The stratigraphic relationships and distribution of 36 named late Quaternary (≤c. 50 000 yr B P.) silicic tephra formations, erupted from 4 volcanic centres—Okataina, Taupo, Maroa, and Tuhua (Mayor Island)—are presented. The stratigraphy and status of several other named late Quaternary tephras are also discussed. This compilation brings together all the data, currently scattered through many publications, to make tephrostratigraphy more accessible and more easily used. The nomenclature of tephra formations is discussed and some rationalisations are suggested. The term “tephrology” is suggested as an appropriate title for the field of tephra studies. The deletion of grain-size (ash, lapi1li), shape (breccia), and lithologic (pumice) terms from all formation names is recommended, as is standardisation on a “Tephra Formation” formal Several tephra layers not previously formally named, or without designated type sections, are defined. The dominant ferromagnesian mineral assemblage of each tephra for...

https://www.tandfonline.com/doi/pdf/10.1080/00288306.1990.10427576

A review of late Quaternary silicic and some other tephra formations from New Zealand: Their stratigraphy, nomenclature, distribution, volume, and age

A new thermodynamic formulation of the Fe−Ti oxide geothermometer/oxygen barometer is developed. The method is based upon recently calibrated models for spinel solid solutions in the quinary system (Fe2+, Mg)(Al,Fe3+,Cr)2O4−(Fe2+, Mg)2TiO4 by Sack and Ghiorso, and rhombohedral oxides in the quaternary system (Fe2+,Mg,Mn)TiO3−Fe2O3 (this paper). The formulation is internally consistent with thermodynamic models for (Fe2+,Mg)-olivine and -orthopyroxene solid solutions and end-member thermodynamic properties tabulated by Berman. The constituent expressions account for compositional and temperature dependent cation ordering and reproduce miscibility gap features in all of the component binaries. The calibration does not account for the excess Gibbs energy resulting from compositional and temperature dependent magnetic ordering in either phase. This limits application of the method to assemblages that equilibrated at temperatures above 600° C. Practical implementation of the proposed geothermometer/oxygen barometer requires minimal use of projection algorthms in accommodating compositions of naturally occurring phases. The new formulation is applied to the estimation of temperature and oxygen fugacity in a wide variety of intermediate to silicic volcanic rocks. In combination with previous work on olivine and orthopyroxene thermodynamics, equilibration pressures are computed for a subset of these volcanics that contain the assemblage quartz, oxides and either ferromagnesian silicate. The calculated log10fO2-T relations are reflected in coexisting ferromagnesian mineral assemblages. Volcanics with the lowest relative oxygen fugacity (Δlog10fO2) are characterized by the assemblage olivine-quartz, those with slightly higher Δ log10fO2 s, by the assemblage orthopyroxene-quartz. The sequence proceeds with the necessary phases biotite-feldspar, then hornblende-quartz-clinopyroxene, and finally at the highest Δ log10fO2 s, sphene-quartz-clinopyroxene. Quantitative analysis of these trends, utilizing thermodynamic data for the constituent phases, establishes that, in most cases, the T-log10fO2value computed from the oxides is consistent with the compositions of coexisting silicate phases, indicating that phenocryst equilibrium was achieved prior to eruption. There is, however, considerable evidence of oxide-silicate disequilibrium in samples collected from more slowly cooled domes and obsidians. In addition, T-log10fO2trends from volcanic rocks that contain biotite and orthopyroxene are interpreted to imply a condition of Fe2+−Mg exchange disequilibrium between orthopyroxene and coexisting ferromagnesian silicates and melt. It is suspected that many biotite-feldspar-quartz-orthopyroxene bearing low temperature volcanic rocks inherit orthopyroxene xenocrysts which crystallized earlier in the cooling history of the magma body.

