地幔包体金云母~(40)Ar/~(39)Ar年龄对玄武岩浆喷发结束时间的约束

山东临朐-昌乐地区新生代岩浆活动强烈,以形成含大量地幔包体的玄武岩为特征.作者在考察与研究昌乐北岩古火山口玄武岩中地幔包体的过程中,发现一些包体中发育有地幔钾交代成因的金云母细脉.鉴于金云母的封闭温度远低于玄武岩浆喷发时的温度以及金云母可以记录喷发的玄武岩浆冷却通过金云母封闭温度的时间,本文尝试通过金云母所记录的~(40)Ar/~(39)Ar年龄来推断玄武岩浆喷溢的结束时间.该火山口玄武岩不同部位的三个地幔岩包体其金云母的~(40)Ar/~(39)Ar坪年龄分别为18.42±0.21Ma、18.65±0.27Ma和18.39±0.36Ma,年龄结果具有很好的一致性,充分说明了该定年手段的有效性.因此可以确定该火山口玄武岩浆喷溢活动约在18.5Ma前结束.由此推测山旺盆地中不整合发育在源于该火山口喷溢的玄武岩之上的化石群的形成时代下限约为18.5Ma.用玄武岩地幔包体金云母的~(40)Ar/~(39)Ar年龄来确定玄武岩年龄是玄武岩定年一个有效的新方法. Abstract： Cenozoic basalts in the Linqu-Changle area, Shandong Province, are characterized by abundant mantle-derived xenoliths. Recently, we discovered that some xenoliths in the Beiyan paleovolcanic crater contain some fine phlogopite veins related to mantle K-metasomatism. Since the closure temperature of phlogopite is much lower than the eruptive temperature of basaltic magma and the phlogopite has potential to record the time when hot magma cool down through the closure temperature of phlogopite, here we try to use the ~(40)Ar/~(39)Ar age of phlogopites to determine the ending time of eruption of basaltic magmas and age of basalt in the Beiyan paleovolcanic crater. Three phlogopite samples from three mantle-derived xenoliths from different positions in the Beiyan paleovocanic crater yielded identical ~(40)Ar/~(39)Ar plateau ages of 18.42±0.21Ma, 18.65±0.27Ma and 18.03±0.63Ma respectively. It is concluded that the basaltic volcanic act