早第三纪玄武岩在苏北盆地以橄榄拉斑玄武岩为主，在地表基本无出露，均为钻井揭示，呈夹层产出。对它们详细的岩相岩石学、微量元素地球化学及锶钕同位素研究表明，玄武岩原始岩浆源于受俯冲洋壳影响的陆下岩石圈地幔较高程度部分熔融并在源区经历了一定程度的单斜辉石和橄榄石的结晶分异，岩浆上升至喷出地表过程中没有发生明显地壳物质的混染，其所表现出的过渡特征暗示区域在古新世时处在一个由活动大陆边缘向大陆板内裂谷转换的关键时期。 隐伏玄武岩 Abstract: The Eogene basalts, predominantly olivine tholeiites which have no surface exposures in north Jiangsu Basin, form thick sequences of many thin flows interfingered with inland lacustrine sediment beneath a sedimentary cover and were sampled through oil well prospecting. Detailed petrographical, petrological, trace element geochemical and Sr and Nd isotopic studies on them show that the primary magma was derived from a subcontinental lithospheric mantle which had been modified by subducted and altered oceanic crust through rather high degree partial melting and later underwent in situ certain degree of clinopyroxene and olivine fractionation.However the parent magma was not obviously contaminated by crustal material during its ascenting to the surface. The transitional characters of the olivine tholeiites indicate that the region was then in an important transforming period from an active continental margin to an intra continental rift in Eogene time. Key words: Eogene Concealed basalt North Jiangsu Basin Geochemistry Source character