Composition of pre-Carboniferous siliciclastic rocks in Tasmania

Twelve hundred whole-rock compositions of the Proterozoic and Paleozoic rocks of Tasmania were compiled from a range of published and unpublished sources. We used these data to establish a chemostratigraphic evolution of the island and to resolve several problems in Tasmanian geology. Six chemostratigraphic mega-sequences were defined for Tasmania. The major events that separate these sequences are the onset of continental rifting (<i>ca</i> 700 Ma), arc–continent collision (<i>ca</i> 515 Ma), post-collisional volcanism (506 Ma) and then the return to continental conditions (500 Ma). Within this large-scale classification, we recognised smaller-scale changes in provenance. Within the Mesoproterozoic, there is a trend to more mafic rocks in the source region, which is interrupted by a short period where granites were prominent in the provenance. The Mesoproterozoic rocks of King Island have a subtly different provenance from the shallow water siliciclastic rocks of the Rocky Cape Group. The distribution of Cr across Tasmania can be attributed to two different events. First, the primitive basalts associated with Cryogenian continental rifting introduced a Cr signature associated with high TiO₂. The ophiolite obduction event in the Cambrian produced a second Cr enrichment event dominated by chromite from supra-subduction-zone peridotite. These two events can be differentiated by the composition of the sedimentary rocks but are more clearly defined by the composition of detrital spinels. Within the East Tasmanian terrane, a trend to less weathered sources reflects the influx of young volcanic detritus at the end of the Silurian. The major tectonic events in Tasmania can be recognised from the composition of sedimentary rocks. The West Tasmania terrane can be subdivided into five chemostratigraphic mega-sequences. There are two major events that introduce high Cr anomalies into Tasmanian strata.