赤峰地区晚中生代火山岩锆石U-Pb年代学及地球化学特征

Zircon U-Pb Age and Geochemical Characteristics of the Late Mesozoic Volcanic Rocks in Chifeng area

内蒙古赤峰地区发育大面积的晚中生代火山岩,是我国东部巨型火山岩带的重要组成部分。锆石U-Pb定年结果显示,火山岩主要形成于晚侏罗世160~147 Ma和早白垩世132~129 Ma两个时期,早期以中酸性火山岩为主,晚期主要为酸性火山岩,局部夹少量的基性火山岩。晚侏罗世早期的安山岩SiO_2含量较低,MgO含量较高,可能是岩石圈地幔部分熔融的产物,流纹岩是安山质熔浆底侵导致下地壳发生部分熔融的产物。晚侏罗世晚期的流纹岩具有与A型花岗岩相似的地球化学特征,表明其形成于伸展构造背景下。早白垩世晚期的流纹岩属于钾玄岩系列,与同时代的玄武岩构成双峰式岩石组合,流纹岩来源于地壳的部分熔融。结合前人研究成果,认为赤峰地区晚中生代的两期火山活动都与蒙古–鄂霍次克缝合带的演化有关,它们分别形成于两次陆壳加厚之后的陆内伸展环境。

The Late Mesozoic volcanic rocks in the Chifeng area, Inner Mongolia, is a very important part of the huge volcanic rock belt in Eastern China. LA-ICP-MS zircon U-Pb dating results showed that most of the volcanic rocks were formed in two periods（160-147 Ma and 132-129 Ma）. The early period of volcanic rocks consist of intermediate-acid volcanic rocks, while those of the late period consist of mainly acidic volcanic rocks and minor locally distributed basic rocks. The early stage of the Late Jurassic andesites are characterized by low SiO_2 and high MgO, suggesting that the andesitic magma could be derived from partial melting of an enriched lithospheric mantle. The contemporary rhyolites could be possibly attributed to the partial melting of the lower crust under the heating of the emplacement of the andesite. The late stage of the Late Jurassic rhyolites are geochemically similar to A-type granite and formed under lithospheric extension. The late stage of the Early Cretaceous rhyolites belongs to shoshonite series and could be derived from partial melting of the crust, constituting bimodal volcanic rock association with the contemporary basalt. Combined with the previous studies, we suggest that the two periods of volcanic rocks should be related to the evolution of the Mongol-Okhotsk Orogeny, they respectively formed in the intracontinental extensional environment after the crust thickening in Middle Jurassic and the early stage of Early Cretaceous.