Petrogenesis of monzonites and leucogranites in the Ama Drime Massif: Implications for sources and differentiation of Himalayan leucogranites

Abstract

Determining the sources and the evolution of the Himalayan leucogranites is crucial to deciphering the teotono-magmatic evolution history of the Himalayan Orogen. Various magmatic rocks, including the monzonites, two-mica leucogranites, and tourmaline leucogranites, are exposed as dykes in the Paleoproterozoic orthogneisses of the Ama Drime Massif (ADM), providing new insights into the Himalayan magmatism. This contribution presents systematic petrological, geochronological, and geochemical studies of these magmatic rocks from the ADM to constrain their emplacement timing, protolith, and petrogenesis. Geochronological results reveal two episodes of magmatism in the ADM, represented by the 13.5 Ma monzonites and the 10 Ma two-mica and tourmaline leucogranites. Despite having different ages, the two episodes of magmatism in the ADM both originated from the melting of the Paleoproterozoic orthogneisses. However, various magmatic rocks underwent different differentiation processes during emplacement. Based on mineral assemblages and geochemical features, the monzonites were formed by the accumulation of plagioclase, biotite, zircon, and monazite. For the leucogranites, the systematic variations in mineral assemblages, whole-rock geochemistry, and mineralogical chemistry demonstrate that the two-mica leucogranites gradually evolved into the tourmaline leucogranites by the separation of plagioclase, biotite, zircon, and monazite. Combining this contribution and previous studies, the Himalayan leucogranites might evolve from the peraluminous melts produced by the melting of Himalayan anatectic rocks according to fractional crystallization, with the formation of complementary cumulate rocks. The involvement of orthogneisses in the source suggests that the Himalayan leucogranites can not be simply classified into S-type granites.