Age of the Acadian deformation and Devonian granites in northernEngland: a review

Nigel H. Woodcock1*, N. Jack Soper2 & Andrew J. Miles31 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK2 Gams Bank, Threshfield, Skipton BD23 5NP3 School of Geography, Geology and the Environment, University of Leicester, University Road, Leicester LE1 7RH, UK*Correspondence: nhw1@cam.ac.uk

Abstract: Field evidence shows that emplacement of Devonian granites in northern England overlaps in space and time withthe end of the supposed Acadian deformation in their country rocks. The age of this Acadian event in England and Wales is inneed of review because of revised Rb-Sr and K-Ar decay constants and recently acquired radiometric ages on the granites.

Published K-Ar and Ar-Ar cleavage ages recalculated to the new decay constants range from 404 to 394 Ma (Emsian, EarlyDevonian). Emplacement of the Skiddaw and Weardale granites at 398.8 ± 0.4 and 399.3 ± 0.7 Ma respectively is indicated byU-Pb zircon ages, and is compatible with the field evidence. However, emplacement of the Shap Granite at a Re-Osmolybdenite age of 405.2 ± 1.8 Ma and at the youngest U-Pb zircon age of 403 ± 8 Mamatches the field evidence less well. Theapparent paradox in these ages is resolved if the K-Ar ages record only the end of millions of years of cleavage formation. Anearlier cluster of K-Ar and Ar-Ar cleavage ages at 426–420 Ma (Ludlow to Prí̌dolí, late Silurian) dates a pre-Acadian resettingevent soon after Iapetus closure, an event of uncertain significance.

Ion microprobe U-Pb zircon ages for the Shap Granite have a mean of 415.6 ± 1.4 Ma but a range of 428–403 Ma,compatible with a long magmatic history. Thermal considerations suggest that this history was not at the upper crustalemplacement site but in a mid-crustal mush zone, now preserved at about 10 km depth as a component of the Lake District andNorth Pennine batholiths.

Received 27 July 2018; revised 14 December 2018; accepted 17 December 2018

The Lower Palaeozoic rocks of northern England and Waleswere mainly deformed by folds and cleavage assigned to theAcadian phase of the Caledonian Orogeny (Soper et al. 1987;McKerrow et al. 2000). When first applied in Britain, thisname implied a time correlation with the type Acadian eventof Devonian age in the northern Appalachians (van Staal et al.1998). The age of the Acadian event in England and Waleshas been constrained by amid-Emsian toGivetian (Devonian)unconformity, by radiometric dating of cleavage-parallelmicas, and by radiometric ages of granites shown to beemplaced during the later stages of deformation (reviews byWoodcock & Soper 2006; Woodcock et al. 2007; Woodcock2012). An age range of 400–390 Ma (mid-Emsian to mid-Eifelian, Devonian) for the Acadian deformation event hasbeen considered to fit the integrated evidence best.Now, four developments prompt a revision of the Acadian

age estimate.

(1) Revised decay constants for the K-Ar and Rb-Srsystems require recalculation of existing cleavageages and some granite ages.

(2) Further radiometric ages on the Shap, Skiddaw andWeardale granites have become available.

(3) Some of the recently acquired U-Pb ages onmagmatic zircons in the Shap Granite pre-date theestimated age of its upper-crustal emplacement.

(4) A pre-Devonian component to the main cleavage innorthern England and Wales needs consideration inthe light of late Silurian ages from the main cleavagein the English Midlands.

Dating the Acadian deformation in Britain and Ireland is alsoimportant in the light of continued attempts to correlatetectonic terranes between the Appalachians and theCaledonides. For instance, Waldron et al. (2014) have useddetrital zircon ages to strengthen the correlation between theLeinster–Lakesman terrane and the Ganderia terrane in theAppalachians (Fig. 1b). Fritschle et al. (2018) have proposeda late Middle Ordovician rather than a Devonian age for theD1 cleavage in SE Ireland and the Isle of Man and, byimplication, elsewhere in the Caledonian sector of Ganderia.Further SE, Waldron et al. (2011) and Pothier et al. (2015)have used detrital zircon dating to match components of theAvalon terrane of England and Wales with the Megumaterrane of Nova Scotia (Fig. 1b).Despite these proposed long-range terrane correlations, all

four of the above studies have recognized that thecorrelations are imperfect and require marked non-cylin-dricity in the terrane organization along the Appalachian–Caledonian orogen. Since the export of the term ‘Acadian’from the Appalachians to the Caledonides, it has alsobecome evident that Acadian events in the two sectors maycorrelate poorly both in time and by formation mechanism.So Woodcock et al. (2007) attributed the Acadian deform-ation in the Caledonides to collision of Iberia/Armorica withAvalonia at about 400–390 Ma. By contrast the Acadianevent in the Appalachians is now ascribed to accretion ofAvalonia to Ganderia at about 420–400 Ma (Fig. 1b),followed by a Neoacadian event due to accretion ofMeguma with Ganderia at 395–350 Ma (van Staal et al.2009; Wilson et al. 2017).

© 2019 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/). Published by The Geological Society of London for the Yorkshire Geological Society. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

Research article Proceedings of the Yorkshire Geological Society

Published online April 8, 2019 https://doi.org/10.1144/pygs2018-009 | Vol. 62 | 2019 | pp. 238–253

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It is important to stress therefore that references in thispaper to the Acadian event and its age apply to the Englandand Wales sector of the orogeny only unless otherwisespecified. Correlation with other parts of the Caledonideswill be discussed at the end of the paper, but better correlationwith the Appalachians will not be attempted.

Regional geological setting

The pre-Upper Devonian geological context of northernEngland is shown in Figure 1a. This area is bisected by theIapetus Suture, which separates terranes with Laurentianaffinity to the NW from those to the SE derived fromGondwana. The intervening Iapetus Ocean closed in mid- tolate Silurian time. Bordering the suture on the Laurentianside is the Central–Southern Uplands terrane, mainly Lower

Palaeozoic sedimentary rocks that formed a deep marineaccretionary complex above the NW dipping Iapetussubduction zone (Leggett et al. 1979). On the Gondwananside of the suture is the Leinster–Lakesman terrane,comprising Lower Palaeozoic sedimentary sequences thatinclude Ordovician magmatic arc rocks. This terrane hasbeen correlated with the Appalachian Ganderia domain (e.g.Waldron et al. 2014; Fritschle et al. 2018), as has the adjacentMonian composite terrane, which contains alsoNeoproterozoic basement rocks. Whether the Monianterrane continues eastward beneath the Upper Palaeozoiccover of northern England or wedges out in that direction isuncertain (Fig. 1a).To the SE of the Monian terrane is the Avalon composite

terrane, amalgamated with the Monian terrane in earlyOrdovician (Floian) time. On Avalonia, Cambrian to Lower

Fig. 1. (a) Geology of the Iapetus Suture zone, showing the Devonian granites of northern England in the context of the other Trans-Suture Suite granites.Most boundaries from British Geological Survey (1996); subsurface granite contours from Kimbell et al. (2010). (b) Palaeocontinental setting of map 1(a)showing the main Appalachian and Caledonian tectonics domains; modified from Waldron et al. (2014).

