A gabbro-granodiorite contact in the Slieve Gullion area and its bearing on Tertiary petrogenesis

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Abstract
In the Slieve Gullion area, Northern Ireland, gabbro occurs in association with each of the two Tertiary structural units, namely : (1) the Ring Complex and (2) the north-westerly extending granophyre mass which Richey (1932, p. 782 and pl. lv) has termed the North-West Belt. In addition, a small gabbro plug outcrops on the lower northern slopes of Camlough Mountain. With the exception of the intrusions associated with the Ring Complex, the gabbro masses have all been shown to be of Tertiary age. Evidence of the Tertiary age of the Camlough Mountain plug was presented by Richey and Thomas in 1932, whilst the Tertiary age of the gabbros associated with the North-West Belt was demonstrated by the writer in 1937. The discovery that the emplacement of the gabbros of the North-West Belt was accompanied by the metasomatism and rheomorphism of the Caledonian granodiorite of the Newry Complex (Reynolds 1937) suggested the possibility that Richey and Thomas might have misinterpreted the evidence from which they inferred a Caledonian age for the gabbros associated with the Ring Complex. A detailed investigation of the contact of one of these gabbros has therefore been undertaken, and the objects of the present communication are (a) to present evidence that the gabbros associated with the Ring Complex are, like those of the North-West Belt, of Tertiary age, and (b) to record the details of the stages whereby Caledonian granodiorite, at its contact with Tertiary gabbro west-south-west of Mullaghbawn Mountain, has become transformed to rock of Tertiary