Journal of the Geological Society, London, Vol. 155, 1998, pp. 541–550. Printed in Great Britain
Garnet provenance studies, juxtaposition of Laurentian marginal terranes and timing of the Grampian Orogeny in Scotland
A. R. HUTCHISON* & G. J. H. OLIVER Crustal Geodynamics Group, School of Geography & Geosciences, University of St Andrews, Purdie Building, North Haugh, St Andrews, Fife, KY16 9ST, UK (e-mail: [email protected]) *Present address: BP Exploration Operating Co. Ltd., Fairburn Industrial Estate, Dyce, Aberdeen AB21 7PB, UK
Abstract: A study of the composition and zoning patterns of detrital garnets from Ordovician greywackes from the Northern Belt of the Southern Uplands terrane of Scotland reveals characteristics of the metamorphic sources very similar to the Dalradian Supergroup of the Scottish Grampian terrane. The radiometric cooling and uplift ages of the Dalradian metamorphic zones and the depositional ages of the Southern Uplands greywackes support the hypothesis for local provenance. Detrital metamorphic garnet, identical to Dalradian garnet, has also been identified in the Upper Ordovican sandstones of the Highland Border Complex. These observations do not support proposals that the Grampian, Midland Valley and Southern Uplands terranes were exotic to each other in the Late Ordovician time. These new results, together with a review of published age dates, clarify the Late Ordovician palaeogeography for this part of the Laurentian margin. The distances between the Grampian, Midland Valley and Southern Uplands terranes may have been similar to the present day. It is concluded that large rivers flowed out of the uplifting mountainous Grampian terrane and across the Midland Valley into a Southern Uplands trench during the Late Ordovician time. The main orogeny (i.e. mountain building) in the Grampian terrane was therefore post-Cambrian, producing the first high mountains and resultant flysch in the Caradoc.
Keywords: Laurentia, Grampian orogeny, provenance, greywackes, palaeogeography.
The usefulness of detrital garnet as a provenance indicator is and when the Iapetus Ocean opened and closed, has been well known (Morton 1985; Houghton & Farrow 1989; subject to speculation (e.g. Elders 1987; McKerrow et al. 1991; Tebbens et al. 1995). Garnet is used in such studies because it see discussion below). can withstand long distances of transport in fluvial–deltaic environments; it is resistant to chemical modification during diagenesis and low-grade metamorphism, and its com- Southern Uplands terrane positional variation can make it specific to source area. The Although the origin of the Southern Uplands is still considered present work is the first description of the provenance of UK controversial, this paper follows the fore-arc accretionary Ordovician detrital garnet based on quantitative electron prism model (McKerrow et al. 1977; Bluck 1983; Walton & microprobe analyses. Compositions and zoning patterns of Oliver 1991). Flysch greywacke sedimentation in the Southern garnets from Ordovician greywackes from the Southern Uplands began in earliest Caradoc (N. gracilis) times and Uplands and Midland Valley terranes are compared with continued through to the Upper Wenlock (Peach & Horne published work and new garnet analyses from the Grampian 1899). Lapworth (1889) interpreted the structure of the terrane. The aim of this paper is therefore to present a specific Southern Uplands as a fan of folds in the form of large provenance for the greywackes, namely the metamorphosed anticlinoria and synclinoria. Using the same Ordovician and Dalradian Supergroup, and to propose a palaeogeography. Silurian biostratigraphic evidence, McKerrow et al. (1977) explained the apparent paradox in which fault-bounded tracts become younger towards the southeast, whilst the beds within Geological setting these tracts young in the opposite direction towards the The Southern Uplands terrane (Fig. 1) is defined as those northwest. This is a pattern seen in modern accretionary tectonostratigraphic units that occur between the Southern prisms. The tectonostratigraphic features of the Southern Uplands Fault and the putative Iapetus Suture. The region Uplands of Scotland are therefore interpreted as the result of a between the Southern Uplands Fault and the Highland continuum of deformation and metamorphism in a developing Boundary Fault is termed the Midland Valley terrane whilst accretionary prism terrane (McKerrow et al. 1977; Oliver & the area of rocks between the Highland Boundary Fault and Leggett 1980). The combined results of Leggett et al. (1983), the Great Glen Fault is termed the Grampian terrane (Stone Freeman et al. (1988), Oliver & McKerrow (1984) and & Kimble 1995). On most continent reconstructions (using Anderson & Oliver (1996) suggest that the Southern Uplands palaeogeography, faunas and palaeomagnetism) for latest crystalline basement is composed of high-grade metamorphic Precambrian and Early Palaeozoic times, all three terranes are equivalents of the prism in thrust contact with mylonitonized juxtaposed against the margin of a Laurentian Supercontinent Lake District-type Borrowdale Volcanics. (Scotese & McKerrow 1990; McKerrow et al. 1991; Soper Alternative models for the Southern Uplands involve 1994; Dalziel et al. 1994; Dalziel 1997). However, the amount Ordovician back-arc or marginal basins and island arcs, with of lateral separation between these terranes, particularly where Silurian successor basins and advancing foreland propagating
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thrust stacks (Morris 1987; Stone et al. 1987; Hutton & Midland Valley terrane Murphy 1987; Kelling et al. 1987; Armstrong et al. 1996) with an island arc separating the Northern and Central Belts being According to Bluck (1984), during the Ordovician, the subsequently excised by the Orlock Bridge Fault (Anderson & Midland Valley terrane was composed of a central Midland Oliver 1986). Valley Block of batholithic and volcanic arc with interarc basins, a southern Ballantrae Igneous Complex with proximal fore-arc basin (coeval with a Northern Belt accretionary prism but now missing due to overthrusting by the Southern Uplands) and a northern, marginal Highland Border basin containing the Highland Border Complex. The latter contains probable pre-Arenig to Arenig ophiolite plus associated Arenig to Late Llanvirn aged deep-sea sediments and lavas unconformably overlain by Caradoc sandstones and lime- stones. These Caradoc sediments were supposedly derived from the Midland Valley Block since the sandy detitus (for example the Bofrishlie Burn and Achray Sandstone For- mations) is granitic in provenance and apparently unlike the Dalradian metamorphics (Curry et al. 1984; Bluck 1984). On the contrary, Robertson & Henderson (1984) argued on the basis of geochemistry that the sediments were derived from a terrigenous terrane similar to that which supplied the Dalradian Supergroup and therefore did not completely rule out the Dalradian as a source. According to Bluck (1984) the Cambrian–Arenig Ballantrae Igneous Complex has a marginal basin-arc setting; con- glomerates of Early Llanvirn, Late Llanvirn and Caradoc ages with fluviatile and marine components transgress northwards over the Complex (see summary in Walton & Oliver 1991). MacNiociall & Smethurst (1994) and Dalziel (1997) include the Ballantrae Igneous Complex as part of a Peri-Laurentian intraoceanic arc.
Grampian Terrane
This terrane is made up of the Dalradian Supergroup of late Proterozoic and early Early Palaeozoic age. These are mostly marine sediments, laid down on a rifted continental margin, and regionally metamorphosed under conditions varying from Barrovian kyanite–sillimanite high-temperature and high- pressure to Buchan cordierite–andalusite low-pressure and high-temperature conditions during the late Proterozoic–early Ordovician (summary in Johnson 1991; Dempster 1985; Dempster et al. 1995). The bulk of the Dalradian has been metamorphosed to garnet grade and above (see Fig. 1). Dempster et al. (1995) place great emphasis on the oldest Rb–Sr mineral ages, 515 Ma from Angus and 505 Ma from the Buchan coast, as the minimum ‘cooling’ ages after a Late Cambrian (or end-Proterozoic) Grampian orogeny. The long delay before cooling in the mid-Ordovician is a problem that this paper throws light on.
Fig. 1. Maps of sample localities 1–21. Upper map shows the Grampian, Midland Valley and Southern Uplands terranes. Note the large extent of garnet-bearing subcrop (i.e. garnet+kyanite+ sillimanite Barrovian zones). BB, Bofrishlie Burn; LCQ, Lime Craig Quarry; GE, Glen Esk; GGF, Great Glen Fault; HBF, Highland Boundary Fault; SUF, Southern Uplands Fault; NB, Northern Belt; CB, Central Belt; SB, Southern Belt; IS, Iapetus Suture; BV, Borrowdale Volcanics; LD, Lake District. Lower map shows the geology of the Ballantrae–Barrhill–Cairnryan area. Map references to sample localities given in text.
