In the previous exercise, I considered spectacular but uncommon lithologies, with sapphirine, etc. Let's look at more common lithologies, such as the metapelites or micaschists commonly found along a MP-HT gradient.

The MP-HT gradient is characterised by a succession of isogrades : chlorite, muscovite, biotite, chloritoïde, garnet, staurolit, kyanite, sillimanite, muscovite (-).These different isograds are isograds of appearance of the mineral written (+), with the exception of the last one, the muscovite isograde (-), which is that of the disappearance of muscovite under the conditions of the high T of the amphibolite facies, and often contemporary with the beginning of anatexis. But to which reaction(s) is this isograde related?

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

In the micaschists, quartz is often abundant and the isograde musc (-) is related to the reaction: Musc + Qtz = Kfs + Sil + V. However, it would be more appropriate to write "musc + Q (-) isograde" in this case! Indeed, there is another reaction of disappearance of muscovite which is written: Musc = Kfs + Crn + V for lithologies under saturated in silica.

Lithological map

Draw the metamorphic map by colouring the different areas according to their paragenesis. This paragenesis is defined by drawing the triangles in the different divariant domains separated by the isograde reactions, either on the map or on the PT diagram.

Choice of the chemical system and the triangular diagram .... The choice of chemical system (or independent constituents) depends on the chemical composition of the minerals considered: musc – corundum– potassium feldspar– sillimanite (or others alumino sillicates) – quartz – water vapor. In this case, it is K2O – Al2O3 - SiO2 - H2O (see this page and also paragraph 7.2 of "Metamorphism and Geodynamics")

Different graphic representations are possible depending on the minerals considered. Here the system chosen is Al2O3 - K2O with SiO2 and H2O in excess: some independent constituents can be considered to be in excess. In the present case, we will consider that the H2O component is in excess and that the V (vapour) phase is always present. SiO2 is often chosen in excess. This is not the case in our example, with a mineral such as corundum (Al2O3) which cannot exist in the presence of quartz.Our diagram will therefore be a triangle whose constituents are : K2O - Al2O3 - SiO2 (with H2O in excess).As the minerals do not cover the whole triangle (especially the more potassic compositions), we can reduce it to its half Kfs - Al2O3 - SiO2.

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

Here is the answer ...

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