The Banks Islands represent the most northern group of volcanic islands in the linear Central Chain of Pliocene - Recent age in the New Hebrides. A study o f the petrological variation in these northern islands provides a basis for the development of a petrogenetic-tectonic model which is applicable to most, if not all, the Central Chain islands. Integration of petrographic, mineralogical and geochemical data on each island in the Banks Group has revealed a systematic petrological variation across the islands, represented chemically by a decrease in K20 (and related incompatible elements) away from the New Hebrides trench. The correlation with depth to the Benioff zone is therefore the reverse of that typically found in island arcs. REE evidence and source modelling based on this data indicates that this variation represents a progressive depletion in LREE/HREE in the upper mantle, laterally away from the trench. The variation also represents a progressive increase in partial melting of the source, in the same direction. However partial melting differences alone could not produce the observed variation. The variation in source composition and degree of partial melting are blended in to a model in which the compositional variation is attributed to an earlier west-facing Miocene subduction event, and the variation in degree of partial melting is a function of the location of the islands with respect to the postulated heat source: the active back-arc basin. The enrichment of the upper mantle in the Miocene is ascribed to the ascent of hydrous fluids enriched in incompatible elements, from the west-dipping Benioff zone, and subsequent reaction with the overlying upper mantle. The model thus envisages approximately contemporaneous development of the Central Chain volcanoes and the back-arc basin in the late Pliocene, with partial melting of hydrous, laterally zoned upper mantle resulting from the convective thermal regime associated with the rifting apart of the back-arc basin. The role of the present subduction regime in magma generation is considered very limited. The mineralogical and geochemical data show that all compositions in the range from basaltic andesite to dacite (ie. > 53wt.% SiO2) in the Banks Islands can be derived from basalts by fractional crystallization of the observed phenocryst phases (including magnetite in the Gaua suite), after allowances have been made for inherited differences between some melt fractions. Together with the paucity of andesites in the Central Chain, this suggests that the primary melts of the Pliocene - Recent volcanism are basaltic. The generation of calc-alkaline magmas in the early Miocene are also believed to have involved basaltic primary melts as both the Miocene Eastern and Western belts are dominantly basaltic.

Blueschists in the South Sepik region of Papua New Guinea are one of several poorly known occurrences along the northern margin of New Guinea's Central Orogenic Belt. It is believed that they formed in a northward-dipping subduction zone, beneath a Palaeogene arc system now accreted along the north coast of New Guinea, and that they now lie within a Tertiary continent-arc collision zone. The South Sepik blueschists, or Tau Blueschist, are situated near the centre of New Guinea, on the northern fall of the island's axial mountains, and are mostly contained in an allochthonous east-west lens (55 x 8 km) known as the Tau body. There are also some much smaller allochthons to the east of the Tau body. The blueschists occur within the late-Mesozoic to Eocene Salumei Formation, which is believed to be a tectonic mixture of mostly politic sediments, derived from the continent to the south, and ophiolite fragments and other volcanogenic rocks, related to the arc to the north. Near the Tau body, the Salumei Formation is metamorphosed from prehnite-pumpellyite to low-greenschist grade. Stratigraphic constraints and isotopic dating indicate an Oligocene to early Miocene metamorphism of the Tau Blueschist, Salumei Formation, and other metamorphic rocks in the region. The blueschists are mostly massive to well-foliated mafic schists, containing abundant blue amphibole, but there are also some intimately associated pelitic-calcareous-graphitic metasediments. Within the Tau body the metamorphic grade increases towards the north, from lawsonite rocks to high-grade epidote blueschists. The lowest grade blueschists are pumpellyitic lawsonitic metabasites, known only from river float. In addition, there are zones and isolated occurrences of coarsely crystalline mafic tectonic blocks, within and just north of the