The Newer Granite problem revisited: a transtensional ... · Wenlock time (Kemp, 1987) and the...

Geol Mag 145 (2 ) 2008 pp 235ndash256 ccopy 2008 Cambridge University Press 235doi101017S0016756807004219 First published online 1 February 2008 Printed in the United Kingdom

The Newer Granite problem revisited a transtensional origin forthe Early Devonian Trans-Suture Suite

P E BROWNlowast P D RYANdagger N J SOPERDagger amp N H WOODCOCKsectlowastSchool of Geography amp Geosciences St Andrews KY16 9AL Scotland UK

daggerDepartment of Earth amp Ocean Sciences National University of Ireland Galway IrelandDaggerGamrsquos Bank Threshfield Skipton N Yorks BD23 5NP England UK

sectDepartment of Earth Sciences University of Cambridge Downing Street Cambridge CB2 3EQ England UK

(Received 5 September 2006 accepted 8 May 2007)

Abstract ndash The origin of the Newer Granites is long-standing problem In the Caledonian orthotectoniczone the intrusions span the period of late orogenic convergence and uplift but attempts to relate themas a group to late Iapetan subduction have been unsuccessful A range of rock types is representedmainly with I-type affinities and granodiorite is the most voluminous In contrast granitic intrusionssouth of the Moniaive shear zone in Scotland and also in the north of England have significantS-type characteristics span the trace of the Iapetus suture and have ages in the range 400ndash390 Masignificantly younger than intrusions to the north We refer to these younger granitic intrusions alongwith others of similar character along-strike to the southwest as the Trans-Suture Suite We explorethe link between the Trans-Suture Suite and recently recognized orogen-wide sinistral transtensionin the Early Devonian period Importantly the Trans-Suture Suite intrusions are accompanied by anintense suite of lamprophyre dykes the origin of which is to be sought in extension decompressionand heating of enriched Avalonian sub-continental lithosphere In some instances the granite intrusionscarry clots of lamprophyric origin and the Criffel body is particularly important in being continuouslyzoned from an I-type with lamprophyric enclaves to an S-type interior We propose that generationof these lamprophyres during transtension advected heat into the base of the crust to produce theS-type component of the Trans-Suture Suite Modelling presented shows that generation of voluminousS-type magmas requires the coincidence of several factors hydrated sub-continental lithosphericmantle preserved during lsquosoftrsquo collision under the Trans-Suture Suite zone thermal relaxation toremove any subduction refrigeration crust composed of juvenile volcanogenic material and Devoniantranstension Our models suggest that if hydration pre-dated transtension then only small graniticbodies could be produced unless the zone of lamprophyre generation extends beyond the rift zoneThe emplacement of the Trans-Suture Suite intrusions overlapped the Acadian deformation periodthat succeeded the transtensional episode during which the granite magmas were generated

Keywords granite transtension trans-suture suite lamprophyre Acadian

1 Introduction

It is axiomatic that when regional-scale geologicalphenomena coincide in space and time they are likelyto represent responses to the same geotectonic causeTwo such are the later stages of Caledonian lsquoNewerGranitersquo magmatism in Scotland and deposition of theLower Old Red sandstone coincidence that has longbeen recognized (eg Thirlwall 198l) but seen as aparadox if the magmatism was associated with oro-genic convergence how could its major developmentcoincide with post-collisional sedimentation

The tectonic setting of the Newer Granite suite ofBritain and Ireland is undoubtedly complex Theseintrusions occur in the orthotectonic Caledonides to thenorth of the Highland Border fault and in the slate beltsto the south on both sides of the Iapetus suture (Fig 1)Their ages lie mainly in the range 435ndash380 Ma post-

lowastAuthor for correspondence petbrownbtinternetcom

dating the Grampian orogeny (with which the lsquoOlderGranitesrsquo are associated) overlapping the latest phasesof Iapetus closure (the Scandian orogeny and lsquosoft-dockingrsquo of Eastern Avalonia) and extending throughthe subsequent Acadian deformation of the slate belts(see Soper et al 1992 and Dewey amp Strachan 2003 fordiscussion of the geometry and timing of these events)The granitic magmas were generated and emplacedin a collage of diverse lithospheric segments and itis scarcely surprising that their origin has proved adifficult petrogenetic problem None of the modelsproposed for parts of the suite (Iapetan subductionmore complex subduction systems slab break-offtranspressive crustal thickening and anatexis post-orogenic decompressional melting) offer a compre-hensive explanation for the whole (see discussions bySoper 1986 Brown 1991 Stephenson amp Highton1999)

Within the Scottish Newer Granites chemical andisotopic criteria enabled Stephens amp Halliday (1984) to

httpsdoiorg101017S0016756807004219Downloaded from httpswwwcambridgeorgcore Open University Library on 17 Jan 2017 at 164016 subject to the Cambridge Core terms of use available at httpswwwcambridgeorgcoreterms

236 P E BROWN AND OTHERS

Figure 1 Outline tectonic map of Britain and Ireland showing the main crustal lineaments discussed in the text and the lsquoNewerrsquoGranites and associated Devonian volcanic rocks

make a threefold division into the Argyll Cairngormand South of Scotland suites demonstrating the provin-cialism of source composition (Fig 1) Stone Kimbellamp Henney (1997) recognized two distinct subgroups inthe South of Scotland suite those occurring north andthose south of the Orlock Bridge fault and associatedMoniaive shear zone (Fig 2) The northern granites areolder (gt 410 Ma) and characterized by low 87Sr86Srinitial ratios while to the south such plutons as FleetCriffel and Cheviot are younger (lt 410 Ma) and havemore radiogenic initial 87Sr86Sr ratios The southernsubgroup of Stone Kimbell amp Henney (1997) wasnamed the Galloway Suite by Highton (1999) It hasmuch in common with the Lake District granites suchas Shap and Skiddaw (see Sections 3a6 and 3a7) thatwere intruded south of the Solway Line the surfacetrace of the N-dipping Iapetus suture We propose

the recognition of a Trans-Suture Suite to include theGalloway and northern England granitic intrusionsprimarily on the basis of age and tectonic setting andalso the common presence of a notable componentof sedimentary origin in the genesis of the magmasThe Trans-Suture Suite can be extended along-striketo include representatives in the Isle of Man (FoxdaleDhoon) and the east of Ireland (principally Newry in theLongford-Down extension of the Southern Uplands andthe large Leinster intrusion to the south of the sutureFig 2) Their origin has long been a puzzle

Obviously the granites of the Lake District andLeinster could not have been generated by northwardsubduction beneath Scotland while the Gallowayplutons although emplaced in the hangingwall of thesuture lie too close to its surface trace to accommodatean arcndashtrench gap (Thirlwall 198l Soper 1986)


The Newer Granite problem 237

Figure 2 Geological map of the Iapetus Suture region in northern Britain and central Ireland The Trans-Suture Suite of granites andtheir probable sub-crops are shown The principle zone of lamprophyre emplacement is marked with diagonal lines and rose diagramsfor the trends of the lamprophyre dykes are given for three regions

Furthermore it is now evident that Iapetan convergenceceased at about the end of Silurian time 20 to 40 Mabefore emplacement of the Trans-Suture Suite granites(Soper amp Woodcock 2003) Therefore if the suiteis related to subduction a post-Iapetan subductionsystem of Early Devonian age would have to beinvoked Northward Rheic subduction appears to beruled out as a source by the close spatial associationof the Trans-Suture Suite with the (fossil) Iapetussuture zone and its absence further south in Wales forexample

A non-subduction origin for the Trans-Suture Suitegranites thus needs to be considered The key lies

in understanding the tectonic setting of the Iapetussuture zone in Early Devonian time when the plutonswere generated and emplaced It has recently beenrecognized that in the northern Caledonides EarlyDevonian time was a period of sinistral transtension(Dewey amp Strachan 2003) In Britain and Ireland thiswas associated with widespread Old Red Sandstonemagnafacies sedimentation and lamprophyric magmat-ism and was terminated by Acadian transpressivedeformation towards the end of Early Devoniantime (Soper amp Woodcock 2003) In this paper weexplore genetic links between these phenomena andthe intrusions of the Trans-Suture Suite



Figure 3 Tectono-stratigraphic framework for Britain during Siluro-Devonian time (after Soper amp Woodcock 2003) Note the onsetof transtension after closure of the Iapetus Ocean at c 420 Ma which ceases at about 400 Ma with Acadian transpression Theemplacement of the Trans-Suture Suite occurs near the end of or soon after the cessation of this phase of transtension

2 Siluro-Devonian tectonics of the Iapetus suturezone in Britain and Ireland

Figure 3 is a tectonostratigraphic summary diagramfor central Britain through Silurian and Devoniantime simplified from Soper amp Woodcock (2003their fig 7) It illustrates that within the sinistrallyoblique Caledonian tectonic context deformation inthe suture zone alternated between transpression andtranstension

2a Silurian convergence to c 420 Ma

There is a well-established link between northwardsubduction of Iapetus lithosphere beneath the Lauren-tian margin (Leggett McKerrow amp Soper 1983)southward progradation of the Southern UplandsLongford-Down accretionary prism in the hangingwallof the suture (Barnes Lintern amp Stone 1989) andformation of the Windermere Supergroup flexuralbasin in northern England in the footwall (Kneller



1991) More contentious has been the question of whenIapetus convergence ended Popular interpretation hasrelated the Acadian deformation in southern Britainto continued convergence and southwardly progradingimbrication of the Avalonian footwall through EarlyDevonian time (Kneller King amp Bell 1993) It isnow apparent that throughout the North AtlanticCaledonides Early Devonian time was period ofsinistral transtension (Dewey amp Strachan 2003)Soper amp Woodcock (2003) have discussed the endingof Iapetus convergence in Britain in that contextAccretion in the Southern Uplands slowed in mid-Wenlock time (Kemp 1987) and the youngest accretedpackage exposed in the Southern Belt is of lateWenlock age Emplacement of late Caledonian minorintrusions spanned the ending of accretion A suite ofK-lamprophyre dykes is concentrated in the CentralBelt of the Southern Uplands Early examples weredeformed by the accretionary deformation but themajority are post-tectonic and have given ages in therange 400ndash418 Ma (Rock Gaskarth amp Rundle 1986)It is inferred that accretion in the Southern Uplandsended at about 420 Ma (Ludlow) and was succeededby a period of sinistral transtension thus marking theend of Iapetus convergence and by definition of theCaledonian orogeny

The presence of early deformed lamprophyre dykesin the Southern Uplands (Barnes Rock amp Gaskarth1986) suggests that the switch from transpressiveaccretion to transtension was not abrupt either the twodeformation modes alternated for a period during lateSilurian time or they were partitioned vertically in thelithosphere so that lamprophyric liquid was generatedat depth and emplaced into the active accretionarycomplex during periods of stress release

2b Devonian transtension c 420ndash400 Ma

The evidence for orogen-wide sinistral transtension inEarly Devonian time has been outlined by Dewey ampStrachan (2003) The great spread of Old RedSandstone magnafacies sediment that covered muchof Britain south of the Scottish Highlands was inferredby Soper amp Woodcock (2003) to have been depositedin coalescing transtensional basins for which a β factorof 15 was suggested

In the present context of lsquolate Caledonianrsquo mag-matism critical evidence is provided by the swarmof K-lamprophyre dykes mentioned above The KndashArages of dykes from the suture zone (Southern Uplandsand Longford-Down in the hangingwall eastern LakeDistrict in the footwall) range from 420 Ma to 400 Madefining a period of lithospheric (trans)tension in theEarly Devonian period that separated the Iapetan andAcadian convergence regimes (Fig 3)

