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An isotopic mixing model for the origin of granitic rocks in southeastern Australia
Earth and PIanetary Science Letters, 70 (1984) 47-60 Else&r Science Publishers B.V., Amsterdam - Printed in The Netherlands
47
An isotopic mixing model for the origin of granitic rocks in southeastern Australia C.M. Gray Geology Department,
La Trobe University, Bundoom, Vie. 3083 (Australia)
Received October 4,1983 Revised version received May 30,1984
A simple mixing process was a significant element in the genesis of many southeastern Australian granitic rocks: major and trace element abundances are near-linearly correlated, Nd and Sr initial isotopic compositions define a simple hyperbolic trend, and many initial 87Sr/86Sr ratios are a regular function of Rb/Sr ratios. Geological arguments and regression of geochemical variation diagrams place limits on the nature of the end members. The highSi end member is meta-sedimentary-derived magma comparable in composition to the Cooma Granodiorite, the one pluton in the region surrounded by, and clearly derived from, regional metamorphic rocks. The lowSi end member is basaltic material with similarities to rare gabbros also found in the area. Isotope dilution calculations based upon initial Sr and Nd isotopic compositions deduced for the end members yield realistic hypothetical Sr and Nd concentrations for the end members in a demonstration of the internal consistency of the model. All of the granitic rocks, from hornblende tonalites to cordierite granodiorites, are a single broad family, products of variable mixing between distinct batches of basaltic material and a uniform partial melt of the regional basement.
1. Introduction
Over the last decade Chappell, White, Compston and co-workers have intensively studied the granitic batholiths of southeastern Australia, producing a wonderfully coherent set of geochemical and isotopic data [l-7]. Their major conclusion has been the postulation of two fundamental compositional classes of granites termed S- and I-types; the two classes are attributed to the partial melting of distinct crustal source rocks of sedimentary (S) and igneous (I) nature respectively [4,8]. The purpose of this paper is to utilise the published analytical results to develop an independent genetic model for the granites based upon the mixing of basaltic and high-Si crustal material. The primary motive for the attempt is to account for the Sr and Nd isotopic compositions of the granites and in particular their initial “7Sr/86Sr ratios which lie between mantle and mean continental crustal val0012-821X/84/$03.00
0 1984 Elsevier Science Publishers B.V.
ues. It must be stressed that all the data presented are the work of others and detailed references are given throughout the text in acknowledgement. The granite bodies investigated are the SiluroDevonian Berridale, Kosciusko, Moruya and Murrumbidgee Batholiths (Fig. 1) for which a general age for magmatism of 420 My is adopted [7]. The batholiths occupy the southeastern end of the Lachlan Orogenic Belt which has a north-south tectonic grain and a time range from Cambrian to Devonian. The country rock to the intrusions is an Ordovician flysch terrain which is the oldest exposed unit in the region, extending over a vast area well-beyond the confines of Fig. 1. The sedimentary sequence is a monotonous repetition of sandstone, greywacke, siltstone and slate interpreted as turbidites and yet to be deciphered stratigraphitally [l]. All four batholiths are composed of many plutons surrounded by contact metamorphic aureoles of the order of 1 km wide. The Berridale
48
Batholith is typical [l] and has an area of approximately 1850 km*; it extends over 120 km parallel to the strike of the erogenic belt and averages 25 km in width. There are 20 separate granite types in numerous elliptical plutons which have long axes of up to 40 km elongate along the length of the batholith. The plutons range from cordierite biotite granodiorites with gneissic xenoliths (S-type, e.g. Cootralantra Granodiorite) to hornblende granodiorites or tonalites with dioritic xenoliths (I-type, e.g. Tara Granodiorite). The seminal area for an understanding of the genesis of the granites is the Cooma Complex (150 km*) which contains relatively high-grade regional metamorphic rocks on strike with, but separate from, the southern end of the Murrumbidgee Batholith (Fig. 1). A narrow concentric metamorphic zonation of chlorite-biotite-andalusite-sillimanite-migmatite zones is centred upon the Cooma Granodiorite [5,11]. The primary age of the metasedimentary rocks cannot be demonstrated palaeontologically within the complex, but the area is generally considered to be part of the TABLE
regional Ordovician terrain [ll]. A Rb-Sr total rock isochron for biotite grade schists gives an Ordovician age of 450 My with an initial “Sr/‘%r ratio of 0.710 [lo]; the maximum model age of these rocks is 630 My and the sequence might be Cambrian in age. The migmatite zone has both gradational and intrusive contacts with the central granodiorite (30 km3) which is a massive mediumto coarse-grained biotite-rich rock containing cordierite, sillimanite and andalusite. Xenoliths are schists and gneisses similar to the country rock. The chemical composition of the granodiorite (Table 1) is notable for its low CaO content (0.95%) which is less then that of the other granitic rocks of the region and more akin to that of the metasedimentary host. The Rb-Sr total rock isotopic study of Pidgeon and Compston [lo] shows that both the high-grade metasedimentary rocks and the granodiorite have identical ages (406 My) and distinctive high initial 87Sr/86Sr ratios (0.718). Field, geochemical and isotopic evidence convincingly demonstrates that the Cooma Granodiorite formed by melting of the local metasedimentary
1
Granitic compositions referred to in the text (major elements in wt.%, trace elements in ppm) Cooma Granodiorite
SiOz TiOz A1203
Fe@, Fe0 MnO MgG CaO Na,O K,G p205
72.00 0.54 13.72 0.59 3.48 0.06 1.76 0.95 1.49 3.73 0.13
Metasedimentaryderived granitic rocks (mean of 8)
70.87 0.50 14.22 0.51 3.09 0.06 1.50 1.11 1.98 4.30 0.21
Cootralantra Granodiorite
Finister Granodiorite
Blind Gabbro
Kosciusko Batholith hornb,ende suite lowSi limit
67.82 0.61 14.47 0.54 4.09 0.07 2.33 3.05 2.06 3.40 0.14
64.54 0.45 14.71 0.98 4.34 0.10 2.77 5.30 1.86 2.55 0.09
47.25 0.77 19.86 3.02 6.75 0.17 6.42 11.41 1.48 0.40 0.05
> 49.2 d 1.1 < 19.7 Q 4.8 < 8.7 d 0.2 < 6.4 < 10.7 > 0.0 > 0.3 0.1
Rb Sr V
153 127 64
169 139 88
132 135 129
15 438 348
211 < 501 < 398
Cr Ni
56 24
65 20
60 12
34 15
< 91 < 23
15 1
121
Reference [S ]
B
’ R.C. Price, personal communication, 1983.
[5
1
high-Si limit < 71.9 > 0.2 > 13.3 > 0.4 al.4 > 0.1 >l.O 2 3.3 d 4.4 d 3.7 0.1 (140 a111 > 26 20 >l
hornblende-free suite low+ limit > 45.4 d 2.2 4 21.2 ( 4.0 Q 11.7 d 0.2 g 7.3 g 8.0 > 0.0 > 0.0 < 0.4 2 0.0 < 357 < 317 < 235‘ < 78
high-,% limit < 75.2 > 0.2 2 12.6 > 0.0 21.0 > 0.0 > 0.4 2 0.4 < 2.9 d 5.4 > 0.1 6340 > 35 >12 >O 20
49
sequence or its equivalent at depth. Structurally, the Cooma Complex is anomalous in a terrain where regional metamorphism seldom exceeds chlorite grade, and all other granitic plutons are surrounded by contact metamorphic aureoles. A simple explanation has Cooma as a localised thermal high perhaps due to injection of basaltic magma, the excess heat causing ultrametamorphism at unusually shallow depths. A plausible alternative view considers the area to be the location of diapiric upwelling which has carried not only the granodiorite magma to a high level in the crust, but also a narrow envelope of its host metamorphic rocks from the site of generation [12]; if so, the high-grade rocks may be somewhat older than the exposed Ordovician. The general conclusion to be drawn from the Cooma Complex is that this is the one locality in the region where the genesis of a granitic rock can be determined directly. It is proposed that the partial melting of metasedimentary material akin to that at Cooma is common to all the other granites under consideration and that Cooma-type magma is an important component in their genesis.
2. Linear geochemical variation The most significant geochemical discovery made on the southeastern Australian granite terrain by Chappell, White and others is the identification of numerous cases of near linear variation in elemental abundances as a function of silica content. Such variation is most pronounced in groups of spatially related plutons and two examples from the Kosciusko Batholith [2] will serve as illustrations. The hornblende-bearing rocks of the batholith are predominantly tonalites in the silica range 60-67X which form well-defined alignments (Fig. 2); significantly, the 67% which form well-defined alignments (Fig. 2); significantly, the trends also include gabbros as far removed as 47% silica. The remaining hornblende-free rocks, granodiorites to granites, outline other arrays often with distinctly different orientations (Fig. 2). The correlations vary considerably in quality; they are particularly pronounced for some major elements (Ca, Mg, Ti) and ferromagnesian trace elements
(V, Cr, Ni), and are at least consistent with linearity for most other elements. Such distinct chemical associations of plutons are widespread in the region and are termed suites [4]. The plutons of a single suite may be dispersed in a strip along the elongation of a batholith. The chemical differences between suites may be considerable and have led Chappell and White [4,8] to postulate two types of granites and two source materials from which they are derived by partial melting. I-type granites such as the homblendic suite described above have high Na,O/K,O ratios, moderate Fe3+/Fe2+ ratios, high Ca contents in mafic rocks and Al,O,/ (Na,O + K,O + CaO) ratios less than 1.05; they are considered to be derived from a pre-existing igneous source [4]. S-type granites such as the hornblende-free suite of the Kosciusko Batholith have low Na,O/K,O and Fe3+/Fe2+ ratios, low Ca contents and Al,O,/(Na,O + K,O + CaO) ratios greater than 1.05; they are taken to be derived from a metasedimentary source [4]. The most convincing interpretation of linear variation invokes simple mixing between two end members. White and Chappell [13] proposed that granitic magmas form by partial melting of crustal source rocks producing a partial melt and residuum (restite). In their model the end members of the mixing system are partial melt and restite and variation within individual suites and samples from specific plutons is due to separation (unmixing) of these components. This paper explores an alternative view that the spectrum of granitic compositions derives from mixing between unrelated end members at or beyond the extremities of the data on the variation diagrams. The two end members are interpreted to be a basaltic material and a granitic melt. The evidence in support of this approach will now be presented. Linear regression of the silica variation diagrams can place limits on the silica content of the end members in the mixing system. For example, as silica contents increase in a suite, many ferromagnesian elements decrease in abundance with substantial negative gradients, and ultimately some trends project to zero and cut the silica axis. Plainly, none of these elements can have negative
50
TABLE Input
2
data for Sr isotope
Pluton
dilution
calculations
Samples used
Murrumbidgee Batholith Bolairo Granodiorite Callemondah Granodiorite Clear Range Granodiorite Stewartsfield Granodiorite Willoona Tonalite Kosciwko Batholith Gaden Tonalite Grosses Plain Tonalite Ingebyrah Granodiorite Jillamatong Granodiorite Jindabyne Tonalite Kalkite Adamellite Pendergast Tonalite Round Flat Tonalite Be&dale Batholith Bimbimbie Granodiorite Cootralantra Granodiorite BB19 (west) BB83 (east) Currowong Granodiorite Dalgety Granodiorite Finister Granodiorite Iona Tonalite Merumbago Granodiorite, Tara Granodiorite Moruya Batholith Moruya Tonalite Tuross Head Tonalite
2 15
Samples rejected
Mean Rb
Mean Sr
Mean initial R7Rb/s6Sr
Initial “‘Sr/ s6 Sr
Mean SiO,
x
Reference
0
2 4 1 1
177 152 172 171 173
138 145 139 155 146
3.74 3.06 4.01 3.22 3.45
0.7145 0.7136 a 0.7130 = 0.7122 0.7147 a
68.35 68.40 68.40 67.00 68.34
0.16 0.15 0.15 0.21 0.16
[17,23] [17,23] [17,23]
0 1 0 0 0 0 0 0
98 85 170 188 97 197 109 77
216 236 171 138 222 111 246 224
1.32 1.05 2.89 3.97 1.27 5.17 1.29 1.00
0.7067 0.7065 0.7106 0.7150 0.7069 0.7153 0.7069 0.7066
65.30 62.44 68.41 66.82 63.87 71.34 66.09 63.38
0.29 0.41 0.15 0.22 0.35 0.03 0.25 0.37
[1,6] [2,7] [2,7] [2,7] [1,6] [1,7] [1,6] [1,6]
0 0
128 165
239 134
1.56 3.58
0.7087
67.20 67.49
0.20 0.19
[6,7] [1,5,7]
66.54 68.54 65.38 63.32 65.21 67.14
0.27 0.15 0.28 0.37 0.29 0.21
[6] [1,7] [6] [6] [6] [1,6]
[17,23] [17,23]
0 2 1 1 0 1
116 173 128 95 131 86
243 167 129 271 143 258
1.39 3.01 2.89 1.02 2.66 0.97
0.7121 0.7138 0.7076 0.7099 0.7114 h 0.7068 0.7108 ’ 0.7062
5 4
1 0
70 50
419 462
0.49 0.31
0.7045 0.7041
64.19 62.24
0.33 0.42
[3,6] [3,6]
7
0 1 0
180 228 219
152 134 122
3.45 4.95 5.22
0.7094 0.7105 0.7095
72.59 71.72 72.27
_ 0.01
L9.171 [9,17]
2 0 0 0 0 0
200 164 196 205 236 124
156 138 110 34 97 186
3.73 3.46 5.19 17.5 7.08 1.94
0.7067 0.7068 ’ 0.7049 0.7048 0.7109 0.7054
72.62 74.06 75.86 76.29 73.48 74.83
0
132
212
1.81
0.7051
71.43
Fractionatedplutons Murrumbidgee Batholith Shannons Flat Adamellite North South Tharwa Adamellite
12 4
Berridale Batholith Buckleys Lake Adamellite Delegate Adamellite Maffra Adamellite Namungo Adamellite Numbla Vale Adamellite Wullwye Granodiorite Moruya Batholith Nelligen Granodiorite ’ Individual ’ Individual
data pooled data pooled
4 at 415 My. at 420 My.
