Mesoproterozoic age for andalusite in the lower Rocky Cape Group, northwest Tasmania

Abstract

New U–Th–Pb chemical data from both electron probe microanalysis and U–Pb laser ablation-inductively coupled plasma mass spectrometry on monazite inclusions in giant andalusite porphyroblasts indicate that the lowermost subdivision of the Rocky Cape Group in northwest Tasmania underwent metamorphism at ca 1100 Ma. The Proterozoic Rocky Cape Group is a ∼10 km-thick succession of quartz arenite, siltstone and pelite. These sediments were deposited between ca 1450 and ca 1010 Ma. Metamorphism within the Rocky Cape Group ranges from the prehnite–pumpellyite to upper greenschist facies, locally reaching garnet grade. In the lowermost formation of the Rocky Cape Group, andalusite-bearing mineral assemblages in the Pedder River Siltstone formed prior to or synchronously with monazite development. The andalusite-bearing mineral assemblage indicates relatively low-pressure conditions of <200 MPa, consistent with the stratigraphic thickness of the Rocky Cape Group (∼10 km). However, the presence of the intermediate-temperature biotite–andalusite mineral assemblages (500 °C) in the study area suggests the contribution of additional heat sources. Garnet porphyroblasts were observed ∼2 km away. Similar-age medium-grade metamorphism and magmatism at about 1100 Ma have been observed regionally in the South Tasman Rise and East Tasman Plateau, and in areas that are thought to have been contiguous with northwestern Tasmania such as Dronning Maud Land, central Arizona (Yavapai–Mazatzal) and Belt–Purcell Basin in the southwestern and northwestern Laurentian crustal sequences. The pressure and temperature relationships, mineral associations and regional data suggest additional heat flow was present through the lower part of the Rocky Cape Group. The higher geothermal gradient could be attributed to burial during basin development and deposition of the upper Rocky Cape Group or associated magmatism on the margin of Laurentia during assembly of Rodinia.

KEY POINTS

1. Low-pressure, intermediate-temperature mineral associations occur in the Rocky Cape Group.

2. Heat sources are unrelated to nearby Devonian granite.

3. Robust ages reveal the first discovery of 1100 Ma metamorphism in Tasmania.

4. These provide data for a missing link in Rodinian paleogeographic reconstructions.

Keywords:

• monazite
• Mesoproterozoic
• metamorphism
• Rodinia
• andalusite

Acknowledgements

We are grateful to Dave Seymour and Carl Jackman for use of the samples they collected on the Balfour and Kenneth map sheets. Andrew Eastaugh assisted during the field sampling and mapping campaign. Jo-Anne Bowerman is thanked for initial drafting of Figure 1. Andrew McNeill is acknowledged for supporting this study. Maxwell Morissette and Jeffrey Oalmann are thanked for assistance with the LA-ICP-MS analysis. We would like to thank the reviewers for their time and effort given in reviewing the manuscript. The valuable comments and suggestions helped to improve the manuscript. Published with permission of the Director of Mines Alastair Morton.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article and/or its supplemental data.

Additional information

Funding

The Tasmanian government-funded West Coast Geoscience Initiative provided financial support for fieldwork and analysis completed as a part of this study. The LA-ICP-MS analysis was funded by Geoscience Australia’s Exploring for the Future Program.