Accretion of juvenile crust at the Early Palaeozoic Antarctic margin of Gondwana: geochemical and...
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Accretion of juvenile crust at the Early Palaeozoic Antarctic margin of Gondwana: geochemical and geochronological evidence from granulite xenoliths M. Gemelli, 1 S. Rocchi, 1 G. Di Vincenzo 2 and M. Petrelli 3 1 Dipartimento di Scienze della Terra, Universita ` di Pisa, Via S.Maria 53, 56126 Pisa, Italy; 2 Istituto di Geoscienze e Georisorse, CNR, Via Moruzzi 1, 56127 Pisa, Italy; 3 Dipartimento di Scienze della Terra, Universita ` di Perugia, P.zza Universita `, 06123 Perugia, Italy Introduction For ancient active margins, petrolog- ical evidence on the nature of the intermediate-lower crust can be gath- ered from either granulite terrains or deep-seated xenoliths in volcanic rocks (e.g. Downes and Leyreloup, 1986; Al-Mishvat and Nasir, 2004). The temporal evolution of the deep crust is mostly constrained by geo- chronological data on xenolith whole rocks, as ion- or laser-probe mineral data are still scarce (e.g. Boger et al., 2006). A key to the geodynamic interpre- tation of the Early Palaeozoic Antarc- tic margin in Victoria Land (Fig. 1a) is the nature and age of the deep crust of the Robertson Bay terrane, the marginÕs outermost lithotectonic (Weaver et al., 1984). A number of hypotheses have been formulated on the kinematics and dynamics of jux- taposition ⁄ accretion of fault-bounded lithotectonic units (Stump, 1995) and includes a variable number of sub- ducting plates with varied dip direc- tion (Weaver et al., 1984; Kleinschmidt and Tessensohn, 1987). In these models, a key role is invari- ably played by the Robertson Bay terrane, which is covered with thick turbidite sequences that hide the underlying crust. This terrane has been considered as being either (i) an exotic continental block, based on petrological investigations on Devo- nian granites (Borg and DePaolo, 1991), or (ii) of an oceanic outboard nature, based on aeromagnetic studies (Finn et al., 1999), or as (iii) an ocean ridge-arc-backarc, based on prelimin- ary petrological investigations on crustal xenoliths from a single locality (Berg and Wu, 1992). To collect direct evidence on the nature and age of the deep crust in this convergent setting, we integrated whole-rock geochemical data with zircon spot U–Pb geochro- nological investigations on granulite xenoliths. Geological setting The studied xenoliths are found in the products of the Cenozoic plutonic– volcanic alkaline province of the West Antarctic rift (Le Masurier and Thomson, 1990; Rocchi et al., 2002). Cretaceous–Cenozoic rift activity led to the uplift of the Transantarctic Mountains (Fitzgerald and Stump, 1997) (Fig. 1a), which represent the roots of the Palaeozoic Ross Orogen. This Orogen and its northward con- tinuation in Australia (Delamerian Orogen) are linked to the Early Palaeozoic convergence between the palaeo-Pacific oceanic plate and the Antarctic-Australian Gondwana mar- gin (Stump, 1995). In Victoria Land, convergence resulted in the colli- sional–accretional assembly of three fault-bounded lithotectonic units, the Wilson, Bowers and Robertson Bay terranes (Fig. 1b), whose nature is still a matter of debate (Weaver et al., 1984; Borg et al., 1987; Kleinschmidt and Tessensohn, 1987; Rocchi et al., 1998; Finn et al., 1999; Goodge, 2002; Tessensohn and Henjes-Kunst, 2005; Federico et al., 2006). The Wilson terrane consists of both polydeformed metamorphic rocks (Kleinschmidt and Tessensohn, 1987) and granulite remnants (Talarico et al., 1995) intruded by calcalkaline tonalites, potassic granites and bi- modal peraluminous-lamprophyric dykes between c. 530 and 490– 480 Ma (Borg et al., 1987; Di Vince- nzo and Rocchi, 1999; Goodge, 2002; Rocchi et al., 2004; Bomparola et al., 2007). Amphibolite- to greenschist- facies regional metamorphism in the Lanterman Range (Fig. 1b) occurred at c. 490–480 Ma, slightly after the c. 500 Ma eclogite metamorphism (Goodge and Dallmeyer, 1996; Di Vincenzo et al., 1997; Di Vincenzo and Palmeri, 2001). The Bowers terrane consists of Middle–Late Cambrian magmatic arc volcanic–vol- caniclastic rocks (Weaver et al., 1984; Stump, 1995) showing metamorphic overprint of variable intensity (Cri- spini et al., 2007). ABSTRACT Geodynamic models for the Antarctic sector of the active Early Palaeozoic Palaeo-Pacific margin of Gondwana are based on the nature and age of the deep crust of the Robertson Bay terrane, the outermost lithotectonic unit of the margin. As this crustal block is covered with thick turbidite deposits, the only way to probe the deep crust is through the analysis of granulite xenoliths from Cenozoic scoria cones. Low-K felsic xenoliths yield the oldest (Middle Cambrian) laser-probe U–Pb ages on zircon areas with igneous growth zoning. This finding, along with the positive whole-rock e Nd(500Ma) , suggests that these felsic rocks derived from a juvenile magma formed during the Early Palaeozoic Ross orogenic cycle. Mafic xenoliths have geochemical-isotopic compositions similar to those of modern primitive island arcs, suggesting the involvement of subducted oceanic crust in their magma genesis and accretion of juvenile crust at the Antarctic margin of Gondwana. Terra Nova, 21, 151–161, 2009 Correspondence: Maurizio Gemelli, Dipar- timento di Scienze della Terra, Universita` di Pisa, Via S.Maria 53, 56126 Pisa, Italy. Tel.: +390502215796; fax: +3905022158 00; e-mail: [email protected] Ó 2009 Blackwell Publishing Ltd 151 doi: 10.1111/j.1365-3121.2009.00868.x
Transcript of Accretion of juvenile crust at the Early Palaeozoic Antarctic margin of Gondwana: geochemical and...
