Volcanic geology, geochemistry and geochronology of the Kapowai caldera complex, Coromandel volcanic zone, New Zealand
Permanent link to Research Commons versionhttps://hdl.handle.net/10289/14971
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.
The University of Waikato
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