Australian Journal of Earth Sciences (1999) 46, 827–836

DISCUSSION

B. W. CHAPPELL,1 A. J. R. WHITE,2 I. S. WILLIAMS,3

D. WYBORN,1 J. M. HERGT2 AND J. D. WOODHEAD2

1ARC Key Centre for the Geochemistry and Metallogeny of theContinents (GEMOC), Department of Geology, AustralianNational University, Canberra, ACT 0200, Australia.

2Victorian Institute of Earth and Planetary Sciences, Schoolof Earth Sciences, University of Melbourne, Parkville, Vic.3052, Australia.

3Research School of Earth Sciences, Australian NationalUniversity, Canberra, ACT 0200, Australia.

Introduction

The restite model (White & Chappell 1977; Chappell et al.1987) was developed during studies of granites in south-eastern Australia to account for compositional, petro-graphic, zircon age inheritance and other features of manygranites which cannot be explained satisfactorily by theclassical models of petrogenesis. The restite model recog-nises the fact that unmelted but magmatically equilibratedsource material (restite) may be entrained by a partial melt,as a magma, and that variation in the degree of separationof those two components, leading to differences in the ratioof melt to restite, is responsible for the variation in com-position in many suites of granites. Within such suites, themore mafic granites have compositions that approachthose of the source rocks. The model therefore provides a way of ascertaining compositional features of the deeper crust.

Despite the clear evidence for the presence of restite andfor the restite model in many granites of the Lachlan FoldBelt, Collins (1998) regarded the model as generally in-applicable to petrogenesis of the granites of that belt.However, we would submit that the paper used many argu-ments that are not relevant to the validity or otherwise ofthe restite model, that it used flawed petrogenetic argu-ments, and that in some instances it misquoted the state-ments of earlier authors. Also, the terminology ‘restite(single-source) model’ coined by Collins (1998) is mislead-ing as it is not analogous to the ‘two-source’ and ‘three-source’ models discussed elsewhere in that paper. Therestite model has as a corollary that the granites of theLachlan Fold Belt were produced from source rocks ofmany compositions.

A flawed model of granite petrogenesis

Writing of fractional crystallisation as a process to producecompositional variation in the Moruya Suite, Collins (1998p. 493) stated that ‘Collins (1996 p. 173) demonstrated that itwas possible’. This implies that evolution of the MoruyaSuite could have started from a completely molten state andthat the range of rock types observed formed through pre-cipitation of crystals and their removal. Collins (1996, 1998)favoured the formation of the bulk of the granites of theLachlan Fold Belt from melts produced by fractional crystallisation and that is the process which he consideredin detail for the Moruya Suite (Collins 1996). We will evalu-ate this hypothesis in detail elsewhere, but two points arenoted here. First, Collins (1996) cited the similarity betweenthe chemical compositions of the Bingie Bingie gabbroicdiorites and calculated cumulates for the Moruya Suite asevidence to support his model. That interpretation is notconsistent with the differences in initial Sr isotopic com-position between the Tuross Head Tonalite and the gab-broic diorites reported by Keay et al. (1997), which requiredistinct sources. Second, much of the Collins (1996) argu-ment was concerned with comparing compositions ofrocks from the Moruya Suite with those obtained by model-ling fractional crystallisation. The results of some of thosecalculations were shown in figure 2 of that paper. For thethree elements shown in that figure, only the calculatedcompositions for Sr are at all close to those of the analysedrocks. For Ba and Rb, the calculated trends are significantlydisplaced from those observed in the rocks. Hence, whetherthe data plotted in that figure provide any support at all forthe model that was being tested is questionable.Interestingly, Collins (1998 p. 493) criticised Chappell (1996b)for using partition coefficients that require assumed tem-peratures of crystallisation, from which we presume thatCollins (1996) did not. Such coefficients are always a func-tion of temperature and for Ba and Sr in plagioclase, themajor fractionating phase in these systems, the effects ofboth mineral composition and temperature are extremeand the coefficients would have changed as fractionationprogressed. This alone means that the modelling of Collins(1996) is incorrect and hence irrelevant.

The hypothesis that the initial magmas were com-pletely molten (Collins 1998) is not consistent with the wide-spread presence of older inherited zircon in the LachlanFold Belt granites (Williams et al. 1990). Collins (1996 p. 172)

Discussion and ReplyEvaluation of petrogenetic models for Lachlan Fold Belt granitoids: implications for crustal architecture and tectonic models