Accretion of juvenile crust at the Early Palaeozoic Antarcticmargin of Gondwana: geochemical and geochronologicalevidence from granulite xenoliths
M. Gemelli,1 S. Rocchi,1 G. Di Vincenzo2 and M. Petrelli31Dipartimento di Scienze della Terra, Universita di Pisa, Via S.Maria 53, 56126 Pisa, Italy; 2Istituto di Geoscienze e Georisorse, CNR, Via
Moruzzi 1, 56127 Pisa, Italy; 3Dipartimento di Scienze della Terra, Universita di Perugia, P.zza Universita, 06123 Perugia, Italy
Introduction
For ancient active margins, petrolog-ical evidence on the nature of theintermediate-lower crust can be gath-ered from either granulite terrains ordeep-seated xenoliths in volcanicrocks (e.g. Downes and Leyreloup,1986; Al-Mishvat and Nasir, 2004).The temporal evolution of the deepcrust is mostly constrained by geo-chronological data on xenolith wholerocks, as ion- or laser-probe mineraldata are still scarce (e.g. Boger et al.,2006).A key to the geodynamic interpre-
tation of the Early Palaeozoic Antarc-tic margin in Victoria Land (Fig. 1a)is the nature and age of the deep crustof the Robertson Bay terrane, themargin�s outermost lithotectonic(Weaver et al., 1984). A number ofhypotheses have been formulated onthe kinematics and dynamics of jux-taposition ⁄accretion of fault-boundedlithotectonic units (Stump, 1995) andincludes a variable number of sub-ducting plates with varied dip direc-tion (Weaver et al., 1984;Kleinschmidt and Tessensohn, 1987).In these models, a key role is invari-ably played by the Robertson Bay
terrane, which is covered with thickturbidite sequences that hide theunderlying crust. This terrane hasbeen considered as being either (i) anexotic continental block, based onpetrological investigations on Devo-nian granites (Borg and DePaolo,1991), or (ii) of an oceanic outboardnature, based on aeromagnetic studies(Finn et al., 1999), or as (iii) an oceanridge-arc-backarc, based on prelimin-ary petrological investigations oncrustal xenoliths from a single locality(Berg and Wu, 1992). To collect directevidence on the nature and age of thedeep crust in this convergent setting,we integrated whole-rock geochemicaldata with zircon spot U–Pb geochro-nological investigations on granulitexenoliths.
Geological setting
The studied xenoliths are found in theproducts of the Cenozoic plutonic–volcanic alkaline province of the WestAntarctic rift (Le Masurier andThomson, 1990; Rocchi et al., 2002).Cretaceous–Cenozoic rift activity ledto the uplift of the TransantarcticMountains (Fitzgerald and Stump,1997) (Fig. 1a), which represent theroots of the Palaeozoic Ross Orogen.This Orogen and its northward con-tinuation in Australia (DelamerianOrogen) are linked to the EarlyPalaeozoic convergence between thepalaeo-Pacific oceanic plate and the
Antarctic-Australian Gondwana mar-gin (Stump, 1995). In Victoria Land,convergence resulted in the colli-sional–accretional assembly of threefault-bounded lithotectonic units, theWilson, Bowers and Robertson Bayterranes (Fig. 1b), whose nature is stilla matter of debate (Weaver et al.,1984; Borg et al., 1987; Kleinschmidtand Tessensohn, 1987; Rocchi et al.,1998; Finn et al., 1999; Goodge, 2002;Tessensohn and Henjes-Kunst, 2005;Federico et al., 2006).The Wilson terrane consists of both
polydeformed metamorphic rocks(Kleinschmidt and Tessensohn, 1987)and granulite remnants (Talaricoet al., 1995) intruded by calcalkalinetonalites, potassic granites and bi-modal peraluminous-lamprophyricdykes between c. 530 and 490–480 Ma (Borg et al., 1987; Di Vince-nzo and Rocchi, 1999; Goodge, 2002;Rocchi et al., 2004; Bomparola et al.,2007). Amphibolite- to greenschist-facies regional metamorphism in theLanterman Range (Fig. 1b) occurredat c. 490–480 Ma, slightly after the c.500 Ma eclogite metamorphism(Goodge and Dallmeyer, 1996; DiVincenzo et al., 1997; Di Vincenzoand Palmeri, 2001). The Bowersterrane consists of Middle–LateCambrian magmatic arc volcanic–vol-caniclastic rocks (Weaver et al., 1984;Stump, 1995) showing metamorphicoverprint of variable intensity (Cri-spini et al., 2007).
ABSTRACT
Geodynamic models for the Antarctic sector of the active EarlyPalaeozoic Palaeo-Pacific margin of Gondwana are based on thenature and age of the deep crust of the Robertson Bay terrane,the outermost lithotectonic unit of the margin. As this crustalblock is covered with thick turbidite deposits, the only way toprobe the deep crust is through the analysis of granulitexenoliths from Cenozoic scoria cones. Low-K felsic xenolithsyield the oldest (Middle Cambrian) laser-probe U–Pb ages onzircon areas with igneous growth zoning. This finding, along
with the positive whole-rock eNd(500Ma), suggests that thesefelsic rocks derived from a juvenile magma formed during theEarly Palaeozoic Ross orogenic cycle. Mafic xenoliths havegeochemical-isotopic compositions similar to those of modernprimitive island arcs, suggesting the involvement of subductedoceanic crust in their magma genesis and accretion of juvenilecrust at the Antarctic margin of Gondwana.
