Early Neoproterozoic granitic gneisses in the Chinese Eastern Tianshan: Petrogenesis and tectonic implications
Graphical abstract
Introduction
Accretionary orogens occurring at intraoceanic or continental margin convergent plate boundaries are major sites of crustal growth through continuing subduction and accretion (Cawood et al., 2009). The Central Asian Orogenic Belt (CAOB), one of the largest Paleozoic accretionary orogens in the world, was formed by multiple collages of microcontinents, island arcs, oceanic islands, ophiolites and accretionary complexes (Xiao et al., 2008, Xiao et al., 2010a, Xiao et al., 2013, Windley et al., 2007, Wilhem et al., 2012). The Chinese Tianshan Orogen occupies the southwestern part of the Central Asian Orogenic Belt (CAOB) and thus its study provides key information for understanding the accretion of the Tarim Craton to Eurasia and the final closure of the Paleo-Asian Ocean (Fig. 1; Gao et al., 1998, Gao et al., 2009, Jahn et al., 2000, Xiao et al., 2004, Charvet et al., 2007, Ge et al., 2014). Among the several tectonic units in the Chinese Tianshan Orogen, the Central Tianshan Arc Terrane (CTA) and the Yili Block are considered as ancient microcontinents with a Precambrian crystalline and/or metamorphic basement, which have played a significant role in the process of the accretionary orogen (Xiao et al., 2004, Xiao et al., 2013, Alexeiev et al., 2011, He et al., 2014a, Wang et al., 2014a).
At present, the origin of these Precambrian crustal fragments in the Chinese Tianshan and their tectonic evolution remain controversial (Jahn et al., 2000, Hu et al., 2000, He et al., 2012a, He et al., 2014a, Ma et al., 2012, Lei et al., 2013, Huang et al., 2014, Wang et al., 2014a, Wang et al., 2014b). In recent years, early Neoproterozoic granitic orthogneisses have been recognized from several areas of the CTA, from east to west: Xingxingxia, Alatage, and Baluntai (Fig. 1). The documented protoliths’ crystallization ages of these orthogneisses range between 970 and 915 Ma (Table 1) and thus represent a major tectonothermal episode in the Precambrian history of the CTA (Hu et al., 2010, Ma et al., 2012, He et al., 2014a). These rocks potentially carry valuable information not only for the reconstruction of the early tectonic evolution of the Chinese Tianshan, but also for understanding the various components of the crustal basement in the CTA and thus the nature of Phanerozoic continental growth in the Chinese Tianshan. Previous studies have generally correlated this granitic magmatism with the assembly of Rodinia (e.g., Chen et al., 2009, Hu et al., 2010, Ma et al., 2012, Peng et al., 2012). Their petrogenesis, tectonic setting and tectonothermal evolution, however, have not been well constrained up to now.
In this study, we present bulk-geochemical, in situ zircon U–Pb as well as zircon Hf isotope data of early Neoproterozoic granitic gneisses from the Xingxingxia and Alatage areas in the CTA (Fig. 1). Furthermore, these data are supplemented by previously published data of contemporary granitic gneisses from the Xingxingxia, Alatage and Baluntai areas in the CTA (Table 1) in order to provide further constraints on the Precambrian tectonic evolution of the Chinese Tianshan. Our results suggest that the protoliths of these granitic gneisses belong to S-type granites, formed in an active continental margin, and the CTA was located at the margin of the supercontinent Rodinia between 970 and 915 Ma.
Section snippets
Geological setting
The Chinese Tianshan Orogen is sandwiched between the Tarim Craton to the south and the Junggar Basin to the north (Fig. 1). Generally, it can be divided into the Western and Eastern Tianshan roughly by the longitude 90° or along the Tuokexun–Kumishi Road (Yuan et al., 2010, Xiao et al., 2013). Several main tectonic units are enclosed in the Chinese Tianshan, namely the North Tianshan, the Yili Block, the CTA and the South Tianshan (Gao et al., 1998, Gao et al., 2009, Xiao et al., 2004, Xiao et
Sample descriptions
The studied samples were collected from road-cutting outcrops in the Alatage and Xingxingxia areas, Xinjiang Uyghur Autonomous Region (Fig. 1). They show varying degrees of a gneissic foliation and migmatization (Fig. 2). Furthermore, the Xingxingxia granitic gneiss is typically crosscut by a late Paleozoic (ca. 270 Ma) fine-grained granitic dyke (Fig. 2b). Photomicrographs of representative samples are shown in Fig. 3 and representative mineral compositions are listed in Table 2.
The Alatage
Analytical methods
Mineral compositions were determined using a JEOL JXA 8800R microprobe with a 20 kV accelerating voltage and 20 nA beam current at the Institute of Mineral Resources, Chinese Academy of Geological Sciences (CAGS).
Bulk-rock chemical analyses were performed in the National Geological Analysis Center of China, CAGS. Major elements were determined using a Rigaku-3080 XRF with analytical uncertainties of less than 5%. Trace elements were analyzed using a Thermo Fisher TJA X-series ICP-MS and the
Zircon U–Pb ages and Hf isotopic compositions
CL images of representative zircon grains are shown in Fig. 4. Their U–Pb ages and REE compositions are listed in Supplementary Tables 1 and 2, respectively. The zircon Hf analyses (Table 3) were performed on domains which were also used for U–Pb dating.
Origin of the granitic protoliths
According to different magma sources and formation mechanisms, granitoids in the Earth’s continental crust are genetically divided into M-, I-, S- and A-types (Chappell and White, 1974, Chappell and White, 2001, Collins et al., 1982, Chappell, 1999, Frost et al., 2001). In the [Zr + Nb + Ce + Y] versus FeOT/MgO discrimination diagram (Fig. 11a), most analyses of the early Neoproterozoic Xingxingxia, Alatage and Baluntai granitic gneisses plot in the fields of M-, I-, S-type or fractionated granites,
Summary
The investigated early Neoproterozoic granitic gneisses occur widespread in the CTA of the Chinese Eastern Tianshan. Petrographical, whole-rock geochemical and zircon Hf isotopic studies revealed an S-type granite affinity for their protoliths and that their formation involved the reworking of the older sedimentary crust in the Chinese Eastern Tianshan. Furthermore, the early Neoproterozoic granitic gneisses are thought to have formed in a subduction-related tectonic setting of an active
Acknowledgments
We are thankful to Prof. W.J. Xiao and two anonymous reviewers for their constructive comments and suggestions. This work was supported by the Program of Excellent Young Geologists of China Geological Survey (QNYC2-2012-21) and the Chinese Geological Survey Project (12120114057401, 1212011407980). This publication is a contribution to the IGCP Project 592.
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