安徽伏川蛇绿岩套的Nd-Sr-O同位素研究  

Nd, Sr, AND O ISOTOPIC STUDY ON FUCHUAN OPHIOLITE SUITE IN ANHUI PROVINCE

作　　者：沈渭洲[1] 邹海波[1] 楚雪君 周新民[1] 杨杰东[2] 王银喜[2] 

机构地区：[1]南京大学地球科学系 [2]南京大学现代分析中心

出　　处：《地质科学》1992年第4期333-341,共9页Chinese Journal of Geology(Scientia Geologica Sinica)

基　　金：国家自然科学基金

摘　　要：安徽歙县伏川的蛇绿岩套形成于中-晚元古宙,其Nd、Sr和O同位素组成是:ε_(Nd)(T)=+0.7-+3.8,ε_(Sr)(T)=+30.7-+53.9,δ^(18)O=3.2-11.0‰。据地质学和同位素地球化学特征,该岩套位于杨子板块南缘、江南古岛弧的弧后小洋盆地轴部。ε_(Nd)(T)值的变化是由于蛇绿岩形成过程中受到下伏不成熟硅铝质基底地壳的混染引起的;ε_(Sr)(T)和δ^(18)O的变化,是在蛇绿岩形成时或形成后不久遭受海水热液蚀变的结果。Fuchuan Ophiolite (FCO)with an original sequence in Anhui Province is composed of harzburgite, cumulate pyroxenite, gabbro, diorite, spthte and Kera tophyre. From its Sm-Nd isochron of 1024±30Ma, FCO is one of the oldest ophio lites in China. FCO has the lower ε_(Nd)(T) value (+0.7—+3.8)in larger dispersion than that of the mantle. In consideration of existance of sialic basement in this area in connection with the good linear distribution of plots of three whole rocks(gabbro, spilite and keratophyre) from FCO in diagram (^(143)Nd/^(144)4Nd)-1/Nd, it is suggested that the Nd isotopic composition is attributed to the contamination of pre-existing crust in course of formation of FCO. FCO has the (^(87)Sr/^(86)Sr)_i value of 0.705468—0.707096 (or ε_(Sr)(T) from +30.7 to +53.9), which is significantly higher than that of the depleted mantle in the time of FCO formation. According to isotopic analysis, the enrichment of radioactive Sr in FCO is attributed to the seawater hydrothermal alteration rather than the crustal contamination. The evidence is as follows: (1) the crystalline basement in the studied areas is predominantly volcanic-sedimentary rocks with lower maturity and basic-intermediate volcanic rocks. These rocks have (^(87)Sr/^(86)Sr)_i value (0.70395±) similar to that of the depleted mantle (0.702) and have ε_(Nd)(T) value (≤—0.4)significantly different from that of the depleted mantle(+7.0). Although crustal contamination may alter ε_(Nd)(T) value in FCO, it can not increase (^(87)Sr/^(86)Sr) value to 0.707096; (2) because the seawater has low Nd (3×10^(-6)ppm), high Sr (8ppm) and ^(87)Sr/^(86)Sr ratio (0.708), the hydrothermal interaction of seawater with ophiolite can distinctly change Sr isotopic composition, while Nd isotopic composition remains unaltered, which results in the horizontal array of FCO plots in ε_(Nd)(T)—ε_(Sr)(T) diagram. FCO has ,δ^(18)O=3.2—11.0％, among which the plutonic rocks have δ^(18)O (3.2—5.4％) lower than the mantle value (5.7％), whereas the volca

关 键 词：蛇绿岩 岩套 同位素 钕 锶 氧 

分 类 号：P588.3[天文地球—岩石学]