A Tectonomagmatic Model for the Genesis and Emplacement of Caledonian Calc-Alkaline Lamprophyres

Journal ofrhe Geological Sociery. London, Vol. 153, 1996, pp. 613-623, 4 figs, 1 table Printed in Northern Ireland

A tectonomagmatic model for the genesis and emplacement of Caledonian calc-alkaline lamprophyres

ALAN P. M. VAUGHAN Department of Geology, Trinity College, Dublin 2, Ireland Present address: British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET, UK

Abstract: Petrogeneticallyrelated, Caledonian, calc-alkalinelamprophyric and shoshonitic dykes in eastern Ireland appear coeval with sinistral transtension. These dykes are exposed in a 10 km wide zone trending SW from Clogher Head, that strikes anticlockwise of the surface trace of the Iapetus Suture. Lamprophyre dykes intruded sinistral Riedel shears with a mean strike-trend at 040". Riedel shears that host lamprophyre dykes strike anticlockwise of primary sinistral shears by 35". consistent with sinistral transtension: primary shears strike 070". parallel to the mean strike trend of shoshonitic dykes.Sinistral. primary shears mayhave controlled the emplacement of Caledonian,shoshonitic, intrusive rocks. Using lamprophyres from Clogher Head as examples, a new model is proposed for lamprophyre genesis and emplacement related to simple shear. In this model. transient decompression of metasomatized mantle during simple shear triggers release of volatile-rich potassic magma, which ascends to crustal levels along steep strike-slip faults. An association between lamprophyre magmatism and deep faulting suggests that relatively deep (>60 km) seismicity may precede magma ascent. This model can account for several paradoxes of Caledonian magmatism.

Keywords: Ireland. lapetus. strike-slip faults, shoshonite.

The question of the origin of calc-alkaline lamprophyres and Mexico (Lange & Carmichael 1991), minette, kersantite and associated K-rich plutons is a global one (Rock 1991) with spessartitelavas, associated with basalticandesite and implicationsfor magma genesis andthe relationship of shoshoniticvolcanism, are emplaced 160 km from the magmatism to tectonics. In this paper, using Caledonian Middle America Trench, 70 km closer to the trench than the calc-alkaline lamprophyres from the Iapetus Suture zone in main axis of andesiticvolcanism. It is probably no easternIreland as an example, I presenta wrench-related coincidence that these lamprophyres appear to be emplaced modelfor the genesis andemplacement of calc-alkaline in a zone of dextraltranstension along strike from the lamprophyres.This model accountsfor some anomalous Tamayo Fracture Zone (Lange & Carmichael 1991, fig. l). features of Caledonian calc-alkaline lamprophyres and may Lamprophyres havealso been identified in Atlanticand resolve several of the paradoxes of Caledonian magmatism IndianOcean deep-sea settings, particularly in association posed by Rock e,' a/. (1986). The five paradoxes of with transform faults in old oceanic crust (Jones er al. 1991). Caledonian magmatism posed by Rock et al. (1986) focus on Siluro-Devonian,calc-alkaline lamprophyres are abun- problems of space, time and chemistry widely expressed in dant throughout the Longford-Down massif of Ireland, the calc-alkaline igneous rocks exposed over a very large area in SouthernUplands of Scotland,and the north of England northwestEurope and eastern North America that were (Rock 1991). Lamprophyres of similar age are also found in emplacedduring closure of theIapetus Ocean. Similar Caledonian-aged rocks of the Meguma tectonostratigraphic problematicrelationships are evident in Cenozoic igneous zone of Nova Scotia (Kempster er al. 1989), which accreted rocks of the Great Basin of the western US (Ormerod et al. to Western Avalonia in Siluro-Devonian times (Gibbons & 1985). The model may also have application to lamprophyre Murphy 1995). Thegeographic range of Caledonian genesis more widely. Usage of chronostratigraphic names is calc-alkaline lamprophyres coversroughly 100000 km2 afterWhittaker et al. (1991) with age/stage names and (Rock et al. 1986), and they exhibit spatial chemical trends numericages of boundariesafter theIUGS Global that convergeon the approximate sub-surfacetrace of the Stratigraphic Chart of Cowie & Bassett (1989). IapetusSuture (Rock et al. 1988). In easternIreland, The tectonic significance of calc-alkaline lamprophyres is calc-alkaline lamprophyre dykes are abundant in the vicinity currentlyambiguous; some authors even go so faras to of Clogher Head (Fig. l), and, with less abundant suggest thatsynthetic a tectonomagmatic model for shoshoniticrocks, are exposed in a 5-10 km wide zone lamprophyre genesis is unlikely(e.g. Wilson 1989). The extending inland about 30 km WSW (where Lower emplacement of calc-alkaline lamprophyres is notneces- Palaeozoic rocks are lost beneath Upper Palaeozoic cover). sarily contemporaneous with subduction,and they often Lamprophyres of this zone intruded Ordovician and Silurian intrudeextensional or wrenchregimes (Thompson er al. rocks. North of this zonetothe Upper Palaeozoic 1989; Sloman 1989). Cenozoic to recent,possibly Kingscourt Outlier(Jackson 1955) lamprophyresare more subduction-related,calc-alkaline lamprophyres from the sparse. A zone of abundant lamprophyres, of similar trend Sunda Arc in Indonesia and the Sierra Nevada batholith in and width,has also been recognizedwithin the south- the westernUS are 300-400 km behindthe trench (Rock ernmost part of the Southern Uplands of Scotland (Rock er 1984). However, calc-alkaline lamprophyresdo intrude al. 1986: Shand er al. 1994),north of theSouthern muchcloser to trenches.For example, in southwestern Belt/Central Belt boundary (Kemp 1986). 613

