THE DISSOLUTION-PRECIPITATION PROCESS AS A LINK BETWEEN MICROSTRUCTURE, METAMORPHIC PETROLOGY, GEOCHRONOLOGY, AND TECTONICS IN GREENSCHIST FACIES ROCKS, NEW ENGLAND

Pressure solution is a well-known deformation mechanism, but dissolution-replacement is also increasingly recognized as a significant reaction mechanism in metamorphic processes. Thus the dissolution-precipitation (d-p) process is an important link between deformation and petrology. Some of the work of deformation (dW) is transformed to heat (-dQ), but some energy is stored in minerals as elastic strain energy, ‘plastic’ strain energy associated with dislocations and twins and, with grain-size occurs, as surface energy, all contributing to the metastability of the deformed mineral. This energy drives the development of microstructures, but it also destroys solid-aqueous fluid equilibrium and thereby drives replacement reactions and recrystallization. One important example of this d-p process is the development of cleavages in chlorite-grade slates and phyllites. Here deformation may cause the crenulation of earlier micas. The concomitant increase in strain energy of these deformed phyllosilicates leads to their metastability, dissolution, and eventual replacement by micas crystallizing in new cleavages. In the chlorite zone the temperature attending this strain-induced replacement is typically below the closure temperature for Ar diffusion in muscovite, and so the new muscovite preserves its age of crystallization. This age of cleavage-formation can be determined, helping to define the tectonic setting, and allowing regional correlations. Our studies of several regions in New England are converging to suggest that an Alleghanian shortening event has involved low-grade rocks in both eastern New England and in the Connecticut River valley (CRV), but also ductilely deforms high-grade rocks in the central ‘Acadian’ high. Kinematic indicators associated with these structures show dextral motion in the east, but divergent motions in the CRV. These observations lead to the hypothesis that extrusion tectonics affected all of eastern New England in the latest Paleozoic.