J. W. Cole

Structural control and origin of volcanism in the Taupo volcanic zone, New Zealand

Taupor volcanic zone (TVZ) is the currently active volcanic arc and back-arc basin of the Taupo-Hikurangi arc-trench system, North Island, New Zealand. The volcanic arc is best developed at the southern (Tongariro volcanic centre) end of the TVZ, while on the eastern side of the TVZ it is represented mainly by dacite volcanoes, and in the Bay of Plenty andesite/dacite volcanoes occur on either side of the Whakatane graben. The back-arc basin is best developed in the central part of the TVZ and comprises bimodal rhyolite and high-alumina basalt volcanism. Widespread ignimbrite eruptions have occurred from this area in the past 0.6 Ma. Normal faults occur in both arc and back-arc basin. They are generally steeply dipping (>40°) and strike between 040° and 080°. In the back-arc basin, fault zones are en echelon and have the same trend as alignments of rhyolite domes and basalt vents. Open fissures have formed during historic earthquakes along some of the faults, and geodetic measurements on the north side of Lake Taupo suggest extension of 14±4 mm/year. In the Bay of Plenty and ML=6.3 earthquake occurred on 2 March 1987. Modelling of known structure in the area together with data derived from this earthquake suggests block faulting with faults dipping 45°±10° NW and a similar dip is suggested by seismic profiling of faults offshore of the Bay of Plenty where extension is estimated to be 5±2 mm/year. To the east of the TVZ, the North Island shear belt (NISB) is a zone of reverse-dextral, strike-slip faults, the surface expression of which terminates at the eastern end of the TVZ. On the opposite side of the TVZ in the offshore western Bay of Plenty and on line with the NISB is the Mayor Island fault belt. If the two fault belts were once continuous, as seems likely, strike-slip faults probably extend through the basement of the TVZ. When extension associated with the arc and back-arc basin is combined with these strike-slip faults, the resulting transtension provides a suitable tectonic environment for caldera formation and voluminous ignimbrite eruptions in the back-arc basin. The types of volcano in the TVZ are considered to be related to the source of magma and overlying crustal structure. Lavas of the arc are probably formed by a multistage process involving (1) subsolidus slab dehydration, (2) anatexis of the mantle wedge, (3) fractionation and minor crustal assimilation and (4) magma mixing. High-alumina basalts of the back-arc basin may be derived by partial melting of peridotite at the top of the mantle wedge, while rhyolitic magmas are thought to come from partial melting of lavas and subvolcanic reservoirs associated with the southern end of the Coromandel volcanic zone. Extreme thinning associated with transtension in the back-arc basin will favour the eruption of large-volume, gas-rich ignimbrites accompanied by caldera formation.

Evolution of the Taupo-Hikurangi subduction system

Abstract The present day Taupo-Hikurangi subduction system is a southward extension of the Tonga-Kermadec Arc system into a sediment-rich continental margin environment. It consists of a shallow structural trench (the Hikurangi Trough), a 150 km wide, imbricate thrust controlled accretionary borderland (the continental slope, shelf, and coastal hills of eastern North Island), a frontal ridge (the main “greywacke” ranges of North Island), and a volcanic arc and marginal basin (the Taupo Volcanic Zone). Structural elements become progressively more elevated and subduction more oblique towards the south. The whole NNE-trending system is truncated at a largely strike-slip, transform boundary that extends along the southwestern part of the Hikurangi Trough and the Hope fault of South Island to the main Alpine Fault. The volcanic arc is 200–270 km from the structural trench and comprises a NNE trending chain of andesite-dacite volcanoes extending along the eastern side of the Taupo Volcanic Zone. Most of the andesites are olivine-bearing and have been erupted within the last 50,000 years. It is suggested the Taupo-Hikurangi margin has evolved by rotation of accretionary elements, from an original NW-trending subduction system north of New Zealand. The older elements of the prism were associated with subduction of a re-entrant of the Pacific Plate (and perhaps the South Fiji Basin) in Mid Tertiary times. They subsequently became separated from their NW-trending volcanic arc by dextral strike-slip movement along curved faults east of the main “greywacke” ranges. During the Plio-Pleistocene, oblique subduction and accretion intensified as the Taupo-Hikurangi margin rotated into line with the NNE-trending Kermadec system and a marginal basin was developed along a similar trend to form the Taupo Volcanic Zone. Within the last 50,000 years olivine-bearing andesite volcanism has commenced along the eastern side of the Taupo Volcanic Zone.

