大别-苏鲁超高压变质带内部的浅变质岩

摘要

There are sparotic occurrences of low-grade metamorphic rocks as tectonic relict flakes within the Dabie-Sulu ultrahigh pressure (UHP) orogen. They are mainly composed of metaclastics, phyllites and marbles. They having, suffered from dynamic metamorphism at low greenschist-facies and fold-and-fault deformation, are in tectonic contacts with the surrounding UHP rocks. Micro-paleontologic fossils from the low-grade metamorphic rocks suggest that their sedimentary protoliths were deposited in a shallow marine environment at Sinian in the northern margin of Yangze plate. Isotopic datings indicate that their igneous protoliths are of Late Proterozoic ages and experienced tectonic events at Caledonian and Indosinian like the surrounding UHP eclogites and gneisses. Oxygen isotope analysis shows that the low-grade metamorphic rocks within the Dabie orogen are depleted in 18O like the surrounding UHP rocks and thus their protoliths underwent hydrothermal alteration by ancient meteoric water at high temperatures. It appears that the low-grade metamorphic rocks were originally the Sinian sedimentary rocks and enclosed volcaniclastic rocks in the northern margin of the Yangtze plate, and have the geological occurrence and formation mechanism similar to accretionary wedge that was scraped from the subducting Yangtze plate and accumulated in the subduction zone. Therefore, they may be a part of the tectonic accretionary wedge of continental plate subduction.%大别苏鲁超高压变质带内部零星出露有若干呈构造残片状产出的浅变质岩,主要以变质碎屑岩-千枚岩-大理岩组合为代表,遭受过低绿片岩相变质和脆-韧性变形作用的改造,与围岩均为构造接触(断层或韧性剪切带)。微古生物化石研究表明,其原岩为震旦纪前后扬子板块北缘的浅海相沉积。同位素年代学研究指示,它们经历过加里东期和印支期构造热事件的影响,与区域超高压岩石经受的构造热事件时间一致; 氧同位素研究得到,部分浅变质岩原岩曾遭受过高温大气降水热液蚀变,与区域超高压岩石的氧同位素特征一致。这些浅变质岩的原岩为扬子板块北缘震旦系沉积岩及其中的火山碎屑岩,构造上具有板块俯冲过程中的刮削岩片-构造加积楔的产状和形成机制,因此可能是大陆板块俯冲加积楔的一部分。