Microtextures and Reaction Mechanisms of Carbonate Rocks: A Comparison Between the Thermoaureoles of Ballachulish and Monzoni (N. Italy)

Abstract

The kinetics of metamorphic reactions have become an important aspect of recent investigations. Attempts to determine the step which controls reaction progress are based primarily on experimental results and theoretical calculations (Fisher 1978; Walther and Wood 1984; Rubie and Thompson 1985; Lasaga 1986; Ridley and Thompson 1986). Field-related studies are mainly concerned with the formation of reaction bands (Vidale 1969; Fisher 1970; Joesten 1974, 1977; Sanford 1980; Walther 1983) and are based on detailed textural investigations. The latter treatment is also the basis of our study, which focuses on endothermic decarbonation reactions in the thermoaureoles of the intrusions of Ballachulish and Monzoni. A detailed petrographic and petrological examination was carried out; the results correlated with the temperature-time history in the thermoaureoles, which are characterized by varying heating rates dependent on the distance from the contact. The comparison of the microtextures in these two aureoles, which formed under different pressure conditions, permits the development of a more general theory of the evolution of microtextures during heating. The aim of this chapter is to work out the systematics of the evolution of the microtextures, to determine possible reaction mechanisms and to discuss the effect of changing boundary conditions ($T, {X_{C{O_2}}}$ and volume of fluid phase) on their stabilization.