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A study of the magnetization of 12 block samples of the Huntly and Insch masses of the Younger Gabbros of Aberdeenshire, Scotland, has shown that these bodies have a complex multicomponent magnetization. Combined AF and thermal treatment has isolated both polarities of two nearly horizontal axes directed NNW-SSE and NNE-SSW although only the first of these axes is obvious in the NRM directions. Microscope examination, electron probe analysis and Curie point studies indicate the magnetization to be due mainly to magnetite and haematite needles exsolved within pyroxene crystals and haematite formed by oxidation of a few primary magnetite grains. It is inferred that the Younger Gabbros were magnetized between their R b S r emplacement age of 189 f 17 Ma and their K-Ar cooling age of 468 f 4 Ma, by a combination of thermochemical and exsolution processes. The two axes of magnetization yield pole positions of 189.6" E. 27.8" N (dp = 4.7", dm = 9.4") and 151.6" E, 25.5" N (dp = 4.0", dm = 7.9"). In view of the unsatisfactory scatter in British Ordovician pole positions it is suggested that these be assessed giving consideration to possible unresolved multicomponent magnetizations as well as tectonic relationships, precise time of magnetization and evidence that the field was dipolar in nature.

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SummaryComparison of Old Red Sandstone palaeomagnetism for NW Scotland and Norway lead to the hypothesis of a 200–300 km sinistral movement along the Great Glen Fault. The displacement is thought to be of late Caledonian (late Devonian) age. The new palaeogeographic reconstruction of Northern Britain (prior to late Caledonian times) seems to have interesting geological implications.