湘东南汝城盆地基性岩元素地球化学及其大地构造环境

汝城盆地基性岩由辉绿岩、玄武岩和玄武质火山碎屑岩组成 ,属于低钾拉斑玄武岩系。玄武岩全岩K Ar年龄为 12 8.4± 4 .2Ma ,辉绿岩全岩K Ar年龄为 112 .1± 3.2Ma。元素地球化学分析表明 ,火山岩系具有相同的岩浆源区 ,其形成以部分熔融方式为主。岩石微量元素出现大离子亲石元素 (LILE)的富集和Ta、Nb、Ti的亏损。强不相容元素比值反映岩浆源区明显偏离原始地幔组分 ,具有富集型地幔岩浆源区特征。岩浆源区受到地壳物质混染和来自消减残留板片析出流体或熔体交代的改造作用。汝城盆地基性岩形成于陆内拉张带 (初始裂谷 )构造环境 ,其强烈拉张时期与华南南岭地区的主要拉张时期 (12 0Ma)相对应。

ELEMENT GEOCHEMISTRY AND TECTONIC SETTING OF THE BASIC ROCKS IN RUCHENG BASIN, SOUTHEASTERN HUNAN PROVINCE, CHINA

Basic rocks in the Rucheng Basin are composed of diabases, basalts and basaltic pyroclastic rocks and belong to low potassic tholeiite series. The whole-rock K-Ar age of the diabase is 112.1±3.2Ma and the whole-rock K-Ar age of the basalt is 128.4±4.2Ma. Based on the element geochemistry data, the basic rock series has the same magma source and the rocks mainly resulted from partial melting. The trace elements show large-ion lithophile elements (Rb, Ba, Th, U) enrichment and high field strength elements (Ta、Nb、Ti) depletion. The ratios of incompatible elements reflected the magma source deviated obviously from the primary mantle with respect to its composition and has the characteristics of the enriched metasomatic mantle. The magma source was reworked mainly by crustal assimilation and metasomatism of fluid or melt from relict oceanic plate. The basic rocks in the Rucheng Basin were formed in the tectonic setting of continental extension belt (or initial rift) and its intense extension epoch is corresponding to the main extension epoch (120Ma) in the Nanling region, South China.