Geochemical investigations of microgranitoid enclaves in the S-type Cowra Granodiorite, Lachlan Fold Belt, SE Australia

Lithos 56 Ž2001. 165–186 www.elsevier.nlrlocaterlithos

Geochemical investigations of microgranitoid enclaves in the S-type Cowra Granodiorite, Lachlan Fold Belt, SE Australia q Tod E. Waight a,b,) , Roland Maas b, Ian A. Nicholls c b c

a Danish Lithosphere Centre, Øster Voldgade 10, L. 1350 Copenhagen K, Denmark Victorian Institute of Earth and Planetary Sciences, School of Earth Sciences, La Trobe UniÕersity, Bundoora, VIC 3083, Australia Victorian Institute of Earth and Planetary Sciences, Department of Earth Sciences, Monash UniÕersity, Clayton, VIC 3186, Australia

Received 10 December 1999; accepted 7 July 2000

Abstract The Cowra Granodiorite is a relatively mafic, enclave-rich, S-type pluton in the Lachlan Fold Belt, which has been cited as a type example of a restitic origin for all varieties of enclaves in Lachlan Fold Belt S-type granites. Microgranitoid enclaves from the pluton are subordinate to metasedimentary varieties and can be subdivided into two groups according to their mafic mineral assemblage: pyroxene microtonalites and biotite microgranites. No geochemical or isotopic distinction can be made between the two varieties. Petrographic evidence Žacicular apatites, xenocrysts from the host granite. suggests an origin as mingled, more mafic, magmas, which have been variably contaminated by the more felsic host magma. This is supported by the fact that the microgranitoid enclaves have isotopic compositions Ž87Srr86 SrŽi. s 0.7095 to 0.7144, ´ NdŽi. s y9.2 to y6.9. that are generally more primitive than, or similar to, those of the host granite Ž87Srr86 SrŽi. s 0.7142, ´ NdŽi. s y8.8.. The spread in isotopic compositions, like their trace element compositions, is considered to be the consequence of variable degrees of diffusive exchange between the felsic and more mafic magmas during slow cooling. Several studied metasedimentary enclaves are not in isotopic equilibrium with their host granite and therefore cannot represent pristine samples of the bulk source region of the granite. Instead, they represent portions of a lithologically and compositionally diverse source terrane or accidental xenoliths entrapped during emplacement. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Enclaves; Granite, Magma-mingling, Geochemistry; Isotopes; Lachlan Fold Belt

1. Introduction

q

Electronic supplements available on the journal’s homepage: http:rrwww.elsevier.comrlocaterlithos ) Corresponding author. Danish Lithosphere Centre, Øster Voldgade 10, 1350 Copenhagen K, Denmark. Fax: q45-33-11-0878. E-mail address: [email protected] ŽT.E. Waight..

Solid material within granitoid magmas may consist of primary crystals formed from the melt phase, unmelted crystals from the source Žrestite. and other larger polyminerallic inclusions, collectively known as enclaves ŽDidier, 1973; Didier and Barbarin, 1991.. Enclaves within granitoids may be incorpo-

0024-4937r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 Ž 0 0 . 0 0 0 5 3 - 0

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T.E. Waight et al.r Lithos 56 (2001) 165–186

rated blocks of country rock and material from deeper levels of the crust Žxenoliths., residues from partial melting events and unmelted portions of the source region Žrestites., early formed portions of the granite bodyrmagma chamber Žautoliths. and remnants of mixed and mingled magmas. Detailed studies of enclaves of all types can, therefore, provide important information on source rocks, unexposed midcrustal rocks, and petrogenetic processes not readily available from the host granitoids Že.g. references in Didier and Barbarin, 1991; Anderson et al., 1998.. Of all the enclave varieties present in granitic rocks, the microgranitoid enclaves Ža.k.a. microgranular enclaves or mafic inclusions. have been the most controversial and the subject of several detailed studies Že.g. Didier and Barbarin, 1991.. Microgranitoid enclaves are centimetre- to metre-scale ellipsoidal, fine-grained, more mafic inclusions within granites, which commonly have characteristics suggesting a plastic rheology at the time of incorporation into the granitic magma. Microgranitoid enclaves contain similar mineral assemblages to their host granite, but higher proportions of ferromagnesian minerals and plagioclase and lower alkali feldspar and quartz ŽEberz and Nicholls, 1988.. Moreover, they are characterised by distinctive microstructures commonly interpreted as being igneous ŽVernon, 1990.. Microgranitoid enclaves have been interpreted as representing Ž1. AcognateB fragments of cumulates or early formed crystals from the host magma Že.g. Clemens and Wall, 1988., Ž2. globules of a more mafic magma that have been injected into and mingled with the host magma Že.g. Didier, 1973; Vernon, 1984, 1990. and Ž3. fragments of the source region that have either undergone partial melt extraction or were too refractory to melt or to allow melt segregation ŽPrice, 1983; Chen et al., 1989; White et al., 1991; Chappell and White, 1991; Chappell et al., 1987.. Distinguishing among these alternatives is sometimes difficult, particularly, in granites where microgranitoid enclaves occur as isolated ovoid bodies and where obvious features that can be related to magma mingling Žmixing zones, double enclaves, synmagmatic dikes. are absent. The debate has been particularly strong in the Lachlan Fold Belt of SE Australia, where the restite model for evolution of granitic rocks Žand the origin of microgranitoid enclaves. has been developed and debated Že.g. Chap-

pell et al., 1987; Wall et al., 1987; Clemens, 1989; Collins, 1991, 1998; Chappell, 1996.. For example, White et al. Ž1991. pointed to close similarities in composition and mineral assemblages between enclaves and host granites and a lack of distinct basalticrandesitic major and trace element signatures in microgranitoid enclaves. They argued that such chemical and mineralogical similarities are more consistent with an origin as residual material from the source region. Proponents of the magma-mingling model attribute the compositional similarities between enclaves and hosts to chemical exchange and equilibration between coexisting melts in a slowly cooling plutonic body Že.g. Eberz and Nicholls, 1990; Holden et al., 1991.. This appears to be supported by numerous isotopic studies Že.g. Holden et al., 1987; Allen, 1991; Metcalf et al., 1995.. A resolution of the origin of enclaves in any given pluton is important in clarifying whether late magma mixing and mingling were involved in the plutons origin. The S-type Cowra Granodiorite has been presented as a type example for a restitic origin of all enclaves, including microgranitoid varieties ŽWhite et al., 1991; Wyborn et al., 1991; Chappell et al., 1993., and this interpretation has also been applied to other microgranitoid enclaves in S-type granites of the Lachlan Fold Belt Že.g. Chen et al., 1989; White et al., 1991.. In contrast, Vernon Ž1983. used microstructural observations to argue that the microgranitoid enclaves in the Cowra Granodiorite represent frozen globules of a more mafic magma that had undergone mixing with its more felsic host. Other studies of S-type granites in SE Australia have reached similar conclusions Že.g. Elburg and Nicholls, 1995; Elburg, 1996a; Maas et al., 1997; Waight et al., 2000.. In this study, we present new geochemical and RbrSr and SmrNd isotopic results for enclaves in the Cowra Granodiorite, and submit further evidence for a mixed mafic magma blob origin for the microgranitoid enclaves, and against an origin as partially melted residual source material.

