Metamorphic P-T path and zircon U-Pb dating of HP mafic granulites in the Yushugou granulite-peridotite complex, Chinese South Tianshan, NW China

Accepted Manuscript Metamorphic P-T path and zircon U-Pb dating of HP mafic granulites in the Yushugou granulite-peridotite complex, Chinese South Tianshan Lu Zhang, Lifei Zhang, Bin Xia, Zeng Lü PII: DOI: Reference:

S1367-9120(17)30283-3 http://dx.doi.org/10.1016/j.jseaes.2017.05.034 JAES 3099

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Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

28 January 2017 27 May 2017 28 May 2017

Please cite this article as: Zhang, L., Zhang, L., Xia, B., Lü, Z., Metamorphic P-T path and zircon U-Pb dating of HP mafic granulites in the Yushugou granulite-peridotite complex, Chinese South Tianshan, Journal of Asian Earth Sciences (2017), doi: http://dx.doi.org/10.1016/j.jseaes.2017.05.034

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Metamorphic P-T path and zircon U-Pb dating of HP mafic granulites in the Yushugou granulite-peridotite complex, Chinese South Tianshan Lu Zhang1, Lifei Zhang1, *, Bin Xia2, Zeng Lü1 1 The Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China (*corresponding author: [email protected]) 2 State Key Laboratory of Geological Processes and Mineral Resources, College of Earth Sciences, China University of Geosciences, Wuhan 430074, China Abstract The co-occurrence of granulite and peridotite complex represents relics of the paleo-suture zone and provides an optimal opportunity for better understanding of orogeny between two blocks. In this study, we carried out petrological and U-Pb zircon dating investigation on the HP mafic granulites associated with peridotite complex at Yushugou in Chinese South Tianshan. The studied samples include garnet-bearing high-pressure mafic granulites which can be subdivided into two types: Type I orthopyroxene-free and Type II orthopyroxene-bearing granulites and amphibolite. Type I granulite (Y21-2) has a mineral assemblage of garnet (33 vol.%), clinopyroxene (32 vol.%) and plagioclase (30 vol.%); and Type II granulite (Y18-8) has a mineral assemblage of garnet (22 vol.%), clinopyroxene (10 vol.%), orthopyroxene (14 vol.%), plagioclase (45 vol.%) and quartz. Garnet in both granulites exhibits core-rim structure characterized by increasing grossular and decreasing pyrope from core to rim. Petrographic observations and phase equilibrium modeling using THERMOCALC in the NCFMASHTO system for the mafic granulites (Y21-2 and Y18-8) show three stages of metamorphism: Stage I (granulite facies) was recognized by the large porphyroblastic garnet core, with P-T conditions of 9.8-10.4 Kbar and 860-900 oC (Y21-2) and 9.9-10.6 Kbar and 875-890 oC (Y18-8), respectively; Stage II (HP granulite facies) has peak P-T conditions of 12.1 Kbar at 755 oC (Y21-2) and 13.8 Kbar at 815 oC (Y18-8) using mineral assemblages combining with garnet rim compositions with maximum grossular and minimum pyrope contents; Stage III (amphibolite facies) was characterized by the development 1

of calcic amphibole in granulites with temperature of 446-563 oC. Therefore, an anticlockwise P-T path characterized by simultaneous temperature-decreasing and pressure-increasing was inferred for the Yushugou HP mafic granulite. Studies of zircon morphology and inclusions, combined with zircon U-Pb dating and REE geochemistry indicate that their protolith’s ages of the mafic granulites were ~430 Ma, while the metamorphism could occur at three stages with ages of ~390 Ma, ~340 Ma and ~320 Ma, which may correspond to Stage I, II and III, respectively. We interpret the HP mafic granulites from the Yushugou granulite-peridotite complex to be formed by the cooling subduction of the lower crustal rocks from the hanging wall of central Tianshan block during the northward subduction of the south Tianshan paleo-ocean from Devonian to Carboniferous. Keywords: Anticlockwise P-T-t path, HP mafic granulites, P-T pseudosection, U-Pb zircon dating, Chinese South Tianshan 1. Introduction The association of crustal and mantle rocks is a characteristic feature of HP-UHP orogenic systems and plays an important role in understanding the processes at the mantle-curst interface (Brueckner, 1998; Jahn et al., 1999; Kusbach et al., 2015). The granulite-peridotite complex that represents the rocks from the lower crust and the upper mantle respectively, is rarely reported coexisting in the subduction zone. Among the reported outcrops of the granulite and the peridotite associations, such as the European Variscides (Kamei et al., 2010; Nahodilová et al., 2014; Faryad and Žák, 2016), the Alpine belt (Montel et al., 2000; Chalouan and Michard, 2004; Chalouan et al., 2008; Afiri et al., 2011), the South Tianshan (Wang et al., 1999a; Li et al., 2011; Ji et al., 2014, the formation mechanism of the associations remains a complicated issue. So far, several models have been proposed to explain their formation mechanism, including: 1) both of the granulite and peridotite complex came from the subducted slab and underwent identical subduction and exhumation processes (Wang et al., 1999a; Faryad et al., 2015); 2) the peridotite belonged to a part of the ophiolite, while the granulite may come from the lower crust of another block, both finally collided together through sheering actions (Shu et al., 1998; Yang et al., 2011); 3) the 2

association of the granulite and peridotite has no relationship to the subduction, but represents the continental crust and the upper mantle separately and was formed during later exhumation within the shear zone or due to a diapiric ascent (Montel et al., 2000; Kamei et al., 2010; Afiri et al., 2011; Ji et al., 2014; Czertowicz et al., 2016). In this paper, we study the metamorphic evolutions using phase equilibrium modeling method and present new zircon U-Pb ages for HP mafic granulites from the Yushugou granulite-peridotite complex to rebuilt the P-T-t path of the mafic granulite and discuss the formation mechanisms of the Yushugou granulite-peridotite association. Our study will also shed new light on the tectonic evolution for the Paleozoic Chinese South Tianshan. Mineral abbreviations by Whitney and Evans (2010) are followed. 2. Geological background The Chinese Tianshan orogenic belt is situated between the Junggar block to the north and the Tarim block to the south and extends east-west for about 1500 km (Fig. 1a). It reflects the collision between the Tarim and Junggar blocks in Late Palaeozoic (Coleman, 1989; Gao et al., 1998). There are two suture zones that have been recognized in Chinese Tianshan: the South Tianshan suture zone between the Tarim and Yili-central Tianshan blocks, which formed probably in the Late Devonian-Early Carboniferous; the northern Tianshan suture zone between the Yili-central Tianshan block and the north Tianshan island arc along the southern margin of the Junggar block, which formed probably in Late Carboniferous-Early Permian (Windley et al., 1990; Allen et al., 1992). The South Tianshan orogenic belt lies between the Tarim block to the south and the Yili-central Tianshan block to the north. It is generally accepted that the south Tianshan orogenic belt was formed due to the closure of the south Tianshan ocean followed by collision between the Tarim block and the Yili-central Tianshan block during the Late Paleozoic (Han et al., 2011; Zhang et al., 2013). Based on the occurrence of HP-UHP belt together with abutting low-P granulite-facies rocks to the north, a paired metamorphic belt has been proposed in the southwestern Tianshan due to the subduction of the south Tianshan paleo-ocean beneath the Yili-central Tianshan block (Li and Zhang, 2004; Zhang et al., 2007, 2013; 3

Xia et al., 2014a). The subduction of the south Tianshan paleo-oceanic crust underneath the Yili-central Tianshan block may start at Early Silurian and last until Early Carboniferous (Gao et al., 2008; Han et al., 2011; Xia et al., 2014a, 2014b), creating a magmatic arc along the south margin of the Yili-central Tianshan block (Yang et al., 2006; Zhu et al., 2006, 2009; Zhu et al., 2006; Yang and Zhou, 2009; Xu et al., 2010; Long et al., 2011; Xu et al., 2013; Ma et al., 2014). The Yushugou granulite-peridotite complex is located at the east of the northern margin of the south Tianshan. It comprises a granulite unit and an abutting peridotite unit (Fig. 1). In previous studies, most researchers interpreted the peridotite unit in Yushugou and the eastward ultramafic rocks in Tonghuashan and Liuhuangshan to represent an ophiolite belt (Wu et al., 1992; Wang et al., 1999a; Wang et al., 1999; Xu et al., 2011; Yang et al., 2011). On the other hand, some work has been done to the granulite unit to the north of the ultramafic rocks and its origin was still in dispute (Shu et al., 1996; Wang et al., 1999a; Yang et al., 2011; Ji et al., 2014; Zhang et al., 2016). The granulite unit comprises mainly of mafic and felsic granulites interbedded with layers of amphibolites. Marble lenses can be found in some places (Fig. 2c). These rocks are generally strongly foliated. The mafic granulite can be further subdivided into three types according to their petrographical characteristics: Grt-Cpx-Pl granulite, Grt-Cpx-Opx-Pl-Qz granulite and Cpx-Opx-Pl-Qz granulite. The felsic granulite can be subdivided into two types: Grt-Opx-Ksp-Pl-Qz and Grt-Pl-Qz granulite. 3. Methods 3.1. Electron probe micro analyses Electron probe micro analyses of minerals were performed with a Jeol JXA-8100 super-probe at the Ministry of Education Key Laboratory of Orogenic Belts and Crustal Evolution, Peking University. It was operated at 15 KV acceleration voltage, 10 nA beam current and 2 μm beam size. For calibration, natural and synthetic mineral standards were used. Final results were reduced by the PRZ correction program supplied by the manufacturer. For major elements, the relative analytical 4

uncertainties are <2%. Representative mineral compositions are presented in Table 2. 3.2. XRF analyses Whole-rock compositions of the samples were obtained using an RIX-2100 X-ray fluorescence (XRF) spectrometer on fused glass discs made of whole-rock powder (<200 mesh) and lithium metaborate at the MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University. Standard basalt GSR3 was used for calibration. Whole-rock compositions are presented in Table 1. 3.3. LA-ICP-MS dating Zircons were prepared by conventional heavy liquid and magnetic techniques and were handpicked under a binocular microscope, mounted onto epoxy resin disks and polished to expose the train centers. The choice of analytical sites based on transmitted and reflected light microscopy to avoid cracks and inclusions and cathodoluminescence (CL) imaging to examine internal structures prior to U-Pb isotopic analysis. CL images were obtained using a Quanta 200F ESEM with a 2-min scanning time at conditions of 15 kV and 120 μA at Peking University. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon dating was made by an Agilent 7500ce ICP-MS equipped with an 193 nm excimer laser ablation system (COMPexPro 102) at the MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University. Instrumental conditions and measurement procedures are similar to those described by Yuan et al. (2004). The diameter of the laser spot size was 32 μm. Considering the correction of isotope fractionation effects, zircon Plesovice (337.3 ± 0.4 Ma; Slama et al., 2008) was used as an external standard and 91500 (1064.1±3.2 Ma; Wiedenbeck et al., 1995) was a monitoring standard. GLITTER 4.4.2 was used to calculate the U-Pb isotopic compositions. Common lead was corrected following the procedure of Andersen (2002). Data processing was done with the ISOPOLT 4.15 (Ludwig, 2003). Geochemical analysis of zircons was conducted simultaneously with the U-Pb measurements. The results of zircon U-Pb dating are shown in Table 4 and presented on U-Pb concordia plots with 1σ uncertainties in Fig. 8. 5

