The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks

0016-7037/8S/f3.00

Geochimica et Cosmcchimtca ACIU Vol. 49. pp. 369-384 0 Pcrgamon Press Ltd. 1985. Pnnted in U.S.A.

+ .IlO

The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks H.-D. HENSEL’,M. T. MKULLOCH’ and B. W. CHAPPELL’ ‘Department of Geology, AustralianNational University Canberra, A.C.T. Australia. *Research School of Earth Sciences, AustralianNational University, Canberra,A.C.T., Australia. (Received November 4, 1983; accepted in revised form October 23, 1984)

Abstract-Nd and Sr isotopic compositions are reported for the granitic suites which comprise the late Palaeoxoic to earliest Mesozoic New England Batholith of eastern Australia. Some of the granitic suites are typically l-type in their mineralogy, chemistry and isotopic compositions, implying a derivation from igneous (infracrustal) source rocks, whereas other suites have characteristics consistent with a derivation from a protolith which was predominantly sedimentary and relatively felsic (S-types). The I-type granitoids of the Nundle Suite have cNdvalues (+3.3 to +6.1) that are amongst the most primitive yet documented for a relatively felsic (SiOr - 65%) plutonic suite and them values imply a derivation from either a depleted upper mantle source or, more probably, a complex source region involving both volcanic-arc rocks and detrital material. Their compositions am distinctly more primitive than those of the New England Super-Suite which constitutes the Permian ‘core’ of the batholith. This extensive Super-Suite (comprising granitoids traditionally designated as I-type) has an overall rangein initial Nd and Sr isotopic compositions of - 1.7to +4.6 and 0.70458 to 0.70624 respectively, although the majority of plutons have initial Nd isotopic compositions which fag into a very narrow range (+ I.0 21.5 c units). This limited range is remarkable considering the extreme iithological diversity and range in chemical composition of the analysed samples (Si02 47%74%) and implies a source region of considerable volume having reasonably uniform isotopic compositions but variation in chemistry. A similarly uniform source isotopically is also indicated for the S-type gmnitoids of the Carboniferous Hillgrove Suite and CarboniferousPermian Bundarra Suite with initial 6,.,,, values of +0.8 to +2.3 and initial %r/%r compositions of 0.70474 to 0.70577 showing only limited ranges. Five pelites, three ‘felsic’ and four ‘mafic’ greywackes, representing typical country rocks from different stratigraphic levels have initial cNd values (-1.7 to +6.6) and initial %r/%r compositions (0.70378 to 0.70585) which essentially mirror the compositional variation in the granitoids. A chemical, mineralogical and isotopic bimodality in these sediments indicates two very distinctive sources, one felsic (rhyodacitic) the other relatively mai% (andesitic), which were separated spatially and temporally in many but not all areas of early- to mid-Palaeozoic New England. A model is presented in which the S-type granitoids are derived from a predominantly felsic source, i.e. pelites and ‘felsic’ greywackes, whereas some of the granitoids belonging to the New England Super-Suite may have been derived from source rocks consisting of both felsic and mafic sedimentary components. The distinction between many Stype and l-type granitoids in New England is unclear for two principal reasons:(a) because the granitoids and their respective source rocks are relatively young geologically so that their isotopic systems have not evolved to any considerable extent, and (b) because of the intrinsic igneous chemical compositions of any sedimentary component that may be involved in their genesis.

1. INTRODUCllON

A FUNDAMENTAL problem faced by igenous petrologists is the characterisation of granitoid source rocks. Mineralogical, chemical and field characteristics are generally sufficient for a first-order recognition of the source rock type (CHAPPELL and WHITE, 1974: Is HIHARA, 1977). Isotopic data may provide a further refinement in our understanding of source rocks and place new constraints on the evolution of the continental crust into which the granitic rocks were emplaced. The eastern part of the Australian continent (Tasman Orogenic Province, Fig. 1) was intruded during the Palaeozoic by very large volumes of granitic rocks. Numerous lines of evidence indicate that some of these granitic rocks (the S-types) were derived from crystalline basement, sedimentary rocks derived from such terranes, younger but weathered (ix ma369

ture) metasediments, volcanogenic sedimentary rocks or a section containing some combination of these (COMPSTON and CHAPPELL, 1979; SHAW and FLOOD, 198 1; HENSEL,1982; CHAPPELL,1984). This is consistent with the general conclusion reached by other workers on granitic rocks (e.g. ALL~GREand BEN OTHMAN, 1980; HAMILTONet al., 1980; DEPAOLO, 1980) that crustal components may be important participants in granitoid genesis. The recognition of these crustal components in many studies has been relatively straight-forward, in that the granitoids had high initial *7Sr/86Srand relatively low ‘43Nd/‘UNd ratios. In contrast, granitic rocks with I-type characteristics and more mantle-like isotopic ratios, i.e. low initial 87Sr/%r and high ‘43Nd/‘UNd, have been interpreted as having been derived either directly from the mantle or from crystalline igneous material to which the contribution of a recycled (sedimentary) component was negligible (MCCULLCKHand CHAPPELL, 1982; CHAPPELL, 1984).

370

H. D. Hensel. M. T. McCulloch and 8. W. Chappell

eluded that the granitoids belonging to at least some of the Permian New England Super-Suite [equivalent to the ‘New England Batholith sensu strictc)’ (WILKINSON, 1969) and traditionally designated as ‘ftype’] were derived from crystalline igneous source material with little or no involvement of supracr~sta~ rocks by either contamination or assimilation. This paper has three objectives; first, to document the Nd and Sr isotopic data for a representative selection of rocks from the New England Batholith, and second, to use this information to test the hypothesis that sedimentary rocks derived from Devonian volcanic arcs (immediately west and east of the present position of the ~tholith) are appropriate source material for these granitoids. Third, this paper attempts to demonstrate the difficulty in ‘pigeonholing’ granitoids. For example, whilst it is relatively easy to identify granitoids derived from metasedimentary crustal material there is no simple scheme for determining the precise source rocks of granitoids with I-type mineralogical. chemical or isotopic characteristics. 2. REGIONAL GEOLOGICAL

FIG. I. Map of eastern Australia showing the location of the New England and La&an Fold Belts within the Tasman Orogenic Provinoc. Boundaries of province and sub-province after RUTLAND(1976). The New England Batholith (boxed) is shown in relation to the two principal divisions (N and S) of the New England Fold Belt (shaded). Inset (a) shows the location of the New Enand Batholith in relation to the southern (S) portion of the New England Fold Belt and inset (b) shows the ~eosraphical division of this southern portion into two lithostratigraphic zones (A and B). Heavy lines indicate major faults.

OUTLINE

The New England Fold Belt (Fig, 1) is a major structural element within the extensive Tasman Orogenic Province in eastern Australia. This Palaeozoic fald belt is itself composite (LEITCH, 1974) and consists of a smaller northern part separated from the relatively larger soutbem part by the Mesozoic Clarence-Moreton and Eromanga Basins. In this report, only rocks from the southern part are considered. Recent summaries of plate tectonic models for the development of the New England Fold Belt have been presented by LEITCH( I974), RUNNEGAR( 1974) and SCHEIBNER ( 1976). LEITCH (1974) divided the southern portion of the New England Fold Belt into two major structural and lithostratigraphic zones, A and B (Fig. I, inset b). The two zones are separated by the Peel Fault, a steeply dipping arcuate fault zone containing many large fault-bounded lenses, pods of massive and s&&se serpentkite (har&urgite and wehrlite), mafic intrusives (gabbros. dolerites, rodingites and diorites), and exotic sedimentary blocks ranging in age From Ordovician

to Permian. K/Ar data on nephrites associated with blocks of ~~ntinite (LANPHEREand HCXKLEY,1976) suggest an

in some parts of Austratia the distinction between the S- and I-type granitoids is relatively simple. For example, S-types from the Lachfan Fold Belt (Fig. 1) typically have high initial *‘Sr/%r ratios (>0.708), IOW t&j VaiUeS (< -8), r’sO > 10, high molecular A1203fNa20 + K20 -I- CaO (> 1.1) and mineralogical features which reflect the chemically mature nature of the sedimentary parent material (CHAPPELL and WHITE, 1974; O’NEIL and CHAPPELL, 1977; COMPSTON and CHAPPELL, 1979, MKXJLLOCH and CHAPPELL, 1982). In New England the S-type granitoids have Sr isotopic signatures which are indistinguishable from granitoids with I-type characteristics. This is consistent with the views of HENSEL (1982) who suggested that both the S- and I-type granitoids of New England might have been derived from similar source rocks. This interpretation is at variance with other workers’ intetp@ations (e.g. CHAPPELL, 1978; SHAW and FLOOD, 1981) who independently con-

emplacement age of -274 m.y. Zone A (Fig. I ) is a narrow. approximately

meridional belt on the western side of the Peel Fault. It is characteris& by zeolite to prehnite-pumpellyite facies metamorphism, low-angle thrusting and only moderate folding. The rocks of Zone A, estimated to be a sequence 12-13 km thick (LEITCH. 1974; CAWOOD, 1982) are predominantly volcanogenic, having been derived mainly from an Early- to MidPalaeozoic western source, presumably a volcanic chain situated along the western margin of this zone (CHAPPELL, I % I ). Widespread ash-fall tufi and continuous but relatively thin ash-flows indicate that volcanism was contemporaneous with sedimentation during the Devonian and Carbonifenws. East of the Peel Fault (Zone B) the rocks are generally strongly deformed, tightly folded and have undeqone regional metamorphism of at ieast prehnite-pumpellyite grade. Tbe majority of rocks are volcanogenic mywackes, siltstones and mudstones, interbedded in the central and southern parts with locally abundant basaltic lavas and cherts. The New England Batholith is a prominent component of the New England Fold Belt, covering an area of almost 20.000 sq km (Fig. 2). It is the largest of a number of N-S trending granitoid complexes occurring along the eastern

The New England Batholith

B NE

Bundarra

371

Suite

New England Super-Suite

N

Nmdle

T

Transitional

Suile

$3

Post- oroffenic

pranitoids granitoids

FIG. 2. Map of the southern portion of the New England Batholith showing outlines of the individual plutons within the major intrusive suites (boundaries marked by dashed lines). Dots indicate sample locations and numbers correspond with sam&s in Tables 1 and 2. Inset map shows location of map area in relation to Australia.

