矿床地球化学国家重点实验室知识库

Mantle-derived rocks in large igneous provinces yield large variations in their chemical and isotopic compositions as well as redox conditions; however, the origin of these heterogeneities remains unclear. We present new geochemical and Sr–Nd–Hf–O isotopic data for the Hongge mafic–ultramafic intrusion and associated syenite in the Emeishan large igneous province (ELIP), and combine them with previously published data from coeval intrusive and volcanic rocks in the ELIP, to constrain the generation of the ELIP and associated world-class Fe–Ti–V oxide deposits. The rocks from the marginal zone of the Hongge intrusion have high-Ti basaltic-like compositions with high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7084–0.7096) and unradiogenic ε_Nd (−7.7 to −5.9) and ε_Hf (−6.0 to −2.4) values, and represent the most enriched endmember of the high-Ti basaltic series observed in the ELIP to date. The marginal zone rocks, temporally and spatially associated basalts, and syenites yield depleted Zr and Hf contents with Zr/Sm (7.1–38.3) and Hf/Nd (0.05–0.12) ratios between those of marine sediment and OIB. The NdHf isotopic compositions of the marginal zone rocks plot above the terrestrial array, suggesting decoupling between Nd and Hf isotopic compositions. Mixing between subducted oceanic crusts, marine sediments and mantle peridotite accounts for the enriched SrNd isotopic compositions, ZrHf depletion, and the decoupling between Nd and Hf isotopic compositions in these samples as well as in other mafic–felsic intrusions of the ELIP. Zircon grains from the syenites yield more radiogenic ε_Hf values (+3.7 − +6.6) than the marginal zone rocks and have δ¹⁸O values of 3.9–5.7‰, most of which are much lower than mantle-zircon values, suggesting that oceanic crust altered at high temperature was involved in the mantle source. The integrated Sr–Nd–Hf–O isotopic compositions of these mafic–felsic intrusions can be explained by a model for the generation of the ELIP whereby a mantle plume interacted with the subducted Paleo-Tethyan oceanic lithospheric slab and overlying marine sediment. Our results suggest that the inclusion of the recycled altered oceanic slab in the mantle source of the ELIP led to its geochemical and, potentially, redox heterogeneity.