桑干地区高压麻粒岩中石榴石和斜长石的连续激光探针^(40)Ar^-^(39)Ar等时年龄及其地质意义  

Continuous laser-probe^(40) Ar^-^(39) Ar age dating on garnet and plagioclase: constraint to metamorphism of high pressure basic granulite from Sanggan area, North China craton

作　　者：胡世玲[1] 郭敬辉[1] 戴谟 蒲志平[2] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029 [2]中国科学院广州地球化学研究所,广州510640

出　　处：《岩石学报》1999年第4期518-523,共6页Acta Petrologica Sinica

基　　金：国家自然科学基金课题资助;基金重点课题资助

摘　　要：采用微区激光探针４０Ａｒ－３９Ａｒ定年方法， 对华北桑干地区高压基性麻粒岩中变质石榴石和斜长石直接进行了原位微区年代测定。石榴石变斑晶是高压麻粒岩相变质作用形成的矿物，石榴石周围后成合晶反应边组合中的斜长石是石榴石减压分解的产物。石榴石斑晶的４０Ａｒ－３９Ａｒ等时线年龄为２５１０ Ｍａ， 证明高压变质作用发生在太古宙末。斜长石４０Ａｒ－３９Ａｒ等时线年龄为１９６８Ｍａ， 代表石榴石在中压麻粒岩相条件下分解的时代。它们之间年龄相差大于５００Ｍａ， 说明高压麻粒岩可能没有经历近等温减压的ＰＴ轨迹。后成合晶组合很可能代表中压麻粒岩相变质作用的叠加。这一结果对探讨华北克拉通桑干地区早期地壳的构造演化具有重要意义。Continuous laser probe 40 Ar 39 Ar technique has been taken to carry out in situ analysis onto the metamorphic garnet and plagioclase from high pressure basic granulites in Sanggan area of the North China craton Garnet porphyroblasts was formed in the high pressure granulite facies episode In the symplectite assemblage arround garnet, plagioclase is one of the garnet breakdown products Ar analysis of garnet porphyroblasts defines an 40 Ar 39 Ar isochron which gives out an age of 2510Ma, that indicates the high pressure granulite facies metamorphic age So, the Archaean high pressure granulite metamorphism has been confirmed by this age dating Another 40 Ar 39 Ar isochron age of 1968Ma has been obtained from Ar data of plagioclase That should represent the age of garnet breakdown reaction The >500Ma gap between the age of high pressure metamorphism and garnet breakdown does not support the isothermal decompression P T path given by petrological view The symplectite assemblages are more likely to be formed during another medium pressure metamorphism overprint This conclusion will give a strong constraint on the crustal evolution of Sanggan area in the North China craton

关 键 词：麻粒岩 石榴石 斜长石 氩40-氩39 等时年龄 

分 类 号：P588.345[天文地球—岩石学] P584[天文地球—地质学]