Petrology and geochemistry of maungatautari, a medium-K andesite-dacite volcano

Abstract

Maungatautari is an isolated andesitedacite composite volcano of Pliocene—Pleistocene age located 70 km northwest of the Taupo Volcanic Zone, New Zealand. It is composed of a suite of pyroxene andesite, labradorite andesite, and hornblende dacite lavas, volcanic breccias, and Iatestageradial dikes. Maungatautari lavas have largeK2O contents that project within the medium-Kandesite suites. Petrogenetic modelling of major and trace element abundances, combined with the presence of disequilibrium phenocryst assemblages, suggests that the pyroxene andesites are the result of mixing of andesitic magma with a primitive basaltic magma. One group of labradorite andesites and hornblende dacites is related by fractional crystallisation, but this was in part an open-system process and may have been slightly modified by such factors as magma mixing. Another group appears to be the product of mixing between end members that lie somewhere within the continuous fractionation trend from pyroxene andesite to hornblende dacite. The occurrence of accessory and accidental xenoliths also suggests that crustal contamination and assimilation is most likely although this is yet to be isotopically confirmed. It is considered that Maungatautari was a subductionrelated volcano situated behind the volcanic front 1.8 m.y. ago. It is geochemically and tectonically distinct from the Alexandra Volcanies or any of the other volcanic rocks of similar age northwest of the Taupo Volcanic Zone. © 1986 Taylor & Francis Group, LLC.