a1 British Geological Survey, Kingsley Dunham Centre, Keyworth, Nottingham NG12 5GG, UK
a2 Scottish Universities Environmental Research Centre, Rankine Avenue, East Kilbride, Scotland G75 OQF, UK
a3 Massachusetts Institute of Technology, Dept. Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139-4307, USA
40Ar–39Ar dating by a combination of spot analysis and step-heating has been carried out on mica fabrics from slaty cleavage and a shear zone in the Neoproterozoic to Cambrian rocks of Charnwood Forest, Leicestershire. The results indicate that crust adjacent to the eastern margin of the Midlands Microcraton was affected by localized epizonal metamorphism and penetrative deformation between 425 and 416 Ma (Silurian to earliest Devonian), somewhat earlier than the Acadian Phase of the Caledonian orogeny recorded elsewhere in southern Britain. The Charnwood cleavage geometry suggests that the deformation arose within a dextral transpressional regime along the eastern margin of the Midlands Microcraton. This tectonism may have overlapped with pre-Acadian sinistral transtensional movements documented for the Welsh Basin and Lake District areas, to the west and north of the Midlands Microcraton.
(Received May 03 2007)
(Accepted October 30 2007)
(Online publication May 20 2008)