2. Regional geology The Lachlan Fold Belt is part of a large orogenic belt extending from beneath the Permian–Triassic

T.E. Waight et al.r Lithos 56 (2001) 165–186

Sydney–Bowen Basin to Tasmania ŽFig. 1., with equivalent rocks recognised in New Zealand Že.g. Cooper and Tulloch, 1992; Muir et al., 1996; Waight et al., 1997. and Antarctica ŽBorg et al., 1987; Weaver et al., 1991.. The belt is dominated by Ordovician–Silurian turbiditic sediments that have been affected by four semicontinuous orogenic pulses between 440 and 340 Ma ŽGray et al., 1997.. Intense felsic magmatism in the Silurian and Devonian Ž; 430–390 Ma. resulted in between 20% and 36% current exposure of granitic rocks in the eastern portion of the belt, with less preservation of felsic volcanics ŽWhite and Chappell, 1983; Chappell et

167

al., 1993.. The Cowra Granodiorite is an elongate body located ; 180 km NNE of Canberra and is well-exposed in the township of Cowra ŽFig. 1.. The pluton intrudes Ordovician slates to the east and the Silurian Canowindra Volcanics, believed to be comagmatic with the pluton, to the north and west. The Canowindra Volcanics have been correlated with the late Wenlockian Ž; 425–420 Ma. Hawkins Suite Volcanics ŽChappell et al., 1993.. Based on mineralogical observations Žparticularly, similar garnet compositions., Chappell et al. Ž1993. further argued that the Hawkins Volcanics and the Cowra Granodiorite are cogenetic. However, initial 87 Srr86 Sr iso-

Fig. 1. ŽA. Location of Cowra Granodiorite Žsolid infill. with respect to main granitoid occurrences of the Lachlan Fold Belt, SE Australia Žmodified from White and Chappell, 1983.; ŽB. Simplified geology of the Canberra to Cowra region illustrating location of Cowra Granodiorite and nearby Silurian felsic volcanics modified from Chappell et al. Ž1993..

168

T.E. Waight et al.r Lithos 56 (2001) 165–186

tope ratios of 0.7126–0.7136 for several samples of Hawkins Volcanics collected in the Cowra region ŽWaight and Maas, unpublished data. are lower than in the Cowra Granodiorite Ž0.7150., bringing a simple cogenetic link between the Cowra and Hawkins Suite into doubt. The Laidlaw Suite Volcanics overlay the Hawkins Volcanics and have been dated at 420.7 " 2.2 Ma by averaging several KAr biotite and Rb–Sr mineral–whole rock ages ŽWyborn et al., 1982., providing a minimum age limit for the Cowra Granodiorite. Preliminary SHRIMP U–Pb data for melt-precipitated rims on several inherited zircon cores in enclaves from the Cowra Granodiorite yield an age of approximately 410 Ma ŽI.S. Williams, personal communication, November 1999.. Zircon ages for other, similar S-type granitoids in SE Australia vary from 425 to 430 Ma ŽWilliams et al., 1983; Chappell et al., 1991; Williams, 1992; Maas et al., 1998, submitted for publication. and, therefore, appear somewhat older than Cowra. Based on the available data, an age of 415 Ma is inferred for the granodiorite and is used to calculate initial Sr–Nd isotope ratios; a variation of "10 Ma has no affect on our conclusions.

3. Cowra Granodiorite and its enclaves The Cowra Granodiorite is a medium to coarsegrained pluton comprising quartz, normally zoned subhedral plagioclase ŽAn 55 – 30 . commonly with albite-rich rims, alkali feldspar Žcommonly sericitised., red-brown biotite with abundant zircon inclusions, cordierite, rare garnet Žcommonly rimmed by cordierite., stubby equant apatite and relatively abundant secondary chlorite and muscovite. The pluton is characterised by abundant enclaves of contrasting mineral assemblages and origins. Expanding on an earlier study by Stevens Ž1952., Chappell et al. Ž1993. identified five different varieties of enclaves in the Cowra Granodiorite: Ž1. quartz-rich hornfels Žpsammites. with plagioclase, biotite" K-feldspar, considered to be derived from shallow Silurian sediments; Ž2. mica-rich schistose and surmicaceous enclaves, dominated by cordierite, biotite and garnet; Ž3. common quartz lumps similar to those found in many S-type granites in the Lachlan Fold Belt and which most probably represent quartz vein material

from country-rock sediments or the source region; Ž4. pale green calc-silicates comprising epidote, plagioclase, quartz " actinolite, calcite and K-feldspar and Ž5. microgranitoid enclaves. Microgranitoid enclaves are present in approximately equal proportions to metasedimentary enclaves ŽR. Vernon, personal communication. and are the focus of this study. The microgranitoid enclaves range in size from a few centimetres to 50 cm in diameter and are finer grained and more mafic than their host granite. They are generally massive, although many are zoned with a biotite-rich rim. Based on petrographic studies, we distinguish two varieties of microgranitoid enclave in the Cowra Granodiorite, termed herein pyroxene microtonalites and biotite microgranites. The pyroxene microtonalites are typical of microgranitoid enclaves found in many S-type granites worldwide. Photomicrographs of selected, representative enclaves examined in this study can be seen on the Lithos website at http:rrwww.elsevier.comrlocaterlithos under AElectronic SupplementsB. The pyroxene microtonalites comprise subhedral to euhedral crystals of plagioclase, biotite and orthopyroxene projecting into a pseudo-doleritic or poikilitic matrix of quartz Žfor details on texture, see Vernon, 1990.. Plagioclase is characteristically lath-shaped, with a consistent ; 3:1 length to breadth ratio, and displays normal, oscillatory zoning and Carlsbad twinning. Anorthite contents generally range from 65% to 35%, but some areas are considerably more calcic Žup to An 94 ; Wyborn et al., 1991.. Orthopyroxene is generally subhedral, prismatic and compositionally zoned; Wyborn et al. Ž1991. reported orthopyroxene compositions ranging from Mg 52 – 87 and a limited number of biotite compositions of Mg 53 – 66 , although it is not clear if these are analyses of biotite euhedra or biotite replacing orthopyroxene. The matrix is characterised by numerous, very fine-grained acicular crystals of apatite, accessory opaque grains and zircon. Rare tourmaline was also observed in the pyroxene–microtonalitic enclaves. Many of the microtonalites contain crystals significantly larger than average Žgenerally plagioclase and quartz, but also garnet, cordierite and alkali feldspar.. Most of these larger crystals are similar in appearance and composition to their equivalents in the host granite. They show disequilibrium textures Že.g. sieved textures. and are best interpreted as having been transferred

T.E. Waight et al.r Lithos 56 (2001) 165–186

from the host granite into the enclave, while the latter was partially molten Že.g. Vernon, 1986; Barbarin, 1990; Waight et al., 2000.. The biotite microgranites are similar in appearance to the pyroxene microtonalites, but are characterised by a lack of orthopyroxene, greater abundance of biotite, the presence of alkali feldspar and groundmass textures that may be pseudo-doleritic, but are more typically granitic. Xenocrystic plagioclase, quartz, etc. Žsee above., appear to be less abundant than in the pyroxene microtonalites. Many of the microgranitoid enclaves are a compound of these two types. They are zoned with an outer hydrated reaction rim of quartz, plagioclase and biotite, surrounding a typical orthopyroxenebearing pseudo-doleritic microtonalitic core, suggesting that the biotite microgranite enclaves may represent hydrated variants of the orthopyroxene microtonalites Žas also suggested by Wyborn et al., 1991.. Several metasedimentary enclaves were also examined. CI-18 is a calc-silicate xenolith composed of a fine-grained equigranular aggregate of epidote, actinolite, plagioclase and quartz with calcite, secondary chlorite and accessory apatite and zircon. This and other calc-silicate enclaves are unlikely to have formed at temperatures much above 5008C given their metamorphic mineral assemblages ŽI. Buick, personal communication. and most probably represent xenoliths of originally marly rocks from the exposed country rocks or greater depths, which did not equilibrate fully with their host granite magma. Quartz-rich hornfels ŽCI-3., biotite-rich metasediment ŽCI-19., and garnet-bearing metapelite ŽCI-11. presumably represent shallower and deeper level country rocks, respectively. The metapelite enclave we examined consists of a fine-grained mosaic of quartz, feldspar, mica containing large garnet crystals altering to sericite and surrounded by rims of coarse-grained quartz–K-feldspar and muscovite, which may represent the initial stages of dehydration melting. Three other enclaves examined were difficult to group with the other enclaves: they are characterised by extensive alteration and distinctive chemistries, and are referred to here as altered enclaves. Enclave CI-5 broadly resembles the biotite microgranites and comprises biotite Žpartly altered to chlorite., altered

169

plagioclase and quartz in a granitic texture. Minor garnet partly altered to chlorite is also present. Enclave TW9-2 has 5-mm chlorite-rich rims with the remainder dominated by fine-grained red-brown biotite Ž0.5 mm., and abundant sericitised and saussuritised plagioclase crystals in a granitic texture. Enclave TW11-4 is a feldspar-rich enclave with an obvious chlorite-rich rim. It shows a sharp contact with the host granite, but is highly altered, with sericitised feldspar, chloritised biotite partly altered to epidote and opaque minerals in a matrix of quartz. The Cowra Granodiorite has textures and mineralogy very similar to the Deddick Granodiorite, a pluton of comparable size in the southern Kosciusko Batholith, some 400 km south of Cowra ŽMaas et al., 1997, 1998, 1999; Nicholls et al., 1999.. Both plutons are enclave-rich, are dominated by enclaves of metasedimentary origin, and carry a small volume of microgranitoid enclaves. Although Wyborn et al. Ž1991. and Chappell et al. Ž1993. argued that all enclaves, including the microgranitoid types, in the Cowra Granodiorite are restitic in origin, Maas et al. Ž1997. argued, on chemical and isotopic grounds, that microgranitoid enclaves in the Deddick Granodiorite were derived from globules of hybrid magma mingled with, and contaminated by, the host granitoid; similar conclusions are reached in our examination of the Cowra Granodiorite.

