Contrib Mineral Petrol (2001) 142: 127±146 DOI 10.1007/s004100100287

G. Demarchi á P. Antonini á E.M. Piccirillo G. Orsi á L. Civetta á M. D'Antonio

Signi®cance of orthopyroxene and major element constraints on the petrogenesis of Ferrar tholeiites from southern Prince Albert Mountains, Victoria Land, Antarctica Received: 12 July 2000 / Accepted: 23 May 2001 / Published online: 14 July 2001 Ó Springer-Verlag 2001

Abstract The least evolved Jurassic Ferrar tholeiites from southern Prince Albert Mountains (Antarctica) are characterized by the occurrence of orthopyroxene (opx), a mineralogical feature virtually absent in the tholeiites from the large igneous provinces of Karoo (South Africa) and Parana (Brazil). Petrography suggests that opx is the early phase in the sequence of crystallization and mineral chemistry indicates that it is in equilibrium with the host rock. In general, MELTS modeling predicts that opx is the liquidus phase in the Ferrar tholeiites with MgO higher than 7 wt% at P=1.5±5 kbar, H2O=0±1 wt% and fO2 ˆ QFM 1 log unit conditions. MELTS results also show that the early crystallization of opx is primarily controlled by high SiO2 and high SiO2/CaO, chemical characteristics typical of the Ferrar tholeiites, but not shown by the Karoo and Parana analogs with similar MgO content. Major element geochemistry of the least evolved Ferrar tholeiite has been modeled through fractional crystallization and fractional crystallization coupled with crustal assimilation processes, starting from natural peridotite-derived experimental melts. Mass balance and MELTS modelG. Demarchi (&) á E.M. Piccirillo Dipartimento di Scienze della Terra, UniversitaÁ di Trieste, Via E. Weiss 8, 34127 Trieste, Italy E-mail: [email protected] P. Antonini Dipartimento di Ingegneria Chimica, dell' Ambiente e delle Materie Prime, UniversitaÁ di Trieste, P.le Europa 1, 34127 Trieste, Italy G. Orsi Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy L. Civetta Dipartimento die Scienze Fisiche, UniversitaÁ di Napoli, Mte S. Angelo, Via Cinzia, 80126 Napoli, Italy M. D'Antonio Dipartimento di Scienze della Terra, UniversitaÁ di Napoli, Largo S. Marcellino 10, Napoli, Italy Editorial responsibility: T.L. Grove

ing support the argument that theoretical magma compositions suitable to be primary to the least evolved Ferrar tholeiites are compatible with hydrous (H2O=0.3±0.5 wt%) and anhydrous melts obtained at 10±15 kbar by high melting degrees (>25%) of fertile and depleted spinel lherzolites, respectively, and later contaminated by the high-grade metamorphic rocks from the Victoria Land crystalline basement. The genesis of primary Ferrar tholeiites does not necessarily re¯ect the generally assumed depleted source mantle being also compatible with a fertile one.

Introduction Large-scale magmatic events produced large volumes of continental ¯ood basalts (CFB) associated with the break-up of the Gondwana supercontinent during the Mesozoic (Macdougall 1988; Storey et al. 1992; Mahoney and Con 1997). The CFB of Parana (SE-Brazil, mainly 133±132 Ma; Renne et al. 1992; Turner et al. 1994; Stewart et al. 1996), Karoo (South Africa, about 180 Ma; EncarnacioÂn et al. 1996; Duncan et al. 1997; Marsh et al. 1997), and Ferrar (Antarctica, about 180 Ma; Heimann et al. 1994; EncarnacioÂn et al. 1996; Fleming et al. 1997; Elliot et al. 1999) constitute the three major CFB provinces of the southern hemisphere. The Ferrar magmatic province (e.g., Hergt et al. 1991) extends in a linear belt from Antarctica to Tasmania and New Zealand (Mortimer et al. 1995) and tends to be parallel to the proto-Paci®c margin of Gondwana (e.g., Cox 1988). Ferrar magmatism belongs to the Gondwana low-Ti province (Cox 1988) and di€ers from the other two CFB provinces for its systematically high initial 87 Sr/86Sr (>0.707), low 143Nd/144Nd (<0.5124), and high SiO2 content (usually>52 wt%), even in the least evolved rocks (MgO=9±10 wt%; e.g., Gunn 1966; Hergt et al. 1989). These features, accompanied by high contents of large ion lithophile and light rare earth elements, have been attributed to (1) mantle source

Fig. 1 Geological sketch map of northern Victoria Land (modi®ed from Lombardo et al. 1991) showing the outcrops of the investigated Ferrar Dolerite and Kirkpatrick Basalt in southern Prince Albert Mountains. Numbers in brackets refer to the last digits of specimen label VG1000 reported in Tables 1, 2, 3 and 4. LZ samples are from Antonini et al. (1999). S dolerite sill; L lava ¯ow; P pillow lava

