Two Stages of Granite Formation in Accretional and Collisional Settings on the Example of Sredinny Range, Kamchatka, NE Russia

New U-Pb SHRIMP zircon datings, analysis of tectonic setting and composition peculiarities of two granite complexes indicate that they mark stages of tectonic evolution of metamorphic complexes of Malka Rise of Sredinniy Range and are indicators of newly formed continental crust of Kamchatka. Granites are divided into gneissosed and equigranular types. Gneissosed granites intrude metamorphic formations of Kolpakovka Group. Equigranular granites intrude gneissosed granites, complexes of Kolpakovka and Kamchatka groups. U-Pb SHRIMP zircon datings of granites indicate two stages of granite formation: Campanian (~78-80 m.a.) and Early Eocene (52±2 m.a.). Petrography and petrochemical features of both types of granites make them similar to S-type granites. REE patterns allow distinguishing among equigranular and gneissosed granites two rock groups differ in their source and melting depth. The first stage of granite magmatism, formation of gneissosed granites (78-80 m.a.), is related to accretional setting at the Kamchatka margin of Eurasia. Granite magmatism in accretionary prism may be due to underplating of mafic material at the base of accretionary prism as a result of mantle wedge melting above subduction zone. The second stage of granite magmatism, formation of equigranular granites (52±2 m.a.), coincides in time with collision between Achayvayam-Valaginskaya ensimatic island arc and Kamchatka margin of Eurasia. After 55 m.y. ago during collision the quick thrusting of island arc slices over heterogeneous formations of the margin occurred. Intensive and quick transformations of the structure, including deep submergence, quick (3-5 m.a.) heating of the crust took place. It is resulted in metamorphism of high T and moderate P in the deep part of collisional zone and melting of granites. Quick heating of the crust, probably, was related to asthenosphere uprise as a result of Eurasian slab break-off, subducting beneath the arc. According to U-Pb SHRIMP zircon datings migmatization, partial melting and intrusion of equigranular granites occurred simultaneously. Work is supported by RFBR (N07-05-00255), FTsNTP leading scientific schools (NSh- 3172.2008.5).