Tephrochronology: a New Zealand case study

Earth-Science Reviews 49 Ž2000. 223–259 www.elsevier.comrlocaterearscirev

Tephrochronology: a New Zealand case study Phil Shane

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Department of Geology, UniÕersity of Auckland, PriÕate Bag 92019, Auckland, New Zealand Received 12 April 1999; accepted 27 September 1999

Abstract Tephrochronology is the study of volcanic ash Žtephra. beds for the purpose of correlating and dating volcanic and other geologic events. Large magnitude silicic eruptions can disperse tephra up to thousands of kilometres from the vent, producing a near instantaneous marker horizon. In addition to their geochronological value, tephra beds are a major source of data on the eruption frequency and geochemistry of large rhyolitic volcanoes. The Quaternary Taupo volcanic zone ŽTVZ. in New Zealand is one of the most frequently active rhyolitic centres on Earth, and for much of the 20th century its tephra beds have been the focus of study. As a result, fingerprinting and dating techniques have been fine-tuned, and tephra beds now under-pin the late Cenozoic chronology for a wide variety of disciplines from volcanology to sequence stratigraphy and archaeology. The key to tephrochronology is the identification and correlation of tephra horizons. In the proximal setting Ž- 50 km from vent., tephra beds can often be identified by their lithology, stratigraphic position and ferromagnesian mineralogy. Farther from source these features became less diagnostic as units become thinner and mineral depleted, and geochemical fingerprinting must then be employed. Grain-specific techniques are commonly required to assess the compositional homogeneity of the tephra, and to avoid xenolithic and detrital contaminants. Criteria that are valuable for identifying both source volcano and individual eruptive events from grain-specific electron microprobe techniques include: glass and Fe–Ti oxide chemistry, and eruption temperature and oxygen fugacity Žestimated from oxide equilibrium pairs.. Deposits of the contemporaneously active Taupo, Okataina, Maroa and Mayor Island caldera centres in the TVZ can be distinguished on the basis of such criteria. If a tephra lacks contaminants, trace- and rare earth element ŽREE. compositions of purified glass separates provide additional criteria. Similar approaches can be used on samples of the chilled, nonwelded bases of ignimbrites, while welded zones often display a characteristic thermal remnant magnetism ŽTRM. direction that can assist in correlation. Late Miocene and Pliocene rhyolitic tephra from the Coromandel region in New Zealand are compositionally indistinguishable from Quaternary TVZ deposits, but deposits from other SW Pacific provinces such as Antarctica and the Tonga–Kermadec arc are compositionally distinct. In the TVZ, rhyolitic tephra erupted during intervals of 10–20 ka from the same volcano are compositionally similar, while over longer periods or following large-volume eruptions the tephra are compositionally and mineralogically distinct. Most TVZ fall deposits are compositionally homogeneous, and there are no temporal compositional trends for individual volcanoes. Tephra beds are also valuable because they can be dated by a variety of methods, and therefore can provide direct age control for sequences that are not normally amenable to radiometric dating. For late Cenozoic tephra beds that are beyond the range of 14C Žca. 40 ka., and are fine-grained andror lack K-rich phenocrysts, the isothermal plateau fission-track ŽITPFT. method using glass can provide an accurate and precise age. 40Ar– 39Ar can be employed for high-resolution dating, however this may require single crystal laser

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Corresponding author. Fax: 64-9-373-7435; e-mail: [email protected]

0012-8252r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 5 2 Ž 9 9 . 0 0 0 5 8 - 6

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fusion to detect relict and partially degassed crystals that are common in pyroclastic deposits, even within single pumice blocks. The New Zealand tephrostratigraphic record is not complete in space or time. Only the post-64 ka record is well established. However, there are chronologically well-constrained tephra beds dating back to 10 Ma. Some tephra beds cover most of the North Island of New Zealand, are found in the South Island, and extend as much as 1400 km from vent into the Pacific Ocean. Widespread andror stratigraphically important units include: Kaharoa Ž665 years BP.; Kawakawa Ž22 ka.; Rangitawa Ž0.33 Ma.; Potaka Ž1 Ma. and Pakihikura Ž1.6 Ma.. Ages on these and other tephra beds provide a framework for global climatic and sea-level change recorded in New Zealand, and allow direct correlation between the marine and terrestrial realms. q 2000 Elsevier Science B.V. All rights reserved. Keywords: tephrochronology; volcanic ash; volcanology; geochronology; correlation; Taupo volcanic zone

1. Introduction 1.1. Tephrochronology Tephra beds are deposits of nonconsolidated pyroclastic ejecta that can be dispersed up to thousands of kilometres from their source volcano. Their emplacement can occur in a matter of hours, days or weeks, and they may be preserved in a wide variety of volcanic and sedimentary facies. Therefore, tephra beds are essentially isochronous geologic marker horizons that are valuable for correlation and geochronology. Tephra beds are known in deposits of Ordovician age Že.g., Kolata et al., 1996. through to historic time. The age of tephra beds has played a critical role in themes as diverse as the chronology of the Cenozoic time scale and mammal assemblages Že.g., Swisher and Prothero, 1990., to the age of hominid fossils Že.g., Walter, 1994., and the paleoenvironmental reconstruction of Antarctica ŽMarchant et al., 1996.. The most widely dispersed tephra beds are the products of large magnitude plinian and ignimbrite-forming rhyolitic eruptions. Such eruptions are destructive near the vent area, and the resulting tephra is often the main record of the event. Thus tephra beds are also the source of information on the eruption frequency and geochemistry of silicic volcanoes. Modern studies using tephra correlation as a stratigraphic tool stem from the pioneering work of Thorarinsson Ž1944. in Iceland, who coined the term tephra for material erupted explosively through the air. He also introduced the term tephrochronology for stratigraphic and chronologic studies using tephra beds. Later, all pyroclastic materials including nonwelded deposits of pyroclastic flows Žignimbrites. were included in the definition of tephra ŽThorarinsson, 1974.. The term tephrostratigraphy is widely

used in studies that have erected stratigraphic frameworks by correlating tephra beds, in some cases regardless of their age. While tephrochronometry has been used where tephra provide an age for enclosing strata. Tephrology has been suggested as an encompassing term for all tephra studies Že.g., Froggatt and Lowe, 1990.. The original term tephrochronology includes aspects of numeric age and stratigraphic correlation, and is thus used in this paper. There are two important facets to the use of tephra in stratigraphic studies: Ž1. the ability to directly date the tephra by a variety of radiometric methods and thus temporally constrain the enclosing strata; and Ž2. the ability to correlate tephra beds by lithologic, mineralogic and geochemical methods regardless of their age. The second facet has become increasingly important because suitable material for dating may not be present at all outcrops, and because correlation techniques are typically faster and cheaper than radiometric dating. In addition, unlike geochronological data, lithological and geochemical features can be characteristic of a particular eruption. The study of tephra has been important in many parts of the world including North America Že.g., Izett, 1981; Sarna-Wojcicki et al., 1987; Perkins et al., 1998.; Europe ŽBogaard and Schmincke, 1985; Dugmore et al., 1995; Pelcher et al., 1995.; Japan ŽMachida and Arai, 1992., East Africa Že.g., SarnaWojcicki et al., 1985., deep-sea cores Že.g., Kennett, 1981.; Antarctica ŽPalais et al., 1988. and New Zealand Že.g., Froggatt and Lowe, 1990.. Previous reviews dealing with tephrochronology and techniques include Westgate and Gorton Ž1981., SarnaWojcicki and Davis Ž1991. and Froggatt Ž1992.. Hildreth and Mahood Ž1985. summarised problems with correlating pyroclastic flow deposits.

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1.2. Tephra studies in New Zealand Tephra in the North Island of New Zealand have been investigated for much of the 20th century starting with early soil surveys for agricultural purposes Že.g., Grange, 1931; Taylor, 1933.. These studies soon expanded into a stratigraphy for late Quaternary studies Že.g., Vucetich and Pullar, 1964, 1969, 1973.. New Zealand Quaternary sequences and those of the surrounding oceans over 1000 km distant are well endowed with silicic tephra beds erupted from the Taupo Volcanic Zone ŽTVZ. in central North Island ŽFig. 1.. The TVZ is considered to be the most frequently active, large rhyolitic centre on Earth ŽWilson et al., 1995a., and has been the locus of numerous large Ž) 100 km3 . caldera-forming ignimbrite and plinian eruptions. Prior to the Quaternary, large-scale rhyolite volcanism was centred in the Coromandel Volcanic Zone ŽFig. 1. starting at ca. 10 Ma ŽAdams et al., 1994..

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A historic account of tephra studies in New Zealand was presented by Lowe Ž1990. and Froggatt and Lowe Ž1990. summarised the stratigraphy of post-65 ka rhyolitic tephra beds. These works predate the rapid growth in the 1990s of chronological studies using new fission-track ŽFT., 40Ar– 39Ar and magnetostratigraphic methods that have greatly revised New Zealand’s late Cenozoic stratigraphy and eruptive history, and extended the tephra record back to the late Miocene. In addition, there is no summary of the geochemical characteristics of the New Zealand tephra with respect to their identification and correlation. The aim of this review is to highlight techniques applicable to correlating and dating late Cenozoic rhyolitic tephra beds and identifying their source. By using the late Cenozoic of New Zealand as a case study, this review also summarises the state of New Zealand tephrochronology which is becoming increasing important to diverse disciplines such as archaeology ŽNewnham et al., 1998; Shane et al., 1998a.; palynology ŽNewnham and Lowe, 1991.; biostratigraphy ŽShane et al., 1995a.; cyclostratigraphy ŽCarter and Naish, 1998.; volcanology ŽWilson, 1993.; tectonics ŽFroggatt and Howorth, 1980.; and marine geology ŽCarter et al., 1995..

2. Dispersal and distribution of tephra deposits

Fig. 1. Map of North Island, New Zealand showing the location of late Cenozoic volcanic centres and sedimentary basins mentioned in the text.

The value of tephra beds as stratigraphic markers and recorders of volcanism is largely due to their wide dispersal. Rhyolitic tephra are the most valuable in such studies because they result from high elevation plinian columns, and can be more widely distributed by co-ignimbrite ash columns away from the vent. One of the most widely dispersed fall deposits known is that of the 74 ka Youngest Toba Tuff eruption in northern Sumatra that resulted in macroscopic fall deposits containing sand-sized ash in western India, some 3000 km distant ŽRose and Chesner, 1987; Shane et al., 1995b.. The dispersal of pyroclastic material around a volcanic region is strongly controlled by prevailing winds, topography and bathymetry. As a result, fall deposits may be distributed in different directions to pyroclastic flow and remobilised deposits Že.g., Sigurdsson et al., 1980.. The products of violent plinian eruptions are widely and uniformly dispersed down-

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wind of the vent producing a topography-mantling fall deposit that typically thins and fines in a regular manner with increasing distance from source. In New Zealand, the predominant tropospheric westerly winds promote dispersal mainly into the east coast region of North Island and beyond into the Pacific Ocean, up to 1400 km distant ŽWatkins and Huang, 1976; Carter et al., 1995.. Numerous rhyolitic eruptions have produced tephra that mantle the landscape as macroscopic horizons over much of the central North Island. Cores collected from lakes and bogs have extended the known fallout distribution into southern Že.g., Howorth et al., 1980. and northern North Island Že.g., Lowe, 1988a,b.. Thus essentially all of the North Island has experienced tephra fall during the Quaternary ŽFigs. 2 and 3.. A few macroscopic tephra beds have also been found in northern South Island more than 600 km south of the Taupo region in a direction perpendicular to prevailing tropospheric winds and are the products of the most intense and large magnitude eruptions Ž Kohn, 1979; Mew et al., 1986; Eden and Froggatt, 1988; Shane, 1994.. Some tephra beds are also found hundreds of kms from source to the N and NW of North Island in the Tasman Sea, in addition to having an eastern dispersal in the Pacific Ocean Že.g., Nelson et al., 1986. ŽFig. 2.. This distribution requires tephra ejection into high-altitude Žca. 20 km. SE trade winds above

Fig. 2. Minimum known extent of macroscopic tephra associated with some large Quaternary eruptions from the TVZ. Data sources: 0.3 Ma Rangitawa ŽFroggatt et al., 1986., 1.0 Ma Potaka ŽShane et al., 1996., 22 ka Kawakawa ŽCarter et al., 1994., 64 ka Rotoehu ŽPullar and Birrell, 1973., and 665 years BP Kaharoa ŽPullar et al., 1977..

Fig. 3. Distribution of Quaternary rhyolitic pyroclastic and associated reworked deposits in North Island.

the predominant westerlies thus allowing simultaneous emplacement to both the east and west. However, uniform blankets of tephra are not found in all depositional environments. Where sedimentary basins are located out of the main atmospheric dispersal axis, the tephra record is dominated by fluvially transported deposits and reflects only the events that were deposited in the associated catchment close to source. Early Pleistocene fluvial systems deposited thick Ž) 1 m., coarse-grained tephra as much as 250 km from source in a discontinuous, semiradial pattern surrounding the TVZ ŽFig. 3. Že.g., Shane, 1991; Shane et al., 1996.. Where fluvial transport and deposition is the dominant process, closely spaced Ž1–10 km apart. contemporaneous sequences contain different numbers of tephra beds andror different tephra beds, and many have widely variable stratigraphic thickness and grain size ŽShane, 1991., thus hindering field-based correlation. Fluvial systems can deposit nearly pure tephra in units exceeding 20 m in thickness at distances up to 250 km from

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the vent. The purity of the tephra and its large volume suggests such emplacement events occur shortly after eruption when a considerable volume of unconsolidated material is still present in the catchment and before soil or vegetation development can anchor the pyroclastic material. Therefore, for stratigraphic purposes many fluvial tephra deposits can be considered near-contemporaneous with the fall or flow event. However, their distribution is restricted to fluvial transport routes. Pre-Quaternary deep marine forearc basin sequences in the East Coast region of North Island display similar tephra dispersal patterns. Many tephra beds in Miocene and Pliocene sequences have been emplaced via sediment gravity flow to produce a fan complex ŽShane et al., 1998b.. Their localised dispersal and lack of stratigraphic continuity suggests a point source discharge far from the volcanic source Ž) 300 km., presumably after transportation within a submarine channel ŽShane et al., 1998b.. In such sedimentary environments, tephra beds cannot be traced over wide geographic areas.

