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Distribution of Calcium in a garnet and adjacent matrix minerals in a specimen from the Ordovician Moretown Formation, Plainfield, Massachusetts.

A complex metamorphic history is revealed. S-shaped trails of minerals in the inner part of the garnet were trapped as this mineral grew, and were part of an earlier fabric not directly related to the lower left to upper right external fabric. Compositional change in the garnet closely parallels the fabric change. A higher-Ca internal zone ends abruptly at the edge of the zone of inclusions defining the older fabric. Subsequent growth of inclusion-free garnet occurred first with little Ca, then much more, then little again. This sequence may record either a smooth single-stage metamorphic history (excursion in pressure and temperature as, for example, that part of the crust was involved in mountain-building) or a multi-stage history (two different events at different times in Earth history). In any case, chemical equilibria involving the outermost rim of the garnet, and the matrix minerals biotite, muscovite, paragonite, chlorite, plagioclase, ilmenite and quartz record final equilibration at ca. 520 degrees C and 7 kilobars pressure, or approximately 23 kilometers depth in the crust.

For more information about this image contact: Michael L. Williams