PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF A DIABASE DIKE FROM NORTH HARPSWELL, MAINE: A PRELIMINARY REPORT

Feeder dikes and flood basalts across northeastern North America are believed to be closely related to the creation of new oceanic crust during the Jurassic opening of the Northern Atlantic Ocean. Such feeder dikes extend all the way from Southern New England to coastal Maine and even into Nova Scotia (McHone et al., 1995). In Connecticut, the Higganum diabase dike cuts many different lithologies as it traverses Southern New England and is believed to have fed the earliest lava flow in the Mesozoic Hartford basin. The Christmas Cove dike in Maine, an extension of the Higganum, is exposed along the southern coast of Maine (McHone et al., 1995, pers. comm., 2002).

The petrography and geochemistry of one such exposure of the Christmas Cove dike from North Harpswell, Maine is the subject of this study. At this location the nearly 60-ft-wide dike intrudes the Ordovician Cape Elizabeth schist which is part of the early Paleozoic sequence of metamorphic rocks from the Casco Bay Group. Neither of the dike contacts with the schist are exposed at this location. The dike, which dips approximately 40° to the south, exhibits the typical spheroidal weathering and rusty brown color observed in diabase. Near the contacts with the schist the diabase is fine-grained and porphyritic whereas the grain size increases towards the interior of the dike. Close to the contact with the schist, phenocrysts of augite, bronzite, and plagioclase make up approximately 25% of the rock. Glomeroporphyritic clusters of augite and plagioclase are also present.

Chemically, the "chilled margin" of the Christmas Cove dike at Harpswell is similar to that of the Higganum dike in Connecticut (Asher, 1995) and from Christmas Cove, the type locality for this dike (McHone, unpublished data). SiO2, Al2O3, CaO, and Na2O contents show no systematic variation with distance across the width of the exposure. However there is a slight increase in the abundance of MgO, Cr, and Ni towards the interior of the dike exposure suggesting the presence of a greater percent of mafic minerals or a more primitive magma composition.