Highly fractionated leucogranites in the eastern Himalayan Cuonadong dome and related magmatic Be–Nb–Ta and hydrothermal Be–W–Sn mineralization

Abstract

Leucogranitic rocks, including two-mica granite, muscovite granite, and granitic pegmatite (aplite), crop out in the eastern Himalayan Cuonadong dome, southern Tibet. These rocks have high SiO2 (∼73 wt.%), Al2O3 (∼16 wt.%), and total alkali contents (∼9 wt.%), and are peraluminous with A/CNK ratios of 1.0–1.1. Whole-rock major and trace element data and mineral compositions reveal strong differentiation and fractionation trends for the two-mica granite, muscovite granite, and pegmatite–aplite dikes. Albite, muscovite, and garnet contents increased as the leucogranitic magma evolved, whereas K-feldspar and biotite contents decreased. Muscovite in aplite has higher Rb2O (∼0.4 wt.%), Nb (∼219 ppm), and W + Sn (∼737 ppm) contents, and lower Nb/Ta ratios (∼6) than muscovite in the two-mica and muscovite granites. Garnet varies in composition from almandine in the leucogranites to spessartine in the aplites. The muscovite granite and pegmatite are associated with rare-metal mineralization. Rare-metal-bearing minerals include beryl, columbite-group minerals, tapiolite, Nb–Ta-bearing rutile, cassiterite, and wolframite. Skarn and altered (i.e., chloritized) rocks along the margins of the leucogranites are the products of hydrothermal reaction between leucogranite and marble. Beryl, scheelite, and cassiterite occur in the skarn and altered rocks. Rare-metal mineralization in the Cuonadong dome can be divided into two types: magmatic Be–Nb–Ta mineralization related to pegmatite–aplite dikes and hydrothermal Be–W–Sn mineralization hosted in the skarn and associated with the two-mica and muscovite granites. Monazite U–Th–Pb dating reveals two episodes of leucogranitic magmatism in the Cuonadong dome at ca. 20 and 17 Ma. Columbite U–Pb dating indicates two stages of rare-metal mineralization at ca. 17 and 14 Ma (including tapiolite). It is inferred that magmatism was related to the exhumation of the Higher Himalayan Crystalline Basement, with the degree of differentiation of the associated leucogranites being the key factor controlling the type of mineralization.