Oxygen isotope evidence for crustal assimilation and magma mixing in the Granite Harbour Intrusives, Northern Victoria Land, Antarctica

Lithos 67 (2003) 135 – 151 www.elsevier.com/locate/lithos

Oxygen isotope evidence for crustal assimilation and magma mixing in the Granite Harbour Intrusives, Northern Victoria Land, Antarctica L. Dallai a,*, C. Ghezzo b, Z.D. Sharp c a

Istituto di Geologia Ambientale e Geoingegneria, CNR, Universita` di Roma ‘‘La Sapienza’’, P.le A. Moro 5, 00187 Rome, Italy b Dipartimento di Scienze della Terra, Universita` di Siena, Via Laterina 8, 53100 Siena, Italy c Department of Earth and Planetary Sciences, Northrop Hall, University of New Mexico, Albuquerque, NM 87131, USA Received 26 April 2002; accepted 1 December 2002

Abstract The stable isotope composition (O,H) of whole-rock and mineral separates of Cambrian-Ordovician gabbros, diorites, granodiorites and granites forming the Mt. Abbott composite intrusions (Northern Victoria Land, Antarctica) was measured to constrain the origin and evolution of the magmas postdating the Ross Orogen. The d18O values of olivine gabbros plot in the field of slightly evolved mantle-derived melts (d18OWR = 6.8 – 7.4x). The O-isotope character of the mantle source inferred from the d18O values of cumulous olivine in gabbros (5.7 – 6.8x) is enriched in18O compared to modern arc-related magmas. Geochemical data and concurrent high d18O values, and initial strontium (87Sr/86Sr = 0.7060) and neodymium (143Nd/144Nd = 0.5122) isotope ratios indicate that the olivine gabbros formed by crustal contamination of a primary calcalkaline basaltic melt. The diorites have high d18O values, among the highest ever measured for dioritic rocks (8.7 – 10.3x), and Sr-isotope ratios that partially overlap with the adjacent and mingled felsic lithologies (0.708 – 0.710). The diorites have pyroxene with high, nearly constant d18O values (8.2 – 8.6x) that are independent from the silica content of the rocks; thus, they did not increase in response of the chemical evolution of the rocks. The diorites originated from the same primary calcalkaline basalt experiencing different amounts of crustal contamination, and underwent different degrees of mixing with the adjacent granites, producing granodioritic facies and quartz/feldspar xenocrystic diorites. The d18O, 87Sr/86Sr and 143Nd/144Nd compositions of the granites and granodiorites overlap (10.8 – 12.1x, 0.7096 – 0.7108, 0.5119 – 0.5120). They are distinct from the values of the mafic rocks and indicate that gabbros and granites were not cogenetic. The granites are a separate melt component likely derived from nonmodal partial melting of fertile meta-igneous protoliths. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Oxygen isotope; Crustal contamination; Magma mixing; Antarctica

1. Introduction

* Corresponding author. Fax: +39-6-446-8632. E-mail address: [email protected] (L. Dallai).

Composite plutonic sequences are manifestations of magmas with contrasting composition emplaced contemporaneously and in spatial relationships

0024-4937/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0024-4937(02)00267-0

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L. Dallai et al. / Lithos 67 (2003) 135–151

(Wiebe, 1980; Foster and Hyndman, 1990; D’Lesmos, 1996). During magma intrusion and emplacement, the original geochemical characteristics of the magmas may be extensively modified by selective contamination and/or incomplete mixing between juxtaposed melts (e.g. Blichert-Toft et al., 1992; Turner, 1996). Whole-rock geochemical investigation provides only partial evidence of petrogenetic evolution of the magmas; better constraints can be obtained by studying cumulous and high-temperature minerals. This work focuses on the stable isotope compositions of rocks and selected minerals from the composite intrusive sequence cropping out in the area of Mt. Abbott, Northern Victoria Land (Fig. 1), with the aim of resolving their origin and petrogenetic evolution. Stable isotope data were used to investigate equilibrium conditions among magmatic phases, check for subsolidus alteration that may result from the

emplacement of adjacent intrusions and reconstruct the primary oxygen isotope composition of the mafic rock types. The d18O data were considered with new and previously published Sr and Nd isotope data in order to constrain the mantle – crust interaction of magmas emplaced at the end of the Ross Orogen.

2. Geological setting The Ross –Delamerian Orogen formed as a 4000km-long mobile belt during the early Palaeozoic due to the subduction of the palaeo-Pacific ocean under Gondwana (Dalziel, 1992; Moores, 1991). The remnants of this orogen are presently exposed on the border of the East Antarctic Craton along the Trans-Antarctic Mountains and in SE Australia. In Antarctica, Northern Victoria Land represents the accreted margin of the

Fig. 1. Geological sketch map of the Mt. Abbott area (modified after Rocchi et al., 1997).

L. Dallai et al. / Lithos 67 (2003) 135–151

East Antarctic Craton facing the palaeo-Pacific ocean (Fig. 1). It is situated at the original junction between SE Australia and Antarctica (Borg et al., 1987; Stump, 1995) and consists of three NW – SE-oriented tectonometamorphic terranes: the Wilson Terrane, the Bowers Terrane and the Robertson Bay Terrane (Bradshaw et

137

al., 1985; Stump et al., 1983). The Bowers Terrane and the Robertson Bay Terrane are considered to be allocthonous (Bradshaw et al., 1985; Kleinschmidt and Tessensohn, 1987): they are formed by low-grade metamorphic rocks of Cambrian– Ordovician age and were extensively intruded by the Devonian plutons.

