Late-stage southwards subduction of the Mongol-Okhotsk oceanic slab and implications for porphyry Cu Mo mineralization: Constraints from igneous rocks associated with the Fukeshan deposit, NE China

Abstract Multistage igneous rocks identified in the newly discovered Fukeshan porphyry Cu Mo deposit, NE China give new insights into the late-stage southwards subduction of the Mongol-Okhotsk oceanic slab (MOOS) and Cu Mo mineralization during the late Mesozoic. The U Pb ages of zircons from medium-grained monzogranite, coarse-grained monzogranite, ore-related quartz diorite porphyry, quartz monzonite, ganodiorite porphyry, diorite porphyry and andesitic porphyry associated with the Fukeshan deposit are 192.7 ± 1.9, 192.2 ± 2.7, 148.7 ± 0.8, 148.8 ± 0.9, 144.1 ± 1.1, 144.9 ± 0.9, and 144.8 ± 1.3 Ma, respectively. The Early Jurassic coarse-grained monzogranite, Late Jurassic quartz monzonite and quartz diorite porphyry, and Early Cretaceous granodiorite porphyry are characterized by low Yb and Y contents, and high Sr/Y ratios, indicating adakite affinities. The whole-rock geochemistry, and Sr Nd and zircon Hf isotopic features of the coarse-grained monzogranite and granodiorite porphyry indicate they were derived from the partial melting of a thickened basaltic lower crust. On the other hand, the quartz monzonite and quartz diorite porphyry probably originated from the melting of oceanic crust together with assimilation of enriched mantle and continental crust. The medium-grained monzogranites were possibly derived from a basaltic lower crust at shallower depths, and the diorite porphyry and andesitic porphyry probably originated from a metasomatized mantle source. Considering the Early Jurassic–Early Cretaceous magmatism and regional tectonic evolution of the Erguna Block, we propose that the MOOS was subducted towards the south during the Early Jurassic (195–170 Ma), slab retreat occurred during the early Late Jurassic (165–155 Ma), oceanic ridge subduction took place during the Late Jurassic (150–147 Ma). The final closure of the Mongol-Okhotsk Ocean occurred during the Early Cretaceous (145–143 Ma). The southwards subduction of a ridge on the MOOS provided a favorable tectonic setting for Cu Mo mineralization, and this is accordance with the discovery of many Late Jurassic–Early Cretaceous porphyry Cu Mo deposits in the eastern part of the Erguna Block.