华北克拉通东部晚中生代至早新生代OIB型玄武岩的水含量和氧同位素组成

The changes in composition of basalts are usually used to deduce the evolution history of lithosphere. Several recent works suggest that from the late Mesozoic to early Cenozoic, the recycled components in the mantle source of basalts in North China Craton (NCC) changed gradually from the upper part to lower part of the oceanic crust, and this kind of evolution reflects the continuous thinning of the NCC lithospheric mantle. However, the water content of mantle source and the specific melting process, through which the recycled oceanic crust (ROC) occurred in the mantle source (say, directly melting of the ROC, the melting of the secondary-stage pyroxenite formed by hybridization of ROC melts and peridotite, or the residual melt of the reaction between ROC melt and the peridotite), would also effect the correlation between the basalts’ composition and the type of ROC components. These factors were not considered by the former study. This project would study the magma water contents and the specific mantle process for the genesis of the basalts in the eastern NCC from 106-40Ma, by the water content and O isotopic composition and trace element concentration of clinopyroxene. The result would provide more constraints on the evolution history of NCC.

玄武岩的组成随时间的变化常被用来推断岩石圈的演化。 最近的研究认为华北克拉通晚中生代至早新生代起源于软流圈的玄武岩源区中的再循环组分逐渐从上部洋壳转变为下部洋壳，并认为这种变化反映了克拉通岩石圈地幔的减薄过程。然而，源区水含量和洋壳组分参与到玄武岩浆形成过程的方式（洋壳直接熔融后的熔体?洋壳部分熔融的熔体与橄榄岩反应形成的二阶段辉石岩再融?还是洋壳熔体与橄榄岩反应后的残余熔体?）都会影响玄武岩的成分和再循环组分之间的关系，而这些因素在前人的模型里并没有得到重视。本项目将选取华北克拉通东部106-40Ma间的OIB型玄武岩为研究对象，用单斜辉石斑晶反演的方法定量其水含量，并用单斜辉石斑晶的原位氧同位素组成和微量元素含量探讨源区的组成和熔融过程，从而为准确利用玄武岩的成分变化推断岩石圈的演化提供制约。

以华北克拉通东部106Ma至60Ma以来的OIB型玄武岩为研究对象，通过对岩浆水含量、矿物氧同位素组成以及微量元素的原位分析，探讨了俯冲太平洋板块对华北早白垩以来的玄武岩源区的物质供应状态。得到的新认识包括：1）从106Ma至晚新生代，OIB型玄武岩的地幔源区都含有来自太平洋板块的俯冲洋壳物质加入；在60Ma以前，再循环物质以上部洋壳物质为主，而在新生代以后则呈现出上部和下部洋壳交替出现的情况，因此这种物质贡献是动态的；2）玄武岩的氧同位素与同时代，同地区的岩石圈地幔氧同位素总表现出相反的趋势，因此玄武岩的源区不在岩石圈地幔，而在软流圈；3）从106Ma到新生代，玄武岩的水含量大都高于正常的MORB和OIB的水含量范围，达到了岛弧玄武岩的水平，这说明自早白垩以来，太平洋板块对中国东部，特别是华北克拉通东部上地幔持续的释放了富水的熔流体。本工作追踪玄武岩氧同位素组成随时间的演化，并将其与同时代岩石圈地幔的值相比较，为探讨再循环组分的存在与否和作用方式提供了最为直接的同位素证据。