Petrological and geochronological evidence for Paleoproterozoic granulite-facies metamorphism of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton

Accepted Manuscript Petrological and geochronological evidence for Paleoproterozoic granulite-facies metamorphism of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton Ping-Hua Liu, Fu-Lai Liu, Zhong-Hua Tian, Jia Cai, Lei Ji, Fang Wang PII: DOI: Reference:

S0301-9268(18)30557-6 https://doi.org/10.1016/j.precamres.2019.03.002 PRECAM 5301

To appear in:

Precambrian Research

Received Date: Revised Date: Accepted Date:

28 October 2018 26 February 2019 10 March 2019

Please cite this article as: P-H. Liu, F-L. Liu, Z-H. Tian, J. Cai, L. Ji, F. Wang, Petrological and geochronological evidence for Paleoproterozoic granulite-facies metamorphism of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton, Precambrian Research (2019), doi: https://doi.org/10.1016/j.precamres.2019.03.002

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Petrological and geochronological evidence for Paleoproterozoic granulite-facies metamorphism of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton

Ping-Hua Liua, b,*, Fu-Lai Liua, b, Zhong-Hua Tiana, b, Jia Caia, b, Lei Jia, b, Fang Wanga, b a

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

b

Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, China

*

Corresponding author: Dr. Ping-Hua Liu

Institute of Geology, Chinese Academy of Geological Sciences 26 Baiwanzhuang Road, Beijing 100037, China E-mail address: [email protected] Tel: + 86 10 15011557374; fax: + 86 10 68994781

Abstract To better understand the formation of high-temperature metamorphic rocks, we present a detailed petrological and geochronological study of recently discovered cordierite-bearing granulites of the South Liaohe Group in the Jiao-Liao-Ji Belt, North China Craton. Petrographic observations indicate that four distinct mineral assemblages are present in the granulites: pre-peak amphibolite facies (M1), peak granulite facies (M2), post-peak decompression (M3), and late cooling retrogression (M4). M1 is preserved as fine-grained inclusions in the cores of garnet grains, represented by quartz + plagioclase + biotite + ilmenite. M2 is interpreted to have comprised garnet + sillimanite + plagioclase + quartz + biotite + ilmenite + melt. M3 was characterized by the formation of cordierite + sillimanite symplectites and cordierite + quartz coronas replacing garnet. M4 is indicated by the formation of 1

staurolite, accompanied by the crystallization of melt. A combination of multi-equilibria geothermobarometers and pseudosection modeling constrains the P–T conditions of the M1, M2, M3, and M4 stages to P = 0.66–0.71 GPa and T = 620–650°C, P = 0.96–1.10 GPa and T = 790–840°C, P = 0.62–0.65 GPa and T = 725–785 °C, and P = 0.43–0.55 GPa and T = 595–625°C, respectively. Zircon and monazite U–Pb dating yields three distinct and meaningful ages for the granulites: (1) 2200–2100 Ma for the granulite protoliths, (2) ca. 1945 Ma for the near-peak metamorphism, and (3) 1851–1839 Ma for the post-peak and late retrogressive metamorphism. Thus, a clockwise P–T–t path is determined for the cordierite-bearing granulites of the South Liaohe Group. Petrological and geochronological evidence from regional metamorphic rocks in the Jiao-Liao-Ji Belt suggests that the belt experienced a continuous orogenesis from 1950 to 1800 Ma. Keywords: Granulite-facies metamorphism; South Liaohe Group; P–T–t path; Jiao-Liao-Ji Belt; North China Craton

1. Introduction High-grade granulite-facies rocks, and particularly pelitic and semi-pelitic granulites, are an important component of orogenic belts worldwide (e.g., Kohn, 2014; Harley, 1989; O’Brien, 2008; O’Brien and Rötzler, 2003). In most cases, pelitic and semi-pelitic

granulites

record

clockwise

P–T–t

paths

involving

post-peak

near-isothermal decompression and near-isobaric cooling (e.g., Cai et al., 2014, 2015, 2017a,b,c; Carswell and O’Brien, 1993; Clarke et al., 2000; Harley, 1989; Kohn, 2014; Tam et al., 2012a,b; Nicoli et al., 2015; Yin et al., 2014, 2015; Zhang et al., 2015, 2017). Compared with mafic granulites, which can form in both anorogenic and orogenic settings (O’Brien and Rötzler, 2003), pelitic and semi-pelitic granulites with

2

clockwise P–T–t paths are characteristic of an orogenic setting, given that subduction and collision are required to transport pelitic and semi-pelitic sedimentary protoliths to middle–lower crustal depths where they experience granulite-facies metamorphism. Therefore, pelitic and semi-pelitic granulites with clockwise P–T–t paths can be regarded as hallmarks of subduction and collisional tectonics (e.g., Carswell and O’Brien, 1993; Harley, 1989; Kohn, 2014; Nicoli et al., 2015; O’Brien and Rötzler, 2003). In the North China Craton (NCC), extensive Paleoproterozoic mafic, pelitic, and semi-pelitic granulites are present in the Trans-North China Orogen (TNCO), the Khondalite Belt (KB), and the Jiao-Liao-Ji Belt (JLJB) (Guo et al., 2012; Zhao et al., 2001, 2012; Zhai et al., 1993; Zhou et al., 2004, 2008, 2017). In the past decade, various studies have examined the metamorphic P–T–t paths of these mafic, pelitic, and semi-pelitic granulites from the NCC (e.g., Duan et al., 2015, 2017; Qian et al., 2018; Tang et al., 2017; Zhao et al., 2012; Zhou et al., 2017). Combined with petrological, structural, geochemical, and geochronological data, these studies have led to a broad consensus that the NCC underwent subduction and collisional processes that led to the formation of the extensive mafic, pelitic, and semi-pelitic granulites during 1950–1800 Ma. However, the spatial–temporal evolution and tectonic setting of the Paleoproterozoic high-grade granulite-facies rocks and related Paleoproterozoic tectonic belts remain controversial (Wei et al., 2018; Zhao et al., 2012; Zhai and Santosh, 2011; Zhao and Zhai, 2013; Zhou et al., 2017). In particular, the tectonic setting of the Paleoproterozoic JLJB is a matter of debate (Li and Zhao, 2007; Li et al.,

3

2011, 2012; Liu et al., 2015a; Wu et al., 2016; Zhao et al., 2012). Some studies have proposed that the formation and evolution of the JLJB involved the opening and closing of an intra-continental rift in the Eastern Block of the NCC (Li et al., 2001, 2003, 2004, 2005, 2006, 2011, 2012; Peng et al., 2014; Wang et al., 2016, 2017b; Zhang et al., 1988), whereas others consider that the JLJB involved the development of an island arc and its collision with continental blocks during 1950–1800 Ma (Bai, 1993; Bai et al., 1998; Chen et al., 2016; Faure et al., 2004; Lu et al., 2006; Li et al., 2018a,b; Xu et al., 2018a,b). These differing views reflect a paucity of geological data for the JLJB. For example, it is widely accepted that the JLJB can be subdivided into northern and southern zones (Li et al., 2011; Zhao et al., 2012), and that these zones are characterized by clockwise (northern) and anticlockwise (southern) P–T–t paths (He and Ye, 1998; Li et al., 2001; Lu et al., 1996; Zhao et al., 2012). However, there remains considerable uncertainty over the P–T–t path and precise timing of metamorphism in the Liaohe Group within the JLJB, because the P–T conditions have only been estimated using conventional geothermobarometers (Zhao et al., 2012). In this paper, we report on newly discovered cordierite-bearing (semi-pelitic) granulites from the South Liaohe Group within the JLJB, and present detailed petrological, mineralogical, and geochronological data for these granulites. The aim is to clarify the metamorphic P–T conditions and ages of the granulites, and compare their P–T–t paths with other regional metamorphic rocks in the JLJB. In particular, we derive phase equilibria constraints on the P–T evolution using the latest version of the Perple_X software package (Connolly, 1990, 2005, 2009; version 6.6.7) using the

4

internally consistent thermodynamic database of Holland and Powell (2011) and updated (hp11ver.dat file) mineral activity–composition (a–x) relationships (White et al., 2014). We also present high-precision laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U–Pb ages, rare earth element (REE) data, and mineral inclusions data for zircon and monazite to constrain the timing of the different metamorphic stages recorded by the granulites. Finally, we integrate our results with existing data to reconstruct a continuous and complete P–T–t path for the granulites and constrain the relationship between their metamorphic evolution and Paleoproterozoic orogenesis of the JLJB.

2. Geological background 2.1 Regional geological setting The NCC, which is the largest craton in China with a total area of ca. 1.5  106 km2, contains Archean to Paleoproterozoic basement that can be divided into the Eastern and Western blocks (e.g., Zhao et al., 2005, 2012). The basement of the Eastern Block can be subdivided into the northwestern Longgang Block and southeastern Langrim Block, separated by the Paleoproterozoic JLJB (Fig. 1; Zhao et al., 2012; Zhao and Zhai, 2013). The boundaries of these tectonic units are characterized by a series of regional fault zones or ductile shear zones (Li et al., 2011, 2012; Tian et al., 2017; Zhao et al., 2012). The JLJB trends approximately NE–SW, is 100–200 km wide, and extends for 1200 km from southern Jilin Province through Liaodong Peninsula and into Jiaodong

5

Peninsula. The JLJB probably extends into North Korea to the northeast and crosses the Tan-Lu Fault into the Wuhe area of Anhui Province to the southwest (Fig.1; Guo and Li, 2009; Liu et al., 2018; Zhao et al., 2012). The belt consists mainly of voluminous metasedimentary, and associated metavolcanic rocks, including the Macheonayeong Group in North Korea, the Ji’an and Laoling groups in southern Jilin Province, the North and South Liaohe groups on the Liaodong Peninsula, the Fenzishan and Jingshan groups on the Jiaodong Peninsula, and the Wuhe Group in Anhui Province (Fig. 1; Li et al., 2011; Zhao et al., 2005, 2012). In addition, voluminous 2900–2500 Ma reworked dioritic and tonalite–trondhjemite–granodiorite (TTG) gneisses and Paleoproterozoic granitoid and mafic intrusions are widely distributed throughout the JLJB (Jahn et al., 2008; Xie et al., 2014; Liu et al., 2013a, 2015a; Zhang et al., 1988). The Paleoproterozoic granitoid plutons are 2200–2100 Ma granites and 1900–1800 Ma alkaline syenites and porphyritic granites (Hao et al., 2004; Lan et al., 2015; Li and Zhao, 2007; Li et al., 2018a; Liu et al., 2017a; Wang et al., 2017d; Yang et al., 2007, 2015a,b, 2017), and the mafic intrusions consist of gabbros and dolerites (Bai et al., 1998; Liu et al., 2013b,c, 2017c,d; Meng et al., 2014; Wang et al., 2016; Xu et al., 2018a,b; Yuan et al., 2015; Zhang et al., 1988). Geochronological data show that most of the metasedimentary–metavolcanic successions in the JLJB formed during 2200–2100 Ma and were metamorphosed and deformed from 1950–1850 Ma (Bai et al., 1998; Cai et al., 2017c; Liu et al., 1998, 2010a, 2013c, 2015a, 2017d; Lu et al., 1996; Tam et al., 2012a,b,c; Tian et al., 2017; Wang et al., 2017a; Zhang et al.,

6

1988).

2.2 Geology of the Sanjiazi area The Sanjiazi area is located in the northern segment of Liaodong Peninsula (Supplementary

Fig.

S1)

and

comprises

Paleoproterozoic

metasedimentary–metavolcanic successions and related igneous rocks (Liu et al., 2017c; Yang et al., 2017), together with Mesozoic granitic plutons (Wang et al., 2017a). Based on lithological, stratigraphic, metamorphic, and tectonic characteristics, the metasedimentary–metavolcanic successions in the Sanjiazi area can be further subdivided from base to top into the Li’eryu, Gaojiayu, Dashiqiao, and Gaixian formations (Li et al., 2011, 2018b; Lu et al., 2006; Luo et al., 2004, 2008; Wang et al., 2017a; Zhang et al., 1988). The lowermost Li’eryu and Gaojiayu formations in the Sanjiazi area consist mainly of metamorphosed boron-bearing volcanic–sedimentary successions through to fine-grained felsic–pelitic gneisses, amphibolites, and mica–quartz schists. The Dashiqiao Formation is composed mainly of dolomitic marbles intercalated with thin layers of carbonaceous slates and pelitic schists (Fig. 2; Bai et al., 1998; Zhang et al., 1988). The Gaixian Group in the Sanjiazi area comprises mainly phyllites and garnetor sillimanite-bearing pelitic schists intercalated with quartzites and marbles, which are equivalent to those of the Dadongcha and Dalizi formations in the Ji’an and Laoling groups in southern Jilin Province. Recent zircon U–Pb dating has shown that (i) the detrital zircons from the

7

Li’eryu, Gaojiayu, Dashiqiao, and Gaixian formations in the Sanjiazi area have 207

Pb/206Pb age peaks at ~2130 Ma, ~2520 Ma, ~2160 Ma, and ~2500 Ma, and ~2100

Ma respectively (Wang et al., 2017a); (ii) the major sources of the South Liaohe Group in the Sanjiazi area are the 2200–2100 Ma Liaoji Granitoids and 2550–2500 Ma basement rocks (Wang et al., 2017a); and (iii) the South Liaohe Group in the Sanjiazi area was deposited at 2200–2100 Ma, based on the ages of youngest detrital zircons and later 2200–2100 Ma meta-mafic rocks that intruded the metasedimentary rocks of the South Liaohe Group (Liu et al., 2017c; Wang et al., 2017a). In addition, detailed field mapping and structural analysis has shown that the South Liaohe Group in the Sanjiazi area has undergone at least four phases of structural deformation (Tian et al., 2017). D1 structures include a bedding-parallel S1 foliation, a penetrative axial plane foliation (S1), and micro- to mesoscopic folds (F1). D2 structures include a crenulation cleavage and a large-scale ductile shear thrust. D3 deformation produced regional gentle re-folds related to 1900–1800 Ma post-collisional exhumation of the JLJB (Li et al., 2005). D4 deformation resulted in a weak foliation that can be correlated with the Late Cretaceous extension that affected the entire eastern NCC (Lin et al., 2013).

3. Analytical methods 3.1 Whole-rock major element analysis A fresh sample was selected for geochemical analysis. The whole-rock sample was crushed in an agate mill to ≤200 mesh, following the removal of altered surfaces.

8

Major elements were analyzed using an Axios X-ray fluorescence (XRF) spectrometer (XR-1500 spectrometer) on fused glass disks produced with a Claisse M4 gas fluxer at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing, China. FeO contents were determined by colorimetry (chemical titration with potassium permanganate), on solutions produced by digestion in a multi-acid mixture (HF–H2SO4–H3BO4) in a non-oxidizing environment. The major element data are listed in Supplementary Table S1.

3.2 Electron microprobe mineral analysis Chemical compositions of the major minerals in the semi-pelitic granulite sample 17KD97-1 were determined using a JEOL JXA-8100 electron microprobe (EMP) at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. The operating conditions were 15 kV accelerating voltage, 10 nA beam current, and 1–5 μm beam diameter. Natural silicate and oxide standards were used for calibration and the raw data were corrected using the ZAF program. Back-scattered electron (BSE) images were taken to document specific textural features. X-ray elemental maps of garnet were also generated with the EMP. Structural formulae of minerals and molar fractions of mineral components were calculated from EMP analyses using the AX computer program (Qian et al., 2018). Mineral abbreviations are after Whitney and Evans (2010).

3.3 Cathodoluminescence and back-scattered electron imaging

9

Zircon and monazite grains were separated from 10 kg of the semi-pelitic sample using standard heavy liquid and magnetic techniques, with subsequent handpicking under a binocular microscope. The selected crystals were embedded in 25 mm epoxy disks and polished down to approximately half their thickness. All of the zircons and monazites were subjected to transmitted and reflected light photomicrography, mineral inclusion analysis, cathodoluminescence (CL) imaging of zircons, and BSE imaging of monazites to reveal internal structures prior to U–Th–Pb dating. Mineral inclusions within and on the exposed surfaces of the zircons and monazites were identified using laser Raman spectroscopy (RANISHAW RM-1000) with the 514.5 nm line of an Ar-ion laser and by scanning electron microscopy (SEM; ZEISS ULTRA-PLUS) equipped with a 50 mm2 Max OXFORD energy dispersive spectrometer at the Institute of Geology, Chinese Academy of Geological Sciences. The internal zoning of the zircon and monazite grains was revealed by CL and BSE imaging, respectively.

3.4 LA–ICP–MS zircon and monazite U–Pb dating, and rare earth element analysis U–Pb dating and REE analyses of zircon and monazite were conducted simultaneously by LA–ICP–MS at Wuhan Sample Solution Analytical Technology Company, Wuhan, China. Detailed operating conditions for the laser ablation system, ICP–MS instrument, and data reduction are given by Liu et al. (2010b). Laser ablation was performed using a GeoLas 2005. An Agilent 7700e ICP–MS instrument was used to measure ion signal intensities. Helium was used as a carrier gas and Ar was used as the make–up gas and mixed with the carrier gas via a T-connector before entering the ICP–MS. A “wire” signal smoothing device is part of this laser ablation system, 10

which produces smooth signals even at very low laser repetition rates down to 1 Hz (Hu et al., 2012). Each analysis involved a background measurement of 20–30 s (gas blank) followed by 50 s of data acquisition whilst ablating the sample. In-house Excel-based software ICPMSDataCal (Ver. 10.0) was used to perform off-line selection and integration of background and analyte signals, and time-drift correction and quantitative calibration for the trace element analysis and U–Pb dating (Liu et al., 2010b; Zong et al., 2017). Zircon 91500 and monazite 44069 were used as external standards for U–Pb dating, and were analyzed twice every five analyses. Time-dependent drift in U–Th–Pb isotopic ratios was corrected using linear interpolation (with time) for every five analyses according to the measured 91500 and 44069 analyses (Liu et al., 2010b). Concordia diagrams were plotted and weighted-mean calculations made using Isoplot/Ex_ver3 (Ludwig, 2003).

4. Sample descriptions and petrography The studied cordierite-bearing granulites were collected from the Dashiqiao Formation of the South Liaohe Group in the Sanjiazi area (Fig. 2). These granulites with deformed granitic leucosomes are gray to dark gray in color, and display lepidoblastic fabrics and a gneissic structure defined by the preferred alignment of biotite and needle-like sillimanite (Supplementary Fig. S2). Two granulite samples were selected from the Sanjiazi area for detailed petrographic studies (Fig. 3). The samples consist mainly of garnet, biotite, cordierite, sillimanite, plagioclase, staurolite,

11

quartz, and Fe–Ti oxides (ilmenite, magnetite, and rutile), with accessory tourmaline, spinel, chlorite, zircon, apatite, monazite, and xenotime. On the basis of microstructures and reaction textures between mineral phases, four mineral assemblages were recognized in the granulites: pre-peak (M1), peak (M2), post-peak (M3), and late cooling (M4).

4.1 Pre-peak assemblage (M1) The pre-peak assemblage is represented by mineral inclusions preserved in the cores of porphyroblastic garnets (Fig. 4a–b). The fine-grained mineral inclusions are mostly quartz, plagioclase, and ilmenite, with rare biotite. These fine-grained inclusions have lengths of 10–150 μm (Fig. 4a–b). Given these inclusions are only found in the cores of garnet grains, the cores are considered to have developed coevally with the enclosed minerals. Therefore, the inferred mineral assemblage of the M1 stage is garnet and fine-grained inclusions of quartz + plagioclase + biotite + ilmenite.

4.2 Peak assemblage (M2) The peak granulite-facies assemblage is evident from the disappearance of muscovite and staurolite, the appearance of sillimanite + plagioclase + melt, and further growth of garnet, which is represented by inclusion-free garnet or inclusion-free garnet mantle domains (Figs 4–5). Sillimanite is present as large tabular crystals that are commonly partially replaced by late sericite, and plagioclase is large,

12

anhedral, and contains inclusions of fine-grained quartz (Supplementary Fig. S3). Sillimanite and plagioclase are most likely to have been produced from the following dehydration melting reaction (Yardley, 1989; Spear et al., 1999; White et al., 2001): muscovite + biotite + staurolite + quartz → sillimanite + feldspar + garnet + melt (1)

4.3 Post-peak assemblage (M3) The

post-peak,

near-isothermal

decompression

assemblage

(M3)

was

characterized by the formation of cordierite (Figs 5–6), which contains garnet, cordierite, sillimanite, biotite, plagioclase, quartz, ilmenite, magnetite, and melt. Two textural types of cordierite were identified in the Sanjiazi granulites (Supplementary Fig. S3). The first type is found as a corona (with or without quartz and sillimanite) surrounding garnet grains (Figs 5–6). Similar decompression microstructures in other pelitic and semi-pelitic granulites have been suggested to form by the following reaction (e.g., Cenki et al., 2002; Currie, 1971; Harley and Hensen, 1990; Santosh, 1987; Spear et al., 1999; Yin et al., 2015): garnet + sillimanite + quartz → cordierite + melt

(2)

The second type of cordierite occurs as fibrous symplectic intergrowths with sillimanite and as large prismatic single cordierite crystals (Supplementary Fig. S2). Petrographic investigations revealed that the fibrous sillimanite + cordierite symplectites occur in a garnet-free matrix (Supplementary Fig. S2), which suggests that the fibrous sillimanite + cordierite symplectites might have formed by the following reaction (Breton and Thompson, 1988; Carrington and Harley, 1995; Harris 13

and Massey, 1994; Hensen and Green, 1973; Lu et al., 1996; Patiño Douce and Johnston, 1991; White et al., 2001): biotite + sillimanite + quartz→ cordierite + melt

(3)

Reactions (1)–(3) have been observed in many other granulite-facies pelitic and semi-pelitic gneisses worldwide, suggesting that the host rocks have undergone intensive partial melting (Harley and Hensen, 1990; Kohn, 2014; Morimoto et al., 2004; Spear et al., 1999; Yin et al., 2014, 2015). Furthermore, the following three lines of microstructural evidence imply that partial melting of the Sanjiazi granulites occurred mainly in the post-peak near-isothermal decompression stage. (1) A notable microstructural feature of the Sanjiazi granulites is the presence of large lobate inclusions of quartz and plagioclase in porphyroblastic garnets, which are believed to be solid products of the melting reaction (Guilmette et al., 2011; Groppo et al., 2012; Rubatto et al., 2013; Waters, 2001; Zhang et al., 2017; Figs 4–5). (2) Some microstructures which is consistent with pseudomorphing of melt-filled pores are locally present in cordierite-bearing domains (Fig. 4f), such as thin films of plagioclase and quartz with low dihedral angles adjacent to refractory solids (e.g., Groppo et al., 2012, 2013; Holness and Sawyer, 2008; Holness et al., 2011). (3) Plagioclase + quartz intergrowths are recognized in the 1884–1803 Ma monazites (see below)

that

formed

during

the

near-isothermal

decompression.

Similar

microstructures in other pelitic and semi-pelitic granulites from the high-grade metamorphic complexes have been interpreted as being consistent with back-reactions involving melt crystallization (e.g., Kriegsman and Alvarez-Valero, 2010; Rocha et al.,

14

2017; Waters, 2001).

4.4 Late cooling assemblage (M4) The late cooling assemblage was marked by the formation of staurolite in the intensively retrogressive domains. Supplementary Figures S2–S3 show the retrogressive staurolite overgrowths on biotite and sillimanite, which contain fine-grained quartz and cordierite. In addition, staurolite occurs as the secondary inclusions in the rims of porphyroblastic garnets (Supplementary Fig. S3). In some domains, vermicular intergrowths of rutile are recognized within coarse-grained ilmenite. The symplectitic rutile has been inferred to form during low-pressure

amphibolite-facies

retrogressive

metamorphism

of

eclogites,

amphibolites, and granulites by the following retrogressive reaction (Fig. 5; Yang, 2004; Cruz-Uribe et al., 2014; Guo et al., 2017): ilmenite + oxygen (in fluid) → magnetite + rutile

(5)

Thus, the inferred late cooling assemblage (M4) is represented by garnet + cordierite + staurolite + sillimanite + plagioclase + quartz + magnetite + ilmenite.

5. Mineral chemistry Garnet, biotite, plagioclase, cordierite, sillimanite, and staurolite are the key minerals in the Sanjiazi granulites. Therefore, these minerals were analyzed by EMP, and representative mineral compositions are listed in Supplementary Tables S2–S7 and shown in Figs 7–8.

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5.1 Garnet Garnet compositions are given in Supplementary Table S2 and show in Fig. 7a. Four distinct textural types of garnet were analyzed: type I = garnet cores containing fine-grained mineral inclusions; type II = garnet rims surrounded by cordierite + quartz coronas; type III = garnet rims in contact with biotite, plagioclase, quartz, and sillimanite in the matrix; type IV = garnet inclusions within the metamorphic monazites. Of note, biotite, plagioclase, quartz, and sillimanite in contact with garnet rims (type III) is interpreted as the M3 or M4 mineral assemblage according to the petrographic observations and mineral compositions. All the analyzed garnets from the two samples and monazites are dominated by almandine (69–79 mol. %) and pyrope (16–25 mol. %) components, with minor grossular (2.31–3.19 mol. %) and spessartine (1.76–3.24 mol. %). The garnets from type I to III show a significant decrease in pyrope components, with Prp = 23–25 mol. % in type I and Prp = 16–18 mol. % in type III. Two garnet grains were selected for compositional mapping and Mg, Fe, Mn, and Ca profiling (Fig. 8; Supplementary Fig. S4). The garnets contain abundant fine-grained mineral inclusions in the core, and its rim is surrounded by biotite + quartz + plagioclase. The XGrs [Ca/(Ca+Mn+Mg+Fe)] value of the analyzed garnet is basically constant from core to rim. However, weak compositional zoning, with a flat profile from core to rim and slightly higher XAlm [Fe/(Ca+Mn+Mg+Fe)] and lower XPrp [Mg/(Ca+Mn+Mg+Fe)] of the rim is observed, which is interpreted to be the

16

result of ion diffusion with surrounding ferromagnesian minerals (e.g., biotite) after peak metamorphism (Spear, 1993).

5.2 Plagioclase Five types of plagioclase were recognized according to texture, shape, and grain size: type I = inclusion-type plagioclase within garnet cores; types II and III = cores and rims, respectively, of matrix-type plagioclase in contact with quartz and biotite in the matrix; type IV = melt-type plagioclase that crystallized from melt between other coarse-grained minerals (Fig. 4d); type V = inclusions within the metamorphic monazites. Representative chemical compositions of the five plagioclase types are summarized in Supplementary Table S3 and Fig. 7b. All five types are albite-rich (An18–30 and Ab70–81); however, the cores of matrix-type plagioclase are relatively anorthite-rich (An28–30), whereas the other types have higher albite contents (Ab75–81). As shown in Supplementary Fig. S3, the matrix-type plagioclase (An28–30) is interpreted to as part of the residual mineral assemblage of peak metamorphism, according to our petrographic observations.

5.3 Biotite Representative chemical compositions of biotite are presented in Supplementary Table S4 and Fig. 7c. Four textural types of biotite were analyzed: type I = inclusion-type biotite within garnet cores; type II = biotite in contact with plagioclase, sillimanite, and staurolite in the matrix; type III = symplectitic biotite within the 17

cordierite + sillimanite symplectites in the matrix; type Ⅳ = inclusions within the metamorphic monazites. The different textural types of biotite have different XMg [XMg=Mg/(Mg+Fe)] values (0.56–0.75) and TiO2 contents (0.74–1.71 wt. %), depending on their proximity to other ferromagnesian minerals. Notably, the type II and IV biotites have the lowest XMg values, ranging from 0.56 to 0.62. The XMg values of the type III biotites have a small compositional range (0.64–0.68), whereas those of two inclusion-type biotites within garnet cores show a larger variation (0.67–0.75).

5.4 Cordierite Two textural types of cordierite were analyzed: type I = matrix-type cordierites from cordierite + sillimanite symplectites in the matrix; and type II = cordierite coronas surrounding garnet crystals. Representative compositions of cordierites are listed in Supplementary Table S5 and shown in Fig.7d. The analyzed cordierites are Mg-rich and show a slight difference in XMg values between matrix- and symplectite-type, which range from 0.73 to 0.83 and 0.72 to 0.83, respectively. This probably indicates that they formed under similar metamorphic conditions.

5.5 Other minerals The Sanjiazi granulites also contain staurolite, sillimanite, rutile, ilmenite, magnetite, tourmaline, spinel, chlorite, zircon, and monazite. Representative chemical compositions of staurolites and sillimanites are presented in Supplementary Tables S6–S7, respectively. The analyzed staurolites are relatively Mg-rich with XMg values

18

of 0.22–0.25, whereas the analyzed sillimanites have low FeO contents of 0.46 to 0.65 wt. %.

6. Phase equilibria modeling and P–T calculations The successful application of conventional thermobarometry to high-grade, anatectic rocks is limited by the extensive resetting of garnet and biotite compositions during late cooling and by the fact that most or even all the biotites may be a product of retrogression (Groppo et al., 2010; Harley, 1989; Li and Wei, 2017; Powell and Holland, 2008; Zhang et al., 2017). Recent improvements in thermodynamic databases, melt and solid solution models (White et al., 2007), and the development of methods for taking into account possible melt loss during the rock evolution suggest that P–T pseudosections are the most suitable approach to constrain the P–T evolution of the granulite-facies and anatectic rocks in the NCC (e.g., Cai et al., 2014, 2015, 2017a,b,c; Gou et al., 2016; Guo et al., 2012; Jiao et al., 2013a,b, 2015, 2017; Tam et al., 2012a,b,c). A P–T pseudosection for a semi-pelitic granulite sample (16KD97-1) was modeled with the PERPLE_X computer program package (Connolly, 2009; version from October 2016) and the internally consistent thermodynamic dataset of Holland and Powell (2011) in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O– TiO2–O2 (NCKFMASHTO). The following solid solution models were used: clinopyroxene (Green et al., 2007); garnet and ilmenite (White et al., 2007); orthopyroxene, spinel, staurolite, and cordierite (Holland and Powell, 1998); silicate

19

melt (White et al., 2007), biotite (Tajčmanová et al., 2009), white mica (Coggon and Holland, 2002); feldspar (Holland and Powell, 2003). Pure phases included andalusite, kyanite, sillimanite, quartz, and H2O. The bulk chemical composition of sample 16KD97-1 was determined by wavelength-dispersive XRF spectrometry (Rigaku RIX 2100). The amount of FeO was determined by titration and then Fe2O3 was calculated by difference. H2O was estimated from the loss on ignition. When compared with the average composition of amphibolite-facies semi-pelitic schists (Ague, 1991; Stevens et al., 1997), the compositions of sample 16KD97-1 are consistent with the average composition of amphibolite-facies semi-pelitic schists and imply that the loss of a siliceous melt from the Sanjiazi granulites was limited. Thus, this sample can be used to approximately determine the prograde, peak, and post-peak retrogression evolution. A appropriate H2O and O contents for the P–T pseudosection were evaluated by T–XH2O and P–XO pseudosections (Fig. 9), respectively, because these diagrams allow the effect of these components on the phase equilibria to be assessed over a range of compositions (e.g., Korhonen et al., 2012, 2014). A T–XH2O pseudosection was calculated at 0.55 GPa for a XO value of 0.30 (Fig. 9a). Both the pressure of 0.55 GPa and XO = 0.30 were selected based on repeated phase diagram calculations for the pelitic and semi-pelitic granulite (Cai et al., 2017b; White et al., 2007). This T–XH2O pseudosection was used to determine an appropriate H2O content so that the observed mineral assemblage is stable just above the post-melt-loss solidus, which is assumed to reflect the conditions where the stable

20

assemblage (Grt–Crd–Bt–Pl–Sil–Ilm–Mag–Qz–Liq) would have been in equilibrium with the remaining melt (Korhonen et al., 2012; White et al., 2014). A value for XH2O of 1.70 wt. % was chosen according to the XGrs [Ca/(Ca+Mn+Mg+Fe)] and XPrp [Mg/(Ca+Mn+Mg+Fe)] values of garnet in contact with the cordierite (Fig. 9a). The H2O content corresponding to this XH2O value was used to assess the effect of O using a P–XO pseudosection (Fig. 8b). Figure 9b shows a P–XO pseudosection at 770°C using the adjusted H2O content constrained from Fig. 9a. The temperature of 770°C is consistent with the temperature range of the observed mineral assemblage (Fig. 8). Based on the observation that the ilmenite and magnetite are in contact with cordierite, a XO value of 0.30 was selected as an appropriate O content, which corresponds to the mid-point of the ilmenite–magnetite–cordierite-bearing assemblage (Fig. 9b).

6.1 P–T conditions of the M1 assemblage The calculated P–T pseudosection, which uses the H2O and O contents constrained by the T–XH2O and P–XO pseudosections, respectively, allows the mineral assemblages to be evaluated over a range of P–T conditions. As discussed above, the proposed mineral assemblage of the pre-peak stage (M1) is garnet + biotite + plagioclase + quartz + ilmenite. The pre-peak mineral assemblage is represented by the field of Grt–Bt–Ms–Pl–Qz–Ilm–St–H2O, which occurs between 600–660°C and 0.60–0.85 GPa. In most cases, muscovite and staurolite are two of the key and common pelitic minerals present under medium-pressure amphibolite-facies

21

metamorphic conditions, but they are major reactants in most metamorphic reactions and might have been entirely consumed in the Sanjiazi granulites during peak granulite-facies metamorphism (Kohn, 2014; Spear et al., 1999; Wei et al., 2004; White et al., 2011; Zou et al., 2017). Taking the average condition of peak metamorphism of the staurolite-bearing garnet schists (Richardson, 1969; Wei et al., 2004; White et al., 2007; Zou et al., 2017), the P–T conditions of the pre-peak amphibolite-facies metamorphism of the Sanjiazi granulites can be inferred to be P = 0.66–0.71 GPa and T = 620–650°C (Fig. 10).

6.2 P–T conditions of the M2 assemblage The typical M2 assemblage includes garnet, biotite, sillimanite, plagioclase, ilmenite, quartz, and melt. The observed mineral assemblage matches with the Bt–Sil–Grt–Pl–Ilm–Liq–Qz field of the P–T pseudosection model. In this field, plagioclase shows a negative correlation between the XAn(Pl) [Ca/(Ca+Na+K)] isopleths and pressure. Thus, the P–T conditions for the peak (M2) metamorphism can be well constrained at 0.96–1.10 GPa and 790–840°C by peak XAn(Pl) isopleths that range from 0.28 to 0.30 (Figs 10 and 11a).

6.3 P–T conditions of the M3 assemblage The near-isothermal decompression assemblage (M3) of cordierite + sillimanite + garnet + plagioclase + quartz + ilmenite + magnetite + melt fits the Grt–Bt–Crd–Pl–Sil–Ilm–Mag–Qz–Liq field. In this field, garnet rims in contact with

22

the cordierite-bearing symplectites show a positive correlation between the XPrp (0.22–0.25) values of garnet and pressures. The composition (XPrp = 0.22–0.25) of garnet in contact with the cordierite-bearing symplectite constrains the P–T conditions of the M3 stage to be 0.61–0.64 GPa and 730–790°C in the pseudosection (Figs 10 and

11).

Furthermore,

using

garnet–cordierite–sillimanite–quartz

geothermobarometers (Hensen and Green, 1973; Hutcheson et al., 1974), we obtained a P–T range of P = 0.63–0.66 GPa and T = 720–780°C. Therefore, we estimate the P–T conditions of the post-peak decompression assemblage (M3) to have been P = 0.62–0.65 GPa and T = 725–785°C.

6.4 P–T conditions of the M4 assemblage The inferred late cooling retrogressive assemblage (M4) is represented by garnet + cordierite + staurolite + sillimanite + plagioclase + quartz + magnetite + ilmenite. The corresponding modeled mineral assemblage in the pseudosection is located in the Bt–St–Sil–Crd–Grt–Pl–Ilm–Mag–H2O field with P–T conditions of 0.43–0.55 GPa and 600–630°C. In addition, the metamorphic temperature and pressure of the M4 stage is further constrained at 0.42–0.47 GPa and 590–620°C by the revised Ti-in-biotite and garnet–biotite–plagioclase–quartz geobarometer (Wu and Cheng, 2006; Wu and Chen, 2015). Taking all the above observations into account, the P–T conditions of the late cooling assemblage (M4) are estimated to have been P = 0.43–0.55 GPa and T =595–625°C.

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7. Zircon and monazite geochronology and geochemistry 7.1 Zircon and monazite morphology and structure One sample (16KD97-1) was selected for zircon U–Pb dating. Most zircon grains are euhedral to subhedral, prismatic to round and columnar and purple red in color. The zircons have lengths of 70–150 μm and length-to-width ratios of 2:1 to 1:1 (Fig. 12). In CL images, most zircon grains lack detrital cores and display concentric or sector zonation, which is typical of zircon that grows during melt-present high-grade metamorphism (e.g., Vavra et al., 1999). In contrast, the other grains show clear core–rim structures. They comprise low luminescence and occasionally oscillatory zoned cores, interpreted as detrital grains, and sector-zoned or featureless metamorphic rims that are typically of moderate luminescence (Fig. 12). Numerous mineral inclusions were observed in the dated zircons, including sillimanite, biotite, plagioclase, quartz, and apatite (Fig. 12). Similar to the zircons, most monazites in this sample are round, oval, or irregular in shape, purple red or red in color, 80 to 200 μm in length, and with length-to-width ratios between 1:1 and 3:1. In the BSE images, most of the dated monazites are homogeneous or weakly zoned and exhibit light gray luminescence. They contain inclusions of garnet, sillimanite, biotite, plagioclase, ilmenite, and quartz, but because these phases were present throughout the P–T trajectory, they are not indicative of any specific metamorphic stage (Fig. 13).

7.2 Results of zircon and monazite U–Pb dating

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Three significant age groups were identified in the different domains of zircons from sample 16KD97-1, which are presented graphically on

206

Pb/238U–207Pb/235U

diagrams with 2σ errors in Fig. 14 (data are listed in Supplementary Table S8). Eight spot analyses in the detrital cores yielded high U and Th contents of 125.64–363.96 and 45.66–257.93 ppm, respectively, with relatively high Th/U ratios of 0.26–0.95 (Supplementary Table S8). These zircons have variable 207Pb/206Pb ages of 2522 ± 34 to 2124 ± 41 Ma (Fig. 14). In contrast, six spot analyses on metamorphic domains yielded U and Th contents of 54.63–231.51 and 0.92–11.87 ppm, respectively, with extremely low Th/U ratios of 0.02–0.06 (Supplementary Table S8) and relatively young

207

Pb/206Pb ages of 1954 ± 50 to 1931 ± 66 Ma, with a weighted-mean age of

1945 ± 40 Ma (Fig. 14c). In addition, 57 spot analyses on metamorphic domains yielded U and Th contents of 52.52–312.46 and 0.80–10.79 ppm, respectively, with extremely low Th/U ratios of 0.01–0.02 (Supplementary Table S8) and

207

Pb/206Pb

ages of 1909 ± 50 to 1803 ± 35 Ma, with a weighted-mean age of 1851 ± 13 Ma (Fig. 14d). The U–Pb age data for 33 analyzed spots on the various monazite domains from the Sanjiazi granulite sample 16KD97-1 are given in Supplementary Table S9 and presented graphically on

206

Pb/238U–207Pb/235U diagrams with 2σ errors in Fig. 15a.

Thirty-three spot analyses yielded consistent

207

Pb/206Pb ages of 1884 ± 34 to 1803 ±

29 Ma, with a weighted-mean age of 1839 ± 11 Ma. The age of 1839 ± 11 Ma is within error of the mean age of the second group of metamorphic zircons (1851 ± 13 Ma).

25

7.3 Zircon and monazite REE compositions The REE data for 35 analyzed spots on the different zircon domains from the Sanjiazi granulite sample 16KD97-1 are summarized in Supplementary Table S10 and presented graphically on a REE diagram in Fig. 16. All analyzed zircon grains are enriched in heavy REE (HREE) compared to light REE (LREE), and pronounced positive Ce anomalies (Ce/Ce* = 1–270; Fig.16) and negative Eu anomalies (Eu/Eu* = 0.17–0.82; Fig. 16). Six detrital cores with 207Pb/206Pb ages of 2522 ± 34 to 2124 ± 41 Ma have very high HREE contents compared to metamorphic grains and display prominent positively sloping HREE patterns from Gd to Lu (GdN/LuN = 0.03–0.07), which are typical of zircon growth in garnet-free igneous rocks (e.g., Whitehouse and Platt, 2003; Jiao et al., 2017). In contrast, the younger metamorphic zircon grains with 207

Pb/206Pb ages between 1909 ± 50 and 1803 ± 35 Ma have much flatter HREE

patterns consistent with zircon growth in garnet-bearing rocks, further supporting the interpretation that these are metamorphic zircons. The REE data for 34 analyzed spots on the different monazite domains from the granulite sample 16KD97-1 are given in Supplementary Tables S9 and presented graphically on a REE diagram in Fig. 15b. All the analyses have high HREE contents and exhibit moderate negative Ce anomalies (Ce/Ce* = 0.45–0.48; Fig.15b). REE patterns of the analyzed monazite grains are strongly LREE-enriched, with concentrations of 300,000–400,000 times chondrite values, as compared with HREE, and have variable negative Eu anomalies (Eu/Eu* = 0.42–0.78; Fig. 15b). Yttrium

26

contents of the monazite grains vary over a wide range from 1972 to 20,142 ppm and show a positive correlation with HREE contents and no correlation with ages (Supplementary Table S9).

8. Discussion 8.1 Timing of deposition of the South Liaohe Group The timing of deposition of the South Liaohe Group has conventionally been ascribed to the Paleoproterozoic, based on stratigraphic correlations, and imprecise geochronological data (Bai et al., 1998; Jiang, 1987; Zhang et al., 1988). This conclusion is strongly supported by recent detrital zircon U–Pb dating (e.g., Li et al., 2015; Liu et al., 2015a; Lu et al., 2006; Luo et al., 2004, 2008; Wan et al., 2006; Wang et al., 2017a). Lu et al. (2006) and Wan et al. (2006) reported SHRIMP U–Pb ages for detrital and metamorphic zircons from the South Liaohe Group, and suggested that the South Liaohe Group was deposited between 2120 and 1850 Ma. Luo et al. (2004, 2008) reported LA–ICP–MS U–Pb ages for detrital zircons from the North and South Liaohe groups, and concluded that the protoliths of both groups were deposited from 2000 to 1900 Ma, shortly after emplacement of the 2200–2100 Ma Liaoji Granitoids (Li and Zhao, 2007). Recently, Meng et al. (2013), Liu et al. (2015a), Li et al. (2015), and Wang et al. (2017a) used the same technique and dated >2000 detrital and >500 metamorphic zircons from the Liaohe Group, and suggested that the South and North Liaohe groups were deposited between 2150 and 1950 Ma. In this study, detrital and metamorphic zircons grains from the Dashiqiao Formation of the South Liaohe Group were analyzed by LA–ICP–MS U–Pb dating. The detrital zircon U–Pb ages of the Sanjiazi granulites in the Dashiqiao Formation have two age peaks at ca. 2520 and ca. 2200–2100 Ma, similar to the results of other 27

studies (e.g., Luo et al., 2008; Wang et al., 2017a). This indicates that the provenance of the Dashiqiao Formation is likely the 2550–2500 Ma basement rocks and 2200–2100 Ma granitoids. Combined with the zircon age data from Wang et al. (2017a), the youngest zircon ages of 2200–2100 Ma can be regarded as the maximum depositional age of the Dashiqiao Formation. The oldest metamorphic zircon weighted-mean age of 1945 ± 15 Ma provides the minimum age of deposition. Recently, Liu et al. (2017c) and Xu et al. (2018a,b) undertook detailed geological mapping of the North and South Liaohe groups in the Sanjiazi–Gongchangling area, and found that 2200–2100 Ma meta-mafic rocks (sills, dikes, and veins) are widely intruded into the metasedimentary rocks of the Langzishan, Lieryu, Gaojiayu and Dashiqiao formations. Similarly, silicic volcanic rocks in the Lieryu and Gaojiayu formations of the North Liaohe Group have been identified and igneous zircons from these rocks yield consistent 207Pb/206Pb ages of 2200–2100 Ma (Chen et al., 2017; Xu et al., 2019). Therefore, the South Liaohe Group in the Sanjiazi area is most likely to have been deposited at 2200–2100 Ma, rather than 2100–1950 Ma as suggested in previous studies.

8.2 Linking metamorphic zircon and monazite ages to metamorphic stages Figure17 summarizes the numerous geochronological studies that have been carried out on the Archean to Paleoproterozoic basement rocks of the JLJB, which indicate that the JLJB underwent regional metamorphism during 1950–1800 Ma (e.g., Liu et al., 2015a,b). However, accurate interpretation of these metamorphic ages obtained from accessory phases (zircon or monazite) is challenging as they can grow at different stages, including during subsolidus prograde metamorphism (e.g., Kohn et

28

al., 2005; Wing et al., 2003), anatexis (e.g., Dumond et al., 2015; Rubatto et al., 2013), retrogressive fluid alteration (Taylor et al., 2014), and high-temperature deformation (Erickson et al., 2015). In this study, some general criteria were used to define the timing of zircon and monazite formation with respect to other minerals and metamorphic evolution. Firstly, mineral inclusions and corresponding chemical compositions of zircon and monazite were taken to reflect the mineral assemblage in which the zircon and monazite grew. Secondly, monazite HREE–Y and zircon HREE signatures were used to indicate the relative timing of zircon, monazite, xenotime, and garnet growth. Thirdly, negative Eu anomalies in monazite REE patterns were used as an indicator of feldspar modal abundances (Groppo et al., 2012; Rubatto et al., 2006, 2013). Based on the above criteria, we consider that two distinct stages of metamorphic zircon growth took place in the Sanjiazi granulites (Figs 18–20). The youngest metamorphic ages obtained for the granulites in this study are 1880–1820 Ma, as recorded by zircons and monazites. The following lines of evidence lead us to propose that the 1880–1820 Ma ages most likely represent the timing of the post-peak decompression, melting, and retrogressive cooling. (I) Some of the 1880–1820 Ma monazite and zircon domains contain abundant garnet, sillimanite, biotite, plagioclase, quartz, ilmenite, and magnetite, as well as polymineralic inclusions of feldspar + quartz, which are interpreted as ‘nanogranites’ and represent crystallized melt (Figs 18–20; Cesare et al., 2009, 2015; Ferrero et al. 2012). The nanogranites in monazite were trapped during or after the supra-solidus conditions at ca. 750–700°C. Importantly, the anorthite contents (Supplementary

29

Table S3; An = 23–25) of plagioclase from the 1880–1820 Ma monazite domains are broadly consistent with those of melt-type plagioclases (Supplementary Table S3; An = 22–23). In addition, the pyrope component (16–18 mol. %) of the garnets within the 1880–1820 Ma monazite domains are also broadly consistent with those (17–18 mol. %) of garnet rims in contact with cordierite, biotite, plagioclase, quartz, and sillimanite in the matrix (Supplementary Table S2). (II) In thin sections, anhedral monazite grains that display a similar morphology to the dated monazites often coexist with cordierite, sillimanite, and biotite (Fig. 20). This implies that these dated monazite grains grew during the post-peak decompression stage. (III) These metamorphic ages are consistent with the 1900–1800 Ma metamorphic ages of some retrogressive hypersthene-bearing zircon domains in the Jiaobei high-pressure mafic granulites (Liu et al., 2013b, 2017d). (IV) The metamorphic ages are also consistent with the crystallization ages of granitic leucosomes, S-type granites, syenites, and granitic pegmatites within the JLJB formed during a post-collisional exhumation stage (e.g., Dong et al., 2011; Liu et al., 2014, 2017a,b; Yang et al., 2007, 2015b, 2017). The older 1945 Ma ages of the analyzed granulites are only preserved in the metamorphic zircons. The older metamorphic ages are consistent with the 1950–1900 Ma ages recorded by some near-peak zircon domains with high-pressure granulite-facies mineral inclusions (garnet, clinopyroxene, and rutile) in the Jiaobei high-pressure mafic granulites (Liu et al., 2013b, 2017d). As such, we speculate that the timing of the near-peak, medium-pressure granulite-facies metamorphism of the Sanjiazi granulites was 1945 Ma. However, more research is required to confidently

30

interpret the geological meaning of the 1945 Ma metamorphic ages of the Sanjiazi granulites.

8.3 P–T–t path and tectonic implications 8.3.1 P–T–t path of the Sanjiazi granulites On the basis of petrographic observations and P–T calculations, combined with zircon and monazite U–Pb dating and REE compositions, four metamorphic stages, individual P–T conditions, and corresponding metamorphic ages are inferred for the Sanjiazi granulites. The pre-peak stage (M1) is most likely represented by the mineral inclusions of biotite + plagioclase + ilmenite + quartz in garnet, which formed under P–T conditions of P = 0.66–0.71 GPa and T = 620–650°C. This inference is further supported by a recent study of the Jiaobei pelitic granulites within the JLJB (Zou et al., 2017). The peak granulite-facies stage (M2) formed a mineral assemblage of garnet, sillimanite, biotite, plagioclase, ilmenite, quartz, and melt in the matrix under P–T conditions of P = 0.96–1.10 GPa and T = 790–840°C, based on the compositions of near-peak plagioclase in the P–T pseudosection. The peak pressure and temperature are consistent with muscovite dehydration melting equilibria accompanying melt formation. Some mineral-free metamorphic zircon domains record relatively old ages of 1945 Ma, which are thought to represent the timing of near-peak granulite-facies metamorphism. Following the peak stage, the Sanjiazi granulites experienced two retrogressive stages. The first stages involved near-isothermal decompression with slight cooling and resulted in the formation of cordierite + quartz corona around garnet porphyroblasts (M3), which occurred at P = 0.62–0.65 GPa and T = 725–785°C, as 31

constrained by the P–T conditions of the Bt–Crd–Grt–Pl–Sil–Ilm–Mag–Liq field in the pseudosection and Xprp values of garnet rims in contact with cordierite + quartz coronae. The late near-isobaric cooling (IBC) assemblage (M4) was marked by the formation of staurolite and biotite in the matrix, which occurred at P = 0.43–0.55 GPa and T = 595–625°C. Some retrogressive zircons and monazites that formed in the retrogressive stages (M3 and M4) record relatively young ages of 1880–1820 Ma, representing the timing of the post-peak granulite- to amphibolite-facies regression. This implies a prolonged residence period for the rocks at mid-crustal levels (England and Thompson, 1984; Harley, 1989; Qian et al., 2016, 2018; Thompson and England, 1984; Wei et al., 2014, 2018). Taken together, the mineral assemblages and their P–T conditions, combined with available geochronological data, define a clockwise P–T–t path involving near-isothermal decompression and near-isobaric cooling for the Sanjiazi granulites within the South Liaohe Group. Notably, like most IBC granulites in Phanerozoic orogenic belts, the Sanjiazi granulites were not exposed at Earth’s surface as a result of the tectonism responsible for metamorphism, but in fact resided in the mid-crust for a long time (100 Myr) following metamorphism. The eventual exhumation of the Sanjiazi granulites only resulted from later tectonic–magmatic events that were unrelated to their formation (Harley, 1989; Qian et al., 2018; Wei et al., 2018). The peak and post-peak P–T conditions estimated for the Sanjiazi granulites yielded thermal gradients of 23°C/km and 35°C/km, respectively (Fig. 21), and thus represent intermediate- and low-P/T-type regional metamorphism, respectively (Maruyama et al., 2010; Miyashiro, 1961). The metamorphic conditions of the granulites that are characteristic of peak middle-pressure granulite-facies of the intermediate P/T-type and post-peak low-pressure granulite- to amphibolite-facies of

