Abstract
Coastal sections between Whiritoa and Whangamata, eastern Coromandel, New Zealand, have exposed a Late Miocene volcanic and pyroclastic sequence which has been interpreted using a caldera facies model. The present-day coastline transects the middle of the caldera, which has been named here the Tunaiti caldera. The sequence comprises: (1) precaldera andesite and dacite lavas; (2) caldera-forming dacitic eruptions and interbedded plinian tuffs; (3) a variety of tilted and deformed moat deposits including extrusive and intrusive preresurgent rhyolitic volcanics, diatomaceous lake sediments, and postcollapse mesobreccias; (4) intracalderaresurgent biotite rhyolite domes and flows; (5) postcaldera dikes and lavas of hornblende rhyolite; and terminated with (6) postcaldera dacite lavas. There is a trend in composition with time from early andesites and dacites through to voluminous intracaldera rhyolites, which reverted to less silicic, small volume ring-fracture rhyolites and dacites. Incompatible element ratios between the andesites, dacites, and rhyolites indicate that they have not been derived from a single evolving magma chamber. The andesites have compositions (low Nb, high Ba/La) typical of subduction-related magmas, and these magmas ascended to upper crustal levels where they mixed with discrete rhyolitic magmas beneath the caldera to produce the dacites. Hydrothermal alteration or mineralisation associated with the caldera is unknown. A caldera facies model approach could be a useful tool in the future for recognising eroded ancient calderas in Coromandel and Tauranga, and possibly also for locating associated epithermal or subaqueous volcanogenic exhalative massive sulphide mineralisation.

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