SUMMARY The late Caledonian structure of the Lower Palaeozoic slate belts which lie to the south of the Iapetus suture in Britain is not ‘Caledonoid’ (NE-SW) but characterised by arcuate trends. The significance of the major cleavage arc of northern England is the subject of this paper. Its exposed part, in the Lake District and adjacent inliers, is described and its regional extent inferred from the control exerted by Caledonian basement trends on early Carboniferous sedimentation patterns. The arc is shown to be a major feature of the orogen, marking a change from a NE-SW ‘Appalachian’ trend to the ESE-WNW ‘Tornquist’ trend of northern Germany and Poland. Evidence for the age of deformation in the British slate belts is reviewed. It is shown that the deformation was not ‘end-Silurian’ as previously supposed, but of early Devonian age, climaxing in the Emsian and approximately synchronous with the Acadian orogeny of Canada. The systematic variation in cleavage/fold transection angles around the arc is described and interpreted in terms of transpressive strains associated with the northward movement of a basement block, the Midlands Massif, which acted as a rigid indenter during accretion of the southern British terrane (Eastern Avalonia) onto the Laurentian margin. These new data on the timing and geometry of the Acadian accretion event in Britain go some way to resolving the current controversy concerning late Ordovician vs. Devonian closure of Iapetus.

Abstract:A new lithostratigraphy is presented for the Skiddaw Group (lower Ordovician) of the English Lake District. Two stratigraphical belts are described.

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The Crummock Water aureole, an ENE-trending elongate zone of bleached and recrystallized Skiddaw Group rocks, 24 km in length and up to 3 km wide, is a zone in which pervasive metasomatism has modified the composition of the dominantly siltstone and mudstone lithologies. The bleached rocks show a substantial net gain of As, B, K and Rb and loss of Cl, Ni, S, Zn, H 2 O and C. Carbon loss is responsible for the bleaching. There are smaller and more localized net losses of Cu, Fe, Li and Mn, and gains of Ca, F and Si, whilst Co, Pb and REE are at least locally redistributed. Many chalcophile elements show evidence of initial widespread depletion and subsequent local enrichment. The mineralogy of the rocks is little affected by the geochemical changes. Like their counterparts outside of the bleached zone, the metasomatized rocks consist essentially of quartz, chlorite, muscovite, paragonite and rutile. Small aggregates and porphyroblasts of white mica and chlorite are developed. The metasomatism, which was accompanied by tourmaline veining, is superimposed on a contact metamorphic event. It post-dates the main Caledonian cleavage but pre-dates late Caledonian minor folds. Rb-Sr whole rock isochrons suggest that the metasomatic event occurred at c. 400 Ma and was thus associated with the Lower Devonian Shap-Skiddaw granite magmatism and not the earlier Eskdale Granite or Ennerdale Granophyre magmatic events. Modelling of Bouguer anomalies indicates that geological and geochemical constraints are most simply satisfied if the metasomatism is attributed to a buried, elongate, highly evolved granitic body intruded along the northern margin of a major granitic-granodioritic component of the Lake District batholith. The bleached zone is associated with a major lineament, which may reflect basement control on the location and form of the buried intrusion. Loss of metals from the bleached rocks is related to penecontemporaneous and subsequent hydrothermal vein mineralization and demonstrates that Skiddaw Group sedimentary rocks were a source of ore metals in the Lake District.

Major folds with associated thrust faults and a major olistostrome are described from the Skiddaw Group, a Lower Ordovician turbidite sequence in the Lake District of NW England. The style and geometry of the structures are shown to be compatible with their generation as submarine slumps or slide masses although they are much larger than any slump structures hitherto described from Britain. The predominant strain is shown to be simple shear. Spatial and temporal variations in strain permit a developmental model to be erected. The opposing vergence of the structures across the mapped area indicates a relatively narrow, probably fault controlled, depositional basin.