Terra Nova, 21, 151–161, 2009
Correspondence: Maurizio Gemelli, Dipar-
timento di Scienze della Terra, Universita
di Pisa, Via S.Maria 53, 56126 Pisa, Italy.
Tel.: +390502215796; fax: +3905022158
00; e-mail: [email protected]
� 2009 Blackwell Publishing Ltd 151
doi: 10.1111/j.1365-3121.2009.00868.x
The Robertson Bay terrane is a40 000 km2-wide crustal sector cov-ered with a thick, folded series ofslightly metamorphosed turbiditicgreywackes and mudstones (Borget al., 1987; Rossetti et al., 2006)intruded by the Middle–Late Devo-nian Admiralty granites (Fiorettiet al., 1997). Based on palaeontologi-cal (Tessensohn and Henjes-Kunst,2005) and geochronological (Henjes-Kunst, 2003) constraints, deformationin both the Bowers and RobertsonBay terranes occurred later than490–485 Ma. A continental sourcehas been suggested for the RobertsonBay turbidites (Stump, 1995; Rolandet al., 2004), whereas the nature of
the underlying basement (whethercontinental or oceanic) remains un-known.
Geochemical and geochronologicalconstraints
Petrography and geochemistry
Crustal xenoliths are known fromseveral Cenozoic volcanoes of theWest Antarctic rift (Berg et al.,1989; Wysoczanski et al., 1995). Innorthern Victoria Land, crustal xeno-liths were found as 2- to 20-cm-sizedcores to bombs (Fig. 2a,b) in scoriacones of the coastal Adare (Bergand Wu, 1992), Hallett and Daniell
peninsulas, as well as up to 200 kmsouth-westward, inland of the Victoryand Admiralty mountains (Fig. 1b).The xenolith population includesdominant felsic and mafic, mostlygranulitic types, along with minoralkaline cumulates (most likely Ceno-zoic, not relevant to this study;Table 1). Although the mineralogicalassemblage of these xenoliths ham-pers geobarometric estimates, theirgranulitic nature, temperature esti-mates of 730–830 �C (two-pyroxenegeothermometer: Brey and Koheler,1990), and the lack of olivine in maficsamples indicate that they are repre-sentative of an intermediate-deepcrust.Felsic granulites are the most com-
mon type of xenoliths (Fig. 2a); theyare usually medium- to coarse-grained granoblastic and show localductile deformation features (Fig. 2c).The mineral assemblage consists ofplagioclase (An7–19), quartz, ortho-pyroxene (Mg# = 0.37–0.57) andK-feldspar, along with rare apatiteand zircon. These granulites are SiO2-rich (64–77 wt%, Table 1) and K2O-poor (0.4–1.6 wt%). Rare earthelement (REE) patterns (Fig. 3a) aremoderately fractionated. The onlyexception is sample DS4A ⁄6, whichshows heavy REE enrichment likelyrelated to zircon cumulates occurringas large scattered crystals. REEenrichment levels are variable andcorrelate negatively with Eu ⁄Eu*(Fig. 3a), which is in turn positivelycorrelated with Sr ⁄Ce, Al2O3 andCaO. These relationships suggest theoccurrence of plagioclase cumulus insamples with the lowest REE con-tents. The REE patterns most likelyrepresentative of igneous melts arethus those with a negligible Euanomaly, like those of island arcrhyolites (Lai et al., 2000). Relation-ships among incompatible elements,when compared with those of gran-ites from various tectonic settings,also best match those of low-K tho-leiitic oceanic arcs (Fig. 4). One sam-ple shows a REE distribution(Fig. 3b) resembling those of somemetasedimentary xenoliths (Downesand Leyreloup, 1986; Sachs andHansteen, 2000). A few felsic xeno-liths contain K-feldspar as large (upto 1 cm) subhedral microcline crys-tals, in a typical hypidiomorphicgranitic texture (Fig. 2d). One gra-
(a)
(b)
Fig. 1 (a) Location map for the Transantarctic Mountains and Victoria Land with anEarly Palaeozoic reconstruction of Antarctica-Australia-Tasmania; the DelamerianOrogen includes Adelaidean successions. (b) Northern Victoria Land portion of theAntarctic margin of Gondwana and its constituent fault-bounded (thin dashed lines)lithotectonic units. The thick dashed line delimits the approximate area of theCambrian primitive island arc inferred in this work; Devonian Admiralty granitesfrom this area are characterized by high eNd values (Borg et al., 1987). West of thearc, the low eNd values of Devonian granites indicate that the continental crust wassignificantly involved in their genesis. The eNd values for the Admiralty granites(stars) reported in the ellipse are recalculated at an age of 500 Ma to allowcomparison between their hypothetical crustal source and the studied crustalxenoliths.
Granulite xenoliths from Gondwana Antarctic margin • M. Gemelli et al. Terra Nova, Vol 21, No. 3, 151–161
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152 � 2009 Blackwell Publishing Ltd
Fig. 2 Photographs and microphotographs of granulite xenolith samples: (a) felsic xenoliths found as volcanic bomb cores;(b) mafic xenoliths with a crust of host volcanic scoria; (c) felsic xenolith with local ductile deformation features (mortar textureand quartz ribbons); (d) felsic xenolith with subhedral microcline crystal; (e) mafic xenolith from Cape McCormick; (f) maficxenolith from Nameless Glacier. It is worth noting that within a few of the felsic xenoliths, thin glass veins rich in clinopyroxeneand plagioclase crystals occur as infiltrations connected to the basaltic material of the host bomb. The xenolith portions affected bythese veins were discarded during sample preparation for whole-rock chemical analyses. The intergranular, colourless to paleyellow, subaphiric glass films present in some felsic xenoliths probably derive from partial melting during xenolith ascent.
