Journal of Petrology

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Request Permissions Google Scholar Articles by Bryant, C. J. Articles by Chappell, B. W. Search for Related Content Social Bookmarking          What's this?

Journal of Petrology | Volume 38 | Number 8 | Pages 975-1001 | 1997
© Oxford University Press 1997

Clarence River Supersuite: 250 Ma Cordilleran Tonalitic I-type Intrusions in Eastern Australia

Colleen J. Bryant^*, Richard J. Arculus and Bruce W. Chappell

Key Centre for the Geochemical Evolution and Metallogeny of Continents (CEMOC), Department of Geology, The Australian National University Canberra, A.C.T. 0200, Australia

Received April 3, 1995; Revised typescript accepted March 14, 1997

	ABSTRACT

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
The Clarence River Supersuite (CRS) is one of the three late Permian I-type supersuites in the southern New England Orogen of eastern Australia. It comprises 12 small (mostly <100 km²) intrusions that occur at the northeastern and southern extremities of the batholith. The intrusions are compositionally diverse, ranging from gabbro to monzogranite, but are dominated by tonalite, granodiorite and diorite. They have low abundances of alkalis, large ion lithophile elements, high field strength elements, and light rare earth elements (LREE) relative to granodioritic I-type intrusions. They are also amongst the most isotopically primitive plutonic rocks in eastern Australia, typically having initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7031–0.7042 and _Nd values of +6.2 to +1.6. In these aspects they are similar to the Mesozoic tonalitic association in the American Cordillera, and in particular to the western Peninsular Ranges batholith. Considerable chemical and isotopic diversity within the CRS points to variable conditions of formation and the involvement of multiple sources. Most intrusions are characterized by LREE enrichments, moderate negative Eu anomalies and relatively constant chondrite-normalized middle to heavy rare earth element (MREE to HREE) abundances. Such REE patterns and the presence of early formed pyroxenes are consistent with formation involving high degrees of dehydration melting of amphibolitic source rocks at pressures <0.8 GPa, producing a melt in equilibrium with a granulitic residuum. Despite similar pressures of formation, the MREE depletions and absence of negative Eu anomalies in the high-Si Kaloe Granodiorite group indicate the stabilization of amphibole, during either partial melting or crystallization, under conditions of higher f(H₂O). In contrast, the Duncans Creek Trondhjemite has steep REE patterns and small positive Eu anomalies indicating the stabilization of garnet at depths >26km. Both higher f(H₂O) and higher pressure led to the destabilization of plagioclase, generating magmas with higher abundances of Al, Ga and Sr. At least three isotopically distinct sources were involved in the petrogenesis of the CRS, but the extent to which they contribute varies between plutons. Most intrusions have incorporated an isotopically primitive component that may represent either young isotopically primitive crust or mantle-derived magma. The other sources include granulitic materials with very low initial ⁸⁷Sr/⁸⁶Sr but more evolved _Nd and isotopically evolved upper-crustal material.

KEY WORDS: Eastern Australia; Cordilleran I-type; isotopically primitive; southern New England Orogen; tonalite

	Introduction

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
An important component of studies of granitic rocks is to develop an understanding of how geochemical characteristics relate to their particular source rocks, petrogenetic processes and tectonic environments (e.g. Pitcher, 1982; Pearce et al., 1984; Chappell & Stephens, 1988). The Cordilleran (Pitcher, 1982) or tonalitic (Chappell & Stephens, 1988) I-type rocks (referred to here as the CTI), which are such a prominent feature of Mesozoic magmatism in North and South America, form one specific and important type of granitic rock. The most famous and well-documented examples of this type of intrusion are from the Peninsular Ranges batholith of North America (Larsen, 1948; Gromet & Silver, 1987; Silver & Chappell, 1988) and the Coastal batholith of Peru (Pitcher, 1982, 1993; Atherton, 1990, 1993). Those intrusions, and the CTI generally, are characterized by primitive and typically arc-like chemical and isotopic signatures, and a predominance of tonalitic to low-K granodioritic compositions. They are distinguished from mantle-derived (M-type) plutonic rocks by greater compositional, geochemical and isotopic diversity (Pitcher, 1982, 1993).

The petrogenesis of the CTI is not well understood. They occur in continental margin arcs where the crust is potentially very complex and heterogeneous. Because of the relatively young and isotopically primitive nature of the crustal source materials, it is difficult to ascertain whether they form by normal subduction-related processes, or by recycling of arc material or basaltic underplate. Understanding their source(s) is fundamental in assessing the rates of continental growth, crustal evolution and the redistribution of crustal materials.

The intrusions forming the Clarence River Supersuite (CRS) of the southern New England Oregon (NEO) in eastern Australia provide an opportunity to document CTI outside the American Cordillera. The CRS plutons are geochemically and isotopically distinct from other I-type suites in the southern NEO. It was on the basis of these distinct characteristics that Shaw & Flood, (1981) recognized the CRS as a separate suite. However, given the chemical variability of intrusions examined in this study it is now termed a supersuite (a group of plutons that are compositionally very similar overall, but differ in detail; Wyborn et al., 1987).

Given the rarity of CTI in eastern Australia, it is perhaps surprising that the CRS has received so little attention. The most detailed studies of CRS intrusions have concentrated on plutons in the southern portion of the New England Batholith (NEB) (Hensel, 1982; Hensel et al., 1985; Mason, 1986; Eggins & Hensen, 1987). Previously published chemical analyses are limited to those of Shaw & Flood, (1981; five major element analyses) and Eggins & Hensen, (1987; eight major and trace element analyses). Rare earth element (REE) data have not previously been published for the CRS plutons. This paper presents a detailed summary of the mineralogical, geochemical and isotopic features of the northern and, to a lesser extent, the southern CRS intrusions, and examines the factors involved in their petrogenesis.

	Regional Geology

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
The NEO is the youngest component of the Australian continent, forming a structurally distinct belt that extends for 1300 km along its eastern margin (Fig. 1a). It is a collage of accreted terranes generated in continental arc, island arc, forearc and subduction complex–accretionary wedge settings (Flood & Aitchison, 1988). Active subduction and terrane accretion occurred along, or adjacent to, the margin of Gondwana during the Devonian and Carboniferous (Aitchison et al., 1992; Fergusson & Leitch, 1993). During the Permian, subsequent to amalgamation and accretion, some of the terranes underwent an unconstrained degree of post-accretionary dispersion as a result of widespread north to north-west trending strike-slip deformation (Aitchison & Flood, 1992; Fergusson & Leitch, 1993). Deformation during the Permian (290 Ma, Fergusson & Leitch, 1993; 260 Ma, Collins et al., 1993) produced large-scale folding and east–west thickening of the accretionary wedge. The Texas–Coffs Harbour Megafold, the major structure formed during this event, is thought to have resulted from 500 km of lateral displacement (Murray et al., 1987).

View larger version (46K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 1. Locality figures (a and b) and a map (c) showing distribution of plutons of the northern Clarence River Supersuite.

 
The detailed geology and tectonic evolution of the NEO is complex, although most workers agree that there was a major change in the style of subduction from intra-oceanic (Devonian) to continental marginal arc (Carboniferous) volcanism [see Aitchison et al., (1992), Fergusson & Leitch, (1993) and Collins et al., (1993) for reviews]. The intra-oceanic arc assemblage, which includes altered volcanic arc andesite and associated pyroclastic and epiclastic rocks, minor limestone and radiolarian siltstone, occurs to the west of the Peel Fault (Fig. 1b) within the Gamilaroi Terrane (Flood & Aitchison, 1992). This is unconformably overlain by ignimbritic and volcaniclastic material from the ‘Andean’ phase of the arc (McPhie, 1987; Flood & Aitchison, 1992). The bulk of the southern NEO and the region east of the Peel Fault forms the dismembered Anaiwan Terrane, (Flood & Aitchison, 1988), a unit that contains sub-vertically dipping sequences of basalt, ribbon-bedded tuffaceous chert, tuffaceous siltstone and volcaniclastic sandstone. This terrane is widely interpreted to be an ancient subduction complex (Murray et al., 1987), tectonically analogous to the Shimanto Terrane of southwestern Japan (e.g. Taira et al., 1982) and the Franciscan Complex in California (Blake & Murchey, 1988).The eastern margin of the exposed southern NEO is marked by numerous smaller, variably deformed and metamorphosed terranes of diverse origin (Flood & Aitchison, 1988). In the north, CRS plutons intrude three of these terranes; Gidabal, Yugambal and Bundjalung (Fig. 1c). The Gidabal Terrane comprises mainly Early to Late Carboniferous shallow marine strata, including siltstone, volcanic and felsic plutonic-derived sandstone, and Early Permian felsic to intermediate volcanic rocks, quartzo-feldspathic sandstone and mudstone (Flood & Aitchison, 1988). The Yugambal Terrane (mélange in part) consists of tuffaceous mudstone, fine-grained mafic to intermediate rocks, volcaniclastic sandstones, argillite, conglomerate, limestone breccia and chert (Fergusson, 1984). The tectonic setting of this terrane is somewhat problematical, being variably interpreted as a fragment of a remnant arc (Day et al., 1978), a forearc basin (Leitch, 1975), an exotic intraoceanic arc (Fergusson, 1988), or a back-arc basin (Korsch & Harrington, 1981). With the exception of Korsch & Harrington, (1981), who argued for a Late Carboniferous–Early Permian age, most workers consider this tectonostratigraphic unit to be of Siluro-Devonian age. The Bundjalung Terrane located immediately to the east comprises a serpentinized mass of harzburgite, with lesser amounts of dunite and pyroxenite, intruded by boninitic dykes. This terrane possibly represents a fragment of forearc material (Qureshi et al., 1993).

	New England Batholith

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
Granitic rocks in the southern NEO (collectively referred to as the NEB) are predominantly intruded east of the Peel Fault. Plutonism was mostly restricted to two major pulses. S-type intrusions of the Hillgrove and Copeton (Bundarra) supersuites, located predominantly in the central and western portions of the NEB, were emplaced during the Late Carboniferous–Early Permian (Fig. 1b) (Shaw & Flood, 1981). The CRS and the other I-type supersuites are predominantly of Late Permian–Early Triassic age, and postdate the last known occurrences of ‘Andean’-style continental margin volcanic rocks (McPhie, 1987) in the southern NEO (Aitchison & Flood, 1992).

