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Abstract

The Ordovician to Lower Silurian Forest Reefs Volcanics in the Cadia – Neville region, northeastern
Lachlan Orogen, represent the product of at least two shoshonitic volcanic centres intercalated with a
volcaniclastic apron. The two episodes of shoshonitic volcanism in the Forest Reefs Volcanics are
separated by between 15 and 20 million years. The Forest Reefs Volcanics are informally divided into
lower and upper parts, separated by an inferred unconformity that broadly coincides in age with a
limestone-forming hiatus in volcanism (449 – 447 Ma) and emplacement nearby of medium-K calcalkaline
dacitic intrusions (448 – 445 Ma). The lower part includes shoshonitic basaltic andesite, feldsparrich
sandstone, volcanic lithic breccia and lesser black mudstone; polymictic volcanic conglomerate
with sandstone matrix and calcareous sandstone are important near the top. The basaltic facies
association occurs near the base of the lower Forest Reefs Volcanics and could represent a submarine
basaltic volcano at least 12 km in diameter. A major change in provenance for the upper Forest Reefs
Volcanics is reflected in the increased abundance of ferromagnesian crystals and coarse mafic
volcanic fragments, particularly in the mafic volcanic sandstone, polymictic volcanic conglomerate
with coarse volcanic matrix and polymictic hornblende andesite breccia. The trachyandesite facies
association records another episode of shoshonitic volcanism that probably occurred late in the
accumulation of the upper Forest Reefs Volcanics and could represent an intrusive complex or the
initial, largely shallow intrusive stages of a cone volcano. Highly porphyritic basalt to basaltic andesite
intrusions were emplaced as sills before the volcaniclastic succession was lithified. The final magmatic
activity generated coarsely equigranular, mafic to intermediate intrusions. The trachyandesite facies
association, the highly porphyritic basalt to basaltic andesite intrusions and the coarsely equigranular
intrusions were emplaced successively in a relatively short time (about 443 – 439 Ma). The complex
facies architecture of the Forest Reefs Volcanics thus represents the product of prolonged broadly arcrelated
shoshonitic volcanism, separated by limestone deposition and the subsequent emplacement
of medium-K calc-alkaline dacitic intrusions.

Item Type:	Article
Authors/Creators:	Squire, RJ and McPhie, J
Keywords:	Cadia, Forest Reefs Volcanics, intra-oceanic volcanism, Lachlan Orogen, Ordovician,
Journal or Publication Title:	Australian Journal of Earth Sciences
ISSN:	0812-0099
DOI / ID Number:	https://doi.org/10.1080/08120090601146995
Additional Information:	The definitive published version is available online at: http://www.tandf.co.uk/journals
Item Statistics:	View statistics for this item

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