Microlithon alteration associated with development of solution cleavage in argillaceous limestone: textural, trace-elemental and stable-isotopic observations

Comparison of microlithons (the rock between adjacent cleavage domains) in strongly cleaved argillaceous limestone with samples of uncleaved limestone from the same stratigraphic horizon (Lower Devonian Kalkberg Formation, Hudson Valley of eastern New York) indicates that the process of cleavage formation results in significant changes in microlithon fabric and in the chemistry of microlithon calcite. Petrographic observation indicates that at least 50% of the calcite in microlithons is recrystallized as calcite fibers whose long axes are subparallel to cleavage domains. The fibrous calcite occurs as overgrowths on larger grains, in thin bed-parallel veins, and in 'diffuse extension zones' (areas of the microlithons in which inequant crystals of microcrystalline calcite are aligned parallel to cleavage domains). Chemical analyses show that calcite in microlithons of strongly cleaved limestone is enriched in Mn and Sr relative to calcite in uncleaved limestone. δ¹⁸O and possibly δ¹³C values for calcite in strongly cleaved rock are significantly smaller than values for calcite in uncleaved rock. These results indicate that microlithon calcite is completely recrystallized (i.e. much more calcite is recrystallized than is visible in thin section) and that it underwent exchange with a fluid external to the local rock system. Calculation of the water/rock ratio suggests that 0.1-2.6 volumes of water may have exchanged with the calcite during cleavage formation. Comparison of the minimum integrated and the inferred instantaneous water/rock ratios suggests that bulk fluid flow occurred during microlithon recrystallization. Comparison of the amount of shortening across cleavage domains with the amount of extension in microlithons, and of the volume of calcite dissolved in domains with the amount of fibrous (presumably added) calcite in microlithons, suggests that microlithons are a major sink for the calcite dissolved in domains during cleavage formation, and that at this locality, development of disjunctive solution cleavage in the Kalkberg Formation resulted in volume-constant plane strain.