Fig. 2. (a) Field relationships and mingling structures between the gabbros and the granites in the Mt. Abbott area (Adelie Cove). The rocks at the contact have lobate margins, and mafic masses are dispersed into the granite. (b) Contact between a diorite bearing K-feldspar megracrysts and the granite; the darker intrusive facies within the granite has a granodiorite-type composition.

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In the Wilson Terrane, the basement is characterized by a metasedimentary sequence defining a westward increase of metamorphic grade from greenschists to granulite facies rocks (Carmignani et al., 1989). This metamorphic basement was intruded by late orogenic plutons during the Cambro-Ordovician, collectively named Granite Harbour Intrusives (Gunn and Warren, 1962). These intrusions constitute a calc-alkaline magmatic suite and define the magmatic arc related to the Palaeozoic subduction of the palaeoPacific ocean (Borg et al., 1986). Their orogenic geochemical signature and NE to SW trend in Sr- and Ndisotope ratios have been interpreted as an increase of crustal component related to westward directed subduction of the palaeo-Pacific beneath the East Antarctic Craton (Kleinschmidt and Tessensohn, 1987; Borg and De Paolo, 1991; Rocchi et al., 1998). The Mt. Abbott plutons form a well-exposed pseudo-triangular composite intrusion of about 90 km2 (Fig. 1), which is referred to as Terra Nova Bay Intrusive Complex (Rocchi et al., 1997). Three main rock types have been described in the Mt. Abbott area: (1) coarse-grained gabbros and diorites; (2) hybrid granodiorites; and (3) syenogranite with microcline megacrysts (Di Vincenzo et al., 1997). Granites dominate the complex (>65%), the diorites form the more abundant mafic rock type (15 –20%), while gabbros form very limited outcrops ( f 5%). The intermediate granodiorite facies are widespread, although spatially limited. Mafic– felsic interfaces are irregular, deeply interdigited with metric mafic enclaves that are often dismembered into the granite. Field relationships and mingling structures (Fig. 2a) indicate a nearly coeval emplacement of the mafic and the felsic intrusives, with the granite emplaced very shortly after the gabbros and partly incorporating it. The occurrence of quartz and feldspar xenocrysts within the diorites next to the granite (Fig. 2b) and the lobate margins of the mafic facies suggest that thermal equilibration and incomplete mixing occurred between the mafic and the felsic rocks. Geochronological dating of the granite yielded an age of 508 F 5 Ma (Armienti et al., 1990a). On geochemical basis, the Terra Nova Bay Intrusive Complex defines a ‘‘high-K’’ calc-alkaline shoshonitic magmatic suite (Ghezzo et al., 1989). Sr– Nd isotopic variations between the mafic and felsic rocks have been observed by Di Vincenzo and Rocchi (1999), who interpreted the evolution of Mt. Abbott

intrusives in terms of assimilation– crystal fractionation (incorporating sedimentary metamorphic rocks) and magma refilling of continental arc basalt, plus mixing with crustal melts independently generated.

3. Samples and Mineral Chemistry Eighteen new olivine gabbro and diorite samples and twelve samples from previous studies (Armienti et al., 1990b; Di Vincenzo and Rocchi, 1999) were investigated. Representative chemical analyses are reported in Table 1. Most samples were collected tens of meters from the granite – gabbro contact, while three samples (AC5, DR13, AF11) were collected at the contact in order to check for possible subsolidus alteration effects related to fluid infiltration from the adjacent granite. Widespread hydration effects were observed during microscopic observation: olivine is altered and/or replaced by orthopyroxene – magnetite – biotite aggregates, hornblende grew at expenses of pyroxene and there is minor subsolidus plagioclase sericitization. Olivine gabbros contain unzoned olivine crystals of Fo60 – 62; cumulous olivine crystals of samples DS4, DR12, DR13, AC7 are also unzoned with Fo70 (Table 2). In the gabbros, large enstatitic orthopyroxene (Wo < 5%, En f 55 – 62%, Fs f 35 – 43%) exsolution lamellae occur within clinopyroxenes (Wo f 45 –48%, En f 42 –44%, Fs f 8– 12%). In the diorites, the medium-grained augitic clinopyroxene (Wo f 35 – 48%, En f 27 – 36%, Fs f 16 –32%) host fine linear exsolution domains of orthopyroxene (Wo < 2 –4%, En f 36– 48%, Fs f 49– 61%). Thermometric estimates from exsolution-free clinopyroxenes and orthopyroxenes analyses (Lindsley, 1980) indicate chemical equilibrium at a temperature of 800 jC both in gabbros and diorites (Dallai, 1998, unpublished). Elongated zoned plagioclase is widespread in gabbros (core: An65, rim: An50) and diorites (core: An60, rim: An40). Large, normally zoned (core: An45, rim: An35) plagioclase mega-xenocrysts sometimes occur in the diorites, where the xenocrysts are rimmed by fine-grained orthopyroxene and clinopyroxene. Kfeldspar occurs only in the diorites, in the groundmass (Or87 – 90) and as large phenocrysts (Or90 – 92). Biotite is always present in the mafic facies, whereas amphib-

L. Dallai et al. / Lithos 67 (2003) 135–151

139

Table 1 Representative chemical analyses of the gabbros (G), diorites (D), granodiorites (GD) and granites (Gr) of the Granite Harbour Intrusives of the Mt. Abbott area Sample

DR12 11197

DR13 11197

AC7 26188

DS4 231293

AC6 26188

AF11 10187

AF6 10187

C6 17188

AF16 10187

DS15 10194

DS17 10194

DS3 291293

DS20 10194

DS1 10194

Rock type

G

G

G

G

G

G

D

D

D

D

GD

GD

Gr

Gr

SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI V Cr Co Ni Y Zr Nb Ba La Ce Nd Sm Eu Pb Th

