The Relation between Rb-Sr, Ar-Ar Geochronometers and Oxygen Isotopic Equilibrium of Intrusions from Eastern Anhui Province, China
The Relation between Rb-Sr, Ar-Ar Geochronometers and Oxygen Isotopic Equilibrium of Intrusions from Eastern Anhui Province, China
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发表时间：
2003
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通讯作者：
Chengzhu Jiang
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作者：
Chengzhu Jiang
为了将矿物氧同位素的热力学平衡与花岗岩类Rb-Sr矿物等时线年龄的有效性联系起来，测定了皖东两块石英二长岩的矿物和全岩样品的氧、Rb-Sr同位素比值以及角闪石和黑云母的Ar-Ar年龄。屯仓岩体和横山岩体的石英（Q）角闪石（Hbl）氧同位素温度分别为545±25 ℃和530±25 ℃，Q黑云母（Bt）氧同位素温度分别为460±20 ℃和390±15 ℃，Q斜长石（Pl）氧同位素温度分别为385±40 ℃和285±35 ℃。O同位素温度关系表明，在冷却过程中，花岗岩类矿物之间存在退变质平衡。同时获得了具有地质意义的Rb-Sr等时线年龄。屯昌岩体给整个岩石（w。（r.）Hbl Pl Rb Sr等时线年龄为128.4± 4.7Ma，与Hbl Ar Ar坪年龄125.51± 0.55Ma一致; P1 Hb 1Bt年龄为120.3± 2.6Ma，与118.0± 0.1Ma的Bt-Ar-Ar坪年龄一致。对于恒山岩体，w.r. Hbl Pl Rb Sr等时线年龄为108±15 Ma，明显年轻于Hbl Ar Ar坪年龄125.7± 1.8Ma。118.6± 1.2Ma的Hbl Pl Bt年龄比120.0± 0.2Ma的Bt Ar Ar坪年龄年轻。年轻的Rb-Sr等时线年龄具有较大的不确定性，是由于长石的蚀变和岩石中Rb/Sr比值的微小变化。因此，氧同位素体系的平衡状态是获得具有地质意义的Rb-Sr等时线年龄的先决条件。然而，要获得良好的等时线，需要进行详细的矿物学研究。利用氧同位素温度建立的冷却曲线计算的冷却速率约为10~20 ℃/Ma，比利用阻塞温度估算的冷却速率20~35 ℃/Ma低10~15 ℃/Ma，其原因值得进一步研究。
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) hornblende (Hbl) oxygen isotopic temperature of 545±25 ℃ and 530±25 ℃, Q biotite (Bt) temperature of 460±20 ℃ and 390±15 ℃ and Q plagioclase (Pl) temperature of 385±40 ℃ and 285±35 ℃. The relation of O isotopic temperatures suggested a retrograde equilibrium among minerals during cooling of the granitoids. Meanwhile, geologically meaningful Rb Sr isochron ages were obtained. The Tunchang 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. Pl Hbl Bt age of 120.3±2.6 Ma, identical to the Bt Ar Ar plateau age of 118.0± 0.1 Ma within uncertainty. For the Hengshan intrusion, the w.r. Hbl Pl Rb Sr isochron age of 108±15 Ma 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 isochron 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 isochron. 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.