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Contrib Mineral Petrol (2016) 171:35DOI 10.1007/s00410-016-1251-y

ORIGINAL PAPER

The Tynong pluton, its mafic synplutonic sheets and igneous microgranular enclaves: the nature of the mantle connection in I‑type granitic magmas

J. D. Clemens1 · K. Regmi2 · I. A. Nicholls3 · R. Weinberg3 · R. Maas4

Received: 24 November 2015 / Accepted: 7 March 2016 / Published online: 26 March 2016 © Springer-Verlag Berlin Heidelberg 2016

quartz dioritic sheets. Despite this evidence of direct man-tle input into the Tynong magma system, the main granodi-oritic series do not appear to have been formed by magma mixing processes. Of any I-type granite in the region, the Tynong pluton has perhaps the most direct connection with mantle magmas. Nevertheless, the main mantle connection here is probably in the mantle-derived protolith for these crustal magmas and in the mantle thermal event that gave rise to melting of the deep crust in the Selwyn Block. This degree of mantle connectedness seems typical for I-type granitic rocks worldwide.

Keywords Granitic rocks · Synplutonic sheets · Microgranular enclaves · Petrogenesis · Tynong batholith · Central Victoria

Introduction

This paper stems from an ongoing debate about how gra-nitic rocks, commonly emplaced into the upper crust, relate to their deep source terranes, and how compositional vari-ability arises within such granitic plutons. There are sev-eral competing ideas for the origins of chemical variations in granitic series, chief among which are fractional crys-tallisation, restite unmixing, magma mixing and peritectic assemblage entrainment (PAE). See, for example, the dis-cussion and critique in Clemens and Stevens (2012). After decades of research into the nature of the source rocks for granitic magmas, and most particularly into their isotopic characteristics, the community seemed to have reached a good measure of consensus that most granitic magmas that were emplaced in large batholiths, within or at the margins of continents, have a substantial crustal component. That crustal component commonly arises through partial melting

Abstract In the Lachlan Orogen of south-eastern Aus-tralia, the high-level, postorogenic, 368-Ma, I-type Tynong pluton contains granitic to granodioritic rocks that crystal-lised from a variety of mainly crustally derived magmas emplaced in the shallow crust, in an extensional regime. The isotopic characteristics of the main plutonic rocks are relatively unevolved (87Sr/86Srt ~ 0.705–0.706 and εNdt ~ −0.4 to 0.6), suggesting source rocks not long sepa-rated from the mantle. We infer that arc mafic to interme-diate rocks and associated immature greywackes formed the main crustal source rocks and that these are located in the largely unexposed Neoproterozoic–Cambrian Selwyn Block that forms the basement. As exposed near its south-ern margin, the pluton also contains minor, pillowed sheet-like intrusions of quartz dioritic rock that show mainly min-gling structures with the enclosing granodiorites, as well as some hybrid pods and fairly abundant igneous microgran-ular enclaves that we infer to have been derived from the

Communicated by Hans Keppler.

Electronic supplementary material The online version of this article (doi:10.1007/s00410-016-1251-y) contains supplementary material, which is available to authorized users.

* J. D. Clemens jclemens@sun.ac.za

1 Department of Earth Sciences, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa

2 Department of Geology, University of Namibia, Private Bag 13301, Windhoek, Namibia

3 School of Earth, Atmosphere and Environment, Monash University, Clayton, VIC 3800, Australia

4 School of Earth Sciences, The University of Melbourne, Parkville, VIC 3010, Australia

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of pre-existing crustal rocks as they are heated from amphi-bolite- to granulite-facies conditions (e.g. Clemens 1990; Thompson 1990). More specifically, those granites with chemical and isotopic features that mark them out as S-types (Chappell and White 1974, 2001) are required to have nearly purely metasedimentary crustal sources. More debate has always surrounded the I-type granites, with some arguing for a direct and sometimes major mantle input to the magmas (e.g. DePaolo et al. 1992; Douce 1999; Soesoo 2000; Sisson et al. 2004; Gray and Kemp 2009; Nandedkar et al. 2014; Keller et al. 2015) and others pointing out that what could be interpreted as a relatively juvenile mantle isotope signature, or a mixed crust-mantle source, could be explained just as readily in terms of partial melting of crus-tal igneous rocks, themselves not long extracted from the mantle (e.g. Whalen et al. 2002; Waight et al. 2007; Clem-ens et al. 2011a, b). Nevertheless, most workers seemed to accept that the I-type granites could have a variety of rela-tionships with mantle magmas, some close and some more distant. However, recently, more extreme views have sur-faced, as perhaps best exemplified by Keller et al. (2015), published in no less a journal than Nature. These authors carried out a survey of the major-oxide and trace-element compositions of igneous rocks, worldwide, and arrived at the conclusion that the vast majority of felsic magmas must be formed by differentiation of mantle-derived magmas—‘fractional crystallisation, rather than crustal melting, is predominantly responsible for the production of intermedi-ate and felsic magmas’ (op. cit, p. 301). Their view may have been affected significantly by the fact that they did not examine the isotopic evidence, which, though equivocal in some respects, demands that the crustal component be considered. In counterpoint to the findings of Keller et al. (2015), Glazner et al. (2015) also used a large data set to conclude essentially the opposite, that volcanic and plu-tonic felsic rocks are chemically equivalent and that these magmas carry large crustal components. Nevertheless, there seems to be a growing perception that granitic mag-mas are very closely chemically related to mantle-derived mafic magmas (e.g. Nandedkar et al. 2014). This will be discussed further below.

The most obvious potential mantle connection, that seems to be present in many S- and I-type granites, is the presence of igneous-textured microgranular enclaves (ME). These usually rounded, dark-coloured bodies typically have somewhat lower SiO2 contents than their host granitic rocks and commonly also have more primitive isotope signatures. Thus, these ME do seem to indicate the involvement of mantle-derived magmas, albeit much hybridised with crus-tal material (e.g. Vernon 1984, 1990; Eberz and Nicholls 1988; Elburg and Nicholls 1995; Elburg 1996; Wiebe et al. 1997; Clemens and Bezuidenhout 2014). The debate then revolves around the stage at which the ME were included

in their host granitic magmas, how and where the ME became hybridised and whether the presence of the mag-mas parental to the ME produced significant compositional changes in the host magmas. In other words, are the ME mostly a magma-mingling phenomenon or are they a sign of significant magma mixing? From the references cited above, the consensus seems to be that the presence of ME signals mantle magma involvement in the provision of heat for crustal melting and that, at the level of emplace-ment, the interaction with the host magmas is generally mingling rather than mixing. The heterogeneous distribu-tion of enclaves within their host magmas provides further evidence of mingling rather than mixing (Ventura et al. 2006). Based on field observations and the previous studies referred to earlier, Clemens and Bezuidenhout (2014) pro-duced a model for ME origins that may have general appli-cability. The conclusion is that the ME represent a definite mantle connection with the production of granitic magmas but that they do not normally signify a significant role for crust-mantle magma mixing in the genesis of their host gra-nitic rocks.

