Questions about myrmekite in deformed rocks

hmvnal at .%lrulrural L.,oh~gv, Vi~l I ; Nl~ q. pp 1")TqIqpqW~, Iqql

I)llal ~l.J I/ql 'f,()t (lip ~ (I I11)

P n n l e d irl (.lle,.ll Bntalrl

ii , Iq'H F'i't~,]rrl,~rl F'I,',,,, pl*

Q u e s t i o n s a b o u t m y r m e k i l e in d e f o r m e d r o c k s R H

VERNON

Scho{dol Earth Sclences, Mac'quarlel.lnlverslly, Sydney, N S W 2 1 ( ~ , A u , d r a h a

(Re¢elved 28 Mar~ It Iqt~), a,'ct'pted m tel,t=~,,d /arm 18,4ptl/ Iqql ) Abstract--Several queshuris ahoul Ihe developmenl ol myrmeklle in d e h ; r m e d lel'.;ic and mclapehltu rL~uk,, remain unanswered AIIhough a genelal relalionnhip between d e l o [ m a l i u n and rnyrmekile appear.,., h~ be well eslabhshed, Ihe lacI Ihal myrmeklle t.,qow.,, into relallvely non d e h w m e d K leldspar, wh;le being ,~lmullaneou,dv delormed and recr'yslallured al Ihe r e a r ol Ihe grow'lng lobe',, ,,el~,eSls Ihal Mlam energy may nol be a malor conlnbulnl Io Ihe advance at Ihe growlh inlertace ol the myrmekHe c o h m y Furlliermote, be~ au.,,¢ myrmeklle lypically nucleal¢.s on exiMing plakrloclase, rulher Ihan in K leldspar, .Mram energy may hal ¢onlnbule dtreclly I~ nu¢leahon ol myrmekile eilher, al leaM in many inslantes However, slram energy probably ¢tminhule,, =ndtleclly by lacdtlalmg access ol fluids Io k~owlh sties, Iheleby a l l e n n g Ihe local chemical e n v m m m e n l and ,,o pfomoling develupmenl ol myrmeklle Because myrmektle reery,slalhzes readily m d e l o r m e d lelstc i,=ck.,,, tl =,, one ol Iht-' ma=n c o n l n b u l o r s Io Ihe developmt.'nl t~l h)ha

INTRODUCTION

solid state replacement processes accompanying defor mahon, and many people have postulated that myrmek THIS NO'T'I:.asks some questions about processes respon ire grows at sites ol locally high stress, see Simpson & sible for the development ol myrmekile in deformed Winlsch (1989, p 201)for relerences However, Hib felsic and metapelilic rocks This =s a long standing, bard (1987) suggested that myrmeklte crysialhzes from complex problem, reviewed by Simpson & Wintsch melt dunng deformation of mcomplelely crystallized (1980), and definihve answers Io many of the questions granite. This proposition will be examined first, alter are not available 'Though confident mterpretahons are which various questions about sohd state growth of unwarranted at this stage, judicious questions, even =1 myrmekile will he discussed sumewhal speculative, may help to guide future re search 'The main questions are concerned with (I) whether myrmekite grows in the solid state, rather than DOES MYRMEKITE CRYSTALLIZE FR()M A by cryslallization from a melt, and (2) whether slraln MELT? energy is a dominanl contnbutlng lactor to either the growth or the nuclealion of myrmekile, or whether Hibbard (1979) inlerred thai myrmeklle in mesosco. deformation contributes indirectly by promoling access p,cally non delormed grantlotds is due to cryslallizauon ol fluids. from a water saturated, pressure quenched melt, as Myrmeklte is best known in mesoscopically non opposed to more Cl.mvenllonal hypotheses mvolvmg delormed granitoids, Ior which Simpson & Wmtseh replacement ol K feldspar m the solid stale (e,g, Phtlhps (1989, p. 273) suggested that myrmekite may form in 1974, 1980) Htbbard 11987) also suggested thai myr response to local stress differences on K feldspar grain mekite in deformed grantlold rocks is due h) t'ryslalltz boundaries during subsohdus coohng. Myrmekitic inter alton of small amc)unls ol water salurated magma in a growlhs in several delormed granilotds have also been largely crystallized rock, In response to 'micro pressure described (e g. Binns Iq66, Phillips et al. 1972, Vernon et quenching' during delormat,~n. Hibbard's hypothesis is al. Iq83, Simpson 1985, Simpson & Wlntsch 1989 and based on his observatlun of crystal laces and zoning in references therein), and my observations suggesl thal it the plagioclase 191'some myrmekiUc aggregates (Hibbard is very common in such rocks. 'This note is concerned Iq79, fig 5), as well as on his general model ol late mainly with these occurrences, though some reference =s magmalic crystallization tn granilolds (Hibbard 1979) also made to mesoscoplcally non del'ormed gramtoids Relerrlng to Jahns & Burnham (Iq69), he suggested that Myrmekite also occurs in metapehhc gneisses (e.g. Ver K is partitioned strongly into an aqueous phase, which non 1078, 1979, Nold 1984, Vernon et al. 19911). ends eoprecipitalion of K leldspar with plagioclase and Most commonly, the myrmeklte occurs as lobes or quartz., allows cry.stalhzatlon ol myrmeklte lr1~m the colonies projecting into porphyroclasls ol K feldspar remaining melt dunng pressure quenching, and eventu (typically mierochne) from the margins of the clast, ally promotes precipitation around the mvrmekite of K which may show marginal internal recrysialhzation and feldspar tram the K-nch aqueous phase However, ml~re neocryslallization to fine-grained aggregates (Fig. I). In recent experimental work has shown that the K/Na ratio places, myrmekile also occurs as tnnges along Iormer in the aqueous phase is never greater than m the coexisl Iraclures in K-feldspar (Figs. 2a & b). Generally the mg silicate melt, unless the melt is also in equdibnum development ol myrmektte m these rocks is attributed to with a K-poor, Na rich m.neral, such as hornblende or 979