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.

Structure, petrology, and genesis of Cenozoic volcanism, Taupo Volcanic Zone, New Zealand—a review

Abstract The Taupo Volcanic Zone extends for approximately 300 km north-eastwards across the central North Island, New Zealand. Within the zone are five volcanic centres: Tongariro, which includes over 95% andesites; Taupo, Maroa, and Okataina, rhyolitic centres, each probably associated with multiple caldera collapse; and Rotorua, a simple collapse structure, sited on the western side of Taupo Volcanic Zone. The lavas of the Taupo Volcanic Zone are all calc-alkaline. Two types of basalt occur; a low-alumina basalt within the Tongariro Volcanic Centre and a high-alumina basalt in the Rotorua-Taupo area. Andesites can be divided into “normal” (57–63% Si02) and low-Si (53–57% Si02) types. Pyroxene low-Si andesites probably represent cumulate fractions of the “normal” andesite, but olivine low-Si andesite may be derived from a separate younger magma. Two types of dacite can also be distinguished. Bay of Plenty lavas show a complete gradation from dacite to andesite, and lavas from the Rotorua-Taupo region sh...

Basalt triggering of the c. AD 1305 Kaharoa rhyolite eruption, Tarawera Volcanic Complex, New Zealand

Abstract The ∼AD 1305 Kaharoa eruption episode of Tarawera Volcanic Complex was the largest (∼4 km3 magma) eruption in New Zealand in the last 1000 years. High-silica (∼76% SiO2) rhyolite was erupted from seven vents along an 8-km linear zone to form pumiceous pyroclastic deposits and lava domes. Initial vent-clearing explosions were followed by a series of plinian pumice eruptions that spread tephra downwind, and pyroclastic density current deposits over the volcano slopes. The early pyroclastic eruptions were followed by extrusion of a dome in the summit vent and the migration of further plinian activity to adjacent vents. The episode finished with the extrusion of three summit lava domes, accompanied by block-and-ash flows. Two main types of rhyolite were erupted: (1) a low-Zr, low-Sr, high-Rb type comprising the early plinian pyroclastics and (2) a high-Zr, high-Sr, low-Rb type comprising the late eruptives including the summit domes. Basaltic material (basalt and basaltic andesite) is commonly present as free clasts within the Kaharoa pyroclastic deposits, along with rare clasts of granodiorite, diorite, gabbro and olivine clinopyroxenite. The basaltic clasts are often veneered by Kaharoa rhyolite. Basaltic material is also common as inclusions within rhyolite pumices, along with olivine and augite xenocrysts derived from the basalt. The basaltic clasts and inclusions contain complexly zoned plagioclase and corroded olivine (commonly with a reaction rim) consistent with disequilibrium caused by magma mixing. Some basaltic clasts and inclusions are hornblende-free; others contain groundmass hornblende. ‘Plumes’ of brown glass often extend from hornblende-bearing basaltic inclusions into the surrounding rhyolite. These plumes demonstrate magma mixing, as does the presence of rhyolitic glass and quartz xenocrysts in some basaltic inclusions. Most basaltic inclusions have crenulate boundaries, suggesting they were fluidal when coming into contact with the rhyolite magma. Diorite, gabbro and olivine clinopyroxenite clasts have mineralogy similar to the basalts, commonly contain patches of fine-grained basaltic material, and are considered co-magmatic with the basalts. Intrusion of basalt magma into the base of the rhyolite magma chamber appears to have occurred for some time before the Kaharoa eruptions began, allowing considerable mixing with the rhyolite magma, transfer of water to some basalt inclusions and formation of hornblende in their groundmass. Conversely, the hornblende-free basalt inclusions did not have time to react with the rhyolite magma and must have been intruded shortly before the start of the eruption. Intrusion of this hornblende-free basalt appears to have triggered the Kaharoa eruption, after the magma chamber had been primed by the earlier intrusions.

