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

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

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),t_(DM)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。

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 SiO_2,Al_2O_3,K_2O,and low CaO,MgO. Their high Sr/Y,La/Yb,and low Y,Yb values show an adakitic geochemical affinity. The results of K_2O/Na_2O 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 t_（DM）（ 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.