Petrogenesis of Dalradian Albite Porphyroblast Schists

J. geol. Soc. London, Vol. 140, 1983, pp. 601-618, 11 figs, 6 tables. Printed in Northern Ireland.

Petrogenesis of Dalradian albite porphyroblast schists

K. P. Watkins

SUMMARY:A linear belt of schistswithin which abundant albite porphyroblasts are developed outcrops over approximately 2000 km2 of the SW Scottish and NE Irish Dalradian. Other features which characterize these rocks are chloritization, the common occurrence of magnetite and development of quartz segregations. The albite porphyroblast schists occur in thebiotite and garnet Barrovian metamorphic zones and occupythe crests of regional F3 antiforms. Textural,mineralogical and petrologicaldata indicate that retrogression of original (Barrovian) metamorphic assemblages and concomitant hydrogen metasomatism occurred. As aconsequence of thesereactions pH gradients were induced between quartz-albite and mica-garnet-richbands in therocks which resulted in redistribution and recrystallization of albiteas porphyroblasts. No input of sodium on a regionalscale is necessary to explain porphyroblast growth. Aninflux of water wasresponsible for these processes. It is likelythat the water was a product of Barrovian regional metamorphism and accumulatedin the crests of F3 antiforms as a result of restricted fluid flow.

Albite porphyroblasts are developed in a linear belt of Mapping regionaldimensions which occursin the biotite and garnet Barrovian metamorphic zones of SW Scotland About 250 km2 of theAPZ and surrounding and NE Ireland (Fig. 1). These rocks are anomalous in terrain in the Balquhidder-Crianlarich region of the the classical metamorphiczonal sequence of Tilley W central Scottish Dalradian have been mapped (Fig. (1925),although Bailey (1923) triedto incorporate 2). The major structures were determined by recon- them in his sequence. Their metamorphic significance naissance mappingalong traverses about 0.5-1 km is poorly understood. The term ‘albite porphyroblast apart. The two important metamorphic features, the zone’ (APZ) will be used when referring to this belt of APZ boundary and the garnet isograd, were mapped schists. by traversing over them at intervals of a few hundred Since they were first described in the Cowal Memoir metres, examining the outcrop everyfew metres with a of theGeological Survey of Scotland (Gunn et al. handlens.Their positions weresubsequently verified 1897, pp. 39 and 299) they have been the subject of by petrographicexamination of over 400 specimens muchresearch and discussion (Cunningham-Craig collected along these traverses. 1904; Hill et al. 1905; Bailey 1923; Tilley 1925; McCallien 1929; Bailey & McCallien 1934; Reynolds Sampling 1942; Harker 1950; Trendall 1953, 1961, 1962; Jones 1961, 1962; Bowes & Convery 1966; Wilson & Robie Samples for chemical analysis were collected along 1966). Two schools of thought on the genesis of albite two broad traverses, one on either side of the Bridge porphyroblast schists emerged from this work: of Balgie Fault (Fig. 2). All analysed specimens were (1) Albiteporphyroblast schists formed as a conse- fresh,i.e. they exhibited no signsof weathering or quence of regional sodium metasomatism. leaching in thin section. (2) Albite porphyroblast schists formed without addition of sodium: (a) by metamorphism of Na-rich sediments; (b) by a type of ‘hydrothermal metamorphism’; Nature and disposition of the (c) by transfer of albite from quartz-feldspar-rich albite porphyroblast zone bands to mica-rich bands. Little or no chemical data were used by most of these Albite zone lithologies authors to support their theories. This study combines chemical data with petrographic observations and field Four lithologies occur within the APZ: semipelitic mappingin an attempt to resolve the conflicting schist, oxidized schist, greenschist (originally a volcani- hypotheses. clastic sediment)and metadolerite (= epidiorite). 0016-7649/83/0700-0601$02.00 0 1983 The Geological Society

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LP (ORNAMENTYyISOGRAD GARNET ON HIGHGRADE SIDE) 0 ALBITEPORPHYROBLAST SCHISTS ..-" ALBITEPORPHYROBLAST ZONE BOUNDARY

FIG.1. Structural and metamorphic compilation map of SW Scotland and NE Ireland. AA, Ardrishaig anticline (Fl); AS, Aberfoyle synform (Fl); BMS, Ben More synform (F2); BIS, Ben Lui synform (F2); BMA, Ben More antiform (F3); BLS, Ben Lawers synform (F3); TM, Tarbert monoform (F3); BLM, Ben Ledi monoform (F3); CA, Cowal antiform (F3); HBF, Highland Boundary Fault.

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.-UI ;I 0 cc m L

FIG.2. Simplified tectonic-metamorphic map of the Balquhidder-Crianlarich region.

