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Metasedimentary units of the Cambro-Ordovician Ross Orogen in northern Victoria Land and Oates Land: Implications for their provenance and geotectonic setting from geochemical and Nd-Sr isotope data

Authors:
  • retired (from Federal Geological Survey of Germany, Hannover)

Abstract and Figures

Metasediments in the three early Palaeozoic Ross orogenic terranes in northern Victoria Land and Oates Land (Antarctica) are geochemically classified as immature litharenites to wackes and moderately mature shales. Highly mature lithotypes with Chemical Index of Weathering values of ≥ 95 are typically absent. Geochemical and Rb-Sr and Sm-Nd isotope results indicate that the turbiditic metasediments of the Cambro-Ordovician Robertson Bay Group in the eastern Robertson Bay Terrane represent a very homogeneous series lacking significant compositional variations. Major variations are only found in chemical parameters which reflect differences in degree of chemical weathering of their protoliths and in mechanical sorting of the detritus. Geochemical data, 87Sr/86Sr t=490Ma ratios of 0.7120 - 0.7174, εNd,t=490Ma values of -7.6 to -10.3 and single-stage Nd-model ages of 1.7 - 1.9 Ga are indicative of an origin from a chemically evolved crustal source of on average late Palaeoproterozoic formation age. There is no evidence for significant sedimentary infill from primitive "ophiolitic" sources. Metasediments of the Middle Cambrian Molar Formation (Bowers Terrane) are compositionally strongly heterogeneous. Their major and trace element data and Sm-Nd isotope data (εNd,t=500Ma values of -14.3 to -1.2 and single-stage Nd-model ages of 1.7 - 2.1 Ga) can be explained by mixing of sedimentary input from an evolved crustal source of at least early Palaeoproterozoic formation age and from a primitive basaltic source. The chemical heterogeneity of metasediments from the Wilson Terrane is largely inherited from compositional variations of their precursor rocks as indicated by the Ni vs TiO2 diagram. Single-stage Nd-model ages of 1.6 -2.2 Ga for samples from more western inboard areas of the Wilson Terrane (εNd,t=510Ma -7.0 to -14.3) indicate a relatively high proportion of material derived from a crustal source with on average early Palaeoproterozoic formation age. Metasedimentary series in an eastern, more outboard position (εNd,t=510Ma -5.4 to -10.0; single-stage Nd model ages 1.4 - 1.9) on the contrary document stronger influence of a more primitive source with younger formation ages. The chemical and isotopic characteristics of metasediments from the Bowers and Wilson terranes can be explained by variable contributions from two contrasting sources: a cratonic continental crust similar to the Antarctic Shield exposed in Georg V Land and Terre Adélie some hundred kilometers west of the study area and a primitive basaltic source probably represented by the Cambrian island-arc of the Bowers Terrane. While the data for metasediments of the Robertson Bay Terrane are also compatible with an origin from an Antarctic-Shield-type source, there is no direct evidence from their geochemistry or isotope geochemistry for an island-arc component in these series.
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Terra Antartica
2003, 10(3), 105-128
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in
Northern Victoria Land and Oates Land:
Implications for Their Provenance and Geotectonic Setting from
Geochemical and Nd-Sr Isotope Data
F. HENJES-KUNST1* & U. SCHÜSSLER2
1Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover - Germany
2Institut für Mineralogie, Universität Würzburg, Am Hubland, D-97074 Würzburg – Germany
Received 11 November 2002; accepted in revised form 30 October 2003
Abstract – Metasediments in the three early Palaeozoic Ross orogenic terranes in northern Victoria Land and
Oates Land (Antarctica) are geochemically classified as immature litharenites to wackes and moderately
mature shales. Highly mature lithotypes with Chemical Index of Weathering values of > 95 are typically
absent. Geochemical and Rb-Sr and Sm-Nd isotope results indicate that the turbiditic metasediments of the
Cambro-Ordovician Robertson Bay Group in the eastern Robertson Bay Terrane represent a very
homogeneous series lacking significant compositional variations. Major variations are only found in chemical
parameters which reflect differences in degree of chemical weathering of their protoliths and in mechanical
sorting of the detritus. Geochemical data, 87Sr/ 86Sr t=490 Ma ratios of 0.7120 – 0.7174, εNd, t=490 Ma values of
-7.6 to –10.3 and single-stage Nd-model ages of 1.7 – 1.9 Ga are indicative of an origin from a chemically
evolved crustal source of on average late Palaeoproterozoic formation age. There is no evidence for
significant sedimentary infill from primitive “ophiolitic” sources. Metasediments of the Middle Cambrian
Molar Formation (Bowers Terrane) are compositionally strongly heterogeneous. Their major and trace
element data and Sm-Nd isotope data (εNd, t=500 Ma values of -14.3 to -1.2 and single-stage Nd-model ages of
1.7 – 2.1 Ga) can be explained by mixing of sedimentary input from an evolved crustal source of at least
early Palaeoproterozoic formation age and from a primitive basaltic source. The chemical heterogeneity of
metasediments from the Wilson Terrane is largely inherited from compositional variations of their precursor
rocks as indicated by the Ni vs TiO2diagram. Single-stage Nd-model ages of 1.6 -2.2 Ga for samples from
more western inboard areas of the Wilson Terrane (εNd, t=510 Ma -7.0 to -14.3) indicate a relatively high
proportion of material derived from a crustal source with on average early Palaeoproterozoic formation age.
Metasedimentary series in an eastern, more outboard position (εNd, t=510 Ma -5.4 to -10.0; single-stage Nd
model ages 1.4 – 1.9) on the contrary document stronger influence of a more primitive source with younger
formation ages. The chemical and isotopic characteristics of metasediments from the Bowers and Wilson
terranes can be explained by variable contributions from two contrasting sources: a cratonic continental crust
similar to the Antarctic Shield exposed in Georg V Land and Terre Adélie some hundred kilometers west of
the study area and a primitive basaltic source probably represented by the Cambrian island-arc of the Bowers
Terrane. While the data for metasediments of the Robertson Bay Terrane are also compatible with an origin
from an Antarctic-Shield-type source, there is no direct evidence from their geochemistry or isotope
geochemistry for an island-arc component in these series.
*Corresponding author (henjes-kunst@bgr.de )
INTRODUCTION
The Ross-Delamerian Orogenic Belt in Antarctica
and SE Australia marks the position of the palaeo-
Pacific continental margin of East Gondwana which
was initiated during Neoproterozoic breakup of
Rodinia and subsequent rifting between Gondwana
and its potential former counterpart Laurentia
(Moores, 1991; Dalziel, 1991; Powell et al., 1994;
Unrug, 1996; Veevers et al., 1997; Foden et al.,
2001). Sedimentary successions reflecting the different
geotectonic stages in the evolution of the palaeo-
Pacific margin of East Gondwana in Neoproterozoic
to early Palaeozoic times are well documented from
the Delamerian Orogen in SE Australia. Here, they
accommodate Neoproterozoic shallow water
sedimentary sequences of the Adelaide Geosyncline
which are overlain by Cambrian platform deposits of
the Normanville Group and Cambrian flysch-type
deposits of the Kanmantoo Group (Preiss, 1987;
Haines & Flöttmann, 1998; Flöttmann et al., 1998).
This evolution is interpreted to reflect the
F. Henjes-Kunst & U. Schüssler106
development from a passive-margin type setting
related to continental break-up to a sedimentary
environment close to the evolving Delamerian
Orogen. To the east of the Adelaide Syncline in a
more outboard position, sedimentary sequences of
Cambrian age are found in close association with
greenstones of oceanic to island-arc affinity in the
area of Mt. Stavely (e.g. Moore et al., 1998).
Sedimentation in the area of the Adelaide Syncline
and the greenstone belt was terminated by/during the
Cambro-Ordovician Delamerian Orogeny.
