Tectonophysics 332 ,2001) 23±49 www.elsevier.com/locate/tecto Reconstruction of the Grampian episode in Scotland: its place in the Caledonian Orogeny Grahame J.H. Oliver Crustal Geodynamics Group, School of Geography and Geosciences, University of St. Andrews, North Haugh, St. Andrews, Scotland, KY16 9ST UK Abstract A study of the composition of detrital garnets from Ordovician siliciclastics from the Midland Valley and Southern Uplands terranes of Scotland reveals characteristics of the metamorphic sources very similar to the Grampian terrane plus a `lost' blueschist±eclogite terrane. The radiometric ages of detrital muscovite from the Southern Uplands overlaps the ages of the metamorphic muscovite from the Grampian terrane. The depositional age of the earliest Midland Valley detritus is Llanvirn; in the Southern Uplands it is Caradoc. These observations support the hypothesis that ¯ysch was formed when the Grampian terrane was suddenly uplifted into mountains which were immediately eroded into neighboring basins and trenches. Thus the Grampian, Midland Valley and Southern Uplands terranes were not exotic to each other in the Upper Ordovician. Therefore the main compressional orogeny ,i.e. Peri-Laurentian island arc versus Laurentian continental margin collision) in the Grampian terrane in Scotland was post-Cambrian and began in the late Arenig. New radiometric age dates for Grampian terrane syn- metamorphic granites and gabbros and post-metamorphic granites in Scotland and Ireland plus mineral cooling ages support the case for a relatively short ,15 Ma catastrophic arc-continent collisional orogeny between ,480 and ,465 Ma. Thus the Caledonian Orogeny is an example of one which was episodic with a short catastrophic collisional tectonometamorphic and magmatic ,Grampian) episode followed by a long period of isostatic adjustment, decompression melting, erosion, subduction and batholith formation that lasted 70 Ma. Orogeny stopped when Avalonia docked ,i.e. the Scottish Scandian episode). q 2001 Elsevier Science B.V. All rights reserved. Keywords: Caledonian; Grampian; orogeny 1. Introduction allows for a newly re®ned integrated geological model for the British Laurentian margin. The aim of this paper A review of old and new precise radiometric age dates is to con®rm that the Scottish Grampian episode ,i.e. and new fossil evidence, with reference to a more Orogeny cf. Lambert and McKerrow, 1976) was an precise Phanerozoic time scale1 ,Tucker and McKer- early collisional episode of the greater Caledonian row, 1995) combined with a new interpretation for the Orogeny ,cf. Suess, 1888) in Britain. origin of the Midland Valley and Southern Uplands The terms Caledonian and Grampian are now so Lower Palaeozoic ¯ysch ,Hutchison and Oliver, 1998) well entrenched in the literature it is worth reviewing their origin. Caledonia is the Roman name for the Scottish Highlands north of the Midland Valley E-mail address: [email protected] ,G.J.H. Oliver). ,Fig. 1). The adjective term Caledonian was ®rst 1 Note that the time scale adopted here follows the recommenda- tions of the Sub-commission on Ordovician Stratigraphy ,Williams, used geologically by Suess ,1888) to mean a region 1994) which subsumes the Llandeilo into the Llanvirn. and epoch of mountain building. He stated ,translation 0040-1951/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S0040-1951,00)00248-1 24 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 25 by Sollas, 1906): ªthese pre-Devonian mountains, different beds which were found in the Highlands which proceed from Norway and form the whole of were accumulated, these were ridged up so as to Scotland we call the `Caledonian mountains'¼the form the Grampian geanticlinal¼. The great central Caledonian mountains are continued through a great massive must have risen over what is now the Midland part of Ireland and Walesº. General usage has led Valley¼. The greater part of this massive has, of Caledonian Orogeny to mean `the area of north-west course, been carried down between the boundary Europe where thick sequences of late Pre-Cambrian faults¼ and lies concealed below the mantle of and early Palaeozoic sediments, were from Ordovi- younger Palaeozoic rocks¼º. Thus Macnair pre- cian to Devonian times, subjected to repeated phases empted Bailey's ,1922) Nappe theory and the contro- of earth movements leading to folding, regional meta- versy over whether or not Grampian rocks extend morphism and granite magmatism' ,Gary et al., south of the Highland Boundary Fault. 