46.3 0.9 16.2 2.6 7.5 0.2 14.5 8.1 1.9 0.9 0.2 0.5 148 934 25 279 24 131 6 122 13 28 18.2 17.7 – 23.0 28.0

46.4 0.9 15.7 1.2 8.7 0.2 14.1 8.5 1.9 0.8 0.2 0.5 167 830 20 242 24 130 6 118 15 23 4.2 4.2 – 23.0 29.0

46.9 1.0 16.1 1.2 8.6 0.2 13.9 8.1 2.0 0.9 0.2 1.0 153 879 69 256 24 136 7 201 16 32 19.0 4.4 – 20.0 6.0

47.0 0.9 15.7 2.3 7.4 0.2 14.9 7.6 2.1 0.9 0.2 0.8 81 781 77 392 16 113 6 202 13 31 15.5 3.4 0.8 4.9 2.0

47.1 1.1 17.7 4.6 6.2 0.1 8.3 10.3 2.4 0.9 0.2 1.0 158 257 48 86 22 96 5 232 11 25 14.0 3.2 1.2 7.0 0.3

48.7 2.2 14.7 3.2 10.2 0.1 5.9 8.7 2.4 2.2 0.4 1.3 235 75 43 22 44 169 12 364 26 56 31.6 7.7 2.0 29.0 3.1

51.1 2.2 15.8 1.4 9.6 0.1 5.3 8.3 2.7 1.7 0.7 1.1 184 96 48 19 43 90 17 709 30 65 33.0 7.6 2.7 28.0 4.9

53.4 3.2 14.7 1.7 9.7 0.1 3.2 6.9 2.5 2.9 0.7 0.9 220 41 45 7 47 321 10 947 42 96 54.3 11.8 – 33.0 7.0

55.8 2.8 14.6 1.2 9.0 0.2 2.9 6.5 2.2 3.1 0.8 0.9 188 9 43 7 49 216 13 719 56 107 61.0 13.0 – 35.0 1.0

55.8 2.5 14.8 1.3 8.5 0.1 3.3 6.0 2.6 3.2 0.7 1.0 98 36 30 15 48 428 27 1000 50 112 59.1 11.7 2.7 18.5 2.9

60.2 2.0 14.2 1.9 6.6 0.1 2.4 4.7 2.4 3.8 0.7 1.0 70 27 21 10 50 380 24 932 62 136 63.7 12.2 2.3 24.0 15.8

64.0 1.4 14.0 1.1 5.7 0.1 1.2 3.6 2.7 5.0 0.4 0.9 37 5 15 5 59 361 28 855 59 163 78.8 15.4 1.8 24.3 6.1

67.9 0.7 15.0 0.6 3.1 0.0 0.6 1.9 2.6 6.6 0.2 0.8 19 4 5 3 34 306 22 797 70 169 71.2 12.9 1.1 24.4 23.2

70.3 0.5 14.7 0.3 2.4 0.0 0.4 1.4 2.3 6.9 0.1 0.6 9 2 4 3 19 368 16 1030 50 120 42.6 7.1 1.2 21.9 11.0

All samples were analysed at the University of Siena for major elements, Rb, Sr and Zr by XRF on pressed powder pellets, following the method of Franzini et al. (1975) and Leoni and Saitta (1976). FeO was determined by titration, MgO and Na2O by AAS and loss on ignition by gravimetry at 960 jC after preheating at 110 jC. Remaining trace elements and REE were determined by ICP-MS at the XRAL Laboratories (Toronto, Canada).

Table 2 Calculated melt equilibrium MgO (wt.%) using the equation: logCliq = logCOl + 1.87
3740/T (K) (Roeder and Emslie, 1970) Sample

AC6 26188

DR12 11197

DR13 11197

DS4 231293

Olivine MgO (wt.%) Whole-rock MgO (wt.%) Calculated MgO (wt.%) in the melt at T = 1250 jC

29.29

38.07

35.46

35.67

8.34

14.49

14.06

14.93

7.6 – 9.1

9.8 – 11.8

9.2 – 11.0

9.2 – 11.1

ole (Mg-rich hornblende) occurs only in the samples located at the mafic – felsic boundary. The complete set of mineral chemical analyses can be obtained from the senior author.

4. Analytical methods Oxygen and hydrogen isotope analyses on wholerock and mineral separates were made at the Isotope Laboratory of the University of Lausanne. Mineral separates were obtained by standard gravimetric and

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L. Dallai et al. / Lithos 67 (2003) 135–151

magnetic methods and final handpicking under a microscope. Orthorhombic and monoclinic pyroxenes were analyzed as a single phase because the equilibrium 18O fractionation between pyroxenes at magmatic temperature is small (Chacko et al., 2001; Eiler, 2001). Oxygen measurements on whole-rock powders were performed using the bromine pentafluoride method of Clayton and Mayeda (1963). Laser fluorination of mineral separates and gas purification followed the procedure described by Sharp (1995). Both the analyses of mineral separates and whole-rock powders were compared with those of an internal standard, calibrated relative to NBS-28 (d18OSMOW = 9.6x ), and no data correction was needed. Almost all samples have been duplicated with analytical precision F 0.2xor better.

