<p>This thesis undertakes a detailed case study of the processes and timescales of arc andesite-dacite magma generation and lava flow emplacement at a continental composite volcano. This has been achieved through the collection and integration of high-resolution field, geochronological and geochemical datasets for lava flows that form the edifice of Ruapehu.  The influence of syn-eruptive lava-ice interaction on the distribution and preservation of lava flows on glaciated composite volcanoes is investigated by characterising the morphology and fracture characteristics of effusive products at Ruapehu. Ice-bounded and ice-dammed lava flows display over-thickened (50–100 m-high) margins adjacent to or within glaciated valleys, are intercalated with till and have lateral margins that are pervasively fractured by quench-contraction cooling joints. These characteristics can be accounted for by impoundment and chilling of lava flows that were emplaced against large flank glaciers. In contrast, lava flows located within valleys have minimal moraine cover and glacial striae and are characterised by fracture networks indicative of only localised and minor interaction with ice/snow. These lavas were emplaced onto a relatively ice-free edifice following glacial retreat since ~18 ka.  New high-precision ⁴⁰Ar/³⁹Ar eruption ages and whole-rock major element geochemistry for lava flows are interpreted in the context of geologic mapping, volcano-ice interaction processes and previous chronostratigraphic studies. This provides a high-resolution eruptive history and edifice evolution model for Ruapehu. Sub-glacial to ice-marginal effusive eruption of basaltic-andesite and andesite constructed the northern portion of the exposed edifice between ~200 and 150 ka (Te Herenga Formation) and the wide southeastern planèze as well as parts of the northern, eastern and western flanks of Ruapehu between ~166 and 80 ka (Wahianoa Formation). No ages were returned for lava flows for the period from 80–50 ka, indicating one or a combination of: an eruptive hiatus; subsequent erosion and burial of lavas; or syn-eruptive glacial conveyance of lava flows to the ring-plain. The greater part of the modern edifice was constructed via effusion of lava flows of the syn-glacial Mangawhero Formation (50–15 ka) and post-glacial Whakapapa Formation (<15 ka). Syn-glacial edifice growth occurred primarily via effusion of andesite-dacite lava flows that formed ice-bounded ridges adjacent to valleyfilling glaciers. Post-glacial summit cones were constructed in the presence of remnant upper flank glaciers between 15 and 10 ka. Debuttressing of two northern summit cones and a southern summit cone as ice underwent continued post-glacial retreat resulted in two major Holocene sector collapses and deposition of debris avalanche deposits on the northern and south-eastern flanks of Ruapehu, respectively. The northern collapse scar was infilled by a new cone comprising <10 ka lava flows that form the modern upper northern and eastern flanks of the volcano. Late Holocene to historic eruptive activity has occurred through Crater Lake, which occupies the site of the collapsed southern cone.  New whole-rock major and trace element compositions for lavas and their mineral and melt inclusion geochemical characteristics are evaluated within the context of the improved chronostratigraphic framework. The new constraints are combined with existing whole-rock isotopic data to establish the long-term development of the magma generation system beneath Ruapehu. Basaltic-andesite lavas erupted between ~200 and 150 ka contain low-K₂O (2–3 wt. %) melt inclusions and have whole-rock compositions characterised by low incompatible element (K, Rb, Ba, Th, U) abundances and high ¹⁴³Nd/¹⁴⁴Nd-low ⁸⁷Sr/⁸⁶Sr when compared to younger eruptive products. In particular, basaltic-andesite to dacite lavas that were erupted between 50–35 ka define a high-K/Ca trend over a range of ~8 wt. % SiO₂ as well as elevated incompatible trace element contents when compared to all other documented eruptive products from Ruapehu. Rhyodacitic to rhyolitic melt inclusions, interstitial glass and melt pockets in partially fused feldspathic xenoliths contained within the dacite lavas from this latter period contain high K₂O (5–6 wt. %) and Rb contents (250–280 ppm). The whole-rock and glass characteristics of 50–35 ka lavas reflect the generation and assimilation of partial melts of the greywacke-argillite basement within the magma system beneath Ruapehu during this period. Selective partial melting and assimilation of fertile, K- and Rb-rich mineral phases (e.g. biotite) within the meta-sedimentary mineral assemblage is inferred to explain the enriched nature of these melts. A reversion to progressively less silicic and less potassic lavas with lower incompatible element abundances erupted since 26 ka is matched by the recurrent incorporation of crystals that trapped low-K₂O melt inclusions. The trend is interpreted to reflect the exhaustion of fertile phases within assimilated continental source rocks as the crust was progressively heated during long-term thermal conditioning of the arc lithosphere beneath Ruapehu.</p>

