Lithology and eruptive history of the Whakamaru ignimbrites in the Maraetai area of the Taupo volcanic zone, New Zealand

Abstract

About 1,000 square miles of the western part of the Taupo Volcanic Zone is covered by the Whakamaru Ignimbrites, a formation consisting of more than a dozen coarse-textured tuff sheets. They are of Pleistocene age, and were erupted over an irregular topographic surface after a prolonged pause in acid ignimbrite volcanism in this province. Locally their total thickness exceeds 1,000 ft. The rocks are generally lightly welded and contain from 10 to 45% of phenocrysts—andesine, quartz, hypersthene, sanidine, magnetite, biotite, and hornblende. In the northern half of the Whakamaru outcrop area the Maraetai Member is exposed. Here it forms the basal member, and consists of the most highly welded rock in the formation. It is up to 500 ft thick and shows two types of horizontal zonation or stratification: (1) Lithologic zoning into glassy, felsitic, and tuffaceous zones, determined by cooling environment; and (2) Ash-flow (or primary) layering, caused by spasmodic eruption or deposition. The Maraetai Ignimbrite Member is a cooling unit composed of two primary ash-flow units, but further subdivision is possible towards its distal margin. At Whakamaru, near its presumed source of eruption, only two units are discernible, although their contact is gradational. At Maraetai, 6 miles distant, the lower unit is readily divisible into two or possibly three sub-units. Beyond Waipapa, 7 miles still further, similar subdivisions are yet more pronounced, because of colour and erosional differences. The upper flow unit, in contrast to the lower, has a higher content of pumice and crystal fragments and maintains its “monolithic” character throughout. The lithologic zonation, typical of welded ignimbrites, is superimposed on the ashflow layering and indicates that the Maraetai eruptions occurred during a short time interval. Succeeding ignimbrites are thin and poorly welded, suggesting that the Whakamaru eruptions were, in general, recurrent ebullitions of declining volume and temperature, and presumably, declining magma volatile pressure. Phenocryst content increases markedly upward in the Maraetai Member, indicating an eruption of hot liquid magma followed by magma at increasingly advanced stages of crystallisation.