Palaeomagnetism of igneous rocks of the Lake District (Caledonian) terrane, Northern England: palaeozoic motions and deformation at a leading edge of Avalonia
✍ Scribed by J. D. A. Piper
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 899 KB
- Volume
- 32
- Category
- Article
- ISSN
- 0072-1050
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✦ Synopsis
The Lake District terrane of northern England comprises Upper Cambrian±Silurian sediments and volcanics accumulated at the northern margin of the Avalonian Plate during growth and demise of the Iapetus Ocean. Ocean closure and suturing resulted in Late Ordovician and Acadian tectonism and were accompanied by emplacement of a large regional batholith. Palaeomagnetic study of intrusive igneous rocks, including application of thermal demagnetization, ®eld tests and principal component analysis, identi®es a history of Ordovician to Devonian magnetization. Late plutons (Shap and Skiddaw granites and/or aureoles) record a shallow dipolar (A3) axis (mean declination/inclination (DaI 278a 178) dating from emplacement in late Early Devonian times (c. 395 Ma). Although this axis is recorded as a sporadic overprint in older rocks, no pervasive remagnetization is attributable to batholith emplacement. Instead, the Carrock Fell Complex Layered Gabbros have a mid-to late Ordovician (A1) remanence (DaI 17Á4a À 58Á18, 36 samples, a 95 4Á88) predating regional F2 folding. Later events in this igneous complex comprise the Carrock Fell Granophyre with a post-folding Ordovician remanence, and Round Knott Dolerite with a remanence linked to hydrothermal alteration late in the Ordovician magmatic episode. A Late Ordovician (Ashgill) palaeo®eld is also de®ned by remanence (A2) in the Threlkeld±St John's Microgranite and aureole (438 Ma, DaI 236Á5a63Á38, 41 samples, a 95 4Á78). Other intrusions carrying a remanence predating the Acadian deformation include the Great Cockup Picrite (458 Ma, DaI 43Á2a À 31Á88, 31 samples, a 95 7Á78) and basic intrusives in the aureole of the Eskdale Granite (429 Ma, DaI 174Á5a25Á88, 32 samples, a 95 8Á88).
Collectively the palaeomagnetic data from this terrane identify a hairpin in the apparent polar wander path during Late Ordovician (Caradoc±Ashgill) times corresponding to `soft' closure of the Iapetus suture and accompanying deformation. The same motion is recognized in contemporaneous data from the Welsh Caledonides where declinations are rotated by c. 558 relative to contemporaneous results from the Lake District. Adjustment for this (probable late Acadian) rotation beings fold trends of the Paratectonic Caledonides into alignment and identi®es a parallel mid-to late Ordovician destructive plate margin comprising forearc (Lake District) and backarc (North Wales). This arc was oriented latitudinally in mid-southerly latitudes during formation and the bulk of the magmatism occurred during a single normal-polarity chron. The relationships between magnetization and folding in both the Lake District and Welsh Borderlands identify the importance of Late Ordovician deformation along this arc during collision of Avalonia and Laurentia. Arc-related volcanism was succeeded in Silurian times by parallel foreland basins embracing the Welsh Basin and southern Lake District as the Laurentian Plate overrode the Avalonian Plate.