大兴安岭中北段塔尔气地区早白垩世白音高老组火山岩地球化学特征及意义

Geochemical characteristics and geological implication of volcanic rocks in Early Cretaceous Baiyingaolao Formation from Taerqi area,middle-north part of Da Hinggan mountains

塔尔气地区早白垩世白音高老组火山岩主要由流纹岩、流纹质（含角砾）岩屑晶屑凝灰岩及流纹质岩屑晶屑凝灰熔岩等组成。岩石地球化学研究表明,火山岩具有富铝（Al2O3含量13.37%-15.79%）、富碱（Na2O＋K2O含量6.10%-8.56%）、贫镁钙的特点;稀土元素丰度总量较高,轻、重稀土元素分馏明显[（La/Yb）N=3.72-17.9],且HREE有较明显的凹陷;微量元素以相对富集K、Rb、Ba、LREE等大离子亲石元素,而相对亏损Sr元素以及高场强元素Nb、Ta、Ti为特征。结合前人研究结果表明,塔尔气地区白音高老组火山岩来源于基性下地壳物质的部分熔融。结合火山岩所具有的A型流纹岩的特点,暗示为伸展构造体制下的产物,可能与蒙古—鄂霍茨克洋闭合后的伸展环境有关。

The volcanic rocks in Baiyingaolao Formation of Taerqi area are composed of rhyolite,lithic crystal tuff and rhyolitic lithic crystal tuffolava. The geochemical study suggests that the rocks are rich in Al2O3（ Al2O3=13. 37% - 15. 79%） and alkalis（ Na2 O ＋ K2 O = 6. 10% - 8. 56%）,while poor in magnesium and calcium. All samples are characterized by relatively high total REE contents,HREE and LREE have distinct fractionation [（ La /Yb）N= 3. 72 - 17. 9] while the HREE has an obvious sag. The trace element are characterized evidently by enrichment of LILEs（ K,Rb,Ba and LREE）,strong depletion of Sr and HFSEs（ Nb,Ta,Ti）. Combining with the previous studies,we suggest that the volcanic rocks in Baiyingaolao Formation from Taerqi area were originated from partial melting of mafic lower crustal materials,and the rocks have the characteristics of A-type rhyolite,implying they were formed in an extensional setting,which may be caused by the closure of Mongol--Okhotsk Ocean.