THE STRUCTURAL CHARACTERISTICS OF THE LONGGANG VOLCANO GROUP IN JILIN

The Longgang volcano group lies in the southeastern part of Jilinprovince, in the middle sector of the Longgang Mountains,covering an area of 1170 sq km.It tectonically belongs to the central part of the Teiling-Jingyu uplift on the northern margin of the Sino-Korean paraplatform.The basement rocks exposed n the area are migmatite,quartzite and migmatitic granite.There are a total of 167 volcanic cones of varying sizes.The longgang volcano group is mainly controlled by NE-and NW-trending faults and the craters are located at the intersection of the two above-mentioned sets of fault. Four eruption phases may be distinguished.The first phase took place in the middle part of the middle Pleistocene （the Dayanzishan Formation）.There are 33 volcanic cones distributed in the area west of the Erlongwan-Longquanwan fault.The eruptive materials are made up of volcanic lavas,agglomerates and bombs.The second phase occurred in the late Pleistocene （the Xinkailing For- mation）.The eruptive activities were concentrated in theXinkailing-Sihai area in the eastern part.There are altogether 134 volcanic cones.The eruptive materials are represented by basaltic volcanic lavas.The third phase happened in the early Holocene （the Sihai Formation）.It was an additional phase of rejuvenation of part of early-stage volcanoes.The eruptive materials are dominated by volcanic scoria.The fourth phase occurred extensively in the terminal Holocene （the Jinlongdingzi Formation）,but the intensity of eruption attenuated obviously.The flood basalt is dominant.