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Journal of the Geological Society; 1996; v. 153; issue.2; p. 191-195;
DOI: 10.1144/gsjgs.153.2.0191
© 1996 Geological Society of London

Article

Geochronological evidence for post-Variscan cooling and uplift of the Carnmenellis granite, SW England

Y. CHEN1,2, M. A. ZENTILLI3, A. H. CLARK1, E. FARRAR1, A. M. GRIST3 & J. WILLIS-RICHARDS4,5

1 Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6, Canada
2 Present address: Geochronology Laboratory, Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada
3 Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada
4 CSM Associates Ltd., Rosemanowas, Penryn, Cornwall TR10 9DU, UK
5 Present address: Research Institute for Fracture Technology, Faculty of Engineering, Aabayama Campus, Tohoku University, Sendai 980, Japan

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 (2{sigma}) 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.

Key Words: SW England • 40Ar39Ar • fission-track dating • granites • cooling




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