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Tracts with NW lapetus I palaeobiogeographic affinltles l (including Grangegeeth Tract) Tracts with SE lapetus 0 Lr. palaeobiogeographlc afflnities to Mid. Sil. I (including Rathkenny Tract) Carrickdexter Lavas m Lr.Dev. ? l

Fig. 1. Simplified geology map of the southeast Longford-Down Massif (after Lenz & Vaughan 1994). Symbols (see key) indicate the distribution of calc- 0 study the within localities rock alkaline L area. C, Clonrnaggadan; CD, Carrick- h dexter: K, Killineer; N, Navan.

Analytical methods opaque minerals.Shattered-looking glomeruloporphyritic Tenlamprophyre dyke, and 16 shoshoniticintrusive and volcanic aggregates of clinopyroxene are evident in some cases. rocksamples were analysed by XRF at TrinityCollege Dublin Spherulitic-texturedquartz or hypidiomorphic-textured (Vaughan 1991). Whole rock major element analyses were analysed quartz xenocrysts and 0.5 mm, carbonate filled globular str- on glass beads; minor and trace elements were analysed on pressed uctures(Rock 1984) are abundant. Biotite phenocrysts are powder pellets; all analyses were done according to the method of chloritized or bleached in some cases. Colourless to brown Potts et al. (1984). The data are presented in Table 1. pleochroism of biotite is present in twocases andsome biotitephenocrysts exhibit growth lamellae when seen in basalsection. In some few casesbiotite phenocrysts have Petrography stronglypleochroic, green cores. Carbonateand chlorite pseudomorphs of olivine (Hatch et al. 1972) are also seen. Lamprophyres The groundmass feldspar is identifiable in only three cases, Calc-alkalinelamprophyres are commonly heteromorphic where it is oligoclase(An3(,). Theconstituent minerals are with chemicallyequivalent mineralogies andthere is present in the following approximate percentages:biotite completeoverlap between types (Rock 1984). In hand lO-SO%; chlorite 5%; quartz; 5%; groundmass 40%. specimen the lamprophyres are very variable. Augiteminettelspessartite (Streckeisen 1980; Rock Minettelaugite kersantite (Streckeisen 1980; Rock 1984). These lamprophyres are abundant south of Clogher 1984). Theselamprophyres are brown-weathering blue- Head. They resemble mafic-mineral-rich syenite (see below) black,commonly vesicular hypabyssal igneous rock. They anda small degree of wallrockalteration is associated. comprise, abundant, flow-oriented in some cases, euhedral These dykes are strongly altered, black or very dark green, tosubhedral, 0.1-S mmbiotite phenocrysts in analtered hypabyssal rock, comprising chlorite pseudomorphs of biot- groundmass of microliticfeldspar, chlorite, carbonateand ite; clinopyroxene; hornblende; carbonate pseudomorphs of

