胶东早元古代荆山群富铝片麻岩中矿物共生的P-T-X关系及其演化

The PTX Phase Relations and Their Evolution for theEquilibrium Assemblages of Alrich Gneisses fromEarly Proterozoic Jingshan Group in East Shandong

根据显微构造和矿物微区化学成分的详细研究，确定了胶东早元古代荆山群富铝片麻岩中存在平衡组合Ｇｒｔ－Ｃｒｄ－Ｂｔ－Ｓｉｌ－Ｋｆｓ－Ｐｌ－Ｑｔｚ，反映变质峰期Ｇｒｔ＋Ｓｉｌ＋Ｑｔｚ→Ｃｒｄ，Ｂｔ＋Ｓｉｌ＋Ｑｔｚ→Ｇｒｔ＋Ｃｒｄ＋Ｋｆｓ＋Ｈ２Ｏ和Ｐｌ（Ａｎ）→Ｇｒｔ（Ｇｒｓ）＋Ｓｉｌ＋Ｑｔｚ①等转换反应和Ｆｅ－Ｍｇ交换反应均基本保持平衡状态。实验资料和地质温压计估算确定其峰期Ｐ－Ｔ－Ｘ关系：Ｐ为０．６０～０．７０ＧＰａ，Ｔ为７００℃～７５０℃，ＸＧｒｔＭｇ为０．２１～０．２６，ＸＣｒｔＣａ为０．０４４～０．０５３，ＸＣｒｄＭｇ为０．６５～０．７２，ＸＢｔＭｇ为０．５０～０．５７，ＸＰｌＡｎ为０．２８５～０．３８８。但Ｇｒｔ、Ｂｔ和Ｐｌ又都有明显的微区成分变化，反映峰期后降温过程各种交换反应的效应，其封闭温度在５００℃～６００℃之间，压力为０．４０～０．５０ＧＰａ。研究确立逆时针的ＰＴｔ轨迹，其样式反映早元古代陆缘壳内拗陷带的闭合过程。

The detailed examination of microstructural evidence and microarea compostion of component mineral in Alrich gneisses from the Jingshan Group in east Shandong indicates the existence of equilibrium assemblage GrtCrdSilBtPlKfsQtz, and shows that the three independent net transfer reactions Grt+Sil+Qtz→Crd, Bt+Sil+Qtz→Grt+Crd+Kfs+H_2O and Pl(An)→Grt(Grs)+Sil+Qtz, and exchange reaction FeMg(Grt)=FeMg(Bt) have reached equilibria during peak metamorphism In the light of available experimantal data and results of geothermobarometry, the PTX relations for such assemblage were determined, ieP=0.60～0.70GPa, T=700℃～750℃,XGrtMg=0.21～0.26,XGrtCa=0.044～0.053,XCrdMg=0.65～0.72,XBtMg=0.50～0.57,XPlAn=0.285～0.388 However, the microarea variation of composition within Grt,Bt and Pl are rather obvious, and they reflect the effect of exchange reaction and diffusion in postpeak regressive stage This study suggests that the “closure temperature” of FeMg exchange reaction between GrtBt is about 500℃～600℃ at pressure 0.40-0.50GPa On this basis, a counter clockwise PTt path is constructed, and its style reflects the closing process of an ensialic depression on the continental margin in early Proterozoic, which is very different from typical collisional orogenic belt