Heterogeneous mantle associated with asthenosphere and Indian slab metasomatism: Constraints on fertilization of porphyry Cu mineralization in Tibetan orogen

Abstract

Previous models related to ultrapotassic mantle and post-collisional porphyry Cu mineralization have been challenged by the newly identified ultrapotassic lamprophyres (14.31 ± 0.61 Ma), diorite porphyries (14.82 ± 0.21 Ma) and high Sr/Y monzogranite porphyries (14.19 ± 0.23 Ma) from Dabu deposit in the Gangdese, Tibet. We found that the continental crust and lithospheric mantle beneath Gangdese were highly heterogeneous. The western Gangdese (80°E to 89°E) are mainly characterized by enriched lithospheric mantle with εNd(t) ≤ -10 and 87Sr/86Sr ≥ 0.71, and relatively normal crustal thickness and maturation (∼55–65 km), whereas eastern Gangdese (89°E to 95°E) are dominated by relatively depleted lithospheric mantle with εNd(t) ≥ -10 and 87Sr/86Sr ≤ 0.71, and much more thickened and mature crust (∼62–75 km). Based on three-component modeling, the geodynamic mechanism in the western Gangdese is subducted Indian slab metasomatism induced by its roll-back and subsequent break-off, whereas the eastern Gangdese is upwelling asthenosphere metasomatism resulted from convective delamination of thickened lithospheric mantle root. The lamprophyres at Dabu with εNd(t) values of −11.34 to −10.59 represent the very rare and well-preserved ancient lithospheric mantle without significant metasomatism and modification. Instead of previous model that focused on connection with Indian continental plate, we prefer to the generation of porphyry Cu deposits in Tibetan orogen were optimal alignments and combinations of multiple magmatic and dynamic processes, involving metasomatism of subducted Neo-Tethyan slab, metamorphic dehydration and thickening of lower crust, long-lived melting-assimilation-storage-homogenization (MASH) hot zone in the lower crust, and further fertilization of multiple magma chambers (∼30 km, ∼10 km, 1–6 km) in the upper crust.