title: Crustal and mantle influences and U–Th–Ra disequilibrium in andesitic lavas of Ngauruhoe volcano, New Zealand

authors: Price, Richard C.

subjects: andesite

abstract: The andesitic volcano Ngauruhoe, which is located within the Tongariro Volcanic Complex at the southern end of the Taupo Volcanic Zone in North Island, New Zealand, has been constructed over the past 5 ka and last erupted in 1975. Nearby Ruapehu volcano has a much longer eruptive history extending back beyond 230 ka B.P. The magmas erupted at both volcanoes have been predominantly medium-K basaltic andesites and andesites, which evolved through polybaric crystal fractionation and assimilation processes that took place within complex, dispersed magmatic storage systems. Despite their close spatial proximity, the two volcanoes show geochemical contrasts suggesting that in each case both the mantle-derived parental magmas and the crustal assimilants were different.
Variations in major and trace element data for Ngauruhoe lavas indicate control by crystal fractionation and assimilation (AFC) but the data are difficult to reconcile with derivation from a single batch of compositionally unique, mantle-derived parental basalt. Geochemical variation can be approximated by generalised AFC models using average basement or crustal xenolith compositions but precise mathematical modelling is limited because of the sensitivity of models to the selection of a particular parental composition and the difficulty of determining the exact nature of the crustal assimilant compositions that might have been involved in each specific case.
U–Th isotopic data for Ngauruhoe volcanic rocks show disequilibrium that defines a positively inclined array lying to the right of the equiline on a ²³⁰Th/²³²Th versus ²³⁸U/²³² Th diagram. U-series data for Ngauruhoe and post 1945 AD Ruapehu eruptives show similar patterns of disequilibrium but the Ngauruhoe data define an array with a different slope and a different intercept on the equiline. ²³⁰Th/²³⁸ U ratios in Ngauruhoe lavas range from close to secular equilibrium (0.979) to 0.864 with the youngest (post 1870 AD) eruptives showing the least disequilibrium. ²³⁰Th/²³⁸U and ⁸⁷Sr/⁸⁶Sr ratios are correlated; the samples with the highest ⁸⁷Sr/⁸⁶Sr values show the least disequilibrium. There is also a correlation between eruption age and Nd and Pb isotopic composition; the oldest samples tend to show the least radiogenic isotopic ratios. These data, in combination with major and trace element abundance data indicate that Ngauruhoe U–Th disequilibrium is determined by variations in parental magmatic compositions and AFC. Unlike the mature, long-lived Ruapehu system, Ngauruhoe's eruptive cycles can be directly connected to periodic magma recharge from the lower crust and mantle. For Ngauruhoe eruptives, ²²⁶Ra/²³⁰Th varies from 1.218 to 1.492. This compares with a range of 0.972 to 1.186 for Ruapehu lavas erupted between 1945 and 1996. Reconciling the ²²⁶Ra/²³⁰Th data with decay during fractional crystallisation requires relatively low rates of fractionation (1–5 × 10⁻⁵/year) and short (1000–2000 years) fractionation times.

text: Crustal and mantle influences and U–Th–Ra disequilibrium in andesitic lavas of Ngauruhoe volcano, New Zealand The andesitic volcano Ngauruhoe, which is located within the Tongariro Volcanic Complex at the southern end of the Taupo Volcanic Zone in North Island, New Zealand, has been constructed over the past 5 ka and last erupted in 1975. Nearby Ruapehu volcano has a much longer eruptive history extending back beyond 230 ka B.P. The magmas erupted at both volcanoes have been predominantly medium-K basaltic andesites and andesites, which evolved through polybaric crystal fractionation and assimilation processes that took place within complex, dispersed magmatic storage systems. Despite their close spatial proximity, the two volcanoes show geochemical contrasts suggesting that in each case both the mantle-derived parental magmas and the crustal assimilants were different.
Variations in major and trace element data for Ngauruhoe lavas indicate control by crystal fractionation and assimilation (AFC) but the data are difficult to reconcile with derivation from a single batch of compositionally unique, mantle-derived parental basalt. Geochemical variation can be approximated by generalised AFC models using average basement or crustal xenolith compositions but precise mathematical modelling is limited because of the sensitivity of models to the selection of a particular parental composition and the difficulty of determining the exact nature of the crustal assimilant compositions that might have been involved in each specific case.
U–Th isotopic data for Ngauruhoe volcanic rocks show disequilibrium that defines a positively inclined array lying to the right of the equiline on a ²³⁰Th/²³²Th versus ²³⁸U/²³² Th diagram. U-series data for Ngauruhoe and post 1945 AD Ruapehu eruptives show similar patterns of disequilibrium but the Ngauruhoe data define an array with a different slope and a different intercept on the equiline. ²³⁰Th/²³⁸ U ratios in Ngauruhoe lavas range from close to secular equilibrium (0.979) to 0.864 with the youngest (post 1870 AD) eruptives showing the least disequilibrium. ²³⁰Th/²³⁸U and ⁸⁷Sr/⁸⁶Sr ratios are correlated; the samples with the highest ⁸⁷Sr/⁸⁶Sr values show the least disequilibrium. There is also a correlation between eruption age and Nd and Pb isotopic composition; the oldest samples tend to show the least radiogenic isotopic ratios. These data, in combination with major and trace element abundance data indicate that Ngauruhoe U–Th disequilibrium is determined by variations in parental magmatic compositions and AFC. Unlike the mature, long-lived Ruapehu system, Ngauruhoe's eruptive cycles can be directly connected to periodic magma recharge from the lower crust and mantle. For Ngauruhoe eruptives, ²²⁶Ra/²³⁰Th varies from 1.218 to 1.492. This compares with a range of 0.972 to 1.186 for Ruapehu lavas erupted between 1945 and 1996. Reconciling the ²²⁶Ra/²³⁰Th data with decay during fractional crystallisation requires relatively low rates of fractionation (1–5 × 10⁻⁵/year) and short (1000–2000 years) fractionation times.

