Johnson, Accepted Article Journal the Grampianterrane, metamorphi low–pressure, high–temperature regional for locality the type Block, oftheBuchan evolution the to understanding criticalinformation andInzieHeadcontain between Fraserburgh along theScottishcoast Rocks exposed ABSTRACT title: Short Scotland North StAndrews,KY169SY, Street, WA 6845,Australia([email protected]) University,GPO BoxU1987,Perth 2 1 T. E.JOHNSON Grampian migmatites intheBuchanBlock, NEScotland This article is protected by copyright. All rights reserved. reserved. Allrights copyright. by protected is This article doi: 10.1111/jmg.12147 lead to differences between and Version this version of the Record. Please cite as thisarticle may which and proofreading process, pagination copyediting, typesetting, the been through hasThis article accepted forpublicati been gradient records agradientincrease temperature records of field metamorphic modelled The in Mn. rocks source the depleted aplites) garnet-bearing providing that peritecticmelts thethe first garnet producedwith formedby (represented appear satisfactorily reproducestheobserved represen a plausible as pelite mid-Dalradian Head and thatthe Inzie event, tectonothermal mid-Ordovicianthe result theoldermetamorphism, (Grampian)an of than rather anatexis was that alternative interpretation an supports Thisobservation zone network. shear themetamorphicthat straddles field gradient continuous a preserve Thus,therocks absent. are fabrics shear where Fraserburgh, around occurred pegmatites, associated and aplites garnet-bearing formationof the associated with was which metapelitic rocks, in melting fieldobservationGrampian However, orogenesis. during current position into their were thrust Precambrianallochthonous basement rocksthat as regarded have Block, been Buchan the within units gneissose with other along Head, me evidenceforpartial preserve unequivocal and facies to granulite amphibolite from upper in grade increase rocks metasedimentary The metamorphism. post-dating late structure, as a regarded is commonly open foldthat large-scale a Anticline, Buchan core the intothe of Head,passes aroundInzie highest grades the at of and, shear zones network aregional longsectiontraverses The~8km Caledonides. Department of Earth & Environmental Science, University of St Andrews, IrvineUniversity Building, StAndrews, & of Science, Environmental of Earth Department TheInstituteGeology, ofApplied Department of T. Metamorphic and Kirkland, Grampian migmatites in theBuchanBlock Grampian migmatitesin Geology. C. 1* and , C.L.KIRKLAND Reddy, 33

one of the major tectonostratigraphicof theScottish components ofthe one (7): pp. S. and 695–709. Fischer, 1 , S. M.REDDY , S. S. atleast150°C(fromaround 650°Cnear 2015. on and undergone full peer review but has not full peerreviewbuthas undergone and on tativemodelling protolith, phaseequilibria lting. The diatexite migmatites Thediatexite around Inzielting. ance and disappearance of key minerals key of ance anddisappearance gneisses are autochthonous. Using an average an average Using autochthonous. are gneisses Grampian for Geoscience Research (TIGeR), Curtin (TIGeR),Curtin Research Geoscience for s show thatinsitu partials show theonsetof sm, and its relationship with the rest of rest withthe its relationship sm, and migmatites 1 &S.FISCHER in the Buchan 2 Block, NE Scotland. Accepted Article Based on a Rb–Sr whole rock age of 691 ± 39 Ma (Sturt 691 ±39Ma(Sturt of wholeage rock Rb–Sr a Based on metamorphic) structure (Stephenson (post- alate tobe considered fold open large-scale a Anticline, Buchan core the the of east the towards grade metamorphic in increase Therocks network. zone shear the regional straddle rocks that metasedimentary grade (Stephenson regardedasautochthonous commonly terraneallochthonous (Sturt an torepresent Block, ofthe Buchan rocks theoverlying and,with age Precambrian evidence, and geochronological structural basis of the (Fig.1a).On zone shear network with the coincident are broadly that number oflocalities Carty Anticline. Buchan ofthe age the and gneisses metamorphic ofmigmatitic the history the betterunderstanding aim of the modelling with equilibria withphase combined observations field using and InzieFraserburgh Head &Sturt, Viete orogenesis (Ramsay 1979; thrust rocks ofPrecambriansliver basement (Ashworth, 1975; Baker & Droop, 1983;Johnson (Ashworth, 1975;Baker &Droop, metamorphism inthemid-Ordovician ( 1979; Viete metamorphism inthe Neoproterozic (c. 1000–700 Ma) (Viete debated still rocks is 471 Ma(Dempster atand ultramaficaround mafic rocks andtem 1994)thatare spatially 1986; Goodman, (Ashcroft zones ofshear system a regional Grampian Highlands. in the rocks elsewhere 1979),metamorphism forBarrovian whichcharacterises that & Hudson, required than (Harte ( ( forhigh–temperature locality the type contains Block Buchan terrane. The oftheGrampian rest the to compared history metamorphic and structural sedimentological, Grampian in thenorth-east bound crustalpackage structurally- a discrete is Block 1925). The Buchan 1893,1912;Read,1923;Tilley, (Barrow, metamorphism to ourunderstandingof fundamental thathavebeen Dalradian Supergroup me by Fault,aredominated Boundary Highland the by and tothesouth the Fault Great boundtothenorthby Glen The GrampianHighlands, INTRODUCTION manganese. migmatite; metapelitic Key words: metamorphism. Buchan the heat supplied for that gabbroicintrusions produced voluminous which upwelling, asthenospheric and thinning ofcrustal anexpression likely is Anticline Buchan The melting. withpartial synchronous Anticline developed suggesting theBuchan of 800°CatFraserburgh excess InzieHead to in This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article 206 ) regional metamorphismP) regional 1952) that (Read,1923, Pb/ etal. 238 The ~8 km long coastal section between Fraserburgh and Inzie Head exposes high andInzie Fraserburgh Head long coastalsection between The ~8km The rocks of the Buchan Block define a broad horseshoe-shaped outcrop, which hosts which hosts outcrop, a broad horseshoe-shaped define Block the Buchan The rocksof U zircon age of 988 ± 23 Ma, the Inzie Head gneisses have been interpretedas a been U zircontheHeadgneisses ± have age Inzie of988 23Ma, , 2012) (Fig. 1a). Within the Buchan Block, migmatiticBlock, at a cropout gneisses (Fig.1a).Within theBuchan 2012) , et al. et Phase equilibria modelling;mePhase Grampian equilibria , 2010; Viete, 2010;

et al. et etal. et al , 2014). While some propose that some or all are productsof or allare some Whilesome proposethat , 2014). , 1977; Ramsay & Sturt, 1979). Although the region is now 1979).Althoughtheregion &Sturt, Ramsay , 1977; ., 2014), others suggest partial melting occurred suggestpartial due2014), others to ., et al c. 470 Ma) as parttheGrampian 470Ma) Orogeny of ., 2013b), are intermingled with sheets of diorite. sheets ofdiorite. areintermingled with ., 2013b), et al etal. into their current positionduring Grampiantheir current into ., 2014). We 2014). therocksbetween re-examine ., , where granulite facies migmatitic gneisses in gneisses migmatitic facies granulite where , tasedimentary rocks of the Neoproterozoic rocksoftheNeoproterozoic tasedimentary , 1984; Kneller, &Leslie,1984; Fettes ) but is isobaric at pressures of 2.7–2.8 kbar, of2.7–2.8 isobaric) butisat pressures porally associated porally with the intrusion of etal. Highlands that exhibits in differences its the gneisses have been interpreted to beof interpreted been have the gneisses etal. is associated with a warmer withageotherm is associated tamorphism; Dalradian Block; Buchan

, 2001b; Johnson , 2001b; (Sturt etal. , 2013b), the the statusof gneissic , 2013b), et al

., 1977) and a single grain SIMS SIMS grain asingle ., 1977)and et al , 2002; Oliver , 2002;Oliver ., 1977; Ramsay &Sturt, ., 1977;Ramsay etal. T et al ), low–pressure ), low–pressure , 2003). ., 2002; ., 2002; etal. , Accepted Article D (Viete advanced has been Grampian deformation Stephenson by the Block, Buchan ofthe Highlands,including north-eastGrampian geology and ofthe Stephenson by are provided terrane Viete &Baxter, 2007; (Ague thickenedcrust significantly thermal relaxation by of crust than heatfrom thelower rather of advection 2005; Vieteetal Dewey, tectonothermal event between event tectonothermal cleavage is particularly well developed me in developed well particularly is cleavage spacedpressurea upright complex inwhich solution folds tight open to locally exhibit Viete& 1995; Gould, Stephenson 1981; Roberts, 1956;Johnson,1962;Treagus& & Watson, (Sutton Block Buchan et al 1994; Stephenson 1994; Stephenson disrupted is (Fettes highly stratigraphy which the magmas,mafic ultramaficand across intrusionof and deformation locus forsubsequent Dalradian depositionsediments a ofthe activeduring andwas was Hill Lineament Duchray NNE–SSW coherent. west,the Portsoy– Tothe generally is statigraphy Dalradian which the of side either graniteplutons, voluminous with associated is Lineament Deeside east–west the (‘lineaments’) thatit separate therestof from Grampianterranesouth, 1a).Tothe (Fig. Group. Highland Southern tothe belonging metaturbidites core ofthe the in levels higher stratigraphic ascr rocks, aregenerally metavolcanic and metalimestones minor with and psammites metapelites graphitic semipelites, comprise which largely the Block, Buchan within levels stratigraphic rocksatlower problematic, commonly is terrane inthe Grampian elsewhere strata Dalradian correlation with precise 1a).Although Anticline (Fig. theBuchan and the TurriffSyncline uprightfolds, open, broad, horseshoe-shaped outcroppatternof a broad define Grampian Highlands north-east the in Block Buchan the The rocksof oftheBuchan Geology Block 2007) and the youngest regional Ar/Ar cooling ages for the Dalradian ( ages forthe Ar/Arcooling regional youngest 2007) andthe Terrane ( age inthe the latest magmatism of arcValley Midland lastedGrampian1984). The someShackleton,million Orogeny 30 as years, constrained by oceanic of accretion to related metamorphism and regional deformation 1976),which causedpolyphase & McKerrow, (Lambert Orogeny Grampian the mid-Ordovician by were affected rocks Most to granitic. from ultramafic in composition igneous rocksranging by areintruded which volcanic units, and carbonate withsome rocks, sedimentary clastic by dominated rocks Palaeozoic early to Neoproterozoic metamorphosedmid-the DalradianSupergroup, deformedand isasequence of terrane, totheBritish Isles.major ofthis partof formthecomponent Orogeny The northern rocks, isoneofseveralmajor crustal bl andintrusive Dalradian comprising itsbasement Supergroup, The Grampianthe terrane, REGIONAL GEOLOGY This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article 1 –D ., 1985;Vieteetal 4 regional nomenclature (e.g. Stephenson regionalnomenclature (e.g.Stephenson Recent reviews on the origin and evolution of rocks within the wider Grampian Grampian wider the within ofrocks and evolution ontheorigin reviews Recent There has been much debate regarding the timing of large-scale folds within the within folds of large-scale timing the regarding much debate been There has The Buchan Block is bounded to the south and west by major crustal structures by southandwest the to isbounded Block The Buchan et al et . (2013b). Although a more complex scheme for classifying sequences of forclassifying more scheme Although a complex . (2013b). et al ., 2013b). ., 2013b). ., 2013). Barrovian metamorphism occurredduringashort-lived Barrovian 2013). ., ., 2013), which is interpreted to have been driven largely by largely been driven to have is interpreted 2013),which ., c . 473 and 465 Ma(Oliver and465 . 473 et al et al. et Dalradian rocks whose distribution is controlled by two two by controlled is distribution whose rocks Dalradian ibed to the Argyll Group (Fettes Group Argyll the to ibed ., 2010). Rocks of the Southern Highland Group HighlandGroup Southern ., 2010).Rocksofthe ocks that amalgamated that ocks duringtheCaledonian (2013a), Tanner (2014)andDewey Tanner (2013a), Buchan Block consist of a thick sequence of thick ofa sequence consist Block Buchan tapsammite horizons. Read (1955) considered tapsammite considered Read(1955) horizons. et al arcs to the Laurentian margin (Dewey & (Dewey margin Laurentian tothe arcs et al et al et al ., 2013). ., 2013a). ., 2013a). ., 1986;Fettes ., 2010), here we follow the traditional the here follow traditional we ., 2010),

et al ., 2000;Baxter c . 490–489 Ma; Chew Ma;Chew . 490–489 et al et al. c . 461Ma; Dempster ., 1991;Goodman, ., , 1991). Rocks at Rocks , 1991). et al. et et al et ., 2002; ., 2002; (2015), (2015), et al ., Accepted Article metagabbros at datedmetagabbros ±0.6Ma with Portsoy were synchronous at 471.3 D emplaced facies amphibolite Upper zones. associated theshear with andtemporally areSuite spatially Dempster extension (Ashcroftcomponent withasignificantofpure shearflattening and involving shortening both complexdetail history recorda although inshearzones the Lineament indicate early thru west-directed metamorphicstratigraphical, and structural di (Ashcroft Buchan Block much ofthe that bound zones shear limbBoyndie the Syncline, of regarded somemajor (regional by asearly a F subvertical lie the regionalsteep dipand therocks becomes on generalshallow Banff, the regional D late interpretedasrelatively (probably aregenerally Both folds totheNNE. plunge gently whichtentatively subsequentworkers consider (1956), & Farquhar andRead Read(1955) BanffNappeby ofthe the hinge-zone as regarded was exposed gneisses, withinpoorly whichsits ofthisfold, surface Theaxial 2013b). where th and Fraserburgh, here between andalusite indicating relatively lowd relatively indicating andalusite then ofcordierite appearance prograde withthe east, and south the west, to increases facies (b record greenschist Highland Group occursthe TurriffSyncline,where Block rocks in core oftheSouthern Buchan Dempster Leslie, 1984; & (Kneller 471Ma around at coeval metamorphismapproximately were Stewart, 1960; Johnson, 1962; Fettes, 1970). 1970). 1962; Fettes, Johnson, Stewart, 1960; Kneller & Leslie, 1984). Thus, magmatism,Kneller Thus, & D Leslie, 1984). these rocks to sit on the upper limb of a major F (regional SE-vergingrecumbent upper limba the of to siton these rocks This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article Watson, 1956; Treagus & Roberts, 1981) and by others1981) andby asalater(F & Roberts, TreagusWatson, 1956; called the Banff Nappe, although the small-scalealthough the ofF Nappe, thevergence Banff called core of the great fold”. fold”. great core ofthe migmatizedmovement of the from andupward “the whicheastward resulted Banff Nappe, early, someform ofsyn-metamorphic gneissdomelower limbthat roseintoofRead’sthe et al pre-D F (Fig. 1a), around Collieston coast theeast NW. On Duchray Hill Lineament (Fettes (Fettes Lineament Hill Duchray Portsoy– Zone,which formssectiontheShear Portsoy northernmost ofthe km-wide of the~1 aseastern limit the Ellesregarded are (1931), Formation,termed Thrustby now Portsoy the Psammite marginCowhythe the of western the at fabrics Thrust-related Banff Nappe. as interpreted whichhe Line Read(1955), of mark the deformed rocks oftheBoyne Formation,east position (Fig.1),highly ofPortsoy Psammite theCowhythe edge of eastern the On Block. Buchan the within occur zones (e.g.Stephenson simple shear of component Tay Nappe (Stephenson Nappe Tay Buchan Block (Carty, 2001;Dempster (Carty, Block Buchan the in elsewhere rocks metagabbroic for determined ages precise ofless the range within falls by ceased thathad shearing 1 folds occur in rocks that are regionally inverted, and a major early fold closure must occur must occur foldclosure major early a inverted, and are regionally rocks that in occur folds ., 2013b). However, Read & Farquhar (1956, p131) considered the Buchan Anticlineas Buchan (1956,p131)consideredthe Read However, &Farquhar 2013b). ., 3 , synchronous withregional D , synchronous Syntectonic mafic and ultramafic igneous rocks of the north-east Grampian Basic Basic Grampian thenorth-east of rocks igneous and ultramafic mafic Syntectonic The large wavelength Turriff Syncline and Buchan Anticline (Fig. 1a) have axes that axes 1a)have (Fig. Anticline andBuchan Syncline Turriff wavelength The large D Porphyroblast growth and the peak of Buchan metamorphism occurred post-D occurred metamorphism Buchan of thepeak and growth Porphyroblast 2 et al. et structures within the Grampian terrane are generally associated with a strong strong a with associated generally Grampian are terrane the within structures et al 4 ) structures that post-date metamorphism (Stephenson & Gould, 1995; Stephenson 1995;Stephenson metamorphism &Gould, that (Stephenson ) structures post-date ., 1995; Carty , 1995; Carty, 2001; Carty, , 1995; Viete et al c ., 2013a). On the On western., 2013a). theTurriffSyncline, limbwestof of et al . 471Ma (Carty et al ., 2012). The lowest grade regional metamorphismin the regional grade Thelowest ., 2012). ., 1986; Goodman, 1994), part of the regional set of regional set partthe of 1986;Goodman, 1994), ., 2 P deformation 1981; (Johnson,&Roberts, 1962; Treagus /d et al et e succession is right way up (Stephenson isrightway e succession T conditions that characterise Buchan-type Buchan-type characterise that conditions et al ., 2002; Oliver ., 2002; sting (Baker, 1987; Beddoe-Stephens, 1990), 1990), 1987;Beddoe-Stephens, (Baker, sting iotite-zone) conditions. Metamorphic grade conditions. grade Metamorphic iotite-zone) et al a major ‘slide’ (low-angle fault) underlying themajora (low-anglefault) ‘slide’ underlying scontinuities across the Portsoy–Duchray Hill the across Portsoy–Duchray scontinuities et al 2 ., 2010). ., 2010). ed to be a northern continuation of the (F ofthe a northerncontinuation ed tobe deformation and the climax of Buchan ofBuchan climax andthe deformation ., 2013a), and a number of major of shear and anumber ., 2013a), ., 2012). This age for the Portsoy Gabbro Portsoy forthe 2012).Thisage ., recumbent, tight to isoclinal, east-verging east-verging isoclinal, tight to recumbent,

etal. et al 1 , 2008). , 2008). folds is generally towards the towards the folds isgenerally ., 1984) (Fig.1a).Major., 1984) 3 ) structure (Johnson& ) structure et al 1 ) fold (Sutton & ., 1984; ., 1984; 1 ) fold, et al 2

1 ., and and 1 ) Accepted Article the asthenosphere the asthenosphere (Viete lithosmantlemeltsinterpretedas by derived are Grampian Basic thenorth-east Suite data, rocksof Based ongeochemical above). (see ofc. most ages with recording &Gallagher,1984), unwarranted (Munro probably of ontheintensity largely series younger based Read(1919)intoanolder and by intrusiveand ultramaficweresubdivided mafic rocks confined to are and Grampian orogenesis continental breakup. and granites during crustalrifting emplaced type’ ledTanner data isotopic Geochemical and Block 1966). (Chinner, Buchan ofthe rocks outside Dalradian characterizes and calcsilicate (Johnson metapelite interbedded thinly dominated by rocksare Group) Argyll uppermost Formation; (Fig.1),Ta KinnairdHead,Fraserburgh On Fraserburgh RELATIONS FIELD more (Oliver,2001). ratios input juvenile reflecting Argyll GroupArgyll sediments Subgroup) (Tayvallich (Halliday with basaltic synchronous (Barreiro, 1998), Grampian terrane pre-date withintheGrampian Anumberintrusions ofgranite the 600Maagesfor andKeithGranitesemplacement ofaround Portsoy has yielded Tanner deformation (Tanner,1996; transition tohigher d and the kyanite by andalusite of replacement for evidence preserve rocks metapelitic Westthe & 1985). of Portsoy–Duchray Hill (Droop Charnley, Lineamenthorizontal sheet sub- more orlesscontinuous as a mafic–ultramafic intruded rockswere thatthe implying and as xenoliths maficwithin the and ultramafic 1983;Johnson & Droop, °C(Baker of800 excess in temperatures with consistent assemblages facies granulite contain Many Basic Suite. Grampians North-east ofthe rocks and intrusive zones shear regional the with associated are spatially that gneisses migmatitic as rocksoccur metamorphic regional highest-grade evolution The Comm.). (D.Pattison, Pers. polymetamorphic a rocks record thatthe 1991) or, alternatively, 1979;Pattison &Tracy, (Harte &Hudson, pressures may indicatehigher limb oftheTurriffSyncline, western metapelitic onthe rocks Staurolite, 1979). metamorphism occurs in which &Hudson, 1966; Harte (Chinner, This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article metagabbros andmetamorphismmetagabbros regional 1978; Kneller &Aftalion, 1987; Oliver etal of ages crystallization give Agedata 1984). 1978; Harmon, magmas &Aftalion, (Pidgeon the crustalconsistent with a within substantial compone nt bodies are metaluminous peraluminous to weakly (‘I-type’) are bodies cordierite, andalusite and prismatic sillimanite in a matrix rich in biotite, quartz, feldspar and feldspar quartz, inbiotite, matrix rich a in prismaticsillimanite and andalusite cordierite, have initial initial have ( early relatively these, after emplaced rocks(Johnson metasedimentary melting Dalradian of having partial formed granites the via with Grampian consistent Grampian-age granites are pera are Grampian-age granites or post-date with, synchronous are Block Buchan the rockswithin Most intrusive

87 Sr/ c 86 . 460 Ma show no association with mafic magmatism (Oliver, 2001). Of Of 2001). maficmagmatism (Oliver, associationwith showno . 460Ma Sr ratios identical to Grampian granites, whereas later( granites, Grampianwhereas identical to Sr ratios P /d c . 460–435 Ma) intrusions are strongly peraluminousare (‘S-type’) and . Ma)intrusions strongly 460–435 T etal. etal. et al conditions associated with Barrovian metamorphism, which metamorphism, which with Barrovian conditions associated ., 2010). ., , 2001a). Hornfelsaureoles occurringinthe roof-zones, inner , 2001a). , 2000). Metapelite layers (Fig. 2a,b) contain porphyroblasts of (Fig.2a,b)contain Metapelitelayers , 2000). et al , 2006). In the Buchan Block, U–Pb dating of zircon dating of U–Pb Block, the etIn Buchan al., 2006). ., 2003). Undeformed (Late Caledonian)granites Undeformed(Late ., 2003). luminous andluminous have initial

the north-east Grampian Highlands. Although north-east Although Grampianthe Highlands. (Carty (Carty et al et andthevolcanism deposition of uppermost yvallich Subgroup(locally theStrichen yvallich ., 2000), synchronous with intrusion of the ofthe withintrusion synchronous 2000), ., pheric extension and decompression melting pheric extensionand of decompression . (2006) to interpret these intrusions as ‘A- tointrusions as interpret . (2006) these deformationis thisand alteration, subdivision intrusions recordpressures kbar, of4.5–5.0 et al ., 2012). Geochemical evidence is is Geochemical evidence 2012). .,

et al. et and c . 470 Ma (Pidgeon&Aftalion, 470 . , 1989; Dempster , 1989;Dempster have lower initial lower initial have 87 Sr/ 86 Sr values0.71–0.72 of c . 435–400 Ma). 435–400

et al 470 Ma 470 87 Sr/ ., 2002). 86 Sr Accepted Article developed via reaction with H reaction via developed wollas contain pegmatites adjacentto the bands sheets ofpegmatite, suggesti mineralogicalmicrostructural) continuity and of spessartine-rich garnet, Head and further to the east.aplitesThe contai structures are re-oriented by gentle open folds, probably F probably folds, gentleopen by re-oriented are structures close association with pegmatite suggests high association with pegmatitesuggestshigh close aplites ofthe degree ofpartialmelting, theirand composition (garnet-bearing) peraluminous andsegregation migration forming successively features are interpreted to record in situ partialmelting of themetapelitic with meltrocks, and, largerveins withsuccessively interconnect metapeliticthe adjacentaplite 2000). The to rocks tight to isoclinal recumbent folds that verge towards the NW; S towards theNW; that verge folds recumbent isoclinal tight to the east (Ashcroft (Ashcroft the east network ofshear zonesmapped is crossing the beneaththe Tayvallich S stratigraphically developedaround Portsoy isnotpresent strongly diopside, diopside, quartz and plagioclase (Johnson and/or calcic amphibole. Palemarble mo bands clinozoisite modal diopsidewith of abundance a high colour reflecting in rocks aregreen rocks are attemperatures above the terminal the that confirming porphyroblastsofcorundum, (quartz-absent)contain stubby horizons 1c).Silica-undersaturated Shore(Fig. side ofBroadsea themuscovite, on first west appears retrograde replacedby tocompletely is partially which fibrolite. Prismaticsillimanite, This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article subsequent metamorphism. The second generation of folds(regional F of generation metamorphism.The second subsequent structures. Minerals that grew during peak metamorphism peak S during Minerals that grew define aweak structures. earlier the coaxial with are broadly that more uprightfolds forming smaller-scale, developed, and flattenedschollen. north-west andisdefine towards the angles steep of metamorphismof beenattained having pre- tosyn-D fabric, probably D regional fabric, probably tointense weak variably A stratigraphy. crude ‘ghost’ a define that trains disaggregated occurin commonly that calc-silicate diopside-rich and psammite semi-pelite, metapelite, diatexites biotite-rich abundant containing Kneller (1987b)andJohnson detail by east to Bay on the side Inzie of Fraserburgh Haven from West rocks dominate Formation Head Inzie migmatitesof the Metasedimentary Cairnbulg toInzieHead incorrect. east, is who(2001b), suggested this tothe Bay isograd Fraserburgh lay somewher within e unexposed earlier (Fig.1c).The and Broadsea ShoreKinnaird Head Johnson of interpretation metapelite,isograd) of small intheform of ve in (i.e.melting the ‘melt’ forinsitu partial firstevidence convincing The metapelitic rocks. the Buchan Block (Dempster, Buchan Theintense regional D the 1985). Irregularaplite sheet-like bodiesof andpegm Theat rocks Fraserburghaffected are threeby regional deformation ‘events’. F et al ., 1984; Fig. 1). 1984; ., rare biotite and tourmaline, and are(i.e. inpetrographic and tourmaline, rare biotite 2 , is developed within the migmatites that dips at moderate to moderate at that dips migmatites the within , isdeveloped ng a genetic relationship (Johnson relationship ng agenetic 2 O-rich fluids derived from the pegmatites (Johnson fluidspegmatites O-rich the from derived et al ubgroup (Kneller, 1987a). However, the regional regional the 1987a).However, ubgroup (Kneller, et al et schlieren and schollen of cordierite-spotted of and schollen schlieren . (2001a,b, 2003), are mostly retrogressedmostly strongly are . (2001a,b,2003), Head (Fig. 1d). The migmatites, described in described migmatites, (Fig.1d).The Head breakdown of muscovite. of Mostcalc-silicatebreakdown (Marchildon & Brown, 2003)withirregular& (Marchildon inlets of granitic leucosome, appears between between appears leucosome, ofgranitic inlets n small (0.2–0.5 mmn small (0.2–0.5 diameter) grains euhedral ., 2000). ., a(H coast at Fraserburgh Bay, a short distance to a shortdistance Bay, at Fraserburgh coast stly contain abundant calcite withscapolite, abundant contain stly tonite, grossular vesuvianitethat and garnet ultimately,theaplites (Fig.2a,b).These larger sheet-like bodies. The inferred low inferred low largersheet-likebodies.The d by the alignment of biotite-rich of schlieren thealignment d by at Fraserburgh, being restricted to levels to beingrestricted at Fraserburgh, s contain a network of small veinlets that of smallthat containa network s veinlets 2 O) near-solidusanatexis ofthe 3 atite, up to 1 m thick, occur on Kinnairdmthick, occur atite, upto1 (Treagus & Roberts, 1981). Allearlier 1981). &Roberts, (Treagus