Hydrogen isotope compositions of hydrous silicates were determined by vacuum fusion according to the procedure of Vennemann and O’Neil (1993). The analyses were adjusted according to the yD values of thes NBS-30 biotite standard (yD =
65 x) in the course of the measurements due to a systematic shift observed in the yD values of the standards. The average reproducibility is F 4xfor standard and duplicate mineral analyses. Both 18O/16O and D/H ratios were determined with a Finnigan MAT 251 mass spectrometer. The results are reported in Table 3 using standard delta notation relative to SMOW. Sr- and Nd-isotope compositions for 10 rock samples from the Mt. Abbott mafic rock types were measured at the University of Bern, Laboratory of

Table 3 WR and mineral oxygen isotope composition and calculated fractionation (D) values in the Mt. Abbott intrusive rocks Rock Sample type

d18OWR d18OQtz d18OBt d18OKfs d18OPl d18OPx d18OOl d18OHbl DPl-Bt DQtz-Bt DPl-Px DQtz-Px DQtz-Pl DQtz-Kfs yDBt

G G G G G G G G G D D D D D D D D D D D D D GD GD GD GD Gr Gr Gr Gr

7.1 6.8 7.0 7.2 7.3 7.4 8.3 8.8 8.1 9.7 9.9 9.8 9.5 9.3 9.3 9.6 9.7 10.3 10.3 8.7 8.8 10.1 11.1 11.4 10.8 12.3 11.5 11.4 11.4 10.9

AC6 26188 AC7 26188 DS4 231293 DR12 11197 DR13 11197 DS18 101094 AC5 26188 AF11 10187 DR11 11197 CF14 9188 AF1 10187 AF4 10187 AF6 10187 L1 25186 AF16 10187 C7 17188 C6 17188 DS15 10194 DS22 10194 AC6 18188 AC7 18188 AF2 10187 DS17 10194 DS3 291293 DS4 10194 DS21 10194 DS1 10194 DS20 10194 L16 281285 L1-10 261285

5.5 5.5 5.4

11.5

11.9 11.8 11.5 9.7 12.1 12.2 12.2 12.4 12.8

12.7 12.7 13.9 12.9 13.0

5.5 5.5 5.6

5.6 10.1

7.4 6.2

7.2 7.5 6.7 7.1

7.9 7.6 7.4 7.8 8.3 7.8 8.5 8.9

6.8 6.6 6.0 6.2 6.2 6.4 6.4 7.0

9.3 9.6 9.7 8.5 10.3 9.9 10.0

8.6 8.6 8.2 8.8 8.5 8.5

10.5

8.6

6.4 6.3 5.7 5.8 5.9

2.4 2.1 2.0

2.3 6.8 6.4

1.1 1.0 1.4 1.6 2.4


87.0
74.3


69.3
105.6
84.8

1.4 3.0 3.3

4.2

5.9

2.1 1.9

4.5

2.6

6.0

0.6 1.0 1.5

3.2 3.2 3.3

1.6 1.4 1.6

3.3 3.7 3.8

2.6 2.2 1.8 1.2 1.8 2.3 2.2

1.9

4.1

2.3


93.0


86.7

5.0

10.4 10.5 11.6 10.9 11.5

10.9 11.4 11.3 11.4

4.4

7.5 7.5 6.5 8.5 9.2

3.7

5.5

1.9

4.0 4.6 4.3

6.4 6.2 5.9

2.5 1.7 1.6

The yD values of selected biotite samples are also reported. Mineral abbreviations according to Kretz (1983).

2.3 2.2 2.3 2.0 1.5


63.7
83.5
81.4
73.5

L. Dallai et al. / Lithos 67 (2003) 135–151

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Table 4 Whole-rock Rb – Sr element concentration and O – Sr isotope composition of the Mt. Abbott intrusive rocks Sample AC7 26188 DR12 11197 DR13 11197 AC6a 26188 AC5 26188 DS4b 231293 AF11 10187 DS18b 101094 AF1 10187 AF4 10187 DS15 10194 AF16 10187 C6 17188 DS22b 10194 DS3b 291293 DS21b 10194 DS20b 10194 DS1b 10194 L16a 281295 L1-10a 261285 a b

Rock Rb Sr type (ppm) (ppm)

87

G G G G G G G G D D D D D D GD GD Gr Gr Gr Gr

0.463 0.460 0.422 0.250 0.219 0.404 0.810 0.294 0.530 0.434 0.624 1.081 0.749 1.610 2.650 3.080 4.610 4.790 3.750 4.570

31.3 29.6 28.3 24.0 24.0 25.8 76.6 26.8 59.9 49.6 71.2 107.6 80.1 158 170.0 212.0 242.0 241.0 203.0 213.0

195.6 186.2 194.2 278.0 317.0 185.0 273.7 264.0 326.9 331.6 330.2 288.2 309.7 284 189.0 199.0 152.0 146.0 157.0 135.0

Rb/ (87Sr/ Sr 86Sr)m

86

0.708816 0.709091 0.708582 0.708240 0.708490 0.708628 0.712163 0.708523 0.713521 0.712588 0.715830 0.716787 0.715196 0.721073 0.728495 0.733064 0.743087 0.744664 0.737540 0.743120

F 2r (87Sr/ 86 Sr)t

Sm Nd (ppm) (ppm)

147 144

144

(144Nd/ Nd)m

F 2r (144Nd/ eNd 144 Nd)

19 25 75 10 10 8 35 10 7 7 11 102 20 15 9 18 9 11 5 5

4.36 4.21 4.17 – – 6.74 7.65 6.13 – – – 13.02 11.83 14.50 15.88 15.09 15.42 9.17 8.50 –

0.13888 0.13961 0.14205 – – 0.13930 0.14656 0.15020 – – – 0.12897 0.13172 0.12610 0.12670 0.12210 0.108500 0.099400 0.137000 –

0.512274 0.512251 0.512255 – – 0.512278 0.512330 0.512402 – – – 0.512084 0.512121 0.512099 0.512031 0.512007 0.5119570 0.5119270 0.5120300 –

17 18 18 – – 8 18 5 – – – 22 17 8 5 7 6 6 3 –

0.70545 0.70575 0.70551 0.70644 0.70653 0.70570 0.70627 0.70640 0.70966 0.70944 0.71129 0.70893 0.70976 0.70945 0.70960 0.71080 0.70970 0.71000 0.71027 0.71800

