ZIRCON FISSION-TRACK COOLING AGES OF THE SREDINNIY RANGE METAMORPHIC COMPLEXES, KAMCHATKA: IMPLICATIONS FOR POST-COLLISIONAL EXHUMATION

The Sredinniy Range is the single largest exposure of high-grade metamorphic rocks on the Kamchatka Peninsula (Russian Far East) and it sits amongst a collage of far-traveled oceanic and island-arc terranes accreted in the Tertiary. The metamorphic rocks sit astride an Eocene suture zone that marks the collision of the Cretaceous Olyutorsky terrane (oceanic island arc) with the NE Asian mainland (Andean-style continental arc). In the collision zone, the margin of the continental arc was marked by a very thick package of deep-water forearc strata of the Ukelayat and Lesnaya Flysch. This study is part of a broader program aimed at understanding the metamorphic and post-metamorphic cooling of the Sredinniy Range and how that exhumation relates to post-orogenic uplift and exhumation. Samples for zircon fission-track analysis were collected from a elevation transect of the poorly dated Kamchatka complex (metasediments) near the Zolotaya River on the NE side of the Sredinniy Range. Rock was sampled in a 1000 m vertical transect at 200 m increments, and the rocks sampled are amphibolite grade, quartz-biotite metapelites with psammitic beds. Recently reported U/Pb (SHRIMP) zircon ages from this metasedimentary unit are as young as 55 Ma, and the overall age distribution is similar to the unmetamorphosed Ukelayat and Lesnaya flysch, well exposed to the north. New zircon fission-track ages from these samples indicate Late Eocene to Early Oligocene cooling (c. 30-35 Ma) that likely records post-orogenic exhumation. Some of the lowest schists and some nearby intrusives have been fully or partially reset in the Early to Middle Miocene indicating a later weak thermal overprint. Further dating will help reveal if cooling of metamorphic rocks in the Sredinniy Range is related to post-orogenic exhumation with a possible later thermal event in the Miocene.