Geochemistry and Chronology of Tectonic Blocks in Serpentinite Mélange of the Southern New England Fold Belt, NSW, Australia

Gondwana Research, V 7, No. 3, pp. 817-831. 02004 International Associationfor Gondwana Research, Japan. ISSN: 1342-937X

Geochemistry and Chronology of Tectonic Blocks in Serpentinite M6lange of the Southern New England Fold Belt, NSW, Australia S. Sanol, R. Offle?, H. Hyodo3and T. Watanabe4* Earth Science Laboratoy, Faculty of Education, Ehime University, Matsuyama 790-8577, Japan,E-mail: [email protected] Discipline of Earth Sciences, School of Environmental 6 Life Sciences, University of Newcastle, Callaghan,NS W2308, Australia Research Institute of Natural Sciences, Okayama University of Science, 2-1 Ridai-cho, Okayama 700-0005,Japan Division of Earth and Planetary Sciences, Hokkaido University, Sapporo, Japan (Manuscript received April 1,2003; accepted January 13,2004)

Abstract Tectonic blocks are widely distributed throughout the serpentinite melange of the Peel-Manning Fault System, southern New England Fold Belt, Australia. ’ h o meta-diorite blocks from the Pigna Barney area have distinctly differing chemical signatures. One resembles boninite, containing high Ni and Cr, low rare earth element (REEs), and high field strength element (HFSEs) contents. Further, it exhibits an enriched chondrite normalized LREE pattern and an atypical &Nd536Ma value of -4. Well-defined isochrons for this sample gave Sm-Nd and Rb-Sr ages of 536k38 Ma and 426238 Ma respectively. The former represents the emplacement age and the latter resetting of the Rb-Sr system during a later metamorphic event. The initial Nd isotopic composition suggests an enriched source. The other meta-diorite block has a flat REE pattern, a T i m ratio >10 and Y, La and Nb values typical of tholeiitic magmas erupted in an island arc setting. By contrast, blueschists from this locality and from the Glenrock Station area show depleted REE patterns, Ti/V = 20-50, and ENd,,,Ma = +8, features characteristic of normal mid-ocean ridge basalts (N-MORB). Meta-diorites from the Glenrock Station area have moderately depleted signatures (&Nd,,,,, = +4.5) and Ti/V ratios and REE patterns typical of calc-alkaline and tholeiitic island arc magmas. These contrast with hornblende cumulate rocks from this location, which have very high Cr and Ni, low Zr and Y, and show slight LREE-enriched chondrite normalized Wo,,En,, ,FsS,,) in these rocks reveal an patterns. Chromites (100 Cr/Cr+Al = 85) and clinopyroxenes (Wo,,En,,.,Fs,,,island arc affinity. The ENd420Ma values vary from +2 to +4 and Rb-Sr ages from 425544 Ma to 411 k 1 5 Ma. The core of a single grain of cumulus hornblende gave a poorly defined ,0ArP9Ar age of 394 Ma in contrast to younger ages (235-263 Ma) obtained from other grains. The younger ages are attributed to resetting of the K-Ar system during a metamorphic event. This study has provided further evidence for an Early Paleozoic arc-trench system along the eastern Gondwana margin.

Key words: Tectonic blocks, Early Palaeozoic, geochemistry, geochronology, southern New England Fold Belt.

Introduction The New England Fold Belt (NEFB) extends approximately 1600 km along the eastern margin of the Australian continent and from west to east is made up of a volcanic arc, fore-arc basin and subduction-accretion complex (Leitch, 1975; Murray et al., 1987; Coney et al., 1990). In the southern New England Fold Belt (SNEFB), Late Silurian to Carboniferous arc and fore-arcbasin sequences of the Tamworth Belt are juxtaposed against the Silurian to Early Carboniferous subduction-accretion complex sequences of the Tablelands Complex by the Peel-Manning Fault System (PMFS). Within the PMFS, tectonic blocks of varying composition, size and age occur in a serpentinite mClange. In the northern part of the PMFS, plagiogranites of Early

* Deceased

Cambrian age (U-Pb zircon; 530 Ma; Aitchison and Ireland, 1995) and eclogite (U-Pb zircon; 536 Ma; Fanning et al., 2002) are present and in the southern part, a zircon U-Pb age of 436 Ma has been reported from a plagioclasepoor “amphibolite” mass (tonalite/hornblendite) in the Pigna Barney area (Kimbrough et al., 1993). Furthermore, phengite K-Ar ages of 482-467 Ma from blueschist blocks (Fukui et al., 1995) and an 40Ar/39Arage of ca. 400 Ma from the core and ca. 240 Ma from the rim of a hornblende in a hornblende cumulate rock from the Pigna Barney and Glenrock Station areas, have also been reported (Hyodo et al., 1999a). Preliminary major and trace element studies of blueschists similar to those dated by Fukui et al. (1995), and of other tectonic blocks, have been carried out by Cross (1983), Aitchison and Ireland (1995), and Offler

818

S. SANO ET AL.

(1999) in these areas and in serpentinites from other parts of the PMFS. The tectonic blocks of early Paleozoic age concentrated in the PMFS provide an opportunity to obtain a better understanding of the tectonic history of the eastern margin of the Gondwana supercontinent during the early Palaeozoic but this will only emerge from further geochemical data, particularly REE and isotope (Sm-Nd; Rb-Sr) data. ‘These should provide an insight into the tectonic setting in which the protoliths of the tectonic blocks were formed, the composition of the protoliths, and possible sources of the magmas from which the protoliths were derived. This paper reports new Rb-Sr and Sm-Nd isotope data, 40Ar/39Arage results and the geochemistry of tectonic blocks in the serpentinite melange from the Pigna Barney and Glenrock Station areas in the southern part of the PMFS. The possible tectonic setting, geochronology and origin of the tectonic blocks are discussed.

Geology The study area is located in the southern part of the SNEFB (Fig. 1).The area is underlain by the Gamilaroi, Weraerai, Djungati and Anaiwan terranes (Fig. 1; Aitchison et al., 1997). The Gamilaroi terrane is made up of Late

Silurian-Devonianarc volcanics and volcanogenicsediments (Flood and Aitchison, 1988,1992;Aitchison and Flood, 1995; Stratford and Aitchison, 1996; Offler, 1982a; Offler and Gamble, 2002), and Cambrian and Ordovician sedimentary rocks of arc provenance lying unconformably below the later Paleozoic rocks (Leitch and Cawood, 1987; Stewart, 1995). Geochemical studies suggest that the Late SilurianEarly Devonian sequences of the Gamilaroi terrane formed in an intra-oceanic island arc setting that was rifted in the Middle to Late Devonian (Aitchison et al., 1993; Stratford and Aitchison, 1997; Offler and Gamble 2002). The Weraerai terrane crops out along the PMFS and is characterized by disrupted, partly or completely serpentinized, suprasubduction zone ultramafic rocks and tectonic blocks of varying size and composition. They include eclogite, blueschists, greenschists, meta-basalts, amphibolites, hornblende cumulate rocks (Shaw and Flood, 1974; Allan and Leitch, 1992; Offler, 1982b; Cross, 1983). The composition of chromites in the ultramafic rocks resembles those in supra-subduction zone ophiolites (Yang and Seccombe, 1997). The Djungati and Anaiwan terranes form part of the subduction-accretion complex of the SNEFB; the former is dominated by red-bedded cherts, and the latter by arc-derived volcaniclastic sediments (Aitchison et al., 1997 and references therein).

