Petrological-Geochemical Characteristics of Volcanic Ash Sediments in Yanchang Formation in Ordos Basin

Abstract

Volcanic ash sediment is widely distributed in Yanchang Formation in Ordos basin,in order to study lithogeochemistry characteristics and formational mechanism of volcanic ashes sediment,the paper use many analysis methods,such as observe the cores,microscope,XRF,ICP-MS,SEM et al.. The results indicate volcanic ash sediment containing much crystal and shard fragments,including tuff and bentonite,the illite and andreattite can be seen under the SEM; SiO2 content between 50.29%-79.82%,rich potassium,K2O+Na2O at 3.20%-7.88%,the negative correlation present in relationship of SiO2 with Al2O3,TFe2O3 and MgO;∑REE=99.82×10-6~550.15×10-6,and rich in LREE,deplete in HREE,the curve rightward incline,Eu negative abnormity and the δEu between 0.147-0.837,there is no evidently Ce abnormity,and K,Rb,Ba,Th are of positive abnormity. In the illustration of Nb/Y-Zr/TiO2,the data dots are mainly located at the zone of andesite-rhyolite,which suggest that the volcanic ash is primary from middle-acid rocks. Lithogeochemistry and sedimentology illustrate the formational mechanisms of volcanic ash include:falling down from air and carried by water. Through the diagram of tectonic setting,the volcanic ash is primary from volcanic arc and calc-alkalic lavas,which is intimately related the evolution of Qinling orogenic belt.