大同三叠纪超钾质岩石的Mg同位素研究：示踪再循环碳酸盐对地幔的交代作用

幔源岩浆岩的Mg同位素研究，对于示踪与再循环碳酸盐有关的地幔交代作用具有重要意义。然而，Mg同位素组成还受到岩浆分异演化过程中多个因素的影响，此类地幔交代作用还需进一步研究的论证。超钾质岩石、尤其是华北克拉通北缘三叠纪的大同超钾质岩石由于其独特的地球化学特征，非常适合开展此类研究。本项目拟在全岩地球化学、Sr-Nd-O同位素研究及锆石微区分析的基础上，重点对大同超钾质岩石及与之共生的火成碳酸岩开展详细的全岩、单矿物等的Mg同位素研究，并将其与区域发现的地幔捕虏体进行对比，以探索与古亚洲洋俯冲有关的再循环碳酸盐对地幔的交代改造，进而从超钾质岩石的角度对此类地幔交代作用予以更好地认识。此项研究对于揭示硅酸盐与碳酸盐熔体的液态不混溶在岩石成因中的作用、以及这一过程中是否会发生Mg同位素分馏皆具有十分重要的意义，也可以检验华北克拉通北缘是否存在由古亚洲洋俯冲引起的再循环碳酸盐交代作用。

Magnesium (Mg) isotopes of igneous rocks have been widely used to trace mantle metasomatism caused by recycled carbonates. However, more studies are still needed in that it is suggested the Mg isotopic composition may also be controlled by some previously unrecognized factors in magmatic differentiation and evolution. Meanwhile, it is also argued whether the mantle in the northern NCC has been affected by recycled sedimentary carbonate via the subduction of Paleo-Asian oceanic slab, besides the frequently suggested contribution from the subducted Pacific oceanic crust. Ultrapotassic rocks, especially the Triassic Datong ultrapotassic rocks in the northern North China Craton (NCC), are proposed to be the best candidate in the study of sedimentary-derived carbonated mantle metasomatism by Mg isotopes analyses. For this reason, the Triassic Datong ultrapotassic rocks and associated carbonatites are chosen to conduct both whole rock and mineral Mg isotopes study in order to reveal the influence of sediments-derived carbonated metasomatism related to the subduction of Paleo-Asian oceanic slab. Whole rock geochemistry, Sr-Nd-O isotopes and zircon compositions will also be analyzed and discussed together. The study has great potential to verify the existence of recycled carbonates metasomatism in the deep mantle because of the subduction of Paleo-Asian oceanic slab and to clarify its role in the petrogenesis of ultrapotassic rocks, if the Datong ultrapotassic rocks are combined with mantle xenoliths hosted in the Cenozoic basalt nearby. What’s more, the study will also explore whether the ultrapotassic and carbonatitic magms are formed by liquid immiscibility from a common parental magma. If this is indeed the case, the behavior of Mg isotopes fractionation during this process will also be discussed.