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 ˆ Great Glen Fault, HBF ˆ Highland Boundary Fault, SUF ˆ Southern Upland Fault, ISZ ˆ Iapetus Suture zone. A ˆ Aberdeen Granite, Kt ˆ Kennethmont Granite, St ˆ Strichen Granite, B ˆ Buchan, section dated by Dempster et al. ,1995). Main ®gure shows the distribution of metamorphic isograds and granites in the Grampian terrane. ,Bv ˆ 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 suspect 2. Signi®cance of syntectonic turbidites ¯ysch) and allochthonous with respect to each other until proven otherwise. Accordingly, Gibbons and Gayer Studies in presently active mountain belts such as the ,1985) named various British Caledonian terranes Himalayas show that recent ¯ysch-type sediments including the Grampian terrane which in Scotland ,syntectonic turbidites) in the foreland basins and fore- is the area of the Grampian Mountains between arc accretionary complexes contain metamorphic and two boundary faults: the Great Glen Fault to the igneous muscovite and K±feldspar detritus with north west and the Highland Boundary Fault to the 40Ar/39Ar radiometric age dates nearly the same as the south east ,Fig. 1). The metasediments and metavol- age of the sediments they lie in ,Copeland and Harrison, canics of the Grampian terrane make up the Dalra- 1990). In other words, metamorphic and igneous rocks dian Supergroup ,Harris and Pitcher, 1975). from within the roots of a orogenic belt formed at a Dempster ,1985) and Dempster et al. ,1995) have destructive plate margin have been crystallised, cooled, carried out a comprehensive mineral dating exhumed, eroded, transported and deposited in neigh- programme in the Grampian terrane. On the basis boring ¯ysch basins within periods of less than ,5Ma. of Rb±Sr whole rock phyllite and metamorphic Garnet grade metamorphic rocks along the muscovite ages ,,515 Ma) they proposed that the Alpine Fault in New Zealand have K±Ar muscovite peak of Caledonian metamorphism was Middle ages of less than 4 Ma and are presently being Cambrian. This agreed with previous Cambrian K± eroded into the Tasman Sea ,Shepherd et al., Ar whole rock ages obtained by Harper ,1967), the 1975). It follows that under favourable circum- Cambrian ages from the syn-regional metamorphic stances, the appearance of high-grade coarse meta- Newer Gabbros ,Pankhurst, 1970) and the Middle morphic detritus in fossiliferous turbidite ¯ysch Cambrian age of the Leny limestone but not the basins can date the tectonometamorphic climax Arenig age for the Macduff Slates ,Downie et al., and plot the exhumation history of an orogenic 1971) which obviously could not be metamorphosed belt. Further control is possible if radiometric age before they were deposited. The bulk of the musco- dates from the orogenic belt can be compared with vite ,cooling) ages in the Grampian terrane, includ- the same kinds of age dates from the metamorphic ing the Macduff Slates, are of Caradoc to Ashgill age detritus in the fossiliferous turbidite sequence. If ,Pankhurst, 1974; Dempster, 1985; Dempster et al., the tectonic phase of the Caledonian Orogeny was 1995). Cambrian or Upper Proterozoic then there should be Rogers et al. ,1989) dated the Ben Vuirich granite signi®cant turbidite ¯ysch deposits of that age: the at 590 ^ 2 Ma ,zircon, U±Pb isotope dilution). This geological maps of Scotland and Ireland do not granite was considered by Rogers et al. ,1989) to have record this. The most famous turbidite ¯ysch cross cut the early D1 Tay Nappe and D2 structures in deposits in Scotland are actually Ordovician and the Dalradian but was folded and recrystallised by D3. Silurian in age and are found in the Southern With this logic the Caledonian Orogeny is now a Uplands terrane ,Fig. 1). G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 27 3. Mica dating in the Southern Uplands and the the Southern Uplands terrane ,Kelley and Bluck, Dalradian 1989, 1992) are plotted in Fig. 2. As in the Himalayan analogue, most Southern Upland tracts have detrital 3.1. 40Ar± 39Ar fusion and K±Ar muscovite results muscovite ages virtually the same age as the sedi- ments that they were deposited in. Thus detrital The results of 40Ar± 39Ar fusion dating of individual muscovite was supplied to the Southern Uplands detrital muscovite grains and muscovite from one terrane from an active orogenic belt starting in the garnet mica schist boulder from a traverse through Caradoc and continuing until the Wenlock. Kelley

Fig. 2. Comparison of muscovite K±Ar and 40Ar± 39Ar fusion age dates from the Southern Uplands and Grampian terranes. Right hand side of ®gure: 40Ar± 39Ar fusion age dates of detrital muscovite from the Southern Uplands terrane compared to the graptolite ages ,black rectangles indicate error bars) for the sediments they occur in ,Kelley and Bluck, 1992), arranged from north to south in tracts ,1) and ,2) etc. See Fig. 1. Left-hand side of the ®gure: K±Ar metamorphic muscovite ages from the Grampian terrane ,Dempster, 1985) arranged from north to south in Angus and Perthshire transects. The K±Ar hornblende age ,478 ^ 7 Ma, Bluck et al., 1980: box indicates the error bar) for the metamorphic sole of the Ballantrae Ophiolite is the age of obduction. Note that all the Grampian terrane ages and all but two ,spurious?) ages from the Southern Upland terrane have the same or younger ages ,within error) as the Ballantrae metamorphic sole. These data suggest that the Southern Uplands muscovite detritus was derived from the Grampian terrane and that Grampian metamorphic K±Ar muscovite closure/cooling began immediately after obduction. See text for more discussion. 28 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 and Bluck ,1989) acknowledged this and noted that If, for the sake of argument, the oldest K±Ar the ages were very similar to K±Ar ages from the muscovite ages in the lowest grade rocks are to be Grampian terrane ,Fig. 2). ,In theory, 40Ar± 39Ar taken at face value to mean that the ®rst tectonome- fusion and K±Ar dating of the same muscovite sample tamorphic event in the Dalradian began in the Arenig should give the same age.) However, Kelley and ,i.e. the Grampian episode s.s. Dewey, 1971) then it is Bluck ,1989) maintained that the ,100 km distance necessary to scrutinise the radiometric evidence that involved for sediment to be transported from the suggested Cambrian or earlier orogenesis. Grampian terrane across the Midland Valley and into the Southern Uplands terrane was impossibly 3.2. Dalradian Rb±Sr muscovite ages large: therefore the mica could not have come from the Grampian terrane and therefore the Grampian, Fig. 3 shows the Rb±Sr muscovite ages from the Midland Valley and Southern Upland terranes were Dalradian analysed by Dempster ,1985) and Demp- exotic to each other during this time. ster et al. ,1995). Note that only four samples are Hutchison and Oliver ,1998) challenged this `older' than the metamorphic sole of the Ballantrae assumption and examined the heavy mineral contents Ophiolite: two samples are from the Barrovian chlor- of the Caradoc ¯ysch from the Midland Valley and ite zone and probably have inherited components that Southern Uplands terranes: they found an abundance were not completely reset during low grade meta- of Barrovian metamorphic garnet, identical in compo- morphism; the other two samples have low Rb±Sr sition and zoning to Barrovian garnet in its type loca- ratios which Evans and Soper ,1997) suggest did not tion in the Grampian terrane. The coincidence of mica equilibrate with their whole rocks, either loosing Rb of the same age and garnet of the same composition and/or gaining Sr during metamorphism. Further- was used as compelling evidence that the three more, these Cambrian `ages' are older than the fossi- terranes were juxtaposed from the Caradoc onwards. liferous Ordovician Macduff Slates ,Molyneux, 1998) The lack of ,1000 Ma Grenvillian/Sveconorwegian and the Middle Cambrian Leny Limestone ,Pringle, detrital muscovite or Grenville-type granulite facies 1940). Ignoring these `bad' Cambrian `ages' allows detrital garnet in the Southern Uplands terrane miti- the conclusion that the Grampian terrane started to gates against large ,500 km strike-slip movements pass through the Rb±Sr muscovite blocking tempera- between these Grampian, Midland Valley and South- ture of 5008C ,JaÈger, 1979) in the Arenig, at the same ern Uplands terranes: larger than 500 km of strike-slip time as muscovite started to pass through its K±Ar faulting would have brought in ,1000 Ma detrital blocking temperature of 3508C ,Fig. 2). Most Rb±Sr muscovite and garnet from either Newfoundland or muscovite ages are Llanvirn to Caradoc; there are no Norway. Silurian ages. Presumably the present level of expo- It is more than curious that none of the Dalradian sure in the Grampian terrane had uplifted and cooled metamorphic muscovite K±Ar age dates are older below the 5008C closure temperature by the end of the than the K±Ar hornblende age of the Ballantrae Ordovician. metamorphic sole ,478 ^ 8 Ma, Bluck et al., 1980) Giletti et al. ,1961) Rb±Sr dated a large muscovite and that the age of the ®rst muscovite cooling `book' from a syn-metamorphic pegmatite from Glen through its 3508C closure temperature ,JaÈger, Clova at 480 ^ 15 Ma ,i.e. Arenig-Llanvirn). Musco- 1979) is the same with in error at 480 Ma, i.e. in vite books are considered by van Breemen and the Arenig. Obduction and the ®rst K±Ar muscovite Piasecki ,1983) to have Rb±Sr closing temperatures cooling are related in time. Fig. 2 shows that musco- of up to 6008C, i.e. they can record the age of syn- vite continued to cool through 3508C during the rest metamorphic temperatures. Thus the available Rb±Sr of the Ordovician, the Llandovery and until the mica dates can be used as evidence for a Grampian Wenlock. In fact the oldest Grampian muscovite orogenic episode that caused regional metamorphism was sampled from the chlorite zone of the Perthshire and exhumation in the early Ordovician. Incidentally, transect ,Fig. 1) where the maximum metamorphic there is one Rb±Sr muscovite age from a schist temperature could have been around the 3508C clos- boulder from the Llandovery of the Southern Uplands ing temperature. terrane: it gives 460 ^ 5 Ma ,van Breemen et al., G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 29

Fig. 3. Rb±Sr ages for Dalradian metamorphic muscovite, data from Angus and Perthshire ,Dempster, 1985) and from Buchan ,Dempster et al., 1995) ,see locations on Fig. 1) arranged from north to south and from high to low grade respectively. Note that only four samples have apparent ages older than the Ballantrae metamorphic sole ,Bluck et al., 1980): these samples are suspect ,see text for discussion). One detrital muscovite sample from the Southern Uplands has the same age as the average Grampian metamorphic muscovite ,van Breemen et al., 1984). These data again suggest that the Southern Uplands muscovite detritus was derived from the Grampian terrane and that Grampian metamorphic Rb±Sr muscovite closure/cooling began immediately after obduction. Macduff Slates have a Rb±Sr WR isochron age of 452 ^ 7 Ma ,Pankhurst, 1974) similar to the youngest Rb±Sr muscovite ages.