32

the low P/T-type are consistent with those in Phanerozoic convergent margin settings, such as Pacific- and Himalayan-type orogens. In such orogens, medium- to high-pressure granulite-facies rocks and Barrovian-type metamorphic zones are common (e.g., Maruyama et al., 2010; Kohn, 2014; Wang et al., 2015, 2016, 2017b, 2018). Thus, we suggest that the formation and evolution of the Sanjiazi granulites were related to late Paleoproterozoic orogenesis.

8.3.2 Implications for regional metamorphism of the JLJB Increasingly, new metamorphic data indicates that the JLJB experienced greenschist- to granulite-facies metamorphism and medium- to high-temperature ductile deformation during 1950–1800 Ma. However, the nature and tectonic setting of the metamorphic P–T–t paths of the JLJB remain controversial (Bai et al., 1998; He et al., 1998; Liu et al., 2015a,b, 2017c,d; Li et al., 2001; Lu et al., 2006; Tam et al., 2012a,b,c; Zhao et al., 2012; Zhou et al., 2004, 2008). Lu et al. (1996) estimated peak metamorphic P–T conditions of T = 700–750°C and P = 0.55–0.60GPa, and T = 700–750°C and P = 0.50–0.60GPa for Al-rich schists and gneisses in the Jingshan and South Liaohe groups, respectively, using the garnet–biotite and garnet–plagioclase–biotite–quartz geothermobarometers, which defined anticlockwise P–T paths involving near-isobaric cooling. Subsequently, on the basis of a detailed petrological investigation, combined with the P–T estimates from traditional geothermobarometers, He and Ye (1998) obtained similar peak metamorphic P–T conditions of T= 600–640°C and P = 0.64–0.73 GPa, T = 530–560°C and P = 0.60–0.70 GPa, and clockwise P–T paths involving heating with 33

increasing pressure, near-isothermal decompression, and near-isobaric cooling for the North Liaohe and Laoling groups. In contrast, they also obtained peak metamorphic P–T conditions of T = 670–710°C and P = 0.60–0.68 GPa, and T = 700–750°C and P = 0.52–0.65 GPa for the South Liaohe and Ji’an groups, respectively, and defined anticlockwise P–T paths involving heating with increasing pressure and near-isobaric cooling. Furthermore, He and Ye (1998) speculated that the clockwise P–T paths of the North Liaohe and Laoling groups were related to tectonic thickening by the formation of nappes, and that the anticlockwise P–T paths of the South Liaohe and Ji’an groups were related to the emplacement of a large amount of gneissic granites that caused the temperature increase. Li et al. (2001) reported similar clockwise and anticlockwise P–T paths for the North and South Liaohe groups, respectively. In addition, Zhao et al. (2012) obtained peak metamorphic P–T conditions of T = 650–700°C and P = 0.60–0.70 GPa, and clockwise P–T paths for the Fenzishan Group. As such, it has long been considered that the Fenzishan, North Liaohe, and Laoling groups have clockwise P–T paths, but the Jingshan, South Liaohe, and Ji’an groups have anticlockwise P–T paths (Liu et al., 2015a; Zhao et al., 2012). However, recent studies have identified abundant mafic, pelitic and semi-pelitic granulites in the Jingshan Group and TTG gneisses from the Jiaobei Terrane (Liu et al., 2010, 2012, 2013b, 2017d; Tam et al., 2011, 2012a,b,c; Wang et al., 2010; Zhou et al., 2004, 2008; Zou et al., 2017), which consistently record clockwise P–T–t paths with an isothermal decompression retrogression, and peak P–T conditions of P = 0.85–1.55 GPa and T = 800–880 °C (Fig. 21). Even more recently, many outcrops of

34

mafic, pelitic and semi-pelitic granulites have been found within the Ji’an Group, which are also characterized by clockwise isothermal decompression P–T paths, with peak P–T conditions of P = 1.00 GPa and T = 860–870°C (Cai et al., 2017c). In the present study, the Sanjiazi granulites from the South Liaohe Group have also been shown to record complete clockwise P–T–t paths with peak P–T conditions of P = 0.96–1.10 GPa and T = 790–840°C. Similarly, Liu et al. (2017c) reported that garnet amphibolites from the South Liaohe Group in the Sanjiazi area also record complete clockwise P–T–t paths with peak P–T conditions of P = 0.98–1.01 GPa and T = 690–710°C (Supplementary Figs S1 and S5). All the petrographic evidence indicates that the abundant meta-mafic and metasedimentary rocks in the Jingshan, South Liaohe, and Ji’an groups have experienced upper amphibolite- to granulite-facies metamorphism and display consistent clockwise P–T–t paths rather than anticlockwise P–T–t paths as previously suggested. Numerous geochronological data further reveal that various high-grade metamorphic rocks in the JLJB experienced peak metamorphism at 1950–1900 Ma, and post-peak isothermal decompression and subsequent amphibolite-facies retrogression at 1900–1800 Ma (e.g., Liu et al., 2015a,b, 2017a,b,c,d).

8.3.3 Implications for the tectonic setting of the JLJB The JLJB is an important Paleoproterozoic tectonic belt in the NCC and provides a unique opportunity to investigate the Paleoproterozoic geology of East Asia. Considerable progress has been made in recent years on petrological,

35

geochemical, structural, geochronological, and metamorphic studies of the JLJB (e.g., Li et al., 2011, 2012; Liu et al., 2015a; Zhao et al., 2012). However, the spatial and temporal evolution of the JLJB has remained controversial, with several tectonic models having been proposed (e.g., Bai, 1993; Bai et al., 1998; Faure et al., 2004; Li et al., 2011; Zhang et al., 1988; Zhao and Zhai, 2013; Zhao et al., 2012), including rift opening and closing (Li et al., 2001, 2011, 2012; Li and Zhao, 2007; Zhang et al., 1988); arc–continent collision (Bai, 1993; Bai et al., 1998; Chen et al., 2016; Faure et al., 2004; Li and Chen, 2014; Li et al., 2015, 2018b; Lu et al., 2006; Meng et al., 2014, 2017a,b; Wang et al., 2015; Xu et al., 2018); rifting, subduction, and collision (Tam et al., 2012a,b,c; Zhou et al., 2008; Zhao et al., 2012); the ‘Korean Arc’ (Peng et al., 2014; Wang et al., 2016, 2017d; Li et al., 2018a); divergent double subduction (Yuan et al., 2015); and Andean-type retro-arc model (Kusky et al., 2016). The rift opening and closing model was initially proposed by Zhang et al. (1988) and has been repeatedly modified by other studies (Li et al., 2001, 2003, 2004, 2005, 2006, 2011, 2012; Liu et al., 1997). According to this model, the Longgang and Nangrim blocks were originally situated on a single continental block that experienced early intercontinental rifting at 2200–1900 Ma. This led to the emplacement of 2200–2100 Ma A-type granitoids and mafic rocks, and the formation of a rift basin in which the sedimentary and volcanic successions formed. Subsequently, the rift basin closed, causing deformation and greenschist- to amphibolite-facies metamorphism of the sedimentary and volcanic successions (Li and Zhao, 2007; Li et al., 2003, 2004, 2005, 2006, 2011, 2012; Liu et al., 1997; Luo et

36

al., 2004, 2008; Zhang et al., 1988). This model can reasonably explain the following geological features of the JLJB (Li et al., 2011; Zhang et al., 1988). (I) The geochemical and geochronological features of the late Archean TTG gneisses of the Nangrim and Longgang blocks are similar (Zhang et al., 1988; Wang et al., 2017c). (Ⅱ) Large volumes of A-type granites and bimodal volcanic assemblages, represented by the meta-mafic rocks and meta-rhyolites, were recognized in the JLJB (Li et al., 2011; Zhang et al., 1988). (Ⅲ) The Ji’an, South Liaohe, and Jingshan groups record low-pressure-type and anticlockwise P–T paths (Lu et al., 1996; He and Ye, 1998; Li et al., 2001). (IV) The borate deposits in the JLJB are non-marine origin (Peng and Palmer, 1995). However, the following geological and geochemical features of the JLJB are not consistent with the rift opening and closing model. (Ⅰ) The Paleoproterozoic volcanic–sedimentary association within the JLJB was formed at 2200–2100 Ma (Liu et al., 2017c; Xu et al., 2018a,b), and the sedimentary rocks were derived mainly from 2200–2100 Ma igneous rocks, rather than Archean crustal rocks (Liu et al., 2015a). This is more typical of convergent margin sedimentation than rift basin sedimentation, given that the latter is dominated by the erosion of Archean rocks (Cawood et al., 2012). (Ⅱ) Mafic rocks from continental rifts typically show geochemical features similar to those of ocean island basalts (Wilson, 1989), whereas the 2200–2100 Ma meta-mafic rocks in the JLJB have geochemical features similar to those of enriched mid-ocean ridge basalts (Xu et al., 2018a,b). (Ⅲ) Metamorphic P–T–t paths of crustal rocks from continental rifts commonly display anticlockwise P–T–t paths, whereas new metamorphic data indicate that the Jingshan, Fenzishan,