Lamprophyre dyke trends in the suture zone canbe analysed in relation to the general model fortranstension developed by Dewey (2002) which

involves the combination of coaxial and non-coaxialstrains Mean orientations are consistent with a sinistralnon-coaxial lsquoCaledonide-parallelrsquo strain componentplus a coaxial lsquoCaledonide-normalrsquo component (seediscussion by Soper amp Woodcock 2003) Thisconfirms that both the hanging- and footwalls ofthe suture zone in Britain experienced the periodof sinistral transtension in Early Devonian time thatDewey amp Strachan (2003) identified regionally

2c Acadian deformation c 400ndash390 Ma

Compared to its major development in the northernAppalachians (eg Bradley et al 2000 Murphy ampKeppie 2005) the Acadian deformation in the slatebelts of southern Britain and Ireland represents a short-lived event that produced little more than anchimeta-morphic rocks at the present surface Geochronometricevidence for its age has been reviewed by Soper ampWoodcock (2003 their fig 8) KndashAr dating of illitein cleaved mudrocks in the Central Welsh Basingave mean of 399 plusmn 3 Ma (ten determinations Evans1996) ArndashAr dating of cleavage mica in strain fringesin Welsh mudrocks has given a high precision age of3961 plusmn l4 Ma (late Emsian Sherlock et al 2003)

The emplacement of Trans-Suture Suite graniteplutons in the Lake District overlapped the Acadiandeformation The structural relationships of individualintrusions are detailed below together with the some-what inadequate geochronological evidence (Section 3)(modern zircon studies are lacking) that is currentlyavailable for their age of emplacement Howevertheir mean age of 397 plusmn 2 Ma (five determinations) isconsistent with KndashAr date of 397 plusmn 7 Ma on cleavage-parallel illite from a Silurian bentonite (Merriman et al1995) indicating a late Emsian age for the Acadiandeformation in NW England

Stratigraphically the ending of Acadian deform-ation is not precisely constrained in Britain Thepreserved Upper Old Red Sandstone overstep sequencecommences in the Famennian but perhaps earlier inthe Lake District (Fig 3) In the west of Irelandhowever the Acadian unconformity occurs betweenthe Dingle and Smerwick groups of late Silurianto Pragian or early Emsian age and the overlyingEifelian Pointagare Group (Richmond amp Williams2000) The Acadian tectonometamorphism in Britainand Ireland thus appears to have occupied a briefperiod between 400 and 390 Ma close to the LowerndashMiddle Devonian boundary with no evidence that thedeformation prograded in any particular direction

2d Evidence of enhanced heat flow

There is evidence that the slate belt deformation tookplace under enhanced heat flow In the Welsh Basinestimates of the palaeogeothermal gradient at the timeof peak Acadian metamorphism range from 36 Ckmminus1 at a depth equivalent to a lithostatic pressure of



030ndash036 GPa (based on fluid inclusion measure-ments Bottrell et al 1990) to 52 C kmminus1 (based onmetabasite mineral assemblages Bevins amp Merriman1988) Direct determinations have not been made onSilurian slates in NW England where the presumptionis that low gradients of 25 C kmminus1 or less are likelyto have characterized the Silurian flexural basin phase(Merriman 2002) However as argued by Soper ampWoodcock (2003) this gradient must have been greatlyenhanced during Early Devonian time otherwise animplausible thickness of Lower Old Red Sandstonecover must be invoked to produce the anchizonalmetamorphism seen in late Silurian slates at the presentsurface A link between transtension and enhancedheat flow may lie in the Early Devonian lamprophyresthis possibility is explored quantitatively in Section 6

3 The Trans-Suture Suite of granite intrusions inBritain and Ireland

Numerous authors have remarked on similaritiesbetween the granitic intrusions in the north of Englandand southern Scotland (eg Harmon amp Halliday 1980Halliday 1984 Harmon et al 1984 Stephens 1988Thirwall 1989 Highton 1999) noting in particularevidence for the importance of an input of sedimentaryorigin there being peraluminous two-mica granites inboth areas It should be noted that the Lake Districtintrusions of Eskdale and Ennerdale are to be excludedfrom these comparisons having been shown to beof Ordovician rather than Acadian age (Millward ampEvans 2003) Information on individual intrusions isconsidered below where an unfortunate disparity isevident between the comprehensive amount of dataavailable for the Galloway Suite and the sparser datafor those intrusions to the south of the suture lineHighton (1999) summarized the Galloway plutons ashaving 87Sr86Sr ratios typically 0705ndash 0707 δ18O inthe range 8 to 12 permil and εNd values suggestive of inputfrom Silurian turbiditic sedimentary sequences

3a Age composition and structural relationships

Modern zircon-based age determinations are largelylacking for the members of the trans-suture suite ofintrusions Enough is known however of their ageand structural relationships to place them in their EarlyDevonian tectonic context (Fig 3) It has been proposedthat large granitic intrusions are emplaced over 106

or even 107 years by the incremental assembly ofsmall batches of magma in sheets or dykes (ColemanGray amp Glazner 2004 Glazner et al 2004) Thelarge Leinster intrusion is thought to be composedof sheets intruded over a substantial period of time(Reavy 2001) On the other hand concentrically zonedplutons such as Criffel (Section 3a2) are likely tohave formed by the continuous or rapidly episodicinjection of compositionally changing magma over atime span that lies within the precision errors typically

associated with KndashAr and RbndashSr age determinationsThe majority of the trans-suture intrusions wereemplaced in anchimetamorphic country rocks andin our view their lsquocooling agesrsquo provide reasonableindication of their time of emplacement

As has become common though not universallyaccepted practice in discussing the Newer Graniteswe use the I and S nomenclature (Chappell amp White1974) For the I-type we regard the origin as beingeither in refused basaltic or lamprophyric material orin the fractional crystallization of such mantle-derivedmagmas

3a1 Cairnsmore of Fleet Intrusion

This pluton was intruded into early Llandovery (GalaGroup) strata in the Central Belt of the SouthernUplands immediately south of the Orlock Bridge fault(Barnes Phillips amp Boland 1995 Figs 2 3) A bulkfraction UndashPb zircon age of 396 plusmn 6 Ma (Pidgeon ampAftalion 1978) is within error of the RbndashSr mineralndashwhole rock age of 392 plusmn 2 Ma (Halliday Stephens ampHarmon 1980) These ages are reported by BarnesPhillips amp Boland (1995) to have been confirmed by amore recent unpublished UndashPb zircon age

Emplacement post-dates the accretionary deforma-tion of the Central Belt and initiation of the Moniaiveshear zone A fabric related to ductile reactivationof the shear zone wraps cordierite porphyroblastsin the aureole but is cut by the granite contact(Barnes Phillips amp Boland 1995 Stone Kimbell ampHenney 1997) If the reported age reliably reflects thetime of emplacement these relationships indicate thatreactivation of the Moniaive shear zone was Acadianand the Fleet pluton was emplaced syntectonically

The Fleet two-mica pluton has S-type chemicaland isotopic characteristics A predominantly crustalorigin is indicated by an initial 87Sr86Sr ratio of07062 to 07083 and an εNdt of minus30 to minus34 Thehighly evolved oxygen isotopes with 18O around 11 permilindicate major sedimentary input (Halliday Stephens ampHarmon 1980 Halliday 1984 Stephens amp Halliday1984) Stephens amp Halliday (1984) suggested that thesource of the Fleet magmas was the same as that of theLake District plutons with the implication that magmagenesis took place after closure of the Iapetus OceanDerivation from Avalonian crust underthrust beneaththe Southern Uplands in the footwall of the suture isalso supported by Pb isotopes (Thirwall 1989) whichmatch those found in Ordovician Skiddaw Group ofthe Lake District

3a2 The Criffel pluton

The Criffel pluton was emplaced into LlandoveryndashWenlock strata in the southern part of the SouthernUplands accretionary complex stitching the tract-bounding fault between the Hawick and Riccartongroups (Fig 2) Features such as enclave alignment



and mineral foliations and rotation of envelope fabricsinto parallelism with the margin indicate diapiricemplacement and ballooning (Stephens 1999)

An RbndashSr age of 397 plusmn 2 Ma (Halliday Stephens ampHarmon 1980) is within error of the 395 plusmn 9 Ma RbndashSr age of the nearby Newmains lamprophyre dyke Thisdyke has the same initial 87Sr86Sr ratio (070514 plusmn 5)as the outer mafic part of the pluton (Macdonaldet al 1986) and the relationship between coevallamprophyric and granitic magmatism gives importantinsights into the petrogenesis of the Trans-Suture Suiteas is discussed in Sections 3 and 4

The Criffel pluton exhibits strong concentric zoningfrom an early outer hornblendic granodiorite ofI-type to an inner two-mica granite with marked S-typeaffinity (Stephens et al 1985 Stephens 1992) Isotopicratios (Halliday Stephens amp Harmon 1980 Halliday1984) vary from the outer zone inwards 87Sr86Sr from07052 to 07073 εNd from minus06 to minus31 and δ18Ofrom 85 to 119 permil Stephens et al (1985) demon-strated correlated behaviour of rare earth elementabundances with isotopic variability and showed thatthe data can only be interpreted in terms of substantialand progressively increasing input of crustally derivedanatectic magma towards the pluton interior

Mafic inclusions are common in the outer part of theCriffel intrusion and it has been shown that these rep-resent syn-plutonic injections of lamprophyric magmainto the host granodioritic magma (Holden Halliday ampStephens 1987 Holden et al 1991 Stephens Holdenamp Henney 1991) The simultaneous existence ofmantle and crustal melts demonstrates the importanceof magmatic advection in the generation and ascent ofanatectic granitic magma in the Criffel pluton

3a3 The Cheviot intrusion

This intrusion is located close to the trace of the Iapetussuture (Fig 2) and intrudes comagmatic lava flowsof the Cheviot Volcanic Formation (Thirwall 1981)Thirwall (1988) reported a mean RbndashSr biotite ageof 395 plusmn 29 Ma for three quartz monzonites from theintrusion while the most reliable age obtained for theextrusive rocks is a biotitendashapatite age of 395 plusmn 38 Maon a trachyte flow confirming the contemporaneity ofthe intrusive and extrusive rocks Thirwall regardedthe Cheviot Volcanic Formation as chemically distinctfrom the Old Red Sandstone volcanic rocks of theMidland Valley which he related to subduction

Harmon amp Halliday (1980) attributed the low δ18O of471ndash478 of the Cheviot granite to post-emplacementexchange with depleted meteoric groundwater Theinitial 87Sr86Sr of 07061 was presumed to have beenunaffected and if so to limit the input of radiogenicSr from crustal source The εNdt of minus42 (Halliday1984) is however somewhat more negative than for theS-type Fleet two-mica granite or the central part of theCriffel body

3a4 Minor granites

Small and poorly known granitic bodies at Portencorkiein the Rhinns of Galloway and at Kirkmabreck nearCreetown were also included by Highton (1999) in theGalloway Suite of intrusions

