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Pdf/17/2/561/5265443/561.Pdf 561 by Guest on 30 September 2021 Research Paper Research Paper GEOSPHERE Suprasubduction zone ophiolite fragments in the central Appalachian orogen: Evidence for mantle and Moho in the GEOSPHERE, v. 17, no. 2 Baltimore Mafic Complex (Maryland, USA) https://doi.org/10.1130/GES02289.1 George L. Guice1, Michael R. Ackerson1, Robert M. Holder2,3, Freya R. George2, Joseph F. Browning-Hanson2, Jerry L. Burgess2, Dionysis I. Foustoukos4, Naomi A. Becker2, Wendy R. Nelson5, and Daniel R. Viete2 11 figures; 3 tables; 1 supplemental file 1Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, 10th Street and Constitution Avenue NW, Washington, D.C. 20560, USA 2Department of Earth and Planetary Sciences, Johns Hopkins University, 3400 N Charles Street, Baltimore, Maryland 21218, USA CORRESPONDENCE: [email protected] 3Department of Earth and Environmental Sciences, University of Michigan, 1100 N University Avenue, Ann Arbor, Michigan 48109, USA 4Earth and Planets Lab, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, USA 5 CITATION: Guice, G.L., Ackerson, M.R., Holder, R.M., Department of Physics, Astronomy, and Geoscience, Towson University, 8000 York Road, Towson, Maryland 21252, USA George, F.R., Browning-Hanson, J.F., Burgess, J.L., Foustoukos, D.I., Becker, N.A., Nelson, W.R., and Viete, D.R., 2021, Suprasubduction zone ophiolite ABSTRACT peridotites and serpentinites. The Hollofield Ultra- has significant implications for the geodynamic fragments in the central Appalachian orogen: Evidence for mantle and Moho in the Baltimore Mafic Complex mafite likely represents the “layered ultramafics” processes that formed the Appalachian orogen. (Maryland, USA): Geosphere, v. 17, no. 2, p. 561–581, Suprasubduction zone (SSZ) ophiolites of the that form the Moho. Interpretation of the Baltimore For example, in the northernmost Appalachians https://doi.org/10.1130/GES02289.1. northern Appalachians (eastern North America) Mafic Complex as an Iapetus Ocean–derived SSZ (Newfoundland and Québec, Canada), the ultra- have provided key constraints on the fundamental ophiolite in the central Appalachian orogen raises mafic-mafic bodies are widely interpreted as Science Editor: Andrea Hampel tectonic processes responsible for the evolution of the possibility that a broadly coeval suite of ophi- near- complete to significantly tectonized supra- Associate Editor: Alan Whittington the Appalachian orogen. The central and southern olites is preserved along thousands of kilometers subduction zone ophiolites that have been Received 22 May 2020 Appalachians, which extend from southern New of orogenic strike. tectonically juxtaposed with the surrounding rocks Revision received 4 September 2020 York to Alabama (USA), also contain numerous ultra- (Church and Stevens, 1971; Williams, 1977; Huot Accepted 22 December 2020 mafic-mafic bodies that have been interpreted as et al., 2002; Kim et al., 2003; Coish and Gardner, ophiolite fragments; however, this interpretation is ■ INTRODUCTION 2004; Kurth-Velz et al., 2004; Lissenberg et al., 2005; Published online 5 February 2021 a matter of debate, with the origin(s) of such occur- Monteiro et al., 2008; Escayola et al., 2011). This rences also attributed to layered intrusions. These The Appalachian orogen in eastern North interpretation constrains the ultramafic-mafic bod- disparate proposed origins, alongside the range of America—a product of the protracted closure of ies as representing forearc (or back-arc) oceanic possible magmatic affinities, have varied potential the Iapetus and Rheic Oceans during the Paleozoic lithosphere that formed due to seafloor spreading implications for the magmatic and tectonic evolu- Era—contains numerous discrete ultramafic-mafic in a subduction zone setting (e.g., Whattam and tion of the central and southern Appalachian orogen bodies along its 3000 km length (Misra and Keller, Stern, 2011; Stern et al., 2012). and its relationship with the northern Appalachian 1978). Such bodies are generally a minor compo- In the central and southern Appalachians, the orogen. We present the results of field observations, nent of orogenic belts such as the Appalachians, petrogenesis and origin(s) of the comparatively petrography, bulk-rock geochemistry, and spinel but determining their origin(s) can place important small, yet numerous, ultramafic-mafic bodies are mineral chemistry for ultramafic portions of the constraints on broader magmatic and tectonic his- poorly understood (Crowley, 1976; Misra and Keller, Baltimore Mafic Complex, which refers to a series tories (e.g., Lissenberg et al., 2005). End-member 1978; Hanan and Sinha, 1989; Sinha et al., 1997; of ultramafic-mafic bodies that are discontinuously interpretations of these bodies are that