Introduction

Armorica is one of a number of terranes (collectively termed peri-Gondwanan) that were located along the northern Gondwanan margin during the late Neoproterozoic (Fig. 1). Together, these terranes record a protracted (c. 780–600 Ma) history of subduction followed at c. 600–550 Ma by a diachronous transition to a continental San Andreas-type transform fault environment, which terminated orogenic activity. In Armorica, this period of orogenic activity is referred to as the Cadomian Orogeny (Bertrand 1921). Development of a stable platform environment followed in the Early Cambrian (e.g. Murphy and Nance 1989, 1991; Taylor and Strachan 1990; Nance et al. 1991, 2002, 2007; Murphy et al. 2000; Keppie et al. 2003). One of these terranes, Avalonia, rifted from the Gondwanan margin during the early Ordovician (Cocks and Torsvik 2002). The subsequent northward drift of Avalonia was associated with the closure of the Iapetus Ocean and the eventual collision of Avalonia with Baltica and Laurentia during the late Ordovician to Silurian to result in the Caledonian orogeny (Pickering et al. 1988; Soper et al. 1992). As Avalonia moved northwards, its southern trailing margin faced a progressively widening oceanic tract known as the Rheic Ocean (e.g. van Staal et al. 1998; Cocks and Torsvik 2002; Stampfli and Borel 2002). The later northward movement of Gondwana and its marginal terranes resulted in closure of the Rheic Ocean during the Devonian-Carboniferous to result in the Variscan orogeny (e.g. Matte 1986). The Rheic Ocean in NW Europe probably closed by south-directed subduction and the collision of Armorica with the southern margin of Avalonia resulted in a north-vergent fold and thrust belt in SW England and southern Ireland (Fig. 1; Holder and Leveridge 1986). The Rheic suture is thought to be represented in southwest England (Avalonia) by the ophiolitic rocks of the Lizard Complex (Fig. 1) (Nance et al. 2010 and references therein).

Fig. 1
figure 1

Geological map of the Central European Variscides (modified from Ballevre et al. 2009)

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Dating of detrital zircons is a powerful tool that can be used to establish the likely provenance of clastic sedimentary rocks and hence assists in the development of palaeogeographic reconstructions (e.g. Rainbird et al. 2001; Fernández-Suárez et al. 2002a, b; Cawood et al. 2003, 2004, 2007; Murphy et al. 2004; Samson et al. 2005). This is particularly useful where terranes have been detached from their source regions by transcurrent faulting and/or continental rifting and ocean formation. Along the northern Gondwanan margin, for example, the contrasting tectonothermal histories of the Amazonian and West African cratons provide a test for deducing the original location of terranes along that margin (e.g. Fernández-Suárez et al. 2002a, b; Samson et al. 2005). In this paper, we present detrital zircon data from Cambrian and Ordovician clastic sedimentary rocks in Armorica (Normandy, NW France) and compare these data with those for rock units of a similar age in other peri-Gondwanan terranes. We also present detrital zircon data from clastic sedimentary rocks within the footwall of the Lizard Complex in SW England. These rock units are thought to have been derived from erosion of the over-riding Armorican microplate during the closure of the Rheic Ocean (Holder and Leveridge 1986; Dörr et al. 1999), and we further test this hypothesis by the comparison of their detrital zircon suites with those characteristic of autochthonous Armorican strata.

General geology

NW France (Armorica)

NW France and the Channel Islands together form the type area of the late Neoproterozoic Cadomian orogenic belt (e.g. Chantraine et al. 1994; Egal et al. 1996; Strachan et al. 1996; Linnemann et al. 2008). This belt is dominated by calc-alkaline plutons and volcanics and variably deformed and metamorphosed volcano-sedimentary sequences of the Brioverian Supergroup (Fig. 2). Small areas of Palaeoproterozoic Icartian basement have yielded U–Pb zircon ages of c. 2.2–1.8 Ga for the igneous protoliths (Calvez and Vidal 1978; Samson and D’Lemos 1998). These basement rocks have been correlated with the c. 2.0 Ga Eburnian belt of NW Africa and may represent a detached fragment of the Gondwanan margin (e.g. Cogné 1990; Rabu et al. 1990). Evolution of the Cadomian belt can be divided into four broad phases: (a) early subduction-related magmatism (750 and 670–660 Ma); (b) syn-kinematic calc-alkaline plutonism (615–600 Ma); rifting and accumulation of the Brioverian Supergroup (590–570 Ma) and (c) regional deformation and metamorphism and continued magmatism (570–540 Ma) (Linnemann et al. 2008 and references therein).

Fig. 2
figure 2

Generalized geological map of the North Armorican Massif (modified from Linnemann et al. 2008) and sample locations (ROC 1, AM 1, AM 2). FSZ Fresnaye shear zone, PCSZ Plouer-cancale shear zone. Inset shows the North Armorican shear zone (NASZ) and South Armorican shear zone (SASZ). C Coutances, GM Guingamp migmatites, NTB North Trégor Batholith, PC Penthièvre Complex, PP Plourivo-Plouézec, SGG Saint Germain le-Gaillaird

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The Cadomian belt is overlain unconformably by transgressive sequences of Cambrian sandstones, siltstones and limestones. These rocks were deposited in fluvial-deltaic to shallow marine environments in basins that were separated from each other by broad landmasses that represented the residual topography after the final stages of Cadomian thickening (Doré 1994). Local occurrences of felsic volcanics are thought to represent the final stages of subduction-related magmatism (Chantraine et al. 1994). During the Arenig, a major marine transgression flooded most of Armorica and deposited an areally extensive quartz sandstone known as the Grès Armoricain Formation (Robardet et al. 1994). This formation is known informally as the ‘Armorican Quartzite’ and correlative units were also deposited in areas thought to have been contiguous with Armorica such as Iberia, Bohemia, Corsica, Turkey and parts of NW Africa.

One sample of a graywacke turbidite from the Brioverian Supergroup, Sample ROC 1, was collected in Normandy in NW France (Fig. 2). Two samples of the Lower Palaeozoic succession in NW France were also collected in Normandy (Fig. 2). Sample AM 1 is a quartzite from Lower Cambrian Le Rozel Formation. Sample AM 2 is a quartz sandstone collected from the Grès Armoricain Formation.

SW England (Avalonia)

The geology of SW England (Fig. 3) is dominated by Devonian and Carboniferous sedimentary successions with minor volcanic units. These rocks were deformed and metamorphosed at low to medium grades during the development of the Variscan fold and thrust belt (e.g. Shackleton et al. 1982; Coward and McClay 1983; Leveridge et al. 1984; Sanderson 1984; Seago and Chapman 1988). Deformation commenced at the end of the Devonian or during the early Carboniferous and migrated northwards. As the wave of deformation spreads northwards, the thickening orogenic wedge loaded the lithosphere and flexed down the foreland. The Devonian rift basin successions that were deposited on the southern passive margin of Avalonia were succeeded by Carboniferous clastic sediments that accumulated in foreland basins which were also progressively deformed (Hartley 1993). The post-tectonic S-type granites of the Cornubian Batholith were emplaced during the late Carboniferous to early Permian (Chen et al. 1993; Chesley et al. 1993).

Fig. 3
figure 3

Generalized map of the Lizard area and sample locations (LIZ 1, LIZ 3) (modified from Barnes and Andrews 1986)

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The structurally highest levels of the Variscan belt in SW England are represented by undated orthogneisses of the Eddystone reefs which are assumed to be part of the over-riding Armorican plate (Holder and Leveridge 1986). The underlying Lizard Complex has long been recognized as a tectonically dismembered ophiolite (e.g. Bromley 1979; Styles and Kirby 1980; Vearncombe 1980; Barnes and Andrews 1984, 1986; Floyd 1984; Gibbons and Thompson 1991; Roberts et al. 1993). Its age is best constrained by a U–Pb zircon age of 397 ± 2 Ma obtained from a plagiogranite (Clark et al. 1998). A southerly inclined crustal-scale structure is interpreted as a Variscan (Rheic) suture which at surface corresponds to the Lizard Complex thrust system (BIRPS and ECORS 1986; Le Gall 1990).

