sequential growth of cordierite and andalusite porphyroblasts,

7/30/2019 Sequential Growth of Cordierite and Andalusite Porphyroblasts,

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J . metamorphic Geol. 1988, 6. 255-269

Sequential growth of cordierite and andalusite porphyroblasts,Cooma Com plex, Australia: microstructural evidence of a pro-grade reactionR. . V E R N O N , School of Eurtli Sciences, Mucqirurie U ai ~e rs it y, ydney .NSW 2109, Austruliu

Abstract. Low-pressure prograd e metamor-phism of pelitic rocks in the Cooma Complex,south-east A ustralia, has produced cordier-ite-andalusite schists at intermediate grades.The first foliation ( S , ) is preserved largely asinclusion trails i n cordierite porphyroblasts.Microstructural evidence indicates that thecordierite porphyroblasts grew during the earlystages of development of a crenulation-foliation( S , ) and that andalusite porphyroblasts grewduring the development of a later crenulation-foliation ( S 3 ) . Microstructural evidence alsoindicates that the andalusite was a product ofthe prograde reaction: cordierite + muscovite+andalusite + biotite + quartz. The occur-rence of the p roducts of this reaction in beardstructures between cordierite microboudins for-med by extension in S3 confirms that th e anda-lusite grew during the development of S3. Theinvestigation shows that porphyroblast-matrixrelationships can preserve the orientation of anearly S-surface tha t has been largely obliteratedfrom the matrix, as well as providing relativelydirect evidence of sequential mineral growthand metamorphic reactions.Key-words: andalusite; Cooma Complex,south-east Australia; cordierite; porphyro-blasts; sequential growth; syndeformationalgrowth.INTRODUCTIONClear evidence relevant to the problem of sim-ultaneous versus sequential growth of porphy-roblasts in schists is not common, and yet suchevidence is important for understanding meta-morphic processes. especially in relation todeformation. This paper presents detailedmicrostructural evidence for sequential growthof cordierite and andalusite porphyroblasts insome metapelitic (kn otted ) schists in the

Slacks Creek area of the Cooma Complex.southeastern Australia. It is an application ofdetailed microstructural studies, involving por-phyroblast-matrix relationships, to (1) theevaluation of simultaneous versus sequentialgrowth of porphyroblasts. (2 ) the timing of por-phyroblast growth relative to the developmentof foliations, and (3) the elucidation of pro-grade metamorphic reactions.The main conclusions are that cordierite por-phyroblasts preserve as inclusion trails theshape and orientation of the earliest tectonicfoliation, which has been largely obliteratedfrom the matrix and that cordierite porphyro-blasts grew during the.second phase of folding,after which they reacted with muscovite toproduce andalusite porphyroblasts during thethird deformation phase.CORDIERITE-ANDALUSITE SCHISTS OFTHE COOMA COMPLEXThe Cooma metamorphic complex occursabout 110 km south of Canberra in southeast-ern Australia, and has been the subject ofdetailed metamorphic and structural investi-gations for many years (e.g. Joplin, 1942; Ho p-wood, 1966. 1976; Granath, 1976, 1978; Ver-non , 1978,1979; Mason, 1984). Th e progressiveisograd sequence, from west to east, is: chlor-ite, biotite, cordierite, andalusite, and K-feld-spar (Mason, 1984). The mineral assemblagesappear to have resulted from a progressiveincrease in temperature, without much crustalthickening. Symplectic aggregates of andalus-ite, biotite and quartz replacing cordierite inhigh-grade gneisses indicate that retrogradecooling occurred initially without decrease inpressure (Vernon 1979, 1982).Cordierite porphyroblasts appear about0.6 km west of the first appearance ofandalusite (Ho pw ood , 1976, pp. 354, 356;


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256 R . H . VernoriMaso n, 1984, p. 17), both occurring in a matrixof quartz , muscovite and biotite. Th e cordieritehas been extensively (generally completely)replaced by micaceous aggregates rich in whitemica, biotite and chlorit e (Figs 1-10). Smallcordierite relics in the micaceous aggregateswere reported by Joplin (1942, pp. 163, 169)and residual sector twin shapes were reportedby Mason (1984, p . 17). Gener ally themicaceous intergrowths have mimeticallyenhanced an included foliation in the cordier-ite, which assists in microstructural interpret-ations. Many cordierite porphyroblasts haveresidual cores surrounded by the micaceousaggregates, the cores being optically isotropic,owing to late replacement by extremely fine-grained chloritic (pinite) material (Figs 1, 4,5 and 10). They contain very small inclusionsof quartz and opaque minerals that are alignedin parallel trails. These inclusion trails areinvariably parallel to those delineated by thesurrounding, replacive coarser grainedmicaceous aggregates (Fig. 5).

