Age, origin and extent of the Newer Volcanic Province in the Melbourne region, Australia
AuthorHeath, Michael Anthony
AffiliationSchool of Earth Sciences
Document TypePhD thesis
Access StatusOpen Access
© 2019 Michael Anthony Heath
Abstract Intraplate basaltic volcanism is present on every continent, untethered to any specific tectonic setting. The ca. 4.6 Ma – 5 ka Newer Volcanic Province (NVP) in south-eastern Australia is a chemically and morphologically diverse intraplate basaltic province. Its diversity, preservation and accessibility make it an ideal natural laboratory for investigating the causes and evolution of intraplate magmatism worldwide. Previous studies in the NVP have put forward competing magmatic models, but their validity is dependent on a limited geochronological dataset. Furthermore, previous geochronology has seldom been supported by geochemical or geomorphological studies on the same volcanic products, such that there is often a disconnect between the absolute ages of NVP rocks and the extent and composition of their associated lava flows or eruption points. This study utilised a holistic approach to lava flow mapping, with a focus on the diagnostic petrographic and geochemical features of individual basaltic lava flows selected for geochronology. The current study area of Melbourne, which is located at the eastern margin of the NVP, was selected for its age range and geochemical complexity, rivalling those of the entire NVP. Drill core was utilised to trace lava flows at depths of up to 80 m, thereby unravelling complex flow networks and facilitating the construction of a detailed lava flow map for the Melbourne area. Trace element geochemistry was utilised to distinguish between petrographically similar flows, and to investigate the eruption of three chemically distinct magma batches erupted consecutively from Mount Fraser. This mapping and geochemical work is complemented by new, high-precision 40Ar/39Ar age constraints, spanning ~7.9 Ma – 0.8 Ma. The new geochronological constraints on Melbourne lava flows reveal that the earliest activity (~7.9 – 3.8 Ma) was dominated by small-volume eruptions that predominantly produced alkali basalts. From oldest to youngest, these included the Bald Hill, Mount Ridley, Tullamarine, Crowe Hill, Spring Hill, Summerhill Rd, Redstone Hill and Aitken Hill lava flows. After ~3.8 Ma, large-volume eruptions dominated, with Fenton Hill, Mount Kororoit, Tulloch Hill and Mount Fraser producing lavas generally of tholeiitic composition. This progression from alkali basalt to tholeiitic volcanism over time is contrary to the purported province-wide progression from tholeiitic to alkali-rich lavas, indicating either that geochemical evolution in Melbourne was distinct from that of the NVP, or that the conclusions of province-wide studies are based on an unrepresentative sample set. An age of 7.931 +/- 0.038 Ma for the Bald Hill Lava Flow far exceeds the generally accepted maximum age of NVP activity (ca. 4.6 Ma). This lava flow, along with those erupted from Pretty Sally, Green Hill and Mount Cooper, is also geochemically distinct from other Victorian NVP products, but is not dissimilar to lavas of the Cosgrove Leucitite suite, the purported products of a long-lived mantle plume. This raises the possibility that the ‘Cosgrove Plume’ traversed the latitude of Melbourne just after 7.9 Ma, casting doubt on its possible role in the initiation of NVP activity some 3.3 million years later. 11 of the 38 samples selected for 40Ar/39Ar dating produced concordant results, with the remainder exhibiting varying degrees of discordance. The underlying causes of this discordance and implications for accurate age determinations are examined and modelled in age spectra and inverse isochron space. A correlation is found between the proportion of radiogenic 40Ar (40Ar*) released and the nature of discordance exhibited by a sample. 39Ar recoil is suggested as the most likely cause of discordance in high-40Ar* samples, whereas low-40Ar* samples exhibit discordance consistent with the modelled effects of mass-dependent fractionation. Based on modelling results, isochron rotation is the main impact of isotopic disturbance on an inverse isochron plot (39Ar/40Ar vs 36Ar/40Ar), leading to a negative correlation between 40Ar/39Ar ages and (40Ar/36Ar)i values. A new framework for the treatment of 40Ar/39Ar data from basaltic rocks is submitted, optimising the interpretation of inverse isochrons and informing the allocation of age constraints. Finally, a new method of 40Ar/39Ar data treatment, here named the multi-isochron approach to 40Ar/39Ar dating, is introduced. A multi-isochron regression utilises the combined output of all possible inverse isochrons from data of a single aliquant to determine its ideal isochron and eruption age. This method has the potential of transforming 40Ar/39Ar data treatment, allowing eruption ages to be calculated even when isotopic disturbance is severe, and to be reported in cases where this was previously not possible.
KeywordsIntraplate volcanism; Newer Volcanic Province; 40Ar/39Ar dating; Basalt; Geochemistry; Isochron; Melbourne
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