青藏高原北羌塘盆地结扎乡一带二叠系尕笛考组火山岩的特征及构造环境

对位于北羌塘盆地结扎乡一带二叠系尕笛考组火山岩进行岩石学、岩石化学、微量元素的分析及研究,为恢复二叠纪北羌塘盆地性质与古构造环境提供岩石地球化学约束.主要岩石类型为玄武岩、安山岩、英安岩及少量流纹岩、火山角砾岩等,SiO2=48.49%～74.52%,平均60.87%,Al2O3多数在14.5～18.86之间,K2O含量平均为1.99,显示钙碱性系列特征.LREE/HREE=4.31～14.60,平均8.74,(La/Yb)n=3.72～16.16,平均9.59,具有LREE富集的特征,δEu=0.61～1.11,平均0.80,多数样品δEu<1.0,表明铕呈弱负异常或不显异常.火山岩La×Nb/Zr×Zr>0.005,平均0.025,Th#215;Ta/Hf#215;Hf>0.098,平均为0.210,与典型的岛弧火山岩具相似性.由上述特征表明,北羌塘地块在早中二叠世处于岛弧环境的构造背景.

Geochemical Characteristics and Tectonic of Volcanic Rocks of the Permian Gadikao Formation in the Jiezha Area,Northern Qiangtang,Qinghai-Tibet Plateau

This paper deals with the petro logical, petrochemical and trace element ana lyses and studies of the volcanic rocks of Permian Gadikao Formation in Jiezha area no rthern Qiangtang basin. It provides the geochemical condition to understand the character of the north of Qiangtang basin and tectonic environment. The rock types mainly include basalt, andesite, dacite and littlerhyolites, lava brecciaetc. The rocks have the following chemical composition: SiO2,48.49%-74.52%, with ameanvalue of 60.87%, Al2O3 majority between 14.5%-18.86%, K2O mean contents 1.99%. This indicates the volcanic rocks with the character of autunite alkalescence siries, LREE/HREE=4.31-14.60, with a mean value of 8.74, (La/Yb)n=3.72-16.16, with amean value of 9.59, enrichment of LREE, δEu=0.61-1.11, with a mean value of 0.80, majority samples δEu<1.0, indicate Eu pronounced weak negative abnormalty or nopronounced Eu abnormalty. La×Nb/Zr×Zr>0.005, with ameanvalue of 0.025, Th×Ta/Hf×Hf>0.098, with ameanvalue of 0.210. These chemical composition sare similar to typical volcanic rocks of island arc. The above-mentioned characteristics suggest that thep lot of north Qiangtang locate in tectonic background of island arc environment in Early Permian.