Post-subduction porphyry Cu magmas in the Sanjiang region of southwestern China formed by fractionation of lithospheric mantle–derived mafic magmas

For porphyry Cu deposits that formed during oceanic slab subduction, there is a general consensus that the ore-forming magmas evolved through fractionation of mafic magmas that were generated by slab fluid (±melt)–fluxed melting of the asthenospheric mantle wedge. This model, however, is not applicable to post-subduction porphyry Cu deposits because they formed distinctly after cessation of oceanic slab subduction. A popular model proposes that post-subduction porphyry Cu magmas were produced by partial melting of lower-crustal, sulfide-rich arc cumulates, with or without minor contributions from potassic mafic magmas. To reappraise the crustal melting model, we focused on one of the largest post-subduction porphyry Cu belts on Earth, which formed during the India-Asia collision in the Sanjiang region of southwestern China. Detailed petrographic studies and new Nd-Sr isotopic data from non-metasomatized versus metasomatized lower-crustal xenoliths suggest that previous models based on crustal melting rest upon wrong radiogenic isotope constraints due to pervasive metasomatism of the xenoliths. Based on trace-element quantitative modeling and regional-scale geochemical trends of magmatic rocks, we demonstrate that the Sanjiang post-subduction porphyry Cu magmas were produced by fractionation of potassic mafic magmas derived from lithospheric mantle. This study highlights that post-subduction porphyry Cu magmas attain their K-rich composition, and all the ore-forming ingredients, from subduction-modified lithospheric mantle, the existence of which may be a prerequisite for the formation of porphyry Cu deposits in post-subduction settings.