Abstract

The Lizard Complex of SW England has recently been interpreted as representing a fragment of oceanic lithosphere that was thrust on to the continental crust of S Cornwall in the Devonian as a result of tectonic processes at a convergent plate margin. A palaeomagnetic study has been carried out on the ultramafics, gabbros and dykes of the E coast section to examine possible tectonic movements that might have been associated with this history of formation. A component of stable magnetization having a shallow to intermediate negative inclination and a SSW declination has been identified in each of these lithologies. This direction is similar to the Permian geomagnetic field direction for SW England, and one interpretation of the results is that the Lizard rocks were remagnetized in the early Permian, perhaps as a result of heating associated with emplacement of the Variscan batholiths. However, no mineralogical or other evidence for widespread heating of the Lizard Complex in the Permian has yet been identified. Available Rb-Sr and K-Ar geochronological data indicate a probable Devonian crystallization age, with no evidence for a Permian overprint. Therefore it is considered equally likely that this magnetization is primary, and dates from the time of original formation of the rocks. After application of a 90° tilt correction to restore the E Coast section of the complex to its postulated original orientation in the oceanic lithosphere the corresponding palaeomagnetic pole position is brought into close coincidence with the mean UK pole for the interval Silurian to Devonian. If the stable magnetization is primary, and was acquired during the original formation of the complex by sea floor spreading at a mid-ocean ridge, then it may be inferred that movement in a latitudinal sense prior to emplacement was small, and below the resolution of the present palaeomagnetic data (10° to 15°).

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