Geochemistry of a peraluminous granitoid suite from North-eastern Victoria, South-eastern Australia

Gmhmmm

0 bOmon

et Cosmochimica Am Vol. 47, pp. 3 I42

00167037/83/01003I-12S03.0010

RemLId.1983.Rintcdin U.S.A.

Geochemistry of a peraluminous granitoid suite from North-eastern Victoria, South-eastern Australia RKHARD c.

PRICE

Department of Geology,La TrobeUniversity, Bundoora,Victoria3083, Australia (Received January 8, 1982; accepted in revised formSeptember 22, 1982) Abstract-The KoctongSuiteof Silurian, 2-x&a gmnitoids wasderivedfroma mctasedimentary source and emplaced into Ordovician sediments and metasediments along the eastern margin of the Western Whole-rock geochemical considerations preclude derivation Metamorphic Belt of South-eastern Australia. of the magmas represented by the granitoids from exposed Ordovician metasediments. The magmas were generated by partial melting of material similar in composition to gamet-cordierite gneisses exposed in the adjacent metamorphic belt. Melting at pressums in excess of 5 Kb and temperatures about 75O’C produced peraluminous magmas and, when the degree of partial melting approached 25-3096, these magmas became mobile and moved vertically into the overlying Ordovician sediments. During movement from the source mgion to the xone of emplacement, separation of the melt and refractory residue components of the magma resulted in a range of compositions so that whole-rock analyses of the gmnitoids are linearly related on major and trace element variation diagrams. Pmcesms such as crystal fractionation and crystal accumulation may have operated locally. The magmas were largely composed of solid material throughout their emplacement histories and the amount of melt may not have exceeded 30-4596 at any stage. Memsedimentaty inclusions am a ntlection of source heterogeneity. After emplacement of the magmas, in situ qstalhmtion of a relatively anhydrous assemblage of minerals led to water contents in residual, intemrysmlhne, melts sufficiently high for muscovite to begin crystallization at pressures around 4 Kb. Subsequent saturation of intercrystaliine residual melt and loss of the resultant volatile phase caused the development of eutectoid intergrowths involving muscovitebiotitequartz and alkali feldspar. INTXODUCTION THISpaper presents whole rock and mineral chemical

data for a suite of muscovite-biotite granitoids from the Western Metamorphic Belt of South-eastern Australia. Particular attention has been given 10 the chemistry of inclusions which are abundant in most members of the suite in an attempt to understand their origin and how they relate to the host granitoids. All the granitoids considered in this paper contain muscovite and sillimanite. Some contain andalusite and many contain evidence that cordierite was present at the magmatic stage. One of the objects of this study is to understand the conditions of genesis and the crystallization history of strongly peraluminous granitoids. Models for the origin of granitoids in South-eastern Australia have emphaaised the importance of source rock composition and have explained the characteristics of the granitoid suites in terms of partial melting and separation of melt and refractory residues (e.g. CHAPPELL and WHITE, 1974; WHMIZ et al., 1977; WHITE and CHAPPELL, 1977; HINE et al., 1978; FLOOD and SHAW, 1975). This contrasts with many North American studies which, although recognizing the importance of source composition, emphasise crystal fractionation as an important process controlling variation in granitoid suites (e.g. PRESNALL and BATEMAN,1973; BATEMANand NCKXLEBERG, 1978; FREY et al., 1978; BATEMANand CHAPPELL, 1979). Recently (e.g. PHILLIPS ef ol., 1981) crystal fractionation has been proposed as the dominant 31

process controlling variation in some South-east Australian granitoid suites. Studies of volcanic rocks have led to the introduction of alternative models for the origin of high-SiOs igneous suites based on processes such as simultaneous fractional crysmlhition and assimilation (e.g. DEZPAOLO,198 1; GROVE et al., 1982); magma mixing (e.g. EICHELBERGER, 1975); and thetmogravitationaldiffusion (e.g. SHAW et al., 1976; HILDRETH,1981).

The variability of granitoid chemistry, mineralogy, geological setting and rock association is such that no one model can have complete and general application and in most situations an interplay of processes is probably the case. One can only reu@ze the dominant processes by considering all the models in the light of a complete data base which is thoroughly understood. The present study is an attempt to understand the origin of a specific suite of peraluminous granitoids which are closely associated with regional metamorphism of low pressure, high temperature type. Geological setting In south+astem Austraha two zones of Palaeoxoic [email protected] of low pressure-high temperature type, occur. The Western Belt (Fig. 1) is continuous from central New South Wales into eastern Victoria and is contained within the Wagga Zone (VANDENBERG, 1978); a major structural element of the Tasman Fold Belt System. The Wagga Zone contains sediments ranging in age from Lower Ordovician (I(ILPATKICKandFLEMING,1980) to Devonian which were deformed and metamorphosed in late Ordovician to early Silurian times. Granitoids which were generated and emplaced both during and after the metamorphism, range from

32

R. u Price Petrography of the Koetong Slate

FIG. 1. Location of the Koetong Suite in relation to the Western Metamorphic Belt of South-east Autitraiia Also shown are the major granitic batholiths. 1. Moruya Batholith. 2. Bega Batholith. 3. Munumbidgee Batholith. 4. Berridale Batholith. 5. KosciuakoBathUh. 6. Conyong Batbolith. Area covered by Fig. 2 is also shown. A = Albury; B = Bega; C = Cooma.

