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.