margin of Australia (Tasman Orogenic Province) and it intrudes stratified rocks dominantly belonging to Zone B with a small transgression into Zone A. The batholith comprises at least six petrographically, mineralogically and chemically distinct plutonic suites (summar54 by SHAW and FLWQ 198 1) ranging in age from Carboniferous (Hillgrove) through Carboniferou&ermian (Rmdarra) and Permian (New England and Nundie) to Triassic (New England Post-Orogenic). In addition to these granitoids, a very distinctive series of chemically and isotopically primitive tholeiitic intrusives, dominated by olivine gabbros with diorites, and kisser granophyres (HENSEL, 1982; HENSELet

al.. 1982) were emplaced during the Permian along the entire eastern margin of the New England Batholith. The occurrenceof these mafic rocks is probably related directly to deep crustal fractures, presumably created during the Early to Mid-Permian orogeny which upiifted and deformed the gmnitoids of the Hi&rove Suite (HENSEL, 1982). 3. FIELD AND PEI’BOCR4PHIC CHABACTEIUSTLCS Of the six suites only the Hillgrove and Bundarra Suite granitoids exhibit field and petrographic characteristics that

372

H. D. Henset. M. 1. McCulloch and B. W. Chappel!

are obviously consistent with an S-type classification. The Nundle Suite granitoids and some plutons from the New England Super-Suite have features which are typically Itype; however. others from the New England Super-Suite (and those designated as transitional) show features that are appropriate to both S- and I-type granitoid classes. The New England Post-Orogenic granitoids also are a diverse group in that they possess characteristics of both I-type and A-type (COLLINSn ai., 1982). ~jl~grove Suite. All 21 members of this suite are either adamellite or granodiorite. They are typically fine- to medium-grained, sparsely potphyritic and have a well-developed tectonite fabric supenmposed onto an earlier foliation. Mineralogical features include ubiquitous graphite, co-existing reddish-brown and green biotite, two generations of plagioclase feldspar, highly ordered alkali feldspar (microcline). ilmenite as the sole iron-titanium oxide, co-existing colourless and blue varieties of quartz, the presence of Fe-Mg and calciferous amphiboles and modally minor almandine-rich garnet. Bundurra Suite. In contrast to the previous suite, Bundarra Suite granitoids are predominantly adamellite, quite felsic (-73% SiOz), fine- to very coarse-grained and extremely porphyritic. They lack amphibole, have only one generation of plagioclase and quartz and typically contain either fresh or extensively pinitised cordietite. Most samples display prominent rapakivi textures. Biotite occurs in aggregates with ilmenite and is not uncommonly intergrown with muscovite. Mafic samples sometimes contain small amounts of almandine-rich garnet. The textural and chemical uniformity is notable along the 220 km of this elongate N-S trending suite. Ken England Super-Suite. This Super-Suite constitutes the batholithic ‘core’ and is character&d by much greater textural. mjne~l~~l and chemical variation than any of the other major suites, It embraces a number of distinct. individual suites in its 400 km extent and also includes extrusives. Most intrusives are sparsely porphyritic although some samples contain abundant aikah feldspar phenocrysts up to 5 cm long. The fe~omagnesian minerals have two contrasting modes of occurrence: firstly, as solitary grains displaying well-formed crystal outlines (indicating complete contact with the melt phase). and secondly. as clusters of anhedral grains. One very prominent textural feature displayed by some indjvidual suites is the distinctive zoning in plagioclase. in which the cores are quite calcic and clouded (due to abundant alteration products) whilst the surrounding rims are clear and unaltered. Sphene is a prominent accessory. accompanied by acicular apatite and occasionally multiply zoned allanite. With only a few exceptions. itmenite is a ubiquitous oxide phase, joined in some individual suites by K-poor magnetite. Nundle Suite. The most important textural feature of the fine- to medium-grained granodiorites and tonalites from this suite is that all minerals appear to have crystallised rapidly from a melt. Strongly zoned plagiociase, a marked paucity of alkali feldspar and a dominance of magnetite over ilmenite typify these rocks. The quartz, and particularly the alkali feldspar, are clearly interstitial to the early-formed piagioclase (and pyroxenes where present). Prismatic sphene. common thorite inclusions, euhedral apatite and relatively large, perfectly-shaped prismatic to acicular zircons dominate the accessories. New England Post-Orogenic Granitoids. Members of this group show a considerable range in mine~l~cal. petrographic and held characteristics. Some are very felsic (-74% SiOa), contain abundant alkali feldspar, and occur as large, roughly circular masses. Others are very similar to plutons of the New England Super-Suite. The major exception is the Billys Creek Tonalite, a small elliptical pluton which consists of a number of separate intrusive phases ranging from mafic diorite to leucoadamellite. The dominant tonalitic variant is fine-grained, sparsely porphyritic. poor in alkali

feldspar and, m common with the leucocratic plutons of this association, shows evidence of considerable late-stage hydrothermal activitv. ~ran~jt~~na~ Gra&& The Gara Adamellite is one ol two plutons which share many of the characteristics of the Hillgrove Suite and the New England Super-Suite. It is lineto medium-grained. sparsely porphyritic and contains small amounts of orthopyroxene and a brown amphibole in addition to a ~ddish-brown biotite. The abundances of blue quartz, graphite, actinohtic amphibole, pink fluorite and unordered alkali feldspar are quite variable within the pluton. Magnetite and sphene are conspicuously absent despite the considerable variation in the (bulk rock) ratio of FezOs/ FeO. Although the mineralogical variations tend to be subtle and apparently nonsystematic. textures change dramatically from the southern margm. where rocks display the strong cataclastic foliation typical of the Hillgrove Suite. to the northern margin where they are massive and contain miarolitic cavities. 4. ANALYTICAL

PROCEDURES

The analytical procedures followed in this report have been detailed in MCCULLOCHand CHAPPELL(1982). Briefly, about 100 mg of rock powder was spiked with mixed ‘“‘Sm/ ‘%Nd and 8JRb/s’Sr tracers and dissolved in teflon bombs over a period of 4 days (at 205°C) using HF, HC104 and HCI. After centrifuging, Rb. Sr and REE were separated initially using 5 g cation exchange columns, then purified on smailer columns. Sm and Nd were separated using 5.2 M dimethyl-lactic acid as the elutriant. Mass analyses for Rb. Sr. Sm and Nd were carried out on three different mass-spectrometers at the Australian National University. Interferences of i’*Ce and ‘“Sm to the measurement (as metal) of ‘~3Nd/‘uNd were closely monitored and were negligible. Corrections to the “Sr/% measurement resulting from “Rb was generally less than 0.01%. After normalisation of “Sr/‘*Sr to 0.1194, the Sr isotope results were standardised to eliminate machine bias to a value of 0.71027 for NBS 987. Following WASSERBURGrf al. (1983). Nd isotopic ratios were normalised to ‘%Nd/ ‘42Nd = 0.636151. Age uncertainties are quoted at 95% confidence level; other uncertainties in isotopic measurements as listed in Table 1. The t notation follows that of MCCULLOCH and WASSERBURC; (1978) and where quoted refers only to inittul values. L’n1c.r.rspeqtied, unppubiished data ret&~ to work carned out hl, IIDH

5. RESULTS Hillgrave Suite. The Sr and Nd isotopic data for plutons belonging to the Hillgrove Suite are listed in Table I and displayed on a diagram of tSt vs cNdin Fig. 3. The analysed samples are widely dist~but~ geographically (see Fig. 2). span the known range of chemical and lithological compositions, and are believed to be representative of the entire Hillgrove Suite. Detailed discussion of the age of this suite appears elsewhere (HENSEL, 1982). Briefly, the oldest biotite age is 293 It 6 m,y. This must be regarded as a minimum estimate of the crystahisation age because the Rb-Sr ages of ail measured Hiilgrove Suite biotites have been reset by variable amounts during one or more later thermal events. A total-rock Rb-Sr isochron, using a much larger sample population (33) than the eleven in Table 1. gives an age of 312 + 10 m.y. (HENSEL, 1982). This estimate of the crystalfisation age involves the assumption that all members of the suite were emplaced at the same time. The

313

Rundorm

England

Round lu11ys

2.422 4.433

0.71611 0.72404

PC%+-oaq7enic

cmi

6.64 6.97

30.8 30.7

0.131 0.138

0.511667 0.511714

0.7

184.6 130.3

18.4 29.8 35.8

0.131

1 2

1.8 2.2 0.9 0.9 -0.1 0.2 1.8 0.9 1.2 1.1 0.1

3 4

955 951 1188 1126

5 6

930 957 P4E 987 1012

7 8 9 10 11

1107 lil6

I2

697 830

14 15

1028

16

<29On.y.)

Bmal~aeIx GlMIW+*r hw

16.7 40.5 11.3 7.6 19.8 13.4 13.1 4.9

154.7 199.8

Suite

24.2 30.‘

23.7 18.3

0.71670 0.84166 0.75304 0.71325 0.71623 0.72157 0.71585 0.72689

G419 6385

llenry River TU Garibaldi

0.511795 0.511858 0.511742 0.511745 0.511726 0.511743 0.5118oi 0.511741 0.511759 0.511767 0.511699

5.39 6.38

184.0 131.0 175.0 105.7

32.56 12.07 1.879 2.360 3.680 2.380 4.902

177.1 25.1 67.5 189.2

2.569 1.639 3.240 2.516

0.150 0.128 0.128 0.145 0.143 0.135 0.128 0.130 0.135 0.127

31.3 26.8 28.7 28.4

4.4 5.3 17.8

151.8 171.2 153.8

853 DuNu2X HA CT5 CT13 *NIX KMM TQEA mA In2 G,%

6.75 6.63 6.08 6.02 4.43 7.05 6.89

0.71607 0.71203 0.71997

134.8 97.1 172.4 154.2 278.7 280.9 123.1 147.0 166.0 144.0 182.1

Dundurrebia 0 Hillgrove II II 1ng1eb. Xwber1ey T*be-Zy

R”2 DSGRl

Hount.in Creek

toia

su !ite

-0.9 -0.2

13

(232n.y.)

