Abstract
An empirical method is described whereby the sequence of textural changes in pelitic rocks from one zone to the next may be reconciled with the balanced metamorphic reaction inferred to have been in progress. It consists in deducing from the textures of a single thin section a set of metasomatic cation-exchange reactions, which proceed in different microscopic domains of the rock, but which add up on the scale of the whole thin section to give the balanced metamorphic reaction. Each metasomatic subsystem is closed to aluminum, but open to the more mobile cations, which are free to diffuse from one subsystem to another, subject to the requirement of short-range electrostatic neutrality, and to the assumption that the system is closed on the scale of the whole thin section. Evidence in support of the central postulate that aluminum is relatively immobile is found in
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1.
the preservation of bedding laminations, on a finer scale than the staurolite porphyroblasts which transect them without disrupting them.
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The fact that quartz, the only abundant aluminum-free mineral in pelitic rocks, is by far the most common mineral in veins and “pressure shadows”.
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The fact that the reactions so deduced provide reasonably precise descriptions of such common textures as the sillimanite needles in biotite and quartz, and the abundant quartz “inclusions” in staurolite.
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Carmichael, D.M. On the mechanism of prograde metamorphic reactions in quartz-bearing pelitic rocks. Contr. Mineral. and Petrol. 20, 244–267 (1969). https://doi.org/10.1007/BF00377479
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DOI: https://doi.org/10.1007/BF00377479