4. Petrography The HP mafic granulites from Yushugou are generally massive foliated due to the ductile shearing (Fig.3a), and the garnet-bearing samples can be subdivided into two types according to the mineral characteristics. Type I orthopyroxene-free granulite, represented by sample Y21-2, displays a porphyroblastic texture and has a mineral assemblage of garnet (33 vol.%), clinopyroxene (32 vol.%), plagioclase (30 vol.%) and amphibole (2 vol.%), with minor ilmenite and calcite (Fig.3b). Large porphyroblastic garnet (3-4 mm in diameter, denoted as GrtA, Fig.3b) is distributed in a matrix of fine-grained garnet (denoted as Grt B, Fig.3b), clinopyroxene, plagioclase, amphibole and ilmenite, may represent two metamorphic stages. Some garnet grains contain inclusions of clinopyroxene and plagioclase. Some clinopyroxene grains contain inclusions of plagioclase. Type II orthopyroxene-bearing granulite is represented by sample Y18-8 and also shows a porphyroblastic texture. It has a mineral assemblage of garnet (22 vol.%), clinopyroxene (10 vol.%), orthopyroxene (14 vol.%), plagioclase (45 vol.%) and quartz (6 vol.%), with minor rutile, ilmenite, amphibole and calcite (Fig.3c,d). Based on grain sizes and occurrences, three types of garnet (GrtA, GrtB, Grtc), two types of orthopyroxene (OpxA, OpxB) and two types of plagioclase (PlA, PlB) were distinguished (Fig.3c-g). GrtA and OpxA forms coarse-grained (0.5-1 mm in diameter) porphyroblasts in a matrix of fine-grained garnet (GrtB), orthopyroxene (OpxB), clinopyroxene, plagioclase (PlA), quartz, rutile and ilmenite (Fig.3c,d). Intergrowth of Grtc and quartz develops around OpxA (Fig.3e,f). Clinopyroxene usually occurs as small grains in the matrix or around Opx A (Fig.3c). Some garnet grains contain inclusions of plagioclase. All these petrographic characteristics described above illustrate two metamorphic stages. Amphibole shows intergrowth with plagioclase (PlB) around garnet (Fig.3g), which is interpreted to be a later phase.

The amphibolite Y14-14 is mainly composed of amphibole and plagioclase (Fig.3h) and was interpreted to have formed due to retrograde amphibolite facies metamorphism during exhumation. 6

5. Mineral chemistry 5.1. Garnet Garnet from two representative mafic granulite samples Y21-2 and Y18-8 generally exhibits apparent compositional zoning for Grs and Prp. For Type I granulite Y21-2, GrtA exhibits a two-stage compositional zoning (Table 2; Fig.4a,c): the Ca-poor and Mg-rich core is characterized by almost constant grossular (19-22 mol.%), pyrope (21-22 mol.%), almandine (53-56 mol.%) and spessartine (2-3 mol.%) contents, while the rim shows an obvious increase in the grossular content (23-31 mol.%) and decrease in the pyrope content (21-17 mol.%). GrtB displays a chemical zonation characterized by a Ca-rich and Mg-poor rim, comparable to the rim of GrtA (Fig.4a,d). For Type II granulite Y18-8, GrtA also shows a two-stage compositional zoning (Table 2; Fig.4b,e): the Ca-poor and Mg-rich core is characterized by almost constant grossular (10-11 mol.%), pyrope (36-38 mol.%), almandine (49-51 mol.%) and spessartine (1 mol.%) contents, while the rim shows a subtle increase in the grossular content (12-17 mol.%) and decrease in the pyrope content (33-29 mol.%). GrtB displays a chemical zonation characterized by a Ca-rich and Mg-poor rim similar to the rim composition of GrtA (Fig.4b,f). Grtc has more Ca but less Mg compared to the compositions of the outermost rim of GrtA and GrtB (Fig.4b). 5.2. Other minerals For Type I granulite Y21-2, clinopyroxene has XMg (= Mg / (Mg + Fe2+)) ranging from 0.64 to 0.78, belonging to diopside (Fig.4g). The An (= Ca / (Ca + Na)) content of plagioclase ranges from 0.40 to 0.45, belonging to andesine. Amphibole has (Ca)M4 of 1.72 to 1.92 p.f.u., (Na+K) A of 0.54 to 0.88 p.f.u., Ti of 0.16 to 0.27 p.f.u., Si of 6.02 to 6.40 p.f.u., XMg of 0.52 to 0.64 and belongs to pargasite according to Leake et al. (1997) (Fig.4i). For Type II granulite Y18-8, clinopyroxene has XMg of 0.68 to 0.78 and belongs to diopside (Fig.4h). The OpxA has XMg of 0.57 to 0.69 and OpxB has

XMg of 0.59 to

0.67, both belonging to hypersthene (Fig.4h). The An contents of PlA and PlB range from 0.42 to 0.49 and 0.71 to 0.88, respectively. 7

For amphibolite Y14-14, the An content of plagioclase ranges from 0.63 to 0.78. Amphibole has (Ca)M4 of 1.74 to 1.99 p.f.u., (Na+K) A of 0.04 to 0.24 p.f.u., Ti of 0.02 to 0.07 p.f.u., Si of 6.96 to 7.76 p.f.u. and XMg of 0.74 to 0.87 and belongs to actinolite or magnesiohornblende according to Leake et al. (1997) (Fig.4j). To summary up, based on the above petrographic observation and mineral compositions, three generations of mineral paragenesis can be identified for the mafic granulites as shown in figure 5: Stage I (garnet ± clinopyroxene ± plagioclase ± amphibole + ilmenite for Y21-2, garnet + orthopyroxene ± plagioclase ± amphibole + quartz + ilmenite for Y18-8), Stage II (garnet + clinopyroxene + plagioclase + amphibole ± quartz ± rutile + ilmenite for Y21-2 and garnet + clinopyroxene + plagioclase ± amphibole + quartz + rutile + ilmenite for Y18-8) and Stage III ( plagioclase + amphibole ± quartz ± ilmenite for Y18-8 and Y14-14). 6. Phase equilibrium modeling P-T pseudosections were calculated using the software THERMOCALC 3.33 (Powell et al., 1998; updated July 2009) and the November 2003 updated version of the Holland & Powell (1998) dataset (file tcds55.txt). P-T pseudosections were constructed for two representative mafic granulites (Y21-2 and Y18-8) under the system Na2O–CaO–FeO–MgO–Al2O3-SiO2–H2O–TiO2–O (NCFMASHTO). Activitycomposition relationships were those presented for garnet (White et al., 2007), clinopyroxene (Green et al., 2007), orthopyroxene (White et al., 2002), amphibole (Diener et al., 2007), epidote (Holland and Powell, 1998), plagioclase (Holland and Powell, 2003) and ilmenite (White et al., 2000). Rutile, quartz and H2O were treated as pure end-member phases and H2O was assumed to be in excess. The XRF-based whole-rock compositions were used for the modeling of the initial growth of garnet cores (since some calcite is present in the matrix, the non-effective component of CaO incorporated in calcite was deducted from the original XRF data accordingly). Because that the bulk composition may not necessarily involve entire mineral grains (Carson et al., 1999; Wei et al., 2003), an effective bulk-rock compositions were calculated for the modeling of Stage II granulite facies metamorphism by subtracting the core composition of GrtA for Y21-2 8

and OpxA composition and the core of Grt A composition for Y18-8 from the XRF-based bulk composition, respectively. As described in Wei et al. (2009), the effective bulk-rock compositions were generated by integrating mineral compositions and modal abundance data for the phases present. Only half of the model abundance of zoned garnet grains was taken into account. 6.1. P-T pseudosections for orthopyroxene-free (sample Y21-2) The NCFMASHTO P-T pseudosection calculated for Stage I of the orthopyroxene-free granulite Y21-2 using the XRF-based bulk composition (Table 1) is presented in Fig.6a, which is contoured with isopleths of z (g) = Ca/(Ca+Mg+Fe 2+) and x (g) = Fe2+/(Fe2++Mg). The measured core compositions of GrtA correspond to a P-T regime of 9.8-10.4 Kbar and 860-900 oC in the stability field of garnet + clinopyroxene + plagioclase + amphibole + ilmenite. The NCFMASHTO P-T pseudosection shown in Fig.6b was calculated for Stage II of Y21-2 with an effective bulk-rock composition obtained by subtracting the core of GrtA composition from the XRF-based bulk composition (Table 1) and is contoured with isopleths of z(g) and x(g). A P-T trajectory is obtained from compositions of Grt B core to rim, which shows an anticlockwise P-T path with a pressure peak at 12.1 Kbar and 755 oC from 10 Kbar and 800 oC. Pmax is in mineral assemblage of garnet + clinopyroxene + plagioclase + amphibole + rutile + quartz. 6.2. P-T pseudosections for orthopyroxene-bearing (sample Y18-8) The NCFMASHTO P-T pseudosection calculated for Stage I of the orthopyroxene-bearing granulite Y18-8 is presented in Fig.6c using the XRF-based bulk composition (Table 1), which is contoured with isopleths of z (g) and x (g). The measured core compositions of Grt A correspond to P-T conditions of 9.9-10.6 Kbar and 875-890 oC in the stability field of garnet + orthopyroxene + plagioclase + amphibole + ilmenite + quartz. The NCFMASHTO P-T pseudosection shown in Fig.6d was calculated for Stage II of Y18-8 with an effective bulk-rock composition obtained by subtracting OpxA and the core of Grt A compositions from the XRF-based bulk composition (Table 1) and is contoured with isopleths of z(g) and x(g). A P-T trajectory is obtained from 9

compositions of Grt B core to rim, which shows an anticlockwise P-T path with a pressure peak at 13.8 Kbar and 815 oC from 12.6 Kbar and 855 oC. Pmax is in mineral assemblage of garnet + clinopyroxene + plagioclase + amphibole + rutile + quartz, which is in accord with the emergence of clinopyroxene during Stage II that we observed on the thin slice. 6.3. Amphibole thermobarometry for amphibolite (sample Y14-14) The amphibolite Y14-14 is mainly composed of amphibole and plagioclase (Fig.3h). And the amphibole thermobarometry of Gerya et al. (1997) was applied to the P-T calculation of amphibolite Y14-14. The metamorphic temperature conditions of 446-563 oC were obtained for amphibolite (sample Y14-14). 7. U-Pb zircon dating 7.1. Zircon morphology, mineral inclusions and U-Pb geochronology Zircons from Type I orthopyroxene-free granulite Y21-2 and Y25-2 are stubby or oval, 100–150 μm long, transparent and colorless. On CL images, a small portion of the grains in Y21-2 contains rounded rims bearing weakly luminescent cores with oscillatory zoning interpreted to be of magmatic origin, while another small portion contains rounded rims bearing highly luminescent cores which suggests two-stage metamorphic zircon growth (Fig.7). All grains in Y25-2 don’t contain any magmatic zircon core (Fig.7). The metamorphic zircons in Y21-2 and Y25-2 are either unzoned or weakly zoned, which should be formed in granulite facies (Vavra et al., 1999; Wu and Zheng, 2004). 42 analyzed spots on 41 grains of zircon were obtained for sample Y21-2. Among them, 8 spots of zircon core yield a weighted mean

