摘要
本文对大别山下地壳岩石的主要组成及其变质岩石学和年代学特征进行了系统总结,并首次报道了作者等最近在罗田穹隆中发现的镁铁质下地壳岩石俯冲变质成因的榴辉岩及其有关片麻岩的 SHRIMP 锆石 U-Pb 同位素年代学的最新研究成果。大别山这种不寻常的镁铁质下地壳俯冲成因榴辉岩为研究大陆深俯冲和陆壳物质再循环以及对地幔不均一性的影响提供了重要对象和可能性。根据阴极发光图像及矿物包体组合,罗田榴辉岩中继承锆石可分为两种即具有岩浆结晶环带的岩浆锆石和含有石榴子石+紫苏辉石+斜长石等麻粒岩相变质矿物包体的变质锆石,而且这两种继承锆石常具有舍石榴子石+绿辉石等榴辉岩相矿物包体的增生边。SHRIMP U-Pb 定年结果表明,继承岩浆锆石的^(206)Pb/^(238)U 年龄为791±9Ma、含麻粒岩相矿物包体的继承变质锆石的^(206)Pb/^(238)U 年龄为794±10Ma、含榴辉岩相矿物包体的锆石增生边的^(206)Pb/^(238)U 年龄值为212±10Ma。罗田榴辉岩的围岩——含石榴子石英云闪长质片麻岩中锆石的 CL 图像具有特征的核-幔-边结构即表现为继承岩浆锆石核-高压变质幔-退变质增生边,高压变质幔给出罗田片麻岩中印支期高压变质的精确年龄为218±3Ma。因此,罗田榴辉岩的原岩时代与早期麻粒岩相变质时代一致,均为晚元古代,证明华南陆块北缘晚元古代岩浆板底垫托的存在。而且,大别山下地壳岩石参与了印支期大陆深俯冲,但与南大别带具有不同的折返过程即前者经历了麻粒岩相退变质过程。此外,北大别带榴辉岩的印支期峰期变质时代(212±4Ma)比南大别带年轻(226±3Ma),这可用大陆叠瓦式深俯冲模式来解释。
he main rock assemblages and their metamorphic petrologic and geochronological data of the lower crustal rocks from the Dabie Mountains were summarized in this paper. The SHRIMP zircon U-Pb dating results of the eclogites from the subducted mafic lower continental crust and related gneiss was firstly reported in the Luotian dome. This kind of unusual eclogite provides an opportunity to study the deep subduction of mafic lower continental crust and its recycling and effects on the mantle heterogeneity. The cathodoluminescene （CL） images and mineral inclusions from the eclogite reveal that there are two types of inherited zircons, one is magmatic origin with clear oscillatory growth zonation and another is metamorphic origin with relic garnet, hypersthene and plagioclase minerals of granulite-facies. Both zircons generally contain very thin overgrowth rim with omphacite ＋ garnet ＋ rutile assemblage of eclogite-facies. The cores of magmatic and metamorphic zircons give ages of 791 ± 9Ma and 794 ±10Ma, respectively. The zircon rim gives a concordant age of 212 ±10Ma. The CL images reveal that the internal structures of most zircons from garnetbearing tonalitic gneiss in the Luotian dome consist of core, mantle and rim domains, and the zircon mantle gives a precise zircon U-Pb age of 218 ± 3 Ma, which is consistent with the peak metamorphic age of the eclogite. It is suggested that the lower crustal rocks from the Dabie Mountains experienced the continental deep subduction at Triassic, but their exhumational process were different from the southern Dabie zone, which the former is of overprinting of granulite-facies. In addition, the peak metamorphic age （212 ±4 Ma） of eclogite from the northern Dabie zone was younger than the age （226 ±3 Ma） of UHP eclogite from the southern Dabie zone, which can be explained by the model of imbricate continental deep subduction.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2005年第4期1059-1066,共8页
Acta Petrologica Sinica
基金
国家重点基础研究发展规划项目(G1999075503)
国家"973"专项项目(2003CB716500)
关键词
麻粒岩
榴辉岩
下地壳
大陆深俯冲
大别山
granulite
eclogite
lower continental crust
continental deep subduction
Dabie Mountains
