KUDRYAVYI VOLCANO AND THE EVOLUTION OF MEDVEZHIYA CALDERA (ITURUP I., KURIL IIS.)

Abstract

Medvezhiya caldera is a polygenetic volcanic structure including the Medvezhii, Srednii, Kudryavyi, and Menshoi Brat volcanoes and several flood basalt vents. The discovery of a new volcano, Tukap, with a rhenium ore occurrence, is reported in this paper. The development of this volcanic group began in the Early Pleistocene with the formation of shield volcanoes and plateau basalt flows. The absolute age of the upper basalt flows is 1.03±0.6 Ma. A caldera was formed during the Early and the beginning of the Middle Pleistocene within the middle of the volcanic massif, and volcanic activity changed to pyroclastic explosions of acid, dacite-rhyolite, composition. Ignimbrite of this phase yielded an age of 0.41±1 Ma. The post-caldera Pleistocene-Holocene activity produced two volcanic cones: Tukap and Kudryavyi. The caldera ranges between 14 and 18 km across, is ∼600 m deep, and is supplemented with a half-caldera 10-12 km in size. The main telescoped caldera contains two similar internal structures. The total volume of the ignimbrite is ∼12 km3 around the caldera and ∼100 km inside, tufaceous sediments included. The rock volume ratio of the three phases is 60:15:12. The addition of the acid rocks buried in the caldera changes this ratio in favor of the medium-acid rocks. Based on these new data, the age and composition of the rocks in northern Iturup I. were revised: the pumice-ignimbrite sequence of Medvezhiya Bay was dated Early-Middle Pleistocene instead of its previous interpretation as the stratotype of the Late Miocene-Early Pliocene Kamui Formation.