Bimodal geochemical evolution at Sheveluch stratovolcano, Kamchatka, Russia: Consequence of a complex subduction at the junction of the Kuril Kamchatka and Aleutian island arcs

Adak-type andesites erupted as domes at Sheveluch volcano (Kamchatka Russia) have been associated withpartial melting of the Pacific plate subducting under Kamchatka. The heat for melting at the plate edge hasbeen proposed to be provided by an asthenosphere upwelling north of the junction of Kuril Kamchatka andAleutian island arcs. However, Sheveluch volcano is composed of two volcano-stratigraphic units bounded bya caldera episode. The lava sequence from the pre-caldera stage of Sheveluch has not previously beenconsidered in the petrogenetic model and in the consequent geodynamic reconstruction.New petrochemical data from the post-caldera domes and the pre-caldera lavas, together with a review of thepetrochemical characteristics of northern Kamchatka volcanics reveal a more complex story. Rocks atSheveluch display a bimodal geochemical and petrological signature, which cannot be derived from acommon parent magma: the pre-caldera lavas are high-Al2O3 basaltic andesites, similar to the products ofKliuchevskoi and other volcanoes in northern Kamchatka, and can be associated with partial melting of themantle wedge hydrated by the subducting slab. The post-caldera andesitic domes, with high Mg#, Cr and Nisignature, are slab primary melts which have assimilated the peridotite in the overlying mantle wedge.A possible mechanism to explain the observations is that beneath Sheveluch two distinct slabs with differentsubduction angle and pressure/temperature paths are responsible for the two geochemical signatures. Areappraisal of geophysical data indicates that between the western Aleutians and northern Kamchatka thereis a transform zone where the Pacific plate is torn apart and decoupled. It is proposed that a portion of the slabwas led to subduct at low angle north of the transform zone and travelled west for a long distance. Thefrictionally heated metabasalt in the slab can yield acidic melts at the amphibolite–eclogite transition ~70 kmbeneath Sheveluch. The ascending melts interact with the peridotite to produce the high Mg-andesiteserupted by the present-day volcano. South of the junction, the dehydration of the Pacific slab undergoing highangle subduction would cause partialmelting of the peridotite in the mantle wedge and produce melts whichwill evolve as high Al2O3 basaltic andesites erupted in the pre-caldera phase.Sheveluch volcano can therefore be considered the only known example in the world in which twoparadigmatic models for magma genesis at colliding margins are not mutually exclusive but are bothnecessary to explain the products of two distinct evolutionary phases.