Albany-Fraser Orogen, Western Australia – Transformation of the Margin of a Craton from the Archaean During Formation of a Basin and Magmatism in the Proterozoic

Australia: The Land Where Time Began

Albany-Fraser Orogen, Western Australia – Transformation of the Margin of a Craton from the Archaean During Formation of a Basin and Magmatism in the Proterozoic

According to Spaggiari et al. the Albany-Fraser Orogen is an example of a modification that occurred in the Proterozoic of a margin of a craton from the Archaean that is well preserved. Distinct changes in tectonic regimes which resulted in significant transformations of the crust from the Archaean are reflected in the formation of 2 successive basin systems that was accompanied by magmatism along the southern and southeastern margin of the Yilgarn Craton in Western Australia. It is indicated by provenance analysis from detrital zircons that the first basin system – the Barren Basin from about 1,815-1,600 Ma – which was predominantly filled with detritus from the Neoarchaean derived from the Yilgarn Craton, and detritus of Palaeoproterozoic age that was derived from coeval voluminous magmatism that was mostly felsic. A largely extensional tectonic setting, which is consistent with either a continental rift basin or a long-lived backarc basin system, is indicated by the abundance of sediment that was derived locally was deposited onto a reworked substrate of the Archaean Yilgarn Craton, coupled with the isotopic signature of magmas that were progressively more juvenile. A passive margin and oceanic basin along the edge of the craton, the Arid Basin from about 1,600-1,305 Ma – and formation of the ocean continent transition in the most heavily modified, outboard region of the margin of the craton (the eastern Nornalup Zone) resulted from the extensional regime. A change in tectonic regime is denoted by an ocean-arc formation, the Loongana Arc dating to about 1,410 Ma, which is preserved in the adjoining Madura Province. Accretion of the Loongana Arc to the edge of the craton, which is recorded by the presence of exotic detritus derived from this arc and deposited into the uppermost Arid Basin (a foreland basin) resulted from oceanic subduction. Crustal thickening and the earliest phase of magmatism of Stage I of the Albany-Fraser Orogeny at 1,330 Ma was triggered by this accretion event. The orogen was placed into a backarc setting, into which the remainder of the Recherche Supersuite mafic and felsic magmas were intruded as a result of a change to west-dipping subduction beneath the accreted portion of the Loongana Arc. The Albany-Fraser Orogen is therefore regarded as a craton margin of Archaean age that preserves a long history of transformation during the Proterozoic that was dominated by extensional processes resulting in the formation of orogen-wide basin systems, accompanied by magmatism. Important constraints on the reconstructions of Nuna and Rodinia supercontinents are provided by the recognition that the Albany-Fraser Orogen has always been part of the West Australian Craton (WAC), and the tectonic regimes that have affected it.

Conclusions

The Albany-Fraser Orogeny preserves 2 basins that are regionally extensive systems – the Barren Basin and the Arid Basin – and their evolution records the modification of the southern and southeastern margin of the Yilgarn Craton of Archaean age and changes in tectonic regime that occurred from about 1,850 Ma to 1,300 Ma.

Deposition of sedimentary rocks that were predominantly mature, quartz-rich occurred through 3 main phases; before about 1,800 Ma, prior to about 1,700 Ma and 1,600 Ma, all of which coincide with the magmatic events that progressively modified the Archaean crust. It is indicated by analysis of detrital zircons and interpretations of their provenance that the Yilgarn Craton hinterland shed detritus as the margin was extended, and mixed with detritus that was derived locally from syn-magmatic rocks. Spaggiari et al. interpret the basin as having either formed in a continental rift setting or alternatively, in a backarc setting in which the subduction zone and magmatic-arc are a substantial distance outboard from the Yilgarn Craton.

Extension continued after about 1,650 Ma leading to the formation of the Arid Basin, an ocean basin that was flanked by a passive margin that developed on the strongly modified Yilgarn Craton margin (Biranup Zone and Nornalup Zone ocean-continent transition). The Arid basin, in contrast to the Barren Basin, contains a greater variety of lithologies, and is dominated by about 1,455 Ma until 1,375 Ma by detritus that does not correspond with any sources known from the Albany-Fraser Orogen. Also, zircons that date to this time contain the most juvenile Lu-Hf isotope signature that has been recorded in the Albany-Fraser orogen, which points to a crust that was newly formed and of different character. Spaggiari et al. interpret that source as an oceanic arc that formed within the adjacent Madura Province – the Loongana Arc from about 1,410 Ma – which has a similar isotope and age signature as the major component of the zircon detritus in the Arid Basin.

A change to convergence is signified by the presence of the Loongana Arc, with an ocean-ocean subduction zone that is east-dipping located within the ocean-ocean basin (Madura Province). By about 1,330 Ma basin closure resulted in a soft collision and accretion of the ocean arc onto the eastern, passive margin, edge of the Albany-Fraser Orogen to form a suture that is defined by the Rodona Shear Zone. Detritus was allowed by this to shed to the west towards the craton and hinterland into a foreland basin (uppermost Arid Basin). Stage I of the Albany-Fraser Orogeny was marked by the accretion event.

West-dipping subduction was initiated beneath the accreted oceanic arc as convergence continued, consuming oceanic crust of the Madura Province to the east, and placing the Albany-Fraser Orogen into a backarc setting, and in this the remainder of the intrusions of the Recherche Supersuite were emplaced. The Albany-Fraser Orogen is suggested by the new tectonic models that it should not now be considered to be simply a Mesoproterozoic collisional belt; rather it should be seen as an extensional and accretionary orogen that was built on modified, Archaean crust of the West Australian Craton (WAC). Spaggiari et al. propose that the collision zone with the Mawson Craton and the South Australian Craton (SAC) is located to the east in the Madura Province or the Coompana Province, beneath the Eucla Basin and the Bight Basin.

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