4 , ascribed to differential uplift of uplift todifferential , ascribed 2 shearing that is particularly isparticularly that shearing 1 et al is recrystallized by by isrecrystallized ., 2000; Fig. 2a). Marble Fig.2a). ., 2000; 3 ) are more locally morelocally ) are 3 fabric, the peak fabric,the et al et al 1 ., . forms Accepted Article metapelitic 2c,d. Fig. in are shownin rocks orthopyroxene and ofgarnet andmelting the firstappearance in) (‘melt’ partial forsitu evidence the first recording 1c).Isograds edge ofBroadseaShore (Fig. onthewestern firstappears Sillimanite (forlocalitiesto InzieHeadis 1c,d): below rocks fromgiven seeFraserburgh Fig. metapelitic subaluminous) ‘normal’ (i.e. silica-saturated within assemblage metamorphic inferredprogradethe Based onfieldrelations, Summary migmatitessynchronous with partialmeltingin the (Johnson intrusionofthedioritewas with2f),indicatingthat granitichybridised leucosome (Fig. mingled 1d).Inallcases thedioritesareintimately abundant (Fig. also and/or foliation are few tens ofcentimetres to several hundredme insize 2c). Sheetsfrom a ofdioriteranging rocks (Fig. the source residual from segregated continuity with, the host migm inpetrographic and sheetsareinterconnected to, section.Thegranite occur throughoutthe mineral (orboth). peak orretrogradea it isunclear highest ifbiotiteis at grades 2e),the and (Fig. retrogressed strongly are section, migmatised in the as elsewhere here, Therocks pseudomorphs. the orthopyroxene of appearance m ofthefirst (upgrade) southeast rocks 50–100 after (Johnson orthopyroxene aggregates prismaticmm ri long) stubby (up to30 etal (Johnson Fig. 1d isogradon in the garnet by shown village, ofInverallochy extent south-eastern to the close Head occurs andInzie Cairnbulg section between the in ofgarnet appearance The first cordierite-spottedmeschollen of rounded abundant contain chlorite,and therocks in rich aggregates darkgreen and biotite by cordierite-bearing leucosome (Fig mingles grey withthe mmin diameter) (upto20 ofgarnet porphyroblasts conspicuous Point of sillimanite. around Further upgrade, biotite-rich metapelite contain abundant ovoid 2c,d). Schollenof mica whitechlorite(Fig and of fine-grained aggregates by pseudomorphed containing small mm(up to2 euhedral across) This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article All migmatised leucosome All grey-coloured and semi-peliticcontain pelitic rocks Irregular patches andsheetsc ofgranite       

., 2001b). At the highest grades, garnet is absent and leucosomes contain large contain leucosomes and absent garnetis grades, the., 2001b).At highest opx + cd + ksp + pl + q + melt ± (e.g.InzieHead) +plqbi cd +ksp opx + StCombs) isograd near mmelt SEoftheopx-in plto q+50–100 ksp+ (up + opx +cdbi + g + melt Point of Whitelinks) +plq + ksp (e.g. cd bi g + + Haven) West q+meltksp +pl(e.g. + cd +bi Head) Kinnaird melt q+ (e.g. +pl sill +ksp cd +bi Shore) (e.g.Broadsea q +pl sill +ksp cd +bi BroadseaShore) (e.g.west of pl+q ksp+ + and cd +bi et al atites suggesting they represent in source leucosome that representin sourceleucosomethat atites suggestingthey tapelite and larger blocksand larger ofcalcareous tapelite psammite (Fig. 2d). . 2d). Leucosome garnet is partially Leucosome garnetis . 2d). tocompletely replaced ., 2001ab) (Fig. 2e). The last occurrence of garnet occurs in ofgarnet 2e).Thelast occurrence 2001ab)(Fig. ., Whitelinks, a second leucosome type containing type a secondleucosomeWhitelinks, tres across and oriented sub-parallel to theoriented acrossand tres sub-parallel pophyroblasts of cordierite, but mostlack ofcordierite,but pophyroblasts grains of cordierite, most are which cordierite, of of grains sequence of changes in the main silicate peak peak silicate main inthe changes of sequence ontaining diffuse, semi-digested schollen ch in chlorite interpreted as pseudomorphs in aspseudomorphs chloritech interpreted

et al ., 2000). ., 2000). Accepted Article prograde appearance of garnet intersect thegarnet intersect H of prograde appearance the fieldsrecording stability the 3b). region, Withinthis °C(Fig. 670–690 3.0 kbarand satisfactorily reproducenear-solidus phaserela in Fig.S1. are contoured forothervariables Fig.4.Phasediagrams in are shown the composition of garnet in terms of the mole fraction of the spessartine component, component, spessartine ofthe mole fraction the of terms in ofgarnet composition the mol.%)melt of (asforthesame and garnet abundance calculated orless).The pressure (~3kbar °C)andlow (>750 athigh temperature only high- 3a. For this H K pressures inferredfornear-solidus asse maximum atpressures(Fig. 3a).This pressure atorbelow 3kbar to only issimilar constraint but oforthopyroxene, appearance modelling the predicts the highertemperatures & 4b).At pressure kbar) andhightemperature°C)(Figs3a low (<1.5 (>750 atvery conditionsexcept garn any these notcontain rocksHowever, do generally lack sillim Inverallochy and Cairnbulg Head (Fig.1d).Cordierite-rich Cairnbulg andInzie explain withinthe section between cannot observations higher-grade with observation. are consistent °C,allwhich of garnet at680–685 melting andform onsetofpartial the °C before andalusitecordierite,and will crossth biotite rich arrow thatin rocks the predicts evolution inFig.3b), pressure (grey subsolidus 2.8 kbar, the solidus becomes fluid absent above 2.8–2.9 kbar. At low temperatures the H the kbar.Atlowtemperatures above2.8–2.9 absent solidusbecomesfluid 2.8 kbar,the range considered, garnet is stable at all but the lowest atlowest is the all but garnet stable range considered, H A AverageGroup Argyll metapelite mol.%)(in arein shown Table1. generated on 06/02/14) and recently calibrated and recently 06/02/14) on generated datasetas end-memberuse data (ds62 thermodynamic the Holland&Powell(2011) of amount of iron thatisFe ofiron amount metapelitic (n=38)of (middle Dalradian) rocks major average the on are based calculations Phaseequilibria rock composition. average bulk using arepresentative reproduced mineralogical observedin thefield changes of overall sequence the Phase whether equilibriamodelling wasexamine undertaken to MODELLING EQUILIBRIA PHASE This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article Calculations using Calculations content is artificially low, leading to some H to low,leading artificially is content crosses the H the crosses isconsiste Head. This on orclosetoKinnaird occur both ofwhich aplitesheets, meltingand formationofthegarnet-bearing onset ofpartial Investigator (PearceInvestigator TC using ofphases andcomposition (‘mode’) the for werecontoured abundance diagrams 2 2 P–T tz, ilmenite and magnetite,tz, ilmenite and quar is shown plagioclase, in contain Fig. all fields inwhich O, O–FeO–MgO–Al T pseudosection for the average Argyll Group metapelitemol.%composition Group with3 Argyll average pseudosectionthe for , cordierite is stable in all fields except at high- all fields in isstable , cordierite The phase diagram for the average Argyll Group metapelite (Fig. 3a)can (Fig. metapelite Group Argyll average forthe diagram The phase By contrast, the phase diagram for the average Argyll Groupmetapelite 3a) Argyll contrast,By (Fig. diagramaverage the phase for 2 2 O-saturated solidus at pressures of 2.7–2.8 kbar. Along a prograde path atO-saturatedthis Alongaprogradepath solidusatpressures kbar. of2.7–2.8 O content, which was chosen to suffici be O content,whichwaschosen THERMOCALC , 2015). Bulk compositions usedin et al., 2015).Bulkcompositions 2 O 3 –SiO 2 O 3 is based on wet chemical analyses of34thesamples. analyses chemical isbasedonwet 2 –H metapelitic rocks within the lower-grade migmatites rocks withinthelower-grade metapelitic between 2 (Powell& Holland,1988)areinthe MnO–Na O–TiO mblages around Kinnaird Head (Fig. 3). Head Kinnaird (Fig. mblagesaround 2 –O (MnNCKFMASHTO) model peliteand system (MnNCKFMASHTO) –O and summarised above can be satisfactorily satisfactorily canbe above summarised and element oxide bulk composition of Argyll Group ofArgyll Group element bulkcomposition oxide 2 e andalusite toandalusite e sillimanite transition at 670–675 O-absent subsolidus assemblages. For theP–T assemblages.O-absent For subsolidus 2 nt with a metamorphic field gradient that gradient metamorphic field with a nt et, which is predicted at all suprasolidus atall et, whichispredicted O-saturated (wet) solidus, consistent with the with consistent solidus, (wet) O-saturated ationquantities ofsmall ofspessartine-rich Atherton & Brotherton (1982), for &which the Atherton Brotherton(1982), tions, as shown in an expanded area infrom2.5– anexpanded tions, asshown a–X anite, consistent with the model predictions. the withanite, consistent models (White models and low- P and P P–T –T ent to just saturate the solidus at 2.7– solidus the saturate just to ent

