Jurassic ages from intrusives and extrusives within the Forties igneous province

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Abstract
The Forties igneous province lies at the intersection of three major graben systems within the North Sea, namely the Viking Graben, Central Graben and Moray Firth Basin (Fig. 1). The first exploration well to encounter the Forties igneous province was the Forties oilfield discovery well, 21/10-1 drilled in October 1970, and subsequent drilling has revealed the full lateral extent of the province (Fig. 1). The thickest sequence of igneous rocks (1492 m) encountered in the limited number of holes drilled so far has been recorded from well 21/3b-3 (Fig. 1). [Woodhall and Knox (1979)][1] and [Dixon et al. (1981)][2] suggested 3–4 km of volcanics exist in places whereas [Howitt et al. (1975)][3] suggested up to 1.5 km. Recent geophysical modelling (J. L. Swallow in prep.) indicates that the thickness rarely exceeds 2.5 km, with maximum thicknesses close to vents and on the downthrow sides of faults active at the time of volcanic activity. Igneous rocks of the Forties province possess strong alkaline affinities and are undersaturated in silica ([Gibb and Kanaris–Sotiriou 1976][4]; [Fall et al. 1982][5]). The major rock types described include alkaline olivine basalts, ankaramites and various extrusive hawaiite types ([Fall et al. 1982][5]). The vast majority of Forties igneous province rocks are extrusive in nature, but [Fall et al. (1982)][5] describe some intrusive hawaiites and mugearites from well 21/3-1A (Fig. 1). A Middle Jurassic minimum age for the whole igneous province has in the past been generally accepted on the basis of conventional K-Ar . . . [1]: #ref-12 [2]: #ref-2 [3]: #ref-9 [4]: #ref-6 [5]: #ref-3