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Re-evaluation of contact relationships between Ordovician volcanic belts and the quartz-rich turbidites of the Lachlan Orogen

Meffre, S, Scott, RJ, Glen, RA and Squire, RJ 2007 , 'Re-evaluation of contact relationships between Ordovician volcanic belts and the quartz-rich turbidites of the Lachlan Orogen' , Australian Journal of Earth Sciences, vol. 2-3, no. 54 , pp. 363-383 , doi:

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Some published tectonic reconstructions of the eastern Lachlan Orogen in New South Wales have
shown Ordovician volcanic and volcaniclastic rocks of the Macquarie Arc conformably overlying or
interfingering with a coeval Ordovician quartz-rich turbidite sequence. Re-examination of key contacts
between the volcanic and quartz-rich successions has found no evidence to support this interpretation,
and suggests that the two packages are separate tectonostratigraphic terranes. The contacts
between these two coeval successions are generally marked by major faults containing mylonites,
cataclasites and, at some locations, fragments of mid-ocean ridge-type pillow basalt and chert. The
quartz-rich turbidites are generally highly deformed and of higher metamorphic grade than the
adjacent volcanics. At Oberon and Mudgee, contacts are faulted but there are no mylonites or
significant differences in metamorphic grade. At Palmers Oaky and Black Springs, Silurian quartz-rich
sandstones overlying the Ordovician volcanics have been mistakenly assigned to the Ordovician in
previous studies. Throughout the Lachlan Orogen, there is no mixing of framework grains. Quartz-rich
turbidite successions are dominated by quartz with lesser feldspar and rare tourmaline, zircon and
monazite derived from recycled continental sources. In contrast, the volcaniclastic sandstones contain
feldspar, clinopyroxene and lithic fragments derived from subduction-related clinopyroxene-phyric
basalt and plagioclase-phyric andesite. Detrital-zircon populations also differ, with separate U/Pb age
populations and almost no overlap. Comparison of the Ordovician sequences of the Lachlan Orogen
with modern turbidites from continental- and arc-related sedimentary basins suggests that complete
separation of sedimentary sources is only possible if the sandstones were deposited hundreds of
kilometres apart, in separate tectonic environments. The two sequences were juxtaposed along major
faults in the Late Ordovician or Early Silurian, probably when the Macquarie Arc collided with a thick
Ordovician sedimentary wedge located on the Gondwanan continental margin.

Item Type: Article
Authors/Creators:Meffre, S and Scott, RJ and Glen, RA and Squire, RJ
Keywords: Benambran, collision, detrital zircon, island arc, Lachlan Orogen, Ordovician, turbidites.
Journal or Publication Title: Australian Journal of Earth Sciences
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