浙西南古元古界八都群是目前华夏地块最古老的变质基底,以往研究认为其变质程度仅达角闪岩相。近来在对遂昌地区八都群富铝片麻岩的研究过程中,发现了具有"石榴石＋夕线石＋正/反条纹长石＋黑云母"特征组合的泥质麻粒岩,表明该地体曾经历麻粒岩相变质改造。通过岩相学与矿物化学分析,确定该岩石经历了3个阶段的演化过程,即:早期进变质阶段(M1),形成"石榴石＋黑云母＋白云母＋夕线石＋斜长石＋石英"的矿物组合;变质峰期阶段(M2-3),形成"石榴石＋夕线石＋三元长石＋黑云母＋石英"的矿物组合;峰期后降压冷却阶段(M4),形成"黑云母＋白云母＋新生斜长石＋石英"的矿物组合。岩石中石榴石普遍发育与降温过程有关的扩散成分环带和与降压过程有关的斜长石后生合晶。通过石榴石-黑云母温度计和GASP压力计估算变质峰期的温压条件为 800～850℃、0.6～0.7 GPa,峰期后退变质阶段的温压条件为560～590℃、0.25～0.33 GPa,具有顺时针样式的pT演化轨迹,反映一种陆壳碰撞增厚、后又拉伸减薄的动力学过程。
The Paleoproterozoic Badu Group in southwestern Zhejiang Province is the oldest metamorphic basement in the Cathaysia block, and its metamorphic grade was previously considered to be only of amphibolite facies. Recently, pelitic granulite with the typical mineral assemblage of garnet+sillimanite+perthite+antiperthite+biotite was discovered in the Al-rich gneiss of Suichang area. According to petrographic observation, micro-mineral composition and calculating results from garnet-biotite thermometer and GASP barometer, three stages of metamorphism were recognized in the pelitic granulite. The early prograde metamorphic stage (M1) was detected on the basis of inclusions of biotite, muscovite and sillimanite in garnet. The peak metamorphic stage (M2-3) is characterized by the presence of garnet, biotite, sillimanite and ternary feldspar, with a probable pT condition of 0.6～0.7 GPa at ca. 800～850℃. The post-peak retrograde metamorphic stage (M4) was recognized according to the mineral assemblage of biotite, garnet, fresh plagioclase and muscovite, with a pT condition of 560～590℃ and 0.25～0.33 GPa. The garnet from the pelitic granulite developed both diffusion zoning and symplectite of plagioclase related to the lowering of temperature and pressure, respectively. A clock-wise pT path has been constructed for the pelitic granulite which is consistent with the thinning process of the thickened continental crust.