The biotite in these replacive aggregates hasthe same chemical composition as that in thematrix of th e rocks, as indicated by microprobeanalysis (Granath. 1976, p. 94, table 2.3) andis of a similar grain size to the matrix biotite.Therefore, though conceivably it could havebeen formed in a general retrograde metamor-phism during development of a later crenula-tion-foliation (e.g . Gra nat h, 1976, p. 94). it ismore likely to represent primary inclusions(Mason, 1984, p. 17) or a prograde reactionproduct (Wyborn. 1977). This paper presentsmicrostructural evidence that the replacive bio-tite is of prograde metamorphic origin, havingbeen formed in the reaction: cordierite +muscovite +andalusite + biotite + quartz.Some biotite grains may be primary inclusions(Mason, 1984, p. 17). but much, if not all thecordierite evidently grew when the matrix grainsize was very small, in view of the minute size ofthe inclusions i n the cores (Fig. 5). Therefore,most of the included biotite probably grew afterthe cordierite. The size of quar tz inclusions in

Fig. 1. Cordierite-andalusite schist from Slacks Creek. Coo ma . showing quartz-rich beds (to p and bottom.S,, enoting bedding) separated by a metapelitic bed with abundant porphyroblasts of altered cordierite.some of which have optically isotropic cores (C) now composed of extremely fine-grained chloriticaggregates. The porphyroblasts preserve the original orientation of S, s inclusion trails. The maindifferentiated crenulation-foliation is S , , S2 emaining as an undifferentiated mica-quartz foliation in thequartz-rich beds and as a differentiated foliation between S3 olia in the metapelitic bed (micaceousfoliation denoted by M and trending roughly horizontal in the ph oto ). Figures 2-5 are enlargem ents ofcordierite porphyroblasts and their environs from the same rock. in the same orientation as this figure andshowing the S, rails more clearly. Figure 1 I shows schematically how the microstructure of this rock mayhave been developed. Crossed polars: base of photo 24 mm.


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Sequetitial growth of porphyroblasts, Coorna Cotnplex 257the replacive micaceous aggregates cannot beused as a guide, as they may have been formedin the reaction that produced the aggregates,as in the above equation.On the other hand. the chlorite in theseaggregates is most likely to be retrograde, inview of the fact that it does not occur as aprimary mineral in the matrix and is evidentlynot a compatible member of the cordierite-bearing prograde assemblage. This applies notonly to the fine-grained material in t he isotropiccores, but also to the larger grains in the mainreplacive aggregates; these probably owe theirlarger grain size to pseudomorphism of biotite.S-SURFACESComparative labelling of S-surfaces incordierite-andalusite schists of the CoomaComplex is shown in Table 1. Bedding is com-monly preserved and is here referred to as'Sl,', in keeping with current practice. Hopw ood(1966. 1976) labelled it 'Sl ' . and thus calledthe next-formed foliation (a slaty cleavage orschistosity) 'S,', whereas nowadays it would becalled 'S l ' . The next-formed S-surface. recog-nized by Hopwood (1966, 1976). is a differen-tiated crenulation-foliation, which he labelled&'. The nomenclature of Granath (1976) ismore conventional. except for his use of 'SS'fo r 'S,,'.Microstructural evidence discussed i n thispaper suggests that a previously unrecognizedcrenulation-foliation (S2) pre-dates the promi-nent differentiated crenulation-foliation that isobvious i n the field (S3) . which was called 'Sz'by Granath (1976) and Mason (1984) and S3'.fortuitously, by Ho pw ood (1966. 1976).SEQUENTIAL VERSUS SIMULTANEOUSGROWTHHopwood (1976, pp. 354.356, table V) inferredthat the andalusite porphyroblasts in theCooma metapelites grew later than themicaceous aggregates (described previously)that replaced older cordierite porphyroblasts.H e suggested that the cordierite porphyroblastsgrew after the formation of 'S , ' (S , in thispap er, as shown in Ta ble 1) during an inferred'first metamorphic event'. Further, he con-sidered that, during the development of '&'(Sz i n this paper), these porphyroblasts were'retrogressed' (i.e. replaced by micaceousaggregates) and that the resulting micaceousaggregates were deformed, both these events

occurring during the first stage of an inferred'second meta morp hic event '. H e suggested thatthe micaceous aggregates were furtherdeformed by the formation of 'S3" (kink-folds)during the first stage of a 'third metamorphicevent '. Hopwood inferred that the andalusiteporphyroblasts grew during the second stageof the second 'event', locally in the alteredcordierite.However, cooling and reheating, to produceseparate metamorphic events, is unlikely. inview of the relatively short cooling time (about20 Ma) for the Coo ma Granodior i te (a gneissicgranitoid in the highest-grade part of theCooma Complex) indicated by the isotopicstudies of Tetley (1979).Gra nath (1978, p. 117) inferred that thecordierite and andalusite grew simultaneouslyduring a coaxial part of the deformation his-tory responsible for producing 's?' (S , in thispaper) or else post-5, ' . and that ret rogradealteration of the cordierite porphyroblastsoccurred durin g the formation of S3' (S2 in thispaper), producing greenschist facies assem-blages. Mason (1984. p. 21) suggested that thecordierite and andalusite formed in close suc-cession in the same prograde metamorphicevent, and that the micaceous alteration ofcordierite occurred post-kinematically, leavingandalusite unaltered.The fact that cordierite first appears at a lowergrade than where it is joined by andalusite (Hop-wood, 1976, pp. 354, 356; Mason. 1984, p. 17)suggests sequential growth of these porphyroblas-tic minerals, although th ey may still be reg ardedas having formed in th e same prograde metamo r-phic even t. Th e microstructural evidence pre-sented below confirms this, and indicates thatandalusite grew, together with the biotite andquartz of the pseudomorphous micaceous aggre-gates, as a product of a prograde reaction involv-ing cordierite and muscovite.PORPHY ROBLAST-MATRIXMICROSTRUCTURAL RELATIONSHIPSAND TH E RELATIVE TIMING OFDEFORMATION AND PORPHYROBLASTGROWTHInitial examination of the Coomacordierite-andalusite metapelites reveals threemain S-surfaces, namely bedding (Sl,), anapproxim ately b edding-parallel foliation (whichnormally would be called ( 'S, ') . and a later.prom inent. differentiated crenulation-foliation


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258 R . H . Vernoti

Fig. 2. Photomicrograph and identifying sketch at the same scale. showing an altered cordieriteporphyroblast (C, stippled) preserving S , , with adjacent S2 M-domains and S2 urving into prominentS3 M-domains. A porphyroblast of unaltered andalusite ( A ) has grown in one of the S2 M-domains.showing curved S2 inclusion trails that are very oblique to the S , trails in the adjacent cordierite. Theandalusite S2 trails are also more strongly curved and composed of larger inclusions than those in thecordierite. Same rock as that shown in Fig. I an d in the same orientation as Fig. I . Plane-polarized light;base of photo 4.4 mm.


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Sequential growth of porphyroblasts, Cooma Complex 259

Fig. 3. Photomicrograph and identifying sketch at the same scale. showing a replaced cordieriteporphyroblast (C , stippled) preserving S , . with adjacent S , M-domains (M ) containing unaltered andalusite(A ) . A l so shown are prominent Sz M-domains. into which S , is rotated. Strain shadows ( S S ) formedaround the cordierite during development of S r are also rotated towards the S, orientation. Same thinsection as that shown in Fig. 1 and in the same orientation as shown in Fig. I . Plane-polarized light; baseof photo 4 .4 mm.


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260 R . H . Vernon

Fig. 4. Similar to Fig. 3 . showing S 2 M-domains ( M ) and syn-S, strain shadows (about a replaced cordicrite(C) porphyroblast) rotated towards the S.l orientation. Also shown is a cordierite corc that survivcd thereplacement by relatively coarse-grained micaceous aggregates. hut which later was replaccd by extremelyfine-grained, optically isotropic (Fig. 5) chloritic material. Plane-polarized light; base of photo 12 mm.


