of granulite terrains from HP–HT granulites stage (<1.4 GPa) to MP granulite stage (~0.8 GPa) spent more than 90Ma, indicating a specific “hot” subduction–collision process at that time (Zhou et al., 2017). Wei et al. (2014) and Wei (2018) summarized the progress on metamorphism and geochronology of Hengshan–Wutai–Fuping area and suggests that the final orogenic collision with crustal

thickening

and

kyanite-type

metamorphism

happened

at

~1.95Ga

corresponding to the pressure peak of the P–T path, and during 1.93–1.80Ga the thickened crust experienced rapid uplifting and slow cooling corresponding to the isobaric cooling (IBC) process after decompression. Detailed petrographic, mineral chemical, and thermobarometric studies of the northern Hengshan mafic granulites has been carried out by Zhao et al. (2001a) and O'brien et al. (2005). Zhao et al. (2001a) identified HP and MP granulites distinguished by orthopyroxene-bearing or not for the peak assemblage (M2), which consists of cpx + g + pl + q ± hb in the HP granulites and opx + cpx + g + pl + q in the MP granulites. Their P–T conditions were estimated to be 13.5–15.5 kbar/770–840 °C for the HP granulites and 9–11 kbar/820–870 °C for the MP granulites, using the average P–T calculation of THERMOCALC (version 2.75, Powell and Holland, 1994; Holland and Powell, 1998). Both types of granulites were experienced near-isothermal decompression (M3) and final decompression-cooling (M4), defining a clockwise P–T path. O'brien et al. (2005) identified two variants of mafic HP

granulites in northern Hengshan: antiperthite-bearing with coarse diablastic clinopyroxene-plagioclase

texture,

and

more

delicate,

finger-print

textured

symplectitic clinopyroxene + plagioclase with lower Mg-content garnet. They emphasized that orthopyroxene is texturally younger than clinopyroxene and replaces garnet in both variants, representing a decompression into the MP granulite field before cooling further under amphibolite facies conditions. Early prograde facies represented by rutile inclusions within the cores of garnet porphyroblasts and jadeitic clinopyroxene pseudomorphs were also recognized, but its P–T conditions cannot be quantitatively determined because of the absence of the major minerals (Zhao et al., 2001a; O'brien et al., 2005). It is still highly contentious whether they undergo a prograde metamorphism related to subduction-collision regime. Moreover, as multi-generation mineral assemblages suggest ubiquitious unequilibrium and heterogeneity in mafic granulites, the geothermobarometric results are fraught with uncertainty (O'brien et al., 2005). It requires re-evaluating the metamorphic evolution from the point of view of phase equilibria modelling. In this paper, we present petrographic, mineral chemical, phase equilibria modelling and geochronology studies on three representative mafic granulites from the northern Hengshan, involving all variants aforementioned, in order to quantificationally investigate the metamorphic evolution details and discuss their tectonic implications.

Mineral abbreviations used are: g, garnet; cpx, clinopyroxene; bi, biotite; q, quartz; cu, cummingtonite; ep, epidote; hb, hornblende; melt, silicate melt; ilm, ilmenite; mt, magnetite; opx, orthopyroxene; pl, plagioclase; ru, rutile.

2. Geological setting The Hengshan complex is located in the central part of the Trans-North China Orogen and is separated into the northern and southern Hengshan units by the Zhujiafang shear zone (Fig. 1b). The northern Hengshan complex consists mainly of intensively migmatized TTG gneisses with numerous HP mafic granulite blocks and granitic intrusions. HP mafic granulites occur as boudins or lenses in TTG gneisses, with granulite facies assemblages g + cpx + pl ± hb ± opx ± bi (+ q + ru + ilm) only preserved in the cores and amphibolite facies overprinting from the margins (Fig. 2a, b). Some boudins are retrograded to amphibolites completely (Fig. 2c), involving assemblages hb + pl ± bi ± cpx (+ ilm). Locally, melt patches can be observed on some outcrops (Kröner et al., 2006). These mafic boudins were previously thought to be originated from intrusive dykes with an emplacement age of ~1.92 Ga and metamorphic ages of 1.85–1.88 Ga (Kröner et al., 2006). However, some of them may represent the basic members of the Archean TTG gneisses (Trap et al., 2007). Zhang et al. (2013) and Wei et al. (2014) argue that most of the mafic boudins were originated from 2.3–2.0Ga mafic intrusives which experienced HP granulite facies metamorphism at ~1.95 Ga and a long post-peak cooling process from 1.92 to 1.85

Ga. This view is supported by redating of the same outcrop to Kröner et al. (2006) from which the zircons exhibit metamorphic features and yield concordant ages ranging continuously from 1.96 to 1.77 Ga (Qian et al., 2017). The TTG gneisses are dated to have emplacement age of 2.48–2.52 Ga representing arc magmatism in the Neoarchean (Kröner et al., 2005a, 2006), and metamorphic ages of 1.85–1.92 Ga (Kröner et al., 2005a; Zhang et al., 2013). They have mineral assemblages of amphibolite facies and develop intense partial melting, producing numerous granitic leucosomes, veins and small intrusions. Some granitic rocks share similar ages of 1.85–1.88 Ga (Kröner et al., 2006; Faure et al., 2007) with the granulites, while others show older ages of 2.05–2.35 Ga and ca. 2.5 Ga (Kröner et al., 2005b, 2006; Zhao et al., 2011). The southern Hengshan complex consists of TTG gneisses, supracrustal sequences, weak-deformed mafic dykes and granitic rocks. Different from that in northern Hengshan, there is no obvious partial melting leucosomes and veins in the TTG gneisses of southern Hengshan. Accordingly, the supracrustal sequences experienced amphibolite facies metamorphism with the peak conditions increasing northwards (Qian and Wei, 2016), involving garnet amphibolites, orthoamphibolites, metamorphic felsic volcanic and sedimentary rocks. Supracrustal sequences in the lower Wutai Subgroup and the south part of southern Hengshan complex were metamorphosed at low amphibolite facies with metamorphic age 1.96–1.90 Ga (Wang et al., 2000; Kröner et al., 2005b; Pang et al., 2010; Qian et al., 2013, 2015; Qian and

Wei, 2016). The garnet amphibolites in supracrustal sequences or as lenses and boudins in TTG gneisses from the north part of southern Hengshan complex were metamorphosed at high amphibolite facies, with metamorphic age ~1.92 Ga (Qian and Wei, 2016). The weak-deformed mafic dykes cutting the principal foliations of host TTG gneisses have a protolith age of ~2.06 Ga (Peng et al., 2012). Four representative samples of northern Hengshan mafic granulites collected from a NW–SE trending valley named Large Stone Valley (Dashiyu) along the road between Shahe and Yingxian (Fig. 1b), including a coarse-grained HP granulite H944 (Fig. 2b) on a deep riverbed at 39°28'02"N, 113°21'43"E, a fine-grained HP granulite H934 (Fig. 2d) in a gully behind a “Noodle House” at 39°27'4.41"N, 113°23'11.54"E, and two adjacent similar two-pyroxene granulite samples H947 and H1024 along a road terrace, at 39°28'53.9"N, 113°20'14.6"E and 39°28'45.54"N, 113°20'17.81"E, respectively.

3. Petrography and mineral chemistry The chemical compositions of minerals were analysed using a JEOL JXA-8100 electron microprobe at Peking University. The operating conditions were 15kV accelerating voltage, a 10nA beam current and a beam diameter set to 1 µm for all minerals. Natural and synthetic minerals of SPI Company were used for standardization. Matrix corrections were carried out using the PRZ correction program. Representative mineral analyses of three samples are presented in Tables 1 to 3, respectively.

