【作者】 岳永君； 何国琦；

【Author】 Yue Yongjun (Lithos phere Research Centre,Chinese Academy of Geological Sciences) He Guoqi (Department of Geology,Peking University)

【机构】 中国地质科学院； 北京大学地质系；

【摘要】 二八地一带的下二叠统火山岩可划分为两套岩石组合：下部的弱碱性玄武质火山岩组合和上部的弱碱性流纹质火山岩组合．岩石地球化学资料表明,玄武质火山岩组合呈橄榄玄武岩→安山玄武岩→安粗岩演化,为上地幔物质部分熔融的产物；流纹质火山岩组合呈石英粗面岩→流纹岩、碱性流纹岩演化,主要为地壳物质平衡部分熔融的产物．两套岩石组合之间缺乏过渡的中间组份,构成双峰式火山岩组合,指示着研究区在早二叠世时期处于裂谷环境．更多还原

【Abstract】 The Lower Permian volcanic rocks of Erbadi area can be divided into two lithological assemblages,the weakly alkaline basaltic assemblage in the lower part and the weakly al- kaline rhyolitic assemblage in the upper part.The transitional components between them are lacking.In the R₁-R₂ diagram of petrochemical classification,the basaltic volcanic rocks show a trend of evolution from olivine basalt to andesitic basalt and latite.All their chondrite normalized REE distribution curves incline to the right and are.parallel each oth- er,showing weak Eu anomaly.In comparison with N-type MORB,the basaltic rocks in the study area are enriched in K,Rb,Ba,Th,etc.which are incompatible with mantle min- erals and poor in Y,Yb,etc.which are compatible with mantle minerals.These character- istics demonstrate that the basaltic volcanic rocks in the study area are product of partial melting of uppermost mantle materials and evolved mainly through fractional crystalization of clinopyroxene.In the R₁-R₂ diagram,the rhyolitic volcanic rocks mainly plot in the fields of quartz traehyte,rhyolite and alkali rhyolite,and two phases of evolution from quartz tra- chyte to rhyolite and alkali rhyolite may be distinguished.The chondrite normalized REE distribution curves incline to the right and show very strong negative Eu anomaly.The trace element geochemical patterns of the rhyolitic rocks are similar to those of the in- traplate crust-derived magmatic rocks in Skaegaard which were formed during the attenu- ating process of the continental crust.The above-mentioned characteristics imply that the rhyolitic volcanic rocks are product of partial melting of relatively HzO-depleted crustal materials.Thus the mantle-derived basaltic rocks and the crust-derived rhyolitic rocks make up a bimodal volcanic lithological assemblage,which indicates that the study area was in a rift setting in the Early Permian.更多还原