In some granitic complexes, the mantle connection is rather more obvious than the presence of a minor volume of ME. In these cases, the granitic magmas seem to have coex-isted with and been cointruded with relatively large bodies typically of dioritic magma. Though of significant volume, the dioritic portion is usually still quite minor in relation to the coexisting felsic portions of the intrusions. The con-tacts between the dioritic bodies (commonly synplutonic dykes) and the granites typically show either pillows of the more mafic rock, bulging into the more felsic host, or complex scalloped contacts in which the two magmas seem to be interfingering on the centimetre scale. In some cases, there is also evident local hybridisation between these more mafic bodies and the surrounding granitic rocks, strongly suggesting a limited degree of magma mixing.

Turning now to the origins of chemical and mineral-ogical variation in granitic series, the restite unmixing model of Chappell et al. (1987) states that the variety among granites arises from different degrees of separa-tion between a low-T melt and the solid mineral assem-blage remaining after melting, defined as ‘restite’ (pre-existing minerals as well as peritectic minerals formed in the melting reactions). This model has been widely debated (e.g. Wall et al. 1987; Clemens 2003; Vernon 2007) and now seems to be generally out of favour, for a variety of reasons specified in the cited works. An alterna-tive model of magma mixing, where magma variety is a result of mixing between a mantle-derived mafic magma, and a crust-derived felsic magma, accompanied by magma fractionation, became widely accepted in the 1990s (e.g. Collins 1996, 1998; Gray 1984; Gray and Kemp 2009; Keay et al. 1997). However, most magma interactions that

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are observable in the field indicate mingling rather than mixing, and there also appear to be serious energetic and rheological reservations about the ability of mafic magmas to assimilate crustal rocks or for felsic and mafic magmas to mix on any large scale, at least at pluton emplacement depths (e.g. Sparks and Marshall 1986; Bremond d’Ars and Davy 1991; Glazner 2007). Simply put, when tested (e.g. Clemens et al. 2009), the amounts of assimilation that are required to produce the compositional spectra of gra-nitic suites are not balanced by the amounts of energy or parent magma volume available.

Recently, however, the debate over the causes of com-positional variation in granitic magmas has been widened with the proposal of PAE as a common mechanism (Clem-ens et al. 2011a, b; Clemens and Stevens 2012). This model contends that differential entrainment of peritectic phases, and small grains of accessory minerals, by magmas as they leave the source area, are the main controls on the observed variety in granitic rocks. Clemens et al. (2011a, b) and Cle-mens and Stevens (2012) were careful, however, to point out that, although PAE appears to dominate, a variety of other processes can and do contribute to the overall compo-sitional variability observed in granitic associations.

Granitic rocks of the Lachlan Orogen, in eastern Aus-tralia, have played a key role in this discussion, thanks ini-tially to the work of Chappell and White (1974) who clas-sified them into I- and S-types, based on their mineralogy and chemistry. More recently, granitoids of the Lachlan became the focus of a comprehensive survey of Hf isotopes in zircons (Belousova 2005). Kemp et al. (2007) inferred the petrogenesis of I-type granitoids of eastern Australia from Hf and O isotope compositions of magmatic zircons in the rocks. They found that the Hf and O isotope compo-sitions are correlated and show characteristics of both the mantle and crustal metasedimentary rocks. Hence, it was suggested that the I-type granitic magmas were derived through assimilation of metasedimentary rocks by man-tle-derived melts. Indeed, in certain circumstances, I-type magmas could be produced in this way, and perhaps even by fractionation of some particular kinds of mantle mag-mas (e.g. Soesoo 2000; Sisson et al. 2004; Nandedkar et al. 2014). Clemens et al. (2011a, b) sounded a note of cau-tion over the experimental methods used by Sisson et al. (2004), bringing into question the validity of their infer-ences on the effectiveness of fractionation as an origin for granitic magmas. Indeed, such a generalised interpretation is open to question for other reasons. First, mantle-like isotope ratios can be derived either directly from mantle melts or by melting of pre-existing, mantle-derived rocks, especially if the ages of extraction from the mantle and of subsequent partial melting are not greatly different. Sec-ond, as shown by Nandedkar et al. (2014), as well as by modelling, the fractionation of mafic magmas to produce

granitic residua also involves production of about four times the volume of ultramafic–mafic cumulate rocks. As has often been pointed out, we do not see such cumu-lates. The common solution is to place these large vol-umes of cumulate rock out of observational reach, in the deep crust or mantle. However, there should also be very substantial amounts of intermediate rocks, which are also commonly missing, and are not as easily disposed of using density arguments. Third, as neatly demonstrated by stud-ies like those of Nandedkar et al. (2014), by the time felsic (≥66 wt% SiO2) residual magmas are produced, the tem-peratures are <750 °C. Even then, these liquids would have about 68 wt% SiO2 and would not be representative of the vast volumes of granitic magma that formed plutons such as Tynong, in which the low-Al series has a median SiO2 content of 73.97 wt% (54 analyses). Fourth, the composi-tions of the magmas formed by basalt fractionation are rel-atively sodic, especially if the parent basalt is tholeiitic. In contrast, the vast majority of the Tynong felsic magmas are high-K calcalkaline (e.g. Fig. 6f). Fifth, studies have shown that the geochemical trends in both I- and S-type granitic rock suites seldom extrapolate back towards conceivable mafic progenitors (e.g. Clemens and Benn 2010; Clemens and Birch 2012; Clemens and Bezuidenhout 2014; Cle-mens and Phillips 2014; the present work). Finally, it has been shown by experiment [e.g. data sources in Roberts and Clemens (1993)] and modelling (e.g. Clemens et al. 2011a, b) that magma source rocks such as andesites and dacites, with up to 70 % greywacke admixture will partially melt to produce I-type magmas. Such rock packages are common in arc-related tectonic settings and, if such rocks were present in a geological column and they reached their solidi, they would certainly partially melt to produce volu-minous I-type granitic magmas. The isotope variability of I-type granitic rocks could indeed be explained by mantle-crust magma mixing, but is just as well explained as min-gled or partially mixed magma fractions derived from het-erogeneous crustal protoliths (e.g. Villaros et al. 2012).

