Paleozoic reconstruction and tectonic evolution of north Xinjiang, NW China: Implications for the lateral growth of central Asia. Wen-Jiao Xiao, Chun-Ming Han.
Chapter 12-17
12-17
Paleozoic reconstruction and tectonic evolution of north Xinjiang, NW China: Implications for the lateral growth of central Asia Wen-Jiao Xiao, Chun-Ming Han State Key Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China Chao Yuan Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, Guangdong, China Han-Lin Chen, Zi-Long Li Faculty of Sciences, Zhejiang University, Hangzhou 310027, Zhejiang, China Min Sun, Guo-Chun Zhao Department of Earth Sciences, The University of Hong Kong, Hong Kong, China Ke-Zhang Qin, Ji-Liang Li, Shu Sun Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
Abstract. The geology and tectonics of North Xinjiang can be divided into the Chinese Altay-East Junggar, West Junggar, and Tianshan-Tarim domains, each of which is composed of Andeantype magmatic arc or island arc, accretionary wedge, and ophiolitic slice, showing archipelago paleogeography. The Chinese Altay-East Junggar domain was more closely located to the Angaran craton (Siberia), while the Tianshan-Tarim domain was near the opposite side of the early Paleozoic Paleoasian ocean. The West Junggar domain occupied an intermediate position near the Kazakhstan block in the early Paleozoic Paleoasian basin. The Tianshan-Tarim and West Junggar domains drifted northwards and approached the Chinese Altay-East Junggar active margin of the Angaran craton in the late Paleozoic. Subsequent amalgamation of these domains squeezed the archipelago systems of these domains, leading to termination of the Paleoasian ocean and formation of a complicated orogenic collage between Angaran craton and the Tarim block by the late Carboniferous or the early Permian. These multiple accretion processes significantly contributed to the lateral growth of Central Asia.
nism of the orogenesis as to whether there was a longlived, single subduction system (Rotarash et al. 1982; Carroll et al. 1990; Sengör et al. 1993) or a collage of various terranes via different subduction systems (Coleman 1989, 1994; Mossakovsky et al. 1993; Buslov et al. 2001; Windley et al. 2002; Badarch et al. 2002). The major debate centers on the time, nature and tectonic setting of the various tectonostratigraphic units. This paper presents a new version of tectonic subdivision and a reconstruction and tectonic model for North Xinjiang in the framework of southern Altaids. The geology and tectonics of North Xinjiang can be divided into the Chinese Altay-East Junggar, West Junggar, and Tianshan-Tarim domains, roughly from N to S, which are described in the following sections.
Keywords. North Xinjiang, Intra-oceanic subduction, ophiolite, accretion, Central Asia
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Introduction
The North Xinjiang occupies a key tectonic position of the Altaids (Sengör et al. 1993; Sengör and Natal’in 1996; XNGMR 1993; Xiao et al. 2004a, 2004b) or Central Asian Orogenic Belt (Mossakovskiy 1993; Jahn 2001; Dobretsov 2003), and its almost complete geological records and excellent exposure of ophiolite, arc, and accretionary wedge have made it a ideal natural laboratory to investigate subduction and accretion of juvenile material from the Neoproterozoic through the Paleozoic (Sengör et al. 1993; Sengör and Natal’in 1996; Yin and Nie 1996; Jahn 2001). However, there is a strong debate about the mecha-
Chinese Altay-East Junggar
The Chinese Altay-East Junggar domain marks the site where one branch of the Paleoasian ocean, the Junggar ocean, was consumed (Sengör et al. 1993; Filippova et al. 2001). Although different terranes in this region have been distinguished (Li et al. 1990, 2003; Xiao et al. 1991; Windley et al. 2002; Xiao et al. 2004a), they are all similar in that all have an island arc/subduction zone with a brief southward younging (Li et al. 1990, 2003; O’Hara et al. 1997; Niu et al. 1999; Xu et al. 2001; Xiao et al. 2004a). Based on terrane analysis and zircon geochronology in the Chinese Altay region, Windley et al. (2002) concluded that the Chinese Altay was a Cambro-Ordovician continental magmatic arc. Similar SHRIMP zircon dates for the Aermantai ophiolitic mélange (Jian et al. 2003) and com-
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bined geology and geochemistry work (Jin et al. 2003; Wang et al. 2003) all indicate that East Junggar was a coeval island arc. However, ages as young as the Devonian have also obtained in the Aermantai and Kelameili ophiolites (Jian et al. 2003), which points to a late stage of subduction-accretion process (Liu et al. 1993; He et al. 1994). Juxtaposition of these tectonic units indicate multiple subduction-accretion systems amalga-mated by subsequent oblique convergence, including remnants of island arc, subduction complexes, seamount and ophiolites, forming an enlarged Cordilleran-type margin by Mid-Carboniferous time (Xiao et al. 2004a).
