Volcanic Geology of Maungatautari: An Andesitic-Dacitic Composite Cone, Central Waikato, New Zealand

Abstract

Maungatautari is an extinct andesitic-dacitic composite cone volcano, situated 35 km southeast of Hamilton, that rises prominently in the Waikato region. A single radiometric age of 1.8 Ma, indicates volcanic activity here was contemporaneous with subduction-related volcanism of the nearby Alexandra Volcanics and the early Taupo Volcanic Zone. The geology and volcanism of Maungatautari was previously addressed (1950s-80s) in context of the broader volcanic geology of the Waikato region, however, only a single dedicated volcanic study, undertaken over 30 years ago, provided insight into the chemical composition and origin of the magmas. This study applied modern volcanological techniques and concepts to reconstruct the volcanic history of Maungatautari and how it may relate to volcanism elsewhere in the Waikato Region around 2 Ma. The work incorporates field and desktop geomorphic analysis, volcanic stratigraphy and facies analysis, petrographic and mineralogical studies, and elemental and isotopic geochemical analysis. Maungatautari overlies Jurassic Manaia Hill group rocks of the Waipapa Terrane and abuts a range of the same rocks immediately to the west. It is surrounded to the north, east and south by a thick ignimbrite plateau which overlies the lowermost flanks of the mountain, originating from the Mangakino caldera to the southeast. Catastrophic failure of the northeastern flank produced a 0.28 km3 rock avalanche deposit over an area of approximately 1.6 km2. The deposits of rock falls, slides and debris flows were also commonly observed and demonstrate that numerous mass-wasting processes have continued to occur into the present. Lavas at Maungatautari consist of non-vesicular labradorite, pyroxene and hornblende andesites and hornblende dacites along with olivine basalt at the small cone at Kairangi 7 km to the northwest. Whole rock geochemical and strontium and neodymium analyses demonstrate these lavas were derived from at least three distinct mantle-melt sources: the fractionation of subduction-related, deep, garnet-bearing, depleted mantle (most andesites and dacites); a shallower, garnet-free, upper mantle (low SiO2 hornblende andesites) and an area of deeper enriched mantle (Kairangi olivine basalt). This study considers Maungatautari to reflect andesitic volcanism occurring along the edge of the continental tip of the Colville volcanic arc c. 2 Ma with the near-by Alexandra Volcanics being contemporaneous, behind-arc subduction-related volcanoes. The olivine basalt at Kairangi is more reflective of intraplate over subduction-related melts and is unrelated to magmatism at Maungatautari. This study is the first to document a record of explosive volcanism at Maungatautari. Eruption styles which occurred at Maungatautari include the effusion of lava resulting in the generation of lava flows and domes, and vulcanian to sub-plinian explosive eruptions which produced both high and low particle concentration pyroclastic density currents.