大别山南缘~2.5Ga钾质花岗岩的发现及其意义

Discovery of~2.5Ga high-K granites in southern margin of the Dabie Orogen and its implications

太古代-古元古代地壳在扬子克拉通的出露极为有限,且成分主要为TTG质片麻岩,本文报告了大别山南缘地区新发现的2.5~2.0Ga钾质花岗岩。为明确该陆壳岩石的性质,揭示扬子克拉通的早期演化历史,对花岗岩及同期基性岩进行了锆石U-Pb年代学、元素-同位素地球化学分析及对比研究。两期钾质花岗岩的形成年龄分别为2443~2463Ma、1995~2048Ma,其中前者的Sr含量和Sr/Y值中等,具有-0.1的εNd(t)值和略早于成岩年龄的2.87Ga模式年龄;后者的Sr含量和Sr/Y值相对前者降低,K、Rb、U、∑REE含量和LREE/HREE值升高,具有明显的Nb、Ta亏损,以及-6.8~-9.4的εNd(t)值和远大于成岩年龄的3.03~3.24Ga模式年龄。研究表明,前者代表的地壳成因主要受地幔垂向添加的模式主导,而后者可能已转变为横向增生的板块构造模式,反映了扬子克拉通的主要生长期在太古代,而元古代以后代之以俯冲碰撞导致的壳内重熔和再造。对比显示,大别山南缘所在区域于新太古代-古元古代已经出现相对成熟的大陆地壳,钾质花岗岩的出现表明此时的扬子陆块已经完成克拉通化。大别山南缘与核部具有相似的成岩和变质作用历史,暗示它们整体属于扬子北缘或相关微陆块的组成部分。

The Neoarchean-Paleoproterozoic crust fragments in the Yangtze Craton are very limited,and they are mainly TTG gneiss.This paper reports the discovery of 2.5~2.0Ga high-K granites in the south margin of the Dabie Orogen.In order to clarify the nature of these rocks and reveal the evolution process of the Yangtze Craton,zircon U-Pb chronology,trace elements and Nd isotope geochemistry analysis were conducted on these granites and basic rocks.The results show that there are two suits of high-K granite in the Dabie Orogen with 2443~2463Ma and 1995~2048Ma ages.The Sr content and Sr/Y value of the~2.5Ga high-K granite are medium,withεNd(t)value of-0.1 and model age of 2.87Ga which is slightly earlier than the diagenesis age.The Sr contents and Sr/Y ratios of the~2.0Ga granites decrease,while the K,Rb,U,ΣREE and LREE/HREE values increase,with an obvious Nb-Ta depletion as well as negativeεNd(t)values of-6.8~-9.4 and 3.03~3.24Ga model ages.The formation of the~2.5Ga crust is mainly the vertical model,while the~2.0Ga crust have changed to the plate tectonic model,reflecting that the main growth period of the Yangtze Craton was in Archean,while the Proterozoic was dominated by subduction and reconstruction in the crust.The high-K granites indicate that there was a mature continent in the southern margin of the Dabie Orogen,while a completed cratonization process in the Neoarchean-Paleoproterozoic.Moreover,the southern margin and the core of the Dabie Orogen had similar diagenetic and metamorphic history,suggesting that they both belong to the northern margin of the Yangtze Craton or related micro-terranes.