Fabrics and structure of metamorphic flagstones and implications for industrial quality

NGU-BULL436,2000-PAGE 137

Fabrics and structure of metamorphic flagstones and implications for industrial quality

TOMHELDAL&BJ0RN LUND

Held al,T. &Lund,B. 2000:Fabricsandstructureof metamorphicflagstones and implicationsforindust rial quality.

Norgesqeoloq iskeundersekelseBulleti n436,137-146.

Metamorphic flagston esare characterisedby aregularly spacedmicafoli ationseparated byquartzo-feldspathic domains,alongwhichtherocks canbecleavedinto commercialslabs. Inthethrust nappesof theScand inavian Caledonides, such deposits are essentially develo ped from psammitic rocks in high-strain zones. Recent invest igationshave shownthattheindustrialqualityof manyflagston edeposits can belinkedtoseveralaspectsof the tectonom etamor phicdevelopmentof therocks,andespeciallytorapidverticaland lateralvariationsinstrain.

Fieldrelations andmicrotexturesfromthreedifferentflagstonedeposits are describedandused as examples.Finally, ast ruct uraland texturalcharacte risationofflagst ones ispropo sed.

TomHe/dolandBjern Lund,Norgesgeologiskeundersekelse,N-7497Tron d heim,Norway.

Introduction

For many cent uries, met am orph ic flagston e¹ has been an import ant,traditi on al const ructio nmateria lin Norw ay.These rocksareeasilycleaved into slabsalong mica-richfoli ati on planes wit h simple tools, and have, throu gh out history, founda wide rangeofapp licat io ns, suchasroofin gtiles,pav- ing slabsand blocks.Earli er,flagston e wasthe poor man's buildin g material,since there were many easily accessib le depositswhichcouldbe explo ited more or lessfreely.Tod ay, how ever, flagston e production has becom e a larg e-scale industry,using adva nced quarrying and processingtechn ol- ogy.Norw egi anflagstoneprodu cts,likeroofin g and floorin g tilesand thicker slabs forcladdin g and paving,hav ein recent years reached a grad uallyexpanding market inScandi navi a, Centra landSout hern Europ e and theFarEast.

Asthestoneindustry growsandevolves from trad it ion al, sma ll-scaleoperat io ns tow ardslargerscale,theneedfor geo- logi cal inform at ion on the quality of depositsand future reserves isincreasing . Thisleadsusto an important question: which geologica l features are imp ortant in contro lli ng the qualityofthe flagsto ne,and howcanwe esta blishameth od- icalapproach regardin g our understanding ofthesetypesof deposits?In this paper,we will focusononeim po rta ntaspect regardi ng flagstonequalit y,nam ely,themechani sms behi nd

1. Ouruseoftheterm'flagst one'needs some explanat io n.In Norwe- gian,theterm 'skifer'isusedforallrockswhichcontainaclosely spacedlepidoblast icfoliation.Commercial'skifer'canbephysically cleavedintoslabs alongthefoliation.Forrockswithclearly defined quartzofeldspathic domains betw een the mica-layers, the prefix

'kvarts-'isused.Somecom paniesusethe name commercial'quartz-

ite'for theserocks,inordertodistin gu ishthem from'softer'slatein themarket,even if,ingeolog icaltermi nology,theycannotbe classi- fiedas such.InEng lish,the term'flagstone' generallyappliesfor non-metamorphic, thin-bedded sandsto nes. However, we have chosentheterm'metamorphicflagstone'tobethemost suitable for their metamorphic equivalents wit h a pronoun ced, spaced schistosity,toavoidconfusio nwit hslateandschist.

Fig.1.Location of the Alta,Dovreand Oppdalflagstonedeposits.

theform at ion of flagston ecleavage,and howan understand- ing of thesemechanismscanhelp usin obta ining abetter predic t io n of flagston equalit y and investigation of dep osits.

Fieldrelati on s and microstru cturesfromthreedifferent flag- stonedep osits (Alta,Oppd al and Dovre;Fig.1)willbeusedto resolvethesequest ions.

NGU-BULL436,2000- PAGE 138

What is a commercial flagstone?

A commercial flagstonedeposit has awell defined,spaced, micafoliation ,along which the rock can be easily cleaved.

The most continuousand obvious micalayers are planar and parallel, and thespacing between them equalsthe average thickness of commercial slabs,usuallybetween 1 and 3 cm (Fig. 2). Betwee n the mica layers are domains consisting mainly of quartz and feld spars.Thus, slabs are essentially quartzo-feldspat hicincompos it io n,coate dwitha thinfilmof micas.

The quality offlagstone depositsdepends on a series of features.Firstly, it relates to the need for easy cleavable slabs of suitable size and thickness,depending onthe end-product in question.Furthermore,the slabsmust be planar,have uni- formsurfacesandthicknesses,andshowno surfacestaining or other feat ures which could influence the customer's

Fig.2.Phot ographsof typical flagstone sections at(a) Oppdal, (b)Dovre and(c)Alta.Notewiderspacingofcleavageandmorediffu secleavage planes in(a)thanin(b)and(c).