19.0 18.2 17.7 – – 31.8 31.6 24.7 – – – 61.0 54.3 69.6 75.7 74.7 85.9 55.8 38.0 –

Sm/ Nd

0.51181 0.51178 0.51178 – – 0.51181 0.51184 0.51190 – – – 0.51165 0.51168 0.51168 0.51161 0.51160 0.51159 0.51159 0.51157 –

t


3.3 F 0.2
3.8 F 0.2
3.9 F 0.2 – –
3.3 F 0.2
2.7 F 0.2
1.6 F 0.1 – – –
6.4 F 0.2
5.9 F 0.2
5.9 F 0.2
7.3 F 0.1
7.5 F 0.1
7.6 F 0.1
7.6 F 0.1
7.8 F 0.1 –

TDM (Ga) 1.63 1.69 1.73 – – 1.56 1.69 1.53 – – – 1.77 1.76 1.64 1.77 1.72 1.57 1.48 1.81 –

Rb, Sr concentration and Sr isotope data from Armienti et al. (1990b). Rb, Sr concentration and Sr isotope data from Di Vincenzo and Rocchi (1999).

Isotope Geochemistry. The separation procedure for Rb – Sr analysis was standard cation exchange. Nd – Sm separation procedure, measurement technique and oxygen correction according to the method described in Chavagnac et al. (1999). Both Sr- and Nd-isotope ratios were measured by thermal ionization on a single collector VG Sector instrument. The average 87Sr/86Sr ratios measured for NBS-987 was 0.710255 F 0.000030 (2r, n = 10); the 143Nd/144Nd ratios measured for the La Jolla Standard was 0.511866 F 0.000028 (2r, n = 5). The measured Rb, Sr, 87Sr/86Sr and 143Nd/144Nd values are reported in Table 4. The initial Sr- and Nd-isotopic ratios were calculated at 508 Ma according to the isochron age obtained by Armienti et al. (1990a).

5. Results Petrographic, geochemical and isotopic evidence allowed the mafic rocks to be characterized as three different rock types, which we will refer to as olivine – gabbros, mafic diorites and SiO2-rich diorites mingled with the granite. Harker-type diagrams show

quite continuous covariations of silica with major element content, with linear trends generally observed from the mafic diorites (SiO2>50 wt.%) to the granites (Fig. 3). The chemical composition of SiO2-rich diorite is poorly correlated with changes in the nature, abundance and composition of mafic minerals. This is interpreted to result from increasing modal abundance of quartz and feldspar xenocrysts from diorite to granite. ‘‘Bell-shaped’’ patterns are typic of the SiO2 – CaO, SiO2 – TiO2, SiO2 – Sr, SiO2 – Y, SiO2 – Eu, SiO2 –Nb, SiO2 – Nd covariations, and rule out that diorite formed by simple mixing processes between gabbros and granites. Primitive-mantle normalized multielement diagrams (Sun and Mc Donough, 1989) show marked similarities between gabbros and diorites and granites and granodiorites, respectively (Fig. 4). Gabbros and diorites exhibit positive anomalies for K and Zr, whereas only diorites have small negative anomalies for Ti, Nb and Sr. Such anomalies are generally observed in mafic rocks derived from basalts contaminated by crustal material (Sun and Mc Donough, 1989), and/or as subduction-related basaltic magmas with calc-alkaline affinity (Wilson, 1989; Mc Culloch

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Fig. 3. Harker-type diagrams for the Mt. Abbott intrusives. Fields in the SiO2 vs. K2O (wt.%) plot according to Peccerillo and Taylor (1976). The trend shown by the measured rocks starts from the calc-alkaline s.s. field and crosses into the ‘‘high-K’’ one. The felsic lithotypes plot in the shoshonitic field. (n) Gabbros, (5) diorites, ( ) granodiorites, (o) granites.

.

L. Dallai et al. / Lithos 67 (2003) 135–151

143

Fig. 4. Primary-mantle normalized multielement diagrams of the intrusive rocks of the Mt. Abbott area (symbols as in Fig. 2). Reference patterns: (a) E-MORB (Sun and Mc Donough, 1989), (b) calc-alkaline basaltic andesites (Wilson, 1989), (c) assumed parental magma after AFC model calculation (see text for discussion), (d) TAM crustal xenoliths (Kalamarides et al., 1987), (e) bulk continental crust (Taylor and Mc Lennan, 1985), (f) Wilson Terrane granulitic crust (Talarico et al., 1995).

and Gamble, 1991). Samples DS4, DR12, DR13 and AC7 have high Cr, Co, Th and Ni concentrations, which is likely a result of their formation as cumulates. The whole-rock O-isotope compositions of the lithologic units cropping out at Mt. Abbott define three distinct ranges: gabbros having d18 O values = 6.8 – 8.7x ; compositionally variable diorites with d18O values = 8.7 – 10.3x; and granodiorites and granites with d18O values = 10.8 –12.1x. In the gabbros, d18O values range from 5.7xto 6.8xfor olivine, from 5.9xto 6.8xfor pyroxenes and from 7.4xto 9.0xfor plagioclase. These 18O/16O ratios for olivine are considerably higher than recorded for olivine from modern MORB and OIB (d18OOl = 5.0– 5.4x), and above the d18O values from modern arcrelated magmas (5.4 –6.4x; Eiler et al., 1996; Eiler, 2001). The amphibolitized samples of gabbro collected near the granite contact have some d18Omineral values, which lie outside these ranges (e.g. d18OPl in AC5 = 8.5, d18OPx in AF11 = 7.0). Biotite has very constant d18O values of 5.5x. The d18O values of mineral separates from the dioritic rocks have small variations, with the exception of sample L1, a decimetric microgranular enclave within the granitic body. The d18O values vary from 8.2xto 8.6xfor pyroxenes, from 9.3xto 10.5x

