TY - JOUR

T1 - The anatomy of an andesite volcano: A time-stratigraphic study of andesite petrogenesis and crustal evolution at Ruapehu volcano, New Zealand

AU - Price, R.C.

AU - Gamble, J.A.

AU - Smith, I.E.M.

AU - Maas, R.

AU - Waight, Tod Earle

AU - Stewart, R.B.

AU - Woodhead, J.

PY - 2012

Y1 - 2012

N2 - Ruapehu, New Zealand’s largest active andesite volcano is located at the southern tip of the Taupo Volcanic Zone (TVZ), the main locus of subduction-related volcanism in the North Island. Geophysical data indicate that crustal thickness transitions from <25 km within the TVZ to 40 km beneath Ruapehu. The volcano is built on a basement of Mesozoic metagreywacke and geophysical evidence together with xenoliths contained in lavas indicates that this is underlain by oceanic, meta-igneous lower crust. The present-day Ruapehu edifice has been constructed by a series of eruptive events that produced a succession of lava flow-dominated stratigraphic units. In order from oldest to youngest, these are the: Te Herenga (250- 180 ka); Wahianoa (160-115 ka); Mangawhero (55- 45 ka and 20- 30 ka); and Whakapapa (15-2 ka) Formations. The dominant rock types are plagioclase- and pyroxene-phyric basaltic andesite and andesite. Dacite also occurs but only one basalt flow has been identified. There have been progressive changes in the minor and trace element chemistry and isotopic composition of Ruapehu eruptives over time. In comparison with rocks from younger formations, Te Herenga eruptives have lower K2O abundances and a relatively restricted range in major, trace element and Sr/Nd isotopic composition. Post- Te Herenga andesites and dacites define a Sr-Nd isotopic array that overlaps with the field for TVZ rhyolites and basalts but Te Herenga Formation lavas and the Ruapehu basalt have higher 143Nd/144Nd isotopic ratios. The isotopic, major and trace element composition of Te Herenga andesite can be replicated by models involving mixing of an intra-oceanic andesite with a crustal composition derived from a meta-igneous composition. Post-Te Herenga andesites show considerable variation in major and trace element and Sr and Nd isotopic compositions (87Sr/86Sr ranges from 0.7049 to 0.7060 and 143Nd/144Nd from 0.51264 to 0.51282). The range of compositions can be modeled by assimilation fractional crystallization involving meta-greywacke as the assimilant, closed system fractionation or by mixing of intra-oceanic andesite or basalt and a meta-greywacke crustal composition. Plagioclase and pyroxene compositions vary over wide ranges within individual rocks and few of these have compositions consistent with equilibration with a melt having the composition of either the host rock or groundmass. The 87Sr/86Sr isotopic compositions of plagioclase also vary significantly within single whole rock samples. Glass inclusions and groundmasses of andesitic rocks all have dacitic or rhyolitic major and trace element compositions. The application of various mineral geothermometers and geobarometers indicates pre-eruption temperatures between 950 and 1190ºC and pressures ranging from 1 to 0.2 GPa. These pressure estimates are consistent with those obtained from xenolith mineral assemblages and geophysical information. Plagioclase hygrometry and the paucity of amphibole are indications that melts were relatively dry (<4 wt% H2O). Magmas represented by Ruapehu andesites were dacitic or rhyolitic melts carrying complex crystal and lithic cargoes derived from the mantle and at least two crustal sources. They have evolved through a complex interplay between assimilation, crystal fractionation, crustal anatexis and magma mixing. Parental magmas were sourced in both the mantle and crust, but erupted compositions very strongly reflect modification by intracrustal processes. Geochemical variation in systematically sampled lava flow sequences is consistent with random tapping of a complex plumbing system in which magma has been stored on varying time scales within a plexus of dispersed reservoirs. Each magma batch is likely to have had a unique history with different sized magma storages evolving on varying time scales with a specific combination of AFC and mixing processes.