1984) which is the mean Rb±Sr age of Grampian younger than the Ballantrae Ophiolite metamorphic metamorphic muscovite ,Fig. 3). sole K±Ar hornblende age of 478 ^ 8 Ma ,Bluck et al., 1980). Both samples have relatively high Sr and 87 86 3.3. Dalradian Rb±Sr biotite ages low Rb with Rb/Sr ratios of ,7.5 and Sr/ Rb ratios ,22 which suggests to Evans and Soper ,1997) that Fig. 4 shows that all except two of the Dalradian these biotites did not equilibrate with their whole Rb±Sr biotite ages analysed by Dempster ,1985) are rocks, either loosing Rb and/or gaining Sr during or 30 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49

Fig. 4. Rb/Sr ages for Dalradian metamorphic biotite: data from Angus and Perthshire ,Dempster, 1985) and from Buchan ,in Dempster et al., 1995) arranged from north to south and from high to low grade respectively. Note that only two samples have suspect `ages' older than the Ballantrae metamorphic sole ,see text for discussion). The data again suggest that the Grampian metamorphic Rb±Sr biotite closure/cooling began immediately after obduction. Note the one group of cooling ages between 450 and 440 Ma and another at 415±400 Ma: see text for discussion. after metamorphism. Biotite began to cool through its 3.4. Dalradian K±Ar biotite ages 3008C Rb±Sr closure temperature ,JaÈger, 1979) during the early Llanvirn; the bulk of the samples closed during Fig. 5 shows that Dalradian K±Ar biotite ages the Caradoc±Lower Llandovery period whilst still quite analysed by Dempster ,1985). There is a continuous a few closed in the Lower Devonian. Thus comparing trend of younger ages towards the south from the Rb±Sr and K±Ar mica ages suggests that exhumation Arenig through to the Lower Devonian. The oldest and cooling continued in the Silurian, cooling down age is the same ,within error) as the K±Ar hornblende from ,5008C down through 3008C. The Devonian age for the metamorphic sole of the Ballantrae ophio- ,410 Ma ages might be related to intrusion of the lite ,Bluck et al., 1980). These data suggest that Dalra- Newer Granite batholith at this time ,see below). dian biotite ®rst cooled through its K±Ar closure G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 31

Fig. 5. K±Ar ages for Dalradian biotite. Data from Dempster ,1985). Arranged from north to south and from high to low grade. This data suggests that biotite began to cool in the Arenig, immediately after the obduction of the Ballantrae Complex. Note the continuous cooling curve from the center to the southern margin of the Dalradian. temperature of 3008C ,JaÈger, 1979) in the Arenig, just summarised the argument that the Taconic Orogeny like muscovite K±Ar and muscovite Rb±Sr. in North America was caused by the collision between the Iapetian Peri-Laurentian island arcs and the Laur- entian margin. Island arc collision and ophiolite 4. Age of collision and orogeny obduction in Newfoundland is dated stratigraphically by the Llanvirn age of the ®rst ophiolite and arc detri- Dalziel ,1997) and Van Staal et al. ,1998) have tus ,Colman-Sadd et al., 1992). The 40Ar± 39Ar age of 32 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 hornblende from the Bay of Islands Ophiolite meta- between 481 ^ 29 and 451 ^ 8 Ma in Ballantrae morphic sole is 469 ^ 5 Ma ,Dallmeyer and Williams, collected from conglomerates of virtually the same 1975), the same as the stratigraphic age for the ophiolite age as the boulders. Further evidence for the collision detritus. The K±Ar hornblende age for the metamorphic and exhumation of a volcanic arc is the abundance of sole of the Ballantrae Ophiolite is the same within error conglomeratic andesitic detritus in the earliest Llanvirn- at 478 ^ 8 Ma ,Bluck et al., 1980) and is interpreted Caradoc ¯ysch of the Southern Uplands Terrane, e.g. the here as the age of contemporaneous island arc collision Tappins Group in Glen App has a 2 km thick sequence and obduction in Scotland. Note in Fig. 2 that all the of these coarse molasse-type conglomerates ,Larminie, Grampian terrane muscovite ages and all but two ,spur- 1988). Acid, intermediate and basic volcanic and pluto- ious?) Southern Upland terrane muscovites) have the nic detritus continues to be a common component in same or younger K±Ar and 40Ar± 39Ar ages ,within Ashgill and Llandovery ¯ysch of the Southern Uplands error) compared to the hornblende K±Ar age of the ,Stone, 1995). Ballantrae metamorphic sole: indeed, high grade Dalra- dian muscovite ®rst started to pass through its 3008CK± Ar closure temperature in the Arenig. The same picture 5. Detrital garnet in ¯ysch is seen in Figs. 3 and 4 where the earliest reliable Rb±Sr muscovite and biotite ages begin in the Arenig and Hutchison and Oliver ,1998) have shown how Lower Llanvirn, respectively, coincidentally with detrital garnet in ¯ysch can be used as a sensitive obduction in the Arenig: i.e. obduction and regional indicator of provenance. Fig. 6a expands on their tectonometamorphism are linked by collision. Fig. 2b and distinguishes garnet from: ,1) low and It is notable that there are a large number of detrital high grade Barrovian metamorphic zones from the muscovite 40Ar± 39Ar fusion ages in the Southern type localities in the Dalradian from the Grampian Uplands that are the same age as the Ballantrae meta- terrane ,data from Ashworth and Chinner, 1978; morphic sole: this might be evidence that the Dalra- Atherton, 1965, 1969; Atherton and Edmunds, 1966; dian was metamorphosed and almost immediately Chinner, 1960, 1961; Dempster, 1983; Droop and exhumed at the time of collision and obduction, just Harte, 1995; Harte and Graham, 1975; Phillips et as is happening now in the Himalayas and New al., 1993, 1994; Sturt, 1962; Watkins, 1987; Hutchi- Zealand ,see discussion above). son and Oliver, 1998) ,2) low temperature eclogites Evidence for a Lower Palaeozoic volcanic±plutonic ,Mùrk et al., 1988; Santallier, 1994; Perchuk and arc in the Midland Valley is rather cryptic: Longman Philipott, 1997); ,3) New Caledonia and Sanbagawa and Bluck ,1979) Rb±Sr WR-mineral dated boulders at blueschists ,Brothers and Yokoyama, 1982; Black,

Fig. 6. Composition of garnet in the Grampian, Midland Valley, and Southern Uplands terranes. ,a) Discrimination diagram showing ®eld boundaries for garnets from ,1) low and high grade Barrovian metamorphic zones from the type localities in the Dalradian from the Grampian terrane; ,2) low temperature eclogites and blueschists; ,3) Lewisian granulites; ,4) Midland Valley high temperature eclogite and granulite facies xenoliths in diatremes; ,5) calc-alkaline granites; ,6) calc-alkaline volcanics; ,7) mantle eclogites; ,8) metamorphic sole of the Ballantrae Ophiolite; ,9) Ballantrae blueschists; and ,10) high grade pyroxene calc-silicates. References given in the text. ,b) Detrital garnet from the Kirkland Conglomerate, Ballantrae. ,c) Detrital garnet from three samples of the Bennan Conglomerate, Ballantrae. ,d) Detrital garnet from the Kilranny Conglomerate, Ballantrae. ,e) Detrital garnet from the Ardwell Group ,crosses) and Whitehouse Group ,®lled and open circles), Ballantrae. ,f) Detrital garnet from the Craigskelly Conglomerate, Ballantrae. ,g) Detrital garnet from the Quartz Conglomerate ,®lled circles) and Scart Grit ,open circles), Ballantrae. ,h) Detrital garnet from the Highland Border Series, ®lled circles ˆ Achray Sandstone from near Aberfoyle, open circles ˆ Bofrishlie Burn greywacke from near Aberfoyle, crosses ˆ Margie sandstone from Glen Esk. ,i): detrital garnet from Southern Uplands tract 1, Rhins of Galloway, Corswall Formation. ,j) Detrital garnet from Southern Uplands tract 2, Rhins of Galloway, Kirkcolm Formation. ,k) Detrital garnet from Southern Uplands tract 2, Rhins of Galloway, Galdenoch Formation. ,l) Detrital garnet from Southern Uplands tract 3, Rhins of Galloway, three samples from the Portpatrick Formation. ,m) Detrital garnet from Southern Uplands tract 4, Rhins of Galloway, Shinnel Formation. ,n) Detrital garnet from Southern Uplands tract 5, Rhins of Galloway, two samples from low in the Gala Group. ,o) Detrital garnet from Southern Uplands tract 6, Rhins of Galloway, four samples from the middle of the Gala Group. ,p) Detrital garnet from Southern Uplands tract 7, Rhins of Galloway, high in the Gala Group. ,q) Detrital garnet from the Ree Burn Formation, Hagshaw Hills, Midland Valley. ,r) Detrital garnet from the Cowie Formation, from near Stonehaven. Locations are given in Fig. 1. See text for discussion. G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 33 34 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49

Fig. 6. ,continued)

