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1 Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
2 Department of Geology, Oxford Brookes University, Gipsy Lane, Oxford OX3 0BP, UK
Present address: Burlington Resources (Irish Sea) Ltd, 1 Canada Square, Canary Wharf, London E14 5AA, UK
3 Robertson Research International, Llanrhos, Llandudno, North Wales, LL30 1SA, UK
4 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
The pre- and syn-Caledonian rocks of the Isle of Man are now known to comprise three distinct units: the early Ordovician Manx Group, the mid-Silurian Dalby Group and the ?late Silurian-early Devonian Peel Sandstones.
The Manx Group is dominated by Arenig deep-marine turbidites and debrites deposited in oxygenated basins on the northwest-facing margin of Avalonia. Its organization into a sand-rich lower part and a mud-rich upper part invites comparison with the Skiddaw Group (Lake District) and Ribband Group (Leinster) and points to control by margin-wide events, in part eustatic sea-level changes. Episodes of mass-wasting and Fe-Mn fluid exhalation also correlate along the margin. A mid-late Ordovician volcanic arc is missing above the Manx Group, although parts of its intrusive substructure may be preserved.
The Dalby Group comprises northwest-derived turbidites, sedimented into an anoxic basin during Wenlock (mid-Silurian) time. These turbidites were deposited in a successor basin above the Iapetus suture zone. The Dalby Group sits with a tectonic contact on the Manx Group.
No evidence has been found of a pre-Silurian cleavage. The main Caledonian D1 and D2 shortening phases are post-Wenlock, comparable in age with those further along the margin in the Lake District and Leinster. The Peel Sandstones preserve a Lower ‘Old Red Sandstone’ sequence, mostly removed by post-Caledonian erosion elsewhere along this outboard part of the Avalonian margin. The unit does not host a definite Caledonian cleavage, and it must have been deposited late in the deformation history. The granitic intrusions into the Manx Group range from early in D1 to late in D2. The intrusions generate only local aureoles, and the high metamorphic grade in parts of the Manx Group may be enhanced by favourable protolith compositions.