We acknowledge that different granitic complexes may exhibit contrasting origins for the heterogeneities that they display. Recent contributions on the origins of granitic and silicic volcanic complexes in the Victorian sector of the Lachlan Orogen have mainly emphasised source heteroge-neity and the role of PAE (e.g. Clemens et al. 2011a, b; Cle-mens and Birch 2012; Clemens and Bezuidenhout 2014; Clemens and Phillips 2014). Most of this work has been on S-type rocks. However, the Clemens and Bezuiden-hout (2014) paper concerned the genesis of I-type granitic rocks and their igneous-textured ME, presenting a pos-sible general model for the origin of such ME in granitic rocks. Here we present an example of a potentially differ-ent kind of I-type pluton, related to postorogenic extension like the others in the geographical region, but with what

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appear to be stronger, more direct mantle connections. We examine what the petrological and chemical evidence sug-gests regarding the origins of the magmas, the nature of the protolith/s and the causes of variation among the rocks.

Regional geology

The Lachlan Orogen formed along the Pacific margin of Gondwana from the Early Cambrian to Early Carbonifer-ous (Willman et al. 2010) and occupies much of eastern Australia (Fig. 1). It was deformed at the end of the Early Ordovician (Benambran Orogeny, ~460–430 Ma), as well as in the Middle Devonian (Tabberabberan Orogeny, ~400–380 Ma) and also in the Early Carboniferous (Kanimblan Orogeny, ~360–340 Ma). In the south of the Orogen lies the Melbourne Zone, bounded on the east and west by major faults along which Cambrian sea-floor and arc rocks are exposed. The Melbourne Zone is, however, dominated by Silurian to Devonian deep- to shallow-marine sedi-ments that were weakly deformed into open, upright folds and metamorphosed to lowermost greenschist facies dur-ing the Tabberabberan Orogeny. This Zone was intruded by numerous I- and S-type granitic plutons, and there are also several voluminous silicic volcanic complexes and

a mafic dyke swarm of the same general age (~370 Ma; Richards and Singleton 1981; Bierlein et al. 2001; Foster et al. 1998). The tectonic environment of this magmatism is postorogenic extensional, and the volcanic components are commonly associated with terrestrial red-bed basins. Beneath the exposed Palaeozoic rocks of the Melbourne Zone, geophysical studies have identified the presence of a substantial mass of Neoproterozoic–Cambrian continen-tal crust known as the Selwyn Block (Cayley et al. 2002). Charnockitic xenoliths in Late Tertiary basaltic rocks in the Melbourne Zone have been dated to 580 Ma, and Allchurch et al. (2008) presumed these to have been derived from the Selwyn Block. However, some consider that these xenoliths are derived from the Permian glacial deposits that under-lie the Tertiary lavas and that they represent rock units that are now in Antarctica rather than in the Selwyn Block (Clive Willman of Geoscience Victoria, pers. comm. 2006). Occurrences of tholeiitic metabasalt and metagabbro on the coast at Waratah Bay (Crawford et al. 1988) and Phillip Island (Henry and Birch 1992) and somewhat farther inland

Fig. 1 Map of eastern Australia showing the location of the Tasman Orogen

Fig. 2 Geological sketch map showing the locations of major gra-nitic plutons and silicic volcanic complexes, of Late Devonian age, within the Melbourne Zone and adjacent portions of the Bendigo and Tabberabbera Zones. The granitic plutons of the Tynong Province are indicated in the south. The Tynong pluton (the largest in the batho-lith) is labelled

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in the Barrabool Hills (Morand 1995) are possible rare exposed parts of the Selwyn Block. It has been argued that the geochemical and isotopic characteristics of the Late Devonian S-type granitic and silicic volcanic rocks of the Melbourne Zone are incompatible with derivation of the magmas through partial melting of the Palaeozoic series (e.g. Wyborn and Chappell 1979) and that there must be a pre-Ordovician source terrane involved. Thus, the Selwyn Block is not only important as a factor relating to the sedi-mentation and subsequent deformation of the Melbourne Zone during the Tabberabberan, but it is highly likely to contain the protoliths of the crustal components of the gra-nitic magmas (e.g. Clemens et al. 2011a, b; Clemens and Birch 2012; Clemens and Bezuidenhout 2014; Clemens and Phillips 2014).

The Tynong pluton

The I-type Tynong Province granitoids (Fig. 2) are char-acterised by the presence of hornblende and biotite as the main mafic minerals, with minor primary clinopyroxene and orthopyroxene, commonly pseudomorphed by cum-mingtonite, hornblende and biotite. The opaque accessory mineral is normally ilmenite. The Tynong batholith, the largest of a group of Late Devonian (~370 Ma) granitic complexes, east of Melbourne, includes at least five plu-tons (Lysterfield, Tynong, Toorongo, Tanjil Bren and Baw Baw; Fig. 3). All the felsic volcanism and plutonism of this age here is late postorogenic and seems to be related to

extensional tectonics, with the development of large terres-trial, red-bed basins together with some mafic volcanism. Clemens and Bezuidenhout (2014) described the Lyster-field pluton and its petrogenesis in some detail. The Lys-terfield pluton contains at least four separate magma series derived from contrasting lithological elements of the source terrane. Clemens and Bezuidenhout (2014) attributed the compositional variations within the series to PAE. The gra-nitic (s.l.) rocks in the batholith are metaluminous–weakly peraluminous. Here we concentrate on the largest pluton in the batholith—the Tynong pluton. Electronic Appendix EA1 contains a selection of field and hand-specimen photo-graphs showing salient features of the rocks.

