武当地块耀岭河群中两类不同性质的酸性火山岩研究

Geochemical Characteristics for Two Types of Acid Volcanic Rocks of the Yaolinghe Group in the Wudang Block

耀岭河群火山岩中可划分出两类不同性质的酸性火山岩:酸性岩A主要属碱流岩-白碱流岩类,酸性岩B主要属流纹英安岩/英安岩类。总体上看二者主量元素组成差异不大:SiO2含量为65.00%～79.85%,(Na2O+K2O)为5.26%～9.68%。强烈富集Th、Nb、Ta、La、Ce、Nd、Zr、Sm和Hf等微量元素,强烈亏损Ba、K、Sr、P、Ti等元素,稀土元素特征表现为∑REE很高(339.94～1152.85μg/g),具有强烈的铕负异常(δEu=0.18～0.32),与典型的大陆裂谷流纹岩的稀土曲线一致。酸性火山岩B显示K、Rb、Ba、Th和LREE的正异常和Sr、P、Ti和Nb的负异常,Eu具弱负异常(δEu=0.64～0.88),表现出与酸性火山岩A迥然不同的微量元素及稀土元素特征,而与耀岭河群基性火山岩相似,显示其成因可能与之有相同源区。

The acid volcanic rocks of the Yaolinghe Group can be divided into two types：A-group and B-group,belonging to pantellerite/comendite and rhyodacite/dacite,respectively.The major elemental characteristics of the two types of rocks are quite similar.Their SiO2 and （Na2O＋K2O） contents range from 65.00% to 79.85% and 5.26% to 9.68%,respectively.The A-group acid volcanic rocks are rich in Th,Nb,Ta,La,Ce,Nd,Zr,Sm,Hf and depleted in Ba,K,Sr,P,Ti.They are high in ∑REE（339.94～1152.85μg/g） and show notable Eu negative anomalies （Eu/Eu=0.18～0.32） on the REE patterns,which are similar to those of typical rhyolites in the continent rift setting.The B-group acid volcanic rocks have high contents of K,Rb,Ba,Th and LREE,and show negative anomalies of Sr,P,Ti,Nb and slight negative Eu anomalies （Eu/Eu=0.64～0.88）.The characteristics of rare earth and other trace elements of the B-group rocks are obviously different from those of the A-group rocks,but similar to those of their basic counterparts,suggesting that the B-type volcanic rocks may have a close relationship with the basic volcanic rocks originally.