Honours Thesis Abstract
Alex Farrar
University of Melbourne, Victoria
2006 Terra Search - AIG Honours Bursary Winner
The hypothesis that a Proterozoic continental crustal block or the ‘Selwyn Block’ forms the basement to central Victoria is controversial. The proximity of the majority of
Victoria’s major 440Ma gold deposits to the inferred western margin of this block makes understanding this structure critical to the success of future gold exploration in
Victoria.
This research report utilised the methods of Illite Crystallinity (IC) and b lattice parameter spacing ( b0 ) of illite to measure the peak thermal and barometric conditions respectively of the rocks in the western Lachlan Fold Belt, in the region proximal to the inferred western margin of the Selwyn Block. This was done in the hope to better define the western margin of the Selwyn Block. The IC and b0 data collected support the presence of the Selwyn Block, due to major thermal and barometric trends observed in the western Lachlan Fold Belt. Higher grade (Epizone) sediments were found to lie to the west of the Whitelaw Fault whilst lower grade (Anchizone) sediments were found to lie to the east of the Whitelaw Fault. The change from Epizone conditions to Anchizone conditions was found to be extremely sharp, and occurs over the distance of a few hundred metres. It was therefore determined that this thermal boundary represents where the western margin of the Selwyn Block existed at the time of peak deformation at ~440Ma, as the Selwyn Block would have shielded its overlying sediments from peak deformation.
The barometric data suggest the presence of thickened lithosphere to the west of the
Selwyn Block. This is interpreted to have formed in a compressional setting between the converging Selwyn Block and the Australian Craton. Late in the Benambran Orogeny the orogenic belt is thinned from erosive processes resulting in asthenospheric upwelling, which produced the Early Devonian I – type granites. The later deformation of the Melbourne Zone and production of Late Devonian S – type granites is attributed to the formation of a decollement along the top of the Selwyn Block.