Chronological and palaeomagnetic constraints on widespread welded ignimbrites of the Taupo volcanic zone,New Zealand

 Large volume (100–1000 km³), widespread rhyolitic ignimbrites are the main products of the Taupo volcanic zone (TVZ) of New Zealand, one of the most active silicic volcanic regions on Earth. Several factors have made correlation and the eruptive history of the ignimbrites difficult to resolve, including limited exposure and chronological data, broadly similar lithologies and the lack of stratigraphic successions visible in the field. We have used the isothermal plateau fission track (ITPFT) method on glass shards from the non-welded basal zones to obtain new eruption ages for the widespread units: Ongatiti (1.25±0.12 Ma), Whakamaru group (0.34±0.03 Ma), Matahina (0.34±0.02 Ma), Chimp (0.33±0.02 Ma), Kaingaroa (0.31±0.01 Ma) and Mamaku (0.23±0.01 Ma) ignimbrites. These glasses show little evidence of geochemical alteration and allow the units to be fingerprinted for correlation. The glass ages we have obtained for the late Quaternary units provide an independent check on chronological data obtained from phenocryst phases. The ITPFT method is a useful dating approach for sanidine-poor eruptives which limit the application of ⁴⁰Ar/³⁹Ar. Errors as limited as 10–30 ka can be obtained from the weighted mean of several age determinations. The thermoremanent magnetic (TRM) direction recorded in the units provides a means of correlation over a wide area of the TVZ, because each ignimbrite can be distinguished by its unique record of palaeosecular variation. These data indicate that the four separately mapped members of the Whakamaru group represent the same phase of activity, occurring within a period of 100 years. The TRM data indicate that the widespread Ahuroa ignimbrite erupted during an excursion in Earth's magnetic field, perhaps associated with the Cobb Mountain subchron (ca. 1.2 Ma). The youngest widespread welded unit, Mamaku ignimbrite (ca. 0.23 Ma), also erupted during an excursion and may represent a southern hemisphere record of the Pringle Falls geomagnetic episode found in the western United States. The palaeomagnetic and ITPFT data for the widespread late Quaternary ignimbrites suggest a major period of caldera formation at 0.34–0.30 Ma. This interval represents the eruption of multiple units from the Whakamaru caldera, followed by the formation of the Okataina and Reporoa calderas in rapid succession. Received: 20 November 1995 / Accepted: 8 May 1996     

Geological structure of hydrothermal fields in the taupo volcanic zone,New Zealand

Wairakei hydrothermal field is underlain by an acid volcanic sequence consisting of the following units: Recent pumice cover, Wairakei Breccia, Huka Falls Formation, Haparangi Rhyolite, Waiora Formation, Waiora Valley Andesite, Wairakei Ignimbrites, Ohakuri Group. The stratified volcanic sequence is draped over a basement horst and thickens eastward and westward into adjoining volcanotectonic depressions. These major depressions have grown progressively during the Quaternary by differential subsidence along active faults and were not the direct result of collapse following major ignimbrite and rhyolite eruptions. The bulk of the steam production is obtained from a thick aquifer of pumice breccias (Waiora aquifer), which is capped by lacustrine shales of the Huka Falls Formation. The best production is obtained from fault zones intersecting the lower part of this aquifer. The Ohakuri Group, underlying the Wairakei Ignimbrites, constitutes a lower, pumice breccia aquifer, not yet directly exploited by drill-holes. Hydrothermal water at 265°C is fed from the lower to the upper aquifer through linear fissures in the Wairakei Ignimbrites, principally at the cress of a small structural dome. These fissures are related to active, north-east striking, normal faults, having a small dextral transcurrent component. Major zones of heat liberation have been localised at the intersection of secondary, north-west cross faults. Fossil, hydrothermal, mud-flow, conglomerates intercalated in the mid-Pleistocene Huka Falls Formation, suggest that hydrothermal activity at Wairakei is at least 500,000 years old. Isopachs on the early Pleistocene Waiora Valley Andesite indicate that the andesite plug and associated flows were crupted at the same fault intersections, now controlling heat flow into the Waiora aquifer. The inferred great age of the hydrothermal system poses problems in maintaining heat flow, and a model of a large, semi-permanent granitic batholith leaking super-critical hydrothermal fluids up active fault zones to heat near-surface aquifers of meteoric water is proposed. Temperature and pressure equilibria of such a magma body may be maintained by gaseous diffusion. Structural and heat flow evidence suggest that the most probable location of the magma body is to the south of the hydrothermal field below a large, late Pleistocene, rhyolitic eruptive centre. Other hydrothermal fields in the Taupo Volcanic Zone — Orakeikorako, Waiotapu and Kawerau — have similar geological sequences and structures to Wairakei. Important features in common include:

1. 1)
   Proximity to a late Pleistocene, rhyolitic eruptive centre typified by ignimbrites, non-welded ash flows and rhyolite domes and sills, which is considered to directly overlie a large, semipermanent magma chamber.     
   
   

Origin of the rhyolitic rocks of the taupo volcanic zone, New Zealand

Major element, Rb, Sr, Ba, Cr and V analyses as well as 13 new rare earth element (REE) analyses are presented for the greywacke basement surrounding the Taupo Volcanic Zone (TVZ). On this basis the basement rocks are divided into a Western Basement of approximately andesitic composition ( 62% SiO₂) and an Eastern Basement of approximately granodiorite composition ( 75% SiO₂). These analyses, 5 new REE analyses for the rhyolites, and published data for the volcanic rocks of TVZ are used to investigate the petrogenesis of rhyolitic rocks in the area.Least-squares mixing calculations for major elements show that 88% fractional crystallisation of high-alumina basalt produces a liquid of rhyolitic bulk composition, but Rayleigh fractionation models show that the trace element concentrations of the rhyolites are inconsistent with basalt fractionation. 57% fractionation of the assemblage plagioclase (35.6%), orthopyroxene (9.7%), clinopyroxene (7.8%), ilmenite (0.6%) and magnetite (3.4%) from a plagioclase-pyroxene andesite can produce liquids of rhyolitic bulk composition. REE concentrations produced by this model are consistent with those observed in the rhyolites but predicted Ba and Rb values are lower and V concentrations are higher than those in the rhyolites. Andesite fractionation also produces an unrealistic fractionation of the Cr/V ratio.A non-modal melting model involving 35% melting of a granulitic assemblage (plagioclase + quartz + clinopyroxene + orthopyroxene + biotite + magnetite + cordierite) with a bulk composition equivalent to the Western Basement can reproduce the REE pattern of the rhyolites as well as the concentrations of Rb and Ba. Sr values remain anomalously high, but the Cr/V ratio does not indicate fractionation. Absolute values of Cr and V are within the uncertainties of published crystal—liquid partition coefficients. The rhyolites have relatively flat REE patterns (La/Yb 7.5), as do the greywackes (La/Yb 8.2), so it is therefore unlikely that the rhyolites equilibrated with a garnet or amphibole-bearing assemblage.     

Lithic types in ignimbrites as a guide to the evolution of a caldera complex, Taupo volcanic centre, New Zealand

