中国东南部早侏罗世岩浆作用与地球动力学意义研究

During the late Mesozoic times, extensive and intensive magmatism took place in southeast China. However, its geodynamic setting and tectonic regime remains controversial. It is generally accepted that the Early Jurassic represents a magmatically inactive period, which corresponds to the tectonic regime change from the Tethyan tectonic domain to the Pacific tectonic domain. However, a series of the Early Jurassic igneous rocks have been reported in southeast China with recent precise chronology being carried out.In this work, we plan to investigate a detailed SHRIMP zircon U-Pb chronology, mineral chemistry, and major, trace elemental, and Sr-Nd-Pb-O-Hf isotopic geochemistry for the Early Jurassic basalt-andesite-rhyolite and gabbro (diorite)-granodiorite-granite as well as trachybasalt-shoshonite-latite-trachyte-syenite in southeast China, in an attempt to better constrain both the petrogenetic processes and their significance in understanding the geodynamic setting of late Mesozoic tectonics and magmatism in southeast China. These new data allow us to explore the following topics of international interest. (1) What is the nature of the source of the magmas which formed basaltic and granitic as well as potassic rocks? (2) In what kind of tectonic settings and geodynamics were the basaltic and granitic as well as potassic rocks formed? (3) Were they formed associated with the subduction of the Palaeo-Pacific plate? If yes,when did the Palaeo-Pacific plate subduct

中国东南部在晚中生代时期发生了广泛而强烈的岩浆作用，但关于其形成的地球动力学机制一直争论不休。前人通常认为早侏罗世是岩浆活动相对平静期，它代表了特提斯构造域向太平洋构造域的转变期。随着近几年较高精度年代学的开展，在中国东南部相继鉴别出了一系列早侏罗世岩浆活动。本项目拟在申请者已有工作基础上对该区早侏罗世玄武岩-安山岩-流纹岩和辉长岩（闪长岩）-花岗闪长岩-花岗岩以及粗面玄武岩-橄榄玄武粗安岩-安粗岩-粗面岩-正长岩开展综合性研究，采用SHRIMP锆石U-Pb定年以及矿物化学、元素地球化学和Sr-Nd-Pb-O-Hf多元同位素联合示踪的方法，阐明它们的源区和岩浆演化过程，探讨其形成的构造环境和地球动力学机制，从而为国内外众多地质学家长期关注的中国东南部晚中生代大规模岩浆作用机理，特别是对其是否与古太平洋板块俯冲有关、古太平洋板块究竟什么时候开始俯冲这样一些重大科学问题的认识作出重要贡献。

中国东南部是环太平洋构造成矿带的重要组成部分，以晚中生代大规模岩浆作用和成矿作用为特征。目前通常认为岩浆作用与古太平洋板块俯冲相关联，并提出了一系列构造演化模式，包括正常俯冲模式、平缓俯冲模式和平板俯冲模式等。前人通常认为早侏罗世是该区岩浆活动宁静期，代表古特提斯构造域向太平洋构造域的转换时期。本次研究在赣南和闽西北鉴别出了一系列早侏罗世镁铁质岩石和A型花岗岩，其中A型花岗岩呈北东向展布，构成一条早侏罗世A型花岗岩带。结合前期研究成果，发现中国东南部还存在另外3条A型花岗岩带，分别为晚侏罗世A型花岗岩带、早白垩世A型花岗岩带和晚白垩世A型花岗岩带，它们与早侏罗世A型花岗岩带一样均呈北东向展布，平行于现今海岸线，其中晚侏罗世A型花岗岩带分布于更靠内陆一侧，早白垩世A型花岗岩带几乎与晚侏罗世A型花岗岩带重叠，晚白垩世A型花岗岩带则分布于沿海地区。通过系统的镁铁质和花岗质岩石成因研究，认为中国东南部晚中生代构造-岩浆活动可用古太平洋板块的反复俯冲-后退模式来解释：古太平洋板块在早侏罗世辛涅缪尔期（~197 Ma）已俯冲至赣南，随后（~197-191 Ma）板片折返，并在大约191-188 Ma板片发生断离。紧随其后俯冲板片回弹，重新发生前进式俯冲，并在大约178 Ma俯冲至湘南，在大约174 Ma板片重新开始折返，俯冲板片由内陆逐步向沿海地区后退。这一古太平洋板块反复俯冲-后退模式对于进一步理解中国东南部晚中生代大规模岩浆作用和成矿作用的深部动力学机制具有重要意义。本次研究还首次在区内鉴别出晚三叠世卡尼期（~228 Ma）镁铁质岩，并通过岩石成因研究，认为其侵位于华南印支碰撞造山作用伸展期，从而将华南古特提斯构造域向太平洋构造域的转换期限较精确地限定在晚三叠世瑞提期。此外，在项目研究过程中，对疑是早侏罗世岩体开展较精确的年代学研究后发现，有一些镁铁质和花岗质岩体形成于早古生代和新元古代，通过岩石成因和动力学机制研究，为华南早古生代和新元古代构造演化提供了新的制约。