青海玉树赵卡隆铁铜多金属矿床安山岩地球化学特征及其成因

李欢^{1, 2}，奚小双¹，吴城明³，张代斌⁴

(1. 中南大学 地球科学与信息物理学院，有色金属成矿预测教育部重点实验室，湖南 长沙，410083；
2. 九州大学工学府地球资源系统，日本 福冈，819-0395；
3. 广西国土资源规划院，广西 南宁，530022；
4. 青海诺贝尔矿业有限公司，青海 西宁，810008)

摘 要：赵卡隆铁铜多金属矿区发育一套与成矿密切相关的安山岩。为了研究其成因，对安山岩的主量、微量、稀土元素及同位素进行分析。研究结果表明：安山岩以英安岩类为主，主成分以低硅(SiO₂质量分数为47.79%~ 61.51%，平均为55.41%)、高铝(Al₂O₃质量分数为15.44%)、高钠(Na₂O平均质量分数为4.81%)、低钾(K₂O平均质量分数为1.60%)含量为特征，岩石分异指数(DI)为39.63~72.52，火山岩QAPF图落入英安岩、安山岩区；轻稀土元素富集，Eu无异常，Ce呈弱的负异常，Y和Yb元素有显著的亏损(w(Y)＜20 μg/g, w(Yb)＜1.93 μg/g)；硫同位素比值δ³⁴S_CDT范围为-1.60‰~15.32‰；地球化学数据分析显示出其大洋安山岩的特征，属于亚碱性－拉斑玄武岩系列，喷发环境为海相，具有岛弧安山岩的性质及弧后扩张盆地火山岩的双重性质；微量元素以富集大离子亲石元素和亏损高场强元素为特征，且高场强元素的比值(w(Zr)/w(Nb)，w(Hf)/w(Ta))具有类似MORB的特性；安山岩构造位置应偏于岛弧中的内弧火山活动，安山岩岩浆应为地幔来源，并混染了大陆壳物质，为壳幔两种岩浆混合的产物，其形成与岛弧东侧的大洋消减作用及岛弧西侧的弧后扩张作用有关；安山岩虽然在某种程度上表现出岛弧安山岩的特征，但实际揭示了弧后扩张盆地的大地构造背景。

关键词：赵卡隆矿床；安山岩；岛弧；弧后盆地

Geochemistry and genesis of andesite from Zhaokalong Fe-Cu polymetallic deposit,Yushu, Qinghai

LI Huan^{1, 2}, XI Xiao-shuang¹, WU Cheng-ming³, ZHANG Dai-bin⁴

(1. School of Geosciences and Info-physics, Central South University, Changsha 410083, China;
Key Laboratory of Non-ferrous Metal Ore Forecast, Ministry of Education, Changsha 410083, China;
2. Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan;
3. The Planning Institute of Land and Resources of Guangxi, Nanning 530022, China;
4. Nobel Mining Co. Ltd., Xining 810008, China)

Abstract:The Zhaokalong andesite from the back-arc rift sedimentary basin is closely related to the Fe-Cu polymetallic mineralization. In order to study the petrogenesis, the major, trace, rare earth elements and isotopes were analyzed. The results show that the andesite belongs to dacite, and it has an uncommon chemical composition with a characteristic of low silica (SiO₂ 47.79%-61.51%, average 55.41%), high alumina (Al₂O₃ average 15.44%), high sodium (Na₂O average 4.81%) and low potassium (K₂O average 1.60%). Rock differentiation index (DI) is 39.63-72.52, and the spots of Q-A-P-F diagram situate in the dacite-andesite zone. In addition, the andesite has an enrichment of light-rare-earth elements (LREE), and its LREE/HREE ratio is 3.97-9.13. Eu shows no anomaly (δEu 0.92-1.10), while Ce shows weak negative anomalies (δCe 0.68-0.84), Y and Yb are depleted greatly (w(Y)＜20 μg/g, w(Yb)＜1.93 μg/g). Furthermore, the andesite has high abundances of large-lithophile trace elements ( e.g. Ba, Th, Rb) and low abundances of high-strength-field elements (HFSE, e.g. Ta, Nb, P, Ti), and the ratios of HFSE (e.g. w(Zr)/w(Nb), w(Hf)/w(Ta)) have similar characteristics to MORB. δ³⁴S_CDT (sulfur isotope ratios) of the andesite range from -1.60 ‰ to 15.32 ‰. Geochemistry indicates that the andesite erupted in a marine environment, with the characteristics of ocean andesite, belongs to the subalkaline-tholeiite series. The andesite has a dual nature of island-arc andesite and back-arc volcanic,and the eruption location of the andesite may be biased towards to inner arc (back-arc rift). Andesite magma, which is related to oceanic subduction and arc spreading, should be crust-mantle mixing magma originated from the mantle and contaminated by the continental crust. The andesite shows the characteristics of island-arc andesite to some extents, but in fact it originates in a tectonic setting of back-arc expansion basin.

Key words:Zhaokalong deposit; andesite; island arc; back-arc basin