Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny

GR-01379; No of Pages 14 Gondwana Research xxx (2015) xxx–xxx

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Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny Li Zhang ⁎, Junfeng Zhang, Zhenmin Jin ⁎ State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China

a r t i c l e

i n f o

Article history: Received 28 May 2014 Received in revised form 25 December 2014 Accepted 28 December 2014 Available online xxxx Handling Editor: W.J. Xiao Keywords: UHT granulite Magmatic arc Nominally anhydrous minerals Accretionary orogeny Tianshan orogen

a b s t r a c t The South Tianshan Accretionary Complex (STAC), forming the southern segment of the Central Asian Orogenic Belt, underwent a long-lived and subduction-related accretionary orogenic process. The high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks within this complex are traditionally considered to be metamorphic ophiolitic slices. In this paper, we report a detailed study of petrology and water content of nominally anhydrous minerals (NAMs) of granulites from the Yushugou HP massif occurring as a fault-bounded tectonic slab in the Paleozoic accretionary complex. The studied granulites consist of garnet, orthopyroxene, plagioclase, K-feldspar, quartz, biotite, ilmenite and rutile and show distinct mylonitic foliation. The augen garnet is dominated by almandine and pyrope components, and has compositional zoning with increasing grossular content from the core to rim of the grain. The augen orthopyroxene has high Al2O3 content (up to 7.91 wt.%), and shows compositional zoning characterized by a decreasing Al2O3 content from core to rim. Phase equilibria modeling indicates that the granulite underwent ultrahigh-temperature (UHT) (N 930 °C) and HP (10.5–14.5 kbar) metamorphism and partial melting under a high geothermal gradient of ca. 24 °C/km, and a possible prograde process characterized by heating and burial. Analyses of Fourier transform infrared spectroscopy indicate that hydrogen was incorporated in all NAMs of the granulites in the manner structural OH and sub-microscopic fluid inclusions and that the average water content (H2O weight) is in the range of 63–215 ppm in garnet, 1–54 ppm in orthopyroxene, 172–533 ppm in feldspar and 34–66 ppm in quartz. The present results show that the Yushugou massif probably derived from the deep root of hot continental magmatic arc. The trace amounts of water in NAMs obviously affected ductile deformation of the near-dry granulites. This study indicates that the thickened lower crust of the Paleozoic Tianshan accretionary orogen is characterized by high-thermal flow, UHT granulite-facies metamorphism, anatexis, ductile deformation and coeval magmatism and crustal growth. © 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction The architecture of the Central Asian Orogenic Belt (CAOB) and the geodynamic implications for Phanerozoic continental growth have long been a major concern of the international community (Mossakovsky et al., 1993; Şengör et al., 1993; Xiao & Santosh, 2014 and references therein). Subduction-related accretion mainly in the Paleozoic gave rise to the present 2400 km-long Tianshan orogenic collage that extends from the Aral Sea eastwards through Uzbekistan, Tajikistan, Kyrgyzstan, and to Xinjiang in China (Xiao et al., 2013; Xiao & Santosh, 2014 and references therein). The northern part of the orogenic collage was developed by consumption of the Junggar–Balkash Ocean, forming the North Tianshan Accretionary Complex; the southern part of the orogenic collage was developed by consumption of the South Tianshan Ocean (PaleoAsian Ocean) which gave rise to the formation of the South Tianshan (Kokshaal–Kumishi) Accretionary Complex, that separates the Central Tianshan from the Tarim craton (Fig. 1). This accretionary complex is ⁎ Corresponding authors. E-mail addresses: [email protected] (L. Zhang), [email protected] (Z. Jin).

characterized by a general southward and oceanward accretion by northward subduction in the early Paleozoic to Permian time. The initial docking of the southerly Tarim craton to this accretionary complex occurred in the Late Carboniferous–Early Permian in the eastern part of the Tianshan and in the Late Permian in the western part (Xiao et al., 2013). The South Tianshan Accretionary Complex (STAC) is composed of various components with different origins, including microcontinents, magmatic arcs, oceanic plateaus, seamounts, ophiolitic mélanges and accretionary complexes (Windley et al., 1990; Jahn et al., 2004; Windley et al., 2007; Rojas-Agramonte et al., 2011; Kröner et al., 2013; Xiao et al., 2013). These rocks have partly undergone high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism with various ages (Gao et al., 1999; Gao & Klemd, 2001, 2003; Klemd et al., 2005; Hegner et al., 2010; Su et al., 2010; Q.L. Li et al., 2011). There have been different views about the tectonic significance of these HP/UHP rocks in the STAC. Some workers have interpreted the ages of these HP/UHP rocks as syn-tectonic, or as a result of amalgamation of many terranes or arc-collision (Charvet et al., 2007; Wang et al., 2011), thus dating collision of the Tarim Craton with the southern Siberian

http://dx.doi.org/10.1016/j.gr.2014.12.009 1342-937X/© 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

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Fig. 1. (a) Tectonic map of the Chinese Western Tianshan showing major Early Paleozoic tectonic assemblages and structures (modified after Xiao et al. (2013)), and the location of b. Inset shows tectonic framework of the Central Asian Orogenic Belt and the location of a. (b) A sketch geological map of the Yushugou massif (modified after Zhou et al. (2004)).

accretionary system to the north. Some other investigators argued that these HP/UHP rocks occur as blocks (Zhang et al., 2007; Xiao et al., 2008), derived from different depths of the slab and mixed within a subduction channel during exhumation (e.g., Klemd et al., 2011). In this regard, the HP and UHP rocks should be predating the collision time of the Tarim Craton and Siberian accretionary system. In addition, Xiao et al. (2004, 2013) interpreted some units as backarc or arc-related sequences in the early Paleozoic and a forearc and accretionary complex in the Late Paleozoic, respectively. The formation of HP and UHP rocks was related to the subduction of the South Tianshan ocean plate instead of the collision of the Tarim craton. Therefore, more detailed investigations are required before the origin of HP and UHP rocks from the STAC is resolved (Xiao et al., 2013). In this paper, we report a detailed study of petrology and water content of nominally anhydrous minerals of the granulites from the Yushugou massif, typical of HP granulites in the STAC. The present

results show that the Yushugou granulites have undergone UHT and HP metamorphism and associated anatexis, the nominally anhydrous minerals (NAMs) contain certain amount of water. We conclude that the granulites formed in the deep root of hot magmatic arc, and water in NAMs obviously affected ductile deformation of thickened lower crust of magmatic arc. 2. Geological setting and samples The Yushugou HP metamorphic massif, located at the eastern segment of the STAC (Fig. 1), was traditionally considered to be a granulite-facies metamorphic and deformed ophiolitic slice (e.g., R.S. Wang et al., 1999a, 1999b; J.L. Wang et al., 1999; Dong et al., 2001; Zhou et al., 2004). However, Shu et al. (2004) proposed that the protolith of the Yushugou granulites was formed in a volcanic arc setting. Most recently, Ji (2013) argued that this HP massif derived

