Geocheemical variations in a suite of granitoids and gabbros from Southland,New Zealand

The Longwoods Complex of Southland, New Zealand is part of an extensive terrane consisting of intrusives, volcanics, and sediments, which outcrops in the southern and north-western portions of the South Island. This terrane represents a volcanic arc which was active from Permian to Jurassic times (Grindley, 1958; Challis, 1968, 1969; Coombs et al., 1976). Between Pahia Point and Oraka Point on the southern coast of the South Island a section across the Longwoods Complex is well exposed and intrusives ranging in composition from ultrabasic cumulate rock, high-Al gabbro and gabbroic diorite to quartz diorite and granite outcrop. Two models have been considered for the origin of the rocks of the Pahia Point-Oraka Point section: (a) the rocks constitute one suite, the members of which are related by a crystal fractionation process; (b) the rocks constitute two suites which are not directly related. The ultrabasic rocks, and quartz diorites are complementary and are derived from a high-Al gabbro parent by crystal fractionation involving pyroxene, olivine, plagioclase and hornblende, but considerations of viscosity and the geochemistry of the granite preclude derivation of the high-Si rocks by continuation of the crystal fractionation model. Furthermore, the quartz-diorites are of two types: xenolith bearing foliated quartz-diorites and xenolith deficient unfoliated types. The latter rock type appears to group with the gabbros on variation diagrams and partitioning of Ti between mica and amphibole supports the view that two distinct suites of rocks are involved: (a) a suite derived by fractional crystallization from a high-Al gabbro parent and consisting of cumulate ultramafic rocks, high-Al gabbro, gabbroic diorite and quartz-diorite; (b) a suite of foliated quartz diorites, formed by partial melting of lower crustal igneous rocks. The xenoliths in the foliated quartz-diorites represent modified residue left after partial melting. Melt and residue have unmixed to varying degrees during diapiric rise and a range of compositions has resulted. The association of the two suites is tectonic. Gabbroic melts are generated in the lithosphere during plate subduction beneath a continental margin and rise of these melts into the lower continental crust results in partial melting and generation of quartz-diorite magmas.     

Petrographic and coalification variations in the Eastern Southland lignites, New Zealand

The Upper Oligocene to Lower Miocene Gore Lignite Measures of Eastern Southland accumulated in humid temperature to subtropical wet forest swamps in a coastal delta-plain paleoenvironment. The lignites have relatively low ash and low sulphur contents. They are xylitic, contain abundant resinite and range from poorly to strongly gelified. Petrographically, they are relatively uniform and dominated by humic macerals.The Eastern Southland Lignite Field can be divided into a southern (lower-rank) and a northern (higher-rank) area. The lower-rank lignites have bed moisture contents of 55–61% whereas the higher-rank lignites have bed moistures of 42–46%. In both areas the lignite seams are within 200 m of the surface but are probably separated by faults and have undergone separate burial and tectonic histories. Burial depths are believed to range from 250 m for lower-rank to 1100 m for higher-rank lignites. The rank increase from south to north is largely the result of compaction-induced reduction in porosity and consequent dewatering and is not accompanied by any notable increase in gelification. For these lignites, there is a linear relationship between bed moisture and fixed carbon but a significant shift away from the linear relationship between volatile matter and specific energy and bed moisture. Also, the higher-rank northern lignites have higher H:C and lower O:C atomic ratios than the southern lower-rank lignites. These geochemical changes are heat- and depth-related and represent changes in functional group composition that are independent of observed gelification.     

Gold and sulphide minerals in Tertiary quartz pebble conglomerate gold placers, Southland, New Zealand