Terra Nova, Vol 21, No. 3, 151–161 M. Gemelli et al. • Granulite xenoliths from Gondwana Antarctic margin
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Tab
le1
Majorandtrace
elem
ents.
Loca
liti
es
Fels
icxe
no
lith
sM
afic
xen
oli
ths
Alk
alin
exe
no
lith
sH
ost
lava
26
.01
.02
DS
4A
⁄7C
MC
26
.01
.02
DS
4A
⁄29
CM
C
26
.01
.02
DS
4A
⁄2C
MC
26
.01
.02
DS
4A
⁄6C
MC
26
.01
.02
DS
4A
⁄3C
MC
18
.01
.02
DS
1A
⁄1R
R
23
.01
.02
DS
16
MB
26
.01
.02
DS
4A
⁄34
CM
C
27
.01
.02
DS
15
⁄1N
G
27
.01
.02
DS
15
⁄2N
G
27
.01
.02
DS
15
⁄4N
G
26
.01
.02
DS
4B
⁄1C
MC
26
.01
.02
DS
4B
⁄2C
MC
26
.01
.02
DS
4B
⁄3C
MC
27
.01
.02
DS
13
⁄7N
G
18
.01
.02
DS
1C
⁄1R
R
11
.01
.02
DS
13
AP
SiO
275
.63
72.4
074
.37
73.2
475
.60
72.4
277
.16
63.9
446
.86
50.1
147
.85
45.4
950
.19
50.0
662
.07
36.8
342
.04
TiO
20.
250.
460.
240.
320.
220.
160.
390.
610.
471.
140.
480.
870.
800.
550.
203.
983.
49
Al 2
O3
12.0
313
.66
12.6
112
.96
12.8
114
.73
7.68
16.7
624
.63
24.6
919
.74
13.5
313
.38
16.1
718
.82
17.3
314
.37
Fe2O
3T
3.25
3.38
3.32
4.52
3.41
1.64
6.16
5.03
4.50
4.48
6.10
16.7
013
.50
11.8
14.
1814
.09
12.5
9
MnO
0.07
0.04
0.10
0.12
0.10
0.04
0.09
0.08
0.06
0.05
0.08
0.25
0.26
0.23
0.13
0.15
0.22
MgO
0.44
1.06
0.36
1.35
0.30
0.56
4.12
2.75
6.28
2.44
7.63
8.09
6.94
6.56
0.27
5.38
7.99
CaO
1.85
3.48
1.46
2.31
1.46
1.35
1.90
5.56
14.5
611
.32
14.1
011
.24
10.8
410
.05
1.67
14.2
410
.39
Na 2
O4.
164.
304.
664.
134.
753.
210.
583.
831.
863.
931.
752.
542.
783.
336.
012.
534.
41
K2O
0.88
0.36
1.59
0.39
1.64
4.58
0.32
0.37
0.07
0.40
0.37
0.24
0.30
0.29
5.23
0.51
2.00
P 2O
50.
060.
090.
040.
040.
040.
110.
040.
110.
040.
100.
060.
060.
120.
080.
101.
560.
87
Tota
l10
0.35
100.
4599
.36
99.8
010
1.02
100.
9410
0.79
99.2
599
.93
99.8
299
.27
98.8
299
.15
99.6
499
.67
97.2
198
.71
L.O
.I.1.
731.
220.
610.
420.
692.
142.
350.
210.
601.
161.
11-0
.19
0.04
0.51
0.99
0.61
0.34
Be
1.66
<d.
l.1.
580.
295
1.66
60.
37<
d.l.
0.35
0.34
0.19
10.
286
1.15
0.44
0.43
0.79
3.06
Sc4.
3<
d.l.
6.6
14.0
6.8
<d.
l.17
.5<
d.l.
14.6
10.6
22.2
6052
22.8
1.03
15.2
22.9
V8.
439
4.7
30.7
3.5
15.0
113
8680
105
156
504
287
123
19.4
287
239
Cr
8.1
<d.
l.0.
972.
370.
50<
d.l.
274
<d.
l.10
84.
344
615
210
510
.82.
050.
6012
6
Co
3.15
8.5
2.37
6.7
1.59
2.40
19.0
16.4
26.2
16.4
3563
4322
.63.
3338
45
Ni
13.7
9.4
5.7
5.4
4.9
1.50
866.
980
10.3
126
115
4914
.61.
439.
211
9
Cu
18.6
4330
.214
.227
.97
3529
.514
.114
.322
.128
.838
14.0
12.0
9747
Ga
13.6
11.4
14.4
12.0
14.8
16.2
8.3
17.3
12.3
18.7
13.9
14.9
15.0
9.7
22.0
21.0
20.3
Rb
7.5
2.90
25.1
2.93
26.2
269
1.99
7.6
3.9
1.29
1.48
2.28
3.5
3.9
965.
052
Sr63
8476
108
7279
100
185
549
1148
622
262
138
115
258
1771
1285
Y13
.94.
622
.537
25.7
21.9
31.2
3.6
4.2
3.8
5.6
20.0
26.9
9.8
6.9
3540
Zr18
215
220
814
921
112
135
15.9
1733
2311
275
148
161
391
Nb
3.29
2.90
4.4
2.91
4.7
16.5
8.4
5.7
1.23
3.16
1.31
3.8
5.6
1.54
18.0
4913
7
Cs
0.05
90.
