胶北高温高压麻粒岩地体变质-岩浆演化与早期构造体制

The Jiaobei high grade terrane is a unique place in the North China Craton where both high-temperature (HT) granulites (khondalite series) and high-pressure (HP) granulites outcrop besides the meta-ultramafic rocks. These three rock types show close overlapping and paragenetic relations in a high grade belt in the Jiaobei terrane. Preliminary studies show that all these three types of rocks experienced HP granulite facies metamorphism during peak metamorphic stage and decompression during retrograde metamorphic stage, and their protolith might be sedimentary rocks, mafic dykes and plutonic rocks, respectively. Zircon U-Pb geochronological data of these rocks from several preliminary studies roughly peak at ~2.5 Ga, ~1.95 Ga, ~1.85-1.83 Ga and ~1.8 Ga. Except the age of ~2.5 Ga, which can both be interpreted as representing protolith age and inheritance age yielded from zircons captured from country rocks, the latter three age peaks have been interpreted as representing HTHP granulite facies metamorphism, HTMP granulite facies metamorphism and amphibolite facies retrogression, respectively. Four episodes of magmatism associated with the granulite facies metamorphism have been roughly identified which are the ~2.5 Ga TTG gneisses, occurring as the country rocks of the HP granulites, the ~2.1-2.0 Ga granites and gabbro, the 1.85-1.83 Ga granite veins and gabbro and the ~1.8 Ga pegmatite. Among them, the magmatic rocks of the former two episodes all recorded granulite facies metamorphism, indicating their formation before the ~1.85-1.83 Ga granulite facies metamorphism, while those of the latter two episodes not whose petrogenesis might be closely related to the metamorphism. Previous studies also show that the PT conditions of the metamorphism and associated magmatism in the Jiaobei terrane both resemble those of the Phanerozoic orogen and differ from them greatly. These study results provide not only a solid basis for further researches, but also good chances to constrain the tectonic regime during early Precambrian. This research will lay special emphasis on the following three aspects: 1) petrogenesis of mafic HP granulites and meta-ultramafic rocks and sources of their protoliths; 2) detailed metamorphic evolution of the three kinds of metamorphic rocks (metamorphic stages and PT conditions, P-T paths, metamorphic geochronology, creep strength) and relations; 3) detailed relations between granulites and their country rocks, the gneissic rocks. Through comprehensive studies on tectonic implications of the three types of metamorphic rocks, metamorphic PT gradients, uplifting speed, multiple metamorphic and magmatic evolutionary processes and a thorough comparison with modern orogenic processes, this research will provide important evidence for the establishing of Paleoproterozoic tectonic regime.

胶北是华北唯一的高温泥质麻粒岩、高压基性麻粒岩和变质超镁铁质岩共同出露的地区，它们的分布呈现重叠与共生的岩带。研究显示三类岩石都经历了高压麻粒岩相峰期变质以及两期降压的退变质过程，锆石有25、19.5、18.5和18亿年的年龄峰值。相关的岩浆作用被识别出4期：麻粒岩的TTG围岩（25亿年）、21-20亿年花岗岩和辉长岩、18.5亿年脉状花岗岩和辉长岩和18亿年伟晶岩。胶北地体的变质作用和相关的岩浆作用，与现代造山过程有相似点又有很大差别。这些为该区的进一步研究提供了扎实基础，又为早前寒武纪的构造体制的突破提供了极好的机遇。本项目将在：高压基性麻粒岩与变超镁铁质岩的成因及联系、三类变质岩石的变质演化与联系、麻粒岩地体与围岩片麻岩地体的关系三个方面为重点，通过对三类变质岩分带的构造意义、温压梯度与抬升速率和多期变质-岩浆演化的过程、与现代造山过程的差异性比较研究，为早期构造体制的建立提供依据。

本项目在执行期间完成了任务书中的野外考察和实验室研究，开展了广泛的国际合作，并进行了华北中部北段和朝鲜半岛两个临近地区麻粒岩的对比研究，取得了预期成果。主要进展如下：1. 确定了变质的沉积岩系、高压基性麻粒岩和超镁铁质麻粒岩在峰期变质之前已经是具有成因联系的地质体。后两者以火成岩墙、岩脉和岩体状侵入到沉积岩层以及古老太古宙的围岩中，然后它们一起经受了高温高压麻粒岩相变质，并一起抬升到地表。胶北含超镁铁岩的麻粒岩地体代表了中朝克拉通古元古代抬升最深的下地壳地质体。研究还表明，在中朝克拉通没有发现与麻粒岩相变质作用有关的地幔岩石，这对于理解古元古代构造体制具有指示意义。2. 胶北的泥质麻粒岩研究进一步确立了峰期变质之前的进变质过程，并证实部分泥质麻粒岩经历了超高温-高压麻粒岩相变质，其峰期之后经历退变到角闪岩相的变质过程，这与中朝克拉通广泛存在的古元古代变质作用一致，为进一步理解早前寒武纪地质构造与动力学提供了证据。研究还对泥质麻粒岩的变质过程、变质年龄、抬升速率和温压计算等方法提出了改进意见。3. 研究确定在胶北存在>2.1 Ga钾质花岗岩-正长岩以及辉长岩，它们形成于伸展环境，推测代表了麻粒岩相变质之前的古元古代裂谷事件。而1.9-1.8 Ga的紫苏花岗岩以及壳熔的花岗岩与麻粒岩相变质作用有关，麻粒岩变质事件通常被解释为古老的俯冲-碰撞事件，而壳熔花岗岩是抬升过程中伴随降压-流体-升温影响的地质效应。4. 研究还对胶北麻粒岩地体的古老基底进行了研究，建立了地质事件与岩浆-变质年代学格架。这增强了对古元古代变质事件的地质背景的理解，也为理解中朝克拉通的早期陆壳形成演化提供了数据。项目在执行中发表了文章42篇，第一标注27篇（含SCI论文21篇）。召开了国际学术讨论会一次，在国际与全国性学术会议上宣读大会论文多次，并注意科普活动以及年轻人培养，毕业了6名博士研究生和出站4名博士后。