LATE ARCHAEAN TECTONOTHERMAL HISTORY OF THE GAWLER CRATON; GEOCHEMICAL AND ISOTOPIC CONSTRAINTS

The southern Australian Gawler Craton is an extensive crystalline basement province of Late Archaean to Mesoproterozoic rocks. The Craton preserves a complex tectonothermal history of Late Archaean magmatism and sedimentation, and earliest Palaeoproterozoic to Mesoproterozoic tectonothermal events. Globally, the late Archaean is marked by a period of super continental tectonic cycles dominantly between ca. 2.78-2.59 Ga which preserve prodigious metallogenic provinces, however to date there has been comparatively little attempt to develop a coherent understanding of the geodynamic development of the Gawler Craton.

The geochemical composition of 2560 to 2500 Ma felsic volcanic and granitic rocks across the craton exhibit the hallmarks of typical late Archaean subduction related arc-type rocks, including negative Nb and Ti anomalies and TTG compositions. The age range of these inferred arc-related rocks coincides with the eruption ca. 2510 Ma ultramafic komatiites in the region, which show geochemical characteristics similar to typical plume related late Achaean komatiites (e.g. Superior Province) including Al-depleted and undepleted compositions, near chondritic Al2O3/TiO2 and flat REE patterns. Coincident with the development of apparent arc felsic rocks and temporally associated plume-related mafic/ultramafics was the deposition of a series of sedimentary successions derived from reworked bimodal crust with depleted mantle model ages (TDM) that range between ca. 3200-2800 Ma.

The co-existence of arc-like felsic rocks, plume-related mafic/ultramafics and coeval sediments derived largely from ca. 2700 Ma crust suggests that the late Archaean development of the proto-Gawler Craton was shaped by a convergent margin situated on or adjacent to a late Archaean continental domain. Within this context the generation of plume-related mafic/ultramafic rocks and the accumulation of sedimentary packages in part derived from a bimodal source(s) may have been related to rifting of the arc, conceivably linked to a retreating subduction system. At around 2500 Ma, the convergent regime became collisional, leading to the formation of the 2480-2420 Ma Sleafordian orogenic system, which affected much of the proto-Gawler Craton and its equivalents in the East Antarctic Shield.