Eocene crustal thickening in the Tethyan Himalaya: Insights from Barrovian metamorphism and granite geochemistry from the Ramba area

Abstract

Magmatism, structures, and metamorphism in the Ramba dome of the Tethyan Himalaya were investigated to shed light on orogenic processes during the early stages of the India-Asia collision. Deformed granite dikes in the dome envelope yield zircon U-Pb ages of ca. 45 Ma. These Eocene granites have adakitic, Na-rich compositions (K2O/Na2O = 0.20−0.61), weak to no Eu anomaly, enrichment in Sr, depletion in heavy rare earth elements and Y, and low MgO and Mg# contents. These characteristics contrast with the Miocene potassic granites in the core of the dome and suggest that the Eocene adakites were derived from the high-pressure melting of crustal amphibolites in a thick crust. The mica schists of the dome envelope have an early foliation (S1) that is overprinted by upright folds (F2). Phase-equilibria modeling of garnet and staurolite mica schists suggests a Barrovian-type, prograde P-T evolution in association with S1, with peak conditions of 6.7−7.2 kbar/590−605 °C and 7.3−7.8 kbar/650−670 °C, respectively, which are typical of crustal thickening metamorphism. Monazites from S1-dominated staurolite mica schists yield metamorphic ages of ca. 51−49 Ma, while those from the late foliation (S2) that transposed S1 give younger ages of ca. 10 Ma. The integration of geochemical, structural, metamorphic, and geochronological data suggests that peak Barrovian D1 metamorphism and adakitic magmatism occurred in the Eocene in response to crustal thickening. The results provide critical constraints for addressing the crustal shortening deficit of the region.