More than twenty igneous intrusions, of widely varying composition and mainly in explosion-breccia pipes, cut the Appin Quartzite, Appin Limestone, Appin Phyllites and Cuil Bay Slates of the Dalradian Series which show evidence of a long and complex structural history. Early phases of tectonics and metamorphism (garnet-amphibolite facies) were followed by the dominant deformation of the region, referred to as main phase. The major structure of the Cuil Bay Synform, together with a series of parasitic folds, were developed during this phase together with a strong, retrogressive, regional cleavage. Late phase deformation acting prior to the development of the pipes produced steeply plunging open folds and box folds, of brittle type. The superimposition of these structures on main phase parasitic antiforms, in banded sequences of competent and incompetent units, produced dome-shaped structures with pipe-shaped zones of intensely fractured and brecciated rock. Late phase deformation after igneous intrusion was associated with the Great Glen Fault. The structural elements which exercised the dominant controls on the location and form of the pipes were the main phase regional cleavage, the main phase parasitic antiforms, the late phase steeply plunging open folds and the axial planes of the late phase box folds and open folds.
A generalized sequence of events for a number of the pipes is: (1) explosive activity with shattering and brecciation, (2) emplacement of variable but generally hornblende-rich igneous types, representing crystallization at high to moderate gas pressure, (3) the emplacement of augite diorites or kentallenites representing crystallization at low gas pressure and (4) the emplacement of generally small bodies of leucodiorite–granodiorite, possibly representing secondary hybrid liquids. The other pipes show one or more of these events and the sequence is that expected in volcanic activity associated with the build up and release of pressure on the breaching of structural traps (formed by the Appin Quartzite). Explosive activity took place on the sudden release of gas when the gas-charged basic magma moved into the dome-shaped interference structures with their pipe-shaped zones of much fractured and brecciated rock.
