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The geology, geochemistry and structure of the Mount Darwin-South Darwin Peak area, western Tasmania

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Jones, AT (1993) The geology, geochemistry and structure of the Mount Darwin-South Darwin Peak area, western Tasmania. Honours thesis, University of Tasmania.

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Abstract

The Cambrian Darwin Granite intrudes calc-alkaline rhyolites of the Central Volcanic Complex
on the Darwin Plateau, Western Tasmania. Two distinct granite phases are recognised, an
equigranular granite and a granodiorite. A biotite grade contact aureole is preserved in the
Central Volcanic Complex immediate to the Darwin Granite. Debris flow deposits of
volcaniclastic conglomerates and sandstones, and coherent dacite lavas of the Mid - Late
Cambrian Tyndall Group unconformably overlie the Darwin Granite and Central Volcanic
Complex, and are in turn overlain unconformably by pebble to boulder conglomerates of the
siliciclastic Owen Conglomerate.
Stratigraphic and structural evidence recognise three deformation periods within the Mt Darwin
- South Darwin Peak area: the Mid - Late Cambrian, Late Cambrian - Early Ordovician, and the
Devonian Tabberabberan Orogeny. Mid - Late Cambrian deformation, evidenced by granitic
and foliated volcanic clasts in basal Tyndall Group conglomerate, indicates catastrophic uplift
and subsequent unroofing of the granite prior to Tyndall Group deposition. This unconformity
represents a significant Cambrian hiatus in the southern Mount Read Volcanics. A second
unconformity between the Tyndall Group and the Owen Conglomerate marks cessation of
Tyndall Group deposition with the onset of deposition of large volumes of siliceous detritus.
The two Devonian Tabberabberan-related deformations are characterised by, NW and Ntrending
dextral strike slip faulting and locally intense N-trending cleavage development. The
Devonian structures dominate the region and have obscured the effects of earlier deformations.
Devonian metamorphism to greenschist facies, characterised by chlorite-sericite assemblages,
occurs throughout the area with intense development in shear zones.
Epigenetic, granite-related, vein and disseminated Cu-Au mineralisation occurs within and
adjacent to the Darwin Granite. Sulphur isotope data indicates mixing of Cambrian seawater
with magmatic-hydrothermal fluids in a hydrothermal convection cell generated during
emplacement of the Darwin Granite. Depletion of Na20 and CaO along with K20 and FeO
enrichment have occw;red during pervasive K-feldspar, sericite and chlorite alteration.
Geochemical analyses of the Central Volcanic Complex and the magnetite series Darwin Granite
suggest that these units were emplaced within an extensional basin (back-arc?), at an Andeantype
margin. Granitic magma was generated through partial melting of an upper mantle to
lower crustal source. REE data suggests that the CVC and Darwin Granite are not comagmatic,
and field relationships preclude a genetic relationship between the Tyndall Group
and the Darwin Granite. Elemental discrimination trends illustrates some similarities between
the Darwin Granite and the most evolved phases of the Murchison Granite. However, the
Darwin Granite displays no Eu anomaly in contrast to the negative Eu anomaly and HREE
enrichment in the Murchison Granite, suggesting that the two granite systems are not
genetically related.

Item Type: Thesis (Honours)
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Date Deposited: 11 Aug 2011 01:20
Last Modified: 11 Mar 2016 05:55
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