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Previous work on Southern Uplands greywacke One clast from the Craigputtock conglomerate was subse- provenance quently reported as having a muscovite Rb–Sr age of 463&5 Ma (van Breemen pers. comm. to Kelley & Bluck In the early work on the Southern Uplands of Scotland, Peach 1989). & Horne (1899) used rock fragments in conglomerates as a method of defining provenance. For example they observed diabase, serpentine, quartz-felsite, chert, limestone and grey- wacke in the Corsewall Conglomerate. Walton (1956) studied Garnet analysis the Finnarts and Glen App conglomerates, finding clasts of Sampling for this work was carried out in the Southern granite, microgranite, spilite, gabbro, dolerite, hornblende- Uplands greywackes from the Corsewall Formation (Grid andesite, epidosite, serpentinite, amphibolite, greywacke, silt- Reference [NX0590 7810], sample locality 1, Fig. 1), the stone, shale, chert, glaucophane schist, mica schist, quartzite Finnarts Member of the Corsewall Formation and vein quartz. He also found pyroxenes within the ([NX0455 7407], sample locality 2, Fig. 1), the Kirkcolm greywackes of the Marchburn Formation. The presence of Formation ([NX0535 7210, NX0680 6960, NX2302 8151]; serpentinite, gabbro, dolerite, spilite and chert indicated an sample localities 3,4&5,Fig. 1), and the Galdenoch Member ophiolitic source, which Walton (1956) interpreted to be of the Kirkcolm Formation ([NX2297 8148], sample locality 6, eroded from the rocks of Ballantrae. This interpretation was Fig. 1). Greywacke was also collected from the Highland supported by the turbidite flow directions being observed to be Border Complex from a road cut near the Bofrishlie Burn from the northwest. Boulders in conglomerates could not have ([NS5055 9905], sample locality BB, Fig. 1), from the Achray travelled far, therefore suggesting a local source. Glaucophane Sandstone from near Lime Craig Quarry ([NS5334 0184], schist, andesite and plagiogranite clasts were observed to sample locality LCQ, Fig. 1), and from the Margie Series in be similar to rocks exposed near Ballantrae. Walton (1956) Glen Esk ([NO7258 5926], sample locality GE, Fig. 1). concluded that the Ordovician conglomerates were locally Samples were also taken from the southern edge of the derived, though he could not find local sources for boulders of garnet zone of the Dalradian at Stonehaven ([NS8945 8935], granite with microcline and microperthite. Kelling (1961, 1962) sample locality 7, Fig. 1) where the garnet is known to have also observed turbidites with a provenance of mixed spilitic crystallised at low pressure and be Mn-rich (Droop & Harte volcanic arc–plutonic–metamorphic sources in the Upper 1995). Ordovician rocks of the Rhins of Galloway. Samples of 10–15 kg were collected, jaw crushed, disc milled, The relatively high serpentinite content, whole rock chemical and then passed over a Wilfley concentrating table. The sieved analyses with elevated values of Cr and Ni and the com- heavy mineral separates were then further divided using tetra- position of detrital chromite from the greywackes of the bromoethane heavy liquid and an electromagnetic separator. Tappins Group indicate a strong ophiolitic signature (Duller & Garnet was hand picked from a generally zircon-rich separate Floyd 1995; Floyd in press). However, the temptation to cite and mounted in araldite blocks for electron microprobe analy- the Ballantrae Ophiolite as a local source, thus constraining sis. The average grain size of the garnet is medium sand-size, strike slip movements on the Southern Uplands Fault, has at 0.25–0.5 mm. Garnets from the Dalradian metamorphic been resisted as similar ophiolites occur along most of the samples were analysed in polished section. Additional Laurentian margin (Coleman-Sadd et al. 1992). Dalradian data were taken from Ashworth & Chinner (1978), Elders (1987) produced Rb–Sr mineral–whole rock ages of Atherton (1965, 1969), Atherton & Edmunds (1966), Chinner 1265–475 Ma from granitic clasts at Corsewall Point. He (1960, 1961), Dempster (1983), Droop & Harte (1995), Harte argued that possible granitic sources for the older ages were & Graham (1975), Phillips et al. (1993, 1994), Sturt (1962) and found only in Newfoundland, thus constraining the source and Watkins (1987). These sample localities, 7 to 21 respectively, suggesting sinistral emplacement of the Southern Uplands are shown on Fig. 1. A full list of the 316 garnet analyses terrane with large (c. 1500 km) strike-slip displacements. discussed in this paper is available on computer disk from the Kelley & Bluck (1989) suggested on the basis of 40Ar/39Ar mica authors on request. dating that this is unlikely due to an apparent lack of Grenville mica ages in the Southern Uplands whilst Grenville ages are abundant in the Humber terrane in Newfoundland (Keppie & Dallmeyer 1988). Kelley & Bluck (1989, 1992) also discounted Results of garnet analysis a Dalradian source, arguing that even though the ages match The detrital core analyses were first plotted on FeO+MgO the Dalradian, the sedimentological problems of transporting versus MnO+CaO weight percent graphs (not illustrated here) metamorphic boulders across the 100 km wide Midland Valley of the type used by Sturt (1962) in a study of the composition terrane are insurmountable. This is despite the fact that Kelley of pelitic garnets relative to the grade of metamorphism. These & Bluck (1992) report a 447&6Ma40Ar–39Ar muscovite age graphs show that Southern Uplands and Highland Border from a garnet mica shist boulder from the Northern Belt. detrital garnets have a similar range of grades to those in the McKerrow et al. (1991) also discounted a Dalradian source, Grampian metamorphic terrane. The fact that the Fe–Mg and considering sources as far away as in a Mongolian arc or Mn–Ca end members are plotted as a sum total reduces the Baltica. The Mongolian arc source was favoured on the basis ability of this plot to show subtle variations of composition of a better palaeogeographical fit. required to define detailed provenance. For this reason the Bluck (1984) claimed that no detritus of certain Dalradian compositions were plotted on a triangular plot using molecular type has been found in sediments older than Upper Old Red values of Fe+Mn, Mg, and Ca calculated on the basis of 24 Sandstone south of the Highland Boundary Fault, despite the oxygens (Fig. 2). The compositions were normalized with report of c. 460 Ma Rb–Sr mineral–whole rock dates for respect to Fe, Mn, Mg, and Ca so that they could be directly muscovites and biotites in metamorphic clasts from the compared with the plots presented by Droop & Harte (1995). Silurian of the Southern Uplands (van Breemen et al. 1984). These triangular plots are discussed below.
