New insights into the origin of Middle to Late Permian volcaniclastics (Nb-Zr-REY-Ga-rich horizons) from eastern Yunnan, SW China

Abstract

Volcaniclastic horizons near the Guadalupian-Lopingian Boundary (G-LB) from southwest China have attracted much attention, not only due to the End-Guadalupian mass extinction near the boundary, but also to the unique and regionally extensive Nb-Zr-REY-Ga mineralization with potential economic significance (REY = rare earth elements and Y). Previous studies have suggested that these horizons were derived from volcanism associated with the terminal phases of the Emeishan mantle plume, which however was not the only contemporaneous volcanic activity in southeast Asia at this time (~260 Ma). Here we present a systematic analysis focusing on the petrological, mineralogical, and geochemical features of the volcaniclastics from the Yiliang and Weixin boreholes from the lowest strata of the Late Permian Xuanwei and Longtan formations, respectively, eastern Yunnan, SW China. In addition, previous geochemical data obtained for the correlative layers near the investigated boreholes were used for reference. The mineralogy of the volcaniclastics is dominated by mixed-layer illite/smectite, illite, berthierine, kaolinite, and quartz, and to a lesser extent, albite, calcite, siderite, anatase, and hematite, along with traces of chamosite, rutile, and florencite. Additionally, zircon, pyrite, chalcopyrite, sphalerite, galena, rhabdophane, parisite, goyazite, and barite were identified under SEM-EDS. The values of Al2O3/TiO2 in the volcaniclastic samples are mostly greater than eight, consistent with the characteristics of intermediate-felsic rocks. The Nb/Yb and Zr/TiO2 vs. Al2O3/TiO2 diagrams, and the chondrite-normalized REY distribution patterns of the samples collectively indicate that the Nb-Zr-REY-Ga mineralization was not only derived from the ELIP alkaline rocks, but also from the arc-related intermediate-felsic magmatic rocks from the Paleo-Tethys. However, the latter source for Nb-Zr-REY-Ga mineralization in SW China has not been previously reported. The two sources of clastic materials, i.e., from both within-plate/anorogenic magmatism and arc-related/orogenic magmatism, agree with previously-reported discrimination diagrams of εHf(t) vs. age and Hf/Th vs. Th/Nb in the zircon grains from the correlative claystones. In addition, the modes of occurrence of critical metals (Nb, Zr, REY, Ga) are identified in detail in this study. The results show that most of the HREE and almost all the Zr and Hf are in zircon; the LREE are dominantly in REE-bearing carbonates and phosphates; the Nb is largely in former FeTi oxide phases; and the Ga is predominantly in the clays.