塔里木盆地东南缘安南坝地区透辉石碳酸岩成因及其地质意义

Petrogenesis and Geological Significance of Diopside Carbonatite in Annanba Area,the Southeastern Margin of Tarim Basin,China

碳酸岩作为自然界出露很少但具有重要地质意义的岩浆岩之一,通常被认为是幔源成因,而少有壳源成因的报道。塔里木盆地东南缘安南坝地区出露的透辉石碳酸岩呈不规则状、脉状侵入围岩之中,岩石内部可见围岩捕虏体,并具致密块状构造、细晶粒状结构,表明其为火成碳酸岩而非沉积碳酸盐岩。岩石地球化学特征显示,透辉石碳酸岩富CaO(含量(质量分数,下同)为24.63%~33.20%)、SiO_(2)(33.11%~40.07%),贫MgO(2.25%~3.21%)、FeO^(T)(1.38%~2.03%),稀土元素总含量((45.44~91.47)×10^(-6))和轻稀土元素总含量((42.83~86.06)×10^(-6))显著偏低,相对富集Rb、Ba、Th、Sr等大离子亲石元素,亏损Nb、Ta、Zr、Hf等高场强元素,明显不同于幔源成因碳酸岩,而与壳源成因碳酸岩类似。透辉石碳酸岩中锆石发育振荡环带结构,具明显的Ce正异常、弱Eu负异常及低Th/U值(平均值为0.24),指示其为变质深熔锆石。碳酸岩锆石U-Pb-Lu-Hf同位素研究表明,锆石Hf同位素模式年龄具有约2.59 Ga的年龄峰值,岩石成岩年龄为(1910±13)Ma,分别与米兰岩群形成时代和麻粒岩相变质时代一致。结合前人研究成果综合分析认为,安南坝地区透辉石碳酸岩可能是麻粒岩相变质条件下米兰岩群含透辉石大理岩深熔作用的产物,为Columbia超大陆聚合事件在塔里木盆地东南缘的响应。

Carbonatites,as one of the magmatic rocks with rare outcrop and geologically important,are conventionally considered as mantle-derived rocks,while crust-derived carbonatite has been rarely reported.The diopside carbonatites exposed in Annanba area,the southeastern margin of Tarim Basin are irregularly and vein-like intruded into the surrounding rocks.The surrounding rocks xenolith can be seen inside the rocks with dense massive structure and fine-grained structure,indicating that they are carbonatites rather than carbonate rocks.The lithogeochemistry characteristics show that the diopside carbonatites are rich in CaO(the content is 24.63%-33.20%)and SiO_(2)(33.11%-40.07%),while poor in MgO(2.25%-3.21%)and FeO^(T)(1.38%-2.03%),the contents of REE((45.44-91.47)×10^(-6))and LREE((42.83-86.06)×10^(-6))are significantly low.The LILEs(such as Rb,Ba,Th and Sr)are relatively enriched,and HFSEs(such as Nb,Ta,Zr and Hf)are relatively depleted.All these characteristics point to the crust-derived carbonatite,but not the mantle-derived carbonatite.Zircons from diopside carbonatites have oscillatory zoning,obvious positive Ce anomaly,weak negative Eu anomaly and low Th/U ratios,indicating that they are anatectic.Zircon U-Pb-Lu-Hf isotopes show that Hf model age has peak at~2.59 Ga,and the diagenetic age of diopside carbonatites is(1910±13)Ma,which are consistent with the formation age and the granulite-facies metamorphic age of Milan Group,respectively.Combined with previous researches,it is concluded that the diopside carbonatites in Annanba area may be the product of anatexis of diopside-bearing marble in Milan Group under granulite-facies metamorphism,and is the response of Columbia supercontinent assenbly event in the southeastern margin of Tarim Basin.