摘要:
通过系统的高密度采样,选取了470件来自华南走廊带的细粒碎屑沉积岩,涵盖了中元古到白垩系地层,对其主要元素、微量元素、稀土元素进行了精确分析。研究结果显示,所有样品都存在明显的稀土元素的分异和Eu的负异常,其中(La/Yb) N=10.0,Eu/Eu*=0.67。相对上地壳平均组成,样品明显亏损Ca、Na和Sr,并具有较高的化学风化指数,暗示样品受到了较为强烈的风化作用影响,并且经历了长距离的搬运过程;主要元素和微量元素图解暗示样品的源岩可能包括花岗质、安山质和长英质岩石。样品的地球化学特征显示,样品可以代表较大的源区,并且W、Sn的含量可以反映走廊带上W、Sn分布特征。在走廊带上,W、Sn的含量自东南向西北递减,在江山-绍兴断裂带附近突变,该分布特征与华南地区的钨锡成矿作用非常吻合。碎屑沉积岩中W、Sn的含量明显高于上地壳平均含量,其中W、Sn含量的峰值存在于中元古代-奥陶系地层,而华夏地块从中元古代开始便具有高钨锡含量,可能是该地区形成全世界最大的钨锡成矿省的重要原因。
关键词:
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地球化学
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稀土元素
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碎屑沉积岩
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华南走廊带
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钨
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锡
Abstract:
South China is comprised of Yangtze and Cathaysia blocks bounded by the NEE-SWW extending Jiangshan-Shaoxing fault. In order to have a better understanding of the abundance and spatial-temporal distribution of each element in South China, the authors have conducted a geochemical investigation along a 2500 km transect in South China since 2008. 470 samples of fine-grained clastic sedimentary rocks from Middle Proterozoic to Cretaceous in age were collected from the Yangtze block to Cathaysia block at a spacing of 1 site/1~4 km. Analyses of 76 elements have provided us with an insight into the composition and distribution of major and minor ele-ments in sedimentary rocks from Middle Proterozoic to Cretaceous. All the samples are characterized by significant fractionation of LREE and HREE and negative Eu anomalies (Eu/Eu*=0.67 on average). The significant depletion of Ca, Na and Sr and the high CIW values suggest intermediate to intense chemical weathering of the source rocks. Discrimination diagrams involving La, Th, Sc, Co, and Hf indicate a multiple lithological source composed of granitic, andesitic and felsic volcanic rocks. The geochemical features show that the samples were derived from distant and large provenances, and the concentrations of tungsten and tin of the samples can provide robust estimates of the composition of their region. Along the transect, the values of W and Sn decrease from southeast to northwest and descend abruptly near the Jiangshan-Shaoxing fault. The distribution of W and Sn matches well with the characteristics of W-Sn mineral-ization in the Cathaysia Block. The Cathaysia Block has high W and Sn relative to UCC, and peak values are existent in the Middle Proterozoic -Ordovician rocks , reflecting the high background in this block , which might explain the reason for the formation of the world's largest W, Sn metallogenic province.
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