Georgetown Mafic Complex

Poorly exposed complex of tonalite, dark quartz diorite, gabbro, amphibolite, and undifferentiated basic rocks.
State Maryland
Name Georgetown Mafic Complex
Geologic age Late Precambrian (?) - Early Paleozoic
Lithologic constituents
Igneous > Plutonic > Granitic > Tonalite
Igneous > Plutonic > Dioritic > Quartz-diorite
Igneous > Plutonic > Gabbroic > Quartz-gabbro
Metamorphic > Metaigneous > Metaintrusive > Metaultramafic > Metapyroxenite
Metamorphic > Amphibolite
Metamorphic > Metaigneous > Serpentinite
Metamorphic > Schisttalc schist
Comments secondary unit description from USGS Geologic Names Lexicon (ref MD004): The Georgetown Intrusive Suite is divided into six mappable units in the map area and consists of a biotite tonalite, garnetiferous biotite-hornblende tonalite, biotite-hornblende tonalite, quartz gabbro and quartz diorite, metapyroxenite and hornblendite, and serpentinite and talc schist. The Georgetown is predominantly made up of the mafic biotite-hornblende tonalite, and the quartz gabbro and biotite tonalite make up smaller plutons. Possibly five intrusive phases can be identified within this intrusive suite that intrudes surrounding metasedimentary rocks. The Georgetown Intrusive Suite is of Early Ordovician age.

Cleaves, E.T., Edwards, J., Jr., and Glaser, J.D., 1968, Geologic Map of Maryland: Maryland Geological Survey, Baltimore, Maryland, scale 1:250,000.

In 1995 the Water Resources Division of the U.S. Geological Survey digitized the geologic map of Maryland from the 1968 paper map (Reference MD001). Information about products from the Maryland Geological Survey can be found at

USGS Geologic Names lexicon found at:

NGMDB product
Counties Montgomery