Early Eocene magmatism in the Sredinnyi Range, Kamchatka: Composition and geodynamic aspects

Migmatization and granite-forming processes were widespread in the southern Sredinnyi Range of the Kamchatka Peninsula in the Early Eocene (at approximately 52 ± 2 Ma). The paper presents data on the composition and genesis of the Early Eocene granitoids. The Malka Rise contains both equigranular peraluminous garnet-bearing granites, on the one hand, and migmatites and tonalites and trondhjemites (TTG), on the other. The petrography and petrochemistry of most granites in the Malka Rise in the Sredinnyi Range (high SiO 2 concentrations, the presence of muscovite and garnet, the proportions of their Al saturation index ASI and SiO 2 , FeO t + MgO + TiO 2 , and SiO 2 , Al 2 O 3 /TiO 2 , and CaO/Na 2 O), and the composition of biotite in these rocks highlight their similarities with S-granites. The character of the REE patterns and the Sr and Y concentrations suggest that the granites and TTG were formed via the melting of sources of two types: metasediments and metabasites. The metasedimentary nature of the protolith of most of the granitoids also follows from similarities between the REE patterns of the granitoids and host metaterrigenous rocks of the Kolpakova and Kamchatka groups. The variations in the Rb/Ba and Rb/Sr ratios of the granites imply that their protoliths could be sedimentary rocks both depleted and enriched in pelite components. The facts that, along with S-granites, some of the granites are TTG, which likely had mafic protoliths, make the Early Eocene granites generally similar to S-granites of the Cordilleran type. The collision of the Achaivayam-Valaginskii ensimatic island arc with the Kamchatka margin of Eurasia started at 55–53 Ma and predated Early Eocene magmatism. In the course of this collision, arc complexes were obducted over continental marginal rocks, and this resulted in their rapid subsidence, crustal heating, magma generation, and the derivation of the granites, tonalites, and trondhjemites at 52 ± 2 Ma at temperatures of 645–815°C. This rapid heating (duirng no more than 3–5 Ma) required an additional heat source, which was likely the mantle. The latter heated the bottom of the crust at the detachment of the slab. The influx of mantle material resulted in intrusions of the norite-cortlandite association, which was coeval with the granites and was accompanied by Cu-Ni sulfide mineralization. The composition of the granitoids and data on the intrusions of the norite-cortlandite association suggest that mantle material was involved in Early Eocene syncollisional magma generation in Kamchatka. Newly obtained U-Pb zircon SHRIMP dates of the granitoids and recently published data on the age of the norite-cortlandite intrusions indicate that they are coeval and make it possible to recognize an Early Eocene phase of magmatic activity in Kamchatka.