保山地体卧牛寺组玄武质岩石年代学、成因及其地球动力学意义研究

The tectonic mechanism responsible for detachment of the Sibumasu terrane from the Gondwana margin Early Permain remains poorly understood and debated. The detachment of the terranes is usually accompanied by the basaltic magmatism. The coeval basaltic rocks of the Woniusi formation took place in the Baoshan terrane, north Sibumasu terrane. Studies on the chronology and geochemistry of these rocks may shed new lights into the problem. Based on the field geological investigation and profile survey, this project plans to provide systematic studies on the baddeleyite or zircon U-Pb dating, Ar-Ar dating of whole rock or minerals, the major element analyses of the olivines and their melt inclusion, whole-rock geochemical analyses as well as Sr-Nd isotopic measurement of the basaltic rocks. We aim to precisely determine their age and geochemical characteristics, to explore their time and space distribution, to determine the nature of their mantle source and primary magma and to inquire into their geodynamic setting. All these results aim to provide more constraints on the tectonic mechanism responsible for detachment of the Sibumasu terrane.

Sibumasu地体（滇缅泰马地体）在早二叠世时从冈瓦纳超大陆北缘裂离的地球动力学机制目前仍存在着不同的观点。Sibumasu地体北部的次级地体（保山地体）中分布着大量与裂离同时期的卧牛寺组玄武质岩石。对这些岩石开展年代学与成因研究，将有助于限定Sibumasu地体裂离的地球动力学机制。本次研究拟选择卧牛寺组玄武质岩石为研究对象，开展系统的野外地质调查与剖面实测。拟选取代表性岩石样品开展LA-ICM-MS斜锆石或SIMS锆石U-Pb定年、全岩或单矿物Ar-Ar定年、橄榄石及其熔体包裹体主量元素分析、全岩主微量元素与Sr-Nd同位素分析。在此基础上，本次研究将精确厘定卧牛寺组玄武质岩石的区域年代学格架与喷发时限，查明这些岩石的地球化学特征及其时空分布格局，阐明它们的地幔源区特征与原始岩浆性质，探讨其形成的构造环境。以期为制约Sibumasu地体裂离的地球动力学机制提供新的关键信息。

Sibumasu地体在早二叠世从冈瓦纳超大陆(澳大利亚板块)的西北缘裂离的地球动力学机制存在不同观点。在本项目中，我们对保山地体六库卧牛寺组玄武质岩石开展了系统的野外地质调查与剖面实测，对代表性岩石样品开展了岩石学和岩石地球化学研究以揭示其岩石成因，为制约Sibumasu地体裂离的地球动力学机制提供新的关键信息。研究表明，六库卧牛寺组玄武质岩石主要由致密块状玄武岩和少量辉绿岩组成，这些岩石具有较低的TiO2含量（0.83-2.22 wt%）。在TAS图上，这些样品投点位于玄武岩与玄武岩-安山岩的范围内，在Zr/Ti-Nb/Y 二元图解上,它们的投点主要位于亚碱性与碱性的玄武岩范围内。这些样品均具有相对富集的轻稀土元素含量，表现出微弱的Eu负异常或正异常，呈现出介于OIB与E-MORB的配分曲线特征。在原始地幔标准化的蛛网图解上，这些样品表现出较为明显的Nb-Ta亏损，而Pb则表现出正异常。它们的(87Sr/86Sri)i值分布于0.705088-0.710877之间，而εNd(t)为-9.20至-2.04之间。详尽的全岩主微量元素与Sr-Nd同位素地球化学研究表明，包括六库卧牛寺组玄武质岩石在内的保山地体的卧牛寺组玄武质岩石是起源于地幔柱的岩浆遭受不同程度的地壳混染的产物。地幔柱是导致Sibumasu地体裂离的地球动力学原因。