Stratigraphy, geochemistry and origin of products of complex volcanic centres, newer volcanics basaltic field, Victoria

2017-03-01T04:45:10Z (GMT) by Boyce, Julie Ann
Monogenetic volcanoes are the most common form of continental basaltic volcanism on Earth and hence present great risk upon eruption in heavily populated areas. Recent studies of individual monogenetic volcanic centres demonstrate that, far from being simple in nature as traditionally assumed, large variations exist between centres in terms of the types of magmas involved in their formation, the sources of those magmas, their stratigraphic architectures, and the timing of the eruption products. The Pliocene–Recent Newer Volcanics Province (NVP) and in particular the Mt Rouse magmatic volcanic complex were studied in detail using desktop, fieldwork and laboratory based techniques in order to gain further insights into the processes and occurrences of monogenetic volcanism in southeast Australia, as well as the complexity of monogenetic volcanism in general. The techniques used in this study can be applied to other monogenetic volcanic fields. A database of NVP volcanoes was produced, documenting >729 vents from >437 volcanoes. A new classification scheme was proposed for the volcanoes of the NVP, whereby volcanic centres are either simple or complex, with subtypes including scoria cones, maars, ash cones and domes, plus magmatic, phreatomagmatic and maar–cone volcanic complexes. An average eruption frequency for the province is estimated to be 1:10 800 yrs. The volcanoes of the Hamilton area in the Western Plains subprovince were examined in more detail, using desk and field-based studies. Sixteen eruption centres were recognised, including three previously unrecorded volcanoes. Three phases of volcanism were confirmed, with ages of >4 Ma (Phase 1), ~2 Ma (Phase 2) and <0.5 Ma (Phase 3). Geochemically, compositions become increasingly alkaline from Phase 1 to 2, with Phase 3 eruptions covering the entire geochemical range and extending into increasingly incompatible element-enriched compositions. The complex stratigraphic architecture of Mt Rouse was investigated in detail using field mapping and sampling for petrography. Eight main facies were documented, of which two are coherent and six fragmental. The eruption styles giving rise to the facies include Hawaiian, Strombolian, micro-Plinian, violent-Strombolian and phreatomagmatic. Major and trace element geochemical data were generated together with Sr–Nd–Pb isotope data for samples selected on the basis of the stratigraphic framework of Mt Rouse. The data define three magma batches, Batches A, B and C all of which are geochemically similar to Ocean Island Basalts. Each batch features LREE-enrichment, increasing from batch A to C; there is no evidence of crustal contamination. The Sr–Nd–Pb isotope data define two groupings, one comprising magma batch A and a second for batches B and C. Estimates of the depths of mantle melting from primary magma compositions suggest that the magmas were sourced from a zone extending across the lithosphere– asthenosphere boundary. Batch A was sourced from the deep lithosphere at a pressure of 1.7 GPa, corresponding to 55.5 km depth, and batches B and C from the shallow asthenosphere at 1.88 GPa/61km and 1.94 GPa/63 km, respectively. A complex eruption sequence outlined for Mt Rouse includes sequential eruption of magma batches A, C and B, then simultaneous eruption of batches A and B, over four stages with multiple sub-stages and no evidence of time-breaks in eruption. This study has shown that monogenetic volcanoes may be far from simple, often showing great complexity in terms of their eruption styles and magma genesis. This will have significant implications for studies of monogenetic volcanism and the emergent field of study of complex monogenetic volcanoes both in the NVP and worldwide.