PETROCHEMICAL CHARACTERISTICS AND TECTONIC SETTING OF THE GRAY GNEISSESIN NORTH DABIE TERRANE

Gray gneiss is the principle component of the north Dabie metamorphic complex. On SiO2 variationdiagram most of major oxide display a smooth trend from tonalitic gneiss to trondhjemitic or granitic rocks,implying a magmatic differentiation model for the origin of the gray gneiss. Gray gneisses have lower ΣREE content with low grade fractionation of light REE/heavy REE. All rocks are enriched in large ionlithophile elements and depleted in high field strength elements and Nb. More basic rocks have highercontents of Cs, Ba, Rb and Sr and less depletion of Nb and Ti compared with meta-granitoids. Theincreasing of Nb depletion, the decreasing of Sr enrichment, and the transitional metal element Tibecoming greatly depleted with the increase of the SiO2 in rocks imply the fraction ofmafic minerals andplagioclase. Major element and trace element discriminations provide that the gray gneiss is of the calcalkaline association formed in a volcanic arc. The forming process of the gray gneiss is closely related to acollisional events. From the subduction before collision, through post-collision uplift, to late orogenicstage, the composition of the gray gneisses was evolving continuously. PetTochemical and geochemicalcharacteristics of the gray gneiss further strengthen the hypothesis that the Northern Dabie Terrain is apossible Andean-type arc.