concordant gneissic types associated with migmatiWs to diS_ tinctly disco&m typeJ with narrow contact aureoks (TATTAM, 1929; Buvrs, 196% PtWE and TAYLOR, 1977). The granitoids which are the subject of this study (the Koetong Suite) outcrop along the eaatem margin of the Wpcp Zone approximately40hmeastofthecityofAiburyintheupper Murray VaUey (figs. 1 and 2) and are part of the Conyow Batholith (Fg. 1). They intrude low grade slates and phyllites which npresent the eastern margin of the Western Metamorphic Belt. A whole rock Rb/Sr isochron obtained fromthecOnyonlBothdithbyBR~~ondLsoco~l9~2) givea an age of 421 m.y. (using the decay constant of STEIGERand JAGER, 1977). The Koetong Suite is surrounded by a contact aureole which, in placer, is up to 1 km wide. P&tic units, within the aureo&, are repmaentui by knottad achiata which consist of the assemblage muacovite + biotite + quartz + albitic plagio&u, The “knots” which are up to 1 cm in diameter, are pseudomorphs after c&&he and consist of biotite + chlorite + muacovite + quartz. Art&k&e occurs in a frw localities within a metre ofthe gmnite-sediment contact but isgfmemIIyabaentfrommostoftheatueok.Thecontact between gmn$oids and mctaaediments is marked by extensive pegmatitu and apIites. The western mar@ of the Koetong Suite is bounded by a major fault zone (the TalIangatta Creek Fault) which at its north-western end is marked by a broad mylonite zone (up to 1 km in width). The Tallonepno Creek Fault has a long and complex history which overlapped with the emplacement of the Koetong gmnitoids. The granitoids of the Koetong Suite are all strongly peraluminous, t-mica adametitcs (using the nomenclat~ of STRECKEISEN,1973). Inthe6eldtbeyaresimilarinappeamnceandoutcropcharact&it& which makes recognition of individual plutons di5cuit. On the basis of variations in gmin-size and foiiation, meta&imentary screens, and outcrop pattern 6 sep arate plutons have been mapped (Fig. 2), although a larger number may be present. The rock units grouped as the Koetong Suite are cor&dered to be r&ted to a single melting event on the m that they are of the same age (BROOKSand Leczo, 1972; PRICEand COLON, unpub. data), they are ail strongly peraiuminous and have similar major and tmce element chemistry, and they are all minemlogically and texturally similar. Considerable can has been taken to collect fresh samples for geochemical analysis. In the case of whole rock samples 5-10 kgs. of sample were crushed and this material was quartered down, using a teaon splitter, t@ii 50 g of sample remained for final crushing in a tungsten carbide swing mill.

A typical sample of Koetong Suite gramtold consms oi coarse-grained alkali feldspar, strained qua, oscillatorzoned piagioclasc (AnTAn& biotite, muscovite and apatite. Myrmekite is abundant as isolated patches In rhr groundmass and as protrubant growths on feldspar crystals. Biotite ranges in modal abundance from around 6% to 18%. and muscovite, which tends to be more abundant in the lower SiO2 samples ranges in modal abundance from 5% to 16%. Muscovite occurs as both large (up to 2 Lrn) plates and as fine ragged aggregates. Biotite and muscovite also occur in intricate vermicular intergrowths and as intergrowths with quartz. Representative mica analyses are presented in Table I. Biotites of both the host rocks and metasedimentary inclusions (see below) are s&rophylIite-eastonita solid solutions. Muscovites have compositions and textures similar to primary muscovites from elsewhere (MIUER et al., 198 1). Sillimanite is common in most samples and is usually pant as fine mats of needles which appear to be overgrown by muscovite. Occasionally siltimanite occurs as coarse prisms or aoorcoates of prisms. Andalusite is found only in some samples of the Granya Adamellite (No. I in Fig. 21. Cordiezite is not present but coarse pgprr%at~s of muscovite and biotite which appear to be pseudomorphine an earlier mineral are common and ue similar in morphology to pardally replaced cord&&e cry&& which occur in inclusions. Muacovites pseudomorph& con&rite have compositions which d&r from those occurring as large plates and presumed to be primary (Tabk I).

,

I

1

zl

Schist

/jJ

Mykanits

P

Porohyry Dyke

‘

,

&Ill.. pJ

.

-

,,

,“‘;

; 1

MiQmatlt.

Sample

FIG. 2. Geological Map of the Koetong area. individuai units of the Koetong Suite are numbered 1 to 6. The Granya Adamellite is unit’ 1. The Yabba Granite is similar to the Koetong Suite granitoids. The Thologokmg Granite is a later (Devonian) hi level granitoid which is, chemically, quite different from the Koetong Suite. Analyzed sample localities are also shown on the map. The Upper Murray highways are shown.

SfUz

Si AlI’ Ai”1

Cl

FeO* MnO b.0 cao Na25 KZQ F

A1203

TiOZ

2.544 0.866 0.321 2.455 Q.Q% 1,913

0.045 1.782

0.149

0.061 1.805

0.189 I

I 8 15

4 - II - 16

for

Tablr

0.290 0.014

0.050 1.831

5.384 2.616 0.822 0,399 2 ‘593 0.037 1.706

on the

95.44

4

f.

0.3x2 0.006

O-OS4 1.886

il.346 2.305 0.036 1.707

0,919

5,426 2.574

bssis

96.72

0.18 9.76 Q.bS 6.05

0.28 7.56

19.58 19.79

1.58

35.8b

A5-5

of

0.046 1.847

0.036 1.767

0.160 -

1,675

0.277 Q.vM

o.fm

2.622

0.032 0.309

5.431. 2.564

CM, F),

95.43

a,15 9.42 0.51 Q,O2

35.32 2.30 18.76 20.31 o,;za 7.31

5.486 2.514 0.971 0.181 2.717 0.040 3.78Q

24 (0,

95.89

0.12 9.08 0.33

35.97 3.37 19.38 21.29 0.31 7.83

9X

130

6 A5-5/

5

As-51

0.157 0.008

0.055 1.832

2.625 1.004 0.264 2.528 0.040 1.767

5.375

95.47

0.19 9.41 0.32 0.03

35.20 3.50 20.16 19.79 0.31 7.76

9

0.062 -

0.134 I.670

0.137 1.614 0,059 .a

#,07b

3.693

I

1 I49 0,346 Q*QO4 Q*lJQ

h.25t

94.42

0.52 9.83 OkI4

45.24 1.34 34.50 L,JQ 0.04 0.65

AS-VA

6.251 1.749 3.629 0.101 Q.236 0.002 0.187

93.36

0.52 9.69 0.15

33.&9 1.20 0.01 0.93

O.V4

46.17

8 45-119

7

-

0.163 1.830

6.143 1.857 3.617 0.137 0.108 Q.Qw 0.123

93.54

0.62 10-56

a.55 34.20 0.95 0.02 0.61

45.74

A5-22

10

REPRESENTATTVE MlCA ANALYSES

A5-9-l/ 57

1:

0.084 Q.OQ3

94.76

0.61 10.0% 0.20 0.01

0.58

46.41. 1.19 34.70 0.95 0.03

A5-5

L1

0.022 -

0,080 1.689

6.238 1.762 3.807 0.057 0 .oRi Q.RQ4 0,083

93.87

0.31 9.88 O-Q5

46.58 0.56 35.28 0.76 0.03 0.42

A5-5J 1.30

12

13

0.049 0.005

0.168 1.729

0.094

6.191 1.809 3.838 0.007 0,092

95 -09

0.65 10.22 0.22 0.02

0.48

46.69 0.07 36.13 0.83

A5-5J 9X

Biotltes in Koctong Suite gsanitaids. 5 7 siotitea in Xnclusions from Xmtong Suite graniteids. Ifrtscnvites in Koerorig Sufte granitoids, X2 - 14 Muscavites fr? fn&_&ons from Koetang Suite granitolds. Muscr~vite ~~~~d~rn~r~~i~~ cordiexite in Koetong Suite gmnitoids,

Explanation

5.456

2.599 0.779 5.311 2.VOti 0.038 1.709

formulae

5.bQS

Structural

96,66

0.17 9.33 0.60 0.05

0.15 9.11 il.31

95.67

35,OO 2.85 18.96 20.16 0.28 7.44

35.61 3.45 18.88 19.16 0.28 8.38

3

A5-22

35.16 2.78 18.66 21.06 a.29 I.&V L 0.2cl 9.a. 0.39

2

AS-7A

1.

hf-119

Table

14

Q.088 0.001

93.99

0.60 10.32 CL*21

0.58

0.88

35.so

45.90

Af-9l/97

15

0.113

0.148 1.746

6.289 1.711 3.753 0.004 0.109 a .QQ6 0.147

93.73

0.56 10.24 0.26

34.32 0.97 0.05 0.73

0.54

45.56

A5-119

16

6.175 1.825 3.7711

93.64

35 .O? 1.06 0.07 O.Vl. _ 0.60 10,91

45.62

As-22

R. C. Price

34

Trace eiement chemistry of the Koetong Surre On FeO* variation diagrams. Ba, Sr, Zr. L . SC.i Cr. Ni, Cu, and Zn show good positive iinear vanation (Fig. 5) while Rb shows strong negative linear correlation. The FeO* variation diagram fur Ga is remarkably similar to that for A&03. Pb shows a poor negative correlation with respect to FeO* j-,:501 1.; 5

345

ILmenlte

Mn/Fe

FIG. 3. Chemical variation in ilmenite. Mn and Fe show a negative comlation in ilmenite of both host granitoids (open symbols) and inclusions (closed symbols). Partitioning of Fe and Mn between ilmenite and biotite is systematic in inclusions and host granitoids and both ilmenite and biotite in granitoids have higher Mn/Fe ratios than those in the inclusions. Error bars show total variation for all the

relevant analytical data.

Apatite, tourmaline, ilmenite and zircon are commonly present as minor phases. Fwre 3 shows that the ilmenites of inclusions and host rocks are compositionally similar. Partitioning of Mn between biotite and ilmenite emphasks the chemical and miner&&al uniformity of the suite (Fig. 3). Ruorite am& very rareIy, topaz occur in some samples of the Granya Adamellite. Metasedimentary inclusions are abundant in all members

of the Koetong Suite. Most inclusions&e rounded, although a few anz angular, and inclusions range in size from a few millimctres up to 3 or 4 metres. Tluee distinct endmember types are readily recogniz& aItJIough gradations occur between these three types: a) Quart&s and metasiltstones which consist of an evenq-ained polygonal aggregate of quartz, albitic-plagiodase and alkali feldspar, with minor biotite and apatie, b) Unfoiiated biotite-rich inclusions consisting of biotite, plagiodase, ail&i feldspar and apatite. Cordieritc, garnet, a&&site, sillimanite and muscovite also occur in some examples of this group; c) Schistose biotite-rich inclusions consisting of biotite, muscovite, alkali

feldspar, quartz and apatite. Sillimanite is o&n present as coarse needles and also as fibrous mats. WHOLE

ROCK GEOCHEMISTRY

Table 2 is a compilation of major and trace element geochemistry for selected samples of granitoids and inclusions from the Koetong Suite. Complete data lists are available from the author on request. Major element chemistry of the Koetong Suite With respect to variation in total iron expressed as Fe0 (FeO”)’ TiOz, MgO and CaO show a strong positive linear correlation (Fig. 4) for samples of the Koetong Suite. In contrast, SiO2, NazO and K20 are negatively correlated with FeO* and scatter of the data points about the regression line is marked. There is very little change in AlzOs abundance among the samples, while PzO5 shows a poor negative correiation with FeO*. ’ FeO* has been chosen for variation diagrams because it accounts for more of the variability of the gmnitoids than does SiOz.

