Eocene magmatism in the Tethyan Himalaya, southern Tibet

Abstract

Recent studies in the Yardoi gneiss dome of the Tethyan Himalaya, southern Tibet demonstrated that the Himalayan orogenic belt experienced a major episode of crustal melting in the Mid-Eocene (c. 44–40 Ma), represented by the genesis of a suite of high Sr/Y two-mica granites (TMGs) accompanied by leucogranites. Geochemical and U–Pb age data indicate that (1)boththe leucograniteandtheporphyriticleucograniteformedat c. 44–41 Maare characterized by similar Sr–Nd–Hf isotope systematics to those in the less evolved TMGs; (2) these three suites of rocks might represent a Mid-Eocene igneous complex derived from partial melting of amphibolite under crustal thickened conditions; and (3) in contrast with the TMGs, leucogranites experienced hydrothermal-magma interactions as well as plagioclase fractional crystallization as revealed from pronounced negative Eu anomalies, much lower Sr concentrations, and elevated common Pb and U concentrations in the latest magmatic zircon overgrowth. Our data suggest that magmatic differentiation of more primitive magmas could have played a critical role in the generation of leucogranites. Documentation of partial melting of deep crustal rocks during the ear- liestphaseofthetectonicevolutionoftheHimalayanorogenimpliesthatthelarge-scalecollisional orogenic belt could experience intensive partial melting under contraction and thickening con- ditions,whichcouldprofoundly affect thegeophysicalandgeochemicalpropertiesas well as rheo- logical properties and facilitate the initiation of movement or flow of deep crustal rocks.