Tau body. These include high-grade blueschist, eclogite and amphibolite. There is a complete spectrum from blueschist to eclogite. Whole-rock chemistry indicates, that all basic rocks are probably metamorphosed mafic ophiolite. The mineralogy and mineral chemistry is similar to other circum-Pacific blueschists, particularly those in New Caledonia. The blue amphibole is glaucophane or crossite, but ferroglaucophane occurs in the lawsonite zone. It is sometimes accompanied by Ca-amphibole, ranging from actinolite, in the lawsonite zone, to barroisite, in the epidote zone. Some metabasites and metasediments contain grossularitic almandine garnet, spessartine rich at low grade to pyropic at high grade. Sodic pyroxene is not particularly common in the blueschists, and is generally an acmitic variety, although omphacite occurs in eclogites. Paragonite is common in the epidote zone, often associated with phengite, while chlorite and albite are present at all grades, generally in small amounts. Calcite, dolomite and chloritoid occur in metasediments of the epidote zone. The unique assemblage lawsonite-glaucophane-chloritoid was found. Apart from zoning in minerals, and minor retrograde alteration, most rocks appear to be equilibrium assemblages, and obey the phase rule. The conditions of metamorphism are estimated from available experimental studies, and calculations based on mineral thermodynamic data. Equilibria involving lawsonite, epidote, grossular, albite, paragonite and quartz suggest 395°C and 7.5 kbar at the disappearance of lawsonite. Metasediments in the middle of the epidote zone yield a minimum of 420°C from the calcite-dolomite geothermometer, and 444° ± 20°C from coexistence of chloritoid, garnet, albite paragonite and quartz. A pressure of 10-11 kbar at 500°C is calculated from garnet, epidote, albite, paragonite and quartz in the highest grade blueschists. Corrections to the muscovite-paragonite solvus for pressure and non-binary components give plausible temperatures and pressures for coexisting white micas. Mineral stabilities, and equilibria between paragonite, jadeite, albite, grossular and zoisite, indicate a maximum of 550°C and 15 kbar for eclogites, and that current versions of the garnet-clinopyroxene thermometer are not giving accurate results in eclogites with high jadeite or acmite in the pyroxene. From buffer equilibria between calcite and Ca-Al silicates in metasediments, the non-ideal mole fraction of CO2 is estimated to be about 0.02 in the epidote zone, and 0.002 in the lawsonite zone. Graphite, pyrite and pyrrhotite in metasediments allow calculation of fluid composition in the system C-H-O-S, and show that very minor CH and H2S were present. Water activities may have been high in some eclogites. The metamorphic conditions require that both blueschists and eclogites were formed in a subduction system, and support the continent-arc collision hypothesis. However, some form of active and rapid transport is needed to bring these rocks back to shallow levels, and the term "retrojection" is coined.

The Mahoenui Group is a body of Oligocene and early Miocene marine clastic sedimentary rock in which two formations are recognized. The first, the Taumatamaire Formation (Happy, 1971) consists of up to 1000m of blue grey calcareous mudstone together with two minor limestones, the Awakino Limestone Member (Hay, 1967) and the Black Creek Limestone Member (new name). The second Formation, the Taumarunui Formation (Nelson & Hume, 1977) is made up of 1000m of flysch. Two facies types are seen in the flysch, the proximal and distal flysch facies. 350 species of Foraminifera are recorded in 167 samples from 47 sections. Their systematics are discussed and many are illustrated using scanning electron photomicrographs. 11 new species are recognised in the genera, Almaena, Anomalina, Epistominella, Gavelinopsis, Globocassidulina, Guttulina, Lamarckina, Lenticulina, Melonis and Verneuilina. One new subspecies of Bolivina reticulata is also recognised. These are not formally named here but will be described in papers to be published later. An appraisal of numerical techniques in taxonomy is made while considering the Globigerina woodi "group" from the Mahoenui. This supports the validity of 5 species of planktonic Foraminifera from New Zealand and illustrates the advantages and disadvantages of numerical classification. The paleoecology and paleobathymetry of the samples is investigated using both conventional and numerical methods. These two approaches are compared, contrasted and then integrated to form a paleogeographic reconstruction.