Geochemical variation among the incluswns 01 :k~ Koetong Suite The inclusions analyzed from the Koetong Suite show considerable variation in chemistry (Figs. JI and 5). For example, SiOz varies from around 50% up to about 80%. Although broad trends can be recognized for many elements, scatter of the data is considerable, and even where variation is similar to that observed for the host rocks, the inclusion analyses do not lie on the regression line for the host rock analyses (e.g. TiOt, MgO, Sc, V, Cr, Ni and Zn; Figs. 4 and 5). In some cases although approximately linear correlation is observed for the inclusion data on FeO* variation diagrams the slope has opposite sign to the slope of the regression line for the host rock analyses (r.g CaO, KzO and Rb; Fii. 4 and 5) and for many elements the inclusion data is extremely scattered f z.g. AlzO3, NalO, PzO~, Ba, Sr, Pb, Y: Figs. 4 and .ii. Rare earth chemistry Representative samples of Koetong suite gramtoids and inclusions have been analyzed for the rare earth elements. The granitoids have typical “sedimentary” rare earth element patterns (Fig. 6). The light REE are enriched relative to the heavy REE and a negative Eu anomaly is evident. The inclusions analyzed all have similar patterns and, in comparison with the host rocks, total rare earth elements are slightly more abundant and the Eu anomaiy more marked. Among the granitoid samples La and Ce are positively correlated with FeO*. The most iron rich granitoids, with the lowest SiOz content show rhe highest abundance of La and Ce. DISCUSSION The linear variation for the Koetong Sune !Ilustrated in Figs. 4 and 5 is difficult to reconcile with a crystal fractionation model. As crystallization proceeds phases which are solid solutions should change composition systematically and this should result in curved trends on the variation diagrams. For ex. ample, a fractionation model involving plagioclase (PRESNALL and BATEMAN, 1973) cannot readily ex-

plain linear variation on an FeO* versus CaO diagram (Fig. 4) because the CaO content of the pla@oclase being removed decreases as fractionation pruceeds and a curved trend should result. Viscosity considerations (SHAW, 1965: BURNHAM. 19’9) also present problems for the crystal fractionation mode1 since it is expected that separation of small Isolated

35

Paraluminous granitoid suite Table 2:

REPRESENTATIVE ANALYSES OF KOETONG SUITE CFaNITOIDS AND INCLUSIONS

1

Bnplanatia”:

3

‘

5

6

7

8

9

AS-119

A5-32

A5-b6

AS-22

As-s

AS-66 xl

AS-7x

A5-51 IOn

AS-91 il

A5-9A

69.35 0.56 14.71 0.57 3.19 0.06 1.49 1.57 2.16 4.21 0.35 1.21 0.22 0.10 0.08

69.46 0.58 14.55 0.24 3.38 0.05 1.45 1.47 2.28 4.43 0.19 1.40 0.08 0.19 0.03

69.73 0.53 14.37 0.34 3.15 0.04 1.47 1.55 2.43 4.24 0.19 1.27 0.12 0.20 -

70.16 0.45 14.84 0.26 2.70 0.05 1.23 1.33 2.24 4.05 0.20 1.26 0.27 0.16 0.03

71.66 0.44 14.37 0.11 2.93 0.06 1.13 1.10 2.18 4.,4 0.22 1.24 0.11 O.OI -

50.23 0.96 26.32 1.43 6.36 0.13 3.06 0.“ 1.04 5.71. 0.19 2.80 0.23 0.12 0.03

50.94 1.05

1.06 7.18 0.13 3.47 0.40 0.78 7.00 0.15 3.09 0.16 0.13 -

55.33 2.08 16.88 1.63 9.55 0.13 3.15 1.04 1.18 5.38 0.44 1.47 0.26 0.05 0.04

62.63 0.89 16.56 1.22 5.46 0.11 2.42 0.21 0.66 5.80 0.14 2.96 0.19 0.19 0.02

76.84 0.57 10.88 0.56 2.55 0.0‘ 1.03 1.60 2.90 1.47 0.10 0.97 0.13 0.07 0.01

99.79

99.77

99.63

100.00

99.93

99.10

99.92

99.17

99.43

99.81

920 541 204 129 38 20

624 222 146 ‘6 21

947 422 238 129 43 19

711 49L 260 122 49 16 4 16? 16 29 8.1 40 31 19 12 56 21 28 61

419 274 99 34 13 4 157 17 21 8.2 41 42 36 9 66 21 24 52

209 382 45 21 27 6 171 28 49 19.3 160 137 48 6 161 39 52 104

325 449 k2 20 24 4 179 33 15 ll3.i 150 133 56 7 179 45 52 84

4

4

4

215 18 29 8.6 53 37 24 10 69 19 40 81

220 18 29 8.6 52 41 26 19 66 20 37 78

195 20 28 9.7 48 36 21 19 70 22 39 76

24.46

112 557 39 13 25 5.2 388 67 57 35.6 225 74 65 31 247 38 48 101

10

90 123 113 17 21 3 290 15 25 8.3 47 54 22 2 63 1L 40 79

355 416 63 33 20 2.7 237 20 34 14.4 116 105 51 29 121 23 37 76

1 - 5 Representative analyses of hoetong Suite granitoids. 6 - i 5chistose 6llllnanite-~"sco"ite_biaf*te rype inclusions. 8 - 9 Granular biotite type Analytical methods are described inclusions. 10 Ouartzlte rype 1nclvsion in Appendix.

crystals from a granitic melt should be relatively inefficient. It is evident that separation of solid material from melt has not taken place to any significant extent in the Koetong magmas because the granitoids contain many metasedimentary inclusions most of

2

8

4

Fe60

1e

which are more mafic, and hence more dense, than the host rocks. If the inclusions have not separated from the magma during rise and emplacement it is unlikely that individual crystals have been physically separated on a large scale.

2

4

8

I0

FiO

FIG. 4. Variation of major elements against total iron expressed as FeO. Open circles are granitoids analyses; crosses are analyses of inclusions; and triangles are garnet cordierite gneiss analyses.. Regression lines are for the granitoid data alone.