Fig. 1. Geology of the Pigna Barney and Glenrock Station areas (modified from Aitchison et al., 1997).

Gondwana Research, V. 7, No. 3, 2004

GEOCHEMISTRYAND CHRONOLOGY OF NEW ENGLAND FOLD BELT, AUSTRALIA

Petrography Samples of tectonic blocks from serpentinite melange of the Weraerai terrane were collected from the Pigna Barney and Glenrock Station areas (Fig. 1). Most are of igneous origin and retain their magmatic textures. However, they have been altered to varying degrees by later metamorphic events. Shear veins filled by prehnite % albite k epidote & calcite are ubiquitous in all samples. Brief descriptions of these rocks are given below.

Meta-diorite (Pigna Barney) (72904) Meta-diorite is composed mainly of medium-grained, green, poikilitic, semi-euhedral to anhedral, magnesiohornblende which is partially replaced by cummingtonite (Table 1). The magnesio-hornblende commonly contains inclusions of euhedral plagioclase, rounded quartz and Cr-rich magnetite (Cr,O, = 3.08%; Table 1).Degraded biotite, anhedral to euhedral quartz and plagioclase, and accessory zircon and apatite, occur in the matrix. Chlorite, epidote, white mica and actinolite replace the magmatic minerals.

Meta-diorite (Pigna Barney) (72908) This meta-diorite is similar to 72904 both texturally and mineralogically. Deep green hornblende with brown cores, turbid plagioclase and quartz are the dominant magmatic phases; apatite is an accessory. Secondary epidote commonly replaces plagioclase and actinolite replaces hornblende.

Meta-diorite (Glenrock Station) (72207, 72208B) Both samples consist of interlocking aggregates of subhedral to euhedral plagioclase (An43-64) interdispersed with green magnesio-hornblende or tschermakite; vanadium-bearing ilmenite is an accessory. In 72207 the magnesio-hornblende is intensely altered to a pale bluishgreen, less Ti-rich magnesio-hornblende or tschermakite. Secondary chlorite and albite occur at the grain boundaries of and fractures within the plagioclase.

Hornblende cumulate rock (Glenrock Station) (72201; 72203A,B; 72204B; 72206; 801 8) The hornblende cumulate rock forms a relatively large mass (ca 100 m x 800 m) embedded in a serpentinite matrix. Microscopically, green actinolitic-hornblende or actinolite appear as euhedral crystals varying in size from a few mm to 1cm. Brown cores of Cr and Ti-rich magnesiohornblende (TiO, = 1.08-1.88%; Cr,O, = 0.51-0.75%) are observed in some samples (Table 1). The matrix surrounding these crystals comprises fine-grained, euhedral Cr-bearing diopside (Cr,O, = 0.08-0.40%) and less common green hornblende, internally strained quartz Gondwana Research, V. 7, No. 3, 2004

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+

and sodic plagioclase. Chromite (100 Cr/(Cr Al) = 85; Table l),pyrite, chalcopyrite, apatite, biotite and zircon are accessory minerals. Alteration of the magmatic minerals with the formation of an actinolite-chloritemuscovite-titanite-albite assemblage is a feature of some samples. Na-poor K-feldspar is also found as a late stage, interstitial alteration product. A similar rock type described by Cross (1983) as a plagioclase-poor “amphibolite,” occurs in the western part of the Pigna Barney area.

Blueschist (Glenrock Station) (72302; 11909) This sample shows a crenulated schistosity (S,) defined by glaucophane, pumpellyite, titanite and minor chlorite. Locally, layers parallel to S, contain quartz, albite, pumpellyite, actinolite and titanite, and scattered throughout the rock are strained, coarser-grained crystals of glaucophane exhibiting subgrain development, twinning and well developed undulose extinction.

Blueschist (Pigna Barney) (73007a,c) These blueschists are similar to those in 72302. Schistosity (S,) is crenulated and defined by glaucophane, titanite and clinozoisite. Rare chlorite and pre-S, strained porphyroblasts of glaucophane are also present. Pyrite and magnetite are accessories.

Analytical Procedures Major and trace elements analyses Major and trace elements (Zr, Y, Ni, Cr, Nb, Ba) were determined by XRF analyses at Ehime University using a PHILLIPS PW2400. The oxide values were determined on a water-free basis. H,O- and H,O+ contents were determined by fire-assay procedure. The analytical procedure and precision and accuracy are described in Yoshizaki et al. (1996) and Hori and Higuchi (1996). Rare earth element (WE) analyses were carried out using a PERKIN-ELMER ELAN6000 ICP-MS at Ehime University. Other trace elements were determined by instrumental neutron activation analyses. Analytical procedures are reported by Sano et al. (1996).

Isotopic analyses Isotopic analyses were performed on a Finnigan MAT262 TIMS equipped with multi-collector at Niigata University and Hokkaido University. The 87Sr/86Srand 143Nd/144Nd ratios were normalized to 86Sr/88Sr= 0.1194 and 146Nd/144Nd = 0.7219, respectively. Concentrations of Rb, Sr, Sm and Nd were determined by an isotopic dilution method. Rb-Sr and Sm-Nd isochron ages and initial Sr and Nd isotope ratios were calculated using the computer program of Kawano (1991) which incorporates

3.45 90.61 4 0.009

0.003 0.108 1.878 0.990 0.003 0.009

3.08 90.16

4

0.004

0.009 0.097 1.882 0.996 0.004 0.008

23 7.961 0.094 0.039 0.055 0.002 0.005 0.000 2.240 0.073 4.415 0.176 0.042 0.000 15.008

55.64 0.02 0.56 18.72 0.60 20.7 1.15 0.15 0.00 0.04 97.58 23 7.517 0.435 0.435 0.000 0.004 0.004 0.955 0.229 0.021 3.835 1.696 0.156 0.016 14.868

53.42 0.04 2.62 10.06 0.18 18.28 11.25 0.57 0.09 0.04 96.55 23 7.134 1.046 0.866 0.180 0.115 0.057 0.341 0.568 0.022 3.716 1.822 0.374 0.038 15.235