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Albiteporphyroblasts and associated features are In Antrim,Kintyre and Cowal the centre of the thoughtto have developed in all fourrock types by APZoutcrop coincides with the axis of theCowal thesame process.Only semipelitic schists are con- Antiform(F3). From NE Cowal to Loch Voil the sidered in this paper as more informationis available on CowalAntiform loses its identity.The major F3 these and on stratigraphically equivalent rocks outside structure tothe NE of this in theBalquhidder- the APZ. Crianlarichregion is asynform-antiform pair. The APZ semipelitic schists are distinguishedprimarily north-western component of the pair is the well known by theporphyroblastic nature of thefeldspar, the BenLawers Synform (Elles 1926), the south-eastern porphyroblasts being ellipsoidal in shape with average component is complementary a ENE-plunging, longdimension of 1 mm.Outside the APZthe asymmetric antiform (the Ben More Antiform-Wat- feldspar, although equally abundant, is inconspicuous, kins 1981). The APZ outcrop is centred on the axis of occurringas grains of similar size andshape to the this antiform, whichis co-axial with theTarbert quartzgrains with which it is associated.Other Monoform.It is clearfrom Figs 1 and 2 and the distinguishing features are as follows: preceding discussion that the APZ coincides with the (1)There is intensequartz veining parallel tothe axial regions of major F3 antiformal structures. It is foliation. This is one of the most striking features alsoevident that albite porphyroblast schists grade of these rocks (see Gunn et al. 1897, p. 45); the into‘normal’ pelitic rocks with increasingtectonic veins are generallylens shaped,about 1-3cm elevationwithin these antiforms, viz. theENE ter- thick and 10-30cm long. mination of the APZ. Gunn et al. (1897) alsonoted (2)Chlorite is abundantthroughout the APZ and thatalbite porphyroblast schists gradeinto ‘normal’ oftenforms thick rims around garnet porphyro- rocks at higher levels. Albite porphyroblast schists do blasts. Thegreen colour this mineralimparts to not occur to the E of the Bridge of Balgie Fault (BBF) therocks provides a background which accentu- system. atesthe character of thefeldspar. Outside the Thesouth-western termination of theAPZ in APZ chloriteis, of course,common in chlorite Antrim is unexposed.It is possiblethat the albite andbiotite grade rocks but rapidly decreases in porphyroblast schists continuebeneath the Antrim abundance beyong the garnet isograd. Volcanicsand reappearfurther theto SW in (3) Magnetite commonly occurs as grains up to 5 mm the Ox Mountains and North Mayo (Trendall 1953). across(average c. 2.5mm) in theAPZ, but is absent in rocks outside it. Relation to Barrovianmetamorphism (4) Graphite does not occur in the APZ, although it is a common constituent of rocks outside it. The APZ is superimposed on the Barrovian zonal sequence;the southern margin traverses Tilley’s Relation to stratigraphy (1925) biotitezone, while thenorthern margin traversesthe biotite and garnet zones. The APZ is From Fig. 1 it is clear that the APZ outcrop is not associated with thegarnet isograd over almost the controlled by thestratigraphy. In the W central entire length of its outcrop. In the area mapped (Fig. Highlands the APZ occurs wholly within the Pitlochry 2) the isograd lies within theAPZ, and in Antrim Schist Formation. InKintyre its northernboundary garnetsare common in albite porphyroblast schists crossesthe Loch Tay Limestone intothe Ben Lui (Wilson & Robie 1966) although no isograd has been Schist Formation (Gunn et al. 1897, p. 41; McCallien mapped there. In Cowal and northern Kintyre, Tilley 1929, pp. 415 and 420; Reynolds 1942, fig. 2); and in (1925) mapped the isograd at or just N of the northern NEAntrim, Wilson & Robie(1966, fig. 7)have limit of the albite porphyroblast schists (Fig. 1). marked this boundary3 km N of theLoch Tay However, in the Balquhidder-Crianlarich region, Tren- Limestone. dall (1953), Jones (1964) andWatkins (1981)all mapped the garnet isograd several kilometres to the S Relation to structure of Tilley’s line, actually in the central APZ. There is The structural scheme adopted in this paper is that alsoreason to believe that the garnet isograd may described by Roberts & Treagus(1977a,b, 1979), occur within the albite porphyroblast schists in Cowal where the Tay Nappe is emplaced during D1-D2 and andnorthern Kintyre. Garnets havebeen noted at refoldedabout upright, ENE-trending, axial planes several localities in the northern part of the APZ in during D3. these regions (e.g. Gunn et al. 1897; Brown 1969).The In thearea studied major F2 andF3 folds are development of garnet is generally obscured by later present. A regional recumbent synform delineated by growth of porphyroblastic albite. Thus, the position of the oxidized schist unit (Fig. 2) occurs within the APZ. the isograd as mapped by Tilley (1925) in Cowal and F3 folds of markedly different wavelength and ampli- northern Kintyre must be regarded as suspect, and it is tudeare developed on either side of theBridge of likely that it occurs several kilometres to the SEwithin Balgie Fault (Fig. 2). the zone of albite porphyroblast schists.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/4/601/4887847/gsjgs.140.4.0601.pdf by guest on 02 October 2021 Dalradian albite porphyroblast schists 605 Petrography Albite textures In theAPZ, albites are not confined mainly to In semipelitic schists outsidethe APZ the main quartz-richbands, as in thesurrounding terrain, but minerals are roughlysegregated into discontinuous are associatedwith both mica andquartz. The mica-rich and quartz-feldspar-rich bands. The associa- porphyroblastscommonly contain inclusions of opa- tion of quartz and feldspar is common in greenschist- quephases, minute garnets, epidote, micas and fine facies terrains (e.g. Turner 1941). The feldspar grains dusty grains (unidentified) which are often aligned to are anhedral and of similar or slightly larger size than form planar or crumpled inclusion trails. At the edges accompanying quartzgrains; lamellar twinning is of manygrains theinternal fabric (Si) is turned common.Biotite occurs sporadically S of thegarnet towards parallelism with the grain rim indicating grain isogradbut is common in thegarnet zone. Garnet rotationduring a late stage of albitegrowth. In the porphyroblastsare equant, subhedral and generally majority of cases Si lies ata marked angle tothe wrapped by the mica fabric. enclosing schistosity (S,) and also to the long dimen- In the APZ rounded,inequant, rarelytwinned sion of the albite grain (which is generally parallel to albiteprophyroblasts are evenly distributed in the S,), as illustrated in Fig. 3. This relationship indicates schists. There is still an overall segregation of quartz that the albites were rotated relative to the mica fabric andmicas intodiscontinuous bands, although this is subsequentto their growth (see also Jones 1961, p. less distinct thanoutside the APZ. The micas are 57). The micas must have been continuously recrystal- generallycoarse-grained and the fabric they form lizing during this process,resulting in thepresent abutsagainst the albite porphyroblasts, although it texturalrelationship inwhich micasgenerally abut mayalso partially wrap these grains. Biotite occurs against albite grains. very rarely and when present is partially retrograded As the southern APZ boundary is approached (from to chloriteand rutile. Garnet occurs in 2forms, as the S) small elliptical grains of clear albite appear in partially to completelyretrograded porphyroblasts, the mica-rich bands of the rock, theirlong axes aligned and as minute pristine grains. Magnetite and rutile are in thefoliation. The anhedral grainspreviously the most common FeiTi oxide phases, this association described still exist in the more quartz-rich bands of beingunusual in metamorphicrocks (Mielke & the rock. The grain size and proportion of recrystal- Schreyer1972). Acrenulation cleavage associated lized albiteincreases towards the APZ concomitant with F3 is oftenwell-developed in rocksfrom both with a decrease in the proportion of anhedral grains in inside and outside the APZ. However, no deformed the quartz-rich bands. This change is accompanied by micagrains are observed and therefore recrystalliza- a general increase in the grain size of other minerals tion must have accompanied the development of this cleavage.

