华北北缘西段狼山地区早石炭世基性-超基性岩的矿物化学研究

The Early Carboniferous mafic-ultramafic rocks from the Langshan area in northwestern North China Craton (NCC) are widely accepted to be originated from the enriched lithospheric mantle beneath the northern margin of the NCC, which had been significantly modified by the prolonged subduction of the Paleo-Asian ocean. However, the contribution of subduction recycling components (the subducted sediments and the slab itself) in the magma source, as well as the impact of crustal contamination during the magma ascent and emplacement, are unclear. Based on previous works, this project mainly focuses on the mineral geochemistry of the olivine and the zircon, because olivine crystallizes at an early stage when the basaltic magma system is not saturated with SiO2, whereas zircon crystallizes when the magma has become saturated with SiO2. An integrated study of olivine O isotope and zircon Hf-O isotope will be presented to unravel the nature of the source mantle for the Langshan mafic-ultramafic rocks, and to distinguish the effect of the contamination at crustal depth. In conjunction with whole rock Sr-Nd-Pb isotope, as well as EMPA and LA-ICP-MS of typical minerals (mainly pyroxene, hornblende and plagioclase), we will figure out the contribution of subduction recycling components in the magma source, the particularly magmatic processes, as well as the petrogenetic relationships for the Langshan mafic-ultramafic rocks. This project will provides effectively mineral geochemistry information for the geodynamic evolution processes of the Late Paleozoic continental arc magmatism at the northern margin of the NCC.

华北北缘西段狼山地区的早石炭世基性-超基性岩被普遍认为是起源于俯冲改造后的富集型岩石圈地幔，但是，俯冲再循环组分在岩浆源区中的具体贡献以及后期地壳同化混染作用的影响仍缺乏精细有效的约束。本项目拟在已有研究基础上，从矿物角度入手，以不同岩石类型中的早期矿物橄榄石和晚期矿物锆石为重点研究对象，通过系统的橄榄石O同位素、锆石Hf-O同位素分析，再配合全岩Sr-Nd-Pb同位素以及辉石、角闪石、斜长石等典型矿物的原位微区主微量元素测试，来约束各岩石类型的岩浆源区属性及其中俯冲再循环组分的贡献，识别后期地壳混染作用所产生的成分改造，并建立不同岩石类型之间的成因联系，还原狼山地区早石炭世幔源岩浆作用的演化发展路径。本项目的实施可为进一步认识华北北缘晚古生代陆缘弧的动态演化过程提供有效的矿物化学信息。

华北板块北缘西段狼山弧中的石炭纪镁铁质-超镁铁质杂岩的成因对理解俯冲带内幔源岩浆物质属性及其在壳内不同深度岩浆储库中的演化分异过程和动力学行为具有重要意义。本项目按照研究计划，通过矿物原位微区和全岩元素和同位素地球化学分析，重点剖析了狼山弧中早石炭世潮格侵入杂岩所记录的地幔源区属性及其在深部-浅部岩浆房内的结晶分异过程和岩浆成分演变路径之间的联系。研究结果揭示了以下几个重要问题：（1）潮格侵入杂岩中各不同类型的镁铁质-超镁铁质岩和长英质侵入体是同一幔源富水玄武质岩浆壳内结晶分异不同阶段的产物；（2）系统的矿物结晶物理化学条件计算和基于质量平衡原理的模拟结果表明，在深部和浅部岩浆储库发生的变压结晶分异过程控制着潮格侵入杂岩中镁铁质-超镁铁质堆晶岩和长英质侵入体的形成；（3）高压和低压实验岩石学数据以及全球弧岩浆大数据对比发现，初始岩浆沿不同的高压和低压分离结晶路径的演化主导了潮格侵入杂岩中中-酸性岩的形成过程。另外，本项目对狼山弧中的早二叠世侵入杂岩的预研究发现这些二叠纪弧岩浆在浅部地壳的晶体-熔体分离作用对于华北板块西北缘成熟大陆地壳生长有着突出贡献。本项目取得的成果和认识将有助于进一步理解陆缘弧岩浆在深部-浅部地壳岩浆储库中的分异演化过程、弧岩浆成分多样性以及现代汇聚板块边缘的大陆地壳生长机制等重要科学问题。