Petrogenesis of the late Oligocene granites in Milin area,the eastern Gangdese arc
Cenozoic granites are widely exposed in the Gangdese magmatic arc of the southern Tibetan Plateau.H owever,the origin of these granites remains controversial.We report geochronology and geochemistry study about the late Oligocene two-mica granites from the Milin area in the eastern Gangdese arc.Whole-rock chemical analyses show that these granites belong to medium-high potassium calc-alkaline series,weakly peraluminous.The samples are enriched in light rare earth elements and large ion lithophile elements such as Rb,Th and U,and depleted in heavy rare earth elements and high field strength elements such as Nb,Ta and Ti.All samples exhibit high Sr and low Y,with high Sr/Y ratio(37～85),and chemical affinity to adakites.The two-mica granites have the crystallization age of～26.6 Ma.Zircon Hf isotopes show that these granites have low and positive εHf(t)values(+0.4 to+7.3).Taking previous data into account,we conclude that the late Oligocene granites from the Milin area were derived from partial melting of the Eocene arc-type magmatic rocks in the thickened lower crust,but with contribution of ancient crustal materials.The continental collision-induced crustal shortening and thickening resulted in the partial melting of the thickened lower crust,and formation of the widespread late Oligocene adakitic granites.
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