ALBERT

Description: The lithospheric mantle beneath West Antarctica has been characterized using petrology, whole-rock and mineral major element geochemistry, whole-rock trace element chemistry and Mössbauer spectroscopy data obtained on a suite of peridotite (lherzolite and harzburgite) and pyroxenite xenoliths from the Mount Morning eruptive centre, Southern Victoria Land. The timing of pyroxenite formation in Victoria Land overlaps with subduction of the Palaeo-Pacific plate beneath the Gondwana margin and pyroxenite is likely to have formed when fluids derived from, or modified by, melting of the subducting, eclogitic, oceanic crustal plate percolated through peridotite of the lithospheric mantle. Subsequent melting of lithospheric pyroxenite veins similar to those represented in the Mount Morning xenolith suite has contributed to the enriched trace element (and isotope) signatures seen in Cenozoic volcanic rocks from Mount Morning, elsewhere in Victoria Land and Zealandia. In general, the harzburgite xenoliths reflect between 20 and 30% melt depletion. Their depleted element budgets are consistent with Archaean cratonization ages and they have mantle-normalized trace element patterns comparable with typical subcontinental lithospheric mantle. The spinel lherzolite mineral data suggest a similar amount of depletion to that recorded in the harzburgites (20–30%), whereas plagioclase lherzolite mineral data suggest 〈15% melt depletion. The lherzolite (spinel and plagioclase) xenolith whole-rocks have compositions indicating 〈20% melt depletion, consistent with Proterozoic to Phanerozoic cratonization ages, and have mantle-normalized trace element patterns comparable with typical depleted mid-ocean ridge mantle. All peridotite xenoliths have undergone a number of melt–rock reaction events. Melting took place mainly in the spinel peridotite stability field, but one plagioclase peridotite group containing high-sodium clinopyroxenes is best modelled by melting in the garnet field. Median oxygen fugacity estimates based on Mössbauer spectroscopy measurements of spinel and pyroxene for spinel-facies conditions in the rifted Antarctic lithosphere are –0·6 log fO 2 at Mount Morning and –1·0 ± 0·1 (1) log fO 2 for all of Victoria Land, relative to the fayalite–magnetite–quartz buffer. These values are in good agreement with a calculated global median value of –0·9 ± 0·1 (1) log fO 2 for mantle spinel-facies rocks from continental rift systems.

Description: Taranaki (Mt. Egmont) in the western North Island of New Zealand is a high-K andesite volcano with an eruptive history extending over more than 200 kyr. In general, petrological research has concentrated on the post-10 ka record of the modern edifice. This study focuses on the earlier history, which is recorded in 11 major pre-7 ka debris avalanche deposits. Each of these formed as a result of a catastrophic collapse of the edifice of the time. The clast assemblages of these deposits provide insights into the chemical compositions of magmas erupted during the earlier stages of activity of the volcano and form the basis for a new chemo-stratigraphic analysis of the pre-10 ka volcanic succession. Sample suites from the studied debris avalanche deposits show a progressive enrichment in K 2 O and large ion lithophile elements (LILE), reflecting a gradual evolution to high-K andesite. The early magmatic system (pre-100 ka) produced a wide range of compositions including relatively primitive basalts and basaltic andesites. These rocks contain phenocryst assemblages that indicate crystallization within the lower crust or mantle, including a broad range of clinopyroxene compositions, high-Al 2 O 3 hornblende, olivine and phlogopite. A higher proportion of high-silica compositions in the younger sample suites and the appearance of late-stage, low-pressure mineral phases, such as high-TiO 2 hornblende, biotite and Fe-rich orthopyroxene, reflect a gradual shift to more evolved magmas with time. These new data are interpreted to reflect a multi-stage origin for Taranaki andesites. Parental magmas were generated within a lower crustal ‘hot zone’, which formed as a result of repeated intrusions of primitive melts into the lower crust. The geochemical and mineralogical evidence indicates that prior to 100 ka this zone was relatively thin and cold, so that primitive magmas were able to rise rapidly through the crust without significant interaction and modification. As the hot zone evolved, larger proportions of intruded and underplated mafic material were partially remelted, and interaction of these melts with fractionating mantle-derived magmas generated progressively more K- and LILE-enriched compositions. A complex and dispersed magma assembly and storage system developed in the upper crust where the hot-zone melts were further modified by fractional crystallization and magma mixing and mingling.

Description: Debris avalanches caused by the collapse of volcanic flanks pose a great risk to inhabited areas and may permanently change the surrounding landscape and its drainage systems. In this research, we explored the interplay between a debris avalanche and a tectonically uplifting surrounding landscape, providing insights into the long-term consequences of volcanic edifice failures. Exposures of coarse volcaniclastic sediments along the Hautapu River ~50 km southeast of Mount Ruapehu, New Zealand, show evidence of the largest known collapse event of the stratovolcano, which was followed by a vigorous regrowth phase that produced numerous pyroclastic eruptions and pumice-rich lahars. Similar diamicton deposits are exposed within the river catchment adjacent to the west. Cover-bed stratigraphy and geochemical correlation of andesitic lava blocks within the debris-avalanche deposit with dated lavas exposed on the cone indicate that deposition occurred between 125 and 150 ka. The collapse took place during the shift from a glacial to an interglacial climate, when glaciers on the cone were in retreat, and high pore-water pressures combined with deep hydrothermal alteration weakened the cone. In addition, collapse may have been accompanied by magmatic unrest. The ~2–3 km 3 debris avalanche inundated an area of 〉260 km 2 and entered the proto-Hautapu catchment, where it was channelized within the deeply entrenched valley. Mass-wasting events associated with postcollapse volcanism continued to be channeled into the proto–Hautapu River for another ~10 k.y., producing long-runout lahars. Subsequently, the river catchment was isolated from the volcano by incision of the intervening Whangaehu River into the proximal volcaniclastic sediments, accompanied by regional faulting and graben deepening around Ruapehu. At present, the volcaniclastic deposits form a distinctive plateau on the highest topographic elevation within the Hautapu Valley, forming a reversed topography caused by preferred incision of the Hautapu River into softer Late Tertiary sediments concurrent with constant uplift.

Description: 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 increases from 〈25 km within the TVZ to 40 km beneath Ruapehu. The volcano is built on a basement of Mesozoic meta-greywacke, 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 eruptive rocks over time. In comparison with rocks from younger formations, Te Herenga eruptive rocks have lower K 2 O abundances and a relatively restricted range in major and trace element and Nd–Sr 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 143 Nd/ 144 Nd ratios. The isotopic, and major and trace element composition of Te Herenga andesite can be replicated by models involving mixing of an intra-oceanic andesite with a crustal component derived from a meta-igneous composition. Post-Te Herenga andesites show considerable variation in major and trace element and Sr and Nd isotopic compositions ( 87 Sr/ 86 Sr ranges from 0·7049 to 0·7060 and 143 Nd/ 144 Nd from 0·51264 to 0·51282). The range of compositions can be modeled by assimilation–fractional crystallization (AFC) 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 single rocks and few of these have compositions consistent with equilibration with a melt having the composition of either the host-rock or groundmass. The 87 Sr/ 86 Sr 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 1190°C 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 % H 2 O). 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.