Emplacement processes of ignimbrites in the Ongatiti Valley, southeast Te Kuiti

摘要

The Mangakino volcanic centre (MVC) is the oldest rhyolitic volcano of the Taupo Volcanic Zone (TVZ) located on the westernmost boundary. Ten eruptive deposits have been derived from the MVC and this study focuses on the 1.21 ± 0.04 Ma Ongatiti Ignimbrite and the 1.20 ± 0.04 Ma Unit D fall and overlying ignimbrite deposit. The Taupo volcanic centre (TVC), located at the southern boundary of the central TVZ is currently active and is considered to be the most active and productive rhyolitic centre on Earth. There have been multiple eruptions derived from the TVC over the past 350 – 320 ka, however the most recent Taupo eruption dated at 232 AD is the only deposit studied in this research. The study area is situated within the Ongatiti Valley, 15 km SE of Te Kuiti. It is 28 km west of the Mangakino caldera rim, and is located to the west of the upstanding Mesozoic basement block of the Rangitoto Range. A 1:25 000 geological map of the volcanic geology exposed within the Ongatiti Valley is produced. Detailed stratigraphc logs, petrographic descriptions and quantitative analysis on the mineralogy are given for all of the exposed eruptive deposits. The geochemical composition of the Ongatiti Ignimbrite and Unit D are determined through XRF, Electron microprobe and ICPMS analyses. Stratigraphic sections of the Ongatiti Ignimbrite in well exposed 25 m high outcrops in the Ongatiti Valley show variations in pumice, crystal, and lithic abundance, both vertically and laterally. Two units can be distinguished within the Ongatiti Ignimbrite overlying a poorly exposed lowermost fall deposit: a lower densely to partially welded, crystal-rich, pumice-poor flow sequence, and an upper densely to partially welded pumice and crystal-rich flow sequence. The boundary between the two units is wavy and gradational, and varies in altitude over relatively short distances. This undulating boundary is considered to result from interaction of pyroclastic density currents flowing both over and around the high-standing Rangitoto Range, and then covering a highly irregular pre-ignimbrite topography in the King Country. In the Ongatiti Valley, Unit D consists of several phreatoplinian fall deposits, a fine-grained ignimbrite and an overlying coarser-grained ignimbrite with a combined thickness of 3 m. A stratigraphic log is presented and the characteristic textures, structure, petrography and geochemistry is described. A six stage process is suggested for the change in the conditions/styles and intensities between the different phases of the eruption. A 15 m high valley-ponded section of the 232 AD Taupo Ignimbrite occurs in the Waipa River Valley, that contains fossilised tree trunks in position of growth. A possible origin involving variation in dynamic pressure and subsequent destructive power of pyroclastic density currents is developed.