Geochemical evidence of the Pliocene bimodel volcanic rocks in Yingjiang area of western Yunnan Province and its tectonic significance

The Pliocene volcanic rocks in Yingjiang area constitute an important part of Tengchong volcanic rocks, but they are rarely involved in previous studies. In this paper, through the study of petrochemistry and K-Ar geochronology of the volcanic rocks and a comparison with the eastern Tengchong Pliocene volcanic rocks in the Longjiang River Valley area, the authors consider that the Pliocene volcanic rocks in Yingjiang and Tengchong areas seem to be a kind of rift volcanic rocks of the post-tectonic orogenic belt formed under the effect of extension that appeared in the continental tectonic background. With high Al₂O₃ (15.54%~17.44%) and high K₂O (0.94%~3.50%) petrochemical characteristics, the rocks should belong to high potassium calc-alkaline volcanic rock series. They formed in an extensional environment and belong to bimodel volcanic rocks (basalt DI=33~50, dacite DI=62~76). The rocks have the geochemical characteristics of arc volcanic rocks, but they formed in a tensional environment in post-tectonic orogenic belt and had nothing to do with the plate subduction. The rift volcanic rocks of orogenic belts formed in the early orogenic belt and hence were magmatic products in the process of the development of the orogenic belt at a certain stage.