Characteristics and genesis of the Pingdingshan megaaugen gnelssic granite in the eastern Tianshan Mountain areas
东天山平顶山巨眼球状片麻状花岗岩特征及成因

Taking the Pingdingshan granite as an example, this paper investigates characteristics and genesis of augen gneissic granites of the late Jinning Cycle in the eastern Tianshan Mountain areas. Field relations, petrography, petrochemistry, rare earth elements, trace elements and isotopes indicate that the Pingdingshan granite, with a Rb-Sr isochron age of 927 Ma, was formed essentially by in situ transformation of metamorphosed island arc calc-alkaline volcanosedimentary rocks. Driving factors including heat could be ascribed to post-collisional delamination of the Tianshan lithosphere. Delamination-induced underplating and intraplating caused ductile shearing of the overlying crustal rock, such that partial melting, melt and fluid migration, and chemical reactions were highly enhanced, whereas melts and fluids themselves promoted further ductile shearing and transformed the deformed and metamorphosed rocks at higher levels into the gneissic granite. Formation of the this granite is resultant from a complex feed back of partial melting, ductile shearing and fluid activity. Petrogenesis being dominated by interaction of deep-sourced melts and fluids with protoliths in accompany with minor partial melting, geochemistry of such a kind of in situ gneissic granites is constrained by both protoliths and external melts and fluids. Underplating and intraplating are important manners of vertical accretion of the crust during late- to post-orogenic compresion-exention conversion, and gneissic granites like that at Pingdingshan are produced just by such a type of vertical accretion.