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Volcanology, geochemistry and structure of the Ordovician Cargo Volcanics in the Cargo – Walli region, central New South Wales


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Simpson, CJ, Scott, RJ, Crawford, AJ and Meffre, S 2007 , 'Volcanology, geochemistry and structure of the Ordovician Cargo Volcanics in the Cargo – Walli region, central New South Wales' , Australian Journal of Earth Sciences, vol. 54, no. 2- 3 , pp. 315-352 , doi: 10.1080/08120090701221706.

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The Middle to lower Upper Ordovician Cargo Volcanics occur in a structural outlier immediately west of
the contiguous Molong Volcanic Belt of the Ordovician Macquarie Arc. The sequence of basaltic to
dacitic lavas, lava breccias and associated volcaniclastic rocks with medium-K calc-alkaline affinities
has been interpreted as the subaqueous portion of a major intra-oceanic arc stratovolcano. The oldest
part of the succession consists of massive to pillowed, poorly vesicular, aphyric to moderately
plagioclaseþclinopyxroxene-phyric basalt and andesite lavas and hypabyssal sills. The main part of
the interpreted volcanic edifice is composed of massive and pillowed vesicular andesite and dacite,
flanked by lenses of hyaloclastite, pillow breccias and interbedded with debris-flow and turbiditic
deposits of crystal-rich and pebbly volcanic sandstones, siltstone and minor conglomerate. Uplift and
erosion of the edifice prior to deposition of the overlying Eastonian Barrajin Group limestones is
indicated by local aprons of thickly bedded subaqueous volcanic conglomerate and pebbly
sandstone which appear to be separated from both the underlying andesite-dominant pile and
overlying limestones by angular unconformities. The Cargo Volcanics, and the neighbouring Walli
Volcanics further south, are clearly distinguished from other Macquarie Arc lavas by their high (425)
Zr/Nb values. However, the Walli Volcanics are readily differentiated from the Cargo Volcanics by their
higher P2O5 contents and likely high-K calc-alkaline to shoshonitic affinities. The Cargo Volcanics are
intruded by Cu– Au mineralised, plagioclaseþhornblendeþquartz-phyric dacites with medium-K calc-
alkaline affinities. These dacites have lower TiO2 and higher MgO contents at any SiO2 level compared
to the Cargo Volcanics, and are compositionally similar to Late Ordovician to Early Silurian (453 –
441 Ma) dacites at Copper Hill and in the Narromine Complex. However, at Cargo, clasts derived from
the dacites are locally abundant in volcaniclastic conglomerate near the top of the Cargo Volcanics,
indicating that the dacites were intruded and exhumed prior to deposition of the Barrajin Group
limestones, which commenced at ca 454 Ma in this area. The dacites at Cargo are intruded by small,
apparently unmineralised monzonitic intrusions with shoshonitic affinities, which also appear to pre-date
deposition of the Barrajin Group. LA-ICPMS U– Pb dating of detrital zircons from the Ranch Member at
the base of the Daylesford Limestone (basal Barrajin Group) revealed a dominant population with an
average age of 453.0+4.1 Ma (identical within error to the mid-Ea1 age for the host sediments based
on fossil assemblages) and subordinate populations with average ages of 480 Ma and 505 Ma. The
453 Ma zircons were most probably derived from either the intrusive dacites or monzonites, suggesting
that the Cargo region was rapidly exhumed following emplacement of the felsic intrusions. The angular
unconformities near and at the top of the Cargo Volcanics and the broadly coincident change in
magma chemistry suggest that this sector of the Macquarie Arc underwent major tectonic upheaval
commencing in the latest Gisbornian – early Eastonian. The Cargo Volcanics are interpreted to have
undergone major eastward translation at this time (from an original position now buried beneath the
Cowra Trough). Their present juxtaposition with the contiguous Molong Volcanic Belt probably did not
occur until the late Early Silurian during the final stages of the Benambran Orogeny.

Item Type: Article
Authors/Creators:Simpson, CJ and Scott, RJ and Crawford, AJ and Meffre, S
Keywords: Cargo Volcanics, geochemistry, Macquarie Arc, Ordovician, stratigraphy, stratovolcano.
Journal or Publication Title: Australian Journal of Earth Sciences
ISSN: 0812-0099
DOI / ID Number: 10.1080/08120090701221706
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