50.25 1.08 6.25 7.66 0.18 17.56 11.98 1.36 0.21 0.51 97.04 23 6.886 1.311 1.114 0.197 0.202 0.085 0.295 0.701 0.027 3.493 1.830 0.408 0.066 15.304

48.1 1.88 7.77 8.32 0.22 16.37 11.93 1.47 0.36 0.75 97.17 23 7.575 0.633 0.425 0.208 0.043 0.035 0.102 0.718 0.025 3.869 1.895 0.183 0.020 15.098

53.74 0.41 3.81 6.96 0.21 18.41 12.55 0.67 0.11 0.31 97.18 23 7.433 0.629 0.567 0.062 0.030 0.032 0.616 0.235 0.025 3.999 1.799 0.198 0.000 14.997

53.17 0.29 3.82 7.28 0.21 19.19 12.01 0.73 0.00 0.29 96.99

0.014 1.404 0.324 0.901 0.078 0.035

0.246

4

48.99 98.82

0.50 5.75 40.39 2.55 0.64

PB GR GR GR GR GR MetaHbHbHbHbHbdiorite cumulate cumulate cumulate cumulate cumulate 72904 8018 8018 8018 8018 72201A ferrian- magnesio- magnesio- actinolite tremolitic- chromite tremolite hornblende hornblende (green hb) hornblende (brown hb) (brown hb) (green hb)

6 1.981 0.041 0.019 0.023 0.000 0.012 0.017 0.126 0.008 0.849 0.944 0.027 0.000 4.006

54.05 0.00 0.96 4.67 0.27 15.55 24.03 0.38 0.00 0.40 100.31

6 1.990 0.025 0.010 0.015 0.000 0.002 0.011 0.134 0.010 0.847 0.967 0.018 0.000 4.005

54.05 0.00 0.58 4.70 0.33 15.44 24.51 0.25 0.01 0.08 99.95

97.79 23

7.845 1.566 0.155 1.411 0.011 0.429 1.040 0.019 2.090 0.300 1.648 0.046 14.993

23 7.010 1.759 0.990 0.768 0.000 1.891 0.000 0.000 2.443 0.509 1.109 0.000 14.720

56.68 0.11 9.60 12.69 0.16 10.13 2.02 6.14 0.26

Blueschist 11909 glaucophane

GR

94.53

49.27 0.00 10.49 15.89 0.00 11.52 3.34 4.02 0.00

GR GR GR HbHbcumulatq cumulate Blueschist 72201A 72201A 11909 clinopyro- clinopyro- ferrixene xene barroisite

3.000 3.000 3.002 0.663 0.863 Mg/(Mg+Fez+) 0.894 0.833 0.843 0.945 0.870 1.000 0.037 0.668 0.944 0.867 Cr/(Al+ Cr) 0.851 Ferric and ferrous iron contents in magnetite,spinel and clinopyroxene were calculated by fitting to stoichiometry . For the calculation of site occupation of amphiboles the program PROBEAMPH (Tindle and Webb, 1994) was used.

0.09 0.19 86.63 0.10 0.15

0.29 0.08 86.45 0.12 0.14

PB PB PB PB MetaMetaMetaMetadiorite diorite diorite diorite 72904 72904 72904 72904 magnesio- Cr-bearing Cr-bearing Cumming hornblende magnetite magnetite to-nite

50.85 SiO, TiO, 0.41 A1203 4.94 RO 10.5 MnO 0.03 MgO 17.42 CaO 11.64 Na,O 1.07 0.26 KZO 0.06 CrP, Total 97.18 Structural formulae 0 23 Si 7.201 A1 0.824 Al'" 0.799 Al" 0.025 Ti 0.044 Cr 0.007 Fe3+ 0.807 Fez+ 0.436 Mn 0.004 Mg 3.677 Ca 1.766 Na 0.294 K 0.047 Total cations 15.107

Sample Mineral

Location Rock

Table 1. Reprentative mineral compositions in rocks from the Pigna Barney (PB) and Glenrock Station (GR) areas.

h? 0

W

GEOCHEMISTRY AND CHRONOLOGY OF NEW ENGLAND FOLD BELT, AUSTRALIA

the equation of York (1966) and the decay constants of hs7Rb=1.42 x lO-”/y (Steiger and Jager, 1977) and h147Sm=6.54x 10-12/y(Lugmair and Marti, 1978). Detailed analytical procedures are reported by Miyazaki and Shuto (1998), Yuhara et al. (2000) and Orihashi et al. (1997). 40Ar/39Aranalyses

For 4”Ar/39Arage determinations, mineral grains of 0.3-0.5 mm size were placed on an aluminum tray and five to six trays were stacked and secured with nuts. The sample holder was’ then vacuum-sealed in a quartz ampoule, and irradiated in the core of the 5 MW nuclear reactor of Kyoto University (KUR) for 5 hours (Hyodo et al., 1999b). The total fluence was approximately 7.0 x 1017n/cm2. A typical J-value is 5 x for 3gr hornblende (Turner et al., 1971; Roddick, 1983), and correction factors for calcium and potassium are (36Ar/39Ar)Ca= 5.5 x lo-“, (39Ar/37Ar)Ca= 1.0 x for CaSi,, (“Ar/”Ar)K = 2.1 x 10.’ for synthetic KA1Si30, glass, respectively. Each grain was step-heated using a n argon ion continuous laser. Temperature during heating was monitored through an infrared thermometer produced by CHINO Co. To estimate the correct temperature, the emissivity of each grain was measured during baking for ultra-high vacuum approximately at 200°C. Mafic grains show little change in emissivity except just below their melting points. Accuracy of the temperature measurements is better than 55°C. Extracted argon gas was purified for 5 to 10 minutes with Aluminum-Zirconium getters (saes stlOl). A typical blank cm3STP, 2 x cm3STP, for each isotopes were 5 x 3x cm3STP, 2 ~ l O -cm3 l ~ STP and 1 x lo-’,cm3STP for 36Ar,37Ar,38Ar,39Arand 40Ar,respectively.Argon isotopic ratios were determined by a modified rare gas mass spectrometer built by Nagao et al. (1984).

EDS The compositions of minerals were determined using a JEOL JSM-840 SEM (University of Newcastle) attached to which is an Oxford ISIS 200 EDS system, operating at 2.5 mA and 15 kV. Data reduction was carried out on an on-line computer using SEMQUANT. Back-scattered electron images were obtained from a PHILIPS XL30 SEM (University of Newcastle).