3a5 The Skiddaw granite

The Skiddaw biotite granite is the most northerly ofthe Lake District granite intrusions and has a widemetamorphic aureole in Ordovician country rocksof the Skiddaw Group It is a boitite granite withlesser amounts of muscovite In the aureole andalusiteovergrows the S1 Acadian cleavage but is locallyweakly wrapped by it and is further deformed by S2(Soper amp Roberts 1971) The emplacement age is lesswell constrained geochronologically but a KndashAr biotiteage of 392 plusmn 4 Ma (Shepherd et al 1976) and an RbndashSrisochron of 399 plusmn 8 (Rundle 1992) are consistent withan lsquointra-Acadianrsquo age setting for the intrusion Thereis a paucity of other isotopic data for the intrusion

3a6 The Shap granite

This well-known K-feldspar megacryst granite wasemplaced in Caradoc volcanic rocks of the BorrowdaleGroup and its aureole affects strata as young as Ludlowin the overlying Windermere Supergroup Contactmetamorphism post-dates the Acadian cleavage butcogenetic microgranite dykes are locally cleaved attheir margins indicating that the magmatism was alsolsquointer-Acadianrsquo (Soper amp Kneller 1990)

The Shap intrusion was the subject of early RbndashSr and KndashAr dating which yielded ages in the range380ndash410 Ma with large precision errors (summarizedby Brown Miller amp Soper 1964) A discordant bulkfraction UndashPb zircon age of 390 plusmn 6 Ma was reportedby Pidgeon amp Aftalion (1978) The best availableestimate of the emplacement age of the intrusion seemsto be the 394 plusmn 3 Ma obtained by Wadge et al (1978)from whole rockndashfeldspar isochron and supported byKndashAr biotite age of 397 plusmn 7 Ma

The Shap granite has high δ18O of 11 permil (Harmonamp Halliday 1980) moderately high 87Sr86Sr of 0707(Wadge et al 1978) and εNd of minus20 These figureshave been variously interpreted as being due tohydrothermal crustal fluids acting on a granite withI-type mineralogy (Halliday 1984) and as obviousevidence of strong sedimentary input (Harmon ampHalliday 1980)

The intrusion contains abundant megacryst-bearingmicrodioritic inclusions which are distinct from thecountry rock xenoliths that are also sparsely presentin the granite The microdiorite clots are thought toresult from injection of contemporaneous basic magma(Grantham 1928) which Harker as long ago as 1909suggested was related to the regional K-lamprophyredyke swarm Unfortunately subsequent studies havenot pursued this important aspect concentrating on



the feldspar megacrysts (eg Cox et al 1996 Lee ampParsons 1997)

3a7 The Crummock intrusion

The presence of a subsurface granitic intrusion in thewestern Lake District is inferred from an extensivezone of bleaching and tourmaline mineralization in theSkiddaw Group (Cooper et al 1988) The alteration islater than the main deformation but pre-dates reversedisplacement on the associated Causey Pike fault sothe intrusion is thought to be lsquolate Acadianrsquo Cooperet al (1988) reported that 15 whole-rock samples gavea RbndashSr regression line equivalent to 395 plusmn 12 MaThe quoted isochron age of 401 plusmn 3 Ma obtained byprogressive elimination of outlying points is moreprecise but not necessarily a more accurate estimateof the emplacement age

3a8 The Weardale intrusion

The presence of a sub-Carboniferous granitic plutonin the northern Pennines was detected geophysicallyand proved by drilling (Rookhope borehole Dunhamet al 1965) It is compositionally similar to the Shapand Skiddaw intrusions (Millward 2002) but unlikethem its upper part has shallow-dipping gneissosefoliation (Dunham 1990) Fitch amp Miller (1965)reported a KndashAr muscovite age for the Weardaleintrusion of 396 plusmn 6 Ma A RbndashSr age of 410 plusmn 10 Mawas obtained by Holland amp Lambert (1970) andrecalculated by Dunham (1974) as 394 plusmn 34 Ma Areliable emplacement age would show whether theWeardale intrusion should be included in the Trans-Suture Suite perhaps as an early ( pre-Acadian)member It may be noted that the other large subsurfacePennine granite Wensleydale which gave a RbndashSr ageof 410 plusmn 10 Ma (Dunham 1974) is now thought tobe Ordovician on the grounds of its similarity to thesub-volcanic Lake District intrusions of Eskdale andEnnerdale (Millward 2002)

3a9 Isle of Man granites

The Dhoon granodiorite and Foxdale and Oatlandsgranites have small outcrops immediately south ofthe trace of the suture (Fig 2) and are intruded intoOrdovician slates of the Manx Group Fabric studieson their aureoles indicate that the Dhoon body wasemplaced during D1 and Foxdale after D2 in thelocal deformation sequence which is presumed to beAcadian (Power amp Barnes 1999)

3a10 Eastern Irish granites

The Leinster granite is the largest Caledonian graniteexposed in the British Isles It is a peraluminous two-mica granite believed to have been derived from partialmelting of an immature dominantly volcaniclasticprotolith (Elsdon amp Kennan 1979) UndashPb and monazite

geochronology dates the intrusion at 405 plusmn 2 Ma(OrsquoConnor Aftalion amp Kennan 1989) Structuralstudies of the batholith (M OrsquoMahony unpub PhDthesis NUI Cork Ireland 2001) have shown thatit was emplaced into low-grade Lower Palaeozoicmetasediments in an active extensional crustal linea-ment by process of amalgamation of multiple sheets ofmagma Petrographic studies (Grogan amp Reavy 2002)have shown magma mixing between successive sheetsAppinites and lamprophyres are locally abundant alongthe northwestern margin of the pluton and are alsoassociated with a major shear zone the East Carlowdeformation zone that bounds its eastern margin(McConnell et al 1994) The penetrative fabric in theEast Carlow deformation zone deforms the regionalfoliation post-dates the emplacement of the appinitesand lamprophyres and is contemporaneous withcontact metamorphism associated with the Leinstergranite (McArdle amp Kennedy 1987)

The Newry igneous complex intruded and con-tact metamorphosed low-grade metasediments of theLongford-Down massif The central part of theintrusion has given a UndashPb zircon age of 423 plusmn 7 Ma(Meighan et al 2003) The geologically older north-eastern part of the complex has a concordant titanite ageof 4104 plusmn 13 that has been interpreted as a cooling age(Meighan et al 2003) The Newry complex probablypre-dates the final closure of the Iapetus ocean and wedo not consider it as part of the Trans-Suture Suite

3b Tectonic setting of the Trans-Suture Suite granites

In summary the isotopic ages and structural re-lationships of the Trans-Suture Suite of granitesshow that they were emplaced at relatively shallow(anchimetamorphic) crustal levels in both the hanging-and footwalls of the Iapetus suture during Acadiandeformation This occurred late in Early Devoniantime within the period 390ndash400 Ma The deformationwas transpressive and no doubt episodic stress releaseby seismic slip may have facilitated the ascent ofgranite magma However although the Trans-SutureSuite granites were lsquosynorogenicrsquo as regards theirfinal emplacement their association with the EarlyDevonian lamprophyre suite explored below suggeststhat their generation and ascent were associated withthe preceding period of transtension

4 Lamprophyre dykes

Mica-phyric and hornblende-phyric lamprophyredykes are present throughout Scotland and extend intonorthern England (eg Rock et al 1988 Shand et al1994 Canning et al 1996 1998) In the region of theTrans-Suture Suite of plutons the main concentrationof dykes is in a 10 km wide zone extending fromthe Central Belt of the Southern Uplands into EasternIreland (Rock Cooper amp Gaskarth 1986 Vaughan



1996) The zone trends significantly anticlockwise tothe regional strike (Fig 2) and within it upper crustalextension averages 1 and locally as much as 6 (Barnes Rock amp Gaskarth 1986) Regionally micalamprophyres dominate the zone but close to theCriffel pluton hornblende lamprophyres are common

Dated examples from the Lake District and SouthernUplands (Nixon Rex amp Condliffe 1984 Rock Gas-karth amp Rundle 1986) have given Early Devonian KndashAr ages but some undated early dykes in the SouthernUplands overlapped late stages of the accretionarydeformation and are likely to be of latest Silurian ageHowever as outlined above their main developmentwas associated with the Early Devonian period ofsinistral transtension that followed Iapetus closure

The biotite lamprophyres include examples withhigh Mg numbers suggesting derivation from prim-itive mantle melts unaffected by fractionation orassimilation The origin of such volatile-rich potassicmagmas is thought to be in small degrees of meltingof metasomatized sub-continental lithospheric mantle(eg Harry amp Leeman 1995 Canning et al 1996) In anenvironment of extensional tectonics and lithosphericthinning melting may be triggered by decompressionand the input of asthenospheric heat Also in theIapetus suture zone decompression melting may beattributed to the influence of lithosphere-penetratingshear zones (Vaughan 1996) which both concentratedtranstensional strain in early Devonian time andprovided channels for H2O and CO2 metasomatism

A genetic link between lamprophyricappinitic mag-matism and the Scottish Newer Granites has longbeen considered Atherton amp Ghani (2002) specific-ally suggested remelting of a lamprophyric crustalunderplate as a source of I-type magmas in the ArgyllSuite of the Newer Granites However fractionationof primitive lamprophyricappinitic magma cannotalone have generated the huge volume of NewerGranite plutons in the Highlands it is necessaryto invoke substantial assimilation of crustal material(eg Macdonald et al 1986 Rock amp Hunter 1987Fowler 1988 Fowler amp Henney 1996 Canning et al1996 Fowler et al 2001) In the case of the Trans-Suture Suite intrusions the volumetric importanceof S-type magma to the point of being the majorcomponent of the Fleet pluton demonstrates theexistence of independent mantle (I-type) and crustal(S-type) magmas and mingling and mixing of these isevident in many members of the Trans-Suture Suite

A mantle input to the Criffel pluton is seen in themafic inclusions which are common in the outer parts ofthe intrusion These have been interpreted by HoldenHalliday amp Stephens (1987) and Stephens Holden ampHenney (1991) as syn-plutonic injections of mantle-derived lamprophyric magma they are evidence of thesimultaneous existence of mantle and crustal melts andby inference for the heat source responsible for crustalmelting and facilitating the rise of S-type magmas

Similar but less well-researched relationships betweenthe Shap granite and its mafic inclusions point to thesame dual magmatic source

A mechanism is thus required for the simultaneousproduction of mantle and crustal melts during the EarlyDevonian period of transtension While decompressionmelting of lithospheric mantle is a well-establishedmechanism under realistic boundary conditions sig-nificant melting of the lower crust is not (Ryan ampSoper 2001) It is not possible to quantify the relativeimportance of fractionation of lamprophyric magmaversus fusion of lamprophyric underplate in the genesisof the I-type component of the Trans-Suture Suite butas explored below it is our proposal that advectionof heat into the lower crust by lamprophyric magmas(Section 6) was responsible for crustal melting andthe I-type transitional to S-type character of the Trans-Suture Suite plutons

5 End-Caledonian lithospheric domains

A starting point is required for thermal modellingto investigate the proposed petrogenetic relationshipbetween Early Devonian transtension lamprophyricmagmatism and generation of the Trans-Suture Suitegranites Figure 4 is a lithospheric cross-section ofScotland and northern England that attempts to depictmantle domains and their thermal structure at the endof Caledonian convergence say 420 Ma The sectionis notionally along the UK Geotraverse North line(Pharaoh et al 1996) It represents a two-dimensionaltime slice through a complex convergence system andis necessarily speculative While the upper part ofsuch a section can be based on surface geology andgeophysical data the configuration of the lithosphericmantle which is critical to the modelling has to beinferred from Caledonian convergence history itselfinferential

