Early Jurassic Mafic Magmatism in the Eastern Tethyan Himalaya, Southern Tibet

An Early Jurassic (∼196 Ma, laser ablation multiple-collector ICP-MS zircon U-Pb ages) suite of diabase dikes and sills has been identified within the Late Triassic Langjiexue Group, part of the Tethyan sedimentary sequence that is composed of shale and fine-grained sandstone. More primitive rocks (MgO>8.0 wt%) from this suite are characterized by mid-ocean ridge basalt–like rare earth element distribution patterns and isotope (Sr and Nd) compositions, as well as relatively high contents of mantle-compatible elements (e.g., Cr, Ni, and Co), which indicates that they were derived from partial melting of depleted mantle. Younger (∼140 Ma) mafic magmatism in the region has been proposed to represent the initial breakup of the eastern Gondwana continent and the opening of the Indian Ocean. We propose that the ∼196 Ma diabases developed in a continental extension setting and represent the products of melting of depleted mantle within or on the margin of the Neo-Tethyan Ocean.