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1 Lithos 155 (2012) Contents lists available at SciVerse ScienceDirect Lithos journal homepage: Continental vertical growth in the transitional zone between South Tianshan and Tarim, western Xinjiang, NW China: Insight from the Permian Halajun A1-type granitic magmatism He Huang a, Zhaochong Zhang a,, Timothy Kusky b, M. Santosh a, Shu Zhang c, Dongyang Zhang a, Junlai Liu a, Zhidan Zhao a a State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing , China b State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Wuhan , China c Geological Survey of Anhui Province, Hefei , China article info abstract Article history: Received 28 April 2012 Accepted 21 August 2012 Available online 29 August 2012 Keywords: Central Asian Orogenic Belt South Tianshan Tarim A-type granite Continental growth Mantle plume The South Tianshan Collisional Belt (STCB) and northern margin of the Tarim Block (NTB) are key areas for understanding the prolonged tectonic evolution of the Central Asian Orogenic Belt (CAOB). The Halajun region in Xinjiang province, NW China is located within the tectonic transition zone between STCB and Tarim Blocks. Several granitic intrusions and one mafic ultramafic complex (Piqiang complex) are exposed in this region. Zircon U Pb dating, whole-rock major oxide, trace element and Nd isotopic data are presented for the Huoshibulake, Tamu, Kezi'ertuo and Halajun II granitic intrusions in this area. New LA-ICP-MS U Pb age for Kezi'ertuo intrusion, coupled with previously published SHIRMP U Pb ages for Huoshibulake and Halajun II intrusions and Piqiang complex, reveals that all the igneous rocks in the Halajun region are coeval (~275 Ma). Geochemically, the four granitic intrusions show high contents of SiO 2,K 2 O and total alkalis and possess trace element patterns characterized by Rb, Nb, Ta, Zr and Hf enrichment and significantly negative Ba, Sr, P, Eu and Ti anomalies. These features strongly favor an A1-type affinity for the Halajun granitic intrusions. Among the four intrusions, the Kezi'ertuo, Tamu and Halajun II intrusions possess positive to slightly negative ε Nd (t) values ranging from 0.9 to +0.6, whereas the Huoshibulake intrusion displays less depleted ε Nd (t) values of 2.6 to 2.9. Our new elemental and isotopic data suggest that the four granitic intrusions were generated by the partial melting of a common Neoproterozoic gabbroic source, probably as a result of the ~275 Ma underplating of the asthenosphere mantle-derived magmas. The variable involvement of the mantle components accounts for the range of ε Nd (t) values. After the generation of the parental magma, alkali feldspar, arfvedsonite, biotite, Fe Ti oxides and zircon seem to have fractionated prior to the final emplacement of the granitic magmas. In combination with the regional geological history, we propose that the ~275 Ma A1-type granitic magmatism in the Halajun region and other areas of the NTB provides a good proxy record for the vertical continental crustal growth in the southern margin of the CAOB during the Permian. Our study, in combination with other geological evidence, indicates that these A1-type felsic and OIB-like mafic ultramafic rocks, with ages from ~282 Ma to ~275 Ma, in the southern margin of the CAOB are parts of the Permian Tarim large igneous province and could be genetically related to the Tarim mantle plume Elsevier B.V. All rights reserved. 1. Introduction The Central Asian Orogenic Belt (CAOB), also known as the Altaid tectonic collage, is one of the world's largest accretionary orogens with considerable juvenile crustal growth during Phanerozoic (Rojas- Agramonte et al., 2011; Şengör and Natal'in, 1996a,b; Windley et al., 2007; Xiao et al., 2010, 2012). In contrast to typical collisional orogenic belts (e.g., Alps and Himalayas), the CAOB comprises Corresponding author. Tel.: ; fax: address: zczhang@cugb.edu.cn (Z. Zhang). numerous tectonic terranes with different amalgamation history, probably including fragments of many island arcs, seamounts, accretionary prisms and ophiolites, interspersed with blocks of older continental crust and slivers of oceanic crust (Cai et al., 2011, 2012; Long et al., 2012; Xu et al., 2012; Zhang et al., 2010a). Due to the difficulty in identifying the nature of individual terranes, the tectonic evolution of this huge and complex accretionary belt is still poorly understood. Western Xinjiang, NW China, located in the southwestern part of the CAOB (Fig. 1a), is tectonically composed of, from north to south, Altay, Junggar Block, North Tianshan Collisional Belt (NTCB), Central /$ see front matter 2012 Elsevier B.V. All rights reserved.

2 50 H. Huang et al. / Lithos 155 (2012) Fig. 1. (a) Tectonic sketch map of the Central Asia Orogenic Belt showing the location of South Tianshan Collisional Belt and the northern margin of the Tarim Block. (b) Geological map of the South Tianshan Collisional Belt and northern margin of the Tarim Block. Modified from Gao et al. (2011) and Huang et al. (2012). Tianshan Block (CTB), South Tianshan Collisional Belt (STCB) and the northern margin of the Tarim Block (NTB). Among these tectonic units, STCB and NTB constitute the southern margin of the CAOB. Since almost all paleogeographic studies put the Tarim Block as the last block to dock with the CAOB, the formation of STCB, as a result of the collision between Tarim and Central Tianshan Blocks, represents the termination of the prolonged accretionary orogeny of the CAOB (Xiao et al., 2010, and references therein). Thus, the STCB and NTC are key areas for understanding the tectonic history of the CAOB. In recent years, several studies have attempted to unravel the enigmatic tectonic history of the STCB and NTB (e.g., Gao et al., 2011; Long et al., 2011a; Yang et al., 2007; Zhang et al., 2009, and references therein). However, the nature and tectonic evolution of STCB and NTB is still debated, and some contrasting tectonic models have been proposed. The controversy mainly focuses on the nature of their tectonic affinities during the Permian, which were variously proposed as arc-related, collision-related, or anorogenic (Long et al., 2011a; Zhang et al., 2008, 2010b). Moreover, understanding of the accretionary process is still handicapped by the lack of a fully developed geological record. Granitoid rocks are widely distributed in the STCB as well as in the NTB, and most of them are of Late Paleozoic age (Gao et al., 2011; Zhang et al., 2009, and references therein). These rocks not only serve as a diagnostic geodynamic tracer for constraining the tectonic evolution, but also represent an important mechanism of continental growth. The Halajun region, cut by numerous NE-SW-extending faults that form the southern border of the STCB, is considered to be situated in the tectonic transition zone between STCB and NTB. Granitic intrusions along the NE-SW-extending faults are exposed for more than 150 km 2 in area. In this paper, we report new laser ablation ICP-MS U Pb zircon age, bulk-rock major and trace element, and Nd isotopic data for three granitic intrusions (Kezi'ertuo, Huoshibulake and Tamu intrusions) in Halajun region. Based on the results, together with previously published data of igneous rocks exposed in Halajun region (e.g., Halajun II intrusion and Piqiang complex) and other areas of the STCB and NTB, we attempt to: 1) track the sources and petrogenesis of the Halajun granitic intrusions, 2) provide important constraints on the tectonic setting and 3) address the mechanism of the continental vertical accretion of the CAOB. 2. Geological setting Among the tectonic units in Western Xinjiang, the Altay, NTCB and STCB are Paleozoic accretionary belts whereas the Junggar Block, CTB and the NTB are microcontinental blocks or terranes (Li et al., 2006). Our study focuses on the area including STCB and NTB, which represent the southern margin of the CAOB. The STCB is bounded by the Northern Tarim suture (NTS) to the south and the Southern Central Tianshan suture (SCTS) to the north. It has been envisaged that this collisional belt represents a collage of two microcontinental blocks amalgamated during the Late Paleozoic: the Tarim Block as a passive continental margin in the south, and Central Tianshan Block as an active continental margin in the north (Gao et al., 1998; Zheng et al., 2006). Before the amalgamation, the two blocks were separated by the Paleozoic South Tianshan Ocean, a branch of the large Paleo-Asian Ocean. Produced by the discontinuous northern subduction of the Paleozoic South Tianshan Ocean and subsequent final collision between the two blocks, ophiolites and ophiolitic mélanges are sporadically exposed along the Southern Central Tianshan suture (SCTS) that separates the STCB from CTB (Fig. 1a; Gao et al., 1998, 2011; Han et al., 2010; Long et al., 2011a). Recent geochronological studies of ophiolites and ophiolitic mélanges along the SCTS have suggested that the Paleozoic South Tianshan Ocean might have opened not earlier than the Cambrian and that the Central Tianshan and Tarim Blocks were part of the Rodinia supercontinent throughout the latest Proterozoic (Gao et al., 1998, 2011, and references therein). Thus, the STCB and NTB share a common Precambrian basement, represented by Archean complexes sporadically exposed in the NTB, and the Paleoproterozoic Xingditagh Formation and Mid-Proterozoic Akesu Formation, both of which are exposed in the central part of the STCB (Fig. 1b). Lower to Middle Paleozoic marine chert, limestone, and flysch mainly crop out in the northern part of the STCB and the central part of the NTB. Upper Paleozoic limestone, sandstone, and shale with minor volcanic rocks, are widespread in the two units (Fig. 1b). Notably, the Permian strata in the northern and central parts of the STCB are characterized by typical terrestrial volcanic rocks (Xiaokantilike Formation) that unconformably overlie the strongly folded upper Carboniferous marine carbonate rocks (Huang et al., 2012). Nevertheless, the Permian

