黔东南凯里-榕江-从江前寒武系甚低级变质作用研究

Very low-grade metamorphism of the Precambrian along profile Kaili-Rongjiang-Congjiang in southeastern Guizhou Province,China

根据碎屑岩甚低级变质作用原理,运用伊利石结晶度、绿泥石结晶度、伊利石-白云母多型、伊利石(白云母)b_0值、绿泥石温度计等碎屑岩低温变质研究的方法,分析研究了黔东南前寒武系67个<2μm的粘土样品,得到黔东南凯里-榕江-从江一带下江群伊利石结晶度为0.221~0.607°Δ2θCu Kα、绿泥石结晶度为0.239~1.393°Δ2θCu Kα,变质带伊利石-白云母多型均为2M_1型,伊利石(白云母)b_0值为0.9001~0.9048nm,平均0.9021nm。根据伊利石(白云母)b_0值的累积频率曲线得出研究区下江群遭受的变质作用为典型的中压变质类型。由Cathelineau(1988)绿泥石温度计估算出下江群浅变质带峰期变质温度为362~382℃。以上结果表明凯里-榕江-从江一带下江群经历了成岩-高近变质-浅变质作用,基于Warr和Rice标样的成岩/变质界限,剖面可划分为一个成岩带、两个近变质带、一个转变带和一个浅变质带。结合华南地区新元古代构造运动,推断新元古界下江群经历的成岩-变质作用与晋宁运动引起的扬子地块与华夏地块碰撞拼贴有关。

According to the theories of very low grade metamorphism for clastic rocks, using illite crystallinity, chlorite crystallinity, illite-muscovite polytypes, the b0 cell-dimension of illite （muscovite）, chlorite geothermometer, etc., we analyzed 67 Precambrian samples of 〈 2μm fractions along profile Kaili-Rongjiang-Congjiang in southeastern Guizhou Province, and obtained these very low to low-grade metamorphism indicating data from the Xiajiang Group. These data include, illite crystallinity ranging from 0.221 to 0.607°Δ2θCuKα, chlorite crystallinity from 0.239 to 1.393°Δ2θCuKα, illite-muscovite polytypes that are all of the 2M1 polytypes in anchizone and epizone, the b0 cell-dimension of illite （muscovite） range from 0.9001 to 0.9048nm, and on average as 0.9021nm. According to the cumulative frequency curves of the b0 cell-dimension of illite （muscovite）, the pressure condition of epimetamorphism along the profile was derived as a typical intermediate-pressure type. The peak metamorphic temperature of the epizone was evaluated at 362 to 382℃ by the chlorite geothermometer of Cathelineau （1988）. The above results show that the Xiajiang Group along profile Kaili-Rongjiang-Congjiang underwent diagenesis, high anchimetamorphism and epimetamorphism. Base on a set of illite crystallinity standards of anchizonal boundaries by Warr and Rice, the profile was divided into one diagenetic zone, two epizones, a transition belt and one anchizone. Combined with the tectonic movement of the Neoproterozoic in South China, it can be inferred that diagenesis-metamorphism of the Neoproterozoic Xiajiang Group was connected with the collision between the Yangtze block and Cathaysia block caused by Jinning Movement.