228.7 160.2

33.6 283.1

19.78 1.635

0,76985 0.70967

-3.5

4.27 4.35

17.7 22.2

0.146 0.119

0.511989 0.511773

4.5 1.1

LeucOdaYllite Adamuite

166.7

151.5

3.180

0.71930

19.6

6.77

31.2

0. I31

0.511716

0.2

Adurllfte

66.4 38.7 82.0 ‘6.5 103.5

712.9 620.0 591.1 396.3 246.3

0.289 0.180 0.400 0.335 1.212

0.70494 0; 76457 0.70546 0.70483 0.70889

2.62 1.98 3.4L 3.02 4.18

15.2 11.3 20.1 23.1 18.3

0.104 0.106 0.103 0.132 0.138

0.511903 0.511903 0.511846 0.512039 0.511s96

4.6

561

4.3 3.3 6.1 3.3

573 630 510 796

611.2 303.9 650.5 146.1 141.7 215.5 368.8 225.0 242.5 337.6 316.9 270.0 376.2 155.9 464.2 890.3 621.9 45.1 257.6 430.2 300.3 305.5

0.627 1.406 0.962 2.641 4.663 3.195 1.489 2.011 1.992 0.856 0.619 1.256 0.818 1.234 1.230 0.591 I.123 18.08 1.663 0.w 1.112 1.357

50.2 29.7 49.2 27.8 28.2 33.9 25.9 29.0 27.2 28.8 33.0 29.8 30.6 16.7 33.6 37.0 24.9 25.1 24.9 32.6 31.4 27.4

0.125 0.119 0.114 0.136 0.118 0.123 0.109 0.122 0.123 0.120 0.149 0.119 0.121 0.138 0.106 0.101 0.102 0.1%

0.511779 0.511743

1.3 0.8

0.511726 0.511966 0.511615 0.511826 0.511753 0.511736 0.511739 0.511704 0.511813 0.511701 0.511785 0.511784 0.511742 0.511792 0.511761 0.511807

0.6 4.6 -1.7 2.2 1.3

0.118 0.117 0.123 0.122

0.511754 0.511783 0.511698 0.511719

TmmitimaZ(29Om.y.) Gara NwrdEs

HH142 Suite

(265m.y.)

“t. Ephraim Gage Top rlunc.ns Creek Barrini$on Tops EofkiaIe New Englmrd

mu04 CJxs HH302 BTB RIGIM

Super-Suite

Kentucky Terrible Vale Highlands GlenbUrnie man River 1‘ Walchs Road Wards ,,i~take 9, KhartO”Il 0 “ilhhalF.Eb*be Back Creak BendePmer * noonbi Inlet 1, l.ooulga * Sbrlivr I* Aberfoyle Oundee

Y.!JO PTVPT HM208 GG-I OP3.A OllwLx WRA w42 WA39 KT KTXIG UT f.CXT3 G223 G447 6432 640 1 6415 SET1 sHT2 88284 OR

-7.9 -8.3 -7.5 -12.9 -9.0

(265m.y.) 132.8 148.0 216.7 133.3 228.6 238.3 190.1 156.6 167.3 100.0 67.9 117.4 106.6 202.2 197.6 180.1 241.9 281.0 148.5 125.1 117.1 143.5

0.70860 0.71113 0.70846 0.71581 0.72224 3.71636 0.71045 0.71264 0.71247 0.709Y) 0.70846 0.71078 0.70870 0.72355 0.7OSll 0.70681 0.70886 0.76988 0.71194

24.0 22.0 5.9 19.5 7.2 0.5 5.1 10.1 6.8 22.6 23.3 22.2 16.1 1.0 8.9 1.4 2.1 15.4 16.9

0.70995

18.0

o.rn83.s 0.70980

13.0 2.9

10.36 5.86 9.29 6.24 5.53 6.99 4.35 5.84 5.54 5.71 8.14 5.86 6.14 3.81 5.92 5.93 4.19 5.65 4.02 6.27 6.40 5.52

Salnpla CT13 and INCA ulculated at 290 m.y. lBendemeer Adaelllfe individual total-rock Rb-Sr lsocbrons. tNd zmdel age using dpplrted men~lc (IM) ‘mr-ters : “‘Sm/‘“Nd

I

Errors

equal

in

thhe “Srf”Sr

17 18 19 20 21

and

“‘Rdl’”

Nd IDCasur==ents

.fc

generally

and

mean initial *‘Sr/%t for the 33 samples is 0.70526 +- BOO35, with some scatter in excess of experimental error (MSWD = 8.6). The 3 I2 ? IO m.y. age and initial ratio are believed to be better estimates than the less precise age of 289 + 25 m.y. and initial 87Sr/ %r of 0.7064 + .0009 for a 7-point (6 pluton) totalrock isochron reported by FLQODand SHAW(1977). It must b-e emphasised that the two samples having “Rb/“Sr in excess of 10 (CT 13 and INGA) have larger un~~inti~ in their calculated initial *‘Sr/ *%r and were excluded from the isochron because field and petrographic evidence suggest that their Rb/ Sr systems have been significantly disturbed by later thermal and deformation events. The total range in initial TV vatues is less than 2 t units (from -0. I to + I .8). This limited variation in tNd (and tsr) is significant because it requires the source rocks to have bad a reasonably uniform isotopic and chemical composition over a considerabje (-2000 km*) extent and to have been of roughly the

Looanm 0.225 co,

Leucoadaellite

sad or

“‘Nd/“‘Nd bctrer

than,

1 I

871 873

1.6 1.0 1.2

af

22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

057 658 1047 799 785 909 913 938 1077 933 826 992 779 687 730

0.6 0.6 0.0 1.2 0.D

calculated I

OiQlite Rhyodacitc

250

m.y.

m

the

buis

:: 39 40 41 42 55 of

0.51235. lo-’

urd

2 x

lo-+

(20)

rc,pcctivaly.

same age. Two relatively mafic inclusions, representing

chemical and rnine~~~l extensions of the granitoids, have cNd values (+2.3 and +0.9) that am essentially identical to their hosts. Because these inclusions are thought to be derived from the same source material as their hosts the differences in their isotopic composition am attributed to local variations in the composition of the source material. Bundam Suite. The best estimates for the age and initial *‘S#‘%r of the suite are 287 .+ 10 m.y. and 0.7062 rt BOOIrespectively, from a Model 1 regmssion of 16 total-rock samples (COMPSTON,unpubl. data), including the two listed in Table 1. This is a very tight group and testifies either to highly uniform source material in 87Sr/%r, or to the efficacy of mixing during fusion. The 287 r: 10 m.y. total-rock age is consistent with a Rb-Sr biotite age of 281 m.y. for this suite (unpubl. data) but difi&x from the K-Ar determinations of biotite by KLEEMAN (1975f which suggest minimum crystallisation ages of 304-

374

H. D. Hensel. M. T. McCulloch

0.704 8

___..__~.~

and B. W. Chappeli

0.708

0.712

. .-

l

Hillgrove Suite

(310m.y.)

i’

TransitIonal granitoids

(295m.y.)

A Bundarra Suite

(290m.y.)

0 New England Super-Suite

f265m.y.)

l

Nundle Suite

0 Post-erogenic

FIG. 3.

__

(265m.y.) granitoids

Nd and Sr isotopic com~sitions of the major intrusive suites from the New England The two anomaious granitoids (Nos. 5 and 25) which plot outside the fields of their assumed respective suites may have difkent crystailisation ages. The ‘mantle array’, as defined by the bulk of oceanic basal& has been included primarily for the Nundle Suite to demonstrate the possibility that Initial

Batholith.

oceanic or volcanic arc material

may have been an important

309 m.y. The two anaiysed samples from the western S-type suite (Fig. 2) rqxcscnt the extremes in chemical

and mineralogical compositions that we have studied, in an otherwise generally uniform suite. The initial cNd values and initial 87Sr/esSr ratios range from -0.2 to -0.9 and 0.70574 to 0.70612 respectively. Although these values differ only slightly from those of the Hillgrove Suite granitoids the overlap between the two S-type suites is minimal. New England Super-Suite. Although the rock types analysed for Sr and Nd isotopes span essentially the entire compositional range (from meianocratic gabbroic diorite with 47% SiOz to Ieucoa~meIlite with >74% SiO& the spread of initial cNdand initial “Srj “Sr are remarkably limited. The initial eNd values range from - I .7 for the Oban River Leucoadamellite to +2.4 for the Inlet Monzonite, a pluton at the southwestern margin of the New England Batholith. Because the Glenbumie Adamellite (eNd = +4.6) has traditionally been assigned to this Super-Suite it has also been included here; however, its anomalous Nd composition supports mineralogical and chemical data which suggest that there may not be a genetic association between it and the surrounding members of the Super-Suite. Rb-Sr total-rock ages for the Super-Suite, based both on arbitrary geographical divisions (north, central and south) and on individual plutons, fall into a narrow range of 265 * 13 m.y. These total-rock ages are compatible with Rb-Sr biotite ages from both individual plutons (250 +- 2 m.y.) and biotite isochrons from different (com~site) regions (HENSEL,1982). Whilst it is not possible to resolve subtle temporal differences within the Super-

component

in the source region.

Suite using the Rb-Sr technique, it is clear from the initial “S#?jr ratios of some individual suites that the source rock compositions must have differed slightly from place to place. For example, plutons from the &alla-Kentucky area just south of Armidale (Fig. 2) have signifi~ntly higher initial g’Sr/%r ratios (0.7058 f .0003) than those of the (northern) Wards Mistake (0.7049 f .0003) and (southern) Moonbi areas (0.7052 + .0002). This is further supported by the non-systematic covariation of initial “Sr/“Sr and initial tNd between these. areas. Several areas of fine-grained porphyritic rocks ocsur scattered throu~out the area of the New England Super-Suite. Irrespective of whether they represent remnants of ignimbrite flows (FLOOD et ai., 1977), subvolcanic intrusives or caldera infills (HENSEL, 1982), there is genera1 agreement that they are closely related genetically to the associated intrusives. Nd and Sr isotopic analyses of the Back Creek Tonalite, Terrible Vale Microporphyritic Tonalite and the Aberfoyle Porphyrite strongly support this conclusion, as both initial 6Nd and calculated initial s7Sr/“Sr (Table 1) are indistinguishable from the associated plutons of the New England Super-Suite. Nund~e Suite, The Rb-Sr biotite ages for the Gags Top Trondhjemite (255 m.y.) and Barrington Tops Granodiorite (262 m.y.) provide a minimum emplacement age for this suite (unpubl. data). A precise estimate of the total-rock crystallisation age (using Rb-Sr) could not be obtained because of very low Rb/Sr ratios and limited dispersion. However, t&NCtural and textural evidence suggest a crystalllisation age for at least one member of this suite (Barring-ton

