Nevis Landscape Partnership - Digital Mapping and Three ......2017/12/19 · Ben Nevis Igneous Complex and other late Silurian to Early Devonian plutonic and volcanic rocks. Main

Digital Mapping and Three-Dimensional Model Building ofthe Ben Nevis Igneous Complex, Southwest Highlands,

Scotland: New Insights into the Emplacement andPreservation of Postorogenic Magmatism

R. J. Muir* and A. P. M. Vaughan

Midland Valley, 2 West Regent Street, Glasgow G2 1RW, United Kingdom

AB STRACT

Caledonian postorogenic magmatic processes are not well understood. New digital mapping and three-dimensionalmodeling of the end-Caledonian (late Silurian to Early Devonian) Ben Nevis Igneous Complex in the SW Highlands ofScotland provide quantitative estimates of magmatic rock volumes for the first time and argue against local sources forthe volcanic rocks or caldera interpretations for structural evolution. Reexamination of the Ben Nevis Intrusive RingTuff, key evidence for caldera collapse, shows it to be a restrictedmarginal facies of the trondhjemitic InnerGranite, andthere is no evidence for a ring fault, previously argued as evidence for cauldron subsidence. The volcanic rocks at thecore of the complex appear to form a 1.3-km3 roof pendant in the Inner Granite, and paleoflow evidence suggests thatthey had a distal volcanic source to theNW.Compositional comparisons between the InnerGranite and themonzoniticOuter Granite indicate that they are unlikely to have had a common petrogenesis or be coeval. The plutonic rocks forma concentric, composite, 6.4 # 8.5-km tabular body, elliptical in plan, with a total volume in the range 44–105 km3.Large areas of late Precambrian Dalradian Supergroup metasedimentary country rocks form parts of a steeply dippingcarapace and roof pendants at the edge of the complex. Comparisonwith calc-alkaline volcanic rocks and plutons of thecomparable Cenozoic San Juan Volcanic Complex in Colorado suggests that the plutonic rocks may have grown aslaccoliths during regional caldera volcanism. Structural data suggest that the laccoliths inflated toward the SE, fed byNE-SW dikes in the core of the tightly folded Appin Syncline. This reevaluation of a classic area of world geology shedslight on a lost volcanic landscape that once covered much of the SW Highlands of Scotland.

Online enhancements: supplemental PDF.

Introduction

Plutonic and volcanic rocks of late Silurian to EarlyDevonian age are well exposed in the SW Highlandsof Scotland (fig. 1) and have been the subject of exten-sive petrographic, geochemical, isotopic, and struc-tural studies formore thanacentury (e.g.,Cloughet al.1909; Bailey and Maufe 1916; Stephens and Halliday1984; Jacques and Reavy 1994; Strachan et al. 2002;Neilson et al. 2009). The plutonic rocks, collectivelyreferred to as the Caledonian granites, were emplacedtoward the end of the Caledonian Orogeny, after clo-sure of the Iapetus Ocean between Laurentia andBaltica/Avalonia (Soper 1986; Dewey et al. 2015).Isotopic dating indicates that postorogenic magma-

tism (e.g., Clemens et al. 2009) began around 434Maand persisted for at least 25My from the late Silurianto the Early Devonian (Rogers and Dunning 1991;Oliver et al. 2008; Neilson et al. 2009; Conliffe et al.2010; Porter and Selby 2010). The plutonic rockssouth of the Great Glen Fault, which belong to theArgyll andNorthern Highlands Suite of Stephens andHalliday (1984), include a range of rock types fromappinite through hornblende diorite to granodioriteand monzogranite. Many of the plutons are zonedwithmaficoutermargins and relativelyhomogeneouscentral cores of monzogranite. Estimates of the depthof emplacement of the granites range frommidcrustallevels (∼10 km) at Ballachulish to shallow (!3 km),subvolcanic plutons atGlenCoe, Etive, andBenNevis(Bailey and Maufe 1960; Droop and Treloar 1981;Pattison and Harte 1997; Kokelaar and Moore 2006).

Manuscript received March 15, 2017; accepted June 30,2017; electronically published September 20, 2017.

* Author for correspondence; e-mail: [email protected].

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[The Journal of Geology, 2017, volume 125, p. 607–636] q 2017 by The University of Chicago.All rights reserved. 0022-1376/2017/12506-0001$15.00. DOI: 10.1086/693858

Figure 1. Geological map of the SW Highlands of Scotland (Lochaber and Lorn districts) showing the location of theBen Nevis Igneous Complex and other late Silurian to Early Devonian plutonic and volcanic rocks. Main faults andcrustal lineaments are from Fettes et al. (1986) and Stephenson and Gould (1995). Late Caledonian NE-SW-trendingdike swarms are omitted for clarity, but see Bailey and Maufe (1960) for further details.

There are four major outcrops of late Silurian toEarly Devonian volcanic and related sedimentaryrocks that are spatially associated with the Cale-donian granites in the SWHighlands (fig. 1): the LornPlateau lavas, the Glencoe Volcanic Formation, thevolcanic pile at Ben Nevis, and a small screen of al-tered volcanic material at the SEmargin of the EtiveComplex. These four separate areas are thought tobe remnants of an extensive volcanic province thatcovered much of the Lochaber and Lorn districts ofthe Scottish Highlands but has subsequently beenremoved by uplift and erosion (Morris et al. 2008;Neilson et al. 2009; Upton 2015). The volcanic se-quences at Glen Coe and at BenNevis were regardedby earlyworkers as classic examples of deeply erodedcaldera volcanoes (Clough et al. 1909; Maufe 1910;Bailey and Maufe 1916). Subsidence was thought tohave occurred on ring faults, which allowed the vol-canic centers to descend rapidly into the underlyingmagma chambers.More recentwork atGlenCoe hasdemonstrated that the deposition of the volcanic andsedimentary rockswas controlled by a set ofNW-SE-and NE-SW-trending fault zones that created accom-modation space and enabled piecemeal subsidenceof the Glen Coe Graben (Moore and Kokelaar 1997).The ring fault at Glen Coe has been shown to be adiscontinuous structure that became active only to-ward the end of volcanic activity in the region (Ko-kelaar and Moore 2006).This article presents the results of newdigitalfield

mapping and three-dimensional (3D) model buildingof the BenNevis Igneous Complex, which lies 20 kmto the north of Glen Coe. This is the first attemptto systematically examine thefield relationships andgeometry of the igneous complex since the originalmapping by the British Geological Survey in theearly part of the twentieth century. The results of thenewmapping have provided important insights intothe emplacement and preservation of the plutonicand volcanic rocks in this classic area. The complexhas previously been regarded as one of the type areasfor cauldron (plutonic) subsidence (Maufe 1910; Bai-ley and Maufe 1916), facilitated by a circular ringfault. Here we use the new field observations to eval-uate the ring-fault model, and we examine the influ-ence of preexisting structures and fabrics in the sur-rounding Dalradian country rocks in controlling theemplacement site and shape of the igneous complex.

Location and Previous Work

The Ben Nevis Igneous Complex is situated on thesouth side of the Great Glen Fault, immediately eastof Fort William, and is centered on Britain’s highest

mountain, which reaches a height of 1345 m abovesea level (fig. 2). The complex has a generally circularoutcrop pattern, covers an area of ∼42 km2, and isdominated by granitic rocks, with sedimentary and vol-canic units forming a small, elliptical outcrop aroundthe summit of the mountain. There are four maincomponents within the complex: the central volca-nic pile; the Inner Granite, which encloses the vol-canic pile on all sides; the Outer Granite; and a suiteofNE-SW-trendingmafic and felsic dikes that cut theOuter Granite but are of limited extent within theInner Granite (Bailey andMaufe 1916, 1960). Smallerigneous bodies of diorite and granite lying to the northand south of Ben Nevis at Coille Lianachain andMul-lach nan Coirean, respectively, are probably coevalwith the main part of the complex (Anderson 1935;Bailey and Maufe 1960).The Ben Nevis area was first mapped systemati-

cally in the early part of the twentieth century, withthe results of this fieldwork published as a 1∶63,360scale (one inch to the mile) geological map (BenNevis, sheet 53) and an accompanying memoir ti-tled Geology of Ben Nevis and Glencoe (Bailey andMaufe 1916, 1960). The northern part of the complexwas remapped by the British Geological Survey be-tween 1949 and 1963 and forms part of the adjacent1∶50,000 scale map (Loch Lochy, sheet 62E; John-stone et al. 1975). There has been no systematic geo-logical survey of the wider Ben Nevis area since thisearly mapping, and the one-and-a-half-page descrip-tion of the volcanic pile in the original survey mem-oir (Bailey and Maufe 1916, 1960) remains the onlypublishedaccountof thevolcanic rocks exposedaroundthe summit region of Ben Nevis. Anderson (1935)described the field relationships of some of the plu-tonic rocks at the outer margins of the Ben NevisIgneous Complex, including the intrusion at CoilleLianachain, but his study focused on the petrographyof the intrusions and their field relationships. Has-lam (1968) and Burt (1994) undertook detailed pet-rographic and geochemical studies of the plutonicand volcanic rocks, and Burt (1994) subdivided thevolcanic pile into four separate formations, domi-nated in the upper part by andesitic lava flows. Burtand Brown (1997) described a marginal intrusion atthe edge of the volcanic pile, the BenNevis IntrusiveRing Tuff, which was thought to have developedduring caldera subsidence.There are no published isotopic age data from the

igneous rockswithin the complex or paleontologicaldata from the associated sedimentary units, but thepetrography and geochemistry of the plutonic andvolcanic rocks bear close similarities to those of thelate Silurian to Early Devonian Argyll and NorthernHighlands Suite rocks (Stephens and Halliday 1984),

Journal of Geology 609B EN N E V I S I GN EOU S COM P L E X

and these rocks are part of the 425–380 Ma late Cal-edonian magmatism affecting Great Britain and Ire-land (Miles et al. 2016). Chemically, the plutonic andvolcanic rocks at Ben Nevis are calc-alkaline, with

high Na2O, marked enrichments in Sr and Ba, andlow concentrations of Nb, Th, and Rb (Haslam 1968;Thirlwall 1981; Burt 1994; Trewin and Thirlwall2002).

Figure 2. Geological map of the Ben Nevis Igneous Complex based on new digital field mapping. Grid referencenumbers are based on the United Kingdom Ordnance Survey 1-km National Grid (prefix NN in the Lochaber area).CIC Hut p Charles Inglis Clark Memorial Hut.

610 R . J . MU I R AND A . P . M . V AUGHAN

Methodology

Digital Field Mapping. Midland Valley’s digitalfield-mapping applications FieldMove Clino, usingan iPhone5, andFieldMove, using an iPad,were usedto collect digital field data, including structural andsedimentologicalmeasurements, field notes, and pho-tographs from the Ben Nevis area. The digital fielddata were then loaded into Midland Valley’s mainsoftware application Move for model building andfurther analysis. One of the main advantages of work-ing entirely within a digital environment is that datacan be very quickly, reliably, and accurately locatedwith the internalGPS in the smartphoneor tablet, andthere are no errors introduced by digitizing data andtranscribing information from a hard-copy field slip(Muir 2015). All of the six-figure grid reference num-bers referred to in the text and diagrams in this articleare based on the United Kingdom Ordnance Survey1-kmNational Grid with prefix NN in the Lochaberarea.Attention during the new fieldwork program fo-

cused on mapping the location and orientation of

lithological boundaries within the igneous complexand the surrounding country rock in order to estab-lish the geometry of the major fold structures andigneous contacts. The results of the new mappingare summarized on the geological map in figure 2,and thismap has been draped over a digital elevationmodel (DEM) in figure 3.

