The Glen Kyllachy Granite and Its Bearing on the Nature of the Caledonian Orogeny in Scotland
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J. geol. Soc. London, 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 Scotland 0. van Breemen & M. A. J. Piasecki SUMMARY:The Tomatin (Findhorn) 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 Highland Boundary Fault. For Silurian events in the N Highlands we propose a 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-isotope ratios of the earlier Scottish Highlands 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/1/47/4888051/gsjgs.140.1.0047.pdf by guest on 23 September 2021 48 0. van Breemen and M. A. J. Piasecki 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 belt and 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 major structure 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 Caledonian orogenies (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 Lower Palaeozoic 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.