acknowledged that restite zircon is present in those gran-ites when he stated that ‘the unequivocal evidence forrestite is reduced to inherited zircon and possibly calcic plagioclase cores’. Williams et al. (1988) found inherited zircon in all granites (now 22 samples) that they examinedfrom the Bega and Moruya Batholiths. All of those zirconcores are surrounded by thick rims that were precipitatedfrom the melt, which suggests that they were not acciden-tally incorporated at a late stage. Much more critically, thepresence of older zircons shows that the mineral was sat-urated in all melts involved in production of the MoruyaSuite. This remains true even if the zircons were acciden-tally incorporated and not restite. Collins (1996) chose sam-ple MG17 from the Tuross Head Tonalite (60.78% SiO2 and180 ppm Zr) as his representative originally completelymolten mafic end-member of the Moruya Suite. MG17 hasa calculated zircon saturation temperature of 754°C (usingthe equations of Watson & Harrison 1983) which, within thelimits of the method and its calibration, is the maximumtemperature at which zircon crystals could exist in a meltof that composition. However, a rock with the compositionof MG17 would require a temperature of at least 1000°C tobe completely molten, with the precise temperature depend-ing on the H2O content. The presence of old zircon in gran-ites of the Moruya Suite shows unequivocally that thoserocks could not have been completely, or even largely,molten, conflicting with the model of Collins (1996).Furthermore, the old zircon tells us that the melt involvedin production of the Moruya Suite, being low temperaturemust therefore have been felsic in composition, which inturn implies that the Moruya Suite magmas also containedlarge amounts of restite. Chappell et al. (1998) have termedsuch suites ‘low-temperature’, and these comprise morethan 90% of the I-type granites of the Lachlan Fold Belt.The fact that most of the I-type granites of the Lachlan FoldBelt formed at low magmatic temperature indicates thatthey formed from the partial melting of older quartzo-feldspathic crust and were not directly subduction-related,contrary to the view of Collins (1998).

Arguments against the restite model

From the title of the Collins (1998) paper one might expecta compelling case to be made both against the restite modeland in favour of alternative models. However, only two ofthe eight ‘problems’ of applying the restite model to gran-ites of the Lachlan Fold Belt advanced by Collins (1998 pp.486–487) are even relevant to that model. Furthermore, theinformation that Collins (1998) cited in these two cases isin fact completely consistent with the restite model.Nowhere did that paper provide evidence that validly chal-lenges the basic concept of that model.

We are of the view that Collins (1998) frequently usedarguments that are not relevant to the validity or otherwiseof the restite model. For example (p. 494), he stated that ‘ThePb isotopic evidence discussed above provides strong sup-port for granitoid-mixing models, rather than the restitemodel, which requires contrasting sources’. The restitemodel is a mechanism for explaining the compositionalvariation within a suite of granites. It does not require con-trasting sources, but it happens that granites do form froma variety of sources and that these may be contrasting.

Whether or not the data presented by Collins (1998) for mix-ing processes, such as isotope data, zircon age patterns, orsimilarities in average I- and S-type granite compositions,favour mixing, is debatable. Either way, they are irrelevantto the validity of the restite model.

We will now briefly consider each of the eight ‘problems’under the headings used by Collins (1998 pp. 486–487).

(1) Appropriate minerals are absent in minimum-melt-composition suites. Collins (1998 p. 486) stated that ‘theKadoona Dacite, a member of the Glenbog Suite, does notcontain K-feldspar phenocrysts. This should not be sobecause minimum melts should contain quartz, plagioclaseand K-feldspar as liquidus minerals’. Whether the melt was‘minimum’ or ‘non-minimum’ temperature is not relevantto any consideration of the validity of the restite model aswe have always recognised that the melt component of arestite-bearing magma may be of either type (Chappell etal. 1987 pp. 1130–1131).

(2) Chemical trends for S-type granite suites do not project back to any major sediment composition on theAustralian continent. Collins (1998 p. 486) used the fact thatthe compositions of the most mafic S-type granites do notmatch those of any known major sediment composition asan argument against the restite model, which it is not.Evidence for the restite model applying to these granitesis very strong, so ways of accounting for this paradox mustbe examined. We note elsewhere that some metasediment-ary enclaves in the mafic S-type granites, derived fromdeeper in the crust, do show critical aspects of the requiredcompositional features not seen in most of the exposed sedimentary rocks.

(3) All petrographic evidence for restite is inferred tohave been lost. In our view there is clear petrographic evi-dence for restite, although when that evidence has beentaken in isolation, it has not been conclusive in terms ofthe volume of restite crystals required by the model.However, we note that plagioclase cores in many granites,the presence of clots in which the mafic minerals appearto have a recrystallised texture (noted by Collins 1998), theoccurrence of oriented sillimanite needles in the centre ofcordierite crystals, and the presence of old zircon cores, forexample, are all petrographic features consistent with arestite origin. More importantly, there is no textural orpetrographic evidence against the presence of restiteamong the low-temperature granites of the Lachlan Fold Belt.

Collins (1998 p. 486) stated that Vernon (1983), among others ‘showed that the enclave textures were igneous,rather than metamorphic, in character’. However, Vernononly showed that a melt had been present in those enclaves.A crystal can grow in contact with a melt at any pointbetween the liquidus and solidus of a rock. Part of thatregion of growth is igneous, but at less than a fraction ofabout one-third melt the material is not a magma becauseit is incapable of moving. If the amount of melt neverexceeded that fraction, the rock is not igneous, and the textures are pseudo-igneous.

(4) Isotopic evidence suggests that some suites are opensystems that require input from several source components.Collins (1998 p. 487) stated that ‘Rossiter and Gray (1996)have demonstrated that the Bullenbalong Supersuite showsconsiderable increase in initial 87Sr/86Sr with increasing

828 Discussion and Reply

SiO2’. We have Sr isotopic data for 37 samples of granite thathave been assigned to the Bullenbalong Suite using othercriteria. In addition, four isotopic analyses of the DeddickGranodiorite were reported by Maas et al. (1997). These dataare plotted against SiO2 values in Figure 1. It can be seenthat the statement of Rossiter and Gray (1996) is incorrect.Writing of their presumed increase in initial 87Sr/86Sr withSiO2 in the Bullenbalong Suite, Rossiter and Gray (1996)stated that ‘The restite-unmixing model can explain suchvariation within a suite only if the melt had higher initial87Sr/86Sr than the restite. Given that quartzofeldspathicrocks with low Rb87/Sr86 (and thus relatively low 87Sr/86Srat the time of melting) would be more likely to melt thanpelitic rocks with higher Rb87/Sr86 such a scenario is mostimprobable.’ Since, as we have seen, the initial 87Sr/86Sr ofthe Bullenbalong Suite does decrease with increasing SiO2, Rossiter and Gray (1996) have inadvertently argued insupport of the restite model.

