东准噶尔扎河坝超高压变质成因石英菱镁岩的^(40)Ar/^(39)Ar同位素年代学信息及地质意义  

^(40)Ar/^(39) Ar geochronology of the ultrahigh-pressure metamorphic quartz-magnesitite in Zaheba, eastern Junggar, Xinjiang.

作　　者：牛贺才[1] 单强[1] 张海祥[1] 于学元[1] 

机构地区：[1]中国科学院广州地球化学研究所,广州510640

出　　处：《岩石学报》2007年第7期1627-1634,共8页Acta Petrologica Sinica

基　　金：国家自然科学基金(40673036);中国科学院知识创新工程重要方向项目(KZCX2-YW-107);国家科技支撑计划重点项目(2006BAB07B02)

摘　　要：在扎河坝-阿尔曼泰蛇绿岩岩块内分布着高压-超高压成因的二辉橄榄岩、石榴辉石岩、石英菱镁岩及榴闪岩。石英菱镁岩内多硅白云母的 Si(pfu)值均大于3.35,最高可达3.77,是典型高压-超高压成因的矿物。石英菱镁岩及其围岩蛇纹石化二辉橄榄岩的 K_2O 含量极低,且多硅白云母的 Na/(Na+K)比值小于0.04,这些地球化学特征显示,扎河坝多硅白云母不应含有过剩 Ar,是一个理想的^(40)Ar/^(39)Ar 定年对象。精确的^(40)Ar/^(39)Ar 年代学研究结果表明,扎河坝石英菱镁岩中多硅白云母的^(40)Ar/^(39)Ar 同位素年龄为281.6±2.5Ma。而矿物化学特征表明,扎河坝石英菱镁岩中的多硅白云母曾经历了退变质作用的改造,因此,它代表的应该是超高压变质石英菱镁岩的折返年龄。多硅白云母的^(40)Ar/^(39)Ar 年代学研究结果表明,扎河坝-阿尔曼泰蛇绿混杂岩内超高压石英菱镁岩的折返事件应发生在早二叠世。High-ultrahigh pressure lherzolite, garnet-pyroxenite, quartz-magnesitite and retrograde eclogite occur in the ZahebaAlmantai ophiolites. The Si content （pfu） of phengite in quartz-magnesitites is more than 3.35 with the maximum up to 3.77, indicating they are the products of high-ultrahigh pressure metamorphism. K20 contents of quartz-magnesitite and its wall rocks i. e. serpenfinized lherzolite are extremely low, and the ratio of Na/（ Na ＋ K） of phengite is less than 0. 04. These geochemical characteristics show that the phengite should not contain any excess Ar and hence be an ideal mineral for ^40Ar/^39Ar dating. The precise ^40Ar/^39Ar dating of the phengite in quartz-magnesitite is 281.6 ± 2.5Ma. The characteristics of mineral chemistry display the phengite had undergone retrograde metamorphism which represents the exhumation age of metamorphism. This study shows that the exhumation of the Zaheba-Almantai ophiolites took place in early Permian.

关 键 词：扎河坝-阿尔曼泰蛇绿岩 石英菱镁岩 超高压变质 多硅白云母^40Ar/^39Ar定年 

分 类 号：P597.3[天文地球—地球化学]