Thermodynamic modelling of the reaction muscovite+cordierite→Al₂SiO₅+biotite+quartz+ H₂O: constraints from natural assemblages and implications for the metapelitic petrogenetic grid

Abstract

The reaction muscovite+cordierite→biotite+Al2SiO5 +quartz+H2O is of considerable importance in the low pressure metamorphism of pelitic rocks: (1) its operation is implied in the widespread assemblage Ms + Crd +And± Sil + Bt + Qtz, a common mineral assemblage in contact aureoles and low pressure regional terranes; (2) it is potentially an important equilibrium for pressure estimation in low pressure assemblages lacking garnet; and (3) it has been used to distinguish between clockwise and anticlockwise P–T paths in low pressure metamorphic settings. Experiments and thermodynamic databases provide conflicting constraints on the slope and position of the reaction, with most thermodynamic databases predicting a positive slope for the reaction. Evidence from mineral assemblages and microtextures from a large number of natural prograde sequences, in particular contact aureoles, is most consistent with a negative slope (andalusite and/or sillimanite occurs upgrade of, and may show evidence for replacement of, cordierite). Mineral compositional trends as a function of grade are variable but taken as a whole are more consistent with a negative slope than a positive slope. Thermodynamic modelling of reaction and associated equilibria results in a low pressure metapelitic petrogenetic grid in the system K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) which satisfies most of the natural and experimental constraints. Contouring of the Fe–Mg divariant interval represented by reaction allows for pressure estimation in garnet‐absent andalusite+cordierite‐bearing schists and hornfelses. The revised topology of reaction allows for improved analysis of P–T paths from mineral assemblage sequences and microtextures in the same rocks.

Polymetamorphism and ductile deformation of staurolite–cordierite schist of the Bossòst dome: indication for Variscan extension in the Axial Zone of the central Pyrenees

“…With additional graphite, the location of reaction (7) shifts to lower pressures. The textural relationships and porphyroblast growth sequences observed in the Bossòst dome correspond to muscovite-cordierite-staurolite-biotite assemblages discussed by Pattison, Spear & Cheney (1999) and Pattison et al (2002), strongly suggesting a polymetamorphic origin of the Bossòst assemblages.…”

“…In the Bossòst samples the ratios are 0.08 and 0.15 (Table 1). On the KFMASH grid of Figure 13 these rocks would pass through reactions (3) and (4) to form andalusite and along a decompression path through reaction (7), which has a shallow negative slope, to form cordierite after andalusite (Pattison et al 2002). Several factors control the location of reaction (7): the applied sillimanite-andalusite equilibrium model, the Mg/Fe ratio of biotite and the presence of graphite.…”

Melt Reintegration Modelling: Testing against a Subsolidus Reference Assemblage

“…Therefore, at this stage, we cannot incorporate orthoamphibole into the modelling with ds62 with any confidence. It should also be noted that low-P subsolidus equilibria involving porphyroblastic cordierite like in this study have been known to be problematic to model in ds62 e.g., [9,57,71]. Instead, the final composition produced after melt reintegration modelling (Figure 6b) was used to model the subsolidus assemblages in the previous THERMOCALC dataset ds55 [72], to investigate the effects of incorporating orthoamphibole into the modelling e.g., [69,73].…”

Contact metamorphism and crystal size distribution studies in the Shivar aureole, NW Iran

“…The intersection of the curve for Reaction 7, which has a positive dP/dT slope, with that of Reaction 8, which has a negative slope, gives a maximum pressure of c. 2.2 kbar for the contact metamorphism in the Shivar aureole, according to the petrogenetic grids of Spear and Cheney (1989) (see Figure 7) and Wei et al (2004) (also see Harte and Hudson 1979). However, the P-T position of the equilibrium curve for Reaction 8 is still somewhat contentious (Pattison et al 2002) and, consequently, so is the pressure of this crucial intersection; other published P-T estimates for this point include 2.8 kbar (Droop and Treloar 1981), 3.0 kbar or 2.1 kbar (Pattison et al 2002).…”