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  

The K-Ca Age of the Eclogite from Bixiling, Dabie Mountains: A New Evidence of the Excess Argon in Phengite

作　　者：彭子成[1] 王俊新[1] 李曙光[1] 刘桂建[1] 朱术云[1] 张巽[1] 

机构地区：[1]中国科学技术大学地球和空间科学学院

出　　处：《高校地质学报》2002年第4期390-398,共9页Geological Journal of China Universities

基　　金：国家自然科学基金项目 (No .49973 0 11);国家重点基础研究发展规划项目 (G19990 43 40 1);中科院知识创新工程项目 (KZCX2 SW 118);中国科学技术大学资源和环境研究基地项目资助

摘　　要：40K可以进行分支衰变成40 Ar和40 Ca。在正常情况下 ,这对40 K 40 Ar或3 9Ar 40 Ar年龄与40 K 40 Ca年龄在误差范围内应该一致。测定了国家钾标样的含量、国家钾长石 (W 0 3116 )和球团矿 (W 72 0 0 )的钾钙含量、国内K Ar法标准矿物黑云母 (ZBH 2 5 )和角闪石 (ZBJ)的钾钙含量以及Tachiti标样的 (40 Ca/42 Ca)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 (40 Ca/42 Ca)和 (40 K/42 Ca)的测定。由15组数据构成的等时线斜率S =0 .12 0 8± 0 .0 4 2 3,相应的K Ca年龄为 2 2 8± 75Ma。然而多硅白云母和绿辉石的3 9Ar 40 Ar等时线年龄分别为 80 7.1± 32 .6Ma和 4 5 6 .7± 2 7.2Ma ,在误差范围内明显地高于40 K 40 Ca等时线年龄 ,这给出了前者存在过剩氩的直接证椐。此外 ,K Ca等时线的初始值 (40 Ca/42 Ca) i 为 15 1.4 2 4± 0 .0 13,相应的 ξCa为 +2 7。由于地幔岩的 ξCa≈ 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 。40 K is subjected to branched decay to form 40 Ar by electron capture and 40 Ca by beta (β -) decay. In general cases, the 40 K- 40 Ar or 39 Ar- 40 Ar and 40 K- 40 Ca 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 40 K- 40 Ca isochron was made for 30 data of 40 Ca/ 42 Ca and 40 K/ 42 Ca in phengite, omphacite and garnet in eclogite from Bixiling, Dabie Mountains and showed a slope of S=0.1208±0.0423, corresponding to a 40 K- 40 Ca age of 228±75 Ma. This is close to the Rb Sr age of 212 Ma and Sm Nd age of 239 Ma in the error range. However, the 39 Ar- 40 Ar 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 40 K - 40 Ca age. It provides a direct evidence for phengite and omphacite with excess argon due to both isotopic systems of K-Ar and K-Ca belonging to the branched decays. In addition, the primary value ( 40 Ca/ 42 Ca)i of the K Ca isochron is equal to 151.424±0.013, and corresponds to ξ Ca =+27. Because the mantle rocks have ξ 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.

关 键 词：多硅白云母 钾钙标样测定 榴辉岩 K-Ca矿物等时线年龄 过剩氩证据 大别山 

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