Tectonics of the North Qilian orogen, NW ... Tectonics of the North Qilian orogen, NW China Shuguang

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  • Gondwana Research 23 (2013) 1378–1401

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    Tectonics of the North Qilian orogen, NW China

    Shuguang Song a,⁎, Yaoling Niu b,c, Li Su d, Xiaohong Xia a

    a MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, China b School of Earth Sciences, Lanzhou University, Lanzhou 730000, China c Department of Earth Sciences, Durham University, Durham DH1 3LE, UK d Geological Lab Center, China University of Geosciences, Beijing 100083, China

    ⁎ Corresponding author. Tel.: +86 10 62767729. E-mail address: sgsong@pku.edu.cn (S. Song).

    1342-937X/$ – see front matter © 2012 International A doi:10.1016/j.gr.2012.02.004

    a b s t r a c t

    a r t i c l e i n f o

    Article history: Received 12 June 2011 Received in revised form 31 January 2012 Accepted 3 February 2012 Available online 14 February 2012

    Keywords: Oceanic suture zone Ophiolite Arc and back-arc basin High-pressure metamorphism Subduction and collision Qilian Orogen

    The Qilian Orogen at the northern margin of the Tibetan Plateau is a type suture zone that recorded a com- plete history from continental breakup to ocean basin evolution, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. The Qilian Ocean, often interpreted as represent- ing the “Proto-Tethyan Ocean”, may actually be an eastern branch of the worldwide “Iapetus Ocean” between the two continents of Baltica and Laurentia, opened at ≥710 Ma as a consequence of breakup of superconti- nent Rodinia. Initiation of the subduction in the Qilian Ocean probably occurred at ~520 Ma with the development of an Andean-type active continental margin represented by infant arc magmatism of ~517–490 Ma. In the begin- ning of Ordovician (~490 Ma), part of the active margin was split from the continental Alashan block and the Andean-type active margin had thus evolved to western Pacific-type trench–arc–back-arc system repre- sented by the MORB-like crust (i.e., SSZ-type ophiolite belt) formed in a back-arc basin setting in the time pe- riod of ~490–445 Ma. During this time, the subducting oceanic lithosphere underwent LT-HP metamorphism along a cold geotherm of ~6–7 °C/km. The Qilian Ocean was closed at the end of the Ordovician (~445 Ma). Continental blocks started to collide and the northern edge of the Qilian–Qaidam block was underthrust/dragged beneath the Alashan block by the downgoing oceanic lithosphere to depths of ~100–200 km at about 435–420 Ma. Intensive orogenic activities occurred in the late Silurian and early Devonian in response to the exhumation of the subducted crustal ma- terials. Briefly, the Qilian Orogen is conceptually a type example of the workings of plate tectonics from continental breakup to the development and evolution of an ocean basin, to the initiation of oceanic subduction and for- mation of arc and back-arc system, and to the final continental collision/subduction and exhumation.

    © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

    1. Introduction

    Plate tectonics through opening and closing of an ocean basin, which is known as the Wilson Cycle (Burke et al., 1976), is the major tectonic scenario on Earth's outer layer and has occurred many times during the Phanerozoic (Condie, 1997). The fragmenta- tion of supercontinent Rodinia in the Neoproterozoic and re- amalgamation into another less long-lived supercontinent Pangaea in the Paleozoic (e.g., Torsvik et al., 1996) manifested such a global cycle. The contemporaneous event occurred widespread in a com- pound orogenic system in China, i.e., “the Central China Orogenic Belt” (e.g., Jiang et al., 2000), which extends in the W–E direction for approximate 5000 km along the central China mountain chains in- cluding Qilian Mountains, Qinling Mountains and Kunlun Mountains.

    ssociation for Gondwana Research.

    TheQilianmountain system,whichwas named “the SkyMountains” by ancient Hunnish and Chinese people, is located at the northern mar- gin of the Qinghai-Tibetan Plateau, northwestern China. This mountain system, striking in NW–SE direction, is ~100 km wide, ~1000 km long and ~2800 to 5570 m high in altitude. It is offset to the northwest by the Altyn-Tagh Fault, the largest sinistral strike-slip fault in NW China (Fig. 1).

    Because of the physiographic difficulty, this mountain system had been poorly studied before the 20th century. From 1920s to 1940s, some Chinese and overseas pioneers set feet in this region for explo- ration and geological survey on stratigraphy (Yuan, 1925; Bexell, 1935; Hou and Sun, 1935; Sun, 1936; Li, 1946a, 1948a; Guo, 1948), bi- ology (Chi, 1935; Young, 1935; Wang, 1937), igneous petrology (Du Rietz, 1940; Li, 1946b), glacier geology (Weng and Lee, 1946), meta- morphism (Sung, 1949; Chen, 1950), tectonics (Lee, 1939; Huang, 1945; Li, 1948a, 1948b; Liang, 1949) and coal and placer gold deposits (Sun, 1940, 1941; He, 1946). Since the establishment of the People's Republic of China in 1949, several geological survey teams have been dispatched to work in the Qilian Mountains for geological

    Published by Elsevier B.V. All rights reserved.

    http://dx.doi.org/10.1016/j.gr.2012.02.004 mailto:sgsong@pku.edu.cn http://dx.doi.org/10.1016/j.gr.2012.02.004 http://www.sciencedirect.com/science/journal/1342937X

  • Fig. 1. Schematic maps showing major tectonic units of China.

