- Graphical Representation of the Parageneses of the Metabasites and Metapelites -

Graphical representations are proposed to plot the parageneses of the most common metamorphic rocks. So, the ACF diagram is used to represent the parageneses of magmatic mafic rocks (metabasalt, metagabbro), the metabasites. The A'KF and AFM diagrams are used to represent the parageneses of pelitic rocks, the metapelites.

An amphibolite (left) and a metapelite (right) have different parageneses although both rocks have recrystallized under the same P T conditions: those of the amphibolite facies (microphotographs under PPL)

Parageneses of Metabasites : It is rare that only 3 chemical constituents (whose variations can be represented in a triangle; see previous exemple) reflect the composition of a rock. Here is the composition of a metabasite; eleven elements have significant concentrations. Five of them account for nearly 95% of the total. These are SiO2, Al2O3, FeO, MgO, CaO. For simplification, we will neglect the others.

The tetrahedron A(Al2O3) - C(CaO) - F(FeO+MgO)- S(SiO2) is representative of the composition of magmatic basic rocks. Some minerals of these rocks are plotted in the figure: Qtz for quartz, Opx for orthopyroxene, Cpx for clinopyroxene, Pl for plagioclase, Grt for garnet and Spl for spinel. The tie lines between these different minerals make it possible to define 4-mineral parageneses. On the left figure, these are: Qtz-Opx-Grt-Pl ; Qtz-Cpx-Opx-Pl ; Opx-Cpx-Pl-Grt ; Spl-Opx-Grt-Cpx ; Spl-Cpx-Grt-Pl (the tie lines involving the Spl are not shown for clarity, unless you use the 3D visualisation). On the right figure, Qtz-Opx-Grt-Cpx ; Qtz-Cpx-Grt-Pl ; Opx-Cpx-Pl-Grt ; Spl-Opx-Grt-Cpx ; Spl-Cpx-Grt-Pl. The 2 figures represent different metamorphic conditions. These are not easy to read. Therefore, we prefer to use a projection of this tetrahedron, from its summit S (where the Qtz is located), on its base: the ACF triangle . A convenient solution is the 3D visualisation!

However, it is difficult to graphically represent the variations of 5 constituents. It is therefore necessary to reduce this figure. The first solution is to consider FeO and MgO as a single component (Fe+Mg)O. Indeed, the addition of the FeO (or MgO) component to a purely magnesian (or ferric) system does not increase the number of minerals (as the phase rule states) :if iron is added to a forsterite (magnesian olivine), not two minerals are produced, but a "ferro-magnesian solid solution" olivine. There are still 4 components whose variations can be represented in a tetrahedron. However, it is difficult to visualise this figure on the 2-dimensional space of a sheet of paper. The solution is to project the volume of this tetrahedron onto a triangle. But the phase rule stipulates that 4 minerals constitute the paragenesis (= divariant assemblage with M=C) of the rocks to be represented in these figures, which is impossible without crossing tie lines. The solution is to choose one of the minerals of the paragenesis as the projection pole.In the case of the ACF triangle used to represent basic rocks, Quartz is the projection pole. For this reason, this mineral must absolutely be part of the paragenesis of the rocks presented in this diagram. This condition is recalled by writing Q next to the diagram.

The triangular diagram A(Al2O3) - C(CaO) - F(FeO+MgO) is a projection of the tetraedron S(SiO2)ACF from the apex S where the quartz is located (Q). For the sake of clarity, only the parageneses containing the Q are shown.As an example, I have drawn the paragenesis O-C-P-G from the previous figure: the dashed line CG is "hidden" by the planes O-C-P and O-G-P.
The 2 triangles represent respectively the Low P and Intermediate P granulite facies. We switch from one to the other by changing the O-P tie line to the C-G line: we cross the univariant equilibrium Opx + Pl = Cpx + Gt + Q.

On this triangle, the triangles Opx-Gt-Pl and Opx-Cpx-Pl represent the parageneses Opx-Gt-Pl-Q and Opx-Cpx-Pl-Q respectively. On the other hand, one cannot represent the parageneses without Q of the lower part of the tetrahedron without making the figure difficult to read. It is therefore imposed to represent only the parageneses with Q. As a result, this projection is extremely restrictive, as it makes it impossible to represent the large lower volume of the tetrahedron SiO2-Al2O3-" F "-CaO, under the Opx-Cpx-Pl and Opx-Gt-Pl planes, i.e. basic rocks under saturated in silica and ultrabasic rocks. For such rocks, it is always possible to change the parameters of the projection. For example, the SCF plane is chosen as the projection plane and corundum or spinel as the projection pole, provided that these minerals are present in the rocks studied. The 3D visualisation is also a convenient tool to use!

Parageneses of Metapelites :The same approach as above allows us to retain 5 major elements from the analysis of metapelite in the table below. These are SiO2, Al2O3, FeO, MgO, K2O. The A'KF diagram is a projection of the SiO2-Al2O3-" F"-K2O tetrahedron onto the AKF triangle from the SiO2 projection pole. As in the ACF diagram, iron and magnesium are considered as one element.

In the AFM diagram, iron and magnesium are considered as two independent components. As quartz is a ubiquitous and abundant mineral in metapelites, silica is assumed to be in excess and is disregarded in the calculation, provided that only rocks containing quartz are represented.The four remaining elements Al2O3- FeO-MgO-K2O are represented in a tetrahedron for which a surface and a projection pole coinciding with a mineral must be defined. The base of the AFM tetrahedron serves as the projection plane; there is no common mineral of the metapelites at the opposite pole: K2O (in fact, K2SiO3, since silica is in excess). Muscovite, on the other hand, is a common mineral on the A-K line. The AFM diagram is used to represent the paragenesis with Q and Musc. Note that the lines AF and AM of triangle AFM extend beyond F and M respectively: This is related to the fact that the "muscovite" projection pole is "low" in the tetrahedron. As a result, some minerals (eg biotite) and rocks with low potassium and low alumina content are projected outside the triangle. This geometrical problem has no consequence on the petrological interpretation. See the AFMK tetrahedron in 3D?

Under deep amphibolite and granulite facies conditions, muscovite is unstable (in the presence of Q) and is replaced by another potassium mineral: potassium feldspar through the reaction Musc + Q = SiAl + FK + V (SiAl : alumino-silicates). Beyond this reaction, the projection is made from the potassium feldspar which is also on the A-K line. This mineral is closer to the K-pole than muscovite; as a result, no more minerals and rocks are projected outside the triangle called A'FM (to distinguish it from the AFM triangle). See the AFMK tetrahedron in 3D?

See the AFM diagrams of a MP-HT metamorphic gradient .

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