华南湘赣永州-萍乡早古生代放射虫硅质岩特征及沉积环境研究

Two suites of radiolarian siliceous rocks have been found in the Early Paleozoic strata in Yongzhou-Pingxiang area of central Hunan and Jiangxi provinces in 2012, namely Lower Cambrian siliceous shales, siliceous rocks and Middle Ordovician cherts. With siliceous shell and unique living environment, radiolaria has great importance to the study on water depth, basin palaeolatitude and type, and basin tectonic background in the depositional environment. Little study on this suite of siliceous rocks has been carried out so far, restricting profound understanding on the depositional environment of South China. Therefore, the discovery of radiolaria in siliceous rocks has substantially improved the significance of the study on these suites of siliceous rocks. As the pivotal belt between Yangtze and Cathaysia block, Yongzhou -Pingxiang is a key area for the study on features of radiolarian siliceous rocks and sedimentary environment. The project is intended to reveal the composition and distribution features of radiolaria through sampling and sedimentologic survey of Lower Cambrian and Middle Ordovician siliceous rocks bearing radiolaria in the area. Moreover, sedimentary geochemical study has a supplementary role. The features of petrology, sedimentary facies, sedimentary formation and geochemical features of this kind of rocks will be learned, which together will provide scientific information and clues to the Early Paleozoic sedimentary environment in this area.

2012年申请人在湘赣中部永州—萍乡一带早古生代发现2套含放射虫的硅质岩层，分别为：下寒武统硅质页岩、硅质岩，中奥陶统硅质岩。放射虫的硅质壳和独特生存环境，使其对于盆地古水深、古纬度、盆地类型及大地构造背景等方面的研究具有重要意义。由于这2套硅质岩的研究较为薄弱，限制了对华南早古生代沉积环境的深入了解，因而放射虫的发现更增加了这2套硅质岩的研究价值。湘赣中部永州—萍乡一带是扬子陆块向华夏陆块过渡的纽带，是详细研究华南含放射虫硅质岩特征和沉积环境的关键区带。本项目拟通过对永州—萍乡地区下寒武统及中奥陶统含放射虫的硅质岩进行详细采集和沉积学调查，揭示放射虫的组成、分布基本特征，辅以沉积地球化学工作，掌握该类岩石的岩石学特征、沉积相、沉积建造特征及地球化学特征，为永州—萍乡一带早古生代沉积环境研究提供新的科学信息和线索。

项目以前期发现疑似放射虫的湘赣永州—萍乡地区中—上奥陶统烟溪组、对耳石组及下寒武统牛蹄塘组、牛角河组为研究对象，对放射虫的发育、硅质岩的特征、成因及沉积环境等进行了研究。.永州—萍乡地区下寒武统地层以黑色、灰黑色、深灰色碳硅质板岩为主，夹深灰色、青灰色中—厚层不等粒变质长石石英杂砂岩、石英杂砂岩，底部见硅质岩、石煤，多见黄铁矿，水平层理发育，在娄底、邵阳、衡阳、萍乡地区发现疑似放射虫的化石，放射虫密度较低，目估最大密度达35%。该层中含疑似放射虫的硅质岩Al/（Al+Fe+Mn）、K2O/Na2O、微量元素与Al2O3离散关系、U/Th、LaN/CeN、Ce/Ce*等证明生物化学成因。研究区早寒武世硅质板岩、硅质岩SiO2含量平均80.08%、TiO2平均0.58，Al2O3/（Al2O3+Fe2O3）为0.67~0.99、Ce/Ce*0.54~1.02。结合地质特征指示研究区下寒武统处于低能、平静、半深水的沉积环境，东南地区更靠近陆源区，总体为被动大陆边缘的环境。.研究区中奥陶世—晚奥陶世早期以黑色、灰黑色、深灰色薄层硅质岩为主，夹硅质板岩，碳质含量丰富，发育水平层理，首次在多个剖面的硅质岩中发现放射虫化石，在湖南中部娄底新化和衡阳祁东剖面中泡制出放射虫Protoentactinia sp.，部分层段形成放射虫岩，且在湖南江西中部地区密度较大。硅质岩Al/（Al+Fe+Mn）＞0.45，K2O/Na2O为1.55~42.11，微量元素Ni、V、B、Cr与Al2O3关系部分线性、部分离散，U/Th平均约0.72。研究区中部稀土元素含量偏高，东南部Ce元素略呈负异常，研究区ƩLREE≈ƩHREE，表明衡阳到赣州一带受一定火山、热水活动的影响。结合地质特征可见，生物Si和陆源Si为本区硅质岩Si的重要来源，生物化学成因特征明显。硅质岩δ30Si含量-0.6到1.18，SiO2含量平均89.08%，TiO2平均0.27，Al2O3/（Al2O3+Fe2O3）为0.65~0.97，MnO/TiO2比值为0.01~0.16，LaN/CeN为0.99~1.27，Ce/Ce*为0.85~1.10，（La/Yb）N为0.91~1.66。综合地质特征分析，与下寒武统相比，本区沉积环境变化不大，仍为大陆边缘的半深水环境，水体加深，少数数据指示为深水平原环境，东南方向仍更靠近陆源供给区。