Tomoharu Miyamoto

Sapphirine+quartz association in garnet: implication for ultrahigh-temperature metamorphism at Rundvågshetta, Lützow-Holm Complex, East Antarctica

Abstract We report the occurrence of sapphirine+quartz association within garnet porphyroblast in the garnet–orthopyroxene–sillimanite granulite (Grt–Opx–Sill granulite) from Rundvågshetta in the Lützow-Holm Complex, East Antarctica. The granulites in the study area show a characteristic mineral assemblage consisting of orthopyroxene+sillimanite+quartz. The presence of sapphirine and quartz inclusions within garnet in the sapphirine-bearing Grt–Opx–Sill granulite suggests that metamorphic conditions changed from the stability field of orthopyroxene+sillimanite+quartz to that of sapphirine+quartz during the garnet growth. Peak metamorphic temperature conditions of about 1000–1100 °C are obtained by ternary feldspar thermometry for these granulites. Similar temperatures were also estimated from the Al-in-orthopyroxene geothermometer. The granulites are also characterized by coarse-grained garnet, being partly surrounded by a fine-grained symplectite composed of orthopyroxene and cordierite, whereas sapphirine+cordierite symplectitic intergrowth occurs in the matrix. These textures imply that the area underwent isothermal decompression subsequent to ultrahigh-temperature metamorphism. The sapphirine-bearing Grt–Opx–Sill granulite is likely to be the restitic product of partial melting and shows signs of segregation and movement of melt.

Pre-metamorphic carbon, oxygen and strontium isotope signature of high-grade marbles from the lützow-holm complex, east antarctica: Apparent age constraints of carbonate deposition

Abstract C, O and Sr isotope geochemistry of high-grade marbles from the Lützow-Holm Complex, East Antarctica, has given clues on the depositional ages and post-depositional alterations. Dolomitic and calcitic marbles occur as thin layers with varying thickness (up to 100 m) in several outcrops in eastern Dronning Maud Land, most of which underwent post-depositional geochemical alterations. In particular, the Sr and O isotope alterations are extensive, with 87Sr/86Sr(550 Ma) ratios as high as 0.758 and δ18O values as low as −5‰. These data suggest that multiple stages of fluid–rock interaction processes during diagenesis, prograde to peak and retrograde metamorphic events have altered the depositional isotopic signatures. However, some of the marble layers, exceptionally, preserve pre-metamorphic geochemical characteristics, such as low Sr isotope ratios, high δ18O and δ13C values, and well-equilibrated unaltered trace and rare earth element patterns. Lowest 87Sr/86Sr isotopic ratios of 0.7066 and 0.7053 with high δ13C and δ18O values suggest an apparent age of deposition around 730–830 Ma, although total geochemical resetting of carbonates by seawater of this age cannot be ruled out. The apparent depositional ages are consistent with carbonate deposition in the ‘Mozambique Ocean’ that separated East and West Gondwana.

Fe2+-Mg partitioning experiments between orthopyroxene and spinel using ultrahigh-temperature granulite from the Napier Complex, East Antarctica

Abstract Temperature dependence of the Fe2+–Mg exchange between orthopyroxene (Opx) and spinel (Spl), was experimentally determined at 9–13 kbar and 900–1200 °C using an ultrahigh-temperature (UHT) granulite collected from the Napier Complex in Enderby Land, East Antarctica. The Fe2+–Mg distribution coefficient, is empirically obtained as where X is the cationic mole fraction, and pressure P and temperature T are in kbar and Kelvin, respectively. The new geothermometer was applied to various natural UHT and associated high-grade metamorphic rocks from East Antarctica and other regions of the world. The results indicate temperatures between 735 and 902 °C at pressures in the range of 5–14 kbar. This geothermometer utilizing spinel does not give peak metamorphic condition, because it is relatively easy for spinel to re-equilibrate during the cooling stage of metamorphism. Hence, we conclude that the geothermometer is suitable for evaluating the closure temperature for the KD between aluminous orthopyroxene and spinel during retrograde metamorphism rather than the thermal peak.

Post-peak (<530 Ma) thermal history of Lützow-Holm Complex, East Antarctica, based on Rb–Sr and Sm–Nd mineral chronology

Abstract Rb–Sr and Sm–Nd mineral dating of metamorphic rocks from Skallen, Skallevikshalsen and Rundvågshetta, in the southwestern part of the Lützow-Holm Complex, Dronning Maud Land, assists in constructing a thermal history after peak metamorphism. The results fall into two groups: (1) a record of regional cooling after peak metamorphism (524–488 Ma); (2) local resetting 50–80 Ma after peak metamorphism (474–446 Ma). This grouping is consistently observed in published ages from various localities in the Lützow-Holm Complex. A Sm–Nd age of 524 Ma is indistinguishable from published zircon and monazite ages. Ages of 511 and 488 Ma are related to cooling after peak metamorphism. The younger age group overlaps with ages of post-metamorphic magmatism and related hydrothermal activity reported from localities throughout East Antarctica. This intracontinental, post-orogenic igneous activity continued after the tectonic assembly of Gondwana.