青藏高原南部麦嘎地区超钾质火山岩成因与地幔交代作用

The mantle-derived ultrapotassic magmatism in Tibetan plateau provides a prime opportunity to probe into the nature of mantle lithosphere, to investigate the deep carbon recycling processes, and to reveal the geodynamic mechanism responsible for plateau uplift. This project, which is based on our previous research, will present a systematic study on the high-MgO and low-SiO2 ultrapotassic rocks from Maiga area, southern Tibet. Zircon U-Pb dating and in situ Hf-O isotopic analyses will improve our understanding of the temporal and spatial distributions of postcollisional magmatism in Lhasa terrane, and provide reliable constraints on the influence of crustal contamination on the evolution of ultrapotassic magma. A combination of petrography and mineral in situ analysis will allow us to elucidate the magmatic evolution, to calculate the pressure and temperature of magmatic generation, and to estimate the properties of primary ultrapotassic magma. With the aid of Sr-Nd-Ca-Mg isotopic analyses, we will be able to clarify the mantle metasomatism occurred beneath south Tibet during the obliteration of Neo-Tethyan ocean and subsequent subduction of Indian continental lithosphere. In summary, this project will be able to provide new petrological and geochemical constraints on the geodynamic processes occurred beneath Tibetan plateau in the Miocene.

青藏高原幔源超钾质岩浆作用的研究对于揭示岩石圈地幔物质组成、探索地球深部碳循环过程、反演与高原隆升相关的深部动力学过程具有重要意义。本项目在前期研究的基础上，聚焦于青藏高原南部麦嘎地区具有高镁、低硅特征的碰撞后超钾质岩浆记录。通过锆石U-Pb定年和原位Hf-O同位素分析，完善拉萨地块碰撞后岩浆活动的时空格架，约束地壳混染作用对超钾质岩石地球化学组成的影响。把镜下观察和矿物的原位微区分析结合起来，阐明超钾质岩浆的演化过程，估算岩浆形成的温度和压力，恢复原始岩浆的组成。在此基础上，应用Sr-Nd-Ca-Mg同位素详细刻画新特提斯洋壳和印度大陆地壳物质在俯冲过程中对藏南岩石圈地幔的交代改造过程。本项目成果可以为构筑中新世高原深部动力学过程提供新的岩石学和地球化学依据。

青藏高原幔源超钾质火山岩和同时代幔源碱性岩浆岩为探讨高原深部地幔物质组成和揭示高原隆升和侧向生长过程提供了重要窗口。本项目选择麦噶中新世超钾质火山岩和同时代幔源岩石为研究对象，开展了细致的野外调查采样工作，和系统的矿物学、岩石学和主量-微量元素地球化学分析、锆石高精度U-Pb定年和Hf-O同位素分析、全岩Sr-Nd-Pb同位素和Ca-Mg同位素分析等室内综合综合分析测试工作。结果表明：（1）确定了青藏高原麦噶超钾质火山岩（15-17Ma）的形成时代、地球化学特征和岩石成因，提出这种最富镁的碰撞后富钾岩浆岩的地幔源区经历过碳酸盐交代作用，为揭示藏南岩石圈地幔演化提供了新证据；（2）准确厘定了青藏高原东北缘西秦岭黄长岩的形成时代并且阐明其岩石成因，深入刻画了古特提斯洋含碳酸盐沉积物的俯冲交代对高原深部地幔的改造过程；（3）通过对藏东南新生代幔源碱性岩浆岩进行系统的Mg同位素分析，揭示碳酸盐交代和高原东南缘的深部地幔不均一性；（4）探讨了希腊Lesvos岛钾玄质火山岩的成岩时代、岩浆源区性质和演化过程，为深入探讨地中海东部地区幔源碱性岩浆作用提供了重要参考。上述研究成果不仅完善了青藏高原碰撞后幔源碱性岩浆作用的岩石成因和时空格架，对揭示碰撞后阶段高原地幔演化和动力学机制提供了重要依据。