Thiosulphate in surficial geothermal waters, North Island, New Zealand

Thiosulphate is present in hot springs, streams and thermal pools of the Taupo Volcanic Zone and Ngawha, New Zealand, at concentrations of 1.2 (±1.3) × 10⁻⁵ M to 7.05 (±0.12) × 10⁻⁴M. Formed as a metastable product of sulphide oxidation, thiosulphate is buffered in the presence of elemental S according to, . Unless all sulphide present has been bacterially oxidised to sulphate, a steady state concentration of thiosulphate is maintained. As a soft base thiosulphate is capable of complexing several transition metals. The thermodynamically predicted speciation of Ag in Champagne Pool, for example, indicates a Ag(S₂O₃)₂⁻³ activity similar to that of AgCl₂⁻ though less than that of Ag(HS)₂⁻.     

Origin of the Torlesse Terrane and Coeval Rocks,North Island,New Zealand

On the basis of differing areal extent, age, petrographic modes, and bulk chemical composition, the sandstones of the northern quarter of the Torlesse terrane are subdivided into four new petrofacies. A comparison of these petrofacies with existing South Island Torlesse classifications indicates continuation of the Triassic Rakaia subterrane and Late Jurassic–to–early Cretaceous Pahau subterrane into the central part of the North Island (as Axial-A and Axial-B petrofacies, respectively). The Waioeka petrofacies defines a new and provisional Late Jurassic-to–early Cretaceous Waioeka subterrane that is not present in the South Island. The Omaio petrofacies is common to deformed Albian basement sequences in the Torlesse of both islands, and in the Houhora Complex of Northland.

The composite Torlesse terrane evolved by Early Jurassic accretion of allochthonous Rakaia rocks followed by parautochthonous deposition of Pahau and Waioeka sandstones. Waioeka sandstones are compositionally similar to sandstones in the coeval eastern Waipapa terrane, but may have been dextrally displaced from their original depositional site by up to 300 km since the middle Cretaceous.     

Upper mantle P-wave velocities in the southeast North Island, New Zealand

Reversed Pn velocities have been determined from regional earthquake sources for the Wairarapa region in the southeast North Island, New Zealand. Values were 8.64 (±0.13) km/s in the easternmost part of the region and 8.90 (±0.13) km/s in the central Wairarapa, which are consistent with subducted Pacific plate lying at shallow depth under the region. A value of 8.22 km/s between Wellington and the Wairarapa region is interpreted as a possible difference in the velocity structure above the refractor between the two localities.     

Rhyolitic volcanic corridors in magmatic arcs: comparing North Wales and North Island, New Zealand

Exceptionally voluminous arc-related rhyolitic eruptions from clusters of caldera complexes, as seen in Snowdonia, North Wales (mid-Caradoc), and North Island, New Zealand (late Neogene-Quaternary), are characteristically confined within transient, fault-controlled corridors in continental crust. New Zealand rhyolitic corridors (Coromandel, Central, Taupo) have developed in response to the spearheading of an oceanic arc into continental crust, combined with subduction rollback-induced extension during clockwise rotation pivoting around central North Island. Inherited high heat flow from earlier arc magmatism, intracrustal plastic deformation, and mantle-derived magma ponding and fractionation beneath a less dense, fracture-toughened crust, all contribute synergistically to crustal fusion and catastrophic volcanism. A similar scenario is suggested for the Snowdonia volcanic corridor where at least six major rhyolitic centres were restricted in space and time (Soudleyan-Woolstonian). After the climactic Snowdonian eruptions, arc magmatism was extinguished in Wales: a fate predicted for New Zealand rhyolitic volcanism as subduction rollback continues.     

