The Reporoa Caldera, Taupo Volcanic Zone: source of the Kaingaroa Ignimbrites

"Sandy black" ignimbrites within the Taupo Volcanic Zone, New Zealand, are often incorrectly correlated. One of these ignimbrites was considered to be the lateral equivalent of the Kaingaroa Ignimbrite, but an integrated study of field geology, lithic componentry, mineralogy, and geochemistry of juvenile pumices has shown the ignimbrite is a separate unit, here termed the Kawerau Ignimbrite, with a source in the southern part of the Okataina caldera complex. The ignimbrite comprises two subunits that locally grade into each other: a lower partially welded "sandy black" ignimbrite and an upper tan-buff pumice-poor ignimbrite. Mineralogy and whole-rock pumice geochemistry indicate significant compositional variation with two magma types: a high-silica rhyolite (type A) and a dacite (type B). Both magma types show anomalously high Zr, Zn, and Hf, with type-B pumice having similar values to weakly peralkaline rhyolites. The study highlights the importance of integrating all study methods before correlating units of apparent similar lithology.

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“…1, 2). Subsequent authors followed this interpretation (e.g., Bailey & Carr 1994;Nairn et al 1994). Kohn (pers.…”

Section: Kawerau Ignimbritementioning

confidence: 97%

Kawerau Ignimbrite: A 0.24 Ma ignimbrite erupted from the Okataina caldera complex, Taupo Volcanic Zone, New Zealand

Beresford

Cole

2000

New Zealand Journal of Geology and Geophysics

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“…6) Healy et al 1964;Graham et al 1995;Houghton et al 1987). Caldera boundaries are after Nairn et al (1994). Boundary of the young TVZ (<0.34 Ma) is modified after Wilson et al (1995) and Villamor & Berryman (2001 1995).…”

Section: B) Okataina Domain (Rhyolite Dominant)mentioning

confidence: 99%

Extensional fault kinematics within the Taupo Volcanic Zone, New Zealand: Soft‐linked segmentation of a continental rift system

Rowland

Sibson

2001

New Zealand Journal of Geology and Geophysics

150

130

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The Taupo Volcanic Zone (TVZ) is a young (<2 Ma) arc-backarc system in continental crust at the southern end of the Havre Trough characterised by intense volcanism and geothermal activity. An extensional fault belt (known locally as the Taupo Fault Belt) can be traced semicontinuously within the TVZ from Ruapehu in the southwest to the Bay of Plenty coast in the northeast. Contained within the belt are numerous active volcanic centres (andesitic, basaltic, rhyolitic) and geothermal systems. Kinematic studies demonstrate that normal faulting predominates within the fault belt, which we interpret as a distributed rift system (here termed the Ruaumoko Rift System), within which rift axes are locally defined by crudely symmetric dispositions of northwest-and southeast-dipping faults. The entire fault belt is partitioned along-strike into offset rift segments defined by association with discrete rift axes. Fault dips, where measurable, are generally steep (8 > 60°). Systematic fracture sets occur in all rock types in the TVZ and generally show a strong northeast-southwest trend, indicating a component of extensional strain distributed throughout the rock mass. Extension is predominantly orthogonal to the rift axes except in "accommodation zones" where interaction between offset rifting segments perturbs the stress field, locally enhancing permeability. The varying trends of extensional structures lie perpendicular to least principal stress trajectories in a locally heterogeneous stress field with normal dip-slip as the predominant fault mechanism.

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“…The Paeroa Fault is normal, strikes about 40 • and dips steeply to the west, with a minimum vertical offset of 450 m. Nairn and Hull (1986) indicate that the fault has offset ash layers from Recent volcanic eruptions in the TVZ during the last 1800 years. Nairn et al (1994) infer that movement on the Paeroa Fault began about 240,000 years ago, so the average rate of displacement across the fault zone is ∼2.5 m/1000 years. The fault movement has influenced surface manifestations, hydrology and hydrothermal alteration within the Te Kopia geothermal system .…”

Section: Geologymentioning

confidence: 99%

“…Bignall and Browne (1994) suggested that quartz crystals exposed in eastern parts of the Te Kopia thermal area had been uplifted at least 300 m since they formed, based on fluid inclusion analysis, presence of relict quartz-adularia-illite alteration mineral assemblage in nearby surface rocks, and +600 m vertical displacement on the Paeroa Fault. Nairn et al (1994) indicate that movement on the Paeroa Fault began about 240,000 years ago, while Bignall and Browne (1994) suggest that the Te Kopia geothermal system may have been active for as long as 120,000 years, so the hydrology of the system and its thermal regime would have changed greatly during this time.…”

Section: Hydrothermal Alterationmentioning

confidence: 99%

Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging

BIGNALL

2004

Geothermics

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Scanning electron microscopy-cathodoluminescence (SEM-CL) imaging of hydrothermal quartz exposed by weathering in the Te Kopia geothermal field (New Zealand) has revealed a history of crystal growth, dissolution, overprinting and fracturing that cannot be detected using other observational techniques (e.g. transmitted or reflected light microscopy, back-scattered electron imaging or secondary electron imaging). The crystals initially grew as CL-dark quartz, at least 350 m below their present location on the Paeroa Fault scarp, in a neutral pH, 215 ± 10 • C liquid reservoir (inferred from the analysis of primary liquid fluid inclusions: mean T h of 213 • C; 0.2-0.4 wt.% NaCl eq. ). Relict quartz-adularia-illite alteration occurs at the surface, in the vicinity of the quartz crystals, and in drillcores from the nearby TK-1 exploration well. Repeated movement on the Paeroa Fault uplifted pyroclastic rocks hosting the quartz crystals, but also provided pathways for "pulses" of hot fluids to move through the system. Quartz precipitation occurred at the edge of the crystals as the reservoir fluids cooled, as indicated by micron-scale alternating CL-dark/CL-bright quartz growth bands, which contain fluid inclusions with T h values of 210 ± 40 • C. Pressure fluctuations were the likely cause of dissolution, marked by corroded crystal edges, with subsequent precipitation of quartz into open space. SEM-CL imaging shows that the quartz crystals contain healed fractures, which trapped low salinity fluids with T h values of 201 ± 6 • C. Low-pH fluids in the near-surface * Corresponding author. G. Bignall et al. / Geothermics 33 (2004) 615-635 setting also rounded the quartz crystals, and coated them with kaolinite and CL-grey amorphous "silica residue".

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The Reporoa Caldera, Taupo Volcanic Zone: source of the Kaingaroa Ignimbrites

Cited by 73 publications

References 15 publications

Kawerau Ignimbrite: A 0.24 Ma ignimbrite erupted from the Okataina caldera complex, Taupo Volcanic Zone, New Zealand

Kawerau Ignimbrite: A 0.24 Ma ignimbrite erupted from the Okataina caldera complex, Taupo Volcanic Zone, New Zealand

Extensional fault kinematics within the Taupo Volcanic Zone, New Zealand: Soft‐linked segmentation of a continental rift system

Fluid-rock interaction processes in the Te Kopia geothermal field (New Zealand) revealed by SEM-CL imaging

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