qX(I

R H VF',RNON

Iourmalme (Burnham 1982, Burnham & Nekvasil IqXh) Evenllc~mdiltonsaresmlablelorparllhonmgot K mlc~ Ihe mell, how can the aqueous phase prectp=tale lale K I'eldspar alter II is lost I'rc)m the system dunng pressure quenching'? Furlhermore, the parliltonmg probably would nol be slrung enough Io prevenl K. leldspar Irom prec=pilatmg from Ihe sihcate mell (Burn. ham Iq82) Therefore, s~me K leldspar should cryslal. lize, ah_mg wnth quarlz and sodnc plagnoclase, rnght h) Ihe sohdus, rather Ihan K leldspar following plagioclase and quarlz, as in Hibbard's model. In supporl ol his hypothesis, Hibbard (Iq7q) stated that myrmekile is abundanl m aphles, which he con lended are Iormed by cryslallizaticm of pressure quenched, waler saluraled magma This mterprelattem of Ihe ongin of aphles may well be correcl, bul myrmek lie ts nol especially common in undeformed aplites, as indicated by extensive reviews of igneous petrography (e g. Johannsen Iq't2, pp. ql-q3), ~mply=ng Ihal Ih=s =s not an elleclwe argumenl. Myrmektte in aphtes may sl=ll be inlerpreled as being replacive, on the available ev=dence Many myrmekillc aggregates are lobate, withoul obvious cryslal laces (F.gs Ib & c) However, the obser rations of Hibbard (It~79) and S=mpson (It~85) that the plagi.'oclase component ol some myrmekil=c aggregales locally has slraighl boundaries Ihal may be crystal faces (F'=g. Ib) can be explained by the presence of fluid along the advancing mlerl'ace between the myrmek=le and Kleldspar, and need nol imply crystallization tram a melt. In lact, flu=d films generally may be revolved in Ihe development of cryslal faces in minerals in metamorphic rocks, although Ih=s is still speculahve (e g. Vernon 1976). 'The complex dtffus=on of components necessary Ior Ihe aggregate Io gl"ow (Mongkollip & Ashworlh 1983) may necessilate the presence ol fluid along the boundary between myrmek=le and K feldspar, the access of such a fired being favoured by an approx=malely I(1% reduction in volume accompanying the replacement ol K leldspar by myrn~ekile (Simpson & Wmtsch 1080, p. 271) Furthermore, tl' myrmek=te cryslalhzes from a melt, why doesn't ils pla~'qoclase componenl always develop cryslal laces, as leldspar invarmbly does dunng Iree growth =n a mell? W=lh regard to partly molten granito=ds undergoing delormalion, Hibbard ('1987, p. 1059) suggested thai Ih,s delormal,on would enhance 'micro pressurequenchmg', so thai myrmekile could prec=p,late from the remaming melt. Th=s idea is opposed by the argu ments agamsl H=bbard's general hypothesis presenled above, as well as by the occurrence ol myrmekile as replac=ve Irmges along former fraclures =n K leldspar (F=gs. 2a & b). In any event, whether or not pressure quenching would occur m deforming graniloids rela lively deep in Ihe crusl is arguable. Moreover, as pointed oul by S.mpson & Wmlsch (1989), the presence ol mell is oul of Ihe quesl=on for graniloids undergoing subsoli dus deformal=on millions of years after they have cryslalIized For example, =n relrograde zones at Broken Hill, Auslraha, myrmek=le was Iormed in lelsw rocks al lower

