Decompressional spinel + plagioclase symplectite from Tenmondai Rock, Lützow–Holm Complex, East Antarctica: Implications for the garnet–aluminosilicate–spinel–plagioclase geobarometer

抄録

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, Lützow–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 = (a_Grs⁵ a_Alm) / (a_Hc³ a_An¹⁵). 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 °C. Spinel + plagioclase symplectite was produced during the decompressional stage of this metamorphic evolution, at about 450 MPa and 700 °C.