Chalcophile element geochemistry of the Baima layered intrusion, Emeishan Large Igneous Province, SW China: implications for sulfur saturation history and genetic relationship with high-Ti basalts

摘要

The Permian Baima mafic layered intrusion, believed to be related to the S-undersaturated Emeishan high-Ti basalts, hosts a giant Fe-Ti-V oxide deposit in the lower part of the intrusion. Uniformly high Cu/Pd (1.9 × 10~6-6.1 × 10~4) and low Pd/Zr (<0.1) indicate that the Baima parental magma experienced prior sulfide seg regation. Mantle-liked δ~(34)S values and low S/Se values indicate negligible external sulfur addition. Primitive mantle-normalized PGE patterns and MELTS calculations indicate that extensive fractional crystallization (～59 %) of chromite, olivine and pyroxene at depth drove the primitive picritic magma to S saturation. Strong positive correlation between IPGE and PPGE and between PGE and V, Cr and S suggest that magmatic sulfide is the dominant mineral controlling the distribution of PGE in the Baima intrusion. A positive correlation between S and Cr, FeO_T + TiO_2 and V content, together with MELTS cal culations, indicate that the parental magma of the Baima intrusion reached a second stage of S saturation in the shallower Baima magma chamber, which was likely trig gered by decreasing Fe~(2+) accompanying magnetite precipitation. Primitive mantle-normalized PGE patterns for Baima intrusion rocks display similar trends to high-Ti basalts inside the Panxi area, suggesting that they are co-magmatic, and following a similar differentiation trend. However, the lavas erupted before they reached sulfide saturation. The more evolved nature of high-Ti basalts outside the Panxi area indicate that they experienced more extensive pre-emption fractional crystallization. Further fractional crystallization process led these lavas show more PGE fractionated feature.