36 Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories

This session welcomes contributions  applying modern approaches in metamorphic petrology, geochronology, and structural geology to unravel the dynamics of convergent margin processes from the Appalachians and beyond. We encourage submission from Early Career participants.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Room 22/23

Rapid Establishment of Arc Magmatism Following Subduction Initiation in the Iapetus Ocean: Insights from the Baltimore Ophiolite, Central Appalachians, USA

Paper number: 36-2
Program: Technical Sessions
Time: 8:25 AM – 8:45 AM

Authors

Abstract

Much of the city of Baltimore, Maryland, USA is built on a 25x50 km, juvenile metaigneous complex known as the Baltimore Mafic Complex (BMC). Mapping by the Maryland Geologic Survey (MGS) in the 1960s and 1970s identified a stratigraphy within the BMC, including (from bottom to top): a layered ultramafic section comprising metapyroxenites and metaperidotites (Hollofield Layered Ultramafite); a layered metagabbroic section (Mount Washington Amphibolite); a km-thick sequence of bimodal metavolcanics (James Run Formation); and metasediments (Jones Falls Schist). Together, these units were interpreted by MGS geologists to constitute the crustal portion of an ophiolite that was emplaced onto the Baltimore Terrane of the Laurentian margin.

New whole-rock major, minor, trace and rare earth element (T–REE) geochemistry and high-precision geochronology data suggest that the BMC records a rapid (≤ 1 My) magmatic evolution from forearc seafloor spreading to the emergence of arc processes during subduction initiation. Amphibolite geochemistry (n=27 samples) spanning the volcanic stratigraphy of the James Run Formation captures an evolution in melting processes during development of the BMC. The Druid Hill Amphibolite, located at the base of the James Run Formation, has relatively high alkalis and TiO2 as well as low MgO. In contrast, mafic units of the Carroll Gneiss, located at the top of the James Run Formation, contain the lowest alkalis and TiO2 as well as highest MgO. This geochemical evolution, from forearc basaltic to boninitic/low-Ti basaltic compositions, which is supported by T–REE geochemistry, is comparable to that recorded in the modern Izu–Bonin–Mariana forearc as well as suprasubduction zone ophiolites associated with closure of the Tethys Ocean. In these younger examples, the observed geochemical evolution is considered to record subduction initiation and the onset of arc melting processes.

High-precision dating of the James Run Formation stratigraphy reveals that evolution in juvenile magma geochemistry and inferred melting processes occurred over a short period of ≤ 1 My at 471–470 Ma. The geochemistry and geochronology results provide valuable constraints on the timescales of tectonomagmatic evolution during subduction initiation, suggesting that arc processes may establish quickly after initial forearc spreading. The Baltimore Ophiolite records a voluminous yet exceptionally brief history of oceanic tectonism and associated magmatism during establishment of convergence in its sector of the Iapetus Ocean.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411139

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