4. Methods Samples used in this study include recrushing of rock specimens originally collected and described by White et al. Ž1991., Wyborn et al. Ž1991. and Chappell et al. Ž1993. ŽCI and WB series., with additional samples collected by the authors ŽTW series.. After removal of adhering host granite and altered surfaces, samples were crushed and milled for geochemical analysis. For several samples, insufficient material was available for a full chemical analysis and, thus, only major elements were analysed. Major and trace elements were determined by conventional XRF techniques at La Trobe University and by ICPMS at Monash University Žsee Price et al., 1997 for procedural details., respectively. Data for the CI- and WB-series samples Žmicrogranitoid enclaves and host granite. presented by Chappell et al. Ž1993. have

170

Sample Type

TW11-3 Bt mgrn

CI-1 Bt mgrn

) CI-7 Bt mgrn

TW11-7 Bt mgrn

C1-13 Bt mgrn

) CI-4 Px mton

C1-15 Px mton

TW9-1 Px mton

C1-14 Px mton

) CI-14 Px mton

) CI-17 Px mton

) CI-16 Px mton

) CI-9 Px mton

C1-17 Px mton

SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 FeO Fe 2 O 3Žtot. MnO MgO CaO Na 2 O K 2O P2 O5 SO3 LOI Total ASI Sc Co Ni Cu Zn Rb Sr Y Zr Nb

59.61 0.58 15.63 8.94 – 8.94 0.16 4.55 5.27 1.41 1.94 0.09 0.21 1.33 99.71 1.12 31 50 39 124 119 162 230 22 108 9

60.14 0.74 16.38 1.55 5.14 7.26 0.11 3.74 7.12 0.91 1.52 0.09 0.09 2.20 99.73 1.02 40 – 10 19 82 153 178 18 69 7

63.07 0.85 15.75 1.28 5.28 7.15 0.11 2.82 5.86 2.04 1.26 0.14 0.04 1.35 99.85 1.02 23 – 13 32 96 79 211 25 148 10

63.27 1.00 15.01 7.26 – 7.26 0.10 3.26 5.50 2.12 1.18 0.14 0.36 0.62 99.83 1.02 20 24 34 23 75 99 199 42 255 13

66.27 0.91 14.41 5.12 – 5.12 0.06 2.15 2.83 2.45 3.51 0.17 0.01 1.76 99.66 1.11 16 13 27 – 44 189 150 50 307 15

59.67 0.61 16.12 1.53 5.79 7.96 0.13 4.23 7.60 0.70 0.92 0.07 0.05 2.52 99.94 1.01 39 – 7 21 81 110 167 21 103 5

60.76 0.46 16.20 6.62 – 6.62 0.11 4.70 7.35 0.80 1.07 0.09 0.10 0.97 99.22 1.02 18 18 19 37 45 46 222 21 108 7

60.88 0.53 15.54 8.56 – 8.56 0.12 4.19 5.38 1.47 1.38 0.11 0.10 1.37 99.64 1.14 – – – – – – – – – –

61.10 0.66 15.81 7.16 – 7.16 0.11 3.98 6.80 1.11 1.22 0.12 0.12 1.21 99.40 1.02 20 23 24 32 65 105 193 26 182 8

61.13 0.65 16.00 1.08 5.36 7.04 0.12 3.95 6.93 1.16 1.21 0.11 0.08 1.70 99.48 1.01 33 – 20 22 81 92 179 26 143 7

61.58 0.99 15.65 1.07 5.41 7.08 0.11 3.76 5.71 1.96 1.50 0.13 0.06 1.78 99.71 1.03 32 – 31 25 83 120 145 31 191 10

61.83 0.93 15.24 1.06 6.50 8.28 0.13 2.91 6.31 1.51 1.05 0.14 0.07 1.92 99.60 1.01 32 – 6 24 96 57 180 33 161 9

61.90 0.50 14.99 1.03 5.20 6.81 0.12 4.69 6.42 0.96 1.25 0.09 0.08 2.65 99.88 1.03 34 – 28 31 76 91 163 22 118 7

61.96 1.01 15.38 7.43 – 7.43 0.11 3.97 5.80 1.89 1.31 0.13 0.12 0.65 99.75 1.02 16 24 39 21 75 87 158 31 208 11

T.E. Waight et al.r Lithos 56 (2001) 165–186

Table 1 Major and trace elemental data for biotite microgranite enclaves ŽBt mgrn., pyroxene microtonalite enclaves ŽPx mton., altered enclaves Žaltered., host granite Žgranite., and a metasedimentary enclave Žmetased. from the Cowra Granodiorite

2 3 17 321 47 6 1 20.2 41.3 4.9 18.2 3.9 1.5 3.8 0.6 3.6 0.8 2.2 0.4 2.6 0.4 3.3 0.8 1.21 3.27 1.13 6.05

– 5 – 245 11 6 2 12 29 – 12 – – – – – – – – – – – – – – – –

– 3 – 230 19 10 3 26 53 – 24 5.1 1.6 – 0.9 – 1 – – – 2.8 0.4 – – 0.94 – –

2 2 22 208 18 14 3 36.6 77.6 9.5 35.8 7.8 1.7 7.7 1.2 7.2 1.5 4.2 0.6 4.3 0.6 6.6 0.9 0.68 2.97 1.48 7.01

3 8 11 591 21 17 3 44 90.4 11.4 42.6 8.9 1.6 8.8 1.4 8 1.7 4.6 0.7 4.6 0.7 7.8 1.1 0.54 3.11 1.54 7.68

– 3 – 62 11 5 2 13 28 – 12 – – – – – – – – – – – – – – – –

3 2 14 180 7 5 2 15.8 34.9 4.2 15.8 3.6 1 3.6 0.6 3.5 0.7 2.1 0.3 2.2 0.3 2.9 0.6 0.84 2.73 1.40 6.12

– – – – – – – – – – – – – – – – – – – – – – – – – – –

2 1 21 217 13 10 3 24.6 51.4 6.3 23.6 5 1.2 4.9 0.8 4.5 1 2.7 0.4 2.8 0.4 4.7 0.7 0.72 3.12 1.44 7.23

– 4 – 185 13 10 3 23 48 – 22 4.9 1.18 – 1 – 1 – – 2.9 0.4 – – 0.68 – – –

– 6 – 345 17 13 2 28 61 – 25 – – – – – – – – – – – – – – – –

– 3 – 255 16 11 3 25 55 – 23 – – – – – – – – – – – – – – – –

– 3 – 270 12 10 2 19 41 – 16 – – – – – – – – – – – – – – – –

2 3 18 336 15 10 2 29.5 62.9 7.8 29.2 6.3 1.4 6.3 1 5.8 1.1 3.2 0.5 3.3 0.5 5.3 0.8 0.68 2.94 1.60 7.53

Average Cowra represents an average of 10 analyses of Cowra Granodiorite presented by Chappell et al. Ž1993.. Data for samples prefixed with ) taken from Chappell et al. Ž1993.. Samples CI-14, CI-16a and b and CI-17, use our data except for V, Cr, Ga and As ŽXRF data of Chappell et al., 1993.. ASIs Aluminum saturation index s molar ŽAl 2 O 3 .rmolar ŽCaOqNa 2 OqK 2 O..

