Late Carboniferous to early Permian partial melting of the metasedimentary rocks and crustal reworking in the Central Asian Orogenic Belt: Evidence from garnet-bearing rhyolites in the Chinese South Tianshan

    Late Carboniferous to early Permian partial melting of the metasedimentary rocks and crustal reworking in the Central Asian Orogenic Belt: Evidence from garnet-bearing rhyolites in the Chinese South Tianshan Zhiguo Cheng, Zhaochong Zhang, M. Santosh, Zhenyu Zhao, Lili Chen PII: DOI: Reference:

S0024-4937(17)30123-8 doi:10.1016/j.lithos.2017.03.017 LITHOS 4266

To appear in:

LITHOS

Received date: Accepted date:

26 August 2016 12 March 2017

Please cite this article as: Cheng, Zhiguo, Zhang, Zhaochong, Santosh, M., Zhao, Zhenyu, Chen, Lili, Late Carboniferous to early Permian partial melting of the metasedimentary rocks and crustal reworking in the Central Asian Orogenic Belt: Evidence from garnet-bearing rhyolites in the Chinese South Tianshan, LITHOS (2017), doi:10.1016/j.lithos.2017.03.017

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ACCEPTED MANUSCRIPT Late Carboniferous to early Permian partial melting of the metasedimentary rocks and crustal reworking in the Central Asian

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South Tianshan

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Orogenic Belt: Evidence from garnet-bearing rhyolites in the Chinese

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Zhiguo Chenga, Zhaochong Zhanga, *, M. Santosha, b,c, Zhenyu Zhaoa, Lili Chena a State Key Laboratory of Geological Processes and Mineral Resources, China University of

b

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Geosciences, Beijing 100083, China

Centre for Tectonics, Resources and Exploration, Department of Earth Sciences, University of

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Adelaide, SA 5005, Australia

Department of Geology, Northwest University, Northern Taibai Str. 229, Xi'an 710069,

China

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* Corresponding author. E-mail: [email protected], Telephone: 13910168892, Fax: 010-

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82322195.

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ABSTRACT: Peraluminous granitic rocks provide important insights on crustal reworking processes. Here we study the garnet-bearing rhyolites of the Early Permian Xiaotikanlike Formation exposed in the Heiyingshan region located in the South Tianshan region in Xinjiang, NW China. Two layers of garnet-bearing rhyolites were recognized, which discordantly overly the strongly deformed Late Carboniferous strata. The rocks generally display porphyritic texture with alkali-feldspar, quartz, plagioclase, biotite and garnet as the major phenocryst phases. Zircon LAICP-MS U-Pb analyses yields ages of 281.5±0.7Ma for the upper layer of rhyolite (ULR) and 299.9±1.9Ma for the lower layer (LLR). The ULR shows SiO2 in narrow range of 71.08 to 72.39wt.%, and Fe2O3 from 0.99 to 1.69 wt.% with Fe2O3/FeO ranging from 1.68 to 2.68. The normalized trace element patterns are characterized by prominent troughs for Nb, Ta, Sr, P and Ti and relatively sloping rare earth element patterns. They have radioactive Sr-Nd isotopic compositions [(87Sr/86Sr)i = 0.7128 to 0.7131 and εNd = -13.09 to -12.44] and negative εHf(t) values of -9.97 to -1.27. The LLR exhibits similar trace and rare earth element patterns, εNd (-12.68 to -

ACCEPTED MANUSCRIPT 11.70) and εHf(t) values (-7.56 to -2.6) to the ULR, but have higher (87Sr/86Sr)i = 0.7166-0.7269 and lower Fe2O3/FeO ratios (0.17-1.73). Geochemical and isotopic characteristics indicate that these rhyolites are dominantly alkali-calcic and strongly peraluminous, which were most likely derived

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from muscovite dehydration melting of metagreywacke. We correlate this with the tectonic

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processes following the collision between the Central Tianshan-Yili Block and the Tarim Craton. Both the Zr saturation temperatures and Al2O3/TiO2 ratios suggest an increase in temperature of

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the source during 300Ma to 280Ma, which may be attributed to heat input from the underplating of mantle-derived basaltic magmas. The field relation between the rhyolite and the Late

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Carboniferous strata favors the model that the final collision culminated during Late Carboniferous before 300Ma. In combination with the occurrence of felsic lavas in other regions

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such as the Xiaotikanlike Formation from Wensu, Laohutai and Boziguo’er region, we define a > 200km peraluminous magma belt in the central part of Chinese South Tianshan, which provides

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new insights into the crustal reworking process in the Central Asian Orogenic Belt.

1. Introduction

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Keywords: garnet-bearing rhyolite; CAOB; Tianshan; peraluminous granite; S-type

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The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens in the world and is also endowed with important mineral resources (Windley et al., 2007; Mao et al., 2008; Kröner et al., 2013, 2014; Xiao et al., 2015). The CAOB was formed by the amalgamation of numerous island arc assemblages, ophiolites and possibly some microcontinents (e.g. Jahn et al., 2004; Xiao et al., 2004; Gao et al., 2011; Xiao et al., 2015). Although previous investigations of the CAOB have identified multiple episodes of accretionary and collisional events, several important issues related to the tectonic history of this orogen remain controversial (Wang et al., 2011; Kröner et al., 2014; Xiao and Santosh, 2014; Huang et al., 2015; Scheltens et al., 2015; Han et al., 2016). The major debates include two aspects: 1) the timing of final collision of the CAOB, with current models showing a range from Late Devonian (Windley et al., 1990; Charvet et al., 2011; Wang et al., 2011) through Carboniferous (Gao et al., 2011; Klemd et al., 2011; Yang et al., 2013; Zhang et al., 2013; Jiang et al., 2014) to Middle Triassic (Zhang et al., 2007); and 2) the crustal growth and recycling history (Jahn et al., 2000; Windley et al., 2007; Kröner et al., 2013,

ACCEPTED MANUSCRIPT 2014). Previous studies indicate that ca. 50% or more of the crust in the CAOB is juvenile, and thus this orogen was considered to represent the largest Phanerozoic crustal growth on this planet (e.g. Sengör et al., 1993; Jahn et al., 2000). Moreover, typical S-type granites were rarely reported

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in the CAOB, which seems to support the hypothesis of the high proportion of juvenile crust

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(Zhou et al., 2007; Cai et al., 2011; Gao et al., 2011; Gou et al., 2015; Xia et al., 2016). However, based on a recent review of the Nd-Hf isotopic data for the granitic rocks within CAOB, Kröner et

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al. (2014) argued that some parts of CAOB, especially the Kyrgyz North Tianshan, northern and central Mongolia and Chinese Central Tianshan, preserve evidence for crustal reworking instead

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of extensive new growth, and thus the proportion of juvenile crust generated during the construction of this Phanerozoic orogen has been grossly overestimated. If this is the case, crustal

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reworking must have also played a vital role in the construction of the CAOB. The South Tianshan Orogenic Belt (STOB), located between the Central Tianshan (CTS)-

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Yilli Block to the north and Tarim Craton to the south, was traditionally regarded to have formed at the final stage of amalgamation in the CAOB following the prolonged accretionary and

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collisional orogeny (Xiao et al., 2013; Seltmann et al., 2011). The Carboniferous-Permian intermediate-felsic igneous rocks which are widely distributed in the STOB can be used to

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evaluate the role of crustal reworking (Jahn et al., 2000; Seltmann et al., 2011; Huang et al., 2013; Kröner et al., 2014). Our recent detailed field investigations in the Heiyingshan area of the Central STOB led to the discovery of two layers of garnet-bearing rhyolites (S-type) within the Early Permian Xiaotikanlike Formation, which discordantly overlie the metamorphosed marine-facies sandstone of the Late Carboniferous Kangkelin Formation. They are in turn partly covered by the middle-lower Triassic Ehuobulak Formation. The Heiyingshan rhyolites provide a rare opportunity to investigate the geodynamic processes and to assess the role of the crustal reworking during the evolution of the CAOB. In this contribution, we report zircon U-Pb ages, mineralogy, major and trace element geochemistry and Sr-Nd-Hf isotopic compositions for the Heiyingshan garnet-bearing rhyolites. Based on the results, we attempt to constrain the geodynamic processes and provide new evidence for crustal reworking in the CAOB. 2. Geological background

ACCEPTED MANUSCRIPT The CAOB occupies a prominent position in the Asian continent, and is surrounded by the East European and Siberian Cratons to the north and the Karakum Craton, Tarim Craton and North China Craton to the south (Fig. 1a; Sengör et al., 1993; Xiao et al., 2013). It preserves the history

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of prolonged evolution in the Paleo-Asian Oceanic realm from ca. 1000Ma to 280Ma, involving

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the amalgamation of various terranes and the oceanic domains (Jahn et al., 2004; Windley et al., 2007; Kröner et al., 2014). Previous studies provide detailed descriptions of the geological

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evolution of the CAOB (Xiao et al., 2013; Han et al., 2011; Long et al., 2011; Wang et al., 2011; Seltmann et al., 2011), and here we mainly focus on the STOB and give a brief introduction for its

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geological background.

