20 Mesozoic Magmatism, Postrift Tectonics, and Passive Margin Development of Northeastern North America

The northeastern region experienced protracted Mesozoic (Triassic-Cretaceous) magmatism associated with continental rifting and subsequent passive margin development. We welcome contributions related to the occurrence, tectonics, or driving mechanisms of these syn- and postrift events.

Monday, 23 March 2026
8:00 AM - 12:00 PM
CCC, Room 27

A Proposed Redefinition of the Coastal New England Magmatic Province

Paper number: 20-1
Program: Technical Sessions
Time: 8:05 AM – 8:25 AM

Authors

Abstract

Mesozoic rifting along the Eastern North American Margin occurred over a prolonged period resulting in the opening of the Atlantic Ocean. Sedimentary basins show evidence for early rifting beginning in the Middle to Late Triassic, coincident with small-scale magmatism of the Coastal New England (CNE) province. The CNE province is defined as a broad region 60 km from the coast that includes mafic dikes from Rhode Island, Massachusetts, New Hampshire, and Maine, as well as felsic plutons from the Agamenticus Complex, Abbott Mountain and the Litchfield Pluton in Maine. 

Previous K-Ar and Ar-Ar ages for CNE magmatism range from 246 - 210 Ma (e.g. Foland and Faul, 1977; Pe-Piper et al., 1992), however, such ages can be subject to uncertainty during exhumation or low-grade metamorphism.  While the Agamenticus Complex was recently dated by U-Pb zircon to be 238.88±0.11 Ma (Hussey et al., 2016), zircons from the Litchfield Pluton record emplacement at 321±2 Ma (West et al., 2016).

We present a new high precision U-Pb zircon age of 206 ± 5 Ma from Abbott Mountain in SW Maine. The age, coupled with geochemistry, suggest it is related to the early alkaline magmatism of the nearby White Mountain Magmatic Suite (e.g. Kinney et al., 2022) rather than the CNE.

Of the three plutons proposed to define the geographic range of the CNE, only the Agamenticus Complex falls within the 240-210 Ma age range on which the province was defined.  Our previously reported U-Pb zircon age of 238.07±0.09 Ma from the Medford Dike in eastern Massachusetts (Ridge et al., 2024), however, is nearly identical to the Agamenticus Complex.  Both the Medford Dike and Agamenticus Complex fall along a narrow well-defined regional trend of N23E, which also intersects similar dikes in RI, NH, and ME. 

We propose that CNE magmatism may have occurred as a single <1 Myr event along a narrowly defined geographic area, rather than as a broadly distributed province.  The narrow N23E trend of CNE dikes is similar to the N23-30E trend of Triassic-age faults to the west (e.g. Wise et al., 1982), suggesting that CNE magmatism likely formed in response to regional extension during rift initiation near the site of eventual breakup.

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

doi: 10.1130/abs/2026NE-411180

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