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 volcanic rock (spilite) has δ^(18)O (+11.0％) higher than that of the mantle. The δ^(18)O depletion is a result of high-temperature seawater hydrothermal alteration, while the δ^(18)O enrichment is due to tow-temperature hydrothermal exchange. In accordence with altered mineral assemblage, seawater hydrothermal alteration has taken place at the temperature of 400℃ in the plutonic rocks and led to the ^(18)O depletion. In contrast to the plutonic rocks, ^(18)O enrichments in volcanic rocks is a results of seawater hydrothermal alteration at the temperature from 100℃ to 200℃. Basing on the isotopic characteristics, geological data and geochemical information, we suggest that FCO was formed in backarc basin environm ent in Jiangnan Paleo-island Arc on the southeastern margin of Yangtze Plate. In its early stage, the basic magmas originated from the mantle was contaminated to various degrees by unmatured sialic basement under the small oceanic basin, which brought about a large dispersion of Nd isotope. In the late stage, seawater hydrothermal alteration took place in FCO which caused a distinct change of Sr and O isotopic composition.
出处
《地质科学》
CAS
CSCD
北大核心
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
关键词
蛇绿岩
岩套
同位素
钕
锶
氧
ophiolite suite, Nd-Sr-O isotopes, Fuchuan