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0: VI

'2. VI

* 3 ~8 j5 l

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olivine; and quartz and chlorite in some cases. The ground- ized and chloritized, extensively fractured, sub-to euhedral, mass is composed of altered, 0.2-1 mm feldspar microlites 1-3 mm clinopyroxene, minor perthite, chlorite, carbonate, that are generally unidentifiable (plagioclase in one case). ilmenite,and abundant needles of apatite. K-feldspar is generallyweakly sericitized and in some casesencloses Sub-volcanic larnprophyres crystals of epidotizedclinopyroxene. Chlorite appearsto have pseudomorphed a ferromagnesian mineral that from its Somelamprophyre intrusions are associated with gas- outline was probablyhornblende. No freshhornblende is escape-typebrecciation of thecountry rock (Fisher & evident. No mica, quartz or cancrinite is evident, although Schmincke1984), pervasive alteration resulting in replace- quartz occurs in quartz/K-feldspar veins in a perthite-rich, ment of pyrite by jarrusite,and recrysta!lization of the mafic mineral-free.syenite at ClonmagaddanQuarry (Fig. countryrock with thedevelopment of hornfelsand 1). The constituentminerals are present in approximately associated chlorite growth. Large pyrite crystals and talc are the followingproportions: K-feldspar 50%; epidotized evident in some cases and a widespreadset of clinopyroxene 2.5%; carbonateand chlorite 10%; ilmenite quartz/carbonate veins with pyrite,talc and local extreme 10%; apatite 5%. alteration of greywacke wallrock is evident at Clogher Head (Fig. 1; Vaughan 1991). The veins appear to have resulted from a volcanically or hydrothermallydriven fluid phase Volcanic rocks (H,O and/or CO,)associated with lamprophyre emplace- Lavas of theCarrickdexter Formation (Fig. 1)(Vaughan ment. Lamprophyre intrusions associated with these features 1991) are red, green or, where seen least altered, black with exhibit flow fabrics; in manycases they contain xenoliths a lilac hue,porphyritic extrusiverock. The brecciated toe and xenocrysts of quartz, and in some cases occur as pods region of individual flowsis mineralized by coarsely with inone case an aureole 8m thick (Vaughan 1991). A crystallinecalcite, green chloriteand red hematite flakes. brecciapipe several metresacross intruded greywackes at The lavascomprise sub-euhedral, 0.25-3 mm orthopyro- ClogherHead (Fig. 1). Thebreccia, whichis cleavedand xene:sub-euhedral, 0.1-2 mm clinopyroxene;elongate to intruded by a laterlamprophyre dyke, is composed of equant, sub-euhedral, 0.5-3 mm olivine; set in a groundmass angularfragments of greengreywacke sandstone and of flow oriented, 0.1-0.4 mm, oligoclase toandesine siltstone in a matrix of quartz, brown and white carbonate, (An,,-An,,,) microlites andiron oxide (in mostcases andabundant 0.1-S mmcubes of pyrite. Adjacent hematite).Orthopyroxene phenocrysts are in some cases greywacke is metasomaticallyaltered. The breccia is partially to totally pseudomorphed by chlorite, and exhibit probably a sub-volcanic feature associated with volatile-rich verylowbirefringence. These superficially resemble lamprophyre. K-feldspar.In some casesorthopyroxene phenocrysts are mantled by oikocrysts of clinopyroxene. Strong zonation is Microgranite evident in some clinopyroxene phenocrysts, with chloritized coresand freshrims in some cases.Fresh olivine The Killineer(Fig. 1) microgranite is a buff-weathering, phenocrystsare evident in some casesbut are generally green,coarse-grained, hypabyssalrock. comprisesIt outlinedby haematite grains with bleachedphenocryst sericitized,sub-euhedral, <5 mm, K-feldsparphenocrysts cores. In some lava units,olivine phenocrysts are (probablysanidine); minor, anhedral,

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Clogher Head (Vaughan 1991).In dykes greater than 1 m thicknessfrom 0.5-6m andare minettes, augite minettes, thick,a spacedcleavage is developedthat is refracted at augite kersantites and spessartites (Streckeisen 1980; Rock dyke margins. In thin-section, deformed biotite phenocrysts 1984). Chemically they are predominantly basaltic (Fig. 3a). exhibitinternal veinsets of white micaindicating simple Inmany cases, these dykes intruded steeply dipping, shear deformation. post-Wenlock,pre-Ludlow sinistral shearsstriking NE to NNE (Fig. 2c and f). These faults have offsets ranging from 1 m to 100 m, based on matching distinctive bedding units or Group 2 lamprophyres foldaxes. Not all areintruded by lamprophyre dykes. These calc-alkaline lamprophyredykes are undeformed. Offsets of 2-5 m are the mode. Riedel shears, that form en They have planar walls and cut calc-alkaline dykes deformed echelon tothe shear plane,and primary shears (after by dextral transpression (Vaughan 1991). A dyke of this set Tchalenko 1970), that form parallel to the shear plane, can cutsdextrally reactivated S1 in cleaved mudstoneat beidentified in Fig. 2c. Therange of Riedel-shearstrike Glaspistol, Co. Louth (Vaughan 1991). trends indicates shears formed under varying sinistral shear conditionsfrom transpression to transtension.Primary- shearsshow a mode at 070 (Fig.2c). Riedel-shears that Shoshonitic dykes contain lamprophyredykes form anen echelon fissure Intrusions of thisgroup cut folds and cleavage of D1 system (Ramsay & Huber 1983, pp. 48-50) with a roughly (Vaughan & Johnston 1992). Dykes of this group are folded ENEzone trend (Rothery 1988) (parallel to the by D2; S2 cleavage is developed in a microgranite dyke at Primary-sheartrend illustrated in Fig.2c andrepresenting Salterstown,Co. Louth (Fig. 1) (Vaughan 1991). North of the trend of a notional plane of simple shear), and a S-angle Navan, Co. Meath (Fig. l), a post-D1 (Vaughan & Johnston of 35" (the angle between Riedel-shears and the zone trend, 1992),sheet-like syenite body with shallow southwesterly Rothery 1988). S-angles less than 45" indicate a component dip is offset by steep, northeast-trending sinistral shears that of extension to simple shear(Ramsay & Huber 1983, pp. host Group 1 lamprophyre dykes (Vaughan 1991). 48-50). This fissure-system geometry indicates weak sinistral transtension during lamprophyre emplacement. Group 2 calc-alkaline lamprophyre dykes have variable Dyke geometry trends. They are minettes (Rock 1984). and less than 1 m Group 1 calc-alkaline lamprophyres in most cases are thick. These dykes were not analysed. They are important steeply-dipping(Fig. 2a), striking northeast to north- since they provide a means of timing structural events, but northeast with amode at 040" (Fig.3d). Theyrange in form a minor component of the lamprophyres overall.

Outer Ring = 7.5% N

ursatl.5.10. 15 and 20% and 15% 7.5.10 and 12.5%

Fig. 2. (a). (b) and (c) Rose diagrams of strike-trend for lamprophyre dykes, shoshonitic dykcs, and sinistral shcars respectively: (d), (e) and (f) corresponding contoured lower-hemispheric, equal-area projections of poles for (a), (b) and (c).