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Devonian sedimentary rocks with Ordovician volcanic rocksrest on a Neoproterozoic basement. Detrital zircon andstratigraphic correlations have suggested to Waldron et al.(2011) that the Cambrian sequences in North Wales matchthose in the Meguma terrane of Nova Scotia. On thishypothesis, Megumia and Avalonia were also amalgamatedby early Ordovician (Floian) time.The Acadian deformation of Devonian (Emsian–Eifelian)

age is recognized in the terranes SE of the Iapetus Suture(reviews by Woodcock & Soper 2006; Woodcock 2012).The deformation is dominated by upright folds trendingbetween NE–SW and east–west, with a steep slaty cleavagethat is axial planar or weakly clockwise-transecting to thefolds (Soper et al. 1987; review by Woodcock & Soper2006). The main Acadian deformation is sporadicallyaffected by a later, gently dipping crenulation cleavage andrelated folds, particularly in the Isle of Man and the northernpart of the Lake District. The Acadian deformation can betraced into Ireland, but the deformation history there iscomplicated by recognition of a late Middle Ordovicianevent of uncertain extent (Fritschle et al. 2018). TheLaurentian margin accretionary complex (the ScottishSouthern Uplands terrane and the Central terrane inIreland) was formed diachronously from late Ordovician toWenlock time, although localized Devonian (possiblyAcadian) deformation occurs on the Moniaive shear zone(review by Stone et al. 2012).Granites of the Trans-Suture Suite were intruded in a zone

about 100 km wide either side of the Iapetus suture (Fig. 1a)about 20 myr after Iapetus had closed (Brown et al. 2008).The debate about this spatial and temporal pattern has beensummarized by Miles et al. (2016), who favoured an originby crustal melting above a zone of delaminated lithospheredue to drop-off of the subducting Iapetus slab.

The geology of Northern England

The outcrop geology of northern England (Fig. 2) isdominated by Late Devonian and younger sedimentaryrocks deposited above the angular unconformity that recordsuplift and erosion during and after the Acadian deformation.East of the Pennine–Dent–South Craven fault system, thesepost-unconformity rocks dip gently eastward towards theNorth Sea Mesozoic basins, complicated only by three majoreast–west fault belts. By contrast, in the west of the region,the post-unconformity cover wraps around the LowerPalaeozoic inlier of the Lake District Massif, upfaulted atits western flank against the Mesozoic Irish Sea Basin. SmallLower Palaeozoic inliers occur along the uplifted footwallsof the North Craven and Pennine faults and in the AlstonBlock, confirming that components of Lake District geologycontinue at depth to the east of the Pennine–Dent fault system(Stone et al. 2010).Plutonic rocks crop out prominently in the western and

northern Lake District (Fig. 2), but these are late Ordovician(Sandbian–Katian) intrusions coeval with the arc volcanicrocks of the Borrowdale Volcanic Group (review byMillward 2002). Ordovician plutonic rocks probably alsomake up some of the subsurface batholiths deduced fromgravity modelling beneath the northern half of the LakeDistrict, Wensleydale and the North Pennines (Fig. 2). The

Devonian granites of interest in this paper have small areas ofoutcrop at Skiddaw and Shap. However metasomaticaureoles at Crummock Water and Ulpha probably overlieDevonian granites and the buried Haweswater granite couldjust as well be Devonian as Ordovician (Millward 2002).Gravity modelling (Kimbell et al. 2010) shows that theSkiddaw and Shap plutons are steep-sided vertical cylindersrooting into the tabular Lake District batholith at 8–10 kmdepth. The same modelling defines the North Penninebatholith to the east, which provides the roots for similarcylindrical plutons. One of these plutons – the WeardaleGranite – has yielded Devonian ages from borehole samples(Selby et al. 2008; Kimbell et al. 2010).The pluton referred to most frequently in this paper is that

at Shap (Fig. 3). The country rock to the Shap granitecomprises the Borrowdale Volcanic Group and sedimentaryrocks of the Windermere Supergroup, which were alreadysteeply dipping at the time of granite emplacement. Thepluton was emplaced mostly into the Borrowdale VolcanicGroup, with a contact aureole that pervades the lower fewunits of the Windermere Supergroup. The emplacing granitemay have exploited a pre-existing fault network, and is cut byat least one north–south striking fault. The Acadian cleavagein the country rocks is deflected around the pluton from itsregional NE–SW strike.

Field relations of the Trans-Suture granite plutons

The field relations around an idealised Trans-Suture plutonare shown schematically in Figure 4. Key relations for mostindividual plutons are summarized by Brown et al. (2008)and for the Southern Uplands terrane by Hines et al. (2018),and are not repeated here. Rather, the main pluton attributesare discussed below with particular reference to the northernEngland examples. It must be emphasized that, of theseexamples, only the Shap and Skiddaw plutons are exposed atoutcrop. The Ulpha and Crummock Water plutons areinferred from their exposed metasomatic aureoles. Theplutons above the North Pennine batholith are known fromgravity modelling and from two boreholes. Their originalcontact relations are not preserved because the plutons werepartly unroofed during the post-Acadian uplift and wereunconformably overlain by Carboniferous rocks.Pluton size varies for the whole Trans-Suture suite with

long-axis diameters of 1–75 km (Fig. 5a) and maybe up to150 km for the unexposed Tweeddale pluton. The northernEngland plutons span the middle part of this size range, withShap, Skiddaw and the smaller North Pennine plutons at thelower (3–12 km) end, and Weardale pluton in the mid-range(20–30 km).Pluton shape is typically ovoid, with an axial ratio mostly

in the range 1.5–2.5 (Fig. 5a). The northern England plutonsfollow this pattern, except for the highly ovoid (5:1)Crummock Water aureole.Pluton orientation is shown in a rose diagram, scaled by

the length of the pluton. The poorly known Tweeddalepluton is omitted (Fig. 5b). The long axis of each plutontends to be sub-parallel to the regional strike of bedding andcleavage, where this can be determined. The Skiddaw pluton,along with the Crummock and Ulpha aureoles, is elongatedalong strike, but the long axis of the Shap pluton is clockwise