315

The New England Batholith Tops Granodiorite) between those of the m&rove Suite and New England Super-Suite (HENSEL et al., 1983). The range of initial “Sr/%r ratios for the Nundle Suite (0.7035 1 to 0.70396) is significantly lower than for any other suite from the New England Batholith and parallels some of the granodiorites, tonal&s and quartz diorites from the western United States (DEPAOLO, 1980). The mantle-like character is also refleeted in the Nd isotopes (Table I). For example, the Barrington Tops Granodiorite has the highest tNd (+6.1) yet reported for any granitoid from Australia. The relationship of the Rockisle Gmnodiorite (Anal. 21, Table 1) to the Nundle Suite remains equivocal. Its lower Sr and higher Rb contents, relatively older Rb-Sr biotite age (277 m.y.) and the fact that it is not closely associated with the Peel Fault suggest that it may constitute a separate intrusive suite, with characteristics transitional between the Nundle and Hillgrove Suites. New England Post-Orogenic Granitoids. The two analysed samples from this group of plutons are part of a Cu-Sn-Mobearing granitoid association emplaced into, and east of, the Carboniferous Hillgrove Suite during the early Triassic, i.e. about 20 m.y. after the culmination of intense erogenic activity in New England. The gross differences between the two analysed plutons, as expressed by their mineralogy, chemistry and field characteristics, are also evident in their Nd and Sr isotopic composition-the tNd of the Round Mountain Leucoadamellite is +4.5 compared to + 1.1 for the Billys Creek Quartz Monzonite. Initial “Sr/*“Sr for the latter, calculated on the basis of a 230 m.y. biotite age (unpubl. data), is 0.70433. For a similar age (BINNS, l966), the Round Mountain mass had an initial “Sr/%r of 0.7052; however, this initial ratio is extremely sensitive to the age estimation because of its very high “Sr/%r. Transitional Granitoids. The transitional characteristics of the Gam Adamellite, as outlined previously, are also evident in the Nd and Sr isotopic compositions. Both values (+0.2 and 0.70618, respectively)

Smple Rmber

Rb ----ppm----

WGV UPS7 Hill14 wH330 MS157 !a4 In?197 !arZSS ““136

109.5 114.2 151.2 153.6 92.1 130.0 32.7 132.5 225.7

28.8 27.4 10.5

sr

are indistinguishable from intrusives belonging to the Hillgrove Suite and New England Super-Suite. Sedimentary Rocks. The Nd and Sr isotopic results for 12 sedimentary rocks or their metamorphosed equivalents are presented separately in Table 2 and illustrated on a cNd-b plot in Fig. 4. The sediments are from both Zones A and B (Fig. 1). Those from Zone A are vitric, elastic and lithic greywackes of mainly andesitic composition, while those from Zone B are ‘mafic’ and ‘felsic’ greywackes, siltstones and shales. Zone A sedimentary rocks all have low initial *7Sr/86Sr ratios (0.7038 to 0.7046) and initial CM values (at 375 m.y.) of between +5.1 and +6.6. In sharp contrast, Zone B sedimentary rocks have initial +.,d values (with one notable exception) close to Bulk Earth, ranging from - 1.7 to +0.6. The overall range in their initial *‘Sr/%r composition is large (0.7027 to 0.7054) although some of this variation (particularly the lowest and highest ratios) may reasonably be attributed to uncertainties in the age estimation of these two samples. In a study of sedimentary rocks from New England, HENSEL (1982) determined RbSr total rock ages for units from the Wongwibinda Complex and the Moona Plains area and showed them to be significantly different. The ages for the respective areas (387 + 12 m.y. (2~) and 351 + 9 m.y.) are broadly equivalent to the stratigraphic ages of the Zone A sedimentary rocks (CROOK, 1961; CHAPPELL,1961) and embrace the arbitrary 375 m.y. age used in initial ratio calculations. Suggestions that these regional ages may in fact be provenance ages rather than sedimentation ages receive little support from (a) the persuasive field and petrographic observations that volcanism and sedimentation of the eroded volcanic detritus were essentially contemporaneous, and (b) the statistical treatment of the Rb/ Sr data using the method of CAMERONef al. (1981) which allows for the possibility that the provenance may have been significantly older. The difference in the Nd isotopic compositions between the sedimentary rocks from Zones A and B is significant as it coincides with a change in the

“RbP6Sr

“Srl’6Sr

299.1 355.7 256.9 367.4 269.‘ 240.9 ‘JO., 241.6 120.0

1.057 0.925 1.701 1.207 0.987 1.560 0.219 1.546 5.444

0.70942 0.70902 0.71295 0.71111 0.70966 0.71418 0.70554 0.71219 0.72963

-8.1 -,.S -6.8 4.5 0.6 21.4 0.3 -5.8 -2.0

4.80 4.09 6.28 6.23 5.42 a. 75 3.87 6.33 5.92

24.8 28.1 31.1 30.5 27.0 44.6 18.4 M.1 28.4

0.117 0.105 0.122 0.124 O.IZI 0.119 0.127 0.127 0.126

0.511666 0.511657 0.511669 0.5‘1664 O.SllS69 0.511602 0.511968 0.511569 0.511666

0.z 0.6 0.1 0.3 -1.4 -0.6 6.5 -1.7 -0.3

176.5 1020.0 467.8

0.473 0.076 0.065

0.70727 0.70‘38 0.70421

5.7 -5.3 -6.9

4.12 2.95 3.33

15.6 12.4 14.7

0.160 0.14‘ 0.137

0.512079 0.511981 0.511949

6.6 5.6 5.

E Sr

_“_,,m_rf_

“‘Smf”*Nd

“‘Ndll”*Nd

cNd

Rock

Type

Crcyv.ckc Crcywcte Sh.h Sh.le Grcywch “er.ptlire Crryv.cke Shale Shale

vitric p,rcyv.ck. Greyweke

I Greymckc

965 MO44 1007 1005 1144 1075 s6b 1214 1053

636 695 695

4) I5 46 47 b6 49 50 51

52 53 34

t+. I). Hensei. M. T. McCullochand B. W. C‘happeli PSrle6SrI1~ 0.704

0.708

0.712 I

-40

-20

0

20

40

60

80

100

120

6%

FIG. 4. initial Nd and Sr isotopic compositions of New England sediments calculated at 375 m.y. Filled circles are ‘mafic’ greywackes and bullseyes are ‘felsic’ sediments. Analyses52-54 are from Zone A, the remainder are from Zone B. The fields defined by the dashed lines and the dotted lines repnsent the displacements of the sedimentsfrom TjT5m.) to TJMm.y and TM ,,,, respectively, comsponding to the time of fusion for the Hill&rove Suite and the New England Super-Suite. The cross-hatched field (Nundle Suite) and the shadedfield (Hi&rove and Bundarra Suites, New England Super-Suite and transitional granitoids, at their respective intrusive ages) are included for comparison.

nature of the provenance, from one which supplied predominantly andesitic detritus (Zone A) to one in which rhyodacites and rhyohtes were prominent (Zone B). It also emphasises the utility of the Nd isotopic system in efucidating source rock characteristics of sedimentary rocks especially when Sr isotopes cannot discriminate. The exception to the narrow range of tNd values for sedimentary rocks from Zone B is a ‘mafic’ greywacke from the Sandon Association (KORSCH, 1977)near Walcha (Fig. 2) which has an initial tNd of +6.6. Although this result was surmising, mainly because of its resemblance petrographically to other greywackes from Zone B, combined with the fact that it comes from the same outcrop as a pelite with an initial cNdof -1.7, it was not entirely unexpected as unconfo~it~~ are not uncommon in the stratigraphic succession of New England, many remaining unrecognised because of the extreme structural and lithological complexity of the region. 6. DISCUSSION

Determining whether or not a pluton has been derived from sedimentary source rocks (and therefore !&type) is generally straightfonvard. Using field, petrographic, mineralogical, chemical and isotopic criteria CHAPPEU and WHITE ( 1974) and subsequent workers (e.g. GRIFFIN er cd.,1978; HINE et ai..1978) were able to unambiguously categorise S-type granitoids

in many batholiths within the Lachlan Fold Belt and distinguish them from other varieties. The basis of their success rested with the fact that the source rocks of the LachIan S-types differed markedly from the source rocks of the I-types. In New England there is general consensus that the S-type Bundarra and Hillgrove Suites were also derived from sedimentary source rocks (GREEN, 1976; O’NEIL et al.. 1977: CHAPPELL, 1978; SHAW and FLOOD, 198 1). However, in contrast to the S-type source rocks from the Lachlan Fold Belt the New England sedimentary source rocks had a bulk chemical composition not significantly different from the ‘felsic’ sedimentary rocks presently exposed in many parts of this region (HENSEL, 1973, 1982). As shown in Fig. 4 the Sr and Nd isotopic compositions for the New England sedimentary rocks (calculated at the times of granite formation) correspond remarkably closely to those of the S-type granitoids, thereby supporting a simple and direct genetic relationship, provided of course that sedimentary rocks similar to those analysed extended to the zone of magma ~n~~~o~. tG&wgh it is not possible to deduce precisely the source rock compositions for the New England S-type suites there is abundant independent evidence such as field observations, petrographic data and mineral phase equilibria that the country rocks surrounding the Hillgrove Suite attained temperatures and pressures that were efficiently high for partial melting to occur on at least a local scale (BINNS, 1966; STEPHENSONand HENSEL, 1979,198 1)

