The structure and eruptive history of Rotorua Caldera, Taupo Volcanic Zone, New Zealand

Abstract

Mamaku Ignimbrite erupted 220 -230 ka, is a voluminous deposit that is exposed over c. 3100 km² predominantly NNE, NW, and SW of Rotorua Caldera, and has a maximum measured outflow thickness of 145 m. Rotorua Caldera is a 21x22 km complex collapse structure in south west Bay of Plenty, Taupo Volcanic Zone, New Zealand. The Mamaku Ignimbrite sequence can be divided into basal tephra and main ignimbrite sequences. The basal tephra sequence comprises pyroclastic fall and density current deposits and records progression of the eruption from wet to dry. The main ignimbrite sequence has been separated into lower, middle and upper parts based on crystal content, welding, and extent of vapour phase alteration and devitrification. Boundaries between lower, middle, and upper ignimbrite are always gradational, and lithic and pumice content and size also vary stratigraphically throughout the deposit. Lateral variations in upper Mamaku Ignimbrite include areas of high lithic concentration, and lithic lag breccias, at and within the Rotorua Caldera margin with size and concentration of lithic fragments decreasing away from the caldera. At medial distances the ignimbrite is internally massive with occasional zones of pumice clast concentration, and varying lithic content. Boundaries of these lithic and pumice concentration zones are gradational and no sharp flow unit boundaries exist. At distal localities a crystal-rich band is present at the base of the ignimbrite and crystal-rich lenses are present at higher levels. These crystal-rich lenses suggest that distally the pyroclastic flow was moving in a dominantly non-particulate manner. XRF analysis has revealed three silicic pumice types in Mamaku Ignimbrite that range from dacite to high silica rhyolite (66-76 wt% SiO₂). No large compositional gaps exist between the pumice types and they provide evidence for the evacuation of a gradationally zoned magma chamber. The more silicic pumice compositions were probably derived by 20 % crystal fractionation of the dacitic magma. Andesitic blebs in upper Mamaku Ignimbrite are a fourth juvenile component which had a different petrogenesis to the silicic pumice types. The andesitic magma probably resided as a sill towards the base of the dacitic magma. Maximum lithic size variation shows Rotorua Caldera to be the source of upper Mamaku Ignimbrite. Gradational contacts between lower, middle, and upper ignimbrite suggest that the ignimbrite was deposited during a single eruptive event from one source. Coexistence of the three pumice types at all stratigraphic levels is further evidence for a single eruptive source. Variations in lithic content, and coexistence of different pumice types through the ignimbrite stratigraphy, indicate that caldera collapse occurred throughout the eruption, but particularly during the eruption of middle Mamaku Ignimbrite and in later stages of the eruption of upper Mamaku Ignimbrite. Rhyolite domes of Rotorua Caldera can be separated into seven groups, two of which are related to adjacent volcanic centres. Fragments of rhyolite lava are also a major component of lithic Jag breccias at caldera margin sites. The domes that are related to Rotorua record the eruption of five different rhyolitic magma bodies. The largest rhyolite dome complexes are located around the area of deepest caldera subsidence. These domes have similar phenocryst assemblages and phenocryst chemistries to silicic Mamaku Ignimbrite pumice clasts suggesting that they are from the same magma. Rhyolite lava fragments in the lag breccias also have similar phenocryst assemblages

and phenocryst chemistry to Mamaku pumice clasts and may represent initial extrusions from the Mamaku magma chamber. All other rhyolite lavas in Rotorua Caldera have different phenocryst assemblages to Mamaku Ignimbrite pumice clasts and are probably not consanguineous with them. Mokai Ignimbrite outcrops in an 8 km wide (east to west) band between lakes Taupo and Whakamaru. It has the same unusual paleomagnetic direction as Mamaku Ignimbrite, is the same age, and has similar pumice chemistry. Upper Mokai Ignimbrite is vapour phase altered and has a similar appearance to upper Mamaku Ignimbrite. Three distinct units define Mokai Ignimbrite's stratigraphy, each separated by ash deposits. Internal variations suggest that Mokai Ignimbrite formed a compound cooling unit. The three flow unit compound cooling unit stratigraphy, phenocryst assemblage of juvenile mafic fragments, presence of mafic blebs at all stratigraphic levels, and thickness of Mokai Ignimbrite suggest that it is distinct from Mamaku Ignimbrite. Rotorua Caldera is described as a rhyolitic, single event, asymmetric, multiple block, single locus caldera on the basis of published geophysical data, caldera geomorphology and geology, location and thickness of Mamaku Ignimbrite and the nature of intracaldera rhyolite domes. The timing of collapse is deduced, from Mamaku Ignimbrite stratigraphy, to have started at least by the time that middle Mamaku Ignimbrite erupted, but probably occurred throughout the eruption of the main ignimbrite sequence. The caldera formed during a single eruption, but cannot be well described by current caldera classifications, as it has characteristics that can be attributed to trapdoor, downsag, piecemeal and piston processes of collapse. Taupo Volcanic Zone is dominated by NE-SW striking faults with more widely spaced NW-SE striking structures. This fault pattern inhibits the formation of simple caldera structures. The regional fault pattern is interpreted to have cut the Rotorua Caldera floor into a number of blocks that subsided to varying depths towards a single collapse locus.