The Metamorphism of Minor Intrusions Associated with the Newer Granites of the Western Highlands of Scotland

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Letters to the Editors

THE METAMORPfflSM OF MINOR INTRUSIONS ASSOCIATED WITH THE NEWER GRANITES OF THE WESTERN HIGHLANDS OF SCOTLAND

Sms,—Dr Dearnley (1967) has suggested that the minor intrusions northwest of the Great Glen were deformed and metamorphosed during the later stages of deposition of the Lower Old Red Sandstone formation. I should like to comment on the assumption upon which this date is based, that is, that the ' main suite' of intrusions northwest of the Great Glen can be correlated with the Etive dyke swarm. I have been working for some years in the Appin district and have been able to establish the following sequence of events: (1) the emplacement of numerous small plugs of pyroxene-diorite and appinitic diorite and lamprophyre sheets (2) the emplacement of the Ballachulish complex and (3) the emplacement of the Etive complex. Therefore, the Etive complex dyke swarm is quite distinct from the appinitic intrusions of this part of Scodand. Consequently there can be little reason for assuming that this porphyrite dyke swarm can be correlated with a swarm of minor intrusions in Morar and Moidart, which include important lamprophyric, appinitic and ultrabasic rock types as well as the porphyrites and which, furthermore, are among the earlier and not the later members of the local sequence of intrusions. It is worth considering if a different approach to correlation would be more useful in view of the present state of field knowledge and the stage of development of radiometric techniques. It is well-established that these intrusions of diorite, lamprophyre and granite post-date the main phases of metamorphism in Scotland, so the objectives of research must surely be to establish (1) the spatial extent of igneous and late metamorphic events, i.e. whether they affected wide areas of the Highlands or were restricted to distinct provinces at any given instance of time, (2) a more precise understanding of the nature and relationship between igneous and late metamorphic events and (3) a close correlation with the stratigraphic time scale. None of these objectives is achieved by assuming that apparently similar igneous or metamorphic events were contemporary in widely separated areas (65 miles or more in this case). However, if detailed sequences of events can be established from field evidence in particular areas and a programme of radiometric dating carried out on members of each sequence, a correlation of events in different areas may then be made with a high degree of confidence on the basis of radiometric dates of proved significance. The available determinations of radiometric dates of Caledonian igneous rocks Downloaded from http://sjg.lyellcollection.org/ by guest on September 26, 2021

LETTERS 371 are confined to granitic complexes and are of little significance in this context, but certain broad correlations between the sequences in the Appin and Moidart— Morar areas can be made. In Appin, the end of igneous activity is clearly defined as 400 m.y. on the K/Ar scale by determinations on the Etive complex (Miller and Brown 1965, table 4; Harper 1967, table 2); an average of age determinations from all Caledonian granites cannot be used once it is accepted that such complexes are not all contemporary. The earlier phases of activity must be significantly earlier than 400 m.y. if sufficient time is to be available for the successive emplace ment of two major central complexes. Furthermore there is evidence from quartz- diorite and granitic boulders in Old Red Sandstone conglomerates (Bailey 1960, pp. 130,131 and 144; Kynaston and Hill 1908, p. 69 and my own observations) that some plutonic complexes were deeply eroded before the deposition of the Glen Coe and Lome lava series which pre-date the granitic members of the Etive complex. It also follows that the Lome and Glen Coe lava series cannot be the sur face representatives of the pyroxene-diorite and appinitic diorite plugs, but may be representatives of dioritic magmas emplaced in the early history of the Etive complex, such as that now forming the Quarry diorite (Nockolds 1934; Anderson 1937). There are no radiometric dates available for the earliest dioritic plugs, but Harper's work (1967) indicates that maximum limits may be provided by the 440-470 m.y. dates obtained by him from Dalradian slates close to the sub-Old Red Sandstone erosion surface in Argyll. In Morar and Moidart, Dr Dearnley has shown that in a traverse from west to east the recrystallisation of the minor intrusions begins where the K/Ar country rock dates have fallen to 430 m.y. This correspondence would suggest to me that, regardless of whether the eastwards decline in country rock dates is the result of overprinting or progressive cooling, the recrystallisation of the minor intrusions was related to the isotopic ' event' and that the minor intrusions were emplaced at least 430 m.y. ago. This date would be consistent with Miller and Brown's (1965) dates of 421, 407 and 433 m.y. for the Strontian-complex tonalite, which carries xenoliths of the lamprophyres, and would thereby avoid the difficulty inherent in Dr Dearnley's proposed sequence. As the ' main suite' of dykes northwest of the Great Glen are over 430 m.y. old they cannot be correlated with the Etive dyke swarm which is only 400 m.y. old. If there had been contemporary igneous activity in Appin then it is represented by some of the pre-Etive complex intrusions, which are at present poorly dated, but which lie within the broad limits 400-470 m.y. The Loch Caoldair complex (Anderson 1947), 40 miles north-east of Appin, provides at the present time the closest approach to the sequence north west of the Great Glen, although this Loch Caoldair sequence cannot be directly correlated with the Appin sequence. Here, a partly foliated appinite plug is cut by pegmatites of the Loch Caoldair complex, which cooled 430 m.y. ago (Harper 1967, 1968; Smith 1968) and which was later cut by members of the Etive complex dyke swarm.

