Platinum-group element geochemistry of the continental flood basalts in the central Emeisihan Large Igneous Province, SW China

Abstract

In the central zone of the Emeishan Large Igneous Province (ELIP), southwestern China, the Permian Emeishan continental flood basalts (ECFB) can be divided into high- and low-Ti basalts and alkaline basalts. At the southern margins of the central ELIP, the low- and high-Ti basalts comprise the lower and upper parts of the ECFB sequences, respectively; the alkaline basalts occur in the lower part of the ECFB sequence in a few locations. The high-Ti basalts are high in Pd (4-16 ppb) and Pt (7–18 ppb), but have large variations in Ir (0.04–0.88 ppb). The alkaline basalts have low TiO 2 contents (1.8–2.15 wt.%) and are depleted in PGE (< 1 ppb Pt and Pd with exception of one sample). The low-Ti basalts may be platinum-group element (PGE) depleted or undepleted. The PGE-depleted low-Ti basalts and alkaline basalts are marked by < 2 ppb Pd, and < 3 ppb Pt, whereas the PGE-undepleted low-Ti basalts contain 5–18 ppb Pd and have large variations in Pt (3–19 ppb) and Ir (0.02–0.36 ppb). For the PGE-depleted low-Ti basalts and alkaline basalts, Pd and Pt decrease significantly with decreasing of Ir. Very high Cu/Pd ratios (> 30,000) and very low Pt/Y ratios (230 to as low as 6) strongly suggest that the PGE-depletion in the low-Ti basalts and alkaline basalts is related to sulfide removal. The positive relationship between Pt and Pd depletion and increases in Zr/Nb and Th/Nb ratios indicate that crustal contamination played a role in driving the low-Ti basaltic magmas to S-saturation. In contrast, for the PGE-undepleted high- and low-Ti basalts, Pd decreases slightly with decreasing MgO and Ir, and Pd/Cr ratios increase with decreasing Pt/Y, indicating that fractionation occurred under S-undersaturated conditions. Some PGE-undepleted low-Ti basalts have very high Pd/Pt ratios (up to 6, much higher than that of the primitive mantle ~ 0.6) and extremely low Ir concentrations (< 0.03 ppb), suggesting that the magmas experienced fractionation of olivine, chromite, pyroxene and PGE minerals rich in Os, Ir, Ru, and Pt. The remaining high-Ti basalts have moderate Pd/Pt ratios (< 2.3) and very high Ir concentrations, up to 0.88 ppb, indicating that they possibly contain chromite and PGE minerals rich in Os, Ir, and Ru.