Petrogenesis of ore-bearing porphyry in non-subduction setting: a case study of the Eocene potassic intrusions in the western Yangtze Block

In the western Yangtze Block, abundant Eocene (~38–34 Ma) potassic adakite-like intrusions and associated porphyry copper deposits are exposed in non-subduction setting, including Machangjing, Beiya, Binchuan, Habo and Tongchang intrusions. All these ore-bearing porphyries share many geochemical characteristics of adakite such as depletion in heavy rare earth elements (HREEs), enrichment in Sr and Ba, absence of negative Eu anomalies, high SiO₂, Al₂O₃, Sr/Y, La/Yb and low Y, Yb contents. They also exhibit affinities of potassic rocks, e.g., alkali-rich, high K₂O/Na₂O ratios and enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs). Their Sr-Nd isotopic ratios are similar to coeval shoshonitic lamprophyres. Geochemical data indicate that they were probably produced by partial melting of newly underplated potassic rocks sourced from a modified and enriched lithospheric mantle. These underplated rocks have elevated oxygen fugacity, water and copper contents, with high metallogenic potential. We propose that all the studied potassic rocks were emplaced in a post-collisional setting, associated with the local removal of lithospheric mantle.