The Research Of Geochemistry And U-Pb Detrital Zircon Geochronology Of Neoproterozoic Glaciogenic Sediments In Northern Xinjiang, NW China

While the early Neoproterozoic Era witnessed the breakup of Rodinia, the assembly of the mega-continent Gondwana heralded the end of the era. The evolving palaeogeography likely played a key role in the occurrence of a series of major geological events during and after that time, including global glaciations, deposition of unique cap carbonates and intimately related negative 613C anomaly, re-appearance of banded iron formation and the rapid evolution of complex life on Earth. The Snowball Earth hypothesis brought by Kirschvink and improved by Hoffman provides a framework for understanding these events, but many questions remain as to the nature of the glaciations. In order to research the related glaciogenic sediments in northern Xinjiang, NW China, we sampled sections in Guozigou arean (Northwest Tianshan Mountain) and Aksu area (Southwest Tianshan Mountain) and made elements geochemistry and U-Pb detrital zircon geochronology researches, which could indicates the time of the Neoproterozoic glaciations, the tectonic setting of the sediments, the paleoweathering conditions of the sediments, the rock sources of the sediments and the geological tectonic evolution.Combining with the previous researches, our geochemical data and our detrital zircon U-Pb data, we concluded that:(1) There was an Ediacaran glaciation (630-542 Ma) in Guozigou area, and a pre-Sturtian glaciation (before 726 Ma) and a Stutian glaciation (726-660 Ma) glaciation in Aksu area.(2) The chemical index of alternation (CIA) of the related sediments indicates that the glaciations deposited under a dry and cold paleoenvironment and experienced relatively lower paleoweathering conditions, while the inter(pre/post)-glaciations deposited under a wet and warm paleoenvironment and experienced relatively stronger paleoweathering conditions.(3) The geochemical data shows that the tectonic setting developed from continental arc (active continental margin) in Cryogenian to passive continental margin in Ediacaran in Aksu area. While the tectonic setting developed from passive continental margin in the beginning of Ediacaran to active continental margin in the end of Ediacaran.(4) The geochemical data suggests that the rock sources of Guozigou related sediments were affected by only felsic rocks, while the rock sources of Aksu related sediments were affected by felsic rocks and little mafic rocks.(5) The Aksu Group blueschist is coeval with the Qiaoenbrak Formation, in contrast with previous models for the tectonic evolution of the terrane. And the Aksu terrane should be an independent microcontinent and collided with Tarim Block between 700 Ma and 650 Ma.(6) The Guozigou area experienced a magma event between 650-640 Ma, which were not proved by any volcanic rocks in geology.