摘要
针对肯尼亚境内东非大裂谷罗毕谷地黑棉土(Black Cotton Soil,BCS)的成分及矿物学特征,综合前人有关该地区BCS构造地质学、沉积岩石学、古地理气候、水文及更新世考古研究结果,分析了BCS地质成因,给出了BCS在半封闭条件下富含碱性水体系火山岩—河湖相沉积的形成机制。分析发现:内罗毕南环线的现代型BCS主要矿物是钙质蒙脱石,含有伊利石化混层矿物与少量的接触变质透长石、石英与沸石等;较古地层黏土岩,现代型BCS的Mg含量大大减少,Al、Ca含量升高;随地质年代更新(0.7~0.9 Ma),BCS有机质含量偏高,Fe_(2)O_(3)含量稍低,硅酸盐板层所带负电荷下降,单元结构由三八面体向二八面体转变。罗毕谷地BCS沉积岩物质源于粗面岩—玄武岩及碎屑岩,以基性岩为主,其沉积岩发育时间始于3.3~0.8 Ma,即晚白垩世(Turonian-Maastrichtian阶),跨基性橄榄绿玄武岩、Limuru及Bishoftu粗面岩至第四纪火山玻璃质碎屑沉积物。发育过程大致遵循火山玻璃→高硅沸石→蒙脱石的路径,沉积水环境为先酸后碱序列,富Si相随半封闭环境而排泄。古地理气候及考古学研究结果显示:东非地域呈湿润→半干旱演变,气候因素促进了BCS沉积发育。
This article focuses on compositional and mineralogical characteristics of Black Cotton Soil(BCS)in the Nairobi Valley of East African Rift Valley in Kenya.Based on previous researches,e.g.,structural geology,sedimentary petrology,paleogeographic climate,hydrology,the Pleistocene archaeology related to BCS geological genesis was analyzed,and formation mechanism of BCS rich in alkaline water system or volcanic rocks river lake sediments under semi-closed conditions was proposed.The main mineral of modern BCS in Nairobi South Ring Road was calcareous montmorillonite,which contains illite mixed layer minerals and a small amount of contact metamorphic plagioclase,quartz and zeolite.Compared to ancient clays,the Mg content of modern BCS has significantly decreased,and contents of Al and Ca were increased.At geological age of 0.7–0.9 Ma,the organic content of BCS was relatively high,and the Fe_(2)O_(3) content was slightly lower,the negative charge carried by silicate plates decreases,and the unit structure changes from trioctahedron to dioctahedron.The sedimentary rocks of BCS in Nairobi Valley were derived from coarse-grained basalt and pyroclastic rock.The development of such sedimentary rocks began from 3.3-0.8 Ma,which was validated in the Late Cretaceous(Turonian Maastrichtian stage),spanning from basic olivine green basalt,Limuru and Bishoftu coarse-grained rocks to the Quaternary volcanic glassy clastic sediments.The development process follows path of volcanic glass→high silica zeolite→montmorillonite,and the sedimentary water environment follows a sequence of acid at the initial stage and then alkali at the final stage.The intermediates rich in Si were excreted with the semi-closed environment.Results of paleogeographic climate and archaeological research show that the East African region has evolved from humid to semi-arid,and climatic factors have also promoted the development of BCS sedimentation.
作者
苗世顶
王兴宇
王雪莲
MIAO Shiding;WANG Xingyu;WANG Xuelian(School of Materials Science and Engineering,Jilin University,Changchun 130022,China;Côte d'Ivoire Office,China Road and Bridge Corporation,Beijing 100011,China;Sichuan Provincial Natural Resources Experimental Testing and Research Center,Chengdu 610081,China)
出处
《矿冶》
2025年第3期363-370,共8页
Mining And Metallurgy
基金
中交股份建设科技项目(2015ZJ214704)
国家自然基金资助项目(52374266)
吉林省科技发展计划项目(202302010032GX)。
关键词
罗毕谷地
黑棉土
矿物学特征
地质成因
Nairobi Valley
black cotton soil
mineralogical characteristics
geological genesis