The Devonian rocks that lie in the footwall of the Lizard Complex preserve a record of the closure of the Rheic Ocean (Nance et al. 2010). A major period of extensional rifting was initiated during the middle Devonian (Bluck et al. 1988, 1992). East–west-trending normal faults downthrowing to the south separated marine shelf and fluvial basins in the north of Cornwall from the deep-water Gramscatho Basin to the south. The latter is dominated by the turbiditic sandstones and mudstones of the Gramscatho Group. The basin was filled from the south by turbidites derived from the erosion of advancing nappes (Floyd and Leveridge 1987). The turbidite successions include sedimentary mélanges of the Meneage Formation (Barnes 1983), which also contain blocks of MORB-type volcanics and mica schist as well as large (200 m) quartzite blocks with Ordovician faunas that are closely comparable with the Grès Armoricain of northwest France (Sadler 1974). Two granite pebbles within the Meneage Formation yielded U–Pb zircon lower intercept ages of 373 ± 6 and 422 ± 4 Ma, which were interpreted as protolith ages (Dörr et al. 1999). The upper intercept ages of 2,606 ± 40 and 2,445 ± 19 Ma were thought to indicate the presence of an inherited Neoarchaean or Palaeoproterozoic source terrane. Together the data suggest derivation from a magmatic arc that developed on the leading edge of the Armorican plate (Dörr et al. 1999).

Two samples were analysed for the present study, both collected from the Meneage Formation north of Porthallow (Figs. 3, 4a). Sample LIZ 1 is a Devonian sandstone that forms the matrix of a debris-flow deposit (Fig. 4c). Sample LIZ 3 is an exotic quartzite block (Fig. 4b), part of the suite correlated by previous workers with the Ordovician Grès Armorican in Normandy.

Fig. 4
figure 4

Photographs from the sampling site in the Lizard area (samples LIZ 1 and LIZ 3, Meneage Formation, Gramscatho Group, Devonian, see also Fig. 3). a Meneage Formation and olistolith of a Lower Ordovician quartzite (Nare Cove, c. 1.3 km N of Porthallow), b Olistolith of the Lower Ordovician quartzite, sample LIZ 3), c Mélange deposit of the Meneage Formation, sample LIZ 1 was taken from the sandstone matrix)

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Analytical techniques

Zircon preparation

Zircon concentrates were separated at the Museum für Mineralogie und Geologie (Senckenberg Naturhistorische Sammlungen Dresden). Fresh 1 kg samples of sandstone were crushed and sieved and then a heavy mineral separate was concentrated by the use of a heavy liquid (lithium heteropolytungstates in water). A final concentration was made by magnetic separation in a Frantz isodynamic separator. Selection of the zircon grains for U–Pb dating was achieved by hand picking under a binocular microscope. All zircon grains are either rounded or sub-rounded. Zircon grains of all sizes and morphological types were selected for single grain analysis by LA-ICP-MS. Zircon crystals were set in synthetic resin mounts, polished to approximately half their thickness and cleaned in a warm dilute HNO3 ultrasonic bath followed by rinsing in de-ionized water. Cathodoluminescence images of selected zircon grains are presented in Fig. 5.

Fig. 5
figure 5

Cathodoluminescence images of selected zircon grains from samples ROC 1, LIZ 1 and LIZ 3. For grains younger than 1 Ga, the 206Pb/238Pb age is quoted. For grains older than 1 Ga, the 207Pb/206Pb age is used

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LA-ICP-MS U–Pb dating: U–Pb age determination of single grains was determined by LA-ICP-MS at the Natural History Museum, London, using a New-Wave UP213 frequency quintupled solid-state Nd:YAG laser (λ = 213 nm) coupled to a PlasmaQuad 3 quadrupole ICP-MS. Samples and standard were placed in an airtight chamber which was flushed by helium gas carrying the ablated material to the ICP-MS, mixed with Ar prior to injection to the plasma torch. U–Pb and Pb–Pb ratios of the unknowns were determined relative to that of the 91500 zircon standard with certified ID-TIMS ages of 1,062.4 ± 0.4 Ma for 206Pb/238U and 1,065.4 ± 0.3 Ma for 207Pb/206Pb (Wiedenbeck et al. 1995). Collection of data spanned up to 180 s per analysis and includes a gas background taken during the initial c. 60 s. To reduce the extent of inter-element laser-induced fractionation, the sample was moved relative to the laser beam along a line. The nominal diameter of the laser beam was 60 μm for the standard and 30 or 45 μm for the unknowns. Pulse energy of the laser was 0.03–0.06 mJ per pulse for the unknowns and 0.09 mJ per pulse for the standards with an energy density of 3.5 J/cm2 and a repetition rate of 20 Hz. Only well-preserved zones within individual grains were analysed (Fig. 5), in order to escape metamorphic rims and altered domains. Further discussion of the analytical protocols used in this study can be found in Fernández-Suárez et al. (2002a) and Jeffries et al. (2003). Raw data reduction was performed using LAMTRACE, a macro-based spreadsheet written by Simon Jackson (Macquarie University, Australia). Calculations and plotting of concordia diagrams were achieved using Isoplot/Ex rev. 2.49 (Ludwig 2001), probability density plots and histograms were prepared by AgeDisplay (Sircombe 2004). For grains younger than 1 Ga, the 206Pb/238Pb age is quoted. For grains older than 1 Ga, the 207Pb/206Pb age is used. Only grains concordant in the range 85–115 % were used in the probability plots and histograms.

