Mantle wedge enrichment beneath southern Tibet during the late stage (100–45 Ma) of oceanic subduction: Geochemical constraints from mantle-derived intrusions

Abstract

The Miocene K-rich mafic rocks in the Lhasa Terrane of the southern Tibetan Plateau are considered to be related to either the subduction of the Neo-Tethyan oceanic slab or Indian continental slab. In order to investigate how subduction of the Neo-Tethyan oceanic slab has geochemically affected the mantle beneath southern Tibet, we obtained geochronological and geochemical data for the late Early Cretaceous Cangguo gabbros (97 Ma) and early Eocene Langka diorites (51 Ma) in southern Tibet. Although both the Cangguo gabbros and Langka diorites are characterized by arc-like trace element patterns, with enrichment in light rare earth and large-ion lithophile elements (e.g., Ba, Pb, and U), depletion in high-field-strength elements (e.g., Nb and Ta), and depleted Sr–Nd–Hf isotopic compositions, there are some other major geochemical differences (e.g., Ba/La and Th/La ratios) between these two suites of rocks. These geochemical characteristics of the Cangguo gabbros and Langka diorites indicate they were produced by melting of the depleted mid-ocean ridge mantle (DMM) that was enriched by oceanic sediment-derived fluids and melts, respectively. Our data and previous studies suggest that dehydration and melting of subducted Neo-Tethyan oceanic sediments beneath the Lhasa Terrane occurred in the late stage (100–45 Ma) of Neo-Tethyan oceanic slab subduction, which may have enriched the mantle beneath the Lhasa Terrane. However, the mantle beneath the Lhasa Terrane that was enriched by the subduction of oceanic slab during 100–45 Ma is still isotopically more depleted than the source of Miocene post-collisional K-rich mafic rocks in the Lhasa Terrane. This indicates that the enriched component in the mantle source of the K-rich mafic rocks cannot be simply attributed to materials derived from the Neo-Tethyan oceanic slab, and other additional materials are required.