Sample H944 is a coarse-grained HP granulite and consists of garnet (24 vol.%), clinopyroxene (15 vol.%), plagioclase (23 vol.%), hornblende (20 vol.%), quartz (10 vol.%), biotite (6%), ilmenite (2%), and small amount of K-feldspar in plagioclase with occasional presence of sulphide. Garnet occurs as porphyroblasts of 2–5 mm across (Fig. 2b, Fig. 3a) and envelopes abundant quartz along with minor hornblende, plagioclase, titanite, ilmenite and apatite especially in core to mantle regions. One representative compositional profile (Fig. 4a) reveals typical garnet growth zoning with high Xsps = ~0.1 in the core which decreases gradually outward (Spear, 1995) then rises a little at the absolute rim. From garnet core outward, Xgr increases slightly from a value of 0.31 in the core to a peak of 0.37 in the mantle before decreasing more sharply towards the absolute rim (Xgr = 0.25). The zoning pattern for Xpy changes inconspicuously from core to inner mantle, but starts to increase from a value of 0.08 to a near-rim peak 0.16. The little changing in trend of Xsps and Xpy at the outermost may be influenced by diffusion. Surrounding garnet porphyroblasts there are obvious plagioclase-rich reaction coronas in a texture referred to as ‘white-eye socket’ (Fig. 2b, c, Fig. 3a), and fine clinopyroxene grains sometimes grow with plagioclase coronas separating them from matrix minerals especially quartz. Coarse clinopyroxene contains lamellae and blebs of plagioclase appearing as a diablastic intergrowth (Fig. 3b) instead of a fine-grained ‘vermicular-like’ symplectite texture, also indicating the previous jadeite-rich clinopyroxene. The composition of plagioclase coronas around garnet shows XAn = 0.46–0.50, while some anhedral

plagioclase in the matrix shows distinct compositional zoning with XAn increasing outwards from 0.27 to 0.41, and the plagioclase lamellae and blebs in clinopyroxene show an XAn increasing trend from 0.31 to 0.41 outwards, too. In addition, minor antiperthite occurs in matrix. Clinopyroxene showing diopside composition contains little Na (0.02–0.03 pfu) with XEn = 0.46–0.49, XWo = 0.34–0.38, XFs = 0.13–0.20. The coronitic clinopyroxene has XMg = 0.63, lower than the coarse diablastic type with XMg = 0.71–0.74. Abundant amphibole, or hornblende as sensu lato, with pargasite to ferro-pargasite compositions (Hawthorne et al., 2012) occurring as replacement or overgrowth on clinopyroxene and coarse grains enclosing other minerals (Fig. 3a, b) together with biotite are petrographically late and should be dominative retrograde products probably related to fluid infiltration. The above petrographic observations suggest that four stages of mineral assemblages are present in the coarse-grained HP mafic granulites. The first pre-peak stage (M1) is preserved in the core to mantle of garnet and its inclusions, the second peak stage (M2) consists of garnet (probable mantle to rim), previous jadeite-rich clinopyroxene, quartz with or without plagioclase, the third stage (M3) is represented by the decomposition products including clinopyroxene and plagioclase intergrowth and plagioclase coronas, and the last stage (M4) is related to late hornblende and biotite. Sample H934 is a fine-grained HP granulite and composed of garnet (32 vol.%) clinopyroxene (23 vol.%), plagioclase (15 vol.%), hornblende (8 vol.%), orthopyroxene (7 vol.%), quartz (10 vol.%), ilmenite (3%) and small amount of rutile

with occasional presence of titanite, sulphide and biotite. It has inequigranular texture with the grain size for most minerals ranging from 0.2–1.2 mm. Garnet surrounded by plagioclase corona as ‘white-eye socket’ (Fig. 3c), and contains a few inclusions in core to mantle region such as rutile and its retrograde products ilmenite and titanite, plagioclase, minor quartz, clinopyroxene, hornblende and calcite (Fig. 2d, Fig. 3c). It shows a compositional profile (Fig. 4b) that has nearly homogeneous core with slightly higher Xsps = 0.03–0.04 and Xgr = 0.26–0.29 but lower Xpy = 0.11–0.13 than in mantle and rim. From mantle to rim, Xgr decreases to 0.18, while Xpy first rises to the peak 0.17 in the outer mantle to inner rim and then decrease slightly to the outermost rim. On the opposite, Xsps first decreases and then slightly rises at the outermost rim. The pseudomorphs of previous lath-like jadite-rich clinopyroxene occur in the form of fine-grained vermiform symplectitic intergrowths of clinopyroxene and plagioclase. Clinopyroxene and orthopyroxene grow in two domains: the corona around garnet and symplectite. Minor later hornblende overprints both kinds of clinopyroxene (Fig. 3d). The plagioclase inclusion in garnet has XAn = 0.28, and the matrix plagioclase shows XAn increasing from core 0.30 to rim 0.45. The symplectic plagioclase has XAn = 0.41, and the coronitic plagioclase shows XAn = 0.39–0.47. The symplectic and coronitic clinopyroxenes share similar diopside composition with XEn = 0.31–0.34, XWo = 0.46–0.48, XFs = 0.18–0.24, but a little difference in XMg = 0.56–0.64 and 0.59–0.65, respectively. Orthopyroxene contains XMg = 0.41–0.44, and later hornblende (sensu lato) shows ferro-hornblende to

ferro-pargasite composition (Hawthorne et al., 2012). The above petrographic observations also suggest four stages of mineral assemblages similar to that in sample H944, the difference are the rutile inclusion in garnet in M1/M2, the development of orthopyroxene in M3 which potentially equilibrates with the adjacent clinopyroxene thus defines a two-pyroxene granulite assemblage for M3, and there is no biotite in M4 stage. Sample H947 is a two-pyroxene granulite and contains clinopyroxene (35 vol.%), orthopyroxene (12 vol.%), plagioclase (30 vol.%), garnet (8 vol.%), hornblende (5 vol.%), ilmenite + magnetite (5 vol.%) and biotite (5 vol.%). Garnet is mostly replaced by orthopyroxene + plagioclase symplectites that present pseudomorphs after garnet, sometimes contain residual garnet (Fig. 3f), or occurs as anhedral grains surrounded by plagioclase coronas. The composition of this residual garnet is commonly similar to the rim of garnet porphyroblasts in HP granulites, showing Xgr = 0.20–0.23 and Xpy = 0.16–0.19 without significant zoning. The plagioclase in symplectite with orthopyroxene has

XAn = 0.81–0.84, the coronitic plagioclase

shows XAn = 0.51–0.53 , while the matrix plagioclase is zoned with XAn increasing from core 0.46 to rim 0.77. In matrix, clinopyroxene and orthopyroxene texturally equilibrate (Fig. 3e), sometimes showing intergrowth with skeletal ilmenite aggregates. The orthopyroxene in symplectite and in matrix shares similar composition with XMg = 0.50–0.53. Clinopyroxene shows diopside composition with XEn = 0.31–0.34, XWo = 0.46–0.48, XFs = 0.18–0.24, while the grains intergrown with

ilmenite contain less XMg = 0.67 than the others with XMg = 0.72–0.75. Pargasitic hornblende texturally equilibrates with pyroxene or later, and locally, later biotite overprints previous minerals. The observed main assemblage in this sample consisting of clinopyroxene, orthopyroxene, plagioclase, ilmenite, magnetite and hornblende is recognized as M2, which is preceded by a garnet-bearing assemblage (M1) indicated by garnet relics and its pseudomorphs, and followed by a subsequent retrograde assemblage (M3) represented by the late growth of hornblende and biotite.

