PETROGRAPHIC-PETROLOGIC CHARACTERISTICS OF BASE-SURGE FACIES IN MESOZOIC VOLCANIC ROCKS, EAST GUANGDONG

摘要

A base-surge, named by volcanologists, is a rapid flow of the ring and low clouds which consist of water vapor (or water) and elastics of wall rocks or magmas, tightly along the ground surface from a center where the phreatic (magmatic) explosions occurred. The ancient base-surge deposits are a suite of stratified volcaniclastic rocks and are easily mistaken as normal sedimentary clastic rocks for their fine-grained, varicoloured and stratification developed diposits. Volcanic rocks in East Guangdong are divided into two (the upper and the lower) volcanic series and four volcanic cycles, and the lower series consists of the first and second cycles and is in late Jurassic age. The rocks of base-surge facies occupied a certain ratio in the first and second cycles and often occurred in intermediate to acidic volcaniclastic rocks. It frequently occurred in an erupto-sedimentary sequence of the third cycle and discontinuously spread around the volcano-tectonic depressions (the Caoan area) and sedimentary volcanic depressions (the Jiexi and Heyuan areas), and was consistent with the stratified volcanic rocks in occurrence.