The Northernmost Effects of the Neo-Tethys Oceanic Slab Subduction Under the Lhasa Terrane: Evidence from the Mazin Rhyolite Porphyry

Abstract

The India–Asia collision represents the most significant geological event in the formation of the Tibetan plateau. The subsidence of the Neo-Tethys oceanic slab and the closure of the ocean basin were precursors of the India–Asia collision. The Linzizong volcanic formations, which range in age from the late Cretaceous to early Cenozoic (70–40 Ma), are widely distributed across the Lhasa terrane and are considered products of the closure of the Neo-Tethys oceanic basin and the India–Asia collision. Here, we report a newly identified series of rhyolite porphyries, which share similar age and geochemical features with typical Linzizong volcanic formations. These porphyries are the northernmost extension of Linzizong volcanic formations discovered to date. Zircon U-Pb dating suggests that they formed between 58.8 and 56.1 Ma. These porphyries are characterized by high SiO2 (75.04%–77.82%), total alkali (K2O: 4.71%–5.03%), and Na2O (2.54%–3.63%) values; relatively low Al2O3 (12.30%–13.62%) and MgO (0.13%–0.33%) values; and low Mg# values (15.8–25.7). They also exhibit strong enrichment in light rare earth elements ([La/Yb]N = 3.76–11.08); negative Eu anomalies (Eu/Eu* = 0.10–0.32); Rb, Ba, Th, U, and Pb enrichments; as well as Nb and Ta depletions. The samples have relatively low εNd(t) values (−6.0 to −3.8) and variable zircon εHf(t) values (−6.3 to +3.6). These features suggest they originated from the remelting of the juvenile lower crust of the North Lhasa terrane under high-temperature and extensional conditions. We propose that the Mazin rhyolite porphyries resulted from mantle-derived magma diapirism, triggering juvenile lower crust remelting during Neo-Tethys oceanic slab rollback at the onset of the India–Asia collision. These findings provide new insights into the magmatic processes associated with early collisional tectonics.