Geochronology of the volcano-plutonic complex of the Verkhneurmi ore field, Khabarovsk Krai, Russia: K-Ar, Ar-39-Ar-40, and Rb-Sr isotopic data

New data on geochronology of igneous rocks of the Verkhneurmi ore field (Badzhal tin-bearing district, Khabarovsk Krai) were obtained. The large Pravourmi tungsten-tin deposit and a number of smaller potential tin-bearing occurrences are situated in this district. Eleven samples representing main phases and facies of the Badzhal volcano-plutonic complex were dated by isotopic analyses (K-Ar, Ar-39-Ar-40 with step heating, and Rb-Sr). The age of the Verkhneurmi granite massif, to which the tin mineralization is related, was determined according to geological data. Based on isotopic dates obtained, a tentative time scale was proposed for the igneous rocks of the Verkhneurmi ore field. Volcanic rocks (rhyolites and dacites) originated during the earliest phase (earlier than 100 Ma). Later, extrusive and subvolcanic bodies formed: the Urmi rhyolite laccolith and minor granite-porphyry bodies (94-97 Ma). Intrusive magmatism of the next phase which involved injection and crystallization of the Verkhneurmi biotite granites (92-94 Ma). Minor diorite porphyrite and andesite bodies and granite-porphyry intrusive dikes, which cross all igneous rocks of the ore field, formed during the last phase (88-90 Ma). The subvolcanic and intrusive phases lasted about 10 Ma, and in Turonian time (88-90 Ma) the magmatic activity in the region ceased. Thus, the Badzhal volcano-plutonic complex should be regarded as Lower-Upper Cretaceous rather than Upper Cretaceous as was assumed before.