"Rock of the Month # 32, posted for February 2004" --- Appinite from the Strontian area, on the north shore of Loch Sunart, which extends westwards to form the southern shores of the Ardnamurchan peninsula. The region is west of Fort William and north of the Morvern district. GCW Old Series sample 117.7. Digital image, 07 April 2004. With 10-pence coin (24 mm diameter) for scale.

Appinites are medium- to coarse-grained plutonic rocks, generally in small intrusive bodies, rich in hornblende in a matrix of intermediate plagioclase feldspar and/or alkali feldspars, with or without quartz. This example is composed largely of amphibole (hornblende) and feldspars. The appinite suite of rocks is equivalent to hornblende-rich calc-alkaline lamprophyres, known as vogesite (rich in K-feldspar) and spessartite (rich in plagioclase). They were originally named for the Appin district near Ballachulish in western Scotland. The petrographic and geochemical terminology of these rocks rapidly becomes esoteric, as hinted at above - thus a good dictionary (Le Maitre, 1989) comes in handy!

This striking rock is most probably of Caledonian `Newer Granite' age, Silurian-Devonian, circa 430-390 Ma (Phemister, 1948; Rock et al., 1988b; Muir et al., 1997). The British appinites are found in parts of Scotland, Ireland and England. European rocks of this type are of late Ordovician, Silurian or early Devonian (that is, Caledonian) and Carboniferous-Permian (Hercynian) ages. They are found on the isle of Colonsay, in Argyllshire, the Lake District, the Channel Islands and elsewhere. Often elevated in large-ion lithophile elements (LILE) such as Ba, Sr and REE, they may be associated with Cu mineralization, and in some cases with anomalous values in Ag-Au-Te and other potentially economic elements. The Scottish examples include breccia pipes, formed by the build-up and release of pressure associated with the breaching of brittle host strata (Bowes and Wright, 1967). The relationship to lamprophyres, and their possible economic significance, was explored in detail by the late Nick Rock (e.g., Rock, 1984; Rock et al., 1988a,b). Examples of similar rocks also occur in North America, such as the small Quetico intrusions of northwest Ontario (MacTavish, 1999); Michigan; New York; California and Nova Scotia. Worldwide, similar rocks are widespread, including examples in the Guiana shield (Guyana); south and east Greenland, central-eastern Europe and eastern Australia.

Subvolcanic intrusives of the appinite suite include pyroxenite, hornblendite, kentallenite (another Scottish rock name: a dark monzonite to diorite with olivine, augite, plagioclase, micas and K-feldspar), diorite and granodiorite. These rocks are associated with much more voluminous granitic plutons in the British Caledonides (Bowes and Kosler, 1993). Archean diorite, monzodiorite and trachyandesite of northwest Ontario and northern Minnesota have been likened to the "sanukitoid" (high-Mg andesite) suite of Japan, as well as to the intrusive Caledonian appinites of Scotland (Stern et al., 1989). The Archean sanukitoids have higher Mg numbers than coeval lamprophyres with which they are commonly associated, and thus were not derived from lamprophyric parents. The sanukitoids may have evolved by direct partial melting of LILE-enriched mantle peridotite at shallow depths under hydrous conditions (Stern et al., 1989).

References

BOWES,DR and KOSLER,J (1993) Geochemical comparison of the subvolcanic appinite suite of the British Caledonides and the durbachite suite of the central European Hercynides: evidence for associated shoshonitic and granitic magmatism. Mineral.Petrol. 48, 47-63.

BOWES,DR and WRIGHT,AE (1967) The explosion-breccia pipes near Kentallen, Scotland, and their geological setting. Trans.Roy.Soc.Edinburgh 97 no.5, 109-143.

LE MAITRE,RW (editor) (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.

PHEMISTER,J (1948) Scotland: the Northern Highlands. Geological Survey and Museum, HMSO, Edinburgh. 2nd edition, 94pp.

MacTAVISH,AD (1999) The Mafic-Ultramafic Intrusions of the Atikokan-Quetico Area, Northwestern Ontario. OGS OFR 5997, 154pp. plus 8 maps.

MUIR,RJ, IRELAND,TR, BENTLEY,MR, FITCHES,WR and MALTMAN,AJ (1997) A Caledonian age for the Kiloran Bay appinite intrusion on Colonsay, Inner Hebrides. Scot.J.Geol. 33, 75-83.

ROCK,NMS (1984) Nature and origin of calc-alkaline lamprophyres: minettes, vogesites, kersantites and spessartites. Trans.Roy.Soc.Edinburgh: Earth Sciences 74, 193-227.

ROCK,NMS, GROVES,DI and RAMSAY,RR (1988a) Lamprophyres: a girl's best friend? In `Advances in Understanding Precambrian Gold Deposits, Volume II' (Ho,SE and Groves,DI editors), Geology Department and University Extension, University of Western Australia, Publ. No.12, 360pp., 295-308.

ROCK,NMS, GASKARTH,JW, HENNEY,PJ and SHAND,P (1988b) 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.

STERN,RA, HANSON,GN and SHIREY,SB (1989) Petrogenesis of mantle-derived, LILE-enriched Archean monzodiorites and trachyandesites (sanukitoids) in southwestern Superior Province. CJES 26, 1688-1712.