University of Tasmania
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The emplacement processes of the St Marys porphyry in eastern Tasmania

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posted on 2023-05-27, 09:16 authored by Gallagher, TM
The St Marys Porphyry (SMP) is unique within the Lachlan Fold Belt, as it is the only interpreted, large-scale, crystal-rich ignimbrite to have been emplaced in the Eastern Tasmania terrane during the middle Devonian. The unit's origin has been contentious due to its granite-like texture, porphyritic nature and abundant crystals. In the 1980s, Nic Turner of Mineral Resources Tasmania mapped the SMP. He interpreted the unit as a massive, welded ash flow tuff, part of which was still connected to its feeder - but this interpretation has been controversial. Confirming the presence of Devonian volcanism is key to better understanding the crustal architecture of eastern Tasmania. Furthermore, this has implications for understanding the evolution of the Eastern Tasmania terrane within the larger setting of the Lachlan Fold Belt. The main aim of this thesis was to determine whether the SMP is volcanic or intrusive. If the SMP is volcanic, it is then important to identify its volcanic architecture. This project revisits Turner's interpretation of the SMP, using modern techniques, by: 1. Characterising and estimating the extent of distinct facies. 2. Mapping the type, size and abundance of lithic clasts. 3. Analysing the size, aspect ratio, texture and distribution of enigmatic features describedhere as White Igneous Lenticular Domains (WILDs) but which Turner called'schlieren', to understand their origins. 4. Radiometrically dating the SMP and surrounding units using U-Pb ICPMS zirconanalysis. The main findings of this project were: 1. The SMP contains six distinct facies, four of which are volcaniclastic and two of whichare intrusive. 2. The distribution of lithic clasts in the SMP is inhomogeneous, with distinct zones ofhigh lithic abundance mapped along the coastal outcrop of the SMP. Smaller and morebroken sedimentary lithic clasts were associated with zones of high lithic abundance. 3. WILDs are a component of Facies C, along the coastal outcrop and inland. Althoughvisually apparent in outcrop, they are difficult to distinguish from Facies C under themicroscope, as they contain the same mineralogy and granophyric textures. However, WILDs are distinct petrographically from the bulk of Facies C in that they are more porphyritic, contain less broken crystals and are more groundmass supported. 4. U-Pb zircon dating found that all units of the SMP are of a similar age: between 397 -389 Ma. The underlying Scamander Formation is only slightly older (400.5 +/- 2.4 Ma),effectively identical to two phases of the Catos Creek Dyke (401- 398 Ma). ThePiccaninny Creek Granite is also contemporaneous with the SMP, but slightly youngerthan the Catos Creek Dyke (397 - 393 Ma). Together, these findings were used to interpret the facies, recommend a mode of emplacement and propose the volcanic architecture. Results from this project indicate that the SMP was emplaced as multiple pulses of ignimbrite flows that ponded in a subsided caldera. The extreme thickness of the pile (over 1,670 m thick) insulated the bulk of the unit, causing welding and recrystallisation. Flow unit boundaries are difficult to identify now because they are masked by welding. The presence of lithic rich zones and the orientation of WILDs are indicators for reconstructing the volcanic architecture. Facies C represents the most insulated portions of the SMP; here, welding textures became devitrified, and the matrix recrystallised into a granophyric texture. The Piccaninny Creek Granite is part of the intra-caldera complex and intruded via the Southern Caldera Fault. This interpretation is in accordance with Nic Turner's interpretation. This project has re-confirmed that middle Devonian volcanism occurred in the Eastern Tasmania terrane. Furthermore, the new results define a slightly older age of the Tabberabberan Orogeny in eastern Tasmania (~391 Ma), indicating the Eastern Tasmania terrane is temporally distinct from the rest of the Lachlan fold belt.


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