As the southern margin of the CAOB, the STOB was formed through Late Paleozoic

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collision between the Kazakhstan-Central Tianshan-Yili Block and the Karakum-Tarim Craton together with the closure of the Paleozoic South Tianshan Ocean (Fig. 1b; Gao et al., 2009; Han et

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al., 2011; Huang et al., 2013, 2016). The STOB is an intensely deformed fold and thrust belt including forearc accretionary complexes and passive margin sedimentary rocks of the Karakum-

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Tarim continent. Separated by the Talas-Ferghana Fault, the STOB is subdivided into eastern and western parts. The western part extends from Uzbekistan to Kyrgyzstan, whereas the eastern part

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is mainly located in the Kyrgyzstan and Xinjiang province in China. The western part in Uzbekistan, Tajikistan and Kyrgyzstan is separated from the Middle Tianshan terranes by the Southern Tianshan Suture, which is defined by the Early Ordovician to Early Carboniferous ophiolites (Kurenkov and Aristov, 1995; Chen et al., 1999). It mainly includes the Kyzylkum and Alay segments, which are composed of a Neoproterozoic and a Precambrian crystalline basement respectively, with a thick Phanerozoic sedimentary cover (Seltmann et al., 2011). The strata units consist of Silurian to Carboniferous pelagic sediments, intraplate volcanics and thick carbonate platforms. Moreover, voluminous post-collisional intrusions of diverse compositions occur in the Kyzylkum and Alay segments, but typical S-type granites are rare (Konopelko et al., 2007; Seltmann et al., 2011). The eastern part of STOB is bound by the South Tianshan Suture to the north and the Northern Tarim Fault to the south. Some high pressure to ultrahigh pressure (HP/UHP) metamorphic rocks including blueschist, eclogite and greenschist have been identified along this

ACCEPTED MANUSCRIPT suture, which are suggested to be associated with the discontinuous northern subduction of the Paleozoic South Tianshan Ocean (Gao et al., 2011; Zhang et al., 2013). The basement rocks of the eastern STOB are represented by Paleoproterozoic Xingditagh Group and Mesoproterozoic Akesu

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Group (Yang and Zhou, 2009). Paleozoic strata are mainly Ediacaran to Lower Cambrian black

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shales and Cambrian to Carboniferous clastic-carbonate-volcanic successions containing siltstone, sandstone, limestone, cherts and volcanic interlayers (Allen et al., 1992; Jiang et al., 2014).

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Permian strata mainly occur in the central section of the Chinese STOB, and include those in the Heiyingshan, Wensu, Laohutai and Boziguo’er regions (Luo et al., 2008; Huang et al., 2015).

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They are grouped as the Xiaotikanlike Formation and generally composed of basaltic and felsic lavas, pyroclastic rocks together with the interbedded siltstones, sandstones and conglomerates.

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Previous zircon U-Pb dating yielded ages of the rhyolite from the Wensu, Laohutai and Bozibo’er regions in the range of 285Ma to 290Ma (Luo et al., 2008; Huang et al., 2015). The outcrops of

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the intrusive rocks are less compared to those in the western part of the STOB, occupying only a small proportion (5%) of the total area (Huang et al., 2012a). Based on the previous

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geochronological data, these intrusions are considered to have been emplaced during Late Silurian to Early Permian (Huang et al., 2015 and the references therein).

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The Heiyingshan region is located in the central part of the Chinese STOB (Fig. 1b). The stratigraphic successions are composed of the Upper Ordovician Yinanlike Group, Middle-Lower Silurian Hetongshala Group, Upper Silurian-Lower Devonian Aertengkes Formation, Upper Silurian Wupataerkan Formation, Early Carboniferous Yeyungou Formation, Late Carboniferous Kalasu Formation and Kangkelin Formation, Lower Permian Xiaotikanlike Formation, MiddleLower Triassic Ehuobulak Formation, Upper Triassic Huangshanjie Formation and Lower Jurassic Taliqike, Ahe, Yangxia Formations (Fig. 2). The Xiaotikanlike Formation discordantly overlies the Upper Silurian-Lower Devonian Aertengkes Formation, Late Carboniferous Kalasu Formation and Kangkelin Formation, and in turn is uncomfortably overlain by the lower Triassic Ehuobulake Formation (Fig. 2). The Aertengkes Formation is well exposed in the central part of the study area, and is predominately composed of limestone, cherts, mafic lavas and siltstone with a thickness of 890m. The Late Carboniferous Kalasu Formation contains ~424m siltstone, sandstone and limestone, and the Late Carboniferous Kangkelin Formation refers to a series of ~783m thick

ACCEPTED MANUSCRIPT shallow marine siltstone, sandstone and limestone. Both the Kalasu and Kangkelin Formations are strongly folded. The Xiaotikanlike Formation is mainly exposed in the central and southwestern part in the Heiyingshan region. It is more than 526m in thick and consists predominately of large

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volumes of terrestrial pyroclastic rocks, felsic lavas and minor sandstone and sandy

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conglomerates. The exposed area of the Xiaotikanlike Formation is ̚20km2, within which two rhyolite layers covering ca. 5km2 have been identified in our field investigation. The lower layer of

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rhyolite (LLR) is gray in color with a thickness of ~50m, whereas the upper layer (ULR) is red with a thickness of ~20m (Fig. 4a, b and c). The lower gray rhyolite is the major phase, which

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uncomfortably overlies the Kangkelin Formation. Our field observations show that the two layers of rhyolites have clear contact relationship (Fig. 4b), and are fresh without any deformation or

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alteration (Fig. 4d). The Lower Triassic Ehuobulake Formation shows a thickness of ~347 m and is composed dominantly of purple-red to grayish-green conglomerate, mudstone and sandstone,

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well exposed in the central part of the study area. Several granitic plutons are distributed in the northern part of the study area with an emplacement age of 430Ma (Zhao et al., 2015). Throughout

3. Petrography

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the study area, E-W trending faults are well developed (Fig. 2).

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The ULR is red in hand specimen and shows porphyritic texture with 15-20% phenocrysts. The phenocrysts are generally composed by alkali-feldspar (45-55%), quartz (40-45%), plagioclase (3-5%), biotite (1-3%), garnet (2-3%) and ilmenite (2-3%; Fig. 4a and b), within which some alkali-feldspar and quartz grains occur as megacrysts (Fig. 4c). The alkali-feldspar laths are euhedral tabular, with a size range of 0.75×0.9mm to 1.5×2mm and diameter of the megacrysts ranging from 2×4mm to 3×7mm. Minor alkali-feldspar grains have experienced argillic alteration and chloritization. Quartz is granular and ranges in size from 0.4×4mm to 4×5mm and show well developed cracks. Plagioclase occurs as subhedral to euhedral phenocrysts ranging in size from 0.05×0.1mm to 0.4×1.7mm. Biotite occurs as lamellae and varies from 0.3×1mm to 0.3×0.3mm in diameter. Some biotite crystals have been altered to chlorite. Zircon and apatite generally occur as inclusions within biotite. Garnet occurs as euhedral to subhedral grains with cracks, and is 0.7mm-2mm across without any mineral inclusions. Ilmenite occurs as euhedral to anhedral grains ranging in size from 0.1×0.3mm to 0.3×0.4mm, and is typically altered

ACCEPTED MANUSCRIPT to be hematite. Occasionally, ilmenite is enclosed by quartz. The groundmass of the rhyolite exhibits cryptocrystalline texture and is composed of felsic minerals. The LLR is grayish and typically exhibits porphyritic texture. The contents of phenocrysts

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vary from 10% to 15% and are composed of alkali-feldspar (40-45%), quartz (45-50%),

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plagioclase (3-5%), biotite (3-5%) and garnet (2-3%) together with minor rutile and apatite (Fig. 4f). Alkali-feldspar occurs as euhedral tabular grains with a size range of 0.3×0.5mm to 1.5×2mm.