amph=bohle lac=es cond=l=ons, Iollowmg peak granuhle lactes melamorphtsm (Phdhps el el Iq'72) Myrrnektte is also conrmpn m lelsic gneisses and myhmilcs produced m shear zones in the Anmaljtra Range, ~.'enlral Auslra lia, durmg Ihe mid Palaeozoic 'Ahce Springs Orogeny' (Coll,ns & 'T'eysster Iq,~q). 'The myhmtlic r~cks were Iormed by Ihe dclormalton ol' grantloJds Ihal had been metamorphosed al granuhle lac,es tonal=lions some I,.1()() Ma previously, and the lemperatures dunng Ihe mylonitit' deformalion were h~o low lu permit remelt,ng With regard to deforming gramlo=ds, Hibbard { Iq87, pp 546-547) staled Ihat myrmekile ~ccurs mainly in 'pressure.shadow' (slram shadow'S) areas, ralher Ihan m zones normal to the local direcln3n ol maximum shorlemng, suggesting migration ~1' magma Io low-strain or low.pressure zones. However, observations (tl Ver non et al (1~83, hg ~), Simpson (1985) and L,a Tour (1087) indicate Ihal, on Ihe conlrary, myrmektle occurs me,st commonly in zones parallel Io the Iohalion, normal Io local zones of max=mum shortening Moreover, =n delormed gramto,ds Irom the Pap=)ose Flal pluton, Cahlornia, which was used as a prime example by Hibbard (1087), myrmek=le occurs very commonly as replacive lobes concenlraled along zones perpend=cular to the mferTed local direct=on of max=mum shortening or even completely surrounds K leldspar porphyrq~clasls (Fig. Ic). 'Therefore, a replac=ve, rather than a magmalic, origin Ior myrmekite =s preferable, ludging ham the ava,lable evidence

WHAT ARE TH.E ROLES OF STRESS, STRAIN AND FLUID COMPOSITION? S=mpson & Wmtsch (1989) mlerTed Ihal local m creases m both stress and strain localize the developmenl o1' myrmekile. 'They suggesled thai temperature, pressure and chem=cal aclivtlaes c~mtr, fl the occurrence of myrmekile, but that stra,n energy localizes Ihe reac lion at sales of high normal st ress These sties occur along boundanes of K leldspar porphyroclasls that are paral. lel to Iolia =n the detorTning rock 'This interprelal=on =s part,cularly strong where K feldspar boundanes Iringed with myrmekile are parallel to S surlaces al 45" Io Ihe plane u[ shear m S-C gnetsses, as these surlaces develop normal Io the d,reclion of maximum local compressive slress (c g. S=mpson It~85, Simpson & Wmtsch 198q) However, myrmekite may also occur in siles where Ihe inlerence of Iormer high compress=re slresses cannot be delended (Fig. la). Moreover, wilh advancing growth, myr'mekHe may complelely fnnge K leldspar porphyTo clasls (Fig Ic), although Ihis myrmekile growth could be Iostered by locally high normal compresswe stress Ihal changes its onentatlon with progress*re del'or mat=on. However, it would be difficult to exclude the possible effects ol shear slress m all these SllUUltons. Furthermore, myrmek=le also occurs along Iraclures (F'lgs. 2a & b), hal only those oriented in such a way Ihal tl,gh compressive slress could occur across Ihem, bul

~1~ I Iill"[ ', I11' Irl ~,J~'l~ I[lllq'll

I, ~'l '

