The Metamorphism of Minor Intrusions Associated with the Newer Granites of the Western Highlands of Scotland
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Downloaded from http://sjg.lyellcollection.org/ by guest on September 26, 2021 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. 2A Downloaded from http://sjg.lyellcollection.org/ by guest on September 26, 2021 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.