Gallagher Till1, Bottrill Ralph1, Carey Rebecca1, Cumming Grace1, Orth Karin1
1University of Tasmania, Hobart, Australia
In the Eastern Tasmania Terrane (ETT), the southern-most extent of the Lachlan Fold Belt, the St Marys Porphyrite (SMP) is thought to represent the only incidence of Devonian volcanism south of mainland Australia. The lack of volcanic succession contrasts with Victoria and New South Wales, where Devonian magmatism is associated with numerous caldera-related volcanic successions.
Turner (1986) described tell-tale textures thought to be volcanic in origin and interpreted the SMP as a dacitic, welded ash-flow tuff. He considered it to be an extrusive equivalent of the hornblende-biotite granodiorite plutons of the Blue Tier Batholith. He interpreted highly welded zones and devitrification textures, but perplexing extrusive and intrusive contact relationships, and granophyric-like crystallisation textures have puzzled subsequent workers. The application of modern volcanology techniques aims to re-evaluate past conclusions and introduce new knowledge and evidence to develop a revised emplacement model for this enigmatic unit.
The distribution and characteristics of texturally diverse polymictic lithic clasts are used to identify variations within the SMP. Several different clast types and a number of textural features have been identified. The most abundant clast type observed throughout the internal part of the SMP is described as ‘white igneous lenticular domains’ or WILDS. The WILDS are tentatively interpreted to represent phenocryst rich relict pumiceous domains and or possible cognate igneous inclusions which form a fabric visible on weathered surfaces. These domains appear to dip consistently to the southwest. This study estimates the original eruption volume of the SMP by using the aspect ratio of both fiamme, WILDS and the surface area of the unit (based on new and previous geological mapping by Turner, (1986).
The interpretation of a volcanic ignimbrite and measurements of the flattening ratios for both fiamme and WILDS enable us to calculate pre-compaction thicknesses of the SMP. Thickness coupled with the area of emplacement enables us to calculate a minimum eruption volume of 920+ km3 It is likely that pre-erosion volumes of the SMP is far greater, as work to understand the geometry of the unit is ongoing. Preliminary volume estimates are speculative but suggest, if the SMP is of a volcanic origin, that it could represent the product(s) of a super eruption (>1000km3 erupted material). Further calculations to estimate the original volume of the SMP are warranted and will assist us to estimate the eruption magnitude related to the emplacement of the unit.
Understanding the eruption and emplacement processes of the SMP expands our understanding of the Devonian upper crustal architecture of eastern Gondwana and provide insight into the plumbing system of eastern-Tasmanian granites.
Till Gallagher is a UTAS honours student and is working on a research project that looks at the emplacement processes of the St Marys Porphyrite in North-eastern Tasmania. The project is supervised by Karin Orth and Rebecca Carey from UTAS and Grace Cumming and Ralph Bottrill from MRT.