Petrology and geochemistry of the K-bentonites at the Ordovician-Silurian transition in XD2 well, Daguan, Yunnan Province

Many K-bentonites have been recognized from the Wufeng-Longmaxi Formations (Upper Ordovician-Lower Silurian) in the Yangtze Block,but only a few of them on the western margin of the Yangtze Block are reported. The mineralogical and geochemical studies of K-bentonites in the Wufeng-Longmaxi Formations through Xindi 2 well in Daguan area of Yunnan province were carried out to confirm the original magma type and its tectonic setting. The mineralogical characteristics show that the potassium bentonite is mainly composed of clay minerals and non-clay minerals,in which the clay minerals are composed of illite and illite-montmorillonite mixed beds. The non-clay minerals are mainly quartz,feldspar,calcite,dolomite and pyrite. It is geochemically characterized by high K₂O and low TiO₂,relative enrichment of Rb,Ba,Th and U and depletion of Ti and P elements. The Ti/Th values indicates acidic volcanic ash character. Compared with the chondrite,the total rare earth elements is (49.86-209.43)×10^-6 with slight rich LREE and negative Eu amomaly,without Ce abnormity. In Nb/Y-Zr/TiO₂ diagram,the data dots are mainly plotted in the andesite and trachy andesite range,which shows that the volcanic ash is mostly from middle-acid rocks. Various chemical discrimination diagrams and trace elements imply that K-bentonites were possibly derived from an island arc environment,and the volcanic ash was probably related to the subduction and closure of the Qinling Ocean on the northern border of Yangtze Plate in the Early Paleozoic.