Taupo volcanic centre is one of two active rhyolite centres in the Taupo Volcanic Zone (TVZ), and has been sporadically active over the past ca. 300 ka. At least four large-scale ignimbrites have erupted from the centre, including the well documented 26.5 ka Oruanui ignimbrite and 1.8 ka Taupo ignimbrite. Because stratigraphy of earlier ignimbrites and their sources are masked by later volcanism, disrupted by regional tectonics and obscured by poor exposure, indirect methods must be applied in order to determine their source regions. In this paper detailed componentry, density and petrology of lithic fragments from three ignimbrites (Rangatira Point, Oruanui, Taupo) are used to reveal aspects of the sub-Taupo caldera geology, including the evolution of the Taupo volcanic centre, to assist in ignimbrite correlation and to evaluate structures within the Taupo caldera complex. Lithic fragments identify a complex subsurface geology. The Rangatira Point ignimbrite sampled dominantly rhyolite lavas, plus a variety of welded ignimbrites, rare high-silica dacites and a single dolerite. Most lithic fragments in the Oruanui ignimbrite are andesite with minor rhyolite, welded ignimbrite, dacite and rounded greywacke, while in the Taupo ignimbrite, rhyolite is again the dominant lithic component with subordinate welded ignimbrites, andesite, and greywacke. The densities of lithic fragments indicate similar ranges of values for all lava types, and thus density is a poor indicator of lithology. Care must, therefore, be taken before interpreting subcrustal stratigraphy using density as the sole criterion. The petrography and geochemistry of lithic types are more specific, and the variation can be used to identify sources for the ignimbrites. Both pumice chemistry and rhyolite lithic fragments from the Rangatira Point ignimbrite are comparable to domes exposed at the southern end of the Western Dome Complex and, combined with limited outcrop information, suggest the most likely source for this unit is in the northern part of the Taupo caldera complex. The dominance of andesite lithic fragments in the Oruanui ignimbrite suggests a major andesite cone existed beneath the source area, and the different lithic suites between Oruanui and Taupo ignimbrites suggest these ignimbrites came, at least in part, from mutually exclusive collapse structures. We believe that the Oruanui caldera is sited principally in the northwestern part of present-day Lake Taupo and the Taupo caldera in the northeastern part. Identification of abundant ignimbrite lithics in the Taupo ignimbrite, which are considered to represent an intracaldera facies of an earlier ignimbrite, that is not exposed at the surface, suggest there was a further (pre-Oruanui) ignimbrite caldera in the Taupo ignimbrite eruptive vent region.     

Nature and origin of cone-forming volcanic breccias in the Te Herenga Formation, Ruapehu, New Zealand

 Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine, unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580  °C and are interpreted as aa flow breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche bedding beneath flows, were emplaced at varying temperatures between 200 and 550  °C and are interpreted as forming part of block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor, well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550  °C. These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature, although some deposits contain clasts possibly emplaced at temperatures as high as 525  °C. These breccias are interpreted as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences. Received: 1 October 1998 / Accepted: 28 December 1998     

Origin of dacites of Taupo Volcanic Zone, New Zealand

Dacites form a relatively small proportion of lavas in the Taupo Volcanic Zone, New Zealand (5km³), and occur mainly on the eastern side. In this paper their origin is considered in terms of three processes: (a) partial melting of crustal rocks; (b) fractional crystallisation of basalt and andesite; and (c) sub-surface mixing of basic and acid magma. Modelling techniques are used to calculate the most acceptable degree of fractional crystallisation and magma mixing to fit major-element data, and these values are used to compare calculated and observed trace-element values. The success or failure of the model is determined by the closeness of the two sets of values. For partial-melt models, trace-element values alone are calculated by the batch-melting equation.Results indicate that White Island dacite can best be modelled by fractional crystallisation; Manawahe by fractional crystallisation plus limited crustal contamination; Maungaongaonga by partial melting of Western Basement greywacke, and Tauhara by partial melting of this greywacke together with minor mixing with a more basic magma. Results from Parekauau and Horohoro indicate that these lavas are unlikely to have formed by any of the processes examined.     

Volcanic hazard assessment for Ruapehu composite volcano,taupo volcanic zone,New Zealand