Ftg l (a) Aul.~,en ~m.'a!.,,, Ir~m II'H. L.akc-' Mag,~iq~l'q: arL-a II~.lh,', ~.,hL~wtril;, n'iiL rl~'hne m,'r~r~t-nhnl, r, prphvr~'la,,I rh.,I h~.~,,I'~'en I~pc~..llly rcpla~'~'d by h~h~'~, ipl m~,,rniektte (.'rl~,.,,.,ed pq~lar,,, ha,,e t~l pli~fl~hJph ,..I .-I mm (h) Myrrrlq'kih' Iq~l','~, Iriril;iri!; i] p~rplwu~pLla,,I ~11 K leld,.,par iri Ihe F'apq~L~-' Flal plul~m, (.ahh~rnt~.i Th~ myrmckll~-" Iia,, ~J-i,]rp f:cnr'r~.llly r~um;Ic~'l h~url¢iarl~.-,, a~,ain'.,I IIie K leld;,par, hi.Jr h~*'allv Ih~' I~uridarie~, ,..ire' ,,Ir;Ji.~.',hf ,Jnd p*~,,~,nhlv ~ ry.'.,talh~f,n'aphn~. 'Th,~ rri~,rmcl, ll~r,,,,,,,~,,, tnl~ hn~' ~-~ratrl,.d. r~'~ ry,.,lalhz,:,l ag, l.',re~.~le.,, ~1 quarlz ,Jrl,'l pla!.,.l,~cl~],,t. (,,~m,' wllii rc~,l~.Ju~.il £llJarlz hl~J'~,., Ir,~rrl I[1(-. I,~rmer m~,,rm~'k~l~') tn II'le rrlalrl~ (.'r~,,he,l [,~lar',, h~J,,,-' ,~1 pl'~,~l~ltr~l'~h I '~ mm (~') .',,rrl~.dl r,'ll~' ~1 K I~'ld,,p,~r ~,~mF, l~-'h'h' l,IJrr~urld~d I'~,. I,d~.ll~' n-lvrrrl¢l,.~l,: F',.ip,~,.,,. FIJI plul,m (_'Jhh~rrll,~ ( r,~,,,,~'~.'l F,~lar,, h,.J,,,- ~l [~h,~h~l;r,.Ipl'l .' 7 rrlm

I-,l!; .P (,J) Rqpl,',~'n~.,~: Irurll..,,~ ~J m'~,imt'kil~:' ,Jhm~, ,J fl~nnL~r IrJ~'lur~ ~ in ITIIL'r(~L hne PapLH~.~ ~ H a l plufl~n, (.';JJillprnld ~CPlTII~ ITiiJ,~ (~','ilt:' ,JJ~,;~ ;P~'~ IJl~,, ~Ji(Pri~, IhL ~ Jr,.iL'lul( ~ , ,Irid ~,qp IhL' JAIL'JJ ITI~,'[ m~ J~,ll(~ JqPl'lTlirl~, rq~,lCliL~l'l Ill;.Iy IIJW' Inw~Jved Ihw ~rqbc.JiJL'lllln oJ mu',l'llVlh' ( A,,l'lw, prlh IL~7.' Phfllnp!, t,tul I'~72) (.'r,p,.,,,~-'d pLflur,,, l',,J,,1' ~I ph,~l~h,~uph .~ ? rrllTl if-l) Nrrlull Ii~h~',,, ,~I m v r m e k i l ( ~ Iriril.,,ml~, ;J z~ri~ ul r~'~ry,,lalhzal,~n al~nL.,, u h~rmLor lldLIUlW nn rru~ l; ~pL~l l hll l; ' K le. ldspar, l,JIc~ JIJl'~,~,l~' phJlon, F~ll'ull~., l~'llarle. (-'ahl~rnla E'x,,~l~,cd lamell,Jc ~I all'~nle nn ll'ic' K h~hJ,,l~,Jr l.qlnllrllJC ii!.~I'II ur, ill lhl ~ rrlyl'm~'kilc ~, ,,u~,~.~"~llrl~ ~ lh,'.ll lhe h~rrrl~'r iTlVrrri~l~ilq ~ lhal l'la,~ lpc~n re~ [w, lalllzed I~ l~rdrH~hliJ',,ll~ plu!.',l¢~ l a w dnd qlJiJrl~ In re.~ID;~rl~',e h~ m ~ v c r r l c n l ,Jh~nl.~ ll'ic' veM (. r ~ . , e d l:~l.~r~.,, h-,,,~~ ~I ph~l~.~,raph I ~ m m (~) ~,.~ry ,,rn~Jll r¢'h~' ~I K h'Iil~p,Jr irln~?~'d hv m v r m e k I l ~ , i ' a p ~ , , e F,lal phJh~ll. (-'i.llllqprrln,J Fnn~~ l:r;JMvd fl~ha ~I rc:.c'rv,,lalhz~.~d an~l p d l l l y recr~,,~.,lalhzt'd mvrmc~k~le lead awa,~' nnl~ ll'ff, malrl~ (.'r~,,,.,~'d p~lar', l'~a!,~~ ~I ph~l~.;r,,l'fl~ I ~ m m

