Thematic Articles

Himalayan leucogranites were once overlooked for rare-metal resources because they initially were thought to have formed by in-situ partial melting of underlying high-grade metamorphic rocks. However, recent findings have revealed widespread rare-metal mineralizations of Be, Nb/Ta, Li/ Rb/Cs, and W/Sn associated with leucogranites in the area, suggesting these mineralizations resulted from extensive fractionation of leucogranitic magmas during long-distance magma transport along the low-angle South Tibetan Detachment System. When combined with coeval Au-Sb-Pb/Zn mineralizations in the Himalayas of the Indian plate, and porphyry Cu-Mo mineralizations in the Gangdese of the Asian plate, a specific Himalayan-type mineralization is proposed to describe the metallogenesis related to the exhumation of the subducted Indian continent, coinciding with the uplift of the Himalayan mountains.

The geochemical characterization of Himalayan leucogranites offers important insights into both their petrogenesis and Himalayan orogenic processes. Himalayan leucogranites are characterized by strongly peraluminous compositions that are comparable to melts derived from anatexis of sedimentary rocks. Their radiogenic (Sr, Nd, and Hf) isotopic compositions point to metasediments from the Higher Himalayan Sequence of the Indian plate as the primary source rocks, with minor contributions from other lithologies. Himalayan leucogranites display considerable variability in trace element ratios (e.g., Rb/Sr, Nb/Ta, Zr/Hf, and Eu/Eu*) and significant fractionation of non-traditional stable isotopes (e.g., Mg, K, Zn, Rb, and Ba), which provide key constraints on the respective roles of source heterogeneity, crystal fractionation, magma–fluid interaction, and crustal melting in their formation.

The High Himalayan leucogranites (HHL) are produced by muscovite breakdown of a metapelitic source, at temperatures below 800°C, with initial melt water contents of ~5–7 wt.%. The tourmaline-rich HHL variety is colder, possibly a fractionation product of the hotter two-mica HHL. HHL lack restites such as iron-rich garnet, which, when present, is Mn-rich, signaling fractionation processes. The low redox state of HHL mirrors that of their graphite-bearing source, yet there is evidence of a significant increase in fO2 during crystallization of some HHL. Their relationships with regional deformation call for late emplacement of the main bodies, which must have cooled at 3–4 kb to allow muscovite crystallization, which in turn imposes stringent constraints on unroofing rates of the collisional chain.

A popular model of Himalayan metamorphic and structural evolution argues that partial melting of deeply buried rocks triggered crustal weakening, ductile flow, orogenic collapse, and genesis of leucogranites. Here, we review the origins and evolution of partial melts and leucogranites to demonstrate that they are largely incidental to deformation. Although a pulse of orogenic collapse and leucogranite crystallization occurred at 15–25 Ma, pervasive partial melts formed as much as 20 My earlier. Thus, leucogranites date extraction and transport, not necessarily melting onset. Extensional structures and distributed extensional strain occur in many rocks that lack partial melt and leucogranites, indicating these are not prerequisite to facilitate orogenic collapse. Most mass transfer appears to occur via thrusting, even in partially molten rocks.

Himalayan peraluminous leucogranites were derived from in-situ melting of sillimanite + K-feldspar-bearing pelite-migmatite, and were transported via layer-parallel sill complexes and cross-cutting dykes to feed giant sills up to 5 km thick. Partially melted Himalayan middle crust was extruded southwards between two large-scale, north-dipping, synchronous ductile shear zones: the Main Central Thrust (MCT) below and the low-angle normal fault South Tibetan Detachment (STD) above. U-Th/Pb monazite dating constrains granite melting to ~25–18.5 Ma in Manaslu and ~24–13 Ma in Everest-Makalu. The Manaslu sheeted sill complex was emplaced by progressive underplating with the oldest intrusions structurally above younger intrusions. Heat was dominantly derived by internal radioactive heating from crustal thickening with little or no contribution from shear heating along the MCT or from the mantle.

Himalayan leucogranites crop out intermittently over 2000 km along the crest of the world’s youngest and largest mountain range. They are derived from partial melting of continental crust during a classic continental collisional orogeny. Studies of these leucogranites have significantly advanced knowledge of crustal anatexis, felsic magmatic differentiation, and the tectonic evolution of the Himalayan-Tibetan orogen. This Elements thematic issue provides an overview of the petrogenesis and significance of the Himalayan leucogranites including field relations, source rocks, petrology, geochemistry, tectonics, and links to orogenesis and economic resources. It not only summarizes the state-of-the-art research on Himalayan leucogranites but also demonstrates how a multidisciplinary approach can help constrain the origins and evolution of granites, their associated mineralizations, and related geodynamic development.