AN EVALUATION OF THE BROAD-TERRANE FLOOD BASALT HYPOTHESIS: A COMPARATIVE STUDY OF THE ONWAY DIKE (NH) AND THE TALCOTT BASALT (CT)

Early Jurassic tholeiitic lavas occur in Mesozoic basins along the eastern seaboard of the United States and Canada. It has long been noted that the lavas across the region share strong geochemical, petrographic, stratigraphic and radiometric age similarities. The observation that interbasinal dikes also share these same characteristics has led to the proposal that the Early Jurassic tholeiites erupted in flood basalt abundances over a broad terrane of northeastern North America (McHone, 1996). A recent geochemical study of lavas and dikes from Atlantic Canada prompted Pe-Piper and Piper (1999) to question the broad-terrane hypothesis because of Zr/Y and Nd and Sr isotopic differences between their rocks and those farther south. The question of the viability of the broad-terrane hypothesis may be one of scale; how extensive were the inferred comagmatic eruptions and were the interbasinal dikes the sources of the lavas? We conducted a geochemical study of the Onway dike of southeastern New Hampshire to compare its composition with the presumed eruptive equivalent, the Talcott Basalt of the Hartford Basin, Connecticut. Comparisons of our analyses of the Onway dike with those in the literature for the Talcott Basalt show the two are indistinguishable in all measured major and trace elements. Additionally, the mineralogy and mineral compositions are indistinguishable. These similarities support the suggestion that the Onway dike is a continuation of the Holden (MA) and Higganum (CT) dike system that erupted as the Talcott Basalt in Connecticut. Our data are therefore permissive of a broad-terrane hypothesis, at least over a scale of a few hundred kilometers.