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Alkali-feldspar is also present as megacrysts which vary from 4×8mm to 4×15mm in diameter (Fig. 4d). Some grains show deuteric alteration and were replaced by kaolin and chlorite. The

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alkali-feldspar envelopes garnet, quartz and biotite grains (Fig. 4d). Quartz occurs as subhedral to euhedral granular grains with cracks, and is 0.8mm-3×3.5mm in across. Plagioclase occurs as

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euhedral tabular grains with size ranging from 0.15×0.2mm to 0.5×0.3mm. Biotite lamellae range in diameter from 0.1×0.2mm to 0.8×2mm. Some biotites were altered to chlorite and carry zircon

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and apatite inclusions. Garnet occurs as granular grains with cracks, with diameter ranging from 0.4mm to 1.2mm. No mineral inclusions have been found in the garnet, and occasionally the

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garnet is associated with biotite (Fig. 4e). The groundmass is cryptocrystalline and consists predominantly of microcrystalline felsic minerals (feldspar and quartz).

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Petrographic observation suggests that zircon, apatite, garnet, biotite, plagioclase, quartz and alkali-feldspar crystallized consecutively. Compared to the LLR, the ULR has a higher content of phenocryst phases. Furthermore, the ULR contains ilmenite and has a higher alkali-feldspar content but lower biotite and garnet contents. 4. Analytical methods 4.1 Mineral major element analyses The electron microprobe analyses (EMPA) was conducted at the institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing. Major elements of the minerals in the rhyolites were obtained using a JXA-8230 electron microprobe with the wavelength dispersive technique. Analysis process is standardized on the national standards including natural minerals and synthetic oxides. The operating conditions were 15kv, 20nA with a beam of 5μm. The results were corrected using the ZAF method. 4.2 Zircon U-Pb dating

ACCEPTED MANUSCRIPT Zircon grains were separated from both the upper and lower layers of the rhyolites for Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb age dating. Transmitted, reflected light images together with cathodoluminescence (CL) images are performed

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to reveal the internal structures of zircon prior to U-Pb dating. The zircon U-Pb dating by the LA-

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ICP-MS were conducted at Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, China. Analytical procedures and instrumental conditions were reported by Hou

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et al. (2009). A Finnigan Neptune MC-ICP-MS instrument was used for U-Th-Pb dating, in which Helium was applied as a carrier gas. Zircon GJ1 with an age of 610.0±1.7 Ma (Elhlou et al., 2006)

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was used as the standard. Plesovice zircon (337.13±0.37 Ma; Sláma et al., 2008) was applied to calibrate the machine. Moreover, the measured

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Pb/238U ratios were corrected using external

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standard zircon M172 (U=923 ppm, Th=439 ppm, Th/U=0.475; Nasdala et al., 2008). The common Pb calibration is according to the method of Andersen (2002), and the weighted mean

4.3 Zircon Lu-Hf isotopes

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calculation and concordia diagrams were made by Isoplot program (ver3.0; Ludwig, 2003).

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Zircon Lu-Hf isotopic ratios was analyzed in-situ by a Neptune MC-ICP-MS with a Newwave UP213 laser ablation system at the Institute of Mineral Resources, Chinese Academy of

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Geological Sciences, Beijing. Instrumental conditions and analytical procedures were described by Wu et al. (2006) and Hou et al. (2007). The laser ablation beam diameter is 40μm in our study and Helium was applied as a carrier gas.

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Lu/175Lu=0.02658 and

determined to correct the isobaric interferences of Moreover, Yb isotopes were normalized to

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Lu and

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Yb/173Yb=0.796218 ratios were

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Yb on

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Hf (Chu et al., 2002).

Yb/173Yb of 1.35274 (Chu et al., 2002) via an

exponential law for the instrumental mass bias correction, while Hf isotopes were normalized to 179

Hf/177Hf of 0.7325. For the mass bias behavior of Lu, it is suggested to be the same with that of

Yb. Zircon GJ-1 was employed as the reference standard. Zircon GJ-1 has a weighted mean 176

Hf/177Hf value of 0.282013±8 (2σ, n=10) during our routine analyses, which is in agreement

with the weighted mean 176Hf/177Hf ratio of 0.282013±19 (2σ) reported by Elhlou et al. (2006). 4.4 Whole-rock major and trace elements analysis Bulk-rock major and trace elements analysis were performed at the National Research Centre for Geoanalysis, Beijing. Major element analysis was analyzed by XRF (X-ray fluorescence

ACCEPTED MANUSCRIPT spectrometer) by the Phillips PW4400 (XRF-1500). The analytical uncertainties are better than 1% based on the certified standards GSR-3 (basalt) and GSR-2 (andesite). The FeO content was obtained by the conventional wet chemical methods, while the loss on ignition (LOI) was

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calculated after heating the samples at 980ć for ~ 30 min. Trace elements were analyzed by the PE300D ICP-MS with an X-series instrument. Two Chinese national standards (GSR-2 and GSR3) were used to check the accuracy, which is generally better than 5% for most trace elements in

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this study. Detailed description of the analyzed procedure followed Norrish and Chappell (1997)

4.5 Whole-rock Rb-Sr and Sm-Nd isotopes

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for major elements and Qi et al. (2000) for trace elements, respectively.

Rb-Sr and Sm-Nd isotope ratios were determined using thermal ionization mass spectrometer

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(TIMS) at the Beijing Nuclear Industry Geological Analysis and Test Center, China. The powder of each sample was totally dissolved in a mixture of HF and HNO3 in the Tlflon beakers, and then

the

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Sr/86Sr and

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Sr and Nd concentrations were purified by the cation exchange columns. Based on the analysis, 143

Nd/144Nd isotopic ratios were corrected to

Sr/88Sr=0.1194 and

Nd/144Nd=0.7219, respectively. The Nd standard La Jolla is given as 143Nd/144Nd=0.511856±10

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146

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(2σ), and the Sr standard NBS987 is given as

Sr/88Sr=0.710248±12 (2σ) during the period of

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data acquisition.

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5. Results

5.1 Mineral Chemistry 5.1.1 Feldspar

Both alkali feldspar and plagioclase occur as phenocryst in the Heiyingshan rhyolites. In the ULR, the alkali feldspar phenocrysts are classified into albite, Na-orthoclase and orthoclase (Fig. 5a). The albite exhibits restricted compositions of An0.10-4.62Ab88.54-99.81Or0-9.67, whereas the Kfeldspar shows a wide compositional range of An0-1.46Ab2.62-36.64Or61.90-97.38 (Supplementary Table A.1). The plagioclase phenocrysts are mainly oligoclase with the composition of An25.8626.57Ab66.33-66.37Or7.77-12.10.

On the other hand, the alkali feldspar phenocrysts in the LLR consist of

albite and orthoclase with compositions of An0.14-1.08Ab98.63-99.86Or0-0.44 and An0-1.75Ab2.8226.90Or71.35-97.07,

respectively. The plagioclase phenocrysts are mainly dominated by andesine

˄An33.39-41.74Ab53.39-62.36Or4.26-4.87˅.

