| Abstract: | The magmatic province of the northern Lhasa Terrane includes an Early Cretaceous (120–130 Ma) plutonic event, and a Late Cretaceous (80–110 Ma) volcanic event. The plutonic association constitutes an older suite of granodiorites, monzogranites and tonalites and a younger peraluminous leucogranite facies. Plutonism occurred about 20 Ma after obduction of the Banggong ophiolite, following closure between the Lhasa and Qiantang Terranes.The earlier suite is of broadly calc-alkaline in composition but differs from arc-related magmas in that only more evolved compositions are represented (SiO2 > 58%) and Rb/Zr ratios are elevated relative to the Gangdese batholith to the south. Trace-element and isotopic constraints are consistent with derivation from a Late Proterozoic amphibole-bearing crustal source requiring temperatures > 950°C during anatexis. The leucogranites require a pelitic source which is tentatively identified as the Nyaingentanglha basement exposed south of the plutonic province. Unlike the High Himalaya leucogranites, trace elements and field relations require a high degree of melting at source (> 50%) suggesting fluid-absent melting at temperatures > 850°C. Such high crustal temperatures indicate convective heat transfer from the mantle.Thermal constraints together with a tectonic setting of post-emplacement uplift followed by a marine transgression in the northern Lhasa Terrane can not be reconciled with a model of tectonically thickened crust but are consistent with post-collision attenuation of the lithosphere. |