5. Phase equilibria modelling and P–T evolution For modelling the mineral assemblages and compositions discussed above, the model system NCKFMASHTO is selected to calculate P–T pseudosections for the three mafic granulite samples. Fluid phase in subsolidus conditions is assumed to be pure H2O and melt is present in all the suprasolidus assemblages. MnO is neglected in the model system as it constitutes only small proportions of the total rock and mainly present in garnet. The bulk rock compositions for the phase equilibria calculation were taken directly from the whole-rock XRF analyses performed at the Tianjin Institute of Geology and Mineral Resources. These data were then normalized into mole proportions in the model system. MnO was supposed completely contained in spessartine and deducted as its formula (MnO)3·Al2O3·(SiO2)3. CO2, P2O5 and relevant components were also deducted as calcite (CaO·CO2) and apatite (CaO)5·(P2O5)1.5·(H2O)0.5. The H2O content was chosen to just saturate the mineral

assemblages immediately subsolidus at 10–12 kbar (White et al., 2001). The bulk compositions of three samples are presented in Table 4. Pseudosection calculations were performed using THERMOCALC 3.33 (Powell et al., 1998; updated July 2006), with the November 2003 updated version of the Holland and Powell (1998) data set (file tcds55.txt). Activity–composition relationships are those presented for garnet, biotite and melt (White et al., 2007), epidote (Holland and Powell, 1998), chlorite (Holland et al., 1998), clinopyroxene (Green et al., 2007), hornblende (Diener et al., 2007), plagioclase (Holland and Powell, 2003), orthopyroxene (White et al., 2002), muscovite (Coggon and Holland, 2002), and magnetite, ilmenite and hematite (White et al., 2000).

Rutile and quartz

are pure end-member phases. Pseudosections for a coarse-grained HP granulite (sample H944) The NCKFMASHTO P–T pseudosection calculated for a coarse-grained HP granulite sample H944 is presented in Fig. 6. The principal phase relationships in the P–T range 2–20 kbar, 600–900 °C are characterized by pressure-dependent boundaries marking the appearance of garnet and rutile towards high pressure and the appearance of ilmenite and plagioclase towards low pressure. Orthopyroxene is stable in low pressure and high temperature area, and hornblende disappears towards high temperature and high pressure. The observed peak assemblage (M2) involving g + cpx + q  pl + ru/ilm is predicted to be stable in P–T conditions > 760 °C at  15 kbar where clinopyroxene is predicted to be omphacite with j(cpx) = 0.35, and biotite is

modelled to be present in a trivial amount although it is not definite from petrographic observation. The core–mantle zoning in garnet (Fig. 4a) is modelled to yield a P–T vector dominated by heating from 630°C to 700 °C with slight compression within 12–14 kbar in the fields adjacent to the water-saturated solidus. The measured garnet rim compositions are plotted to yield lower pressure and higher temperature conditions at 9–10 kbar/750 °C, which are considered to be insignificant because the Xgr and Xpy values may have been differently modified during the post-peak decompression and cooling. However, the minimum Xgr = 0.25 from garnet rims can be used to constrain the maximum peak temperature being less than 830 °C because the Xgr value decreases both along the pre-peak prograde path and along the post-peak decompression path. The core-rim zoning with deceasing XAn from 0.27 to 0.41 in matrix plagioclase is consistent with a decompression vector from 14 to 11 kbar. A similar core-rim zoning with deceasing XAn from 0.31 to 0.41 in symplectitic plagioclase is modelled to be consistent with this decompression vector, while the higher XAn = 0.46–0.50 from coronitic plagioclase around garnet is plotted at lower pressure ~10 kbar. The observed M4 assemblage characteristic of hornblende and biotite together with clinopyroxene and plagioclase, which is the same as the amphibolite assemblage in the margins of granulite boudins, may correspond to P–T conditions 720–760 °C at 6–8 kbar, indicating a cooling process should take place after the decompression with a near-solidus assemblage produced.

Pseudosections for a fine-grained HP granulite (sample H934) The NCKFMASHTO P–T pseudosection calculated for a fine-grained HP granulite (sample H934) is presented in Fig. 7. The observed peak assemblage (M2) involving g + cpx  pl + ru  ilm (+q +melt) is predicted to be stable in P–T conditions > 760 °C and 15 kbar. The core–mantle zoning in garnet (Fig. 4b) is modelled to yield P–T vector dominated by heating from 680–720 °C at ~12.5 kbar. The measured near-rim compositions are plotted as an array pointing to lower pressure and higher temperature conditions ~9 kbar/~760 °C, which are also considered to be insignificant because of the different modification of Xgr and Xpy during the post-peak decompression and cooling. The inclusion-type plagioclase with the minimum XAn = 0.28 records a pressure ~12.5 kbar/800 C. The core-rim zoning with deceasing XAn from 0.30 to 0.45 in matrix plagioclase is consistent with a decompression vector from ~12 to 9 kbar. The symplectitic and coronitic plagioclase with

XAn = 0.39–0.47 is modelled to be consistent with the lower part of

decompression vector at 9–10 kbar. The observed assemblage M3 characterized by the coexistence of cpx + opx + pl probably with garnet is stable in P–T conditions >800 °C at 7–10 kbar, consistent with the measured x(opx) = 0.41–0.44 and x(cpx) = 0.56–0.65. The observed later growth of hornblende (M4) is consistent with an isobaric cooling process to T < ~800 C. Pseudosections for a two-pyroxene granulite (sample H947) A NCKFMASHTO P–T pseudosection calculated for a two-pyroxene granulite

sample H947 is presented in Fig. 8. The observed two-pyroxene assemblage (M2) involving cpx + opx + hb + pl + ilm + mt (+ melt) is predicted to be stable in a P–T range of 6–8 kbar and > 800 C. The measured minimum x(cpx) = 0.67, maximum x(opx) = 0.53, and XAn = 0.77 from matrix plagioclase rims yield a consistent temperature estimate of 840–860 C in the stability field of the two-pyroxene assemblage, indicating an equilibrium tendency may have reached among the relevant minerals. The observed garnet-bearing assemblage (M1) is predicted to be stable in higher pressure fields > 8 kbar and the measured minimum XAn = 0.46 in the core of a matrix plagioclase grain is modelled to give a pressure estimate of 12 kbar at 830 C, defining a decompression vector. The XAn = 0.51–0.53 in coronitic plagioclase is modelled to be consistent with this decompression vector as a short section around 11 kbar, but the higher XAn = 0.81–0.84 from symplectitic plagioclase cannot be plotted in Fig. 8 and may represent compositions controlled by local domains. The observed later growth of biotite (M3) is consistent with a near-isobaric cooling path with T < 800 C. It should be noted that the amphibolite assemblages in the margins of the granulite boudins including hb + pl + ilm + q + bi ± cpx may correspond to a P–T condition of 6–8 kbar/740–770 C.

6. Zircon age dating and thermometer 6.1 Analytical methods Zircon grains from sample H934 and H1024 were separated by conventional

heavy liquid and magnetic separation followed by handpicking under a binocular microscope. Selected grains were mounted in an epoxy resin, polished down to expose the grain centre, photographed in transmitted and reflected light, and imaged using cathodoluminescence (CL). Zircon U–Pb isotopic and trace element analyses were performed using LA–ICP–MS method by an Agilent 7500ce ICP–MS equipped with a 193 nm excimer laserablation system (COMPexPro 102) at the Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University. Zircon PLE (Sláma et al., 2008) and 91500 was used as the standard and the standard silicate glass NIST was used to optimize the machine. The concentrations of U, Th and Pb elements were calibrated by using 29Si as an internal calibrant and NIST 610 as an external reference standard.

207

Pb /206Pb,

206

Pb /238U and

207

Pb /235U ratios and apparent ages were

calculated using the GLITTER 4.4 (Van Actherbergh et al., 2001). The age calculations and concordia plots were made using Isoplot (ver. 4.15, updated of Ludwig, 2003).

