The Magnetization of Lower Carboniferous Rocks from the North‐west Border of the Dartmoor Granite, Devonshire

Summary The directions of magnetization and magnetic properties of lower Carboniferous rocks from the north‐western border of the Dartmoor granite are as complex as the geological history of the area. In the Okehampton district and particularly in the Meldon Quarries it was found that the directions of magnetization are grouped in a way which cannot be explained by either the folding or by reheating in situ . The directions are also anomalous when compared with the results obtained elsewhere for the Carboniferous and early Permian. Evidence from thermal and a. c. demagnetization experiments, thermomagnetic balance results and the mineralogy suggests that the remanent magnetization of many of the igneous and sedimentary rocks is due to pyrrhotite, formed during the intrusion of the Dartmoor granite. The pyrrhotite gives rise to an anisotropy of susceptibility which is probably responsible for some of the anomalous directions of magnetization, but it is conceivable that post‐granite fault movements also caused some remagnetization. Pyrrhotite is present in lesser amounts in rocks of the Tavistock district. The mean directions for four sites have been found to be similar, relative to the present horizontal, despite great differences in the field positions of the rocks. Their total mean (189° ‐21°) is similar to the result obtained from the Exeter lavas, Devonshire, which are identical in age to the Dartmoor granite. It is believed that the remagnetization was the combined result of heating due to the intrusion of the granite and the simultaneous introduction of pyrrhotite by metasomatic action.