TOM HELDAL&BJ0RN LUND

impression of the final products.To maintain a profit able operation,quarry waste must be redu ced to a minim um. Consequently, the spacing of joint s andveins should be as wid e as possible, fol d s shouldbe absent or only sporadically present in a quarry site,and the geometry of thedeposit should give easyaccess to sufficientreserves wit hou t neces- sitati ng theremo valoftoo muchwasterock.

Even though the three flagsto nes in question have approximatelythe same appearance,i.e. grey wit hamica- ceous schistosit y surface, there are important differences whichareapp reciate d by andwell known to quarrymen and users.TheAltaflagstone sareknown tobe extre mely hard anddurableandareprod uced as thinslabs.At the other end ofthe scale,the Oppdal flagstonesgenerallyhave a wider spaced schistosity,are easier to work with and are mor e porous.TheDovre flagstonesfallsomew here inbetweenthe other two,though are closertotheAlta thantheOppd alflag- stone .The major differ encesinaveragetechnicalpropert ies aresummarisedin Tab le1.

These differencesare alsoreflect ed inthe productionof thethree typ es.TheAltaflagst ones are wid ely used asroof- ing tiles and thintilesfor flooring.This typeis considere dto be expensiveto cutwithdiscsaws and topolish.TheOppdal flagstones are hardlyeverused forroofing,as the slabs tend to be to thick and heavy,and absorb more water. On the otherhand,Oppdalflagstonesare consideredto be excellent for thick slabs forpaving and build ing bloc ks.Animportant method for formatting and shapingslabs is splittingperpen - dicular to the cleavage, which is easily done with simp le tools.This isactuallynot possibleto achieve inthecaseofthe Alta flagstones.

Alta Dovre Oppdal

Average slab thickness 10-20mm 10-25mm 15-40mm Sawingproperties extremelyhard hard soft

Colouron polished sur- darkgreyto darkgrey light grey

face black

Abilityto splitacross difficult difficult excellent foliation

Waterabsorption extremelylow low medium

Bending strength high high medium

Table 1. Some propertiesfortheAlta,Dovreand Oppdalflagstones.

Structural evolution

Althoug hwecannot recall any papers dealin g specifically wit h the geol og yof metamorphicflagstones,many unpu b- lished reports andfieldwo rkovermany years by NGU geolo- gistshave consistentlysuggested that workable flagstone deposit s develop from psammites(arkose or sandsto ne with a pelit iccomponent)in high-strain zones characterised by strong flattening and thinning of original layers. Typically, metamorphic grade has reached midd le to upper green- schist facies.The Norwegian flagstone depo sits may, in fact,

TOMHELDAL&BJ0RNLUND

beunifo rmlyassociatedwiththrusts, shearedcontactzones or the highlystraine dlong limbs ofmajorfolds.The three flagstone deposits in question occur wit hin Caledonian thrustnappes indiffe rent partsof thecountry.

Asthis art icle highlightsthe mechanismof deformation as aflagston e-forming process,the timing of deform ational events and controversies concerning this will not be dis- cussed here.Furthermore ,thisis notan attempt to correlate deform ation episodes from AltatoDovre,and the number- ing of such deform ati on episodes below must be regarded onlyasatoolto emphasise thatthepsammites at thethree localit ies in question have undergone approximate ly the same patte rnofdeform ational evolution.Fromtheworkof several aut ho rs(Zwaan&Rob ert s 1978,Guezou 1978,Rob- erts1985),itseems wellesta blishedthat at leastthreeto four episodesof Caledon iandefor mation can be identifi ed at all three locati on s.Theearly,ductil edeform ati on, commonly referred to asD₁and/or D₂, involved, according to these aut hors,thrusting ,folding andsubseq uent gravitation alflat- tening,reachingthethermalmaximaduring theD₂episode.

Zwaan &Roberts (1978) described early D₂thrustingwit h the developmentof blastomylon it esfollowed byagravitational flatt ening deform ation wit hin theKaiakNappe.Krill (1980) describeda similar developm ent fromtheOppdal area,while ourown mapp ing of theDovreflagstoneindicates aD₂stage ofthrusting also in thatarea(Lund 1979,Heldal&Sturt1996).

As afinalresultof thisearly deform ation ,thest rong,penetra- tive 'flagst one' foliat ion (Fig.2) isdeveloped in high-strain zones, such as in the thinned long limbs of folds,along sheared primary contacts and within thrust zones.

