碧口地块富铁硅岩REE及Nd-Sr同位素组成及其古环境意义  

REE AND Nd-Sr ISOTOPIC COMPOSITIONS OF THE IRON-RICH SILICEOUS ROCK IN BIKOU TERRANE:IMPLICATION TO ANCIENT SEDIMENTARY ENVIRONMENT

作　　者：宋史刚[1] 丁振举[1] 姚书振[1] 周宗桂[1] 

机构地区：[1]中国地质大学资源学院

出　　处：《矿物岩石》2008年第3期57-63,共7页Mineralogy and Petrology

基　　金：国土资源部“九五”地质科技公关项目(95-02-004);国家攀登计划预选项目(编号95-预-39)

摘　　要：对碧口地块富铁硅岩进行稀土元素及同位素数据测试证实其δEu=1.02~8.79,δCe=0.44~1.26,(La/Yb)N=0.071~1.285,除个别样品外,大部分样品同时发育程度不等的Eu正异常、Ce负异常和LREE相对亏损的特征,其稀土样式总体上与海底热水流体类似,表明富铁硅岩为热水喷流成因。富铁硅岩Y/Ho比值在22~47之间,与Ce/Ce*存在明显的负相关关系,反映二者变化受流体、海水混合过程控制。根据富铁硅岩Ce/Ce*和Y/Ho的拟合方程,预测古海水端元的Ce/Ce*比值为0.55左右。富铁硅岩的Nd同位素初始比值εNd(t)为4.9^-5.1,与Ce/Yb比值呈粗略负相关关系,表明富铁硅岩稀土主要来自古海底热水流体和海水两个端元。87Sr/86Sr比值0.710 053~0.717 020,不仅高于对应同时代地幔Sr比值,也较新元古代海水富放射Sr,表明形成富铁硅岩海水以高放射Sr为特征。结合古海水的Ce负异常和高放射Sr的特征,推断富铁硅岩形成于陆缘到深海盆地之间的过渡位置。The REE distribution patterns of iron-rich siliceous rock samples in Bikou terrane showed commonly with Eu positive anomalies,δEu= 1.02～8.79, varied Ce negative anomalies, δCe=0.44～1.26,and relative depleted LREE and enriched HREE, （La/Yb）N= 0. 071～ 1. 285. These parameters are similar to those of hydrothermal vent fluid on modern seafloor and indicate that the siliceous rocks were hydrothermat exhalative sedimentation generated by the convectional mixing of seawater and hydrothermal fluid on ancient seafloor. Y/Ho ratio in siliceous rock samples ranges from 22 to 47, correlating with Ce/Ce^＊ ratios obviously, reflecting that both ratios were controlled by two end members ofhydrothermal fluid and seawater. Moreover, based on the fitting equation,the ancient seawater end member Ce/Ce^＊ value （0.55） was deduced. The initial Eaxe（t） values of iron-rich siliceous rock compared with contemporaneous mantle are between 4.9 to -5.1,roughly negative correlation with Ce/Ce^＊ ratios, indicating that siliceous rock REE mainly comes from hydrothermal fluid and seawater. Meanwhile,the Sr isotopic composition with ^87Sr/^86Sr ratios from 0. 710 053 to 0. 717 020 were higher than not only contemporaneous mantle Sr ratio but also contemporaneous seawater Sr. These characteristics revealed that the ancient seawater forming the siliceous rock is of high radiogenic Sr. Based on the Ce negative anomaly and Sr isotope composition, the rock-forming setting of iron-rich siliceous rock was considered as the transition from continental margin to ocean basin.

关 键 词：稀土元素 同位素 富铁硅质岩 碧口地块 

分 类 号：P597.2[天文地球—地球化学]