The CRS is volumetrically a minor component of the NEB, covering an area of 910 km² (6% of the batholith). It consists of 12 relatively small, compositionally distinct plutons, which are typically <100 km² and occur in the northeastern and southern parts of the batholith (Fig. 1b). The five CRS intrusions that occur in the northeastern portion of the southern NEO are elongated in a NNW direction, parallel to the major regional faults and to the margin of the Clarence–Moreton Basin. The southern CRS intrusions form the Nundle Suite of Hensel et al., (1985). These plutons intrude the Gamilaroi and Anaiwan terranes, and are volumetrically less significant than their northern counterparts, forming <30% of the observed CRS outcrops.

	Ages of the Clarence River Supersuite

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
Until recently, there have been few radiometric dates published for the CRS intrusions, particularly for the northern plutons. Shaw & Flood, (1993) proposed that the CRS intrusions were emplaced between 255 and 244 Ma, which corresponds to peak plutonic activity in the southern NEO during which 70% of Late Permian–Early Triassic plutons were emplaced. More specifically, the Dumbudgery Creek and Towgon Grange granodiorites are dated at 249–250 Ma (Rb/Sr-biotite age, Shaw & Flood, 1993) and 258 Ma (⁴⁰Ar/³⁹Ar-hornblende age, Bryant et al., 1997), respectively. With a reported ⁴⁰Ar/³⁹Ar-hornblende age of 250 Ma (Bryant et al., 1997), the Jenny Lind Granodiorite is indistinguishable in age from those two intrusions. The Kaloe Granodiorite is substantially older, having a ⁴⁰Ar/³⁹Ar (hornblende) age of 291 Ma (Bryant et al., 1997).

The southern CRS plutons also appear to vary in age. The Duncans Creek Trondhjemite and Gogs Top Granodiorite have Rb/Sr ages of 248–246 Ma (Shaw & Flood, 1993) and 255 Ma (Hensel et al., 1985), respectively, equivalent to the majority of the northern CRS intrusions. With reported ages of 262 Ma (Rb/Sr dating; Hensel et al., 1985), 269–265 Ma (Roberts & Engel, 1987), 265±8 Ma (U/Pb, zircon; Collins et al., 1993), and 281 Ma (²⁰⁷Pb/²⁰⁶Pb, zircon; Kimbrough et al., 1993), the Barrington Tops Granodiorite appears to be marginally older than other southern CRS intrusions.

	Composition of the Clarence River Supersuite

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
The CRS intrusions are compositionally diverse, ranging from gabbro to monzogranite (Fig. 2), with the proportions of different rock types varying between plutons.

View larger version (11K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 2. Streckeisen diagrams for intrusions of the northern Clarence River Supersuite. The data of Eggins & Hensen, (1987) from Barrington Tops (Omadale Brook and Gummi Plain plutons) are included for comparison. sg, syenogranite; mg, monzogranite; gd, granodiorite; t, tonalite; qm, quartz monzonite; qmd, quartz monzodiorite; m, monzonite; md, monzodiorite; qd, quartz diorite; qg, quartz gabbro; d, diorite; g, gabbro.

 
Northern CRS intrusions
The Towgon Grange Granodiorite is the most mafic intrusion of the CRS, comprising dominantly tonalite, and lesser amounts of quartz gabbro, quartz diorite, minor granodiorite, with rare quartz norite (Fig. 2). It is associated with several clinopyroxene- and/or orthopyroxene-bearing basaltic andesite to dacite units. The Kaloe Granodiorite is predominantly tonalite with lesser volumes of granodiorite. There are also fine-grained felsic rocks whose field relationships with the host intrusion are unknown, because of extremely poor outcrop and, consequently, a lack of detailed mapping. Given that the two are compositionally similar, the fine-grained rocks probably represent either dykes or perhaps a marginal variant. The Kaloe Granodiorite is also intruded by several gabbroic stocks (<20 m across) and by numerous felsic and mafic dykes. The Jenny Lind Granodiorite consists of quartz gabbro, quartz diorite, quartz monzodiorite, tonalite and granodiorite, with no apparent dominant composition. Rocks of the Bruxner Monzogranite form two compositionally and texturally distinct units, hydrothermally altered leucomonzogranite and quartz diorite to granodiorite.

The Dumbudgery Creek Granodiorite is the most compositionally diverse intrusion in the CRS. It contains two distinct units, a tonalite–trondhjemite and a more abundant high-K granodiorite that is locally fractionated to a fine-grained leucomonzogranite. Enclosed within the intrusion are several elongate zones of mafic rocks that include dolerite, porphyritic hornblende gabbro, biotite-free quartz gabbro, and foliated and massive hornblende±biotite quartz diorite. The margins of the dolerite zones are characterized by abundant, small, angular microgranular enclaves and/or by narrow zones of hybridized tonalite–quartz diorite. The abundance, angularity and grainsize of the enclaves indicate that they were formed by magma mingling close to their original site of formation, implying that they were coeval with the host intrusion. This is confirmed by ⁴⁰Ar/³⁹Ar (hornblende) dates of 251 and 254 Ma for quartz diorite samples from the mafic zones (Bryant et al., 1997). The intrusion of the basaltic magma was possibly facilitated by the adjacent terrane boundary. The occurrence is a coincidental rather than integral feature of the CRS intrusions, and as such the petrology of these rocks is not examined in this paper.

Southern CRS intrusions
The Barrington Tops Granodiorite intrusion (which includes Omadale Brook and Gummi Plains plutons) is a composite intrusion consisting of 75% augite–hypersthene granodiorite, 15% hornblende–biotite granodiorite, 10% quartz diorite and minor aplite (Eggins & Hensen, 1987). Other units are less well documented, but consist dominantly of tonalite–trondhjemite (Hensel, 1982).

	Petrography of the Clarence River Supersuite

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
Northern CRS intrusions
The northern CRS intrusions are medium grained (grainsize 2 mm), equigranular and typically comprise plagioclase, hornblende (±variably altered clinopyroxene cores or less commonly orthopyroxene cores) and biotite, with interstitial quartz and K-feldspar. Magnetite, ilmenite, zircon, apatite, ±titanite (usually secondary) are the principal accessory minerals. Allanite is restricted to the high-K granodiorite–leucomonzogranite unit of the Dumbudgery Creek Granodiorite and the monzogranitic compositions of the Bruxner pluton.

Plagioclase
Plagioclase forms euhedral to subhedral, strongly normally zoned (±delicate oscillatory zoning) crystals. They are 0.5–4 mm in length, with individual grains being characterized by a large compositional range from An_50–70 to An_20–35 (Table 1). Plagioclase from the Kaloe Granodiorite is more sodic (An_38–45 to An_14–23), consistent with the higher whole-rock Na contents of that intrusion (see geochemistry section). Other minor inter- and intra-pluton variations in plagioclase compositions (summarized in Table 1) parallel variations in bulk-rock compositions, with the highest An contents being observed in the most mafic intrusions (e.g. Jenny Lind and Towgon Grange granodiorites). Calcic cores, where present, have compositions that also tend to reflect the bulk-rock trends; for example, the Jenny Lind and Kaloe granodiorites have ‘calcic core’ compositions of An_77–89 and An_49–58, respectively (Table 1).

View this table: [in this window] [in a new window]   Table 1: Summary of mineral chemistry from northern CRS intrusions

 
Ferromagnesian minerals
Biotite and hornblende are the most abundant ferromagnesian minerals in the northern CRS intrusions, but the ratios of the two are highly variable. In most cases, hornblende exceeds biotite in more mafic compositions, but abundances are similar in the more felsic variants. However, biotite is the most abundant ferromagnesian mineral in the high-K granodiorite–leucomonzogranite of the Dumbudgery Creek Granodiorite and the monzogranite of the Bruxner intrusion, consistent with the lower Ca and higher K contents of those rocks. In contrast, the higher biotite/hornblende ratios in the Kaloe Granodiorite are attributed to higher Al, as this intrusion does not have high K contents. Biotite occurs both as irregular grains around the margins of pyroxene and hornblende aggregates, and as isolated euhedral to subhedral crystals.

Hornblende barometry calculations made using the method of Andersen & Smith, (1995) and assuming a temperature of 700°C, indicate typical crystallization pressures (104 analyses) of <0.24 GPa (2.4 kbar), with an average of 0.14 GPa. The temperature of 700°C was chosen as it is lower than the interpreted peak magmatic temperatures within the hornblende stability field, and eliminates both scatter and negative pressure estimates. The pressures calculated for cores are within error of those from the rims, consistent with the similar mg-number [mg-number=Mg/(Mg+Fe)x100] of the hornblende (Table 1), implying that the magmas only became H₂O saturated at a shallow depth.

Clinopyroxene is present in all intrusions in varying proportions, being very minor in the Kaloe and Dumbudgery Creek granodiorites (excluding mafic zones). The most mafic intrusions (e.g. Towgon Grange and Jenny Lind granodiorites, and the quartz diorite–granodiorite unit of the Bruxner Monzogranite) contain the highest abundances of clinopyroxene. Overall the abundance of clinopyroxene within a particular intrusion decreases with increasing Si, but is not systematically correlated with either quartz or K-feldspar contents. Clinopyroxene is typically rimmed by magmatic hornblende, and variably replaced by patchy blue–green actinolitic hornblende. Many clinopyroxene grains are also deuterically altered to a fibrous, colourless to pale green amphibole, probably actinolite. Fresh clinopyroxene has mg-number of 69.9–75.1, and compositions of Wo_37.1–46.0En_38.4–43.1Fs_14.6–19.7 (Table 1). Cr₂O₃ is below detection in all cases (<0.10%). TiO₂ abundances are low (<0.44%), as are the pressure-sensitive components Na₂O (<0.33%) and Al₂O₃ (<2.1%).

Orthopyroxene is restricted to more mafic rocks (<65% SiO₂) with low abundances of hydrous minerals. Consequently, its occurrence in the northern CRS intrusions is relatively minor, occurring in 5–8% of rocks from the Towgon Grange and Jenny Lind granodiorites and the Bruxner Monzogranite. In rocks with the appropriate composition, it may form a major ferromagnesian phase, such as in the Towgon Grange Granodiorite where quartz norite compositions are locally attained. Orthopyroxene is characterized by a larger compositional range and lower mg-number than clinopyroxene (mg-number of 56.9–69.0, En_53.4–67.1Fs_30.2–44.7Wo_1.1–2.7). This is also a feature of pyroxene inclusions trapped within plagioclase grains. Given that the differences are typically so substantial, it is unlikely that the clinopyroxene and ortho–pyroxene were in equilibrium. We consider it unlikely that the mg-number offset is due to differential subsolidus re-equilibration, and argue that the differences are real, and that they result from progressive crystallization from a fractionating melt.

