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Petrology of the Baltimore gabbro, Maryland

Norman Herz
Petrology of the Baltimore gabbro, Maryland
Bulletin of the Geological Society of America (September 1951) 62 (9): 979-1016


The Baltimore gabbro, made classic by G. H. Williams, is found to conform to modern petrologic theories. Two plagioclases, An (sub 78-83) and An (sub 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-SiO (sub 2) -H (sub 2) O 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.

ISSN: 1050-9747
Serial Title: Bulletin of the Geological Society of America
Serial Volume: 62
Serial Issue: 9
Title: Petrology of the Baltimore gabbro, Maryland
Author(s): Herz, Norman
Pages: 979-1016
Published: 195109
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Accession Number: 1959-013927
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. geol. map
N37°55'00" - N39°43'00", W79°30'00" - W75°04'60"
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from Bibliography and Index of North American Geology, U. S. Geological Survey, Reston, VA, United States
Update Code: 1959
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