Highly fractionated Himalayan leucogranites and associated rare-metal mineralization

Abstract

The formation of the Himalaya was associated with the exhumation of high-grade metamorphosed rocks of the Higher Himalayan sequence (HHS) complex, which underwent amphibolite-, granulite- to eclogite-facies metamorphism and anatexis. Occurring along the Himalayan mountain crest in the south and the Lhagoi Kangri mountain in the north, the Himalayan leucogranites contain varying proportions of biotite (<5%), muscovite, tourmaline, and garnet, and are typically equigranular in texture with variable structures from foliated to massive. These peraluminous granites have been interpreted as purely crust-derived melts, without any input from the mantle, and may therefore record the timing of continental collision. However, they were emplaced between 44 and 7 Ma, much later than the timing of collision between India and Asia (c. 60 Ma). Although typically interpreted as products of in situ partial melting of the HHS during subduction and exhumation, we suggest that the Himalayan leucogranites underwent intense crystal fractionation, as recorded by crystal layering, sedimentary-like cross-bedding, and the occurrence of pegmatites with varying grain sizes. Geochemically, these leucogranites are low in siderophile but high in lithophile elements, with significant negative europium (Eu) anomalies, non-chondritic Nb/Ta and Zr/Hf ratios, and rare earth element (REE) tetrad effects. Many of the leucogranites and pegmatites contain rare-metal minerals, such as beryl and chrysoberyl; columbite–tantalite, tapiolite, and pyrochlore–microlite; rutile and fergusonite; and zinnwaldite, lepidolite, spodumene, and petalite, supporting an origin involving a high degree of magmatic fractionation. We suggest that the primary magma was generated through partial melting of subducted Indian crust due to input of heat from depth. The generation of magma triggered the exhumation of the Indian crust and subsequent formation of the Himalayan mountain chain. The magma ascended with the Indian crust along the South Tibetan Detachment System (STDS), during which time it underwent significant fractionation. Anatectic melts generated from exhumed Indian crust through decompression melting are distinct from the leucogranites, indicating that exhumation of the Indian crust had a limited contribution to the generation and evolution of the leucogranites. Extensional faulting facilitated a high degree of magma fractionation and rare-metal mineralization.