J. geol. Soc. , Vol. 140, 1983, pp. 47-62, 4 figs., 3 tables. Printed in Northern Ireland

The Glen Kyllachy Granite and its bearing on the nature of the Caledonian Orogeny in

0. van Breemen & M. A. J. Piasecki

SUMMARY:The Tomatin () Granite in the NW GrampianHighlands has been separatedinto two distinct complexes: alate-tectonic Glen Kyllachy Granite (tectonically foliated) and a post-tectonic Findhorn Granite (flow foliated). For the Glen Kyllachy complex, Rb-Sr analyses of muscovites from the granite and from an associated suite of cross-foliated pegmatites yield an emplacement age of 443T:5 Ma. Whole-rock Rb-Sr data support field and textural evidence that the pegmatite and granite emplacement was late-tectonic to the last (F3) phase of Caledonian folding. Initial granite X7Sr/X6Srof 0.7176 supports field and geochemical evidence of derivationfrom upper crustal metasediments first metamorphosedduring the Grenville event. For the Findhorn Granite, concordant Rb-Sr and K-Ar mineral data establish an age of 413 2 5 Ma, and an initial *'SriX6Sr of c. 0.706 indicates a lower crustal and/or mantle source. Thisage and isotopiccontrast between these granites is characteristic of the whole Grampian region, in which a plutonic hiatus between c. 440 and 415Ma coincides with the peak of sedimentary accretion in the Southern Uplands, and may be explained in terms of the lack of hydrous materials passing down the associated subduction zone. Structural, metamorphic and radiometric evidence suggests (a) that the late-F3 Glen Kyllachy pegmatites are comparable with the 442 f 7 Ma old, syn-F3 pegmatites in the N Highlands- both pegmatite suites are situated in the axial zone of the metamorphic Caledonides displaced by the GreatGlen Fault; and (b) thatthe Caledonian, c. 454 Ma old 'peak' tectonic- metamorphic event (D2) previously documented in the N Highlands coincides with a Carado- cian basin closure along the Boundary Fault. For Silurian events in the NHighlands we proposea separate subduction history; in this region, thepeak of calc-alkaline plutonism was accompanied by continuous folding and westward thrusting at the same time as a plutonic hiatus in the Grampian Highlands. Current evidence suggests that the c. 160 km or greater sinistral movements on the Great Glen Fault were not initiated until late Silurian-early Devonian time.

The early evidence for diachronism of Palaeozoic de- Glen Fault. In view of the likely sinistral movements formation (Dewey 1969) and post-metamorphic cool- along the Great Glen Fault, the most northerly part ing (Dewey & Pankhurst 1970) across the Scottish of this region, in the Upper Findhorn (Figs 1 & 2), Caledonides has recently been confirmed (van Bree- isof most direct interest. Here, the earliest phase men et al. 1979a,b). Radiometric age data indicate that of Caledoniangranite magmatism is a recently in Aberdeenshire (Fig. 1) the peak of metamorphism recognized Glen Kyllachy Granite and related minor and tectonism occurred slightly before 490 Ma (Pank- intrusions, which can be shown to be late-tectonic to hurst 1970) and ended before 463 f 5 Ma (van Bree- the last (F3) phase of intensive local deformation. The men & Boyd 1972), while N of the Great Glen Fault a late-tectonic intrusions were later followed by a suite tectono-metamorphic climax occurred at 455 f 5 Ma of voluminous post-tectonic granites, which are com- (van Breemen et al. 1979a). Explanations have either parable tothe ubiquitous, large Silurian-Devonian invoked progressive thrusting towards the foreland or granites of Scotland. the juxtapositioning of anomalousmetamorphic ter- The present paper attempts tointegrate the geology, rains along the Great Glen Fault (cf. Kennedy 1946). geochronology andisotope geochemistry of the For instance, a c. 160 km or greater post-metamorphic Upper Findhorn area into the regional pattern of the sinistral shift along the Great Glen Fault (Winchester Scottish Caledonides. Some of the results of previous 1973; Storetvedt 1974) is consistent with the patterns fieldwork in theUpper Findhorn are revised in the of cover-basement distribution in the Moine on either light of new field data from this and neighbouring side of this fault (Piasecki et al. 1981). Another possi- areasand structural-textural rock relationships are bleexplanation for metamorphic diachronism in the described. The initial Sr-isotoperatios of the earlier may be found in the evidence for and later granites are contrasted, while the age data repeated marginal basin opening and closing along the lead to a temporal comparison of tectonic and plutonic northwestern margin of Iapetus (Longman et al. 1979). events across and along the axes of the orogen. In orderto test the abovehypotheses, more Diachronous orogeny has been related to changes in needs to be known of the geology and geochronology subduction along the NW margin of Iapetus and to of the Grampian region immediately S of the Great differential responses in the Highlands. 001~7649/83/010C-0047$02.000 1983 The Geological Society

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m Tertiary 8 Mesozoic Old Red Sandstone

a Moine Thrust zone Caledonian granitic intrusions

m SouthernHighland Group) A rgyll Group Argyll Dalrodian AppinGroup

Largely paragneisses, Argyll GP. or pre- Dalradian

m Grampian Division 1

U Moinc Central Highland Division Glenfinnan Division Sutherland Migmatitc Corn p lex m Granlttc gneisses Lewisian inliers i 1 Foreland (Lewisian 8, B Torridonian) BF' ' A6 - Thrust or slide -. - Fault

