内蒙古中部红格尔地区白音高老组流纹岩地球化学特征及成因

内蒙古中部红格尔地区白音高老组主要由流纹岩组成, 含少量英安岩和粗安岩。流纹岩具有高硅、铝和钾,低钙镁的特点；富集大离子亲石元素K、Rb、Th和U, 高场强元素Nb、Ta、Ti和P强烈亏损, 具负异常；高w(Sr)/w(Y)和w(La)/w(Yb), 低w(Y)和w(Yb)。稀土元素总量较低, 轻重稀土元素分馏强烈, 具有中等的铕负异常。εNd(t)值为正值(+088~+266), tDM变化于5883~7164 Ma之间。主量、微量元素地球化学和Sr Nd Pb同位素组成显示红格尔地区白音高老组流纹岩为高钾钙碱性C型埃达克质岩, 其形成与蒙古—鄂霍茨克洋的闭合碰撞有关, 形成于早白垩世造山后加厚岩石圈跨塌阶段的板内伸展环境, 是加厚下地壳部分熔融的产物。岩浆源区部分熔融残留相为石榴子石, 在岩浆上升演化过程中经历了斜长石、钛铁矿和磷灰石的分离结晶作用。

Geochemisty and Petrogenesis of the Rhyolites from the Baiyingaolao Formation in the Honggeer Area,Central Inner Mongolia

The Baiyingaolao Formation in the Honggeer area of central Inner Mongolia is mainly composed of rhyolites, with minor dacite and latite. The rhyolites are characterized by high SiO2, Al2O3, K2O, and low CaO, MgO. Their high Sr/Y, La/Yb, and low Y, Yb values show an adakitic geochemical affinity. The results of K2O/Na2O and A/CNK reveal that the rhyolites belong to a peraluminous high K calc alkalic type. The rocks are obviously enriched in LILEs such as K, Rb, Th, U, and depleted in HFSEs such as Nb, Ta, Ti and P with obvious negative anomalies. The total of REE content is relatively low, with enrichment in LREE and medium negative Eu anomaly. The Sr Nd Pb isotopic compositions show positive εNd(t)(+088 to +266), and low tDM(t) (5883 to 7164 Ma). The above geochemical characteristics reveal that the rhyolites of the Baiyingaolao Formation in the Honggeer area belong to high K calc alkine and C type adakitic rocks, and they were the products of partial melting of the thickened continental crust. The rocks were formed in an intraplate extensional tectonic setting after post orogenic thickened lithospheric gravitational collapse at Early Cretaceous, which was related with the collision after closure of Mongol Okhotsk Ocean. Garnet was the residual phase during the partial melting, and fractional crystallization with the removal of plagioclase, ilmenite and apatite played an important role in magma evolution.