EMPLACEMENT STYLES OF CALEDONIAN GRANITOIDS IN BRITAIN AND IRELAND

The British and Irish Caledonian granitoids were emplaced into a complex tectonic mosaic of terranes assembled during the orthogonal collision of Laurentia with Baltica (Scandian) and the oblique collision of Avalonia with Laurentia after final Iapetus closure. The structures which controlled magma siting, ascent and arrival configurations were mainly pre-existing structures which were reactivated during late orogenic times. Data are presented for granitoids emplaced in several tectonic zones moving NW to SE across the British Isles, effectively from Laurentia, through the amalgamated oceanic arcs and accretionary complexes to the Avalonian margin, in an attempt to constrain the gross orogenic strain regime syn- and post-granite emplacement.

The Ross of Mull granite in the NW Highland block is sited at the intersection of steep Caledonian structures but emplacement was facilitated by eastward-directed extensional collapse of Scandian Moine nappes. The emplacement of other NW Highland granitoids may be controlled in part by reactivations of much older lineaments in the Laurentian basement.

The Grampian block forms a very distinctive zone where dominantly Dalradian metasediments suffered the Grampian but apparently escaped the Scandian event and Iapetus closure deformation was accommodated in transpressional shear zones. Granitoids in this sector (SW Highlands and NW Ireland (Donegal)) indicate the interaction of steep older lineaments and Caledonian shear zones as the dominant control on siting, ascent and emplacement, often creating pull-aparts or extensional jogs at intersections.

The granitoids south of the HBF include those emplaced within Ordovician/Silurian accretionary wedges (Southern Uplands, Scotland; Longford-Down, Ireland) and along accreted terrane sutures (Galway). Reactivation of steep structures seem to be important controls on ascent and emplacement although other deeper NW trending structures of uncertain affinity seem to be important in siting.

The Leinster granite in the SE Ireland (Leinster Massif) is controlled in all aspects of its siting, ascent and emplacement by a SE-dipping anastomosing shear zone system, representing extensional reactivation of a suggested earlier collisional structure, developed within Ordovician Avalonian margin accretionary sediments.