Whole document restricted, see Access Instructions file below for details of how to access the print copy. / Molybdenite mineralisation in North-west Nelson is associated with small granite intrusives. Molybdenite occurs within the Separation Point Granite Batholith (Cretaceous) at Mt Evans (Canaan). Other occurrences are within lower Palaeozoic metasediments near the eastern margin of the Karamea Granite Batholith (late Palaeozoic-ear1y Mesozoic) - at Eliot Creek, Roaring Lion River, and Karamea Bend; or within the Karamea Batholith, at Taipo Spur and Mt Radiant. The molybdenite mineralisation and its genetically associated intrusives have been dated (K - Ar) as Cretaceous (~110 m.y.) at Eliot Creek and Taipo Spur. The Karamea Bend occurrence is of the same age. With the exception of Taipo Spur, where molybdenite is associated with pyrite-magnetite, a basemetal-bismuth mineral assemblage is associated with the molybdenite. At Mt Radiant and Mt Evans bismuth minerals (emplectite and aikinite respectively) occur in mutual association with molybdenite in a single paragenesis. At Karamea Bend and Eliot Creek. a later paragenesis of base metal sulphides and bismuth minerals (aikinite, bismuth-bearing acanthite, and bismuth-zinc fahlore at Karamea Bend; schapbachite, joseite-type sulphosalts and native bismuth at Eliot Creek), is superimposed on, or peripheral to, the earlier molybdenite mineralisation. Mapping of hydrothermal alteration at Eliot Creek, Karamea Bend and Taipo Spur shows the presence at all three of an inner potassic potash feldspar zone (with hydrothermal biotite at Taipo and Karamea Bend). At Karamea Bend and Eliot Creek this is surrounded by a phyllic-type albite-muscovite zone; whereas at Taipo the potassic zone is margined by an epidote-albite sericite zone of propylitic type, in which the molybdenite mineralisation is concentrated. In contrast, it is preferentially associated with the potassic zone in the other two cases. Studies of fluid inclusions in sulphide-associated quartz in the various deposits indicate that hydrothermal alteration and formation of disseminated molybdenite mineralisation occurred at temperatures ranging from 330° to 390°C, occasionally up to ~450°. Molybdenite veins. at Mt Radiant and Mt Evans were, similarly, formed at c. 370°C, whereas vein molybdenite at Eliot Creek was deposited at somewhat lower temperatures, 270° to 330°C. Base metal sulphides and bismuth sulphosalts at Eliot Creek were formed at much lower temperatures, ~200°C. Fluid inclusions further show that the hydrothermal fluids had low salinities (<26 wt.% NaCl eq.) and high CO2 activities (by ubiquitous presence of liquid CO2), for the molybdenite and base-metal-bismuth depositional stages. During molybdenite mineralisation in the higher range of temperatures, fluids were at or above the critical point, as indicated by dry-vapour inclusions. Alteration mineralogy and sulphide assemblages indicate that the hydrothermal solutions depositing molybdenite were moderately to weakly acidic, and that deposition of base-metalbismuth mineralisation was related to pH changes resulting from changes in CO2 activity consequent on fracturing and pressure release. The granitic intrusives which have given rise to the molybdenite mineralisation are characterised by several unusual chemical features: as regards major elements, the unaltered intrusives are adamellites with very high Na2O/K2O ratios, having a chemical composition comparable to trondhjemites. Trace element compositions are also unusual, particularly in the presence of extremely high strontium and rather high barium. In the unmineralised parts of the adamellites, molybdenum is at specialised levels at the three localities investigated (Eliot Creek, Karamea Bend, and Taipo Spur) While Eliot Creek also shows tin specialisation. Consideration of the distribution, situation and size of the molybdenum-bearing intrusives, and of their peculiar chemistry, collectively indicate a probable origin by differentiation from basaltic lithospheric material in the deeper parts of a subduction-zone environment.

The general properties of micropulsations are reviewed, together with some theories of their origin. A particular study is made of the hydromagnetic wave theory and of the probability of these waves being produced by gyrating charged particles in the magnetosphere. The conditions are discussed under which gyration-induced hydromagnetic waves have frequencies within the micropulsation range. Assuming the production of Alfvén waves by some process in the magnetosphere, a layer model is developed to investigate the transmission of these waves down along the Earth’s magnetic field lines to the ionosphere. One feature of this model is a realistic ionospheric termination, in which a phase shift and amplitude reduction may be introduced into back reflected waves. Calculations using the model indicate a definite harmonic structure associated with micropulsations and only a small variation of frequency with geomagnetic latitude; they also provide a good explanation of the frequency structure and diurnal variation of ‘pearls’.