华北克拉通板内拉张性岩浆作用与三个超大陆裂解及深部地球动力学

对华北克拉通碱性岩等板内拉张背景岩浆岩进行了大量的同位素年代学及空间分布的研究,并结合前人的有关研究成果得出：华北克拉通板内拉张性岩浆作用主要集中在３个时段,即古元古代末—中元古代早期（1 850~1 600 Ma）新元古代中–晚期（900~600 Ma）、和古生代末—新生代（250 Ma~现今）。这３个时段的拉张性岩浆作用,由老到新依次称之为第一拉张作用阶段、第二拉张作用阶段和第三拉张作用阶段岩浆作用,3个阶段拉张性岩浆作用出现的频率强度和空间分布的广度均有明显差别。第三阶段拉张性岩浆作用出现的频率强度最大,空间分布也最广;第一阶段拉张性岩浆作用次之;而第二阶段拉张性岩浆作用出现的频率强度最弱,并且空间分布也很局限。特别值得提出的是三个阶段拉张性岩浆作用在时间上分别与哥伦比亚（Columbia）、罗迪尼亚（Rodinia）及潘基亚（Pangaea）三个超级大陆的拉张裂解时间基本一致。这可能说明华北克拉通对三个超级大陆的拉张裂解都有不同程度的反映。对于导致超级大陆拉张裂解的深部地质过程地球动力学,一般认为是超级热地幔柱活动。推测华北克拉通距导致三个超级大陆拉张裂解的超级热地幔柱活动中心,在第三拉张阶段时可能最近,第一拉张阶段时也较近,但在第二拉张阶段时相对较远。     

Body-waveforms and source parameters of some moderate-sized earthquakes near North Island, New Zealand

P-wave first motion and synthetic seismogram analysis of P- and SH-waveforms recorded at teleseismic distances on the WWSSN are used to estimate source parameters of seven of the largest earthquakes (6.1 ≤ m_b ≤ 6.3) that occurred in the vicinity of North Island, New Zealand since 1965. The source parameters of three other (m_b ≥ 6.1) events determined outside of this study are included and considered in the final analysis. Four of the earthquakes occurred at shallow depths (< 20 km), of which three were located within and to the north of North Island. Two of the shallow events show strike-slip and normal focal mechanisms with T-axes oriented in a manner consistent with their location in an area of known back-arc extension. One of the shallow events occurred in northern South Island and shows a reverse-type mechanism indicating horizontal contraction of the crust in an easterly azimuth. Six events occurred at intermediate depths (h = 39 to 195 km) of which five exhibit thrust mechanisms with T-axes consistently oriented near vertical. In the light of previously published plate tectonic models, the near vertical orientation of T-axes of the intermediate-depth events may be used to infer that the southern Kermadec plate boundary immediately north of North Island is not strongly coupled, and hence, not likely capable of producing great earthquakes. A similar inference cannot be made for the section of the Hikurangi Margin adjacent to North Island since the intermediate-depth events considered in this study lie to the north of this segment of the plate boundary.     

太平洋古陆的破碎消亡与中新生代华北克拉通板内构造岩浆活化

回顾了太平洋撞击成因假说的主要内容, 重点围绕太平洋古陆消亡的内外动力学机制进行讨论, 认为P/ T之交发生在太平洋地区的撞击事件撞裂了岩石圈板块, 导致岩石圈下的地幔对流方式发生重大转折.在这种全新的对流方式驱动下, 太平洋古陆板块伴随新生洋壳板块的俯冲和碰撞而逐渐消亡并拼接到环太平洋大陆边缘, 与此同时, 环太平洋构造域开始形成.这种以太平洋构造域为中心的深部构造热体制深刻地影响了环太平洋大陆板块在中新生代的构造岩浆活动.      

Measurements of beach pebble attrition in Palliser Bay, southern North Island, New Zealand

The beaches of Palliser Bay, exposed to oceanic waves from the south, are composed of mixed sand and gravel derived from mountain ranges of greywacke and argillite that rise above both sides of the bay. Beach gravel tracer experiments, each using up to 75 tonnes of limestone pebbles, were conducted at three sites. Increase in the roundness of these tracer pebbles at each site during the year of observation was compared with the increase in roundness and rate of weight loss of limestone and greywacke pebbles in a laboratory tumbler. The comparison enabled estimation of the rate of attrition of the natural beach pebbles. A weight loss of 41% per year was found for pebbles at the most exposed site and 15 and 7% at the other two sites.     