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

L. Zhang et al. / Gondwana Research xxx (2015) xxx–xxx

from a subducted lithospheric slab near its Moho discontinuity during late Paleozoic collision between the Tarim and Kazakhstan blocks. Zircon SHRIMP U–Pb dating studies showed that the Yushugou granulites were formed in the Devonian of ca. 390 Ma (Zhou et al., 2004) or 390–401 Ma (T.F. Li et al., 2011). Metamorphic conditions and tectonic mechanism of the Yushugou massif remain controversial (e.g., R.S. Wang et al., 1999a; J.L. Wang et al., 1999; Wang et al., 2003; Shu et al., 2004; Zhou et al., 2004; T.F. Li et al., 2011). The Yushugou massif occurs as a fault-bounded thrust slab in NW-SE extension, and is about 10 km long and 1–3 km wide in Devonian volcanic sedimentary rocks, and partly intruded by the late Devonian granites (Fig. 1). Field occurrence, rock assemblage, petrographic and geochemical studies have shown that the massif consists mainly of four rock units (Zhou et al., 2004), namely, (1) metamorphosed peridotite unit, representing residual mantle peridotite; (2) two-pyroxene granulite unit, its protolith is mafic cumulate; (3) garnet two-pyroxene granulite unit with gabbros as its protolith; (4) interlayered intermediate-mafic granulite unit, its protolith is mafic igneous rocks with sedimentary intercalations. The studied mafic and pelitic granulites occur in the interlayered intermediate-mafic granulite unit in the northwestern part of the massif (Fig. 1b). GPS coordinates of the samples 07XT14-1 and 07XT14-17 are 42°16′59″N and 87°54′38″E, and 42°16′46″N and 87°54′41″E, and other three samples collected between the two samples. 3. Analytical methods Mineral compositions were analyzed using a JEOL JXA 8900 electron microprobe (EPM) with a 15 kV accelerating voltage, 5 nA beam current, 5 μm probe diameter, and count time of 10 s for peak and background, at the Institute of Geology, Chinese Academy of Geological Sciences. Natural or synthetic standards were used for EPM analysis and ZAF corrections were carried out. Whole-rock compositions were analyzed at the National Geological Analysis Center of China, Beijing. Oxides of major elements including loss on ignition (LOI), were determined by X-ray fluorescence (XRF) (Rigaku-3080) with an analytical uncertainty of b0.5%. Thin sections were doubly polished to a thickness of 0.2–0.1 mm for microscopic Fourier transform infrared spectroscopy (Micro-FTIR) analysis. Unpolarized infrared absorbance spectra were collected with a Nicolet 6700 FTIR spectrometer coupled with a Continuum microscope at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, using a KBr beam splitter and a liquid-nitrogen cooled MCT-A detector. Each spectrum was obtained from 4500 to 1000 cm−1 at 4 cm−1 resolution and was accumulated over 128 scans. In order to select analytical areas free of cracks, grain boundaries, and visible inclusions under the microscope, the size of square aperture was set down to 30 × 30 μm. To minimize the uncertainty from unpolarized light for optically anisotropic minerals, 6–57 grains for each mineral in the same sample were recorded and analyzed, and the averaged value was used to define water content of the corresponding mineral in that sample as estimated previously (e.g., Asimow et al., 2006; Katayama et al., 2006; Xia et al., 2006; Grant et al., 2007; Kovács et al., 2008; Sambridge et al., 2008; Yang et al., 2008). A modified form of the Beer–Lambert law was used to calculate the structural OH concentration: Δ¼Ictγ

ð1Þ

Where Δ is the total integrated area (cm−1) of absorption bands in the region of interest, I is the integral specific absorption coefficient (1/ppm⋅cm2), c is the concentration of hydrogen species (expressed as ppm H2O by weight), t is the thickness of the sample (cm), and γ is the orientation factor discussed by Paterson (1982). In this study, the integral OH absorption region is 3800–3000 cm−1, and the integral specific coefficients are 14.84 for orthopyroxene (Bell et al., 1995), 2.38

3

for garnet (Rossman & Aines, 1991) and 13.51 for quartz (Thomas et al., 2009). The orientation factor of 1 for garnet and of 1/3 for orthopyroxene and quartz were used to calculate the water content (Paterson, 1982). The concentration of non-structural hydrous species in feldspar was calculated by the Beer–Lambert law: c¼

18:02  A 6  10 tρε

ð2Þ

Where c is the concentration of hydrous species (expressed as ppm H2O by weight), 18.02 is the molecular weight of H2O, A is the height of the absorption band, t is the thickness of the sample (cm), ρ is the density of the sample (g/L), we used a value of ρ of 2600 g/L to represent the density of feldspar from Yushugou granulites. ε is the molar absorption coefficient (L/mol·cm). For sub-microscopic fluid inclusions, a value of ε = 115 L/mol·cm was used in the calculation for feldspar (Clunie et al., 1966; Johnson & Rossman, 2004). To determine the concentration of sub-microscopic fluid inclusions in quartz, we use the following relation: A¼εct

ð3Þ

Where A is the absorbance at 3400 cm−1, t is the thickness of the sample (cm), c is the concentration of water (mol/L) and ε is the molar absorption coefficient (L/mol·cm). For sub-microscopic fluid inclusions, a value of ε = 81 (L/mol·cm) was used in the calculation for quartz (Thompson, 1965; Nakashima et al., 1995; Ito & Nakashima, 2002). 4. Petrology The studied granulites consist mainly of garnet (30–35 vol.%), plagioclase, K-feldspar (~30–35 vol.%) and quartz (22–25 vol.%) with minor orthopyroxene (2–10 vol.%), biotite (1–5 vol.%), rutile (~1 vol.%) and trace amount of ilmenite and zircon, and show distinct mylonitic structure (Figs. 2 and 3). The elongated and oriented porphyroblastic garnet, orthopyroxene and feldspar occur as augen mostly with asymmetric tails, and the fine-grained quartz, feldspar and garnet grains respectively occur as alternative thin layers or ribbons around the porphyroblasts. The porphyroblastic orthopyroxene exhibits kink band (Fig. 2d), and the augen feldspar often shows subgrains and core–mantle structure. These microstructures indicate that the granulites underwent ductile deformation under granulite-facies conditions. It is noted that the garnet porphyroblasts contain inclusions of biotite, plagioclase, quartz, rutile and ilmenite (Fig. 2) as well as multiphase inclusion of plagioclase, quartz and K-feldspar (Fig. 2f). The included plagioclase is irregular-shaped and contains round K-feldspar and vermicular quartz grains; they possibly represent crystallized former melt whereas the hosting garnet porphyroblast was a peritectic phase (Indares et al., 2008; Groppo et al., 2010; Guilmette et al., 2011; Groppo et al., 2012; Rubatto et al., 2013). This indicates that the granulites experienced partial melting, as further shown by the following phase equilibria modeling. However, late ductile deformation has largely altered the primary anatectic textures. Mineral chemical compositions from a pelitic granulite (sample 07XT14-1) are listed in Tables 1–4. Garnets are characterized by having high almandine (0.42–0.56) and pyrope (0.28–0.54), low grossular (0.03–0.18) and minor spessartine (0.01–0.03) contents (Table 1). Microprobe traverse of garnet porphyroblasts shows a weak compositional zoning, characterized by increasing grossular content from the core to rim of the grain (Fig. 3). The wide cores of garnet porphyroblasts have relatively high pyrope (0.49–0.54) and low grossular (0.031–0.043) contents, while the rims of garnet porphyroblasts and the fine-grained garnets in the matrix have similar compositions, characterized by high grossular (0.039–0.065) and low pyrope (0.44–0.52) contents. Orthopyroxene grains analyzed have similar FeO (16.08– 17.55 wt.%) and MgO (24.10–26.56 wt.%), but variable and high

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

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Fig. 2. Photomicrographs of the Yushugou granulites. (a and b) The granulites show distinct mylonitic foliation; elongate garnets and orthopyroxene porphyroblasts embedded in a matrix of alternating quartz- (white) and feldspar-rich (slight gray) layers, and surrounded by a mantle of fine-grained aggregates of garnet. The red lines in (a) refer to the locations of compositional profiles of Fig. 3. (c) Elongate orthopyroxene grain with oblique cleavage set in a matrix of polycrystalline plagioclase, quartz and garnet. The red lines refer to locations of the compositional profiles of Fig. 4. (d) Kinked orthopyroxene porphyroblast embedded in a matrix of fined-grained quartz, feldspar and garnet. (e) Porphyroblastic garnet with biotite inclusions embedded in alternating quartz- and feldspar-rich and garnet-rich bands. (f) Multi-mineral inclusion of plagioclase, quartz and K-feldspar within garnet; the plagioclase and quartz forming myrmekite. Mineral abbreviations are Bt = biotite, Gt = garnet, Kf = K-feldspar, Pl = plagioclase and Opx = clinopyroxene, Qz = quartz. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Al2O3 (3.75–7.91 wt.%) contents, corresponding to enstatite. Moreover, the porphyroblastic orthopyroxene shows distinct compositional zoning, characterized by decreasing Al2O3 content from the core to rim (Table 2; Fig. 4). Biotite occurred as inclusions within garnet has high TiO2 of 3.56–4.22 wt.% (Table 3). Plagioclase occurred both in the matrix and as inclusions has similar Na2O (6.88–7.91 wt.%) and CaO (6.19–7.70 wt.%) contents and accordingly An is between 0.31 and 0.37 (Table 4). The petrological features show that the granulites record two generations of mineral assemblage. The cores of porphyroblastic garnet and orthopyroxene, and inclusions of biotite, plagioclase, quartz, rutile and ilmenite within garnet form an early prograde mineral assemblage (Gt + Opx + Bt + Pl + Qz + Rt + Ilm; mineral abbreviations refer to the caption of Fig. 5); whereas the rims of garnet and orthopyroxene porphyroblasts and matrix minerals plagioclase, quartz and rutile represent a peak-metamorphic assemblage (Gt + Opx + Pl + Qz + Rt). In addition, some plagioclases and quartzes as well as all K-feldspars likely represent the crystallized former melt enclosed within the peritectic garnet. 5. Metamorphic P–T conditions of the granulite Metamorphic conditions of the pelitic granulite (sample 07XT14-1) were estimated quantitatively from P–T pseudosection and mineral

isopleth thermobarometry. The pseudosection was calculated with the Perple_X computer program package (Connolly, 2005; version from August 2012) and the internally consistent thermodynamic data set of Holland and Powell (1998, updated 2002). The activity-solution models used in the pseudosection calculation are clino- and orthoamphibole (Diener et al., 2007), clinopyroxene (Green et al., 2007), garnet (White et al., 2007), cordierite (Holland & Powell, 1998), biotite (Tajčmanová et al., 2009), white mica (Coggon & Holland, 2002) and feldspar (Fuhrman & Lindsley, 1988). P–T pseudosection was calculated in the system MnO–Na2O–CaO– K2O –FeO–MgO–Al2O3–SiO2–H2O–TiO2 (MnNCKFMASHT) in a P–T range of 5–18 kbar and 700–1150 °C. The rock chemical composition, including H2O content, was determined by bulk rock analyses and listed in Fig. 5. The calculated pseudosection shows that garnet is always stable, biotite disappears at high T of N 880–990 °C, and orthopyroxene is stable at high T of N 870 °C at above 8.5 kbar (Fig. 5). The solidus of the system is located at T = 840–870 °C (Fig. 5b). The observed early mineral assemblage of Gt + Opx + Pl + Bt + Qz + Rt + Ilm is stable in a narrow P–T field of 915–945 °C and 10–11.5 kbar at the presence of melt (Fig. 5), while the peak mineral assemblage of Gt + Opx + Pl + Qz + Rt is stable in a wide P–T field of N950 °C and N10.8 kbar at the presence of melt (Fig. 5). For the core of garnet porphyroblast, the composition isopleths of XCa = 0.031–0.043 exactly cross the stability field of the early mineral

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

L. Zhang et al. / Gondwana Research xxx (2015) xxx–xxx

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Fig. 3. (a and b) BES images of the granulites. Garnet porphyroblasts embedded in the matrix of fine-grained quartz, K-feldspar, plagioclase and garnet aggregates. The two diagrams are the local amplification of Fig. 2a. The white-dashed lines refer to locations of garnet compositional profiles shown in the lower part of this diagram. (c and d) Compositional profiles of garnet porphyroblasts with an increasing grossular content from the core to rim. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

In summary, this modeling indicates that the Yushugou pelitic granulite underwent UHT (N940 °C) and HP (11.5–14 kbar) metamorphism under high geothermal gradient of ca. 24 °C/km, and partial melting generated via biotite breakdown melting; the predicted melt mode is ca. 6–10 vol.% in the stability field of the peak mineral assemblage (Fig. 5b). In addition, the prograde process of the Yushugou granulites probably characterized by both temperature and pressure increase (Fig. 5). Thus P–T path is consistent with increasing CaO contents from the garnet core to rim (Figs. 3 and 5b) and decreasing Al2O3 contents

assemblage; while the isopleths of XMg = 0.49–0.54 indicate a wide temperature range with a minimum T of ca. 980 °C (Fig. 5b). For the garnet rim and fine-grained matrix garnet, the composition isopleths of XCa = 0.039–0.065 exactly cross the stability field of the observed peak assemblage, indicating a metamorphic pressure of ca. 11.5–14 kbar at ca. 975 °C, and the isopleths of XMg = 0.44–0.52 give a wide temperature range of 860–N1150 °C at ca. 12 kbar (Fig. 5b). In addition, the calculated modal isopleths (1–5%) of biotite exactly cross the stability field of the observed early assemblage (Fig. 5a).

Table 1 Chemical compositions of representative garnet of the pelitic granulite (sample 07XT14-1).