1973); ,4) Lewisian granulites ,O'Hara, 1977); ,5) ,Harrison and Hutchison, 1987; Bogoch et al., Midland Valley high-temperature eclogite and gran- 1997); ,7) calc-alkaline volcanics ,Beddoestephens ulite facies xenoliths in diatremes ,Upton et al., and Mason, 1991); ,8) mantle eclogites ,Smellie and 1984); ,6) calc-alkaline granites and pegmatites Stone, 1984); ,9) metamorphic sole of the Ballantrae G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 35 Ophiolite ,Treloar et al., 1980); ,10) Ballantrae Ballantrae crossite `blueschist' ,cf. Fig. 6a) and blueschists ,Smellie and Stone, 1984); and ,11) presumably came from a nearby subduction zone. high-grade pyroxene calc-silicates ,Winchester, Figs. 6k, 5l and 5o show that blueschist-eclogite pers. comm.). garnet is sporadically present in Caradoc and Llan- Winchester ,pers. comm.) has pointed out that dovery ¯ysch in the Southern Uplands. This cryptic medium grade regionally metamorphosed garnets blueschist-eclogite subduction zone is no longer from Moine calc-silicates partially overlap blueschist exposed in Scotland. Rare examples of detrital garnet garnet compositions; however, associated high-grade from the Ballantrae cover indicate S-type granite and garnet±pyroxene calc-silicates have a ®eld of their mantle sources ,Fig. 6c, e and g). own ,see Fig. 6a). In any case, the volume of garne- tiferous calcsilicates in the Moine ,and Dalradian) is 5.2. Detrital garnet from the Southern Uplands miniscule. Garnet compositions are plotted in Fig. 6 terrane using molecular values of Fe 1 Mn, Mg, and Ca calculated on the basis of 24 oxygens and normalised Fig. 6i±p summarises the results of the analysis of with respect to T ˆ Fe 1 Mn 1 Mg 1 Ca so that they detrital garnet separated from the heavy mineral frac- could be directly compared with the plots presented tions of ¯ysch turbidite greywackes from the Southern by Droop and Harte ,1995). Uplands terrane. The oldest Tract 1 Caradocian Most of the analysed detrital garnet grains are sand sample ,Fig. 6l) has a population of almandine-rich sized and are angular fragments or have recognisable Barrovian-type low and medium grade garnet. There dodecahedral faces; very few are rounded. This is also a sub-population comparable to garnet from together with their relative abundance in most heavy blueschist-eclogite subduction zone complexes. mineral separates suggests that these garnets are ®rst Hutchison and Oliver ,1998) incorrectly assigned a cycle detritus, eroded from an actively forming moun- possible Grenville garnet granulite source for this tain chain. The maximum age of the muscovite detri- sub-population: granulite garnets are more tus ,Fig. 2) coexisting with the detrital garnet is a pyropic than these. New analyses from the Southern taken as the age of the metamorphic garnet ,i.e. Uplands are presented in Fig. 6l±p. Tract 2 Arenig). Caradocian and younger samples ,Fig. 6j±p) contain the same compositions plus a signi®cant component 5.1. Detrital garnet in Ballantrae cover sediments of grossular-almandine garnets comparable to highest grade Barrovian zones ,Fig. 6a): i.e. ®rst low and Lambert and McKerrow ,1976, Table 2) noted that medium grade metamorphic rocks were exhumed, metamorphic detritus ®rst appears in the Midland then the higher grades. Thus by the end of the Cara- Valley with a note of detrital andalusite in the Llan- doc, the Southern Uplands terrane was receiving the virn Kirkland Conglomerate lying unconformably on detritus exhumed from the roots of an adjacent the Ballantrae Ophiolite and used this as evidence for orogenic belt and an associated subduction zone. the upper age limit of the Grampian Orogeny. Fig. Tracts 3±6 continue these trends up to the late Llan- 6b±g presents new analyses of ,abundant) garnets dovery. from samples of Ballantrae molasse and ¯ysch uncon- Upper Llandovery Tract 7 has very little detrital formably overlying the Ballantrae Ophiolite: clearly Barrovian garnet ,Fig. 6p) and Tracts 8, 9 ,Upper Barrovian metamorphic garnets dominate the popula- Llandovery) and 10 ,Wenlock) have none at all. tions. This con®rms the conclusion ,Lambert and According to palaeocurrent data ,Stone, 1995), Tract McKerrow, 1976) that high grade metamorphic detri- 8 Hawick Group ¯ysch was predominantly derived tus was being eroded from the neighboring Grampian from a southerly source, from the opposite direction terrane starting in the Llanvirn; younger sediments to where the Dalradian now lies. It is possible that this show that Barrovian garnet continues to arrive until southerly derived material may have been eroded the Llandovery ,Fig. 6g). Blueschist and eclogite from the northern margin of Avalonia, which in Brit- garnet is present in the Upper Llanvirn-Caradoc ain is notably lacking in Barrovian-type garnet rocks. Bennan Conglomerate: this is unlike garnet from the Presumably, by the end of the Llandovery, either the 36 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 mountain belt to the north which was previously Incidentally, northerly derived unfoliated granite providing high grade metamorphic detritus had been clasts from the Pragian Dunnotter Group near Stone- largely eroded away or a new sedimentary basin was haven are as old as 459 ^ 2.4 Ma and as young as opened up between the Grampian and Southern 412 ^ 4 Ma ,Rb±Sr mineral-WR ages, Haughton et Uplands terranes. al., 1990). Thus it is possible that the Midland Valley terrane opened up basins, starting in the Upper Llan- 5.3. Detrital garnet from the Highland Border Series dovery, which starved and cut off supplies of Gram- pian metamorphic and plutonic detritus to the Hutchison and Oliver ,1998) found detrital Barro- Southern Uplands terrane during the Upper Llandov- vian garnet ,see Fig. 6h) in greywacke ,¯ysch?) from ery and Wenlock. By Lower Devonian times the Assemblage 4 of the Highland Border Series, Midland Valley was well established as a graben although the Caradoc age of all these rocks is less with internal pull-apart basins ,Smith, 1995). than certain ,Curry et al., 1984). New garnet data The signi®cance of Barrovian garnet in Midland from the Achray Sandstone from near Aberfoyle has Valley and Southern Upland molasse and ¯ysch and been added to Fig. 6h. Assemblage 4 is thought to be its Grampian provenance is that it gives an almost unconformable on Assemblages 1±3 of the Highland continuous record of uplift and erosion of the Gram- Border ophiolite ,Curry et al., 1984). Again, the pian terrane from the Llanvirn through to the Lower suggestion is that the Barrovian garnet was eroded Devonian. from the neighboring Grampian terrane in the Cara- doc. Robertson and Henderson ,1984) have already demonstrated that the Margie psammites and pelites from Assemblage 4 in Glen Esk have geochemical 6. Grampian granite and gabbro ages characteristics that were inherited from a source like the Dalradian. The classic geochronological procedure in orogenic belts is to date magma bodies from various tectonic 5.4. Detrital garnet from the Midland Valley inliers situations. This method has been vigorously applied in and Lower Old Red Sandstone Scotland using mainly Rb±Sr, K±Ar and bulk zircon U±Pb techniques. In more recent years precise U/Pb The Lower Palaeozoic Midland Valley Inliers are methods have been used and it is now timely to review shown on Fig. 1. The Ree Burn Formation is an Upper the data. Fig. 7 is a new compilation of radiometric Llandovery turbidite ¯ysch unit, low down in the stra- ages for the post-Cambrian Scottish granites. As quite tigraphy of the Hagshaw Hills ,Fig. 1): higher a few have been dated by Rb±Sr and K±Ar techniques Wenlock units show a transition into Old Red Sand- it is not certain if these represent intrusion and crystal- stone ¯uviatile conditions ,Walton and Oliver, 1991). lisation ages. For example the Stathspey Granite has The Cowie Formation is a Lower Old Red Sandstone, been recently U±Pb monazite dated at 443 Ma Wenlock-Ludlow, ¯uviatile deposit from Stonehaven ,Noble, 1998): previously it was Rb±Sr `dated' at ,Marshall, 1991). Both formations have abundant 404 Ma ,Brown, 1975). Where there are examples detrital garnet with the same range of compositions like this then the Rb±Sr or K±Ar dates have been as seen in the neighboring Dalradian ,Fig. 5q and r). deleted.

