Neoproterozoic tectonic transition from subduction-related convergence to continental extension of the Tarim Block, NW China

Abstract

The Tarim Block, one of the three largest cratons in China, plays an important role in the reconstruction of the Rodinia supercontinent. Knowledge of the Neoproterozoic tectonic evolution of the Tarim Block, especially the tectonic transition from convergence to rifting, remains unclear. The Aksu terrane, an integral part of the Tarim Block, is marked by the occurrence of Neoproterozoic blueschists, conglomerates, and mafic rocks that may provide important constraints on its tectonic history. We present an integrated study involving detailed field observations, whole-rock geochemistry, and zircon U-Pb geochronology for rhyolitic clasts in a Neoproterozoic conglomerate, a syenite pluton and mafic rocks in the Aksu region. Zircon U-Pb dating of the rhyolite yielded a crystallization age of 840 ± 4 Ma. Geochemically, the rhyolite is characterized by enrichment of light rare earth elements and depletion of high field strength elements, indicating a subduction-related arc setting. Syenites interspersed with mafic intrusions were crystallized at ~755 Ma, representing fractional crystallization products from intra-plate mantle-derived basaltic magma. Based on a compilation of previous studies, we find that the Late Neoproterozoic–Early Cambrian mafic rocks in the Aksu region can be divided into three phases. Phase I mafic intrusions (~760–745 Ma) occurring as dikes intruded into the blueschist-bearing Aksu Group. Phase II mafic intrusions (~755 Ma) intruded into the Neoproterozoic Qiaoenbulake Formation. Phase III mafic rocks (~520 Ma) are sills or basalts hosted within the Sugaitebulake Formation. Detailed studies suggest that subduction of oceanic lithosphere in the Aksu area persisted until at least ~840 Ma. The transition from ocean-continent subduction to continental extension took place around ca. 760 Ma. At 755 Ma, the northwestern Tarim region experienced significant extension.