37

North Liaohe, South Liaohe, Ji’an, and Laoling groups in the JLJB have clockwise P–T–t paths. The arc–continent collision model was first proposed by Bai (1993), who postulated that the Liaonan–Nangrim Block was an island arc, the Longgang Block was an Archean continental block, and the North and South Liaohe groups represented a back-arc basin that closed to form the JLJB during 1950–1850 Ma arc–continent collision. Subsequently, the model has been refined and modified (Chen et al., 2016; Faure et al., 2004; He and Ye, 1998; Li et al., 2018a,b; Lu et al., 2006; Peng et al., 2014; Tian et al., 2017; Yuan et al., 2015). The arc–continent collision model is supported by the following geological features of the JLJB. (Ⅰ) The Archean Longgang and Nangrim blocks differ in terms of their rock assemblages, ages, and metamorphism. The Longgang Block yields ages of 3800–2500 Ma, is dominated by granitic rocks, and underwent greenschist- to granulite-facies metamorphism (Dong et al., 2017; Liu et al., 2017e; Lu et al., 2006), whereas the Nangrim Block yields ages of 2550–2450 Ma, is dominated by quartz diorites and amphibolites (Wang et al., 2017c), and underwent amphibolite-facies metamorphism. (Ⅱ) The 2200–2100 Ma meta-igneous rocks within the JLJB display the geochemical characteristics of arc magmatism (Faure et al., 2004; Li and Chen, 2014; Lu et al., 2006; Meng et al., 2014; Xu et al., 2018a,b). (Ⅲ) The metamorphic rocks of the Jingshan, North Liaohe, and Laoling groups show clockwise P–T paths (He and Ye, 1998; Li et al., 2001; Zhao et al., 2012). However, the arc–continent collision model is not consistent with the absence of oceanic crust in the JLJB, which is typically found in Phanerozoic

38

arc–continent collisional belts (Kusky et al., 2016; Xiao et al., 2015). Although we are unable to reconstruct the initial (2200–2100 Ma) tectonic setting prior to collision, we have clearly shown that the Sanjiazi granulites from the South Liaohe Group underwent tectonic–thermal processes similar to those of the pelitic and semi-pelitic granulites from the Jingshan and Ji’an groups (Fig. 21; Cai et al., 2017c; Liu et al., 2010a, 2013b, 2017d; Tam et al., 2012a,b,c), characterized by near-isothermal decompression following peak granulite-facies metamorphism and a clockwise P–T–t path during 1950–1850 Ma. Such a P–T–t path suggests that, similar to Cenozoic granulites from the Himalayan orogenic belt (e.g., Kohn et al., 2014; Wang et al., 2017b; Zhang et al., 2017), the Sanjiazi granulites experienced initial crustal thickening followed by rapid exhumation, uplift, and rapid erosion. In brief, the development of the whole JLJB must have involved subduction- or collision-related tectonic processes, and could not have formed by simple closure of a rift system (Fig.21). Even though the JLJB represents a 2200–2100 Ma rift basin (e.g., Li and Zhao, 2007; Li et al., 2003, 2004, 2005, 2006; Liu et al., 1997; Luo et al., 2004, 2008; Zhang et al., 1988), this rift basin must have developed into an incipient ocean basin, where the oceanic crust was subducted, leading to the final closure of the incipient ocean basin during the formation of the mafic, pelitic, and semi-pelitic granulites (Tam et al., 2012a,b,c; Zhao et al., 2012).

9. Conclusions Our

comprehensive

petrographic,

39

mineralogical,

and

geochronological

investigations of the Sanjiazi granulites from the South Liaohe Group within the JLJB, together with the results of previous studies, have led to the following main conclusions. 1. Four distinct mineral assemblages (M1–M4) are recognized in the Sanjiazi granulites. The pre-peak amphibolite-facies assemblage (M1) is preserved as fine-grained mineral inclusions within the cores of garnet grains and is represented by quartz + plagioclase + biotite + ilmenite. The inferred peak granulite-facies assemblage (M2) comprises garnet + sillimanite + plagioclase + quartz + biotite + ilmenite + melt in the matrix. The post-peak near-isothermal decompression assemblage (M3) was characterized by the formation of cordierite + sillimanite symplectites and cordierite + quartz coronas replacing garnet porphyroblasts. The late cooling retrogressive assemblage (M4) comprises staurolite + garnet + sillimanite + plagioclase + quartz + biotite + cordierite + ilmenite + magnetite in the matrix, accompanied by the crystallization of melt. 2. A combination of multi-equilibria geothermobarometers and pseudosection modeling in the NCKFMASHTO system constrains the P–T conditions of the M1, M2, M3, and M4 stages to P = 0.66–0.71 GPa and T = 620–650°C, P = 0.96–1.10 GPa and T = 790–840°C, P = 0.62–0.65 GPa and T = 725–785°C, and P = 0.43–0.55 GPa and T = 595–625°C, respectively. 3. U–Pb geochronology and REE compositions of zircon and monazite, combined with CL and BSE images and analyses of mineral inclusions in different zircon and monazite domains, revealed that the protolith of the Sanjiazi granulites was

40

deposited at 2200–2100 Ma. In addition, the timing of peak granulite-facies metamorphism is ca. 1945 Ma. The post-peak and late retrogressive metamorphism occurred during 1938–1851 Ma. 4. The mineral assemblages, P–T conditions, and geochronological data define a clockwise P–T–t path for the Sanjiazi granulites that involved near-isothermal decompression and near-isobaric cooling following peak metamorphism. These new metamorphic data, combined with the results of recent studies, further indicate that like the Jingshan and Ji’an groups within the JLJB, the South Liaohe Group also records a complete clockwise P–T–t path, rather than an anticlockwise P–T–t path as suggested in previous studies. The P–T–t path further indicates that the JLJB experienced a continuous orogenesis from 1950 to 1800 Ma.

Acknowledgements We thank Hua Xiang for advice regarding Perple_X calculations. We also thank Qian Mao and Yuguang Ma for assistance with electron microprobe analyses, and Zhaochu Hu and Keqing Zong for assistance with LA–ICP–MS dating. Guochun Zhao (Editor), Sanzhong Li, and one anonymous reviewer are thanked for helpful comments and constructive suggestions that allowed us to significantly improve this manuscript. This study was jointly supported by the National Natural Science Foundation of China (41430210, 41890833, and 41672191) and the Geological Investigation Project of the China Geological Survey (DD20160121).

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Figure captions Fig. 1. Simplified geological map of the Paleoproterozoic Jiao-Liao-Ji Belt, North China Craton, and the study area (modified from Zhao et al., 2005, 2012). The location of the study area (Fig. 2) is indicated by a rectangle.

Fig. 2. Simplified geological map of the South Liaohe Group in the Sanjiazi area showing the geological setting and locations of the Sanjiazi granulites.

Fig. 3. Representative field and thin section photographs show the occurrences of the Sanjiazi granulites. (a)–(d) Cordierite-bearing granulites associated with deformed granitic leucosomes in the Sanjiazi area, South Liaohe Group. (e)–(f) The Sanjiazi granulites comprising mainly garnet + biotite + sillimanite + cordierite + plagioclase + quartz + ilmenite + magnetite, with granitic leucosomes visible in thin section.

Fig. 4. Representative photomicrographs and BSE images showing mineral assemblages and microtextures of the Sanjiazi granulites. (a)–(b) Mineral inclusions of quartz, plagioclase, biotite, and ilmenite in the cores of garnet porphyroblasts, (BSE). (c) Sillimanite, plagioclase, biotite, ilmenite, and quartz in the matrix, with staurolite in garnet porphyroblasts (plane-polarized light; PPL). (d) Relatively coarse-grained sillimanite occurring as fibrous crystals in the matrix, and in contact with matrix-type biotite, plagioclase, and quartz (crossed-polarized light; CPL). (e) Plagioclase corona around a garnet porphyroblast and cordierite + sillimanite symplectite and coarse-grained quartz in the matrix (CPL). (f) Detailed view of pseudomorphs of melt-filled pores consisting of an optically continuous crystal of plagioclase with low dihedral angles against quartz (CPL). 72

Fig. 5. Representative photomicrographs showing mineral assemblages and microtextures of the Sanjiazi granulites. (a)–(b) Cordierite + quartz/sillimanite coronae surrounding garnet porphyroblasts and cordierite + sillimanite symplectites in the matrix (CPL). (c) Cordierite + sillimanite symplectites with biotites surrounding relatively large Fe–Ti oxides in the matrix (CPL). (d) Cordierite + sillimanite symplectites with quartz + biotite (CPL). (e) Late retrogressive staurolite, biotite, and Fe–Ti oxides in the matrix (PPL). (f) Late retrogressive biotite, and staurolite, with plagioclase surrounding a garnet porphyroblast (PPL).

Fig.6. Representative BSE images showing mineral assemblages and microtextures of the Sanjiazi granulites. (a) Cordierite + quartz coronae surrounding garnet porphyroblasts. (b) Staurolite + quartz symplectites, with biotite, Fe–Ti oxides, and plagioclase in the matrix. (c)–(d) Fe–Ti oxides (magnetite, rutile, and ilmenite) coexisting with garnet, biotite, cordierite, plagioclase, and quartz. (e)–(f) Fine-grained symplectitic intergrowths of rutiles in relatively coarse-grained ilmenite.

Fig. 7. Compositional diagrams showing the key metamorphic minerals of the Sanjiazi granulites. (a) Ternary Prp–(Alm + Sps)–Grs diagrams showing variations in the chemical composition of garnets. (b) Ternary Or–An–Ab diagrams showing the variation in chemical composition of plagioclase grains. (c) XMg vs. Ti (p.f.u.) diagram for biotites. (d) Si (p.f.u.) vs. XMg for cordierites.

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Fig. 8. Chemical zoning profile of a garnet porphyroblast in the granulite sample 16KD97-1 showing the (a) location of the profile and (b) variations in pyrope, almandine, grossularite, and spessartine components.

Fig.9. Phase equilibria modeling results for granulite sample 16KD97-1. (a) T–XH2O pseudosection at 0.55 GPa. Red dashed line denotes the H2O content selected to construct P–XO and P–T pseudosections. (b) P–XO pseudosection at 770°C. Red dashed line denotes the XO value selected to construct the P–T pseudosection.