Model Building. Data used for model building con-sisted of the digital field data, a georeferenced geo-logicalmap image (fig. 2), and the relevant part of theOrdnance Survey 1∶25,000 Colour Raster DEM ofthe United Kingdom (fig. 3). Three-dimensional sur-faces representing the fold structures in the Dalra-dian country rocks and the geometries of the differ-ent units that make up the igneous complex weregenerated with the constrainedmodel-building toolsinMidlandValley’sMove software suite. The resultsof the 3D model-building process are shown in fig-ure 4. The fold structures in the country rocks wereproduced by extending surface dip measurements ofbedding above and below the DEM, defined as theground surface, and using dip isogons (Ramsay 1967)to create geologically realistic horizon geometries and

Figure 3. Geological map of the Ben Nevis Igneous Complex draped in Move over a digital elevation model extractedfrom the Ordnance Survey 1∶25,000-scale Colour Raster of the United Kingdom. See figure 2 for key. The red arrowon the compass rose points north, and grid reference numbers are based on the United KingdomOrdnance Survey 1-kmNational Grid (prefix NN in the Lochaber area). CIC Hut p Charles Inglis Clark Memorial Hut.


fold structures along multiple two-dimensional (2D)cross sections. A 3D surface representing the foldstructure was then generated from the 2D sectiondata with a surface kriging algorithm. The outer mar-gins of the plutonic bodies that make up the igneouscomplex, the Inner andOuterGranites, and theoutermargin of the volcanic pile were created by extend-ing dip and strike measurements of the exposed con-tacts. The terrain is heavily glaciated, and, on thebasis of the abundance of roof pendants and the pres-ence of wall-rock screens, the current level of ex-posure is assumed to be close to the roof zone of theintrusion. The intersection of the outcrop trace withtheDEMwas also used to verify the orientation of thecontacts in 3D space via structure contours. For theInner Granite, the floor and roof of the plutonic bodywere created fromaseries of 2Dcross sections throughthe igneous complex that honored the available dipmeasurements. The roof of the Outer Granite and the

floor of thevolcanic pilewere generatedwith the samemethod, with a variety of surface-creation algorithmsbeing used to produce the final 3D surfaces. An in-teractive 3D PDF version of the Move model illus-trated in figure 4 is available online

Modeling Results

The resulting model (fig. 4) is centered just north ofthe summit of Ben Nevis, at grid reference NN 170730. It is bounded by a north-south rectangle de-fined by the geological map in figure 3 and covers anarea of 255.6 km2, withmaximumdimensions north-south of 16.3 km and east-west of 14.7 km. TheDEMrepresenting the land surface ranges in elevation fromsea level to the summit of BenNevis at 1345m.Awayfrom Ben Nevis the land surface shows rugged topog-raphy to the SE, with many subsidiary peaks up to

Figure 4. Three-dimensional model of the Ben Nevis area generated in Move. The model is at the same orientationand scale as the geological map draped over the digital elevation model in figure 3. The top edge of the volcanic pile(highlighted in yellow) sits at a height of 1400 m above sea level. The volcanic pile forms an elliptical bowl enclosedby the Inner Granite magmas (pink), with the base of the bowl sitting at a depth of 400–500 m above sea level. The redarrow on the compass rose points north.


1000 m, whereas to the NW the topography is moresubdued, with maximum elevations of ∼500 m.Geological surfaces such as stratigraphic horizons,

faults, and igneous contacts are modeled as triangu-lar meshes made up of individual elements approxi-mately70macross.TheGreatGlenFaultZone formsthe NW structural boundary and has been modeledas a single vertical plane striking 2207 (fig. 4). SE oftheGreatGlen Fault Zone, theAppin Syncline (fig. 4)is modeled as two surfaces separated by the Ben Ne-vis Igneous Complex. The NE surface is 34 km2,forming a tight to isoclinal fold with an axial planedipping steeply to the SE at 757–807. The axial planeis curved in plan, striking on average 2407 and con-cave to the SE. The half-wavelength of the fold rangesfrom 810 m in the NE to 1820 m in the SW. Foldamplitude is greater than 3 km. The SW surface is50 km2, forming a tight to parallel fold with an axialplane striking 2307 and dipping 807 to the SE. Thehalf-wavelength is consistent along strike at 2500 m,and amplitude is greater than 3 km. SE of the BenNevis IgneousComplex, the Stob Ban Synform (fig. 4)consists of a single 25-km2 folded surface forming anopen fold, plunging 77 to the NE with a vertical axialplane striking on average 2257, curved in plan andconcave to the NW.The granites of the Ben Nevis Igneous Complex

have been modeled as partial or complete quasi-cylindricalmeshes plunging 707 to the SE and domedat or above the current topographic surface. Two sat-ellite bodies to the Ben Nevis Igneous Complexhave beenmodeled, theMullachnanCoirean graniteto the SW and the Coille Lianachain diorite to theNE. TheNE part of theMullach nanCoirean graniteis modeled as a hemicylinder, convex to the SW,with a long axis in map view striking ∼2307. TheCoille Lianachain diorite is modeled as an irregularcylinder with a long axis in map view striking ∼1507and a gently domed top. The intrusion has amodeledmap surface area of 1.85 km2. The Outer Granite ismodeled as an irregular cylinder partially open to thesouth,where it is truncated by the InnerGranite. It iscrudely elliptical in plan, with a semimajor axis of8.4 km in map view striking ∼1887 and a semiminoraxis of 6.5 km. The intrusion has a modeled mapsurface area of ∼21 km2 (which includes the outer di-orite). The InnerGranite ismodeled as a tabular body,crudely elliptical in plan, with a semimajor axis of5.8 km in map view striking ∼197 and a semiminoraxis of 4.3 km. The intrusion has a modeled mapsurface area of 14.7 km2, or ∼18.2 km2 including thearea covered by the volcanic pile. The roof pendantcontaining the volcanic pile of the Summit Forma-tion is modeled as a small, flat-bottomed bowl, ellip-tical in plan, with a semimajor axis of 2.3 km in map

view striking ∼1207 and semiminor axis of 1.8 km.The bowl has amodeledmap surface area of 3.5 km2.Dike swarms are represented only as lines on themap in figure 2 and have not been modeled in threedimensions.

The Ben Nevis Igneous Complex

The geology of the Ben Nevis Igneous Complex ispresented here as a combined description of the litho-stratigraphy and structure starting from the countryrocks and working in toward the center of the com-plex. New observations are identified where appro-priate.

Country Rocks. The plutonic rocks in the Ben Ne-vis Igneous Complex intruded preexisting volcanicrocks and tightly folded, late Precambrian (1600Ma),garnet-grade metasedimentary rocks belonging totheDalradian Supergroup (fig. 2). The volcanic rocksare described in more detail below. The metamor-phic country rocks were deformed and metamor-phosed at ∼470 Ma during the Grampian event ofthe Caledonian Orogeny (Soper et al. 1999; Stephen-son et al. 2013). There is general agreement on theoverall geometry of the major folds within the Dal-radian rocks in the SW Highlands, but there is con-tinuing debate over the relative ages of the fold struc-tures and their kinematic development (seediscussionsin Stephenson and Gould 1995; Stephenson et al.2013).Grampian Group rocks on the NW side of the com-

plex, the Eilde Flags, are micaceous quartz-feldsparschistsmapped as dipping steeply to the SE at 407–807.The Dalradian rocks to the east of the complex con-sist of folded slate, limestone, schist, and quartzitebelonging to the Appin Group (Stephenson et al.2013). The boundary between Grampian Group andAppin Group rocks, the Fort William Slide (Bailey1911), is a ductile shear zone, variously interpretedas an extensional or compressional structure and atectonized unconformity (Bailey and Maufe 1960;Soper and Anderton 1984; Glover 1993). The FortWilliam Slide is not exposed in the study area butwas mapped as dipping steeply to the NW at 1607along the shores of Loch Linnhe to the SW of FortWilliam. Farther north, near Spean Bridge (fig. 1), theslide is observed dipping steeply to the SE at 1757.TheNW side of the igneous complex cuts through

the surface trace of the Fort William Slide proposedby Bailey (1911; shown in figs. 1, 2) and the steeplimbs of the Appin Syncline (labeled “AS” on fig. 2).The SE side of the complex isflanked by the Stob BanSynform (“SBS” in fig. 2). These major folds weremapped in this study as tight, isoclinal structurestrending NE-SW; however, no field evidence was


observed for a large antiformal structure between thetwo synforms. The relative ages of the Appin Syn-cline and the Stob Ban Synform are not clear, withdifferent workers assigning the folds to either D1 orD2 of the Grampian Orogeny, depending on the in-terpretation of minor fold structures and way-upcriteria in the steep limbs (Stephenson and Gould1995).

The Dalradian rocks surrounding the igneous com-plex show the effects of contact metamorphism upto 3 km from the outer margin (Bailey and Maufe1960). The thermal aureole is marked by the wide-spread development of cordierite and andalusite inall lithologies, and zones of hornfels texture severalmeters wide are observed in the country rocks clos-est to the igneous margin.

Geometry of the Outer Granite Margin. Mappingof the contact between the igneous complex and theDalradian country rocks in this study shows thatit dips steeply outward at ∼707 around most of themargin, but on the NW side of Ben Nevis betweenAchintee and the Allt an t-Sneachda (fig. 2), thecontact dips steeply inward to the SE at ∼707 (fig. 5).The outer margin was drawn as a solid line by theearly surveyworkers, but newobservations presentedhere show that, in detail, the contact is defined bynumerous thin sheets and veins of igneous material(both diorite and granite) trending generally NE-SW,subparallel to the main tectonic fabric in the Dal-radian country rocks. The veins and sheets at theouter margin were mapped, ranging in width from afew centimeters to nearly 3m, andwere observed onall sides of the complex. Accessible examples can beobserved in the lower reaches of theAllt a’Mhuilinn(grid ref. NN141 757), in the streambed of theAllt ant-Sneachda (grid ref. NN 180 764), and in the streamsdraining the western slopes of Ben Nevis betweenAchintee and Polldubh inGlenNevis (fig. 6A). Theseearly NE-SW-trending diorite and granite sheets aredistinct from the later NE-SW-trending mafic andfelsic dikes that cut through the Outer Granite andthe Dalradian country rocks.

On the NW side of the complex, mappingmade itpossible to identify a 50-m-wide zone of alternat-ing strips of Dalradian country rock and NE-SW-trending igneous sheets. The steep-sided sheets andveins merge and coalesce rapidly over a few meterstoward the main body of the igneous complex.

Large fragments of country-rock carapace (up to600 m2) and roof pendants of Dalradian metasedi-mentary rockweremappedat several localities aroundthe outermargin of the complex. These indicate thatthe current level of exposure is close to the roof zoneof the intrusion (fig. 5). The largest carapace frag-ment, with a surface area of ∼0.6 km2, was mapped

on the steep slopes of Carn Dearg (S) above Polldubhin Glen Nevis. Toward the top of the carapace frag-ment, the contact between the Dalradian (LevenSchist) and the Inner Granite on Carn Dearg (S) wasrecorded as dipping to the south at ∼457, parallel tothe general slope of the hillside between Carn Dearg(S) and Bealach Cumhann. Mapping shows that thedip of the contact then gradually steepens down-hill toward the base of the crags in Glen Nevis. AtBealach Cumhann (grid ref. NN 177 700), the con-tact between the Dalradian and the Inner Granitewas recorded as dipping shallowly to the SE at !107,and there are also several small (up to 10 m2) roofpendants of Dalradian country rock enclosed by gra-nitic material at this locality.

