Late Cretaceous (100–89 Ma) magnesian charnockites with adakitic affinities in the Milin area, eastern Gangdese: Partial melting of subducted oceanic crust and implications for crustal growth in southern Tibet

Abstract

Rapid Mesozoic–Early Cenozoic crustal growth in the Gangdese area, southern Tibet, has commonly been, attributed to pre-collisional and syn-collisional underplating of mantle-derived magmas. Here, we report on, adakitic magnesian charnockites (i.e., hypersthene-bearing diorites and granodiorites) near Milin, in eastern, Gangdese, that provide new insights into the crustal growth process of the region. Zircon U–Pb analyses of' seven charnockite samples indicate that they were generated in the Late Cretaceous (100–89 Ma). They exhibit variable SiO2 (53.9 to 65.7 wt.%) contents, high Na2O/K2O (1.6 to 14.4) and Sr/Y (27.2 to 138.7) ratios, low Y (6.5 to 18.5 ppm), heavy rare earth element (e.g., Yb = 0.6 to 1.6 ppm) and Th (0.20–2.39 ppm) contents and Th/La (0.02–0.23) ratios, with relatively high Mg# (46 to 56) and MgO (2.0 to 4.5 wt.%) values. They are characterized isotopically by high and slightly variable εNd(t) (+ 2.4 to + 4.0) and εHf(t) (+ 10.1 to + 15.8) values with relatively low and consistent (87Sr/86Sr)i (0.7042 to 0.7043) ratios.Their pyroxenes have high crystallization temperatures (876 to 949 °C). The Milin charnockites were most probably produced by partial melting of subducted Neo-Tethyan oceanic crust that was followed by adakitic melt–mantle interaction, minor crustal assimilation and fractional crystallization of amphibole + plagioclase. The upwelling asthenosphere, triggered by the roll-back of subducted Neo-Tethyan oceanic lithosphere, provided the heat for slab melting. Therefore, we suggest that, in addition to pre-collisional and syn-collisional underplating of mantle-derived magmas, the recycling of subducted oceanic crust has also played an important role in continental crustal growth in southern Tibet.