Volcanic geology and geochemistry of Waiotapu Ignimbrite, Taupo Volcanic Zone, New Zealand. (1996)
Waiotapu Ignimbrite (0.710 ± 0.06 Ma) is a predominantly densely welded, purple-grey coloured, pumice rich lenticulite, which is exposed on both eastern and western flanks of Taupo Volcanic Zone. The unit is uniform in terms of lithology and mineralogy over its entire extent and has been deposited as a single flow unit. The unit contains abundant pumice clasts which are often highly attenuated (aspect ratios of c.1 :30) and are evenly distributed throughout the deposit. Lithic fragments are rare, never exceeding 1% of total rock volume at an outcrop and no proximal facies, such as lithic lag breccias, have been identified. The deposit is densely welded to the base and only in more distal exposure does the ignimbrite become partially welded at the top of the deposit. Post-depositional devitrification is pervasive throughout the deposit, often destroying original vitroclastic texture in the matrix. Vapour phase alteration is extensive in welded and partially welded facies of the deposit. Pumices within Waiotapu Ignimbrite appear to have been derived from two distinct magma batches, with differing Rb concentrations, that originated along different fractionation trends. Type-A pumices have significantly lower Rb than the subordinate type-B pumices. The presence of the pumices may represent the simultaneous evisceration of two spatially discrete magma chambers or the type-B chamber may have been intruded into type-A body, the magmas subsequently mingling prior to, or during, the eruption. The source of Waiotapu Ignimbrite is poorly constrained, largely owing to the lack of meaningful maximum lithic data, and poor exposure of the unit. The distribution of the ignimbrite suggests that it was erupted from within Kapenga volcanic centre. If so the most proximal exposures of Waiotapu Ignimbrite are approximately 10km from the vent. Intensive and voluminous silicic volcanism, beginning with the eruption of the 0.33 Ma Whakamaru Group Ignimbrite eruptions, and extensive faulting within Kapenga volcanic centre will have obscured any intra-caldera Waiotapu Ignimbrite. The mechanism of eruption suggests that the source may not have been a caldera in the strictest sense, but instead a series of near linear fissures aligned with the trend of regional faulting. Waiotapu Ignimbrite was generated in one sustained eruption and produced an energetic and high temperature pyroclastic flow. The lack of any recognised preceding plinian deposit, coupled with the energetic nature and paucity of lithics suggests eruption by an unusual mechanism. The eruption most likely resulted from the large scale collapse of a caldera block into the underlying chamber resulting in high discharge rates, which were no conducive to the development of a convecting column, and minimal vent erosion, resulting in negligible entrainment of lithics. The density of welding and recrystallisation textures suggest that the flow retained heat to considerable distances which allowed the ignimbrite to weld densely to the base. The deposit was most likely progressively aggraded from the base, with material being supplied from an overriding particulate flow.
RightsCopyright Alistair B. H. Ritchie
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