扬子板块北缘早—中志留世硅质岩成因及古沉积环境的地球化学研究  

Origin and Paleodepositional Environment of the Early-Middle Silurian Bedded Chert on the Northern Margin of the Yangtze Block:Evidence from Geochemical Study

作　　者：张岩[1] 漆富成[2] 陈文[1] 叶会寿[3] ZHANG Yan QI Fucheng CHEN Wen YE Huishou(Institute of Geology, Chinese Academy of Geological Sciences, 100037, Beijing , China Beijing Research Institute of Uranium Geology, 100029, Beijing , China Institute of Mineral Resources, Chinese Academy of Geological Sciences, 100037, Beijing, China)

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]核工业北京地质研究院,北京100029 [3]中国地质科学院矿产资源研究所,北京100037

出　　处：《地质学报》2017年第10期2322-2350,共29页Acta Geologica Sinica

基　　金：中国黑色岩系非常规铀资源矿集区成矿条件分析及资源预测评价(编号201493);湘中地区黑色岩系非常规铀资源调查及远景评价(编号201768);中国地质科学院基本科研业务费项目(编号YYWF201707)共同资助的成果

摘　　要：本文对四川省若尔盖县北部-甘肃省迭部县中-下志留统[兰多维列统(Llandovery)-温洛克统(Wenlock)]富有机碳硅质岩进行了无机化学研究,目的是探讨其成因及形成环境,并建立研究区硅质岩形成及相关元素富集模式。主要对1个钻孔、2个剖面共28件硅质岩的岩相学、扫描电镜、主量、微量、稀土元素、有机碳及Si、O稳定同位素特征进行了研究。(1)岩相学、扫描电镜分析,Fe/Ti、Al/(Al+Fe+Mn)(0.08~0.79)均值0.31、(Fe+Mn)/Ti,(La/Yb)N(0.12~1.41)均值0.54、δEu(0.81~3.36)均值1.55、∑REE(7.59~253.48)均值82.90,Si同位素(-0.2‰~2.1‰)均值0.66‰、O同位素(13.5‰~21.8‰)均值19.53‰、同位素对成岩温度的反演,显示硅质岩为热水沉积成因,并伴有生物作用(Si、O稳定同位素显示),益哇硅质岩形成时陆源物质供给适量,导致与碳酸盐相关生物繁盛,少量样品受陆源物质影响显著。(2)岩相学、扫描电镜分析,MnO_2/TiO_2、Al2O3/(Al2O3+Fe2O3)、Al/(Al+Fe)(0.08~0.80)均值0.33、Th/U、V(28.6×10^(-6)~2261×10^(-6))均值454.42×10^(-6)、Ti/V,δCe(0.65~1.58)均值1.01、(La/Ce)N(0.60~1.314)均值0.97、有机碳(TOC)-微量元素二元图解,显示研究区硅质岩形成于大陆边缘的热水喷流导致海底局部水动力较强的缺氧环境[特别是δCe、(La/Ce)N、稀土配分模式],部分硅质岩由于富集热液元素导致其具有洋脊、大洋盆地硅质岩的特征。结合前人研究成果,认为研究区硅质岩形成于扬子板块北缘(古商丹洋南缘)热水喷流系统发育(导致海底水动能较强的)、缺氧状态的陆缘拉张裂陷环境,这种环境有利于U、V、Mo、Cu、Ni等变价元素的富集,扬子板块北缘早古生代(寒武纪-志留纪)黑色页岩系产自扩张的大陆边缘缺氧-还原环境。In this study, we performed an inorganic geochemical study of Early-Middle Silurian bedded organic C-enriched cherts in the region between northern Ruoergai （Zoige） County and Diebu （Tewo） County, central China. The goal of this study is to determine the origin and paleodepositional environment of the chert beds and to establish a depositional model for the Yangchanggou Formation-Lalong Formation in the study area. An integrated study, involving petrographic analysis, scanning electron microscopy （SEM） analysis, major, trace and rare earth element analyses, organic carbon content analysis, and Si, and O isotopic analyses, has been performed on twenty-eight chert samples from the Yangchanggou Formation, Taer Formation and Lalong Formation of the Bailongjiang Group. Based on the petrographic, SEM, Fe/ Ti, Al/（Al＋Fe＋Mn） （0. 08-0. 79; average 0.31）, （Fe＋Mn）/Ti （ 4. 20 - 205. 23 ; average 61.94）, （La/Yb）N（0.12-1. 41; average 0.54）, δEu （0.81-3.36; average 1.55）, ∑REE （7.59-253.48; average 82.90）, Si isotope （--0.2‰-2.1‰; average 0.66‰）, and O isotope （13.5‰-21.8‰） average 19.53‰） results and the diagenetic temperatures inverted from the O and Si isotopic data, the chert samples are primarily hydrothermal in origin, with some evidence of biogenic chert （based on the Si and O isotopic compositions）. During the deposition of the Taer Formation, terrigenous material mixed with the paleo-seawater, promoting the growth of carbonate-producing organisms. Minor samples exhibit effect of terrigenous detrital material. Additionally, the petrographic, SEM, MnO2/TiO2, Al2O3/（Al2O3＋Fe2O3） （0.11-0.84; average 0.37）, Al/（Al＋Fe） （0.08-0.80; average 0.33）, Th/U, V （28.6× 10-6- 2261× 10 -6; average 454.42 ×10 -6）, Ti/V, δCe （0. 65-1. 58; average 1.01）, and （La/Ce）N（0.60-1.314; average 0. 97） results and the REE, TOC, and trace element （U, V, Mo, Cu, Ni） patterns indicate that the chert samples were deposited on a co

关 键 词：硅质岩 成因 古构造环境 早-中志留世 扬子板块北缘 

分 类 号：P588.244[天文地球—岩石学] P59[天文地球—地质学]