The MP-HT metamorphic gradient ends with the disappearance of muscovite through two reactions.

The diversity in the parageneses of the rocks on either side of the isogrades is immediately noticeable. Certain rocks with different paragenesis on one side of an isograde have the same paragenesis on the other side of the same isograde; and vice versa of course.Some lithologies do not 'see' such an isograde.We understand the interest of multiplicating the number of samples of various compositions in order to characterise the metamorphic evolution of a region in the greatest possible detail.

Carte métamorphique

An isograde (of appearance or disappearance) is often defined by the name of a single mineral: in the present case, one conceives the confusion that can arise if one refers to the reaction Musc + Qtz = Kfs + Sil + V for the isograde of disappearance of muscovite (musc -). Rocks B, C and D still contain muscovite after such a "musc (-)" isograde, which is better called "musc+Q (-)" isograde!

It is better to define an "isograde" by the precise mineralogical reaction that produces (or disappears) a mineral or a mineral association and to speak of "the limit of appearance of such and such a mineral ..." when the isograde reaction is not identified.

qtz : quartz ; musc : muscovite ; sill : sillimanite ; kfs : potassium feldspar ; crn : corundum ; V : vapor.

See also the AFM Diagrams of a MP-HT metamorphic Gradient .

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