Preliminary Fabric Analysis of the Fanad Granite, Co. Donegal, Ireland.

C.D.O’Connell and R.J.Reavy

Department of Geology, National University of Ireland, Cork, Ireland.

caitoconnell@yahoo.ie

Much recent research (Hutton and Alsop 1996) has focused on structural controls on granitoid emplacement in Donegal, Ireland. Early work (Pitcher and Berger 1972) suggested that the Fanad Granite was emplaced by passive stoping as shown by a complex roof zone in western Rosguill. This work noted the presence of the so-called "Melmore Migmatite" in eastern Rosguill.

Recent structural mapping has identified the presence of a pervasive post-RCMP foliation in the Melmore area; this has a variable intensity, a very steep to vertical orientation and a strike of 030. Magmatic enclaves and Dalradian metasedimentary xenoliths show dextral shear within this foliation.

It is therefore apparent that the "migmatite" is in fact part of the roof zone, which has undergone extensive post-RCMP dextral shear. The recognition of subvertical, sheeted contacts within this shear zone suggests that the shear zone may also have been active as an ascent control in this region.

Hutton and Alsop (1996) have shown convincingly that the siting, ascent and emplacement of the granitoid bodies that comprise the rest of the Donegal Batholith were controlled by the N12E Donegal Lineament and the Caledonian Main Donegal Shear Zone.

The recognition here of a second shear zone active in the Rosguill area during magmatism, with a similar orientation to that of the Donegal Lineament, suggests that plutonism in Donegal may be ultimately controlled by more than one set of crustal structures, as demonstrated along strike in Argyllshire by Jacques and Reavy (1994).

References:

PITCHER, W.S. & BERGER, A.R. 1972. The Geology of Donegal – A Study of Granite Emplacement and Unroofing. Wiley-Interscience, New York and London.

HUTTON, D.H.W. & ALSOP, G.I 1996. The Caledonian strike-swing and associated lineaments  in NW Ireland and adjacent areas; sedimentation, deformation and igneous intrusion patterns. Journal of the Geological Society, London, 153, 345-360.

JACQUES, J.M. & REAVY, R.J. 1994. Caledonian plutonism and major lineaments in the SW Scottish Highlands. Journal of the Geological Society, London, 151, 955-969.