The Pokai and Chimp ignimbrites of NW Taupo Volcanic Zone

• Main Author: Karhunen, Ritva Annikki
• Language: en
• Published: University of Canterbury. Geology 2011
• Online Access: http://hdl.handle.net/10092/5791

Taupo Volcanic Zone (TVZ) is the largest active volcanic belt in New Zealand, and has erupted >10.000 km3 of dominantly rhyolitic magma during the last 1.6 m.y. This study concerns the field relations, volcanology and petrology of two post-Whakamaru (330 ka) - pre-Mamaku (140 ka) ignimbrites, informally named as the Pokai and Chimp ignimbrites, occurring in a ca. 360 km2 area SW and W from Rotorua in the north-western TVZ. The Pokai Ignimbrite has a minimum volume of ca. 33 km3 DRE, whereas the older Chimp Ignimbrite has a minimum volume of only ca. 5 km3 DRE. Of the two ignimbrites the younger Pokai Ignimbrite is better preserved and is thus the main emphasis in this thesis. The Chimp Ignimbrite is relatively pumice- and crystal-poor (1-2 vol. % phenocrysts), and the exposed flow units are relatively thin (4-6 m). A short plinian phase preceded the Chimp Ignimbrite, whereas the Pokai Ignimbrite is marked by a number of pre-ignimbrite air-fall pumice and ash layers. The Pokai Ignimbrite represents a multiple flow unit ignimbrite, with single flow units usually ranging from 6-30 m. Thick deposits (>20 m thick) are usually welded in the upper middle part of the deposit. Ground deposits, i.e. layer 1 deposits, are rare. Field evidence suggest that the Pokai Ignimbrite originated from the Kapenga Volcanic Centre, a multiple caldera structure in the northern central TVZ. Two pumice types occur in the Pokai Ignimbrite; a crystal-poor type (2-3 % phenocrysts) and a crystal-rich type (6-12 % phenocrysts). Plagioclase is the dominant phenocryst throughout, with minor amounts of orthopyroxene, Fe-Ti oxides and quartz, which occurs in ca. 30 % of the pumices. Hornblende and clinopyroxene are present occasionally. The Pokai Ignimbrite ranges from mildly to strongly peraluminous, whereas the Chimp Ignimbrite is mildly peraluminous, both coinciding with other TVZ rhyolitic ignimbrites, but clearly differing from the rhyolitic lavas which are usually metaluminous to only mildly peraluminous. Whereas most TVZ rhyolitic eruptives have been regarded as relatively homogeneous, the Pokai Ignimbrite shows significant geochemical variation. The magma chamber was compositionally zoned from crystal-poor, high silica, low Sr (77 % SiO2 , 50 ppm Sr) rhyolitic top to more crystal-rich, low silica, high Sr (70 % SiO2 , 130 ppm Sr) rhyolite at the deeper levels. Prior to the eruption vigorous mixing of magma from different levels occurred, producing different pumice types in the airfall deposits, and multiple phenocryst populations in single pumice clasts. As the eruption progressed successively deeper levels of the magma chamber were tapped, the last eruption products representing the less evolved, crystal-rich magma. Least squares and Rayleigh fractionation models indicate that the Pokai, and the Chimp magmas most probably generated by AFC from TVZ andesitic magmas contaminated by Mesozoic basement sediments.