PGE and geochemistry of Wajilitag ultramafic cryptoexplosive brecciated rocks from Tarim Basin: Implication for petrogenesis.
塔里木瓦吉里塔格超镁铁质隐爆角砾岩铂族元素和微量元素地球化学特征及其岩石成因探讨

Wajilitag ultramafic cryptoexplosive brecciated rocks, as one of the most important rock types of the Tarim Permian Large Igneous Province, were studied on platinum group elements (PGE), major, trace and rare earth elements in this study. The results show that these rocks have Os, Ir, Ru, Rh, Pt and Pd contents of 0.36×10-9~1.08×10-9, 0.23×10-9 ~0.44×10-9, 0.29×10-9~0.92×10-9, 0.11×10-9~0.18×10-9, 1.88×10-9 ~3.16×10-9 and 1.39×10-9~3.52×10-9 respectively, which are lower than those of the primitive mantle and can be compared with the Hawaiian picrites. The Pd/Ir ratios range from 3.6 to 11.9 and the PGE patterns take a shape of positive inclined curve, showing a trend of differentiation and a PGE affinity of basic-ultrabasic rocks under within-plate environment. Generally, the brecciated rocks have higher and concentrated Cu/Pd ratios (5.1×104 ~12.1×104) compared to the primitive mantle, indicating that no obvious sulfur oversaturation and segregation occurred during the magma upwelling, but the magma source may have some residual sulfide during its process of partial melting. The positive correlations for IPGE vs. MgO and negative correlations (or no obvious correlation) for PPGE vs. MgO suggest that the PGE differentiation was probably controlled by differentiation of olivine. Geochemically, the brecciated rocks are much enriched in total REE amounts (964.1×10-6~1299×10-6) and have strong fractionation between LREE and HREE . On the primitive mantle normalized spidergram, the brecciated rocks show humped distribution patterns, with LILE enrichment, slight depletion of Nb and Ta, and negative Zr and Hf anomalies, suggesting that the rocks may have undergone mantle metasomatism. However, similar PGE patterns from the breccias and matrix show that mantle metasomatism might not be a main factor for the PGE fractionantion of the brecciated rocks, indicating that the PGE may be existed in the sulphide inclusions in the silicate minerals.