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Fig. 2. Garnet Ca/T–Mg/T–Fe/T+Mn/T triangular plots. (a) The area of the triangular plot displayed in (b)–(g) inclusively; (b) compositional fields of garnets from the Dalradian, the Lewisian, the Midland Valley diatremes and the Ballantrae Ophiolite metamorphic sole; (c) the Kirkcolm Formation; (d) the Galdenoch Member of the Kirkcolm Formation; (e)the Corsewall Formation; (f) the Finnarts Member of the Corsewall Formation; (g) Highland Border Complex detrital garnets. All values are molecular values calculated on the basis of 24 oxygens and normalised according to the proceedure of Droop & Harte (1995) where T=Fe+Mn+Mg+Ca. Note the strong similarities between the detrital garnet and the Dalradian garnet and the disimilarity with other potential sources.
Fig. 3. A comparison of zoning patterns in euhedral garnets displaying compositions similar to the Dalradian from the Tappins Group (a) and the Kirkcolm Formation (c) with zoning patterns from the Aberfeldy transect (b&d). From Dempster (1983).
Comparison of garnet zoning patterns comm. 1983), Watkins (1987) and Goodman (pers. comm.). These are shown in Fig. 3. The similarities include generally The garnets that showed the greatest variations of composition raised Ca levels at rims and double Fe-maxima in the cores as from core to rim were selected for a study of zoning patterns. seen in the Aberfeldy area (Dempster 1983). Samples from Euhedral crystals were chosen for analysis as these were more Stonehaven and Glenshee display flat traces which were seen in likely to have intact rims. The zoning patterns of Southern most of the detrital garnets. These apparently flat patterns Uplands detrital garnets were compared with Dalradian could however have been caused by the removal of the zoned examples from Atherton & Edmunds (1966), Dempster (pers. rims during transport, reducing the variation observed.
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Discussion sources for the Southern Uplands greywackes. The Dalradian garnet-bearing rocks display rapid uplift rates at a time Discussion of garnet results from the Southern Uplands that allows them to be exhumed at the onset of greywacke terrane sedimentation in the Southern Uplands (see Fig. 4). McKerrow et al. (1991), using Dempster’s (1985) results, Analyses of detrital garnet from the Northern Belt formations suggest that the beginning of greywacke sedimentation around shows strong similarities with the Scottish Dalradian when 460 Ma was a period of little uplift in Dalradian times, plotted on triangular diagrams using molecular values of preventing the Dalradian strata from providing detritus. Fe+Mn, Mg, and Ca (Fig. 2), the most complete overlap Figure 4 shows that this is incorrect: the central Dalradian in composition being seen in the Kirkcolm Formation and Angus transect (Dempster 1985) displays rapid cooling rates the Galdenoch Member. The earliest deposited Corsewall from around 462 Ma through to around 448 Ma (i.e. Late Formation and the Finnarts Member display some garnet Llanvirn–Caradoc) between the kyanite and sillimanite iso- compositions characteristic of higher grades than those seen in grads, and moderate uplift rates from as early as 480 Ma just the Dalradian: Fig. 2 shows three high-grade detrital grains north of the staurolite isograd in the southern Dalradian that are not obviously comparable to Dalradian or Lewisian transect (Dempster 1985). The Ar–Ar fusion ages of detrital examples. They plot nearest Grenville-aged quartzofeldspathic muscovite from the Northern Belt vary from Late Cambrian gneiss garnets from xenoliths in Carboniferous vents in the (coincident with Dalradian peak metamorphism according to Midland Valley (Halliday et al. 1993). The lack of high-Mn Dempster 1985, but see Evans & Soper (1997) who challange garnets in the Corsewall Formation suggests that there was the validity of the pre-Arenig age) through to Caradoc, almost no contribution from a high-Mn garnet zone such as the exactly the same range of K–Ar muscovite ages found in the Stonehaven area of the Dalradian (Droop & Harte 1995). Dalradian. The mica age ranges from the Northern Belt and Their absence may be explained by a river system which simply the Dalradian are the same, so regardless of their interpretation, did not flow over and sample the eastern Dalradian. There is a the link is feasible (see Fig. 4). Note that fusion 40Ar–39Ar and notable lack of Lewisian-type granulite facies garnet detritus in K–Ar ages from the same muscovite should be the same. the Northern Belt: consequent rivers flowing SE from a rising Walton (1955) reports that euhedral garnet is an abundant Grampian orogen might not have tapped Lewisan basement. heavy mineral in many Central Belt turbidites. We have not Those flowing NW might have deposited orogenic detritus on analysed this garnet yet but we would like to speculate that this inboard parts of the Durness passive margin sequence which too is of Barrovian type. Late Ordovician–Early Silurian rapid formed the cover to the