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O Total Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na Prp Grs Alm Spe And

Porphyroblast-1

Porphyroblast-2

Matrix

Rim

Rim

Core

Core

Core

Rim

Rim

Rim

Rim

Core

Core

Rim

39.97 0.02 22.15 0.01 21.30 0.54 13.48 2.40 0.00 99.87 2.988 0.001 1.951 0.001 0.070 1.262 0.034 1.502 0.192 0.000 0.502 0.064 0.422 0.011 0.034

40.20 0.03 21.85 0.04 21.86 0.51 14.09 1.45 0.02 100.03 2.997 0.001 1.920 0.002 0.082 1.280 0.032 1.566 0.116 0.003 0.523 0.039 0.428 0.011 0.041

39.10 0.01 21.80 0.07 22.31 0.55 14.17 1.20 0.01 99.22 2.940 0.001 1.932 0.004 0.184 1.219 0.035 1.588 0.097 0.001 0.540 0.033 0.415 0.012 0.087

39.99 0.02 22.02 0.02 22.13 0.53 14.00 1.16 0.00 99.87 2.989 0.001 1.940 0.001 0.078 1.305 0.034 1.560 0.093 0.000 0.522 0.031 0.436 0.011 0.039

40.18 0.00 21.99 0.02 21.57 0.50 13.89 1.60 0.04 99.81 3.003 0.000 1.937 0.001 0.061 1.287 0.032 1.547 0.128 0.005 0.517 0.043 0.430 0.011 0.030

39.99 0.00 21.96 0.05 21.86 0.47 13.65 2.02 0.02 100.02 2.987 0.000 1.932 0.003 0.093 1.272 0.030 1.519 0.162 0.003 0.509 0.054 0.426 0.010 0.046

39.89 0.01 22.16 0.01 22.29 0.54 13.16 2.30 0.00 100.37 2.977 0.001 1.949 0.001 0.095 1.296 0.034 1.464 0.184 0.000 0.492 0.062 0.435 0.012 0.046

39.27 0.04 21.57 0.02 22.35 0.53 13.26 2.16 0.00 99.21 2.964 0.002 1.919 0.001 0.147 1.265 0.034 1.492 0.175 0.000 0.503 0.059 0.426 0.011 0.071

39.35 0.01 21.69 0.03 22.46 0.46 13.01 2.14 0.03 99.19 2.974 0.001 1.932 0.001 0.122 1.297 0.029 1.465 0.173 0.004 0.494 0.058 0.437 0.010 0.059

39.46 0.00 21.65 0.04 22.69 0.49 13.60 1.24 0.00 99.16 2.978 0.000 1.926 0.002 0.115 1.318 0.032 1.530 0.101 0.000 0.513 0.034 0.442 0.011 0.056

39.99 0.03 21.94 0.10 22.29 0.51 13.80 1.24 0.01 99.97 2.990 0.001 1.933 0.006 0.084 1.309 0.033 1.538 0.099 0.001 0.516 0.033 0.439 0.011 0.042

39.90 0.00 21.96 0.03 22.38 0.54 13.21 2.01 0.00 100.06 2.988 0.000 1.938 0.002 0.086 1.315 0.034 1.475 0.161 0.000 0.494 0.054 0.441 0.011 0.043

40.19 0.04 21.92 0.08 20.98 0.48 13.28 2.46 0.00 99.44 3.019 0.002 1.940 0.005 0.014 1.304 0.030 1.487 0.198 0.000 0.493 0.065 0.432 0.010 0.007

40.62 0.00 21.87 0.04 21.29 0.52 13.50 2.09 0.01 99.95 3.035 0.000 1.926 0.002 0.005 1.325 0.033 1.504 0.167 0.001 0.496 0.055 0.438 0.011 0.002

39.37 0.04 21.51 0.00 24.35 0.55 11.63 1.88 0.00 99.33 3.002 0.002 1.933 0.000 0.059 1.493 0.036 1.322 0.154 0.000 0.440 0.051 0.497 0.012 0.030

39.49 0.01 21.42 0.11 22.11 0.45 13.80 1.90 0.01 99.32 2.971 0.000 1.899 0.006 0.156 1.235 0.028 1.548 0.153 0.001 0.522 0.052 0.417 0.010 0.076

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

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Table 2 Chemical compositions of representative orthopyroxene of the granulite (sample 07XT14-1). Profile-1

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total Si Ti Al Fe3+ Fe2+ Cr Mg Mn Ca Na K Cations Wo En Fs

Profile-2

Rim

Core

Core

Core

Core

Rim

Rim

Core

Core

Core

Core

Rim

51.34 0.06 5.39 0.06 17.55 0.01 25.66 0.09 0.06 0.00 100.35 1.851 0.002 0.229 0.000 0.529 0.002 1.380 0.000 0.003 0.004 0.000 4.000 0.18 72.13 27.69

50.64 0.05 6.96 0.14 17.37 0.02 25.02 0.10 0.06 0.01 100.52 1.824 0.001 0.295 0.000 0.523 0.004 1.344 0.001 0.004 0.004 0.000 4.000 0.21 71.80 28.00

51.13 0.00 7.91 0.05 16.66 0.06 24.78 0.08 0.06 0.00 100.75 1.832 0.000 0.334 0.000 0.499 0.001 1.324 0.002 0.003 0.004 0.000 4.000 0.17 72.42 27.41

50.71 0.00 7.51 0.12 17.18 0.00 25.26 0.07 0.07 0.03 101.04 1.812 0.000 0.316 0.000 0.513 0.003 1.346 0.000 0.003 0.005 0.001 3.999 0.14 72.28 27.58

51.59 0.00 6.85 0.09 17.30 0.07 24.71 0.13 0.04 0.03 100.81 1.853 0.000 0.290 0.000 0.520 0.003 1.323 0.002 0.005 0.003 0.001 3.999 0.27 71.52 28.21

52.37 0.00 3.75 0.01 16.96 0.00 26.56 0.07 0.00 0.00 99.82 1.893 0.000 0.160 0.000 0.513 0.000 1.432 0.000 0.003 0.000 0.000 4.000 0.14 73.52 26.34

50.64 0.28 6.76 0.05 17.27 0.04 24.61 0.13 0.00 0.01 99.85 1.839 0.008 0.289 0.000 0.524 0.001 1.332 0.001 0.005 0.000 0.000 4.000 0.27 71.51 28.22

50.64 0.02 7.85 0.04 16.86 0.08 24.10 0.05 0.04 0.00 99.93 1.839 0.001 0.336 0.000 0.512 0.001 1.305 0.002 0.002 0.003 0.000 4.000 0.11 71.64 28.25

50.55 0.06 7.53 0.04 16.47 0.02 24.81 0.04 0.00 0.04 99.71 1.832 0.002 0.321 0.000 0.499 0.001 1.341 0.001 0.002 0.000 0.002 3.998 0.08 72.78 27.14

50.52 0.00 7.73 0.06 16.63 0.03 24.57 0.08 0.00 0.02 99.66 1.831 0.000 0.330 0.000 0.504 0.002 1.328 0.001 0.003 0.000 0.001 3.999 0.17 72.32 27.51