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/1/47/4888051/gsjgs.140.1.0047.pdf by guest on 23 September 2021 The Glen KyllachyGranite andCaledonian the Orogeny 49 We follow Lambert & McKerrow (1976) in using the p. 43) while the large ‘Fl’ folds of the Upper Glen termGrampian Orogeny forthe event which con- Kyllachy (Piasecki 1975, fig. 1) may be composite and verted the Dalradian sedimentary basin into a meta- belong to more than one orogenic event. morphic fold beltand which ended in early Ordovi- On the other hand, in the structurally and stratig- cian time;and CaledonianOrogeny for events of raphically higher rocks of the Grampian Division, S of Ordovician to MiddleDevonian age. This study is Glen Mazeran, the major NE-trending ‘Fl’ folds are concerned mainly with the prolongedCaledonian most probably of Grampian age. The tectonic effects of activity. the Grampian Orogeny in all the rocks of the Find- horn-Slochd region must be considerable, since the region is crossed by the Grampian Steep Belt (Fig. 1). The metasediments of the This majorstructure hasbeen described as a‘Root Upper Findhorn Zone’ by Thomas (1979) who traced it from Loch Awe to Kinlochlaggan and suggested that it may represent a The detailed fieldwork in theUpper Findhorn suture of Grampian age. It is identified in the region of (Piasecki 1975) needs to be put into the context of the the present study by the presence of a complex, very morerecent model of Late Precambrian cover- steeply inclined belt which can be traced to Kinloch- basementrelationships forthe Central (Grampian) laggan (unpublishedwork): it contains tectonically- Highland region (Piasecki 1980). In this model, mig- sliced synforms of Grampian Division rocks, including matitic and gneissose psammites and semipelites of marbles, within theCentral Highland Division (the probable Grenville age (theCentral Highland Divi- structure originally referred to as the ‘Loch Laggan- sion) are separated by a complex tectonic break (the Monadhliath Syncline Complex’ in fig. 1 of Piasecki Grampian Slide) from an overlying cover assemblage 1975). This steep belt may extend further N into the of psammites and semipelites of post-Grenville age steeply inclined rocks of the Glenfinnan Division N of (the Grampian Division). The latter suffered deforma- theGreat Glen Fault (Piasecki et al. 1981). Later tion and amphibolite facies metamorphism at c. 800- structures, such as the open to close, near upright ‘€2’ 750 Ma (the Morarian event), prior to the Grampian folds with southeasterly (‘cross-fold’) axial trends and and Caledonianorogenies (Piasecki & van Breemen the close, near upright ‘F3’ folds with an overall NNE 1979~).Following this recent work, a correlation be- axial trend, are most probably of Caledonian age (see tween the rocks of the Central Highland Division and below). the Glenfinnan Division N of the Great GlenFault has As is the case with the structures, it is difficult to been proposed on lithological, metamorphic andstruct- separate the effects of metamorphism produced by the ural grounds (Piasecki & van Breemen 19796; Piasecki Precambrian and LowerPalaeozoic orogenic events. et al. 1981). Undoubtedly,the oldest visible folds are associated Inthe Upper Findhorn region, all the metasedi- with the highest grade of metamorphism; but in the ments of Glen Kyllachy and most of the rocks to the N rocks of the Central Highland Division, the peak of are psammites, semipelites and occasional quartzites, Grampian metamorphism is likely to be obscured by of theCentral Highland Division. Flaggy, non- the previous regional migmatization of the Grenville migmatitic rocks of the Grampian Division become Orogeny (c. 1100 Ma) and also by the metamorphism dominant a short distance S of Glen Mazeran (Fig. 2), of the c. 750 Ma (Morarian) event. However, as the and also occur within a steep belt along the River Morarianmetamorphism is observed to decreasein Findhorn. grade rapidly above the Grampian Slide, its effect on The three-phasefold chronology for the UpperFind- the cover rocks of the Grampian Division some dis- horn region originally proposed by Piasecki (1975) tance above the slide becomes weaker than that of the now also requires modification. Thus, in the rocks of laterGrampian event (Piasecki 1980). Hence,the Glen Kyllachy, the earliestintrafolial and isoclinal recentresults of calc-silicate geothermometryand folds associated with concordant migmatization are old geobarometry obtained by Wells (1979) from nearby structures of probable Grenville age (cf. Piasecki 1980, rocks which belong to the GrampianDivision, suggest-

FIG. 1. Distribution of granitic and related intrusions in relation to the main subdivisions of the Moine and the Dalradian. Newer Gabbros of Aberdeenshireand Banffshire are omittedfor clarity. The 160 km sinistral displacement on the Great Glen Fault proposed by Winchester (1973) is tentatively increased to allow for possible separate subduction histories for the Northern and Central Highlands prior to late Silurian wrench faulting. Inset: present configuration of the main regions and of the major tectonic features of Scotland. Abbreviations of localities, tectonic features and intrusions mentioned in text-Localities: B, Belhelvie; G, Glenfinnan; K, Kinlochlaggan; Ki, Kincraig; P, Portsoy; Q, Loch Quoich; S1, Slochd. Tectonic features: GrS, Grampian Slide; MT, Moine Thrust; SBS, Sgurr BeagSlide; SF, Strathglass Fault;StF, Strathconnon Fault; SS, Swordly Slide. Intrusions: A, Abershirder; BN, Ben Nevis; Bo, Loch Borrolan; BV, Ben Vuirich; C, Cluanie; Ca, Cairngorm; D, Glen Dessary; E, Etive; F, Findhorn; G1, Glencoe;HF, Hill of Fare;Ke, Kennethmont; Lo, Lochnagar; R, Ratagan;RM, Rannoch Moor; Ro, Rogart; ROM, Ross of Mull; S, Strontian; St, Strichen.

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Semipelitic 8 Biotite-hornblende granodiorlte, with -e- Fault pelitic gneiss dioritic (L appinitic types Sample number Quartzite J. 8 location Biotite granodiorite