Results Geochemistry Whole-rock major element compositions of tectonic blocks are shown in table 2 and figure 2. In the Pigna Gondwana Research, V. 7, No. 3, 2004

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Barney area, meta-diorite sample 72904 is characterized by high SiO, and MgO, low A1,0, and unusually low TiO,. Further, Cr and Ni contents are high and HFSE (particularly Y and Zr), and REE contents are low (Table 2). Further, the Yb, Sr/Y and [La/Ybl, values of 1.65, 26 and 8 respectively are low. These features of the rocks are similar to those of boninites (Rogers et al., 1989; Bloomer and Hawkins, 1987) and resemble the boninitic dacites reported by Crawford and Cameron (1985). The Tim-Ti/Sc ratios (Tim = 3.12; Ti/Sc = 20) support this interpretation (Hickey and Frey, 1982) but the light REE enrichment apparent in the chondrite normalized pattern ([La/Lu], = 8.1; Fig. 4a) and La, Y, Nb contents are more characteristic of calc-alkaline rocks (Fig. 5). By contrast 72908, a meta-diorite from nearby (Fig. 1) is more TiO,, CaO and Al,03-rich and contains less SiO, and MgO. In addition, it shows a flat chondrite normalized REE pattern ([La/Lu], = 1.2; Fig. 4c), and Ni and Cr contents are low. A tholeiitic affinity is suggested for this sample on the SiO, - FeO”/MgO diagram of Miyashiro (1973; Fig. 2a). Meta-diorites 72207 and 72208B show quite different compositions, the former having the whole rock chemistry of a high alumina basalt, the latter that of an andesite. Chondrite normalized REE patterns also differ, 72207 having a flat MORB-like pattern ([La/Lu], = 0.8) with a positive Eu anomaly and 72208B a LREE enriched pattern ([La/Lu],=4.1; Fig. 4d). The REE patterns and SiO, - FeO*/MgO relationships (Fig. 2a) suggest that 72207 and 72208B have tholeiitic and calc-alkaline magmatic affinities respectively. In the Glenrock Station area, the hornblende cumulate samples (72201A,72203A,B) have similar major element compositions. Sample 72204B is more A1,0,, K 2 0 and Na,O-rich, due to the greater abundance of primary plagioclase and secondary K-feldspar in this sample. Particularly significant is the presence of very high Cr and Ni contents (Table 2). REE patterns exhibit a slight LREE enrichment ([La/Lu], = 4.3; Fig. 4b). Apart from differences in CaO and SiO, contents, the blueschists have similar major element composition (Table 2). They have a tholeiitic affinity (Fig. 2a), exhibit NMORB-like REE patterns and have TiO, - FeO”/MgO and TimMORB ratios (Figs. 2b, 3,4). Cross (1983) and Offler (1999) have noted N-MORB affinities for blueschists from the Pigna Barney and Glenrock Station areas. However, blueschists with shoshonitic affinities have also been recorded from the Glenrock Station area (Offler, 1999). Sr and Nd isotopic compositions and ages

In general, it is difficult to determine ages of ophiolitic rocks by Rb-Sr or Sm-Nd methods because ‘of element redistribution (e.g., Rb, Sr) during alteration and the small

S. SANO ET AL.

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Blueschist (73007A, C, 72302) Meta-diorite

65

(72904)

,/'

60

Meta-diorite (72908)

6

i7j 55

50 Skaergaard intrusion

45

/

I

0

3

2

1

4

5

FeO*/MgO

0

0

1

2

3

4

FeO*/MgO

Fig. 2. Major element compositions. (a) SiO, vs. FeO"/MgO diagram. Boundary between calc-alkali series (CA) and tholeiite series (TH) is from Miyashiro (1973). (b) TiO, vs. FeO*/MgO diagram. Fields of oceanic island tholeiite (OIT), mid-oceanic ridge basalt (MORB) and island arc tholeiite (IAT) are from Shuto et al. (1985).

range of Rb/Sr or Sm/Nd ratios. To overcome these problems, we have separated clinopyroxene, hornblende and felsic fractions from hornblende cumulates (72201A - 72206), and four fractions (Felsic-1, Felsic-2, Green and Black) from the meta-diorite (72904) in which the grain size of individual minerals was too small to allow pure separates to be prepared. Sr and Nd isotope results are shown in tables 3 and 4. Meta-diorite (72904) shows a7Sr/a6Srratios varying from 0.7069 to 0.7084 and 87Rb/86Srratios from 0.13 to 0.38. These define an isochron which yields an age of 426+38 Ma with an initial a7Sr/a6Srratio of 0.7060

(Fig. 6). In contrast to the Rb-Sr isochron age, the Sm-Nd system gives an older age of 536+38 Ma with initial 143Nd/144Nd = 0.51174 (ENd,36Ma = -4.1; Fig. 7). Slightly lower whole rock s7Sr/a6Srand higher 143Nd/144Nd ratios are recorded by 72908 (Tables 3 and 4). The hornblende cumulate from Glenrock Station area gave Rb-Sr ages of 339+9, 411+15 and 425+44 Ma (Table 5; Fig. 6a,b). The 339 Ma age obtained from 72201A has been determined from an isochron defined by wholerock, clinopyroxene, hornblende and the felsic fraction; the last plays a critical role in defining the age. &Nd420Ma varies from +2 to + 4 (Fig. 8). The Sr-Nd isotopic compositions of meta-diorites 72207 and 72208B are plotted on an &Nd- eSr diagram (Fig. 8) and have been corrected using tentative age of 400 Ma. They show a moderately depleted Nd isotopic composition (ENd400Ma = +4.5), which contrasts with the value of &Nd = -4.1 determined from meta-diorite 72904 (Fig. 8) but comparable with the eNd values determined from the hornblende cumulate rocks. The blueschists from Glenrock Station and Pigna Barney have an &Nd4a0Ma value of approximately +8 (Fig. 8), assuming that the blueschists from both study areas have ages of ca. 480 Ma similar to those obtained by Fukui et al. (K-Ar 467-482 Ma; 1995).

40Ar/39Ar age determinationfor hornblende cumulate from Glenrock Station - 0

2

4

6

8

10

12

14 16

18

Ti (ppm)/lOOO Fig. 3. Ti/lOOO vs. V diagram. Areas of arc-tholeiite, boninite MORB and ocean island and alkalic basalt are from Shervais (1982). Symbols as in figure 2.