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Age of calc-alkaline dyke intrusion $? 14 It is evident from Scotland (Rock et al. 1986) that lamprophyreintrusion was continuous over about 25 Ma (Ludlow to mid-Early Devonian). This is assumed to be the case in eastern Ireland. There are also suites of deformed andundeformed lamprophyres in Co.Down (Reynolds +8l 1931, T.B. Anderson pers.comm. 1991), andthe English Lake District (Arthurton & Wadge 1981). Dykes of both setsat Clogher Head, particularly deformed dykes,have intruded parallel to the post-Wenlock, pre-Ludlow foliation 2t in some cases. Lamprophyre dykes arepost-dated by the o"""""""""""' pre-Early Carboniferous, probably Early Devonian, layered 35 39 43 47 51 55 59 63 67 71 75 79 trachybasaltic and trachyandesiticvolcanic rocks of the CarrickdexterFormation at Carrickdexter quarry in Co. a SiO, Wt Yo Louth (Fig. 1) (Vaughan 1991). Noradiometric dates are availablefor Caledonian calc-alkaline lamprophyric or shoshonitic rocks from eastern 8 Ireland.Rock et al. (1986),working on assumedlysimilar lamprophyre dykes north of the Iapetus Suture zone in the 7 SouthernUplands of Scotland,derived eight K/Ar biotite/hornblende ages ranging from 418-400 Ma (Ludlow tomid-Early Devonian) fromundeformed dykes. These 6 post-tectonicdykes are likely tobe comparable in age to calc-alkaline lamprophyres of eastern Ireland that post-date $? Late Siluriansinistral transpressive deformation(D1 of t Vaughan & Johnston 1992). Rock et a/. (1986) statedthat z5 t the youngest age was derived from a dyke at St. Abbs Head 34 Banakite in the southeast Southern Uplands that field evidence also 1 Shoshonite I/ indicatedwas the youngest, although complete overlap of age data exists from Berwickshire and Kircudbrightshire (P. 3 Stonepers. comm. 1992). All of thedykes post-date a Llandovery-Wenlock-aged sedimentarysequence, and the 2 St AbbsHead dyke predatesthe 397 Ma Criffel Pluton (Halliday et al. 1979; Harmon et al. 1984). Macdonald et al. (1986) derived a Rb-Sr whole-rock isochron age of 395 Ma 1 (mid-EarlyDevonian) from lamprophyres and cumulates * A A LOW-K fromthe Newmains Dyke in theSE Southern Uplands. m andesite *I Although field evidence suggeststhat the broadzone of 0 47 50 55 60 lamprophyres identified in easternIreland straddles the Iapetus Suture zone, it seems reasonable to assume that the b SiO, Wt O/O Irish and Scottish lamprophyres belong to the same suite. In the absence of radiometric ages from Irish lamprophyres, it Fig. 3. (a) Total alkali-silica diagram (Le Bas et al. 1985) and (b) is anecessary requirement, although notideal, to assume K,0-Si02 diagram (with fields after Pecerillo & Taylor 1976), for lamprophyre dykes (filled circles); lavas of the Carrickdexter age similarity with Scottish lamprophyres (Macdonald et al. Formation (filled triangles); Killineer microgranite dykes (filled 1986; Rock et a/. 1986) to time lamprophyre emplacement in diamonds); and Clonmaggadan Syenite (filled stars). High-silica eastern Ireland. (>69%) Killineer microgranite dykes not plotted in (b). Compositions recalculated to 100% with volatiles. F, foidites: Pc, Picrobasalt; U,, tephrite: U,. phonotephrite: U,, tephriphonolite; Regional evidence for late Caledonian syn-sinistral Ph, phonolite; B, basalt: S,, trachybasalt: .S2, trachyandesite shear magmatism (mugearite, shoshonite): S3, trachyandesite (benmoreite, latite). T, As describedabove. deformed calc-alkaline lamprophyre alkali-trachyte. trachyte; 0,,basaltic andesite: Oz. andesite; O,, dacite; R, alkali-rhyolite, rhyolite. dykes post-date sinistral transpression, but were coeval with continuingsinistral simple shear or transtension. In the Southern Uplands of Scotland, lamprophyres that post-date sinistraltranspression, but were coeval with continuing Shoshoniticintrusives strike predominantly ENE with brittle-ductilesinistral shear, havebeen datedat 418- moderate dips to the south and southwest (Fig. 2b and e). 395 Ma (Rock et al. 1986; Macdonald et a/. 1986).This Theyrange in thickness frommicrogranite dykes 0.5-8m suggeststhat sinistral transpression had ceased by 418 Ma thick, to intrusive sheets of theClonmaggadan Syenite (Ludlow) at the earliest, although the argumentof Barnes et (Vaughan 1991), thatare up to 50 m thick.Shoshonitic al. (1989) allowsthe zone of activetranspression to have intrusive dykesand sheets trend sub-parallel to sinistral migratedsouthwards with time. Southward migration of primary-shears(Fig. 2c) but were not found intruded in deformation may over-simplificationanbe if strain shears with this orientation. partitioning was active.With strain partitioning (e.g.