Southern CRS intrusions
The southern intrusions are characterized by a similar mineral assemblage to the northern CRS plutons. However, in the case of the Omadale Brook pluton, pyroxenes form the dominant or only ferromagnesian minerals present (Eggins & Hensen, 1987). Although the clinopyroxene in the Barrington Tops Granodiorite is on average more magnesian (mg-number=60–82) than those in the northern CRS plutons, other mineral compositions are similar. Orthopyroxene within the host intrusion (mg-number=56–65), and as inclusions within plagioclase crystals, has lower mg-numbers than the coexisting clinopyroxene [see data of Eggins & Hensen, (1987), and Mason, (1986)].

	Geochemistry of the Clarence River Supersuite

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
Major and trace element data for 26 rocks of the CRS are given in Tables 2 and 3. Sample numbers listed in those tables refer to the collections of the University of New England (R prefix) and the Australian National University (A prefix). A total of 94 samples have been analysed and all of these data are utilized in Figs 3 and 4. Averages of samples from each intrusion are shown in Table 4. Major element data were obtained by X-ray fluorescence (XRF) spectrometry using fused discs [modified technique of Norrish & Hutton, (1969)]. The trace elements Rb, Sr, Ba, Zr, Nb, Y, V, Cr, Ni, Cu, Zn, Ga, As and Pb were measured on pressed powder pellets using the X-ray spectrometric methods described by Chappell, (1991). The elements Cs, Hf, Ta, REE, Sc, Co, W, Th and U were determined by instrumental neutron activation analysis (Chappell & Hergt, 1989).

View this table: [in this window] [in a new window]   Table 2: Chemical analyses of rocks from the Jenny Lind, Towgon Grange and Kaloe intrusions

 

View this table: [in this window] [in a new window]   Table 3: Chemical analyses of rocks from the Bruxner, Dumbudgery Creek, Duncans Creek, Gummi Plain and Omadale Brook intrusions

 

View larger version (20K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 3. Harker diagrams for selected elements in units of the Clarence River Supersuite. , Towgon Grange; , Jenny Lind; , Bruxner; , high-K Dumbudgery Creek; , low-K Dumbudgery Creek; , Kaloe; , Omadale Brook; , Gummi Plain; x, Duncans Creek Trondhjemite; +, Mt Ephraim.

 

View larger version (10K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 4. Harker diagrams for selected elements, illustrating the similar composition of the Kaloe, Duncans Creek and Mt Ephraim plutons (symbols as for Fig. 3) and their distinctive chemical character with respect to other intrusions of the Clarence River Supersuite ().

 

View this table: [in this window] [in a new window]   Table 4: Comparison of normalized averages and 1 standard deviations (in parentheses) for intrusions of the Clarence River Supersuite, and I-type intrusions from the western Peninsular Ranges batholith and Lachlan Fold Belt

 
With the exception of the Dumbudgery Creek Granodiorite, the other major elements for a particular intrusion are moderately correlated with SiO₂ (Figs 3 and 4). The overall scatter of the CRS on trace element diagrams is principally due to interpluton variations, with individual intrusions typically defining relatively coherent trends (Figs 3 and 4). Exceptions to this include the Dumbudgery Creek Granodiorite and, to a lesser extent, the Kaloe Granodiorite, which contain two or more chemically distinct units. Chemical variability within and between plutons of the CRS is summarized below, but the discussion of their implications for their respective sources is deferred to the petrogenesis section.

Northern CRS intrusions
The Towgon Grange and Jenny Lind plutons have very similar major and trace element compositions. They define the dominant CRS trends (e.g. low abundances of incompatible elements) and provide a reference for other plutons.

The Dumbudgery Creek Granodiorite is the most chemically heterogeneous intrusion in the CRS, consisting of at least two distinct units (excluding rocks from the mafic zones). These are termed the low-K and high-K groups, and correspond to the tonalite–trondhjemite and granodiorite–leucomonzogranite units, respectively. The high-K Dumbudgery Creek Granodiorite is a chemically coherent group that is characterized by high abundances of Ga, Rb, Pb, Th (Fig. 3), Nb and U. This unit has lower Ca, Y (Fig. 3), and slightly lower Fe_total than the Towgon Grange Granodiorite at equivalent Si contents. In contrast, the low-K group is scattered on most Harker diagrams. On the basis of field relationships, petrography and chemical composition, the majority of these samples are not considered to be directly genetically related, but were grouped together because of their low K contents. This group is compositionally similar to the Towgon Grange and Jenny Lind granodiorites.

The Bruxner Monzogranite also has elevated concentrations of K, Rb, Th, Pb (Fig. 3), the light REE (LREE; Fig. 5) and U. However, with the exception of Pb, the concentrations are not as high as those in the Dumbudgery Creek Granodiorite at equivalent SiO₂. The abundances of other elements are equivalent to those in the Towgon Grange and Jenny Lind granodiorites.

View larger version (23K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 5. Plots of rare earth elements in selected intrusions of the Clarence River Supersuite, normalized to C1 chondrite of Sun & McDonough, (1989). Sample numbers are listed downwards with increasing SiO2 contents. REE data for the western (plagioclase+pyroxene source) and central (garnet-bearing source) Peninsular Ranges batholith of Gromet & Silver, (1987) are included for comparison.

 
The Kaloe Granodiorite is chemically distinct from other northern CRS intrusions, containing higher Al (Fig. 4), Ga, Sr (Fig. 3) and Na, and lower Sc, V (Fig. 4) and Fe_total at equivalent Si contents. Within the Kaloe Granodiorite, there are two compositionally distinct units:

1. A low-Si Group (63.3–64.6% SiO₂), characterized by higher concentrations of Ti, P, Zr, Nb, Y (Fig. 3) and LREE than other members of the Kaloe Granodiorite. The abundances of these elements are also higher than in other northern CRS intrusions.
   
2. A high-Si group (67.1–70.1% SiO₂) that includes tonalite to granodiorite intrusives, felsic dykes and the associated fine-grained felsic rocks characterized by low abundances of Y (Fig. 3) and the REE. This group is higher in Sr and Na than the low-Si group.
   

Comparison with southern CRS intrusions
The Duncans Creek Trondhjemite and the Mt Ephraim intrusion are similar in composition to the high-Si Kaloe group, having higher abundances of Al (Fig. 4) and Sr (Fig. 3), and lower Sc, V (Fig. 4) and Y (Fig. 3) contents than elsewhere in the CRS. However, these two intrusions are characterized by higher Sr, Th, Pb (Fig. 3), U, P, Ba and the LREE (Fig. 5) and lower Na than the Kaloe Granodiorite.

The Gummi Plain pluton is compositionally similar to the low-Si Kaloe group, containing higher abundances of Ga, Y, Zr (Fig. 3), Nb, Ti and P, and, to a lesser extent, slightly higher LREE (Fig. 5) and Na than most northern CRS intrusions. The Omadale Brook Granodiorite is chemically most similar to the Towgon Grange Granodiorite although it has slightly higher Ga, Sr (Fig. 3) and P, and marginally lower Sc and Nb contents.

REE patterns and abundances
All CRS intrusions are characterized by an enrichment in the LREE (La to Nd) relative to the heavy REE (HREE=Ho to Lu) on a chondrite-normalized basis (Fig. 5). Most samples have relatively flat middle REE (MREE=Sm to Tb) to HREE patterns, and a moderate negative Eu anomaly. Deviations from this general case, and more subtle variations between or within plutons, are examined below.

The Towgon Grange Granodiorite is characterized by low LREE abundances, having La_N/Lu_N [chondrite-normalized values using C1 chondrite of Sun & McDonough, (1989)] ratios of between 2.7 and 3.9. Samples R72163B, R72098 [GenBank] and R70203 [GenBank] have typical CRS-shaped REE patterns. Sample R72096 [GenBank] has a very similar pattern, but with much higher total REE abundances and a pronounced negative Eu anomaly. In contrast, R72172 [GenBank] has low REE contents and no Eu anomaly. The rocks of the high-K Dumbudgery Creek Granodiorite are characterized by high LREE abundances, and large negative Eu anomalies (Fig. 5). La_N/Lu_N ratios in that intrusion range between 6.2 and 13.3, with La varying between 80 and 140 times chondrite values. Rocks from the low- and high-Si groups of the Kaloe Granodiorite have strongly contrasting REE patterns (Fig. 5). The low-Si group patterns are similar to those typical of most other CRS intrusions. In contrast, the high-Si group have considerably lower REE contents, higher La_N/Lu_N (5.0–8.3 compared with 3.7–4.8 for the low-Si group), with higher LREE/MREE ratios and little or no Eu anomaly.

The Duncans Creek Trondhjemite has fundamentally different REE abundances with respect to other CRS intrusions. Notably, the chondrite-normalized patterns are very steep (Fig. 5), with La_N/Lu_N ratios of 16.9 and 24.8, and samples have small positive Eu anomalies.

Isotope geochemistry
In their study of the NEB, Shaw & Flood, (1981) found that the CRS have an average initial ⁸⁷Sr/⁸⁶Sr ratio (Sr_i) of 0.7037, with four analyses ranging between 0.7034 and 0.7042. Hensel et al., (1985) obtained similar results for the southern CRS intrusions, having Sr_i values of 0.70351–0.70396 and _Nd values of +3.3 to +6.1. However, both of these studies were regional and principally concerned with the geochemical and isotopic variations within the NEB. There are few published isotopic analyses for many CRS intrusions, particularly the northern ones. New Sr and Nd results for the northern CRS intrusions are presented in Table 5 and Fig. 6a–d.

View this table: [in this window] [in a new window]   Table 5: Sr and Nd isotopic data for the intrusions of the northern Clarence River Supersuite

 

View larger version (13K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 6. Variation diagrams for Sri and Nd with increasing SiO2, and Nd and Nd model ages (TDM) with Sri. Symbols are the same as in Fig. 3. In the inset (top right), the field occupied by the Clarence River Supersuite is shaded. Also plotted are analyses of sedimentary rocks from Hensel et al., (1985), west () and east () of the Peel Fault, corrected to 250 Ma.