We determined 4oAr/39Arages from single grains of cumulus hornblende and secondary K-feldspar. The results are shown in table 5 and figure 9. Green cumulate hornblendes show ages between 235 and 263 Ma (Fig. 9a, b, c) but, one hornblende sample, possibly from a brown Gondwana Research, V. 7, No. 3, 2004

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Table 2. Major and trace element compositions of tectonic blocks in the Pigna Barney (PB) and Glenrock Station (GR) areas. Location Rock Sample SiO, TiO, Fez03 MnO MgO CaO Na,O KP

p20, Total H,OH,O+ SC' v2

Cr2 CO' NiZ CUZ Rb2 Srz Y2 Zrz NbZ La3 Ce3 PI3 Nd3 Sm3 Eu3 Gd' Tb3 DY3 ~

0

Er3 Tm3 Yb3

Lu3

3

PB Metadiorite 72904

PB Metadiorite 72908

GR-N Metadiorite 72207

GR-N Metadiorite 72208b

GR-S GR-S GR-S GR-S GR-S HbHbHbHbHbcumulate cumulate cumulate cumulate cumulate 72201a 72203a 72203b 72204b 72206

65.32 0.08 10.02 9.10 0.14 8.52 3.57 2.53 0.97 0.05 100.30 0.21 1.89 24 154 559 50 97 39 21 157 6 29 1.4 3.14 8.11 0.69 3.08 0.64 0.19 0.78 0.08 0.51 0.09 0.24 0.02 0.28 0.04

56.47 0.57 15.23 11.93 0.20 3.92 8.25 2.75 0.34 0.06 99.71 0.17 1.96 42 362 24 45 15 99 7 173 12 39 1.2 2.90 8.53 0.93 4.51 1.53 0.58 2.09 0.34 2.87 0.60 1.59 0.22 1.50 0.25

48.24 0.57 18.37 12.16 0.21 7.65 9.27 2.56 0.39 0.04 99.45 0.18 2.54 43 364 77 43 40 106 7 519 8 27 0.3 1.06 3.01 0.38 2.26 0.87 0.51 1.21 0.20 1.57 0.30 0.80 0.13 0.75 0.13

56.82 0.57 19.58 7.68 0.18 2.83 6.37 5.16 0.72 0.15 100.05 0.17 1.44 15 188 17 43 8 48 7 736 14 81 4.9 10.72 30.98 3.16 14.58 3.81 1.14 4.00 0.54 3.52 0.63 1.73 0.23 1.71 0.27

58.33 0.21 5.97 5.14 0.18 14.25 13.29 1.15 1.14 0.05 99.69 0.09 0.76 39 104 2412 52 164 6 22 47 11 26 2.6 6.47 18.65 1.93 8.48 1.97 0.64 2.50 0.31 2.13 0.39 0.88 0.13 0.90 0.17

56.07 0.32 4.40 7.64 0.25 16.29 13.48 0.96 0.17 0.04 99.62 0.11 0.82 46 127 1953 59 159 6

4 51 9 29 1.9 3.80 12.63 1.51 6.57 1.84 0.59 2.30 0.33 2.02 0.42 1.11 0.15 0.93 0.16

54.79 57.88 0.59 0.25 5.52 9.67 9.56 5.84 0.24 0.18 16.21 11.48 11.51 9.83 1.07 2.60 0.24 1.47 0.06 0.12 99.78 99.31 0.13 0.13 2.50 1.08 58 32 233 108 1535 1076 62 44 164 151 11 11 3 39 53 170 11 14 39 32 3.0 1.4 5.35 4.95 18.42 14.18 2.09 1.42 10.93 6.59 3.12 1.59 0.96 0.54 4.00 2.06 0.55 0.25 3.79 1.73 0.69 0.33 1.88 0.76 0.25 0.12 1.78 0.80 0.29 0.12

55.11 0.29 4.04 7.69 0.20 17.49 14.19 0.71 0.33 0.06 100.10 0.10 0.92 47 158 2336 54 189 35 8 66 9 16 1.5 3.47 10.77 1.17 5.77 1.67 0.49 1.87 0.27 1.73 0.36 0.84 0.11 0.79 0.12

PB Blue schist 73007a 51.55 1.55 13.22 13.78 0.19 6.51 8.53 3.63 0.06 0.10 99.11 0.11 3.13 46 368 132 52 64 43 2 91 24 102 2.0 2.39 9.75 1.35 8.28 3.46 1.34 5.59 0.90 7.87 1.59 4.15 0.49 3.37 0.50

PB Blue schist 73007c 51.35 1.64 13.35 14.68 0.19 6.18 7.12 4.30 0.14 0.10 99.05 0.10 0.89 46 364 74 56 62 78 5 87 26 111 2.2 3.04 11.04 1.52 9.69 4.16 1.52 6.25 1.03 8.23 1.64 4.46 0.56 3.50 0.58

GR-S Blue schist 72302 47.73 1.05 14.52 12.42 0.22 8.21 12.87 2.09 0.05 0.05 99.19 0.18 3.86 47 323 181 56 92 19 2 15 20 57 1.4 1.54 5.91 0.80 4.95 2.25 0.86 3.83 0.66 5.78 1.18 3.28 0.43 2.98 0.43

Analytical methods are as follows: 1. Instrumental Neutron Activation Analyses, 2. XRF, 3. ICP-MS.

core yields an age of approximately 394 Ma (Fig. 9d) at high temperature fractions. Since some of the hornblendes in the thin section have brown cores surrounded by green hornblende, the older age possibly came from the brown core. K-feldspar shows similar ages to those of green hornblende (Fig. 9e, f).

Discussion Tectonic setting In order to determine the tectonic setting of the samples, we have used the discrimination diagram of Shervais (1982; Fig. 3) and Cabanis and Lecolle (1989; Fig. 5). These diagrams, however, are based on basalts from Gondwana Research, I? 7, No.3, 2004

various tectonic settings and cannot be used for more siliceous rocks such as 72904. For a rock of this composition (Table 2), the discrimination diagrams for granites of Pearce et al. (1984) are more appropriate. The Rb and Y+Nb values of 21 and 7.4 respectively for this sample suggest a volcanic arc setting. The meta-diorite (72904) shows a low Ti/V ratio, low HFSE and REE contents, and high Ni and Cr, characteristics of boninites in intra oceanic arc settings (Crawford et al., 1989). The La, Y and Nb concentrations support an arc setting (Fig. 5). Sample 72904 has possibly been derived from a magma emplaced in a fore-arc during subduction of an active spreading center according to current models (Crawford et al., 1989). Meta-diorite 72908 also appears

S. SANO ET

824

100

SmEu Gd Tb Dy Ho Er TmYb Lu

La Ce Pr Nd I

I

I

I

I

I

I

I

I

I

I

I

La Ce Pr Nd kI I I I

I

;a)']

Nepoui boninite, New Caledonia

AL.