6.2 Analytical results Zircons in sample H934 are predominantly long to short-prismatic with rounded terminations (Fig. 9a). Most grains have simple internal growth structures with sector zoning, blurred banded zoning or homogeneous, but variable luminescence. Some grains contain subhedral, relatively high luminescence cores. A few grains show or partly show oscillatory magmatic zoning. Twenty nine analyses from 27 zircon grains

obtained are presented in Table S1 and Fig. 10a. Th content ranges from 1.0 to 5.8 µg/g, and U content ranges from 38 to 283 µg/g with very low Th/U ratio 0.02–0.05. Twenty four analyses yield a near-concordant cluster of

206

Pb/207Pb age ranging

continuously from 1897 to 1830 Ma, with a weighted mean 206Pb/207Pb age of 1862 ± 11 Ma (MSWD = 0.4). The two analyses from core show relatively older ages than those from rim, but all distributed in this continuous cluster. Other 5 analyses from rounded and high luminescence grains yield a younger weighted mean 206Pb/207Pb age of 1799 ± 26 Ma (MSWD = 0.015) and an intercept age of 1803 ± 76 Ma (MSWD = 0.02). All these zircons show consistent REE patterns with positive Ce anomalies and without obvious Eu anomalies (Fig. 10b). Significantly HREE depletion comparing with MREE indicates abundant garnet existence during these zircons growth. Zircon grains in a two-pyroxene granulite sample H1024 are equant, stubby or irregular in shape with rounded terminations (Fig. 9b). Most grains exhibit rare or patchy zoning with low luminescence, commonly interpreted as metamorphic origin. A few grains have inherited irregular cores with higher luminescence, being the relics of magmatic zircons, and broad low luminescence metamorphic overgrowths. Thirty six analyses from 32 zircon grains obtained are presented in Table S1 and Fig. 11c. Six analyses from magmatic cores show various degrees of lead loss and discordant plots, and cannot yield reliable ages. Thirty analyses on the grains or rims with low luminescence show Th = 2.87–27 µg/g, U = 108–212 µg/g and Th/U = 0.02–0.16, most of which are concordant of near concordant, except 5 analyses that are variably

discordant but define a linear arrangement (Fig. 10c). If regressed together with the concordant or near concordant data (MSWD = 1.5), a discordia line with an upper intercept age of 1835.2 ± 6.4 Ma, and a weighted mean

206

Pb/207Pb age of 1837.3 ±

6.8 Ma (MSWD = 0.92) are yielded. The relic cores of magmatic zircon have relatively higher REE contents with positive Eu anomalies and HREE enrichment, while the metamorphic zircons have relatively low REE contents without obvious Eu anomalies and are depleted in HREE due to the garnet existence (Fig. 10d). The Ti contents in metamorphic zircons from the two samples are generally low, varying from 1.09 to 2.59 µg/g in sample H934 and 1.96 to 6.24 in sample H1024. Crystallization temperatures for zircon were calculated using the revised Ti-in-zircon thermometer (Fig. 11) of Ferry and Watson (2007), which was assigned a precision of ±2.2 % (for °C), taking into account both the analytical uncertainty on Ti measurements (±15 %) and the error inherent in the calibration (Watson et al., 2006). The activity of TiO2 was assumed to be 0.6 considering the wide presence of ilmenite although rutile is found as inclusion in garnet in sample H934. The zircon crystallization temperatures of the two samples are computed to be 607–691 °C with average 654 °C for H934 and 651–750 °C with average 701 °C for H1024, respectively.

7. Discussion and conclusions 7.1 P–T estimates and paths of the northern Hengshan HP mafic granulites The P–T estimates from pseudosection of three mafic granulite samples from northern Hengshan are summarized in Fig. 12. In contrast to previous studies of HP mafic granulites in the northern Hengshan that have no appropriate way to determine the early prograde process because of the absence of available thermobarometries (Zhao et al., 2001a), the compositional profiles of garnet together with pseudosection modelling in this study provide quantitative constraints on the P–T conditions of the prograde stage. The typical ‘bell-shaped’ Xsps profiles in garnet core to mantle are rare in the granulites for the high temperature and diffusion rate, and the large-grained porphyroblasts tend to have well protected the original growth zoning from modification. The garnet Xgr and Xpy zoning from sample H944 corresponding to the best Xsps ‘bell-shaped’ part from the core to mantle plot on the pseudosection and indicate a prograde path from 12 kbar/630 °C to 13~14 kbar/710 °C. For the fine-grained garnet in sample H934 which is not big enough to preserve a good ‘bell-shaped’ Xsps profile but still contain relatively higher Xsps in the core, the plots of relevant garnet compositions also show a prograde path but with higher temperature from 670 °C to 730 °C at 12 kbar. In the HP granulite samples, it is reasonable that the records of peak condition are probably absent in garnet (Florence and Spear, 1991; Li and Wei, 2016). For instance, along the prograde path in the pseudosection for sample H944, a large volume of

garnet (1140 mole %) grow before the hb-out boundary, and thus, record this part of prograde path. Then the garnet growth is retarded after hornblende disappears and a small volume of garnet growth occurs near the peak, which are easy to be consumed away during the decompression as indicated by the formation of plagioclase coronas. The compositions of former jadeite-rich clinopyroxene are also totally reset with minor jadeite content now. Although the lowest XAn in plagioclase indicates relatively higher pressure, there is still a possibility of plagioclase-absent for the higher peak pressure. Hence, only a probable P–T range could be deduced on the basis of the mineral assemblage implied from petrography, which is 15 kbar and 760–830 C for samples H944, where the maximum temperature was constrained from the maximum Xgr from garnet rim because the Xgr values show a decreasing trend along the prograde vector in Fig. 6, and would be further decreased along the decompression path. There is enough evidence to support that a post-peak decompression occurred in the HP granulites, including the formation of cpx + pl symplectites after jadeite-rich clinopyroxene, plagioclase coronas around garnet, the core-rim zoning with XAn increasing in matrix plagioclase and the growth of orthopyroxene. This decompression evolution probably coupled with slight heating led the HP granulites to their Tmax stage. However, this metamorphic evolution should have proceeded under fluid-absent conditions and the most decompression assemblages did not reach a good equilibrium state. Thus, the P–T conditions of the Tmax stage for the HP

granulites samples cannot be well constrained. Only in the two-pyroxene granulite samples, a series of compositions of minerals potentially reached an equilibrium state that defined a condition of 6–8 kbar, 840–860 °C in the stability field of the two-pyroxene assemblage without garnet (Fig.8). The phase modelling indicates there should be considerable amount of melt at such high temperature, which may react with the anhydrous granulite assemblages to produce the hydrous phases during cooling. However, the HP granulites are proved to be fluid (or melt)-absent during their decompression to the Tmax stage as indicated by the textures of, for instance, symplectites and coronas, and the amphibolization in a granulite boudin becomes more intensive from core to margin, indicating potential exotic fluid infiltration. The fluids are probably derived from the melt crystallization from the surrounding TTG gneisses (Fig. 2a, c). The P–T conditions of the amphibolites from the margins of HP granulite boudins can be well constrained in Figs. 6 and 8, to be 6–8 kbar/750 C, defining an isobaric cooling path for the post-Tmax stage. In consequence, by taking these episodes record by variants of northern Hengshan mafic granulites together, a relatively complete P–T evolution history of this area could be obtained (Fig. 12), including the pre-peak compression and heating to the peak stage, post-peak decompression with slight heating to the Tmax stage and late isobaric cooling to the amphibolite facies.

