Magmatic zircon Lu–Hf isotopic record of juvenile addition and crustal reworking in the Gawler Craton, Australia

Abstract

New in situ zircon Lu–Hf isotopic data are presented from magmatic rocks distributed across the Gawler Craton, Australia. These rocks range in composition from granite to gabbro, with the majority being granite or granodiorite and moderately peraluminous in composition. The new Lu–Hf isotopic data, together with previously published data, provide insight into the magmatic evolution of the craton and crust and mantle interaction through time. Increased juvenile content of magmatic rocks correlate with periods of extensional tectonism, in particular basin formation and associated magmatism during the Neoarchean to earliest Paleoproterozoic (c. 2555–2480Ma), Middle Paleoproterozoic (c. 2020–1710Ma) and Late Paleoproterozoic (c. 1630–160Ma). In contrast, magmatic rocks associated with periods of orogenic activity show greater proportions of crustal derivation, particularly the magmatic rocks generated during the c. 1730–1690Ma Kimban Orogeny. The final two major magmatic events of the Gawler Craton at c. 1630–1604Ma and c. 1595–1575Ma both represent periods of juvenile input into the Gawler Craton, with εHf(t) values extending to as positive as +8. However, widespread crustal melting at this time is also indicated by the presence of more evolved εHf(t) values to −6.5. The mixing between crust and mantle sources during these two youngest magmatic events is also indicated by the range in two stage depleted mantle model ages (TDMc) between 1.76Ga and 2.51Ga. Significant mantle input into the crust, particularly during formation of the c. 1595–1575Ma Hiltaba Suite and Gawler Range Volcanics, likely facilitated the widespread crustal magmatism of this time period. Viewed spatially, average εHf(t) and TDMc values highlight three of the major shear zones within the Gawler Craton as potentially being isotopic as well as structural boundaries. Differences in isotopic composition across the Coorabbie Shear Zone in the western Gawler Craton, the Middle Bore Fault in the northern Gawler Craton and, to a lesser extent, the Kalinjala Shear Zone in the southern Gawler Craton, broadly correspond to crustal and even lithospheric-scale discontinuities evident in geophysical studies. Therefore, these shear zones may approximate some of the first order crustal domains within the Gawler Craton.