皖东侵入岩矿物Rb－Sr和Ar－Ar年代学及其与氧同位素平衡之间的关系

皖东侵入岩矿物Rb－Sr和Ar－Ar年代学及其与氧同位素平衡之间的关系

陈江峰1, 谢智1, 郑永飞1, 龚冰1, 陈文2, 周泰禧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. 中国地质科学院地质研究所，北京 100037

收稿日期:2003-06-20 修回日期:2003-06-20 出版日期:2003-06-20 发布日期:2003-06-20

The Relation between Rb-Sr，Ar-Ar Geochronometers and Oxygen Isotopic Equilibrium of Intrusions from Eastern Anhui Province，China

CHEN Jiang-feng1, XIE Zhi1, ZHENG Yong-fei1, GONG Bing1, CHEN Wen2, ZHOU Tai-xi1, Zhang Xun1

1. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; 2. Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China

Received:2003-06-20 Revised:2003-06-20 Online:2003-06-20 Published:2003-06-20

摘要/Abstract

摘要： 为探讨花岗质岩石中矿物O同位素平衡与Rb-Sr矿物等时线年龄有效性之间的关系，测定了皖东屯仓与横山两个石英二长岩体中单矿物的Rb-Sr和O同位素组成以及角闪石和黑云母的Ar-Ar年龄。屯仓岩体的石英—角闪石对温度为545±25℃，石英—黑云母对温度为460±20℃，石英—斜长石对温度385±40℃。横山岩体的石英—角闪石对温度为530±25℃，石英—黑云母对温度为390±15℃和石英—斜长石对温度285±35℃。反映封闭体系中矿物在冷却过程中的退化交换再平衡。矿物间的氧同位素平衡对应于有地质意义的Rb-Sr矿物等时线年龄。屯仓岩体的全岩—角闪石—斜长石Rb-Sr等时线年龄(128．4±4．7Ma)与角闪石的Ar-Ar年龄(125．51±0．55Ma)在误差范围内一致，全岩—角闪石—斜长石—黑云母Rb-Sr等时线年龄(120．3±2．6Ma)与黑云母的Ar-Ar年龄(118．0±0．1Ma)一致。横山岩体的全岩—角闪石—斜长石Rb-Sr矿物等时线年龄(108±15Ma)比角闪石的Ar-Ar年龄(125．7±1．8Ma)低，全岩—角闪石—斜长石—黑云母Rb-Ar等时线年龄(118．6±1．2Ma)小于黑云母的Ar-Ar年龄(120．0±0．2Ma)。Rb-Sr等时线年龄小于Ar-Ar年龄是由于长石的蚀变引起的。大的等时线误差是由于Rb/Sr比值变化范围太小。研究实例表明，在花岗岩类定年时，O同位素的平衡状态是Rb/Sr等时线年龄可靠的前提条件，但详细的矿物学研究仍不可少。用O同位素平衡温度代替封闭温度构筑了冷却曲线，得到两岩体的冷却速率为10—20℃/Ma，比用放射成因同位素的扩散封闭温度估计的冷却速率20—35℃/Ma低约10—15℃/Ma，其原因尚待研究

Abstract: In order to relate the thermodynamic equilibrium of oxygen isotopes of the minerals and the validity of the Rb-Sr mineral isochron age for granitoids，oxygen，and Rb-Sr isotopic ratios of mineral and whole rock samples，as well as Ar-Ar ages of hornblendes and biotites were determined for two quartz monzonites from eastern Anhui province．The Tuncang and Hengshan bodies gave quartz (Q)-homblende (Hb1) oxygen isotopic temperature of 545±25℃ and 530±25℃ ，Q-biotite(Bt)temperature of 460±20℃ and 390±15℃ and Q-plagioclase(P1)temperature of 385±40℃ and 285±35℃ ．The relation of 0 isotopic temperatures suggested a retrograde equilibrium among minerals during cooling of the granitoids． Meanwhile，geologically meaningful Rb-Sr isochron ages were obtained ．The Tunchavg intrusion gave whole rock (w．r．)-Hbl-Pl Rb-Sr isochron age of 128．4±4．7 Ma．identical to the Hbl Ar-Ar plateau age of 125.51±0.55 Ma，and w．r．-P1-Hbl-Bt age of 120.3±2.6 M a，identical to the Bt At-At plateau age of 118.0±0.1 M a within uncertainty．For the Hengshan intrusion。the w．r．-Hbl-Pl Rb-Sr isoehron age of 108±15 M a is significantly younger than the Hbl Ar-Ar plateau age of 125.7±1.8 Ma, and the w ．r．-Hbl-Pl-Bt age of 118.6±1.2 Ma younger than the Bt Ar-Ar plateau age of 120.0±0.2 Ma．The young Rb-Sr isoehron ages with large uncertainties are due to alteration of the feldspar and small variation in Rb/Sr ratios in the rock．Therefore．we conclude that the equilibrium state of the oxygen isotope systems is a prerequisite for the geologically meaningful Rb-Sr isochron ages of the granitoids ．However，detailed mineralogical investigation is needed for obtaining a good isoehron．The cooling rate of about 10～20℃ ／Ma obtained by cooling curve built using oxygen isotope temperatures is 10～15℃ ／Ma lower than that of 20～35℃ ／Ma estimated by blocking temperatures，the reason for it merits further investigation

陈江峰, 谢智, 郑永飞, 龚冰, 陈文, 周泰禧, 张巽. 皖东侵入岩矿物Rb－Sr和Ar－Ar年代学及其与氧同位素平衡之间的关系[J]. J4.

CHEN Jiang-feng1, XIE Zhi1, ZHENG Yong-fei1, GONG Bing1, CHEN Wen2, ZHOU Tai-xi1, Zhang Xun1. The Relation between Rb-Sr，Ar-Ar Geochronometers and Oxygen Isotopic Equilibrium of Intrusions from Eastern Anhui Province，China[J]. J4.