SmEu Gd Tb Dy Ho Er TmYb Lu I

I

I

I

I

I

I

I

I

I

10

1

Cape Vogel boninite

+72904

Meta-diorite 0.1

I

I

1

I

l

l

l

l

l

l

l

l

l

l

l

l

10

+73007A

1Blueschist La Ce Pr Nd

----

73007C 72302

SmEu Gd Tb Dy Ho Er TmYb Lu

Fig. 4. Chondrite-normalized rare earth elements (REE) patterns for tectonic blocks. Normalization values are those of Anders and Grevesse (1989). Data of REE patterns of boninites from Cape Vogel and New Caledonia are Hickey and Frey (1982) and Cameron (1989), respectively.

to have formed in an island arc environment. It shows a flat chondrite normalized REE pattern, a T i n ratio just > 10 (Fig. 3) and Y, La, Nb values which plot in the volcanic arc tholeiite field of Cabanis and Lecolle (1989; Fig. 5). An arc setting is suggested for the hornblende cumulate rocks based on the island arc affinities shown by the composition of chromite (Al,0,=5.75 %, TiO, = 0.5 To, 100 Cr/(Cr+Al)=85; Table 1; Kamenetsky et al., 2001) and the clinopyroxenes that are similar to those in island arc cumulate complex (Table 1; Fig. 5 in Spandler et al., 2003). The blueschists (73007A, C; 72302) plot in the MORB field (Figs. 3 and 5) which accords with the ENd value of +8 (Fig. 8). The remaining rocks with the exception of 72208B, plot in the field of island arc-tholeiites (IAT).

72208B plots within the calc-alkaline arc field in the discrimination diagrams of Shervais (1982) and Cabanis and Lecolle (1989; Figs. 3 and 5) confirming an arc setting for this rock.

Age and origin of meta-diorite 72904f r o m Pigna Barney Well defined Rb-Sr and Sm-Nd isochrons have been obtained from 72904 which yielcled a Rb-Sr age of 426k39 Ma and a Sm-Nd age of 536+38 Ma. We attribute the differences in ages to the alteration which resulted in the replacement of magmatic minerals by actinolite, chlorite, epidote and white mica. Due to the differences in closure temperature of the Rb-Sr and Sm-Nd systems, and the different chemical behaviour of RS and Sr during metamorphism or alteration, the Rb-Sr system has reset Gondwana Research, V. 7, No. 3, 2004

GEOCHEMISTRYAND CHRONOLOGY OF NEW ENGLAND FOLD BELT, AUSTRALIA

825

-

Fig. 5. The La/lO-Y/lS-Nb/8 discrimination diagram of Cabanis and Lecolle (1989). Field 1 contains volcanic-arc basalts, field 2 continental basalts and field 3 oceanic basalts. The subdivisions of the fields are as follows: lA, calc-alkali basalts; lC, volcanicarc tholeiites; 1 B is an area of overlap between 1A and 1C; 2A, continental basalts; 2B, back-arc basin basalts; 3A, alkali basalts from intercontinental rifts; 3B, 3C, E-type MORB (38 enriched, 3 C weakly enriched), 3D, N-type MORB. Symbols as in figure 2.

during this metamorphic alteration event. We regard the older age of 536 Ma as the emplacement age of the protolith. The chemical composition of the meta-diorite is unusual in that it shows high SiO, (65%), MgO (8.5%), Ni (97 ppm) and Cr (559 ppm) and low Zr (29 ppm), TiO, (0.08%), Al,O, (10%) and REE contents. As shown in figure 3, the meta-diorite has a low T i m ratio that is comparable with the composition of boninites from the Marianas (Shervais, 1982). Additionally,the meta-diorite shows a chondrite-normalized LREE pattern (Fig. 4a; 10 x chondrite - LREE; 1-2 x chondrite - HREE), similar to the REE patterns reported by Hickey and Frey (1982) and Cameron (1989) for boninites from DSDP Site 458 and New Caledonia, and ratios of CaO/A1,0, (0.36), CaO/ TiO, (45), Al,O,/TiO, (125) and Ti/Zr (17) characteristic of boninites (Rogers and Saunders, 1989). Therefore, we conclude that the chemical features of the meta-diorite are similar to those of boninites. However, the Nd isotope results indicate that they have originated from an enriched source. Boninites derived from an enriched source are uncommon and require the mixing of a depleted mantle wedge with sediment melt and a melt and fluid derived from MORB (Munker, 2000). Age and origin of the hornblende cumulate bodyfrom the Glenrock Station area

0.720

0.715

Rb-Sr mineral/whole-rock isochrons for samples from three localities in the body are presented in figure 6. 72201A shows the youngest age of 339?9 Ma which was derived from an isochron determined from the felsic fraction clinopyroxene -lhornblende +whole rock assemblage. On the other hand, the feldspar-free rock (72206) yields a much older age of 4 2 5 k 4 4 Ma (clinopyroxene + hornblende whole-rock assemblage). The difference in ages clearly depends on the presence or absence of felsic minerals. Secondary alteration of plagioclase to white mica, albite and/or prehnite is apparent in most samples. This suggests that the younger isochron age is a result of the secondary alteration. Significantly, the older age of 425 Ma is similar to the zo7Pb/206Pb zircon age of 4 3 6 2 9 Ma determined by Kimbrough et al. (1993) for a sample from the Pola Fogal Hornblendite-Tonalite Suite (Cross, 1983) in the Pigna Barney area. This suite contains hornblende bearing cumulates similar to those from the Glenrock Station area. Kimbrough et al. (1993) interpreted the age of 436 Ma to be the minimum age for the igneous emplacement of the suite. We also consider that the 425 Ma age dates the time of emplacement. The 40Ar/39Ar age of approximately 394 Ma was obtained from a hand picked fragment of a hornblende grain. However, the sample may

+

$2

>

2 0.710

+

0.705

B

0

0.1

0.2

0.3

R b/

0.4

0.5

r

Fig. 6. 87Rb/86Sr- 87Sr/86Srisochrons for rocks and mineral separates from the Pigna Barney and Glenrock Station areas. Figure 6B is an enlarged and rescaled section of the boxed area in figure 6A.

Gondwana Research, V. 7, No. 3, 2004

S. SANO ET AL.

826

Table 3. Kb, Sr, Sm and Nd concentrations and Sr and Nd isotopic compositions (Pigna Barney). Sample 72904 (Meta-diorite) whole-rock felsic-1 felsic-2 green black 72908 (Meta-diorite) whole-rock 73007A (Blueschist) whole-rock 73007C (Blueschist) whole-rock

Rb, ppm

Sr, ppm

20.5 27.3 20.6 2.38 1.96

163 208 177 39.3 43.6 183

5.57 0.634 3.18

94.3 109

87Rb/s6Sr

-C

0.3635 0.3796 0.3368 0.1753 0.1297

0.708213 0.708365 0.708108 0.707024 0.706890

(14) (14) (13) (14) (14)

0.723 0.0808 0.381 0.828 4.01

0.0879

0.704761

(14)

