U-Pb SHRIMP zircon ages of the mafic-ultramafic rocks from Chigu Co of South Tibet and their geological significance

Abstract: The nearly EW-trending mafic dyke swarms are widely distributed in South Tibet, and the layered pyroxenite occurs in the gabbro. Two weighted average ages of gabbro and pyroxenite were obtained by SHRIMP zircon U-Pb dating, i.e., (131.71±2.7)Ma (2σ, MSDW=2.8) and (135.0±2.4)Ma (2σ, MSDW=2.1）respectively. The crystallization of the ultramafic rocks took place 4 Ma earlier than that of the mafic ones. Element geochemical study shows that, in gabbro, SiO2 is 50.85%, TiO2 is 1.45%, MgO is 10.05%, TFe2O3 is 8.57%, and Mg# is 71.33, whereas in pyroxenite, SiO2 is 48.28%, TiO2 is 2.02%, MgO is 18.68%, TFe2O3 is 13.08%, and Mg# is 78.76. Their K2O content is high (0.68 and 0.81), Eu loss is not obvious, Nb, Ta and other elements are depleted, and the patterns of REE distribution are similar to those of OIB. The formation of the mafic-ultramafic rocks was related to the mantle thermal events occurring in the same period, and might have been contaminated by crustal materials in the rising process.