36

FIG. 5. Vat&ion of trace ekments a@nst toti iron expressedas FeO. Syr&ols as rlt Fig. 4

The~~~~~~ diffusion model proposed by l%amtEm (e.g. 198I) to explain extensive fractionation of high-Si02 voicanics has also been successfully applied to concentrically zoned plutons (LUDINGTON, 198 I). The Koetong Suite consists of more than one pluton; zoning is not observed in the members of the suite; and extreme enrichments in large ion lithophile elements do not occur. Therrn~v~mtia~ dif%sion may have localized exnressiun, as indeed may crystal fractionation, but the overall linear variation observed for the Koetong Suite requires another explanation. Linear variation in volcanic suites has been ascribed to magma mixing (EICHELBJ%GER,1975; ANDERSON, 19%; GROVE Pf ai., 1982). In many “itype” granitoid suites where ma& inclusions of dioritic or basaitjc wmposition are common, gabbroic rocks an cioaeiy associated and hybrid rocks occur along comacts (e,g. ERIKSON, 1977; GIUFFIN ef al.. 1978; PIuCEafld SINTON, 1978), magma mixing may

2 i

!

La

Ce

iv

Nd

SmEu

FIG. 6. Choadrite nom&ized

.X

Tb

3y

*O

Er

fb

rare Earth element panerns

for Koetong Suite graxutoi& and inclusions. Open symbols are host graniteid analyses, closed circler are analyses of schistosc and grmmlar biotite-rich in&ions, and the triangles represent analyses for a quart&e ?ype inclusion.

be aa important process controlhng v~a~o~. In contrast to those suites, the Koetong granitoids are strongly peraluminous, have very high initiai ratios of 87Sr/86Sr and lack igneous-like mafic inclusions. The essentially linear nature of geochemical vanation in the Koetong Suite is consistent with the restite-unmixing model of WWIIX and CHAPPELL ( i 97% Acax&ng to r.he modd, when granitoid melt IS formed, me&&g is not complete so that the resulting magma is composed of mcit and cry%& of refractory minerals (restite). The melt, bulk restite and source rock compositions lie on a straight line on any variation diagram. Separation of the restite material will occur to varying degrees during migration and emplacement of the magtnas and a range of compositions and hence rest&ant rock types will result COMPSTON and CHAFPELL (t9?% ar@I~ that 1525% melting is the range normally involved in the generation of granitoid magmas. Degrees of partia! melting below this range mean, at the viscosities normally encountered in granitic melts, that the melt cannot move out of its source matrix and above this limit the whole mass of melt and residue is sufficiently mobile to move upward away from the source region, VAN DER MULEN and PATERSONt 147% consider that at meit fractions of 30-35s the strength of a partially melted rock is reduced suffcientfy for it to deform, flow and rise at a rate consistent with movement and emplacement of a granitoid piuton. Thus granitoid plutons could be very largeiy soiid material at the early stages of migration and emplacement and be very close in composition to the bulk source. As the magmas rise+ decreasing pressure will result in more extensive partial melting with 3 consequent decrease in viscosity and density of the magma and processes such as fractionai cqstahization and thermogravitation diffision will becomr more significant.

Paraluminous

37

granitoid suite

Limitations on source rock chemistry for the Koetong Suite The strongiy peraluminous chemistry of the Koetong Suite suggests derivation from a aluminous sedimentary source (CHAPPELL and WHITE. 1974: MILLER and BRADRSH, 1980; CLEMENSand WALL. 1981: KISTLER et al., 1981; LEE et al., 1981). Derivation from a meta-sedimentary source is also indicated by the initial “Sr/%r ratio of the suite which is relatively high (0.7163; BROOKS and LEGGO, 1972). Application of the restite unmixing model enables limi~tions to be placed on the chemistry of the source rock from which the Koetong Suite was generated. The composition of the source rock must lie on the extension of the regression line through the analysis points on any variation diagnlm and beyond the most mafic of the samples. Examination of the variation dia8ram for Rb plotted a8ainst FeO* (Fig. 5) indicates that the source rock had a maximum FeO* content of 6.1 wt. % since Rb cannot be less than 0. The source material must be at least as mafic as the most ma& granitoid of the suite, which provides a minimum value for FeO* (3.7 wt. %). Applying these limits for FeO* to the other variation diagrams enables limits for the abundance of other elements in the source rock to be estimated (Table 3. Column 2). Similar arguments can be used to estimate a composition for the melt component (Table 3. Column 1). The suggestion has been made (e.g. BROOKS and

Table 3: SOURCEROCKS

LEGGQ, 1972; PRICE and TAYLOR, 1977) that the peraluminous granitoids of the Corryong Batholith were derived from the Ordovician me&sediments of the Wagga Zone. Figure 7 shows the compositions of analyzed samples of Ordovician sediments and me&sediments from South-eastern Austraha on a CaO versus Sr variation diagram and comparison is made with the Koetong Suite. The Ordovician sediments and meta-sediments have CaO and Sr abundance levels which are too low in comparison with the limitations estimated for the source material for the Koetong Suite. The granitoids must have derived from an older. less chemicaily mature source underlying the Ordovician meta-sediments within the Wagga Zone. Such material could be present in the higher grade zones of the adjacent metamorphic belt. Garnet-cordierite gneisses which outcrop to the west of the Koetong Suite in the Wagga Zone have compositions which lie within the source rock compositional limits listed for most elements in Table 3. Analyses of three garnet cordierite gneisses listed in TabIe 3 are strikingly similar to the source composition estimated using the restite model (Table 3, Figs. 4 and 5). If the gamet-cordierite gneisses of the Wagga Zone are possible parental material for the Koetong Suite magmas, the P. T conditions under which the gneisses were metamorphosed may provide some information about the geothermal gradient prevailing in the me~mo~hic belt from which the magmas were presumably generated. FERRY (1980) used the assemblage gamet-plagioclase-biotite-quartz-silli-