(5) S-and I-type granites have remarkably similar average compositions, as pointed out by Collins and Vernon(1994). We agree that the two granite types of the LachlanFold Belt have similar compositions, apart from those elements which are lost when feldspars are weathered toclay minerals (Ca, Na and Sr), or for Rb and Ba which areretained in clay minerals during such weathering, and alsoPb. These similarities are not relevant to a considerationof the restite model. They are to be expected, as we will discuss elsewhere.

(6) S- and I-type granitoids of the eastern Lachlan FoldBelt fall on the same well-defined eNd–Sr isotopic array.Based on isotopic data alone, this Nd–Sr isotopic array couldbe broadly accounted for in terms of two (Gray 1984) or three(Keay et al. 1997) end-members, but such simple solutionsare not consistent with the observed variations in chem-ical compositions (McCulloch & Chappell 1982; Chappell1998). This problem has no direct bearing on the validity ofthe restite model, which says nothing directly about howgranite source rocks themselves were produced.

(7) Zircon inheritance patterns between S-type granites,I-type granites and Ordovician sedimentary rocks are verysimilar. Williams (1995) has shown that the patterns of ageinheritance in S-type and I-type granites, and detrital zircon in the Ordovician sediments, are very similar, butthe amount of zircon showing such inheritance is vastlydifferent between the two granite types. Williams et al. (1992p. 503) pointed out that ‘Zircons with inherited cores arerare in I-type granites, but virtually every zircon in the S-types contains an older core’. This implies that the sedi-ment component in the I-type granites, at least as indicatedby the amount of inherited zircon, is trivial, a conclusionsustained by the observation that zircon was saturated inall of the low-temperature I-type magmas.

(8) A Cordilleran mountain chain is the basement to theOrdovician turbidites in the eastern Lachlan Fold Belt. Wehave never proposed such a basement to the easternLachlan Fold Belt. The tonalites of the Moruya and CobargoSuites, at the very eastern part of the Lachlan Fold Belt,have close compositional similarities to the distinctivetonalites of the Cordillera. The magmas from which thoseLachlan Fold Belt tonalites formed were at low magmatictemperatures and contained large amounts of restite.Collins mistakenly (1998 p. 486) took the ‘remagmatism’model of Chappell and Stephens (1988) and the inferencethat the source rocks of some of the I-type granites of theeastern Lachlan Fold Belt were similar to tonalites of theCordillera, to imply that the eastern Lachlan Fold Belt wasa Cordilleran mountain belt at ca 600 Ma. ‘Cordilleran’ asapplied to tonalites refers to aspects of their chemical com-positions, which characterises many granites occurring inthe American Cordillera. Used as a petrologic term, it doesnot imply a cordillera, or mountain range. Our usage of‘Cordilleran’ follows from that of Pitcher (1982) who recognised the distinct I-(Caledonian) and I-(Cordilleran)granite types, and Chappell and Stephens (1988) used itexactly in that sense. They could equally have referred to‘Cordilleran’ rocks as ‘calcic’, ‘high Na/K’, or ‘low Rb’, etc.,but instead followed Pitcher’s terminology.

Errors in interpretation of isotopic data

Collins (1998) appears to have misunderstood the inter-pretation of Sm–Nd model ages made by previous authors.He stated (p. 486) that ‘The depleted mantle Nd model agesfor the S-type granites have been considered by McCullochand Chappell (1982 p. 63) to give the ages of the sourcerocks’. Referring to ‘Model ages of the S-type granite source rocks . . .’, what McCulloch and Chappell (1982 p. 63)actually stated was ‘We interpret these ages as represent-ing a Sm–Nd age of the sedimentary source rocks~1000–1100 m.y. (1400–1500 m.y. before the present time)’.Earlier in that paper (p. 60), those authors stated that ‘TheSm–Nd model age represents the time in the past when themajor REE fractionation in the components of a rockoccurred, that is during their derivation from a mantlereservoir’. Confusion has perhaps arisen becauseMcCulloch and Chappell (1982 abstract), following a refer-ence to the range in eNd values of the S-type granites, statedthat ‘These characteristics are indicative of an ~1400-m.y.sedimentary or metasedimentary source for S-types’.Taken alone, that statement might be ambiguous to those

Evaluation of granitoid models 829

Figure 1 Plot of Sr isotopic composition at 430 Ma for granitesof the Bullenbalong Suite of the Kosciuszko Batholith. The 10open circles are for the Jillamatong Granodiorite and the 27 filledcircles are for 13 other lithological units within the suite; the starsare four data points from the Deddick Granodiorite (Maas et al.1997). Note the wide range of isotopic compositions forJillamatong and the general decrease in the isotopic values withincrease in SiO2 for other members of the suite.

incognisant of Nd isotope systematics. The Sm–Nd age isof course not that of the source rocks, but a model estimateafter making various assumptions about the average ageof derivation of the source components of the S-type gran-ites from the mantle. The Nd model ages thus reflect theages of the components that make up the sediments, not ofthe sediments themselves. Collins (1998 p. 486) went on tostate, that ‘If granites are taken as “images” of their source,the Nd model ages must give the age of the source.’ Thereis no basis for that statement. We have indeed argued thatsome granites form images of their source rocks, referringto compositional features, but never that Sm–Nd model agesare the ages of the source rocks, for to do so would be ridicu-lous. If an Ordovician age sedimentary rock of the LachlanFold Belt had been partly melted through contact with aTertiary basalt, would the fact that the melted sedimentretained its 1500 Ma Nd model age imply that the sedimentthat was melted was Proterozoic in age and not Ordovician?