year: 2010

title: Evolution of the Giant Foresets Formation, northern Taranaki Basin, New Zealand

authors: Hansen, Rochelle J.

subjects: Taranaki Basin

abstract: Plio-Pleistocene aggradation and progradation has resulted in the rapid outbuilding of the continental shelf margin, northern Taranaki Basin. Seismic reflection profiles reveal that this outbuilding is characterised by bold clinoforms which offlap in a basinward direction. This stacked succession of clinoforms, collectively termed the Giant Foresets Formation, obtains thicknesses of over 2 km in places, and has had a significant effect on the thermal regime of the region. This integrated study was initiated to document the Late Neogene evolution of this formation, and thereby gain insights on sedimentary distribution patterns, timing of sedimentation, and controls on progradation and aggradation. Latest Miocene extension in the northern Taranaki Basin, related to rotation of the Hikurangi subduction zone, greatly influenced sedimentation patterns in the Pliocene. Palinspastic reconstruction shows that initial extension of the Northern Graben occurred before Giant Foresets Formation sedimentation began. Sediment, sourced from erosion to the east, was preferentially funneled into the newly created Northern Graben during the late Miocene and early Pliocene, while areas to the north and west underwent a period of sediment starvation. During the late Pliocene, and into the Pleistocene, sediment accumulation outpaced graben extension, and by the end of the Mangapanian, the graben was overtopped. During this period, the progradational front associated with the outbuilding of the continental shelf-slope margin advanced northwards. Throughout the Nukumaruan, continuing to the present day, shelf migration was extremely rapid. While at least seven cyclical sea level changes, with an approximate periodicity of 400 ka (fourth-order) have been identified, overall, depths shallowed from dominantly bathyal, to dominantly shelfal.

text: Evolution of the Giant Foresets Formation, northern Taranaki Basin, New Zealand Plio-Pleistocene aggradation and progradation has resulted in the rapid outbuilding of the continental shelf margin, northern Taranaki Basin. Seismic reflection profiles reveal that this outbuilding is characterised by bold clinoforms which offlap in a basinward direction. This stacked succession of clinoforms, collectively termed the Giant Foresets Formation, obtains thicknesses of over 2 km in places, and has had a significant effect on the thermal regime of the region. This integrated study was initiated to document the Late Neogene evolution of this formation, and thereby gain insights on sedimentary distribution patterns, timing of sedimentation, and controls on progradation and aggradation. Latest Miocene extension in the northern Taranaki Basin, related to rotation of the Hikurangi subduction zone, greatly influenced sedimentation patterns in the Pliocene. Palinspastic reconstruction shows that initial extension of the Northern Graben occurred before Giant Foresets Formation sedimentation began. Sediment, sourced from erosion to the east, was preferentially funneled into the newly created Northern Graben during the late Miocene and early Pliocene, while areas to the north and west underwent a period of sediment starvation. During the late Pliocene, and into the Pleistocene, sediment accumulation outpaced graben extension, and by the end of the Mangapanian, the graben was overtopped. During this period, the progradational front associated with the outbuilding of the continental shelf-slope margin advanced northwards. Throughout the Nukumaruan, continuing to the present day, shelf migration was extremely rapid. While at least seven cyclical sea level changes, with an approximate periodicity of 400 ka (fourth-order) have been identified, overall, depths shallowed from dominantly bathyal, to dominantly shelfal.