[9,17,231
_ _
WI VI WI
_
151 [51 [61
0.02
[3,6]
51
concentrations hence the points of intersection on the silica axis must give upper limits to the silica contents of the high-silica end member. If all such elements were equally well correlated then the lowest silica intersection would be a strict upper limit. Because the correlations do vary substantially in quality it becomes necessary to select the most likely limit by favouring the best correlation, one preferably supported by the intersection of other elements. In the case of the hornblendic suite from the Kosciusko Batholith (Fig. 2) there are several elements with correlation coefficients in excess of 0.9; the cut-off silica percentages are TiO, at 80.7, Fe,O, at 75.8, MgO at 73.7, CaO at 80.2 and V at 71.9. The lowest well-defined limit is selected, V at less than or equal to 72% silica; it is confirmed by the poorer correlations of Cr and Ni at 70.2% and 71.3% silica for correlation coefficients of - 0.76 and -0.84, respectively. In a similar manner, a lower limit to the silica content of the low-silica end member may be derived from trends with positive gradients. The appropriate elements are K, Rb and Th although these are often poorly correlated with silica. In this instance Rb with a correlation coefficient of 0.80 gives a lower limit of 45.2% SiO,. The general approach may be extended beyond regression against silica to a regression matrix involving all elements (here 21 elements) to obtain the limiting compositions of the end members. The compositional limits for the two suites from the Kosciusko Batholith described earlier are given in Table 1. For both suites the high-silica end member is clearly defined at first sight because several well-correlated elemental variations converge to zero between 71% and 80% silica (e.g. Ca, V-Fig. 2). Plainly the end member has a composition approximating the minimum in the granite system [13]. The literal conclusion would place the end member at the highest silica value found in actual rocks, namely 76%. However, this cannot be true for all suites as the hornblende-bearing suite of the Kosciusko Batholith does not project to such a high value (Table 1). In an attempt to unify the origin of most granites in the region a common end member is sought; as a result its silica content cannot greatly exceed the 71% limit defined by the hornblendic rocks. The evidence of the Cooma
Complex is that at least one granodiorite formed directly by the melting of the regional metasedimentary rocks. Such a melt is proposed as the high-silica end member. The primary support for this hypothesis is the compatibility of the major element chemical composition deduced for the end member as well as a general consistency with the trends of the variation diagrams (Table 1, Fig. 2). The Cooma Granodiorite (72% SiO,) will be taken as characteristic of metasedimentary-derived melts for the region recognising that this single pluton need not be perfectly representative. To generalise, the average composition of 8 similar granites and gneisses from the Wagga Metamorphic belt approximately 150 km to the west of Cooma is given in Table 1; a similar low silica content of 71% is found. Any granites with silica contents in excess of 72% must then be considered to have undergone geochemical fractionation away from the mixing system towards a granitic minimum melt composition at 76-77% silica. Accordingly, two mechanisms, mixing and fractionation, must be considered when describing geochemical variation, and the likely superimposition of two trends causes the strictly linear interpretation above to be an approximation only. Recognition of fractionation in plutons with silica contents less than 72% is difficult, hence the implications of fractionation are considered further in several discussions below (note that there are no indications of significant fractionation in the suites under consideration here). The low-silica end member could lie at any point on the variation diagrams between the lowsilica limits deduced above (45-49% SiO,) and those granodiorites and tonalites with the lowest silica contents. Minor occurrences of rocks of basaltic composition’ in severa of the batholiths suggest that the mixing system extends to silica values as low as 50%. Hence the calculated lowsilica limits probably approximate the end member. As some confirmation, two gabbros from the Kosciusko region [2] plot on the continuation of the elemental trends of the hornblendic suite from this batholith (Fig. 2), even sharing distinctive features of the trace element chemistry such as low Ni abundances. An analysis of one, the Blind Gabbro, has a close correspondence to the low-
52
silica limit of the hornblendic suite (Table 1). A single basalt cannot be the end member for both suites as some elemental trends diverge to low silica (e.g. CaO in Fig. 2). Accordingly, while there are considerable similarities between the end members for the Kosciusko Batholith suites (Table l), high-alumina basalt of variable composition is taken as the second end member.
Fig. 1. Granitic batholiths of part of southeastern Australia. Granitic rocks are shown in black: 2 = Cooma Granodiorite; 2 = Murrumbidgee Batholith; 3 = Kosciusko Batholith; 4 = Berridale Batholith; 5 = Moruya Batholith. Country rocks are shown in white and are dominantly Ordovician flysch of north-south strike. Cainozoic basalts are marked with the V symbol.
Sr
o
300
-
PPm
0 l
am.
“O.
l
.+ + 4q
,+ +q++ + ++
300 400
V PPm
-
0
Rb PPm 200
-
++ +
+ a++ $+a + $ +
+
n
200
.
. “.a
l* es*% u: 0 50
!L2
# + *+
70 %
Fig. 2. Silica variation diagrams for two suites of granitic rocks from the Kosciusko Batholith [2]. Solid circles = hornblendic suite; open circles = gabbros considered part of the hornblendic suite [2]; crosses = hornblende-free suite; solid square = Cooma Granodiorite.
53
The geochemical data are consistent with a model for granite genesis involving simple mixing between metasedimentary-derived magma (highsilica end member) and basaltic material (low-silica end member).
3. Isotopic definition of the mixing system 3. I. Nd-Sr isotopic correlation diagram Neodymium and strontium isotopic composition show a covariance in nature that is best expressed on a plot of initial 143Nd/‘44Nd ratio versus initial 87Sr/86Sr ratio. On such a diagram isotopic mixing between two end members generates intermediates which lie along a hyperbola between the end members [14]. Hence the detection of the distinctive hyperbolic trend is strong evidence for a mixing system. The initial Nd and Sr isotopic compositions of rocks from the Berri-
‘-Nd/“‘Nd 05112
. F
05108
1
l.....,...‘J 0700
0720
Fig. 3. Initial 143Nd/ ‘“Nd versus initial “Sr/s%r for granitic rocks from the Berridale and Kosciusko Batholiths. Data are from McCulloch and Chappell [7]: small solid circles = granitic rocks; solid square = Cooma Granodiorite; open square = gneisses from the Cooma Complex. The mantle field is an estimate of the equivalent at 420 My of modem oceanic basalts assuming that the Sm-Nd and Rb-Sr systems developed their present heterogeneity by divergence from a single-stage Earth system 1800 My ago. The dashed line marks the field of most oceanic and island arc basalt data (e.g. [18-201); the large solid circle is the arbitrary centroid of the field used in calculations. The data point to the left of the mantle field was omitted from the calculation of the best-fit hyperbola.
dale and Kosciusko Batholiths form a surprisingly coherent continuous curve that clearly approximates a hyperbola [7] (Fig. 3). The only reasonable interpretation is that to a first approximation the data depict an essentially two-component mixing system with end members that are isotopically discrete and which lie at the limits of the data or on the further projection of the curve. Fig. 3 is the most elegant expression of the mixing system in the granites. As a guide to the location of the end members a best fit hyperbola is passed through the points in Fig. 3. This construction is a guide only and not a literal expression of mixing because the scatter in data, while small, could conceal several superimposed hyperbolas; the curvature of the hyperbolas is governed by the ratio of the Sr/Nd ratios of the end members [14], hence small variation in the concentrations of Sr and Nd in the end members will produce a family of hyperbolas of slightly different curvature. One end member must lie in the realm of high initial 143Nd/ lUNd ratios and low initial 87Sr/86Sr ratios. The only rocks known to occupy this region are oceanic basalts which define the oblique “mantle” array and island arc basalts which are coincident with the array or displaced slightly to the right of it [14-161. The majority of these rocks lie in the more restricted field as shown (Fig. 3) which is taken as representative of common basalts. The best-fit hyperbola for the granites abuts this field suggesting that basaltic materials are the end member; that the curve does not pass into the field is not significant given the scatter in the granite data (Fig. 3). The end member is defined as the centroid of data for most basalts-initial 143Nd/144Nd ratio of 0.51161 and initial 87Sr/86Sr of 0.7033. The determination of the other end member is more subjective as there are no distinctive rock types with low initial 143Nd/144Nd and high initial 87Sr/86Sr ratios. The only general deduction is that this will be a continental crustal material. The extreme data are in fact the Cooma Granodiorite and a gneiss from its metamorphic envelope, and it is proposed that these rocks are representative of the actual end member on the geological-geochemical grounds described above-initial 143Nd/ ‘“Nd = 0.51083 and initial 87Sr/86Sr = 0.7179.
