Discovery of Permian crustal carbonatite in the northwestern margin of Tarim Basin and its geological significance

Abstract

碳酸岩是自然界较为特殊的一类火成岩，由于其在上侵过程中不易受到地壳物质的混染，可以很好地保留源区原始特征，因而在揭示岩浆源区属性和地球深部碳循环等方面具有重要的研究价值。塔里木盆地西北缘阿克苏-乌什南部地区发育多条侵位到新元古界南华系中的火成碳酸岩岩墙或岩脉。这些岩脉的锆石U-Pb测年结果显示出多个年龄峰值，其中最年轻一组（3颗）岩浆锆石的谐和加权年龄为272±4Ma，另外还有400Ma、450Ma、790Ma、850Ma等年龄峰，而这些锆石可能均为碳酸岩岩浆捕获成因。结合区域上发育大量早二叠世辉绿岩岩床及岩墙等侵入事件，推测这些碳酸岩很可能形成于早二叠世（~270Ma）。碳酸岩主要由方解石（90%以上）构成，并含少量的白云石、重晶石、天青石、赤铁矿等其它矿物，可见方解石与白云石之间明显的出溶结构以及野外宏观上碳酸岩包裹围岩等现象。元素地球化学分析显示，碳酸岩CaO含量高（44.40%~50.40%），SiO₂含量低（1.83%~7.97%）；稀土元素总量很低（ΣREE=21.67×10^-6~91.32×10^-6），轻重稀土分异明显（LREE/HREE=3.90~8.40），具有中等负铕异常（δEu=0.61~0.68）和中等至弱负铈异常（δCe=0.59~0.98）；大离子亲石元素Sr、Rb、Ba等相对富集，Nb、Ta、Ti等高场强元素相对亏损；稳定同位素δ¹³C_V-PDB值高（-2.69‰~-2.93‰）、δ¹⁸O_V-SMOW值高（17.33‰~17.86‰）。这些特征与壳源碳酸岩特征相似，而与幔源碳酸岩差别较大。Sr-Nd同位素结果显示高的（⁸⁷Sr/⁸⁶Sr）_i值（0.710106~0.710558），低的ε_Nd（t）值（-8.46~-12.80），也进一步证实碳酸岩为地壳来源。综合分析认为这些碳酸岩很可能是塔里木二叠纪大火成岩省晚期岩浆事件的组成部分，系由塔里木西北缘地幔柱减压熔融后形成高温的基性岩浆向上侵位运移时，导致下地壳的碳酸盐储库部分熔融形成碳酸岩岩浆，并沿着断裂快速向上侵位形成。本文研究不仅丰富了塔里木大火成岩省的岩石组成认识，而且有助于增进对二叠纪深部复杂壳幔作用过程的理解。;Igneous carbonatites is a kind of special igneous rock in nature. Because they are not easily contaminated by crustal materials in the process of upwelling, they can well retain the original characteristics of the magma source. Therefore, they have important research value in revealing the properties of the magma source and the carbon cycle of the deep earth. Several carbonatite dikes or dykes are found intruded into the Neoproterozoic Nanhua System in the southern Aksu-Wushi area, the northwestern margin of Tarim Basin. Zircon U-Pb geochronology dating of these carbonatites yield multiple peak ages, including a youngest one at 272±4Ma obtained by three zircons, and other ones at 400Ma, 450Ma, 790Ma, 850Ma, respectively. However, all these zircons were probably captured by carbonate magma. Considering the intrusive events of massive Early Permian diabase sills and dikes in the region, we speculate that these carbonatite dikes were probably formed in the Early Permian (~270Ma). The carbonatites are mainly composed of calcite (over 90%) and a small amount of dolomite, barite, celestite, hematite and other minerals. In addition, the carbonatites have obvious exsolution structures between calcite and dolomite, and the country rock breccias are found wrapped by the carbonatites in the field. The element geochemistry analysis results show that carbonatites have high CaO content (44.40%~50.40%), low SiO₂ content (1.83%~7.97%), extremely low total rare earth elements content (21.67×10^-6~91.32×10^-6) with obvious differentiation of LREE and HREE (LREE/HREE=3.90~8.40) and weak negative Eu and Ce anomalies (δEu=0.61~0.68; δCe=0.59~0.98). And the large ion lithophile elements like Sr, Rb, Ba are relatively enriched, and high field-strength elements such as Nb, Ta, Ti are relatively depleted. The stable isotope of δ¹³C_V-PDB and δ¹⁸O_V-SMOW are high (-2.69~-2.93‰ and 17.33~17.86‰, respectively). All these characteristics are similar to the crustal carbonatite, but quite different from the mantle carbonatite. The Sr-Nd isotopic results of high (⁸⁷Sr/⁸⁶Sr)_i values (0.710106~0.710558) and low ε_Nd(t) values (-8.46~-12.80) also confirm that the Aksu carbonatites are derived from the crust. After comprehensive analysis, we propose that these carbonatites are probably the components of the late magmatic event in the Permian Tarim Large Igneous Province. When the mantle plume in the northwestern margin of Tarim was decompressed, it would form high-temperature basic magma and emplaced upward. These basic magma would heated the carbonate reservoir in the lower crust and partially melted it to form carbonatites magma, which was rapidly emplaced upward along the extension fault. This study not only enriched the petrological compositions of the Permian Tarim Large Igneous Province, but also promoted the understanding of the complex crust-mantle interaction processes in the deep area of Permian Tarim Basin.