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

Zircon U-Pb chronology of Yuyan granitic gneiss in Southwestern Zhejiang:a discussion on ancestral basement materials of Cathaysia Massif

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2.5Ga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.