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Sequeiitial gro ~ l t l i f porphyroblasrs, Cooniu Coniplex 261

Fig. 5. Same field as Fig. 4. showing the isotropic core in the cordicritc po rphyroblast. with straight S ,inclusion trails made up mainly of minute quartz grains and fcwcr. larger opaque grains (Fig. 4) that ilrcparallel to the trails outlined by mimetic growth of thc surrounding mic;iccous aggrcgatcs. Crossed polars:base of photo 12 mm.(which consequently normally would be calledS2') . as shown in Fig. 1. However, closerexamination shows that straight inclusion trailsin cordierite porphyroblasts may be veryoblique to SII. being even perpendicular to S,,in rocks from the hinge zones of large F, folds(Fig. 1). Because the primary inclusions in thecordierite cores are so small, as described pre-viously, t he co rdierite evidently grew when therock's grain size was very small. so that

inclusion trails in the cordierite probably rep-resent the first foliation developed after bed-ding. i.e. s , . This interpretation is supportedby the invariable straightness of the trails.Obliquity between S , inclusion trails and S(, nthe matrix would not be expected in schistsfrom the limbs of large F, folds.The next-formed foliation, S, . is now rep-resented by the main mica-quartz matrix foli-ation that is nearly parallel to SII in Fig. 1. andTable 1. Comparative labelling of S-surfaces. Coonia Complex

Hopwood Grnnath Mason This(1966) (1976) ( IYH- I ) paperBedding S , ss s, S, ,First cleavage s, s, S , s,First differentiatedSecond differentiated

crenulation-foliationcrenulation-foliation S; S 2 S,

Kinks or folds of main S , ' S ; S3 S ,crenulation-foliation: locallya new crenulation-foliation i nmica-rich dom ains(Mason. 1984. p. 82 )


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262 R.H. Vernonwhich is at stage 6 of Bell & Rubenach (1983),so that no crenulation hinges remain. S2 is alsorepresented by differentiated mica-rich folia(M-domains) and quartz-rich folia (Q-domains)preserved between and truncated by the promi-nent S3 M-domains (Figs 1-5). The inferredsequence of events is sketched in Fig. 11.Relicsof S2 M-domains adja cent to cordierite porphy-roblasts are common (Figs 2-5, 10). Localstrain shadows formed around cordierite por-phyroblasts during the development of S2 arepreserved, commonly distorted by S3 (Figs2-5), and these may show remnants of matrixS , crenulated by S2. A few similar zones oflower S2 strain appear, at least in two dimen-sions, to be independent of cordierite porphy-roblasts.Inclusions in the andalusite (Figs 2, 3, 12and 13) are typically larger than those in thecordierite cores, suggesting that andalusitegrew after the matrix grain size had becomeconsiderably larger than when the cordieritegrew. Furthermore, inclusion trails in andalus-ite generally are much more strongly curvedthan those in cordierite (Figs 2, 13), and arecommonly very oblique (even at right angles)

to those in adjacent cordierite grains (Fig. 2) .All these features suggest that the andalusitegrew later than the cordierite. This is alsoindirectly supported by the general lack ofalteration of andalusite, compared with thealmost complete micaceous alteration of cordi-erite (Figs 2 and 3). Although this could beexplained by greater resistance to retrogradealteration of andalusite if the micaceous aggre-gates in cordierite are regarded as retrograde(Mason, 1984), evidence presented below sug-gests that the aggregates are mostly prograde.Relationships between cordierite and S2 arebest shown (Figs 6-8) in cordierite schistsdevoid of and alusite , which show only incipienteffects of S3. nd which occur at lower gradetha n th e cordierite-andalusite schist shown inFig. 1. The general straightness of the S ,inclusion trails in cordierite (Figs 2 4 , ogetherwith slight curvatures of the trails at the edgesof some porphyroblasts (Figs 7 and 8), suggestthat t he cordierite grew during the early stagesof development of S2, using the reasoning ofBell & Rubenach (1983) and Bell, Fleming 8~Rubenach (1986). The local preservation ofmillipede struc ture (Bell & Rubena ch, 1980),

Fig. 6. Porphyroblasts of altered cordierite with straight to slightly curved. approximately parallel S ,inclusion trails, about which anastomose S2M-domains in which S, has been strongly rotated into the S ,orientation. In the S, Q-domains, S , is crenulated, but preserves its overall original orientation. The Sz M-domains show incipient S , kinks and crenulations. Figures 7 and 8 show enlargements of cordieriteporphyroblasts and their environs from this rock, which is a cordierite schist at a lower grade than thecordierite-andalusite schists of Figs 1-5. Plane-polarized light: base of photo 24 mm .


