Mineralogy and Petrogenesis of the Whakamaru Ignimbrite in the Maraetai area of the Taupo volcanic zone, New Zealand

Abstract

The Whakamaru Ignimbrite crops out over an area of nearly 1,000 square miles, between Putaruru and Lake Taupo. In this paper, it is treated as two distinct sheets, the lower sheet (Sheet 1) being up to 300 ft thick, and the upper sheet (Sheet 2) being about 100 ft thick. The Whakamaru Ignimbrite contains phenocrysts of plagioclase, quartz, sanidine, hypersthene, magnetite, hornblende, and biotite set in a vitroclastic matrix (mostly devitrified). At the base of Sheet 2 is a lenticulite, about 5 ft thick, characterised by black glassy lenticules enclosed by devitrified ignimbrite. Chemical analyses of all the phases (except quartz), from this lenticulite, are presented. The glassy lenticules are greatly enriched in potassium and silica relative to the ignimbrite, and also contain a distinct mineralogical assemblage, i.e., quartz, biotite, plagioclase, and magnetite only. Through Sheet 1 there is an upward decrease in normative Or, and increase in normative Ab and An of the total rock compositions. In addition there is an upward change in mineralogy through Sheet 1, in particular, a progressive upward increase in total crystal content, in the sizes of the phenocrysts of plagioclase, quartz, hornblende, and biotite, and in the amount of resorption of quartz. There is a progressive upward decrease in the plagioclase/quartz ratio. The mineralogy and chemistry of Sheet 2 is essentially similar to the topmost part of Sheet 1. These variations as seen in the two sheets of the ignimbrite are inferred to be a reversal of their sequence in the original magma chamber prior to the eruption of the ignimbrite. Sanidine occurs at the top of Sheet 1 and throughout Sheet 2, although the residual glasses at the base of Sheet 1 lie on the feldspar cotectic. Biotite, however, occurs throughout both sheets. This is attributed to an upward increase in vapour pressure in the original magma chamber, the lower vapour pressure favouring the crystallisation of sanidine deeper within the magma body. Similarly, the inferred downward decreasing plagioclase/quartz ratio in the magma chamber prior to eruption is also attributed to the decreasing vapour pressure. From the compositions of the residual glasses with respect to the boundary curves, in the system Ab—Or—Q—H₂O, the following approximate vapour pressures are suggested: 1,000 kg/cm² at the base of Sheet 1, 750 kg/cm² for the ignimbrite at the base of Sheet 2, and 2,000 kg/cm² for the lenticules from the base of Sheet 2. Liquidus temperatures suggested for the ignimbrite by reference to the same system are 720–740°c; Barth's two-feldspar method gives 750°c. The origin of the potassium-rich lenticules, and of the upward enrichment in potassium within the magma body prior to the eruption of Sheet 1, are both explained as a result of alkali transfer In supercritical water, as a result of a recurrent “boiling” process. This is considered to have been brought about by upward movements of the magma in response to previous large-scale eruptions. Although diffusion and mixing would tend to equalise the chemical gradient, renewed boiling would keep the gradient established. The ultimate origin of the ignimbrite magma is believed to be deep-seated fusion of greywacke-type sedimentary rocks, in accordance with the accepted ideas.