Lewisian–Torridonian. So a lack of cooling ages (450–440 Ma) were measured by Dempster (1985) Lewisian detritus is predictable. Rivers draining into the in the central transect of the Dalradian in Perthshire: this present-day Midland Valley do not sample Lewisian bedrock. might be the source for the Central Belt detrital garnet. Kelley There is a lack of detrital Ca-rich compositions which & Bluck (1992) found that detrital muscovites fron the Central characterize the metamorphic sole of the Ballantrae Ophiolite Belt were virtually the same age as those found in the Northern (Fig. 2b). This is perhaps not surprising considering the small Belt (see Fig. 4). size of garnetiferous outcrops at Ballantrae (Treloar et al. 1980). The suggestion that the Southern Upland detrital mica ages Much of the discussion on Southern Uplands provenance match the Mongolian Arc requires an apparent transport has been based on radiometric dating (Elders 1987; Kelley & distance of around 2500–3000 km from a position adjacent to Bluck 1989, 1992; McKerrow et al. 1991). The possibility of the east coast of central Greenland (McKerrow et al. 1991). Grenville-aged basement in the Midland Valley cannot be It is quite unrealistic to accept that proximal greywacke dismissed: metamorphic garnet granulite xenoliths from turbidites such as those found in the Northern Belt could have Carboniferous Midland Valley volcanic necks yield Rb–Sr been transported so far. Their proximal nature suggests a whole rock ‘ages’ of 1101&63 Ma (MSWD=8) and Sm–Nd much more local source. The occurrence of schist clasts in the model ages of c. 1100 Ma (Davies et al. 1984). These match Kirkcolm Formation also reflects immaturity. The consistently the Corsewall plutonic boulder dates of Elders (1987). The north-westerly flow directions (Walton 1956; Kelling 1962) possibility of a Midland Valley basement source is one that within conglomeratic members also suggest non-Mongolian Elders (1987) does not consider. The possibility of Grenville derived flows: according to McKerrow et al. (1991) Mongolia aged boulders sourced in the local Midland Valley basement is lay to the northeast. consistent with the model of a Dalradian source for further Watson (1984) did not discount the possibility of deposition travelled sand-sized garnet. of Dalradian detritus in the Southern Uplands but argued that Despite a match between detrital mica ages from the the volume of detritus that would be produced by the erosion Southern Uplands and the Dalradian (see Fig. 4), Kelley & of 25–30 km of Dalradian crust might yield a pile of detritus of Bluck (1989, 1992) suggested that the Midland Valley was too unrealistic thickness if it was all deposited in the Southern wide in the Ordovician to allow boulders to be preserved Uplands alone. The comparative sizes of the Grampian terrane during c. 100 km of transport. Alternatively, they inferred the and the combined Midland Valley and Southern Uplands presence of Dalradian blocks within the Midland Valley terranes are similar (Fig. 1). The diversion of some of the although Dalradian-like xenoliths have yet to be found in any sediment into basins now hidden in the Midland Valley (and Midland Valley vents (Davies et al. 1984). other hidden basins in the North and Irish Sea) does not The coincidence of Northern Belt plutonic boulder ages with conflict with model described below. Grenville aged basement to the Midland Valley (Davies et al. A further argument against a Dalradian source was 1984; Elders 1987), the coincidence of mica dates from the presented by Stone & Evans (1995) on the basis of a com- Northern Belt greywackes with Dalradian mica ages (Kelley & parison of åNd440–460 Ma values from the greywackes and Bluck 1989, 1992; Dempster 1985), together with the strong the åNd400 Ma values of the Dalradian. However, a close correlation of detrital garnet compositions with the Dalradian look at the data reveals that the average value for the now offer a strong argument for the involvement of local quartzofeldspathic (basement) derived Kircolm Formation
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Fig. 4. Stratigraphical column illustrating: (1) the ages of greywacke sedimentation in the Southern Uplands; (2) ages of peak Dalradian metamorphism, rapid cooling and exhumation of garnet bearing rocks in the Grampian terrane (Rb–Sr and K–Ar muscovite ages from Dempster 1985 and Dempster et al. 1995), and (3) 40Ar–39Ar fusion ages of detrital muscovite in Northern and Central Belt greywackes (Kelley & Bluck 1989, 1992). Note how the Northern and Central Belt 40Ar–39Ar detrital muscovite ages match the Grampian K–Ar muscovite cooling ages. The age of peak Dalradian metamorphism is taken from Dempster (1985) and Dempster et al. (1995) although Evans & Soper (1997) challenge this. Ballantrae stratigraphy from Bluck (1985) and Armstrong et al. (1996); Highland Border Complex stratigraphy from Curry et al. (1884); Southern Uplands stratigraphy from Armstrong et al. (1996) and Leggett et al. (1979). Time-scale taken from Tucker & McKerrow (1995).
