title: The anatomy of an andesite volcano: a time-stratigraphic study of andesite petrogenesis and crustal evolution at Ruapehu Volcano, New Zealand

authors: Price, Richard C.

subjects: andesite

abstract: Ruapehu, New Zealand’s largest active andesite volcano, is located at the southern tip of the Taupo Volcanic Zone (TVZ), the main locus of subduction-related volcanism in the North Island. Geophysical data indicate that crustal thickness increases from <25 km within the TVZ to 40 km beneath Ruapehu. The volcano is built on a basement of Mesozoic meta-greywacke, and geophysical evidence together with xenoliths contained in lavas indicates that this is underlain by oceanic, meta-igneous lower crust. The present-day Ruapehu edifice has been constructed by a series of eruptive events that produced a succession of lava flow-dominated stratigraphic units. In order from oldest to youngest, these are the Te Herenga (250–180 ka), Wahianoa (160–115 ka), Mangawhero (55–45 ka and 20–30 ka), and Whakapapa (15–2 ka) Formations. The dominant rock types are plagioclase- and pyroxene-phyric basaltic andesite and andesite. Dacite also occurs but only one basalt flow has been identified. There have been progressive changes in the minor and trace element chemistry and isotopic composition of Ruapehu eruptive rocks over time. In comparison with rocks from younger formations, Te Herenga eruptive rocks have lower K₂O abundances and a relatively restricted range in major and trace element and Nd–Sr isotopic composition. Post-Te Herenga andesites and dacites define a Sr–Nd isotopic array that overlaps with the field for TVZ rhyolites and basalts, but Te Herenga Formation lavas and the Ruapehu basalt have higher ¹⁴³Nd/¹⁴⁴Nd ratios. The isotopic, and major and trace element composition of Te Herenga andesite can be replicated by models involving mixing of an intra-oceanic andesite with a crustal component derived from a meta-igneous composition. Post-Te Herenga andesites show considerable variation in major and trace element and Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr ranges from 0•7049 to 0•7060 and ¹⁴³Nd/¹⁴⁴Nd from 0•51264 to 0•51282). The range of compositions can be modeled by assimilation–fractional crystallization (AFC) involving meta-greywacke as the assimilant, closed-system fractionation, or by mixing of intra-oceanic andesite or basalt and a meta-greywacke crustal composition. Plagioclase and pyroxene compositions vary over wide ranges within single rocks and few of these have compositions consistent with equilibration with a melt having the composition of either the host-rock or groundmass. The ⁸⁷Sr/⁸⁶Sr compositions of plagioclase also vary significantly within single whole-rock samples. Glass inclusions and groundmasses of andesitic rocks all have dacitic or rhyolitic major and trace element compositions. The application of various mineral geothermometers and geobarometers indicates pre-eruption temperatures between 950 and 1190°C and pressures ranging from 1 to 0•2 GPa. These pressure estimates are consistent with those obtained from xenolith mineral assemblages and geophysical information. Plagioclase hygrometry and the paucity of amphibole are indications that melts were relatively dry (< 4 wt % H₂O). Magmas represented by Ruapehu andesites were dacitic or rhyolitic melts carrying complex crystal and lithic cargoes derived from the mantle and at least two crustal sources. They have evolved through a complex interplay between assimilation, crystal fractionation, crustal anatexis and magma mixing. Parental magmas were sourced in both the mantle and crust, but erupted compositions very strongly reflect modification by intracrustal processes. Geochemical variation in systematically sampled lava flow sequences is consistent with random tapping of a complex plumbing system in which magma has been stored on varying time scales within a plexus of dispersed reservoirs. Each magma batch is likely to have had a unique history with different sized magma storages evolving on varying time scales with a specific combination of AFC and mixing processes.

