桐柏造山带榴辉岩早古生代高压变质作用的发现  

Discovery of Early Paleozoic high-pressure metamorphism of eclogite in Tongbai orogen

作　　者：李乐倩 张宏福[1] LI LeQian;ZHANG HongFu(State Key Laboratory of Continental Dynamics,Department of Geology,Northwest University,Xi’an 710069,China)

机构地区：[1]西北大学地质学系,大陆动力学国家重点实验室,西安710069

出　　处：《岩石学报》2021年第12期3575-3590,共16页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41688103)资助.

摘　　要：桐柏造山带是连接秦岭造山带和红安-大别造山带的枢纽,一直被认为是三叠纪变质带。本文通过对其南部高压榴辉岩带内榴辉岩和白云母石英片岩的全岩化学、锆石U-Pb年代学和Lu-Hf同位素地球化学研究,首次确定该地区的榴辉岩存在早古生代变质作用。榴辉岩主要由石榴石、绿辉石和角闪石组成,含少量多硅白云母、石英和金红石等。榴辉岩中锆石的^(206)Pb/^(238)U加权平均年龄为452±5Ma,这些锆石具有低Th/U比值、低^(176)Lu/^(177) Hf比值,并具有平坦的HREE配分型式和无Eu负异常的特征,即为典型榴辉岩相的变质锆石。因此,452Ma代表该榴辉岩的变质时间。此外,白云母石英片岩中的锆石SIMS定年也获得450±93Ma的下交点年龄,证明白云母石英片岩与榴辉岩皆记录了早古生代的变质事件。榴辉岩的主微量元素及锆石Hf同位素特征反映榴辉岩原岩形成于板内环境,并且在侵位过程中遭到古老地壳物质的混染。结合桐柏造山带内不同岩石类型中早古生代的变质年龄,表明早古生代的变质作用在桐柏造山带内是普遍存在的。同时,桐柏造山带早古生代榴辉岩相变质事件的发现,进一步说明其经历了从古生代到中生代多阶段的构造演化。Tongbai orogen,situated at the junction between Qinling and Hong’an-Dabie orogens,has been considered a Triassic metamorphic belt.Based on the whole-rock geochemistry,zircon U-Pb chronology and Lu-Hf isotopic geochemistry of the eclogite and mica quartz schist outcropped in the southern high-pressure metamorphic zone,we first confirm the existence of Early Paleozoic eclogite metamorphism in Tongbai orogen.The eclogite is mainly composed of garnet,omphacite and amphibole,as well as minor amounts of phengite,quartz and rutile.The weighted mean age of ^(206)Pb/^(238)U obtained from eclogite metamorphic zircons is 452±5Ma.These zircons have low Th/U and ^(176)Lu/^(177)Hf ratios,and display a flat HREE distribution pattern without any notable Eu negative anomaly.Therefore,these zircons are typical metamorphic origins of eclogite facies.The U-Pb age of 452Ma should be interpreted as the age of eclogite facies metamorphism.In addition,the lower intercept age of 450±93Ma of zircons from mica quartz schist also represents the metamorphic age.The consistent metamorphic ages of eclogite and mica quartz schist indicate that the two studied samples record metamorphic event of the Early Paleozoic.Whole-rock geochemistry and Hf isotopic compositions indicate that the protolith of the Tongbai eclogite was intra-plate basalt,which was contaminated by the ancient crustal materials during the emplacement processes.Combined with the Early Paleozoic metamorphic ages of different rock types in the Tongbai orogen,we can conclude that the metamorphism of the Early Paleozoic is widespread in the Tongbai orogen.Furthermore,the discovery of the Early Paleozoic eclogite facies metamorphic event implies that the Tongbai orogen has undergone a multi-stage tectonic evolution from Paleozoic to Mesozoic.

关 键 词：桐柏造山带 榴辉岩 锆石U-PB定年 锆石Lu-Hf同位素 

分 类 号：P588.348[天文地球—岩石学] P597.3[天文地球—地质学]