Conventional K-Ar dates and 40Ar/39Ar age spectra were obtained for the basalt flows of the Triassic-Jurassic Hartford Basin in Connecticut and the Newark Basin in 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 ± 8, 189 ± 6, and 230 ± 12 m.y. B.P., respectively, whereas those from the First, Second, and Third Watchung Flows (proceeding from bottom to top) are 191 ± 8,194 ± 4, and 185 ± 4 m.y. B.P., respectively. The 40Ar/39Ar age spectrum for the Talcott Flow, after a minor initial increment of gas yielding a date of 166 ± 11 m.y. B.P., exhibits a well-defined plateau at 187 ± 10 m.y. The well-defined plateau obtained suggests that the date represents a reliable estimate of the cooling age of the Talcott Flow. In contrast, the 40Ar/39Ar age spectrum for the Hampden Flow yields a pattern of dates that decrease from an initial value of 284 ± 34 m.y. B.P., for gas released at the lowest temperature, to a small plateau at ∼ 202 m.y. B.P. for gas released at the highest temperatures. The 40Ar/39Ar release pattern obtained for the Hampden Flow suggests that the conventional K-Ar date of 230 ± 12 m.y. B.P. obtained for the plagioclase separate from this flow reflects the presence of excess radiogenic 40Ar. Conventional whole-rock K-Ar dates obtained for samples taken from a variety of locations within the Hampden Flow range from 171 ± 6 to 279 ± 14 m.y. B.P. Hydrothermally altered coarse- and medium-grained samples from the flow interior exhibit higher dates than those for finer-grained samples from the base of the flow. This suggests that the addition of excess radiogenic 40Ar is associated with hydrothermal alteration of the emplaced basalt rather than trapping as the result of rapid cooling at the time of emplacement. The 40Ar/39Ar release pattern obtained for the Hampden Flow may be typical of samples to which excess 40Ar has been added by hydrothermal alteration. Excluding the date for the Hampden Flow, the dates for the flows of the Hartford and Newark Basins range from 185 ± 4 to 194 ± 4 m.y. B.P. This range is consistent with paleontologic information that restricts the age of the basalts to older Early Jurassic.

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