conditions and at very low- atvery conditions and range shown in Fig. 3a, as well as as well inFig.3a, shown range the phase equilibria calculationsequilibria the phase T and orthopyroxene occurs andorthopyroxene etal. , 2014a,b). Phase 2014a,b).Phase , 2 O–CaO– X P (spss), and 2 O

Accepted Article to the west of the shear zone network within rocks that lack fabrics related to regional D toregional fabricsrelated thatlack withinrocks zone network theshear west of to the evidence the field is import ofcritical However, zonesViete shear(e.g. set the of regional andwithin along sequenceDalradian interpretation that the gneisses are allocht Form Gneiss Head migmatitesthe Inzie with the metapeliteGroup Argyll the average may not be that indicates phase relations observed and calculated between correspondence of The lack 3 kbarorless,the appearance of observation. Althoughthe (Fig.4c). = 0.28] 0.32; Johnson 0.32; Johnson spessartine, in rich is at Fraserburgh sheets theaplite (Fig.4b,c).Garnetwithin theaplites garnet within peritectic mol.%)(<1 of quantities ofsmall formation the with consistent garnet, of appearance first the with coincides onset of partial meltingat H the onset ofpartial cordierite, biotite and andalusite that cross the andalusite to sillimanite transition before the predicts high-temperature subsolidus rocksrichin pressures these at prograde evolution reproduces near-solidus phase relations around Phasemetapelite equilibria Group modelling composition Argyll based anon average ArgyllNear-solidus theaverage phaseGroupmetapelite on relationsbased P–T–X the Modelling DISCUSSION This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article of the higher grade migmatites. migmatites. grade of thehigher theapl garnet-bearingmelts(now representedby firstformed the with that formed garnet of spessartine-rich or isolation, removal, physical Wedocumented. suggest that islesswell theresidue effect on Spencer, 2014),thepotential granites peraluminous the composition (e.g.Stevens effect onof ofperitectictohave entrainment asignificant isknown phases Although phases. of hydrate removal phasesthatrocks by ofperitectic Schmolke Konrad- 1988;Evans, 2004; (Spear, scalesexceedinglength the sizeofporphyroblasts on to diffusionalequilibration are allow enough temperatures high until prograde evolution subsequent their affect that composition bulk effective inthe changes may lead to rocks intoof Mn ea Fractionation 1980). 1979; Hudson, & Hudson, (Harte Block Buchan the in pressure,including low at metamorphosed rocks low-grade garnetinsimilarly arepressures, smallntostabilize ofMnO alsoknow quantities andamphibolite(greenschist Barrovian facies) 1995; Mahar garnet 1992; &Harte, of Droop & stability (Symmes criticaleffect onthe Ferry, a rocks, has metapelitic 1991)in wt%; Ague, minor (0.1–0.3 component as a which ispresent Manganese, inperitectic ofMnO garnet Fractionation 1979). Hudson, the low– extension to representingahigher-grade InzieHeadsection with the deformation, consistent At higher temperatures, phase relations for the average composition do not match donot composition fortheaverage phaserelations At highertemperatures, metamorphic field gradient developed throughout the Buchan Block (Harte& Block metamorphic theBuchan P developed throughout fieldgradient et al etal. et al ., 2005). However, metamorphic However,may ., 2005). fractionation also occurinsuprasolidus P–T , 1997; White , 1997; ., 2000] matches calculat ., 2000] wellthose stability field for garnet is muchla for garnetis field stability evolution 2 O-saturated solidus at 680–685 °C. Crossing of theO-saturated °C.Crossingof solidus at solidus 680–685 et al the measured composition ofwhich [ composition measured the ., 2014b). Although garnet is stablein subsolidus Althoughgarnet ., 2014b). orthopyroxene is orthopyroxene predicted at high- honous and were tectonically emplacedtectonically and were the intohonous are produced with melt by incongruent breakdown incongruentmelt breakdown produced by withare rocks metamorphosed at significantly highermetamorphosedat significantly rocks Fraserburgh at 2.7–2.8 kbar (Fig. 3a,b). A A Fraserburgh (Fig.3a,b). at kbar 2.7–2.8 for the onset of partial melting,occursof partial which fortheonset ation. This might in This used be supportofthe ation. an appropriate composition for the bulk of fortheappropriate of an composition bulk ites) can explain the mineralogical evolution evolution mineralogical the canexplain ites) rly formedrly (cores) ofgarnet insubsolidus ed for theed for firstformed garnet[

rger than field observations allow. than fieldobservations rger etal. , 2007; Dorais & Dorais , 2007; X(spss) = 0.30– T at a pressure of at a pressure et al ., 2014). ., 2014). (spss) X(spss) 2