54
3.2. Rb-Sr isochron diagram Two-component mixing is manifested on a RbSr isochron diagram by a linear array of mixes between the end members. While linearity is maintained for any end members with the defined *‘Rb/*%r and *‘Sr/*%r ratios, the location of intermediates is only a regular function of degree of mixing if Sr concentrations are specified as well. To represent the granite system it is necessary to plot mean “Sr/ *%r and 87Rb/86Sr ratios at the time of formation. The mean *‘Sr/*%r ratio is the initial ratio derived from an isochron age determination and this will approximate the average value of a well-mixed magma. The drawback to the approach is the estimation of the mean *‘Rb/*%r ratio because this parameter is susceptible to change after the formation of a magma batch; for example crystal fractionation is most likely to increase Rb/Sr ratios and displace points to the right of the mixing line. The mean *‘Rb/*%r ratios for individual plutons are obtained by averaging all the available Rb and Sr concentrations (Table 2). In many cases plutons are chemically homogeneous and the mean values are easily defined. In a few instances rocks have been rejected as unrepresentative local variants usually in the sense of increased fractionation as shown by low Sr and high SiO, and Rb contents; the numbers rejected for each body are given in Table 2. The isochron diagram of initial *‘Sr/*%r ratio versus mean 87Rb/86Sr ratio can be resolved into a broad diagonal band of analyses and some points scattered to the right of the band as distinguished by different symbols in Fig. 4. The band (23 plutons) is interpreted as a primary mixing trajectory; the other points (10 plutons) are believed to have been displaced from the band by fractionation subsequent to mixing. Beginning with the mixing line, one end member is clearly defined by data adjacent to the *‘Sr/*%r axis. It has a very low *‘Rb/*%r ratio (< 0.2), an initial *‘Sr/ *%r ratio between 0.703 and 0.704 and is isotopically coherent; the initial ratio is consistent with that inferred from the Nd-Sr diagram. All three characteristics strongly suggest a basaltic or perhaps andesitic material. The high Rb/Sr ratio end member could lie
anywhere on the projection of the trend and cannot be uniquely specified; it may well be somewhat inhomogeneous in these parameters because scatter in the data does increase with Rb/Sr ratio. Again the Cooma Granodiorite lies at the extremity of the trend (mean of 3 analyses). Furthermore, the mean of 8 high-grade metasedimentary rocks from the Cooma Complex occupies a similar position. It is again proposed that Cooma-like materials represent the end member. Those data points displaced to the right of the mixing trajectory in Fig. 4 are considered to have undergone some form of magmatic geochemical fractionation. They have several common features consistent with fractionation. Xenoliths are rare or absent in all, and most plutons are described as felsic. Silica contents are in excess of 71%, much higher than for most plutons on the mixing trend (Table 2) and usually in excess of the 72% interpreted for the high-silica end member. In all, 10 plutons can be separated clearly from those on the mixing trajectory and will not be considered further. It is possible that some of the other plutons have also suffered minor degrees of fractionation and that this accounts from some of the scatter in Fig. 4. The Sm-Nd data [7] can be treated in a like
.
0.718
l
0.714 8'Sr/86Sr
.
.
.
.
. l
.
.
.
. 0
. .
. .
0
0
0 0.706
.
.
0. 0.710
q
0
0
0
.* 20-
0.702.
-I 0
1
2
3
4
5
B'Rb/8BSr
Fig. 4. Initial s7Sr/s6Sr versus initial 87Rb/86Sr for granitic rocks from southeastern Australia. Solid circles = plutons on the primary mixing trajectory; open circles = fractionated plutons; solid square = mean of 3 analyses of the Cooma Granodiorite 17,111; open square = mean of 8 analyses of high-grade metamorphic rocks from the Cooma Complex [!,ll].
55
manner and are also crudely consistent with the pattern of mixing and fractionation. The plot is poorly correlated and of limited use probably because the Sm/Nd ratios of the basaltic end members had substantial range; by contrast, in the Rb/Sr system all basalts have Rb/Sr ratios close to zero.
4. Numerical modelling of isotopic mixing Isotopic mixing systems as outlined above can be described algebraically to a good approximation by two equations. Firstly the amounts of the element involved must be balanced: (l-x)-C,
+x-c,=
metasedimentaryderived magma
Secondly, equated:
the
co
basalt
Degree of
mixing(x)
0.10
isotopic
abundances
must
be
basalt
granite
where x = degree of mixing ranging between 0 and 1; C = concentration of element, ppm Sr or Nd; R = isotopic ratio, “Sr/%r or 143Nd/‘44Nd; and the subscripts M = metasedimentary-derived magma; B = basalt; and G = granite. Overall, there are seven variables and the simultaneous equations may be solved if two unknowns are carried. However, because x, the degree of mixing, lies between 0 and 1 by definition, valid solutiqns for two unknowns may be found for assigned values of x-in a sense three unknowns can be tolerated. In the present case the concentrations and isotopic ratios in the granites are known by direct measurement. As well, the isotopic ratios of the end members have been deduced from the isotopic mixing diagrams. This leaves three unknowns, x the degree of mixing and the concentrations in metasedimentary-derived melt and basalt. It is then possible to rigorously calculate these concentrations for various assigned values of x as presented in the following examples. Input data for the granites may be found in Table 2. The Bolairo Granodiorite of the Murrumbidgee
High-Si end member
Basaltic end member
(ppm)
(ppm)
Bolairo Granodioriie 111 0.05 118 0.10 132 0.20 151 0.30 116 0.40 212 0.50 265 0.60 353 0.70 529 0.80 1059 0.90 Jindabyne Tonalite 0.05
granite
(l-x)~C,R,+x~C,R,=CGRG metasedimentaryderived magma
representative granite systems
0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90
58 61 68 78 91 109 137 182 247 547
643 321 161 107 80 64 54 46 40 36
3345 1673 836 558 418 335 279 239 209 186
Batholith contains abundant schistose xenoliths [17] and has a high initial 87Sr/86Sr ratio of 0.7145. The Sr concentrations calculated for its “metasedimentary” and basalt progenitors are given in Table 3. Unfortunately, there is a wide range of acceptable solutions when the results are compared with possible concentrations: the spread of Sr concentration observed in the Cooma Complex allows an upper limit of 200 ppm for the high-silica melt giving corresponding x values from 0 to 0.5; the basalt could reasonably have a Sr content between 100 and 1000 ppm with x from 0 to 0.35. No specific solution for x is possible, but at this stage it can be said that the isotope dilution relationship is consistent with the mixing model because plausible concentrations are obtained. In order to derive a specific solution x must be estimated independently. The linear variation of major elements provides the means-given the abundance of an element in both end members and in the granodiorite x is calculated by simple
56
proportion. Silica is the most suitable parameter as it is the most abundant oxide and because it can be reliably estimated in the end members in the light of the earlier discussion. The high-silica end lies at 72% silica, and the likely range of basalts from 47% to 50% is arbitrarily averaged at 48.5%. The mean silica content of the Bolairo Granodiorite is 68.4%, hence x is calculated to be 0.16; uncertainty in x is probably about 0.05. The explicit solutions for the Sr contents of the end members in the Bolairo Granodiorite system are metasedimentary-derived melt 126 ppm and basalt 201 ppm; the latter figure is very sensitive to the value for x (Table 3). Because basalts have a considerable range of Sr contents 201 ppm is a reasonable result, but certainly not definitive. The high-silica end member at 126 ppm can be compared favourably with three observational esti-
mates (a) Cooma Granodiorite-124 ppm; (b) mean of 11 metasedimentary rocks in the Cooma Complex-121 ppm [7,10]; (c) mean of 14 Ordovician-derived granites-126 ppm [7]. The Murrumbidgee Batholith is rather uniform in composition and similar results to those for the Bolairo Granodiorite are obtained for four other plutons (Table 4); the calculated high-silica end member is well defined with a mean 122 ppm Sr and the basalt is more variable at 200-311 ppm. It is taken to be highly significant that the Murrumbidgee Batholith immediately to the north along strike from the Cooma Complex (Fig. 1) yields Sr parameters for its metasedimentary-derived component identical to those in the Cooma Complex where granite generation can be observed. The Jindabyne Tonalite of the Kosciusko Batholith is a contrasting situation, with the rock a
TABLE 4 Explicit solutions for end member Sr and Nd concentrations Pluton
Murrumbidgee
x
High-% end member
Basaltic end member
Sr(ppm)
Sr(ppm)
Nd(ppm)
Nd(ppm)
Batholith
Bolaiio Granodiorite Callemondah Granodiorite Clear Range Granodiorite Stewartsfield Granodiorite Willoona Tonalite
0.16 0.15 0.15 0.21 0.16
126 120 109 120 136
0.29 0.41 0.15 0.22 0.35 0.03 0.25 0.37
71 88 101 87 84 94 81 80
0.20 0.19
111
201 285 311 288 200
Kosciusko Batholith
Gaden Tonalite Grosses Plain Tonalite Ingebyrah Granodiorite Jillamatong Granodiorite Jindabyue Tonalite Kalkite Adamellite Pendergast Tonalite Round Flat Tonalite
15 34 28 20 27
571 449 570 318 478 659 741 469
13 22 3 15 60
Berridale Batholith
Bimbimbie Granodiorite Cootralantra Granodiorite BB19 (west) BB83 (east) Currowong Granodiorite Dalgety Granodiorite Finister Granodiorite Iona Tonalite Merumbago Granodiorite Tara Granodiorite
0.27 0.15 0.28 0.37 0.29 0.21
100 119 99 89 99 103 104 67
30 31
24 31 26 21 26 19
753 280 198 634 610 207 558 239 976
24 5
23 38 5 24 1 46
57
member of a hornblendic suite, containing diorite xenoliths, and having lower silica contents and initial 87Sr/86Sr ratios. The possible Sr concentrations in the end members (Table 3) have an even greater range of feasible solutions than before. The mean silica content of 63.9% corresponds to an x value of 0.35; accordingly the basaltic end member has 478 ppm Sr and the “metasedimentary” 84 ppm with the former number again sensitive to X. Comparable results are obtained in seven other plutons from the batholith which represent both the suites described earlier and which run the gamut of granitic compositions (Table 4). The basaltic component is again quite variable in Sr concentration (318-741 ppm); the two gabbros from the area (47.3 and 52.1% SiO, [2]) which lie on the projection of the geochemical trends of the granites at least have consistent Sr contents (438 and 290 ppm respectively). The Sr concentrations of the high-silica component are distinctly lower than the 122 ppm found in the Cooma and Murrumbidgee setting, but are conceivable in a metasedimentary rock-mean 86 ppm. The important conclusions for the Kosciusko Batholith are that the outcome of the calculations is independent of rock type and that a single metasedimentary-derived component is present. The plutons of the Berridale Batholith are intermediate and record both the high- and low-Sr metasedimentary sources identified above (Table 4). This is geologically reasonable because while the greater part of the batholith is on strike with the Murrumbidgee Batholith (high Sr source) some western plutons lie on the continuation of the Kosciusko Batholith (low Sr source). The Cootralantra Granodiorite is recognisably one rock type in hand specimen although numerous plutons exist. The two initial 87Sr/86Sr ratios available differ considerably, in the east 0.7138 and in the west 0.7121; the equivalent Sr concentrations in the high-silica component are 119 and 100 ppm respectively. These results are in harmony with their position relative to the Murrumbidgee and Kosciusko Batholiths and it is concluded that there is a systematic change in the composition of metasedimentary-derived material across the region. No calculations are presented for the Moruya
Batholith because all rocks have very low initial 87Sr/86Sr ratios and the outcome is unacceptably sensitive to the choice of isotopic compositions in the end members. With appropriate juggling, using less radiogenic end members, solutions consistent with the model can be obtained. No conclusions can be drawn because such rocks are arithmetically unsuitable for this approach. An identical isotope dilution method can be applied to the Nd data [7] although there are fewer measurements which are limited to the Kosciusko and Berridale Batholiths. As a brief example, the Jindabyne Tonalite for its specific degree of mixing (x = 0.35) gives Nd concentrations of 15 ppm in the basalt and 20 ppm in the high-silica end member. Other plutons give similar results (Table 4) with the basalt heterogeneous, varying from 1 to 46 ppm Nd, and the metasedimentary-derived component more uniform and averaging 26 ppm. The deduced Nd concentrations are reasonable; basalts do have considerable span of Nd concentrations which includes these values; the highsilica result compares well with one analysis of 23 ppm reported for the Cooma Granodiorite [5].
