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A distinctive assemblage of upper greenschist to lower amphibolite facies metasedimentary rocks with intercalated mafic and ultramafic rocks forms a major tectonostratigraphic unit, the Liberty Complex, within the central Maryland Piedmont. The Liberty Complex is composed of two lithotectonic units, the Morgan Run Formation and the Sykesville Formation. The Morgan Run Formation is a thick sequence of pelitic schist with intercalated quartzite, amphibolite, and detrital ultramafite; metagraywacke and intercalated amphibolite; serpentinite and associated ultramafic rock. The Sykesville Formation is a thick, poorly layered sequence of uniform, medium-grained quartzofeldspathic granulite and metadiamictite. Metamorphosed clasts in the diamictite are everywhere matrix supported, range in size from granules to slabs at least tens of meters in length, and commonly display internal metamorphic foliations that are truncated by the clast margins. Petrographic study of clast lithologies from exposures around Liberty Lake in Carroll and Baltimore Counties indicates that all clasts are represented by Morgan Run Formation bedrock lithologies in the immediate area.

The Morgan Run and Sykesville Formations are both polydeformed; however, the Morgan Run displays several phases of deformation that are not expressed in the Sykesville Formation or adjacent Piedmont terranes. Metagraywacke sequences in the Morgan Run commonly exhibit relict soft-sediment structures, and pelitic schist-quartzite sequences contain stratiform zones of block-in-matrix structure, which we interpret to be tectonic mélange. The Sykesville Formation was deposited after initial metamorphism of the Morgan Run Formation as a sequence of mass-flow and slide deposits characteristic of sedimentary mélanges.

The lithologic assemblage and the internal structural features of the Morgan Run Formation are strongly suggestive of an origin within a subduction complex. We interpret the Morgan Run as having been assembled in an accretionary wedge fronting a continent-facing volcanic arc in Cambrian time. The Morgan Run may represent underplated oceanic and trench-fill material that was deformed into tectonic mélange and broken formation during subduction and then rapidly uplifted as the continental margin of North America collided with the accretionary wedge. The Morgan Run Formation supplied coarse metamorphic detritus to the Sykesville deposits during collision and uplift. Both units were strongly deformed, locally interleaved, and regionally metamorphosed during final suturing of the arc to the continental margin. This final stage of development represents the classical Taconic orogeny. It resulted in the assembly of the Liberty Complex, a polygenetic mélange, from the Morgan Run and Sykesville Formations.

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