MINERAL GROWTH ASSOCIATED WITH, BUT POTENTIALLY POST-DATING, A LATE CRENULATION CLEAVAGE IN TACONIC SLATE

Isotopic ages attained from grains that lie in a cleavage orientation are, with caveats, widely held to approximate the age of the cleavage-forming deformation and, at many localities world-wide, tectonized rocks contain multiple cleavage generations. The Taconic slate belt has at least two cleavages which yield "Taconic" ages (e.g., Harper, 1968), and "Acadian" ages (e.g., Chan and Crespi, 1999). The microstructure of Taconic slates from L. Bomoseen, VT, includes phyllosilicate grains that cross-cut a crenulation cleavage (the proposed "Acadian" cleavage, here referred to as S₂) without deflection from the slaty cleavage (S₁) orientation. This necessitates a hypothesis that some of the grains in the slate post-date S₂ -producing deformation (Means, person. commun.). S₁ is axial planar to the Cedar Point recumbent syncline, a roughly cylindrical fold in Mid-Granville (Mettawee) slate and discordantly overlying Brown's Pond wacke. S₂ is a conjugate set at high angles to S₁ with a kink-band morphology. Poles to S₂ maintain a tight grouping in stereoprojection, though locally they anastamose and pinch-out. There is enrichment of magnetite-dominated oxides within S₂, indicating possible incongruent dissolution. The fabric associated with S₂ may be best described as owing to more general dissolution-re-precipitation of grains and diffusive mass transfer of interstitial fluid. The presence of epitaxial chlorite indicates near-greenschist peak metamorphic conditions, and non-chloritic phyllosilicates in the slate have a low- and high- K population, indicating a Tsermack-like, Paragonite-Phengite solid solution across a solvus. The working hypothesis is that there are two, if not three, compositional groups of phyllosilicates and associated oxides that grew at different times in local equilibrium with interstitial fluids, some mimetically tracking the S₁ orientation. With few clear-cut cases of pressure solution, and no sharp compositional contrast between S₂ and S₁ -oriented grains, a relationship between S₂ -forming deformation and young mineral growth is unresolved. These observations complicate, but do not invalidate, interpretations of ages from these cleavages, and have implications for non-specific fluid-rock problems.