In further discussing the implication of Nd ages, Collins(1998 p. 486) stated that ‘A 200 million year age-range wasconsidered to reflect isotopic and chemical differenceswithin source-rock sediments of “differing . . . maturitiesand feldspar contents” (Chappell & White 1992 p. 17)’. In factChappell and White (1992) did not refer to a 200 million yearage-range; they would not have taken a difference in Nd iso-topic compositions of two e-units so literally. In that sectionof their paper, those authors did not refer to Nd isotopes atall! They were comparing the Sr isotopic composition of theCooma Granodiorite, which they stated matched that of thesurrounding sediments, but not that of the BullenbalongSuite of deeper derivation. Finally, the statement by Collins(1998 p. 486), that ‘strict application of the restite model suggests that a large Mesoproterozoic–Palaeoproterozoiccontinental (metasedimentary) basement underlies theLachlan Fold Belt’ is erroneous. Of course the restite modelmakes no such requirement.

Collins (1998 p. 494) stated that ‘S- and I-type granites ofthe Lachlan Fold Belt show striking similarities in Pb iso-tope ratios’. However, while they are similar, they are quitedistinct. The two S-type granites analysed by McCullochand Woodhead (1993) have higher 207Pb/204Pb than the I-typesand therefore look more ‘crustal’. It is not possible to definethe extent of crustal influence in the I-type granites with-out knowing the Pb content of the primitive end-member.If this were very low then even a minute quantity of crustalPb would produce the observed compositions. Collins (1998p. 494) stated that ‘A high µ crustal reservoir of Ordoviciansedimentary rocks should have a uniform compositionbecause sedimentation processes homogenise the variousprotoliths’. That statement is at odds with the fact that thePb isotope data for six Ordovician sediments obtained byMcCulloch and Woodhead (1993) are extremely hetero-geneous. This is in contrast with the homogeneity of Pb isotopes within the granites.

Composition of sediments and enclaves in theBullenbalong Suite

There are errors in the discussion by Collins (1998) ofenclaves in the S-type granites and the Ordovician sedi-ments, which detract from his conclusions. In figure 5 ofthat paper he plotted 37 points representing ‘Lachlan Fold

Belt sediment data from Wyborn and Chappell (1983)’. Sincethose earlier authors listed only 15 analyses of Ordovicianand 10 Silurian sediments from the Lachlan Fold Belt, witha maximum CaO content of 0.78% and the data plotted by Collins (1998) includes one point with ~1.75% CaO,the provenance of the data plotted by Collins (1998) isuncertain.

Collins (1998 figure 5) also plotted 13 enclaves which helabelled ‘Bullenbalong enclaves’, 10 having SiO2 contentswith a maximum of 57.01% SiO2, the other three beingsiliceous enclaves. Those data are from Chen et al. (1989)who described eight of the lower SiO2 enclaves as ‘mica-rich, schistose inclusions’. Only three of those schistoseenclaves are from the Jillamatong Granodiorite and henceare contained within granites of the BullenbalongSupersuite. They have CaO contents of 1.78%, 1.23% and1.80%, all significantly higher than those of the Ordoviciansediments. The other five schistose enclaves are from theKoetong and Granya plutons, and have an average CaO content of 0.36% and a range from 0.15% to 0.61%. The other two low-SiO2 enclaves that were plotted by Collins(1998) were from the Granya pluton, classified as ‘quartzo-feldspathic inclusions’ by Chen et al. (1989) and they con-tain 0.79 and 0.62% CaO. The latter seven values areindistinguishable from CaO levels in the Ordovician sedi-ments. Two points should be noted about the host rocks ofthose low-CaO enclaves. First, the Koetong and Granyaunits are not part of the Bullenbalong Supersuite but of theKoetong Supersuite, which occurs further to the west in theWagga Batholith, so they are not ‘Bullenbalong enclaves’,which fact Collins (1996 p. 174) had previously recognised.Second, those two granite units are felsic and their com-positions show that they crystallised from melts which hadundergone fractional crystallisation of quartz andfeldspars, with Rb contents in our analysed samples rang-ing from 226 to 415 ppm for Koetong and from 284 to 430 ppmfor Granya. More mafic granites of the Koetong Supersuitesuch as the Wantabadgery Granodiorite contain peliticenclaves, which are probably lithic restite fragments, butsuch material was lost prior to, or during, the fractionalcrystallisation that produced the felsic granites. Theenclaves that are present in those felsic rocks must havebeen incorporated as xenoliths after that fractionation, andit is therefore not surprising that they possess the distinc-tive low CaO contents of the surrounding Ordovician sediments.

