辽西地区早白垩世义县组幔源火山岩的成因：橄榄石和熔体包裹体Pb同位素约束

Investigating the petrogenesis of the Early Cretaceous mantle-derived igneous rocks, especially their origin of the lower crust-like geochemistry flavors, is very crucial in understanding the thinning and destruction mechanism of the North China Craton. Because the bulk rocks are hard to unravel the origin of the crust components in the mantle-derived magmas, olivine phenocrysts and melt inclusions entrapped by them can directly reflect the flavors of the mantle-derived magmas at the early stage, effectively excluding the possibility of the crustal assimilation. This project planed to select the mantle-derived volcanic rocks in the Early Cretaceous Yixian formation as the object, using olivine phenocrysts and melt inclusions entrapped by them, especially Pb isotope compositions of the melt inclusions, together with the bulk rocks, to explore their petrogenesis: revealing the origin of their lower crust-like geochemistry features, reconstructing their primary magma compositions and identifying their source lithology. The expected results of the project will be helpful in testing the dynamic mechanisms of the thinning and destruction of the North China Craton.

研究早白垩世幔源火成岩的成因，尤其是似下地壳地球化学特征的来源，对理解华北克拉通减薄和破坏的过程和机制具有重要意义。针对全岩难以揭示幔源岩浆中地壳组分的来源问题，橄榄石斑晶及被其捕获的熔体包裹体可以直接反映早期幔源岩浆特征,有效排除地壳混染的干扰。本项目拟选择辽西地区早白垩世义县组幔源火山岩为研究对象，利用橄榄石斑晶及被其捕获的熔体包裹体，尤其是熔体包裹体Pb同位素组成，结合全岩，探讨义县组幔源火山岩的成因：查明似下地壳地球化学属性的来源；重建原生岩浆的组成；识别源区岩性。本项目的预期成果有助于验证华北克拉通减薄和破坏的动力学机制。

研究早白垩世幔源火成岩的成因，尤其是似下地壳地球化学特征的来源，对理解华北克拉通减薄和破坏的过程和机制具有重要意义。针对全岩难以揭示幔源岩浆中地壳组分的来源问题，橄榄石斑晶及被其捕获的熔体包裹体可以直接反映早期幔源岩浆特征。本项目对辽西义县组黄半吉沟火山岩的研究发现，该火山岩的SiO2>53wt%，MgO>8，Mg#>68（Mg# = mol Mg/(Mg+Fe2+)），为典型的高镁安山岩，属于亚碱性系列；微量元素组成上与大陆下地壳非常相似。然而，熔体包裹体MgO>6wt%，SiO2<53wt%，属于碱性系列。这些现象表明黄半吉沟高镁安山岩的全岩和熔体包裹体成分存在显著差异。熔体包裹体Pb同位素组成随着寄主橄榄石Fo降低而增加，结合全岩微量元素特征，我们认为黄半吉沟高镁安山岩在地壳深度的岩浆演化过程中加入了来自下地壳的酸性熔体，是壳幔相互作用的结果。辽西义县组新开岭火山岩为低Ni高镁安山岩，但是橄榄石具有高Ni和陡倾的Fo-Ni特征，指示其源区为斜方辉石岩。根据橄榄石的实际结晶温度比无水条件的结晶温度低87-152℃，结合橄榄石低Ca特征，估算了高镁安山岩的水含量（2.6-6 wt%）。因此，新开岭高镁安山岩的源区为富水斜方辉石岩，是俯冲板片释放的流体/熔体改造岩石圈地幔的结果。我们认为俯冲作用可能是导致华北克拉通岩石圈破坏和减薄的重要原因。上述成果有助于提高我们对板内高镁安山岩的成因，俯冲改造岩石圈地幔及其与克拉通减薄和破坏过程的认识。