朝鲜甑山“群”变质岩中锆石-榍石-金红石U-Pb体系:古元古代-中生代构造-热事件记录

Zircon-titanite-rutile U-Pb system from metamorphic rocks of Jungshan “Group ” in Korea: Implications of tectono-thermal events from Paleoproterozoic to Mesozoic

朝鲜甑山地区甑山＂群＂（杂岩）的主要岩石类型包括石榴云母片麻岩、石榴角闪岩,并有少量大理岩。其原岩为泥质岩为主的碎屑岩,并含有少量火山岩和碳酸盐岩,经历了角闪岩相到麻粒岩相变质作用。本文对甑山＂群＂2件石榴云母片麻岩和1件石榴角闪岩样品进行了锆石-榍石-金红石U-Pb体系年代学研究。石榴云母片麻岩中分选出的锆石均为变质新生锆石,Pb-Pb加权平均年龄为1850±5Ma,没有发现继承锆石组分,暗示原岩为缺乏碎屑锆石的泥质岩。石榴角闪岩中的榍石根据成因不同,可分为两期,即早期麻粒岩相变质生长的榍石和后期热事件生长榍石。早期榍石具有高U、高Th/U比值特点,U-Pb分析构成一条不一致线,上交点年龄1831±5Ma代表了早期变质热事件冷却到榍石U-Pb体系封闭温度时代,下交点年龄155±3Ma代表后期热事件造成Pb丢失时代;另一种榍石具有低U低Th/U比值特点,U-Pb年龄为单一的153±3Ma,为中生代构造热事件的新生榍石。两类样品中的金红石少量颗粒保存有早期U-Pb年龄信息,绝大多数在～155Ma发生完全重置,并直到～110Ma一直处于U-Pb体系开放状态。以上数据说明,甑山＂群＂在～1850Ma前发生高级变质,1850～1830Ma开始抬升出露,并成为中新元古界沉积岩物源与沉积基底,中生代受到朝鲜广泛发育的岩浆-热事件的影响。

The Jungshan ‘Group ＇（ Complex） in the Korean Peninsula is mainly composed of garnet mica gneiss, garnet amphibolite and some marble.Its protolith rocks are pelite-dominated clastic rocks with a few volcanic and carbonates,which were metamorphosed to amphibolite to granulite facies.In this paper,we present our results on the U-Pb system of zircon,titanite and rutile from two pelitic gneissic samples and one garnet amphibolite sample from the Jungshan Group.Zircons form the two garnet mica gneissic samples are of typical metamorphic zircons with average age of 1850 ± 5 Ma.No zircon cores were identified from the zircons analyzed,indicating that zircons must have been scarce in the protolith of the pelitic gneiss.Two different groups of titanite grains can be identified from the garnet amphibolite based on their petrogenesis and they are the early stage and the late stage titanite grains.Early stage titanite grains were formed at amphibolite to granulite facies and they are characterized by high U contents and high Th/U ratios.Their U-Pb ages plot on a well-defined discordia whose upper intercept age is 1831 ± 5 Ma and lower intercept age is 155 ± 3 Ma.The former age represents the time at which the temperature of the rock reduced close to the closure temperature of titanite while the latter represents time of the Mesozoic tectonothermal event disturbance.The late stage titanite grains are characterized by low U contents and low Th/U ratios.These titanite grains give one age of 153 ± 3 Ma which also represents time of the Mesozoic tectonothermal disturbance.Rutile grains from the two different rocks preserved little early geochronological information while most of their U-Pb systems were reset at ～ 155 Ma.However,the U-Pb system of rutile was not closed until ～ 110 Ma.The ～ 110 Ma age is probably closely related to the widespread emplacement of ～ 110 Ma granites in vicinity.The data indicate that the Jungshan Complex experienced high-grade metamorphosm prior to/close to ～ 1850 Ma and exhumed since ca.1850 ～ 1830 Ma and thus became the provenance,as well as the basement of the Meso-Neoproterozoic strata.The Jungshan Complex also recorded the extensive igneous-thermal events during Meoszoic.