FIG. 2. Simplified geological map of the Glen Kyllachy area in the Upper Findhorn region. Geographical position indicated in Fig. 1. Abbreviations: BG, biotite gneiss transitional into granite; CO, Carn Oighreagan. Folds in NE corner are ‘F3’ folds. Stereogram (plots in lower hemisphere) shows their geometry for the S and E slopes of Carn na Farr Bheinne. Contours at 1, 3, 5, 7% of 133 poles to composition banding in metasediments; small dots, F3 axes; large dots, F3 axial surfaces. The land is privately owned, and permission for access is required from the Glen Kyllachy, Farr and Glenmazeran Lodges.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/1/47/4888051/gsjgs.140.1.0047.pdf by guest on 23 September 2021 The Glen Kyllachy Graniteand the CaledonianOrogeny 51 ing a maximum metamorphic temperature of c. 550°C into polygonal, granularaggregates (Fig. 3-5). This and pressure of as high as c. 9 kb, are interpreted as foliation is,again, broadly subparallel tothe axial the peakconditions for theGrampian event. This surfaces of the F3 folds in the hostmetasediments. temperature is consistent with the view derived from The consistent geometrical relationships between the field relationships that, at the present level of expo- elements of theF3 folds, the foliated early minor sure, conditions of large scale partial melting were not intrusions andthe weak cataclastic foliation in the reached at any stage of LowerPalaeozoic orogeny, Glen Kyllachy Granite indicate thatthe intrusions and that the bulk of the concordant migmatization in wereemplaced late in theF3 stress field, probably the Upper Findhorn area almost certainly dates back soon after the peak of folding. to Grenville time. In the rocks of the Central Highland The Glen Kyllachy Granite is an inhomogeneous, Division, further minor migmatization occurred be- composite body.It contains earlier units of more tween the F2 and F3 fold phases, and after F3 such distinctly foliated, coarse-grainedquartzose biotite- migmatization took the form of concordant to discor- granodiorite (Fig. 3-9, locally alternating with quartz- dant sheets, veinlets and pools of granodiorite associ- rich,leucocratic variants; and a later, coarsely- ated with the emplacement of the Glen Kyllachy Gra- porphyritic facies which is more weakly foliated (Fig. nite complex (Piasecki 1975, fig. 6). 3-4). The granite lacks sharply defined xenoliths, but contains rafts (‘BG’in Fig. 2) in which cores of coarsened, pelitic biotite gneisses pass outwards The Glen KyllachyGranite through zones of continuous transitions several metres complex wide, into medium-grained, biotite-rich granodiorites with bandednebulitic structures (Fig. 3-6, 3-7), and Previously, five granodioritic units were mapped in the then into the coarse quartzose granodiorite. Ill-defined Upper Findhorn region (G1 to G5in fig. 7 of Piasecki inhomogeneous zones and ‘schlieren’ of such nebulitic, 1975) and all were thought to represent the stages of biotite rich granodiorite types resembling ‘granitized’ emplacement of theTomatin (Findhorn) Granite. pelitic gneisses are abundant throughout the granite. Additional fieldwork has led to the separation of this The early minor intrusions of foliated granodiorites granite into two distinct assemblages-the Glen Kyl- form sharp contacts with the host metasediments, but lachy Granite complex and the Findhorn Granite com- also containinternal zones which are nebulitic and plex. which appear to be transitional into the thinly-banded The Glen Kyllachy Granite complex comprises an psammitic rocks of the Central Highland Division. early suite of cross-foliated minor intrusions of grano- Zircon populations from the Glen Kyllachy Granite diorite and related pegmatites, which were followed by and the minorfoliated granodiorites resemble the the emplacement of a weakly-foliated granodiorite of stubby, rounded zircons in the host psammites, except some 8 km2 extent (the Glen Kyllachy Granite) associ- for the more common presence of overgrowths; and ated with cross-cutting aplitesand pegmatites. Later their morphology contrasts sharply with the elongate, members of the complex areminor bodies of fine- euhedral zircons in the granites of the later Findhorn grained, unfoliated granodiorite and pegmatite. Granite complex. The field relationships described The earlyminor intrusions are dykes,sheets and above, combined with the zircon morphology, indicate small elongate stocks of fine to medium-grained, foli- a probable derivation of the granodioritic melts in ated biotite granodiorite. These are closely associated Glen Kyllachy from the metasediments of the Central with parallel-walled, muscovite- and garnet-bearing Highland Division, at some depth below the present pegmatite veins. The emplacement of all these minor surface. intrusions appears to have been related to the geom- The Findhorn Granite complex (Fig. 2), which suc- etry of thenear upright ‘F3’ folds in the metasedi- ceeded the Glen Kyllachy Granite complex, is com- ments (Figs 3-1-3-3, and stereogram in Fig. 2). Thus, posed mainly of biotite granodiorite. This was in- the pegmatite veins and most of the granodiorite dykes truded soon after some smaller bodies of associated were emplaced along planes which were near normal biotite-hornblendegranodiorites with dioritic and to the hinge lines of the F3 folds. Other minor bodies appiniticvariants. All the rocks of this complex are of granodiorite were emplaced subparallel to F3 axial xenolithic, flow foliated and post-tectonic. surfaces. All these early minor intrusions truncate the All the graniticintrusions of the Upper Findhorn F3 foldswithout themselves being folded, but they are region wereemplaced within the broad zone of the cross-foliated subparallel to the axial surfaces of the F3 GrampianSteep Belt and have north-easterlya folds. elongation. This steep belt may have acted as a zone The Glen Kyllachy Granite also has a crude folia- of weakness along which repeated phases of granitic tion. It is defined by abundant, weakly augen-shaped magma werefunnelled. Onthe larger scale, it may ‘studs’ of highly strained quartz (Fig. 3-4), which in have controlled the rise of magmas on both sides of somezones have been granulated and recrystallized the Great Glen Fault, from the complexes of Etive

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/1/47/4888051/gsjgs.140.1.0047.pdf by guest on 23 September 2021 The Glen Kyllachy GraniteCaledonian andOrogeny the 53 and Glen Coe in the SW, with their associated NE- analysed by the K-Ar method (D. R. Lux, using the trendingmicrodiorite dykes, to beyond the Cluanie facilities at East Kilbride set up by R. M. Macintyre). Granite N of the Great Glen Fault (Fig. 1). Sample locations are marked on Fig. 2. The Rb-Sr analytical data are given in Table 1, K-Ar in Table 2, and the isotopic ratios are plotted in Fig. 4. Results of current Rb-Sr From the late-F3pegmatites, large muscovite books, isotope work some 0.5 cm thick and 2-3cm wide, were taken and their outer rims of c. 1 mm were removed. The ages of Rb-Srisotope dilution analyses follow the analytical the muscovite books range from 446 to 422 f 5 Ma, methods of Blaxland et al. (1978). The average s7Sr/ with a strong clustering of dates at 44-42 Ma (Table “Sr valuefor NBS 987 standardSrC03 during the 1). runshas been 0.71028 f 0.00003. Seven analyses of Eight whole-rock samples of the Glen Kyllachy Gra- NBS 607 potassium feldspar standard yielded average nite were collected in an area 200 m across, near the 87Rb/86Srand s7Sr/s6Srvalues of 24.21 f 0.22 (20) and northwestern margin of the intrusion (Fig. 2). These 1.2006 f 0.0012 (24, respectively. Twenty-three included c. 25 kg samples of the quartzose and por- analyses of an internal laboratory rock powder stan- phyritic granodiorites and three c. 10 kg samples from dard have, over the past eight years, yielded an analy- the associated cross-cutting aplites. The sample of tical uncertainty in the RbiSr ratio of *1.0% (20). All leucocratic granodiorite came from the centre of the ages reported or quoted in this study have been calcu- intrusion. The RbiSr ratios for these main granodiorite lated with the decay constants as recommended by types, excluding the aplites, range from 0.153 to 0.217 Steiger & Jager (1977). Two-sigma uncertainties are (Table 1) and the scatter of data points is too large to quoted throughout. allow a meaningful age calculation. A regression cal- The following types of materials, listed in a decreas- culation for the data points including the aplites yields ing order of age as deduced from their field rela- an age of 443 Ma and an a priori error of 4 Ma (York tionships, were collected and analysed: 1969). Atthe level of precision quoted,there is, (a) psammitic and pelitic host metasediments of the however,large a amount of geological scatter Central Highland Division; (MSWD = 112) and the ‘scatter’ error is f40 Ma. The (b) muscovite books from the late-F3 pegmatites; initial s7Sr/s6Sr is 0.7176 * 0.0004 (scatter error). (c) whole-rock samples fromthe early foliated grano- Three psammites and three semipelites from c. 5 kg diorites (from the collection of Dr A. G. Fraser): samples of the adjacent metasediments of the Central (d) whole-rock samplesfrom the Glen Kyllachy Highland Division yielded data points which plot Graniteand grains of its primary muscovite and mainly above the Glen Kyllachy ‘errochron’ (Fig. 4), biotite; with Rb/Sr ratios intermediate between those of the (e) whole-rock samples from aplites associated with main granite and the later aplites of the same suite. the Glen Kyllachy Granite; The pelites have somewhat higher Rb/Sr and g7Sr/86Sr (f) whole-rock samples,and grains of primary ratios than the psammites. biotite andhornblende from coarse a biotite- Samples of the minor intrusions of foliated grano- hornblende granodioritefrom the Findhorn Granite diorites (2K2 and 22K1) plot between the data points complex in GlenMazeran. The hornblendeswere of the metasediments and of the Glen Kyllachy Gra-

