Abstract

Apatite fission-track (AFT) dates for granitic samples from surface exposures and deep boreholes in the 293.3 ± 1.2 Ma Carnmenellis granite, west Cornwall, decrease radically from 130 ± 28 (2σ) Ma at surface to 26 ± 6 Ma at c. 2500 m depth, a trend accompanied by a decrease in mean track length from 12.99 ± 0.80μm to 9.83 ± 0.62 μm. In contrast, 40Ar–39Ar apparent age plateaux for muscovite decrease only from 288.5 ± 1.0 Ma at surface to 286.1 ± 1.0 Ma at c. 2500 m depth. The small difference in the muscovite ages may record the presence of a slightly younger intrusive phase at depth, but it is apparent that the preserved upper 2.5 km of the pluton cooled rapidly through c. 320°C. However, thermal modelling of the AFT data delimits a distinct fast-cooling event at c. 137–155 Ma, exhibiting a satisfactory correlation with a major regional uplift episode in Late Jurassic/Early Cretaceous times recorded by the sedimentary basins flanking the Cornubian massif.

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