MULTIPLE CLEAVAGE DEVELOPMENT IN THE WORCESTER FORMATION OF EASTERN MASSACHUSETTS BY REPEATED DISSOLUTION AND REPLACEMENT PROCESSES

The development of continuous, spaced, and discrete cleavages in meta-sedimentary rocks from the poly-metamorphic and multiply deformed Worcester Formation each occurred by a pressure-solution process. Coarse-grained, randomly oriented phyllosilicates are cut by continuous cleavage folia; deformed, continuous cleavages are overprinted by spaced cleavage domains and discrete cleavage traces; older, deformed spaced cleavage domains are transected by younger spaced and discrete cleavages. Detailed optical and BSE microscopy reveal significant mineralogical differences between: (a) phyllosilicate-rich domains and quartzofeldspathic domains, (b) successive generations of cleavage development, and (c) variably developed cleavage domains of the same generation. Muscovite content grows at the expense of chlorite, quartz, and plagioclase in the youngest cleavages. Muscovite content also grows in cleavage domains in which the muscovite (00l) direction is most parallel to the domain boundaries. Concomitant with this evolution is an increasing grain elongation and development of beards on plagioclase.

Several characteristics of these cleavages indicate a dissolution/precipitation process was involved with each stage of development. In particular, quartz and plagioclase grains become narrower without increasing in length, indicating dissolution. An exception is some plagioclase grains where cores (An₂₈) preserve more calcic beards (An₃₄); but these also reflect precipitation of locally dissolved material.

Selective dissolution of stronger framework silicates in a weaker phyllosilicate-rich host is refuted by the simultaneous removal of chlorite from the developing mica domains. Finally, bent or crenulated micas from an earlier cleavage are also truncated by the muscovites of the later cleavage generation. Thus, one muscovite-bearing assemblage is replaced by a later muscovite-dominated assemblage that defines a new cleavage. With identical assemblages, the driving force for the replacement must be strain energy, which increases the solubility of strained grains in earlier cleavages and leads to the precipitation of new, strain-free grains in the new cleavage orientation.