Late Paleozoic tectonic transition from subduction to collision in the Chinese Altai and Tianshan (Central Asia): New geochronological constraints

The evolution of the largest accretionary orogen in the world, the Central Asian Orogenic Belt (CAOB), involved a prolonged accretion history since the Neoproterozoic, followed by a collisional phase in response to the closure of the Paleo-Asian Ocean in the latest Paleozoic. The exact process for the tectonic transition from subduction to collision is still poorly constrained. Here we address this issue by investigating the late Paleozoic tectonic evolution of the Chinese Altai and Tianshan orogens in the western CAOB. We provide new geochronological data from two areas of the Chinese Altai and Tianshan orogens, which allow us to link polyphase deformation with orogenic processes. In the Fuyun area of the Chinese Altai Orogen, we conducted monazite U-Pb dating on four samples that show pervasive foliations with the originally sub-horizontal orientation (DS2/DQ2). The monazite U-Pb ages cluster at ∼284 to 281 Ma, which interpreted to represent the time of sub-horizontal foliations (DS2/DQ2) that may result from orogen-parallel extension related to the collision of the Chinese Altai Orogen with the East Junggar Terrane. Farther south, in the Gangou area of the Chinese Tianshan Orogen, we obtained a muscovite 40Ar/39Ar plateau age of 256.6 ± 0.6 Ma for a mica schist from the dextral South Central Tianshan Shear Zone. This age confirms the Permian activity of dextral strike-slip deformation (DCT4) in the Chinese Tianshan Orogen. In contrast, three mylonitic schist/granitoid samples from the dextral Main Tianshan Shear Zone are characterized by 40Ar/39Ar plateau ages of 353.9 ± 1.9 Ma (biotite), 353.9 ± 1.5 Ma (biotite) and 352.1 ± 0.7 Ma (muscovite). We interpret these early Carboniferous ages to either represent a pre-Permian dextral shearing event, or to record an early Carboniferous tectono-thermal event with recrystallized micas not reset during the Permian strike-slip deformation (DCT4). An additional 40Ar/39Ar plateau age of 280.9 ± 0.5 Ma (hornblende) from a mafic dike (dolerite) that crosscuts macroscopic folds (DST2) in the southern Chinese Tianshan Orogen, provides a minimum time constraint for these folds. This age supports the simultaneous folding deformation (DST2) with dextral shearing (DCT4) in the Chinese Tianshan Orogen. Combined with a comprehensive synthesis of available geological and geochronological data, we argue that orogen-parallel extension and transpressional tectonics might have played a significant role in the late Paleozoic arc/continental amalgamation of the western CAOB.