Fig. 7. Summary diagram showing the radiometric age dates for post-Cambrian/pre-Carboniferous granite and lava from Scotland. Age dates taken from the compilations in Brown ,1991) and Stephenson and Gould ,1995), supplemented from Oliver et al. ,1998), Noble ,1998), Rogers and Dunning ,1991), Haughton et al. ,1990), Thirwall ,1988), van Breemen and Bluck ,1981), Longman and Bluck ,1979) and Pankhurst and Pidgeon ,1976). The peralkaline Ben Loyal and Borrolan Granites associated with Moine thrusting have open symbols to distinguish them from the early Ordovician syn-metamorphic S-type ,symbols with sloped shading), from the later Ordovician decompression S-type ,symbols with vertical shading), from Siluro-Devonian subduction I-type granites ,®lled symbols) and from the Midland Valley arc granite boulders ,horizontal shading). Mon ˆ monazite, Zr ˆ zircon, Xen ˆ Xenotime, Musc ˆ muscovite, Tit ˆ titanite, bio ˆ biotite, WR ˆ whole rock, min ˆ mineral. See text for discussion. G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 37 38 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 6.1. Older granites Lower Ordovician. They are contemporaneous with the earliest Rb±Sr and K±Ar muscovite and biotite Barrow ,1893) named granites in the `Older' gran- mineral ages from the Grampian terrane ,see ite suite as those that intruded the already deformed Figs. 2±5). Dalradian but were subsequently deformed and meta- The trace element chemistry ,Thompson, 1985) and morphosed. Since the publication of the 590 ^ 2Ma low 87Sr± 86Sr initial ratio ,,0.706, Pankhurst, 1970) Ben Vuirich ,Older) Granite zircon age ,Rogers et al., of the Newer Gabbros is compatible with their origin 1989) much interest has been shown in its structural in the roots of a calc-alkaline arc. The highP initial Sr status ,Tanner and Leslie, 1994). Tanner ,1996) ratios ,0.7112 and 0.7117) and low Nd values studied the hornfels fabrics in the aureole and ,212.5 and 210.6) for the Aberdeen and Strichen proposed that they pre-dated the large scale D1 fold S-type granites ,Haughton et al., 1990; Stephens, structures: the granite could have been intruded as the 1988) together suggest lower crustal syn-meta- Laurentian margin was rifted apart to form the Iapetus morphic melting as an origin ,Richardson and Powell, Ocean, nearly at the same time that dike swarms and 1976), possibly encouraged by the pressure of crustal thick MORB lavas were formed at Tayvallich in the thickening caused by arc-continent collision, and the stretched crust at 595 ^ 5 Ma ,upper intercept U±Pb addition of heat caused by the invasion of arc gabbro zircon, Halliday et al., 1989). The Ben Vuirich Gran- intrusions. ite therefore predates the Grampian episode of island Up till now the oldest dated post-metamorphic, arc collision by ,120 Ma ,see below). post-tectonic granite in the Grampian terrane has been the Kennethmont Granite ,Fig. 1 inset). The 6.2. Grampian granites and Newer Gabbros Rb±Sr WR isochron age is 452 ^ 4 Ma, MSWD ˆ 0.23 ,recalculated from Pankhurst, 1974). Brown ,1991) distinguished a suite of `Grampian Oliver et al. ,1998) report a similar preliminary granites' as those Ordovician granites which were 206Pb± 207Pb evaporation age of 458 ^ 0.9 Ma on syn-to-late tectonic. Pankhurst and Sutherland zircon from the same granite. This is an undeformed ,1982) called them `Group One Newer Granites'. ®ne-grained pink granite which has intruded low- The Aberdeen Granite ,Fig. 1) is a syn-D3, syn-meta- grade Macduff Slates. Thus the Kennethmont Granite morphic, coarse grained granite with migmatitic is a high level intrusion that marks the end of the aureoles in high grade regionally metamorphosed Grampian episode of orogenesis ,Fig. 7). It has a country rock and has a concordant U±Pb monazite high Rb±Sr initial ratio ,0.7146 ^ 0.0012, recalcu- age of 470 ^ 1 Ma ,Kneller and Aftalion, 1987). lated from Pankhurst, 1974) suggesting a high degree The Strichen Granite is deformed along its eastern of crustal component in its parent melt. These post- contact with migmatites and has a near concordant tectonic, Caradoc to Llandovery, granites could be U±Pb monazite age of 476 ^ 5 Ma ,Pidgeon and examples of syn-exhumation/decompression S-type Aftalion, 1978). crustal melts ,England and Thompson, 1986; Read ,1961) used the term `Younger Basic' for the Stephens, 1988). Continuous exhumation and erosion gabbros of Aberdeenshire. The early Ordovician of the Grampian terrane during this time is indicated Grampian granites ages are the same ,within error) by the huge in¯ux of Barrovian-type metamorphic as the 468 ^ 8 Ma U±Pb zircon age of from the detritus into the then active Southern Uplands accre- syn-metamorphic Newer Gabbros that cut the tionary prism. Macduff Slates ,Rogers et al., 1994). This zircon date is preferred to the Cambrian WR Rb±Sr age of 6.3. Newer Scottish granites 490 ^ 15 Ma age obtained by Pankhurst ,1970) which has previously been used as evidence for a Cambrian The younger or newer granites are those that have age for the Dalradian tectonometamorphism, e.g. not been deformed ,Read, 1961). A new compilation Dempster et al. ,1995). These syn-metamorphic gran- of Scottish granite ,and acid volcanic) age dates given ite and gabbro intrusions date the regional meta- in Fig. 7 shows that they are post-Llanvirn. Fig. 8 morphism and early large scale fold structures as compares the age of Scottish granites with their G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 39 have classi®ed the Llandovery and Lower Devonian low 87Sr/86Sr initial ratio granites into three geochemi- cally and geographically different suites, namely the Cairngorm, Argyll, and South of Scotland. Despite these general differences, Brown ,1979) has shown that they have speci®c geochemical characteristics of modern continental arcs: e.g. high Rb/Sr, high K/Na, low K/Rb, relatively low Sr initial ratios ,i.e. average ,0.706) and emplaced in a tensional regime. They are calc-alkaline, I-type granites ,Stephens, 1988; Thirwall, 1988). These granites are very differ- ent to the Ordovician S-type granites described above. Halliday et al. ,1979) pointed out that the presence of inherited 1200±1800 Ma zircon in many of these granites from north of the Highland Boundary Fault, indicates that there must have been a certain amount of lower crustal component added to these melts. Presently exposed levels of Dalradian crust have initial Sr ratios of ,0.718 ,Pankhurst, 1974; van Bree- men and Piasecki, 1983) and therefore could not have made important contributions to low initial Sr ratio ,0.706) granite melts. Melt from suitably low initial Sr ratio Lewisian-type gneisses in the lower crust must have been small to non-existent since the latter have much lower eNd400 values ,of , 2 19, Frost and O'Nions, 1985) than the Newer Granites ,eNd400 values of 210 to 13, Halliday and Stephens, 1984). However, low Sr initial ratios and high eNd440 values for the Newer Granites indicate possible mantle contributions. Clearly, a systematic study of the Newer granite isotope geochemistry is required but nevertheless the preferred environment for the melt- ing of Newer granites is an Andean-style northerly Fig. 8. Compilation of age of Grampian terrane granites and lavas dipping subduction zone, as ®rst proposed by versus their 87Sr± 86Sr initial ratios. Data from Table 2 in Stephenson and Gould ,1995). Note that there is a signi®cant change in initial Dewey ,1971), and endorsed by Brown ,1979), van ratios at 434 ^ 9 Ma from high ,S-type ratios, average 0.716) to Breemen and Bluck ,1981), Soper ,1986) and Thir- lower ,I-type, average 0.706) ratios associated with a change from wall ,1988). This subduction zone formed separately decompression melting to subduction zone melting. See text for from and much later than the Ordovician island arc 87 86 discussion. ,The question mark indicates the high Sr± Sr initial collision described above. The block fault melting ratio, foliated Kamnay granite which on textural and geological grounds is like other early Ordovician granites although its Rb±Sr models favoured by Stillman and Francis ,1979), mineral age is 411 ^ 7 Ma, see Stephenson and Gould, 1995). Watson ,1984) and Stone et al. ,1987) are not preferred here. In this model, subduction processes 87Sr/86Sr initial ratios. It is interesting to note that in the mantle produced granite melts, and faults in there is sudden change in the Upper Llandovery ,i.e the crust controlled where granites were emplaced, the 434 ^ 9 Ma Corrieyairack granite) when initial not the other way round ,Jacques and Reavy, 1994). ratios suddenly drop from relatively high ,crustal The faulting was controlled by the geometry of the melt, S-types?) ratios to much lower ,subduction subduction. There are no basic dyke swarms and zone, I-types?) ratios. Stephens and Halliday ,1984) insigni®cant volumes of gabbro of Newer Granite 40 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 Grampian terrane by mid-Llandovery times, or it was inactive, or it was too shallow dipping to initiate granite formation, or the plate boundary had a domi- nantly strike-slip motion and wet rocks were not being subducted. Most reconstructions show little separa- tion of Avalonia and Laurentia in the Mid±Late Silurian ,e.g. Van Staal et al., 1998). Therefore, convergence must have been slow. Furthermore,

there is an apparent lack of high-Na2O, adakitic magma in the Grampian terrane granites: according to Herve et al. ,1999) this means that relatively old and cold rather than hot and young oceanic crust was being subducted ,i.e. .20 Ma).

6.4. Old Red Sandstone and the Midland Valley

Low 87Sr± 86Sr I-type ,subduction zone) magma- Fig. 9. Summary diagram showing correlation of Newer granite tism resumed in the mid-Llandovery and became ages and the contemporaneous formation of Lower Old Red Sand- stone extensional basins in the Grampian and Midland Valley established in the Wenlock ,Fig. 7). This is coincident terranes. Data and references discussed in the text. GT ˆ Grampian with the opening of the Midland Valley graben and terrane, MVT ˆ Midland Valley terrane, LORS ˆ Lower Old Red the ®rst Old Red Sandstone deposition at Stonehaven Sandstone. Symbols as in Fig. 7. ,Fig. 9; Marshall, 1991): the Stonehaven Group is full of plutonic and high grade Barrovian metamorphic as age in the Grampian terrane: so lower crustal granite well as the acid volcanic detritus ,Robinson et al., melting caused by copious underplating by mantle 1998). However, the bulk of Scottish Newer calc- ,hot spot) derived basic magma at this time is not alkaline granites and lavas were crystallised in the supported. Pragian ,,410 Ma) at the same time that intermon- It is notable that there is a lack of Grampian tane Lower Old Red Sandstone extensional basins high 87Sr/86Sr initial ratio granites younger than formed in the Grampian terrane, e.g. the Rhynie 434 ^ 9 Ma ,Fig. 7): presumably decompression basin ,Hirst and Maulik, 1926). This was also the melting stopped in these late Llandovery times in time when mega-conglomerates containing pluto- the Grampian terrane. The change in Scottish granite nic±volcanic and high grade Barrovian metamorphic petrogenesis in the late-Llandovery ,within the age clasts were formed at Dunnottar and Crawton, south error bars) is the time when the Moine Thrust was of Stonehaven ,Fig. 9). Haughton et al. ,1990) dated initiated ,430 ^ 4 Ma, van Breemen et al., 1979). an unfoliated granite boulder from the Crawton Group This is also the time in the Southern Uplands terrane at 412 ^ 4 Ma ,Rb±Sr WR-mineral age). The when the tectonics changed from orthogonal NW- Crawton Group is dated by the Lintrathen Ignimbrite directed under-thrusting to W-directed sinistrally at 415.5 ^ 5.8 Ma ,Rb±Sr biotite, recalculated in oblique under-thrusting ,Stone, 1995). Finally, this Thirwall, 1988) and a rhyolite lava boulder at is also the time that the supply of Barrovian garnet- 410.6 ^ 5.6 Ma ,Rb±Sr plagioclase-biotite mineral bearing detritus ended in the Southern Uplands and age recalculated in Thirwall, 1988). The close switched to the Midland Valley. The late-Llandovery ages of acid plutonism, acid volcanism and is therefore an important age when the geodynamics erosion indicate very rapid exhumation processes. of granite production ,plate motions?) in the Scottish Calcalkaline plutonism in Scotland occurred on a region fundamentally changed ,see discussion below). batholithic scale, so much so that the crust thick- It is possible that the Iapetus subduction zone carry- ened and uplifted itself isostatically by batholithic ing the wet rocks necessary for mantle melting buoyancy causing erosion ,Robinson and Oliver, ,Gillully, 1973) either had not reached under the 1998). Regional crustal extension may have added G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 41 to the exhumation of the batholith and this might have 478 ^ 8 Ma metamorphic sole ,Bluck et al., 1980). been related to increasing roll back of the subduction Syn-metamorphic pegmatites formed in Glen Clova zone with increasing age ,Molnar and Atwater, 1978). at 480 ^ 15 Ma ,Giletti et al., 1961). The syn-meta- morphic, syn-D3, Strichen and Aberdeen Granites and Newer Gabbros were intruded at 475 ^ 5Ma 7. Comparison of Scotland and western Ireland ,Pidgeon and Aftalion, 1978), 470 ^ 1 Ma ,Kneller and Aftalion, 1987) and 468 ^ 8 Ma ,Rogers et al., Fig. 10 is a summary of the stratigraphic and 1994), respectively. Metamorphic muscovite and precise radiometric age dating for Scotland during biotite ®rst passed through the 5008C Rb±Sr and the the period 490±430 Ma. The Macduff Slates are 3008C K±Ar blocking temperatures at ,480 Ma, i.e. certainly Ordovician ,Molyneux, 1998) and possibly immediately after collision and the peaking of meta- Arenig ,Downie et al., 1971) and therefore as young morphism. High grade Barrovian metamorphic detri- as ,485 Ma. These are the youngest regionally meta- tus ®rst appeared in the Midland Valley, mixed with morphosed sediments in the Dalradian Supergroup. Ballantrae ophiolite detritus in the ,465 Ma old Kirk- The collision of island arcs and obduction of ophiolite land Conglomerate. Post-metamorphic, post-D4 unfo- ends sedimentation on the Durness shelf on the Laur- liated granite started to intrude the Grampian terrane entian passive margin in the Llanvirn ,Dewey, 1969). from at least 458 ^ 0.9 Ma ,i.e. the age of the The collision is dated by the Ballantrae Ophiolite Kennethmont Granite, Oliver et al., 1998), at the