 
Sr and Nd were separated using standard ion-exchange column techniques at the University of New England. The separates were analysed by thermal ionization mass spectrometry (VG354 spectrometer) at the Centre for Isotope Studies at North Ryde, N.S.W. Analysis of reference standards averaged values of ⁸⁷Sr/⁸⁶Sr=0.71027±1 for NBS987 and ¹⁴³Nd/¹⁴⁴Nd=0.511111±10 for the O'Nions Nd standard. The ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd were normalized to ⁸⁶Sr/⁸⁸Sr=0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd=0.7219, respectively. Sr_i and _Nd were calculated assuming ages of: 260 Ma, Towgon Grange Granodiorite; 253 Ma, Dumbudgery Creek Granodiorite; 250 Ma, Bruxner Monzogranite and Jenny Lind Granodiorite; 291 Ma, Kaloe Granodiorite. The data of Hensel et al., (1985) for the Duncans Creek Trondhjemite, the Mt Ephraim intrusion, and Omadale Brook pluton were recalculated using ages of 248 Ma, 248 Ma and 265 Ma, respectively, and are plotted in Fig. 6a–c.

The Towgon Grange Granodiorite is characterized by essentially uniform Sr_i (0.70325–0.70350), with slightly more variable _Nd (+6.26 to +4.70). In contrast, the Jenny Lind Granodiorite is more variable and isotopically evolved. Contrary to ‘normal’ isotopic systems (see DePaolo, 1981a), the Sr_i decreases from 0.70406 to 0.70338 with increasing Si (Fig. 6a), with a concomitant increase in _Nd from +2.11 to +5.29 (Fig. 6b). The Sr_i in the Kaloe Granodiorite also decreases with increasing Si (Fig. 6a), ranging between 0.70314 and 0.70372. The Sr_i of 0.70298 for R72003 [GenBank] , a fine-grained marginal variant or dyke associated with the Kaloe Granodiorite, is consistent with this trend. With the exception of R72026 [GenBank] , collected from the margin of the intrusion and probably contaminated by the adjacent wall rocks, there is remarkably little variation in _Nd, and no correlation with Si (Fig. 6b).

The isotopic composition of the low-K Dumbudgery Creek Granodiorite contrasts strongly with the high-K group from the same intrusion. The low-K group is isotopically equivalent to the Towgon Grange Granodiorite, whereas the high-K group is more evolved, with Sr_i ranging between 0.70352 and 0.70489 and _Nd from +4.24 to +0.43. With the exception of R72315 [GenBank] , both isotopic systems became more evolved with increasing fractionation (Fig. 6a).

Single isotopic results from the Bruxner Monzogranite and Omadale Brook plutons are similar to those of the Towgon Grange Granodiorite. The Duncans Creek and Mt Ephraim intrusions [recalculated data of Hensel et al., (1985)] have similar _Nd, but higher Sr_i than the chemically similar Kaloe Granodiorite.

	Comparisons with Other Supersuites

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
The CRS is chemically distinct from other I-type supersuites in the NEB, having lower abundances of incompatible elements such as P, K, Rb, Nb (Fig. 7), Pb, Th and U. It also has lower abundances of Ba, LREE (Fig. 7), and, to a lesser extent, Sr and Zr. On average, the CRS are more mafic than the I-type suites of the Lachlan Fold Belt (LFB), with a preponderance of low-K rocks (Table 4) and more primitive isotopic compositions. Unlike the I-type suites of the LFB, the CRS has a well-defined arc signature, most notably in the low HFSE abundances. This is most evident for Nb (Table 4; Fig. 7), which is depleted relative to the other HFSE (e.g. Zr and Ti). The CRS also has lower LREE than is typical for the LFB I-type granites.

View larger version (27K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 7. Variation diagrams of p.p.m. Nb vs % SiO2 and p.p.m. La vs % TiO2 for the plutons of the Clarence River Supersuite (), other I-type supersuites of the New England Batholith (Moonbi Supersuite, ; Uralla Supersuite, ; B. W. Chappell, unpublished data, 1994), and Devonian I-type granites of the Lachlan Fold Belt (; B. W. Chappell, unpublished data, 1994).

 
The CRS has mineralogical, geochemical and isotopic characteristics that are more typical of the Mesozoic CTI of the America Cordillera such as the Peninsular Ranges batholith (PRB) (Silver & Chappell, 1988) and the Coastal batholith of Peru (Pitcher, 1982; Atherton, 1990, 1993). The PRB has a comparable compositional diversity to the CRS, comprising gabbro, quartz gabbro, quartz diorite, tonalite (dominant) and low-K granodiorite, with minor leucogranodiorite, leucotonalite and monzogranite. Major and trace element pattern abundances of the CRS and western PRB are also very similar (Table 4; Fig. 8), although the CRS is characterized by significantly lower abundances of Cr and Ni. Both have high Na₂O/K₂O ratios, a distinctive feature of CTI (Chappell & Stephens, 1988). Despite the primitive isotopic nature of the CRS relative to other granitic rocks of eastern Australia, the Sr_i and _Nd values are comparable with those of the western PRB [generally Sr_i=0.703–0.705 (Silver & Chappell, 1988) and _Nd=0 to +8.2 (DePaolo, 1981a)]. The intrusions from these two regions also have very similar REE patterns and abundances (Gromet & Silver, 1987; Fig. 5).

View larger version (20K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 8. Spider diagram comparing the average trace element compositions of northern Clarence River intrusions (Table 4), and Zones A, B and C from the Peninsular Ranges batholith (shaded field) (Silver & Chappell, 1988) normalized to N-type MORB (Saunders & Tarney, 1984; Sun & McDonough, 1989).

 

	Petrogenesis

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
There are strong compositional similarities between the CRS and other CTI. Because of that, in the first instance we will examine more broadly the petrogenesis of that larger group. Petrogenetic models for the CTI must account for their compositional diversity, primitive isotopic characteristics, arc-like signatures and their spatial association with both modern and more ancient arc settings. Proposed models range between two possible end-members, fractional crystallization of arc basalts (e.g. Eggins & Hensen, 1987) and partial melting of crustal materials, such as subarc crust, continental lithosphere or basaltic underplate (e.g. Gromet & Silver, 1987; Chappell & Stephens, 1988). Intermediate models invoke some combination of the two sources including magma mixing (Reid et al., 1983) or AFC–contamination processes (DePaolo, 1981a; Boily et al., 1989). Additionally, more complex processes such as melting–assimilation–storage–homogenization must be considered (Hildreth & Moorbath, 1988).

Fractional crystallization of basalt
Eggins & Hensen, (1987) argued that the hot, dry and isotopically primitive nature of the Gummi Plain and Omadale Brook plutons is most adequately explained by the fractional crystallization of basalt. They proposed that the complementary cumulates are located at the Mohorovicic discontinuity, where the magmas were forced to pond and fractionate as a result of density contrast between the mantle and crust. This process would be difficult to detect geochemically if it occurred at a pressure below that required for the formation of garnet. There are several factors that argue against this process, when occurring in isolation, forming the CRS as a whole. In most plutons it is impossible to account for the observed isotopic variations without invoking a significant crustal component. The CRS intrusions are also chemically and compositionally more evolved than M-type suites, which are thought to evolve principally by fractional crystallization of basalts (Whalen, 1985).

Partial melting of a basaltic underplate or subarc crust
On the basis of a detailed REE study, combined with other geochemical and isotopic data, Gromet & Silver, (1987) concluded that the intrusions of the western PRB were derived by partial melting of a basaltic–gabbroic underplate. Similarly, Atherton, (1990, 1993) proposed that the predominantly tonalitic Peruvian Coastal batholith resulted from the rapid recycling of relatively young basaltic crust within a rifted continental margin. Chappell & Stephens, (1988) argued that the CTI are the first-stage product of crustal remagmatization, which ultimately produces a range of I-type suites.

Partial melting within the crust is generally thought to take place under fluid-absent conditions, with reaction and incongruent melting being volumetrically dominated by hydrous minerals such as mica and amphibole (e.g. Clemens & Vielzeuf, 1987; Beard & Lofgren, 1989). Amphibolite dehydrational melting experiments conducted at 750–1100°C, over a wide variety of pressures, generate granite-granodiorite-tonalite magmas (e.g. Rushmer, 1991; Beard & Lofgren, 1991; Rapp et al., 1991; Wolf & Wyllie, 1994; Patiño Douce & Beard, 1995), which are similar to or indistinguishable from natural compositions (Beard & Lofgren, 1991). The volume and nature of the resultant melts are strongly influenced by the starting composition, temperature, pressure and H₂O content in hydrated minerals in the source, and physical dynamics of the melting process (Bergantz & Dawes, 1994). Heat ultimately controls the degree of partial melting, and consequently the residual mineral assemblage. During dehydration melting experiments, hornblende is totally consumed at 925–1000°C, leaving a granulitic residue of clinopyroxene, orthopyroxene, plagioclase and Fe–Ti oxides with garnet at higher pressures (Rushmer, 1991; Beard & Lofgren, 1991; Wolf & Wyllie, 1994). Melting to higher temperatures partially consumes the remaining minerals. Although partial melting during this latter stage is volumetrically dominated by plagioclase, the residual mineral assemblage remains unchanged for a considerable isobaric temperature increase.

The residual mineral assemblage of the source, and minerals fractionating from the generated magma, collectively determine the observed geochemical characteristics, and in particular the REE patterns of the resultant rocks. In most CRS intrusions, accessory minerals cannot have influenced the observed REE patterns. The inflexions in Zr (Fig. 3) and P on Harker diagrams for the CRS (excluding the Kaloe and Gummi Plain intrusions) indicate that zircon and apatite remain undersaturated until 65% and 68% SiO₂, respectively (see Watson, 1979; Green & Watson, 1982). Titanite is usually secondary, and allanite is restricted to the LREE-rich compositions of the high-K Dumbudgery Creek Granodiorite and Bruxner Monzogranite. Consequently, the observed REE patterns must reflect the major minerals of either the residual source material and/or the fractionating mineral assemblage.