~~ FIG. 3. Pegmatites and granodiorites of the Glen Kyllachy suite. Scale, where shown by a match: the lengths of the match and of the match-head are 3.5 cm and 4 mm, respectively. Late syn-F3 pegmatites. 3-1: Typical fabric of afoliated pegmatite. The foliation in thepegmatite (S3) is continuous with the composite foliation in the psammite (top), here transposed into parallelism with S3 (c5 fig. Sa in Piasecki 1975). Locality near CF4 in Fig. 2. 3-2: A foliated pegmatite (one of a conjugate pair) cuts obliquely across F3 folds. Foliation in the pegmatite rerams continuity with the axial surface foliation (S3) of the folds. Locality: CF4 in Fig. 2. 3-3: Typical pegmatite vein normal to the hinges to the lineation L3 (match on left), and to the axial surface foliation (S3) of F3 folds. S3 continues through the pegmatite (its trace shown by match on the pegmatite). Locality near CF4 in Fig. 2. Granodiorites. 3-4: Porphyritic microcline granodiorite. Large microcline in a coarse matrix of oligoclase (white) and weakly augened ‘studs’ of large, strained grains of quartz (grey, as in front of match-head). The weak overall foliation,parallel tothe matchstick, is subparallel to S3 in themetasediments. Locality GKlO in Fig. 2. 3-5: Quartzose granodiorite. Photomicrograph showing part of a stringer of folded micas (centre right: muscovite with bubble, and biotite below), separating areas of granulated-polygonized quartz. A microcline is present at top left. The granular aggregates of quartz form large, augen shaped ‘studs’ in the more deformed zones of the granodiorite. at the margins of which transitions into the less well augened, strained quartz grains illustrated in 3-4 are well developed. In hand specimen, the less deformed quartzose granodiorites resemble the matrix of the rock in 3-4. Locality: top of Carn Oighreagan (Fig. 2). 3-6: Inhomogeneous, quartzose granodiorite of nebulitic aspect, with bands rich in biotite, transitional into nearby biotite gneiss. Locality: W flank of “BG” in the bank of the main river. 3-7: Close-up of a region close to the area of 3-6.

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Rb Sr RblSr 87Rb186Sr 87SrI"Sr Apparent Sample @pm) @pm) weight ratio atomic ratio atomic ratio age (Ma) Central Highland Division Psammites CF 2K5 whole-rock 93.3 300 0.311 0.9013 0.72439 f 6 CF 22.1 whole-rock 98.8 365 0.270 0.7832 0.72013 f 4 CF 33K1 whole-rock 97.5 316 0.308 0.8925 0.72404 f 4 Pelites CF 33K3 whole-rock 134 341 0.392 1.136 0.72779 f 5 CF 33K4 whole-rock 139 318 0.436 1.263 0.72749 f 4 CF 35C whole-rock 152 328 0.463 1.343 0.72775 f 6

Glen Kyllachy Complex Pegmatite veins Ri = 0.7176 f 4 CF 4 muscovite 429 10.3 41.7 130.5 1.5390 f 1 442 f 5 253 A1 muscovite 346 8.80 39.4 122.4 1.4711 f 2 433 * 5 253 A2 muscovite 342 8.74 39. l 121.8 1.4854443 f 6 f 5 253 A3 muscovite 357 10.0 35.7 110.2 1.3794422 f 10 * 5 253 A4 muscovite 344 8.88 38.7 120.6 1.4803 f 3 444 f 5 253 B1 muscovite 344 8.75 39.3 122.4 1.4949446 f 4 f 5 Granodiorite dykes CF 2K2 whole-rock 85.0 362 0.235 0.6807 0.72214 f 12 CF 22K1 whole-rock 85.7 314 0.273 0.7919 0.72453 f 4 Quartzose granodiorites GK5 whole-rock 62.7 409 0.153 0.4439 0.72050 f 4 GK6 whole-rock 61.8 362 0.171 0.4947 0.72110 f 3 GK6 biotite 440 12.2 36.1 111.0 1.3488 f 2 399 f 4 Leucocratic granodiorites GK14 whole-rock 65.9 324 0.203 0.5887 0.72114 f 5 GK14 muscovite 297 67.8 4.37 12.76 0.79578 f 9 431 f 5 GK14 biotite 264 12.6 20.9 62.69 1.0867 f 3 413 f 5 Porphyritic microcline granodiorites GK7 whole-rock 74.5 343 0.217 0.6286 0.72120 f 2 GKlO whole-rock 75.1 399 0.188 0.5452 0.72091 f 3 Aplites GK12A whole-rock 87.6 100 0.874 2.536 0.73378 f 3 88.3 101 0.878 2.546 0.73375 f 7 GK12 whole-rock 75.8 138 0.549 1.593 0.72778 f 3 GK13 whole-rock 78.6 116 0.679 1.968 0.72986 f 4

Findhorn Complex Biotite-hornblende granodiorites 429 311 whole-rock 85.2 1170 0.0729 0.2108 0.70739 f 5 429 311 biotite 308 18.9 16.3 48.52 0.99237 f 16 414 f 5 429 312 whole-rock 64.8 999 0.0648 0.1876 0.70649 f 5 429 312 biotite 289 28.9 10.0 29.41 0.87856 413f 14 f 5

TABLE2: K-Ar isotopic analyses (Findhorn Complex)

Sample 40Ar* X 10-' (scclg) 40Ar*140ArTo,al(%)

429 3/1 hornblende84.0 1.394 0.787 403 f 9 429 312 hornblende 0.622 79.9 1.119 412 f 9

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0.7207/ -

Ordovlcian

/ gmnltesAberdeen Of and / 4 1 t ,' Banffshire

Mantle values i: 0.70 ""'"""""""'""'05 1.0 1.5 2.0 2.5 FIG. 4. Rb-Sr isotope ratio plot. Granitic rocks, circles; pelites and psammites from Glen Kyllachy, triangles and inverted triangles, respectively; semi-pelites from Glenfinnan, squares; psammite from Loch Quoich, diamond. The dashed line labelled 1030 Ma is the lower extension of the Ardgour granitic gneiss isochron of Brook et al. (1976) which is discussed in the text