Fig. 10. Summary of stratigraphic and radiometric evidence for a short ,15 Ma period for Grampian episode of the Caledonian Orogeny in Scotland. Details are discussed in the text. 42 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49

Fig. 11. Summary of stratigraphic and radiometric evidence for a short ,10 Ma period for the Grampian episode of the Caledonian Orogeny in Ireland. Compare this with Fig. 10 which is a similar compilation for Scotland: there is a very strong correlation. Age dates from Friedrich et al. ,1997, 1999)), stratigraphy from Ryan and Dewey ,1991) and Ryan, pers. comm. Details are discussed in the text. Bio ˆ biotite, Mon ˆ monazite, Zr ˆ zircon, Xen ˆ Xenotime, Musc ˆ muscovite, Hbl ˆ hornblende. same time that the bulk of Grampian metamorphic of the orogeny in the west of Ireland was caused by micas passed through their blocking temperatures the collision of the Loch Nafooey arc with the Laur- ,Dempster, 1985; Dempster et al., 1995). entian margin. Syn-metamorphic, pre-early D3 Loch The Strichen and Kennethmont Granite age dates Wheelaun Metagabbros were intruded at 470.1 ^ are important because they bracket the peak and end 1.4Ma.The®rstdetritalhighgradeBarrovianmeta- of an orogenic episode in the Grampian terrane in Scot- morphic detritus arrived in the South Mayo Trough land between 476 ^ 5 and 458 ^ 0.9 Ma. This in the Middle Llanvirn ,i.e. ,465 Ma, Ryan and tectonometamorphism episode must post-date the Dewey, 1991). Late Post-D4 ,unfoliated) Oughterard sedimentation of the youngest Dalradian strata, i.e. Granite was intruded at 462.5 ^ 1.2 Ma. A quartz the Macduff Slates which are de®nitely Ordovician diorite has a mean U±Pb zircon age of ,Molyneux, 1998) and possibly Arenig, i.e. 466.5 ^ 0.6 Ma and monazite from metapelites and ,480 Ma, based on microfossils ,Downie et al., 1971). late pegmatites give ages from ,468to463Ma. Fig. 11 summarises the stratigraphic and radio- 39Ar± 40Ar hornblende ,460±453 Ma), muscovite metric evidence from western Ireland as described ,455 Ma) and biotite ,450 Ma) ages document the by Ryan and Dewey ,1991) and Friedrich et al. exhumation. The close comparison with the Scottish ,1997, 1999), respectively. The Ordovician episode data in Fig. 10 is obvious. G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 43 8. Synthesis Southern Upland terrane molasse and turbidite deposition. Mountains the size of the Himalayas Fig. 12 is a series of cartoon cross sections that might have formed. The time of gabbro and granite attempt to synthesis the geodynamic evolution of the intrusion is much earlier then the main phase of Scottish Laurentian margin during the period between subduction zone magmatism that produced the ,750 and 410 Ma. This model is inspired by Dewey Newer Granites after the Llandovery ,see below). ,1969, 1971), Van Breemen and Bluck ,1981) and This early arc magmatism may be related to the Ryan and Dewey ,1991) but has been re®ned using closure and subduction of the Highland Border more recently published data and the re-interpretation back-arc basin ,Fig. 12d) rather than subduction of of older published data ,discussed above). According the northern Iapetus Ocean ,Fig. 12g). This tectono- to Dalziel ,1997), the pre-orogenic Dalradian history metamorphic Grampian episode is the Grampian commenced with rifting of Rodinia when Laurentia ªeventº in the sense of Dewey ,1971, p. 227) and separated from South America forming the Southern the Grampian ªOrogenyº in the sense of Lambert Iapetus Ocean in the Sturtian ,Fig. 12a). This was and McKerrow ,1976, p. 289): ª¼ that set of followed by the formation of Vendian Tayvallich processes which converted the Dalradian and related MORB-type volcanics at 595 ^ 4 Ma. The Ben Vuir- Arenig sediments into the metasedimentary fold belt ich Granite was intruded into the stretching margin at extending from the NW coast of Ireland to the NE 590 ^ 2 Ma ,Fig. 12b). Sedimentation continued on coast of Scotland; it occurred during Arenig and Llan- a passive margin through the Middle Cambrian virn time.º These processes were diachronous: lasting ,Fig. 12c) with deposition of Leny Limestone and between 480 and 465 Ma in Scotland and between 470 Arenig Macduff fan turbidites ,Fig. 12d) until the and 460 Ma in Ireland. collision of a Peri-Laurentian fully oceanic Midland The docking of the Midland Valley arc terrane Valley Arc in the Middle Arenig when the Ballantrae against the Grampian terrane is probably dated as at Complex was obducted at 487 ^ 8 Ma ,Fig. 12e). least Llanivrn since Grampian terrane-type high grade This model is not without critics and problems: e.g. metamorphic detritus is found in Midland Valley Prave and Alsop ,1998) argue that the lower Dalra- molasse ,Kirkland Conglomerate) of that age. The dian is actually an orogenic foreland ¯ysch following Southern Uplands terrane ®rst received Grampian a ,830±800 Ma old Knoydartian-Morarian Orogeny metamorphic-type ¯ysch in the Caradoc. Thus all and that a continuous sedimentary sequence lasting three terranes were juxtaposed by Caradoc times and through the rest of the Dalradian for all of ,260 Ma have not signi®cantly shifted apart since ,see Hutch- is unlikely. Furthermore, the status of the Midland ison and Oliver, 1998). Valley terrane is poorly known for this time: there ,2) The second episode is one of early Caradocian may even have been a Grenvillian basement accord- to late Ashgillian plutonism ,but with no regional ing to the evidence from poorly dated ,U±Pb upper folding) in the now deformed Laurentian margin intercept zircon ages) granulite facies xenoliths found ,Fig. 12f). This was caused by isostatic uplift and in Carboniferous vents ,Halliday et al., 1984). decompression melting during exhumation ,e.g. the The collision of the Midland Valley arc with the Kennethmont Granite). This was coincident with Laurentian margin initiated the Caledonian Orogeny. Southern Uplands terrane turbidite deposition. There The Caledonian Orogeny can be divided into three is no formal name for this episode of isostatic uplift. episodes: ,3) There was a noticeable change from S to I-type ,1) The ®rst episode in the Lower Ordovician was a of magmatic activity in the Llandovery which contin- short ,,15 Ma) catastrophic Peri-Laurentian island ued into the to Early Devonian: voluminous plutonism arc versus Laurentian continental margin collisional on a batholithic scale occurred on both sides of the orogeny ,Fig. 11e) coincident with docking of the Iapetus Suture ,Fig. 12g). Again there was no regional Midland Valley arc, Ballantrae ophiolite obduction, folding in Scotland. Thus by 430 Ma a subduction Nappe tectonics and Barrovian regional metamorph- zone had reached under the Grampian ,and North ism, intrusion of Newer Gabbro arc rocks and crustal West Highland) terranes and there followed a contin- melting, and the beginning of Midland Valley and uous period of plutonism on a batholithic scale. 44 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 45 Indeed, calc-alkaline plutonism occurred on such a apparently happy to accept the Scandinavian Caledo- scale that the crust thickened and uplifted itself isosta- nides as a name. However, the details of the Scandian tically by batholithic buoyancy ,Robinson and Oliver, episode in Scandinavia show that Scotland was rather 1998). The resulting erosion formed several kilo- peripheral to the main extensional events ,Osmundsen meters of molasse in the Midland Valley. Subduction and Andersen, 2001). zone geometry and roll back caused sinistral strike- slip and extensional ,i.e. transtensional) faulting; this 9. Conclusions was a major factor in locating intrusions, volcanism and molasse basins. In this model, the bulk of the ,1) The main compressional phase and regional uplift and erosion in Scotland in the Siluro-Devonian Barrovian metamorphism and crustal melting asso- was not caused by a continental collision between ciated with Himalayan-scale crustal thickening in Laurentia and Baltica as inferred by Soper ,1986) the Caledonian Orogeny of Scotland was the Gram- and Soper et al. ,1992). pian episode. It began with Laurentia versus Midland This episode of batholith emplacement lasted Valley island arc collision and obduction of the ,40 Ma until the ®nal juxtapositioning of Ballantrae and Highland Border Ophiolites, starting Laurentian, Avalonian and Baltica margins in the in the Arenig ,478 ^ 8 Ma). The Grampian episode Lower Devonian at ,395 Ma. Soft docking of the in Scotland lasted for a mere 15 Ma between 480 and Lake District terrane ,the leading edge of Avalonia) 465 Ma. formed the Iapetus Suture and ended subduction and ,2) Grampian terrane isostatic uplift, decompres- plutonism under Laurentia ,and Avalonia). Compar- sional melting and erosion of turbiditic sequences ison with the Himalayas shows that plutonic activity into the Southern Uplands terrane occurred between in the ,2000 km long Andean-type Transhimalayan 465 and 435 Ma. This implies that all three terranes arc was abruptly stopped when India docked against were juxtaposed since the Caradoc. Tibet at ,45 Ma ,Copeland et al., 1995). Deforma- ,3) By ,435 Ma, Andean-type subduction had tion in the Canadian Maritime Provinces at this time reached deep enough under the Grampian terrane for was regional and penetrative and is known as the batholith construction to commence, ceasing at Acadian Orogeny ,Rast and Skehan, 1993). In Scot- ,395 Ma when Avalonia docked. Batholithic buoy- land, at the present levels of erosion, deformation ancy was an important component of crustal uplift and was brittle and concentrated in fault zones. The erosion at this time. This Scottish Scandian episode of name Caledonian Orogeny has been proposed for the Caledonian orogeny occurred ,40 Ma after the this episode in Scotland ,Lambert and McKerrow, Grampian episode ®nished. 1976) but this is confusing for the majority who ,4) It is perhaps ironic that the Arenig-Llanvirn consider this to be the name for the combined Ordo- Grampian episode is named after the Mons Graupius vician±Devonian orogenic events in the British battle ®eld, located on the ¯anks of the Lower Devo- Isles, Scandinavia, E Greenland, Northern Germany nian Bennachie Granite. and Poland ,Oliver et al., 1993). To save confusion it ,5) Those that model orogeny numerically should might be possible to compare these Early Devonian take into account that the 80 Ma long Caledonian events with similar events that are better expressed in Orogeny is an example of one which was episodic Scandinavia ,Gee and Roberts, 1983) and to call this the with a short catastrophic collisional tectono- Scottish Scandian episode. After all, Scandinavians are metamorphism and magmatism lasting 15 Ma only