Auriferous quartz pebble conglomerates (QPC) formed during Tertiary sedimentary recycling in the Waimumu district, Southland, New Zealand. These sediments contain fine-grained gold of detrital origin with abundant surface textures and gold-forms associated with authigenic gold remobilisation. Most authigenic gold contains no detectable silver and occurs as overgrowths on detrital Au–Ag and Au–Ag–Hg alloys that contain up to 13 wt.% Ag, and 9 wt.% Hg. Fine-grained Au–Ag and Au–Ag–Hg alloys are compositionally heterogeneous, exhibiting both well-defined silver-depleted and silver-enriched rims. Rare coarse Au–Ag alloy is intergrown with quartz and is homogenous. Discrete grains of authigenic, porous, sheet-like gold occur in carbonaceous mudstone within a QPC sequence. Some QPC contain abundant sulphide minerals. Some of these sulphides (pyrite and arsenopyrite) are of long-distance detrital origin, presumably from the Otago Schist, whereas the bulk of the sulphide suite is marcasite of variably transported diagenetic origin, derived from the erosion of QPC and underlying Tertiary sediments. There has also been authigenic deposition of sulphide minerals in the QPC themselves. These diagenetic sulphides include framboidal and anhedral marcasite, and framboidal and euhedral pyrite. Sulphur isotope data for the sulphide minerals range from − 45‰ to + 18‰ (relative to VCDT). Sulphur isotope data for euhedral detrital pyrite and arsenopyrite range from − 9‰ to − 1‰ and are most likely derived from the Otago Schist to the north. Both framboidal and anhedral marcasite have lower values (< − 20‰) reflecting microbial sulphate reduction as a source for the precursor hydrogen sulphide. Anhedral marcasite contains elevated concentrations of Ni, Co, As and Cr, commonly with compositional banding of these metals.Both the gold and diagenetic sulphides from the Belle-Brook QPC are compositionally similar to gold and sulphides from Archaean QPC. Porous, sheet-like authigenic gold is morphologically similar to gold associated with carbonaceous material in the Witwatersrand. In addition, Southland marcasite textures resemble the rounded and banded pyrite in Witwatersrand QPC placers. There is abundant evidence from these Tertiary QPC in southern New Zealand for sedimentary transport of sulphide minerals and post-depositional sulphide mineralisation in the surficial environment despite an oxygen-rich atmosphere. These young deposits thus provide an example of authigenic gold and sulphide textures formed during diagenesis in unmetamorphosed placers. Many of these textures are similar to those commonly ascribed to metamorphic processes in Archaean auriferous QPC.     

Debris flow deposits of early miocene age,deadman stream,Marlborough, New Zealand

Detailed analysis is presented of a conformable succession of Early Miocene conglomerates and sandstones lying between massive marine mudstones. The coarse sediments reflect deposition by a spectrum of subaqueous debris-flow mechanisms during an early pulse of tectonism that ultimately resulted in Plio-Pleistocene eversion of the Kaikoura Mountains.Sparse pebbly mudstones and rare sandy conglomerates show disoriented clasts and reflect high-viscosity flows and slurry-creep flow mechanisms. Other deposits have little mud matrix, hence appear to reflect low-viscosity flow processes. The largest clasts in these have a preferred planar orientation, probably reflecting dispersive grain pressure, and a preferred long-axis orientation parallel to flow direction. Common sorted sandstones and some conglomeratic sandstones show diffuse parallel lamination; with rare exceptions neither grading nor traction structures are present. Other conglomeratic sandstones show trough cross-bedding which we attribute to entrained bedload movement during intersurge episodes of debris flow.Microfossil data from the mudstones indicate sedimentation in an environment of outer neritic to upper bathyal aspect. Most detritus is abraded, suggesting derivation from terrestrial or inner neritic sources, but angular calcilutitic clasts and irregular sandstone and mudstone clasts and rafts were probably derived from submarine erosion between the emergent source area and the site of accumulation. Deposits generally appear to infill broad shallow channels. Paleocurrent and fabric analysis indicate a markedly uniform flow direction throughout succession, and suggest that the locus of channeling remained relatively fixed in space throughout accumulation of hundreds of metres of superimposed, commonly amalgamated debris-flow deposits. Although lateral control away from the measured sequence is limited, we infer that the locus of deposition lay shoreward of any submarine canyon or fan.     