Structural control of volcanism in the McMurdo Volcanic Group, Antarctica

Volcanoes of the McMurdo Volcanic Group occur in four volcanic provinces: Balleny, Hallett, Melbourne and Erebus. The Balleny and Hallet provinces are distributed along the Balleny Fracture Zone and Hallett Fracture respectively. Stratovolcanoes within the Melbourne province may be associated with north to northwest-trending grabens and faults in northern Victoria Land. The Erebus volcanic province is located at the intersection of the Rennick Fault and northeast trending faults along the central Transantarctic Mountains. Within the Erebus province, volcanic centres around Mt. Erebus and Mt. Discovery possess radial symmetry which may be related to radial fractures at approximately 120° to each other.

Rhyolite magma processes of the ∼AD 1315 Kaharoa eruption episode, Tarawera volcano, New Zealand

Abstract Products of the ∼5 km3 (DRE), ∼5-yr duration Kaharoa eruption episode display two main high-silica rhyolite compositions; T1 erupted early (as plinian pyroclastics), and T2 erupted late (mostly as lavas). The T1 and T2 eruptive types are defined by crystal contents and compositional variations in whole rock, glass, plagioclase and biotite. Stratigraphically intermediate pyroclastic deposits have an intermediate composition (T1+2). A small volume of rhyodacite pyroclastics, mingled with injected basalt, was also erupted. The Kaharoa rhyolites were erupted from multiple sources spread along an 8-km linear vent zone, but the changes in eruptive compositions were largely controlled by position in the eruption sequence and magma discharge rates, rather than vent locations. Data from the Kaharoa eruptive types, vent locations, eruption sequence and discharge rates can be combined with concepts of magma chamber evacuation processes to produce a preliminary dimensional model of the pre-eruption rhyolite magma body. Our model magma body is sill-like, ∼8 km long, 1 km wide, 1.4 km thick, and located at ∼6–7 km depth in the upper crust. T1 magma overlay T2 magma in the upper levels of the chamber, with each magma layer internally mixed to a homogeneous composition along an axial extent defined by the vent locations. An underlying third rhyolite magma (T3) is recognised as the silicic end-member that was modified by basalt to form the rhyodacite eruptives. The rhyolite magma stratification survived multiple injections of basalt magma, which primed and finally triggered the Kaharoa eruptions. The T1+2 eruptives resulted from syn-eruption mingling in the conduit of the two main rhyolite magma types. Thickness of the T1 layer in the model can be estimated at 0.25 km; the T2 layer was somewhat thicker. Thicknesses of the underlying T3 and basalt layers are uncertain. Post-eruption geothermal heat flow indicates a residual magma volume of ≥6 km3, suggesting that the pre-eruption magma volume was ≥11 km3.

The Whakamaru group ignimbrites, Taupo Volcanic Zone, New Zealand: evidence for reverse tapping of a zoned silicic magmatic system

