The Old Gabbro of Lochan An Aodainn

An interesting mafic intrusive rock from Ardnamurchan,
in the Tertiary Volcanic District, in western Scotland

gabbro HS [459 kb] gabbro HS [308 kb]

Figs. 1-2: A nice hand specimen of the "Older Gabbro". Gabbro is a plutonic igneous rock, which crystallizes from cooling magma within the Earth's crust. Gabbro is composed of crystals of similar size (typically 1 to 10 mm) as those which comprise granite. However, compared to granite, gabbroic magma is relatively enriched in elements such as iron and magnesium, calcium and titanium, and impoverished in other elements such as silicon and potassium. The essential minerals are pale plagioclase feldspars and dark clinopyroxene. Grain size in this sample is roughly 2 mm.

This black, granular rock cooled from magma injected into the oldest units within the second of three volcanic centres, the bedrock of the rugged western part of the peninsula of Ardnamurchan. It was reheated when later Centre 2 and subsequent Centre 3 intrusions were emplaced, with the result that the feldspars, now "baked", have become clouded and turbid with exsolved iron oxides, a secondary feature that accounts for the particularly dark aspect of the rock. Black clinopyroxene grains stand proud on weathered surfaces, which may be orange-flecked with secondary iron oxides (perhaps reflecting oxidation of primary oxides, and/or olivine). This sample (552 grams, 10x8x4 cm --- prior to cutting of new thin section in 2021) was collected on 18 July 1975. It is quite strongly magnetic (magnetic susceptibility measured at 36x10-3 SI units).

Lochan an Aodainn (Little Loch of the Slope according to Gribble, or Lily Loch, of Richey and Thomas) lies just west of the road which runs northwest from Kilchoan to Sanna Bay (NM 457660: Ordnance Survey, 1971). The sample location is estimated (pre-GPS!) at NM 44446661, in the western half of an arcuate outcrop, in plan concave to the northeast, where the core of centre 2 lay for a geologically short time, before being obscured or obliterated when Centre 3 was developed in all its subcircular glory.


"Rock of the Month # 244, posted for October 2021" ---

Introduction

The peninsula of Ardnamurchan lies on the west coast of Scotland, at roughly 56° 39-47' N latitude. It forms the northwestern promontory of the county of Argyllshire, on the north side of Loch Sunart. To the south lies Mull, to the north Rhum and Skye, and other storied islands of the Inner Hebrides. Ardnamurchan lacks the striking, higher peaks of Skye, Rhum and Mull, but is cloaked in beautiful hills, bogs and little lakes (lochans).

Mull and Ardnamurchan are major volcanic centres dating to the opening of the North Atlantic Ocean, some 60 million years ago. The basaltic lavas of this major rifting episode are widespread in western Scotland (Geological Survey, 1957), and to the south, across the North Channel of the Irish Sea in Antrim, Northern Ireland (Pitcher, 1970).

The igneous and associated metamorphic rocks of Ardnamurchan have featured in many geology and petrology textbooks since the 1880s, and probably earlier. The rocks of the peninsula are nicely described in Gribble (1976) and Emeleus and Gyopari (1992) and makes cameo appearances in reference works such as Craig (1965, pp.429-433) and Gillen (2013). A lovely non-technical summary by McKirdy (2017) is highly recommended: as a bonus, air photos of the area can be studied (ibid., cover, p.29).

The pioneering mapping and interpretation of the igneous geology of the peninsula by Richey and Thomas (1930) unravelled a complex sequence of intrusions, emplaced in three successive, overlapping volcanic centres. As in Mull, there are many minor intrusions, various interpreted as ring dykes, cone sheets and bodies of other geometries (Parsons, 2012). Volcanic centres 1, 2 and 3 lie respectively in the southeast and east, the west, and the north-centre of the western half of the peninsula (Emeleus and Gyopari, 1992, pp.113-142). Centre 1, best seen around Ben Hiant, displays a range of volcanic vents, volcanic rocks and intrusives (including dolerite [diabase], diorite and gabbro). Centre 2 (including our sample) exhibits a dizzying array of gabbroic rocks. Centre 3, which overprints the earlier centres, is composed of more gabbros (including varieties known as eucrites), leading to the youngest rocks, more siliceous types identified as tonalite and quartz monzonite. The term eucrite is now restricted to a class of achondrite meteorite composed of calcium-rich feldspar (Grove, 1977) and clinopyroxene, a diverse clan of "space gabbros" believed to originate on asteroid 4 Vesta. Historically, the terrestrial use of "eucrite" is preserved in the names of several units of gabbroic rocks at Ardnamurchan.

Lead isotopes (Geldmacher et al., 2002) suggest that, whereas relatively primitive magmas on Rhum have a mantle signature, with contamination by Lewisian basement in more evolved rocks, at Ardnamurchan the magmas in the cone sheets assimilated first Lewisian gneiss and then, upon emplacement, their immediate (and younger) Moine country rocks.

