The emplacement and fault history of the Coolac Serpentinite, Lachlan Fold Belt, southeastern Australia

Abstract

The Tumut Synclinorial Zone, located in the Lachlan Fold Belt, southeastern Australia, forms a fault-bounded belt of early Silurian volcanics and flyschoid metasediments of the Tumut Trough. The Mooney Mooney Fault System, containing an extensive ultramafic belt known as the Coolac Serpentinite, forms the eastern margin of the trough. The ultramafics together with mafic volcanics and intrusive grabbroic rocks have been previously interpreted as early Palaeozoic oceanic crust, dismembered and obducted during late Silurian deformation of the Silurian trough sequence. Recent work, however, has unravelled a more complex history involving several periods of movement. The ultramafics and early Silurian volcanics are intruded by early Silurian gabbro and syn-kinematic late Silurian granodiorite. These intrusive relationships indicate that the ultramafic rocks were present in approximately their present structural position prior to deformation of the trough sequence. The ultramafics therefore cannot represent early Silurian oceanic crust, obducted during the Late Silurian deformation. They probably represent either early Silurian or Cambrian-Ordovician mantle-derived material emplaced within a strike-slip fault zone during early Silurian oblique extension.