Fragments of country-rock carapace were alsomapped along the east side of Aonach Mor, imme-diately east of the summit (grid ref. NN194 729), andfarther north atAonach anNid (grid ref.NN195750).At the highest point on both of these carapace frag-ments, the contact between the Dalradian and theOuter Granite is observed to be subhorizontal, andit gradually steepens down the east side of AonachMor. Large roof pendants of Dalradian country rocks(Leven Schist), several tens of meters across, weremapped along the east side of Aonach Mor, on theridge south of Coire an Lochan, and farther northbetween Sgurr Finnisg-aig and the Allt Choille Rais.

The dominant tectonic fabric in theDalradianmeta-sedimentary rocks surrounding Ben Nevis was re-corded as dipping steeply to the SE at 507–707. How-ever, mapping of the fabric shows that there wasconsiderable disruption along the contact around theSE side of the complex between Carn Dearg (S) andAonach Mor (fig. 5). Brittle faulting and chaotic frac-turing of the country rocks is observed up to 75 mfrom the main contact.

Outer Granite. The Outer Granite is a compositeintrusion (e.g., Maufe 1910); it forms slightly morethan half of the igneous complex by area (fig. 2) andis modeled covering an area of ∼21 km2 (fig. 4). Pre-vious workers (Maufe 1910; Anderson 1935; Baileyand Maufe 1960; Haslam 1968; Burt 1994) have at-tempted to subdivide the Outer Granite into a num-ber of separate intrusive phases and have tended toemphasize the crudely concentric nature of the in-trusion, progressing inward from hornblendic dio-rite (oldest) to porphyritic monzogranite (youngest).Subtle variations in texture and petrography, a com-bination of sharp and merging contacts, and a lackof clean exposure make it difficult to map out theextent of the separate igneous phases with any cer-tainty. As a consequence, several different interpre-tations of the distribution of the various subunitshave been proposed (Maufe 1910; Anderson 1935;


Bailey andMaufe 1960;Haslam1968; Burt 1994). Forthe purposes of this study, the outer dioritic rockshave been grouped as a singlemappable unit that canbe readily distinguished from the coarse-grained por-

phyritic monzogranite that forms the bulk of theOuter Granite.Fine- tomedium-grainedquartzdioritesweremapped

along theNW side of the complex betweenAchintee

Figure 5. Structural-elements map of the Ben Nevis Igneous Complex showing the orientation of igneous contactsand the location of large fragments of country-rock carapace (S) and roof pendants (R). Grid reference numbers arebased on the United Kingdom Ordnance Survey 1-km National Grid (prefix NN in the Lochaber area). CIC Hut pCharles Inglis Clark Memorial Hut.


and the Allt an t-Sneachda and on the east side ofAonach Mor (fig. 2). Previously unrecorded expo-sures of diorite also occur on the lower slopes ofCarnDearg (S) in Glen Nevis (fig. 2). Magma-minglingtextures are common in the dioritic phases. Thequartz diorites are predominantly dark gray and non-porphyritic and contain biotite, clinopyroxene, ortho-pyroxene, andhornblende as themainmaficminerals.Field relationships at the outer margins indicate thatthe quartz dioriteswere emplaced as steep-sided,NE-SW-trending sheets and veins.

The main mass of the Outer Granite (∼80%) is aporphyriticmonzogranite containing large, euhedralphenocrysts of pink alkali feldspar and plagioclase(1–2 cm in length) set in a finer-grained groundmass.The primary mafic minerals are biotite and horn-blende. The contact between the main porphyriticmonzogranite and the quartz diorites,which is shown

as a dotted line in figures 2, 3, 5, was everywheremapped as gradational over several tens ofmeters andis not a sharp, intrusive contact. Two small occur-rences of appinite recorded by the early survey work-ers in the western part of the Outer Granite are in-terpreted in this study as hornblende-rich patches ofdiorite, rather than discrete appinite intrusions orbreccia pipes.

Numerous small enclavesoffine- tomedium-grainedquartz diorite (a few tens of centimeters across) wererecorded within the main mass of porphyritic mon-zogranite, and xenoliths of Dalradian country rock(fig. 6B) displaying a range of assimilation featuresare commonwithin the outer diorites. The long axesof the enclaves and the country-rock xenoliths gen-erally show no consistent orientation, but in severalzones on the NW side of the complex they weremapped as displaying a strong NW-SE alignment, at

Figure 6. A, NE-SW-trending sheets of coarse-grained porphyritic monzogranite (pink) cutting quartz diorite (darkgray) on the west side of Ben Nevis (grid reference NN 142 703; United Kingdom Ordnance Survey 1-km NationalGrid). B, Xenolith of Dalradian schist within quartz diorite in the Allt a’ Mhuilinn on the NW side of Ben Nevis (gridref. NN 141 757). The long axis of the xenolith is aligned NW-SE, at right angles to the main tectonic fabric in thecountry rocks surrounding the igneous complex. C, Contact between the Inner Granite (pink, left-hand side of theimage) and the volcanic pile (below red rucksack) in the Allt a’ Mhuilinn, ∼50 m upstream from the Charles InglisClark Memorial Hut (grid ref. NN 168 721). Contact dips to the SW at ∼407. D, Flow-banded rhyolite at the edge of theInner Granite, toward the head of Coire Leis (grid ref. NN 171 713).


right angles to the orientation of the early sheets ofdiorite and granite at the outer margins. Examples ofaligned mafic enclaves and xenoliths were recordedin the quartz diorites exposed in the lower reachesof the Allt a’ Mhuilinn (grid ref. NN 141 757) and inthe porphyritic monzogranite exposed on either sideof the Allt an t-Sneachda on the north side of thecomplex (grid ref. NN 185 762).

NE-SW-TrendingDike Swarm. TheDalradian coun-try rocks and the Outer Granite are cut by an exten-sive swarm of steep-sided NE-SW-trending mafic(dioritic and lamprophyric) and felsic (microgranitic)dikes in approximately equal proportions (fig. 2). Bai-ley andMaufe (1960) referred to these collectively asthe Ben Nevis dike swarm.Mapping shows that the dikes range inwidth from

a few tens of centimeters to a maximum of 5 m, andmany can be traced laterally for several tens of me-ters. Cross-cutting relationships indicate that therewas more than one episode of dike emplacement,and some of the felsic dikes on the north side of thecomplex were mapped showing flow banding wherethere are wall-rock constrictions. On the west sideof Aonach Mor, a thin (∼1.2-m), shallowly dippingmafic sheet, which extends for nearly 500 m acrossthe hillside, is observed to be connected from belowto at least three NE-SW-trending dikes and then con-nected to at least two dikes above. The early surveyworkers noted that the Inner Granite appears to trun-cate the outcrop pattern of the NE-SW dike swarm inthe Outer Granite, but there are several examples ofboth mafic and felsic dikes cutting the Inner Granitein the Allt Daim (Bailey and Maufe 1960), and previ-ously unrecorded examples were observed on the south-ern flanks of Carn Dearg (S). NE-SW-trending dikeswere not observed cutting the volcanic pile.

Inner Granite. The Inner Granite is modeled tocover an area of ∼18 km2 in the southern part of thecomplex and encloses the volcanic pile on all sides.Cross-cutting relationships indicate that the InnerGranite is younger than both the Outer Granite andthe volcanic pile andwas emplaced after themajorityof the NE-SW-trending mafic and felsic dikes hadbeen intruded. Inhand specimen, the InnerGranite ispale pink or pale orange and is a relatively homoge-nous, fine- to medium-grained, plagioclase-rich trond-hjemite. Themainmaficmineral is biotite, but thereis also a small outcrop of amphibole-biotite-bearinggranite in thenorthernpart of this unit (Haslam1968;Burt and Brown 1997).A large patch (∼50 m2) of coarse porphyritic gran-

ite within the Inner Granite on the SW side of CarnDearg (S) was interpreted by Haslam (1968) as a roofpendant of theOuterGranite. Several similar patchesof coarse-grained granite, ranging inwidth from a few

meters to several tens of meters, were identified andmapped on the southern slopes of Carn Dearg (S).These coarse patches blend andmerge gradually intothe fine- to medium-grained Inner Granite. Fine-grained felsic veins of aplite ranging in width from afewcentimeters to 0.5mare also commonly observedwithin the Inner Granite, especially in the southernpart of the pluton.

Geometry and Style of the Inner Granite Margin.Themost accessible contact between the Outer Gran-ite and the InnerGranite is observed on themountaintrack between Achintee and Lochan Meall an t-Suidhe (grid ref. NN 142 718). In the exposures on thewest side of the track, the InnerGranite lies under theporphyriticmonzogranite, and the contact is irregularbut mapped dipping generally outward to the west at!457. Farther north, in the Allt Daim, the contact isobserved to be inclined steeply inward at ∼607 to theSW (grid ref.NN176748). The contrast in the attitudeof the contact between these two localities suggeststhat the InnerGranite has a tabular form rather than asteep-sided stock.

Volcanic-Pile Basin Geometry. The volcanic pilethat forms the steep cliffs on the North Face and sum-mit region of Ben Nevis was described by the earlysurveyworkers (Bailey andMaufe 1916) as consistingof hornblende andesite lavas and unstratifiedmassesof agglomerate, accompanied by small amounts offine-grained sedimentary rocks. Exposures of schistin the streambed of the Allt a’ Mhuilinn on the NEside of BenNevis and in Five Finger Gully on the SWside of the mountain (fig. 7) were previously inter-preted as Dalradian basement underlying the volca-nic sequence. The volcanic rocks were assigned tothe upper Silurian/Early Devonian (lower Old RedSandstone) by the survey workers, but no paleonto-logical samples have been recovered from the se-quence. The volcanic pile was described as exhibit-ing a symmetrical basin-like structure, with steeplydipping beds at the margins becoming more shal-lowly dipping toward the center of the pile.New field mapping and digital modeling has re-

vealed that the rockswithin the volcanic pile consistof a thick sequence (1600 m) of sedimentary rocksand volcaniclastic material that lies within a small,elliptical basin 2.3 km # 1.8 km. The oldest sedi-ments in the basin are preserved on the NE side ofBen Nevis, and mapping shows that they form twothin lenses: one in the streambed of the Allt a’Mhui-linn, ∼250 m upstream from the Charles Inglis ClarkMemorial Hut (CIC hut), the other farther to the SEin Coire Leis at the base of the Little Brenva Face(fig. 7). Bedding in both lenses dips steeply to thewest, at 1707, but shows considerable disruption andsoft-sediment deformation. Bedding within the over-


lying volcaniclastic rocks, which form 195% of thevolcanic pile, dips consistently to the SE at 207–307.There is no evidence for a symmetrical structure tothe basin.

Volcanic-Pile Stratigraphy. The volcanic pile con-tains minor igneous intrusions that make up !1%of the total volume, but there are no examples ofandesitic lava flows or a central volcanic conduit.

Figure 7. Geological map of the volcanic pile, Ben Nevis. Grid reference numbers are based on the United KingdomOrdnance Survey 1-km National Grid (prefix NN in the Lochaber area). CIC Hut p Charles Inglis Clark MemorialHut.


