内蒙中部苏左旗晚石炭世-早二叠世镁铁质侵入岩成因及构造意义

It has been a controversial issue that the late Paleozoic magmatism in Central Inner Mongolia was associated with the subduction of the Paleo Asian Ocean plate or the post-collisional extension related to the closure of the Paleo Asian Ocean. The Sonid Zuoqi region is located in the southwestern segment of the Xing’an-Airgin Sum Massif. Fortunately, the late Carboniferous to early Permian mafic intrusive rocks in this region provide insights to reveal the late Paleozoic tectonic nature and evolution of the Central Inner Mongolia. This project studies geological occurrence, rock association, geochronology, mineralogy, and element-isotope geochemistry of the late Carboniferous to early Permian mafic intrusive rocks in Sonid Zuoqi, together with the regional comparisons of the coeval igneous rock associations, sedimentary formations, metamorphic and deformation events in Central Inner Mongolia, to determine the spatial-temporal distribution of these late Carboniferous to early Permian mafic intrusive rocks, the nature of their magma sources and their tectonic settings. This program is of important significance for revealing not only the late Carboniferous to early Permian tectonic setting of the southwestern Xing’an-Airgin Sum Massif, but also the late Paleozoic tectonic evolutionary history of the central-western Xing’an-Mongolia Orogenic Belt (XMOB).

内蒙中部晚古生代火成岩的形成是与古亚洲洋洋壳的俯冲作用有关还是与古亚洲洋闭合后的伸展有关一直存在争议。苏尼特左旗地区位于兴安-艾力格庙地块西南部，该区晚石炭世-早二叠世镁铁质侵入岩的存在为解决内蒙中部晚古生代的构造属性与演化提供了可能。鉴于此，本项研究通过对苏左旗地区晚石炭世-早二叠世镁铁质侵入岩系统的野外地质调查、岩石组合、年代学、矿物学以及元素-同位素地球化学研究，结合区域上同期火成岩组合的时空变异、沉积建造和变质-变形事件分析，查明兴安-艾力格庙地块西南部晚石炭世-早二叠世镁铁质侵入岩的时空分布、岩浆源区性质及其形成的动力学背景。该项研究不仅对揭示兴安-艾力格庙地块西南部晚石炭世-早二叠世期间的构造属性，而且对揭示兴蒙造山带中西部晚古生代的构造演化历史均具有重要意义。

古亚洲洋构造体系晚古生代的演化历史一直是地学领域研究的热点与争论的焦点问题之一。本项目对兴安-艾力格庙地块西部苏左旗地区晚石炭世-二叠纪镁铁质侵入岩岩石组合、时空分布、地幔源区性质及形成的动力学背景展开了研究，同时对镁铁质侵入岩的围岩（早石炭世火山岩和元古宙变质沉积岩）和中元古代基底花岗岩开展了U-Pb年代学和地球化学研究。结果表明：1）苏左旗地区晚石炭世-二叠纪镁铁质侵入岩可划分为3期：316-310 Ma高铝堆晶角闪辉长岩和辉长闪长岩、306-295 Ma角闪石岩和角闪辉长岩、280-265 Ma橄榄辉长岩和钛铁矿辉长岩，从晚石炭世到二叠纪镁铁质侵入岩的分布有向研究区北部迁移的趋势；2）三期镁铁质侵入岩的岩浆源区岩石性质存在碳酸盐化橄榄岩、辉石岩、橄榄岩的明显差异，且地幔源区经历了从早期富集到280 Ma强烈亏损的转变；3）研究区晚石炭世到早二叠世15 Ma的岩浆间歇期及同位素的突变表明早二叠世（280 Ma之前）发生了显著的岩石圈减薄作用；4）兴安-艾力格庙地块西部早石炭世早期-中期处于持续伸展作用之下；5）兴安-艾力格庙地块西部1.4-0.8 Ga期间可能与劳亚大陆存在亲缘性，存在Columbia超大陆周缘俯冲作用的岩浆记录、Rodinia汇聚阶段格林威尔造山和垮塌伸展的沉积记录。上述研究成果不仅为兴安-艾力格庙地块西部二叠纪岩石圈伸展的深部动力学过程提供了重要制约，而且为内蒙中部晚古生代区域成矿物质来源及背景的研究也提供了重要的参考价值，同时对研究中亚造山带东段微陆块的起源及全球超大陆重建也具有重要的理论意义。