Geochemistry and tectonic settings of meta-igneous rocks in the Arthur Lineament and surrounding area, northwest Tasmania

Abstract

The Arthur Lineament is a Cambrian age high-strain metamorphic belt that transects northwestern Tasmania and forms the eastern margin of the Mesoproterozoic and Neoproterozoic Rocky Cape Block. It formed as a result of Middle Cambrian arc–continent collision, and is composed of both allochthonous slices (Bowry Formation and Reece amphibolite), the para-autochthonous ‘eastern’ Ahrberg Group and the autochthonous ‘western’ Ahrberg Group. Amphibolites and mafic schists of the ‘eastern’ Ahrberg Group rocks show a temporal change in composition from transitional alkaline basalt to dominant E-MORB-type rift tholeiites, whereas the ‘western’ Ahrberg Group metabasic rocks are typical E-MORB rift tholeiites. The lithostratigraphy of the Ahrberg Group matches very well that of the Upper Neoproterozoic (650–580 Ma) Togari Group of the Smithton Trough and King Island. In contrast, amphibolites of the allochthonous Bowry Formation, which also show E-MORB-type patterns typical of rift-type tholeiites, are intruded by granitoid sheets dated at 777 ± 7 Ma, similar to the Precambrian granites exposed on King Island (760 ± 12 Ma). The Bowry Formation granitoids have distinctive high Ti, Zr, Nb compositions best matched by granites produced by extended fractionation of rift basalts with significant crustal assimilation. This suggests that the Bowry Formation metabasic rocks may correlate with the Willouran flood basalts in South Australia, and together record the ca 780 Ma breakup event hypothesised to mark the breakup of the Rodinian supercontinent.