Permian–Triassic Volcanic Rocks, Central Part of the Okhotsk Terrane: Petrography, Geochemistry and Geochronology

The Okhotsk terrane is one of the few regions of Russia that still remain poorly studied. In the central part of this terrane, successions of tuffs and ignimbrites have been identified; they formed within a previously unstudied segment of a Permian–Triassic volcanic arc. Most of the studied rocks are represented by interbedded tuffs and ignimbrites of different grain-size classes, from silt-sized and psammitic to fine-agglomeratic. Paleosoil layers with abundant plant detritus occur between volcanic successions. These rocks formed in an Andean-type active continental-margin setting developed directly on the Okhotsk terrane. The volcanic rocks are andesitic, dacitic and rhyolitic in composition. They most likely formed as a result of mixing between magmatic melts derived from the mantle wedge and from the melting slab; as a result, they show a positive europium anomaly and patterns similar both to the E-MORB trend and to the trend of the average upper crustal composition. U–Pb dating of the tuffs yielded ages of ca. 270, 267, 263 and 248 Ma, corresponding to the Permian and Middle Triassic. The studied volcanic rocks are probably part of the large Okhotsk–Taigonos belt, extending from Siberia and the Okhotsk terrane toward the Omolon terrane and farther to the northeast.