Identification of fractionation processes in the Himalayan leucogranites-Case study from the Nyalam region

Abstract

As the product of a collisional belt, Himalayan leucogranites provide insights into the geodynamic and thermal evolution of tectonically thickened crust. Nevertheless, the petrogenesis of these leucogranites is still a much-debated topic. They are typically interpreted as the primary melt formed by partial melting of the Greater Himalayan Crystalline Complex (GHC). There is, however, another view that they are the products of fractional crystallization of less evolved parent magmas. This study aims to explore these competing petrogenetic models. We collected biotite granite, two-mica granite, tourmaline granite, and garnet granite in the Nyalam region for mineralogical and geochemical studies. The granites occur as small laccoliths, sills, and dikes in the upper GHC and the South Tibet Detachment System (STDS). The plagioclase in biotite granite is oligoclase and andesine (An >20) but is more sodic (An <20) in other rocks. From biotite granite to two-mica granite to tourmaline granite, Mg and Ti in biotite and Zr/Hf in zircon gradually decrease, and Al^VI and X_Fe in biotite and Hf in zircon increase correspondingly. Garnet is typically euhedral and spessartine-rich (spessartine >30 wt%), and it only appears in garnet granite, suggesting an increase of Mn/(Fe + Mg) in the melt. These variations suggest that biotite granite is the least evolved rock, and that two-mica granite, tourmaline granite, and garnet granite are increasingly fractionated. This fractionation trend is supported by trace element compositions. Rb-Sr-Ba trace element modeling suggests that the feldspars (plagioclase and k-feldspar) are the main fractionated minerals. ZrHf modeling indicates that the decrease of Zr/Hf ratio in the melt is related to the fraction of zircon fractionated, which is controlled by the change of temperature and melt composition. Additionally, very high Rb/Sr (>20), low Zr/Hf (<20), and strongly negative Eu anomaly (0.2) in garnet granite strongly suggest that the melt experienced extensive fractionation. Compiled Himalayan leucogranite geochemistry presents a similar pattern to our data and modeling result, suggesting that the variety of Himalayan leucogranites might relate to fractional crystallization. In addition, the composition of leucogranites in the Nyalam is a function of spatial position and age. The leucogranites that were produced during the activity of STDS and developed close to STDS have more evolved compositions, indicating that the development of STDS might have played an essential role in the differentiation of leucogranite in the Himalaya.