Sills of olivine-microgabbro and olivine-dolerite 0.5–180 m thick intrude Lower Liassic (Hettangian–Sinemurian) sediments in the Fastnet Basin, a SW extension of the N Celtic Sea Basin, about Lat. 50°N, Long. 10°W. The evidence of 3 wells and 2300 km of seismic reflection profiles indicates 6 major sills and sill-complexes, and 3 possible igneous centres or plugs. A high-level sill may intrude Lower Cretaceous sediments, and one plug apparently deforms Cretaceous reflectors. K-Ar ages on separated biotites from a thick sill intruding Liassic sediments gave 170 ± 4 Ma (Bajocian). This sill contains 30% olivine (FO70-73), 20% clinopyroxene (magnesian salite), 45% plagioclase (An60-78), &, with accessory magnetite, ilmenite, edenitic hornblende, Ti-phlogopite, apatite, zircon, spinel (in olivine) and sulphides. Alteration to serpentine minerals, chlorite, sericitic mica and leucoxene is sporadic.

The magmatism in the Fastnet province thus appears to have been active from the mid-Jurassic to the early Tertiary, related initially to the time of initial rifting between the European and American plates, and subsequently to their final separation between Greenland and Rockall and associated Thulean magmatism.

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