EVALUATING THE EVOLUTION OF A PASSIVE MARGIN: COMPARING JURASSIC MAGMATISM IN NEW HAMPSHIRE AND VIRGINIA

The Eastern North American Margin (ENAM) records at least one complete Wilson cycle, with both continental collision and break-up co-spatially preserved. Various tectonic events associated with continental collision, and thus the amalgamation of Pangea, span the ENAM, including the Taconian, Acadian, and Alleghanian orogenies. The rifting of Pangea is associated with the Central Magmatic Atlantic Province (CAMP), erupting 200 Ma. Since rifting, the ENAM has developed into a tectonically passive margin, with rift volcanism migrating east to the present location of the Mid-Atlantic Ridge. However, this passive margin has been quite volcanically active since rifting, with magmatic pulses in the Virginias (late Jurassic and Eocene), New York (late Jurassic), New England (Cretaceous), and Bermuda (Oligocene). These intraplate magmatic events provide insight into the dynamic evolution of the post-rift mantle from lithospheric instabilities (e.g. Virginia, Mazza et al., 2014; 2017) to the recycling of young, volatile rich lithologies (e.g. Bermuda, Mazza et al., 2019).

Both rift and post-rift magmatic activity is well preserved in New England with several magmatic pulses spanning ~120 Ma. In particular, New Hampshire preserves these events: CAMP dikes are found throughout New Hampshire, the bulk of the White Mountain magma series (WMMS) has been dated between 198 and 180 Ma (Kinney et al., 2022), and the New England-Quebec Igneous Province records magmatism at 124-123 Ma (Kinney et al., 2021). The new ages from the WMMS raises questions about mantle dynamics associated with rifting, and the longevity of magmatism in the early development of a passive margin. While the WMMS is in part contemporary with CAMP, the younger dates exceed rifting by 20 Ma. Could the longevity of the WMMS be associated with similar types of lithospheric instabilities that triggered the late Jurassic magmatism in Virginia? By exploring the geochemistry of the alkaline magmas both in Virginia and the WMMS we can begin to better understand the mantle dynamics associated with the early evolution of a passive margin. New data from Passaconaway syenites are discussed with context of the phonolites from Virginia to explore lithospheric foundering and delamination as a local mechanism to sustain post-rift magmatism.