秦岭拉鸡庙镁铁质岩体锆石LA-ICP-MS年代学研究

LA-ICP-MS zircon dating from the Lajimiao mafic complex in the Qinling orogenic belt

秦岭拉鸡庙镁铁质岩体位于北秦岭南缘,主要由辉长岩(80%)、苏长辉长岩(15%)和少量闪长岩等侵入杂岩组成。对采自该岩体闪长岩的锆石进行阴极发光图像、微区原位LA-ICP-MS微量元素分析和U-Pb定年。CL图像显示这些锆石可以分为两类,一类锆石呈长柱状,具有明显的岩浆生长环带;另一类则呈浑圆状,阴极发光图像复杂,部分颗粒岩浆生长环带较模糊,个别样品外围存在一窄的亮色环边,推测为后期地质事件影响的结果。对26颗锆石核部和生长边进行28次U-Pb同位素分析,获得两组^(206)Pb/^(238)U年龄,分别为973±60Ma和422±7Ma。分析结果显示,所有样品具有高的Th,U,REE含量,明显富集HREE,其Th/U比值普遍高于0.6,表明这些锆石应属于岩浆成因。其中,422±7Ma应该代表拉鸡庙镁铁质岩体的成岩时代,这可能与古生代扬子陆块或者是具有扬子板块属性的微陆块和华北陆块的碰撞有关,该碰撞导致了秦岭洋的闭合;而973±60Ma应为捕获锆石年龄,代表北秦岭早期与Rodinia超大陆拼合有关的岩浆事件。考虑到没有检测到典型的华北克拉通的年龄,推测元古代北秦岭更接近华南板块。

The Lajimiao mafic complex, located on the south limb of the North Qinling orogenic belt, consists mainly of gabbro (80%)and norite-gabbro (15%) with minor diorite. A combined study of cathodoluminescence images (CL), in situ LA-ICP-MS U-Pb isotope dating and trace element analysis were carried out for zircons from the Lajimiao diorite. Cathodoluminescence images show two kinds of zircon grains, prismatic with clear magmatic zonations and round with complicated internal structure, some of which have no magmatic zonation. All together 28 U-Pb analyses have been taken on cores and overgrowths of 26 zircon grains. Two groups of ages were obtained, with weighted average (206)Pb/(238)U ages of 973 +/- 60Ma for round zircons, and 422 +/- 7Ma for prismatic ones, respectively. All analyses show high concentrations of Th, U, and HREE, with positive Ce anomaly and negative Eu anomaly. The high Th/U ratios indicate these zircons are magmatic in origin. The younger group (422 +/- 7Ma) represents the formation age of the Lajimiao gabbro complex, which is probably relative to Paleozoic collision of the North China Block with the South China Block or a micro-block separated from it, which led to the closure of the Qinling Ocean. The older group (973 +/- 60Ma) is likely to be inherited zircon, formed in magmatic event(s) during the assemblage of Rodinia supercontinent. Given that no typical North China craton ages were detected, we speculate that the North Qinling was closer to the South China Block than the North China Block in the Proterozoic.