安徽碧溪岭含柯石英榴辉岩的U－Pb与^（40）Ar／^（39）年代学研究  

A U-Pb AND ￣(40)Ar/￣(39)Ar GEOCHRONOLOGIC STUDY OF BIXLING COESITE-BEARING ECLOGTE FROM ANHUI PROVINCE,CHINA

作　　者：曹荣龙[1] 朱寿华[1] 

机构地区：[1]中国科学院广州地球化学研究所

出　　处：《地球化学》1995年第2期152-161,共10页Geochimica

摘　　要：安徽大别、苏北和山东胶南是世界上第三个发现含柯石英榴辉岩的地区。在关于苏鲁皖超高压变质带大地构造背景的学术讨论中，其时代问题成为当前争论的焦点。本文用锆石Ｕ－Ｐｂ法得到碧溪岭榴辉岩体谐和图上交点年龄２７７４．５±２４Ｍａ以及ｔ（^（２０７）Ｐｂ／^（２０６）Ｔｂ）平均表面年龄２６６２Ｍａ，提供了榴辉岩的原岩和超高压变质作用可能形成于晚太古代的信息；并用多硅白云母Ａｒ－Ａｒ法测得６６１．７±１３Ｍａ的退变质年龄，也表明本区榴辉岩可能是前寒武纪的产物；此外，还测得围岩大别群斜长片麻岩中的白云母以及榴辉岩中晚期交代的普通角闪石的Ａｒ－Ａｒ等时线年龄分别为１９２．５５±３．８５和２３０．７０±４．６１Ｍａ，代表后榴辉岩（ｐｏｓｔｅｃｌｏｇｉｔｅ）的印支期区域动力变质作用改造年龄。笔者对已报道的榴辉岩Ｓｍ－Ｎｄ年龄的地质意义进行了讨论，并指出了含石榴子石Ｓｍ－Ｎｄ矿物等时线定年法潜在的弊端。The coesite-bearing eclogite samples were collected from the Bixiling body of Yuexi County,Anhui Province, part of the Dabie Mountains. The mineral paragenesis of garnet+omphacite+kyanite+coesite in eclogite indicates an ultra-high pressure and medium temperature metamorphic environment.The zircon grains from an original and fresh sample selected for U-Pb dating are round and elongately columnar. Four fractions with different grain-size and crystal shape have been analyzed.The calculated t(207/206) apparent ages are 2615.4±0.3,2652.2±0.1, 2680±0.2,and 2699.6±0.1 Ma,with an average of 2661.9 Ma. The discordia curve intercepts the concordia one at an upper intercept age of 2774￣(+24)_(-22)Ma and a lower intercept age of 452￣(+78)_(-79)Ma. The upper intercept point may indicate the formation age of the eclogite during ultra-high pressure metamorphism, while the lower intercept one represents the age of a late-stage thermal event.In addition, Phengite from eclogite yields an Ar-Ar isochron age of 662±13Ma,which represents the age of retrograde metamorphism during the Precambrian. It is believed that the Dabie coesite-bearing eclogite probably formed in the late Archean.The Ar-Ar isochron dating method got an age of l92.6±3.8 Ma for muscovite from the host gneiss and an age of 230.7±4.6 Ma for hornblende from the eclogite.They indicate that both the host gneiss and the eclogite have been subjected to regional dynamic metamorphism during the Indosinan orogeny.

关 键 词：榴辉岩 含柯石英榴辉岩 铀 铅 氩同位素 年代学 

分 类 号：P588.3[天文地球—岩石学] P597[天文地球—地质学]