Palaeomagnetism of the Ross of Mull granite complex, western Scotland: lower Palaeozoic apparent polar wander of the Orthotectonic Caledonides

The Ross of Mull granite complex was emplaced near the western margin of the Orthotectonic Caledonides of Scotland in early–mid‐Silurian times. Anisotropy of magnetic susceptibility reflects the post‐tectonic character of the pluton and contrasts with magnetic fabrics in adjoining country rocks which record the regional D3 deformation. Palaeomagnetic study identifies two dominant magnetizations. An ‘A’ component is magnetite‐resident with distributed blocking temperatures (100–500 °C) and normal polarity ( D/I =339/−50°, 30 samples, α 95 =5.6°, palaeomagnetic pole at 12°E, 1°S, dp/dm =5.0/7.5°); it was probably acquired during initial cooling. A ‘B’ magnetization (unblocking temperatures 500–580 °C, D/I =195/+21°, 40 samples, α 95 =4.3°, palaeomagnetic pole at 339°E, 22°S, dp/dm =2.4/4.5°) is of dual polarity and comparable to magnetizations widely recorded in the Younger Granite Suite of the Scottish Caledonides. It does not correlate with the timing of regional isotopic closure as defined by K/Ar thermochron ages and appears to have been imparted during a regional thermochemical event at low temperatures (≈250–150 °C) and a late stage of cooling of the orogen (≈420–410 Ma). Metamorphic facies in the aureole and adjoining country rocks record examples of both A and B magnetizations, together with a low‐blocking‐temperature (0–350 °C) component acquired during late Palaeozoic/early Mesozoic rifting in the Hebridean Basin.  Palaeomagnetic poles from the Orthotectonic Caledonides define a late to post‐tectonic magnetization record equivalent to a general east to west apparent polar wander trend of late Ordovician–Lower Devonian age. A short‐term reversal of this trend is identified during the earlier part of Silurian times so that mid–late Silurian poles are located in a similar position to late Ordovician poles. The overall path coincides with the contemporaneous record from the Paratectonic Caledonides of England and Wales from c . 455 Ma. It is therefore concluded that the three‐plate model of the Caledonides (incorporating the collision of Laurentia and Baltica in mid‐Ordovician times to produce the Orthotectonic Caledonides, and the closure of the Iapetus Ocean between Baltica–Laurentia and Avalonia in late Ordovician times to produce the Paratectonic Caledonides) was unified prior to the oldest magnetizations preserved in the Orthotectonic Caledonides. Certain second‐order discrepancies remain and may either reflect the incompleteness of the present database or the effects of subsequent regional rotation and strike‐slip movements prior to full welding of the orogen by late Lower Devonian times.