地学前缘 ›› 2011, Vol. 18 ›› Issue (2): 17-25.

• 论文 • 上一篇    下一篇

高级变质作用对锆石U-Pb同位素体系的影响：胶东栖霞地区变质闪长岩锆石定年

万渝生,刘敦一,董春艳,刘守偈,王世进,杨恩秀   

  1. 1. 中国地质科学院 地质研究所北京离子探针中心， 北京 100037
    2. 山东省地质调查院， 山东 济南 250013
  • 收稿日期:2010-09-20 修回日期:2011-01-06 出版日期:2011-03-15 发布日期:2011-04-15
  • 作者简介:万渝生(1956—)，男，博士，研究员，地球化学专业，主要从事早前寒武纪研究。E-mail：wanyusheng@bjshrimp.cn
  • 基金资助:

    中国地质构造区划与区域地质调查综合集成项目(1212010811033)；中国地质调查局重点资助项目(1212010711815)

The impact of highgrade metamorphism on the UThPb system of zircons：A case study of zircon dating of metadiorite in Qixia Area， eastern Shandong.

The impact of highgrade metamorphism on the UThPb system of zircons：A case study of zircon dating of metadiorite in Qixia Area， eastern Shandong.   

  1. 1.  Beijing SHRIMP Center， Institute of Geology， Chinese Academy of Geological Sciences， Beijing 100037， China
    2.  Shandong Geological Survey Institute， Jinan 250013， China
  • Received:2010-09-20 Revised:2011-01-06 Online:2011-03-15 Published:2011-04-15

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摘要：

文中对高级变质作用(高角闪岩相麻粒岩相)条件下“干的”岩石体系中锆石U、Th、Pb行为进行了研究。样品为胶东栖霞地区太古宙变质基底的高角闪岩相麻粒岩相变质闪长岩。矿物组合为单斜辉石+斜长石+钾长石+黑云母+石英。锆石呈近短轴状、等轴状和不规则状，阴极发光下具板状、杉树叶状、扇形结构，部分具封闭环带结构。锆石中存在斜长石+钾长石+石英+黑云母、石英+角闪石+斜方辉石+单斜辉石等矿物包体。在34颗锆石上进行了50个数据点分析。锆石Th/U 值普遍大于05，仅个别小于01。在UPb年龄谐和图上，数据点沿谐和线连续分布于225～254 Ga，最年轻锆石年龄约为195 Ga。同一锆石的不同位置，锆石年龄和组成也可存在很大变化。研究表明：(1) 岩石遭受过两期高级变质作用，时间分别为新太古代晚期(大约250 Ga)和古元古代晚期(大约195 Ga或稍晚)；(2)“干的”岩石体系发生高级变质作用，可引起锆石UThPb体系部分开放，数据点沿谐和线分散分布，在这种情况下，位于中间的数据点的年龄无地质意义；(3)高Th/U 值是高级变质锆石的组成特征。

关键词: 锆石, UThPb体系, 铅丢失, 高级变质作用, SHRIMP定年, 早前寒武纪, 胶东

Abstract:

This paper presents a study of the UThPb behavior of zircons in a “dry” rock system during highgrade metamorphism in the Archean basement of eastern Shandong. The studied sample has a mineral assemblage of plagioclase+Kfeldspar+clinopyroxene+biotite+quartz and its protolith is considered to be diorite. The zircons are stubby， equant or irregular in shape and show firleaf， sector， banded or oscillatory zoning. They contain inclusions， including the mineral assemblages of clinopyroxene+orthopyroxene+hornblende+quartz and plagioclase+Kfeldspar+biotite+quartz. Fifty SHRIMP analyses were carried out on 34 zircon grains and commonly yielded high Th/U ratios(mostly>05). Most analyses are distributed along a concordia from 254 to 225 Ga， with the youngest age of about 195 Ga. Compositions and ages show large variations even in an individual zircon grain. Combined with early studies， conclusions have been drawn as follows： （1) the diorite underwent two episodes of highgrade metamorphism both at the end of the Neoarchean and the Paleoproterozoic(about 250 Ga and about 195 Ga or slightly later)； （2) highgrade metamorphism in a “dry” rock system may partially reset the UThPb system of zircons， and in this case， the ages between the oldest and youngest are chronologically meaningless； （3) high Th/U ratios may be common features of the zircons formed during highgrade metamorphic conditions.

Key words: zircon； UThPb system； lead loss； highgrade metamorphism； SHRIMP dating； , Early Precambrian； , eastern Shandong

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