Relationships between metamorphism and deformation

R.H. Vernon, School of Earth Sciences, Macquarie University, Sydney, NSW 2109, Australia

Relationships between metamorphism and deformation are complex and variable. Effects of deformation (with consequent enhanced access of water) on retrograde metamorphic reactions and vice versa have long been appreciated. Examples include retrograde schist zones at Broken Hill and eclogite facies shear zones in the western Musgrave Block in Australia. Effects of prograde metamorphic reactions on deformation and vice versa are equally important. As water is pervasively released on the grain scale by dehydration reactions, it has a potential weakening effect through large rock volumes, compared with the relatively local effect of water in retrograde metamorphism. Weakening of the rocks can occur by: (1) hydraulic fracturing, which reduces the effective mean stress, especially along grain boundaries, (2) increasing diffusivity of grain boundaries, assisting grain-boundary sliding (Etheridge & Wilkie, 1979), and (3) "water weakening" inside grains. In LPHT regional metamorphism, granites may cause metamorphism to commence prior to foliation-forming deformation (Vernon, Collins & Paterson, 1993). The heating and pervasive release of water in the resulting prograde reactions may promote the deformation, which in turn may assist reactions.

Metamorphic reactions may produce weaker or stronger minerals than those originally present, and so may induce reaction softening or reaction hardening. Examples of reaction softening include the replacement of feldspar phenocrysts by fine-grained micaceous aggregates in metavolcanic rocks, so that the mica is readily deformed and becomes incorporated in the matrix, leaving strong quartz phenocrysts relatively undeformed (Vernon, 1986). Examples of reaction hardening include most porphyroblastic minerals in metapelitic rocks, which force deformation to be partitioned into the matrix. Because of their size and strength, they tend to resist deformation/recrystallization, and so may escape (except for their margins) participation in later prograde reactions, thereby altering the chemical composition of the effective reacting system. Porphyroblast/matrix relationships may assist in the elucidation of P-T-D-t paths (Reinhardt & Rubenach, 1989; Vernon, 1988), though pitfalls in this approach have been pointed out by Johnson & Vernon (1995a, b) and Vernon (1996).

Zones of intense deformation can cause local chemical changes; e.g., concentrations of mica or sillimanite in limb domains in crenulated schists (e.g., Vernon, 1987). If dissolved components leave the system, the chemical composition of the effective reacting system may change.

Symplectites are good indicators of the solid products of metamorphic reactions. Brodie (1995) inferred that symplectic intergrowths of plagioclase, orthopyroxene and spinel oriented parallel to the foliation and regional stretching direction, grew in response to decreasing pressure in dilatant sites during granulite facies extension. Myrmekite in felsic rocks typically appears to be related to deformation (Vernon et al., 1983; Simpson, 1985; Simpson & Wintsch, 1989). However, because it nucleates on plagioclase and grows into relatively non-deformed K-feldspar while being deformed and recrystallized at the rear of the growing lobes, strain energy may not contribute directly to either the nucleation or growth of the myrmekite (Vernon, 1991). However, strain energy probably contributes indirectly by facilitating access of fluids to growth sites, altering the local chemical environment. Because myrmekite recrystallizes readily, it can be a major contributor to the development of foliations in felsic rocks.

Deformed granites provide good examples of the interaction of metamorphic reactions and deformation (e.g., Kerrich et al., 1980). Where deformed, the Wologorong Batholith (SE Australia) shows evidence of reactions involving the replacement of igneous plagioclase by white mica, epidote, and albite, the replacement of igneous biotite by new biotite, muscovite and sphene, with or without epidote, and the replacement of igneous K-feldspar by myrmekite and muscovite (Vernon et al., 1983). Fracturing of the strong feldspar grains has allowed penetration by water, promoting hydration reactions and forming finer-grained aggregates that are deformed and recrystallized into folia in high-strain mylonitic zones. K released in the replacement of K-feldspar may be used in the replacement of plagioclase, which may have released Ca used in the replacement of biotite. Flame perthite has been ascribed to similar cyclic reactions at greenschist facies conditions (Pryer & Robin, 1995). An interesting question is whether mylonitic zones in massive rocks, such as granites, are initiated by penetration of water along fractures (Segall & Simpson, 1986) or whether the deformation permits access of water for the reactions, or both.

Kerrich, R., Allison, I., Barnett, R.L., Moss, S. & Starkey, J. 1980. Contrib. Mineral. Petrol., 73, 221-242.

Vernon, R.H., Collins, W.J. & Paterson, S.R. 1993: Tectonophysics, 219, 241-256.

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