大别山东部花岗片麻岩的锆石U-Pb年龄

花岗片麻岩是大别山区除超高压变质杂岩外的另一种重要岩石类型,本文测得南大别二长花岗片麻岩中单颗粒锆石U-Pb不一致线的上交点年龄为789±43 Ma.位于五河一水吼韧性剪切带南缘的糜棱岩化二长花岗片麻岩中锆石的²⁰⁷Pb/²⁰⁶Pb表面年龄接近一致年龄,为715～777 Ma,平均757 Ma,U-Pb不一致线的上交点年龄为815±70 Ma,下交点年龄为482±167 Ma.北大别石英二长片麻岩中锆石²⁰⁷Pb/206表面年龄的变化范围较广,介于594～700 Ma,平均为649 Ma,U-Pb不一致线的上、下交点年龄分别为814±97 Ma和477±77 Ma.这说明这些正片麻岩的侵位时间范围大致为750～850 Ma的晚元古代;南、北大别正片麻岩中的锆石年代学显示它们可能具有相似的形成与演化历史;约480 Ma前后的加里东运动对大别山地区可能也有影响.

U/Pb ZIRCON DATING OF GRANITIC GNEISSES IN EASTERN DABIE MOUNTAINS, CENTRAL CHINA

In addition to ultrahigh pressure(UHP) metamorphic complex, the granitic gneiss is another important rock type in southeastern Dabie Mountains. The granitic gneiss yields 789±43 Ma based on the U-Pb zircon concordia upper intercept. The ages for mylonitization granitic gneiss to the south of Wuhe Shuihou ductile shear fault are 815±70 Ma and 482±167 Ma for upper and lower intercepts, respectively, apparent ages of ²⁰⁷Pb/ ²⁰⁶Pb ranges 715～777 Ma, with average age of 757 Ma. The ages for mylonitization quartz two feldspar gneiss north to the ductile shear fault are 814±97 Ma based on upper intercept and 477±70 Ma on lower intercept, and apparent ages of ²⁰⁷Pb/ ²⁰⁶Pb ranges 594～700 Ma, with average age of 649 Ma. These dates demonstrated the intrusive ages of protolith of the granitic gneiss being about 750～820 Ma. The zircon U-Pb date of gneiss from southern Dabie and northern Dabie Mountains show that they have similar forming and evolution history; and the Caledonia orogenesis occurred about 480 Ma ago which influenced all the Qinling area, and also the west part of the Qinling Dabie orogen.