新疆西准噶尔紫苏花岗岩的岩石成因和构造背景研究：对古亚洲洋最终闭合时限的制约

When the Central Asian Orogenic Belt (CAOB) was finally closed is a recent hot debate, which focuses on whether the ocean existed in the late Carboniferous-early Permian (C3-P1). West Junggar, located in the central part of the CAOB, hosts voluminous C3-P1 igneous rocks, but their tectonic setting still remains controversial. One group suggests that no ocean existed and West Junggar was in a post-collisional setting during the C3-P1 period, while the other group argues that the ocean still existed, and ridge subduction was operative during the C3-P1 period. Since post-collision and ridge subduction undergo under different conditions in terms of temperature, pressure and oxygen fugacity, the tectonic setting can be determined once these parameters are constrained. The Miaoergou charnockite contains orthopyroxene, clinopyroxene, fayalite, plagioclase and quartz. In plagioclase and quartz, CO2 inclusions occur. Through EPMA and Laser Raman microspectroscopy, this study is to obtain the compositions of minerals and inclusions, in order to constrain the petrogenesis of the charnockite. Combined with zircon U-Pb and Lu-Hf analysis as well as studies of whole rock geochemistry, the tectonic setting of charnockite will be illustrated. This study will not only provide constraints on the tectonic framework of West Junggar, but will also provide important insights into understanding when the CAOB finally closed.

古亚洲洋的最终闭合时限是近年来争论的一个热点，其焦点是晚石炭-早二叠世洋盆是否存在。西准噶尔位于中亚造山带中部，晚石炭-早二叠世岩浆岩发育。前人对这些岩浆岩的形成背景有不同的认识。一种观点认为晚石炭-早二叠世洋盆已消失，为后碰撞环境，而另一种观点则认为那时还存在洋盆，为洋脊俯冲环境。由于后碰撞和洋脊俯冲具有不同的温度、压力、氧逸度等特征，所以查明晚石炭-早二叠世岩浆岩形成时的上述参数，能为揭示当时的构造环境提供重要依据。西准噶尔庙尔沟紫苏花岗岩发育斜方辉石、单斜辉石、铁橄榄石、长石、石英等矿物；其中，石英和长石中含有CO2包裹体。通过矿物成分分析以及流体包裹分析，可以探测紫苏花岗岩形成的温度、压力、氧逸度等，阐明其岩石成因。结合锆石U-Pb和Lu-Hf同位素、全岩地球化学研究，讨论紫苏花岗岩的产出背景。本研究不但能进一步限定西准噶尔的构造格架，而且能为古亚洲洋的最终闭合时限提供重要依据。

本课题以西准噶尔庙尔沟紫苏花岗岩为研究对象，详细的岩石地球化学、锆石U-Pb、Lu-Hf同位素研究表明，西准噶尔庙尔沟紫苏花岗岩起源于热的（~800℃）、年轻的下地壳，其构造环境为洋脊俯冲。该研究表明晚石炭世西准噶尔仍处于俯冲环境，因此，本课题的研究成果揭示古亚洲洋的最终闭合时间应晚于晚石炭世。