Complex volcanic facies architecture of the Forest Reefs Volcanics near Cadia, New South Wales, associated with prolonged arc-related volcanism

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 calc-alkaline dacitic intrusions (448 – 445 Ma). The lower part includes shoshonitic basaltic andesite, feldspar-rich 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 arc-related shoshonitic volcanism, separated by limestone deposition and the subsequent emplacement of medium-K calc-alkaline dacitic intrusions.