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Holdsworth & Strachan 1991),sinistral shears wouldhave with noother contemporaneous igneousrocks apart from beenactive in a broad, across-strike zone of distributed veinsassociated with thelamprophyres themselves (Asso- simple shear throughout the age range described above. ciation L) e.g. Gondwana coalfields of India (Rock 1991) (v) Evidence from elsewhere in theCaledonian supports a they may more rarely form regionalswarms unconnected zone of distributed sinistral shear, and indicates widespread with contemporaneous igneousactivity (Association R). fault activity from: Associations A, B and C may also occur together, and, in 422 Ma- (Early Ludlow) to, at the latest, 400 Ma (Early manycases, calc-alkaline lamprophyre dykes are most Devonian) along the Rhue Fault in the NW of Scotland abundantaround plutons that are rather K-rich or (Powell & Glendinning 1990); quartz-poor i.e. shoshonitic (Rock 1991). 425 Ma (lateWenlock) to 410-395 Ma (EarlyDevo- Calc-alkaline lamprophyre dykes in easternIreland fall nian)on the Great Glen Fault, bracketed between into associations A, B and L (Rock 1984). (i) Lamprophyres sinistral, syn-emplacement deformation of the Ratagain in association Aare petrogeneticallyassociated with the Granite(Hutton & McErlean 1991) and cross-cutting ClonmaggadanSyenite, bestexposed atClonmaggadan lamprophyres (Rock 1988); quarry, north of Navan, Co. Meath (Fig. 1) (Vaughan 1991), pre-410 Ma (earliestDevonian) on the Highland and possibly with pre-Carboniferous a granite, (the Boundary Fault, from the distribution of the Lintrathen Kentstown Pluton, Phillips et al. 1988) inferred to exist from ignimbrite (Thirlwall 1989); anegative bouguer anomaly beneathCarboniferous cover pre-398 Ma (Early Devonian) on the Southern Uplands rocks to the south of the surface trace of the Iapetus Suture Fault in the west of Ireland, from the age of the Galway Zone (Fig. 4). The sigmoidal shape of the gravity anomaly granitewhere stitchesit theSkird Rocks fault (W.E.A. Phillipspers. comm. 1991) is consistent with (McKerrow & Elders 1989): emplacement of thisgranitoid during sinistral shear as pre-408 Ma (earliestDevonian) to 392Ma (Early suggested by Jacques & Reavy (1994) for Scottish Devonian) in the Southern Uplands, based on the ages Caledonian plutons. A link to lamprophyres of Group 1 (see of post-tectonic granitoid plutons (Halliday er al. 1980; above) is suggested.(ii) Association B is more clearcut; Barnes et al. 1989): calc-alkaline lamprophyresare associated with asuite of Wenlock to Ludlow times in the Iapetus Suture Zone in 1-5 m thickshoshonitic microgranite dykes (Fig. 3), best easternIreland, crudelybased on the youngestrocks exposedat Killineer quarry,Co. Louth (Fig. 1)(Vaughan cut by undeformedlamprophyres south of theTinure 1991), andare chemicallysimilar to theshoshonitic Fault (Vaughan 1991); trachybasalts andtrachyandesites of theCarrickdexter Ludlow times to 410-390Ma (Early Devonian) in the Formation (Fig. 3) (Vaughan 1991). These lavas bear LakeDistrict, based on the ages of weaklycleaved, remarkable petrographic similarity to basaltic andesites and microgranitedykes that post-date the main phase of shoshoniticlavas fromsouthwestern Mexico(Lange & sinistral transpression (Soper & Kneller 1990); Carmichael1990) thatappear to below-volatile hetero- 435 Ma (earlyLlandovery) to c. 410Ma (earliest morphs of nearby lamprophyric lavas. The Mexican basaltic Devonian) in the Avalon Terrane, New Brunswick, andesites are characterized by lack of plagioclase phenocry- Canada,bracketed between a bimodal dyke swarm sts, presence of modalolivine, and clinopyroxene-rimmed deformed by sinistral shear and dextral mylonite (Doig quartz xenocrysts (Lange & Carmichael 1990), although the et al. 1990). Carrickdexter Lavas contain groundmass plagioclase microl- What this sampleintends to show is evidencefor a ites (An4(,-Anh5) that are more calcic (Vaughan 1991). (iii) widespread, broadlycoeval, episode of sinistral shear, in Association L is represented by the abundant lamprophyres, some casestranspressional, throughout the Caledonides andsparse subvolcanic pipes, with associatedtalc, chlorite from Llandovery times to Early Devonian. This episode of and carbonate veins at Clogher Head (Vaughan 1991). sinistral shearappears to overlap with equallywidespread igneous activity (Jacques & Reavy 1994). A model for lamprophyre generation and emplacement Petrogenesis of lamprophyres and relationship to Althoughnumerous fault-control modelshave been other late Caledonian intrusive rocks suggestedfor granite emplacementand petrogenesis (e.g. Rock (1991) suggested that calc-alkaline lamprophyre has a D’Lemos et al. 1992; Ingram & Hutton 1994; Jacques & two componentsource: deep(i)a mantle lamproitic Reavy1994), few fault-controlmodels exist forthe component and (ii) a continental crust component (indicated generation or emplacement of basic magmas (e.g. Quick et by thepresence of quartzosexenoliths, Rock et al. 1986). al. 1994) apartfrom the well-recognizedassociation with Five calc-alkaline lamprophyre field associations have been large degrees of extension,and general geophysical or recognised and classified by Rock (1991): (i) calc-alkaline geochemicalmodels based on large-scaletectonics (e.g. lamprophyres may be associated with calc-alkaline granitoid McKenzie & Bickle 1988). As illustratedabove, the plutons or volcanicrocks (Association Aor post-granite) association of magmaemplacement, particularly of calc- e.g. the Criffelgranite in thesouthwest Southern Uplands alkaline lamprophyres, with major sinistral wrench faults is (Macdonald et al. 1986);(ii) they may beassociated with characteristic of Caledonian igneousactivity (Rogers & shoshoniticplutonic or volcanicrocks (Association B) e.g. Dunning 1991). The link between sinistralshear deforma- southwestern Mexico (Lange & Carmichael 1990); (iii) they tion and lamprophyric magmatism can be partly explained may beassociated with appinite-brecciapipe complexes by the model of McKenzie (1989) who suggested that small (Association C or pre- to early granite) e.g. north Donegal, degree K-richmelt fractions percolating upthrough Ireland (Elsdon & Todd 1989); (iv) they may be associated low-permeabilityconvecting mantle pondwithin the with, for example, alkaline or ultramafic lamprophyres, but mechanical boundary layer of the lithospheric mantle below