Zhao et al. (2001a) calculated the P–T conditions of HP granulites and MP granulites using THERMOCALC average P–T method and suggested that they are both the peak conditions of two P–T paths for each variant respectively. However, it is more likely that these variants are apt to record the P–T conditions of certain metamorphic stages respectively due to the variation of bulk composition, fluid abundance, stress, or local environment. Moreover, orthopyroxene in HP granulites is texturally later and not equilibrate with early clinopyroxene and garnet according to both petrographic observation and phase modelling. In addition, the zoning plagioclase in two-pyroxene granulite sample H947 also records a decompression vector from 12 to 8 kbar, sharing similar decompression paths with HP granulites. 7.2 Significance of age data and tectonic implications The main clusters of metamorphic zircon analyses for the HP granulite sample and two-pyroxene granulite sample from northern Hengshan complex are ~1.85 Ga. Seemingly, they are coeval with the previous studies of metamorphic zircon U–Pb ages from granulites in TNCO, and most scholars agree that the ages of ~1.85 Ga represent the collision between the Western and Eastern blocks of NCC (e.g. Zhao et al., 2012). However, controversies on the significance of this metamorphic zircon age and the time relating to main episode of collisional orogeny in TNCO are still disputable. Which stage of metamorphic P–T path does the U–Pb age of metamorphic zircon analysis represent? It is a quite important issue of linking geochronology and

metamorphism for figuring out the tectonic evolution. In metamorphic processes, the growth of metamorphic zircon is controlled by fluid/melt behaviour. Under the subsolidus conditions, the age of metamorphic zircon can be interpreted as the time of metamorphic pressure peak or temperature peak. This is because the process before metamorphic peaks is mainly controlled by dehydration reactions, releasing fluids that are favourable for zircon growth; during the process after metamorphic peaks, however, the rocks are in the fluid-absent condition without dehydration reactions, and it is hard to grow zircon (Guiraud et al., 2001). For the high-grade metamorphic processes under suprasolidus conditions, the prograde metamorphic evolution produces anatectic melt before metamorphic peak, and zircon tends to dissolve in melt without growth and age record, since the higher solubility of Zr in melt (Watson and Harrison, 1984; Roberts and Finger, 1997); only during cooling, zircon could grow and record age with melt crystallization. Hence, in melt-bearing high-grade metamorphic rocks, newly grown zircon should record the time of cooling stage coupling with melt crystallization, in the other words, the retrograde metamorphic age when the rock cooling across the solidus, rather than the peak metamorphic age (Kelsey and Powell, 2011). As various ways and degrees of melt extraction, segregation and crystallization may occur in rocks of different bulk compositions and structural positions, zircon from a high-grade terrane may record a series of ages, especially when the rocks have a long residence in the hot middle to deep crustal level or experience sluggish cooling

at suprasolidus conditions (Watson, 1996). This is probably the reason why metamorphic zircon ages from one sample generally range in a wide continuous spectrum on the concordia curve, for instance, from 1.90 to 1.83 Ga in sample H934, similar to those in the garnet amphibolites from southern Hengshan complex (1.96–1.90 Ga and 1.97–1.87 Ga, Qian and Wei, 2016) and in the HP mafic granulites (1.96–1.77 Ga) and garnet-absent amphibolite (1.88–1.79 Ga) from northern Hengshan complex (Qian et al., 2017). Actually, if high temperatures were preserved and melting lasted a long period, new growth as well as modifications of zircon could have occurred at any time, and in some cases, zircon age populations are smeared out showing a ‘geological scatter’ (Rubatto et al., 2013). However, most of zircon from a high-grade terrane should be close to the age when the cooling path traverses the solidus. For example, if the age 1.85–1.87 Ga (Kröner et al., 2006) for granitic veins which cut mafic granulites in the northern Hengshan complex is considered to represent melt crystallization, most mafic granulites and amphibolites in the same region that yield the similar ages of 1.84–1.89 Ga (Kröner et al., 2006; Qian et al., 2017; this study) may not correspond to the peak stage of metamorphism. This inference can also be supported from the results of Ti-in-zircon thermometer, mostly ranging from 650–700 °C corresponding to the conditions of mafic and granitic solidi in Fig. 12. Even though the zircon REE patterns show strong garnet impressions, it may suggest that garnet did exist when zircon grew or zircon grew from a melt that is

ever in equilibrium with garnet, but did not evidence that zircon and garnet grow simultaneously. How long is the time of cooling to soidus later than the metamorphic peak in a orogen? It is related to the tectonic environment and structural positions, which manifest as different P–T paths (Wei et al., 2014). For the rocks from different metamorphic belts in one orogen, the higher the metamorphic grade is, the younger the metamorphic age records (Qian et al., 2017). Besides, for that from suprasolidus high-grade metamorphic belt in orogen, it depends on the post-peak decompression process. Wei (2016) summarized three scenarios of post-peak decompression: decompression with considerable cooling across solidus, isothermal decompression to a shallow crust followed by cooling across solidus, and isothermal decompression perhaps coupled with slight heating to a middle–lower crust followed by isobaric cooling across solidus. For the former two scenarios, rocks may cool very fast with the solidus ages not too later than the peak. But for the third scenario, rocks may record cooling ages much later than the peak, mainly dependent on how much the temperature after decompression is higher than the solidus. A summary of metamorphic P–T paths and relevant metamorphic age in Hengshan–Wutai area is presented in Fig. 13. From south to north, the metamorphic grade increases from lower amphibolite facies to granulite facies, and their pressure peaks show similar apparent thermal gradient of ~15 °C /km representing metamorphic products at different crustal depths in one orogeny, while their

metamorphic ages decrease as 1.95→1.92→1.85 Ga correspondingly (Qian et al., 2017). Considering the peak metamorphic conditions and P–T paths, the peak temperature of rocks from lower Wutai Subgroup and the south part of southern Hengshan complex which is low amphibolite facies and below the solidus, and their metamorphic zircon ages of 1.95 Ga should be close to the age of the peak pressure stage (Qian et al., 2015). For the metamorphic rocks in the north part of southern Hengshan complex which is high amphibolite facies, their P–T paths after peak pressure stage traverse the solidus during decompression or close to solidus after decompression, and their metamorphic ages of 1.92 Ga should record the post-peak decompression age. The northern Hengshan granulites reached peak temperature after post-peak decompression, which is much higher than the solidus temperature, and their metamorphic ages of mostly 1.85 Ga with a few up to 1.92 Ga represent the time of cooling to solidus after the temperature peak. The Hengshan–Wutai area is a typical collisional orogen characterized by crust thickening in TNCO, in which kyanite-type metamorphic belts are developed (Thompson and England, 1984; Miyashiro, 1994), and the pressure peak of metamorphism may correspond to the thickest stage of crust during collisional orogeny. According to the metamorphic P–T paths and age data discussed above, the main episode of collisional orogeny is proposed to correspond to the peak time of the lower amphibolite facies metamorphism of kyanite-type from lower Wutai Subgroup and the south part of southern Hengshan complex, which is ~1.95 Ga or earlier (Qian

et al., 2013, 2015), and other younger ages represent the uplifting and cooling time of different metamorphic terrains, which could last to 1.80 Ga (Zhang et al., 2013; Wei et al., 2014). However, the interpretation of a long-lived orogenic process which may last a period over 100 Ma (Wei et al., 2014) is still somewhat unreasonable. Linked the metamorphic P–T path and a crustal thickening-dominated orogeny together, a prograde evolution with P–T increasing to the pressure peak followed by an isothermal decompression (ITD) to a certain crustal level, may correspond to the orogenic evolution from subduction–collision (or thickening) to isostatic uplifting, that is to say the thickened crust reached the state of isostasy and the orogeny finished. The subsequent isobaric cooling (IBC) represents the post-orogenic thermal relaxation in the isostatic crust, which could record different metamorphic ages, but irrelevant to the preceding orogenic event (Wei, 2016). Therefore, the ~1.95 Ga orogeny in the Hengshan–Wutai area was over at least 1.92 Ga, and the 1.92–1.80 Ga represents the post-orogenic uplifting-cooling ages of metamorphic terrains or the overprinting of another thermal-tectonic event (Wei, 2018).

Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant 41402056 and 41430207. We thank Guiming Shu and Fang Ma for their help in experimental analyses, and Jiahui Qian, Hang Chu and Shuang Zhang for their involvement in field and experimental work. Profs. T. Tsujimori and A. Perchuk are

much appreciated for their critical and constructive comments, and Prof. Guochun Zhao is appreciated for his editorial work.

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Figure captions Fig. 1 (a) A sketch map showing the framework of North China Craton and location of Fig. 1b. (b) An overview map of Hengshan terrain and relevant northern Wutai area (modified after Kröner et al. 2006 and Qian et al., 2013) showing major rock units and sample location. Number labels ‘1’–‘5’ show the sample location corresponding to the P–T paths in Fig. 13. Fig. 2 Field outcrops((a), (c)) and hand specimens((b)H944, (d)H934) of HP mafic granulites from the northern Hengshan Complex. (a) A classical HP mafic granulite boudin (~3.5 m in diameter) surrounded by migmatitic granodioritic gneiss. (b) Close-up of a HP mafic granulite boudin H944 showing conspicuous cm-sized spots composed of garnet with plagioclase coronas as ‘white-eye socket’ texture, and outwards from core of the boudin more garnet grains are replaced by plagiocalse. (c) A small HP mafic granulite boudin in TTG gneiss in which all garnet porphyroblasts are retrograded to plagioclase pseudomorphs. (d) Fine-grained mafic granulite H934 showing mm-sized garnet sitting in a matrix of clinopyroxene and plagioclase. Fig. 3 Petrographic features of mafic granulites from the northern Hengshan. (a, b) Coarse-grained HP mafic granulite H944. (c) Fine-grained HP mafic granulite H934.

(d) cpx + pl symplectite in H934. (e) Two-pyroxene granulite H947. (f) BSE, opx+pl symplectite as pseudomorph after garnet in H947. Fig. 4 Representative compositional zoning profiles of garnet. (a) 5 mm in diameter, H944. (b) 1.2 mm in diameter, H934. Xalm = Fe2+/(Fe2++Mg+Ca+Mn), Xpy = Mg/(Fe2++Mg+Ca+Mn), Xsps = Mn/(Fe2++Mg+Ca+Mn), Xgr = Ca/(Fe2++Mg+Ca+Mn). Fig. 5 Plagioclase compositions XAn(pl) = Ca/(K+Na+Ca) of different types in the mafic granulite samples. Fig. 6 P–T pseudosection for a coarse-grained HP mafic granulite sample H944 in the system NCKFMASHTO with quartz in excess. The H2O-saturated solidus is labelled with ‘+ melt’ and the H2O-out boundary is labelled with ‘+ H2O’. Divariant fields are shown as blank, and for the higher variance fields, the darker the shade, the higher the variance. The pseudosection is contoured with isopleths of garnet c(g) = Xgr = Ca/(Ca+Fe2++Mg) and m(g) = Xpy = Mg/(Ca+Fe2++Mg). MnO in garnet is neglected since it is not involved in modelling, which only slightly effects the c(g) and m(g) values in the core. Isopleths of XAn(pl) (=Na/(Na+Ca)) for different plagioclase variants and j(cpx) (= Na/(Na+Ca)) for jadeite in clinopyroxene are shown for the corresponding mineral assemblages. Projections of the garnet compositions are shown as small circles where the solid, hollow and dashed circles correspond to the core, mantle and rim respectively. Fig. 7 P–T pseudosection for a fine-grained HP mafic granulite sample H934 in the system NCKFMASHTO with quartz in excess. The additional isopleths shown in this pseudosection are x(cpx) = Mg/(Fe2++Mg) in clinopyroxene and x(opx) = Mg/(Fe2++Mg) in orthopyroxene for the corresponding mineral assemblages. Other explanations are the same as in Fig. 6.

Fig. 8 P–T pseudosection for a two-pyroxene granulite sample H947 in the system NCKFMASHTO with quartz in excess. Other explanations are the same as in Figs. 6 and 7. Fig. 9 Cathodoluminescence images of selected zircons of mafic granulite samples H934 (a) and H1024 (b) from the northern Hengshan. The circles show the position of LA–ICP–MS analysis with 207Pb/206Pb ages in Ma. Fig. 10 Concordia diagrams of zircon U–Pb dating results and chondrite-normalized REE patterns of zircons of mafic granulite samples H934 (a-b) and H1024 (c-d) from the northern Hengshan. Data of chondrite were cited from Sun and Mcdonough (1989). Fig. 11 Histogram of Zircon crystallization temperatures calculated by the Ti-in-zircon thermometer (Ferry and Watson, 2007) for mafic granulite samples from the northern Hengshan. Fig. 12 P–T paths for mafic granulites from the northern Hengshan. The boundaries for metamorphic facies are cited from Brown (2001). The granitic and basic solidi are cited from Johannes and Holtz (1996) and Vielzeuf and Schmidt (2001), respectively. The transition lines of Al2SiO5 are calculated using THERMOCALC. Fig. 13 The summarized P–T–t paths for metamorphic rocks from the Hengshan–Wutai area. The P–T paths are shown for the garnet-orthoamphibole rocks (‘1’) from the south part of southern Hengshan complex in Qian et al. (2015), the garnet amphibolites from the Lower Wutai Subgroup (‘2’), and the north part of southern Hengshan complex (‘3’–‘5’) in Qian and Wei (2016), and northern Hengshan HP mafic granulites (‘6’) from this study. Other explanations are the same as in Fig. 12.

0

200 400km

s Yi n

ha

(a)

0

1867±23 (Kröner et al., 2006) 1849±9 (Qian et al., 2017) 1850±3 (Kröner et al., 2006)

k loc e lt n B te B d a li n o ck Kh o l s B do Or

Beiloukou

Jiao-Liao-Ji Belt

WESTERN BLOCK

u iy sh D a H947, H1024 Yingxian H944 1915±4,1914±2 (Kröner et al., 2006) Xiaoshiyu H934 1963–1769 (Qian et al., 2017) 1917±10

Northern Hengshan

EASTERN BLOCK

TRANS-NORTH CHINA OROGEN

20km

u So

th

er

nH

g en

sh

an

5

i Yixingzha

ing

2060 (Peng et al., 2012)

up ish

L

4 3

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39°30′

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(Zhang et al, 2013)

10

N

113°30′

Fig.1b

1921±12 (Qian and Wei, 2016) 1923±7 (Qian and Wei, 2016)

uiling

1

he

Sha

1964±25 (Qian et al., 2015) 1901±28 (Pang et al., 2010)

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(Qian and Wei, 2016)

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2

Da

1912±9

(Kröner et al., 2005b)

(b)

H

u

o tu

riv

er

113°00′

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113°30′ 39°00′

Northern Hengshan TTG gniesses and high-pressure granulites (Granulite facies)

Middle Wutai Subgroup supracrustal sequence (Greenschist facies)

Zhujiafang shear zone

Southern Hengshan TTG gniesses and garnet amphibolites (High and Low amphibolite facies)

Wutai granitoid rocks

Weak-deformed mafic dykes

Lower Wutai Subgroup supracrustal sequence (High and Low amphibolite facies)

