玄武岩源区岩性的识别是岩石学研究的最基本问题之一.一直以来,通常认为地幔橄榄岩是部分熔融产生玄武质岩浆的最主要的源区母岩.然而近年来大量的岩石学和地球化学研究表明,地幔在岩性和地球化学组成上具有很大的不均一性.本文系统地总结了不同的超镁铁质岩石部分熔融产生的熔体所具有的独特的、可识别的地球化学特征,并以此来判别海南岛玄武岩源区母岩的岩性.海南岛玄武岩具有较低的CaO,以及较高的Fe/Mn,Zn/Mn及Zn/Fe比值.与橄榄岩部分熔融形成的熔体结晶的橄榄石相比,海南岛玄武岩中的橄榄石斑晶具有较低的Ca和Mn,以及较高的Ni和Fe/Mn比值.实验岩石学的数据表明,干的地幔橄榄岩,橄榄岩+C02以及角闪石岩部分熔融形成的熔体都不能作为海南岛玄武岩的源区母岩,而辉石岩熔体则能很好地解释海南岛玄武岩各种主量元素及橄榄石的化学组成特征.原始地幔标准化的不相容元素蛛网图上,海南岛玄武岩呈现Nb-Ta,Rb,Ba,Sr的正异常以及Th,U的负异常特征,指示了玄武岩的源区有再循环洋壳物质的参与.%Identification of the source lithology for basalts is one of the fundamental issues in petrological studies.For decades,it was commonly considered that mantle peridotite was the only source lithology for basaltic magmas.However,numerous recent petrological and geochemical studies indicate that the mantle source is highly heterogeneous.In this paper,we summarize the different geochemical feathers of melts derived from partial melting of different ultramafic rocks.The results will be used to identify the source lithology for the Hainan basalts.Compared to partial melts of peridotite,the Hainan basalts have lower CaO contents and higher Fe/Mn,Zn/Fe,Zn/Mn ratios.Meanwhile,the olivine phenocrysts from the Hainan basalts have lower Ca,Mn and higher Ni,Fe/Mn than those of the olivines crystallized from partial melts of peridotite.The results of the melting experiments indicate that partial melts of dry mantle peridotite,peridotite +CO2,and hornblendite cannot match the chemical compositions of the Hainan basalts,while the pyroxenite melts can well match their geochemical characteristics.The OIB-like trace element compositions,such as Rb,Ba,Sr,Nb,Ta positive anomalies and Th,U negative anomalies of the Hainan basalts suggest that a recycled oceanic crust component was involved in the source of the Hainan basalts.
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