51.44 0.14 6.95 0.10 16.39 0.13 25.18 0.13 0.03 0.00 100.63 1.848 0.004 0.294 0.000 0.492 0.003 1.349 0.004 0.005 0.002 0.000 4.000 0.27 72.90 26.83

52.08 0.00 4.92 0.07 16.08 0.09 26.44 0.13 0.02 0.00 99.87 1.876 0.000 0.209 0.000 0.484 0.002 1.419 0.003 0.005 0.001 0.000 4.000 0.26 74.26 25.48

from the orthopyroxene core to rim (Figs. 4 and 5a) as well as increasing modal content of garnet (Fig. 5a), shown by growth of peritectic garnet during melting. Metamorphic conditions of the Yushugou mafic granulites have been estimated by conventional thermobarometry. A combination of Gt–Cpx thermometry and Gt–Cpx–Pl–Qz barometry yielded P–T conditions of 795–964 °C and 9.7–14.2 kbar (R.S. Wang et al., 1999a), and 800–870 °C and 8.8–11 kbar (Shu et al., 2004). For the Weiyai mafic granulites from the adjacent area of the STAC, metamorphic conditions of 910–1025 °C and 10.8–11.2 kbar were estimated using similar geothermobarometries (Shu et al., 2004). These studies provided primary constraints on UHT metamorphism of the Yushugou granulites. Garnet + orthopyroxene is not a classical UHT mineral assemblage, but many studies have shown that UHT conditions are preserved by

garnet + aluminous orthopyroxene associations in metapelitic rocks (e.g., Harley, 1998a; Harley & Motoyoshi, 2000; Nandakumar & Harley, 2000; Brandt et al., 2003; Pattison et al., 2003; Ishii et al., 2006; Santosh et al., 2012). In the studies of Harley (1998a) and Brandt et al. (2003), orthopyroxene coexisting with garnet has alumina contents up to 7.5–8.5 wt.% and 8–11 wt.%, which recorded P–T conditions of ca. 1050 °C and 12 kbar, and ca. 970 °C and 9.5 kbar, respectively. The present modeling also shows that the Gt + Opx (+ Pl + Qz + Rt) association is stable above 950 °C (Fig. 5). Moreover, orthopyroxene of the Yushugou pelitic granulite has Al2O3 content of up to 7.9 wt.%, indicating a temperature of ca. 950 °C at 10–12 kbar based on the correlation between Al solubility in orthopyroxene and temperature (Harley & Motoyoshi, 2000). These provide further evidence for UHT metamorphism of the Yushugou granulites.

Table 3 Chemical compositions of biotite of the granulite (07XT14-1).

6. Water in NAMs of the granulites

No.

1

2

3

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na20 K2O Total Si Al Ti Fe3+ Fe2+ Cr Mn Mg Ca Na K Cations

39.23 3.56 15.17 0.14 7.36 0.02 18.62 0.01 0.14 10.32 94.57 5.957 2.713 0.407 0.000 0.935 0.017 0.003 4.215 0.002 0.041 1.999 16.289

39.33 4.22 15.83 0.08 7.43 0.00 18.51 0.00 0.19 9.96 95.55 5.895 2.794 0.476 0.000 0.931 0.009 0.000 4.136 0.000 0.055 1.905 16.201

39.23 3.71 15.31 0.04 7.87 0.00 18.29 0.05 0.23 10.49 95.22 5.937 2.729 0.422 0.000 0.996 0.005 0.000 4.126 0.008 0.067 2.025 16.315

NAMs from five granulite samples have been analyzed for their hydrogen content by Micro-FTIR. Detailed results are presented in Appendix Table 1 and briefly summarized in Table 5. 6.1. Garnet The absorption bands in the OH region of the infrared spectra of garnet include three groups: 3610–3590 cm− 1, ~ 3560 cm−1 and 3440–3420 cm−1 (Fig. 6a). The first two bands confirm that OH component is present in the garnet grains examined (Aines & Rossman, 1984a; Rossman & Aines, 1991) while the last band is characteristic of weakly bound H2O (Rossman & Aines, 1991). The concentration of OH component in garnet is calculated by Eq. (1), using the integrated area of the first two bands. The garnet from five samples has highly variable OH content ranging from 0 to 1716 ppm (Table 5; Appendix Table 1). It is noted that the fine-grained garnet in the matrix and the rims of coarse-grained porphyroblastic garnet have relatively high water content with the average H2O content ranging from 301 to 448 ppm (Table 5) while the cores of porphyroblastic garnet are almost waterfree (Fig. 6a; Appendix Table 1).

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

L. Zhang et al. / Gondwana Research xxx (2015) xxx–xxx

7

Table 4 Chemical compositions of representative plagioclase of the granulite (07XT14-1). Matrix SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total Si Ti Al Fe Mn Mg Ca Na K Ab An Or

60.32 0.01 24.72 0.05 0.04 0.00 6.31 7.46 0.16 99.08 2.718 0.000 1.313 0.002 0.002 0.000 0.305 0.652 0.009 0.67 0.32 0.01

Inclusion 59.27 0.00 25.70 0.02 0.01 0.00 7.63 6.88 0.31 99.83 2.658 0.000 1.358 0.001 0.001 0.000 0.367 0.598 0.018 0.61 0.37 0.02

60.78 0.00 24.41 0.10 0.00 0.00 6.28 7.57 0.26 99.44 2.729 0.000 1.292 0.004 0.000 0.000 0.302 0.659 0.015 0.67 0.31 0.02

60.80 0.02 24.62 0.03 0.00 0.00 7.70 6.95 0.27 100.41 2.716 0.001 1.296 0.001 0.000 0.000 0.368 0.602 0.016 0.61 0.37 0.02

6.2. Orthopyroxene The spectra of orthopyroxene commonly display three bands in O\H stretching vibration region (3000–3800 cm−1): ~ 3550 cm− 1, ~ 3500 cm−1 and 3420–3410 cm−1 (Fig. 6b). All three of these bands are identical with typical OH absorption bands in orthopyroxene (Skogby et al., 1990; Bell et al., 1995; Peslier et al., 2002; Skogby, 2006; Xia et al., 2006; Yang et al., 2008; Sundvall & Stalder, 2011). Eq. (1) was used to calculate OH concentration in orthopyroxene. The orthopyroxene grains from three samples have variable water content ranging from 0 to 199 ppm with average values of 1, 19 and 54 ppm (Table 5), respectively. 6.3. Feldspar The spectra of feldspar are characterized by the presence of very broad bands with a main peak at about 3400 cm− 1 and a shoulder at 3270 cm− 1. Some of these spectra have relatively narrow bands at 3630–3530 cm − 1 (Fig. 6c). According to previous studies, the very broad bands centered at about 3400 cm− 1 with a shoulder at 3270 cm − 1 attributable to fluid inclusions (Johnson & Rossman, 2003, 2004). Therefore, the main hydrous species for feldspar from the granulites is sub-microscopic fluid inclusions. The narrow bands from 3630 to 3530 cm− 1 are probably due to alteration products of feldspar (Potter & Rossman, 1977; Shoval et al., 2002). Eq. (2) was used to calculate the concentration of sub-microscopic fluid