Myrmeklle in delormed rock,,, als~ in stlual,m,~ where high o,mpresslve sires,,, w~uld be unhkely Therel'ure, a quest,.m may be asked as t~ whelher sir'am and c, msequenl recrystalhzatlon may prove h) he more =mpurtant than compress,ve stress tn promullnglhedevelopment ol mvrmeklte If the pub hshed evidence indlcaling a connection between delor malton and Ihe development of myrmeklle is accepted, Ihe I'olhlwing three questions are relevanl

IS STRAIN ENERGY A DIRECT, MAJOR CAUSE OF MYRMEKITE GROWTH? Is strain energy in Ihe Iorm ol high concentrations ol tangled dislocations (Simpson & Winlsch It)Sq, p 271) the mare, direct conlnhuling laclor to the development of myrmekite or is chemical environment the more direct cause, deh~rmalion assisting by promohng access of fluids? Simpson & Wintsch (1~89, pp. 20q--271) suggested Ihal local increases in strain energy would cause replacemenl ol K I'eldspar by plagloclase and quartz, via components m a fluid. However, Hanmer (1982) suggesled that delicate vermicular mtergrowths could not survive much strain, and consequently in. ferred that myrmeklte ts posl-deh~rmatlonal For myr mekite Inn[=,'qngK leldspar porphyroclasls, mlcroslruc lural evidence shows that progressive deformation and neocrystalhzatlon destroys myrmeklUc intergrowlhs, converting Ihem to polygcmal aggregates of quartz, and pla~noclase in fine grained Ioha (Vernon et al. Iq83, Simpson lq85, Mu~re Iq87), asshown in Figs. I(b) & (c) and 2(b) & (c) 'This, combined with other evidence (e.g Vernon el al Iq83, Simpson & Winlsch Iq89), suggests thai myrmekite in de.formed racks is a product of reac lions accompanying deformation and so is syndel'orma tional, nol post de[ormallonal. However, it also indicates that myrmeklle lobes cannot exist (and hence cannot grow) in the high strain environments thai occur at the margins ol K I'eldspar porphyroclasts. Instead, they appear Io grow hy replacement of K leldspar in an environment protecled from deformation, the growth occurnng al the Ironl ol the lobes, where the strain should be minimal, and away from the deforming, recrystallizlng edge of the porphyroclasl (Figs. Ib & c). In other words, the growth front moves ahead ol an advancing I'ront ol deformation and recrystalhzatlonneocryslalhzation that simultaneously destroys the mvr meklte behind Ihe advancing lobe, so that the pro portion ol myrmeklte to residual K teldspar tends to increase with progressive delormallon (Figs. Ic and 2c) In extreme examples, mvrmekile fringes advancing from opposite sides ol a K,leldspar porphyroclast may meel, resulting in an aggregate ol myrmeklle enclosed by matrix t'oha. This implies thai slraln energy is nol a direct cause of the growth of myrmekile, unless slram energy associated with translormallon twins in microchne and/or lamellae of exsolved albile is effective, which possiblhly is dis cussed in Ihe next section Regardless ol the Ioregomg uncertainhes, deformation may well he a malor redirect

uX3

contnhutmg lactor Io the growlh ut mvrmeklte, by l acnlnlalnng access ~1 fluids t~ the gr,~wlh Ir, mt, thereby altering, Ihe local chemical envlnmmenl and prq~mollng development ul the myrmekite (e g Simpson & Wintsch I~Nq, p '271), with or without a m~table contnbution trom slrain energy At this stage, the nature of Ihe chemical changes, as well as suur~.'es and sinks for chemical components, are matters Ior modelling (e.g Simps~m & Wlntsch IqSq), clear evidence being dlfficull to obtain Similar reasoning applies to myrmeklte occurnng as Innges replacing K feldspar along Iormer fractures in K leldspar (Philhps el al Iq72, Vernon el,rl. Iq83, pp. 1~,4135, fig. 0D), as shown in Figs. 2(a) & (h). Some of the myrmeklhc Iohes are curved, suggesting growth and possible duclde deh~rmalion uf Ihe aggregates dunng movement along the I'racture (.-'onlinued movement along Ihe fracture or Iormer fracture induces delor. malion and recrystalhzatlon ol the myrrnektte closest to the snte ot the Iraclure, the recrystallized aggregates being partly Innged with myrmekite advancing into Ihe undel'ormed K leldspar (Fig 2b) Therelore, as Ior myrmektte fnnging p~rphyroclasts, the growth lace ol the colony progresses away from the Iracture into rela lively weakly delormed K leldspar Indeed, cemcen tratlon ol tangled dislocations in K leldspar adlacent to a Iraclure is less likely Ihan in K leldspar adlacent tu an active Iohum inw.flvlng plastic delormatlon ol the mar gin ol the K feldspar in a mylonite or augen gneiss Again, the inlerence is Ihat the growth ol the myrme, kile is n~l controlled directly by strain energy in K leldspar, Ihough Ihe Iracture presumably provided access Ior fluids Ihal supplied nulrlenl components [or the myr meklte and removed excess components 'The I'oregomg dmcussion relers to growth of myrmek lie The question of the possible contribulton of strain energy Io m w h , a m m ol myrmek=te is discussed in the next section.