Petrogenesis of the Greenhills Complex,Southland, New Zealand: magmatic differentiation and cumulate formation at the roots of a Permian island-arc volcano

A Permian (~265 Ma) intrusive complex which formed as a magmatic feeder reservoir to an immature island-arc volcano is fortuitously exposed in southern New Zealand. Known as the Greenhills Complex, this intrusion was emplaced at shallow crustal levels and consists of two layered bodies which were later intruded by a variety of dykes. Cumulates, which include dunite, olivine clinopyroxenite, olivine gabbro, and hornblende gabbro-norite, are related products of parent-magma fractionation. Both primary (magmatic) and secondary platinum-group minerals occur within dunite at one locality. Using the composition of cumulus minerals, mafic dykes and melt inclusions, we have determined that the parent magmas of the complex were hydrous, low-K island-arc tholeiites of ankaramitic affinities. Progressive magmatic differentiation of this parent magma generated fractionated melt of high-alumina basalt composition which is now preserved only as dykes which cut the Complex. Field evidence and cumulus mineral profiles reveal that the magma chambers experienced turbulent magmatic conditions during cumulate-rock formation. Recharge of the chambers by primitive magma is likely to have coincided with eruption of residual melt at the surface. Similar processes are inferred to account for volcanic-rock compositions in other parts of this arc terrane and in modern island-arc systems.     

Origin of red beds in a Cambrian flysch sequence, Canadian Appalachians, Quebec

The basal unit of this flysch sequence consists of about 700 m of rhythmically interlayered red and green claystone intercalated with feldspathic sandstone and lithic conglomerate. Chemical analyses show that both types of claystones have approximately the same Fe₂O₃ content (8%). The presence within green claystone of inarticulate brachiopods, thin laminations of phosphate and pyrite, coupled with a high Mn content (Fe/Mn : 27) together with the composition of the chlorites, suggest that green mud was deposited slowly in a marine environment under reducing conditions. The hematite content of red claystone averages 4%. The Fe/Mn is high (71), and the clay fraction contains chamosite in addition to chlorite and illite. These characteristics favour a paralic oxidizing environment for the red muds. Rapid resedimentation of red sediment from a paralic environment into the green lithotope, rather than a lateral variation of facies, is suggested by: (1) the distribution of the two hemirhythms in the stratigraphic column, (2) the presence in the red hemirhythm of large slump structures, contorted laminations, and pebbly claystone, and (3) by the internal sedimentary structures of the coarse fraction.     

Evidence for early miocene wrench faulting in the Marlborough fault system,New Zealand: structural implications

In New Zealand, the Marlborough strike-slip faults link the Hikurangi subduction zone to the Alpine fault collision zone. Stratigraphic and structural analysis in the Marlborough region constrain the inception of the current strike-slip tectonics.Six major Neogene basins are investigated. Their infill is composed of marine and freshwater sediments up to 3 km thick; they are characterised by coarse facies derived from the basins bounding relief, high sedimentation rates and asymmetric geometries. Proposed factors that controlled the basins' generation are the initial geometry of the strike-slip faults and the progressive strike-slip motion. Two groups of basins are presented: the early Miocene (23 My) basins were generated under wrench tectonics above releasing-jogs between basement faults. The late Miocene (11 My) basins were initiated by halfgrabens tilted along straighter faults during a transtensive stage. Development of faults during Cretaceous to Oligocene times facilitated the following propagation of wrench tectonics. The Pliocene (5 My) to current increasing convergence has shortened the basins and distorted the Miocene array of faults. This study indicates that the Marlborough Fault System is an old feature that connected part of the Hikurangi margin to the Alpine fault since the subduction and collision initiation.     

Formation mechanisms of illite,chlorite and mixed-layer illite-chlorite in Triassic volcanogenic sediments from the Southland Syncline,New Zealand

Clay minerals from the three principal kinds of zeolitic sediments from the type area for zeolite facies alteration, the Triassic Murihiku Supergroup, Southland, New Zealand, have been studied by TEM. Bentonitic tuff consists largely of smectite and heulandite with minor illite; they occur as replacements of glass shards and are inferred to be direct alteration products of tuff alteration. Both analcime- and laumontite-rich tuffs contain chlorite, illite and mixed-layer illite-chlorite, including 11 mixed-layer sequences. Subhedral to euhedral phyllosilicate crystal shapes and other textural features imply that phyllosilicates crystallized from solution derived in part by dissolution of precursor smectite. Intralayer transitions involving illite and chlorite are inferred to be products of crystallization rather than direct alteration and replacement. Petrographically similar bentonitic and analcimized tuffs overlap each other in the stratigraphic section, supporting earlier observations that there is no systematic change in smectite relative to the illite plus chlorite derived from smectite in sections up to 8.5 km thick. The data imply that smectite may be metastable relative to illite plus chlorite. Permeability and fluid chemistry are inferred to be as significant as temperature in promoting reactions in clay minerals as well as zeolites during burial metamorphism.Contribution No. 436, from The Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109     

