|dc.description.abstract||Volcanic activity began in the north Coromandel Volcanic Zone (CVZ) at 18 Ma, migrated southward during the Miocene to Pliocene, and continued until the Pleistocene (1.9 Ma). The Waihi Caldera was active during the Pliocene and has been infilled with lake sediments (Romanga Formation) and three rhyolitic ignimbrites of the Whitianga Group, the Corbett, Waikino and Owharoa ignimbrites. This study constrains the volcanic history and processes involved in the formation of three volcanic units in eastern Waihi; the Corbett Ignimbrite, Ratarua Ignimbrite and Hikurangi Rhyolite. Detailed stratigraphic logs, petrographic studies, geochemical analyses and U-Pb dating of zircons are presented for each unit.
The Corbett Ignimbrite is a widespread deposit which is constrained to eastern Waihi. It has a maximum exposed thickness of 18 m at its type section, and is a creamy-buff, pumice-rich, crystal-rich, moderately-welded ignimbrite. The basal zone of the ignimbrite is pumice-rich (40-50%), crystal-rich (20-30%) and contains two distinct lithic concentration zones. At one locality the ignimbrite consists of a 9 m thick densely welded zone with abundant fiamme. The matrix is composed of fine ash with shard textures only visible under the scanning electron microscope. Crystals comprise plagioclase, quartz, hornblende, orthopyroxene, augite, titanomagnetite, ilmenite and zircon. Lithics are predominantly andesite with minor rhyolite, dacite and greywacke. Lithic concentration zones suggest that collapse and erosion of the vent occurred several times during the eruption, and these were emplaced by rapid depositional pulses. The upper half of the outcrop represents a more steady flow. Pumice and glass shard composition shows a transition from andesitic to rhyolitic. The source of the Corbett Ignimbrite was thought to have been from either a silicic centre in the vicinity of the Bowentown Rhyolite or from the Waihi Caldera. However, the U-Pb age 6.09 + 0.34 Ma is consistent with older source vents located elsewhere, with two other possible vent locations assessed.
The Corbett Ignimbrite pyroclastic flow was constrained by the welded, fiamme and crystal-rich, dacitic Ratarua Ignimbrite which has been dated at 6.79 + 0.42 Ma. The Ratarua Ignimbrite is exposed on hillsides as angular blocks ranging from 30 cm to several metres in size. Crystals comprise plagioclase, hornblende, orthopyroxene, augite, titanomagnetite, ilmenite and zircon. Lithics are andesite, dacite, rhyolite and rare sandstone.
The spherulitic Hikurangi Rhyolite dome belongs to the Homunga Rhyolite formation and overlies the Corbett Ignimbrite in southeastern Waihi. This rhyolite dome contains quartz, plagioclase, biotite, titanomagnetite and ilmenite phenocrysts. Petrographical analysis of granophyric intergrowth crystals indicates the presence of a granite body below this dome, which is the first evidence of granite present in southern CVZ. U-Pb dating of zircons determined an age of 4.53 + 0.13 Ma for this dome showing a younging in age of the Homunga Rhyolite formation southwards.
The units included in this study show a progression in volcanism with a transition from andesite dominant to rhyolitic. The Ratarua Ignimbrite formed during an earlier period dominated by andesite volcanism. This ignimbrite pre-dates the Corbett Ignimbrite, which formed during later andesitic volcanism in the eastern Waihi area, predating the Waihi Caldera. The youngest unit in this study, the Hikurangi Rhyolite, shows younger rhyolitic volcanism in southeastern Waihi.||