DOES STRAIN ENERGY ASSIST NUCLEATION OF MYRMEKITE OR DOES HETEROGENEOUS NUCLEATION PREDOMINATE?

Conceivably, strain energy could assisl nucleation ol the myrmeklle, in as much as the aggregales tend to be concentrated adjacent to hlgh-slram zones in lelsic mylonitic rocks, as discussed previously For example, Simpson & Wintsch (1989, p 271) suggesled thai high concentrations ol tangled dislocations in K-leldspar along boundanes sublected to high normal stress destroy the K-leldspar-plagioclase--quartz equ,librium that per tams elsewhere in the rock and so induce replacement Though no evidence ot the dislocations ts available, this mechanism may apply to some or all myrmeklte beanng rocks. However, in most myrmeklte beanng, mesosco pleally non-del'ormed granltoids, the plagioclase ol myr mekltlc aggregates is optically continuous with pnmarv pla~.,noclase, implying that the plag]oclase componenl nucleated helerogeneously on existing plagioclase 'This

q~,4

R H. VEI~N(.)N

als~ apphes I~ deh~rmed lelMc and mclapehl~c rocks m which mvrmek=le is ~n contact w=th plag~oclase How ever, can ~l apply h~ myrmekile on K leldspar-K leldspar gram boundarws or along mlragranular Irac lures nn K leldspar'~ Sodnc plag,)clase (locally myrmekt I=c) m 'swapped nms' along K leldspar-K feldspar b~undarws, wtlh Ihe same cryslallographw onenlalton as exsolved alb~le lamellae nn Ihe opposile K I'eldspar gram, imphes Ihal helerogencous nucleation on plag0o clase (m Ihts instance exsolved plag=uclase, rather than pr=mary plagioclase)m=l=aled growth ol the nm plagtoc lase Th=s mechamsm could also apply Io myrmekde occumng along Iraclures =n microperlhihc K feldspar (e g Vernon el~d 1983), allhough recryslallizat=on and growlh ol muscov=lc commonly obscure Ihe relation sh,ps In lad, mtcrostruclural evidence of the nucleation subslrate for myrmckile ,n delormed rocks =scommonly obscured by increasing deformalion-recrystalhzalton and/or replacemenl, and so the queslion ol the contn button ol slram energy to nucleal,on ol myrmek=te remains unanswered. Observations of optical conlmuily between myrmek=tw and pnmary quartz =n some rocks (Slel & Breedveld 19q(I) suggest thai nucleation on quartz may imtiale the growlh ol some myrmekil=c colonies. In myrmekllc beanng, rnesoscopically non delormcd gramloids, the K feldspar commonly has strata energy at the boundanes ol' ( 1) del'ormaiion twins caused by the Iransl'ormalton ol' orlhoclase to m=crochne and (2) sere= coherent lamellae of exsolved alb,le. However, Ihis slram energy m the K leldspar ts inadequate to promule nuclealion ol myrrnekite, because ~t nucleates al gram or fracture boundaries, rather Ihan inside r'mcroperthtl|c mwrochne grams. In summary., a cnnlribulion I'rom strain energy may nol be necessary I'nr the nucleation ol myrmekHe =n many rocks, tn wh=ch helerogeneous nucleal=on on exist mg pnmary plag=oclase (less commonly quartz) or exsolved plagioclase may be el lechve