Petrology of Ocellar Lamprophyres from Western Otago, New Zealand

The southern part of a lamprophyre dyke swarm from West Otago,New Zealand comprises monchiquites and camptonites. Dykes arecompositionally similar to volatile-rich nephelinite or nephelinebasanite, and contain leucocratic ocelli and irregular veinsegregations of nepheline benmoreite composition. Ocelli arecommonly zoned with an upper globule or centrally disposed coreof analcime-carbonate and an alkali feldspar-kaersutite-richlining. Virtually identical zoning is developed in minerals common toboth lamprophyre host and ocellus. Kaersutite is progressivelyenriched in Fe, Mn, Al_vi, and Na + K, and depleted in Mg, andTi towards the rim. Titanaugite is concentrically zoned to aegirine-augite,and is sector zoned with prism sectors enriched in Ti and Al(and to a lesser extent in Fe and Na) compared to basal sectors.Brown micas are zoned from titanphlogopite cores to titanbiotiteand titaniferous lepidomelane rims. In monchiquites glass occurs as an intersertal phase and asan ocellus infilling. The glass is phonolitic and contrastssharply with the composition of the ultrabasic host, indicatingthat production of extreme differentiates without intermediatecompositions is possible by fractional crystallization. Identicalglass compositions in both lamprophyre groundmass and ocelluspreclude liquid immiscibility. Instead, it is suggested thatocelli are segregations of residual liquids drawn into gas vesiclesas a consequence of reduction of the vapour phase, volume duringcooling. The analcime-carbonate globule within the ocellus isinterpreted as the condensed vapour phase. At Haast River, in the northern part of the swarm the dyke associationof peridotite, camptonite, trachyte, tinguaite, and carbonatiteis readily explained in terms of this model. Fractional crystallizationof camptonite results in phonolitic late stage liquids which,if separated from the lamprophyre host crystallize as sodalitetinguaites, leaving a more ultrabasic residuum whose compositionis equivalent to damkjernitic peridotite. Vesiculation of thecamptonite results in separation of a volatile phase (fluidimmiscibility) which is carbonate and alkali-rich, the sourceof both carbonatites and the agent responsible for fenitization.     

Accretion and exhumation at a Variscan active margin, recorded in the Saxothuringian flysch

The Saxothuringian flysch basin, on the north flank of the Central European Variscides, was fed and eventually overthrust by the northwestern, active margin of the Tepla-Barrandian terrane. Clast spectra, mineral composition and isotopic ages of detrital mica and zircon have been analyzed in order to constrain accretion and exhumation of rocks in the orogenic wedge. The earliest clastic sediments preserved are of early Famennian age (ca. 370?Ma). They are exposed immediately to the NW of the suture, and belong to the par-autochthon of the foreland. Besides ultramafic (?ophiolite) material, these rocks contain clasts derived from Early Paleozoic continental slope sediments, originally deposited at the NW margin of the Saxothuringian basin. These findings, together with the paleogeographic position of the Famennian clastics debris on the northwestern passive margin, indicate that the Saxothuringian narrow ocean had been closed by that time. Microprobe analyses of detrital hornblendes suggest derivation from the “Randamphibolit” unit, now present in the middle part of the Saxothuringian allochthon (Münchberg nappes). Detrital zircons of metamorphic rocks formed a little earlier (ca. 380?Ma) indicate rapid recycling at the tectonic front. The middle part of the flysch sequence (ca. early to middle Viséan), both in the par-autochthon and in the allochthon, contains abundant clasts of Paleozoic rocks derived from the northwestern slope and rise, together with debris of Cadomian basement, 500-Ma granitoids and 380?Ma (early Variscan) crystalline rocks. All of these source rocks were still available in the youngest part of the flysch (c. middle to late Viséan), but some clasts record, in addition, accretion of the northwestern shelf. Our findings permit deduction of minimum rates of tectonic shortening well in excess of 10–30?mm per year, and rates of exhumation of ca. 3?mm/a, and possibly more.     

