Petrology and petrogenesis of volcanic rocks from the Taupo Volcanic Zone: a review

Abstract

Taupo Volcanic Zone (TVZ) volcanic rocks comprise three major compositional series: high-alumina basalt (HAB), andesite, and rhyolite, plus a fourth, minor, dacitic series resulting from mixing of andesite and rhyolite magma. Relatively rare HABs originate as near-primary magma from depleted, chemically and isotopically homogeneous mantle. They erupted exclusively through thin, extensional crust, and have evolved by limited crystal fractionation and crustal assimilation. Andesite volcanoes broadly define the present-day active volcanic front, particularly in the southern and northern sectors of the TVZ. The rocks are generally high-silica, but range in composition from basaltic andesite to dacite. Nine petrologically distinct andesite types are recognised; none are directly related to HAB, the majority originating as AFC derivatives of low-alumina basalt. Rhyolite is volumetrically dominant in the TVZ (> 90% by volume), occurring predominantly in the central part but also offshore to the north as lava, ignimbrite and airfall deposits. Although geochemical and isotopic data, and experimental studies have placed some constraints on petrogenetic models, these remain controversial. Crustal anatexis of meta-greywacke basement can be dismissed as a major source, but basement rocks may be involved through secondary contamination. Least-squares mixing models using geochemistry and a variety of stable and radiogenic isotopes show that TVZ rhyolite could be generated by AFC of a mafic parent. However, these models cannot easily explain the apparent absence of large volumes of intermediate rocks and mafic residues. Melting of pre-existing volcanics or lower crustal granulites is also possible, but the existence of such rocks beneath the TVZ is not proven. Although HAB, andesite and rhyolite have coherent major-element compositions, and their occurrence can be explained in terms of crystal fractionation/crustal contamination/AFC models, all three have distinctive trace-element and isotopic characteristics that cannot be convincingly explained by any single-stage petrogenetic model.