Some 2500 specimens of Monotis from more than 500 localities within New Zealand were studied by statistical and traditional descriptive methods. From this analysis 20 taxa are recognised as constituting the New Zealand Monotis fauna, with the future possible addition of two further taxa. M. richmondiana Zittel, claimed by some overseas workers as a junior synonym of M. ochotica (Keyserling), is established as validly separable and is seen to include Trechmann's (1918) Pseudomonotis ochotica var. acutecostata as a subspecies. The presence of the New Caledonian M. routhieri and M. ochotica gigantea (Avias, 1953) is confirmed and the latter is transferred to M. subcircularis Gabb. Monotis salinaria var. hemispherica and var. intermedia (of Trechmann, 1918) are reinstated as valid taxa and given full specific rank. M. calvata Marwick (1953) is retained as a full species. In addition, the following species and subspecies are proposed: awakinoensis, kiritehereensis, maniapotoi, marwicki, murihikuensis, murihikuensis taringatura, pinensis aotearoa, rauparaha, rauparaha aries, rauparaha mokaui, subcircularis discordans, wairakae, and warepana. Only Pseudomonotis richmondiana var. truncata Frech (1908) amongst names previously proposed for New Zealand Monotis is rejected as a nomen dubium; it is probably synonymous with M. richmondiana acutecostata (Trechmann). The species-group concept of Westermann (1973b) is formalised by the proposal of five subgeneric divisions to accommodate the 55 definite and up to 5 possible taxa included in the genus. The subgenera Monotis, Entomonotis Marwick (type: M. richmondiana), Eomonotis n.subgen. (type: Pseudomonotis scutiformis var. typica Kiparisova), Inflatomonotis n.subgen. (type: Monotis salinaria var. hemispherica Trechmann), and Maorimonotis n.subgen. (type: Monotis routhieri) are differentiated on the basis of adult size, prominence and sculpture of the posterior auricle, the degree to which it is separated from the disc, shell inflation, radial ribbing and shell thickness. Eomonotis is the oldest subgenus and the stem-stock for the others; it probably evolved from Otapiria. It is unclear from morphologic evidence whether Monotis (s.s.) and Entomonotis developed one from the other or both from Eomonotis independently. Maorimonotis is confidently believed to have sprung from an Entomonotis similar to M. pachypleura (Teller) and Inflatomonotis is most likely to have evolved from an Eomonotis with strongly inflated left valve (e.g. M. jakutica (Teller) or M. iwaiensis Ichikawa). Eomonotis and Entomonotis are geographically the most widespread, the former being absent only from the Andean area of Monotis distribution and Entomonotis not having been recorded in the western or central Tethys. Monotis (s.s.) is common only in the western Tethys. Inflatomonotis and Maorimonotis are confined to New Zealand-New Caledonia except for one record of the former in British Columbia. In many regions the stratigraphic sequence of Monotis is too inadequately known for evolutionary sequences to be deduced but some examples of chronologic and geographic subspeciation are pinpointed. The only clearly defined lineage is that within Maorimonotis from maniapotoi through awakinoensis and routhieri to calvata. The genus can be seen to have a high taxonomic diversity and rapid rate of evolution. Apart from three questionable lower Triassic and lower Jurassic species, Monotis is confined to the Norian Stage, and possibly to the Columbianus and Suessi Zones of the mid and upper Norian. Monotis is postulated as having been epibyssate throughout life and normally epiplanktonic on drifting or floating marine vegetation, with coexisting taxa occupying different microenvironments on different parts of the plants. A serious objection to this mode of life is the massive algal production necessary to provide a substrate for the tremendous numbers of individuals involved in Monotis shellbeds. Reduction of the byssal ear in Maorimonotis is seen as a significant event and coupled with the trend to an equivalved condition is interpreted as indicating change to an endobyssate benthic existence for M. routhieri and calvata; this adequately explains the restricted distribution of this subgenus. Detailed stratigraphic analysis of Monotis in Murihiku sequences necessitates rejection of the Rocky Dome section as stratotype for the Warepan Stage on the grounds that although M. richmondiana has been cited as the marker fossil for the stage, past usage has clearly equated the base of the stage with the incoming of the genus, and richmondiana is not the oldest member of the genus to be found in Murihiku sections. Earlier members are absent from the stratotype which therefore must be relocated in order to retain the stage in its accepted scope. The Kiritehere coastal section suffers the disadvantage of penecontemporaneous submarine slumping, but is proposed as stratotype for a new Marakopan Stage because it is the thickest, best exposed, clearest and best known sequence of Monotis beds and adjacent strata in New Zealand. Within this stage a sequence of five formal and three informal chronozones is defined. The lower Marakopan Mokaui and Murihikuensis Chronozones can be grouped together as an Eomonotis chronozone when faunas are too poor for better correlation, and similarly the overlying Acutecostata, Richmondiana and Calvata Chronozones can be grouped as an Entomonotis chronozone. In addition a topmost Marakopan chronozone can be recognised from the absence of Monotis and basal Otapirian indicators and the presence of a fauna which at present seems common to the two stages. Monotis is the most common fossil in the Torlesse Super-group and the lower four Chronozones, and probably also the Calvata Chronozone, are all represented. Most forms present in Murihiku rocks are also present in Torlesse, but two common Torlesse forms are unknown in Murihiku sequences and this is believed to indicate both communication and barriers between the two regions in Marakopan times. Monotis-bearing Torlesse lithologies are more varied than Murihiku and indicate different depositional regimes. Whilst Murihiku Marakopan strata are all of normal clastic lithologies and show a southwards increase in grain-size, the Torlesse picture is more complex. Firstly, an increasingly deeper and offshore situation is suggested by some deposits with Murihiku faunas: inner or middle neritic origin for Monotis-bearing clasts in a (?) late Jurassic conglomerate at Morrinsville which possibly accumulated in the outer neritic zone or slightly deeper, outer neritic for in situ strata near Aria, slightly further offshore accumulation for ? middle Jurassic mélange in the Oroua Valley and Marakopan slump deposits in the Otaki Gorge, and bathyal or even abyssal deposition for argillites with scattered Monotis in the Mt Arthur-Bealey area. The Aria Monotis beds lie west of the Waipa Fault, yet fit best into Kear's (1967, 1971) Morrinsville Facies, and indicate that the Hakarimata Anticline, Waipa Fault and the facies relations postulated by Kear date from the mid-Jurassic and that there was no feature like the modern continental edge and slope during upper Triassic times in the region. Close paleogeographic relations between these areas and the Murihiku region in Marakopan times is indicated and is not disproven by any sedimentologic evidence yet known. One lithologic association not represented in and showing distinct geographic separation from the Murihiku area is the shell limestone-submarine lava-chert association, which is that containing Monotis species endemic to the Torlesse. This is postulated as having been formed in localised offshore shoal areas (guyots) formed by submarine volcanism; even these regions had some connection with Murihiku seas.

