华北克拉通东南部新生代大陆玄武岩斑晶稀有气体和氢同位素以及水含量研究

There are sporadic occurrences of Cenozoic continental basalts in the eastern part of the North China Craton. Previous studies have laid fundamentals for deciphering the subcontinental lithospheric mantle and its underlying asthenospheric mantle. However, it is little known about the contents and property of volatile materials in the mantle with respect to destruction of the North China Craton. This project focuses on noble gas and hydrogen isotopes as well as water contents of phenocryst minerals from Cenozoic basalts in Shandong, Jiangsu and Anhui provinces in the southeastern part of the North China Craton. Aims of this study are to recognize the effects of mantle source property and magmatic differentiation on these geochemical parameters, finding the relationships of phenocryst noble gas and hydrogen isotope compositions and water contents to common geochemical variables in Cenozoic lithospheric mantle of eastern China. This study is also combined with those on Cenozoic mantle xenoliths and megacrystals. In view of the debate whether the continent lower crust or the oceanic crust is the crust component in the mantle source of Cenozoic basalts, this study is combined with whole-rock major-trace elements and radiogenic Sr-Nd-Pb isotopes as well as phenocryst oxygen isotopes to constrain the origin of crustal components in mantle sources and the geodynamics of crust-mantle interaction.

华北克拉通东部零星出露有不同成分的新生代大陆玄武岩，前人的研究为认识大陆岩石圈地幔及其下伏软流圈地幔的性质奠定了重要的地球化学基础。但是就克拉通岩石圈破坏来说，地幔中挥发性物质的含量和性质是个尚未解决的问题。本项目拟以华北克拉通东南部山东、江苏、安徽三省的新生代玄武岩为研究对象，开展斑晶矿物稀有气体和氢同位素以及水含量研究，探讨地幔源区成分和玄武质岩浆演化对这些挥发性地球化学参数的影响。结合前人对中国东部新生代地幔包体和幔源巨晶矿物的相关研究，查明新生代岩石圈地幔的稀有气体同位素组成、水含量及氢同位素组成。针对当前研究中对新生代玄武岩地幔源区中地壳组分为大陆下地壳还是大洋地壳的争议，结合玄武岩全岩主微量元素、放射成因Sr-Nd-Pb同位素及斑晶矿物氧同位素组成，制约其地幔源区地球化学组成及其形成的地球动力学机制。

本项目主要研究了中国东部新生代玄武岩中斑晶矿物的稀有气体、水和氢同位素，发现1. 稀有气体带有大气信号(40Ar/36Ar远低于MORB而接近大气值)；2. 地幔源区较亏损地幔富水(290 ppm，高于亏损地幔的<200 ppm)；3. 氢同位素(-145 – -94‰)组成低于亏损地幔值(-72±10‰)。这些都表明了中国东部新生代玄武岩源区是亏损地幔和地壳组分混合的产物，后者来自于受到水岩相互作用，并在弧下经历脱水作用的俯冲大洋板片。另外还总结了中国东部地幔包体和玄武岩的水含量和氢同位素组成，强调了俯冲大洋板片对地幔中含水量和氢同位素的影响。这些研究均表明俯冲地壳再循环是影响地幔地球化学性质的重要因素。