辽西凌源河坎子碱性杂岩体地球化学特征及地质意义

Geochemical characteristics and geological implications of the Hekanzi alkaline complex in Lingyuan County,western Liaoning Province

分析测定了河坎子碱性杂岩体的稀土、微量元素和Sr、Nd、Pb同位素数据 ,对该杂岩体形成的构造背景、源区特征及意义进行了讨论。河坎子碱性杂岩体稀土元素总量较高 ,富集LREE和大离子亲石元素 ,Eu呈弱负异常或异常不明显 ,Nb、Ta、K、P、Ti和Y呈负异常 ,过渡族元素含量偏低。稀土和微量元素分布模式与辽东凤城碱性杂岩体相似。主侵入期 4个全岩加 1个黑云母样品的全岩矿物Rb_Sr等时线年龄为 2 2 1.4± 8.5 (2σ)Ma ,属于印支期碱性岩浆活动。该岩体的εSr(t)均为正值 ,平均 7.2 3,ISr=0 .70 4 77± 0 .0 0 0 15 ,εNd(t)均为负值。微量元素和Sr、Nd及Pb示踪同位素特征显示其源区与EMⅠ型富集地幔端员有关。河坎子碱性杂岩体地球化学特征不仅揭示了华北地台北缘东部上地幔在三叠纪时具有富集性 ,而且也显示了辽东地区比辽西地区更富集的特点。河坎子碱性杂岩体形成于板内拉张环境 ,岩浆作用以批式部分熔融占主导地位。

REE, trace element and Sr, Nd and Pb isotopic data of the Hekanzi alkaline complex are presented in this paper, together with a discussion on its relationship with the regional tectonic regime and its geological implications. Rocks of this complex have high REE contents. In the chondrite-normalized REE diagram, these rocks show LREE enrichment and have weak or very indistinct negative Eu anomalies. The PM-normalized trace element diagram exhibits LREE and LILE (Rb, Ba, Th and Sr) enrichment with negative Nb, Ta, K, P, Ti and Y anomalies. Transitional element contents of the complex are very low compared with data of the primitive mantle. REE and trace element characteristics of the Hekanzi alkaline complex are very similar to those of the Fengcheng alkaline complex in eastern Liaoning Province. A five point whole rock-mineral Rb-Sr isochron for the main intrusive stage of the complex yields an age of 221.4±8.5(2σ) Ma, implying that the Hekanzi alkaline complex results from activity of Indosinian alkaline magma. All ε Sr( t ) values of the Hekanzi alkaline complex are positive with an average of 7.23, and the initial ^(87)Sr/^(86)Sr value obtained from the Rb-Sr whole rock-mineral isochron is 0.704?77±0.000?15. However, all the εNd( t ) values are negative. Trace element compositions and Nd, Sr, Pb isotopic geochemical characteristics of the Hekanzi alkaline complex show that its source has features similar to those of the EMⅠ end member. These features suggests that the upper mantle of the eastern North edge of North China platform was fertile in Triassic period and, what is more, the upper mantle under the eastern part was more fertile than that under the western part of Liaoning Province. Variation diagrams of major and trace elements imply that the Hekanzi alkaline complex was formed in an intra-plate environment, and its major magmatic process is dominated by batch melting.