Fractured rhyolite lava domes and flows occur between 0 and 1000 m.b.g.l. in the Kawerau geothermal field (New Zealand), with several rhyolite bodies intersected by geothermal drilling. Differentiating the rhyolites has helped resolve an important part of the Kawerau stratigraphy, and enhanced our knowledge of the geological history of the Taupo Volcanic Zone. Onepu rhyolites underlie the Matahina ignimbrite and are composed of a series of flows interspersed locally with rhyolite breccia, tuff and fluviatile pumiceous sediments of Onepu Ash, particularly in peripheral parts of the dome complex. The Caxton Rhyolite has been inferred to be extruded from multiple vents, and forms a large rhyolite complex interbedded with Kawerau Andesite and ignimbrite units. Concordant quartz porphyry dikes occur in some wells (e.g. KA28, KA30 and KA31), where they separate ignimbrite and andesite between ~500 – 1000 m depth.

Recent age determinations obtained from these rhyolite groups using U-Pb dating has indicated a complex magmatic (and structural) history at Kawerau. The Caxton Rhyolites represent a common series of intrusions (rather than surface flows) with inferred ages clustering around 400 ka. Some of these intrusions have ages similar to those of the buried Onepu Rhyolite lava flows (~400 ka, from stratigraphic relationships to the 320 ka Matahina ignimbrite) and are inferred to be part of a feeder system for the rhyolite. The ages provide insight into the temporal evolution of rhyolitic magmatic activity at Kawerau, with the intrusive complexes representing past heat sources that are likely to have rejuvenated the geothermal system beneath Kawerau.

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