Fig. 12. Cartoon cross sections: ,a)±,g), illustrating the geodynamic evolution of the Scottish Laurentian margin during the period between ,750 and ,410 Ma. Details given in the text. Lew ˆ Lewisian, Torrid ˆ Torridonian, G ˆ Grenville, M ˆ Moine/Morarian, K ˆ Knoydartian. ,TVL) ˆ Tayvallich Lavas, BVG ˆ Ben Vuirich Granite, St ˆ Strichen Granite, Kmt ˆ Kennethmont Granite, Ab ˆ Aberchirder Granite, GD ˆ Glen Dessary Syenite, NG ˆ Newer Gabbros, DAL ˆ Dalradian, HBS ˆ Highland Border Series, NHT ˆ Northern Highlands terrane, GT ˆ Grampian terrane, MVT ˆ Midland Valley terrane, LDT ˆ Lake District Terrane, HBF ˆ Highland Bound- ary Fault, SUF ˆ Southern Upland Fault, GGF ˆ Great Glen Fault, MT ˆ Moine Thrust, HBs ˆ Highland Boundary Slide, IS ˆ Iapetus Suture, SL ˆ Sea level. 46 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49

,i.e. the Grampian episode) followed by a 25 Ma cance of garnet-bearing intrusions within the Borrowdale period of isostatic adjustment, decompression melt- Volcanic Group, Eskdale Area. Geol. Mag. 128, 505±516. ing, erosion, and then another 40 Ma of subduction Black, P.M., 1973. Mineralogy of New Caledonian metamorphic rocks. ,I): Garnets from the Ouegoa District. Contrib. Mineral. and batholith formation. The Caledonian Orogeny Petrol. 38, 221±235. stopped when Avalonia docked. Bluck, B.J., Halliday, A.N., Aftalion, M., Macintyre, R.M., 1980. Age and origin of the Ballantrae ophiolite and its signi®cance to the Caledonian orogeny and the Ordovician time scale. Geology Acknowledgements 8, 492±495. Bogoch, R., Bourne, J., Shirav, M., Harnois, L., 1997. Petrochem- istry of a Late Precambrian garnetiferous granite and aplite, Financial support was provided by the St. southern Israel. Miner. Mag. 61, 111±122. Andrews University Welch Fund. This paper has van Breemen, O., Bluck, B.J., 1981. Episodic granite plutons in the bene®ted from discussions with members of the Scottish Caledonides. Nature 291, 113±117. Crustal Geodynamics Group at St. Andrews, espe- van Breemen, O., Piasecki, M.A.J., 1983. The Glen Kyllachy Gran- ite and its bearing on the nature of the Caledonian Orogeny in cially G.I. Alsop, A. Prave, R.A.J. Robinson and Scotland. J. Geol. Soc. Lond. 140, 47±62. W.E. Stephens. The following offered enlighten- van Breemen, O., Aftalion, M., Johnson, M.R.W., 1979. Age of the ment: J.F. Dewey, I.W.D. Dalziel, D.W.H. Hutton, Loch Borrolan complex, Assynt, and the late movements along P. Ryan, M.P. Searle, J. Soper, J.A. Winchester and the Moine thrust zone. J. Geol. Soc. Lond. 136, 489±496. many participants at the meetings held for the van Breemen, O., Halliday, A.N., Bluck, B.J., Haughton, P., 1984. Age and derivation of metamorphic clasts in conglomerates in 650th Anniversary of Charles University, Prague, and around the Midland Valley ,abstract). Trans. R. Soc. Edinb.: September 1998 and the 27th Annual Meeting of Earth Sci. 75, 300. the Tectonic Studies Group at St Andrews, Decem- Brothers, R.N., Yokoyama, K., 1982. Comparison of high-pressure ber 1998. P. Ryan and T. Anderson have materially schist belts of New Caledonia and Sanbagawa, Japan. Contrib. improved the manuscript with their constructive Mineral. Petrol. 79, 219±229. reviews. Acknowledgments are due to G. Sander- Brown, G.C., 1979. Geochemical and geophysical constraints on the origin and the evolution of Caledonian granites. In: Harris, A.L., man for drawing Figures 1 and 12. However, Holland, C.H., Leake, B.E. ,Eds.), The Caledonides of the Brit- special thanks go to D.A. Herd who separated, ish Isles Reviewed. Scottish Academic Press, Edinburgh, pp. analysed and plotted hundreds and hundreds of 645±652. garnet grains. Brown, J.F., 1975. Rb±Sr studies and related geochemistry of the Caledonian calc-alkaline igneous rocks of N. W. Argyllshire. DPhil thesis, Oxford University, unpublished. Brown, P.E., 1991. Caledonian and earlier magmatism. In: Craig, References G.Y. ,Ed.), Geology of Scotland, 3rd ed., Geological Society of London, pp. 229±295. Ashworth, J.R., Chinner, G.A., 1978. Coexisting garnet and cordier- Chinner, G.A., 1960. Pelitic gneisses with varying ferrous/ferric ite in migmatites from the Scottish Caledonides. Contrib. ratios from Glen Clova, Angus, Scotland. J. Petrol. 1, 128±267. Mineral. Petrol. 65, 379±394. Chinner, G.A., 1961. Almandine in thermal aureoles. J. Petrol. 3, Atherton, M.P., 1965. The composition of garnet in regionally meta- 316±340. morphosed rocks. In: Pitcher, W., Flinn, D. ,Eds.), The Controls Colman-Sadd, S.P., Stone, P., Swinden, H.S., Barnes, R.P., 1992. of Metamorphism. Oliver and Boyd, London, pp. 281±290. Parallel geological development in the Dunnage Zone of Atherton, M.P., 1969. The variation in garnet, biotite and chlorite Newfoundland and the Lower Palaeozoic terranes of southern composition in medium grade pelitic rocks from the Dalradian, Scotland: an assessment. Trans. R. Soc. Edinb.: Earth Sci. 83, Scotland with particular reference to the zonation in garnet. 571±594. Contrib. Mineral. Petrol. 18, 347±371. Coney, P.J., Jones, D.L., Monger, J.W.L., 1980. Cordillerian Atherton, M.P., Edmunds, W.M., 1966. An electron microprobe suspect terrains. Nature 288, 329±333. study of some zoned garnets from metamorphic rocks. E. P. S. Copeland, P., Harrison, T.M., 1990. Episodic rapid uplift in the L. 1, 185±193. Himalaya revealed by 40Ar/39Ar analysis of detrital K±feldspar Bailey, E.B., 1922. The structure of the south-west Highlands of and muscovite, Bengal fan. Geology 18, 354±357. Scotland. Q. J. Geol. Soc. Lond. 78, 62±127. Copeland, P., Harrison, T.M., Yun, P., Kidd, W.S.F., Roden, M., Barrow, G., 1893. On the intrusion of muscovite biotite gneiss in the Yuquan, Z., 1995. Thermal evolution of the Gangdese batholith, south-east Highlands of Scotland and its accompanying meta- Southern Tibet: a history of unroo®ng. Tectonics 14, 223±236. morphism. Q. J. Geol. Soc. Lond. 49, 330±358. Curry, G.B., Bluck, B.J., Burton, C.J., Ingham, J.M., Siveter, D.J., Beddoestephens, B., Mason, I., 1991. The volcanogenetic signi®- Williams, A., 1984. Age, evolution and tectonic history of the G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 47