3 H. Huang et al. / Lithos 155 (2012) sedimentary strata are well exposed in the NTB and the southern margin of the STCB, and conformably overlie the upper Carboniferous marine carbonate rocks. The outcrops of intrusive rocks, most of which are granitoids, comprise ~5% of the total area of the STCB and NTB. Five major tectono-thermal events have been identified by previous studies as follows (Yang et al., 2007; Zhang et al., 2009): 1) Neoproterozoic Cambrian, 2) Early Paleozoic, 3) Middle Paleozoic, 4) Late Carboniferous to Early Permian and 5) Cenozoic (mafic dykes). However, most of the igneous rocks were formed during the fourth pulse, i.e., late Carboniferous to early Permian. 3. Geology and petrography of Halajun granitic intrusions The Halajun region is geographically close to Atushi city in the western part of Xinjiang Uyghur Autonomous Region. It is cut by numerous NE-SW-extending faults that are components of Northern Tarim Suture (NTS), which is regarded as the boundary between STCB and NTB. Accordingly, the region is considered to be located in the tectonic transition zone. Paleozoic strata, mainly composed of Carboniferous marine carbonate rocks and Permian sandstones, are well exposed in the northern and central parts of the study area, whereas the southern part of the region is predominantly covered by desert (Fig. 2). More than eight granitic intrusions and one mafic ultramafic intrusive complex (Piqiang complex) occur along the NE-SWextending faults in the Halajun region. Besides, some mafic dykes cut the Paleozoic strata and granitic intrusions. Four representative intrusions (Huoshibulake, Tamu, Kezi'ertuo and Halajun II intrusions) are chosen for this study. The salient characteristics of these intrusions and the Piqiang mafic ultramafic complex are described as follows: The Huoshibulake intrusion, located in the central part of the Halajun region, occupies an area of ~40 km 2. It intrudes the Permian sedimentary rocks with a m wide hornfels belt developed along the margin of the intrusion. Typical major constituents are medium- to coarse-grained alkali feldspar (30 35 vol.% orthoclase and vol.% perthite), quartz (40 50 vol.%), arfvedsonite (~3 vol.%), biotite (~2 vol.%), and accessory minerals including zircon, apatite and Fe Ti oxides. The alkali feldspar shows kaolinitisation and is locally replaced by secondary muscovite. Quartz-, tourmaline- and fluoritebearing veins and geodes are well-developed within the intrusion, and they are believed to be the products of late-stage hydrothermal activity (Fig. 3a). The Tamu pluton intrudes the Permian sedimentary strata on the west and is covered by the Tarim desert on the east. A ~40 m wide hornfels belt has been developed along the western margin of the Tamu intrusion. Similar to Huoshibulake intrusion, the Tamu intrusion is pinkish in color with medium- to coarse-grained textures, and predominantly consists of alkali feldspar (30 35 vol.% orthoclase and vol.% perthite), quartz (40 50 vol.%), arfvedsonite (~3 vol.%), biotite (~2 vol.%), and accessory minerals including zircon, apatite and Fe Ti oxides. Similar to that in the Huoshibulake intrusion, alkali feldspar in this intrusion is commonly altered to kaolinite and is locally replaced by secondary muscovite (Fig. 3c). Veins and geodes composed of quartz, tourmaline and fluorite are abundant within the Tamu intrusion (Fig. 3b). The Kezi'ertuo intrusion, located in the southern margin of the Halajun region, has an outcrop area of ~20 km 2 and is surrounded by the Tarim desert. It is pink or gray in fresh outcrops and has a coarse grained texture. This intrusion mainly consists of alkali feldspar (~20 vol.% orthoclase and ~20 vol.% perthite), quartz (~40 vol.%), arfvedsonite (~10 vol.%), biotite (~10 vol.%) and accessory minerals (e.g., zircon, apatite and Fe Ti oxides). In contrast to other granitic intrusions described in this paper, the Kezi'ertuo intrusion is characterized by relatively high amounts of mafic minerals (e.g., arfvedsonite and biotite, Fig. 3d) as well as the absence of quartz-, tourmaline- and fluorite-bearing veins and geodes. The Halajun II intrusion is located immediately south of the Kezi'ertuo intrusion and is also surrounded by the Tarim desert. According to Zhang et al. (2010b), the rock-forming minerals are quartz (~40 vol.%), alkali feldspar (~60 vol.%) and very minor amphibole. The oxide (magnetite)-bearing Piqiang intrusive complex is exposed in the eastern part of the Halajun region with an outcrop area of ca. 25 km 2. Field observations and previous studies (e.g., Rui Fig. 2. Geological map of the Halajun region. 1 Cambrian; 2 Ordovician; 3 Silurian; 4 Devonian; 5 Carboniferous; 6 Permian; 7 Granitic intrusion; 8 Piqiang complex; 9 Cenozoic; 10 Fault. HSB Huoshibulake intrusion; TM Tamu intrusion; KZE Kezi'ertuo intrusion; HLJ II Halajun II intrusion.