5. Discussion
A hybrid or mixing origin for some granitic rocks was first indicated by initial 87Sr/86Sr ratios intermediate between mantle and mean continental crustal values [18]. Correlations between Sr initial ratios and St80 values [19,20] were more direct evidence of mixing and indicated that individual plutons must have formed from different combinations of end member chemical compositions. Most Nd-Sr data, while consistent with mixing, do not give well-defined hyperbolic trends. A global survey [21] led to the conclusion that mixing between a mantle-derived diorite magma and crustal rock could be involved. A slightly curved trend for the Sierra Nevada Batholith is consistent with interaction between island arc magmas and Palaeozoic or Precambrian basement [22]. The granitic rocks of southeastern Australia are of particular significance because they preserve a more coherent geochemical and isotopic pattern than other cases in the literature. As a result a
58
mixing system can be discerned clearly; the end members are interpreted here to be a granitic melt derived from metasedimentary rocks comparable to those at Cooma and basaltic material. The main requirement for coherence in a mixing system is compositional homogeneity in the end members, a condition which must have been peculiarly satisfied in the Lachlan Orogenic Belt. The composition of an individual granite body was determined by four factors: (a) the nature of the metasedimentary-derived granitic melt; (b) the nature of the basalt; (c) the degree of mixing of granitic melt and basalt; and (d) the extent of geochemical fractionation superimposed upon mixing. The metasedimentary-derived melt component was common to all the granite systems and was the unifying factor in their compositions as shown by the similarity of Cooma-type granitic rocks over a considerable area (Table 1). This might seem surprising given the ultimate metasedimentary derivation of the rocks, but the primary cause was the regional homogeneity of the basement flysch sequence formed by marine currents mixing and depositing sediment in sheets on a vast scale. Any local chemical variations that might have developed would have been erased subsequently by mechanical mixing during partial melting and magma migration. The basaltic component can be expected to have been isotopically uniform given the limited range of isotopic compositions in common mantle-derived rocks. Combined with a welldefined overall composition for the sedimentary rocks this will guarantee coherent isotopic mixing trends. However, the major element composition of basalts is variable and it must be supposed that essentially one basalt type was involved in the genesis of an individual granite suite in order to create linear geochemical variation. On the other hand, different basalts generated the various suites with their calcium content of particular significance; in the Kosciusko Batholith a high-Ca basalt gave rise to the high-Ca suite that precipitated hornblende over much of its compositional range; a low-Ca basalt produced the other suite in which hornblende does not occur. Thus the geological circumstances of this terrain do seem to have favoured the development of a remarkably simple
mixing system amenable to numerical treatment. However, the model as presented can only be an approximation to a process as complex as granite formation. In particular, there must have been local variation in the compositions of the end members which will limit the literal interpretation of the calculations. The spectrum of granitic compositions within a suite was produced by variable mixing between the two end members. In a general way the sequence from adamellite to granodiorite to tonalite corresponds to increasing basalt component. A good example is the relationship between the Cooma, Cootralantra and Finister Granodiorites (Table 1) of which the first two are cordierite-aluminosilicate-bearing (S-type) and the third hornblendebearing (I-type). The Cooma Granodiorite is the end member of the mixing system and the other plutons progress from this composition towards the basaltic end with the distinctive common feature of a very low Sr content in the basalt component (- 200 ppm-Table 4). The change to hornblende in the mode of the Finister Granodiorite is simply due to a slight increase in Ca coupled with a significant decrease in SiO, along the trend. Previously, no link has been recognised between the Finister Granodiorite and the other bodies and Finister has been regarded as an unrelated I-type [5]. In fact, it is unnecessary to postulate fundamentally different granite types using mineralogy when a mixing relationship is considered; any suite will develop modal hornblende at some stage asI the basaltic component increases. The subsequent history of batches of granitic magma made in this way can be remarkably simple. The Rb-Sr isochron plot (Fig. 4) demonstrates that numerous magma bodies once formed are unmodified by processes such as crystal fractionation or contamination that can change Rb/Sr ratios. Of 33 plutons investigated 23 retain their original Rb/Sr ratios and must have had simple histories from generation to emplacement. Obviously, there is some late and local internal fractionation in these cases as evidenced by occasional rocks with anomalous Rb/Sr ratios and the existence of aplite dykes. Grossly fractionated entire plutons are common nonetheless, and must be recognised if the geochemistry of granites is to be
59
understood (the isochron diagram-Fig. 4-is a simple means of their identification). Such rocks originate by mixing to a set composition on linear geochemical trends between 48.5% and 72% silica as described above. Crystal fractionation then drives this composition towards the minimum melt of the granite system which lies close to 76% silica; the trajectory may be straight or curved depending upon the mechanism of fractionation. The difference between trends directed towards 72% and 76% silica is often small and in many cases it is difficult to distinguish pure mixing from mixing plus fractionation using elemental abundances alone. However, in cases where geochemical variation has a high gradient significant divergence may result. The complications introduced by the superimposition of mixing and fractionation paths greatly restrict the literal use of such trends. The physical processes of granite formation are a matter of speculation. A simple view begins with the eastern margin of Australia during the early Palaeozoic composed of a massive accumulation of flysch. Subduction beneath the margin caused basaltic magmas to rise into the crust as an arcuate strip, the magmas invading the turbidites to form dolerite dykes and gabbro intrusions which decreased in abundance towards the surface. Heat from the cooling basaltic material induced hightemperature/low-pressure regional metamorphism and ultimately partial melting of the metasedimentary rocks to form granite magmas. Where basaltic material was absent (at the highest levels?) pure metasedimentary-derived melts were formed. At depth, increasing amounts of basaltic material as solid dykes or as freshly-injected magma became involved in crustal melting producing granitic magma batches of varied mixture compositions. The physical and thermal difficulties of mixing limited the amount of the basaltic component to less than 40% in most instances and truly intermediate compositions were rare. Changes in the pattern of subduction and basalt generation with time produced parallel strips of crustal melting, each strip the site of a granitic geochemical suite. The magma bodies were actually a composite of granitic melt, fragments of refractory horizons from the metasedimentary rocks (quartzite, biotite-rich schists), and unmixed basaltic material. The latter
exotic materials became xenoliths and their fate was determined by the composition of the melt. If the melt was peraluminous, basic xenoliths were preferentially destroyed by the reaction hornblende + melt + biotite and metasedimentary xenoliths survived; if the melt was metaluminous hornblendic xenoliths were retained and metasedimentary types dissolved. Magmatic flow during diapiric ascent homogenised magma batches and crystallisation processes developed the rocks we see today.
References
9
10
11
12
13 14
A.J.R. White, IS. Williams and B.W. Chappell, Geology of the Berridale 1: 1OOOOO sheet 8625, 138 pp., Geological Survey of New South Wales, 1977. R. Hine, IS. Williams, B.W. Chappell and A.J.R. White, Contrasts between I- and S-type granitoids of the Kosciusko Batholith, J. Geol. Sot. Aust. 25, 219-234. 1978. T.J. Griffin, A.J.R. White and B.W. Chappell, The Moruya Batholith and geochemical contrasts between the Moruya and Jindabyne suites, J. Geol. Sot. Aust. 25, 235-247, 1978. A.J.R. White and B.W. Chappell, Granitoid types and their distribution in the Lachlan Fold Belt, southeastern Australia, Geol. Sot. Am. Mem. 159, 21-34, 1983. B.W. Chappell and A.J.R. White, Plutonic rocks of the Lachlan Mobile Zone, 25th Int. Geol. Congr., Field Excursions Guide 13C, 40 pp., 1976. W. Compston and B.W. Chappell, Sr-isotope evolution of granitoid source rocks, in: The Earth: Its Origin, Structure and Evolution, M.W. McElhinny, ed., pp. 377-426, Academic Press, London, 1979. M.T. McCulloch and B.W. Chappell, Nd isotopic characteristics of S- and I-type granites, Earth Planet. Sci. Lett. 58, 51-64, 1982. B.W. Chappell and A.J.R. White, Two contrasting granite types, Pacific Geol. 8, 173-174, 1974. J.C. Roddick and W. Compston, Radiometric evidence for the age of emplacement and cooling of the Murrumbidgee Batholith, J. Geol. Sot. Aust. 23, 223-233, 1976. R.T. Pidgeon and W. Compston, The age and origin of the Cooma Granite and its associated metamorphic zones, New South Wales, J. Petrol. 6, 193-222, 1965. G.A. Joplin, Petrological studies in the Ordovician of New South Wales, I. The Cooma Complex, Proc. Linn. Sot. N.S.W. 67, 156-196, 1942. R.H. Flood and R.H. Vernon, The Cooma Granodiorite, Australia: an example of in situ crustal anatexis?, Geology 6, 81-84, 1978. A.J.R. White and B.W. Chappell, Ultrametamorphism and granitoid genesis, Tectonophysics 43, 7-22, 1977. D.J. DePaolo and G.J. Wasserburg, Petrogenetic mixing
60
15
16
17 18
19
models and Nd-Sr isotopic patterns, Geochim. Cosmochim. Acta 43, 615-627, 1979. R.K. G’Nions, P.J. Hamilton and N.M. Evensen, Variations in ‘“Nd/ ‘“Nd and s7Sr/86Sr ratios in oceanic basalts, Earth Planet. Sci. Lett. 34, 13-22, 1977. M.T. McCulloch and M.R. Perfit, 143Nd/‘elNd, “Sr/‘%r and trace element constraints on the petrogenesis of Aleutian island arc magmas, Earth Planet. Sci. Lett. 56, 167-179, 1981. A.S. Joyce, Petrogenesis of the Murrumbidgee Batholith, A.C.T., J. Geol. Sot. Aust. 20, 179-197, 1973. P.H. Hurley, P.C. Bateman, H.W. Fairbaim and W.H. Pinson Jr., Investigation of initial 87Sr/86Sr ratios in the Sierra Nevada Plutonic Province, Geol. Sot. Am. Bull. 76, 165-174, 1965. L.T. Silver, H.P. Taylor, Jr. and B. Chappell, Some petrological, geochemical and geochronological observations of the Peninsular Ranges Batholith near the international
border of the U.S.A. and Mexico, in: Mesozoic Crystalline Rocks, P.L. Abbot and V.R. Todd, eds., pp. 83-110, Department of Geological Sciences, San Diego State University, San Diego, Calif., 1979. 20 R.S. Harmon and A.N. Halliday, Oxygen and strontium isotope relationship8 in the British late Caledonian granites, Nature 283, 21-25, 1980. 21 C.J. Allegre and D. Ben Othman, Nd-Sr isotopic relationship in granitoid rocks and continental crust development: a chemical approach to orogenesis, Nature 286, 335-342, 1980. 22 D.J. DePaolo, A neodymium and strontium isotopic study of the Mesozoic talc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California, J. Geophys. Res. 86, 10470-10488, 1981. 23 J.C. Roddick and W. Compston, Strontium isotopic equilibration: a solution to a paradox, Earth Planet. Sci. Lett. 34, 238-246,1977.