Ruapehu is a very active andesitic composite volcano which has erupted five times in the past 10 years. Historical events have included phreatomagmatic eruptions through a hot crater lake and two dome-building episodes. Ski-field facilities, road and rail bridges, alpine huts and portions of a major hydroelectrical power scheme have been damaged or destroyed by these eruptions. Destruction of a rail bridge by a lahar in 1953 caused the loss of 151 lives. Other potential hazards, with Holocene analogues, include Strombolian and sub-Plinian explosive eruptions, lava extrusion from summit or flank vents and collapse of portions of the volcano. The greatest hazards would result from renewed phreatomagmatic activity in Crater Lake or collapse of its weak southeastern wall. Three types of hazard zones can be defined for the phreatomagmatic events: inner zones of extreme risk from ballistic blocks and surges, outer zones of disruption to services from fall deposits and zones of risk from lahars, which consist of tongues down major river valleys. Ruapehu is prone to destructive lahars because of the presence of 10⁷ m³ of hot acid water in Crater Lake and because of the surrounding summit glaciers and ice fields. The greatest risks at Ruapehu are to thousands of skiers on the ski field which crosses a northern lahar path. Three early warning schemes have been established to deal with the lahar problems. Collapse of the southeastern confining wall would release much of the lake into an eastern lahar path causing widespread damage. This is a long-term risk which could only be mitigated by drainage of the lake.     

Earthquake swarms and volcanism in New Zealand

The term « swarm » is used to describe a group of related earthquakes, concentrated in space and time, without an obvious principal event. Large shallow earthquakes are often followed by aftershocks, but the pattern in which aftershocks occur differs in detail from that of a swarm. Sequences of New Zealand earthquakes that have been called swarms differ markedly from one another. The most vigorous of them, near Taupo in 1922, appears to have been an ordinary tectonic earthquake accompanied by foreshocks and aftershocks, and by surface faulting. No fault movements accompanied the 1964 swarm in the same area. Other localities that have experienced swarms include Great Barrier Island, Matamata, Kawerau, and Opunake. Swarms are considered by some writers to be characteristic of volcanic regions. Although all New Zealand swarms have occurred in areas of Quaternary volcanism, there are still no observations showing what part, if any, volcanism plays in the generation of earthquake swarms.     

Genesis of lavas of the Taupo Volcanic Zone, North Island, New Zealand

The Taupo Volcanic Zone forms part of the Taupo-Hikurangi subduction system, and comprises five volcanic centres: Tongariro, Taupo, Maroa, Okataina and Rotorua. Tongariro Volcanic Centre is formed almost entirely of andesite while the other four centres contain predominantly rhyolitic volcanics and later fissure eruptions of high-Al basalt. Estimated total volume of each lava type are as follows: 2 km³ of high-Al basalt (< 0.1%); 260 km³ of andesite (< 2.5%); 5 km³ of dacite (< 0.1%); > 10,000 km³ of rhyolite and ignimbrite (> 97.4%).The location of the andesites and vent alignments suggest a source from a subduction zone underlying the area. However, the lavas differ chemically from island-arc andesites such as those of Tonga; in particular by having higher contents of the alkali elements, light REE and Sr and Pb isotopes. This suggests some crustal contamination, and it is considered that this may occur beneath the wide accretionary prism of the subduction system. Amphibolite of the subduction zone will break down between 80 and 100 km and a partial melt will rise. A multi-stage process of magma genesis is then likely to occur. High-Al basalts are thought to be derived from partial melting of a garnet-free peridotite near the top of the mantle wedge overlying the subduction zone, locations of the vents controlled largely by faults within the crust. Rhyolites and ignimbrites were probably derived from partial melting of Mesozoic greywacke and argillite under the Taupo Volcanic Zone. Initial partial melting may have been due to hydration of the base of the crust; the “water” having come from dehydration of the downgoing slab. The partial melts would rise to form granodiorite plutons and final release of the magma to form rhyolites and ignimbrites was allowed because of extension within the Taupo graben.Dacites of the Bay of Plenty probably resulted from mixing of andesitic magma with small amounts of rhyolitic magma, but those on the eastern side of the Rotorua-Taupo area were more likely formed by a higher degree of partial melting of the Mesozoic greywacke-argillite basement. This may be due to intrusion of andesite magma on this side of the Taupo volcanic zone.     

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.     

