K-Ar dates and (super 40) Ar/ (super 39) Ar age spectra for Mesozoic basalt flows of the Hartford Basin, Connecticut, and the Newark Basin, New Jersey

Conventional K-Ar dates for plagioclase separates from the bottommost Talcott, the middle Holyoke, and the topmost Hampden Flows of the Hartford Basin are 186 + or - 8, 189 + or - 6, and 230 + or - 12 m.y. B.P., respectively, whereas those from the First, Second, and Third Watchung flows (proceeding from bottom to top) are 191 + or - 8, 194 + or - 4, and 185 + or - 4 m.y. B.P., respectively. The (super 40) Ar/ (super 39) Ar age spectrum for the Talcott Flow exhibits a well defined plateau at 187 + or - 10 m.y., representing a reliable estimate of the cooling age of the Talcott Flow. In contrast, the (super 40) Ar/ (super 39) Ar age spectrum for the Hampden Flow yields a pattern of dates that decrease from an initial value of 284 + or - 34 m.y. B.P., for gas released at the lowest temperature, to a small plateau at approximately 202 m.y. B.P. for gas released at the highest temperatures. The (super 40) Ar/ (super 39) Ar release pattern obtained for the Hampden Flow suggests that the conventional K-Ar date of 230 + or - 12 m.y. B.P. obtained for the plagioclase separate from this flow reflects the presence of excess radiogenic (super 40) Ar. Conventional whole-rock K-Ar dates range from 171 + or - 6 to 279 + or - 14 m.y. B.P. Excluding the date for the Hampden Flow, the dates for the flows of the Hartford and Newark Basins range from 185 + or - 4 to 194 + or - 4 m.y. B.P.--Modified journal abstract.