Duration and Geochemical Evolution of Triassic and Jurassic Magmatism Along the Eastern North American Margin

Our understanding of the impact of melt generation and the interplay between magmatism and mechanical stretching during progressive rifting leading to seafloor spreading remains rudimentary. The Eastern North American Margin (ENAM) provides an excellent location to study the influence of rift magmatism on continental break-up considering the preservation of ∼30 Myr of syn-rift strata and voluminous basaltic dikes, sills, and flows. Previous studies mainly focused on magmatism preserved in ENAM rift basins, emphasizing Central Atlantic Magmatic Province activity. Aeromagnetic data sets show pervasive magmatism across the ENAM proximal domain in the form of dikes that largely remain undated. We present in situ apatite U-Pb geochronology and whole-rock geochemical data from diabase dikes along the ENAM to determine the temporal and chemical evolution of Mesozoic dike emplacement and evaluate whether these magmas were emplaced rapidly at ∼201 Ma or in episodic pulses during rifting and break-up. New in situ apatite U-Pb analyses collectively indicate multiple magmatism pulses along the proximal domain of the ENAM, clustering around ∼201, ∼180, and ∼150 Ma. A first pulse at the Triassic/Jurassic boundary is likely due to decompression melting of an enriched mantle, a second smaller pulse in the Early Jurassic potentially correlative to the Blake Spur Magnetic Anomaly and lithospheric breakup, and a third small pulse in the Early Jurassic potentially correlative to the transition to symmetric seafloor spreading. These results indicate that prolonged off-axis magmatism is likely due to slow spreading rates driving delocalization of extension away from the rift axis into the proximal domain.