GEOCHEMISTRY AND PROVENANCE OF MAFIC DIKES, WESTERN MAINE

In his 1965 report on the Geology of the Bryant Pond quadrangle, Maine, Guidotti mapped some 65 mafic dikes within this field area. Subsequent fieldwork has revealed a preponderance of dikes all over western Maine. They intrude the high-grade metamorphic country rocks, the 382 Ma Songo granodiorite, the 296 Ma Sebago granite and its associated pegmatitic bodies. At present their age and provenance are unclear although they could be related to a number of sources. This study examines the petrography and geochemistry of these mafic dikes to enable us to pinpoint their provenance.

The dikes are predominantly aphyric basalts although some are plagioclase-phyric. They display chilled margins with their hosts and in the center of the larger examples they are diabasic. There appears to be no correlation between the size/composition of the dikes and the rock unit they intrude. They trend predominantly NNE – SSW and range in size from 0.5m to 5m, though most commonly they are around a meter wide. Their mineralogy is plagioclase, pyroxene (some of which may be titanaugite) and abundant opaques. Most are fresh though chloritic alteration is observed. As no cross-cutting relationships are apparent we believe that these dikes constitute a single series and propose naming them the Western Maine dike series.

Geochemically they plot in the basalt field on the TAS diagram with SiO₂ wt % ranging from 46% to 52%. However, some dikes plot very close to the trachy basalt field and therefore they range from sub-alkaline to alkaline. On other bivariate plots, such as TiO₂ V MgO they have higher TiO₂ contents coincident with the Coastal New England magma series as documented by Dorais et al. (2005) and McHone (1992). On the standard basalt discrimination diagrams they plot in the Within Plate Basalt field. Therefore it would seem that the dikes of the Western Maine dike series show more geochemical affinity with the Coastal New England magma series, which greatly extends the geographic area over which this series is observed.