for plagioclases and from 11.9xto 12.8xfor quartz. In contrast, the d18OBt values vary considerably, from 5.5xin the mafic diorites to 7.4xin the silica-rich facies. The granodiorites and the granites have very homogeneous mineral isotopic compositions, with the exception of sample DS21 which exhibits isotopic disequilibrium among the different phases. The d18O values range from 12.7xto 13.9xfor quartz, from 10.9xto 11.4xfor plagioclase and from 10.4xto 11.6xfor K-feldspar. Biotite d18O values are homogeneous, from 6.7xto 7.5x . The yD values of biotites are in the ‘‘normal’’ compositional range of intrusive rocks, from
90x to
70x . In the gabbros, the yDBt values vary from
69xto
87x , with only sample AC5 having a low yDBt value (
105x), likely due to incipient subsolidus alteration. Assuming a biotite –water equilibrium fractionation value between
20xand
40x(Suzuoki and Epstein, 1976), the yD values of the equilibrium water of biotites in the mafic and felsic rocks fall in the field of magmatic values as defined by Sheppard (1986). The diorites, granites and granodiorites show a slightly broader but similar yD range, and vary from
93xto
64x. These values suggest that the Mt. Abbott intrusives inter-

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acted with limited amounts of fluids (low W/R ratios), and that a fluid phase dominated by the magmatic component circulated at the mafic – felsic interface during magma interaction and upon cooling. Initial Sr- and Nd-isotope ratios of gabbros have narrow ranges, from 0.70545 to 0.70653 and from 0.511780 to 0.511900, respectively, which are ‘‘enriched’’ compared to primary mantle values (87Sr – 86 Sr = 0.702 – 0.704, 143Nd/144Nd = 0.5132 – 0.5128; Faure, 1986). The diorites are more radiogenic in Sr and Nd isotopic composition than the gabbros: their ( 87Sr/ 86 Sr) i ratios are more scattered than (143Nd/144Nd)i ratios ( f 0.51210) and shifted more towards crustal values (from 0.70892 to 0.71129). Similar Sr- and Nd-isotope ratios were measured in granites and granodiorites; these results are consistent with the previously published data from Di Vincenzo and Rocchi (1999).

6. Discussion: O-isotope constraints on petrological processes and sources A clear break in the O-isotope variation with fractionation is observed in the SiO2 –d18Owr diagram (Fig. 5a), indicating that the Mt. Abbott intrusive suite originated by mechanisms other that of simple closedsystem crystal fractionation. The covariation trends for SiO2 vs. d18Ominerals (Fig. 5b) shows that there is a general consistency for the minerals: most of the mineral d18O values mimic the whole-rock values, and the observed 18O-enrichment in minerals is from biotite to pyroxene, plagioclase, and quartz. This order is consistent with the intrusive rocks having cooled unperturbed by low-temperature subsolidus hydrothermal alteration (Taylor, 1977). In a d– d graph (Criss and Taylor, 1986; Gregory et al., 1989), lines of constant temperature can be drawn 6 by means of the equation: 1000lna ¼ a10 T 2 þ bfda
db , where a is the oxygen isotope fractionation factor between phase ‘‘a’’ and phase ‘‘b’’, and a and b are temperature coefficients. Using the temperature coefficients of Chiba et al. (1989), the temperatures estimated on the basis of the D18OPl – Px and D18OQtz – Kfs are 650 –700 jC in mafic rocks, and 550– 600 jC in the granites and granodiorites, respectively (Fig. 6). These are the temperatures expected from subsolidus oxygen isotope re-equilibration in mafic and acidic

Fig. 5. (a) Covariation of d18O with SiO2 vs. d18O for whole-rock samples (symbols as in Fig. 2), and (b) mineral separates d18O values – SiO2. (E) K-feldspar, (D) quartz, (x) olivine, (w) pyroxene, (+) biotite, (  ) plagioclase. The d18Opx – SiO2 relationships define two distinct evolution trends for the gabbros and diorites.

igneous rocks (Giletti, 1986; Taylor and Sheppard, 1986). The measured D18O values for pyroxene and olivine are 0.3– 0.4x(Fig. 6c), comparable to the equilibrium fractionation values between olivine and pyroxene in mantle xenoliths (Mattey et al., 1994). These D18O values are likely due to the removal of the cumulous olivine from the melt prior to pyroxenes (and plagioclase) crystallization. Only the sample from the gabbro – granite contact (AC5) show negative ( = disequilibrium) d18OPx – d18OOl values, due to the partial alteration of olivine. The d18OOl values of unaltered gabbros were used to estimate the oxygen isotope composition of the parental magma by means of the olivine –melt oxygen isotope fractionation at high temperatures (e.g. Kyser

