|
Tectonics and Metamorphism |
1 School of Earth, Environmental and Physical Sciences, University of Portsmouth, Burnaby Road, Portsmouth PO1 3QL, UK
2 British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK
The early Ordovician rocks of the Isle of Man, comprising fine-grained clastic sedimentary rocks at low regional metamorphic grade, contain a range of variably developed porphyroblasts. These are used to establish the relationship between the metamorphism and early Devonian deformation. Evidence for pre-deformation mineral growth is limited to diagenetic chlorite-mica stacks which may occur in most lithostratigraphical units. The first deformation (D1) is marked by a pervasive cleavage, related to folding at various scales. Subsequent events formed crenulation cleavages in association with generally minor folding, including a gently dipping fabric developed relatively widely in response to the second phase of deformation (D2). The main metamorphic peak, represented by growth of ilmenite, Mn-rich garnet, chloritoid and cordierite in muddy facies in the Manx Group, occurred after D1 and before the onset of D2. Local heating contributions are also recognized in restricted metamorphic aureoles around dioritic dykes and intermediate size granitic intrusions of syn-D1 (Dhoon Granodiorite), syn-D2 (Black Hut Dyke) and post-D2 (Crosby and Foxdale Granite) ages.
Porphyroblast phases are best developed along the spine of the island and previous workers have suggested a metamorphic pattern related to underlying granitic intrusions. However, the timing of the growth of porphyroblasts within and outside the aureoles around exposed granites in this zone are very different, militating against any simple relationship between porphyroblast growth and igneous activity. Metamorphic mineral maps reveal a strong lithological control on the distribution of porphyroblasts. Chloritoid, in particular, is largely restricted to two prominent carbonaceous black mudstone units, the Barrule and Glen Rushen Formations, which strike northeast-southwest along the central part of the island. Cordierite is developed in the mudrocks of the Injebreck and Maughold Formations which crop out either side of the Barrule Formation. Further out across-strike, the absence of porphyroblasts (other than ilmenite) in the turbidite sandstone-rich parts of the sequence may again be, at least in part, related to composition. Here, however, low grades in the outermost tectonic tracts may be a real feature resulting from early D2 extensional reactivation and subsidence relative to the elevated central tracts.