they repre- Peterson and Ryan, 2009). Several occurrences exposed in Maryland and southern Pennsylvania sent ophiolite fragments (Crowley, 1976) or layered are interpreted as the dismembered fragments of (USA). Our data indicate that the Baltimore Mafic intrusions (Sinha et al., 1997; Kerrigan et al., 2017). ophiolites, including: amphibolites and associated Complex comprises SSZ ophiolite fragments. The These two first-order interpretations require very units of the Dadeville Complex, Alabama (Stow et Soldiers Delight Ultramafite displays geochemi- different tectono-magmatic origins: obduction or al., 1984); the Hamlett Grove meta-igneous suite, cal characteristics—including highly depleted emplacement of oceanic lithosphere onto or into South Carolina (Mittwede, 1989); the Buck Creek bulk-rock trace element patterns and high Cr# of continental lithosphere; or the intrusion of mafic ultramafic body and associated ultramafic- mafic spinel—characteristic of subduction-related mantle magma in a continental setting. bodies within the Ashe and Alligator Back Meta- This paper is published under the terms of the Beyond these basic interpretations, the mag- morphic Suites, North Carolina (McElhaney and CC-BY-NC license. George Guice https://orcid.org/0000-0003-0599-0907 matic affinity of the ultramafic-mafic bodies also McSween, 1983; Misra and Conte, 1991; Tenthorey © 2021 The Authors GEOSPHERE | Volume 17 | Number 2 Guice et al. | Suprasubduction zone ophiolite fragments in the central Appalachian orogen Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/2/561/5265443/561.pdf 561 by guest on 30 September 2021 Research Paper et al., 1996; Raymond et al., 2003, 2016; Peterson have been confidently recognized in ultramaf- contrast, the Iapetan and Peri-Gondwanan realms and Ryan, 2009); the Piney Branch Complex, Vir- ic-mafic complexes of the northern Appalachian show considerable variability and have been fur- ginia (Drake and Morgan, 1981); and the Baltimore orogen. We also consider the magmatic affinity of ther subdivided into a series of terranes that record Mafic Complex (BMC), Maryland–southern Penn- the BMC and discuss potential implications for the unique ages, lithologies, metamorphic grades, and sylvania (Crowley, 1976). However, none of the evolution of the Appalachian orogen. structural styles (e.g., Williams and Hatcher, 1982; proposed ophiolites in the central and southern Horton et al., 1989; Faill, 1997; Hatcher, 2004, 2010; Appalachians have been definitively shown to Sinha et al., 2012). In the central and southern contain residual mantle rocks (Shank and Mar- ■ GEOLOGICAL FRAMEWORK Appalachians, the Iapetan realm comprises volca- quez, 2014). no-sedimentary rocks of arc and oceanic affinities, Mantle rocks, which comprise the lowermost The Appalachian Orogen while the Peri-Gondwanan realm records Gondwa- portions of oceanic lithosphere (e.g., Dilek and nan arc magmatism and sedimentation (Hibbard et Furnes, 2011, 2014, and references therein), exhibit The Appalachian orogen is a northeast-south- al., 1998, 2007b). In places, these Neoproterozoic– characteristically depleted trace element abun- west–trending belt of Mesoproterozoic to Paleozoic Paleozoic rocks are overlain by Carboniferous clastic dances and are therefore the most chemically rocks that are exposed over a distance of 3000 km in sedimentary rocks, Mesozoic volcano-sedimentary unique portion of ophiolites (e.g., Paulick et al., North America, from Alabama in the south to New- units, and the dominantly Cenozoic sediments of 2006; Godard et al., 2008). This geochemical sig- foundland in the north (Rodgers, 1968; Aleinikoff et the Atlantic coastal plain (Fig. 1A). nature results from the preferential concentration al., 2002; Hibbard et al., 2007b; Hatcher, 2010; Hor- of incompatible trace elements in the melt during ton et al., 2010; Sinha et al., 2012). It is commonly partial melting, resulting in their extraction from, separated into northern, central, and southern seg- The Baltimore Mafic Complex (BMC) and subsequent depletion in, the upper mantle (e.g., ments (see Fig. 1) that reflect distinctions in terms Godard et al., 2008; Stern et al., 2012). Unequivocal of protolith age, lithology, metamorphism, and The BMC of the central Appalachians (located demonstration of the presence of residual mantle deformation (Rodgers, 1970; Hibbard et al., 2007b; within the Iapetan realm; Fig. 1A) is a series of rocks in any of these central or southern Appala- Hatcher, 2010; Sinha et al., 2012; Bosbyshell et al., ultramafic-mafic bodies that are discontinuously chian ultramafic-mafic complexes would provide 2016). Along its entire strike, the Appalachian oro- exposed in Maryland and southern Pennsylva- robust evidence in favor of an ophiolite
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