Himalayan leucogranites: A review of geochemical and isotopic characteristics, timing of formation, genesis, and rare metal mineralization

Abstract

As prototypes of crust-derived remelted S-type granite, Himalayan leucogranites have considerable relevance in crustal evolution and associated metallogeny. However, given their highly differentiated character, the petrogenesis of these rocks has remained controversial. This study reviews the geochemical and isotopic data from >2000 samples of Himalayan Cenozoic granitoids to evaluate their genesis. The Himalayan leucogranites include two major belts: the northern zone occurs in the Tethyan Himalayas and gneiss domes, and the southern zone is exposed mainly at the top of the Higher Himalayas. The ages of these rocks show a southward younging trend from 49 Ma to 1 Ma. The main rock types are two-mica granites and garnet-tourmaline-bearing muscovite granites, whereas Eocene and Miocene intermediate-mafic and adakitic rocks are developed mainly in the northern belt. The leucogranites originated from incongruent disequilibrium partial melting of the Greater Himalayan Crystalline Complex and underwent a high degree of differentiation and fractional crystallization. The rocks are strongly peraluminous and characterized by high Si, K, and Na; low Ca, Fe, Mg, Ti, and Mn; and low rare earth elements with obvious tetrad effects and negative Eu anomalies. A more striking feature of these granites is their high Rb/Sr and Y/Ho values and low Th/U, Nb/Ta, Zr/Hf, and K/Rb values. The Sr–Nd–Pb–Hf isotopes indicate that the proportion of older crustal material in the magmatic source area gradually increased from the Eocene to the Pliocene. The Cenozoic magmatic activity can be divided into five stages—49-40 Ma, 39–29 Ma, 28–15 Ma, 14–7 Ma, and 6–0.7 Ma—and their geotectonic settings are related to the break-off of the Neo-Tethyan oceanic plate, low-angle underthrusting of the Indian continental lithosphere, break-off or rollback of the Indian plate, tearing of the Indian lithospheric slab or delamination of the lithospheric mantle, and rapid extrusion and uplift of the Eastern and Western Himalayan Syntaxis resulting from flat subduction of the Indian continent, respectively. Stream sediment geochemical data from previous work show high positive anomalies for some rare metals in the Himalayas. The average contents of Li, Be, Sn, Ta and Cs in pegmatite are 2466 ppm, 93 ppm, 29 ppm, 14 ppm and 61 ppm, respectively. All these elements have enrichment coefficients higher than 10 compared to the upper continental crust. Recent progress in ore prospecting shows that the Himalayan Cenozoic leucogranite is emerging as a new world-class rare metal metallogenic belt.