Writing of the possible isotopically evolved end-member in his three component mixing model, Collins (1998p. 489) stated that ‘Collins (1996) considered these possibil-ities and concluded that Ordovician sedimentary rocks ofthe Lachlan Fold Belt were the likely end-member, basedon the chemical compatibility of S-type metasedimentaryenclaves with Lachlan Fold Belt turbidites’. That was anunfortunate choice, given that those enclaves, which arechemically compatible with the sedimentary rocks, aredemonstrably accidental inclusions of those sediments.

Some specific misquotations

Collins (1998 p. 485) stated that ‘Chappell (1984 p. 698) sug-gested that the most mafic granite matches the source-rockcomposition for that suite’. In fact, Chappell (1984) referred

830 Discussion and Reply

to a ‘model source composition’, which implies quite a dif-ferent level of certainty, representing the best that could beachieved with the available data. To quote directly fromChappell (1984): ‘Thus, in a granite suite the most mafic pos-sible composition (most restite-rich) will be the same as thesource composition, within the limits of the model. Thusin any suite, the most mafic granite composition can betaken as the model source composition’. Collins (1998 p. 485)cited minimum SiO2 values for various suites of the Begaand Berridale Batholiths as though the restite modelimplies that they are actual source compositions. We are also mystified by his succeeding statement that ‘thesampling program focused on obtaining the most maficgranitoids of each suite’. That was never a factor in oursampling.

Referring to compositional variation within the Bullen-balong Suite, Collins (1998 p. 493) stated ‘Chappell (1996b)argued that because the variation cannot be caused by frac-tional crystallisation, it must result from restite unmixing’.This is incorrect and misleading. The closest that Chappell(1996b) came to that was to write (p. 166) ‘the observed vari-ations in Sr abundances in the Glenbog and Moruya suites,which cannot be accounted for by fractional crystallisation,can be readily explained as due to variations in the pro-portions of melt and restite in the different samples ofgranite’. It is true that Chappell (1996b) concluded that therestite model was the only viable mechanism for generat-ing variation in the Moruya Suite, but additional evidencewas cited. Reasons were given for the mechanisms ofmagma mixing and/or mingling and of assimilation or con-tamination being inapplicable. Also, the various other linesof evidence that directly support the restite model were dis-cussed. Modelling the compositions of the granite, andshowing that variations in the abundances of specific ele-ments are not consistent with fractional crystallisation,was only part of the argument in that paper.

White et al. (1976) recognised that the S-type granites ofthe Lachlan Fold Belt have a sharp eastern limit of occur-rence, which they termed the I–S line. Referring to that line,Collins (1998 p. 486) stated that ‘This line was inferred byWhite et al. (1976) to be the eastern limit of Proterozoicbasement’, when the earlier authors had only suggestedthat as a possibility. White et al. (1976 p. 111) stated that ‘theline probably coincides with the eastern margin of a verythick block of continental crust, possibly crystallineshield’. In the abstract of that paper, those authors wrotethat ‘changes in both tectonics and granitoid lithology suggest that the I–S line marks the eastern boundary ofcrystalline basement, possibly of Precambrian age’.

Conclusions

The paper of Collins (1998) contains many inaccuracies andit misquotes and misinterprets earlier papers. It has pres-ented no evidence to show that the restite model cannot bewidely applied to granites of the Lachlan Fold Belt. Thatmechanism remains the preferred model for the origin ofcompositional variation within most granite suites of thatbelt. The veracity of the model is confirmed by recent argu-ments based on zircon saturation. Most of the I-type gran-ites of the Lachlan Fold Belt formed at low magmatictemperatures and contrast with those granites from some

other regions, which formed at high temperatures and hadan origin closely related to subduction. Most of the S-typegranites of the Lachlan Fold Belt formed from sedimentarysource rocks that are more feldspathic than those exposedat the surface. We will discuss these issues in more detailin a forthcoming communication.

REPLY

W. J. COLLINSDepartment of Geology, University of Newcastle, Newcastle,NSW 2308, Australia.

Introduction

Chemical variation in granite plutons can be produced byrestite unmixing, crystal fractionation, filter pressing,flow sorting, residual phase separation, assimilation,magma mixing, to name a few processes. Plutons are fossil magma chambers likely to be subjected to any or allof the above processes. Physical interaction betweenmagma batches and chemical buffering within these largemolten systems tend to homogenise the chemical effects sothat sublinear arrays result. To ascribe the variation to oneprincipal cause, as with the closed-system restite model,rather than to a number of ‘fractionation’ processes, someof which may reflect open-system behaviour, is a major dif-ference in philosophy between the conflicting petrologicalmodels. Collins (1998) dismissed the restite model as gen-erally inapplicable to the Lachlan Fold Belt because chem-ical variation in granites is not simply or mainly the resultof restite unmixing. No one is arguing about the existenceof restite, just its influence on chemical variation, and theimplications that follow.

The implications of the two models are markedly dif-ferent, particularly for inferred source compositions. If therestite-unmixing model is followed, the linear chemicalarray of granite suites should ‘image’ their source rocks(see Collins 1998 figure 2a), as advocated repeatedly byChappell (1979, 1984, 1994). If a variety of processes oper-ate, the linear chemical variation of suites generally haslittle bearing on the composition of the source(s), whichmay comprise a number of components, none of which necessarily bear similarity to the granites (Collins 1998 figure 2c). Chappell (1996a p. 467) accepted that mixing of source components and later fractionation of that mixed parental magma is a possibility, which is a majoraspect of the three-component mixing model, and he admitted that sometimes it seems to be required.