with abundant detrital metamorphic garnet is Galdenoch Member is significantly different at "3.2 (n=6). åNd464 Ma= "10.2 (n=5). This is virtually the same as the An explanation for these results is that juvenile volcanic arc average Dalradian åNd400 Ma value ("11.7, n=5). The detritus has swamped the Galdenoch Member whereas average åNd464 Ma value for the andesitic volcanic arc-derived Dalradian-type basement detritus dominates the Kircolm
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Formation. However, significant amounts of Barrovian Alternative palaeogeography (Dalradian)-type garnet have found their way into the According to Copeland & Harrison (1990), 40Ar/39Ar dating of Galdenoch Member (Fig. 2c). That the Galdenoch Member detrital K-feldspar and muscovite shows that in every level in turbidites have volcanic arc pyroxene, contain metamorphic drill holes in the Bengal fan, the age of deposition is identical garnet, and display SE derived palaeocurrent indicators (Stone to at least one pair of detrital mineral ages. Therefore a et al. 1987), is quite paradoxical unless one accepts Leggett’s significant portion of the Bengal fan is first cycle detritus from (1987) hypothesis that Galdenoch turbidites were originally the Himalayas. Most greywacke sequences have a provenace in axial (NE–SW) flows that were diverted to the NW by the actively uplifting sources. From this it follows that if the age of topography of the ocean trench. the greywackes in the Southern Uplands is Ordovician and The average åNd400 Ma value for Lewisian granulite gneises Silurian then there is a good chance that the age of the source is "19 (n=5, Frost & O’Nions 1985). The much higher values uplift will be the same and the detrital components in the pile found in the Northern Belt suggest that significant Lewisian will carry this age signature. Kelley & Bluck (1989, 1992) make sources were not involved, a point borne out by the lack of this linkage very clear. granulite facies detrital garnets in the Northern Belt so far It is therefore proposed that a deluge of first cycle orogenic sampled. detritus flooded southwards from the Grampian terrane during the Caradoc, with garnet compositions and mica ages identical to the Dalradian. It follows that the Grampian orogeny must Discussion of garnet results from the Highland Border have immediately predated the Caradoc, i.e. it must be Complex late Arenig–Llanvirn. Inspection of Fig. 4 shows that there is The fact that Ordovician sandstones from the Bofrishlie Burn, about 15 Ma between the time (c. 475 Ma, Arenig–Llanvirn Achray Sandstone Formation and Glen Esk Margie Series boundary) that the bulk of Dalradian garnet–mica schist have yielded detrital metamorphic garnet identical to the passed through the K–Ar closure temperature and the time Dalradian in the neighbouring Grampian terrane (Fig. 2g) is (c. 460 Llanvirn–Caradoc boundary) that identical micas and surprising in view of the comments by Bluck (1984, p. 278) e.g. garnets first arrived in the Southern Uplands. ‘Neither in the Margie Series nor the rest of the Highland Although Northen Belt greywackes are often dominated Border Complex is metamorphic debris of Dalradian aspect by fresh volcanic arc detritus, there is always a strong meta- recorded’ and Curry et al. (1984, p.131) e.g. ‘the Scottish morphic component, i.e. detrital garnet and muscovite, the Dalradian metamorphic block is an alien terrane with respect latter with Arenig to Caradoc radiometric ages. This suggests to the Midland Valley arc-massif and its adjacent Highland that active volcanic arc(s) and the actively metamorphosing Border basin until Devonian times’. They used the apparent Laurentian continental margin came together (i.e. collided) lack of metamorphic detritus and the abundance of acid ig- within the Arenig–Caradoc (470–450 Ma) period producing neous detritus to promote a non-Dalradian ‘Midland Valley’ high mountains. On this evidence, orogeny in the Grampian provenance. The Achray Sandstone is in fact rich in detrital terrane was therefore mostly post-Cambrian and pre-Caradoc. white mica, plagioclase and microcline and although it is Armstrong et al. (1996) have investigated the sedimentary reasonable to assume that this is derived from granite, a meta- succession overlying the Ballantrae Igneous Complex and in morphic source for some of the mica cannot be discounted. the neighbouring Northern Belt. They suggested extension and Bluck (1984, 1985) had great difficulty in resolving the subsidence of a shallow