At Matawai, Raukumara Peninsula a high angle unconformity separates Cretaceous conglomerates, sandstones and siltstones from the underlying Urewera Greywacke, at least a part of which is upper Jurassic. This unconformity is believed to be of considerable significance. It represents a considerable time gap, a major deformation phase, and the initiation of a new regional sedimentation cycle. Mapping and sedimentological studies enable the recognition of two major lithostratigraphic units within Cretaceous strata at Matawai. The Aptian - Coniacian MATAWAI GROUP (new name) includes the Koranga, Te Wera and Karekare Formations. It is overlain unconformably by the Maastrichtian - ?Landenian WHAREKOPAE GROUP (new name), consisting of the Tahora (revived name) and Rakauroa Formations. The low angle unconformity between the Matawai and Wharekopae Groups is believed to represent only a minor phase of deformation and renewed marine transgression. It is local, not regional, and does not mark a significant time gap. A further unconformity separates Koranga Formation shallow marine coarse clastic sediments from texturally more mature sediments of the overlying Te Wera and Karekare Formations. It too is interpreted as being of only minor significance; it marks only a short period of non-deposition and not a major change in sedimentation pattern. Six biostratigraphic zones, all based on short-ranging species of either Inoceramus or Aucellina have been mapped in Matawai Group strata. Diverse shallow water faunas of the Koranga, Te Wera and lower Karekare Formations are succeeded by upper Karekare assemblages containing few fossils other than Inoceramus. Koranga Formation sandstones are texturally and compositionally immature. Like thick-bedded Urewera sandstones, they are of very mixed provenance, being derived from acid plutonic - high grade metamorphic, sedimentary and volcanic sources. Thus although the oldest Matawai Group strata are stratigraphically discrete from the Urewera Greywacke, they were derived largely from similar sources. Petrographic study of sandstones and X-ray study of siltstones suggest Karekare Formation sediments were derived largely from underlying Urewera and Matawai strata. Reworking of unlithified Matawai Group sediments provided much of the detritus for the glauconitic quartzose Tahora sandstone and the siliceous Rakauroa siltstones and shales. Distribution and composition of clays and feldspars in Matawai and Wharekopae rocks indicate a proportion of the sediment was derived from a granitic source to the south or southeast. Although zeolites and authigenic epidote are common in Urewera rocks at Matawai, extensive zeolitisation is virtually restricted to the sandstones and conglomerates overlying the Urewera Greywacke - Matawai Group unconformity. Development of zeolites is not, therefore, wholly a function of depth of burial. Volcanogenic Urewera and Koranga sandstones are both suitable hosts for zeolite formation, and restriction of extensive zeolitisation to the latter indicates the importance of permeability in controlling zeolite distribution. The relatively permeable Koranga Formation was a paleoaquifer because of its stratigraphic position between relatively impermeable Urewera and Karekare strata. From consideration of regional facies patterns, Wharekopae strata at Matawai and their coal measure and marginal marine equivalents in the South Island are believed to be the compositionally mature end products of a sedimentation cycle initiated in Aptian (Korangan) time. Matawai Group strata are considered to post-data the major deformation pulse known as the Rangitata Orogeny. Study of published and unpublished work on coeval rocks elsewhere in New Zealand suggest a fundamental and regional break between Aptian, Albian or even younger sediments, and older rocks. The Matawai Group and its coeval equivalents should, therefore, be included in the Kaikoura Sequence, and not, as has been proposed, in the underlying Rangitata Sequence.

