PETROLOGY, GEOCHEMISTRY, AND TECTONIC SIGNIFICANCE OF MAFIC DIKES IN SALEM, MA

Petrological and geochemical analysis of mafic dikes provides necessary information regarding the mantle-lithosphere interaction and overall geodynamic setting during magma formation. Numerous mafic dikes have been reported in the basement rocks of the Avalon terrane in Massachusetts. We present new petrological and geochemical results from mafic dikes intruding the Salem Gabbro-Diorite at the Salem Willows Park and Winter Island locations in Salem, MA. The dikes trend NNE and their width range between 10 to 60 cm. They are predominantly aphyric, although some have large (0.5-1 cm long) plagioclase lathes. In most dikes, these plagioclase phenocrysts are randomly oriented and occur throughout the outcrops. The dikes are composed of plagioclase, biotite, pyroxene, amphibole, and abundant opaque minerals. When plotted on a TAS diagram, dikes are tephrite/basanite and trachy-basalt with SiO₂ weight percent ranging from 42% to 47%. On a tectonic discriminate diagram (Merchede, 1986), they plot in the within plate tholeiite field. Trace element analyses are in process and will provide insights into the tectonic setting and nature of these magmatic units.