Left: Sample 118.09, crinanite cut by a thin sheet of tachylyte (a black basaltic glass, typically at the chilled margins of dykes and sills injected into cold country rocks). Mass 338.8 grams, 8 x 5 x 5 cm. Magnetic susceptibility 29.0 x 10-3 SI units. Right: Sample 118.10, crinanite. 358.4 grams, 10 x 6 x 2 cm, 15.8 x 10-3 SI units.
The 4 samples shown here were all collected on 10 April 1974, from the Kingscross Point area, approximate UTM co-ordinates NS 056283. This area is on the southern half of the east coast of Arran, southeast of Lamlash, in the lee of Holy Island, separating Whiting Bay to the south from Lamlash Bay to the north (O.S., 1956). The Point, as with other nearby features such as Dippin Head to the south and Clauchlands Point to the north, is a topographic expression of an underlying resistant sill.
"Rock of the Month # 243, posted for September 2021" ---
Nature of the rock
Crinanite is one of the many geographically-based rock names that have been coined over the past 2 or 3 centuries, especially in Europe, home of the original generation of petrographers. These rock names are hard to remember, and so there has been a move to standardize nomenclature, while simultaneously making it easier to assign a name by mineralogical or chemical analyses. These samples display a granular, holocrystalline (all crystals and no glass, as far as can be seen) igneous rock, the crinanite, which is a variety of dolerite (diabase) or fine-grained gabbro (see Macgregor et al., 1972, pp.141-149; and notes on the related Dippin sill by McKerrow and Atkins, 1989, pp.81-87). Sir John Flett coined the term in 1911 for a suite of northwest-strending analcime dolerite dykes which are quite extensive in Argyllshire, centred on Loch Crinan on the west coast. Flett also described dykes of this lithology offshore, on the islands of Jura and Colonsay (Walker, 1934). Loch Crinan, and the seaward end of the Crinan canal, lie on the east shore of the Sound of Jura, just east of the northeast end of that island. Crinanite and teschenite appear in the igneous rock catalogue of Johannsen (1938). The eponymous rock type is, you might say, a little specialized, and some such as Wilkinson (1955) argued that its use should be discontinued.
Definitions of the various rock names may be found in Le Maitre et al. (1989, pp.58, 60, 61). Here we find that:
Figs. 3-5: More examples of crinanite. All samples are appreciably magnetic due to their iron oxide (magnetite) content,
a feature they hold in common with many varieties of basalt,
diabase and gabbro.
Left and centre: Sample 118.11 (exterior and interior). 781.5 grams, 9 x 6 x 6 cm, magnetic susceptibility 18.1 x 10-3 SI units. Right: Sample 118.21, 94.4 grams, 3 x 5 x 2 cm, magnetic susceptibility 37.1 x 10-3 SI units. This little sample displays a sharp margin between a typical local dolerite (crinanite) and what appears to be a felsic variant (more siliceous, rich in pale pink alkali feldspar), displaying the ophitic texture so typical of crinanite and the wider diabase clan. The white patches are vugs infilled by zeolite, most probably analcime (the mineral is soft, it does not react to dilute acid, and the crystal habit is not consistent with common minerals like quartz and calcite). Traces of carbonate are present in the matrix of the pink rock.
Locality Notes and Wider Context
Crinanite is a local name, but the occurrence of analcime-bearing volcanic and subvolcanic rocks is not limited to Scotland (e.g., in southeast Australia: Binns, 1969; Irving, 1974).
Arran and its curious intrusions are described by many authors, including those of major reviews such as Craig (1965) and Emeleus and Gyopari (1992). The nearby Dippin sill, on the southeast corner of Arran, is a differentiated mafic intrusion, 43 m thick, composed of variants of analcime olivine dolerite, especially crinanite, bordered by thin marginal teschenite facies, with minor units of augite teschenite and basic pegmatite at various levels (Henderson and Gibb, 1977; Gibb and Henderson, 1978).
Crinanite is often found in thin dykes, as described by Flett and successors. Possibly the largest crinanite body in Scotland is a thick sill which dominates the rugged landscape of the Shiant Isles, north of Skye. Two of the islands are formed of a 168-metre-thick topless crinanitic sill injected into Jurassic sediments. A picritic facies at the base is succeeded upwards by picrodolerite (olivine-rich crinanite). The olivine percentage decreases upwards and the rock passes into crinanite. The third main island (Eilean Mhuire) is mainly augite teschenite: this may be a downfaulted block from a higher level in the main sill than is preserved on Garbh Eilean and Eilean-an-Tighe (Gibb, 1973). Further discussion of the Shiant Islands sill, with four possible phases of intrusion, can be found in Walker (1930), Mohammed (1982), Gibb and Henderson (1984, 2006), Latypov and Chistyakova (2009) and Holness et al. (2017). Picrite, picrodolerite and crinanite are also the main components of the Little Minch sill complex in northern Skye. These are related, formed from an alkali-olivine basalt magma with circa 10% MgO. The Little Minch sill complex occurs on Trotternish, but also encompasses the Shiant Isles and Raasay, extensive magmatism over an area of some 4,000 km2 (Gibson and Jones, 1991) **.