The D₂event wasfollowedbyalater stage ofsemi-d uct ile to brittle deform ation,hereafter referred to as D₃.Principal str uct uresareessent ially coaxialwit h D_2,but occurredat a high ercrustal level andat lowermetamorph icgrade.Struc- tures includesmall- and large-scalefolds wit hsteeperaxial planes thanthe earlier folds,thrusts andste epreversefaults, kink-bandsandjoints,especiallyinthe areasof foldhinges.In peliti clayers, acrenul ation cleavageiscommonlydeveloped, especially in fold hingezones.A later,D₄event is character - isedbynorm alfault s,fractu re zonesand regional,open folds wit hsteepaxial planes.

Thus, the tecton ometamorphic evolution involvesthe generation and shaping of the flagst ones during shearing and flatt eningatapproximate ly thesame timeasthepeak of metamorphi smwasreached.Subsequentfoldingand thrust- ing at lowertemp eratur es wit hin the samest resssyst emand, finally,non-coaxialfault ingand folding undereven 'colder' condit ionsaffected theflagst ones to various degrees.The struct ures of the early duct iledeform ationcanbe regarded asconstr uctivefor produ cing good-quality flagstone depo s- it s, whilst the later st ruct ures,where present , combi ne to dest roy the flagsto ne qualitybyjointingand fracturingalong fault s and in foldhing eareas, and by bucklingandcrenulat- ingof the52flagstonefoliation.In the following,wefocuson how thevariati onsof important qualit y aspectswit hin the flagstones, such as c1eavability, slab thickness and other physical prop erti es, can be st udied in thelight of these early, ductiledeformationalevents.

NGU-BULL 436,2000-PAGE139

Field relations and microstructures

The Alta flagstone

TheAltaflagstonesoccur wit hintheNalganasNappe(Zw aan

&Gauti er1980). which isasubdivi sionof the Kaiak Napp e ComplexwithintheMiddle Allochthon of Finnmark(Robe rts 1985,Ramsayet. al.1985).The NalganasNappeconsists ofa succession of meta-arkosesand mica schists restin g upon allocht ho nous, Precambrian gneisses wit hin the Naifjell Group(Zwaan&Gautier 1980).The ext ract able flagston eunit occurs in thecent ral part ofthe napp e,with non-exploit able meta-arkosesbeneat handaboveinthe nort hern part of the area(Fig.3). The uppermost part of the nappe comprises micaschists wit h intercalatedpsammit ic layers.

The workable flagston eunit varies in thicknessfrom 40 to 100 m, and locallygradeslaterally into schistose psammit es with apoorlydeveloped schistosi ty.Thevariatio nsinthick- ness seem to be the resultofstra in, indicated byapositi ve

N

~ W-Q- E

_ Normalfault

~T1TUslfault _ _Road

.c::::::::>Lake/river

- Elevationcurves (interval 20m)

Fig.3.Geolog icalmaps of(a) thePeska-Langvannquarryarea and(b)the Stiliaarea,bot hwithin the Alta flagstonedeposit (from Heldal etal.

1997).

NGU-BULL436,200 0-PAGE140 TOM HELDAL &BJ0RN LUND

~

~ ~

.s

I ^I

~

~

I

~ eo

•

0:;

1

;;

'-c_~

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I

•

·C

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~

a)

Foliation

IOcm

Thrus t

20c J11

lOcm

b)

. - - - .. '--

/\ - - - - .,..--- Foliation

2 . .----

~

^{;- = - -}

~ ~

c)

Fig . 6.MinorD2st ruc turesfromthe Altaflagstone.(a) small,boudinaged duplex, (b)minor thrustswhichare rooted and roofedin along the S-fo- liat ion planes, (c)minorth rustwit hth rust-bend box fold,and(d) poly- phasal thrusting andfolding(early thrusts(T1)are folded and cut bylat er thrusts (T2)develop in gfro mthefold s).Q=quartz veins.

50 100 150

Thicknessofflagstoneformation(m}

C Fig.4.Averagespacing ofcleavage(slabthickness)plottedag ainst total th icknessof flagstoneunit atthree differentlocations withi nthe Alta flagst one deposit.Points equals average spacingand line range of meas- urement s.A:Detsika,B: Stilla,C:Peska.

orrelati on between formati on thicknessandaveragespacing of the schistos ity(Fig. 4).Locally,the formati on is seen tothin awayfrom the hinge areasoflargeF₂fold s(Fig. 5).The base oftheflagst one issharply def inedby a seriesof 10to 30 cm thick layers of coarse-g rainedbiotite schist, intercalatedwith bluish-black quartzites. Intheeastern part of the area,the same type of succession is repeated approximate ly10 m up in the flagstone unit.The upper contact of theflagstone is transit ional into beddedpsammites wit h a diffuse andwidely spaced schistosity.