Mineral assemblages Common mineral assemblages for semipelitic schists are: (i) S of theAPZ: quartz + albite + phengite + chlorite + rutile f biotite f calcite f tourmaline. (ii) APZ, S of thegarnet isograd: quartz + albite + phengite + chlorite + rutile f epidote f magnetite f ilmenite f calcite f tourmaline. (iii) APZ, N of thegarnet isograd: quartz + albite + phengite + chlorite + rutile f garnet(1) f garnet(2) f magnetite f ilmenite f epidote f calcite f tourmaline. (iv) N of theAPZ: quartz + (albite-oligoclase) + phengite + biotite + garnet(1) + chlorite f ilmenite f rutile f calcite f tourmaline. (garnet(1):garnet porphyroblasts; garnet(2) : small 1 mm garnet euhedra). l 1 InPitlochry Schist semipelites E of the BBF the FIG. 3. Albiteporphyroblasts aligned in an F3 mineral assemblages are: S of the garnet isograd-as schistosity comprising chlorite,phengite, quartz, for (i)above; N of thegarnet isograd-as for (iv) magnetite and rutile. Si, internalfabric; S,, en- above. closing schistosity.

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and by thedevelopment of quartzsegregations and Textural synthesis largemagnetite grains. In the APZ all albite is of The clearly displayed textures of micas, albites and porphyroblastic habit and no anhedral, lamellar twin- garnets in albite porphyroblast schists enable construc- ned albite occurs. tion of a mineral growth history: As the northern boundary is approached (from the (i)generation of S2 foliationsurface (= nappe N) the small anhedralgrains associated with quartz emplacement); give way to larger anhedral grains in the mica bands. (ii) growth of garnet porphyroblasts (over S2) under These porphyroblasts have less regular shapes than the fairly static conditions; grainsat the southern limit of the APZ and are in (iii) growth of albite porphyroblasts (over S2) initially apparenttextural equilibrium withbiotite-garnet under fairly staticconditions and genesis of assemblages. Small lamellar twinned grains of feldspar secondstage garnets. Initial retrogression of may still exist in quartz-rich bands at this stage. Just garnetporphyroblasts and biotite late in this inside the northern APZ boundary all the albites are stage. There may be some overlap between stages porphyroblastic and the schists are heavily chloritized ii and iii; from retrogression of garnet and biotite. The textural (iv) rotation of albiteporphyroblasts late in their changes at both the southern and northern boundaries growthhistory by F3movements. Continuous of the APZ occur over an interval of 200-500 m. recrystallization of micas and partial retrogression of garnet and biotite; Garnet textures (v) continued rotation of albite porphyroblasts (after Garnet occurs in three forms in the APZ: cessation of growth) during the culmination of F3 (i) large (average 1 mm), subhedral, usually partially movements.Continued crystallization of micas retrograded porphyroblasts; and retrogression of garnet and biotite. (ii) small (<0.3 mm), subhedral to euhedral, usually pristine matrix grains; and Rock chemistry (iii) small (<0.2mm), subhedral to euhedral, usually pristine inclusions in albite porphyroblasts. 18 specimens of semipelitic albite porphyroblast schist The porphyroblasts occur in varying statesof preserva- have been analysed for major elements by XRF and tionranging from small (<0.2 mm) coreremnants, wet chemical techniques. For comparison, 19 semipeli- surrounded by large amorphous masses of chlorite, to tic PitlochrySchists from within a few kilometres N pristine grains (>lmm) in apparent textural equilib- and S of the APZ and from E of the BBF were also rium with the albite-matrix assemblage. All gradations analysed. Specimen locations are indicated in Fig. 2. between these extremes occur; well preserved grains An averagecomposition for each group of rocks is occur only in the extreme northern parts of the APZ. given in Table 1. Garnetporphyroblasts are sometimes partially in- TABLE1. Comparison of Pitlochry Schist semipelites cluded by large albites. Inclusiontrails in the porphyro- from the APZ with those from outside it blasts arerare but where observed are planar or crumpled; it is generallythought that Barrovian Mean compositions of Pitlochry metamorphismclimaxed in atectonically quiescent Schist semipelites: period between D2 and D3 (Johnson 1963; Atherton outside the in the 1977;Bradbury 1979; Fettes 1979, and references APZ (19) APZ (18) ~~ therein). 65.98 61.15 The three garnet types may occur individually or in 0.76 1.00 any combination in the same specimen. It seems, from 15.24 17.56 these textures, that garnet was produced in the APZ 1.34 2.11 duringtwo episodes of growth.The first produced 0.08 0.09 porphyroblasts which in the central parts of the APZ MgO 2.01 2.17 havesubsequently been retrograded. The second CaO 1.19 1.14 produced small grains, many of which are included in Na,O 2.20 2.32 albite.The first growthepisode preceded albite K20 2.54 3.27 P,% 0.10 0.19 porphyroblast growth and is the same as that observed Fe0 4.55 4.56 tothe E of theBBF, i.e. it representsthe main volatiles 3.80 4.20 Barrovian metamorphic event. The second episode of Total 99.79 99.76 garnet growth must have coincided with the develop- M/MF X 100 44.05 45.90 ment of albiteporphyroblasts. Small matrix grains, O.R. 21.07 29.44 althoughgenerally larger than grains included in All Fe as Fe0 5.76 6.45 albite, are of the same appearance, possibly indicating M/MF = Mol MgO/(MgO + FeO) that the grains were still growing when included. O.R. = Mol (2Fe,03/(2Fe,0, + FeO)) X 100

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I 10 20 30 40 50

OXIDATIONRATIO FIG.4. Comparison of bulk oxidation ratios of semipelitic Pitlochry Schists, (A) inside and (B) outside, the APZ. Mean values are indicated.

