Volcanic-sedimentary complex of Otrozhnaya sheet in the Ust-Belaya Terrane, western Koryakia

Abstract

New data on the structure and composition of the rock complexes making up the Otrozhnaya sheet of the UstBelaya Terrane are discussed. This sheet is curre ntly regarded as a complete section of the Early Devonian oceanic crust. The U-Pb age of zircon (547 ± 17 Ma) from plagiogranite porphyry that cuts through the ultrama� fic-gabbroic complex indicates that the country rocks are much older. Basalts of volcanic complex are similar to MORBtype rocks. The contact between volcanic and volc anosedimentary complexes is tectonic. No volcanic- cherty association of rocks typical of ophiolites has been established at the base of the sedimentary sequence. The U-Pb age of detrital zircons from the base of this section is 571 Ma. As follows from the composition of terrigenous rocks and the suprasubduction origin of andesitic lavas and tuffs, the rocks of the lower part of volcanosedimentary complex were deposited at the marginal part of the basin near convergent boundary. The remains of conodont ele� ments make it possible to estimate the age of the upper part of the volcanosedimentary complex as Late Devonian. The composition of terrigenous rocks shows that the provenance has abruptly changed. Sporadic rock fragments evidence the erosion of ophiolitic sections. Silurian rocks became the main source of clastic material. This is cor� roborated by redeposited microfauna species and U-Pb age of detrital zircons (~432 Ma). The geochemistry of dacite pebbles from conglomerate indicates that this volcanic rock was formed in a suprasuduction setting. The Ordovician-Silurian island arc could have been a source.