Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original Paper

Vladimír Žáček

Retrograded eclogite from the Staré Město Belt, NE margin of the Bohemian Massif

Journal of the Czech Geological Society, volume 41 (1996), issue 3-4, 167 - 175

Strongly retrograded eclogite was found at the NE edge of the Staré Město Belt most probably representing a suture zone, in the eastern rim the Orlice-Klodsko Unit. While primary garnet is preserved, (Alm 51-59, Grs 17-31, Pyr 13-17, Sps 1.3-2.1), clinopyroxene I is completely replaced by clinopyroxene II (Jd 18) - plagioclase I (An 10-14) symplectite. Continuing retrogression led to growth of hornblende + plagioclase II (An 17-42) at the expense of symplectite. Amphibolite sampled nearby displays a prograde zoning of garnet in the range of amphibolite facies, (Alm 56-62, Grs 24-30, Pyr 5-10 Sps 0.4-10); the assemblage includes hornblende, plagioclase (An 12-31), quartz and biotite. Thermobarometry indicates that local equilibrium of rocks of different previous P-T histories has been attained at conditions of high amphibolite facies at 650-725-C and 8-11 Kb. The highest pressure record of 11.5-12.5 Kb at 600-680-C, indicating conditions transitional between eclogite and amphibolite facies, yielded the sample of retrograded eclogite with preserved symplectite. The partial P-T path of retrograded eclogite shows a moderate uplift between 40-30 km and a re-equilibration at depth of about 30 km.