江西南部震旦—寒武纪寻乌岩组变沉积岩地球化学特征及构造意义

寻乌岩组变沉积岩发育在江西南部新元古代—早古生代南城—寻乌盆地内,时代为震旦—寒武纪。寻乌岩组的Si O2含量变化较大,除1个含量较低的样品外,平均为63.01%,具有相对较高的K2O、Al2O3及(TFe O+Mg O)值,较低的Na2O、Ca O。相容元素含量与后太古代澳大利亚页岩(PAAS)十分接近,高于中国东部上地壳,与其较高的(TFe O+Mg O)特征相吻合,说明源区具有一定数量的中基性组分。稀土元素总量高于上地壳及PAAS,但其球粒陨石标准化配分模式与典型的上地壳及PAAS相似,表现为明显向右倾斜,轻稀土富集,重稀土平坦,铕负异常显著,铈负异常不明显。岩石地球化学特征显示其原岩属于一种中等成熟度的陆源碎屑岩,物源主要为被改造的上地壳长英质和花岗质物源区,少部分物源为中基性火山岩。高场强元素及稀土元素等不活动元素地球化学特征表明其沉积环境为浅海—半深海,其构造环境属于发育有裂谷系的被动大陆边缘,该认识从沉积岩地球化学方面为华南早古生代构造属性提供了新资料。

The geochemical characteristics and tectonic significance of metasedimentary rocks in Sinian-Cambrian Xunwu rock group, southern Jiangxi Province

Metasedimentary rocks in Sinian-Cambrian Xunwu rock group of southern Jiangxi Province show obvious variation of SiO₂ values. Except for one sample with a relatively lower concentration, the average SiO₂ content is 63.01%. These rocks show comparatively high K₂O, Al₂O₃ and (TFeO+MgO) values and low Na₂O and CaO values. Compared with the concentrations of compatible trace elements, they are very close to values of Archaean Australian Shales (PASS), but their values are dramatically higher than values of the upper crust, and are consistent with the characteristics of high (TFeO+MgO) values, indicating that a certain amount of intermediate-basic components existed in the source region. Total REE concentrations are higher than those of the upper crust and PAAS. Nevertheless, , the chondrite-normalized REE patterns exhibit a slanting rightward shape with enriched LREE, flat HREE, obvious negative Eu anomalies and less obvious negative Ce anomalies, similar to features of the typical upper crust and PAAS. Based on the geochemical characteristics, the protolith should be terrigenous clastic rocks with intermediate maturity, and the provenance was mainly dominated by reconstructed felsic and granitic sediments from the upper crust, with partly contributed by certain amounts of intermediate-basic components. Immobile elements like high field-strength elements and rare earth elements reveal that the sedimentary environment was bathyal to abyssal and the tectonic background was the passive continental margin with rift systems. The results obtained by the authors shed some light on the tectonic attributes of southern China in Early Paleozoic from the perspective of sedimentary geochemistry.