Magmatic Layering and Magnetic Fabrics in the Palaeogene Carlingford Later Gabbros, Co. Louth, Ireland

Abstract:A detailed magnetic fabric study of the annular partially layered Palaeogene Carlingford 'Later Gabbros', Co. Louth, is combined with field observations to constrain existing emplacement models for the intrusion. The Anisotropy of Magnetic Susceptibility (AMS) fabrics are generally quite inconsistently oriented. Only in three of seven traverses of the intrusion is consistency in orientation observed, suggesting that little can be drawn from the data by way of interpretation. However, the traverse in the north of the intrusion, at Ballinteskin, corresponds to an area of considerable syn-magmatic deformation of layering. Magnetic fabrics in this area exhibit consistently oriented inward dipping and inward plunging magnetic foliations and lineations respectively. Comparison of this data with a recent AMS study of the Great Eucrite of Ardnamurchan, NW Scotland, suggests that though central subsidence may have occurred in the Later Gabbros too, it was not as important as in Ardnamurchan. This is supported to some degree by the presence of a shallowly dipping (∼20°) intrusive contact between the Later Gabbros and an extensive granophyre intrusion, which also suggests that the granophyre is the older intrusion. This conclusion is inconsistent with previous interpretations of the granophyre as the youngest intrusion in the Carlingford igneous centre.