The area studied, covers about 150 square kilometers in the northern end of New Caledonia in the lower parts of the Diahot River near Ouégoa. The rocks consist dominantly of a metamorphosed Cretaceous sedimentary-igneous sequence of carbonaceous pelites with intercalated basaltic rocks and rhyolitic tuffs. The Cretaceous sequence is flanked to the southwest by Eocene rocks consisting mainly of siliceous argillites, phtanites (massive cherts) and limestones. High-pressure metamorphism, radiometrically dated at 38-21 m.y. (Oligocene-lowermost Miocene) by Coleman (1967), increases progressively in grade towards the northeast in a continuous sequence from lawsonite-albite facies through glaucophanitic greenschists to eclogitic glaucophanitic albite-epidote amphibolites. Lowest, grade rocks occur in the southwest near the Cretaceous-Tertiary boundary and the highest grade rocks are exposed along the east coast and as tectonically-emplaced blocks in fault zones around Ouégoa. Regional metamorphic assemblages are defined with respect, to four zones in pelitic parents which in order of increasing metamorphic grade are:- (1) lowest grade rocks, (2) lawsonite zone, (3) transitional zone, (4) epidote zone. Metamorphic isograds are mapped for paraschist lawsonite, Na-amphibole, garnet and epidote; and for pumpellyite, Na-amphibole, lawsonite and omphacite in metabasalts. In the Cretaceous rocks the regional strike of the foliation is northwest-southeast with dips to the southwest, although small-scale steeply-plunging folds are abundant. On a regional scale the major structure is an open to isoclinal, asymmetric, reclined fold with a sinistral vergence, trending southwest, and it plunges steeply down the dip of the regional foliation. This folding occurred largely synchronously with high-pressure metamorphism. During retrogressive metamorphism, large-scale trans-current sinistral faulting occurred, striking northwest-southeast and dipping southwest. In the region north and east of Ouégoa these faults characteristically occur as zones up to 2 km wide occupied by high-grade ortho- and paragneisses (glaucophanites) and serpentinites, and form a complex anastomosing network. This faulting, accompanied by broad flexuring, has removed some parts of the metamorphic sequence resulting in constrictions and broad, flexuring of the isograd patterns in certain areas of the field. Small-scale kink and chevron folding has occurred late in the tectonic history and post-date both the steeply-plunging folds and the metamorphism. Stratiform Cu-Pb-Zn mineralization with minor amounts of Au and Ag occur at five relatively major sites (Pilou, Mérétrice, Fern-Hill, Balade and Murat mines) and at numerous smaller localities throughout the field. The mineralization is restricted to definite stratigraphic horizons in the Cretaceous sequence where black carbonaceous phyllites are associated and interbedded with rhyolitic metatuffs. The sulphide ores are well laminated, show relict sedimentary textures, and occur as layers and lenses which are conformable with the host rocks. The deposits are thought to be sedimentary-volcanic in origin and derived from acid volcanic metalliferous exhalations which have undergone chemical precipitation and sedimentation in localized, black shale, euxinic environments on the sea floor. All of the deposits have been subjected to and variously modified by the metamorphism. An attempt is made to explain the large-scare structural features observed in the Diahot region in terms of plate tectonic theory.

The rhyolitic Matahina eruption sequence outcrops in the northeast of the Taupo Volcanic Zone of New Zealand. The sequence consists of an airfall deposit overlain in places by up to four ignimbrites erupted from the Okataina Volcanic Centre. The airfall consists, at its base, of crystal poor (< 7%) pumice with a phenocryst assemblage of plagioclase, quartz, hypersthene, magnetite and ilmenite with rare zircon and apatite. Towards the top of the airfall unit crystal rich (> 7%) pumice is present along with crystal poor pumice and thus forms a bimodal population. The crystal rich pumice has the same mineralogy as the crystal poor pumice plus hornblende. The pumice ranges from rhyolite to high-silica rhyolite with the rhyolite containing high TiO2, MgO, CaO, Zr and Sr and low K2O, Ba and Rb in comparison to the high silica rhyolite. The mixture of high silica rhyolite and rhyolite pumice is present throughout the remainder of the eruption with more high silica rhyolite than rhyolite erupted and with no significant change in proportions. The variation in chemistry and mineralogy indicates that the magma was compositionally zoned. Random and oscillatory zoning with evidence of resorption episodes in plagioclase and hypersthene suggest that the magma was turbulently convecting. Zoning in the magma was therefore probably characterised by fluid interfaces between fluid dynamically distinct layers. Stability relations of hornblende and pressure estimates suggest that the hornblende containing rhyolite lay above the high silica rhyolite. The variation is interpreted as having formed by rapid crystallisation and absorption of water on the margins of the intrusion. Least square calculations indicate that fractional crystallisation is a viable mechanism although diffusion through the fluid interface(s) must have occurred with redistribution of components by convection in each layer. The vent of the Matahina eruption tapped the side of the magma chamber close to the interface between the rhyolite and high silica rhyolite. Turbulence during the eruption lead to limited mixing of the zoned magma with the formation of pumice with overlapping mineral composition ranges and rare banded pumice. Indicators such as a lack of an associated veneer deposit, the multi-flow nature of the eruption sequence and strong welding suggest that the eruption was of moderate violence (for its type). Modelling of welding suggests depositional temperatures of between 650 and 700°C in the Rangitaiki Valley. The presence of unexsolved titanomagnetites and osumilite support such high depositional temperatures. Almandine garnet of vapour phase origin indicates that little air was incorporated into the eruption column. Devitrification was characterised by the formation of axiolitic and spherulitic intergrowths of cristobalite and alkali feldspar. SEM observations suggest that the density of nucleating points determines the textures in the devitrifying ignimbrite. Degassing of the ignimbrite was characterised by corrosion of glass and subsequent deposition of minerals such as tridymite and alkali feldspar. Si and Zr were the least mobile elements. The Matahina eruption sequence is an example of a zoned pluton erupted early in its evolution.