Nature of the rock

This sample was collected way back in 1975, on a summer sojourn that resulted in an undistinguished bachelor's thesis with a map covered in tiny crabbed writing - mine. The petrography in that little report was probably better than the mapping, and indeed over the years I have spent more time on the former than the latter...

As mentioned above, the Old Gabbro is much darker than typical gabbros of similar bulk mineralogy. A thin section (Wilson, 1975) revealed something of the nature of this mysteriously black rock. A visual tour of the section reveals that the sample is roughly 80% plagioclase feldspar, 9% pyroxene (augite), 6% opaque phases, 5% secondary sheet silicates ("chlorite") and a trace of epidote. The feldspar is turbid with fine opaque inclusions and sericitic white mica. Three styles of twinning (albite, Carlsbad-albite and pericline) are evident in the crystals, and the estimated composition is An60 (labradorite). Most of the augite is present as two large grains, the host component of an ophitic texture, in which feldspar laths are enclosed in the pyroxene.The rest of the rock has an intergranular texture of feldspar laths in a matrix rich in chlorite and opaque minerals. The latter are presumed to be mostly oxides of iron and titanium, dominated by magnetite: a new polished thin section should provide further detail.

The margins of the Old Gabbro are largely determined by younger intrusions. In one sector it appears chilled against older agglomerates and basaltic lavas. It is a relatively early gabbro of Centre 2, and contains some granophyric material and fine-grained basic inclusions (Gribble, 1976, pp.36-37). The rock (originally a predominant olivine diabase and sundry varieties) is variably crushed, shattered and altered, darkened by opaque inclusions in the feldspar and alteration of ferromagnesian minerals (Emeleus and Gyopari, 1992, p.132). Today the last word shall come from H.H. Thomas (Richey and Thomas, 1930, pp.238-241). The Old Gabbro intrudes the volcanics, and in turn is cut by later intrusive units, such as the Grigadale granophyre and the Garbh-dhail quartz gabbro. It may be a ring dyke, and predates most of the cone sheets. The rock type varies from coarse olivine gabbro to olivine dolerite to troctolite (allivalite), and locally contains augite granophyre, though whether the latter is a late stage evolution of the gabbro, or a subsequent injection of melt, is unclear.

In prep.: petrographic notes and photomicrograph(s) from a new thin section.


References

Craig,GY (editor) (1965) The Geology of Scotland. Oliver & Boyd, Edinburgh and London / Archon Books, Hamden, CT, 556pp. plus map.

Emeleus,CH and Gyopari,MC (1992) British Tertiary Volcanic Province. Chapman & Hall, 259pp.

Geldmacher,J, Troll,VR, Emeleus,CH and Donaldson,CH (2002) Pb-isotope evidence for contrasting crustal contamination of primitive to evolved magmas from Ardnamurchan and Rum: implications for the structure of the underlying crust. Scot.J.Geol. 38, 55-61.

Geological Survey (1957) Geological Survey `Ten-Mile' Map, Sheet One. Geol.Surv.Great Britain, 1:625,000 scale map, Scotland and Northern England, 2nd edition.

Gillen,C (2013) Geology and Landscapes of Scotland. 2nd edition, Terra Publishing, update of 1st edition of 2003, 260pp.

Gribble,CD (editor) (1976) Ardnamurchan, a Guide to Geological Excursions. Edinburgh Geol.Soc., 120pp.

Grove,TL (1977) Structural characterization of labradorite-bytownite plagioclase from volcanic, plutonic and metamorphic environments. Contrib.Mineral.Petrol. 64, 273-302.

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

Ordnance Survey (1971) Sound of Mull. Ordnance Survey one-inch map 45, 1:63,360 scale, fully revised 1954.

Parsons,I (2012) Three rings for the elven-kings... Elements 8 no.1, 80, February.

Pitcher,WS (compiler) (1970) Geological Map of Northern Ireland. Geological Survey of Northern Ireland / Department of Commerce, Northern Ireland, solid edition, 1:250,000 scale.

Richey,JE and Thomas,HH (1930) Ardnamurchan, north-west Mull and Coll. British Geological Survey memoir for sheet S51 and part of sheet S52, reprinted 1987.

Wilson,GC (1975) The Tertiary Igneous Geology of part of northwest Ardnamurchan, Argyllshire. BA Thesis, University of Oxford, 33pp. plus map (1:10,000 scale).

Graham Wilson, posted 03, 09, 10, 17 October, 2021

See the Rock of the Month Thematic Index

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

Class/Group/Family 11 rocks from 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
Done, or coming soon, five thin sections in prep. (late 2021):
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 - "YOU ARE HERE"
Volcanic / minor intrusive -- #245 --- Craignurite, Isle of Mull, Scotland
Minor intrusive -- #246 --- Lamprophyre, Iona, Scotland