The New England Batholith producing veins whose chemical compositiutts are very close to those of the granitoids. With an initial 87Sr/86Srratio of 0.70526 5 AlOO (cf:FLOOD and SHAVE, 1977) and an average tNd of + 1.0 the Hillgrove Suite and, to a lesser extent the Bun&rm Suite, represent the isotOpically least evolved !&type Phanerozoic granitoids yet documented. For this to occur the granitoids must have essentially inherited the isotopic composition of the intermediate to felsic volcanogenic sedimentary source rocks. 6.2 Source rocks for other New England granitoids New England Super-Suite. Whereas it is a relatively easy procedure to establish that the New England S type granitoids were derived from sedimentary source rocks, considerable debate surrounds the nature and type of the source rocks which yielded the granitoids of the New England Super-Suite. For example, FLED ( 197 1) proposed a subcrustal origin for the tonalites primarily on the basis that the temperatures required for their generation as partial melts were too high to be achieved within the crust. CHAPPELL(1978) concluded that the likely source rocks for the Moonbi granitoids (southern New England, Fig. 2, Table 1) were an ‘older plutonic source produced by underplating of the crust in the New England Fold Belt during an earlier subduction event’. The character of this source is implicitly very uniform as evidenced by ‘the extremely good linear correlations between different elements within suites and the chemical discontinuities between suites’. More recently SHAW and FLOOD( 198 1) suggested that the &alla-Kentucky granitoids were derived by ‘partial melting of an interface region consisting of pelitic metasedimentary rocks of the type that gave rise to the S-type suites and a metaigneous source region of the type that gave rise to the Moonbi Suite’, so that the composition of the granitoids is related directly to the ratio of these two components. The Nd and, to a lesser extent, the Sr isotopic data provide important constraints on the latter model. Foremost, such a model is required to address the fact that the sedimentary and igneous components in their source region must be capable of producing the extreme compositional and lithological diversity of intrusives whose Nd and Sr isotopic compositions are remarkably uniform over the entire 400 km longitudinal extent of this Super-Suite. Indeed, any models requiring the assimilation of crustal material by mantle-derived magmas to produce the granitoids of the New England Super-Suite face significant prob lems. Not only must these models satisfy the constraints of extreme compositional variety yet uniformity of Nd and Sr isotopic compositions they must use source rocks whose compositions are believed to be reasonably well constrained. For example, it has been established (HENSEL, 1982) that at least part of the upper mantle under New England is isotopically and chemically highly depleted, having produced a

377

suite of tholeiitic olivitt&bearing, chemically impoverished magmas with cNd values of +8.1 to +10.5 (HENSEL et al., 1981b). Similarly, the source rocks from which the Nundle Suite granitoids were derived (the metaigneous component of Shaw and Flood) are known also to have had Nd isotopic compositions of depleted upper mantle. Appealing to the lower crust as the source for the igneous component also has drawbacks. Although its constitution in New England is poorly known some insight into its composition may be gained from xenolith and inclusion assemblages in pipes and sills. Many of these inclusions are lower crustal granulites of intermediate to mafic compositions (WILKINSON, 1975). If these inclusions with their basaltic chemistry are truly representative of the lower crust in New England mixtures of this material with upper mantle magmas through assimilation are highly unlikely to yield granitoids whose isotopic compositions are similar to those of the New England Super-Suite. Only mixtures of upper mantle magmas and the most isotopically and chemically evolved upper crustal material for New England (Stype granites or ‘felsic’ greywackes) adequately satisfy the isotopic constraints. However, physical mixtures of this kind in which the components are so different are not likely to produce an extremely wide compositional spectrum of granitic derivatives with uniform isotopic compositions. Nevertheless, and largely based on Nd and Sr isotopic data, it is commonly accepted that granitoids elsewhere consist of at least two isotopically and chemically distinct components (ALLURE and BEN OTHMAN, 1980; DEPAOLO, 1981; NOHDA and WASSERBURG, 198 1; MCCULL~CH and CHAPPELL, 1982). One of the components

in this source is assumed to have a history of LREE depletion, i.e. depleted upper mantle, with positive CNd,whereas the other is continent-derived with a considerable range in +&,. In a survey on granitoids from a spectrum of tectonic environments. ALL~GRE and BEN OTHMAN ( 1980) concluded that Nd and Sr isotope systematics in many young (Palaeozoic) granitoids may be adequately explained only if recycled older continental crust is an essential component of the source. MCCULLOCHand CHAPPELL( 1982) showed that the variable tNd (+0.4 t0 -8.9) in I-type granitoids from the Berridale and Kosciusko Batholiths could be modelled by mixing a crustal component with a depleted mantle-like component. However, because their binary mixing model failed to withstand rigorous testing using selected major and trace elements, they concluded that either (a) additional components may have been present in the source rocks, or (b) these Itype granitoids may have been derived from new crust that had been generated continually from the mantle over considerable time. Nundle Suite. Based solely on the mineralogy, chemistry and Sr isotopic data, models for the origin of this suite might be expected to favour a direct derivation by partial melting of, or fractionation

378

H. D. Hensel. M. T. McCulloch and B. W. Chappell

from, a reasonably uniform upper mantle source. However, despite the very low values of B7Sr/%$r (0.7036~.7~) such simple models are not entirely consistent with the wide variations in c,,,,,values (+3.3 to $6.1). MORB-like or ocean-floor basalts alone, such as those occurring extensively in southern and south-eastern New England and along the entire Peel Fault, may be excluded as possible source material because of their signi~cantly higher tNd values (+8.4 to +9.0), compared to the granitoids (unpubl. data). Hydrothermally altered oceanic crust or oceanic sediment as a substantial component in the source rocks is not consistent with their low Sr isotopic compositions. Similarly, subgmnitic igneous rocks (e.g. mafic tonalites, quartz diorites) underplated early in the history of New England (e.g. during the Proterozoic) must appeal to unreasonably low Rb/Sr ratios and to a history of LREE depletion. The variation in CNdvalues suggests a composite source whose combined components have high cr.& values, low Rb/Sr ratios coupled with low initial *‘Sr/ *%r, and be capable of producing granitic meits with high NarO (>,4%), SiO? (-68%) and A& (- 16%). The source rocks which seem most likely to produce the observed ranges in chemical and isotopic compositions arc intermediate volcanic rocks probably of island-arc character. Sedimentary rocks derived from these volcanics. such as the ‘mafic’ greywackes from Zone A, or a combination of such sedimentary and volcanic rocks would also constitute suitable source rocks. If the detrital material was similar isotopically to, for example the 8reywackes from Zone B, and variable proportions of this material contributed to the formation of the granitic magmas. differences in +,d VaheS between piUtOnS could Wd RSUh yet may not be detected by Sr isotopes. Thus plutons like the Rockisle Granodiorite (biotite age of 277 m.y.) with its high &O (3. I’%), SQ (69.3%) and IOU tNd (+3.3) mi# refIect the involvement of a substantial amount of this relatively more-evolved sedimentae component. New England

Post-Orogenic Granitoids. As the SiOz contents of granitoids approach minimum melt compositions (-76% Si02) the distinction between S-type and I-type becomes progressively more difficult. Chemical parameters and petrographic features which may clearly distinguish between these granitoid types at lower Si02 may overlap and are therefore inadequate as discriminants in highly siliceous intrusive% Furthermore, the extreme enrichment in Rb, relative to Sr, often precludes precise determination of their initial *‘Sr/*%r ratios. Some felsic 8ranitoids have particular mineralogical and chemical characteristics which suggest that they are A-type, i.e. derived from source rocks that have already been depleted in granitic components during an earlier generation of I-type granitoids (LQISELLE and WONES. 1979: Cot.LtNS ef al.. 1982). Diagnostic features include small amounts of sodic amphibole, annite-rich biotite and fluorite, an abundance of large highly-charged cations

such as Ga, Nb, Sn. Zr and REE, and their appearance late in the erogenic history of a region. Although the Round Mountain ~u~a~mellite (RMLA) was one of the last plutons to lx emplaced into the New England Fold Belt and is highly siliceous it bears Only a passing similarity to A-type granites (COLLINS et a/.. 1982). The high CNdvalue (+4.6) and low initial 87Sr/86Sr ratio (0.7052) of the RMLA provide some constraints on the composition of the source rocks. Foremost. it requires that the source rocks had low Rb/Sr and a history of net LREE depletion. Intermediate and mafic volcanic rocks or their detrital equivalents such as the ‘mafic’ greywackes from Zone A, mixtures of sedimentary rocks from Zones A and B, residue left behind after the generation of the Nundle Suite, a mixture of oceanic crust and overlying sediments or lower crustal material which has not been exposed all constitute suitable source rocks for this pluton in terms of their bulk Nd and Sr isotopic compositions. Whether the chemistry of such felsic rocks as the RMLA is consistent with its derivation from such mafc source rocks is less certain. Indeed, the processes which generated such large volumes of felsic magma remain enigmatic. 6.3 Sediment Provenances Zone A (‘weslern zone]. The Nd and Sr isotopic compositions of the immature elastic sedimentary rocks from Zone A support abundant chemical and petrographic evidence (CHAPPELL, 196 1; LEITCH, 1975) that these rocks were derived from an active proximal volcanic arc. The prominence of this lon~tudin~ly trending volcanic arc which flanked the western margin of New England is emphasized by the fact that there is an extreme paucity in the Zone A sedimentary sequence of detritus from continental terrains adjacent to the New England region. The efficiency of this arc in preventing the influx of older crustal detritus from the central and western zones of New South Wales indicates that it may have formed an integral part of the New England Fold Belt for much of the Early- to Mid-Palaeozoic. Alternatively, and based principally on the rather limited compositional variation of elastic rocks within such a prominent and extensive arc, it is speculated that the arc may have developed in response to a series of major tectonic events which culminated in the movement of the New England Block into its present relative geographical position during the late Silurian or Early &.vonian, Born a location well away from the earlier continental margin. The high positive tHd vaiues of Zone A rocks contrast sharply from sediments or sedimentary rocks from other evironments. For example, apart from a Baja California shale with an CNd vahle Of +0.7 (MCCLJLLOCHand WASSERBURG, 1978) most analysed sedimentary rocks reflect the infiuence of a continent-derived LREE-enriched component (MCCULLOCH and CHAPPELL. 1982; FARMER and

379

The New England Batholith DEPAOLO, 1983).Even a wide assortment of pelagic clays and ferromanganese deposits from the ~pnrcifie; Indian and Atlantic oceans (O’NIONS et al., 1978; GOLDSTEIN and O’NIONS, 198 1) rarely exceed cNd = 0, despite their obviously close association in a number of instances with ocean-floor volcanics. The occurrence of a thick sequence of essentially unmodified Early- to Mid-Palaeozoic marine sedimentary rocks (Zone A) overlying probable oceanic crust is therefore very significant in view of the numerous studies which have demonstrated that the isotopic and chemical characteristics of many arc magmas may be attributed to the participation in magma genesis of a small (l-2%) proportion of sedimentary component (HAWKESWORTH et al., 1977, 1979a; KAY et al., 1978; KARIG and KAY, 1981; COHEN and O’NIONS, 1982). Although some of the isotopic constraints, for example Pb, may be satisfied by this component there is commonly a pronounced overabundance of trace elements in arc magmas which cannot be explained by the contribution of oceanic sediment alone. An alternative, but only partially viable solution that is favoured by some authors (e.g. HAWKESWORTHet al., 1979b), centres on the introduction into the magmatic source regions of silicasaturated and incompatible-element enriched liquids due to the dehydration and partial melting of altered oceanic crust (NICHOLLS and RINGWOOD, 1973). ARCULUS and JOHNSON (198 1) suggested that lower crustal material enriched in Sr, Ba and

