FIELD RELATIONS, PETROGRAPHY AND GEOCHEMISTRY OF MAFIC DIKES, SCHOODIC PENINSULA, COASTAL MAINE

A number of diabase and basaltic dikes outcrop along the almost continual coastal exposures of the Schoodic Peninsula, coastal Maine. They intrude the Siluro-Devonian Gouldsboro granite and are interpreted to be related to the opening of the North Atlantic during Mesozoic rifting. This petrographic and geochemical study focuses on two areas of dike concentration; Grindstone Neck and NW of Winterharbor, and extends the geochemical database for the Schoodic dike swarm.

The dikes of the study areas display the range of compositions and cross-cutting relationships evident throughout the Schoodic swarm. The diabase dikes, commonly 5-20m wide, have aphanitic chilled margins that grade over a few cms into coarser grained diabase. The thinner basaltic dikes (<1-2m) which cross-cut the diabase dikes are usually completely aphanitic though may contain some isolated plagioclase phenocryst rafts. Both dike sets are clearly post-granite emplacement (evidenced by granite xenoliths and xenocrysts and the control of brittle fractures on their orientation), however some syn-plutonic dikes are also observed. These typically show more interaction with the granitic host and have lobate margins and xenoliths of granite and, less commonly, meta-sedimentary rock.

Two main sets of Mesozoic dikes are distinguished in the Northern New England – the Central New England (CNE) and Eastern North American (ENA) suites. Data compiled in Dorais et al. (2005) show that their geochemical signatures differ, e.g. the CNE set have higher TiO₂ wt. % and Zr ppm for equivalent MgO wt. % and Mg numbers, respectively, than dikes of the ENA suite. Our preliminary data demonstrate that representatives of both the CNE and ENA dikes are present in the Schoodic dike swarm. This data extends the geochemical coverage of both Mesozoic and syn-plutonic mafic magmatism along the central Maine coast.