T.E. Waight et al.r Lithos 56 (2001) 165–186

Mo Sn Cs Ba Pb Th U La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta EurEu ) ŽLarSm. N ŽGdrLu. N ŽLarLu. N

171

172

) C1-16A TW11-2 TW11-6 C1-16B TW10 WB143 TW11-1 C1-12 CI-10 TW9-2 TW11-4 ) CI-5 C1-5 TW11-H Average Cowra CI-19 Px mton Px mton Px mton Px mton Px mton Px mton Px mton Px mton Px mton Altered Altered Altered Altered Granite Granite Metased

62.18 0.96 15.13 8.54 – 8.54 0.13 2.96 6.18 1.56 0.98 0.15 0.14 0.36 99.27 1.02 9 22 11 22 86 65 213 25 222 10

63.25 1.03 14.96 7.26 – 7.26 0.11 3.22 5.57 2.05 1.07 0.14 0.11 0.54 99.30 1.02 9 41 25 19 72 95 163 32 251 11

63.81 1.03 14.50 7.40 – 7.40 0.13 3.00 3.58 1.86 1.95 0.17 0.17 2.24 99.84 1.24 – – – – – – – – – –

63.85 0.91 15.00 7.63 – 7.63 0.11 2.60 5.14 2.01 1.54 0.15 0.13 0.66 99.74 1.05 5 15 12 23 67 37 195 29 208 11

64.12 0.79 15.07 6.74 – 6.74 0.10 3.23 4.25 2.21 1.72 0.14 0.14 1.31 99.83 1.14 20 21 31 36 74 144 168 35 235 12

64.16 0.43 14.80 6.46 – 6.46 0.12 4.62 6.33 0.90 1.01 0.07 0.11 0.36 99.37 1.05 25 55 22 40 62 68 152 28 128 8

65.23 0.86 14.82 5.93 – 5.93 0.08 2.60 3.73 2.19 2.30 0.15 0.07 1.73 99.69 1.15 16 40 21 21 108 159 188 30 262 12

65.32 0.47 12.82 6.88 – 6.88 0.11 5.92 4.20 1.37 1.23 0.10 0.08 0.68 99.19 1.14 22 27 97 27 79 110 155 25 182 9

65.45 0.47 13.87 0.98 5.34 6.91 0.11 3.69 5.00 1.57 1.17 0.10 0.06 2.03 99.84 1.07 27 – 20 24 94 66 191 19 134 7

51.84 0.92 22.78 7.31 – 7.31 0.09 3.45 2.97 3.27 4.35 0.12 0.00 2.61 99.71 1.47 – – – – – – – – – –

53.03 0.85 20.61 7.73 – 7.73 0.12 3.30 3.61 3.61 3.04 0.12 0.03 3.00 99.06 1.30 – – – – – – – – – –

58.23 1.07 18.26 1.51 4.60 6.62 0.09 2.20 4.79 3.31 2.46 0.22 0.03 2.95 99.72 1.09 23 – 8 44 91 168 258 32 205 18

59.80 0.96 18.00 6.11 – 6.11 0.08 2.11 4.42 3.25 2.65 0.22 0.00 1.76 99.35 1.11 – – – – – – – – – –

67.26 0.67 14.37 4.96 – 4.96 0.08 2.10 2.37 1.97 3.47 0.16 0.03 2.00 99.43 1.27 10 27 19 27 53 100 104 32 246 15

67.82 0.65 14.44 0.81 3.74 4.96 0.07 2.02 2.35 2.01 3.57 0.15 0.02 2.35 100.41 1.26 15 15 20 18 82 179 129 31 192 12

76.60 0.56 9.95 3.73 – 3.73 0.07 1.37 2.10 1.43 2.06 0.18 0.02 1.09 99.16 1.19 – – – – – – – – – –

T.E. Waight et al.r Lithos 56 (2001) 165–186

Table 1 Ž continued .

2 2 19 298 18 13 3 34.1 70.4 8.6 32.4 6.9 1.5 6.8 1.1 6.4 1.3 3.6 0.6 3.7 0.5 6.9 0.9 0.64 3.09 1.55 7.78

– – – – – – – – – – – – – – – – – – – – – – – – – – –

4 2 14 189 8 7 2 25.2 59.1 7 26.8 5.8 1.7 5.8 0.9 5.5 1.1 3.1 0.5 3.4 0.5 5.2 0.8 0.91 2.73 1.39 6.04

2 4 18 273 22 12 2 33.8 70.5 8.5 31.3 6.5 1.3 6.2 1 5.7 1.2 3.3 0.5 3.7 0.6 5.8 0.9 0.60 3.27 1.40 7.58

1 2 6 308 31 11 4 27.6 57 6.9 24.2 5 0.9 4.9 0.8 4.6 0.9 2.6 0.4 2.6 0.4 4.1 1 0.56 3.50 1.48 8.44

2 3 10 387 45 13 2 33.3 69.5 8.7 32.6 6.9 1.6 6.4 1 5.4 1 2.7 0.4 2.6 0.4 7.3 1.1 0.73 3.05 2.03 10.47

2 1 19 318 11 10 2 26.5 53.8 6.4 23.3 4.6 1.2 4.4 0.7 4 0.8 2.4 0.4 2.6 0.4 4.5 0.7 0.79 3.64 1.40 8.32

– 3 – 340 13 6 2 19 39 – 17 – – – – – – – – – – – – – – – –

– – – – – – – – – – – – – – – – – – – – – – – – – – –

– – – – – – – – – – – – – – – – – – – – – – – – – – –

– 8 – 415 153 13 4 36 71 – 31 – – – – – – – – – – – – – – – –

– – – – – – – – – – – – – – – – – – – – – – – – – – –

1 7 7 450 25 13 2 36.8 69.8 9.4 34.2 6.9 1.2 6.3 0.9 5.3 1 2.8 0.4 2.9 0.4 6.9 1.2 0.57 3.36 1.82 10.68

– 7 – 514 26 21 3 31.2 70.3 – 25.9 6.9 – – – – – – – 2.9 – 6.9 1.2 – – – –

– – – – – – – – – – – – – – – – – – – – – – – – – – –

T.E. Waight et al.r Lithos 56 (2001) 165–186

4 2 11 225 17 8 3 25.6 52.4 6.5 24.4 5.2 1.5 5.3 0.9 5.1 1.1 3 0.5 3.2 0.5 5.8 0.7 0.85 3.10 1.38 6.63

173

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T.E. Waight et al.r Lithos 56 (2001) 165–186

also been incorporated into the data. Several repeat analyses were made of samples previously analysed by Chappell et al. Ž1993. and the results show good agreement for both major and trace elements, despite our analyses being made on separate powders. Where duplicate analyses are available, we use the data from the Melbourne labs in preference, as these powders were also used for the ICPMS and isotopic analyses. RbrSr and SmrNd isotope analyses were carried out at La Trobe University using a Finnigan MAT 262 mass spectrometer, following methods described by Waight et al. Ž1998.. Sample dissolution for both trace element ŽICPMS. and isotope dilution were done in high-pressure teflon bombs to ensure complete dissolution of refractory minerals.