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Sequential growth of porphyroblasts, C o o m a Complex 263

I

Fig. 7. Photomicrograph and identifying sketch at the same scale, showing a porphyroblast of cordierite(C, stippled) with slightly curved inclusion trails ( S , ) curving into a matrix foliation ( S , ) , which is morestrongly crenulated than in the porphyroblast. S 2 domains of strong, non-coaxial deformation (M) are richin muscovite, due to solution and removal of quartz, and show S , strongly rotated into S2. These domainscontrast markedly with less deforme d domain s ( 0 ) in which S , has been only weakly crenulated. Thoughthe rotation of S , in Sz M-domains is very strong, the rotation of S , in the Q-domains and cordieriteporphyroblast is negligible. so that S , in these domains retains its overall pre-S, orientation. The M- andQ-domains constitute a differentiated S 2 crenulation-foliation a pproxim ately at right angles to S , (as shownin Fig. 1) . The M-domains show small S , crenulations. In higher-grade rocks showing a strongly developedS3, th e S2 M-domains remain only as vestiges between abundant S, M-domains (Figs 1-5, 1 1 ) . Same rockas shown in Fig. 6 . Plane-polarized light; base of photo 4.4 mm.


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264 R .H . Vernon

Fig. 8. Photomicrograph and identifying sketch at the same scale. showing several porphyroblasts ofcordierite (C , stippled) with S , inclusion trails that curve into a matrix foliation ( S , ) . which is morestrongly crenulated than i n the porphyroblasts. S2M- and Q-domains trend approximately normal to thepre-S1 orientation of S,, hich is best preserved in the porphyroblasts (cf. Fig. I). utside theporphyroblasts, the folding of S , is more intense than in Fig. 7. so that. though four of the porphyroblastspreserve the pre-S, orientation of S ,. one (to p right) shows an oblique S , orientation. due probably togrowth of the cordierite in a Q-domain that had previously undergone relatively strong rotation during thedevelopment of S1. T he S , inclusion trails in the central cordierite porphyroblast show a 'millipede'geometry (Bell & Rubenach, 1980). indicating t h a t coaxial deformation occurrcd in the Q-domain. in whichthe porphyroblast grew during the early stages of S, development; additional rotation due to S,crenulations occurred in this domain since growth of the cordicrite. Samc rock as shown i n Fig. 6 . Plane-polarized light; base of photo 4.4 mm.


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Sequeritiul growl? of porphyroblusts, Coornu Comnplex 265as shown in Fig. 8, suggests that the cordieritegrew in zones of low, commonly coaxial defor-mation (S, Q-domains), separated by zones ofhigh, non-coaxial deformation ( S 2 M-domains),which are now preserved as muscovite-rich S2 folia adjacent to cordierite porphyro-blasts (Figs 2-5 and 11) .Strongly curved S2 inclusion trails in theandalusite porphyroblasts (Figs 2. 13) suggestthat either the andalusite grew after the devel-opment of S 3 , which previously had folded S 2 ,or relatively late during the development of S3 .T he fact that som e inclusion trails are relativelystraight (Fig. 12) suggests eith er that someandalusite grew during the early stages of thedevelopment of S 3 , or that it grew in low-strainzones during S 3 . The overall evidence suggeststhat the andalusite grew syn-S3, probably late,in view of the commonly strongly curved S 2trails and the general absence of deformationof the micaceous aggregates with which theandalusite is commonly associated. as discussedin the section on metamorphic reactions. Theserelationships apply even where cordierite isabsent (Figs 12 and 13) due to bulk compos-itional controls, suggesting that a ndalusite post-dates cordierite everywhere.NON-ROTATION OF PORPHYROBLASTSTh e typically large angle between th e alignmentof inclusion trails in cordierite and the Sz matrixfoliation (e .g. Figs 2-8) could be taken to implyrotation of the porphyroblasts, with respect togeographical co-ordinates, which is the usualexplanation of this relationship. However, S ,trails in cordierite porphyroblasts are typicallyparallel or approximately parallel (Figs 1 and6) across areas at least as large as a 6 X 4 cmthin section, even where matrix foliations arestrongly folded. This suggests minimal or norotation of the porphyroblasts, although, ofcourse, it does indicate rotation of the matrixfoliation, relative to the inclusion trails. O n thebasis of this kind of observation, Ramsay (1962)and Fyson (1975, 1980) suggested that porphy-roblasts may remain static during flattening,with consequent rotation of the matrix. This isthe essence of more formalized models involv-ing zones of coaxial versus non-coaxial defor-mation (e .g. Co bbo ld, 1977; Bell, 1981). Ram-say (1962) also explained how strain shadowsmay develop adjacent to porphyroblasts duringthis process, and how porphyroblasts that growduring the deformation commonly have straightinclusion trails at their ce ntres and trails curvinginto matrix folia at their edges. These features

