Cambrian reworking of the southern Australian Proterozoic Curnamona Province: constraints from regional shear-zone systems

Abstract

Palaeoproterozoic to early Mesoproterozoic metamorphic rocks of the Curnamona Province, southern Australia, are crosscut by a system of regional-scale shear zones that locally dominate the terrain. Combined metamorphic and geochronological data from localities across the southern Curnamona Province indicate that the peak metamorphic shear-zone assemblages formed during the Cambrian (c. 500 Ma) Delamerian Orogeny, and not during the waning stages of the c. 1600 Ma Olarian Orogeny as has been previously asserted. A combination of monazite chemical U–Th–Pb and garnet Sm–Nd geochronology indicates that shear-zone fabrics formed between 497 and 517 Ma. Peak metamorphic conditions obtained from prograde garnet–staurolite–biotite–muscovite–chlorite–quartz assemblages are between 530 and 600 °C at pressures of around 5 kbar. The apparent absence of significant up-pressure prograde paths recorded by the mineral assemblages, together with modest (10–20%) Delamerian shortening, suggests that attainment of burial to depths of around 18 km was largely a function of pre-Delamerian sedimentation over the interval from c. 700 to 530 Ma. The spatial association between the pattern of basement metamorphism and reactivation during the Delamerian Orogeny is interpreted to reflect in part the distribution of pre-Delamerian sedimentation, and highlights the potential importance of pre-orogenic processes such as basin development in controlling the style and pattern of later terrain reactivation and reworking.