Argon geochronology of the Crown Hill Andesite, Mt Read Volcanics, Tasmania

The Crown Hill Andesite is one of several shallow subvolcanic hornblende‐phyric andesite/dacite intrusions, associated with similar lavas, which intrude both the Yolande River Sequence and Central Volcanic Complex units of the Mt Read Volcanics in the Queenstown area. The age of these bodies is constrained by late Middle or early Late Cambrian fossils in the overlying Tyndall Group. Geochronology, using the incremental heating ⁴⁰Ar/³⁹Ar method, on a visually homogeneous hornblende separate from the Crown Hill Andesite shows that it is an aggregate of four mineralogical phases, possibly representing one or more recrystallization events that caused loss of argon. Data from degassing between 970°C and 1150°C are consistent with derivation from a binary mixture of two phases, and lie on an isochron of 489 ± 9Ma on a ³⁶Ar/⁴⁰Ar versus ³⁹Ar/⁴⁰Ar plot. This age is too young to be magmatic, and probably dates recrystallization during an Early Ordovician metamorphic event, perhaps related to the waning phases of Late Cambrian/Early Ordovician tectonic activity in western Tasmania (Jukesian Orogeny). However, the data do not exclude partial resetting during the Middle to Late Devonian Tabberabberan Orogeny and subsequent granitoid emplacement, which may also be responsible for the high temperatures indicated by conodont geothermometry of Ordovician limestone from western Tasmania. The youngest overprint is younger than 280 Ma and may represent mild reheating associated with intrusion of Jurassic dolerite, or simply partial degradation of Devonian phyllosilicate alteration products.

Key words:

• ⁴⁰Ar/³⁹Ar dating
• Cambrian
• Crown Hill Andesite
• geochronology
• Mt Read Volcanics
• Ordovician
• Tasmania

Notes

Present address: Mineralogisches Institut, Abteilung für Isotopengeologie, Universität Bern, Erlachstrasse 9a, CH‐3012 Bern, Switzerland.