text: The anatomy of an andesite volcano: a time-stratigraphic study of andesite petrogenesis and crustal evolution at Ruapehu Volcano, New Zealand Ruapehu, New Zealand’s largest active andesite volcano, is located at the southern tip of the Taupo Volcanic Zone (TVZ), the main locus of subduction-related volcanism in the North Island. Geophysical data indicate that crustal thickness increases from <25 km within the TVZ to 40 km beneath Ruapehu. The volcano is built on a basement of Mesozoic meta-greywacke, and geophysical evidence together with xenoliths contained in lavas indicates that this is underlain by oceanic, meta-igneous lower crust. The present-day Ruapehu edifice has been constructed by a series of eruptive events that produced a succession of lava flow-dominated stratigraphic units. In order from oldest to youngest, these are the Te Herenga (250–180 ka), Wahianoa (160–115 ka), Mangawhero (55–45 ka and 20–30 ka), and Whakapapa (15–2 ka) Formations. The dominant rock types are plagioclase- and pyroxene-phyric basaltic andesite and andesite. Dacite also occurs but only one basalt flow has been identified. There have been progressive changes in the minor and trace element chemistry and isotopic composition of Ruapehu eruptive rocks over time. In comparison with rocks from younger formations, Te Herenga eruptive rocks have lower K₂O abundances and a relatively restricted range in major and trace element and Nd–Sr isotopic composition. Post-Te Herenga andesites and dacites define a Sr–Nd isotopic array that overlaps with the field for TVZ rhyolites and basalts, but Te Herenga Formation lavas and the Ruapehu basalt have higher ¹⁴³Nd/¹⁴⁴Nd ratios. The isotopic, and major and trace element composition of Te Herenga andesite can be replicated by models involving mixing of an intra-oceanic andesite with a crustal component derived from a meta-igneous composition. Post-Te Herenga andesites show considerable variation in major and trace element and Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr ranges from 0•7049 to 0•7060 and ¹⁴³Nd/¹⁴⁴Nd from 0•51264 to 0•51282). The range of compositions can be modeled by assimilation–fractional crystallization (AFC) involving meta-greywacke as the assimilant, closed-system fractionation, or by mixing of intra-oceanic andesite or basalt and a meta-greywacke crustal composition. Plagioclase and pyroxene compositions vary over wide ranges within single rocks and few of these have compositions consistent with equilibration with a melt having the composition of either the host-rock or groundmass. The ⁸⁷Sr/⁸⁶Sr compositions of plagioclase also vary significantly within single whole-rock samples. Glass inclusions and groundmasses of andesitic rocks all have dacitic or rhyolitic major and trace element compositions. The application of various mineral geothermometers and geobarometers indicates pre-eruption temperatures between 950 and 1190°C and pressures ranging from 1 to 0•2 GPa. These pressure estimates are consistent with those obtained from xenolith mineral assemblages and geophysical information. Plagioclase hygrometry and the paucity of amphibole are indications that melts were relatively dry (< 4 wt % H₂O). Magmas represented by Ruapehu andesites were dacitic or rhyolitic melts carrying complex crystal and lithic cargoes derived from the mantle and at least two crustal sources. They have evolved through a complex interplay between assimilation, crystal fractionation, crustal anatexis and magma mixing. Parental magmas were sourced in both the mantle and crust, but erupted compositions very strongly reflect modification by intracrustal processes. Geochemical variation in systematically sampled lava flow sequences is consistent with random tapping of a complex plumbing system in which magma has been stored on varying time scales within a plexus of dispersed reservoirs. Each magma batch is likely to have had a unique history with different sized magma storages evolving on varying time scales with a specific combination of AFC and mixing processes.

year: 2012

title: Ecology and conservation of Alseuosmia quercifolia (Alseuosmiaceae) in the Waikato region, New Zealand

authors: Merrett, M. F.