Browne et al. (2002), following Burt (1994), subdi-vided the volcanic pile into two separate formations:the Allt a’ Mhuilinn Mudstone Formation, which isthe oldest unit and which they argued was overlyingDalradian basement, and the overlying Ben NevisVolcanic Formation. The sequence has a combinedstratigraphic thickness of more than 600 m. Thesestratigraphic names have been retained during ourstudy, but the distribution of the formations andtheir origins have been revised in the light of newfield data. A new geological map of the volcanic pileis shown infigure 7, togetherwith a lithostratigraphiccolumn in figure 8. The distribution of the differentunits within the volcanic pile is also illustrated onan annotated photograph of the North Face of BenNevis in figure 9.Dalradian Basement. Previous workers (Maufe

1910; Burt and Brown 1997) have described thesediments at the base of the volcanic pile as uncon-formably overlying Dalradian schist in the stream-bed of the Allt a’ Mhuilinn. However, detailed ex-amination of these exposures has revealed that the

rocks of the volcanic pile are never seen in contactwith the Dalradian at this location. There are fourlow-lying, isolated blocks of schist a few metersacross on the south side of the Allt a’Mhuilinn (gridref. NN 171 719), but the blocks are entirely sur-rounded by glacial drift. The pervasive tectonic fab-ric within the blocks of schist is a crenulation fabric,with the axes of the minor folds trending NW-SE, atright angles to the main fabric observed in the Dal-radian country rocks surrounding Ben Nevis. TheDalradian rocks at this locality are reinterpretedhereas a single megablock (10 m# 8m) or blocks withina debris flow rather than in situ basement or morerecent ice-rafted blocks. Two smaller blocks of pre-viously unrecorded Dalradian schist (1 m # 1.5 m)entirely enclosed by mudstone within the Allt a’MhuilinnMudstone Formationweremapped fartherdownstream (grid ref. NN 170 720), supporting theobservation that the larger blocks are clasts within asedimentary unit and are not in situ basement.In Five Finger Gully on the SW side of Ben Nevis,

Maufe (1910) mapped out a thin slice of Dalradian

Figure 8. Lithostratigraphic column for the volcanic pile, Ben Nevis.


country rock at the contact between the Inner Gran-ite and the volcanic pile. Detailed examination ofthe field relationships at this locality has revealedthat the Dalradian rocks, which are intensely brec-ciated and form a narrow lens 5 m # 20 m, sit en-tirelywithin an altered felsic igneous intrusionwithinthe volcanic pile. Contact relationships on the southside of the Dalradian block are obscured by a heavilyweathered Tertiary igneous dike, but it is clear thatthe brecciated schist is not in situ basement and iseither a large xenolith or a megablock higher up inthe volcanic pile (fig. 8).

TheAllt a’MhuilinnMudstone Formation. Theoldest unit at the base of the volcanic pile, the Allt a’Mhuilinn Mudstone Formation, is recorded as crop-ping out in two locations on the north side of BenNevis: in the streambedof theAllt a’Mhuilinn∼250m

upstream from the CIC hut (grid ref. NN 170 720)and farther east at the base of the Little Brenva Face(grid ref. NN 172 715). The formation comprisesmainlymudstone and laminated siltstone,with lensesof conglomerate dominated by quartzite clasts. Atthefirst locality,∼250mupstream from theCIChut,theAllt a’MhuilinnMudstone Formation is observedto lie in direct vertical contact with the Inner Graniteon the south side of small waterfall. Approximately1mofmudstone dips steeply to the west at 1707 andis overlain by 2 m of pebble to cobble conglomerate,with angular clasts of Dalradian quartzite and schistranging in size from 5 to 15 cm. The conglomerate atthis locality is overlain by coarse debris flows con-taining large clasts of volcanicmaterial at the base ofthe Ben Nevis Volcanic Formation. Loading struc-tures are developed at the top of the Allt a’Mhuilinn

Figure 9. Photograph taken from the summit of Carn Mor Dearg (grid reference NN 177 722; United KingdomOrdnance Survey 1-km National Grid) showing the distribution of lithostratigraphic units in the volcanic pile on theNorth Face of Ben Nevis. Numbers in white boxes refer to the units within the volcanic pile: (1) Allt a’ MhuilinnMudstone Formation, (2) Coire na Ciste Member, and (3) Summit Member. Blue lines indicate the main ridges on theNorth Face. The green line indicates the extent of the Allt a’ Mhuilinn Mudstone Formation at the base of the LittleBrenva Face. The yellow dashed line indicates the approximate location of the gradational boundary between theCoire na Ciste Member and the Summit Member. Brown lines indicate the location of the main ash flow tuff unitswithin the Summit Member.


Mudstone Formation that indicate that the sedimentswere not lithified when the overlying volcaniclasticdebris flows were deposited.The largest outcrop of the Allt a’ Mhuilinn Mud-

stone Formation is recorded on the crags at the baseof the Little Brenva Face, immediately below theclimbers’ track to the first platform on the NE But-tress (fig. 9). A mud-dominated sequence of lami-nated sediments (fig. 10A) forms a steep-sided lens150 m long and 40 m across at the widest point. Atleast three large lenses of clast- to matrix-supportedmassive cobble conglomerate (individual lenses upto 5 m # 40 m) are contained within the laminatedsediments. Bedding is mapped as dipping steeply tothe SW at 1707 but shows considerable disruptionand is overturned in places. Numerous examples ofsoft-sediment deformation and water-escape struc-tures were observed within the mudstones and silt-

stones. Clast size in the conglomerate lenses rangesfrom sand to large cobbles up to 25 cm in diameter,with most clasts (approx. 95%) consisting of angularto subrounded quartzite fragments (fig. 10B). Clastsof schist are also present, but these tend to be moreangular and smaller in size (0.1–15 cm). No igneousclastswere observedwithin the conglomerate lenses.The lowermost units of mudstone and conglom-

erate at the base of the crags on theLittle Brenva Faceare mapped to be in direct contact with banded rhy-olitic rock at the edge of the InnerGranite, and a thinsheet of pale pink rhyolite (∼30 cm wide) with nar-row chilled margins can be traced for several metersinto the lower part of the volcanic pile. The Allt a’MhuilinnMudstone Formation appears to indicate afreshwater lacustrine environment,with thequartzite-dominated conglomerates representing subaqueousdebris flows.

Figure 10. A, Steeply dipping beds of laminated siltstone and mudstone within the Allt a’ Mhuilinn MudstoneFormation on the Little Brenva Face (grid reference NN 172 715; United Kingdom Ordnance Survey 1-km NationalGrid). B, Conglomerate with clasts of Dalradian quartzite and schist within the Allt a’Mhuilinn Mudstone Formationon the Little Brenva Face (grid ref. NN 172 715). C, Large clasts of volcanic material within the Coire na CisteMember (Ben Nevis Volcanic Formation) on the glacially polished slabs behind the Charles Inglis Clark MemorialHut (CIC hut; grid ref. NN 167 722). D, Sheet of banded rhyolite with thin chilled margin cutting the Coire na CisteMember (Ben Nevis Volcanic Formation) on the glacially polished slabs behind the CIC hut. (grid ref. NN 168 721).


The Ben Nevis Volcanic Formation. The appear-ance of igneous detritus within the massive un-sorted conglomerates overlying themud-dominatedsediments on the Little Brenva Face is taken tomarkthe base of the overlying Ben Nevis Volcanic For-mation. The formation comprises twomembers andis dominated in the lower part by coarse volcaniclas-tic conglomerates with a total thickness of around250m. The upper part consists of extensive sheets ofblock-and-ashflow deposits with a total thickness ofat least 350 m.

Coire na Ciste Member. The oldest unit, theCoire na CisteMember, is recorded at the base of theNorth Face and is composed of massive unsortedvolcaniclastic conglomerates (fig. 10C) dominatedby andesite, dacite, and rhyodacite clasts (190%),withsubordinate rhyolite and eutaxitic ignimbrite clasts.Fine millimeter- and centimeter-scale banding andlayering is commonly observed inmany of the clasts.Sedimentary and metamorphic clasts are rare andmake up !1% of the total volume. Clast size variesfrom a few millimeters to nearly 2 m. The clasts areangular to subrounded, and both matrix- and clast-supported textures are common. The conglomeratesare poorly sorted, but faint traces of bedding definedby thin layers of sand- and grit-sized material can beobserved on the glacially polished slabs adjacent tothe CIC hut. Bedding in these outcrops dips shal-lowly to the SE at ∼207. Individual conglomerateunits range in thickness from !1 m to at least 30 m,but contacts between units are often hard to locateon weathered surfaces. It has not been possible tomap out the geometry of separate flow units withinthe Coire na Ciste Member, but local topographicvariations are clearly present, with flows infillinghollows and terminating against intraformationalridges and channel sides. In general, however, all ofthe flows dip consistently to the SE at !307.

The Coire na Ciste Member has a total thicknessof at least 250 m and forms the lower half of theNorth Face between the NE Buttress and the north-ern slopes of Carn Dearg (N).

Summit Member. Mapping of the upper half ofthe cliffs forming theNorthFaceofBenNevis and thehigh crags on the south side of the mountain above950 m shows that they consist of massive sheets ofblock-and-ash flow deposits with a total thicknessin excess of 350m. Extensive weathering and growthof lichen on the surface of the North Face make it dif-ficult to determine the geometry of individual flowunits. However, on the clean exposures on the southside of Ben Nevis and on the summit plateau, it canbe seen that individual beds range in thickness from1 to 20 m. The flows dip consistently to the SE ataround 207–307 (fig. 11A).

Blockswithin theflow deposits are angular, rangein size froma few centimeters to nearly 1m (fig. 11B),and in places show imbrication indicating flow fromNW to SE. Both clast- andmatrix-supported texturesare present within the same flow unit. The blockswithin the deposits are predominantly (190%) daciteand rhyodacite in composition, rather than andesite(Haslam 1968; Burt 1994). In hand specimen, pheno-crysts of plagioclase feldspar 3–4 mm in length arecommon and are set in a finer-grained groundmassof plagioclase (An15–An50), hornblende, biotite, andiron ore.

The contact with the underlying Coire na CisteMember is hard to locate on weathered surfaces, butobservation suggests that it has an irregular topog-raphy and is not a sharp, planar contact. Interbeddedwithin the SummitMember are severalfiner-grainedlaminated tuff units ranging in thickness from a fewcentimeters to nearly 20 m. The largest outcrops oftuffaceousmaterial are observed at the base of NorthTrident Buttress and near the top of Ledge Route(fig. 11C). Small outcrops of tuff also occur on theNE Buttress, just above the forty-foot corner, and onTower Ridge, ∼50 m above Tower Gap.

The tuff unit at North Trident Buttress directlyoverlies the Coire na Ciste Member, but the contactis not exposed.The tuff can be traced for nearly 150macross the front face of the buttress from Coire naCiste toNumber 5Gully (fig. 9) and consists offinelylaminated ashfall deposits, reworked by mass-flowprocesses and soft-sediment deformation. Bedding inthe tuff is mapped dipping at 257–307 to the SE, andcross bedding and slumping in the sediments indi-cate flow and downslopemovement fromNW to SE.Bedding in the thick tuff unit near the top of LedgeRoute also shows a similar orientation.

