"Rock of the Month # 30, posted for December 2003" --- Ross of Mull granite, GCW Old Series sample 118.95. Digital image, 07 April 2004. With 10-pence coin (24 mm diameter) for scale.

The Ross of Mull granite is of latest Silurian or early Devonian age, circa 414 Ma. It forms a prominent mass of probable total extent 50 km², half hidden beneath the waters of the Atlantic, in southwest Mull (Jones, 1997). This is in the same age range as widespread lamprophyre dykes, dated at circa 430-390 Ma, across parts of Scotland, Ireland and northern England. Caledonian minor intrusions, including also felsite, microdiorite and appinite, occur in parts of Assynt, Ben Nevis, Strontian, Iona and the Ross of Mull, and the Cross Fell inlier (which includes the Shap granite in northwest England (Phemister, 1948; Rock et al., 1988).

The typical Ross of Mull granite is coarse (grain size circa 2-5 mm) and massive, pink in colour. It is a K-feldspar granite with pink K-feldspar predominant over white plagioclase, plus abundant brown-black biotite mica and smoky quartz. Lustrous brown sphene (titanite) is a common accessory mineral.

References

JONES,R (1997) Mull in the Making. Rosalind Jones, Craigmore, Aros, Isle of Mull, 41pp.

PHEMISTER,J (1948) Scotland: the Northern Highlands. Geological Survey and Museum, HMSO, Edinburgh. 2nd edition, 94pp.

ROCK,NMS, GASKARTH,JW, HENNEY,PJ and SHAND,P (1988) Late Caledonian dyke-swarms of Northern Britain: some preliminary petrogenetic and tectonic implications of their province-wide distribution and chemical variation. Can.Mineral. 26, 3-22.