40Ar/39Ar ages of CAMP in North America: implications for the Triassic-Jurassic boundary and the 40K decay constant bias.

摘要

The Central Atlantic magmatic province (CAMP) is one of the largest igneous provinces on Earth (N107 km2),spanning four continents. Recent high-precision 40Ar/39Ar dating of mineral separates has providedimportant constraints on the age, duration, and geodynamic history of CAMP. Yet the North American CAMPis strikingly under-represented in this dating effort.Here we present 13 new statistically robust plateau, mini-plateau and isochron ages obtained on plagioclaseand sericite separates from lava flows from the Fundy (n=10; Nova Scotia, Canada), Hartford and Deerfield(n=3; U.S.A.) basins. Ages mostly range from 198.6±1.1 to 201.0±1.4 Ma (2σ), with 1 date substantiallyyounger at 190.6±1.0 Ma. Careful statistical regression shows that ages from the upper (199.7.0±1.5 Ma)and bottom (200.1±0.9 Ma) units of the lava pile in the Fundy basin are statistically indistinguishable,confirming a short duration of emplacement (≪1.6 Ma; ≤1 Ma). Three ages obtained on the Hartford(198.6±2.0 Ma and 199.8±1.1 Ma) and Deerfield (199.3±1.2 Ma) basins were measured on sericite fromthe upper lava flow units. We interpret these dates as reflecting syn-emplacement hydrothermal activitywithin these units. Collectively, CAMP ages gathered so far suggest a short duration of the main magmaticactivity (2–3 Ma), but also suggest the possibility of a temporal migration of the active magmatic centers fromnorth to south. Such a migration challenges a plume model that would postulate a radial outward migrationof the magmatism and is more compatible with other models, such as the supercontinent global warminghypothesis. When compared to the age of the Triassic–Jurassic boundary, the filtered CAMP age databasesuggests that the onset of the magmatic activity precedes the limit by at least few hundred thousand years,thereby suggesting a causal relationship between CAMP and the end of Triassic mass extinction.An age at 191 Ma possibly suggests a minor CAMP late tailing activity (190–194 Ma) which has been observedalready for dykes and sills in Africa and Brazil. We speculate that, if genuine, this late activity can be due to amajor extensional event, possibly heralding the oceanization process at ~190 Ma. Comparison between highquality U/Pb and 40Ar/39Ar ages of pegmatite lenses from the North Mountain basalts confirms a ~1% biasbetween the two chronometers. This discrepancy is likely attributed to the miscalibration of the 40K decayconstants, in particular the electron capture branch.