nite, lying slightly above the isochron for the aplites staurolite in regional metamorphicterrains. This and granodiorites of the latter body (Fig. 4). Substan- temperature may be close to 550°C (Richardson 1968; tially lower Rb/Sr and 87Sr/86Srratios were obtained Hoschek 1969). For large muscovite books, the block- forthe samples of the youngerbiotite-hornblende ing temperatures may be even higher, as Wells (1979) granodiorite from the Findhorn Complex. reported 'maximum' calc-silicate temperatures of 60C A 431 f 5 Ma age was obtained from a medium- 640°C nearto the area from which Piasecki & van grainedprimary muscovite fromthe Glen Kyllachy Breemen (1979~)reported PrecambrianRb-Sr ages Granite (GK 14 in Table l), which is in good agree- from muscovites in theGrampian Slide. As argued ment with a 433 Ma muscovite age reported for the earlier, these temperatures, obtainedfrom calc- same granite body by Halliday et al. (1979, their silicates in theGrampian Division, almost certainly sample numberRC 722). The biotite ages of relate to the Lower Palaeozoic Grampian event and 413 f 5 Ma reported here fromboth the Glen Kyl- apparently have not reset the Rb-Sr muscovite clocks. lachy and the Findhorncomplexes (Table 1) areslight- In theUpper Findhornregion, Wells (1979) re- ly older than the 400 Ma biotite age obtained by the corded a lower, maximum calc-silicate temperature of above authors. 550°C. Hence, the concentration of Rb-Sr muscovite Within the biotite-hornblendegranodiorite of the ages at446442 f 5 Ma obtained from the large Findhorn Granite complex, our Rb-Sr biotite ages of muscovite books in the Glen Kyllachy pegmatites, is 414 and 413 f 5 Maare inagreement with K-Ar interpreted in terms of dating the pegmatite emplace- hornblende ages of 403 and 412 f 9 Ma (Table 2). ment duringa late stage of F3 deformation, i.e. at 444 f 4 Ma. The downward scatter of the muscovite Interpretation of age ages may berelated tothe partialresetting by the thermal effects of the large volume of the later mag- According to Purdy & Jager (1976) the blocking mas of the Findhorn Granite complex. Such more temperature of the Rb-Sr isotopic system in muscovite localized, thermal effects could also explain the is close to the temperature of the first appearance of c. 430 Ma ages for the fine-grained muscovites in the