204
0.07
20.
159
0.08
921
.00.
047
0.60
0.22
10.
017
0.01
90.
049
0.04
70.
112
0.09
90.
092
0.69
Ba
236
115
422
9643
454
722
342
3613
844
8246
27.0
702
283
715
La14
.45.
518
.16.
618
.633
3.6
5.3
2.35
4.6
2.77
5.0
5.6
2.27
3657
92
Ce
25.7
10.3
3413
.137
704.
79.
84.
98.
66.
110
.914
.05.
657
126
180
Pr2.
911.
183.
91.
604.
37.
60.
461.
100.
651.
060.
851.
382.
070.
855.
416
.520
.3
Nd
11.3
5.2
15.2
6.6
17.2
25.5
1.87
4.1
2.95
4.5
4.1
5.9
9.6
4.1
18.1
7077
Sm2.
320.
943.
201.
313.
84.
80.
720.
750.
771.
001.
151.
652.
861.
142.
6713
.113
.3
Eu0.
900.
881.
011.
161.
040.
520.
780.
540.
381.
060.
560.
830.
940.
481.
154.
24.
2
Gd
2.19
0.99
3.33
1.58
3.9
4.2
2.36
0.66
0.81
0.92
1.18
2.26
3.5
1.37
1.96
10.5
10.5
Tb0.
360.
152
0.55
0.42
0.64
0.72
0.60
0.11
00.
134
0.14
10.
191
0.44
0.67
0.24
70.
264
1.46
1.51
Dy
2.22
0.87
3.4
4.1
4.1
3.8
4.6
0.61
0.75
0.78
1.10
3.14
4.4
1.61
1.35
7.4
7.9
Ho
0.50
0.17
20.
801.
300.
910.
731.
070.
131
0.14
50.
150.
216
0.74
0.97
0.36
0.25
81.
311.
49
Er1.
370.
472.
475.
02.
722.
113.
190.
370.
350.
360.
502.
112.
831.
010.
672.
973.
6
Tm0.
205
0.08
10.
380.
990.
410.
320.
520.
062
0.04
80.
046
0.07
10.
323
0.44
0.15
20.
105
0.38
0.51
Granulite xenoliths from Gondwana Antarctic margin • M. Gemelli et al. Terra Nova, Vol 21, No. 3, 151–161
.............................................................................................................................................................
154 � 2009 Blackwell Publishing Ltd
nitic-textured xenolith has the highestK2O content (4.6 wt%) and a distri-bution of REE (Fig. 3b) and incom-patible elements (Fig. 5) typical ofgranites from the continental crust.Mafic granulite xenoliths (Fig. 2b)
show a medium- to coarse-grained,granoblastic to polygonal texture(Fig. 2e,f) and mostly consist oforthopyroxene (Mg# = 0.53–0.57),clinopyroxene (Mg# = 0.53–0.70)and plagioclase (An44-63), with acces-sory apatite, magnetite and ilmenite.The basic composition (SiO2 = 45.5–50.2 wt.%) and N-MORB normal-ized incompatible trace element dis-tribution of these xenoliths (Fig. 5)are similar to those of modern oce-anic-arc basalts (Pearce et al., 1995)and show a low-K tholeiitic (CapeMcCormick samples) to calcalkaline(Nameless Glacier) affinity. Traceelement distributions compared to afertile MORB mantle composition(Fig. 6) are very similar to those ofPacific island arcs such as Vanuatuand Marianas (Cape McCormicksamples) or Tonga (Nameless Gla-cier). Trace element distributionssimilar to those of primitive islandarcs are likewise found in oceanicridge segments at small distancesfrom subduction systems (e.g. SouthSandwich island arc-oceanic ridge,Leat et al., 2004; Fig. 7). Theseobservations suggest that the mag-mas associated with the mafic xeno-liths may have been generated in aprimitive island arc or in an oceanicridge setting linked to active subduc-tion.
U–Pb zircon geochronology
Only two low-K felsic granulite xeno-liths yielded zircon separates in suffi-cient quantity for study.Cathodoluminescence imaging re-vealed complex structures (Fig. 8),with internal areas characterized bymore or less well-defined oscillatoryzoning surrounded by either oscilla-tory-zoned or featureless light greyrims up to �50 lm in thickness. Thelatter are locally connected to theinner portions of crystals through anetwork of microveins.Within-error U–Pb concordant
data (Table S1) collected by laser-ablation-inductively coupled plasma-mass spectrometry (methods inTable S1 and Alagna et al., 2008)Ta
ble
1(Continued)
Loca
liti
es
Fels
icxe
no
lith
sM
afic
xen
oli
ths
Alk
alin
exe
no
lith
sH
ost
lava
26
.01
.02
DS
4A
⁄7C
MC
26
.01
.02
DS
4A
⁄29
CM
C
26
.01
.02
DS
4A
⁄2C
MC
26
.01
.02
DS
4A
⁄6C
MC
26
.01
.02
DS
4A
⁄3C
MC
18
.01
.02
DS
1A
⁄1R
R
23
.01
.02
DS
16
MB
26
.01
.02
DS
4A
⁄34
CM
C
27
.01
.02
DS
15
⁄1N
G
27
.01
.02
DS
15
⁄2N
G
27
.01
.02
DS
15
⁄4N
G
26
.01
.02
DS
4B
⁄1C
MC
26
.01
.02
DS
4B
⁄2C
MC
26
.01
.02
DS
4B
⁄3C
MC
27
.01
.02
DS
13
⁄7N
G
18
.01
.02
DS
1C
⁄1R
R
11
.01
.02
DS
13
AP
Yb
1.32
0.54
2.60
6.8
2.79
2.08
3.27
0.42
0.29
00.
