A simple cartographic exercise allows to understand the influence of the chemical composition of rocks on their paragenesis. It also helps to appreciate, in a concrete way, the usefulness and power of the graphic representation of these parageneses.

Draw the metamorphic map by colouring the different sectors according to their paragenesis.

Here is the answer ...

Metamorphic map

In the northwestern area of the map (top left), rocks A and B have the same paragenesis: Opx + Sill + Qtz. The difference in chemical composition between the 2 rocks can go undetected, as it is only visualised by a difference in the proportions of the 2 minerals Sill and Opx.When crossing the isograde of the Opx + Sill + Sp = Sapph reaction, the mineralogical composition of the 2 rocks remains the same: these do not "see" this isograde reaction. On the other hand, the paragenesis of the C rock changes when crossing this same isograde. Conversely, by going through the Opx + Sill = Sapph + Qtz reaction, the paragenesis of rock C does not change whereas the 2 rocks A and B have 2 different parageneses, but nevertheless obtained under the same metamorphic conditions!

We understand the importance of having a large number of samples of various compositions in order to characterise the metamorphic evolution of a region as accurately as possible. 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 only refers to the isograde of the Sapphirine.

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. In the present case, the limit of appearance of sapphirine is the combination of the 2 isograds.

See also the isograd "musc (-)" ?

Go back to the exercise or "Metam. and Geodyn.",  au Course of Metamorphism, Pétrologie Endogène or to the Home page ?