Mineralogical evidence for fractionation processes in the Himalayan leucogranites of the Ramba Dome, southern Tibet

Abstract

Abstract Himalayan leucogranites provide some of the best examples in the world of highly felsic and strongly peraluminous granites, and they have been paid special attention with regard to investigating the evolution of continental crust. Here, we describe our mineralogical investigations of a late-Miocene leucogranite complex within the Ramba Dome of the Tethyan Himalaya Belt that have helped us decipher the petrogenesis of these high-silica peraluminous granites. The Ramba leucogranites range from two-mica granites (biotite + muscovite) to muscovite leucogranites (muscovite ± garnet ± tourmaline), with the two-mica granites constituting the main part of the pluton and the muscovite leucogranites commonly occurring as late-stage dikes that are located within the pluton or invaded the country rocks. Variations in the mineral assemblages of the two types of leucogranite are associated with systematic changes in bulk-rock geochemistry, and the distinctive mineralogical features demonstrate a genetic relationship between the two types of leucogranite that involved fractional crystallization, with the muscovite leucogranites being more evolved than the two-mica granites. Chemical data for the minerals in the two-mica granites record a long period of differentiation during which a normal granitic melt developed into a highly evolved magmatic system with high flux components (e.g., H2O, F, B). In contrast, the mineralogical data for the muscovite leucogranites record the crystallization of a very late residual melt, i.e., a volatile-rich, highly fluid melt. According to recent studies of differentiation processes in silicic magma reservoirs, we can interpret the Ramba leucogranites as having undergone a high degree of differentiation by in situ fractional crystallization in a crystal mush environment. The two-mica granites represent a congealed crystal mush that was composed of ‘cumulate crystals’ and a trapped interstitial liquid, and the muscovite leucogranites represent the almost pure liquid that was extracted from the crystal mush. Modeling using the trace elements Sr and Ba shows that the extraction probably occurred when the crystallinity of the mush was ~59–64%, at least for the most evolved muscovite leucogranite sample. The two-mica granite corresponds to a residual crystal mush that had a terminal porosity of ~20–26% filled with a trapped interstitial liquid. Collectively, fractional crystallization can account for the geological, mineralogical, petrological, and geochemical variations between the two types of leucogranite. This work highlights the important role of crystal fractionation in the generation of Himalayan leucogranites.