Garnet-Orthopyroxene Bearing Granulites from Enderby Land, Antarctica: Metamorphic Pressure Temperature-Time Evolution of the Archaean Napier Complex

Abstract

Garnet-orthopyroxene bearing granulite assemblages from the Archaean Napier Complex, Enderby Land, Antarctica, display a variety of exsolution, recrystallization and corona textures which result both from near-isobaric cooling from the peak of metamorphism and from later overprinting. Compositional data on distinct generations of phases and on zoning patterns in coexisting minerals, have been used to estimate (a) peak metamorphic conditions attained between the first and second major deformation phases (Dl and D2); (b) cooling paths from this peak, and (c) ambient metamorphic conditions at the time of a later deformation (D3). Experimentally calibrated geothermobarometers indicate initial metamorphism at 900–950°C and 7–10 kb during and subsequent to Dl and D2, at 3100–3000 Ma. The presently exposed granulites indicate a regional increase in the pressures of this metamorphism south-west to the Scott Mountains-Casey Bay region, where minimum crustal thicknesses of 10 kb were attained at c. 3000 Ma. Subsequently, the Napier Complex granulites evolved through a prolonged period of near-isobaric cooling prior to further metamorphism at 600–750 and 4–8 kb during D3 at c. 2500 Ma. The near-isobaric pressure-temperature-time path (P-T-t) suggests that the Napier Complex acted as an essentially stable craton as early as 3000 Ma, and that the major magmatic and tectonic crustal thickening events associated with Dl preceded the thermal peak represented by the earliest recognized metamorphism.