3. Field characteristics Most of the late Pleistocene tephrostratigraphy established for central North Island was based on identifying and correlating tephra beds on the basis of their lithological characteristics and stratigraphic position Že.g., Vucetich and Pullar, 1964, 1969.. Most of the tephra erupted in the last ca. 64 ka represent shower bedded units separated by paleosols that occur in a regular and predictable succession. Using soil survey techniques that involved recording properties such as colour, texture, sorting, clast compositions, lithic content and composition, and bedding characteristics, simple stratigraphies were developed by ’counting down’ units from the top Žthe present soil. of the sequence. Close to vent Ž- 50 km. tephra units commonly display characteristic bedding sequences reflecting changes in eruption style, such as pumice supported plinian phases alternating with poorly sorted, fine grained phreatomagmatic phases, that can be identified over considerable distances. Isopach maps have been developed for most of the fall units - 64 ka Že.g., Pullar and Birrell, 1973.. Therefore, thickness, location and

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stratigraphic position can in some cases be sufficient criteria for tephra identification within the TVZ. However, the lithological character of eruptive units can change rapidly over short distances. Crystal, lithic and glass proportions in fall deposits change with distance from vent due to atmospheric sorting. Energetically emplaced ignimbrites can be thick and massive where ponded in topographic depressions, but thin and stratified over paleohighs. Similarly, parts of the unit can be fines depleted due to changing flow processes and topography Že.g., Wilson and Walker, 1985.. Turbulent flow mechanisms commonly sort and concentrate lithic clasts into parts of the ignimbrite thus depleting them from elsewhere in the deposit. Farther from the vent Ž) 50–100 km. most fall deposits are thin Že.g., 1 mm–20 cm. and can lack distinguishing bedding that reflect eruptive phase changes. At these distances, ignimbrites are fine grained, generally thin, nonwelded deposits highly depleted in potentially characteristic crystal and lithic components. In such settings, field characteristics are of limited use in identification. Similarly, lithological distinctions become impractical in sedimentary basin sequences of southern North Island that contain numerous events and commonly represent 0.5–2 Ma of deposition. In addition, tephra deposits ) 65 ka in New Zealand are fragmentary in outcrop distribution having been disrupted by erosional and tectonic processes ŽShane et al., 1996., therefore isopach maps generally cannot be constructed.

4. Ferromagnesian mineralogy The relative abundance of various ferromagnesian minerals Že.g., pyroxenes, amphiboles, mica. within silicic tephra beds have long been used as an aid in the identification of post-64 ka events in New Zealand Že.g., Kohn, 1973; Vucetich and Howorth, 1976.. The minerals can be easily examined as loose grains under a binocular microscope after the tephra sample has been crushed and wet sieved to remove particles - 63 mm in size. Distal tephra beds Ž) 100 km from source. are often mineral poor due to atmospheric sorting during transport. For such samples, the ferromagnesian minerals can be concentrated using heavy liquids or electromagnetic methods. If required, stan-

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dard grain mounts andror thin sections can be prepared to aid mineral identification using a petrographic microscope. The examination of the ferromagnesian mineral assemblage of a post-64 ka tephra bed is the first laboratory step toward identifying the source volcano and event. Problems with the use of mineralogies include the total dissolution of some minerals Že.g., biotite. and preferential dissolution of orthopyroxene relative to hornblende in acid peat settings resulting in incorrect ratio estimates ŽHodder et al., 1991.. Platy minerals such as biotite are easily winnowed from deposits that have been subjected to wind or water action. Therefore the presence of a particular mineral is more instructive than its absence. Lowe Ž1988a. and Froggatt and Lowe Ž1990. have summarised the ferromagnesian mineral assemblages for late Pleistocene tephra beds in New Zealand ŽTable 1.. As well as helping to identify the source volcano, the occurrence of particular minerals can aid in eliminating candidate eruptive events. For example, during the last 64 ka only three widely dispersed tephra beds contain the mineral cummingtonite: Rotoehu Žca. 64 ka.; Rotoma Ž8.5 ka. and Whakatane Ž4.8 ka. ŽFroggatt and Lowe, 1990.. All were erupted from the Haroharo Complex of Okataina Centre. Biotite is normally abundant in only four post-22 ka tephra beds Žall from Okataina.: Okareka Ž18 ka., Rerewhakaaitu Ž14.7 ka., Rotorua Ž13 ka. and Kaharoa Ž665 years BP.. The occurrence of sodic phases such as aegirine, riebeckite, ferrohedenbergite and aenigmatite are indicative of Mayor Is-

land eruptions. Only five post-64 ka tephra beds from this volcano have been recognised in deep-sea cores ŽPillans and Wright, 1992. andror in the North Island ŽLowe, 1988a.. Of these, only the Tuhua tephra Ž6.2 ka. is widely dispersed and recorded as a thick, coarse grained layer in the North Island. The use of ferromagnesian mineralogies to identify tephra beds over longer time intervals Ž100 ka–1 Ma. is not practical because of the large number of eruptive events recorded in New Zealand sedimentary sequences, and the near ubiquitous occurrence of hypersthene and hornblende. For tephra beds in the interval ca. 2–0.5 Ma, approximately 25% contain biotite accompanied with high-K glass ŽK 2 O ) 3.9 wt.%. with NarK - 1 ŽShane et al., 1996.. At present, knowledge of sources does not permit assignment of mineralogies to particular vents for pre64 ka eruptions.

5. Glass geochemistry 5.1. Bulk samples and glass separates A wide range of major, trace and rare earth elements can be determined rapidly and precisely by modern instrumental methods such as X-ray fluorescence ŽXRF., instrumental neutron activation analysis ŽINAA. and inductively coupled plasma mass spectrometry ŽICP-MS.. However, these techniques require bulk samples or glass separates of 100 mg–10 g. Bulk samples of tephra are of limited use to

Table 1 Ferromagnesian mineral assemblages of post-64 ka tephra beds in New Zealand Rhyolites Hypersthene" augite" hornblende Hornblendeq hypersthene" augite Hyperstheneq hornblendeq biotite Cummingtoniteq hypersthene" hornblende Augiteq hypersthene" hornblende Aegirine" riebeckite" aenigmatite" olivine

Volcano Post-22 ka Taupo Okataina; pre-22 ka Taupo Okataina Okataina ŽHaroharo. Okataina Mayor Island

Andesites Orthopyroxeneq clinopyroxene" olivine" hornblende Clinopyroxeneq hornblende" orthopyroxene

Tongariro Egmont

Minerals in order of abundance. Diagnostic mineral in bold. Following Lowe Ž1988a. and Froggatt and Lowe Ž1990..

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geochemical fingerprinting studies because the composition can be affected by variable phenocryst content. Therefore, a sample must be of sufficient size to be representative of the original magmatic components. For a single pumice clast, sizes ) 4 cm are required otherwise crystals may have been lost during eruptive fragmentation. Bulk samples of ash or fine lapilli are problematic as phenocrysts are commonly enriched or depleted by pyroclastic flow mechanisms, and depleted in fall deposits by atmospheric sorting. Fall deposits and samples of ignimbrite matrix are inherently prone to impurities such as xenocrysts and xenoliths. Therefore, the use of nongrain-discrete samples requires the purification of the glass phase. However, separation techniques that usually involve electromagnetic or density properties of the components of a tephra cannot remove contaminant glass shards and shards with mineral micro-inclusions. Glass separates techniques also involve the analysis of whole shards that may have a weathered coating from diagenetic processes. Such weathering must be efficiently removed in a chemical pretreatment. The fundamental limitation in the use of glass separates for tephra correlation is that the glass phase may not be homogeneous either due to post-eruption sedimentary mixing Že.g., Shane, 1991. or compositional gradients within the magma Že.g., Hildreth, 1981.. Therefore, the homogeneity of the glass phase must be assessed by a grain-specific technique prior to the use of separates. 5.2. Glass alteration and hydration Glass shards are prone to alteration after prolonged exposure in subaerial settings and in ignimbrites that have undergone slow cooling, welding, andror vapour phase alteration ŽScott, 1971.. Alkali exchange is a common alteration characteristic involving the enrichment of K and loss of Na in glass. In TVZ calc–alkaline rhyolites, typical abundances of Na 2 O and K 2 O in fresh isotropic glass are ) ca. 3 wt.% and - 4.5 wt.%, respectively Že.g., Froggatt, 1983; Lowe, 1988a; Shane and Froggatt, 1992; Shane et al., 1996; and others.. Glass from the matrix and pumice clasts of poorly welded to intensely welded ignimbrites of the TVZ will commonly display Na 2 O contents of 1–3 wt.%, and K 2 O contents of 5–6

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wt.%, reflecting alkali mobilisation. These glasses may also display low concentrations of FeO and CaO Ž- 0.6 wt.%.. Therefore, welded ignimbrites are generally not suitable for geochemical fingerprinting. Some welded ignimbrites have a basal chilled zone containing nonwelded, fresh glass that has not suffered prolonged cooling and the percolation of fluids. Glass from such chilled zones have been used to fingerprint ignimbrite sheets Že.g., Black et al., 1996.. Glass also becomes hydrated within a few hundred to a thousand years after eruption, but remains isotropic. Typical electron microprobe analysis of New Zealand late Cenozoic glasses have analytical totals in the range 91–99%, typically 94–95%. The discrepancy from 100% is largely due to water not analysed and this is supported by loss on ignition studies that show a good agreement with electron microprobe analytical totals ŽFroggatt, 1983.. Modern silicic tephra such as that erupted from Mt. St. Helens in 1980 are anhydrous ŽSarna-Wojcicki et al., 1981.. Therefore, much of the water contained in older tephra glass is considered to be the result of secondary hydration. Stable isotope studies of this hydration water in New Zealand tephra indicate a meteoric origin ŽIngraham, pers. commun., 1998.. Glass in the tephra beds becomes hydrated to different degrees depending on depositional environment. Recalculation of the analyses to 100% on a water-free basis results in consistent elemental abundances suggesting that the water is accommodated within the glass structure without chemical alteration. Also, discriminant function analysis shows that such normalisation provides valid statistical results ŽStokes et al., 1992.. 5.3. IndiÕidual glass shard analysis One of the most widely used techniques for chemically characterising glass within tephra beds is individual shard analysis by electron microprobe following a procedure similar to that used in early studies by Smith and Westgate Ž1969.. The efficiency of the technique for fingerprinting pyroclastic fall and flow deposits in New Zealand was established by Froggatt Ž1983., and has since been widely used to characterise tephra beds in lake cores Že.g., Lowe, 1988a.; deep-sea cores ŽNelson et al., 1985.; early Quater-

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nary fluvial and lacustrine sequences Že.g., Black, 1992; Shane et al., 1996.; Miocene and Pliocene bathyal sequences ŽShane et al., 1995a, 1998b.; and proximal ignimbrites in the TVZ ŽBlack et al., 1996.. Shards in the size range 63–500 mm are separated from bulk tephra samples or pumice lapilli or blocks where available. The use of pumice blocks limits the potential of contamination from accidental ejecta and post-eruption depositional processes. However, such coarse material is commonly not available in distal settings Ž) 50 km from vent.. The glass shards are commonly separated from a crushed bulk sample via electromagnetic or heavy liquid methods. However, the complete purification is not required as individual shards are selected optically during analysis. The electron microprobe analysis of polished sections allows the freshly exposed interior of the shard to be analysed thus avoiding any weathered surface coating. Most workers have analysed 10–20 Žor more. different shards per sample in an attempt to assess its homogeneity or search for evidence of magmatic gradients or magma mixing. If the shard population is compositionally homogeneous, then the analyses can be presented as a mean and standard deviation ŽTable 2. that reflects the average composition of the melt phase. The analyses are recalculated to 100% to avoid the effects of variable secondary hydration. In New Zealand, many Miocene to Holocene tephra beds have been emplaced into rapidly deposited andror reducing environments such as swamps, lakes and deep-sea settings and only exposed in the latest Holocene by river downcutting or uplift on shore platforms. As a result, glasses as old as 10 Ma are optically isotropic and do not show evidence of alkali exchange or loss Že.g., Shane et al., 1998b.. Tephra beds deposited in rapidly aggrading fluvial systems and in loess also contain fresh glass ŽShane, 1991.. 5.4. Volcanic proÕenance Nearly all Miocene to Holocene tephra beds found in sedimentary basins in the North Island are calc– alkaline rhyolites with SiO 2 in the range 72–78 wt.% and K 2 O in the range 2.6–4.5 wt.%, recalculated to 100% on a volatile-free basis. A remarkably similar compositional range is displayed in the tephra beds regardless of age ŽFig. 4A. ŽShane et al., 1998b..

