Structural Setting and Geochronological Significance of the West Highland Granitic Gneiss, a Deformed Early Granite Within Proterozoic, Moine Rocks of NW Scotland
Total Page：16
File Type：pdf, Size：1020Kb
J. geol. Soc. London, Vol. 142, 1985, pp. 663-675, 7 figs. Printed in Northern Ireland Structural setting and geochronological significance of the West Highland Granitic Gneiss, a deformed early granite within Proterozoic, Moine rocks of NW Scotland D. Barr, A. M. Roberts,” A. J. Highton,? L. M. Parson$ & A. L. Harrisg British Geological Survey, 19 Grange Terrace, Edinburgh EH9 2LF; * Britoil plc, Exploration Department, Grosvenor Centre, 6C-80 GordonStreet, Glasgow; t British GeologicalSurvey, Murchison House, West Mains Road, Edinburgh EH93LA; $ Institute of Oceanographic Sciences, Brook Road, Wormley, Godalming,Surrey GU85UB; 3 JaneHerdman Laboratories of Geology, University of Liverpool, Brownlow Street, Liverpool L69 3BX; UK. SUMMARY: Are-investigation of the six major outcrops of granitic gneiss within the Glenfinnan and Loch Eil divisions of the Moine has shown that they are a suite of deformed and metamorphosed granite intrusions. They record all the tectonic events recognized in their host metasediments, while the absence of thermal aureoles suggests that their emplacement was synmetamorphic, i.e. syn-D, rather than pre-D,. Their dominant gneissic fabric is of S2 age, but in low-strain augen an earlier S, fabric is preserved which is cut by MPI leucocratic segregations contemporaneous with, but compositionally distinct from, those in the pelitic regional migmatites of the Moine. The deformed granites underwent local MP2 remelting and have been widely intruded by later, Caledonian pegmatites. The gneiss has a uniformly granitic (sensu stricto) bulk composition, regardless of country-rock lithology or location within the Glenfinnan and Loch Eil divisions. It has strongly S-type chemical affinities, being reduced and peraluminous with a high initial 87Sr/86Srratio. It was probably produced at least in part by partial melting of Moine-likerocks at depth. A published Rb/Sr whole-rock isochron of c. 1028Ma probably dates intrusion of the Ardgour body and confirms the reality of a ‘Grenvillian’ event in the Glenfinnan and Loch Eil divisions of the Moine. One of themore controversial issues in geological geological interpretation is often less ambiguous than interpretations of the Moine of NW Scotland is the that of dates from metasediments. Several deformed distinction betweenPrecambrian and Caledonian intrusions have been used as geochronological markers orogenicevents. Radiometric dates obtained from within the Moine, for example the Carn Chuinneag metasediments, and frompegmatites within them, granite (550 f 20 Ma; Pidgeon & Johnson 1974), the convincingly demonstrate an important Caledonian Glen Dessary syenite (456 * 5 Ma; van Breemen et metamorphicevent (e.g. van Breemen et al. 1974, al. 19796) and the Loch Borrolan syenite (430 f 4 Ma; 1978;Brewer et al. 1979). Olderdates suggest that van Breemen et al. 1979~).The Ardgour granitic ‘Morarian’ (70&800 Ma) or ‘Grenvillian’ (c. 1000 Ma) gneiss (Fig. 1) is the only syntectonic ‘igneous’ body events arerepresented (e.g. Lambert 1969; van within the Moine to have provided convincing Gren- Breemen et al. 1974, 1978; Brook et al. 1977; Piasecki villian radiometric dates (1028 f 43 Ma; Brook et al. & van Breemen 1983). With respect to the southern 1976, recalculated with A s7Rb = 1.42 X 10-” a-l by part of the N Highland Moine, extreme viewpoints are Brewer et al. 1979;Aftalion & van Breemen 1980). provided by Brewer et al. (1979) and Powell et al. The Quoich granitic gneiss is probably also Grenvillian (1983), who argue that all major units haveexperi- (Piasecki & van Breemen 1979). The usefulness of enced Grenvillian orogenesis, the Morarian represent- these dates has been limited by uncertainty over the ing an episode of non-orogenicpegmatite intrusion, nature and origin of the granitic gneiss. If it represents and Piasecki & van Breemen (1979,1983), who argue pre-existing granitic basement, then the associated for widespread Morarian tectonism but would restrict Moine is post-Grenvillian. If it was produced in situ by the Grenvillian event to one tectonostratigraphic unit, metasomatism, then significant Grenvillian orogenic the Glenfinnan Division, which by inferenceforms activity is implied. If it is intrusive, then the Moine basement to the remainder of the Moine. sediments are older than 1028 Ma, but the case for In general, radiometric dates obtained from igneous Grenvillian orogenic activity is notproven if the bodies can only provide upper and lower limits to the granite can plausibly be interpreted asentirely pre- age of tectonometamorphicevents. However, their orogenic. We have studied in detail three of the major Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/663/4888455/gsjgs.142.4.0663.pdf by guest on 26 September 2021 664 D.Burr et al. graphic units: from west to east, the Morar, Glenfin- nan and Loch Eil divisions. The Glenfinnan Division comprises interbanded pelites and psammites and passes up stratigraphically intothe dominantly psammitlc Loch Eil Division (Roberts & Harris 1983), but has beentectonically emplaced onto thedominant- ly psammitic Morar Division by the Sgurr Beag slide, an early Caledonian ductile thrust (Tanner et al. 1970; Rathbone et al. 1983). Six major bodies of granitic gneiss have been recognized in the Glenfinnan and Loch Eil divisions (Fig. l), along with anumber of smaller satellite intrusions. The six named bodies of granitic gneiss and their surrounding metasediments carry three well-developed sets of structures, D1-D3. The earliest, D1 structures are represented by sparse, intrafolial F1 isoclines, and a bedding-parallel S1 fabric which is crenulated in F2 fold cores. D2 structures are widespread on all scales and were originally sub-recumbent. Where subsequent strain is low, F2 folds are observed to be tight to isoclinal and intensely curvilinear abouta N-S- trendingextension lineation (Holdsworth & Roberts 1984). Exceptin F2 fold cores, S2 is subparallel to banding, and the mesoscopic 'foliation' mapped in the metasediments is compositea So/S11S2 fabric. D3 structures are upright with NNE-SSW-trending axial planes. In the Loch Arkaig area (Fig. l),Roberts et al. (1984) have demonstrated that D1 and D2 pre-date the c. 456 Ma Glen Dessary syenite (van Breemen et al. 19796), while D3 is later than c. 456 Ma, i.e. Caledo- nian. The Loch Quoich line (Fig. 1; Clifford 1957) has been shown by Roberts & Harris (1983) to separate two domains of contrasting D3 strain. To the west, in the 'steep belt', F3 folds are tight and steeply plunging Loch EilDivision with a strong axial planar S3 fabric; only in low-strain GlenfinnanDivision F3 fold cores is the pre-D3 disposition of stratigraphy .. and structure preserved. To the east, in the 'flat belt', ... Morar Division(including Lewisian rocks) D3 ductile strain is low,and sub-recumbent D2 structures dominate. The Loch Quoich line lies close FIG.1. Distribution of the West Highland granitic to, but does not precisely coincide with, the lithostra- gneiss (modified from Johnstone (1975) fig. 6). tigraphic boundary between the Glenfinnan and Loch Eil divisions. bodies of granitic gneiss shownin Fig. 1 (Ardgour, Metamorphic grade inthe Glenfinnan Division is high Loch Quoich,and Fort Augustus) and visited three everywhere:kyanite and sillimanite are present in others(Gulvain, Loch Arkaig,and Glen Doe). In suitable lithologies and calc-silicates are pyroxene- combination with existing data, our results confirm the andlorbytownite-bearing (Winchester 1974; Fettes igneous origin of the granitic gneiss, and enable the 1979). Sillimanite + K-feldspar assemblages are, radiometric dates to be placed in a definite geological however,restricted to a small area inSW Ardgour context. (Dalziel & Brown 1965; Tyler & Ashworth 1982). The Loch Eil Division appears to be of somewhat lower Regional setting metamorphic grade (Winchester 1974), although this is due, atleast in part, to Caledonianretrogression of The N Highland Moine comprises thatpart of the calc-silicate mineral assemblages (Johnstone et al. Caledonian orogenic belt lying between the marginal 1969; Tanner 1976). The high-grade mineral assemb- Moinethrust belt andthe transcurrent Great Glen lages are uniformly early (MS1-MP1: syn- to post-D1, fault; only the southern portion will be discussed in sensu Sturt & Harris 1961), and broadly coeval with this paper (Fig. 1; Johnstone 1975). It was subdivided the regional migmatization of pelites and, less com- by Johnstone et al. (1969) into three majortectonostrati- monly, other lithologies. Most pelites in the Glen- Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/4/663/4888455/gsjgs.142.4.0663.pdf by guest on 26 September 2021 W est Highland granitic gneissgraniticHighland The West 665 finnan Division, and somein the Loch Eil Division distinct kind of magma emplacedamong high-grade (e.g. in the Garry Banded Unit of Roberts & Harris metamorphic rocks’. Pidgeon & Aftalion (1978) 1983), are represented by lit-par-lit migmatitic gneisses demonstrated thatinherited, ‘detrital’ zircons are with trondhjemitoid(K-feldspar-free) leucosomes. common in other, undoubtedly intrusive Scottish These formed isochemically, by subsolidus segregation granites. Gould (1966) concluded, on the basis of over within rocks having favourable bulk chemistry (Barr 100 whole-rock analyses, that the Ardgour gneiss has a 1985), particularly plagioclase-rich pelites (sensu Zato) uniform granitic (sensu stricto) composition and is of ‘metagreywacke’ composition (Butler 1965). Litho- quite