The Delamerian Orogenic Belt in SE Australia
has traditionally been correlated with the Ross Orogen
in northern Victoria Land and Oates Land at the
Pacific end of the Transantarctic Mountains (TAM) in
East Antarctica (Stump et al., 1986; Flöttmann et al.,
1993). According to this model, the Neoproterozoic
Adelaidian and Cambrian sequences of SE Australia
are to be correlated with the basement rocks of the
inboard Wilson Terrane in Antarctica while the Mt.
Stavely greenstone belt in SE Australia probably finds
its counterpart in the volcanosedimentary Bowers
Terrane of northern Victoria Land. In both regions,
these zones are fault-bound to the east by terranes
built up by turbiditic sediments of early Palaeozoic
age (Robertson Bay Terrane in Antarctica; Stawell
zone of the Lachlan Fold Belt in Australia).
Sedimentological and palaeontological observations in
the Cambrian sedimentary sequences in the Bowers
and Robertson Bay terranes provide convincing
evidence in favor of a correlation with early
Palaeozoic strata in outboard zones of the Delamerian
Orogen. The age and origin of basement rocks in the
Wilson Terrane, however, are poorly constrained. A
correlation with the Neoproterozoic to Cambrian
rocks of the inboard Delamerian Orogen remains
thus speculative and is based on indirect evidence as
for instance Nd isotope signature or age pattern of
detrital zircons (Turner et al., 1993; Ireland et al.,
1995, 1999).
From the early geological investigations in
northern Victoria Land and Oates Land it became
clear that the basement rocks of the Wilson Terrane
originated to a very large extent from clastic
sedimentary precursors (see GANOVEX-Team 1987
for review). Limitations to outcrop exposures due to
snow and ice cover as well as Ross-orogenic
polyphase deformation make it impossible to
constrain stratigraphic relationships within and
between the different metasedimentary units.
Comparison of the metasedimentary units in northern
Victoria Land and Oates Land is further complicated
by the variable metamorphic recrystallization of
basement rocks in the Wilson Terrane which
culminated in the formation of high- to very-high-
grade migmatites. This precludes the classical
sediment-petrographical approach often used in the
study of (meta-)sedimentary sequences. The chemical
composition of metasedimentary rocks, however, has
great potential for reconstructing their geological
history. Geochemistry of clastic (meta-)sediments has
often been used to define their protoliths (Pettijohn et
al., 1972; Blatt et al., 1980), to constrain the
geotectonic setting of the sedimentary basin (Bhatia,
1983, 1985; Roser & Korsch, 1986, 1988; McLennan
et al., 1990) and to identify protolith components in
the sedimentary infills (Bjorlykke, 1974; Hiscott,
1984; Warfter & Graham, 1989; Floyd et al., 1991;
Fralick & Kronberg, 1997). In addition to
geochemical studies, Nd isotope investigations of
metasediments have proven as a potential tool to
discriminate between source components of different
origin and crustal age (Turner et al., 1993; Farmer &
Ball, 1997). Following this geochemical-isotopic
approach in the study of metasediments, the objective
of the present study is:
a) to investigate and compare the protolith
lithologies of metasedimentary units of the three
tectonometamorphic terranes in northern Victoria
Land and Oates Land,
b) to evaluate the bearing of these data on the
provenance and plate tectonic setting of the
investigated units, and
c) to discuss plate tectonic implications for the
early-Ross-orogenic evolution of the palaeo-
Pacific margin of East Gondwana.
GEOLOGICAL SETTING AND
SAMPLING DETAILS
GENERAL REMARKS
Northern Victoria Land and Oates Land, located at
the Pacific end of the Transantarctic Mountains in
East Antarctica, are traditionally subdivided into three
tectonostratigraphic units bounded by NNW-SSE
trending faults (Fig. 1). These are from east to west
the Robertson Bay Terrane (RBT), the Bowers
Terrane (BT) and the Wilson Terrane (WT). The
western “inboard” WT is interpreted as the active
palaeo-Pacific continental margin of the Precambrian
Antarctic Craton. The two “outboard” BT and RBT
were accreted to the WT by plate convergence and
craton-ward directed subduction of oceanic crust
(Kleinschmidt & Tessensohn 1987, Tessensohn 1997,
Flöttmann et al. 1998) and thus may be interpreted as
true allochthonous terranes. Formation of the active
continental margin, accretion of the terranes as well
as metamorphism and deformation within the terranes
occurred during the Ross Orogeny in Cambro-
Ordovician times (e.g., Stump 1995; Tessensohn
1997; Bassett et al. 2002). A detailed description of
the geology of northern Victoria Land and Oates
Land as well as of the various geological units within
the different terranes can be found elsewhere (e.g.,
GANOVEX-Team, 1987; Stump, 1995). In the
following, only metasedimentary units investigated in
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 107
this study will be described. The metasedimentary
rocks are arbitrarily divided into “fine-grained”
lithologies (“meta-argillites”) including slates,
siltstones and their higher-grade metamorphic
equivalents and “coarse-grained” lithologies
(“metapsammites”) including sandstones, greywackes
and their metamorphic equivalents. Samples were
collected during the 1988/89 GANOVEX V, the
1990/91 GANOVEX VI, the 1992/93 GANOVEX VII
and the joint German-Italian 1999/2000 expeditions to
northern Victoria Land and Oates Land.
Robertson Bay Terrane
The outboard RBT is mainly built up by the
Robertson Bay Group (referred to as rb), a
monotonous turbiditic sequence of alternating
metapsammites and -argillites (Harrington et al., 1964;
Field & Findlay, 1983). The protoliths of the series
were deposited in distal fans to basin plains with
NNW-ward directed transport (Wright, 1981, 1985;
Wright et al., 1984). A maximum Late Cambrian to
early Ordovician sedimentation age is re-emphasized
by more recent dating of detrital zircon (Flöttmann,
pers. communication; Ireland et al., 1998; Fioretti et
al., 2003) and by 40Ar-39Ar laser dating of detrital
mica (Henjes-Kunst, 2003) from samples of different
areas of the RBT. Since the series were affected by
large-scale folding and very-low to low-grade
metamorphism in the course of the Ross Orogeny
(e.g., Buggisch & Kleinschmidt, 1991; Wright &
Dallmeyer, 1991), an early Palaeozoic syn-Ross-
orogenic sedimentation age of the Robertson Bay
Group can be concluded. Lithologically, the
metapsammites are greywackes with abundant clasts
of quartz and lithics while the meta-argillites (slates,
phyllites) represent silty mudstones rather than true
claystones. Graded bedding is common. Volcanic or
major calcareous intercalations are unknown. Based
on a preliminary study of the clastic components of
the metasediments, Wright (1981) argued for a
considerably uniform provenance of the Robertson
Bay Group from a mainly mature continental source
Fig. 1 - Schematic map showing the Ross-orogenic geology of the three fault-bounded terranes of northern Victoria Land and Oates Land
(Robertson Bay Terrane, Bowers Terrane, Wilson Terrane). Post-Cambro-Ordovician lithologies are undifferentiated. Location of the study
area in Antarctica is indicated in the inset (upper right). Sample locations in the various metasedimentary units are depicted by different
symbols as explained in the lower inset.
Abbreviations: ArN, Archangel Nunataks; BeM, Berg Mountains; BoM, Bowers Mountains; DaR, Daniels Range; HeH, Helliwell Hills;
KaH, Kavrayskiy Hills; LaM, Lazarev Mountains; LaR, Lanterman Range; MCB, McCain Bluff; MoR, Morozumi Range; OuN, Outback
Nunataks, SaR, Salamander Range; WiH, Wilson Hills.
F. Henjes-Kunst & U. Schüssler108
of medium to high-grade rocks. For this study, a total
of 36 samples of psammitic and argillitic
metasediments collected at 14 sites spread over the
northern part of the RBT (Fig. 1) were considered.