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/1/47/4888051/gsjgs.140.1.0047.pdf by guest on 23 September 2021 56 0. van Breemen and M. A. J. Piasecki Glen Kyllachy Granite (Table 1; see also Halliday et the Moy Granite). Though the upper intersection age al. 1979). The Rb-Sr muscovite ages for the pegmatites is in the range 1500-1600 Ma, this may be an apparent andfor the granite, and their respective upperand age masking more than one event, as the distribution lower uncertainties provide limits which are consider- of the U-Pb zircon data points is entirely consistent ably tighter thanthe whole-rock scatter error of with aderivation from the Grenville psammites (cf. f40 Ma, i.e., the Glen Kyllachy Granite must have Aftalion & van Breemen 1980). beenemplaced 443-f:5 Ma ago. The Rb-Srbiotite A similar pattern is revealed by the major element ages for this granite, in the range 413-400 Ma chemistry (unpublished data of Dr A. G. Fraser). The (Table l), indicate the timeat which regional corundum normative, early foliated granodiorites and temperatures had subsided to c. 300°C (Purdy & Jager the Glen Kyllachy Granite are, chemically, extremely 1976), though the downward scatter to 400 Ma may uniform, with Si02 contents in the range 70-75% and also reflect more local thermal events. the molecular ratio A1203/Ca0 + K20 + Na20 in the For the Findhorn Granite complex, the concordance range of 1.02-1.09. Almost identical values are found of Rb-Sr biotite and of K-Ar biotite and hornblende in the psammites and themigmatized psammites of the ages obtainedfrom the biotite-hornblendegrano- Central Highland Division. Only in the pelites do the diorite (Tables 1 & 2) are significant: the estimated Si02 values concentrate at around60%, while the blocking temperatures of Rb-Sr and K-Ar in biotite above ratio ranges from 1.3 to 1.5. The hypothesis of (Purdy & Jager 1976) and of Ar inferropargasitic crustal melting doesnot, however, seem to be sup- hornblende end-members(O'Nions et al. 1969) are ported by the Rb/Sr (Table 1) and Rb/K ratios in the c. 300°C; whereas in themore common pargasitic rocks, which are reduced in the granodiorites relative hornblendes it is c. 500°C (O'Nions et al. 1969; Gerl- to the surrounding metasediments (i.e. from c. 0.3 to ing et al.1965). Where these systems reflect post- 0.2 and from to &, respectively). This anomaly can metamorphic cooling, the K-Ar hornblende ages nor- be explained, on the reasonable assumption that the mally pre-date biotite ages: for instance, for the Glen- metasediments of the Central Highland Division had Dessary syenite situated within the steep belt N of the been depleted inlithophile elements by Grenville Great Glen Fault (Fig. l), this discordance is in the anatexis more extensively at depth, corresponding order of 18 Ma (van Breemen et al. 1979~).The with the sourceregions of subsequentCaledonian hornblendes in our samples have ratio a magmas, than at the level of the present exposure. 100[Mg2++Fe2++AI(V')]/[Mg2++Fe3C+AI(V')+Fe2+]There is, as yet, little Rb-Sr data available for the of 0.66 (average of four determinations from the same Central Highland Division. However, inveiw of the locality by Dr A. G. Fraser),and hence lie in the high evidence for a c. 160 km or greater sinistral displace- blocking temperature range of O'Nions et al. (1969). ment on the Great Glen Fault, the Central Highland Thus, the concordance of mineral ages is interpreted Division can be taken as an extension of the Grenville in terms of dating the emplacement of the Findhorn complex represented by the Glenfinnan Division (Fig. Granite complex at c. 413 f 5 Ma into rocks already 1; Piasecki et al. 1981). Four outcrops of Glenfinnan cooled substantially below 400°C. Division metasediments from near Glenfinnan (Fig. 1) have been extensively sampled and analysed for their averageRb-Sr isotopic composition (Aftalion & van Origin of the magmas Breemen 1980). Theseare plotted in Fig. 4 as four representative data points, while unpublished data The high initial 87Sr/86Srratio of 0.7176 f 4 for the froma 1 mthin slab traverse in the Glenfinnan main types of the Glen Kyllachy Granite (close to the metasediments at Loch Quoich yields the fifth point. value for the surrounding rocks at that time) suggests a Also plotted is the lower extension of the derivation at a high level in the crust. Such a deriva- 1030 50 Ma whole-rock isochron from the Ardgour tion is entirely consistent with the transitional migma- granitic gneiss at Glenfinnan, with Rb/Sr ratios in the tite-granite temporal relationship observed in the field range 0.83 to 1.47 (Brook et al. 1976). It is apparent which also suggests melting some distance below the from Fig. 4 that the Glen Kyllachy Granite data points presently-exposed surface (Piasecki 1975). Other sup- lie at the lower end of a general alignment of Grenville porting evidence comes from the scatter of data points paragneisses. This relationship supports the indepen- which is compatible with incomplete homogenization dent field, geochemical and isotopic evidence that the during ascent. Glen Kyllachy Granite received amajor input from Crustal melting is further supported by zircon mor- metasediments of theCentral Highland Division, phology (this study) and by the U-Pb isotope data of which may have been partially depleted in Rb during Pidgeon & Aftalion (1978). Zircons in the Glen Kyl- an earlier orogeny, at a somewhat lower tectonic level. lachy Granite contain the highest proportion of inher- In contrast, the initial 87Sr/R6Srratios of c. 0.706 for ited lead of any of theLate Caledoniangranites the more voluminous magmas of the Findhorn Granite recorded (see fig. 10 in Pidgeon & Aftalion 1978, in complex indicate substantially deeper source regions which their sample RC 722 was mistakenly assigned to (cf. van Breemen & Bluck 1981).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/1/47/4888051/gsjgs.140.1.0047.pdf by guest on 23 September 2021 The Glen Kyllachy Granite and the Caledonian Orogeny 57 Tectonic diachronism across event of late Ordovician age. Instead, this zone had the Caledonian trend undergone nappe formation (Roberts 1974; Bradbury et al. 1979; Thomas 1979), great tectonic thickening (Wells & Richardson 1979) and intense metamorphism In the Moine outcrops, the pattern of distribution of (Bradbury et al. 1976) during the preceding Grampian post-Grenvillecover (theGrampian, Loch Eil and event.The preservation of Grampian metamorphic possibly theMorar Divisions of Fig. 1) and of the ages in this zone can be explained if this indurated pile Grenville basement(the Central Highland, the responded largely by uplift during the c. 455 Ma Glenfinnan Divisions and probably the Sutherland Caradocian event, withoutintensetectono- Migmatite Complex), and the apparent northerly con- metamorphic activity. tinuation of the Grampian Steep Belt within this base- The Caledonian events in the N Highlands and NW ment (Piasecki et al. 1981), substantiates the sugges- Grampians are possibly best interpreted in terms of a tion that at some stage after the Grampian Orogeny stress release related to the island arc collision along theNorthern and the Grampian Highlandregions the Highland Boundary Fault and directed towardsthe were displaced by 160 km or more (Fig. 1; Winchester NW foreland along a series of slides, with associated 1973; Storetvedt 1974). Prior to the present study, the folding and metamorphism.N of theGreat Glen case could be argued for major tectonic and metamor- Fault, the movements on the Sgurr Beag Slide which phic diachronismacross theGreat Glen Fault, to placed the rocks of the Glenfinnan Division over those explain the delay between the peaks of metamorphism of theMorar Division are thought to be associated anddeformation in Northernthe Highlands with the F2Caledonian (sensu luto) phase(Tanner (c. 455 Ma) relative to Aberdeenshire (S490 Ma) and 1971; Powell 1974). In the Sutherland Migmatite Com- the Ben Vuirich region (c. 514-491 Ma, Pankhurst & plex, the age of the movements on the Swordly Slide Pidgeon 1976; Bradbury et al. 1976). However, such a (Fig. l), a major dislocation also associated with ‘F2’ hypothesis is not supported by the presence of coeval, folding, has recently been obtained at c. 450 Ma, by c. 445 Ma old pegmatites associated with the last dating large muscovite books in pegmatites syntectonic phase of widespread Caledonian folding (‘F3’ in Table with the sliding (unpublished results in collaboration 3) on both sides of theGreat Glen Fault-in the with Drs V. E. and S. J. Moorhouse). S of the Great Glenfinnanregion (van Breemen et al. 1974) and in GlenFault, Palaeozoicmovements along the older Glen Kyllachy (this work). Furthermore, the relation Grampian Slide have also been documented (Piasecki between this folding and the late discordant migma- & van Breemen 1979a; Piasecki 1980). tization in the Glen Kyllachy region indicates that the The abovehypothesis appearsto depend on a lastwidespread deformation occurredduring down- substantial NE continuation of the Midland Valley and temperature conditions (table 2in Piasecki 1975);a the Highland Boundary Fault. However, the marginal relationship similar to that established in the Glenfin- basin closure along the Highland Boundary Fault may nan region (Dalziel & Brown 1965; van Breemen et al. not have been the direct cause of compression in the N 1979a). Highlands and of uplift in the Grampian Highlands. The earlier,intense folding (‘F2’ in Table 3), sliding, The same effects could have resulted from a shallow- and associated high grade metamorphic activity N of ing of the subduction zone during the late stages of the the Great Glen Fault, has been dated at 455 k 5 Ma closure of Iapetus (van Breemen & Bluck 198l), a (van Breemen et al. 1979~).S of the fault, a multiple phenomenon which can account for the continuing late fold sequence has been described from the region of Ordovician to Silurian northwesterly compression (cf. Kincraig (Fig. 1; Piasecki 1980, p. 53). In this sequ- Molnar & Atwater 1978). The evidence ‘ presented ence, the F3-F5 structures probably relate to Gram- substantiates further the structural unity between the pian events, and the FGF7 to theCaledonian (unpub- Northernand Central Highlandsprior to a sinistral lished work). As the latter correspond with the ‘F2’ shift along the Great Glen Fault (Fig. 1) and extends and ‘F3’ structures in Glen Kyllachy, it seems that the this unity to c. 440 Ma. Differences in theplutonic ‘F2’ folding in theUpper Findhorn region may be history of the opposing segments (below and Table 3) broadly coeval with the eventN of theGreat Glen suggest thatthe two regionswere not brought into Fault dated at 455 f 5 Ma. The new data are consis- their present configuration until substantially later. tent with a hypothesis of synchronous c. 46W40 Ma (Caledonian)tectono-metamorphic evolution along the now displaced axial zone of the orogen. Caledonian plutonic and tectonic The initiation of Caledonian tectonism in this axial history ‘Nof the Great Glen zoneappears to have coincided with aCaradocian Fault closure of a marginal basin along the Highland Bound- ary Fault (Longman et al. 1979). Yet, in the zone of Evidencefrom theNorthern Highlands suggests a the main Dalradian outcrop bordering this fault, no continuum of tectonic activity from the ‘D2’ deforma- evidence has been found for an intense tectono-thermal tion at c. 455 Ma to the latest movements along the

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/1/47/4888051/gsjgs.140.1.0047.pdf by guest on 23 September 2021 58 0. van Breemen and M. A. J. Piasecki TABLE3. Schematic chronology of plutonism, tectonism and related metamorphism in the Scottish Highlands

NORTH OF GREATGLEN FAULT SOUTH OF GREAT GLEN FAULT

At, CENTRAL REGION GRAMPIAN STEEP BELT IAPETUS MARGlh WESTERN& FORELANO REGION [ I [ (s:E8EspEt[LT[ I 1 11 I I

N-S cornpresslan LARGE I MINETTE U 2 FINDHORN GRANITEGRANITES b 0 SUITE 0 0 m “

ROSS OF MULLGRANITE NW-SE cOrnpreSS’On building late mmments L no Of { MainOYhrust recorded accretlanary -4 prlsrn ,“ W piutonlsrn STRONTIANGRANITE 16 In the MICRODIORITES, FELSIC PORPHYRITES Southern ,o CLUANIE GRANITE I Uplands c l m

