Metabasites from the central Vor-Spessart, North-West Bavaria Part 2: Comparison of different geothermometers and geobarometers

The occurrence of metabasites, forming two distinct belts in the central part of the Spessart Crystalline Complex, offers the possibility to test various geothermometers and geobarometers on rocks from a restricted area. Amphibolites, hornblende gneisses and calcsilicate rocks in the south-eastern belt are associated with metapelites displaying the assemblage staurolite - garnet - biotite -muscovite - kyanite/sillimanite - plagioclase - quartz. This assemble provides an independent P-T estimate of 5-7 kbar and 570-650 °C for the metamorphic peak, against which consistency of geothermobarometric results obtained from metabasic rocks can be compared. Amphibohtes of the northwestern belt are intercalated within metapelites devoid of staurolite and thus may have attained somewhat lower peak metamorphic conditions. For the southeastern metabasite belt, temperatures obtained from the garnet-hornblende (GRAHAM and POWELL 1984), Fe-Ti oxide (ANDERSEN and LINDSLEY 1988) and garnet-clinopyoxene geothermometers (ELLIS and GREEN 1979; POWELL 1985) are in good agreement with each other and compare well with those derived from mineral assemblages in the associated metapelites. However, the garnet-clinopyroxene temperatures derived from the model of KROGH (1988) are significantly lower. Temperature estimates using garnet-biotite thermometers with non-corrected InK_D (FERRY and SPEAR 1978; PERCHUK and LAVRENT'EVA 1983) are similar to those with corrected InK_D (KLEEMANN and REINHARDT 1994) and fit well to the "preferred" temperature range. By contrast, the InK_D corrected mode, of PATIÑO DOUCE et al. (1993) overestimates temperatures. The different formulations of the plagioclase-hornblende thermometer yield either lower (SPEAR 1980; PLYUSNINA 1982) or higher temperatures (BLUNDY and HOLLAND 1990; HOLLAND and BLUNDY 1994). Results from the garnet-hornblende thermometer of PERCHUK et al. (1985) and he garnetclinopyroxene thermometers of SENGUPTA et al. (1988) and PATTISON and NEWTON (1998) are inconsistent and tend to underestimate temperatures. For the amphibolites of the northwestern metabasite belt, which are associated with staurolite-free metapehtes, some but not all geothermometers yield somewhat lower temperatures. Pressures estimated using the plagioclase-hornblende (PLYUSNINA 1982) and the garnetclinonyroxene-plagioclase geobarometers (NEWTON and PERKINS 1982; ECKERT et al. 1991) are consistent with the pressure range derived from mineral assemblages in the adjacent metapelites. By contrast, the garnet-hornblende-plagioclase-quartz barometer of KOHN and SPEAR (1990) and the garnet-clinopyroxene-plagioclase-quartz barometer of MOECHER et al. (1989) yield higher pressures of up to 11 kbar.