One sample (G8-27a; McKenzie Ntk.) was collected
within the Millen Schists, which is a stronger
deformed zone of metasediments of unclear terrane
assignment at the boundary of the RBT against the
BT. The sample from McKenzie Ntk., however, does
not show evidence for pervasive polyphase
deformation and quartz veining typical of the Millen
Schists. It is therefore structurally well comparable to
samples from other parts of the RBT. The sampling
scheme for the Robertson Bay Group should enable
detection of any lateral variations in the composition
of the metasediments.
Typical Millen Schists from the westernmost part
of RBT were not included in this study since they are
characterized by abundant quartz veining. As SiO2
appears to be the most mobile chemical component
during metamorphism (Ague, 1994) this veining
strongly indicates the influence of post-sedimentary
element mobility.
Bowers Terrane
In the BT, three different metasedimentary units
having been formed in contrasting depositional
environments are known (e.g., GANOVEX-Team,
1987). These units comprise stratigraphically from
bottom to top:
(i) the Cambrian slope- to shelf-facies Molar
Formation which is intercalated with primitive
island-arc volcanics (Sledgers Group),
(ii) the shallow marine Cambrian to early Ordovician
Mariner Formation which rests in part
conformably (Wodzicki & Robert, 1986) on the
metasediments and metavolcanics of the Sledgers
Group, and
(iii) the fluviatile to deltaic early Ordovician Leap
Year Group which rests unconformably on both,
the Sledgers Group and the Mariner Formation.
Since the amount of samples for both, the Mariner
Formation and the Leap Year Group is very small,
they have not been considered in this study.
The Molar Formation (bmo) is a turbiditic
sequence consisting of alternating greywackes and
silty mudstones with minor conglomerates and
intercalated volcaniclastic rocks (Laird et al., 1982). A
Middle Cambrian depositional age has been re-
emphazised by more recent fossil investigations
(Wolfart 1994; Cooper et al. 1996) and is consistent
with the results of SHRIMP dating of zircon (Bassett
et al., 2002) and 40Ar-39Ar laser dating of detrital
mica (Henjes-Kunst, 2003). The transport direction
was to the southeast with palaeoslopes predominantly
oriented to the southwest (Laird et al., 1982;
Wodzicki & Robert, 1986). All previous studies agree
in that the clastic sediments of the Molar Formation
were fed mainly by two sources: a magmatic arc
probably represented by the intercalated island-arc
volcanics and a continental landmass. The
investigated 25 samples (13 metapsammites and 12
meta-argillites) were collected in the area between the
southeastern Solidarity Range (upper Sledger Glacier)
in the central BT and the northern Explorer Range in
the northern BT (Fig. 1). They include specimens
from both, the western slope and eastern shelf facies
of Wodzicki & Robert (1986).
Wilson Terrane
The basement of the western inboard WT is built
up by a collage of units consisting predominantly of
metasedimentary rocks which were intruded in Ross
orogenic times by large volumes of the Granite
Harbour Intrusives. Lithologically these units may be
grouped in monotonous turbiditic series consisting
mostly of clastic metasediments on the one hand and
more variegated shallow-water series containing
clastic metasediments, metamorphosed limestones and
marls on the other hand (GANOVEX Team, 1987).
Intercalations of metavolcanics of basic to acidic
compositions are present in some units of both
groups. They are, however, found only in subordinate
amounts. Regional metamorphic degree varies from
unit to unit from low grade via medium grade to high
and very-high grade but is relatively constant within a
given unit. Steep metamorphic gradients in some
areas may in part be related to the thermal influence
of nearby Granite Harbour Intrusives. The
stratigraphic relations between the different units are
not constrained for the central and northern part of
the WT. Major thrust systems in the northwestern part
of the WT (Flöttmann & Kleinschmidt, 1991) separate
units with contrasting metamorphic degrees and were
activated during final tectonometamorphism of the
Ross Orogeny (Schüssler & Henjes-Kunst, 1994;
Henjes-Kunst, unpubl. results). These structures were
interpreted to indicate that blocks of metasedimentary
rocks with different metamorphic degree representing
different crustal levels of the WT basement were
tectonically exhumed to various degrees along the
thrust zones during the late stage of the Ross
Orogeny (Flöttmann & Kleinschmidt, 1991; Schüssler,
1996). In Late Cambrian to early Ordovician times,
the metasedimentary units of the WT were intruded
by the late-Ross orogenic Granite Harbour Intrusives.
In this study, only metasedimentary units occurring
north of about 73°S in the WT were investigated. The
description of the units closely follows the
nomenclature of GANOVEX Team (1987).
Low- to medium-grade units in the central to
northern WT comprise the Morozumi Phyllites (wsm),
Rennick Schists (wsr), metasediments of the Berg
Group (ws/BM) and from the small outcrop of
McCain Bluff (ws/MC). These units have in common
that primary sedimentary features similar to those
observed in the Molar Formation of the BT and in
the Roberston Bay Group of the RBT are mostly well
preserved in outcrop to thin section scale. However,
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 109
unlike most of the metasediments from the BT and
RBT, all low- to medium-grade units in the WT have
experienced to various degrees a static contact
metamorphism during the emplacement of the Granite
Harbour Intrusives. The Morozumi Phyllites (wsm)
which crop out at Morozumi Range and at small
isolated nunataks north and south of it (Lonely One,
Onlooker Ntk.) are an alternating sequence of
metagreywackes to metasiltstones. Grading bedding is
common. Characteristic primary sedimentary
structures and the absence of calcareous intercalations
indicate that the protolith of the Morozumi Phyllites
have been formed in a turbiditic environment.
Volcanic intercalations are unknown. The age of
deposition is not constrained by fossils. However,
SHRIMP dating of detrital zircons (Flöttmann, pers.
comm.) from the southern Morozumi Range indicate
that the Morozumi Phyllites contain clastic input from
source rocks of Cambrian age thus argueing for a
maximum Cambrian sedimentation age of the
turbiditic sequence. Since the metasediments were
intruded by a late-Ross-orogenic pluton of the Granite
Harbour Intrusives, a Cambrian to early Ordovician
sedimentation age can be inferred. 12 specimen (9
metapsammites, 3 meta-argillites) from various
outcrops of this unit were investigated (Fig. 1). One
samples was taken from a metasedimentary xenolith
in the Granite Harbour Intrusive at the northern
Morozumi Range. Lithologically, petrographically and
structurally, this metasedimentary xenolith compares
well to the Morozumi Phyllites. Therefore, it is
interpreted to represent a small raft of the country
rocks incorporated into the roof zone of the intrusion.
This is supported by the geochemical data.
Rennick Schists (wsr) according to the definition
of Sturm & Carryer (1970) occur in the area from
south of Sequence Hill (southwestern WT) to the
Daniels Range (northwestern WT). Lithologically,
Rennick Schists are fine- to medium-grained schists
and gneisses originating from argillitic to psammitic
metasediments which are often associated with layers
and lenses of calc-silicate rocks. Metavolcanic
intercalations have so far not been described.
Lamination is well preserved; even relict graded
bedding can be found in some places. The
stratigraphic age of the Rennick Schists is not
constrained by fossil findings. However, a Cambrian
to early Ordovician sedimentation age of the
metasediments is constrained by Cambrian ages of
detrital minerals in the rocks (Henjes-Kunst 2003;
Flöttmann, pers. comm.) on the one hand and the
presence of abundant late-Ross-orogenic crosscutting
Granite Harbour Intrusives on the other hand.
Stronger deformation of the series in some areas as
for instance in the Welcome Mountains and the
southern Daniels Range can probably be related to a
very proximal position to branches of major Ross-
orogenic thrust systems (Läufer et al., 2003; see also
Flöttmann & Kleinschmidt, 1991). A total of 15
samples (14 metapsammites, 1 meta-argillite)
collected in the area of Outback Nunataks, Helliwell
Hills and southern Daniels Range were investigated
(Fig. 1). Some of the specimens from the Outback
Nunataks were sampled from large metasedimentary
rafts in roofs zones of Granite Harbour Intrusives
Here, granitic veins of variable width and density
crosscut metasediments, which can be traced back
into typical Rennick Schist series.