Why then, the argument, when common ground seemsto exist? The different models strongly influence conceptsfor Lachlan Fold Belt tectonic evolution and crustal archi-tecture. Chappell and coworkers persist with the notion thatmost Lachlan Fold Belt granites are restite-dominated andthus insist that ‘mafic granites have compositions thatapproach those of the source rocks’, which implies whole-sale recycling of the ‘imaged’ crustal source materials ina non-subduction (but otherwise unspecified) environ-ment where a cryptic origin for the heat source exists. Themulti-source (3-component mixing) model allows for crustalheating by, and mixing with, mantle-derived magmas in a

Evaluation of granitoid models 831

subduction environment. The restite model requires aProterozoic continental basement to the Lachlan Fold Belt(Chappell et al. 1988), whereas the multi-source modelrequires an oceanic substrate, upon which the vast deep-water Ordovician turbidite sequences of the Lachlan FoldBelt were deposited (Collins 1998).

One must ask: Is it likely that mantle-derived basalticmagmas, which provide the heat to generate the widespreadvoluminous granites of the Lachlan Fold Belt, do not inter-act with those silicic magmas during partial melting toproduce mixed parental magmas? Is it likely that theparental magmas will then rise through the crust withoutassimilating parts of the wall rock, particularly in thedeeper, hotter crust where migmatites might exist? Is itlikely that a single fractionation process, that of restiteunmixing, is likely to be the primary cause of chemicalvariation in those granitic magmas? This is the ‘closed-system’ model that one must accept if granites are to beimages of their source rocks, yet the literature is now fullof examples of open-system behaviour in granitic magmas,from the partial melting stage (Sawyer 1994, 1996; Brown etal. 1995), through ascent and emplacement (Davidson 1996;Mason et al. 1996; Poli et al. 1996; Wiebe 1996), to the finalcrystallisation stage (Hanson 1996).

The discussion by Chappell et al. is more a defence ofthe restite model than an attack on the three-componentmixing model. It focuses on perceived misquotes and mis-leading statements made by myself, which reflect variationsin ‘degree of certainty’ in interpretation, rather than out-right misunderstandings. I will address the criticisms assequenced, then show how chemical variation in one of the‘type’ restite-controlled granites, the Moruya Suite, is aproduct of ‘open-system’ magma mixing.

A flawed model of granite petrogenesis?

Chappell et al. misquote Collins (1996, 1998) by claiming hehypothesised that the ‘initial magmas were completelymolten’. This misquotation then becomes their platform to argue why Collins has a flawed model. Collins (1996) modelled chemical variation in the Moruya Suite as crystal fractionation to show that the linear chemical vari-ation could be produced by that process. Collins did not suggest that the magmas were ever complete melts. In fact,Collins (1998 p. 493) stated that ‘S-type . . . magmas con-tained crystals and probably small entrained restitic frag-ments. The I-type granites also contain small mafic clotswhich are refractory, residual or mixed material. These aregranitic magmas, not melts’. Thus the arguments aboutchoice of partition coefficients, zircon inheritance and zircon saturation temperatures become irrelevant. Collins(1996) was merely demonstrating what Wall et al. (1987) hademphasised and Clemens (1989) had reiterated: linearchemical variation in granitoid suites can be produced byprocesses other than restite unmixing.

Collins (1996, 1998) can be justifiably criticised for notstating that the word ‘fractionation’ was used in a generalcontext, rather than to describe a specific process. However,this has no bearing on the three-component mixing model,as it relates to the second-order problem of chemical vari-ation, not source composition.

Arguments against the restite model

The eight perceived ‘problems’ are discussed below.(1) Appropriate minerals are absent in minimum-melt-

composition suites. Chappell and coworkers have alwaysstressed the importance of using geochemical variation todiscern petrological process in granite suites. Given thatemphasis, the geochemical variation suggests that theGlenbog Suite (Bega Batholith) should be a ‘minimum-melt’, rather than a ‘non-minimum melt’, which indicatesthat the melt should contain K-feldspar, according to therestite model. The assertion by Chappell et al. (1998) thatall Bega Batholith granites are ‘low-temperature’ types alsoimplies that K-feldspar should exist in the melt phase. Theabsence of K-feldspar in the Kadoona Dacite, comagmaticwith the Glenbog Suite, suggests that a fundamental flawexists in the restite model.

(2) Chemical trends for S-type granite suites do not project back to any major sediment composition on theAustralian continent. Chappell et al. admit that a ‘paradox’exists with S-type granites imaging their source rocks.

(3) All petrographic evidence for restite is inferred tohave been lost. All workers agree that restite exists in gran-ites, but the amount is debatable. Chappell et al. state thatthe demonstrable amount required by the restite model is‘not . . . conclusive’, a tacit admission that most of thepetrographic evidence for restite is lost. In the MoruyaSuite, point-counts of calcic cores and zircon in the mostmafic (restite-rich?) tonalite amount to <2% of the entirerock! More felsic rocks should contain less restite. As such,the restite model can only be based on chemical variation diagrams, which is problematic (see point 1).

