大兴安岭北段八大关地区早侏罗世流纹岩锆石U-Pb定年与岩石成因

Zircon U-Pb dating and petrogenesis of the Early Jurassic rhyolite in Badaguan area, northern Da Hinggan Mountains

大兴安岭北段八大关地区首次发现早侏罗世火山岩,该类岩石主要由流纹岩组成,含少量英安岩和流纹质凝灰岩,LA-ICPMS锆石U-Pb定年结果显示其形成于185~190 Ma,岩石地球化学研究表明,流纹岩具有高硅富碱,贫镁钙和高FeO^T/MgO比值的特征,属于高钾钙碱性系列;稀土总量不高(ΣREE=103.80×10^(-6)~194.31×10^(-6)),轻重稀土分馏较明显((La/Yb)N=7.12~10.22),中等负Eu异常(δEu=0.34~0.74),微量元素以富集Rb、Ba、Th、U、K和LREE,强烈亏损Sr、P、Ti,相对亏损Nb、Ta为特征,显示地壳来源,结合较低的Ga/Al比值,该区流纹岩具有I型花岗岩特点,在(Y+Nb)-Rb和(Yb+Ta)-Rb判别图解上显示其形成于活动大陆边缘环境。综合研究表明,八大关地区早侏罗世之前为活动陆缘环境,蒙古—鄂霍茨克大洋板块向额尔古纳地块之下俯冲。

The newly-discovered Early Jurassic rhyolite in Badaguan area of Da Hinggan Mountains is of great significance in investigating the tectonic evolution history of Northeast China.The rocks are mainly composed of rhyolites,with a small amount of dacitic and rhyolitic tuffs.LA-ICPMS shows the rhyolites were formed in Early Jurassic,and their formation ages range from185Ma to190Ma.Geochemical studies show that the rhyolites are characterized by rich silicon and alkalis,poor calcium and magnesium and high FeOT/MgO ratios.Hence the rhyolites belong to high-potassium calc-alkaline rocks.All samples have the similar characteristics of REE patterns,which are characterized by relatively poor total REE content(ΣREE=103.80×10-6-194.31×10-6),significant fractionation of HREE and REE[(La/Yb)]N=7.12-10.22)and moderate Eu negative anomalies(δEu=0.34-0.74).The rhyolites are characterized by enrichment of Rb,Th,U,K and LREE,strong depletion of Ba,Sr and HFSE(e.g.,Nb,Ta,P and Ti),and relative loss of Nb,Ta.The relatively low Ga/Al ratios indicate that the rhyolites are I-type granites of high differentiation.The discrimination diagrams of(Y+Nb)-Rb and(Yb+Ta)-Rb show that the rhyolites were formed on the active continental margin.The comprehensive studies show that the Badaguan area was located on an active continental margin before Early Jurassic,which was affected by the subduction of Mongol-Okhotsk plate towards metamorphic rocks in the Erguna Block.