新疆克拉玛依高硅碱性玄武岩中碳酸盐矿物成因及其对岩石成因的约束

The identification of the subducted recycled carbonate in the mantle source of the high-Si alkali basalts can significantly contribute to the study of the deep carbon recycle. There are two different types of high-Si alkali basalts observed in Karamay Jurassic strata, i.e., siderite/calcite-bearing olivine basalt and carbonate minerals-free basalt. In this program, we will focus on the siderite/calcite-bearing alkali olivine basalt and investigate the genesis of the carbonate minerals (i.e., siderite and calcite) based on the detail petrographic observations and the associated minerals chemistry by EMPA and LA-ICP-MS. Studies on the melt inclusions enclosed by olivine grains in the alkali basalts could give us an opportunity to understand the physical and geochemical information on the parental melts, while the whole-rock Mg-Zn-C-Sr-Nd isotope compositions might throw light on the source of the magmatic carbonate minerals in the Karamay alkali basalts. Based on these results, we will further discuss whether the subducted recycled carbonates were involved into the mantle source of the alkali basalts. Additionally, the co-occurrence of some carbonate mineral-free alkali basalts and the siderite/calcite-bearing olivine basalts in the same outcrops in Karamay provide us a rare but perfect opportunity to reveal the genetic links between these two types of high-Si alkali basalt by comparing their geochemical and isotopic compositions. Based on that, we can unravel whether the subducted recycled carbonates are essential to the formation of alkali basalts. Our studies in this program will not only provide crucial constraints to the petrogenesis of the alkali basalts, but also provide important clues to the research of deep carbon recycling.

高硅碱性玄武岩地幔源区俯冲再循环沉积碳酸盐的识别对深部碳循环的研究具有重要的意义。克拉玛依侏罗纪地层中发育两种类型的高硅碱性玄武岩：含原生碳酸盐矿物的橄榄玄武岩和不含碳酸盐矿物的玄武岩。本项目以克拉玛依含碳酸盐矿物碱性橄榄玄武岩为研究对象，通过详细的岩石学、矿物化学（EMPA和LA-ICP-MS）研究揭示碳酸盐矿物的成因；利用橄榄石中熔融包裹体恢复玄武岩母岩浆成分，并结合全岩Mg-Zn-C-Sr-Nd同位素以及前人高温高压实验成果，约束高硅碱性橄榄玄武岩中碳的来源，揭示是否有俯冲再循环沉积碳酸盐的参与；通过对比含碳酸盐矿物橄榄玄武岩与同一露头发育的不含碳酸盐矿物玄武岩的地球化学特征，查明两类岩石成因上的区别和联系，探讨俯冲再循环沉积碳酸盐是否为高硅碱性玄武岩形成的必要条件，为阐明高硅碱性玄武岩的岩石成因提供一个典型案例，同时也为全球碳循环的向下部分提供新的信息。

板内玄武岩具有较大的成分变化范围，通常被划分为碱性玄武岩和拉斑玄武岩，且在TAS图解上，两者表现出成分连续性。然而长期以来，这两类玄武岩之间的成分连续性及其内在的成因联系仍存在较大争议。位于中亚造山带南部西准噶尔地体的早侏罗世克拉玛依玄武岩是一个形成于稳定板内环境的小体积玄武岩。岩石具有无斑隐晶质结构，暗示其岩石成分更接近初始幔源熔体成分。不同于传统意义上根据化学和岩相学上划分的典型碱性玄武岩和拉斑玄武岩，克拉玛依玄武岩表现出从碱性向拉斑玄武岩过渡的性质，因此可为理解板内玄武岩的地球化学连续性提供重要约束。与中国东部碱性玄武岩相似，克拉玛依玄武岩具有相对正常地幔较轻的Mg和较重的Zn同位素特征（δ26Mg = -0.54‰~-0.34‰，δ66Zn = 0.36~0.46‰），暗示形成中具有俯冲再循环沉积碳酸盐的参与。然而与此同时，其具有相对较低的(87Sr/86Sr)t 比值（0.7047–0.7051），正的 εNd(t) 值 (3.3–4.2) 和 Nb-Ta-Ti-Zr-Hf 的正异常，反映其地幔源区并不存在碳酸盐的参与。为了解释这一矛盾，我们提出在板片俯冲过程中，在压力>5 GPa条件下，大洋沉积碳酸盐和硅酸盐洋壳（MORB)之间发生脱碳反应，该过程将碳酸盐中的C以CO2形式释放进入地幔，而其中的Mg和轻Mg同位素，以及Zn和重Zn同位素则被转移至深俯冲的榴辉岩中。此外，结合样品地球化学特征和研究区地质特征，通过和前人熔体-橄榄岩反应实验结果进行对比，我们认为克拉玛依碱性-拉斑玄武岩主要形成于脱碳榴辉岩部分熔融的熔体在上升过程中与周围亚固相地幔橄榄岩反应形成。本研究不仅反映了俯冲洋壳可以作为板内碱性玄武岩的主要源区，而且揭示了板内碱性玄武岩还可由拉斑质熔体通过与地幔橄榄岩之间的反应演化形成。