128

VG1039 Sill opx ph/c

56.16 0.10 1.23 0.32 10.37 0.26 30.17 2.47 0.01 101.09

0.43 4.71 79.55 15.74

SiO2 TiO2 Al2O3 Cr2O3 FeOt MnO MgO CaO Na2O Sum

Fe2O3 Ca Mg Fe*

0.23 4.90 76.72 18.38

55.71 0.11 0.83 0.12 12.07 0.20 28.72 2.55 0.04 100.35

VG1039 Sill opx mph/c

0.00 4.46 79.19 16.35

56.47 0.03 0.76 0.17 10.67 0.09 29.32 2.30 0.00 99.83

VG1026 Sill opx ph/c

0.00 4.92 78.68 16.40

56.31 0.11 1.26 0.25 10.66 0.24 29.32 2.55 0.00 100.70

VG1026 Sill opx ph/r

0.79 4.90 79.48 15.61

55.48 0.16 1.34 0.25 10.21 0.26 29.95 2.57 0.01 100.22

VG1048 Sill c.m. opx mph/c

1.02 4.59 79.21 16.20

55.43 0.09 1.33 0.20 10.63 0.28 29.88 2.42 0.00 100.26

VG1028 Lava opx mph/c

0.00 9.54 61.99 28.47

53.74 0.12 0.75 0.01 17.91 0.40 22.35 4.78 0.00 100.06

VG1039 Sill pgn m-opx

0.00 8.67 61.53 29.80

53.65 0.26 0.75 0.07 18.54 0.50 22.07 4.33 0.02 100.20

VG1026 Sill pgn m-opx

0.82 10.89 54.47 34.64

52.59 0.21 0.78 0.14 21.65 0.44 19.49 5.43 0.06 100.79

VG1026 Sill pgn m-opx

0.38 9.45 64.86 25.68

54.06 0.18 1.09 0.00 16.34 0.37 23.66 4.80 0.02 100.52

VG1026 Sill pgn ph/c

0.00 9.20 62.44 28.36

53.48 0.16 0.63 0.08 17.74 0.34 22.33 4.58 0.06 99.38

VG1026 Sill pgn ph/r

0.33 11.79 48.59 39.62

51.66 0.25 0.71 0.05 24.09 0.52 16.94 5.72 0.04 99.99

VG1026 Sill pgn m-aug

0.00 11.36 50.35 38.29

52.07 0.25 0.77 0.00 23.29 0.48 17.54 5.51 0.08 99.99

LZ57A Sill pgn mph/c

0.00 13.07 31.66 55.27

49.44 0.54 0.67 0.02 32.13 0.55 10.51 6.05 0.09 100.00

LZ57A Sill pgn mph/r

0.30 9.89 73.83 16.28

55.68 0.10 1.15 0.19 10.64 0.21 27.63 5.14 0.05 100.80

VG1048 Sill c.m. pgn mph/c

0.15 11.78 62.39 25.83

53.72 0.19 0.94 0.06 16.31 0.36 22.56 5.92 0.01 100.09

VG1028 Lava pgn m-opx

0.78 11.47 72.61 15.92

55.30 0.11 1.07 0.15 10.29 0.31 27.14 5.96 0.06 100.39

VG1028 Lava pgn m-opx

0.80 8.38 75.01 16.61

55.60 0.11 1.05 0.19 10.82 0.33 28.23 4.39 0.04 100.76

VG1028 Lava pgn mph/c

1.18 9.34 67.74 22.92

53.98 0.17 1.45 0.11 14.63 0.39 24.95 4.79 0.03 100.50

VG1019 Pillow pgn mph/c

0.15 10.85 63.37 25.78

54.24 0.13 0.77 0.00 16.40 0.36 23.10 5.49 0.03 100.53

VG1019 Pillow pgn mph/r

0.51 12.46 52.03 35.51

51.58 0.35 1.47 0.00 21.67 0.48 18.23 6.08 0.00 99.88

VG1019 Pillow pgn micro

0.00 8.54 63.09 28.36

53.48 0.20 0.81 0.04 17.65 0.46 22.65 4.27 0.01 99.58

VG1052 Pillow pgn mph/r

54.16 0.22 1.28 0.11 7.43 0.30 18.08 19.29 0.08

54.05 0.19 1.48 0.19 6.36 0.21 18.44 19.37 0.11

100.39 0.00 38.61 51.15 10.24

SiO2 TiO2 Al2O3 Cr2O3 FeOt MnO MgO CaO Na2O

Sum Fe2O3 Ca Mg Fe*

100.96 0.08 38.21 49.87 11.92

VG1039 Sill aug ph/r

VG1039 Sill aug ph/c

Sample Rock type

100.86 0.69 35.92 45.53 18.55

53.07 0.28 1.32 0.05 11.43 0.41 16.29 17.90 0.11

VG1039 Sill aug micro

99.15 0.00 36.11 50.67 13.22

53.52 0.19 1.38 0.17 7.98 0.33 17.82 17.66 0.10

VG1026 Sill aug m-opx

100.46 0.84 37.78 52.45 9.76

53.78 0.17 1.62 0.39 6.18 0.15 19.00 19.04 0.13

VG1026 Sill aug ph/c

100.27 0.98 34.50 46.49 19.00

52.71 0.37 1.22 0.08 11.80 0.28 16.55 17.10 0.16

VG1026 Sill aug ph/r

99.99 0.30 37.12 46.69 16.19

53.01 0.30 1.22 0.09 9.96 0.32 16.61 18.37 0.11

LZ6B Sill aug mph/c

100.00 0.97 34.33 41.30 24.37

51.76 0.30 1.21 0.04 14.87 0.42 14.53 16.80 0.07

LZ6B Sill aug mph/r

100.00 0.32 33.57 32.60 33.83

50.37 0.67 1.29 0.01 20.17 0.38 11.11 15.92 0.08

LZ57A Sill aug micro

100.41 0.62 24.43 60.23 15.34

54.12 0.17 1.49 0.14 9.74 0.24 22.02 12.42 0.06

VG1048 Sill c.m. aug m-opx

100.82 0.82 24.15 61.62 14.23

54.43 0.17 1.56 0.20 8.97 0.35 22.69 12.38 0.08

VG1048 Sill c.m. aug m-pgn

100.17 0.81 35.32 51.35 13.32

53.24 0.25 1.86 0.02 8.26 0.28 18.46 17.69 0.11

VG1048 Sill c.m. aug mph/c

99.53 0.00 25.71 48.42 25.87

52.38 0.33 1.19 0.03 15.79 0.32 16.93 12.50 0.06

VG1048 Sill c.m. aug micro

100.80 1.56 36.35 50.25 13.39

52.96 0.34 2.17 0.06 8.39 0.27 18.21 18.32 0.10

VG1028 Lava aug m-opx

101.01 0.65 39.42 50.08 10.50

54.14 0.22 1.29 0.12 6.66 0.17 18.29 20.04 0.09

VG1028 Lava aug mph/c

100.31 0.00 34.05 52.70 13.25

54.03 0.18 1.33 0.08 8.30 0.22 19.03 17.12 0.02

VG1028 Lava aug micro

100.68 1.82 36.31 48.26 15.43

53.07 0.24 1.30 0.06 9.72 0.29 17.53 18.35 0.12

VG1019 Pillow aug mph/r

101.13 0.67 28.38 49.92 21.70

53.22 0.34 1.26 0.03 13.60 0.36 18.02 14.26 0.04

VG1019 Pillow aug mph/c

99.98 1.36 23.38 49.54 27.08

51.72 0.41 1.68 0.00 16.58 0.47 17.52 11.51 0.09

VG1019 Pillow aug micro

100.10 1.60 34.34 41.93 23.74

51.26 0.50 1.91 0.02 14.38 0.44 14.69 16.74 0.16

VG1052 Pillow aug micro

99.63 0.79 29.42 47.76 22.82

52.23 0.30 1.36 0.00 14.00 0.33 16.85 14.45 0.12

VG1052 Pillow aug micro

99.43 0.00 24.66 50.67 24.66

52.90 0.31 1.28 0.04 14.86 0.40 17.61 11.94 0.10

VG1052 Pillow aug micro

Table 2 Microprobe analyses of augite of Ferrar tholeiites from southern Prince Albert Mountains. Fe2O3 calculated according to Papike et al. (1974); Fe*=Fe2+ + Fe3+ + Mn. ph/ c, ph/r core and rim composition of phenocrysts; mph/c, mph/r core and rim composition of microphenocrysts; micro composition of groundmass microlites; m-opx, m-pgn composition of augite mantling orthopyroxene and pigeonite, respectively; sill c.m. sill-chilled margin

Rock type

Sample

Table 1 Microprobe analyses of orthopyroxene and pigeonite of Ferrar tholeiites from southern Prince Albert Mountains. Fe2O3 calculated according to Papike et al. (1974); Fe*=Fe2+ + Fe3+ + Mn. ph/c, ph/r core and rim composition of phenocrysts; mph/c, mph/r core and rim composition of microphenocrysts; micro composition of groundmass microlites; m-opx, m-aug, composition of pigeonite mantling orthopyroxene and augite, respectively; sill c.m. sill-chilled margin. All minerals were analysed with a CAMECA wavelength-dispersive electron microprobe at the University of Padova using operating conditions of 20 kV, 20 nA, and a 2-lm-diameter electron beam

129

99.87 69.37 29.31 1.31 100.00 71.67 27.42 0.91 99.86 64.96 33.52 1.52 101.23 81.60 17.99 0.40 100.72 75.04 23.96 1.01 100.11 82.34 17.06 0.60 100.23 71.73 27.27 1.00 101.29 66.28 32.10 1.62 100.17 74.59 24.60 0.80 100.54 56.86 41.35 1.80 100.52 55.62 42.77 1.61 100.08 82.81 16.61 0.59 101.38 68.05 30.77 1.18 100.77 87.12 12.29 0.59 Sum An (wt%) Ab (wt%) Or (wt%)

102.40 63.18 35.50 1.32

micro 51.71 0.00 29.43 0.00 0.79 0.00 0.23 14.02 3.47 0.22 micro 51.28 0.05 29.95 0.05 0.63 0.00 0.19 14.46 3.24 0.16 micro 53.32 0.05 28.01 0.04 0.80 0.03 0.22 13.15 3.99 0.26 mph/c 49.25 0.02 32.00 0.00 0.89 0.01 0.26 16.60 2.15 0.06 mph/r 50.77 0.02 30.83 0.00 0.55 0.00 0.23 15.29 2.86 0.17 mph/c 48.02 0.00 32.30 0.03 0.60 0.00 0.23 16.81 2.04 0.09 micro 51.18 0.03 29.96 0.04 0.65 0.00 0.22 14.70 3.28 0.17 mph/r 53.35 0.07 29.16 0.04 0.80 0.05 0.11 13.57 3.86 0.28 mph/c 50.86 0.02 30.19 0.06 0.45 0.02 0.29 15.19 2.93 0.15 micro 54.93 0.04 27.68 0.00 0.71 0.04 0.05 11.77 5.01 0.31 ph/r 55.03 0.05 27.82 0.03 0.61 0.02 0.04 11.48 5.17 0.28 ph/c 47.61 0.01 32.48 0.02 0.55 0.00 0.15 17.16 2.02 0.10 ph/r 52.55 0.06 29.59 0.01 0.72 0.00 0.17 14.28 3.79 0.22 ph/c 47.07 0.00 33.49 0.00 0.39 0.04 0.20 17.99 1.49 0.10 SiO2 TiO2 Al2O3 Cr2O3 FeOt MnO MgO CaO Na2O K2 O

micro 54.31 0.10 29.32 0.01 0.90 0.00 0.14 13.09 4.31 0.23

VG1052 Pillow VG1052 Pillow VG1019 Pillow VG1019 Pillow VG1028 Lava VG1028 Lava VG1048 Sill c.m. VG1048 Sill c.m. VG1048 Sill c.m. VG1026 Sill VG1026 Sill VG1026 Sill VG1039 Sill VG1039 Sill VG1039 Sill Sample Rock type

Table 3 Microprobe analyses of plagioclase of Ferrar tholeiites from southern Prince Albert Mountains. ph/c, ph/r core and rim composition of phenocrysts; mph/c, mph/r core and rim composition of microphenocrysts; micro composition of groundmass microlites; sill c.m. sill-chilled margin

130

contaminated by crustal components during ancient subduction processes (Hergt et al. 1989; Brewer et al. 1992; Molzahn et al. 1996; Hergt 2000), or by ``global lithosphere recycling'' (Menzies and Kyle 1990), or (2) basalt magmas crustally contaminated during their uprising to the surface (Faure et al. 1974, 1982; Fleming et al. 1995; Antonini et al. 1999, 2000). The current petrogenetic models, mainly based on trace element and isotopic geochemistry, are (1) 3% addition of subducted sediments (PAAS; post-Archean Australian shale; Taylor and McLennan 1985) to a depleted MORB mantle source. The primary tholeiitic melts would not be appreciably modi®ed by low-pressure crustal contamination during their ascent to the surface (Hergt et al. 1989; Molzahn et al. 1996); (2) assimilation combined with fractional crystallization (AFC) processes, whereby the primary Ferrar magmas, generated by high melting degrees of E-type Dupal MORB mantle, were contaminated by the granulitic rocks of the Victoria Land crystalline basement (Antonini et al. 1999, 2000). In general, little attention has been paid to the signi®cance of the Ferrar major element geochemistry and the occurrence of orthopyroxene in the least evolved Ferrar rocks. Hergt et al. (1991) suggested that the high SiO2 of the Ferrar tholeiites cannot be totally ascribed to fractional crystallization, representing instead a primary feature caused by, in part, melting of the hydrous mantle. Turner and Hawkesworth (1995), revisiting the importance of the major element geochemistry to constrain the mantle source regions of many CFB provinces, concluded that the observed major element composition of many CFB, including Ferrar and Tasmania tholeiites, would result from ``melting of depleted peridotite at volatile-present solidus, under ¯uid absent conditions''. However, a model that accounts for the whole major element geochemistry of Ferrar is not known to the authors of this paper. In this paper we present new geochemical and mineralogical data for Ferrar tholeiites from southern Prince Albert Mountains and evaluate the signi®cance of orthopyroxene as a liquidus phase. It will be shown that the early crystallization of orthopyroxene in the least evolved tholeiites is primarily controlled by high SiO2/CaO, and particularly by high SiO2 content of Ferrar magmas. Focusing on the origin of the SiO2-rich signature of the least evolved Ferrar tholeiites, our approach has been to use experimental melts of natural peridotites as primary magmas for modeling the Ferrar major element geochemistry through fractional crystallization (FC) and assimilation coupled with fractional crystallization (AFC). Anhydrous and hydrous experimental picritic melts produced by fertile peridotite (Hirose and Kushiro 1993; Hirose and Kawamoto 1995) and anhydrous melts produced by a depleted peridotite (Jaques and Green 1980) have been employed in order to take into account the role of hydrous melting and di€erent peridotite compositions on the genesis of Ferrar tholeiites. We will show that, starting from these picritic melts, the AFC process is

131

necessary for ®tting the major element composition of the least evolved Ferrar tholeiite, and that both depleted and fertile peridotite are suitable Ferrar mantle sources.

