Platinum-group elements and geochemical characteristics of the Permian continental flood basalts in the Tarim Basin, northwest China: Implications for the evolution of the Tarim Large Igneous Province

Permian continental flood basalts (CFBs), as a main component for Tarim Large Igneous Province (TLIP), are widely distributed in the western and central parts of the Tarim Basin. Here we present a systematic study of platinum-group elements (PGEs) combined with conventional major, trace and Sr-Nd isotopic geochemistry to characterize the Tarim CFBs from the Yingan section in the Keping area, west of the Tarim Basin. The basaltic units with a total thickness of ca. 400 m in the section can be divided into two basaltic sequences: the lower Kupukuziman and the upper Kaipaizileike sequences. Both of the sequences display extremely low PGE concentrations, with Os. It, Ru, Rh, Pt and Pd contents of 0.014-0.106, 0.007-0.072, 0.035-0253, 0.011-0.078, 0.043-0.149 and 0.026-0.124 pot,. respectively. The Keping basalts exhibit a marked increase in Cu/Pd ratios (>10(5)) albeit with a narrow range of lower Pd/Ir ratios (<50), different from the PGE-undepleted basalts of the Siberian Traps, Emeishan Large Igneous Province (ELIP), East Greenland CFBs and Deccan Traps. Furthermore, the Tarim CFBs in Keping and elsewhere in the basin have very low Cu/Zr ratios (<05 commonly), indicating that their parent magmas were significantly depleted in chalcophile elements. The Tarim CFB magmas are estimated to have formed by <5% partial melting, and were probably S-saturated during partial melting, leaving residual sulfide in the mantle source. The differences in Th/Yb and (Th/Nb)(N) ratios between the Kupukuziman and Kaipaizileike sequences suggest that the Keping basalts incorporated variable degrees of assimilated crustal components from the Precambrian basement in the Tarim Basin. Modeling indicates that the earlier erupted Kupukuziman lavas witnessed relatively larger extent of assimilation than the later Kaipaizileike lavas. Nevertheless, the crustal assimilation did not trigger sulfur saturation and sulfide segregation for the basaltic magma in the crust. Furthermore, a magma mixing process between the relatively primitive and more evolved residual magmas of the same lineage during magma chamber replenishment is suggested according to the PGEs and Sr-Nd isotope variations in the Kaipaizileike sequence. This process might have also induced limited sulfur saturation and sulfide segregation in the late stage of the magmatic evolution. A comparison of the chalcophile element characteristics between the basalts in the Tarim Basin and the mafic-ultramafic rocks in the Eastern Tianshan and Beishan Rift areas indicates that the degree of partial melting plays an important role on the PGE geochemistry and potential PGE-(Ni-Cu) ore deposits in the TLIP. (C) 2012 Elsevier B.V. All rights reserved.