Lamprophyre from Iona

A Caledonian minor intrusion in western Scotland

118.087 HS [287 kb]

Fig. 1: Photograph of a sample of red lamprophyre from a dyke on the west coast of Iona. A massive, equigranular, holocrystalline intrusive rock. Sample 118.087, weight 749 grams, 9 x 6 x 5 cm, magnetic susceptibility 20.9x10-3 SI units. This piece was collected in situ on 16 March 1976, approximate location NM263230, near the Spouting Cave along the southern shore of the bay known as Camas Cuil an t-Saimh (Ordnance Survey, 1962).


"Rock of the Month # 246, posted for December 2021" ---

Introduction

Iona is much older than the early Tertiary (Paleogene) volcanic pile that constitutes much of the far larger island of Mull, located just to the east across the 1-2 -km -wide Sound of Iona (Bailey et al., 1924). The Sound is itself the expression of a regional structure, the northeast-southwest Colonsay fault. Iona is accessible via Oban, Craignure, and a scenic drive across rugged east-central Mull and the westward-reaching peninsula of the Ross of Mull, followed by a short ferry ride from the coastal hamlet of Fionnphort. Though small, Iona offers the student an alluring mix of geology and geomorphology, ecology and archaeology (see, e.g., Jones, 1997; Taylor, 2005, 2006; McKirdy, 2017; and Westland, 2021). Iona is one of the many geological gems to appear in country-wide reviews such as Whittow (1977) and Gillen (2013). Iona's diverse geology includes Lewisian (Archean) gneisses, younger Precambrian sediments, a marble quarry and the machair ecosystem, based on soils enriched in shell sand from the adjacent beaches (see the wonderful new guide to the natural history of Mull and adjacent isles, by Littlewood and Jones, 2021).

Lamprophyres

are a rather complex suite of lithologies, of widely variable composition and mineralogy, that form minor intrusions such as dykes and sills (Nockolds et al., 1978, pp.203-211). Many varieties have been distinguished, based on their chemical and/or mineralogical nature. Lamprophyre is "A name for a distinctive group of rocks which are strongly porphyritic in mafic [ferromagnesian] minerals, typically biotite, amphiboles and pyroxenes, with any feldspars being confined to the groundmass" (Le Maitre et al., 1989, p.83).

The late Nick Rock, an indefatigable researcher of the lamprophyre clan, noted 88 dykes of late Caledonian (circa 430-390 Ma) age on Iona (an isle barely 6 x 2.5 km in size), 36 of which were lamprophyres (Rock et al., 1988). The numerous dykes, and also minor intrusions of broadly related rocks such as appinite, are broadly contemporaneous with the Newer Granite suite of Caledonian intrusions, such as the Ross of Mull granite, a prominent pluton located immediately east of Iona. The Ross of Mull granite has been dated at 414±3 Ma, and it seems likely that the lamprophyre dykes in the vicinity are just a few years younger. Rock et al. (1988, pp.12-13) tabulate compositions of many lamprophyres. Cherry-picking their data, the mean composition of the 36 Iona dykes includes 52.61 wt.% SiO2, 16.70 wt.% Al2O3, 7.93 wt.% Fe2O3, 5.44 wt.% CaO, 5.00 wt.% MgO, plus appreciable Na, K, Ti and P and, amongst the trace elements, 1331 ppm Ba, 917 ppm Sr and 222 ppm Zr. This is near the low end of so-called intermediate compositions (52-66% silica, more than gabbro, less than a granite). Based on this chemistry, we might expect a rock containing two feldspars, biotite mica and other ferromagnesian minerals, and accesssory minerals such as the oxides magnetite and ilmenite, carbonates, the phosphate apatite, and zircon (zirconium dioxide). What do we find in this sample, this particular dyke (?).

118.087 PM1 [xxx kb] 118.087 PM2 [xxx kb]

Figs. 2-3: IN PREP. Photomicrographs of a sample of Caledonian lamprophyre from Iona. Sample 118.087 is a fine-grained, granular, holocrystalline lamprophyre which occurs as a narrow dyke. This dyke formed by cooling of magma injected into a brittle fracture which propagated through far-older basement rocks, long before the Tertiary volcanism that dominates the scenery of nearby Mull.

In prep.: Petrographic notes and photomicrographs from a new thin section.


