【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 5 4 5± 2 5℃ ,石英 -黑云母对温度为 4 6 0± 2 0℃ ,石英 -斜长石对温度 385± 4 0℃。横山岩体的石英 -角闪石对温度为 5 30± 2 5℃ ,石英 -黑云母对温度为 390± 15℃和石英 -斜长石对温度 2 85± 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 (12 8.4± 4 .7Ma)与角闪石的Ar Ar年龄 (12 5 .5 1± 0 .5 5Ma)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 (12 0 .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 (10 8± 15Ma)比角闪石的Ar Ar年龄 (12 5 .7± 1.8Ma)低 ,全岩 -角闪石 -斜长石 -黑云母Rb Sr等时线年龄 (118.6± 1.2Ma)小于黑云母的Ar Ar年龄 (12 0 .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比值变化范围太小更多还原

【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) 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.更多还原