Geochronology and geochemistry of Mesoproterozoic granitoids in the Lhasa terrane, south Tibet: Implications for the early evolution of Lhasa terrane

Abstract

The early history of the Lhasa terrane remains poorly constrained due to the poor exposure of the Proterozoic rocks. We report here U–Pb zircon ages, geochemical and Hf isotopic data for granite gneisses and biotite gneisses from the Bomi Complex in the eastern part of the Lhasa terrane, south Tibet. Petrological and geochemical data suggest that the protoliths of the granite gneisses and the biotite gneisses could be granites and tonalites, respectively. LA-ICPMS U–Pb zircon analyses yielded ages of 1343±27Ma (MSWD=0.3) and 1276±22Ma (MSWD=0.4) for two granite gneisses, and a consistent age of ca. 1250Ma for two biotite gneisses. These ages are interpreted as the magma crystallization time of both the gneisses protoliths, and thus the Bomi Complex represents the oldest rocks found in the Lhasa terrane. Our data indicate that the Mesoproterozoic detrital zircons from the Paleozoic metasedimentary rocks in the Lhasa terrane could be derived from the Lhasa terrane itself or the Tethyan Himalaya, rather than necessarily from the Albany-Fraser belt in the Australia. Geochemical characteristics show that the granite gneisses have an aluminous A-type granite affinity. The two granite gneisses dated in this study have zircon ɛHf(t) values between +4.0 and +1.8 and between +2.6 and +0.2, respectively. They have identical two-stage Hf model ages of ∼2.0Ga. We suggest that the protoliths of the granite gneisses were produced by protracted high temperature partial melting of a felsic intracrustal source in an extensional setting. In contrast, the biotite gneisses have similar geochemical characteristics to those of calc-alkaline granitoids that probably formed in a subduction-related environment. Zircons from the two dated biotite gneisses have relatively higher ɛHf(t) values of +8.1 to +3.6 and +10.5 to +5.7, respectively, indicating a juvenile mantle contribution to their magma source. Earlier magmatism at ∼1343–1276Ma may formed in a continental rift setting related to the final breakup of supercontinent Columbia, while subsequent magmatism of ∼1250Ma resulted from subduction of ocean slab during the assemblage of Rodinia. We thus infer that the Bomi Complex was related to the contact zone between the Eastern Ghats Belt and the Archean cratons in southeastern India during the Mesoproterozoic