Structural analysis and 40Ar/ 39Ar thermochronology of Proterozoic rocks in Sailimu area (NW China): Implication to polyphase tectonics of the North Chinese Tianshan

Abstract

Understanding the tectonothermal history of the Central Asian Orogenic Belt (CAOB), is of significance in revealing the amalgamation of microcontinents or continental fragments during the accretion and collisional processes of the CAOB. A Precambrian metamorphic basement of the continental block may have been involved in the accretion and collisional processes and thus recorded complete tectonothermal information that is essential in understanding the tectonic evolution of the studied area. The Precambrian Wenquan Group in the Northwest Chinese Tianshan is commonly thought as the basement of the Yili Block. Here we try to reveal the tectonothermal history of this basement by detailed structural analyses and associated Ar–Ar datings. Four main events are recognized, the first one (D1) corresponds to ductile deformation characterized by NNW–SSE stretching lineation and top-to-the south sense of shear. 40Ar/ 39Ar datings on amphibole from an amphibolite and on muscovite from an orthogneiss and a migmatite constrain the age of the D1 event as ∼443–413 Ma. The second event (D2) is represented by north-verging folding that variably overprinted the D1 structures. Muscovite on D2 foliation in a micaschist yielded a 40Ar/ 39Ar intercept age of 338 ± 4 Ma providing an oldest limit for the D2 event. The D3 event is marked by localized strike-slip ductile shearing producing sub-E–W trending steep foliation with shallow lineation, occurring at around 289 ± 12 Ma. Finally, the D4 event is widespread brittle thrusting and open-upright folding. The Proterozoic rocks in the Wenquan Group underwent and recorded main tectono-thermal events relating to the Paleozoic orogenic processes of the CAOB. The D1 event corresponds to the Early Paleozoic continent-arc accretion forming the Kazakhstan–Yili microcontinent (North Tianshan), the D2 event could be correlated to the Late Paleozoic southward subduction of Junggar–Balkash Ocean beneath the Kazakhstan–Yili block and final amalgamation of the Tianshan belt, and the D3 event reflects the Permian post-collisional transcurrent tectonics in the North Chinese Tianshan.