内蒙古中部苏尼特左旗地区阿巴嘎组火山岩地球化学特征及成因

Geochemistry and petrogenesis of the volcanic rocks from the Abaga Formation in Sonid Left Banner,central Inner Mongolia

内蒙古中部苏尼特左旗地区阿巴嘎组火山岩主要由安山岩组成。岩石富碱、高钾和铝、低镁。富集大离子亲石元素(LILEs)Rb、Ba、U、K和轻稀土元素(LREE),相对亏损高场强元素(HFSEs),具明显的Nb、Ta和Ti负异常和Pb正异常。稀土元素总量较高,轻、重稀土元素分馏强烈且属轻稀土元素富集型,具弱负Eu异常。主量、微量元素地球化学特征表明,阿巴嘎组安山岩为钾质火山岩,岩浆上升演化过程中经历了斜长石和铁镁矿物的分离结晶作用,无地壳物质的混染。阿巴嘎组钾质火山岩的形成与新生代太平洋板片俯冲密切相关,其岩浆来源于滞留的俯冲太平洋板片释放流体交代的富集陆下岩石圈地幔,是在板内伸展体制下含金云母石榴子石二辉橄榄岩低程度部分熔融的产物。这种板内伸展体制可能是新生代滞留于地幔过渡带中的太平洋板片俯冲后撤引起的。

The volcanic rocks of the Abaga Formation in Sonid Left Banner area of central Inner Mongolia are mainly composed of andesites. These andesites have high alkali, potassium and aluminum, but low magnesium, and are obviously enriched in LILEs such as Rb, Ba, U, K and LREE, and depleted in HFSEs such as Nb, Ta, Ti with obvious negative anomalies. They have high ∑REE values, and show intense REE fractionation with LREE enrichment and weak negative Eu anomalies. Their geochemical characteristics suggest that they belong to potassic volcanic rocks, and were likely generated from a small degree of partial melting of phlogopite-bearing garnet lherzolite in an enriched sub-lithospheric mantle. The enriched sub-lithospheric mantle was metasomatized by fluids released from the stagnant Pacific slab in the mantle transition zone. Geochemical features also reveal that fractional crystallization with the removal of plagioclase and ferromagnesian minerals played an important role in the evolution, and the potassic magma was not affected by crustal contamination as it passed through the thick continental crust. These potassic rocks were formed in an intraplate extensional tectonic setting resulting from the rollback of the stagnant Pacific slab in the mantle transition zone.