Buchitic metagreywacke xenoliths from Mount Ngauruhoe,Taupo Volcanic Zone,New Zealand

Buchitic sedimentary xenoliths, a few centimetres to several decimetres diameter, occur in Recent andesite from Mount Ngauruhoe, Tongariro Volcanic Center, Taupo Volcanic Zone, New Zealand. Bulk chemistry and Sr isotope compositions of the xenoliths indicate that they are greywacke and argillite derived from Mesozoic Torlesse terrane basement that partly underlies the Taupo Volcanic Zone. The xenoliths contain up to 80% glass with quartz, apatite and zircon remaining as unmelted phases. Glasses within the xenoliths are peraluminous (A/CNK = 1.0 − 1.4), have high normative corundum (2–7%), appreciable FeO (2–4 wt.%), MgO (0.2–1.5 wt.%), TiO₂ (0.17–0.84 wt.%), relatively high normative An (1.0–5.3%), and do not represent S-type granitic melts. In the argillite the glass has higher amounts of AI₂O₃, FeO, MgO, CaO and K₂O, and has less SiO₂ and Na₂O than glass in the greywacke. Silica-rich glass (up to 80 wt.% SiO₂) surrounds partially melted quartz. Variable glass chemistry reflects the heterogeneous (layered) nature of the xenoliths. Cordierite (Mg/(Mg + Fe + Mn) = 0.78-0.58), orthopyroxene (En_43–56), Mg-rich ilmenite, rutile, pleonaste, V-Cr-Ti spinel, and pyrrhotite occur in the glass of the xenoliths. The dominant cordierite, orthopyroxene, spinel assemblage can be accounted for by disequilibrium breakdown reactions under low oxidation conditions < QFM) involving phengite and chlorite which are abundant in Torlesse greywacke and argillite cropping out along the eastern side of the Taupo Volcanic Zone. Comparison with glass compositions and phase relations of disequilibrium melting experiments on Torlesse greywacke and argillite indicates a minimum temperature of 775°C and a maximum pressure of 1.5 kbar for fusion of the xenoliths that underwent a rapid rate of heating at a depth of less than 5 km and a cooling period constrained by the time of quenching when they were erupted.     

Crustal heat transfer in the Taupo Volcanic Zone (New Zealand): comparison with other volcanic arcs and explanatory heat source models

The Taupo Volcanic Zone (TVZ) is a 200-km-long volcanic arc segment which developed ≤2 Ma ago within the continental crust of the North Island of New Zealand and lies at the southern end of the much larger Tonga-Kermadec arc system. The total crustal heat transfer of the TVZ is at present c. 2600 MW/100 km, most of the heat being transferred by convective geothermal systems. The rate of transfer is anomalously high in comparison to that of other active arcs, and arguably the highest world wide for such a setting. Heat transfer of other active arcs appear to vary almost linearly with subduction speed (about 150 MW/100 km for 10 mm/yr). The mass rate of common type arc extrusions (basalts, andesites, dacites) also increases almost linearly with subduction speed. This allows separation of the TVZ heat transfer into a “normal” component, associated with extrusions and intrusions of andesites and dacites (about 600 MW/100 km), and an “anomalous” component of about 2000 MW/100 km, related to extrusions and intrusions of rhyolitic melts whose generation is not directly controlled by subduction processes.Rhyolitic melts in the TVZ are partial melts of dominantly crustal origin. Comparison with other arcs indicates that the long-term extrusion rate of TVZ rhyolites (about 400 kg/s per 100 km) is also the highest world wide for this setting. The occurrence of voluminous Quaternary rhyolitic pyroclastics is a rare phenomenon and appears to be associated with a few arc segments (TVZ, Sumatra, Kyushu) that undergo significant crustal deformation.Various models have been proposed to explain the phenomenon of the anomalously high heat transfer within the TVZ. Models which require only heat transfer from plumes and subcrustal melts, either ponded at the crust/mantle boundary or intruding a spreading crust, are not suitable because the associated heat transfer at the contact is too low by a factor 2 to explain the required transfer rate of about 0.8 W/m² representing the “anomalous” crustal heat component of the TVZ. Heat generation by focussed plastic deformation within the ductile lithosphere is an alternative mechanism to explain “endogenous crustal heating” which yields heating rates that are also too low by a factor of two, although important parameters (average yield strength of lithosphere and opening rate of the TVZ) are not well known. A further search for a suitable combination of heat source models is required.     

