华北北缘集宁–怀安下地壳组成及演化：来自包体及地体麻粒岩相岩石的制约

Granulite xenoliths and terranes offer direct glimpses into the composition and evolution of the lower crust. These two kinds of deep-seated crustal rocks are exposed in the northern margin of the North China Craton (NCC). Although some investigations have been carried out on these rocks, there are still great disputes about the tectonic subdivision and crustal growth and modification. Recently we newly discovered abundant crustal xenoliths from the Cenozoic basalts at the Nangaoya Town and Yangyuan County in the Jining–Huai’an areas， which are situated at the conjunction area for different tectonic models of the NCC. Several high-grade metamorphic terranes also crop out in the study area. These granulite xenoliths and terranes can offer important constraints on the composition, structure and process of the lower crust in the northern margin of the NCC. In this project, the deep-seated xenoliths newly found and typical granulite terranes from the Jining–Huai’an areas are selected as main objects. On the basis of detailed field investigation, and in-house petrography and mineral internal structure observation, the systematic studies on zircon U–Pb dating, mineral chemistry, whole-rock major-element, trace-element and Sr–Nd–Pb isotopes, and zircon Hf–O isotopes will be carried out in order to clarify the formation ages, composition, petrogenesis and metamporhic evolution of these crustal rocks. Combined with published results, we will constrain the temporal and spatial distribution of magmatism and metamorphism at the middle segment of the northern margin of the NCC, and try to constitute the lithology–composition–age structure profile of the lower crust. Finally, we will discuss Precambrian tectonic evolution, and then unravel the complex accretion and reworking process of lower crust in the northern margin of the NCC.

包体和地体麻粒岩相岩石是研究下地壳组成和演化的重要窗口。这两种深部壳源岩石在华北北缘同时出露。前人开展过一些研究，成果对构造块体划分及地壳增生再造过程等远未形成共识。最近我们在集宁–怀安地区的南高崖及阳原新生代玄武岩中发现了多种壳源岩石包体，这一地区属华北北缘中段不同构造单元划分的交汇部位，该区也有高级变质地体岩石出露，它们可为限定深部地壳组成、结构和过程提供重要约束。项目拟以集宁–怀安地区新发现的壳源岩石包体及其周边深变质地体岩石为对象，在翔实的野外工作和室内岩相学观察基础上，通过锆石U–Pb定年，和矿物化学、全岩元素及Sr–Nd–Pb同位素和锆石Hf–O同位素的配套研究，查明这些壳源岩石的物质组成、形成时代、岩石成因及变质演化。结合已有资料，厘清研究区岩浆活动–变质作用时空分布规律，根据研究区下地壳岩性–成分–年龄结构关系，探讨华北北缘前寒武纪构造演化历史，剖析地壳复杂增生再造过程。

包体和地体麻粒岩相岩石是研究下地壳组成和演化的重要窗口。深部壳源包体在华北北缘有所发现，古老地体麻粒岩在华北北缘中部的高级杂岩体中都有广泛出露。我们在集宁-怀安地区的南高崖及阳原新生代玄武岩中发现了多种壳源岩石包体，这一地区属华北北缘中段不同构造单元划分的交汇部位，同时在该区及华北中部的恒山杂岩体中都有高级变质地体岩石出露，它们可为限定深部地壳组成、结构和过程以及华北克拉通前寒武纪演化提供重要约束。项目以集宁-怀安地区新发现的壳源岩石包体、恒山-宣化杂岩体中的深变质地体岩石及阳原地区的埃达克岩为对象，在翔实的野外工作和室内岩相学观察基础上，通过锆石U-Pb定年，和矿物化学、全岩元素及Sr-Nd-Pb同位素和锆石Hf-O同位素的配套研究，查明这些壳源岩石的物质组成、形成时代、岩石成因及变质演化。项目主要获得了以下几点认识：（1）恒山杂岩体的基性麻粒岩记录了顺时针的P-T轨迹，同时恒山杂岩体记录了太古代-古元古代多期次的构造岩浆-变质事件（2.5, 2.1和1.95-1.80 Ga）和两期地壳生长事件（2.8-2.7和2.1-2.0 Ga）；（2）西望山地区高压基性麻粒岩显示了高T/P型地热梯度 (775°C/Gpa–1500°C /Gpa)，表明了东西陆快碰撞后的大陆抬升导致了岩石圈的减薄和软流圈的上涌，可能为~1.85 Ga的高级变质作用提供了额外的热源；（3）南高崖地区的下地壳在时间和成分上都有分带性：下地壳上部为新太古代-古元古代中酸性岩石；中部为古元古代和新生代的中性含石榴石麻粒岩相混合层；深部为受到间歇性岩浆底侵强烈改造的中生代基性麻粒岩层；（4）阳原髫髻山组火山岩喷发于144.6 Ma，其具有高Sr/Y，La/Yb的埃达克岩地球化学特征，综合分析认为其可能为加厚下地壳部分熔融产物，而古太平洋板片的俯冲-后撤可能造成其喷发的主要因素。