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

• Published in: Chemical geology Vol. 262; no. 3; pp. 246 - 261
• Main Authors: Song, Xie-Yan, Keays, Reid R., Xiao, Long, Qi, Huo-Wen, Ihlenfeld, Christian
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 30-05-2009
• Subjects: Alloy; Central Emeishan large igneous province; Differentiation; Fractionation; Platinum-group elements; Sulfide separation; Fractionation; Sulfide separation; Central Emeishan large igneous province; Differentiation; Alloy; Platinum-group elements
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2009.01.021

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.