206

Pb/238U age of

432.6 ± 5.3 Ma (MSWD = 2.4; Fig.8), which is interpreted to represent the time of the zircon cores crystallized from a mafic magma. The remaining spots obtained from the metamorphic zircons yield weighted mean ages of 391.6 ± 2.6 Ma (MSWD = 1.2; Fig.8) and 340.0 ± 2.0 Ma (MSWD = 1.4; Fig.8) respectively. 23 analyzed spots on 23 grains of zircon were obtained for sample Y25-2, which yield a weighted mean 206

Pb/238U age of 343.1 ± 2.9 Ma (MSWD = 0.39; Fig.8). Among the zircon grains

with the ages about 340 Ma in Y21-2, some garnet and dioposite inclusions have been identified based on the Laser Raman and electron microscope analyses (Fig.9). The 10

compositions of garnet inclusion in zircons (x (grs) = 0.23-0.28) are similar to Grt B and the rim of Grt A (Fig.9, Table 3). Therefore, the metamorphic ages of 340 Ma should be the ages of second granulite facies (high pressure granulite facies) metamorphism. Zircons from Type II orthopyroxene-bearing granulite Y18-8 are also stubby or oval, 150–200μm long, transparent and colorless. On CL images, a small portion of the grains in Y18-8 contain rounded rims bearing weakly luminescent cores with highly luminescent rims showing vague or no internal structure (Fig.7). 45 analyzed spots on 38 grains of zircon were obtained for sample Y18-8. 4 analyzed spots on cores give a weighted mean 206Pb/238U age of 433.9 ± 5.1 Ma (MSWD = 0.05; Fig.8). Other analyzed spots on metamorphic zircons give a weighted mean 206Pb/238U age of 391.6 ± 1.6 Ma (MSWD = 0.40; Fig.8), among which some mineral inclusions such as plagioclase, apatite and quartz have also been identified. Zircons from amphibolite Y14-14 are prismatic, subhedral to euhedral, 200–250 μm long, with aspect ratios of 2:1 to 3:1. On CL images, they show no or weakly zoned internal structure (Fig.7). Based on the Laser Raman and electron microscope analyses, some mineral inclusions such as amphibole and plagioclase have been identified (Table 3). 25 analyzed spots on 25 zircon grains were obtained for sample Y14-14, which yield a weighted mean

206

Pb/238U age of 321.6 ± 2.2 Ma (MSWD =

2.0; Fig.8). These ages are interpreted as the ages of amphibolite facies retrograde metamorphism. 7.2. Th-U and REE chemistry of zircon The Th and U concentrations of different zircon domains are shown in Fig.10 and Table 4. Three groups of zircons have been recognized in sample Y21-2. The first group zircon has magmatic cores with oscillatory zoning and Th/U ratios of 0.3–1.1 (Fig.10a), characteristics pointing to a magmatic origin. They show typical igneous REE pattern with a strong positive Ce anomaly, an apparent negative Eu anomaly and a steep light to heavy REE slope (Fig.11a). The second group zircon is metamorphic origin, which yields a weighted mean

206

Pb/238U age of 391.6 ± 2.6 Ma. Zircons of 11

this group have Th/U ratios of 0.2-0.9 (Fig.10a). They have slightly lower REE contents than the magmatic zircon cores (Fig.11a). The third group zircon is also metamorphic origin, which yields a weighted mean 206Pb/238U age of 340.0 ± 2.0 Ma. Zircons of the third group have relatively lower Th/U ratios of 0.04-0.22 compared to the first and second group (Fig.10a). They exhibit relatively flat middle to heavy REE patterns (Fig.11a), hinting that they are crystallized under the presence of the HREE-bearing mineral garnet (Schaltegger et al., 1999; Rubatto et al., 2003; Whitehouse and Platt, 2003; Wu and Zheng, 2004). In sample Y25-2, zircon (yield a weighted mean

206

Pb/238U age of 343.1 ± 2.9

Ma) has Th/U ratios of 0.01-0.07, similar to the third group in sample Y21-2 (Fig.10b). They also display the similar REE patterns with the third group zircon in Y21-2 (Fig.11b). In Y18-8, the zircon cores have Th/U ratios of 0.15-0.63 (Fig.10c). They show typical igneous REE pattern with a strong positive Ce anomaly, an apparent negative Eu anomaly and a steep light to heavy REE slope, similar to the first group zircon in Y21-2 (Fig.11c). The others have Th/U ratios of 0.37-1.09 (Fig.10c). They have slightly lower REE contents than the magmatic zircon cores, similar to the second group zircon in Y21-2 (Fig.11c). In Y14-14, metamorphic zircons (yield a weighted mean 206Pb/238U age of 321.6 ± 2.2 Ma) have Th/U ratios of 0.08-0.26 (Fig.10d). They show a negative Eu anomaly and a relatively high HREE concentration (Fig.11d). 8. Discussion 8.1. Metamorphic evolution of HP mafic granulite from the Chinese South Tianshan In previous publications, some geologists use conventional thermobarometer to calculate the pressure-temperature (P-T) conditions of the HP mafic granulite. The calculated peak P-T conditions of the mafic granulite are 800-870 oC and 8.8-11.3 Kbar (Shu et al., 2004), 795-964 oC and 9.7-14.2 Kbar for the high pressure granulite facies (garnet-diopside-plagioclase±quartz assemblage) (Wang et al., 1999b), and 724-826

o

C and 6.4-8.8 Kbar for the medium pressure granulite facies 12

(garnet-orthopyroxene-diopside-plagioclase-quartz assemblage) (Li et al., 2011). Wang et al. (1999b) and Li et al. (2011) consider that the mafic granulite in Yushugou experienced a clockwise P-T path. Zhang et al. (2016) proposed that the felsic granulite underwent UHT (T > 930 oC) and HP (10.5-14.5 Kbar) metamorphism, which may record a possible prograde process characterized by heating during subduction. However, because of granulite uncertainty principle (Frost and Chacko, 1989; Pattison, 2003) and the existence of multi stage mineral assemblages in HP granulite (Zhao et al., 2001), it is very difficult to estimate the pressure-temperature (P-T) conditions of metamorphism by conventional thermobarometer (Powell and Holland, 2008). In contrast, pseudosections involve a forward calculation of mineral equilibria for a given rock composition. They do not merely give the conditions of formation of a mineral assemblage, and commonly allow sections of P-T paths to be deduced from the way that mineral proportions and mineral compositions are interpreted to have evolved during the textural evolution of a rock (Powell and Holland, 2008). By integrating petrographic observations, phase equilibria modelling and conventional thermobarometry, an anticlockwise P-T path (Fig.12) comprising three stages [Stage I (granulite facies), Stage II (HP granulite facies) and Stage III (retrograde amphibolite facies)] has been derived for the mafic granulites in Chinese south Tianshan. Stage I (granulite facies) is reflected by GrtA cores in Y21-2 and GrtA cores and OpxA

in

Y18-8.

Pseudosections

predict

the

garnet-clinopyroxene-plagioclase-amphibole-ilmenite

mineral in

assemblage Y21-2

of and

garnet-orthopyroxene-plagioclase-amphibole-ilmenite-quartz in Y18-8. For this stage, compositional isopleths of Grt A cores point to 9.8-10.4 Kbar and 860-900 oC for Y21-2 and 9.9-10.6 Kbar and 875-890 oC for Y18-8. Stage II (HP granulite facies) is represented by Grt A rims and the matrix mineral assemblages.

Pseudosections

predict

the

mineral

assemblage

of

garnet-clinopyroxene-plagioclase-amphibole-rutile-quartz in Y21-2 and garnetclinopyroxene-plagioclase-amphibole-rutile-quartz in Y18-8. GrtA rims and GrtB from 13

both samples exhibit subtle increase of the grossular and decrease of pyrope. Phase equilibria modelling reveals that they show an anticlockwise P-T path and the P-T conditions at Pmax are12.1 Kbar at 755 oC for Y21-2 and 13.8 Kbar at 815 oC for Y18-8. Stage III (amphibolite facies) is reflected not only by amphibole staying around garnet, diopsite and in the matrix of granulites, but also the existence of amphibolite in granulite unit. The metamorphic condition revealed by amphibole thermobarometry for the amphibolite Y14-14 is 446-563 oC. Recently, more and more HP metamorphic rocks characterized by anticlockwise P-T path have been reported in subduction zone (Oh and Liou, 1990; Wakabayashi, 1990; Smith et al., 1999; Willner, 2004; Garcíacasco et al., 2006; Page et al., 2006; Tsujimori et al., 2006; Garcia-Casco et al., 2007; Blanco-Quintero et al., 2011; Hyppolito et al., 2014; Ukar and Cloos, 2014; Czertowicz et al., 2016). Because of relatively high geothermal gradients, most of the HP metamorphic rocks with anticlockwise P-T path may represent the onset of subduction (Oh and Liou, 1990; Willner, 2004; Page et al., 2006; Tsujimori et al., 2006; Blanco-Quintero et al., 2011). However, there exits other explanation for HP metamorphic rocks along anticlockwise P-T path experienced temperature decreasing and pressure increasing simultaneously, such as deeper burial or participating in the subduction (Wakabayashi, 1990; Czertowicz et al., 2016). Our petrographic observations and phase equilibria modelling for two representative granulites from the Chinese south Tianshan show an anticlockwise P-T path (Fig.12) comprising three-stage metamorphic evolution. Stage I has a high geothermal gradient of ~26 oC/km suggesting heating due to the proximity of a pluton, which may be related to arc above the subduction zone (Fig.13a). Stage II is characterized by the temperature decreasing and pressure increasing simultaneously, which recorded a lower geothermal gradient of ~18 oC/km. These HP mafic granulites should enter into a new environment similar to the subduction zone and become cooler and compression during this stage (Fig.13b), which can be witnessed by the extensive ductile sheer in the HP mafic granulite unit. Stage III may represent the overprint of retrograde amphibolite facies during the 14

exhumation of the granulite unit (Fig.13c). 8.2. Zircon age interpretation According to the earlier studies, the test results of granulite-facies metamorphic ages are Sm-Nd isochron age of 315 Ma (Wang et al., 1999b), zircon SHRIMP U-Pb ages of 392 Ma and 390 Ma (Zhou et al., 2004), zircon SHRIMP U-Pb ages of 390-401 Ma (Li et al., 2011),

