Paper Number: 708
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
1 State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Northern Taibai Str. 229, Xi'an 710069, China
2State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China
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Unlike the Bushveld complex, in which economic Fe-Ti oxide layers commonly occur in the upper section of these layered intrusions [1], the Permian Baima mafic layered intrusion, which is believed to be related to the S-undersaturated Emeishan high-Ti basalts, hosts a giant Fe-Ti oxide deposit in the lower or middle parts of the intrusion [2, 3, 4].
Uniformly high Cu/Pd (1.9×106 - 6.1×104) and low Pd/Zr (<0.1) indicate that the Baima parental magma experienced prior sulfide segregation. Mantle-liked δ34S values and low S/Se values indicate negligible external sulfur addition. Primitive mantle-normalized PGE patterns of the Baima rocks indicate that their parental magmas were initially S-undersaturate, and experienced significant fractionation of silicate minerals before reaching sulfide saturation. MELTS modeling indicates that the parental magmas were generated by ~60% fractional crystallization of olivine, pyroxene, and chromite from a high-Ti picritic magma at a deep crustal levels (5 kbar) [4]. Hence, we suggest that S saturation and sulfide liquid immiscibility in the deep crust resulted from relatively high S concentration in the residual magma, which was triggered by advanced fractional crystallization of silicate minerals.
Strong positive correlations 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, FeOT+TiO2, and V, together with MELTS calculations, 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 triggered by decreasing Fe2+ accompanying magnetite precipitation.
Primitive mantle-normalized PGE patterns for Baima intrusion rocks display similar trends to high-Ti basalts inside the Panxi area [5, 6, 7], suggesting that they are comagmatic, 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 [7, 8] indicate that they experienced more extensive pre-eruption fractional crystallization. Further fractional crystallization process led these lavas show more PGE fractionated feature.
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