METAMORPHIC EVOLUTION OF GARNET-SPINEL PERIDOTITES FROM THE VARISCAN SCHWARZWALD (GERMANY)

Garnet-spinel peridotites form small, isolated, variably retrogressed bodies within the low-pressure high-temperature gneisses and migmatite s of the Variscan basement of the Schwarzwald, southwest Germany. Deta iled mineralogical and textural studies as well as geothermobarometric calculations on samples from three occurrences are presented. Two of the garnet-spinel peridotites have equilibrated at 680-770 degrees C, 1.4-1.8 GPa within the garnet-spinel peridotite stability field, one o f the samples having experienced an earlier stage within the spinel pe ridotite stability field (790 degrees C, <1.8 GPa). The third sample, with only garnet and spinel preserved, probably equilibrated within th e garnet peridotite stability field at higher pressures. These finding s are in line with the distinction of two groups of ultramafic garnet- bearing high-pressure rocks with different equilibration conditions wi thin the Schwarzwald (670-740 degrees C, 1.4-1.8 GPa and 740-850 degre es C, 3.2-4.3 GPa) which has previously been established (Kalt et al. 1995). The equilibration conditions of 670-770 degrees C and 1.4-1.8 G Pa for garnet-spinel peridotites from the Central Schwarzwald Gneiss C omplex (CSGC) are similar to those for eclogites of the Schwarzwald an d also correspond quite well to those for garnet-spinel peridotites fr om the Moldanubian zone of the Vosges mountains and of eclogites from the Moldanubian s.str. of the Bohemian Massif.