The commonly muscovite- and sometimes garnet-bear-ing, high-Al granodiorites, granites, microgranites and aplites in the centre of the Tynong pluton contain no ME, but the porphyritic low-Al granodiorites contain quartz dioritic enclaves and pods, and pillowed sheets of quartz dioritic rock intrude them (see, e.g., Electronic Appendix EA1, Figs. EA1.2, 3, 4, 6). The ME are more mafic than their hosts, and they vary from fine-grained to almost as coarse-grained as their host granodiorites. The dioritic pods and pillowed sheets are finer-grained but contain zones with coarser grain sizes interpreted as largely mechani-cal hybrids between the diorites and the host granodiorites (e.g. Electronic Appendix EA1, Fig. EA1.4). Some of the coarser-grained ‘hybrid’ tonalite and mafic granodiorite pods also contain plagioclase and quartz ocelli (e.g. Elec-tronic Appendix EA1, Fig. EA1.5) that seem to have been derived from the surrounding host felsic magmas. Apart

Fig. 3 Map showing the Tynong batholith, its component plutons and the extent of its contact metamorphic aureole in the surrounding Siluro–Devonian metasediments

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from the quartz dioritic bodies, the low-Al granodiorites were also intruded by dykes of porphyritic microgranite with mineralogical characteristics similar to the granodior-ites, though with lower mafic mineral content and lacking hornblende. The numerous, very late dykes of aplite, which belong to the high-Al series, vary in width from a few cen-timetres to perhaps 1 m. Some of these contain tourmaline and occasionally tourmaline–quartz nodules of centimetric size, surrounded by pale-coloured ‘bleached’ halos rich in alkali feldspar (e.g. Electronic Appendix EA1, Fig. EA1.7).

Mechanical mixing has been proposed as a major pro-cess producing large-scale variations within the Tynong pluton (Cramer 1979). Cramer delineated four types of rocks—biotite monzogranite with or without hornblende, medium-grained biotite granite with or without garnet or muscovite, fine-grained equigranular biotite granite with or without garnet and muscovite, and porphyritic rocks, all with apparent transitional contacts with each other. The Tynong pluton is zoned, with the generally leucocratic muscovite-bearing high-Al rocks in its core, at the higher topographic elevations.

Petrography of the Tynong pluton

Electronic Appendix EA2 presents petrographic descrip-tions of all Tynong rock series, and photomicrographs illustrating the mineralogy of and microtextures in the granodioritic rocks, aplitic dykes, quartz dioritic sheets and ME.

Conclusions from petrography

The Tynong Province granitic (s.l.) rocks are characterised by the presence of clinopyroxene and hornblende, typical of I-type granitoids, and the presence of reddish brown bio-tite and accessory ilmenite—features considered more typi-cal of S-type rocks. The Tynong pluton itself exhibits most of these features (but with pyroxenes occurring only in the ME). These characteristics are interpreted as showing that the magmas were rather reduced I-types. The mantled feld-spar textures (Electronic Appendix EA1, Fig. EA1.1) dem-onstrate that plagioclase began crystallisation before alkali feldspar but that, as the magma evolved further, parts of the system swung back to plagioclase precipitation. As shown by Abbott (1978) anti-rapakivi texture (plagioclase mantled by K-feldspar) is an occasional consequence of isobaric fractional crystallisation. The formation of rapakivi tex-ture, however, has been ascribed to decompression of H2O-undersaturated magma (e.g. Nekvasil 1991). Such an origin is unlikely in this case, as K-feldspar is texturally late in the crystallisation sequence, and these overgrowths seem to have formed at emplacement level. Similar effects on

feldspar phase equilibria can be produced through lowering of melt H2O content (which is unlikely during crystallisa-tion) or by heating of the magma (which is also unlikely, in view of the small volumes of presumably higher-tempera-ture quartz dioritic magma present within the batholith, and even within the North Tynong Quarry itself). It has been suggested that the sodic plagioclase in the rims of rapakivi mantled feldspars represents recrystallisation of plagioclase exsolved from the perthitic alkali feldspars (e.g. Dempster et al. 1994). This is a possible origin here. However, since the rocks contain adjacent crystals of feldspars both with and without these overgrowths, the cause is most likely to be localised compositional shifts in the magma, with small equilibration volumes. In other words, the feldspars were responding to local equilibrium conditions within an over-all context of disequilibrium during late crystallisation. Mantled feldspars have been interpreted as features indicat-ing magma mixing, but the late appearance of K-feldspar in the crystallisation sequence of these rocks and the other chemical and isotopic evidence that we present suggest that this is not the case, at least for the Tynong pluton.

Other significant features of these rocks are that they contain some textures that are characteristic of small-scale magma hybridisation, such as enclaves containing host-derived xenocrysts, coarser-grained, possibly hybrid mar-gins on some enclaves, and quartz and plagioclase ocelli in some ‘hybrid’ tonalite to mafic granodiorite pods within the granodiorite. Rather than being a sign of local hybridisa-tion, the formation of the coarser margins on some enclaves may be related to the release of H2O towards the host magma, from these thermally quenched magma globules. As suggested by Pistone et al. (2016), the loss of this H2O is likely to result in increased melt proportion in the outer zone, through local lowering of the solidus, and to provide positive feedback in the form of chemical quenching of the enclave core, through an increase in its solidus tempera-ture. Fine grain size, acicular apatite crystals and lath-like plagioclase in the pillowed quartz diorites and enclaves indicate quench crystallisation in the shallow plutonic envi-ronment. The absence of any flow foliation in either the pillowed quartz dioritic sheets or the enclaves indicates that most crystallisation of these magmas occurred after emplacement.

Chemical characteristics of the Tynong pluton

Data sources and analytical techniques

The major-, minor- and trace-element analyses of the rocks of the Tynong pluton are presented in Electronic Appendix EA3, Table EA3.1. Data sources are various, but all major and minor oxides were analysed by XRF,

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at La Trobe University for analyses from Cramer (1979), Rossiter (1973) and Rossiter (2001), at James Cook Uni-versity for Regmi (2012), at the University of Wollongong for samples with a TYN prefix and at the University of Stellenbosch for the balance. Trace elements were ana-lysed by XRF, except for the samples from Regmi, and the TYN samples, which were analysed for REE by solu-tion quadrupole ICPMS at Monash University, and the samples with TY prefixes, for which trace elements were analysed at Stellenbosch by laser-ablation ICPMS in the fused discs that were used for the XRF analyses. Elec-tronic Appendix EA4 contains information on the analyti-cal methods used. Samples with the CM prefix are from the PhD thesis (La Trobe University) of Cramer (1979), and those with plain numbers 85–89 and with the LFB prefixes are from the MSc and PhD theses (La Trobe University) of Rossiter (1973, 2001). The eight samples with a TYN prefix were collected by Maas and Nicholls (coauthors here) for a separate project and analysed at the University of Wollongong. Most of the balance are from the PhD thesis (Monash University) of Regmi (2012), one of the coauthors here. The rest, with the prefix TY, were collected by Clemens (a coauthor here) and analysed at

Stellenbosch University and the University of Cape Town (for tracer isotopes). The Regmi (2012) samples were ana-lysed for Sr and Nd isotope ratios at Melbourne Univer-sity, by multi-collector solution ICP-MS (see Maas et al. 2005), and the samples with TY prefixes were analysed by a similar technique at the University of Cape Town (see, e.g., Clemens and Bezuidenhout 2014, Electronic Appendix EA3). The rock chemistry (Electronic Appen-dix 3, Table EA3.1) and isotopic characteristics (Elec-tronic Appendix 5, Table EA5.1) are portrayed in Figs. 4, 5, 6, 7, 8 and 9.