Depending on location wit hin the deposit area, slabs tendto come outin thicknesses ranging between 7 and30 mm.Wit hinaspecificquarryarea,the range is morelimit ed, dividing the deposit into 't hin-slab' and 'thick-slab' areas, according to theabove-mentio ned lateral variationsin for- mation andslab thickness.How ever,individualbeds withina quarrymay showlenticularshapes,giving a mesoscopicvar- iation inslab thicknessbutwit hin a narrowerrange than on the regional scale.

D₂ structures such as small- and large-scale thrust s, thrust-b endfolds,biotite-rich shear-bands and ashear (C)- foliation in micaceous layers (Fig. 6)are connected to the thrusting during the early st ages of the deformation .How-

QlL1l1Y

Road

Flagstone (workable) Psanuuite and micasclust 2700 metres

( )

=-

? ".- -- - --- ---

_ _ _3

Fig.5.Sketch profile ofthe Peskadepositarea,Alta,seen fromthe east.Theflagsto ne unit canbe seen th in ningtoward s the south.awa yfrom early fold structures,as illustratedinthelower draw ing.

TOMHELDAL

s

ever, boudinaged fold hinges, pinch-and-swell st ruct ures and circular spots of altered residues of sulphidesand/or oxidesindicate a change fromessenti ally pureshear to a sim- ple shear regimetowards the end of the D2episode.

Two inte rcalat ing subtypes of flagstone are observed, onewhich hasan almost entirelyquartzofeldspath iccompo- sit ion in betwee nthe micalaminae(Fig. la)and one wit h abundant lepidoblasticmica.Thelatteris generally preferred among quarrymen,since the 'armouring' mica makes the rock more elastic and less brittle and fractured than the former.

Thetext ure of quartzinthin-sect ions(Fig. la)is classified asporphyroclast ic, annealed mylonitic (5pry 1969). where granobl ast ic-elongated grains occurinalmostfibrous,paral- lel aligned (52)aggregates averaging 0.1 mm in thickness.

NGU-BULL436,2000- PAGE14 1

Imm

Fig.7.Photom icrogr aphsof flagstonetextures,crossed polar s.Scalebar applies forallimages. (a)Annealedmylonitic, S-C texturefromamica- poorvariet yof the Altaflagstone(5andC foliatio nsaremarked).(b)An- nealedmylonit ic,S-Ctexturefrom a mica-rich variety of theDovreflag- stone. (clQuartziticflagston efromDovre,cleavageplaneto therightof centre.(d)Typicalgranobl astictextur efromthe Oppdal flagstones.(e) Granoblastic-elongatetextur efrom theEnganvarietyoftheOppdalflag- stones.Concentratedmicalaminaerepresentcleavageplanes.

The alignmentcoincideswiththe NW-5Estretchinglineation, L2.Feldspars occuras larger, rounded porphyroclasts,up to 1 mm in size.Lepidoblastic micas(less than 1 mm long)areori- ented within 52or, alternatively,along shear-bands (C-folia- tion)at approximately15°to 52'Quartzaggregates arespo- radicallyseen parallel to the lattermicaorientation,butthe dimens ional orientation of the grainsgenerallyfollow s52, indicating a late-D2annealingalong 52'

Thus, both field relations and microtexturesindicate that the formation of the schistosity during D2isthe result of simultaneousflattening and shearing /thrusting,terminating in an episode of gravitationalflattening which overprinted androtated shear (C)-str uctures. Allin all,the generally very low-angletoalmost parallel relations betw een 5 and Cst ruc- turesindicate an extremethinning of the flagstoneformation.

NGU-BULL436,2000-PAGE 142 TOM HELDAL

s

The Dovre flagstones

The Dovre flagstonedeposit is situated at Storvassberget, approx imately 20 km NE of the village of Dovre in Gud- brandsdalen.The flagstone unit belongs to the extensive psammitic-quartziti csucsessions in the pre-Ordovician Hei- dalGroup (Strand 1951,Sturt et al. 1991).wit hi n the Otta Nappe(St urtet al.1997). The HeidalGroup rest s unconform- ablyupon the Hevrinqen gneiss, which represents the basal part of thenappe .On top of theHeidalGroup,above a pri- mary,depositional unconform ity,are thelow -g rade phyllites of theMiddle Ordovician Sel Group.Locally, rocks of ophi- oliti c affi n ity(t heVagamo Ophioli te)occur between thetwo metasedime nta rysuccessions,havingbeen thruston top of the Heidal GroupinEarly Ordoviciantimes,priorto the dep- ositionofthe Sel Group(St urt et al. 1991).