Relative teleseismic travel-time residuals, North Island, New Zealand, and their relation to upper-mantle structure

Relative travel-time residuals computed from clear P-wave arrivals at fourteen seismograph stations in the North Island, New Zealand, from five deep-focus events in the Banda Sea region, show large spatial variations of up to 3 sec. The variations can be explained by higher than normal velocities in the oceanic lithosphere which is underthrust to depths of 350 km beneath the North Island. After correction for crustal structure, the residuals imply an average P-wave velocity about 11% higher than in the surrounding mantle. The lack of suitable source events at azimuths other than northwest prevents a more detailed investigation by this means.     

The volatile component of quaternary ignimbrite magmas from the North Island,New Zealand

Ignimbrites from the central North Island consist mainly of glass or its devitrified product (70–95%); their phenocryst mineralogy is varied and includes plag., hyp., ti-mag., ilm., aug., hblende, biot., san., qtz, ol., with accessory apatite, zircon and pyrrhotite. The Fe-Mg minerals can be used to divide the ignimbrites into four groups with hyp.+aug. reflecting high quench temperatures and biot.+hblende +hyp.+aug., low quench temperatures. Oxygen fugacities lie above the QMF buffer curve and even in ignimbrites with low crystal contents the solid phases apparently buffered fO₂. Some ignimbrites contain the assemblage actinolite, gedrite, magnetite and hematite, reflecting post-eruption oxidation. The mineralogy also allows estimation of using pyrrhotite and thence , . The assemblage biotite-sanidine can be used to estimate and thence . Water fugacity is calculated in a variety of ways using both biotite and hornblende as well as the combining reaction . It is high and approaches P _total in most ignimbrites (~4kb) but is lower in unwelded pumice breccias. Comparison of temperature estimates using mineral geothermometers for the various phenocryst phases suggests that the ignimbrite magmas showed temperature differences of 60–100 °C and pressure differences of several kilobars. Individual magma chambers therefore, would have extended over several kilometres vertically. The chemical potential of water may have been constant through the magma.     

Petrological and trace element geochemical features of the Okete Volcanics,western North Island,New Zealand

The Okete Volcanics form small volume monogenetic volcanoes situated around the flanks of larger tholeiitic cones of the Plio-Pleistocene Alexandra Volcanics, in the back-arc tectonic environment of western North Island, New Zealand. The lavas and tuffs of the Okete Volcanics have compositions which include basanites, alkali olivine basalts, olivine tholeiites, and hawaiites. Most rocks have Mg numbers >66, >250 p.p.m. Ni, >500 p.p.m. Cr, and often contain ultramafic xenoliths, which indicate that they are very close to being primary magmas. The Okete Volcanics show geochemical trends, from basanite to hawaiite, of progressive depletion of both compatible and incompatible trace elements, progressive increase in Al₂O₃, and heavy REE and Y enrichment with crossingover REE patterns in the hawaiites. These geochemical trends can be accounted for by varying degrees of partial melting of a light REE enriched garnet peridotite with subsequent modification of the melts near source or during ascent by fractional crystallization of olivine and minor clinopyroxene. Mass balance calculations cannot quantitatively constrain the degree of partial melting or fractional crystallization, but nevertheless indicate that the Okete alkali olivine basalts, olivine tholeiites, and hawaiites have been derived by successively larger degrees of partial melting relative to basanites, and have also been progressively more modified by fractional crystallization than have the basanites. Sources of the alkalic melts lay at depths corresponding to >20 kb, and most of the ultramafic xenoliths, apart from some which may be cognate cumulates, are unrelated to the magmas that brought them to the surface. Magmas have changed in composition with time from older smaller-volume volcanoes of basanite or alkali olivine basalt compositions, to younger and more voluminous volcanoes which contain hawaiites. The geochemical trends shown by the Okete Volcanics and their spatial association with voluminous tholeiitic volcanism, are features which are different from those observed elsewhere in the Pliocene to Recent basaltic fields of northern North Island, and may be related to their unique tectonic setting, situated in a distinct structural domain.     

