喜马拉雅然巴淡色花岗岩中石英微量元素的成分特征及其指示意义

为进一步推进喜马拉雅淡色花岗岩稀有金属成矿问题的研究，需全面厘清喜马拉雅淡色花岗岩的岩石学成因和岩浆分异演化过程，并寻找可以判别和指示其岩浆结晶分异程度及成矿潜力的矿物-岩石-地球化学指标。近年研究发现，石英晶格中的微量元素成分特征可以很好的反映岩浆来源和分异演化过程。为此，本文对喜马拉雅然巴岩体中两种演化程度不同的花岗岩（演化程度较低的二云母花岗岩和典型高分异成因的白云母花岗岩）中的石英进行了微量元素成分分析。结果显示，然巴淡色花岗岩石英中含量较高的元素包括Li、Be、Na、K、Al、Ca、Sc、Ti、Ge，这些元素含量在两类淡色花岗岩中存在较大的重叠，但白云母花岗岩中石英普遍具有更高的Be和更低的Ti含量，同时Li、Na、Al的含量高值均出现在白云母花岗岩中。另外，相对二云母花岗岩，白云母花岗岩中石英具有明显增高的Al/Ti（>10）和Ge/Ti（>0.1）比值。综上，本文认为石英中的Be、Ti元素含量以及Al/Ti、Ge/Ti比值有潜力成为指示喜马拉雅淡色花岗岩分异演化程度和稀有金属矿化的有效指标。根据石英中Ti含量估算然巴二云母花岗岩的结晶温度为666~491℃，白云母花岗岩的结晶温度为559~358℃。该结果证实喜马拉雅淡色花岗岩在成岩过程中经历了显著的降温，其岩浆体系可以演化至低于一般花岗岩体系固相线的温度，这为深入理解喜马拉雅淡色花岗岩的岩浆演化过程提供了重要的物理学约束。

Trace elemental characteristics and implications of the quartz from Ramba leucogranites in Himalaya

To comprehensive understanding the rare-metal mineralization in the Himalayan leucogranites, it is necessary to clarify their petrogenesis and magmatic evolution processeses. Besides, a further aim is to search several mineralogical-petrological-geochemical indexes to indicate and quantitatively describe the degree of magma differentiation and metallogenic potential. It has been demonstrated that trace elements in the quartz lattice can be used to trace the origin and evolutional history of the magma. In this study, the trace-element contents in quartz from two types of leucogranites with different degrees of differentiation (e.g., the less evolved two-mica granite and the highly fractionated muscovite granite in Ramba pluton) were investigated. Elements Li, Be, Na, K, Al, Ca, Sc, Ti and Ge are preferentially incorporated into the quartz lattice, and the concentrations of these elements generally overlap between the two types of leucogranites. It is notable that Be is always present with higher concentrations in the quartz from the highly fractionated muscovite granite, while the Ti content is significantly higher in the quartz from the less evolved two-mica granite. Li, Na and Al all reached the highest contents in the muscovite granite. Besides, the quartz from the muscovite granite show obviously higher Al/Ti (>10) and Ge/Ti (>0.1) ratios than that from the two-mica granite. Obviously, the distribution characteristics of the concentrations of Be and Ti and the ratios of Al/Ti and Ge/Ti are potential indexes to indicate the degree of differentiation of Himalayan leucogranites. Using the Ti-in-quartz thermos-barometer, the two-mica granite and muscovite granite are estimated to crystallize at 666~491℃ and at 559~358℃, respectively, which are significantly lower than the solidification temperature of an ordinary granitie system. It provides an important constraint on the physical condition of the magmatic system.