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372 LETTERS

REFERENCES ANDERSON, J. G. C. 1937. The Etive Granite Complex. Quart. Jlgeol. Soc. Lond. 93, 487-533 1947. The Kinlochlaggan Syncline, Southern Inverness-shire. Trans, geol. Soc. Glasg. 21, 97-115. BAILEY, E. B. 1960. The Geology of Ben Nevis and Glen Coe. Mem. Geol. Suru. U.K. DEARNLEY, R. 1967. Metamorphism of minor intrusions associated with the Newer Granites of theWestern Highlands of Scotland. Scott. J. Geol. 3, 449-457. HARPER, C. T. 1967. The geological interpretation of potassium-argon ages of metamorphic rocks from the Scottish Caledonides. Scott. J. Geol. 3, 46-66. 1968. The geological interpretation of potassium-argon ages of metamorphic rocks from the Scottish Caledonides. (Letter). Scott. J. Geol. 4, 90. KYNASTON, H. and HILL, J. B. 1908. The Geology of the Country near Oban and Dalmally. Mem. Geol. Surv. U.K. MILLER, J. A. and BROWN, P. E. 1965. Potassium Argon Age Studies in Scotland. Geol. Mag. 102,106-134. NOCKOLDS, S. R. 1934. The Contaminated Tonalites of Loch Awe. Quart. Jl geol. Soc. Lond. 90, 302-321. SMITH, T. E. 1968. The geological interpretation of potassium-argon ages of metamorphic rocks from the Scottish Caledonides. (Letter). Scott. J. Geol. 4, 87-90. I. M. PLATTEN Department of Geology, University of Liverpool. MS received 14th May 1968

Dr R. DEARNLEY (Institute of Geological Sciences) replies: The suggestion in Mr Platten's letter is basically that the Etive and Nevis dyke swarms cannot be corre lated with the minor intrusions north of the Great Glen, since the latter ' are over 430 m.y. old'. However, I would seriously question whether in fact these minor intrusions are over 430 m.y. old on a number of grounds including the following: (i) Mr Platten has taken the 421 db 19 m.y. and 433± 19 m.y. ages obtained on one specimen of tonalite from Strontian (Miller and Brown 1965) to be consistent with the idea of emplacement of the minor intrusions prior to 430 m.y. ago. However, these twa high ages (relative to the other Strontian and Foyers ages) are open to considerable doubt as they are early determinations with a large experimental scatter and the 433 ± 19 m.y. date was obtained by the total volume method which is known to yield somewhat high results (Brown et ah 1968, p. 275). (ii) Other dates, from another tonalite and from an adamellite in the Strontian complex are respectively 407±18 m.y. and 381 ±17 m.y. (Miller and Brown 1965). These dates are similar to the more recent determinations using the improved method of argon determination of Grasty and Miller (1965) by Brown et at (1968), ranging from 397±8 m.y. to 406±8 m.y. for the downfaulted and displaced eastern portion of the complex at Foyers. It would seem better to treat the higher ages with some caution and to rely more on these later determinations Downloaded from http://sjg.lyellcollection.org/ by guest on September 26, 2021

