Chemical U – Th – Pb monazite dating and the Proterozoic history of King Island, southeast Australia*

Abstract

The Proterozoic stratigraphy of Tasmania has some common aspects with South Australia but the Wickham Orogeny (760 Ma) has been a major contrasting feature.We have used chemical U-Th-Pb monazite dating to clarify the age relationships Monazites are typically concordant, with the measured common Pb component much less than the analytical error in electron probe micro -analysis (EPMA) (Cocherie et al. 1998, Scherrer et al. 2000. In-situ analysis and the high spatial resolution of EPMA allows the correlation of monazite age with the structural environment of the monazite grains and this is a major advantage in dating of metamorphic events (Williams & Jercinovic 2002). Finally, monazite has a high closure temperature so that in medium grade metamorphic rocks the grains close as they grow and resetting is controlled by recrystallisation: driven by strain, partial melting or hydrothermal activity (Cocherie et al. 1998, Seydoux-Guillame et al. 2002. For these reasons in -situ chemical dating of monazite by EPMA is revolutionising the dating of low to medium grade, regional metamorphic rocks. The major disadvantage of EPMA chemical age dating is that the high detection limits on Pb (~100 ppm) restrict the application to older events and/or high-Th monazite grains. Quantitative analyses of monazite grains were obtained using a Cameca SX50 electron probe microanalyser operat ed at 20kV and 100nA. The errors quoted in this work were estimated from counting statistics. The counting errors have been propagated through the age calculation using the rules for normally distributed er rors (e.g. Barford 1985). They do not include any systematic errors associated with calibration, or the errors in decay constants. The EPMA calibration was checked regularly using a monazite from the Wilson Lake Terrane, Canada (Thermal ionisation mass spectrometry (TIMS) age of 1000 2 (95% confidence) Ma; G. A. Jenner, pers. comm.) and reproduces the TIMS age to within error. Based on this result, we estimate that the systematic error is less than 1%. Weighted means and probability plots were calculated using ISOPLOT v2.49 (Ludwig 2001). For individual spot analyses, the 1 error is quoted in the tables. All other ages are given with errors shown at the 95% confidence level.King Island, located to the northwest of Tasmania, is composed of several different (meta-) sedimentary and igneous sequences (Fig. 2 ). The western half of the island is

Linking mainland Australia and Tasmania using ambient seismic noise tomography: Implications for the tectonic evolution of the east Gondwana margin

“…Albeit only a small island in Bass Strait, King Island predominately comprises Precambrian rocks and is situated in a key position with major implications for tectonic reconstruction in SE Australia. The oldest rocks of King Island are Mesoproterozoic metasediments that crop out at the southern tip of the island (Black et al, 2004;Berry et al, 2005;Calver, 2007). It is important to note here that no corresponding rocks are known in Tasmania and Victoria (Calver, 2007).…”

“…Palaeomagnetic evidence in the form of magnetic remanence (Li et al, 1997) has also been used. Geochemical investigations include apatite fission track (O'Sullivan et al, 2000) and detrital zircon age analysis (Berry et al, 2001), U-Th-Pb monazite dating (Berry et al, 2005(Berry et al, , 2007(Berry et al, , 2008) and a variety of other geochemical measurements (Reed et al, 2002;Clemens and Benn, 2010). Only a few interpretations rely on an integrated approach whereby multiple datasets are used to provide a tighter set of constraints (Cayley et al, 2002;Direen and Crawford, 2003;Teasdale et al, 2003).…”

The palaeomagnetically viable, long-lived and all-inclusive Rodinia supercontinent reconstruction

“…Here the term 'Mawsonland' is formally introduced as a more succinct synonym to 'Mawson Continent.' The Albany-Fraser belt is truncated on its western end by the late Neoproterozoic Pinjarra orogen (Fitzsimons 2003) and on its eastern end by the Palaeozoic Lachlan-Thompson accretionary orogen (Li & Powell 2001), although some local vestiges of Mesoproterozoic orogeny or magmatism are documented in northern Queensland (Blewett & Black 1998) and around Tasmania (Berry et al 2005;Fioretti et al 2005). These truncations can provide important piercing points for Rodinia reconstructions, because the Albany-Fraser-Musgrave orogen contains, along its entire length, two episodes of tectonomagmatic activity dated at c. 1320 and c. 1200-1150 Ma (White et al 1999;Clark et al 2000).…”

Evolution of a Cambrian active continental margin: The Delamerian–Lachlan connection in southeastern Australia from a zircon perspective

“…The study of the Tasmanian eclogite was undertaken to confirm if its zircons grew during collision-related eclogite facies metamorphism or whether they formed in some other event unrelated to high-pressure metamorphism. Although metamorphism at ~510 Ma has been shown by chemical U-Th-Pb monazite dating to be widespread in Tasmania (Berry et al, 2005) and therefore part of the Delamerian(-Tyennan) Orogeny, it has been assumed rather than demonstrated that the ~510 Ma SHRIMP U-Pb zircon age on eclogite in the Franklin Metamorphic Complex reflects the timing of high-pressure metamorphism. We have therefore undertaken rare earth element (REE) analysis of these zircons to see if they have a low HREE abundance and no negative Eu anomaly signature that is diagnostic of zircons grown during eclogite facies metamorphism (Rubatto, 2002).…”