Paleozoic tectonic evolution of the Yili Block, western Chinese Tianshan

Abstract

The Yili block is a triangular area bordered by sutures and fault zones in the western Chinese Tianshan belt. It is often considered as a part of the Central Tianshan micro-continent with Proterozoic basement extending westward into Kazakhstan and Kirgizstan, but this interpretation is questionable. This paper aims to synthesize the available data, discusses the meaning of the tectonic boundaries and proposes a model for the Paleozoic evolution of the Yili block. Alike the entire Tianshan belt, the Yili block underwent a polyphase evolution including subduction of oceanic crust and collision with micro-continents and volcanic arcs. The southern boundary of the Yili block is formed of Proterozoic basement and Early Paleozoic platform sediments, tectonically overlain by oceanic high-pressure metamorphic rocks and ophiolite. It has been involved in a south-dipping subduction associated with the closure of the Tianshan Ocean and the subsequent collision with a micro-continent correlated with Central Tianshan. This tectonic event resulted in top-to-the-north ductile thrusting observed in oceanic HP metamorphic rocks and Proterozoic basement as well. During the Late Paleozoic, the northern boundary of the Yili block was an active continental margin related to the southward subduction of the North Tianshan oceanic basin, this boundary is represented by Late Carboniferous turbidite and ophiolitic mélange. The southern and northern boundaries have been both reworked by Permian strike-slip faults.

Geochemistry, zircon U–Pb ages and Lu–Hf isotopes of early Paleozoic plutons in the northwestern Chinese Tianshan: Petrogenesis and geological implications

“…These assemblages mainly consist of Nb-enriched basalts and adakitic rocks and show typical characteristics of magmas formed in a subduction zone, therefore demonstrating that southward subduction of the Junggar Ocean most likely began during the late Paleozoic (Long et al, 2011a;Tang et al, 2010a;Wang et al, 2006aWang et al, , 2007bWindley et al, 1990;Xiao et al, 2004aXiao et al, , 2008Xiao et al, , 2010Zhang et al, 2009Zhang et al, , 2012Zhu et al, 2012a). This is also supported by extensively exposed late Paleozoic accretionary complexes in the northern part of the Yili Block (Gao et al, 1997;Wang et al, 2008;XBGMR, 1993;Xiao et al, 2010Xiao et al, , 2013. In the middle Ordovician, however, calc-alkaline volcanic rocks interlayered with the marine strata indicate an active margin tectonic setting and reveal that the middle Ordovician is a key period for the southern PaleoAsian Ocean when extension (ocean opening) changed to convergence (ocean closure) (Gao et al, 1997;XBGMR, 1993).…”

“…Silurian arc-related calc-alkaline volcanic rocks, volcanic-sedimentary rocks and flysch are also observed in the northern part of the KYB and in the central part of the NMT (Gao et al, 1997;Wang et al, 2008;XBGMR, 1993). Late Paleozoic to Mesozoic strata are widely distributed in the northwestern Tianshan, and most have Devonian to Carboniferous ages.…”

“…1). In this area, Precambrian basement rocks occur extensively and mainly consist of granitic gneiss, amphibolite, migmatite, marble, schist, quartzite, limestone and slate (Chen et al, 1999a;Hu et al, 2000Hu et al, , 2008Hu et al, , 2010Wang et al, 2008;XBGMR, 1993). Early Paleozoic supracrustal rocks are absent, but late Paleozoic strata including sandstones, limestones and conglomerates are mostly exposed in the southern part.…”

“…In the northwestern Chinese Tianshan, Cambrian to early Ordovician strata consist of marine limestone, shale, siltstone and sandstone and are typical sedimentary rocks deposited on a passive continental margin (Gao et al, 1997;Wang et al, 2008;XBGMR, 1993;Zhu et al, 2004). Voluminous late Paleozoic arc-related volcanic and intrusive rocks were discovered in the northern part of the Yili Block Long, 2007a;Long et al, 2011a;Tang et al, 2010a;Wang et al, 2006aWang et al, ,b, 2007aZhai et al, 2006;Zhang et al, 2008Zhang et al, , 2009Zhang et al, , 2012, e.g., the Aktau-Wenquan volcanic rocks (320 Ma), Lamasu granitoids (366 Ma), Kekesai plutons (288-305 Ma).…”

Paleozoic tectonics of the southern Chinese Tianshan: Insights from structural, chronological and geochemical studies of the Heiyingshan ophiolitic mélange (NW China)

“…The first one is the age of the orogeny which some authors considered as to be late Paleozoic (e.g., Gao et al, 1998Gao et al, , 2009Wang et al, 2010) but, alternatively, others ague for Triassic (e.g., Zhang et al, 2007;Xiao et al, 2008). The second issue concerns the polarity of subduction of the paleo-Tianshan Ocean, a north-directed subduction was proposed by Windley et al (1990), Xiao et al (2004Xiao et al ( , 2008, Gao et al (1998) and Gao and Klemd (2003); whereas, a southward subdution is more likely consistent with newly published data (Charvet et al, 2007;Wang et al, 2008;Lin et al, 2009); and a compromise of bi-directional subduction was suggested by Gao et al (2009).…”