Age constraints on terrane-scale shear zones in the Gawler Craton, southern Australia

Electron Probe Micro Analysis (EPMA) chemical dating of monazite from crustal scale shear zones in the late Archaean to Mesoproterozoic Gawler Craton, southern Australia, highlights a long history of shear zone development associated with assembly and reworking of the craton. In the southeast Gawler Craton, dextral transpression was focussed into the >250 km long Kalinjala Shear Zone, which dominates the structural character of that part of the craton. Monazite in upper amphibolite facies shear fabrics within the Kalinjala Shear Zone give a mean age of 1682 ± 10 Ma. This is indistinguishable from an age of 1693 ± 16 Ma obtained from monazite inclusions within garnet porphyroclasts within the shear fabric. In the western Gawler Craton, monazite populations from two separate locations along the >400 km long Tallacootra Shear Zone give ages of 1680 ± 37 and 1679 ± 39 Ma, respectively, suggesting that both the Kalinjala and Tallacootra Shear Zone systems record regional-scale strain partitioning toward the end of the ca. 1730-1700 Ma Kimban Orogeny. In the northwest Gawler Craton, the Karari Fault Zone can be traced for >700 km, and separates late Archaean rocks from an assembly of tectonic terranes that comprise the northern and northwest Gawler Craton. Monazites from amphibolite-grade mylonites associated with the Karari Fault Zone have a mean age of 1631 ± 12 Ma. This age is identical to the timing of regional arc-like magmatism represented by the St. Peter Suite in the central Gawler Craton, and suggests that exhumation of the ca. 1650-1640 Ma ultra high-grade granulites of the Nawa Domain in the northwest Gawler Craton was linked to the development of the St. Peter Suite magmatic margin. In the central Gawler Craton, monazite in the >200 km long Yerda Shear Zone, which sheared granites belonging to the ca. 1580 Ma Hiltaba Suite, have a mean age of 1503 ± 16 Ma. Similar ages of 1516 ± 18 and 1508 ± 14 Ma are obtained from monazite inclusions in garnet porphyroclasts within mylonites along the >300 km long Coorabie Fault Zone in the western Gawler Craton. Younger ages of 1468 ± 12 and 1457 ± 22 Ma from the amphibolite-grade mylonitic fabrics in the Coorabie Fault Zone are similar to 40Ar-39Ar ages from the western Gawler Craton, and suggest that reactivation of the Coorabie Shear Zone in part was associated with regional cooling and stabilisation of the Gawler Craton at ca. 1450 Ma. The history of deformation on the terrane-scale shear zones in the Gawler Craton therefore does not reflect a single period of terrane assembly or reworking, rather it points to a complex pattern of strain partitioning during tectonic events in the late Palaeoproterozoic to early Mesoproterozoic.