Palaeomagnetic study of the Cairnsmoor of Fleet Granite and Criffel-Dalbeattie granodiorite contact aureoles: Caledonian tectonics of the Southern Uplands of Scotland and Devonian palaeogeography

Abstract

The plutons of Cairnsmoor of Fleet (392±2 Ma) and Criffel-Dalbeattie (397±2 Ma, both mineral isochron ages) comprise two of four major post-tectonic granitic complexes emplaced into the Southern Uplands, an Ordovician–Silurian back-arc and foreland basin complex formed at the northern margin of the Iapetus Suture. To expand the palaeomagnetic record of the Southern Uplands we have studied palaeomagnetism and magnetic fabrics in traverses spanning contacts of these intrusions with host mudrocks. A uniform anisotropy of magnetic susceptibility (AMS) fabric across the Cairnsmoor of Fleet contact has been enhanced by recrystallization into hornfels near the contact and records a late Acadian regional stress operative during, or soon after, emplacement of the pluton in Middle Devonian times. Magnetization during slow cooling recorded a dual polarity (‘A’) remanence in granite and hornfels with mean direction D/I = 92/−2° (α95 = 6.5°) yielding a palaeopole (Q = 6) at 2°N, 265°E linked to cooling at c. 392 Ma. Subsidiary magnetizations are overprints imparted during Variscan tectonism (‘B’, D/I = 194/6°) and Jurassic rifting within the adjoining Irish Sea Basin (‘C’, c. 160–140 Ma, D/I = 172/−52°). The Criffel-Dalbeattie pluton has more complex AMS fabrics recording both deformation and emplacement effects. Hematite of secondary hydrothermal origin is a significant feature of the rock magnetic record in the aureole, which is otherwise dominated by paramagnetism. The granodiorite is more strongly magnetized than the country rocks, accounting for a positive aeromagnetic anomaly. A fairly dispersed dual polarity remanence (mean D/I = 115/55°, α95 = 18°) in granodiorite and late tectonic porphyrite dykes is probably the oldest magnetization preserved in this pluton because it correlates with an excursion of Britain into southerly palaeolatitudes at c. 410 Ma and indicates an Early Devonian emplacement age. The palaeofield at c. 397 Ma, the currently accepted isotopic age, is recorded by a minority overprinted remanence (mean D/I = 272/2°, α95 = 12°) similar to the record in the Cairnsmoor of Fleet pluton and granites from the adjoining Lake District terrane. Granite complexes of the Southern Uplands Block collectively record regional rotation and excursion of Britain into southerly latitudes between c. 410 and 390 Ma. Comparable Silurian–Devonian palaeomagnetic poles identify common apparent polar wander (APW) in paratectonic and orthotectonic terranes from the Variscan Front in the south to the Laurentian foreland in the north following climactic Acadian deformation. APW between 430 and 390 Ma embracing the (post-closure) history of the Caledonian orogen is a loop executed at rates much higher than typical rates of plate motion and appears to record a component of true polar wander. The ∼110° arc length is identical to polar shift identified between mid-Silurian and Lower–Middle Devonian poles from Gondwana. The two paths superimpose to show that the western margin of Gondwana was in proximity to the SE margin of Laurentia during Acadian deformation in Early–Middle Devonian times and remote from the Caledonides; the residual Rheic Ocean subsequently closed by a combination of pivotal and left lateral strike-slip motions.