Progressive Low-Grade Metamorphism of a Black Shale Formation, Central Swiss Alps, with Special Reference to Pyrophyllite and Margarite Bearing Assemblages
The unmetamorphosed equivalents of the regionally metamorphosed clays and marls that make up the Alpine Liassic black shale formation consist of illite, irregular mixed-layer illite/montmorillonite, chlorite, kaolinite, quartz, calcite, and dolomite, with accessory feldspars and organic material. At higher grade, in the anchizonal slates, pyrophyllite is present and is thought to have formed at the expense of kaolinite; paragonite and a mixed-layer paragonite/muscovite presumably formed from the mixed-layer illite/montmorillonite. Anchimetamorphic illite is poorer in Fe and Mg than at the diagenetic stage, having lost these elements during the formation of chlorite. Detrital feldspar has disappeared. In epimetamorphic phyllites, chloritoid and margarite appear by the reactions pyrophyllite + chlorite = chloritoid + quartz + H2O and pyrophyllite + calcite ± paragonite = margarite + quartz + H2O + CO2, respectively. At the epi-mesozone transition, paragonite and chloritoid seem to become incompatible in the presence of carbonates and yield the following breakdown products: plagioclase, margarite, clinozoisite (and minor zoisite), and biotite. The maximum distribution of margarite is at the epizone-mesozone boundary; at higher metamorphic grade margarite is consumed by a continuous reaction producing plagioclase. Most of the observed assemblages in the anchi-and epizone can be treated in the two subsystems MgO (or FeO)-Na2O−CaO−Al2O3−(KAl3O5−SiO2−H2O−CO2). Chemographic analyses show that the variance of assemblages decreases with increasing metamorphic grade. Physical conditions are estimated from calibrated mineral reactions and other petrographic data. The composition of the fluid phase was low in XCO2 throughout the metamorphic profile, whereas XCH4 was very high, particularly in the anchizone where aH2O was probably as low as 0.2. P-T conditions along the metamorphic profile are 1-2 kb/200-300 °C in the anchizone (Glarus Alps), and 5 kb/500-550 °C at the epi-mesozone transition (Lukmanier area). Calculated geothermal gradients decrease from 50 °C/km in the anchimetamorphic Glarus Alps to 30 °C/km at the epi-mesozone transition of the Lukmanier are