Holocene coastal evolution and uplift mechanisms of the northeastern Raukumara Peninsula,North Island,New Zealand

The coastal geomorphology of the northeastern Raukumara Peninsula, New Zealand, is examined with the aim of determining the mechanisms of Holocene coastal uplift. Elevation and coverbed stratigraphic data from previously interpreted coseismic marine terraces at Horoera and Waipapa indicate that, despite the surface morphology, there is no evidence that these terraces are of marine or coseismic origin. Early Holocene transgressive marine deposits at Hicks Bay indicate significant differences between the thickness of preserved intertidal infill sediments and the amount of space created by eustatic sea level rise, therefore uplift did occur during early Holocene evolution of the estuary. The palaeoecology and stratigraphy of the sequence shows no evidence of sudden land elevation changes. Beach ridge sequences at Te Araroa slope gradually toward the present day coast with no evidence of coseismic steps. The evolution of the beach ridges was probably controlled by sediment supply in the context of a background continuous uplift rate. No individual dataset uniquely resolves the uplift mechanism. However, from the integration of all evidence we conclude that Holocene coastal uplift of this region has been driven by a gradual, aseismic mechanism. An important implication of this is that tectonic uplift mechanisms do vary along the East Coast of the North Island. This contrasts with conclusions of previous studies, which have inferred Holocene coastal uplift along the length of the margin was achieved by coseismic events. This is the first global example of aseismic processes accommodating uplift at rates of >1 mm yr⁻¹ adjacent to a subduction zone and it provides a valuable comparison to subduction zones dominated by great earthquakes.     

Trace and minor element geochemistry of the rhyolitic volcanic rocks,Central North Island,New Zealand

Analytical data are presented for the following elements: Cs, Rb, Ba, K, Sr, Ca, Na, Fe, Mg, Cu, Co, Ni, Li, Sc, V, Cr, Ga, Al, Si, La, Y, and Zr. Eight samples were analysed by the spark source method for rare earths, Tl, Pb, Hf, Sn, Nb, Mo, Bi, and In. In addition to data on rhyolitic volcanics, a small number of intermediate volcanics and eugeosynclinal sediments were analysed for comparative purposes. The following features are shown by the trace element data:

1. The rhyolitic rocks have consistently lower concentrations of most trace and minor elements when compared with recent estimates of average concentrations in granites. None of the criteria for strong fractionation (e.g. low K/Rb, Ba/Rb and K/Cs ratios) are present.
2. The data do not indicate any systematic differences between the rhyolitic lavas and ignimbrites, although the very young rhyolitic pumices are consistently more “basic” in their element concentrations compared to the other rhyolitic analyses.
3. The residual glasses (and devitrified matrices) are depleted, relative to the total rock compositions, in Fe, Mg, Ca, Sr, V, Sc, and Al, and enriched in Cs, Rb, K, Ba, and Si. Zr is depleted in the residual glasses separated from rhyolites, but not in the andesitic residual matrices.
4. The rare earth fractionation patterns of the rhyolitic and andesitic extrusives are very similar, being intermediate between chondritic and sedimentary patterns i.e., there is no evidence of strong fractionation. The rhyolitic patterns also indicate a slight Eu depletion.
5. Comparable trace and minor element behaviour (with the possible exception of Zr) seems to exist through the rhyolite-andesite compositional range. This is supported by the whole rock-residual liquid trends for the various elements studied, which broadly coincide with the observed whole rock trends, both through the rhyolitic-andesitic compositonal range, and within the rhyolitic compositional range.

The data are finally discussed in the light of the possible origin of the rhyolitic magmas. It is believed that the analytical data presented are qualitatively consistent with the recently proposed idea that the magmas are derived by partial fusion of the associated Triassic-Jurassic eugeosynclinal greywacke-argillite sedimentary sequence.     