There are no fundamental differences in major oxide content of glass erupted from the late Miocene and Pliocene Coromandel Volcanic Zone and the Quaternary TVZ. In contrast, late Pleistocene tephra from the peralkaline Mayor Island volcano can be easily distinguished from other late Cenozoic rhyolites by their low Al 2 O 3 contents - 10 wt.%, and high FeO Ž) 5 wt.%. and Na 2 O Ž) 5 wt.%. contents ŽFig. 4B.. Electron microprobe analysis ŽEMA. of major oxides in glass allow tephra beds from different tectonovolcanic provinces in the SW Pacific and Antarctic regions to be distinguished from New Zealand sourced tephra. For example, widespread sea-rafted pumice found in Holocene coastal deposits and on the modern beach in New Zealand have a distinctive tholeiitic oceanic island composition suggesting they are exotic to New Zealand. These searafted rhyolitic pumices ŽSiO 2 ) 70 wt.%. are characterised by low K 2 O Ž0.5–1.75 wt.%. and variable high FeO Žca. 2–5 wt.%. and CaO Žca. 2.4–4.0 wt.%. compared with local TVZ-derived tephra with K 2 O ) 2.7 wt.% and FeO generally - 2.3 wt.% ŽFig. 4B.. The sea-rafted pumices have an origin in the Tonga–Kermadec arc where historic pumice rafts display a similar chemical composition ŽShane et al., 1998a.. Widespread Quaternary rhyolitic glass shards are found in deep-sea sediments at latitudes ) 608S in circum-Antarctic seas of the Southern Ocean. These glasses were originally assigned to the TVZ because of their rhyolitic composition Že.g., Kyle and Seward, 1984.. However, an EMA investigation revealed the deep-sea glasses have a high Peralkaline Index ŽPI s molar Na 2 O q K 2 OrAl 2 O 3 . of 0.93– 2.19, compared with typical TVZ glasses Ž0.84–0.87. ŽShane and Froggatt, 1992.. In addition, they are distinguished from TVZ glasses by their low CaO content of - 0.5 wt.% and high and variable FeO contents of 1.5–6.0 wt.% ŽFig. 4B.. The composition of the Southern Ocean glasses is compatible with that of pyroclastic rocks from the large alkalic volcanoes in West Antarctica ŽShane and Froggatt, 1992.. 5.5. Source Õolcano Only the post-64 ka interval is well enough known for particular TVZ-derived tephra beds to be as-

Table 2 Glass composition of representative tephra beds in New Zealand Al 2 O 3

TiO 2

77.33 Ž0.17. 74.93 Ž0.34. 72.88 Ž0.41. 77.33 Ž0.45. 76.34 Ž0.23. 77.86 Ž0.17. 78.15 Ž0.32. 77.63 Ž0.30. 77.53 Ž0.44.

12.61 Ž0.11. 13.42 Ž0.19. 14 Ž0.18. 12.42 Ž0.32. 13.24 Ž0.1. 12.61 Ž0.09. 12.41 Ž0.19. 12.20 Ž0.21. 12.43 Ž0.11.

0.13 Ž0.05. 0.28 Ž0.06. 0.48 Ž0.04. 0.17 Ž0.07. 0.24 Ž0.07. 0.18 Ž0.03. 0.14 Ž0.04. 0.11 Ž0.04. 0.10 Ž0.02.

Sea-rafted pumice Loisels 68.93 Ž0.58.

14.59 Ž0.91.

Mayor Island Tuhua 72.92 Ž0.36.

TVZ rhyolites Kaharoa Taupo Unit c Kawakawa Mangaone Rotoehu Rangitawa Potaka Pakihikura

FeO

MnO

MgO

0.86 Ž0.06. 2.11 Ž0.09. 2.84 Ž0.22. 1.27 Ž0.20. 1.26 Ž0.07. 0.99 Ž0.05. 0.97 Ž0.1. 1.11 Ž0.19. 1.30 Ž0.08.

0.06 Ž0.04. 0.06 Ž0.04. 0.09 Ž0.05. – 0.11 Ž0.04. 0.07 Ž0.05. – – –

0.05 Ž0.05. 0.23 Ž0.05. 0.49 Ž0.16. 0.15 Ž0.04. 0.17 Ž0.06. 0.1 Ž0.05. 0.12 Ž0.03. 0.11 Ž0.03. 0.10 Ž0.02.

0.57 Ž0.1.

4.81 Ž0.68.

0.16 Ž0.07.

9.91 Ž0.23.

0.26 Ž0.04.

5.91 Ž0.09.

CaO

Na 2 O

K 2O

Cl

H 2O

0.6 Ž0.04. 1.5 Ž0.1. 2.28 Ž0.12. 1.14 Ž0.09. 1.19 Ž0.06. 0.87 Ž0.03. 0.8 Ž0.06. 0.91 Ž0.14. 1.21 Ž0.07.

4.03 Ž0.07. 4.42 Ž0.13. 4.21 Ž0.12. 4.06 Ž0.23. 4.65 Ž0.1. 4.04 Ž0.11. 3.21 Ž0.15. 3.72 Ž0.11. 3.75 Ž0.20.

4.12 Ž0.04. 2.87 Ž0.05. 2.65 Ž0.04. 3.25 Ž0.10. 2.86 Ž0.1. 3.32 Ž0.07. 4.22 Ž0.14. 4.02 Ž0.23. 3.42 Ž0.13.

0.19 Ž0.02. 0.19 Ž0.03. 0.16 Ž0.02. 0.20 Ž0.04. 0.22 Ž0.03. 0.22 Ž0.03. – 0.21 Ž0.05. 0.15 Ž0.02.

2.96 Ž0.64. 4.41 Ž1.57. 3.8 Ž1.26. 5.87 Ž1.13. 6.69 Ž1.49. 8.2 Ž1.72. 4.83 Ž2.73. 4.88 Ž1.02. 7.22 Ž1.46.

0.79 Ž0.25.

3.74 Ž0.34.

4.68 Ž0.23.

1.24 Ž0.09.

0.5 Ž0.04.

3.42 Ž0.53.

0.14 Ž0.03.

0.08 Ž0.05.

0.26 Ž0.05.

5.91 Ž0.14.

4.35 Ž0.06.

0.27 Ž0.03.

3.42 Ž0.68.

Tongariro Unnamed

59.51 Ž0.37.

17.65 Ž0.34.

0.91 Ž0.06.

7.48 Ž0.31.

0.13 Ž0.05.

2.59 Ž0.13.

6.49 Ž0.19.

3.53 Ž0.11.

1.67 Ž0.1.

0.11 Ž0.03.

2.09 Ž0.71.

Egmont Unnamed

65.27 Ž0.37.

17.26 Ž0.13.

0.64 Ž0.06.

3.33 Ž0.12.

0.16 Ž0.05.

0.95 Ž0.06.

2.94 Ž0.18.

5 Ž0.2.

4.28 Ž0.3.

0.3 Ž0.03.

4.39 Ž1.37.

Auckland Field Rangitoto 43.36 Ž0.30.

16.13 Ž0.25.

3.24 Ž0.10.

12.67 Ž0.18.

0.17 Ž0.05.

4.47 Ž0.39.

11.35 Ž0.43.

5.52 Ž0.55.

1.91 Ž0.11.

0.11 Ž0.03.

1.10 Ž0.17.

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

SiO 2

Presented as a mean Žstandard deviation. of 10–20 shards determined by electron microprobe analysis Žwt.%.. Analysts: Shane and Froggatt. Recalculated to 100% on a volatile free basis. Water by difference. All Fe as FeO. Tongariro tephra from Lake Poukawa core 97-1 Ždepth 158.45 m.; Egmont tephra from Lake Poukawa core 97-1 Ždepth 142.86 m.. Rangitoto also contains 1.08"0.18 wt.% P2 O5 .

231

232

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

Fig. 4. ŽA. Major oxide glass composition of late Cenozoic tephra beds in North Island. Each data point represents the mean of 10–15 shard analyses by electron microprobe. ŽB. Glass composition of tephra beds from different tectonovolcanic provinces in the SW Pacific region. Additional data sources: Western Antarctica ŽShane and Froggatt, 1992., Mayor Island ŽLowe, 1988a; Barclay et al., 1996., and Kermadec arc ŽHawkins, 1985; Shane et al., 1988a..

signed to source vents on the basis of dispersal. Older tephra beds still await the identification of proximal deposits linked to particular vents. The 22 ka Kawakawa eruption from Taupo centre provides a convenient marker to divide the post-64 ka tephra sequence, and also marks a major change in the chemistry and mineralogy of Taupo eruptive units. Tephra beds erupted from Taupo in the last 22 ka can easily be distinguished from Okataina units on the basis of EMA data using simple binary plots of oxides, although individual eruptive events in this short time interval are difficult to distinguish Že.g., Lowe, 1986; Froggatt and Rogers, 1990.. Taupo tephra beds Ž- 22 ka. are distinguished by their lower SiO 2 and higher FeO contents compared to Okataina units ŽFig. 5.. Most of the post-22 ka tephra beds from Taupo are characterised by a monotonous glass composition of SiO 2 s ca. 75–76 wt.% and FeO s ca. 1.7–2.0 wt.%. The exceptions are three early Ž10–20 ka. eruptive units of limited dispersal referred to as A, V and c ŽWilson, 1993.. These tephra beds have low SiO 2 contents Žca. 72–73 wt.%. and high FeO Žca. 2.5–3.0 wt.%. and CaO Ž) 2.0 wt.%.. The compositional differences between Taupo and Okataina tephra are also reflected in their ferromagnesian mineralogies. Post-22 ka Taupo tephra beds are hypersthene-dominated compared to hornblende Ž" biotite. rhyolites from Okataina. The 22 ka Kawakawa eruption and pre-22 ka events from Taupo display hornblende Ž" biotite.-dominated mineralogies and can be also be distinguished from post-22

ka Taupo events on the basis of their higher SiO 2 contents and accompanying lower FeO and CaO contents in the glass phase ŽFig. 5.. In these respects, the pre-22 ka Taupo eruptions are very similar to

Fig. 5. Major oxide glass composition of tephra beds erupted from the contemporaneously active Taupo and Okataina centres ŽFroggatt and Shane, unpublished data.. Data represents individual shard analyses.

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

many tephra beds from Okataina and are thus difficult to distinguish from Okataina-derived tephra. Most Okataina sourced tephra are characterised by high SiO 2 contents Žca. 77–78 wt.%. and low FeO contents Žca. 1 wt.%.. The exceptions are four older tephra: Mangaone, Hauparu, Maketu and Te Mahoe, that have distinctively lower SiO 2 contents and thus are enriched in other oxides such as FeO and CaO. The later three are also characterised by an augitedominated mineralogy. Stokes and Lowe Ž1988. used discriminant function analysis to classify different source volcanoes including Taupo and Okataina, using the range of major oxides typically available from EMA. Their statistical analysis supported conclusions reached from the use of binary plots such as SiO 2 vs. FeO. 5.6. IndiÕidual eruptiÕe eÕents Analyses of individual shards reveal that most rhyolitic tephra beds erupted from the TVZ are compositionally homogeneous Že.g., Froggatt, 1983; Shane et al., 1996.. Little compositional variation is found within individual high SiO 2 Ž) 75 wt.%. rhyolite tephra beds, e.g., SiO 2 Ž"- 0.40 wt.%.; FeO Ž"- 0.15 wt.%.; CaO Ž"- 0.10 wt.%. and K 2 O Ž"- 0.30 wt.%. ŽTable 2.. This variability is comparable to that of multiple analyses of glass standards by electron microprobe ŽShane and Froggatt, 1992. and is within analytical uncertainties. Analyses from different eruptions show greater differences in composition: SiO 2 72–78 wt.%; FeO 0.8–2.5 wt.%;

233

CaO 0.7–1.6 wt.%; and K 2 O 2.7–4.4 wt.% ŽTable 2.. As a result, numerous Miocene to Holocene tephra beds including distal fall and reworked deposits and nonwelded bases of proximal ignimbrites can be distinguished by as few as two to three oxides, such as FeO and CaO ŽFig. 6A.. Individual tephra compositions are not unique and in many stratigraphic sequences the same composition may be encountered at various stratigraphic levels. However, within short stratigraphic intervals the tephra beds may be recognised by their composition in relation to the composition of other units in close proximity. Tephra beds erupted from the same volcano over very short time intervals Žca. 10–20 ka. are difficult to distinguish on the basis of glass chemistry alone. For example, many of the Okataina or Taupo centre eruptions of the last 22 ka are compositionally similar ŽFig. 6B.. Stokes et al. Ž1992. were able to statistically classify some of these tephra beds by discriminant function analysis using major oxides. Sequential trends in composition that would reflect tapping of a closed magma system are not evident in tephra records from either the TVZ ŽShane et al., 1996. or Coromandel volcanic zone ŽShane et al., 1998b.. Instead, the homogeneous glass phase in most tephra beds suggests that they are the product of discrete magma batches that have not developed significant pre-eruptive chemical gradients. Similar conclusions were reached by Sutton et al. Ž1995. who examined whole pumice compositions and isotopes from the Taupo centre.

Fig. 6. Examples of the glass composition in rhyolitic tephra beds determined by electron microprobe analysis. ŽA. A sequence of tephra beds in Oroua stream, Wanganui basin Žca. 1.8–1.5 Ma., demonstrating the ability to distinguish eruptive events Žsee Shane, 1991.. ŽB. Compositions of tephra beds from Okataina centre in the last ca. 15 ka, showing the similarity of compositions erupted over short time intervals ŽShane, unpublished data..

