ON THE TECTONIC SIGNIFICANCE OF ARC VOLCANISM IN NORTHERN KAMCHATKA

The Vyvenka volcanic field records a period of Neogene, subduction-related volcanism in northern Kamchatka. Most models describing the tectonic evolution of the northwest Pacific do not account for this type of Neogene volcanism because the main locus of Pacific/Kula-North American convergence switched to the Aleutian Ridge during Eocene time. The Vyyenka volcanism, as well as oceanic spreading and crust formation within the Komandorsky Basin, demonstrate that this region remained tectonically and volcanically active in Neogene times. We report petrologic, geochemical, and K-Ar age data for the approximately 15 Ma Golovin and 6-8 Ma Valovayam volcanic rocks, two andesite suites within the Vyyvenka volcanic field. The Golovin suite consists of medium to high-K andesites with strong arc-like trace-elements signatures, while the Valovayam suite consists of medium-K andesites with weaker arc-like trace-element signatures. The Valovayam andesites also contain some trace-element ratios indicative of melting of the subducted oceanic crust. These include high Sr/Y (30-50) and Zr/Sm greater than the chondritic value of 28. The Golovin andesites have overlapping Sr/Y (25-45) and lower Zr/Sm. The compositional differences between the Golovin and Valovayam andesites correlate with Neogene tectonic evolution of the Komandorsky region. In northern Kamchatka, subduction waned as spreading stopped in the Komandorsky Basin and newly generated oceanic crust entered the subduction zone. Thus, the trace-element signals of slab melts in the younger Valovayam rocks indicates melting of the young, hot Komandorsky Basin crust that entered the subduction zone and subsequent metasomatism of the mantle wedge. The weaker subduction signature of the Valovayam suite, which distinguishes it from the Golovin suite, records the decreasing vigor of subduction processes with time.