60.22 0.01 24.74 0.04 0.02 0.00 6.70 7.40 0.10 99.24 2.711 0.000 1.313 0.002 0.001 0.000 0.323 0.646 0.006 0.66 0.33 0.01

60.58 0.00 24.63 0.01 0.00 0.00 6.19 7.40 0.14 99.04 2.735 0.000 1.310 0.000 0.000 0.000 0.299 0.648 0.008 0.68 0.31 0.01

60.85 0.01 24.45 0.00 0.00 0.00 6.24 7.74 0.15 99.44 2.728 0.000 1.292 0.000 0.000 0.000 0.300 0.672 0.009 0.69 0.31 0.01

59.03 0.02 25.01 0.12 0.04 0.07 6.76 7.91 0.68 99.77 2.631 0.001 1.313 0.004 0.001 0.005 0.323 0.683 0.039 0.65 0.31 0.04

58.74 0.05 24.58 0.11 0.04 0.05 6.92 7.41 1.32 99.48 2.636 0.002 1.300 0.004 0.001 0.003 0.333 0.645 0.075 0.61 0.32 0.07

inclusions in feldspar. The feldspar grains from five granulite samples have variable but relatively high water content with average values ranging from 173 to 534 ppm (Table 5). 6.4. Quartz The FTIR spectra of quartz are characterized by the presence of a sharp peak at 3380 cm−1 superimposed on the broad 3400 cm−1 band (Fig. 6d). Previous studies showed that the sharp band absorptions in the range of 3000–3800 cm−1 are due to OH groups associated with other H+ or monovalent cations including Li+, Na+, K+, Cu+, and Ag+, and hydroxyl associated with Al3+ (Kats, 1962; Aines & Rossman, 1984b; Rovetta et al., 1986; Miyoshi et al., 2005; Johnson, 2006). For instance, the major band at 3380 cm−1 is due to H+ occurring in a charge-compensating role for Al3+ in a Si4+ site (Kats, 1962; Aines & Rossman, 1984b; Ito & Nakashima, 2002) and they are stable to above 1000 °C (Aines & Rossman, 1984b). Natural quartz can also hold H2O in sub-microscopic fluid inclusions (Kronenberg & Wolf, 1990). The broad band centered at 3400 cm−1 is considered to be due to O\H stretching vibrations of “liquid-like” molecular water (Aines & Rossman, 1984b; Rossman, 1988; Kronenberg & wolf, 1990; Nakashima et al., 1995; Ito & Nakashima, 2002). Therefore, the broad bands at 3400 cm−1 are ascribed to sub-microscopic fluid inclusions. Eqs.(1) and (3) were used to calculate the concentration of OH groups and of sub-microscopic fluid inclusions in quartz. The quartz grains from four samples contain variable water content with similar mean values of 35–66 ppm (Table 5).

Fig. 4. Compositional profiles of the orthopyroxene with a decreasing Al2O3 content from the core to rim. The profile locations are shown in Fig. 2c.

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

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L. Zhang et al. / Gondwana Research xxx (2015) xxx–xxx

peritectic phase that grow under relatively low pressure condition and associated with partial melting, commonly are water-free because the H2O is strongly partitioned into silicate melt (see discussion below). By contrast, the fine-grained garnet in the matrix and the rims of porphyroblastic garnet have relatively high water content as they were formed under HP condition during the peak-metamorphism. Available studies have shown that the water content of NAMs increases systematically with increasing metamorphic pressure (e.g., Lu & Keppler, 1997; Withers et al., 1998; Rauch & Keppler, 2002; G.D. Bromiley et al., 2004; G. Bromiley et al., 2004; Katayama et al., 2006; Zheng, 2009).

7. Discussion 7.1. Origin of the Yushugou UHT granulites

Fig. 5. P–T pseudosection of the pelitic granulite. The observed mineral assemblages are shown by the bold red fonts. v and subsequent number represent variance. The red, blue and dashed lines in (a) show modal contents of biotite and garnet, and isopleths of Al content (per formula unit) in orthopyroxene, respectively. The red, green and dashed lines in (b) refer to isopleths of melt mode, XCa [Ca/(Ca + Fe + Mg + Mn)] and XMg [Mg/(Ca + Fe + Mg + Mn)] values of garnet. The red and thick lines with arrow represent the speculated prograde P–T path of the granulite. Mineral abbreviations are Am = amphibole, Bt = biotite, Cpx = clinopyroxene, Gt = garnet, Ilm = ilmenite, Kf = K-feldspar, Ky = kyanite, Ms = muscovite, Pl = plagioclase, Opx = orthopyroxene, Qz = quartz, Rt = rutile, Sil = sillimanite and Sp = spinel; Me = melt. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

In summary, this study shows that the NAMs of the HP granulites contain significant amounts of water that were incorporated in the manner of structural OH and sub-microscopic fluid inclusions. But, the water content among different grains or even within a single grain is obviously heterogeneous. For quartz and feldspar, such heterogeneous are probably caused by sub-microscopic fluid inclusions that usually occur along microstructures and as isolate clusters within the hosting mineral grains. The cores of the porphyroblastic garnet, representing

Most of the previous studies proposed that the Yushugou massif is a granulite-facies metamorphic ophiolitic slice, which once subducted to the lower crustal depth of 40–50 km during late Paleozoic subduction and collision orogeny (e.g., R.S. Wang et al., 1999a, 1999b; J.L. Wang et al., 1999; Dong et al., 2001; Zhou et al., 2004). However, we suggest that the Yushugou massif formed in a tectonic setting of magmatic arc, representing a lower crust to upper mantle section of the deep root of an accretionary orogen. The relevant evidences are as follows: (1) Petrochemical data indicate that the protolith of Yushugou mafic granulites is gabbro formed in a volcanic arc tectonic setting (Shu et al., 2004). (2) The rock associations and metamorphic features of the Yushugou massif are similar to those of magmatic arc roots in active continental margins. Typical island arcs contain an upper mantle section of ultramafic rocks (harzburgite, dunite and pyroxenite) and a lower crust section of HP mafic granulites (meta-gabbros and gabbronorite) (e.g., DeBari & Coleman, 1989; Yamamoto & Nakamura, 1996; Burg et al., 1998; Garrido et al., 2006; Greene et al., 2006; Farris, 2009; Jagoutz et al., 2009; Bosch et al., 2011; Jagoutz et al., 2011). By contrast, continental magmatic arcs contain an upper mantle section of metaultramafic rocks (spinel peridotite and garnet pyroxenite) and a lower crust section of mafic granulites (meta-gabbros) as well as schist, marble and calc-silicate rocks (meta-supracrustal rocks) (Saleeby et al., 2003; Zandt et al., 2004; Miller & Snoke, 2009; Zhang et al., 2010a, 2013; Palin et al., 2014). Therefore, the Yushugou massif is likely to represent a continental arc root. (3) Magmatic arc roots commonly experienced HT to UHT metamorphism. For example, the granulites of the Kohistan arc root, SW Tibet, were formed under conditions of 800–1000 °C and 11–15 kbar (Yoshino & Okudaira, 2004); the metagabbros of the Amalaoulaou arc, NE Mali, underwent metamorphism and anatexis under conditions of 850–1050 °C and N 10 kbar (Berger et al., 2009); the Gangdese arc root, SE Tibet, recorded a metamorphic temperature of 800–900 °C (Zhang et al., 2010a, 2013). (4) Prograde processes of magmatic arc granulites are commonly characterized by heating and burial. Thus P–T paths related to the accretion and loading of large amounts of mantle-derived mafic magmas not only provide heat for the metamorphism but also result in distinct thickening of lower crust of magmatic arcs (Bohlen, 1987, 1991; Brown & Walker, 1993; Brown, 1996; Yoshino et al., 1998; Yoshino & Okudaira, 2004; Dhuime et al., 2009). (5) HP/UHP metamorphic rocks in subduction and collision zones are characterized by low geothermal gradient (4–15 °C/km; Chopin, 2003; Zhang et al., 2003; Liou et al., 2009; Gilotti, 2013). By contrast, granulites in magmatic arc roots commonly witnessed a high geothermal gradient of N 20 °C/km (Bohlen, 1987; Ellis, 1987; Harley, 1989; Bohlen, 1991; Ringuette et al., 1999; Müntener et al., 2000; Bhowmik et al., 2005; Brandt et al., 2006; Zhang et al., 2013; Zhao & Zhai, 2013). In general, plate convergence and magma accretion resulting in crustal thickening and growth, and associated HT/UHT granulite-facies metamorphism and anatexis may be one of the most important processes in subduction-related magmatic arcs (e.g., Gibson et al., 1988; Brown, 1996; Clarke et al., 2000; Collins, 2002a, 2002b; Hollis et al., 2003;

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

L. Zhang et al. / Gondwana Research xxx (2015) xxx–xxx

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Table 5 Water contents of NAMs of the granulites. Smaple Garnet

Orthopyroxene

Feldspar

Quartz

Average Maximum Minimum Spots Average Maximum Minimum Spots Average Maximum Minimum Spots Average Maximum Minimum Spots

07XT14-1

07XT14-3

07XT14-5-1

07XT14-16

07XT14-17

418 910 0 33

423 1043 0 45 1 31 0 21 284 633 102 20 35 50 25 6

448 1716 0 57

301 501 0 21 19 95 0 32 173 427 34 19 45 102 11 8

420 956 0 27 55 199 0 15 217 814 48 23

277 759 24 11 37 122 0 32

534 793 349 6 66 208 23 17

Note: the average H2O contents of garnet were calculated based on the analytica spots for the matrix garnet and the rim of porphyroblastic garnet.

Hyndman et al., 2005; Currie & Hyndman, 2006; Klepeis et al., 2007; Scott et al., 2009; Stowell et al., 2010; Yin et al., 2014). 7.2. Effect of melt loss on water content in NAMs Numerous studies have documented the occurrence of water in NAMs in the mantle and crust (e.g., Aines & Rossman, 1984b; Beran, 1987; Bell & Rossman, 1992; Katayama et al., 2005; Johnson, 2006; Smyth, 2006). In crustal rocks, quartz typically contains up to 40 ppm structural OH, and up to 8000 ppm H2O in fluid inclusions (Kronenberg & Wolf, 1990). Alkali feldspars host up to 1000 ppm structural H2O and OH, and N1000 ppm H2O in fluid inclusions, and plagioclase hosts up to 300 ppm structural OH, and N1000 ppm H2O in fluid inclusions (Johnson, 2006). Xia et al. (2006) reported that clinopyroxene, orthopyroxene, and plagioclase

from granulite xenoliths in Cenozoic basalts from the North China Craton have water content of 2360 ppm, 1170 ppm and 880 ppm, respectively. Structural water content of garnet, omphacite and rutile of UHP eclogites in the Dabie orogen is in the range of 92–1735 ppm, 115–695 ppm and 980–1490 ppm, respectively (Zhang et al., 2001; Xia et al., 2005; Gong et al., 2007; Sheng et al., 2007). NAMs of UHP eclogites of the Kokchetav massif contain 50 to 1650 ppm H2O (Katayama et al., 2006). This study shows that the NAMs of the Yushugou granulites have lower water content as compared with the crustal rocks described above. Fluids in normal subduction regimes are dominantly hydrous, as also attested by petrological, geochemical and geophysical studies in subduction systems (e.g., Zheng et al., 2003; Zhao et al., 2007; Zhang et al., 2008; Hasegawa et al., 2009; Maruyama et al., 2009; Zheng, 2009; Li et al., 2013). However, most HP granulite facies rocks forming

Fig. 6. Representative infrared spectra of structural and non-structural hydrous species in NAMs of the Yushugou granulites. (a) Spectra of structural OH in garnet. Lines 1 to 3 refer to the fine-grained matrix garnet, the rim and core of the porphyroblastic garnet of sample 07XT14-5, respectively; Lines 4 and 5 refer to the fine-grained matrix garnet and the core of porphyroblastic garnet of sample 07XT14-1, respectively. (b) Spectra of structural OH in orthopyroxene. (c) Spectra of non-structural sub-microscopic fluid inclusions in feldspar. (d) Spectra of structural OH and non-structural sub-microscopic fluid inclusions in quartz. All the spectra were normalized to 1 cm thickness but were vertically offset.

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

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the lower continental crust of active orogenic belt lack hydrous phase. This main feature is a consequence of partial melting in which H2 O is strongly partitioned into silicate melt. If no melt is lost from granulite facies rocks, high-grade anhydrous assemblages will be retrogressed on cooling via the reversal of the partial melting reactions to hydrous assemblages typical of the amphibolite facies (White et al., 2001). The common occurrence of weakly retrogressed granulite facies assemblages is consistent with substantial melt loss from the majority of granulite facies rocks (White & Powell, 2002). Phase equilibria modeling indicate that the degree of partial melting of normal pelitic granulites with H2O content of ~ 1.8% will be more than 25–30 vol.% of the whole rock volume when the metamorphic temperature reaches 900 °C between 10 and 14 kbar (White et al., 2001, 2007; Guilmette et al., 2011; Zhang et al., 2013). For the Yushugou pelitic granulites, the estimated melt mode is ca. 6–10 vol.% during the peak-metamorphic stage, which is comparable to a critical threshold of 7–10 vol.% of melt loss in anatectic rocks (Brown, 2007a; Rosenberg & Handy, 2005). Therefore the predicted melt mode represents small amount of remnant melt while voluminous melt (up to 15 vol.%) has been extracted from the rock system. As a result, the Yushugou pelitic granulites lack hydrous minerals, and the granulitic NAMs have relatively low water content. In addition, the extracted voluminous melt formed magmatic source for widespread Paleozoic granitoids in the STAC.

