江南造山带西段梵净山新元古代玄武岩岩浆演化与构造背景研究

Subject to the assimilation and contamination of the lithosphere mantle and crust, continental flood basalts often show the characteristics of trace element similar to those of island arc basalts, thus affecting the identification of tectonic setting. The latest research shows that there are huge differences between the continental flood and ocean island basalts caused by mantle plume and the island arc basalts caused by subduction, in terms of mantle potential temperature in the source region, water content of primitive magma, and major and trace elements and isotopic composition. Because of uncertain melting conditions, characteristics and process of the basaltic magma, there is still great controversy (Caused by mantle plume? Or subduction?...) over the tectonic setting of the Neoproterozoic igneous rocks in the Jiangnan orogen, severely restricting the researches on the tectonic evolution of South China continent. Taking the Neoproterozoic basalts in Fanjing Mountain region as the object, this study will investigate the melting conditions, characteristics and process of magma to reveal the magmatic evolution, by adopting analysis of major and trace elements and Sr-Nd isotopes, electron microprobe of phenocrysts including olivine, Cr-spinel, plagioclase, Fourier transform infrared spectroscopy of clinopyroxene phenocrysts, combined with a variety of numerical simulation methods. Through the mantle potential temperature, water content of primitive magma, and geochemical composition, the tectonic setting can be comprehensively identified, thereby contributing to the researches on the tectonic evolution in the Jiangnan orogen and South China continent.

大陆溢流玄武岩常受到岩石圈地幔和地壳同化混染作用，出现类似于岛弧玄武岩的微量元素特征，从而影响构造背景判别。最新的研究表明，地幔柱成因的大陆溢流玄武岩和洋岛玄武岩与俯冲成因的岛弧玄武岩，在源区地幔潜在温度、原始岩浆水含量、主微量元素和同位素组成等方面均存在巨大差异。江南造山带新元古代岩浆岩，由于岩浆形成条件、特征及过程不明，构造背景仍存在较大争议（地幔柱成因？俯冲成因？...），严重制约了华南大陆构造演化研究。本项目将以江南造山带西段梵净山地区新元古代玄武岩为研究对象，采用全岩主微量元素、Sr-Nd同位素分析、橄榄石、铬尖晶石、斜长石斑晶电子探针分析、单斜辉石斑晶傅里叶变换红外光谱分析、结合多种计算模拟方法，探究玄武岩浆形成条件、特征及过程，并通过其源区地幔潜在温度、原始岩浆水含量和地球化学成分，综合判断其大地构造背景，促进江南造山带及华南大陆构造演化研究。

新元古代的地幔热状态以及地球动力学对于探究罗迪尼亚超大陆演化十分关键。作为罗迪尼亚超大陆的组成部分，扬子板块在新元古代的地幔热状态以及构造背景还不明朗。本次研究以江南造山带拉伸纪中期玄武质岩浆为对象，通过建立系统的全岩主微量和Sr-Nd同位素数据库，探究岩浆演化、计算原始岩浆成分、估算地幔潜能温度以及查明岩石形成构造背景。研究表明，在拉伸纪中期地层（梵净山群及其相当层位）之中，广泛发育845~825 Ma的钙碱性苦橄岩-玄武岩-玄武安山岩（A类）以及860~830 Ma拉斑玄武岩（B类）两类岩石。地球化学和同位素特征表明A类玄武岩形成于富集地幔。橄榄石和单斜辉石的分离结晶、俯冲板片脱水流体对岩浆演化具有重要影响。A类岩石的原始岩浆为苦橄质，其地幔潜能温度为1451℃。该温度与受俯冲板片回撤作用影响的地幔楔温度相似，但显著低于（119℃）受地幔柱影响的热地幔。鉴于江南造山带同时出现具有岛弧地化特点的A类玄武质岩石与具有弧后盆地地化特点的B类玄武岩、以及与玄武岩同时期的碎屑岩具有双向物源的证据，本研究提出A类玄武岩形成于后弧（rear arc）环境、B类玄武岩形成于弧后（back arc）环境，进而提出罗迪尼亚超大陆的边部在拉伸纪中期发育了一个复杂的俯冲相关弧盆体系的认识。