Genesis and tectonic setting of Middle Permian OIB-type mafic rocks in the Sumdo area, southern Lhasa terrane

Abstract

This paper reports newly discovered mafic rocks, including cumulate gabbros, meta-gabbro, and meta-basalt, from the Sumdo area of the southern Lhasa terrane. Fifteen samples of cumulate gabbros, meta-gabbro, and meta-basalt collected near Wenmulang in the Sumdo area have similar geochemical characteristics, including relatively low SiO2 and MgO contents and high TiO2, P2O5, and total alkali contents. The samples are strongly enriched in light rare-earth elements (LREE), particularly in Nb (vs. Th) and Ta (vs. U), and are depleted in heavy rare-earth elements (HREE). Zircon UPb ages obtained from cumulate gabbros and meta-gabbro are 260.6 ± 3.1 Ma and 268.5 ± 1.9 Ma, respectively. The εHf(t) values of the meta-gabbro vary between −1.9 and + 1.6. We conclude that a combination of metasomatism and variable degrees of partial melting of a mantle plume explain the geochemical characteristics and origin of the Wenmulang oceanic island basalt (OIB)-type mafic rocks. There was no incorporation of continental crust or subcontinental lithospheric mantle (SCLM) during their formation. The geochemical characteristics of the Wenmulang OIB-type mafic rocks are similar to those of Hawaiian alkali basalt, suggesting that they formed in an oceanic island environment, most likely at the base of an oceanic island. We suggest that the Paleo-Tethys Ocean in the Sumdo area was a mature ocean basin during the Middle Permian. An upwelling mantle plume caused volcanism in the ocean basin and led to the formation of the Wenmulang OIB-type mafic rocks.