<p>The Macquarie Ridge Complex (MRC) forms the submarine expression of the Australia‐Pacific plate boundary south of New Zealand, comprising a rugged bathymetry made up of numerous seamounts along its length. Tectonic plate reconstructions show that the plate boundary evolved from divergent to transpressional relative plate motion from ca. 40 – 6 Ma. However, only limited geological observation of the products of past seafloor spreading and present transpressional deformation has been achieved. This study presents new high-resolution multibeam, photographic, petrologic and geochemical data for 10 seamounts located along the MRC in order to elucidate the current nature and evolution of the plate boundary. Seamounts are oriented parallel to the plate boundary, characterized by elongate forms, and deformed by transform faulting. Three guyot‐type seamounts display summit plateaux that were formed by wave and current erosion. MRC seafloor is composed of alkaline to sub‐alkaline basaltic pillow, massive and sheet lava flows, lava talus, volcaniclastic breccia, diabase and gabbro. This oceanic crust was formed during effusive mid‐ocean ridge volcanism at the relic Macquarie spreading centre and has since been sheared, accreted and exhumed along the modern transpressional plate boundary. Major element systematics indicate samples originated from spatially distinct magmatic sources and have since been juxtaposed at seamounts due to transpressional relative plate motion. MRC seamounts have formed as discrete elevations as a result of dip‐slip and strike‐slip faulting of the ridge axis. Thus, MRC seamounts are volcanic in origin but are now the morphological manifestations of tectonic and geomorphic processes. Petrologic and geochemical characteristics of volcanic glass samples from the MRC indicate that both effusive and explosive eruption styles operated at the relic Macquarie spreading centre. Primitive and sub‐alkaline to transitional basaltic magma that rose efficiently to the seafloor was erupted effusively and cooled to form lava flows with low vesicle and phenocryst contents or was granulated on contact with seawater to form hyaloclasts deposited in volcaniclastic breccias. More alkaline magmas that underwent crystal fractionation and volatile exsolution in shallow reservoirs were fragmented and erupted during submarine hawaiian‐type eruptions. Such a scenario is likely to have occurred during the final stages of magmatism at the Australia‐Pacific plate boundary south of New Zealand when seafloor spreading was ultraslow or had ceased, which induced low degrees of partial melting and retarded magma ascent rates. All MRC samples display enriched mid‐ocean ridge basalt (E‐MORB) trace element characteristics. The sample suite can be divided into two groups, with Group 1 samples distinguished from Group 2 samples by their lower concentrations of highly incompatible trace elements, flatter LREE slopes, higher MgO contents and lower alkali element contents. Group 1 basalts were derived from low degree partial melting of spinel lherzolite generated during the late stages of mid‐ocean ridge volcanism at the plate boundary when seafloor spreading rates were slow to ultraslow (full spreading rate < 20 mm yr⁻¹). Group 2 basalts were derived from low degree partial melting of spinel lherzolite, mixed with small amounts of very low degree partial melting of garnet lherzolite, during post‐spreading volcanism at the MRC. Remnant heat from previous seafloor spreading induced buoyant ascent of the sub‐ridge mantle and enriched heterogeneities were preferentially tapped by the ensuing low melt fractions. Magma ascent was stalled due to the cessation of extension at the ridge and the melts underwent crystal fractionation prior to eruption, which accounts for the lower MgO contents of Group 2 basalts. The pervasive incompatible element‐enrichment of MRC basalts and similarity to lavas from fossil spreading ridges in the eastern Pacific Ocean may reflect regional enrichment of the Pacific upper mantle.</p>