ACCEPTED MANUSCRIPT 5.1.2 Garnet The chemical composition of garnets from the Heiyingshan rhyolite is listed in Supplementary Table A.2. Generally, the garnets from the upper and lower layer of rhyolite

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contain dominantly almandine, pyrope and grossular in varying proportions (Fig. 5b). The garnet

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in the ULR is rich in Al2O3 (20.79-21.44 wt. %) and FeO (33.34-35.41wt. %), and poor in MnO (0.37-1.95 wt. %). They show a moderate compositional range (And0.31-4.34 Pyr8.07-21.30 Spe0.84-4.45

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Gro6.44-13.38 Alm66.96-78.55) with low MgO (3.09-5.47 wt. %) and CaO (4.95-5.14 wt. %) contents (Fig. 5b). The garnet in the LLR has lower FeO (29.52-36.75 wt. %), CaO (1.11 to 4.16 wt. %),

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MnO (0.24-1. 03 wt. %) and higher MgO (3.32 to 6.03 wt. %) and Al2O3 (20.59-21.27 wt. %) contents with compositions of And0.45-4.68 Pyr13.34-23.44 Spe0.53-2.28 Gro2.49-11.06 Alm64.06-82.22.

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Compared to the garnet in the leucogranite from the Himalayan Orogen and Eastern Kunlun Orogen, the garnets in the Heiyingshan rhyolite show higher pyrope and lower spessartite content

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(Fig. 5b; Gao et al., 2012; Shi, 2014) with higher MgO and lower MnO. 5.1.3 Biotite

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The biotite phenocrysts in the ULR and LLR exhibit similar and restricted compositional range (Supplementary Table A.3; Fig. 5c). They generally plot in the boundary between the

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magnesian biotite and ferribiotite, characterized by high TiO2 (5.08-7.12 wt. % and 4.67-6.46 wt. %) contents and MgO (7.83-9.48 wt. % and 7.52-9.33 wt. %) with Mg/(FeT+Mg) ratio of 0.380.46 wt. % and 0.37-0.43 wt. %, respectively. Computations based on Ti-saturation temperature (TTi) proposed by Henry et al. (2005) for biotite in ULR yield temperature in the range of 740 to 768ć, whereas the TTi from the LLR vary from 708 to 767ć. In the Lgf(O2) vs. TTi diagram, the ULR displays a higher oxygen fugacity than the LLR (Fig. 5d). The fugacity of the LLR mainly varies from 10-15 to 10-14, and that of ULR ranges from 10-15 to 10-12.5. In the plot of Fe3+-Fe2+Mg2+ (Fig. 5e), all the biotite data fall in the HM (hematite-magnetite) field. 5.1.4 Apatite and Fe-Ti oxides The EMPA data of apatite and Fe-Ti oxides are given in Supplementary Tables A.4-5. Apatite in the ULR shows consistent composition for P2O5 (40.03-40.94 wt. %) and CaO (52.59-55.65 wt. %) with minor F (2.29-3.14 wt. %) contents. The apatite in the LLR has 40.35-41.16 wt. % for P2O5 and 53.64-54.04 wt. % for CaO. They are also fluorapatites with F contents ranging from

ACCEPTED MANUSCRIPT 2.25 to 2.84 wt. %. The Fe-Ti oxides occur only in the ULR and are mainly magnetite and ilmenite, where the ilmenite contains 53.74-56.49 wt. % TiO2 and 42.03 to 43.83 wt. % FeO. 5.2 Zircon U-Pb ages

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The zircon grains from both the ULR and LLR in the Heiyingshan region show a size range

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of 50-300μm with a width/length ratio of 1:1-1:3. As shown in CL images, most of the zircon grains exhibits homogeneous planar or oscillatory growth zoning, which is one of the typical

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characters for magmatic zircon (Fig. 6a and c). Occasionally, some grains have cores that are unzoned with strong luminescence. The diameter of the cores ranges from 10 to 40μm with clear

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resorption structure, and might represent inherited grains. It is worth to note that only the most likely magmatic zircons were analyzed to determine the ages of the rhyolites. The U-Pb analytical

grains from ULR yielded

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results are listed in Supplementary Table A.1. Twenty three of twenty four spot analyses of zircon Pb/238U ages ranging from 279 to 284Ma with a weighted mean

Pb/238U ages of 281.5±0.7Ma (MSWD=0.68; Fig. 6a and b). This age could be considered as

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the eruption age of the ULR. One zircon grain gives an older

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Pb/238U age of 325Ma, which

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indicates inheritance from the magma source or a xenocryst. Twenty four spots were analyzed for the LLR, among which nineteen spots are clustered around 206Pb/238U ages in the range of 294 to

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306Ma, with a weighted mean age of 299.9±1.9Ma (MSWD=4.0; Fig. 6c and d). These zircon ages are concordant, and thus the weighted mean age is considered to represent the eruption age of the magma. The other five zircon grains exhibit 206Pb/238U ages of 310, 312, 313, 325 and 334Ma, respectively, suggesting inherited or entrained origin. 5.3 Whole rock major and trace elements 5.3.1 Major elements The major and trace element compositions of seven representative samples of the ULR and eight samples of the LLR are listed in Supplementary Table A.2. The results show that all the samples are fresh with their LOI ranging from 0.91 to 1.39wt. %. The ULR displays SiO2 abundance ranging from 71.08 to 72.39wt.%, 13.97 to 14.67wt.% for Al2O3, 0.59 to 0.74wt.% for FeO, 0.99 to 1.69wt.% for Fe2O3, 0.74 to 1.54wt.% for CaO, 0.29 to 0.75 wt. % for MgO, 2.58 to 3.82 wt.% for Na2O, 4.5 and 5.48wt.% for K2O. The Fe2O3/FeO ratios range from 1.85 to 2.68. As shown in SiO2 vs. Na2O+K2O diagram, the ULR samples plot in the field of rhyolite (Fig. 7). In

ACCEPTED MANUSCRIPT the plot of SiO2 vs. Na2O+K2O-CaO and SiO2 vs. K2O, the ULR falls in the alkali-calcic series and high-K calc-alkaline to shoshonitic series, respectively (Fig. 8a and b). They have high A/CNK [Al2O3/(Na2O+K2O-CaO)] values varying from 1.08 to 1.19, indicating peraluminous to

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strongly peraluminous characteristic (Fig. 8c).

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The LLR samples exhibit SiO2 from 70.90 to 72.62 wt.%, Al2O3 from 13.78 to 15.07wt. %, Fe2O3 from 0.26 to 0.97 wt.%, FeO from 0.56 to 1.53 wt.%, MgO from 0.29 to 0.61 wt.%, CaO

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from 0.67 to 1.04 wt.%, Na2O from 2.34 to 3.39 wt.% and K2O from 5.38 to 6.41wt.%. Compared to the ULR, the LLR samples typically have higher FeO and lower Fe2O3 contents, with lower

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Fe2O3/FeO ratios ranging from 0.17 to 1.73. On the SiO2 vs. Na2O+K2O diagram, they are classified as the rhyolite (Fig. 7). In the SiO2 vs. Na2O+K2O-CaO and SiO2 vs. K2O diagrams,

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they fall in the alkali-calcic series and high-K calc-alkaline to shoshonitic series, respectively, similar to the ULR (Fig. 8a and b). The A/CNK ratios range from 1.10 to 1.19 showing strongly

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peraluminous characteristics (Fig. 8c). Compared to the felsic lavas of the Xiaotikanlike Formation from the other regions, e.g. Laohutai and Wensu, the samples in Heiyingshan Region

5.3.2 Trace elements

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have lower SiO2 contents and A/CNK ratios but higher total alkali contents (Figs. 7 and 8c).

