峡东地区闪长岩风化剖面元素和Sr-Nd同位素特征及其地球化学意义

Element and Sr-Nd isotopic geochemistry profiles of weathered dioritic rocks in the eastern Three Gorges and its implication

以峡东地区闪长岩风化剖面为研究对象,探讨了风化过程中主元素、微量元素的活动性和Sr-Nd同位素体系的稳定性。研究表明:(1)Ti是估算元素活动性的理想参考元素,表征风化更准确;(2)由基岩(D)至风化壳顶层(A),CIA和CIW逐渐升高;风化过程中主元素的活动性顺序为:Na、Ca>Si、P>Mg、K>Al>Fe(Fe3+)、Mn、Ti;(3)微量元素中,Zr、Hf、Sc、V和REE相对稳定;亲硫元素Cu、Mo和Zn变化不明显,Pb呈富集趋势;不同剖面大离子亲石元素Sr、Ba和Rb变化不同;(4)剖面1和2风化层中LREE相对亏损,而剖面3风化层中LREE相对富集;由基岩(D)至风化壳顶层(A),δEu和δCe逐渐下降,指示风化过程中相对富集Eu的斜长石优先蚀变并流失,以及氧逸度增高导致部分Ce氧化成+4价,并呈可溶络合物态溶解流失;(5)风化过程导致各风化层Rb-Sr同位素体系的开放,随风化程度增加(87Rb/86Sr)0增高而(147Sm/144Nd)0降低,指示外来高放射成因Sr和低放射成因Nd的加入;相对基岩,不同风化层εSr和εNd的变化范围扩大,且εNd总体呈下降趋势,Nd同位素模式年龄在误差范围内基本相同,指示基岩特征。

As the important period of supergene elements＇ geochemistry circulation, rock weathering has important connection with the survival and the living environment of mankind. It is ideal to study the migration behaviors of elements under the surface condition in the eastern Three Gorge, located in Yichang, Hubei. This paper reports element and Sr-Nd isotopic geochemistry profiles of weathered Huangling dioritic rock mass in eastern Three Gorges. The samples were collected from three profiles, and their element mobility and Sr-Nd isotopic system stability are discussed. It demonstrates that （1） Ti is the ideal reference element for calculating element activity and indicating weathering characters; （2） CIA and CIW value increases from bedrock （D） to the top weathered layer （A）; the order of major elements activity is Na, Ca 〉 Si, P 〉 Mg, K 〉 A1 〉 Fe, Mn, Ti; （3） For trace elements, Zr, Hf, Sc, V, Y and REE are relatively stable, whereas chalcophile elements Cu, Mo and Zn are varied indistinctively and Pb is enriched. Variations of Sr, Ba and Rb are different in different profiles; （4） LREE of profile 1 and 2 is relatively enriched, whereas LREE of profile 3 is relatively depleted; the rising trends of δEu and δCe from bedrock to top weathered layers indicate preferential alteration and loss of Eu-enriched plagioclase, high oxygen fugacity resulting in transition of Ce^3＋ to Ce^4＋ and alteration and loss of soluble complex duringweathering; （5） the rising trend of initial Sr isotope ratios with increasing weathering and the involvement of extra high radiogenic Sr indicates that the Rb-Sr isotope system is an open system during weathering of dioritic rocks.The decrease of initial Nd isotope ratios with increasing weathering indicates the involvement of extra low radiogenic Nd; compared with their bedrocks, the weathering layers show wider ranges of εsr and εnd values with overall decreasing εnd. However, relatively the constant Nd isotopic model ages within error likely reflect the characteritics of bedrocks.