大别山太湖地区宿松变质杂岩中石榴斜长角闪岩的P-T轨迹及反映的俯冲过程

依据对大别山太湖地区宿松变质杂岩中的石榴斜长角闪岩的岩相学、矿物成份分析和P-T条件评价,将变质作用分为Ⅰ、Ⅱ、Ⅲ和Ⅳ4个阶段,其中Ⅰ、Ⅱ阶段经历了绿帘角闪岩相的变质;Ⅲ阶段为峰期变质,变质条件为T=580℃,P=1.31GPa,为角闪石—榴辉岩相变质;Ⅳ阶段为绿片岩相变质。构建的P-T轨迹显示了顺时针的演化特征,反映了宿松变质杂岩分阶段的、不等速的俯冲和折返过程:早期短暂的迅速俯冲→峰期缓慢俯冲→晚期可能的相对缓慢抬升。研究表明宿松变质杂岩的峰期变质条件偏离“俯冲梯度”线,显示了大幅增温、压力变化轻微的特征,这一增温可能是由于受到扰动的热结构恢复所致。结合前人对高压榴辉岩的峰期变质条件的研究,认为宿松变质杂岩与高压-超高压榴辉岩单元至少有0.5GPa的压力差,约15km厚的地壳损失,两者为太湖—马庙断层(F₂)所分隔。

P-T PATH OF GARNET AMPHIBOLITES FROM THE SUSONG METAMORPHIC COMPLEX ACROSS TAIHU AREA IN THE DABIE MOUNTAINS, AND ITS IMPLICATIONS FOR SUBDUCTION

On the basis of the analyses of petrology,mineral compositions and P-T condition estimations of the garnet amphibolites from the Susong metamorphic complex across Taihu area of Anhui province in the Dabie Mountains,four metamorphic stages could be identified as Ⅰ-,Ⅱ-,Ⅲ-and IV-stages.The Ⅰ-and Ⅱ-stages were in the metamorphism of epidote-amphibolite facies;the P-T conditions for Ⅲ-stage were T=580 ℃,P=1.31 GPa,which belonged to the metamorphism of amphibolite-eclogitic facies;Ⅳ-stage was in the metamorphism of greenschist facies.The P-T path displayed the characteristics of clockwise evolution and reflected the multi-stages and inhomogeneous rate of subduction and exhumation of northern marginal part of the Yangtze plate for the Susong metamorphic rocks:the rapid transient subduction in early stage → the slow subduction in the peak stage → the relatively slow uplift in late stage.The results showed that the peak P-T conditions for the Susong complex rocks drifted off the “subducted gradient” line,since temperature increased rapidly and pressure varied little.The increasing temperature might result from the resume of the disturbed thermal structure.Combined with the results of P-T condition for high-pressure eclogites,0.5 GPa gap existed between the Susong metamorphic complex and the HP-UHP eclogitic unit,and the both being separated by the Taihu-Mamiao fault,about 15 km thickness crust had lost.