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.