268
0.40
2.10
2.80
0.97
0.70
2.14
2.96
Lu0.
212
0.09
10.
441.
070.
450.
310.
480.
071
0.04
10.
034
0.05
20.
311
0.41
0.14
40.
118
0.28
60.
41
Ta0.
172
<d.
l.0.
231
0.13
00.
241
2.31
0.44
0.30
0.11
80.
235
0.08
30.
164
0.35
0.08
51.
153.
108.
5
Tl0.
060
<d.
l.0.
033
0.04
70.
030.
200
0.25
1<
d.l.
0.02
80.
013
0.00
20.
018
0.00
50.
009
0.05
90.
015
0.03
4
Pb12
.41.
8028
.54.
328
.64.
32.
880.
600.
910.
861.
350.
903.
418
.75.
61.
393.
9
Th0.
440.
208
0.34
0.05
40.
261
230.
191
0.51
0.14
00.
148
0.17
40.
183
0.34
0.06
13.
291.
9510
.6
U0.
154
0.10
50.
510.
153
0.50
3.1
0.05
90.
104
0.29
20.
111
0.08
70.
058
0.12
50.
058
0.87
0.56
3.00
Maj
orel
emen
tsw
ere
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edon
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via
stan
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XR
Fat
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artim
ento
diSc
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ede
llaTe
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Uni
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ityof
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ngan
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00X
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ectr
omet
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ollo
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al.(
2003
).Lo
sson
igni
tion
was
dete
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etry
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00�C
afte
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e-he
atin
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110
�C.T
race
elem
ents
wer
ede
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ined
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ples
atD
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(Fis
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edin
D�O
razi
o(1
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and
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.(2
002)
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ion
limit.
Loca
lity
abbr
evia
tions
:(C
MC
)C
ape
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orm
ick,
(NG
)N
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nsul
a.
Terra Nova, Vol 21, No. 3, 151–161 M. Gemelli et al. • Granulite xenoliths from Gondwana Antarctic margin
.............................................................................................................................................................
� 2009 Blackwell Publishing Ltd 155
yielded comparable 206Pb ⁄ 238U agepatterns for the two samples: coreand rim areas characterized by vari-ably well-defined growth zoning sys-tematically yielded the oldest ages (inthe range 520–460 Ma; Figs 8 and 9),whereas featureless rims and internallight-coloured microveins yieldedyounger ages clustering c. 370–350 Ma (Figs 8 and 9). This agepattern could be ascribed to a pre-Ross origin, followed by Ross-agegranulite facies metamorphism andsubsequent Devonian reworking.However, the lack of pre-Ross agessuggests that the zircon age patternresults from the magmatic origin ofthese felsic rocks during the Rossorogenic cycle, followed by granulitefacies metamorphism during the Rosscycle itself and final reworking duringDevonian intrusive activity. A similartwo-stage magmatic-granulitizationprocess in a supra-subduction zonesetting during a single subductioncycle has been documented for zirconsin Cenozoic granulites from Japan(Kemp et al., 2007).
Sm–Nd isotope data
Initial eNd values for felsic sampleswere calculated to 500 Ma on thebasis of U–Pb data (Table 2). Maficsamples were assumed to have thesame initial crystallization age, be-cause comparable Nd isotopic com-positions were found for EarlyPalaeozoic igneous rocks only. Boththe Devonian Admiralty Intrusives(Borg et al., 1987) and the JurassicFerrar basalts (Hergt et al., 1991;Antonini et al., 1999) exhibit signifi-cantly lower initial eNd values,whereas Cenozoic alkaline basaltshave higher values and different geo-chemical features (Rocchi et al.,2002).The two mafic xenoliths from
Cape McCormick and NamelessGlacier yield eNd(500Ma) values(1.5–5.6) that are much higher thanthose typical of old continentalcrusts, thereby indicating their man-tle origin. However, the fact thateNd(500Ma) values are definitely lowerthat those of a Depleted Mantle at500 Ma (DePaolo et al., 1991) sug-gests the involvement of a subduc-tion component. The two low-Kfelsic granulites from Cape McCor-mick used for U–Pb zircon dating
DS 4A/6 26.01.02DS 4A/7 26.01.02DS 4A/2 26.01.02DS 4A/3 26.01.02DS 4A/34 26.01.02DS 4A/29 26.01.02
Roc
k/ch
ondr
ite
1000(a)
(b)
100
10
1
100
10
1
0.1
Roc
k/ch
ondr
ite
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
DS 16 23.01.02DS 1A/1 18.01.02
Fig. 3 Rare earth element (REE) patterns of felsic granulite xenoliths normalized tochondrite values. Normalization factors after Sun and McDonough, 1989. In the text,the Eu anomaly is described by the ratio Eu ⁄Eu* = Eun ⁄ (Smn*Gdn)
1 ⁄ 2.
0.01
0.1
1
10
100
0.1 1 10 100 1000Rb
Rb n
/Ba
n VAG
ORG
WPG
COLG
SSP
Low-K felsic xenoliths
High-K felsic xenolith
Fig. 4 Rbn ⁄Ban vs. Rb for low-K felsic xenoliths. Normalization factors andreference values for granites from different settings are after Pearce et al. (1984).ORG (Ocean Ridge Granites), VAG (Volcanic Arc Granites), WPG (Within PlateGranites), COLG (Collision Granites). SSP is a dacite from a low-K tholeiite series ofthe South Sandwich island arc (Pearce et al., 1995).
Granulite xenoliths from Gondwana Antarctic margin • M. Gemelli et al. Terra Nova, Vol 21, No. 3, 151–161
.............................................................................................................................................................