The emplacementof the Otta Nappe is ofScandian age, and occurred under middlegreenschist-faciesmetamorphic conditions.How eve r,it has been shownbySturt et al.(1997) thattheHeid alGrou phad already attainedamedium-gra de mineral paragenesisprio r to sedimenta tio n ofthe overlying Sel Group. Within the Heidal psammites,flagstone is locally developed along shear zones,related to thrusts or to the sheared contacts between different rock units, and in the longlimbs of earlyfolds. In theStorvassbergetarea,the Hei- dalGroup thinsout towards N-NE,and the rocks inthe area are generally int ensively deformed. The productiveflagstone unit occurs sandwichedbetween the underlying Hovrinqen gneiss and an overlying,locallydeveloped,thrust sheet of the same gneiss(Fig.8).Thus,the low er contact of the flag-

LEGE 0

CiJ

o Greyflagstone

~Quartz mylon itc

c::::c

\ Strike anddip tocleavage

N

A

jOt!m

..,'" Norm al fault

Lithologicalboundary

: nIli '

••••• Dirtroad

• Lake/river

= ' Eleva tion curv es (interva l 5 metr es )

stone may representa tectonicallymodif iedprimary uncon- formitywhilstthe upper boundaryis a thrust.

The baseof the flagstones is well exposed,and quartz- mica mylon ite s areseenresting upon phyllonitesand phyllo- nitic augen gneiss. These mylonites grade upwards into workable,grey flag stones with awell developed,spaced foli- ati on (10-30 mm).The upperpart of theflagstone unit con- sists of dull white , quartzit ic flagstones, with a markedly lower contentof micaand feldspar than in the greyvariety.

The intern alstructuresof the flagstone are stronglyremi- niscent of those described from Alta. Individual beds show pinch-and-swell structure,with correspo ndi ng variation in slab thickness(Fig.9).We have,how ever,insuffi cientobser- vationsto reach any conclu sion regarding lateral variationsin theformationthickness,due tothe lack of outcrops.

Inthin-section,thetext ure and grainsize of the greyflag- stone appears quit esim ilarto theAlta flagstone(Fig.7b).but the relict mylon it ic fabric of the quartz aggregates is less obvious. The text ureof thequart z in theDovre flagstones may,more correctly,be described asgranoblasti c-elongate.

Asin Alta,elongatequartz aggregatesfollowthe NW-SE,L₂ stretc hing lineation,and a (-foliationlocally developsas-SE dip ping intersectionlineation on the schistositysurface(Fig.

10).Thus,theD₂structuresat Dovre may indicat e a slight ly lower,butst ill significant,degree ofthinning than at Alta,or

Fig.9.Pinch-and -swe ll ofindividualbeds withinthe Dovre flagstonede- posit.

Fig.8.Geologicalmap of theDovreflagstone deposit(from Held al &

stun1997).

Fig.10.Lineatio nsin the Dovreflagston e.L] =S-(intersectionlineation. L₂=stret ching lineation .

TOMHELDAL&BJ0RNLUND

alternatively,ast ronger overprinting of D₂ textures during later deformation.However, we have no observationsthat indicate any significant overprintingtexturescorresponding with the orientationsof D₃stru ctural elements.

The upper, dullwhiteflagstones differ significa nt ly from thegreyones,consisti ngalmostent irelyof quart z,wit h only min or amo unts of feldspar and lepidob lasticmica (Fig.7c).

Even on the schistos itysurface, themicasdo not form cont in- uouslayers.This obviouslyreflects thefactthatthere is alim- it ed amo unt of mica-formi ng materia l available in a pure quartzite.Quartz occursinthe pure quartzitic parts between the mica laminaeas<0.4 mm diameter grainswit hagrano- blastic, annealed texture.In the micaceous laminae, how- ever,the grainsize is smaller(0.1 mm)and the aggregates show a higherdegree of preferred orientation.This indicates ast ronger post-tectonicannealingin the former case.

On thenatu ralst onemarket,the quartziticflagst ones are considered to be ahighl y inte resti ng produ ct dueto their almostwhitecolour. However, ashasbeen shown,the schis- toseproperties are poor,and the verylow conte ntofmica makesthe rockbrittleand closely jointed.

The Oppdal flagstone

TheOppdal areainvolvesan autochthonousand parautoch- thonous Palaeoproterozoic basement with younger sedi- mentarycover sequence,overlain by a seriesof Caledonian nappescomposed of ProterozoicandEarly Palaeozoic rocks (Krill 1980 and 1986,Fig.11).The flagsto nedeposits occur wit hin theSzetra Nappe,which consists primarily of highl y defo rmed psamm it es of Neopr ot erozoic age.Deform eddol- erite dykes are abundant in theSeetraNapp e;thelack ofsuch dykes in the ot herrock unit s in the arealed Krill (1980, 1986) to conclude that the Seetra Nappe is complete ly alloch-

NGU-BULL436, 2000 -PAGE143

thonous inthe Oppdal area and can possibly be correlated withtheSarv Nappe further east(Gee et al. 1985).

Krill(1980) concluded that the thrusting which gaverise tothe tectonostratigraphic succession ofthe areatook place during the early stagesof the main Caledo nian (Scand ian) orogeny. Later deformation and metamorphism obscured these early str uct ures and the rock units were folded in major, recumbent fold s.Theflagstone quarriesshown in Fig.

