Cenozoic crust-mantle interaction in response to Indian slab tearing: Insights from zircons in rhyolites in the Namco, northern of Yadong-Gulu rift

Abstract

The crust-mantle interaction provides the driving forces and juvenile components for the surface uplift, crustal reworking, and the extensive and intense magmatism in the Tibetan Plateau. However, the spatial correlation between the Cenozoic volcanic rocks and the N-S trending rifts keeps the role of the subducted Indian slab in the upwelling of mantle materials and the crust-mantle interaction remain highly controversial. New zircon U-Pb dating, combined with trace element and Hf isotope studies, and mineral inclusion analysis of the Cenozoic Namco rhyolites, which are emplaced along the Yadong-Gulu rift (YGR), can provide additional constraints on this issue. The zircons from the Namco rhyolites exhibit high Th/U ratios (0.3 to 4.8), combined with the zircon morphology and the presence of calcite and anorthoclase inclusions, indicating a magmatic origin within a continental crust setting. These zircons exhibit two prominent age peaks (ca. 52 Ma with ca. 115 Ma) within a broad age range spanning from 49 to 3198 Ma. The younger age peak suggests the formation age of the Namco rhyolites, while the older age peak corresponds to the record of magmatic activity of the Zenong Group. The Early Cenozoic syn-magmatic zircons exhibit negative ε_Hf(t) (−9.8 to −1.5) and high ΔFMQ (−0.28 to 3.97) values, implying that the Namco rhyolites originated from the partial melting of the ancient lower crust in Central Lhasa. Furthermore, these zircons are also characterized by elevated zircon saturation temperatures (704° to 1107 °C) and transitional Hf isotopic composition, which suggest the involvement of upwelling asthenosphere mantle in the formation of rhyolites. The crustal thickness in the Namco area exhibits a thinning process at ca. 52 Ma. This variation can be attributed to the tearing of the subducted Indian slab. The tearing penetrated the entire Indian slab, enhancing local mantle convection, which in turn thinned the subcontinental lithospheric mantle in the Namco area and provided a vertical pathway for the upwelling asthenosphere material. Therefore, we propose that the tearing of the subducted Indian slab had a substantially more significant impact on the crust-mantle interaction beneath the Tibetan plateau and on the formation of the YGR system.