Minor Intrusionswithin theVolcanicPile. Thereare no examples of the NE-SW-trending dike swarmcutting the volcanic pile, but a small number of dis-tinctiveminormafic and felsic intrusionsweremappedon the north side of BenNevis.Narrow sheets of palepink, fine- to medium-grained granite and bandedrhyolite can be traced from the main mass of theInner Granite into the volcanic pile for several tensof meters. The sheets range in width from 0.2 to 1mand exhibit narrow chilled margins a few millime-terswide (fig. 10D). Flowbanding is common inmanyof the sheets.Accessible examples canbe foundwithinthe Coire na Ciste Member on the glacially polishedslabs behind the CIC hut and on the rock platformbelow the Organ Pipes. Thin sheets of orange weath-ering rhyolite, 20–30 cmwide, can also be found spo-radically within the volcanic pile; one example onthe NE Buttress can be traced laterally from the firstplatform toward Orion Face for ∼70 m. No sheets of


granite or rhyolitewere recordedwithin the volcanicpile on the south side of Ben Nevis, apart from an al-tered sheet of felsic igneous material enclosing brec-ciated Dalradian schist in Five Finger Gully.Near the base of Observatory Gully, a small, ir-

regular intrusion of fine-grained mafic igneous rock∼10 m wide was mapped cutting through the lowerpart of the Coire na Ciste Member. The mafic rockcontains small phenocrysts of plagioclase feldspar (2–4mm in length) and is closely similar in petrographyto the blocks of dacite within the Summit Member.An orange-gray weathering mafic sheet ∼5 m thick

was observed at the top ofNumber 5Gully and formsa semicircular pile of scree∼100m in diameter on thesummit plateau. The sheet appears to be fed frombelow by a narrow dike ∼1mwide. Finally, a heavilyweathered Tertiary dolerite dike trendingWNW-ESEcan be traced for more than 4 km from themountaintrack below Lochan Meall an t’Suidhe across FiveFingerGully to the east side ofCoire Eoghainn (fig. 7).

Contact between the Volcanic Pile and the InnerGranite. The contact between the volcanic pile andthe Inner Granite was interpreted by the early sur-vey workers as a circular ring fault associated with

Figure 11. A, Photograph of the South Face of Ben Nevis and the back wall of Coire Eoghainn showing block-and-ashflow deposits of the Summit Member (Ben Nevis Volcanic Formation). The base of the cliffs on the left-hand side ofthe image (close to the shadow) are ∼950 m above sea level, and the summit plateau on the right-hand side is ∼1300 mabove sea level. Block-and-ash flows dip at ∼207 to the SE (toward the right-hand side of the South Face). B, Largeblocks of porphyritic dacite within the Summit Member (Ben Nevis Volcanic Formation) on the South Face of BenNevis (grid reference NN 163 709; United Kingdom Ordnance Survey 1-km National Grid). The two large clasts at thecenter of the image are ∼0.75 m across. C, Laminated air-fall tuff horizons within the Summit Member (Ben NevisVolcanic Formation) near the top of Ledge Route on the north side of Carn Dearg (N; grid ref. NN 159 723).


a caldera-collapse event. Maufe (1910) and Baileyand Maufe (1916) described narrow bands of “flintycrush rock” and flow banding in the granite close tothe contact at several localities around the summitregion of Ben Nevis. These features were thought tohave been produced by intense mechanical shearingas the volcanic pile descended rapidly into the un-derlying magma chamber. In contrast, Burt andBrown (1997) suggested that banded rhyolite at theedge of the Inner Granite is a separate intrusion thatcuts both the Inner Granite and the volcanic pile.Burt and Brown (1997) proposed a newname, the BenNevis Intrusive Ring Tuff, for this marginal intru-sion and suggested that it represents the remainsof an ignimbrite conduit formed during caldera col-lapse.

The most accessible exposures of the igneous con-tact crop out on the north side of Ben Nevis and canbe followed from the steep cliffs below Castle Ridge,across the base of the North Face, and along thestreambedof theAllt a’Mhuilinn to the head ofCoireLeis (fig. 7). The Inner Granite commonly displays a20–30-cm-wide chilled margin against the volcanicpile, which exhibits a narrow zone of hornfels at itsouter edge. The contact is always sharp and planarand dips steeply inward to the SW at 1707. Upstreamfrom the CIC hut, the contact is recorded as dippingmore shallowly to the SW at !457 (fig. 6C). Veins andsheets of the Inner Granite can be observed pene-trating the volcanic pile and exhibit thin chilled mar-gins (∼5 mm wide) and follow planar (brittle) jointswithin the pile.

A narrow belt of dark-gray and pink banded rhyo-lite !4mwide is observed intermittently at the edgeof the Inner Granite adjacent to the volcanic pile(fig. 6D) between the CIC hut and the head of CoireLeis (fig. 5). This banded rhyolitic unit was observedto merge into the main body of the Inner Graniteover a distance of !1m. The banding within the unitis generally subparallel to the contact with the vol-canic pile, but in some exposures, notably towardthe head of Coire Leis, chaotic folding and magma-mingling textures can also be observed. The bandingis defined by aligned layers and streaks of mafic andfelsic minerals ranging in width from a few millime-ters to 5 cm. The main phenocryst recorded is euhe-dral plagioclase up to 2 mm in length, with subordi-nate biotite, occasional amphibole, and rare quartz. Inthin section the darker bands have a higher concen-tration of mafic minerals. The rock is heavily altered,but there is no obvious mineral alignment in eitherthe mafic or felsic bands.

The contact between the Inner Granite and thevolcanic pile is not exposed on the western slopes ofBen Nevis, but it can be mapped out to within a few

meters in the Red Burn and, by tracing the change inrock color, in the extensive screefields between FiveFinger Gully and the slopes of Carn Dearg (N).

On the south side of Ben Nevis in Coire Eoghainn,the contact is exposed in four steep-sided gullies thatform the main drainage channels from the summitcrags (∼950 m above sea level). In all four gullies,there is clean rockexposed for severalmeters oneitherside of the contact. There is a thin chilled margin, afew millimeters wide, in the granite in the western-most gully. No banded rhyolite, “flinty crush rock,”fault, or separate marginal intrusion was observednear any of the contacts. In all of the gullies in CoireEoghainn, the unmodified igneous contact betweenthe InnerGranite and the volcanic pile is observed asdipping steeply outward to the south at 1707.

Interpretation and Discussion

Igneous Rock Volumes. The new 3D model of theBen Nevis Igneous Complex can be used to deter-mine accurate estimates for pluton and volcanic rockareas. The data used do not place any constraints onpluton thickness, which is needed to place an esti-mate on pluton volume. Observation data globallysuggest that plutons are stacked tabular bodies withthicknessT related to horizontal lengthL by a power-law relationship, where T p (0:65 0:15)# L0:650:1

(Cruden and McCaffrey 2001). The Outer Granitepluton has a tabular shape, with map dimensions of6.4 km # 8.5 km. On the basis of the power-lawrelationship detailed above and averaging the mapdimensions to give L as 7.5 km, the Outer Granite iscalculated to have a vertical thickness in the range1.2–3.0 km. It ismodeled to cover an area of∼21 km2

and is estimated to have a minimum remaining vol-ume (after erosion) in the range 26–64km3. The innerpluton has a tabular shape, with map dimensions of4.5 km # 5.5 km. On the basis of the power-lawrelationship detailed above and averaging the mapdimensions to give L as 5 km, the Inner Granite iscalculated to have a vertical thickness in the range1.0–2.3 km. It ismodeled to cover an area of ∼18 km2

and is estimated to have a minimum remaining vol-ume (after erosion) in the range 18–41 km3. The totalvolume of the plutonic complex is in the range 44–105 km3.

As modeled in three dimensions (fig. 4), the shapeof the contact and the base of the volcanic pile re-sembles a circular bathtub or bowl with steep sidesand a more gently sloping back dipping to the SWunder the North Face. When the shallower segmentof the contact is projected to the SW, it becomesapparent that the base of the volcanic pile is probably


not far below the current land surface and lies at adepth of 400–500 m above sea level. The pile has di-mensions 2.3 km # 1.8 km, with a modeled area of3.5 km2 around the summit of Ben Nevis, and is ap-proximately 600m thick. The model provides a min-imum remaining volume (after erosion) of 1.3 km3

for these rocks.Conceptual Models for Emplacement of the Plutonic

Rocks and Preservation of the Volcanic Pile. The pro-cesses driving granitoid magma emplacement andtheir crustal controls are a topic of long-standing de-bate and often controversy (e.g., Reynolds 1947; Read1957; Petford 1996; Pitcher 1997; Bouchez et al. 2013).Although transformational processes for granite for-mation (Reynolds 1947) have been consigned to his-tory, it is only in the past decade that a consensus hasemerged in favor of incremental, dike-fed laccolithmechanisms for pluton formation over diapirism (e.g.,Brown 2013).

Ben Nevis Igneous Complex Emplacement byCauldron Subsidence. Early surveyworkers (Maufe1910; Bailey and Maufe 1916) envisaged that forma-tion of the Ben Nevis Igneous Complex began withemplacement of the Outer Granite through gravita-tional sinking of a circular block of crust, in a processreferred to as cauldron subsidence, probably inspiredby similar proposals for the volcanic rocks at GlenCoe by Clough et al. (1909), the plutonic equivalentof caldera subsidence or collapse. As the crustal blocksubsided, it was argued that magma rose around themargins of the block to fill the void created at highercrustal levels and created the circular pluton of theOuter Granite (fig. 12A). The NE-SW-trending maficand felsic dikes that cut the Outer Granite, but are oflimited extent in the InnerGranite, were then arguedto have been emplaced during a phase of regionalNW-SE extension. Further subsidence of the centralblock of crust followed to allow emplacement of the

Figure 12. Emplacement models for the plutonic rocks forming the Ben Nevis Igneous Complex: A, Cauldron sub-sidence, modified after Bailey andMaufe (1916); B, Jacques and Reavy (1994);C, newmodel for emplacement of the BenNevis Igneous Complex involving ascent of early diorite magmas as steep-sided sheets trending NE-SW in the core ofthe Appin Syncline. D, Vertical ascent of magma followed by lateral flow from NW to SE to form the Outer Granite.


Inner Granite, and finally the overlying volcanic pilecollapsed into the still-molten Inner Granite magmachamber on a circular ring fault.

Reappraisal of the Ben Nevis Intrusive Ring Tuff.Afundamental componentof the cauldron-subsidencemodel is the existence of a ring fault, mapped as“flinty crush rock” by early workers (e.g., Bailey andMaufe 1916) or as an ignimbrite conduit, also calledthe Ben Nevis Intrusive Ring Tuff (Burt and Brown1997), forwhich supportingfield evidencewas drawnfrom mapping the banded rhyolite.

Petrographic evidence argues against a cataclasticorigin for the banded rhyolite. Although early sur-veyworkers (e.g., Bailey andMaufe 1916) referred tothe banded rhyolite as “flinty crush rock,” the crys-talline texture of these rocks does not resemble thedistinctive glassy textured veins of pseudotachylyteassociated with the caldera-bounding “ring fault” ex-posed farther south at Glen Coe (Kokelaar andMoore2006; Muir 2017; fig. 3).

New mapping shows that the banded rhyolite atthe edge of the Inner Granite is restricted to the NEside of Ben Nevis (fig. 7). It does not form a separatecircular intrusion surrounding the volcanic pile, cast-ing doubt on circular-ring-fault or ignimbrite-conduitinterpretations. In addition, there is no field evidencefor a ring fault between the Inner Granite and thevolcanic pile. The banding in the rhyolite at the edgeof the Inner Granite more closely resembles igneouslayering produced by laminar flow of viscous magmaand was probably generated during late stages of theascent and emplacement of the InnerGranitemagmas.