0.0194

0.703917

0.0843

0.704556

have included some green amphibole as the spectrum is poorly defined (Fig. 9d), and shows younger ages in the low temperature fractions. The oldest 40Ar/39Arage is possibly related to the 425-411 Ma event indicated by the Rb/Sr mineral isochrons. For other hornblende results, the presence of older ages in the last few high temperature fractions may also be attributed to the presence of brown cores. This interpretation must be regarded with caution and can only be confirmed by a spot age analysis of an uncontaminated brown core in a thin section. The 235 to 263 Ma ages for the green actinolitic hornblende (Fig. 9a-c) and approximately 223-243 Ma for the K-feldspar (Fig. 9e, f) suggest a resetting of the K-Ar system. This resetting may have occurred during the contractional, 265-230 Ma Hunter-Bowen Orogeny that affected the NEFB. The variable ages from hornblende grains may be attributed to simultaneous gas releases from green (reset) and brown (un-reset) portion of the hornblende grains. The question now arises as to the source of the magma from which these rocks were derived. The presence of chromite (Al,O, = 5.75; TiO, = 0.5; 100 Cr/(Cr+Al) = 85)

0.5130

U

z

5

a z

E

Nd, ppm

147Sm/144Nd143Nd/'44Nd 2

3.42 0.615 1.88 3.58 15.3

0.1280 0.0795 0.1225 0.1397 0.1588

0.512200 0.512019 0.512175 0.512240 0.512293

1.58

5.32

0.1794

0.512858 (13)

(14)

3.66

9.79

0.2257

0.513154 (10)

(13)

4.62

0.2164

0.513109 (12)

12.9

(14) (24) (12) (12) (12)

suggest a magma of tholeiitic composition has been derived from the melting of a highly refractory source in the mantle lithosphere. To explain the presence of relict cores of Ti and Cr-rich magnesio-hornblende surrounded by less Cr and Ti-rich rims of magnesio-hornblende, the subsequent crystallization of diopside, quartz and plagioclase and the ENd values (-2 to +4), magma mixing of an enriched component and the tholeiitic magma is required. The cumulate texture of these rocks and the low Al,O, contents of the clinopyroxenes (Al,O, = 0.39-1.03 Yo; Al"'= 0-0.023; Table 1) suggest that the magma mixing and crystal settling took place in a shallow reservoir beneath the arc.

Geochemical characteristics of blueschists The blueschists show high Ti/V ratios and LREE depleted chondrite normalized patterns typical of N-MORB " 1 " " 1 " " 1 " " 1 ' " '

10 -

.:1

Sr and Nd isotope results of tectonic blocks in Serpentinite matrix from Pigna Barney and Glenrock Station areas

BIUeSChlSt (480~a)

-

Hb Cumulate (420Ma) Glenrock Station

Glenrock Station Meta dlorite (536Ma) Pqna Barney

-5

0.5125

-10

9

72904

4

Fel-l

0.05

Sm, ppm

87Sr/s6Sr

0.10

0.15

0.20

0.25

'47Sm/'44Nd Fig. 7. 147Sm/'44Nd-143Nd/144Ndisochron for rocks and mineral separates from the Pigna Barney and Glenrock Station areas.

t

f(t)

V 71201A Hb cumelate 72203A L B Hb Cumulate

*0 A A

0

-

W 0 0- 0

I

-100

-

E(P)

-50

,

,

,

,

I

0

I

I

722048 Hb-cumulate 72206 Hb Cumulate 72207 8 722088 meta dlorlte 72904 rneta-diante 72908 mete.-dmde

I

100

50

150

200

250

ESI

Fig. 8. Epsilon Sr and Nd diagram for rocks and the separated minerals in serpentinite mClange from Pigna Barney and Glenrock Station areas. Solid symbols are present day isotopic results and open symbols are mean age corrected isotopic compositions, based on the ages obtained from the isochrons.

Gondwana Research, V. 7, No. 3, 2004

827

GEOCHEMISTRYAND CHRONOLOGY OF NEW ENGLAND FOLD BELT, AUSTRALIA Table 4. Rb, Sr, Sm and Nd concentrations and Sr and Nd isotopic compositions (Glenrock Station). Sample

87Rb/86Sr

72201A (Hb-cumulate) whole-rock felsic CPX Hb 72203A (Hb-cumulate) whole-rock Hb 72203B (Hb-cumulate) whole-rock Hb 72204B (Hb-cumulate) whole-rock felsic Hb 72206 (Hb-cumulate) whole-rock CPX Hb 72207 (Meta-diorite) whole-rock 72208B (Meta-diorite) whole-rock 72302 (Blueschist) whole-rock

87Sr/86Sr

&

Sm,ppm

147Sm/144Nd

47.1 66.5 32.0 44.2

1.3498 3.5460 0.3397 0.1396

0.710205 0.721096 0.705554 0.704644

(15) (33) (13) (14)

2.12 0.262 1.25 2.83

9.73 1.28 5.73 12.8

0.1318 0.1238 0.1319 0.1341

0.5 12520 0.5 12490 0.512526 0.512516

2.42 0.859

52.0 11.8

0.1344 0.2104

0.704594 0.704084

(14) (14)

1.89 2.58

7.84 10.6

0.1457 0.1479

0.512614 0.512615

2.30 0.843

53.9 45.7

0.1233 0.0534

0.704499 0.704104

(14) (14)

3.34 3.30

12.9 12.8

0.1571 0.1557

0.512664 0.512635

4.01 107 1.19

18.0 463 37.6

0.6466 0.6698 0.0917

0.707447 0.707653 0.704236

(13j (14) (14)

1.81 0.119 2.74

8.10 0.699 11.9

0.1351 0.1028 0.1394

0.512693 0.512577 0.512690

7.12 4.13 1.19

68.1 52.2 52.4

0.3024 0.2291 0.0656

0.705527 0.705156 0.704110

(19) (14) (11)

1.79 0.715 2.36

7.08 2.69 9.33

0.1528 0.1609 0.1527

0.512732 0.512723 0.512720

2.89

0.1989

0.512878

0.1321

0.512708

0.2361

0.513131

22.0 81.4 3.75 2.14

5.99

583

0.0297

0.704001

(9)

0.951

6.35

765

0.0240

0.704008

(14)

3.97

0.0551

0.705723

(14)

2.33

0.831

43.6

300

-

200

-<

100

18.2 5.96

500

- ( C ) 8018Hb3 7 300 -

(b)8018Hb2

400

400

1

>

235 f 5 Ma

-

-ol 00 0.2

100

0.6 0.8 1.0 Fraction of 39Arreleased

0.2

0.4

0.6

0.8

1.0 - 0

500

2 400 v

a,

r n 300

0

Gondwana Research, V. 7, No. 3, 2004

"

243 5 3 Ma

"

"

"

'

0.2 0.4 0.6 0.8 1.0 Fractionof 39Arreleased

0

Fraction of 39Arreleased

.

