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‘Lamprophyre’ minor intrusions of Colonsay – a comment

S I R S Borradaile's (1986) paper is extremely welcome, as the first published study of the Colonsay ' lamprophyres' since Cunningham-Craig, Bailey & Wright (1911). This comment in no way questions the great value of Borradaile's careful and detailed field descriptions, but rather emphasises the urgent need for structural geologists to collaborate with igneous petrologists (all too rare phenomenon!) in studying such rocks, if the many problems identified by Borradaile himself (e.g. p. 670) are to be solved, and the larger-scale ideas he raises are to be substantiated. Borradaile relies on incomplete, largely outdated literature citation-for example, Watson's (1964) account of the 'appinites' is greatly modified by Watson (1984). Berger's (1971) study offelsic dykes is less relevant than numerous, partly structural, papers on mafic dykes equivalent to those of Colonsay (e.g. Reynolds, 1931; Blyth, 1949; Rust, 1965; French, 1978; Smith, 1979; Leake & Cooper, 1983), or indeed than the one published study on the Colonsay plugs themselves (Reynolds, 1936). Borradaile's omission of all this work has broader implications, notably the question of how many igneous suites (i.e. discrete events, sensu Rock, 1981) are represented on Colonsay. Borradaile (p. 666) correctly records three structurally distinct intrusions: (1) undeformed, discordant, subvertical dykes; (2) undeformed, discordant syenitic.appinitic plugs (Scalasaig, Kiloran and Balnahard); (3) deformed, concordant to discordant bodies, which I term sheets (Smith, 1979), to avoid confusion with the other two types. However, based on petrographical examination of my own collection of over 80 Colonsay 'lamprophyres' (British Geological Survey registered rocks S71163-71247), type (1) dykes probably belong to Smith's (1979) post-tectonic (395-410 Ma) Minette Suite. Type (2) plugs have long been linked (e.g. Bailey, 1960) with the SB 400 Ma Appinite Suite of the Appin-Ballachulish area (Wright & Bowes, 1979). Type (3) sheets, by contrast, most probably belong to Smith's (1979) syn-tectonic Microdiorite Suite, emplaced around 424+ 13 Ma (unpubl. BGS K-Ardata). In the Ross of Mull (the nearest area to Colonsay documented recently), type (1) dykes at c. 405 Ma postdate the c. 415 Ma granite, whereas type (3) sheets arc metamorphosed by this pluton (Rock & Hunter, 1987). In short, the Colonsay dykes almost certainly include representatives of at least three minor intrusive suites, differing by 20 Ma or more in age, and Borradaile's (p. 665) statement that they 'belong to the Appinite suite recognized throughout the British Caledonides' perpetuates two misconceptions from the 1960s: (a) it ignores the modern separation of'appinites' into the Microdiorite Suite (formerly the West Highland Appinite Suite of MacGregor & Kennedy, 1931), represented by the Colonsay mafic dykes, and the unrelated, Appin-Ballachulish Appinite suite, represented by the Colonsay plugs; (b) it copies from Johnson and Dalzicl (1966) the incorrect assumption (Smith, 1979) that the foliated Colonsay dykes (Microdiorite Suite) actually represent foliated variants of the unfoliatcd dykes (when the latter in fact represent a third, unrelated, Minette Suite). In his larger-scale interpretation of the western Caledonides (pp. 669-70), Borradaile does not consider the problem of the Colonsay Group metasediments forming Colonsay and the Islay Rhinns; three views as to their age are extant: (a) They are Torridonian, as indicated on all British Geological Survey memoirs and maps so far published. (b) They are Dalradian (Stewart, 1962, 1975); possibly, Colonsay Limestone = Ballachulish Limestone (Rock, 1986). (c) They belong to a new, hitherto unrecognized stratigraphical grouping (e.g. Bentley, Fitches & Maltman, 1986). On view (a), 'Torridonian' (Colonsay Group) unconformably overlies uncontested Lewisian (in northeast Colonsay and the Rhinns of Islay), and is tectonically separated from the Islay Dalradian by the Loch Gruinart Fault. On view (b), by contrast, Dalradian is juxtaposed directly on Lewisian in Colonsay and Islay, and extends northwest of the Great Glen Fault system. On view (c), Colonsay and Islay constitute a 'suspect terrane' (Bentley, Fitches & Maltman, 1986), tectonically separate from the mainland Caledonides. Borradaile's regional extrapolations (fig. 9) and his use of the Colonsay fabrics as 'regional palaeostress indicators' are both premature, given these radically different alternatives. Finally, Borradaile appreciates (pp. 669-70) that his 'pan-metamorphic' interpretation of the Colonsay intrusion fabrics poses major problems. I do not deny a metamorphic origin for fabrics such as those actually seen to crenulate earlier flow foliations (p. 665). However, given that the fabrics are 'confined to the igneous rock over most of the area' (p. 665), and 'do not pass into the country rocks' (p. 670), the prima facie conclusion must surely be that some of them have an igneous component. Borradaile's statement that they are 'clearly of secondary nature' (p. 665) is not adequately justified by petrographical description (pp. 666-7). Borradaile assumes (p. 666) that a chloritecarbonate-mica mineralogy automatically implies metamorphism, but in lamprophyres such assemblages are commonly autometasomatic, generated by exceptionally high H2O and CO2 in the parent magma (Rock, 1987). Lamprophyes can contain very abundant biotite (30-60 modal %); efficient flow foliation of such abundant mica can produce a 'pseudo-schistosity' in an unmet amorphosed rock. Among > 2000 Caledonian 'lamprophyres' I have examined, flow foliated, 'schistose' examples include many from the paratectonic Caledonides (e.g. Barnes, Rock & Gaskarth 1986, p. 107; Rock, Cooper & Gaskarth 1986, fig. 3). Borradaile himself only uses 'schistosity' 'for want of a better term' (p. 666). His differences with Wilkinson (1907, p. 61) over the interpretation of one Colonsay intrusion, where the fabric in a ' folded' (terminated) body is not only discordant with that in the country-rocks, but also unrelated to an apparent fold closure, provide yet another example where an alternative (igneous) interpretation could usefully have been considered. Overall, Borradaile's assumption (p.