Significant differences between the compositions of bearingrocks E of theBBF occur in the Ben Lui PitlochrySchist semipelites inside andoutside the Schist Formation.Thus, relating the chemistries of APZ are: garnetbearing assemblages in theAPZ withthose (i) rocks from the APZ are poorer in SiOz and richer outside it involvescomparison of BenLui and in all other oxides except CaO (Table l); PitlochrySchist mineralogies. This comparison is (ii) theoxidation ratio (rnol (2Fe203/(2Fe203+ thought to be valid since: (a) the compositions of Ben FeO) X 100) of APZ rocks is distinctly higher Luiand Pitlochry Schists outside theAPZ in this than that of rocks outside the zone (Fig. 4). region are similar (Fig. 6); (b) in thegarnet zone, Bulk compositions of the two groups of rocks are semipelitebulk composition exerts little controlon plotted on AFMand AKF diagrams in Fig. 5. The matrix mineral chemistry (Watkins 1981). Albite and only significant difference between the two groups on FeiTi oxides have simple chemistries (albites contain a the AFM diagram is that rocks from the APZ plot on few per cent Ca in the rims, FeiTi oxides a few per average closer to the A-M side of the diagram, due to cent Si and/or Al) which do not vary across the APZ; higheroxidation ratios and hence mol (MgO/MgO they are not described further. + FeO) ratios ("F). There is no significant difference in the fields occupied on the AKF diagram. WMF ratios E of theBBF "F ratios of thechlorites are Mineral chemistry influenced by bulk composition S of the garnet isograd and by garnetrim M/MF ratios (which are directly The chemicalcompositions of themajor mineral related to metamorphic grade) N of the isograd (Fig. phases in each of the 37 specimenshave been 7). M/MFratios of chloritesinthe APZ vary determined by electron-probe (Watkins 1981). In the independently of garnet porphyroblast chemistry, this followingsections aspects of mineralchemistry re- phase being out of equilibrium with the matrix (Fig. levant to thepetrogenesis of albiteporphyroblast 8). Garnet rim M/MF ratios increase northwards with schists aredescribed. APZ phase chemistries are metamorphicgrade E of theBBF; in theAPZ, compared with those outside the zone. Most garnet- however, no increase is recorded (Fig. 9).

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0.2y'7 \

FIG.5. Bulk compositions of 18 sernipelitic Pitlochry Schists from the APZ (0)and 19 semipelitic Pitlochry Schists from outside the APZ (0)plotted on portions of AFM and AKF diagrams. Mineral abbreviations: gnt., garnet; chl., chlorite; pheng., phengite; Garnet-chlorite and chlorite-phengite (solid solution) tie lines are shown.

(Fe, Mg)Si = Al, AI in phengites Garnet Most of the white micas analysed have more than Garnet is chemically(and texturally) the most three times as much Si as tetrahedral A1 and contain complexphase in thealbite porphyroblast schists. appreciable amounts of Fe and Mg and can therefore Each of the3 morphologicaltypes described above be classified asphengites (Deer et al. 1962). The may have different chemical compositions, even within caledonitecomponent (K(Fe, Mg)AlSi4010(OH)2) in thesame specimen. E of theBBF clearlydefined phengite is generallythought to decrease(by the trends have been recorded in garnet rim compositions tschermaksubstitution) with increasing metamorphic with increasing grade. In the APZ there are no clear grade in the greenschist andamphibolite facies trends in these parameters (Fig. 9). Core compositions (Lambert 1959; Butler 1967; Guidotti 1973;Hoffer of garnet porphyroblasts from the APZ and E of the 1978; Fletcher & Greenwood 1979). S of the APZ the BBF are similar (Table 2); zoning profiles of garnets celadonitecomponent decreases towards the N; this from these areas are, except for the extreme rims, also trend is reversed in thesouthern APZ, where the similar. Thus it is likely that theygrew by similar celadonitecomponent increases to a maximum (Fig. processesand under similarmetamorphic conditions 10). Itgradually decreases again in thecentral and forthe greater part of theirperiod of formation. northernparts of theAPZ. There is no significant However, either at a late stagein their growth history, change in the trend across the northern boundary. The orsubsequent to theirgrowth, metamorphic condi- celadonite components of muscovites outside the APZ tions in the APZ changed sufficiently to produce the also decrease withincreasing grade; in general, observed differences in rim composition. however,celadonite components are lower in these Grainsincluded in albite are compositionally dis- muscovites. tinct fromporphyroblasts (Table 3)and exhibit

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FIG. 6. Bulk compositions of 31 semipelitic Ben Lui Schists (0)and 19 semipelitic Pitlochry Schists (X) from outside the APZ plotted on portions of the AFM and AKF diagrams.

complex zoning behaviour, without consistent pattern. From the preceding discussionit is clear that, as well TheMn and Mg contents of includedgrains are as being texturally distinct, the two APZ garnet phases generally much higher than thoseof porphyroblasts in are alsochemically distinct. Porphyroblasticgarnets the same specimen. The Fe content is correspondingly would not be in equilibrium with second stage garnets lower in included grains while the Ca content appears and would therefore react with them (other phases in tobe similar. Includedgrains in the same specimen the rock would also be involved in such a reaction). mayexhibit different types of zoningalthough bulk This may explain why there seem to be no consistent grain compositions (i.e. (core + rim compositions)/2) trendsingarnet porphyroblast rim compositions aregenerally similar. Bulkgrain compositions are northwards from the isograd across the APZ. remarkably similar throughout the APZ; average end The high Mn content of the second stage garnets is membercomponents are 49.5% almandine, 22.9% significant, considering thecommon occurrence of spessartine, 21.5% grossular, 4.9% pyrope and 1.2% magnetite in albiteporphyroblast schists andthe andradite (Fe3+ was calculated by assuming a theore- relatively high oxidationratios of theserocks. Hsu tical divalent cation content). The same calculbtion for (1968) showedexperimentally that the Mn content porphyroblasts yields 63.9% almandine, 8.8% spessar- of garnetincreases with the oxygen fugacity of the tine, 22.5% grossular, 3.4% pyrope and 1.4% andra- fluid phase. He stated that ‘with increasing oxidation dite, although some variation of bulk grain composi- states almandine must incorporate increasing amounts tion is found across the APZ. Small matrix grains are of spessartine and pyrope components to maintain its compositionally similar to included grains and exhibit stability’. Thus, the Mn content enlarges the stability thesame complexity of zoning behaviour,although field of almandine-rich garnet in f(02)-T space. Hsu zoning patterns of included and small matrix grains in (1968) also demonstrated that Mn-rich garnets nucle- the same specimen may differ. ate at much lower temperatures than almandine-rich