Pb relative to Rb, Th and U is a more appropriate choice to satisfy both the chemical and isotopic abundances in arc

volcanics. An equally satisfactory choice for the ‘contaminant’ is mafic, pIag&&+rich greywacke derived from an isotopically depleted source, for example those from Zone A. The highly positive tNd values and low “Sr/%r of these greywackes are virtually indistinguishable (Fig. 5) from (time-corrected) modem day and older arc volcanics such as New Britain (DEPAOLO and JOHNSON, 1979),Aleutians (McCULLOCH and PERFIT, 1982), South Sandwich Islands (HAWKESWORTH et al.. 1977), the Late Silurian of Scotland (THIRLWALL, 1982)

and the Ecuadorian

portion of the Andean arc (HAWKESWORTH et al.,

1979b). Provided the New England region represents a Palaeozoic am-trench system at which oceanic lithosphere (including overlying sediments) was consumed (LEITCH, 1975; SCHEIBNER, 1976; CAWOOD, 1982) it is clear that a substantial contribution to the upper mantle of Zone A greywackes would significantly influence the chemical (but not isotopic) composition of volcanic and intrusive magmas generated at a continental margin. In particular these magrnas should be characterised by a pronounced Sr ‘spike’, i.e. enrichment in Sr, and a less pronounced enrichment in Pb and Ba. It follows that the accretion of sufficiently large volumes of these volcanogenic sedimentary rocks to the base of the crust would provide an excellent reservoir from which to generate granitoids with I-type characteristics, not unlike the granites of the Nundle Suite. Zone B (eastern zone). The provenance of both ‘mafic’ and ‘felsic’ sedimentary rocks from this zone is also volcanic, derived from a northerly source

FIG. 5. Comparison between New England ‘mafic’ greywackes and magmatic rock8 of diKerent age8 from a wide range of tectonic environment8 in term8 of Nd and Sr isotopic compositions. ALsoincluded for comparison are granitoids from the Nundle Suite (New England Batholith), Bega Batholith (Lachlan Fold Belt), Peru and Japan. Two samples (D), joined by dashed line, repent extreme compositions of granulite inclusions from the Delegate breccia pipe (southeast Australia), calculated at time of fusion of surrounding granitoids.

180

H. D. Hens& M. T. McCulloch and B. W. C’happell

(BORSCH, i977), presumably a volcanic arc. However, the composition of the detritus from this arc is, in general, significantly more fefsic, indicating a dacitic to rhyoiitic source (LEITCH, 1972). More importantly, there is a considerable difference in the Nd isotopic compositions of the Zone B sedimentary rocks compared to the greywackes from Zone A. This difference (-6 c units) may be interpreted as reflecting the presence of an old com~nent. Indeed, the average Nd isotopic composition of the Zone B rocks (-Of is consistent with a 20-300/o contribution of detrital material, with tNd values typical of the Early Proterczoic Australian provinces (-20 to -25; MCCULLOCH and HENSEL, in prep.). to depleted volcanic arc detritus such as the Zone A rocks. However, the presence of such a large proportion of old detrital material is inconsistent with (a) extensive petrographic evidence which suggests that the contribution of detritus derived from a non-volcanic source is minimal (LEITCH, 1972; KORSCH. 1976, 1981) and fb) Sr isotopic data. The low initial 87Sr/s6Srratios (-0.7045) which are similar to Zone A, combined with relatively high Rb/Sr. place severe limitations on the proportion and composition of an old feisic component in the Zone B sedimentary sequence because such a component (intrinsicaIiy high in Rb/Sr) must necessarily evolve “Sr at a rate which would result in initial “Sr/**Sr ratios considerably higher than those of either the sedimentary rocks or any granitoids derived from them. it follows that the Nd and Sr isotopic compositions therefore must be reflecting the nature and composition of the parental volcanic source rocks. Provided the Zone B sedimentary rocks truly approximate the isotopic compositions of the parental volcanic source rocks, i.c. there has been negligible isotopic fractionation or fractionation between REE’s during sedimentation and diagenesis ( MCCULLCK‘H and WASSERBURG.1978; TAYLOR and MCLEUU~N. 198 I). their essentially chondritic Isotopic compositions indicate either (a) a derivatron for the parental volcanic material from undeplcted tprimordial:‘) mantle. or (b) a derivation from volcamcs which became contaminated during ascent b) a component with low Rb/Sr but high negative cLldvalues. Since it has already been established that the Paiaeozoic New England upper mantle produced large volumes of highly depleted voicanics. a derivation from undepieted mantle must appeal to a second. and strongly contrasting, upper mantle source region under New England capable of producing similarly large volumes of relatively feisic volcanics whose isotopic com~sitions are close to chondritic. A more favoured interpretation, however. is the interaction between depleted volcanic magma and old lower crustai material. e.g. intermediate granulite. to produce volcanic rocks with isotopic compositions and chemical characteristics that are consistent with those of the Zone B sedimentary rocks. This is analogous to the Sunda and Banda Arcs of Indonesia where it has been demonstrated on the basis of Sr and Nd isotopes

(WHITFORD et al., 1981; MCCULLKH ef ai., )9x3) and petrological studies ofcordierite-bearing inciustons (ARCULUSand JOHWON, 198 1) that continental material. probably derived from the Paiaeozoic and Proterozoic terrains of northern Australia, has been incorporated by the volcanic magma during its rise from the upper mantle 7. A PR~CA~B~AN LOWER CRUST IN NEW ENGLAND? A number of workers have speculated that the New England Fold Belt is, or has been, underlain at some stage by old crust. For example, RUTLAND ( 1976) suggested that Precambrian continental crust such as that occurring under the bulk of the Lachfan Fold Belt might well extend under New England. although rifting and subsequent displacement. presumably eastwards, during the Palaeozoic, may have disrupted its continuity. It is important therefore to note that the Nd and especially Sr isotopic data on the granitoids and the sedimentary rocks from New England severely limit the presence of involvement of old continental crust (with granitic or average upper crustal composition) in the granitoid source regions. For example. the oldest T&$ model age of any New England granitoid is less than 1200 m.y. (Tabie 1f. Conversely, the isotopic data do not preclude that lower crust, consisting of intermediate to mafic granuiites with i47Sm/‘44Nd ratios of -0.15 and *‘Rb/%r of ~0.5. could provide material to an evolving volcanic arc system that may significantly influence. for example, the original Nd isotopic composition of the magma. Granulites of inte~ediate to matic composition but exhibiting a wide dispersion of Sm/Nd ratios and measured cNd values (-8 to + 16: unpubl. data) are known to occur as inclusions in breccia pipes from Delegate (Lachian Fold Belt). Similar lower crustal granulites, some preserving evidence of moderate-pressure tholeiitic fmctionation (W~LKINSO~~ and TAYLOR, 1980). are also exposed in New England (LOVERING,1964; WILKINSON,1975: WILKINSON and TAYLOR. 1980; STOLZ. 1984). However, their essentially basaltic compositions preclude any significant contribution to granite magma characteristics. 8. NEW ENGLAND

GRANITOIDS CIRCUM-PACIFIC CONTEXT

IN A

The granitoids and associated coeval extrusives from the Late Palaeozoic New England Batholith are comparable in many of their chemical and isotopic characteristics with modern granitoids and their extrusive equivalents from the numerous erogenic belts that encircle the Pacific Ocean (Fig. 5). For example. DEPAOLO ( 1980) described granitoids and mafic intrusives from the Sierra Nevada and Peninsular Ranges Batholiths with a range in Nd and Sr isotopic com~itions that is more extensive, but which overlaps. that of the New England granitoids. A commonly

381

The New England Batholith accepted petrogenetic model for many west&% U. 3. granitoids (KISTLERand PETERMAN, 1973; TAYLOR and SILVER, 1978; DEPAOLO, 1981; FARMER and DEPAOLO, 1983) is one involving mantle- or lower crust-derived melts that were variably contaminated by the country rocks they have traversed or into which they were emplaced, with the result that there is a direct relationship between the isotopic composition (and model age) of the granitoids, the amount of contamination and the age of the country rock. Japanese granitoids are also very similar to those from New England, particularly in their regional tectonic setting and development and their Sr isotopic characteristics (SHIBATA and ISHIHARA, 1979). Furthermore, two distinct granitoid classes are recognised (ISHIHARA, 1977) according to source rock compositions and/or physical conditions during crystallisation. The initial Nd isotopic compositions are more negative (- 16 to -3) than those from New England (ALL~GRE and BEN OTHMAN, 1980; NOHDA and WASSERBURG, 198 1) indicating an older crustal, probably continental, input with low Rb/Sr. Recent Nd isotopic measurements (unpubl. data) have confirmed the presence of old crust in some parts of Japan (Hida Metamorphic Belt) with TN* model ages of around 1700 m.y. Similarly a significant crustal input is also suggested for some Late Palaeozoic Peruvian granites (tNd = -5) and recent volcanics (-5 to -1; ALL~?GREand BEN OTHMAN, 1980) although determinations of Late Mesozoic/Early Tertiary Peruvian granites (unpubl. data) show that this influence can be quite small. Equally relevant to this discussion is the relationship between the New England Fold Belt and the southern part of the Tasman Orogenic Province (particularly the Lachlan Fold Belt). In earlier studies by CHAPPELL and WHITE ( 1974) it was shown that the granitoids from the Lachlan Fold Belt possess chemical and mineralogical characteristics which permitted a twofold classification according to source rock type and from which it was possible to infer the lateral extent and trend of presumably older lower crustal source rocks. Rb-Sr source rock isochrons (COMPSTON and CHAPPELL,1979), U-Pb studies of zircons (WILLIAMS et al., 1983) and Sm-Nd determinations (MCCULLOCH and CHAPPELL,1982) of both S- and I-type granitoids, confirm the presence of Mid-Proterozoic lower crust under the Lachlan Fold Belt. T&&, model ages for many of these granitoids cluster around 1250 m.y. However, this age must be regarded as a minimum if a component of the protolith has been derived from a LREEdepleted mantle source, in which case T% model ages may be as old as 1550 m.y. The range of tNd in the S-types from the Lachlan Fold Belt is from -6.1 to -9.8, i.e. considerably more negative than their counterparts from the New England Batholith and clearly reflects the involvement during fusion of older source material. This range is nevertheless relatively restricted compared to the wide range in initial *‘Sr/“Sr (0.7094 ‘to 0.7 184) and

