浙西南玉岩花岗片麻岩锆石U-Pb年代学-华夏地块古老基底物质的探讨

摘要

通过对浙西南遂昌县玉岩古元古代花岗片麻岩锆石 LA-ICP-MS U-Pb 年代学研究,以探讨和揭示华夏地区基底的物质组成。玉岩花岗片麻岩锆石可分为两类：第一类锆石年龄相对年轻,其207 Pb/206 Pb年龄介于597~1917 Ma之间,结合稀土元素特征,表明为变质成因锆石,在锆石谐和图上,15个年轻锆石点整体上构成了一条不一致线,其上交点年龄为1904±19Ma,代表了片麻岩形成的变质时间,表明华夏地区存在古老的古元古代晚期岩浆岩-变质岩基底,下交点年龄为262±47Ma,可能记录了华夏地块晚二叠世―早三叠世构造-变质作用的响应。第二类锆石年龄相对较老,锆石207 Pb/206 Pb年龄介于2087~2523Ma之间,锆石发育核幔结构并有明显的振荡环带,较老的锆石年龄表明华夏地区可能存在太古宙末至2.5 Ga的古老基底物质。%This paper tried to discuss and expose the composition of the basement materials of Cathaysia Mas-sif through a study on zircon LA-ICP-MS U-Pb chronology of Paleoproterozoic Yuyan granitic gneiss in Suichang of Southwestern Zhejiang.Yuyan granitic gneiss can be categorized into two types:a younger one and an older one.The younger type has younger zircon age and its 207 Pb/206 Pb age falls into the range of 5 9 7-1 9 1 7 Ma.Combined with REE characteristics,it shows that the zircon is of metamorphic genesis.On the concordia diagram of zircon,1 5 younger zircon points form an inconcordant curve:the upper intersection point age is 1904±19 Ma which represents the metamorphic time of formation of gneiss,and it indicates that late Paleoproterozoic magmatite-metamorphic rock basement occurs in Cathaysia area;the lower intersection point age is 262±47 Ma,which might record the response of late Permian-early Triassic tectonic metamor-phism.On the other,the older type has older zircon age and its 207 Pb/206 Pb age ranges within 2087-2523Ma,and the core-mantle structure is developed in zircon with obvious oscillatory zoning,and older zir-con age suggests that ancient basement materials existed since the end of Archaeozoic era to 2.5Ga might oc-cur in Cathaysia area.