Vein arrays, hydraulic fractures and pressure-solution structures in a deformed flysch sequence S.W. England

Vein arrays and pressure-solution cleavages are common in the sandstone units of the deformed flysch. Both structures were established prior to the folding of the sediments, and they continued to evolve during this folding. Tensile fractures and conjugate sets of shear fractures (of wrench fault type) were formed. En-echelon arrays of veins are closely associated with the formation of these principal veins. The geometry and relations of the veins are described in detail and their relation to the principal stresses at the time of formation is discussed.

Forking at vein terminations, branch fractures and offset structures in veins are described. The analogies between these and structures produced in rock deformation experiments and found in magmatic dykes are discussed. The branch fracture provides a record of the orientation of the maximum principal stress at the time of its formation.

The veins are infilled with quartz and siderite displaying drusy growth fabrics, indicating that crystal growth occurred into a fluid-filled cavity. It is suggested that the veins originated as hydraulic fractures in a flysch sequence that had developed high pore-fluid pressures during sedimentation. The material in the veins was derived by pressure-solution activity in the sandstone units, which produced a spaced pressure-solution cleavage throughout the region. The relations between veins and pressure-solution cleavages are described. Both small- and large-scale solution transfer of material was involved.     

Glacial conglomerates of the Appalachian Plateau,New York

Three generations of Quaternary conglomerates formed by cementation of glacial gravels are found in drift of the Appalachian Plateau in south-central New York. Postglacial conglomerates have been locally cemented by carbonate minerals within limestone-bearing (erratic-rich) gravels of Late Wisconsin (Woodfordian) age. Conglomerates cemented during the Mid-Wisconsin interstade occur as numerous fragments in erratic-rich Late Wisconsin drift, and as a single in situ mound near Syracuse. Mid-Wisconsin conglomerate contains fragments of a still older conglomerate believed to be of possible pre-Wisconsin (Sangamon) age. Both postglacial and Mid-Wisconsin conglomerates were cemented in the same manner, as a result of diagenetic processes in the vadose zones of limestone-bearing terraces. Information gleaned from conglomerate fragments indicates that Early Wisconsin drift was similar to Late Wisconsin drift; the Early and Late Wisconsin glaciations and deglaciations of the plateau probably proceeded in much the same fashion.     

Biogeochemical exploration for tungsten at Barrytown,New Zealand

Biogeochemical prospecting for tungsten was carried out at a mineralised area near Barrytown, New Zealand. The tungsten content of the ash of the shallow-rooted tree ferns Cyathea medullaris and Dicksonia squarrosa correlated well with the concentrations of this element in the soil. Sampling of these species therefore provides a possible method of detecting general soil anomalies and moreover is considerably faster than soil sampling under local conditions.Trend surface analysis of the concentrations of tungsten in the trunks of all species of common large evergreen trees sampled on belt transects, could be used to delineate extensions of known tungsten mineralisation in bedrock, and was to some extent superior to soil geochemistry in this respect.     

Ir anomalies and other elemental markers near the Palaeocene–Eocene boundary in a flysch sequence from the Western Tethys (Slovenia)

Ir abundance anomalies, platinum-group elements (PGE) enrichment and increased concentrations of meteoritic (Ni, Fe, Co) and nonmeteoritic (Sb, As, Zn and Cu) elements were found in the Palaeocene–Eocene (P/E) boundary interval in a flysch sequence from the Western Tethys (Goriška Brda section, W. Slovenia). This records one of the most important calcareous deep benthic extinctions in the history of the Earth. Although the observed geochemical patterns could indicate complex sources for these metals, such as weathering of the continental and oceanic crust, volcanic processes, as well as diagenetic mobilization and redistribution, we cannot preclude the highly speculative possibility that the observed Ir abundances may also indicate extraterrestrial contamination.     

An occurrence of ardennite in quartz veins in piemontite schist,Western Otago,New Zealand