ESTIMATESANTJPOSSIBLE

SOURCEROCKS FOR THE KOETONGSUITE 1

TiOl SiO2

0

Al203 FeO*

14.0 0.4

w

cao NazO W PPOS

I

73.76 - 0.12

2

PA SI Pb Th zr Nb Y 0 CK Ni CU

O-188 413-460 24..52 41- 52 ii- 6 20- 58 Cl7 o- 11 o- 5 o- 12 4- 14 0" 5

La ce

20- 22 o- 10 o- 19

Ba

, Zn Ga

3% LB

4

I3

/ 5

0.60 65.69 l.* j "7:;; 1 "1;:2;

- 14.6 - 1.10 0 - 0.9 0.18 - 0.68 3.0 - 3.4 5.2 - 5.5 0.3

14.6 3.7 1.5 -

14.7 j 14.45 6.1 I 4.07 2.5 1.88 I 1.6 2.6 1.68 1.8 2.2 1.83 3.5 4.2 4.20 qo.2 .34

L

t-

I i-

I

.bO 15.35 5.89 2.25 1.44 1.69 1.44 .lb

I

I

)540

5:lE 2.15 1.68 1.63 3.75 .3B

68.05

708

659

468 145 135 27 26 247 11 4i 12P 78 46 67 100 19 56 107

Etrpianarion: 1. Comvosirionsl limitsfor melf in KoefcnbSuite 2. Compositionallimitsfor source rock of Xoetonpmagmas. 3. Garnet-cordierite gneiss (23646). Sethangs anal. 1. Willlams(1969). 4. Garnet-cordlerite gneiss (23645). Hum Wetr Quarry,anal. N. Williams (1969). 5. Garnet-cordierite gneiss (23536). Upper Snndv Creek,anal. Q.J. McKay (1969).

38

R. C. Pnce

net-cordierite gneisses which are considered posslbie parental material for the Koetong magmas contain 15-25% biotite. If the arguments developed by Van der Molen and Paterson ( 1979) and Compston .mti Chappell (1979) are valid, 20-30% melting would appear to be a reasonable limit for mobilisation OI a granitoid magma. If the gamet-cordierite gneisses are source rocks for the Koetong Suite magmas. then the melt in the parental magma would contain 3-: wt. % Hz0 depending on the extent of melting and original biotite content. Water contents in the melt must have increased to around 8% during the evet lution of the Koetong Suite melts, for muscovite to have crystallized. This could occur only if H20 was concentrated in the melt by crystallization of prinFIG. 7. Comparisonof Sr and Ca in Kcetong Suite gran- cipally anhydrous phases. Anhydrous phases crystaiitoids (open circlea)and analyzed Ordoviciansediment and lized in situ with only a small degree of separation metasedimentsFromSouth-easternAustmlia (crosses).Also shown are analyses of gamet-cordieritegneisses(triangles). of melt and crystals, but remaining intercrystalline melts became progressively more hydrous until anThe regression line is for the granitoids alone. Analyses of Ordo&ian sediments and metasediments are from ELZE hydrous mafic phases such as garnet and orthopy(1978); ZUCCALA ( 1978); KILPATRKK (1979); and from roxene began to react to form biotite. Eventually CHAPPELL and WYBORN(unpubl. data). uHfl was sufficiently high for muscovite to crystallize. Jahns and Bumham ( 1969 j have shown that 60-70% manite and thermodynamic treatments by GHENT crystallization of plagioclase, alkali feldspar, cordier( 1976) and FERRY and SPEAR ( 1978) to determine ite, orthopyroxene and possibly biotite at a pressure P, T coqlitions, and application of Ehis method, with of around 4-5 Kb could lead to concentrations of mineral data for the garnet cordierite gneisses wnHtO in the remaining intercrystalline melt consistent sidered in this study (MCKAY, 1969; WILLIAMS, with primary crystallization of muscovite. rZ possible 1969) gives P and T of 5 Kb and 760°C respectively. This pressure is consistent with the range determined using the assembiage garnet-cordierite-sillimanitequartz (NEWTONand WOOD, 1979) with an assumed temperature of 760”. The P and T estimated for equilibration of the gamet-cordierite gneisses might be taken as minimal values for the melting event

which gave rise to the Koetong Suite. and indicate a steep geothermal gradient (Fig. 8). Conditions of cr.vstallization for the Koetong Suite magmas The occurrence of muscovite places a lower pressure limit on the crystallization of the Koetong Suite of 3.75 Kb. and temperature is narrowly constrained (Fig. 8). Two feldspar geothermometry (STORMER. 1975: WHII-NEY and STORMER, 1977) gives temperatures ranging up to 670°C; feldspar temperatures

must be regarded as lower limits. The granitoids of the Koetong Suite are highly reduced. The occurrence of almost pure ilmenite and the absence of magnetite coexisting with iron-rich biotite implies oxygen fugacities less than IO-” bars at the temperatures under consideration (BUDDINGTON and LINDSLEY. 1964: WONES and EUGSTER, 1965). Highly reducing conditions are consistent with a sedimentary source which would contain appeciable carbonaceous material. Bumham ( 1967) pointed out that water contents in excess of 8-9 wt. B and pressures in CXCLZ of about 4 Kb are necessary for the primary crystallization of muscovite from a granitoid melt. The gar-

FIG. 8. Diagrammanc representanon of physrcal condotions for generation and emplacement of Koetong Suite magmas. A represents the lower limits for P and Tof melting which gave rise to Koetong Suite magma, as determined

from the garnet cordierite gneisses of the Wagga Zone. s and a are emplacement and crystallization paths for the sillimanite-bearing and andalusite-bearing graniroids eespcctively. M = muscovite + quartz - AlzSiOJ + K-feldspar

