TRIASSIC AND JURASSIC MAGMATISM DURING CONTINENTAL BREAKUP ALONG THE EASTERN NORTH AMERICAN MARGIN

Our understanding of progressive rifting and continental break-up has greatly increased over the past two decades. But questions remain regarding the impact of syn-rift melt generation on the transition from mechanical stretching to seafloor spreading. The U.S. Eastern North American Margin (ENAM) preserves ~30 Myr of Triassic lithospheric stretching and syn-rift sedimentation prior to the emplacement of Central Atlantic Magmatic Province (CAMP) magmatism. CAMP magmatism is followed by ~50 Myr of rifting leading to steady-state seafloor spreading. Numerous regional dikes preserved in the ENAM proximal rift domain, recognized both in outcrop and aeromagnetic datasets, are attributed to CAMP, but these dikes remain largely undated. This study utilizes in-situ apatite U-Pb geochronology and whole-rock geochemistry to constrain the temporal and spatial geochemical evolution of Mesozoic dike emplacement along the ENAM.

Here we present new, spatially extensive, in-situ apatite U-Pb geochronology and corresponding whole-rock geochemical data for 16 diabase dikes and sills along the ENAM. We integrate these data into a regional tectonic model spanning the transition from Triassic diffuse stretching to Jurassic lithospheric breakup. Apatite U-Pb results from the Palisades Sill yielded an age of 201.4 ± 8.3 Ma, which agrees with the previous ⁴⁰Ar/³⁹Ar and U-Pb ages. Our new in-situ apatite U-Pb ages indicate three main pulses of magmatism occurred during protracted continental breakup along the ENAM. An initial pulse of magmatism at ~205-195 Ma is attributed to CAMP with whole-rock geochemistry comparable to previously reported geochemical data for CAMP-related rocks, including silica ranging from 46-53 wt. %, total alkalis between 2-4 wt. %, and an enrichment of LREEs relative to HREEs. A second magmatic pulse with slightly higher silica and total alkalis relative to the CAMP-related phase occurs between ~181-175 Ma. Finally, a third pulse with generally higher silica and total alkalis occurs between ~166-145 Ma. These new data demonstrate episodic, protracted, and low-volume syn-rift off-axis magmatism persisted ~50 Myr after the onset of CAMP. This off-axis magmatism is likely related to slow magma-limited spreading during the prolonged transition to symmetric seafloor spreading along the ENAM.