Abstract The Whakamaru group ignimbrites are widespread voluminous welded ignimbrites which crop out along the eastern and western margins of the Taupo Volcanic Zone (TVZ), New Zealand. The ignimbrites have a combined volume exceeding 1000 km3, and were erupted from a large caldera in the central TVZ around 340 ka, following a c. 350 ka hiatus in caldera-forming activity in TVZ. Analysis of individual pumice clasts identifies five distinct magma types (rhyolite types A to D, and high alumina basalt) and significant gradients in temperature, water content, and Sr isotopic composition in the pre-eruptive Whakamaru magmatic system. There is a marked variation in mineral assemblage with composition; type A low-silica rhyolite pumices contain plagioclase, quartz, orthopyroxene, hornblende, biotite, and magnetite/ilmenite with distinctive large rounded quartz phenocrysts. High-silica (types B and C) pumices contain quartz (smaller, subhedral phenocrysts), plagioclase, sanidine, biotite, and magnetite/ilmenite. Type D pumices are rich in plagioclase and biotite phenocrysts, and have anomalously high Rb contents (>200 ppm) relative to all other pumice types. Rhyolite types B and C are related to type A magma by a two-stage crystal fractionation process, probably by side wall crystallisation and convective fractionation. The first stage involved 30–40% fractionation of a plagioclase-dominated (sanidine-free) assemblage to produce a type B magma, which in turn underwent fractionation of a plagioclase/quartz/sanidine assemblage to produce the highly evolved, but relatively Ba-depleted, type C magmas. Stratigraphic variations in modal proportions of mineral phases, and calculated Fe–Ti oxide equilibrium temperatures indicate that eruptions commenced with the hottest, least evolved magmas, and more evolved magmas became important at a later stage in the eruption along with a high alumina basalt component. This reverse-zoned sequence precludes simple sequential tapping of a large zoned magma chamber, and indicates a complex magma chamber configuration and/or withdrawal dynamics during eruption. Type D magma, which appears to be unrelated to either types A or B by crystal fractionation, may have formed a separate subjacent chamber that was ruptured and incorporated into the eruption. The Whakamaru magma system provides clear evidence that (less evolved) low silica rhyolites undergo significant fractionation at shallow crustal levels in central TVZ, to produce the generally more evolved rhyolites more commonly erupted at the surface, and suggests large ignimbrite eruptions may tap multiple magma chambers.

Structure and eruptive history of the Tarawera Volcanic Complex

Abstract The Tarawera Volcanic Complex comprises 11 rhyolite domes and associated flows and one plug. These extrusions, together with pyroclastic debris, were formed during the Rerewhakaaitu eruption (14,700 ± 400 yr before 1960), the Waiohau eruption (11,250 ± 250 yr before 1960), and the Kaharoa eruption (930 ± 70 yr before 1960). The Tarawera eruption of 10 June 1886 was basaltic, forming the Tarawera Basalt, and terminating in a violently explosive phase which left a linear series of craters cutting three of the rhyolite domes, and providing good internal sections.

Andesites of the Tongariro volcanic centre, North Island, New Zealand

Abstract The Tongariro volcanic centre comprises four major andesite massifs, Kakaramea, Pihanga, Tongariro and Ruapehu, and four smaller cones and flows, Maungakatote, Pukeonake, Hauhungatahi and Ohakune. Older lavas from Kakaramea, Pihanga and Tongariro were erupted from a series of vents aligned NW-SE and more recent lavas from vents aligned NNE-SSW. The most voluminous lava types in the centre are labradorite and labradorite-pyroxene andesite containing phenocrysts of plagioclase (An 68 —An 48 ), orthopyroxene (mainly bronzite) and clinopyroxene, in a fine to medium-grained groundmass. Smaller amounts of pyroxene andesite (from Pihanga), olivine andesite (from NNE-trending vents), and hornblende andesite also occur. Pigeonite surrounds orthopyroxene in some of the pyroxene andesites. Average major and trace element compositions of each petrographic type indicate that all lavas are “normal” calc-alkaline andesites and low-Si andesites. Low-Si andesites, represented mainly by pyroxene and olivine andesites, are typified by higher FeO, MnO, MgO, Sr, Cu, Ni, V, Sc and Cr and lower Al 2 O 3 , Na 2 O, K 2 O, Rb, Ba, Zr and Pb than “normal” andesites. Some low-Si andesites are regarded as phenocryst-rich cumulates. The location of the Tongariro andesites and alignments of eruptive vents suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island arc andesites such as those of Tonga by having higher Na 2 O, K 2 O, Rb, Ba and Zr. This suggests some crustal contamination, but the overall chemistry of the andesites is inconsistent with an origin by mixing, en route to the surface, of high-alumina basalt with either Mesozoic greywacke-argillite or with a partial melt of the greywacke-argillite equivalent in composition to Taupo rhyolite. A model is presented in which oceanic crust assimilates Mesozoic and Cenozoic sediments at the eastern side of the North Island, and is subducted to produce amphibolite. This amphibolite would subsequently break down, hydrating the overlying upper mantle and lower crust, and, in steeply dipping subduction zones such as that under Tongariro, would produce phlogopite eclogite below 90 km. The latter will partially melt at 150–200 km and the resultant magma will fractionate in the upper mantle or lower crust to produce andesite.