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et al., 1981; Garcia et al., 1998; Eiler et al., 2000). Oxygen isotope investigation on ocean-arc lavas (Eiler et al., 2000) and empirical models (Matthews et al., 1998) indicate melt – olivine fractionation of 0.4– 0.5x, whereas Ito and Stern (1986) calculated a Dmelt – olivine f 0.7x, with little or no temperature dependence (see also Chiba et al., 1989). The use of the latter value of 0.7xminimizes the crystallization temperature uncertainties. The calculated d18O values are between 6.4xand 7.5x, and statistically undistinguishable from the estimates obtained by different melt – olivine fractionation (measurement uncertainty F 0.2x). It is worth noting that only the unaltered noncumulite gabbro (sample AC6) matches the isotopic value calculated on the basis of the olivine –melt fractionation. These values are slightly higher than the d18O values of modern magmas from active continental margins (Harmon and Hoefs, 1995), and suggest that the Mt. Abbott gabbroic magma was already modified before olivine accumulation. The increase of d18O value in the residual melt (diorites?) was maximized during crystal fractionation (Matsuisa et al., 1973) due to olivine extraction, and subsequent fractionation of oxide minerals, inferred from FeO and TiO2 depletion, likely produced in a further 18O increase. The d18O values of Mt. Abbott gabbros are above the compositional range inferred for mantle materials in different geodynamic environments (e.g. Mattey et al., 1994; Harmon and Hoefs, 1995; Eiler et al., 2000), such that they do not represent the O-isotope composition of their mantle source. Further distinction between source contamination and crustal assimilation is uncertain; however, it is unlikely that concurrent d18OOl values, high Sr- and low Nd-isotope compositions result from mantle metasomatism. Moreover, the forsterite content in cumulous olivine ( f 70%) accounts for olivine gabbros to be partly evolved magmas. From Fig. 7, it is clear that O- and Sr-

Fig. 6. (a) d18Oplagioclase vs. d18Opyroxene plot showing the two compositional fields of gabbros and diorites, respectively. (b) d18Oplagioclase vs. d18Oquartz plot: the felsic lithologies plot on the same fractionation line (D = 2) indicating similar subsolidus isotope exchange upon cooling. (c) d18Opyroxene vs. d18Oolivine plot: the gabbro samples (AC5) at the contact with the granite has negative fractionations, indicative of high-T disequilibrium isotope exchange.

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isotope compositions are positively correlated, whereas the d18O values and the Sr content, and the Sr- and Nd-isotope compositions define negative covariations. Systematic behaviour is also found for O-, Sr- and Nd-isotope data with respect to most of the major and trace elements contents. In the (87Sr/86Sr)i vs. d18O diagram, the d18O values of olivine gabbros and diorites cluster in two separate fields (Fig. 7a), defining a slightly downward concave trajectory similar to those interpreted as a result of crustal contamination (e.g. James, 1981; Davidson and Harmon, 1989). Modeled parental magma compositions, assuming contamination of normal slab-derived fluid from altered oceanic crust and 5– 10% of sediments (i.e. Dorendorf et al., 2000: d18O = 6 – 10x; Sr = 400 ppm; 87Sr/86Sr = 0.7039) differ significantly from the measured d18O and 87Sr/86Sr values of Mt. Abbott olivine gabbros (Fig. 7a and b). Instead, these latter values can be matched by assimilation and fractional crystallization (AFC) modeling (De Paolo, 1981) of a primary magma contaminated by lower crustal materials (Fig. 7a, b, c) such as the crustal xenoliths from the McMurdo Sound area (Kalamarides et al., 1987) and/or the granulite-facies rocks presently exposed in the Mt. Abbott area (Palmeri, 1997). Preliminary data on these latter rocks show O-, Sr- and Nd-isotope compositions from 10xto 14x, from 0.714 to 0.726 and from 0.51130 to 0.5115, respectively (Armienti et al., 1990b; Talarico et al., 1995; Dallai et al., 2002). AFC curves, which reproduce the measured trace elements, Sr-, Nd- and O-isotope compositions of olivine gabbros and diorites, are calculated assuming a unique starting mafic magma assimilated a granulitic end member with 150 Sr ppm, 87Sr/86Sr = 0.7200 and 143Nd/144Nd = 0.5115 (Fig. 7a, b, c). The AFCbased compositions over a broad range of ‘‘assimilated material vs. crystallized material’’ ratio (r = 0.4– 0.8)

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match those measured in this work. As plagioclase was not the main phase controlling the fractionating assemblage, the bulk Kd for Sr was set at < 1. Mt. Abbott diorites have O-, Sr- and Nd-isotope compositions (d18O = 9.6 F 0.5x, (87Sr/86Sr)510Ma = 0.7096 F 0.00096, (143Nd/144Nd)510Ma = 0.51167 F 0.000012) that cannot be produced either by selective contamination of gabbros from the adjacent granite (d18O f 11.3 x, 87Sr/86Sr f 0.710, 143Nd/144Nd f 0.5120) or by digestion of granite portions by the mafic magma, even though these processes occurred and produced hybrid rocks. The higher self diffusivities of Sr vs. O in mafic magmas (Muehlenbachs and Kushiro, 1974; Canil and Muehlenbachs, 1990; Lesher, 1994; Kyser et al., 1998) rule out the possibility that self-diffusion mechanisms at the granite contact produced high and invariant d18O values of high-temperature phases like pyroxenes (d18O = 8.5 F 0.16x) over a kilometric scale and scattered 87 Sr/86Sr ratios. Therefore, the diorites can be considered the result of higher degrees of crustal contamination experienced by the same olivine-rich magma. Because the d18O values of pyroxenes show no correlation with the SiO2 content of the rocks (Fig. 5b), they did not increase with increasing chemical and isotopic evolution of the host magma, and the dioritic melt with high d18O and 87Sr/86Sr values was produced by crustal assimilation before mingling. The estimated amount of assimilation is close to 40%, suggesting that a considerable amount of crustal material was assimilated. In this scenario, the compositions of the more felsic diorites was generated by simple mixing between the Mt. Abbott granite and the AFC-derived mafic diorites. The disequilibrium textures observed in the diorites, such as quartz/feldspar megacrysts in a medium-grained matrix and reaction coronas of pyroxene/biotite on quartz and feldspar