The argument about the ability of metamorphic rocks(restite) to develop igneous textures in a magma is ill-informed. Means and Park (1994) have shown that quench-like textures (typical of mafic enclaves) are easily modifiedby grain-boundary migration, a term they called ‘texturalmetamorphism’. Means and Park (1994 p. 325) concludedthat ‘metamorphism can begin in igneous systems wellbefore the igneous (melt present) history is over’, not viceversa as argued by Chappell et al. Metamorphic textures aremuch more stable than igneous textures, because they rep-resent a lower energy state, so restite should retain its meta-morphic character in granite magmas. This certainlyapplies to schistose and gneissic enclaves in S-type granites, which may represent relics from the sourceregion, but they contrast with the quench textures of maficmicrogranular enclaves, which reflect freezing of hotmafic magma globules (e.g. Vernon 1990). The preservationof these quench textures suggests that mafic enclaves wereincorporated late in the crystallisation history of themagma, not at its inception in the source region as required by the restite model. Chappell and coworkers havecommonly asserted that textures are meaningless, but thisis mainly because quench features in mafic enclaves do notsupport their model.

(4) Isotopic evidence suggests that some suites are opensystems, which require input from several source components.The diagram illustrated by Chappell et al. (Figure 1)demonstrates exactly what Collins (1998) has stated in (4).If source-isotopic variability exists in the BullenbalongSupersuite, source-chemical variability must also exist.

832 Discussion and Reply

Thus, how can granites ‘image’ their source rocks, as con-sistently stated by Chappell (1979, 1994, 1996a p. 168), if thegranitic magma has not homogenised the various com-ponents? The chemical trends will reflect an array of mag-mas derived from different sources and an ‘error line’ isproduced that will not project to any specific source com-position. At best, it provides information on the characterof a dominant source component, but not on the specificsource composition. This explains point (2).

(5) S- and I-type granites have remarkably similar average compositions. Chappell et al. suggest that the similarity is to be expected, but one must ask why wouldS-type granites, supposedly derived from sediments sourcedfrom the Precambrian Australian craton, be similar to I-type granites supposedly sourced from younger mantle,via two-stage melting? The compositional similarity couldimply that the S-type source is the I-type granite sourceminus the weathered feldspar component, but this wouldmean that the Nd model ages of S-type granites be the sameage or younger than I-type granites, yet the opposite is thecase.

(6) S- and I-type granitoids of the eastern Lachlan FoldBelt fall on the same well-defined eNd–Sr isotopic array.Chappell et al. simply assert that the point is not relevant,but a similar question to (5) must be asked. Why would S-type granites, inferred to be derived from craton-derivedsediments with Proterozoic Nd-model ages, lie on the sameisotopic array as I-type granites indirectly derived fromunderplated, mantle-derived sources, unless they had acommon source component? Keay et al. (1997) showed that the common source component was Ordovician sediment.

(7) Zircon inheritance patterns between S-type granites,I-type granites and Ordovician sedimentary rocks are verysimilar. I-type granites should contain much less inheritedzircon than S-types because the two major source compon-ents, mantle and Neoproterozoic–Cambrian greenstones,are generally too mafic to be zircon saturated. Nonetheless,the inherited zircon populations of S- and I-types are similar, suggesting a shared component. Given that theinheritance pattern of both granites types is very similarto the Ordovician sediments, the evidence is compellingthat the common component is the Ordovician sediment,as shown chemically by Collins (1996, 1998). This reinforcesthe relevance of points (5) and (6) and is supported by thePb isotopic evidence.

(8) A Cordilleran mountain-chain is the basement tothe Ordovician turbidites in the eastern Lachlan Fold Belt.I agree that Chappell and coworkers never proposed a 600Ma Cordilleran mountain belt in the east Lachlan FoldBelt, but it is an unstated implication of the ‘remagma-tism’ model (Chappell & Stephens 1988), which hypothe-sises that ‘Caledonian’ I-types (= Lachlan Fold Belt I-types)were derived from ‘Cordilleran’ I-types by wholesalecrustal recycling. Although these inferred source rocksare usually ‘calcic’, ‘high Na/K’, ‘low Rb’, etc., they typ-ically occur throughout Cordilleran mountain belts,which is why Pitcher (1982) called them ‘Cordilleran’rocks. The tectonic implication of the remagmatismmodel remains valid.

Errors in interpretation of isotopic data

Chappell et al. explain why Collins (1996, 1998) claimed thatthe ages of the inferred source rocks are Proterozoic,according to the restite model. They quote their ownpapers which show that they stated the sedimentary sourcerocks were of Proterozoic (1500–1400 Ma) age. They alsostate that an Ordovician sediment with a 1500 Ma Nd-modelage should retain that age if melted in the Tertiary. Exactly!Similarly, if an Ordovician sediment, derived from theProterozoic Craton, melted in the Silurian to produce an S-type granite, the granite would also retain the Proterozoicage, which is precisely my point: the Proterozoic Nd-modelages of the Lachlan Fold Belt S-type granites reflect incor-poration of Ordovician sediments, not Proterozoic sourcerocks.