Geological setting Three metamorphic ``Terrains'', separated by N±S to NW±SE major faults, form the crystalline basement in Victoria Land (Fig. 1). They are, from the Ross Sea towards the East Antarctic craton, the Robertson Bay (RBT), the Bowers (BT), and the Wilson (WT) Terrains (Bradshaw 1987; Lombardo et al. 1991). RBT consists of Cambrian±Ordovician low-grade metagraywackes (Burrett and Findlay 1984). BT consists of low-grade metavolcanics (Weaver et al. 1984) overlain by lowgrade quartzo-feldspathic metasediments of Middle Cambrian to Early Ordovician age. WT is mainly formed by medium-grade schists and gneisses, including a metasedimentary sequence of Late Precambrian± Cambrian age (Casnedi and Pertusati 1991), older polymetamorphic migmatites with remnants of calc-alkaline meta-igneous rocks, ma®c granulites, and highgrade metasedimentary rocks. RBT and BT are intruded by the Devonian granitic plutons of the Admiralty Intrusives suite (400 Ma; Borg et al. 1987), whereas WT is intruded by the Late-Cambrian±Ordovician Granite Harbor Intrusives (450±550 Ma; Borg et al. 1987; Fig. 2 Ca±Mg±Fe* (Fe2+ + Mn + Fe3+) pyroxene compositions (after Poldervaart and Hess 1951, modi®ed) of the investigated rocks from Prince Albert Mts. a Coarse-grained dolerites (samples VG1039, VG1026, LZ57A, and LZ6B); b chilled margin dolerite (triangles:VG1048), lava ¯ow (®lled circles:VG1028) and pillow lavas (half-®lled circles:VG1019, VG1052). The arrow indicates the ``quench trend'' of Muir and Mattey (1982). The compositional pyroxene trends of Dufek intrusion (dashed lines; Himmelberg and Ford 1976) and Thern Promontory and Archambault Ridge (shaded ®elds; Brotzu et al. 1988) are shown for comparison

Armienti et al. 1990). The BT and WT basement is overlain by the Permo-Triassic continental sedimentary sequence of the Beacon Supergroup and by the Middle Jurassic magmatic rocks of the Ferrar Supergroup. The latter consists of sills (Ferrar Dolerite) intruding the Beacon Supergroup, lava ¯ows (Kirkpatrick Basalt), and minor pyroclastic rocks (Mawson Formation and Exposure Hill Formation) separating the Kirkpatrick Basalt from the Beacon sediments (Elliot et al. 1986; Elliot and Larson 1993; Roland and WoÈrner 1996). Late Cenozoic alkaline volcanic rocks, related to the West Antarctic Rift System, outcrop in a linear belt parallel to the Ross Sea coast in Victoria Land. In this work we studied the sills of the Ferrar Dolerite and the lavas of the Kirkpatrick Basalt from the southern sector of Prince Albert Mountains, which extends south of David Glacier. In this area, the nunataks most distal from the coast line are formed from the Kirkpatrick Basalt (WoÈrner 1992), whereas the Ferrar Dolerite mainly outcrops towards the Ross Sea. Samples were collected by G. Orsi during the 1994±1995 Antarctic Italian Expedition. The location of samples is shown in the sketch map of Fig. 1.

Classi®cation and petrography The studied Ferrar tholeiites mostly correspond to basaltic andesites and subordinate andesites according to

132 Table 4 Major (wt%) and trace (ppm) element abundances of representative Ferrar tholeiites from southern Prince Albert Mountains. mg# at. 100*Mg/(Mg+Fe2+), assuming Fe2O3/ FeO=0.15. Analyses were carried out at the Dipartimento di Scienze della Terra, University of Trieste, on pressed powder pellets by Sample Rock type SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 Sum mg# Cr Ni Rb Sr Y Zr Nb Ba La Ce Nd

wavelength dispersive X-ray ¯uorescence with PW1404 XRF spectrometer and the PhilipsÒ (1994) procedure for the correction of matrix e€ects; analytical error is less than 5% and 10% for major and trace elements, respectively

VG1039 VG1025 VG1026 VG1004 VG1006 VG1003 VG1048 VG1015 VG1016 VG1029 VG1028 VG1019 VG1052 Sill Sill Sill Sill Sill Sill c.m. Sill c.m. Sill c.m. Lava Lava Lava Pillow Pillow 53.03 0.44 14.44 8.57 0.17 9.68 11.42 1.60 0.58 0.07

53.75 0.46 16.76 7.93 0.16 7.25 11.14 1.81 0.65 0.08

53.88 0.51 15.46 8.84 0.17 7.68 10.69 1.91 0.77 0.09

55.67 0.81 14.28 9.93 0.17 6.04 8.76 2.79 1.43 0.12

59.66 1.05 14.25 10.33 0.15 2.61 7.05 2.61 2.05 0.23

100.00 99.99 100.00 69.6 64.9 63.7 373 249 258 135 95 102 22 26 29 102 115 108 14 17 21 186 112 109 <10 <10 <10 111 131 165 10 <10 <10 22 21 21 10 10 11

100.00 55.2 90 66 57 174 27 157 10 318 16 36 15

99.99 33.8 14 16 78 168 39 247 12 433 22 51 26

54.11 0.62 15.00 8.85 0.17 7.46 10.81 1.81 1.06 0.10

54.47 0.68 14.51 9.64 0.19 6.97 11.09 1.79 0.56 0.10

54.50 0.67 14.73 9.70 0.19 6.78 11.09 1.84 0.40 0.09

55.97 0.95 14.83 10.73 0.18 4.25 9.62 2.13 1.22 0.12

56.55 1.07 13.65 11.89 0.20 4.30 8.83 2.41 1.00 0.10

99.99 99.99 100.00 100.00 100.00 63.0 60.1 57.6 63.3 59.4 126 146 114 148 119 78 75 68 83 81 49 15 32 24 31 126 133 141 171 131 21 20 23 23 22 121 123 135 126 122 <10 <10 <10 <10 <10 274 174 181 437 163 12 11 14 12 11 23 23 29 27 31 12 15 11 13 12

99.99 58.6 121 71 35 133 24 125 <10 153 11 27 17

100.00 44.5 31 33 50 147 29 191 10 253 20 39 20

100.00 42.2 32 34 60 149 30 196 11 279 19 44 19

the TAS classi®cation of LeBas et al. (1986). Lava ¯ow and pillow samples are ®ne-grained aphyric rocks with variable amount of holo-cryptocrystalline materials and red-brown glass. Textures are hyalo-ophitic, intersertal, sub-ophitic, and intergranular. Lava ¯ow samples are basaltic andesites composed of pigeonite, augite, plagioclase, and minor Fe±Ti oxide microlites disseminated in the matrix. Scarce orthopyroxene microphenocrysts, commonly mantled by pigeonite, are present. Pillow samples have more evolved basaltic andesite composition. Their mineralogical assemblage consists of acicular crystals of pigeonite, augite, plagioclase, and minor Fe±Ti oxide grains set in a red-brown quenched glass. Dolerites have medium- to coarse-grained sub-ophitic texture, ®ne-grained in the chilled margins, and a groundmass made up of cryptocrystalline materials and ®ne-grained quartz±alkali feldspar intergrowths. Dolerites range from basaltic andesites to andesites and are composed of pigeonite, commonly zoned augite and plagioclase, and Fe±Ti oxide microlites disseminated in the interstitial matrix. Dolerite chilled margins and almost all the medium- to coarse-grained dolerites with basaltic andesite composition are characterized by the presence of orthopyroxene microphenocrysts and phenocrysts, respectively, which are commonly mantled by pigeonite and/or augite and lack plagioclase inclusions. In the medium- to coarse-grained dolerites with the least evolved basaltic andesite compositions the modal con-

53.89 0.71 15.25 9.21 0.21 6.84 11.45 1.72 0.60 0.11

54.42 0.70 15.27 9.47 0.19 6.35 11.02 1.69 0.79 0.10

53.80 0.66 14.81 9.05 0.20 7.70 11.18 1.62 0.88 0.10

tent of orthopyroxene phenocrysts is up to 15 vol%. Augite commonly shows a pigeonite core or pigeonite intergrowths, and usually contains plagioclase laths. Dolerites of andesite composition are orthopyroxenefree, coarse-grained rocks, characterized by abundant matrix and Fe±Ti oxides. The textural relationships among pyroxenes suggest that, in the less evolved basaltic andesites, the orthopyroxene is the liquidus phase, followed by pigeonite, and then by augite and plagioclase.

Mineral chemistry The samples for microanalysis were selected on the basis of texture and bulk-rock composition. Representative analyses of pyroxenes and plagioclase are listed in Tables 1, 2, and 3. Pyroxene compositions are plotted in Fig. 2, where the samples of pillow lavas, lava ¯ows, and chilled margin dolerites are distinct from those of the medium- and coarse-grained dolerites. Orthopyroxene (opx) has a rather uniform composition, ranging from Ca4Mg80Fe16 to Ca5Mg77Fe18 and only occurs in the least evolved basaltic andesites (mg# 70±55; see later). These opx compositions, very similar to those reported by Elliot et al. (1995) for the Ferrar rocks of Mesa Range, are higher in Mg than the early crystallized opx from the Dufek Intrusion (Himmelberg and Ford 1976).

133

Fig. 3 Selected major (wt%) and trace (ppm) elements versus mg# (at. 100*Mg/[Mg+Fe2+] assuming Fe2O3/FeO=0.15) for the investigated Prince Albert Mts. rocks. Open triangles coarsegrained dolerites; ®lled triangles chilled margin dolerites; ®lled circles ¯ows (mg# 60) and pillow lavas (mg# 40±50)

Pigeonite (Pgn) has variable composition and is characterized by a marked Fe-enrichment trend, from Ca8Mg75Fe17 to Ca13Mg32Fe55. The Pgn highest in Mg, which have the same Mg/Fe ratios of the coexisting opx (Fig. 2b), are the cores of discrete grains from the least evolved lava ¯ow (VG1028) and chilled margin dolerite (VG1048) samples. Pgn mantling opx and needle crystals in the quenched glass of the pillow lavas usually are highest in Fe (Ca10Mg60Fe30±Ca12Mg52Fe36).

Augite (aug) generally exhibits a wide range of compositions from Ca39Mg51Fe10 to Ca33Mg33Fe34. Aug of the dolerites de®ne a Ca±Mg depletion and Fe-enrichment trend, which ®ts that shown by Brotzu et al. (1988, 1992) for the Ferrar tholeiites from Thern Promontory and Archambault Ridge (Fig. 2a). The Mg-rich compositions correspond to crystal cores from the least evolved basaltic andesites, whereas the Fe-rich compositions represent crystal rims and microlites of the most evolved andesites. By contrast, aug from pillow lavas, lava ¯ows, and chilled margin dolerites (Fig. 2b) have compositions trending to the sub-calcic augite ®eld (Ca29Mg48Fe23± Ca23Mg49Fe28) similar to those reported by Elliot et al. (1995). These compositions follow the ``quench trend'' of

134

29) and are characterized by a mg# gap between 54 and 37, where the pillow lava samples (mg# 47±42) plot. Note that both the dolerites and lava ¯ows with mg# of 70±55 correspond to opx-bearing basaltic andesites. In general, with mg# decreasing, the studied samples show SiO2, TiO2, FeOt, P2O5 and alkali increasing, and CaO and Al2O3 decreasing. Incompatible trace elements exhibit negative correlations with mg#, whereas positive correlations are shown by Cr and Ni. Sr shows a slightly incompatible behavior in the mg# range 70±55 (opxbearing basaltic andesites), whereas it remains virtually constant for mg# less than 47. Similar Sr behavior was observed in the Ferrar tholeiites from the Mesa Range (Elliot et al. 1995).

