The initial slab rollback of Neo-Tethys Ocean: Constrain from Gongga adakitic rocks and enclaves in the late Cretaceous

The Neo-Tethys oceanic slab rollback and ridge subduction are two competing geodynamic processes responsible for the early stage of late Cretaceous magmatic activity. The spatial and temporal distribution of geochemical variability in igneous rocks can be used for reconstructing specific geodynamic events. We presented the whole-rock geochemistry, zircon UPb ages, mineral chemistry data of the Gongga adakitic host rocks and coeval dioritic enclaves emplaced in the southern Lhasa terrane. The Gongga adakitic host rocks have low Mg# (43.7– 44.9), but high in K2O/Na2O > 0.6, indicating they may be derived from the juvenile lower crust. According to geochemical modeling, the adakitic host rocks were formed by about 40% partial melting of the metamorphic gabbroic diorite with adakitic affinity. The typical magmatic texture and reverse oscillatory zoning of plagioclase in dioritic enclaves imply that they may be generated during the process of magma mixing. By the integrated analysis of new geochemical data of the Gongga intrusive rocks and compiled spatio-temporal distribution and geochemical data of Cretaceous magmatism in the southern Lhasa terrane, we proposed that the initial Neo-Tethyan oceanic slab rollback occurred at ∼98 Ma, causing the production of the Gongga host rocks and magmatic encalves. The Neo-Tethys ridge subduction and slab rollback jointly took part in the production of the magmatic flare-up during the late Cretaceous.