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

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 midDevonian 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 CambrianEarly 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.

Complex regional fault history of the Badger Head region, northern Tasmania

“…It has been assumed to be Precambrian on the basis of lithologic and structural correlation with known Precambrian rocks such as the Burnie Formation (Komyshan 1977; Gee & Legge 1979), but this correlation can be questioned. Adams et al (1985) obtained a K-Ar whole rock age of 512 ± 7 Ma for multideformed pelite at Badger Head, but this age may reflect the age of the source rock of the detrital micas, the age of metamorphism or any combination of the two.…”

Blue amphibole, Arthur Metamorphic Complex, Tasmania: Composition and regional tectonic setting

“…The Togari Group has well‐sorted, nearshore, basal deposits, whereas the basal Ahrberg Group is a proximal turbidite sequence. A minimum age for the tholeiitic volcanics of ca 600 Ma is inferred from K–Ar total rock ages of apparently comagmatic dykes in the Rocky Cape Group (Adams et al 1985; Crawford & Berry 1992). A maximum age limit for the volcanics of ca 700 Ma is indirectly provided by the isotope chemostratigraphy of the lower dolomite formation (Calver 1996, 1998; Calver & Walter 2000).…”

Base metal exploration of the Mount Read Volcanics, western Tasmania; Pt. II, Lead isotope signatures and genetic implications

“…In contrast to the crystallization and metamorphic ages, U-Pb ages in zircons provide evidence for the (Adams et al, 1985). It would appear that pressures and temperatures prevailing during the Cambrian and Devonian were below the conditions necessary to reset the U-Pb ages in zircons; only granulite facies metamorphic conditions appear sufficient to reset U-Pb zircon ages (e.g., Gulson and Krogh, 1975;Gebauer and Griinenfelder, 1976, 1977, 1979Gulson, 1984a) and even under these conditions, only in some cases.…”