Signi®cance of orthopyroxene occurrence in the Ferrar tholeiites Fig. 4 FeO/MgO(opx) of opx core compositions versus whole-rock FeO/MgO(w.r) of the investigated chilled margin dolerite (VG1048) and lava ¯ow (VG1028) from Prince Albert Mts. Whole rock Fe2O3/FeO computed by MELTS (Ghiorso and Sack 1995) as a function of composition, liquidus temperature, pressure and fO2 (after Kress and Carmichael 1991). fO2 ˆ QFM 1 log unit bu€er, open symbols: P=1 kbar and dry condition; ®lled symbols: P=5 kbar and 1.0 H2O wt%. FeO of opx calculated according to Papike et al. (1974). The equilibrium KD lines after Hunter (1998) are shown for comparison. Ferrar lava sample 81-2-16 (diamond) from Elliot et al. (1995)

Muir and Mattey (1982), which is consistent with rapid cooling of the host rocks (Smith and Lindsley 1971). Both the Ca-poor and the Ca-rich pyroxenes of Ferrar rocks are characterized by low Al2O3 (and AlVI atoms per formula unit; Brotzu et al. 1988, 1992; Hornig 1993; Elliot et al. 1995 and present study). The Al2O3 content of pyroxenes in coarse-grained dolerites is similar to that in pyroxenes from ®ne-grained chilled margin dolerite and lava samples. Because the latter display quench textures and a quench compositional trend, the Al2O3 of pyroxene likely re¯ects low-pressure crystallization rather than low-pressure re-equilibration. Plagioclase (pl) ranges from An87 to An56 in dolerites, and from An82 to An65 in the lava samples. As previously observed by Elliot et al. (1995) for the pl from the Mesa Range tholeiites, the pl compositional range shown by cores and rims within a single sample approaches that of the whole sample set (e.g., sample VG1026: An83±An56).

Geochemistry Whole rock samples have been analyzed by wavelength dispersive X-ray ¯uorescence. Representative analyses are listed in Table 4. All the investigated samples belong to the low Fe (FeOt<13 wt%) MFCT group of Fleming et al. (1992). The contents of selected major and trace elements versus mg# [100*Mg/(Mg+Fe2+)] are shown in Fig. 3. The dolerites show a wide range of mg# (70±

Petrography and mg# variation diagrams suggest that the evolution from basaltic andesites to the most evolved andesites was dominated by fractional crystallization of pyroxenes and plagioclase. The geochemical behavior of Sr, the mineral crystallization sequences and textural features indicate that the early stages of the di€erentiation were dominated by the fractionation of pyroxenes over that of the plagioclase, whose fractionation became essential in the later di€erentiation stages. As previously noted, the least evolved tholeiites studied here are characterized by early-crystallized opx, which are in equilibrium with their host rocks for a KDopx/melt (FeO/ MgO) ranging from 0.25 to 0.29 (Fig. 4). The presence of early-crystallized opx is a common feature of the least evolved Ferrar rocks (Gunn 1966; Himmelberg and Ford 1976; Elliot et al. 1995), but is absent in the similarly evolved tholeiites of the other Gondwana Mesozoic igneous provinces (Parana and Karoo). MELTS modeling It is known that the crystallization of opx as the liquidus phase, given a magma composition, is favored by high pressure (P) and low values of oxygen fugacity (fO2 ) and H2O. This is predicted (see below) also by the MELTS program (Ghiorso and Sack 1995). We have used the fractional crystallization (FC) modeling of MELTS for evaluating which of these factors may have played a major role on the early crystallization of opx in the Ferrar magmas. MELTS calculations have been carried out at low P (1±5 kbar) under dry and H2O-undersaturated conditions (H2O up to 1 wt%). Oxygen fugacity was assumed to vary according to the QFM-1 log unit curve, as suggested by fO2 values obtained by Brotzu et al. (1988) for Ferrar tholeiites. The role of P and H2O on the early opx crystallization is illustrated in Fig. 5 where the least evolved (mg# 71) Ferrar tholeiite (TPS31, Table 5; Wilhelm and WoÈrner 1996) from the investigated area (Thumb Point

135

Fig. 5 Magma temperature (°C) versus % crystallized mass (c.m.) per 10 °C decrease, showing the relative proportions of fractionating minerals calculated by MELTS program (Ghiorso and Sack 1995) for di€erent pressure (P) and H2O content, starting from TPS31 composition (Thumb Point Sill; Wilhelm and WoÈrner 1996). fO2 ˆ QFM 1 log unit bu€er. aug augite; ol olivine; opx orthopyroxene; pgn pigeonite; pl plagioclase

sill) was used as the starting composition. MELTS modeling shows that pressure increase favors the early crystallization of opx, whereas the increase of H2O promotes that of olivine. It follows that opx crystallization as the liquidus phase requires higher pressure for increasing H2O content. Note that increase of both P and H2O does not favor an early crystallization of pigeonite and delays that of plagioclase. Similar results (not shown) have also been obtained for the Ferrar sillchilled margin from Darwin Glacier (DG49: mg# 68, Table 5; Hergt et al. 1989). It should be noted that for P>1 kbar the crystallization of opx as the liquidus phase is not very sensitive to pressure at near anhydrous (H2O=0±0.5 wt%) conditions. The latter data suggest a primary role of magma composition on early opx crystallization in the least evolved Ferrar tholeiites. The in¯uence of magma composition has been evaluated using the least evolved Ferrar chilled dolerite and

lava (Gunn 1966; Hergt et al. 1989; Elliot et al. 1995; Wilhelm and WoÈrner 1996; present study) as starting compositions. FC MELTS results are consistent with the petrographic features observed in our samples and indicate that Ca-poor pyroxene is the liquidus phase for pressure more than 1 kbar (Fig. 6). Under dry conditions, opx crystallizes as liquidus phase only from Ferrar magmas with MgO>8 wt% (TPS31, DG49), whereas pigeonite crystallizes from liquids with lower MgO (Fig. 6a). The concomitant increase of P and H2O does not promote the early crystallization of pigeonite, suggesting that under H2O-undersaturated conditions opx may also crystallize from Ferrar magmas with MgO <8 wt% (Figs. 6b, c). The early opx crystallization also depends on SiO2/CaO of magmas, i.e., for similar MgO, high SiO2/CaO values favor opx appearance on the liquidus (Figs. 6b, c). The importance of the SiO2±CaO± MgO relationships to obtain opx as the liquidus phase is apparent from Fig. 7, where the MELTS liquid lines of descent obtained by fractional crystallization at P=1.5 kbar and dry conditions are compared, starting from the volatile-free compositions of the least evolved low-Ti tholeiites from Ferrar (TPS31, DG49), Karoo (B69, ON35), and Parana (1032) CFB provinces (Table 5). It is apparent that opx is the liquidus phase

136 Table 5 Recalculated as 100% anhydrous major element compositions of the least evolved tholeiites of the low-Ti Gondwana provinces used in the MELTS-FC modeling of Fig. 7. Data source: Ferrar, TPS31 base of Thumb Point Sill, Wilhelm and WoÈrner (1996); Ferrar, DG49 dolerite chilled margin of Darwin Glacier Sill, Hergt et al. (1989); ParanaÂ, 1032 lava, GuataÂ-Bom Jardin, Piccirillo and Mel® (1988); Karoo, B69 lava, Leribe, Basutoland Basalt, Cox and Hornung (1966); Karoo, ON35 lava, Ma®ka Lisiu, Lesotho Formation, Marsh et al. (1997). mg# as in Table 4 Sample CFB province

TPS31 Ferrar

DG49 Ferrar

SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 mg# SiO2/CaO

53.43 0.48 14.65 8.41 0.16 10.15 10.55 1.52 0.58 0.07 71.0 5.1

53.00 0.50 16.42 8.42 0.17 8.68 10.64 1.64 0.47 0.06 67.6 5.0

1032 Parana 52.05 0.79 15.77 8.26 0.15 8.62 11.79 1.92 0.51 0.12 67.9 4.4

B69 Karoo

ON-35 Karoo

52.46 1.01 14.66 10.02 0.17 8.19 10.50 2.20 0.61 0.17 62.3 5.0

51.11 0.9 15.3 9.95 0.17 8.73 10.99 2.25 0.45 0.15 64.0 4.7

only if the melts have MgO ³8 wt% and SiO2/CaO >4.7, and that high SiO2 content, rather than low CaO of the magmas, is essential for the early opx crystallization (cf. the curves of sample ON35 in the insets of Fig. 7). In summary, the occurrence of early-crystallized opx in the least evolved Ferrar tholeiites is primarily related to their high SiO2 content.

The origin of high SiO2 in the least evolved Ferrar tholeiites Major element modeling of Ferrar tholeiites needs to take into account the results of petrogenetic models based on trace element and isotopic compositions. According to these models the high SiO2 of Ferrar tholeiites could represent (1) a crustal signature caused by AFC processes of primary magmas generated by a E-MORB Dupal-type mantle source (Antonini et al. 1999, 2000); (2) a feature of primary magmas derived from hydrous melting of depleted mantle source contaminated by PAAS-crustal material (Hergt et al. 1989; Molzhan et al. 1996). Because the PAAS only has a limited in¯uence on major element composition of a depleted mantle, hydrous melting has been invoked because FC alone cannot account for the SiO2-rich compositions of Ferrar tholeiites (Hergt et al. 1991). Although these models postulate di€erent mantle sources and di€erentiation processes, both agree in the requirement for high melting degrees (>15%) for the origin of Ferrar magmas. Therefore, it is important to evaluate the extent to which a fertile and depleted peridotite that has undergone high degrees of dry and hydrous partial melting may contribute to SiO2-rich primary magmas, and then estimate the additional e€ects of fractional crystallization and crustal assimilation on the SiO2-rich signature of the least evolved Ferrar tholeiites.

Fig. 6 Whole-rock SiO2/CaO versus MgO wt% of the least evolved Ferrar chilled margin dolerite and lava samples illustrating, for increasing pressure (P) and H2O content, the role of these parameters on the crystallization of opx as liquidus phase by MELTS (Ghiorso and Sack 1995) assuming fO2 ˆ QFM 1 log unit bu€er. Arbitrary dotted lines separate the compositions suitable for opx crystallization (®lled symbols, star and cross) from the others (open symbols). Circles opx-bearing rocks from the present study, numbers refer to the last digits of specimen label VG1000; squares aphyric and hypersthene-bearing (En82) tholeiites from Gunn (1966); diamond Tasmania dolerite 84-111 sample from Hergt et al. (1989); triangles lava samples from Elliot et al. (1995); cross DG49 sample of Darwin Glacier from Hergt et al. (1989); star TPS31 sample of Thumb Point Sill from Wilhelm and WoÈrner (1996)

Peridotite melting experiments Numerous experiments have been carried out to determine the composition of anhydrous peridotite derived melts (Ito and Kennedy 1967; Kushiro 1968; Presnall et al. 1979; Jaques and Green 1980; Stolper 1980;

137

Fig. 7 SiO2/CaO versus MgO (wt%) of liquids evolving by fractional crystallization calculated by MELTS (Ghiorso and Sack 1995). Liquid lines of descent (steps for 10 °C decrease) computed at dry conditions, P=1.5 kbar and fO2 ˆ QFM 1 log unit bu€er. Starting compositions (see Table 5) are the least evolved tholeiites of Ferrar (circles TPS31, Wilhelm and WoÈrner 1996; DG49, Hergt et al. 1989), Parana (squares 1032, Piccirillo and Mel® 1988) and Karoo (triangles B69, Basutoland Basalt, Cox and Hornung 1966; ON35, Lesotho Formation, Marsh et al. 1997). Filled symbols MELTS liquid compositions in equilibrium with orthopyroxene. Numbers in brackets are the starting liquidus temperatures. The insets show that, for similar MgO (wt%), high SiO2 (wt%) rather than low CaO (wt%) promotes early orthopyroxene crystallization. Opx liquidus ®elds (shaded areas) are arbitrarily drawn

