Isotope Dating of Lherzolite-Type Ophiolites in the Tamvatnei Massif, Koryak-Kamchatka Fold Region: Zircon-Based SHRIMP U-Pb Data on Quartz Dolerite

摘要

The Tamvatnei ophiolite massif is located in the external part of the Koryak-Kamchatka accretionary orogen, within the limits of the Anadyr-Koryak Fold System (Late Cretaceous accretion), composed largely of complexes of enzymatic island arcs and backarc basins referred to the Jurassic-Early Neocomian. Lherzolite type ophiolites of the Tamvatnei massif, located in the harzburgite province of the Mainitsa and Algan-Velikorechenskii terranes and being a regional anomaly in terms of geological structure and rock composition, were not dated by paleontological methods. The geological data indicate only their pre-Senonian age of formation, and K-Ar datings for mafites by the rock bulk compositions cover nearly the whole Early Cretaceous. The isotope dating often accessory zircon grains from quartz diorite, which finished the formation of magnesian series of gabbro-diabases belonging to the Tamvatnei hypoabyssal ophiolite complex and that of magnesian andesites from the lower stratum of the volcanogenetic complex, yielded a concordant age of 138.9 ± 1.9 Ma. These results, as well as the previously acquired date for amphibole from amphibole microgabbro belonging to the hypoabyssal complex (120.0 ± 2.4 Ma; after P. U. Layer), indicate that Tamvatnei lherzolite-type ophiolites formed in the Valanginian-Aptian, i.e., later than the Late Mesozoic suprasubduction ophiolites from the adjacent Mainitsa and Algan terranes, whose volcanogenic-siliceous strata are dated at the Jurassic-Valanginian. The obtained results prove the model for Tamvatnei ophiolite generation in an introarc basin founded during breakup of the crust of the Mainitsa-Algan island-arc system.