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

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 SiO 2 and high MgO, Cr, and Ni contents with high ε Nd (t) values of +3.3 to +8.0, variable and positive zircon ε Hf (t) of +0.9 to +11.2, and low zircon δ 18 O 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 SiO 2 and low MgO contents with much lower ε Nd (t) of −8.6 to −7.3, variable and negative zircon ε Hf (t) of −10.4 to −1.3, and high zircon δ 18 O 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.