| 华北克拉通南缘约2.5 Ga构造变质事件及意义 |
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| 引用本文： | 曹正琦,翟文建,蒋幸福,胡正祥,曾佐勋,蔡逸涛,张雄,徐少朋,郭君功.华北克拉通南缘约2.5 Ga构造变质事件及意义[J].地球科学,2016,41(4):570-585. |
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| 作者姓名： | 曹正琦 翟文建 蒋幸福 胡正祥 曾佐勋 蔡逸涛 张雄 徐少朋 郭君功 |
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| 作者单位： | 1.中国地质大学地球科学学院，湖北武汉 430074 |
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| 基金项目： | 中国地质调查局项目1212011220497 |
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| 摘 要： | 林山岩群是华北克拉通中部造山带南缘前寒武纪结晶基底的一部分，它对于探讨中部造山带碰撞时代及其在克拉通南缘的分布具有重要意义.选择林山岩群主体的变基性岩(斜长角闪片岩)为主要研究对象，利用锆石U-Pb定年和岩石地球化学方法，获得的结果表明斜长角闪片岩样品锆石多呈长柱状，且外形不规则，具弱分带、斑杂状分带和扇形分带结构，其中少量锆石保留有早期原岩残留的岩浆核.锆石核部具岩浆振荡环带特征，大部分Th/U比值范围为0.10~9.24，且外形不规则，边部具溶蚀结构；绝大部分锆石具弱分带、斑杂状分带和扇形分带结构，以及低Th/U比值范围为0~0.09，个别比值为0.12，显示为典型的变质锆石特征.LA-ICP-MS锆石U-Pb定年结果表明该斜长角闪片岩的核部残留加权平均年龄为2 528±43 Ma(MSWD=1.5，n=10)，经历变质作用的加权平均时限为2 474±22 Ma(MSWD=1.3，n=21).全岩地球化学结果显示林山岩群变基性岩贫硅(47.70%~50.77%，平均值为48.90%)和低钾(0.56%~1.78%，平均值为1.13%)，轻、重稀土分异较弱，亏损微量元素Nb、Th、Ti、Zr等高场强元素，且具有低的微量元素Zr含量(＜130×10-6)和Zr/Y比值(＜4).这些特征表明林山岩群形成于俯冲作用背景下的岛弧环境.因此，上述证据共同反映了华北克拉通东、西陆块碰撞产物的中部造山带经历了新太古代晚期-古元古代早期的俯冲碰撞造山作用.
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| 关 键 词： | 古元古代早期 变质事件 林山岩群 华北克拉通 中部造山带 同位素 构造地质学 |
| 收稿时间： | 2015-07-12 |
About 2.5 Ga Tectono-Metamorphic Event in Southern Margin of North China Craton and Its Significance |
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| Abstract: | Linshan Group, located in the southern margin of the central orogenic belt of the North China Craton, is a Precambrian basement. It is important to study the collision era of the central orogenic belt and the distribution of the southern margin of the North China Craton. A study was carried out on the metabasite (plagioclase-amphibole schist) of the Linshan Group for the purpose of constraining the collisional time of the central orogenic belt and its distribution through the zircon U-Pb dating and rock geochemical methods. The results obtained show that most of zircon grains from plagioclase-amphibole schist are long columnar, euhedral to subhedral, and show typical core-rim structure with irregular shape, weak zoning, taxitic zoning and sector zoning structure. Some zircons preserve the inherited cores of the early magma. Zircon rims display characteristics of dissolution texture with low Th/U ratios (most of them less than 0.09), however, zircons' cores show typical oscillatory zoning structure and relatively high Th/U ratios ranging from 0.10 to 9.24. The evidence suggests that the zircons of plagioclase-amphibole schist could be metamorphic origin. Relict cores and rims of the zircons from the plagioclase-amphibole schist yield ages of 2 528±43 Ma (MSWD=1.5, n=10) and 2 474±22 Ma (MSWD=1.3, n=21), respectively. The former data represent residual age of protolith, and the latter indicate a circa 2.5 Ga metamorphic event in the central orogenic belt of the North China Craton. Whole-rock geochemical results show that metabasite samples from the Linshan Group have relatively low contents of SiO2 (47.70%-50.77%, average=48.90%) and K2O (0.56%-1.78%, average=1.13%), and are depleted in high field strength elements (eg., Nb, Th, Ti and Zr) with flat REE patterns. Moreover, the geochemical samples have low content of Zr (less than 130×10-6) and ratios of Zr/Y (less than 4), indicating that the Linshan Group could have been formed in the subducted-related island arc environment. Therefore, the above evidence show that the central orogenic belt that formed by the collision orogenesis between eastern block and western block of the North China Craton experienced late Neoarchean to early Paleoproterozoic subduction-collisional event. |
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