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The ULR has total rare earth element (REE) content ranging from 206.5 to 231.1 ppm. In the chondrite-normalized REE diagram, the samples are characterized by the relative enrichment in light rare earth elements (LREEs) and low heavy rare earth elements (HREE) with moderate (La/Yb)N ratios from 33.73 to 43.54 (Fig. 9a). Obvious negative Eu anomalies are observed with Eu/Eu* values [Eu/((Sm+Gd)/2)] of 0.17 to 0.18. The rhyolites show significant enrichment in large-ion lithophile elements (LILEs) and low concentrations of high field strength elements (HFSEs). For example, they are mostly enriched in Rb, Th and K, with low contents of Nb, Ta and Ti. As shown in primitive mantle-normalized trace element spidergrams, the samples show remarkable troughs at Nb, Ta, Sr, P and Ti (Fig. 9b). The rhyolite from the LLR has total REE of 197.6-220.6 and exhibits patterns similar to these of the ULR. They are characterized by high contents of LILEs and low concentrations of HFSEs, with strong negative anomalies of Nb, Ta, Sr, P and Ti (Fig. 9b). They show moderately fractionated REE patterns [(La/Yb)N=34.17-60.17] with prominent negative Eu [Eu/Eu*=0.16-0.19] anomalies (Fig. 9a). Compared to the rhyolitic lavas

ACCEPTED MANUSCRIPT from the Wensu and Laohutai region, our samples are characterized by the low contents of the HREE (Fig. 9). The zircon CL images and U-Pb dating have revealed that the rhyolites from Heiyingshan

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region contain inherited zircon grains or discrete cores indicating the saturation of Zr element in

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the melts. The Zr saturation temperature (TZr) proposed by Miller et al. (2003) can therefore be adopted to estimate the upper limit of temperatures for zircon crystallization. The calculated TZr

5.4 Whole rock Sr-Nd and zircon Lu-Hf isotopes

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values range from 794-824ć and 780-801ć for the ULR and LLR, respectively.

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Measured and age-corrected Rb-Sr and Sm-Nd isotopic ratios are presented in the Supplementary Table A.3 and shown in Fig.10a where the data are plotted in conjunction with

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those from other Permian granitoids in STOB and the basaltic lava in the Xiaotikanlike Formation for comparison. The ULR have (87Sr/86Sr)i ranging from 0.7128 to 0.7179 and εNd values ranging

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from -12.44 to -13.09. The LLR samples have slightly higher (87Sr/86Sr)i ranging from 0.71660.7269 and εNd values ranging from -11.70 to -12.68. Compared to the other granitoids in the

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STOB and the basaltic rocks in the Xiaotikanlike Formation, our rhyolite samples show much lower εNd and higher (87Sr/86Sr)i values (Fig. 10a).

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Twenty two zircon grains were chosen for Lu-Hf analyses from the ULR and 23 grains from LLR, and were analyzed on the same spots from where the U-Pb ages were analyzed. The data are reported in Supplementary Table A.4 and shown in Fig. 10b. For the ULR, the magmatic zircon grains with ca. 281.5Ma crystallization ages have negative εHf(t) values from -1.27 to -9.97. The single-stage Hf model ages (TDM) for the zircon are 965-1305Ma, while two-stage Hf model ages (T2DM) are 1381-1935Ma. The inherited zircon (325Ma) exhibits εHf(t) value of -1.86, TDM value of 1027Ma and T2DM of 1452Ma. The LLR have variable Hf isotopic compositions with εHf(t) values of -7.56 to -2.6, TDM and T2DM values of 1040- 1286Ma and 1478-1794Ma, respectively. The entrained zircon (310-334Ma) shows εHf(t) values of -7.56 to -2.6, TDM values of 1052-1217Ma and T2DM ranging from 1502 to 1749Ma. Compared to the felsic lavas in the Xiaotikanlike Formation from the Boziguo’er region (-8.7 to -0.3), the Heiyingshan rhyolites display similar εHf(t) compositions, but have lower values than those of the Permian A-type granitoids in the STOB (4.6 to 9; Fig. 10b).

ACCEPTED MANUSCRIPT In summary, although the ULR and LLR show some similar geochemical characteristics, such as the trace element patterns and Nd-Hf isotopic compositions, the ULR samples display lower (87Sr/86Sr)i values and higher Fe2O3/FeO ratios compared to the LLR.

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6. Discussion

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6.1 Origin of the garnet

Garnet is not a common rock-forming mineral in the igneous rocks but is a useful

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geochemical tracer for the petrogenesis (Dahlquist et al., 2007). Apart from their occurrence in some mantle rocks such as kimberlites, garnet is also present in crustal pegmatites and aplite

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dykes (Dahlquist et al., 2007; Cheng et al., 2014). Garnet is also a common constituent of some felsic peraluminous granitoids (e.g. S-type granite; Hogan, 1996; Kebede et al., 2001). Garnet in

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granite is generated by the following mechanisms: (1) assimilation from the wallrocks during the magma accent (Warren, 1970; Allan and Clarke, 1981; Lantai, 1991); (2) refractory residual

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phases of the magma source (Zeng et al., 2005; Stevens et al., 2007; René and Stelling, 2007); (3) low-pressure phenocrysts from the differential high MnO magmas, mainly for the Mn-rich garnet

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(i.e. spessartite; Cawthorn and Brown, 1976; Abbott, 1981; Miller and Stoddard, 1981; René and Stelling, 2007); and (4) high-pressure (>7kbar) precipitates from the felsic magmas (Green, 1976,

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1977; René and Stelling, 2007).

The garnet in the Heiyingshan rhyolite cannot be of xenocrystic origin from the wallrocks or a residual phase of the source, as no garnet-bearing xenoliths have been observed in the rhyolite. Moreover, no mineral inclusions, especially metamorphic minerals, are present in the garnet. Thus, the xenocrystic origin for the garnet can be precluded. The euhedral to subhedral forms, lack of reaction rims and the association between the garnet and biotite indicate a magmatic origin for the garnet in the Heiyinshan rhyolite (Zhou et al., 2015). Previous experimental studies have revealed that only Mn-rich garnet crystallizes at P<5-7 kbar in the intermediate to silicic melts, and the other types of garnets form only at pressures higher than 7kbar (Green and Ringwood, 1968; Green, 1977, 1992; Clements and Wall, 1984). For example, based on the melting experiments for the andesitic and rhyodacitic magmas, Green and Ringwood (1968) proposed that almandine could crystallize at pressures of 9-18kbar and is absent at pressures <9 kbar. As noted above, the garnets in Heiyingshan rhyolite are mainly composed of almandine and pyrope and are

ACCEPTED MANUSCRIPT characterized by the low MnO (0.24-1.95 wt. %) contents, indicative of pressure >7kbar for the garnet crystallization. This is also supported by the occurrence of the quartz and alkali feldspar megacrysts, which generally mark high pressure conditions (Binns et al., 1970; Bell et al., 2004).

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6.2 Petrogenesis of the Heiyingshan rhyolites

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6.2.1 Fractional crystallization

The compositional variation between the ULR and LLR can be attributed to be the result of:

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1) fractional crystallization (Tartèse and Boulvais, 2010) and 2) partial melting of heterogeneous sources (Brown and Pressley, 1999). The geochemical variations within the Heiyingshan rhyolites

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can hardly be explained by the fractional crystallization model, since no geochemical trends were identified in the Harker diagrams (not illustrated). In fact, the major and trace elements display a

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very restricted range, which is different from the crystallization trend of evolved magmas. The lack of the evolved ratios of FeOT/MgO (Fig. 11) and significant tetrad effect in the normalized

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REE patterns (Fig.9a), are also not in conformity with the typical characters of highly fractionated granitoid magmas (Whalen et al., 1987; Gou et al., 2016 and inferences therein). Therefore, we

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consider that the Heiyingshan rhyolites had not experienced strong fractional crystallization, and that the different geochemical characteristics between the ULR and LLR may be attributed to the

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source regions.