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/4/601/4887847/gsjgs.140.4.0601.pdf by guest on 02 October 2021 610 K. P. Watkins frommetamorphic temperature, bulk AI content controls occurrence of biotite in pelitic rocks. On the e AKF diagramthe chlorite-phengite tie line of the a chlorite-phengite-biotite trianglesweeps across in- a. creasingly aluminous rock compositions as the tscher- a maksubstitution renders phengite more aluminous with increasing grade. If the bulk composition plots to a* the left of the chlorite-phengite tie line on an AKF diagram,then biotite should occur in rocks of appropriatemetamorphic grade (i.e. at least biotite e zone). A significant proportion of albite porphyroblast schists plot tothe left of thepertinent chlorite- phengite tie line and yet do not contain biotite (Fig. 11). Rocks from outside the APZ which plot to the left L 1 of thepertinent chlorite-phengite tie line contain 6 10 14 biotite.This indicates that the metamorphic grade finally reached in theAPZ was insufficient to garnet rim qMFx100 stabilize biotite in rocks of favourable composition. FIG. 7. Relationsbetween garnet rim M/MF and chlorite M/MF in rocks E of theBBF. All iron assumed to be ferrous. Petrogenesis of albite garnets; this is consistent with the generally retrogres- porphyroblastschists sive character of thealbite porphyroblast forming episode.Thus, the second stage of garnetgrowth Fromthe preceding discussion the following conclu- probably occurred under conditions of higher oxygen sionsregarding conditions of albiteporphyroblast fugacity and lower temperature than the first. growth can be made: (i)Albite porphyroblasts developed during retrogression of Barrovianepidote-amphibolite and Status of biotite uppergreenschist-facies assemblages to lower Biotite occurs only rarely in the extreme northern greenschist-facies assemblages. The metamorphic parts of the APZ. Mather (1970) showed thatapart grade finally attained was insufficient to stabilize biotite in rocks of suitable composition; (ii) the oxygen fugacity of the fluid phase increased duringthese reactions as indicated by thepre- sence of magnetiteand high Mn garnet in the productassemblages. As aresult, the oxygen 0 contents of albite porphyroblast schists are grea- terthan those of equivalentrocks outside the APZ. Compositions of garnet porphyroblasts are 0 similar to those outside the APZ, which grew in a 0 graphitebuffered environment; thus, it is prob- 0 0 able that graphite originally existed in the albite porphyroblast schists. The absence of graphite 00 andcommon occurrence of magnetite in these rocks, along with evidencefor hydration, indi- catesthat reactions such as 2H20 + C+C02 + 4H+ and H20+ 3FeO-.Fe304 + 2H+ probably occurred; (iii) thereaction process was notcompleted in the garnet zone as relics of the originalassemblage 40 0 ! I , l (garnet) remain. S of the garnet isograd, howev- 4 6 8 10 er, it is likely that the reaction was completed as garnetrim M/MF XIOO the onlydisequilibrium texture observed is the FIG. 8. Relations between garnet rim M/MFand inclusion of minutephases in albiteporphyro- chlorite M/MF in APZ rocks. All iron assumed to blasts; beferrous. Note the difference in scale between (iv) albiteporphyroblasts are associated with both Figs 7 and 8. mica-rich andquartz-rich bands in the APZ,

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0 e 0

e 0 0 a00 e 00

0’ , 1 l- 0 1 2 3 KM 0 1 2 3 KM distanceperpendicular to the garnet distance perpendicular to thegarnet isograd surface E of the BBF isograd surface W of the BBF

FIG.9. Garnetporphyroblast rim M/MF trends E of theBBF and N of the APZ (O), andin the APZ (0). There is no discernible gradient in APZ MiMF values. All iron is assumed to be ferrous.

whereasalbite outside is associatedonly with TABLE2. Meangarnet porphyroblast core quartz-rich bands. Thus, it is likely that much of compositions thealbite (or itsconstituent elements) was transferredfrom quartz-rich bands to mica-rich outside the in rhe bandsduring the reaction process. Quartz seg- APZ(I0) APZ (10) MgO 0.56 0.45 AlKh 21.15 21.07 SiOz 38.16 38. 05 CaO 8.67 9.07 3-71 mm MnO 5.37 6.31 Fe0 27.68 26.27 Total 101.59 101.22 MiMF X 100 3.48 2.96 3.6 -I Ob. 0 - regations, possibly remnant from such a process, Y, O + I 00 are commonly observed. These conclusions form the basis for a petrogenetic model for the albite porphyroblast schists, the essen- + 0 tial features of which are: Y (9 metamorphic differentiation involving segregation 0. of quartzand transfer of much of thealbite 3l contained in the rocks from quartz-rich to mica- rich bands; 14J . 0 i (ii) metamorphicreactions responsible for retrogres- 2.4 2.5 2.6 2.7 sion of epidote-arnphibolite and upper greenschist- A1 faciesassemblages to lowergreenschist-facies Cations per I1 oxygenr assemblages involving a concomitantincrease in the oxi.dation state of the rocks.

FIG. 10. Tschermak substitution (Fe, Mg)Si = AI, AI) in phengite across the APZ: (U), garnet-free Metamorphic differentiation rocksoutside the APZ; (U), garnet-free APZ rocks; (e),garnet-bearing rocks outside the APZ; Usefulinformation regarding the growth of albite (0),garnet-bearing APZ rocks. All iron assumed porphyroblasts in mica-rich bands can be derived by to be ferrous. comparing the bulk compositions of APZ rocks with

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TABLE3. Mean core and rim compositions those of rocks from the surrounding terrainin terms of of garnetsincluded in albite porphyro- correlations between pairs of major elements. In order blasts to make this comparisoncorrelation, coefficients between each different pairof major elements for each Cores (5) Rims (4) rock group are calculated and displayed in a matrix. The basis for using correlation matrices in studies of MgO 1.20 MgO 0.73 retrogressive metamorphism is that if a suite of rocks A1203 21.23 21.18 undergoesisochemical metamorphism, there will be SiOz 38.12 38.35 nochange in any of thecorrelation coefficients CaO 7.40 CaO 8.14 betweenelement pairs other than that produced by MnO 10.66 9.91 variability in sample populations (Beach 1976; Beach Fe0 22.89 23.16 & Tarney 1978). For the present study it was thought Total 101.50 101.47 useful to employthis technique to examinethe M/MF X 100 8.55 5.32 correlationchanges which must occur due to the transfer of albite to mica-rich bands and also to show chemically that mass transfer took place. Comparisons of averagecompositions give little indication of any

MENGITCS

os1

AS6 A62

FIG. 11. Phengite-chlorite phase relations in albiteporphyroblast schists. (0),mineral composition; (A), bulk composition. Bulk compositionsplot: tothe right of thephengite-chlorite join: 143645* to 143650,143655 and 143656, and 143662 to 143664; to the left of the phengite-chlorite join: 143644, 143651, 143653, 143665 and 143666. (*Harker Collection numbers, Dept. Earth Sciences, Univ. Cambridge. Prefix 1436- omitted on diagram for clarity).

