Formation of a third volcanic chain in Kamchatka: generation of unusual subduction-related magmas |
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| Authors: | Yoshiyuki Tatsumi Tetsu Kogiso Susumu Nohda |
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| Institution: | (1) School of Earth Sciences, IHS, Kyoto University, Yoshida-Nihon-Matsu, 606-01 Kyoto, Japan;(2) Department of Geology, Kyoto University, 606-01 Kyoto, Japan;(3) Kyoto Sangyo University, Kamigamo, 603 Kyoto, Japan |
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| Abstract: | The unusual development of three volcanic chains, all parallel to the trend of the subduction trench, is observed in Kamchatka at the northern edge of the Kurile arc. Elsewhere on the Earth volcanic arcs dominantly consist of only two such chains. In the Kurile arc, magmatism in the third volcanic chain, which is farthest from the trench, is also unusual in that lavas show concentrations of incompatible elements intermediate between those of the two trenchward chains. This observation can be explained by relatively shallow segregation of primary magmas and high degrees of partial melting of magmas in the third chain, compared to the conditions of magma separation expected from a simple application of the general acrossarc variation. Initial magmas in such an atypical third chain may be produced by melting of K-amphibolebearing peridotite in the down-dragged layer at the base of the mantle wedge under anomalously hightemperature conditions. Such an unusual melting event may be associated with the particular tectonic setting of the Kamchatka region, i.e. the presence of subductiontransform boundary. Such a mechanism is consistent with the across-arc variation in Rb/K ratios in the Kamchatka lavas: lowest in the third chain rocks and highest in the second chain rocks. |
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