5. Results 5.1. Major elements Major and trace element data for enclaves and host granitoid from the Cowra Granodiorite are presented in Table 1. The Cowra Granodiorite is peraluminous Žaluminum saturation index ŽASI. ; 1.2–1.3. and has SiO 2 Ž; 68 wt.%., restricted MgrFe ratios Ž100 P MgOrMgO q Fe tot s 27.5–29.9. and low Na 2 OrK 2 O ratios Ž; 0.5–0.6. ŽFig. 2. typical of S-type granitoids of the Lachlan Fold Belt ŽChappell and White, 1992.. The Cowra Granodiorite and the supposed coeval Hawkins Suite Volcanics are grouped within the Bullenbalong Supersuite, which extends from Cowra to Bass Strait for almost the full exposed length of the Lachlan Fold Belt ŽWhite et al., 1991; Chappell et al., 1993.. The restricted composition of the granodiorite is striking ŽSiO 2 s 67.3– 68.3%., given the large number of enclaves Žfrom subcentimetre to several metres scale. in the intrusion. The compositions of the microgranitoid enclaves range from mildly to strongly peraluminous ŽASI s 1.01–1.24. and generally have ASI lower than the host granodiorite. Microgranitoid enclaves have lower SiO 2 Ž59.6–66.3%. and higher Na 2 OrK 2 O than the host granodiorite ŽFig. 2.. Al 2 O 3 Fe 2 O 3Žtotal. , MgO, CaO, and MnO decrease, and Na 2 O and P2 O5 increase with increasing SiO 2 ; K 2 O shows no obvi-

Fig. 2. Plots of ASI and Na 2 OrK 2 O vs. SiO 2 . Note that enclaves have lower ASI and higher Na 2 OrK 2 O than host granodiorite. Shaded field represents compositions of microtonalitic enclaves from the Deddick Granodiorite ŽMaas et al., 1997., tie lines join estimated source and melt compositions for the Bullenbalong Suite Žtaken from Chappell, 1984.. See text for discussion.

ous trend ŽFig. 3.. Almost complete compositional overlap exists for the pyroxene microtonalite and biotite microgranite enclaves, despite their distinct mineral assemblages. In common with many microgranitoid enclave suites, the Cowra enclaves define chemical trends that are broadly collinear with the host granite for some elements Že.g. Al, Fe, Mg, Ca., but they are not necessarily collinear with the trends established for the Bullenbalong Suite of S-type

T.E. Waight et al.r Lithos 56 (2001) 165–186 Fig. 3. Plots of selected major and trace elements against SiO 2 for enclaves and host Cowra Granodiorite. Tie lines join estimated source and melt compositions for the Bullenbalong Suite Žtaken from Chappell, 1984.. See text for discussion. Symbols as for Fig. 2.

175

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T.E. Waight et al.r Lithos 56 (2001) 165–186

granites ŽFigs. 2 and 3., to which the Cowra Granodiorite belongs ŽChappell, 1984.. Overall, major element compositions of the microgranitoid enclaves have andesitic–dacitic affinities Že.g. with respect to Na 2 O, K 2 O and SiO 2 contents.. The three ‘altered’ enclaves ŽTW11-4, TW9-2, CI-5. have the lowest SiO 2 contents and two of them have considerably higher ASI than the microgranitoid enclaves. Their alkalies are characteristically high and Fe, Mg and Ca contents are low. For most elements, the altered enclaves do not lie on the chemical trends defined by the microgranitoid enclaves. The low SiO 2 and CaO, high alkalies and relatively high LOI values of these enclaves are consistent with petrographic evidence for extreme alteration Žpossibly hydrothermal. involving Si removal and the addition of alkalis and water. A single metasedimentary enclave ŽCI-19. has high SiO 2 Ž76.6%., moderate ASI Ž1.20. and relatively high CaO Ž2%. for such a SiO 2-rich sediment Že.g. Wyborn and Chappell, 1983.. 5.2. Trace elements Although there is considerable scatter, the enclaves show broad patterns of decreasing Sc, Sr, V and Th, and increasing Rb, Ba, Zr, Ce, Y, Pb, Th and U contents with increasing SiO 2 . Again, no clear distinction exists between the biotite microgranite and pyroxene microtonalite enclaves. Enclave REE patterns broadly resemble those for the host granodiorite, but show variation in detail ŽFig. 4.. The host granodiorite has the highest LarLu and GdrLu, and pronounced Eu depletion. Two of the biotite microgranite enclaves have LREE-enriched patterns similar to the host granite, but with considerably higher abundances of HREE. A third biotite microgranite ŽTW11-3. has distinctly lower total REE contents and a positive Eu anomaly. The pyroxene microtonalite enclaves also show similar REE abundances and patterns to the host granite and have somewhat lower REE contents to the biotite microgranites. One sample ŽCL-15. has somewhat lower REE abundances than the other pyroxene microtonalites. Overall, REE contents correlate with SiO 2 , and EurEu ) decreases with increasing SiO 2 and REE contents. There is no discernable correlation between SiO 2

Fig. 4. REE patterns for pyroxene microtonalitic enclaves Žtop., biotite microgranitic enclaves Žbottom. and host Cowra Granodiorite Žfilled triangle in both plots.. Data normalised to chondrite values of Taylor and McLennan Ž1985..

and ratios such as ŽLarLu. N , ŽLarSm. N or Gdr Lu. N . 5.3. Rb–Sr and Sm–Nd isotopes Initial Sr and Nd isotope ratios for the single sample of Cowra Granodiorite analysed are relatively evolved Ž87 Srr86 SrŽ415. s 0.7150; ´ NdŽ415. s

T.E. Waight et al.r Lithos 56 (2001) 165–186

177

Table 2 Rb–Sr and Sm–Nd isotope data for Cowra Granodiorite and enclaves Žabbreviations as in Table 1. Sample

Rb

Sr

87 86

Rbr Sr

87 86

Srr Srm

87 86

Srr Sr415

Nd

Sm

147 144

Smr Nd m

143 144

Ndr Nd m

´ Nd 415

TDM

Cowra granodiorite TW11-H granite

177.3

146.8

3.505

0.73567

0.71495

35.7

7.0

0.1182

0.511964

y8.99

1.68

Microgranular enclaÕes TW11-7 Bt mgrn TW11-3 Bt mgrn C1-15 Px mton WB143 Px mton TW11-1 Px mton C1-17 Px mton C1-16A Px mton C1-14 Px mton TW11-2 Px mton TW9-1 Px mton C1-12 Px mton TW10 Px mton C1-16B Px mton TW11-6 Px mton

113.2 156.7 94.3 59.1 201.3 105.3 57.1 97.0 88.9 120.4 95.8 141.6 101.4 158.0

176.1 204.3 205.7 131.6 195.0 154.3 201.3 188.3 158.2 177.1 141.8 154.3 219.2 151.1

1.863 2.222 1.328 1.300 2.992 1.978 0.8209 1.492 1.627 1.971 1.958 2.660 1.340 3.026

0.72415 0.72671 0.71768 0.71862 0.72937 0.72324 0.71651 0.72056 0.72249 0.72460 0.72462 0.72890 0.72149 0.73249

0.71314 0.71357 0.70983 0.71093 0.71168 0.71155 0.71166 0.71175 0.71288 0.71295 0.71304 0.71318 0.71357 0.71462

31.0 14.6 14.3 19.4 34.8 28.7 25.7 21.3 30.9 13.7 20.8 29.3 28.5 41.5

6.6 3.0 3.2 4.0 7.1 6.0 5.5 4.6 6.6 3.2 4.2 5.9 5.9 6.2

0.1285 0.1249 0.1351 0.1258 0.1230 0.1267 0.1294 0.1302 0.1295 0.1399 0.1211 0.1223 0.1262 0.0911

0.512052 0.512046 0.512059 0.512027 0.512006 0.512041 0.512090 0.512095 0.512008 0.512006 0.511986 0.512012 0.512051 0.511902

y7.82 y7.74 y8.03 y8.16 y8.42 y7.94 y7.13 y7.07 y8.73 y9.32 y8.72 y8.27 y7.72 y8.76

1.73 1.67 1.85 1.72 1.70 1.71 1.68 1.69 1.82 2.05 1.70 1.68 1.69 1.41

Altered enclaÕes C1-5 altered TW9-2 altered TW11-4 altered

185.0 281.5 222.5

261.2 178.0 249.1

2.053 4.589 2.589

0.72697 0.74179 0.73040

0.71484 0.71467 0.71510

36.2 59.5 29.7

7.1 10.3 8.7

0.1189 0.1045 0.1769

0.511941 0.511920 0.512009

y9.47 y9.12 y11.22

1.73 1.55 3.70

Metasedimentary enclaÕes CI-11 metased 333.2 CI-3 metased 67.2 CI-19 metased 99.0 CI-18 metased 11.5

