华北西部陆块狼山地区下地壳组成与演化

As a link between mantle and crust, the lower crust records abundant information of the interaction between them, and thus is an important target for the study of the formation and evolution of the continent. The igneous rocks in the Eastern Block and the Trans-North China Orogen (TNCO) of the North China Craton (NCC) carried a large number of granulite xenoliths, which greatly promoted the understanding of the formation and evolution of the lower crust under these areas. However, granulite xenoliths in the Western Block of the NCC are rarely found, seriously restricting the understanding of the composition and structure of the lower crust beneath this block. Recently, we newly discovered abundant granulite xenoliths from the basalts in the Langshan region of the Western Block. They are the westernmost xenoliths in the NCC by far, and thus can offer significant constraints on the composition and structure of the lower crust beneath this block. In this project, the lower crust xenoliths newly found in the Langshan region are selected as main objects. Based on the detailed fieldwork and petrography observation, the systematic analyses on mineral chemistry, whole-rock major- and trace-element and Sr-Nd-Pb isotopes, zircon U-Pb dating and Hf-O isotopes and geophysical methods (such as seismic velocity, density and magnetism), will be employed in order to define the features, depths and ages of these granulite xenoliths and investigate the lower crustal accretion and reworking below the Langshan region. Combining the results of geophysical exploration, this project will establish the lithology-composition-age section of the lower crust beneath this region. Compared with the typical lower crustal sections established in the Eastern Block and the TNCO, the lower crustal composition and structure under different tectonic units of the NCC will be discussed.

作为地幔与地壳的联结载体，下地壳记录着壳-幔作用的丰富信息，是大陆形成演化研究的重要对象。华北东部陆块和中部带的火成岩携带有大量的麻粒岩包体，大大促进了对该区下地壳形成与演化的了解。然而西部陆块的麻粒岩包体出露很少，严重制约了对其下地壳组成与结构的认识。最近在西部陆块狼山地区的玄武岩中发现了多种麻粒岩包体，它们是目前所知华北最西边的包体，可为限定西部陆块下地壳组成与结构提供重要约束。项目拟以狼山地区新发现的下地壳包体为对象，在详细野外工作和室内岩相学观察基础上，通过锆石U-Pb定年及Hf-O同位素、矿物化学、全岩主微量元素及Sr-Nd-Pb同位素和物性(波速、密度和磁性)等的配套研究，进行定性、定深和定年及下地壳增生再造过程研究。结合地球物理探测成果，建立狼山地区下地壳的岩性–成分–年龄结构剖面。与华北东部陆块和中部带已建立的典型的下地壳剖面对比，探讨不同构造位置下下地壳的组成与结构特征。

华北东部陆块和中部带的火成岩携带有大量的下地壳包体，大大促进了对该区下地壳形成与演化的了解。然而西部陆块的下地壳包体出露很少，严重制约了对其下地壳组成与结构的认识。最近在华北西部陆块的狼山地区发现了丰富的深源岩石包体。它们包括橄榄岩，辉石岩，基性麻粒岩，中性麻粒岩和酸性麻粒岩等。本项目首先对狼山下地壳包体开展了岩相学，矿物学，地球化学，及锆石U-Pb年龄和Hf同位素分析，发现狼山地区下地壳残留有古太古代陆核，且它们来源于始太古代地幔的抽取。为了更好地理解这一新发现，本项目还对华北典型地区下地壳的组成、结构及形成过程进行了系统的总结，结果表明华北下地壳具有分层的特点，且上老下新，暗示可能与底侵作用有关。它还经历了自冥古宙到新生代复杂的形成与演化过程。其次对来源更深的狼山橄榄岩和辉石岩包体开展了稳定Cr同位素的研究，并与华北典型地区辉石岩包体对比，揭示辉石岩包体具有相对低的δ53Cr值，可能代表与幔源铬铁矿岩在Cr同位素上互补的轻Cr同位素储库。对狼山橄榄岩包体还进行了全岩粉末及四相矿物的穆斯堡尔谱学的研究，限定了它们的Fe3+/ΣFe比值，揭示狼山岩石圈地幔整体表现为较还原状态，推测与深部地幔低ƒO2熔体上升交代有关。最后为了更好地理解狼山地区下地壳的形成与演化，本项目还扩展到深部地壳演化和深部过程的研究，因此还开展了对华北南缘信阳地区古元古代和中生代花岗岩，以及紧邻华北的西秦岭中生代正长岩的岩相学，矿物学，地球化学，锆石U-Pb年龄和Hf同位素分析，以及全岩Sr-Nd-Pb同位素的综合研究，揭示华北南缘古元古代深部地壳具有垂向的双层结构和晚中生代深部地壳的减薄与再造过程，以及早中生代阿尼玛卿洋北向俯冲的深部过程记录。