Geochemical Types, Petrology, and Genesis of Late Cenozoic Volcanic Rocks from the Kurile-Kamchatka Island-Arc System

The Kurile-Kamchatka Late Cenozoic volcanic rocks can be categorized as magmatic series of island arc and within-plate geochemical types, respectively. These types clearly can be identified on geochemical discriminant diagrams. Across-arc geochemical, mineralogical, and Sr-Nd-isotope zoning is characteristic of island-arc volcanics, but is not found for within-plate volcanics. Such zoning probably is the result of modification of fluid-phase composition, which originates in the underlying subducting plate (as a result of dehydration of water-bearing secondary minerals) and rises into the zone of island-arc magma generation in the mantle wedge. Along-arc Sr, Be, H., and 0 isotopic zoning is noted for island-arc volcanics. Probably it is associated with various levels of island-arc magma contamination. 10Be data for modern island-arc lavas attest to the fact that young pelagic sediments from the subducting plate participate in island-arc magma genesis. Within-plate lavas are found in the region north of Avacha Bay. They may precede island-arc volcanics (East Kamchatka), coexist with them during the late stage of volcanic activity (Central Range), or be completely unrelated to island-arc volcanics in the far rear arc (West Kamchatka). The appearance of within-plate lavas probably is connected with deep faults that accompanied the formation of the northern Kurile-Kamchatka trench and the associated new subduction zone in this part of the island-arc system, an event caused by the accretion of the Kronotskiy terrane to eastern Kamchatka in Middle Miocene time.