Palaeoenvironmental reconstruction from the last interglacial using phytolith analysis,southeastern North Island,New Zealand

Phytoliths extracted from loess resting conformably on lacustrine sediments in southeastern North Island, New Zealand, provide a nearly continuous vegetation–climate record spanning the time period from the last interglacial (Marine Oxygen Isotope Stage 5) to the present. Phytoliths reveal changes in vegetation patterns following changes in climate. Correlation between tree–shrub phytolith fluctuations and the SPECMAP oxygen isotope curve, between Marine Oxygen Isotope Stages 1 and 5, suggest that changes in the ratio of arboreal to non‐arboreal phytoliths directly result from changes in climate. These data generally support the existing pollen and diatom record. This study confirms the usefulness of phytolith fossils for providing environmental and climate information from the Quaternary sedimentary record, especially in cases where the deposits contain no fossil pollen or diatoms. Copyright © 2000 John Wiley & Sons, Ltd.     

Frequency-magnitude,depth, and time relations for earthquakes in an island arc: North Island,New Zealand

Analysis of over 1400 earthquakes in the North Island of New Zealand from 1955 to 1969, comprising all shocks with m_l ? 4.3 for shallow, and M_L ? 4.0 for deep events, reveals several empirical relationships between the depth and the equivalent radius of the area occupied by shocks, the number and density of the shocks, and the coefficient b and the maximum magnitude. The coefficient b increases linearly with depth from 1.0 for shallow earthquakes to 1.4 for those at a depth of 120 km, and then decreases to 0.75 at 300—350 km. The variation with depth shows clear inverse correlation with the distribution of maximum stress along the downgoing slab, calculated for several slab models by Smith and Toksöz. Similarly, the maximum magnitude at different depths correlates distinctly with the distribution of the principal stress. Time variations of the coefficient b and the rate of earthquake occurrence, for both shallow and deep earthquakes, have an oscillatory character, with a period of 7–8 years. These variations also imply that shallow and deep seismicity are mutually dependent.     

Very Low-grade Metamorphism in Basement Greywacke Terranes of the Northern and Central North Island, New Zealand

Metamorphism in the late Permian to early Cretaceous North Island basement greywackes has been investigated using petrography and clay mineral crystallinity. Several terranes are represented in the North Island greywackes and the study area includes Murihiku, Manaia Hill, Bay of Islands and Omahuta terranes and the Mélange Zone. Very low-grade metamorphic events in the greywackes have produced mineral assemblages of zeolite to pumpellyite-actinolite greywacke facies. Zeolite facies greywackes are characterized by the assemblage Zeo (Lmt, Anl, Hul)+Qtz±Ab±Cal± Chl±I±I/S* observed in the entire Murihiku terrane and in the eastern part of the Bay of Islands terrane and the Mélange Zone. The entire Manaia Hill, most of the Bay of Islands, the eastern area of the Omahuta terranes and the central part of the Mélange Zone are at prehnite-pumpellyite facies with mineral assemblages of Prh+Qtz+Chl+Pmp+Ab+± Ill±Cal±Lmt. Pumpellyite-actinolite facies with the mineral assemblage of Pmp±Act+Qtz+Ab+Chl±Ep±Ill±Cal±Chl occurs in the western part of the Mélange Zone and the Omahuta terrane.

Illite (IC) and chlorite (ChC) crystallinity values of greywackes are very similar and range from diagenetic zone to anchizone. Metamorphic conditions indicated by the IC and ChC and mineral facies are in excellent agreement and correlate as follows: crystallinity diagenetic-zone with the zeolite mineral facies, crystallinity lower anchizone with prehnite-pumpellyite mineral facies and crystallinity upper anchizone with pumpellyite-actinolite mineral facies. The general increase in the metamorphic grade from east to west, except in Murihiku terrane, is compatible with the sequence of accretion expected in a subduction environment.     