234

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

5.7. Multiple glass populations within tephra beds Tephra horizons containing more than one population of compositionally distinct glass shards are relatively common in distal sequences where the tephra have been remobilised by fluvial or marine processes ŽFroggatt, 1983; Black, 1992; Shane, 1991; Shane et al., 1996, 1998b.. Compositional heterogeneity found within tephra beds include: Ž1. two or more discrete populations of similar proportion; Ž2. the occurrence of one or two outlier shards; or Ž3. a near continuum of shard compositions ŽFig. 7.. The origin of such heterogeneity may result from accidental ejecta from the vent or post-eruption entrainment of xenolithic material into pyroclastic flows; magma-mixing Že.g., Carey and Sigurdsson, 1978.; or post-eruption sedimentary mixing with older nonconsolidated tephra in catchment areas or by recycling in a tidal environment. Post-eruption sedimentary mixing is favoured for many of the occurrences of multiple glass populations in New Zealand late Cenozoic sequences because compositional heterogeneity is rarely encountered in primary fall or flow deposits, but is common in deposits displaying sedimentary bed-

forms and nonvolcaniclastic detritus ŽShane, 1991.. A sedimentary mixing origin can be confirmed for some tephra beds as they consist of compositions that can be matched to underlying homogeneous tephra beds found in the same sequence ŽFig. 7D.. Such relationships could arise if the older tephra beds were eroded in the head waters of fluvial systems and then redeposited higher in the stratigraphic sequence farther downstream. Recognition of compositionally heterogeneous tephra beds resulting from sedimentary mixing by grain-specific analysis is important as they do not represent new eruptive events and they could produce anomalous geochronological and geochemical results. 5.8. REE and trace-element characterisation As for major oxides, a wide range of REE and trace elements in glass can be used to fingerprint individual eruptive events and identify the source volcano. Although extensively used elsewhere Že.g., Sarna-Wojcicki et al., 1984., trace-element data has not been widely employed in New Zealand tephros-

Fig. 7. Examples of compositional heterogeneity found in individual tephra beds as revealed by electron microprobe analysis: ŽA. two discrete compositional groups; ŽB. a dominant shard population and a single outlier shard; and ŽC. a near continuum of compositions. In ŽD. tephra 244 is composed of three shard populations that compositionally match three older tephra beds found in the same section. In ascending stratigraphic order: 288, 287, 286 and 244 Ždata from Shane et al., 1996.. All examples come from early Pleistocene fluvial sequences of the southern North Island Že.g., Shane, 1991..

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

235

Fig. 8. Trace-element composition Ždetermined by INAA. of glass in tephra beds erupted by the contemporaneously active Taupo and Okataina centres, demonstrating the ability to identify source for post-22 ka units ŽFroggatt, unpublished data..

tratigraphy. Where EMA data shows that a tephra bed is compositionally homogeneous, techniques such as XRF and INAA can be readily employed on pure glass separates. In most TVZ tephra beds, REE and trace elements vary inversely with SiO 2 content analogous to the major oxides, and there is no significant difference in signature between the Miocene– Pliocene Coromandel volcanic zone and the Quaternary TVZ ŽShane et al., 1998b..

A variety of REE and trace elements can be used to distinguish tephra from the contemporaneous active Taupo and Okataina centres erupted during the last 22 ka. Taupo tephra beds display higher abundances of Sm, Eu, Tb, Lu, Hf and Sc Že.g., Fig. 8, Table 3.. In contrast, Okataina tephra can be distinguished by high Th and Ba contents ŽTable 3.. Pre-22 ka Taupo tephra beds Že.g., Kawakawa tephra. are compositionally similar to Okataina centre tephra

Table 3 REE and trace-element composition Žppm. of glass in New Zealand tephra beds La

Ce

Nd

Eu

Tb

4.67 4.21 4.04 4.04 3.59 4.38

0.53 0.56 0.62 0.53 0.59 0.86

0.66 0.63 0.6 0.56 0.59 0.6

19.7 19.7

5.12 5.54 5.56 5.06 5 4.9

1.06 1.03 0.99 0.84 0.79 0.85

15.6 15.7 19.2 13.8

3.83 3.39 4.2 3.44

66.5

18.17

Okataina post-22 ka Kaharoa 29.3 Whakatane 25.2 Rotoma 24.8 Rerewhakaiitu 27.3 Okareka 24.5 Te Rere 23.2

63.5 52.9 52 52 52.8 50.5

20.1 17.8 19.6

Taupo post-22 ka Taupo Ig Taupo lapilli Waimihia Opepe Poronui Karapiti

22.7 24.6 24.7 22.8 24.1 23.3

53.7 55.6 55.4 49.9 59.9 48.8

20.2 23.2 22.2

Older TVZ Kawakawa Rangitawa Potaka Pakihikura

21.8 24.9 26.4 21.9

44.2 49.3 57.2 49.6

Mayor Island Tuhua

78.6

189

15.7

Sm

Yb

Lu

U

Th

3.1 3.2 3.1 2.8 2.8 3

0.54 0.51 0.46 0.48 0.47 0.51

2.4 2.3 2.7 2.3 2.2 1.7

13.8 12.4 11.2 12.5 12.4 9.2

3.2 3.2 3.6 3.4 3.6 4.4

0.84 0.81 0.89 0.72 0.75 0.86

3 3.3 3.2 3.1 3.6 2.9

0.53 0.57 0.59 0.6 0.57 0.57

2.2 2.2 2.3 2.1 2.1 1.8

10.5 10.3 10.3 10.4 10.3 9.6

0.63 0.36 0.58 0.64

0.64 0.5 0.56 0.46

2.8 2.2 2.63 2.47

0.44 0.46 0.4 0.37

2.7 3.5 2.5 2.2

1.61

3.13

12.15

2.09

4.2

Compositions determined by INAA on glass separates. Analysts: Froggatt and Shane.

Hf

Sc

Ta

Cs

Ba

3.6 3.7 3.8 3.8 3.2 4.3

0.6 0.6 0.6 0.6 0.6 0.7

2.6 2.5 2.3 3.3 2.7 2

1150 1190 450 1170 1150 1100

5.7 6.2 6 5.9 5.7 5

10.1 11.1 10.7 9.3 8.5 7.6

0.7 0.7 0.7 0.7 0.6 0.6

2.3 2.3 2.4 2.2 2.4 2.1

710 770 720 660 760 640

11.1 15.7 14.7 12.1

3.8 3.5 3.5 3.2

5.4 4.3 3.5 4.7

0.5 0.8 0.8 0.6

2.9 3.5 6.8 5.7

860 1200 733 760

15.9

24.8

0.31

6.2

3.9

1213

236

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

Fig. 9. Examples of geochemical characterisation of glass in tephra beds using trace-element data. Each data point represents a different sample locality. Rb–Sr data determined by XRF Žfrom Shane and Froggatt, 1994., and Th–Eu data determined by INAA Žfrom Shane, unpublished data; and P. Froggatt, pers. commun., 1998..

and are thus difficult to distinguish ŽFig. 8.. The Tuhua tephra from the peralkaline Mayor Island volcano is distinctive from all TVZ rhyolites by its significantly higher abundances of all REE and other elements such as U, Th and Hf ŽTable 3.. Many tephra beds erupted from a common volcano over a short period of time such as the Holocene sequence of Taupo volcano cannot be distinguished by REE or trace element data ŽTable 3.. The ability to distinguish tephra from the Okataina and Taupo centres and individual eruptions from the same volcano is about the same as can be achieved by major oxides from electron microprobe analysis. Binary plots of elements such as Rb and Sr can be used to distinguish some events erupted over long time periods ŽFig. 9.. However, to use such plots effectively, representative compositional fields need to be defined Žrather than a single analysis.. This requires the preparation of a number of pure separates. For XRF, this commonly requires ) 4 g of glass per sample. Statistical analysis of compositional data of TVZ rhyolites show that Ba, Y, Rb and Sr have a high discriminating power, but a similar degree of discrimination is possible using major oxide data ŽShane and Froggatt, 1994.. In contrast, some tephra beds in the western US erupted from a common intra-continental caldera can be recognised only on the basis of their trace element signature ŽSarna-Wojcicki et al., 1984, 1987.. In tephra erupted from stable continental settings, these trace-element characteristics may reflect the lower frequency of eruptions and long magma residence time in the crust, as well as differences in source material.

5.9. Geochemical fingerprinting of nonrhyolitic tephra The two andesitic centres, Tongariro Volcanic Centre and Egmont ŽTaranaki. Volcano, have produced numerous late Pleistocene and Holocene tephra beds. Because the glass is highly weathered or not present in many proximal andesitic tephra, the composition of various mineral phases have been used in attempts identify the source volcano Že.g., Kohn and Neall, 1973; Cronin et al., 1996a. and individual tephra beds Že.g., Cronin et al., 1996b.. However, there are significant geochemical differences in lava and pyroclastic rocks that distinguish the Tongariro centre from Egmont volcano ŽFig. 10.. Tongariro

Fig. 10. Composition of proximal lavas of Egmont volcano ŽPrice et al., 1992, 1999. and of lavas and pyroclastic rocks of the Tongariro Centre ŽGraham and Hackett, 1987; Nakagawa et al., 1998. compared to glass in distal tephra assigned to these volcanoes ŽLowe, 1988a; Froggatt and Rogers, 1990; Eden and Froggatt, 1996.. Med-K and high-K fields from Le Maitre et al. Ž1989..

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

magmas are typical medium-K, calc–alkaline basalts and andesites ŽGraham and Hackett, 1987; Nakagawa et al., 1998., while Egmont magmas belong to the high-K series Že.g., Price et al., 1992; Price et al., 1999.. Egmont lavas contain abundant clinopyroxene and hornblende, both of which are rarer in Tongariro eruptive products. Therefore, the composition of glassy tephra deposits should reflect these affinities. Andesitic and dacitic tephra beds containing fresh glass from the Tongariro centre and Egmont volcano have been reported in distal sedimentary basins of North Island. The andesitic tephra beds are occasionally preserved in peat bogs Že.g., Froggatt and Rogers, 1990. and lake sediments ŽLowe, 1988a,b; Eden et al., 1993. that have been sampled via coring. The tephra beds are generally thin relative to their rhyolitic counterparts and can go undetected due to their dark color relative to enclosing organic sediments. Andesitic and dacitic tephra are commonly mineralrich and glass-poor compared to rhyolitic tephra beds. A wide compositional range of glass has been reported for distal tephra assigned to the Tongariro and Egmont centres ŽSiO 2 ca. 55–) 70 wt.%. corresponding to andesite, dacite and rhyodacite. Lowe Ž1988a. assigned tephra beds with SiO 2 contents of ca. 60 wt.% to Tongariro, and high SiO 2 beds to Egmont Žca. 70 wt.%.. The Egmont units are also characterised by high K 2 O Ž) 4 wt.%.. However, there is no comprehensive database for tephra from the two centres and therefore direct correlation is still uncertain without precise radiometric age control. Tephra assigned to Tongariro centre eruptions by Lowe Ž1988a. and Froggatt and Rogers Ž1990. plot within the high-Si range of reported proximal whole rock lava and pyroclastic deposit compositions ŽFig. 10.. The distal tephra assigned to Egmont by Lowe Ž1988a. have higher SiO 2 and K 2 O than reported proximal lavas, but plot on the same compositional trend suggesting they may represent highly differentiated magmatic fractions from the same source ŽFig. 10.. The within-sample compositional variability for glass within some andesitic tephra beds is no greater than most rhyolite units ŽSiO 2 - "0.5 wt.%., although other andesite tephra display a wider compositional range Žup to 6 wt.% SiO 2 . ŽFig. 11.. This compositional variability is likely to reflect magmatic gradients because it is found in primary fall

237

Fig. 11. Examples of the glass composition of nonrhyolitic tephra beds determined by electron microprobe analysis ŽShane, unpublished data.. Žtop. Compositions of 10 different andesiticrdacitic tephra beds in a core collected from Lake Poukawa in southern North Island. Sample numbers refer to depth in metres from top of core. Žbottom. Composition of four basaltic tephra beds. Rangitoto Žca. 600 years BP.; Mt. Eden Žca. 14 ka. and Mt. Wellington Žca. 10 ka. from the Auckland Volcanic Field. Mt. Tarawera Ž1886. from Okataina Centre.

deposits. However, mixing of closely spaced eruptions in slowly accumulating peat sequences cannot be ruled out of the source of compositional variation. Major oxide glass chemistry depicted on simple SiO 2 vs. K 2 O or SiO 2 vs. FeO binary plots are sufficient to fingerprint some andesite units analogous to rhyolite tephra ŽFig. 11.. The glass composition of basaltic tephra in New Zealand has received almost no attention in the literature, although basaltic volcanoes have been active throughout the Quaternary until the last few thousand years in the Auckland ŽAllen and Smith, 1994. and Kaikohe-Bay of Islands Volcanic Fields ŽKear, 1961; Smith et al., 1993., and historically in the TVZ such as the Tarawera eruption in 1886. Much of the basaltic pyroclastic material in the Auckland Volcanic Field is phreatomagmatic or phreatic and contains a large proportion of accidental ejecta, some of which is hydrothermally altered. The

238

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

subaerially exposed deposits are commonly weathered to clay. However, glassy basaltic ejecta has been found in cores Že.g., Newnham and Lowe, 1991.. Basaltic ejecta is often microcrystalline containing euhedal crystals of plagioclase, olivine and Fe–Ti oxides dispersed in a glass groundmass. This increases the difficulty in obtaining homogeneous glass analyses. Reconnaissance analyses of basaltic tephra from a few volcanoes shows the potential for geochemical fingerprinting by electron microprobe ŽFig. 11.. Similar to some andesitic tephra, the basaltic glasses can be more chemically variable ŽSiO 2 range ) 2–3 wt.%. than those of large volume rhyolites.