182.8 92.5 116.5 413.4

5.292 2.104 2.462 0.0807

0.74409 0.72695 0.72953 0.71757

0.71282 0.71451 0.71498 0.71710

67.9 23.3 36.8 32.9

11.7 4.7 7.1 6.8

0.1044 0.1216 0.1164 0.1257

0.511898 0.511886 0.511906 0.511907

y9.55 y10.70 y10.03 y10.50

1.57 1.86 1.74 1.90

Initial isotopic ratios are calculated at 415 Ma with propagated errors Ž2SD. of "0.00010 for

y9.0, TDM s 1.68 Ga; Table 2. and similar to those for other Silurian S-type granitoids in the eastern part of the Lachlan Fold Belt Že.g. McCulloch and Chappell, 1982.. The microgranitoid enclaves are also relatively evolved with 87 Srr86 SrŽ415. ranging from 0.7098 to 0.7146 and ´ NdŽ415. ranging from y9.3 to y7.1 ŽFig. 5.. No isotopic distinction can be made between the biotite microgranitic and pyroxene microtonalitic enclaves, although the latter extend to the most primitive compositions in the Cowra Granodiorite. The altered enclaves have a restricted range of radiogenic Sr isotopic signatures Ž0.7147–0.7151. overlapping with those of the host granite and the metasediments, but trend towards more negative ´ NdŽ415. Žy9.1 to y11.2.. Four metasedimentary enclaves consistently show lower

87

Srr86 SrŽi. and "0.5 ´ units for ´ NdŽi. .

´ NdŽ415. Žy9.6 to y10.7. than the host granite and other enclaves. 87 Srr86 SrŽ415. ratios in these enclaves overlap with the microgranitoid enclaves and the host granite, but extend to more radiogenic compositions Ž0.7128–0.7171; Fig. 5..

6. Discussion 6.1. Origin of microgranitoid enclaÕes 6.1.1. Origin from mingled hybrid magma blobs The mode of occurrence and composition of microgranitoid enclaves in the Cowra Granodiorite is

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Fig. 5. Initial Nd–Sr isotopic compositions for the Cowra Granodiorite and its enclaves at 415 Ma. Fields for the Deddick granite and its enclaves shown for comparison Žfrom Maas et al., 1997.. Field from Lachlan Fold Belt sediments from O’Halloran and Maas Žunpublished data..

similar to those of enclave populations in many S-type granites in SE Australia and elsewhere. They comprise < 1% of the outcrop and lack an obvious association with mafic dikes, stocks or recognisable magma-mingling zones; their chemical Žespecially trace element. and mineralogical compositions are similar to those of the host granite; and they occur together with similar to higher abundances Žup to several percent of outcrop. of metasedimentary enclaves of deep-seated origin Že.g. Maas et al., 1997.. Although these characteristics have been used by some to infer that all enclaves in S-type granites are metasedimentary in origin Že.g. White et al., 1991., microstructural observations of the microgranitoid enclaves from Cowra and other SE Australian granites ŽVernon, 1984, 1990. appear to be more consistent with the comingled hybrid magma blob model developed in Europe and North America Že.g. Vernon, 1983 and references therein; Bacon, 1986;

Holden et al., 1987; Barbarin, 1988, 1990; Didier and Barbarin, 1991; Metcalf et al., 1995.. Following this model, the Cowra enclaves are interpreted as being derived from variably hybridised, more mafic magmas Žoriginally of broadly basaltic or andesitic composition. intruded into, and mingled with, the more felsic host magma. Other possibilities include an origin as comagmatic early differentiates eroded from magma chamber walls, or as accidental xenoliths of older igneous rocks. The following observations support the comingled hybrid magma blob model: Ži. local double enclaves Žtypical microgranitoid enclaves discordantly enclosing obvious metasedimentary enclaves. indicating similar rheological characteristics for host granodiorite and enclaves, i.e. they could have coexisted as magmas; Žii. the chemical compositions of the enclaves differ from those of the host granodiorite, do not define trends simply related to the host granite, and are unlike those of

T.E. Waight et al.r Lithos 56 (2001) 165–186

other enclaves of obvious sedimentary origin; Žiii. the enclaves contain xenocrystic plagioclase, quartz, garnet, cordierite and alkali feldspar most probably derived from the host granite magma; and Živ. Nd–Sr initial isotopic ratios are less evolved than those for the host granodiorite and the majority of metasedimentary enclaves, a characteristic pattern that is also observed in enclave populations for which field evidence strongly supports magma mixing and mingling Že.g. Holden et al., 1987; Eberz et al., 1990; Elburg and Nicholls, 1995; Keay and Collins, 1995; Metcalf et al., 1995; Elburg, 1996a; Poli et al., 1996.. A comagmatic origin is unlikely, because of the isotopic and chemical differences between enclaves and host granodiorite. Furthermore, the presence of host-derived xenocrysts in the enclaves suggests both coexisted as magmas, implying that the enclaves cannot be igneous xenoliths Ži.e. solid at the time of incorporation.. 6.1.2. Origin from metasedimentary precursors A metasedimentary origin of all enclaves in S-type has long been favoured by the proponents of the restite model for granite petrogenesis Že.g. White et al., 1991.. The enclaves were thought to represent refractory ŽPrice, 1983; Chen et al., 1989. or restitic ŽWyborn et al., 1991. portions of the Žmetasedimentary. granite source. The latter model was developed with particular reference to the Cowra enclaves and it is, therefore, appropriate to review this particular model in the light of the new data presented here. Wyborn et al. Ž1991. and Chappell et al. Ž1993. dismissed a magmatic Žbasaltic or andesitic. origin for the Cowra microgranitoid enclaves because only one of their samples, enclave CI-5, had an andesitic composition. All other samples contained too little Na 2 O at the appropriate SiO 2 , and all were peraluminous. Noting that enclave compositions Žparticularly, ASI, P, Zr, Nb and EurEu ) . vary with the anorthite content of plagioclase Žapparently ignoring obvious plagioclase zoning evident in thin section., Wyborn et al. Ž1991. interpreted the enclaves as residues from crustal melting of feldspathic–pelitic metasediments of variable fertility. Though this model appears consistent with the data presented by these authors, replotting of their data on ASI vs. P2 O5 , Nb, Zr, EurEu ) diagrams shows the purported correlations to be strongly dependent on the

179

data for just one sample, CI-5. This sample shows considerable alteration of all feldspar crystals, Žwhich was noted by Wyborn et al., 1991., and has, therefore, been included by us in the group of ‘altered enclaves’, characterised by particularly strong alkali and SiO 2 mobility. In any case, if our data are plotted with those of Wyborn et al. Ž1991., including CI-5, the trends degenerate to clusters, despite the additional dispersion in ASI from 1.01 to 1.29. Therefore, this more extensive data set Žparticularly, for EurEu ) . does not support the model by Wyborn et al. Ž1991.. Furthermore, the high CaO contents and relatively high ´ NdŽ415. of the microgranitoid enclaves are not found in either the Paleozoic Lachlan Fold Belt turbidities or in psammitic–pelitic enclaves within Lachlan Fold Belt granites Že.g. Maas et al., 1997; Anderson, 1997.. The absence of metamorphic or residual sedimentary fabrics in all microgranitoid enclaves we studied, as opposed to obviously metasedimentary enclaves Že.g. Maas et al., 1997; Anderson et al., 1998., is difficult to relate to a metasedimentaryrresidual source model for microgranitoid enclaves. Any originally banded sedimentary material would convert to schist, gneiss or stromatic migmatite or diatexite during metamorphism. Residual mineral grains in the migmatite would be in equilibrium with melt and substantial grain coarsening would be expected Že.g. Sawyer, 1996.. No such textures are seen in the microgranitoid enclaves either at Cowra or elsewhere, and fine grainsize is a well-known characteristic of microgranitoid enclaves. White et al. Ž1999, p. 420. have recently argued that all microgranitoid enclaves represent ‘calc-silicate lenses that are common in turbidite sequences’, yet provide little evidence to substantiate this claim. Arguments against this suggestion include the lack of sedimentary fabrics, the lack of typical calc-silicate minerals, presence of igneous microstructures, the relatively high and variable ´ NdŽ415. in microgranitoid enclave suites Žrelative to Lachlan Fold Belt sedimentary rocks., the trend of microgranitoid enclave compositions towards the host granite Žwhich would not be expected in calc-silicate rocks., and the field evidence for magma mixing and mingling associated with many microgranitoid enclave suites. The mineral assemblages of calc-silicate metamorphic rocks Že.g. Buick et al., 1993; Owen et al., 1994;

