Evolution of Kakaramea and Maungakatote volcanoes Tongariro volcanic centre New Zealand

Abstract

Kakaramea and Maungakatote volcanoes form the northwestern part of the Tongriro Volcanic Centre, and andesitic centre which lies at the southwestern end of the Taupo Volcanic Zone. Kakaramea and Maungakatote vents are aligned in a NW-SE direction, suggesting that both are related to an older trend which extends in a northwest direction to Waiheke Is. A series of recent NNE-SSW trending faults transect the area - evidence of the present graben structure which forms the Taupo Volcanic Zone. Kakaramea andesites are porphyritic labradorite and labradorite pyroxene andesites, with one newly-described hornblende andesite flow. Maungakatote andesites are primarily porphyritic pyroxene andesites, although this suite also includes labradorite, labradorite-pyroxene, and hornblende andesite flows. Maungakatote pyroxene (and hornblende) andesites differ from the labradorite andesites in having plagioclase as a groundmass phase only. Microprobe studies on two Kakaramea andesites show that the phenocryst phases include labradorite (An58-70), augite (av. En40Fs19Wo41), hypersthene (av. En62Fs35Wo3) and titaniferous magnetite. Maungakatote pyroxene and hornblende andesites (one sample of each) contain partially resorbed olivine (Fo81-88) in the cores of some pyroxenes. Orthopyroxenes are bronzite, with Fe content increasing from core to rim (En79Fs18Wo3 to En71Fs25Wo4). Clinopyroxenes have cores of diopside to endiopside, with rims of augite (En49Fs6Wo45 to En43Fs15Wo42). Hornblende is pargasitic with heavily oxidized rims. Groundmass plagioclase is mainly bytownite (av. An72). Crystallisation conditions for Kakaramea and Maungakatote andesites were different. The crystallisation sequence in Kakaramea andesites, in addition to the presence of reverse zoning, suggest formation under relatively anhydrous conditions. In contrast, the presence of olivine and pyroxene on the liquidus of Maungakatote andesites, plus the occurrence of hornblende in one flow, indicates that they crystallised under hydrous conditions (H20> 3%). Chemically Kakaramea and Maungakatote andesites range from basic to acid medium-K orogenic andesites. Major element chemistry shows typical colc-alkaline trends, while trace element chemistry suggests moderate low pressure fractionation involving plagioclase, magnetite, and olivine. REE normalised patterns are strongly enriched in LREEs while HREE patterns are flat at 10-15 x chondrite. Fractionation within each sequence is shown by an increase in total REE abundances, although the lack of substantial Eu anomalies suggests that plagioclase may not be the fractionating phase involved. Ce anomalies are present in the two hornblende andesites - this may be the result of hornblende fractionation, although experimental distribution coefficients do not support this hypothesis. Models of andesite genesis are examined, and a multistage model invoking contamination of mantle peridotite by a partial melt of the descending slab is considered the most feasible. The formation of the Taupo Volcanic Zone andesites through the assimilation of crustal material (either greywacke) is considered improbable on the basis of REE abundances in the phases involved. Subduction of sediment is a possible source of high incompatible element abundances and strongly fractionated LREEs. Partial melting of the subducted slab (and minor sediment material) at 100-150 km and subsequent contamination of the overlying mantle material would provide a source material for the andesite. Partial fusion of this contaminated material probably occurred at relatively high levels (within the spinel stability field) and the magma then rose to undergo low pressure fractionation at crustal levels. Labradorite-pyroxene andesites of the Tongariro Volcanic Centre probably represent the primary andesitic magma formed from slab subduction. Pyroxene andesites of Maungakatote appear to have formed under local conditions of high PH2o. Water could conceivably concentrate at the top of a magma chamber, and would thus explain the early eruption of pyroxene andesites in the Tongariro area.