6. Fe–Ti oxides 6.1. Composition Spinel Žtitanomagnetite. in TVZ tephra beds was one of the earliest phases targeted for geochemical fingerprinting. Kohn Ž1970. used partial analyses of spinel bulk separates from emission spectrometry to recognise individual tephra beds. Since these extensive early studies Že.g., Kohn and Neall, 1973; Topping and Kohn, 1973., Fe–Ti oxides in TVZ rhyolites have received little attention except as an adjunct in a few tephra studies Že.g., Lowe, 1988a,b. and in petrological studies Že.g., Dunbar et al., 1989; Ghiorso and Sack, 1991.. The realisation of the potential for contaminant crystals Že.g., Kohn, 1979. and the ubiquitous occurrence of apatite and glass inclusions in the Fe–Ti oxides prompted a move to grain-specific techniques and the use of the glass phase to avoid detrital contamination in distal tephra that were commonly mineral poor ŽFroggatt, 1983.. However, the use of Fe–Ti oxides for correlation in the TVZ has recently been revisited using grainspecific techniques ŽShane, 1998a.. The spinel phase in TVZ rhyolites occur mostly in the compositional range X Usp 0.20–0.40, and the rhombohedral phase Žilmenite. in the range X Ilm 0.77–0.97. TiO 2 in the spinel phase is the most variable major oxide, ranging between ca. 6.5–14.5 wt.% for different eruptions ŽTable 4.. MgO, MnO, and Al 2 O 3 may vary by a factor ) 2 in both oxide phases between different tephra beds. However, the

compositional range within individual eruptive units is narrow Že.g., Table 4.. The composition of the spinel phase in a tephra bed can be used to identify the source volcano. For example, post-22 ka Taupo and Okataina centre tephra can be distinguished on a binary plot of TiO 2 vs. Fe 2 O 3 ŽFig. 12.. The relatively Fe-rich glass of Taupo tephra beds Žcompared to Okataina. is accompanied by Fe-poor and Ti-rich spinels. In accord with the major- and trace-element composition of the co-magmatic glasses, the compositions of the Fe–Ti oxides of pre-22 ka Taupo tephra are similar to those of Okataina tephra. However, Fe–Ti oxides in pre-22 ka Taupo tephra can be distinguished on the basis of their lower MnO contents relative to Okataina tephra ŽFig. 12.. In this respect, Fe–Ti oxides are more distinctive than glasses. The composition of the spinel phase in a tephra is not universally unique, but can be used to distinguish many individual eruptive events. The spinel phase is particularly useful in discriminating between eruptions of the last 65 ka from the Okataina centre ŽShane, 1998a., that contain glasses that are broadly compositionally uniform ŽFroggatt and Rogers, 1990; Stokes et al., 1992.. Some of these tephra beds can be distinguished on simple binary plots of the TiO 2 and Fe 2 O 3 contents of the spinels ŽFig. 12.. The minor oxides of Al 2 O 3 , MnO and MgO in co-existing spinelrrhombohedral pairs are particularly powerful in identifying different eruptive units ŽFig. 13.. 6.2. Eruption temperatures and fO2 The composition of equilibrium spinelrrhombohedral pairs within a tephra bed allows the eruption temperature ŽT . and oxygen fugacity Ž f O 2 . of the magma to be estimated Že.g., Ghiorso and Sack, 1992.. Such calculations have been used in petrological studies of TVZ rhyolites Že.g., Ewart et al., 1975; Dunbar et al., 1989; Ghiorso and Sack, 1991.. Lowe Ž1988b. showed that T and f O 2 could be used to potentially distinguish andesitic tephra beds, but these parameters have been only recently applied as an identification tool for rhyolitic tephra ŽShane, 1998a.. Because tephra is erupted and emplaced as nonconsolidated material, there is a great potential for mineral contamination. This is particularly important for T–f O 2 studies that require equilibrium pairs

Table 4 Fe–Ti oxide compositions in rhyolitic tephra beds from TVZ 1

3

4

S.D.

Mean

S.D.

Mean

S.D.

0.38 8.39 1.19 83.43 1.11 0.47 95.08 37.8 50.71 100.05

0.06 0.07 0.07 0.45 0.05 0.1 0.53 0.22 0.52 0.54 5

0.43 12.88 1.91 78.76 0.88 1.04 96.07 41.56 41.35 100.05

0.08 0.33 0.07 0.46 0.03 0.14 0.31 0.33 0.52 0.53 5

0.4 14.49 1.82 78.68 0.83 0.74 96.95 43.77 38.78 100.84

0.03 0.13 0.05 0.15 0.07 0.07 0.19 0.4 0.51 0.21 7

0.15 0.5 0.12 0.87 0.67 0.14 0.65 0.48 0.62 0.67 5

0.34 47.16 0.24 48.69 1.16 1.79 99.35 38.45 11.38 100.52

0.07 0.41 0.05 0.69 0.09 0.09 0.25 0.17 0.94 0.34 5

0.3 48.82 0.19 48.34 1.22 1.36 100.21 40.6 8.61 101.09

0.03 0.29 0.05 0.33 0.07 0.05 0.52 0.37 0.32 0.51 5

Rhombohedral phase SiO 2 0.36 TiO 2 47.1 Al 2 O 3 0.14 FeO a 48.15 MnO 1.64 MgO 1.28 total 98.67 FeO 38.83 Fe 2 O 3 10.36 Total 99.72 n

Mean

5

6

7

8

S.D.

Mean

S.D.

Mean

S.D.

Mean

S.D.

0.4 7.35 1.47 83.64 0.86 0.65 94.42 36.4 52.5 99.62

0.03 0.13 0.04 1 0.03 0.05 1.07 0.8 0.73 1.14 6

0.5 13.7 2.33 76.53 0.49 1.61 95.16 41.64 38.78 99.05

0.08 0.26 0.11 0.23 0.11 0.09 0.07 0.35 0.38 0.05 5

0.38 10.38 1.61 82.04 0.52 0.56 95.49 40.14 46.57 100.15

0.05 0.14 0.06 0.37 0.08 0.05 0.47 0.32 0.33 0.49 4

0.36 8.45 1.86 82.57 0.99 1.07 95.3 37.16 50.47 100.35

0.05 0.18 0.11 0.42 0.06 0.05 0.58 0.36 0.3 0.59 6

0.33 46.81 0.22 48.94 1.6 1.48 99.37 38.16 11.98 100.57

0.07 0.6 0.02 0.51 0.08 0.05 0.39 0.69 1.21 0.39 5

0.41 45.5 0.37 48.79 0.57 2.48 98.13 36.29 13.89 99.52

0.05 0.44 0.06 0.26 0.05 0.1 0.64 0.55 0.68 0.63 5

0.33 47.67 0.19 48.9 1.21 1.3 99.6 39.38 10.59 100.66

0.05 0.16 0.05 0.22 0.07 0.08 0.17 0.6 0.81 0.22 4

0.31 44.91 0.25 50.01 1.49 2.01 98.96 35.71 15.89 100.58

0.06 0.19 0.06 0.14 0.07 0.08 0.18 0.27 0.33 0.19 7

9

Mean

10

S.D.

Mean

S.D.

Mean

S.D.

0.37 10.55 2.3 79.45 0.7 1.64 95 38.34 45.68 99.58

0.04 0.42 0.15 0.7 0.05 0.13 0.53 0.33 0.85 0.55 5

0.48 6.96 1.52 83.48 0.78 0.73 93.93 36.28 52.45 99.19

0.03 0.26 0.08 0.31 0.09 0.18 0.3 0.59 0.62 0.31 5

0.4 9.13 1.39 82.94 0.82 0.49 95.18 38.76 49.09 100.1

0.07 0.09 0.05 0.24 0.05 0.08 0.39 0.26 0.15 0.39 5

0.29 43.36 0.36 51.3 0.74 2.46 98.51 34.21 18.99 100.4

0.04 0.09 0.09 0.07 0.03 0.15 0.11 0.3 0.28 0.12 5

0.42 45.5 0.21 49.16 1.4 1.59 98.21 37.09 13.41 99.62

0.05 1.73 0.05 0.71 0.08 0.34 0.79 2.24 3.27 0.45 5

0.32 47.87 0.16 48.18 1.67 1.2 99.33 39.53 9.61 100.37

0.04 0.18 0.06 0.3 0.1 0.07 0.23 0.18 0.43 0.24 5

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

Spinel phase SiO 2 TiO 2 Al 2 O 3 FeO a MnO MgO total FeO Fe 2 O 3 Total n

2

Mean

1, Kaharoa; 2, Taupo fall; 3, Waimahia; 4, Rotoma; 5, V ; 6, Oruanui; 7, Mangaone; 8, Hauparu; 9, Rotoehu; 10, Earthquake Flat. Analysis Žin wt.%. represent a mean and standard deviation on n crystals. a FeOs Fe analysed as FeO.

239

240

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

lihood of equilibrium and ensures a co-magmatic origin. Fe–Ti oxides that have suffered exsolution are chemically variable on a microscale and cannot be used for simple T–f O 2 estimates. Exsolved crystals are commonly encountered in vapour-phase altered and intensely welded ignimbrites. In some cases, the exsolution is evident only after careful X-ray backscatter imaging ŽShane, 1998a.. In contrast, many nonwelded flow units and nonwelded basal chill zones of welded ignimbrites contain homogeneous Fe–Ti oxides similar to those in fall deposits that can be used for T–f O 2 estimates. Tephra erupted from Taupo and Okataina in the last 64 ka display a range of temperatures Ž690– 9908C. and ylog f O 2 Ž16.5–9.5. ŽFig. 14.. Temperatures show a linear negative correlation with SiO 2 content in the glass phase and a relationship with ferromagnesian mineral assemblage, while f O 2 correlates with both dominant ferromagnesian mineral present and the eruption centre ŽShane, 1998a.. Both Taupo and Okataina hornblende-dominated tephra display higher f O 2 values than hypersthene-dominated Taupo tephra. Cummingtonite-bearing tephra from Okataina display the highest f O 2 values. Augite-bearing and augite-dominated tephra display the highest temperatures. However, superimposed on these mineralogic trends are higher f O 2 values for all Okataina tephra compared to Taupo tephra of the same eruptive temperature ŽFig. 14.. In this respect,

Fig. 12. The composition of spinel in rhyolitic tephra showing the potential to distinguish eruptive centre and individual eruptive events ŽShane, 1998a.. ŽA and B. Comparison of tephra from the Taupo and Okataina centres. ŽC. Individual post-65 ka tephra beds from the Okataina centre.

of crystals. Bacon and Hirschmann Ž1988. have proposed a simple equilibrium test that involves plotting log MgrMn Žatomic. composition of spinel and rhombohedral pairs. In many TVZ rhyolites, spinelrrhombohedral pairs can be found attached to orthopyroxene phenocrysts which increases the like-

Fig. 13. The minor-element composition of Fe–Ti oxide pairs in post-65 ka tephra beds erupted from Okataina centre ŽShane, 1998a.. The spinel phase is Al-rich and the rhombohedral phase is Al-poor. The compositions are related to eruption temperature Žcalculated following Ghiorso and Sack, 1991..

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

Fig. 14. Temperature and oxygen fugacity Ž f O 2 . estimated from Fe–Ti oxides in TVZ tephra beds showing differences between eruptive centre and dominant mafic mineral ŽShane, 1998a.. Fayalite–magnetite–quartz ŽFMQ., NiNiO and magnetite–hematite ŽMH. buffer curves shown for reference only. Opx s orthopyroxene, Hbs hornblende, Cm s cummingtonite, Cpx s clinopyroxene.

products from the two centres can be distinguished by a T vs. f O 2 plot. It appears that f O 2 can be a long-lived fundamental characteristic of a volcanic centre. For a particular volcanic centre, T–f O 2 trends may change with time reflecting major changes in source following voluminous caldera-forming eruptions. For example, pre-22 ka tephra and the 22 ka

241

Kawakawa event define a separate T–f O 2 trend that is ) 0.5 log units more oxidised than subsequent eruptions from the centre. Thus estimates of T and f O 2 can be potentially used to assign a tephra bed to a temporal phase of the volcano. Individual tephra beds from the TVZ commonly display narrow ranges in T Žca. 308C. and log f O 2 Ž- 0.5.. This limited variation is about the same as multiple oxide pair estimates from a single pumice clast ŽShane, 1998a., suggesting that the variation is due largely to analytical uncertainties. Individual eruptive events can be recognised on plots of T vs. f O 2 ŽFig. 15., where f O 2 is expressed as log units relative to the fayalite–magnetite–quartz ŽFMQ. buffer Ž Dlog f O 2 .. Not all tephra beds from a common volcano were erupted under unique T–f O 2 conditions, However, these variables do allow further discrimination of units that are compositionally similar in major and trace elements.

7. Silicate mineral geochemistry Orthopyroxene Žmostly hypersthene. is the most common ferromagnesian mineral in New Zealand late Cenozoic tephra beds. X-ray backscatter imaging reveals that the crystals are commonly homogeneous and show little evidence of zoning. The composition of most orthopyroxenes within TVZ tephra beds

Fig. 15. Temperature and Dlog f O 2 ŽFMQ. estimates for individual tephra beds erupted from the Okataina centre showing their homogeneity in physical properties and differences between eruptions ŽShane, 1998a..

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generally overlap and do not distinguish individual eruptions. Tephra erupted from the Okataina centre in the last 64 ka display a wider total range in orthopyroxene composition Žca. En 45 to En 65 . than tephra erupted from Taupo centre Žca. En 43 to En 54 .. The orthopyroxene compositions determined from core analyses show a correlation to Fe–Ti oxide temperatures in Okataina centre tephra ŽFig. 16.. Orthopyroxene compositions increase from ca. En 45 to En 65 with increasing temperature over the range ca. 700–9808C. Systematic variations are less evident in Taupo centre orthopyroxenes; however, limited analyses from pre-22 ka units appear to parallel the Okataina centre trend ŽFig. 16.. Ghiorso and Sack Ž1991. have previously noted a correlation between T–P and orthopyroxene composition for TVZ rhyolites. A combination of eruption temperature and orthopyroxene composition can thus be used to distinguish eruptive centres for post-64 ka rhyolitic tephra, but not individual eruptions. Lowe Ž1988b. showed that clinopyroxenes in Egmont-derived andesitic tephra are more Ca-rich Žsalite and Ca-rich augite. compared to those of Tongariro-derived andesite tephra Žtypical augite.. Hornblende from Egmont tephra are generally more Ca-rich ŽEden et al., 1993. and K-rich ŽDonoghue and Neall, 1996. than that in Tongariro tephra. There are more significant differences in mineral compositions between andesitic and rhyolite tephra. Hornblendes from the Egmont and Tongariro andesite centres are more pargasitic than those in rhyolites

Fig. 16. Composition of host orthopyroxene crystals plotted against estimated eruption temperatures from Fe–Ti oxides in the tephra ŽShane, 1998a.. Each data point represents a single pyroxene core analysis. Temperature is averaged from several oxide pairs.

from TVZ ŽFroggatt and Rogers, 1990.. Olivine in Tongariro tephra beds are forsteritic and those in peralkaline Mayor Island rhyolite tephra are fayalitic Že.g., Lowe, 1988a.. In general, the composition of ferromagnesian minerals do not allow the identification of individual eruptive events.

