RAPID EMPLACEMENT OF GARNET LEUCOGRANITE IN THE RAMBA DOME, TIBET

The Ramba Dome, located at the intersection of the Tethyan Himalayan leucogranite belt and the Yadong–Gulu Rift, hosts several generations of granitic intrusions. In this dome, the 8-Ma two-mica granites and crosscutting garnet leucogranite dykes are the youngest members of Himalayan leucogranites. They are highly fractionated and are likely derived from dehydration melting of metapelite. The garnet leucogranite features garnet, muscovite, beryl, and sporadic occurrences of tourmaline. The geochemical signatures of the garnet granite indicate that it is more evolved than the coeval two-mica granite. Zircons from the leucogranites contain extremely high concentrations of U, which causes them to become partially amorphous due to radiation damage. Despite this, these zircons yield concordant U–Pb ages that are consistent with the U-Th-Pb ages of xenotime and monazite (Liu et al., 2014 Lithos). Thus, the leucogranite emplacement, cooling and exhumation must have taken place rapidly, a hypothesis tested by the P-T-t history recorded by the garnet–staurolite schists in the contact aureole (<1 Myr, Chu et al., 2022 J Metamorphc Geol). The chemical maps of garnet phenocrysts in the leucogranite reveal concentric, euhedral and sharp core-mantle zones in Fe, Mn, and Ca, along with a discontinuous veneer of high-Ca peritectic rim. The concentrations of the rare earth elements (REEs) are predominantly higher in the core than the mantle, with mid-REEs more enriched than heavy-REEs in the core. Diffusion simulation yields ~0.25 Myr at the core-mantle boundary, and ~0.1 Myr at the rim, assuming a solidus temperature ~650 °C. The emplacement of these notably young granites within the belt likely resulted from Yadong-Gulu rifting, induced by the upwelling of the asthenosphere through a tear in the subducting Indian slab. The rapid emplacement and cooling of the 8-Ma Ramba leucogranites marked the eventual cessation of Himalayan leucogranite magmatism, and the corresponding termination of such deep heat supply.