Early Eocene diorite-porphyry dykes and granitic dykes from the Nyemo pluton, southern Tibet: Implication for the Neo-Tethyan slab breakoff

The process of oceanic subduction to continental collision is of significant importance for the comprehension of orogenic belt formation and continental crust growth. Nevertheless, the  geodynamic evolution of the Neo-Tethyan slab during the transition from subduction to collision remains central to understanding the Tibetan Plateau's formation. This study presents whole-rock geochemistry and zircon U-Pb-Hf isotopes for Eocene diorite-porphyry and granite dykes from the Nyemo area, southern Lhasa subterrane. Both dyke suites were emplaced at 47-46 Ma and exhibit subduction-related arc affinities with positive εHf(t) values (+9.0 to +17.2). The diorite-porphyry dykes (high Mg#, Cr, and Ni) originated from the partial melting of the subducted Neo-Tethyan slab, followed by interaction with the overlying mantle peridotite. In contrast, the granite dykes were derived from the melting of juvenile Gangdese crust triggered by increased thermal flux. Combined with regional magmatic records, we propose a diachronous slab breakoff model propagating from west to east. The coexistence of high-Mg adakitic diorite-porphyry dykes and juvenile crust-derived granite dykes signifies a "slab window" environment and the completion of Neo-Tethyan slab breakoff beneath the central segment of the southern Lhasa subterrane.