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Arran - Judd's Dykes

(Other classic sites on Arran include Hutton's Unconformity)

The Isle of Arran has been a Mecca for geologists for many years due to its wonderfully varied geology. Many of the island's rocks are Palaeogene igneous rocks, formed at the time when the North Atlantic Ocean was just beginning to open. Many famous geologists can be associated with the island including James Hutton, Robert Jameson (1774 - 1854) and John Wesley Judd (1879 - 1914) who visited the area in 1893. Judd famously studied many of Arran's igneous rocks, including the Palaeogene dyke swarms found across the island. It is after him that five major composite dykes were named. Judd's Dykes demonstrate classic examples of composite (acid-basic) dykes and sills.

Judd's Dykes - the map from Judd's 1893 description of the dykes, illustrating their nature and distribution. © BGS/NERC

The dykes are exposed on the shore southwest of Tormore, intruding into Triassic sandstones. Composed of quartz-feldspar porphyry, pitchstones and tholeiitic dolerite, the basic (tholeiitic) components have been partially hybridized by phenocrysts and matrix from the acid magma (as xenocrysts). The dyke swarms are thought to have arisen from late stage (approximately 58Ma) intrusive activity following the intrusion of the Northern Granite earlier in the Palaeocene.

The nature of intrusion is thought to be as follows. The rising basic magma came into contact with a differentiated, partially crystallised acid magma. Mixing did not occur due to differences in viscosity, although it was at this point that the basic magma became partly hybridized. Continuing upwards through the crust, the basic magma was intruded as dolerite dykes and sills. This was followed by the intrusion of the acid magma into the unconsolidated centres of the dolerites.

Note: The Arran volcanic centre forms part of the North Atlantic Palaeogene Igneous Province, along with the other centres of Skye, Mull, Ardnamurchan, Rum and St. Kilda.

Further reading:

Emeleus, C.H. & Gyopari, M.C. 1992. British Tertiary Volcanic Province, Geological Conservation Review, Series No. 4. Joint Nature Conservation Committee, Peterborough, 259 pp.

Emeleus, C.H & Bell, B.R. 2005. British regional geology: the Palaeogene volcanic districts of Scotland (Fourth edition). (British Geological Survey, Nottingham.

McKerrow, W.S. & Atkins, F.B. 1989. Isle of Arran - A Field Guide for Students of Geology. 2nd edn. The Geologists' Association.

Tyrrell, G.W. 1928. The Geology of Arran (Memoirs of the Geological Survey, Scotland). His Majesty's Stationery Office, Edinburgh.