References

Bailey,EB, Clough,CT, Wright,WB, Richey,JE and Wilson,GV (1924) Tertiary and Post-Tertiary Geology of Mull, Loch Aline and Oban. Memoir for parts of geological sheets 43, 44, 51 and 52 (Scotland). British Geological Survey Memoir, HMSO, Edinburgh, vii+449pp., reprinted 1987 [with contributions by E.M. Anderson, H.B. Mauffe, G.W. Lee, B. Lightfoot, T.O. Bosworth and G.A. Burnett, plus petrology by H.H. Thomas and E.B. Bailey, chemical analyses by E.G. Radley and F.R. Ennos, and palaeobotany by A.C. Seward and R.E. Holttum].

Gillen,C (2013) Geology and Landscapes of Scotland. Dunedin Academic Press Limited, Edinburgh and London, 2nd edition, viii+246pp. [an update of the Terra Publishing 1st edition of 2003].

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

Le Maitre,RW, Bateman,P, Dudek,A, Keller,J, Lameyre,J, Le Bas,MJ, Sabine,PA, Schmid,R, Sorensen,H, Streckeisen,A, Woolley,AR and Zanettin,B (1989) A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Blackwell Scientific Publications Ltd, Oxford, 193pp.

Littlewood,S and Jones,M (2021) Wild Mull: A Natural History of the Island and its People. Pelagic Publishing, Exeter, 300pp.

McKirdy,A (2017) Mull, Iona and Ardnamurchan: Landscapes in Stone. Birlinn Ltd, Edinburgh., printed and bound by Latimer Trend, Plymouth. 48pp.

Nockolds,SR, Knox,RWO'B and Chinner,GA (1978) Petrology for Students. Cambridge University Press, 435pp.

Ordnance Survey (1962) Iona & Colonsay. Ordnance Survey one-inch map 51, 1:63,360 scale, fully revised 1953-1954.

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.

Taylor,T (2005) The Time Team Guide to the Archaeological Sites of Britain & Ireland. Transworld Publishers / Random House Group Ltd, 320pp.

Taylor,T (editor) (2006) What Happened When: Everything you need to know about Britain's Past since 650,000 BC. Channel 4 Books, 320pp.

Westland,J (2021) Mull rocks: why is Mull geology so special (?). Geowalks, Edinburgh, talk via Zoom, 30 March. See James Westland's excellent dedicated web site on Mull Geology.

Whittow,JB (1977) Geology and Scenery in Scotland. Penguin, 362pp.

Graham Wilson, posted 11 October, 2021, updated 30-31 October, 10,30 November 2021,
index update 07 March / 03 April / 28 May 2022

See the Rock of the Month Thematic Index

or, visit the Turnstone "Rock of the Month" Chronological Archives!

Class/Group/Family 13 entries for Scotland
The "Rock of the Month"
Granite --- #30 --- Ross of Mull granite, Isle of Mull, Scotland
Troctolite --- #31 --- Allivalite, Isle of Rhum, Scotland
Diorite --- #32 --- Appinite, Strontian area, Scotland
Marble --- #51 --- Forsterite marble, Glenelg, Scotland
Marble -- #112 --- Brucite ophicalcite (marble), Isle of Skye, Scotland
Monzonite -- #239 --- Kentallenite, a dark monzonite, Ballachulish area, Scotland
These are ready (photomicrographs coming, early 2022):
Volcanic glass -- #242 --- Pitchstones from the Isle of Arran, Scotland
Diabase -- #243 --- Crinanite (analcime olivine dolerite / diabase), Isle of Arran, Scotland
Gabbro -- #244 --- "Old Gabbro" of Ardnamurchan, Scotland
Volcanic / minor intrusive pitchstones -- #245 --- Craignurite, Isle of Mull, Scotland
Minor intrusive -- #246 --- Lamprophyre, Iona, Scotland - "YOU ARE HERE"
Glacial erratics (notes) -- #249 --- Erratics in and from Scotland, including Arran
Pyromorphite (Pb phosphate) -- #252 --- Pyromorphite from Co. Wicklow, Ireland and Leadhills, Scotland
Proposed, later in 2022, -- ---- --- to partially redress the Ign:Sed:Met ratio (!)
Clastic sediment -- #--- --- Brodick breccia, New Red Sandstone, Arran, Scotland
Carbonate sediment -- #--- --- Cambrian sediment, west coast of Sleat, Skye, Scotland
Clastic sediment -- #--- --- Torridonian sediment, Liathach, Sutherland, Scotland
Eclogite -- #--- --- Eclogite, Glenelg or Loch Duich, Scotland