Roger M. Briggs

Volcanic and structural evolution of Taupo Volcanic Zone, New Zealand: a review

The Taupo Volcanic Zone (TVZ) in the central North Island is the main focus of young volcanism in New Zealand. Andesitic activity started at c. 2 Ma, joined by voluminous rhyolitic (plus minor basaltic and dacitic) activity from c. 1.6 Ma. The TVZ is c. 300 km long (200 km on land) and up to 60 km wide, as defined by vent positions and caldera structural boundaries. The total volume of TVZ volcanic deposits is uncertain because a sub-volcanic basement has not been identified, but present data suggest bulk volumes of 15–20,000 km3, and that faulted metasediments form most of the immediate subvolcanic basement. Rhyolite (≥15,000 km3 bulk volume, typically 70–77% SiO2) is the dominant magma erupted in the TVZ (mostly as calderaforming ignimbrite eruptions), andesite is an order of magnitude less abundant, and basalt and dacite are minor in volume (< 100 km3 each). The history of the TVZ is here divided into ‘old TVZ’ from 2.0 Ma to 0.34 Ma, and ‘young TVZ’ from 0.34 Ma onwards, separated by the Whakamaru eruptions, which obscured much of the evidence for older activity within the zone. The TVZ shows a pronounced segmentation into northeastern and southwestern andesite-dominated extremities with composite cones and no calderas, and a central 125-km-long rhyolite-dominated segment. Eight rhyolitic caldera centres have so far been identified in the central segment, of which two (Mangakino and Kapenga) are composite features, and more centres will probably be delineated as further data accumulate. These centres account for 34 inferred caldera-forming ignimbrite eruptions, in the c. 1.6-Ma lifetime of the central TVZ. The modern central TVZ is the most frequently active and productive silicic volcanic system on Earth, erupting rhyolite at c. 0.28 m3 s−1, and available information suggests this has been so for at least the past 0.34 Ma. The rhyolites show no major compositional changes with time, though the extent of magma chamber zonation may have changed with the incoming of rifting and crustal extension in the past c. 0.9 Ma. Within the central TVZ, non-rhyolitic compositions have been erupted apparently irregularly in time and space; in particular there is no evidence for a geographic separation of basalts from andesites. Between 0.9 and 0.34 Ma, a major episode of uplift affected areas around the TVZ, while at the same time the main focus of activity may have migrated eastwards within the TVZ accompanying rifting along the axis of the zone. The modern TVZ is rifting at rates between 7 and 18 mm a−1 and restoration of the thin (15km) ‘crust’ (Vp ≤ 6.1 km s−1) beneath the central TVZ to its pre-rifting thickness (25 km) implies that rifting at such rates may have begun only at c. 0.9 Ma. The TVZ is a rifted arc, but its longitudinally segmented nature, high thermal flux and voluminous rhyolitic volcanism make it unique on Earth.

Chronology and dynamics of a large silicic magmatic system: Central Taupo Volcanic Zone, New Zealand

The central Taupo Volcanic Zone in New Zealand is a region of intense Quaternary silicic volcanism accompanying rapid extension of continental crust. At least 34 caldera-forming ignimbrite eruptions have produced a complex sequence of relatively short-lived, nested, and/or overlapping volcanic centers over 1.6 m.y. Silicic volcanism at Taupo is similar to the Yellowstone system in size, longevity, thermal flux, and magma output rate. However, Taupo contrasts with Yellowstone in the exceptionally high frequency, but small size, of caldera-forming eruptions. This contrast reflects the thin, rifted nature of the crust, which precludes the development of long-term magmatic cycles at Taupo. 11 refs., 4 figs., 1 tab.

Petrology and petrogenesis of volcanic rocks from the Taupo Volcanic Zone: a review

Abstract Taupo Volcanic Zone (TVZ) volcanic rocks comprise three major compositional series: high-alumina basalt (HAB), andesite, and rhyolite, plus a fourth, minor, dacitic series resulting from mixing of andesite and rhyolite magma. Relatively rare HABs originate as near-primary magma from depleted, chemically and isotopically homogeneous mantle. They erupted exclusively through thin, extensional crust, and have evolved by limited crystal fractionation and crustal assimilation. Andesite volcanoes broadly define the present-day active volcanic front, particularly in the southern and northern sectors of the TVZ. The rocks are generally high-silica, but range in composition from basaltic andesite to dacite. Nine petrologically distinct andesite types are recognised; none are directly related to HAB, the majority originating as AFC derivatives of low-alumina basalt. Rhyolite is volumetrically dominant in the TVZ (> 90% by volume), occurring predominantly in the central part but also offshore to the north as lava, ignimbrite and airfall deposits. Although geochemical and isotopic data, and experimental studies have placed some constraints on petrogenetic models, these remain controversial. Crustal anatexis of meta-greywacke basement can be dismissed as a major source, but basement rocks may be involved through secondary contamination. Least-squares mixing models using geochemistry and a variety of stable and radiogenic isotopes show that TVZ rhyolite could be generated by AFC of a mafic parent. However, these models cannot easily explain the apparent absence of large volumes of intermediate rocks and mafic residues. Melting of pre-existing volcanics or lower crustal granulites is also possible, but the existence of such rocks beneath the TVZ is not proven. Although HAB, andesite and rhyolite have coherent major-element compositions, and their occurrence can be explained in terms of crystal fractionation/crustal contamination/AFC models, all three have distinctive trace-element and isotopic characteristics that cannot be convincingly explained by any single-stage petrogenetic model.

