Miocene olivine leucitites in southern Tibet. Implications for the recycling of carbonates into the mantle during continental subduction

We present a comprehensive geochronological, mineralogical, and geochemical study on previously unknown olivine leucitites occurring in the Maiga area of southern Tibet to examine the possibility of carbonate recycling during continental subduction. The Miocene (similar to 16 Ma) Maiga olivine leucitites are characterized by low SiO2 (44.6-46.3 wt%), high MgO (14.0-15.7 wt%), and ultra-calcic (CaO/Al2O3 = 1.1-1.2) compositions, along with arc-like incompatible element patterns (e.g., enrichment in large-ion lithophile and light rare earth elements, and depletion in high-field-strength elements). These rocks also have extremely radiogenic Sr-87/Sr-86((i)) (0.715647-0.715844), unradiogenic epsilon Nd-(i) (-11.6 to -11.2) and epsilon Hf-(i) (-10.5 to -10.0), and moderately radiogenic Pb-206/Pb-204((i)) (18.44-18.46) isotopic compositions. The studied samples are also characterized by slightly heavy O isotope ratios (clinopyroxene delta O-18 = +6.7 parts per thousand to +8.7 parts per thousand) and relatively low water contents (0.75-1.75 wt%). The geochemical features of the Maiga olivine leucitites are compatible with an origin from a carbonated peridotite mantle source. Additionally, based on the regional tectonic evolution and mid-ocean-ridge-basalt-like Zn isotopic compositions (delta Zn-66 = +0.26 parts per thousand to +0.30 parts per thousand), we hypothesize these magmas formed from interactions between ambient mantle peridotite and recycled Ca-rich carbonate sediments during Indian continental subduction. The transition from the neighboring Miocene lamproite-like rocks in southern Tibet to olivine leucititic magmas in Maiga is assumed to reflect the tapping of two different mantle sources. The first was modified by silicate-rich sediments, which generated lamproite-like rocks, whereas the Maiga mantle source was modified by carbonate-rich sediments. This study and previous research on other K2O-rich subduction-related magmas indicate that non-negligible amounts of carbonates can be recycled deep into the mantle during continental subduction, which may play an important role in global carbon recycling.