40

Ar-39Ar isochron ages of 368 Ma and 360 Ma and

Sm-Nd isochron age of 310 Ma (Wang et al., 2003). As pointed out by Wu and Zheng (2004), zircon U-Pb dating is the most commonly used method for geochronology because of its high U and Th contents, low common Pb, and high mineral stability. Because of the high closure temperature of U-Pb diffusion in zircon (Lee et al., 1997; Cherniak and Watson, 2001), the U-Pb radiometric system is mainly applied to date magmatic and high temperature stages of metamorphic rocks. Zircon U-Pb analyses undertaken in this paper reveals four distinct age groups: Middle Silurian (~430 Ma), Middle Devonian (~390 Ma), Early Carboniferous (~340 Ma), Late Carboniferous (~320 Ma). 8 cores of zircons from Y21-2 show oscillatory zoning indicative of magmatic origin and give a weighted mean

206

Pb/238U age of

432.6 ± 5.3 Ma. 4 cores of zircons from Y18-8 give a weighted mean

206

Pb/238U age

of 433.9 ± 5.1 Ma, consistent with the 8 cores of zircons from Y21-2. Therefore, ~430 Ma may represent the time of zircons crystallized from a magma. The more old cores may be inherited, which can also explain the existence of older cores in earlier publication (Li et al., 2011; Yang et al., 2011). The second group of zircons from Y21-2 and Y18-8 give a same weighted mean 206

Pb/238U age of 391.6 ± 2.6 Ma and 391.6 ± 1.6 Ma, which represents the time of

Stage I granulite facies metamorphism. These ages are consistent with the previous results that measured by SHRIMP zircon U-Pb isotopic dating method (Zhou et al., 2004; Li et al., 2011). The third group of zircons from Y21-2, some of which contain garnet inclusions with x (grs) = 0.23-0.28 (similar to Grt B and the rim of Grt A), gives a weighted mean 206

Pb/238U age of 340.0 ± 2.0 Ma. The zircons from Y25-2 also give a weighted mean 15

206

Pb/238U age of 343.1 ± 2.9 Ma. Consequently, ~340 Ma ages are the ages of the HP

granulite facies metamorphism. Zircons from amphibolite Y14-14 give a weighted mean 206Pb/238U age of 321.6 ± 2.2 Ma, which represents the time of retrograde amphibolite facies metamorphism overprinted granulite block. 8.3. Tectonic implications in the South Tianshan orogenic belt Based on the occurrence of HP-UHP belt together with discovery of coeval low-P granulite-facies rocks to the north, a paired metamorphic belt tectonic model has been proposed for the south Tianshan (Li and Zhang, 2004; Zhang et al., 2007, 2013; Xia et al., 2014a). The northward subduction of the South Tianshan Paleo-Ocean underneath Yili-central Tianshan may start at Early Silurian and last until Early Carboniferous producing a series of continental arc along the southern margin of Yili-central Tianshan block (Gao et al., 2008; Han et al., 2011; Xia et al., 2014a, 2014b). There are a lot of mafic to felsic magmatic plutons related to continental arc from Early Silurian to Late Carboniferous reported in the South Tianshan, such as ~ 400 Ma migmatite in Kekesu Valley, Akeyazi Valley and Muzhaerte Valley (Xia et al., 2014a); two periods of arc magmatism occurred at 440-390 Ma and 360-320 Ma along the active continental margin of the Paleo-Kazakhstan continent (Xia et al., 2014b); 480-330 Ma granitoids exposed along the Kekesu river and the Bikai river across the central Tianshan block (Gao et al., 2008), 450-400 Ma granitoids in Baluntai region (Ma et al., 2014), 429-415 Ma arc-related gabbroic rocks from Senmtas, Kekesuhe and Qiakebu areas (Xu et al., 2013), ~ 430 Ma Jingbulake mafic-ultramafic intrusion which related to the subduction of the South Tianshan oceanic lithosphere (Yang and Zhou, 2009), 416-405 Ma dioritic-granitic plutons occurred in north of Baluntai area, central Tianshan (Yang et al., 2006), 361-313 Ma volcanic

rocks

from

Tekesdaban,

Dahalajunshan,

Xinyuan

and

Laerdundaban-Yuximolegai (Zhu et al., 2009), 436-366 Ma granitic intrusions in Nalati area (Zhu et al., 2006), 333-326 Ma gneissic porphyritic tonalite and garnet-bearing gneissic granitoids in Muzhaerte Valley (Gou et al., 2012). The mafic granulite in Yushugou formed at ~430 Ma, in according with the arc 16

magmas in the south Tianshan, which illustrates that the mafic granulite may come from the hanging wall of the subduction zone. The anticlockwise P-T path modelled above for the mafic granulites (Fig.12) shows that the metamorphic evolution is characterized by experiencing three stages of metamorphism. We propose a following tectonic evolution for the origin of HP mafic granulite in Yushugou. Stage I granulite facies metamorphism happened at ~390 Ma that may be related to the Devonian arc magmatic intrusion. Stage II (compression with cooling) took place at ~340 Ma and the granulites were possibly involved into the subduction slab of the paleo South Tianshan oceanic crust experienced high pressure granulite facies metamorphism, which may be related to the subduction erosion (Hacker et al., 2011; Gerya and Stöckhert, 2006). Stage III amphibolite facies metamorphism occurred at ~320 Ma during exhumation. The serpentinized peridotite unit associated with the granulite unit in Yushugou, affinities of the ultramafic rocks in Tonghuashan and Liuhuangshan in the southeast Tianshan, may represent the relics of the paleo south Tianshan oceanic crust (Wu et al., 1992; Wang et al., 1999a, 1999b; Xu et al., 2011; Yang et al., 2011). 9. Conclusions The HP mafic granulites

in Yushugou can be grouped into two types:

orthopyroxene-free and orthopyroxene-bearing granulites. Petrographic observations and phase equilibrium modeling suggest they have experienced three stages of metamorphism: granulite facies with 9.8-10.6 Kbar and 860-900 oC, HP granulite facies with the P-T conditions of 12.1-13.8 Kbar and 755-815 oC and amphibolite facies of 446-563 oC. The studies of U-Pb zircon dating and morphology, mineral inclusions, Th/U ratios and REE patterns of zircons from mafic granulites show that their protolith’s ages are ~430 Ma, reflecting Silurian magmatism, but the metamorphic ages are ~390 Ma, ~340 Ma and ~320 Ma, corresponding to granulite facies, HP granulite facies and amphibolite facies metamorphic stages respectively. Consequently, the Yushugou HP granulite has recorded an anticlockwise P-T path which should be formed by the northward super-subduction of south Tianshan paleo ocean from Devonian to Late Carboniferous. 17

Acknowledgements We greatly thank Prof. Chunjing Wei and Shuguang Song for their helpful discussions. We also thank Xiaoli Li for assistance with microprobe analyses and Fang Ma for zircon LA-ICP-MS dating. We also thank Huanglu Yu for his help in the field. Prof. Chunming Wu and an anonymous reviewer are acknowledgement for their careful reviews and constructive suggestions. This study was financially supported by the National Natural Science Foundation of China (Grants 41330210, 41520104004) and Major State Basic Research Development Program (Grant 2015CB856105).

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metapelitic granulites of the Musgrave Block, central Australia: constraints from mineral equilibria calculations in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2-Fe2O3. Journal of Metamorphic Geology 20, 41-55. White, R.W., Powell, R., Holland, T.J.B., 2007. Progress relating to calculation of partial melting equilibria for metapelites. Journal of Metamorphic Geology 25, 511-527. White, R.W., Powell, R., Holland, T.J.B., Worley, B.A., 2000. The effect of TiO 2 and Fe2O3 on metapelitic assemblages at greenschist and amphibolite facies conditions: mineral equilibria calculations in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3. Journal of Metamorphic Geology 18, 497-511. Whitehouse, M.J., Platt, J.P., 2003. Dating high-grade metamorphism—constraints from rare-earth elements in zircon and garnet. Contributions to Mineralogy and Petrology 145, 61-74. Whitney, D.L., Evans, B.W., 2009. Abbreviations for names of rock-forming minerals. American Mineralogist 95, 185-187. Wiedenbeck, M., AllÉ, P., Corfu, F., Griffin, W.L., Meier, M., Oberli, F., Quadt, A.V., Roddick, J.C., Spiegel, W., 1995. Three Natural Zircon Standards for U-Th-Pb, Lu-Hf, Trace Element And Ree Analyses. Geostandards and Geoanalytical Research 19, 1-23. Willner, A.P., Glodny, J., Gerya, T.V., Godoy, E., Massonne, H.J., 2004. A counterclockwise PTt path of high-pressure/low-temperature rocks from the Coastal Cordillera accretionary complex of south-central Chile: constraints for the earliest stage of subduction mass flow. Lithos 75, 283-310. Windley, B.F., Allen, M.B., Zhang, C., Zhao, Z.Y., Wang, G.R., 1990. Paleozoic accretion and cenozoic redeformation of the chinese tien shan range, central asia. Geology 18, 128. Wu, W.K., Jiang, C.Y., Yang, F., Li, L.C., 1992. The Yushugou-Tonghuashan structural mixtite in Xinjiang. Journal of Xi'an College of Geology 14, 8-13. Wu, Y.B., Zheng, Y.F., 2004. Genesis of zircon and its constraints on interpretation of U-Pb age. Chinese Science Bulletin 49, 1554-1569. XBGMR (Xinjiang Bureau of Geology and Mineral Resources), 1959. Geological map of Kumux sheet (K-45-17), scale 1: 200,000. XBGMR (Xinjiang Bureau of Geology and Mineral Resources), 1960. Geological map of Baoertu sheet (K-45-16), scale 1: 200,000. 25

