Geochemical zoning and eruptive mixing in ignimbrites from Mangakino volcano, Taupo Volcanic Zone, New Zealand

Abstract

The Mangakino volcanic centre is a rhyolitic caldera complex situated in the western part of Taupo Volcanic Zone. Eruptive units ascribed to Mangakino volcanic centre have produced a sequence of voluminous (> 1,100 km 3 ), widespread welded and non-welded ignimbrites, phreatomagmatic fall deposits, and minor lava domes, mostly erupted in two concentrated periods of caldera-forming eruptions from 1.62 to 1.51 Ma, and 1.23 to 0.91 Ma. Geochemical, mineralogical, and Sr and Nd isotopic data are presented for the five main welded ignimbrites (Ngaroma, Ongatiti, Ahuroa. Rocky Hill, and Marshall), with particular emphasis on the Ongatiti, Ahuroa and Marshall ignimbrites. Most previous studies of the rhyolitic eruptives from the Taupo Volcanic Zone have concluded there is little, if any, compositional variation in individual units. The chemistry of the pumice clasts in the welded Mangakino ignimbrites indicate that they show significant compositional variations from low- to high-silica rhyolite (69–77% SiO 2 ), and from dacite to high-silica rhyolite (65–76% SiO 2 ) in the Ahuroa. The pumices have widely variable major- and trace-element compositions at any given stratigraphic level and there is no systematic variation in the composition of pumice with stratigraphic height. This implies that the Mangakino magma chambers were originally compositionally zoned, but the zonation was disrupted by pre- and syn-eruptive mixing processes. The ignimbrites cannot represent simple inverted magma chambers. The original compositional variation is considered to have been produced in part by crystal-liquid fractionation involving all the phenocryst (plagioclase, ferrohypersthene, hornblende, Fe-Ti oxides) and accessory (zircon, apatite) phases, although other magmatic processes such as magma mixing also probably occurred. Evidence from REE abundances and isotopic data suggest that the Mangakino magmas were derived from multiple sources, which included: (1) partial melting of a crustal source similar in composition to a western basement greywacke or its metamorphic equivalent; (2) a plagioclase-rich plutonic or metamorphic source similar to an anorthosite or trondhjemite: and (3) a minor upper mantle mafic source. The Ahuroa Ignimbrite contains discrete rhyolitic and dacitic pumices that have contrasting REE abundances, incompatible-element ratios, and Sr isotopic compositions indicating that they were derived from different sources, but these pumice clasts occur adjacent to each other within the same flow unit, implying that there has also been mixing of discrete bodies of magma prior to or during eruption. The existence of compositional zoning in Mangakino magmas, as opposed to the generally homogeneous rhyolite magmas in the eastern centres within the Taupo Volcanic Zone, could be explained by lower heat flows. Mangakino predated or lay west of the main region of triggering and convection-inducing effects of rising mantle-derived magmas, and also the dominant zone of faulting and crustal extension within the Taupo Volcanic Zone. This lower level of magmatic and tectonic activity may have permitted compositional zoning and more stable stratified conditions to develop in Mangakino magma chambers.