EARLY MESOZOIC ALKALINE MAFIC DYKES, SOUTHWESTERN NOVA SCOTIA, CANADA, AND THEIR BEARING ON TRIASSIC-JURASSIC MAGMATISM

Abstract

Dykes of lamprophyre and alkaline olivine diabase cut the Cambro-Ordovician Goldenville Formation and the Carboniferous Wedgeport Pluton near Plymouth, southwestern Nova Scotia. 40Ar/39Ar geochronology yielded hornblende–biotite ages ranging between 231 ± 3 and 222 ± 3 Ma (mean 227 Ma) for the lamprophyres, and biotite ages of 209 ± 6 and 203 ± 15 Ma for the olivine diabase. Minerals present in the dykes include olivine (Fo83), augite with a rim of titanaugite, kaersutite and titaniferous phlogopite. The whole-rock composition of both the lamprophyre and olivine diabase shows greater LILE and HFSE enrichment in the lamprophyres, but the relative abundances of these elements are remarkably similar in the two rock types, suggesting that they are comagmatic. Values of eNd (~+4) and eSr (~+5) are much more primitive than values for the widespread early Jurassic tholeiites. Pb isotope compositions for the lamprophyres fall near the Northern Hemisphere Reference Line; those of the olivine diabase are intermediate between the lamprophyres and more radiogenic Pb compositions typical of the Triassic alkaline dykes at Seabrook, New Hampshire, and the early Jurassic tholeiites; they may have experienced some contamination in the lower crust. The lamprophyres resemble, in age and chemical composition, other alkaline rocks of the Coastal New England province. The olivine diabases, apparently synchronous with the main Eastern North American tholeiites, represent renewed partial melting of the same mantle source, triggered by the thermal effect of the regional tholeiitic magma.