A CUMULATE ORIGIN OF HIMALAYAN LEUCOGRANITES: A POTASSIUM ISOTOPE PERSPECTIVE

Fractional crystallization plays a critical role in generating the highly differentiated continental crust. However, whether efficient crystal-melt separation can occur in viscous felsic magmas remains a long-standing debate due to the difficulty in discriminating between differentiated melts and complementary cumulates. Here, we reported our work on constraining petrogenesis of Himalayan leucogranites using K isotope geochemistry. We found large (~1‰) K isotopic variation in strongly peraluminous high-silica leucogranites, with K isotopes correlated with trace elemental proxies (e.g., Sr, Rb/Sr and Eu/Eu*) for plagioclase crystallization. Our quantitative modelling results require up to ~60−90% fractional crystallization to account for the progressively light K isotopic composition of the fractionated leucogranites, while plagioclase accumulation results in the enrichment of heavy K isotopes in cumulate leucogranites. Our findings strongly support significant fractional crystallization of high-silica magmas and highlight the great potential of K isotopes in studying felsic magma differentiation, an area where Dr. Chris Yakymchuk has contributed significantly to.