Abstract: The Beishan perlite in the Santai area belongs to the first member of the Yingcheng Formation and is exposed in an area of 0.62 km², and its thickness is over 30 m. The perlite is associated with bentonite, hydrothermal breccia, rhyolite in typical geological sections. The perlite can be divided into pillow and orbicular, fluidal massive，breccia perlites and perlitic agglomerate. The perlite contains phenocrysts of amphibole and pyroxene, and its groundmass is of clear spherulitic and microlitic textures, indicating intensive devitrificaltion. In ascending order, the rock sequence is perlite→bentonite→hydrothermal breccia→rhyolite, and the facies sequence is volcanic conduit facies→effusive facies. The perlite is formed by fast under water cooling. The petrogenesis of perlite consists of the fellowing three stages: the first stage is a collapse volcanic lake-explosive phase. A collapse volcanic lake was formed during the early Cretaceous in the Santai area and tuff with developed horizontal beddings were formed in the stage. The second stage includes three substages. Substage of extrusion under water was featured by the volcanic eruption happened in the collapse volcanic lake, and large amount of perlite (volcanic glassy lava ) and some rhyolite lavas were formed because of quick cooling of magma under water of the basin in the substage. The hydrothermal substage was featured by the formation of rhyolitic hydrothermal breccia while the explosive-effusive substage is characterized by volcanic agglomerate, tuff and rhyolite. The third stage is called partial bentonitie stage. The perlite was hydrated by the groundwater of atmospheric origin under alkaline condition during inter-volcanic eruption stage. Only partial perlite (surface or fractures) was altered into bentonite because there was not enough water medium under relatively closed condition for the perlite.

Key words: the Yingcheng Formation, perlite, Songliao Basin