The active Orakeikorako-Te Kopia geothermal system was drilled in the mid-1960’s, down to 1405m, as part of a programme to investigate its electrical generation capability. Four wells were completed at Orakeikorako (23km NNE of Taupo) and two at Te Kopia, 9.5km further northeast. The exploration drilling provided information on the present day hydrological and thermal regime which is as hot as 265°C (1137m drilled depth (-801m RL) in OK-2). Major flows into the wells occurred at depths down to 850m, although poor permeability and decline in mass output discouraged development. The waters discharged were of near neutral pH and had low salinities (highest Cl content from OK-2 ≈546mg/kg), low discharge enthalpies and indicated water temperatures (TSiO2 and TNaKCa) of 2l0°C to 240°C. A hydrologic model proposed here envisages a hot water reservoir in the OK-2 area (northeastern part of the Orakeikorako thermal area) with a lateral flow supplying water to the Red Hill (OK-4 area) in the southern part of the system and a concealed northeast flow which reaches the surface at Te Kopia. The Orakeikorako thermal area occupies a surface area of about 1.8km2, mainly on the east bank of the Waikato River, where dilute chloride-bicarbonate water discharges along faults and fractures in association with an extensive silica sinter sheet, boiling springs and geysers. The occurrence of a mordenite-smectite assemblage at shallow depths, plus the oxygen and hydrogen isotopic composition of surface discharge waters, indicate that the ascending chloride fluids are diluted by near surface (heated?) groundwaters. The δD shift from local groundwater composition may be evidence for a magmatic component to the convecting hydrothermal system. Incursion of fluids from the relatively cool (<l20°C) steam-heated carapace into deep levels of the system and its mixing with the alkali-chloride fluids, produces argillic alteration, sealing and will eventually result in the demise of the system. Old silica sinter on the west bank, at the foot of the Tutukau Rhyolite Dome, is covered by Oruanui Ash (22,700 years B.P.) and demonstrates that the hydrologic character of this part of the system has changed due to a combination of fault movement, changes in the height of the watertable and sealing. The Te Kopia thermal area is located along 2.5km of the Paeroa Fault scarp, a major structural feature inferred to be controlling migration of deep hydrothermal fluids in the Orakeikorako-Te Kopia area. Surface activity at Te Kopia is characterised by acid alteration (including fumaroles, warm acid pools and steaming ground), although neutral pH alkali-chloride fluids discharged here within the last 3000 years and deposited silica sinter (C14 age on wood enveloped by sinter is 3026 +/- 43years B.P.). New thermal areas in the past --year have begun to develop in the northwestern part of Te Kopia, whilst cold hydrothermally altered ground (hosting a mordenite + clay assemblage) records a decline in activity in the southern part of the Te Kopia thermal area. The system is hosted by a generally SE dipping sequence of Pliocene to Quaternary ignimbrite, tuff and rhyolite lavas of the Taupo Volcanic Zone. Point counting, electron microprobe analyses of surviving primary phases (Fe, Mg, Al and Ti contents of hornblende and biotite), together with X-ray fluorescence and neutron activation analysis were used to distinguish three extensive ignimbrites encountered in the Orakeikorako-Te Kopia drillholes: (0.33Ma, sanidine-bearing) Paeroa, (0.35Ma) Te Kopia and (undated) Akatarewa Ignimbrites, despite their having been hydrothermally altered. Ignimbrite recognition is made on the basis of a combination of immobile trace and rare earth element abundances and ratios: Ta and Yb (ppm), P2O5(%) and the ratios Zr/Yb, Zr/Y, Yb/Hf, La/Lu, Nb/Hf, Zr/Nb, Zr/Lu, Yb/Ta, Ta/Lu, La/Tb and Nb/Ta. The Paeroa Ignimbrite is distinguished by is Eu anomaly ((Eu/Eu*)cn is 0.48 to 0.54), whereas the Te Kopia and Akatarewa Ignimbrites are characterised by their flatter REE Spidergrams ((Eu/Eu*)cn is ~1.0 and ~0.8 