Palaeomagnetic and rock magnetic reliability criteria in ophiolitic rocks: A case study from the Palaeozoic Ballantrae Ophiolite, Scotland

Palaeomagnetic studies of the Ballantrae Ophiolite, southwest Scotland, are assessed in the light of "palaeomagnetic" and "rock magnetic" reliability criteria. The remanence directions are palaeomagnetically reliable but have only limited significance for apparent polar wander path construction. "Rock magnetic" reliability criteria are found to be ambiguous when treated in isolation. Original grain size effects are detected in magnetic properties across three sections through pillow lavas. We propose that a systematic study of spatial magnetic property variations within pillow lavas can provide an additional field test in the assessment of reliability. Thermomagnetic analyses and microscopic observations suggest three remanence-carrying magnetic phases exist within the extrusive layer of the ophiolite. These are almost pure magnetite, hematite and titanomaghemite. Palaeomagnetic, rock magnetic and geological observations are best satisfied by a pre-obduction magnetisation age. This study links the remagnetisation of an ultra-oceanic conglomerate to local hydrothermal circulation rather than to a pervasive remagnetisation event as previously suggested.