Accepted Article with this phase stable only below 3.5 kbar and above 750 °C. 750°C. 3.5above kbarand below only phasestable with this high at relatively stable in the ofgarnet, is stability 3)only which composition(Fig. group Argyll for theaverage those from differsignificantly relations phase S2). Calculated Fig. also inFig.7(see shown meltthe X mol.%)and and garnet (in of abundance Fig. 6.Thecalculated migmatites higher-grade inthe phase relations spessartine-rich garnet remo A High-grade phase relations dehydration.stillsubsolidus undergoing in the modelling (0.3 mol.%modelling H (0.3 in the this value until temperatures of around 750 °C (Figs 4a & S3a). ThissuggestsH ofaround750°C(Figs4a &S3a). temperatures this valueuntil This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article H mol.%). aroundmodelled higherthan1 marginally Thissmall (by ofadditional amount were cannot currently cannot currently be modelled. (e.g.F, Cl) that components minor of small quantities to due peak metamorphic at the stable some whether biotite and/or was grewlater retrogression, during and metamorphic the peak at exhausted was biotite whether unclear garnet.Itis as as well biotite lack to are predicted arrow(grey in 6).Atitshigh- Fig. Formationat kbar metamorphicHead Gneiss 2.7–2.8 fortheInzie fieldgradient isobaric garnet. Th lacking but containing orthopyroxene rock by metamorphic represented fieldgradient, high– The isograd. theorthopyroxene-in isogradfollowedby in the garnet metamo prograde the Thereafter lack sillimanite. fractions beyond around 7% (Rosenberg & Handy, 2005). For the minimal H 2005). Forthe aroundfractions beyond 7%(Rosenberg&Handy, meltmelt ispredictedat in whichitmeltof the connectivity formed. rocks, Indeforming °C). 790–800 of garnet(at the disappearance by followed is 780°C),which above alittle at isograd (orthopyroxene-in around 760°C)is predictedat of orthopyroxene isograd priortothe formation mol.%0.5 formed early peritectic Along garnet. ataround occurs field observations crossingsolidus onFig. the isobaric arrow 5)thatsatisfiespath (Fig.5).An prograde (grey garnet on removal thespessartine-rich of reducedby greatly is temperatures athigher garnet (Table1). around 95% by are which reduced contents, onMnO effect a significant buthas composition), the original to effect just2%relative that are contents (thelargest isonFeO reducedby bulk composition thecomponents within ofother concentrations ofgarneteffectamount aon the trivial has within anaplite sheet on Kinnaird (Johnson Head EPMA) microanalysis; fromgrain a electronprobe (by measured that garnetis the removed mol.% remains( MnO 0.005 such that later;Fig.S3) only shown as ofthesolidphases, the stability effect on hasnosignificant this as bulk composition, mol.%) ofspessartine-rich garnet removed 3 (at used inFig. thatvaries fromcalculated (Fig.5) thecom 2 P–T O was probably supplied by nearby rocks with slightly higher temperature solidi that were that were solidi higher temperature rocks withslightly nearby supplied by was probably O pseudosection for the average Argyll Group metasedimentary rock with 0.5 mol.% 0.5 rockwith metasedimentary Group Argyll average the pseudosection for Around mostCairnbulg, metapelitic migmatites To test this hypothesis, an isobaric (2.75kbar) an isobaric hypothesis, To testthis Importantly, removalperitectic garnetre of of stability unchanged,the is practically stability Although thephases ofall other = 0 with 0.09 mol.% MnO) to a composition with a small quantity (0.6 (0.6 mol.% to asmallcomposition with with MnO) quantity 0.09 X =0 P- T ; orthopyroxene stability is extended slightly pressure, ;to higher slightly orthopyroxene stability isextended ved thecomposition (i.e. at 2 O is predicted at the wet solidus), melt exceed isatfractionsdonot the wetsolidus), predicted O T = 0.8, which equates to the effectiveequates tothe removal 0.8,whichofaround X = end, orthopyroxene and cordierite-bearing assemblages (for simplicity no melt removed was the from average position ofthe between Cairnbulg and Inzie Head is shown in is shown Head Cairnbulg andInzie between s at Inzie Head, is constrained to fields tofields constrained is Head, s atInzie e observations are consistent with a near- with consistent are e observations this path, new garnet growth (thegarnet-in newgarnetthis path, growth rphic field gradient is constrained to cross to is constrained rphic gradient field et al quires effective segregation and migration effective and quires segregation T ., 2000). The removal ofthis The small ., 2000). removal –X(-garnet) pseudosection was are rich biotitein cordierite but and = 0.8 onFig.5) appropriate for X =0.8

X =1 Argyll Group metapelite Group Argyll ). The composition of composition ). The (spss) ofgarnetare(spss) T endofthe 2 O content used O content 2 O contents O contents Accepted Article and partial meltingsyn-D and partial (i.e. metamorphism earlier with, must be than, orsynchronous Anticline theBuchan 10 km. Thus, Fraserburgh recordsmetamorphismand Inzie Head 1979). & Hudson, (Harte series facies metamorphic regional low-pressure Buchan classic of the thatmid-Ordovician thehigh-gradeend form metamorphosedduring Grampian the orogeny emplacedrepresentnot tectonically but gneisses arerocks basement, slivers ofPrecambrian Buchan thecoreof into network, passing degrees of partial melting resultedinflux an H degrees ofpartial from of 7b,c). For the7b,c). For H EPMA)(by fromof Whitelinksat leucosomePoint [ (Fig withobservation consistent Assuming the calculated location of th conditions and from amphibolite to transition the frommetamorphic tosuprasolidus subsolidus records The fieldgradient of2.7–2.8kbar. 800 °Catpressures of toexcess °C in around ranging650 field from gradient metamorphic 6), arean isobaric (Fig. consistentwith depleted inMn composition using a grades at higher garnet andorthopyroxene of and disappearance appearance the by imposed 3),andthose metapelite (Fig. composition Group Argyll average using the Fraserburgh modelling phaseequilibriaconstraints imposedThe by onnear-solidus pressuresat andregionalimplications fieldgradient metamorphic An isobaric metamorphic imposedthe constraints on field gradient(seeFig.S3). onthe consequently and or orthopyroxene, ofgarnet stability temperature or on thepressure positive d any ifthemetamorphic records fieldgradient 0.4 kbar(2 ).Consequently, in Fig.6are shown garnet-outboundaries and the associated with orthopyroxene-in data thermodynamic garnet. Th of and disappearance orthopyroxene of theappearance prograde by is constrained metamorphic Head at Inzie of the gradient field end bar(Fig. 6).to Thehigh-grade up 3.5 havehigher,perhaps may somewhat been pressures This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article produce thesyn-D upwelling of andrelated to lithospheric extension mostlikely this antiformregional (1956). In represents scalethermal interpretation, a fold the generally been destroyed (Fig. 2c-e). Johnson (Fig.2c-e).Johnson beendestroyed generally have features and structural sedimentary pre-anatectic inwhich melt) former representing (broadly leucosome in rich are diatexites and Inzie Head Cainbulg rocks between predicted atthealong modelled grades fieldhighest 7a).(Fig. gradient However, the mol.%melt 15–20 maximum is a of kbar, at2.7–2.8 just saturatesufficientthe solidus to neither H 2c).Importantly, higher(seeFig. significantly tobe inferred are heremelt fractions difficult, chosen H mol.%melt forthe produce <10 isograd in the garnet than lower grades rocksat suprasolidus that show calculations example, For followedhere. interpretation an zone network, shear lie above the intersection thisH the of reactionabove and the lie must they modelled, as lowerthan beensignificantly cannot have metamorphic fieldgradient pseudosection is correct, pressures at the low-grade (high- low-grade atthe iscorrect,pressures pseudosection Importantly, the metamorphic the fieldImportantly, straddlesthe zone shear gradient regional The isobaric metamorphic The isobaric fieldgradient Along this field gradient a maximum of around 1 mol.% garnet is predicted, ispredicted, around 1mol.%maximum garnet aof Along thisfield gradient

2 2 P/d O content (Fig. 7a). Although constraining constraining 7a).Although O content(Fig. O influx and the resultingmelt increaseO the influx and in T it is extremely small. itsmall. extremely is 2 O-content in the modelled bulk com modelled bulk O-content inthe 2 (meta)gabbrosof the Gram north-east 2 ), as originally proposed Readas originally ), andRead&Farquhar (1955) by . 2d), whose composition matches grains composition . 2d),wellthose whose analysed granulite faciesconditions. This Anticline. suggests thatthemigmatitic e andalusite–sillimanite reactionthe in andalusite–sillimanite e et al e calculated uncertainties on the end member on the endmember uncertainties e calculated indicates that the section between between thatsection the indicates the underlying asthenosphere that melted to that asthenosphere the underlying . (2001b) suggested that the high inferred thethat inferred . (2001b)suggested high 2 at a near-constant crustal depth of around of crustal depth at anear-constant O-saturatedhowever, solidus (~2.6kbar); (spss) 0.03–0.11; Johnson, 1999] (Fig. X(spss) Johnson,1999](Fig. 0.03–0.11; position, which was chosen to be tochosen whichwasbe position, fraction nor lossmelt of haveany effect the precise degree of partial meltingpartialthe precisedegree of is 2 O-rich fluid phase focussed along the focussedalong fluidphase O-rich

pian Basic the thatpian provided Suite T subsolidus)endofthe P–T

Accepted Article CONCLUSIONS is interpreisograds (Fig.1a),and Ashcroft, W. A., Kneller, B. C., Ague, J. J., 1991. Evidence for major mass transfer and volume REFERENCES editorial Hudson. N.F.C. ever-proficient handling. For for his toM.Brown and improvementversion, a significant intheled tofinal that comments constructive fortheir and (again) D. Viete Pattison D. R.Palin, Wethank Block. Buchan the the within originofgneisses andabout fordiscussionsmapFig. 1a became that the of copy editable an forsupplying D.Viete to grateful are We field. the in fruitful discussions for and arethanked C.Gärtner J.Carty toTJ isindebted R.Whiteforfundingfieldwork. ACKNOWLEDGEMENTS Anticline is not the same generation as the Tu generation same not the is Anticline extensiveroof oftheanticline in resulting the in granulite facies conditions reached fig. 1.7).Metamorphic Rollin,(Trewin & 2002, itsstrong along easternmargin,extendingto offshorenortheast particularly is the but Block mGal) Buchan characterises the anomaly that(+10–20 a positivegravity with consistent heat forBuchanmetamorphism (Vieteetal This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article Ague, J. J. & Baxter, E. F., 200      