Sedimentation and volcanism in a Permian arc-related basin,southern New Zealand

An exceptionally well-exposed, ancient, intra-arc basin in the Permian Takitimu Group of New Zealand contains 14 km of interbedded primary volcanic and marine volcaniclastic rocks of basaltic to rhyodacitic composition. These are the products of subaerial and submarine arc volcanism and closely associated turbidite sedimentation. The Takitimu oceanic arc/basin setting formed a dynamic closed sedimentary system in which large volumes of volcaniclastic material generated at the arc was rapidly redeposited in marine basins flanking the eruptive centres. Volcanism probably included (1) moderate- to deep-water extrusion of lava and deposition of hyaloclastite, (2) extrusive and explosive eruptions from shallow marine to marginally emergent volcanoes in or on the margin of the basin, and (3) Plinian and phreato-Plinian eruptions from more distant subaerial vents along the arc. Much of the newly erupted material was rapidly transported to the adjacent marine basin by debris flows, slumping and sliding. Hemipelagic sedimentation predominated on the outer margin of the basin, infrequently interrupted by deposition of ash from the most explosive arc volcanism and the arrival of extremely dilute turbidites. Turbidite sedimentation prevailed in the remainder of the basin, producing a thick prograding volcaniclastic apron adjacent to the arc. The volcaniclastic strata closely resemble classic turbidite deposits, and show similar lateral facies variations to submarine fan deposits. Study of such sequences provides insight into poorly understood processes in modern arc-related basins.     

Asymmetric,multiple-block collapse at Rotorua Caldera,Taupo Volcanic Zone,New Zealand

Calderas worldwide have been classified according to their dominant collapse styles, although there is a good deal of speculation about the processes involved. Recent laboratory experiments have tried to constrain these processes by modelling magma withdrawal and observing the effects on overlying materials. However, many other factors also contribute to final caldera morphology. Rotorua Caldera formed during the eruption of the Mamaku Ignimbrite. Collapse structure and evolution of Rotorua Caldera is interpreted based its geophysical response, geology and geomorphology, and the stratigraphy of the Mamaku Ignimbrite. Rotorua Caldera is situated at the edge of the extensional Taupo Volcanic Zone, in which major faults strike NE-SW. A second, less dominant fault set strikes NW-SE. These two fault sets have a strong influence on the morphology of Rotorua Caldera. No one style of collapse can be applied to Rotorua Caldera; it was formed during a single eruption, but subsided as many blocks and shows features of trapdoor, piecemeal and downsag types of collapse. Here Rotorua Caldera is described, according to its composition, activity and geometry, as a rhyolitic, single event, asymmetric, multiple-block, single locus collapse structure. The Mamaku Ignimbrite is the only ignimbrite to have erupted from Rotorua Caldera. Extracaldera thickness of the Mamaku Ignimbrite is up to 145 m, whereas inside the caldera it may be greater than 1 km thick. The Mamaku Ignimbrite can be separated into a basal tephra sequence and main ignimbrite sequence. The main ignimbrite sequence contains no observable flow unit boundaries but can be split into lower, middle and upper parts (LMI, mMI, uMI respectively) based on crystal content, welding, jointing, devitrification and vapour phase alteration. Juvenile clasts within the ignimbrite comprise three consanguineous silicic pumice types and andesitic fragments. Only the most evolved pumice type occurs in the basal tephra sequence. All three pumice types occur together throughout the main ignimbrite sequence, whereas the andesitic fragments are only present in uMI. Lithic lag breccias in uMI indicate a late stage of caldera collapse. Concentration of lithic fragments increases towards the middle of the ignimbrite, and may also reflect increased subsidence rate during an earlier stage. Collapse of Rotorua Caldera is thought to have occurred throughout the eruption of the main ignimbrite sequence of the Mamaku Ignimbrite, allowing simultaneous eruption of all the different pumice types and causing the abrupt transition from deposition of the basal tephra sequence to the main ignimbrite sequence.     

