Preliminary Thermal Modelling of the Massif Anorthosite-Charnockitic Gneiss Interface from Gruber Mountains, Central Dronning Maud Land, East Antarctica

The Gruber massif-type anorthosite, with marginal noritic anorthosite, represents a lopolithic magmatic intrusion within the orogen exposed in the central Dronning Maud Land (cDML). The meta anorthosite, emplaced during the Dlb deformation event into the adjacent charnockitic gneiss (which had undergone an initial deformation event (Dla) equilibrated along with the charnockitic gneiss during granulite facies conditions (M₁ = 787 ± 50°C, 7.4 ± 1kbar) followed by near isobaric cooling, M₂ event The gneiss and anorthosite were then deformed and partly retrogressed during a M₃ metamorphism (hydration during amphibolite facies conditions) accompanied by weak to moderate D₂deformation.Cooling of the anorthosite pluton (using a parameterized model with high initial crystallinity, a lower crustal contact temperature of ∼800°C with an average radius of 17 km) from 1200 to 840°C would require c. 45 Ma, following which the cooling rate would fall to <1 °C Ma^-1. An initial transient elevated geotherm of interest which fits the extracted P,T conditions during the M₁metamorphism was calculated, simulating the equilibration of the interface. Next, cooling and uplift of this interface with realistic combinations of thermal properties of the crust, heat generation, low uplift rates and boundary conditions was made. Calculation of the cooling and uplift of the equilibrated anorthosite and chamockitic gneiss interface produces an initial near isobaric cooling path for c. 30 Ma at or near the depth of equlibration.Using available geophysical data from the cDML orogen, the causes for the thermal perturbation during the granulite facies metamorphism are discussed and it is proposed that the granulite facies metamorphism was caused by a combination of crustal thickening and magmatic underplating during the collision (i.e. crustal thickening followed by extension) of the Kalahari (equivalent) Craton and a proposed East Antarctica Craton during the Grenvillian/Kibaran event.