渤海湾盆地济阳坳陷早第三纪钻孔玄武岩年代学和地球化学研究

Basic magma is an important probe of deep geological processes, which can be used to inverse the evolution of lithosphere. However, the history of Mesozoic-Cenozoic basic magma in eastern North China Craton (NCC) is not completely recoganized, particularly poor on the Early Tertiary basic magma. Oil drilling had revealed that a large volume of basalts erupted in the Bohai Bay basin during the Early Tertiary. In this project, detail geochronology, petrology and geochemistry will be performed on the drill-core basalts from the Jiyang sag. Ar-Ar geochronology dating on whole rock and minerals will be carried out to constrain the eruption time of the basalts, and reveal temporal and spatial distribution of Cenozoic basic magma in the NCC. The bulk major elements, trace elements and Sr-Nd-Pb-Hf-Os-B isotopes and in-situ major elements, trace elements and O isotope on minerals will be carried out to constrain the source characteristics. The geodynamic models of the lithospheric thinning of the NCC will also be tested based on these data. The thickness of lithosphere will be modeled according to the chemical composition of the basalts, with aims to construct the thickness-time framework of the lithosphere during the Late Mesozoic-Cenozoic and supply constrains on the time, process, and dynamic mechanism of the transformation from thinning to thickening of lithosphere beneath NCC. Based on the comparison between Cenozoic basic magma evolution in different areas of the eastern China, study on the temporal and spatial inhomogeneous of transition from thinning to thickening of lithosphere will be carried out.

基性岩浆是深部地质过程的重要探针，能够用于反演岩石圈演化，然而目前对华北东部中-新生代基性岩浆演化历史的认识并不完整，尤其早第三纪基性岩浆活动研究偏弱。油田钻探显示渤海湾盆地存在大量的早第三纪玄武岩，本申请拟对济阳坳陷钻孔玄武岩进行详细的年代学、岩石学和地球化学研究，通过全岩和单矿物Ar-Ar年代学研究，限定玄武岩的喷发时代，厘定华北新生代基性岩浆的时空分布规律；通过全岩主量元素、微量元素、Sr-Nd-Hf-Pb-Os-B多元同位素分析，结合矿物斑晶原位主量、微量元素和O同位素分析，限定岩浆源区的性质，验证岩华北克拉通石圈减薄的动力学模型；根据玄武岩的化学组成，反演岩石圈厚度，构建晚中生代-新生代岩石圈厚度随时间演化的格架，为岩石圈减薄向增生转换的具体时限、过程和动力学机制提供制约；通过中国东部不同区域新生代玄武岩演化的对比研究，探讨中国东部岩石圈减薄向增生转换的时空不均一性。

本项目对山东和渤海湾盆地新生代玄武岩进行了系统的研究，发现这些玄武岩都是由富集的EM1组分和一个亏损的组分组成。渤海湾盆地伸展之后的玄武岩（≤23Ma），化学组成与SiO2含量密切相关。高硅玄武岩显示EM1型微量元素和同位素组成特征；低硅玄武岩具有HIMU (高U/Pb) 型微量元素特征，但其206Pb/204Pb较典型HIMU玄武岩偏低。通过全岩及橄榄石熔融包裹体以及Hf-Nd-B同位素研究揭示，高硅玄武岩来源于一个石榴石辉石岩地幔，具有印度洋型地幔域同位素特征，且具有相对较高的δ11B值 (-4.9~-1.4)，符合古老再循环的沉积物+洋壳特征（> 1Ga）；而低硅玄武岩来源于一个含碳酸盐的辉石岩+橄榄岩地幔源区，具有太平洋地幔域特征，且其δ11B值较低 (δ11B = -6.9 ~ -3.9)，类似于典型的HIMU地幔源区。进一步研究发现渤海湾盆地伸展期玄武岩(50–40Ma)同样含有EM1组分，但是相对于伸展后玄武岩具有高SiO2、Al2O3低MgO、Fe2O3T含量，以及较弱的中重稀土分异程度，表明其岩石成因及起源深度类似于洋中脊玄武岩，这些EM1组分应存在于较浅的上部软流圈地幔。以上研究揭示华北东部软流圈地幔化学和矿物学组成上的垂向不均一性：浅部地幔存在零星分布的石榴石辉石岩，滞留的太平洋板块对东亚大地幔楔不均一性的物质贡献是来源于地幔过渡带的碳酸质熔体，玄武岩的化学组成受到岩石圈厚度和源区不均一性的共同制约，碳酸质熔体对浅部地幔的交代及其触发的熔融，是浅部地幔具有低波速的原因。