<p>The Macquarie Ridge Complex (MRC) forms the submarine expression of the Australia‐Pacific plate boundary south of New Zealand, comprising a rugged bathymetry made up of numerous seamounts along its length. Tectonic plate reconstructions show that the plate boundary evolved from divergent to transpressional relative plate motion from ca. 40 – 6 Ma. However, only limited geological observation of the products of past seafloor spreading and present transpressional deformation has been achieved. This study presents new high-resolution multibeam, photographic, petrologic and geochemical data for 10 seamounts located along the MRC in order to elucidate the current nature and evolution of the plate boundary. Seamounts are oriented parallel to the plate boundary, characterized by elongate forms, and deformed by transform faulting. Three guyot‐type seamounts display summit plateaux that were formed by wave and current erosion. MRC seafloor is composed of alkaline to sub‐alkaline basaltic pillow, massive and sheet lava flows, lava talus, volcaniclastic breccia, diabase and gabbro. This oceanic crust was formed during effusive mid‐ocean ridge volcanism at the relic Macquarie spreading centre and has since been sheared, accreted and exhumed along the modern transpressional plate boundary. Major element systematics indicate samples originated from spatially distinct magmatic sources and have since been juxtaposed at seamounts due to transpressional relative plate motion. MRC seamounts have formed as discrete elevations as a result of dip‐slip and strike‐slip faulting of the ridge axis. Thus, MRC seamounts are volcanic in origin but are now the morphological manifestations of tectonic and geomorphic processes. Petrologic and geochemical characteristics of volcanic glass samples from the MRC indicate that both effusive and explosive eruption styles operated at the relic Macquarie spreading centre. Primitive and sub‐alkaline to transitional basaltic magma that rose efficiently to the seafloor was erupted effusively and cooled to form lava flows with low vesicle and phenocryst contents or was granulated on contact with seawater to form hyaloclasts deposited in volcaniclastic breccias. More alkaline magmas that underwent crystal fractionation and volatile exsolution in shallow reservoirs were fragmented and erupted during submarine hawaiian‐type eruptions. Such a scenario is likely to have occurred during the final stages of magmatism at the Australia‐Pacific plate boundary south of New Zealand when seafloor spreading was ultraslow or had ceased, which induced low degrees of partial melting and retarded magma ascent rates. All MRC samples display enriched mid‐ocean ridge basalt (E‐MORB) trace element characteristics. The sample suite can be divided into two groups, with Group 1 samples distinguished from Group 2 samples by their lower concentrations of highly incompatible trace elements, flatter LREE slopes, higher MgO contents and lower alkali element contents. Group 1 basalts were derived from low degree partial melting of spinel lherzolite generated during the late stages of mid‐ocean ridge volcanism at the plate boundary when seafloor spreading rates were slow to ultraslow (full spreading rate < 20 mm yr⁻¹). Group 2 basalts were derived from low degree partial melting of spinel lherzolite, mixed with small amounts of very low degree partial melting of garnet lherzolite, during post‐spreading volcanism at the MRC. Remnant heat from previous seafloor spreading induced buoyant ascent of the sub‐ridge mantle and enriched heterogeneities were preferentially tapped by the ensuing low melt fractions. Magma ascent was stalled due to the cessation of extension at the ridge and the melts underwent crystal fractionation prior to eruption, which accounts for the lower MgO contents of Group 2 basalts. The pervasive incompatible element‐enrichment of MRC basalts and similarity to lavas from fossil spreading ridges in the eastern Pacific Ocean may reflect regional enrichment of the Pacific upper mantle.</p>