HOW DOES MYRMEKITE DEFORM AND CONTRIBUTE TO FOLIATION DEVELOPMENT? In delormed granik, ds, myrmektte lobes commonly pass continuously mh) areas ol polygonal (recrystal hzed) plah~oclase and quartz (Fig. 2c), via mlermedtale stages m which some of Ihe quartz remains as blebs (Vernon e/al. 1983), ow,ng Io progressive recrystalhza lion o1' myrrnekite (Vernon el al. 1983, Simpson 198fi, M.ore 19X7) "rhe aggregates pass laterally into fine grained Iol.a (shear bands) m augen gnelsses and mylo nmc rocks (Vernon etal. 1~8'3, p. 13~), as shown in F'lg. 2(c) In places, the aggregates curve rata Ihe Iolia, suggesting d~splacement along the folia dunng the recrystalhzal.on and continued growth nl myrmeklle (Vernon el a/ I983, p. 13~, S=mpson It~8.~, S=mpson & Wmtsch 1~89, p 204). In some rocks, the myrmek=le recrystalhzes more readdy than most ol the olher miner als and =s one of Ihe mare conlnbutors h) the deveh)p

nienl ol hd=a (Vernon el al I983). 'T'he quest=ons may be asked, is th=s abdity connecled w=lh ils fine gram size, and, tl' s~), does I he large mlerfacial area ol' the myrmeki t=c mterg, r~wlh allow flu=ds to penclrale and prumole b, mndary movcmenl during, recrystallizal=on'~

CONCLUSIONS Several somewhal controversml queshons relevanl Io the developmenl of myrrneklte in deformed lelsic and melapelilic rocks remain unanswered. Though a re lationship between myrmeklle and deformation has been estabhshed and an ingenious model relating strain and chemical react,on has been proposed (Simpson & Wmtsch 1989), tl is not yel clear that Ihese relationships exlend Io myrmeklte m mesoscopically non-d¢lormed gramlolds, although tt is a possibdity. In addition, the exact nature and de[wee ol the contribution ol lattice slram energy Io the nucleation and growth o1' myrrnekite remains to be clanfied. The lacl thai myrmekile grows mlo relatively non deformed K-leldspar, wh,le being simultaneously delormed and recrystallized at Ihe rear ol' Ihe growing lobes, suggests the possibility thai strata energy may not be a major conlribulor Io the advance of the gTowth interlace of Ihe myrmekile colony. Further more, the tact thai myrmekile typically nucleates on existing plagioclase (pnmary or exsolved), ralher than reside K leldspar-.-even m mwroperth=tic mlcrocline, which should have some internal strain energy, owing to the presence ol deformahon Iwms and exsoluhon lamellae--suggests Ihal strata energy may hal contrlb ule directly 1o nuclealion ul myrrnektle etlher, at least in many instances. However, deformation appears to con Iribute mdlreclly, by facditating access or flu,ds to growth sites, thereby altering the local chemical en v,ronmenl and promoting the development ol myrmek =re. Myrmek=le appears to recrystallize relatively readdy m deformed fels=c rocks, and so ts one of the main contnbutors to the development of Iolia. Myrmekile =s being increasingly recogmzed ,n deformed felstc and metapel=l=c rocks; perhaps some ol them will reveal answers Io Ihe questions posed in this paper ,4L knnwledgement.~--I Ihank Seoll R Palerson and Roberl P W,nl,~h hu helpful comrnenls The work was financed by Ausltalian Rescmch Council Granl No A '~841'57I(~ and a Li S -Auslralia Bdaleral Sc'lence and 'Tec hnc',hlgy Ciranl

REFERENCES A,,hwcwlh, ,J R IH'72. Mvrrnekde,,, ol ex,,,nlul.m and replacemenl onh,~ns Geol Mag IO~, 4h---h2 Binn,.,, R A 19('~ Chanilic inlrustons and te/,.,~onal melamorphw rocks ol Permian age hom the Wonbnvtbinda dtsln~.l, north ea.,slern New Soulh Wales ) Ptoc R ,qo¢ N S W ~ , .~-'3h Burnham, C W 1982 Magmas and hydrnlhermal fluids In Geo thernL~try o/Hvdrothermal Ore Depos:u (ediled by Barnes, H L ) Wdey, New Yurk, 71- I "~h Burnham, (' W & Nekvasd, H 198h E,qudibnum properl,es e,I bn'ande pegmahle magff'nas Am Miner 71,2'~9-'2h'~ C'ollins, W J & Tevssiet, C' 1989 C'ruslal ~-ale duclde laull syslems in Ihe Arunla Inher, Lenlral Au,,,Itaha 'rectonophv,s'w,s 158, 4q--hh

