Mo isotope evidence for the significance of subducted continental crust in formation of post-collisional porphyry Cu deposits

Abstract

Porphyry systems in continental collision belts contain substantial copper (Cu) and molybdenum (Mo) resources. However, unraveling their magma source compositions poses a significant challenge due to the superposition of previous oceanic subduction fingerprints. We report Mo isotope data on post-collisional (Oligocene–Miocene) Cu (Mo)-bearing porphyries (CBPs) and related barren-ore rocks (including Eocene granites and Miocene high-Mg diorites and ore-barren granites) in the eastern Gangdese belt, southern Tibet. The Eocene granites with high δ^98/95Mo values (0.37–0.58 ‰) and K₂O, Th contents originated from anoxic sediment-modified juvenile lower crust related to oceanic subduction. The Miocene high-Mg diorites with shoshonitic characteristic have extremely low δ^98/95Mo of −1.20 ‰ to −0.92 ‰, possibly deriving from a lithospheric mantle metasomatized by slab melts from subducted Indian crust. By contrast, the CBPs show moderate and variable δ^98/95Mo values (−0.85 ‰ to 0.34 ‰), reflecting either source or magma mixing of juvenile Asian lithosphere and subducting continental crust-derived melt components. The Miocene ore-barren granites have element compositions similar to the CBPs but are more Na-rich and have lower Th contents, slightly depleted Sr-Nd isotopes, and heavy Mo isotopes (−0.14 ‰ to 0.23 ‰), implying lower amounts of continental components in their magma source. Our Mo isotope data thus provides solid evidence for the incorporation of subducted Indian slab melts into the CBP magmas. Furthermore the CBPs contain more Indian components than the Miocene ore-barren granites, suggesting that the input of subducted continental crust melts is critical in the formation of post-collisional porphyry systems possibly by oxidizing the overlying fertile lithosphere.