The volcanic geology of the Western Tauranga Basin

Abstract

The western Tauranga basin is bounded to the north by the Tauranga Harbour and to the west by the Kaimai Range. It contains rhyolitic domes, ignimbrites, fluvial deposits and tephras. The oldest units are the Minden Rhyolites which are hypersthene, hornblende and biotite rhyolites. These form the dome complexes Minden Peak, Manawata and Kaikaikaroro. These domes are surrounded by the overlying Waiteariki Ignimbrite which is a dacitic ignimbrite made up of three flow units and contains co-eruptive lithics ,vithin its basal unit. The Te Puna Ignimbrite is a rhyolitic ignimbrite which is found in the coastal cliffs around westerm Tauranga Harbour. Flu vial material which is prodominantly of volcanic origin lies unconformably below and on top of the Te Puna Ignimbrite. A number of tephras overlie the rhyolites and ignimbrites in the area. These are: Pahoia Tephras, Hamilton Ash, Rotoehu Ash, Mangaone, Oruanui, Okareka, Mamaku, Taupo and Kaharoa Tephras. Whole rock geochemistry shows that western Tauranga basin rocks are calcalkaline, and have resulted from arc volcanism. The rocks have formed predominantly from crystal fractionation, with the rhyolites also resulting from crustal melting. The Te Puna Ignimbrite contains pumice that ranges in Si02 content which indicates that the magma chamber was zoned. Upon eruption the magma chamber under went simple inversion which is seen when comparing element composition and stratigraphic height. This eruption may have caused a caldera which is located east of Katikati. The Tauranga basin lies between the Coromandel Volcanic Zone and the Taupo Volcanic Zone, and is a transitional zone in terms of age and geochemistry, shown by transitional incompatible trace element ratios.