With the exception of the high-Si Kaloe and Duncans Creek intrusions, the CRS has REE patterns characterized by an enrichment in LREE, relatively flat MREE to HREE, and a moderate negative Eu anomaly. This general pattern is very similar to the CTI of the western PRB (Gromet & Silver, 1987) and Chilliwack batholith, North Cascades (Tepper et al., 1993). On the basis of modelling, both Gromet & Silver, (1987) and Tepper et al., (1993) argued that the REE patterns result from partial melting of a mafic lower-crustal source, where the resulting magma was in equilibrium with a gabbroic residue. Both the presence of liquidus pyroxene and the REE patterns in most CRS intrusions (LREE enrichment, flat MREE–HREE and moderate negative Eu anomalies) support such a model. Additionally, the mineral compositions observed in the CRS correspond to those generated in dehydration melting experiments. Plagioclase compositions within the experiments of Beard & Lofgren, (1991) are in the range An_26–73, varying strongly as a function of both temperature and starting composition. Beard & Lofgren reported typical core to rim zoning of An_53–45, with six of their charges having zoning of An_64–53. The broader compositional spectrum in the CRS intrusions (typically cores of An_50–70 and rims of An_20–35) probably reflects both the more protracted fractionation history and the diversity of the starting compositions. Beard & Lofgren, (1991) indicated that orthopyroxene and clinopyroxene generated at 1000°C have mg-numbers of 68–76 and 69–73, respectively. Although clinopyroxene compositions are equivalent to those observed in the CRS, orthopyroxene is markedly more magnesian.

The MREE are depleted in the high-Si Kaloe Granodiorite, implying the involvement of hornblende, either as a residual or as a fractionating mineral. The REE patterns of the high-Si Kaloe Granodiorite strongly resemble those observed by Tepper et al., (1993) in the leucocratic intrusions of the Chilliwack batholith. Those workers proposed that the magmas were generated in equilibrium with an amphibolitic residuum, at f(H₂O) of 0.2–0.3 GPa.

The smooth and curved REE patterns, one with a slight positive Eu anomaly, for samples R72172 [GenBank] and R70211 [GenBank] of the Towgon Grange and Jenny Lind granodiorites, cannot be readily attributed to a residual amphibole, as both samples contain abundant pyroxene. It would be possible to generate a melt with liquidus pyroxene and such an REE pattern through fractional rather than batch melting. However, it is not apparent in samples containing only marginally higher SiO₂ contents. A more plausible explanation is that these samples contain cumulate pyroxene and some plagioclase, or formed from liquids that underwent extensive clinopyroxene fractionation.

Pressure of partial melting
Hornblende geobarometry, which indicates equilibration at low pressure and is clearly related to emplacement, provides no evidence for the depth of partial melting. Except for the Duncans Creek Trondhjemite, there is no indication that garnet was either a residual or a fractionating mineral in the CRS. Garnet was absent in dehydration melting experiments conducted at 0.8 GPa (Rushmer, 1991; Rapp et al., 1991) and present in those conducted at 1 GPa (Wolf & Wyllie, 1994), but the location of the garnet-in curve is composition dependent. On that basis, we argue that most CRS intrusions were generated at pressures <0.8 GPa. Although unlikely, they could have been generated at pressures up to 1 GPa, as garnet may be totally consumed during dehydration melting at temperatures of 1000°C (Wolf & Wyllie, 1994).

The significantly steeper REE pattern of the Duncans Creek Trondhjemite implies the involvement of small amounts of garnet as either a fractionating or a residual mineral (see Green, 1992). This indicates that partial melting for that intrusion occurred at pressures >0.8 GPa.

Temperature of the melt
Pyroxene equilibria temperatures reported for the Barrington Tops Granodiorite range from 1000°C to 500°C (Mason, 1986; Eggins & Hensen, 1987), with an associated pyroxene dacite having calculated temperatures of 950–1050°C (Mason, 1986). However, like the northern CRS intrusions the clinopyroxene and orthopyroxene have significantly discordant mg-numbers, and therefore are unlikely to be in equilibrium. Despite this, the calculated temperatures are probably close to the actual magmatic temperatures. One unifying conclusion of CRS studies is that the rocks crystallized from relatively liquid magmas, containing minimal residual material (e.g. Hensel, 1982; Mason, 1986; Eggins & Hensen, 1987; Bryant, 1992). Comparison with experimental results indicates that the tonalitic CRS compositions would have liquidus temperatures of 1000–1100°C (e.g. Wyllie et al., 1976). Such temperatures are consistent with REE patterns controlled by a residual granulitic mineral assemblage, as experimental studies indicate that amphibole would have been exhausted in the source at 925–1000°C (Rushmer, 1991; Beard & Lofgren, 1991; Wolf & Wyllie, 1994).

Dehydration melting of amphibolite at 1000°C typically generates 40–50 vol. % melt (Rushmer, 1991; Beard & Lofgren, 1991; Wolf & Wyllie, 1994). Using a starting composition containing 48.4% SiO₂, and temperatures of 1000°C and 975°C, the dehydration melting experiments of Wolf & Wyllie, (1994) generated liquids of 51.8 and 57.5% SiO₂, respectively. Similarly, Rushmer, (1991) produced a partial melt with 54.8% SiO₂, at 950°C from a tholeiite containing 51.7% SiO₂, and 1.36% H₂O. These results imply that it is possible to generate even the most mafic CRS rocks, as melts, by dehydrational melting of amphibolitic sources, given sufficiently high temperatures.

Source of heat
When considering the problem of crustal melting, a major problem is explaining how the crust attains the very high temperatures (1000°C) required. The low volume of primitive tonalitic magmas in the NEB, and the relatively small size of the intrusions, may in part reflect the difficulty in attaining these temperatures. Given formation pressures of 0.8 and 1 GPa, the required geothermal gradients are 42°C/km and 33°C/km, respectively. Such high gradients are not uncommon beneath modern volcanic fronts (Honda & Uyeda, 1983) and marginal basins. For example, Atherton, (1990) reported that the Huarmey marginal basin locally attained geothermal gradients as high as 300°C/km. He proposed that the tonalites of the Peruvian Coastal batholith formed by low-pressure melting (5–10 km) caused by axial vertical fracturing and subsequent adiabatic decompression of relatively new crust within this basin.

The exact cause of partial melting of large volumes of lower and/or mid-crustal sources during the Permian–Triassic plutonic event in the southern NEO is poorly understood. Shaw et al., (1991) indicated that the igneous activity took place in a rift or graben structure similar to the Taupo Volcanic Zone or the Rio Grande Rift. However, this rifting event was not as extreme as that proposed by Atherton, (1990, 1993), and it is doubtful whether in the present case adiabatic decompression alone could generate a tonalitic melt in equilibrium with a granulitic residuum. In the absence of adiabatic decompression, multiple intrusions of basaltic melt would be required to thermally mature the crust to a state where regional-scale melt generation is possible (Bergantz & Dawes, 1994).

The presence of coeval basaltic magmas within the Dumbudgery Creek Granodiorite strongly supports the underplating hypothesis for the generation of the CRS intrusions. However, the amount to which these magmas have physically and chemically contributed to the CRS intrusions is poorly understood. Bergantz & Dawes, (1994) indicated that initially strongly contrasting thermal conditions between the country rock and the basaltic magma decay, forming a stepped thermal profile. Latent heat released during fractional crystallization of the basalt is conductively transferred to the country rock, resulting in partial melting. Initially, the basaltic magma will mix with the partial melt, but because of density and viscosity contrasts, the two become chemically isolated. Modelling by Campbell & Turner, (1987) and Bergantz & Dawes, (1994) indicates that partial melting of the lower crust can occur with minimal mass input from the basaltic melt. However, any process that destabilizes the boundary layer may result in some magma mixing or mingling between the two. Given the relatively young and isotopically primitive nature of the crustal materials required in the petrogenesis of most CRS plutons, it is not possible to evaluate the relative contribution of the crustal and mantle components, especially if homogenization had taken place before emplacement. It is also possible that the degree of involvement of mantle-derived magmas varies from one pluton to another. The chemically depleted nature and primitive isotopic characteristics of the Towgon Grange Granodiorite, and the absence of hydrous minerals in the Omadale Brook pluton, possibly result from the addition of a mantle-derived component.

With the exception of the Dumbudgery Creek Granodiorite, microgranular enclaves are very minor in CRS intrusions, implying limited interaction between basaltic and more felsic magmas at shallow crustal levels. Although some magma mixing or mingling is evident in the heterogeneous low-K Dumbudgery Creek Granodiorite, this is not characteristic of the remaining low-K group rocks, or the CRS as a whole. This implies that if any mantle component was involved in the petrogenesis, it was incorporated at deeper crustal levels, probably during the melting event.

Role of fractional crystallization
Strong normal zoning in plagioclase, the presence of pyroxene rimmed by hornblende and subsequently biotite, and interstitial quartz and K-feldspar, imply progressive crystallization from an evolving liquid. On Harker diagrams, TiO₂ and P₂O₅ have inflexions at 65% and 68% SiO₂, respectively, which may correspond to the saturation and subsequent fractional crystallization of Fe–Ti oxides and apatite. Other more typical chemical indicators of fractional crystallization are absent. For example, there is no rapid decrease in the abundance of highly compatible elements, such as Ni and Cr, with increasing Si.

However, this does exclude the possibility of significant opaque oxide fractionation before the emplacement of the CRS intrusions. Such a process could potentially explain the lower abundances of Cr and Ni in the CRS relative to the western PRB CTI. In the case of incompatible elements (e.g. Rb), the CRS intrusions never attain the haplogranite (eutectic) compositions required for the extreme fractionation of these elements. Although there is clearly textural and lesser chemical evidence supporting fractional crystallization, the isotopic variability within many intrusions implies that this process alone cannot have controlled their complete chemical diversity.

Petrogenetic discussion with reference to individual plutons
We will now consider the chemical and isotopic variations within the CRS and their petrogenetic implications.

Towgon Grange Granodiorite
The Sr_i values are essentially uniform throughout, defining a total rock isochron of 245±21 Ma, within analytical error of the ⁴⁰Ar/³⁹Ar age. There is considerably more variation in the _Nd and T_DM (+6.26 to +4.70 and 474–811 Ma, respectively; Table 5), with the most mafic (R72172 [GenBank] ) and most felsic (R72096 [GenBank] ) samples having the lowest and highest values, respectively. We propose that the Towgon Grange Granodiorite incorporated a relatively young and isotopically primitive component, possibly a mantle-derived melt, plus an older crustal component with time-integrated low Sm/Nd and Rb/Sr (DePaolo, 1981a; Farmer & DePaolo, 1984), as observed in some lower-crustal granulites (Cohen et al., 1984; Chen & Arculus, 1995).