6.2.2 Source region

As shown in the Fig.11b, most of the Heiyingshan rhyolites show relatively low 10000×Ga/Al values, excluding the A-type affinity (Fig. 11b). The presence of garnet, and the geochemical data show a peraluminous character, broadly comparable with a typical S-type magmatic source (Miller, 1985; Sylvester, 1998). Furthermore, the garnet exhibits low CaO and MnO and coexists with the plagioclase, biotite, quartz and K-feldspar, which are also consistent with the mineral compositions and mineral associations of the S-type magmas (Green, 1992; Harangi et al., 2001). The two-stage Hf model ages for the upper and lower layers of rhyolite range from 1381 to 1935Ma and 1478-1794Ma, respectively, which suggest that the source region consists of Mesoproterozoic to Paleoproterozoic basement rocks, indicating the involvement of the Tarim Craton. Compared to the Sr-Nd isotopes of the mafic constituents from the Tarim Craton ((87Sr/86Sr)t=300Ma = 0.7042–0.7100, εNd(t=300Ma) = -11.90 to 0.87; Zhang et al., 2009, 2011), these

ACCEPTED MANUSCRIPT rhyolites show much more radioactive Sr-Nd isotopic compositions, precluding the possibility of the mafic constituents as the major sources for magmas. Alternatively, experiments had confirmed that partial melting of sedimentary rocks (i.e. argillaceous sediments and greywackes) could

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generate peraluminous and potassic granite melts (Patiňo Douce and Johnston, 1991; Skjerlie and

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Johnston, 1996), which is also supported by the high K2O contents (4.50-6.41wt.%), greywackelike trace element ratios (e.g. Rb/Ba=0.11-0.42, Rb/Sr=0.91-3.64), together with high (87Sr/86Sr)i

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(0.7128 - 0.7269) and negative εNd(t) (-13.09 to -11.70) and εHf(t) (-1.27 to -9.97) values. It is thus likely that the Heiyingshan rhyolites formed by partial melting of the metasedimentary rocks.

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The CaO/Na2O values of the S-type magmas mainly depend on the plagioclase and clay ratios (Sylvester, 1998). It is well established that greywacke is enriched in plagioclase and pelite

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has a high proportion of clay. The melts produced from pelite would have higher CaO/Na2O (>0.3) than melts derived from the greywacke (CaO/Na2O<0.3; Sylvester, 1998; Dahlquist et al.,

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2007). The CaO/Na2O ratios of the LLR vary from 0.20 to 0.39, and the ratios of the ULR range from 0.18 to 0.60. Although the CaO/Na2O ratios have a wide variation, most of the samples have

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CaO/Na2O values less than 0.3 indicating a greywacke source. In the diagram of Rb/Ba vs. Rb/Sr and Al2O3/(MgO+FeOT)molar vs. CaO/(MgO+FeOT)molar, all of the samples fall in the field of clay-

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poor sources with higher plagioclase content, and coincide with the greywacke-derived melts (Fig. 11c and d). The negative anomalies of the Ba, Ta, Nb, Sr, Eu and Ti in the rhyolite samples are in good agreement with the presence of residual plagioclase, biotite and K-feldspar in the source region. The low HREE contents are indicative of a significant role of residual garnet during partial melting. For the anatexis of the metagreywacke, Vielzeuf and Holloway (1988) proposed the typical reactions, e.g. Biotite + Sillimanite + Quartz + Plagioclase = Garnet ± K-feldspar + Melts, Muscovite + Quartz + Plagioclase = K-feldspar + Sillimanite + Melts. Additionally, the low Sr/Ba ratios (0.09-0.17) and the preferably negative relationship between the Rb/Sr and Ba reveal that the muscovite dehydration melting under the H2O-undersaturated condition is the major mechanism of the partial melting (Fig. 12; Harris and Inger, 1992). The TZr is consistent with the temperature required for the breakdown of muscovite, which could occur at <800ć (Miller et al., 2003). Sylvester (1998) suggested that peraluminous granites can be subdivided into two types

ACCEPTED MANUSCRIPT according to their petrogenesis: high-temperature and high-pressure. The former represented by those in European Alps and Himalayas, were derived from the overthickened crust under relatively lower temperature (<875ć), whereas the latter are mainly distributed in the Hercynides and

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Lachlan Fold Belt related to the high temperature crustal anatexis (>875ć). As shown by the TZr and TTi thermometer, the temperatures of the Heiyingshan rhyolite magmas are considerably lower

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than 875 ć , which suggested a high-pressure affinity within the overthickened crust. The Al2O3/TiO2 ratios can be used to estimate the relative temperatures of S-type granites (Sylvester,

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1998). The ratio decreases steadily with increasing temperature, because of the stability of aluminous refractory phases (e.g. garnet) and breakdown of Ti-bearing phases (e.g. biotite and

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ilmenite; Patiňo Douce and Johnston, 1991; Skjerlie and Johnston, 1996). Hence, it is concluded that the peraluminous granite with low Al2O3/TiO2 ratios are generated under higher temperature conditions than those with high ratios. In our study, the Al2O3/TiO2 ratios of the LLR display high

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values ranging from 42.26 to 47.52, and were possibly produced under high pressure conditions, subsequent to the collisional event. The heat for the partial melting to generate the LLR might

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have been derived by the radiogenic decay of K, U and Th in the over thickened crust (Sylvester,

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1998). In addition, the ratios of the ULR vary from 40.09 to 44.45, which are relatively lower than those of the LLR, suggesting an elevated temperature for the source from 300Ma to 280Ma, which are also in good agreement with the TZr. The Early Permian Xiaotikanlike Formation contains not only the rhyolitic volcanic rocks but also some basaltic lavas (~285Ma), as shown by the outcrops from Boziguo’er (Fig. 1; Huang et al., 2015). The basaltic rocks are dominantly tholeiitic and characterized by high Mg# (41-61), slightly enriched Sr-Nd isotope signature (ISr = 0.70495 – 0.70624 and εNd(t) = − 0.5 to + 0.6) and ocean island basalt (OIB)-affinity trace element patterns. They are suggested to be derived from an upwelling asthenospheric mantle source in a postcollision extension (Huang et al., 2015). The regional ~280Ma rhyolites also show a high TZr (755-882ć for Boziguo’er, 851-906ć for Laohutai and 887-933ć for Wensu rhyolite; Huang et al., 2015). Therefore, the stratigraphic location of the basalt beneath the rhyolite in the Xiaotikanlike Formation suggests the possibility that the partial melting of the 280Ma ULR was triggered by the underplating of mantle-derived magmas resulting in a relatively high temperature

ACCEPTED MANUSCRIPT condition. 6.3 Implications for the tectonic setting Peraluminous S-type magmas are derived from the partial melting of the thickened crust

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related to continental collision (Pearce et al., 1984; Harris et al., 1986). However, this does not

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mean that all the S-type granites typically form in the collisional environment, even though they are plotted in the syn-collisional field in some discrimination diagrams (Pearce et al., 1984;

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Förster et al., 1997). Recent studies argued that the peraluminous granites could also be generated after the major collision-related metamorphic events and are part of the ‘post-collisional’

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extensional environment (Finger et al., 1997). As suggested by Pearce et al. (1984) and Förster et al. (1997), the compositions of the granite are principally controlled by the sources and melting

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conditions (e.g. pressure, temperature and oxygen), instead of their geological settings. Therefore, the overall geological evolution history should be carefully taken into consideration when

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evaluating their genesis.

