Natural and experimental constraints on ultra-high temperature metamorphism

Simple petrogenetic grids based on experiments on metapelitic bulk compositions (e.g. Hensen and Green, 1973) have proved very successful in elucidating the metamorphic histories of granulite facies terrains. However, these are notably limited by the poorly constrained thermodynamic properties of sapphirine (e.g. Holland and Powell, 1998), an important phase in aluminous and particularly in silica-deficient aluminous granulite facies rocks. The entropy and enthalpy of sapphirine have been determined at 1573K and 1.2 GPa by experimental determination of the position of the MgO-Al 203-Si02 (MAS) divariant reaction: Aluminous orthopyroxene sapphirine + quartz 4MgAl2SiO6 Mg4 Al 8Si 202o 2SiO2 (expressed here as the endmember univariant reaction in the system Mg 4Al8Si 202o-SiO2) This data falls within error of extrapolated heat capacity data on natural sapphirine (Kiseleva and Topor. 1975), but is significantly lower than predicted from existing experimental work in the MAS system using synthetic sapphirine. This discrepancy is attributed to differences in short and long range ordering between natural and synthetic sapphirine. The results indicate that natural sapphirine and sapphirine-bearing assemblages will be stabilised to lower temperatures and higher pressures than the limits that have been experimentally determined for synthetic sapphirine and synthetic sapphirine-bearing assemblages. Independent determination of the heat capacity of the sapphirine used in this study is currently underway. It is expected that extrapolation of this data to the results of this study will result in the derivation of a heat capacity function for natural sapphirine from 01573K. Incorporation of this into existing thermodynamic datasets should allow the quantitative determination of the positions of sapphirine-bearing reactions in petrogenetic grids relevant to ultra-high temperature (UHT) metamorphism in natural systems. The Archean Napier Complex of Antarctica is one of the best documented J.JHT terrains, long recognised as having experienced temperatures in excess of 1000°C followed by a long period of near-isobaric cooling at deep crustal levels. However, the early history of the terrain, the timing of deformation and metamorphism, and the tectonic processes responsible for the generation of the extreme temperatures of metamorphism have not been resolved (e.g. Ellis, 1987; Harley, 1989; Sandiford, 1989; Hensen and Motoyoshi, 1992). Mineral textural relationships linked to deformation features from a range of localities in and around Amundsen Bay are consistent with peak metamorphic conditions of 1000-1100°C at 0.8-1.1 GPa during intense lower crustal extension. Rare decompression textures from widely spaced localities attest to decompression of the whole terrain to depths equivalent to the base of a normal thickness crust after peak metamorphism while still under UHT conditions. The timing of textural development indicates that intense lower crustal extension and UHT metamorphism occurred synchronously with crustal thickening. Retrograde reaction textures may have been produced either by isobaric cooling, or by later lower temperature granulite facies metamorphic event/s, or both. Mineral reaction textures and structural features support a tectonic model involving lithospheric thinning for the development of UHT metamorphism in the Napier Complex. This model involves the detachment of a part or all of the mantle lithosphere during continental collision, allowing upwelling of hot asthenospheric material directly beneath the crust. This in turn results in intense extensional deformation of the lower crust.