title: Volcanic geology, geochemistry and geochronology of the Kapowai caldera complex, Coromandel volcanic zone, New Zealand

authors: Krippner, Stephen J.P.

abstract: Kapowai Caldera Complex (KCC) is a late Miocene (ca. 8.5-4.9 Ma) caldera volcano situated in the central Coromandel Volcanic Zone (CVZ), North Island, New Zealand. It is the largest (10 x 11 km diameter) onland caldera known within the CVZ, and is the source of the most widespread rhyolitic pyroclastic sequence (Wharepapa Eruptive Sequence; WES) within this zone. This study investigates the structural and petrological evolution of the KCC based upon detailed geological mapping, mineralogical, petrological and geochemical analysis, and K/Ar dating. 

KCC is defined by a combination of remote sensing imagery (Landsat-MSS and SAR), geophysical anomalies (magnetic and gravity) and geological facies. Intracaldera KCC structures identified include structural boundaries, faults and intrusives. Observed structural boundaries are high-angle normal faults with minimum displacements varying from 40 - 100m. Faults have NW, N and NE orientations, and extend laterally for ca. 1-8 km. Displacements are highly variable, but are typically concealed by thick pyroclastics and lava flows. The intracaldera succession overlies pre-caldera andesitic-dacitic formations (lava, tuff and breccia) and consists of a sequence of thick andesitic-rhyolitic pyroclastic deposits (ignimbrite and tuff), numerous rhyolitic lava domes, flows and dikes, and andesitic-dacitic lavas and dikes. Interbedded within this sequence are volcaniclastic to carbonaceous siltstones and sandstones. Several episodes of epithermal alteration and associated mineralisation are recorded at the KCC and there is significant control on the location of alteration zones by both caldera-related structures (structural boundaries, faults) and regional NE-trending faults. 

Two distinctive, genetically unrelated calc-alkaline magma suites of basaltic andesite to low silica andesite (56 - 64 wt.% SiO₂), and low to high silica rhyolite (71 - 78 wt.% SiO₂) were erupted. Both suites are characterised by enriched LILE relative to HFSE, and negative Nb anomalies consistent with a continental-arc subduction-related origin. REE abundances are similar in both pre-caldera and caldera-related andesitic-dacitic lavas, although between rhyolite lavas there are significant differences in HREE abundances relative to LREE. Andesitic-dacitic source magmas were generated within the mantle wedge and modified during migration to the base of the crust. Subsequent differentiation occurred by crystal fractionation, crustal assimilation, and hybridisation during migration to high-level magma reservoirs. Rhyolitic source magmas are suggested to be generated by partial melting of continental crust. Differentiation of parental magmas then occurred by crystal fractionation and crustal assimilation, during ascent and storage in high-level silicic magma chambers.

An intracaldera facies association defined at the KCC includes thick infilling proximal and medial pyroclastic facies, effusive facies, and minor lacustrine, hydrothermal alteration and mass wasting facies. Facies vary in geometry, distribution, thickness and lithology, but are all linked to active and passive caldera processes.
Exocaldera facies association includes caldera rim and distal pyroclastic subfacies.

Compared with most Taupo Volcanic Zone (TVZ) calderas, the KCC shows some similarities, but has smaller dimensions and erupted smaller magmatic volumes (ca.< 30 km³) over a longer time interval (ca. 3.6 Ma). These features imply differences in magma volumes, crustal thickness, and lower rates of subduction and extension within the late Miocene CVZ arc system compared to the modern TVZ system.

text: Volcanic geology, geochemistry and geochronology of the Kapowai caldera complex, Coromandel volcanic zone, New Zealand Kapowai Caldera Complex (KCC) is a late Miocene (ca. 8.5-4.9 Ma) caldera volcano situated in the central Coromandel Volcanic Zone (CVZ), North Island, New Zealand. It is the largest (10 x 11 km diameter) onland caldera known within the CVZ, and is the source of the most widespread rhyolitic pyroclastic sequence (Wharepapa Eruptive Sequence; WES) within this zone. This study investigates the structural and petrological evolution of the KCC based upon detailed geological mapping, mineralogical, petrological and geochemical analysis, and K/Ar dating. 

KCC is defined by a combination of remote sensing imagery (Landsat-MSS and SAR), geophysical anomalies (magnetic and gravity) and geological facies. Intracaldera KCC structures identified include structural boundaries, faults and intrusives. Observed structural boundaries are high-angle normal faults with minimum displacements varying from 40 - 100m. Faults have NW, N and NE orientations, and extend laterally for ca. 1-8 km. Displacements are highly variable, but are typically concealed by thick pyroclastics and lava flows. The intracaldera succession overlies pre-caldera andesitic-dacitic formations (lava, tuff and breccia) and consists of a sequence of thick andesitic-rhyolitic pyroclastic deposits (ignimbrite and tuff), numerous rhyolitic lava domes, flows and dikes, and andesitic-dacitic lavas and dikes. Interbedded within this sequence are volcaniclastic to carbonaceous siltstones and sandstones. Several episodes of epithermal alteration and associated mineralisation are recorded at the KCC and there is significant control on the location of alteration zones by both caldera-related structures (structural boundaries, faults) and regional NE-trending faults. 

Two distinctive, genetically unrelated calc-alkaline magma suites of basaltic andesite to low silica andesite (56 - 64 wt.% SiO₂), and low to high silica rhyolite (71 - 78 wt.% SiO₂) were erupted. Both suites are characterised by enriched LILE relative to HFSE, and negative Nb anomalies consistent with a continental-arc subduction-related origin. REE abundances are similar in both pre-caldera and caldera-related andesitic-dacitic lavas, although between rhyolite lavas there are significant differences in HREE abundances relative to LREE. Andesitic-dacitic source magmas were generated within the mantle wedge and modified during migration to the base of the crust. Subsequent differentiation occurred by crystal fractionation, crustal assimilation, and hybridisation during migration to high-level magma reservoirs. Rhyolitic source magmas are suggested to be generated by partial melting of continental crust. Differentiation of parental magmas then occurred by crystal fractionation and crustal assimilation, during ascent and storage in high-level silicic magma chambers.