5a Caledonian convergence history

The later stages of the Caledonian orogeny are attrib-uted to convergence between three plates LaurentiaBaltica and Avalonia (Fig 5) The timing and geometryof BalticandashAvalonia interaction is debated but notof critical importance here It is widely agreed thatin the BritishndashIrish sector Iapetus closure involvedsubduction to the south (in the present frame) beneathEastern Avalonia until late Ordovician time andnorthward beneath Laurentia until late Silurian time

Generally southward subduction in the Ordovicianbeneath Eastern Avalonia (the Borrowdale subductionzone) led to the development of arc-marginal basinpairs in the Lake District and Wales (Kokelaar 1988)and Leinster (McConnell 2000) culminating in aburst of volcanicity in the Caradoc at about 450 Ma(Millward amp Evans 2003) This and the subsequent



Figure 4 Lithospheric cross-section drawn approximately along the line of the UK Geotraverse North line depicting mantle domainsand their thermal structure at c 420 Ma The geometry of the Avalonian lithosphere is derived from assuming that the ridge wasdestroyed around 450 Ma (Woodcock 2000) The southern extent of Iapetan lithosphere is unknown and may be as far north as theGreat Glen Fault or as far south as the Southern Uplands Fault The base of the lithosphere is taken as 1300 C The local depressionof the 300 C and 700 C geotherms is drawn to reflect the cessation of subduction at this time

volcanic shut-down have been attributed by Woodcock(2000 figs 105 109) to subduction of an Iapetanridge-transform system probably followed by slabbreak-off The Avalonian lithospheric mantle lay inthe hangingwall of this system and as such wouldhave been fluxed by volatiles during these events Thenorthwards extent of the Avalonian lithospheric mantleis unknown but it is assumed to extend at least as faras the Midland Valley

Lithospheric convergence geometry during theensuing closure of Iapetus in the Silurian periodpresents a four-dimensional problem in which themain uncertainty is whether Baltica and Avaloniahad different displacement vectors with respect toLaurentia or whether they had already combinedinto a single plate during late Ordovician collision(eg Trench amp Torsvik 1992) A resolution ofthe problem is not essential in the present contextbecause we are concerned principally with AvaloniandashLaurentia interaction in the BritishndashIrish sector Hereit is widely agreed that northward convergence ofIapetan lithosphere with Laurentia was partitioned intoNW-dipping subduction system and sinistral slip onNEndashSW (Caledonoid) terrane-bounding faults in thehangingwall principally the Great Glen and HighlandBorder

5b Seismic evidence

The Iapetus suture zone has been imaged on BIRPSdeep seismic reflection profiles and can be tracedfor some 900 km from the Atlantic continental shelfwest of Ireland to the North Sea (Klemperer Ryan ampSnyder 1991 and references therein) It is associated

with a N-dipping zone of middle and lower crustalreflectivity consistent with northward underthrustingof Avalonian crust beneath the Laurentian marginduring final closure The suture zone is perhaps bestimaged on the NEC line located close offshore ofeastern Britain (Freeman Klemperer amp Hobbs 1988)A boundary (IN on Fig 4) can be recognized betweenreflective presumably Avalonian crust to the southand unreflective mid-crust to the north On a depth-converted section the boundary dips at about 20 to thenorth and projects up-section to intersect the base ofthe Upper Palaeozoic cover on the Solway Line thegeologically defined trace of the Iapetus suture (Soperet al 1992) A similar reflectivity boundary (IW) is seenon the WINCH line to the west in the North Channelwhich had previously been identified by Hall et al(1984) as marking the suture We have interpolatedbetween IW and IN to fix the crustal position of thesuture on Figure 5

The deeper trajectory of the suture has not beenimaged directly but there are grounds to suppose thatin the present-day lithosphere it flattens northwardsOn the NEC line the IN boundary appears to mergewith a zone of strong subhorizontal reflectivity in thelower crust which includes two bright gently N-dippingreflections (P1 and P2 Fig 4) that are coincident withthe Moho at 30 km depth These have been interpretedas Iapetus crust (Freeman Klemperer amp Hobbs 1988)or imbricated basement and sedimentary cover fromthe continentndashocean transition at the leading edge ofAvalonia (Soper et al 1992) We adopt the latterinterpretation in Figure 4 and assume therefore thatAvalonian mantle must extend at least as far north as theIN P1 and P2 reflectors Avalonian sedimentary rocks



Figure 5 Tectonic diagram showing the likely positions ofthe three plates Avalonia Baltica and Laurentia during lateSilurianndashearly Devonian time

could be the source of the high 207Pb204Pb ratios ofcertain Southern Uplands granites which as mentionedabove Thirlwall (1989) matched with the OrdovicianSkiddaw Group of northern England

On the WIRE profiles offshore of western Ireland(Klemperer Ryan amp Snyder 1991) strong crustalreflections in the suture zone dip north at about 30 andfurther north there are patches of gently dipping sub-Moho reflectivity which presumably relate to subhori-zontal mantle fabric (Klemperer Ryan amp Snyder 1991their fig 3) However flat subduction could not havegenerated the LlandoveryndashWenlock magmatism ofnorthern Connemara (Menuge Williams amp OrsquoConnor1995) now some 100 km north of the suture Even ifthe Silurian arcndashtrench gap was significantly greater arelatively steep subduction zone is required

Our inference is that normally inclined subduction inearly Silurian time became shallower as progressivelyyounger lithosphere entered the system and conver-gence slowed to zero at 420 Ma with modest collisional

deformation due in part to the buoyancy effect ofthe Avalonian margin The dip of the subduction zoneat 420 Ma adopted in Figure 4 assumes that furtherflattening of the uppermost mantle fabric occurredduring subsequent episodes of lithospheric extensionprincipally in Early Devonian and Permo-Triassic time

6 Thermal modelling

Our proposal is that the Trans-Suture Suite graniteswere produced by melting in the lower part of theAvalonian crust where it forms the footwall of theIapetus suture by the advection of heat from risinglamprophyric magma The lamprophyre melts weregenerated by decompression melting of deep enrichedand hydrated Avalonian lithospheric mantle duringEarly Devonian transtension We use two-dimensionaltransient finite element models to test whether thisproposal is physically realistic The modelling isperformed in three stages thermal relaxation ofthe initial 420 Ma template to 400 Ma followed bylithospheric extension to produce a pull-apart basin21 km wide to determine the volume of associatedlamprophyric melt produced under realistic strain ratesand then the emplacement of this melt into the lowercrust to explore the generation of granite melt Theduration of the rifting events and strain rates werevaried to produce the required Beta factor of 15(Table 3) a reasonable value for the overlying Devonianbasins (Soper amp Woodcock 2003) We follow themethodology outlined in Ryan amp Soper (2001) withsome amendments discussed in this section Assumedvalues of the input parameters used in the modellingare listed in Table 1

Figure 6a represents the initial lithospheric thermalstructure at 420 Ma derived from Figure 4 andFigure 6b shows the thermal structure after staticrelaxation for 20 Ma following the method of Ryan ampDewey (1997) This provides the starting conditionsfor subsequent finite element models of extensionlamprophyre intrusion and consequent generation ofgranite magma Models were run using a crust with30 km 35 km and 40 km thickness these bracketthe likely values estimated by adding the erodedmaterial onto the present crustal thickness (see Soper ampWoodcock 2003) The models predict that the Mohotemperature at the centre of the site of the rift willrise due to thermal relaxation and radiogenic heatingbetween 420 and 400 Ma from 424 C to 481 C orfrom 494 C to 551 C and from 499 C to 577 C forcontinental crust of 30 km 35 km and 40 km thicknessrespectively This is an important prediction becausethe temperature of the lower crust significantly affectsthe amount of granitic melt that can be generated bya given aliquot of injected mafic magma (see forexample Annen amp Sparks 2002) If the crust hadremained at the refrigerated temperatures associatedwith an accretionary prism then it would be hard



Table 1 Parameters used in the modelling

Moho depth (km) 40 35 30

Thickness of lithosphere (km) Set by program sim 135Heat productivity of upper half of crust (W mminus3) 135Heat productivity of lower half of crust (W mminus3) 063Heat productivity of mantle (W mminus3) 30times10minus2

Temperature at base of lithosphere (C) 1333Fixed temperature at 560 km (C) 1800Diffusivity of crust (m2 sminus1) 778 times 10minus7

Depth dependent diffusivity of mantle (m2 sminus1) 745ndash113 times 10minus7

Onset and duration of stretching (Ma) 20 2ndash13Strain rates (sminus1) 66 times 10minus15 33 times 10minus15 10 times 10minus15

Mean density of crust (kg mminus3) 2780Mean density of mantle (kg mminus3) 3350Mean density of asthenosphere (kg mminus3) 3203 3220 3240Topography before rifting (m OD) 23 12 minus10Initial porosity of rift sands 049Compaction coefficient of sands (kmminus1) 027Mean density of sandstones (kg mminus3) 2650Initial porosity of thermal subsidence muds 063Compaction coefficient of muds (kmminus1) 051Mean density of shales (kg mminus3) 2720Density of interstitial water (kg mminus3) 1030Coefficient of expansion (Cminus1) 25 times 10minus5

Compressibility (Paminus1) 10minus11

Thermal constant of lithosphere (Ma) 50Specific latent heat of crust (J kgminus1 Kminus1) 30 times 105

Specific latent heat of lamprophyre (J kgminus1 Kminus1) 50 times 105

Dimensions of crustal elements melted (m) 500 wide times 250 highTime step for estimation of lamprophyre and

sediments in basin01 Ma

List of assumptions used in this analysis All densities are given in values at NTP The asthenosphericdensities were adjusted so that the basins initiated at or near sea-level these basins were then sedimentloaded to produce a maximum value for the rift sediment deposited

to generate any substantial volume of granite by ourchosen mechanism

Lamprophyric magma is regarded as resulting from alow-fraction melting of metasomatized subcontinentallithospheric mantle or of metasomatic veins withinsuch mantle (eg McKenzie 1989) they are volatile-rich and have high abundances of light rare earthand large ion lithophile elements Such magmas areproduced by low-fraction melting of hydrous peridotite(see for example Hirose amp Kawamoto 1995) Theconcept of decompression melting as used to producelamprophyre magma in our modelling requires thatthe lower Avalonian lithosphere was enriched andhydrated The Avalonian subcontinental lithospherewas above the southerly Iapetan subduction zoneduring the Ordovician and may have been enrichedat that time Also as proposed by Vaughan (1996)deep-seated transtensional shear zones (reactivatedtransform faults) are thought to have facilitateddecompression and movement of volatiles in thelower lithosphere The main concentration of dykeslies anticlockwise to the regional structural trend butclockwise to the dyke maximum (Fig 2) suggestingthat the controlling shear zone was neither inheritedfrom crustal structure nor initiated under the EarlyDevonian transtensional regime but existed as a zone ofweakness in the deep lithosphere which was reactivatedin transtension

A further problem is that the lamprophyre solidus hasnot been fully determined experimentally However ex-perimental studies (Wyllie 1995 Hirose amp Kawamoto1995) show that solidus temperatures for peridotitewith only 25 to 30 of water are significantly lowered(Fig 7) A hydrated basaltic melt results as the wateris partitioned into the initial melt increments andthe source peridotite then becomes dehydrated It isdifficult to select an exact solidus temperature to modelas this depends on the degree of water saturation How-ever Hirose amp Kawamoto (1995) showed that 5 meltfraction will be generated at 1100 C and 1 GPa Thusfor wet basaltic magma to ascend to the Moho about10 GPa it must have a temperature of at least 1100 CA significant amount of the lower lithospheric mantle isat or above 1100 C melt should therefore originate assoon as hydration takes place and subsequent batchesof melt may be produced in regions below the solidus inresponse to other tectonic events