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systematicrelations between elements that mightbe standard error of ll(n - 3)0.5and may be regarded as established as a result of such a process. significant if it exceedsthis value. Significant differ- It is essential that the two groups of rocks compared ences in correlationbetween element pairs can be were initially similar (i.e. that they were originally of determined by comparing the difference in Zvalues thesame geochemical population). For this reason, between the twospecimen groups with the standard onlysemipelitic Pitlochry Schists areused in the error of their differences, i.e. correlation matrix. Lateral variation in the geochem- Iz1 - z21 (li(n1 - 3) l/(n2 - 3))”.5 istry of this stratigraphic formation is thought to be > + minimal in this area(Watkins 1981). The statistical Differences in Z values between the two specimen procedurefor constructing a correlation matrix and groupsare given in Table 5. Thestandard error of testing the significance of the correlation coefficients it theirdifferences is calculatedas 0.36. Changes in contains is described in Beach & Tarney(1978, pp. correlation are regarded as significant only if (1) the 327-31). difference in Z values is greater than 0.36; and(2) one Amatrix which compares correlation coefficients of the correlation coefficients for that element pair in between element pairs in 18 APZ semipelitic Pitlochry the correlation matrix is significant at the 99.9% level. Schists with those for 19 semipelitic Pitlochry Schists Changes of correlation may resultfrom either a from outside the APZ is shown in Table 4. In order to straightforwardincrease or decrease in positive or determine if any significant changes in correlation negative correlation, or achange from negative to occurredduring retrogression it is necessary to positive correlation, or viceversa. Significant differ- transform the correlation coefficients into parameters ences in correlationare underlined in Table 5, the which can be compared statistically. Fisher transforms: sense of correlation change being indicatedby the type - Z = 0.5 log, (1 r)/(l - r) of underlining. Large values of IZ1 Z21 which are not + significant are due to the subtraction of two insigni- are parameters which can be derived from correlation ficant correlations of opposite sign. If thealbite coefficients andcompared totest if differences porphyroblast schists wereisochemically metamor- between them are significant. Z is distributed with a phosedthere would be no significant changes in

TABLE4. Pitlochry Schist correlation matrix

19 semipelitic Pitlochry Schists from outside the APZ

SiA1 Ti Fe2+Fe3+ Mn Mg Cu Na K PH Si -0.31-0.80-0.30 -0.94 -0.76 -0.63-0.84-0.28 -0.79 0.44 0.36 YI ...... Ti 0.69-0.90 _0,60_ -0.24Q.3Q-0.33 0.44 0.51 0.46 c.z ...... 4.61 0.56 AI 0.390.86 -0.98 0.740.25 0.88 -0.45g.33 -0.46 0.40 0.16 X ...... 30.21 Fe3+0.49 0.12 0.42 0.41 -0.46 -(l22 0.38-0.27 Q.32 0.18 S Fe2+ -G 0.560.78 0.42 -0.29 0.50 -0.280.750.17 0.40 -0.32 is ...... Mn 0.18-0.20 -0.23 0.42 0.37 0.44 -0.15 0.16 0.11 0.250.06 .Mo Mg -.0:?8. 0:68 0.17 0,76-0.15 0.28 -0.150.17 0.24 j.59 Ca 0.04 -0.02 -0.13 0.37 -0.35 0.21 -0.110.21 -0.35 0.37 -0.13-0.02 0.04.g Ca -0.10 -0.21-0.41-0.36 Na -0.04 0.18 -0.02-0.040.18 Na -0.07 -0.13-0.03-0.400.04 SE -0.69-0.730.18...... ZK 0.930.72 -0.89 ...... 0.210.30 0.49 0.51 -0.250.03 0.720.13 ....

significance n r level underlining C;(X-x).(Y-Y) correlation coefficient, r = [C;(X-X)~]”~.~;(~-~)*]”~19 > 10.53 I 99.0% - 19 > 1 0.57 1 99.5% ___ where X and y are the two variables X and 9 are their means; 19 > 10.66 I 99.9% .... n is the number of specimens 18 > 10.55 I 99.0% - r = + 1.O : perfect positive correlation 18 > I 0.60 1 99.5% ___ r = 0: random distribution 18 > I 0.68 I 99.9% .... r = - 1.O : perfect negative correlation

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TABLE5. Differences in Fisher transforms between the two rock groups

Ti AI Fe2+Fe3+ Mn Mg Na Ca K P H

Si _9,45_ 0.18 0:490.010.15 0.340.51 0.34 0.31 0.12 Ti 0.380.050.260.43 0.450.21 0.230.210.42 0.32 AI 0.04 0.30 0.01 0.42 0.460.37 0.29 0.08 0.28 Fe3+ 0.420.04 0.69 !.O!. 0.150.21 0.01 0.09 Fe2+ 0.330.02 0.160.05 0.120.08 0.31 0.11 0.24 0.19 0.10 0.47 0.05 0.47Mn 0.10 0.19 0.24 0.11 Mg 0.26 0.09 0.32 0.04 0.19 Ca 0.03 0.40 1.31 0.07 Na !):G$ 0.12 0:55 K 0.300.22 P 0.07 underlining scheme: -significant increase in positive correlation in albite schists - - - significant decrease in positive correlation in albite schists _._._significant increase in negative correlation in albite schists . . . . significant decrease in negative correlation in albite schists

