PETROLOGY, GEOCHEMISTRY, AND TECTONIC SIGNIFICANCE OF CROSSCUTTING DIKES IN NAHANT, MA

During the late Proterozoic the Avalon Terrane rifted away from the Gondwana supercontinent. As the terrane was moving towards the modified margin of Laurentia, around 488 Ma the Nahant Gabbro intruded. This was later intruded by a series of dikes in the late Devonian. We use drone imagery and field survey to map multiple sets of crosscutting dikes outcropping along the Castle Rock shoreline in Nahant, MA. In a key area we see five crosscutting dikes, which are identified from oldest to youngest as A, B and E (which do not crosscut each other therefore we cannot establish their temporal relationships), D, and C based on their crosscutting relationships. Dike A (not sampled) is thin, phaneritic, and trending at 054°. Dike B is aphanitic-porphyritic, trending at 038°, 30-45 cm thick. Dike E is aphanitic-porphyritic, 20-27 cm thick, and trending at 086°. Dike D is aphanitic, 25-33 cm thick, and trending at 150°. The youngest, dike C, is aphanitic, 11-16 cm thick, and trending at 096°. Hence, in this area, we identified a minimum of four sets of intrusive events. The dikes are aphanitic to aphanitic-porphyritic and therefore the minerals are too small to be determined using a petrographic microscope. Dikes B, C, D, and E were collected to be analyzed for whole rock geochemistry using XRF and LA-ICP-MS. Dikes B, C and D are basaltic tholeiites while dike E is a trachybasalt. Dike D is more primitive but younger than dike B suggesting a complex magmatic history with multiple stages of magmatic recharge. When plotted on a variety of tectonic discriminant diagrams, the Nahant dikes consistently plot into a within-plate or oceanic arc setting; however given that they are not subalkaline the former is our current preferred interpretation. Trace element analyses are in process and will provide insights into the tectonic setting and the nature of these magmatic events.