156 � 2009 Blackwell Publishing Ltd
also have a positive eNd(500Ma) (1.5–2.1) like the mafic granulite from thesame locality. Nd model ages for
felsic samples (1.04–1.09 Ga) cannotbe considered the ages of differenti-ation of their source from a depletedmantle. Based on their inferred
origin from a subduction-modifiedmantle, they should instead be re-garded as maximum magma genera-tion ages. This observation rules outa Neoproterozoic (or older) genesisof these felsic melts, in keeping withthe lack of any evidence of precam-brian zircons. In summary, bothmafic and felsic granulites representmostly juvenile materials derivedfrom the mantle during the Rossorogenic cycle.
Discussion and conclusions
Petrographic and geochemical fea-tures of different types of crustalxenoliths testify the significant com-positional heterogeneity of the inter-mediate-lower crust of the easternRobertson Bay terrane. Both maficand felsic granulite xenoliths aregeochemically distinct from mafic-enderbitic and felsic-metasedimentarygranulites cropping out in the Wil-son terrane (Talarico et al., 1995),thereby suggesting that the deepcrust in the two areas differs innature. Mafic xenoliths have traceelement distributions and Nd isotopecompositions similar to those ofprimitive island arcs. Geochemicalcomparison with the South Sandwichisland arc-oceanic ridge (Leat et al.,2004) suggests the involvement ofa young subducted oceanic crust inan intraoceanic island arc or asupra-subduction-zone segment ofan oceanic ridge.Along the whole Antarctic margin
of Gondwana, such an isotopic sig-nature is only found for the TigerGabbro intrusion [eNd(535 Ma) = 3.0,Rocchi et al., 1999; Fig. 1b]; theinitial eNd values for the entire intru-sive association of comparable age inthe Wilson margin are much lessradiogenic (£)1.5, Fig. 9). Evidencefor the generation of Cambrian gran-ites through the partial recycling of apre-existing crust is documented onthe Pacific side of the Robertson Bayterrane (Surgeon Island, Fig. 1b),where a granite contains Early-Mid-dle Cambrian zircon populations aswell as Proterozoic relics (Fiorettiet al., 2005). The eNd values (recal-culated at 500 Ma to allow compar-ison between their hypotheticalsource and the studied xenoliths) ofDevonian granites from the western,Pacific side of the Robertson Bay
CsTl
RbBa
ThU
TaNb
LaCe
PrSr
NdZr
HfSm
EuGd Tb
DyY
HoEr
TmYb
Lu
0.1
1
10Roc
k / N
-MO
RB
0.1
1
10
Mafic xenoliths-NG
Mafic xenoliths-CMC
Ti
Fig. 5 N-MORB normalized multielemental diagrams for mafic granulite xenolithsfrom the Nameless Glacier (NG) and Cape McCormick (CMC) areas. Normalizationfactors after Sun and McDonough (1989).
Tonga arc
Mafic xenoliths-NG
Mafic xenoliths-CMC
Vanuatu arc
Marianas arc
Nb Zr
Ti Y
Yb Ca
Al Ga
V Sc
Mn Fe
Co Mg
Cr Ni
Roc
k/F
MM
R
ock/
FM
M
100
10
1
0.1
10
1
0.1
0.01
Fig. 6 FMM (Fertile MORB Mantle)normalized multielemental diagrams formafic xenoliths from Cape McCormick(CMC) and Nameless Glacier (NG).Normalization factors after Pearce andParkinson (1993).
Mafic xenoliths-NG
Mafic xenoliths-CMC
E2
Central ESR
E10
Regio
nal M
ORB-O
IB a
rray
Volcanic Arc
Ba/
Yb
Nb/Yb
1000
100
10
11 10 100 0.1
Fig. 7 The Ba ⁄Yb vs. Nb ⁄Yb plot formafic xenoliths also reports the regionalMORB-OIB array defined by samplesfrom the South America-Antarctic ridge,the volcanic arc field for the SouthSandwich island arc, and the composi-tional fields for ridge sectors (E2, E10)closest to the subduction zone (Leatet al., 2004). ESR, East Scotia Ridge;NG, Nameless Glacier; CMC, CapeMcCormick.
Terra Nova, Vol 21, No. 3, 151–161 M. Gemelli et al. • Granulite xenoliths from Gondwana Antarctic margin
.............................................................................................................................................................
� 2009 Blackwell Publishing Ltd 157
terrane (Borg et al., 1987) range from)3.4 to )6.2, indicating that thecontinental crust was significantlyinvolved in their genesis. In contrast,the area facing the Ross Sea, inferredto be underlain by a thinner crust(Borg et al., 1987), hosts Devoniangranites with eNd values close to zero,much like those of our xenoliths(Fig. 1b).Overall, the data highlight the
complex nature of the deep crust ofthe Robertson Bay terrane, whichconsists of fragments of (i) an oldercontinental crust on the Pacific Oceanside and (ii) a Ross-age juvenilecomponent generated in an oceanicsubduction setting on the Ross Seaside (Fig. 1b: area delimited by thedashed line). A comparable change inthe nature of the basement, fromcontinental (west) to oceanic (east), isdocumented in the Tasmanides of SEAustralia (Grey and Foster, 2004).The inferred oceanic subduction onthe Ross Sea side of the RobertsonBay terrane could have had an oppo-site dip with respect to subductionbelow the Wilson margin, as inferredfor coeval south-eastern Australia-Tasmania (Gibson, 1987; Flottmannet al., 1998; Munker and Crawford,2000). Nevertheless, a commonsouth-westward dip for both subduc-tion zones is most likely in the overallframework of convergence betweenthe palaeo-Pacific plate and theAntarctic margin.In conclusion, we infer that the
deep crust of the Robertson Bayterrane was constructed during theRoss Orogeny through accretion of asupra-subduction-zone oceanic crustcarrying primitive island arc(s), alongwith fragments of continental crust,to the active Antarctic margin ofGondwana. In this scenario, the RossOrogen was not produced by colli-sion against the margin of largecontinents or exotic continental colli-ders (Federico et al., 2006), nor bythe accretion of a forearc oceaniclithosphere (Finn et al., 1999). Weinstead infer that transient couplingbetween the lower and upper plates(Cawood and Buchan, 2007) gener-ated multiple docking of small-sizedfragments of continental crust and aNS-trending island arc forming ahigh angle with respect to the NW-trending Bowers island arc–forearc–(backarc) system.
