Nature of the Late Cretaceous mantle source beneath the western Lhasa terrane, southern Tibet: Insights from the newly discovered mafic intrusion

Recent studies indicate that the mantle source of eastern Lhasa terrane was still depleted during the late stage of oceanic subduction (100-50 Ma). However, the mantle source beneath the western Lhasa terrane during this period was poorly constrained mainly due to the rare exposure of mafic rocks in this region. Here, we report the detailed petrological, geochronological, geochemical, and Sr-Nd-Hf-Pb isotopic data for recently identified gabbroic rocks in the Taruocuo area, western Lhasa terrane, southern Tibet. The zircon U-Pb dating results indicate that the Taruocuo gabbroic rocks were emplaced during the Late Cretaceous (similar to & nbsp;90 Ma). These gabbroic rocks have low SiO2 (50.9-55.0 wt%), and moderately high MgO (4.10-5.60 wt%) contents with arc-like trace-element patterns, such as enrichment in large ion lithophile elements, light rare earth elements, and depletion in high field strength elements. Meanwhile, they have high Th/La, low Ba/La values, and exhibit relatively enriched and uniform Sr-Nd-Hf-Pb isotopic compositions (Sr-87/Sr-86((i)) = 0.7048-0.7058, epsilon Nd-(t) =-1.8 to-1.2, epsilon Hf-(t) = +3.1 to +3.8, and Pb-206/Pb-204 = 18.60 to 18.66). Such geochemical features of the Taruocuo gabbroic rocks suggest that they would originate from an enriched and fertile mantle source that was generated by the meta-somatic reaction between the mantle wedge peridotite and the subducted Neo-Tethyan oceanic sediment-derived melts. Taking the regional tectonic evolution into consideration, we favor the model of Neo-Tethyan oceanic-ridge subduction having taken place to account for the Late Cretaceous (similar to & nbsp;90 Ma) magmatic event in the Lhasa terrane. Under such geological background, partial melting of this enriched mantle would have produced the Taruocuo gabbroic rocks in the western Lhasa terrane. Given that the Late Cretaceous Taruocuo gabbroic rocks have the relatively enriched Sr-Nd-Hf isotopic compositions, distinctive from the contemporaneous mafic rocks in the eastern Lhasa terrane that have the depleted Sr-Nd-Hf isotopic compositions, we propose that the mantle source beneath the Lhasa terrane was probably isotopically segmented during the Late Cretaceous (similar to & nbsp;90 Ma).