Planetary peritecticthe first garnetwith formed (H spessartine-rich of production whichpredicts metapelitecomposition, Group Argyll average usingan modelled satisfactorily be rocks can metapelitic Near solidus granulite facies and straddle the regionalset ofshear zones granuliteand straddle facies to amphibolite from grade metamorphic in range Head Fraserburgh and Inzie metasedimentary rocks expos Group Argyll 760–762. autochthonous Dalradian in the north‐east Scottish Caledonides. by Sr diffusion in apatite and multicomponent diffusion in garn metamorphism of pelites. decompression melting of the asthenosphere. the asthenosphere. melting of decompression following middleand uppercrust the of heating advective and lithosphere of the to extension high related thermal a regional Turriff Itrepresents Syncline. a pre-tosyn-me is Anticline The Buchan tectonically emplaced into the Dalradian succession succession the Dalradian into tectonically emplaced Viete Oliver (e.g. event Grampian tectonothermal Ordovician mid magmatically-driven localised and abrief as partof melted partially The rocks around 650 °C at Fraserburgh to in excess of 800 °C at Inzie Head Head around 650°CatFraserburgh toin excessof800°CatInzie metamorphic from gradient (2.7–2.8kbar) ranging isobaric field The rocksrecordan first melts with the garnet produced ofpertitectic quantities of small removal dueto physical depleted composition inmodelled relatively be a Mn canHigher-grade rocksusing et al

., 2013) and are not allochthonous slivers of gneiss slivers Precambrianallochthonous andare not 2013) ., Science

Letters 7. Brief thermal pulses during mountain building recorded 4. Major shear zones and Leslie, A. G. & Munro, M., 198 ted to be an earlier structure. tobe ted , 261 Geology , 500–516. partial melting of fertile rocks. The Buchan The Buchan rocks. melting offertile partial ., 2010). A large basic mass depthsin at shallow Alargebasic 2010). ., , 19 rriff Syncline,which fold , 855–858. 2 tamorphic structure andearlier tamorphic structure than the O-rich) melts ed along thecoastal section between

et al strain during regional ., 2000; Dewey, 2005; 2000;Dewey, ., s lower-grade regional et.

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Accepted Article Figure S3. mol.% metapelite garnet0.5 Group composition spessartine-rich removed. with Figure S2. metapelite Group compositionBrotherton (1982). ofAtherton & Figure S1. website: publisher’s Informationmay befound in Additional Supporting INFORMATION SUPPORTING White, R. W., Powell, R. & Johns White, R. W., Powell, R., Holland, T. J. B., Johnson, T. E. & G Viete, D. R., Richards, S. W., Lister, G. S., Oliver, G. J. H. Viete, D., Oliver, G. & Wilde, S Viete, D. R., Oliver, G. J., Fra Trewin, N. H. & Rollin, K. E., Treagus, J. E. & Roberts, J. L., Tilley, C. E., 1925. A prelimina Tanner, P. W. G., Leslie, A. G. & Gillespie, M. R., 2006. Struc Tanner, P. W. G., 2014. A kinema Received 19 May 2015; 2015. June revisionaccepted 19 May Received 19 Bulkcompositions the ofFig.S3. construction usedin S1. Table composition. variable H Tanner, P. W. G., 1996. Signific This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article Journal metapelites revisited: New a–x relations for manganese‐bearing Caledonian orogen, Scotland’. path of high‐pressure Barrovian metamorphic zones near Cairn Le metapelitic systems. mineral activity‐composition relations for thermodynamic calcul Special scale extension during Grampian orogenesis in Scotland. In: sources for the Barrovian metamorphism, Scotland. The Trewin, N. H. (ed.) Dalradian of Scotland. 100–112. Highlands of Scotland. related intrusion. of the Ben Vuirich granite Pluto Geological Magazine aureole of the 590 Ma Ben Vuirich Granite, Perthshire, Scotland 2 (a) Argyll average of phases(mol.%) forthe abundance maps showingthe Contour Argyll average phases(mol.%) forthe abundance of maps showingthe Contour O contents and melt loss, respectively, on phase relations for the garnet-depleted onphase garnet-depleted meltloss, forthe respectively, relations contents and O

T–X Publication of ,

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, Accepted Article Dalradian) metapelites of Atherton & Brotherton (1982; table 5) with a reduced 5)with table H (1982; & Brotherton of Atherton metapelitesDalradian) (middle Group ofArgyll composition average the modelling using equilibria Fig. 3.Phase diatexites. fromthe derived leucosome segregated represents that granite mingling betweenHead. (f)Magma thelargedior in these northwest butnot (atlowergrades) to ashortdistance the pseudomorphs withorthopyroxene Garnet occurs orthopyroxene. closeCombs. toSt Stubby dark-green prismatic pseudomorphs areinterpreted toreplace fragments ofpalecalcareous psammite(e (top). diatexites ofcord contain smallschollen rounded These cordierite abundant mingles nogarnet. but containing leucosomeinset) grey with(see 1). ‘Flame’-likeleucosome containing large por largergranite sheets (left). (d) rich schieren. Irregularleucosomecontaini veins metapelite,of residual calcareo semipeliteand east onthe WestHaven diatexitearound Typical melting metapelite aplite.of andfeedinto, interconnect the garnetiferous the with, (c) irregularcontains abundant fibrolite. ve Thin metapelitecontainsof porphyroblasts small metapelite. The aplitecontains abundant smal metapelitic at(Fig. 1). rocks Head Kinnaird withindark, sheet biotite-rich aplite–pegmatite photos.(a)Composite Fig. 2.Field 2013. right) (anddatabase Copyright © Crown data Survey Ordnance m isograd.Contains the 50–100 southeastorthopyroxene of Garnet disappears orthopyroxene. at grades, and, high of garnet appearance prograde theFormation first record andalusite cross the andalusite to sillimanite transition before crossing the H the crossing before transition tosillimanite theandalusite cross andalusite is not extrapolated inland due to lack dueto inland ofe extrapolated is not Sheets ofdioriteand gran Crinan Subgroup). GneissFormation (probably Inziemap (d)Geological ofthe Head harbour. and the Head Broadsea Shore and KinnairdGarnet-bearing Head. aplites areabundant Kinnaird between clearthe west.Thefirstevidence forpartial maroundrocks 500 isfurtherto (upperDalradian) Highland Group with Southern boundary The around Fraserburgh. rocks middle (locally Strichen Dalradian Group Formation) Argyll of uppermost map (c)Geological areawithin Scotland. study thelocation ofthe shows d. Inset inand themaps c Fraserburgh showingshown ofthedetailedand StCombs location Viete (modified after Highlands Grampian mapof thenortheast Fig. 1.(a)Simplified geological CAPTIONS FIGURE This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article H the fieldwith observations. Along preferred grad consistent arrow)isshownthat is metamorphic (grey kbar) (2.75 fieldgradient isobaric An less.pressures of3kbar or (b)Expandedregion kbar).Theprogradegrowth oforthopyroxene athigh- (<1.5 lowest pressures the atall but with orthopyroxene (see athighertemperaturecoexists garnet b), phase relations solidus well near reproduces conditions. tosuprasolidus Althoughthis diagram subsolidus (3 mol.%). (a) 2 O content is artificially low)predictsas content isartificially O et al ., 2010; fig. 3 with additional data from Read, 1952). (b) Coastal section between between Coastal (b) section Read, 1952). datafrom with additional 2010; fig.3 ., P–T pseudosection from 1.0–5.5 kbar and 660–820 °C spanninghigh– kbarand660–820 1.0–5.5 pseudosection from Typical garnet-bearing diatexite at Point of Whitelinks (Fig. ofWhitelinks atPoint diatexite garnet-bearing Typical xposure). Isogradswithin semblages cordierite,and biotite containing rocks or those at higher grades aroundInzie orthosehigher grades rocks at cordierite and (metastable) andalusite and andalusite and (metastable) cordierite melting (the ‘melt’-in isograd) lies between (b) Close up of contact between aplite and and contact (b)Closeaplite of between up ite are abundant (although the position ofthese (although abundant are ite ins of graniteof (leucosome)ins derivedfromin situ l (0.5 mm) grains of euhedral garnet; the the garnet; ofeuhedral mm) grains (0.5 l us psammite are abundant, along with biotite- us psammite alongwith areabundant, ) Typical coarse-grained granofelsic diatexite diatexite granofelsic coarse-grained ) Typical of (a)showing near of solidus phaserelations. phyroblasts of partially retrogressedgarnet phyroblasts ofpartially side of Fraserburgh Bay (Fig.1). sideofFraserburgh Bay Schollen ng retrogressed cordierng retrogressed ient the subsolidus evolution (for which the (forwhichthe evolution the subsolidus ient ierite-spotted metapelite ite sheetnearStCombsa porphyriticand

the HeadGneiss Inzie ite interconnect with (bottom) andlarger 2 T O-saturated requires 2 O content T

