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Journal of the Geological Society

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Regional metamorphism of the Vassalboro Formation, south-central Maine, USA: a case study of the role of fluid in metamorphic petrogenesis

Progressively metamorphosed carbonate rocks of the Vassalboro Formation serve a' natural laboratory for the investigation of fluid/rock interactions during metamorphism. An integrated study of their whole-rock chemistry, mineralogy, mineral chemistry and mineral abundances has led to the following results. (a) Prograde mineral reactions involved hydrolysis as well as dehydration and decarbonation. The hydrolysis reactions have resulted in an almost quantitative extraction of K and Na from high-grade metacarbonates. (b) Prograde mineral reactions buffered the X_CO2 of coexisting metamorphic fluid during almost all of the rocks’ metamorphic history. (c) The carbonate rocks were infiltrated by large volumes of H₂O-rich fluid while the buffering reactions progressed during the metamorphic event. Some high-grade metacarbonates were infiltrated by at least 3 rock volumes of fluid. Combined buffering and infiltration appears to be a general metamorphic phenomenon in carbonate-bearing rocks to depths of 30 km in the crust. (d) There is an excellent positive correlation between the heat budget of individual metacarbonate rock samples and calculated fluid/rock ratios. Convective heat transfer by metamorphic fluids is therefore likely to be an important item in the heat budget of metamorphic terrains.

Consideration of these results leads to a model of regional metamorphism which involves infiltration of rock by large volumes of acid, H₂O-rich fluid (large-scale acid infiltration metasomatism?). In this model, the infiltrating fluid exerts enormous control over the mineralogical evolution of the carbonate rocks during the metamorphic event: it drives mineral reactions, it changes whole-rock chemistry and it supplies heat.

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