Determination of pre-eruptive H2O,F and Cl contents of silicic magmas using melt inclusions: Examples from Taupo volcanic center,New Zealand

Water, F, and Cl contents of melt inclusions in phenocrysts from the 2-ka-old Taupo and Hatepe plinian tephras, and the 22-ka-old Okaia tephra from the Taupo volcanic center, New Zealand, were measured by electron and ion microprobe. Major and trace element chemistry of the inclusions is similar to that of bulk rock, supporting our assumption that volatile contents of inclusions are representative of the magma in which the crystals grew. Inclusions in the 2-ka Taupo plinian tephra contain a mean of 4.3 wt% H₂O, 450 ppm F, and 1700 ppm Cl; from the Hatepe plinian tephra 4.3 wt% H₂O, 430 ppm F, and 1700 ppm Cl; and from the Okaia tephra 5.9 wt% H₂O, 470 ppm F, and 2100 ppm Cl. Sulfur was below the detection limit of 200 ppm. The constant H₂O, F and Cl from a number of stratigraphic horizons in the tephra deposits suggest that the Taupo and Hatepe plinian tephras (>8.2 km³ magma volume) were derived from a magma body that did not contain a strong volatile gradient. By inference, there is no pre-eruptive volatile difference between these plinian eruptions and a phrea-toplinian eruption which occurred between the two. Virtually no major element zonation is seen in this eruptive sequence. Although the Okaia tephra was also erupted from the Taupo volcanic center, probably from a similar vent area, its higher volatile contents and distinct composition as compared to the Taupo tephras show that it was derived from a different, and possibly deeper, magma body.     

Variations in the chemical and isotopic composition of fluids discharged from the Taupo Volcanic Zone, New Zealand

Contents of H₂O, CO₂ and Cl in well discharges from six explored geothermal systems of the Taupo Volcanic Zone, New Zealand, point to the existence of two distinct source fluids. The fluid present in discharges from systems along the eastern boundary is characterised by high CO₂ contents, 1.6 ± 0.5 , at mole ratios of 3.9 ± 1.5. High (0.06) and (12) weight ratios in these waters suggest that all four constituents are derived from associated andesitic rock. Geothermal discharges in the western parts of the TVZ, dominated by rhyolitic magmatism, are characterised by low CO₂ contents, 0.12 ± 0.05 , and low (0.14 ± 0.1) ratios. Again, relative Cl, B, Li and Cs contents agree with those of this potential source rock. High and ratios in the east are typical of fluids affected by the addition of volatiles released from subducted marine sediments. For the western systems, these ratios resemble more closely those expected for mantle-derived volatiles. The isotopic compositions of all deep waters point to the presence of variable amounts of a magmatic component, some 14 ± 5% in the eastern and 6 ± 2% in the western systems. The observed variations are explained in terms of interaction of volatiles released from the subducted sediments with material of the mantle wedge to form a volatile-charged, high-alumina basalt. Its convective rise, in a direction opposite to that of the down-going slab, leads to high enrichment in volatiles of the magmas generated beneath the eastern parts of the TVZ and increases their ability to intrude the continental crust. Further fractional crystallisation and assimilation leads to the formation of volatile-rich andesitic melts, partly extruded to form the volcanoes of the andesitic arc, partly intruded to act as source rocks for the high-gas geothermal systems. Batches of high-alumina basalt, depleted in subducted volatiles, travel farther west to pond beneath a zone of crustal extension. Following extensive fractionation, highly siliceous melts, carrying predominantly mantle-type volatiles, rise beneath the western part of the TVZ to supply both heat and volatiles to the geothermal systems there.     