Geochemical zoning and eruptive mixing in ignimbrites from Mangakino volcano, Taupo Volcanic Zone, New Zealand

The Mangakino volcanic centre is a rhyolitic caldera complex situated in the western part of Taupo Volcanic Zone. Eruptive units ascribed to Mangakino volcanic centre have produced a sequence of voluminous (> 1,100 km3), widespread welded and non-welded ignimbrites, phreatomagmatic fall deposits, and minor lava domes, mostly erupted in two concentrated periods of caldera-forming eruptions from 1.62 to 1.51 Ma, and 1.23 to 0.91 Ma. Geochemical, mineralogical, and Sr and Nd isotopic data are presented for the five main welded ignimbrites (Ngaroma, Ongatiti, Ahuroa. Rocky Hill, and Marshall), with particular emphasis on the Ongatiti, Ahuroa and Marshall ignimbrites. Most previous studies of the rhyolitic eruptives from the Taupo Volcanic Zone have concluded there is little, if any, compositional variation in individual units. The chemistry of the pumice clasts in the welded Mangakino ignimbrites indicate that they show significant compositional variations from low- to high-silica rhyolite (69–77% SiO2), and from dacite to high-silica rhyolite (65–76% SiO2) in the Ahuroa. The pumices have widely variable major- and trace-element compositions at any given stratigraphic level and there is no systematic variation in the composition of pumice with stratigraphic height. This implies that the Mangakino magma chambers were originally compositionally zoned, but the zonation was disrupted by pre- and syn-eruptive mixing processes. The ignimbrites cannot represent simple inverted magma chambers. The original compositional variation is considered to have been produced in part by crystal-liquid fractionation involving all the phenocryst (plagioclase, ferrohypersthene, hornblende, Fe-Ti oxides) and accessory (zircon, apatite) phases, although other magmatic processes such as magma mixing also probably occurred. Evidence from REE abundances and isotopic data suggest that the Mangakino magmas were derived from multiple sources, which included: (1) partial melting of a crustal source similar in composition to a western basement greywacke or its metamorphic equivalent; (2) a plagioclase-rich plutonic or metamorphic source similar to an anorthosite or trondhjemite: and (3) a minor upper mantle mafic source. The Ahuroa Ignimbrite contains discrete rhyolitic and dacitic pumices that have contrasting REE abundances, incompatible-element ratios, and Sr isotopic compositions indicating that they were derived from different sources, but these pumice clasts occur adjacent to each other within the same flow unit, implying that there has also been mixing of discrete bodies of magma prior to or during eruption. The existence of compositional zoning in Mangakino magmas, as opposed to the generally homogeneous rhyolite magmas in the eastern centres within the Taupo Volcanic Zone, could be explained by lower heat flows. Mangakino predated or lay west of the main region of triggering and convection-inducing effects of rising mantle-derived magmas, and also the dominant zone of faulting and crustal extension within the Taupo Volcanic Zone. This lower level of magmatic and tectonic activity may have permitted compositional zoning and more stable stratified conditions to develop in Mangakino magma chambers.

40Ar/39Ar ages of silicic volcanic rocks in the Tauranga‐Kaimai area, New Zealand: Dating the transition between volcanism in the Coromandel Arc and the Taupo Volcanic Zone

Abstract Subduction‐related volcanism in the northern part of the North Island of New Zealand shifted abruptly during the late Pliocene. This study focuses on the transition, in time and space, from the NNW‐oriented Miocene‐Pliocene Coromandel Volcanic Zone to the northeast‐oriented active Taupo Volcanic Zone. The volcanic rocks marking this transition are exposed in the Tauranga Basin and adjacent Kaimai Range, and associated here with the recently defined Tauranga and Kaimai Volcanic Centres, respectively. New 40Ar/39Ar age determinations indicate that the transition occurred between 1.90 and 1.55 Ma, that is between the youngest age of silicic volcanism in the Tauranga‐Kaimai area, and the age of the oldest silicic volcanism in the Taupo Volcanic Zone. This interpretation is generally consistent with recent plate models and with the initiation of the Kermadec Arc within the last 2 m.y.

Ages of the Pliocene—Pleistocene Alexandra and Ngatutura Volcanics, western North Island, New Zealand, and some geological implications

Abstract The Alexandra and Ngatutura Volcanics are the two southernmost of the Pliocene-Quaternary volcanic fields of western and northern North Island. New Zealand. northwest of Taupo Volcanic Zone (TVZ). The Ngatutura Basalts are an alkalic basaltic field comprising monogenetic volcanoes. The Alexandra Volcanics consist of three basaltic magma series: an alkalic (Okete Volcanics), calcalkalic (Karioi, Pirongia, Kakepuku, and Te Kawa Volcanics), and a minor potassic series. Twenty new K-Arages are presented for the Alexandra Volcanics and 9 new ages for the Ngatutura Basalts. Ages of the Alexandra Volcanics range from 2.74 to 1.60 Ma, and the ages of all three magma series overlap. Ages of the Ngatutura Basalts range from 1.83 to 1.54 Ma. Each basaltic field has a restricted time range and there is a progressive younging in age of the basaltic fields of western North Island from the Alexandra Volcanics in the south, to Ngatutum, to South Auckland, and then to the Auckland field in the north. Neither of t...