Magma groups

As mentioned earlier, we have identified several different magma groups within the pluton. These include the volu-minous low-Al and high-Al series granodiorites and gran-ites, minor high- and low-Na granodiorites and high-Ni granodiorites, late microgranitic and aplitic dykes, low-vol-ume pillowed quartz dioritic sheets, ‘hybrid’ pods and ME in the granodiorites, and one biotite cumulate enclave. The chemical and tracer-isotopic characteristics of each of these series of rock types are described below.

Fig. 4 Plots illustrating the chemical contrasts between the rocks of the low- and high-Al series in the Tynong pluton—a ASI [mol. Al2O3/(CaO − 3.33P2O5 + Na2O + K2O)], b Sr and c P2O5 plotted against SiO2 content, and d the REE spectra for the two series. Aplitic

dykes are present in both series and are plotted with thick black rings. In a the dashed line separates strongly from weakly peraluminous rocks

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Low‑ and high‑Al series

These granitic rocks form the main bulk of the Tynong pluton. As described above, they belong to the high-Al series commonly contain muscovite and some have acces-sory magmatic garnet. They occupy the centre of the plu-ton and probably represent a sheet emplaced near the roof of the original magma chamber. The aplitic dykes that cut the rest of the rocks in the pluton can also be divided into those that belong to the high-Al series and those that belong to the low-Al series. The main features that dis-tinguish these two series are the relatively high concen-trations of P2O5 and Sr, and the high ASI [mol. Al2O3/(CaO − 3.33P2O5 + Na2O + K2O)] in the high-Al series (Fig. 4). In terms of tracer isotopes, these two series pre-sent no significant contrasts (Fig. 5a), with small ranges for both 87Sr/86Srt (0.7055–0.7058) and εNdt (−0.5 to +0.4), suggesting relatively unevolved magma sources—probably metaigneous rocks with no great length of crus-tal residence at the time of magma genesis. The zircon,

U–Pb, laser-ablation ICP-MS dates obtained for two of the Tynong rocks (Regmi 2012) are as follows: for the low-Al granodiorite sample TYN5 the date is 353 ± 6 Ma (MSWD = 0.21, p = 0.9993), and for high-Al granite 2803-8 the date is 368 ± 7 Ma (MSWD = 0.73, p = 0.76). This suggests that the possibility that these two series rep-resent magmas emplaced perhaps a few millions of years apart. However, from the available data, the statistical sig-nificance of the age difference cannot be determined easily.

High‑ and low‑Na granodiorites

Neither of these two series appears to be voluminous, and both have been identified only from the analytical data. Two high-Na rocks were analysed by Regmi (2012) and one low-Na rock by Cramer (1979). As the names suggest, these two unusual sorts of Tynong magmas have Na2O con-tents outside the ranges shown by the main series in the pluton (the high- and low-Al rocks). There are no tracer isotope data for the low-Na rock (sample 86), but high-Na rock 2803-5 is isotopically distinct from any other ana-lysed Tynong rocks. As shown in Fig. 5a, 2803-5 has far more evolved characteristics, with 87Sr/86Srt of 0.7073 and εNdt of −1.8, although these values cannot be considered particularly evolved among the I-type rocks of the central Victorian region as a whole. Additional chemical character-istics that set these two series apart from the high- and low-Al rocks include low TiO2, FM (FeOT + MnO + MgO, where FeOT = total Fe as FeO) and CaO, and high Al2O3 and Pb in the high-Na series, low Ba and Zr in both series, and low Sr in the low-Na rock. These features are illus-trated in Figs. 6 and 7.

High‑Ni granodiorite

A single analysed sample of a high-Ni granodiorite appears among the data from Regmi (2012). Apart from the high Ni and Cr contents (108 and 205 ppm, respectively), the other chemical characteristics that are unique to this coarse-grained granodiorite (sample 2802-2) include low Al2O3, ASI, K2O and Ba, and high FM, Mg# [100 Mg/(Mg + Fe)], Ni and CaO (Figs. 6, 7, 8).

Cumulate quartz monzodiorite enclave

The chemistry of sample TY16 reflects its probable origin as a fragment of a cumulate body within the low-Al grano-diorite magmas. It is highly enriched in TiO2, FM, K2O, Rb, Zr, Nb, V and REE (except Eu) and impoverished in SiO2, Al2O3, CaO, Na2O, Sr and Ba (Figs. 6, 7). These are all characteristics consistent with accumulation of biotite (with its inclusions of accessory zircon and ilmenite) and deple-tion in quartz and plagioclase, features noted in Electronic

Fig. 5 Tracer isotope plots for analysed rocks of the Tynong pluton. a Isotope correlation diagram (87Sr/86Srt vs. εNdt, where t = 368 Ma) and b a Sr isotope mixing diagram (87Sr/86Srt vs. 1000/Sr concentra-tion in ppm)

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Appendix EA2. Although the error is quite large, it is clear that the one sample of this rock group (sample TY16) has the lowest 87Sr/86Srt (0.7047) of any of the analysed rocks from the Tynong pluton, though its εNdt value of −0.1 is consistent with those of the other rocks of the low-Al series (Fig. 5a). This is a curious relationship that implies that the specific magma from which biotite accumulated to form this rock was slightly less evolved in its Rb–Sr isotope sys-tem than the other analysed Tynong rocks, for which the lowest 87Sr/86Srt is 0.7055.

Granodioritic enclave

For many elements and oxides, the relatively coarse-grained granodioritic enclave sample TY3 plots as an extension of the trend for the high-Al series of host rocks, though it was collected as an enclave in a rock belonging to the low-Al series. However, there are notable excep-tions to its similarity with the high-Al rocks. The enclave has low TiO2 (relative to its FM content) and very low Ba, it is richer in Sr, Zr and Nb and is far more ferroan than its

Fig. 6 Major-oxide Harker plots for the rocks of the Tynong pluton. a TiO2, b Al2O3, c FM (FeOT + MnO + MgO, where FeOT = total Fe as FeO), d CaO, e Na2O and f K2O, showing the fields for the tholeiitic, medium-K, high-K and shoshonitic series

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host granodiorites, with an Mg# of 29. The chondrite-nor-malised REE plot for this enclave is unlike the spectra for any of the coarse-grained granitic to granodioritic rocks, in that it has no significant Eu anomaly. The pattern is actu-ally most similar to those of the more mafic ME, with a slightly concave upward shape in the middle to heavy REE (Fig. 9b). Figure 5a shows that this enclave has 87Sr/86Srt and εNdt (0.7059 and +0.1, respectively) within the range shown by the other, more mafic enclaves, and indeed the host rocks.

