Three chemical variations in pelitic minerals have been examined through a well-defined sequence of prograde reactions in the 3-kbar thermal aureole surrounding the Devonian Ballachulish Igneous Complex, in Argyllshire, Scotland: (1) Mg/(Mg + Fe) partitioning between coexisting cordierite, biotite, and chlorite, (2) F in biotite, and (3) Si content, as controlled by the Tschermaks exchange [(Fe,Mg)Si = 2A1], in coexisting muscovite, biotite, and chlorite. The Mg/(Mg + Fe) variations are controlled mainly by the metamorphic grade via continuous reactions, the F variations mainly by primary bulk compositional differences, and the Si variations by a combination of the two.

Using a T-XFe-Mg diagram constructed in the ideal pelitic system K2O-FeO-MgO-Al2O3-Si02-H2O, the theoretical prediction of the movement of Mg/(Mg + Fe) tie lines through a sequence of continuous reactions in both quartz-bearing and quartz-absent assemblages fits well with the measured compositions, with two major exceptions: specimens that come from graphitic slates, which contain minerals more magnesian than predicted, and specimens that contain biotite with significant F (>0.7 wt%), which are also anomalously magnesian. The first exception is due to the lowering of aH2O due to the presence of C-bearing fluid species, which shifts the stability of some of the reactions to more Mg-rich compositions, whereas the second is due to the stabilizing effect of F on Mg-rich biotite.

The Tschermaks exchange [(Fe,Mg)Si = 2A1] operates sympathetically in coexisting muscovite, biotite, and chlorite throughout a wide range of assemblages. This pattern highlights the importance of treating (Fe.Mg)-1Si-1A12 as a general exchange vector that operates in all amenable phases (e.g., mica) in an analogous fashion to the sympathetic Fe-Mg variations in coexisting ferromagnesian phases. Reactions involving (Fe,Mg)Si = 2A1 may be evaluated using Schreinemakers analysis, leading to the construction of P-T and isobaric T-XAl-Si diagrams. The application of T-XAl-Si diagrams to assemblages from Ballachulish succinctly accounts both for the observed distribution of biotite-bearing and biotite-absent regional assemblages and for the measured variations in (Fe,Mg)Si = 2A1 in a variety of contact metamorphic and regional assemblages.

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