Melting a melt-metasomatized subcontinental lithospheric mantle: Evidence from Oligocene lamproites within the Gangdese batholith, southern Tibet

Lamproite represents one of the most exotic mantle-derived magmas and provides insights into geochemical properties of subcontinental lithospheric mantle (SCLM). Zircon UPb ages and whole-rock element and Sr-Nd-Pb isotopic data on four lamproite dikes within the Qulin complex, Gangdese batholith, southern Tibet demonstrate that these dikes formed at ∼30 Ma, ∼4 myr earlier than those potassic to ultrapotassic rocks occurred along the N-S trending rift zones. The Nyemo lamproites have low SiO2 and Al2O3, but high K2O, MgO, and CaO. They also have high Sr and Ba concentrations, enriched in light rare earth element (LREE) but strong depleted in heavy rare earth element (HREE) with (La/Yb)N = 43.9–136, and enriched in LILE, as well as negative anomalies in Nb, Ta and Ti. The Nyemo lamproites have unradiogenic ɛNd(t) values ranging from −9.0 to −15.6, and 206Pb/204Pb(t) = 18.64–18.83, 207Pb/204Pb(t) = 15.77–15.81 and 208Pb/204Pb(t) = 39.53–39.83. They were derived from low-degree partial melting of SCLM which had been metasomatized by silicic melt and minor carbonated melt. Elevated Nb/Ta and Th/U ratios might be resulted from element transfer and metasomatism by silicic melt from the subducting sediments during the prolonged and episodic subduction of the Tethyan oceanic lithosphere followed by the Indian continental lithosphere. High Ba and Sr concentrations and super-chondritic Zr/Hf ratios are related to a carbonate metasomatism. The contribution from the melt-modified SCLM in the Nyemo lamproites is much higher than in the other Tethyan Realm Lamproites (TRL). Both the age and geochemical differences of the Nyemo lamproites from the other potassic to ultrapotassic rocks in southern Tibet could be explained by the preferential melting of fertile and enriched components in the SCLM due to melt metasomatism during the initial opening of the Southern Tibet Rift System (STRS). Some of high Sr/Y and high Ba rocks might be derived from partial melting of thickened mafic lower crust modified by various degrees of infiltration of lamproitic magma. Most importantly, the Nyemo lamproite dikes within the Yadong-Gulu Rift suggest the initiation of the N-S trending STRS could be as early as ∼30 Ma. •The Nyemo lamproites were derived from melting of melt-metasomatized SCLM.•The Nyemo lamproites have elevated Nb/Ta and Th/U ratios.•Melt of metasomatism was derived from melting of subducted sediments.•The initiation of the Southern Tibet Rift System could be as early as ∼30