FIELD AND PETROGRAPHIC STUDY OF THE BALTIMORE MAFIC COMPLEX, CECIL COUNTY, MARYLAND

The Baltimore Mafic Complex (BMC) extends from Baltimore MD northeast into southeastern Pennsylvania. Generally, the BMC is described as a layered intrusion or intrusions consisting of ultramafic bodies at its base and intermediate bodies at its peak. The origin of this formation is widely debated: various arguments support that it has a subduction related origin, while others imply a continental rift. A recent study suggests that it is a supra-subduction zone ophiolite. To better understand the nature of the complex, a field and petrographic study was conducted along a 3000 foot cross-section along the Octararo Creek in Cecil County, Maryland. As in other areas of the Baltimore Mafic Complex, a gradation from mafic to intermediate rock is evident; however, felsic rock is also identified at the southern terminus of this cross-section. To the north, rocks are dominantly mafic consisting of poikilitic gabbros with abundant pyroxene and relict olivine oikocrysts. Southward, samples display an increase in quartz and hornblende abundance. Continuing southward, the rocks become more felsic consisting of abundant quartz, as well as plagioclase, and biotite. Most samples show some degree of retrograde alteration. Plagioclase grains in the more felsic samples exhibit saussuritization. Metadiamictite is also abundant in the southern end of the section. Foliated metadiamictite samples contain sedimentary inclusions greater than 10 centimeters in size. Clasts are predominantly meta-sandstones and biotite schists. While field evidence of deformation is limited, thin sections show grain boundary migration, micro-shearing, subgrain formation, and microfractures. Textural features within the samples suggest that the area experienced at least two stages of deformation. Preliminary results indicate that this cross-section along the Octararo Creek is the margin of a mafic igneous body intruding into quartz-rich sedimentary rocks.