![]() |
||||||||||||||
Australia: The Land Where Time Began |
||||||||||||||
Australian
Western Margin – Evolution During the Supercontinent of Rodinia and
Gondwana According to Wilde it appears the proto-Darling
Fault Zone and the Darling Fault, its successor that extend 1,000 km
along the western margin of the continent, have been active at several
periods in the geological past. At ~ 2,570 Ma deformation began which
affected granitoids of Late Archaean age along the western margin of the
Yilgarn Craton. Events related to the accretion and breakup that was
associated with the supercontinent cycles of Gondwana and Rodinia are
reflected in much of the later activity. Northampton and Mullingarra Complexes in the north
are part of a high-grade Grenvillian orogenic belt that is located to
the west of the Darling Fault, which is referred to as the Pinjarra
Orogen. They form part of a global collisional event that resulted in
the amalgamation of Rodinia, having undergone granulite facies
metamorphism ~1,080 Ma. These rocks extend to the south beneath the
Phanerozoic sedimentary cover of the Perth Basin, where they are
constrained by the Darling Fault and to the west by the Dunsborough
Fault, the latter marking the boundary of the Leeuwin Complex. The Leeuwin Complex, which is traditionally
considered to be part of the Pinjarra Orogen, is a fragment of the pan
African crust. Its composition is predominantly of upper amphibolite to
granulite facies felsic orthogneisses that are derived from A-type
anorogenic granitoids. It has been established by conventional and
SHRIMP U-Pb zircon geochronology that the granitoids evolved between
~780 Ma and ~520 Ma and were metamorphosed ~615 Ma. These events have
been equated with the breakup of Rodinia. The juvenile nature of the
crust and the lack of evidence for the involvement of Archaean
continental material that was pre-existing are supported by Sm-Nd whole
rock data. The Dunsborough and Darling Faults were reactivated
during the Phanerozoic as normal faults that define the inner arm of a
major rift system within eastern Gondwana and controlling sedimentation
in the Perth Basin that at present overlies the Grenvillian terrane. By
the Late Jurassic major normal movement of the Darling Fault ceased and
in the Early Cretaceous continental breakup occurred along fractures
that were closely related to the western boundary of the Leeuwin Complex
that defined the eastern margin of the outer arm of the rift system. At
~132 Ma breakup began between Australia and Greater India and was
followed at 130 Ma and 123 Ma by the eruption of the Bunbury Basalt.
Wilde suggests this was probably the result of hotspot activity beneath
Eastern Gondwana and may have been a reflection of the Kerguelen Plume,
though the evidence is equivocal. Based on the petrographic, geochemical and isotopic
characteristics, as well as with the likely contiguity of the Eastern
Gondwana continents since the assembly of Rodinia, Wilde says it is
argued that the Leeuwin Complex evolved within an intracrustal rift and
is not an exotic terrane. It should be considered a separate terrane as
it is distinct from the portions of the Pinjarra Orogen that is
adjacent. Wilde recommends that the term ‘Pinjarra Orogen’ be confined
to rocks that record the Grenvillian events, which thereby excludes
those rocks (the Leeuwin Complex) that evolved during the Pan-African
Orogeny that occurred later.
|
|
|||||||||||||
|
||||||||||||||
Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |