The Thomson Fold Belt in Queensland revisited

Abstract

The Thomson Fold Belt (TFB) covers a large area of Queensland, mostly under cover where it forms the basement to the Eromanga Basin and to basins beneath the Eromanga Basin. The basement was considered to be metamorphosed quartzose turbidites of Ordovician age intruded by Silurian and Devonian granites. They were thought to be of similar age to rocks in the Lachlan Fold Belt, although a structural discordance had been recognised between the two fold belts. In 2004 some SHRIMP U-Pb dating was carried out on two granites, a volcanic rock at the base of the Devonian Adavale Basin, and a volcanic rock from beneath the Carboniferous ? Triassic Galilee Basin. The granites provided Middle Ordovician and mid Silurian ages ? older than expected. The volcanic rocks from the Adavale Basin produced an Early Ordovician age and those from beneath the Galilee Basin a Middle to Late Devonian age. Additional dating in 2005 focussed on volcanic rocks. The presence of Early Ordovician volcanic rocks was confirmed. An early Devonian volcanic sequence was identified in the Adavale Basin. Middle Cambrian volcanic rocks were identified in the far west. The presence of Early Ordovician volcanic rocks overlying the meta-sediments in the eastern TFB meant that the meta-sediments were deposited and metamorphosed prior to this. The meta- sediments are now tentatively correlated with late Neoproterozoic to Middle Cambrian rocks in the Anakie Inlier and the Charters Towers Province which were deformed in the Late Cambrian. Rocks of the Cambrian to Late Ordovician Warburton Basin were probably metamorphosed in the Silurian. Further studies are required to unravel the geological history of the TFB.