In the Late Paleozoic, collision occurred between the CTS-Tianshan Block and Tarim Craton

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during the closure of the South Tianshan Ocean. However, the timing of final collision between the Central Tianshan-Yili Block and Tarim Craton remains controversial, and ranges from Late

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Devonian to Middle Triassic (Windley et al., 1990; Zhang et al., 2007; Charvet et al., 2011; Gao et al., 2011; Han et al., 2011; Klemd et al., 2011; Xiao et al., 2013; Jiang et al., 2014). Based on the regional unconformity between Late Devonian and Early Carboniferous sedimentary rocks, coupled with the ophiolites with Late Silurian to Early Carboniferous ages, some workers proposed that the collisional event occurred between Late Devonian and Early Carboniferous (Windley et al., 1990; Charvet et al., 2011; Wang et al., 2011). However, the peak UHP metamorphism has been dated between 330 and 310 Ma in the STOB (Gao et al., 2011; Klemd et al., 2011, 2015; Yang et al., 2013; Zhang et al., 2013; Jiang et al., 2014). Furthermore, some much younger ophiolites with Late Permian-Middle Triassic ages have also been recognized, which makes the collision time much more disputable (Zhang et al., 2007). Han et al. (2016) argued that these Permian ophiolites occur only in the Chinese Eastern Tianshan and are not representative for the whole Tianshan orogen, especially for the western part of STOB in China. The Heiyingshan rhyolites discordantly lie over the strongly folded Late Carboniferous

ACCEPTED MANUSCRIPT Kangkelin Formation that consists of marine sediments (sandstone and limestone). The deformation of the Late Carboniferous strata is potentially correlated to the collisional event between the Central Tianshan-Yili Block and the Tarim Craton. If this is the case, the final

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collision is expected to have occurred after the Late Carboniferous Kangkelin Formation and

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before the ca. 300Ma rhyolite. As discussed above, the ca.300Ma rhyolite was derived from the thickened crust, followed by slightly crustal thinning at ca.280Ma as represented by the rhyolite.

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The thinning of the crust indicates that the ca.300Ma rhyolite was generated subsequent to the collisional event followed by the onset of post-collisional extension. Hence, our study favors the

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model that the final collision happened in the end of Late Carboniferous. This model is also supported by the regional widespread A-type granites (290-280Ma) in the STOB, which indicate

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that the STOB was experiencing extension from Early Permian (e.g. Konopelko et al., 2007; Seltmann et al., 2011; Huang et al., 2014).

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6.4 CAOB: crustal growth or reworking?

The generation of peraluminous granites has close links with crustal evolution history (Patiňo

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Douce, 1999). The S-type granites are widely distributed in major orogens of the world including the European Alps, Himalayas and Hercynides (Sylvester, 1998). In contrast to these collisional

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orogenic belts, S-type granites were rarely reported within the CAOB, with only rare occurrences in the Altai Mountain, Muzhaerte area of Central Tianshan, and Kekesu valley in SW Tianshan (Zhou et al., 2007; Cai et al., 2011; Gao et al., 2011; Gou et al., 2015; Xia et al., 2016). Several workers proposed that the CAOB as a typical accretionary orogen is characterized by dominant juvenile crustal growth (e.g. Sengör et al., 1993; Jahn et al., 2000; Xiao et al., 2004; Gao et al., 2009). However, recent reassessment of continental growth during the accretionary history of the CAOB shows that the proportion of juvenile crust had been grossly overestimated and some parts of CAOB represent crustal reworking (Kröner et al., 2014). The involvement of crustal components is further substantiated in recent studies (Zhang et al., 2016). The discovery of the typical S-type rhyolite in the Heiyingshan region provides new evidence for the crustal reworking in the CAOB. A Permian mafic-felsic volcanic belt extends for more than 200km along Wensu County to Heiyingshan region in the Central South Tianshan in China, which is named as Xiaotikanlike

ACCEPTED MANUSCRIPT Formation by mapping geologists. Generally, the Xiaotikanlike Formation is composed of basaltic and rhyolitic lavas, pyroclastic rocks and some interbedded sedimentary rocks, such as siltstones, sandstones and conglomerates. Among these, the rhyolite lavas mainly occur in the Wensu County,

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Laohutai and Heiyingshan regions and show eruption ages of 285Ma to 290Ma (Luo et al., 2008;

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Huang et al., 2015). Due to low- to moderate-degree of kaolinization or secondary replacement of feldspars and high TZr (851ć-933ć), Huang et al. (2015) suggested that the aluminum saturation

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index (ASI) values may be elevated by the post magmatic alteration and proposed an igneous or meta-igneous source for the rhyolites in the Laohutai and Wensu areas. However, from a

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revaluation of the published data, we observe that the alteration of the samples from Laohutai and Wensu is not heavy with LOI values of 1.54-1.88 and 2.02-2.42, respectively, and most of the ASI

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index are >1.1 even after recalculation to 100% (Fig. 8c). In this case, the effect of alteration is expected to be very limited. Moreover, the high TZr is not a diagnostic criterion for the crustal

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sources, because even the S-type magma can be generated at high temperature condition (>875ć; Sylvester, 1998). As suggested above, the relatively high temperature may be related to the

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underplating of mantle-derived basaltic magmas. Hence, we argue that the geochemistry of the

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rhyolites from the Wensu and Laohutai regions showing features of peraluminous character (Fig. 8c; ASI>1.1 for Laohutai and Wensu) and sedimentary rocks –like trace element ratios (e.g. greywacke and shale, Rb/Ba=0.06-0.61, Rb/Sr=0.96-9.21 for Laohutai and Wensu; Luo et al., 2008; Huang et al., 2015) indicate a S-type affinity, and were possibly derived from the partial melting of the metasedimentary rocks through muscovite-dehydration reaction (Fig. 12). In the diagram of Rb/Ba vs. Rb/Sr and Al2O3/(MgO+FeOT)molar vs. CaO/(MgO+FeOT)molar, the samples show a wide range of source compositions from clay-rich to clay-poor, indicating heterogeneous sources for the rhyolites in the Xiaotikanlike Formation (Fig. 11c and d). Combined with our results, we suggest a 200km long peraluminous magma belt in the central part of STOB, which provides strong support for the vital role of crustal reworking in the CAOB. 7. Conclusion The LA-ICP-MS U-Pb zircon dating of newly reported layers of garnet-bearing rhyolites from Heiyingshan in the central part of the Chinese South Tianshan shows that the lower and upper layers erupted at ca. 300Ma and ca. 280Ma, respectively. Their mineralogical and

ACCEPTED MANUSCRIPT geochemical characteristics indicate that the rhyolites are typical S-type lavas that were derived from sources dominated by metagraywacke. They are considered to be generated by partial melting of the overthickened crust in a post-collisional setting. The ca.300Ma LLR is

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characterized by low TZr and Al2O3/TiO2 ratios with a ‘high pressure’ affinity, suggesting that they

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formed not long after the collision between CTS-Yili Block and the Tarim Craton. In contrast, the ULR has relatively higher TZr and lower Al2O3/TiO2 values, indicating that they were generated at

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a higher temperature condition compared to the LLR. We correlate this to the underplating basalts of the Xiaotikanlike Formation, beneath the ~280Ma rhyolite in the Chinese South Tianshan. The

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field relation shows that the ca. 300Ma LLR discordantly overlies the strongly folded Late Carboniferous Kangkelin Formation, and suggests that the final collision between the CTS-Yili

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Block and Tarim Craton occurred after the formation of the late Carboniferous Kangkelin Formation and before the eruption of ca. 300Ma rhyolite. We also confirm the occurrence of a

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peraluminous magma belt in the central Chinese South Tianshan from Wensu County to Heiyingshan region, which provides insights into crust reworking during the formation of the

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Acknowledgments

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CAOB.

We thank Editor Prof. S. L. Chung and two anonymous referees for their valuable comments which helped in improving our paper. Financial support for this work was provided by the National Nature Science Foundation of China (41390442 and 41472060).

Appendix A. Supplementary material Supplemental data to this article can be found online at.

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pressure Magmatic System: Megacrysts from the Monastery Kimberlite, South Africa. Journal of Petrology 45(8), 1539-1564.

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Binns, R.A., Duggan, M.B., Wilkinson, J.F.G., 1970. High pressure megacrysts in alkaline lavas from northeastern New South Wales. American Journal of Science 269(2), 132-168.

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Zhang, C.L., Zou, H.B., 2013. Permian A-type granites in Tarim and western part of Central Asian Orogenic Belt (CA)B): Genetically related to a common Permian mantle plume? Lithos 172-

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Zhang, L.F., Du, J.X., Lü, Z., Yang, X., Gou, L.L., Xia, B., Chen, Z.Y.,Wei, C.J., Song, S.G.,

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2013. A huge oceanic-type UHP metamorphic belt in southwestern Tianshan, China: peak metamorphic age and PT path. Chinese Science Bulletin 58, 4378-4383. Doi:

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http://dx.doi.org/10.1007/s11434-013-6074-x.