LETTERS 373 (which in any case are similar to 7 out of 9 of the earlier values) for the age of the Strontian and Foyers complexes which range from 381-409 m.y. (iii) Additional ages, which give considerable support to this conclusion come from the Moine Schists close to or at the contact of the Foyers granite where values of 392±18 m.y. and 409±18 m.y. are recorded (Miller and Brown 1965); also Moine schists from Salen, only 6 miles W. of the Strontian tonalite-schist contact are dated at 410± 18 m.y. and Moine Schists from the same locality as the dated tonalites at Strontian are 412±19 m.y. old. The data from (i) and (ii) suggest that the main suite of minor intrusions is older than about 381-410 m.y. (iv) A maximum age estimate for the emplacement of the main suite of minor intrusions is suggested by the fact that they are dominantly, if not entirely, post F3 in age (Johnson and Dalziel 1966). F3 has been dated in the L. Monar and L. Hourn areas as between 413 m.y. and 425 m.y. (Brown et al. 1965) correspond ing to the main Moine age peak. Thus it appears that the main suite is post 413-425 m.y. and pre about 381-410 m.y. old. (v) Further support for this age range for the minor intrusions is suggested from the Ross of Mull where minor intrusions ' of Scottish Lower Old Red Sandstone facies cut the granite' (Bailey 1925) but were probably intruded shortly after the consolidation of the granite (Clough 1911). The Ross of Mull granite has been dated by Miller and Brown (1965) at 399±14 m.y. and by Brown et at (1968) between 406±8 m.y. and 409±9 m.y., and therefore the ages of these minor intrusions are closely comparable with the range suggested in (iv) for the main suite minor intrusions. The emplacement of the minor intrusions is closely connected with the granites, being somewhat earlier than the Strontian—Foyers complexes (381-409 m.y.) and somewhat later than the Ross of Mull (399-409 m.y.) and Cluanie granites—similar dykes cutting the Glenelg—Ratagan complex (Clough 1910; Nicholls 1951) may also be related. The phase of metamorphism which some of these minor intrusions exhibit must be later than this range of dates but earlier than the emplacement of the E.—W. red felsites. On this basis I would like to stress that Mr Platten is incorrect in saying that my estimated date of metamorphism is based on the assumption that the main suite of minor intrusions N. of the Glen can be correlated with the Etive dyke swarm. The estimate is independent of whether these suites can be correlated or not. I would agree that the Etive (and probably the Nevis) dyke swarm is quite distinct from the appinites and associated intrusions in Appin which are clearly somewhat earlier, but one cannot conclude from this that ' consequently there can be little reason for assuming that this [Etive] porphyrite dyke swarm can be correlated with a swarm of minor intrusions in Morar and Moidart which include important lamprophyric, appinitic and ultrabasic rock types as well as the porphyrites and which, furthermore, are among the earlier and not the later members Downloaded from http://sjg.lyellcollection.org/ by guest on September 26, 2021

374 LETTERS of the local sequence of intrusions '. Firstly the minor intrusions cover a much larger area than Morar and Moidart, they occur from Morvern to at least Cluanie. Secondly ultrabasic rocks are very rare and these plus the appinites account for only about 15 per cent of a count of 600 sliced specimens from the Loch Eil (1" Sheet 62) area, the remaining 85 per cent falling under the headings of micro- diorite and porphyrite in the Survey classification. Lastly these minor intrusions are not always amongst the earlier members of the local sequences, they are later than, for instance, the Cluanie and Ross of Mull granites (and the Glenelg- Rattagan granite if its associated dykes are considered approximately equivalent). If it is possible to make a general and approximate correlation of phases of igneous activity across the Great Glen it would be most reasonable to suggest an approximate correlation between the largest suites of minor intrusions on either side, that is between the main suite of minor intrusions north of the Great Glen (post 413-425 m.y., and pre 381-410 m.y.) and the Etive and Nevis swarms south of the Great Glen (approximately 400 m.y. old). Both groups are dominantly porphyrites and microdiorites with much smaller numbers of appinites and more basic types. If one does not agree with this suggested correlation then the only other suite of minor intrusions which might be considered as a general correlative to the main suite of minor intrusions north of the Great Glen would be the Appinite-Lamprophyre suite of Appin and adjacent areas. This correlation would seem unlikely as the main suite north of the Great Glen is a much larger suite and also there are quite different proportions of rock types in the suites. North of the Great Glen porphyrites and microdiorites are very abundant, south of the Great Glen these types are very scarce and the bulk of the suite is composed of appinites and lamprophyres. Whichever of these correlations is taken, or even if neither is correct, the independent evidence outlined above strongly indicates a Lower Old Red Sandstone date for the emplacement and metamorphism of the main suite of minor intrusions north of the Great Glen.