Results

U–Pb data of detrital zircon grains are represented in the concordia, histograms and binned frequency plots of Figs. 6, 7, 8, 9, 10 and in the supplementary data Tables 1, 2, 3, 4, 5. The tables also contain information on litho- and biostratigraphy, sample location and co-ordinates.

Fig. 6
figure 6

U–Pb ages of detrital zircon grains from sample ROC 1 (graywacke, Upper Brioverian, Post-Bedded Chert Brioverian, Ediacaran, Rocreux near Bretteville s. Laize, Normandy, Armorican Massif). Concordia diagram (a) and combined binned frequency and probability density distribution plots of detrital zircon grains in the range of 400–3,200 Ma (b) and of 400–800 Ma (c)

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Fig. 7
figure 7

U–Pb ages of detrital zircon grains from sample AM 1 (sandstone, Lower Cambrian, Le Rozel formation, Le Rozel, Normandy, Armorican Massif). Concordia diagram (a) and combined binned frequency and probability density distribution plots of detrital zircon grains in the range of 400–2,900 Ma (b) and of 400–1,100 Ma (c)

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Fig. 8
figure 8

U–Pb ages of detrital zircon grains from sample AM 2 (quartzite, Lower Ordovician (Arenigian), Armorican quartzite formation, Les Rieux, Normandy, Armorican Massif). Concordia diagram (a) and combined binned frequency and probability density distribution plots of detrital zircon grains in the range of 400–3,000 Ma (b) and of 400–1,100 Ma (c)

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Fig. 9
figure 9

U–Pb ages of detrital zircon grains from sample LIZ 1 (greywacke matrix of the mélange deposit, Devonian, Meneage Formation, Gramscatho Group, Nare Cove, c. 1.3 km north of Porthallow, Cornwall, UK). Concordia diagram (a) and combined binned frequency and probability density distribution plots of detrital zircon grains in the range of 400–3,000 Ma (b) and of 400–1,100 Ma (c)

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Fig. 10
figure 10

U–Pb ages of detrital zircon grains from sample LIZ 3 (quartzite olistolith correlated with the Armorican quartzite in the mélange deposit of the Devonian Meneage Formation, Gramscatho Group, Nare Cove, c. 1.3 km north of Porthallow, Cornwall, UK) (see also Fig. 4). Concordia diagram (a) and combined binned frequency and probability density distribution plots of detrital zircon grains in the range of 400–3,000 Ma (b) and of 400–800 Ma (c)

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Table 1 LA-ICP-MS detrital zircon data of sample ROC 1, n = 48/60, concordant in the range of 85–115 %, greywacke turbidite from the Upper Brioverian (Post-Bedded Chert Brioverian), Bocaine Zone, Rocreux near Bretteville s. Laize, Normandy, France, (sample location: road cut at Rocreux in the valley of the Laize river, co-ordinates: 49°03′16.03″N, 0°20′41.28″W)
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Table 2 LA-ICP-MS detrital zircon data for sample AM 1, n = 40/60, concordant in the range of 85–115 %, quartzite from the Le Rozel Formation, Lower Cambrian, Normandy, Armorican Massif, France (sample location: beach south of Le Rozel, co-ordinates: 49°28′46″N, 01°50′39″W)
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Table 3 LA-ICP-MS detrital zircon data for sample AM 2, n = 39/60, concordant in the range of 85–115 %, quartzite from the Armorican Quartzite Formation, Lower Ordovician (Arenig), Normandy, Armorican Massif, France (sample location: c. 2 km SW of the city of Les Pieux; co-ordinates: 49°30′26″N, 1°50′21″W)
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Table 4 LA-ICP-MS detrital zircon data of sample LIZ 1, n = 54/60, concordant in the range of 85–115 %, a sandstone from the matrix of the Meneage Formation, Gramscatho Group, age of sedimentation is Devonian, Cornwall, Great Britain (sample location: c. 1.3 km north of Porthallow, co-ordinates: 50°04′40″N, 5°04′37″W)
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Table 5 LA-ICP-MS detrital zircon data for sample LIZ 3, n = 50/60, concordant in the range of 85–115 %, a Lower Ordovician quartzite block in the Meneage Formation, Gramscatho group, age of sedimentation is Devonian, Cornwall, Great Britain (sample location c. 1.3 km north of Porthallow, co-ordinates: 50°04′41″N, 5°04′40″W)
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Sample ROC 1 is dominated by late Neoproterozoic grains (30 out of 48) that lie close to or on concordia and range in age from 547 ± 8 to 685 ± 11 Ma (Fig. 6). Of the remaining analyses, most are variably discordant. Thirteen yield Palaeoproterozoic ages with relative age peaks of c. 1,850, 2,100 and 2,400 Ma, four are Neoarchaean and the oldest grain is dated at 3,113 ± 31 Ma.