Metasediments of the Berg Group (ws/BM) occur
in the coastal area of Oates Land (“Oates Coast”) in
the northwesternmost part of the WT close to its
suspected boudary to the Antarctic Craton. Originally
described by Soviet workers (Klimov & Soloviev,
1960; Ravich et al. 1965) the Berg Group has been
reinvestigated in detail more recently (Skinner et al.
1996; Adams 1996). It is a turbiditic sequence of
psammitic to pelitic rocks with intercalations of
nodules, lenses and layers of carbonate rocks. A
section of about 50 m on one of the exposed ridges
consists predominately of carbonate beds. Contrary to
a Riphean stratigraphic age which was proposed on
the basis of fossil data (Iltchenko 1972), more recent
geochronological data obtained on detrital minerals in
the metasediments (Henjes-Kunst 2003; Flöttmann,
pers. comm.) and on the crosscutting Berg Granite
(Adams 1996) constrain a Cambrian sedimentation
age of the Berg Group. 13 psammitic and 1 argillitic
samples collected from the area of Berg Mountains
and Archangel Nunataks have been investigated.
Included are two specimen of well-laminated
metapsammites collected from large rafts in the
Granite Harbour Intrusion at Outrider Nunatak
(Archangel Nunataks). Lithologically and
petrographically, these metapsammites compare well
to the metasediments forming the country rocks of the
intrusion and, therefore, are interpreted as rafts of the
Berg Group incorporated into the roof zone of the
intrusion.
Metasediments comparable in lithology and
sedimentary inventory to the low- to medium-grade
series of the WT described above are also found at
McCain Bluff (ws/MC) in the northeastern USARP
Mountains (northern WT; Fig. 1). Because of the
small outcrop area only a very limited sedimentary
section can be observed. Since the metasediments at
McCain Bluff are located directly east of the eastern
branch (Wilson Fault) of the late-Ross orogenic thrust
systems in the WT, they may be correlated with the
Morozumi Phyllites and likely also with the Rennick
Schists further to the south. The depositional age is
so far not constrained. Five metapsammites and four
meta-argillites from McCain Bluff were investigated.
Medium- to very-high-grade metasedimentary units
in the WT are characterized by complete regional
metamorphic recrystallisation thus obliterating any
primary sedimentary fabrics and minerals.
Nevertheless, sedimentary protolith can well be
deduced because of typical rock associations and
F. Henjes-Kunst & U. Schüssler110
relict sedimentary layering and in part gradation. The
Lanterman Metamorphics (wsl) which build up the
Lanterman and Salamander ranges close to the
tectonic boundary of the WT against the BT (Fig. 1)
were described in detail by Roland et al. (1984) and
Talarico et al. (1998). They consist of psammitic to
pelitic schists and gneisses with some intercalations of
amphibolites suggesting that the rocks originated from
clastic metasediments with some minor volcanic
intercalations. The sedimentation age is so far not
constrained. Preliminary investigations on the
amphibolites indicate that they compare
geochemically and isotopically well to primitive
basaltic rocks of the BT volcanics (Henjes-Kunst,
unpubl. results). In the western Lanterman Range,
calc-silicate bands and pods become more abundant.
Thirteen samples of overall psammitic composition
from the Lanterman Range and from the northern part
of the Salamander Range were investigated. In
addition one gneiss was included which was collected
at a small isolated nunatak in the northern Evans
Névé in the southern prolongation of the Salamander
Range. Lithologically and petrographically, the
gneisses from this small nunatak compare well to the
metamorphic rocks from the Salamander and
Lanterman ranges.
Metasedimentary rocks of Kavrayskiy Hills
(wg/KH) were considered as typical representatives
for the medium to high-grade in part migmatitic
gneisses found in large areas in the northern part of
the WT (GANOVEX Team 1987). According to
Schubert et al. (1984) rock series at Kavrayskiy Hills
comprise fine-grained biotite-plagioclase gneisses
intermediate in colour alternating with light-coloured
gneisses with low biotite contents interpreted as
former pelitic to arenaceous sediments. Coarse-grained
gneisses containing feldspar megacrysts were assumed
to derive from greywacke source rocks. Primary
sedimentary and relict graded bedding can be
observed. Similar to the Lanterman Metamorphics the
series include some minor intercalations of
amphibolites. No age constraint can be given for the
deposition age of the likely clastic protoliths of the
gneisses. For this study, 9 samples some of which
were already studied by Schubert et al. (1984) were
investigated.
The high- to very-high-grade and mostly
migmatitic basement of the Wilson Hills area
(wg/WH) consists of extensive series of psammitic
metasediments with local variations to more quartzitic
or more pelitic compositions (Schüssler 1996;
Schüssler 2000; Schüssler et al. 1999). Differences in
metamorphic grade allow to distinguish a very-high-
grade central zone from high-grade eastern and
western zones (Schüssler 1996). Whereas
metasediments in the latter two zones are extremely
monotonous, calc-silicate and amphibolitic
intercalations are relatively abundant in the central
zone resulting in a more variegated series. According
to the results of recent SHRIMP dating (Henjes-Kunst
et al. 2003, in prep.) detrital zircons of latest
Neoproterozoic age are present in the very-high-grade
rocks thus argueing for a maximum Cambrian
sedimentation age of their protoliths. A minimum age
of deposition is constrained by a ~500 Ma age for
high- to very-high-grade metamorphism of the rocks
(Henjes-Kunst et al. 2003, in prep.). A total of 36
samples including four argillitic varieties were
investigated from the Wilson Hills area (Fig. 1).
GEOCHEMISTRY
ANALYTICAL METHODS
Whole-rock preparation and geochemical analyses
were performed at the Bundesanstalt für
Geowissenschaften und Rohstoffe (BGR), Hannover
and at the Institut für Mineralogie, Würzburg. Whole-
rock powders were prepared using steel jaw crushers
and an agate mortar (BGR Hannover) or a tungsten-
carbide mortar (Würzburg). About 2 kg of sample
material free of weathering crusts for coarser grained
specimens (e.g. metagreywackes and medium to high-
grade gneisses) and about 0.5 to 1 kg for finer
grained samples (e.g. meta-argillites to metapelites)
were used. Concentrations of major and trace
elements were determined by X-Ray Fluorescence
Spectrometry (XRF) on lithium borate glass fusion
disks (Burhke et al., 1998) using automated Philips
PW 1480 (Hannover, Würzburg) and PW 2400
(Hannover). Sample powders were mixed with lithium
borate in the ratio of 1:5 at the BGR Hannover and
1:6 at the Institut für Mineralogie, Würzburg. For the
trace elements, detection limits (and used tube/line)
are 20 ppm for Ce (Rh/Lβ1) and La (Rh/Lα), 5 ppm
for Ba (Cr/Lα) and Th (Rh/Lα), and 2-3 ppm for Co
(Rh/Kα), Cr (Rh/Kα), Ga (Rh/Kα), Nb (Rh/Kα), Ni
(Rh/Kα), Rb (Rh/Kα), Sc (Cr/Kα), Sr (Rh/Kα), V
(Rh/Kα), Y (Rh/Kα), Zn (Rh/Kα) and Zr (Rh/Kα) at
the BGR Hannover and 20 ppm for Ba, 15 ppm for
Zr, 10 ppm for Rb, Sc, Sr, V, Cr, 8 ppm for Y, and
5 ppm for Ga, Ni, Th, and Zn (all but Ba and Th:
Rh/Kα; Ba and Th: Rh//Lα) at the Institut für
Mineralogie, Würzburg.
