河南刘山岩铜锌矿区细碧-石英角斑质含矿火山岩系的构造环境

Palaeotectonic setting of ore-bearing spilite-quartz keratophyreseries in Liushanyan copper-zinc mine,Henan

刘山岩铜锌矿含矿岩系属于早古生代二郎坪群海相细碧-石英角斑岩系。在SiO2-(K2O+Na2O)岩石分类图上,细碧岩属于玄武岩和粗面玄武岩,石英角斑岩和石英钠长斑岩属于酸性流纹岩类,岩石组合为双峰式类型。大多数细碧岩和超基性岩的FeO*/MgO比值为0.2～2.93。在SiO2-(FeO*/MgO)图解上,它们属于拉斑玄武岩和钙碱性系列。火山岩系的微量元素模式以亏损Rb,Th,Nb,Sr,P,Ti和富集Ba,K及LREE为特征。在微量元素构造判别图上,火山岩样品集中在火山岛弧和弧后盆地的范围内。该火山岩系底部超镁铁岩的REE分布模式为轻稀土富集型,有小的Eu正异常;细碧岩-石英角斑岩也是轻稀土强富集型,Ce和Eu有轻微的正异常;硅质岩也有Eu正异常,表明与火山喷发有关;本区硅质岩δCe值为0.87,显示不是深海硅质岩的特征。火山岩系的地球化学特征表明,其形成的大地构造环境为弧后盆地型。

The ore-bearing formation of the Liushanyan copper-zinc deposit belongs to an Early Paleozoic submarine spilite-quartz keratophyre series(Erlanping Group),showing a bimodal volcanic assemblage.On the SiO_2-(K_2O+Na_2O) plot,the spilite is projected in the area of basalt and trachybasalt,whereas the quartz keratophyre and quartz-albite porphyry are projected in the area of rhyolite.On the SiO_2 -(FeO~*/MgO)plot,most spilites and ultrabasic rocks are projected in the area of tholeiites and calc-alkalic series with the FeO~*/MgO ratio ranging from 0.2 to 2.93.The trace element compositions of volcanic rocks are characterized by depletion in Rb,Th,Nb,Sr,P,Ti and rich in Ba,K,LREE.On tectonic discrimination plots of trace elements,all samples of volcanic rocks fall into the field of volcanic island arc and back arc basin.REE patterns of spilites,keratophyres and ultramafic rocks under the volcanic rocks show strong enrichment in LREE with unobvious Ce and Eu positive anomalies.Siliceous rocks have Eu positive anomalies and Ce negtive anomalies(δCe=0.87),showing a not-deep-ocean origin.All the geochemical characteristics mentioned above indicate that the ore-bearing spilite-quartz keratophyre series in this mine were formed in a back arc basin.