Subtle oscillatory zoning in garnet from regional metamorphic phyllites and mica schists, western Erzgebirge, Germany

Growth zoning patterns of garnet grains in phyllite and mica schist from th e Gamet-Phyllite Unit and the Mica-Schist/ Eclogite Unit of the Western Erz gebirge, in Saxony, Germany, have been studied in detail by electron microp robe and are characterized with X-ray-intensity mapping images (MAPS) and q uantitative chemical analyses. Zoned grains of garnet in the phyllite show a continuous decrease in spessartine component from core (28 mole %) to rim (19 mole %), and, over about 100 mu m at the rim, a discontinuous, oscilla tory increase, decrease and further increase to 30 mole %. These changes ar e correlated with an antithetic, oscillatory zoning of comparable amplitude in the almandine component (47% in the core, 56% toward the rim, 49% at th e extreme rim). Available data for diffusion rates in garnet suggest that n o significant modification of growth zoning attributable to later intracrys talline diffusion has occurred. In two samples of mica schist, oscillatory zoning with respect to the grossular component is correlated with an antith etic pattern of the almandine component (e.g., sample E 514: 17.0 mole % gr ossular component in the core, decreasing to 4.7 mole %, then increasing to 6.8 mole % and decreasing again to 4.3 mole % at the extreme rim). Althoug h the patterns of zoning in themselves do not yield any evidence of modific ation after growth, available data on diffusion do not entirely rule out th is possibility. In agreement with the observed textural relationships, we a ttribute the oscillatory changes in garnet composition to specific continuo us reactions during regional metamorphism, indicating a complex growth-and- resorption history of the garnet resulting from small-scale variations in t he rate of decompression of the rocks. No evidence in support of an open-sy stem behavior of the fluid phase, such as highly variable and irregular pat terns of oscillatory zoning, could be found. Where oscillatory zoning occur s, the number of oscillations and their relative changes in composition are the same throughout that sample, and may even be correlated between sample s collected many kilometers apart.