RESULTS
General remarks
The geochemical data base used in this study
comprises major and trace element compositions of a
total of 175 samples from northern Victoria Land and
Oates Land. In table 1 only the average compositions
and the relevant standard deviations of metasediments
from the investigated units are listed. The complete
geochemical data set, including sample localities and
coordinates, is available online at http://www.mna.it/
english/Publications/TAP/terranta.html. The loss-on-
ignition (LOI) values of the samples are variable. All
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 111
but two specimens from the WT have low LOI values
of less than 2 wt.%. The RBT samples show variable
LOI between 2 and 8 wt.%. No correlation of LOI
with CaO or MgO is evident for metasediments from
the RBT or the WT. A correction for modal carbonate
content has therefore not been applied. The LOI value
for BT metasediments ranges from 2 – 15 wt.% and
is positively correlated with CaO and MgO
abundances. Inspection of thin sections, however,
reveals that clastic components in addition to
interstitial carbonate may be responsible for this
correlation. Therefore, a correction of the chemical
data for biogenic carbonate has not been applied.
Only one BT sample has been disregarded because of
its very high LOI content of approximately 15 wt.%.
The remaining specimen have values less than 7
wt.%. For graphical presentation, the chemical data of
all samples have been recalculated on a LOI-free
basis to 100 %.
Chemical classification of the metasedimentary
rocks
A general relationship between the original
mineralogy and grain size of sandstones to mudstones
Tab. 1 - Major and selected trace element values (mean values and respective 1 standard deviations) of psammitic and argillitic
metasediments from the Robertson Bay, Bowers and Wilson terranes of the Ross Orogenic Belt in East Antarctica (rb: Robertson Bay
Group; bmo: Molar Formation; wsl: Lanterman Metamorphics; wsm: Morozumi Phyllites; wsr: Rennick Schists; ws/BM: Berg Group;
ws/MC: metasediments at McCain Bluff; wg/KH: Wilson Gneisses at Kavrayskiy Hills; wg/WH: Wilson Gneisses at Wilson Hills; compare
text for details).
F. Henjes-Kunst & U. Schüssler112
and the chemical composition of the rock has not yet
been found. Therefore, a classification of clastic
sediments similar to conventional mineralogical
criteria, as for instance the relative amount of
fragments of quartz, feldspar, and lithics cannot be
obtained from chemical discrimination diagrams.
Instead, the latter diagrams make use of chemical
parameters which mimic the maturity of (meta-)
sedimentary rocks in terms of their relative amounts
of quartz vs. feldspar and clay (e.g. their SiO2/Al2O3
ratio) on the one hand and of clay minerals vs.
feldspar (e.g. Na2O/K2O and Fe2O3/K2O ratios) on the
other hand. The chemical data of metasediments from
northern Victoria Land and Oates Land are plotted in
a classification diagram using log(Fe2O3/K2O) vs
log(SiO2/Al2O3) after Herron (1988) (Figs. 2 a-d).
Psammitic metasediments from the RBT (rb) show
only very restricted variations in their Fe2O3/K2O and
SiO2/Al2O3ratios resulting in a tight cluster of data
points on both sides of the boundary line between the
wacke and litharenite fields (Fig. 2a). Argillitic
metasediments from the RBT range in composition
from wackes to typical shales. Noticeable is that both
metapsammites and meta-argillites exhibit only
restricted variations in the Fe2O3/K2O ratio compared
to samples from all other units. Psammitic varieties of
samples from the BT (bmo; Fig. 2b) and from the
different units in the WT (Fig. 2 c,d) show very
similar variation fields. They all are characterized by,
on average, slightly lower maturity in terms of their
SiO2/Al2O3 ratio. While most of the BT and WT
metapsammites plot in the wacke field there is a
considerable amount of data points plotting in the
neighbouring fields of arkose, litharenite, Fe-sand or
shale close to the wacke field. The position of one
wsr sample in the upper part of the Fe-sand field
results from its very low potassium content. Argillitic
varieties of samples from the BT and the WT are
distinguished from metapsammites of these terranes
mainly by lower SiO2/Al2O3ratios and plot in the
fields of shale to Fe-shale (Fig. 2 b,c,d). Most of the
meta-argillites from the WT (e.g., wg/WH, wsm,
ws/MC, wg/KH) are however relatively immature and
plot close to the boundary of shale to wacke.
The maturity of clastic sediments is mainly
controlled by the degree of chemical weathering
Fig. 2 - Protolith classification diagrams for metapsammites and —argillites from the 3 terranes of northern Victoria Land and Oates Land
using log(Fe2O3/K2O) vs log(SiO2/Al2O3) after Herron (1988); a) Robertson Bay Group (rb) (Robertson Bay Terrane), b) Molar Formation
(bmo) (Bowers Terrane), c) Morozumi Phyllites (wsm), Rennick Schists (wsr), Berg Group (ws/BM) and from McCain Bluff (ws/MC) (all
Wilson Terrane), d) Lanterman Metamorphics (wsl) and from Kavrayskiy Hills (wg/KH) and Wilson Hills (wg/WH) (all Wilson Terrane).
Field boundaries and rock names according to Herron (1988); psam., metapsammitic varieties; argill., meta-argillitic varieties.
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 113
either directly of the source rocks or during transport
of the detritus. The Chemical Index of Weathering
(CIW = 100*Al2O3/[Al2O3+ CaO + Na2O]; molecular
proportions) which was introduced by Harnois (1988)
is plotted for the RBT, BT, and WT metasediments
against the K2O/ Na2O ratio in figure 3. In this
diagram, samples from the different metasedimentary
units roughly follow a trend from low to high CIW
values at increasing K2O/ Na2O ratios which mimics
the increase of clay minerals at the expense of
feldspar due to chemical disintegration of the
framework minerals. During this process, selective
leaching and removal of Na and Ca results in a
relative increase of Al2O3. Because of the absorption
of K+on clay minerals, K is regarded here as an
immobile element. Almost all psammitic samples
from the investigated units of the RBT, BT, and WT
have CIW values typical of unweathered to only
moderately weathered igneous rocks (Fig. 3) (Harnois
1988). Very low CIW values of < 50 obtained for
some samples from the BT and WT (Fig. 3 b-d) can
be explained by elevated CaO concentrations due to
uncorrected biogenic carbonate contents. Meta-
argillites from all 3 terranes are indicative of
moderately to strongly weathered source rocks.
Metapelites derived from highly mature pelagic shales
with CIW values of > 90 and K2O/ Na2O > 10 (e.g.,
Mingram 1998) are however not present in all 3
terranes. This is in line with field evidence that fine-
grained rocks in the low-grade units of the 3 terranes
were derived from silty sandstones rather than shales.
Interestingly, a considerable amount of samples of
bmo meta-argillites cannot be distinguished by their
CIW values and K2O/ Na2O ratios from psammitic
varieties of the formation (Fig. 3b).
Provenance and tectonic setting of the
metasedimentary rocks
Major and trace element compositions of
metasedimentary rocks are routinely used to infer the
composition and type of source rocks involved, the
depositional environment and therewith the plate
Fig. 3 - Relation between CIW and K2O/Na2O ratio for metapsammites and —argillites from metasedimentary units of northern Victoria
Land and Oates Land. Boundary for unweathered igneous rocks (CIW < 60) and strongly weathered igneous rocks (CIW > 80) according
to Harnois (1988); a) Robertson Bay Group (rb) (Robertson Bay Terrane), b) Molar Formation (bmo) (Bowers Terrane), c) Morozumi
Phyllites (wsm), Rennick Schists (wsr), Berg Group (ws/BM) and from McCain Bluff (ws/MC) (all Wilson Terrane), d) Lanterman
Metamorphics (wsl) and from Kavrayskiy Hills (wg/KH) and Wilson Hills (wg/WH) (all Wilson Terrane). Field boundaries and rock names
according to Herron (1988); psam., metapsammitic varieties; argill., meta-argillitic varieties. CIW = 100*Al2O3/[Al2O3+ CaO + Na2O];
molecular proportions.