Rhodolith-bearing limestones as transgressive marker beds: fossil and modern examples from North Island, New Zealand

Rhodoliths are nodular structures composed mainly of the superimposed thalli of calcareous red algae. Because their development is controlled by an array of ecological parameters, rhodoliths are a valuable source of palaeoenvironmental information. However, despite their common use in palaeoecological reconstructions, the stratigraphic significance of rhodolith accumulations seldom has been addressed in detail. In a study of Cenozoic rhodolith‐bearing deposits from the North Island of New Zealand, rhodolithic units, usually of limited lateral extent, typically occur above major unconformities at the base of deepening upwards successions. Two types of transgressive rhodolith‐bearing deposits may be distinguished on the basis of texture and rhodolith internal structure: (i) type A deposits are clast‐supported rhodolithic rudstones containing abundant pebbles and cobbles reworked from the substrate, and are characterized by rhodoliths with a compact concentric to columnar internal structure and a high nucleus to algal cover ratio; (ii) type B deposits are rhodolithic floatstones with a matrix usually consisting of bryozoan fragments, benthic foraminifera and echinoid fragments or terrigenous silty fine sand. The rhodoliths of type B units usually have a loose internal framework with irregular to branched crusts. The two contrasting rhodolith‐bearing units are interpreted as characteristic facies of transgressive systems tract deposits, analogous to shell concentrations formed under conditions of low net sedimentation. Type A deposits are correlated with relatively high‐energy settings and/or narrow submerged palaeotopographic lows, whereas type B deposits are interpreted as forming in lower‐energy settings. The association between transgression and development of rhodolithic facies is confirmed by observations of a modern rhodolith production site at Whangaparaoa Peninsula in North Island, where algal nodules grow above a ravinement surface cut during the Holocene sea‐level rise, and also by a review of published fossil examples, many of which show stratigraphic and compositional attributes analogous to those of the New Zealand occurrences. The review indicates that transgressive rhodolith accumulations develop more commonly in, but are not restricted to, non‐tropical settings. It is suggested that a combination of factors, such as low net sedimentary input, nature of the substrate, sea‐level rise and inherited physiography contribute to determine the relationship between rhodolith‐bearing deposits and transgressive settings.     

Trace element geochemistry of the rhyolitic volcanic rocks,Central North Island,New Zealand. Phenocryst data

Data are presented for K, Ba, Sr, Rb, Li, Ga, Mg, Mn, and Fe for twelve rhyolitic plagioclases (An₂₈-An₄₆), one dacitic (An₅₃), and three andesitic plagioclases (An₆₈-An₈₁). Additional data are presented for Ga, Gr, V, Ni, Co, Sc, Y, La, Sr, and Ba for two augites, nine hypersthenes, and five hornblendes separated from the same rocks. Distribution factors have been calculated, using these data, and previously published results for coexisting groundmass compositions (=liquids).The plagioclases show a positive correlation between, and a progressive increase in K and Ba (range 0.09–0.58% and 61–610 p.p.m. respectively) with increasing Ab-content. Sr (range 465–880 p.p.m.) shows a well defined maximum between An₄₀-An₅₅. The plagioclases have extremely high K/Rb ratios (mostly > 1,000).This volcanic series is characterised by relatively Mg-rich pyroxenes and hornblendes. The augites contain higher Sc, Cr, Y, Sr, and Y relative to their coexisting hypersthenes, while the hornblendes exhibit higher Sc, V, Ba, Sr, Y, and La relative to coexisting hypersthenes. Very marked differences in concentrations of these elements exist between the rhyolitic and andesitic ferromagnesian phenocrysts. There is also evidence of a systematic distribution of Sc, V, Cr, Y, Co, and Ni between coexisting hypersthenes and hornblendes, and between these minerals and their coexisting whole rock and groundmass compositions.The data are discussed from a petrological viewpoint, as they are interpreted to indicate that the phenocrysts crystallised in the magmas in which they are found, and are not xenocrystic. No evidence of hybridisation or contamination, subsequent to the onset of crystallisation, is found.