Sample AM 1 is dominated by late Neoproterozoic to early Cambrian grains (35 out of 40) that mostly lie close to or on concordia and range in age from 514 ± 10 to 627 ± 5 Ma (Fig. 7). The remaining five analyses are somewhat discordant. Four Palaeoproterozoic grains yield ages of c. 1,800 Ma(3), c. 2,300 Ma(1), and the oldest grain is latest Neoarchaean and dated at 2,522 ± 15 Ma.

Sample AM 2 is dominated by late Neoproterozoic to middle Cambrian grains (27 out of 40) close to or on concordia and ranging in age from 515 ± 7 to 695 ± 9 Ma (Fig. 8). An older Neoproterozoic cluster features three concordant or slightly discordant analyses at c. 905–997 Ma. Of the remaining ten analyses, nine are Palaeoproterozoic with relative age peaks of c. 1,800, 1,850, 1,900, 2,000 and 2,150 Ma, and the oldest grain is Neoarchaean and dated at 2,636 ± 35 Ma.

Sample LIZ 1 is dominated by late Neoproterozoic to late Cambrian grains (29 out of 54) close to or on concordia and ranging in age from 496 ± 22 to 709 ± 18 Ma (Fig. 9). Two Mesoproterozoic grains yield ages of c. 1,028 and 1,431 Ma. Of the remaining nineteen analyses, eighteen are Palaeoproterozoic with major relative age peaks at c. 2,000 and 2,100 Ma, and the oldest grain is Neoarchaean and dated at 2,593 ± 39 Ma.

Sample LIZ 3 is dominated by late Neoproterozoic to early Cambrian grains (37 out of 50) close to or on concordia and ranging in age from 531 ± 5 to 646 ± 14 Ma (Fig. 10). Two Mesoproterozoic grains yield ages of c. 1,338 and 1,485 Ma. Of the remaining eleven slightly discordant analyses, ten are Palaeoproterozoic with major relative age peaks at c. 2,000 and 2,100 Ma, and the oldest grain is Neoarchaean and dated at 2,641 ± 25 Ma.

Discussion and conclusions

Detrital zircons analysed from the three samples obtained from the Armorican Massif have a distinctive bimodal age distribution, dominated by late Neoproterozoic to middle Cambrian (c. 710–518 Ma) and Palaeoproterozoic (c. 1,800–2,200 Ma) groupings (Fig. 11). Additionally, the samples contain minor amounts of Mesoproterozoic to early Neoproterozoic detritus. There has been much debate surrounding what is a meaningful statistical population in detrital zircon analysis (e.g. Dodson et al. 1988; Sircombe 2000). According to Dodson et al. (1988), at least 59 randomly selected grains need to be measured to reduce the probability of missing a provenance component to 5 %. In our study, we measured 60 grains per sample, 40–50 of which yielded analyses that were in the range of concordance of 85–115 % and incorporated into data presentation and interpretation. Taking the three Armorican samples together (ROC 1, AM 1 and AM 2), this equates to 127 analyses, and the two Lizard samples (LIZ 1 and LIZ 2) provide a further 104 analyses. We are therefore confident that we have identified all major provenance components within the two sets of samples.