An intracaldera facies association defined at the KCC includes thick infilling proximal and medial pyroclastic facies, effusive facies, and minor lacustrine, hydrothermal alteration and mass wasting facies. Facies vary in geometry, distribution, thickness and lithology, but are all linked to active and passive caldera processes.
Exocaldera facies association includes caldera rim and distal pyroclastic subfacies.

Compared with most Taupo Volcanic Zone (TVZ) calderas, the KCC shows some similarities, but has smaller dimensions and erupted smaller magmatic volumes (ca.< 30 km³) over a longer time interval (ca. 3.6 Ma). These features imply differences in magma volumes, crustal thickness, and lower rates of subduction and extension within the late Miocene CVZ arc system compared to the modern TVZ system.

year: 2000

title: Evaluation of the introduction history and genetic diversity of a serially introduced fish population in New Zealand

authors: Purcell, K. M.

subjects: invasive

abstract: Understanding the introduction history and the impact of founder events on invasive species is crucial to understanding the evolutionary mechanisms driving successful invasions. Recently, there has been increased discussion of the ‘‘paradox’’ of invasions, the high success of introduced populations that presumably have limited genetic diversity associated with founder events. The western mosquitofish Gambusia affinis is an ideal species for evaluating this paradox, because it has been widely introduced from its native range in central Texas, USA. This species was introduced to the North Island of New Zealand, circa 1930, and has since invaded aquatic habitats across the North Island. We conducted a microsatellite assay of populations from both the native and introduced range to verify the documented history of invasion and to assess the impact of serial introduction events on the genetic diversity of recently established New Zealand populations. The molecular data were consistent with the documented introduction history. In addition, we found sharp reductions in the allelic richness and the heterozygosity of the introduced populations relative to the original native populations, indicating the presence of founder effects. We also observed the development of strong genetic structure within the introduced range, which is absent within the native range. Finally, we applied approximate Bayesian computation to the introduction scenario to estimate the long-term effective population sizes for the sampled populations.

text: Evaluation of the introduction history and genetic diversity of a serially introduced fish population in New Zealand Understanding the introduction history and the impact of founder events on invasive species is crucial to understanding the evolutionary mechanisms driving successful invasions. Recently, there has been increased discussion of the ‘‘paradox’’ of invasions, the high success of introduced populations that presumably have limited genetic diversity associated with founder events. The western mosquitofish Gambusia affinis is an ideal species for evaluating this paradox, because it has been widely introduced from its native range in central Texas, USA. This species was introduced to the North Island of New Zealand, circa 1930, and has since invaded aquatic habitats across the North Island. We conducted a microsatellite assay of populations from both the native and introduced range to verify the documented history of invasion and to assess the impact of serial introduction events on the genetic diversity of recently established New Zealand populations. The molecular data were consistent with the documented introduction history. In addition, we found sharp reductions in the allelic richness and the heterozygosity of the introduced populations relative to the original native populations, indicating the presence of founder effects. We also observed the development of strong genetic structure within the introduced range, which is absent within the native range. Finally, we applied approximate Bayesian computation to the introduction scenario to estimate the long-term effective population sizes for the sampled populations.

year: 2012

title: New observations on the stratigraphy and radiocarbon dates at the Cross Creek site, Opito, Coromandel Peninsula

authors: Furey, Louise

subjects: New Zealand

abstract: This paper re-examines stratigraphy and radiocarbon dates at Cross Creek in Sarah's Gully. Three new radiocarbon dates are presented for Layer 9, the earliest, and previously undated, occupation. This investigation is part of a programme of archaeological work being carried out on the Coromandel Peninsula. Although there are several individual research projects underway, they have a common theme related to the Polynesian settlement period on the Coromandel Peninsula. The two seasons of excavation at Tairua are being written up by Matthew Campbell of CFG Heritage on behalf of Roger Green. Louise Furey, also CFG Heritage, is researching a thematic study on early sites for the Department of Conservation, Archaeological research in the Opito area includes documenting a pollen sequence for Opito under a grant obtained from the Green Foundation for Polynesian Research: Pam Chester, Louise Furey and Brenda Sewell are participants. In addition, positively identifying the Kaharoa Ash in the Opito-Sarah's Gully area is a priority.

text: New observations on the stratigraphy and radiocarbon dates at the Cross Creek site, Opito, Coromandel Peninsula This paper re-examines stratigraphy and radiocarbon dates at Cross Creek in Sarah's Gully. Three new radiocarbon dates are presented for Layer 9, the earliest, and previously undated, occupation. This investigation is part of a programme of archaeological work being carried out on the Coromandel Peninsula. Although there are several individual research projects underway, they have a common theme related to the Polynesian settlement period on the Coromandel Peninsula. The two seasons of excavation at Tairua are being written up by Matthew Campbell of CFG Heritage on behalf of Roger Green. Louise Furey, also CFG Heritage, is researching a thematic study on early sites for the Department of Conservation, Archaeological research in the Opito area includes documenting a pollen sequence for Opito under a grant obtained from the Green Foundation for Polynesian Research: Pam Chester, Louise Furey and Brenda Sewell are participants. In addition, positively identifying the Kaharoa Ash in the Opito-Sarah's Gully area is a priority.

year: 2008