大兴安岭十八站—韩家园地区晚中生代火山岩年龄、地球化学特征及其构造意义

Zircon U-Pb ages and geochemical characteristics of Late Mesozoic volcanic rocks from Shibazhan-Hanjiayanzi area of Da Hinggan Mountains and their tectonic significance

大兴安岭十八站—韩家园地区发育晚中生代基性-中酸性火山岩。选取粗面安山岩、粗面岩、流纹岩进行年代学和岩石地球化学研究。粗面安山岩LA-ICP-MS锆石U-Pb年龄为125.2±0.9 Ma,为早白垩世火山作用的产物。岩石地球化学特征表明,中酸性火山岩属于高钾钙碱性-钾玄岩系列。岩石稀土元素总量∑REE=121.42×10^(-6)~154.43×10^(-6),轻、重稀土元素分异明显(La/Yb)_N=19.25~31.98,在稀土元素配分图上显示右倾型,除一个流纹岩样品具显著负Eu异常外,多数样品无明显Eu负异常(δEu=0.45~0.90)。岩石具弱富集大离子亲石元素Ba、Sr,明显亏损高场强元素Nb、Ta、Ti的特征。结合区域资料,认为十八站—韩家园地区早白垩世中酸性火山岩形成于蒙古-鄂霍茨克洋闭合机制下后碰撞伸展背景。粗面安山岩、粗面岩和流纹岩系同源岩浆部分熔融与结晶分异作用的结果,岩浆来源于受俯冲流体交代的富集岩石圈地幔。

There exist a set of basic to intermediate-acid volcanic rocks in Shibazhan-Hanjiayuan area of Da Hinggan Mountains.In this paper,chronology and petrogeochemistry of trachyandensite,trachyte and rhyolite were discussed.LA-ICP-MS zircon U-Pb dating of trachyandensite yielded an age of 125.2±0.9Ma,revealing that it was a product of the Early Cretaceous volcanism.Geo-chemical characteristics suggest this set of volcanic rocks should belong to high-K calc-alkaline to shoshonite series.Volcanic rocks possess characteristics of REE composition as follows:ΣREE are 121.42×10-6~154.43×10-6,the fractionation between LREE and HREE is obvious with(La/Yb)N ranging from 19.25 to 31.98,the REE patterns show rightward inclining,and most samples present a weak Eu anomaly(δEu=0.45~0.90)except one rhyolite.The trace elements geochemistry is characterized by weak enrichment of LILE(Ba,Sr),accompanied by prominent negative HFSE(Nb,Ta,Ti).Combining previous regional achievements with this study,the authors have reached the conclusion that the Early Cretaceous intermediate-acid volcanic rocks were formed in a post-collision extension environment,which was related to closure of Mongolia-Okhotsk Ocean.Trachyandensite,trachyte and rhyolite were formed by partial melting and fractional crystallization of comagma,which was derived from an enrichment of lithospheric mantle and probably experienced metasomatism of subduction fluid.