大兴安岭中段索伦地区白音高老组火山岩锆石U-Pb年龄、地球化学特征及构造意义

Zircon U-Pb ages and geochemical characteristics of volcanic rocks in Baiyingaolao Formation of Suolun area within central Da Hinggan Mountains and their tectonic implications

白音高老组火山岩位于大兴安岭中段科右前旗索伦地区,主要岩性为流纹岩和流纹质晶屑凝灰岩。LA-ICP-MS锆石U-Pb同位素分析表明,3组样品的锆石U-Pb年龄分别为127±2Ma、133±2Ma和123±1Ma,即白音高老组火山岩形成于133~123Ma,为早白垩世岩浆活动的产物。岩石学和岩石地球化学特征显示,白音高老组火山岩类似于高分异的I型花岗岩,有斜长石和少量角闪石部分熔融残留。锆石的^(176)Hf/^(177)Hf值介于0.282854~0.283026之间,ε_(Hf)(t)为较高的正值,变化于+5.5^+11.5之间,Hf二阶段模式年龄(T_(DM2))为828~439Ma,表明兴安地块地壳的主体增生年代为新元古代—显生宙。通过对比东北地区(以邻区为主)同时代岩浆-构造活动,研究区内白音高老组火山岩形成于伸展构造环境,这种伸展构造环境的形成可能与古太平洋板块俯冲于欧亚大陆之下的弧后伸展环境有关。

The volcanic rocks in Baiyingaolao Formation are located in Suolun area of Horqin Right Wing Front Banner within centralDa Hinggan Mountains.Lithologically,the Baiyingaolao Formation is composed mainly of rhyolites and rhyolitic tuffs.Three groups ofzircon samples yielded U-Pb ages of127±2Ma,133±2Ma and123±1Ma respectively.The results of LA-ICP-MS zircon U–Pb datingindicate that the volcanic rocks in the Baiyingaolao Formation of the study area were formed during the Early Cretaceous period with ag?es of133~123Ma.Petrological and geochemical characteristics of these volcanic rocks suggest that they are all highly fractionated rockssimilar to I-type granite rocks,and their parental magmas were likely derived from the partial melting of lower crustal materials with pla?gioclase and a little hornblende as the residual phases.In addition,the volcanic rocks sampled in this paper,tectonically located in theXing'an terrain,have high initial176Hf/177Hf ratios(0.282854~0.283026)and positiveεHf(t)values(+5.5^+11.5).These data,combinedwith young Hf two-stage model ages of828~439Ma,suggest that the crustal growth of the Xing'an terrain occurred during Neopro?terozoic and Phanerozoic periods.The above results,combined with previous studies of the contemporaneous magma-tectonic activi?ties in Northeast China,imply that the generation of the Early Cretaceous volcanic rocks in the central Da Hinggan Mountains was relat?ed to the extensional environment caused by the subduction of the Paleo-Pacific plate beneath the Eurasian continent.