GEOCHEMICAL AFFINITY OF MESOZOIC DIABASE DIKE SEGMENTS IN BETHANY, CT

In south central Connecticut mafic magmatism associated with Triassic-Jurassic continental rifting is represented by three spatially and chemically distinct dike systems (Bridgeport-Pelham, Buttress-Ware, and Fairhaven-Higganum) which, based on petrographic and chemical data, have been interpreted to feed the three basaltic lava flows found in the Hartford (Talcott, Holyoke, and Hampden) and Pomperaug (East Hill, Orenaug, and South Brook) basins (Philpotts and Martello, 1985; Burton et al., 2005). The dike systems trend north-northeast and show well developed sinistral en echelon segmentation. Individual dike segments are commonly 0.5 to 10 km in length with offsets perpendicular to strike typically limited less that 0.5 km.

We have examined several geographically isolated diabase dike segments in Bethany, CT which trend north-northeast and are intruded into the Golden Hill Schist, Harrison Gneiss and Ordovician granites just west of the Hartford Basin. The Bethany dike segments are typically 3-5 meters wide and can be traced continuously in outcrop for up to several hundreds of meters. Geologic mapping of these intrusive bodies reveals some variations from the outcrop pattern represented on the geological compilation map published by the State of Connecticut (Rodgers, 1985). Geographic separation of the Bethany dike segments from the major dike systems of the region makes their magmatic association ambiguous. They trend parallel to, but are ~2-5 km east of major segments of the Bridgeport-Pelham dike system exposed adjacent to Rock Rimmon in Beacon Falls, are ~4 km west of the West Rock Ridge sill which is comagmatic with the Fairhaven-Higganum dike, and are ~8 km west of and parallel to the Buttress-Ware dike system. Chemical data, on the other hand, provide a clearer picture of magmatic association. Samples from the Bethany dike segments are relatively unaltered and show a narrow range of chemical compositions which averages: SiO₂ =50.1%, TiO₂=1.35%, Al₂O₃=13.6%, Fe₂O₃^T= 16.0%, MgO = 5.9%, CaO= 10.1%, Na₂O=2.0%, K₂O= 0.40%, P₂O₅=0.14%, LOI=0.08%. The compositions fall within the much broader array of compositions reported for the Bridgeport-Pelham dike system by Philpotts and Martello (1985) and clearly tie the Bethany dike segments to that intrusive suite.