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oblique by 35° (Fig. 3). The strike-parallel geometry reflectsemplacement into already folded and therefore anisotropicrocks, but some post-emplacement flattening is evident.Internal foliation is seen in some plutons only. Where

present, it parallels the pluton margin. It tends to be strongestat the margin and to weaken inwards, as well documented inthe Rookhope borehole into the Weardale Granite (Dunhamet al. 1965). This mica fabric is likely to be deformational,but the Shap Granite locally has a weak magmatic alignmentof the pink K-feldspar phenocrysts that are such a distinctivefeature of the pluton. The Skiddaw pluton has a marginalsheeting and mica-foliated greisen pods.Xenoliths are only reported in a few Trans-Suture plutons,

where they tend to parallel any magmatic foliation in thegranite. At Shap, sparse xenoliths can carry the S1 cleavage,disoriented with respect to country rocks (Soper & Kneller1990), proving that a component of the cleavage pre-dated

formation of part of the pluton. More common than xenolithsin some plutons are ovoid mafic enclaves (Fig. 6a),representing inclusions of coeval mafic melt that did notfully mix with the granitic magma. In the Shap pluton, thesemafic enclaves are particularly common. The mafic meltsentrained early-formed K-feldspar phenocrysts, proving thatmafic and silicic melts co-existed.Country rock fabric, comprising steep bedding and the S1

cleavage, tends to be locally truncated, particularly at the NEand SW ends of each pluton (Fig. 6b), but elsewhere it isdeflected to become parallel with and wrap the pluton(Fig. 3). This geometry is well seen at Shap (Fig. 3), whereemplacement must be later than the folding of bedding tosteep dips and formation of some component of the steepcleavage. However, shortening must have continued for thebedding and cleavage fabrics to wrap around the consolidat-ing pluton. At Skiddaw, contact relations are only well

Fig. 2. Geological map of northern England showing the outcrop geology and main faults (modified from British Geological Survey 1996) overlain onthickness contours of subsurface granite (from Kimbell et al. 2010). Location of map shown on Figure 1.

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exposed at the northern margin of the pluton. However, theAcadian cleavage did not develop here because the countryrocks had previously been hornfelsed by the Ordovicianemplacement of the Carrock Fell intrusive complex.Apophyses to the pluton and cogenetic dykes may transect

(Fig. 6c) or follow the composite bedding/cleavage fabric,showing that their intrusion post-dated some component ofthe Acadian deformation. However, some co-genetic dykesat Shap bear a weak cleavage in their margins (Soper &Kneller 1990) (Fig. 6f ), showing that their intrusion pre-dated a component of the shortening. Aplite veins in the

Weardale granite have a muscovite alignment of uncertain,but possibly deformational, origin (Dunham et al. 1965).A metamorphic aureole is present around most Trans-

Suture plutons. To the NW of the suture, the S1 fabricpredates mineral growth in the aureoles. In northern England,the S1 cleavage, inferred to be Acadian, predates themetasomatic aureoles at Crummock and Ulpha. At Shap,micas overgrow this cleavage, first mimetically thenrandomly (Boulter & Soper 1973), and calcareous siltstonesare randomly recrystallized (Soper &Kneller 1990) (Fig. 6e).At Skiddaw, andalusite porphyroblasts in the aureole

Fig. 3. Geological map of the area around the Shap pluton. Location marked on Figure 2. Based on British Geological Survey (2004, 2007), with granitethickness contours from Kimbell et al. (2010)

Fig. 4. Schematic map of the field relations in and around a typical Trans-Suture Suite pluton.

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mostly post-date S1 (Fig. 6d), but porphyroblasts in the inneraureole zones are partly wrapped by the continuingdevelopment of the deformation fabric (Soper & Roberts1971). The aureole minerals pre-date the sub-horizontal S2crenulation cleavage.In summary, the evidence from the exposed Trans-Suture

plutons of northern England indicates that the plutons wereemplaced during the later stages of Acadian S1 cleavageformation (Brown et al. 2008), but before that shorteningdeformation had ended completely. Consequently, radio-metric ages from the granites are an important factor inconstraining the age of the Acadian deformation event, forwhich radiometric ages are limited.

Dating Acadian deformation

Stratigraphic constraints on the age of the Acadiandeformation

Soper et al. (1987) assembled the evidence that the angularunconformity marking syn- and post-Acadian uplift acrossEngland, Wales and Ireland is intra-Devonian rather thanend-Silurian. The stratigraphic constraints on this uncon-formity are summarized on Figure 7.The Acadian deformation is most intense in the former

Lower Palaeozoic basinal rocks across Wales and throughoutthe Leinster–Lakesman terrane. Here, Mid-Devonian or latersedimentary units unconformably overlie folded and cleavedmarine Cambrian to Silurian or Lower Devonian rocks. Innorthern England, the unconformity spans all of Early toMidDevonian time, although the metamorphic grade below theunconformity implies a missing Lower Devonian cover atleast 3.5 km thick (Soper & Woodcock 2003). The MidlandPlatform in SE Wales escaped intense Acadian deformation,but still preserves a gentle angular unconformity spanning atleast intra-Emsian through Frasnian time, at most 395–372 Ma (Fig. 7). Unconformities in the Dingle area of SWIreland may constrain the Acadian deformation more tightly,perhaps to at most mid-Emsian through Eifelian time (400–388 Ma, Fig. 7), but the ages and relationships of crucial

units are debated (Richmond & Williams 2000; Meere &Mulchrone 2006; Todd 2015).An unconformity that potentially includes the Acadian

event also exists beneath East Anglia (Woodcock & Pharaoh1993), but here it spans the whole of Early and Mid-Devonian time (419–383 Ma).

Radiometric ages: presentation and decay constants

The sections that follow review radiometric age determina-tions from the Trans-Suture granites and from their cleavedcountry rocks, both regionally then locally to northernEngland. Regionally aggregated ages for the relevantmagmatic and tectonic events are shown as probabilitydistribution functions (Fig. 8), mostly those of Miles et al.(2016), who detail their sources and derivation method.There is ongoing discussion of the appropriate decay

constants to be used for both the Rb-Sr and K-Ar/Ar-Ardating methods. We have used revised constants for the mainage spectra, but have shown the unadjusted spectra outlinedby pecked lines for comparison (Fig. 8). The K-Ar ages thatuse the constant of 5.543 × 10−10 a−1 (Steiger & Jäger 1977)have been adjusted upwards by the 1% suggested by Dickin(2005, pp. 275–276) and supported by comparisons withzircon U-Pb ages (Jourdan et al. 2009; Naumenko-Dez̀eset al. 2018). Rb-Sr ages have been adjusted upwards by 1.9%corresponding to a lowered decay constant of 1.393 × 10−11

(Nebel et al. 2011). These adjustments are significant at thelevel of age resolution attempted in this paper.

