PETROGRAPHIC AND GEOCHEMICAL ANALYSIS OF A DIABASE DIKE FROM NORTH HARPSWELL, MAINE: A PRELIMINARY REPORT
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of the Higganum dike in Connecticut (Asher, 1995) and from Christmas Cove, the type locality for this dike (McHone, unpublished data).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.