47
An isotopic mixing model for the origin of granitic rocks in southeastern Australia C.M. Gray Geology Department,
La Trobe University, Bundoom, Vie. 3083 (Australia)
Received October 4,1983 Revised version received May 30,1984
A simple mixing process was a significant element in the genesis of many southeastern Australian granitic rocks: major and trace element abundances are near-linearly correlated, Nd and Sr initial isotopic compositions define a simple hyperbolic trend, and many initial 87Sr/86Sr ratios are a regular function of Rb/Sr ratios. Geological arguments and regression of geochemical variation diagrams place limits on the nature of the end members. The highSi end member is meta-sedimentary-derived magma comparable in composition to the Cooma Granodiorite, the one pluton in the region surrounded by, and clearly derived from, regional metamorphic rocks. The lowSi end member is basaltic material with similarities to rare gabbros also found in the area. Isotope dilution calculations based upon initial Sr and Nd isotopic compositions deduced for the end members yield realistic hypothetical Sr and Nd concentrations for the end members in a demonstration of the internal consistency of the model. All of the granitic rocks, from hornblende tonalites to cordierite granodiorites, are a single broad family, products of variable mixing between distinct batches of basaltic material and a uniform partial melt of the regional basement.
1. Introduction
Over the last decade Chappell, White, Compston and co-workers have intensively studied the granitic batholiths of southeastern Australia, producing a wonderfully coherent set of geochemical and isotopic data [l-7]. Their major conclusion has been the postulation of two fundamental compositional classes of granites termed S- and I-types; the two classes are attributed to the partial melting of distinct crustal source rocks of sedimentary (S) and igneous (I) nature respectively [4,8]. The purpose of this paper is to utilise the published analytical results to develop an independent genetic model for the granites based upon the mixing of basaltic and high-Si crustal material. The primary motive for the attempt is to account for the Sr and Nd isotopic compositions of the granites and in particular their initial “7Sr/86Sr ratios which lie between mantle and mean continental crustal val0012-821X/84/$03.00
0 1984 Elsevier Science Publishers B.V.
ues. It must be stressed that all the data presented are the work of others and detailed references are given throughout the text in acknowledgement. The granite bodies investigated are the SiluroDevonian Berridale, Kosciusko, Moruya and Murrumbidgee Batholiths (Fig. 1) for which a general age for magmatism of 420 My is adopted [7]. The batholiths occupy the southeastern end of the Lachlan Orogenic Belt which has a north-south tectonic grain and a time range from Cambrian to Devonian. The country rock to the intrusions is an Ordovician flysch terrain which is the oldest exposed unit in the region, extending over a vast area well-beyond the confines of Fig. 1. The sedimentary sequence is a monotonous repetition of sandstone, greywacke, siltstone and slate interpreted as turbidites and yet to be deciphered stratigraphitally [l]. All four batholiths are composed of many plutons surrounded by contact metamorphic aureoles of the order of 1 km wide. The Berridale
48
Batholith is typical [l] and has an area of approximately 1850 km*; it extends over 120 km parallel to the strike of the erogenic belt and averages 25 km in width. There are 20 separate granite types in numerous elliptical plutons which have long axes of up to 40 km elongate along the length of the batholith. The plutons range from cordierite biotite granodiorites with gneissic xenoliths (S-type, e.g. Cootralantra Granodiorite) to hornblende granodiorites or tonalites with dioritic xenoliths (I-type, e.g. Tara Granodiorite). The seminal area for an understanding of the genesis of the granites is the Cooma Complex (150 km*) which contains relatively high-grade regional metamorphic rocks on strike with, but separate from, the southern end of the Murrumbidgee Batholith (Fig. 1). A narrow concentric metamorphic zonation of chlorite-biotite-andalusite-sillimanite-migmatite zones is centred upon the Cooma Granodiorite [5,11]. The primary age of the metasedimentary rocks cannot be demonstrated palaeontologically within the complex, but the area is generally considered to be part of the TABLE
regional Ordovician terrain [ll]. A Rb-Sr total rock isochron for biotite grade schists gives an Ordovician age of 450 My with an initial “Sr/‘%r ratio of 0.710 [lo]; the maximum model age of these rocks is 630 My and the sequence might be Cambrian in age. The migmatite zone has both gradational and intrusive contacts with the central granodiorite (30 km3) which is a massive mediumto coarse-grained biotite-rich rock containing cordierite, sillimanite and andalusite. Xenoliths are schists and gneisses similar to the country rock. The chemical composition of the granodiorite (Table 1) is notable for its low CaO content (0.95%) which is less then that of the other granitic rocks of the region and more akin to that of the metasedimentary host. The Rb-Sr total rock isotopic study of Pidgeon and Compston [lo] shows that both the high-grade metasedimentary rocks and the granodiorite have identical ages (406 My) and distinctive high initial 87Sr/86Sr ratios (0.718). Field, geochemical and isotopic evidence convincingly demonstrates that the Cooma Granodiorite formed by melting of the local metasedimentary
1
Granitic compositions referred to in the text (major elements in wt.%, trace elements in ppm) Cooma Granodiorite
SiOz TiOz A1203
Fe@, Fe0 MnO MgG CaO Na,O K,G p205
72.00 0.54 13.72 0.59 3.48 0.06 1.76 0.95 1.49 3.73 0.13
Metasedimentaryderived granitic rocks (mean of 8)
70.87 0.50 14.22 0.51 3.09 0.06 1.50 1.11 1.98 4.30 0.21
Cootralantra Granodiorite
Finister Granodiorite
Blind Gabbro
Kosciusko Batholith hornb,ende suite lowSi limit
67.82 0.61 14.47 0.54 4.09 0.07 2.33 3.05 2.06 3.40 0.14
64.54 0.45 14.71 0.98 4.34 0.10 2.77 5.30 1.86 2.55 0.09
47.25 0.77 19.86 3.02 6.75 0.17 6.42 11.41 1.48 0.40 0.05
> 49.2 d 1.1 < 19.7 Q 4.8 < 8.7 d 0.2 < 6.4 < 10.7 > 0.0 > 0.3 0.1
Rb Sr V
153 127 64
169 139 88
132 135 129
15 438 348
211 < 501 < 398
Cr Ni
56 24
65 20
60 12
34 15
< 91 < 23
15 1
121
Reference [S ]
B
’ R.C. Price, personal communication, 1983.
[5
1
high-Si limit < 71.9 > 0.2 > 13.3 > 0.4 al.4 > 0.1 >l.O 2 3.3 d 4.4 d 3.7 0.1 (140 a111 > 26 20 >l
hornblende-free suite low+ limit > 45.4 d 2.2 4 21.2 ( 4.0 Q 11.7 d 0.2 g 7.3 g 8.0 > 0.0 > 0.0 < 0.4 2 0.0 < 357 < 317 < 235‘ < 78
high-,% limit < 75.2 > 0.2 2 12.6 > 0.0 21.0 > 0.0 > 0.4 2 0.4 < 2.9 d 5.4 > 0.1 6340 > 35 >12 >O 20
49
sequence or its equivalent at depth. Structurally, the Cooma Complex is anomalous in a terrain where regional metamorphism seldom exceeds chlorite grade, and all other granitic plutons are surrounded by contact metamorphic aureoles. A simple explanation has Cooma as a localised thermal high perhaps due to injection of basaltic magma, the excess heat causing ultrametamorphism at unusually shallow depths. A plausible alternative view considers the area to be the location of diapiric upwelling which has carried not only the granodiorite magma to a high level in the crust, but also a narrow envelope of its host metamorphic rocks from the site of generation [12]; if so, the high-grade rocks may be somewhat older than the exposed Ordovician. The general conclusion to be drawn from the Cooma Complex is that this is the one locality in the region where the genesis of a granitic rock can be determined directly. It is proposed that the partial melting of metasedimentary material akin to that at Cooma is common to all the other granites under consideration and that Cooma-type magma is an important component in their genesis.