The nature of the marginal banded rhyolite whereit is developed provides key information on the re-lationship between the InnerGranite and the volcanicpile. As described above, the Inner Granite commonlyexhibits a thin chilled margin against the volcanicpile, and there is no evidence of granitic magma inter-acting with wet sediment or unconsolidated volca-nicmaterial. The contact is always sharp and planar,and the volcanic rocks close to the contact are horn-felsed. These lines of evidence indicate that the sed-imentary andvolcaniclastic rockswithin thevolcanicpile were cold and lithified at the time of emplace-ment of the Inner Granite magma. The gradationalnature of the internal contact with the banded rhyo-lite and the Inner Granite and the observation thatveins and sheets of the banded rhyolite that penetratethe volcanic pile on the north side of Ben Nevis havethin chilled margins and follow planar (brittle) jointswithin the pile suggest that the banded rhyolite ismore likely to be a comagmatic phase, probably causedby chilling of the magma close to the margin, ratherthan a later cross-cutting intrusive body.

In summary, there is no field or petrographic evi-dence for a ring fault or a separate marginal intru-sion surrounding the volcanic pile at BenNevis. Thebanded rhyolite, which is found only close to thecontact on the NE side of the mountain, is inter-preted as a finer-grained comagmatic phase of theInner Granite, similar in composition to amphiboleand biotite granite in the extreme northern part ofthe pluton (Burt and Brown 1997). It is suggested,therefore, that the concept of a ring fault at BenNevis and the name Ben Nevis Intrusive Ring Tuffshould be abandoned.

Dike-Controlled Pluton Emplacement Mechanism.More recent studies on granite emplacement mech-anisms have demonstrated that granite plutons, in-cluding those with near-circular outcrop patterns atthe present level of exposure, have been constructedfrommultiple pulses of magma ascending as narrowdikes and sheets and feeding a laccolith or sill (e.g.,Hutton 1992; Petford et al. 1993; Cruden 1998; Mc-Carthy et al. 2015), often with a strong structuralcontrol on the location of the magma ascent path-way and the final site of emplacement. Transport ofmagma via a series of steep-sided feeder dikes allowsrapid vertical ascent of the magma to a level wherethe magma can spread laterally, often along a majordiscontinuity such as the brittle-ductile transition(Brown 2013), or even at the base of the water table(Leake andCobbing 1993). In three dimensions,manyplutons are actually stacked, relatively thin (≤3 km),flat-lying tabular bodies rather than steep-sided cyl-inders that extend for many kilometers below theEarth’s surface (e.g., McCaffrey and Petford 1997; Cru-den et al. 1999; Stevenson et al. 2007; Horsman et al.2009; Miller et al. 2009).

Reevaluation of a previously interpreted ring com-plex forming thePaleogeneEasternMournepluton inNorthern Ireland using anisotropy of magnetic sus-ceptibility (AMS) data favors a laccolithic-inflationemplacement model over cauldron subsidence (Ste-venson et al. 2007). Similarly, reassessment of the“classic” ring dike in the Paleogene Slieve Gullionring complex using magmatic fabric and AMS dataindicates that it is a former volcanic rhyolite ashflowdomed during later laccolith emplacement and in-flation (Stevenson et al. 2008). Evidence for calderaresurgence—for example, the ∼33 Ma Bonanza Cal-dera of the San Juan Volcanic Field in Colorado (Lip-man et al. 2015)—shows features consistent with in-flation of laccolithsmodeled to be at depths of 1–5kmbeneath thecaldera roof (BrothelandeandMerle 2015),which in the case of the 15 # 17-km Bonanza Cal-dera initially collapsed to a depth of 3.5 km beforeresurging by a similar amount. The Bonanza Caldera


resurgent dome is steep sided, with dips of up to 807on its flanks. Even more recently, the 2011 pyro-clastic eruption of Cordón Caulle in Chile was as-sociated with surface deformation indicating intru-sion of a 0.8-km3 laccolith body at a 200-m depth(Castro et al. 2016).

Evidence for Emplacement Mechanism. There isabundant evidence for dike-related granitoid magmaemplacement at Ben Nevis. The Dalradian countryrocks and the Outer Granite are cut by an extensiveswarm of steep-sidedNE-SW-trendingmafic (dioriticand lamprophyric) and felsic (microgranitic) dikes inapproximately equal proportions (fig. 2). Bailey andMaufe (1960) referred to these collectively as the BenNevis dike swarm and suggested that they are coevalwith the large swarmof lateCaledoniandikes centeredon the Etive Igneous Complex to the south (fig. 1).Matching features on the opposing sides of the dikewalls indicate that the majority (90%) of the dikesin the BenNevis areawere emplacedduringorthogonalopening of a pervasive set of NE-SW-trending frac-tures. However, offset margins of some of the earlydioritic and granitic sheets intruding the Dalradianrocks at the outer edge of the Ben Nevis Igneous Com-plex indicate that there was a dextral component ofshear during emplacement. This is in contrast to thelateCaledonian EtiveDike Swarm,which is thoughtto have been emplaced during sinistral transpression(Morris et al. 2008). The widespread occurrence ofNE-SW-trendingmafic and felsicdikes cutting throughall of the plutonic rocks indicates that the same tec-tonic stresses responsible for generating the initialfeeder dike system for the Outer Granite remainedactive for an extended period of time. The locationand orientation of the feeder system for the InnerGranite pluton have not been identified, although itsorientation (fig. 2) and the observation that it is incontact with the Dalradian country rocks along thesouthernmargin of the igneous complex provide someevidence that the Inner Granitemay have grown froma series of dikes striking more NNE than inferredfeeder dikes for the Outer Granite.Examination of the carapace and roof pendants of

Dalradian metasedimentary rock shows them to beextensively brecciated and net-veined by graniticmaterial. These steep-sided sheets and veins, whichmerge and coalesce rapidly over a fewmeters towardthe main body of the igneous complex, may be ves-tigial evidence of the pre- or syninflation emplace-ment mechanism and may represent relics of feederdikes for the Outer Granite.Pre–full crystallization fabrics record ductile flow

within a pluton during the final stages of solidifica-tion (e.g., Ingram and Hutton 1994) and can be used

to determine flow directions in a magma chamber,possibly identifying the location of feeder dikes andconduits. In addition, new tectonic fabrics will oftendevelop in the surrounding country rocks in responseto the emplacement and inflation of a laccolith pre-cursor to a pluton (e.g., Stevenson 2009). Examplesof pre–full crystallization fabrics within the Innerand Outer Granites are rare, but the long axes ofmafic enclaves and country-rock xenoliths are alignedNW-SE in the Outer Granite on the NW side of thecomplex. This observation, together with the nar-row envelope of deformed and brecciated countryrocks along the SEmargin, suggest that the complexwas filled and inflated in a southeasterly direction(fig. 12C, 12D). Late-stage submagmatic deformationof the pluton associated with flow of magma fromNW to SE would account for the alignment of theenclaves and xenoliths as well as the structures ob-served around the SE margin. Large-scale structuralpatterns in the country rock and the steep dips ofthe granite carapace of Dalradian metasedimentaryrock also support doming associatedwith a laccolith-inflationmodel, particularly in light of evidence fromthe Bonanza Caldera (Lipman et al. 2013). Furtherevidence comes from field evidence that the earliestintrusive phases of the Outer Granite were fine- tomedium-grained quartz diorites, suggesting either quiteshallow emplacement or emplacement into cold coun-try rock. The circular outcrop pattern of the igneouscomplex and the curvature in plan of the axial sur-faces of the Appin Syncline and Stob Ban Synformin the country rock may represent deformation dur-ing inflation of a laccolith to form a tabular pluton atthe final site of emplacement (e.g., McCarthy et al.2015). Consequently, the plutons probably compriseat least two subhorizontal laccolithic bodies (theOuter and Inner Granites), rather than steep-sidedstocks formed by cauldron subsidence.On the basis of the field observations made during

this study, a cauldron-subsidence model is not sup-ported. An alternative model is proposed where em-placement of the Ben Nevis Igneous Complex ini-tiateswith emplacement ofmultipleNE-SW-trendingfeeder dikes filling and inflating a laccolith or sillprogressively in a SE direction. In this model, theoldest components in the Outer Granite, the dioriticphases on the NW side of the complex, were em-placed as narrow, steep-sided dikes in the core of theAppin Syncline (fig. 12C). Bowes and Wright (1967)have previously noted that theAppinite Suite,whichis the earliest phase of magmatism associated withthe Caledonian granites, is also located in the coreof the Appin Syncline in the type area SW of BenNevis at Kentallen (fig. 1). Controls on emplacement


of the Inner Granite are less clear, but the elongatedcircular shape inmapviewof theOuterGranite (fig. 2)is consistent with a continued process of inflationduring laccolithic emplacement of the Inner Granite.

Pluton Petrogenesis. Modeling of pluton compo-sition can be used as an indicator of the depth andnature of themagmatic source and can inmany casesalso provide information on the tectonic setting ofpluton petrogenesis (Chappell et al. 1988; Janoušeket al. 2000). The intrusive rocks at Ben Nevis canbe divided into two distinct chemical compositionalclasses. The porphyriticmonzogranite and appinite ofthe Outer Granite indicate that they have a potassicmagma composition common to widely recognizedcomponents of late Silurian to Early Devonian mag-matism in Scotland (Neilson et al. 2009; Miles et al.2016). These rocks are modeled to have a componentsourced in metasomatized sublithospheric mantletapping a Mesoproterozoic subduction component(Neilson et al. 2009). The plagioclase-rich trondhjem-ite composition of the later Inner Granite suggeststhat it had a very different origin from the OuterGranite. Rocks of this type are more sodic in com-position and form part of the trondhjemite-tonalite-granodiorite suite often informally called adakites(Martin et al. 2005). They are modeled to be derivedfrom partial melts of hydrated basaltic crust underhigh pressure (argued on modeling grounds to be inthe garnet stability field; Martin et al. 2005, 2014).On the basis of these inferences, sources for theOuterand Inner Granites are unlikely to have come fromthe same part of the crustal/mantle columnand, giventhe absence of evidence for intermediate composi-tion or mixed magmas, are unlikely to have beenemplaced at the same time. This would provide sup-porting evidence for a very deep-seated and long-lived crustal conduit for magmatism at Ben Nevis.

As amore recent example of spatially associated po-tassic and sodic pluton emplacement, in the juvenileIzu-Bonin Arc, Neogene magmatic complexes con-taining contemporaneous trondhjemitic and monzo-granitic plutons have been observed (Saito and Tani2017). Geochemical analysis and modeling of theplutonic rocks indicate that these require very dif-ferent lower-crustal sources. The trondhjemitic rocksare interpreted to be sourced in juvenile basaltic lowercrust of the Izu-Bonin Arc; later monzogranitic rocksare interpreted to be sourced in hybrid K-rich rear-arc lower crust of the Izu-Bonin Arc and metasedi-mentary rocks of the Honshu Arc after arc collision.The Izu-Bonin Arc case implies a spatially varyingsource as a result of tectonic amalgamation, whichis not unreasonable in the case of the Caledonian ofScotland and for the magmatic rocks at Ben Nevis.The heat source for late Caledonian magmatism is

argued to be gravitationally driven delamination ofthe lithospheric mantle following Iapetus closure(Miles et al. 2016), which hadwidespread effects andis likely to have resulted in granite magma genera-tion from a range of sources.