An alternative approach is to use the wet peridotitesolidus (Wyllie 1991 1995) (Fig 7) Again we assumethat 5 melt is produced when the temperature exceedsthat of the solidus for a given pressure However thissolidus has both positive slope at low pressure andnegative slope at high pressures with maximum temper-ature of 1100 C at about 15 GPa (Fig 7) Magma pro-duced below this point will freeze on ascent unless it hastemperature at or in excess of 1100 C In our model it



Figure 6 (a) Assumed initial thermal structure of the lithosphere110 km either side of the Iapetus suture at 420 Ma based uponFigure 4 (b) Modelled thermal structure of the lithosphere at400 Ma at the start of rifting These diagrams were preparedusing GMT software (Wessel amp Smith 1991)

Figure 7 Solidus curve(s) for wet and dry mantle peridotitesfrom 0ndash120 km (40 GPa) The arrow shows the interruptedascent path of magma produced at 1080 C at 4 GPa It freezesagain at 2 GPa However if this magma were produced at13 GPa it could ascend to a Moho at 1 GPa (dashed arrow)

is then registered as being trapped and available for re-melting if suitable conditions arise Both solidi shownin Figure 7 produce similar amounts of lamprophyricmelt for the conditions assumed This is because duringstretching with Beta factors of about 15 temperaturesof 1100 C are never exceeded at pressures of less than14 GPa Therefore all melt produced at gt 14 GPaabove 1100 C can ascend If temperatures of 1050 Cwere exceeded between 10 GPa and 14 GPa then themodels would differ (Fig 7)

The next step is to assume that this region is thensubjected to transtension developing pull-aparts of21 km width We present models for one such rift thatinitiates at 100 km into the model space The predictedcrustal temperature structures across the model arefairly uniform by 400 Ma but the mantle has not fullythermally relaxed Selecting 100 km gives a conservat-ive estimate as this area has the thinnest lithosphereand will produce the least lamprophyre on rifting Inmodern transtension zones strain rates can approach10minus14 sminus1 (Dewey 2002) Consequently we assumetwo relatively high strain rates of 66 times 10minus15 sminus1 and33 times 10minus15 sminus1 orthogonal to the zonersquos boundaries inaddition to the more lsquonormalrsquo rate of 10minus15 sminus1 We thenrun two-dimensional finite element models followingthe method of Ryan amp Soper (2001) to calculate theamount of lamprophyric melt generated during riftingfor each strain rate (Table 2) The area of lamprophyricmelt is calculated by re-evaluating the temperature ofthe nodes bounding each element between 1 GPa and4 GPa after each time step If the node temperatureexceeds the selected solidus at that pressure thenthat element is assumed to produce 5 melt at thattemperature and become refractory the melt is movedup instantaneously until it freezes (see Fig 7) or reachesthe Moho Such a model assumes that there is no heatexchange between the melt and the surrounding mantlerocks

Two models for the hydration of the mantle wereused Firstly the mantle was hydrated at 400 Ma atthe onset of transtensional rifting and development ofdeep penetrating shear zones Secondly the mantle washydrated during final closure of the ocean at 420 MaThe appropriate area of lamprophyric melt generated atgiven times (Table 3 Fig 8) is then emplaced as 20 kmlong sills 4 km above the base of the crust (Fig 9a)Lamprophyres generated outside these time windowswere assumed to have ascended to higher levels inother words they were not emplaced in the lower crustand did not contribute to the generation of graniticmagma However they may have assisted in the ascentof the magma by increasing heat flow We identified sixphases of intrusion for all models in which the locus ofintrusion was assumed to move downwards with timeThe nodes which accommodated the lamprophyre lsquosillrsquohad their vertical spacing increased and the interveningelements received the appropriate amount of advectedheat at given time intervals (Table 2)



Figure 8 Areas of lamprophyric melt generated and ableto ascend to the Moho with time assuming 5 melting oflithospheric mantle Melt model assumes the wet peridotitesolidus of Wyllie (1991 1995) Melt areas are shown in alogarithmic scale (a) Hydration of the mantle at 420 Ma anda strain rate of 33 times 10minus15 sminus1 most melt is generated during

Table 2 Lamprophyre produced with time by rift models

40 km 35 km 30 km

Sill (m) T (C) Sill (m) T (C) Sill (m) T (C) Time (Ma)

Hydration at 400 Ma rifting at 400 MaVery fast rift (66 times 10minus15 sminus1)9940 11890 11730 12010 11430 11992 39992000 11970 1970 12070 1650 12050 39971950 12050 1650 12140 1630 12126 39951650 12120 1950 12220 1970 12203 39931970 12240 1650 12280 690 12220 39911310 12250 50 12280 30 12210 3989Fast rift (33 times 10minus15 sminus1)9940 11890 11730 12010 11430 11992 39992000 11970 1630 12070 3280 12050 39951950 12050 1650 12140 2660 12126 39911650 12120 1630 12220 390 12203 39871970 12240 1650 12280 310 12220 39831310 12250 690 12280 340 12210 3978Normal rift (10 times 10minus15 sminus1)9940 11890 11730 12010 11430 11992 39991310 12250 50 12280 30 12210 39891630 12050 1540 12140 1570 12126 39751630 12120 1920 12220 1711 12203 39621650 12240 990 12280 1008 12220 39511950 12250 1300 12280 47 12210 3936

Hydration at 420 Ma rifting at 400 MaVery fast rift (66 times 10minus15 sminus1)1650 11720 1520 12010 1630 11992 39991630 11810 1750 12070 1650 12050 39971730 11890 1640 12140 1630 12126 39951870 11970 1650 12220 1010 12203 39931680 12050 670 12280 50 12220 3991

330 12060 40 12280 350 12210 3989Fast rift (33 times 10minus1 5 sminus1)1650 11710 1310 12010 1630 11992 39991630 11790 1630 12070 1650 12050 39951640 11870 1970 12140 1630 12126 39911670 11960 1630 12220 1010 12203 39871930 12030 1010 12280 380 12220 3983

370 12070 370 12280 370 12210 3978Normal rift (10 times 10minus15 sminus1)1630 11890 1310 12010 1630 11992 39991320 11970 1310 12070 1653 12050 39871640 12050 1310 12140 1650 12126 39751630 12120 1310 12220 1651 12203 39621630 12240 1650 12280 290 12220 39511000 12250 360 12280 510 12210 3936

Thicknesses and temperatures of lamprophyric magma emplacedas 20 km wide sills into the rift zone at the nodes equivalent to4 km above the Moho before rifting The thickness decreases by1β t and the width increases by β t as rifting progresses β t is thebeta factor at a given time Each successive package was emplacedbelow the previous one

The crust is assumed to contain considerableamounts of hydrated juvenile material and bothmuscovite- and biotite-dominated melting models (seeFigs 9b c 10b) are evaluated (Zen 1995 Ryan ampSoper 2001) However tests showed that the area of

rifting over a period of some 20 Ma after the onset of rifting(b) Hydration of the mantle at 420 Ma and a strain rate of10minus15 sminus1 most melt is generated during rifting over a periodof some 70 Ma after the onset of rifting (c) Hydration of themantle at 400 Ma and a strain rate of 66 times 10minus15 sminus1 note thatthe majority of melt is generated during the first time step afterthe onset of rifting the time of hydration



Table 3 Results of melting models for 21 km wide basin

Strain rate (times 10minus15 sminus1)

Parameter 660 330 100

Beta factor 152 152 150Rift interval (Ma) 200 400 1300Rift sediments (km) for 30 km Moho 238 278 334Rift sediments (km) for 35 km Moho 252 301 361Rift sediments (km) for 40 km Moho 285 356 406Lamprophyre melt generated during rifting at 400 MaLamprophyre (km2) for 40 km Moho hydrated at 400 Ma 3963 3984 3639Area not emplaced in sills 199 22 017Lamprophyre (km2) for 35 km Moho hydrated at 400 Ma 3942 3934 3639Area not emplaced in sills 142 138 133Lamprophyre (km2) for 30 km Moho hydrated at 400 Ma 3916 3917 3614Area not emplaced in sills 436 235 455Lamprophyre (km2) for 40 km Moho hydrated at 420 Ma 1989 2009 1988Area not emplaced in sills 211 231 218Lamprophyre (km2) for 35 km Moho hydrated at 420 Ma 1945 1998 1688Area not emplaced in sills 491 414 238Lamprophyre (km2) for 30 km Moho hydrated at 420 Ma 1934 1963 1663Area not emplaced in sills 67 629 187Biotite dominated melting with hydration at 400 MaGranite (km2) ndash 40 km Moho time step = 000625 Ma 936 722 624Predicted with time step of 0 Ma 964 745 644Granite (km2) ndash 35 km Moho time step = 000625 Ma 802 663 610Predicted with time step of 0 Ma 835 688 629Granite (km2) ndash 30 km Moho time step = 000625 Ma 463 459 424Predicted with time step of 0 Ma 496 477 442Muscovite dominated melting with hydration at 400 MaGranite (km2) ndash 40 km Moho time step = 000625 Ma 3000 2289 1941Predicted with time step of 0 Ma 3058 2298 1970Granite (km2) ndash 35 km Moho time step = 000625 Ma 2453 2008 1854Predicted with time step of 0 Ma 2510 2038 1834Granite (km2) ndash 30 km Moho time step = 000625 Ma 1390 1369 1229Predicted with time step of 0 Ma 1444 1376 1257Biotite dominated melting with hydration at 420 MaGranite (km2) ndash 40 km Moho time step = 000625 Ma 151 137 091Granite (km2) ndash 35 km Moho time step = 000625 Ma 089 067 052Granite (km2) ndash 30 km Moho time step = 000625 Ma 034 039 039Muscovite dominated melting with hydration at 420 MaGranite (km2) ndash 40 km Moho time step = 000625 Ma 507 453 324Granite (km2) ndash 35 km Moho time step = 000625 Ma 280 228 212Granite (km2) ndash 30 km Moho time step = 000625 Ma 131 129 130

Results of melting models used in this analysis based upon on the method of Ryan amp Soper (2001) butusing variable time steps to estimate the likely maximum melt volume The granite models were constructedusing the method of Zen (1995) and also based upon data from Johannes amp Holtz (1990)

granitic melt produced by a given model was affectedby the size of the time step used If a time step is toolong a crustal node may pass into and out of the meltfield during this step and no melt will be recorded at theend of the step This effect is illustrated in Figure 10awhich shows a speculative temperature versus time plotfor a crustal node adjacent to a lamprophyre intrusionTo record the maximum possible melt the time stepshould coincide with the maximum temperature fora given node For example time steps of 15 05or 025 would record the maximum possible meltof 35 at time 15 while steps of 1 or 2 wouldrecord maximum melt of 15 at time 2 but a stepof 3 would record no melt (Fig 10a) A plot ofmaximum melt produced versus time step is givenin Figure 10c There are several hundred nodes thatmelt in one step therefore it would be expected thatsuch behaviour would be approximately linear over

a range of time steps excluding the very short andthe very long To allow for this effect a range oftime steps (005 Ma to 000625 Ma) was used andthe area of granite magma produced in an infinitelysmall time step was estimated using simple linearextrapolation (Fig 10d) The linear fit in Figure 10cslightly overestimates the maximum melt for this nodeIt is therefore assumed that the amount of meltproduced in any one set of models lies between thevalue for the minimum time step (000625 Ma) andthe value extrapolated to a zero-duration time step Alltwelve models show strongly linear relationships withcorrelation coefficients approaching minus100 and littledifference between the area of melt modelled for timesteps of 000625 Ma and 00 Ma