Accepted Article Fig. 5.T–X spessartine component, of volume Calculatedcomposition percent. (c) approximate to mol.%oxide normalisedto one metapelite Group (Fig.3a)as Argyll average of(a) Fig. 4.Calculatedabundance aplites. productionthe garnet-bearing with the of the formationcoincides with °C,whichconsistentat prograde solidus ofgarnet, 680–685 This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article Fig. Fig. 6; 7 Fig. 5(rhs;X Fig. 5(lhs; X Fig. 3a,b; Fig. 4 1. Table Fig. 6.P–T around 790–800°C. at predicted is ofgarnet the consumption around 785°Cand prograde at ispredicted orthopyroxene garnet (i.e.isograd)ispredictedatappearanceprograde thegarnetinaround 760°C,the of theFor evol suprasolidusprograde rich garnet. MnO, for example 0.09 mol.% MnO in evolution asignificant(X =0).Therequires reduction suprasolidus with composition average the is reproducedby evolution subsolidus The field observations. with metamorphic fieldconsistent gradientsisobaric arrowsshow content. Thegrey MnO withdecreasing highertemperature tosignificantly increases of orthopyroxene, appearance only significant effect isthe stability of ga of agraincore fromthe EPMA) (by measured garnet removed ( mol.%;metapelite =0.09 (MnO component, in of garnet composition Calculated (c) percent. approximateto mol.% volume to oneoxide normalised 6as modelledin Fig. composition meltand (b) of(a) Fig. 7.Calculatedabundance gradie field metamorphic anisobaric arrow shows metapelitewithmol.% 0.5 spessartine-rich garnet removed Thegrey (X=0.8inFig.5). Normalisedcompositions bulk used inthecalculation phase of diagramsmol.%. in

= 1) = 0) = 0) (MnO) pseudosection at 2.75 kbar varying from the average Argyll Group averagefrom Group Argyll the (MnO) pseudosection kbarvarying at 2.75 pseudosection from 1.0–5.5 kbar and 660–820 °C for the average Argyll Group Group Argyll average pseudosection 660–820°Cforthe kbarand from 1.0–5.5 X (spss), = molar(spss), = Fe+Mg Mn/(Mn+ +Ca). H = 1 with 0.005 mol.% MnO). The composition of the garnet is that is garnet ofthe Thecomposition mol.% MnO). with0.005 X =1 3.110 3.110 3.108 3.108 2 O = 0.8, which corresponds to removalcorresponds around 0.5mol.% spessartine- which of 0.8, X = ##### ##### ##### ##### SiO (spss), X(spss), = Mn/(Mn +FeMgCa). 2

##### ##### ##### ##### Al 2 O 3

= 0) to a composition with 0.6 mol.% 0.6 spessartine-rich with tocomposition a 0) X = CaO CaO 1.378 1.379 1.373 1.373 meltand (b)garnetforthe MgO MgO 4.712 4.717 4.693 4.693 rnet that, at temperatures below the prograde the prograde below at temperatures that, rnet FeO ution of this composition, the appearance of theappearance composition, of this ution within an aplite sheetHead.The Kinnaird aplite on an within 7.079 7.049 7.200 7.200 garnet in terms of the mole fraction of the molefraction of ofthe garnetterms in terms of the mole fraction of the spessartine ofthespessartine fraction mole of the terms garnet forthe garnet-(andMnO-) depleted nt that is consistent with field observations. field with observations. is consistent that nt K 2.434 2.436 2.424 2.424 2 O

Na 2.038 2.042 2.020 2.020 2 O P–T TiO 0.777 0.777 0.777 0.777 2 pseudosectionthe for

MnO MnO 0.023 0.005 0.093 0.093 0.788 0.788 0.787 0.787 O TOTAL 100 100 100 100 Accepted Article

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T) (c) Broadsea NJ (a) Shore

ky

40

27 00

30

F3 ‘melt’ in NK

26

Kinnaird Head 68

30 ky Portsoy

28 A

92(T) 25 st 68 12 st F3 F1 TURRIFF SYNCLI and

Banff c 75 d FRASERBURGH 40

S3 NE Bedding andS1 Strichen Formation

16 F3

l sil

cd and Rosehearty

Cainbulg Point Fold hinge Fraserburgh

3.25 km

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and N sill (d) Peterhead 03

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04 60 CAIRNBULG Figure 1

60

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05 Edinburgh study area 41 Aber- garnet in deen

60 c. Fraserburgh Bay Whitelinks Point of Whitelinks

1000 m

43 Bay 65 41 N INVERALLOCHY CAIRNBULG/ 06 1 km Cairnbulg Point NORTH ST COMBS SEA Whitelinks Bay Inzie HeadGneissFormationInzie Granite Diorite orthopyroxene in Dip andstrikeoffoliation ST COMBS Head Inzie N 48 d. 41 Head Inzie 44

Accepted Article This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article

Accepted Article (kbar) This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article P 1.5 2.5 3.5 4.5 5.5 2 3 4 5 6 8 0 2 4 6 8 0 820 800 780 760 740 720 700 680 660 MnNCKFMASHTO (+pl,q,ilm,mt)

cd biksp cd biand and cd bi sill ksp

Average Argyll Group

ksp H

s ill H 2 b. g-in O 2 O g cd bikspH g bisillksp

g cd bi sill ksp s o l i d u s u d i l o s cd bisillkspliq g cd bikspH 2 O 2 O liq

s o l i d u s g bisillkspliq g cd bikspliq

g cd bi sill ksp liq bi kspliq g opx cd opx cd kspliq T

(˚C) opx-in g cd kspliq g opx cd g cd sill ksp liq ksp liq (a) ( kbar) P 2.5 2.6 2.7 2.8 2.9

3 670 and ksp cd bi cd cd biandkspH cd bisillksp 675 cd bisillkspH cd bikspH 2

O

and

ll ksp liqH si g cd bi 2 O 2

680 O

2 O g-in

ksp H

g cd bi

s

o l i l o cd bisill

ksp liq

d u s 2 O s o l i d 685 g cd bikspliq g cd biksp

liq H u s T sill kspliq 2 g cd bi

O (˚C) (b)

g-in 690

Accepted Article (kbar) This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article (kbar) (kbar) P P P 2 3 4 5 2 3 4 5 2 3 4 5 (a) liquid(mol.%) (b) garnet(mol.%) (c) garnet-X(spss) 8 0 2 4 6 8 800 780 760 740 720 700 680 Figure 4 T (°C) 5 10 15 20 25 30 35 0.05 0.10 0.15 0.20 0.25 0.30 1 2 3 4 5 6 7 8 9

Accepted Article T This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article (˚C) 660 680 700 720 740 760 780 800 820 0 MnNCKFMASHTO (+pl,q,ilm,mt) opx-in g cd biksp @ 2.75 kbar @ 2.75kbar Average Argyll Group cd bisillksp liq H ...... 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 2 O g cd bikspliq fractionation of peritectic garnet ofperitectic fractionation g opx cd bikspliq g opx cd kspliq cd bisillkspH cd biandksp Figure 5 cd bikspliq

g-in

2 O g-out X (-garnet) cd bikspH opx cd bi opx cd

s o l i d u s ksp liq ksp liq 2 O 1

Accepted Article (kbar) This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article P 1.5 2.5 3.5 4.5 5.5 2 3 4 5 6 8 0 2 4 6 8 0 820 800 780 760 740 720 700 680 660

MnNCKFMASHTO (+pl,q,ilm,mt)

and and

depleted inspss-rich garnet Average Argyll Group

cd biand sill cd bisill ksp H d ksp H cd bikspH 2 2 cd bisillksp O O

g bisillksp liq-in 2 O cd bikspH

s o l i d u s u d i l o s cd bi sill ksp liq s o l i d u s cd bikspliq 2 O liq

Figure 6 g cd bi sill ksp opx cd bi ksp liq g bisillkspliq

g cd bi sill ksp liq g cd bikspliq

bi kspliq g opx cd g-in

opx cd kspliq g-out T

(˚C) opx-in g cd kspliq g cd sill ksp liq g opx cd ksp liq

Accepted Article This article is protected by copyright. All rights reserved. Allrights copyright. by protected is This article (kbar) (kbar) (kbar) P P P 2 3 4 5 2 3 4 5 2 3 4 5 (c) garnet-X(spss) (b) garnet(mol.%) (a) liquid(mol.%) 8 0 2 4 6 8 800 780 760 740 720 700 680 Figure 7 T (°C) 0.01 1 2 3 4 5 6 7 8 0.02 0.03 0.04 0.05 0.06 0.07 0.08 5 10 15 20 25 30 35