<p>This thesis undertakes a detailed case study of the processes and timescales of arc andesite-dacite magma generation and lava flow emplacement at a continental composite volcano. This has been achieved through the collection and integration of high-resolution field, geochronological and geochemical datasets for lava flows that form the edifice of Ruapehu.  The influence of syn-eruptive lava-ice interaction on the distribution and preservation of lava flows on glaciated composite volcanoes is investigated by characterising the morphology and fracture characteristics of effusive products at Ruapehu. Ice-bounded and ice-dammed lava flows display over-thickened (50–100 m-high) margins adjacent to or within glaciated valleys, are intercalated with till and have lateral margins that are pervasively fractured by quench-contraction cooling joints. These characteristics can be accounted for by impoundment and chilling of lava flows that were emplaced against large flank glaciers. In contrast, lava flows located within valleys have minimal moraine cover and glacial striae and are characterised by fracture networks indicative of only localised and minor interaction with ice/snow. These lavas were emplaced onto a relatively ice-free edifice following glacial retreat since ~18 ka.  New high-precision ⁴⁰Ar/³⁹Ar eruption ages and whole-rock major element geochemistry for lava flows are interpreted in the context of geologic mapping, volcano-ice interaction processes and previous chronostratigraphic studies. This provides a high-resolution eruptive history and edifice evolution model for Ruapehu. Sub-glacial to ice-marginal effusive eruption of basaltic-andesite and andesite constructed the northern portion of the exposed edifice between ~200 and 150 ka (Te Herenga Formation) and the wide southeastern planèze as well as parts of the northern, eastern and western flanks of Ruapehu between ~166 and 80 ka (Wahianoa Formation). No ages were returned for lava flows for the period from 80–50 ka, indicating one or a combination of: an eruptive hiatus; subsequent erosion and burial of lavas; or syn-eruptive glacial conveyance of lava flows to the ring-plain. The greater part of the modern edifice was constructed via effusion of lava flows of the syn-glacial Mangawhero Formation (50–15 ka) and post-glacial Whakapapa Formation (<15 ka). Syn-glacial edifice growth occurred primarily via effusion of andesite-dacite lava flows that formed ice-bounded ridges adjacent to valleyfilling glaciers. Post-glacial summit cones were constructed in the presence of remnant upper flank glaciers between 15 and 10 ka. Debuttressing of two northern summit cones and a southern summit cone as ice underwent continued post-glacial retreat resulted in two major Holocene sector collapses and deposition of debris avalanche deposits on the northern and south-eastern flanks of Ruapehu, respectively. The northern collapse scar was infilled by a new cone comprising <10 ka lava flows that form the modern upper northern and eastern flanks of the volcano. Late Holocene to historic eruptive activity has occurred through Crater Lake, which occupies the site of the collapsed southern cone.  New whole-rock major and trace element compositions for lavas and their mineral and melt inclusion geochemical characteristics are evaluated within the context of the improved chronostratigraphic framework. The new constraints are combined with existing whole-rock isotopic data to establish the long-term development of the magma generation system beneath Ruapehu. Basaltic-andesite lavas erupted between ~200 and 150 ka contain low-K₂O (2–3 wt. %) melt inclusions and have whole-rock compositions characterised by low incompatible element (K, Rb, Ba, Th, U) abundances and high ¹⁴³Nd/¹⁴⁴Nd-low ⁸⁷Sr/⁸⁶Sr when compared to younger eruptive products. In particular, basaltic-andesite to dacite lavas that were erupted between 50–35 ka define a high-K/Ca trend over a range of ~8 wt. % SiO₂ as well as elevated incompatible trace element contents when compared to all other documented eruptive products from Ruapehu. Rhyodacitic to rhyolitic melt inclusions, interstitial glass and melt pockets in partially fused feldspathic xenoliths contained within the dacite lavas from this latter period contain high K₂O (5–6 wt. %) and Rb contents (250–280 ppm). The whole-rock and glass characteristics of 50–35 ka lavas reflect the generation and assimilation of partial melts of the greywacke-argillite basement within the magma system beneath Ruapehu during this period. Selective partial melting and assimilation of fertile, K- and Rb-rich mineral phases (e.g. biotite) within the meta-sedimentary mineral assemblage is inferred to explain the enriched nature of these melts. A reversion to progressively less silicic and less potassic lavas with lower incompatible element abundances erupted since 26 ka is matched by the recurrent incorporation of crystals that trapped low-K₂O melt inclusions. The trend is interpreted to reflect the exhaustion of fertile phases within assimilated continental source rocks as the crust was progressively heated during long-term thermal conditioning of the arc lithosphere beneath Ruapehu.</p>