内蒙古扎兰屯地区白音高老组火山岩地球化学、年代学及其地质意义

VOLCANIC ROCKS OF THE BAIYINGAOLAO FORMATION IN THE SOUTHWEST OF ZHALANTUN, INNER MONGOLIA:Geochemistry, Geochronology and Tectonic Implications

扎兰屯西南部白音高老组火山岩的岩相学鉴定为流纹岩组,少量英安岩.LA-ICPMS锆石U-Pb定年显示,流纹岩形成于125～129 Ma的早白垩世.岩石地球化学研究表明,火山岩高硅、富碱,为过铝质高钾钙碱性系列;稀土丰度较高（∑REE=128.35×10-6～169.26×10-6）,轻重稀土分馏明显（（La/Yb）N=12.17～16.42）,弱负Eu异常（δEu=0.43～0.70）;大离子亲石元素（LILE）Rb、Th、K等相对富集,出现Ba的亏损,高场强元素（HFSE） Sr、P、Ti强烈亏损,Nb、Ta相对亏损,与A型花岗岩特征相似.Rb/Sr=0.58～2.06,Ti/Y=6.50～17.98,Ti/Zr=7.49～40.87,基性相容组分Cr、Co、Ni含量较低,Mg#较低,具有壳源岩浆特征.综合考虑扎兰屯白音高老组火山岩的上述特征并结合他人研究成果,认为扎兰屯地区的这套火山岩源于地壳的部分熔融,形成于非造山板内伸展环境.

The volcanic rocks of Baiyingaolao formation in the southwest of Zhalantun are petrographically composed of rhyolite,with minor dacite.The zircon U-Pb dating results show that they were formed in Early Cretaceous from 125 to 129 Ma.The geochemical study suggests that the rhyolite is rich in silicon and alkali,belonging to peraluminous high-K calcalkaline series.The rocks are also characterized by obviously right-declined REE patterns and slightly negative Eu anomalies with high total REE contents.On the primitive mantle normalized multi-elements diagram,the volcanic rocks of Baiyingaolao Formation exhibit enrichments of Rb,Th and K and depletions of Ba and HFSEs such as Nb,Ta,Ti,Sr and P,similar to the characteristics of A-type granite.They have low Cr,Ni,Co and Mg# concentrations,with Rb/Sr ratios of 0.58-2.06,Ti/Y ratios of 6.50-17.98 and Ti/Zr ratios of 7.49-40.87,indicating a crust magma source,instead of mantle source.From the above characteristics,it is proposes that the volcanic rocks of Baiyingaolao Formation were derived from the partial melting of the crust and formed in the non-orogenic intraplate extensional environment.