were interpreted as being typical of syndeform-ational porphyroblasts by Bell & Rubenach(1983) and Bell et af. (1986). Bell (1985) andVernon (1988) have discussed other examplesof static porphyroblasts in more generalappraisals of rotation versus non-rotation inporphyroblasts.In the Cooma metapelites, not only was theoriginal S , orientation preserved in cordierite por-phyroblasts during the development of S2 , bu tboth this orientation and the adjacent S2 matrixorientation have been preserved in Q-domainsduring the development of S 3 (Figs 2-5). Simi-larly, curved inclusion trails in andalusite porphy-roblasts may be explained by overgrowth of Szfolded during the development of S 3 , as explainedpreviously, rather than by rotation of the porphy-roblasts during or after growth.METAMORPHIC REACTIONIn cordierite-bearing metapelites at Cooma,andalusite invariably occurs close or adjacentto cordierite, either in muscovite-rich Sz M-dom ains (Figs 2-5) or , with biotite and qu artz ,in syn-S3 microb oudinag e 'beards' (Fig. 10) andS 3 folia that truncate and dismember cordieriteporphyroblasts (Fig. 9) , as shown diagramm at-ically in Fig. 11. Some andalusite also occurswith biotite and quartz in aggregates that havereplaced cordierite.A likely metamorphic reaction to explainthese microstructural relationships is: cordierite+muscovite andalusite + biotite + quartz.This reaction was suggested as a possibility byGr an ath (1976, pp. 93-94) and Wyborn (1977),but they found insufficient evidence for i t .Mason ((1984), p . 29) also suggested it as anexplanation for some of the andalusite. Theoptical and chemical similarity of the biotite inthese aggregates and the matrix biotite (Gran-ath, 1976, p. 94, table 2.3) indicates that thewhole rock underwent reconstitution at thisstage (syn-S3, according to the microstructuralevidence of inclusion trails in andalusite, dis-cussed previously). Syn-S3 reaction is also sup-ported by the alignment of biotite in 'beard'structures formed by microboudinage of cordi-erite during the development of S 3 . The numer-ous quartz inclusions in some andalusite por-phyroblasts may also be products of the abovereaction.Locally the micaceous aggregates replacingcordierite are weakly cren ulated , possibly by S,(Table 1) or by a late r phase of S 3 development.Individual mica (or pseudomorphous chlorite)flakes are not deformed, indicating that this


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266 R .H . Vernon

Fig. 9. Similar to Figs 2 and 3. but also showing an S , zone (running top-left to bottom-right) breakingthrough an altered cordierite porphyroblast. Andalusite. biotite and quartz have grown i n this zone. whichmay have involved some boudinage of the cordierite. Plane-polarized light: base of photo 4.4mm.

Fig. 10. Porphyroblast of altered cordierite that has undergone boudinage during S , . the neck having beensimultaneously filled with relatively coarse-grained andalusite. biotite and quartz, the biotite tending to bealigned parallel to Sz which is parallel to the base of the photo). This suggests syn-S, growth of th eandalusite, biotite and quartz. The left-hand boudin shows a central relic of cordierite that has escaped thegeneral micaceous replacement, but which was later altered to extremely fine-grained chloritic material.Same rock as that shown in Fig. I . Plane-polarized light; base of photo 4.4 mm.


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Sequential growth of porphyroblasts, Cooma Complex 267

to S I trails in cordierite/----A t B t Q i n "bet&d"structure betweenmicroboudins o fcord ier ite

S2 M-domain truncatedb y S3 M-domainICordierite dismemberedby S3 olium, w i t hreaction to A t B + O

