Geochronology of various fault bound units at Savage River: a melange of different terranes

Miss Grace Cumming1, Mr Ralph Bottrill1, Dr Clive Calver2, Assoc. Prof. Sebastien Meffre2

1Mineral Resources Tasmania, Rosny Park, Australia, 2School of Earth Sciences, University of Tasmania, Hobart, Australia

To support geological mapping of the poorly dated Arthur Metamorphic Complex (in the Savage River Area), the detrital zircon provenance of some major geological units were characterised. Detrital zircon was obtained from several units within the Ahrberg Group, Timbs Group (including Bowry Fmn. & Fulfords Ck Schist), Armstrong Ck schist and Keith Schist. All samples were dated by the U-Pb method using laser ablation inductively coupled mass spectrometry (LA-ICPMS).

Results from the Donaldson Formation (basal Ahrberg Group) indicate that deposition occurred after ~ 1343 Ma. The detrital zircon age spectra are comparable to those previously obtained from the Forest Conglomerate (Rocky Cape Group) and Oonah Formation, and support a previously suggested correlation with these units (Mulder et. al., 2015). However, this age is much older than the youngest zircons obtained from the Oonah Formation and correlatives elsewhere (~734 Ma). Our results from the Bernafai Volcanics (upper Ahrberg Group) indicate deposition occurred after ~569 Ma which, along with field relationships, supports a correlation with the Kanunnah Subgroup of the Togari Group.

In contrast, samples from the Bowry Formation (Timbs Group) cannot be easily related to other units in Tasmania (based purely on the zircon spectra alone). A 717 ± 18 Ma age (with 2σ error) of albitised meta-sediment gives a revised maximum age for the Bowry Fmn. Four age assemblages have been characterised from the Bowry Fmn.  Two of these have restricted ages, and the remainder show complex and varied age spectra. Samples with restricted age profiles between 780 and 750 Ma were most likely deposited during Neoproterozoic rifting, and sediments with diverse components were probably derived from intraplate sedimentation during these rifting events. Furthermore, samples from the Timbs Group cannot be easily correlated with the Ahrberg Group, an idea proposed by previous workers (Turner & Crawford 1993). The western section of the Timbs Group was interpreted by Holm and Berry (2002) as a parautochthonous slice of the Ahrberg Group, and they referred to it as the ‘eastern’ Ahrberg Group, yet this relationship was not confirmed by detrital zircon geochronology due to the paucity of zircon in this group. However, there are strong peaks of 1400 and 1700 Ma zircons in additional units in the Timbs Group (Armstrong Creek Schist and Fulfords Creek Schist). These peak ages are comparable with the Keith Schist, and ‘upper’ and ‘lower’ Rocky Cape Group lithologies.

The varied zircon age spectra in samples from Savage River is foreseeable, given that they were obtained from narrow fault-enclosed blocks in a melange zone, in the highest strain portion of the Arthur Metamorphic Complex. The Timbs Group likely represents an assemblage of fault bounded allochthonous fragments of Meso to Neoproterozoic siliclastic, carbonate rich and rifted intra-plate basins. These fragments were deformed, albitised and faulted together during the Early-middle Cambrian collisional event and emplaced against the Late Neoproterozoic Bernafai Volcanics.

References

Holm, O. H.; Berry, R. F 2002. Structural history of the Arthur Lineament, northwest Tasmania: an analysis of critical outcrops. Australian Journal of Earth Sciences, Volume 49, pp. 167-185

Mulder J. A., Halpin J. A, Daczko N. R., 2015. Mesoproterozoic Tasmania: Witness to the East Antarctica-Laurentia connection within Nuna, In: Geology doi:10.1130/G36850.1

 Turner N.J. & Crawford A.J. 1993. General features and chemical analyses of mafic and other rocks, Corinna geological map quadrangle. Report Mineral Resources Tasmania 1993/23.


Biography

Grace currently works as a geologist for the Geological Survey Branch at Mineral Resources Tasmania. Grace has spent the last 10+ years making geological maps.

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