新疆阿尔金南部迪木那里克铁矿赋矿地层的形成时代及其地质意义

摘要

新疆迪木那里克铁矿赋矿地层是南阿尔金蛇绿构造混杂岩带地层的重要组成部分.该铁矿矿石类型主体可分为块状磁铁矿和条带状磁铁矿,矿石围岩均为绢云钠长千枚岩,矿石矿物组合基本一致,为磁铁矿+赤铁矿+钠长石+绢云母(或绿泥石),石英极少(＜5％),层控明显,为浅海相(火山)沉积型铁矿.LA-ICP-MS法测得其赋矿地层底部火山角砾岩中锆石的U-Pb年龄为622.6Ma±1.4Ma,南阿尔金洋盆的闭合时代为500Ma,确定赋矿地层与铁矿床的形成时代介于500～ 623Ma之间.赋矿地层下部形成于裂谷环境的火山角砾岩等与其上部的浅海相沉积地层可能分别代表南阿尔金初始洋盆与洋盆发育期间洋盆边缘相的沉积产物,表明南阿尔金在伊迪卡拉纪-寒武纪期间有洋盆存在.%Ore-bearing strata of the Dimunalike iron deposit constitute an important stratigraphic part of the south Altun continental—type subduction—collision complex. The emphasis of the study is to ascertain features of the iron ore and rock composition of the ore-bearing strata and to confirm the formation age of the ore-bearing strata. Ores of the Dimunalike iron deposit can be classified into two types: massive magnetite ore and banded magnetite ore. They are both hosted by sericite albite phyl-lite with very similar mineral compositions. The Dimunalike iron deposit is a stratabound marine (volcanic) sedimentary iron deposit with the mineral composition of magnetite, hematite, albite, sericite (or chlorite) and rare quartz (<5%). LA-ICP-MSrnU—Pb dating of zircons from the bottom volcanic breccia yielded an age of 622.6Ma±1.4Ma and closure epoch 500Ma for South Alum ocean basin, thus suggesting that the ore-bearing strata and the iron deposit were formed during 500~623Ma. The agglomerate volcanic breccia, volcanic breccia and volcanic tuff formed in the rifting tectonic setting underneath the ore -bearing strata and the shallow marine sedimentary unit above them probably represent the sedimentary products of the initialization of the oceanic basin and the margin facies during the development of the basin, respectively. The results show that the ocean basin in South Altun existed during Ediacaran—Cambrian period.