Pillowed quartz dioritic sheets and mafic to intermediate enclaves

These two sets of samples are dealt with together because, from both field and microscopical studies, it is clear that the ME were derived from the same sorts of dioritic mag-mas as formed the sheets. Indeed, many enclaves seem to have been derived directly from these sheets, though there has certainly been interaction between the enclave and host low-Al granodiorite magmas.

Fig. 7 Harker plots for selected trace elements in the analysed rocks of the Tynong pluton. a Pb, b Ba, c Sr, d Zr, e Cr and f Ni

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The sheets and ME have SiO2 contents between 56 and 61 wt%, with a large compositional gap between them and the most mafic of the granodiorite host rocks (68 wt% SiO2). Although some major-oxide Harker plots appear to suggest a possible mixing trend between the diorites and enclaves on the one hand and the host low-Al granodiorites on the other (e.g. TiO2, FM and CaO in Fig. 6a, c, d), some other plots show that such a scenario is highly unlikely (e.g. Al2O3, Na2O and K2O in Fig. 6b, e, f). Note also in Fig. 6 that the ME and sheets commonly exhibit a high degree of scatter in the data, while the low-Al granodioritic host rocks define a good, quasi-linear trend. Similar relations are apparent in the trace-element variations depicted in Fig. 7a, b, d, e, where the plots for Pb, Ba, Cr and Zr form roughly linear trends for the low-Al granodiorite points, but these trends do not form any sort of prolongation of the trends defined by the points for the ME and sheets. In this regard, the variation in Ba concentrations (Fig. 7b) presents

perhaps the best example of this lack of relationship. Addi-tionally, the variation in ASI (Fig. 8b) would appear to rule out mixing as the origin of the low-Al host trend. However, note that, in many of the variation diagrams, the distribu-tions of points for the ME and sheets do suggest that the variation in these magmas may have been influenced by mixing interactions with the host low-Al granodiorite mag-mas. Thus, on the grounds of elemental concentrations, although magma mixing seems unlikely to have materially affected the granodioritic host magmas, the compositions of the smaller-volume ME and sheets may well have been modified by a degree of mixing with some of the more mafic low-Al granodiorite magmas.

In terms of isotope variations, Fig. 5b also provides no support for a simple mixing model. The points for the low-Al granodiorite hosts lie at virtually the same value for 87Sr/86Srt although the εNdt values indicate some het-erogeneity. In any case, there is no apparent mixing trend between these and the cluster of points for the ME and sheets. The scatter of points on the isotope correlation dia-gram of Fig. 5a rather suggests emplacement of a variety

Fig. 8 Harker-style plots for calculated chemical parameters in the analysed rocks of the Tynong pluton, plotted against SiO2 concen-tration. a Mg# [100 Mg/(Mg + Fe)] and b ASI. In b the solid black line separates metaluminous from peraluminous compositions and the dashed line separates weakly from strongly peraluminous composi-tions

Fig. 9 Chondrite-normalised REE plots for the rocks of the Tynong pluton. REE spectra for a the low-Al granodiorite series, the high-Al series and high-Na granodiorites, and b the quartz dioritic sheets and igneous microgranular enclaves (ME), the relatively coarser-grained granodioritic enclave (TY3) and the two rocks from the ‘hybrid’ ton-alitic and mafic granodioritic pods

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of compositionally distinct magma batches. The REE vari-ations also provide evidence of this heterogeneity and the general lack of any great chemical or isotopic influence of the mafic to intermediate and felsic magmas on each other. In Fig. 9b, note that none of the ME or quartz dior-itic sheets has any significant Eu anomaly, in contrast to the generally very prominent negative anomalies shown by the low-Al granodiorites (Fig. 9a).

The coarser-grained, more felsic margins on some enclaves are represented by sample TY3 (Electronic Appendix 3, Table EA3.1 and plotted as a pale green tri-angle in various geochemical figures). It appears that these zones are richer in SiO2, Na2O and Zr and poorer in TiO2, MgO, K2O, transition elements and Ba than the cores of the enclaves. Such variations suggest in situ fractionation as the origin of these zones, possibly influenced by the loss of H2O from enclave cores to the rims (see ‘Conclusions from petrography’ section). This is in keeping with low Mg# and the 87Sr/86Srt values being like those of the enclaves but higher than in the host granodiorites, as well as the fact that TY3 does not fit on any putative mixing trend between these enclaves and their hosts (e.g. Figs. 5, 6f, 7b–d, 8).

‘Hybrid’ tonalite and mafic granodiorite pods

The distinctive features of this group of rocks (as exempli-fied by samples 2801 and 2802-3) include low TiO2 relative to their FM contents, low Al2O3, very low ASI, high FM and Mg#, and very high Ni and Cr contents (see Figs. 6, 7, 8). Ignoring the biotite cumulate enclave, these ‘hybrid’ rocks seem to have the most primitive isotope signature in the pluton; sample 2802-3 has an 87Sr/86Srt of 0.7054 and an εNdt of +1.2 (Fig. 5a). These rocks do not plot on any possible mixing trends between the quartz dioritic sheets, the quartz monzodioritic ME and the low-Al granodiorites. This is especially apparent in the Al2O3, FM, ASI, Mg#, Cr and Ni variation diagrams of Figs. 6b, c, 7e, f, 8.

