Deformation and regional metamorphism of the Dalradian, Northern Ireland

Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.

M R Cooper and T P Johnston

Deformation of the Dalradian

Plate tectonic amalgamation of the Tyrone Igneous Complex on the Laurentian continental margin during the Grampian Orogeny. (P947796)

Geological map of the Pre-Dalradian Basement and Dalradian rocks in Co. Fermanagh and south Co. Donegal (16). (P947792)

The Dalradian rocks in Northern Ireland were intensely deformed and metamorphosed during the Grampian Orogeny (P947796). At least four separate phases of deformation and associated structures are recognised in the Dalradian rocks in Northern Ireland. Folds and cleavages resulting from the earliest deformation (D₁) are, at best, barely discernible and are generally obscured in a composite fabric defined by bedding surfaces (S_s) and the sub-parallel first schistosity (S₁). D₂ was the dominant deformation of the Grampian Orogeny in Northern Ireland and was associated with the formation of major regional southeast-facing recumbent anticlines, the Sperrin Fold in the Sperrin Mountains and the Altmore Anticline in northeast Co. Antrim. These structures are analogous to the Tay Nappe in Scotland, which formed as a result of D₂ flattening, tightening and extension of an earlier D₁ anticline . A penetrative schistosity, which developed throughout the southern part of the Sperrin Mountains and in northeast Co. Antrim, strongly crenulates the earlier formed composite S_s/S₁ surfaces.

In the south Sperrin Mountains, the D₃ deformation resulted in southeasterly-verging minor folds and a number of low-angle, northwards inclined thrust surfaces such as the Omagh Thrust Fault, which transported Dalradian rocks to the SSE over the early Ordovician Tyrone Igneous Complex . In west Co. Tyrone and central and south Co. Donegal the Ballybofey Nappe is the dominant D₃ structure controlling the disposition of Dalradian strata (P947792). This sub-recumbent structure closes and faces towards the southeast and is disrupted by several northeast-trending late Caledonian faults . The lowest structural levels of the Dalradian succession in west Co. Tyrone are in tectonic contact with the Lough Derg Group along the Lough Derg Slide , .

Post- D₃ structures in Northern Ireland mainly take the form of localised sets of kink bands and late stage brittle fractures.

Regional metamorphism

The pattern of regional metamorphism in the Dalradian Supergroup in Northern Ireland reflects a thermal and pressure gradient increasing from lower greenschist facies in the north to lower amphibolite facies in the south, close to the Omagh Thrust Fault . Metamorphic minerals including albite, muscovite, biotite, epidote, chlorite and actinolite occur throughout a large part of the Sperrin Mountains and are indicative of greenschist facies metamorphism. However, in the southern part of the Sperrin Mountains and in the Lack Inlier, garnet (almandine) porphyroblasts are indicative of slightly higher grade metamorphism in the upper greenschist to lower amphibolite facies. The garnets commonly have chloritic alteration rims indicating a later retrogressive phase possibly linked to late Caledonian movements on the Omagh Thrust Fault.