南大别角闪岩相退变质与热液活动:朱家冲榴辉岩与闪石脉Ar-Ar年代学研究

Amphibolite-facies overprinting and a hydrothermal activity in southern Dabie terrane:Insight from Ar-Ar dating of Zhujiachong eclogite and metamorphic amphibole vein

南大别山朱家冲榴辉岩中产有一条岩脉。电子探针分析结果表明,朱家冲榴辉岩中较大颗粒的白云母和岩脉中的白云母均为钠云母;而榴辉岩中细小颗粒的白云母则为多硅白云母;岩脉中的蓝绿色柱状矿物为闪石(amphibole)。采用激光阶段加热技术和真空阶段击碎技术,对朱家冲冷榴辉岩体和岩脉进行了Ar-Ar定年研究。选自榴辉岩和闪石脉的两个钠云母激光加热Ar-Ar年龄均约为200Ma,选自闪石脉的闪石真空击碎流体包裹体Ar-Ar年龄也约为200Ma,代表了热液活动钠云母化发生的时间;而提取流体包裹体后的闪石矿物粉末阶段加热Ar-Ar等时线年龄约为243Ma,则可能代表了朱家冲榴辉岩角闪岩相退变质作用发生的最小年龄。

A metamorphic amphibole vein occurs within the Zhujiachong eclogite which was defined as Type Ⅲ eclogites by Wang et al. （1989, 1990, 1992） or cold eclogites by Okay （1993） and Castelli et al. （1998）. Ar-Ar dating results by laser stepwise heating indicate that all the paragonites from the eclogite （DB-42MS） and the vein （DB-38MS） were formed at ～ 200 Ma. Amphibole DB-38Amp from the vein by Ar-Ar in vacuo crushing technique yields concordant plateau age of （201±3） Ma and isochron age of （203±19） Ma corresponding to an initial ^40Ar/ ^36Ar ratio of 293±24 based on the analysis data of steps 13±25. The crushed powders of amphibole DB-38Amp were further analyzed by Ar-Ar stepwise heating with an external furnace, and concordant plateau age of （242±2） Ma and isochron age of （243±3） Ma are obtained. These Ar-Ar dating results might suggest that the amphibolite- facies overprinting took place earlier than 243 Ma, followed by wide hydrothermal activities in ～ 200 Ma in the Dabie terrane.