Acid volcanism of various geotectonic situations (on the example of orogenic volcanic belts in Asia)

Abstract

Acid volcanism (rocks with silica more than 64 wt%) is the most widely represented in the orogenic volcanic belts (continental margin and kollisional) and ensialic island arcs on the continental stage in their development. At an early stage of formation of these structures acid volcanics are part of differentiated series basalt-andesite-dacite-rhyolite formation, at a later stage they form bimodal (contrasting silica) formation of high alkalinity. The heyday of acid volcanism coincides with middle-orogenic stage as rhyolite-rhyodacitic eruptions, often in the form of ignimbrites. Variations of the the content of potassium and sodium in rocks are changing in different stages. The most constant ratio of potassium and sodium are characterize for middle-orogenic volcanics while early and especially later-orogenic volcanic rocks vary widely in content of these parameters. In general, the acid volcanic orogenic belts more alkaline and potassium than rock ensialic island arcs, except sometimes occur at the front of orogenic belts of small island- structures ultrapotassic rhyolites. Petrochemical peculiarities of volcanics vary considerably across the strike of orogenic belts. Low-potassium branch of calc-alkaline series dominated in the frontal zones, the rocks of high-K branches of this series as far as shoshonite-latite petrochemical series developed in the rear areas of orogenic belts. The emergence of acid volcanic rocks in a large volume and their alkalinity correlated with the maturity of the continental crust, with an overall impact on the granite-metamorphic layer heat a deep basaltic magma, transmagmatic fluids and reomobilized of “crust” volatile components.