Synopsis: The dolerites and gabbros of the Tertiary central complex of Slieve Gullion, NE Ireland, represent a suite of tholeiitic magmas (varying from tholeiitic basalt to tholeiitic andesite, Gamble 1979a) which crystallized in the root-zone of a shield volcano. The ubiquity of biotite suggests that the magmas were hydrous and, in addition, both silicate (olivine) and oxide (Ti-magnetite) phases show evidence for oxidation at high temperatures.

The chemistry of the ferromagnesian minerals (olivine, pyroxene, biotite) and plagioclase feld-spars behaves in a fashion which is in keeping with the tholeiitic affinity of the suite. In the ferro-magnesian minerals moderate iron enrichment accompanies increasing differentiation whilst the plagioclase feldspars display strong compositional zoning indicative of fluctuating P-T conditions during crystallization.

The pyroxenes in some texturally complex doler-ites and gabbos show several generations of growth accompanied by subsolidus exsolution. These complex textures have been linked to complications in the crystallization history caused by the coexistence of cooler, volatile-rich granitic magma (Gamble 1979b).

The available field evidence (vent agglomerates and lava flows marginal to the encompassing ring-dyke complex, Emeleus, 1962, together with widespread explosive interaction between granitic and basaltic magmas in the central complex, Gamble, 1979b) suggests a shallow level of em-placement for the Central Complex magmas. When considered in conjunction with nominal calculations relating to the amount of cover removed since 58 my B.P. (best estimated age for Slieve Gullion from Evans et al., 1973; MacIntyre, 1973) it is unlikely that P_tot exceeded 2 kb, more likely 1 kb. Calculations of temperature and f_O2 based on coexisting mineral equilibria yield a range of values consistent with the fluctuating conditions under which the mineral assemblages crystallized and equilibriated. Provided that the assumptions neces-sitated by the dynamic conditions under which the magmas equilibriated are fully realized, it would appear that the basic magmas of the Slieve Gullion Central Complex crystallized about 1050–1100 °C under moderately high oxygen fugacity.