4 52 H. Huang et al. / Lithos 155 (2012) Fig. 3. (a) Quartz-, tourmaline- and fluorite-bearing geodes developed within the Huoshibulake intrusion. (b) Veins composed of quartz, tourmaline and fluorite are well developed within the Tamu intrusion. (c) Alkali feldspar in the Tamu intrusion commonly altered to kaolinite and locally replaced by secondary muscovite. (cross-polarized light). (d) Kezi'ertuo intrusion containing relative high amounts of mafic minerals in comparison with the other studied intrusions (cross-polarized light). Arf Arfvedsonite; Bi Biotite; Or orthoclase; Pt Perthite; Qz Quartz; Ms (secondary) muscovite. et al., 2002; Zhang et al., 2010b) suggest that the Piqiang complex is composed mainly of gabbro with minor olivine-bearing gabbro, clinopyroxenite, dolerite and nepheline-bearing syenite. Detailed petrographic description of Halajun II intrusion and Piqiang complex has been previously published by Zhang et al. (2010b) and is not repeated in this paper. 4. Analytical methods A representative granite sample (YT-HH-7) from the Kezi'ertuo intrusion was chosen for age determination by LA-ICP-MS U Pb zircon method in this study. Zircon separation was carried out using conventional magnetic and density techniques to concentrate non-magnetic, heavy fractions. Zircon grains were then hand-picked under a binocular microscope. Internal structures of the zircon grains were examined using transmitted electron, backscattered electron (BSE) and cathode luminescence (CL) prior to U Pb isotopic analyses. The BES and CL imaging was carried out on a LEO1450VP scanning electron microscope with a MiniCL detector at the Institute of Geology, Chinese Academy of Geological Sciences. These images have been used to identify different stages of zircon growth, and to select the positions for LA-ICPMS analyses (Fig. 4a). The U Pb isotopic analyses for samples were obtained with an Elan 6100 DRC ICP-MS equipped with 193 nm Excimer lasers, housed at the Key Laboratory of Continental Collision and Plateau Uplift, Chinese Academy of Sciences, Beijing, China. U Th Pb ratios were determined relative to the Plesovice standard zircon, and the absolute abundances of U, Th and rare earth elements (REEs) were determined using the NIST 612 standard glass. A mean age of 338.2±1.5 Ma was obtained for the Plesovice zircon standard. The spot diameter was 30 μm. Corrections for common-pb were made using the method of Andersen (2002). Data were processed using the GLITTER and ISOPLOT (Ludwig, 2003) programs. Errors on individual analyses by LA-ICPMS are quoted at the 95% (1σ) confidence level. The details of the analytical procedures have been described by Yuan et al. (2004). After petrographic examination, 24 fresh samples from the complex were crushed and powdered in an agate mill for geochemical analysis. Nine samples are from the Huoshibulake intrusion, eight samples are from the Tamu intrusion and the other seven samples are from the Kezi'ertuo intrusion. Major oxides of samples of the Huoshibulake intrusion were analyzed by X-ray fluorescence analysis (XRF; PHILIPS PW1480) using fused glass disks at the Institute of Geology and Geophysics, Chinese Academy of Sciences. Uncertainties for most major oxides are b2 wt.%, for MnO and P 2 O 5 b5 wt.%, and the totals are within 100±1 wt.%. Major elements of other samples were determined by XRF in the State Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, with an analytical uncertainties ranging from 1 to 3%. Trace (including rare earth) element analyses were conducted in the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences in Beijing, China, determined by inductively coupled plasma mass spectrometry (ICP-MS). For trace element determination, about 50 mg of powder was dissolved for about 7 days at ca. 100 C using HF HNO 3 (10:1) mixtures in screw-top Teflon beakers, followed by evaporation to dryness. The material was dissolved in 7N HNO 3 and taken to incipient dryness again, and then was re-dissolved in 2% HNO 3 to a sample/solution weight ratio of 1:1000. The analytical errors vary from 5% to 10% depending on the concentration of any given element. An internal standard was used for monitoring drift during analysis; further details have been given by Gao et al. (2008). Nd isotopic compositions were determined on a Nu Plasma HR multicollector-inductively coupled plasma mass spectrometry (MC- ICP-MS) at the National Research Center for Geoanalysis, Chinese Academy of Geological Sciences in Beijing, China. Total chemical blanks were b100 pg for Nd, and 143 Nd/ 144 Nd ratios were normalized to 146 Nd/ 144 Nd= Results 5.1. LA-ICP-MS U Pb zircon data The zircon grains of the sample YT-HH-7 are colorless and/or pale yellow, transparent and commonly euhedral. They range in size from

5 H. Huang et al. / Lithos 155 (2012) Fig. 4. (a) Representative CL images showing the internal structure of zircons of the sample YT-HH-7 from the Kezi'ertuo intrusion. The U Pb analytical sites are marked and the number refers to the analytical data presented in Table 1. (b) U Pb concordia diagrams of zircons. (c) Zircon chondrite-normalized REE diagram. Chondritic values are from Sun and McDonough (1989). 53

6 54 H. Huang et al. / Lithos 155 (2012) to 300 μm with length/width ratios from 1:1 to 4:1, and most grains have oscillatory zoning (Fig. 4a). Thirty-five grains were analyzed (Table 1). Chondrite-normalized REE diagram of the zircons shows enrichment of heavy rare earth elements (HREEs) relative to light rare earth elements (LREEs), negative Eu anomaly and positive Ce anomaly in most zircons (Fig. 4c). In addition, they are characterized by relatively low U ( ppm) and moderate Th ( ppm) contents, with Th/U ratios of The morphologic and chemical characteristics of zircon grains in this rock clearly indicate a magmatic origin. The 206 Pb/ 238 U ages range from 264 to 278 Ma, forming a coherent group with a weighted mean 206 Pb/ 238 U age of 273±1 Ma (MSWD=0.74, 2σ, Fig. 4b). The weighted mean 206 Pb/ 238 Uageof the sample can be interpreted to be the crystallization age of the Kezi'ertuo intrusion. Based on the Ti concentrations in zircon grains ( ppm, Table 1), the temperatures of zircon crystallization can be estimated to be between 684 and 857 C (Watson et al., 2006) Major and trace elements Major elements The major and trace element data of the three studied intrusions are presented in Table 2. The loss on ignition (L.O.I.) for all samples was ~1 wt.%. As expected from the high modal abundance of alkali feldspar and quartz, all samples from the three intrusions show high contents of SiO 2 ( wt.%), K 2 O ( wt.%) and total alkalis ( wt.%). In Fig. 5a, all the samples exhibit an alkaline affinity. The Huoshibulake intrusion shows to wt.% SiO 2, 0.96 to 1.92 wt.% Fe 2 O 3T, 0.03 to 0.17 wt.% MgO, 0.48 to 0.95 wt.% CaO, and high K 2 O ( wt.%), Na 2 O ( wt.%) and Na 2 O+ K 2 O ( wt.%) contents. As a result of the high Fe* [FeO T / (FeO T +MgO)] values ( ), samples of this intrusion mainly plot into the ferroan field on a SiO 2 vs. Fe* diagram (Fig. 5b). Their Al 2 O 3 contents are between wt.% and wt.%, with A/CNK [molar Al 2 O 3 /(CaO+Na 2 O+K 2 O)] values ranging from 0.95 to 1.04, exhibiting metaluminous and slightly peraluminous affinities on the plot of A/CNK versus A/NK [molar Al 2 O 3 /(Na 2 O+K 2 O] (Fig. 5c). The slightly peraluminous affinities of some samples could be ascribed to the presence of the secondary muscovite as seen in thin sections. The Tamu intrusion shows to wt.% SiO 2, wt.% to wt.% Al 2 O 3, 0.20 to 2.02 wt.% Fe 2 O 3T, 0.02 to 0.22 wt.% MgO and 0.10 wt.% to 2.60 wt.% CaO. In addition, samples of this intrusion have high K 2 O ( wt.%) and moderate Na 2 O ( wt.%) and K 2 O+Na 2 O ( wt.%) contents. With two exceptions that display low Fe* values (0.47 and 0.45), majority of the Tamu samples have high Fe* values ( ) and plot into the ferroan field on a SiO 2 vs. Fe* diagram (Fig. 5b). The A/CNK values varying from 0.80 to 1.03 suggest metaluminous to weakly peraluminous affinities for Tamu samples (Fig. 5c). The presence of secondary muscovite is probably responsible for the slightly peraluminous nature of a few samples. In accordance with the absence of quartz-, tourmaline- and fluorite-bearing veins and geodes as well as relatively high amounts of mafic minerals such as arfvedsonite and biotite, the Kezi'ertuo intrusion is geochemically less evolved in comparison with other studied intrusions, as indicated by the low SiO 2 contents ( wt.%) as well as high Fe 2 O 3T ( wt.%), MgO ( wt.%) and TiO 2 ( wt.%) contents relative to other granitic intrusions. The rocks show to wt.% Al 2 O 3 and 0.97 to 1.38 wt.% CaO. High K 2 O ( wt.%) and moderate Na 2 O ( wt.%) and K 2 O+Na 2 O ( wt.%) contents are Table 1 LA-ICP-MS U Pb analytical results for sample YT-HH-07 from the Kezi'ertuo intrusion. Spot Pb (ppm) Th (ppm) U (ppm) Th/U 207 Pb/ 235 U ±1σ 206 Pb/ 238 U ±1σ t( 206 Pb/ 238 U) (Ma) ±1σ Ti (ppm)