1972). Many early workers were impressed by the Macnair's geological use of the term Grampian was extent of the Lower Old Red Sandstone molasse in ignored until Dewey ,1969, 1971) distinguished a Britain ,presumed then to be of Lower Devonian age) distinctive Ordovician history in the Caledonian of and assumed that since this molasse was rich in western Ireland and Scotland and proposed a `Grampian igneous and metamorphic detritus then the main event'. His thesis was based on stratigraphic criteria: orogeny occurred during late Silurian, and early Durness shelf sedimentation was interrupted in the Devonian times ,Wills, 1929). Associated igneous early Llanvirn Ð this dates the start of regional tecto- intrusions, mostly granites, are often called `Caledo- nometamorphism; the ®rst appearance of coarse, high- nian' ,Hatch and Wells, 1937). grade metamorphic detritus of mid-Llanvirn age in the The Grampian Mountains are that part of Caledonia South Mayo Trough provides the younger age limit for that lie between the Midland Valley and the Great the tectonometamorphism. The maximum stratigraphic Glen ,Fig. 1). The name Grampian is derived from age constraint in Ireland is the presence of Cambrian graupius, the Latinised form of the Celtic word Protospongia in the Upper Dalradian of Clare Island grup, meaning bump. Mons Graupius was a battle ,Rushton and Phillips, 1973). Therefore, the Grampian ®eld most likely located around the granite hill fort event was post-Cambrian and pre-Caradoc. on Bennachie, 35 km west of Aberdeen, where in 83 A Grampian Orogeny ,ss) was then proposed by AD a Roman army commanded by Agricola, alleg- Lambert and McKerrow ,1976) as `¼ that set of edly defeated a Celtic army led by Calgacus processes which converted the Dalradian and related ,Maxwell, 1990). Scottish historians claim the result Arenig sediments into the metasedimentary fold belt was a stalemate: the carnage on both sides was so extending from the NW coast of Ireland to the NE great that the Romans and their mercenaries retreated coast of Scotland; it occurred during Arenig and Llan- south to their ships and the Celts dispersed into the virn time.' Lambert and McKerrow ,1976) used a Grampian Mountains ,Pitbaldo, 1935; Whittington, combination of fossil and radiometric evidence to pers. comm.). support this conclusion, e.g. the Upper Dalradian Macnair ,1908, p. 153) was perhaps the ®rst to use Leny Limestone with Pagetides is uncontroversially the term Grampian in a geological context: ªafter the Middle Cambrian while the Macduff Slates contain formation of the Grampian geosynclinal, in which the rather controversial ,see Harris, 1991) Arenig acritarchs Fig. 1. Map of Scotland showing ,inset) the distribution of Caledonian terranes, the bounding faults: GGF Great Glen Fault, HBF Highland Boundary Fault, SUF Southern Upland Fault, ISZ Iapetus Suture zone. A Aberdeen Granite, Kt Kennethmont Granite, St Strichen Granite, B Buchan, section dated by Dempster et al. ,1995). Main ®gure shows the distribution of metamorphic isograds and granites in the Grampian terrane. ,Bv Ben Vuirich Granite; the names of the 52 main Newer Siluro-Devonian granite masses in Scotland are given in Brown, 1991). Note the distribution of the Lower Palaeozoic Midland Valley Inliers, the position of the Ballantrae Igneous Complex and its cover in the Midland Valley terrane, and the occurrence of Siluro-Devonian volcanics. The Southern Uplands terrane shows the distribution of the Corsewall ,C), Kircolm ,K), Portpatrick ,P), Shinnel ,S), Gala ,G), Hawick ,H) and Riccarton ,R) groups and formations ,Stone, 1995) in tracts 1±10 ,Leggett et al., 1979), plus the main Siluro-Devonian granites and lavas. Note the Angus and Perthshire traverses dated by Dempster ,1985). 26 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 ,Downie et al., 1971): therefore the Grampian protracted affair that lasted ,200 Ma starting in the Orogeny was post-Arenig and pre-Llanvirn. Lambert lower Vendian. and McKerrow ,1976, p. 289) proposed that the `set of More recently there have been reports ca. Upper Ordovician to Middle Devonian events be 840±800 Ma dates for tectonometamorphism in the termed the Caledonian Orogeny', a concept that has Central Highland Division south of the Great Glen not been adopted. The term Caledonian Orogeny still Fault ,Noble, 1998). These dates con®rm the ideas has time ,Ordovician to Devonian) and place conno- of Piasecki ,1980a,b) about older tectonic cycles tations ,the British Isles, Scandinavian, E. Greenland, than the Caledonian in the Grampian terrane. Since Svaalbad, Northern Germany and Poland; Oliver et these events are older than the deposition of the Dalra- al., 1993). dian Supergroup they do not affect the hypothesis Coney et al. ,1980) popularised the terrane presented here. concept: i.e. tectonically bound terranes of appar- ently different tectonostratigraphic and meta- magmatic histories are to be considered