Fig. 11. Diagrammatic sketches to show inferred sequence of metamorphic and deformational events inCooma metapelites at the microstructural scale, i n an orientation approximately at right angles to thatshown in Fig. 1. The final stage of development is analogous to the situation depicted in Fig. I . Cordieriteporphyroblasts grow in the earliest stages of crenulation of S , . and preserve the orientation of S, aspredominantly straight inclusion trails that may curve into S2 folia at the edges of the porphyroblasts. Thecordierite is confined to SzQ-domains (quartz-rich domains). which preserve crenulated S, trails in strainshadows that form adjacent to the porphyroblasts as S1 micaceous folia (M-domains) develop morestrongly. Eventually, these Sz M-domains become crenulated and kinked by S , (cf. Figs 68). bove theandalusite isograd, the cordierite (C) reacts with muscovite (M ), producing andalusite (A ), biotite (B ) andquartz (Q), which grow in 'beard' structures between microboudins of cordierite, in residual S2M-domainsadjacent to cordierite, or in new S, folia that truncate or dismember cordierite porphyroblasts. as well asby replacing cordierite. At this stage, S 3 has developed into a differentiated crenulation-foliation and is thestrongest foliation i n the rock (Fig. 1) . Sz emains as residual M-domains against cordierite. residual Q-domains, and strongly curved inclusion trails in andalusite. S , is preserved only as inclusion trails incordierite.

crenulation occurred while the minerals of theaggregates were still stable.As pointed out by Mason (1984). the P-Tslope of the reaction: cordierite + muscovite

+andalusite + biotite + quartz is shallow,either slightly positive or negative (Schreyer &Seifert, 1969, p. 382). Such a reaction surface

that the reactants and products may coexist ina relatively wide divariant P-T field (Wall inMaso n, 1984, p. 29), which may explain th epresence of muscovite intergrown with, and ofa similar size to, the biotite in the micaceousaggregates replacing cordierite.CONCLUSIONSould be crossed during low-pressure metamor-ohism in which temoerature rise was

accompanied by little or no increase in confin- Microstructural observation s show tha t ining pressure. Phase rule consideration s suggest cordierite-andalusite schists at Coom a the


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268 R.H . Vernon

Fig. 12. Photomicrograph showing an- andalusite porphyroblast (A ) with straight t o slightly curved inclusiontrails (Sz) hat curve into the matrix foliation (S,) at the edges of the porphyrobiast; this relationshipsuggests syn-S,, growth of the andalusitc. The identification of the inclusion trails as S 2 is confirmed by thesize of the inclusions (which is similar to that of inclusions in definite SI rails in andalusite in other rocks.as shown in Fig. 2) . and by the fact that the trails are parallel to S 2 M-folia that have bcen crenulated byS , (e.g. at top of photograph). Strong anastomosing of S., about the elongate porphyroblast has produced asyn-S, strain shadow on each side (to p and bo ttom) of the andalusite. A prominent S,l M-domain truncatesS 2 a t right. Andalusite schist. without cordierite. Plane-polarized light: base of photo 4.4 mm.

Fig. 13. Similar to Fig. 12. but showing S 2 inclusion trails that are more strongly curved than those inFig. 12. which curvature is more typical of andalusitc in these rocks (as shown in Fig. 2) . Overgrowth of S,folds by the two andalusite porphyroblasts has preserved a continuously and markedly curved S 2 . The maindifferentiated foliation in the matrix is S 2 . which has been folded and kinked by S,. Plane-polarized light;base of photo 4. 4 mm.


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Sequential growth of porphyroblasts, Coonia Complex 269earliest tectonic foliation is preserved largely inaltered, syn-S2 cordierite porphyro blasts andcannot be de tected in the field. Su ch porphyro-blasts occur in the hinge-zones of large F, folds,where S I (preserved in the porphyroblasts) isstrongly oblique to S,,. In the FI limbs, approxi-mate parallelism of S 1 and S,,obscures evidenceof this kind. Unless inclusion trails in porphyro-blasts in FI hinge-zones are examined, thematrix foliation ap proximately parallel to bed-ding may be mistakenly labelled Sl , hen itis in fact S2 (Fig. 1). This implies that somefolds previously labelled Fzmay in fact beF3 folds, but does not affect the overall foldgeometry. Microstructural evidence also indi-cates that andalusite grew as a result of a pro-grade reaction between cordierite and muscov-ite, during the development of the maincrenulation-foliation, S3. The study shows thatdetailed examination of porphyroblast-matrixmicrostructural relaionships can reveal a pre-viously unsuspected S-surface, as well as pro-viding relatively direct evidence of sequentialmineral growth and metamorphic reactions.ACKNOWLEDGEMENTSI thank B.E. Hobbs . R . Mason and R.J . Knipefor helpful comments. The work was financedby a Macquarie University research grant.REFERENCESBell , T .H. , 1981. Foliation development: the contri-bution, geometry. and significance of progressivebulk inhomogeneous shortening. Tecronophysics.75, 273296.Bell , T.H., 1985. Deformation partitioning and por-phyroblast rotation in metamorphic rocks: a radicalre-interpretation, Journal of MeramorphicGeology, 3, 109-1 18.Bell , T.H., Fleming, P.D. &L Rubenach. M.J.. 1986.Porphyroblast nucleation. growth, and dissolution

