大别山碧溪岭榴辉岩450Ma年龄信息:石榴子石流体包裹体^(40)Ar-^(39)Ar定年初步结果

The 450 Ma message and excess 40Ar within garnet from the Bixiling eclogite in Dabie Shan:Interpreted from the ^(40)Ar-^(39)Ar dating results by crushing

高压-超高压变质岩常含有过剩40Ar,排除过剩40Ar的干扰是获得可靠40Ar-39Ar年龄的关键。本文试图从流体包裹体40Ar-39Ar定年的角度,探讨榴辉岩40Ar-39Ar定年技术与过剩40Ar问题。采用真空击碎法提取流体包裹体,对碧溪岭浅色榴辉岩石榴子石进行40Ar-39Ar定年,获得了下降型年龄谱:最初9个阶段年龄谱总体上呈逐渐下降,表观年龄从2226Ma下降到753Ma,这是次生包裹体(含过剩40Ar)与原生包裹体(不含过剩40Ar)混合的结果;第10阶段至第22阶段(末阶段),年龄谱呈平缓波动,表观年龄变化小,对应坪年龄为(449±18)Ma(2σ,39Ar占59.5%),这是微小的原生流体包裹体的贡献,此13个数据点在36Ar/40Ar-39Ar/40Ar图上构成线性关系很好的反等时线,对应的等时线年龄(448±34)Ma代表了石榴子石的结晶年龄,40Ar/36Ar初始比值(292±5)表明原生包裹体不含过剩40Ar。

Excess 40Ar is present in the HP-UHP metamorphic rocks. It is cru cial to exclude the excess 40Ar in order to obtain the true metamorphic ages of these rocks. In this study, we try to date the age and investigate the excess 40 Ar of the garnet from a Bixiling eclogite in Dabie Shan with the 40Ar-39Ar tec hniques. The coexisting garnet and phengite were selected from a hand-specimen of light-color eclogite. The garnet was determined by crushing and phengite b y laser-stepped heating. The 40Ar-39Ar dating results of garnet by crushing form a falling age s pectrum (Fig.2a) marked the highest apparent ages of ～2.2 Ga at the 2nd and 3r d stages, then the apparent ages falling step by step, and finally a plateau app earing with a plateau age of (449±18) Ma in the last thirteen stages (39Ar rel eased 59.5%). The data points of garnet exhibit obviously two groups correspon ding to the secondary and primary fluid inclusions and their mixture. The second ary fluid inclusions contain excess 40Ar and their data points scatter on the in verse isochron diagram of 36Ar/40Ar vs. 39Ar/40Ar (Fig.2b), while the primary fl uid inclusions do not contain excess 40Ar and their data points yield an excelle nt isochron line corresponding to an age of (448±34) Ma and an initial 40Ar/36 Ar ratio of 292±5. The results indicate that the excess 40Ar entered into the garnet during the post-geological history associated with the fluids which for med the secondary fluid inclusions. The phengite DB-1MS by laser stepped heating yields a complicated two-saddle-shaped age spectrum. On the diagram of 36Ar/40Ar-39Ar/40Ar based on the 40Ar-39Ar dating results, the points show an isochron trend but scatter. A n isochron age of (463±116) Ma with an initial 40Ar/36Ar ratio of 1 843 is obt ained by excluding three points (Fig.3b). Applying this initial ratio to correct the non-radiogenic 40Ar, a lower age spectrum is gained with a total age of ( 468±6) Ma (the dotted lines in Fig.3a). This initial ratio is much larger than that of the modern atmosphere, showing the present of excess 40Ar within the ph engite. If we take the isochron age of 448 Ma of the coexisting garnet by crushi ng (Fig.2b) as its real age to calculate the relative amount of excess 40Ar by t he formula of [(40Ar-40ArR)/40ArR]×100%(where 40Ar=40ArR+40ArE), the excess 40Ar (40ArE) takes up about 33.2%of the radiogenic 40Ar (40ArR) by the in situ potassium decay within the phengite, and the distribution of excess 40Ar is quite unhomogeneous from 8.5%to 62.0%(Fig.3a, the gray line). The later intermixture and unhomogeneous distribution of excess 40Ar within the phe ngite cause the abnormal high 40Ar-39Ar apparent ages and the data point scatt erations on the isochron diagrams. The 40Ar-39Ar isochron and plateau ages of the garnet are concordant wi th those of the coexisting phengite within the experiment errors, showing a Cale donian age message. The geological significance of the Caledonian age should be further investigated.