皖赣相邻鄣公山地区变泥砂质岩石元素组成特征及源区分析

Element characteristics and prominence analysis of meta-argillo-arenaceous rocks in Zhanggongshan area,Anhui-Jiangxi border region

皖赣相邻的鄣公山地区位于江南造山带东段北缘,区内广布一套厚度巨大、低绿片岩相变质的以泥砂质细碎屑岩为主含少量火山物质的复理石建造体,大量高精度同位素测年数据显示,其形成于822～850 Ma的新元古代。其以斜切本区的景德镇—江潭构造带为界,南部为溪口岩群,北部属双桥山群,二者REE、高场强元素(HFSE),如Nb、Ta、Zr、Hf及Nd同位素等具有相似的变化特征,显示区内浅变质碎屑岩具相同或相近的"原始"地壳物质组成特征;(La/Yb)N=4.66～13.63,δEu=0.09～0.3,(Gd/Yb)N=1.22～1.66,表明区内浅变质碎屑岩物源主要来自长英质岩石组成的后太古宙上地壳。碎屑锆石U-Pb年龄主要集中在826～2600 Ma,并出现显著的5个峰值,大致为2500 Ma,2000～2200 Ma,1800 Ma,960 Ma,827～841 Ma,其中以年轻碎屑锆石占绝对多数,大量的年轻碎屑锆石U-Pb年龄与碎屑岩密切共生的火山岩夹层年龄相近,反映浅变质碎屑岩物质主要来源于同时代火成岩类。溪口岩群和双桥山群浅变质碎屑岩大离子亲石元素(LILE)Rb、Cs、Sr、Ba、Th等含量及其相关比值、Sr同位素特征等存在一定差异,可能意味着溪口岩群和双桥山群碎屑岩在成岩过程中经历了不同的地质构造演化。区内浅变质火山-陆缘细碎屑岩形成于与板块构造体制相关构造-沉积环境,溪口岩群显示大陆边缘拉张构造背景下的沉积作用特征,而双桥山群原岩沉积构造环境与大陆岛弧关系较密切,显示板块俯冲阶段的沉积特征。

Located in the northern margin of the Jiangnan orogen（south of the Yangtze River）,Zhanggongshan area of Anhui-Jiangxi border region contains very thick low greenschist-facies metamorphic flysch rocks dominated by argillo-calcareous fine-grained elastic rocks with minor volcanic materials.High precise isotopic dating shows that it was probably formed in Neoproterozoic between 821-840 Ma.It was bordered by obliquely crossed Jindezhen-Yintan structural belt with Xikou Group located in the southern part and Shuangqiaoshan Group in the northern part.REE and HFSE in these two groups,such as Nb,Ta,Zr,Hf and Nd,show similar characteristics,indicating that epimetamorphic rocks within the area have the same or similar original crust compositions,while the data（La/Yb）N=4.66～13.63,δEu=0.09～0.3 and（Gd/Yb）N=1.22～1.66 suggest that these low-grade metamorphic rocks mainly came from Late Archean upper crust composed of felsic rocks.Detrital zircon U-Pb ages are largely 826-2600Ma,showing evidently 5 peaks,i.e.,2500Ma,2000-2200Ma,1800Ma,958Ma and 827-841Ma,dominated by young detrital zircons whose ages are very close to ages of volcanic rocks interbeded with clastic rocks,indicating that epimetamorphic rocks mainly resulted from contemporary igneous rocks.Differences in content and ratios of large-ion lithophile elements such as（LILE）,Rb,Cs,Sr,Ba and Th,together with variations of Sr isotopic characteristics,suggest that Xikou Group and Shuangqiaoshan Group detrital rocks experienced different tectonic evolutions.Epimetamorphic volcanic-margin fine-grained clastic rocks were formed in a plate tectonic system related tectonic-sedimentary environment in which Xikou Group shows depositional characteristics of extensional continental margin while original rocks of Shuangqiaoshan Group display a continental island arc closed tectonic depositional environment,suggesting plate subduction depositional characteristics.