F. Henjes-Kunst & U. Schüssler114
tectonic setting of the sedimentary basins (see
Rollinson 1993 for references). Element characteristics
of lithologies in the source rocks may however be
obliterated by secondary processes like weathering
(see above) and/or mechanical sorting processes
during transport of the detritus. Floyd et al. (1989)
used a plot of the concentrations of the relatively
immobile elements Ti and Ni in the metasediments to
distinguish between primary (e.g., source controlled)
and secondary (e.g., sedimentary reworking) controls
on the composition of metasediments. Using this plot
different trends become evident for metasediments
from the RBT, BT, and WT (Fig. 4). Psammitic and
argillitic metasediments of the RBT plot on both sides
and close to the boundary line between the
compositional fields of immature sediments controlled
by magmatic precursor rocks and of mature sediments
controlled by sedimentary processes. The relatively
low degree of scatter again suggests that these rocks
were derived from a very homogeneous source or that
source-controlled variations were effectively
homogenized during transport and sedimentation of
the clastic components. Slightly elevated
concentrations of Ni and other “basaltophile”
elements (Cr, V; Tab. 1) in meta-argillitic varieties
compared to the metapsammites suggest that these
elements were preferentially ad-/absorbed by clay
minerals and thus concentrated in the fine-grained
sediments. The elevated Ni, Cr and V contents can
thus not be interpreted to indicate an elevated clastic
input from an “ophiolitic” source. This is further
supported by, on average, low Cr/V ratios of < 2.
Metasediments from the Molar Formation in the BT
also exhibit a horizontal spread across the boundary
line which is very similar to the sandstone trend
according to Floyd et al. (1989). However,
comparison with the chemical composition of the
associated Glasgow Volcanics (Henjes-Kunst unpubl.
results) suggests that the trend of the bmo
metasediments in the Ni vs. TiO2diagram is mainly
controlled by the influence of the local volcanogenic
source rather than by sedimentary reworking
(Fig. 4b). For instance, two meta-argillites sampled in
the northern Bowers Mountains have anomalously
low Ni contents of < 10 ppm very similar to Ni
concentrations of chemically strongly evolved
volcanics also present in the northern Bowers Terrane.
Most of the BT metavolcanics, however, have
Fig. 4 - Plot of Ni vs TiO2for metapsammites and -argillites from northern Victoria Land and Oates Land according to Floyd et al. (1989);
a) Robertson Bay Group (rb) (Robertson Bay Terrane), b) Molar Formation (bmo) (Bowers Terrane; variation field of associated
metavolcanics is shown in grey), c) Morozumi Phyllites (wsm), Rennick Schists (wsr), Berg Group (ws/BM) and from McCain Bluff
(ws/MC) (all Wilson Terrane), d) Lanterman Metamorphics (wsl) and from Kavrayskiy Hills (wg/KH) and Wilson Hills (wg/WH) (all Wilson
Terrane).
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 115
relatively primitive basaltic compositions with high Ni
contents. Clastic input from these igneous rocks offers
a likely explanation for the relatively high Ni values
of bmo metapsammites and -argillites. Metasediments
from all investigated units in the WT can collectively
be classified as immature “magmatogenic” sediments
according to the Ni-TiO2diagram (Fig. 4 c,d). Their
data points clearly spread along the trend of
intermediate to acidic magmatic rocks (Floyd et al.
1989) indicating that their compositional variations
are largely inherited from chemically evolved igneous
precursor rocks.
Roser & Korsch (1988) have proposed a
discriminant function diagram which is based on the
concentrations of all major elements exept Si in order
to distinguish between four different types of
provenance of sedimentary rocks. This discriminant
function diagram is shown in figure 5 for the
metasediments from the RBT, BT, and WT. The data
points of the rb metasediments again exhibit a very
restricted scatter and collectively plot in the field of
“quartzose sedimentary provenance” (Fig. 5 a). A very
similar position in the discriminant function diagram
is found for most of the bmo metasediments
(Fig. 5b). There are, however, some significant
outliers. While a group of argillitic samples display a
trend into the field of “mafic igneous provenance”, 2
psammitic and 2 argillitic specimens plot in the field
of “felsic igneous provenance”. A relatively local
provenance from felsic igneous rocks has already
been indicated for the 2 argillitic samples on the basis
of their very low Ni concentrations (Fig. 4b).
Metapsammites and meta-argillites from the WT
cannot be distinguished on the basis of the two
discrimant functions. Data points of samples from the
investigated units display a relatively large scatter and
mostly plot on both sides of the boundary line
separating the fields of felsic-igneous and quartzose-
sedimentary provenances (Fig. 5 c,d). One sample
from the Rennick Schists which plots in the mafic-
igneous-provenance field may contain a significant
input from more primitive magmatic source.
Distinct provenance characteristics of (meta-)
sediments can be used to infer the geotectonic
environment in which the sedimentary basins was
formed. In the K2O/Na2O vs SiO2diagram after Roser
Fig. 5 - Discriminant function diagram for the provenance signature of metapsammites and -argillites from northern Victoria Land and
Oates Land according to Roser and Korsch (1988); a) Robertson Bay Group (rb) (Robertson Bay Terrane), b) Molar Formation (bmo)
(Bowers Terrane), c) Morozumi Phyllites (wsm), Rennick Schists (wsr), Berg Group (ws/BM) and from McCain Bluff (ws/MC) (all Wilson
Terrane), d) Lanterman Metamorphics (wsl) and from Kavrayskiy Hills (wg/KH) and Wilson Hills (wg/WH) (all Wilson Terrane).
DF1 = -1.773TiO2+ 0.607Al2O3+ 0.76Fe2O3(total) – 1.5 MgO + 0.616CaO + 0.50Na2O – 1.224K2O – 9.09
DF2 = 0.445TiO2+ 0.07Al2O3– 0.25Fe2O3(total) – 1.142 MgO + 0.438CaO + 1.475Na2O – 1.426K2O – 6.861
F. Henjes-Kunst & U. Schüssler116
& Korsch (1986) three settings are distinguished:
passive-continental margin, active-continental margin
and oceanic-island arc. Psammitic and argillitic
metasediments from the RBT plot along the boundary
of the passive-continental-margin field to the active-
continental-margin field in this diagram (Fig. 6 a).
Within the BT Molar Formation, psammitic and
argillitic varieties again display different
charcateristics. While all psammitic specimens plot
into the active-continental-margin field, 4 argillitic
samples are offset to lower SiO2concentrations thus
plotting in the oceanic-island-arc field and 3 meta-
argillites plot in or close to the passive-continental-
margin field. Roser & Korsch (1986) showed that the
K2O/Na2O vs. SiO2diagram yields consistent results
for sand-mud couplets derived from a common source
(compare results for the RBT metasediments).
Therefore, this plot can be regarded to behave robust
against chemical influences caused by mechanical
sorting of detrital material and thus different grain
sizes of the sediments. Consequently, it can be
inferred that the different positions of the bmo
metasediments in this diagram are geologically
significant and indicate that their sedimentary basin
was fed from active-continental-margin and oceanic-
island-arc sources with minor contributions from a
passive continental margin. Metasediments of the WT
scatter mostly along the boundary of the fields of
passive to active-continental margins. No significant
differences between various units of the WT are
apparent. As compared to metapsammites of the RBT,
metapsammitic varieties of the WT may indicate a
slightly higher sedimentary input from an active-
continental-margin source.
