The petrogenesis of Tertiary microgranites and granophyres from the Slieve Gullion Central Complex, NE Ireland

Abstract

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.

Anorogenic granite magma genesis: new isotopic data for the southern sector of the British Tertiary Igneous Province

“…Differentiation of the BTIP granitic magmas is now generally accepted (Thompson 1982;Gamble et al 1992) and is the only reasonable explanation for the acid rocks in the province with <5 ppm Sr, Eu/Eu*<0-1 and Rb/Sr> 100. Clearly, this conclusion dictates that many of the felsic rocks cannot be considered as potential primary magma compositions formed by melting of "normal crustal sources", whereas the more primitive granitic rocks must be evaluated in this context.…”

“…= alkali feldspar, * = duplicate determination, t = same sample; Slieve Gullion data are from Gamble et al (1992); Carlingford sample is as in Table 5; Arran data are from an unpublished study with J. D. Hudson and A. P. Dickin.…”

“…on July 5, 2015 specialpapers.gsapubs.org Downloaded from (1) Lundy -0-9-1-9 0-7146 (5) Data sources: this study, (Arran), Harrison et al (1987) (Ailsa Craig), Gamble et al (1992) (Slieve Gullion), Thorpe et al (1990) (Lundy); Nd result for Tardree was determined at Royal Holloway and Bedford New College as in Gamble et al (1992); number of samples in brackets; Carlingford (Q.U.B. 45) and Tardree (Q.U.B.…”

Palaeomagnetic and anisotropy of magnetic susceptibility data bearing on the emplacement of the Western Granite, Isle of Rum, NW Scotland

“…This region has also been noted for its diverse assemblage of magma types, particularly the common occurrence of mixed mafic and felsic compositions (Blake et al 1965;Gamble, 1979;Sparks, 1988;Troll, Donaldson & Emeleus, 2004). Box 9000, Las Vegas, NM 87701, USA §School of Physical and Geographical Sciences, Keele University, Keele ST5 5BG, UK Thompson, 1982;England, 1992;Gamble, Meighan & McCormick, 1992;Meighan, Fallick & Mccormick, 1992;McCormick et al 1993;, and no universally accepted model presently exists to explain their genesis. Box 9000, Las Vegas, NM 87701, USA §School of Physical and Geographical Sciences, Keele University, Keele ST5 5BG, UK Thompson, 1982;England, 1992;Gamble, Meighan & McCormick, 1992;Meighan, Fallick & Mccormick, 1992;McCormick et al 1993;, and no universally accepted model presently exists to explain their genesis.…”

The structure, fabrics and AMS of the Slieve Gullion ring-complex, Northern Ireland: testing the ring-dyke emplacement model

“…The central Palaeogene complex comprises up to 13 horizontal layers of granophyre and dolerite intruded roughly synchronously (from liquid -liquid relationships between felsic and mafic members) at 56.5 + 1 Ma forming Slieve Gullion Mountain ( Fig. 2c) (Richey & Thomas 1932;Reynolds 1951;Gamble et al 1976Gamble et al , 1992Gamble et al , 1999. A slightly older ring-complex surrounds the central complex and forms an approximately 12 km-diameter circular chain of low hills known as the Ring of Gullion.…”