7.3. Effect of trace water in NAMs on deformation of the granulites The studied granulites only contain minor hydrous mineral (biotite b1–5 vol.%), but they underwent intensive ductile deformation, shown by typical mylonitic foliation and other microstructures of plastic deformation, such as the elongate garnet and orthopyroxene porphyroblasts, alternating quartz, and feldspar and garnet-rich layers, and kinked orthopyroxene (Fig. 2). In addition, the associated peridotites from the Yushugou massif also underwent marked ductile deformation. In fact, such an example that dry granulites witnessed strongly ductile deformation is quite rare in orogens (Hanmer, 2000; Kleinschrodt & Duyster, 2002; Lund et al., 2006; Vollbrecht et al., 2006). The ductile deformation mechanism is a function of temperature– pressure–fluid conditions during deformation (Lund et al., 2006; Katayama & Karato, 2008; Muramoto et al., 2011; Xu et al., 2013). Low concentrations (tens to hundreds of parts per million) of water in NAMs of the mantle and crust have been shown to significantly affect the rheology, depth of decompression melting, and many other properties of the mantle and crust (Green, 1973; Griggs, 1974; Mackwell et al., 1985; Karato & Masuda, 1989; Karato, 1990; Kronenberg, 1994; Karato & Jung, 1998; Mei & Kohlstedt, 2000; Aubaud et al., 2004; Simpson & Tommasi, 2005; Johnson, 2006; Kohlstedt, 2006; Vollbrecht et al., 2006). Moreover, water in NAMs may be of particular importance in the lower crust, where hydrous minerals are sparse or locally absent (Seaman et al., 2013). This study reveals that the quartz, feldspar, orthopyroxene and garnet of the Yushugou granulites contain certain amount of water occurring as structural OH and sub-microscopic fluid inclusions. Therefore, except for UHT and HP conditions, the trace amounts of water in NAMs may have played an important role in ductile deformation of the near dry granulites. In addition, the remnant melt within the Yushugou granulites has also promoted ductile deformation because experimental studies of melt-bearing crustal rocks consistently indicate a reduction of rock strength with increasing melt fraction (e.g., van der Molen & Paterson, 1979; Dell'Angelo & Tullis, 1988; Rutter et al., 2006). It is noted that water content of biotite that forms only hydrous mineral in the pelitic granulite does not match with the whole-rock water content of 0.25 wt.% obtained by the whole-rock chemical analyses. Therefore, it is speculated that water in NAMs and in fluid inclusions,

boundaries and fractures of the granulitic minerals contribute largely to the whole-rock water basket. 7.4. Tectonic implications The Tianshan orogenic collage records the long-lived accretion of the Central Asian Orogenic Belt (Windley et al., 2007), forming one of the largest accretionary orogens in the planet (Cawood et al., 2009; Xiao et al., 2013; Xiao & Santosh, 2014 and therein references). Previous works also indicated that the STAC contains not only a series of ophiolitic blocks and their HP/UHP metamorphosed equivalents that represent remnants of subducted oceanic crusts (Xiao et al., 1994; Gao et al., 1995; R.S. Wang et al., 1999a; Dong et al., 2001; Gao & Klemd, 2001; Zhou et al., 2004; Klemd et al., 2005; Su et al., 2010; Q.L. Li et al., 2011; Wang et al., 2011; Xiao et al., 2013), but also voluminous Paleozoic granitoids, typical of arc-related igneous rocks, that mostly represent juvenile additions to the upper crust (Jahn et al., 2004; Xiao et al., 2004; Gao et al., 2009; Long et al., 2011; Huang et al., 2013). This study shows that the Yushugou massif represents a magmatic arc root, and the orogenic lower crust has been thickened to a depth of ca. 40–50 km and underwent UHT and HP metamorphism. Such high temperatures not only resulted in intensive partial melting of the granulites but also triggered remelting of intermediate-mafic plutons newly underplating in the lower crust of magmatic arc. Whole-rock Sr–Nd and zircon Hf isotopic data have shown that the Paleozoic magmatic rocks of the STAC were derived from mixing of melts from the continental crust and depleted-mantle (Huang et al., 2013). UHT metamorphic granulite terrains are windows to the lower crust, as they typically form at depths of 25–45 km, with peak metamorphic conditions above 900 °C and even up to 1150 °C and pressures between 7 and 15 kbar (Harley, 1998a,b, 2004; Kelsey, 2008). Regional UHT metamorphism implies that the crust has achieved an advective geothermal gradient steeper than approximately 20 °C/km (Brown, 2007b, 2009). The accretionary orogenic system is a plausible tectonic setting for generating large-scale UHT metamorphism (Kelsey, 2008). Voluminous mantle-derived magmatic accretion and loading, convective removal of lithospheric mantle following crustal thickening, upwelling of asthenosphere induced by rollback and breakoff of deeply subducted oceanic slab, and mid-oceanic ridge subduction are potential tectonothermal processes that can induce regional scale HT/UHT metamorphism of the lower crust of accretionary orogens (Baba, 1999; Sengupta et al., 1999; Underwood et al., 1999; Iwamori, 2000; Collins, 2002a,b, 2011; Bradley et al., 2003; Brandt et al., 2003; Saleeby et al., 2003; Zandt et al., 2004; Yin et al., 2009; Santosh & Kusky, 2010; Zhang et al., 2010a,b, 2011; Yin et al., 2011; Guo et al., 2012; Santosh et al., 2012; Zhang et al., 2013; Yin et al., 2014). This study provides the first documentation of Paleozoic UHT metamorphism in the Tianshan orogenic collage and new insights into the accretionary orogeny of the STAC although detailed tectonic mechanism remains to be revealed in future studies. 8. Conclusions (1) The granulites from the Yushugou massif, occurring as a tectonic slab within the South Tianshan Accretionary Complex, consist mainly of garnet, orthopyroxene, plagioclase, K-feldspar and quartz with minor biotite, ilmenite and rutile, and show typical mineral assemblage and microstructures of ductile deformation under granulite-facies conditions. (2) The pelitic granulite underwent UHT (N 940 °C) and HP (11.5–14 kbar) metamorphism and partial melting under high geothermal gradient of ca. 24 °C/km, and recorded a possible prograde process characterized by heating and burial. (3) NAMs of the granulites contain certain water in the manner of structural OH and sub-microscopic fluid inclusions. The trace amounts of water in NAMs obviously affected ductile

Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009

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deformation of the near-dry granulites. (4) The Yushugou granulites probably derived from the deep root of a hot continental magmatic arc, together with associated ultramafic rocks, forming a lower crust to upper mantle section of the Paleozoic South Tianshan accretionary orogen.

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Please cite this article as: Zhang, L., et al., Metamorphic P–T–water conditions of the Yushugou granulites from the southeastern Tianshan orogen: Implications for Paleozoic accretionary orogeny, Gondwana Research (2015), http://dx.doi.org/10.1016/j.gr.2014.12.009