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Fig. 4. See upper inset of Fig. 1 for location. Position of lamprophyre-rich zone in eastern Ireland and southern Scotland modified after Rock e? al. (1986) (Ards Peninsula lamprophyres (Reynolds 1931) are excluded as these are structurally further north in the Central Belt than the southeast Scotland lamprophyres. The eastern Ireland lamprophyres coincide with the projected southwest strike of thc lamprophyre-rich zone, based on its orientation in Scotland. and are structurally consistent with an anticlockwise, Caledonian-orogen-obliquetrend). Lower Palaeozoic inliers and distribution of Palaeozoic granites after Murphy (1987). Extension of Central Belt-Southern Belt Boundary into Ireland after Vaughan & Johnston (1992). Trend of Iapetus Suture after Phillips er al. (1976), Vaughan & Johnston (1992). and Lee et al. (1990). Geology not differentiated north of Southern Uplands inlier/Southern Uplands Fault. SUF. Southern Uplands Fault; OBF. Orlock Bridge Fault; CSB. Central Belt-Southern Belt Boundary; IS, lapetus Suture: K. approximate position of Kentstown Pluton after Phillips et al. (1988).

the base of the continental crust (the mechanical boundary The association of lamprophyresand appinites with layer is considered to be part of the ‘sub-continental mantle’ major shear zones within the Caledonides has been cited as of Harry & Leeman 1995, distinguishingit from the rigid evidence thatthese structures penetrate themantle, upperportion of themantle that remains attached to the providing conduits for magma (Rogers & Dunning 1991). I base of thecrust). This layer is relativelycold andheat propose, following Jones et al. (1991), that strike-slip passes through it entirely by conduction. It is insulated from movement on majorshear zones triggers lamprophyric the convecting mantle below it by a thermal boundary layer magmatism. During episodicstrike-slip or transtensional that transmits heat by convection at its base and conduction faultmovements, transient adiabatic decompression may at its top. Successive additions of K-rich melts results in a occur, triggering melting. Experimental petrology studies of zone of the mechanical boundarylayer rich in frozen volatile-rich basic melts are few (e.g. Foley et al. 1986), but potassic intrusions, that can remain isolated for geologically studies of CO,-richphonolite (Hay & Wendlandt 1995) longperiods. Several mechanisms have been proposed for suggest that melting temperature isdepressed, relative to the subsequent re-fusion of this melt to make it available to lower and higher pressures, at 0.7 GPa. This suggests that, rise through the crust as lamprophyric magma. Primarily, it ondeep-rooted shear zones that intersect metasomatized canbereleased inlarge volumes during heating or mantle at the base of lithosphere 60 to 150 km thick (Harry decompression of thesub-continental mantle lithosphere & Leeman 1995), melting is likely to be triggered if rocks (McKenzie 1989). Thompson et al. (1989) proposed that in aredecompressed to thispressure. Experimentaldata addition,large-scale melting may occurasresult a of suggest that simpleshearing facilitates the release of melt delamination of thickened subcontinental mantle beneath an even at low percentage (4-8%) concentrations (Zhen-Ming orogenicbelt, exposing the mechanical boundarylayer to et al. 1994). This is supported by melt-enrichment in shear asthenosphericheating. This mechanism was invoked to zonescutting peridotite in ophiolitecomplexes (e.g. accountfor the uprisein potassic magmas immediately Kelemen & Dick 1995). Melting during simple shear is likely followingcollisional orogeny. Harry & Leeman (1995) to focus melt in conduits (Kelemen et al. 1995). This may suggested thatmelt-metasomatized sub-continental mantle result in hydrostaticoverpressures facilitating rapid melt at the base of lithosphere between 60 and 150 km thick will extraction as suggested by Nicholas (1990) and Kelemen et produce largevolumes of meltat theearliest stages of al. (1995). Althoughthese inferences are predominantly extensionin melt-metasomatized, sub-continental mantle based on experimental data, theystrongly suggest a direct that is at 1300°C or greater. None of the above mechanisms association between simple shearing and volatile-rich mafic directlyaccount theclosefor association between magmatism. The association of magma-release with calc-alkaline lamprophyres and simple shear outlined above, strike-slipfault movements alsoimplies thatlamprophyric nordo they provide a mechanismfor thesimultaneous magmatism may be preceded by relatively deep (>60 km) generationand rapid ascent of lamprophyremagmas that seismicactivity. This couldbe tested in areas of recent pipe- anddiatreme-like lamprophyre bodies suggestmust lamprophyre magmaticactivity, such southwesternas have occurred (Rock 1991). Mexico (Lange & Carmichael 1991), whereabundant

