大兴安岭嫩江地区中生代双峰式火山岩锆石U-Pb定年、地球化学特征及其地质意义

Zircon U-Pb chronology and geochemistry of Mesozoic bimodal volcanic rocks from Nenjiang area in Da Hinggan Mountains and their tectonic implications

本文对嫩江地区中生代双峰式火山岩进行了锆石LA-ICP-MS U-Pb年代学和岩石地球化学研究。测年结果显示嫩江地区中生代双峰式火山岩形成于127.5 Ma的早白垩世晚期。岩石地球化学研究表明,早白垩世晚期火山岩具有双峰式组合特点,基性端员富碱,富含轻稀土元素和大离子亲石元素,亏损重稀土元素和高场强元素(Nb、Ta、Ti、Y),形成于富集的岩石圈地幔的部分熔融和分离结晶作用,形成的过程可能含有少量的陆壳混染。酸性端员显示A型流纹岩的特征,为幔源岩浆底侵,使中下地壳岩石发生部分熔融的成因。双峰式火山岩组合的存在暗示其形成于陆内拉张的构造环境。结合区域上中生代火山岩的空间展布特征,嫩江地区早白垩世晚期双峰式火山岩的形成应与太平洋板块向欧亚大陆的俯冲作用有关。

LA-ICP-MS zircon U-Pb ages and geochemical data of the Mesozoic bimodal volcanic rocks from Nenjiang in the Da Hinggan Mountains were studied in this paper.U-Pb ages of zircons show that the Mesozoic bimodal volcanic rocks were formed at the late stage of the Early Cretaceous （about 127.5 Ma）.The Baiyingaolao Formation acidic volcanic rocks are composed of rhyolites and rhyolitic crystal clastic tuffs.The Meletu Formation basaltic volcanic rocks are composed of trachybasalt,basaltic andesite and basaltic trachyte.Geochemical studies suggest that the late Early Cretaceous volcanic rocks display a bimodal volcanic rock association.The Meletu volcanic rocks belong to alkaline series,have low silicon and are rich in alkalis.The chondrite-normalized REE patterns indicate that the volcanic rocks are enriched in light rare earth elements （LREE） with significant fractionation between HREE and LREE and week Eu anomalies.The trace element geochemistry is characterized by enrichment of large ionic lithophile elements （K,Rb,Ba） and depletion of high field strength elements （Nb,Ta）.These characteristics indicate that the volcanic rocks originated from the remelting and fractionation of metasomatized enriched subcontinental lithospheric mantle,probably mixed with a small amount of crustal materials.The alkaline basalt series was formed under the condition of an intraplate setting,most likely continental rift.The Baiyingaolao volcanic rocks are rich in silicon and alkalis,poor in calcium and magnesium and have characteristic FeOT/MgO ratio.The chondrite-normalized REE patterns indicate that the volcanic rocks are enriched in light rare earth elements （LREE） with significant fractionation between HREE and LREE and strong negative Eu anomalies.The trace element geochemistry is characterized by enrichment of large ionic lithophile elements （K,Rb） and LREE,and depletion of high field strength elements （Nb,Ta,P,Ti）.The trace element geochemistry characteristics of rhyolites show an affinity with the A-type granite,suggesting that the volcanic rocks should have been formed in an extensional setting.These characteristics imply that the low-Sr rhyolitic magma originated from enriched mantle plume that intruded into the lower crust,and made it partially melted.The bimodal volcanic rocks were probably formed in an extensional setting.Combined with spatial and temporal distribution of the Mesozoic volcanic rocks in Northeast China,it is suggested that the formation of the late Early Cretaceous bimodal volcanic rocks might be attributed to subduction of the Paleo-Pacific plate beneath the Eurasian continent.