内蒙古中部红格尔地区玛尼吐组钾玄质火山岩锆石U-Pb年龄、地球化学及其地质意义

Zircon U-Pb age and geochemistry of shoshonitic rocks from the Manitu Formation in the Honggeer area, central Inner Mongolia and their geological significance

内蒙古中部红格尔地区玛尼吐组火山岩主要由粗面英安岩、粗面岩、粗安岩和安山岩组成。LA-ICP-MS锆石U-Pb方法获得该火山岩的形成年龄为133~137 Ma,表明其形成时代为早白垩世。岩石富碱(Na_2O+K_2O=6.22%~8.26%)、高K_2O(2.49%~4.58%)、低TFe_2O_3(3.76%~6.53%)、贫Ti O_2(0.92%~1.61%)、高Al_2O_3(14.27%~15.88%)。富集Rb、Ba、Th、U等大离子亲石元素和轻稀土元素,相对亏损Nb、Ta和Ti等高场强元素,且在微量元素蛛网图上呈现明显的负异常。ε_(Nd)(t)为正值(+0.40^+1.64),t_(DM)变化于694~767Ma之间;有较高的放射性Pb同位素组成,(^(206)Pb/^(204)Pb)_i为18.180~18.498,(^(207)Pb/^(204)Pb)_i为15.503~15.533,(^(208)Pb/^(204)Pb)_i为37.914~38.042。主元素、微量元素地球化学和Sr-Nd-Pb同位素组成显示红格尔地区玛尼吐组火山岩为钾玄质岩,钾玄质岩浆上升演化过程中经历了铁镁矿物、斜长石、钛铁矿、磁铁矿和磷灰石的分离结晶作用,基本未受地壳物质混染。玛尼吐组钾玄质岩源于古亚洲洋俯冲板片释放流体交代的富集岩石圈地幔,是在伸展体制下富集岩石圈地幔减压部分熔融的产物。这种伸展体制是早白垩世蒙古-鄂霍茨克洋闭合后加厚岩石圈重力跨塌引起的,反映了造山后板内伸展环境。

Volcanic rocks of the Manitu Formation from the Honggeer area, central Inner Mongolia are mainly composed of trachydacite, trachyte, trachyandesite and andesite. The results of LA-ICP-MS Zircon U-Pb dating indicate that these volcanic rocks were formed during the Early Cretaceous time with the ages of 133-137 Ma. The volcanic rocks are high in alkali （Na2O ＋ K2O = 6.22%-8.26%）, potassium （K2O = 2.49%-4.58%） and aluminum （Al2O3 = 14.27%-15.88%）, whereas they are low in iron （total Fe2O3 = 3.76%-6.53%） and titanium （TiO2 = 0.92%-1.61%）. These volcanic rocks are obviously enriched in LILEs such as Rb, Ba, Th, U and LREEs, and depleted in HFSEs such as Nb, Ta and Ti with obvious negative anomalies. Their Sr-Nd-Pb isotopic compositions show positive εNd（t） （＋0.40- ＋1.64）, and low tDM （（694-767） Ma）. The geochemical characteristics of these volcanic rocks suggest that they belong to the shoshonitic series, and were likely generated from the partial melting of an enriched lithospheric mantle which was metasomatized by fluids releasing from subducted slabs during the closure of the Paleo-Asian oceans. Elemental and isotopic variations reveal that fractional crystallization with the removal of ferromagnesian minerals, plagioclase, ilmenite, magnetite and apatite played an important role in the evolution of magma, and the shoshonitic magma was not affected by crustal contamination as it passed through the continental crust. These shoshonitic rocks were formed in an intraplate extensional tectonic setting resulting from the gravitational collapse of the orogenically thickened lithosphere at the Early Cretaceous, which was related with the collision after closure of the Mongol-Okhotsk Ocean.