Radiometric age of the Acadian cleavage

Merriman et al. (1995) have directly determined theradiometric age of the northern England cleavage fromcleavage-parallel illite/mica in the Ribblesdale Inlier. Theseminerals gave a K-Ar age of 401 ± 7 Ma and an Ar-Ar totalfusion age of 422 ± 3 Ma, both ages adjusted upwards by 1%(Fig. 8b). Merriman et al. (1995) concluded that these twoages bracketed the possible period of Acadian cleavageformation.

Fig. 5. (a) Plot of length v. width of Trans-Suture plutons or sub-plutons on log-log scale. Locations are on Figure 1. The length/width value is contoured.Outcrop data are from British Geological Survey (1996). Sub-surface plutons are measured at 5 km depth from Kimbell et al. (2010) and Stone et al.(2012). (b) Rose diagram of the orientation of pluton long axes scaled by long axis length, omitting the unproven Tweeddale pluton.

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Fig. 6. Annotated field photographs, each with 5 cm scale bar. (a) Mafic enclave in K-spar porphyritic granite, Shap Pink Quarry [NY 558 084]. (b) Shappluton truncating cleaved Borrowdale Volcanic Group country rock, Longfell Gill [NY 561 101]. (c) Granitic vein cutting cleaved Borrowdale VolcanicGroup country rock, Longfell Gill [NY 561 101]. (d) Randomly oriented andalusite in hornfelsed Skiddaw Group aureole to Skiddaw granite, CaldewValley, Skiddaw [NY 320 320]. (e) Hornfelsed aureole to Shap Granite overprinting cleavage in Coldwell Formation, Packhorse Hill [NY 553 075]. (f )Parkamoor dyke with marginal foliation parallel to Acadian cleavage in country rock, Low Parkamoor [SD 305 924]. (g) Contact of Stage II granite againsta phenocryst-poor schlieren zone of Stage I granite, Shap Pink Quarry [NY 558 084]. (h) Contact as (g) but with a vein of Stage II granite into the schlierenzone, seen in kerbstone in Settle (Yorkshire).

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However, the same two age peaks occur (Fig. 8b) in Ar-Arages from five localities in Wales (Dong et al. 1997)suggesting that both age peaks have some discretesignificance. The younger peak, at about 400 Ma (mid-Emsian, Early Devonian) is seen also in two other Welshdatasets: the K-Ar ages of Evans (1996) and the Ar-Ar agesof Sherlock et al. (2003). All authors agreed that the c.400 Ma ages probably record the culmination of Acadianshortening.The older peak at about 422 Ma (Prí̌dolí, latest Silurian) is

more problematic. Evans (1996) correctly pointed out thatthese ages substantially predate the time of maximum burial,which probably occurred after deposition of some kilometresof Lower Devonian rocks (Soper & Woodcock 2003). Donget al. (1997) saw these Prí̌dolí ages as the beginning ofAcadian shortening and metamorphism, but both Evans(1996) and Merriman et al. (1995) suggested instead that theages record the prograde transition from smectite to illiteduring burial. These early ages closely match the main423 Ma peak of Ar-Ar ages (Fig. 8a) from the NW–SE-striking cleavage in the Charnwood inlier of the EastMidlands (Carney et al. 2008). The latter authors linkedthe late Silurian ages to a pre-Acadian shortening event thatwas due to Iapetus closure, an event recognized as theBrabantian deformation in Belgium (Debacker et al. 2005).Here we regard the 405–395 Ma peak as the best age

estimate for the closure of the K-Ar system in cleavage-parallel illites or white micas as their host rocks were upliftedand cooled late in the Acadian shortening. It will be arguedlater that this shortening probably started earlier than

405 Ma, but not as early as 422 Ma. The cluster ofearlier ages must mark a separate tectonothermal event,immediately after the soft closure of the UK sector of theIapetus Ocean. It is unlikely that a strong component of thecleavage in northern England or Wales formed then,however, as there is no evidence for a two-stage history forthe Acadian fabric.The main slaty cleavage-forming deformation (D1) SE of

the Iapetus Suture is sporadically affected by a later (D2),gently dipping crenulation cleavage and related folds,particularly in the Isle of Man and the northern part of theLake District. This event has not been radiometrically dated.

Dating Siluro-Devonian magmatism

Regional spectra of Trans-Suture granite ages

Before focusing on the Devonian granites of northernEngland, it is helpful to review ages across the Trans-Suture suite as a whole (Fig. 8). Available ages for thegranites and related lamprophyre dykes are shown asprobability distribution functions on Figure 8c–g (fromMiles et al. 2016 with added ages from Miles & Woodcock2018 and Hines et al. 2018). To compensate for the differentsizes of the plutons, the granite data have been weighted bythe outcrop area of each pluton (or by the subsurface area forthe North Pennine batholith). The coloured Rb-Sr and K-Ardistributions have been adjusted upwards by 1.9% and 1%respectively, in line with the revised decay constantsdiscussed above. The unadjusted curves are also shown aspecked lines.

Fig. 7. Summary of key events relevantto the dating of the Acadian deformationin England and Wales. Evidence forAcadian unconformity updated fromWoodcock et al. (2007) and Woodcock(2012).

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The U-Pb zircon ages for the granites have peaks between414 and 399 Ma (Fig. 8g). Only the later half of thismagmatic range overlaps with the Acadian shortening peakbased on adjusted K-Ar cleavage ages (405–395 Ma).Compared with the U-Pb ages, more of the Rb-Sr ages for

the granites (Fig. 8f ) are from older analytical studies, wheredoubts about reliability arise because of less sophisticatedmethods and higher analytical errors. However, the adjustedage spectrum is compatible with that from U-Pb ages, withsimilar peaks at 405 and 399 Ma, but with an even greaterproportion of the ages pre-dating the Acadian event. Bycontrast, aggregated K-Ar ages from the granites are younger

than ages from the other two methods, even after 1%adjustment (Fig. 8e). These ages might record the very end ofgranite cooling, resetting late in the Acadian event, or merelyan unreliability of K-Ar ages in granite biotite.

