Nature and Evolution of Crust in Southern Lhasa, Tibet: Transformation From Microcontinent to Juvenile Terrane

Abstract

The nature and pre-Cenozoic evolution history of crust in southern Lhasa, which is crucial for our understanding of Indo-Asian continental collision and Tibetan uplift during the Cenozoic, remains controversial due to a “missing” pre-Mesozoic magmatic record. In this contribution, we report petrological and geochemical data for newly identified Paleozoic bimodal magmatism in the Zhengga area of southern Tibet. The magmatism comprises Late Devonian-Early Carboniferous (366–353 Ma) amphibolite and two-mica gneissic granite. The protoliths of the Zhengga amphibolite were gabbro and diorite with low SiO2 and high MgO, Cr, and Ni contents with high εNd(t) values of +3.3 to +8.0, variable and positive zircon εHf(t) of +0.9 to +11.2, and low zircon δ18O of 5.7 ± 0.2‰. These protoliths are proposed to have formed by decompression melting of asthenosphere during intracontinental back-arc extension. In contrast, the granite has relatively high SiO2 and low MgO contents with much lower εNd(t) of −8.6 to −7.3, variable and negative zircon εHf(t) of −10.4 to −1.3, and high zircon δ18O of 9.4 ± 0.2‰ values and was most likely derived from an ancient metasedimentary source. This magma subsequently underwent recharge with minor amounts mafic magma followed by fractional crystallization of K-feldspar in middle-upper crust (~10–20 km) magma chambers. Using our new data, in combination with Nd-Hf isotopes, we present the first comprehensive picture of crustal evolution in southern Lhasa. The southern Lhasa sub-block is likely to have been a microcontinent that underwent extensive Phanerozoic crustal reworking and growth, rather than a Mesozoic-Early Tertiary juvenile accretionary arc terrane.