Pressure Solution Creep and Textural Softening in Greenschist Facies Phyllonites

We have found evidence for dissolution-precipitation creep (DPC) in phyllites and phyllonites naturally deformed at greenschist facies conditions. Since the experiments of Kronenberg et al. (1990) and Mares and Kronenberg (1993) micas are known to be among the weakest of rock-forming minerals. They deform by dislocation glide in their basal plane and when these micas are aligned and contiguous in an orientation favorable for glide they tend to localize strain into shear zones. Therefore, these closed-system experiments suggest that dislocation glide should be the dominant deformation mechanism in mica-rich shear zones from near surface through greenschist facies conditions. In contrast, in naturally deformed rocks we have found strong textural and chemical evidence that micas deform by dissolution-precipitation creep in phyllites at upper and lower greenschist facies conditions. In the Littleton Formation (N.H.) we find retrograde muscovite (pg5)-rich folia (Sn) truncating amphibolite facies Na-rich muscovite and biotite grains that define earlier foliations. Na-rich muscovite grains are also selectively replaced along crenulation axes and boudin necks where plastic and elastic strain are highest. In biotite grade regional metamorphic rocks in the Tananao schist of Taiwan muscovite-rich folia (Sn) truncate crenulated muscovite-biotite schists at high angles. In still lower (chlorite) grade phyllonitic fault zones marking terrane boundaries in southern New England (East Derby shear zone) and in Taiwan (Daugan shear zone) crenulated older fabrics are cut by new undeformed muscovite grains in chlorite-free planar folia. Further evidence for recrystallization rather than dislocation glide comes from the 40Ar/39Ar ages of muscovite in the new Sn folia younger than the age of the truncated folia. The younger ages in each case demonstrate that recrystallization was activated at lower shear stresses than dislocation glide, and that the recrystallization occurred at lower greenschist facies conditions below the closure temperature for diffusion of argon in muscovite. The increase in muscovite/chlorite ratios and change in microchemistry of Sn muscovite, the truncating microstructures, and isotopic results are all incompatible with deformation by dislocation creep.