respectively). The correlation of the extensive ignimbrites was satisfactorily effected by a combination of their characteristic bulk rock and pumice chemistry, plus primary mineralogy, to enable the subsurface stratigraphy and structure of the Orakeikorako-Te Kopia geothermal system to be defined. The alteration assemblage below 500m consists of quartz, albite+adularia, with variable abundence and distribution of chlorite, pyrite, calcite, wairakite, epidote, pyrrhotite, titanite, leucoxene, siderite and clinozoisite; illite is a late overprint. Rare almandine occurs in rhyolite in OK-1: 1312.5m; this is the first known occurrence of garnet in an active geothermal system of the TVZ. The surficial alteration assemblage of kaolin, cristobalite, alunite, hematite and jarosite reflects alteration by acid sulphate-steam heated waters. The occurrence and textual relations exhibited by the hydrothermal mineral assemblage define the geochemical structure and thermal evolution of the Orakeikorako-Te Kopia system The activities of components of minerals, determined from electron microprobe analyses and composition-activity relationships (OK-2 discharge: logαK+/αH+=3.6, logαNa+/αH+=4.8) support the petrologic observation that illite is now the stable potassium phase (overprinting adularia), although a state of equilibrium between the sheet silicate and the fluids is clearly not fully reached. The Na/K ratio of the altering fluid is controlled by the albite-adularia reaction (dissolution of albite and replacement by adularia, after andesine), whilst the H2/H2S ratio is buffered by the virtually complete replacement of pyrrhotite by pyrite. The fluids are now slightly undersaturated with respect to calcite, this is shown by etched surfaces on some calcite grains. In the past the deep fluid boiled adiabatically from >300°C to ~250°C as it ascended, resulting in the deposition of adularia, quartz and bladed calcite. The system has cooled, resulting in lower subsurface temperatures (as recorded by fluid inclusion geothermometry) suppressing boiling, and migrated northwards as a consequence of self sealing. The thermal decline and retention of CO2 in the deep alkali-chloride fluid shifted the alteration assemblage from one of albite-adularia stability to illite stability. The homogenisation (Th) temperatures of primary and secondary liquid-rich inclusions in 27 cores from different depths mostly match measured temperature profiles (e.g. OK-1 (shallow levels) and OK-2). Never-the-less, fluid inclusion data support mineral-inferred stability temperatures which indicate that parts of the Orakeikorako-Te Kopia system have cooled appreciably (e.g. OK-1, deep levels) and OK-4 (maximum Tbore=238°C, maximum Th=312°C; epidote abundant). In contrast, the northwestern margin (OK-6 area) has heated (OK-6:1113.4m; Tbore=261°C, Th=210-221°C). Some inclusions in the Te Kopia drillholes have Th values that exceed Tbore by as much as 50°C, and are deduced to have been uplifted by movement on the Paeroa Fault. Freezing data indicate that the trapped fluid was dilute (~0.2 to 1.7 wt% NaCl equivalent) since most Tm values range from -0.1 to -0.5°C. The outflow portion of the Orakeikorako-Te Kopia system has evolved recently, both chemically and physically. Movement on the Paeroa Fault, that uplifted pyroclastic rocks hosting a quartz-adularia-illite assemblage, combined with a lowering of the watertable has resulted in an overprinting of the neutral pH hydrothermal mineral assemblage by a kaolinite-alunite type assemblage which derives from an acid sulphate fluid. Quartz crystals found 150m above the base of the Paeroa Fault scarp host dilute (~1.5wt% NaCl equivalent) fluid inclusions with Th values that range from 180-206°C (average 196°C). Bladed quartz (after calcite) did not contain usable inclusions. It is deduced that the inclusions formed about 120-160m below the ground, which indicates uplift in the order of ~300m. Assuming a constant rate of uplift of 4m/ka (based on the offset of 330ka Paeroa Ignimbrite), the minimum duration of activity at Te Kopia is 75,000 years.