High-pressure melting of metapelite and the formation of Ca-rich granitic melts in the Namche Barwa Massif, southern Tibet

Abstract

A detailed geochemical and geochronological study of anatectic migmatites from the Namche Barwa Massif (NBM), southern Tibet, has been carried out to place important constraints on the thermal and tectonic evolution of the eastern Himalayan syntaxis. SHRIMP zircon U/Pb dating indicates that the granulite-facies metapelite underwent metamorphism at 21.8 ± 0.7 Ma and 24.5 ± 0.7 Ma, respectively. The latter is similar to the timing of partial melting and the formation of Ca-rich leucosomes at ~ 24–25 Ma. These leucosomes are characterized by (1) high CaO, Na 2O, and Na/K ratios; (2) radiogenic Sr ( 87Sr/ 86Sr(t) = 0.7407–0.7904) but unradiogenic Nd (ε Nd(t) = − 7.0 to − 21.2) isotope compositions; (3) depleted HFSE, and (3) variable but depleted HREE relative to their host pelites. Some of the leucosomes show large degrees of Nd isotopic disequilibrium, up to 10 epsilon units with respect to their hosts. These high CaO and Na 2O leucosomes were derived from fluxing melting of metapelite at high pressures. A similar process could have operated during the formation of the Himalayan leucogranites and contributes to the heterogeneities in such granites.