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T.E. Waight et al.r Lithos 56 (2001) 165–186

Shaw and Arima, 1996; Neogi et al., 1998. are strikingly different from those of microgranitoid enclaves. For example, minerals, such as scapolite, metamorphic epidote, wollastonite and calcite, are common in calc-silicates, but are never observed in microgranitoid enclaves. Hydrous minerals are almost completely absent from calc-silicates, whereas the Cowra microtonalite enclaves have abundant biotite. Titanite and garnet are common to both calcsilicate and granitic rocks and, therefore, should remain stable at granitic magma pressures and temperatures; yet these minerals are rare or xenocrystic in microgranitoid enclaves. Calc-silicate rocks have considerably lower Al 2 O 3 , MgO and K 2 O, and much higher CaO than typical microgranitoid enclaves, such as found in the Cowra Granodiorite. RbrSr ratios and 87 Srr86 Sr in calc-silicates are expected to be lower than in carbonate-poor sedimentary rocks, but there is no reason to expect differences in SmrNd ratios and ´ Nd ŽOwen et al., 1994; Maas et al., 1997.. A calc-silicate origin for the Cowra microgranitoid enclaves appears inconsistent with measured ´ Nd values that are systematically higher than those of the psammo–pelitic enclaves in the granodiorite and of the Lachlan Fold Belt Paleozoic sediments in general. This is borne out by enclave CI-18, a calc-silicate nodule, which is mineralogically, chemically and isotopically distinct from the microgranitoid enclaves. It has the low RbrSr ratio expected of Ca-rich Žmarly. sediments; yet ´ NdŽ415. and even 87 Srr86 SrŽ415. are within the range for psammo–pelitic Lachlan Fold Belt sediments and metasedimentary enclaves ŽFig. 5.. Therefore, we conclude that models that propose a sedimentary precursor for microgranitoid enclaves are not supported by the available data. 6.2. Origins and implications of metasedimentary enclaÕes. Metasedimentary enclaves are the most common enclave type in the Cowra Granodiorite, but were not examined in great detail in this study. The four metasedimentary enclaves analysed all have lower ´ NdŽ415. than the host granodiorite and are, therefore, not representative of the bulk metasedimentary source of the granitic magma. The magma source either had other metasedimentary components with higher

´ NdŽ415. Žwhich are not preserved as enclaves or were missed in our sampling. or a Ahigh-´ Nd B component was introduced via magma mixing. The metasedimentary enclaves have isotopic compositions that plot within or near the field of Paleozoic sediments from the Lachlan Fold Belt ŽO’Halloran and Maas, unpublished data. and they most probably represent portions of the sedimentary pile above the granitic source region. 6.3. Origin of the altered xenoliths The three altered xenoliths are difficult to classify as coming from either a metasedimentary or magmatic source. Sample TW9-2 is mica-rich and granitic in texture and is isotopically similar to the host granodiorite. It could be a normal microgranitoid enclave composition that was substantially modified by late magmatic–granitic fluids. A comagmatic Žautolith-type, Wall et al., 1987. origin is also possible on isotopic grounds, but is ruled out by the enclave’s bulk composition. CI-5 and TW11-4 also have granitic textures, but their ´ NdŽ415. values are lower than in the host granodiorite. They could represent xenoliths of older granite from deeper within the transport zone of the host magma, perhaps modified by reaction with fluids derived from the sedimentary country rocks. 6.4. Implications of a mixed magma blob origin for the eÕolution of microgranitoid enclaÕes during residence in a host granite magma Most petrologists now accept that mafic microgranitoid enclaves form through injection of mafic melt into a magma chamber containing cooler, partially crystalline felsic magma Že.g. Huppert et al., 1984; Campbell and Turner, 1985, 1986; Sparks and Marshall, 1986; Koyaguchi and Blake, 1991; Wiebe, 1993, 1994; Snyder and Tait, 1995; Poli et al., 1996., and this is the interpretation we favour for the microgranitoid enclaves in the Cowra Granodiorite. In this model, the enclaves represent small volumes of relatively mafic melt which, once injected into the felsic magma, cool rapidly, partially crystallize and become more viscous to form discrete magma blobs within the chamber. Because thermal diffusion is

T.E. Waight et al.r Lithos 56 (2001) 165–186

10 3 –10 5 times faster than chemical diffusion, initially hotter, more mafic magma batches reach thermal equilibrium with felsic host magma long before chemical equilibrium is attained Že.g. Sparks and Marshall, 1986; Frost and Mahood, 1987; Blake and Koyaguchi, 1991; Fernandez and Barbarin, 1991.. The mafic blob then cools at the same Žgenerally slow. rate as the host magma, allowing for considerable chemical exchange between the contrasting magmas. Chemical diffusion between coexisting mafic and felsic magmas may significantly influence the compositions of microgranitoid enclaves. Field and experimental data consistently show diffusion of alkalies and water to be much faster than that of other components Že.g. Watson, 1982; Watson and Jurewicz, 1984; Johnston and Wyllie, 1988; Baker, 1989, 1990., with similar diffusion coefficients on the order of 10y7 –10y8 cm2 sy1 ŽHofmann, 1980; Karsten et al., 1982; van der Laan et al., 1994.. Rapid diffusional transfer of K and water results in hydration of enclaves, and promotes growth of biotite at the expense of pyroxene ŽDebon, 1991; Wiebe, 1993; Elburg and Nicholls, 1995.. The pyroxene to biotite reaction is responsible for the change from pyroxene microtonalite to biotite microgranite enclaves at Cowra. This is supported by their mineral assemblage, by the presence of zoned enclaves Žpyroxene-rich cores, biotite-rich rims, Stevens, 1952; Wyborn et al., 1991., and by the overall similarity in terms of chemical and isotopic compositions between the two enclave types. Crystallization within the enclave magma also influences enclave composition, by providing a sink for compatible elements ŽJohnston and Wyllie, 1988.. For example, K diffusion from the felsic into the mafic melt promotes growth of biotite, removing K from the mafic melt, maintaining the initial concentration gradient and thereby allowing exchange to continue in the short term. Besides biotite, Cowra enclave melts also precipitated plagioclase, biotite, orthopyroxene, cordierite, apatite and perhaps Fe–Ti after mingling, thus providing sinks for many compatible elements Že.g. P in apatite, Ca in plagioclase.. Growth of these minerals within individual enclaves that had different mutual compositions andror diffusion histories, or in their parental magma prior to injection into the granite magma, may explain much

181

of the scatter observed in major element Harker diagrams for the enclaves Že.g. TiO 2 .. REE patterns for the pyroxene microtonalitic enclaves are likely influenced by equilibration with host magma, but some characteristics expected in a more mafic magma are observed, such as lower total REE Žespecially LREE. and higher EurEu ) in the pyroxene microtonalites. The HREE values vary, and in some samples exceed levels in the host granodiorite, whereas biotite microgranite enclaves have higher total REE Žespecially HREE. than the host. Again, crystallization of zircon and apatite in the enclave melts may have contributed to this effect, by removing ŽH.-REE and maintaining further REE exchange. Similarly, some microgranitoid enclaves having higher Nd and Zr contents than their host granite. If this process was able to continue for long enough, bulk REE concentrations greater than in the host granite may be achievable. REE diffusion would be enhanced by exchange of water, and preferential diffusion of HREE is predicted by their smaller ionic radius and lower field strength relative to the LREE. Strontium and neodymium isotopic compositions undergo diffusional exchange roughly 10 times faster than their corresponding chemical species ŽBaker, 1989, 1990; Lesher, 1990, 1994.. Furthermore, Lesher Ž1994. demonstrated that contrasts in 87 Srr 86 Sr between two magmas are reduced faster than contrasts in 143 Ndr144 Nd, in part because mafic and felsic magmas tend to show greater contrasts in Sr than Nd concentrations ŽSnyder and Tait, 1998., and because diffusivities of elemental Sr and Sr isotopes are higher than those for Nd ŽLesher, 1994.. This decoupled behaviour has been noted in several enclave studies Že.g. Holden et al., 1987, 1991; Pin et al., 1990; Allen, 1991.. The effect may also operate in felsic volcanic rocks that show constant Nd but variable Sr isotope ratios Že.g. Halliday et al., 1984; Worner et al., 1985.; this has been explained as a ¨ result of variable diffusional dispersion rates for exotic Sr and Nd isotopic signatures introduced by injection of basaltic magma into a felsic magma chamber ŽSnyder and Tait, 1998.. Decoupling of Sr and Nd isotopic exchange during irreversible mixing results in sigmoidal mixing paths that initially diverge from bulk mixing curves due to faster equilibration of Sr isotopic compositions. The intersection between the sigmoidal geometry and a typical mix-

