Decompressional spinel plus plagioclase symplectite from Tenmondai Rock, Lutzow-Holm Complex, East Antarctica: Implications for the garnet-aluminosilicate-spinel-plagioclase geobarometer

Spinel + plagioclase symplectites in pelitic metamorphic rocks have been reported from many localities. We report in detail an occurrence of this texture from Tenmondai Rock, Lutzow-Holm Complex, East Antarctica. This texture is produced by the following metamorphic reaction; garnet + sillimanite = spinel + plagioclase. This can be described as the following two net-transfer reactions; 5Grs + Alm + 12Als = 3Hc + 15An, and 5Grs + Prp + 12Als = 3Spl + 15An. We propose new geobarometers (GASpP) based on these equations. For example, one of these is P = (-155790 + 587.4T + RT lnK(Fe)) / 29.378, for garnet + sillimanite + spinel + plagioclase assemblage in the CFASZn-system. Where P is pressure in bar, T is temperature in K, and R represents gas constant. The equilibrium constant is, K-Fe = (alpha(5)(Grs) alpha(Alm))/(alpha(3)(Hc) alpha(15)(An)). Our proposed new geobarometers are free from quartz, corundum, orthopyroxene, and cordierite within these equations. These barometers are useful to estimate pressure conditions for spinel-bearing pelitic metamorphic rocks within a wide pressure-temperature range (andalusite, kyanite, and sillimanite fields). We estimate the metamorphic pressure-temperature (P-T) conditions at Tenmondai Rock by applying the GASP, GRIPS, GRAIL, GASpP geobarometers, and the Zr-in-Rt geothermometer. The metamorphic evolution of the rocks at Tenmondai Rock is characterized by a clockwise P-T-t path. The peak P-T condition is about 820 MPa and 850 degrees C. Spinel + plagioclase symplectite was produced during the decompressional stage of this metamorphic evolution, at about 450 MPa and 700 degrees C.