Falloon and Green 1987; Kinzler and Grove 1992a, 1992b; Hirose and Kushiro 1993; Baker and Stolper 1994; Walter and Presnall 1994; Baker et al. 1995; Kinzler 1997) whereas experimental data on hydrous peridotite melting remain scarce. The ®rst experimental studies on simpli®ed systems (Kushiro et al. 1968; Kushiro 1969, 1972) and subsequently those on the origin of andesite (Kushiro et al. 1972; Nichols and Ringwood 1972, 1973; Green 1973, 1976; Nichols 1974; Mysen and Boettcher 1975a, 1975b ; Nehru and Wyllie 1975) suggest that partial melting of hydrous peridotite produce SiO2-rich melts. Recently, the experimental study of Gaetani and Grove (1998) has, however, documented that the higher SiO2 content of the hydrous melts, compared with the anhydrous melts, is only apparent when the comparison is made on a volatile-free basis. Hydrous magmas with 3±6 wt% of H2O have SiO248±49 wt% (on anhydrous basis), which is only 1 wt% higher than that of anhydrous melts (Gaetani and Grove 1998). Note that these hydrous melts have SiO2 that is considerably lower (3±4 wt%) than those of the least evolved Ferrar rocks. Experimental melting studies on natural peridotite for systems

with less then 3 wt% H2O in the melts are reported by Hirose and Kawamoto (1995). These hydrous melts, compared with the anhydrous melts produced at the same pressure and from the same peridotite (Hirose and Kushiro 1993), allow us to evaluate the e€ects of H2O on the compositions of melts generated from a natural fertile spinel±lherzolite (KLB-1). The in¯uence of the source peridotite on partial melt compositions may be evaluated by comparing the anhydrous experimental melts obtained from natural fertile (KLB-1) and depleted spinel lherzolites (Tinaquillo peridotite; Jaques and Green 1980). In Fig. 8, the least evolved Ferrar tholeiites (MgO >6 wt%) are compared with the experimental melts produced at 10 and 15 kbar from KLB-1 and Tinaquillo spinel lherzolites believed to be representative of natural fertile and depleted peridotites, respectively. Note that these pressures are in agreement with the pressure range estimated for low-Ti Parana basalts (at the lower end of a 10±22-kbar pressure range for Gramado unit; Garland et al. 1996) and for low Ti±Zr picrites of Karoo (13±15 kbar; Sweeney et al. 1991). Following the results given by previous models on Ferrar trace element composition (Hergt et al. 1989; Molzhan et al. 1996; Antonini et al. 1999, 2000) only melts generated by melting degrees higher than 15% have been considered. In Fig. 8a the hydrous (P=10 kbar; Hirose and Kawamoto 1995) and anhydrous (P=10±15 kbar; Hirose and Kushiro 1993) experimental melts produced from KLB-1 spinel lherzolite are shown while those produced by anhydrous partial melting of Tinaquillo spinel lherzolite (P=10±15 kbar; Jaques and Green 1980) are shown in Fig. 8b. The hydrous melts (Fig. 8a) are broadly similar to the anhydrous ones formed at the same pressure and similar degree of melting, as reported by Hirose and

138

Fig. 8 SiO2 versus MgO (wt%) of the least evolved Ferrar tholeiites (®lled circles present study; ®eld literature data, source as in Fig. 6; stars base and uppermost samples of Thumb Point Sill; Wilhelm and WoÈrner 1996; cross DG49; Hergt et al. 1989) compared with experimental primary melts produced by melting degree (f)³15%. a: Hydrous (®lled squares, P=10 kbar; from Hirose and Kawamoto 1995) and anhydrous melts (open squares, P=10 kbar; gray squares, P=15 kbar; from Hirose and Kushiro 1993) produced from the same fertile spinel lherzolite KLB-1. Numbers refer to literature run # (r) experiments. b Anhydrous melts (open triangles, P=10 kbar; gray triangles, P=15 kbar) produced from depleted spinel lherzolite (Tinaquillo peridotite; Jaques and Green 1980; number in brackets refer to % melting). Anhydrous melts from a are shown for comparison (open and gray squares). The calculated primary Tasmania tholeiite (triangle TAS; Hergt et al. 1989), the calculated parental Ferrar picrite (inverted triangle OG; Ortez and Green, unpublished work, in Sweeney et al. 1991) and the Ferrar olivine±tholeiite (diamond sample 26903, lower contact of Painted Cli€ sill; Gunn 1966) are also reported. The dotted arrows show the e€ect of olivine (mg#=89 from picrite basalt KP111, Lebombo; Bristow 1984) and orthopyroxene (sample VG1048, Table 1) addition (bar=10% increment) to TPS31 composition. The ®eld between the arrows constrains the experimental melts which may have generated the TPS31 composition through orthopyroxene and olivine fractional crystallization

Kawamoto (1995). As expected (Hirose and Kushiro 1993; Gaetani and Grove 1998; Hirschmann et al. 1998), SiO2 of the anhydrous melts decreases with increasing melting pressure and refractory nature of the peridotite source, as well as with the decrease of melting degree (Fig. 8b). It is apparent that the least evolved Ferrar tholeiites do not resemble any of the reported picritic experimental melts in terms of MgO and SiO2. Even if direct evidence of picritic magmas in the Ferrar magmatism is lacking, picritic melts occur in the low-Ti Karoo magmatism that Elliot and Fleming (2000) suggest is generated from the same mantle source in the Weddell Sea region. Moreover, Ortez and Green (unpublished work, in Sweeney et al. 1991) proposed a picrite composition as parental Ferrar magma, which is very close to run #43 of the hydrous experimental melt (Hirose and Kawamoto 1995) reported in Fig. 8. This supports a picritic nature for primary Ferrar tholeiites.

Fractional crystallization Major element mass-balance calculations (Stormer and Nicholls 1978) and fractional crystallization±MELTS (Ghiorso and Sack 1995) modeling were made to test whether the least evolved Ferrar tholeiite from Victoria Land (TPS31; Wilhelm and WoÈrner 1996) could be derived by fractional crystallization (FC) from the experimental primary melts shown in Fig. 8. The following FC modeling results refer to the experimental melts that plot between orthopyroxene and olivine fractional crystallization trends, which constrain the melts suitable to generate the TPS31 composition. Mass-balance calculations yielded high values for the sum of the squares of residuals (Sres2=2.3±4.1) starting from depleted Tinaquillo peridotite-derived melts, and lower values (Sres2=0.8±1.4) starting from fertile KLB1 peridotite-derived melts. The best results have been obtained by the KLB-1-derived hydrous melts that have the highest SiO2, i.e., run #44 (r44; H2O=0.5 wt%) and run #42 (r42; H2O=1.7 wt%). TPS31 composition was matched through 26 and 19% of olivine (ol) + orthopyroxene (opx) fractionation, in the proportion ol=8% opx=18%, and ol=11% opx=8%, starting from r44 (Sres2=1.0) and r42 (Sres2=0.8), respectively. FCMELTS liquid lines of descent (lld) are reported in Fig. 9. The lld that more closely approach the TPS31 composition have been obtained starting from the KLB-1-derived hydrous magmas r44 (2 kbar, Sres2=2.6) and r42 (4 kbar, Sres2=1.0) through 24 and 19% of ol + opx fractionation, in the proportions of ol=9% opx=15%, and ol=11% opx=8%, respectively. Higher values of Sres2 have been obtained for lld relative to both KLB-1and Tinaquillo-derived anhydrous magmas (5.6 and 6.8± 7.6, respectively, P=1±1.5 kbar). Note that, in spite of the lower Sres2 value, the r42 composition is not favored as a primary melt for TPS31 because the di€erentiated liquid matching the TPS31 composition would have 2 wt% of H2O. This relatively high H2O delays the crystallization of plagioclase in the subsequent stages of FC. It follows that the di€erentiated tholeiites (e.g., MgO =6 wt%) at Thumb Point Sill would have Al2O3

139 Fig. 9 FC-MELTS (Ghiorso and Sack 1995) liquid lines of descent versus mg# starting from experimental primary melts produced by a fertile KLB1 (Hirose and Kushiro 1993; Hirose and Kawamoto 1995) and b depleted Tinaquillo spinel lherzolites (Jaques and Green 1980). Step of temperature decrease and fO2 as in Fig. 7. Symbols and labels as in Fig. 8

ass./f.r. Sres

2

28 24 19 20 18 17 17 17 11

(57.05) (58.03) (59.59) (59.60) (60.78) (61.14) (61.46) (62.86) (65.40)

(52.59) (59.25) (61.45) (61.62)

(52.59)

Meta-igneous 28BH14 28BH12 21BD12 21BD7 28BH13 28BH16 21BD8 17AB6 21BD6

Intrusive 1BH13 29BP1 1BH17 1BH16

Intrusive 1BH13

29 29 29 30 31 31 31 31 30

31 31 32 31 1.0 0.8 0.7 0.7 0.6 0.5 0.5 0.5 0.4

0.3 0.3 0.2 0.2

0.5 0.4 0.4 0.3 0.3 0.3

0.2 0.2 0.2

0.2 0.3 0.3 0.3 0.5 0.4 0.4 0.4 0.3

0.5 0.5 0.5 0.6

0.5 0.1 0.1 0.2 0.2 0.2

ass./fr. Sres

18 29 0.6 13 30 0.4 13 30 0.4 From r17 to TPS31

10 10 7 7

Metasedimentary felsic 13B28 (66.61) 13B29 (67.86) 13B27 (73.21) 13B30 (74.08)

31 31 30 31 30 31

opx

% ass. % fr.

17

1

19 26 27 27

28 28 28 28 29

15 14 14 14 12 14 15 11 11

28 28 29

31

21 25 27 29 29 30 30

26 23 19

10

9 12 11 10 9 10 9

opx

% ass. % fr.

13

6 2 2 2

2 2 2 2 <1

1 1 <1

<1

6 4 2 1 1 <1 <1

ol

1.2

0.6 0.5 0.4 0.4

0.5 0.5 0.5 0.5 0.4

0.9 0.8 0.7

0.3

0.3 0.4 0.4 0.3 0.3 0.3 0.3

0.5

0.4 0.1 0.2 0.1

0.4 0.4 0.4 0.4 0.3

0.2 0.3 0.3

0.5

0.1 0.1 0.1 0.1 0.2 0.2 0.2

36 27 26

37 37 37 37 23

39

22 20 15 15

25 22 21 20 20

18 19 19

21 21 21 21 20

18

22 22 23 23

21 21 21 21 21

opx

% ass. % fr.