Xia, B., Zhang, L.F., Bader, T., 2014a. Zircon U–Pb ages and Hf isotopic analyses of migmatite from the ‘paired metamorphic belt’ in Chinese SW Tianshan: Constraints on partial melting associated with orogeny. Lithos 192-195, 158-179. Xia, B., Zhang, L.F., Xia, Y., Bader, T., 2014b. The tectonic evolution of the Tianshan Orogenic Belt: Evidence from U–Pb dating of detrital zircons from the Chinese southwestern Tianshan accretionary mélange. Gondwana Research 25, 1627-1643. Xu, X.Y., Wang, H.L., Li, P., Chen, J.L., Ma, Z.P., Zhu, T., Wang, N., Dong, Y.P., 2013. Geochemistry and geochronology of Paleozoic intrusions in the Nalati (Narati) area in western Tianshan, Xinjiang, China: Implications for Paleozoic tectonic evolution. Journal of Asian Earth Sciences 72, 33-62. Xu, X.Y., Wang, H.L., Ma, G.L., Li, P., Chen, J.L., Li, T., 2010. Geochronology and Hf isotope characteristics of the Paleozoic granite in Nalati area, West Tianshan Mountains. Acta Petrologica et Mineralogica 29, 691-706. Xu, X.Z., Yang, J.S., Guo, G.L., Li, T.F., Ren, Y.F., Chen, S.Y., 2011. The Yushugou-Tonghuashan ophiolites in Tianshan, Xinjiang, and their tectonic setting. Acta Petrologica Sinica 27, 96-120. Yang, J.S., Xu, X.Z., Li, T.F., Chen, S.Y., Ren, Y.F., Li, J.Y., Liu, Z., 2011. U-Pb ages of zircons from ophiolite and related rocks in the Kumishi region at the southern margin of Middle Tianshan, Xinjiang: evidence of early Paleozoic oceanic basin. Acta Petrologica Sinica 27, 77-95. Yang, S.H., Zhou, M.F., 2009. Geochemistry of the ~430-Ma Jingbulake mafic–ultramafic intrusion in Western Xinjiang, NW China: Implications for subduction related magmatism in the South Tianshan orogenic belt. Lithos 113, 259-273. Yang, T.N., Li, J.Y., Sun, G.H., Wang, Y.B., 2006. Earlier Devonian active continental arc in Central Tianshan: evidence of geochemical analyses and zircon SHRIMP dating on mylonitized granitic rock. Acta Petrologica Sinica 22, 41-48. Yuan, H.L., Gao, S., Liu, X.M., Li, H.M., Günther, D. and Wu, F.Y., 2004. Accurate U–Pb age and trace element determinations of zircon by laser ablation-inductively coupled plasmamass spectrometry. Geostandards and Geoanalytical Research 28, 353-370. Zhang, L., Zhang, J.F., Jin, Z.M., 2016. Metamorphic P–T–water conditions of the Yushugou 26

granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny. Gondwana Research 29, 264-277. Zhang, L.F., Ai, Y.L., Song, S.G., Liou, J., Wei, C., 2007. A Brief Review of UHP Meta-ophiolitic Rocks, Southwestern Tianshan, Western China. International Geology Review 49, 811-823. Zhang, L.F., Du J.X, Lü Z.,

2013. A huge

oceanic-type UHP metamorphic belt in southwestern Tianshan, China: Peak metamorphic age and P-T path. Chinese Science Bulletin, 58, 4378-4383.

Zhao, G.C., Cawood, P.A., Wilde, S.A., Lu, L.Z., 2001. High-pressure granulites (retrograded eclogites) from the hengshan complex, north china craton: petrology and tectonic implications. Journal of Petrology 42, 1141-1170. Zhou, D.W., Su, L., Jian, P., Wang, R.S., Liu, X.M., Lu, G.X., Wang, J.L., 2004. Zircon U-Pb SHRIMP ages of high-pressure granulite in Yushugou ophiolitic terrane in southern Tianshan and their tectonic implications. Chinese Science Bulletin 49, 1415. Zhu, Y.F., Guo, X., Song, B., Zhang, L.F., Gu, L.B., 2009. Petrology, Sr-Nd-Hf isotopic geochemistry and zircon chronology of the Late Palaeozoic volcanic rocks in the southwestern Tianshan Mountains, Xinjiang, NW China. Journal of the Geological Society 166, 1085-1099. Zhu, Y.F., Zhou, J., Guo, X., 2006. Petrology and Sr-Nd isotopic geochemistry of the Carboniferous volcanic rocks in the western Tianshan Mountains, NW China. Acta Petrologica Sinica 22, 1341-1350. Zhu, Z.X., Wang, K.Z., Zheng, Y.J., Sun, G.H., Zhang, C., Li, Y.P., 2006. Zircon SHRIMP dating of Silurian and Devonian granitic intrusions in the southern Yili block, Xinjiang and preliminary discussion on their tectonic setting. Acta Petrologica Sinica 22, 1193-1200.

27

Figure and table captions Figure captions Fig.1. (a) Geological sketchy map of Chinese South Tianshan (modified from Lü et al., 2012;

XBGMR,

1959,

1960).

(b)

The

geological

map

of

Yushugou

granulite-peridotite complex (modified from XBGMR, 1960), the solid line labeled A - B represents the cross section in Fig.2.

Fig.2. Cross-section of the granulite-peridotite complex in the Yushugou. (a) the tectonic boundary between the HP granulite and the serpentinized peridotite; (b) the interbedded mafic granulite and felsic granulite; (c) the lenses of marble enclosed by felsic granulite. Red stars represent sample localities in this study. Y21-2 and Y25-2 are the Type I mafic granulite, Y18-8 is a Type II mafic granulite, Y14-14 is an amphibolite.

Fig.3. Photomicrographs of HP mafic granulites in the Yushugou (a) The HP mafic granulite (Y18-8) show distinct mylonitic foliation; (b) Large porphyroblastic garnet (GrtA) in a matrix consisting mainly of fine-grained garnet (GrtB), clinopyroxene, plagioclase, amphibole and ilmenite (Y21-2); (c) and (d) Porphyroblastic garnet (GrtA) and orthopyroxene (OpxA) in a matrix of fine-grained garnet (GrtB), orthopyroxene (OpxB), clinopyroxene, plagioclase (PlA), quartz, rutile and ilmenite (Y18-8); (e) and (f) Intergrowth of garnet (Grtc) and quartz around porphyroblastic OpxA (Y18-8); (g) The intergrowth of amphibole and plagioclase (PlB) after garnet (Y18-8); (h) Fine-grained amphibole and plagioclase in the amphibolite Y14-14.

Fig.4. Mineral chemistry diagrams showing variations of garnet, pyroxene, amphibole in HP mafic granulite from the Yushugou, Chinese South Tianshan; (a) and (b) Xprp-Xgrs diagrams showing core-rim variations (arrows) of garnet in Type I mafic 28

granulite (Y21-2) and Type II mafic granulite (Y18-8), respectively; (c) and (d) Compositional profiles of garnets (GrtA and GrtB) in Y21-2; (e) and (f) Compositional profiles of garnets (GrtA and GrtB) in Y18-8; (g) and (h) Wollastonite – enstatite – ferrosilite diagrams of pyroxene from Y21-2 and Y18-8 (Morimoto et al., 1988); (i) and (j) Discrimination diagrams for hornblendes from Y21-2 and Y14-14 (Leake et al., 1997). XGrs = Ca/(Ca + Mg + Fe2+ + Mn), XPrp = Mg/(Ca + Mg + Fe2+ + Mn), XAlm = Fe2+/(Ca + Mg + Fe2+ + Mn), XSps = Mn/(Ca + Mg + Fe2+ + Mn).

Fig.5. Mineral generations for different metamorphic evolutions in Y21-2 and Y18-8. Continuous lines represent minerals present in the samples, whereas dashed lines indicate inferred minerals.

Fig.6. P-T pseudosections calculated for granuiltes Y21-2 and Y18-8 in the NCFMASHTO system. Water is assumed to be in excess. The bulk rock compositions for phase equilibrium modeling are from Table 1. Various shades of colors represent different variants with higher variant fields darker-shaded and low variant fields lighter-shaded. (a) P-T pseudosection for Stage I of Type I mafic granulite (sample Y21-2); (b) P-T pseudosection for Stage II of Type I mafic granulite (sample Y21-2) using an effective bulk composition obtained by subtracting the core of Grt A composition from the XRF-based bulk composition； (c) P-T pseudosection for Stage I of Type II mafic granulite (sample Y18-8)； (d) P-T pseudosection for Stage II of Type II mafic granulite (sample Y18-8) using an effective bulk composition obtained by subtracting OpxA compositions and the core of Grt A composition from the XRF-based bulk composition. All the pseudosections are contoured with isopleths of z (Grt) = Ca/(Ca+Mg+Fe2+) and x (Grt) = Fe2+/(Fe2++Mg) contents in garnet. White circles represent core compositions of GrtA, while black circles represent compositions of GrtB and the rim of Grt A.

Fig.7. Representative CL images of zircons from mafic granulites and amphibolite in the Yushugou. Analytical spots and measured ages are marked. Regardless of their 29

different sizes, the diameters of all circles in this picture represent the analytical spot diameter of 32 μm.

Fig.8. Concordia diagrams for the investigated zircons from mafic granulites and amphibolite in the Yushugou.

Fig.9. Representative photomicrographs under plane-polarized light and BSE (back scattered electron) and corresponding CL images of inclusions in zircons from Y21-2. x(Grs) =Ca/(Ca+Mg+Fe2++Mn), analytical spots and measured ages are marked.

Fig.10. Th-U diagram of zircons from mafic granulites and amphibolite in the Yushugou.

Fig.11. Chondrite-normalized REE patterns of zircons from mafic granulites and amphibolite in the Yushugou.

Fig.12. P-T-t path for mafic granulites and amphibolite in the Yushugou. Stage I granulite facies metamorphism occurred at ~390 Ma with P-T conditions of 9.8-10.6 Kbar, 860-900 oC; Stage II HP granulite facies metamorphism occurred at ~340 Ma with P-T conditions of 12.1-13.8 Kbar, 755-815 oC; Stage III amphibolite facies metamorphism occurred at ~320 Ma with temperature condition of 446-563 oC. Symbols and boundaries of metamorphic facies follow Liou et al. (2004). P-T boxes of Zhang, Wang, Shu and Li refer to the P-T conditions for granulites from Zhang et al. (2016), Wang et at. (1999b), Shu et al. (2004) and Li et al. (2011), respectively.

Fig.13.

Schematic

cartoon

showing

the

tectonic

evolution

of

Yushugou

granulite-peridotite complex, Chinese South Tianshan. Yili-CT, Yili-Central Tianshan block. Green stars represent for the granulite unit, and the black represents the serpentinized oceanic slab. (a) Stage I occurred at ~390 Ma that may be related to the Devonian arc magmatic intrusion. (b) Stage II (compression with cooling) occurred at 30

~340 Ma. The granulites were possibly involved in the subduction slab of the South Tianshan Paleo-Ocean underneath Yili-Central Tianshan block and were undergone high pressure granulite facies metamorphism. (c) Stage III (amphibolite facies) took place at ~320 Ma during exhumation.

Table captions

Table 1. Bulk-rock compositions of the granulites (Y21-2 and Y18-8) from Yushugou, Chinese South Tianshan.

Table 2. Representative electron probe micro-analyses of rock-forming minerals from Y21-2, Y18-8 and Y14-14 from Yushugou, Chinese South Tianshan.

Table 3. Table 3. Representative electron probe micro-analyses of inclusions in zircons from Y21-2 and Y14-14.

Table 4. LA-ICP-MS U-Pb analyses of zircons from the mafic granulites and the amphibolite in Yushugou, Chinese South Tianshan.

