呼伦湖早白垩世碱性流纹岩的地球化学特征及其意义

大兴安岭呼伦湖一带的上库力组第3段流纹岩具有高硅(SiO₂=75.41%～77.32%)、富碱(Na₂O+K₂O=7.98%～8.74%)、富Y、Nb、Zr及低Al、Mg、Ca、Ni、Cr、Ti和高Ga/Al比值等特点,类似于A型花岗岩,而与高度分异的I型和S型花岗岩有明显差别。该流纹岩的地球化学特征类似于A₁型花岗岩,且与该区A型花岗岩同时,很可能是A型花岗岩浆喷出相的产物。流纹岩的全岩Rb-Sr等时线年龄127±5Ma,与伊列克得组玄武岩(125±2Ma)时代一致,两者构成了双峰式火山岩组合,形成于早白垩世岩石圈拉张环境。

GEOCHEMISTRY OF EARLY CRETACEOUS ALKALINE RHYOLITES FROM HULUN LAKE, DAXING''ANLING AND ITS TECTONIC IMPLICATIONS

The upper part of rhyolites within the Shangkuli Formation in the Hulun Lake region, northern Daxing’anling, is characterized by high SiO₂ (75.41%~77.32%), total alkaline-rich (Na₂O+K₂O=7.98%~8.74%), high Ga/Al, enriched in Y, Nb, Zr, and depleted in Al, Mg, Ca, Cr, Ti. Their chemical compositions are very similar to those of A-type granites, but different from highly fractionated I-and S-type granites. These rhyolites, geochemically similar to those of A₁-type granites subdivided by Eby(1992), are the synchronous products of A-type granite in this region. Five rhyolite samples yield a Rb-Sr whole rock isochron age of 127±5 Ma, coeval with the age of 125±2 Ma of Yeliekede basalts in the Daxing’anling. It is therefore suggested that the bimodal magmatism occurred in a lithospheric extensional setting during the Early Cretacoues.