2

2.0 1.4 1.4

1.8 1.8 1.8 1.8 1.2

2.2

1.0 0.9 0.7 0.7

1.2 1.0 1.0 1.0 1.0

0.7 0.8 0.8

0.6 0.6 0.6 0.6 0.3

0.4

0.8 0.6 0.8 0.9

1.0 0.4 0.3 0.2 0.2

ass./fr. Sres

Opx fractionation

Opx+ol fractionation

Opx fractionation 2

From r43 to TPS31

From r44 to TPS31

17 13 11 10 10 10

(SiO2 wt%)

Metasedimentary 12B26 (56.39) 13B45 (58.31) 13B26 (61.76) 13B14 (64.13) 11G42 (66.56) 31B2 (66.78) 12B25 (67.41)

Contaminant granulites

26 19 19

30 30 30 30 21

34

20

20 17 17 17 18 17

9 10 10

15 15 15 15 17

14

19

6 10 13 15 17 16

opx

% ass. % fr.

7 7 7

4 4 4 4 2

3

1

10 7 5 4 3 3

ol

Opx+ol fractionation

1.6 1.1 1.1

1.6 1.6 1.6 1.6 1.1

2.0

1.0

1.3 1.0 0.9 0.9 0.9 0.9

0.1 0.3 0.2

0.3 0.3 0.3 0.0 0.1

0.3

0.8

0.1 0.2 0.1 0.1 0.1 0.1

ass./fr. Sres2

Table 6 Results of compositional transitions from experimental melts produced by the fertile KLB1 peridotite (hydrous melts r44 and r43, Hirose and Kawamoto 1995; anhydrous melts r17, Hirose and Kushiro 1993) to TPS31 (Wilhelm and WoÈrner 1996) by mass balance calculations (Stormer and Nicholls 1978) giving Sres2 values <1 and assimilated amount <40%. Compositions of fractionated phases as in Fig. 8. Compositions of the assimilated crustal contaminants are the granulites of the Victoria Land basement (Talarico et al. 1995). ass. assimilated mass; fr. fractionated mass

140

(52.59) (59.25) (61.45) (61.62)

(52.59)

Intrusive 1BH13

(57.05) (58.03) (59.59) (59.60) (60.78) (61.14) (61.46) (62.86) (65.40)

(66.61) (67.86) (73.21) (74.08)

(56.39) (58.31) (61.76) (64.13) (66.56) (66.78) (67.41)

Metasedimentary 12B26 13B45 13B26 13B14 11G42 31B2 12B25 Metasedimentary felsic 13B28 13B29 13B27 13B30 Meta-igneous 28BH14 28BH12 21BD12 21BD7 28BH13 28BH16 21BD8 17AB6 21BD6 Intrusive 1BH13 29BP1 1BH17 1BH16 9 13

29

5 8 11 10

opx

% fr.

12 8

±

14 12 10 11

ol

24

24

26

From TQ15-f44 to TPS31

37 34

37

26 26 28 26

% ass.

Opx+ol fractionation

Contaminant granulites (SiO2 wt%) From TQ15-f25 to TPS31

0.5

1.8 1.6

1.3

1.4 1.3 1.3 1.2

ass./fr.

0.5

0.4 0.3

0.8

0.7 0.4 0.2 0.6

Sres2

25 18 18

38 31 36 27 25 27 21 20

18 22 18

14 19 17 17 16 16 15

% ass.

18 19 18

20 22 26 22 22 25 23 25

26 15 38

9 15 19 22 23 24 24

opx

% fr.

19 19 19

17 16 14 18 18 16 17 15

15 10 9

26 22 20 17 17 16 16

ol

Opx+ol fractionation

From TQ15-f34 to TPS31

0.7 0.5 0.5

1.0 0.8 0.9 0.7 0.6 0.7 0.5 0.5

0.4 0.9 0.4

0.4 0.5 0.4 0.4 0.4 0.4 0.4

ass./fr.

0.2 0.4 0.3

0.6 0.6 0.4 0.8 0.7 0.5 0.6 0.5

1.0 0.7 0.9

0.3 0.2 0.2 0.2 0.3 0.3 0.4

Sres2

7 15 20 24 24 26 25

opx

% fr.

18 14 10 7 7 6 7

ol

26

28

17

31 18 10 22 19 10 22 18 10 From TQ10-f40 to TPS31

19 24 21 21 19 20 19

% ass.

Opx+ol fractionation

From TQ10-f29 to TPS31

0.6

1.1 0.8 0.8

0.8 0.8 0.7 0.7 0.6 0.6 0.6

ass./fr.

0.5

0.5 0.9 0.7

0.5 0.4 0.4 0.3 0.8 0.9 1.0

Sres2

Table 7 Results of compositional transitions from experimental melts produced depleted Tinaquillo peridotite (Jaques and Green 1980) to TPS31 (Wilhelm and WoÈrner 1996) by mass balance calculations (Stormer and Nicholls 1978) giving Sres2 values <1 and assimilated amount <40%. Compositions of fractionated phases as in Fig. 8. Compositions of the contaminants as in Table 6. ass. assimilated mass; fr. fractionated mass. TQ15-f25, TQ15-f34, TQ15-f44 Tinaquillo peridotite-derived melts at 15 kbar for 25, 34, and 44% melting degree, respectively. TQ10-f29, TQ10-f40 Tinaquillo peridotite-derived melts at 10 kbar for 29 and 40% melting degree, respectively

141

142 Fig. 10 AFC-MELTS (Ghiorso and Sack 1995) liquid lines of descent versus mg# computed using the meta-igneous granulite 21BD6 (Talarico et al. 1995) as contaminant. Starting magmas are experimental melts produced by fertile KLB1 (r43, r44; P=10 kbar; Hirose and Kawamoto 1995) and depleted Tinaquillo spinel lherzolites (at P=15 kbar for 34% melting degree, TQ; Jaques and Green 1980). Step of temperature decrease and fO2 as in Fig. 7. The assimilation is con®ned to the achievement of TPS31 composition (continuous lines AFC process; dashed lines FC process). The total assimilated mass is: 24 g for r43 (3 g per 10 °C decrease; mean of assimilated mass/fractionated mass=1.3); 12 g for r44 (1 g per 10 °C decrease; mean of assimilated mass/fractionated mass=0.5); 22 g for TQ (1 g per 10 °C decrease; mean of assimilated mass/fractionated mass=0.6). The uppermost diagram on the right shows the comparison between the amounts of fractionated minerals (opx ‹ ol) and assimilated mass (21BD6) required by mass balance calculations (mass% XLFRAC) and those required by MELTS (mass% MELTS). Sres2 refer to the values obtained by AFC±MELTS modeling, while those relative to XLFRAC are listed in Tables 6 and 7. Other symbols as in Fig. 9

as high as 18 wt% instead of 13 wt% (cf. Thumb Point Sill tholeiites, Wilhelm and WoÈrner 1996). Therefore, r42 melt will not be considered in further modeling. Figure 9a shows that the KLB-1-derived melts fail to approach K2O (and partly CaO) values of TPS31. On the other hand, Fig. 9b reveals that Tinaquillo-derived melts do not match TPS31 composition not only for K2O but also for CaO, TiO2, and Al2O3. This suggests that additional K-rich components have to be considered in the genesis of the Ferrar tholeiites. According to Hergt et al. (1989), the addition of 3% PAAS crustal

component to a depleted mantle source that has undergone hydrous melting would account for the origin of the major element signature of Ferrar tholeiites. Unfortunately, melting experiments on a mantle source such as that predicted by Hergt et al. (1989) are lacking. Nevertheless, the following considerations can be made: (1) the major contributions to the Sres2 values relative to Tinaquillo-derived melts are due to CaO and Al2O3 (MELTS res2=1.7±3.3 and 1.4±2.6, respectively). These results would not be improved by the addition of a PAAS component to a depleted source, because such a

143

component has a limited in¯uence on the major element composition of the mantle source (Hergt et al. 1991); (2) if the addition of a small amount of H2O to the dry Tinaquillo peridotite had e€ects comparable to those reported for the experimental melting of KLB-1-fertile peridotite (0.1±0.5 wt%; Hirose and Kawamoto 1995), the hydrous melts would be expected to be close in composition to the anhydrous melts for the same degree of melting, and, therefore, the results of the above FC modeling would not change signi®cantly; (3) at constant pressure, the increase of H2O would produce, at constant MgO, melts with higher CaO and Al2O3, and lower FeO (Gaetani and Grove 1998). The above FC-modeling results might improve for Al2O3, but the contrary would occur for CaO. Again, the results of the above FC modeling would not change signi®cantly. Therefore, we would anticipate that, even assuming hydrous partial melting of a depleted mantle source contaminated by PAAS, mass-balance and FC±MELTS results would not be substantially modi®ed. Crustal assimilation The possibility that the major elements of the least evolved Ferrar tholeiites could be the result of crustal assimilation during fractional crystallization of primary basalts was tested by mass-balance calculations. We modeled the transitions from KLB-1- and Tinaquilloderived melts to the TPS31 composition, assuming the granulites of Victoria Land basement (Talarico et al. 1995) to be the contaminants, as proposed by Antonini et al. (1999) to explain the trace element content of the Ferrar rocks. Mass-balance results yielding Sres2 values <1 and assimilated amounts <40% are listed in Tables 6 and 7. As regards the KLB-1-derived melts, the transitions from hydrous magmas r43 and r44 to TPS31 are compatible with the assimilation of almost all the di€erent types of granulites found in Victoria Land (SiO2=52± 74 wt%; Talarico et al. 1995). In comparison, only one transition is possible from anhydrous melts (i.e., r17) to TPS31 through the assimilation of the more ma®c granulite (SiO2=52.6 wt%). Also the transitions from Tinaquillo-derived melts appear to be compatible with the assimilation of many granulites, particularly for the melt generated by 34% partial melting at 15 kbar (TQ15-f34). Note that the transitions from Tinaquilloderived melts require olivine fractionation in addition to orthopyroxene, whereas those from KLB-1-derived hydrous melts are also possible with orthopyroxene fractionation only. The results of Tables 6 and 7 show that relatively low amounts of contaminant (mean=14‹5%; mean of assimilated/fractionated mass: ass/fr=0.5‹0.2) are involved in the transition from r44, whereas higher amounts are required for the transition from r43 (25‹8; mean ass/fr=1.3‹0.4) and from TQ15-f34 (mean=22‹7; mean ass/fr=0.6‹0.2). In any case, the

calculated amounts of assimilated mass, as well as assimilated/fractionated mass are, in general, negatively correlated with SiO2 from the contaminants. The crustal contamination process has been further tested by AFC±MELTS modeling, starting from melts that yielded the most mass-balance solutions (r44, r43, and TQ15-f34) and using, as contaminant, the metaigneous granulite 21BD6 (SiO2=65.4 wt%), already employed by Antonini et al. (1999) for the AFC modeling of the Ferrar trace element patterns. MELTS modeling indicates that the TPS31 composition can be matched (Sres2<1) at pressures of 5 and 2 kbar, starting from KLB-1 and Tinaquillo-derived melts, respectively, through amounts of fractionated minerals and assimilated mass similar to those obtained by mass balance calculations (Fig. 10). The high value of assimilated/fractionated mass (mean 1.3) required for the r43 melt is consistent with the thermal limits and extent of crustal contamination modeled by Reiners et al. (1995). In summary, the AFC model explains quite well the major element composition of the least evolved Ferrar tholeiite starting from experimental melts derived by high melting degrees of both fertile and depleted peridotite at pressures of 10±15 kbar. The fact that the AFC model is compatible with many types of contaminants makes it a plausible process because it is unlikely that only a speci®c contaminant may be responsible for the crustal signature shown by Ferrar magmas whose emplacement occurred over thousand of kilometers.