Fig. 8. Probability distribution functions of radiometric ages from theTrans-Suture granites and related lamprophyre dykes and country rocks.Dashed curves are the unadjusted spectra for Rb-Sr and K-Ar ages. Curvesa–c and f are from Miles et al. (2016), where data sources are specified.Curve (g) however incorporates new ages from Newry (Anderson 2015),Doon (Hines et al. 2018) and from Shap (Miles & Woodcock 2018).Curve (d) uses data from Hines et al. (2018) and curve (e) from Kulpet al. (1960); Brown et al. (1964, 1968); Shepherd et al. (1976) andRundle (1982).

Fig. 9. Plots of radiometric age against closure temperature for the lateCaledonian granites of northern England. Each data point is annotatedwith its source: Al87, Al Jawadi (1987); Co88, Cooper et al. (1988);Da05, Davidson et al. (2005); Ho70, Holland & Lambert (1970); Ki10,Kimbell et al. (2010); Mi18, Miles & Woodcock (2018); Pi78, Pidgeon &Aftalion (1978); Ru82, Rundle (1982); Ru87, Rundle (1987); Ru92,Rundle (1992); Se08, Selby et al. (2008); Sh76, Shepherd et al. (1976);Sh81, Shepherd & Darbyshire (1981); Wa78, Wadge et al. (1978).

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The remaining probability distribution functions are for K-Ar biotite ages (Fig. 8c) and U-Pb zircon ages (Fig. 8d) in thesuite of lamprophyre dykes that intrude much of the Trans-Suture zone. The field and petrological evidence, reviewedby Brown et al. (2008), suggests that these dykes are coevalwith the Trans-Suture granites, probably representing themore mafic mantle melts that triggered the crustal melts forthe granitic magmatism. This genetic link is supported by thesimilarity of the lamprophyre and granite age ranges. Mileset al. (2016) explain the lack of Acadian resetting of thelamprophyre K-Ar ages by the low Ar diffusion rates in Mg-rich biotite.In summary then, the age spectra from both the granites

and related lamprophyres suggest that magmatism firstpreceded then overlapped with the Acadian deformationpeak. This result conflicts with the field evidence to bediscussed below, which shows rather that most plutons wereemplaced late in the Acadian deformation history. Twoexplanations will be developed later in this paper: (1) thatAcadian deformation began some millions of years beforethe date recorded by the closure of the K-Ar system in thecleavage-parallel illite/mica; and (2) that some early zirconages from granites are dating crystallisation in a mid-crustalmush zone, millions of years before upper crustal emplace-ment of that granite.

Detailed age analysis of the northern EnglandTrans-Suture granites

This section looks in more detail at the available ages for thenorthern England plutons of the Trans-Suture suite. Becausethese ages derive from a range of isotopic systems, they havebeen plotted against the approximate closure temperature forthe system concerned (Fig. 9). On these plots, error bars aregiven both for the age determination and for the closuretemperature, using the estimates compiled by Dickin (2005).The plots contain relevant radiometric age determinationspublished since 1970, but caution must be exercised ininterpreting ages from older studies, where analyticaltechniques were less sophisticated and less accurate.Radiometric ages are available from two exposed plutons

(Shap and Skiddaw), from a subsurface pluton (Weardale)and from the aureole of an inferred pluton (CrummockWater). There are no ages from the Ulpha pluton or thepossible Haweswater pluton. The best estimate of emplace-ment age for each pluton, discussed below, is shown on thesummary diagram (Fig. 7) and ranges from 408 to 399 Ma. Itwill be argued later that cooling and final solidification ofmost of the upper crustal plutons took place in less than amillion years. Therefore, this age range shows that eachpluton results from a discrete emplacement event rather thanfrom a synchronous event across the whole of northernEngland.The evidence for the emplacement age of each dated

pluton is now reviewed.The buried Weardale pluton has a U-Pb zircon age

(isotope dilution thermal ionization mass spectrometry; ID-TIMS) of 399.3 ± 0.7 Ma (Fig. 9a) (Kimbell et al. 2010).This age is within error of a Re-Os age of 398.3 ± 1.6 Ma onmolybdenite in a hydrothermal quartz vein (Selby et al.2008) that must post-date granite emplacement. An Rb-Sr

whole rock age from an aplite vein of 397 ± 8 Ma (1.9%adjusted from Holland & Lambert 1970) is compatible withthe U-Pb and Re-Os ages. An Rb-Sr age of 418 ± 10 Ma fromthe main granite published by the same authors might seemanomalously old were it not for the growing evidence fromthe Shap granite that the northern England magmas had along pre-emplacement history in the mid-crust. We agreewith Kimbell et al. (2010) that the U-Pb age of 399.3 ±0.7 Ma is the best estimate of the emplacement age of theWeardale pluton.The Skiddaw pluton had a very similar emplacement age to

the Weardale pluton, based on a U-Pb ID-TIMS age onzircon of 398.8 ± 0.4 Ma (Selby et al. 2008) (Fig. 9b). Thezircons picked for their analysis were small acicular grainsthat probably grew late in the magmatic history. Within errorof the U-Pb age is an adjusted Rb-Sr age of 406 ± 8 Ma ongranite K-feldspar (Rundle 1982) and adjusted K-Ar ages ongranite biotite and muscovite of 396 ± 4 and 398 ± 8 Marespectively (Shepherd et al. 1976; Rundle 1982). Anadjusted Rb-Sr whole-rock age of 390 ± 4 Ma is anomal-ously young (Rundle 1982). Hydrothermal quartz veins thatcross-cut granite and country rock give a significantlyyounger Re-Os age on molybdenite of 392 ± 3 Ma (Selbyet al. 2008). This age is compatible with an Rb-Sr age on veinquartz fluid inclusions of 399 ± 5 Ma (Shepherd &Darbyshire 1981) and with a K-Ar age of 391 ± 2 Ma onmuscovite in greisen marginal to the granite (Rundle 1982).The Shap pluton has a more complex radiometric age

record than the Weardale and Skiddaw plutons. The bestdirect estimate of its emplacement age is the Re-Os age ondisseminated magmatic molybdenite of 405.2 ± 1.8 Ma(Selby et al. 2008) (Fig. 9c). An Rb-Sr whole-rock age of402 ± 3 Ma (Wadge et al. 1978) and a K-Ar age on granitebiotite of 401 ± 7 Ma (Rundle 1992) are both compatiblewith this emplacement age when recalculated to new decayconstants. Nearly compatible are averaged Rb-Sr whole-rockages on the comagmatic Devonian minor intrusive suitearound Shap (Al Jawadi 1987; Rundle 1987), recalculated at400 ± 3 Ma, but an early U-Pb zircon age of 390 ± 7 Ma(Pidgeon & Aftalion 1978) seems anomalously young. Alsowithin the main granite, a more modern Rb-Sr age (Davidsonet al. 2005) on the conspicuous K-feldspar phenocrysts,recalculated at 413 ± 2 Ma, suggests that these grains startedto grow up to 10 million years before the pluton wasemplaced in the upper crust. This hypothesis is supported byU-Pb ages on single zircon grains analysed by ionmicroprobe (Miles et al. 2016; Miles & Woodcock 2018).These grains have a mean age of 415.6 ± 1.4 Ma andindividual ages ranging from 428 ± 8 Ma to 403 ± 10 Ma,suggesting a protracted magmatic history in the lower andmid-crust before rapid upper crustal emplacement. Only theyoungest of these grains puts any constraint on thisemplacement age, and is compatible with the 405 Ma Re-Os age. The new zircon data therefore support the conclusionof Selby et al. (2008) that the Shap pluton was emplacedabout 6 million years earlier than those at Skiddaw andWeardale.The buried Wensleydale pluton has an Rb-Sr age that

