山东东部中生代中基性火山岩的地球化学特征及成因探讨

Geochemistry of the Mesozoic basic-intermediate volcanic rocks in eastern Shandong province(eastern China) and their genesis

本文对山东东部中生代中-基性火山岩的岩石化学、微量元素、稀土元素及铅同位素特征作了系统研究.该区中生代火山岩主要为玄武岩、粗面玄武岩、玄武质粗面安山岩、安山岩和粗面安山岩.玄武岩属钙碱性系列的拉斑玄武岩,微量元素特征显示大离子亲石元素和高场强元素均稍富集;稀土配分模式略呈右倾型,轻、重稀土之间的分馏不明显,基本无Eu异常;铅同位素组成显示玄武岩形成于原始地幔.粗面玄武岩和安山岩类属钙碱-弱碱性系列,微量元素特征显示大离子亲石元素富集,高场强元素含量低;稀土配分模式呈右倾型,轻、重稀土之间的分馏明显,基本无Eu异常;铅同位素组成显示粗面玄武岩形成于亏损地幔,安山岩类形成于富集地幔,具EMI属性.这些特征表明,山东东部中生代火山岩的岩浆来源于不同的源区.形成于早白垩世的安山岩类,产于活动大陆边缘拉张动力背景下,是由富集型地幔部分熔融形成的岩浆直接喷发形成;形成于晚白垩世的粗面玄武岩,也产于活动大陆边缘拉张动力背景下,随着弧后扩张引起拉伸作用,岩石圈伸展,从地幔来源的玄武质岩浆在上升过程中受到了陆壳物质的混染,之后沿着大断裂喷出地表形成本区显示亏损地幔来源特征的粗面玄武岩;形成于晚白垩世的玄武岩产于大陆板内拉张环境,是由原始地幔部分熔融而成的岩浆直接喷发形成.

Geochemical and geological studies on the Mesozoic basic-intermediate volcanic rocks in Tan-Lu fault zone in eastern Shandong province suggest that these vocanic rocks include basalt, trachybasalt, basaltic traebyandesite, andesite and trachyandesite. The basalts belong to tholeiitic series and the other rocks belong to cale-alkahne to weakly alkaline series. The basahs are characterized by slight enrichment of light rare earth elements （LREE）, large-ion lithophile elements （LILE） and high-field strength elements （HFSE） with no obvious Eu depletion, which are consistent with features of intraplate tholeiitic basalts. Both the trachybasalts and andesites are characterized by high LREE and LILE contents and low HFSE contents with no obvious Eu depletion, which are compatible with features of volcanic rocks in active continental margin. The early Cretaceous andesites may be derived from the partial-melting of the enriched mantle in the extensional setting of active continental margin. The late Cretaceous trachybasalts might have come from the partial-melting of the lithospheric mantle and might be contaminated by crust materials in the same setting as andesites. The late Cretaceous basalts in Tan-Lu fault zone and its adjacent areas might he originated mainly from the partial-melting of the original mantle and formed in intracontinental extension.