HIGH-GRADE METAMORPHIC CORE OF AN EOCENE ARC-CONTINENT COLLISION ZONE, SREDINNIY RANGE, KAMCHATKA

The Sredinniy Range, Kamchatka, is the most deeply exhumed part of the 1500 km long Eocene Olyutorsky arc-continent collision zone, a major feature of the northwest circum-Pacific. Widely accepted models posit that the Sredinniy core represents a fragment of Mesozoic or Precambrian continental crust associated with the postulated Okhotsk Sea Microplate. We report new geochronologic data that demonstrate that the Sredinniy core comprises Late Cretaceous and Paleocene sediments of the northeast Asian margin metamorphosed during the Early Eocene Olyutorsky arc collision. Our conclusion is based on SHRIMP U-Pb data from 14 zircon samples and Th-Pb data from 5 monazite samples collected from high-grade metamorphic rocks, granite and tuff from an overlapping sedimentary unit. Detrital zircons from the predominantly metasedimentary Kolpakova Gneiss and Kamchatka Schist preserved in the form of inherited cores in migmatites or “undisturbed” grains in lower-grade rocks reveal a broad range of grain-ages from Archean to Paleocene. We assign Late Cretaceous and Paleocene stratigraphic ages to the protoliths of the Kolpakova Gneiss and Kamchatka Schist, respectively, based on the youngest concordant grain-ages in each sample. A comparison of zircon grain-ages from the Kamchatka Schist and non-metamorphosed age-equivalent sandstones of the northeast Russian margin shows remarkable similarity (P(KS)=46%) of grain-age probability suggesting that the units are stratigraphic correlatives with the same provenance. Rim analyses of zircon from leucosomes in the Kolpakova Gneiss are characterized by very low (0.01) Th/U and produce a mean age of 51.2 ± 0.5 Ma. Monazites from the same sample yield a mean Th/Pb age of 52.4 ± 0.7 Ma pointing to co-precipitation of zircon rims and monazite during peak metamorphism in the Early Eocene. Zircons from a boudinaged late syn-kinematic granite yield a mean age of 51.5 ± 0.7 Ma. Exhumation of the metamorphic core is recorded by abundant metamorphic clasts in the overlying Baraba conglomerate. A basal tuff horizon within the Baraba unit yields a 50.5 ± 1.2 Ma zircon age. These data indicate rapid subduction, metamorphism, and exhumation of the NE Asian margin during the Early Eocene, coincident with collision of the Olyutorsky arc, which structurally overlies the metamorphic core.