METABASITES FROM THE CENTRAL VOR-SPESSART, NORTH-WEST BAVARIA .2. COMPARISON OF DIFFERENT GEOTHERMOMETERS AND GEOBAROMETERS

The occurrence of metabasites, forming two distinct belts in the centr al part of the Spessart Crystalline Complex, offers the possibility to test various geothermometers and geobarometers on rocks from a restri cted area. Amphibolites. hornblende gneisses and calcsilicate rocks in the southeastern belt are associated with metapelites displaying the assemblage staurolite - garnet - biotite muscovite - kyanite/sillimani te - plagioclase - quartz. This assemblage provides an independent P-T estimate of 5-7 kbar and 570-650 degrees C for the metamorphic peak, against which consistency of geothermobarometric results obtained from metabasic rocks can be compared. Amphibolites of the northwestern bel t are intercalated within metapelites devoid of staurolite and thus ma y have attained somewhat lower peak metamorphic conditions. For the so utheastern, metabasite brit, temperatures obtained from the garnet-hor nblende (GRAHAM and POWELL 1984), Fe-Ti oxide (ANDERSEN and LINDSLEY 1 988) and garnet-clinopyroxene geothermometers (ELLIS and GREEN 1979; P OWELL 1985) are in good agreement with each other and compare well wit h those derived from mineral assemblages in the associated metapelites . However, the garnet-clinopyroxene temperatures derived from the mode l of KROGH (1988) are significantly lower. Temperature estimates using garnet-biotite thermometers with non-corrected lnK(D) (FERRY and SPEA R 1978; PERCHUK and LAVRENT'EVA, 1983) are similar to those with corre cted lnK(D) (KLEEMANN and REINHARDT 1994) and fit well to the ''prefer red'' temperature range. By contrast, the lnK(D) corrected model of PA TINO DOUCE et al. (1993) overestimates temperatures, The different for mulations of the plagioclase-hornblende thermometer yield either lower (SPEAR 1980; PLYUSNINA, 1982) or higher temperatures (BLUNDY and HOLL AND 1990; HOLLAND and BLUNDY 1994). Results from the garnet-hornblende thermometer of PERCHUK et al. (1985) and the garnet-clinopyroxene the rmometers of SENGUPTA et al. (1988) and PATTISON and NEWTON (1989) are inconsistent and tend to underestimate temperatures. For the amphibol ites of the northwestern metabasite belt, which are associated with st aurolite-free metapelites, some but not all geothermometers yield some what lower temperatures. Pressures estimated using the plagioclase-hor nblende (PLYUSNINA 1982) and the garnet-clinopyroxene-plagioclase geob arometers (NEWTON and PERKINS 1982 ECKERT et al, 1991) are consistent with the pressure range derived from mineral assemblages in the adjace nt metapelites. By contrast, the garnet-hornblende-plagioclase-quartz barometer of KOHN and SPEAR (1990) and the garnet-clinopyroxene-plagio clase-quartz barometer of MOECHER et al. (1989) yield higher pressures of up to 11 kbar.