THERMOCHRONOLOGY OF ZIRCON FROM THE SREDINNY RANGE, KAMCHAKTA

The Eocene collision of the Olutorsky Island arc into the NE Asiatic margin resulted in the accretion of this exotic terrane to what was then an Andean-style margin dominated by a continental arc. The metamorphic culmination of this collision zone is in the Sredinny Range of southern Kamchataka, where metamorphic rocks from the event have been deeply exhumed. The low-temperature thermochronology of rocks from this range has been complicated by the fact that original cooling ages (Eocene) appear to have been overprinted by a thermal event in the Miocene. We use of zircon fission-track (ZFT) and zircon U+Th/He (ZHe) dating of radiation-damaged grains, and apatite fission-track (AFT) dating techniques to establish the late-stage thermal history of this metamorphic culmination. In addressing this problem, we've designed a double-dating technique that yields two cooling ages per sample; one for the time at which rock passed through the ZFT closure temperature and the second for the time at which the rock passed through the ZHe closure temperature. Zircon fission-track cooling ages resulted in multiple samples retaining two major cooling events at about 38-42 Ma and 18-22 Ma and ZHe cooling ages mainly fall into the latter group. The first event is inferred to represent exhumation following collision, and the second event is related to crustal extension associated with opening of the Sea of Okhotsk, or arc volcanism.