The implications of overstepping for metamorphic assemblage diagrams (MADs)

Abstract

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 300J/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.