Concluding remarks 1. The least evolved investigated Ferrar Dolerite and Kirkpatrick Basalt from the southern Prince Albert Mountains are characterized by the occurrence of early-crystallized orthopyroxene. This feature petrographically distinguishes these tholeiites from those of Parana and Karoo. Textural relationships indicate that opx was the liquidus phase, followed by pigeonite, and then by augite and plagioclase in the crystallization sequence. The mineral chemistry indicates that opx is in compositional equilibrium with the host rock. 2. MELTS modeling tested on the least evolved Ferrar tholeiites gives opx as the liquidus phase. Comparison with the analogs from Parana and Karoo suggests that the primary control on opx crystallization is played by the Ferrar magma composition, and in particular by its high SiO2. 3. Mass-balance calculations and MELTS modeling indicate that the SiO2-rich signature associated with early opx crystallization, and, in general, the major element geochemistry of the least evolved Ferrar tholeiite, can be reproduced by AFC process. Experimental picritic melts derived at 10±15 kbar by high melting degrees (>25%) of natural fertile (KLB-1) and depleted (Tinaquillo) peridotites, under

144

hydrous and dry conditions, respectively, have the appropriate compositions to be the Ferrar primary magmas, assuming the Victoria Land granulites to be the crustal contaminants. Therefore, the Ferrar tholeiites do not necessarily re¯ect the generally assumed depleted source mantle as they are also compatible with a fertile mantle. Acknowledgements We are grateful to A. Cundari, R. Petrini and R. Varne for their useful comments. The authors greatly appreciated the suggestions and the constructive criticism of G. Moore and another anonymous reviewer. Thanks are due to L. Furlan (Trieste) and R. Carampin (Padova) for their technical and analytical collaboration. This work has been carried out with the ®nancial support of the Italian ``Programma Nazionale di Ricerche in Antartide'' and MURST.

References Antonini P, Piccirillo EM, Petrini R, Civetta L, D'Antonio M, Orsi G (1999) Enriched mantle ± Dupal signature in the genesis of the Jurassic Ferrar tholeiites from Prince Albert Mountains (Victoria Land, Antarctica). Contrib Mineral Petrol 136:1±19 Antonini P, Piccirillo EM, Petrini R, Civetta L, D'Antonio M, Orsi G (2000) Reply to the comment by J. Hergt on the paper ``Enriched mantle ± Dupal signature in the genesis of the Jurassic Ferrar tholeiites from Prince Albert Mountains (Victoria Land, Antarctica)'' by Antonini et al. (Contributions to Mineralogy and Petrology 136:1±19, 1999). Contrib Mineral Petrol 139:245±249 Armienti P, Ghezzo C, Innocenti F, Manetti P, Rocchi S, Tonarini S (1990) Isotope geochemistry and petrology of granitoid suites from Granite Harbour Intrusive of Wilson Terrane, North Victoria Land, Antarctica. Eur J Mineral 2:103±123 Baker MB, Stolper EM (1994) Determining the composition of high-pressure mantle melts using diamond aggregates. Geochim Cosmochim Acta 58:2811±2827 Baker MB, Hirschmann MM, Ghiorso MS, Stolper EM (1995) Compositions of near-solidus peridotite melts from experiments and thermodynamic calculations. Nature 375:308±311 Borg SG, Stump E, Chappell BW, McCulloch MT, Wyborn D, Armstrong RL, Holloway JR (1987) Granitoids of northern Victoria Land: implications of chemical and isotopic variations to regional structure and tectonics. Am J Sci 287:127±169 Bradshaw JD (1987) Terrane boundaries in North Victoria Land. Mem Soc Geol It 33:9±15 Brewer TS, Hergt JM, Hawkesworth CJ, Rex D, Storey BC (1992) Coats Land dolerites and the generation of Antarctic continental ¯ood basalts. In: Storey BC, Alabaster T, Pankhurst RJ (eds) Magmatism and the causes of continental break-up. Geol Soc Lond Spec Publ 68:185±208 Bristow JW (1984) Picritic rocks in the north Lebombo and south-east Zimbabwe. In: Erlank AJ (ed) Petrogenesis of the volcanic rocks of the Karoo Province. Geol Soc S Afr Spec Publ 13:105±123 Brotzu P, Capaldi G, Civetta L, Melluso L, Orsi G (1988) Jurassic Ferrar Dolerites and Kirkpatrick Basalts in northern Victoria Land (Antarctica): stratigraphy, geochronology and petrology. Mem Soc Geol It 43:97±116 Brotzu P, Capaldi G, Civetta L, Orsi G, Gallo G, Melluso L (1992) Geochronology and geochemistry of Ferrar rocks from North Victoria Land, Antarctica. Eur J Mineral 4:605±617 Burrett CF, Findlay RH (1984) Cambrian and Ordovician conodonts from the Robertson Bay Group, Antarctica, and their tectonic signi®cance. Nature 307:723±725 Casnedi R, Pertusati PC (1991) Geology of the Priestly Formation (northern Victoria Land, Antarctica) and its signi®cance in the context of early Paleozoic continental reconstruction. Mem Soc Geol It 46:145±156

Cox KG (1988) The Karoo Province. In: Macdougall JD (ed) Continental ¯ood basalts. Kluwer, Boston, pp 239±272 Cox KG, Hornung G (1966) The petrology of the Karroo basalts of Basutoland. Am Mineral 51:1414±1432 Duncan RA, Hooper PR, Rehacek J, Marsh JS, Duncan AR (1997) The timing and duration of the Karoo igneous event, southern Gondwana. J Geophys Res 102:18127±18138 Elliot DH, Fleming TH (2000) Weddell triple junction: the principal focus of Ferrar and Karoo magmatism during initial breakup of Gondwana. Geology 28:539±542 Elliot DH, Larsen D (1993) Mesozoic volcanism in the Transantarctic Mountains depositional environment and tectonic setting, In: Findlay RH, Unrug R, Banks HR, Veevers JJ (eds) Gondwana eight: assembly, evolution and dispersal. AA Balkema, Rotterdam, pp 397±410 Elliot DH, Siders MA, Haban MA (1986) Jurassic tholeiites in the region of the upper Rennick Glacier, north Victoria Land. In: Stump E (ed) Geological investigations in northern Victoria Land, Antarctica. Am Geophys Union, Washington, DC, Antarct Res Ser 46:249±265 Elliot DH, Fleming TH, Haban MA, Siders MA (1995) Petrology and mineralogy of the Kirkpatrik Basalt and Ferrar Dolerite, Mesa Range Region, North Victoria Land, Antarctica. In: Elliot DH, Blaisdell GL (eds). Contributions to Antarctic research IV. Am Geophys Union, Washington, DC, Antarct Res Ser 67:103±141 Elliot DH, Fleming TH, Kyle PR, Foland KA (1999) Long-distance transport of magmas in the Jurassic Ferrar Large Igneous Province, Antarctica. Earth Planet Sci Lett 167:89±104 EncarnacioÂn J, Fleming TH, Elliot DH, Eales HV (1996) Synchronous emplacement of Ferrar and Karoo dolerites and the early breakup of Gondwana. Geology 24:535±538 Falloon TJ, Green DH (1987) Anhydrous partial melting of MORB pyrolite and other peridotite compositions at 10 kbar: implications for the origin of primitive MORB glasses. Mineral Petrol 37:181±219 Faure G, Bowman JR, Elliot DH, Jones LM (1974) Strontium isotope composition and petrogenesis of the Kirkpatrick Basalt, Queen Alexandra Range, Antarctica. Contrib Mineral Petrol 48:153±169 Faure G, Pace KK, Elliot DH (1982) Systematic variations of 87 Sr/86Sr ratios and major element concentrations in the Kirkpatrick Basalt of Mount Falla, Queen Alexandra Range, Transantarctic Mountains. In: Craddock C (ed) Antarctic geosciences, University of Wisconsin Press, Madison, pp 715±723 Fleming TH, Elliot DH, Jones LM, Bowman JR (1992) Chemical and isotopic variations in an iron-rich lava ¯ow from the Kirkpatrick Basalt, north Victoria Land. Contrib Mineral Petrol 111:440±457 Fleming TH, Foland KA, Elliot DH (1995) Isotopic and chemical constraints on the crustal evolution and source signature of Ferrar magmas, north Victoria Land, Antarctica. Contrib Mineral Petrol 121:217±236 Fleming TH, Heimann A, Foland KA, Elliot DH (1997) 40Ar/39Ar geochronology of Ferrar Dolerite sills from the Transantarctic Mountains, Antarctica: implications for the age and origin of the Ferrar magmatic province, Geol Soc Am Bull 109:533±546 Gaetani GA, Grove TL (1998) The in¯uence of water on melting of mantle peridotite. Contrib Mineral Petrol 131:323±346 Garland F, Turner S, Hawkesworth C (1996) Shifts in the source of the Parana basalts through time. Lithos 37:223±243 Ghiorso MS, Sack RO (1995) Chemical mass transfer in magmatic processes. IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid± solid equilibria in magmatic systems at elevated temperatures and pressures. Contrib Mineral Petrol 119:197±212 Green DH (1973) Experimental melting studies on a model upper mantle composition at high pressure under water-saturated and water-undersaturated conditions. Earth Planet Sci Lett 19:37±53 Green DH (1976) Experimental testing of ``equilibrium'' partial melting of peridotite under water-saturated, high-pressure conditions. Can Mineral 14:255±268