Discussion and conclusions on the origins of the Tynong magmas

It is quite clear from their hornblende- and allanite-bear-ing mineralogy, and metaluminous to weakly peralumi-nous chemistry, that the magmas that formed the low-Al series were I-type. This conclusion is supported by the low 87Sr/86Srt of these rocks (0.7055–0.7073) and the relatively unevolved εNdt values of −1.8 to 0.5. A common model for the production of I-type magmas is mixing between a mantle-derived mafic or intermediate magma and crustally derived felsic magmas. As we have seen above, the inter-mediate, quartz dioritic magmas in the Tynong pluton do not appear to represent end members in such a process, as

the chemical and isotopic variations lend scant support for mixing as the origin of the main chemical variations within the pluton. It could, however, be hypothesised that the low-Al series magmas result from mixing with a more mafic end member. Figure 10 shows the Tynong data points and those for calcalkaline basalts and andesites, and effectively

Fig. 10 Harker plots of the variation of a Al2O3, b Na2O and c Ba in the low-Al series rocks of the Tynong pluton (pink dots). The blue lines represent linear fits (with correlation coefficients) and extrapo-lations of the trends defined by the low-Al rock analyses. The small black dots represent the extant database of 266 analyses of calcalka-line basalts (from the Earthchem database, http://www.earthchem.org/), and the small black crosses represent the 7472 analyses of andesites, from the same database. It is apparent that the trends for the Tynong rocks do not extrapolate into the fields where most calcal-kaline basalts and andesites plot. Only the most exceptional of such mafic to intermediate rocks could represent an end member in a puta-tive magma mixing series

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rules out such a model. The extrapolated trends for Al2O3, Na2O and Ba are incompatible with involvement of basaltic or andesitic end members, except for those with the most exceptional compositions. Thus, we have no evidence that the low-Al series is related by a processes of magma mix-ing to any other known rocks in the pluton, or indeed to any unspecified basaltic to andesitic magma. What we do see is minor, emplacement-level hybrid formation in some locations between fragmented pillows of quartz diorite sheets. However, these are very localised phenomena and do not appear to have been involved in producing the over-all chemical variations in the pluton. The idea that deeper-level/near-source mixing might be involved is effectively countered using the relationships shown in Fig. 10 as well. What we do suggest is that partial melts from the some-what heterogeneous package of crustal source rocks for the Tynong felsic magmas must have undergone some measure of mingling and mixing. What we are excluding is mixing with mantle magmas, at any crustal level, to form the felsic series.

In addition to biotite, many of the high-Al series at Tynong contain muscovite and some contain garnet. These rocks too are best thought of as weakly to strongly peralu-minous I-types, as they also appear to have relatively une-volved Sr and Nd isotope characteristics. Sample 2803-8 has an 87Sr/86Srt of 0.7058 and a εNdt of 0.6. If we accept this then the next issue is how they attained their peralu-minous chemistry. Since they are I-types, the peralumi-nous nature of the high-Al Tynong rocks could, in theory, result from fractional crystallisation of hornblende from a less aluminous parent magma (e.g. Cawthorn and Brown 1976; Cawthorn and O’Hara 1976). As Zen (1986) and Chappell et al. (2012) point out, only very small volumes of peraluminous residual magma can be created by this process. The muscovite-bearing high-Al rocks do indeed make up only a very small fraction of the Tynong pluton but, for the following reasons, it seems unlikely that they formed by extreme fractionation of an I-type parent. First, although the high-Al rocks are generally quite felsic, their SiO2 contents (69.8–77.5 wt%) span most of the range shown among the low-Al series (67.9–77.9 wt%), and the chemical trends for the two series run parallel in many Harker plots (see, e.g., Figs. 4, 6). Second, on the Ba–Pb plot devised by Finger and Schiller (2012), both the low- and high-Al series define trends that imply their origin as progressively higher-temperature melts (Fig. 11a). Spe-cifically, the points for the high-Al series do not form the trend expected for fractionation involving feldspars and biotite (i.e. parallel to the bold line in Fig. 11a). This Ba–Pb plot was devised for S-type magmas, but the principle should also apply to I-types, as long as the partial melting reactions that formed them were dominated by the break-down of micas.

There are two credible hypotheses for the origin of the Tynong high-Al magmas. These magmas could repre-sent a series of relatively low-temperature partial melts of an I-type source, leaving behind restitic and/or peritectic hornblende (see, e.g., Fig. 11b). Alternatively, these highly aluminous I-type magmas might have been the products of muscovite breakdown reactions in a part of the I-type source terrane that had been previously altered by hydro-thermal activity. If such alteration did not long predate magma formation then the magmas could still inherit rela-tively unevolved Sr and Nd isotope characteristics.

Other than mentioning the concepts in the introductory section, we have paid little attention to fractional crystal-lisation, restite unmixing and PAE, which are the main

Fig. 11 a Logarithmic plot of Ba versus Pb concentrations in the Tynong low- and high-Al series rocks (pink and yellow dots, respec-tively). This diagram was originally devised by Finger and Schiller (2012) to differentiate between S-type granitic magmas formed by partial melting reactions involving muscovite or biotite breakdown, as well as rocks formed through fractionation processes. Note that neither series of rocks defines an array consistent with fractionation as the origin of the variations. b Plot of apparent zircon saturation temperature against ASI values for the analysed rocks. The tempera-tures were calculated from the zircon saturation model of Watson and Harrison (1983), using the Zr concentrations in the rocks, with the assumptions that the whole-rock compositions approximate to those of the melts and that little of the zircon in the rocks is inherited. Nei-ther of these assumptions is strictly correct, and so the absolute val-ues of the derived temperatures cannot be trusted. However, the main point is that the high-Al rocks yield generally lower temperatures than the low-Al rocks

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alternative models for the formation of chemical variation in granitic suites. We do not propose to reiterate the argu-ments that have been presented in effective refutation of the restite unmixing hypothesis of Chappell et al. (1987). The reader is referred to Wall et al. (1987), Clemens (1989), Vernon (2007) and Clemens and Stevens (2012) for a com-prehensive treatment of this issue. Clemens et al. (2010) and Clemens and Stevens (2012) have detailed the evidence that crystal fractionation is a minor process in the produc-tion of heterogeneities in granitic magmas in general. The common heterogeneity in calculated initial Sr and Nd iso-tope ratios for granitic series in plutons and the mismatches between models based on different elements rule out crys-tal fractionation as the main cause of the variations, though there may be a variety of other possible causes of this. Note also that, as stressed by Clemens and Stevens (2012), frac-tionation certainly does occur, especially on local scales, where filter pressing and flow segregation of crystals seem to be the mechanisms most commonly responsible. Together with local magma–magma interactions, these pro-cesses certainly add noise to the main signal carried from the magma source region.

