AQUEOUS DISSOLUTION-PRECIPITATION AS A LINK BETWEEN MICROSTRUCTURE, PETROLOGY, AND RHEOLOGY

Metamorphic reactions in moist rocks occur via a dissolution-precipitation process (DPP), during which metastable reactants dissolve, and stable products precipitate. However, the work done deforming grains during syntectonic reaction influences the site of metamorphic reactions by adding strain energy to, and thereby increasing the solubility of those grains in the grain boundary fluid. In pressure solution, strain energy alone drives the dissolution of deforming grains and precipitation of strain-free grains.

In moist schists from eastern Massachusetts no thermally driven metamorphic reactions occur. Rather, continuous, spaced and crenulation cleavages contain micas with new compositions that truncate micas, feldspars, and quartz, indicating dissolution of earlier (deformed) grains, and precipitation of strain-free grains in the new cleavage orientation. A benefit of the low grade of these cleavage micas is that the muscovite precipitated below its closure temperature for Ar diffusion, and is thus able to preserve its age.

In drier orthogneisses in New England neither the chemical energy associated with the chemical metastability of orthoclase + amphibole nor the volume of H2O were high enough to drive their retrograde replacement to biotite and epidote to completion. Rather local deformation of selected grains of the amphibole and orthoclase reactants led to their selective dissolution and replacement at sites facing the shortening direction. The redistribution of schistosity-defining reaction products as beards, trails, and ribbons of feldspar, epidote, biotite, and quartz constitutes both a reaction and textural softening that led to considerable weakening of the orthogneiss protolith. The DPP is monitored in time by beards of metamorphic sphene that preserve their U-Pb age, and yield a log(strain rate) of about -14.

In still drier rocks in a kms-thick mylonite/ultramylonite zone in Argentina, a x50 reduction in grain size did not lead to significant softening by DPP because biotite, quartz, and feldspar grains remained randomly oriented, and grain boundary migration was limited by pinned grain boundaries. Apparently not enough H2O entered the rock to facilitate the DPP that led to foliation and ribbon production and textural weakening in the wetter examples.