correlation. These can only result from redistribution assemblage retrograded from biotite. The latter asso- of elements on a scale larger than the specimen size. ciation may be sufficient to increase the Ti-Mg Most of the significant positive correlations for rocks correlation in albiteporphyroblast schists morethan outside the APZ in Table 4 result from association of that of other elements (K, Fe2+) associated with Ti in elements in particularminerals and the preferential phengitesuch that the correlation is statistically location of theseminerals in bands. For example, significant. A positive correlation between Ca and P in there is a strong correlation between Fe and Mg; these albite porphyroblast schists may indicate that apatiteis elementsare located mainlyin micas andgarnets producedduring the retrogressivereactions at the whichoccur in mica-richbands. Rocks with more expense of calcite. However, it is thought that much of mica-richbands thanquartz-feldspar-rich bands will the P in albiteporphyroblast schists is located in containproportionately more Fe and Mg.Strong phengite, as it is relatively abundant in phengite-rich negativecorrelations between Si andmost other rocks. This association has been recorded in oxidized elements (except Na and Ca, due to the preferential gneisses from Glen Clova to the NE (G. A. Chinner, location of albiteand calcite in quartz-richbands) pers. comm.). A positive correlation probably results occurbecause of this bandingeffect. In Table 4 all fromthe occurrence of calcium-bearingminerals correlation coefficients involving Na in albite porphyro-(calcite and epidote) in phengite-rich bands. Thus all blast schists are close tozero, demonstrating that the significant differences in correlationcan be albite is evenly distributed in these rocks. In contrast, explained in terms of a differentiation process which Na is strongly anticorrelated with K and H in rocks involvesmass transfer,on a scalelarger than the outsidethe APZ, indicatinganegative association specimensize, of alarge proportion of albitefrom between albite and mica, viz. the banding effect. quartz-rich to mica-rich portions of the rock. Thereare 10 significantchanges in correlation: Inorder to determine if there was any regional Si-Ti, Si-AI, %-Fe2+, Ti-AI, Mg-AI, Na-K, Na-H, Ti- influx (or depletion) of material as advocated by some Mg and Ca-P. All except two of these involve either previous workers, it is necessary to compare individual Si,Na or AI and canbe explained in terms of the oxideabundances in eachgroup of rocks(Table 1). redistribution of albite(Na, AI) andsegregation of An increase in the abundance of Na20, K20, total Fe quartz.There is noobvious explanation for the as FeO, MgO, MnO, TiOz, P205 and A1203, anda increase in positivecorrelation between Ti and Mg. relativelylarge decrease in Si02are due to the However, it is possible that it is the result of two segregation of quartzand relative enrichment of associations. Ti is contained in phengite and is strongly remainingmaterial. The significance of the small correlated with all the mainelements (except Si) decrease in CaO is unknown. If the differences of the constituting phengite in albite porphyroblast schists. It means are summed according to sign, it is found that is also contained in rutile, which in albite porphyro- there is anet subtraction (segregation) from albite blast schists is oftenassociated with chlorite in an porphyroblast schists of 4.83% Si02 and 0.05% CaO,

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/140/4/601/4887847/gsjgs.140.4.0601.pdf by guest on 02 October 2021 Dalradian albite porphyroblast schists 615 and an addition (enrichment) of 4.72%, in total, of all therefore, not a straightforward retrogressive reaction other oxides. The difference between these figures is butrather ahydrogen metasomatic or hydrolysis well within the errors of the calculation. There is no reaction (Meyer & Hemley 1967, p. 206). The reaction significant enrichment of Na20, other than that caused for garnet-free semipelites is more difficult to formu- by thesegregation of quartz, whichrules out an late,because chlorite and phengite dominate both externalsource (in aregional sense) of Na. K20 is reactants and products and the changes in composition enriched to a greater degree than would be expected between them is small; it is not attempted here. fromstraightforward quartz segregation. However, one of thealbite porphyroblast schists (143651, Mechanism of albite transfer HarkerCollection number, Dept. Earth Sciences, University of Cambridge) is unusually rich in K20 Albite porphyroblasts grew contemporaneouslywith (8.11%). If thisspecimen is omitted from the the hydrolysisof the matrixassemblage. This is calculation a weighted mean of 2.98% is derived for indicated by the presence of included garnets of the K20 in these rocks; this is consistent with the degree same chemical character asthose in the matrix. of enrichment of other oxides. There are no dispro- Transfer of albite from quartz-richto mica-rich regions portionate relative enrichments of any of the oxides of the rock requires a driving force in the form of a which cannotbe explained by thedifferentiation chemical or physical (i.e. tectonic) ,gradient. Textural process. observations suggest that the rate of albite porphyro- Thus it seems valid to assume that the metamorphic blast growth was (at leastinitially) rapid comparedwith process by which albite porphyroblasts and associated therate of anypenetrative deformation, making it minerals formed occurred in a closed system (for all unlikely thatthe drivingmechanism was a physical analysed cations) of, at most, outcrop scale (viz. a few one.The most likely chemicalgradient capable of m3 to a few tens of m3). Therefore the only material causinga mass transfer reaction is furnished by the which can be added to the system is that which is not hydrolysis reactions. As these reactions only produce now present as an individual phase, i.e. the metamor- hydrogen from garnet and micas, a chemical gradient phic fluid. in H+ concentration(i.e. a pH gradient)between quartz-feldspar-rich and mica-garnet-rich bandswill be set up. Metamorphic reactions The mass transfer mechanism must involve reaction It is possible to approximate a reaction for garnet- of albitewith H+. The initial reaction may be an bearing rocks by comparing mineral assemblages and exchange of H+ for Na' in albite leaving behind an compositions E of the BBF with those in the APZ. amorphousresidue of aluminaand silica hydroxides This is done in order to demonstrate the natureof the (cf. Fyfe et al. 1978, p. 95). Theresidue must also reactionrather than to deriveaccurate chemical react atsome stage and betransferred to the mica information.Average phase compositions for the bands.Further than this, details of the process are garnetzones E of the BBF and in thealbite unclear.Clues which may indicatethe detailed porphyroblast schists are used in the reaction (Table chemical nature of the mass transfer are: 6). The volume proportionsof the reactant assemblage (i) the masstransfer process is terminatedwhen are based on an average mode for the garnet zoneE of albites are evenly distributed in the rock (i.e. no the BBF. As a simplifying step the mainbulk of quartz gradient in Na+); and albite in the rock (arbitrarily taken as 50% of bulk (ii) all albite,whether transferred into mica-rich volume) is excluded from the reaction. The reaction bands or not, is recrystallized in porphyroblastic consumeswater and produces hydrogen and is, form:

