Very low-grade metamorphism of the Taveyanne greywacke, Glarus Alps, Switzerland

Detailed textural and chemical data for mineral assemblages on a regional scale are presented for the metaandesitic Eocene-Oligocene Taveyanne greywacke of the Glarus Alps, Eastern Switzerland. Presented data indicate an increase of metamorphic grade from zeolite facies to prehnite-pumpellyite and pumpellyite-actinolite facies. Low-grade outcrops contain laumontite, minor corrensite and pumpellyite (assemblage type 1), whereas outcrops of higher metamorphic grade contain prehnite and two populations of pumpellyite (type 2), prehnite—pumpellyite-(Al)—white mica (type 3), a single outcrop shows pumpellyite-actinolite facies (type 4). From the zeolite to prehnite-pumpellyite/pumpellyite-actinolite facies there are indications for an increase of the chemical equilibrium domain size for the critical paragenesis from a single detrital grain ≤1 mm) in type 1, to a few millimetres in type 2, and to a whole thin section in type 3. Metamorphic P - T conditions were determined by a combination of chlorite thermometry, fluid inclusion and vitrinite reflectance data. Peak temperatures range from 170-190 C for zeolite facies to 270-310 C for prehnite-pumpellyite and pumpellyite-actinolite facies. For the higher temperature range, pressures of 2-3 kbar are derived indicating a geothermal gradient of 24-32 C km^-1. The well-constrained temperature estimations derived for the assemblages provide a useful test of the different empirical calibrations of chlorite thermometers recently proposed. The best correspondence to the temperatures determined here is for the Cathelineau calibration. In addition, in the lower grade samples differences in textures and calculated temperatures provide a mean to distinguish between detrital and newly formed chlorites.