marginal shelf during the Caradoc. apparent paradox whereby the uplift and cooling of the This model allows for the flow of detritus from a Midland Dalradian took place in the period c. 520 to c. 430 Ma thus Valley Arc on to the obducted Ballantrae ophiolite at the same overlapping the time of ophiolite formation and sedimentation time as greywacke sedimentation occurred in the Northern in the now juxtaposed Highland Border Complex (see Fig. 4). Belt. It is proposed to go further here by suggesting that the However, these data can be used to suggest that uplift of the Midland Valley terrane supplied sediment (originating in the Dalradian was indeed going at the time the Highland Border Grampian terrane) into large river systems running across ophiolite was being formed in a neighbouring stretching the Midland Valley (see Fig. 5). The apparent dearth of marginal basin. large Dalradian-type clasts in conglomerates in the Southern It remains hypothetical whether or not the Highland Border Uplands may be explained by the fact that these boulders ophiolite formed in a modern day Gulf of California type of mostly did not survive the journey across the c. 100 km wide situation (i.e. where a spreading ridge is being subducted at Midland Valley. However, there is no problem for sand and right angles to a continental margin with the oceanic fracture silt-sized garnet detritus crossing the gap: the Brahmaputra– zones, causing strike slip faulting parallel to the margin and the Ganges river system delivers sand-size detritus >300 km down- opening of an elongated gulf). Alternatively, Bluck’s model stream into the Bengal fan, whilst the Rhine delivered Alpine (1984, fig. 5) may be more applicable: the Highland Border metamorphic garnet >400 km down to the Netherlands coast ophiolite formed in the stretched gap between a Midland during the Quaternary (Tebbens et al. 1995). A contribution to Valley Arc and the Grampian terrane and is therefore a back the Northern Belt from the Midland Valley is also supported arc basin. Whichever way the Highland Border ophiolite was by the matches between the Grenville-aged basement xenoliths formed, the results of this work on detrital garnet suggests that from Midland Valley volcanic necks and the granitic clasts the Highland Border Complex was probably never very far of the Corsewall Point Conglomerate (Elders 1987; Davies away from the Grampian terrane, at least from the Caradoc et al. 1984). Therefore a modified palaeogeography to that of onwards, some 100 Ma earlier than proposed by Bluck (1984). Bluck (1985) and Armstrong (1996) is proposed whereby a This is also the time that shelf faunas from Baltica, Gondwana Brahmaputra–Ganges-scale fluvial system originating in the and Laurentia started to become cosmopolitan, with the Grampian terrane cut across the Midland Valley to deposit elimination of the wide Iapetus and Tornquist oceans as the proximal submarine fans in the Northern Belt of the Southern three continents converged (Cocks & Fortey 1982; Oliver et al. Uplands, bringing in a mixture of Dalradian metamorphics, 1993). Midland Valley basement, Ballantrae ophiolitic detritus,
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Fig. 5. Reconstruction of the Grampian Midland Valley basin and continental Southern Uplands palaeogeography of the Grampian, Terrane - margin arc with "Grenville" plutonic accretionary prism. Midland Valley, and Southern Uplands terranes during the Upper Ordovician: a the Dalradian. - metamorphic basement. large river complex drains out of the Grampian terrane, crosses the Midland Valley terrane containing a ‘Grenvillian’ crystalline basement, active plutonic–volcanic continental margin arc, Sea level. obducted Ballantrae Ophiolite and presumably suducted Highland Border Complex (HBC). The Achray Sandstone (AS) is shown schematically overlying + Ballantrae + + + + + AS + + + + + + + + + + + + + the HBC receiving local detritus from + + + + + + + + + + + + + + + + + + + + + + + Plutons + + + + both the Grampian and Midland Valley + + + + + Metamorphic sole of + + + + + + + + HBC + + + + + + + ophiolite terranes. Ganges-sized rivers feed + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + submarine fans into the Northern Belt + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + thus bringing Grampian garnet and mica + + + + + + + + + + + + "Grenville" Basement. into the Southern Uplands terrane. A modern-day analogue might be the Highland Boundary Fault. Southern Upland Fault. Pacific–British Columbia region.
