High-T, low-P metamorphism in the Palaeoproterozoic Halls Creek Orogen, northern Australia:: the middle crustal response to a mantle-related transient thermal pulse

High-T, low-P metamorphic rocks of the Palaeoproterozoic central Halls Creek Orogen in northern Australia are characterised by low radiogenic heat production, high upper crustal thermal gradients (locally exceeding 40 degreesC km(-1)) sustained for over 30 Myr; and a large number of layered mafic-ultramafic intrusions with mantle-related geochemical signatures. In order to account for this combination of geological and thermal characteristics, we model the middle crustal response to a transient mantle-related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere. This mechanism has the potential to raise mid-crustal temperatures by 150-400 C within 10-20 Myr following initiation of the mantle temperature anomaly, via conductive dissipation through the crust. The magnitude and timing of maximum temperatures attained depend strongly on the proximity, duration and lateral extent of the thermal anomaly in the mantle lithosphere, and decrease sharply in response to anomalies that are seated deeper than 50-60 km, maintained for < 5 Myr in duration and/or have half-widths < 100 km. Maximum temperatures are also intimately linked to the thermal properties of the model crust, primarily due to their influence on the steady-state (background) thermal gradient. The amplitudes of temperature increases in the crust are principally a function of depth, and are broadly independent of crustal thermal parameters. Mid-crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust, and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly. Although pluton-related thermal signatures significantly dissipate within < 10 Myr (even for very large, high-temperature intrusive bodies), the interaction of pluton- and mantle-related thermal effects has the potential to maintain host rock temperatures in excess of 400-450 degrees C for up to 30 Myr in some parts of the mid-crust. The numerical models presented here support the notion that transient mantle-related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high-T, low-P metamorphic belts, especially in those characterised by low surface heat flow, very high peak metamorphic geothermal gradients and abundant mafic intrusions.