Year

2013

Degree Name

Bachelor of Science (Honours)

ANZSRC / FoR Code

0403 GEOLOGY

Department

School of Earth & Environmental Science

Advisor(s)

Solomon Buckman

Abstract

The Lachlan Orogen of south eastern Australia is a complex orogenic belt that has evolved through various continental growth mechanisms on the eastern margin of Gondwana. Stepwise accretion, cordilleran crustal thickening and exotic terrane accretion have all been proposed for the growth of the Lachlan Orogen through ca 520 to 340 Ma. Felsic volcanism is derived from crustal anatexis and is commonly associated with continental arc and rifted environments and as such provides important clues to the tectonic setting active during time of emplacement. Initial onset of felsic volcanism within the Lachlan Orogen is marked by the eruption of the Canowindra Volcanics at 432 ± 6 Ma coinciding with an end to compressional deformation of the Late Silurian Benambran Orogenic event. The Mullions Range Volcanics occur on the western flank of the Hill End Trough and a new age date of 433.5 ± 4.7 Ma is reported, indicating that both the Canowindra and Mullions Range volcanics erupted coevally during the Early Silurian. Ages for these two units are coeval with both the Canowindra and Carcoar Granodiorite but display different geochemistry to the I-type Carcoar intrusive. Anatexis of buried Ordovician turbidites gives the volcanics distinct S-type geochemical affiliations where CaO, NaO2 and Sr are depleted relative to I-type rocks due to removal of these components during the weathering processes of the buried mature sediments. High Ba in the volcanics indicates crystal fractionation during ascent of the genetic magma. The contrasting geochemistry between the volcanics and the Carcoar Granodiorite are attributed to mid crustal contributions and residence times of magmas in rocks of the Macquarie Arc compared to buried Ordovician turbidite sediments. Zircon U/Pb age dates yielded from the main felsic volcanics of the Dulladerry Volcanics more tightly constrains the termination of felsic volcanism in the Lachlan Orogen to Middle to Late Devonian (385 ± 5 to 377 ± 5 Ma). Rocks of the Dulladerry volcanics show distinct A-type geochemistry with high concentrations of silica (~78-80%), Zr, Y, Nb, Ga and Na2O+K2O. Mineralisation within the felsic volcanics of the region is dominated by volcanogenic massive sulfide deposits within the Mullions Range Volcanics and bounding units with minor magmatic/orogenic characteristics developed in the Canowindra Volcanics. Sulfide inclusions trapped within magmatic zircons of all units sampled indicates mineralisation began, at least in part, in the magmatic phase of the formations. Sulfide inclusions within zircons of the Canowindra Volcanics contained no native lead, indicating that lead mineralisation in quartz veins was likely derived from buried sediments. Exploration potential within felsic volcanics remains high with indications from sulfide inclusions of a fertile source melt and indication of fractionation inferring that with suitable host rock and fluid migration economic mineralisation is likely. By building on current understanding of felsic volcanism better constraints on the timing of tectonic settings in the Lachlan can be made.

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