em@%ses the variable Fractionation of Rb relative to Sr which may occur when the sedimentary source material has experienced at least one weathering cycle or when granitoids have been derived from compositionally variable source rocks. The I-type granitoids from the Lachlan Fold Belt also have a wide range of initial *‘Sr/*%r ratios (0.7045 to 0.7 119); however, it is their wide range of CM values (+4.3 to -8.9) which testifies to a source rock prehistory which appears considerably more complex than that in New England. An indication of this complexity is evident from a reconnaissance Sm-Nd study of several pyroxenite and mafic granulite inclusions contained in explosive breccia pipes from southeastern Australia. tNd values of the inclusions calculated at 420 m.y., the approximate time of granite fusion, range from -7.1 to +8.2. These data combined with geophysical evidence (FINLAYSONet al., 1980) clearly demonstrate the presence in the Lachlan Fold Belt of a lithologically variable and probably mafic lower crust (FERGUSON et al., 1979). A small group of I-type granitoids (Moruya Suite) from the easternmost part of the Lachlan Fold Belt have chemical and isotopic characteristics which distinguish them from the other granitoids in southeastem Australia. Their Rk3tiVely high tNd VaheS (-?-3.0 to +4.3) and uniformly low initial *‘S#‘%r ratios (0.7038 to 0.7041) indicate a distinct and homogeneous type of source-rock, unlike that which generated the isotopically more-evolved granitoids from the remainder of the region. The source rocks for these granitoids may be very similar to those from the Nundle Suite of New England for which a derivation involving isotopically depleted volcanic arc material is favoured. The implication drawn from this analogy is for the possible presence of volcanic arc-like material in the lower crust of the eastern Lachlan Fold Belt and perhaps also further to the west. 9. SUMMARY The Late Palaeozoic New England Batholith is composed of at least 6 chemically distinct suites. Some of these suites exhibit features diagnostic of a derivation from sedimentary source rocks (S-types), whilst others are typically l-type (derived from igneous source rocks). As in other batholiths, the S-types were emplaced early in the magmatic history of the New England region, commencing with the Hillgrove Suite in the Late Carboniferous and followed about 20 m.y. later by the Bundarra Suite. For both suites, a derivation from predominantly felsic volcanogenic sediments, similar to those presently exposed in New England, is entirely consistent with Sr and Nd isotopic data. However, compared with granitoids elsewhere, especially those in which sedimentary rocks comprised a significant component in the protoliths, the S-type New England granitoids are relatively unevolved iso topically. This may be attributed directly to the

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H. D. Hens& M. T. McCulloch and B. W. Chappeii

composition of the volcanogenic sedimentary source rocks. Two factors undoubtedly contribute to this situation; (a) the sediments have not experienced mote than one erosion cycle, and (b) the source region was effectively shielded from an influx of radiogenic Precambrian material by a major volcanic arc along the western margin of New England. These essentiahy unmodified sediments must therefore refiect the chemicaI and isotopic commotion of the parental volcanic arc. Assuming this arc to have been generated from isotopically depleted upper mantle (in common with some modem-day arcs), it follows that preexisting crustal material has been added to it to produce EN,+values of around 0. Because neither Nd or Sr isotopes provide tight constraints on the absolute age and proportion of added materid. it is quite possible that low Rb/Sr lower crustal intermediate to mahc gram&es with Sm/Nd ratios close to chondritic, such as those known to occur in sills and breccias in both the New England and Lachian Fold Belts, constitute the principal con~minant. Where two compositionahy contrasting and not completely contemporaneous arcs shed their detritus from opposing flanks, a wide spectrum of sediment compositions must be expected. If the detritus from one of the arcs was quite mafic, as for example the mafic greywackes from the Tamworth Trough, derivative granitoids coutd reasonably be expected to assume an I-type character. However, it is more likely that partial melting on a regional scale would affect many units within the mixed-lithology source region, resulting in granitoids (New England Super-Suite) that show a wide spectrum of compositions. Furthermore, these granitoids display features transitional between the ‘true’ end-member S- and I-granitoid types, A derivation for the New England Super-Suite from a mixed-lithology source is consistent with Nd and Sr isotopes, with granitoid compositions falling midway between a mixture of felsic and mafic sediments, calculated at the time of fusion. Acknowledgements-HDH is grateful to Dr. W. Compston for permission to use the excellent analytical facilities, particularly during 1975-77 whilst he was a visiting student. We also wish to thank W. Compston for permission to use unpub~~ data and for providing critical and valuable diseussion. This project was supported by an Australian Research Grants Scheme grant to BWC. HDH also wishes to thank Lang Farmer and an unknown reviewer for the many constructive criticisms which have helped to improve this manuscript. The expert and patient typing by Chris Webb and Mary MacDougall and the lucid drafting by

Liiianc Wittig are appreciated. Editorial

hand&~:

L. E. Nyquist REFERENCES

C. J. and BENOTHMAN D. (1980) Nd-Sr isotopic telationsbip in granitoid rocks and continental crust de-

ALL&RE

ve&unent:

a chemical approach to orogenesis. nature

286,335~342. ARCULLB R. J. and

magma sources: a geochemical assessment of the roles OI

slab-derived components and crustal contamination, Get> chern. J. IS, 109-133. BINNSR. A. (1966) Granitic intrusions and regional metamorphic rocks of Permian age from the Wongwibinc& district, northeastern New South Wales. .I. Proc. R S’OC. N.S. W. 99, S-36.

CAMERON M.. COLLERSONK. D., COMP~TON W. and MORTON R. ( 198 I ) The statistical analysis and inteptetation of imperfectly fitted RbSr isochrons from polymetamorphic terrains. Ge~hf~. Cosmochim. Acta 45, 1089-1097. CAWOOD P. (1982) Structural relations in the subduction complex of the Paleozoic New England Fold Belt. eastern Australia. J. Geol. 90, 381-392. CHAPPELLB. W. (1961) The stratigraphy and strtmtural geology of the Manila-Moore Creek district, N.S.W. .I. Proc. R. SK N.5’. W. 95, 63-75. CHAPPELLB. W. (1978) Granitoids from the Moonbi district, New EngJand Batholith, eastern Australia. J. Geuf. See. Am. 25, 267-283. CHAPPELL B. W. (1984) Source rocks of I- and S-type granites in the Lachlan Fold Belt, southeastern Australia. Phil. Trans. R. Sot. Lond. A310, 693-707.

CHAPPELLB. W. and WHILEA. J. R. (1974) Two contrasting granite types. Pac. Geol. 8, 173- 174. COHEN R. S. and O’NIONS R. K. (1982) Identification of recycled continental material in the mantle from Sr, Nd and Pb isotope investigations. Eurrh Planef. SC;. Leu. 61, 73-84. COLLINS W. J., BEAMSS. D., WHITEA. J. R. and CHAPPELL B. W. (1982) Nature and origin of A-type granities with particular reference to southeastern Australia. Cum&. Mineral. Petrol. 80, 189-200.

COMPSTONW. and CHAPPELL B. W. (1979) Sr-isotope evolution of granitoid source rocks. In The Earth: ifs Qrigin. Strucrure and Evolution (ed. M. W. MCELHINNY). pp. 377-426. Academic Press. CROOK K. A. W. (1961) Stratigraphy of the Tamworth Croup (Lower and Middle Devonian), Tamworth-Nundfe district, N.S.W. J. Proc. R. Sot. N.S.K 94, 173-188. DEPAOLO D. J. (1980) Sources of continental crust: neodymium isotope evidence from the Sierra Nevada and Peninsular Ranges. Science 209. 684-687. DEPAOLOD. J. (1981) A neodymium and strontium isotopic study of the Mesozoic talc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California. J. Geoph.rs. Res. 86, 1~7~1~88. DEPAOLOD. J. and JOHNSON R. W. (19791 Magma genesis in the New Britain island arc: constraints from Nd and Sr isotopes and trace-element patterns. Conrrib. Mineral. Petrol. 70, 367-379.

FARMER G. L. and DEPAOLO D. J. (1983) Origin of Mesozoic and Tertiary granite in the western United States and implications for pre-Mesozoiccrustalstructure. I. Nd and Sr isotopic studies in the geocline of the Northern Great Basin. J. Geophys. Res. s8, 3379-3401. FERGU~QN J.. ARCULU~R. J. and JOYCEJ. (1979) Kimberlite and kimberlitic intrusives of southeastern Australia: a review. Bur. Miner. Resour.. Aus!. Geol. Geophys 4, 227241. FINLAYSON D. M., COLLINS C. D. N. and DENHAM D.

(1980) Crustal structure under the Lachlan Fold Belt, southeastern Australia. Phys. Earfh. P&met. In& 21, 32t342. ELAND R. H. (197 1) A study of part of the New England

Bathylith, New South Wales. Ph.D. dissertation, University of New England (unpubl.). FLOOD R. H. and SHAW S. E. (1977) Two ‘S-type’ granite suites with low initial *‘Sr/% ratios from the New England Batholith, Australia. Confrib. Mineral. Petrol. 61, 163-173.

JOHNSONR. W. (19811 island-are

FLCQD R. J-l.. VEaNoN R. H..