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seismicity atthese depths has been recorded (Pardo & This can be accounted for by ponding at depth in the crust Suarez 1995). of chemicallysimilar lamprophyre magma released by Withsuch a model involving adiabatic decompression- shear-relatedadiabatic decompression melting. In this meltingtriggered by movement on steep sinistral shears, situation K-rich granitoidmagmas may be produced by deep-rooted magma conduits are a necessary result of coeval either re-fusion of andesitic magmatic underplate (Roberts igneousactivity. Therelationship between thismechanism & Clemens 1993), or fractionalcrystallization of K-rich and strike-slip faulting can explain the main aspects of the mafic magma and assimilation of melted crust (Leat et al. genesis andemplacement of Siluro-Devoniancalc-alkaline 1987, 1988), although it is notclear which mechanism lamprophyresand associated rocks.Like thelithospheric dominates. Rock (1991) favoured crustal anatexis by ponded mantlesource models proposed by McKenzie(1989) and lamprophyricmagma to generate granitoid melts. Huppert Thompsonet al. (1989) thismodel can account for the & Sparks (1988) modelled sills of basic magma intruding at chemical trendsseen in lamprophyremagmas north and the base of thecontinental crust and suggested that large south of thesuture (Rock et al. 1988; Shand et al. 1994) volumes of silicic melt would be generated, as suggested for because it does not require a contemporaneous subduction the largeMid-Jurassic rhyolite province of southern South zone to generate calc-alkaline lamprophyric melts: i.e. these America (Pankhurst et al. 1995). A field example of crustal can be a product of much older subduction. melting near a mafic intrusion has been described by Elsdon & Todd (1989) from north Donegal; a shallowly dipping to flat-lying appinite sill that was fed by a spessartite dyke has Discussion apparently migmatized the country rock. Applying this model to the lamprophyres of the study area Consistent with thelamprophyre model, Caledonian, suggeststhat the northeast-striking, steep, sinistralwrench K-rich granitoidmagmatism in Scotland is coeval with faultsthey intrudedare probable en echelon structures to sinistral simple shear (Watson 1984; Jacques 8: Reavy 1994). majorshears. En echelonsinistral shearsare spatially- The similarity in trend of primary-shearsand more-silicic relatedto the Iapetus Suture zone in easternIreland shoshonitic dykes from this study (Fig. 2b and c) suggests a (Vaughan 1991). Shears of thistype have a helix-like 3D similarassociation in easternIreland. Primary-shears form geometry(Naylor et a/. 1986) androtate with depthinto afterRiedel-shears in theevolution of enechelon fissure parallel with theprimary shear. Accordingly,a large-scale systems (Tchalenko 1970). Silicic magmas, generated by or wrench fault, plumbing the mantle, is inferred parallel to the fromlamprophyres, that ponded in thecrust, couldhave lamprophyre-rich zone depicted in Fig. 4. The lamprophyre- ascended to shalloweremplacement levels alonglater- rich zonestrikes slightlyanticlockwise of theSouthern formedPrimary-shears fault-controlledas dykes (e.g. Uplands/Longford-Down regional Caledonian strike, strad- Hutton 1988; Petford et al. 1993): evidencethat silicic dling the surface trace of the Iapetus Suture zone(Fig. 4). A magmasmay rise rapidly comes from feldspar geother- similar relationship in Scotland was recognised by Shand et mometry of ash-flow magmas (Whitney & Stormer 1986). al. (1994). An inferreddeep-rooted sinistral shearzone (3) The initjarion of lamprophyre intrusion overlaps with wouldaccount for the calc-alkaline lamprophyre-rich zone the end of sedimentation in the Southern Uplands. This can present in the southern Southern Uplands (Rock et al. 1986) beexplained by the evidence for the initiation of sinistral and extending into eastern Ireland. shearduring subduction accretion, i.e. while trench Theabsence of abundant calc-alkalineIamprophyre sedimentation wasstill active, in thesouthwest Southern intrusionassociated with Acadian deformation (D2 of Uplands(Knipe et al. 1988). Thiswould have released Vaughan & Johnston 1992) is explained by themodel accumulated K-rich melt close thetotrench as in outlinedabove: if a slowly rechargingsource region is southwesternMexico (Lange & Carmichael1990). The envisaged(McKenzie 1989), thenthe relative absence of inferredunderthrusting (subduction) of EasternAvalonia lamprophyres during Acadian deformation probably reflects continentalcrust beneath the Laurentian marginfrom the a depleted source. The absence of abundant lamprophyres mid-Wenlock(Kneller 1991; Vaughan & Johnston 1992) until Variscan times (Leat et al. 1987; Morrison et al. 1987) couldaccount for the initiation of sinistral shearduring at c.290Ma suggests that the reservoir may take 100Ma to accretionand provide sub-continentala mantle source replenish. (EasternAvalonia) for syn-sedimentation lamprophyres The model outlined above can resolve the five paradoxes north and south of the Iapetus suture without placingany of Caledonian magmatism posed by Rock et al. (1986). limitation on their proximity to the trench. (l) Chemicaltrends evident in Siluro-Devonian lavas (4) Calc-alkalinelamprophyrr dykes are wide1.y distrib- north of the Iapetus Suture zone, indicate the presence of a uted (from theNorthern and Western Isles of Scotland to coevalsubduction zone. Thiscanexplainedbe by northernEngland) and extremely chemically uniform. deep-rootedshear zones tapping metasomatized sub- Althoughsecular trends in the chemistry of Caledonian continentallithospheric mantle; chemical trends across the lamprophyres have been recognised (Rock et al. 1988; Shand suture (e.g.Rock etal. 1988) would reflect the thickness et al. 1994), the wide distribution and relative uniformity of (Brodie et al. 1994) and degree of metasomatism (McKenzie calc-alkaline lamprophyres is reflected by the equally widely 1989) of thesub-continental mantle, relics of subduction spreadevidence for sinistral shearoutlined above. It is processesthen nolonger active(post-mid Wenlock). envisaged that this sinistral shear event would have triggered Chemicaltrends controlled by distancefrom the former adiabatic decompression melting of compositionally similar subductionzones (Brodie etal. 1994)would beevident in K-rich frozen melts. magmasmodified by crustalinteraction during ascent, (5) Thelamprophyric zone includes plutonic, sub- although misleading. volcanic arm' volcanicrocks, apparently juxtaposed in time (2) Strongchemical similarities e.rist betweenSouthern and at the same emplacement level in the crust. The model Uplandsgranitoids and those from the north of England. outlinedabove provides aparticularly simple explanation.

