Palaeomagnetism and K/Ar results from the Duncansby volcanic neck, NE Scotland: superimposed magnetizations, age of igneous activity, and tectonic implications

Abstract

The Duncansby volcanic neck, intruding the Middle Devonian red beds of north Caithness, Scotland, has revealed two significantly different axes of magnetization, yielding pole positions at 149°E, 24°N and 126.5°E, 60°N, respectively. The first pole, which is interpreted as corresponding to the oldest magnetization, is in perfect agreement with Devonian polar estimates from west of the Great Glen Fault. It is tentatively suggested therefore that the Duncansby neck correlates with the Late Devonian volcanism in the nearby Orkney Islands though palaeomagnetism allows an upper age estimate of around Middle Carboniferous. The data support an earlier proposition of there being a palaeomagnetic discordance across the Great Glen Fault that can be interpreted in terms of a large-scale late- or post-Devonian transcurrent movement along this fracture zone. The original (? Late Devonian) magnetization has been nearly completely erased by the second phase of magnetization which, according to its pole position, most likely dates from about the Middle Jurassic. The latter magnetization is thought to be a consequence of burial, the coastal districts of Caithness having participated in the general subsidence of the North Sea area in late Palaeozoic and Mesozoic times. The burial magnetization, involving VPTRM and or TCRM processes, is considered to have been “frozen-in” as a result of uplift in connection with the well-documented mid-Jurassic tectonic phase that affected the northern North Sea basin, including the adjacent Moray Firth area. K/Ar analyses of the Duncansby intrusion give apparent ages ranging from 258 to 239 Ma. These dates, which lie between the two geological events inferred from palaeomagnetism, are not seen as true rock ages but rather as the result of a partial Ar loss during burial reheating.