Fe-Ti oxide geothermometry: thermodynamic formulation and the estimation of intensive variables in silicic magmas

This paper attempts to review those salic eruptives in which Si02 >60%, but specifically excluding peralkaline and silica-undersaturated types The approach is based on a compilation of major and trace elements, plus phenocryst occurrence and phenocryst modal data; some 2,890 sets of rock data are included The data are tabulated into five chemical groupings (60-63, 63-66, 66-69, 69-72, >73% SiO2); further breakdown of data is based on geographic and/or tectonic subregions, and further chemical subdivision into low-K, calcalkaline, and high-K series from each subregion In this paper, the general terms siliceous andesite, dacite, and rhyolite are applied to volcanic rocks with SiO2% of 60-63, 63-69, and >69%, respectively, except lavas specified as trachytes, for which a separate set of data are compiled The majority of salic volcanic magmas have been erupted within orogenic zones; ie, the circum-Pacific regions, Indonesia, the Mediterranean arcs, and to a lesser extent, the Caribbean and Scotia arcs An additional group of silicic magmas, dominated by rhyolites, are those of the bimodal mafic-silicic magma associations; examples are drawn from Yellowstone, Mono, Medicine Lake, and Salton Sea in the western USA: Iceland; western Scotland; and southern Queensland (Australia) Further comparative data are provided from the Hawaiian, Galapagos, and Canary Islands, and the unusual high soda-low potash lavas of Deception and Fedarb Islands Cluster analyses of averaged data for the various subregions suggest close correlations between the orogenic silicic magmas of the western USA (excluding Cascades), western South America, and Mediterranean regions Further clustering is found between the eruptives of the Cascades-Aleutians, Middle Americas, and Indonesian subregions The clustering tends to separate these orogenic magmas from the oceanic rhyolites and dacites, and the rhyolites of the bimodal associations The trachytes exhibit no clear correlations with any of these groupings Mineralogically, the calc-alkaline orogenic salic volcanics are characterized by phenocryst assemblages comprising plagioclase, quartz, augite, hypersthene, hornblende, cummingtonite, Fe-Ti oxides, biotite, and rarely sanidine The low-K eruptives lack biotite and sanidine, while the high-K eruptives contain much more abundant sanidine, quartz and biotite, and commonly also sphene and allanite Detailed phenocryst assemblages are tabulated in the text Fe-Ti oxide temperatures range from 640-780° in high-K rhyolites of western USA (excluding those of the bimodal associations); 725-900° in calc-alkaline rhyolites from the SW Pacific; and 830-1120° for orogenic dacites from various circum-Pacific regions These data also suggest that the orogenic silicic magmas from the western USA have equilibrated on a slightly higher fO2 buffer (between NNO and HM buffers) than the orogenic magmas from other regions It is also shown that the averaged compositions of the magmas from western USA, western South America, and Mediterranean regions, and indeed the high-K eruptives from other regions, trend towards and terminate close to the lower pressure “ternary minima” in the quartz-feldspar system, and the 2-feldspar cotectic surface in the feldspar system This is not found for the calc-alkaline and low-K series Rhyolites of bimodal associations often exhibit distinctive phenocryst assemblages and compositions, most notably the progressive development of strongly Fe-enriched pyroxenes, olivines, and where present, amphiboles and biotites Fe-Ti oxide data indicate relatively high equilibration temperatures (800-985°), and equilibration between the QFM and NNO fO2 buffers (ie, lower than most orogenic salic magmas) These rhyolites also frequently exhibit very fractionated trace element abundance patterns

A Review of the Mineralogy and Chemistry of Tertiary-Recent Dacitic, Latitic, Rhyolitic, and Related Salic Volcanic Rocks

