Two stages of granite formation in the Sredinny Range, Kamchatka: Tectonic and geodynamic setting of granitic rocks

Abstract

The newly formed continental crust in southern Kamchatka was created as a result of the Eocene collision of the Cretaceous-Paleocene Achaivayam-Valagin island arc and the northeastern Asian margin. Widespread migmatization and granite formation accompanied this process in the Sredinny Range of Kam- chatka. The tectonic setting and composition of granitic rocks in the Malka Uplift of the Sredinny Range are characterized in detail, and the U-Pb (SHRIMP) zircon ages are discussed. Two main stages of granite forma- tion—Campanian (80-78 Ma ago) and Eocene (52 ± 2 Ma ago) have been established. It may be suggested that granite formation in the Campanian was related to the partial melting of the accretionary wedge due to its under- plating by mafic material or to plunging of the oceanic ridge beneath the accretionary wedge. The Eocene gra- nitic rocks were formed owing to the collision of the Achaivayam-Valagin ensimatic island arc with the Kam- chatka margin of Eurasia. In southern Kamchatka (Malka Uplift of the Sredinny Range), the arc-continent col- lision started 55-53 Ma ago. As a result, the island-arc complexes were thrust over terrigenous sequences of the continental margin. The thickness of the allochthon was sufficient to plunge the autochthon to a considerable depth. The autochthon and the lower portion of the allochthon underwent high-grade metamorphism followed by partial melting and emplacement of granitic magma 52 ± 2 Ma ago. The anomalously rapid heating of the crust was probably caused by the ascent of asthenospheric magma initiated by slab breakoff, while the Eurasian Plate plunged beneath the Achaivayam-Valagin arc.