超高压变质岩多硅白云母的外来^(40)Ar探讨  

Extraneous ^(40)Ar within UHPM phengite:Interpretation for the ^(40)Ar-^(39)Ar results of phengites by laser stepwise heating from Bixiling eclogite,southern Dabieshan

作　　者：邱华宁[1,2] J.R. Wijbrans 

机构地区：[1]中国科学院广州地球化学研究所同位素年代学和地球化学重点实验室,广东广州510640 [2]Department of Isotope Geochemistry,Vrije Universiteit Amsterdam,De Boelelaan 1085,1081 HV Amsterdam,The Netherlands

出　　处：《地球化学》2007年第3期237-246,共10页Geochimica

基　　金：国家自然科学基金(40272039);中国科学院知识创新工程重要方向项目(KZCX2-SW-117);中国科学院广州地球化学研究所知识创新工程领域前沿项目(GIGCX-03-01);中国科学院与荷兰皇家文理科学院合作项目(01CDP026)

摘　　要：采用激光阶段加热40Ar-39Ar定年技术,测定了碧溪岭榴辉岩6个多硅白云母的Ar同位素组成。02BX030MS形成基本平坦的40Ar-39Ar年龄谱,数据点构成很好的正等时线,加权平均年龄和等时线年龄均为697Ma,这一年龄可能接近原岩年龄。DB-1MS、02BX018MS和02BX019MS之40Ar-39Ar年龄谱起伏较大,表观年龄范围为～400Ma至～680Ma。在40Ar/36Ar-39Ar/36Ar正等时线图解上,这3个样品数据点落在下拟合线Reg-Ⅰ和上拟合线Reg-Ⅲ之间。Reg-Ⅰ对应的年龄为436～463Ma,与榴辉岩石榴子石原生流体包裹体年龄基本一致,可能代表了榴辉岩超高压变质作用峰期的年龄;Reg-Ⅲ对应的年龄为541～659Ma,最高年龄值小于02BX030MS的年龄,表明超高压变质作用使多硅白云母产生了不同程度的放射成因40Ar部分丢失。继承40Ar模型可以合理解释高压-超高压变质岩石之多硅白云母40Ar-39Ar年龄明显偏老等地质现象。Extraneous argon found within phengites of （U） HP rocks from various locations has cast doubts on the applicability of the ^40Ar-^39Ar method when dating the phengites from such lithologies. It was argued that under ultrahigh pressure conditions, argon trapped in the pore space in the rock partitions readily into the crystal lattice, thus causing unrealistically high apparent ages. In this study, six phengite samples from the Bixiling eclogite were analyzed by laser stepwise heating. The ^40Ar-^39Ar results of 02BX030MS yields an essential flat age spectrum with total age of （696±1） Ma and weighted mean age of （697±4） Ma for all steps （Fig. 3a）. On normal isochron diagram, a well-defined isochron is obtained for all data except for step 19 （laser power of 1.73W）, corresponding to age of （697±7） Ma, initial^40Ar/36Ar ratio of 275 ±66 and MSWD of 59.9 （Fig. 3b）. The ^40Ar-^39Ar results of DB-1MS, 02BX018MS and 02BX019MS form very complicated age spectra with wide apparent age range from - 400 Ma to - 680 Ma. Their total fusion ages are from 458 Ma to 578 Ma （Fig. 3c, 3e and 3g）. On the normal isochron plots, the data points scatter between two regression lines of Reg-Ⅰ and Reg-Ⅲ （Fig. 3d, 3f and 3h）. These lower regression lines Reg-Ⅰ correspond to ages of 463, 455 and 436 Ma, respectively, which are well concordant to the isochron ages - 450 Ma of these garnets by stepwise crushing in vacuum （Qiu and Wijbrans,2006）. These upper regression lines Reg-Ⅲ correspond to ages of 659, 636 and 541 Ma, respectively, which are younger than the isochron ages of - 697 Ma of 02BX030MS. Compared with the excess argon model, the inherited argon model is more appropriate to explain the different apparent ^40Ar/^39Ar ages of the phengites from eclogites within Dabieshan and Sulu. Ages constrained by the lower regression lines of phengites might be interpreted as representing the metamorphic events, while ages corresponding to the upper regression lines might be younger than and cl

关 键 词：外来^40Ar “过剩”^40Ar ^40Ar-^39Ar定年 榴辉岩 超高压变质作用 大别山 

分 类 号：P597[天文地球—地球化学]