KORDONKA TERRANE - A FRAGMENT OF THE EARLY PALEOZOIC ACTIVE MARGIN
The Kordonka Formation (lower Silurian) forms a lens-shaped tectonic block up to four km wide, extending southward from the Pogranichny village area (Primorsky Krai) for about 30 km. The formation is composed of argillaceous slate, volcanic-sedimentary rocks with horizons of sandstones, siltstones, conglomerates, and cherts, and also contains flows of basalts and andesite-basalts. The results of lithological-geochemical study of the terrigenous rocks of the formation indicate that the Kordonka Formation was accreted in the outer pre-oceanic part of the paleo-island arc system. The eastern part of the formation is intruded and partially assimilated granitoids of Middle Paleozoic and Late Permian age. The early phase of the granitoids was dated by U-Pb zircone 373.5±1.7 and 383.5±1.6 Ma (Late Devonian). The late phase is characterized by 261.2±1.2, 257.4±0.93 and 261.5±1.6 Ma (Wuchiapingian and Capitanian stages of the Middle and Late Permian). The Devonian granitods first identified in this area may be evidence of the Kordonka block accession to the Bureya-Jiamusi-Khanka orogenic belt consolidated during the Ordovician. The Kordonka terrane thus marks an independent Early-Middle Paleozoic episode in the formation and closure of the Central Asian paleo-ocean.