Summary Ardennite of complex composition: (Mn²⁺ _3.488Ca_0.509Ba_0.002)₌₄(Mg_0.916916 Fe³⁺ _0.165 Mn³⁺ _0.099Cu_0.033Ni_0.009Zn_0.006 Ti_0.008Al_4.764)₌₆(As⁵⁺ _0.823V⁵⁺ _0.022P_0.005B_0.069Al_0.042Si_5.039)₌₆O_21.81(OH)_6.17 occurs in crack-seal quartz veins in quartz-albite-piemontite-spessartine-phengitehematite-chlorite-rutile-tourmaline ± calcite schist of the Haast Schist Group near Arrow Junction, western Otago, New Zealand. The Mn²⁺/Mn³⁺-ratio is sensitive to calculations and to accuracy of analyses. Boron is detected in ardennite for the first time. Other properties include = 1.734(3), = 1.735(3), = 1.751(3), 2V_Z = 30(2)°;a = 8.721(1),b = 5.816(1),c = 18.545(3) Å,V = 940.7(2) ų. Associated mineral phases are spessartine, hematite, piemontite containing 0.7% SrO and 0.06% PbO, and phengite. Later-stage vein minerals comprise chlorite, albite, and manganoan calcite which were deposited under less highly oxidizing conditions. Digenite with minor intergrown covellite occurs in small amount with manganoan calcite and quartz in a cross-cutting late-stage veina chalcopyrite and native copper occur in other late-stage veins. Arsenic and other components of the ardennite and associated minerals are derived from highly oxidized ferromanganese oxide- and hydroxide-bearing siliceous pelagic sediments that formed the protolith for the piemontite schist. The veins formed at a relatively early stage after metamorphism peaked in the chlorite zone of the greenschist facies under conditions that have been estimated at about 4.5 kbar, 390 °C.

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Vorkommen von Ardennit in Quarzgängen aus Piemontit-Schiefern, West-Otago, Neuseeland

Zusammenfassung Ardennit mit der Zusammensetzung (Mn²⁺ _3.488Ca_0.509Ba_0.002)₌₄(Mg_0.916Fe³⁺ _0.165Mn³⁺ _0.099Cu_0.033Ni_0.009Zn_0.006 Ti_0.008Al_4.764)₌₆(As⁵⁺ _0.823V⁵⁺ _0.022P_0.005B_0.069Al_0.042Si_5.039)₌₆O_21.81(OH)_6.17 tritt in Crack-seal-Quarzgängen in Quarz-Albit-Piemontit-Spessartin-Phengit-Hämatit-Chlorit-Rutil-Turmalin ± Calcit-Schiefern der Haast Schiefer-Gruppe nahe der Arrow Junction, West-Otago, Neuseeland, auf. Die Proportionen von Mn²⁺/Mn³⁺ hängen von der Kalkulation und der Genauigkeit der Analyse ab. Bor wird zum ersten Mal im Ardennit bestimmt. Andere Eigenschaften sind: = 1.734(3), = 1.735(3), = 1.751(3), 2Vz = 30(2)°; a = 8.721(1), b = 5.816(1), c = 18.545(3) Å, V = 940.7(2) Å3. Assoziierte Mineralphasen sind Spessartin, Hämatit, Piemontit, der 0.7% SrO und 0.06% PbO enthält und Phengit. Spät gebildete Gangmineralien, wie Chlorit, Albit und Mn-Calcit, sind unter geringer oxidierenden Bedingungen entstanden. Digenit mit etwas Covellin tritt in kleinen Mengen zusammen mit Mn-Calcit und Quartz in einem querschlägigen Gang auf, Chalcopyrit und gediegenes Kupfer kommen in anderen späten Gängen vor. Arsen und andere Komponenten des Ardennites and der assoziierten Minerale können von hochoxidierten, Fe-Mn-Oxid- und Hydroxyd-führenden, Sireichen, pelagischen Sedimenten hergeleitet werden, die das Ausgangsgestein für den Piemontit darstellen. Die Gänge wurden in einem relativ frühen Stadium, nach dem Metamorphosehöhepunkt, innerhalb der Chloritzone der Grünschiefer-Fazies, unter ungefähr 4.5 kbar und 390°C, gebildet.

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With 4 Figures     

The interstratified breccias and conglomerates in the Cretaceous flysch of the northern Basque Pyrenees: submarine outflow of diapiric mass

In the Cretaceous flysch of the Northern Pyrenees, polymict conglomerates interbedded in the flysch are described from the vicinity of Orio (near San Sebastián). These contain components derived from rocks of all periods from Paleozoic to Lower Cretaceous. During Late Maastrichtian times a diapir, originated in Keupez evaporites, penetrated the deep-sea Cretaceous flysch which was still undergoing deposition. This resulted in the expulsion of large quantities of diapiric mass. After the solution of the evaporites the pebbles, which had been dragged along with the diapiric masses, slumped into a neighbouring marginal depression where they were redeposited. The reddish strata of Late Maastrichtian and Danian age in this region are interpreted as being out-thrust and transported Keuper shales. It will be shown that the other breccias and conglomerates described by Feuillé and Mathey (1972) from the Late Cretaceous of the Basque Pyrenees probably have the same origin.The comprehensive name “Vascongadian Diapir Zone” is suggested for the diapir field on the northern edge of the Basque sedimentary trough.     

