МАГМАТИЧЕСКИЕ ПОРОДЫ ОСТРОВОДУЖНОЙ СИСТЕМЫ КЕРМАДЕК: ДАННЫЕ О НЕОБЫЧНОМ ТИПЕ МАГМ ПРЕДДУГОВОЙ ОБЛАСТИ

Abstract

Two dredging and profiling traverses were conducted across the Kermadec arc system at 30-33˚S. The system comprises, from the west, the Colville Ridge (remnant arc), Havre Trough (active backarc basin), Kermadec Ridge (active arc), trench slope/forearc, Kermadec Trench, Pacific Plate ocean floor entering the trench. The system has been active since the early Miocene. Basalts and dolerites from the remnant arc and from the upper regions of the active arc are typical island arc tholeiites (IAT). Basalts from an Eocene seamount entering the trench arc typical ocean island basalts. Basalt from the backarc basin is strongly contaminated by IAT, apparently as a result of strong asymmetry in the basin which retains the spreading axis within 40-50 km of the arc. Basalts and dolerites recovered from the lower trench slope, deeper than 7 km, have a geochemistry which is intermediate in nature between IAT and ocean plate basalt. This indicates a previously unknown locus of magma generation beneath the oceanic forearc, in which magma with aspects of both IAT and ocean plate basalt is generated: eruption sites and mechanisms are unknown, and many problems remain to be investigated.