In a muscovite-dominated melting system the areaof granite melt produced ranged from 125 km2 to300 km2 for hydration at 400 Ma and 13 km2 to



Figure 9 (a) Thermal structure of the lithosphere 0025 Ma after emplacement of a sill of lamprophyre 20 km long and 29 km thick atabout 1200 C at a position 4 km above the Moho The model uses a strain rate of 66 times 10minus15 sminus1 affecting a pull-apart of 21 km initialwidth and an initial crustal thickness of 35 km (see Table 2 for more details) (b) Contours showing total proportion of melt formedusing a biotite melting model 25 Ma after the start of rifting at a strain rate of 66 times 10minus15 sminus1 (c) Contours showing proportion ofmelt formed using a muscovite melting model 25 Ma after the start of rifting at a strain rate of 66 times 10minus15 sminus1

51 km2 for hydration at 420 Ma In a biotite-dominatedsystem it ranged from 42 km2 to 94 km2 for a systemhydrated at 400 Ma and from 13 km2 to 04 km2 forone hydrated at 420 Ma Although similar amounts oflamprophyre were emplaced for models with strainrates 66 times 10minus15 sminus1 and 33 times 10minus15 sminus1 the formerproduced about 30 more melt This was becausethe longer time interval between sill emplacementsfor the lower strain rate model allowed the heat todissipate (Table 2) consequently fewer nodes reachedthe minimum melting temperature for granite (675 C)This result mirrors that of the one-dimensional modelsof Annen amp Sparks (2002) where an increasedfrequency of emplacement promotes greater crustalmelting The runs with 40 km Moho produced abouttwice the amount of melt as those with 30 km Mohowhere other conditions were equal This reflects thehigher temperature at the Moho with thicker crust(577 C see above) and emphasizes the importanceof the 20 Ma thermal relaxation stage Had this periodbeen shorter we would predict that correspondinglyless granite magma would have been produced

The total volume of granite produced by such modelsdepends upon the third dimension of the rift As pull-aparts typically have length to breadth ratio between21 to 51 (Aydin amp Nur 1982) the maximum volumeof melt produced within a 20 km wide rift would beup to 3000 km3 for a strain rate of 66 times 10minus15 sminus1 andhydrated underlying mantle If the magma ascendedvertically this would form a pluton of approximately20 km width by 100 km length and 15 km thicknessOther geometries are possible if lateral transport of thegranitic magma occurs

The model used in this analysis is capable oftaking a large range of parameters into account thatmay well vary the final amount of melt producedRyan amp Soper (2001) showed that dykes near themargin of rift can produce up to 30 more meltthan sills at the depocentre Annen amp Sparks (2002)showed that both the rate of magma emplacementand the nature of the lower crust can influence themelt volume However it is not the purpose of thisanalysis to investigate the subtle influences that varioustectonic and geological factors can have on the precisevolume of granitic melt produced Rather it is ourintention to demonstrate that under certain conditionsthe emplacement of lamprophyres into the lower crustcan produce sufficient granite volumes to account forthe Trans-Suture Suite

These models suggest that the whole of the relativelysmall elongated Crummock Water intrusion of the LakeDistrict (Fig 2) with volume of perhaps 102 km3

could be produced at strain rates equal to or inexcess of 10minus15 sminus1 and crust of 30 km or morethickness regardless of when hydration took placelsquoIrsquo-type components from fractionation or mixingof lamprophyric magma would create an S+I-typecomplex which would promote the rise of the graniticmelts to higher crustal levels The large isolated plutonsof the Southern Uplands such as Criffel and Fleetwhich have volumes of the order of 103 km3 can onlybe produced in muscovite-dominated systems with astrain rate ge 10minus15 sminus1 and a crust ge 30 km in thicknessThe 104 km3 Leinster pluton could only be producedby this model with a strain rate of 66 times 10minus15 sminus1a crustal thickness of ge 40 km and a rift of 120 km



Figure 10 (a) Hypothetical thermal evolution of a node adjacent to a lamprophyre intrusion The time axis is linear but with arbitraryvalues (b) Melt fraction curves for a muscovite-dominated melt and a biotite-dominated melt system (c) Plot of maximum meltyielded from the region adjacent to the node shown in (a) plotted against the size of the time step the time interval is in arbitrary units(d) Plots of area of melt generated versus size of time step (005ndash000625 Ma) for rifts with a strain rate of 66 times 10minus15 sminus1 in 35 kmcrust for muscovite and biotite melt models a strain rate of 10minus15 sminus1 in 30 km crust for muscovite and biotite melt models Linearinterpolation lines are plotted for each set of models and results are given in Table 3

length with lateral migration of magma over 10 km ormore

The Trans-Suture Suite plutons typically compriseboth S- and I-type components but it has not beenpossible even in the best-studied example of Criffel

to quantify the relative proportions of these inputs Ifthe crustal component was only 50 then accordingto our model it must represent granitic melt gatheredfrom considerable length of shear zone unless the strainrate was very high and assuming that the mantle was



hydrated at the time of rifting The same applies to thedifferentiated and contaminated lamprophyric melt inthe mafic outer parts of the pluton In such situationour models predict a ratio of muscovite- to biotite-dominated melt of about 31 and volume of excesslamprophyre of the order of 1ndash4 km3 Emplacement ofthe Trans-Suture Suite plutons thus involved substantiallateral magmatic flow andor very high strain rateswithin the putative transtensional shear zones

The model produces the most granite if hydrationwas coeval with transtension and penetrated thelithosphere along pre-existing lines of weakness Itis important to note that at the onset of graniteformation the mantle was hydrated a fact supportedby the presence of the lamprophyre swarms We fullyaccept that the lack of an experimentally determinedlamprophyre solidus to parameterize our model placesconstraints on the accuracy of our models Howeverin a test run using the dry peridotite solidus for35 km thick crust at a strain rate of 66 times 10minus15 sminus1a sill of basaltic melt of only 00024 km thicknesswas produced and no granite magma This supportsour contention that a melting regime similar to thosemodelled is required to produce the observed volumesof lamprophyric magma during transtension

7 Conclusions

A group of granitic intrusions occurring to the southof the Moniaive shear zone in the south of Scotlandin northern England and in Ireland and which spanthe trace of the Iapetus suture is recognized asthe Trans-Suture Suite The ages of these intrusionsare in the range 400ndash390 Ma significantly youngerthan the Newer Granite intrusions in the Caledonideorthotectonic zone to the north In the orthotectoniczone a range of rock types is represented mainly ofI-type whereas in the Trans-Suture Suite memberswith significant S-type characteristics are more con-spicuous The origins of the intrusions occurring inboth the footwall and the hangingwall of the Iapetussuture have been particularly problematical It is con-cluded that the genesis of these granitic intrusions canbe explained in the context of Siluro-Devonian platetectonics and the recognition of orogen-wide sinistraltranstension in the early Devonian period This episodeof extensional tectonics is linked to the depositionof Old Red Sandstone sediments in coalescing basinsacross much of Britain south of the Scottish HighlandsSignificantly the granitic intrusions of the Trans-SutureSuite are accompanied by an intense suite of lampro-phyre dykes the origin of which was in extensiondecompression and melting of subducted enriched andhydrated Avalonian lithospheric mantle Following thelsquosoftrsquo collision of Avalonia with Laurentia there wasperiod of thermal relaxation which removed subduc-tion refrigeration Lamprophyre melting triggered byDevonian transtension led to the advection of heat

into the lower crust of the Trans-Suture Suite zonewhich comprised juvenile volcanogenic material andgenerated the Trans-Suture Suite Numerical modelssuggest that if hydration occurred during transtensionthen large I+S-type granitic bodies could be generatedIf hydration pre-dated transtension then only smallgranitic bodies would be produced unless the zone oflamprophyre generation extends beyond the rift zoneThe volumes of lamprophyre and granite are consistentwith volatile fluxing of the Avalonian lithosphere abovethe Borrowdale subduction zone during Ordoviciantime and hydration along deep-seated transtensionalshear zones during the Early Devonian period A similarmultiphase model which required both subduction andpost-collision processes was proposed by MacDonaldet al (1985) for the lamprophyres of NW England

The generation of I+S granite suites within atranstensive pull-apart zone with a high strain rate willassist emplacement at higher levels For example inthe model for a 35 km thick crust at a strain rate of66 times 10minus15 sminus1 the upper surface of the zone of granitegeneration was transported tectonically upwards fromabout 31 km to about 23 km by crustal thinning beneaththe sedimentary basin The higher the strain rate thefaster this process and the more likely it is to transporta magma body before final consolidation

The model may be generalized to account for post-orogenic S-type granites in other lsquosoftrsquo collision zonesHowever the amount of I-type melt produced by thismodel was limited and thus some other mechanismmust be sought for the generation of the northernI-type suite of the Newer Granites

Acknowledgements We would like to acknowledge a re-view by M B Fowler which helped improve the manuscriptP E B wishes to acknowledge much help and discussion onthis problem over the years from W E Stephens

References

ANNEN C amp SPARKS R S J 2002 Effects of repetitiveemplacement of basalt intrusions on thermal evolutionand melt generation in the crust Earth and PlanetaryScience Letters 203 937ndash55

ATHERTON M P amp GHANI A A 2002 Slab breakoffa model for Caledonian Late Granite syn-collisionalmagmatism in the orthotectonic (metamorphic) zone ofScotland and Donegal Ireland Lithos 62 65ndash85

AYDIN A amp NUR A 1982 Evolution of pull-apart basinsand their scale independence Tectonics 1 91ndash106

BARNES R P LINTERN B C amp STONE P 1989 Timing andregional implications of deformation in the SouthernUplands of Scotland Journal of the Geological SocietyLondon 77 203ndash22

BARNES R P PHILLIPS E R amp BOLAND M P 1995The Orlock Bridge Fault in the Southern Uplands ofsouthwest Scotland a terrane boundary GeologicalMagazine 132 523ndash9

BARNES R P ROCK N M S amp GASKARTH J W 1986Late Caledonian dyke swarms in Southern Scotlandnew field petrological and geochemical data for the



Wigtown Peninsula Galloway Geological Journal 21101ndash25

BEVINS R E amp MERRIMAN R J 1988 Compositionalcontrols on co-existing prehnitendashactinolite and prehnitendashpumpellyite facies assemblages in the Tal y fanmetabasite intrusion North Wales implications forCaledonian metamorphic field gradients Journal ofMetamorphic Geology 6 17ndash39

BOTTRELL S H GREENWOOD P B YARDLEY B W DSHEPHERD T J amp SPIRO B 1990 Metamorphicand post-metamorphic fluid flow in the low-graderocks of the Harlech Dome North Wales Journal ofMetamorphic Geology 8 131ndash43

BRADLEY D C TUCKER R D LUX D HARRIS A Gamp MCGREGOR D C 2000 Migration of the Acadianorogen and foreland basin across the Northern Ap-palachians US Geological Survey Professional Paper1615 49 pp