7 H. Huang et al. / Lithos 155 (2012) also shown. Fe* values for the Kezi'ertuo samples are high, from 0.92 to 0.94, typical of the ferroan granitoids as shown in SiO 2 vs. Fe* diagram (Fig. 5b). The A/CNK values of the Kezi'ertuo intrusion range from 0.88 to 0.98, exhibiting a metaluminous character (Fig. 5c). The geochemical and isotopic data of the Halajun II have been previously reported by Zhang et al. (2010b), and are used here for comparison. The intrusion exhibits nearly identical characteristics in major elements with Huoshibulake intrusion, although the former has slightly higher SiO 2 ( wt.%) and lower Fe 2 O 3T ( wt.%) and P 2 O 5 ( wt.%) contents, with Fe* values from 0.73 to In SiO 2 vs. Fe* and A/CNK vs. A/NK diagrams, the Halajun II samples exhibit ferroan to magnesian and metaluminous to slightly peraluminous affinities, respectively (Fig. 5b and c) Trace elements Samples from the four intrusions are commonly characterized by high concentrations of large ion lithophile elements (LILE) such as Rb and high field strength elements (HFSEs) such as Zr, Hf, Nb and Ta, rare earth elements (REEs, except Eu) and Th (Fig. 6). Another remarkable feature is the strong depletion of Ti coupled with high Nb (and Ta) concentrations. In the chondrite-normalized rare earth element patterns (Fig. 7), those samples show slightly fractionated REE patterns with a restricted variation in (La/Yb) N ratios ( ) as well as pronouncedly negative Eu anomalies (Eu/Eu* = ). The Huoshibulake intrusion contains highest Rb ( ppm), Nb ( ppm) and Ta ( ppm) contents, and lowest Ba ( ppm) and Sr ( ppm) concentrations among the three intrusions studied in this paper (Fig. 6a). Samples of this intrusion have relatively high total REE contents ranging from 144 ppm to 495 ppm, with the most pronounced negative Eu anomalies (Eu/Eu*= ) among the four intrusions (Fig. 7a). The Tamu intrusion exhibits high Rb ( ppm), Nb ( ppm) and Ta ( ppm) and moderate Sr ( ppm) and Ba ( ppm) abundance (Fig. 6b) compared with other granitic intrusions in the Halajun region. Except for sample TM-YT-07 containing low total REE content (25 ppm), which is probably attributed to low-temperature post-magmatic alteration (Parasapoor et al., 2009), most of Tamu samples have high total REE contents of 120 ppm to 643 ppm. Strongly negative Eu anomalies with Eu/Eu* values of 0.03 to 0.23 for Tamu samples are also exhibited in chondrite-normalized rare earth element pattern (Fig. 7b). In contrast to the other studied intrusions, the Kezi'ertuo intrusion shows relatively high Ba ( ppm) and Sr ( ppm) and low Rb ( ppm), Nb ( ppm) and Ta ( ppm) concentrations (Fig. 6c). In addition, among the studied intrusions, the Kezi'ertuo intrusion is characterized by relatively low total REE contents ( ppm), positive Ce anomalies (Ce/Ce*= ) and moderately negative Eu anomalies (Eu/Eu*= ) (Fig. 7c). The Halajun II intrusion shows essentially identical trace elemental patterns with that of the Huoshibulake intrusion, as shown in Figs. 6d and 7d. Detailed trace element chemistry of these rocks is reported in Zhang et al. (2010b) and is not repeated here Whole-rock Nd isotopic compositions Sm Nd isotopic compositions for nine samples (five samples from the Huoshibulake intrusion, two from the Tamu intrusion and two from the Kezi'ertuo intrusion) were obtained for this study. Three samples of Halajun II intrusion were previously reported by Zhang et al. (2010b) which are also shown in Table 3. Samples from Tamu, Kezi'ertuo and Halajun II intrusions have initial 143 Nd/ 144 Nd ratios of , with ε Nd (t) values from 0.9 to The Huoshibulake intrusion has much lower initial 143 Nd/ 144 Nd ratios of , with ε Nd (t) values from 2.8 to 2.6. Since the f Sm/Nd values of the granites are mainly between 0.2 and 0.5, the samples from Huoshibulake, Tamu, Kezi'ertuo and Halajun II intrusions yield meaningful two-stage Nd model ages (T 2DM ) ranging from 1.26 to 1.28 Ga, from 0.99 to 1.03 Ga, from 1.05 to 1.12 Ga and from 1.06 to 1.08 Ga, respectively. 6. Discussion 6.1. Late Carboniferous to Permian magmatism in the southern margin of the CAOB Previous SHRIMP zircon U Pb dating has shown that the Huoshibulake and Halajun II intrusions were emplaced at 278± 3 Ma(Zhang et al., 2010b). The results from present study assign an age of 273±1 Ma for the Kezi'ertuo pluton, which is slightly younger than or approximately coeval with Huoshibulake and Halajun II intrusions within error ranges. The Tamu intrusion has not been dated following the effects of Pb-loss identified by Yang et al. (2001), but its petrographic features and country rocks are nearly identical with those of Huoshibulake intrusion. In combination with the similar geochemical characteristics and close spatial relations, we infer that the Tamu intrusion may be coeval with the other granitic intrusions in the region. In addition, a SHRIMP U Pb crystallization age of 276± 4 Ma for Piqiang mafic ultramafic complex has been reported by Zhang et al. (2010b). Therefore, we infer that the granites and Piqiang mafic ultramafic complex in the study area were emplaced within a short time span and are likely to be genetically related to the same tectono-thermal event. On a regional scale, several zircon U Pb ages ranging from ~300 Ma to ~285 Ma for granitoids and mafic intrusive rocks from the northern and central parts of the STCB have been reported. For example, Konopelko et al. (2007, 2009) reported Cameca U Pb ages varying from ~296 Ma to ~285 Ma for A-type granites in the Kyrgyz part of the STCB. In the Chinese part of the STCB, many early Permian granitic intrusions have been identified, e.g., a LA-ICP-MS age of 284.8±2.0 Ma for strongly peraluminous (SP) granite dykes (Gao et al., 2011), a LA-ICP-MS age of 286.4±2.5 Ma for weakly peraluminous S-type granite from the Yingmailai pluton (Ma et al., 2010), and a LA-ICP-MS age of ~286 Ma for the Chuanwulu bimodal intrusive rocks (Huang et al., 2012). In contrast, the Mangqisu pluton in the Hejing region in the easternmost part of the STCB, which is geochemically similar to adakite, has yielded a LA-ICP-MS U Pb age of ~300 Ma (Zhu et al., 2008). In comparison with those from the northern and central parts of the STCB, the magmatic rocks in the NTB as well as in southern margin of the STCB have relatively younger zircon U Pb ages. For instance, the Mazhashan bimodal magmatic suite, composed of a mafic ultramafic complex and minor syenites, are generally considered to have formed around ~275 Ma based on SHRIMP and LA-ICP-MS zircon U Pb dating (Chen et al., 2010; Li et al., 2007; Yang et al., 2005, 2006b, 2007; Zhang et al., 2008). Another well studied suite, the Wajilitag complex, geographically close to the Mazhashan region, contains a series of ultramafic, mafic and felsic rocks. A LA-ICP-MS zircon U Pb age of ~275 Ma for quartz syenite within the complex has been reported by Zhang et al. (2008). However, our unpublished Cameca U Pb data reveal that the mafic ultramafic rocks in the complex were generated at ~282 Ma, slightly earlier than the syenites, which is also consistent with the field relations. In addition to these intrusive rocks, widespread Permian basalts have recently been discovered in the Keping region. Yang et al. (2006c) reported 40 Ar 39 Ar age of whole-rock basalts from Keping which show a plateau age of 281.8±4.2 Ma. Interestingly, the younger mafic ultramafic rocks, including the Piqiang complex, widely exposed in the NTB and southern margin of the STCB commonly possess oceanic island basalt (OIB)-like geochemical characteristics, implying that they were derived from an OIB-like, asthenospheric mantle source (Zhang et al., 2008, 2010b; Zhou et al., 2009).

