Abstract

Zircons extracted from the Palisades and Gettysburg sills and baddeleyite from the Palisades sill yield consistent U-Pb ages of 201 ±1 Ma. These sills are likely related to the lowermost basalt flows of the Late Triassic-Early Jurassic Newark Supergroup rift basins of the eastern margin of North America (because of the geochemical similarity of their high-Ti quartz tholeiites and because drilling suggests that the Palisades sill directly fed some of the lowermost flows). Because the lowermost flows of the Newark Supergroup are paleonto-logically assigned to the lowermost Hettangian, the 201 ±1 Ma age of the sills should be slightly younger than the age of the Triassic/Jurassic boundary. Our data support the age of 204 ±4 Ma assigned the Triassic/Jurassic boundary and suggest that recent K-Ar dating for the Hettangian flows of the Newark Supergroup, which yielded an average age of 190 ±3 Ma, is about 5% low.

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