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

U-Pb Zircon Dates Suggest Rapid Tectonic Rates: Constraints on the Early-Convergent Evolution of the Iapetan Margin, Central Appalachian Piedmont, USA

Paper number: 36-3
Program: Technical Sessions
Time: 8:45 AM – 9:05 AM

Authors

Abstract

Rocks of the central Appalachian Piedmont record Paleozoic orogenesis from subduction initiation to the final assembly of Pangea. The early convergent history is partly represented by a thick package of metasedimentary rocks in the Baltimore and Potomac Terranes, which is interpreted as an early syn-convergent mélange. These metasediments host numerous kilometer-scale mafic and ultramafic bodies of uncertain origin; historically, they have been interpreted as intrusions, diapirs, fault-bounded blocks, or ophiolite-derived olistoliths. We performed igneous and detrital zircon U-Pb geochronology to investigate the origins of these mafic–ultramafic bodies and their enclosing metasediments. Metasediments from both the Baltimore and Potomac Terranes have detrital zircon age spectra with major mid- to late-Mesoproterozoic peaks, suggesting derivation from the Granite–Rhyolite and Grenville Provinces of Laurentia, respectively. Metasediments from the Potomac Terrane contain minor Neoproterozoic zircons. These have been proposed to represent Gondwanan detrital input, but could have been sourced from local igneous units related to the Cryogenian–Ediacaran breakup of Rodinia. Only one sample yielded Ordovician zircons (n=4 of 150 concordant grains), though it is not yet clear if these are metamorphic neocrystallized zircons. Metasediments from the Baltimore Terrane, located outboard of the Potomac Terrane, contain no zircons younger than c. 900 Ma.

In the Mather Gorge Formation of the Potomac Terrane, the Carr’s Mill metagabbro contains zircons with Ordovician magmatic rims (c. 464 Ma) and inherited cores with ages matching the detrital zircon age spectrum of the enclosing metasediments. These results suggest that the metagabbro was intruded into the Laurentian margin rather than derived as an olistolith from a juvenile oceanic arc.

The data together indicate that the Maryland Piedmont metasediments were primarily derived from Laurentia and were deposited in a marginal basin inboard of the nascent arc. Since obduction of the Baltimore Ophiolite onto the Laurentian margin requires the initial subduction zone to be oceanward-dipping, back-arc extension would require subduction polarity reversal to place Laurentia on the overriding plate. In such a scenario, U-Pb age constraints on the Baltimore Ophiolite (471 Ma) and the Carr’s Mill metagabbro (c. 464 Ma) require that Iapetan subduction initiation, arc establishment, ophiolite obduction, polarity reversal, and back-arc spreading all occur in ≤ 15 My. Further investigation is needed to resolve the complex early-convergent tectonic history of the Maryland Piedmont.

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

doi: 10.1130/abs/2026NE-411179

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