桐柏北部宽坪群变泥质岩的变质作用研究

桐柏北部宽坪群发育一套由斜长角闪岩、变泥质岩、大理岩以及石英岩夹层组成的中级变质岩石。本文选取4个含石榴变泥质岩样品进行了详细的岩石学研究,其中回龙寺组的一个样品含有特征的十字石+蓝晶石组合,而左老庄组的一个样品则含有夕线石。回龙寺组样品中的石榴石均具有明显的进变质成分环带,而左老庄组样品中的石榴石均遭受到晚期低温扩散的影响,所以在计算其形成条件时选取了生长于峰期变质阶段的特定部位。运用传统的石榴石-黑云母-白云母-斜长石-石英地质温压计估算回龙寺组的变质p-T区间为590～610℃、0.95～1.14GPa,左老庄组为630～650℃、0.65～0.89GPa,分别相当于低角闪岩相和高角闪岩相。这一结果与变质反应曲线所限定的温压范围基本一致,并且后者中还存在由蓝晶石区域向相对高温低压的夕线石区域转变的过程。低-高角闪岩相的宽坪群与低角闪岩相的二郎坪群和麻粒岩相的秦岭群紧密伴生,且变质时代近于一致(440～400Ma),说明华北南缘在志留纪—早泥盆世发生了岛弧、微陆块与华北板块大陆边缘的碰撞造山作用。     

P–T estimates from the Nellore schist belt (India) and evidence for superimposed metamorphic events

The Nellore schist belt (NSB) is one of the prominent schist belts of southern India. It is thrust over an unmetamorphosed Proterozoic sedimentary sequence (Cuddapah Basin) in the west and in turn is overthrust by the Eastern Ghats Granulite Terrain (EGGT) in the east. Metamorphic grade has been considered to be either greenschist to amphibolite facies or else to show high- and low-grade groups of rocks. Detailed mineralogical and P, T studies on representative rock types across the high- to low-grade groups (metapelite and metadacite from the high- and low-grade groups respectively) near Vinjamuru reveal that there are high (M1) and medium (M2) grade metamorphic events. While metapelite documents both M1 and M2 events, metadacite shows only the M2 event, thus indicating the presence of high- and medium-grade groups of rocks in the NSB. The stable mineral assemblage during the early prograde part of M1 was Qtz+St₁+Ms₁+Pl₁+Bt±Grt, which subsequently suffered high-grade metamorphic conditions (T∽715–765°C and P∽8·6–9·2 kbar) leading to the demise of St₁ and melting of Ms₁. In contrast, medium-grade metamorphism (M2) is characterized by the stable association of St₂+Qz. Quantitative geothermobarometry suggests T=520–570°C and P=6·1-6·8 kbar, and these results show good consistency with the average P, T estimates obtained from the THERMOCALC program. This metamorphic episode is probably coeval with the 900 Ma granulite facies metamorphic event in the EGGT. The superimposition of M2 over M1 probably has led to widespread retrogression of the high-grade rocks to medium grade, thereby giving the general impression of a greenschist to amphibolite facies gradation. © 1998 John Wiley & Sons, Ltd.