Highland Border Complex, Scotland. Trans. R. Soc. Edinb.: logical study of the metamorphic complexes of the Scottish Earth Sci. 75, 113±133. Highlands. Q. J. Geol. Soc. Lond. 117, 233±264. Dallmeyer, R.D., Williams, H., 1975. 40Ar/39Ar ages from the Bay Gillully, J., 1973. Steady plate motion and episodic orogeny and of Islands metamorphic aureole: their bearing on the timing of magmatism. Bull. Geol. Soc. Am. 84, 499±524. Ordovician ophiolite obduction. Can. J. Earth Sci. 12, Halliday, A.N., Stephens, W.E.S., 1984. Crustal controls on the 1685±1690. genesis of the 400-Ma old Caledonian granites. Phys. Earth Dalziel, I.W.D., 1997. Neoproterozoic-Paleozoic geography and Planet. Int. 35, 89±104. tectonics: review, hypothesis, environmental speculation. Bull. Halliday, A.N., Aftalion, M., van Breemen, O., Jocelyn, J., 1979. Geol. Soc. Am. 109, 16±42. Petrogenesis and signi®cance of Rb±Sr and U±Pb isotopic Dempster, T.J. 1983. Studies of orogenic evolution in the Scottish systems in the 400 Ma old British Isles granitoids and their Dalradian. PhD thesis, University of Edinburgh. hosts. In: Harris, A.L., Holland, C.H., Leake, B.E. ,Eds.), The Dempster, T.J., 1985. Uplift patterns and orogenic evolution of the Caledonides of the British Isles Reviewed. Scottish Academic Scottish Dalradian. J. Geol. Soc. Lond. 142, 111±128. Press, Edinburgh, pp. 653±662. Dempster, T.J., Hudson, N.F., Rogers, G., 1995. Metamorphism and Halliday, A.N., Aftalion, M., Upton, B.G.J., Aspen, P., Jocelyn, J., cooling of the NE Dalradian. J. Geol. Soc. Lond. 152, 383±390. 1984. U±Pb isotope ages from a granulite-facies xenolith from Dewey, J.F., 1969. Evolution of the Appalachian Caledonian Partan Craig in the Midland Valley of Scotland. Trans. R. Soc. Orogen. Nature 222, 124±129. Edinb.: Earth Sci. 75, 71±74. Dewey, J.F., 1971. A model for the Lower Palaeozoic evolution of Halliday, A.N., Graham, C.M., Aftalion, Dymoke, P., 1989. The the southern margin of the early Caledonides of Scotland and depositional age of the Dalradian Supergroup Ð U±Pb and Ireland. Scott. J. Geol. 7, 219±240. Sm±Nd isotopic studies of the Tayvallich volcanics, Scotland. Downie, C., Lister, T.R., Harris, A.R., Fettes, D.J., 1971. A paly- J. Geol. Soc. Lond. 146, 3±6. nological investigation of the Dalradian rocks of Scotland. Harper, C.T., 1967. The geological interpretation of potassium± Report of the Institute of Geological Sciences, No 71/9, 30 pp. argon ages of metamorphic rocks from the Scottish Caledo- Droop, G.T.R., Harte, B., 1995. The effect of Mn on the phase nides. Scott. J. Geol. 3, 46±66. relations of medium grade pelites: constraints from natural Harris, A.R.L., Pitcher, W.S., 1975. The Dalradian Supergroup. In: assemblages on petrogenetic grid topology. J. Petrol. 36, Harris, A.R.L. et al. ,Eds.), A correlation of Precambrian Rocks 1549±1578. in the British Isles. Special Report of the Geological Society of England, P., Thompson A.B., 1986. Some thermal tectonic models London, 6, pp. 52±75. for ultrametamorphism and crustal melting in continental colli- Harris, A.R.L., 1991. The growth and structure of Scotland. In: sion zones. In: Coward, M.J., Ries, A. ,Eds.), Collision Craig, G.Y. ,Ed.), Geology of Scotland, 3rd ed., Geological Tectonics. Geological Society of London, Special Publication Society of London, pp. 1±24. No 19, pp. 83±84. Harrison, T.N., Hutchison, J., 1987. The age and origin of the East- Evans, J.A., Soper, N.J., 1997. Discussion on metamorphism and ern Grampians Newer Granites. Scott. J. Geol. 23, 269±282. cooling of the NE Dalradian and U±Pb and Rb±Sr geogronol- Harte, B., Graham, C.M., 1975. The graphical analysis of greens- ogy of magmatism and metamorphism in the Dalradian of chist to amphibolite facies mineral assemblages in metabasites. Connemara, western Ireland. J. Geol. Soc. Lond. 154, 357±360. J. Petrol. 16, 347±370. Friedrich, A.M., Hodges, K.V., Bowring, S.A., 1997. Geochrono- Hatch, F.H., Wells, A.K., 1937. The Petrology of the Igneous logical constraints on the tectonic evolution of Connemara. Rocks. 9th ed. Allen and Unwin, London ,368 pp.). Abstracts volume of Discussion Meeting on the Caledonides, Haughton, P.D.W., Rogers, G., Halliday, A.N., 1990. Provenance of University of Glasgow, p. 12. Lower Old Red Sandstone conglomerates, SE Kincardineshire: Friedrich, A.M., Bowring, S.A., Martin, M.W., Hodges, K.V.F., evidence for the timing of Caledonian terrane accretion in 1999. Short-lived continental magmatic arc at Connemara, central Scotland. J. Geol. Soc. Lond. 147, 105±120. western Irish Caledonides: Implications for the age of the Gram- Herve, M., Sial, A.N., Stephens, W.E., Ferreira, V.P., 1999. Adaki- pian orogeny. Geology 27, 27±30. tic magmas; modern analogues of Archaean granitoids. Lithos Frost, C.D., O'Nions, R.K., 1985. Caledonian magma genesis and 46, 411±429. crustal recycling. J. Petrol. 26, 515±544. Hirst, S., Maulik, S., 1926. On some Arthropod remains from the Gary, M., McAfee Jr., R., Wolfe, C.L., 1972. Glossary of Geology. Rhynie Chert ,Old Red Sandstone). Geol. Mag. 63, 69±139. American Geological Institute, 805 pp. Hutchison, A.R., Oliver, G.J.H., 1998. Garnet provenance studies, Gee, D., Roberts, D., 1983. Timing of deformation in the Scandi- juxtaposition of Laurentian marginal terranes and timing of the navian Caledonides. In: Schenk, W. ,Ed.), Regional Trends in Grampian Orogeny in Scotland. J. Geol. Soc. Lond. 155, the Geology of the Appalachian-Caledonian Orogen. NATO 541±550. ASI Series cReidel, Dordrecht ,116 pp.). Jacques, J.M., Reavy, R.J., 1994. Caledonian plutonism and major Gibbons, W., Gayer, R.A., 1985. British Caledonian terranes. In: lineaments in the SW Scottish Highlands. J. Geol. Soc. Lond. Gayer, G.A. ,Ed.), The Tectonic Evolution of the Caledonide± 151, 955±969. Appalachian Orogen. Vieweg, Braunschweig, Wiesbaden, pp. JaÈger, E., 1979. Introduction to geochronology. In: JaÈger, E., Hunzi- 3±17. ker, J.C. ,Eds.), Lectures in Isotope Geology. Springer, Berlin, Giletti, B., Moorbath, S., Lambert, R., St, J., 1961. A geochrono- pp. 1±12. 48 G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49