Regional Structural Controls on Caledonian Mag-matism. Severalworkers (e.g.,Watson 1984;Huttonand Reavy 1992; Jacques and Reavy 1994) have sug-gested that the emplacement of the Caledonian gran-ites in the Scottish Highlands was controlled bymajor NE-SW-trending faults, including the GreatGlen, Etive-Laggan, and Ericht-Laidon Faults (fig. 1).There is considerable debate over the timing, senseof movement, and amount of displacement on all ofthese faults, but it is generally accepted that sinistraldisplacements of several tens of kilometers occurredduring the late Silurian/Early Devonian (e.g., Trea-gus 1991; Stewart et al. 1999). The Ben Nevis Igne-ous Complex lies on a NE-SW-trending array of plu-tons that can be traced from Ballachulish in the SWthrough to the Corrieyairack and Allt Crom plutonsin theNE (fig. 1). The plutons are bounded to theNWby the Great Glen Fault Zone and to the SE by theEtive-Laggan Fault.

Some of the Caledonian granites in the SW High-lands lie at the intersections between these NE-SW-trending faultsandanorthogonal setofNW-SE-trendinglineaments. TheNW-SE-trending features, which in-clude the Strath Ossian and Cruachan lineamentshighlighted infigure 1, are thought to represent olderbasement structures, perhaps related to the archi-tecture of the original Dalradian sedimentary basin(Fettes et al. 1986; Graham 1986). Jacques and Reavy(1994) proposed that granitic melts, generated in thelower crust during the final transpressional stages ofthe CaledonianOrogeny, used the intersection pointsbetween the NW- and NE-trending structures asmagma conduits to reach higher crustal levels. Theirmodel requires sinistralmovement on the orthogonalstructures to produce a zone of extension at the pointof intersection (fig. 12B). According to Jacques andReavy (1994), the Ben Nevis Igneous Complex is sit-uated at the intersection between a NE-SW-trendingstructure termed the Ballachulish-Corrieyairack ShearZone and a NW-SE-trending structure termed theRannoch Moor Line. However, Burt and Brown (inBurt et al. 1996) and Pattison and Harte (1997) haveboth pointed out that there does not appear to be anydirect evidence for the existence of these two struc-tures in the vicinity of Ben Nevis; nevertheless, asoutlined above, the compositional differences betweenthe Outer and Inner Granites require a repeatedlyused deep-seated magmatic conduit.

Origin of the Volcanic Rocks. The rocks of thevolcanic pile are very coarse grained and fragmental,


requiring an explosive origin (Fisher and Schmincke2012). This is supported by their intermediate to acidcompositions. The thickness of individual units isup to 10m, and the size of the largest observed clasts,11 m, suggests that the deposits originated as domecollapse (fig. 13) or caldera-related pyroclastic block-and-ash flows rather than pyroclastic surges (FisherandHeiken1982).However, thefiner-grained tuff unitsat North Trident Buttress and on Ledge Route mayhave formed as a pyroclastic surge deposit and provideevidence for a more complex eruptive history. Someof the mapped clasts are large, for example, the 1 #1.5-m blocks of Dalradian schist recorded within theAllt a’ Mhuilinn Mudstone Formation and volcanicclasts up to nearly 2 m in diameter in the Coire naCisteMember of the BenNevis Volcanic Formation.

Comparison with block-and-ash flow deposits sim-ilar in thickness, lateral extent, and block size sug-gests that the volcanic source might have been situ-ated up to several tens of kilometers away (Freundtet al. 1999). The center could lie within the confinesof the present outcrop of the Ben Nevis Igneous Com-plex, perhaps overlying the early dioritic rocks in thecore of the Appin Syncline, or even on the oppositeside of the Great Glen Fault above the current loca-tion of the Glen Loy basic igneous complex (fig. 1). Adistal origin for the volcanic rocks questions whetherthey should be included in the Ben Nevis IgneousComplex or assumed to be part of a larger Caledo-nian volcanic province that includes theLornPlateaulavas and the Glencoe Volcanic Formation (Morriset al. 2008; Neilson et al. 2009; Upton 2015).

Figure 13. Diagram showing development of block-and-ash flow deposits by gravity segregation from a turbulentpyroclastic flow produced by volcanic dome collapse (modified after Fisher and Heiken 1982). Wide downward arrowsshow trends of bulk movement of greatest volume of fragments. The zone preserved within the volcanic pile in BenNevis represents volcaniclastic material deposited on the flank of a larger volcanic structure that lay to the NW of BenNevis.


Lamprophyric dikes of theNE-SW-trending swarmare compositionally similar to clasts of hornblendicandesite within the Ben Nevis volcanic pile (e.g.,Lange and Carmichael 1991). The parental horn-blende andesite lavas might be expected to have lo-cal subvolcanic lamprophyre or hornblende andes-ite dike feeders preserved. The lack of evidence formembers of theNE-SW-trending dike swarmcuttingthe volcanic pile, even though dikes of this type cutthe surrounding country rock, argues against thesedikes representing a local volcanic feeder system toa local vent. The evidence is more consistent withNE-SW-trending dikes feeding a subvolcanicmagmachamber or vent that resulted in the eruption butwas centered elsewhere.

Preservation of the Volcanic Pile. As describedabove, the rocks forming the volcanic pile are dom-inated by volcaniclastic debris flows and pyroclasticblock-and-ash flow deposits, with minor ashfall tuffunits. The deposits are enclosed on all sides by gra-nitic rocks of the Inner Granite pluton. There are nomajor volcanic centers, conduits, or ventswithin thevolcanic pile and no extrusive lava flows. Sedimen-tological data summarized in this article indicatethat the debris flows and volcanic detritus were de-rived from a volcanic center that lay to the NW ofBen Nevis, perhaps several tens of kilometers away.

In addition, there is no field or petrographic ev-idence for a ring fault surrounding the volcanic pileor a separate marginal intrusion that represents anignimbrite conduit, to support the suggestion thatthe volcanic pile at Ben Nevis represents the deeplevels of a caldera that collapsed into the underlyingInnerGranitemagma chamber. In the absence of anyclear evidence for caldera collapse, it seems morelikely that the volcanic pile is simply a large roofpendant or keel of the former volcanic land surfacethat once covered much of the SW Highlands. Vol-canic rock pendants of this type are known from theSierra Nevada batholith in California (Fiske and To-bisch 1978), where the largest is 30 km# 20 km andincorporates a 1-km-thick lens of caldera-collapsebreccia. A volcanic keel would have been enclosedby the Inner Granitemagmas, protecting it from latererosion. This requires the granite magma to haverisen to shallow levels in the crust (toprobably arounda 1-km depth; Haslam 1968). It is not necessary toinvoke any form of caldera-related subsidence toaccount for the field relationships observed at thepresent day.

Comparisons with the Lorn Plateau Lavas and theGlencoe Volcanic Formation. It has been suggestedthat many of the Caledonian granite plutons in theSWHighlands of Scotlandhad extrusive equivalents,fed by the extensive swarms ofNE-SW-trendingmafic

and felsic dikes, and a small number of larger volcaniccenters (Morris et al. 2008;Neilson et al. 2009;Upton2015). In addition to the volcanic pile at Ben Nevis,there are three other remnants of late Silurian toEarly Devonian volcanic rocks in this area: the LornPlateau lavas, the Glencoe Volcanic Formation, anda small screen of altered volcanic material at the SEedge of the Etive Complex (fig. 1). At Glen Coe andon the west side of the Lorn Plateau lavas, the vol-canic sequences unconformably overlie Dalradianmetasedimentary rocks and have basal units con-taining clasts of schist and quartzite, like the clastlithologies observedwithin themudstones at the baseof the volcanic pile at BenNevis. However, there areno other examples of thick block-and-ash flow de-posits on the scale observed at Ben Nevis.

Lorn Plateau Lavas. The Lorn lavas form thelargest outcrop of late Silurian to Early Devonianvolcanic rock in the SWHighlands, covering an areaof ∼300 km2, and consist of ∼800 m of potassic, ba-saltic andesite lavas and minor sediments on thesouth side of the Pass of Brander Fault, composition-ally comparable to andesitic clasts within the Coirena Ciste Member of the Ben Nevis Volcanic Forma-tion (Kynaston et al. 1908; Trewin and Thirlwall2002). Individual lava flows range in thickness from5 to 30 m and often exhibit weathered tops, indi-cating periodsof erosionbetween the individualflows.Many of the flows are laterally extensive and can betraced for several kilometers dipping gently to the SEat !107, an orientation similar to that of the thicksequence of volcaniclastic deposits within the vol-canic pile at Ben Nevis.

The location of the feeder dikes or pipes for theLorn lavas has not been observed, but it is generallyassumed that they were fed by members of the lateCaledonianNE-SW-trending dike swarm (Upton2015).Plutonic rocks to the south of the lava pile aroundKilmelford (fig. 1) are chemically similar to the lavapile andmay represent their plutonic equivalent (Tar-ney and Jones 1994). On the north side of the Pass ofBrander Fault, a narrow belt of andesitic lava formsa screen (the Beinn a’Bhuridh screen) at the SE edgeof the Etive Complex. The lavas in the screen arehornfelsed and heavily altered but are probably coe-val with the Lorn lavas to the south (Highton 1999).

Glencoe Volcanic Formation. At Glen Coe, athick sequence of late Silurian to Early Devonianvolcanic and related sedimentary units (11 km inthickness) is contained within a NW-SE-trendinggraben, 14 km in length and about 8 km across atthe widest point (fig. 1). The graben sits within andis floored by tightly folded Dalradian metasedimen-tary rocks. The rockswithin the graben are dominatedby pyroclastic deposits and consist of at least seven


thick ignimbrite sequences (with individual depositsup to 80 m thick), intercalated with sediments andintruded by numerous andesitic and rhyolitic sheetsand sills (Moore and Kokelaar 1997, 1998; KokelaarandMoore2006). Several eruptive centers andmagmaconduits can be identified within the graben, and thewhole sequence is cut by NE-SW-trending dikes be-longing to the Etive Dike swarm.At the SE end of the graben, early explosive vol-

canic activity produced a thick sequence (∼70 m) oflaminated tuffs and breccias, collectively referred toas the Kingshouse Tuffs (Kokelaar and Moore 2006).It is inferred that the source vent or cone that pro-duced the tuffs lay to the NW, with pyroclastic den-sity currents flowing toward the SE. There is con-siderable evidence for subaqueous deposition, withwidespread liquefaction features such as convolutelaminations andflamestructures.The strata also showevidence of sliding and slumping along low-angledetachment surfaces, like the features observedwithinthe laminated tuffs within the Summit Member atBen Nevis.In contrast to the geology of Ben Nevis described

here, rockswithin theGlenCoeGraben are preservedin a downfaulted block developed during piecemealsubsidence of a major caldera volcano (Kokelaar andMoore 2006). Subsidence was controlled by a set ofearly NW-SE- and NE-SW-trending faults and a laterring-fault system that consists of numerous separatefault strands, jogs, and bends rather than a simpleannular structure. The total amount of subsidencewithin the graben is poorly constrained, with esti-mates ranging from a minimum of 600 m to morethan 1.5 km.The relationship between plutonic and volcanic

