合肥盆地磨墩水库石墨片岩变形变质分析及构造意义探讨

摘要

At the bottom of the Jurassic conglomerate of the Fanghushan Formation in the Modun Reservoir area,central—south Hefei basin,developed a set of graphite schist,with fault contact relationship between them.The graphite schist is different from the Carboniferous—Permian sediments in the Hefei basin,also different from Yangshan coal-bearing measures in Gushi county,Henan province.Analysis of the graphite schist should be rocks of Fuziling group in Beihuaiyang area,its protolith is carbonaceous mudstone or carbonaceous siltsone.Graphit is of organic origin.Relatively low degree of graphitization hut deformation intensity,part of the graphite schist has entered the mylonit or ultra-mylonite stage.These reflect the features of subduction zone deformation and metamorphic.Further to do the analysis of metamorphic facies and temperature and pressure condition,the results show that the graphite schist was in the metamorphism of greenschist facies (estimated temperature 450 ～ 500℃,pressure O.55 ～0.8GPa.According to the microstructural characteristics and metamorphic deformation conditions can infer that the graphite schist should be the sedimentary cover in the northern margin of the Yangtze plate (part of the Fuziling group).It had experienced collision,graphitized in strong tectonic stress and formed the tectonic schist.%合肥盆地中南部肥西县磨墩水库南岸防虎山组侏罗系砾岩下发育一套石墨片岩系,两者之间呈断层接触.该石墨片岩不同于钻井揭示的合肥盆地石炭纪一二叠纪沉积岩,亦不同于河南固始县杨山煤系.经分析认为该石墨片岩应属于北淮阳佛子岭群岩系,原岩为炭质泥岩或炭质粉砂岩.石墨为有机成因,石墨化程度较低,但变形强烈,部分已糜棱岩—超糜棱岩,反映出俯冲—碰撞带变形变质特征.通过对白云母、石墨XRD衍射和电子探针测试分析,认为石墨片岩为绿片岩相,估算其变质温度为450～ 500℃,压力0.55 ～0.8GPa.根据显微构造特征和变形—变质条件推断,该石墨片岩为扬子板块北缘的沉积盖层(佛子岭群部分),在扬子板块与华北板块碰撞过程中受到强烈挤压,变形—变质成糜棱岩、石墨构造片岩.该变形带暗示了扬子板块向华北板块缝合带的位置,即石墨片岩出露区及其附近,部分被新生代沉积所覆盖.