Late Eocene sodic calc-alkaline lamprophyre in Gangdese batholith, southern Tibet: Implications for mantle metasomatism by carbonatitic melt

Abstract

Similar to oceanic lithosphere, the progressive and continuous subduction of continental lithosphere could lead to early dehydration and subsequent melting of the subducting slab. Melting subducted carbonate materials could release carbonatitic melts and trigger melting of the overlying lithospheric mantle and the formation of different types of alkaline and calc-alkaline rocks. In this paper, we present a study on a suite of mafic dikes of sodic calc-alkaline lamprophyre (CAL) composition in the eastern Gangdese batholith, southern Tibet. These dikes, emplaced ca. 36.9 ± 0.3 Ma, are characterized by (1) high CaO and Na2O but low K2O content and hence exceptionally high Na2O/K2O ratios (>2.3 up to 68.1); (2) negative anomalies of Nb-Ta-Zr-Hf-Ti, and (3) strong enrichment in light rare earth elements and large ion lithophile elements. Significantly low Rb/Sr (<0.06) and Dy/Yb (<2.5), coupled with high Ba/Rb (>31 up to 1447), suggest their derivation by low-degree partial melting of amphibole-bearing peridotite within the garnet-facies lherzolite field. Compared to those in depleted mantle, they are also characterized by high initial 87Sr/86Sr (0.7074−0.7078), 207Pb/204Pb(t) (15.74−15.75), and 208Pb/204Pb(t) (39.39−39.41), but relatively low εNd(t) values (−1.8 to +2.4) and 206Pb/204Pb(t) (18.81−18.82) and zircon εHf(t) values (+2.7 to +7.3). In terms of Sr-Nd isotope systematics, they are more enriched than those in Mesozoic−Cenozoic magmatic rocks, but more depleted than younger (after 30 Ma) potassic−ultrapotassic rocks within the Gangdese batholith, which requires a different mantle source. These lamprophyres contain abundant carbonate minerals with a linear correlation between CaO and CO2, which suggests primary carbonate phases and a mantle source region rich in carbonate materials. We propose that sodic CAL was generated by the partial melting of an enriched lithospheric mantle metasomatized by carbonatitic melts derived from the subducted Indian continental crust. The emplacement of lamprophyres is associated with E-W extension induced by local post-collisional delamination of thickened lower continental crust (∼75 km) after the collision between India and Asia.