Geological Society of America

T133 From Rodinia to Pangea: Evolution of the Appalachian-Caledonian Orogen I

Wednesday, 28 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

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

Paper number: T133-9
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

George Guice Presenting Author
Washington, DC
Daniel Viete
Johns Hopkins University, Baltimore, MD
Michael Ackerson
Smithsonian National Museum of Natural History, Washington, DC
Robert Holder
Johns Hopkins University, Baltimore, MD
Robert Holder
University of Michigan, Ann Arbor, MI
Freya George
Baltimore, MD
Joseph Browning-Hanson
Johns Hopkins University, Baltimore, MD
Jerry Burgess
Johns Hopkins University, Baltimore, MD
Dionysis I. Foustoukos
Geophysical Laboratory, Washington, DC
Naomi Becker
Baltimore, MD
Wendy Nelson
Towson University, Towson, MD

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-353736

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