31

Table 1

1

2

Table 1. Bulk-rock compositions of the granulites (Y21-2 and Y18-8) from Yushugou, Chinese South Tianshan. Samples SiO2 Al2O3 TiO2 Fe2O3 CaO MgO K2O Na2O MnO XRF analyses (wt%) Y21-2 45.09 15.36 1.49 15.41 13.53 5.86 0.06 2.23 0.31 Y18-8 49.76 16.59 2.09 14.15 6.66 7.22 0.31 2.55 0.21 Samples SiO2 Al2O3 TiO2 FeO CaO MgO Na2O O Total Stage I bulk-rock compositions (calculated by deducting the calcite composition from the XRF analyses, mol.%) Y21-2 49.43 9.92 1.23 12.71 14.35 9.57 2.37 0.41 100.00 Y18-8 54.18 10.65 1.72 11.59 6.46 11.73 2.69 0.99 100.00 Stage II bulk-rock compositions (calculated from mineral modes and microprobe analyses , mol.%) Y21-2 49.47 10.29 1.64 12.04 14.71 9.09 2.24 0.51 100.00 Y18-8 56.37 10.79 1.77 10.98 7.23 9.81 2.90 0.14 100.00 LOI, loss on ignition.

P2O5 LOI

Total

0.12 0.42 99.88 0.20 0.19 99.93

Table 2

1 2

Table 2. Representative electron probe micro-analyses of rock-formation minerals from Y21-2, Y18-8 and Y14-14 from Yushugou, Chinese South Tianshan. Y21-2 Grt

A

Grt

B

Y18-8 Cpx

Pl

Amp

Grt

A

Grt

B

Grt

C

Y14-14 A

B

Cpx

Opx

Opx

Pl

A

Pl

B

Amp

Amp

Pl

Mineral c

r

c

r

c

r

c

r

SiO2

37.81

38.44

37.80

38.35

49.15

57.55

40.83

38.80

38.05

39.49

39.45

38.92

51.03

50.61

52.85

56.23

50.09

49.05

53.22

49.70

TiO2

0.04

0.14

0.00

0.06

0.38

0.04

2.44

0.00

0.00

0.07

0.03

0.02

0.35

0.08

0.08

0.00

0.00

0.40

0.19

0.04

Al2O3

20.48

21.39

21.00

21.37

4.46

26.85

13.68

21.48

21.38

22.11

22.35

21.22

3.25

1.85

1.03

27.19

31.51

7.44

3.19

31.27

Cr2O3

0.06

0.11

0.05

0.14

0.00

0.02

0.44

0.07

0.02

0.08

0.00

0.05

0.03

0.07

0.11

0.00

0.00

0.05

0.21

0.01

Fe2O3

3.11

1.42

2.06

1.53

5.53

0.10

1.44

2.94

2.54

0.00

0.00

1.39

2.26

2.94

0.46

0.00

0.79

4.18

0.81

0.10

FeO

24.26

22.77

24.25

22.24

6.11

0.00

14.90

23.14

22.88

24.38

23.59

25.30

8.28

24.09

22.96

0.00

0.00

5.73

9.13

0.00

MnO

1.43

0.92

1.00

0.94

0.24

0.00

0.10

0.71

0.99

0.52

0.63

0.74

0.27

0.43

0.23

0.01

0.00

0.19

0.18

0.00

MgO

5.48

4.53

5.18

4.55

11.22

0.00

10.09

9.85

7.68

8.77

8.17

6.77

11.92

19.84

22.22

0.00

0.02

16.80

16.70

0.00

CaO

7.51

11.14

8.32

11.39

22.65

8.72

11.76

3.72

5.93

4.97

5.85

6.53

22.53

0.55

0.30

9.39

14.35

11.28

12.54

14.77

Na2O

0.03

0.01

0.01

0.00

0.81

7.19

2.63

0.05

0.07

0.00

0.03

0.01

0.53

0.00

0.02

6.78

3.26

1.28

0.27

3.27

K2O

0.00

0.00

0.00

0.00

0.00

0.11

0.64

0.00

0.00

0.02

0.01

0.01

0.07

0.00

0.00

0.29

0.08

0.09

0.06

0.01

Total

99.89

100.72

99.46

100.43

100.00

100.58

98.81

100.47

99.29

100.42

100.12

100.82

100.28

100.17

100.21

99.89

100.10

96.07

96.42

99.17

Oxygen

12.00

12.00

12.00

12.00

6.00

8.00

23.00

12.00

12.00

12.00

12.00

12.00

6.00

6.00

6.00

8.00

8.00

23.00

23.00

8.00

Si

2.96

2.97

2.97

2.97

1.84

2.57

6.07

2.95

2.95

3.01

3.01

3.00

1.91

1.91

1.97

2.54

2.28

7.05

7.65

2.29

Ti

0.00

0.01

0.00

0.00

0.01

0.00

0.27

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.04

0.02

0.00

Al

1.89

1.95

1.94

1.95

0.20

1.41

2.40

1.93

1.96

1.99

2.01

1.93

0.14

0.08

0.05

1.45

1.69

1.26

0.54

1.70

Cr

0.00

0.01

0.00

0.01

0.00

0.00

0.05

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.02

0.00

Fe3+

0.18

0.08

0.12

0.09

0.16

0.00

0.16

0.17

0.15

0.00

0.00

0.08

0.06

0.08

0.01

0.00

0.03

0.45

0.09

0.00

Fe2+

1.59

1.47

1.59

1.44

0.19

0.00

1.85

1.47

1.49

1.55

1.51

1.63

0.26

0.76

0.72

0.00

0.00

0.69

1.10

0.00

Mn

0.10

0.06

0.07

0.06

0.01

0.00

0.01

0.05

0.07

0.03

0.04

0.05

0.01

0.01

0.01

0.00

0.00

0.02

0.02

0.00

Mg

0.64

0.52

0.61

0.53

0.63

0.00

2.23

1.12

0.89

1.00

0.93

0.78

0.66

1.12

1.23

0.00

0.00

3.60

3.58

0.00

Ca

0.63

0.92

0.70

0.95

0.91

0.42

1.87

0.30

0.49

0.41

0.48

0.54

0.90

0.02

0.01

0.45

0.70

1.74

1.93

0.73

Na

0.01

0.00

0.00

0.00

0.06

0.62

0.76

0.01

0.01

0.00

0.00

0.00

0.04

0.00

0.00

0.59

0.29

0.36

0.08

0.29

K

0.00

0.00

0.00

0.00

0.00

0.01

0.12

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.02

0.01

0.02

0.01

0.00

Sum

8.00

8.00

8.00

8.00

4.00

5.04

15.85

8.00

8.00

7.99

7.98

8.00

4.00

4.00

4.00

5.05

5.00

15.38

15.07

5.01

XGrs

0.21

0.31

0.24

0.32

0.10

0.17

0.14

0.16

0.18

XPrp

0.22

0.18

0.20

0.18

0.38

0.30

0.33

0.31

0.26

XAlm

0.54

0.49

0.54

0.48

0.50

0.51

0.52

0.51

0.54

XSps

0.03

0.02

0.02

0.02

0.02

0.02

0.01

0.01

0.02 0.43

0.71

Xan XMg

3 4 5

0.40 0.29

0.26

0.28

0.27 2+

0.77

0.43

0.37 2+

0.39

0.38

0.32 2+

0.72

0.59 2+

0.71

0.63

XGrs = Ca/(Ca + Mg + Fe + Mn), XPrp = Mg/(Ca + Mg + Fe + Mn), XAlm = Fe /(Ca + Mg + Fe + Mn), XSps = Mn/(Ca + Mg + Fe2+ + Mn), Xan = Ca/(Ca + Na), XMg = Mg/(Mg + Fe2+). C, core; r, rim. The mineral formulae ferric iron were calculated with the program AX (Holland; http://www.esc.cam.ac.uk/research/research-groups/holland/ax).

Table 3

1

Table 3. Representative electron probe micro-analyses of inclusions in zircons from Y21-2 and Y14-14. Y21-2

Y14-14 Amp Pl

Mineral SiO2 TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O Total Oxygen Si Ti Al Cr

Cpx 50.82 0.47 4.12 0.00 1.36 8.78 0.15 11.53 22.07 0.74 0.00 99.90 6.00 1.90 0.01 0.18 0.00

38.44 0.03 21.13 0.12 1.30 24.80 0.99 4.59 8.48 0.22 0.04 100.01 12.00 3.00 0.00 1.95 0.01

38.46 0.03 21.09 0.00 0.55 24.80 0.98 5.08 8.55 0.04 0.01 99.53 12.00 3.01 0.00 1.95 0.00

38.73 0.08 21.17 0.05 0.86 23.80 1.13 4.41 10.01 0.15 0.02 100.32 12.00 3.01 0.01 1.94 0.00

38.04 0.06 21.07 0.17 2.25 23.02 0.84 4.51 9.71 0.21 0.07 99.73 12.00 2.97 0.00 1.94 0.01

38.65 0.04 21.24 0.03 2.16 24.59 0.94 4.58 7.92 0.36 0.24 100.53 12.00 3.00 0.00 1.94 0.00

38.70 0.10 21.58 0.02 0.00 24.44 0.98 4.20 10.19 0.01 0.00 100.22 12.00 3.01 0.01 1.98 0.00

38.15 0.07 21.35 0.11 0.86 25.11 0.66 4.61 9.01 0.03 0.00 99.88 12.00 2.99 0.00 1.97 0.01

38.20 0.06 21.43 0.02 0.05 23.64 1.54 4.79 9.34 0.00 0.01 99.07 12.00 3.00 0.00 1.99 0.00

50.69 0.39

49.00 0.03

6.96 0.07 4.19 5.84 0.33 17.67 10.96 0.84 0.12 97.64 23.00 7.14 0.04 1.16 0.01

32.36 0.01 0.02 0.00 0.00 0.00 14.88 3.26 0.00 99.57 8.00 2.25 0.00 1.75 0.00

Fe3+

0.04

0.08

0.03

0.05

0.13

0.13

0.00

0.05

0.00

0.44

0.00

2+

0.28

1.62

1.62

1.55

1.51

1.60

1.59

1.64

1.55

0.01 0.64 0.89

0.07 0.53 0.71

0.07 0.59 0.72

0.07 0.51 0.83

0.06 0.53 0.81

0.06 0.53 0.66

0.07 0.49 0.85

0.04 0.54 0.76

0.10 0.56 0.79

0.69 0.04 3.71 1.66

0.00 0.00 0.00 0.73

Fe

Mn Mg Ca

Grt

Na K Sum XGrs XPrp XAlm XSps Xan XMg 2 3 4 5

0.05 0.00 4.00

0.03 0.00 8.00 0.24 0.18

0.01 0.00 8.00 0.24 0.20

0.02 0.00 8.00 0.28 0.17

0.03 0.01 8.00 0.28 0.18

0.05 0.02 8.00 0.23 0.19

0.00 0.00 7.99 0.28 0.16

0.01 0.00 8.00 0.25 0.18

0.00 0.00 8.00 0.26 0.19

0.55 0.02

0.54 0.02

0.52 0.02

0.52 0.02

0.56 0.02

0.53 0.02

0.55 0.01

0.52 0.03

0.23 0.02 15.29

0.29 0.00 5.02

0.72 0.70

0.25 2+

0.27

0.25

0.26 2+

0.25

0.23 2+

0.25

0.27 2+

XGrs = Ca/(Ca + Mg + Fe + Mn), XPrp = Mg/(Ca + Mg + Fe + Mn), XAlm = Fe /(Ca + Mg + Fe + Mn), XSps = Mn/(Ca + Mg + Fe2+ + Mn), Xan = Ca/(Ca + Na), XMg = Mg/(Mg + Fe2+). C, core; r, rim. The mineral formulae ferric iron were calculated with the program AX (Holland; http://www.esc.cam.ac.uk/research/research-groups/holland/ax).

