Geochemistry of Light Colour Clayrock Layers from the Early Cambrian Meishucun Stage in Eastern Yunnan and Their Geological Significance
滇东早寒武世梅树村期浅色粘土岩层的地球化学特征和地质意义

The character of lithology, mineralogy and geochemistry demonstrates that light colour clayrock layers are volcanogenic metabentonites within phosphatic succession of the Early Cambrian Meishucun Stage in eastern Yunnan. These clayrock layers could be taken as markers for the volcanic eruption events and for the event stratigraphy correlation. Metabentonites retain high concentrations of relatively immobile trace elements, particularly Hf, Nb, Y, Th and U compared with non-volcanogenic mudstones, but contain less concentrations of Fe, Ni, Co and Cr compared with the mudstones of the Qiongzhusi Formation. TiO 2/Al 2O 3, Ti/Th and Zr/Hf ratios of the all metabentonites are characterisitic of acidic magmas. Both chondrite normalized REE distribution patterns and middle to large negative Eu anomalies of the metabentonites are similar to those of granites. Discrimination diagrams indicate that these metabentonites possibly derived from rhyolite dacite trachyte magmas in a intraplate setting. The temporal and spatial distributions of metabentonites indicate that volcanic activities occurred very frequently at the west margin of the Yangtze Platform during the Meishucun Stage. These voicanic eruption events were related to the separation of the old continent and to the extension in a intraplate settings.