Palaeomagnetic studies within the Ballantrae Ophiolite; southwest Scotland: magnetotectonic and regional tectonic implications

The Early Arenig Slockenray Formation within the Ballantrae Ophiolite, southwest Scotland, displays a multivectorial magnetisation structure. Two components (S and M) are identified delineated by differing blocking temperature/ coercivity spectra. Component S is removed around 200°C/10 mT, and is regarded to be of recent viscous origin. Component M forms the characteristic formation magnetisation and resides in both magnetite and haematite. Extensive sampling of all exposed lithologies reveals an (in situ) non-Fisherian distribution of the characteristic magnetisation defining an envelope from SE moderate positive to SW shallow negative directions. A negative infra-formation conglomerate test identifies this component as a pervasive overprint.

A second conglomerate test performed in the overlying Benan Conglomerate of Llandeilo age, reveals dispersely directed magnetisation with a stability range equivalent to that of component M. This field test therefore defines a maximum remagnetisation window of 30 million years for the characteristic remanence. “Hard” viscous magnetisations are identified both in the Benan Conglomerate and at some sites within the Slockenray Formation.

Structurally corrected site mean results from the Slockenray Formation define a non-Fisherian distribution and form a small circle partial arc centred on a vertical axis (NW moderate positive to SW moderate positive directions). A combined palaeomagnetic fold and fault test suggests that acquisition of component M pre-dates both folding and faulting.

The resulting palaeolatitude of remanence acquisition (28.8°S) implies a tectonic position close to the southern Laurentian margin for the Ballantrae ophiolite in Arenig times.