2. Linear geochemical variation The most significant geochemical discovery made on the southeastern Australian granite terrain by Chappell, White and others is the identification of numerous cases of near linear variation in elemental abundances as a function of silica content. Such variation is most pronounced in groups of spatially related plutons and two examples from the Kosciusko Batholith [2] will serve as illustrations. The hornblende-bearing rocks of the batholith are predominantly tonalites in the silica range 60-67X which form well-defined alignments (Fig. 2); significantly, the 67% which form well-defined alignments (Fig. 2); significantly, the trends also include gabbros as far removed as 47% silica. The remaining hornblende-free rocks, granodiorites to granites, outline other arrays often with distinctly different orientations (Fig. 2). The correlations vary considerably in quality; they are particularly pronounced for some major elements (Ca, Mg, Ti) and ferromagnesian trace elements
(V, Cr, Ni), and are at least consistent with linearity for most other elements. Such distinct chemical associations of plutons are widespread in the region and are termed suites [4]. The plutons of a single suite may be dispersed in a strip along the elongation of a batholith. The chemical differences between suites may be considerable and have led Chappell and White [4,8] to postulate two types of granites and two source materials from which they are derived by partial melting. I-type granites such as the homblendic suite described above have high Na,O/K,O ratios, moderate Fe3+/Fe2+ ratios, high Ca contents in mafic rocks and Al,O,/ (Na,O + K,O + CaO) ratios less than 1.05; they are considered to be derived from a pre-existing igneous source [4]. S-type granites such as the hornblende-free suite of the Kosciusko Batholith have low Na,O/K,O and Fe3+/Fe2+ ratios, low Ca contents and Al,O,/(Na,O + K,O + CaO) ratios greater than 1.05; they are taken to be derived from a metasedimentary source [4]. The most convincing interpretation of linear variation invokes simple mixing between two end members. White and Chappell [13] proposed that granitic magmas form by partial melting of crustal source rocks producing a partial melt and residuum (restite). In their model the end members of the mixing system are partial melt and restite and variation within individual suites and samples from specific plutons is due to separation (unmixing) of these components. This paper explores an alternative view that the spectrum of granitic compositions derives from mixing between unrelated end members at or beyond the extremities of the data on the variation diagrams. The two end members are interpreted to be a basaltic material and a granitic melt. The evidence in support of this approach will now be presented. Linear regression of the silica variation diagrams can place limits on the silica content of the end members in the mixing system. For example, as silica contents increase in a suite, many ferromagnesian elements decrease in abundance with substantial negative gradients, and ultimately some trends project to zero and cut the silica axis. Plainly, none of these elements can have negative
50
TABLE Input
2
data for Sr isotope
Pluton
dilution
calculations
Samples used
Murrumbidgee Batholith Bolairo Granodiorite Callemondah Granodiorite Clear Range Granodiorite Stewartsfield Granodiorite Willoona Tonalite Kosciwko Batholith Gaden Tonalite Grosses Plain Tonalite Ingebyrah Granodiorite Jillamatong Granodiorite Jindabyne Tonalite Kalkite Adamellite Pendergast Tonalite Round Flat Tonalite Be&dale Batholith Bimbimbie Granodiorite Cootralantra Granodiorite BB19 (west) BB83 (east) Currowong Granodiorite Dalgety Granodiorite Finister Granodiorite Iona Tonalite Merumbago Granodiorite, Tara Granodiorite Moruya Batholith Moruya Tonalite Tuross Head Tonalite
2 15
Samples rejected
Mean Rb
Mean Sr
Mean initial R7Rb/s6Sr
Initial “‘Sr/ s6 Sr
Mean SiO,
x
Reference
0
2 4 1 1
177 152 172 171 173
138 145 139 155 146
3.74 3.06 4.01 3.22 3.45
0.7145 0.7136 a 0.7130 = 0.7122 0.7147 a
68.35 68.40 68.40 67.00 68.34
0.16 0.15 0.15 0.21 0.16
[17,23] [17,23] [17,23]
0 1 0 0 0 0 0 0
98 85 170 188 97 197 109 77
216 236 171 138 222 111 246 224
1.32 1.05 2.89 3.97 1.27 5.17 1.29 1.00
0.7067 0.7065 0.7106 0.7150 0.7069 0.7153 0.7069 0.7066
65.30 62.44 68.41 66.82 63.87 71.34 66.09 63.38
0.29 0.41 0.15 0.22 0.35 0.03 0.25 0.37
[1,6] [2,7] [2,7] [2,7] [1,6] [1,7] [1,6] [1,6]
0 0
128 165
239 134
1.56 3.58
0.7087
67.20 67.49
0.20 0.19
[6,7] [1,5,7]
66.54 68.54 65.38 63.32 65.21 67.14
0.27 0.15 0.28 0.37 0.29 0.21
[6] [1,7] [6] [6] [6] [1,6]
[17,23] [17,23]
0 2 1 1 0 1
116 173 128 95 131 86
243 167 129 271 143 258
1.39 3.01 2.89 1.02 2.66 0.97
0.7121 0.7138 0.7076 0.7099 0.7114 h 0.7068 0.7108 ’ 0.7062
5 4
1 0
70 50
419 462
0.49 0.31
0.7045 0.7041
64.19 62.24
0.33 0.42
[3,6] [3,6]
7
0 1 0
180 228 219
152 134 122
3.45 4.95 5.22
0.7094 0.7105 0.7095
72.59 71.72 72.27
_ 0.01
L9.171 [9,17]
2 0 0 0 0 0
200 164 196 205 236 124
156 138 110 34 97 186
3.73 3.46 5.19 17.5 7.08 1.94
0.7067 0.7068 ’ 0.7049 0.7048 0.7109 0.7054
72.62 74.06 75.86 76.29 73.48 74.83
0
132
212
1.81
0.7051
71.43
Fractionatedplutons Murrumbidgee Batholith Shannons Flat Adamellite North South Tharwa Adamellite
12 4
Berridale Batholith Buckleys Lake Adamellite Delegate Adamellite Maffra Adamellite Namungo Adamellite Numbla Vale Adamellite Wullwye Granodiorite Moruya Batholith Nelligen Granodiorite ’ Individual ’ Individual
data pooled data pooled
4 at 415 My. at 420 My.
[9,17,231
_ _
WI VI WI
_
151 [51 [61
0.02
[3,6]
51
concentrations hence the points of intersection on the silica axis must give upper limits to the silica contents of the high-silica end member. If all such elements were equally well correlated then the lowest silica intersection would be a strict upper limit. Because the correlations do vary substantially in quality it becomes necessary to select the most likely limit by favouring the best correlation, one preferably supported by the intersection of other elements. In the case of the hornblendic suite from the Kosciusko Batholith (Fig. 2) there are several elements with correlation coefficients in excess of 0.9; the cut-off silica percentages are TiO, at 80.7, Fe,O, at 75.8, MgO at 73.7, CaO at 80.2 and V at 71.9. The lowest well-defined limit is selected, V at less than or equal to 72% silica; it is confirmed by the poorer correlations of Cr and Ni at 70.2% and 71.3% silica for correlation coefficients of - 0.76 and -0.84, respectively. In a similar manner, a lower limit to the silica content of the low-silica end member may be derived from trends with positive gradients. The appropriate elements are K, Rb and Th although these are often poorly correlated with silica. In this instance Rb with a correlation coefficient of 0.80 gives a lower limit of 45.2% SiO,. The general approach may be extended beyond regression against silica to a regression matrix involving all elements (here 21 elements) to obtain the limiting compositions of the end members. The compositional limits for the two suites from the Kosciusko Batholith described earlier are given in Table 1. For both suites the high-silica end member is clearly defined at first sight because several well-correlated elemental variations converge to zero between 71% and 80% silica (e.g. Ca, V-Fig. 2). Plainly the end member has a composition approximating the minimum in the granite system [13]. The literal conclusion would place the end member at the highest silica value found in actual rocks, namely 76%. However, this cannot be true for all suites as the hornblende-bearing suite of the Kosciusko Batholith does not project to such a high value (Table 1). In an attempt to unify the origin of most granites in the region a common end member is sought; as a result its silica content cannot greatly exceed the 71% limit defined by the hornblendic rocks. The evidence of the Cooma
Complex is that at least one granodiorite formed directly by the melting of the regional metasedimentary rocks. Such a melt is proposed as the high-silica end member. The primary support for this hypothesis is the compatibility of the major element chemical composition deduced for the end member as well as a general consistency with the trends of the variation diagrams (Table 1, Fig. 2). The Cooma Granodiorite (72% SiO,) will be taken as characteristic of metasedimentary-derived melts for the region recognising that this single pluton need not be perfectly representative. To generalise, the average composition of 8 similar granites and gneisses from the Wagga Metamorphic belt approximately 150 km to the west of Cooma is given in Table 1; a similar low silica content of 71% is found. Any granites with silica contents in excess of 72% must then be considered to have undergone geochemical fractionation away from the mixing system towards a granitic minimum melt composition at 76-77% silica. Accordingly, two mechanisms, mixing and fractionation, must be considered when describing geochemical variation, and the likely superimposition of two trends causes the strictly linear interpretation above to be an approximation only. Recognition of fractionation in plutons with silica contents less than 72% is difficult, hence the implications of fractionation are considered further in several discussions below (note that there are no indications of significant fractionation in the suites under consideration here). The low-silica end member could lie at any point on the variation diagrams between the lowsilica limits deduced above (45-49% SiO,) and those granodiorites and tonalites with the lowest silica contents. Minor occurrences of rocks of basaltic composition’ in severa of the batholiths suggest that the mixing system extends to silica values as low as 50%. Hence the calculated lowsilica limits probably approximate the end member. As some confirmation, two gabbros from the Kosciusko region [2] plot on the continuation of the elemental trends of the hornblendic suite from this batholith (Fig. 2), even sharing distinctive features of the trace element chemistry such as low Ni abundances. An analysis of one, the Blind Gabbro, has a close correspondence to the low-
52
silica limit of the hornblendic suite (Table 1). A single basalt cannot be the end member for both suites as some elemental trends diverge to low silica (e.g. CaO in Fig. 2). Accordingly, while there are considerable similarities between the end members for the Kosciusko Batholith suites (Table l), high-alumina basalt of variable composition is taken as the second end member.
Fig. 1. Granitic batholiths of part of southeastern Australia. Granitic rocks are shown in black: 2 = Cooma Granodiorite; 2 = Murrumbidgee Batholith; 3 = Kosciusko Batholith; 4 = Berridale Batholith; 5 = Moruya Batholith. Country rocks are shown in white and are dominantly Ordovician flysch of north-south strike. Cainozoic basalts are marked with the V symbol.
Sr
o
300
-
PPm
0 l
am.
“O.
l
.+ + 4q
,+ +q++ + ++
300 400
V PPm
-
0
Rb PPm 200
-
++ +
+ a++ $+a + $ +
+
n
200
.
. “.a
l* es*% u: 0 50
!L2
# + *+
70 %
Fig. 2. Silica variation diagrams for two suites of granitic rocks from the Kosciusko Batholith [2]. Solid circles = hornblendic suite; open circles = gabbros considered part of the hornblendic suite [2]; crosses = hornblende-free suite; solid square = Cooma Granodiorite.
53
The geochemical data are consistent with a model for granite genesis involving simple mixing between metasedimentary-derived magma (highsilica end member) and basaltic material (low-silica end member).
3. Isotopic definition of the mixing system 3. I. Nd-Sr isotopic correlation diagram Neodymium and strontium isotopic composition show a covariance in nature that is best expressed on a plot of initial 143Nd/‘44Nd ratio versus initial 87Sr/86Sr ratio. On such a diagram isotopic mixing between two end members generates intermediates which lie along a hyperbola between the end members [14]. Hence the detection of the distinctive hyperbolic trend is strong evidence for a mixing system. The initial Nd and Sr isotopic compositions of rocks from the Berri-
‘-Nd/“‘Nd 05112
. F
05108
1
l.....,...‘J 0700
0720
Fig. 3. Initial 143Nd/ ‘“Nd versus initial “Sr/s%r for granitic rocks from the Berridale and Kosciusko Batholiths. Data are from McCulloch and Chappell [7]: small solid circles = granitic rocks; solid square = Cooma Granodiorite; open square = gneisses from the Cooma Complex. The mantle field is an estimate of the equivalent at 420 My of modem oceanic basalts assuming that the Sm-Nd and Rb-Sr systems developed their present heterogeneity by divergence from a single-stage Earth system 1800 My ago. The dashed line marks the field of most oceanic and island arc basalt data (e.g. [18-201); the large solid circle is the arbitrary centroid of the field used in calculations. The data point to the left of the mantle field was omitted from the calculation of the best-fit hyperbola.
dale and Kosciusko Batholiths form a surprisingly coherent continuous curve that clearly approximates a hyperbola [7] (Fig. 3). The only reasonable interpretation is that to a first approximation the data depict an essentially two-component mixing system with end members that are isotopically discrete and which lie at the limits of the data or on the further projection of the curve. Fig. 3 is the most elegant expression of the mixing system in the granites. As a guide to the location of the end members a best fit hyperbola is passed through the points in Fig. 3. This construction is a guide only and not a literal expression of mixing because the scatter in data, while small, could conceal several superimposed hyperbolas; the curvature of the hyperbolas is governed by the ratio of the Sr/Nd ratios of the end members [14], hence small variation in the concentrations of Sr and Nd in the end members will produce a family of hyperbolas of slightly different curvature. One end member must lie in the realm of high initial 143Nd/ lUNd ratios and low initial 87Sr/86Sr ratios. The only rocks known to occupy this region are oceanic basalts which define the oblique “mantle” array and island arc basalts which are coincident with the array or displaced slightly to the right of it [14-161. The majority of these rocks lie in the more restricted field as shown (Fig. 3) which is taken as representative of common basalts. The best-fit hyperbola for the granites abuts this field suggesting that basaltic materials are the end member; that the curve does not pass into the field is not significant given the scatter in the granite data (Fig. 3). The end member is defined as the centroid of data for most basalts-initial 143Nd/144Nd ratio of 0.51161 and initial 87Sr/86Sr of 0.7033. The determination of the other end member is more subjective as there are no distinctive rock types with low initial 143Nd/144Nd and high initial 87Sr/86Sr ratios. The only general deduction is that this will be a continental crustal material. The extreme data are in fact the Cooma Granodiorite and a gneiss from its metamorphic envelope, and it is proposed that these rocks are representative of the actual end member on the geological-geochemical grounds described above-initial 143Nd/ ‘“Nd = 0.51083 and initial 87Sr/86Sr = 0.7179.
