| 安徽伏川蛇绿岩套的Nd-Sr-O同位素研究 |
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| 引用本文： | 沈渭洲,邹海波,楚雪君,周新民,杨杰东,王银喜.安徽伏川蛇绿岩套的Nd-Sr-O同位素研究[J].地质科学,1992,0(4):333-341. |
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| 作者姓名： | 沈渭洲 邹海波 楚雪君 周新民 杨杰东 王银喜 |
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| 作者单位： | 1. 南京大学地球科学系;2. 南京大学现代分析中心 |
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| 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:εNd(T)=+0.7-+3.8,εSr(T)=+30.7-+53.9,δ18O=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。εNd(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;εSr(T)和δ18O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。
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| 关 键 词： | 蛇绿岩套 Nd-Sr-O同位素 伏川 |
| 收稿时间： | 1990-07-01 |
| 修稿时间： | 1990-07-01; |
Nd, Sr, AND O ISOTOPIC STUDY ON FUCHUAN OPHIOLITE SUITE IN ANHUI PROVINCE |
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| Shen Weizhou Zou Haibo Chu Xuejun Zhou Xinmin.Nd, Sr, AND O ISOTOPIC STUDY ON FUCHUAN OPHIOLITE SUITE IN ANHUI PROVINCE[J].Chinese Journal of Geology,1992,0(4):333-341. |
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| Authors: | Shen Weizhou Zou Haibo Chu Xuejun Zhou Xinmin |
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| Institution: | 1. Department of Earth Sciences, Nanking University;2. Modern Analysis Center, Nanjing University |
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| Abstract: | 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 εNd(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(143Nd/1444Nd)-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(87Sr/86Sr)1 value of 0.705468-0.707096(or εSr(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(87Sr/86Sr)i value(0.70395±)similar to that of the depleted mantle(0.702)and have εNd(T)value(≤-0.4)significantly different from that of the depleted mantle(+7.0). Although crustal contamination may alter εNd(T)value in FCO, it can not increase(87Sr/86Sr)value to 0.707096;(2)because the seawater has low Nd(3×10-6ppm), high Sr(8ppm)and 87Sr/86Sr 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 εNd(T)-εSr(T)diagram. FCO has,δ18O=3.2-11.0%, among which the plutonic rocks have δ18O(3.2-5.4%)lower than the mantle value(5.7%), whereas the volcanic rock(spilite)has δ18O(+11.0%)higher than that of the mantle. The δ18O depletion is a result of high-temperature seawater hydrothermal alteration, while the δ18O 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 18O depletion. In contrast to the plutonic rocks, 18O 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. |
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| Keywords: | ophiolite suite Nd-Sr-O isotopes Fuchuan |
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