THE CYCLICITY OF VOLCANIC ACTIVITIESOF YIXIAN FORMATION IN SIHETUN-YIXIAN AREA,LIAONING PROVINGE

Based on the analysis of petrology and occurrence of tuffite and fossils within it,the volcanic activities of Yixian formation are divided into three cycles.The early stage is a basic and intermediate-basic cycle that is of mainly basaltic or basaltic andesitic volcanic rocks.The middle stage is an acid-intermediate cycle dominated by rhyolitic or dacitic volcanic rocks.The last stage is a basaltic cycle with mainly basaltic or trachytoid rocks.The early stage could be subdivided into four subcycles.And for the first time,we have discovered planty of paleo vertebrate fossils (mainly of Psittacosaurus)in the tuffite of the early stage(basic and intermediate-basic cycle).The tuffites of explosive-sedimentary phase or inland lake phase always occur at the bottoms of each cycle or subcycle."Jehe Biota" fossils,including precious birds,occur in the tuffites and calcareous shale beds of the middle stage(intermediate-acidic cycle).The division of volcanic activity cycles of Yixina formation is of most significant to realize the volcanic-sedimentary action and the relationship between creature hazard and volcanic actions.