Chappell et al. state that the restite model does notrequire a Proterozoic continental basement for the LachlanFold Belt. This is a direct contradiction of the basement ter-rane model (Chappell et al. 1988), itself an outgrowth of therestite model. Chappell et al. (1988 p. 507) stated that ‘gran-ites form an image of their source, so that by studying theman image can be gained of the source in the deeper crust’.They also stated that S-type granites are produced from sedimentary material, so their inferred ‘continental crustor microplates, [which] assembled in the Late Proterozoicor Early Palaeozoic to form the [Lachlan Fold Belt] sub-strate’ (Chappell et al. 1988 p. 505), must be Proterozoic con-tinental basement. This ‘required’ continental basement isthe major reason why many Lachlan Fold Belt tectonic models of the 1980s and early 1990s incorporated aProterozoic continental basement (e.g. Cas 1983; Scheibner1989, 1992; Fergusson & Coney 1992; Glen 1992; Wyborn 1992).

Composition of sediments and enclaves in theBullenbalong Suite

Collins (1998) called some Koetong Supersuite metasedi-mentary enclaves, Bullenbalong Supersuite enclaves, butshould have called them Lachlan Fold Belt metasediment-ary enclaves. The second point of Chappell et al. is moreimportant: that some metasedimentary enclaves exist in fel-sic granites. Chappell et al. argue that the enclaves cannotbe source material because they should have been removedduring fractionation, and therefore are ‘demonstrably acci-dental inclusions’ from the Ordovician sediments. Thisstatement merely assumes that the magma chamber is aclosed system (as required by the restite model), but it is not a proof of accidental inclusion. At the base of theKameruka pluton in the Bega Batholith, high-grademetasedimentary enclaves exist within felsic granitic dykes,which dissipate into coarse felsic layers (crystal mush) inthe main granite body. The field evidence shows that syn-plutonic felsic dykes can carry gneissic fragments into amagma chamber at any stage of its crystallisation history.

Some specific misquotations

The arguments by Chappell et al. about the ‘level ofcertainty’ of source-rock compositions are semantic.

Evaluation of granitoid models 833

For twenty years, Chappell (1979, 1994, 1996b) has repeatedlystated that granites ‘image’ their source rocks, whichimplies a very specific level of certainty, yet it is nowclaimed that compositions only ‘approach those of thesource rocks’. The other examples of misquotations arealso minor problems. If the comment on ‘Proterozoic base-ment’ west of the I–S line is not strictly correct, this strongimplication has certainly carried through in their laterpapers, most notably in the basement terrane model ofChappell et al. (1988).

Magma mixing and the Moruya Suite

Chappell (1996a, b) gave reasons why he thought crystalfractionation, magma mixing and/or mingling, and assimi-lation were not applicable for causing chemical variationin suites of the Bega Batholith, which includes the MoruyaSuite. He concluded that the restite model was the onlyviable mechanism for generating the chemical variation.However, detailed fieldwork, petrography, and systematicgeochemical/isotopic analysis show that magma mixingmust have occurred in the Moruya Suite (Rodriguez 1998).

Figure 2 shows a north–south section through theTuross Head pluton of the Moruya Suite at Tuross Head,with load casts in mafic layers indicating way-up to thenorth, away from the pluton contact (Wiebe & Collins 1998).Three zones are evident. The lowermost Zone A consists of a silicic tonalite (SiO2 ~65–66%) with few mafic enclaves.It is overlain by Zone B, which contains wedge-shaped gabbro/diorite layers and tonalites of generally lower silica content (61–64% SiO2) with abundant mafic enclaves,mostly derived from the mafic layers. The uppermost layer

(Zone C) consists of strongly foliated, very mafic tonalites(57–59% SiO2) and highly dismembered gabbro/diorite layers, which are progressively disaggregated to mineral-aggregate scale, so that in places it is not possible to isolate tonalitic from dioritic rock. These mafic aggregatesare the material called restite by Chappell and coworkers,but it is petrographically and chemically similar to the minerals within the mafic layers, for major elements and over 30 trace elements (M. Tate & W. J. Collins unpubl.data).

The systematic chemical and isotopic trends show thatthe granitic magma chamber became more mafic and iso-topically more primitive with time, with compositions ofthe tonalite and gabbro converging toward an intermedi-ate ‘hybrid’ diorite in Zone C. According to the restitemodel, the chemical trends should become more silicic withtime as restite progressively separates from felsic melt, theopposite to that observed. More importantly, the progres-sive isotopic variation indicates that the magma chamberwas an open system strongly influenced by mafic magmainteraction, not the closed system required by the restitemodel.The Tuross Head Tonalite trends account for approx-imately half of the total chemical variation in the MoruyaSuite, and overlap with those defined by Griffin et al. (1978).Field and petrographic evidence shows that the coexistinglayered mafic and silicic magmas (MASLI system of Wiebe1996) were strongly disrupted in the upper portion duringmagmatic flow and the two components mechanicallymixed to produce the chemical variation (66–57% SiO2) inthe Tuross Head Tonalite. The higher silica (>66% SiO2)granites of the Moruya Suite can be modelled by removalof the mafic aggregates, plus plagioclase (Collins 1996),

834 Discussion and Reply

Figure 2 Chemical and isotopic variation of a stratigraphic section in the Tuross Head Pluton, Moruya Suite, at Tuross Head. Thegeometry of gabbroic lobes (stippled; drawn to scale) in Zone B indicates ‘way-up’ in the pluton is to the north, away from the contact.

from any of the mixed granitic magmas. Thus, chemicalvariation in one of the ‘type’ restite-granite suites (White& Chappell 1977; Chappell 1996a, b; Chappell & Stephens1988) is demonstrably controlled by ‘open-system’ magmamixing, followed by fractionation processes other thanrestite unmixing.

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836 Discussion and Reply