8. Paleomagnetism Secular variation of Earth’s magnetic field recorded within deposits that have a thermal remnant magnetism ŽTRM. provides a criterion for identifying individual eruptive events ŽBlack et al., 1996.. Many welded ignimbrites in the TVZ display very stable TRM after the removal of weak viscous magnetic overprints at low temperature or in weak alternating fields. Mean TRM directions with 95% cones of confidence Ž a 95 . of 2–48 can be obtained from a single site of three to five specimens. This allows many ignimbrites to be distinguished using stereographic projections of site means ŽFig. 17.. Vertical sampling through thick ignimbrite pond deposits Žup to 200 m. has revealed uniform TRM directions indicating that such sequences cooled in sufficient time as to not record any changes in field orientation ŽBlack et al., 1996.. Using TRM directions, Black et al. Ž1996. were able to correlate widely dispersed lobes of the Whakamaru group ignimbrites and show that the entire sequence was erupted within a short time interval Žperhaps - 100 years.. This is supported by recent 40Ar– 39Ar chronology ŽPringle et al., 1992.. Some ignimbrites were erupted during directional excursions in the Earth’s magnetic field, and thus record a distinctive TRM direction that greatly facilitates correlation. The Mamaku ignimbrite records such an excursion ŽFig. 17.. Geochronological data suggest this excursion may represent the Pringle Falls event ŽHerrero-Bervera et al., 1994. dated ca. 0.23 Ma ŽBlack et al., 1996.. Not all welded Quaternary ignimbrites display stable TRM. Hematite formation and other post-depositional weathering processes provide problems for isolating the primary field component, and often result in poor site statistics that prevent correlation. In sedimentary basins, magnetostratigraphy can assist in the correlation of tephra beds. For example,

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

243

nique is not suited to young deposits Ž- 1 Ma. with low K content Že.g., - 0.4 wt.% K 2 O in feldspars and hornblendes.. K-rich phases such as sanidine have only been rarely reported in TVZ deposits. As a result, studies have focused on fission track ŽFT. methods for deposits beyond the age range of 14 C Žcommonly ) 40–50 ka.. Early FT studies employed the ubiquitous glass phase ŽSeward, 1975, 1976.. Over time or due to post-depositional thermal events, fossil FTs anneal or become shorter. These pioneering studies did not correct for partial track fading Žannealing. that occurs at ambient temperatures in glass and that if uncompensated will lead to underestimates of age. Zircons have also been targeted for FT geochronology of TVZ tephra ŽSeward, 1979. as they have higher track annealing temperatures Ž2008C.. However, magnetostratigraphic studies demonstrated that these early FT techniques had produced anomalously young ages for some sequences ŽBlack, 1992.. 9.2. Radiocarbon (14C)

Fig. 17. Lower hemisphere stereographic projection of mean TRM directions and surrounding fields of uncertainty Ž a 95 . for welded ignimbrites of the TVZ. Data from Black et al. Ž1996.. ŽA. Four flow lobes of the Whakamaru group ignimbrites Žca. 0.34 Ma.. ŽB. Widespread late Pleistocene ignimbrites: Whakamaru, Matahina Žca. 0.33 Ma., Kaingaroa Ž0.31 Ma., and Mamaku Žca. 0.23 Ma..

the identification of relatively short lived polarity subchrons such the Jaramillo Normal Subchron Ž1.07–0.99 Ma. has greatly facilitated the construction of a tephrostratigraphy in basins of the southern North Island ŽPillans et al., 1994; Shane et al., 1996..

9. Geochronology 9.1. Early studies Many TVZ Quaternary tephra beds are inherently difficult to date because the traditional K–Ar tech-

The use of organic materials found in association with tephra deposits for 14 C dating has been widely employed in developing the chronology of the most recent rhyolitic eruptions of the Taupo, Okataina, Maroa and Mayor Island centres Žsummarised by Hogg et al., 1987; Froggatt and Lowe, 1990.. Material carbonised during the emplacement of the tephra provides the best material for chronology, but is not always available. Many ages are based on organic material found interbedded with the tephra in peat bogs and lakes Že.g., Lowe, 1988a.. Such ages may not precisely reflect the time of the eruption and are prone to contamination. However, peat and lake sequences do provide a near continuous depositional record that commonly contain multi-sourced tephra beds that allow chronologic relationships to be established Že.g., Hogg et al., 1987; Lowe, 1988a.. The potential accuracy and precision for dating tephra in the 14 C range is great. Froggatt and Lowe Ž1990. had presented a error-weighted mean age of 770 " 20 years BP based on 15 ages for the Kaharoa tephra, the youngest rhyolitic eruption from the Okataina centre. Lowe and Hogg Ž1992. determined an age of 665 " 17 years BP based on four addi-

244

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

tional samples of carbonised wood from proximal Kaharoa deposits. Lowe et al. Ž1998. listed 22 determinations based on charcoal, wood, peat sediments and soil, that range in age from 607 to 1144 years BP, and an age of 665 " 15 years BP was determined via cluster analysis. This age corresponds to a range of calendar ages on radiocarbon calibration curve. These workers estimated a calibrated age of 650–560 cal years BP ŽAD 1300–1390. for the eruption. One of the most dated eruptions is the ca. 2 ka Taupo event, the youngest eruption from the Taupo centre. Froggatt and Lowe Ž1990. presented a mean 14 C age of 1850 " 10 years BP for this tephra from 41 determinations. However, as for many 14 C Holocene ages, assigning the age of Taupo tephra to a calendar date is difficult.Various calibration attempts have placed this eruption at AD 214 Ž1 s AD 138–230. and AD 177 Ž1 s AD 166–195. ŽFroggatt and Lowe, 1990.. Using a floating tree ring chronology from a forest buried by the eruption, Sparks et al. Ž1995. estimated a calibrated age of AD 232 " 15. Zielinski et al. Ž1994. matched a sulfate peak in GISP2 Greenland ice core at AD 181 " 2 to Taupo by assuming an eruption age of ca. AD 177. Despite the uncertainty in calendar year of the eruption, the Taupo event is known to have occured in mid–late summer, on the basis of fossil flora ŽClarkson et al., 1988.. The widespread Kawakawa tephra has been accurately dated at 22590" 230 years BP from four organic fragments carbonised within the Oruanui ignimbrite phase of the eruption ŽWilson et al., 1988.. This age is older than some estimates from enclosing peats and silts at distal localities that may have been affected by contamination from young carbon ŽFroggatt and Lowe, 1990.. 9.3. Isothermal plateau fission-track (ITPFT) In early FT studies of glass in TVZ tephra, the fossil track population had been affected by annealing, while the laboratory induced tracks had not. This resulted in an underestimate of age. Partial track fading can affect young glass that has had a simple cooling history. Black et al. Ž1996. showed that glass in the Mamaku ignimbrite had suffered an average

track size reduction of ca. 16% in 230 ka. It is now possible to correct for partial fission track fading in glass via the ITPFT method ŽWestgate, 1989.. This simple technique involves a single heat treatment of 1508C for 30 days to ensure that the fossil tracks and those induced in the laboratory have been annealed to the same degree. As the age of glass depends on the ratio of fossil track density to induced track density, both populations need to have suffered the same degree of fading to produce an accurate age. This can be demonstrated by track diameter measurements after heat treatment. In addition, it is now recognised that tracks must be etched to an average size of 6–8 mm for a 500 = magnification for representative track revelation ŽSandhu and Westgate, 1995.. The ITPFT technique involves counting tracks over a wide glass surface area and thus numerous shards. Therefore the glass must be compositionally homogeneous. This can be tested by electron microprobe analysis, but this assumes that any contaminant population of glass is compositionally distinguishable. The ITPFT technique is fully summarised in Westgate et al. Ž1997.. The ITPFT method has been widely applied in New Zealand to construct stratigraphic frameworks for distal tephra sequences back to 10 Ma ŽAlloway et al., 1993; Shane et al., 1995a, 1996, 1998b; Shane, 1998b.. For Quaternary tephra, a typical age error is "10%, but this can be greatly reduced by calculating a weighted mean from several age determinations. For example, the Potaka tephra has been dated at 1.00 " 0.03 Ma from six ITPFT determinations ŽShane et al., 1996.. As a result of ITPFT work, the age of many Quaternary sequences has been revised to as much as twice as old as originally determined in early FT studies. The new ITPFT ages are all in good agreement with magnetostratigraphic ages. Glass from the chilled nonwelded base of welded ignimbrite sheets is also suitable for ITPFT studies ŽBlack et al., 1996.. Such zones can contain fresh, isotropic glass unaffected by vapour-phase alteration and devitrification that characterises the main ignimbrite sheet Že.g., Hildreth and Mahood, 1985.. In proximal exposures, the glass shards are commonly large Ž) 250 mm. and thus provide a large surface area for track revelation. This promotes accurate and precise age determinations on young deposits Ž- 0.4 Ma.. The precision of ages from weighted means of

P. Shaner Earth-Science ReÕiews 49 (2000) 223–259

three to four determinations is comparable to that obtained from 40Ar– 39Ar Že.g., Houghton et al., 1995. on plagioclase from the TVZ ignimbrites. Black et al. Ž1996. presented an age of 0.23 " 0.01 Ma for the Mamaku ignimbrite from four ITPFT age determinations, in good agreement with and of same precision as a plagioclase 40Ar– 39Ar age of 0.22 " 0.01 Ma by Houghton et al. Ž1994.. Ages as young as ca. 70 ka have been determined by the ITPFT method ŽChesner et al., 1991., but these were on glass of higher U content than is typical for TVZ tephra. The ITPFT ages are based on glass and thus represent the quench time of the eruption and are unaffected by pre-eruption crystallisation events or incomplete resetting found in some ignimbrites Že.g., Bogaard, 1996.. 9.4. Zircon FT Kohn et al. Ž1992. and Seward and Kohn Ž1997. revisited the use of zircon in FT dating of Quaternary tephra in New Zealand. Problems with early zircon FT studies included the anisotropic nature of track revelation. Only when tracks parallel to the c-axis of the crystal are fully etched can a representative track density be determined. These workers also showed that at least 20 different crystals should be counted before an accurate age can be assessed because of the low fossil track densities in Quaternary zircons, and because of contaminant crystals. They recommended that a running mean of crystal ages be used to assess when a sufficient number of crystals have been counted to determine an accurate age. In addition, problems with neutron dosimetry and decay constants have now been overcome with the use of calibrated zeta values Žsee Hurford and Green, 1983.. Some of the new zircon FT ages reported by Seward and Kohn Ž1997. for tephra beds in the Rangitikei valley are in accord with ITPFT, 40Ar– 39 Ar, and magnetostratigraphic data, but contaminant crystals complicate the age interpretation. For the Mangapipi ash, two populations of zircons with mean ages of 1.38 " 0.11 and 2.15 " 0.17 Ma were found. Seward and Kohn Ž1997. interpreted the younger population as the eruption age, and the older group as detrital or xenocrystic contaminants. Even more variation in zircon crystal ages were determined for

245

Rewa Pumice, a deposit that displays evidence of reworking in an estuarine environment. Different crystal population groupings gave mean ages of 1.21 to 11.1 Ma ŽSeward and Kohn, 1997.. Using statistical approaches, a young age component in the range 0.96–1.04 Ma was isolated and considered to represent the eruption age. However, this is at odds with the stratigraphic position of Rewa Pumice beneath the Jaramillo Normal Subchron Žbase 1.07 Ma. ŽPillans et al., 1994.. 9.5. 4 0Ar– 39Ar This technique has not been widely applied to distal tephra beds in New Zealand, but has been used to construct a new chronology for proximal ignimbrites of the TVZ ŽPringle et al., 1992; Houghton et al., 1995.. These workers performed step-heated Ar release experiments on small feldspar and sanidine separates Ž14–79 mg., and laser fusion analysis on 15–20 mg subsamples from the nonwelded bases of flow units to obtain age spectra and isochrons. The 40Ar– 39Ar technique provides an opportunity to obtain high-precision ages on young tephra deposits. Pringle et al. Ž1992. obtained precision of about "30 ka for Quaternary aged plagioclase and better than 1% on sanidine. For three members of the Whakamaru group ignimbrites they determined a mean age of 332 " 2 ka. This age is within uncertainty limits of more recent zircon FT age determinations ŽKohn et al., 1992.. The use of single crystal laser fusion ŽSCLF. to investigate the 40Ar– 39Ar age distribution of crystal populations in pyroclastic deposits has revealed the common occurrence contaminants and partially degassed relict crystals Že.g., Walter et al., 1991; Bogaard, 1995; Gansecki et al., 1996, 1998.. Individual pumice blocks may contain crystals with a range of ages reflecting incomplete degassing during ingestion into the new eruptive phase ŽGansecki et al., 1996.. Such studies have shown that an accurate age for some tephra deposits can be assessed only by grain-specific techniques. The SCLF 40Ar– 39Ar technique has been applied to tephra deposits as young as Holocene age ŽChen et al., 1996.. Because of the small volume of Ar involved in SCLF, K-rich phases are commonly required. This limits its application to calc–alkaline volcanic provinces such as the TVZ.

