湘东北冷家溪群沉积岩地球化学特征及其构造意义  

Geochemistry and tectonic implication of the sedimentary rocks in Lengjiaxi Group in northeastern Hunan

作　　者：柏道远 蒋启生 李彬 姜文 李银敏 Bai Daoyuan;Jiang Qisheng;Li Bin;Jiang Wen;Li Yinmin(Hunan Institute of Geology Survey,Changsha 410016,China)

机构地区：[1]湖南省地质调查院,长沙410016

出　　处：《地质科技通报》2021年第1期1-13,26,共14页Bulletin of Geological Science and Technology

基　　金：中国地质调查局项目(DD20160031-04);湖南省地质矿产勘探开发局科研项目(2019-17)。

摘　　要：冷家溪群是江南造山带湖南段的最早物质纪录,其沉积构造背景及相关的钦杭结合带位置尚存争议。在野外地质调查基础上,对湘东北金井地区冷家溪群早期-中期相对新鲜砂岩采样进行系统的主量元素和微量元素含量分析,进而按时代先后对砂岩分组并进行地球化学研究,以此探讨沉积期盆地性质及大地构造格局。金井地区冷家溪群砂岩的主量元素组成变化较大,SiO2质量分数总体较低、Al2O3质量分数和Al2O3/SiO2比值较高、K2O/Na2O比值高且变化大。轻稀土富集、重稀土平坦、铕负异常明显等特征暨球粒陨石标准化曲线形态与典型的后太古宙页岩和上陆壳相似。主量元素地球化学特征反映沉积环境为弧后盆地,且早期易家桥组和潘家冲组的成熟度较高,主要来源于北邻构造相对稳定的扬子陆块南缘;中期雷神庙组-黄浒洞组的成熟度较低,可能更多来源于南邻构造相对活动的大陆岛弧。各组地层构造环境微量元素判别图解均显示为大陆岛弧环境,但从微量元素特征对母岩的继承性分析,仍反映出弧后盆地环境;有关微量元素参数的相对大小指示早期沉积环境为活动陆缘、中期沉积环境为大陆岛弧,与主量元素特征反映的信息一致。根据上述地球化学证据,提出冷家溪期构造格局与演化过程:早期受古华南洋板块向北西高角度俯冲影响,弧后软流圈上涌导致岩石圈伸展而形成宽阔的弧后盆地,金井地区处于盆地北部而主要接受北邻扬子陆块来源沉积;中期古华南洋板块俯冲角度变缓并推动大陆岛弧向北西运移,弧后盆地收缩,金井地区因构造迁移而主要接受南邻大陆岛弧来源沉积。结合区域资料,认为弧后盆地南邻大陆岛弧大体在安仁-双牌一线。Lengjiaxi Group is the earliest material record of Jiangnan orogenic belt in Hunan.There exist debates about the sedimentary tectonic setting of Lengjiaxi Group and the related location of Qinzhou-Hangzhou juncture.Based on field investigation,the authors collected fresh samples from the sandstones in lower-middle Lengjiaxi Group from Jinjing,northeastern Hunan,analysed their major and trace element compositions,studied the geochemistry of the sandstones in groups by age,and then probed into the basin type and tectonic framework.The sandstones on the whole are characterized by considerable variations of petrochemical components,lower SiO2 contents,high Al2O3 contents,high Al2O3/SiO2 and K2O/Na2O ratios.The REE distribution patterns are similar to typical post-Archean shales and the upper continental crust,with LREE enrichment,flat HREE,and significant negative Eu-anomalies.Characteristics of major elements suggest that the sedimentary environment was back-arc basin,and that the Yijiaqiao Formation and Panjiachong Formation in early stage possess higher maturity with sediment source from stable Yangtze Block to the north,while the Leishenmiao Formation and Huanghudong Formation in middle stage possess lower maturity with more sediment source from active continental arc to the south.Trace element-diagrams for discrimination of structural environment of all the formations show informations of continental arc,which actually reflects back-arc basin setting according to trace element inheritance from source rocks.Relative numeric size of trace elements parameters suggested that the sedimentary environment in the early stage was active continental margin,and in middle stage was continental arc,which is accordant with the information of major elements.According to the geochemistry evidences,the authors put forward the tectonic framework and evolution in Lengjiaxi Period as follows:in the early stage,upwelling of back-arc asthenosphere mantle caused lithospheric to extend for northwest-ward high-angle subduction of Huanan

关 键 词：冷家溪群 地球化学特征 弧后盆地 构造迁移 湘东北 

分 类 号：P542[天文地球—构造地质学] P549[天文地球—地质学]