I GLEN KYLLACHYGRANITE n I PEGMATITES rnorgmai last c tmsm closure , ductile l c I’E3’widespreadfolds 3 thrusts &!I along the X c 3 .- I Hlghlond

Boundary L SMALL Y SYENITE Fault 2 I I0 GRANITES U) _I;60 Moine Thrust, and there also appears to be no gap in Highlands, a close study of the Rogart Complex (Fig. the plutonichistory. Table 3 compares the plutonic 1; Soper 1963) suggests that further N there was also and tectonic events across the Great Glen Fault; for no hiatusbetween deformation, metamorphismand northern events, it draws on Smith’s (1979) study of magmatism (see also Soper & Wilkinson 1975). minorintrusions, many of which aredated reliably Both Soper & Brown (1965) and Elliott & Johnson and provide useful benchmarks in the post-Grampian (1980) related the westward directed (F4) movements development of the region. The Glen Dessary syenite along the Moine Thrustto aphase of rapid uplift (Fig. 1)dated at 456 f 5 Ma (van Breemen et al. ending in Old Red Sandstone times and pointed out 1979a) appears tocoincide with or slightly pre-date the that thesethrusts need not necessarily have been phase of intense ‘F2’ folding.This syenite was fol- driven by a lateral stress field. Late thrusting move- lowed by a suite of parallel-walled, muscovite-biotite ments, possibly continuing into Old Red Sandstone pegmatites which range in age from post42 to syn-F3, times, have been shown to post-date the emplacement the latter dated at 442 f 7 Ma; and to post-F3, dated of the early sheet of the alkaline Loch Borrolan com- at 434 f 4 Ma (van Breemen et al. 1974, 1979~). The plex at 430 f 3 Ma (van Breemen et al. 1979b; Elliott post-F3pegmatites pre-date asuite of NE-trending & Johnson 1980). However, 200 km further S, the felsic intrusionsassociated with the Cluanie granite, metamorphic aureole of the Ross of Mull Granite followed in turn by asuite of NE-trending micro- (Fig. 1) appears on both sides of an extension of the diorites andthe Strontian complex which hasbeen Moine Thrust (Clough in Cunningham Craig et al. dated at 435-430 Ma (Pidgeon & Aftalion 1978; Halli- 1911; Jehu 1922) and this calc-alkaline intrusion has dayet al. 1979). The majority of the microdiorite been dated at 420 f 4 Ma (Beckinsale & Obradovich dykes dip 35-40’ tothe SE, show evidencefor W- 1973). Thus,the more significant brittle movements lateral shearing and indicate stacking to the W (Smith are likely to haveoccurred during the 430-420 Ma 1979). The amphibolite facies mineral parageneses in interval and may be related to the uninterrupted plu- the microdiorite dykes are consistent with radiometric tonism which, with its associated high heat flux and evidence that intermediate temperature metamorphic ductility at depth, may have led to the detachment of conditions were maintained until c. 430 Ma. In addi- overthrust sheets (Armstrong & Dick 1974). The tion to this evidence from the southernregion of the N WNW-directed thrusting and tectonic burial provides

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/1/47/4888051/gsjgs.140.1.0047.pdf by guest on 23 September 2021 The Glen KyllachyGranite andCaledonian the Orogeny 59 a plausible explanation for the younger cooling ages in from the Midland Valley and the Grampians along the the N Highlands which range down to c. 400 Ma (De- HighlandBoundary Fault, are related to subduction wey & Pankhurst 1970). (Longman et al. 1979; van Breemen & Bluck 198l), According to Smith (1979), the whole region had then the time constant of subduction (Vlaar & Wortel stabilised before the intrusion of asuite of minette 1976) orthe depth of primary melting (Green & dykes,many of which arecentred on theRatagan Ringwood 1968) may have been significant, or the intrusion of mixed alkaline and calc-alkaline affinities. angle of subduction may have become less steep with A single K-Ar analysis onone such dyke from the time(Coney & Reynolds 1977). In our opinion, the Ross of Mull has yielded an age of 406 f 10 Ma above processes were not mutually exclusive, but con- (Beckinsale & Obradovich 1973), though clearly furth- tributed in varying, unquantifiable degrees to the west- er work is required. Most of the dykes of this suite ward migration ot crustal melting. trend WNW and have been displaced sinistrally by In the Grampian Steep Belt, the c. 30 Ma time gap NE-trending faults such as the Strathglass and Strath- between the emplacement of the Glen Kyllachy Gra- connon faults, which are probably related to the Great niteand of themore voluminous magmas of the Glen shear system (Smith 1979; Johnson & Frost Findhorn Granite complex (Table 3) is likely to be of 1977). In both instances, however, the dykes contain regional significance. Thistime gap provides further inclusions of, or cut rocks fractured by earlier move- support for the hypothesis of van Breemen & Bluck ments on the same faults. Thus, by late Silurian-early (1981) thatthe late Ordovicianepisode of granite Devonian time, a new regional stress regime had been emplacement was followed by aphase of magmatic established, which appearsto supersede the calc- quiescence which coincided with the development of alkaline plutonism N of the Great Glen Fault. an accretionary prism in the Southern Uplands. The above authors suggested thatafter Wenlock time, hydrous material was again subducted but in a higher temperature/shallow subduction regime which initiated Caledonian plutonic and the late Silurian-Devonian phase of voluminous calc- tectonic history S of alkaline magmas with low to intermediate R7Sr/x6Sr the Great Glen Fault ratios. Rb-Sr whole-rock data on the large Lochnagar and Though the late-F3 pegmatites of Glen Kyllachy can Hill of Fare granites yield reliable ages of 415 f 3 Ma be linked with similar pegmatites N of the Great Glen and 413 f 3 Ma respectively (Halliday et al. 1979), Fault,the latter are not associated with two mica while the Cairngormgranite has yielded an Rb-Sr granites.Small, two mica,corundum-normative gra- whole-rock isochron age of 406 f 2 Ma (M. Brook & nitesassociated with muscovite-biotite-tourmaline R. J. Pankhurst, quoted by Plant et al. 1980): these pegmatites occur, however, in the Dalradian outcrop ages correspond to the age of the Findhorn Granite. of NE Scotland. The intrusions are entirely post- Other intrusions near the southwestern extension of tectonic, but are somewhat older than the c. 443 Ma theGrampian Steep Belt, such as Etive, Rannoch Glen Kyllachy Granite. The Kennethmont and Aber- Moor, Glen Coe and Ben Nevis can be shown to be chirdergranites have yielded Rb-Sr whole-rock iso- Devonian in age onthe basis of stratigraphicaland chrons of 453 f 4 Ma, and 444 f 9 Ma, respectively complementary U-Pb isotopicevidence (Pidgeon & (Pankhurst 1974). The Strichen Granitehas yielded Aftalion 1978). Granites of similar age occur in the a U-Pb monazite age of 475 f 5 Ma (Pidgeon & Afta- Midland Valley and the Southern Uplands, the latter lion 1978), and tourmaline-bearingpegmatites at being emplaced into the accretionary prism (Halliday Belhelvie andPortsoy have yielded Rb-Sr muscovite et al. 1980). As most of the large plutons have now ages of 463 f 5 Ma (van Breemen & Boyd 1972; M. been dated, further radiometric age work may at the P. Lap in et al., pers.comm.). The granites have most be expected to reveal minor plutonic activity in initial 'Sr/?3 ratiosin the region of 0.714-0.717 the range of 440-415 Ma (Table 3). which are similar to, or slightly lower than those of the The Highland region between the Great Glen Fault Glen Kyllachy Granite (Fig. 4). and the Highland Boundary Fault was the main area If one attributes the origin of the above granites to of a study by Johnson & Frost (1977), who demons- ultrametamorphism of tectonically-thickened crust tratedthe presence of a system of sinistral Riedel (e.g.Richardson & Powell 1976) thenthe c. 20 Ma shears related to the major sinistral movement along E-W time delay relative to the Glen Kyllachy Granite the Great GlenFault. Most of the major faults cut the can be interpreted in terms of the tectonic diachronism Lower Devoniangranites,. but one major fault and discussed above. Additional factors may be the higher some of the minor faults are cut by minor intrusions of (Buchan) heat flow in the NE and possibly the em- Lower Devonian age (Smith 1961). Other granites placement of theAberdeenshire Newer Gabbros at havebeen intruded along NE-SW faultlines, and 490 f 17 Ma (Pankhurst 1970). However, as it is like- related cross-faults are demonstrably older (Smith ly thatthe late Ordoviciangranites, including those 1961). Thus, Johnson & Frost (1977) concluded that