11 arelocatedin the inverte d, highl y strainedlimbof sucha recumben tantifo rm.The superimposed, latest stageofstruc- tural evolution includes upright foldi ng, and bot h reverse and normal faulting.

Theworkable flagstonesin Oppdaloccur at several sepa- rate levels with intervening non-workable psammites and laterally extensivefolded layers.This pattern of quit epro- nounced verticalvariationsin qualityreflectseitheran inho- mogeneou sdist ribut ion ofstrain or primarysediment ologi- cal feat ures,or both , where the folde d layers may reflect primary variations in rheologybetw eenthelayers.

The slab thickness (20- 50 mm) of the Oppdal flagstones is generallygreate rthanin the ot hertwo deposits.How ever, afew except ionsexist,and inthe Enganarea (Fig.11)(Lund etal.1998)the spacingofthe schistosityapproaches the typ- ical range of the Alta and Dovre deposits, namely 8-20mm. This is explained byaregional thinning of the flagstone for- mation.

In thi n-section, the 'average'Oppdal flagstoneismark- edly coarser grained than at Alta and Dov re(Fig.7d).Quartz occursasgrainsup to 0.8mm in size,in a grano blastic to granoblasti c-elongate texture. Feldspars are essentia lly of the samesize,except inspora dicgritty layers.Lepidoblasti c micas approa ch 14mm insize.TheEngantype of flagst one is more fin egrained (q uartz lessthan 0.2mm), anda bimodal

TECTON OSTRAT IGRA PHIC UN ITS

~30

LEG END

~ Strike/dipto mainfoliation

'X Flagstonequarry

' " E6road

T RONGET

D ^{B LA.H0} D ^SIETRA D ^RISBERGET D ^{A.MOTS DAL}

/ L0NSET

N

W -</-E

5

10km

Phyllite,metagreywacke, grecnstone

Garnet-m icaschist,amphibo litc, se rpentinite

Feldspath ic psammitc(t1agstone), metadolerite

Augcngneiss,rapakivigranite, metasabbro anorthosi te

Autochthonous /pa rautochthonous gne isswith metascdimcnta ry cover

Fig.11.Geologic almapofthe Drivda lenflagston equarries,Oppda l,and main lit hologicalunitsin thesurrounding area(afterKrilI1980).

NGU-BULL436.2000 - PAGE144 TOM HELDAL& 8)(2JRN LUND

Shear

zone ~

...

~'-.=---~

... ... ..

wasterati ointhe quarries than small ones. 51ab sizeis closely related to jointing and,thus, also to the intensity of later deformation, i.e.close spacing of joints gives small slabs.

How ever,jointing is not only a function of tectonicsand st ressrelease,as the rheology of the rocksis also of vital impo rtance. For flagstones,rocks with a closely spaced folia- tionandlow contentof mica are generallymore conspicu- ouslyjointed than mica-rich flagstones wit h a widelyspaced folia ti on.The micascont ribute in'armouring'the slabs and reducing thebritt leness of the rock,a feature that is highly appreci ated by thequarrymen.

Fig.12.Pinch-an d-sw ellof flagstone beds in the Oppdal flagstone de- posit. wit h the develop men tofasymmet ric len ses close to a low angle shearzone.

grain-size distribut io n between quartzand feldsparsis seen (Fig.le).The quartz text ure isst ill granoblastic-elongate,but with atendency towards a higherdegreeofpreferred orien- tation than inthe average flagstone.The overall impression ofthe Oppdal-type flagstoneisthat the grainsizeand textu re are closer to those in a primary'sandstone-fabric'than at Alt a and Dovre.

Fabrics and quality

'Soft' or'hard' cleavage

Among quarrymen,itiswellknow n that some flagstonesare 'soft' (easy) to cleave,othersare'hard' (diff icult). Alta and Dovre(grey) flagstones belong essentiallyto the former type, and cleaving is done with short chisels.Oppdal is considered to be more difficult,and theworkers need to penetratethe wholeblocks wit hlong,flat chiselsin order to cleavethem.

Itseemsclear thatthis characteristic oftheflagstones is attributa ble to deform ati on grade and metamor phic diffe r- ent iat ion.The'soft' types have a very sharplydefine dplanar str uct ure (52), are very fine grained with larger porphyro- clasts of feldspar, and have a granoblast ic-elongate to annealed mylonitic texture.The 'hard' flagstones have a coarsergrain size, and granoblastictogranoblasti c-elongate texture with more evenly distributed, lepidob lastic mica throughout the quartzofeldspath ic layers. Thereareno sig- nificantdifferencesin the grainsize for quartzand feldspar, except in grittylayerswhere there arelarger porphyroclast s of feldspar.Thus, when the rocks becom emore anisotropic as a consequenceofincreasing strain,the cleaving proper - ties,not surprising ly, improve.