Zhang, C., Liu, L., Santosh, M., Luo, Q., Zhang, X., 2016. Sediment recycling and crustal growth

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in the Central Asian Orogenic Belt: Evidence from Sr–Nd–Hf isotopes and trace elements in granitoids of the Chinese Altay. Gondwana Research, DOI:

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http://dx.doi.org/10.1016/j.gr.2016.08.009

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Zhao, Z.Y., Zhang, Z,C., Santosh, M., Huang, H., Cheng, Z.G., Ye, J.C., 2015. Early Paleozoic magmatic record from the northern margin of the Tarim Craton: Further insights on the evolution of the Central Asian Orogenic Belt. Gondwana Research Doi: http://dx.doi.org/10.1016/j.gr.2014.04.007. Zhou, G., Zhang, Z.C., Luo, S.B., He, B., Wang, X., Yin, L.J., Zhao, H., Li, A.H., He, Y.K., 2007. Confirmation of high temperature stongly peraluminous Mayin’ebo granites in the south margin of Altay, Xinjiang: age, geochemistry and tectonic implications. Acta Petrologica Sinica 23(8), 1909-1920. (in Chineses with English abstract) Zhou, Y.Y., Zhao, T.P., Zhai, M.G., Gao, J.F., Lan, Z.W., Sun, Q.Y., 2015. Petrogenesis of the 2.1 Ga Lushan garnet-bearing quartz monzonite on the southern margin of the North China Craton and its tectonic implications. Precambrian Research 256, 241-255. Zindler, A., Hart, S.R., 1986. Chemical geodynamics. Annual Review of Earth and Planetary Sciences 14, 493-571.

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Figure Captions: Figure 1 (a) Sketch geological map showing the location of the Central Asian Orogenic Belt

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(modified after Sengör et al., 1993); (b) Geological map of the Chinese western Tianshan

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Orogenic Belt (Modified after Zhao et al., 2015). Note: ķ-North Tianshan Suture; ĸ-Nikolaev D -Dalubayi ophiolite; ƻ H Line; Ĺ-Southern Central Tianshan Suture; ĺ-North Tarim Trust; ƻ

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K -Kulehu ophiolite; ƻ T -Tonghuashan ophiolite; ƻ W -Wuwamen ophiolite; ƻ Y Heiyingshan ophiolite; ƻ

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-Yushugou ophiolite.

Figure 2 (a) Geological map of the Heiyingshan region, SW Chinese Tianshan (Modified after

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Xinjiang Bureau of Geology and Mineral Resources, 1984); (b) Stratigraphic column for the Heiyingshan region (Modified after Xinjiang Bureau of Geology and Mineral Resources, 1984); (c) A-B section and the location of the samples of this study.

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Figure 3 Field photographs. (a) and (b) Field photographs showing the two layers of rhyolite and the contact relationship in the Heiyingshan region; (c) Field photograph showing the lower layer

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of rhyolite; (d) The lower layer of rhyolite unconformably overlies the strongly folded Late

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Carboniferous Kangkelin formations. Figure 4 Photomicrographs of the Heiyingshan rhyolites. (a) Porphyritic texture of the upper layer of the rhyolite, plane-polarized light; (b) The occurrence of the Fe-Ti oxides in the upper layer of rhyolite, plane-polarized light; (c) The occurrence of the quartz megacrysts in the upper layer of rhyolite, plane-polarized light; (d) The akali-feldspar megacryst envelopes biotite in the lower layer of rhyolite, plane-polarized light; (e) The association of the garnet and biotite, planepolarized light; (f) The occurrence of the rutile in the lower layer of rhyolite, plane-polarized light. Abbreviations: Q=quartz, Grt=garnet, Kfs=K-felderspar, Pl=plagioclase, Ilm=ilmenite, Bt=Biotite, Rt=Rutile. Figure 5 (a) An-Ab-Or classification diagram of feldspar; (b) Classification diagram of garnet; (c) The classification diagram of biotite (after Foster, 1960); (d) The Lgf(O2) versus TTi diagram for the biotite+sanidine+magnetite+vapor equilibrium at Ptotal=2070 bar (Wones and Eugster, 1965); (e) Fe3+-Fe2+-Mg2+ diagram for the biotite (Wones and Eugster, 1965). The data source of the garnet from the leucogranite in the Himalayan Orogen and Eastern Kunlun Orogen from Gao et al.

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CL image and concordia diagram of the lower layer of rhyolite; (d) The weighted average Pb/238U age for the lower layer of rhyolite.

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Figure 7 TAS diagrams for the Heiyingshan rhyolite (Le Maiitre, 2002). Data source: the data of

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the felsic rocks in the Xiaotikanlike Formation form Laohutai and Wensu region is from Huang et al. (2015) and Luo et al. (2008).

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Figure 8 (a) The SiO2 vs. Na2O+K2O-CaO diagram (Frost et al., 2001); (b) The K2O vs. SiO2 diagram (Peccerillo and Taylor, 1976); (c) The A/NK vs. A/CNK plots (Maniar and Piccolli,

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1989). Data source: the data of the felsic rocks in the Xiaotikanlike Formation form Laohutai and Wensu region is from Huang et al. (2015) and Luo et al. (2008); the data of Permian A-Type

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granites in the STOB is from Huang et al. (2012a).

Figure 9 (a) Chondrite-normalized REE patterns of the Heiyingshan rhyolites; (b) Primitive

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mantle-normalized trace element patterns of the Heiyingshan rhyolite. Primitive mantle and chondrite values are from Sun and McDonough (1989) and McDonough and Sun (1995),

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respectively. The data of the basaltic rocks in the Xiaotikanlike Formation is from Huang et al. (2015); the data of the felsic rocks in the Xiaotikanlike Formation form Laohutai and Wensu region is from Huang et al. (2015) and Luo et al. (2008). Figure 10 (a) 87Sr/86Sr vs. εNd(t) diagram. Data source: MORB and OIB (Zindler and Hart. 1986); The basaltic rocks in the Xiaotikanlike Formation (Huang et al., 2015); the Permian granitoid in the South Tianshan (Jiang et al., 1999; Huang et al., 2011, 2012b). Late Neoarchean and early Paleoproterozoic basement (Long et al., 2011) and references therein. (b) Age vs. εHf(t) diagram for the analyzed zircon from the Heiyingshan rhyolites. Data source: Hf isotopes of the felsic lava in Xiaotikanlike Formations are from Huang et al. (2015); Hf isotopes of the Permian A-type granitoid in the South Tianshan are from Chuanwulu, Halajun and Bashisuogong plutons (Huang et al., 2012b; Zhang and Zou, 2013; Ma et al., 2016); Evolution of the early Paleoproterozoic crust and Neoarchean basement of Tarim Block is from Long et al. (2010, 2011) and Ge et al. (2012), respectively. CHUR= chondritic uniform reservoir.

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diagram (Altherr et al., 2000).

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Figure 12 Rb/Sr vs. Ba diagram (Inger and Harris, 1993). Ms (VP)=vapor-present muscovite

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melting; Ms (VA)=vapor-absence muscovite melting; Bt (VA)=vapor-present biotite melting.

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Graphical Abstract

Both the upper and lower layer of the Heiyingshan garnet-bearing rhyolites show typical S-type affinity, such as the radioactive 87Sr/86Sr and low ε(Nd) and ε(Hf) values indicating they were derived from the partial melting of the metasedimentary rocks. These characteristics are clearly distinct from those of the Permian A-type granitoids, providing new evidence for the crustal reworking in the CAOB.

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Ø The zircon LA-ICP-MS U-Pb analyses on the two layers of rhyolites yield

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ages of ca. 300Ma and ca. 280Ma, respectively.

Ø The timing of the final collision between the Central Tianshan-Yili Block and the Tarim Craton is constrained for culminating during Upper Carboniferous

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before 300Ma

Ø We define a 200km long S-type magmatic belt for the South Tianshan, which

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provides new evidence for the crustal reworking in the CAOB.