M~/rmekile in deh)rmed

racks

'4145

Philhps, E'. R Iq74 Myrmekile.--*~n¢ hundred years laler L,llh+~.~7, Egglelun, R A ,.ind Bu,.,eck, [' R 1980 'The orlh.clase-mti.roi.hne IXl-lq4 iriverslon a high resolulion "I'E..M sludv and ;,Ira,n analysis t'onlr Phdhps, E, R 19NO On ~ilygenelt~.' myrmektle Gt'ol Mug 17,29- ~,h Mi,ner Petty~ 74, 121,--IVI Phdhps, E" R , Ransom, C) M & Vernon, R H, Iq'72 Myrmek.eand Hanmer, S K IqH2 Mtci,oslrat'lure and geuchemlsfry ol plat:,n.clase musowtle developed by relroF.~ade melamorphtsm at Be.ken Hdl, and mterocline in nalurally delormed giantle J .t~'lrut'l Geol 4, New Sipufl'=Wales Mineral Mag .~4, h7(I-%TM Iq7-214 Htbbard, M I Iq79 Mvrmektleasamarkei,belweenp/eaqueousand Simpson, C' IqH% Delurmalion ol [...,rantl=¢rq..'ks aciu,ss Ihe brtllledu¢lllelranslhon ] StrHct (.;co/ 7 , ¢ d l ~ h l l pnslaqueous phase saluraltor= m [.,wantli~ syslems Bull geol .%c Simpson, (.' & Wmlsch, R P 19Hq E',vtdenLe lot, delormalton Am ~l, liN7-1llh2 mdueed K leldspar replaeemenl by myrmektle ,I metamorph Hibbard, M ) 1987 Deiormahon ol incompletelyi.rvslalhzed maglma (.h'ol 7, 2hl-2'7% systems t.,wanilict.,meissesand their tectonic tmphcaltons .I Geol Slel, H & Breedveld, M IqHq C'ryslall.grapht¢ onenlalton pallerns 95, %4 t-'~hl ul mvrmekllw quartz a labnc memory in quartz nbbun beanng .lahns, R H & Burnham, C W Iqhq Expenmenlal sludies . I gnemses ! Struct Geol 12, 19-28 pegl~altle genesis I A model lot Ihe denvahon and cryslalliz,aliun Vernon, R H IqTh Mi, rummphi,t Proce,~,w,s Murby, London ol panlhc pel,.,,mallles £,'on Gl'ol b4, ~ "t-Stv.l Vernon, R H Iq'7H Pseudomnrphous replacemenl ol coidlenle by .Johannsen, A lil'll2 A Dest nptlve Petrography o/the Igv#eoiL~Rock.i syrriplecltc .nlei,~.wlhs .I andalu,sde, bioltle and quartz L,rthos I I, Volume II The Quart: beurmg Rot k.~ I..Iniversily i'll (."hieago Press, 28 ~- 289 (."hu;ago Vernun, R H Iq79 Formalion ol late sdlimamle by hydrogen La Tour, T E 1987 Gecwherrucal model Io¢ Ihe symple~.l.." Iormallun melasomailsm (base leaching) m some high [.,wadekmemses t,ttho.s ol mv'n'nekile dunng amphibohle knade pru~esswe mylomluralion 12, 14~1%? ol gramle Geol S,,c ,4m Ab.s w Prog 19,741 VemLm, R H , Clarke, G L & Collins, W J 19~1 L¢leal, mid Mongkollip, P & Ashworth, i R Iq8't Quanlilahve eslimahon ol an ¢'ruslal ~.,wanuhle lacies melaml'~rplusm and melllng an example in open syslem symplectlle Iormmg reaclion reslnOed ddlusion ol AI the Mounl SlaHord area, eenlral Auslra.ha In High temperature and Si in coronas around olivine J Petrol 24, h.1%-t~l Metamorphism and Crustal Anutexu Mmerulogi,cal Soci,t,rV Seru,s 2 MLx~re, D E 1987 Syndelormalional metamorphic mvrmekile in (edlledbv Ashw.rlh, J R & Brown, M ) LInwln Hyman, London, gianodionle ol Ihe Sterxa Nevada, Callltlrnia Geol Sot' Am Ab,s 272- ~19 w Prog 19,770 Vemun, R H , Williams, V A & D'Arey, W F 198't Grain size Nold, J L, 1984 MyTmekile in Bell Group melasedlmenlarv rocks--reduellon and Ioliahon developmenl in a dehwmed t,.,n'anttoldbalho norlheasl bllidei' 'zone of Ihe Idahu Balhohlh Am Mi,ner 5il, II1%ltIll h /"t'tttmoph V,s'tt,s92, 12 t- 14h 10'42