8 56 H. Huang et al. / Lithos 155 (2012) Table 2 Major (wt.%) and trace (in ppm) element data for the Huoshibulake, Tamu and Halajun II intrusions. Huoshibulake intrusion Tamu intrusion YTHS-03 YTHS-04 YTHS-08 YTHS-09 YTHS-12 YTHS-14 YTHS-18 YTHS-20 YTHS-21 TM-YT-2 TM-YT-4 Major elements (wt.%) SiO TiO Al 2 O Fe 2 O 3T MnO MgO CaO Na 2 O K 2 O P 2 O LOI Total A/CNK K 2 O+Na 2 O AR Trace elements (ppm) Sc Ga Rb Ba Th U Nb Ta La Ce Pr Sr Nd Zr Hf Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu Ti P Total REE (La/Yb) N Eu/Eu* AR =(Al 2 O 3 +CaO+Na 2 O+K 2 O)/(Al 2 O 3 +CaO Na 2 O K 2 O). (La/Yb) N means the ratio between chondrite-normalized La and Yb. Eu/Eu* =Eu/SQRT (Sm+Gd). In general, all these results indicate that the Late Carboniferous to Permian tectono-thermal events recognized in the southern margin of the CAOB can be temporally subdivided into two stages. The early (~ Ma) and late (~ Ma, peaked at ~275 Ma) stages are likely to be related to different tectonic settings, which will be discussed in detail in a later section Genetic type: an A1-type affinity The Huoshibulake, Tamu and Kezi'ertuo intrusions commonly contain perthite and arfvedsonite, typical of alkaline granitoids worldwide (Bonin, 2007, and references therein). Geochemically, the samples from the four intrusions are highly silicic in composition and plot in metaluminous to slightly peraluminous fields in A/CNK vs. A/NK diagram (Fig. 5c). They have high Zr contents ( ppm) and M values [cation ratio (Na+K+2Ca)/(Al Si)] ranging from 1.3 to 1.8. The Halajun granitic rocks yield zircon saturation temperatures (T Zr ) varying from 788 to 870 C (after Watson and Harrison, 1983), the highest of which are roughly consistent with that of the temperatures estimated using Ti contents of zircons (857 C, see Section 5.1). These temperatures are much higher than those for the I-type granitoids in the STCB (Fig. 8). With a few exceptions, most of the Halajun granitic samples have high Fe* values and plot in the field of typical A-type granites in the SiO 2 vs. Fe* diagram (Fig. 5b). Moreover, these granitic intrusions display high (K 2 O+Na 2 O)/CaO, FeO T /MgO and 10,000 Ga/Al ratios (Fig. 9a, b and c) and show Rb, Nb, Ta, Zr and Hf enrichment and considerably negative Ba, Sr, P, Eu and Ti anomalies on the trace element patterns (Fig. 6). In summary, the geochemical features bear close resemblance to those of A-type granites worldwide (e.g., Bonin, 2007; Frost and Frost, 2011; Frost et al., 1999, 2001; King et al., 2001; Whalen et al., 1987). The concept that A-type granites can be subdivided into two sub-types, as proposed by Eby (1992), has been widely accepted and applied in the study of these rocks. The A1-type granites have chemical characteristics similar to those observed for OIBs and are emplaced in continental rifts or during intraplate evolution. In

9 H. Huang et al. / Lithos 155 (2012) Table 2 Major (wt.%) and trace (in ppm) element data for the Huoshibulake, Tamu and Halajun II intrusions. Kezi'ertuo intrusion TM-YT-7 TM-YT-8 TM-YT-10 TM-YT-11 TM-YT-14 TM-YT-16 YT-HH-1 YT-HH-2 YT-HH-4 YT-HH-5 YT-HH-6 YT-HH-7 YT-HH contrast, A2-type granites are similar to rocks of continental crust or island-arc origins in chemical characteristics and can be formed in arc- or collision-related settings (Eby, 1990, 1992; Bonin, 2007, and references therein). As shown in Fig. 9d, the Halajun granites and Mazhashan syenites plot generally within the A1 field, whereas the A-type granites in Kyrgyz South Tianshan are classified as A2 subgroup. We therefore conclude that all of the Halajun granitic intrusions studied in this paper belong to A1-type granites Petrogenesis General considerations A fundamental question on the origin of A-type granitic magmas is whether such granites are exclusively crust-derived or whether they require significant involvement of mantle materials; or even whether they can be generated directly by fractionation of mantle-derived magmas (Bonin and Giret, 1990; Collins et al., 1992; Creaser et al., 1991; Eby, 1990, 1992; Martin, 2006; Turner et al., 1992; Wu et al., 2002; Yang et al., 2006a). Previous study by Zhang et al. (2010b) proposed that the Halajun granitic intrusions were generated from mantle-derived mafic magmas, represented by the Piqiang complex in the study area, through extensive fractionation. If this is the case, the volume of mafic magma intruding into the upper-crust should be an order of magnitude greater than that of granites (Frost et al., 2002; Turner et al., 1992). However, our field observation reveals that, even though it is possible that some of the mafic ultramafic rocks are not presently exposed, the granitic rocks are evidently larger in volume than the mafic ultramafic rocks. Consequently, the Halajun granitic intrusions seem to have been derived from the remelting of pre-existing source rocks rather than generated directly by extreme fractionation of mafic magmas. The studied intrusions were coevally emplaced and exhibit essentially similar trace element patterns, suggesting that they were probably derived from a common source. The Precambrian two stage Nd model ages (T DM2 = Ga) for these rocks are indicative of the presence of ancient crustal rocks in the source region. Nevertheless,

10 58 H. Huang et al. / Lithos 155 (2012) Fig. 6. Primitive mantle-normalized spidergrams for Halajun granitic intrusions. Values of primitive mantle and OIB are from Sun and McDonough (1989). Fig. 5. Chemical classification diagrams for the Halajun granites and other granitoids in the STCB and NTB. (a) AR versus SiO 2 diagram (after Wright, 1969). AR=(Al 2 O 3 + CaO+Na 2 O+K 2 O)/(Al 2 O 3 +CaO Na 2 O K 2 O) (b) SiO2 vs. Fe* diagram with a field for A-type granites of the world (after Frost et al., 2001). (c) A/CNK versus A/NK diagram (after Maniar and Piccoli, 1989). Data sources: Halajun II intrusion (Zhang et al., 2010b); A-type granites in Kyrgyz South Tianshan (Konopelko et al., 2007); Mazhashan syenites (Sun et al., 2008). to produce such high-temperature, large-volume felsic magma at crustal levels obviously requires the participation of hot mafic magmas. In addition, among the four intrusions, the Kezi'ertuo, Tamu and Halajun II plutons show positive or slightly negative ε Nd (t) values ( 0.9 to +0.6), whereas the Huoshibulake intrusion displays more crust-like ε Nd (t) values ( 2.8 to 2.6). The variable ε Nd (t) values of the intrusions necessitates a complex process that juxtaposes magmas with variable Nd isotopic compositions. Taking into account these features, we infer that depleted, mantle-derived materials were likely involved in the genesis of the Halajun granitic intrusions. In addition, the presence of the temporally and spatially related Piqiang mafic ultramafic complex further emphasizes the ~275 Ma input of the asthenospheric mantle-derived, OIB-like magmas in the study area (Zhang et al., 2010b). Felsic rocks generated through the mixing of mantle- and crust-derived magmas often carry mafic microgranular enclaves (MMEs). The pronounced absence of MMEs in the Halajun granite is a notable feature. A number of reasons can be considered for the lack of MMEs as follows. 1) Insufficient investigation of all the outcrops in the region. 2) Given that the presence of MMEs may represent a process of fast quenching of mafic magma in a dynamic magma chamber (Kaygusuz and Aydınçakı, 2009; Yuan et al., 2010), they are not necessarily formed if the interaction occurs at a slow cooling condition. 3) Considering the high SiO 2 contents, it is reasonable to infer that, at the present erosion level, the outcrops of the studied intrusions may represent evolved magmas which were finally emplaced at shallow crustal levels (see below). As a result, the MMEs, which are expected to form initially in a deeper magma chamber, can be hardly observed in the present outcrops. The foregoing discussion suggests that the Halajun granitic intrusions were likely derived from the partial remelting of Precambrian