Table 4

Table 4. LA-ICP-MS U-Pb analyses of zircons from the mafic granulites and the amphibolite in Yushugou, Chinese South Tianshan. spot

position

Granulite Y21-2 Y21-2-01 meta Y21-2-02 meta Y21-2-03 meta Y21-2-04 core Y21-2-05 meta Y21-2-06 meta Y21-2-07 meta Y21-2-08 meta Y21-2-09 meta Y21-2-10 meta Y21-2-11 core Y21-2-12 core Y21-2-13 core Y21-2-14 core Y21-2-15 rim Y21-2-16 meta Y21-2-17 core Y21-2-18 meta Y21-2-19 meta Y21-2-20 meta Y21-2-21 meta Y21-2-22 meta Y21-2-23 meta

Th

U

Th/U

56.9 2.1 24.3 186.0 123.6 20.1 56.3 1.3 1.0 1.6 102.2 114.4 181.0 289.3 67.9 4.1 640.7 186.5 6.7 12.2 116.7 278.3 31.5

256.6 45.1 272.6 267.5 151.7 141.1 239.8 25.3 18.3 13.4 158.7 195.2 539.9 255.6 166.6 27.1 1131.7 353.0 180.4 182.7 241.7 459.1 108.8

0.22 0.05 0.09 0.70 0.81 0.14 0.23 0.05 0.06 0.12 0.64 0.59 0.34 1.13 0.41 0.15 0.57 0.53 0.04 0.07 0.48 0.61 0.29

207

Pb/206Pb

0.05503 0.05310 0.05404 0.05510 0.05322 0.05484 0.05389 0.05416 0.05374 0.05366 0.05616 0.05430 0.05491 0.05534 0.05400 0.05450 0.05514 0.05413 0.05390 0.05342 0.05416 0.05486 0.05407

1σ 0.00163 0.00483 0.00165 0.00090 0.00134 0.00218 0.00111 0.00432 0.00501 0.00093 0.00114 0.00120 0.00094 0.00112 0.00119 0.00348 0.00089 0.00098 0.00129 0.00123 0.00114 0.00093 0.00158

207

Pb/235U

0.41867 0.39830 0.39023 0.53957 0.45849 0.41111 0.45645 0.40027 0.39778 0.40761 0.54263 0.51731 0.51243 0.52540 0.47202 0.41152 0.52248 0.47209 0.39874 0.40240 0.46100 0.47894 0.47143

1σ 0.01199 0.03588 0.01148 0.00792 0.01095 0.01585 0.00874 0.03134 0.03657 0.00642 0.01016 0.01074 0.00792 0.00984 0.00973 0.02572 0.00761 0.00779 0.00897 0.00867 0.00904 0.00736 0.01324

206

Pb/238U

0.05519 0.05441 0.05239 0.07104 0.06250 0.05438 0.06145 0.05362 0.05370 0.05510 0.07009 0.06911 0.06770 0.06888 0.06341 0.05478 0.06874 0.06327 0.05367 0.05465 0.06175 0.06333 0.06326

1σ

ρ

0.00051 0.00085 0.00060 0.00062 0.00063 0.00065 0.00058 0.00101 0.00099 0.00049 0.00066 0.00064 0.00060 0.00065 0.00061 0.00086 0.00060 0.00057 0.00054 0.00053 0.00059 0.00056 0.00070

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

207

Pb/235U

1σ

355 340 335 438 383 350 382 342 340 347 440 423 420 429 393 350 427 393 341 343 385 397 392

9 26 8 5 8 11 6 23 27 5 7 7 5 7 7 19 5 5 7 6 6 5 9

206

Pb/238U

1σ

346 342 329 442 391 341 384 337 337 346 437 431 422 429 396 344 429 395 337 343 386 396 395

3 5 4 4 4 4 4 6 6 3 4 4 4 4 4 5 4 3 3 3 4 3 4

Y21-2-24 Y21-2-25 Y21-2-26 Y21-2-27 Y21-2-28 Y21-2-29 Y21-2-30 Y21-2-31 Y21-2-32 Y21-2-33 Y21-2-34 Y21-2-35 Y21-2-36 Y21-2-37 Y21-2-38 Y21-2-39 Y21-2-40 Y21-2-41 Y21-2-42

meta meta meta meta meta core core meta meta meta meta meta core meta meta meta meta meta core

1.1 110.8 103.2 94.7 56.4 325.2 106.3 37.5 37.8 4.2 2.7 17.3 317.3 1.0 52.7 77.9 46.2 206.3 247.6

13.1 1002.1 273.5 668.1 97.8 611.8 249.7 129.8 386.0 45.5 26.9 151.5 577.1 6.8 312.3 183.7 85.7 236.9 346.1

0.08 0.11 0.38 0.14 0.58 0.53 0.43 0.29 0.10 0.09 0.10 0.11 0.55 0.15 0.17 0.42 0.54 0.87 0.72

0.05505 0.05318 0.05453 0.05439 0.05488 0.05511 0.05466 0.05447 0.05436 0.05429 0.05333 0.05368 0.05571 0.05425 0.05265 0.05575 0.05559 0.05348 0.05493

0.00094 0.00075 0.00123 0.00090 0.00240 0.00089 0.00134 0.00158 0.00127 0.00100 0.00516 0.00203 0.00099 0.00106 0.00119 0.00151 0.00200 0.00123 0.00116

0.40739 0.39984 0.46422 0.40253 0.47768 0.53298 0.52634 0.46653 0.40989 0.40601 0.39452 0.39692 0.55547 0.40255 0.39004 0.48261 0.47772 0.45828 0.55466

0.00645 0.00513 0.00995 0.00617 0.02045 0.00799 0.01229 0.01298 0.00913 0.00700 0.03768 0.01463 0.00921 0.00737 0.00837 0.01251 0.01664 0.01004 0.01109

0.05369 0.05454 0.06176 0.05369 0.06315 0.07016 0.06986 0.06214 0.05470 0.05426 0.05367 0.05364 0.07234 0.05383 0.05374 0.06280 0.06235 0.06217 0.07326

0.00050 0.00049 0.00064 0.00050 0.00075 0.00065 0.00076 0.00069 0.00055 0.00052 0.00101 0.00062 0.00069 0.00053 0.00055 0.00070 0.00078 0.00064 0.00074

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

347 342 387 343 396 434 429 389 349 346 338 339 449 343 334 400 397 383 448

5 4 7 4 14 5 8 9 7 5 27 11 6 5 6 9 11 7 7

337 342 386 337 395 437 435 389 343 341 337 337 450 338 337 393 390 389 456

3 3 4 3 5 4 5 4 3 3 6 4 4 3 3 4 5 4 4

Granulite Y25-2 Y25-2-01 meta Y25-2-02 meta Y25-2-03 meta Y25-2-04 meta Y25-2-05 meta

4.1 15.5 0.9 33.2 2.9

123.6 339.4 30.4 853.8 128.6

0.03 0.05 0.03 0.04 0.02

0.05248 0.05475 0.05478 0.05432 0.05373

0.00995 0.00433 0.01067 0.00245 0.00860

0.39878 0.42020 0.41970 0.41095 0.39340

0.07503 0.03270 0.08113 0.01808 0.06225

0.05511 0.05566 0.05557 0.05487 0.05310

0.00157 0.00107 0.00161 0.00082 0.00155

0.9 0.9 0.9 0.9 0.9

341 356 356 350 337

54 23 58 13 45

346 349 349 344 334

10 7 10 5 9

Y25-2-06 Y25-2-07 Y25-2-08 Y25-2-09 Y25-2-10 Y25-2-11 Y25-2-12 Y25-2-13 Y25-2-14 Y25-2-15 Y25-2-16 Y25-2-17 Y25-2-18 Y25-2-19 Y25-2-20 Y25-2-21 Y25-2-22 Y25-2-23

meta meta meta meta meta meta meta meta meta meta meta meta meta meta meta meta meta meta

9.5 19.3 5.5 4.3 6.1 16.5 23.5 3.5 23.3 13.3 5.6 5.6 13.0 26.4 3.1 2.6 13.1 0.9

356.7 361.6 169.9 176.9 162.7 437.7 360.1 92.2 442.5 400.7 83.8 196.5 298.2 485.4 143.9 80.7 269.5 61.1

0.03 0.05 0.03 0.02 0.04 0.04 0.07 0.04 0.05 0.03 0.07 0.03 0.04 0.05 0.02 0.03 0.05 0.01

0.05468 0.05228 0.05205 0.05241 0.05359 0.05431 0.05461 0.05464 0.05215 0.05474 0.05430 0.05329 0.05162 0.05482 0.05438 0.05420 0.05184 0.05440

0.00370 0.00742 0.00676 0.00542 0.00794 0.00297 0.00400 0.01538 0.00349 0.00471 0.01219 0.00498 0.00521 0.00365 0.00808 0.01759 0.00454 0.01654

0.40863 0.39208 0.39471 0.39756 0.41038 0.40789 0.41790 0.40380 0.38395 0.40475 0.41276 0.40822 0.38779 0.42007 0.40822 0.40426 0.39019 0.41342

0.02713 0.05534 0.05066 0.04057 0.06024 0.02180 0.03018 0.11307 0.02528 0.03437 0.09196 0.03755 0.03867 0.02743 0.05983 0.13038 0.03367 0.12501

0.05420 0.05438 0.05499 0.05501 0.05553 0.05446 0.05549 0.05359 0.05338 0.05362 0.05511 0.05554 0.05447 0.05556 0.05443 0.05408 0.05458 0.05510

0.00097 0.00102 0.00137 0.00120 0.00145 0.00088 0.00095 0.00179 0.00090 0.00099 0.00184 0.00116 0.00114 0.00098 0.00161 0.00228 0.00109 0.00206

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

348 336 338 340 349 347 355 344 330 345 351 348 333 356 348 345 335 351

20 40 37 29 43 16 22 82 19 25 66 27 28 20 43 94 25 90

340 341 345 345 348 342 348 337 335 337 346 348 342 349 342 340 343 346

6 6 8 7 9 5 6 11 6 6 11 7 7 6 10 14 7 13

Granulite Y18-8 Y18-8-01 core Y18-8-02 rim Y18-8-03 core Y18-8-04 rim Y18-8-05 core Y18-8-06 rim