recalculates as 408 ± 10 Ma (Dunham 1974). This age iswithin error of the emplacement ages of the Shap, Skiddaw orWeardale plutons. However, a Devonian emplacement age

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for the Wensleydale pluton has been questioned (e.g. byMillward et al. 2000) for several reasons: its cleaved margin,its low heat production and its geochemical similarity to theOrdovician plutons of the Lake District. A new U-Pb zirconage of 449 ± 0.52 Ma (C. Thomas et al. pers. comm. 2018)has verified this Ordovician age.The Crummock Water pluton is postulated from its gravity

signature and from the metasomatically bleached aureole inthe overlying Skiddaw Group mudrocks (Millward 2002).This aureole gave an Rb-Sr age that recalculates to 409 ±3 Ma (Cooper et al. 1988) (Fig. 9d), an age only just withinerror of the Shap pluton and significantly older than theSkiddaw and Weardale granites.There are no radiometric ages available from the

subsurface Ulpha and Haweswater plutons.

Discussion of the ages of Devonian plutons

The best estimate of emplacement age for each Devoniangranite pluton in northern England is summarized onFigure 7 and ranges from 408 to 398 Ma. Comparing theseages with the cleavage ages highlights the paradox thatemplacement of the Shap and Crummock Water plutonsradiometrically predates the cleavage that the plutonsphysically truncate or metasomatise in the field. Thereliability of the Crummock Water age might be doubted asan older Rb-Sr whole rock age, but the Shap age is based onmore modern U-Pb, Re-Os and Rb-Sr mineral techniques.The age paradox therefore prompts consideration of what theradiometric ages mean in relation to a possibly protractedperiod of emplacement or cleavage formation. Estimates ofthe duration of emplacement and deformation are attemptedin the next section.The early U-Pb zircon ages from the Shap pluton have

been discussed by Miles & Woodcock (2018). The large(±10 Ma) errors on the ages of individual grains makes itstatistically possible that all these grains form a singlepopulation with mean age of about 416 Ma. However, Miles& Woodcock (2018) considered it more likely that theyrecord protracted crystallisation in a mid-crustal mush zonethroughout much of Early Devonian time, as evidence fromother granitic plutons suggests (e.g. Coleman et al. 2004;Glazner et al. 2004). Miles & Woodcock (2018) envisagethat this mush reservoir was intersected by steep strike-slipfaults at about 405 Ma and that a crystal-rich mush close tothe solidus temperature was piped up into the upper crust at areleasing bend in the strike-slip system. The other pipe-shaped Devonian plutons in northern England were probablyemplaced in a similar way (Kimbell et al. 2010), analogous toother Caledonide examples in Donegal (Hutton 1982), theGrampian terrane (Jacques & Reavy 1994) and moregenerally (Hutton & Reavy 1992). In northern England, thesolidified mush zone is thought to be preserved at about10 km depth as a component of the North Pennine and LakeDistrict batholiths.The exposed Shap pluton has three main textural varieties

(Stages I, II and III, Grantham 1928). The abundance oforthoclase megacrysts increases from 15% in Stage I, to 30%in Stage II and up to 60% in Stage III. Contacts betweenStages I and II are often diffuse, but rafts of Stage I granitewithin the younger Stage II granite suggested to Grantham

that the Stage II variety post-dated Stage I. However, nosignificant difference in zircon ages has been found betweenthe Stage I and II granite (Miles & Woodcock 2018, fig. 5)and we have found contacts where the darker Stage I varietylocally post-dates Stage II (Fig. 6g and h). This evidencesuggests that both varieties coexisted in the same melt body.The typically diffuse contacts suggest that juxtaposition ofthe two varieties happened in the mid-crustal mush zone or,less likely, in very quick succession in the upper crust. TheStage III granite is a minor constituent of the pluton and isfound as 1 cm to 1 m wide dyke-like bodies that cross-cutStages I and II with sharp margins.

Durations of emplacement and deformation

Duration of granite pluton emplacement

The likely duration of cooling of a cylindrical pluton can beestimated from conductive cooling models such as thoseexplained by Best & Christiansen (2001, pp. 198–199),assuming that the pluton was emplaced into its final uppercrustal site in one episode.The characteristic time τ is the time for the temperature

difference between the centre of the pluton and the far-fieldcountry rock to reduce to 1/e (36.8%) of its initial value. For acylindrical pluton

τ ¼ κt=a2

where κ is the thermal conductivity, t = time and a = plutonradius. Figure 10a shows the cooling times plotted againstaverage pluton radius for various values of τ and for anappropriate thermal conductivity for the granite and thecountry rocks of 10−6 m2 s−1. Apart from the large Weardalepluton, the cooling times for the northern England plutonsare low; they would be 95% cooled within 2.5 myr and 63%cooled within a million years. Even Weardale would be 63%cooled in less than 6 myr.For Shap specifically, the average radius at 2 km depth is

about 2 km and the cooling time is given by t = 0.127τ myr.In the area to the west of Shap, illite crystallinity studies(British Geological Survey 1998) show that the country rockhad reached mid-anchizone grade by the time of graniteemplacement, corresponding to a temperature of 250°C(Kisch 1987). The Shap magma probably intruded at atemperature of about 650°C, close to solidus temperature,giving an initial temperature difference with respect to thecountry rock of 400°C. As the pluton cooled and the localwall rocks heated, the temperature difference with the far-field country rock would have reduced to 37% of its initialvalue at τ = 1, corresponding to a duration of 127 000 years.Thirty-seven percent of 400 C is 147°C, implying that thecentre of the pluton would then have cooled to 397°C, wellbelow solidus temperatures. Cooling equivalent to τ = 3,when the differential temperature would have reduced to 5%of its initial value, would have been achieved in only 300 000years.The rapid cooling times for the Shap pluton require a very

short time gap between the upper crustal intrusion of theStage I and Stage II magmas, or require that these twotextural varieties were brought together in the mid-crustalmuch zone, to be emplaced together into the upper crust.