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ing curve gives an indication of the relative proportion of mafic and felsic end-members involved ŽBlichert-Toft et al., 1992; Lesher, 1994.. One major difficulty with the origin of microgranular enclaves is to establish where chemical modification occurred. The end-member possibility is that an unmodified parental basaltic magma is injected into a felsic magma chamber, then breaks up into enclaves, and all exchange and modifications occur in situ following injection. While there is little doubt that isotopic and chemical exchange occurs between an enclave and its host granite Žindicated by zoned enclaves and biotite-rich hydration rims., injection of unmodified basaltic magmas seem to be the exception rather than the norm for several reasons. Many enclave suites display similar levels Žgenerally high. of chemical and isotopic modification, and typical mantle-derived compositions are rarely, if ever, found preserved in enclaves. Injection and subsequent modification of a basaltic magma should instead result in a spectrum of compositions reflecting varying degrees of modification due to differing blob-sizes and resident times in the felsic magma Žrelative to its crystallization history.. Where the injection of mafic magmas into felsic magma chambers is recorded in the field, the former pond at the base of the chamber where they undergo internal differentiation coupled with chemical and physical exchange with the overlying resident silicic magma Že.g. Wiebe, 1993, 1999.. Additionally, in such examples, the injected mafic magma commonly shows petrographic and chemical evidence for having undergone fractionation, contamination and mixing at a remote site prior to injection. These earlier interactions could have occurred in larger bodies of mafic magma pooled at the base of the silicic chamber, or in mafic magma chambers at currently unexposed, deeper crustal levels. Many of these mafic magmas may be the parents of those magmas injected to form microgranitoid enclaves, suggesting that some proportion of the compositional modification of the enclave magma occurred outside the pluton prior to injection. 6.5. EÕidence for, and consequences of, diffusional exchange between coexisting mafic and felsic melts Strontium and neodymium isotope ratios in the Cowra enclaves are variable, ranging from composi-

tions close to those in the host granodiorite to more primitive compositions. Nevertheless, they do not extend to typical mantle values. This is typical of microgranitoid enclave suites around the world Že.g. Holden et al., 1987, 1991; Eberz et al., 1990; Pin et al., 1990; Allen, 1991; Pin, 1991; Elburg and Nicholls, 1995; Metcalf et al., 1995; Elburg 1996a,b; Poli et al., 1996; Maas et al., 1997; Waight et al., 2000. and is attributed to variable equilibration with the host felsic magma. Calculations by Lesher Ž1994. indicate that a 20-cm-diameter enclave takes about 10 5 years to undergo 95% homogenisation in Sr isotopes at 8508C, whereas homogenisation of Nd isotopes takes about seven times longer; bulk chemical equilibration is only achieved after about 6 Ma. Close to complete crystallization, most Sr and Nd is locked into crystallized minerals Že.g. apatite and plagioclase., in which diffusional rates are much slower Že.g. ; 10y1 9 cm2 sy1 , Giletti and Casserly, 1994.. We follow previous workers in attributing the isotopic variability and generally lower 87 Srr86 SrŽ415. and higher ´ NdŽ415. of the microgranitoid enclaves in the Cowra Granodiorite to isotopic equilibration from an initially more isotopically primitive mafic magma. Importantly, the microgranitoid enclaves do not show isotopic compositions more evolved than the host granite. Diffusion coefficients for K 2 O and 87 Srr86 Sr are Žwithin an order of magnitude. similar, 2 = 10y7 Žvan der Laan et al., 1994. and 9 = 10y6 to 8 = 10y7 cm2 sy1 ŽLesher, 1994., respectively, and if the isotopic compositions of the microgranitoid enclaves reflect diffusive equilibration, then a correlation should exist between these two parameters. A plot of K 2 O vs. 87 Srr86 SrŽ415. ŽFig. 6. shows a positive correlation, consistent with mutual transfer of K 2 O and Sr tracers. Those samples that are most equilibrated in Sr isotopes also have K 2 O contents like the host granodiorite, suggesting that equilibration of high diffusivity species has gone nearly to completion. However, the K 2 O– 87 Srr86 Sr relationship is steep at low K 2 O and flattens out at high K 2 O, indicating that Sr isotopic exchange was initially faster, consistent with measured diffusivities ŽLesher, 1994; van der Laan et al., 1994.. The enclaves show little overall variation in ´ Nd , and several have ´ Nd that is within uncertainty in measured ´ Nd of the host granodiorite. On the other hand, 87 Srr86 Sr shows

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Fig. 6. Plot of K 2 O against initial Sr isotopic composition, showing an apparent correlation between two elements with similar diffusivities. Symbols as for Fig. 5.

a much larger range away from the host granodiorite. This contrasts with the observations of others Že.g. Holden et al., 1987, 1991; Pin et al., 1990; Allen, 1991. and with the experimental observations of Lesher Ž1994.. It appears that isotopic equilibration at Cowra proceeded faster for Nd than for Sr. A possible explanation for this is lock-up of Sr in large plagioclase grains crystallizing within enclaves or their parental magma body, while most Nd partitioned into fine-grained acicular apatite and zircon Že.g. Elburg, 1996b.. Because diffusivities of Sr in plagioclase and Nd in apatite are likely roughly equal ŽWatson et al., 1985; Giletti and Casserly, 1994; Elburg, 1996b., equilibration would depend largely on crystal size and would be more rapid for Nd in small apatite grains than for Sr in large plagioclase grains.

7. Conclusions Petrographic, geochemical and isotopic data for microgranitoid enclaves in the Cowra Granodiorite are most compatible with an interpretation that views

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them as products of arrested hybridisation of more mafic, andesitic magma that was mingled with partially crystalline Cowra granite magma. Although purely speculative, a possibility also exists that some modification of the microgranitoid enclaves parental magma occurred in a large pond of mafic magma at the unexposed base of the magma chamber andror in lower level mafic magma chambers prior to injection into the Cowra magma chamber. Once injected, experimental constraints suggest that the enclaves behaved as more viscous bodies within the granitic magma. Chemical and isotopic exchange, both prior to and following injection, resulted in compositions far removed from those of their original parent magma. Major alkali and water diffusion led to the growth of biotite at the expense of pyroxene and has resulted in two distinct petrographic varieties of enclave, dependent on the amount of alkali diffusion. Enclaves that have undergone major alkali exchange are also those that are the most equilibrated in terms of isotopic compositions. The least equilibrated enclave ŽCI-15. Žlowest 87 Srr86 SrŽ415. , SiO 2 , and REE concentrations. is broadly andesitic in composition and most probably represents the closest composition to the original more mafic magma that formed the enclaves, subsequently obscured and overprinted by diffusional exchange.

Acknowledgements We would like to thank A.J.R. White who kindly supplied the CI-series samples reanalysed in this study, and D. Wyborn who supplied sample WB143 and information on finding fresh outcrops of the Canberra Volcanics although it was not always an easy task. Helpful reviews by R. Vernon, C. Allen and B. Barbarin are acknowledged with appreciation. J. Metz and I. McCabe assisted with XRF analyses. This study was funded by an ARC large grant to IAN and RM.

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