11 H. Huang et al. / Lithos 155 (2012) CaO, Fe 2 O 3T,K 2 O+Na 2 O, Ba, Sr, Zr, P, Ti and REEs (see Figs. 5, 6 and 7), for samples from the three intrusions are not the result of magma mixing process. Instead, such variations can be produced by either variable degrees of partial melting or fractional crystallization process during magmatic evolution. Ratios of La/Yb and Zr/Nb are sensitive indicators of the degree of partial melting (Yang et al., 2011), thus the limited ranges of La/Yb ( ) and Zr/Nb ( ) ratios for samples from Kezi'ertuo, Tamu and Halajun II intrusions testify to a fractional crystallization trend (Fig. 10). Accordingly, the elemental variations are likely to reflect fractional crystallization that occurred after the generation of the parental magma. With increasing SiO 2, the Fe 2 O 3T contents decrease, whereas the Nb/Ti ratios increase (Fig. 11a and b). These systematic variations support fractional crystallization of Fe Ti oxides. Experimental studies of A-type granitic melts indicate that plagioclase is stable at near liquid conditions and that alkali feldspar crystallizes at temperatures about C lower than the onset of plagioclase (Clemens et al., 1986). As a result, fractionation processes are locally dominated by alkali feldspar separation in A-type magmas (Eby, 1990, 1992). A similar process is invoked for the Halajun granitic intrusions, as also confirmed by the negative correlation between SiO 2 and K 2 O+N 2 O (Fig. 11c), Sr Ba variation (Fig. 11d) and moderately to strongly negative Ba and Eu anomalies (Fig. 6). Since the Kezi'ertuo intrusion is comparatively enriched in mafic minerals such as biotite and arfvedsonite, which are both rare within other intrusions, the fractionation of moderate amounts of these minerals is inferred. In addition, the fractionation of arfvedsonite is also recorded by the positive correlation between SiO 2 contents and A/CNK values (Fig. 11e, Konopelko et al., 2007). On the SiO 2 vs. Zr diagram (Fig. 11f) the general decrease in Zr with increasing SiO 2 for the Halajun granitic intrusions indicates zircon fractionation. This implies that the calculated zircon saturation temperature of C obtained for the Halajun granitic intrusions may be lower than the initial temperature of primary magma (>900 C). For the Huoshibulake intrusion, the fractional crystallization process is not apparent, since the rock seems to have evolved from a less depleted magma, compositionally different from those of the other intrusions. However, given the high SiO 2 contents and strong Ba, Sr, Eu, and Ti anomalies, a fractional crystallization process, at least involving mafic minerals (e.g., biotite and arfvedsonite), alkali feldspar and Fe Ti oxides, is reasonably inferred. Fig. 7. Chondrite-normalized rare earth element patterns of Halajun granitic intrusions. Normalized values and OIB are from Sun and McDonough (1989). crustal rocks, probably triggered by underplating of asthenospheric mantle-derived magmas as also suggested by the nearby Piqiang mafic ultramafic complex. However, to evaluate the relative contribution of each end-member is difficult, since both the presently studied Permian and Precambrian OIB-like rocks (see below) in the STCB and NTB show fairly variable isotopic and geochemical compositions (Zhang et al., 2010b, 2011; Zhou et al., 2009) Fractional crystallization Given that the samples from Kezi'ertuo, Tamu and Halajun II intrusions show similar Nd isotopic compositions, their parental magmas are expected to have the same degree of contamination (by mantlederived magmas). Thus, variations in some elements, such as SiO 2, Source of granitic intrusions As mentioned in the previous section, the Kezi'ertuo intrusion is chemically least evolved and contains more mafic minerals. Accordingly, this intrusion most likely represents the composition of parental magmas and reflects the geochemical characteristics of their source. The Kezi'ertuo samples possess features typical of metaluminous series (Fig. 5c), manifesting a source region mainly composed of igneous or metaigneous rocks. The rock shows high HREE (Yb>0.9 ppm, up to 4.1 ppm) and Y (>6.9 ppm, up to 40.0 ppm) contents and possesses flat HREE patterns (Fig. 7c), precluding garnet as a residual phase. Plagioclase is typically the reservoir for Eu and Sr in gabbroic rocks and, as a result, depletion of Sr (below 84 ppm) and the significant Eu anomalies (Eu/Eu* = ) in Kezi'ertuo samples could be interpreted to result from the melting of a gabbroic source rock within the stability field of plagioclase (Jiang et al., 2011; Shellnutt and Zhou, 2007). The depletion of Ti coupled with high Nb (and Ta) concentrations is suggestive of Fe Ti oxides in the source residue after melt extraction, with little influence of rutile or titanite. Fe Ti oxides, rutile and titanite are the main Ti enriched mineral phases, with Fe Ti oxides stable at high temperature and low pressure (Liou et al., 1998). In contrast to rutile and titanite which usually have high concentrations of Nb and Ta (Cole and Stewart, 2009; Foley et al., 2002; Green, 1995), Fe Ti oxide contains much lower Nb and

