河北汉诺坝碱性玄武岩的源区和成因

Origin and source of the Hannuoba alkaline basalts

我国华北北部新近纪喷发的汉诺坝玄武岩,岩层出露比较完整,普遍含有超镁铁岩包体和各种高压巨晶。大麻坪代黄沟剖面出露较好,主要岩性为玄武岩,从上到下可分为上三、二、一层和底层。岩相学观察显示这些玄武岩含有1%~5%的捕虏晶且斑晶数量很少。橄榄岩包体为尖晶石二辉橄榄岩,轻稀土略微亏损,轻重稀土无分馏,显示这是只经过少量部分熔融后的原始地幔包体。通过对代黄沟碱性玄武岩中橄榄石、辉石和斜长石等斑晶或捕虏晶的矿物学分析,得出它们的成分变化是玄武岩混合了分解的地幔橄榄岩快速上升造成的。玄武岩的主量元素与二辉橄榄岩相平衡的原生熔体相比,Mg^#(52.0~62.7)以及CaO(7.3%~8.5%)、Ni((82~192)×10^-6)和Cr((65~192)×10^-6)含量都较低。玄武岩的稀土分配模式为轻稀土富集的右倾型,δEu=1.01~1.05,δCe=0.95~1.02,富集高场强元素Nb、Ta、Zr及大离子亲石元素Ba、Sr。玄武岩痕量元素Ba/Rb和Rb/Sr的比值显示源区可能遭受过流体的交代作用。同时,痕量元素显示这一系列玄武岩主要受到部分熔融的控制,大量的结晶分异并未发生,且源区残留石榴子石。分析认为玄武岩岩浆应该是原生岩浆,而不是演化岩浆,影响整个岩浆的形成的过程可能是部分熔融而不是分离结晶。基于目前已发表的实验岩石学结果和理论分析认为汉诺坝玄武岩与辉石岩类相平衡,源区主要矿物相为单斜辉石和石榴子石,可能存在少量的橄榄石。

The Hannuoba basalts,erupted in the Neogene in northern North China,contain not only complete rock outcrops but also widespread ultramafic xenoliths and a variety of high-pressure megacrysts.The Daihuanggou profile,one of the better exposed profiles of the Hannuoba basalts generation,can be divided into the upper three layers and base from top to bottom.Petrographic observations showed that these basalts contained 1%-5%of xenocrysts and trace of phenocrysts.The peridotite xenoliths were spineliherzolite.They were slightly depleted in LREE and had no fractionation of LREE and HREE,which is consistent with primary mantle xenoliths after some partial melting.Through mineralogical analysis of phenocrysts and xenocrysts,such as olivine,pyroxene and plagioclase in the Daihuanggou basalt,we concluded that the compositional changes in phenocrysts or xenocrysts are caused by the rapid rising basalt after mixing with the decomposition product of mantle peridotite.Compared to the native melt in equilibrium with lherzolite,the Mg^#(52.0-62.7),CaO(7.3%-8.5%),Ni((82-192)×10^-6)and Cr((65-192)×10^-6)contents were lower.The REE patterns for basalts showed right-wing type enrichment;HFSE(Nb,Ta and Zr)and LILE(Ba and Sr)were also enriched,withδEu=1.01-1.05 andδCe=0.95-1.02.Trace elemental ratios Ba/Rb and Rb/Sr showed that magmatic source might have been fluid metasomatism.According to trace elemental compositions,this series of rocks is mainly controlled by partial melting,where large scale fractional crystallization does not occur,and the residual source contains garnet.The analysis suggests that the basalt magma should be a primary rather than an evolved magma.The process that affects the formation of the entire magma may be partial melting rather than fractional crystallization.Based on current published experimental results and theoretical petrology analysis,the Hannuoba basalts are in equilibrium with pyroxenite,and the magma source contains mainly clinopyroxene and garnet and possibility a small amount of olivine.