Fig. 11
figure 11

Detrital zircon age distributions: the samples studied in this paper together with zircon age distributions of Baltica, Amazonia, East Avalonia (Brabant massif), Armorica (Cadomian basement in the Saxothuringian and Moldanubian zones, Bohemian Massif), and the West African craton (data compilation from Drost et al. 2011; Linnemann et al. 2004, 2008). Note the general occurrence of Mesoproterozoic zircon ages in Baltica, Amazonia and Avalonia and the contrasting rarity of the same zircon populations in Armorica (Cadomia) and the West African craton

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The samples have essentially the same bimodal age distribution as detrital zircons analysed from other samples of the Brioverian Supergroup (Fernández-Suárez et al. 2002b; Samson et al. 2005). Both groupings can be readily linked with lithologies exposed within the Cadomian belt as well as the West African craton which is characterized by major tectonothermal events at 2.0–2.4 Ga (Fig. 11). Similar detrital zircon age distributions are characteristic of Neoproterozoic successions in SW Iberia (Fernández-Suárez et al. 2002a) and, in particular, the Saxothuringian and Moldanubian zones in central Europe (Fig. 11). On this basis, these crustal blocks are thought to have been located adjacent to the NW Africa segment of the Gondwanan margin during the Neoproterozoic and Lower Palaeozoic (Fernández-Suárez et al. 2002b; Samson et al. 2005).

The detrital zircon signature of Avalonia is quite distinct from that of Armorica in that there is a much larger proportion of Mesoproterozoic detritus (Fig. 11; Collins and Buchan 2004; Murphy et al. 2004; Samson et al. 2005; Strachan et al. 2007). This contrasting signature is consistent with a location close to the Amazonian craton which records Mesoproterozoic tectonothermal activity at c. 1.6 and 1.1 Ga (Murphy et al. 2004; Collins and Buchan 2004; Samson et al. 2005; Strachan et al. 2007). Similar detrital zircon age distributions are characteristic of Neoproterozoic successions in NW Iberia (Fernández-Suárez et al. 2002b) and Bohemia (Friedl et al. 2000; Samson et al. 2005), and by implication these crustal blocks are also thought to have been located adjacent to Amazonia. It has been suggested that a major phase of Cambrian(?) sinistral strike-slip faulting displaced these crustal blocks along the Gondwanan margin into a location between the West African craton and Armorica (Fernández-Suárez et al. 2002b).

The two samples taken from the Devonian Meneage Formation in the footwall of the Lizard ophiolite display an identical provenance to the three samples analysed from Armorica (Fig. 11). This is consistent with: (a) the derivation of these mélange deposits from the over-riding Armorican plate during closure of the Rheic Ocean, collision of Avalonia and Armorica, and obduction of the Lizard ophiolite (Fig. 12; Holder and Leveridge 1986; Dörr et al. 1999) and (b) the correlation of the quartzite blocks within the Meneage Formation with the Ordovician Grès Armoricain Formation (Sadler 1974). The absence of identifiable clasts of Brioverian material in the Meneage Formation suggests that many of the detrital zircons are second cycle, derived from reworking of the Lower Palaeozoic cover to the basement of Armorica. We cannot absolutely preclude the incorporation of Avalonian material within the Meneage Formation, and further material needs to be analysed to provide a broader database. The presence of two Mesoproterozoic grains within sample LIZ 1 might be taken as indicative of at least partial derivation from an Avalonian source. However, sample LIZ 3 also contains two Mesoproterozoic grains but is more firmly linked to Armorica by the palaeontological evidence (Sadler 1974).

Fig. 12
figure 12

A model for the Devonian tectonic evolution of the Lizard ophiolite complex in South Cornwall, depicting the progressive erosion of advancing thrust nappes of the Armorican plate (right) onto the southern margin of Avalonia (left) (from Holder and Leveridge 1986). Cne Carne Formation, Ds Dartmouth Slates, Mg Meadfoot Group, Ms Mylor Slate Formation, Pd Pendower Formation, Pto Portscatho Formation, Ptn Porthtowan Formation, Rbr-Roseland Breccia Formation. Tectonic units: Ck-Carrick Nappe, CT-Carrick Thrust, DP-Dodman Nappe, DT-Dodman Thrust, LT-Lizard Thrust, Lz-Lizard Nappe, N-Normannian Nappe, NT-Normannian Thrust

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Our data set is therefore consistent with the south-directed subduction geometry of the Rheic collision zone invoked by Holder and Leveridge (1986) (Fig. 12). Such a geometry would be expected to have resulted in the generation of a magmatic arc on the over-riding Armorican plate, consistent with the Silurian-Devonian U–Pb zircon ages obtained from granite pebbles in the Meneage Formation by Dörr et al. (1999). However, our new data set does not contain any evidence for detrital zircons of this age. Detrital zircons analysed from the matrix of the Meneage Formation therefore only provide a partial picture of the composition of the over-riding Armorican plate in the vicinity of the Rheic suture.