Thequartziticflagston es at Dovre, how ever,are diffic ult to cleavefor additionalreasons.Thecontent ofmicais actu- ally too lowto givea good cleavage.Thisremind sus of the basic fact that arkoses and/orsandstoneswith a peliticcom- ponentare necessaryprotoliths for 'prod ucing' a flagstone deposit of acceptable quality.

Slab size

The obtainablesize(area)of flagst one slabs is of greatimpo r- tanceinexploitat ion,sincelarge slabsgenerally give a better

Slab thickness

Thethickness ofslabs,correspond ing tothe spacing of the schistosity or cleavage, showsbothvertical andlateralvaria- tion s in the deposits.A gradual, vertica l variation is seen clearlyat Alta and Dovre.In the for mer case,slabthickness increasesupw ard s in theupper part of the commercial unit, until non-comm ercial psammit es are reached. At Dovre, there is atransit ion atthe base of the unitfrom fissile,non- commercialflagstonewithvery closely spaced cleavage(less than5 mm)intothe commercial typeswit h slab thicknesses reachingmorethan 10 mm. In both cases,this can belinked tothe degree of relict myloniticfabric in the rocks.The pic- ture isnot that obvious at Oppdal, where there are more rapid,cyclicchanges between folded layers,thick-slablayers andthin-slab layers.

The lateral variations correspond to variations in thick- ness of both individual layers and slabs('edging slabs')and the flagstone units themselves.On a mesoscopicscale,thick- nessvariatio nsof flagstone beds are seen in most quarries, where thethicker areas have essentially a wider spacing of thefoliationthan the thin ner,thusindicatingthat these vari- ation sare stronglyrelated to strain.In many cases,these structures seem tobeconnected to thelateflatte ning defor- mation, resulti ng in pinch-and-swell struc tures.This was combined wit h high layer-parallel shearstrai n,particularly around the 'swell' areas. Early fold hinges are commonly observed in such areas, indicating that early folds were import ant incontrolling such structures.However,there are also observatio nssupportingthe view that pure shear was a more dominant factor in the formation of lateral thickness variations of flagstone beds. One exampleis shown in Fig.12, showing how the development of a shear zone hasinsti- gated formation of'fish'-shaped beds.

Pinch-and-swell structure in the flagstone unit s as a wholewould appear to have simi larexp lanations,as shown inFig.13.AtAlt a,the correspondence between lateralthin- ning of the formation and slab thicknessis, as shownabove, quit e clear.Thus,there areseveral indicationsthat slabthick- ness appearstobe a function of strain,and that both lateral andverti calvariatio nin straincan have been quitesignificant withinanyonedeposit.

Importance oflinear fabrics

L2intersection lineation s are mainly found in the Altaand Dovre areas, where it results from the int ersection of shear foliat ion wit h the penetrative micaceous 52surface. This

TOMHELDAL&BJ0RNLUND NGU-BU LL436,2000-PAGE14 5

Schistose psammite Psa mmite

~-~~~~-~~

~

Workable t1 agstone ~

Fig.13.Schematicmod elforlateralvariat ionsofslab thickn ess and form ationthickness wit hin a flagston edeposit.

givesthe 52surface a't iled' appearanceremin iscentofslick- ensides.The most obvious conseq uenceforquar rying isthat theslabsare moreeasilycleaved along these'slickensides' than against them.TheL₂stretching lin eat ion isa distinct feat ure ofall three deposits.Itcommon lyconsistsof elongate aggregatesofmicas and quartz,and quartz rods.Thislinea- tion is the second strongest anisotro pic feature ofthe flag- stones,and makesthe bestdirectio n forprim ary blasti ngand vertica lsplit t ingof slabs.

Fabrics and physical properties

Asexperiencedin practica l use andshown inphysical testin g, the Oppdal flagsto ne has ahigh er water abso rptio nvalue thanthose ofAltaandDovr e,impl yin g that larger volumesof wate r more easily penet ratethe pore spacesofthe rock.This has practi calconsequences for the flag ston e, i.e.eventhin slabsof theOp pda l flagston e arenot conside redto be suita - bleas roofing mat erial, as a rock with high er waterabsorp- tion is generally mor e vulnera b le tofrost actio n than less porousrocks.Thepresent st udyestablishes thatthehighl y strai ned and myl on iti cflag stones areless porousthan the less st rainedflagsto nes,since the grain-sizeand layerthick- ness reduct io nduring myl onit isati on cont rib utetored ucti on in the pore volume.Similarconcl usionswere reachedinan earlierstudyonthe durabi lityofnat uralsto ne(Alnees1995).

A positive consequenceofhigh er porosityare the excel- lent cutt ing propert ies of the Oppda l flagsto nes. After eng ravi nga shallowgroovealonga straig htor curved line on thecleavedsurface,the slabscanbe broke nwit hast raight edge along the line.This enab les low-cost product io n of blocksfor claddi ng .The Alta andDovre flagstonescanno tbe treated in this way,and mustbe cutbysaw ing.

