从榴辉岩与围岩的关系论苏鲁榴辉岩的形成与折返

Discussion on the Formation and Exhumation of the Sulu Ecologites on the Basis of the Relationship between the Eclogites and the Country Gneisses

位于华北和扬子两板块碰撞带中的苏鲁榴辉岩形成的温压条件不但是超高压,而且是高温。榴辉岩的PTt轨迹表明其为陆-陆碰撞俯冲带的产物。榴辉岩的区域性围岩花岗质片麻岩为新元古代同碰撞期花岗岩,榴辉岩及其他直接围岩皆呈包体存在于其中,并见新元古代花岗岩呈脉状侵入榴辉岩包体中。区域性围岩新元占代花岗岩的锆石中发现有柯石英、绿辉石等包裹体,表明新元古代花岗岩的组成物质也经受过超高压变质作用,且榴辉岩与围岩新元古代花岗岩的锆石U-Pb体系同位素年龄基本相同。但新元古代花岗岩所记录的变质作用和变形作用期次(或阶段)却少于榴辉岩。据上述可得如下推断:超高压榴辉岩与新元古代花岗岩岩浆是同时在碰撞带底部(俯冲板块前部)形成的;榴辉岩的第一折返阶段是由新元古代花岗岩岩浆携带上升的,其第二折返阶段是和新元古代花岗岩一起由逆冲及区域性隆起而上升,遭受剥蚀。

The Sulu eclogites are located in the collision zone between the north china plate and the Yangtze plate. The p-t conditions of their formation is not only ultrahigh-pressure(UHP) ,but also high temperature (HT). The PTt paths of the eclogites indicate that they are the products of the continent-continent collisional subduc-tion zone. Their regional country rocks, i. e. granitic gneiss, are syncollisional granites of the Neoproterozoic, while the other direct country rocks (joining each other) of the eclogites all occur as enclosures in the granites. The eclogite bodies were intruded by the Neoproterozoic gneissic granite veins, indicating that the solid eclogite enclosures once existed in liquid granitic magma. Coesite and omphacite inclusions have been identified as minute inclusions in zircons from the regional country rocks, namely Neoproterozoic granites, indicating that the constituents of the Neoproterozoic granites also experienced UHP metamorphism, and that the isotopic ages of the zircon U-Pb system of the eclogites and the country rocks are basically equal to each other. However, the Neoproterozoic granites recorded less events (stages) of metamorphism and deformation than the eclogites. The following inferences can be obtained in light of the above: UHP eclogites and the Neoproterozoic granomagma were formed at the same time at the bottom of the collision zone (the front part of the subducted continental crust). The eclogites were carried to uplift by the Neoproterozoic granomagma at the first stage of the exhuma-tion and at the second stage the eclogites were uplifted and degraded along with the Neoproterozoic granites by thrusting and regional uplifting.