华北克拉通东南缘前寒武纪下地壳的幕式生长与多期改造:岩石学、年代学和Hf同位素证据  

Episodic Growth and Multiple Modification of Precambrian Lower Crust in the Southeastern Margin of North China Craton: Petrologic, Geochronological and Hf-isotopic Evidences

作　　者：刘贻灿[1,2] 王安东[1,2] 

机构地区：[1]中国科学院壳幔物质与环境重点实验室,安徽合肥230026 [2]中国科学技术大学地球和空间科学学院,安徽合肥230026

出　　处：《地球科学与环境学报》2012年第4期1-11,共11页Journal of Earth Sciences and Environment

基　　金：国家自然科学基金项目(40634023;40973043);国家自然科学基金重点项目(90814008)

摘　　要：早前寒武纪被认为是大陆地壳形成的重要时期。华北克拉通东南缘前寒武纪下地壳岩石主要以变质基底(五河变质杂岩)和中生代闪长/二长闪长质斑岩中包体形式存在,它们为研究该地区前寒武纪下地壳的形成和演化提供了极好的天然实验室。近期的变质岩石学、岩石地球化学、Hf同位素及锆石U-Pb年代学研究表明:该区前寒武纪下地壳经历了幕式地壳生长与多期改造。强烈的构造-热事件和变质改造时间集中于2.5～2.6、2.1、1.8～1.9Ga以及390、176Ma。其中,形成于2.5～2.6Ga的下地壳岩石,一部分经历了2.1Ga和(或)1.8～1.9Ga高压麻粒岩相变质作用以及390、176Ma的变质改造,类似于形成于2.1Ga并经历了1.8Ga高压麻粒岩相变质作用的岛弧成因的下地壳岩石,表现为高放射成因Pb同位素组成;另一部分岩石则形成于2.55～2.64Ga,可能因处于下地壳上部而仅遭受了2.48～2.49Ga麻粒岩相变质作用,但没有2.1Ga和(或)1.8～1.9Ga变质叠加的岩石学和年代学记录,表现为典型前寒武纪下地壳岩石特点的低放射成因Pb同位素组成。2.7～2.8Ga的继承锆石U-Pb年龄和锆石Hf模式年龄暗示研究区可能还存在更早的地壳生长时期,但尚需进一步研究。The ear The Precambrian y Precambrian is regarded to be a crucial stage for continental crust formation. ower-crustal rocks in the southeastern margin of North China Craton occur as the exposed metamorphic basement （named as the Wuhe metamorphic complex） and xenoliths in the Mesozoic dioritic to monzodioritic porphyry. These rocks provide an excellent natural laboratory to study formation and evolution of the Precambrian lower crust in the region. Integrated investigations on metamorphic petrology, petrologic geochemistry, Hf-isotope and zircon U-Pb geochronology of the rocks suggested that the Precambrian lower crust beneath the studied region experienced an episodic growth and multip tectono-thermo events and metamorphic overprinting mainly occuri 2.5-2.6, 2.1, 1.8-1.9 Ga and 390, 176 Ma. For the 2.5-2.6 Ga them with high radiogenic Pb-isotopic compositions, which were si Ga subduction-related arc setting and underwent 1.8 Ga h metamorphism, suffered from high-pressure granulite-facies metamorphism at 2.1 Ga and（or） 1.8-1.9 Ga, and subsequent metamorphic overprinting at 390 Ma and 176 Ma; however, others had low radiogenic Pb-isotopic compositions for typical Precambrian lower-crustal rocks, and formed at 2.55-2.64 Ga and underwent 2.48-2.49 Ga peak metamorphism of granulite-facies without record of post-peak metamorphic overprint at 2.1 Ga and（or） 1.8-1.9 Ga. Therefore, they formed simultaneously, but probably located at different crustal levels and escaped from subsequent metamorphic overprinting, strongly depending on their formation depths. The 2.7- 2.8 Ga ages defined by inherited zircons and depleted mantle zircon-Hf model ages could record an earlier crustal growth episode in the area.

关 键 词：锆石 U-PB年代学 HF同位素 麻粒岩相变质作用 下地壳 幕式生长 前寒武纪 华北克拉通 

分 类 号：P588.3[天文地球—岩石学]