The geological evolution of the Norseman Area, Western Australia

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: ‚Äö Mafic volcanic rocks and dolerite intrusives; ‚Äö Ultramafic rocks; ‚Äö Mafic conglomerate and sandstone; ‚Äö Fine-grained sedimentary units, dominantly pyritic black shales and thin-bedded, siltstone-shale turbidites; and ‚Äö 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.