AB - Ruapehu, New Zealand’s largest active andesite volcano is located at the southern tip of the Taupo Volcanic Zone (TVZ), the main locus of subduction-related volcanism in the North Island. Geophysical data indicate that crustal thickness transitions from <25 km within the TVZ to 40 km beneath Ruapehu. The volcano is built on a basement of Mesozoic metagreywacke and geophysical evidence together with xenoliths contained in lavas indicates that this is underlain by oceanic, meta-igneous lower crust. The present-day Ruapehu edifice has been constructed by a series of eruptive events that produced a succession of lava flow-dominated stratigraphic units. In order from oldest to youngest, these are the: Te Herenga (250- 180 ka); Wahianoa (160-115 ka); Mangawhero (55- 45 ka and 20- 30 ka); and Whakapapa (15-2 ka) Formations. The dominant rock types are plagioclase- and pyroxene-phyric basaltic andesite and andesite. Dacite also occurs but only one basalt flow has been identified. There have been progressive changes in the minor and trace element chemistry and isotopic composition of Ruapehu eruptives over time. In comparison with rocks from younger formations, Te Herenga eruptives have lower K2O abundances and a relatively restricted range in major, trace element and Sr/Nd isotopic composition. Post- Te Herenga andesites and dacites define a Sr-Nd isotopic array that overlaps with the field for TVZ rhyolites and basalts but Te Herenga Formation lavas and the Ruapehu basalt have higher 143Nd/144Nd isotopic ratios. The isotopic, major and trace element composition of Te Herenga andesite can be replicated by models involving mixing of an intra-oceanic andesite with a crustal composition derived from a meta-igneous composition. Post-Te Herenga andesites show considerable variation in major and trace element and Sr and Nd isotopic compositions (87Sr/86Sr ranges from 0.7049 to 0.7060 and 143Nd/144Nd from 0.51264 to 0.51282). The range of compositions can be modeled by assimilation fractional crystallization involving meta-greywacke as the assimilant, closed system fractionation or by mixing of intra-oceanic andesite or basalt and a meta-greywacke crustal composition. Plagioclase and pyroxene compositions vary over wide ranges within individual rocks and few of these have compositions consistent with equilibration with a melt having the composition of either the host rock or groundmass. The 87Sr/86Sr isotopic compositions of plagioclase also vary significantly within single whole rock samples. Glass inclusions and groundmasses of andesitic rocks all have dacitic or rhyolitic major and trace element compositions. The application of various mineral geothermometers and geobarometers indicates pre-eruption temperatures between 950 and 1190ºC and pressures ranging from 1 to 0.2 GPa. These pressure estimates are consistent with those obtained from xenolith mineral assemblages and geophysical information. Plagioclase hygrometry and the paucity of amphibole are indications that melts were relatively dry (<4 wt% H2O). Magmas represented by Ruapehu andesites were dacitic or rhyolitic melts carrying complex crystal and lithic cargoes derived from the mantle and at least two crustal sources. They have evolved through a complex interplay between assimilation, crystal fractionation, crustal anatexis and magma mixing. Parental magmas were sourced in both the mantle and crust, but erupted compositions very strongly reflect modification by intracrustal processes. Geochemical variation in systematically sampled lava flow sequences is consistent with random tapping of a complex plumbing system in which magma has been stored on varying time scales within a plexus of dispersed reservoirs. Each magma batch is likely to have had a unique history with different sized magma storages evolving on varying time scales with a specific combination of AFC and mixing processes.

KW - Faculty of Science

KW - Andesite

KW - volcano

KW - Taupo volcanic zone

KW - New Zealand

KW - assimilation

KW - fractional crystallization

U2 - 10.1093/petrology/egs050

DO - 10.1093/petrology/egs050

M3 - Journal article

VL - 53

SP - 2139

EP - 2189

JO - Journal of Petrology

JF - Journal of Petrology

SN - 0022-3530

IS - 10

ER -