喜马拉雅夏如淡色花岗岩⁃伟晶岩的矿物学特征

A large number of leucogranites and pegmatites occurred in the Xiaru pluton,Himalayan oregen,mainly including (tourmaline⁃garnet) muscovite granite and granitic pegmatite.Granite and pegmatite which are related to Nb⁃Ta⁃W rare⁃metal mineralization locally,contain Nb⁃Ta⁃W oxide minerals,such as columbite⁃group minerals,qitianlingite,wolframite,and ilmenorutile. In order to investigate the petrological and mineralogical characteristics of the Xiaru granite⁃pegmatite related rare⁃metal mineralization,and gain insight of the rare⁃metal mineralization metallogenisis of the Xiaru pluton,the samples with rare⁃metal mineralization are selected,including muscovite granite with the occurrence of wolframite,muscovite granite with Nb⁃Ta⁃W oxide minerals and pegmatite with the occurrence of columbite⁃group minerals. Meanwhile,the typical muscovite granite without any rare⁃metal minerals (barren muscovite granite hereafter) is selected for comparison. Major and trace elements of whole rock geochemistry,the occurrence,major and trace elements of rock forming minerals and accessory minerals are comparable. The abundances of tourmaline and garnet increase significantly with the occurrence/increased abundance of Nb⁃Ta oxide minerals in the samples. The Xiaru granite⁃pegmatite are rich in SiO2 (72.29~75.53 wt.%),Al2O3 (13.17~15.94 wt.%),total alkali (K2O+Na2O 8.0~9.8 wt.%),but poor in CaO,MgO and FeO. The trace elements concentrations of the granite⁃pegmatite are similar,and the Nb⁃Ta⁃bearing pegmatite is rich in Nb and Ta. The shapes of normalized REE patterns are similar,with obvious negative Eu anomalies. However,the mineralized granite⁃pegmatite have significantly lower total REE concentrations (~21×10-6 on average) than the barren muscovite granite (98×10-6 on average),and they have more obvious REE tetrad effect. Mineralogical works show that the mineral compositions of barren muscovite granite are similar to that of W⁃bearing muscovite granite,while the mineral compositions of Nb⁃Ta⁃W⁃bearing muscovite granite are similar to that of Nb⁃Ta⁃bearing pegmatite. The Al2O3 and FeO contents of muscovite,Mn/(Mn+Fe) ratio and Eu negative anomaly of garnet,the Mg/(Mg+Fe) ratio of tourmaline show a progressive trend of magma evolution from barren muscovite granite and W⁃bearing muscovite granite to Nb⁃Ta⁃W⁃bearing muscovite granite and Nb⁃Ta⁃bearing pegmatite. The occurrences and Zr/Hf ratios of zircons in the Xiaru granite⁃pegmatite with Nb⁃Ta oxide minerals indicate strong fluid activity in the later magma. Therefore,it is suggested that whole⁃rock composition is indicative for the fluid activity,while the composition characteristics of muscovite,tourmaline and garnet are indicative for the process and degree of the magma evolution.

Keywords： leucogranite ; rock⁃forming minerals ; magma evolution ; mineral indicators