A GRAIN SCALE APPROACH TO EQUILIBRIUM DURING CLEAVAGE FORMATION IN CHLORITE AND BIOTITE GRADE PHYLLITES, SE VERMONT

Detailed electron microprobe analysis of phyllosilicates in crenulated phyllites show that grain scale zoning is common, and sympathetic zoning in adjacent minerals is also common. This suggests that phyllosilicates grew from a very small reservoir of components, “fractionating” that reservoir during grain scale crystallization. Multiple analyses along single muscovite, biotite, and chlorite grains (30-100 microns in length) show zoning patterns indicating substitutions of Mg2+ + Si4+ for Al3+ + Al3+. Mica may also be zoned for Fe, but patterns are less uniform.

Samples selected for analysis include chlorite grade cleavages, chlorite grade cleavages overprinted by biotite porphyroblasts, and biotite grade cleavages. In all three cases zoning patterns persisted requiring a similar cleavage forming mechanism regardless of grade. In particular the sample with chlorite grade cleavages overprinted by biotite porphyroblasts suggests the “closure temperature” for the diffusion of Mg, Al, Si, and Fe ions is greater than the temperature at the biotite isograd (~400C).

Although the bulk chemistry of micas form smooth trends in Mg vs. Al, analyses from grains no more than 100-200 microns away from each other show largely different chemical compositions. Furthermore, zoning patterns of adjacent grains of muscovite + chlorite, or muscovite + biotite mimic each other. Moreover, tie lines between pairs of muscovite-chlorite and muscovite-biotite analyses on diagrams showing Si vs. Al cations are generally parallel, showing a close approach to equilibrium between grains, but not across grains, and certainly not among grains in adjacent domains (scale 100 um).

These conclusions contradict ideas that cleavage formation can take place in a chemically open system. If crenulation cleavages formed in a fluid dominated system in which these fabrics may have been paths for fluid migration, then their compositions would have been uniform. On the contrary, the fine scale chemical zoning observed probably reflects a grain-scale reservoir, as expected with a pressure-solution mechanism. Thus the role of fluids is probably limited to one of catalyzing the pressure solution process. Fluids apparently did not drive the overall process of cleavage development, nor did cleavages act as conduits for fluid migration.