Fig. 7. Calculated AFC trajectories for the (87Sr/86Sr)510 – d18OWR, Srppm – d18OWR, (87Sr/86Sr)510 – (143Nd/144Nd)510 correlations using the equation of De Paolo (1981). Assumed values: r (assimilated vs. crystallized material ratio) = 0.5, Kd for Sr = 0.7. Primary magma: d18OWR = 5.5x , (87Sr/86Sr)510 = 0.7040, Srppm = 340, (143Nd/144Nd)510 = 0.51250. Assimilated crust: d18OWR = 12x , (87Sr/86Sr)510 = 0.7200, Srppm = 150, (143Nd/144Nd)510 = 0.5115. Tick marks indicate the percent values of residual melt ( F) during the assimilation and fractional crystallization process (each mark = 0.1). Simple-mixing lines from mafic diorites to granites are also reported (tick marks are 10% end-member percentages in the melt). The Srppm – d18OWR correlation clearly indicates the presence of cumulus facies within the Abbott gabbros, characterized by low Sr concentration (symbols as in Fig. 3). The compositional fields for the average Mantle Source, MORB, OIB, Andean Central Volcanic Zone and Ocean Island Arc magmas are from Faure (1986) and references therein. Fields for Ross Sea lower crustal xenoliths and TAM xenoliths are from Kalamarides et al. (1987); the field for Mt. Abbott granulites is based on the data of Armienti et al. (1990b) and Talarico et al. (1995).

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megacrysts are evidence of incomplete magma mixing in a shallow environment (e.g. Neves and Vauchez, 1995; Wiebe, 1980; D’Lesmos, 1996). As magma mingling occurred, contamination of the mafic melt by the assimilation of the contemporaneous silicic magma may also produce anomalous O-isotope values. With regards to the felsic intrusive rocks, high Sr- and O-isotopic compositions and low eNd values indicate a major contribution of old crustal material to their genesis. Metamorphic rocks presently exposed in the area, which can be claimed as plausible protoliths, are metapelitic migmatites and relict enderbites (Talarico et al., 1995; Palmeri, 1997). However, the migmatites have d18O values from 12.6x to 14.8x, and the peraluminous dykes interpreted as their minimum melts (Frezzotti et al., 1994) show d18 O wr >14, d18 O Qtz = 15.6, d18OPl = 13.8x(Dallai et al., 2002). These values suggest that the granites originated from melting of a source other than the migmatites, as also suggested by Di Vincenzo and Rocchi (1999) on the basis of Sr- and Nd-isotope composition. The intrusive granulites and enderbites occurring within the high temperature – low pressure metamorphic belt in the Wilson Terrane (Carmignani et al., 1989, Talarico and Castelli, 1995) have geochemical and isotopic characteristics similar to the Mt. Abbott granites (d18O = 11.6 –13.3x, (87Sr/86Sr)i = 0.71037, (143Nd/144Nd)i = 0.51166). Reported chemical data for these granulites (Armienti et al., 1990b; Palmeri, 1997; Talarico et al., 1995) show similarities to those of Mt. Abbott granites. It is likely that melting of fertile rock types similar to the igneous granulites was triggered by a thermal perturbation induced by the emplacement of mafic magmas. These melts produced the Mt. Abbott granite, and magmas with granodioritic composition were likely generated by mixing of the granite with the dioritic melts. A simple mixing calculation allows for geochemical and isotopic compositions of granodiorites to be reproduced, whereas a similar calculation using the gabbros as mafic end-members is not adequate (Fig. 7). It is worth noting that where the gabbros and granites are juxtaposed, the contacts are sharp and no intrusive facies with intermediate composition are observed, and the gabbroic rocks show distinctive evidence of amphibole and/or biotite growth

after pyroxene. Therefore, chemical –rheological barriers prevented mixing of the granites and the gabbros, but not between the diorites and the granites, the latter mix producing the granodioric rocks.

7. Conclusions Geochemical data and coupled stable and radiogenic isotope compositions of the composite intrusions of Mt. Abbott support the following conclusions: (a) Olivine gabbros and diorites were derived from calc-alkaline basaltic magmas, which had been contaminated by high 18O – 87Sr crustal rocks. (b) The granites show chemical and isotopic similarities with the granulite-facies acidic igneous rocks from adjacent areas, suggesting they originated from the partial melting of these fertile igneous source rocks. (c) Mineral phases are in isotopic equilibrium and preserve magmatic d18O and yD values. Local high-temperature hydration occurred only at the immediate contacts between the mafic and the felsic facies. The gabbro – granite interfaces are characterized by amphibolitized gabbroic facies, bearing amphibole and biotite grown after pyroxene. In these rocks, the oxygen isotope composition of plagioclase is slightly higher than those of unaltered gabbros, likely due to interaction with the hydrous fluids from the granite. The absence of chemical intermediate rocks at the gabbro – granite contacts likely indicates that rheological difference between the gabbros and granites prevented the chemical – physical mixing. (d) The lack of any correlation between the O- and Srisotope composition in the dioritic samples with the distance from the acid-basic interface constrains that the effects of the diffusion did not alter the isotopic ratios to a significant extent. (e) Mixing and mingling processes took place only between dioritic and granitic magmas, and hybrid facies such as xenocrystic diorites and granodiorites, which generally occur at the diorite – granite contacts, were produced.

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Acknowledgements This work has been supported by the Italian National Research Project in Antarctica (PNRA). The authors are indebted to J. Hunziker for L.D.’s stay at Isotope Lab of the University of Lausanne. We acknowledge R.J. Arculus, A. Longinelli and G. Di Vincenzo for discussions; F. Bussy for microprobe assistance; and S.M. Sheppard for thoughtful review of an early version of this paper. Reviews from R.S. Harmon and O.T. Ramo further improved the manuscript.

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