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Paper No. 133-8
Presentation Time: 3:45 PM

THE BALTIMORE MAFIC COMPLEX, MARYLAND: OPHIOLITE FRAGMENTS IN THE SOUTHERN APPALACHIAN OROGEN?


GUICE, George L.1, ACKERSON, Michael R.1, HOLDER, Robert M.2, GEORGE, Freya R.3, BROWNING-HANSON, Joseph4, BURGESS, Jerry L.4, FOUSTOUKOS, Dionysis I.5, BECKER, Naomi A.4, NELSON, Wendy R.6 and VIETE, Daniel R.3, (1)Department of Mineral Sciences, Smithsonian National Museum of Natural History, Smithsonian Institution, MRC 0119, PO BOX 37012, Washington, DC 20013-7012, (2)Earth and Planetary Sciences, Johns Hopkins University, 301 Olin Hall, 3400 N Charles St, Baltimore, MD 21211; Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, (3)Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21211, (4)Earth and Planetary Sciences, Johns Hopkins University, 301 Olin Hall, 3400 N Charles St, Baltimore, MD 21211, (5)Carnegie Institute of Science Washington, Geophysical Laboratory, 5251 Broad Branch Rd. NW, Washington, DC 20015, (6)Physics, Astronomy and Geosciences, Towson University, 8000 York Road, Towson, MD 21252

Ultramafic–mafic complexes decorate the length of the Appalachian Orogen, from Alabama to Newfoundland. In the northern Appalachians (i.e., north of New York City), several complexes are interpreted as near-complete or highly tectonized ophiolites, placing a constraint upon the magmatic and tectonic evolution in this portion of the orogen. The origin of those complexes in the southern Appalachians, however, remain a matter for considerable debate, having been attributed to continental arc or back-arc basin-related layered intrusions, or ophiolites.

In this study, we present the results of field observations, petrography, bulk-rock geochemistry, and spinel mineral chemistry for the ultramafic portions of the Baltimore Mafic Complex (BMC). The BMC refers to a discontinuous, poorly-exposed, tectonized, and highly altered suite of ultramafic–mafic bodies in Maryland and southern Pennsylvania. Two groups of samples are clearly recognized based on field characteristics, texture and composition. One group displays highly depleted bulk-rock trace element patterns similar to the residual mantle that forms lower oceanic lithosphere. This interpretation is supported by the composition of primary spinel grains, which show distinctly high Cr2O3 (51–59 wt. %) contents and extremely low Fe3+-numbers (< 0.05). Another group of samples — collected from exposures that exhibit distinctive layering on mm-, cm- and dm-scales — display relatively enriched bulk-rock trace elements compositions consistent with crystallization from a melt.

We interpret the BMC as representing various ophiolite fragments, including residual mantle and layered ultramafic portions. This finding suggests close similarities with similar-aged ophiolites in the northern Appalachians, raising the potential implication that the initiation and evolution of Paleozoic tectonic convergence is remarkably consistent along 1000s of kilometers of orogenic strike. Moreover, our methodologies may inform future investigations assessing the origin of highly metamorphosed Proterozoic–Archean ultramafic–mafic bodies; this includes our detailed petrographic and chemical assessment of spinel grains, which enabled confident discrimination between primary and secondary compositions.

Session No. 133

T8. From Rodinia to Pangea: Evolution of the Appalachian-Caledonian Orogen I
Wednesday, 28 October 2020: 1:30 PM-5:30 PM
Geological Society of America Abstracts with Programs. Vol 52, No. 6
doi: 10.1130/abs/2020AM-353736
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