矿床地球化学国家重点实验室知识库

The ∼ 260-Ma Emeishan large igneous province (ELIP) in SW China and northern Vietnam includes high-Ti and low-Ti volcanic rocks and coeval mafic–ultramafic plutons. In the Jinping (SW China)–Song Da (northern Vietnam) district, the southernmost part of the ELIP, basalts and gabbros of the high-Ti series displays a narrow range of εNd(t) values (+ 0.5 to + 1.1) and low primitive mantle-normalized Th/Nb ratios (∼ 1.4), similar to those of an OIB-like enriched mantle source. The rocks have chondrite-normalized REE patterns enriched in LREE and depleted in HREE. This feature, together with low Al₂O₃/TiO₂ ratios (∼ 4), indicates that the primary melts were generated by small degrees of partial melting at garnet stable depths. The low-Ti series includes LREE-depleted and LREE-enriched picrites, basalts, olivine gabbros and gabbros. The LREE-depleted picrites have chondrite-normalized Ce/Yb ratios of 0.4–0.5, very positive εNd(t) values (+ 7.0), and very negative Th anomalies in primitive mantle-normalized trace element patterns. They were formed by high degrees of partial melting of a strongly depleted mantle source at relatively shallow mantle depths. The LREE-enriched picrites have very negative εNd(t) values (− 7.8) that indicate a mantle source locally modified by subducted oceanic crust. Highly variable chemical compositions and εNd(t) values (+ 6.4 to − 10.2) of the low-Ti basalts and gabbros are attributed to variable degrees of upper crustal contamination of the picritic parental magmas. Therefore, the extrusive and intrusive rocks in this region clearly formed from heterogeneous mantle sources. Crustal contamination triggered S-saturation of some low-Ti magmas and resulted in segregation of immiscible sulfide melts and generation of magmatic Ni–Cu–(PGE) sulfide deposits in this region. Eruption of magmas after sulfide segregation formed PGE-depleted low-Ti basalts.