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The geological evolution of the Norseman Area, Western Australia


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Hope, MW 2004 , 'The geological evolution of the Norseman Area, Western Australia', Research Master thesis, University of Tasmania.

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Norseman lies 200 km south of Kalgoorlie within the most southern part of the late
Archaean Eastern Goldfields Province of Western Australia. This study resolves the
stratigraphy and structure of the upper Norseman Terrane and proposes tectonic models to
explain the formation of the terrane and differences with the adjacent Kalgoorlie Terrane.
Problems with the current understanding of the lower stratigraphy of the Norseman Terrane
were identified, but not fully resolved.
Geological mapping at 1:2500-scale of the upper part of the Norseman Terrane at the Polar
Bear Peninsula; revealed five Archaean facies associations:
• Mafic volcanic rocks and dolerite intrusives;
• Ultramafic rocks;
• Mafic conglomerate and sandstone;
• Fine-grained sedimentary units, dominantly pyritic black shales and thin-bedded,
siltstone-shale turbidites; and
• Rhyolite lavas and both in situ and resedimented breccias.
The stratigraphy and geometry of the map patterns, when combined with age dating from the
Eastern Goldfields, suggests that basalts overlying the Norseman komatiite are a thrust
repetition of basalts from below the komatiite. This hypothesis is supported by trace
element geochemistry that found all basalts above and below the komatiite have similar
tholeiitic composition. They do not possess the distinctive siliceous high magnesium
chemistry of the basalts that overlie the komatiite at Kambalda. Basalts from the lower
Penneshaw Formation at the base of the Norseman stratigraphic succession also have a
tholeiitic composition and are interpreted as another thrust repetition of the mafic greenstone
package. The uppermost stratigraphy preserved within the Norseman Terrane is rhyolite at Polar
Bear. Facies analysis of rhyolite and breccias indicates the rock units represent a submarine
lava dome that was emplaced upon unconsolidated black shale, generating peperite breccias.
At the margins of the dome, where it pinches out of the stratigraphy, the equivalent strata are
rhyolite-sediment breccias and sandstone. These represent the lower sections of gravelly
and sandy high-density turbidites resulting from gravitational collapse of sections of the
dome. Underlying fine-grained sediments were incorporated as rip-up clasts.
Two evolved felsic rock suites are distinguished within the Norseman Terrane. The first
suite (Polar Bear rhyolite lava, intrusive Ajax rhyolite porphyry, Harlequin rhyolite
porphyry dyke and the Dundas Granite) is interpreted to have been deiived from fusion of a
siliceous crustal basement source. The second suite (intrusive Polar Bear quartz porphyry
and Harlequin granodiorite) is interpreted to have originated as magma with a high-Al
Trondhjemite-Tonalite-Dacite (TTD) chemistry. Low pressure feldspar fractionation was a
key process in the subsequent development of the members in both suites.
In the adjacent Coolgardie Domain of the Kalgoorlie Terrane, at an equivalent upper
stratigraphic level, dacitic conglomerate, sandstone and siltstone of the Black Flag Beds
(BFB) are present. These rocks represent erosional products of emergent volcanic centres,
which were subsequently deposited as high density turbidites within the Kalgoorlie Basin, a
depression that previous studies indicate is a rift basin developed under an extensional
tectonic regime. These dacitic epiclastic rocks and other intrusive felsic porphyries have
distinctive high-Al TTD suite chemistry, thought to form by melting of young, hot,
subducted oceanic crust. There is no geochemical evidence within these dacites and other
felsic intrusives, of low pressure feldspar fractionation. The stratigraphic and geochemical differences between the felsic rocks within the
Kalgoorlie Terrane and the Norseman Terrane are interpreted to be the result of differing
degrees of extension. The Kalgoorlie Terrane formed the central axis of an island arc. The
arc rifted, forming the Kalgoorlie Basin, which was filled with BFB epiclastic volcanics and
sediments. The extension assisted the ready passage to the surface, of magmas generated
deep in the subduction zone. The Norseman area was marginal to the Kalgoorlie Basin rind
did not undergo substantial extension. Efficient magma conduits were not developed sb that
subduction zone magmas became trapped in crustal magma chambers and underwent
feldspar fractionation. Intrusion and underplating of subduction zone magmas caused
melting of felsic basement forming the rhyolites.
Closure of the Kalgoorlie Basin occurred during N-S directed compression, suggested to
have resulted from continental collision at the subduction zone. This compression caused
large scale thrusts that throw the basaltic greenstories over the komatite in the Norseman
Terrane. It also pushed the entire Norseman area in the north, developing it as an
allochthonous fault-bounded terrane, which split the former Kalgoorlie basin and caused it
to wrap around the Norseman Terrane to the east arid west.

Item Type: Thesis - Research Master
Authors/Creators:Hope, MW
Keywords: Geology, Stratigraphic
Copyright Holders: The Author
Copyright Information:

Copyright 2004 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Available for library use only and limited copying in accordance with the Copyright Act 1968. Thesis (M.Sc.(Ex.Geo.))--University of Tasmania, 2004. Includes bibliographical references. v. 1 - Text. -- v. 2 - Maps

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