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Arran
- Judd's Dykes
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 Tertiary 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 Tertiary 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.
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 Tertiary
Igneous Province, along with the other centres of Skye, Mull, Ardnamurchan,
Rum and St. Kilda.
Further reading:
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.
Emeleus, C.H. & Gyopari, M.C. 1992. British Tertiary Volcanic Province,
Geological Conservation Review, Series No. 4. Joint Nature Conservation Committee,
Peterborough.
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