BROWN P E 1991 Caledonian and earlier magmatism InGeology of Scotland (ed G Y Craig) pp 229ndash96Geological Society of London

BROWN P E MILLER J A amp SOPER N J 1964 Age of theprincipal intrusions of the Lake District Proceedings ofthe Yorkshire Geological Society 34 331ndash42

CANNING J C HENNEY P J MORRISON M A ampGASKARTH J W 1996 Geochemistry of late Caledonianminettes from northern Britain implications for theCaledonian sub-continental lithospheric mantle Min-eralogical Magazine 60 221ndash36

CANNING J C HENNEY P J MORRISON M A VAN

CALSTEREN P W C GASKARTH J W amp SWARBRICKA 1998 The Great Glen Fault a major vertical litho-spheric boundary Journal of the Geological SocietyLondon 155 425ndash8

CHAPPELL B W amp WHITE A J R 1974 Two contrastinggranite types Pacific Geology 8 173ndash4

COLEMAN D S GRAY W amp GLAZNER A F 2004 Rethink-ing the emplacement and evolution of zoned plutonsgeochronologic evidence for incremental assembly ofthe Tuolemne intrusive suite California Geology 32433ndash6

COOPER D C LEE M K FORTEY M J COOPER A Hamp RUNDLE C C 1988 The Crummock Water aureolea zone of metasomatism and source of ore metals in theEnglish Lake District Journal of the Geological SocietyLondon 145 523ndash40

COX R A DEMPSTER T J BELL B R amp ROGERS G1996 Crystallisation of the Shap granite evidence fromzoned K-feldspar megacrysts Journal of the GeologicalSociety London 153 625ndash35

DEWEY J F 2002 Transtension in arcs and orogensInternational Geological Review 44 402ndash39

DEWEY J F amp STRACHAN R A 2003 Changing SilurianndashDevonian relative plate motion in the Caledonidessinistral transpression to sinistral transtension Journalof the Geological Society London 160 219ndash29

DUNHAM K C 1974 Granite beneath the Pennines in NorthYorkshire Proceedings of the Yorkshire GeologicalSociety 40 191ndash4

DUNHAM K C 1990 Geology of the Northern Pennine Orefield Volume 1 Tyne to Stainmore Economic Memoirof the British Geological Survey England and WalesSheets 19 and 25 Second edition

DUNHAM K C DUNHAM A C HODGER B L amp JOHNSONG A L 1965 Granite beneath Visean sedimentswith mineralization at Rookhope northern Pennines

Quarterly Journal of the Geological Society of London121 383ndash417

ELSDON R amp KENNAN P S 1979 Geochemistry of Irishgranites In The Caledonides of the British Isles ndashreviewed (eds A L Harris C H Holland amp B ELeake) pp 713ndash16 Geological Society of LondonSpecial Publication no 8

EVANS J A 1996 Dating the transition of smectite to illitein Palaeozoic mudrocks using the RbndashSr whole-rocktechnique Journal of the Geological Society London153 101ndash8

FITCH F J amp MILLER J A 1965 Age of the Weardalegranite Nature 208 743ndash45

FOWLER M B 1988 Elemental evidence for crustalcontamination of mantle-derived Caledonian syenite bymetasediment anatexis and magma mixing ChemicalGeology 69 1ndash16

FOWLER M B amp HENNEY P J 1996 Mixed Caledonianappinite magmas implications for lamprophyre frac-tionation and high BandashSr granite genesis Contributionsto Mineralogy and Petrology 126 199ndash215

FOWLER M B HENNEY P J DERBYSHIRE D P F ampGREENWOOD P B 2001 Petrogenesis of high BandashSrgranites the Rogart pluton Sutherland Journal of theGeological Society London 158 521ndash34

FREEMAN B KLEMPERER S L amp HOBBS R W 1988The deep structure of northern England and the Iapetussuture zone from BIRPS deep seismic reflection profilesJournal of the Geological Society London 145 727ndash40

GLAZNER A F BARTLEY J M COLEMAN D S amp TAYLORR Z 2004 Are plutons assembled over millions of yearsby amalgamation from small magma chambers GSAToday 14 4ndash11

GRANTHAM D R 1928 The petrology of the Shap graniteProceedings of the Geologists Association 39 299ndash331

GROGAN S E amp REAVY R J 2002 Disequilibrium texturesin the Leinster Granite Complex SE Ireland evidencefor acid-acid mixing Mineralogical Magazine 66 929ndash39

HALL J BROWN J A MATHEWS D H amp WARNERM R 1984 Crustal structure across the Caledonidesfrom the WINCH seismic reflection profile influenceon the evolution of the Midland Valley of ScotlandTransactions of the Royal Society of Edinburgh EarthSciences 75 97ndash109

HALLIDAY A N 1984 Coupled SmndashNd and UndashPb system-atics in late Caledonian granites and the basement underNorthern Britain Nature 307 229ndash33

HALLIDAY A N STEPHENS W E amp HARMON R S1980 RbndashSr and O isotopic relationships in threezoned Caledonian granitic plutons Southern UplandsScotland evidence for varied sources and hybridisationin magmas Journal of the Geological Society London137 329ndash48

HARKER A 1909 Natural History of Igneous RocksLondon Methuen amp Co 384 pp

HARMON R S amp HALLIDAY A N 1980 Oxygen andstrontium isotopic relationships in the late Caledoniangranites Nature 283 21ndash5

HARMON R S HALLIDAY A N CLAYBURN J A P ampSTEPHENS W E 1984 Chemical and isotopic sys-tematics of the Caledonian intrusions of Scotland andnorthern England a guide to magma source region andmagma-crust interaction Philosophical Transactionsof the Royal Society of London A 310 1514 709ndash42



HARRY D L amp LEEMAN W P 1995 Partial meltingof melt metasomatised subcontinental mantle andthe magma source potential of the lower litho-sphere Journal of Geophysical Research 100 10255ndash69

HIGHTON A J 1999 Late Silurian and Devonian graniticintrusions of Scotland In Caledonian Igneous Rocks ofBritain (eds D Stephenson R E Bevins D MillwardA J Highton I Parsons P Stone amp W J Wadsworth)pp 397ndash404 Geological Conservation Review SeriesJoint Nature Conservation Committee

HIROSE K amp KAWAMOTO T 1995 Hydrous partial meltingof lherzolite at 1GPa the effect of H2O on the genesis ofbasaltic magmas Earth and Planetary Science Letters133 463ndash73

HOLDEN P HALLIDAY A N amp STEPHENS W E 1987 Neo-dymium and strontium isotope content of microdioriteenclaves point to mantle input to granitoid productionNature 330 53ndash6

HOLDEN P HALLIDAY A N STEPHENS W E amp HENNEYP J 1991 Chemical and isotopic evidence for majormass transfer between mafic enclaves and felsic magmaChemical Geology 92 135ndash52

HOLLAND J G amp LAMBERT R S 1970 Weardale GraniteIn Geology of Durham County (ed G A L Johnson)pp 103ndash18 Transactions of the Natural History Societyof Northumberland 41

JOHANNES W amp HOLTZ F 1990 Formation and compositionof H2O under saturated melts In High-temperatureMetamorphism and Crustal Anatexis (eds J RAshworth amp M Brown) pp 87ndash104 London UnwinHyman

KEMP A E S 1987 Evolution of Silurian depositionalsystems in the Southern Uplands Scotland In MarineClastic Sedimentology (eds J K Leggett amp G G Zuffa)pp 124ndash55 Graham amp Trotman

KNELLER B C 1991 A foreland basin on the southernmargin of the Iapetus Journal of the Geological SocietyLondon 148 207ndash10

KNELLER B C KING L M amp BELL A M 1993 Forelandbasin development and tectonics on the northwestmargin of eastern Avalonia Geological Magazine 130691ndash7

KLEMPERER S L RYAN P D amp SNYDER D B 1991 A deepseismic reflection transect across the Irish CaledonidesJournal of the Geological Society London 148 149ndash64

KOKELAAR B P 1988 Tectonic controls of Ordovician arcand marginal basin volcanism in Wales Journal of theGeological Society London 145 759ndash75

LEE M R amp PARSONS I 1997 Compositional andmicrotextural zoning in alkali feldspars from the Shapgranite and its geochemical implications Journal of theGeological Society London 154 183ndash8

LEGGET J K MCKERROW W S amp SOPER N J 1983 Amodel for the crustal evolution of southern ScotlandTectonics 2 187ndash210

MACDONALD R ROCK N M S RUNDLE C C ampRUSSELL O J 1986 Relationships between late Cale-donian lamprophyric syenitic and granitic magmas ina differentiated dyke southern Scotland MineralogicalMagazine 50 547ndash57

MACDONALD R THORPE R S GASKARTH J W ampGRINDROD A R 1985 Multi-component origin ofCaledonian lamprophyres of northern England Miner-alogical Magazine 49 485ndash94

MCARDLE P amp KENNEDY M J 1987 The East Carlow de-formation zone and its regional implications GeologicalSurvey of Ireland Bulletin 3 237ndash55

MCCONNELL B 2000 The Ordovician volcanic arc andmarginal basin of Leinster Irish Journal of EarthSciences 18 41ndash9

MCCONNELL B PHILCOX M E SLEEMAN A GSTANLEY G FLEGG A M DALY E P amp WARRENW P 1994 A geological description to accompanythe bedrock geology 1100000 map series Sheet 16Kildare-Wicklow Geological Survey of Ireland 70 pp

MCKENZIE D 1989 Some remarks on the movement ofsmall melt fractions in the mantle Earth and PlanetaryScience Letters 95 53ndash72

MEIGHAN I G HAMILTON M A GAMBLE J A ELLAMR M amp COOPER M R 2003 The Caledonian Newrycomplex NE Ireland new UndashPb ages a subsurfaceextension and magmatic epidote Joint Meeting Geo-logical Society of America ndash Northeastern Section ndashAtlantic Geoscience Society March 27ndash29 2003pp 79 Halifax Nova Scotia

MENUGE J F WILLIAMS D M amp OrsquoCONNOR P D1995 Silurian turbidites used to reconstruct a volcanicterrain and its Mesoproterozoic basement in the IrishCaledonides Journal of the Geological Society London152 269ndash78

MERRIMAN R J 2002 Contrasting clay mineral assemblagesin British Lower Palaeozoic slate belts the influence ofgeotectonic setting Clay Minerals 37 207ndash19

MERRIMAN R J REX D C SOPER N J amp PEACOCKD R 1995 The age of Acadian cleavage in northernEngland UK KndashAr and TEM analysis of a Silurianmetabentonite Proceedings of the Yorkshire GeologicalSociety 50 255ndash65

MILLWARD D 2002 Early Palaeozoic magmatism in theEnglish Lake District Proceedings of the YorkshireGeological Society 54 65ndash93

MILLWARD D amp EVANS J A 2003 UndashPb chronology andduration of late-Ordovician magmatism in the EnglishLake District Journal of the Geological Society London160 773ndash81

MURPHY B J amp KEPPIE J D 2005 The Acadian orogenyin the northern Appalachians International GeologyReview 47 663ndash87

NIXON P H REX D C amp CONDLIFFE E 1984 A noteon the age and petrogenesis of lamprophyre dykesof the Cautley area Yorkshire Dales National ParkTransactions of the Leeds Geological Association 1040ndash5

OrsquoCONNOR P J AFTALION M amp KENNAN P S 1989Isotopic UndashPb ages of zircon and monazite fromthe Leinster Granite southeast Ireland GeologicalMagazine 126 725ndash8

