Studies on the glaciovolcanic and magmatic evolution of Ruapehu Volcano, New Zealand
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.