REFERENCES BAILEY, E. B. in BAILEY, E. B. and ANDERSON, E. M. 1925. The geology of StafFa, Iona and Western Mull. Mem. Geol. Surv. U.K. BROWN, P. E. et al. 1965. Potassium-argon ages of some Dalradian, Moine and related Scottish rocks. Scott. J. Geol. 1,144-151. BROWN, P. E. et al. 1968. Isotopic ages of late Caledonian granite intrusions in the British Isles. Proc. Yorks. geol. Soc. 36, 251-271. CLOUGH, C. T. in PEACH, B. N. et al. 1910. The geology of Glenelg, Lochalsh and the southeast part of Skye. Mem. Geol. Surv. U.K. in CUNNINGHAM CRAIG, E. H. et al. 1911. The geology of Colonsay and Cronsay with parts of the Ross of Mull. Mem. Geol. Surv. U.K. GRASTY, R. L. and MILLER, J. A. 1965. The omegatron: a useful tool for argon isotope investigation. Nature, Lond., 207, 1146-1148. Downloaded from http://sjg.lyellcollection.org/ by guest on September 26, 2021

LETTERS 375

JOHNSON, M. R. W. and DALZIEL, I. W. D. 1966. Metamorphosed lamprophyres and the late thermal history of the Moines. Geol. Mag. 103, 240-249. MILLER, J. A. and BROWN, P. E. 1965. Potassium-argon age studies in Scotland. Geol. Mag. 102,106-134. NICHOLLS, G. D. 1951. The Glenelg-Rattagan igneous complex. Q. Jl geol. Soc. Lond. 106, 309-344. MS received 18th July 1968

THE CALEDONIAN HISTORY OF THE NORTH-EASTERN IRISH SEA REGION

Sms,—Simpson (1967; 1968) has described three phases of folding, Fx, F2 and F3, from the Skiddaw Group in the core of the Lake District anticline. In some ways these episodes of deformation are comparable to those which produced the ¥lt F2 and F3 structures in the Skiddaw Group in the Black Combe inlier (Helm, in Helm, Roberts and Simpson 1963). Fx and F2 in both areas belong to an intra- Lower Ordovician orogenic episode which pre-dates the deposition of the Borrowdale Volcanic Group. Fi (Black Combe) folds, however, have a northerly trend while Fx (Lake District) folds are Caledonoid; F2 (Black Combe) folds are more variable in attitude and tightness than F2 (Lake District) folds. In general, in both areas, F3 minor folds may plunge towards the north-westerly quarter and the associated axial-plane cleavage, S3, usually has a north-westerly trend (Simpson 1967, fig. 4f and g, fig. 6, and fig. 7; Helm 1968, p. 67). In the Black Combe area (Helm 1968) the major, primary end-Silurian phase of folding (F4) produced the north-easterly trending Borrowdale anticline with an associated axial-plane cleavage in the Borrowdale Volcanic Group and brought the Skiddaw Group of the inlier to its present structural level. The response to the F4 movements in the slaty inlier was the development of a suite of minor F4 folds congruous with the Borrowdale anticline and an associated axial-plane cleavage, S4. The latter cuts and displaces S3, and is itself modified by two later end-Silurian cleavages, S6 and S6. The Skiddaw Group of the inlier displays the effects of six phases of deformation while the Borrowdale Volcanic Group displays only three. Hence the tectonic history of the Black Combe area apparently differed from that of the main tract of the Lake District as described by Simpson (1968). In the structural chronology suggested for the Black Combe area (Helm, op. cit.) the F3 structures pre-date the Borrowdale Volcanic Group whereas in the Skiddaw Group of the main tract the F8 structures are equated by Simpson (1968, table 2) with the main-phase Fx (end-Silurian) folding which formed the Lake District anticline. Thus the N.-S. compressive force (Simpson 1967, p. 415) which was responsible for the flexing of the volcanic sequence into the east- northeasterly trending Lake District anticline and which produced the Caledonoid fold-pair around Crummock Water in the Skiddaw Group is also stated to have