Geochemical and isotopic structure of the early Palaeozoic active margin of Gondwana in northern Victoria Land, Antarctica

TECTONOPHYSICS ELSEVIER

Tectonophysics 284 (1998) 261-281

Geochemical and isotopic structure of the early Palaeozoic active margin of Gondwana in northern Victoria Land, Antarctica Sergio R o c c h i a,*, S o n i a Tonarini b, Pietro A r m i e n t i

a, F a b r i z i o I n n o c e n t i a, Piero M a n e t t i c

" Dipartimento di Scienze della Terra, Universitgl di Pisa, via S. Maria, 53, 56126 Pisa, Italy b lstituto di Geocronologia e Geochimica lsotopica, CNR Pisa, via Card. Maffi 36, 56127 Pisa, Italy ~"Dipartimento di Scienze della Terra, Universitgt di Firenze, via G. La Pira 4, 50121 Firenze, Italy

Received 30 January 1997; accepted 14 July 1997

Abstract The regional distribution of geochemical and isotopic compositions of granitoid rocks from a Gondwana continental margin is studied to highlight its structure and geodynamic evolution. The intrusive rocks emplaced during the early Palaeozoic Ross Orogeny in northern Victoria Land (Antarctica) constitute a high-K calc-alkaline association. The geographic patterns of isotope and geochemical data on granitoid rocks allow the distinction of two portions of the continental margin, separated by a sharp discontinuity. The portion towards the palaeo-Pacific Ocean (Oceanward Side) displays strongly regular inland increase of Sr- and decrease of Nd-isotope ratios, coupled with analogous variations in major and trace elements; on this basis we infer a NW-SE-trending margin affected by SW-directed subduction. The portion towards the East Antarctic Craton (Continentward Side) shows a similar regular variation only for Nd isotope compositions, consistent with a hypothesis of a N-S-striking margin with west-ward subduction. In the Oceanward Side, isotope and trace-element characteristics suggest that the granites were generated by extensive interaction of mantle-derived magmas with high-level crustal melts. The origin of Continentward Side intrusives is compatible with a process of interaction between mantle-derived melts and a mafic granulite lower crust. The granitoids of the two crustal sectors share the same Proterozoic Sm-Nd model ages, suggesting that they both belong to the same crustal province. We interpret this arrangement of crustal segments as due to the shift and rotation of a forearc sliver of the Gondwana margin. This movement was likely enhanced by oblique subduction under an irregular margin weakened by the presence of a magmatic arc. © 1998 Elsevier Science B.V. All rights reserved.

Keywords: Antarctica; continental margin; Gondwana; isotope geology; Sr-Nd isotopes

1. Introduction

The study of geochemical and isotopic compositions of mantle-derived magmas fluxing through a crust of varying age and thickness, represents a *Corresponding author. Fax: +39 (50) 500675; E-mail: [email protected]

powerful tool in the exploration of large-scale geodynamic processes such as those responsible for the evolution of orogenic belts and continental margins (Farmer and DePaolo, 1983; Bennett and DePaolo, 1987; Borg et al., 1990). This approach may be invaluable, and is often the only viable one in ancient peneplaned belts, or when the outcrops are scarce and scattered. In East Antarctica both limiting con-

0040-1951/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 4 0 - 1 9 5 1 ( 9 7 ) 0 0 1 7 8 - 9

262

S. Rocchi el al. / Tectonophysics 284 (1998) 261-281

ditions are often encountered, thus the evolution of crystalline basement has been largely investigated relying upon isotopic mapping, to get information about direction of subduction, crustal discontinuities, basement provinces and accreted terranes (Borg and DePaolo, 1991, 1994). The distribution of Sr-Nd isotopic compositions and other geochemical parameters of intrusive rocks, presently exposed in the Transantarctic Mountains of northern Victoria Land (Fig. 1) are used to constrain the tectonomagmatic features of the continental margin, active during the early Palaeozoic Ross Orogeny, to highlight its geotectonic evolution. It is to be noted that the studied region represents a crucial site for the links between the mobile belt bordering the East Antarctic Craton and its Gondwanan extension in Australia (Dalziel, 1992; Dalziel et al., 1994). 2. The Ross Orogeny in northern Victoria Land

According to the SWEAT hypothesis (South-West United States-East AnTarctic connection: Moores, 1991; Dalziel, 1991, 1992), the rifting, and subsequent drift of Laurentia from Gondwana gave way to the spreading of the palaeo-Pacific Ocean. This induced long lasting subduction of palaeo-Pacific oceanic lithosphere under the continental margin of Gondwana during the early Palaeozoic (Dalziel, 1992; Dalziel et al., 1994), generating the Ross-Delamerian Orogen, a 4000-km-long mobile belt, which is now exposed on the border of the East Antarctic Craton and in southeastern Australia. This early Palaeozoic chain was reduced to the Kukri Peneplain in Devonian-Permian times. Later, in Antarctica, during Cretaceous-Cainozoic rifting, the uplift of the craton-ward shoulder of the rift produced a rejuvenation of morphology, roughly paralleling the Ross Orogen. The present day chain of the Transantarctic Mountains (with heights in excess of 4000 m) is the result of the uplift, and exposes the crystalline basement of the early Palaeozoic Ross Orogeny (Stump, 1995, and references therein). Within this basement, in northern Victoria Land, it is now possible to recognize three main terranes. From southwest to northeast they are: the Wilson Terrane, the Bowers Terrane and the Robertson Bay Terrane (Fig. 1; Weaver et al., 1984; Bradshaw et al., 1985; Bradshaw, 1989; Carmignani et al., 1989;

Fl0ttmann and Kleinschmidt, 1991; GANOVEX RN.R.A. ltaliAntartide, 1993). The accretion of the Bowers Terrane plus Robertson Bay Terrane onto the Wilson Terrane generated a NNW-trending suture: timing and style of this accretion are still a matter of debate (Borg and DePaolo, 1991; Musumeci et al., 1994). During the Cambrian-Ordovician Ross Orogeny, in a magmatic arc setting, between about 530 and 480 Ma (Tonarini and Rocchi, 1994), the continental crust of Wilson Terrane was extensively invaded by a suite of granitoid rocks known in the literature as Granite Harbour Intrusives (GHI: Gunn and Warren, 1962). They exhibit calc-alkaline affinity with variable K enrichment (Armienti et al., 1990b). Geochemical polarities observed within GHI in the northeastern portion of Wilson Terrane indicate a southwestward subduction of the palaeo-Pacific oceanic lithosphere (Kleinschmidt and Tessensohn, 1987; Borg et al., 1987). 3. Granite Harbour Intrusives - - general outline

The study area belongs to the Wilson Terrane and extends over 70 x 240 km, along the Ross Sea coast of northern Victoria Land (Fig. 1). Granite Harbour Intrusives (GHI) in this area are made up of granites, granodiorites, tonalites and diorites, as shown in Fig. 2. Geological evidence indicates three main phases of emplacement of GHI. Products of the initial phase are scarce and outcrops are limited to the southern part of the Terra Nova Intrusive Complex and the northwestern tip of the Deep Freeze Range (Fig. 1); they are represented by metaluminous syenogranites and granodiorites bearing strong evidence of subsolidus deformation. A R b - S r whole-rock isochron yielded a value of 530 ± 57 Ma (Tonarini and Rocchi, 1994), which, in spite of the uncertainty, is in agreement with the dating of the older intruded units at 508 ± 6 Ma (Armienti et al., 1990a; Rocchi et al., 1997). In addition, this magmatic phase may be the counterpart in northern Victoria Land of the early arc magmatism documented by Encarnacidn and Grunow (1996) in the southern Victoria Land and Scott Glacier areas. The genesis of this minor association will not be discussed further in this paper.