K‐Ar ages, paleomagnetism, and geochemistry of the South Auckland volcanic field, North Island, New Zealand

Abstract The South Auckland volcanic field is one of the Pliocene‐Quaternary intraplate basaltic fields in northern North Island. It consists of at least 97 monogenetic volcanic centres covering an area of c. 300 km2, 38 km south of Auckland. Fifty‐nine of the volcanic centres are characterised by mainly magmatic or effusive activity that constructed scoria cones and lava flows, while 38 are mainly phreatomagmatic or explosive that produced tuff rings and maars. Rock types consist of basanites, hawaiites, nepheline hawaiites, transitional basalts, and oZ‐tholeiitic basalts, with relatively minor amounts of nephelinites, alkali basalts, Q‐tholeiitic basalts, and nepheline mugearites. Forty‐three new K‐Ar ages are presented, which range from 0.51 to 1.59 Ma, and show two peaks of activity at 0.6 and 1.3 Ma. Paleomagnetic determinations at 26 selected sites agree well with the paleomagnetic reversal time scale and support the K‐Ar age data. Age data from each of the volcanic fields of Okete, Ngatutura, South...

Emplacement history of a low-viscosity, fountain-fed pantelleritic lava flow

Abstract The strongly peralkaline 8-ka flow, Mayor Island, New Zealand, was emplaced as a low-viscosity fountain-fed lava flow. The flow stratigraphy comprises from top to base: a finely vesicular pumiceous carapace: an upper obsidian layer; central crystalline rhyolite; a lower obsidian layer incorporating a crumble breccia; above a fall lapilli unit. Rare relict clasts welded within the lowermost 0.5 m of the lower obsidian layer support a fountain-fed origin for the 8-ka flow. Five physical parameters (density, porosity, microlite size, void and autoclast aspect ratios) and water content, were measured throughout the thickness of the lava flow, and show that prolonged vesiculation occurred in the upper 33% of the flow thickness, and that autoclast and pyroclast shapes, and microlite lengths were influenced by the thermal conditions of the lava after eruption. Crystallisation of the flow interior commenced before the flow stopped moving. Compared with calc-alkaline rhyolites, the high flow temperature (∼ 950°C) and peralkaline chemistry contributed to a less polymerised low-viscosity melt (log η=6.4 Pa s). These data and structural observations of the 8-ka flow suggest that near-complete homogenisation and remobilisation of spatter from a lava fountain occurred on the caldera rim, producing a distally tapering lava sheet that also spilled back down the caldera wall towards the vent.

Pyroclastic deposits and volcanic history of Mayor Island

Abstract The emergent summit of Mayor Island, Bay of Plenty, New Zealand, is a peralkaline rhyolite volcano constructed by: a sequence of lava flows, the Tutaretare Rhyolite Formation (new); and pyroclastic deposits, the Oira Pyroclastite Formation (new). These 2 formations constitute the Mayor Island Group (new). The pyroclastic deposits mantle most of the outer slopes of the island, in places exceeding 100 m in thickness, and also occur interbedded with lava flows of the main cone. The pyroclastics have been informally assigned on the basis of their compositional, welding and textural, and sedimentary structural characteristics to one or other of 15 lithotypes which may be related to palrticular modes of eruption and emplacement, of both airfall (phreatic, phreatomagmatic, phreatoplinian, and plinian types) and pyroclastic flow (ignimbrite, nuee ardente, and base surge types) origins. A sixteenth lithotype comprises epiclastic deposits formed possibly by catastrophic overspill from an ancestral crater l...

Physical volcanology and emplacement history of the Ben Lomond rhyolite lava flow, Taupo Volcanic Centre, New Zealand

Abstract Currently, there are two contrasting models concerning the state in which rhyolite lava reaches the surface: (1) the permeable foam model, where the lava reaches the surface as an expanded foam and collapses into dense obsidian during lava flow; and (2) the traditional model, where decreased load pressure leads to vesiculation of the upper surface of the dome. In order to test these models, textural parameters were documented on outcrops of the Ben Lomond rhyolite dome, which has two flow lobes comprising an eroded dome (c. 100 ka old) cut by the Whangamata Fault. A composite stratigraphy of the upper 60 m of the flow, assembled from three sections (roadcut, airstrip and fault scarp), comprises, irom the flow top: finely vesicular pumice, black aphyric obsidian, and a spherulitic transition zone above a central crystalline rhyolite core. An explosion breccia occurs as a pod at a depth of c. 35 m and cross‐cuts the upper two units us an inverted cone‐shaped deposit—an infilled explosion pit. Eight...