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/1/47/4888051/gsjgs.140.1.0047.pdf by guest on 23 September 2021 60 0.Breemenand van M. A. J. Piasecki this ‘sinistral pattern of faulting was established before plutonic hiatus in the Grampian region, with the peak the main granites’ now present S of the Great Glen of calc-alkaline plutonism N of the Great Glen Fault, Fault. and with thepeak of sedimentary accretion in the Southern Uplands (Table 3). If the subduction of wet Discussion oceanic sediments is essential tothe generation of calc-alkaline magmas (Gilluly 1973) then, during most The dating of the last,NNE-trending fold phase in of the Silurian, the Grampian and the N Highlands Glen Kyllachy at c. 445 Ma demonstratesthat the must have been sited over adjacent segments of the maindiachronism of Caledoniantectonism was not Iapetus subduction zone. possibly separated by a across the Great Glen Fault,but was between the NW transform fault. Even though both regions were still Grampian region and the zone of the main Dalradian facing the same Iapetus Ocean, no accretionary build- outcrop to the E and SE. As argued above, this late up is envisaged in front of the NHighlands. The Ordovician (Taconic?) tectonism can be linked to the 160 km or greaterreconstruction along the Great Glen Caradocian collision of the Grampian mainland with Fault (Fig. 1) postulatedfor the period preceding an island arc centred on the Midland Valley (Longman 440 Ma ago is therefore likely to have persisted until er al. 1979). The change from alatest Precambrian- near the end of the Silurian. LowerOrdovician island arcregime to aprolonged The evidence presented here is consistent with a late phase of late Ordovician-Silurian compression can be Silurian-Devonian sinistral shear regime which juxta- related to a continuous shallowing of the Benioff zone posed the opposingsegments along theGreat Glen asyounger andhotter ocean floor was consumed Fault(Johnson & Frost 1977). It coincided with the (Molnar & Atwater 1978; van Breemen & Bluck demise of plutonism tothe N, but the initiation of 1981). Just as the large Silurian-Devonian granites of calc-alkaline magmatism to the S (Smith 1961, 1979). the Scottish Highlands appear to have much in com- Assuming thatthe sinistral faulting associated with mon with Cordilleran batholiths (Brown 1977), so the N-S compression was established before the emplace- regime of continuousthrusting towards the foreland ment of the main granites in the Grampian Highlands, which continued long afterthe Caradocian collision the latter could be related to subduction from the S. along the Highland BoundaryFault, may also have Thishypothesis is consistent with the nearly E-W much in common with the prolongedcompressional alignment of many of the late Caledonian granites of tectonics of the Cordilleran belts (Molnar & Atwater the E Grampians (Fig. 1). E-W folding has, however, 1978). only been demonstrated for the period following the Thelate Ordovician to Silurian tectonism final closure of Iapetus in Middle Old Red Sandstone (F2, F3, F4 in Table 3) is distinct from the Grampian time (Johnson et al. 1979; Bluck 1980; Thirewall 1981). event with its symmetrical distribution of nappes Irrespective of the direction of the last phase of sub- (Roberts 1974; Thomas 1979). The two are separated duction S of the Great Glen Fault, it appears that the by a phase of tension and spreading which produced N Highland region was shunted behind the Grampian the Arenig ophiolites bounding both sides of the Mid- region-after the magmatism associated with WNW land Valley, and possibly by the emplacement of the subductionfrom a more northerlysector of Iapetus younger gabbros of Aberdeenshire (Bluck 1978; Long- had subsided. man er al. 1979; Bluck et al. 1980). The short-lived mid-Arenigmarginal basin spreading at Ballantrae ACKNOWLEDGMENTS.Professor B. E. Leake is thanked for which has been dated precisely at 483 f 4 Ma (Bluck critically reading the manuscript. The authors are grateful to et al. 1980) providesa natural ‘benchmark’ forthe Dr A. G. Fraserfor the samples of metasediments and minor separation of the Grampian and Caledonian orogenic intrusions, andfor helpful discussions. Mark Piasecki events. Though the existence of local tensional regim- gratefullyacknowledges assistance fromthe Department of es between theBallantrae spreadingevent and the Geology at Hull University, and during the 1981 season, collision along the Highland Boundary Fault cannot be support by NERC Grant No GR3/4519; and also the hospi- ruled out, the early part of the Caledonian Orogeny tality shown to him in the field by Major MacKenzie, Mr was characterized by the closing of marginal basins Philip MacKenzie, Mr and Mrs John Hendry, the family of andembraces the obduction atBallantrae, dated at Lord Elphinstone and Mr lan Watson. Mr John Hutchison and Mr Julian Jocelyn are thanked for their technical help 478 * 8 Ma (Bluck et al. 1980). and Professor H. W. Wilson for his interest and support. The The time interval between the emplacement of the Isotope Geology Unit at SURRC, where the isotopic work Glen Kyllachy Granite at c. 443 Ma and of the Find- was done, is financed by the Scottish Universities andthe horn Granite at 413 f 5 Ma, coincides with a general Natural Environment Research Council. References

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Received 18 January 1982. 0. VAN BREEMEN,Scottish Universities Research andReactor Centre, East Kilbride, Glasgow G75 OQU. Present address: Geological Survey of Canada, 601 Booth Street, Ottawa, Canada K1A OES. M. A. J. PIASECKI,Department of Geology, University of Hull,Hull, North Humberside HU6 7RX.

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