The Waihi Caldera (Coromandel Volcanic Zone) is defined by a 15 km diameter gravity anomaly, which was active during the Pliocene and has been infilled by a 1.5 km-thick succession of volcanic deposits and lake sediments. The Owharoa and Waikino ignimbrites are both suspected to have been a product of the Waihi Caldera (Brathwaite & Christie, 1996). The aim of this study was to determine and compare the eruption and emplacement processes of the Owharoa and Waikino ignimbrites.
The ignimbrites were characterised in the field by facies analysis and dated by U-Pb zircon dating using laser ablation inductively coupled plasma mass spectrometry. Their petrographic characteristics were described by optical microscopy, scanning electron microscopy and X-ray powder diffraction. Geochemical characteristics were identified by electron microprobe analysis (on minerals and glass shards) and X-ray fluorescence spectrometry (on pumice and bulk ignimbrite).
The Owharoa Ignimbrite (3.76 ± 0.05 Ma, Vincent, 2012) is poorly sorted with variable degrees of welding degrees. Facies identified include pumice-rich and lithic-rich facies (O1), flattened pumice rich facies (O2), lithic rich, pumice poor facies (O3), dark grey, densely welded, fiamme rich facies (O4) and pumice rich facies (O5). Juvenile clasts include creamy rounded woody-textured pumice in the east, and dark to black, lensoidal, glassy fiamme in the west with quartz and plagioclase phenocrysts. Lithic clasts include volcanic lithics (rhyolite, andesite, and ignimbrite), and occasional sedimentary lithics (sandstone/siltstone).
The Waikino Ignimbrite (3.48 +/- 0.19 Ma) is a finer-grained, relatively well sorted, massive, glass shard matrix-rich (~93%) ignimbrite that is separated into two facies, W1, a softer, massive yellow basal facies; and W2 a grey, well welded, massive facies. The pumice within the Waikino Ignimbrite (1%) was no larger than coarse ash-sized, white fragments. The Waikino Ignimbrite had plagioclase, quartz, biotite (larger than the Owharoa Ignimbrite) and opaque minerals. Geochemically the Waikino Ignimbrite was rhyolitic with higher alkali content than the Owharoa Ignimbrite.
The Owharoa Ignimbrite represents the deposit of an intra-caldera, pulsating depositional pyroclastic flow that shows subtle variations in pumice and lithic abundance. The massive nature of the Waikino Ignimbrite indicates a consistent, steady pyroclastic flow, derived from an intensely fragmented magma to form a glass shard-rich, pumice-poor deposit. Both ignimbrites were sourced from the Waihi Caldera due to their proximity to the caldera, the lithic characteristics and their similar geochemistry and mineralogy. The closeness in age of the ignimbrites signifies the relationship between the deposits, and relationship they have with the Waihi Caldera. It is possible that the Waikino Ignimbrite was the last major eruption from the Waihi Caldera, therefore the end of the duration of the caldera can be identified as after the Waikino Ignimbrite eruption at 3.48 +/- 0.19 Ma.