54
3.2. Rb-Sr isochron diagram Two-component mixing is manifested on a RbSr isochron diagram by a linear array of mixes between the end members. While linearity is maintained for any end members with the defined *‘Rb/*%r and *‘Sr/*%r ratios, the location of intermediates is only a regular function of degree of mixing if Sr concentrations are specified as well. To represent the granite system it is necessary to plot mean “Sr/ *%r and 87Rb/86Sr ratios at the time of formation. The mean *‘Sr/*%r ratio is the initial ratio derived from an isochron age determination and this will approximate the average value of a well-mixed magma. The drawback to the approach is the estimation of the mean *‘Rb/*%r ratio because this parameter is susceptible to change after the formation of a magma batch; for example crystal fractionation is most likely to increase Rb/Sr ratios and displace points to the right of the mixing line. The mean *‘Rb/*%r ratios for individual plutons are obtained by averaging all the available Rb and Sr concentrations (Table 2). In many cases plutons are chemically homogeneous and the mean values are easily defined. In a few instances rocks have been rejected as unrepresentative local variants usually in the sense of increased fractionation as shown by low Sr and high SiO, and Rb contents; the numbers rejected for each body are given in Table 2. The isochron diagram of initial *‘Sr/*%r ratio versus mean 87Rb/86Sr ratio can be resolved into a broad diagonal band of analyses and some points scattered to the right of the band as distinguished by different symbols in Fig. 4. The band (23 plutons) is interpreted as a primary mixing trajectory; the other points (10 plutons) are believed to have been displaced from the band by fractionation subsequent to mixing. Beginning with the mixing line, one end member is clearly defined by data adjacent to the *‘Sr/*%r axis. It has a very low *‘Rb/*%r ratio (< 0.2), an initial *‘Sr/ *%r ratio between 0.703 and 0.704 and is isotopically coherent; the initial ratio is consistent with that inferred from the Nd-Sr diagram. All three characteristics strongly suggest a basaltic or perhaps andesitic material. The high Rb/Sr ratio end member could lie
anywhere on the projection of the trend and cannot be uniquely specified; it may well be somewhat inhomogeneous in these parameters because scatter in the data does increase with Rb/Sr ratio. Again the Cooma Granodiorite lies at the extremity of the trend (mean of 3 analyses). Furthermore, the mean of 8 high-grade metasedimentary rocks from the Cooma Complex occupies a similar position. It is again proposed that Cooma-like materials represent the end member. Those data points displaced to the right of the mixing trajectory in Fig. 4 are considered to have undergone some form of magmatic geochemical fractionation. They have several common features consistent with fractionation. Xenoliths are rare or absent in all, and most plutons are described as felsic. Silica contents are in excess of 71%, much higher than for most plutons on the mixing trend (Table 2) and usually in excess of the 72% interpreted for the high-silica end member. In all, 10 plutons can be separated clearly from those on the mixing trajectory and will not be considered further. It is possible that some of the other plutons have also suffered minor degrees of fractionation and that this accounts from some of the scatter in Fig. 4. The Sm-Nd data [7] can be treated in a like
.
0.718
l
0.714 8'Sr/86Sr
.
.
.
.
. l
.
.
.
. 0
. .
. .
0
0
0 0.706
.
.
0. 0.710
q
0
0
0
.* 20-
0.702.
-I 0
1
2
3
4
5
B'Rb/8BSr
Fig. 4. Initial s7Sr/s6Sr versus initial 87Rb/86Sr for granitic rocks from southeastern Australia. Solid circles = plutons on the primary mixing trajectory; open circles = fractionated plutons; solid square = mean of 3 analyses of the Cooma Granodiorite 17,111; open square = mean of 8 analyses of high-grade metamorphic rocks from the Cooma Complex [!,ll].
55
manner and are also crudely consistent with the pattern of mixing and fractionation. The plot is poorly correlated and of limited use probably because the Sm/Nd ratios of the basaltic end members had substantial range; by contrast, in the Rb/Sr system all basalts have Rb/Sr ratios close to zero.
4. Numerical modelling of isotopic mixing Isotopic mixing systems as outlined above can be described algebraically to a good approximation by two equations. Firstly the amounts of the element involved must be balanced: (l-x)-C,
+x-c,=
metasedimentaryderived magma
Secondly, equated:
the
co
basalt
Degree of
mixing(x)
0.10
isotopic
abundances
must
be
basalt
granite
where x = degree of mixing ranging between 0 and 1; C = concentration of element, ppm Sr or Nd; R = isotopic ratio, “Sr/%r or 143Nd/‘44Nd; and the subscripts M = metasedimentary-derived magma; B = basalt; and G = granite. Overall, there are seven variables and the simultaneous equations may be solved if two unknowns are carried. However, because x, the degree of mixing, lies between 0 and 1 by definition, valid solutiqns for two unknowns may be found for assigned values of x-in a sense three unknowns can be tolerated. In the present case the concentrations and isotopic ratios in the granites are known by direct measurement. As well, the isotopic ratios of the end members have been deduced from the isotopic mixing diagrams. This leaves three unknowns, x the degree of mixing and the concentrations in metasedimentary-derived melt and basalt. It is then possible to rigorously calculate these concentrations for various assigned values of x as presented in the following examples. Input data for the granites may be found in Table 2. The Bolairo Granodiorite of the Murrumbidgee
High-Si end member
Basaltic end member
(ppm)
(ppm)
Bolairo Granodioriie 111 0.05 118 0.10 132 0.20 151 0.30 116 0.40 212 0.50 265 0.60 353 0.70 529 0.80 1059 0.90 Jindabyne Tonalite 0.05
granite
(l-x)~C,R,+x~C,R,=CGRG metasedimentaryderived magma
representative granite systems
0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90
58 61 68 78 91 109 137 182 247 547
643 321 161 107 80 64 54 46 40 36
3345 1673 836 558 418 335 279 239 209 186
Batholith contains abundant schistose xenoliths [17] and has a high initial 87Sr/86Sr ratio of 0.7145. The Sr concentrations calculated for its “metasedimentary” and basalt progenitors are given in Table 3. Unfortunately, there is a wide range of acceptable solutions when the results are compared with possible concentrations: the spread of Sr concentration observed in the Cooma Complex allows an upper limit of 200 ppm for the high-silica melt giving corresponding x values from 0 to 0.5; the basalt could reasonably have a Sr content between 100 and 1000 ppm with x from 0 to 0.35. No specific solution for x is possible, but at this stage it can be said that the isotope dilution relationship is consistent with the mixing model because plausible concentrations are obtained. In order to derive a specific solution x must be estimated independently. The linear variation of major elements provides the means-given the abundance of an element in both end members and in the granodiorite x is calculated by simple
56
proportion. Silica is the most suitable parameter as it is the most abundant oxide and because it can be reliably estimated in the end members in the light of the earlier discussion. The high-silica end lies at 72% silica, and the likely range of basalts from 47% to 50% is arbitrarily averaged at 48.5%. The mean silica content of the Bolairo Granodiorite is 68.4%, hence x is calculated to be 0.16; uncertainty in x is probably about 0.05. The explicit solutions for the Sr contents of the end members in the Bolairo Granodiorite system are metasedimentary-derived melt 126 ppm and basalt 201 ppm; the latter figure is very sensitive to the value for x (Table 3). Because basalts have a considerable range of Sr contents 201 ppm is a reasonable result, but certainly not definitive. The high-silica end member at 126 ppm can be compared favourably with three observational esti-
mates (a) Cooma Granodiorite-124 ppm; (b) mean of 11 metasedimentary rocks in the Cooma Complex-121 ppm [7,10]; (c) mean of 14 Ordovician-derived granites-126 ppm [7]. The Murrumbidgee Batholith is rather uniform in composition and similar results to those for the Bolairo Granodiorite are obtained for four other plutons (Table 4); the calculated high-silica end member is well defined with a mean 122 ppm Sr and the basalt is more variable at 200-311 ppm. It is taken to be highly significant that the Murrumbidgee Batholith immediately to the north along strike from the Cooma Complex (Fig. 1) yields Sr parameters for its metasedimentary-derived component identical to those in the Cooma Complex where granite generation can be observed. The Jindabyne Tonalite of the Kosciusko Batholith is a contrasting situation, with the rock a
TABLE 4 Explicit solutions for end member Sr and Nd concentrations Pluton
Murrumbidgee
x
High-% end member
Basaltic end member
Sr(ppm)
Sr(ppm)
Nd(ppm)
Nd(ppm)
Batholith
Bolaiio Granodiorite Callemondah Granodiorite Clear Range Granodiorite Stewartsfield Granodiorite Willoona Tonalite
0.16 0.15 0.15 0.21 0.16
126 120 109 120 136
0.29 0.41 0.15 0.22 0.35 0.03 0.25 0.37
71 88 101 87 84 94 81 80
0.20 0.19
111
201 285 311 288 200
Kosciusko Batholith
Gaden Tonalite Grosses Plain Tonalite Ingebyrah Granodiorite Jillamatong Granodiorite Jindabyue Tonalite Kalkite Adamellite Pendergast Tonalite Round Flat Tonalite
15 34 28 20 27
571 449 570 318 478 659 741 469
13 22 3 15 60
Berridale Batholith
Bimbimbie Granodiorite Cootralantra Granodiorite BB19 (west) BB83 (east) Currowong Granodiorite Dalgety Granodiorite Finister Granodiorite Iona Tonalite Merumbago Granodiorite Tara Granodiorite
0.27 0.15 0.28 0.37 0.29 0.21
100 119 99 89 99 103 104 67
30 31
24 31 26 21 26 19
753 280 198 634 610 207 558 239 976
24 5
23 38 5 24 1 46
57
member of a hornblendic suite, containing diorite xenoliths, and having lower silica contents and initial 87Sr/86Sr ratios. The possible Sr concentrations in the end members (Table 3) have an even greater range of feasible solutions than before. The mean silica content of 63.9% corresponds to an x value of 0.35; accordingly the basaltic end member has 478 ppm Sr and the “metasedimentary” 84 ppm with the former number again sensitive to X. Comparable results are obtained in seven other plutons from the batholith which represent both the suites described earlier and which run the gamut of granitic compositions (Table 4). The basaltic component is again quite variable in Sr concentration (318-741 ppm); the two gabbros from the area (47.3 and 52.1% SiO, [2]) which lie on the projection of the geochemical trends of the granites at least have consistent Sr contents (438 and 290 ppm respectively). The Sr concentrations of the high-silica component are distinctly lower than the 122 ppm found in the Cooma and Murrumbidgee setting, but are conceivable in a metasedimentary rock-mean 86 ppm. The important conclusions for the Kosciusko Batholith are that the outcome of the calculations is independent of rock type and that a single metasedimentary-derived component is present. The plutons of the Berridale Batholith are intermediate and record both the high- and low-Sr metasedimentary sources identified above (Table 4). This is geologically reasonable because while the greater part of the batholith is on strike with the Murrumbidgee Batholith (high Sr source) some western plutons lie on the continuation of the Kosciusko Batholith (low Sr source). The Cootralantra Granodiorite is recognisably one rock type in hand specimen although numerous plutons exist. The two initial 87Sr/86Sr ratios available differ considerably, in the east 0.7138 and in the west 0.7121; the equivalent Sr concentrations in the high-silica component are 119 and 100 ppm respectively. These results are in harmony with their position relative to the Murrumbidgee and Kosciusko Batholiths and it is concluded that there is a systematic change in the composition of metasedimentary-derived material across the region. No calculations are presented for the Moruya
Batholith because all rocks have very low initial 87Sr/86Sr ratios and the outcome is unacceptably sensitive to the choice of isotopic compositions in the end members. With appropriate juggling, using less radiogenic end members, solutions consistent with the model can be obtained. No conclusions can be drawn because such rocks are arithmetically unsuitable for this approach. An identical isotope dilution method can be applied to the Nd data [7] although there are fewer measurements which are limited to the Kosciusko and Berridale Batholiths. As a brief example, the Jindabyne Tonalite for its specific degree of mixing (x = 0.35) gives Nd concentrations of 15 ppm in the basalt and 20 ppm in the high-silica end member. Other plutons give similar results (Table 4) with the basalt heterogeneous, varying from 1 to 46 ppm Nd, and the metasedimentary-derived component more uniform and averaging 26 ppm. The deduced Nd concentrations are reasonable; basalts do have considerable span of Nd concentrations which includes these values; the highsilica result compares well with one analysis of 23 ppm reported for the Cooma Granodiorite [5].
