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Australia: The Land Where Time Began |
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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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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |