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Gneiss domes in the Maryland Piedmont near Baltimore contain a wide variety of metamorphic rock types, all metamorphosed to amphibolite grade and collectively named Baltimore Gneiss. Most were derived from interlayered sedimentary and volcanic rocks into which granitic plutons were emplaced in both Precambrian and Paleozoic time. Migmatitic features are common in the Baltimore Gneiss. Most seem to reflect either original sedimentary layering or original layering that has been enhanced by later metamorphic differentiation, but some involved partial melting, perhaps during the Precambrian metamorphism. We find no evidence for widespread melting in rocks at the present level of exposure during the Paleozoic. Limited fluid-driven K-metasomatism may well have occurred locally, but we find no evidence for metasomatism on a regional scale. Pressure-temperature conditions for the Baltimore Gneiss during the Paleozoic metamorphism are estimated to be 7–9 kb and 625–675 °C.

For half a century, the anticlinal structures exposing basement gneiss at Baltimore have been cited as classic examples of mantled gneiss domes, but more recent field studies and geophysical investigations suggest that they are the result of interference between late upright folds and early nappe structures formed by a combination of isoclinal folding along subhorizontal axial surfaces and thrust faults rooted in a regional décollement.

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