碧溪岭岩体中石榴橄榄岩的锆石显微结构及离子探针定年

SIMS U-Pb Dating and CL Images of Zircons from Garnet Peridotite of Bixiling

对碧溪岭岩体中石榴橄榄岩的锆石进行了BSE和CL显微结构分析 ,并在此基础上用离子探针进行了锆石微区U Pb定年。锆石CL图像显示其有明显的核边结构 ,核部具岩浆锆石 ,边部是变质增生锆石的特征。结合锆石的外形特征认为 ,边部的变质锆石是在超高压变质作用中重结晶形成的。离子探针分析结果在一致曲线图中构成不一致线 ,得到上下交点年龄分别为 82 4±39Ma和 2 5 4± 38Ma。结合CL图像认为上交点 82 4± 39Ma的年龄代表其原岩形成年龄 ,下交点2 5 4± 38Ma为其超高压变质作用的年龄。另有少量较老年龄的捕虏晶锆石 。

SIMS U Pb dating and CL images have been done for zircons of garnet peridotite from Bixiling, southern Dabie. The CL images reveal that most zircons have core rim texture. The cores show igneous zircons characteristics. The rims may be recrystallized during ultra high pressure metamorphism. Thirteen points were dated within seven zircons. All the points have Th/U ratios of 0.85∽2.13, and ten of them ≥1.5. These suggest that most zircons come from the garnet peridotite, for it has positive Th anomaly in primitive mantle normalized pattern. Except for 4 points, the other nine analyses define a chord intersecting the concordia curve at 824Ma and 254Ma respectively. According to the zircons' CL images, the upper intercepted age represents the magmatic protolith age, the lower intercepted age defines the metamorphic event. The protolith of Bixiling garnet peridotite eclogite massif was formed at 824±39Ma, and experienced UHP metamorphism at 254±38Ma. Some xenocryst zircons have been found. Their ages are older than that of protolith of the Bixiling massif. These zircons may come from the crustal rock that was contaminated during the magmas rising.