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

Abstract

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 per aluminous 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 ), 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 AchaivayamValaginskii 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 mar ginal 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 intru sions of the norite-cortlandite association, which was coeval with the granites and was accompanied by CuNi sulfide mineralization. The composition of the granitoids and data on the intrusions of the norite-cort landite association suggest that mantle material was involved in Early Eocene syncollisional magma genera tion 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 rec ognize an Early Eocene phase of magmatic activity in Kamchatka. 2 Km Kl An Khv Khm Khz Kr Ir Fig. 3. Schematic chart showing age and structural relations between geological complexes in the southern Sredinnyi Range in Kamchatka (see text for the age of stratigraphic units). Neoautochthon: Br-terrigenous deposits of the Baraba Formation; allochthon: Kr-volcanic and volcanisedimentary de...

Reconstruction of Subduction and Back‐Arc Spreading in the NW Pacific and Aleutian Basin: Clues to Causes of Cretaceous and Eocene Plate Reorganizations

“…U-Pb (SHRIMP) zircon age data of metamorphic and magmatic rocks constrain peak metamorphism, migmatization, and granitic magmatism at 47-55 Ma (Bindeman et al, 2002;Hourigan et al, 2009;Luchitskaya et al, 2008;Luchitskaya & Soloviev, 2012). These events are correlated to the obduction of the Olyutorsky island-arc complex and underlying ophiolite onto the Sredinny complex (Bindeman et al, 2002;Hourigan et al, 2009;Luchitskaya & Soloviev, 2012).…”

“…The youngest phase of metamorphism and felsic magmatism of the Sredinny Complex occurred during the Early Eocene (Bindeman et al, ; Hourigan et al, ; Luchitskaya & Soloviev, ). U‐Pb (SHRIMP) zircon age data of metamorphic and magmatic rocks constrain peak metamorphism, migmatization, and granitic magmatism at 47–55 Ma (Bindeman et al, ; Hourigan et al, ; Luchitskaya et al, ; Luchitskaya & Soloviev, ). These events are correlated to the obduction of the Olyutorsky island‐arc complex and underlying ophiolite onto the Sredinny complex (Bindeman et al, ; Hourigan et al, ; Luchitskaya & Soloviev, ).…”

“…Both units are unconformably overlain by amphibolite-to greenschist-facies metasedimentary schists and gneisses of the Kamchatka Group (Rikhtyer, 1995; Figure 5), suggested to be of Paleocene age (∼60 Ma; Hourigan et al, 2009). The youngest phase of metamorphism and felsic magmatism of the Sredinny Complex occurred during the Early Eocene (Bindeman et al, 2002;Hourigan et al, 2009;Luchitskaya & Soloviev, 2012). U-Pb (SHRIMP) zircon age data of metamorphic and magmatic rocks constrain peak metamorphism, migmatization, and granitic magmatism at 47-55 Ma (Bindeman et al, 2002;Hourigan et al, 2009;Luchitskaya et al, 2008;Luchitskaya & Soloviev, 2012).…”

Eocene accretion at Kamchatka and a pulse of mantle plume magmatism

“…This almost exactly coincides with the time period during which the accretion zone was restyled: 3-5 Ma (Luchitskaya et al, 2008) starting at the onset of collision until subduction resumption in the opposite direction. These geochronologic data (Luchitskaya and Soloviev, 2012) lead to the conclusion that mantle magmas syngenetic with the Ni bearing intrusions of the Dukuk Complex served as the heat source for fast processes of postcollisional granite gen eration. …”

On pole position

“…5), suggested to be of Paleocene age (~60 Ma; Hourigan et al, 2009). The youngest phase of metamorphism and felsic magmatism of the Sredinny Complex occurred during the Early Eocene (Bindeman et al, 2002;Hourigan et al, 2009;Luchitskaya & Soloviev, 2012). U-Pb (SHRIMP) zircon age data of metamorphic and magmatic rocks constrain peak metamorphism, migmatization, and granitic magmatism at 47-55 Ma (Bindeman et al, 2002;Hourigan et al, 2009;Luchitskaya et al, 2008;Luchitskaya & Soloviev, 2012).…”