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10. Geochronological case studies 10.1. Potaka tephra Attempts to obtain an accurate age for the early Pleistocene Potaka tephra exemplify the problems in geochronology for deposits of ca. 1 Ma and younger. In the Rangitikei Valley of central Wanganui basin, Seward Ž1976. showed that Potaka tephra was contained within normal polarity sediments and overlay reversed polarity sediments containing another tephra ŽRewa Pumice.. Seward Ž1976. determined a glass fission track age of 0.61 " 0.06 Ma for Potaka tephra, and 0.74 " 0.09 Ma for Rewa Pumice. It was inferred from these ages that the Potaka tephra occurred within the Brunhes Normal Chron, and that the two tephra beds bracketed the Brunhes– Matuyama boundary Žthen considered to be 0.73 Ma.. The glass ages had not been corrected for partial track fading, however a later FT study using zircon provided an age of 0.64 " 0.18 Ma for Potaka tephra, implying the glass ages may have been fortuitously correct due to unrecognised track fading and inaccurate neutron dose determinations ŽSeward, 1979.. With the use of glass chemistry, Shane and Froggatt Ž1991. correlated Potaka tephra to a unit within the Brunhes Normal Chron in a Pacific Ocean deep-sea cores RC12-215 and RC9-113. This apparently confirmed the magnetostratigraphic position of Potaka tephra because the cores contained a complete record that included the Matuyama Reversed Chron and Jaramillo Normal Subchron lower in the sequence. The deep-sea cores also contained an older tephra bed near the top of the Jaramillo Normal Subchron that compositionally matched the Potaka tephra, but available age data from the Rangitikei valley favoured correlation to the younger tephra Žwithin the Brunhes Normal Chron.. Kamp and Turner Ž1990. presented new magnetostratigraphic studies of coastal sections of the Wanganui basin that suggested strata correlative to the Potaka sequence may be older than originally expected. Subsequently, reversed polarity sediments were discovered immediately above Potaka tephra at sites in the east coast region ŽShane, 1994., suggesting the eruption occurred shortly before the end of the Jaramillo Normal Subchron and not in the Brunhes Normal Chron. 40Ar– 39Ar ages determined by

R.C. Walter Žpers. commun., 1992., from several sites showed that the age of Potaka tephra was closer to 1 Ma. This was confirmed by Pillans et al. Ž1994. for the Rangitikei section where the normal polarity interval containing Potaka tephra was reinterpreted as the Jaramillo Normal Subchron Žcalibrated at 1.07–0.99 Ma., and by ca. 1 Ma ITPFT ages corrected for partial track fading in glass from the Rangitikei section ŽAlloway et al., 1993., and several other sites in basins of the east coast ŽShane et al., 1995a, 1996.. The age is now constrained at 1.04 " 0.03 Ma Žweighted mean. from five 40Ar– 39Ar plagioclase ages, and 1.00 " 0.03 Ma from six ITPFT ages from four localities. Recently, Seward and Kohn Ž1997. reported a revised zircon FT age for Potaka tephra of 0.97 " 0.04 Ma resulting from improvements in track revelation and larger numbers of crystals counted. These ages are in accord with current estimates for the duration of the Jaramillo Normal Subchron Ž1.07–0.99 Ma. ŽIzett and Obradovich, 1994. in which it is contained. 10.2. Rotoehu tephra The time interval ca. 50–200 ka remains problematic for the geochronology of tephra beds in New Zealand because it is beyond the normal dating range of 14 C and but younger than that routinely dated by the FT method on glass or zircon. In addition, calc– alkaline tephra beds generally lack high-K phases that may be amenable to 40Ar– 39Ar dating. The widespread Rotoehu tephra and associated Rotoiti ignimbrite occur within this time range and exemplify the problems in determining an accurate and precise age. Lowe and Hogg Ž1995. have demonstrated that many 14 C determinations - 40 ka are underestimates due to contamination by younger carbon. Estimates beyond the range of 14 C Ž) 40–50 ka. have also been determined for Rotoehu tephra Že.g., Froggatt and Lowe, 1990.. Ota et al. Ž1989. determined a U–Th disequilibrium age of 71 " 6 ka for Rotoehu tephra, although its validity has been questioned due to the small number of data points that constrain the isochron ŽFroggatt and Lowe, 1990.. Buhay et al. Ž1992. reported an ESR age for Rotoiti ignimbrite of 45.2 " 8.2 ka from individual ages in the range ca. 35–62 ka. Wilson et al. Ž1992. estimated an age of 64 " 4 ka for a fall deposit of

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Rotoehu tephra by averaging K–Ar ages on interbedded lava flows on Mayor Island. Other estimates include 52 " 7 ka based on the tephra bed’s stratigraphic position on marine terraces matched to sealevel change records ŽBerryman, 1992.; ca. 55 ka from sedimentation rates in a deep-sea core ŽPillans and Wright, 1992.; and ca. 61 ka based on amino acid racemisation dating of a loess–paleosol sequence ŽKimber et al., 1994.. 11. Rhyolitic tephra record and widespread horizons The current knowledge of the late Cenozoic rhyolitic tephra record in New Zealand is neither complete nor is its resolution uniform in space and time. However, several widespread andror distinctive tephra horizons have allowed a stratigraphic framework to be constructed. The tephra record is divided into three uneven time intervals on the basis of resolution of age control and preservation of the deposits: Ž1. The post-64 ka tephra record is well constrained in age and all deposits can be assigned to source vents. Nearly all tephra beds have distribution andror isopach maps. Ž2. The pre-64 ka Quaternary record Žca. 1.6–0.06 Ma., that represents most of the history of the TVZ. Some tephra beds are well constrained by ITPFT, Ar–Ar and paleomagnetism, and most cannot be assigned to a particular vent with certainty but still can be used for correlation. Ž3. The Miocene and Pliocene record is poorly constrained by radiometric dating and few horizons have been used in stratigraphic studies. The source region is inferred to be Coromandel on the basis of known caldera-forming eruptions at this time. 11.1. Post-64 ka The best known interval post-dates the widespread 64 ka Rotoehu tephra in central North Island, erupted from the Okataina Centre Že.g., Froggatt and Lowe, 1990.. The temporal resolution for much of this period is good because most of the tephra beds are directly datable by 14 C on wood carbonised during the eruption or by enclosing organic matter. The Rotoehu tephra is a useful marker at the base of the sequence and is one of the few tephra beds that

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contains the mineral cummingtonite. The tephra has not been accurately dated directly. However, its age is constrained by K–Ar data on interbedded lava flows at ca. 64 ka ŽWilson et al., 1992.. The Kawakawa tephra erupted at 22.6 ka ŽFroggatt and Lowe, 1990. is an additional field marker for this time interval. It is found the lower third of the regional Ohakea Loess deposit deposited near the Last Glacial Maximum Že.g., Pillans et al., 1993., and is often the only macroscopic tephra found in young loess sequences far from the TVZ. Beyond the zone of macroscopic fallout, the eruption has been recorded as dispersed glass shards as far south as southern South Island ŽEden and Froggatt, 1988.. The Kawakawa tephra is particularly widely dispersed in the Pacific Ocean east of New Zealand and is found as a macroscopic bed in deep sea cores some 1400 km from source ŽCarter et al., 1995.. Four rhyolite producing caldera centres were contemporaneously active during the last 64 ka: Taupo, Okataina, Mayor Island and Maroa ŽFig. 1.. Taupo and Okataina account for nearly all of the tephra fall deposits erupted in the post-64 ka interval ŽFig. 18.. More than 30 named and well-documented rhyolitic tephra beds or mappable ‘‘tephra formations’’ have been defined Že.g., Vucetich and Pullar, 1969; Froggatt and Lowe, 1990. in the post-64 ka period ŽFig. 18.. A tephra formation includes all the eruptive phases Že.g., ignimbrite, fall deposits. associated with the event or eruptive episode and is separated from other tephra formations by a paleosol that represents a considerable period of quiescence. By definition, the formations include the tephra and the overlying paleosol developed into it ŽFroggatt and Lowe, 1990.. For the Taupo Centre stratigraphy of the last ca. 22 ka, Wilson Ž1993. has preferred to further subdivide many tephra formations into individual eruptive events. For example, the Hinemaiaia tephra in now known to consist of 10 eruptive events Žreferred to alphabetically as units I to R. ŽFig. 18., some of which are separated by weak paleosols. In many circumstances, the earlier tephra formation nomenclature can still be used as many of the thin eruptive units identified by Wilson Ž1993. are not widely dispersed or cannot be differentiated in the distal setting. Five tephra beds from Mayor Island have been found dispersed beyond the volcanic edifice in deep-

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Fig. 18. Post-64 ka tephra beds from rhyolitic volcanic centres in the North Island. Data from Froggatt and Lowe Ž1990. and supplemented from Pillans and Wright Ž1992., Wilson Ž1993. and Wilson et al. Ž1995b..

sea cores ŽPillans and Wright, 1992. and on land ŽLowe, 1988a. ŽFig. 18.. However, on Mayor Island there is evidence for at least 17 pyroclastic eruptions during the last 64 ka ŽWilson et al., 1995b., most of which have a small volume and limited dispersal. The youngest rhyolitic event from the Taupo centre is the Taupo tephra erupted at ca. 1.8 ka. It consists of several plinian fall deposits overlain by

an ignimbrite ŽFroggatt, 1981; Wilson and Walker, 1985.. The episode can be correlated in the field by its distinctive sequence of eruptive phases and its stratigraphic position at the top of the Holocene Taupo sequence. The flow unit was energetically emplaced and covers much of the central North Island. The most recent rhyolitic eruption from the TVZ is the Kaharoa tephra erupted at ca. 665 years

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BP ŽLowe et al., 1998.. This eruption was associated with the extrusion of domes in the Okataina centre and the emplacement of plinian fall, surge, ignimbrite and block and ash flow emplacement close to source. The plinian deposit was dispersed on a NNW–SSE trend into northern North Island, unlike most late Quaternary fall deposits that were dispersed by westerly winds to the east ŽFig. 2.. The Kaharoa tephra is important to archeological studies as it occurs close to the first evidence of human settlement in New Zealand in late Holocene sequences ŽNewnham et al., 1998.. The preservation of the unconsolidated tephra deposits erupted during the last 64 ka reflect their youth and the few caldera-forming eruptions that have covered and obliterated older deposits. However, even in this time interval the record may not be complete. For example, few tephra beds are recorded from Taupo centre during glacial periods such as the Last Glacial Maximum when deposits are dominated by loess and contain erosion surfaces. In contrast, numerous events are preserved during the Holocene, a period of continuous paleosol formation in central North Island Že.g., Froggatt and Lowe, 1990; Wilson, 1993.. Although lake cores have failed to identify additional pre-Holocene Taupo eruptions Že.g., Lowe, 1988a., this could be due in part to the distal location of the lakes, as many known events of limited dispersal are commonly not recorded in such settings. 11.2. Pre-64 ka quaternary The pre-0.06 Ma tephra record of the TVZ is significantly less complete. In particular, there is poor age control and sequences are discontinuous for the interval 0.06–0.2 Ma. In northern TVZ, Manning Ž1996. reported numerous rhyolitic tephra beds in this interval, however there is a complete lack of numeric age control. Twenty-one different tephra beds in the interval 0.3–1.9 Ma have been dated by the ITPFT method, and for many their paleomagnetic polarity is also known ŽFig. 19.. In addition, proximal ignimbrites in the TVZ have been constrained by 40Ar– 39Ar ŽHoughton et al., 1995. and FT ŽBlack et al., 1996. methods. In the interval 1.6–0.06 Ma, at least 13 major ignimbrite-forming eruptions have occurred within the TVZ ŽHoughton et al., 1995.. Many of these events were associated with caldera

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formation, which have sequentially obliterated earlier eruptive centres and their deposits. In addition, much of the eastern side of the TVZ is now deeply buried by the youngest ignimbrites, while the western side which is upwind of the most likely fallout zones has been uplifted and eroded. As a result the pyroclastic preservation within the TVZ is biased toward welded ignimbrites. At least 54 different pyroclastic events are recorded in distal sedimentary basins of eastern and southern North Island during the period ca. 1.8–0.5 Ma ŽShane et al., 1996., in comparison to 13 large ignimbrite eruptions in the TVZ ŽHoughton et al., 1995.. The distal basins have collected fallout, fluvially transported volcaniclastic debris and fartraveled pyroclastic flows. These unconsolidated and often thin deposits have a very low preservation potential in the TVZ, and as a result there are few marker horizons linking the distal and the proximal settings. Although the tephra record is not complete, there are several early–middle Pleistocene tephra beds for which stratigraphic and chronological control has been well established. The Rangitawa Žs Mt Curl. tephra is a distinctive coarse grained fall deposit found throughout the North Island, northern South Island and in deep-sea cores in the Pacific Ocean and Tasman Sea ŽFroggatt et al., 1986.. Numerous attempts using various methods have been employed to constrain its age, and there had been much debate over whether the tephra represented two separate events Žsee Pillans et al., 1996.. Recent geochronology provides evidence for only one event and it has been dated at 0.345 " 0.012 Ma from astronomical tuning of a deep-sea oxygen isotope record ŽPillans et al., 1996.. This age is in agreement with recent ITPFT data ŽAlloway et al., 1993. and zircon FT data ŽKohn et al., 1992.. The tephra occurs in loess L10 that is correlated to oxygen isotope stage 10 in the Wanganui basin ŽPillans, 1991., and is found with cold climate paleoflora ŽKohn et al., 1992.. Recently, Carter and Naish Ž1998. have suggested that the base of the New Zealand Haweran Stage be tied to the Rangitawa tephra. Rangitawa tephra has been associated with the voluminous Whakamarugroup Ignimbrites erupted from Whakamaru caldera in the TVZ on the basis of age, geochemistry and mineralogy ŽFroggatt et al., 1986; Pringle et al., 1992..