Jenny Lind Granodiorite
Although the Jenny Lind Granodiorite is geochemically very similar to the Towgon Grange Granodiorite, it is isotopically more diverse and evolved (Fig. 6a–c). It is notable that Sr_i decreases and _Nd increases with increasing Si. The exact cause of this phenomenon is unclear, although there are several geological situations in which this could occur:

1. The Jenny Lind Granodiorite is younger than other CRS plutons, and the assumed age of 250 Ma could have resulted in over-correction of the Sr_i and _Nd values. However, ages of <200 Ma are required to counter the observed isotopic trends. On the basis of field relations alone, such an age is geologically untenable, as the Jenny Lind Granodiorite is unconformably overlain by the Late Triassic–Early Jurassic (200 Ma) Laytons Range Conglomerate (Thomson, 1976). Additionally, the ⁴⁰Ar/³⁹Ar correlation age of 231±7 Ma (Bryant et al., 1997) represents the minimum emplacement age.
   
2. The observed trends result from magma mixing or AFC. Although possible, it would be highly unusual, as it would require the mixing between an isotopically evolved mantle component with very old T_DM (1200 Ma) and primitive crustal sources.
   
3. The observed trends are due to isotopic heterogeneity in the crustal source. This is possible if the source contained intercalated basalt and mafic volcanogenic sediments. Hydrated weathered sediments are more fertile than underplated basaltic rocks (e.g. Clemens & Vielzeuf, 1987), and generally more isotopically evolved. A similar result could be achieved by the partial melting of variably aged crustal source rocks.
   

Kaloe Granodiorite
The Kaloe Granodiorite is somewhat enigmatic in terms of its age and composition, although comparable high Al–Ga–Sr and low Sc–V–Fe–Y characteristics are observed in the Duncans Creek Trondhjemite.

The interpretation of the Kaloe Granodiorite is complicated by the presence of two distinct units, whose exact petrogenetic relationship is unclear. Individually, the high- and low-Si groups rarely show strong linear trends, and they are separated by a small compositional gap. The low-Si group is diopside-normative (di=0.99–1.69%) and has REE patterns consistent with a gabbroic residual mineral assemblage. In contrast, the high-Si group is corundum-normative (C=0.45–1.26%), and has REE patterns that are more consistent with hornblende being a residual or fractionating mineral. However, hornblende alone cannot explain the observed REE patterns. There are several possible alternative explanations for the contrasting compositional characteristics of the high- and low Si units:

1. Both groups were produced by peritectic melting under H₂O-excess conditions. Ellis & Thompson, (1986) indicated that partial melting at 0.5 GPa and H₂O-excess initially yields C-normative compositions, with a crossover to di-normative compositions at higher temperatures (1000°C). Liquidus minerals vary between hornblende and clinopyroxene at low and high temperatures, respectively.
   
2. Both groups were produced by dehydration melting. Wolf & Wyllie, (1994) indicated that dehydrational melting of rocks containing An-rich plagioclase initially yields C-normative liquids, with a crossover to di-normative compositions at 1000°C, following the complete consumption of hornblende at 975°C. Although the presence of An-rich plagioclase does not exclude the formation of trondhjemitic liquids, it is not a prerequisite for the generation of C-normative magmas by dehydrational melting. Patiño Douce & Beard, (1995) also generated C-normative liquids by dehydrational melting of a synthetic quartz amphibolite, in which the plagioclase had a composition of An₃₈. The liquids generated in those experiments were granodioritic and granitic in composition; that does not preclude the formation of tonalitic melts from a more appropriate starting composition with higher Na/K.
   
3. The two groups are related by AFC processes involving extended hornblende fractionation (Cawthorn & O'Hara, 1976). Given that the most mafic sample has 63.3% SiO₂, such a model requires significant fractionation before the contamination event, and an incomplete representation of the compositional spectrum at the surface.
   

Models (1) and (2) require that the felsic C-normative melt was sourced, or had a component derived from, a lower-crustal source with time-integrated low Rb/Sr and Sm/Nd, as observed in some lower-crustal granulites. However, the granulite would have to have undergone retrogression and hydration to produce the REE patterns observed in the high-Si group. In the case of AFC, the granulite would represent the contaminant. With the exception of R72026 [GenBank] , which was contaminated during emplacement, there is little evidence for the involvement of an upper-crustal component.

There is a strong positive correlation between ⁸⁷Sr/⁸⁶Sr and 1/Sr for the Kaloe Granodiorite (r²=0.96), implying some petrogenetic relationship between the low- and high-Si groups. However, the absence of strong correlations on Harker diagrams and for plots of ¹⁴³Nd/¹⁴⁴Nd vs 1/Nd argues against this being a simple two end-member mixing process.

In the case of AFC, it is possible that geochemical differences between the two groups, including lower P, Zr, Ti, Sc, Fe and REE, are explained by the combined fractionation of amphibole, biotite, zircon, apatite±ilmenite. In models (1) and (2), variations in the source compositions, mineralogy and/or contrasting fractionation histories are required to explain differences in the geochemical and REE characteristics of the low- and high-Si groups. For example, the differences in Zr concentrations in the low- and high-Si groups may in part be controlled by the metaluminous vs peraluminous nature of the generated melts (Watson, 1979).

Duncans Creek Trondhjemite and Mt Ephraim intrusion
The Duncans Creek Trondhjemite and Mt Ephraim intrusion, like the Kaloe Granodiorite, are characterized by high Al–Ga–Sr, and low Sc–V–Fe–Y, implying similar processes and/or sources were involved in their petrogenesis. However, the Duncans Creek Trondhjemite is characterized by a steeper REE pattern with a depletion in the HREE and insignificant Eu anomalies, consistent with a source containing residual garnet. The overprinting of this signature by an MREE depletion implies that hornblende was either a residual or fractionating mineral. This is consistent with the hypothesis that the high Al–Ga–Sr, and low Sc–V–Fe–Y characteristics of both this intrusion and the Kaloe Granodiorite are fundamentally linked to the involvement of hornblende.

The presence of residual garnet rather than feldspar, results in higher Ba, Sr, Pb and LREE in the Duncans Creek Trondhjemite than in the Kaloe Granodiorite. However, differences in source mineralogy alone cannot completely explain the disparities in the compositions of the two intrusions. For example, the Duncans Creek Trondhjemite is characterized by higher P, Th and U than the Kaloe Granodiorite. Such variations are thought to reflect regional differences in lower-crustal compositions, as the neighbouring Moonbi Supersuite intrusions are also characterized by high P, Ba, Sr, Th (40 p.p.m. Th at 55% SiO₂), U, Pb and LREE (Chappell, 1994).

High-K Dumbudgery Creek Granodiorite
The high-K Dumbudgery Creek Granodiorite is characterized both by higher and more rapidly increasing abundances of Rb, Cs, Th, U and Pb than other members of the CRS. Those features, together with the higher Fe₂O₃/FeO ratios (average of 1.31) and more evolved isotopic characteristics of this unit, indicate the involvement of an oxidized upper-crustal source component (Fig. 9). The isotopic composition of the most felsic end-member approaches that calculated for sediments to the east of the Peel Fault at 250 Ma (inset, Fig. 6c). It is uncertain whether the sedimentary component was incorporated through partial melting, by AFC processes, or by mixing in the source material. Sample R72315 [GenBank] has higher _Nd and lower Sr_i than other high-K Dumbudgery Creek Granodiorite samples at equivalent SiO₂, and may indicate some mixing between the high- and low-K magmas.

View larger version (13K): [in this window] [in a new window] [Download PowerPoint slide]   Fig. 9. Schematic diagram highlighting the nature of various components involved in the petrogenesis of individual intrusions of the Clarence River Supersuite.

 
Bruxner Monzogranite
The Bruxner Monzogranite has many critical chemical characteristics that are similar to the high-K Dumbudgery Creek Granodiorite, including high abundances of alkalis, Pb, Th and U relative to normal CRS intrusions, together with high Fe₂O₃/FeO ratios (average of 1.30). It may be that the Bruxner Monzogranite has incorporated small amounts of more evolved, and enriched upper-crustal components. The Sr_i-_Nd datum available for this intrusion is similar to data of the Towgon Grange Granodiorite.

	The Crustal Source

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
The relatively large degree of isotopic variation within individual intrusions precludes their formation by simple fractional crystallization of any isotopically uniform melt. This variation implies incomplete homogenization by mixing before or during emplacement, either in lower- or upper-crustal magma chambers. Also, the entire spectrum of the CRS cannot have been produced by simple mixing between variable amounts of a mantle-derived basalt and a single, high Sr_i-low _Nd crustal component. Excluding the Dumbudgery Creek Granodiorite and possibly the Bruxner Monzogranite, the CRS intrusions cannot have not undergone significant degrees of AFC at shallow crustal levels. Hence, the observed isotopic variation must reflect heterogeneity in the lower-crustal sources. Overall, at least three components are required in the petrogenesis of the CRS (Fig. 9); (1) an isotopically primitive component that may represent either mantle-derived magmas or young isotopically primitive crust, (2) LILE-enriched and isotopically evolved upper-crustal material such as the sediments to the east of the Peel Fault, and (3) lower-crustal materials that are isotopically primitive in terms of their Sr_i, but with comparatively low and possibly variable _Nd values, as observed in some granulitic terranes. Also, the isotopically evolved signature of R72026 [GenBank] from the Kaloe Granodiorite indicates that this and possibly other CRS intrusions have locally incorporated small amounts of upper-crustal materials during their emplacement.

The chemical and isotopic variations in the CRS do not correlate with the surface geology. The Towgon Grange, Kaloe and Dumbudgery Creek granodiorites were all emplaced into the same terrane (a Devonian island arc sequence), but have strongly contrasting isotopic and chemical characteristics. The lower-crustal ‘granulitic’ component incorporated in the Kaloe Granodiorite is more extreme than that involved in the Towgon Grange Granodiorite (Fig. 9). Neither lower-crustal component is detected in the Dumbudgery Creek Granodiorite, although it is possible they were overwhelmed by the dominant signature from the upper crust. The involvement of older granulitic components is not entirely consistent with current tectonic models for the NEO, or the isotopically primitive nature of the CRS intrusions, which imply young crustal source components. However, it should be emphasized that the involvement of this crustal component does not require that the entire southern NEO is underlain by a granulitic terrain. Unfolding of the Texas-Coffs Harbour Megafold places the Siluro-Devonian Yugambal Terrane at the western boundary of the currently exposed NEO (Cawood & Leitch, 1985; Collins et al., 1993). It is possible that the partial melting event that formed the Kaloe Granodiorite at 290 Ma sampled an underlying granulitic terrane. The fact that the Towgon Grange and Dumbudgery Creek intrusions are not as isotopically extreme as the Kaloe Granodiorite is perhaps the result of partial or total delamination during the Texas-Coffs Harbour megafolding event.