Dilute gravity current and rain-flushed ash deposits in the 1.8 ka Hatepe Plinian deposit,Taupo, New Zealand

Two groups of poorly sorted ash-rich beds, previously interpreted as rain-flushed ashes, occur in the ca. AD 180 Hatepe Plinian pumice fall deposit at Taupo volcano, New Zealand. Two ash beds with similar dispersal patterns and an aggregate thickness of up to 13 cm make up the lowermost group (A). Group A beds extend 45 km north-east of the vent and cover 290 km². In the southern part of the group A distribution area, a coarse ash to lapilli-size Plinian pumice bed (deposit B) separates the two group A beds. The scarcity of lapilli (material seen elsewhere from the still-depositing pumice fall) in group A beds indicates that they were rapidly transported and deposited. However, this rapid transportation and deposition did not produce cross-bedding, nor did it erode the underlying deposits. It is proposed that thick (>600 m) but dilute gravity currents generated from the collapsing outer margin of the otherwise buoyant Hatepe Plinian eruption column deposited the group A beds. The upper ash beds (group C) consist of one to seven layers, attain an aggregate thickness of 35 cm, and vary considerably in thickness and number of beds with respect to distance from vent. Group C beds contain variable amounts of ash mixed with angular Plinian pumices and are genuine rain-flushed ashes. Several recent eruptions at other volcanoes (Ukinrek Maars, Vulcan, Rabaul, La Soufrère de Guadeloupe and Soufrière, St Vincent) have produced gravity currents similar in style, but much smaller than those envisaged for group A deposits. The overloaded margins of otherwise buoyant eruption plumes generated these gravity currents. Laboratory studies have produced experimental gravity current analogues. Hazards from dilute gravity currents are considerable but often overlooked, thus the recognition of gravity current deposits will contribute to more thorough volcanic hazard assessment of prehistoric eruption sequences.     

Variability of unsaturated Bromide fluxes as measured through a layered volcanic vadose zone in New Zealand

The measured drainage fluxes through a layered volcanic vadose zone exhibited high spatial variability as a consequence of heterogeneous flow conditions. The drainage flux variability was quantified using automated equilibrium tension lysimeters, installed in close‐proximity and resulted in high variability in the Br masses recovered from a conservative tracer experiment. The primary cause of the heterogeneous flow was attributed to textural changes occurring at the interface between volcanic layers, resulting in development of funnel‐flow patterns, and further enhanced by the existence of hydrophobic conditions. The Br recoveries in individual automated equilibrium tension lysimeters were used to determine the corresponding variable sizes of the surface areas contributing drainage to the lysimeters. The tracer experiment confirmed the existence of unsaturated lateral transport occurring at the sloping interface of the coarse Taupo Ignimbrite material with the silty Palaeosol layer at approximately 4.2 m depth. This study demonstrates that measurements of both flux and solute concentrations at multiple locations are essential when heterogeneous flow is suspected to be present, to be able to determine reliable estimates of contaminant leaching through the vadose zone at the plot scale. Copyright © 2013 John Wiley & Sons, Ltd.     

Timing of volcanic, plutonic and geothermal activity at Ngatamariki, New Zealand

Four ⁴⁰Ar/³⁹Ar dates on mineral separates from fresh and hydrothermally altered volcanic and plutonic rocks from the Ngatamariki geothermal field indicate that andesitic volcanism took place in the eastern portion of the Taupo Volcanic Zone (TVZ) prior to 1.2 Ma and probably considerably earlier. These data significantly extend the onset and duration of andesitic volcanism in the east-central TVZ over previous estimates. Intrusive activity is represented at Ngatamariki by a dioritic pluton, the only such pluton yet recognized in the entire TVZ. Hornblende from the pluton yields a crystallization age of near 550 ka. Hydrothermal alteration spatially associated with the pluton produced sericite of a similar age. Overlying and postdating the most intense hydrothermal alteration zone is the Whakamaru Ignimbrite (or its equivalent) which was emplaced at 330 ka. Two distinct geothermal systems may have been active at nearly the same site from 550 ka to present. The most intense activity occurred before 330 ka and was associated with emplacement of the Ngatamariki diorite. This was followed by the less intense system that is currently active. The geothermal regime at Ngatamariki has, therefore, probably been active intermittently for at least 550 ka.