Rb-Sr AND Sm-Nd ISOTOPE COMPOSITIONS
ANALYTICAL METHODS
Rb-Sr and Sm-Nd isotope analyses were
performed at the BGR using the same whole-rock
powders as for the geochemical analyses. About 0.1
to 0.2 g of sample material were decomposed with
HF-HNO3in Teflon vessels after addition of the
isotopic spikes. For Sm-Nd isotope analysis, samples
were disolved at temperatures close to 190 °C in the
Picotrace™ pressure-digestion system DAS™. Rb, Sr,
and the lanthanides were separated on cation-
exchange columns. Sm and Nd were isolated from
Fig. 6 - K2O/Na2O vs SiO2discrimination diagram for the provenance signature of metapsammites and -argillites from northern Victoria
Land and Oates Land according to Roser and Korsch (1986); a) metasediments of the Robertson Bay Group (rb) (Robertson Bay Terrane),
b) metasediments of the Molar Formation (bmo) (Bowers Terrane), c) metasediments of the Morozumi Phyllites (wsm), Rennick Schists
(wsr), Berg Group (ws/BM) and from McCain Bluff (ws/MC) (all Wilson Terrane), d) paragneisses of the Lanterman Metamorphics (wsl)
and from Kavrayskiy Hills (wg/KH) and from Wilson Hills (wg/WH) (all Wilson Terrane).
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 117
each other and adjacent lanthanides in separate
columns using HDEHP-coated Teflon powder (Cerrai
& Testa 1963). Sr, Sm, and Nd were loaded on Re
filaments and run on a double-filament assembly
using a Finnigan MAT 261 multicollector mass
spectrometer in static mode. Rb isotopes were
measured on Re filaments using a VG Micromass
MM30 mass spectrometer. The isotopic ratios were
normalized to 146Nd/144Nd = 0.7219 for Nd and to
86Sr/88Sr = 0.1194 for Sr. 143Nd/144Nd isotope ratios
were further normalized to a LaJolla Nd standard
value of 0.511860. Rb, Sr, Nd, and Sm concentrations
were determined by isotope-dilution techniques.
Procedural blanks for Rb, Sr, Nd, and Sm are less
than 0.1 % of the relevant sample concentration and
are therefore negligible. Uncertainties at the 95 %
confidence level in 87Sr/86Sr and 87Rb/86Sr are 0.01 %
and 1 %, respectively and in 143Nd/144Nd and
147Sm/144Nd, they are 0.005 % and 1.4 %,
respectively. Calculation of the Nd model parameter
εNd is based on 143Nd/144Nd = 0.512638 and
147Sm/144Nd = 0.1967 for a CHUR reference
(“chondritic uniform reservoir”, Jacobsen &
Wasserburg 1980). Single-stage depleted-mantle model
ages (TDM) were calculated assuming 143Nd/144Nd =
0.513151 and 147Sm/144Nd = 0.219 for the present-day
depleted-mantle reservoir. In all calculations, the
IUGS-recommended constants (Steiger & Jäger 1977)
were used. Initial 87Sr/ 86Sr isotope ratios and εNd
values of samples are calculated according to the
available geochronological data base and geological
constraints (see above) at ages of 510 Ma, 500 Ma,
and 490 Ma for metasediments of the Wilson, Bowers
and Robertson Bay terranes, respectively. While
differences in the assumed ages of 10-20 Ma have no
significant effect on the calculated initial εNd values
of all samples and of the initial 87Sr/ 86Sr isotope
ratios of metapsammites with low Rb/Sr ratios, they
will result in significantly different initial 87Sr/ 86Sr
isotope ratios for meta-argillites with elevated Rb/Sr
ratios. However, these age-dependant variations do
only extend the range of calculated ratios of two units
(wsm;wsr) to somewhat lower initial 87Sr/ 86Sr
isotope compositions. Based on the errors quoted
above, initial Sr isotope ratios have absolute
analytical uncertainties varying from 0.0001 for
samples with low 87Rb/86Sr ratios of 1 to 0.0008 for
samples with 87Rb/86Sr ratios of 10. Initial εNd values
have absolute analytical uncertainties of
approximately 0.6. The Rb-Sr and Sm-Nd analytical
data are reported in table 2.
RESULTS
Sr isotope composition
A consequent geological interpretation of the Sr
isotope composition of (meta-)sediments towards
likely source rocks, from which their clastic input
was derived, is hampered by the mobility of the
elements Rb and Sr during for instance chemical
alteration or by biogenic carbonate contributing
variable amounts of oceanic Sr to the Sr budget of
the sediment. The amount of carbonate can be
inferred semiquantitatively from thin section or by
geochemical considerations (see above) at least of
(meta-)sediments, which did not experience significant
metamorphic recrystallisation. However, the influence
of alteration processes having affected the protolith,
the detrital material during sedimentary transport
and/or the (meta-)sediment itself cannot be controlled.
Therefore, metasediments showing evidence of
alteration or containing clastic components with
strong evidence of alteration (e.g. many meta-
sediments of the BT Molar Formation) were not
investigated for their Rb-Sr isotope composition.
Initial 87Sr/ 86Sr isotope ratios of samples of all 3
terranes are listed in table 2 and graphically depicted
in histogram plots in figure 7. Included are the Sr
isotope data of 3 high-grade migmatitic gneisses of
likely metasedimentary origin from the western
Wilson Hills (US 380, 495, 501; Schüssler et al.
1999). The data base for Wilson schists from McCain
Bluff and for the BT Molar Formation is only very
restricted. Disregarding a geologically unrealistic low
ratio of < 0.7 calculated for sample G7-StP3 from the
Bowers Terrane (Tab. 2), the samples from all 3
terranes cover a range from approximately 0.708 to
0.726 which is compatible with an origin of the rocks
from isotopically moderately evolved continental
source rocks (cf. Adams 1996, 1997). It becomes
obvious that the Sr initial ratios of metasediments
from the Robertson Bay Group (n= 11; Fig. 7a) show
less scatter than those of the Morozumi Phyllites (n=
10; Fig. 7c), the Rennick Schists (n= 10; Fig. 7d) or
the high-grade gneisses from the Wilson Hills (n= 7;
Fig. 7h). This is in line with geochemical evidences
indicating relatively homogeneous composition for the
rb metasediments as compared to metapsammites and
meta-argillites from all other units. High initial Sr
isotope ratios of > 0.72 as obtained for part of the
samples from the Wilson Terrane (Figs. 7 d, h) and
Bowers Terrane (Figs. 7b) may indicate a locally
predominant influence of an isotopically more
evolved continental protolith while low ratios of
<0.71 (e.g. Fig. 7 d, g, i) are probably related to
clastic input from more juvenile sources. The initial
Sr isotope ratios for argillitic varieties from two WT
metasedimentary units (wsm: Fig. 7c; wsr: Fig. 7f) are
on average lower than the ratios for metapsammitic
samples of the respective units. This suggests a higher
proportion of material derived from more juvenile
sources in the finer-grained metasediments which is
supported by on average somewhat less evolved Nd
isotope compositions in these samples (see below).
Nd isotope composition
The Sm-Nd isotope system of rocks or minerals is
generally assumed not to be affected in the course of
F. Henjes-Kunst & U. Schüssler118
Tab. 2 - Rb-Sr and Sm-Nd element concentrations and isotope ratios of metasediments from the three tectonostratigraphic terranes of northern Victoria Land and Oates Land (Antarctica). Element
concentrations are determined by isotope-dilution techniques. 87Sr/ 86Srtand εNd. t are calculated for ages of 510 Ma (WT metasediments), 500 Ma (BT metasediments) and 490 Ma (RBT
metasediments). Compare text for details.
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 119
Tab. 2 - Continued.
F. Henjes-Kunst & U. Schüssler120
Tab. 2 - Continued.
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 121
hydrothermal alteration or weathering processes.
Furthermore, addition of biogenic carbonates will not
significantly contribute to the Sm-Nd budget of clastic
sediments. The Sm-Nd isotope signature of (meta-)
sediments thus directly mirrors the relative amounts
of different types of source rocks involved and
therefore is an important tool to unravel their
provenance.