219.6 35.7 63.2 48.4 73.2 50.8

404.9 54.9 428.0 71.3 122.6 82.2

0.54 0.65 0.15 0.68 0.60 0.62

0.05454 0.05510 0.05763 0.05511 0.05508 0.05396

0.00150 0.00405 0.00191 0.00302 0.00124 0.00273

0.52340 0.47796 0.55427 0.47048 0.57409 0.46789

0.01396 0.03458 0.01785 0.02527 0.01245 0.02314

0.06958 0.06293 0.06978 0.06193 0.07560 0.06290

0.00082 0.00105 0.00087 0.00089 0.00084 0.00091

0.9 0.9 0.9 0.9 0.9 0.9

427 397 448 392 461 390

9 24 12 17 8 16

434 393 435 387 470 393

5 6 5 5 5 6

Y18-8-07 Y18-8-08 Y18-8-09 Y18-8-10 Y18-8-11 Y18-8-12 Y18-8-13 Y18-8-14 Y18-8-15 Y18-8-16 Y18-8-17 Y18-8-18 Y18-8-19 Y18-8-20 Y18-8-21 Y18-8-22 Y18-8-23 Y18-8-24 Y18-8-25 Y18-8-26 Y18-8-27 Y18-8-28 Y18-8-29 Y18-8-30 Y18-8-31 Y18-8-32

meta meta meta meta meta meta rim core meta meta meta meta meta meta meta meta meta meta meta meta meta rim core meta rim core

100.0 62.8 122.3 194.4 64.6 38.3 80.3 40.3 42.8 182.8 209.7 135.7 945.0 46.2 35.9 114.4 32.8 374.5 29.5 1585.3 42.6 78.9 115.6 26.7 87.2 165.1

161.9 96.2 125.0 231.8 70.4 45.9 120.6 79.1 68.1 282.6 308.1 154.0 1149.8 72.1 68.6 918.1 40.4 2916.7 36.0 1129.9 65.4 72.6 584.6 37.7 236.8 264.1

0.62 0.65 0.98 0.84 0.92 0.83 0.67 0.51 0.63 0.65 0.68 0.88 0.82 0.64 0.52 0.12 0.81 0.13 0.82 1.40 0.65 1.09 0.20 0.71 0.37 0.63

0.05304 0.05318 0.05323 0.05463 0.05379 0.05431 0.05497 0.05576 0.05514 0.05426 0.05357 0.05382 0.05503 0.05355 0.05540 0.05448 0.05542 0.05472 0.05318 0.05509 0.05596 0.05439 0.06020 0.05504 0.05426 0.05651

0.00174 0.00274 0.00295 0.00147 0.00428 0.00392 0.00208 0.00269 0.00327 0.00138 0.00146 0.00186 0.00094 0.00361 0.00304 0.00103 0.00587 0.00093 0.00483 0.00102 0.00487 0.00359 0.00121 0.00459 0.00161 0.00153

0.45925 0.46151 0.45859 0.47510 0.46217 0.46944 0.47178 0.53332 0.47665 0.47297 0.46460 0.46657 0.47897 0.45880 0.47132 0.46471 0.47802 0.47662 0.46249 0.47928 0.48140 0.47042 0.63493 0.47067 0.45901 0.54193

0.01451 0.02332 0.02500 0.01225 0.03638 0.03329 0.01728 0.02504 0.02774 0.01145 0.01216 0.01558 0.00763 0.03047 0.02531 0.00820 0.05009 0.00757 0.04147 0.00834 0.04151 0.03057 0.01198 0.03862 0.01305 0.01402

0.06281 0.06295 0.06250 0.06309 0.06233 0.06270 0.06226 0.06938 0.06271 0.06323 0.06292 0.06290 0.06314 0.06215 0.06172 0.06189 0.06257 0.06319 0.06309 0.06312 0.06240 0.06274 0.07651 0.06204 0.06137 0.06956

0.00074 0.00085 0.00083 0.00069 0.00095 0.00109 0.00080 0.00101 0.00096 0.00068 0.00067 0.00076 0.00059 0.00092 0.00091 0.00060 0.00121 0.00059 0.00115 0.00060 0.00099 0.00097 0.00076 0.00114 0.00070 0.00078

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

384 385 383 395 386 391 392 434 396 393 387 389 397 383 392 388 397 396 386 398 399 391 499 392 384 440

10 16 17 8 25 23 12 17 19 8 8 11 5 21 17 6 34 5 29 6 28 21 7 27 9 9

393 394 391 394 390 392 389 432 392 395 393 393 395 389 386 387 391 395 394 395 390 392 475 388 384 434

4 5 5 4 6 7 5 6 6 4 4 5 4 6 6 4 7 4 7 4 6 6 5 7 4 5

Y18-8-33 Y18-8-34 Y18-8-35 Y18-8-36 Y18-8-37 Y18-8-38 Y18-8-39 Y18-8-40 Y18-8-41 Y18-8-42 Y18-8-43 Y18-8-44 Y18-8-45

meta meta core rim meta meta meta meta meta meta meta meta meta

124.5 48.0 104.7 56.1 81.4 62.6 54.7 32.3 33.1 44.7 25.9 59.5 89.9

201.5 72.9 179.6 67.3 91.6 103.0 58.0 46.7 53.8 49.2 33.4 99.2 149.8

0.62 0.66 0.58 0.83 0.89 0.61 0.94 0.69 0.61 0.91 0.78 0.60 0.60

0.05512 0.05355 0.05842 0.05299 0.05329 0.05308 0.05401 0.05375 0.05341 0.05334 0.05306 0.05520 0.05347

0.00199 0.00334 0.00137 0.00520 0.00245 0.00318 0.00339 0.00540 0.00397 0.00413 0.00539 0.00285 0.00243

0.46716 0.46644 0.63233 0.45746 0.46105 0.45864 0.46730 0.45880 0.46374 0.45623 0.46138 0.47720 0.46487

0.01628 0.02863 0.01409 0.04450 0.02070 0.02703 0.02881 0.04544 0.03393 0.03474 0.04627 0.02411 0.02068

0.06148 0.06319 0.07853 0.06263 0.06276 0.06269 0.06276 0.06192 0.06299 0.06205 0.06308 0.06272 0.06307

0.00078 0.00094 0.00081 0.00104 0.00086 0.00089 0.00096 0.00131 0.00109 0.00111 0.00130 0.00089 0.00079

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

389 389 498 382 385 383 389 383 387 382 385 396 388

11 20 9 31 14 19 20 32 24 24 32 17 14

385 395 487 392 392 392 392 387 394 388 394 392 394

5 6 5 6 5 5 6 8 7 7 8 5 5

Amphibolite Y14-14 Y14-14-01 meta Y14-14-02 meta Y14-14-03 meta Y14-14-04 meta Y14-14-05 meta Y14-14-06 meta Y14-14-07 meta Y14-14-08 meta Y14-14-09 meta Y14-14-10 meta Y14-14-11 meta

15.2 12.9 55.0 12.8 25.0 12.2 14.1 41.9 12.4 12.3 11.9

154.8 167.4 241.6 113.2 139.7 123.7 124.3 260.2 109.1 109.1 153.6

0.10 0.08 0.23 0.11 0.18 0.10 0.11 0.16 0.11 0.11 0.08

0.05265 0.05351 0.05361 0.05395 0.05157 0.05224 0.05311 0.05381 0.05165 0.05407 0.05442

0.00181 0.00142 0.00123 0.00252 0.00253 0.00169 0.00233 0.00123 0.00162 0.00177 0.00184

0.36541 0.37981 0.37432 0.37024 0.35809 0.36091 0.37836 0.38948 0.37393 0.38683 0.38181

0.01229 0.00973 0.00827 0.01694 0.01736 0.01130 0.01633 0.00858 0.01141 0.01227 0.01258

0.05035 0.05149 0.05066 0.04978 0.05038 0.05012 0.05168 0.05251 0.05252 0.05191 0.05090

0.00060 0.00060 0.00055 0.00066 0.00062 0.00061 0.00064 0.00057 0.00062 0.00064 0.00060

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

316 327 323 320 311 313 326 334 323 332 328

9 7 6 13 13 8 12 6 8 9 9

317 324 319 313 317 315 325 330 330 326 320

4 4 3 4 4 4 4 3 4 4 4

Y14-14-12 Y14-14-13 Y14-14-14 Y14-14-15 Y14-14-16 Y14-14-17 Y14-14-18 Y14-14-19 Y14-14-20 Y14-14-21 Y14-14-22 Y14-14-23 Y14-14-24 Y14-14-25

meta meta meta meta meta meta meta meta meta meta meta meta meta meta

13.8 16.4 38.9 12.6 9.1 13.0 50.1 21.1 14.6 12.8 97.9 19.3 12.3 9.2

108.9 182.7 212.1 109.1 96.2 111.6 240.2 156.0 129.0 119.8 377.5 152.3 107.1 77.1

Note: meta, metamorphic zircon.

0.13 0.09 0.18 0.12 0.09 0.12 0.21 0.14 0.11 0.11 0.26 0.13 0.11 0.12

0.05301 0.05370 0.05377 0.05334 0.05247 0.05239 0.05360 0.05236 0.05396 0.05121 0.05344 0.05381 0.05206 0.05241

0.00285 0.00198 0.00131 0.00184 0.00203 0.00176 0.00124 0.00253 0.00221 0.00194 0.00108 0.00110 0.00195 0.00231

0.37408 0.37169 0.38387 0.38256 0.37900 0.36546 0.37585 0.37550 0.38990 0.35482 0.37497 0.37145 0.36530 0.36887

0.01983 0.01342 0.00899 0.01283 0.01425 0.01191 0.00837 0.01794 0.01564 0.01310 0.00729 0.00728 0.01333 0.01585

0.05120 0.05021 0.05179 0.05203 0.05240 0.05061 0.05087 0.05203 0.05242 0.05027 0.05090 0.05009 0.05091 0.05106

0.00066 0.00058 0.00058 0.00066 0.00069 0.00064 0.00056 0.00062 0.00065 0.00064 0.00054 0.00053 0.00066 0.00073

0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9 0.9

323 321 330 329 326 316 324 324 334 308 323 321 316 319

15 10 7 9 10 9 6 13 11 10 5 5 10 12

322 316 326 327 329 318 320 327 329 316 320 315 320 321

4 4 4 4 4 4 3 4 4 4 3 3 4 4

Graphical abstract

b

32

Highlights:

 Three-stage metamorphic evolution of HP mafic granulites in Yushugou has been recognized based on the petrological study and phase modelling.  The U-Pb zircon ages of three metamorphic stages have been reported in this paper.  An anticlockwise PT path for HP mafic granulites in Yushugou is proposed.

33