Early Cretaceous gabbroic anorthosites and diabases within the Langjiexue terrane, Tethyan Himalaya, southern Tibet: An indication of incipient opening of the Eastern Indian Ocean basin

Early Cretaceous (ca. 136 Ma, secondary ion mass spectrometry zircon U-Pb ages) gabbroic anorthosite and diabase rocks are intruded within the Late Triassic Langjiexue Group of the Tethyan sedimentary sequence in southern Tibet. These gabbroic anorthosite rocks display very high Al2O3 (21.7−23.4 wt%), high CaO (7.44−14.1 wt%), and low MgO (2.62−3.39 wt%) contents. The diabase rocks show high Al2O3 (16.2−16.6 wt%), high CaO (10.9−11.1 wt%), and medium MgO (4.16−4.27 wt%) contents. All the gabbroic anorthosite and diabase samples exhibit enrichment in light rare earth elements but depletion in heavy rare earth elements, together with enrichment in incompatible elements and high field strength elements, as well as high Sr and Pb concentrations. They have high εNd(t) values (+6.6 to +13.0) and εHf(t) values (+7.2 to +16.0). These geochemical characteristics are analogous to typical enriched mid-ocean-ridge basalts, which indicate these samples were derived from mid-ocean-ridge basalt−sourced mantle. Based on petrographic and geochemical observations, we propose that gabbroic anorthosites and diabases were derived from depleted lherzolite mantle where apatite dissolution was involved in melting. Density contrast and plagioclase accumulation were responsible for the formation of the gabbroic anorthosites and diabases. Regional extensional tectonics of the eastern Gondwana at ca. 140 Ma could provide a channel for the lower-density plagioclase-bearing melts upwelling beneath this region. These rocks represent pioneer products of mid-oceanic-ridge magmatism of the Eastern Indian Ocean, which signify that the formation of the Eastern Indian Ocean occurred as early as 136 Ma.