Some causes and consequences of high-temperature, low-pressure metamorphism in the eastern Mt Lofty Ranges, South Australia

Abstract

Andalusite-, and sillimanite-bearing metapelites in the eastern Mt Lofty Ranges preserve evidence for mineral equilibration during convergent deformation at temperatures near 550-600°C and pressures of 300-500 MPa, with lateral gradients in peak temperature of about 10°C/km near the first appearance of fibrolitic sillimanite (i.e. the sillimanite-in isograd). The high-temperature portion of the metamorphic history appears to have been nearly isobaric and the inferred geometry of isograd pattems apparently established during the deformation suggests the duration of deformation to be in the range 0.3-0.3+0.7 million years. These factors imply that the thermal perturbation responsible for peak metamorphism resulted primarily from localized, advective heat transfer in the lithosphere. Advection of heat in granitic magma, at least to the presently exposed crustal levels, is suggested by the intimate relationship between the attainment of peak metamorphic temperatures, convergent deformation and intrusion. The apparently short duration of deformation suggests that thermal weakening of the lithosphere during magma ascent of heat may have triggered the deformation. © 1995, Taylor and Francis Group, LLC.