贵州梵净山前寒武系甚低级变质作用研究

Very low to low-grade metamorphism of Precambrian in Fanjingshan area,Guizhou,China

本文运用伊利石结晶度、绿泥石结晶度、云母b0值和绿泥石地质温度计研究了梵净山前寒武系甚低级变质作用。结果表明经Kisch伊利石结晶度标样校正的伊利石结晶度Kübler指数为0.20~0.99°Δ2θ,绿泥石结晶度árkai指数为0.22~0.63°Δ2θ。由此阐明梵净山前寒武系经历了成岩作用到浅变质作用的影响,迄今依然保留了成岩带、近变质带和浅变质带的岩石记录。成岩岩石具有伊蒙混层特征成岩指示矿物,浅变质带具有钠云母特征变质指示矿物。钾云母(伊利石)b0值范围为:0.9000~0.9045nm;平均0.9018nm,b0值累积频率曲线分布介于典型中压—中低压系列之间,估算的压力为366MPa。峰期变质温度为293~364℃,平均342℃,古变质温压梯度为27℃/km。近变质带界线穿越地层和梵净山背斜轴线,表明甚低级变质作用为成岩-构造运动同期或后期构造增厚引起下部和底部岩石随温压的升高而产生的。

The very low to low-grade metamorphism overprinting on the Precambrian in Fanjingshan area was investigated with illite crystallinity,chlorite crystallinity,b0 value of K-micas(illite)and geothermometry of chlorite.The results demonstrate that illite crystallinity Kübler index(KI),calibrated with Kisch IC set,ranges from 0.20~0.99°Δ2θ,and the árkai index of chlorite crystallinity ranges as 0.22~0.63°Δ2θ,indicating the Precambrian in Fanjingshan area encountered with the geological processes from diagensis to epimetamorphism,and it remains diagenetic,anchizonal and epizonal records in rocks in the area studied.The diagenetic rock contains diagenetic index mineral illite-montmoriolite mixed-layer phase while anchizonal rock bears metamorphic index mineral paragonite.The b0 of K-micas(illite)ranges 0.9000~0.9045 nm,and on average 0.9018 nm or roughly equivalent to 366 MPa.The cumulative frequency curve of b0 stands in between the curves of N.New Hampshire and Otago,indicating the metamorphic pressures condition in between low-intermediate pressure and intermediate pressure facies.Peak metamorphic temperature was evaluated at 293~364℃and average 342℃.Therefore the palaeo-gradient was derived at 27℃/km.The anchizonal boundaries across strata and folding axis of the Fanjingshan anticline,clarifying the syn-to post-sedimentary or structural movement of epimetamorphism,which caused the increasing in depth and the rising of temperature and pressure in rocks at the bottom or lower part of basin due to structural burial.