in regional metamorphic rocks as a function ofdeformation partitioning during foliation develop-ment. Journal of Meiamorphic Geology, 4. 37-68.Bell. T.H. & Rubenach. M.J.. 1980. Crenulationcleavage devel oprn ent-e vide nce for progressive,bulk inhomogeneous shortening from millipedemicrostructures in the Robertson River Metamor-phics. Tecronophysics, 68, T9-TI5.Bell, T.H. & Rubenach . M.J . , 1983. Sequential por-phyroblast growth a nd cre nulation cleavage devel-opment during progressive deformation. Tecroii-Cobbold , P .R . . 1977. Description and origin ofbanded deformation structures. I . Regional strain.

local perturbations, and deformation bands. Can-adian Journal of Earrh Scierices. 14 , 1721-1731.

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Fyson, W.K., 1975. Fabrics and deformation ofArchean rnetasedimentary rocks, Ross Lake-Gordon Lake area, Slave Province, NorthwestTerri tories. Canadian Jouriial of Earrh Sciences,12, 765-776.Fyson. W.K.. 1980. Field fabrics and emplacementof an Archean granitoid pluton. Cleft Lake.Northwest Territories. Catradian Journal of EurthSciences, 17. 325-332.Gr an a th . J .W . . 1976. Petrogenesis of Metamorph-ically Layered Tectonites at Cooma New SouthWales. Uripubl. Ph D thesis. Uiiiversiiy of Sydney.Granath , J .W., 1978. Discussion: Stratigraphy andstructural summary of the Cooma metamorphiccomplex. Journal of ihe Geological Society ofAustralia. 25 , 115-1 18.Hopwood, T .P. , 1966. The Relationship betweenTectonic Style and Metamorphic G rade in theCooma Complex, N.S.W. Unpubl. PhD iliesis.University o f Sydiiey. (Publication No. 75-3724Xerox University Microfilms, Ann Arbor, Michi-gan , USA) .Hopwood. T .P. . 1976. Stratigraphy and structuralsummary of the Cooma metamorphic complex.Journal of ihe Geological Sociery of Ausiralia. 23.

Jo pl in, G .A . , 1942. Petrological studies in the Ordo-vician of New South Wales. I . Th e C o o ma C o m-plex. Proceedings of ihe Linneaii Socieiy of NewSouih Wales. 67 . 156-196.Mason, R .A. . 1984. Structural and MetamorphicEvolution of the Cooma Complex, N.S.W.Unpubl. B.Sc. (Hons) thesis, Monash Uiiiversiry.Melbourne.Rarnsay. J.G.. 1962. The geom etry and mechanics offormation of similar type folds. Journal ofGeology, 7 0 , 309-327.Schreyer, W. & Seifert, F., 1969. Compatibilityrelations of the aluminum silicates in the systemMgGAI2O3-SiO2-H2O an d K20-MgGAl2O1-SiOrH,Oat high pressures. American Journal of Science,

Tetley, N.. 1979. Geochronology and thermal historyof the Cooma Granodiorite (abstract) . Record ofthe Bureau of Mineral Resources. Geology & Ge o-physics. (Ausiralia), 197912. 8b89.Vernon , R.H. , 1978. Pseudomorphous replacementof cordierite by symplectic intergrowths of andalus-ite, biotite and quartz. Liihos, 11. 283-289.Vernon , R .H. , 1979. Formation of late sillimanite byhydrogen metasomatism (base-leaching) in somehigh-grade gneisses. Liihos, 12. 143-152.Vernon. R.H., 1982. Isobaric cooling of two regionalmetamorphic complexes related to igneousintrusions in southeastern Australia. Geology, 10.7681.Vernon, R.H.. 1988. Microstructural evidence ofrotation and non-rotation of mica porphyroblasts.Journal of Meiamorphic Geology, (in press).Wyborn. L.A.I . . 1977. Aspects of the Geology of theSnowy Mountains Region and Their Implicationsfor the Tectonic Evolution of the Lachlan FoldBelt . Unp ubl. PhD ihesis, Ausrraliari National Uni-versity.

345-360.

267, 371-388.

Received 4 Marc h 1987; rev i s ion accepted 8 October 1987

sequential growth of cordierite and andalusite porphyroblasts,

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