    1379S. Song et al. / Gondwana Research 23 (2013) 1378–1401

    investigation, mapping and mineral exploration. Several large ore de- posits were found and some basic studies on regional geology and mineralization were carried out in this period of time (e.g. Lee, 1954; Lu, 1954; Huang, 1955; Sung, 1955; Yan, 1955; Chen, 1957; Tu, 1957a, 1957b; Hu et al., 1958; Hu, 1959; Institute of Geology of Chinese Academy of Sciences, 1960, 1963).

    Since 1970s, the orogenesis of the Qilian Mountains has been rein- terpreted in terms of the Plate Tectonics theory as reflected in the studies of ophiolite associations (Wang and Liu 1976, 1981; Xiao et al., 1978; Feng and He, 1995a; Zhang et al., 1997a, 1997b, 1998; Zhang and Zhou, 2001), high-pressure metamorphic rocks (Xiao et al., 1974; Wu, 1980, 1982, 1984, 1987; Zhang and Liou, 1987; Zhang, 1989; Wu et al., 1990, 1993; Song and Wu, 1992; Wu and Song, 1992; Zhang and Xu, 1995; Song, 1996, 1997; Zhang et al., 1997a, 1997b; Cao and Song, 2009; Cao et al., 2011), volcanic rocks (Xia et al., 1991a, 1991b, 1995a, 1995b, 1996, 2003; Wang et al., 2005), and tectonics/orogenic syntheses (Li et al., 1978; Zuo, 1986; Zuo and Liu, 1987; Liou et al., 1989; Feng and Wu, 1992; Xu et al., 1994; Feng and He, 1996; Sobel and Arnaud, 1999; Yue and Liou, 1999; Yin et al., 2007, 2008; Xiao et al., 2009; Gehrels et al., 2011). The well preserved low grade lawsonite-bearing blueschist and eclo- gite and carpholite-bearing metapelite afford convincing evidence that the North Qilian HP metamorphic belt records cold oceanic lith- osphere subduction with a low geothermal gradient (6–7 °C/km) in the Early Paleozoic (Wu et al., 1990, 1993; Song, 1997; Song et al., 2007; Zhang et al., 2007; Zhang et al., 2009). Therefore, this 1000- km-long orogenic belt has been considered as a type example for an ancient oceanic subduction zone that records the forming and shrink- ing histories of the proto-Tethyan ocean (e.g., Hou et al., 2006; Gehrels et al., 2011) from seafloor spreading and subduction to the ultimate continental collision. This belt was thought to be the final su- ture zone conjunction of two continents between segments of Rodi- nia supercontinent and North China Craton (NCC) (Song et al., 2009a).

    Furthermore, the Qilian Orogen records of the earliest “cold” oce- anic subduction zone on Earth, yet all these remains little known to the international community. The aim of this paper is to provide a comprehensive review of the geological background and spatiotem- poral constraints that allow the reconstruction of the evolution histo- ry of the Qilian Orogen in terms of the plate tectonics theory, i.e., from oceanic subduction to continental collision in the early Phanerozoic. We document with new data in this paper (1) the two distinguished belts of ophiolite genetically associated with normal ocean-ridge

    magmatism and back-arc basin magmatism, respectively, (2) island- arc or continental arc magmatic sequences associated with seafloor subduction, (3) high-pressure (HP) low-temperature (LT) metamor- phic rocks with age data and P-T-t path, (4) the Silurian flysh and Devonian molasses that indicate ocean basin closing and mountain building processes in response to the continental collision event.

    2. Tectonic settings and subunits

    The Qilian orogenic belt is part of the Qinling–Qilian–Kunlun Fold System (Li et al., 1978), or the Central China orogenic belt (Fig. 1). It is located in a joint region among the three major blocks in China, i.e., the North China Craton in the northeast, the Yangtze Craton in the southeast and the Tarim Craton in the northwest. The bulldozer- like, northward compression of the Indian Plate led the movement and rotation along the Altyn-Tagh Fault in the Cenozoic, which have re-shaped both topographic and tectonic patterns of the Qilian and adjacent regions.

    The Qilian–Qaidam region at the northern Qinghai-Tibet Plateau consists of five nearly E–W trending subparallel tectonic units (Fig. 1); from north to south, they are (1) the Alashan block, (2) the North Qilian orogenic belt (oceanic-type suture zone), (3) the Qilian block, (4) the North Qaidam co