Investigations of New Zealand Pyroclastic-Flow Deposits

Abstract

At least 38 distinct pyroclastic-flow formations are recognised in the Central North Island of New Zealand. These deposits comprise both young pyroclastic flows, which are generally small and non-welded, and older "ignimbrites" which are more voluminous and are at least in part welded. Most of these deposits have been erupted within the last 1-1.5 m.y. and can be related to one of four recognised centres of caldera collapse within the Central North Island. This thesis gives the results of a detailed study on one of these deposits, reviews the available data on all the major pyroclastic flows, and discusses the history of the four centres of caldera collapse. Part A contains a detailed study of whakamaru Ignimbrite, one of the most extensive and best exposed of the New Zealand pyroclastic-flow deposits. This ignimbrite extends approximately 2500 km2 to the north, west, and south of its source in Western Lake Taupo. Throughout most of this area, it comprises a simple cooling unit of non-welded to densely welded, rhyolitic tuff which contains up to 52 percent phenol-crysts of (in order of decreasing abundance) plagioclase, quartz, hypersthene, magnetite, sanidine, biotite, hornblende, zircon, apatite, and clinopyroxene, in a generally crystallized groundmass of fine shards and dust. Details of the lithology, texture and mineralogy of the ignimbrite are described for each of 12 vertical sections that are spaced throughout the outcrop area from the source to the distal edges. From this data lateral variations in whakamaru Ignimbrite have been determined. These variations are best demonstrated in sections between whakamaru and Arapuni along the waikato River because detailed correlation is possible in this area. This correlation is based on the recongnition at these sections of a series of pyroclastic-flow sheets of distinctive mineralogy, texture, and paleomagnetic properties. In each pyroclastic-flow sheet, significant, generally systematic lateral variations in lithology and mineralogy are related to an increasing distance from source towards the northwestern distal edges of the ignimbrite. Outside the Waikato River valley, comparable lateral changes are found away the source, towards the northern, western, and southern distal edges of the ignimbrite, A decrease in size and abundance of total phenocrysts, quartz, plagioclase, and pumice fragments away from source probably reflects changing capacity and competence of turbulent flows with distance from source, and results in slightly xenoliths occur near source but this is more likely to reflect a limited distribution of several xenolith-rich flows rather than lateral sorting. As the ignimbrite grades from a single cooling unit near source to a series of separate cooling units on the distal edges, porosity increases and degree of welding and crystallization decrease. These changes probably reflect both a thinning of flows and loss of heat with increasing distance from source. Local variations in this pattern result from an irregular pre-eruptive topography. The lateral and vertical in the specific type of devitrification products found in the ignimbrite suggests the conditions necessary for their formation. Lateral variation in phenocryst percentages are also reflected in the total-rock chemistry of the ignimbrite. Comparison of eight new analyses from the section at whakamaru to published analyses from the waipapa section, which is 18 kms farther from source, shows that significant variations occur in most of the major elements over this distance. These variations are considered to be mainly related to the lateral sorting of phenocrysts during emplacement, but differential post-depositional alteration may have had some effect. Part B summarises the available data on the age, source, strati-graphic relationships, lithology, and petrography of the major pyroclastic-flow deposits and associated volcaniclastic sediments of the Central North Island. A general bibliography and chemical data bibliography are also listed for each of these units. Based on the information from this summary, the sequence and timing of the events in the evolution of the Okataina, Rotorua, Maroa, and Lake Taupo Volcanic Centres are discussed. Appendices list data from the study on Whakamaru Ignimbrite, including, sample locations, modal analyses, size measurements, and physical property determinations. Also included is a discussion of the methods used in this study and their value in the study of welded pyroclastic-flow deposits in general.