The primitive isotopic compositions of the CRS intrusions indicate that the basaltic source components involved in their genesis must have been derived from long-term depleted mantle (i.e. chondrite-normalized Sm/Nd>1). It also implies limited ageing of the protolith before the CRS-forming partial melting event, and indicates that any recycling of LFB crust into these sources was not significant.

	Summary

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
The CRS intrusions contain early-formed clinopyroxene and orthopyroxene, indicating that they were initially relatively dry melts, with the appearance of hornblende and the development of water saturation at shallow crustal levels (P<0.24 GPa). Most intrusions in the CRS are LREE enriched, have relatively flat MREE–HREE and are characterized by moderate negative Eu anomalies. These REE patterns are consistent with a petrogenetic model involving the dehydration melting of amphibolite at temperatures of at least 1000°C, leaving a granulitic mineral assemblage of clinopyroxene, orthopyroxene and Fe–Ti oxides. The REE patterns for high-Si Kaloe Granodiorite and Duncans Creek Trondhjemite differ from this general model, requiring distinctly different petrogenetic histories. The MREE depletions and absence of negative Eu anomalies in the high-Si Kaloe Granodiorite are consistent with the involvement of hornblende, as either a residual or a fractionating phase. In contrast, the steep REE patterns and positive Eu anomalies of the Duncans Creek Trondhjemite require the presence of residual garnet and generation at pressures >0.8 GPa. Both the higher f(H₂O) required for the stabilization of amphibole and higher pressures necessary for the formation of garnet lead to destabilization of plagioclase, consistent with the high-Al–Ga–Sr characteristics observed in these two units. The absence of steep REE patterns in all but the Duncans Creek Trondhjemite indicates that for those intrusions dehydrational melting took place at P<0.8 GPa (depths <26 km).

The partial melting event in the southern NEO is generally thought to be in response to rifting (e.g. Shaw et al., 1991; Collins et al., 1993). In that case, the increased geothermal gradient would result from a combination of crustal thinning, basaltic underplating–intrusion±adiabatic decompression. Although we consider that much of the inter- and intra-pluton isotopic variations is due to heterogeneity within the crust, principally expressed as different source components, a mantle component cannot be discounted. However, the isotopic and chemical signatures for many of the analysed plutons are inconsistent with two end-member mixing or AFC processes. It may be that the interaction between the basaltic melts and the crust is more complex than can be accounted for by such relatively simple models. If basalts have contributed chemically to the petrogenesis of the CRS intrusions, this must have occurred through mixing at deeper crustal levels, probably during the melting event, as there is little evidence of such processes at shallow crustal levels.

	Acknowledgements

 
This project was initially undertaken by C.J.B. as a B.Sc. Honours project at the University of New England. We appreciate the help of Peter Flood during that stage of the study. Nick Ware (Research School of Earth Sciences, ANU) assisted with the microprobe analyses. Rikki Davidson assisted with the Sr and Nd separations. Ian Parkinson, Elaine McPherson and two anonymous reviewers are thanked for their helpful comments, and Charlotte Allen and Phil Blevin for their stimulating discussions. The assistance of Heather Davies is appreciated for improving the presentation of this paper. Our efforts were supported by the Australian Research Council Grants SARC94017 and A39030242 to R.J.A. and A39232908 to B.W.C. Isotopic measurements were made at the Centre for Isotope Studies—a joint ARC–CSIRO supported facility, under ARC grant to R.J.A. A grant to B.W.C. from the Australian Institute of Nuclear Science and Engineering covered the cost of neutron irradiations. This is Publication 61 in the Key Centre for the Geochemical Evolution and Metallogeny of Continents.

---

^* Corresponding author. Telephone: 61-6-2492056. Fax: 61-6-2495544. e-mail: colleen.bryant{at}anu.edu.au

	References

TOP ABSTRACT Introduction Regional Geology New England Batholith Ages of the Clarence... Composition of the Clarence... Petrography of the Clarence... Geochemistry of the Clarence... Comparisons with Other... Petrogenesis The Crustal Source Summary References

 
Aitchison J. C., Flood P. G. Early Permian transform margin development of the southern New England Orogen, eastern Australia (eastern Gondwana). Tectonics (1992) 11:1385–1391.[Web of Science]

Aitchison J. C., Flood P. G., Spiller F. C. P. Tectonic setting and paleoenvironment of terranes in the southern New England Orogen, eastern Australia, as constrained by radiolarian biostratigraphy. Palaeogeography, Palaeoclimatology and Palaeoecology (1992) 94:31–54.

Andersen J. L., Smith D. R. The effects of temperature and fO₂ on the Al-in-hornblende barometer. American Mineralogist (1995) 80:549–559.[Abstract]

Atherton M. P. The Coastal Batholith of Peru: the product of rapid recycling of ‘new’ crust formed within rifted continental margin. Geological Journal (1990) 25:337–349.[Web of Science]

Atherton M. P. Granite magmatism. Journal of the Geological Society of London (1993) 150:1009–1023.[Abstract/Free Full Text]

Beard J. S., Lofgren G. E. Effect of water on the composition of partial melts of greenstone and amphibolite. Science (1989) 244:195–197.[Abstract/Free Full Text]

Beard J. S., Lofgren G. E. Dehydration melting and water-saturated melting of basaltic and andesitic greenstones and amphibolites at 1, 3, and 6.9 kb. Journal of Petrology (1991) 32:365–401.[Abstract/Free Full Text]

Bergantz G. W., Dawes R. Aspects of magma generation and ascent in continental lithosphere. In: Magmatic Systems—Ryan M. P., ed. (1994) New York: Academic Press. 291–317.

Blake M. C. Jr., Murchey B. L. A Californian model for the New England Fold Belt. Quarterly Notes of the Geological Survey New South Wales (1988) 72:1–9.

Boily M., Brooks C., Ludden J. N., James D. E. Chemical and isotopic evolution of the Coastal batholith of southern Peru. Journal of Geophysical Research (1989) 94:12483–12498.

Bryant C. J. Clarence River Suite granitoids: petrography and geochemistry. (1992) Armidale, N.S.W.: University of New England. 251. B.Sc. Thesis.

Bryant C. J., Cosca M. A., Arculus R. J. ⁴⁰Ar/³⁹Ar ages of Clarence River Supersuite intrusions from the northern portion of the New England Batholith, southern New England Orogen. Australian Journal of Earth Sciences (1997) In press.

Campbell I. H., Turner J. S. A laboratory investigation of assimilation at the top of a basaltic magma chamber. Journal of Geology (1987) 95:155–172.[Web of Science]

Cawood P. A., Leitch E. C. Accretion and dispersal tectonics of the southern New England Fold Belt, eastern Australia. In: Tectonostratigraphic Terranes of the Circum-Pacific Region. Earth Sciences Series 1—Howell D. G., Jones D. L., Cox A., Nur A., eds. (1985) Houston, TX: Circum-Pacific Council of Energy and Mineral Resources. 481–492.

Cawthorn R. G., O'Hara M. J. Amphibole fractionation in calc-alkaline magma genesis. American Journal of Science (1976) 276:309–329.[Abstract/Free Full Text]

Chappell B. W. Trace element analysis of rocks by X-ray spectrometry. Advances in X-ray Analysis (1991) 34:263–276.

Chappell B. W. Lachlan and New England: fold belts of contrasting magmatic and tectonic development. In: Journal of Proceedings of the Royal Society New South Wales (1994) 127:47–59.

Chappell B. W., Hergt J. M. The use of known Fe content as a flux monitor in neutron activation analysis. Chemical Geology (1989) 78:151–157.[Web of Science]

Chappell B. W., Stephens W. E. Origin of infracrustal (I-type) granite magmas. Transactions of the Royal Society of Edinburgh, Earth Sciences (1988) 79:71–86.

Chen W., Arculus R. J. Geochemical and isotopic characteristics of lower crustal xenoliths, San Francisco Volcanic Field, Arizona, U.S.A. Lithos (1995) 36:203–225.[Web of Science]

Clemens J. D., Vielzeuf D. Constraints on melting and magma production in the crust. Earth and Planetary Science Letters (1987) 86:287–306.[Web of Science]

Cohen R. S., O'Nions R. K., Dawson J. B. Isotope geochemistry of xenoliths from East Africa: implications for development of mantle reservoirs and their interaction. Earth and Planetary Science Letters (1984) 68:209–220.[Web of Science]

Collins W. J., Offler R., Farrell T. R., Landenberger B. A revised Late Palaeozoic–Early Mesozoic tectonic history for the southern New England Fold Belt. In: New England Orogen, Eastern Australia—Flood P. G., Aitchison J. C., eds. (1993) Armidale, N.S.W.: Department of Geology and Geophysics, University of New England. 69–84.

Day R. W., Murray C. G., Whitaker W. G. The eastern part of the Tasman orogenic zone. Tectonophysics (1978) 48:327–364.[Web of Science]

DePaolo D. J. A neodymium and strontium isotopic study of the Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular Ranges, California. Journal of Geophysical Research (1981a) 86:10470–10488.

DePaolo D. J. Neodymium isotopes in the Colorado Front Range and crust–mantle evolution in the Proterozoic. Nature (1981b) 91:193–196.

Eggins S., Hensen B. J. Evolution of mantle-derived, augite–hypersthene granodiorites by crystal–liquid fractionation: Barrington Tops Batholith, eastern Australia. Lithos (1987) 20:295–310.[Web of Science]

Ellis D. J., Thompson A. B. Subsolidus and partial melting reactions in the quartz-excess CaO+MgO+Al₂O₃+SiO₂+H₂O system under water-excess and water-deficient conditions to 10 kb: some implications for the origin of peraluminous melts from mafic rocks. Journal of Petrology (1986) 27:91–121.[Abstract/Free Full Text]

Farmer G. L., DePaolo D. J. Origin of Mesozoic and Tertiary granite in the western United States and implications for pre-Mesozoic crustal structure: 2. Nd and Sr isotopic studies of unmineralized and Cu- and Mo-mineralized granite in the Precambrian craton. Journal of Geophysical Research (1984) 89:10141–10160.

Fergusson C. L. Tectono-stratigraphy of a Palaeozoic subduction complex in the central Coffs Harbour Block of north-eastern New South Wales. Australian Journal of Earth Sciences (1984) 31:217–236.[Web of Science]

Fergusson C. L. Tectonostratigraphic terranes in the central Coffs Harbour Block, northeastern New South Wales. In: New England Orogen Tectonics and Metallogenesis—Kleeman J. D., ed. (1988) Armidale, N.S.W.: Department of Geology and Geophysics, University of New England. 42–48.

