Granophyres and microgranites of the Tertiary central intrusive complex of Slieve Gullion, NE Ireland are intimately associated with appreciably larger volumes of mafic rocks whose compositions extend between basalt and basaltic andesite. These felsic intrusions show near minimum melt compositions and are characterized by trace element abundances and ratios consistent with evolution by crystal fractionation within a suite of granite magmas. Rb, Cs, Th and U all increase while Sr and Ba and Eu decrease with differentiation, suggesting an important feldspar control on the development of this felsic suite. All rocks are enriched in light rare earth elements relative to heavy rare earth elements and show negative Eu-anomalies (Eu/Eu* = 0.5–0.05), the extent of the anomaly increasing with differentiation. Isotopic ratios of Sr and Nd ( 87 Sr/ 86 Sr 57.6Ma = 0.70983–0.71044 and 143 Nd/ 144 Nd = 0.512379–0.512384) rule out an origin through straightforward crystal fractionation of basaltic magmas, and the Nd isotopes are distinct from those of the Scottish Hebridean centres such as Skye. The δ l8 O and δD values on whole rock and mineral separates show evidence for interaction between the granitoids and a meteoric groundwater system. However, the extent of the interaction is not as extreme as that in the Skye granitoids. We interpret the granites as end products of advanced crystal fractionation of mantle-derived mafic liquids which interacted with lithosphere. The isotopic distinction between our data and those from the Hebridean islands are a further argument for lithospheric provinciality in the NW British Isles.

Summary Full Rb-Sr isotopic data and whole-rock regressions are presented for each of the Mourne Mountains granites (G1–G5 inclusive) and a set of granophyres/microgranites from the Slieve Gullion central complex. The former were intruded at approximately 56 Ma (G1–G4 inclusive) and later (G5), and may all be younger than the major intrusions of the Slieve Gullion and Carlingford central complexes. The initial Sr isotope ratios are >0.7060 for all five Mourne granites, the Slieve Gullion granophyres/microgranites analyzed and the rhyolitic rocks of the Tardree area (Co. Antrim), and crustal involvement is indicated in their petrogenesis. In the E Mourne Centre the first (the least silicic and most Sr-rich) granite has a significantly higher initial ratio (0.7129 ± 2) than G2 (0.7109 ± 3) and G3 (0.7104 ± 6), and the isotopic data provide further support for the recent revision to the mapping of the first and second granites. Preliminary Rb-Sr isotopic data for a comprehensive suite of Tertiary rhyolites from NE Ireland, some separated by >50 km, define a linear array whose explanation is considered.

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It is now generally accepted that British Tertiary granites contain crustal and mantle components. Genesis principally by differentiation of crustally contaminated basaltic magmas is widely held and silicic melts with some remarkable trace element similarities were generated within different upper crust along the St Kilda/Skye -Carlingford zone.New whole-rock (and mineral) O isotope data for the southern sector of the province (N Arran, Ailsa Craig, Mourne Mountains, Slieve Gullion, etc) reveal that <5 18 0 lies in the range +5-1 to -i-9-7%o for most of the analysed granites, meteoric water-rock interaction having been in general less intensive than at Skye and Mull. Nevertheless, highly '"O-depleted country rocks (with ¿ IK O < 0) exist adjacent to the N Arran and Mourne Mountains granite plutons. There is as yet no evidence for the existence of low -18 0 granitic melts in this southern sector where magmatic <5 ls O compositions (up to c. +9-5%o) can be inferred for some of the intrusions.New Nd (and Sr) isotope data indicate that although there is some similarity in initial 87 Sr/ 86 Sr ratios between the northern and southern sector granites, in northeastern Ireland initial e Nd values for the analysed Tertiary acid major intrusions range from -3-9 to -4-5. This is in marked contrast to the Skye granites, some of which have values below -20, reflecting the involvement of different lithosphere.

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