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Duration of the Acadian deformation

The likely duration of the Acadian deformation can beestimated by dividing the accumulated finite strain by aplausible strain rate based on modern contractional ortranspressional orogens.The best indicators of the amount of Acadian shortening in

northern England are the accretionary lapilli shapes in theBorrowdale Volcanic Group measured by Bell (1981). Theselapilli suggest shortening perpendicular to the cleavage of50–70%, a result very similar to wider compilations ofcleavage in the Caledonian belt (Ramsay & Wood 1973).Strain rates in contractional orogens vary from 10−12 to

10−16 s−1 but in strike-slip orogens have a narrower rangefrom 10−14 to 10−15 s−1 (Campbell-Stone 2002). Some strainrates that are well constrained by GPS measurements areshown on Figure 10b, and fall within the range from 1.5 to6.5 × 10−15 s−1. At these rates, 60% Acadian shortening innorthern England would have taken between 3 and 10 myr toaccumulate (Fig. 7).

Significance of the emplacement and deformation rates

The estimates of the duration of emplacement and deform-ation (Fig. 7) are helpful in clarifying that the Acadiandeformation was more protracted than the emplacement ofany one late Devonian pluton. The modelling suggests thatthe Acadian folding and cleavage probably developed over 3to 10 myr, whereas all plutons except for Weardale appear tohave been emplaced in the upper crust and solidified in lessthan a million years. Even Weardale was 63% cooled in lessthan 6 myr (Fig. 10a).The rate estimates therefore suggest an explanation for the

paradox that the K-Ar ages on the Acadian cleavage, locallyand regionally, are younger than the emplacement age of theShap pluton, even though the pluton truncates the cleavage inthe country rocks. The paradox is resolved if the Acadiancleavage took as long as 10 myr to evolve, with the K-Ar agerecording only the end of this deformation, when overburdenwas eroded, the deformed rocks cooled, and the K-Ar systemclosed. The local K-Ar age on the cleavage of 401 ± 7 Maallows deformation to have started as early as 411 Ma and tohave been in progress for 6 myr before the emplacement ofthe Shap pluton at about 405 Ma (Fig. 7).

The cooling estimates above assume that each cylindricalpluton was emplaced in one event, with the textural andgeochronological complexity of a pluton such as Shap beinginherited from the mid-crustal mush zone (Miles &Woodcock 2018). If, alternatively, such plutons were builtup incrementally in the upper crust by a series of sill or dykeintrusions, then the cooling time of each magma batch wouldbe an order of magnitude shorter; tens of thousands of yearsonly (e.g. Barboni et al. 2015). The spread of the U-Pb zirconages in the Shap pluton could then be interpreted as uppercrustal emplacement of successive magma batches over 10million years or more, comparable with larger graniticplutons such as the Tuolumne Suite in California (Colemanet al. 2004; Glazner et al. 2004).This incremental emplacement model is implausible

because of the paucity of sharp chilled contacts betweendifferent magma batches in the Shap pluton. Figure 6g andh shows local examples of sharp contacts, which wouldconventionally be interpreted as chilling of a laterphenocryst-poor batch (‘Stage I’) against an earlierphenocryst-rich batch (‘Stage II’). However the veiningin Figure 6h suggests that the Stage II facies was indeedthe later component. We therefore reinterpret the apparentchills as mica-rich schlieren formed by filter-pressing atthe margin of the Stage I melt (Weinberg et al. 2001;Paterson et al. 2018), probably because it still containedsubstantial interstitial melt. Both magma batches probablycoexisted as crystal-rich mushes in the mid-crust, not in theupper crust.

Discussion

This review has considered the dating of the Acadiandeformation in northern England and of the Devoniangranites that overlapped with that deformation in both spaceand time. The central conclusion is that the cluster of K-Arand Ar-Ar ages recalculated at 404–394 Ma (Emsian, EarlyDevonian; Figs 7 and 9) provides a good estimate of the endof the Acadian deformation. However, that deformation mustalready have been in progress for some millions of years inorder to satisfy the observed field relations wherein theAcadian cleavage is truncated at the margins of the Shap andother granites. The granitic plutons themselves were rapidly

Fig. 10. (a) Plot of duration of cooling against pluton radius for three multiples of the characteristic time. Sizes of northern England plutons are arrowed.(b) Plot of duration of Acadian deformation against strain rate for three values of finite shortening across the cleavage. Arrowed strain rates are fromGPS-constrained studies in active orogenic belts: Himalaya (Allmendinger et al. 2007), Japan (Sagiya et al. 2000) and New Zealand (Beavan & Haines 2001).

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emplaced and cooled into these country rocks, mostly in lessthan a million years.There is, however, a second hypothesis that needs to be

considered: that the main cleavage-forming ‘Acadian’deformation is dated by the earlier cluster of K-Ar and Ar-Ar ages at about 419–426 Ma (late Ludlow and Prí̌dolí, lateSilurian). This hypothesis naturally satisfies the fieldrelations that the cleavage is truncated and thermallyoverprinted at the margins of the Shap and Skiddaw granitesand in the Crummock and Ulpha metasomatic aureoles.However, a late Silurian age for the Acadian deformationpresents the following problems:

a) The late Silurian does not correspond to the regionalAcadian unconformity: the gap in deposition duringwhich pre-Acadian rocks were deformed, upliftedand eroded. Where it is best constrained, in SouthWales and the Welsh Borderland (Avalonia), thisunconformity spans intra-Emsian through Frasniantime, at most 407–372 Ma (Figs 7 and 9). It thereforebegins at least 12 myr after the end of the late Silurianradiometric resetting episode.

b) It might be argued that the unconformity in northernEngland (Leinster–Lakesman terrane or Ganderia) iswider, and begins in Prí̌dolí time. However, illitecrystallinity data show that at the time the Acadiancleavage developed, the Silurian rocks in northernEngland had an overburden of at least 3.5 km ofuppermost Silurian and lower Devonian rocks (Soper&Woodcock 2003), so a late Silurian cleavage age isimplausible.

c) A late Silurian age for the Acadian deformationconflicts with the evidence around the Shap andSkiddaw plutons that shortening continued afteremplacement of the plutons and whilst theirmetamorphic aureoles developed (Soper & Roberts1971; Boulter & Soper 1973; Soper &Kneller 1990).

