Paragonite in marbles from the Tauern Window, Austria: Compositional and thermobaric controls

Abstract

Paragonite coexists with phengite, chlorite, zoisite, tremolite and quartz in dolomitic marbles from near Döllach in the Sonnblick area of the SE Tauern Window, Austria, prompting an investigation into the petrogenetic significance of paragonite in carbonate rocks.Thermodynamic calculations on pure-end-member reactions indicate that the assemblages paragonite+CaCO3+quartz and paragonite+dolomite+quartz are stable over wide ranges of X(CO2) and have P–T stability fields almost as wide as that of paragonite+quartz. P–T pseudosection calculations spanning 0.35–1.4GPa and 400–590°C in the model system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–TiO2–H2O–CO2 yield wide fields for paragonite-bearing assemblages in the Döllach paragonite-marbles at low X(CO2). The bulk compositions of these rocks are characterised by high values of molar Na/(Na+K) and Al/(Na+K) and these parameters are influential in stabilising paragonite-bearing assemblages in marbles. A large proportion of published limestone and dolostone whole-rock compositions would be capable of supporting paragonite at P–T conditions within the greenschist, blueschist or epidote–amphibolite facies. Paragonite is probably much more common in low- and medium-grade marbles than the rarity of reports of its occurrence suggest.Apart from zoisite, the hydrous silicates in the Döllach marbles are characterised by modest fluorine contents, with XF values of coexisting minerals decreasing in the order Tr>Phe>Pa≈Chl.Calcite–dolomite thermometry and pseudosection calculations indicate peak-metamorphic conditions for the Döllach marbles of T=510±20°C, P>0.77GPa and X(CO2)<0.065. The P–T data agree with previous estimates for the area.