A NEW 238 MA CA-ID-TIMS U-PB ZIRCON AGE FOR THE MEDFORD DIKE, MASSACHUSETTS

The Medford Dike is an up to 180 m thick biotite-gabbro dike intruded into volcanic, plutonic, and sedimentary rocks of the Avalon Terrane. Efforts to date the dike using K-Ar and Ar-Ar systems have yielded conflicting results; a K-Ar whole rock age from the 1970s is 190 Ma and an Ar-Ar biotite age is 304 Ma (Ross, 2020). A sample from the center of the dike where it is ~100 m wide and generally coarser grained and more compositionally evolved was collected from an abandoned quarry at Pine Hill in the Middlesex Fells Reservation. We isolated 37 zircon grains and obtained a weighted mean of six CA-ID-TIMS U-Pb zircon dates of 238.07 ± 0.07 Ma (2 sigma).

Our new high precision age for the Medford Dike places it squarely within the Coastal New England (CNE) province of basaltic-granitic magmas emplaced in the shallow crust along the Eastern North American margin from ~240 to 200 Ma (e.g. McHone and Butler, 1984; Whalen, 2019). The CNE is linked to the onset of rifting of Pangea following the Alleghenian orogeny. The only other high precision U-Pb zircon age of CNE magmatism is 238.88 ± 0.11 Ma (2 sigma, Hussey et al., 2016), from the Agamenticus Complex of southeastern Maine, a series of alkali syenites and biotite granites (Brooks, 2019). Given the similarities in age and alkaline geochemistry, it is possible that the Medford Dike and the Agamenticus Complex represent different parts of the same magmatic system. Additionally, we postulate that further high precision dating of CNE-related rocks may show that the duration of the CNE magmatic event was much shorter than originally proposed, similar to other continental large igneous province events such as the subsequent Central Atlantic Magmatic Province (e.g. Blackburn et al., 2013).