Kelley, I., Bluck, B.J., 1989. Detrital mineral ages from the South- Pankhurst, R.J., Sutherland, D.S., 1982. Caledonian granites and ern Uplands using 40Ar± 39Ar laser probe. J. Geol. Soc. Lond. diorites of Scotland and Ireland. In: Sutherland, D.S. ,Ed.), 146, 401±403. Igneous Rocks of the British Isles. Wiley, New York, pp. Kelley, S., Bluck, B.J., 1992. Laser 40Ar/39Ar ages for detrital 141±190. muscovite in the Southern Uplands of Scotland. Chem. Geol. Perchuk, A., Philipott, P., 1997. Rapid cooling and exhumation of ,Isotope Geosci. Sec.) 101, 143±156. eclogitic rocks from the Great Caucasus. Russ. J. Meteorol. Kneller, B., Aftalion, M., 1987. The isotopic and structural age of Geol. 15, 299±310. the Aberdeen granite. J. Geol. Soc. Lond. 144, 717±722. Phillips, E.R., Clark, G.C., Smith, D.I., 1993. Mineralogy, petrology Lambert, R., St, J., McKerrow, W.S., 1976. The Grampian orogeny. and microfabric analysis of the Eilrig Shear Zone, Fort Augus- Scott. J. Geol. 12, 271±292. tus. Scott. J. Geol. 29, 143±158. Larminie, F.G., 1988. British Geological Survey ,Scotland) 1: Phillips, E.R., Key, R.M., Clark, G.C., May, F., Glover, B.W., 50,000 Solid Edition, Sheet 7, Girvan. Chacks®eld, B.C., 1994. Tectonothermal evolution of the Leggett, J.K., Mckerrow, W.S., Eales, M.H., 1979. The Southern Neoproterozoic Grampian and Appin Groups, south-western Uplands: a Lower Palaeozoic accretionary prism. J. Geol. Soc. Monadhliath Mountains, Scotland. J. Geol. Soc. Lond. 151, Lond. 136, 755±770. 971±986. Longman, C.D., Bluck, B.J., 1979. Ordovician conglomerates and Piasecki, M.A.J., 1980a. New light on the Moine rocks of the the evolution of the Midland Valley. Nature 280, 578±581. Central Highlands of Scotland. J. Geol. Soc. Lond. 137, 41±59. Macnair, P., 1908. The Geology and Scenery of the Grampians and Piasecki, M.A.J., New light on the Moine rocks of the Central High- the Valley of Strathmore, vol. II. J MacLehose and Sons, lands 137, 41±59. Glasgow ,256 pp.). Pidgeon, R.T., Aftalion, M., 1978. Cogenetic and inherited zircon Maxwell, G.S., 1990. A Battle Lost: Romans and Caledonians at Mons U±Pb systems in granites: palaeozoic granites of Scotland and Graupius. Edinburgh University Press, Edinburgh ,138 pp.). England. Crustal Evolution in North-west Britain and Adjacent Marshall, J.E.A., 1991. Palynology of the Stonehaven Group, Regions. Geological Society of London, Special Issue 10, pp. Scotland Ð Evidence for Silurian age and its geological 183±220. implications. Geol. Mag. 128, 283±286. Pitbaldo, L.O., 1935. The Roman Invasions. Allen and Unwin, Molyneux, S.G., 1998. An upper Dalradian microfossil reassessed. London ,160 pp.). J. Geol. Soc. Lond. 155, 741±743. Prave, A.R., Alsop, G.I., 1998. Evidence for stratigraphic disconti- Molnar, P., Atwater, T., 1978. Inter-arc spreading and Cordilleran nuities in the Dalradian succession: implications for Neoproter- tectonics as alternates related to the age of subducted oceanic ozoic tectonics. Abstracts Volume, 29th Annual Meeting of the lithosphere. Earth Planet. Sci. Lett. 41, 330±340. Tectonic Studies Group, University of St Andrews, p. 9. Mùrk, M., Kullerud, K., Stabel, A., 1988. Sm±Nd dating of Seve Pringle, J., 1940. The discovery of Cambrian trilobites in the High- eclogites, Norrbotten, Sweden. Evidence for early Caledonian land border rocks near Callander, Perthshire. Adv. Sci. Lond. 1, ,505 Ma) subduction. Contrib. Mineral. Petrol. 99, 344±351. 252. Noble, S., 1998. U±Pb constraints on the tectonometamorphic evolu- Rast, N., Skehan, J.W., 1993. Mid-Paleozoic orogenesis in the tion of the Central Highlands, Scotland. Abstract with Programs, North Atlantic: the Acadian orogeny. Geol. Soc. Am. Spec. Geological Society of America Meeting, Toronto, A215 pp. Pap. 275, 235±247. Osmundsen, P.T., Andersen, T.B., 2001. The middle Devonian Read, H.H., 1961. Aspects of Caledonian magmatism in Britain. basins of western Norway: sedimentary response to large- Proc. Liverpool Manchester Geol. Soc. 2, 653±683. scale transtensional tectonics? Tectonophysics 332, 51±68. Richardson, S.W., Powell, R., 1976. Thermal causes of the Dalra- O'Hara, M.J., 1977. Thermal history of excavation of Archaean dian metamorphism in the central Highlands of Scotland. Scott. gneisses from the base of the continental crust. J. Geol. Soc. J. Geol. 12, 237±268. Lond. 143, 185±200. Robertson, A.H.F., Henderson, W.G., 1984. Geochemical evidence Oliver, G.J.H., Corfu, F., Krogh, T.E., 1993. U±Pb ages from SW for the origins of igneous and sedimentary rocks of the Highland Poland: evidence for a Caledonian suture zone between Baltica Border, Scotland. Trans. R. Soc. Edinb.: Earth Sci. 135, 150. and Gondwana. J. Geol. Soc. Lond. 150, 355±369. Robinson, R.A.J., Rennie, C.A., Oliver, G.J.H., 1998. Palaeocurrent Oliver, G.J.H., Buchwaldt, R., Robinson, R., 1998. How and when data, source terrains and palaeogeographic settings of the Dalra- did the Grampian Orogeny occur in Scotland? Abstracts dian block: the Stonehaven-Dunnnottar Groups revisited. Volume, 29th Annual Meeting of the Tectonics Studies Abstracts Volume, 29th Annual Meeting of the Tectonic Studies Group, University of St. Andrews, 62 pp. Group, University of St Andrews, p. 71. Pankhurst, R.J., 1970. Geochronology of the basic complexes. Robinson, R.A.J., Oliver, G.J.H., 1998. Granite buoyancy and Scott. J. Geol. 6, 83±107. Dalradian terrane uplift. Abstracts Volume, 29th Annual Meet- Pankhurst, R.J., 1974. Rb±Sr whole-rock chronology of Caledonian ing of the Tectonic Studies Group, University of St Andrews, events in northeast Scotland. Bull. Geol. Soc. Am. 85, 345±350. p. 100. Pankhurst, R.J., Pidgeon, R.T., 1976. Inherited isotope systems Rogers, G., Dempster, T.J., Bluck, B.J., Tanner, P.W., 1989. A and the source region history of early Caledonian granites high-precision U±Pb age for the Ben Vuirich granite: implica- in the Dalradian series, Scotland. Earth Planet. Sci. Lett. 31, tions for the evolution of the Dalradian Supergroup. J. Geol. 55±58. Soc. Lond. 146, 789±798. G.J.H. Oliver / Tectonophysics 332 ,2001) 23±49 49

Rogers, G., Dunning, G.R., 1991. Geochronology of appinitic and Stone, P., Floyd, J.D., Barnes, R.P., Lintern, B.C., 1987. A sequen- related granitic magmatism in the W Highlands of Scotland: tial back-arc and foreland basin thrust duplex model for the constraints on the timing of transcurrent fault movement. J. Southern Uplands. J. Geol. Soc. Lond. 144, 753±764. Geol. Soc. Lond. 148, 17±27. Sturt, B.A., 1962. The composition of garnets from pelitic schists in Rogers, G., Paterson, B.A., Dempster, T.L., Redwood, S.D., 1994. relation to the grade of metamorphism. J. Petrol. 3, 181±191. U±Pb geochronology of the Newer gabbros, NE Grampians. Suess, 1888. Der Antlitz der Erde ,quoted in Sollas, 1906). Caledonian Terrane Relationships in Britain. Programme with Tanner, P.W.G., 1996. Signi®cance of the early fabric in the contact Abstracts, British Geological Survey, Keyworth, p. 8. metamorphic aureole of the 590 Ma Ben Vuirich Granite, Perth- Rushton, A.W., Phillips, W.E.A., 1973. A Protospongia from the shire, Scotland. Geol. Mag. 133, 683±695. Dalradian of Clare Island, Co. Mayo, Ireland. Palaeontology 16, Tanner, P.W.G., Leslie, A.G., 1994. A pre-D2 age for the 590 Ma 231±237. Ben Vuirich Granite in the Dalradian of Scotland. J. Geol. Soc. Ryan, P.D., Dewey, J.F., 1991. A geological and tectonic cross- Lond. 151, 209±212. section of the Caledonides of western Ireland. J. Geol. Soc. Thirwall, M.F., 1988. Geochronology of Late Caledonian magma- Lond. 148, 173±180. tism in northern Britain. J. Geol. Soc. Lond. 145, 951±967. Santallier, D., 1994. Mineralogy and crystallisation of the Seve Thompson, R.N., 1985. Model for Grampian tract evolution. Nature eclogites in the Vuoggatjalme area, Swedish Caledonides of 314, 562. Norrbotten. Geol. For. I Stockholm 116, 89±98. Treloar, P.J., Bluck, B.J., Bowes, D.R., Dudek, A., 1980. Hornble- Shepherd, D.S., Adams, C.J., Bird, G.M., 1975. Age of metamorph- nde±garnet metapyroxenite beneath serpentinite in the Ballan- ism and uplift in the Alpine Schist Belt. N. Z. Bull. Geol. Soc. trae Complex of SW Scotland and its bearing on the depth Am. 86, 1147±1153. provenance of obducted oceanic lithosphere. Trans. R. Soc. Smellie, J.L., Stone, P., 1984. Eclogite in the Ballantrae Complex: a Edinb.: Earth Sci. 71, 201±212. garnet±clinopyroxene segregation in mantle harzbuurgite? Tucker, R.D., McKerrow, W.S., 1995. Early Palaeozoic chronology Scott. J. Geol. 20, 315±327. Ð a review in light of new U±Pb zircon ages from Newfound- Smith, R.A., 1995. The Siluro-Devonian evolution of the Midland land and Britain. Can. J. Earth Sci. 32, 368±379. Valley of Scotland. Geol. Mag. 132, 503±513. Upton, B.G.J., Aspen, P., Hunter, R.H., 1984. Xenoliths and their Sollas, 1906. Suess's Face of the Earth II. ii. 82. implications for the deep geology of the Midland Valley of Soper, N.J., 1986. The Newer Granite problem: a geotectonic view. Scotland. Trans. R. Soc. Edinb.: Earth Sci. 75, 65±70. Geol. Mag. 123, 227±236. Van Staal C.R., Dewey, J.F.C., Mac Niocaill, McKerrow, W.S., Soper, N.J., Strachan, R.A., Holdsworth, R.E., Gayer, R.A., Greil- 1998. The Cambrian±Silurian tectonic evolution of the northern ing, R.O., 1992. Sinistral transpression and the Silurian closure Appalachians and the British Caledonides: history of a complex of Iapetus. J. Geol. Soc. Lond. 149, 871±880. west and southwest Paci®c-type segment of Iapetus. In: Blun- Stephens, W.E., 1988. Granitoid plutonism in the Caledonian dell, D.J., Scott, A.C. ,Eds.), Lyell: The Past is the Key to the orogen of Europe. In: Harris, A.L.R., Fettes, D.J. ,Eds.), The Present. Geological Society of London. Special Publication, Caledonian±Appalachian Orogen. Geological Society of 143, pp. 199±242. London, Special Publication No 38, pp. 389±403. Walton, E.K., Oliver, G.J.H., 1991. Lower Palaeozoic Stratigraphy. Stephens, W.E., Halliday, A.N., 1984. Geochemical contrasts between late Caledonian granitoid plutons of northern, central In: Craig, G.Y. ,Ed.), Geology of Scotland, 3rd ed., Geological and southern Scotland. Trans. R. Soc. Edinb.: Earth Sci. 75, Society of London, 612 pp. 259±273. Watkins, K.P., 1987. Biotite- and garnet-forming reactions in Stephenson, D., Gould, D., 1995. British Regional Geology. The inverted metamorphic zones in the Balquhidder region of the Grampian Highlands. 4th ed. HMSO, London ,261 pp.). Scottish Dalradian. Scott. J. Geol. 23, 105±127. Stillman, C.J., Francis, E.H., 1979. Caledonian volcanism in Britain Watson, J.V., 1984. The ending of the Caledonian orogeny in Scot- and Ireland. In: Haris, A.R.L., Holland, C.H., Leake, B.E. land. J. Geol. Soc. Lond. 141, 193±214. ,Eds.), The Caledonides of the British Isles Reviewed. Geolo- Williams, H., 1994. IUGS Commission on Stratigraphy, Subcom- gical Society of London, Special Publication, 8, pp. 557±577. mission on Ordovician Stratigraphy, Ordovician News. No 11. Stone, P., 1995. Geology of the Rhins of Galloway. Memoir of the 44 pp. British Geological Survey, Sheets 1 and 3 ,Scotland), p. 102. Wills, J.J., 1929. Physiographic Evolution of Britain. II, vi, p. 76.