Structural Relationships of the Shap Granite

(Short communication) The Shap Granite crops out over an area of about five square kilometres in the eastern Lake District. The granite, its contact aureole and associated metasomatism have been described by Harker and Marr (1891 Harker and Marr (1893), Grantham (1928) and Firman (1957). These authors established that the intrusion was emplaced after the Ordovician country rocks had undergone folding and cleavage-formation in end-Silurian or earliest Devonian time. This communication describes the effect of the intrusion on the pre-existing cleavage and attempts to draw some conclusions on the mechanism and timing of its emplacement. The stratigraphical sequence in the area is as follows: | Silurian | Coniston Grits | || | | Coldwell Beds | | | Brathay Flags | | | Stockdale Shales | | Ordovician | Ashgill Shales | | | Coniston Limestone | | | Stockdale (Yarlside) Rhyolite | | unconformity | | | Borrowdale Volcanic Group | The Borrowdale Volcanic country rocks have been described by Mitchell (1934). They consist of a variety of lava types, ranging from basic andesite to rhyolite, and pyroclastic rocks which include ash-flow, detrital and probably air-fall tuffs. The lavas are hardly cleaved, while the tuffs outside the aureole exhibit a well-developed planar fabric which appears to be the combination of a fracture cleavage and a truly penetrative slaty cleavage. As the finer tuffs are traced into the aureole, their fabric becomes coarser owing to the mimetic crystallization of platy minerals with basal planes roughly parallel to the cleavage. At several localities close to the contact (for example NY 5618 1008), mimetic crystallization of biotite is accompanied by growth of the mineral in random orientation across the cleavage. This confirms ...