大兴安岭北段根河地区早白垩世火山杂岩特征及成因探讨

Characteristics of Early Cretaceous volcanic complexes in the Genhe area in the northern segment of the Da Hinggan Mountains:with a discussion of their genesis

大兴安岭北段根河地区早白垩世火山杂岩按岩石及其地球化学特征,可划分为3类:第Ⅰ类为流纹质岩石,富Si、碱,低Al、Ti,Na2O/K2O<1,具有明显的负Eu异常(δEu=0.27～0.59),富Rb,贫Sr、Ba;第Ⅱ类为粗面英安质岩石,富Al、Ti,Na2O/K2O>1,具弱的负Eu异常(δEu=0.65～0.95),富Sr、Ba,贫Rb;研究认为第Ⅰ、Ⅱ类岩石产于挤压隆升相伴的拉张至造山后伸展构造环境。第Ⅲ类由安粗岩、歪长粗面岩组成,富Ti、Al,Na2O/K2O≥1.1,Eu无异常(δEu=0.93～1.06),Th/Ta、Ta/Hf值显示大陆板内伸展环境,其形成可能与玄武质岩浆的底侵作用有关。

Early Cretaceous volcanic complexes in the Genhe area in the northern segment of the Da Hinggan Mountains may be divided into three types according to their petrological and geochemical characteristics. Type Ⅰ is composed of rhyolitic rocks, which are characterized by higher Si and alkali contents and lower A1 and Ti contents, Na2O/K2O〈1, a pronounced negative Eu anomaly （δEu=0.27 - 0.59）, enrichment in Rb and depletion in Sr and Ba. Type Ⅱ consists of trachydacitic rocks, which are enriched A1 and Ti, with Na2O/K2O〉1 and a weak negative Eu anomaly （δEu=0.65 - 0.95）, enriched in Sr and Ba and poor in Rb. Types I and Ⅱ probably formed in an extensional to postorogenic extensional environment, accompanied by compressional uplift. Type Ⅲ is composed of latite and benmoreite, which are enriched in A1 and Ti, with Na2O/K2O≥1.1 and no negative Eu anomaly （δEu=0.93 - 1.06）, and Th/Ta and Ta/Hf ratios indicate a continental intraplate extensional environment. Its formation is probably related to underplating of basaltic magma.