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Fig. 19. Summary of pre-64 ka rhyolitic tephra beds in sedimentary basins outside of the TVZ that have published radiometric age control. Paleomagnetic polarity shown as normal ŽN. or reversed ŽR. where known. Data from Shane Ž1998b; Shane et al. Ž1995a; 1996; 1998b..

The early Pleistocene Potaka tephra is as widely dispersed on land as Rangitawa tephra. It is contained within the Jaramillo Normal Subchron Ž1.07– 0.99 Ma. and is variously dated at ca. 1 Ma ŽShane, 1994; Shane et al., 1996.. The Potaka tephra occurs in the terrestrial realm as a fall deposit in loess and

as an ignimbrite. The onland distribution of the ignimbrite that covers much of the North Island ŽWilson et al., 1995b., makes it one of the most far traveled flows known. In fluvial and estuarine depositional environments, the eruption is represented as a catastrophic flood deposit associated with rapid

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aggradation. The fall deposit has been found in deep-sea cores both east of New Zealand in the Pacific Ocean, and to the west in the Tasman Sea, and on land in the South Island. Wilson et al. Ž1995b. correlated the Potaka tephra with Unit E, a phreatomagmatic ignimbrite erupted from the Mangakino Centre. The Pakihikura tephra Ž1.64 Ma. is a widespread horizon in marine and nonmarine basins of the southern North Island ŽShane et al., 1996.. It is an important marker because it occurs a short distance above the Olduvai Subchron in reversed polarity sediments. The tephra is yet to be located in deep-sea cores and its source is unknown. Numerous tephra beds are recorded in southern North Island basins during the interval ca. 1.8–1.5 Ma ŽShane et al., 1996.. This appears to coincide with the initiation of ignimbrite-forming eruptions in the TVZ Že.g., Houghton et al., 1995.. Many of the tephra beds in this time interval have been used to provide a stratigraphic framework for marine cyclothems in the Wanganui basin, which is an exceptional record of global sea-level change Že.g., Carter and Naish, 1998.. 11.3. Miocene and Pliocene The stratigraphy of Miocene and Pliocene tephra beds have received little attention in comparison to Quaternary deposits, although pre-Quaternary tephra beds have been documented because of their position relative to magnetostratigraphic and stage boundaries, and bioevents Že.g., Vella and Collen, 1984; Shane, 1990, 1998b; Shane et al., 1995a, 1998b., or because of their distal occurrence in deep-sea cores ŽNelson et al., 1985; Barnes and Shane, 1992.. Pliocene marine tephra beds have been dated via the ITPFT method to constrain the age of coccolith and foraminifera bioevents, and have demonstrated that some bioevents are diachronous, presumably due to local tectonic recycling ŽShane et al., 1995a.. During the Miocene and Pliocene, caldera-forming rhyolite volcanism occurred in the Coromandel volcanic zone ŽAdams et al., 1994.. The distal products of this activity are now found throughout basins of the east coast region, both in the North and South islands ŽGosson, 1986.. These sedimentary basins contain thick sequences of monotonous outer shelf and bathyal mudstones or turbidite sequences with nu-

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merous tephra beds, many of which have been deposited by sediment gravity flows ŽGosson, 1986.. In one late Miocene sequence that has been examined in detail, distal rhyolitic tephra beds are preserved at an average frequency of ca. 1 per 21 ka ŽShane et al., 1998b.. This is comparable to the distal early Pleistocene tephra record of 1 per 19 ka ŽShane et al., 1996. suggesting rhyolitic volcanism was as frequently active 7–10 Ma ago as during the Quaternary period.

12. Andesite tephrostratigraphy Two andesitic centres have been active throughout much of the late Pleistocene and Holocene in North Island: Tongariro Volcanic Centre and Egmont ŽTaranaki. Volcano. Both comprise several vents andror volcanoes. Although both centres have produced widely dispersed tephra Ž200–300 km from vent., only limited progress Žcompared to rhyolite tephrochronology. has been made on fingerprinting and dating marker horizons. The andesitic volcanoes erupt more frequently than rhyolitic centres and often produce small volume tephra beds of restricted dispersal. In subaerial environments the andesitic and basaltic glasses weather rapidly, and thus proximal deposits on the flanks of the volcanoes can be difficult to geochemically fingerprint. In various sectors around the andesitic volcanoes, tephrostratigraphies based on field characteristics have been established for Egmont Volcano back to ca. 28 ka ŽAlloway et al., 1995.; and in part back to ca. 75 ka for the Tongariro centre Že.g., Topping, 1973; Donoghue et al., 1995; Cronin and Neall, 1997; Nairn et al., 1998.. These studies have made much use of the occurrence of distal rhyolitic tephra beds to constrain the andesitic stratigraphy in conjunction with 14 C dating. The resulting eruptive stratigraphies consist of packages Žcalled formations. of tephra that can represent numerous eruptive events. There are no published studies that geochemically link the voluminous lava flows with distal fall deposits. Andesitic tephrostratigraphies for the last ca. 20 ka have also been established from distal units collected by coring lake sediments and swamps Že.g., Lowe, 1988a,b; Froggatt and Rogers, 1990; Eden and Froggatt, 1996; Eden et al., 1993..

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13. Sea-rafted pumice horizons Sea-rafted pumice occurs as horizons and as scattered clasts in Holocene coastal deposits, mostly on eastern beaches of the North Island. In addition to local reworked deposits from the TVZ, some pumice is exotic to New Zealand. For example, a 1962 eruption in the South Sandwich Islands in the southern Atlantic Ocean produced pumice that was carried by the west-flowing circum-Antarctic current and deposited on southern and western coasts of the South Island ŽCoombs and Landis, 1966.. Attempts have been made to use sea-rafted pumice as stratigraphic markers ŽMcFadgen, 1985; Wellman, 1962., but identifying primary depositional horizons is difficult due the continuous recycling on the foreshore ŽPullar et al., 1977.. Loisels Pumice has been used as a late Holocene stratigraphic marker that occurs above sea-rafted pumice of the 1.8 ka Taupo eruption. Together, the pumice horizons bracket the earliest evidence of human occupation in some coastal regions ŽMcFadgen, 1985.. Froggatt and Lowe Ž1990. summarised 14 C ages for Loisels Pumice and found two clusters: one at 610 " 20 years BP and the other at 1250 " 40 years BP. Osborne et al. Ž1991. suggested that the older ages reflected the first arrival of the pumice in New Zealand, while McFadgen Ž1994. argued that shells from which the older ages were determined had an in-built age reflecting prolonged recycling on the foreshore. Geochemical analyses of the glasses and minerals in the pumices from sites throughout the geographic range of Loisels Pumice reveal considerable inter-site and intra-site variation ŽShane et al., 1998a.. The compositional diversity, especially differences between sites, is consistent with multiple sources and thus presumably multiple emplacement events. Therefore, the Loisels Pumice may not be an isochronous horizon. Caution must be exercised when using reworked pyroclastic material in high energy depositional environments.

14. Summary Tephra beds can be identified and correlated on the basis of their field character, stratigraphic se-

quence and with the use of isopach maps. However, lithologies can change rapidly over short distances and farther from source Ž) 50–100 km. tephra beds typically lack characteristic physical features. For tephra beds that are thin, occur in stratigraphic isolation, or are distal to source, laboratory techniques in addition to stratigraphy must be employed. The first step is the identification of ferromagnesian mineralogy. The presence but not absence of characteristic minerals is an important identification criteria for tephra of the last 64 ka in New Zealand. For example, cummingtonite occurs in only three tephra beds that erupted from the Haroharo vent of Okataina, and biotite occurs in a limited number of units from Okataina and scarcely from Taupo centre Žpre-22 ka.. The presence of sodic phases such as aegirine, riebeckite, ferrohedenbergite and aenigmatite are indicative of peralkaline ŽMayor Island. eruptions. The chemical composition of ferromagnesian minerals is generally not characteristic of individual eruptions. The glass composition of post-64 ka tephra beds in New Zealand allows the volcanic source ŽOkataina, Taupo, Mayor Island, Tongariro, Egmont. to be identified. In some cases the time interval can also be determined. For example, all post-22 ka tephra from Taupo volcano have lower SiO 2 contents than pre-22 ka units. There is no fundamental difference in the composition of tephra from the pre-Quaternary Coromandel volcanic zone and the Quaternary TVZ. Fingerprinting can be achieved by major oxides from EMA, or a variety of trace elements from XRF and INAA. The compositional homogeneity must be checked first by grain-specific techniques, and deposits that have suffered hydrothermal or vapourphase alteration Že.g., welded ignimbrites. are not suitable for geochemical fingerprinting. Not all tephra beds are unique in glass chemistry, and those erupted over short intervals Ž- 20 ka. from the same volcano are often similar with respect to major and trace elements. Widely dispersed tephra beds erupted over longer time periods Ž20 ka–1 Ma. can be readily identified by glass chemistry using simple binary plots Že.g., FeO vs. CaO; SiO 2 vs. K 2 O.. The composition of Fe–Ti oxides also allows the source volcano to be identified ŽTaupo vs. Okataina., as well as many individual eruptive events, by simple binary ŽFe 2 O 3 vs. TiO 2 . and ternary ŽAl–Mn– Mg. plots. Such data allow some eruptions to be

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distinguished that are indistinguishable by glass chemistry. Temperature ŽT . and oxygen fugacity Ž f O 2 . estimated from Fe–Ti oxide pairs, allows eruptive source and individual eruptions to be identified ŽT vs. f O 2 plots.. f O 2 reflects the ferromagnesian mineralogy of the tephra as well as the source area. For a given T, Okataina-sourced tephra always have higher f O 2 values than Taupo-sourced tephra. Welded ignimbrites commonly record a stable TRM and can be correlated by their record of the geomagnetic field direction. In long sedimentary sequences representing 0.5 Ma or more, radiometric methods andror paleomagnetism may need to be employed for positive identification of tephra beds, in addition to stratigraphic and geochemical criteria. For calc–alkaline rhyolitic tephra Ž) 200 ka. that are fine grained and mineral poor, the ITPFT method on glass provides a means to obtain accurate and precise ages. High resolution ages Ž"1–2%. can be obtained by 40Ar– 39Ar when K-rich phases such as sanidine are present. SCLF is sometimes required to avoid xenoliths and partially degassed relict crystals. The current knowledge of the rhyolitic tephra record of New Zealand is neither complete nor uniform in space and time. For the last 64 ka, most eruptions are well documented and source vents are known. The tephra beds provide a stratigraphy for much of the North Island. However, even in this period tephra beds from Taupo centre are recorded more frequently in the Holocene soil forming paleoenvironment than the late Pleistocene where unconformities and loess dominated the sequences, suggesting the record may not be complete. The pre-64 ka record is partially obliterated by caldera-forming eruptions in the proximal realm and tectonic processes in the distal realm, and thus source volcanoes cannot be readily identified. However, some tephra beds are particularly widespread Že.g., 0.3 Ma Rangitawa and 1.0 Ma Potaka., and are well constrained by ITPFT, 40Ar– 39Ar and paleomagnetic methods. The widespread horizons provide a stratigraphic framework for basins of the southern North Island. The Pliocene and Miocene tephra record has received little attention. The tephra beds occur in distal deep-marine basins in the east coast region, tectonically separated from the volcanic source area. Many of the tephra beds have been emplaced by sediment

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gravity flow and the distributions are highly localised.

15. Future directions It has become increasingly important in geochemical and geochronological studies to use grain-specific techniques to avoid xenolithic and detrital contaminants in tephra beds. Currently most studies rely on bulk separates for trace element analysis. However, the development of laser ablation ICP-MS will allow the routine analysis of individual glass shards for a wide range of elements. Although ITPFT ages on tephra have greatly revised the early and middle Quaternary chronology of New Zealand, a greater precision is required to test chronologies erected by cyclostratigraphy Že.g., Carter and Naish, 1998.. Improved precision may be possible through the use of 40Ar– 39Ar dating. This will be hindered by the lack of high-K feldspars, however other phases such as biotite may provide reliable ages. The ITPFT ages will provide a framework to test new age data. Orbital tuning of oxygen isotope records and other cyclic signals in deep-sea cores that contain tephra provides another means of potentially improving age precision. Numerous tephra beds spanning the last 12 Ma have recently been collected in high resolution deep-sea coring offshore of the North Island ŽOcean Drilling Program leg 181.. This provides great potential for constraining widespread tephra horizons in the New Zealand region. Very little has been published on the age, composition and dispersal of basaltic tephra from the Auckland and Kaikohe Volcanic Fields. These tephra beds provide an opportunity to construct a eruptive history for the fields, and potentially provide marker beds for other studies such as archaeology ŽNewnham et al., 1998..

Acknowledgements Paul Froggatt has provided support and has been a source of ideas through the early stages of the authors career. He also kindly provided unpublished

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geochemical data. David Lowe is thanked for comments on an early version of the manuscript. Brad Pillans and Andrei Sarna-Wojcicki reviewed the final version.

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P. Shaner Earth-Science ReÕiews 49 (2000) 223–259 Phil Shane completed his PhD at Victoria University of Wellington in 1993 and then took-up a post-doctoral fellowship at the University of Toronto. Since 1996 he has worked as a research associate and then lecturer at the University of Auckland. Research interests include Ž1. the use of isothermal plateau fission-track dating of volcanic glass and paleomagnetism to construct the chronology of pyroclastic deposits, and Quaternary sedimentary sequences; Ž2. the tephrochronology and eruptive history of the Taupo Volcanic Zone; and Ž3. the geochemical characterisation and geothermometry of large rhyolitic deposits. Current research is focused on the Neogene tephra record of New Zealand from cores collected by the Ocean Drilling Program, and the tephrochronology and volcanology of the Auckland and Okataina volcanic centres.

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