PHARAOH T C MORRIS J H LONG C B amp RYANP D 1996 The Tectonic Map of Britain and Ire-land (11500000 map series) British GeologicalSurvey

PIDGEON R T amp AFTALION M 1978 Cogenetic andinherited zircons UndashPb systems in granites Journal ofGeology 10 183ndash200

POWER G M M amp BARNES R P 1999 Relationshipsbetween metamorphism and structure on the northernedge of eastern Avalonia in the Manx Group Isle ofMan In In sight of the suture the Palaeozoic geologyof the Isle of Man in its Iapetus Ocean context (edsN H Woodcock D G Quirk W R Fitches amp R P



Barnes) pp 289ndash306 Geological Society of LondonSpecial Publication no 160

REAVY R J 2001 Caledonian Granites ndash EmplacementOpen University Geological Society Journal 22 8ndash11

RICHMOND L K amp WILLIAMS B P J 2000 A new terranein the Old Red Sandstone of the Dingle Peninsulasouthwest Ireland In New Perspectives on the Old RedSandstone (eds P F Friend amp B P J Williams) pp 147ndash83 Geological Society of London Special Publicationno 180

ROCK N M S COOPER C amp GASKARTH J W 1986 LateCaledonian subvolcanic vents and associated dykes inthe Kirkudbright area Galloway southwest ScotlandProceedings of the Yorkshire Geological Society 49 29ndash37

ROCK N M S GASKARTH J W amp RUNDLE C C 1986Late Caledonian dyke swarms in southern Scotlanda regional zone of primitive K-rich lamprophyresand associated vents Journal of Geology 94 505ndash21

ROCK N M S GASKARTH J W HENNEY P J amp SHANDP 1988 Late Caledonian dyke swarms of northernBritain some preliminary petrogenetic and tectonicimplications of their province-wide distribution andchemical variation Canadian Mineralogist 26 3ndash22

ROCK N M S amp HUNTER R H 1987 Late Caledoniandyke swarms of northern Britain spatial and temporalintimacy between lamprophyric and granite magmatismaround the Ross of Mull pluton Inner HebridesGeologische Rundschau 76 805ndash26

RUNDLE C C 1992 Review and assessment of isotopic agesfrom the English Lake District Technical Report BritishGeological Survey WA9238 27 pp

RYAN P D amp DEWEY J F 1997 Continental eclogites andthe Wilson Cycle Journal of the Geological SocietyLondon 154 437ndash42

RYAN P D amp SOPER N J 2001 Modelling anatexis in intra-cratonic rift basins an example from the Neoproterozoicof the Scottish Highlands Geological Magazine 138577ndash88

SHAND P GASKARTH J W THIRWALL M F amp ROCK NM S 1994 Late Caledonian lamprophyre dyke swarmsof south-eastern Scotland Mineralogy and Petrology 51277ndash98

SHEPHERD T J BECKINSALE R D RUNDLE C C ampDURHAM J 1976 Genesis of Carrock Fell tungstendeposits Cumbria fluid inclusion and isotopic studyTransactions of the Institution of Mining and Metallurgy85 B63ndash73

SHERLOCK S C KELLY S P ZALASIEWICZ J ASCHOLFIELD D I EVANS J A MERRIMAN R J ampKEMP S J 2003 Precise dating of low temperaturedeformation strain-fringe analysis by Ar40ndashAr39 lasermicroprobe Geology 31 219ndash22

SOPER N J 1986 The Newer Granite problem a geotectonicview Geological Magazine 123 227ndash36

SOPER N J amp KNELLER B C 1990 Cleaved microgranitedykes of the Shap swarm in the Silurian of NW EnglandGeological Journal 25161ndash70

SOPER N J amp ROBERTS D E 1971 Age of cleavage inthe Skiddaw Slates in relation to the Skiddaw aureoleGeological Magazine 108 293ndash302

SOPER N J STRACHAN R A HOLDSWORTH R E GAYERR A amp GREILING R O 1992 Sinistral transpressionand the closure of Iapetus Journal of the GeologicalSociety London 149 871ndash80

SOPER N J amp WOODCOCK N 2003 The lost Lower OldRed Sandstone of England and Wales a record ofpost Iapetan flexure and Early Devonian transtensionGeological Magazine 140 627ndash47

SOPER N J ENGLAND R W SNYDER D B amp RYANP D 1992 The Iapetan suture in England Scotlandand eastern Avalonia Journal of the Geological SocietyLondon 149 697ndash700

STEPHENS W E 1988 Granitoid plutonism in the Caledo-nian orogen of Europe In The CaledonianndashAppalachianOrogen (eds A L Harris amp D J Fettes) pp 389ndash403 Geological Society of London Special Publicationno 38

STEPHENS W E 1992 Spatial compositional and rheolo-gical constraints on the origin of zoning in the Criffelpluton Scotland Transactions of the Royal Society ofEdinburgh Earth Sciences 83 191ndash9

STEPHENS W E 1999 The Criffel Pluton In The CaledonianIgneous rocks of Great Britain (eds D Stephenson R EBevins D Millward A J Highton I Parsons P Stone ampW J Wadsworth) pp 460ndash8 Joint Nature ConservationCommittee Peterborough

STEPHENS W E amp HALLIDAY A N 1984 Geochemicalcontrasts between late Caledonian granitoid plutons ofnorthern central and southern Scotland Transactionsof the Royal Society of Edinburgh Earth Sciences 75259ndash73

STEPHENS W E HOLDEN P amp HENNEY P J 1991Microdioritic enclaves within Scottish Caledonian gran-itoids and their significance for crustal magmatismIn Enclaves and Granite Petrology (eds J Didier ampB Barbarin) pp 125ndash34 Developments in Petrologyno 13 Amsterdam Elsevier

STEPHENS W E WHITELY J E THIRWALL M F ampHALLIDAY A N 1985 The Criffel zoned plutoncorrelated behaviour of rare earth element abundanceswith isotopic systems Contributions to Mineralogy andPetrology 89 226ndash38

STEPHENSON R E amp HIGHTON I 1999 The CaledonianIgneous Rocks of Great Britain an introduction In Cale-donian Igneous Rocks of Britain (eds D StephensonR E Bevins D Millward A J Highton I ParsonsP Stone amp W J Wadsworth) pp 17ndash19 GeologicalConservation Review Series Joint Nature ConservationCommittee

STONE P KIMBELL G S amp HENNEY P J 1997 Basementcontrol on the location of strike-slip shear in theSouthern Uplands of Scotland Journal of the GeologicalSociety London 154 141ndash4

THIRWALL M F 1981 Implications for Caledonian platetectonic models of chemical data from volcanic rocks ofthe British Old Red Sandstone Journal of the GeologicalSociety London 138 123ndash38

THIRWALL M F 1988 Geochronology of Caledonian mag-matism in northern Britain Journal of the GeologicalSociety London 145 951ndash67

THIRWALL M F 1989 Movement on proposed ter-rane boundaries in northern Britain constraints fromOrdovicianndashDevonian igneous rocks Journal of theGeological Society London 146 373ndash6

TRENCH A amp TORSVIK T H 1992 The closure of theIapetus Ocean and Tornquist Sea new palaeomagneticconstraints Journal of the Geological Society London149 867ndash70

VAUGHAN A P M 1996 A tectonomagmatic model for thegenesis and emplacement of Caledonian calc-alkaline


256 The Newer Granite problem

lamprophyres Journal of the Geological Society Lon-don 153 613ndash23

WADGE A J GALE N H BECKINSALE R D amp RUNDLEC C 1978 A RbndashSr isochron age for the Shap graniteProceedings of the Yorkshire Geological Society 42297ndash305

WESSEL P amp SMITH W H F 1991 Free software helps mapand display data EOS Transactions of the AmericanGeophysical Union 72 445ndash6

WOODCOCK N 2000 Ordovician volcanism and sediment-ation on eastern Avalonia In Geological History ofBritain and Ireland (eds N H Woodcock amp R AStrachan) pp 153ndash67 Oxford Blackwell Science

WYLLIE P J 1991 Magmatic consequences of volatilefluxes from the mantle In Progress in Metamorphicand Magmatic Petrology Korzhinskii Memorial Volume(ed L L Perchuk) pp 477ndash503 Cambridge UniversityPress

WYLLIE P J 1995 Experimental petrology of uppermantle materials processes and products Journal ofGeodynamics 20 429ndash68

ZEN E-An 1995 Crustal magma generation and low-pressure high temperature regional metamorphism inan extensional environment possible application to theLachlan Belt Australia American Journal of Science295 851ndash74



Figure 1 Outline tectonic map of Britain and Ireland showing the main crustal lineaments discussed in the text and the lsquoNewerrsquoGranites and associated Devonian volcanic rocks

make a threefold division into the Argyll Cairngormand South of Scotland suites demonstrating the provin-cialism of source composition (Fig 1) Stone Kimbellamp Henney (1997) recognized two distinct subgroups inthe South of Scotland suite those occurring north andthose south of the Orlock Bridge fault and associatedMoniaive shear zone (Fig 2) The northern granites areolder (gt 410 Ma) and characterized by low 87Sr86Srinitial ratios while to the south such plutons as FleetCriffel and Cheviot are younger (lt 410 Ma) and havemore radiogenic initial 87Sr86Sr ratios The southernsubgroup of Stone Kimbell amp Henney (1997) wasnamed the Galloway Suite by Highton (1999) It hasmuch in common with the Lake District granites suchas Shap and Skiddaw (see Sections 3a6 and 3a7) thatwere intruded south of the Solway Line the surfacetrace of the N-dipping Iapetus suture We propose

the recognition of a Trans-Suture Suite to include theGalloway and northern England granitic intrusionsprimarily on the basis of age and tectonic setting andalso the common presence of a notable componentof sedimentary origin in the genesis of the magmasThe Trans-Suture Suite can be extended along-striketo include representatives in the Isle of Man (FoxdaleDhoon) and the east of Ireland (principally Newry in theLongford-Down extension of the Southern Uplands andthe large Leinster intrusion to the south of the sutureFig 2) Their origin has long been a puzzle

Obviously the granites of the Lake District andLeinster could not have been generated by northwardsubduction beneath Scotland while the Gallowayplutons although emplaced in the hangingwall of thesuture lie too close to its surface trace to accommodatean arcndashtrench gap (Thirlwall 198l Soper 1986)






















































3a4 Minor granites
























4 Lamprophyre dykes





















5b Seismic evidence











6 Thermal modelling

































40 km 35 km 30 km





































7 Conclusions






References














































































































































































3a4 Minor granites
























4 Lamprophyre dykes





















5b Seismic evidence











6 Thermal modelling

































40 km 35 km 30 km





































7 Conclusions






References








































































































































4 Lamprophyre dykes





















5b Seismic evidence











6 Thermal modelling

































40 km 35 km 30 km





































7 Conclusions






References












































































































































5b Seismic evidence











6 Thermal modelling

































40 km 35 km 30 km





































7 Conclusions






References
































































































































6 Thermal modelling

































40 km 35 km 30 km





































7 Conclusions






References























































































































































40 km 35 km 30 km





































7 Conclusions






References





















































































































































7 Conclusions






References


























































































































The Newer Granite problem revisited: a transtensional ... · Wenlock time (Kemp, 1987) and the...

Documents

Transcript of The Newer Granite problem revisited: a transtensional ... · Wenlock time (Kemp, 1987) and the...