MONZONITOIDS OF THE BILIKAN-KHULAMRI MAGMATIC ZONE ( North-East Russia ): ROCK COMPOSITION, AGE, GEODYNAMIC SETTING

We analyzed composition of monzonite-syenite intrusive rocks within the Bilikan-Khulamri magmatic zone of the central segment of the Yana-Kolyma Orogenic Belt. These rocks compose small intrusions cutting terrigenous strata of the Inyali-Debin turbidite terrane. The monzonitoids under consideration contain moderate amount of silica and Al2O3, relatively high contents of K2O, Na2O, MgO; by chemical composition they are similar to latite and very close to alkaline-salic intrusive rocks of the Aldan Igneous Province. Concentrations of major and rare elements indicate that these rocks belong to granitoids of convergent continental margins, including syncollisional and those of volcanic arcs. According to U–Pb dating (SIMS), the age of zircon from quartz syenite is 84.0 Ma. It corresponds to the secondary pulse of magmatic activity of the Okhotsk-Chukotka Volcanic Belt. The specific chemical composition of the monzonitoids close to the shoshonite-latite series is due to their location deep in the back arc of the volcanic belt, at more than 200 km from its axial zone, on the sialic bedrock composed by turbidite terrigenous complexes.