喜马拉雅造山带地壳物质部分熔融中放射性同位素行为的研究

Geologists have long been puzzled by two intriguing questions on crustal anatexis: (1) how do important accessory minerals (such as zircon, garnet) affect the geochemical natures of crustally derived melts? (2) whether are there radiogenic isotope (such as Sr, Nd, Hf and Pb) disequilibrium during crustal anatexis? The Himalayan Cenozoic leucogranites are the typical product of crustal anatexis and are characterized by large magnitude of heterogeneities in element as well as radiogenic isotope compositions. These young leucogranites provide an important chance to answer the above questions. In this project, we will focus on the Miocene granites and metamorphic wall-rocks from Malashan-Gyirong, Yardoi-Cona and Everest area, and systematically determine their element and radiogenic isotope (especially Pb and Hf) compositions. These new data will used to investigate the geochemical effects of differential dissolution of garnet, zircon, monazite, and apatite on the element and isotope compositions of melts and the magnitude of radiogenic isotope disequilibrium in the Himalayan leucogranites. Further, combined with experiment data, we will provide theoretical models that combining melting reactions with dissolution of accessory minerals to constrain how dissolution of accessory phases affect the element as well as radiogenic isotope nature of anatectic melts. New results and knowledge achieved from this project will promote our understanding on the distribution process of element and isotope during partial melting of middle-lower crustal materials in orogenic belts, and on the mechanism for the development of radiogenic isotope disequilibrium during crustal anatexis.

在地壳深熔作用研究中，存在两个困惑地球科学家的课题：（1）重要副矿物（如锆石、石榴子石等）的溶解作用如何调制熔体的元素和放射性同位素地球化学特征？（2）地壳物质部分熔融是否存在明显的放射性同位素（如Sr、Nd、Hf和Pb）不平衡？喜马拉雅淡色花岗岩是地壳深熔作用的典型产物，具有元素和Sr-Nd-Hf同位素组成高度不均一的特征，是探讨上述问题的理想对象。本项目拟以马拉山-吉隆、雅拉香波-错那和珠峰地区淡色花岗岩和变质岩为主要研究对象，确定淡色花岗岩和变质围岩的元素和放射性同位素地球化学特征，了解副矿物（石榴子石、锆石、独居石、磷灰石等）在地壳深熔作用中的地球化学效应。结合实验岩石学成果，构建地壳物质深熔作用过程中副矿物控制熔体元素和同位素特征的理论模型。预期成果将有助于深入了解中下地壳岩石部分熔融过程中元素和同位素的分配过程，提高对地壳深熔作用中放射性同位素不平衡的形成机理的认识。

通过对喜马拉雅造山带花岗岩的岩石学、矿物学、地球化学和地质年代学等研究，确定了研究区地壳深熔作用时限和地球化学特征，取得了一系列重要进展和成果，共发表论文15篇，其中SCI论文13篇，主要进展包括：（1）揭示了锆石、独居石、榍石的差异性溶解控制了部分熔融过程中放射性同位素Pb、Hf 和Nd同位素的不平衡，（2）分析了副矿物锆石、独居石、磷灰石、石榴子石和榍石在部分熔融过程中差异溶解的元素地球化学效应，（3）提出了熔体结构的改变控制了地壳物质深熔作用过程中元素的变化和关键金属元素的富集，（4）识别出递进部分熔融作用是南北向裂谷系中重要的部分熔融类型，（5）探讨了喜马拉雅成矿带花岗岩和变沉积岩中关键金属的成矿潜力。这些成果为深化理解大型碰撞造山带壳源岩浆系统的演化特征及其成矿效应提供了关键的实例。同时，新观测结果不仅验证的早期理论模拟和实验研究的预测，而且拓展了不同地壳深熔作用副矿物的差异性溶解可导致熔体元素变异和Sr-Nd-Hf-Pb同位素耦合等效应，为深入理解地壳深熔作用过程中元素和同位素地球化学行为提供了新限定，为喜马拉雅成矿带稀有金属（甚至关键金属）矿产的远景靶区的确立提供限定。