东昆仑早古生代变质演化:来自浪木日石榴斜长角闪岩的记录SCIEI北大核心CSCD

东昆仑造山带近年来被厘定为早古生代高压-超高压变质带。带内广泛出露早古生代的中-高级变质基性岩,这些岩石记录了不同的变质温压和多期的变质年龄,是反演和制约东昆仑早古生代变质演化的重要样品。本文选取东昆仑浪木日地区的石榴斜长角闪岩为研究对象,开展了变质岩石学及锆石年代学研究。石榴斜长角闪岩呈团块状出露在黑云二长片麻岩中,主要组成矿物为石榴子石、角闪石、斜长石、透辉石和石英,含少量黑云母、绿泥石、金红石、钛铁矿和榍石。石榴子石变斑晶的核部含有绿帘石、角闪石、斜长石、金红石和石英包裹体,其成分从核部到边部,锰铝榴石逐渐降低、钙铝榴石和Mg/(Mg+Fe;)比值逐渐升高,为进变质作用形成的环带。岩石中的矿物结构关系和成分特征显示其经历了进变质、峰期变质和退变质三个阶段的变质演化,变质温压分别为:T≈610℃和P≈6.5kbar、T≈700℃和P≈10.5kbar以及T≈650℃和P≈4.5kbar。这三阶段的变质作用构成顺时针的变质P-T轨迹,指示岩石经历进变质升温升压至峰期阶段,随后经历近等温降压的退变质阶段。同时该P-T轨迹特征表明岩石形成于俯冲-碰撞的构造背景。对石榴斜长角闪岩中的锆石进行SIMS U-Pb定年,得到492.8±5.1Ma的谐和年龄。锆石的形态特征与典型的变质锆石一致,其内包裹的石榴子石、角闪石和斜长石组合与岩石的峰期矿物组合一致。因此,锆石在峰期变质阶段结晶,所测年龄~493Ma为角闪岩相峰期变质年龄。本文研究的石榴斜长角闪岩与该区高压-超高压榴辉岩在野外产状、P-T轨迹和变质年龄等方面密切相关,暗示ca.490Ma是该区高压-超高压变质作用的一个重要时间节点。石榴斜长角闪岩和榴辉岩之间的变质差异,表明东昆仑早古生代经历了多阶段的变质作用,不同岩石记录了原特提斯洋俯冲-碰撞过程的不同阶段。本文获得的变质P-T轨迹和变质年龄可为进一步探究东昆仑早古生代高压-超高压变质作用提供限定。

Early Paleozoic metamorphic evolution of the East Kunlun Orogen recorded in Langmuri garnet-amphibolite

The East Kunlun Orogen (EKO) was recently considered an Early Paleozoic HP-UHP metamorphic belt. Many Early Paleozoic intermediate- to high-grade meta-basic rocks exposed in the belt record different metamorphic conditions and ages, and as a result, they are important samples for studying Early Paleozoic metamorphic evolution of the EKO. This paper presents metamorphic petrological and zircon geochronological studies of a garnet-amphibolite in Langmuri area in the eastern section of the EKO. The garnet-amphibolite mainly consists of garnet, plagioclase, amphibole, diopside and quartz, with a small amount of biotite, chlorite, rutile, ilmenite and titanite. The porphyroblastic garnet grains include epidote, amphibole, plagioclase, rutile and quartz in their cores. A compositional profile of the garnet shows a decrease in spessartine and grossular and an increase in Mg/(Mg + Fe2+) from its core to rim. These compositional features suggest that the garnet crystallized in a process of prograde metamorphism. Textural relationship and mineral compositions reveal that the rock experienced three stages of metamorphism, with their metamorphic conditions of T approximate to 610 degrees C and P approximate to 6.Okbar (prograde stage), T approximate to 700 degrees C and P approximate to 10. 5kbar (peak stage), and T approximate to 650 degrees C and P approximate to 4. 5kbar (retrograde stage), respectively. These metamorphic conditions form a clockwise P-T path, with a nearly isothermal decomposition process from the peak stage to the retrograde stage. The characteristics of the P-T path indicate the garnet-amphibolite was formed in the subduction-collision tectonic process. Zircon SIMS U-Pb analysis yield a concordant age of 492. 8 +/- 5. 1Ma. The morphological characteristics of the measured zircons are consistent with those of typical metamorphic zircon. The mineral inclusions (garnet, amphibole and plagioclase) of the zircons are consistent with the peak mineral assemblage of the rock, which suggests that the zircons should crystallize at the peak stage of amphibolite facies. Therefore, the age of similar to 493 Ma is the peak metamorphic age of the garnet-amphibolite. The garnet-amphibolite studied in this paper show close relationship with the HP-UHP eclogites of the same area in the aspects of field occurrence, P-T path, and metamorphic age. The relationship between the two rock types indicates ca. 490Ma should be an important stage of the HP-UHP metamorphism of this area. Meanwhile, the differential metamorphism between the two rock types implies the EKO experienced multi-stage metamorphism in the Early Paleozoic, thus different rocks recorded different stages of the subduction-collision process of Proto-Tethys Ocean. The P-T path and metamorphic age presented in this paper can provide constraints into the further study of the Early Paleozoic metamorphic evolution of the EKO.