500 -(f) 8018K2 400-

-

300-

*

200'

h

a c g 200 zg 100 a

-

0

0.4

>

256 f 9 Ma

200

- 0

0

rf:

223 k 3 Ma

pt.-

100 0-

'

'

'

'

'

'

'

'

'

828

S. SANO ET AL.

Table 5. Ages for rocks from Pigna Barney and Glenrock Station areas. Rb/Sr and Sm/Nd isochron ages Sample 72904 72904 72201A 72204B 72206

Method Meta-diorite Meta-diorite Hb-cumulate Hb-cumulate Hb-cumulate

Rb-Sr mineral Sm-Nd mineral Rb-Sr mineral Rb-Sr mineral Rb-Sr mineral

Age (Ma) 426 536 339 41 1 425

Error

(2)

39 38 9 15 44

MSWD

Initial ratio

0.21 0.51 1.26 0.12 0.15

0.706041 0.5 11744 0.703882 0.703698 0.703726

Error (i.) 0.000164 0.000032 0.000227 0,000114 0.000139

40Ar/39Araaes for minerals for hornblende cumulate Sample

Remarks

Age (Ma)

8018 Hbl 8018 Hb2 8018 Hb3 8018 H 1 8018 K 1 8018 KZ

green hornblende (rim of cumulate crystal) green hornblende (rim of cumulate crystal) green hornblende (rim of cumulate crystal) brown hornblende (core of cumulate crystal) K-feldspar (heterogeneous in matrix) K-feldspar (heterogeneous in matrix)

Error (t)

263 235 256 394 243 223

ReDorted ages * Sample

Method

Hornblendite Metagabbro Blueschist Blueschist Blueschist Glaucophanitic metabasite Glaucophanitic metabasite

Z07Pb/Z06Pb zircon K-Ar phengite K-Ar phengite K-Ar phengite K-Ar phengite K-Ar phengite fine K-Ar phengite coarse

Age (Ma)

Error (?)

436 48 1 470 473 482 467 471

9 10 10 10 9 10 10

* Data sources: 1: Kimbmugh et al. (1993), 2: Fukui et al. (1995).

basalts. These features combined with the fact that they are now blueschists indicates that the protolith formed at a mid-ocean ridge and was conveyed to the subduction zone at the eastern margin of Gondwana. They were subsequently taken to depths sufficient for recrystallization and multiple deformations of the basalts and have produced B-type blueschists (Cordilleran; Maruyama et al., 1996). If the blueschists examined in this study were produced at the same time as those dated by Fukui et al. (481-467Ma; 1995), then they are the result of exhumation during the Early Ordovician as previously suggested by Fukui et al. (1995) and Offler (1999).

Tectonic history at eastern Gondwana margin through Paleozoic deducedfrom tectonic blocks Magmatic and metamorphic ages and the tectonic settings of the tectonic blocks are summarized in figure 10. This shows that subduction events and associated island arc volcanism occurred during the Early Cambrian (530-540 Ma), and Late Ordovician to Early Silurian (425-435 Ma). Island arc boninitic volcanism at 536 Ma associated with subduction and high P-low T metamorphism of MORB-like basalts to eclogite (536418 Ma; Fanning et al., 2002) commenced in the Early Cambrian. This is earlier than the boninitic

magmatism recorded in Tasmania 514+5 Ma; Black et al., 1997) and in Victoria (Botomian; 519-514 Ma; Vandenberg et al., 2000). Subsequent exhumation of MORB-like basalts in the Early Ordovician gave rise to high-pressure blueschist metamorphism (482-467 Ma; Fukui et al., 1995) and arc-related plutonism as typified by the Attunga gabbro (U-Pb zircon; 479 2 11Ma; Fanning et al., 2002). Island-arc magmatism and metamorphism occurred again during the Early Silurian.

Conclusion Tectonic blocks in the serpentinite mdange from the Pigna Barney and Glenrock Station areas along the PMFS in New England Fold Belt, NSW, Australia, include metadiorites, hornblende cumulate, and blueschists. Metadiorite from the Pigna Barney area has a boninitic affinity and is of Early Cambrian age. It formed in an immature island arc setting as a result of mixing of a depleted mantle wedge with sediment melt and fluid and melt derived from MORB. A meta-diorite from the same area has an island arc tholeiite affinity Blueschistsfrom these two areas were derived from MORB-like protoliths and formed in B-type high P/T belts and were probably produced during exhumation in the Early Ordovician. The primary Gondwana Research, V. 7, No. 3, 2004

829

GEOCHEMISTRY AND CHRONOLOGY OF NEW ENGLAND FOLD BELT, AUSTKALIA

600 Ma

I

500 Ma Cambrian

400 Ma

Ordovician

300 Ma

I

Silurian

plutonism event metamorphic event

Woods reef

exhumation event

Attunga

Glenrock Station

Pigna Barney

Fig. 10. Relationship between age and tectonic setting for tectonic blocks in the serpentinite mClange of the SNEFB.

magmatism responsible for the hornblende cumulate body occurred in an island-arc setting at about 430 Ma. The hornblende cumulates and diorites from the Glenrock Station area were formed during the Early Silurian in an island arc setting. The radiometric ages of these tectonic blocks are concentrated in the Early Cambrian (530-540 Ma), Early Ordovician (460-480 Ma), and Early Silurian (425-435 Ma). This suggests that island arc magmatism took place in the New England Fold Belt, and hence at the eastern margin of Gondwana. This is consistent with the tectonic history proposed earlier (Cawood, 1983; Leitch and Cawood, 1987).

Acknowledgments We thank Y. Nakano and T. Takeuchi (Kyoto University) for support during the neutron activation analyses, and H. Kagami, M. Yuhara (Niigata University) and T. Nishiya, Y. Kuramoto (Hokkaido University) for help isotope analyses. We also thank Ken Cross for kindly allowing us to have access to the data in his Ph.D. thesis, Dave Phelan Gondwana Research, V. 7, No. 3, 2004

for assistance with the electron microprobe and Yan Yan Sun for the preparation of thin sections. This work has been carried out in part under the Visiting Researcher’s Program of the Research Reactor Institute, Kyoto University. We thank the staff of the institute, in particular, A. Tanaka and J. Takada for their help during the neutron irradiation experiments. This study was supported by a Grant-in-Aid for Scientific Research (C) from Japan Society for the Promotion of Science (JSPS) to S. Sano (No. 14540429) and (A) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) to T. Watanabe (No. 10041102). We thank E. Leitch and A. Ishiwatari for their reviews and many helpful comments.

References Aitchison, J.C. and Flood, P.G. (1995) Gamilaroi Terrane: A Devonian rifted intra-oceanic island-arc assemblage, NSW, Australia. In: Smellie, J.L. (Ed.), Volcanism associated with extension at consuming plate margins. Geol. SOC. Spec. Pub., V. 81, pp. 155-168.

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