Rb/Sr dating of differentiated cleavage from the upper Adelaidean metasediments at Hallett Cove, southern Adelaide fold belt

Upper Adelaidean psammites from the foreland part of the Late Proterozoic Adelaide fold belt at Hallett Cove have suffered one major deformation which produced upright folds that plunge gently south. Axial to these folds is a well developed differentiation cleavage inferred to have resulted from constant volume, plane-strain deformation by solution transfer and now defined by alternating P- (phyllosilicate-rich) and Q- (quartz-rich) domains. Geochemical analyses show that cleavage development resulted in a 2-4-fold fractionation of Rb/Sr between the P- and Q-domains; thus the age of deformation is datable and isotopic analyses show the P- and Q-domains form a 531 ± 32 Ma isochron. This age is statistically older than the pervasive deformation and metamorphism in the more interior parts of the fold belt, inferred from intrusive ages to have occurred at 485 ± 6 Ma, and thus suggests that deformation either propagated across the fold belt from external to interior zones or that it occurred episodically. The age data allow, but do not demand, that cleavage formation predated the deposition of the overlying Cambrian Kanmantoo Group sediments (<526 Ma). This would lend support to a model in which deposition of the Kanmantoo Group occurred after the onset of convergent deformation in this fold belt.