鲁西地区新太古代晚期构造岩浆热事件及构造环境

The tectonic setting of the North China Craton during the late Neoarchean is still controversial and has long been debated. Apart from voluminous ca. 2.5 Ga rocks, widespread >2.6 Ga rocks are also preserved in western Shandong Province and were not extensively modified in the late Paleoproterozoic. Therefore, western Shandong is an excellent area to study the tectonic setting and evolution of late Neoarchean rocks. The proposed project will investigate Archean igneous rocks in western Shandong to investigate the spatial distribution of >2.6 Ga rocks, their relationship with the younger, ca. 2.5 Ga assemblages, the temperature-pressure conditions of metamorphism, anatexis, and deformation at ca. 2.5 Ga. In addition, detailed field investigations and geochemical analyses also will be undertaken on ~2.5 Ga and older rocks to understand the relationship between different periods of magmatism and to speculate on their possible tectonic environment. SHRIMP U-Pb zircon analyses will be employed to define the age of rock formation, metamorphism, and deformation, whereas Hf- and O-in-zircon as well as whole-rock Nd isotopic data will be used to determine the magma source of igneous rocks and their petrogenesis. All these data will be integrated to develop a comprehensive model for the tectonic setting and evolution during the late Noearchean in western Shandong Province. This project will contribute substantially to better define the Neoarchean tectonic setting of igneous rocks in western Shandong and will also have great significance for reconstructing processes of crustal evolution during the late Neoarchean in the North China Craton and will thus be a contribution to understand the development of the continental crust in the early Earth.

华北克拉通新太古代晚期的构造环境一直存在争议。鲁西地区除大量2.5 Ga岩石之外，还发育较大规模>2.6 Ga地质体，基本未遭受古元古代末构造热事件影响，是研究新太古代晚期构造环境的理想地区。本项目主要研究内容包括：对鲁西地区>2.6 Ga地质体开展野外调查，确定其空间分布、与2.5 Ga地质体之间的关系及2.5 Ga时变质深熔作用、变质温压条件和变形特点；对2.5 Ga地质体进行详细野外考察和地球化学研究，确定不同期次岩浆作用的关系和成因联系，推断其可能的构造属性；利用锆石SHRIMP定年限定各岩石单元和变形作用的时代；利用锆石O、Hf同位素分析和全岩Sm-Nd分析限定各岩浆单元的源区。最终限定鲁西地区新太古代晚期可能的大地构造环境。这一研究不仅可以深化对鲁西地区早前寒武纪地质演化的认识，对华北克拉通新太古代晚期的构造属性和全球早期地壳演化模式也具有重要意义。

项目试图通过研究鲁西地区>2.6 Ga岩石的空间分布、与2.5 Ga地质体之间的关系以及2.5 Ga时变质深熔作用和变形特点，确定不同期次岩浆作用的形成时代、相互关系和成因联系，最终限定鲁西地区新太古代晚期可能的大地构造环境。在项目实施过程中，通过详细的野外地质考察和局部地区的地质填图工作，结合室内锆石SHRIMP U-Pb定年、锆石Hf同位素分析和全岩Nd同位素分析、主微量元素地球化学分析，获得了以下主要认识：（1）鲁西地区新太古代表壳岩主要分为新太古代早期（2.7-2.75 Ga）和新太古代晚期（2.5-2.56 Ga）两期，前者以玄武质岩石为主，后者以碎屑岩-火山碎屑岩为主；（2）新太古代早期表壳岩中广泛发育变形程度不同的枕状构造，构成目前华北克拉通已知最大规模的太古宙枕状玄武岩。其地球化学可以明显分为N-MORB和E-MORB两类，两者记录的是原始地幔的差异，表明在2.7 Ga鲁西地区已经存在明显的地幔分异。（3）在鲁西地区识别出2.6 Ga和2.5 Ga两期变质深熔作用，最终完成了鲁西地区的稳定化。并在2.5 Ga富钾花岗岩中广泛发育早期TTG和变玄武岩的包体。它们记录了鲁西地区新太古代地壳从早期以玄武质为主的洋壳和以TTG为主的不成熟陆壳向成熟陆壳的转变。（4）在济南七星台地区识别出新太古代晚期两期变形作用的叠加。稍早一期变形以右旋挤压作用为主，受到了后期左旋走滑为主的构造作用叠加。这种变形作用的叠加和碰撞造山带中的变形方式非常类似。（5）发现原划的大规模2.6 Ga上港奥长花岗岩主体为2.7 Ga英云闪长岩，2.6 Ga奥长花岗岩只是以较小规模侵入其中。2.7 Ga的英云闪长岩轻重稀土分异程度变化很大，且具有带状分布的特征。（6）鲁西南四海山-店子一带发现保存极好的太古宙末岩浆混合作用，其空间分布具有带状特征，记录了2.51 Ga鲁西地区存在的带状伸展作用。（7）综合各方证据以及与南非太古宙地质进行对比，我们指出在新太古代鲁西地区已经存在较大规模的带状构造，变形样式类似碰撞造山的特征，表明在2.5 Ga左右鲁西地区已经开始发育类似板块构造的机制。

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