Post-collisional ultrapotassic rocks and mantle xenoliths in the Sailipu volcanic field of Lhasa terrane, south Tibet: Petrological and geochemical constraints on mantle source and geodynamic setting

Abstract

Post-collisional ultrapotassic magmatic rocks (15.2–18.8Ma), containing mantle xenoliths, are extensively distributed in the Sailipu volcanic field of the Lhasa terrane in south Tibet. They could be subdivided into high-MgO and low-MgO subgroups based on their petrological and geochemical characteristics. The high-MgO subgroup has olivine-I (Fo87–92), phlogopite and clinopyroxene as phenocryst phases, while the low-MgO subgroup consists mainly of phlogopite, clinopyroxene and olivine-II (Fo77–89). These ultrapotassic magmatic rocks have high MgO (4.6–14.5wt%), Ni (145–346ppm), Cr (289–610ppm) contents, and display enrichment in light rare earth element (REE) over heavy REE and enriched large ion lithophile elements (LILE) relative to high field strength elements (HFSE) with strongly negative Nb-Ta-Ti anomalies in primitive mantle-normalized trace element diagrams. They have extremely radiogenic (87Sr/86Sr)i (0.7167–0.7274) and unradiogenic (143Nd/144Nd)i (0.5118–0.5120), high (207Pb/204Pb)i (15.740–15.816) and (208Pb/204Pb)i (39.661–39.827) at a given (206Pb/204Pb)i (18.363–18.790) with high δ18O values (7.3–9.7‰). Strongly linear correlations between depleted mid-ocean ridge basalt-source mantle (DMM) and the Indian continental crust (HHCS) in Sr-Nd-Pb-O isotopic diagrams indicate that the geochemical features could result from reaction between mantle peridotite and enriched components (fluids and melts) released by the eclogitized Indian continental crust (HHCS) in the mantle wedge. The high-MgO (13.7–14.5wt%) subgroup displays higher (143Nd/144Nd)i, lower (87Sr/86Sr)i and (206Pb/204Pb)i ratios and lower δ18O values compared with the low-MgO (4.6–8.8wt%) subgroup. High Ni (850–4862ppm) contents of olivine phenocrysts and high whole-rock SiO2, NiO, low CaO contents indicate that the low-MgO ultrapotassic magmatic rocks are derived from partial melting of olivine-poor mantle pyroxenite. However, lower Ni concentrations of olivine phenocryst and lower whole-rock SiO2, NiO, higher CaO contents of the high-MgO ultrapotassic rocks may indicate their peridotite mantle source. This could be attributed to different amounts of silicate-rich components added into the mantle sources of the parental magmas in the mantle wedge caused by the northward subduction of the Indian continental lithosphere. The reaction-formed websterite xenoliths, reported for the first time in this study, are made up of anhedral and interlocking clinopyroxene (45–65vol%) and orthopyroxene (30–50vol%) with minor phlogopite (<3vol%) and quartz (<2vol%) and are suggested to be formed by silicate metasomatism of the mantle peridotite. The harzburgites, another major type of mantle xenolith in south Tibet, have a mineral assemblage of olivine (60–75vol%), orthopyroxene (20–35vol%), clinopyroxene (<3vol%), phlogopite (<2vol%) and spinel (<2vol%) and may have experienced subduction-related metasomatism. Combined with two types of ultrapotassic magmas, we propose that compositions of mantle wedge beneath south Tibet may gradually evolve from harzburgite through lherzolite to websterite with strong metasomatism of silicate-rich components in their mantle source region. Partial melting of the enriched mantle sources could be triggered by rollback of Indian continental slab during 25–8Ma in south Tibet.