北大别黄土岭麻粒岩中黑云母过剩Ar的成因解释和Ar同位素热年代学研究启示

应用阶段加热脱气技术测定了北大别黄土岭麻粒岩的黑云母和共存花岗岩岩体中斜长角闪岩包体角闪石的Ar-Ar年龄。角闪石给出的坪年龄为(124.9±4.6)Ma,与区域上早白垩世120~130 Ma的变质-岩浆事件年龄相一致。黑云母给出的坪年龄为(176.9±0.8)Ma,年龄大于Ar封闭温度较高的角闪石的年龄,这表明黄土岭麻粒岩黑云母中含有过剩Ar。较低温度和富流体环境生长的晚世代黑云母是过剩Ar的主要载体。因此,在应用Ar-Ar热年代计重塑多成因同类含钾矿物的岩石地质体的冷却速率时,应用激光探针对代表峰期世代矿物进行原位测定应是正确的选择。

Excess Argon in Biotite from the Felsic Granulite in Huangtuling Area, North Dabie Unit: Genetic Interpretation and Implications to Ar Isotope Chronology

Step-wise heating gas-released Ar-Ar method was applied for dating of biotite from the granulite and amphibole from the amphibolite enclave in the adjacent granitic pluton,Huangtuling area,North Dabieshan.The amphibole indicates a plateau age of(124.9±4.6) Ma,consistent with the timing of the Cretaceous(120—130 Ma)metamorphic and magmatic events regionally.However,the biotite gave a plateau age of(176.9±0.8) Ma,much older than that of the amphibole which has a higher closure temperature for Ar than biotite does,suggesting that excess argon exists in the biotite of the granulite.Late generation biotites growing in lower temperature condition and fluid-rich environment are the main carriers of excess argon.Therefore,when applying the Ar-Ar dating method to reconstruct the cooling rate of geologic rocks containing multi-genetic cogeneric potassium-bearing minerals,the only appropriate choice should be using of laser probe to in situ determining the age of the generation mineral of metamorphic peak.