Back to the south, on Arran, a thin section of sample 118.11 affirms that the rock is largely titanaugite (30%), plagioclase (45%), olivine (10%), analcime (5%) and 10% of accessory minerals, mostly Fe-Ti oxides, sheet silicates (chlorite) and possibly a second species of zeolite (e.g., natrolite).
See also the companion article on pitchstones from Arran (Scottish locality list, below). New thin sections of pitchstone and crinanite, as well as an unusual gabbro and a craignurite will be prepared later this year, to illustrate the distinctive textures and mineralogy of these igneous rocks from the Tertiary Volcanic District.
** The British sector of the Tertiary Volcanic District is manifested largely along the western coast of northwest Scotland and in Northern Ireland (e.g., the columnar basalts of the Giant's Causeway in Antrim). There are limited expressions, mostly minor intrusions, that extend south into England, and westwards into and beneath the North Atlantic ocean. As an indicator of scale, the distance N.N.W. from Ailsa Craig (a small rocky island noted for riebeckite microgranite, 20 km south of Arran) past Arran, Mull, Rhum and the other Small Isles, Ardnamurchan and Skye to the Shiant Isles is some 300 km. Perhaps not incidentally, the extremes of this region are manifested as smaller islets: Ailsa Craig and the three main Shiant Isles each have a maximum dimension little more than 1 km.
Binns,RA (1969) High-pressure megacrysts in basanitic lavas near Armidale, New South Wales. Amer.J.Sci. 267-A, 33-49.
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.
Gibb,FGF (1973) The zoned clinopyroxenes of the Shiant Isles sill, Scotland. J.Petrol. 14, 203-230.
Gibb,FGF and Henderson,CMB (1978) The petrology of the Dippin sill, Isle of Arran. Scot.J.Geol. 14, 1-27.
Gibb,FGF and Henderson,CMB (1984) The structure of the Shiant Isles sill complex, Outer Hebrides. Scot.J.Geol. 20, 21-29.
Gibb,FGF and Henderson,CMB (2006) Chemistry of the Shiant Isles Main Sill, NW Scotland, and wider implications for the petrogenesis of mafic sills. J.Petrol. 47, 191-230.
Gibson,SA and Jones,AP (1991) Igneous stratigraphy and internal structure of the Little Minch sill complex, Trotternish peninsula, northern Skye, Scotland. Geol.Mag. 128, 51-66.
Harker,A (1954) Petrology for Students. Cambridge University Press, 8th edition revised by Tilley,CE, Nockolds,SR and Black,M, 283pp.
Hatch,FH, Wells,AK and Wells,MK (1949) The Petrology of the Igneous Rocks. 10th edition, George Allen and Unwin, 469pp.
Henderson,CMB and Gibb,FGF (1977) Formation of analcime in the Dippin sill, Isle of Arran. Mineral.Mag. 41, 534-537.
Holness,MB, Farr,R and Neufeld,JA (2017) Crystal settling and convection in the Shiant Isles main sill. Contrib.Mineral.Petrol. 172. article 7, 25pp.
Irving,AJ (1974) Megacrysts from the newer basalts and other basaltic rocks of southeastern Australia. BGSA 85, 1503-1514.
Johannsen,A (1938) A Descriptive Petrography of the Igneous Rocks, Volume IV. Part I, The Feldspathoid Rocks, and Part II, The Peridotites and Perknites. University of Chicago Press, 523pp.
Latypov,R and Chistyakova,S (2009) Phase equilibria testing of a multiple pulse mechanism for origin of mafic-ultramafic intrusions: a case example of the Shiant Isles Main Sill, NW Scotland. Geol.Mag. 146, 859-875.
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.
Macgregor,M, Herriot,A and King,BC (1972) Excursion guide to the geology of Arran. Geological Society of Glasgow, 2nd edition, 199pp.
McKerrow,WS and Atkins,FB (1989) Isle of Arran: a field guide for students of geology. Geologists' Association Guide, 2nd edition, 104pp.
Mohammed,ARO (1982) Mineralogy and Petrology of Eilean Mhuire, Shiant Islands. PhD thesis, University of Cambridge.
Ordnance Survey (1956) Isle of Arran. One-inch map sheet 66, 1"63,360 scale, reprinted with minor corrections 1960.
Walker,F (1930) The geology of the Shiant Isles (Hebrides). QJGS 86, 355-398.
Walker,F (1934) The term "crinanite". Geol.Mag. 71, 122-128.
Wilkinson,JFG (1955) The terms teschenite and crinanite. Geol.Mag. 92, 282-290.
Graham Wilson, posted 12,23,30-31 August and 01-04,07,14 September, 2021
or, visit the Turnstone "Rock of the Month" Chronological Archives!
|Class/Group/Family||8 rock types 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|
|Volcanic glass||-- #242 --- Pitchstones from the Isle of Arran, Scotland|
|Diabase||-- #243 --- Crinanite (analcime olivine dolerite / diabase), Isle of Arran, Scotland|
|To come, thin sections in prep. (late 2021):|
|Gabbro||-- #2-- --- "Old Gabbro" of Ardnamurchan, Scotland|
|Volcanic / minor intrusive||-- #2-- --- Craignurite, Isle of Mull, Scotland|
|Minor intrusive||-- #2-- --- Lamprophyre, Iona, Scotland|