year: 2002

title: Adam Porter: a miner who became a ‘self-made man’

authors: Hart, Philip

abstract: Adam Porter won the accolade of being a ‘self-made man’ because of rising from humble beginnings in Scotland. After arriving in New Zealand, aged 12, and working hard for some years, he joined the South Island gold rushes from 1861 onwards, sometimes as a miner and sometimes as an investor, storekeeper and publican. Even as a young man he was involved in local politics. After settling at Thames after the goldfield opened there, he concentrated on prospecting in Ohinemuri and in promoting the interests of the mining industry generally. From 1875 onwards he would be a director of many mining companies, and would encourage the prospecting of new districts, especially in his capacity as an Ohinemuri representative on the county council. Amongst his many policies designed to benefit the community was the promotion of education, including secondary education. By the late 1870s he was based in Auckland.
From 1878 onwards he claimed to know that gold was to be found at Te Aroha, and urged the government to acquire Maori land there for Pakeha farmers and prospectors. In mid-1880 he arranged for a government-subsidized prospecting party to examine the mountain under the leadership of Hone Werahiko, and liaised with both the warden and the government on the latter’s behalf, on occasions implying that he had shared in the discovery of gold. He may have tried to obtain control over the new find, but despite this he remained Werahiko’s agent and would be the executor of his estate. As well as investing in the Te Aroha and Waiorongomai fields, he was involved in the development of mines throughout Hauraki in the 1880s. 
Over time he took an increasingly leading role in promoting the mining industry, as for example a member of the Thames Drainage Board and of the Auckland Chamber of Commerce. For many years he attempted to obtain government assistance for mining, and was also interested in new mining technology. In his last years he was a leading member of the Auckland business community, investing in non-mining ventures and being a good employer (though critics disagreed). He also became involved in local government issues and the temperance movement before speaking his mind on national issues, finally standing for parliament as an independent-minded supporter of his old friend from his West Coast days, Richard John Seddon. 
Harsh working conditions on the West Coast led to poor health and an early death. He left his family a comfortably legacy, and was remembered as having a genial, kindly, and witty personality but also having a good sense of his own importance. He seized opportunities, sometimes in a manner that offended others, but tried to benefit not only himself but also the wider community.

text: Adam Porter: a miner who became a ‘self-made man’ Adam Porter won the accolade of being a ‘self-made man’ because of rising from humble beginnings in Scotland. After arriving in New Zealand, aged 12, and working hard for some years, he joined the South Island gold rushes from 1861 onwards, sometimes as a miner and sometimes as an investor, storekeeper and publican. Even as a young man he was involved in local politics. After settling at Thames after the goldfield opened there, he concentrated on prospecting in Ohinemuri and in promoting the interests of the mining industry generally. From 1875 onwards he would be a director of many mining companies, and would encourage the prospecting of new districts, especially in his capacity as an Ohinemuri representative on the county council. Amongst his many policies designed to benefit the community was the promotion of education, including secondary education. By the late 1870s he was based in Auckland.
From 1878 onwards he claimed to know that gold was to be found at Te Aroha, and urged the government to acquire Maori land there for Pakeha farmers and prospectors. In mid-1880 he arranged for a government-subsidized prospecting party to examine the mountain under the leadership of Hone Werahiko, and liaised with both the warden and the government on the latter’s behalf, on occasions implying that he had shared in the discovery of gold. He may have tried to obtain control over the new find, but despite this he remained Werahiko’s agent and would be the executor of his estate. As well as investing in the Te Aroha and Waiorongomai fields, he was involved in the development of mines throughout Hauraki in the 1880s. 
Over time he took an increasingly leading role in promoting the mining industry, as for example a member of the Thames Drainage Board and of the Auckland Chamber of Commerce. For many years he attempted to obtain government assistance for mining, and was also interested in new mining technology. In his last years he was a leading member of the Auckland business community, investing in non-mining ventures and being a good employer (though critics disagreed). He also became involved in local government issues and the temperance movement before speaking his mind on national issues, finally standing for parliament as an independent-minded supporter of his old friend from his West Coast days, Richard John Seddon. 
Harsh working conditions on the West Coast led to poor health and an early death. He left his family a comfortably legacy, and was remembered as having a genial, kindly, and witty personality but also having a good sense of his own importance. He seized opportunities, sometimes in a manner that offended others, but tried to benefit not only himself but also the wider community.

year: 2016