+ H20 (KERRICK,1972); G = minimum melting curve for field boundaries as determined by RICHARDXJN ef al. (1969) and HOLDAWAY(197 1) mspectively. Phi, and Phlz are the saturated and vapour absent melting curves for phlogopite in the presence of quartz and alkali feldspar (WONES and DODGF 1977’

granite;R and If represent aluminosikate

Pamluminous granitoid suite model is outlined in Fig. 8. 20-35% partial melting of garnet-cordierite gneisses at pressures in excess of 5 Kb and temperatures in excess of 750°C leads to a peraluminous melt which rises, cools, and crystallizes until at pressures of around 4 Kb, the intercrystalline melt, although still not saturated with respect to HzO. is sufficiently hydrous so that muscovite can begin to crystallize.

Intergrowth textures A unique feature of the Koetong Suite granitoids is the occurrence of myrmekite and particuhrly, of analogous textures in biotite and muscovite. Myrmekitic textures have been interpreted as replacement features or as exsolution phenomena (see, for example, ASHWORTH, 1972), but, in the case of the Koetong Suite intergrowth textures, very similar to myrmekite, occur which involve combinations of feldspar, quartz, biotite, and muscovite. Furthermore, al1 such examples of intergrowth developed at a specific point in the ~s~li~tion history of the granitoid and represent either reaction textures or simultaneous growth of all four minerals. At the time of formation of the intergrowth textures the magma was extensively crystallized with only a small amount of interstitial melt remaining. The remnant melt would have approached saturation with respect to volatiles. Loss of a volatile phase, which could occur through microfractures and along larger faults, would cause substantial underwing and initiate relatively rapid crystalBzation of remaining interstitial melt (JAI-INS and BURNHAM, 1969; HIBBARD, 1979). In this respect the close proximity_ of the Tallangatta Creek Fault is important. Pegmatites and aplites extensively developed along the contacts of the plutons and metasomatic replacement of cordierite in adjacent metasediments indicate the migration of large volumes of aqueous fluid following empla~ment of the granitoids.

Andalusite stability Discussion of the primary crystallization of andalusite is complicated by the uncertainties associated with the experimental determination of phase relationships in the alumina-silicate system (Fig. 8). The ~umin~sili~te triple point of RICHARDSONef al. (1969) is consistent with primary crystallization of andalusite (Fig. 8). However, the HOLDAWAV ( 197 1) triple point is presently favoured by many metamorphic petrologists because it is more consistent with other geothermometers and geobarometers. Holdaway’s triple point implies that andalusite cannot he a primary magmatic mineral in granitoids unIess the HzO-saturated granite melting curve can be dramatically depressed by addition of other volatiles such as F (WYLLIE and TU’ITLE, 1961) or B (CHORLTON and MARTIN, 1978). The high concentrations of such volatiles required for suppression of the saturated granite solidus to sufficiently low tem-

39

peratures seems to preclude this possibility as the only factor necessary to reconcile the triple point and the granite solidus. Reconciliation of the Holdaway triple point with the occurrence of muscovite in granitoids may involve an interplay between a number of factors causing departure from the ideality of the relevant experiments. For example, the muscovite stability curve may be shifted to higher temperatures when impure (Na and Ti-bearing) natural muscovite is involved. At the same time the granite solidus may be depressed by the presence of appreciable B and F in the system and the stability field of andalusite may expand in the impure system (e.g. STRENS, 1968). All these factors working together could expand the magmatic stability fields for muscovite and andalusite.

Inclusions in the Koetong Suite The analyzed inclusions from the Koetong Suite show a broad variation in chemistry and for many elements the analyses are widely scattered showing no systematic variation. For some elements (e.g. SiOz, TiOz. MgO, CaO, K.20, Rb, Th, Zr, Sc, Zn, Ga, La and Ce) the inclusions show a scattered systematic linear variation which does not coincide with the regression line for the host rocks on FeO* variation diagrams (Figs. 4 and 5). If the source region from which the granitoids derived was originally part of a sedimentary sequence, it would retain original heterogeneities to high grades of metamorphism. The degree to which individual lithological units will melt depends on the water content (hydrous phases) and on the quartz, alkali feldspar and albite contents. Metagreywacke horizons would undergo the highest degrees of partial melting because they contain high proportions of the granitic components (quartz and feldspar) and also sufficient hydrous phase (biotite) to provide water for the mehing reaction. Quartz& horizons would undergo very small degrees of melting because they are deficient in KrO and also in hydrous phase. Biotite schist horizons in the source region may undergo small degrees of partial melting, but they contain insufficient felsic component for large scale melting and biotite will remain stable. Small amounts of granitic melt cannot be effi~ently extracted from the enclosing solid matrix and up to 15-20% melting the rock remains coherent (Compston and Chappell, 1979). The relationship between melt generation and source heterogeneity is outlined in Fig. 9 for CaO and KzO plotted on FeO* variation diagrams. Since metagreywacke horizons in the source region are the units which undergo large scale partial melting they control the granitoid composition. Other lithological units in the source regions may undergo small levels of partial melting but will remain as coherent horizons which are subsequently mechanically disrupted when the magma generated in the meta-greywacke units, hegins to move.

R.