For these reasons, the Emsian (Early Devonian) age ispreferred here for the main Acadian deformation in northernEngland and Wales. The cause of the end-Silurian cluster ofages is still debatable. The suggestion of Evans (1996) andMerriman et al. (1995) that the ages record the progradetransition from smectite to illite during burial is appealing.However, a late Silurian cluster of Ar-Ar ages with a peak at423 Ma has been obtained from the NW–SE strikingcleavage in the Charnwood inlier of the East Midlands(Carney et al. 2008). These ages seem to record substantiallate Silurian shortening in the Anglian Basin on the northeastmargin of the Midland Platform. The possibility that thisshortening also affected the Welsh Basin and the basins ofthe Lakes-Leinster terrane cannot be discounted. However,any late Silurian shortening seems not to have been enoughto terminate deposition, only to have reduced subsidencerates in these basins and to promote a facies change frommarine to non-marine (Soper & Woodcock 2003). Eastwardfrom the Anglian Basin, the Brabant Massif was alsodeformed in late Silurian and Lower Devonian time (426–393 Ma; Debacker et al. 2005), in the event termed theBrabantian.The D1 and D2 deformation sequence in the Lake District

Massif, supposed here to be Acadian, was correlated by

Simpson (1963, 1967) with the similar sequence on the Isleof Man, a structural correlation broadly supported by morerecent studies (Fitches et al. 1999; Morris et al. 1999; Power& Barnes 1999). The most recent mapping of the Isle of Man(Chadwick et al. 2001) regards D1 and D2 as ‘Acadian’ but oflate Silurian or early Devonian age. D1 and D2 must pre-datethe weakly deformed Peel Sandstone Group, then thought tobe about 400–410 Ma (Pragian to Emsian), on the basis of its29 ± 5 palaeolatitude estimated palaeomagnetically (Piper &Crowley 1999). This age for the Peel Group would becompatible with the 404–394 Ma age for the Acadiandeformation in northern England if the Peel Group was atthe later Emsian end of the palaeomagnetically allowablerange. However, whilst a subsequently identified Scoyeniaichnofacies assemblage indicates a latest Silurian to EarlyDevonian age, a spore microflora suggests a more specificlate Lochkovian to Pragian age for the Peel Group (Crowleyet al. 2009). This age is incompatible with the evidence fromnorthern England, and the necessary conclusion is that D1

and D2 in northern England and the Isle of Man are not timecorrelatives. The northern England ‘Acadian’ deformation is404–394 Ma (Emsian) whereas the Isle of Man deformationis pre-end Pragian, so at least 408 Ma, possibly equivalent tothe 419–426 Ma (late Ludlow and Prí̌dolí, late Silurian)radiometric ages from England andWales already discussed.Interpretation of deformation timing in the Isle of Man has

recently been further complicated by a Late Ordovician U-Pbzircon age of 457.2 ± 1.2 Ma from the Dhoon Granodiorite(Fritschle et al. 2018). These authors quote Power & Barnes(1999) as concluding that this granite was intruded during orafter the D1 deformation, implying that D1 must be anOrdovician event. In fact, Power & Barnes (1999) observedthat porphyroblasts in the aureole to the Dhoon pluton,flattened in the S1 cleavage and replaced by chlorite andbiotite, suggest that ‘…the Dhoon Granodiorite was formedpre- or possibly syn-D1.’ So a late Silurian or Early Devonianage for D1 is still compatible with the new age for the Dhoonpluton. Furthermore, Fritschle et al. (2018) do not mentionthat the D1 to D3 structural sequence in the Ordovician ManxGroup is also present in the Wenlock (mid-Silurian) DalbyGroup, precluding a pre-late Silurian age for the maindeformation on the Isle of Man.A late Silurian or Early Devonian ‘Acadian’ deformation

can be traced into the Balbriggan inlier of eastern Ireland(Murphy 1987), through the Slieve Phelim and GalteeMountains inliers of central Ireland (Phillips 2001) to theDingle Peninsula of the west coast (Todd 2015). To the SW,in the Leinster Massif, U-Pb zircon ages (Fritschle et al.2018) show that the D1 deformation in the Ribband Group isabout 460 Ma (late Middle Ordovician) in age rather thanAcadian as previously thought. Deformation in the uncon-formably overlying Duncannon Group may still be Acadian.

Conclusions

a) A review of the field evidence shows that theDevonian plutons in northern England were intrudedafter most – but not all – of the Acadian folding andcleavage formation had occurred.

b) U-Pb zircon and Re-Os molybdenite ages (Selbyet al. 2008; Kimbell et al. 2010) suggest that the

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Skiddaw and Weardale plutons were emplaced atabout 399 Ma, but the Shap pluton earlier at about405 Ma.

c) The K-Ar age on the Acadian cleavage in northernEngland of 397 ± 7 Ma (Merriman et al. 1995) andK-Ar and Ar-Ar ages from Wales ranging from 400to 390 Ma are just compatible with the emplacementages of Skiddaw and Weardale, but incompatiblewith emplacement of Shap at 405 Ma. Adjustment ofthese ages upwards by 1% in line with the reviseddecay constant increases the local K-Ar age to 401 ±7 Ma and the regional range from 404 to 394 Ma(Emsian, Lower Devonian); this is still younger thanthe Shap emplacement age, but is more compatiblewith it within error.

d) Assessment of the thermal history of the northernEngland plutons show that they mostly cooled withina million years, with only the Weardale plutonhaving a more protracted cooling history.

e) By contrast, the Acadian deformation probably lastedfrom 3 to 10 myr, offering a solution to the ageparadox around the Shap pluton. Folds and cleavagemay have been forming for as much as 10 myr beforethe closure of the K-Ar isotopic system at about401 Ma.

f ) An earlier cluster of K-Ar and Ar-Ar cleavage ages at426–420 Ma (Ludlow to Prí̌dolí, late Silurian) isproblematic, as it falls before the regionalunconformity marking Acadian deformation, upliftand erosion.

g) A cluster of U-Pb zircon ages from the Shap plutonaveraging 415.6 ± 1.4 Ma is thought not to relate topluton emplacement. Rather, these ages areconsidered to record zircon crystallisation in a mid-crustal mush zone kept near solidus temperature formuch of Early Devonian time, and which waseventually the source for the upper crustal plutons

Acknowledgements We thank the Armstrongs Group for access to theShap Pink Quarry, Richard Hinton and the NERC EMMAC group at Edinburghfor help during the acquisition of zircon U-Pb ages, and Chris Thomas and ananonymous reviewer for their helpful comments on an earlier version of thispaper.

Funding This research received no specific grant from any funding agency inthe public, commercial, or not-for-profit sectors.

Scientific editing by Stewart Molyneux

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