rocks at Glen Coe resembles that for the new obser-vations at BenNevis. For example, the eastern end oftheGlenCoeGraben and the volcanic sequencewereintruded by high-level granite plutons, including thehigh-K, calc-alkaline I-type ∼418 Ma Clach Leathadpluton (Neilson et al. 2009). These are of similarcomposition to the Outer Granite at Ben Nevis andpostdate the eruptive history of the volcano andmove-ment on the ring-fault system. These late intrusionswere emplaced at high levels in the crust and containdrusy cavities and large roof pendants of Dalradiancountry rock. The plutonic intrusive relationshipsat the eastern end of the Glen Coe Graben are likethose observed at Ben Nevis, with late posttectonicgranitic bodies reaching high levels in the crust andpreserving the base of the late Silurian to Early De-vonian volcanic landscape.Prevolcanic Environment. The abundance and

variety of volcanic clasts within the volcanic pileindicates that there was an extensive preexisting vol-

canic landscape beyond the wider Ben Nevis area atthe time of deposition. At Ben Nevis, the oldest unitat the base of the volcanic pile, the Allt a’ MhuilinnMudstone Formation, comprises mainly mudstoneand laminated siltstone, with lenses of conglomeratedominated by quartzite clasts. As described here, soft-sediment deformation and water-escape structuresin the mudstones and siltstones and loading struc-tures between the upper part of the Allt a’ MhuilinnMudstone Formation and the overlying Ben Nevis Vol-canic Formation indicate that the sediments werenot lithified when the overlying volcaniclastic debrisflows were deposited. The Allt a’ Mhuilinn Mud-stone Formation appears to indicate a freshwater la-custrine environment, intowhichquartzite-dominatedconglomerates were deposited as subaqueous debrisflows before the onset of volcanism represented bydeposition of the Ben Nevis Volcanic Formation.Insufficient evidence is available to infer any paleo-drainage sense or orientation. In the Ben Nevis Vol-canic Formation, the volcaniclastic debris flows andlahars forming the Coire na Ciste Member show nodistinguishing features that would indicate eithersubaqueous or subaerial deposition. The block-and-ash flow deposits forming the Summit Member areinterpreted as pyroclastic deposits generated as a re-sult of the repeated eruption and collapse of a volca-nic center or dome that lay to theNWof the summitof Ben Nevis (fig. 13).Elsewhere, comparable rocks in the Lorn lava pile

are underlain by fluvial conglomerates that rest un-conformably on theDalradian basement aroundObanand on the island of Kerrera (fig. 1), and, similarlyto the conglomerate lenses observed in the Allt a’MhuilinnMudstone Formation, clasts within the con-glomerates include an abundance of quartzite, lo-cally derived from the Dalradian basement, as wellas andesite and a wide variety of granitic material.Paleocurrent data suggest that the source region forthe clasts lay to theNE (Morton 1979). The presenceof Dalradian clasts within the basal sediments un-derlying the Lorne lavas and at Ben Nevis and GlenCoe indicates that there were probably considerablerelief and well-developed fluvial systems at the timeof deposition.At Glen Coe (Moore and Kokelaar 1997, 1998; Ko-

kelaar and Moore 2006), there is evidence for a long-lived fluvial system with repeated incision and depo-sition of alluvial and lacustrine sediments throughoutthe eruptive history. Sedimentological data indicatethat themain drainage systemflowed along themainaxis of the graben from SE to NW.Wider Comparisons. To find more recent ex-

amples of comparable volcanic and plutonic rocks, itis necessary to look to the Paleogene and Neogene


supervolcanoes of the western United States (e.g.,Geist andRichards 1993; Lake and Farmer 2015). Thepost-Laramide 40,000-km3 Oligo-Miocene San Juanvolcanic field in Colorado (e.g., Lake and Farmer2015), covering an area approximately half that ofScotland, was probably linked to rollback of the Fa-rallon slab, with a progression from andesitic tomorerhyolitic magmas with time (Best et al. 2016). It pre-serves deposits from some of the largest caldera-forming eruptions known, such as the 5000-km3 FishCanyon Tuff (e.g., Bachmann et al. 2002), with cal-deras up to 75 km in diameter (Lipman et al. 2013).Andesitic and dacitic block-and-ash flows are com-mon, with clast sizes in the range recorded from BenNevis. These form deposits up to 1 km thick and upto 200 km3, extending up to 50 km from their sources(e.g., Bachmann et al. 2000; Parat et al. 2005). A la-custrine preenvironment is recorded, confirming a ter-restrial depositional environment (Larsen and Lipman2016). The volcanic rocks are abundantly linked toand intruded by calc-alkaline plutonic complexes(Lipman 1984). Volcanic deposits are preserved inareasof caldera collapse and resurgence (Lipmanet al.2015) and in tectonic grabens (Lipman et al. 1978).There are many similarities between magmatism ofthe San Juan Volcanic Field and the late Caledonianmagmatism of Scotland, implying a large thicknessand area of original deposits that are likely to havecovered the Caledonian landscape. These were prob-ably orders of magnitude larger than the 1.3-km3

volcanic keel preserved at Ben Nevis. It seems prob-able that the keel represents exceptional preserva-tion, and its evidence for the location of source vol-canic centers should be evaluated with caution.

Summary and Conclusions

Detailed field mapping and 3D model building haveprovided important new insights into the geometry,emplacement, and preservation of the Ben Nevis Ig-neous Complex in the SW Highlands of Scotland.The composite intrusion has an elliptical outcroppattern with dimensions of 6.4 km # 8.5 km and isexposed over a wide range in altitude, from 20 mabove sea level in Glen Nevis to 1345 m at the sum-mit of Ben Nevis. The presence of large fragments ofcountry-rock carapace, screens, and roof pendants ofDalradian material around the outer margins of thecomplex indicates that the current level of exposure isclose to the roof zone of the plutonic rocks, whichwere probably laccolithic in form.

The oldest componentwithin the complex, referredto by early workers as the volcanic pile, is a thicksequence (1600 m) of sedimentary rocks and volca-niclasticmaterial. The pilewas previously interpreted

as a thick sequence of andesite lavas and agglom-erates that were downfaulted during caldera subsi-dence. Detailed mapping of the summit region ofBen Nevis presented here, including both the steepNorth andSouth Faces of themountain, has revealedthat the volcanic pile consists largely of volcaniclasticdebris flows and extensive block-and-ash flow depos-itswithminor air-fall tuff units. The volcanic detritusis more likely to have been derived from a volcaniccenter that lay to the NW of Ben Nevis, perhaps sev-eral tens of kilometers away. Igneous blocks withinthe block-and-ash flow units are predominantly da-cites and rhyodacites, and there is no evidence for anyandesite lava flows, vents, or major igneous intru-sions within the volcanic pile. In addition, there isno field or petrographic evidence for a ring fault orignimbrite conduit at the outer edges of the pile, and3D modeling of the igneous complex described heredoes not require it. The volcanic pile is reinterpretedas a large roof pendant or keel of the former late Si-lurian to Early Devonian land surface that once cov-ered much of the SW Highlands. The keel has beenprotected from erosion by the surrounding plutonicrocks that form the bulk of the Ben Nevis IgneousComplex.

The results of new field mapping of the plutonicrocks that surround the volcanic pile indicate thatthe earliest intrusive phases were fine- to medium-grained quartz diorites that were emplaced as steep-sidedNE-SW-trending sheets and dikes in the core ofthe Appin Syncline. The quartz diorites are now re-stricted to the outer margins of the complex, mainlyon theNWside, andmerge inward to a coarse-grainedporphyriticmonzogranite. Together the early dioritesand the porphyritic monzogranite are referred to astheOuterGranite.Alignedmaficenclaves andcountry-rock xenoliths within the Outer Granite also recordthe lateral flow of magma or laccolithic inflationof the pluton in a NW-SE direction, at right anglesto the early NE-SW-trending feeder dikes. The roofzone of the Outer Granite lies close to the summitridges surrounding Ben Nevis (around 1300–1400 mabove sea level), but the depth to the base of thecomplex cannot be determined from the availablefield data.

The youngest plutonic phase, the Inner Granite, isa composite pluton in the SE part of the complex andencloses the volcanic pile on all sides. The plutonsits mainly within the Outer Granite but is in con-tact with the Dalradian country rocks along thesouthernmargin of the igneous complex. The roof ofthe pluton appears to lie not far above the summit ofBen Nevis, and inward-dipping contacts at loweraltitudes suggest that thefloor of the intrusion lies ata depth of ∼1000 m below sea level.


The Dalradian country rocks and all of the plu-tonic phases forming theOuter Granite are cut by anextensive swarmofNE-SW-trendingmafic and felsicdikes known as the BenNevis dike swarm.There area few examples of NE-SW-trending dikes cutting theInner Granite, but no members of the dike swarmhave been identified within the volcanic pile. Offsetmargins of some of the early feeder dikes for theOuter Granite and the main Ben Nevis dike swarmindicate that there was a dextral component of shearduring emplacement. Some of the dikes also exhibitflow banding close to wall-rock restrictions. A re-stricted zone of similar flow banding in the InnerGranite at the eastern edge of the volcanic pile is alsointerpreted as a magmatic fabric developed duringemplacement of the pluton rather than a separatemarginal intrusion or a product of mechanical shear-ing during caldera subsidence.This new study has demonstrated the importance

of detailed field mapping and 3D model buildingto help evaluate models for the geometry and intru-sive history of the Ben Nevis Igneous Complex. Acarefully designed study of the anisotropic magneticmineral susceptibility would help to evaluate thelaccolith-emplacement model presented here to sep-arate diking-related fabrics from those related to in-flation of the pluton. There is still a requirement forhigh-resolution U-Pb zircon dating of the differentcomponents within the igneous complex, particu-larly the outer and inner plutons, and a detailed pet-rographic, geochemical, and isotopic study of a rep-resentative set of igneous clasts from the volcanicpile (e.g., Haughton 1988) would also help to revealthe nature of the lost volcanic landscape that oncecovered much of the SW Highlands of Scotland.

ACKNOW L EDGMENT S

Fieldwork on the North Face of Ben Nevis formedpart of a detailed botanical and geological survey co-ordinated by theNevis Landscape Partnership (NLP).Funding for the three-year North Face Survey (2014–2016) was kindly provided by the Heritage LotteryFund, Scottish Natural Heritage (SNH), and the High-land Council. T. Semple and L. Pate from the NLPand C. Mayne from SNH are especially thanked fortheir support and encouragement during the survey.Support has also been provided by Midland Valley,Cotswold Outdoor, and the outdoor equipment man-ufacturers Mammut and DMM. The Scottish Moun-taineering Club kindly provided access to the CIChut during the survey. Safe access to the North Facewould not have been possiblewithout the skill, energy,and enthusiasm of all of the professional climbers in-volved in the survey: M. Pescod (Abacus MountainGuides),D.King,A.Halewood,S.Kirkhope,D.Buckett,D.Anderson,W.Rowland,C.Holdsworth,A.Hague,J. Cooper, and E. Holt. A. Austin and B. Fyffe, fromthe John Muir Trust, are also thanked for bringingtheir geological skills to the mountain. S. Nicol andher team of trainee volunteer rangers from the NLPmoved several kilometers of rope up and down themountain in all weather and also helped to informand entertain the general public at the same time.D. and C. MacLeod are thanked for introducing R. J.Muir to the NLP and for their hospitality at Roy-bridge. R. Burt, J. Grocott, and all of the MidlandValley team are thanked for numerous discussionson the geology of BenNevis over the past three years.Constructive reviews by J. Dewey, J. Cole, and C. Ste-venson helped to improve the final manuscript.

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