"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 (?).

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.