Late Early Cretaceous magmatic rocks (118–113 Ma) in the middle segment of the Bangong–Nujiang suture zone, Tibetan Plateau: Evidence of lithospheric delamination

Abstract

Early Cretaceous magmatic rocks (including andesites, rhyolites, and granodiorites) occur in the Jiang Co and Zigetang Co areas of the middle segment of the Bangong–Nujiang suture zone, Tibet, and zircon UPb dating reveals that the magmatism took place between 118 and 113Ma. The Zigetang Co andesites have geochemical features of the high-Mg adakitic andesites (HMAA) and are characterized by high K2O (2.95–3.58wt%), Th (12.5–15.0ppm), MgO (2.50–3.31wt%), and Mg# (58–59), and relatively juvenile εHf(t) (+2.7 to +6.4) isotopic compositions. These observations suggest that the andesites were derived from partial melting of the delaminated juvenile lower continental crust of the northern Lhasa terrane. The Zigetang Co granodiorites represent the melts generated by high-degree fractional crystallization of the HMAA magma. The andesites from the Jiang Co area are interpreted as deriving from partial melting of ancient heterogeneous subduction-modified lithospheric mantle. The Jiang Co rhyolites are probably products of crustal anatexis in a heterogeneous source. Taking into account previous data, we propose that these late Early Cretaceous magmatic rocks developed in a postcollisional tectonic setting and that they were related to the delamination of thickened lithosphere following the final Lhasa-Qiangtang amalgamation.