Geochemistry and U-Pb-Hf detrital zircon geochronology of metamorphic rocks in terranes of the West Kunlun Orogen: Protracted subduction in the northernmost Proto-Tethys Ocean
Introduction
The Kunlun Orogenic Belt (KOB) marks an important collisional junction between the India-Eurasian and Tethyan tectonic domains (Fig. 1; e.g. Xiao et al., 2005, Dong et al., 2018 and references therein), and is the least-known portion of the western Central China Orogenic System. The belt incorporates several complex terranes (including the North and South Kunlun) that have undergone a polyphase tectonic history associated with the opening and closure of the Proto- and Paleo-Tethys oceans (Mattern and Schneider, 2000, Xiao et al., 2002, Xiao et al., 2005, Jiang et al., 2013, Dong et al., 2018, Zhang et al., 2019a). The West Kunlun terrane likely collided with the southern margin of Asia during the closure of the Proto-Tethys Ocean in the Early Paleozoic, as inferred from the Cambrian Kudi-Qimanyute suture (Dong et al., 2018, Hu et al., 2016, Mattern and Schneider, 2000, Pan et al., 1996, Xiao et al., 2002, Yin and Harrison, 2000), Early Paleozoic metamorphism, and the existence of late Devonian molasse lying unconformably on the metamorphosed Pre-Devonian sequences (Pan et al., 1996, Jia, 1997, Jiang et al., 2002, Hu et al., 2016, Dong et al., 2018, Zhang et al., 2018a).
Due to the sparse availability of data, diverse tectonic models have been proposed for the Paleozoic evolution of the KOB, its interpretation as a subduction–accretion complex (Yuan et al., 2002, Xiao et al., 2002, Xiao et al., 2005, Zhang et al., 2007, Jiang et al., 2013, Liu et al., 2015, Zhang et al., 2017, Dong et al., 2018), collisional orogen (Zhang et al., 2018c, Zhang et al., 2019b, Wang et al., 2020) or an island arc (Zhu et al., 2016, Zhang et al., 2020). Most studies advocate a southward collisional orogen between the North Kunlun and South Kunlun terranes that led to the closure at ca. 460–400 Ma of the Proto-Tethys Ocean and the amalgamation of the West Kunlun Orogenic Belt (WKOB) with the northern margin of Gondwana (Yuan et al., 2002, Liu et al., 2014, Li et al., 2018, Zhang et al., 2018c, Zhang et al., 2019a, Wang et al., 2019). The final closure of the Proto-Tethys Ocean is constrained at 440–420 Ma in the WKOB (Zhang et al., 2018c, Zhang et al., 2019b, Wang et al., 2020). However, other studies have suggested northward subduction in the EKOB (Dong et al., 2018, Li et al., 2019) and a protracted subduction process involving multiple oceans lasting until 214 Ma (Xiao et al., 2005). There are also controversial proposals involving “soft” and “hard” collisional orogeny in the WKOB (Wang et al., 2020 and references therein). These proposals are linked to sparse subduction-related magmatic rocks with ages greater than 460 Ma. The ambiguous metamorphic basement underlying the Devonian has also hampered study in the KOB. The nature and relationship of these pre-Devonian fragments remain poorly understood, and their subduction history prior to the closure of Proto-Tethys Ocean is uncertain.
Given that the U-Pb-Hf isotopic compositions of detrital zircons together with their whole-rock geochemistry are powerful tools for deciphering the nature of sedimentary provenance and tectonic setting (e.g., McLennan et al., 1993, Griffin et al., 2006, Dickinson and Gehrels, 2009, Cawood et al., 2012), we present LA-ICP-MS U-Pb ages and Lu-Hf isotopic data from detrital zircons from the top of the pre-Devonian metamorphic basement in the WKOB. Combined with whole-rock major and trace element data and regional considerations (Fig. 1b), we discuss the tectonic setting of the WKOB and its implications for a protracted subduction system that records the opening and closure of the Proto-Tethys Ocean.
Section snippets
Geologic background
The Kunlun Orogenic Belt is divided into the East Kunlun Orogenic Belt (EKOB) and the WKOB by the Altun strike-slip fault zone (Dong et al., 2018). The WKOB is generally divided into the North Kunlun, South Kunlun, and Tianshuihai-Taxkorgan terranes separated by the Oytag–Kudi–Qimanyute and Mazha–Kangxiwar sutures, respectively (Fig. 1b; Pan et al., 1996, Jiang et al., 2002, Zhang et al., 2007, Hu et al., 2016). The North Kunlun terrane is generally regarded to be part of the Tarim Craton (e.g.
Sampling and field observations
We collected four metasedimentary samples from the Bulunkuole Group in the Taxkorgan terrane (TS91, TS92, TS82 and TS83), one metasedimentary sample from the basement of the western South Kunlun terrane (KH1), and three metasedimentary samples from the basement of the central South Kunlun terrane (XZ701, XZ024 and XZ017) for zircon geochronology (Fig. 1). In addition, a granite sample (XZ503) was dated from one of the basement-intruding plutons in the central South Kunlun terrane. Geochemical
Analytical methods
Zircon grains were separated by conventional magnetic and density techniques. The grains were then mounted in epoxy resin and polished for analysis. All the grains were documented with transmitted and reflected light micrographs as well as cathodoluminescence (CL) images to reveal their internal structures and to select analytical spots (Fig. 5). Zircon U-Pb analysis was performed using the LA-ICP-MS method at Nanjing FocuMS Technology Co. Ltd. Australian Scientific Instruments RESOlution LR
General features of the detrital zircon grains
Except for a few metamorphic zircons, the detrital zircons share similar characteristics and are colorless and transparent with length/width ratios of 1–3 (Fig. 5). The zircon crystals generally have euhedral to subhedral shape with clear oscillatory zoning suggesting a local or near-source region magmatic origin. Most grains possess relatively high Th/U ratios (range: 0.3–2) typical of an igneous origin (Fig. 6). A few grains display rounded morphology. Some of the zircon crystals show
Depositional ages of the metamorphosed rocks
The Bulunkuole Group is generally interpreted to be a BIF-bearing Paleoproterozoic deposit, but its depositional age is not well defined (e.g. Qiao et al., 2015, Zhang et al., 2018a). Recent studies report Late Neoproterozoic to Cambrian zircon U-Pb ages from this group, including ca. 521.3 Ma bimodal volcanic rocks and ca. 537.2 Ma metaigneous rocks (biotite plagiogneiss) from the iron deposits (Gao et al., 2013, Zheng et al., 2016), and 700–515 Ma detrital zircon U-Pb ages from the
Conclusions
- (1)
Detrital zircons ages reveal that part of the uppermost basement in the WKOB is Early Paleozoic.
- (2)
Early Paleozoic sedimentary units in the WKOB were probably sourced from Kunlun terranes and blocks of Gondwanan affinity rather than from the Tarim Craton.
- (3)
Protracted retreating subduction during the Neoproterozoic and advancing subduction in the Early Paleozoic took place in the WKOB.
- (4)
Progressively advancing subduction at 540–420 Ma with a “soft” collision between the Tarim Craton and southern WKOB
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors are grateful to the reviewers for their constructive comments and suggestions that substantially improved the manuscript. We thank M. Santosh for his constructive comments. We also thank Zhibin Huang, Bin Wang and Zhilin Yang for their help in the field trips. This study has been partly supported by the National Natural Science Foundation of China (91955204), the second Tibetan Plateau Scientific Expedition and Research Program of China (2019QZKK080301) and Science and Technology
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