Fergusson C. L., Leitch E. C. Late Carboniferous to Early Triassic tectonics of the New England Fold Belt, eastern Australia. In: New England Orogen, Eastern Australia—Flood P. G., Aitchison J. C., eds. (1993) Armidale, N.S.W.: Department of Geology and Geophysics, University of New England. 53–67.

Flood P. G., Aitchison J. C. Tectonostratigraphic terranes of the southern part of the New England Orogen. In: New England Orogen Tectonics and Metallogenesis—Kleeman J. D., ed. (1988) Armidale, N.S.W.: Department of Geology and Geophysics, University of New England. 7–10.

Flood P. G., Aitchison J. C. Late Devonian accretion of the Gamilaroi terrane to eastern Gondwana: provenance linkage suggested by the first appearance of Lachlan Fold Belt-derived quartzarenite. Australian Journal of Earth Sciences (1992) 39:539–544.[Web of Science]

Green T. H. Experimental phase equilibrium studies of garnet-bearing I-type volcanics and high level intrusives from Northland, New Zealand. Transactions of the Royal Society of Edinburgh, Earth Sciences (1992) 83:429–438.[Web of Science]

Green T. H., Watson E. B. Crystallization of apatite in natural magmas under high pressure, hydrous conditions, with particular reference to ‘orogenic’ rock series. Contributions to Mineralogy and Petrology (1982) 79:96–105.[Web of Science]

Gromet L. P., Silver L. T. REE variations across the Peninsular Ranges batholith: implications for batholithic petrogenesis and crustal growth in magmatic arcs. Journal of Petrology (1987) 28:75–125.[Abstract/Free Full Text]

Hensel H. D. The mineralogy, petrology and geochronology of granitoids and associated intrusives from the southern portion of the New England Batholith. (1982) Armidale, N.S.W.: University of New England. 273. Ph.D. Thesis.

Hensel H. D., McCulloch M. T., Chappell B. W. The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks. Geochimica et Cosmochimica Acta (1985) 49:369–384.[Web of Science]

Hildreth W., Moorbath S. Crustal contributions to arc magmatism in the Andes of Central Chile. Contributions to Mineralogy and Petrology (1988) 98:455–489.[Web of Science]

Honda S., Uyeda S. Thermal process in subduction zones—a review and preliminary approach on the origin of arc volcanism. In: Arc Volcanism—Physics and Tectonics—Shimozuru D., Yokoyama I., eds. (1983) Tokyo: Terrapublishing. 117–140.

Kimbrough D. L., Cross K. C., Korsch R. J. U–Pb isotopic ages for zircons from the Pola Fogal and Nundle granite suites, southern New England Orogen. In: New England Orogen, Eastern Australia—Flood P. G., Aitchison J. C., eds. (1993) Armidale, N.S.W.: Department of Geology and Geophysics, University of New England. 403–412.

Korsch R. J., Harrington H. J. Stratigraphic and structural synthesis of the New England Orogen. Journal of the Geological Society of Australia (1981) 28:205–226.[Web of Science]

Larsen E. S. Batholith and associated rocks of Corona, Elsinore, and San Luis Rey Quadrangles, Southern California. In: Geological Society of America, Memoir (1948) 29.

Leitch E. C. Plate tectonic interpretation of the Paleozoic history of the New England Fold Belt. Geological Society of America Bulletin (1975) 86:141–144.[Abstract/Free Full Text]

Mason D. R. Magmatic ferromagnesian inclusions in granitoid plagioclase cores, Barrington Tops Granodiorite, New South Wales, Australia. American Mineralogist (1986) 71:1314–1321.[Abstract]

McPhie J. Permo-Carboniferous silicic volcanism and palaeogeography on the western edge of the New England Orogen, north-eastern New South Wales. Journal of the Geological Society of Australia (1987) 31:133–146.

Murray C. G., Fergusson C. L., Flood P. G., Whitaker W. G., Korsch R. J. Plate tectonic model for the Carboniferous evolution of the New England Fold Belt. Australian Journal of Earth Sciences (1987) 34:213–236.[Web of Science]

Norrish K., Hutton J. T. An accurate X-ray spectrographic method for the analysis of a wide range of geological samples. Geochimica et Cosmochimica Acta (1969) 33:431–453.[Web of Science]

Patiño Douce A. E., Beard J. S. Dehydration-melting of biotite gneiss and quartz amphibolite from 3 to 15 kbar. Journal of Petrology (1995) 36:707–738.[Abstract/Free Full Text]

Pearce J. A., Harris N. B. W., Tindle A. G. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. Journal of Petrology (1984) 25:956–983.[Abstract/Free Full Text]

Pitcher W. S. Granite type and tectonic environment. In: Mountain Building Processes—Hsu K. J., ed. (1982) London: Academic Press. 19–40.

Pitcher W. S. The Nature and Origin of Granite (1993) Glasgow: Blackie. 316.

Qureshi S. A., Arculus R. J., Flood P. G. Occurrence and significance of boninite-series rocks in serpentinized harzburgite of the Bundjalung Terrane, New England Orogen. In: New England Orogen, Eastern Australia—Flood P. G., Aitchison J. C., eds. (1993) Armidale, N.S.W.: Department of Geology and Geophysics, University of New England. 157–158.

Rapp R. P., Watson E. B., Miller C. F. Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites. Precambrian Research (1991) 51:1–25.[Web of Science]

Reid J. B., Evans O. C., Fates D. G. Magma mixing in granitic rocks of the central Sierra Nevada, California. Earth and Planetary Science Letters (1983) 66:243–261.[Web of Science]

Roberts J., Engel B. A. Depositional and tectonic history of the southern New England Orogen. Australian Journal of Earth Sciences (1987) 34:1–20.[Web of Science]

Rushmer T. Partial melting of two amphibolites: contrasting experimental results under fluid-absent conditions. Contributions to Mineralogy and Petrology (1991) 107:41–59.[Web of Science]

Saunders A. D., Tarney J. Geochemical characteristics of basaltic volcanism within back-arc basin. In: Marginal Basin Geology. Geological Society Special Publication—Kolelaar B. P., Howells M. F., eds. (1984) 16:59–76.

Shaw S. E., Flood R. H. The New England Batholith, eastern Australia: geochemical variations in time and space. Journal of Geophysical Research (1981) 86:10530–10544.

Shaw S. E., Flood R. H. A compilation of Late Permian and Triassic biotite Rb–Sr from the New England Batholith and areas to the southeast. In: Centre for Isotope Studies, Research Report, 1991–1992 (1993) North Ryde, Sydney: N.S.W.:CSIRO. 151–155.

Shaw S. E., Conaghan P. J., Flood R. H. Late Permian and Triassic igneous activity in the New England Batholith and contemporaneous tephra in the Sydney and Gunnedah Basins. In: Diessel, C. F. K. (convenor). In: Advances in the Study of the Sydney Basin, Twenty Fifth Newcastle Symposium (1991) Newcastle: N.S.W.: The University of Newcastle. 44–51.

Silver L. T., Chappell B. W. The Peninsular Ranges batholith: an insight into the evolution of the Cordilleran batholiths of southwestern North America. Transactions of the Royal Society of Edinburgh, Earth Sciences (1988) 79:105–121.

Sun S.-S., McDonough W. F. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Magmatism in the Ocean Basins. Geological Society Special Publication—Saunders A. D., Norry M. J., eds. (1989) 42:313–345.

Taira A., Okada H., Whitaker J. H., Smith A. J. The Shimanto Belt of Japan: Cretaceous–lower Miocene active-margin sedimentation. In: Trench–Forearc Geology. Geological Society Special Publication—Legget J. K., ed. (1982) 10:5–26.

Tepper J. H., Nelson B. K., Bergantz G. W., Irving A. J. Petrology of the Chilliwack batholith, North Cascades, Washington: generation of calc-alkaline granitoids by melting of mafic lower crust with variable water fugacity. Contributions to Mineralogy and Petrology (1993) 113:333–351.[Web of Science]

Thomson J. Geology of the Drake 1:100 000 sheet 9340 (1976) Sydney: New South Wales Geological Survey, Department of Mines. 185.

Watson E. B. Zircon saturation in felsic liquids: experimental results and applications to trace element geochemistry. Contributions to Mineralogy and Petrology (1979) 70:407–419.[Web of Science]

Whalen J. B. Geochemistry of an island-arc plutonic suite: the Uasilau–Yau Yau intrusive complex, New Britain, P.N.G. Journal of Petrology (1985) 26:603–632.[Abstract/Free Full Text]

Wolf M. B., Wyllie P. J. Dehydration-melting of amphibolite at 10 kbar: the effects of temperature and time. Contributions to Mineralogy and Petrology (1994) 115:369–383.[Web of Science]

Wyborn D., Turner B. S., Chappell B. W. The Boggy Plain Supersuite: a distinctive belt of I-type igneous rocks of potential economic significance in the Lachlan Fold Belt. Australian Journal of Earth Sciences (1987) 34:21–43.[Web of Science]

Wyllie P. J., Huang W.-L., Stern C. R., Maaloe S. Granitic magmas: possible and impossible sources, water contents, and crystallisation sequences. Canadian Journal of Earth Sciences (1976) 13:1007–1019.

CiteULike    Connotea    Del.icio.us    What's this?

This article has been cited by other articles:

				S. E. Shaw and R. H. Flood Zircon Hf Isotopic Evidence for Mixing of Crustal and Silicic Mantle-derived Magmas in a Zoned Granite Pluton, Eastern Australia J. Petrology, January 22, 2009; (2009) egn078v1. [Abstract] [Full Text] [PDF]

This Article Abstract FREE Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Add to My Personal Archive Download to citation manager Request Permissions Google Scholar Articles by Bryant, C. J. Articles by Chappell, B. W. Search for Related Content Social Bookmarking          What's this?

Online ISSN 1460-2415 - Print ISSN 0022-3530

Copyright © 2009 Oxford University Press

Oxford Journals Oxford University Press

• Site Map
• Privacy Policy
• Frequently Asked Questions

Other Oxford University Press sites: Oxford University Press Oxford Journals China Oxford Journals Japan American National Biography Booksellers' Information Service Children's Fiction and Poetry Children's Reference Corporate & Special Sales Dictionaries Dictionary of National Biography Digital Reference English Language Teaching Higher Education Textbooks Humanities International Education Unit Law Medicine Music Online Products Oxford English Dictionary Reference Rights and Permissions Science School Books Social Sciences Very Short Introductions World's Classics