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- Les réactions minéralogiques, indicateurs des conditions métamorphiques
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Les réactions minéralogiques, indicateurs des conditions métamorphiques
Abstract
As a scheme of classification, metamorphic facies as well as subfacies have been abolished and a system of metamorphic zones bounded by well-defined isograds is presented instead. Such isograds are termed isoreaction-grads because they are characterized by mineral assemblages which are due to specific metamorphic reactions; these are indicators of metamorphic conditions.
Reactions that take place in various kinds of common rocks have been taken into account, such as rocks of pelitic, marly, siliceous dolomitic, mafic, and ultramafic compositions.
Petrogenetically significant mineral reactions have been recognized and are graphically shown in figures 3, 4 and 5 for very-low grade, low grade, medium grade and high grade metamorphism. These four metamorphic grades serve as a coarse division of metamorphic conditions and the range of each metamorphic grade is defined again by specific mineral reactions (see fig. 1). In addition, a qualitative distinction of pressure dependent ranges within each metamorphic grade supplies a coarse grid of metamorphic conditions which is very useful in field work (see fig. 2 and table 1).
The sequence of specific isoreaction-grads observed with rising temperature permits a good estimate of pressure, while intersecting isoreaction-grads determine P and T, on condition that Ptotalis equal to Pfluid (see tables 2 through 5).