Camptonite-Monchiquite dyke swarms of Northern Scotland; Age relationships and their implications

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Abstract
Synopsis: New 40 K/ 40 Ar data for the camptonite-monchiquite dyke suite of the Northern Highlands give a late Visean age for the east-west trending Loch Eil–Loch Arkaig (326 ± 8 Ma) and Loch Monar (323 ± 9 Ma) swarms. Three dykes from the Thurso area give ages of 249–268 Ma, confirming earlier work indicating a late Permian age for the NE–SW trending Orkney swarm. In conjunction with published data which show a Stephanian-basal Permian age (291 ± 5 Ma) for the NW–SE Ardgour swarm, the three principal swarm trends of the camptonite-monchiquite suite therefore represent tectonomagmatic events of quite different ages. Different structural controls on orientation appear to have operated for each trend. Thus the Visean east–west dykes may have utilised a prexisting late Caledonian fracture system; the Permo-Carboniferous NW–SE swarm is contemporaneous and co-linear with a well described zone of taphrogenic subsidence and volcanism extending across western Britain while the late Permian NE–SW Orkney dykes follow a Caledonide trend typically associated with Permo-Triassic extensional tectonism and basin formation in that area.