Metamorphic pressure-temperature conditions of the Luetzow-Holm Complex of East Antarctica deduced from Zr-in-rutile geothermometer and Al_2SiO_5 minerals enclosed in garnet

摘要

The metamorphic pressure-temperature (P-T) conditions of high-grade pelitic gneisses from Akarui Point, Skarvsnes, Skallen, and Rundvagshetta in the Luetzow-Holm Complex (LHC), East Antarctica are re-examined by applying the Zr-in-rutile geothermometer to rutile inclusions in garnet enclosing Al_2SiO_5 minerals and to matrix rutile grains. By utilizing the P zoning of garnet to indicate isochronous surface, samples from Akarui Point, Skarvsnes, and Skallen were shown to have experienced almost the same P-T conditions around the kyanite/sillimanite transition boundary (～ 830-850 °C/～ 11 kbar). From Rundvagshetta, higher-T condition (850 ± 15 °C/0.1 kbar to 927 ± 16 °C/12.5 kbar) was confirmed from rutile inclusions in garnet enclosing sillimanite. Matrix rutile yielded similar temperature as inclusion rutile for Akarui Point, Skarvsnes, and Skallen samples. Therefore, the traditional metamorphic zone mapping based on matrix mineral assemblages, which classified Akarui Point as belonging to the transitional zone between the upper-amphibolite and the granulite facies zones, does not reflect the highest metamorphic conditions attained. The P-T-t evolution of the LHC needs to be re-evaluated utilizing detailed petrochronological approaches.