塔里木柯坪地区库普库兹曼组玄武岩锆石LA-ICPMS年代学、Hf同位素特征及其意义

LA-ICPMS U-Pb ages,Hf isotope characteristics of zircons from basalts in the Kupukuziman Formation,Keping area,Tarim Basin

塔里木盆地广泛发育有二叠纪玄武岩,产于库普库兹曼组和开派兹雷克组中。本文对库普库兹曼组玄武岩进行锆石LA-ICPMSU-Pb精确定年,并首次进行该区玄武岩锆石的铪(Hf)同位素分析。库普库兹曼组底部玄武岩的锆石LA-ICPMSU-Pb加权平均年龄为291.9±2.2Ma(MSWD=0.30,n=17),说明玄武岩为早二叠世岩浆活动产物,该年龄值指示了塔里木大火成岩省二叠纪基性岩浆活动的下限年龄;库普库兹曼组顶部玄武岩获得了293.9±4.6Ma的锆石年龄,与底部锆石年龄在误差范围内基本一致,指示了库普库兹曼组玄武质岩浆活动的时间较为短暂,291.9±2.2Ma可以代表塔里木盆地二叠纪普库兹曼组玄武岩的成岩时代。库普库兹曼组玄武岩中存在大量继承性或捕获的老年龄锆石,指示玄武岩浆在上升过程中与地壳物质发生了混染作用。锆石原位微量、稀土元素分析显示,库普库兹曼组玄武岩锆石具有典型岩浆结晶的基性岩锆石特征,且没有发生明显的后期改造作用。库普库兹曼组玄武岩锆石εHf(t)同位素值在-4.84～-0.65之间,具富集岩石圈地幔源区特征,指示了塔里木大火成岩省二叠纪基性岩浆岩源于富集岩石圈地幔,其地幔源区性质明显不同于新疆北部准噶尔地区同时代A型花岗岩的地幔源区。

There are a lot of Permian basalts lay in the Kupukuziman Formation and Kaipaizileike Formation in the Tarim basin. LA-ICPMS U-Pb precise age and in-situ Hf isotope and trace and rare earth element compositions of zircons from the basalts in the Kupukuziman Formation have been analysed in this work. The weighted average age of zircons from the bottom basalt in the group is 291.9±2.2Ma (MSWD=0.30,n=17),it is considered to be the primal time of the mafic magmatic activity in Permian. The zircon age of 293.9±4.6Ma from the top basalt in the Kupukuziman Formation is consistent with the age of the bottom basalts within error,which suggests the basaltic magmatic activity is short-lived. Therefore the petrogenic age of the basalts in the Kupukuziman Formation is around 291.9±2.2Ma. Many inherited or captured zircons hosted in the basalt refer that the basaltic magma experienced crustal contamination during the rising emplacement. The in-situ trace elements and REE show that the zircons from the basalts in the Kupukuziman Formation have typical characters of mafic magmatic origin without any latter interruption such as alteration. The εHf(t) of zircons in the Kupukuziman Formation is-4.84^-0.65,indicate that the basalts were derived from the enriched lithosphere mantle. Based on these results we propose that the Permian mafic magma of basalts in the Tarim Basin were derived from the enriched-type lithosphere mantle which was different from the source mantle of the coeval A-type granite in the Junggar area,northern Xinjiang.