145 Gunn BM (1966) Modal and element variation in Antarctic tholeiites. Geochim Cosmochim Acta 30:881±920 Heimann A, Fleming TH, Elliot DH, Foland KA (1994) A short interval of Jurassic continental ¯ood basalt volcanism in Antarctica as demonstrated by 40Ar/39Ar geochronology. Earth Planet Sci Lett 121:19±41 Hergt JM (2000) Comment on the paper ``Enriched mantle ± Dupal signature in the genesis of the Jurassic Ferrar tholeiites from Prince Albert Mountains (Victoria Land, Antarctica)'' by Antonini et al. (Contributions to Mineralogy and Petrology 136:1± 19, 1999). Contrib Mineral Petrol 139:240±244 Hergt JM, Chappel BW, McCulloch MT, McDougall I, Chivas AR (1989) Geochemical and isotopic constraints on the origin of the Jurassic dolerites of Tasmania. J Petrol 30:841±883 Hergt JM, Peate DW, Hawkesworth CJ (1991) The petrogenesis of Mesozoic Gondwana low-Ti ¯ood basalts. Earth Planet Sci Lett 105:134±148 Himmelberg GR, Ford AB (1976) Pyroxenes of Dufek Intrusion, Antarctica. J Petrol 17:219±243 Hirose K, Kawamoto T (1995) Hydrous partial melting of lherzolite at 1 GPa: the e€ect of H2O on the genesis of basaltic magmas. Earth Planet Sci Lett 133:463±473 Hirose K, Kushiro I (1993) Partial melting of dry peridotites at high pressures: determination of compositions of melts segregated from peridotite using aggregates of diamond. Earth Planet Sci Lett 114:477±489 Hirschmann MM, Ghiorso MS, Wasylenki LE, Asimow PD, Stolper EM (1998) Calculation of peridotite partial melting from thermodynamic model of mineral and melts. I. Review of methods and comparison with experiments. J Petrol 39:1091± 1115 Hornig I (1993) High-Ti and low-Ti tholeiites in the Jurassic Ferrar Group, Antarctica. Geol Jahrb Reihe E47:335±369 Hunter AG (1998) Intracrustal controls on the coexistence of tholeiitic and calc-alkaline magma series at Aso Volcano, SW Japan. J Petrol 39:1255±1284 Ito K, Kennedy GC (1967) Melting and phase relations in a natural peridotite up to 40 kilobars. Am J Sci 265:519±538 Jaques AL, Green DH (1980) Anhydrous melting of peridotite at 0±15 kbar pressure and the genesis of tholeiitic basalts. Contrib Mineral Petrol 73:287±310 Kinzler RJ (1997) Melting of mantle peridotite at pressures approaching the spinel to garnet transitions: application to midocean ridge basalt petrogenesis. J Geophys Res 102:853±874 Kinzler RJ, Grove TL (1992a) Primary magmas of mid-ocean ridge basalts. 1. Experiments and methods. J Geophys Res 97:6885± 6906 Kinzler RJ, Grove TL (1992b) Primary magmas of mid-ocean ridge basalts. 2. Applications. J Geophys Res 97:6907±6926 Kress VC, Carmichael ISE (1991) The compressibility of silicate liquids containing Fe2O3 and the e€ect of composition, temperature, oxygen fugacity and pressure on their redox states. Contrib Mineral Petrol 108:82±92 Kushiro I (1968) Compositions of magmas formed by partial melting of the Earth's upper mantle. J Geophys Res 73:619±634 Kushiro I (1969) The system forsterite±diopside±silica with and without water at high pressures. Am J Sci 267A:269±294 Kushiro I (1972) E€ect of water on the composition of magmas formed at high pressures. J Petrol 13:311±334 Kushiro I, Yoder HS Jr, Nishikawa M (1968) E€ect of water on the melting of enstatite. Geol Soc Am Bull 79:1685±1692 Kushiro I, Shimizu N, Nakamura Y, Akimoto S (1972) Compositions of coexisting liquid and solid phases formed upon melting natural garnet and spinel lherzolites at high pressures: a preliminary report. Earth Planet Sci Lett 14:19±25 Le Bas MJ, Le Maitre RW, Streckeisen A, Zanettin B (1986) Chemical classi®cation of volcanic rocks based on the total alkali±silica diagram. J Petrol 27:745±750 Lombardo B, Pertusati PC, Ricci CA (1991) Some geological and petrological problems in the Wilson Terrane of Borchgrevink Coast (Northern Victoria Land, Antarctica). Mem Soc Geol It 46:135±143

Macdougall JD (ed) (1988) Continental ¯ood basalts. Kluwer, Dordrecht Mahoney JJ, Con MF (eds) (1997) Large igneous provinces. Am Geophys Union, Geophysical Monograph 100, Washington, DC Marsh JS, Hooper PR, Rehacek J, Duncan RA, Duncan AR (1997) Stratigraphy and age of Karoo basalts of Lesotho and implications for correlations within the Karoo igneous province. In Mahoney JJ, Con MF (eds) Large igneous provinces. Am Geophys Union, Geophysical Monograph 100 Washington, DC, pp 247±272 Menzies MA, Kyle PR (1990) Continental volcanism: a crust± mantle probe. In: Menzies MA (ed) Continental mantle. Clarendon Press, Oxford, pp 157±177 Molzahn M, Reisberg L, WoÈrner G (1996) Os, Sr, Nd, Pb, O isotope and trace element data from the Ferrar ¯ood basalts, Antarctica: evidence for an enriched subcontinental lithospheric source. Earth Planet Sci Lett 144:529±546 Mortimer N, Parkinson D, Raine JI, Adams CJ, Graham IJ, Oliver PJ, Palmer K (1995) Ferrar magmatic province rocks discovered in New Zealand: implications for Mesozoic Gondwana geology. Geology 23:185±188 Mysen BO, Boettcher AL (1975a) Melting of hydrous upper mantle: phase equilibria of a natural peridotite at high pressures and high temperatures as a function of controlled activities of water, hydrogen and carbon dioxide. J Petrol 16:520±548 Mysen BO, Boettcher AL (1975b) Melting of a hydrous upper mantle. II. Geochemistry of crystals and liquid formed by anatexis of mantle peridotite at high pressures and high temperatures as a function of water, hydrogen and carbon dioxide. J Petrol 16:549±593 Muir ID, Mattey DA (1982) Pyroxene fractionation in ferrobasalts from the Galapagos spreading centre. Mineral Mag 45:193±200 Nehru CE, Wyllie PJ (1975) Compositions of glasses from St. Paul's peridotite partially melted at 20 kilobars. J Geol 83:455±471 Nichols IA (1974) Liquids in equilibrium with peridotite mineral assemblages at high water pressures. Contrib Mineral Petrol 45:289±316 Nichols IA, Ringwood AE (1972) Production of silica-saturated tholeiitic magmas in island arcs. Earth Planet Sci Lett 17:243± 246 Nichols IA, Ringwood AE (1973) E€ect of water on olivine stability in tholeiites and the production of silica-saturated magmas in the island-arc environment. J Geol 81:285±300 Papike JJ, Cameron K, Baldwin K (1974) Amphiboles and pyroxenes: characterization of other than quadrilateral components and estimate of ferric iron from microprobe data. Bull Geol Soc Am 6:1053±1054 Philips (1994) X40 Software for XRF analysis, Software Operation Manual Piccirillo EM, Mel® AJ (eds) (1988) The Mesozoic ¯ood volcanism of the Parana Basin (Brazil). Instituto AstronoÃmico e Geofõ sico Publishers, SaÄo Paulo, Brazil Poldervaart A, Hess HH (1951) Pyroxenes in the crystallization of basaltic magmas. J Geol 59:472±489 Presnall DC, Dixon TH, O'Donell TH, Dixon SA (1979) Generation of mid-ocean ridge tholeiites. J Petrol 20:3±35 Reiners PW, Nelson BK, Ghiorso MS (1995) Assimilation of felsic crust by basaltic magma: thermal limits and extents of crustal contamination of mantle-derived magmas. Geology 23:563±566 Renne PR, Ernesto M, Pacca IG, Coe RS, Glen JM, Prevot M, Perrin M (1992) The age of Parana ¯ood volcanism, rifting Gondwanaland, and the Jurassic±Cretaceous boundary. Science 258:975±979 Roland NW, WoÈrner G (1996) Kirkpatrick Flows and associated pyroclastics: new occurrences, de®nition and aspects of a Jurassic Transantarctic Rift. Geol Jahrb B89:97±121 Smith D, Lindsley DH (1971) Stable and metastable augite crystallization trends in a single basalt ¯ow. Am Mineral 56:225± 233 Stewart K, Turner S, Kelley S, Hawkesworth C, Kirstein L, Mantovani M (1996) 3-D 40Ar/39Ar geochronology in the Pa-

146 rana continental ¯ood basalt province. Earth Planet Sci Lett 143:95±109 Stolper EM (1980) A phase diagram for mid-ocean ridge basalts: preliminary results and implications for petrogenesis. Contrib Mineral Petrol 74:13±27 Storey BC, Alabaster T, Pankhurst RJ (eds) (1992) Magmatism and the causes of continental break-up. Geol Soc Lond Spec Publ 68 Stormer JC, Nicholls J (1978) XLFRAC: a program for the interactive testing of magmatic di€erentiation model. Computer Geosci 4:143±159 Sweeney RJ, Falloon TJ, Geen DH, Tatsumi Y (1991) The mantle origins of Karoo picrites. Earth Planet Sci Lett 107:256±271 Talarico F, Borsi L, Lombardo B (1995) Relict granulites in the Ross Orogen of Northern Victoria Land (Antarctica). II ± geochemistry and paleo-tectonic implications. Precambrian Res 75:157±174 Taylor SR, McLennan SM (1985) The continental crust: its composition and evolution. Hallam A (ed) Blackwell Scienti®c Publications, Oxford Turner S, Hawkesworth C (1995) The nature of the sub-continental mantle: constraints from the major-element composition of continental ¯ood basalts. Chem Geol 120:295±314

Turner S, Regelous M, Kelley S, Hawkesworth CJ, Mantovani S (1994) Magmatism and continental break-up in the South Atlantic: high precision 40Ar/39Ar geochronology. Earth Planet Sci Lett 121:333±348 Walter MJ, Presnall DC (1994) Melting behavior of simpli®ed lherzolite in the system CaO±MgO±Al2O3±SiO2±Na2O from 7 to 35 kbar. J Petrol 35:329±359 Weaver SD, Bradshaw JD, Laird MG (1984) Geochemistry of Cambrian volcanics of the Bowers Supergroup and implications for the Early Paleozoic tectonic evolution of the northern Victoria land, Antarctica. Earth Planet Sci Lett 69:129± 140 Wilhelm S, WoÈrner G (1996) Crystal size distribution in Jurassic Ferrar ¯ows and sills (Victoria Land, Antarctica): evidence for processes of cooling, nucleation, and crystallization. Contrib Mineral Petrol 125:1±15 WoÈrner G (1992) Kirkpatrick lavas, exposure hill formation and Ferrar sills in the Prince Albert Mountains, Victoria land, Antarctica. PolarfoÈrschung 60(2):87±90