PETROLOGY OF THE BALTIMORE GABBRO, MARYLAND

The Baltimore gabbro, made classic by G. H. Williams, is found to conform to modern petrologic theories. Two plagioclases, An_78–83 and An_64–70 distinguish themselves. The mafic minerals, especially orthopyroxene, show evidences of strong fractionation. Alleged primary brown poikilitic hornblende of Williams and others proves to be a product of uralitization, and primary hornblende is distinguished solely by texture. On this basis, a new rock type, bojite, is defined. The structure of the gabbro complex conforms with an origin in magmatic action, and many structures previously alleged to be metamorphic prove to have an igneous origin. The general structure is that of a funnel with the southeast lip dipping less steeply than the northwest. On the basis of iron enrichment, three distinct periods of crystallization are described: an ultramafic, a gabbroic, and a bojitic. Respective conditions prevalent in the magma chamber are discussed. Differentiation trends of Maryland rocks show absolute enrichment in iron throughout most of the gabbro complex crystallization; alkali enrichment begins with the bojites and is emphasized later in the quartz-bearing rocks. Alteration processes at work in the complex seem to bear out the laboratory system MgO-SiO₂-H₂O of Bowen and Tuttle and analogies are drawn between the two. The sequence of intrusion is believed to be: ultramafic border rocks, gabbros with included ultramafic bands, and quartz-bearing rocks.