变斑晶石榴石^(40)Ar-^(39)Ar年龄谱的含义与华北高压麻粒岩变质时代  

^(40)Ar-^(39)Ar age spectra of garnet porphyroblast: Implications for metamorphic age of high-pressure granulite in the North China craton.

作　　者：郭敬辉[1] 王松山[1] 桑海清[1] 翟明国[1] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029

出　　处：《岩石学报》2001年第3期436-442,共7页Acta Petrologica Sinica

基　　金：国家自然科学基金课题 (4 9972 0 64和 4983 2 0 3 0 );中国科学院知识创新工程项目 (KZCX1-0 7)的资助

摘　　要：采用 4 0 Ar- 39Ar阶段加热法研究华北桑干地区高压基性麻粒岩的变斑晶石榴石 ,获得 4 0 Ar- 39Ar坪年龄 185 2± 8Ma,4 0 Ar- 39Ar等时线年龄 186 2± 37Ma。这组年龄相当于高压麻粒岩的变质作用年龄。这一结果表明 ,虽然变斑晶石榴石中仅仅在显微矿物包体、流体包裹体和晶格缺陷中赋存少量的 K+ ,但其生成的 4 0 Ar*仍然能够形成稳定的年龄谱。如果退变质的影响较小 ,4 0 Ar-Conventional step heating 40 Ar 39 Ar spectra of metamorphic garnet separated from a high pressure granulite sample yielded date of 1852±8Ma. The spectra included 76% 39 Ar released with 40 Ar/ 36 Ar ratio of 4376～10666. The six data in the age spectra defined an 40 Ar 39 Ar isochron which gives an age of 1862±37Ma. Its initial 40 Ar/ 36 Ar is 244, slightly lower than the atmospheric argon ratio. These ages are consistent with metamorphic age of the high pressure granulite determined by Sm Nd garnet + whole rock isochron and zircon U Pb methods. The results revealed the potential of 40 Ar 39 Ar method for dating metamorphic garnet. Although pure metamorphic garnet contains only extremely low content of potassium, there is always some potassium in tiny mineral inclusions, fluid inclusions and lattice defect within garnet. As a good container, garnet protects these inclusions from later disturbance. Therefore, step heating 40 Ar 39 Ar spectra are very useful to date metamorphic age of garnet as long as the retrograde metamorphism was not intensive.High pressure granulites are widely distributed as enclaves and sheet like blocks in highly reworked TTG gneisses and migmatites in Sanggan area in the North China craton. They are considered to be generated during continental collision between eastern and western blocks in the North China craton. The metamorphic garnet 40 Ar 39 Ar ages presented here, together with other age determinations, indicate that the continental collision took place during late Paleoproterozoic.

关 键 词：石榴石 高压麻粒岩 华北克拉通 40Ar-39Ar阶段加热法 氩同位素 变质时代 

分 类 号：P588.34[天文地球—岩石学] P597[天文地球—地质学]