Eclogitic xenoliths from volcanic breccia at Kakanui,New Zealand

Eclogitic xenoliths consisting of tschermakitic augite and pyrope garnet, together with variable amounts of kaersutitic hornblende, are common in a volcanic breccia of Lower Oligocene age at Kakanui, New Zealand. The breccia also contains xenocrysts of these minerals, and xenoliths of peridotite. Modal analyses are given of a number of the eclogitic xenoliths, and chemical analyses of two of them and their component minerals. They are compared with similar xenoliths from Hoggar (Algeria), Salt Lake Crater (Hawaii), and Delegate (Australia), with eclogite xenoliths from kimberlites, and with garnet peridotites. These three types of igneous eclogites can be characterized by the nature of their clinopyroxene: tschermakitic in the xenoliths from basaltic rocks, jadeitic in the xenoliths from kimberlites, and chrome diopside in the garnet peridotites. The eclogitic xenoliths in basaltic rocks probably crystallized in the mantle at depths of about 60 km, but their rarity in contrast to the numerous occurrences of peridotite xenoliths poses some significant problems.     

Mass-emplaced sand-fingers at Mararoa construction site, southern New Zealand

During work on a dam construction site in South Island, New Zealand, outflowing ground water resulted in extensive local mass-transport of the sands being excavated. A series of small scale sand fans was built up, probably mainly by laminar mass-flow processes. The surfaces of the fans were made up of a series of complexly interdigitating sand-fingers that are inferred to have been emplaced by viscous plug-flow. Other sedimentary processes associated with the building and synsedimentary destruction of the fans included rapid grain-flow, liquefaction and progressive slumping. Although small in scale, the Mararoa mass-flow fans may be a close analogue for some of the many‘fluxoturbidite’ or‘proximal flysch’ facies described in the sedimentological literature, and their geologic implications are therefore briefly discussed.     

Lahar-Triggering Mechanisms and Hazard at Ruapehu Volcano,New Zealand

Late Holocene volcanic activity at Ruapehu has been characterizedby the generation of small (<10⁵ m³) to very large (>10⁷ m³) lahars and repeated,small to medium (VEI 1-3) tephra-producing eruptions. The Onetapu Formation groupsall lahar deposits that accumulated during the last 2,000 years on the southeastern Ruapehu ring plain. The andesitic tephras are grouped within the Tufa Trig Formation and are intercalated within the laharic sequence. By correlating these two formations with new radiocarbon ages obtained on interbedded paleosols, we reconstruct a detailed volcanic history of Ruapehu for this period.Clast assemblages identified in the laharic sequences record thelithologies of synchronous tephras and rocks within the source region. These assemblages suggest a strong genetic link between the development of Crater Lake, the variation in eruptivestyles, and the production of lahars.Lahar-triggering mechanisms include: (1) flank collapse ofhydrothermally altered and unstable portions of the cone; (2) phreatic and phreatomagmatic eruptions favoring the generation of snow-rich slurries and hyperconcentrated stream flows; (3) suddenCrater Lake rim collapse, releasing large amounts of water inducing debris flows; and (4) eruptions that generate large volumes of tephra on snow-covered slopes, later remobilized by heavy rain.Two major lahars in the Onetapu sequence had a volume 4 × 10⁷ m³, roughly 1 to 2 orders of magnitude larger than the 1953event leading to the Tangiwai disaster (151 casualties). One of these lahars crossed over a lowinterfluve currently separating the Whangaehu River from a stream feeding the Tongariro River,sometime since peat accumulated between AD 1400 and AD 1660. A repetition of such a large-scaleevent would have devastating consequences on the infrastructure, economy and environment withinthe distal areas of the two catchments. The 1995–1996 eruptions were a timely reminder ofthe hazards posed by the volcano.