In the case of the Tynong pluton, the Sr isotope ratios would be compatible with a fractionation model, but the Nd isotopes are far too variable to permit such a model for the low-Al series (see, e.g., Fig. 5a). Thus, we are left with two processes that are most likely to have been responsible for the observed variations. The first is hetero-geneity in the crustal sources of the magmas, which would explain the existence of the low- and high-Al series and the isotopic variations. The second process is PAE, which would explain the quasi-linear chemical variations within the series. The details of this process have been explained by Clemens et al. (2011a, b) and Clemens and Stevens (2012), who also showed how PAE differs fundamentally from restite unmixing, which is a fractionation mecha-nism and involves all the crystalline phases with which a melt may have coexisted in the magma source rocks. In essence, PAE involves partial melts escaping the source rocks in batches that carry varying amounts of tiny crys-tals of the peritectic minerals that were produced in the melting reactions. These crystals are always entrained in the proportions dictated by the stoichiometry of the partial melting reaction that produced them, and they are mostly completely resorbed during magma ascent and emplace-ment (Clemens et al. 1997; Villaros et al. 2009). Their chemical components are then recycled into the mafic minerals (e.g. pyroxene, hornblende and biotite) and plagi-oclase that crystallise from the magmas. The inferred crus-tal source rocks for I-type granitic magmas, such as those that formed the Tynong low-Al series, are arc andesites and dacites, with some immature greywacke component, metamorphosed and partially melted at granulite-facies

conditions (Clemens et al. 2011a, b). Under these meta-morphic conditions the peritectic minerals likely to be entrained into the rising magmas would be orthopyrox-ene, clinopyroxene, plagioclase and Fe–Ti oxide (mainly ilmenite). Accompanying these would be tiny crystals of refractory accessory minerals, such as zircon, liberated when their host biotite and amphibole crystals broke down in the melting reactions. As shown by Clemens and Ste-vens (2012), granitic series with maficities that correspond to SiO2 contents in the range shown by the Tynong pluton (68–78 wt%) can be explained by 0–30 % PAE.

The ‘hybrid’ tonalitic and mafic granodioritic pods within the low-Al granodiorites have isotopic and other chemical characteristics that suggest derivation from a sep-arate source (or combination of sources) to either the quartz dioritic sheets or the low-Al granodiorite magmas. Their position on geochemical plots suggests that these ‘hybrid’ magmas were not derived through mixing between any of the other analysed Tynong rocks. They have relatively unevolved Sr and Nd isotope ratios and may indeed repre-sent magmas formed by hybridisation processes involving a crustal and a mantle end member. The presence of quartz ocelli, apparently derived from the surrounding low-Al granodiorites, suggests that these distinctive magma com-positions must have been produced largely through inter-actions between small volumes of a relatively unevolved mantle-derived magma and the enclosing low-Al granodi-orite, at or near emplacement level. These ‘hybrid’ rocks contain hornblende and biotite, but could have contained early-magmatic pyroxenes that subsequently reacted out. In any case, their extremely high Cr and Ni contents sug-gest that accumulation of hornblende (or pyroxene) also played a role. It is virtually impossible to say whether these pods physically intruded earlier-emplaced low-Al granodi-orites or were simply emplaced together with these other granodiorites.

Production of the high- and low-Na granodiorite mag-mas may have involved an element of plagioclase feld-spar concentration and impoverishment, respectively, in small-scale fractional crystallisation. However, the com-mon depletion in TiO2 and FM and the isotopic contrasts between the low-Al granodiorites and the high-Na grano-diorite suggest that both these groups of rocks crystallised from small batches of contrasting magma that coexisted with the dominant magma series. We suggest that these minor magmas were most likely the products of partial melting of small-scale heterogeneities in the crustal rocks that formed the source for the Tynong magmas.

The high-Ni granodiorite sample most likely also repre-sents a distinct, minor magma batch. In this case, however, the elevated Cr and Ni contents and high Mg# suggest the possibility that this magma could have been the product of some form of hybridisation between a mantle-derived

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magma and crustal, I-type, partial melts. The most distinc-tive feature (the Cr and Ni contents of 205 and 108 ppm, respectively) could equally well have been produced through accumulation of magmatic pyroxene and mag-netite. Accumulation of about 13 wt% of pyroxene with 1500 ppm Cr together with some magnetite and plagioclase would account for the enrichment in Cr and Ni as well as the high Mg# and low SiO2 of this rock. Thus, although a hybrid origin cannot be discounted, the extant data do not appear to require such a model.

The overall chemistry, fine grain sizes and quench tex-tures of the rocks of the quartz dioritic sheets, and enclaves derived from them, suggest their origin as largely liquid magmas produced from an enriched mantle source. There may well have been some hybridisation, at depth, between these melts and partial melts of the crust, but it would be very difficult to be certain of this since the isotopic charac-teristics of enriched mantle reservoirs are rather similar to those of melts from typical I-type magma sources that have themselves not long been separated from the mantle. What is certain is that there was minor, local, emplacement-level hybridisation between lobes of the quartz dioritic sheets and the intervening granodiorite magma (e.g. Electronic Appendix 1, Fig. EA1.4). Also, there is microscopic evi-dence of reaction between the ME and the enclosing low-Al granodiorite (e.g. Electronic Appendix 2, Figs. EA2.8, 2.11). We stress, however, that these are localised effects and that there is no indication that such interactions were responsible for the main chemical variations in the low-Al series of Tynong magmas.

The Tynong pluton contains abundant evidence for the coexistence and local interaction between magmas derived from the enriched mantle and I-type felsic magmas formed by partial melting of crustal rocks that probably involved simultaneous breakdown of biotite and amphibole (Clem-ens et al. 2011a, b). The volumes of hybrid magma were apparently quite small, being limited to pods of hybrid granodiorite and tonalite, hybridised magma sandwiched between adjacent pillows of quartz dioritic sheets and hybrid zones in enclaves derived from those sheets. Thus, although this pluton shows stronger mantle connections than many other I-type granitic bodies in the region, the bulk of it seems to have been constructed from pulses of magmas derived through partial melting of heterogene-ous deep crust. The main mantle connection here, as else-where in the region’s I-type rocks, is that derived from source rocks that were themselves formed from the mantle. In the case of the Tynong pluton, we may infer that these source rocks, which must be located within the Neoprote-rozoic to Cambrian Selwyn Block, probably included meta-andesites, metadacites and minor metavolcaniclastic rocks that were formed in a continental arc setting that is in con-trast with the continental, postorogenic extensional setting

of the granitic magmatism (see also, e.g., Clemens et al. 2011a, b; Clemens and Bezuidenhout 2014).

Acknowledgments JC acknowledges the South African National Research Foundation for providing support for travel and field and analytical work through its programme of Incentive Funds for Rated Researchers. We thank Tom Sisson and Calvin Barnes who both pro-vided very useful reviews, which assisted us in strengthening our arguments on the efficacy of various mooted petrogenetic processes and in clarifying and extending some of our explanations.

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