TABLE6. Hydrolysis reaction for garnet bearing albite porphyroblast schists

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(iii) quartzsegregations form approximately 5% of thermalgradient throughout a large volume of the the rocks, indicating appreciable mass transfer of Dalradian in this region. silica probablycontemporaneously with that of The fluid released during this metamorphism in the albite. Much of this quartz could be segregated as deeper parts of the pile would probably not be able to a restite phase during the mass transfer of albite- escapeupwards in the ‘normal’way, due tothe to mica-rich bands. thermal barrier created by overlying hotter rocks. In That porphyroblasts should grow rather than more rockswith a strong inclined schistosity or cleavage abundant, smaller grains is likely to be a function of fluid will migratepreferentially along the planes of the rate of nucleation and growth, factors which have anisotropy in an upward direction (Fyfe et al. 1978). many controlling variables (Spry 1969, p. 140). From Suchan anisotropy exists in the form of an S2 the presence of many included phases in albite it can schistositywhich in this regiondipped gently tothe beassumed that growth was fairly rapid(Spry 1969, NW beforeD3. Given theconstraints on fluidflow pp. 171 & 173). outlinedabove, it is likely that fluid generated by dehydration reactions at deep levels could migrate in a Causative process directionmore or less controlled by thestrong S2 schistosity but with an upward component due to the Accountingfor the 50% inert albite andquartz increasing elevation of isotherms to the SE. removedfrom the reactants as a simplifying step, As there are no indicationsof hydrolysis reactions in about 1.7 vol.% of water is required to complete the the terrain E of the BBF, it must be presumed that hydrolysisreaction in garnet-bearingrocks. Slightly fluid flowedwithout appreciable reaction through less thanthis amount wouldactually be used, as these rocks. There must, therefore, besome feature of remnants of garnet porphyroblasts remain in the APZ. the rock mass W of the BBFsufficiently different from If it is assumed that a similar amount of water was that to the E to modify the fluid flow regime such that absorbed by garnet-freealbite porphyroblast schists, hydrolysisreactions could occur. Apart from the then it is possible tomake a roughestimate of the change in tectoniclevel across the BBF, the most amount of water input to the APZ. obvious difference is in the scale and amplitude of the Aminimum volume of albiteporphyroblast schist F3 structures. The terrain E of the BBF is character- can be calculated from the approximate surface areaof ized by aseries of low amplitudeantiforms and outcrop multiplied by the average topographic relief. synforms which contrast with therelatively high Takingthe average relief tobe 700 mand the amplitude and longer wavelength Ben More Antiform approximate outcrop area to be 2000 km2, a figure of to the W of the BBF and the Cowal Antiform further 1400 km3 is derived. Assuming that the volume change to the SW. of the reaction is small compared to the errors in this It is thoughtthat the developing Ben More and crudecalculation, then about 24 km3 of waterare Cowal Antiforms could havepartially trapped the fluid required for the hydrolysis process in the APZ. migratingaway from thedehydrating garnet zone The only reasonable source for this amount of water terrainina way analogoustothe trapping of distributedthrough such largea volume is the hydrocarbons in afold forming in the layer through metamorphic dehydration water from the terrains to which they are migrating. It is possible that, during the theN and S of theAPZ. The garnet-producing later stages of development of this fold, fluid from the reaction in the W-central Dalradian releases about 2 biotite zone to the SE could also have migrated into it. vol.% of water(Watkins 1981) from aterrain of Thestructural disposition of theAPZ, in the axial similardimensions tothe APZ (except in the NE regions of the F3 structures, is consistent with this wherethe garnet zone is much larger).The precise hypothesis. Si textures in albiteporphyroblasts indi- amount of waterreleased from chlorite and biotite cate that the initial rate of F3 fold development must producingreactions is unknownbut is probablyat have been slow compared with rates of crystallization. least a few per cent (Fyfe et nl. 1978). The timing of Either the deformation rate increased, or the crystal- the hydrolysis process is consistent with this hypothesis, lization ratedecreased at a later stage resulting in viz. late syn- to post-garnetgrowth. The garnet rotational Si textures in albite rims. isogradin this regiondips gently tothe NW, the metamorphic zones being inverted, with higher grade Conclusions rocksoverlying lower graderocks. As the garnet isograd surface intersects the axial plane of a regional The following conclusions can be made regarding the F2 recumbent synform in the area studied (Fig. 2) it is nature, dispositionand petrogenesis of Dalradian unlikely thatthe isogradsurface was inverted by albite porphyroblast schists: emplacement of theTay Nappe (F2). There is also (i) Albite porphyroblasts are developed in a zone of textural evidence to indicate that recrystallization took schists which (in the area studied) comprised four placein the D2-D3 interval (see above); it follows lithologies: semipeliticschist, greenschist, oxi- that Barrovian metamorphism occurred in a negative dized schist and metadolerite. The zone occupies

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the axialregions of theBen More and Cowal quence of these reactions provided the chemical Antiforms and extends some 200 km through the driving force necessary for mass transfer of albite. SW Scottish and NE Irish Dalradian, straddling These processesoccurred in closeda system theBarrovian garnet isograd over much of its (excluding the fluid phase) of, at most, outcrop length.The boundaries of thezone cut across scale. lithostratigraphic horizons. (iii) Water required for this process was derived from (ii) Albite porphyroblasts grew during retrogression dehydration fluid migratingfrom theterrain to of Barrovianassemblages (epidote-amphibolite the N (andpossibly S) and was trapped in the andupper greenschist- to lowergreenschist- crests of regional F3 antiforms long enough for facies) and concomitant oxidation. Mass transfer the reactions to occur. of much of the original albite from quartz-rich to mica-richbands anddevelopment of quartz ACKNOWLEDGMENTS. Thiswork was done during the tenure of a NERC research studentship at the Department of Earth segregations accompanied this growth. Retrogres- Sciences, University of Cambridge.Constructive criticism sivereactions consumed H20 andreleased H+ from GrahamChimer, Alec Trendall,Ian Tyler andan resulting in an increase in the oxidation state of anonymous reviewer improved the organization of the paper therocks. pH gradients set up between quartz- and is appreciated. Jackie is thanked for typing various drafts feldspar-richand mica-rich bands as aconse- of the manuscript.

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Received 6 September 1982; revised typescript received 20 January 1983. K. P.WATKINS, Geological Survey of WesternAustralia, Mineral House, 66 Adelaide Terrace, Perth, Western Australia 6000.

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