Thereare alsomarked differences in the polishi ngprop- ertiesof the flagsto nes.Eventhoughthe colourofthenat u- rally cleaved surfaces is medium grey for allthree flagst on e types, Altaand Dovreturndark grey,almostblack,whenpol- ished, whileOppda lturnslight grey. Thismay be explained by the porosity and grain size. Smaller grainsand a better cohesion between the grainsgives adarker colour to pol- ishedsurfaces. Anot her consequence ofthelow porosityin theAlta and Dovreflagstones isthattheyaremuch harder and more expen sive to cut and polish thanOppdal,even where mineralcontents,by volume,areapproxim ate ly the same.

There are notable,but notsig nificantvariations in bend-

ingstrengt hand crushi ng streng t hbetwee n therocks ofthe deposits.Again, theOppdal flagston e occursat the lower endofthe scale,probablydueto itsporosity.How ever, the directional differencesaregreaterin the Altaand Dovreflag- sto nes,due to the st ro ng linear anisotropy develope d in theserocks.

Conclusions

The three flagstone deposits, and other similar deposits wit hin theNorw egi an Caledonides,arepreferent ially devel- opedclosetothru sts orin thest rongly flattenedandsheared lon glim bs ofmaj or folds.Character istica lly,metamorphism was in midd leor upper greenschistfacies.Such cond itions werefavourableto thedevelop me ntof the kindoffol iat ion and mineral growt h necessary for a workable flagstone deposi t.

Flag ston es from Alta.Dovre and Oppda laresuperfic ially simi larwhenviewed inbuildings or paveme nt s.How ever, as a resultof differences in respect ofworkability inquarryand fact ory andot her inservicerequire me nts, they canbe cate- gorised into two main groups:1) the essentiallymyloni tic flagsto nes fromAlta and Dovre, and2)theessentia llygrano- blast ic Oppd alflagston es.Thisis a ref lectio nofthe factthat themetam orph icflagston esrepresentdifferent stagesalo ng aline of low -grade,dyn am oth ermal metamo rp hicdevelop- ment from moderat ely high to very high strai n,where the average variet iesof Oppda land Altarepresentthetw o end- mem bers.Based on the texturalandstructura ldifferences,it is possibleto relateanum ber ofimporta nt'technical'proper- ties of the flagstones to the degree of stra inand to erect a sim pleclassificat io n schemesuch as shown in Fig.14. The 'low-s trai n' end -me m ber is characterised by thicker slabs, poorercleavabi lity, high er porosity and water absorpt ion, low er st rengthand(most likely) abett er abilityfor transverse breakage thanthe'high-strain'variety.Inall three deposits, there are significa nt lateraland vert icalvariat ionsin strain, leadin gtoquit erapid changesin thequalityofthe deposits from unexpl oit abl e psam mi tes throu gh paving -q uality to roof ing-q uality flagst one.Thus, one can find sub -types of flagsto ne in the Oppdalarearemini scent oftheaverage qual- ity of Alta and Dovr e, and vice-versa.This is afeatu re one sho uld bear in mind when invest igati ng other depositsor lookin gfor new ones.Aprof ession almapp ing of important structuralfeaturesandstructural character isat ionof therock

NGU-BULL436 , 2000 -PAGE146 TOM HELDAL

s

Fig.14.Texture sand quality aspects of flagstonesand theirvariat ion sac- cordingto the degreeof strain .

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•

would,necessarily,increasethepossibilityof predicting sub- surface depositqualityaswellasproperties of importancefor the user.

In additiontothe strain-dependent variation inc1eavabil- ity,mostof the prob lems thatthequarrymen encounter are relat ed to the overallst ructura l pattern in the area.Product type,produ ct ion yield,blasting and splitting directions and surfaceappearanceareall factorsthat arerelatedtotheint er- act io n ofseveral deformational episodesandcan besystem- atically described and inte rpreted in order to increase the predictabilityofsubsurfaceflagsto nequality.

Acknowledgements

Theaut ho rswish tothan k Stenslip eriet and Alt askif erAL,Alta,Skifer&

Naturstein avd. Do vreand Skifer&Natu rstein (avd. Oppd al),forsha ring their experien ceon working andusingflag stone,andfor theirgoodsu p- port during field work.We woul dalso liketo expressour grati tu de to sev- eralgeologistswho,duringthe last decades,have investi g at edflagstone deposits in Norway,inclu ding David Roberts,Per Rygh aug ,KlaasB.

Zwaan,Odd 0vereng andJanEgil Wanvik. Do naldRamsay ,DavidRob- erts,thelat e Brian Stu rt,and Alvar Braathen areacknowled g ed for criti- callyread ingthe manuscript.