Zircon dating of Devonian-Carboniferous rocks from the Bombala area, New South Wales

Zircons from two igneous and two sedimentary units in the Bombala area of s outheastern New South Wales have been examined by the sensitive high resolu tion ion microprobe (SHRIMP) to establish a timeframe in which to interpret these rocks. Previous studies have correlated these rocks with Late Devoni an units of the south coast, solely upon the basis of stratigraphy and lith ology as palaeontological evidence was absent. The two igneous units are th e Hospital Porphyry and Paradise Porphyry occurring beneath the sedimentary units. Both give a Frasnian age that can be correlated with the Boyd Volca nic Complex. The sedimentary samples are from the basal and upper sections of the Rosemeath Formation, a fluvial 'redbed' consisting of conglomerate, coarse sandstone, and associated red siltstone and mudstone. Detrital zirco ns from the basal conglomeratic section at Kilbrechin indicate a dominant p rovenance from local Silurian granites and volcanics and a maximum depositi onal age that can be correlated with the Frasnian-Famennian Merrimbula Grou p. However, detrital zircons from the upper coarse sandstone section of the Rosemeath Formation at Endeavour Lookout challenge the positive correlatio n trend with a lack of Silurian-age grains and a presence of grains ranging from Late Devonian to Early Carboniferous in age. These results imply eith er that the south coast correlation is not valid for the upper sequences, o r that the Merrimbula Group sequences also extend upward into the Carbonife rous. The general coarseness of the Rosemeath Formation also suggests a rel atively local provenance. No Early Carboniferous source is known in the imm ediate vicinity suggesting that Early Carboniferous igneous activity in thi s region of the Lachlan Orogen may have been more extensive than is current ly realised.