263

S. Rocchi et al./Tectonophysics 284 (1998) 261-281

164 °

162 °

166 ° L o n g i t u d e

I

I

EAST

170 °

I

~£~ Bowers ~. ~ \ Terrane"

Wilson Terrane

73'

168 °

I

Robertson Bay Terrane

'1 C3 t~

".

73 c

A \ .-.d

\

C

Fr~

74'

~"~.

:

74 °

,

i t

I 1

ROSS Sea

~Ro

Nova Terr~ Intrus~ ~ ~

\

75 c

75 °

DAVID G i

1 62 °

~

0

5,0

1tOOkm

i

164 °

166 °

168 °

170 °

Fig. I. Sketch map of northern Victoria Land. The shaded areas represent the main outcrop zones of Granite Harbour Intrusives in the Wilson Terrane (after Carmignani et al., 1989). The solid line A - B - C is the transect on which the geochemical data are projected (see text and Figs. 6 - 8 fur further details). The dashed line D - E represents the shield (left)-plafform (right) boundary of the East Antarctic Craton, according to Roland (1991). The dashed line F - G represents the high-angle shear zone, reported by Biagini et al. ( 1991 ) along the northeastern side of the Campbell Glacier. The heavy dashed line labelled TCD represents the inferred Tinker-Campbell Discontinuity, separating the Oceanward Side from the Continentward Side of the continental margin (see text and Figs. 6 - 8 and 10). The circles indicate the location of samples of intrusive rocks reported in Tables 1 and 2: filled symbols for samples analysed for both Sr and Nd isotope systematics, open symbols for samples analysed only for Sr isotope systematics. The sample location and coordinates are reported in Appendix A.

S. Rocchi et aim / Tectonophy.sic's 284 (1998) 261-281

264

40 I f ~- --~-- ~ ' ~ ~ ~ G D

o•

•

•

• eoe

30

\,

O,

•

= "

•

\

• •Q •

•

20

\

•

•• ~,

• north-eastern Wilson Terrane • south-western Wilson Terrane

T •

\ \

•

\_

"i

i

\

• ~-ml \

QM 10 •

\

\

0

10

20

30

40

50

60

70

80

90

100

ANOR Fig. 2. Q'-ANOR classification diagram (after Streckeisen and Le Maitre, 1979). Geochemical data and locations for samples analysed for isotope systematics are reported in Appendix A, Appendix B and in Armienti ctal. (1990b): for the other samples they can bc requested from the authors. Field labels: D = diorite. QD = quartz diorite, QMD quartz monzodiorite. QM = quartz monzonite, T = tonalite. GD = granodiorite. MG = monzogranite. SG = syenogranite. AG = alkali feldspar granite. The second, main magmatic phase is represented by intrusive masses with compositions varying from granites and granodiorites, through tonalites, to minor amounts of gabbro-diorites. These rocks are generally characterized by a medium-grained hypidiomorphic texture; sometimes they appear microcline-phyric (mainly in the Deep Freeze R a n g e Terra Nova Bay area). Iso-orientation of minerals and weak anisotropy are observed in many outcrops and are due to magmatic flow. Cumulus processes of plagioclase or accessory minerals are sometimes observed in d i o r i t i c - g a b b r o i c rocks. Quartz, microcline and plagioclase are always j o i n e d by biotite and by hornblende in some syenogranites, in many granodiorites, in all the tonalites and diorites. Accessory zircon and apatite are ubiquitous, sphene is very frequent, allanite is frequent and monazite is sometimes observed in the most evolved rocks. Deuteric minerals are rather scarce, being mainly represented by chlorite after biotite and sericite after plagioclase.

These dominantly calc-alkaline intrusions show variable K enrichment, which in some cases leads to a shoshonitic affinity (e.g., Abbott Unit: Armienti et al., 1990b; Di Vincenzo et al., 1997). Available geochronological data ( R b - S r whole-rock isochron) indicate for this main phase an age around 508 + 6 Ma (Armienti et al., 1990a; Tonarini and Rocchi, 1994). The subsequent fading of igneous activity is characterized by the emplacement of peraluminous leucogranites, some of which showing weak deformation. Their outcrops are mainly located southwest of the Aviator Glacier along a belt striking roughly parallel to the local structural trends of the Ross Orogeny. Minor peraluminous sheets are also found in the southernmost Deep Freeze Range (Biagini et al., 1991). A R b - S r whole-rock isochron age of 481 -4- 10 Ma has been determined by Tonarini and Rocchi (1994) for this late orogenic peraluminous phase.

265

S. Rocchi et al./Tectonophysics 284 (1998) 261-281 Mountaineer Range - north-eastern area Si02=65-67 wt%

lOO

Mountaineer Range south-western a r e a Si02=65-70 wt%

100

10

lO

eli i K

1

RIh b Ba

T

h

= Ta

N

t b Ce

= Hf

t Zr

[ Sm

=Y=b Y

Southern Cross Mts. Si02=65-67 wt%

lO0

0.1

I

I

K

Rb

I

I

I

Ba Yh

I

Ta

I

Nb Ce

I

I

I

I

I

nf

Zr

Sm

Y

Yb

Deep Freeze Range, Terra Nova Intr. C., Prince Albert Mrs. Si02=69-74 wt%

100 -

10~

10-

1 -

.

0.1 K

b

Ba

T

Ta

b

Ce

.

.

Hf

.

Zr

.

0.1

.

Sm

Y

Yb

I

K

I

I

Rb Ba

I

t

Th

Ta

~

l

Nb Ce

I

~

I

J

I

Hf

Zr

Sm

Y

Yb

Fig. 3. Patterns of incompatible trace elements of selected intermediate-acidic rocks, containing >5% modal quartz, normalized to the abundances of the calculated Ocean Ridge Granite (ORG) of Pearce et al. (1984). Data source as in Fig. 2.

The geochemical signature of these products is summarized in Fig. 3, where the content of incompatible elements of intermediate and acidic rocks are normalized to the Ocean Ridge Granite (ORG). All the samples show a pattern typical of Volcanic Arc to Collisional Granites as defined by Pearce et al. (1984). Tables 1 and 2 report Sr- and Nd-isotopic data (both our new data and published data of others) for the study area. The distribution of 878r]86Sr initial isotopic ratios is reported in Fig. 4 together with available data on host metamorphic rocks (Armienti et al., 1990b; Talarico et al., 1995; this work). These ratios cover a wide, continuous range from 0.7040 to 0.7180. The emplacement age is not fully constrained for each pluton, but uncertainties on

the crystallisation age of 4-10 m.y. correspond, for metaluminous granitoids, to discrepancies in initial Sr ratios between 4-0.0001 and +0.0007 (depending on the Rb/Sr ratio), and can be neglected for the purposes of this paper. Corresponding maximum uncertainties on end are 4-0.1, well within the analytical error. In the conventional eyd vS initial 87Sr/86Sr diagram (Fig. 5), the samples define a roughly hyperbolic trend between a component akin to the Bulk Earth and the metamorphic basement cropping out in the area. This suggests that different sources were involved in the origin of GHI; in particular, the frequency of high 87Sr]86Sr and low eyd values, overlapping those of metamorphic rocks, and the most recurrent values of Sr isotopic ratio at 0.710 and of end at --8, put in evidence that recycling of the

266

S, Rocchi et al./Tectonophysics 284 (1998) 261-281

~

• .

.

.

.

.

.

~i

"

"

"

--

r ~ ; ~ - ~ r ~ r ~

?

I

I

!

I

I

I -/

i

~i

' I

I

~

-:

' I

~

--:

~r~ I

I I

I I I

'

/

II 2

Z-~ .=

7-

~

~

-

~ - ~ _

~

-

~

H <

444444 ,~,_~

~ _

~444

~4

# ~

@~

--

z~ z

=

d

oc

II

?

eq ~

L~ =

It ~

II

& 44444444

14444444444444

.... ~ , ~ , ~ , ~

~ 44 44 44 44 44

~ 44 44 44 44 44 1444+1

~ c ~ , ~

~

I

~

,

~

,

q

~

4444444444

4444444444

~

~ .

,

.

.

~~

.

.=

r- 3c

"

d~

~ =

E=

r~

,~

.

.~ 0

=

~

267

S. Rocchi et al./Tectonophysics 284 (1998) 261-281

¢,i I

I

I

I

I

eq ~6 e z ~

I

I

I

"~

H

H Z

oo~

~

z~

~

tt~

~

tt~

tt~

z

z~

~

:

~

o

~ .

. ~ o ~. o o o.~ o o .

. ~ .o ~ - ~ o o

_~

~

.....

~ - - .... H

268

S. Rocchi el al./Tectonophysics 284 (19981 261 281

.704 .706 .708 . 7 1 0 .712 .714

.716 .718

.720 .722

.724

.726

.728 .730

20 18 oo II 16 14

initial

(,/1

o.. 12

E

(/1

10

86Sr/87Sr

[]

metaluminous

association

[]

peraluminous

leucogranites

I

metamorphic

rocks

o I,.,.

E

8

6

4 2 0 •704

.706 .708 . 7 1 0

.712 .714

.716 .718

.720 i722 .724

.726 .728 .730

Fig. 4. Cumulative histogram of initial STSr/86Srof intrusive rocks, subdivided in main metaluminous association {calculated at 510 Ma) and the late peraluminous leucogranites (calculated at 485 Ma). The initial (510 Ma) ratios of high-grade metamorphic rocks of the Wilson Terrane are also reported. crust played an important role in the genesis of these magmas.

4. Isotopic and geochemical zoning across the margin In the northwestern portion of the Wilson Terrane of northern Victoria Land both the structural trends o f the Ross Orogeny and the suture W i l s o n - B o w e r s Terrane, which are presumably parallel to the ancient continental margin, strike N I40°E. Thus, possible patterns in regional distribution of geochemical and Sr, Nd isotopic data can be detected in an orthogonal transect striking N50°E. Isotopic ratios of Sr and Nd, projected onto this section, exhibit regular trends in a belt of about 80 km from the suture of the W i l s o n - B o w e r s Terrane towards the south-

west, down to the Tinker and Campbell Glaciers (Figs. 1 and 6). A similar behaviour is shown by several major and trace elements like K 2 0 , SiO> AleO3, Rb, Ce and some trace-element ratios (Rb/Zr, Th/Yb, Ta/Yb). On the contrary, no regular pattern is observed on this transect for data projected from the portion of Wilson Terrane south of the Campbell Glacier. This led us to explore, for this area, the possibility that regular variations of geochemical parameters could be observed along a section with a different orientation: indeed we found that on a transect oriented about E - W (N105°E), Nd isotopic ratios exhibit a regular trend. No direction was found lbr which the other geochemical parameters could define any kind of pattern. It is worth noting that the E - W direction is perpendicular to the structural trends of the Ross Orogen in other sectors of the

269

S. Rocchi et al./Tectonophysics 284 (1998) 261 281

• main metaluminous association o peraluminous leucogranites 4- metamorphic rocks

SNd 2 0 -2 -4 -6 -8

-lO '

-12 -14 .702

.706

.710

.714

.718

.722 initial

.726

.730

87Sr/86Sr

Fig. 5. Conventional SNd vs ~7Sr/86Srdiagram. The initial ratios are calculated as in Fig. 4.

margin, further to the south, and was adopted by Borg et al. (1990) for an analogous isotopic transect in the central Transantarctic Mountains. Figs. 6 8 report our data projected onto a section N50°E from the boundary between the Wilson and Bowers Terranes to the Tinker-Campbell Glaciers ( A - B segment of the transect, Fig. 1), and on a section N 105°E for the remaining belt of the Wilson Terrane towards the southwest ( B - C segment of the transect, Fig. 1). In the first part of the transect (A-B, Fig. 1) Sr isotopic compositions of GHI belonging to the main emplacement phase show an almost regular increase (0.705 to 0.714) with distance from the Wilson-Bowers Terrane boundary (Fig. 6). A similar behaviour was already observed in northern Wilson Terrane by Borg et al. (1987). In the section B - C of the transect (Fig. 1), 87Sr/86Sr shows small variations which are always within the range 0.7080.711. In the transect A-B, eNd (Fig. 6) gives the same indication found for Sr, with a regular decrease ( - 6 to - 1 2 ) . In the transect B - C , end regularly decreases ( - 4 to - 9 ) . Thus we may conclude that the narrow belt between the Campbell and Tinker

Glaciers, in which the pattern of increasing 87Sr]86Sr and decreasing end undergoes a sudden break, represents a first-order geological discontinuity, that is here called Tinker-Campbell Discontinuity (Fig. 1). This feature separates two distinct portions of the continental margin, with granites bearing different isotopic signatures: an Oceanward Side, towards the early Palaeozoic palaeo-Pacific Ocean, and a Continentward Side, towards the East Antarctic Craton (Fig. 1). Peraluminous leucogranites which exhibit the youngest ages among the GHI, are found astride the Tinker-Campbell Discontinuity and have the highest 87Sr/86Sr ratios (up to 0.730). Their end values are markedly lower than those of all the GHI in the Continentward Side, and fall at the end of the trend shown by the granites of the Oceanward Side (Fig. 6). Similar plots for major and trace elements show a larger degree of scatter, yet systematic patterns are still exhibited by some key elements in spite of superposition of complex processes such as fractional crystallisation and/or crustal contamination, which may occur during emplacement and cooling of these plutons. K20 and Rb contents of granitoids increase

270

S. Rocchi et al./Tectonophysics 284 (1998) 261 281

.7

30j

A

Ocean-Ward Side

,,,

,,,

,],,,

Continent-Ward

B

,u

,,,

i

i

i

i

i

i

I

i

i

Side C i

i

i

i

i

• • metaluminous association o D peraluminous leucogranites • Borg et al. (1987)

87Sr/SSSr j .725

.720 0 0 []

.715

[]

00

i

I

•

_>, ~,~pII ip "5, -m " I r-I

.710

.705

#

tO 0 U) C~

[ l I I l l l l l l l l l l l l

JQ ¢Z

E 0

-5

L G) J¢ ¢-

-6

4 •

-7

•

i

-8

II

1

•

•

•

-9 []

-10 0

-11

• "o -12 -13

'

0

'

,

,

'

'

'

I

40

,

,

,

i

,

,

,

i

80

,

,

,

,

,

,

I

,

,

,

,

,

,

'120

I distance along the transect A,B.C (km)~

,

160

S. Rocchi et al./Tectonophysics 284 (1998) 261-281

A

271

B Continent-Ward ~

Ocean-Ward Side

Side C

TKzO/SiOz i o

mm

.09 o~.

• .07

•

•

•

E_ umunn

•

O~

00

mm

0 C

•

•

[] minim

•~O •0 lqD~OOO • O lie Illn• ~ •:

•

.05

II •

•

oo 0

.03 •

.O1

•

•

l l

I •

•

•

• o

,

0

•

.

i

.

.

,

!

I

.

,

l

l

i

40

.

,

I

80

I

i

#

,

,

,

I

.

,

1 20

i

,

,

p..

.

i

160

distance along the transect A.B-C (kin) Fig. 7. K20/SiO2 of intrusive rocks versus distance along cross-section A - B - C of Fig. I. Filled symbols: main metaluminous association. Symbols as in Fig. 6. Data source as in Fig. 2.

inland in the A - B section of the transect (Oceanward Side, Fig. 7), whereas a random distribution is observed for the Continentward Side. Geographic regularity in the Oceanward Side is also preserved when K20 and Rb contents are normalized to SiO2 (Fig. 7) or Zr, to minimize variations due to the degree of evolution. Further evidence for geochemical zoning along the transect A - B is shown in Fig. 8. Negative Ba anomaly as displayed in Fig. 3 is quantified by the

Ba/Ba* ratio, where Ba* is interpolated between Rb and Th in the ORG-normalized spidergrams. Ba/Ba* plotted versus the geographic position (Fig. 8) varies from 0.85 to 0.05. This variation can be accounted for by an inland increasing involvement of crustal partial melts: (1) in fact, simple batch melting models of Upper Crust source (composition and mineralogy after Taylor and McLennan, 1995), using partition coefficients after Henderson (1982) and Nash and Crecraft (1985) indicate that low fraction melts

Fig. 6. Initial 87Sr]86Sr and SNd of intrusive rocks versus distance as projected along cross-section A - B - C of Fig. 1. This diagram shows the relationships between the geographic position and the isotopic composition of the Granite Harbour Intrusives of the Wilson Terrane. Isotope ratios for literature samples from the study area for which full isotope data and location are reported (Borg et al., 1987), were recalculated at 510 Ma and plotted in the figure.

272

S. Rocchi el al./TeclolToph)'sics 284 (1998) 261 281

A

Ocean-Ward S i d e

BContinent-Ward

I

T

Ba/Ba*

,oo- S i T z

•

•

2.8

7,

2.4

~

2.0

0 •

I

~-

" •"O O0

8

O •

.

0

•

•

•

im,l, "o

.4

O1 i l

0

J

40

•

•

•

a l =•• I1 0

;

•

• •

II

1.2

0

C

•

~

1.6

Side

•

1

•

I'll 0

lUl I-

2

"• ~l

80

I distance along the transect A-B-C

I

•

•

120

i-....160

(km)

Fig. 8. Ba anomaly in the ORG normalized patterns {sec Fig. 3). The Ba anomaly is quantified by' dividing the ORG-normalized Ba content of sample by the calculated Ba* value by interpolation between Rb and Th (ORG-normalized). The value of Ba/Ba* ratio has been normalized to the SiO2 content of the sample to minimize the effect of fractional crystallisadon. Symbols as in Fig. 6. Data source as in Fig. 2.

of the crust will have Ba/Ba* < 0.10; (2) despite of the non-Henrian behaviour of Th during melting of crustal protoliths (Bea, 1996), the enrichment of Rb and depletion of Ba in pelite-derived partial melts (Harris and Inger, 1992) point to a high Rb/Ba, which in turn yields a low Ba/Ba*; (3) some of the peraluminous leucogranites from Wilson Terrane have Ba/Ba* values of 0.03 to 0.07 (Fig. 8). Thus, an increasing involvement of crustal partial melts in the continentward direction may be invoked to explain the geographic patterns of trace elements in the Oceanward Side of the margin. Along the transect B - C (Continentward Side), no correlation between chemical composition and distance from the discontinuity is observed. This sug-

gests that different petrogenetic mechanisms were acting in these two portions of the margin. S m - N d model ages relative to depleted mantle (TDM: DePaolo et al., 1991) are dispersed over a rather wide range (1.6-2.1 Ga, Tables 1 and 2) for both portions of the margin. However, in this region granitoid genesis is mainly related to the interaction of two distinct components of crustal and subcrustal origin (Armienti et al., 1990b): on account of this, model ages must be regarded as mixed ages (Arndt and Goldstein, 1987), with the highest values representing lower limits for the time of crustal residence of the older (= crustal) end-member. Maximum Nd model ages for Oceanward Side granites point to a (minimum) Proterozoic age of 2.1 Ga for the crust

S. Rocchi et al./Tectonophysics 284 (1998) 261-281 Table 3 Summary of geochemical features of main phase GHI from the two portions of the margin

STSr/86Sr end K20/SiO2 Rb (ppm)/SiO2 (wt%) 100 (Ba/Ba*)/SiO2 TDM (Nd)

Oceanward Side

Continentward Side

0.705 ~ 0.714 --6 -~- -- 12 0.02 ~ 0.08 1.0 ~ 4.5 2.1 ~ 0.05 1.7 2.1 Ga

0.708 0.711 --4 +-- --9 0.02 0.10 (no trend) 0.5-4.0 (no trend) 0.0-3.5 (no trend) 1.6-1.9Ga

involved in the genesis of these granites. TDM for Continentward Side granites, are 1.9 Ga, close to the model age of high-grade metamorphic rocks from Deep Freeze Range, which cluster around 1.9-2.0 Ga (Table 2). From these data we infer that the crust involved in the genesis of GHI is early Proterozoic in age in both portions of the margin. Table 3 summarizes the main geochemical features of the main phase GHI of the Wilson Terrane. The crustal contribution to the genesis of granites distinctly increases towards the East Antarctic Craton in the Oceanward Side of the margin. To the southwest, in the Continentward portion of the margin, only the Nd isotope ratio shows a regular geographic variation, on a section striking about E W; here, Sr isotopic compositions cluster around the value 0.710, while all the other parameters are scattered. 5. Discussion

The main differences between Oceanward and Continentward Side magmas consist in the larger variability, among the former, of Sr and Nd isotope ratios and their correlation with the distance along the A - B portion of the transect (Fig. 6). The larger geochemical variability of Oceanward Side granites is coupled with a rough correlation with the SiO2 content on this side of the margin (Fig. 9). Moreover, among granitoids from the Oceanward Side, the high STSr/86Sr ratios partially overlap with those of metamorphic rocks and peraluminous magmas (Fig. 4): this condition may be achieved only by extensive interaction of mantle-derived melts with the crust or with crustal melts, as also suggested by the behaviour of the Ba anomaly.

273

In the Continentward Side, the Sr isotopic composition is more uniform: mafic rocks have SVSr/86Sr around 0.709, a rather high value for gabbroic rocks with SiO2 49-56 wt%; evolved acidic rocks at most reach a SVSr/86Sr value of 0.711 (Table 2, Fig. 9). Moreover, we have to consider that this limited variability of Sr isotopic ratios is associated with a changing Nd isotope composition, which exhibits a well defined polarity, with a decrease of end towards the craton. We conceive that these variations could be the result of a process which is similar to that observed in the Oceanward Side; however, the different distribution of initial eyd and 87Sr/S6Sr for Continentward Side granitoids (Tables 1 and 2), implies that 87Sr/S6Sr of the crustal component would not be very high, probably about equal to the lower values observed among the metamorphic rocks of the crystalline basement of the area (~0.710-0.711; Table 1). We therefore suggest that also on the Continentward Side a subcrustal magma could have progressively interacted with the increasing distance from the margin, but that in this area the contaminant had a different nature. The constraints for this crustal component are represented by the isotopic composition of 87Sr/S6Sr of about 0.710-0.711 and end about --9, and by the Nd model age of the northern Victoria Land crust, around 1.9 Ga. Mafic granulite relics from the Deep Freeze Range (Talarico et al., 1995) show an average 87Sr/86Sr of 0.712, eNa of -8.1 and Nd model age of 1.8 Ga (Table 2), making these products suitable as the lower crustal end-member. This crustal material which interacted with mantle melts during the Ross Orogeny, probably derived from the mantle reservoir and was added to the crust by underplating during early Proterozoic time (around 2 Ga). In fact, taking the elemental abundances reported by Condie (1993) for the early Proterozoic basalts, at the time of the Ross event (here assumed to be 500 Ma) the isotopic compositions of the lower crust had reached values for ENj of - 8 . 4 and 87Sr/86Sr about 0.709. These figures are compatible with the composition of the mafic granulites that could have interacted with mantlederived melts. Depleted felsic granulites from the Deep Freeze Range (Talarico et al., 1995) proved unsuitable as the lower crust component, because of their highly radiogenic Sr isotopic composition (SVSr/~6Sr >0.720, Table 2).

S. Rocchi et aim /Tectonophysics 284 (1998) 261 281

274

,720

_Ocean-Ward

Continent-Ward

Side

Side_

0 0 .716 [] •

.712

0

0

°

ee

• •

OtlII•

ml

•

•

~

ml

Ii

•

i m mmll

•

mm

.708

.704

,

,

,

,

, ,

,

,

,

, ,

,

,

,

,

, ,

,

,

,

,

, ,

,

,

,

, ,

,

,

,

,

, ,

i

D

l

l

m

l

l

l

l

m

m

l

m

l

m

l

l

m

m

l

m

l

i

l

l

l

m

r

l

F

i

m

l

l

•

0 -2 -4 -6 •

e lla e

-8

•

oO • O•

-10

n

o

0

oo

-12 -14

,

45

,

,

,

,

50

,

,

,

,

,

55

,

,

,

,

,

60

,

,

,

,

,

65

,

,

,

,

,

70

,

,

,

,

,

,

75

,

,

,

Si02

,

,

,

,

,

so

,

,

,

,

,

55

,

,

,

,

,

oo

,

,

,

,

,

os

,

,

,

,

,

70

,

,

,

,

,

7s

,

,

,

,

80

Fig. 9. Initial STSr/S~'Srand/"Nd of intrusive rocks versus SiO2 for Ihe two portions of the continental margin. Symbols as in Fig. 6.

An alternative explanation for the regular variation of ENd in the Continentward Side would take into account the possible existence there of a crustal structure with regular variations in lower crust age. However, this hypothesis conflicts with the lacking of any regular variation of Nd model age (calculated according to Faure, 1986) along the B - C transect. The sharp break in the geochemical polarity of GHI astride the Tinker-Campbell Discontinuity, coupled with the change in direction from N E - S W to E-W, is interpreted here as the expression of a tectonic discontinuity between two continental blocks. This discontinuity trends parallel to the shieldplatform transition traced by Roland (1991), with a rough N150°E orientation ( D - E line in Fig. 1): this, in agreement with the different petrogenetic models invoked for the Oceanward Side and the Continentward Side, could imply different thicknesses for the adjoining crustal blocks. Moreover, Biagini et al. (1991) report the occurrence, along the northeastern side of Campbell Glacier, of a high angle shear zone,

of late Ross age, extending along 30 km (F-G line in Fig. I). In addition, this zone may be interpreted as lying on the prolongation of the Wilson Thrust, described by Fl6ttmann and Kleinschmidt (1991) in an adjacent area of northern Victoria Land. Two different hypotheses can be pul forward in order to explain the discontinuity: it may arise from the accretion of an exotic terrane or by the movement along the margin of a sliver belonging to the continental margin itself. Geological and geochemical evidences for a terrane accretion such as terrane suture, contrasting sedimentary-metamorphic history of the basement, and strongly differing model ages were not observed, with the only evidences being represented by the occurrence of contrasting geochemical polarities. The difference /k)und by Borg el al. (1990) and Borg and DePaolo (1994) for the crustal provinces of other sectors of the Transantarctic Mountains are not observed here and the Continentward Side and Oceanward Side crustal sectors share a similar basement with uniform maximum

S. Rocchi et al./Tectonophysics 284 (1998) 261 281 present-day ~ u s North

I/ /

275

!

tfS//8 \

J

I

/ I

/NVZ

rfD / / / / /

oenirs/ T A M

palaeoPacific

plate

Fig. 10. Sketch of the geodynamic model accounting for the contrasting geochemical polarities found in Granite Harbour Intrusives from the Wilson Terrane of northern Victoria Land. The marked area labelled NVL (northern Victoria Land) corresponds to the area of Fig. I. The dashed line labelled TCD corresponds to the Tinker-Campbell Discontinuity. Small grey arrows indicate the inferred direction of subduction before block displacement for NVL (this work) and for the central Transantarctic Mountains (TAM,Borg et al., 1990). Small white arrows in NVL indicate geochemically inferred direction of subduction after shift and rotation of the Oceanward Side of the margin (the sliver) indicated by the black thick arrow. Nd model ages. Thus the hypothesis of a sliver is preferred. In our model (Fig. 10), the G o n d w a n a margin was affected by oblique subduction, and an Oceanward portion (the sliver) of this margin was displaced and rotated anticlockwise by some 50 °, i.e. the angular difference between the B - C and A - B segments o f the transect (Fig. 1). The generation of forearc slivers belonging to a continental margin and moving along the margin itself, is a process which is largely documented along active continental margins: forearc slivers occur in about 50% of modern continental margins over subduction zones (Jarrard, 1986). Particularly, in the central Transantarctic Mountains, the occurrence of significant orogen-parallel displacement of crustal

blocks during the early Palaeozoic is invoked by Rowell and Rees (1989), Borg and DePaolo (1994) and Encarnaci6n and Grunow (1996). The displacement of these blocks likely happened through a series of large strike-slip faulting or strike-slip partitioned transpressive movements (Teyssier et al., 1995). The occurrence of a belt of peraluminous granitoids in the area of the T i n k e r - C a m p b e l l Discontinuity (Biagini et al., 1991) may be related to a continental magmatic arc that weakened the crustal theology, making the movement of the sliver easier. In this case the age of the leucogranites (481 ± 10 Ma) could be related to the age of the sliver motion along the continental margin. In our model (Fig. 10) the shifting and rotation

276

S. Rocchi el al./Tectonophysics 284 (1998) 261-281

of the sliver were enhanced by obliquity of convergence. This type of process is asked for by several authors in various sectors of the Transantarctic Mountains (e.g., Goodge et al.+ 1991, 1993; Janosy and Wilson, 1995; Storey et al., 1996). Particularly, the sinistral oblique subduction invoked by Goodge and Dallmeyer (1996) for the central Transantarctic Mountains at 540-500 Ma (a time span encompassing our main emplacement phase) and by Encarnaci6n and Grunow (1996) may well account for the anticlockwise rotation/shift of the Oceanward Side of the margin. Finally, the faster uplift rate documented by Goodge and Dallmeyer (1996) in the Lanterman Range (northeastern Wilson Terrane, Oceanward Side) with respect to the central Transantarctic Mountains, might be related to the weak link of this area with the southernmost cratonic sectors of southern Victoria Land and the central Transantarctic Mountains. As a variation of our model we consider the possibility that the sliver underwent a simple rotation, with only minor relative shift, in analogy with the situation presently observed in the southern Andes (Cembrano et al., 1996). However, this alternative mechanism would involve the occurrence of a buttress, and a main component of the subduction from the north, in conflict with the polarity of Nd isotope ratios observed within the Continentward Side of the margin and further south in the central Transantarctic Mountains (Borg et al., 1990), and is therefore unsuitable.

6. Summary and conclusions The regional distribution of geochemical and SrNd isotopic data for Granite Harbour Intrusives of

northern Victoria Land reveals a sharp discontinuity (the Tinker-Campbell Discontinuity), roughly coinciding with a belt striking N140°E, where peraluminous granites are dominant. The two portions of the northern Victoria Land crust thus identified, belong to the same early Palaeozoic active continental margin bordering the East Antarctic Craton, as suggested by the similar Nd model ages and by the absence of any geologic evidence of allochthony. The margin was affected by oblique subduction and the Oceanward Side moved as a tectonic sliver along the Tinker-Campbell Discontinuity (Fig. 10). Leftlateral displacement of this block was associated with an anticlockwise rotation, so that along the Tinker-Campbell Discontinuity two portions of the magmatic arc with different geochemical polarities resulted in contact. At the end of the early Palaeozoic Ross Orogeny, the belt astride the Tinker-Campbell Discontinuity resulted as a crustal weakness zone, that was reactivated during the Mesozoic-Cainozoic rifting process in the Ross Sea, when a long-lasting (Eocene to Present) magmatic activity took place just in the discontinuity zone (Tonarini et al., 1997).

Acknowledgements This work is financially supported by the Italian PNRA (Programma Nazionale di Ricerche in Antartide). S.G. Borg and B.-M. Jahn are gratefully acknowledged for their thoughtful and accurate review of the manuscript, which led to improvement and better clarity of the paper.

S. Rocchi et al./Tectonophysics 284 (1998) 261-281

Appendix A. Sample locations and coordinates Latitude (south)

Longitude (east)

7 3 ° 3 7 ' 1 0 '' 73°41'21" 73042'28" 73"27'16" 7 3 0 2 4 ' 3 6 '' 73°24'19" 73°52'39" 7 3 0 4 4 ' 5 4 '' 73°53'31 '' 7 3 0 4 2 ' 3 9 '' 73°34'47" 73016'49 "

165027'24 '' 165025'09 '' 165°47'11" 165°59'31" 166°09'51 '' 166027'37 '' 165°45'48" 165023'45" 165021'32" 165°32'01 '' 165°22'31" 165°58'36 ''

Mountaineer Range 14-1-87 AB7 30-1-88 AC5 3 0 - 1 88 A C 3 2 2 - 1 - 8 9 AC1 5-1-89 BD16 9-1-89 GH13 7-2-89 AG7 7-2-89 AGI0 7-2-89 AG12 1 1 - 2 - 8 9 AH1 11-2-89 AH3 29-1-90 MO16

Geologist Ridge low Aviator G l a c i e r Mr. C a s e y Fitzgerald G1. W side Mt. M u r c h i s o n top east o f Mt. M u r c h i s o n A n d r u s Point south o f M t . M o n t e a g l e C a p e Sibbald Mt. B r a b e c Mt. B r a b e c south o f M t . Kinet

277

Appendix A (continued) Latitude (south)

Longitude (east)

75001'21 '' 75°01'21 '' 75°01'21" 74°10'49" 74°22'18" 73°58'31 '' 7 4 o 5 0 ' 0 0 '' 74°50'00" 74050'00 '' 74050'00" 74050'00"

162°37'02 '' 162°37'02" 162°37'02 '' 162043 , 17" 162°33'00 '' 161047'48 '' 162°35'00" 162o35'00 '' 162°35'00 '' 162°35'00 '' 162035'00"

73o35'52 '' 74°01'35" 73o56'30" 73°51'10" 74002'38 '' 74023'50 '' 7 3 o 4 3 ' 2 0 '' 74°24'20" 74024'45 " 7 4 ° 2 5 ' 1 5 ''

163°49'43 '' 165°16'50 '' 164°32'15" 164°54'45 '' 165°04'01" 165°47 ' 1 5 " 162°39'40 '' 165°47'35 '' 165°46'20 '' 165048'05 ''

Prince Albert Mountains 18-1-86 18-1-86 18-1-86 23-1-86 27 1 - 8 6 21-1-87 LZ-23 LZ-27 LZ-50 LZ-57 LZ-58

B2 B5 B6 GI8 B3 FP21

Mt. G e r l a c h e Mt. G e r l a c h e Mt. G e r l a c h e Timber Peak Mt. B a x t e r Ogden Heights Teall N u n a t a k Teall N u n a t a k Tealt N u n a t a k Teall N u n a t a k Teall N u n a t a k

Peraluminous leucogranites Southern Cross Mountains (Tinke~Aviator Glaciers) 25-1-88 25-1-88 25-1-88 25-1-88 25-1-88

CMll CM31 CM34 CM36 CM46

Co-Pilot G l a c i e r Arrowhead Range Stewart Heights Daughtery Peaks D a l e y Hills

7 3 0 1 2 ' 2 6 '' 7 3 ° 2 6 ' 5 8 '' 73030'02 '' 73°29 ' 1 5 ' ' 73°35'37"

1 6 4 ° 2 3 ' 2 0 '' 164012'00 '' 163°57'09 '' 164020'57" 164053'49"

Southern Cross Mountains (Tinker-Campbell Glaciers) 13-2-86 B 1 13-2-86 13-1-88 13-1-88 13 1 - 8 8 13-1-88 13-1-88 13-1-88

B8 AMI AM4 AM6 AM7 AM24 AM26

P i n c k a r d Table R a n d o m Hills Mt. J i r a c e c k Mt. J i r a c e c k Mt. J i r a c e c k Mt. J i r a c e c k Mr. J i r a c e c k Mt. Jiraceck

74~04'36 '' 7 4 ° 0 9 ' 5 3 '' 73°45'55" 7 3 ° 4 5 ' 4 0 '' 73'45'40" 7 3 ° 4 5 ' 4 0 '' 7 3 ° 4 3 ' 4 7 '' 7 3 ° 4 5 ' 3 9 ''

164°04'42 '' 164°26'43 '' 163056'40 '' 163°55'13 '' 163°55'13 '' 163°55'13 '' 163047'58 '' 163040'20 ''

74°31'43" 7 4 ° 2 8 ' 2 3 '' 74015'49" 7 4 ° 0 1 ' 4 0 '' 73051'49 '' 74024'03 '' 73°46'13" 7 3 ° 4 6 ' 1 3 '' 73°46'13" 73042'52"

163°47'30" 163°30'30 '' 163°25'17 '' 164031'08" 163°10'58 '' 163040'30 '' 161041'36" 1 6 1 ° 4 1 ' 3 6 '' 161041'36" 1 6 3 ° 0 0 ' 0 2 ''

7 4 ° 5 3 ' 0 7 '' 7 4 ° 4 0 ' 3 8 '' 74052'25 '' 74043'23 '' 7 4 ° 4 4 ' 5 2 '' 7 4 ° 4 4 ' 4 4 '' 74°46'05" 74°53'15" 74°53'10"

163044'37 '' 1 6 4 ° 0 3 ' 2 0 '' 1 6 4 ° 3 5 ' 2 0 '' 1 6 3 ° 4 2 ' 1 2 '' 163043 ' 1 3 " 164°06'37" 1 6 3 ° 5 5 ' 5 6 '' 1 6 4 ° 3 6 ' 1 0 '' 163044'20"

Deep Freeze Range 9 - 1 - 8 6 L21 2 1 - 1 - 8 6 B2 23-1-86 Gl6b 13 2 - 8 6 B I 0 1 4 - 1 - 8 7 FP51 15-1-87 AB2 21 1 - 8 7 AB1 21-1-87 AB2 21-1-87 AB5 28-1-87 AB5c

Boomerang Glacier Black Ridge Howard Peaks Mt. E m i s o n Mt. M a n k i n e n Howard Peaks Szanto Spur Szanto Spur Szanto S p u r Archambault Ridge

Terra Nova Intrusive Complex 26-12-85 LI-10 28 1 2 - 8 5 L I 6 2 6 - 1 2 - 8 6 BP1 7 - 1 - 8 6 L3 7 1 - 8 6 L5 2 5 - 1 - 8 6 L11 2 6 - 1 - 8 6 L3 12-1-88 CFI 12-1-88 CF17

Terra N o v a B a y station Gerlachelnlet Inexpressible Island Cape Canwe Cape Canwe N o r t h e r n Foothills N o r t h e r n Foothills Inexpressible Island Inexpressible Island

2 2 - 1 86 L l 2 17-1-87 AB12 29-1-88 C6c 3 1 - 1 - 8 8 CI 3 1 - 1 - 8 8 C3 27-12-86 AF16 2 8 - 1 - 8 7 A B 12 2-1-88 CF10 2-1-88 CF25 3-1-88 AM62

Schulte Hills Hayes Head Tinker G l a c i e r Hayes Head Hayes Head Boomerang Glacier Recoil G l a c i e r Boomerang Glacier Boomerang Glacier Boomerang Glacier

S. Rocdii el ~d./Tectmiophysic.s" 284 (1998) 261-281

278

~ , ~ - ~ ' ~ - , ~ ~ _ _

~

~_~,~

-

~,~'=~,

u,-, e l

r-

,,o o c m

_,-g e-i

~t

i_

~

-1 e : ~2 - : ~

~

~,h ~g - : ~ !

oq

~ . . . . . . . . . . .. . .

,g

r--~-

.~ . . . .

~

. . . .o ~ , ~ -

~

"

?

_~

-

....

~

_

~

-

t-~

~

~ ,.,c,

@

r-i

--

cq

~c

.;

..~

~,~-~=

.......

~-

: . ~ . ~ - ~ , ~ _ ~ _ ~ . ~ o

~.

e~

<

z ~ ' = ~

~

>

0

~~ -

=

~

>z ~

~

=

5= ~

£

_

~

.

i

~

279

S. Rocchi et al./Tectonophysics 284 (1998) 261 281

- ~ - ~ ~ - ~

,

~

-

~

~

"

=

~

~

-

~

-

~

~

..~

~,_~

-~

-

~

~

~

~ . _ c -~

.~.~

-

~ ,"5

. ~ = ~.~,

~

~P2

•r-.= g

~z

~

-

~

,

~

~

_

~,

e© ~ o ~ - ~ ~ = - _

~_

~_

~

~

=

_

~

= ~ = _ N,,, ~

e~ e~

II II ~"

280

S. Rocchi el al./Tectonophysics 284 (1998) 201 281

References Armienti, E, Ghezzo, C., lnnocenti, E.. Manetti, P.. Rocchi. S., Tonarini, S., 1990a. Granite Harbour Intrusives from North Victoria Land between David and Campbell Glaciers: new geochronological data. Zentr. Geol. Pal/iontol. I, 63 74. Armienti. E, Ghezzo, C.. lnnocenti, F., Manetti, P, Rocchi, S., Tonarini, S.. 1990b. Isotope geochemistry and petrology of granitoid suites from Granite Harbour lntrusives of the Wilson Terrane, North Victoria Land. Antarctica. Eur. J. Mineral. 2. 103-123. Arndt, N.T.. Goldstein. N.L., 1987. Use and abuse of crust formation ages. Geology 15,893-895. Bea. F., 1996. Residence of REE, Th, Y and U in granites and crustal protoliths: implications for the chemistry of crustal melts. J. Petrol. 37. 521-552. Bennett, V.C., DePaolo, D.J.. 1987. Proterozoic crustal history of the western United States as determined by neodymium isotopic mapping. Geol. Soc. Am. Bull. 99, 674-685. Biagini, R.. Di Vincenzo, G., Ghezzo. C., 1991. Petrology and geochemistry of peraluminous granitoids from Priestley and Aviator Glacier region, Northern Victoria Land (Antarctica). Mem. Soc. Geol. ltal. 46 (1989), 205-230. Borg, S.G.. DePaolo. D.J., 1991. A tectonic model of the Antarctic Gondwana margin with implications lk)r southeastern Australia: isotopic and geochemical evidence. Tectonophysics 196. 339-358. Borg, S.G., DePaolo, D.J., 1994. Laurentia, Australia, and Antarctica as a late Proterozoic supercontinent: constraints from isotopic mapping. Geology 22. 307-310. Borg, S.G., Stump. E.. Chappell. B.W.. McCulloch. M.T., Wyborn. T., Armstrong, R.L., Holloway, J.R.. 1987. Granitoids of Northern Victoria Land, Antarctica: implications of chemical and isotopic wtriations to regional crustal structure and tectonics. Am. J. Sci. 287, 127 169. Borg. S.G.. DePaolo, D.J.. Smith, B.M.. 1990. Isotopic structure and tectonics of the Central Transantarctic Mountains. J. Geophys. Res. 95. 6647-6667. Borsi. L., Ferrara. G., Tonarini. S., 1989. Geochronological data on Granite Harbour lntrusives from Terra Nova Bay and Priestley Glacier. Victoria Land, Antarctica. Mem. Soc. Geol. ltal. 33 (1987). 161-169. Bradshaw. J.D., 1989. Terrane boundaries in North Victoria Land. Mem. Soc. Geol. ltal. 33 (1987), 9-15. Bradshaw, J.D,, Weaver. S.D.. Laird. M.G.. 1985. Suspect terfanes and Cambrian tectonics in Northern Victoria Laud, Antarctica. In: Howell, D.G. (Ed.), Tectonostratigraphic Terranes of the Circum-Pacific Region. Circum-Pacific Council for Energy and Mineral Resources. Earth Science Series. Houston, TX. pp. 467-479. Carmignani, L., Ghezzo, C., Gosso, G.. Lombardo, B.. Meccheri. M.. Montrasio, A., Pertusati, EC.. Salvini, F.. 1989. Geological map of the area between David and Mariner Glaciers, Victoria Land (Antarctica). Mem. Soc. Geol. hal., 33 (1987). Cembrano, J., Herv6. F., Lavenu. A., 1996. The Liquine-Ofqui fault zone: a long-lived intra-arc fault system in southern Chile. Tectonophysics 259.55 66.

Condie, K.C., 1993, Chemical composition and evolution of the upper continental crust: contrasting results from surface samples and shales. Chem. Geol. 104, 1 37. I)alziel, I.W.D,, 1991. Pacilic Margin of Laurentia and East Antarctica Australia as a conjugate rift pair: evidence and implications lk)r an Eocambrian supercontineni. Geology 19. 598-601. Dalziel, I.W.D.. 1992. Antarctica: a tale of two supercontinents?. Annu. Rev. Earth Planet. Sci. 20, 501 526. Dalziel. I.W.D., Dalla Salda, L.H., Gahagan, I,.M.. 1994. Paleozoic Laurentia (]ondwana interaction and the origin of the Appalachian-Andean mountain system. Geol. Soc. Am. Bull. 106, 243-252. DePaolo. D.J.. Linn. A.M.. Schubert, G., 1991. The continental crustal age distribution: methods of determining mantle separation ages Dom Sm-Nd isotopic data and application io the south-western United States. J. Geophys. Res. 96, 2071-2088. Di Vincenzo. G.. Rocchi. S., Ghezzo, C., Andriessen, RAM.. 1997. Malic and fclsic magmas in the Terra Nova Intrusive Colnplex (Northern Victoria Land. Antarctica). In: Ricci, C.A. (Ed.), The Antarctic Region: Geological Evolution and Processes. Terra Antartica Publication, Siena(in press). Encarnaci6n. J., Grunow, A., 1996. Changing magmatic and tectonic style akmg the paleo-Pacific margin of Gondwana and the onset of early Paleozoic magmatism in Antarctica. Teclonics 15, 1325 1341. Farmer, G.L.. DePaolo, D.J., 1983. Origin of Mesozoic and Tertiary Granite in the W US and implications for Pre-Mesozoic Crustal Structure. 1. Nd and Sr Isotopic studies in the geocline of N. Great Basin. ,1. Gcophys. Res. 88, 3379-3401. Faure. G., [986. Principles of Isotope Geology. Wiley, Ncw York. 589 pp. Fl6ttmann. T., Kleinschmidt. G., 1991. Opposite thrust systems in northern Victoria Land. Antarctica: imprints of Gondwana Palcozoic accretion. (]eology 19, 45 47. Franzini, M., Leoni, L., Saitta, M., 1975. Revisione di una metodologia analitica per t]uorescenza-X, basata sulla c o l rezione completa degli effetti di matricc. Rend. Soc. [la]. Mineral. Petrol. 31,365 378. GANOVEX - RN.R.A. ltaliAntartide, 1993. Geological and structural map of the area between the Aviator (]lacier and Victory Mountains. Northern Victoria Land, Antarctica. LAC, Firenze. Goodge, J.W.. Dallmeyer. R.D.. 1996. Contrasting thermal evo lution within the Ross Orogen, Antarctica: evidence from nfineral 4°Ar/;gAr agcs. J. Geol. 104. 435-458. Goodge, J.W.. Borg. S.G.. Smith, B.K., Bennett. V.C.. 1991. Tectonic significance of ductile shortening and translation along the Antarctic margin of Gondwana. Earth Planet. Sci. Lett. 102.58-70. Goodge. J.W., Hansen, V.L., Peacock, S.M., Smith, B.K., Walker. N.W.. 1993. Kinematic evolution of the Miller Range shear zone, central Transantarctic Mountains, Antarctica, and implications for Neoproterozoic to early PaIeozoic tectonics of the East Antarctic margin of Gondwana. Tectonics 12. 14601478. Gunn, G.M.. Warren, G., 1962. Geology of Victoria Land be-

S. Rocchi et al./Tectonophysics 284 (1998) 261-281 tween Mawson and Mullock Glaciers, Antarctica. N.Z. Geol. Surv. Bull. 71, 85-100. Harris, N.B.W., Inger, S., 1992. Trace element modelling of pelite-derived granites. Contrib. Mineral. Petrol. 110, 46-56. Henderson, E, 1982. Inorganic Geochemistry. Pergamon Press, London, 353 pp. Janosy, R.J., Wilson, T.J., 1995. Tectonic setting of the early Paleozoic dike swarm in southern Victoria Land. 7th Int. Syrup. Antarctic Earth Sciences. Siena (Italy), 10-15 September 1995, Abstr., p. 212. Jarrard. R.D., 1986. Terrane motion by strike-slip faulting of forearc slivers. Geology 14, 780-783. Kleinschmidt, G.. Tessensohn, F., 1987. Early Paleozoic westward directed subduction at the pacific margin of Antarctica. 6th Gondwana Symp., AGU, pp. 89-105. Leoni, L., Saitta, M., 1976. X-ray fluorescence analyses of 29 trace elements in rock and mineral standard. Rend. Soc. Ital. Mineral. Petrol. 32, 497-510. Moores, E.M., 1991. Southwest U.S.-East Antarctic (SWEAT) connection: a hypothesis. Geology 19, 425 428. Musumeci, G., Capponi, G., Meccheri. M., Oggiano, G., 1994. Deformed lntrusives along the Wilson-Bowers Terrane boundary (northern Victoria Land): structural features and tectonic evolution. Terra Antart. 1, 54-58. Nash, W.P., Crecraft. H.R., 1985. Partition coefficients for trace elements in silicic magmas. Geochim. Cosmochim. Acta 49, 2309-2322. Pearce, J.A., Harris, N.B.W., Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of the granitic rocks. J. Petrol. 25. 956 983. Poli, G.. Manetti, P., Peccerillo, A., Cecchi, A.. 1977. Determinazione di alcuni elementi del gruppo delle Terre Rare in rocce silicatiche per attivazione neutronica. Rend. Soc. ltal. Mineral. Petrol. 33, 755-763. Rocchi, S., Di Vincenzo, G., Ghezzo, C.. 1997. Geopetrographic map of Terra Nova Intrusive Complex (northern Victoria Land, Antarctica). In: Ricci, C.A. (Ed.), The Antarctic Region: Geological Evolution and Processes. terra Antartica Publication, Siena (in press).

281

Roland, N.W., 1991. The boundary of the East Antarctic craton on the Pacific margin. In: Thomson, M.R.A., Crame, J.A., Thomson, J.V. (Eds.), Geological evolution of Antarctica. Cambridge University Press. Cambridge, pp. 161-165. Rowell, A.J., Rees, M.N., 1989. Early Palaeozoic history of the upper Beardmore Glacier area: implications for a major Antarctic structural boundary within the Transantarctic Mountains. Antarct. Sci. 1,249-260. Storey, B.C., Macdonald, D.I.M., Dalziel, I.W.D., lsbell, J.L., Millar, I.L., 1996. Early Paleozoic sedimentation, magmatism and deformation in the Pensacola mountains, Antarctica: the significance of the Ross Orogeny. Geol. Soc. Am. Bull. 108, 685-707. Streckeisen, A., Le Maitre, R.W., 1979. A chemical approximation to the modal QAPF classification of igneous rocks. Neues Jahrb. Mineral. Abh. 136, 169-206. Stump, E., 1995. The Ross Orogen of the Transantarctic Mountains. Cambridge University Press. Cambridge, 284 pp. Talarico. F.. Borsi, L., Lombardo, B., 1995. Relict granulites of the Ross Orogen of northern Victoria Land (Antarctica), II. Geochemistry and paleo-tectonic implications. Precambrian Res. 75, 157-174. Taylor, S.R., McLennan, S.M., 1995. The geochemical evolution of the continental crust. Rev. Geophys. 32, 241-265. Teyssier, K., Tikoff, B., Markley, M., 1995. Oblique plate motion and continental tectonics. Geology 23, 447-450. Tonarini, S., Rocchi, S., 1994. Geochronology of CambroOrdovician intrusives in northern Victoria Land: a review. Terra Antart. 1, 46-50. Tonarini, S,, Rocchi, S., Armienti, E, Innocenti, F.. 1997. Constraints on timing of Ross Sea rifting inferred from Cainozoic intrusions from northern Victoria Land, Antarctica. In: C.A. Ricci (Ed.), The Antarctic Region: Geological Evolution and Processes. Terra Antartica Publication. Siena (in press). Weaver, S.D., Bradshaw, J.D., Laird, M.G., 1984. Geochemistry of Cambrian volcanics of the Bowers Supergroup and implication for early Paleozoic tectonic evolution of Northern Victoria Land, Antarctica. Earth Planet. Sci. Lett. 68, 128-140.