Detrital zircon, monazite and tourmaline reveal the magmatic and metamorphic history of the Himalayan orogen from Archean to present

Abstract

The litho-tectonic structure of the Himalaya is the result of multiple orogenic cycles. However, its pre-Cenozoic history is not well understood, which limits our ability to constrain the stratigraphy, structure, and tectonism in the Himalayan orogen. This issue can be potentially addressed by detrital mineral geochronology and isotopic studies, which can be used to reconstruct the magmatic and metamorphic records from past to present. In this study, we collected three river sand samples from the central Himalaya, which originated from the interior of Ama Drime Massif (ADM), Greater Himalayan Sequence (GHS) outside of the ADM, and the boundary between the ADM and GHS, respectively, and then conducted a geochronological study of detrital zircon and monazite, and obtained detrital zircon Hf and tourmaline B isotope data. Detrital zircon of the river sand from the ADM only records the 1.9–1.7 Ga magmatic event, while age groups of 3.5–2.4, 1.1–0.7, and 0.6–0.4 Ga are observed in the GHS outside the ADM. Detrital tourmaline from the GHS has heavy B isotopic signatures with δ11B = −15‰ to −7‰, similar to the values for GHS leucogranites. Detrital tourmaline from the ADM has δ11B = −20‰ to −13‰, similar to the Lesser Himalayan Sequence (LHS). Integrating these results with previous metamorphic and thermochronological works, we interpret that the ADM represents deeply buried LHS rocks beneath the GHS, which underwent a significant uplift assisted by the detachments on its two sides during the Miocene. Furthermore, zircon Hf isotopic variations indicate that the north Indian margin was an active convergent boundary during the Paleoproterozoic, Neoproterozoic, and Paleozoic, in relation to supercontinent amalgamation. Detrital monazite records two major thermal events in the Cenozoic era. A large proportion of detrital monazite yield ages from 25 to 20 Ma, indicating that the collision entered a mature stage at this time, during which thickened Himalayan crust formed and widespread anatexis occurred. A tectonic style transition in the Himalayan hinterland from shortening that thickened the crust to extension that thinned it occurred during the middle to late Miocene (15–10 Ma) in the Himalaya.