subjects: Alseuosmia

abstract: The ecology of Alseuosmia quercifolia, a small endemic shrub, was investigated, focussing on its habitat requirements, population dynamics, phenology and reproductive biology, and conservation status. This species occurs most commonly in lowland native forests of the Waikato region of the North Island (north of latitude 38°05'S), but is also found in scattered populations to North Cape. In the Waikato region it typically occupies shady, well-drained, south or south-east facing lower slopes of hills and ranges at altitudes below 400 m. Population structures show considerable variation amongst seven study sites in the Waikato region, with disjunct size classes a reflection of the presence and abundance of introduced browsing mammals. It is a relatively short-lived (less than 50 years), slow-growing species with a fleshy fruit adapted to bird dispersal, but seed dispersal now appears to be primarily by gravity. Flowering occurs early in spring and is synchronous at both individual and population levels, occurring over a 5-week period, with peak flowering during the second and third weeks. While all populations set seed, reproductive output can be negatively affected by persistent browse and by rain during peak flowering. This species is vulnerable because it is highly palatable to introduced mammals and all plants in a population are within browse height. It has relatively narrow habitat specificity, localised distribution, and limited potential to extend its range. We suggest it fulfils the requirements of the category "declining", using the most recent classification of threatened and uncommon plants of New Zealand.

text: Ecology and conservation of Alseuosmia quercifolia (Alseuosmiaceae) in the Waikato region, New Zealand The ecology of Alseuosmia quercifolia, a small endemic shrub, was investigated, focussing on its habitat requirements, population dynamics, phenology and reproductive biology, and conservation status. This species occurs most commonly in lowland native forests of the Waikato region of the North Island (north of latitude 38°05'S), but is also found in scattered populations to North Cape. In the Waikato region it typically occupies shady, well-drained, south or south-east facing lower slopes of hills and ranges at altitudes below 400 m. Population structures show considerable variation amongst seven study sites in the Waikato region, with disjunct size classes a reflection of the presence and abundance of introduced browsing mammals. It is a relatively short-lived (less than 50 years), slow-growing species with a fleshy fruit adapted to bird dispersal, but seed dispersal now appears to be primarily by gravity. Flowering occurs early in spring and is synchronous at both individual and population levels, occurring over a 5-week period, with peak flowering during the second and third weeks. While all populations set seed, reproductive output can be negatively affected by persistent browse and by rain during peak flowering. This species is vulnerable because it is highly palatable to introduced mammals and all plants in a population are within browse height. It has relatively narrow habitat specificity, localised distribution, and limited potential to extend its range. We suggest it fulfils the requirements of the category "declining", using the most recent classification of threatened and uncommon plants of New Zealand.

year: 2002-03-01

title: Contextualising social enterprise in New Zealand

authors: Grant, Suzanne Lisa Parker

subjects: business enterprise

abstract: Purpose – Social enterprise in New Zealand is still in its infancy, with no recognised framework to inform knowledge of current or future developments. In this exploratory paper, the aim is to consider four influences which are shaping the development of social enterprise in New Zealand.

Design/methodology/approach – A critical-appreciative lens utilising Habermas' concepts of the lifeworld and system informs the consideration of these influences.

Findings – Four distinct cultural and historical influences are proposed as contributing to the scope and “flavour” of social enterprise developing in New Zealand: socio-cultural norms, e.g. “Kiwi ingenuity”; the neoliberal reforms initiated by successive governments during the 1980s; Crown settlements in relation to breaches of the principles of 1840 Treaty of Waitangi; New Zealanders' as international citizens.

Originality/value – The paper shows how feedback and dialogue across the sectors, at local, national and international levels, is now required to determine how other scholars, practitioners and policy makers perceive this proposed initial framework.

text: Contextualising social enterprise in New Zealand Purpose – Social enterprise in New Zealand is still in its infancy, with no recognised framework to inform knowledge of current or future developments. In this exploratory paper, the aim is to consider four influences which are shaping the development of social enterprise in New Zealand.

Design/methodology/approach – A critical-appreciative lens utilising Habermas' concepts of the lifeworld and system informs the consideration of these influences.

Findings – Four distinct cultural and historical influences are proposed as contributing to the scope and “flavour” of social enterprise developing in New Zealand: socio-cultural norms, e.g. “Kiwi ingenuity”; the neoliberal reforms initiated by successive governments during the 1980s; Crown settlements in relation to breaches of the principles of 1840 Treaty of Waitangi; New Zealanders' as international citizens.

Originality/value – The paper shows how feedback and dialogue across the sectors, at local, national and international levels, is now required to determine how other scholars, practitioners and policy makers perceive this proposed initial framework.

year: 2008