SHAW

S. E. and CHAPPELL

383

The New England Batholith

fission tracks A.A.E.C. Journal, Atomic Energy in Australia 18, 3-8. KORSCHR. J. (1977) A ffar&ork for the Paleoz.oic geology of the southern part of the New England Geosyncline. J. Geol. Sot. Aust. 25, 339-355. KORS.CHR. J. (1978) Petrographic variations within thick turbidite sequences: an example from the late Palaeozoic of eastern Australia. Sedimentology 25.247-265. KORXH R. J. (1981) Some tectonic implications of sandstone petrofacies in the Co& Harbour Association, New England Orogen, New South Wales. J. Geol. Sot. Aust. 28, 261269. LANPHERE M. A. and H~CKLEY J. J. (1976) The age of nephrite occurrences in the Great Serpentine Belt of New South Wales. J. Geol. Sot. Aust. 23, 15-17. LEITCH E. C. (1972) The geological development of the HAWKESWORTHC. J., O’NIONS R. K., PANKHURSTR. J., Bellinger-Macleay &on. Ph.D. dissertation, University HAMILTONP. J. and EVENSENN. M. (1977) A geochemical of New England (unpubl.). study of island-arc and back-arc tholeiites from the Scotia LEITCH E. C. (1974) The geological development of the Sea. Earth Planet. Sci. Lett. 36, 253-262. southern part of the New England Fold Belt. J. Geol. Sot. HAWKESWORTHC. J., O’NIONS R. K. and ARCULUS R. J. Aust. 21, 133-156. (1979a) Nd and Sr isotope geochemistry of island arc LEITCH E. C. (1975) Plate tectonic interpretation of the volcanics, Grenada, Lesser Antilles. Earth Planet. Sci. Palaeozoic historv of the New Eneland Fold Belt. Bull. Lett. 45, 237-248. Geol. Sot. Amer.86, 141-144. HA-WORTH C. J., NORRY M. J., RODDICK J. D. and LOISELLEM. C. and WONES D. R. (1979) Characteristics BAKER P. E. (1979b) 143Nd/‘“Nd. *‘Sr/*%r and incomand origin of anorogenic granites. Geol. Sot. Amer. Abst. patible element variations in calcalkaline andesites and with Prog II, 468. plateau lavas from South America. Earth Planet. Sri. LOVERINGJ. F. (1964) The e&&e-bearing basic igneous Lett. 4% 45-57. pipe at Ruby Hill near Bingara, New South Wales. J. HENSEL H.-D. (1973) The petrology, mineralogy and geoProc. Roy. Sot. N.S. W. 97, 73-79. chemistry of plutonic rocks from the Moona Plains MCCULLWH M. T. and WASSERBURGG. J. (1978) Sm-Nd Complex, near Walcha, N.S.W. Honours thesis. University and Rb-Sr chronology of continental crust formation. of New England (unpubl.) Science 200, 1003-101 I. HENSEL H.-D. (1982) The mineralogy. petrology and geochronology of granitoids and associated intrusives from MCCULLOCHM. T. and PERFIT M. R. (1981) “‘Nd/‘“Nd, the northern portion of New England Batholith. Ph.D. “Sr/“%r and trace element constraints on the petrogenesis dissertation, University of New England (unpubl.) of Aleutian Island arc magma% Earth Planet. Sci. Lett. 56, 167-179. HENSEL H.-D., MCCULL~CH M. T.. COMPSTONW. and CHAPPELL B. W. (1981a) The protoliths of the New M~CULLKH M. T. and CHAPPELL B. W. (1982) Nd England Batholith: Evidence from Nd and Sr isotope isotopic characteristics of S- and I-type granites. Earth studies. Rex Sch. Earth Sci.. Australran hhrujnal LiniverPkunPl SCI. Lett. 58, 51-64. sify, Ann. Rep., 196. MCCIILLOCHM. T.. COMPSTON W., ABBOTM. and CHIVAS HENSEL H.-D., COMP~TONW.. CHAPPELL B. W. and TAYA. ( 1983) Noedymium, strontium, lead and oxygen isotopic LOR S. R., (1981b) Primitive tholeitic intrusives in the trace element crmstraints on magma genesis lithe Banda New England Batholith. Res Sch. Earth Sci. Australian island-arc. Wetar. Geol. Sot. Aust. Abstr. 9. 152-153. National University, Ann. Rep.. 199. NICHOLLSI. A. and RINGWOODA. E. (1973) &ct of water on olivine stability in tholeiites and the production of HENSEL H.-D., CHAPPELL B. W.. COMETON W. and SiO+aturated magmas in the island-arc environment. J. MCCULLOCHM. T. (1982) A neodymium and strontium Geol 81. 285-300. isotopic investigation of granitoids and possible source rocks from New England. eastern Australia. Proc. l’oisc~v NOHDA S. and WASSERBURGG. J. (1981) Nd and Sr isotopic study of volcanic rocks from Japan. Earth Planet. Symp. University of New England. 193-200. Sci. Letr. 52, 264-276. HENSEL H.-D., MCCULLOCHM. T. and CHAPPELLB. W. O’NEIL J. R. and CHAPPELL B. W. (1977) Oxygen and (1983) Island-arc source for isotopically primitive granitoids hydrogen isotope relations in the Berridale Batholith. J. from the New England Batholith. Rex Sch. Earth Sci.. Gcol. Sot. London 133, 559-571. Australian National University. .4nn Rep. 148- 149. HINE R., WILLIAMS1. S.. CHAPPELL B. W. and WHITE O’NEIL J. R.. SHAWS. E. and FLOOD R. H. (1977) Oxygen and hydrogen isotope compositions asindicators of granite A. J. R. (1978) Geochemical contrasts between I- and Sgenesis in the New England Batholith, Australia. Contrib. type granitoids of the Kosciusko Batholith. J. Geool.Sot. .&fineral. Petrol. 62, 3 13-328. AUSI. 25, 219-234. O’NIONS R. K., CARTER S. R., COHEN R. S., EVENSEN ISHIHARAS. (1977) The magnetite-series and ilmenite-series N. M. and HAMILTONP. J. (1978) Pb, Nd and Sr isotopes granitic rocks. Min. Geol. 27. 293-305. in oceanic ferromanganese deposits and ocean floor basalta. tiRlG D. E. and KAY R. (1981) Fate of sediments on the Nature 273, 435-438. descending plate at convergent margins. Phil. Trans. Ro.v. RUNNEGARB. N. (1974) The geological framework of New Sot. Lond. 14301. 233-251. England. In I974 Field Conference, New England area, KAY R. W., SUN S.-S and LEE-HU C. N. (1978) Pb and Sr Q/d. Div. Geol. Sot. Ausl.. Brisbane, 9-19. isotopes in volcanic rocks from the Aleutian Islands and RUTLANDR. W. R. (1976) Orogenic evolution of Australia. Pribilof Islands, Alaska. Geochim. Cosmochim. Acta 42, Earth Sci. Rev 12, I6 1-I 96. ^,_ ^__ B. W. (1977) Origin of pyroxene-plagioclase aggregates in a rhyodacite. Contrib. Mineral. Petrol. 60, 299-309, ., GOLLXTEIN S. L. and O’NIONS R. K. (1981) Nd .Piid -6 isotopic relationships in pelagic clays and ferromanganese deposits. Nature 292, 324-327. GREEN T. H. (1976) Experimental generation of cordieriteor garnet-bearing granitic liquids from a pelitic composition. Geology 4.85-88. GIUF~N T. J., WHITE A. J. R. and CHAPPELL B. W. (1978) The Moruya Batholith and geochemical contrasts between the Moruya and Jindabyne suites. J. Geol. SOC.Au.Y~.25, 235-247. HAMILTON P. J., O’NIONS R. K. and PANKHURST R. J. (1980) Isotopic evidence for the provenance of some Caledonian granites. Nature 287, 279-284.

LO.%-L

13.

KISTLER R. W. and PETERMANZ. E. (1973) Variations in Sr, Rb, K, Na and initial *‘Sr/‘%r in Mesozoic granitic

rocks and intruded wall rocks in Central California. Geol. Sot. Amer. Bull. 84, 3489-3512. KLEEMANJ. D. (1975) Geological age measurements using

S~HEIBNER E. (1976)

Explanatory

notes on the tectonic

map of New South Wales, scale I: IO0 000. Geol. Surv. N.S.W., Sydney. SHAW S. E. and FLUID R. H. (1981) The New England Batholith, Eastern Australia: Geochemical variations in time and space. J. Geophys. Res. 86, 10530-10544.

384

W. D. Hen&. M. T. McCulloch and B. W. Chappell

SHIBATAK. and ISHIHAFCAS. (1979) Initial *%/%r ratios of plutonic rocks from Japan. Contrib ~M/neru/ Petal. 70, 38 i-390.

STEPHENSONN. C. and HENSEL H.-D. (1979) intergrown caicic and Fe-Mg amphiboles from the Wongwibinda metamorphic complex, N.S.W., Australia. C’un. Mineral. 17, 11-23. STEPHENSONN. C. and HENSE~H.-D. (198 I) Amphiboiites and related rocks from the Won~ibinda me~mo~hic complex, northern N.S.W., Australia. Lithos 15, 59-75. STOE! A. (I 984) Garnet web&rites and associated uitramafic inclusions from a nepheiine mugearite in the Waicha area, New South Wales. Australia. Min ,Mag. 48, i67179. TAYLOR H. P. and SILVER L. T. (1978) Oxygen isotope relationships in piutonic igneous rocks of the Peninsular Ranges Batholith, Southern and Baja California. U.S. Geol. Surv. Open File Rep. 78-701, 423-426. TAYLORS. R. and MCLENNANS. (1981) The composition and evolution of the continental crust: rare earth element evidence from sedimentary rocks. Phil. Trans. Rev Sot Land. A301, 381-399. THIRLWALLM. F. (1982) Systematic variation in chemistry and Nd-Sr isotopes across a Caledonian caic-aikaiine volcanic arc: implications for source materials. Earth Planet. Sri. Left. 58, 21-50.

WASSER~URGG. J.. JACOBSEN8. N., DEPA~LV F). i . MCCULLOCHM. T. and WEN T. (1983) Precise determr.. nation of Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochim Cf)~rn~hirn. Aciu 45. 231 l-2323. WHITFORD D. J.. WHITE W. M. and JESEK P. A. i IV8I ) Neodymium isotopic composition of Quaternary Island arc lavas from Indonesia. Cicoehim. Cosmochim. .&?a 45, 989-995.

WILKINSONJ. F. G. (1969) The New England Batholith. in The Geology of New South Uhles (ed. G. H. PAcKtiAM). J Geol. SK /lust. 16, 271-278 WILKINSONJ. F. G. (1975) Uitramahc inclusions and high pressure megacrysts from a nephelinite sill. Nandewar Mountains, north-eastern New South Waies. and their bearing on the origin of certain uitramafic inclusions in alkaline volcanic rocks. Conrrib Mineral. Petrol. 51, 23% 262. WILKINSONJ. F. G. and TAYLOR S. R. (1980) Traceelement fractionation trends of tholeiitic magma at moderate pressure: evidence from A I -spine1 uitramafic-mafic inclusion suite. Conrrib. Mineral. Per&. 75, 225-233. WILLIAMS1. S., COM~~TONW. and CHAPPELLB. W. C1983) Zircon and monazite U-Pb systems and the histories of I-type magma% Benidaie 3atholith. Australia. .I Pefrol 24. 76-97.