5. Discussion
A hybrid or mixing origin for some granitic rocks was first indicated by initial 87Sr/86Sr ratios intermediate between mantle and mean continental crustal values [18]. Correlations between Sr initial ratios and St80 values [19,20] were more direct evidence of mixing and indicated that individual plutons must have formed from different combinations of end member chemical compositions. Most Nd-Sr data, while consistent with mixing, do not give well-defined hyperbolic trends. A global survey [21] led to the conclusion that mixing between a mantle-derived diorite magma and crustal rock could be involved. A slightly curved trend for the Sierra Nevada Batholith is consistent with interaction between island arc magmas and Palaeozoic or Precambrian basement [22]. The granitic rocks of southeastern Australia are of particular significance because they preserve a more coherent geochemical and isotopic pattern than other cases in the literature. As a result a
58
mixing system can be discerned clearly; the end members are interpreted here to be a granitic melt derived from metasedimentary rocks comparable to those at Cooma and basaltic material. The main requirement for coherence in a mixing system is compositional homogeneity in the end members, a condition which must have been peculiarly satisfied in the Lachlan Orogenic Belt. The composition of an individual granite body was determined by four factors: (a) the nature of the metasedimentary-derived granitic melt; (b) the nature of the basalt; (c) the degree of mixing of granitic melt and basalt; and (d) the extent of geochemical fractionation superimposed upon mixing. The metasedimentary-derived melt component was common to all the granite systems and was the unifying factor in their compositions as shown by the similarity of Cooma-type granitic rocks over a considerable area (Table 1). This might seem surprising given the ultimate metasedimentary derivation of the rocks, but the primary cause was the regional homogeneity of the basement flysch sequence formed by marine currents mixing and depositing sediment in sheets on a vast scale. Any local chemical variations that might have developed would have been erased subsequently by mechanical mixing during partial melting and magma migration. The basaltic component can be expected to have been isotopically uniform given the limited range of isotopic compositions in common mantle-derived rocks. Combined with a welldefined overall composition for the sedimentary rocks this will guarantee coherent isotopic mixing trends. However, the major element composition of basalts is variable and it must be supposed that essentially one basalt type was involved in the genesis of an individual granite suite in order to create linear geochemical variation. On the other hand, different basalts generated the various suites with their calcium content of particular significance; in the Kosciusko Batholith a high-Ca basalt gave rise to the high-Ca suite that precipitated hornblende over much of its compositional range; a low-Ca basalt produced the other suite in which hornblende does not occur. Thus the geological circumstances of this terrain do seem to have favoured the development of a remarkably simple
mixing system amenable to numerical treatment. However, the model as presented can only be an approximation to a process as complex as granite formation. In particular, there must have been local variation in the compositions of the end members which will limit the literal interpretation of the calculations. The spectrum of granitic compositions within a suite was produced by variable mixing between the two end members. In a general way the sequence from adamellite to granodiorite to tonalite corresponds to increasing basalt component. A good example is the relationship between the Cooma, Cootralantra and Finister Granodiorites (Table 1) of which the first two are cordierite-aluminosilicate-bearing (S-type) and the third hornblendebearing (I-type). The Cooma Granodiorite is the end member of the mixing system and the other plutons progress from this composition towards the basaltic end with the distinctive common feature of a very low Sr content in the basalt component (- 200 ppm-Table 4). The change to hornblende in the mode of the Finister Granodiorite is simply due to a slight increase in Ca coupled with a significant decrease in SiO, along the trend. Previously, no link has been recognised between the Finister Granodiorite and the other bodies and Finister has been regarded as an unrelated I-type [5]. In fact, it is unnecessary to postulate fundamentally different granite types using mineralogy when a mixing relationship is considered; any suite will develop modal hornblende at some stage asI the basaltic component increases. The subsequent history of batches of granitic magma made in this way can be remarkably simple. The Rb-Sr isochron plot (Fig. 4) demonstrates that numerous magma bodies once formed are unmodified by processes such as crystal fractionation or contamination that can change Rb/Sr ratios. Of 33 plutons investigated 23 retain their original Rb/Sr ratios and must have had simple histories from generation to emplacement. Obviously, there is some late and local internal fractionation in these cases as evidenced by occasional rocks with anomalous Rb/Sr ratios and the existence of aplite dykes. Grossly fractionated entire plutons are common nonetheless, and must be recognised if the geochemistry of granites is to be
59
understood (the isochron diagram-Fig. 4-is a simple means of their identification). Such rocks originate by mixing to a set composition on linear geochemical trends between 48.5% and 72% silica as described above. Crystal fractionation then drives this composition towards the minimum melt of the granite system which lies close to 76% silica; the trajectory may be straight or curved depending upon the mechanism of fractionation. The difference between trends directed towards 72% and 76% silica is often small and in many cases it is difficult to distinguish pure mixing from mixing plus fractionation using elemental abundances alone. However, in cases where geochemical variation has a high gradient significant divergence may result. The complications introduced by the superimposition of mixing and fractionation paths greatly restrict the literal use of such trends. The physical processes of granite formation are a matter of speculation. A simple view begins with the eastern margin of Australia during the early Palaeozoic composed of a massive accumulation of flysch. Subduction beneath the margin caused basaltic magmas to rise into the crust as an arcuate strip, the magmas invading the turbidites to form dolerite dykes and gabbro intrusions which decreased in abundance towards the surface. Heat from the cooling basaltic material induced hightemperature/low-pressure regional metamorphism and ultimately partial melting of the metasedimentary rocks to form granite magmas. Where basaltic material was absent (at the highest levels?) pure metasedimentary-derived melts were formed. At depth, increasing amounts of basaltic material as solid dykes or as freshly-injected magma became involved in crustal melting producing granitic magma batches of varied mixture compositions. The physical and thermal difficulties of mixing limited the amount of the basaltic component to less than 40% in most instances and truly intermediate compositions were rare. Changes in the pattern of subduction and basalt generation with time produced parallel strips of crustal melting, each strip the site of a granitic geochemical suite. The magma bodies were actually a composite of granitic melt, fragments of refractory horizons from the metasedimentary rocks (quartzite, biotite-rich schists), and unmixed basaltic material. The latter
exotic materials became xenoliths and their fate was determined by the composition of the melt. If the melt was peraluminous, basic xenoliths were preferentially destroyed by the reaction hornblende + melt + biotite and metasedimentary xenoliths survived; if the melt was metaluminous hornblendic xenoliths were retained and metasedimentary types dissolved. Magmatic flow during diapiric ascent homogenised magma batches and crystallisation processes developed the rocks we see today.
References
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A.J.R. White, IS. Williams and B.W. Chappell, Geology of the Berridale 1: 1OOOOO sheet 8625, 138 pp., Geological Survey of New South Wales, 1977. R. Hine, IS. Williams, B.W. Chappell and A.J.R. White, Contrasts between I- and S-type granitoids of the Kosciusko Batholith, J. Geol. Sot. Aust. 25, 219-234. 1978. T.J. Griffin, A.J.R. White and B.W. Chappell, The Moruya Batholith and geochemical contrasts between the Moruya and Jindabyne suites, J. Geol. Sot. Aust. 25, 235-247, 1978. A.J.R. White and B.W. Chappell, Granitoid types and their distribution in the Lachlan Fold Belt, southeastern Australia, Geol. Sot. Am. Mem. 159, 21-34, 1983. B.W. Chappell and A.J.R. White, Plutonic rocks of the Lachlan Mobile Zone, 25th Int. Geol. Congr., Field Excursions Guide 13C, 40 pp., 1976. W. Compston and B.W. Chappell, Sr-isotope evolution of granitoid source rocks, in: The Earth: Its Origin, Structure and Evolution, M.W. McElhinny, ed., pp. 377-426, Academic Press, London, 1979. M.T. McCulloch and B.W. Chappell, Nd isotopic characteristics of S- and I-type granites, Earth Planet. Sci. Lett. 58, 51-64, 1982. B.W. Chappell and A.J.R. White, Two contrasting granite types, Pacific Geol. 8, 173-174, 1974. J.C. Roddick and W. Compston, Radiometric evidence for the age of emplacement and cooling of the Murrumbidgee Batholith, J. Geol. Sot. Aust. 23, 223-233, 1976. R.T. Pidgeon and W. Compston, The age and origin of the Cooma Granite and its associated metamorphic zones, New South Wales, J. Petrol. 6, 193-222, 1965. G.A. Joplin, Petrological studies in the Ordovician of New South Wales, I. The Cooma Complex, Proc. Linn. Sot. N.S.W. 67, 156-196, 1942. R.H. Flood and R.H. Vernon, The Cooma Granodiorite, Australia: an example of in situ crustal anatexis?, Geology 6, 81-84, 1978. A.J.R. White and B.W. Chappell, Ultrametamorphism and granitoid genesis, Tectonophysics 43, 7-22, 1977. D.J. DePaolo and G.J. Wasserburg, Petrogenetic mixing
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