Reconnaissance isotopic studies bearing on the tectonothermal history of Early Palaeozoic and Late Proterozoic sequences in western Tasmania

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Abstract
Reconnaissance isotopic dating (K‐Ar, Rb‐Sr, U‐Pb) of over 20 different units of Early Palaeozoic and Late Proterozoic age in western and northern Tasmania has assisted in clarifying some ages and relationships but has met with many problems. The Rb‐Sr systems in most Cambrian rocks have been re‐set or partially re‐set by post‐Cambrian tectonothermal events, particularly the mid‐Devonian Tabberabberan Orogeny, and U‐Pb zircon data show considerable scatter due to contamination by older zircon and to post‐crystallization Pb loss. K‐Ar dating, mainly of slates and schists, has been more successful in delineating major tectonothermal events, but spreads of ages due to incomplete re‐setting are again evident in some cases. The investigation has demonstrated some of the difficulties involved in dating in this complex region. Slate ages from the Late Proterozoic Burnie and Oonah Formations and Rocky Cape Group fall in the 630–690 Ma range, and confirm that the Penguin Orogeny was a Proterozoic event which, although protracted, was not related to the generation of the Cambrian Mt Read Volcanics. Slates and schists from the Proterozoic metasedimentary sequences of the Tyennan Region give ages in the 540–610 Ma range, suggesting independence from the Rocky Cape Region and confirming the importance of the D3 event of Råheim and Compston (Lithos 10, 271–90, 1977). A considerable number of Ordovician ages (450–490 Ma) have been recorded from known Cambrian units, not all of which can reasonably be ascribed to partial Tabberabberan re‐setting. Some of these are interpreted as indicating a significant thermal event, associated with some folding and cleavage formation, in the Late Cambrian‐Early Ordovician. Slates and schists produced by the Tabberabberan Orogeny show a spread of ages from about 390 Ma, near the indicated biostratigraphic position of the orogeny, to about 420 Ma (e.g. Rosebery Mine schists), suggesting that the Devonian tectonothermal activity may have been spread over some 30 Ma.