陕甘川交界区碧口群的绿帘石及其岩石学意义

Epidotes from Bikou Group in the Shanxi-Gansu-Sichuan Border Area and Their Petrological Significance

绿帘石广泛分布于碧口群的各类变质岩石中,但以变质基性岩(绿片岩和蓝片岩)为主。绿片岩中绿帘石X_(?s)=0.23—0.37,但多为0.30—0.32,在不同样品中绿帘石成分变化很大,但同一样品中绿帘石变化不明显,说明其成分与全岩成分有关。蓝片岩中绿帘石X_(?s)=0.22—0.36,其成分在同一样品中也有很大变化,一般细粒者ps分子少,而粗粒者ps分子多,且常发育环带结构,向中心ps分子减少,说明蓝片岩可能经历了一个以降压为特征的变质过程。此外,该群也偶见绿纤石,但它们均被绿帘石包裹,不能构成独立变质相。

Epidotes are extensively seen in various metamorphic rocks of the BikouGroup, especially in such metabasites as greenschist and blueschist. X_(ps) valuesof epidotes from greenschist are in the range of 0.23--0.37, mostly 0.30--0.32.Epidotes from different samples show considerable compositional variation,whereas those from the same sample are quite similar in composition, indica-ting that their composition is related. to bulk composition. As for epidotesfrom blueschist, X_(ps)=0.22--0.36, and their composition varies remarkablyeven.in the same sample. In general, fine--grained epidotes contain relativelylow ps while things are just the opposite for coarse--grained ones whichcommonly exhibit zonal structure characterized by the gradual decrease of pstowards the core, suggesting that the bluescbist might have experienced apressuree--decline metamorphic process. In addition, a few pumpellyites areoccasionally observed in the group; nevertheless, they are all enveloped inepidotes and fail to constitute an independent metamorphic facies.