Proterozoic/Mesozoic cover

Sample location

(a)

(b)

g

pl 4cm

(c)

(d)

1cm

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(b)

cpx g pl

bi

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hb

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0.5mm

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rim rim mantle

0.0

1.0

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2.0

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4.0

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rim

(mm) 5.0

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cp

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gh

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ig

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bi g cpx hb ru

bi

bi

ilm ru hb px b ilm h gc bi px ep i g c lm b ui ep br

ep

ep

ep bi hb ilm

g cpx

c0

gc

px

hb

ru

ep bi g cpx hb ru

b ru

gc px hb ru

h g cpx mu bi

bi

ep mu bi g cpx hb ru

mu g cpx hb ru

c0

ep mu g cpx hb ru

bi

P(kbar)

mu g cpx ru

T( °C)

0.37

(a) H934 Intercepts at 1803±76 Ma MSWD = 0.02

0.35

206Pb/238U

(b) H934 1940 1900 1860

0.33

1820 1780

1940 1900

0.31

1860

Mean = 1862±11 Ma n=24 MSWD= 0.40

~1.86Ga

1820

~1.80Ga

1780

0.29

1740

4.6

4.8

5.0

5.2

5.4

5.6

207Pb/235U

5.8

6.0

(c) H1024

La

Ce

Pr

Nd Sm Eu Gd Tb Dy

Ho

Er Tm Yb Lu

(d) H1024

0.36

Intercepts at 1835.2±6.4 [±7.7] Ma MSWD = 1.5

1900 1800

1700

Mean = 1837.3±6.8 n=30 MSWD= 0.92

1600

0.28

box heights are 2σ

1500

1900

Pb207/Pb206

206Pb/238U

0.32

1860

0.24

metamorphic zircon magmatic core of zircon

1820

0.20

1780

3.0

3.4

3.8

4.2

4.6

207Pb/235U

5.0

5.4

5.8

La

Ce

Pr

Nd Sm Eu Gd Tb Dy

Ho

Er Tm Yb Lu

25

H1024 H934

Frequency (N)

20

15

10

5

0

600

625

650

675

700

T (°C)

725

750

Basic solidus

Gran itic solid us

Ja

u +Q te ite lbi e A d

1

15

integrated path of N. Hengshan

o-0

tz ar

Zha

P(kbar)

Eclogite facies

M2 (HPG)

H944

H93

ky l sil

4

Amphibolite M2 (MPG)

10

Epidoteamphibolite H947 M3 M4

5

Granulite

ky and s a n di l l

500

600

700

800

T(°C)

900

~1

.9

5

2

ky l sil

Depth (km)

m 60

40

Wutai

m

i ph

bo

e lit

~1.9

A

Epidoteamphibolite 30°C

5

3 S.Hengshan

1

10

5

C/k

N. Hengshan

4

Ga

15°

2 Ga

tz

Grani tic solidu s

P(kbar)

J

r ua +Q te bite i l e ad A

6 Basic solidus

Eclogite facies

15

1.85-1.92 Ga

Granulite 20

/km

ky and 8 Ga ~1.7 s a n di l l

500

600

700

800

T(°C)

900

Table 1. Selected microprobe analyses for coarse-grained HP mafic granulite sample H944 from northern Hengshan Mineral

g

g

cpx

cpx

pl

pl

pl

pl

pl

hb

g-C

g

g-M

g-R

sym

corona

corona

sym-C

sym-R

matrix-C

matrix-R

matrix

SiO2

37.52

37.92

37.36

50.82

51.66

54.4

60.43

56.61

61.79

58.28

41.92

TiO2

0.06

0.12

0.07

0.09

0.1

0

0.01

0.03

0.03

0

1.75

Al2O3

20.94

21.09

20.79

1.09

1.2

29.16

24.89

27.08

24.05

26.11

10.89

Cr2O3

0

0

0.01

0

0.04

0

0.03

0

0.03

0.02

0

Fe2O3 FeO MnO MgO CaO Na2O

1.09 24.21 2.83 1.99 11.12 0.01

0.98 23.7 0.56 3.16 12.11 0

1.8 25.44 1.15 3.9 8.7 0

4.87 7.87 0.3 12.48 22.62 0.3

1.45 12.1 0.28 11.36 21.53 0.35

0.43 0 0.01 0 10.53 5.72

0.1 0 0 0 6.5 7.67

0.39 0 0.03 0 8.76 6.97

0.1 0 0.05 0 5.75 8.51

0.16 0 0.04 0.01 8.59 6.68

2.44 16.59 0.12 9.28 11.43 1.65

K2O Total

0 99.77

0.01 99.66

0.01 99.23

0.02 100.46

0.01 100.08

0.18 100.43

0.4 100.03

0.17 100.04

0.34 100.65

0.24 100.13

1.25 97.32

Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K O Sum

2.983 0.004 1.963 0 0.065 1.61 0.191 0.236 0.947 0.002 0 12 8

2.985 0.007 1.958 0 0.058 1.561 0.037 0.371 1.022 0 0.001 12 8

2.969 0.004 1.947 0.001 0.107 1.691 0.077 0.462 0.741 0 0.001 12 8

1.916 0.003 0.048 0 0.138 0.248 0.01 0.701 0.914 0.022 0.001 6 4

1.962 0.003 0.054 0.001 0.041 0.384 0.009 0.643 0.876 0.026 0 6 4

2.448 0 1.547 0 0.015 0 0 0 0.508 0.499 0.01 8 5.026

2.69 0 1.306 0.001 0.003 0 0 0 0.31 0.662 0.023 8 4.996

2.546 0.001 1.436 0 0.013 0 0.001 0 0.422 0.608 0.01 8 5.037

2.731 0.001 1.253 0.001 0.003 0 0.002 0 0.272 0.729 0.019 8 5.013

2.607 0 1.377 0.001 0.005 0 0.002 0.001 0.412 0.579 0.014 8 4.998

6.388 0.201 1.956 0 0.279 2.115 0.015 2.108 1.866 0.487 0.243 23 15.658

x(phase) 0.08 0.13 0.16 0.74 0.63 0.50 0.31 0.41 0.27 0.41 y(phase) 0.34 0.35 0.26 g: x=m(g)=Mg/(Fe2++Mg+Ca), y=c(g)=Ca/(Fe2++Mg+Ca); cpx: x(cpx)=Mg/(Fe2++Mg); pl: XAn(pl)= Ca/(K+Na+Ca). ‘-C’, ‘-M’, and ‘-R’ represent core, mantle and rim of relevant minerals.

Table 2. Selected microprobe analyses for fine-grained HP mafic granulite sample H934 from northern Hengshan Minera l

