The implications of overstepping for metamorphic assemblage diagrams (MADs)

Metamorphic assemblage diagrams (MADs, also known as pseudosections) are based on the assumption of chemical equilibrium throughout. Nucleation of a porphyroblast, however, requires a driving force that is achieved through overstepping of the isograd reaction. By making an assumption about the degree of overstepping required to nucleate garnet, aluminosilicate, staurolite, and cordierite, a modified MAD can be constructed that offers insights into metamorphic parageneses beyond those provided by an equilibrium phase diagram.Assuming a representative value of 300 J/mol oxygen of affinity for the nucleation of a porphyroblast, modified MADs have been constructed for a typical pelitic bulk rock composition. The resulting diagrams show a displacement of the garnet isograd to higher temperature (T), a shrinking of the stability field for garnet + chlorite, an expansion of the field for aluminosilicate, and a shrinking of the stability field for cordierite. Furthermore, the size of the stability field for staurolite + garnet depends on which reaction produces aluminosilicate: if aluminosilicate nucleates from an assemblage of garnet + staurolite the field of staurolite + garnet is greatly expanded; if aluminosilicate nucleates from an assemblage of chlorite + muscovite, the field shrinks dramatically and may be eliminated entirely. If garnet is a reactive phase, then kyanite is predicted to nucleate prior to or nearly simultaneously with staurolite in typical Barrovian metamorphic trajectories. Finally, certain equilibrium assemblages present in only small regions of P-T space may not be realized unless sufficient driving force for nucleation or progressive reaction can occur over limited changes in P and T. The key consideration is the amount of affinity required to drive the reaction at the P-T conditions of interest.