Petrography, Geochemical Features and Absolute Dating of the Mesozoic Igneous Rocks of Medvedev and Taezhniy Massifs (Southeast Russia, Aldan Shield)

The paper presents the results of the petrographic and geochemical studies of igneous rocks of the Medvedev and Taezhniy massifs, including their first absolute dating. The massifs are located in central Nimnyr block of the n shield within the Leglier ore cluster of the Evotinskiy ore district (Southeast Russia, Aldan Shield). For the first time, the three-phase structure of the Medvedev massif has been defined, as observed in our expedition and petrographic studies. Rocks from the three phases of the Medvedev massif include quartz syenites, syenites, and monzonites, and rocks from the two phases of the Taezhniy massif include quartz monzonites and syenites. Geochemically, the rocks are close to volcanic island arcs, the formation of which was related by subducted oceanic crust of the Mongol–Okhotsk Ocean. The defined duality of the geochemical compositions of the igneous rocks of the massifs may be due to the presence of both mantle and crustal sources; however, it is most likely that these rocks resulted from the melting of a mixed mantle source or the latter was contaminated by the crust with further differentiation of melts in intermediate crust chambers. Additionally, geochemical characteristics suggest that the analyzed rocks are close to latite and shoshonite derivatives and can be considered as part of the monzonite–syenite formation type. The first identified periods of formation of igneous rocks in the Medvedev massif are 122.0–118.0 Ma and Taezhniy 117.5–114.5 Ma, which correspond to the Early Cretaceous (Aptian)