A VESTIGE OF VERY HIGH-PRESSURE (CA. 28 KBAR) METAMORPHISM IN THE VARISCAN BOHEMIAN MASSIF, CZECH-REPUBLIC

A unique Mg-Al rich rock from the Podolsko Complex of the South Bohemi an Moldanubicum contains a relic assemblage of pyrope-rich garnet + lo w-Al orthopyroxene + quartz that implies very high-pressure (VHP) cond itions of metamorphism. Primary garnet (70 mol% Prp; X-Mg = 0.70-0.73, Grs < 1 mol%) and coarse orthopyroxene (X-Mg = 0.88-0.90) are both ve ry magnesian, and the initial orthopyroxene also preserves very low al umina contents (Al2O3 = 0.8-1.2 wt%). Garnet-orthopyroxene thermobarom etry yields relatively precise P-T estimates of 26 +/- 3 kbar and 830 +/- 30 degrees C using the Harley (1984a) thermometer and Harley (1984 b) barometer, or 28 +/- 3 kbar using the Harley & Green (1982) thermob arometer. These pressures are consistent with the absence of coesite r elies or palisade quartz after coesite, which places an upper pressure limit of about 29 kbar at the estimated temperature. This VHP assembl age is extensively overprinted by symplectites and coronas. Orthopyrox ene (X-Mg = 0.74-0.78, 5-7 wt% Al2O3) and plagioclase (An45) form laye red coronas between quartz and garnet. Garnet is now further replaced by fingerprint and filamentous symplectites involving sapphirine (X-Mg = 0.80-0.81), orthopyroxene (X-Mg = 0.72-0.76, 6-9 wt% Al2O3), cordie rite (X-Mg = 0.90), and locally spinel. Highly resorbed garnet is zone d to much lower X-Mg (0.62-0.55) within 50-100 mu m of these symplecti tes, which formed at granulite-facies pressure-temperature conditions of 8 +/- 1.5 kbar and 800-900 degrees C. Despite its demonstrably cont inental crustal origin, the investigated Mg-Al-rich rock documents an initial phase of VHP metamorphism at pressures exceeding any estimates obtained so far for the crustal eclogites present in the Moldanubian Zone of the Bohemian Massif, which are generally nearer 20 kbar. The p ressures estimated here are more consistent with > 30 kbar pressure es timates obtained on mantle-derived garnet peridotites and garnet pyrox enites, as well as eclogites with reported occurrence of palisade quar tz after coesite in the Saxothuringian Zone. It remains uncertain whet her the lower pressure (< 10 kbar) granulite-facies overprint recorded in the reaction textures reflects simple decompression, with higher-p ressure reactions impeded by sluggish kinetics, or the effects of a di stinct heating event superimposed on the VHT assemblage subsequent to decompression.