g g-C

g

g

g-M

g-R

cpx

cpx

pl

pl

pl

sym

coron a

inclusio n

matrixC

matrixR

51.26

60.08

60.18

56.17

pl

pl

sym

coron a

opx coron a

overpri nt

hb

56.95

55.53

49.83

43.88

TiO2

0.16

0.12

0.06

51.6 3 0.07

0.09

0.11

0

0

0.06

0.02

0

1.29

Al2O3

21.39

21.48

21.22

0.85

1.2

23.61

24.75

26.23

27.38

27.76

0.74

9.82

Cr2O3

0.01

0

0

0

0.01

0.03

0

0

0

0.04

0

0

Fe2O3

0.68

1.03

1.43

0.8

1.64

0.11

0.07

0.31

0.58

1.03

2.88

FeO

26.42

26.4

28.36

MnO

1.67

0.6

0.51

MgO

2.79

4.06

4.22

CaO

9.59

8.82

7.16

Na2O

0.01

0.01

0

1.94 11.1 8 0.1 11.3 9 22.3 9 0.33

K2O

0.01 100.6 2

0.02 100.6 3

0 101.0 2

0 99.8 8

Si

2.98

2.975

2.977

Ti

0.009

0.007

0.004

Al

1.983

1.978

1.956

Cr

SiO2

Total

37.89

38.09

38.07

0.001

0

0

3+

0.04

0.061

0.083

2+

Fe

1.738

1.725

1.855

Mn

0.111

0.04

0.034

Mg

0.327

0.473

0.492

Ca

0.808

0.738

0.6

Na

0.002

0.002

0

K O Sum

0.001 12 8

0.002 12 8

0 12 8

Fe

1.96 3 0.00 2 0.03 8 0 0.05 6 0.35 6 0.00 3 0.64 5 0.91 2 0.02 4 0 6 4

13.29

0

0

0

0

0

32.87

17.03

0.11

0.03

0.01

0

0.02

0

0.45

0.05

10.52

0.52

0.03

0

0.02

0

14.34

8.97

21.54

5.84

6.46

9.28

8.9

10

0.55

11.27

0.35

8.27

7.88

6.26

7.02

5.99

0

1.36

0

0.07

0.18

0.16

0.48

100.2

99.6

98.17

0.26 100.1 8

0

99.17

0.12 100.7 8

99.8

97.03

1.972

2.684

2.69

2.569

2.542

2.502

1.968

6.654

0.003

0.004

0

0

0.002

0.001

0

0.147

0.054

1.243

1.304

1.414

1.441

1.474

0.034

1.755

0

0.001

0

0

0

0.001

0

0

0.023

0.055

0.004

0.002

0.01

0.02

0.03

0.329

0.428

0

0

0

0

0

1.085

2.158

0.004

0.001

0

0

0.001

0

0.015

0.006

0.603

0.035

0.002

0

0.001

0

0.844

2.028

0.888

0.28

0.309

0.455

0.426

0.483

0.023

1.831

0.026

0.716

0.683

0.555

0.608

0.523

0

0.4

0 6 4

0.004 8 5.023

0.01 8 5.003

0.009 8 5.005

0.007 8 5.038

0.015 8 5.019

0 6 4

0.093 23 15.401

x(phas 0.11 0.16 0.17 0.64 0.59 0.28 0.31 0.45 0.41 0.47 0.44 e) y(phas 0.28 0.25 0.20 e) 2+ g: x=m(g)=Mg/(Fe +Mg+Ca), y=c(g)=Ca/(Fe2++Mg+Ca); cpx: x(cpx)=Mg/(Fe2++Mg); opx: x(opx)=Mg/(Fe2++Mg); pl: XAn(pl)= Ca/(K+Na+Ca). ‘-C’, ‘-M’, and ‘-R’ represent core, mantle and rim of relevant minerals.

Table 3. Selected microprobe analyses for two-pyroxene granulite sample H947 from northern Hengshan Mineral

g

g

cpx

cpx

opx

relic

relic

matrix

SiO2

38.29

37.87

49.78

matrix with ilm 51.57

TiO2

0.04

0.01

0.25

0.11

Al2O3

21.38

21

2.81

1.23

Cr2O3

0.01

0.07

0

Fe2O3 FeO MnO MgO CaO Na2O

0.68 25.75 2.62 4.3 7.58 0.01

0.58 26.1 2.63 4.27 6.95 0

5.88 7.85 0.34 13.35 20.67 0.27

K2O Total

0 100.66

0 99.49

0.01 101.21

Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na K O Sum

2.993 0.002 1.97 0.001 0.04 1.683 0.173 0.501 0.635 0.002 0 12 8

3 0.001 1.961 0.004 0.034 1.73 0.176 0.504 0.59 0 0 12 8

1.859 0.007 0.124 0 0.165 0.245 0.011 0.743 0.827 0.02 0 6 4

opx

pl

pl

pl

pl

hb

corona

matrix-C

matrix-R

matrix 41.2

matrix

+pl sym

50.63

50.71

+opx sym 46.07

54.35

55.65

48.6

0.03

0.01

0.02

0

0.03

0

2.34

0.96

1.55

34.19

28.73

27.58

33.3

11.83

0

0

0.05

0

0.04

0.01

0

0

1.13 10.24 0.34 11.92 22.13 0.34

0.29 29.24 0.9 16.65 0.58 0

2.37 27.25 0.91 17.45 0.8 0.07

0.62 0 0 0.04 17.13 1.78

0.17 0 0 0 10.95 5.43

0.14 0 0.05 0.01 9.81 6.12

0.33 0 0.02 0.01 15.88 2.63

2.72 15.96 0.16 9.31 11.52 1.35

0 99.01

0 99.28

0.01 101.18

0.07 99.92

0.28 99.95

0.25 99.65

0.08 100.85

1.68 98.07

1.966 0.003 0.055 0 0.032 0.327 0.011 0.677 0.904 0.025 0 6 4

1.973 0.001 0.044 0 0.008 0.953 0.03 0.967 0.024 0 0 6 4

1.933 0 0.07 0.002 0.068 0.869 0.029 0.991 0.033 0.005 0 6 4

2.124 0.001 1.858 0 0.022 0 0 0.003 0.846 0.159 0.004 8 5.017

2.458 0 1.532 0.001 0.006 0 0 0 0.531 0.476 0.016 8 5.019

2.516 0.001 1.47 0 0.005 0 0.002 0.001 0.475 0.536 0.014 8 5.021

2.208 0 1.783 0 0.011 0 0.001 0.001 0.773 0.232 0.005 8 5.013

6.234 0.266 2.11 0 0.310 2.019 0.021 2.1 1.868 0.396 0.324 23 15.649

x(phase) 0.18 0.18 0.75 0.67 0.50 0.53 83.8 51.9 0.46 0.77 y(phase) 0.23 0.21 g: x=m(g)=Mg/(Fe2++Mg+Ca), y=c(g)=Ca/(Fe2++Mg+Ca); cpx: x(cpx)=Mg/(Fe2++Mg); opx: x(opx)=Mg/(Fe2++Mg); pl: XAn(pl)= Ca/(K+Na+Ca). ‘-C’, ‘-M’, and ‘-R’ represent core, mantle and rim of relevant minerals.

Table 4. Bulk-rock compositions of mafic granulites from northern Hengshan XRF whole rock compositions (wt.%) Sample

SiO2

Al2O3

Fe2O3

FeO

MgO

CaO

Na2O

K2O

TiO2

MnO

P2O5

LOI

Mg#

H944

49.82

13.17

3.12

12.18

6.02

8.54

2.00

1.07

1.60

0.24

0.25

0.60

42

H934

51.44

13.25

2.35

14.88

4.27

7.37

1.80

0.11

2.42

0.26

0.30

0

31

H947

48.19

14.53

1.75

12.00

8.27

10.86

1.82

0.17

0.92

0.23

0.09

0

52

Normalized molar proportion used for phase equilibria modelling Sample

Figs

H2O

SiO2

Al2O3

CaO

MgO

FeO

K2O

Na2O

TiO2

O

H944

Fig.6

5.80

50.60

7.85

8.72

9.15

12.78

0.70

1.98

1.23

1.20

H934

Fig.7

3.41

54.20

8.18

7.65

6.74

15.04

0.08

1.85

1.93

0.94

H947

Fig.8

5.08

48.12

8.52

11.30

12.38

11.38

0.11

1.77

0.70

0.66

LOI = Loss on Ignition.

Highlights

► HP granulites record prograde path before inferred peak stage 760–820 °C/15 kbar. ► Decompression to Tmax stage 840–860 °C/ 6–8 kbar, followed by isobaric cooling. ► Metamorphic ages 1.90–1.83 Ga of the granulites represent the post-orogenic cooling.