and Midland Valley island arc detritus into the Southern Another objection to this new model might be that the small Uplands (see Fig. 5). A modern-day analogue might be area of presently exposed Ordovician rocks in the Midland the Pacific–British Columbia–Washington margin where Valley is not dominated by fluviatile conglomerates of obvious Cordilleran Barrovian metamorphic–granitic basement is over- Dalradian provenance. Conway et al. (1987) carried out a thrust by Mesozoic arcs and ophiolites and is partially covered seismic study that suggested thicknesses of up to 5 km of Lower by a modern day volcanic arc: this is presently being eroded by Palaeozoic and Lower Old Red Sandstone over most of the large river systems such as the Columbia–Snake, which are Midland Valley. Therefore the presently exposed Ordovician in transporting sediments hundreds of kilometres into the Pacific the Midland Valley at Ballantrae may not be representative of trench (Howell 1985). the total: there is potential for a considerable hidden Ordovician Metamorphic schist and gneiss clasts are seen as a minor sequence. Haughton & Halliday (1991) have already speculated component of the Llanvirn Kirkland Conglomerate at that there might be a significant hidden Lower Ordovician Ballantrae and in the Early–Mid-Caradoc Kilranny flysch succession in the Midland Valley based on clasts in the Conglomerate at Girvan (Bluck 1983). These clasts have not Lower Old Red Sandstone on the east coast. yet been tied to the Grampian terrane, but the involvement of There remains the problem of enigmatic ages of boulders a metamorphic source within or near to the Midland Valley in the Highland Border Complex and the Northern Belt: during the Ordovician is consistent with this new model as is Dempster & Bluck (1989) report 1863, 1799 and 1817 Ma the fact that both conglomerates have significant fluviatile mica–whole rock ages for c. 50 cm diameter clasts in the components (Ince 1984). Highland Border Complex whilst Elders (1987) reports Proterozoic ages of up to 1265 Ma from even bigger granite boulders from the Corsewall Conglomerate. These are Objections to the model emphatically not Dalradian cooling ages and not found in local The presence of Barrovian detrital garnet in Ordovician Dalradian lithologies. Perhaps their provenance lies buried in sediments does not specifically identify the Grampian terrane the Midland Valley. However, Northern Belt detrital micas as the source. However, either combined sinistral or dextral ages from greywackes associated with the Corsewall Conglom- strike-slip movement along the Highland Boundary and erate are identical to Dalradian mica cooling ages. Detrital Southern Uplands Faults cannot be much larger than about micas from the Highland Border Complex need to be dated. 500 km: if there had been significant sinistral movement in the Ordovician then detrital white mica should have invaded the Southern Uplands originating from the Grenvillian of Summary Newfoundland, but such Grenvillian aged mica has not been (1) The compositions of Upper Ordovician Southern Uplands found. If there had been significant dextral movement, and Highland Border detrital garnets match metamorphic continuing eastwards along the Tornquist Suture Zone (Oliver garnets from the Grampian terrane. et al. 1993), then detrital white mica should have invaded the (2) 40Ar–39Ar detrital muscovite mica fusion ages from the Southern Uplands originating from the Sveconorwegian/ Northern (and Central) Belt greywackes coincide with K–Ar Grenvillian of SW Baltica by the end of the Ordovician; again and Rb–Sr muscovite ages in the Grampian terrane, suggesting this has not been recorded. Also detrital eclogite or granulite a Dalradian source for the mica. facies detrital garnet is evidently lacking in the Ordovician of (3) Northern Belt greywacke–flysch with mixed provenances the Midland Valley or Southern Uplands terranes; such would indicate that active volcanic arc(s) and the actively meta- be expected to occur had Baltica been in the near vicinity morphosing Laurentian continental margin came together of Scotland during Ordovician times. Palaeomagnetic data (i.e. collided) in the Arenig–Caradoc period. The main orogeny indeed precludes Mid-Ordovician juxtaposition of Laurentia (i.e. mountain building) in the Grampian terrane was therefore (i.e. northern Britain) and Baltica (MacNiocaill & Smethurst post-Cambrian, producing the first high mountains and 1994). resultant flysch in the Caradoc.
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Received 1 April 1997; revised typescript accepted 20 October 1997. Scientific editing by Simon Kelley.
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