The Taupo volcanic zone (TVZ) has been active since 2 Ma and has erupted >104 km3 of dominantly rhyolitic magma during the last 1 m.y. Most of the volcanism is concentrated in a 125×60 km area forming the central TVZ and is expressed largely as six major caldera volcanoes, Rotorua, Okataina, Kapenga, Mangakino, Maroa, and Taupo, marked by localized collapse of the underlying basement and clustering of known or inferred vent sites. These centers have activity spans from 150 to 600 ka and have each erupted at least 300 to 1000 km3 of magma. All centers except Rotorua are known or inferred to have had complex histories of multiple caldera collapse, which have occurred alongside general basement collapse within the TVZ accompanying regional extension. Deep-seated NE trending basement lineations and/or faults have influenced vent sites at Okataina, Maroa, and Taupo. Welded ignimbrites are prominent in the pre-140 ka record; their absence since then is attributed to the effects of surface water on eruption styles rather than to a change in eruptive behavior. Volcanism from the centers has been overwhelmingly rhyolitic (>97% SiO2 69–77 wt%) with minor high-A1 basalt and dacite and traces of andesite, mostly as lithic fragments in ignimbrites from Okataina and Mangakino. Although insignificant in volume, the basalt is important as a low-Si end-member in mixing relationships with the rhyolite (at one extreme generating the dacites) and occasionally as a trigger for the rhyolitic eruptions. The current average rhyolite magma eruption rate from the central TVZ is ∼0.27 m3 s−1, equally divided between Okataina and Taupo, a figure close to the long-term average for the last 1.1 Ma. However, geothermal heat flow data imply that a further 1.4–1.8 m3 s−1 of magma may be intruded within the crust. The ratio of inferred intruded material to erupted material is higher at centers where lava extrusions are volumetrically significant (Okataina, Maroa), and this is correlated with lower phenocryst equilibration temperatures in the eruptives. Evidence for resurgent doming and long-term (>105 years) magma cycles documented at similarly sized rhyolitic calderas in the western United States is absent from the TVZ centers; this is attributed to the young faulted crust of the region, preventing the formation of sufficiently large high-level magma chambers. In overall terms, the central TVZ is comparable in size and longevity to the Yellowstone system, but its individual eruptions have very much shorter recurrence intervals and smaller volumes.

Caldera Volcanoes of the Taupo Volcanic Zone, New Zealand

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

https://www.tandfonline.com/doi/pdf/10.1080/00288306.1979.10424173

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

This account is based on the systematic examination of the phenocrystic mineralogy of the rhyolitic lavas (referred to as rhyolites) associated with the four volcanic centres of the Taupo Volcanic Zone, namely the Mokai Ring Structure, Lake Taupo Volcanic Centre, Rotorua Caldera, and Okataina Volcanic Centre. From the results of over 200 modal analyses, it has been possible to subdivide the Mokai rhyolites into seven groups of varying extent, and the Okataina rhyolites into five complexes. The rhyolites of each group or complex are interpreted to be the eruptive products of the same high-level magma reservoir. Evidence suggests that the foci of eruptions in the Mokai structure have migrated eastwards with time. A close correlation exists between the quartz and plagioclase phenocrysts in the rhyolites from each of the groups and complexes of the Mokai and Okataina centres, and also for the Lake Taupo and Rotorua rhyolites. This is interpreted to indicate that the rhyolites from each group, etc., crystallised under similar water vapour pressure conditions, from magmas of fairly restricted compositions. Differences exist among groups or complexes, in the proportions of plagioclase crystallising before being joined by quartz, this being mainly the result of small differences in magma compositions between these groups or complexes. Significant correlations also exist between the average diameter and modal abundance of both the quartz and plagioclase phenocrysts, and provide additional evidence against a xenocrystic origin of these phenocrysts. It is believed that the rhyolites of the four volcanic centres have similar crystallisation histories before their eruption, with the exception of the Haroharo rhyolites (and their associated pumice deposits) within the Okataina centre. These Haroharo eruptives differ in a number of respects from the other rhyolites of the region, the most interesting difference being the occurrence of phenocrystic cummingtonite (in place of the normal calcic hornblende) in most of the pumice deposits and some of the lavas. It is concluded that the crystallisation of quartz and feldspar in the rhyolites of the Taupo Volcanic Zone, considered in relation to the rhyolite chemistry, can be broadly explained in terms of the Ab-Or-Q-HO system (Tuttle and Bowen, 1958).

https://www.tandfonline.com/doi/pdf/10.1080/00288306.1968.10423663

A. Ewart

Papers

The Petrography of the Central North Island Rhyolitic Lavas