扬子北缘大洪山造山带中新元古代基性岩和浊积岩的年代学和地球化学研究:对弧后盆地演化的指示

Chronological and geochemical studies of the Middle Neoproterozoic mafic rock and turbidite in the Dahongshan orogenic belt on the northern margin of the Yangtze Block:Implications for the evolution of the back-arc basin

扬子北缘~820Ma可能记录了从挤压环境向伸展环境的重要转换,是解决扬子新元古代演化争议的关键时间节点。本研究在扬子北缘大洪山造山带圆潭地区识别出一套基性岩和浊积岩组合。其中基性岩全岩Sm-Nd等时线年龄为845Ma±81Ma(MSWD=1.5),浊积岩碎屑锆石年龄谱包括四个主要峰值年龄(~835Ma、~1597Ma、~2002Ma、~2482Ma)和两个次要峰值年龄(~2680Ma、~2934Ma),综合前人研究成果将浊积岩的形成时代约束在835~820Ma。浊积岩与花山群的形成时代一致,应代表花山群的北向延伸。浊积岩的物源来自扬子北缘桐柏-大别地区,不同于前人对花山群的分析结果,指示花山群可能具有多个物源。以上地层碎屑锆石年龄谱和地层层序特征与弧后盆地相似。本研究基性岩样品具有低的SiO_(2)(45.41%~50.29%)、K 2O(0.03%~1.49%),高的TiO_(2)(1.42%~3.03%),和低的U/Th(0.15~0.29)、Th/Zr(0.003~0.011)和Nb/Y(0.059~0.147)以及正的εNd(t)(+1.18~+1.57),类似N-MORB地球化学特征。基性岩来自于软流圈,经历<3%部分熔融而生成,并在上升过程中受到了一定程度的地壳混染。综合以上分析,本文认为这些基性岩和浊积岩共同构成一套弧后背景环境岩石组合。

The northern margin of the Yangtze Block may have witnessed the tectonic shift from a compressional setting to an extensional setting at~820Ma,which is the key to reveal Neoproterozoic tectonic evolution process of the Yangtze Block.This study identified a set of mafic rock and turbidite assemblage in Yuantan area of Dahongshan orogenic belt in the northern margin of the Yangtze Block.The whole-rock Sm-Nd isochronal age of mafic rock is 845±81Ma(MSWD=1.5),and the turbidite detrital zircon age spectrum includes four major peak ages(~835Ma,~1597Ma,~2002Ma,~2482Ma)and two minor peak ages(~2680Ma,~2934Ma).Together with previous research results,the formation age of turbidite is constrained at 835~820Ma.This is consistent with the formation age of the Huashan Group,and the turbidite should represent the northward extension of the Huashan Group.Zircons of turbidite are mainly sourced from the Tongbai-Dabie area in the northern margin of the Yangtze Block,different from those of the Huashan Group in previous study,which indicates that the Huashan Group may have multiple provenances.Detrital zircon age spectrum and stratigraphic sequence characteristics are similar with those in the back-arc basin.Mafic rocks in this study have low SiO_(2)(45.41%~50.29%),K 2O(0.03%~1.49%),high TiO_(2)(1.42%~3.03%),and low U/Th(0.15~0.29),Th/Zr(0.003~0.011),Nb/Y(0.059~0.147)and positiveεNd(t)(+1.18~+1.57)are similar with N-MORB.These mafic rocks should be derived from the asthenosphere,experienced<3%partial melting and suffered low degree assimilation during the ascent.In conclusion,mafic rock and associated turbidite were suggested to represent a rock assemblage of back-arc setting.