Abstract

The net compositions of neosome plus mafic selvage in two biotitic layered migmatites from the Baltimore Gneiss at Piney Creek, Maryland, closely approximate the paleosome compositions: the migmatization must have occurred in rocks closed to all except possibly volatile components. Anatexis or metamorphic differentiation is indicated as the migmatization mechanism. Parallel tie lines between the neosome, selvage, and paleosome in a modal biotite-microcline plot suggest a subsolidus metamorphic differentiation mechanism in which microcline replaces biotite in the neosome by a reaction such as biotite + 6H+ → microcline + 3 (Fe++, Mg++) + 4H2O; the Fe and Mg released by the reaction diffuse from the neosome to the selvage; biotite replaces microcline in the selvage by the reverse of the reaction. Anatexis (simple partial fusion) in the Piney Creek rocks is also indicated because (1) the closer a paleosome composition is to the granite minimum the more extensive is the migmatization in the rock; and (2) the neosomes are closer to the granite minimum than the paleosomes.

It is postulated that the Piney Creek migmatites formed by metamorphic differentiation induced by anatexis. Anatexis decreases fH2o locally because much water must be dissolved in the first melt, thus initiating the reaction by which biotite breaks down to microcline. The fH2o gradient between the neosome and paleosome maintained by the presence of melt in the neosome drives the metamorphic differentiation and should also create an asio2 gradient leading to the observed quartz migration from the selvage to the neosome.

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