PETROGRAPHY, AGE AND PROVENANCE OF MAFIC DIKES, WESTERN MAINE

Basaltic (sensu latto) dikes intrude the high-grade metamorphic rocks, the Songo granodiorite, the Sebago granite and the Sebago Granite-Migmatite domain of western Maine. Collectively they constitute therefore a major dike swarm, although it is unclear if they comprise one or multiple pulses of dike emplacement. Similarly their provenance is also problematic as they could be related to the CAMP magmatism, assumed to be ~ 200Ma and tholeiitic in composition, or associated with the Coastal New England (CNE) suite, an older (~ 230Ma) mantle plume source of alkaline affinity. This study presents further field, petrographic and geochemical data for dikes that outcrop to the east of Sebago Lake along with preliminary Ar/Ar ages. Combining this data with previous data for the dikes that outcrop to the north enables a critical assessment of the geochemical affinity, age, and provenance of these basalt dikes across western Maine.

The basaltic dikes range in thickness from < 1m to 4m across and have well developed chilled margins. They are mostly aphyric in texture, although some of the larger dikes are coarser grained in their centers and should be termed diabase. Some porphyritic examples are also observed containing small cm-sized, randomly oriented plagioclase lathes. In thin section many of the dikes may contain titanaugite although this needs to be confirmed with microprobe data. Another set of dikes that cross cut the basalt/diabase dikes are trachybasalts compositionally and contain glomerocrysts composed of plagioclase and aergerine (?). These dikes are most likely related to nearby alkaline stocks.

Geochemically the basaltic dikes are mostly alkaline in composition, plot in the “within plate” alkali basalt field on trace element discrimination diagrams and display the high TiO₂ levels observed in dikes of the CNE suite. However, there are notable exceptions to this, with some of the dikes not conforming to this pattern. Indeed new preliminary ⁴⁰Ar/³⁹Ar integrated ages of 228.4± 4.9Ma and 197.0±1Ma may suggest two pulses of dike emplacement. Therefore, while most of the basaltic activity can be related to the older CNE suite there may be some representatives of CAMP magmatism in this part of northern New England.