大别山碧溪岭榴辉岩的K—Ca同位素定年多硅白云母含过剩氩的新证据

大别山碧溪岭榴辉岩的K—Ca同位素定年多硅白云母含过剩氩的新证据

彭子成1 2, 王俊新1, 李曙光1, 刘桂建1, 朱术云1, 张巽1

1. 中国科学技术大学地球和空间科学学院，合肥 230026; 2. 中国科学院地球环境研究所黄土与第四纪地质国家重点实验室，西安 710054

收稿日期:2002-12-20 修回日期:2002-12-20 出版日期:2002-12-20 发布日期:2002-12-20

he K-Ca Age of the Eclogite from Bixiling,Dabie M ountains：A New Evidence of the Excess Argon in Phengite

PENG Zi-cheng1 2, WANG Jun-xin1, LI Shu-guang1, LIU Gui-jian1, ZHU Xue-yun1, ZHANG Xun1

1. The School of Earth and Space Science, University of Science and Technology of China, Hefei 230026, China; 2. State Key laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710054, China

Received:2002-12-20 Revised:2002-12-20 Online:2002-12-20 Published:2002-12-20

摘要/Abstract

摘要： ^40K可以进行分支衰变成^40Ar和^40Ca。在正常情况下，这时^40K-^40Ar或^31Ar-40^Ar年龄与^40K-^40Ca年龄在误差范围内应该一臻，测定了国家钾标样的含量，国家钾长石（W03116）和球团矿（W7200）的钾钙含量，国内K－Ar法标准矿物黑云母（ZBH－25）和角闪石（ZBJ）的钾钙含量以及Tachiti标样的（^40Ca/^42Ca)比值和含量。结果实测值与标称值在误差范围内一致，对大别山碧溪岭超高压变质榴辉岩的多硅白云母，绿辉石和石榴子石进行了（^40Ca/^42Ca)和（^40K/^42Ca)的测定。由15组数据构成的等时线斜率S＝0.1208±0.0423，相应的K－Ca年龄为228±75Ma，然而多硅白云母和绿辉石的^39Ar-^40Ar等时线年龄分别为807.1±32.6Ma和456.7±27.2Ma，在误差范围内明显地高于^40K-^40Ca等时线年龄，这给出了前者存在过剩氩的直接证据，此外，K－Ca等时线的初始值（^40Ca/^42Ca)i为151.424±0.013，相应的ζCa为±27。由于地幔岩的ζCa≈0，所以前者表明榴辉岩的原岩具有典型的大陆地壳性质，这为大别山超高压变质岩体是由大陆物质俯冲至上地幔的深度，然后折返至大陆的形成过程提出了又一个判据。

Abstract: 40K is subjected to branched decay to form 40Ar by electron capture and 40Ca by beta (β-)decay.In general cases,the 40K-40Ar or 39Ar-40Ar and 40K-40Ca ages are consistent in the error range.In order to test the measuring system ,the K and Ca contents and their isotopic compositions in the potassium feldspar(W03116),pellet(W7200),biotite(ZBH-25),hornblende(ZBJ) national standards,and the Tachiti secondary international standard have been determined and their results are certified values in the error range.The 40K-40Ca isochron was made for 30 data of 40Ca／42Ca and 40K／42Ca in phengite, omphacite and garnet in eclogite from Bixiling,Dabie Mountains and showed a slope of S=0.1208±0.0423.corresponding to a 40K-40Ca age of 228±75Ma. This is close to the Rb-Sr age of 212 M a and Sm-Nd age of 239Ma in the eror range.However.the 39Ar-40Ar isochron ages of phengite and omphacite are 807.1± 32.6 Ma and 456.7± 27.2 Ma, respectively,which are obviously older than that of 40K-40Ca age.It provides a direct evidence for phengite and omphacite with excess argon due to both isotopic systems of K-Ar an d K -Ca belonging to the branched decays. In addition.the primary value(40 Ca／40Ca)i of the K-Ca isochron is equal to 151.424±0.013.and corresponds to Ca +27.Because the mantle rocks have Ca=0,it is certified that the original rocks of the eclogite have the continental character. This provides a new criterion that the ultrahigh-pressure metamorphic rocks at Bixiling in Dabie Mountains were formed during the process of subduction of the continental materials to the upper mantle and their exhumation back to the continent.

彭子成 , 王俊新, 李曙光, 刘桂建, 朱术云, 张巽. 大别山碧溪岭榴辉岩的K—Ca同位素定年多硅白云母含过剩氩的新证据[J]. J4.

PENG Zi-cheng1 2, WANG Jun-xin1, LI Shu-guang1, LIU Gui-jian1, ZHU Xue-yun1, ZHANG Xun1. he K-Ca Age of the Eclogite from Bixiling,Dabie M ountains：A New Evidence of the Excess Argon in Phengite[J]. J4.