The Matahina ignimbrite: its evolution including its eruption and post depositional changes

Abstract

The rhyolitic Matahina eruption sequence outcrops in the northeast of the Taupo Volcanic Zone of New Zealand. The sequence consists of an airfall deposit overlain in places by up to four ignimbrites erupted from the Okataina Volcanic Centre. The airfall consists, at its base, of crystal poor (< 7%) pumice with a phenocryst assemblage of plagioclase, quartz, hypersthene, magnetite and ilmenite with rare zircon and apatite. Towards the top of the airfall unit crystal rich (> 7%) pumice is present along with crystal poor pumice and thus forms a bimodal population. The crystal rich pumice has the same mineralogy as the crystal poor pumice plus hornblende. The pumice ranges from rhyolite to high-silica rhyolite with the rhyolite containing high TiO2, MgO, CaO, Zr and Sr and low K2O, Ba and Rb in comparison to the high silica rhyolite. The mixture of high silica rhyolite and rhyolite pumice is present throughout the remainder of the eruption with more high silica rhyolite than rhyolite erupted and with no significant change in proportions. The variation in chemistry and mineralogy indicates that the magma was compositionally zoned. Random and oscillatory zoning with evidence of resorption episodes in plagioclase and hypersthene suggest that the magma was turbulently convecting. Zoning in the magma was therefore probably characterised by fluid interfaces between fluid dynamically distinct layers. Stability relations of hornblende and pressure estimates suggest that the hornblende containing rhyolite lay above the high silica rhyolite. The variation is interpreted as having formed by rapid crystallisation and absorption of water on the margins of the intrusion. Least square calculations indicate that fractional crystallisation is a viable mechanism although diffusion through the fluid interface(s) must have occurred with redistribution of components by convection in each layer. The vent of the Matahina eruption tapped the side of the magma chamber close to the interface between the rhyolite and high silica rhyolite. Turbulence during the eruption lead to limited mixing of the zoned magma with the formation of pumice with overlapping mineral composition ranges and rare banded pumice. Indicators such as a lack of an associated veneer deposit, the multi-flow nature of the eruption sequence and strong welding suggest that the eruption was of moderate violence (for its type). Modelling of welding suggests depositional temperatures of between 650 and 700°C in the Rangitaiki Valley. The presence of unexsolved titanomagnetites and osumilite support such high depositional temperatures. Almandine garnet of vapour phase origin indicates that little air was incorporated into the eruption column. Devitrification was characterised by the formation of axiolitic and spherulitic intergrowths of cristobalite and alkali feldspar. SEM observations suggest that the density of nucleating points determines the textures in the devitrifying ignimbrite. Degassing of the ignimbrite was characterised by corrosion of glass and subsequent deposition of minerals such as tridymite and alkali feldspar. Si and Zr were the least mobile elements. The Matahina eruption sequence is an example of a zoned pluton erupted early in its evolution.