12 60 H. Huang et al. / Lithos 155 (2012) Table 3 Nd isotopic data for the Halajun granitic intrusions. Huoshibulake intrusion Tamu intrusion Kezi'ertuo intrusion Halajun II intrusion (Zhang et al., 2010b) YTHS-4 YTHS-9 YTHS-12 YTHS-14 YTHS-21 TM-YT-11 TM-YT-14 YT-HH-5 YT-HH-8 08KT KT KT03-7 Sm (ppm) Nd (ppm) Sm/Nd ( 147 Sm/ 144 Nd) m ( 143 Nd/ 144 Nd) m Age (Ma) ( 143 Nd/ 144 Nd) i ε Nd (0) f Sm/Nd ε Nd (t) T DM (Ma) T 2DM (Ma) ε Nd (0)=[( 143 Nd/ 144 Nd) m /( 143 Nd/ 144 Nd) CHUR 1] 10,000. f Sm/Nd =[( 147 Sm/ 144 Nd) m /( 147 Sm/ 144 Nd) CHUR ] 1. T DM =1/λ Sm Ln{[( 143 Nd/ 144 Nd) m ]/[( 147 Sm/ 144 Nd) m ]}. T 2DM =T DM (T DM t)[(f cc f sm/nd )/(f cc f DM )]. ( 143 Nd/ 144 Nd) CHUR = and ( 147 Sm/ 144 Nd) CHUR = The decay constant λ Sm = yr 1.f cc = 0.4 and f DM = Ta (Cole and Stewart, 2009; Ding et al., 2009; Li et al., 2012). Therefore, decoupling between Nb (and Ta) and Ti suggests that the original magma might have been generated at high temperature and low pressure (P 4 kbar, Li et al., 2012; Yang et al., 2006a). Experiment studies by Rapp and Watson (1995) demonstrated that low degree of dehydration melting (5 10%) of gabbroic rocks produced high silicic melts with low to moderate Al 2 O 3 content, whereas 20 40% partial melting produced silicic to intermediate melts with high Al 2 O 3 content (e.g., trondhjemitic, tonalitic, granodioritic, quartz dioritic and dioritic). The Kezi'ertuo intrusion is compositionally characterized by high SiO 2 contents of wt.% and moderate Al 2 O 3 contents of wt.%, implying that they could have been generated by 5 10% melting of gabbroic rocks. In addition, according to previous studies (Frost and Frost, 2011; Frost et al., 2001), low-degree partial melting of gabbroic rocks will produce Fe-rich melts and therefore high Fe* values. Consequently, the high Fe* values for the Kezi'ertuo samples ( ) are in good agreement with such model. In general, it appears that most of the observed geochemical features of Kezi'ertuo intrusion can be explained by the model of low-degree partial melting of gabbroic rocks at hightemperature and low-pressure. Fig. 8. Zr versus T Zr diagram for the Halajun granitic intrusions and some I-type granitoids of the STCB. Data sources: Halajun II intrusion (Zhang et al., 2010b); Chuanwulu granitoids (Huang et al., 2012); Mangqisu pluton (Zhu et al., 2008). As mentioned above, the A1-type Halajun granitic intrusions have geochemical characteristics similar to those observed for OIBs. In addition to the two stage Nd model ages, the Precambrian gabbroic rocks, which could be ultimately produced by partial melting of an OIB-like mantle source, could be candidates for the source of these granitic rocks. Considering the presence of OIB-like gabbroic rocks in the NTB and in the southern margin of STCB, a rift-related tectonic setting environment during Neoproterozoic can be envisaged (Long et al., 2011b; Zhang et al., 2011, and references therein). It is reasonable to note that these granitic intrusions were emplaced at a relatively shallow level. Given the involvement of mantle-derived component that was subsequently mixed with the crustal melts, the emplacement of the Halajun granitic intrusions also represent mass transfer from the asthenospheric mantle to the upper continental crust, as will be discussed in the next section Implications for vertical crustal accretion The growth and evolution of the continental crust are a widely discussed topic in relation to the tectonic evolution of the CAOB. There is a general consensus that accretionary tectonics was the major mechanism for the Phanerozoic continental growth of the CAOB (Chen and Jahn, 2002, 2004; Long et al., 2011a; Şengör and Natal'in, 1996a, 1996b; Sengör et al., 1993; Xiao et al., 2009, 2010, 2012). The following two major phases have been identified: 1) syn-subduction continental growth by accretion of arc complexes related to oceanic subduction that mainly occurred before Carboniferous, and 2) subsequent non-arc (syn-/post-collisional and/or anorogenic) vertical continental growth by input of mantle-derived materials. The Carboniferous to Permian granitoids with positive ε Nd (t) values and low initial ( 87 Sr/ 86 Sr) t ratios are widespread in Altay, Mongolia, Junggar, NTCB and CTB, and have been regarded as robust signature for the non-arc vertical continental growth of the northern part of CAOB (Jahn et al., 2000a, 2000b, 2001). Nevertheless, most of the previously studied Hercynian granitoids in the STCB have negative ε Nd (t) values and high initial ( 87 Sr/ 86 Sr) t ratios (Fig. 12), giving rise to debates on the significance of non-arc vertical continental growth. As a result, the Late Paleozoic continental evolution of the southern margin of the CAOB has been proposed to be classified as reworking rather than growth (Gao et al., 2011; Jiang et al., 2004b). Compared with the ~300 Ma to ~285 Ma (early stage, see Section 6.1) igneous rocks commonly showing crust-like Nd isotopic signatures, the magmas emplaced during ~282 to ~275 Ma (late

13 H. Huang et al. / Lithos 155 (2012) Fig. 9. (a) (K 2 O+Na 2 O)/CaO versus 10,000 Ga/Al, (b) Zr versus 10,000 Ga/Al, (c) FeOT/MgO versus Zr+Nb+Ce+Y discrimination diagrams of Whalen et al. (1987) and (d) Nb Y Ce discrimination diagram of Eby (1990), showing the A1-type nature of the Halajun granitic intrusions. stage) apparently display more mantle-like features, as shown in Fig 12a and b. In addition, these late-stage granites and syenites consistently show A1-type granite affinity (Figs. 9d and 10), further attesting to a genetic relationship to the juvenile, asthenospheric mantle-derived and hot magmas. We consequently infer that the early-stage felsic rocks may represent the reworking of ancient lithosphere and, in contrast, the late-stage ones contributed to the continental vertical accretion as a result of significant mass transfer from the mantle to the upper crust Geodynamic setting Subduction-, collision- or rift-related? The geodynamic setting of the Halajun region in the Permian has been a controversial but important issue, given its critical location between the South Tianshan Collisional Belt and Tarim Block. Several models have been proposed, including 1) post-collisional extension (e.g., Long et al., 2011a; Xu et al., 2005; Zhou et al., 2006), 2) intraplate setting (e.g., Zhang et al., 2008, 2010b), and 3) arc environment associated with paleo-tethyan subduction north of the Qinghai Tibet Plateau (e.g., Yang et al., 1995, 2006b). Recent studies indicate that the north-dipping subduction of paleo-tethys along the southern margin of the South Kunlun Terrane lasted from at least the mid-carboniferous to the Early Triassic (Xiao et al., 2002, 2005; Zhang et al., 2008). If the magmatic rocks were genetically related to the subduction, they should have ages varying from Carboniferous to Triassic rather than being a short pulse during the Late Carboniferous to Permian, as identified in the STCB and Tarim Block. Furthermore, since no Late Carboniferous to Permian Na-rich (K 2 O+2 wt.%bna 2 O) igneous rocks have been identified in STCB and NTB (see Huang et al., 2012, and references therein), a subduction-related setting seems to be unlikely. Although considerable debate surrounds the time of the collision between Tarim and Central Tianshan blocks, the general consensus is around Late Carboniferous (Gao et al., 2011, and references therein). Konopelko et al. (2009) suggested that both the CTB and STCB were already in a post-collisional extensional setting before ~300 Ma. Consequently, the Late Carboniferous to Permian magmatism in the NTB and STCB was traditionally regarded to be associated with the post-collisional extension and, accordingly, these late-stage A1-type granites and syenites are considered to have formed during the latest stage of the orogenic evolution (Long et al., 2011a). Although the early-stage magmatism can be genetically related to the post-collisional extension, this mechanism alone cannot be readily used to interpret many features of the igneous rocks formed during the late stage. For example, the late-stage granites and syenites uniformly display A1-type geochemical affinities and mostly exhibit positive to slightly negative ε Nd (t) values, features that are comparable to those of alkaline rocks in intraplate tectonic settings (e.g., Baker et al., 1997; Upadhyay et al., 2006) but significantly distinct from those of post-collisional A2-type granites widely exposed in the Kyrgyz part of the STCB (Konopelko et al., 2007, 2009). Furthermore, Permian sedimentary sequences, mainly composed of terrigenous sandstones with total thickness higher than 2000 m and conformably overlying the Carboniferous marine carbonates, are well-exposed and locally intruded by felsic rocks in the Halajun region and other areas of the NTB (e.g., Mazhashan and Wajilitag regions), further indicating an intraplate rift-related (Jiang et al., 2004a, 2004b; Yang et al., 1996) rather than a collision-related setting. More significantly, as mentioned in Section 6.1, OIB-like, asthenospheric mantle-derived mafic ultramafic rocks, which are spatially, temporally and geochemically related to the A1-type felsic rocks, are widespread in the NTB and in the southern margin of the STCB. The high temperatures required for the large-scale partial melting of the asthenospheric mantle cannot be easily explained by a single post-collisional setting. In general, all of the aforementioned features suggest that the late-stage magmatism seems unlikely to be genetically associated with a collision-related setting. An alternate mechanism which

