The Origin of Intraplate, Monogenetic Volcanism in Southeastern Australia
Abstract
The Newer Volcanic Province (NVP) is the youngest and most extensive occurrence of Cenozoic intraplate volcanism in Australia. This widespread (15,000 km2), low-volume (20,000 km3) basaltic field is the product of a discontinuous series of eruptions that have occurred over the last 5 million years. The most recent (>5000 BP) volcanic centres of the NVP are Mt. Gambier and Mt. Schank. The intraplate setting of the NVP, along with broadly OIB-like, enriched geochemical and isotopic signatures, resulted in the long-time assumption that this volcanism was related to a mantle plume. However, lack of age progression in these monogenetic volcanoes suggests a more complex origin. Another possibility for the origin of the NVP is edge-driven mantle convection. This involves lowering the mantle solidus by introducing lithosphere-derived volatiles and alkalis, via small-scale convection in the asthenospheric upper mantle. This work combines both geophysical and geochemical modelling to test these hypotheses and further explore the cause of intraplate volcanism in southeastern Australia. We undertook magnetotelluric (MT) 2D modelling of 17 sites along two 90 km transects: one across the NVP (from Mt. Gambier) and the other beyond the western edge of the NVP (from Robe). Both lines show low resistivity (<5 Ω.m) at shallow crustal depths (<10 km) due to seawater and high porosity sediments of the Otway Basin. The Robe line suggests the lithosphere beyond the NVP is uniformly resistive (>500 Ω.m) at lower crustal depths and upper mantle depths. However, the Gambier line within the NVP shows a low-resistivity anomaly of 50 Ω.m at >50 km depth, suggesting a temperature anomaly of up to 200˚C, with the possible presence of <14% melt. Geochemical modelling of data from lherzolite, kimberlite and basalt samples, using the pMELTS software program, suggests a starting composition of lherzolite combined with 5-10% enriched kimberlite, containing 0.17-0.35 wt% volatiles (eg: H2O), at 4-15% partial melting for the basalts of the SA NVP. These findings suggest against the possibility of a mantle plume source for this region. Instead, they are consistent with the hypothesis of edge-driven convection for the origin of volcanism in southeastern Australia.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.V51B0353S
- Keywords:
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- 8178 Tectonics and magmatism;
- TECTONOPHYSICS;
- 8415 Intra-plate processes;
- VOLCANOLOGY;
- 8439 Physics and chemistry of magma bodies;
- VOLCANOLOGY;
- 8488 Volcanic hazards and risks;
- VOLCANOLOGY