松辽盆地三台地区下白垩统营城组珍珠岩地质特征与成因

三台地区北山珍珠岩产于营城组一段,出露面积约0.62 km², 厚度超过30 m。野外掌子面可见珍珠岩与膨润土、隐爆角砾岩、流纹岩共生。研究区珍珠岩可分为枕状球状珍珠岩、具流纹构造（块状）珍珠岩、角砾状珍珠岩、珍珠质集块岩。珍珠岩中含角闪石和辉石斑晶，基质脱玻化明显，出现球粒结构和微晶(骸晶)结构。珍珠岩与其他岩石的序列关系为珍珠岩-膨润土-隐爆角砾岩-流纹岩。岩相序列关系为侵出相-火山通道相-喷溢相。研究区珍珠岩是水下快速冷却形成的，其形成可以分为3个阶段。第一阶段（塌陷火山湖-爆发阶段），早白垩世在三台地区形成了陆相塌陷火山湖，并发生火山爆发，形成凝灰岩（水平层理发育）。第二阶段进一步分为3个亚阶段：水下侵出亚阶段，在断陷湖盆中发生火山喷发，由于盆地内水体的骤冷作用，形成大量的珍珠岩（火山玻璃质熔岩）和部分流纹质熔岩；隐爆亚阶段，形成流纹质隐爆角砾岩；喷溢亚阶段，形成流纹岩。第三阶段（部分膨润土化阶段），在火山喷发间歇期由于大气降水补给的地下水，在碱性条件下对珍珠岩进行水化，但珍珠岩层处于相对封闭条件，没有充分的水介质活动，珍珠岩部分（表层、裂缝处）转化为膨润土。

Geological Characteristics and Genesis of Perlite in the Lower Cretaceous Yingcheng Formation in the Santai Area of the Songliao Basin

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.