14 62 H. Huang et al. / Lithos 155 (2012) Fig. 10. (a) La vs. La/Yb and (b) Zr vs. Zr/Nb diagrams (after Yang et al., 2011), showing the trend of fractional crystallization. Symbols are the same as Fig. 9. provided the heat source for the melting of asthenospheric mantle is required A mantle plume model Recently, the Permian Tarim large igneous province (TLIP) has been widely proposed by many authors (e.g., Su et al., 2011, andthereferences therein; Xia et al., 2012; Yu et al., 2011; Zhang et al., 2008, 2010b; 2012; Zhou et al., 2009). The TLIP comprises mafic ultramafic and felsic intrusive rocks and dykes exposed in the Mazhashan and Wajilitag regions as well as flood basalts in the Keping region and in the inner part of the Tarim Block together covering area of ~250,000 km 2 (see Zhou et al., 2009, and the references therein), although the precise ages have not been well defined. The ~275 Ma Halajun igneous rocks consisting of A1-type granitic intrusions and OIB-like Piqiang mafic ultramafic complex,which are geochronologically, geochemically and isotopically comparable to those in Wajilitag and Mazhashan regions, appear to be parts of the TLIP. Many large igneous provinces are thought to originate from the rapid ascent of mantle plumes. In the case of TLIP, such argument has been supported by several lines of evidence. For instance, Zhou et al. (2009) reported that olivine from the ultramafic dyke in the Bachu region has high Fo values (up to 85) and the liquidus temperature of olivine was as high as ~1303 C. Besides, a crustal doming event coincided with the late-stage magmatism (see Zhang et al., 2008). Mantle plumes worldwide are known to be usually close to rift zones (e.g., Emeishan mantle plume and Panxi rift, Wu and Zhang, 2012; Xu et al., 2005, 2008; Siberian mantle plume and Baikal rift, Zorin et al., 2003). Indeed, A-type felsic intrusions in the Panxi rift are widely recognized as a diagnostic feature of the mantle plume (Shellnutt and Iizuka, 2011; Zhou et al., 2009). Compared with the dataset published by Shellnutt et al. (2011) and Shellnutt and Iizuka (2011, 2012), the calculated T Zr values for the Halajun granitic rocks are even higher than those of Panxi felsic rocks which formed by the remelting of the pre-existing rocks (T Zr values for Woshui, Huangcao and Daheishan plutons range from 705 to 741 C, for Ailanghe and Yingpanliangzi plutons from 753 to 781 C). Therefore, in conjunction with the possibility of zircon fractionation, the initial temperature of primary magma of the Halajun granitic intrusions is likely to have been high enough to be associated with the plumerelated magmatism. According to the Permian Tarim mantle plume model, the ~282 Ma basaltic eruptions in Keping region and mafic ultramafic intrusions in Wajilitag region may represent the onset or earlier phase of the large igneous province event associated with the mantle plume. The upwelling mantle plume would continuously provide the high temperatures required for large-scale partial melting of asthenospheric mantle and the large igneous province event is likely to be temporally peaked at ~275 Ma, as witnessed by numerous coeval A1-type felsic rocks and OIB-like mafic ultramafic intrusive rocks and dykes. Given the juvenile Nd isotope signature for the majority of those rocks, the generation of ~275 Ma A1-type granites and syenites seems mostly related to convective heating associated with underplating of mafic magmas from the asthenospheric mantle. Thus, the A1-type felsic rocks could have genetically been related to a Permian Tarim mantle plume. Several magmatic intrusive and extrusive rocks in Central Asia have ages and geochemical features similar to those of the Tarim mantle plume-related rocks. For instance, in the Beishan region, the eastern extension of the Tarim Block, several ~280 Ma ultramafic, mafic tofelsic intrusive rocks show highly positive ε Hf (t)values(upto+17,su et al., 2011). These magmatic activities are thought to be associated with an upwelling mantle plume. Furthermore, the well-exposed mafic ultramafic intrusions in the Huangshan region of the CTB have been dated at ~270 Ma and are considered to be related to mantle plume (Zhou et al., 2004). In addition, some basalts in the Turpan Hami and Sangtanghu basins of the North Tianshan Collisional Belt are also likely to have been derived from a Permian mantle plume (Zhang et al., 2010b; Zhou et al., 2006, 2009). All these suggest that not only the Tarim Block and southern marginofthestcbbutalsootherareasofthecaobwereaffectedbythe Permian mantle plume-related magmatism, although it is still unclear if a single plume or several plumes were involved. In fact, Permian mantle plumes were very active in the Eurasian continent. The plume-related Tarim large igneous activity would have occurred ~15 My before the ~260 Ma Emeishan LIP in southwestern China (Shellnutt and Zhou, 2007; Xu et al., 2008) and ~25 My before the ~250 Ma Siberian traps (Pirajno et al., 2009). The wide distribution of these Permian plumes in Eurasia may represent the early stage of the dipolar Pangean and SW Pacific superplumes (Zhang et al., 2010b), which might have been triggered by heat loss from the core and contributed collectively to the Permian mass extinction (Zhou et al., 2009). 7. Conclusions (1) Field investigation and zircon U Pb dating suggest that the Halajun granitic intrusions and Piqiang complex might have been coevally generated at ~275 Ma and are likely to be associated with the same tectono-thermal event. (2) The Halajun granitic intrusions possess typical A1-type affinity, and were generated by partial melting of Neoproterozoic gabbroic rocks, probably triggered by the ~275 Ma underplating of asthenospheric mantle-derived magmas.

15 H. Huang et al. / Lithos 155 (2012) Fig. 11. (a) SiO 2 versus Fe 2 O 3T, (b) SiO 2 versus Nb/Ti, (c) SiO 2 versus Na 2 O+K 2 O, (d) Sr versus Ba, (e) SiO 2 versus A/CNK and (f) SiO 2 versus Zr diagrams for Halajun granitic intrusions, showing fractionation of Fe Ti oxides, alkaline feldspar, hornblende and zircon. Symbols are the same as Fig. 9. (3) The A1-type granites and syenites exposed in the NTB and the southern margin of the STCB, including the Halajun granitic intrusion, can be considered as evidence for significant mantle contributions to vertical crustal growth in the southern margin of the CAOB. (4) The magmatic rocks generated during ~282 Ma to ~275 Ma show several features that are similar to those of Large Igneous Provinces. We therefore exclude a subduction- and collisionrelated setting, and invoke Permian Tarim mantle plume-related magmatism. Acknowledgments This study was financially supported by the National 305 Project (2011BAB06B02-04) and the NSFC grant ( ). We are grateful to Zhenyu Chen of Institute of Geology, Chinese Academy of Geological Sciences, Lin Ding of the Key Laboratory of Continental Collision and Plateau Uplift, Chinese Academy of Sciences, Suohan Tang of National Research Center for Geoanalysis, Chinese Academy of Geological Sciences and other workers for their assistance in geochemical, isotopic and LA-ICP-MS determinations. Special thanks are given to Hongxing Du and Changrong Feng for their kind help during the field work. Constructive reviews and suggestions by Dr. Greg Shellnutt and one anonymous reviewer, and Editor Prof. Nelson Eby, helped to improve the revised version. References Andersen, T., Correction of common lead in U Pb analyses that do not report Pb Chemical Geology 192, Baker, J.A., Menzies, M.A., Thirwall, M.F., MacPherson, C.G., Petrogenesis of quaternary intraplate volcanism, Sana'a, Yemen: implications for plume lithosphere interaction and polybaric melt hybridization. Journal of Petrology 38, Bonin, B., A-type granites and related rocks: evolution of a concept, problems and prospects. Lithos 97 (1 2), Bonin, B., Giret, A., Plutonic alkaline series: Daly gap and intermediate compositions for liquids filling up crustal magma chambers. Schweizerische Mineralogisch und Petrographische Mitteilungen 70, Cai, K.D., Sun, M., Yuan, C., Zhao, G.C., Xiao, W.J., Long, X.P., Wu, F.Y., Prolonged magmatism, juvenile nature and tectonic evolution of the Chinese Altai, NW China: evidence from zircon U Pb and Hf isotopic study of Paleozoic granitoids. Journal of Asian Earth Sciences 42 (5), Cai, K., Sun, M., Yuan, C., Xiao, W.J., Zhao, G.C., Long, X., Wu, F., Carboniferous mantle derived felsic intrusion in the Chinese Altai, NW China: implications for geodynamic change of the accretionary orogenic belt. Gondwana Research 22,

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