520±17
346±12
500±16
491±15
503±18
360±12476±15
485±16
358±13
335±10
401±12485±16
482±15
504±18
440±16504±16
495±16
496±16
364±13
447±14
349±12
379±12
489±17
386±18
478±16
394±13
366±12
465±16
464±15
457±16
361±14
475±15
488±14
461±16
432±15
405±13
100 µm
1 2 3 4
5 6 7 8
9
10 11 12
13 14 15 16
Fig. 8 Selected SEM-cathodoluminescence images of zircons separated from felsicgranulite xenoliths. Circles represent the location of spots analysed by LA-ICP-MSfor U–Pb systematics. The general 30 lm analytical spot size was reduced to 20 lmfor rim analyses. The 206Pb ⁄ 238U age is reported for each spot. Data from Table S1.
Table 2 Sm–Nd isotopic data.
Sample
Sm
p.p.m.
Nd
p.p.m. 147Sm ⁄ 144Nd
143Nd ⁄ 144NdeNd
500 Ma
T(DM)
GaMeasured ± 2rm
Felsic xenoliths
DS4A ⁄ 3 3.80 17.2 0.1333 0.512508 ± 0.000009 1.5 1.09
DS4A ⁄ 2 3.26 15.4 0.1281 0.512516 ± 0.000009 2.1 1.04
Mafic xenoliths
DS4B ⁄ 2 2.86 9.59 0.1803 0.512663 ± 0.000008 1.5 1.09
DS15 ⁄ 4 1.15 4.15 0.1675 0.512830 ± 0.000009 5.6 0.71
Sm and Nd by ICP-MS. Initial eNd calculated at 500 Ma. T(DM) calculated according to DePaolo et al. (1991).
T(DM) for mafic xenoliths are reported in italics to indicate their poor significance because of their high147Sm ⁄ 144Nd ratios. Sm ⁄ Nd isotopic ratios were determined via TIMS techniques at the IGG–CNR (Pisa,
Italy) using a Finnigan MAT 262 multicollector mass spectrometer running in dynamic mode. Nd was
separated using standard separation techniques after digestion in a PTF Teflon bomb at �200 �C. At the
time of data collection, the JNdi-1 standard (Tanaka et al., 2000) yielded an average 143Nd ⁄ 144Nd of
0.512103 ± 0.000007(±2 SD).
Granulite xenoliths from Gondwana Antarctic margin • M. Gemelli et al. Terra Nova, Vol 21, No. 3, 151–161
.............................................................................................................................................................
158 � 2009 Blackwell Publishing Ltd
Acknowledgements
This study was completed as a part ofItaly�s National Program for Research inAntarctica (PNRA). We thank M. Tamp-oni, M. D�Orazio and S. Tonarini forassistance with XRF, ICP-MS and TIMSanalyses, respectively. Constructive criti-cisms by W. Van Schmus and X. Liuhelped significantly improve the quality ofthe manuscript.
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300 350 400 450 500 550 600
–15
–12
–9
–6
–3
0
+3
+6
+9
Ross main intrusive association
Ross peraluminous granites
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AI-PO
AI-RS
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Depleted mantle
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206 Pb/ 238 U Age (Ma)
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2
3
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5
6 50 µm
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50 µm
7
8
9
Relative probability curve
Number of analyses
Fig. 9 Summary of 206Pb ⁄ 238U zircon ages from U–Pb data concordant within error(and relative probability diagram), along with selected cathodoluminescence imagesof zircon crystals from felsic xenoliths. Source of Nd isotopic data: this work(diamonds); main Ross intrusive association, Ross peraluminous granites andmigmatites (Borg et al., 1987; Rocchi et al., 1998; Di Vincenzo and Rocchi, 1999);Tiger Gabbro (Rocchi et al., 1999); mafic and felsic granulite terrains (Talarico et al.,1995); AI-RS, Admiralty Intrusives from the Ross Sea side of the Robertson Bayterrane (Fig. 1b); and AI-PO, Admiralty Intrusives from the Pacific Ocean side (Borget al., 1987). AI-PO ⁄RS values of eNd are shown at both the emplacement age(360 Ma) and the Ross Age (at 500 Ma). Depleted Mantle evolution line afterDePaolo et al. (1991). Gray stripe represents the overall age interval of igneousactivity in Victoria Land during the Ross Orogeny.
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Received 1 August 2008; revised versionaccepted 27 January 2009
Supporting Information
Additional Supporting Informationmay be found in the online versionof this article:Table S1 U–Pb data for zircons
separated from low-K felsic granulitexenoliths.Please note: Wiley-Blackwell are
not responsible for the content orfunctionality of any supporting mate-rials supplied by the authors. Anyqueries (other than missing material)should be directed to the correspond-ing author for the article.
Terra Nova, Vol 21, No. 3, 151–161 M. Gemelli et al. • Granulite xenoliths from Gondwana Antarctic margin
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