"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.