GEOCHRONOLOGICAL EVIDENCE FOR POST-VARISCAN COOLING AND UPLIFT OF THECARNMENELLIS GRANITE, SW ENGLAND

Apatite fission-track (AFT) dates for granitic samples from surface ex posures and deep boreholes in the 293.3+/-1.2 Ma Carnmenellis granite, west Cornwall, decrease radically from 130+/-28 (2 sigma) Ma at surfa ce to 26+/-6 Ma at c. 2500 m depth, a trend accompanied by a decrease in mean track length from 12.99+/-0.80 mu m to 9.83+/-0.62 mu m. In co ntrast, Ar-40-Ar-39 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. T he small difference in the muscovite ages may record the presence of a slightly younger intrusive phase at depth, but it is apparent that th e preserved upper 2.5 km of the pluton cooled rapidly through c.320 de grees C. However, thermal modelling of the AFT data delimits a distinc t fast-cooling event at c. 137-155 Ma, exhibiting a satisfactory corre lation with a major regional uplift episode in Late Jurassic/Early Cre taceous times recorded by the sedimentary basins flanking the Cornubia n massif.