- AFM Diagrams of a MP-HT Metamorphic Gradient -
The different parageneses illustrated along the MP-HT metamorphic gradient in the "Plotting the Metamorphic gradient " exercise are not the only ones possible, far from it! Depending on small variations in the chemical composition of the rocks, it is possible to have various other parageneses. This is the purpose of the graphical representations (described here) that are proposed to represent the parageneses of metamorphic rocks. The diagrams A'KF et AFM are used to represent the parageneses of pelitic rocks, the metapelites.
The AFM diagrams of a MP-HT Metamorphic Gradient
The figure below represents AFM diagrams that summarize the possible parageneses in the metapelites of a series affected by a MP-HT gradient metamorphism that are illustrated by the succession of thin sections. Each paragenesis contains a maximum of 5 minerals (+ the water vapour phase V): quartz, muscovite (which is the projection pole) and 3 phases at the corners of the white triangles. In the grey fields, the minerals that occur under solid solutions and only 2 minerals are represented in the AFM triangle: the paragenesis of rocks in these grey areas therefore contains only 4 minerals.
Successions of A - F - M triangles representative of metapelite parageneses affected by a Medium Pressure - High Temperature metamorphic gradient. From 1 to 3-4 : GreenSchists facies; the following: Amphibolite facies. The triangle between the "isogrades grt1+ and st1+ is not shown. The red ellipse indicates the most common compositions of metapelites.
cld : chloritoïde, st : staurolite, bt : biotite, chl : chlorite, qtz : quartz, grt : garnet, ky : kyanite, sill : sillimanite, ms : muscovite , V : water vapour
The paragenesis Chl + Bt + Ms + Qtz stable under all the conditions of MP metamorphic gradient
This series of triangles, which correspond to the conditions of greenschist facies (the first 3) and amphibolite facies (the next 4), makes it possible to appreciate the diversity of these parageneses within the same range of P-T conditions (1 triangle) and when the metamorphic conditions increase (the series of triangles). The chemical composition of a metapelite can be located anywhere within the triangle; however, the red ellipse corresponds to the most common compositions of the rocks of the (sandstone-) pelitic series: these are rather low in alumina. Consequently, it is noticeable that the Chl + Bt + Ms + Qtz paragenesis is the most common in greenschist facies conditions. In rare cases, garnet or chloritoid is added. This rarity of chloritoid is considered below and here.
This paragenesis also persists in the conditions of the Amphibolite facies. Towards high T, it becomes rarer and is confined to the most magnesian compositions.This paragenesis can coexist with parageneses containing diagnostic minerals (st, ky, sill).Thus, in triangle 6 for example, the possible parageneses for the rock compositions which correspond to the red ellipse are: Bt + Grt ; St + Bt + Grt ; St + Bt ; Ky + St + Bt ; Ky + Bt ; Ky + Chl +Bt ; Chl +Bt (without omitting to add Ms + Qtz to each paragenesis).
It is therefore clear that the 5 samples in the « Tracing the metamorphic gradient » exercise were not randomly collected. In fact, the characterisation of a metamorphic gradient in the field requires the analysis of a large number of thin sections of carefully selected samples in the field!
The scarcities of aluminous silicates in the greenschist facies
It is noted that the parageneses of the metapelites in the greenschist facies of a MP gradient are poorly varied.(triangles 1 to 3-4). These are mainly Chl + Bt + Ms + Qtz micaschists to which garnet is added at the transition to the amphibolite facies: Chl + Bt + Ms + Grt + Qtz. However, several aluminous minerals are stable at the beginning of greenschist facies conditions (chloritoid, kyanite, staurolite), but they remain rare before the amphibolite facies, because they are confined to the aluminous part of the AFM diagram.To find parageneses containing these aluminous minerals, and thus to characterise the metamorphic evolution of the region in more detail, it is necessary to look for rocks without biotite. These considerations are illustrated by the animation «rarity of the chloritoid».
The meaning of "isograds" and the limits of appearance of a mineral
Each triangle (" divariant field " = paragenesis) is separated from the next by a reaction line (whose intervening minerals are indicated or not on the figure) labelled as an isograde of appearance (+) or disappearance (-) of a mineral. It can be noticed that there are no less than 3 isograds (+) of the staurolite! Usually, staurotide becomes abundant at the beginning of the amphibolite facies and the st3+ reaction is the commonly recognised isograde. This reaction allows the Biot - St association and, therefore, the presence of the latter mineral in low-alumina metapelites (the red ellipse on the figure). But staurolite can exist in greenschist facies rocks, for relatively rare aluminous chemical compositions. The same applies to the Ky + isograde, which is generally located at the beginning of the Amphibolite facies (this is Ky2+ in the figure above), but in no way corresponds to the first possible appearance of this mineral, which is restricted to aluminous compositions, in the conditions of the beginning of the greenschist facies. It can thus be seen that the notion of isograde of appearance (or disappearance) of a mineral is a statistical notion which is a function of the most abundant and most common chemical compositions of the metamorphic series considered.
It is therefore more accurate to distinguish an "isograde" by the precise mineralogical reaction that produces (or removes) a mineral or mineral association and to use the term "limit of appearance of ..." where the isograde reaction is not recognised.
See also "the mapping exercise of disappearance isograds of muscovite" in the high degree conditions of this MP metamorphic gradient.