40

c Pnce source rocks indicate apatite modal abundances

oi less than I?6 and calculations using presently accepted distribution coefficients for apatite in granitlo systems (e.g. WATSON and GREEN. 1982) would rz-

Fe0

K2°

FIG. 9. D+arnmatic explanation for the relationship between variation in host grar~itoidsand inelusions of the Koctong Suite in terms of Ca and &O variation relative to Fe0 (total ion as FeO). The shaded area represeats compositionaI variation in the sourcc region. Composition lying between the limits a-a’, b-b’ melt to a degree which results in diqgre&on of the o~nal lithology and mobilization of the resultant mqmas will result in a small degree of separation of melt and ret&tory solids (rutite). The Koe-

tong Suite gmnitoids (shaded area labclled ‘K’ lie on an unmixing tine bctweea the mdt (open circle) and the bulk m&e. CM&c the limits a-0: b-b’ melting of source rocks islimiud(kdow209b)ondsuchrocksntPintheircoherence in the ma as inclusions. ~da=physicallyW The armws (‘x’) indicate the effects of cry3tal fractionation at higher crustal levels. In the case of the Koetong Suite,

such mare not importantin ControlJiagoverall variation, although they may be of local sigttilicana.

Biotite and ilmenite mineral chemistry imply that the inclusions have, to some extent, equilibrated with the host granitoids and the trace element abundances in granitoids and inclusions, therefore, reflect equilibration between melt and refractory components. Many of the inclusions show higher total rare earth abundances than the host graaitoid, and this would suggest that the bulk solid/melt distribution coefficients for rare earth elements were greater than 1 during the partial melting event which generated the granitoid magmas. This suggestion is supported by La and Ce variation (Fig 5) among the whole rock samples from the Koetong Suite. Bulk distribution coefficients of greater than 1, for the rare earth elements, might arise from two effects: accessory phase influence and variation in mineral/melt distribution coefficients. Accessory phases in the restite, such as apatite or monaxite, might raise substantially the bulk distribution coefficients for the light rare earth elements

quire higher modal abundances of the mineral if the bulk distribution coefficients are to be raised sutficiently. Very minor quantities of the mineral monazite could have a substantial iatluence on rare earth element partitioning between melt and residual solid (MILLER. 1982). An alternative explanation IS that the cumbtned effects of a lower temperature and melt chemistr; cause the melts under consideration here to be much more structured so that presently accepted distnbutioa coefficients for minerals in graaitic systems are inappropriate. Rare earth distribution coefficients for apatite (WATSONand GREEN, 1982) and plagioclase (DRAKE and WEILL, 1975) are strongly temperature dependent. Mineral/melt distribution coefficients presently available for granitic systems are based on studies of rhyolites (see HANSON, 1978, for a review) or on experiments at temperatures well above those under consideration here (WATSON and GREEN, 1982). Furthermore, the effects of composition on melt structure, and hence on mineral/melt distribution coefficients, although known to be extremely important (MYSEN and VIRGO, 1980) are not understood for granitic systems.

CONCLUSIONS The Koetong Suite represents peraluminous magma which was generated at depths of around 15 km by partial melting of a meta-sedimentary sequence. The principal lithology which melted was a gamet-cordierite gneiss representing meta-greywacke unus in the source region. For melting to have occurred, geothermal gradients of the order of 40-45”Ukm would be required and such gradients could only arise if heat were transferred from deeper regions of the crust by the rise of more matic magmas originating in the lower crust or upper mantle (TAUBENECK. 1957. see also summary by HILDRETH, 198 1). Synmetamorphic diorite plutons occur within the Wagga Zone (r.g. CARLYLE. 1975) and may represent some of these more mafic magmas. The Koetong Suite contains primary muscovitc. which formed when residual melts approached saturation as a result of earlier crystallization of mainly anhydrous phases. Saturation of a residual interstitiai melts with Hz0 and subsequent loss of the volatiie phase resulted in the formation of eutectoid intergrowth textures involving muscovite, biotite, alkali feldspar and quartz. Primary andalusite occurs in one of the units of the Koetoag Suite and crystallization of andalusite seems to be favoured by the interaction of several factors, including high boron and fluorine activity in the melt and stabilization of andalusite by impurities (e.g. Ma).

Paraluminous gramtoid suite

the stability of muscovite and andalusite is not consistent. This could be caused in part by application of experiments on pure muscovite and andalusite to natural situations where these minerals are not pure (MILLER el a!., 198 1).

Inclusions in the E&tong Suite represent portions of the metasedimentary source region which did not melt to an extent sufficient for them to provide a major contribution to the composition of the host magma. Acknowledgements-This work benefited from discussions with A. J. R. White, F. A. Frey and V. J. Wall, and from critical reviews of draft manuscripts by 1. I... Grove, J. M. Sinton, F. A. Frey, J. Ferry, P. R. Whitney, and an anonymous reviewer. I. McCabe, K. Palmer, P. Oswald Sealy and M. Shelley provided assistance in the laboratory and T. A. Ryan, P. Brennan and K. PaImer assisted in the field. Ibis work was supported in part by a grant from the Australian Research Grants Committee.

41

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(1965) Comments on viscosity, crystaI settJing and convection in granitic magmas. Amsr. J. Sci. 263, were anrdyzed by game photometsy. Selected samples were 120-152. anaIyxed by spark source mass spectrograpk, using methods SHAW H. R., Shnrm R. I.. and HlLDggTn W. (1976) described by TAYLOR(~~~~, 1971), forrare eartk eiements, Thetmogmvitational mechanisms for ckcmicaI variaSn, Cs, Hf, Bi and MO. F was ana&zed by specific ion efections in zoned magma ckambers. Geai. $oc. Amer. Abstr. trode using metkods described by kxx~~ ( 1970). AI1other Programs #I+I 102. trace element analysis was carried out by x-ray tluorescence STEIGER R. H. and JAGEK E. (1977) Subcommission on specunmetry using pressed powder p&&s and the methods described by No~atszi and f&APPEt& ( 1967). geoc?tronoIogy: convention in the use of decay constants in gee- and ~~~~~. Enrtfr Pfan. Sck tetf, 36, Minerai com~tions were determined using an auto359-362. mated JEOI. JXA-SA ekcfron microprobe iocated in the STORWR J. C. 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