西秦岭中新生代碱性玄武岩及其地幔包体地球化学研究

Silica-undersaturated alkali basalts are generally characterized by low SiO2 and high alkali metal (K2O+Na2O) contents. Recently, more and more experimental petrological studies suggested that partial melting of dry peridotite cannot account for the lithochemical feature of alkali basalts on Earth. The nature of their mantle sources, especially the mantle lithologies, is the main factor that constrain the geochemical composition of alkali basalts. However, this remains controversial. It is also unclear by which mechanism their mantle sources were generated. This project will carry out a detailed study of petrology and geochemistry for Mesozoic-Cenozoic alkalic basalts and mantle xenoliths from the western Qinling orogen. Intended investigations mainly include whole-rock major-trace elements and isotopes, mineral major-trace elements, olivine and pyroxene noble gas isotopes of alkalic basalts, and detailed study of mantle xenoliths (including petrography, mineral major-trace elements and isotopes). Taken together, we expect to look for some identification indexs to identify the mantle lithology of alkalic basalts, elucidate the nature and origin of crustal components during crust-mantle interaction in fossil subduction zone, constrain the geochemical nature of their mantle sources as well as chemical geodynamic mechanism for generating them, and provide insights into the petrogenesis of alkalic basalts.

硅不饱和的碱性玄武岩具有明显低的SiO2含量以及高的碱(K2O+Na2O)含量。近年来，越来越多的实验岩石学研究表明，干的地幔橄榄岩部分熔融不可能形成碱性玄武岩，地幔源区性质(特别是岩石学性质)可能是控制碱性玄武岩地球化学成分的主要因素。然而，目前对于碱性玄武岩地幔源区性质及其形成机制的研究还存在很大争议。本项目拟以西秦岭中-新生代不同产状、不同性质的碱性玄武岩及其地幔包体为研究对象，进行系统的岩石学和地球化学研究。主要通过玄武岩全岩主微量元素与同位素、矿物主微量元素与稀有气体同位素(橄榄石和辉石)，并结合地幔包体岩相学特征以及矿物元素、同位素组成，试图从天然样品角度寻找识别和区分碱性玄武岩地幔源区岩石学性质的鉴定性指标，查明古俯冲带壳幔相互作用交代介质的性质和来源，确定碱性玄武岩地幔源区的地球化学性质及其形成的化学地球动力学机制，进而深化对碱性玄武岩成因的认识。

造山带碱性玄武岩地幔源区性质及其形成机制的研究还存在很大争议。根据项目申请时的研究计划，本项目基本完成了预期目标。通过对西秦岭中新生代碱性玄武岩的系统研究，包括主微量元素、放射性成因Sr-Nd-Hf和Mg同位素等，提出中新生代玄武岩的地幔源区分别受到了硅酸盐熔体和碳酸盐熔体的交代作用，并进一步识别出其地幔源区的岩石学性质可能分别富含角闪石岩和碳酸盐化的橄榄岩。最后，综合造山带镁铁质岩浆岩的地球化学特征，认为其记录了俯冲古洋壳物质再循环。相关研究成果共发表4 篇论文，包括1 篇Journal of Petrology和2 篇Lithos，以及1篇国内核心期刊（地球科学）。此外，项目执行期间，培养一名优青（戴立群），3名博士（陈龙、代富强和郑菲），2 名硕士（汤雨微和郭军）。