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West Junggar
The West Junggar domain comprise of various terranes with island arc/ subduction origin (Coleman 1989; Feng et al. 1989; Allen et al. 1989; Windley et al. 1990; Xiao et al. 1991; Zhang et al. 1993; Buckman and Aitchison 2001, 2004), but due to the severe Mesozoic-Cenozoic overprint the terranes of the West Junggar domain are mostly allochthonous (Allen et al. 1989, 1995), thus making its geology much complicated than the East Junggar domain. Another conspicuous character is that there are more ophiolitic terranes than in the East Junggar domain (Allen et al. 1989; Feng et al. 1989; Kwon et al. 1989; Zhang et al. 1993). These ophiolitic terranes are actually relicts of possible Ordovician-Silurian arc-related basins which acted as basement or substrata of Devonian-Carboniferous arc edifices (Wang et al. 2003; Xiao et al. 2004a). Based on regional geology, radiolarian fossils and isotopic dating, all terranes in the West Junggar were thought to be amalgamated by the end of Carboniferous (Allen et al. 1989; Buckman and Aitchison 2004). There has been a controversial issue concerning the basement of Junggar Basin, which is located between the West and East Junggar domains, and mostly covered by Mesozoic-Cenozoic sediments. Based on geological, geochemical, geochronological, and geophysical data, the basement of the Junggar Basin may be mostly composed of arcs and accretionary complexes or tapped oceanic crust (Carroll et al. 1990; Hu et al. 2000; Chen and Jahn 2002, 2004), although different views such as continental basement exist.
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Tianshan-Tarim
The Tianshan-Tarim domain is conveniently used to describe the southernmost tectonic domain of the Altaids, which includes the Tianshan and Tarim sub-domains. The Tianshan sub-domain comprises, from N to S, of Northern, Central, and Southern Tianshan tectonic entities. The Northern Tianshan was assigned as an Ordovician-Silurian island arc with south-dipping subduction
at its northern side, and evolved into a possibly doublesubduction scenario in the Devonian and Carboniferous. The Central Tianshan was an Andean-type arc during most of its existence in the Paleoasian ocean history. The Southern Tianshan was mainly an accretionary complex including ophiolites, UHP and HP rocks (Xiao et al. 1991; Gao et al. 1995, 2001; Zhang et al. 2002a, 2002b), island arcs and accretionary wedges, which recorded the final elimination of the Paleoasian ocean and the basic architecture in Central Asia in the Late Paleozoic (Windley et al. 1990; Allen et al. 1993; Xiao et al. 2004a, 2004b). The Tarim sub-domain was mainly a continental margin which remained passive during the Paleozoic. Some of its Precambrian basement rocks had been thrust southward over its northern marginal sequences.
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Reconstruction and tectonic model
The North Xinjiang orogenic collage, composed of various Paleozoic tectonic domains, provides one of the almost complete sections from the southern Angaran margin to the northern Tarim block. A summary of its tectonic evolution and reconstruction for this part of Central Asia is given as follows. During the Early Paleozoic, the Chinese Altay-East Junggar domain was more closely located to the Angaran craton (Siberia), while the Tianshan-Tarim domain was near the opposite side of the Paleoasian ocean. The West Junggar domain occupied an intermediate position near the Kazakhstan block in the early Paleozoic Paleoasian basin. The Tianshan-Tarim and West Junggar domains drifted northwards and approached the Chinese AltayEast Junggar active margin of the Angaran craton in the late Paleozoic. Subsequent complicated amalgamation processes of these domains squeezed the archipelago systems of these domains, leading to termination of the Paleoasian ocean and formation of a complicated orogenic collage between Angaran craton and the Tarim block by the late Carboniferous or the early Permian. These multiple accretion processes significantly contributed to the lateral growth of Central Asia. From the summery presented here, we conclude that, although with a general southward younging accretion, there were multiple subduction systems existed in the long, complicated evolution history of the Paleoasian ocean and the formation of the Altaids. A number of the various terranes were different parts of relicts of ancient subduction systems, mostly tectonically incorporated into accretionary complexes. Except from minor Precambrian rocks in some Andean-type magmatic arcs, such as the Chinese Altay and Central Tianshan, no significant old continents had been involved into the orogenic processes of the Altaids (Ma et al. 1997; Han et al. 1997; Qin et al. 1999; Qin 2000; Hu et al. 2000; Jahn 2001; Xiao et al. 2004a, 2004b; Zhang et al. 2004).
Chapter 12-17 · Paleozoic reconstruction and tectonic evolution of north Xinjiang, NW China: Implications for the lateral growth of central Asia
The tectonic evolution and reconstruction of North Xinjiang not only helps to reconcile the long-standing controversy of single Rotarash et al. 1982; Sengör et al. 1993) versus complicated subduction polarities (Massakovsky et al. 1993; Coleman 1989; Windley et al. 1990, 2002), but also sheds lights on global reconstruction in the Paleozoic (Heubeck 2001; Yakubchuk et al. 2001; Torsvik and Cocks 2004).
Acknowledgements We thank Academicians Guang-Chi Tu, Xu-Chang Xiao, and Ting-Dong Li, Professors Guo-Qi He, Fu-Chen Ma, Ying-Jun Ma, Jing-Bin Wang, Jun Gao, Jin-Yi Li, Wei Liu, Bao-Fu Han, Zhao-Jie Guo, Tao Wang, and Zheng-Le Chen for discussion. We appreciate Professors B.F. Windley, Reimar Seltmann and Jing-Wen Mao who helped us by generously giving of their time and expertise. This study was financially supported by the Chinese MOST (2001CB409801, 96-915-07), the Chinese Academy of Sciences (KZCX2-SW-119-01), the Chinese NSF (40172080) and the Hong Kong Research Grants Council (HKU7040/ 04P). This is a contribution to IGCP 473 and 480.
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