Simulation and modelling of an argon age spectrum from biotite in the Yilgarn Craton immediately adjacent to the Albany–Fraser Orogen, Western Australia

Abstract

Here we further show how modelling based on argon geochronology can be used to develop constraints as to the nature of the temperature–time path followed by an individual sample in the natural environment, by demonstrating how to estimate palaeodepth, and its variation through time. The method is of general application to the interpretation of any biotite age spectrum with a well-developed age plateau, as long as there is relatively minor loss of radiogenic argon evident in the initial steps, and as long as those initial steps are not masked by the release of excess argon. Accurate simulation relies on how well the morphology of the measured age spectrum has been delineated, especially in the initial heating steps. The requisite detail depends on precise control of temperature variation during step-heating, and on the aliquot having sufficient mass to allow many steps. Here we numerically model the morphology of a single age spectrum produced by step-heating an aliquot of biotite grains separated from a single sample from the Kalgoorlie Terrane, in the Yilgarn Craton, Western Australia. The sample location is less than 15 km from the inferred location of the fault that separates the Archean Yilgarn Craton from the Albany-Fraser Orogen (AFO). Potentially therefore the age spectrum should reflect thermal disturbance related to the juxtaposition of the AFO against the Yilgarn Craton during the Mesoproterozoic. The absence of significant effects implies that the high-grade portions of the AFO had cooled prior to their juxtaposition, and/or that the sample from the Yilgarn Craton was already at such shallow crustal levels that it was little affected by any thrusting orthogonal to the margin during juxtaposition.