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Oblique movements on shear zones may produce complex Scotland and northernEngland: a guide to magma sourceregions and structuressuch as sinistral shearwrench duplexes (Wood- magma-crust interactions. Philosophical Transactions of (he Royal Society of London. A310,709-742. cock & Fischer 1986). Such oblique movements, coeval, or HARRY.D.L. & LEEMAN.W.P. 1995. Partial melting of melt metasomatized overlapping,with magmatism will juxtaposeigneous rocks subcontinentalmantle and the magma sourcepotential of the lower emplaced at different crustal levels. lithosphere. Journal of Geophysical Research, 100, 10255-10269. HAICH.F.H., WELLS,A.K. & WELLS,M.K. 1972. Petrology of the Igneous Rocks. Allen and Unwin. London. Conclusions HAY. D.E. & WENDLANDI.R.F. 1995. The origin of Kenya rift plateau-type (1) Lamprophyres and shoshonitic rocks in eastern Ireland flood phonolites: results of high-pressure/high temperature experiments are coeval with sinistral transtension. in thesystems phonohte-H20 and phonolite-H+CO,. Journal of Geuphysicul Research, 100, 401-410. (2) A broadENE-trending zone of lamprophyricand HOLDSWOKI-H,R.E. & STKACHAN.R.A. 1991. Interlinked system of ductile shoshonitic rocks trends anticlockwise of the surface trace of strike-slip and thrusting formed by Caledonian sinistral transpression in the Iapetus Suture zone in eastern Ireland. northeast Greenland. Geology. 19,510-513. (3) A model of simple-shear-inducedadiabatic de- HLTPEKT. H.E.& SPARKS,R.S.J. 1988. The generation of granitic magmas by intrusion of basalt intocontinental crust. Journal of Petrology, 29, compression melting of melt-metasomatized sub-continental 599-624. mantle for lamprophyregeneration and emplacement can HL'TION,D.H.W. 19x8. Igneous emplacement in a shear zone termination: the account for many of the features of Caledonian calc-alkaline biotitegranite at Strontian.Scotland. Geological Society of America lamprophyres and shoshonitic rocks. Bulletin. 100, 1392-1399. - & MCERLEAN.M. 1991. ShortPaper: Silurian andEarly Devonian (4) A prediction of thismodel is thatlamprophyric sinistral deformation of the Ratagain granite. Scotland: constraints on the magmatismmay bepreceded by deep (>60 km) seismic age of Caledonian movements on the Great Glen fault system. Journal of activity. the Geological Society. London. 148, 1-4. - & REAVY,R.J. 1992. Strike-slip tectonics andgranite petrogenesis. Tectonics, 11, 960-967. This research was funded as part of EC contract no. MAIM1 322. INGRAM. G.M. & HLTON.D.H.W. 1994. TheGreat Tonalite Sill: Special thanks to D. Doff and K.M. Oun who analysed part of the emplacement into a contractional shear zone and implicatlons for Late CarrickdexterFormation. I am grateful to W.E.A. 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Received 4 April 1995; revised typescript accepted 15 January 1996. Scientific editing by Hugh Rollinson.

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