Fig. 10. P–T pseudosection with the proposed P–T path for granulite sample 16KD97-1.

Fig. 11. Compositional isopleths of (a) plagioclase (XCa) and (b) garnet (XPrp) for Sanjiazi granulite sample 16KD97-1.

Fig. 12. Cathodoluminescence (CL) images and LA–ICP–MS U–Pb ages of host zircons from the Sanjiazi granulites. (a) CL image of zircon grain 16KD97-1_18 from sample 16KD97-1 showing a gray-luminescent inherited detrital core containing inclusions of quartz, a gray-luminescent metamorphic rim, and the 207Pb/206Pb age. (b) CL image of zircon grain 16KD97-1_04 from sample 16KD97-1 showing a gray-luminescent inherited detrital core, gray- and white-luminescent metamorphic rim, and the 207Pb/206Pb age. (c) CL image of zircon grain 16KD97-1_25 from sample

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16KD97-1 showing a gray-luminescent inherited detrital core, black-luminescent metamorphic rim, and the 207Pb/206Pb age. (d) CL image of zircon grain 16KD97-1_10 from sample 16KD97-1 showing a gray-white-luminescent metamorphic domain with inclusions of sillimanite, and the 16KD97-1_83

from

sample

207

Pb/206Pb age. (e) CL image of zircon grain

16KD97-1

metamorphic domain, and the

207

16KD97-1_47

16KD97-1

from

sample

metamorphic domain, and the

207

showing

a

gray-white-luminescent

Pb/206Pb age. (f) CL image of zircon grain showing

a

gray-white-luminescent

Pb/206Pb age. (g) CL image of zircon grain

16KD97-1_43 from sample 16KD97-1 showing a gray-luminescent metamorphic domain, and the

207

Pb/206Pb age. (h) CL image of zircon grain 16KD97-1_50 from

sample 16KD97-1 showing a gray-luminescent metamorphic domain with inclusions of sillimanite, and the

207

Pb/206Pb age. (i) CL image of zircon grain 16KD97-1_65

from sample 16KD97-1 showing a gray -luminescent metamorphic domain, and the 207

Pb/206Pb age. (j) CL image of zircon grain 16KD97-1_24 from sample 16KD97-1

showing a gray-luminescent metamorphic domain, and the 207Pb/206Pb age.

Fig.13. Back-scattered electron (BSE) images and LA–ICP–MS U–Pb ages of monazites from the Sanjiazi granulites. (a) Monazite grain 16KD97-1_02 from sample 16KD97-1 containing inclusions of quartz, with a homogeneous (gray) BSE image from core to rim, and its

207

Pb/206Pb age. (b) Monazite grain 16KD97-1_25 from

sample 16KD97-1 containing inclusions of quartz, with a homogeneous (gray) BSE image from core to rim, and its

207

Pb/206Pb age. (c) Monazite grain 16KD97-1_40

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from sample 16KD97-1 showing a homogeneous (gray) BSE image from core to rim, and its

207

Pb/206Pb age. (d) Monazite grain 16KD97-1_28 from sample 16KD97-1

showing a homogeneous (gray) BSE image from core to rim, and its

207

Pb/206Pb age.

(e) Monazite grain 16KD97-1_31 from sample 16KD97-1 showing a homogeneous (gray) BSE image from core to rim, and its

207

Pb/206Pb age. (f) Monazite grain

16KD97-1_10 from sample 16KD97-1 showing a homogeneous (gray) BSE image from core to rim, and its

207

Pb/206Pb age. (g) Monazite grain 16KD97-1_11 from

sample 16KD97-1 containing inclusion of zircon, with a homogeneous (gray) BSE image from core to rim, and its

207

Pb/206Pb age. (h) Monazite grain 16KD97-1_17

from sample 16KD97-1 containing inclusion of quartz, with a homogeneous (gray) BSE image from core to rim, and its 207Pb/206Pb age. (i) Monazite grain 16KD97-1_30 from sample 16KD97-1 showing a homogeneous (gray) BSE image from core to rim, and its

207

Pb/206Pb age. (j) Monazite grain 16KD97-1_29 from sample 16KD97-1

showing a homogeneous (gray) BSE image from core to rim, and its 207Pb/206Pb age.

Fig. 14.

206

Pb/238U vs.

207

Pb/235U diagrams and histograms for U–Pb analyses of

different zircons from the Sanjiazi granulites. (a)

206

Pb/238 U vs.

207

Pb/235U diagram

for inherited detrital zircons. (b) Age histogram for inherited detrital zircons. (c) 206

Pb/238 U vs. 207Pb/235U diagram for the old metamorphic zircons. (d) 206Pb/238 U vs.

207

Pb/235U diagram for the young metamorphic zircons.

Fig. 15. (a) 206Pb/238 U vs. 207Pb/235U diagram and (b) CI-normalized REE patterns of

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monazites from the Sanjiazi granulites. The chondrite (CI) values are from McDonough and Sun (1995).

Fig. 16. CI-normalized REE patterns of zircons from the Sanjiazi granulites. (a) Inherited detrital zircons. (b) Metamorphic zircons. The chondrite (CI) values are from McDonough and Sun (1995).

Fig. 17. Histograms of metamorphic and anatectic zircon U–Pb ages for the different metamorphic

rocks

of

the

JLJB.

(a)

Meta-sedimentary

rocks.

(b)

Meta-mafic–ultramafic rocks. (c) Granitic leucosomes. The data are cited from Li and Chen (2014), Liu et al. (2014a; 2015a,b; 2017a,b,c,d), Meng et al. ( 2014; 2017a,b), Lu et al. (2006), Luo et al. (2004, 2008), Qin et al. (2014, 2015), Wang et al. (2017a,d) and Xu et al. (2018a,b).

Fig. 18. CL images, LA–ICP–MS U–Pb zircon ages, and energy spectrums of typical mineral inclusions in different zircon domains from Sanjiazi granulite sample 16KD97-1.

Fig. 19. BSE images of metamorphic monazites showing typical mineral inclusions in different monazite domains from Sanjiazi granulite samples 16KD97-1. (a) Garnet. (b) Plagioclase + quartz. (c) Biotite. (d) Sillimanite. (e) Plagioclase. (f) Magnetite + ilmenite. (g) Quartz + zircon. (h) Apatite.

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Fig. 20. BSE images showing the textural locations of representative monazite and zircon grains in sample 16KD97-1. (a)–(c) Anhedral and elongate monazite and zircon in biotite. (d)–(f) Monazite in cordierite. (g)–(i) Monazite surrounded by biotite, cordierite, and sillimanite.

Fig. 21. Inferred P–T paths determined in this study for the Sanjiazi semi-pelitic granulite compared with representative published P–T paths for other rocks throughout the JLJB. 1.1. HP mafic granulite (Liu et al., 1998); 1.2. HP pelitic granulite (Zhou et al., 2004); 1.3. HP pelitic granulite (Wang et al., 2010); 1.4. HP pelitic granulite (Tam et al., 2012a); 1.5. MP pelitic granulite (Tam et al., 2012b); 1.6. HP mafic granulite (Tam et al., 2012c); 1.7. HP mafic granulite (Liu et al., 2013b); 1.8. HP mafic granulite (Liu et al., 2017d); 1.9. HP pelitic granulite (Zou et al., 2017); 2.1. MP pelitic schist of the North Liaohe Group (He and Ye, 1998); 2.2. MP pelitic schist of the Laoling Group (He and Ye, 1998); 2.3. MP pelitic schist of the North Liaohe Group (Li et al., 2001); 2.4. Garnet-bearing amphibolite of the South Liaohe Group (Liu et al., 2017c); 2.5. MP pelitic granulite of the Ji’an Group (Cai et al., 2017c); 2.6. MP semi-pelitic granulite of the South Liaohe Group (this study). The metamorphic facies boundaries are after Vernon and Clarke (2008). The division of the metamorphic facies series is after Miyashiro (1961). The stability fields of Al2SiO5 polymorphs are from Salje (1986). The boundaries between metamorphic facies are after Brown (2001).

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Supplementary Figures and Tables

Fig. S1. Schematic geological map of the Liaohe Group showing the location of the cordierite-bearing granulites and garnet amphibolites of the South Liaohe Group.

Fig. S2. Photomicrograph of a thin section from sample 16KD97-1 showing mineral assemblages and microtextures of the Sanjiazi granulites.

Fig. S3. Representative field-photographs and photomicrographs showing the detailed textures and mineral assemblages of cordierite-bearing granulites and garnet amphibolites of the South Liaohe Group (see text for detailed explanation).

Fig. S4. X-ray maps of Fe, Ca, Mg and Mn in garnet porphyroblasts from sample 16KD97-1 showing variations from core to rim.

Fig. S5. Representative photomicrographs showing the detailed textures and mineral assemblages of the Sanjiazi cordierite-bearing granulites of the South Liaohe Group

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(see text for detailed explanation).

Table S1 Bulk-rock composition (wt. %) of the Sanjiazi granulite (16KD97-1) from the South Liaohe Group

Table S2 Representative compositions of garnet from the Sanjiazi granulite (16KD97-1) in the South Liaohe Group

Table S3 Representative compositions of plagioclase from the Sanjiazi granulite (16KD97-1) in the South Liaohe Group

Table S4 Representative compositions of biotite from the Sanjiazi granulite (16KD97-1) in the South Liaohe Group

Table S5 Representative compositions of cordierite from the Sanjiazi granulite (16KD97-1) in the South Liaohe Group

Table S6 Representative compositions of staurolite from the Sanjiazi granulite (16KD97-1) in the South Liaohe Group

Table S7 Representative compositions of sillimanite from the Sanjiazi granulite (16KD97-1) in the South Liaohe Group

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Table S8 Zircon U–Pb ages and rare-earth elements concentrations (ppm) from the Sanjiazi granulite (16KD97-1) in the South Liaohe Group

Table S9 Monazite U–Pb ages and rare-earth elements concentrations (ppm) from the Sanjiazi granulite (16KD97-1) in the South Liaohe Group

Table S10 Zircon rare-earth element concentrations (ppm) from the Sanjiazi granulite (16KD97-1) in the South Liaohe Group

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Highlights • A clockwise P–T –t path was reconstructed from the cordierite-bearing granulites of the South Liaohe Group. • U–Pb age data reveal 1945 Ma peak and 1851–1839 Ma retrogressive metamorphisms.

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