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Late Variscan granites intruded Brioverian (Upper Proterozoic) and Lower Paleozoic pelitic sequences to the north of the South Armorican shear zone. In the vicinity of the granites, Brioverian garnet micaschists contain pre/syn-S2 assemblages with garnet + staurolite and post-S2 assemblages with staurolite ± andalusite. Andalusite appeared pre/syn- and post-S2 in garnet-free micaschists. The garnets in the Brioverian micaschists are zoned with increasing Mg and decreasing Mn and Ca from core to inner rim. Only poor garnet zonations occur in Paleozoic hornfelses of enclaves in the Rostrenen granite. The results of a microstructurally controlled application of garnet–biotite geothermometers and garnet–plagioclase geobarometers are similar to P–T trends obtained by the Gibbs method of garnet zonation modelling in the system NCFMnMASH. The P–T paths of a pre/syn-S2 regional metamorphism are clockwise between 500–550°C/8 kbar and 700°C/5 kbar, followed by cooling decompression. They contrast with isobaric contact metamorphism between 500 and 700°C at 2.5–3 kbar in Paleozoic hornfelses. This points to a two-stage Variscan metamorphism with a pre-granitic pressure-dominated event in the Brioverian micaschists, followed by Late Variscan contact metamorphism, and suggests the existence of a pre-granitic tectonic boundary between the micaschists and overlying low-grade sequences. [ABSTRACT FROM AUTHOR]

Lenses of ultramafic rocks intercalated within a metagabbro–amphibolite sequence were encountered in the KTB pilot hole. A pervasive metamorphic overprint formed the dominating assemblage calcic amphibole–orthoamphibole–chlorite–talc. In this study the phase relations of ultramafic rocks are investigated in order to (a) constrain the stability field of this assemblage in general, and (b) define the equilibrium pressure–temperature (P–T) conditions of this assemblage in the ultramafic rocks from the KTB borehole. For that purpose, phase equilibria were calculated in the model systems CaO–MgO–Al2O3–SiO2-H2O (CMASH) and CaO–MgO–FeO–Al2O3–SiO2-H2O (CMFASH). Thereby, the continuous compositional change of solid solutions with pressure and temperature was modeled, including the Tschermak’s substitution and the MgFe-1 exchange. Based on these results, petrogenetic grids were constructed, revealing that calcic amphibole–orthoamphibole–chlorite–talc assemblages cover a stability field of <650–770 °C/1→ 14 kbar (CMASH) and <550–650 °C/1→14 kbar (CMFASH), respectively. This explains the widespread occurrence of the considered assemblage. Based on the bulk rock composition of the KTB samples, a special P–T diagram was constructed, limiting the stability field of the calcic amphibole–orthoamphibole–chlorite–talc assemblage. At 580 °C the stability field extends from 6 to 14 kbar pressure, and shrinks to 10–11 kbar at 630 °C. Conventional estimates using the mineral compositions of the KTB samples yield a temperature around 630 °C, at which the calculated stability field of calcic amphibole–orthoamphibole–chlorite–talc extends from 10 to 11 kbar. [ABSTRACT FROM AUTHOR]

Garnet-spinel peridotites form small, isolated, variably retrogressed bodies within the low-pressure high-temperature gneisses and migmatites of the Variscan basement of the Schwarzwald, southwest Germany. Detailed 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°C, 1.4-1.8 GPa within the garnet-spinel peridotite stability field, one of the samples having experienced an earlier stage within the spinel peridotite stability field (790°C, <1.8 GPa). The third sample, with only garnet and spinel preserved, probably equilibrated within the garnet peridotite stability field at higher pressures. These findings are in line with the distinction of two groups of ultramafic garnet-bearing high-pressure rocks with different equilibration conditions within the Schwarzwald (670-740°C, 1.4-1.8 GPa and 740-850°C, 3.2-4.3 GPa) which has previously been established (Kalt et al. 1995). The equilibration conditions of 670-770°C and 1.4-1.8 GPa for garnet-spinel peridotites from the Central Schwarzwald Gneiss Complex (CSGC) are similar to those for eclogites of the Schwarzwald and also correspond quite well to those for garnet-spinel peridotites from the Moldanubian zone of the Vosges mountains and of ecologites from the Moldanubian s.str. of the Bohemian Massif. [ABSTRACT FROM AUTHOR]