Initial εNd values of samples of all 3 terranes are
reported in table 2 and plotted in histograms in
figure 8. Included in this figure are the Nd isotope
data of 3 high-grade paragneisses (US 380, 495, 501)
from the Wilson Hills area (Schüssler et al. 1999).
Samples from the 3 terranes span a range of εNd, t
values from – 1 to -14. Very similar to the Rb-Sr
isotope evidence, rb samples from the RBT show
relatively small variations between εNd, t of -7.6 and -
10.3 (n= 12; Fig. 8a). Their single-stage Nd-model
ages vary between 1.66 Ga (G7-Dyo1) and 1.91 (G7-
MMu1) (Tab. 2) and thus are indicative of a well
mixed sedimentary input from an continental crust
with on average late Palaeoproterozoic formation age.
Metapsammitic and meta-argillitic rb varieties are
indistinguishable with respect to their Nd isotope
compositions. Metasediments of the BT cover the
complete range of εNd, t values denoted above (– 14 to
-1; Fig. 8b) and can be interpreted as mixtures
beween an early Palaeoproterozoic or older
continental source and a primitive source with mantle-
like εNd, t values. The Sledgers Group metavolcanics
which are associated with the bmo metasediments in
the northern Bowers Terrane and which have εNd, t
values in the range of +2 - +7 (Henjes-Kunst, unpubl.
results) represent the likely protolith for the latter
primitive component in the bmo metasediments. The
coarser-grained bmo varieties have on average more
negative εNd, t values and therefore likely contain a
higher proportion of the evolved continental source as
compared to the fine-grained samples of the unit.
Samples from ws/MC and wg/WH metasedimentary
units in the WT display εNd, t values in the range of
approximately -7 to – 11 that is similar to those of
the RBT samples (Fig. 8 e, h). Their single-stage Nd-
model ages range from approximately 1.7 Ga to
1.9 Ga. The wsm,wsr and ws/MC series additionally
contain samples with low εNd, t values of -12 to -14
(Fig. 8 c, d, f) and single-stage Nd-model ages of 2.0
– 2.2 Ga documenting a higher proportion of clastic
Fig. 7 - Histogram plot of initial 87Sr/ 86Sr isotope ratios of metapsammites and meta-argillites from northern Victoria Land and Oates
Land calculated for ages of 510 Ma, 500 Ma, and 490 Ma for the Wilson Terrane, Bowers Terrane and Robertson Bay Terrane,
respectively. Bars for metapsammites are shown in light grey, those for meta-argillites in dark grey; a) Robertson Bay Group (rb)
(Robertson Bay Terrane), b) Molar Formation (bmo) (Bowers Terrane), c) Morozumi Phyllites (wsm), d) Rennick Schists (wsr), e) McCain
Bluff (ws/MC), f) Berg Group (ws/BM), g) Kavrayskiy Hills (wg/KH), h) Wilson Hills (wg/WH), i) Lanterman Metamorphics (wsl) (all
Wilson Terrane).
F. Henjes-Kunst & U. Schüssler122
infill from source rocks of early Palaeproterozoic or
older formation age. On the opposite, εNd, t values of
approximately -6 (Fig. 8 g, i) and single-stage Nd-
model ages of 1.7 – 1.4 Ga in the wg/KH and wsl
series provide evidence for somewhat higher amounts
of input from a more primitive source compared to
the other WT series.
DISCUSSION AND INTERPRETATION
IMPLICATIONS FOR GEOTECTONIC SETTING
AND PROVENANCE
From the geochemical evidence it becomes
obvious that the turbiditic metasediments of the
Roberson Bay Group which cover most of the large
outcrop area of the Robertson Bay Terrane in
northern Victoria Land with a thickness of at least
several thousand meters (e.g. GANOVEX Team 1987)
represent a very homogeneous series of alternating
immature litharenites and wackes to moderately
mature shales. There are no significant variations in
the composition of coarse-grained varieties between
samples from the easternmost RBT compared to the
area close to the boundary against the Bowers Terrane
(Millen Schist Zone) approximately 200 km further
west. Major variations are only found in chemical
parameters which reflect differences in degree of
chemical weathering of the source rocks or their
components and in mechanical sorting of the detritus
and which are well documented in the gradational
evolution from coarse- to fine-grained sediments
typical of turbiditic series. Metasedimentary series of
in part turbiditic character in the western Wilson
Terrane show very similar gradations between
immature litharenites and wackes and moderately
immature shales but are characterized by stronger
compositional variations. Metasediments of the BT
Molar Formation display a variability which for some
chemical elements exceeds that of the WT series.
However, they compare well to metasediments of the
WT and RBT in that the protoliths of the bmo
samples were immature wackes to moderately
immature shales. In all three terranes, very mature
sedimentary rock types are typically absent. This
demonstrates that protracted and deep weathering
either of the source rocks or during transport of the
Fig. 8 - Histogram plot of initial εNd values of metapsammites and -argillites from northern Victoria Land and Oates Land calculated for
ages of 510 Ma, 500 Ma, and 490 Ma for the Wilson Terrane, Bowers Terrane and Robertson Bay Terrane, respectively. Bars for
metapsammites are shown in light grey, those for meta-argillites in dark grey; a) Robertson Bay Group (rb) (Robertson Bay Terrane), b)
Molar Formation (bmo) (Bowers Terrane), c) Morozumi Phyllites (wsm), d) Rennick Schists (wsr), e) McCain Bluff (ws/MC), f) Berg
Group (ws/BM), g) Kavrayskiy Hills (wg/KH), h) Wilson Hills (wg/WH), i) Lanterman Metamorphics (wsl) (all Wilson Terrane).
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 123
detritus has not taken place. It is therefore not likely
that the protoliths of the respective metasedimentary
series formed during periods of tectonic stagnation
with sea-level lowstands characteric of passive margin
settings. Instead, it is plausible that the sedimentary
basins evolved in front of tectonically active
hinterland experiencing rapid exhumation. The
available age data which are summarized above
demonstrate that the protoliths of the rb and bmo
metasediments and of most series in the Wilson
Terrane incorporate strata of Cambrian sedimentation
age. Only for the Lanterman Metamorphics (wsl), the
Wilson gneisses from Kavrayskiy Hills (wg/KH) and
the small outcrop of low-grade metasediments at
McCain Bluff (ws/MC) no age constraints are
available so far. It can therefore be inferred that the
protoliths of most of the metasedimentary series in
the three terranes of northern Victoria Land and Oates
Land were formed in early Palaeozoic time by
erosion of mountain belts that were exhumed in the
late Neoproterozoic to early Cambrian.
The chemical heterogeneity of metasedimentary
units in the WT demonstrates that compositional
variations in the sedimentary infill were not
eliminated by sedimentary mixing processes. It can
therefore be inferred that the protoliths of these series
were deposited in relatively small basins. The
covariation of the Ni and TiO2abundances of the WT
metasediments suggests that their chemical
heterogeneity reflects compositional variations in the
eroded hinterland (“magmatic trend” in Fig. 4 c,d)
and not mechanical sorting for instance during
transport. Local source-rock dependent variations are
also obvious from the Rb-Sr and Sm-Nd isotope
record of the WT metasediments. Single-stage Nd-
model ages of 2.1-2.2 Ga for samples from more
inboard westernmost areas of the Wilson Terrane
(wsr,ws/BM) indicate higher proportion of material
derived from source rocks with significantly older
mantle-separation ages as compared to the majority of
the WT samples which have model ages close to
1.8 Ga. Metasedimentary series in a more outboard
easternmost position (wg/KH;wsl) on the contrary
document somewhat stronger local influence of more
primitive source rocks with younger model ages.
These differences in the nature of the source rocks of
WT metasedimentary series are not obvious from the
provenance and tectonic discrimination diagrams
(Figs. 5 c,d & 6 c,d, respectively). Only for the
Wilson gneisses from Kavrayskiy Hills, the position
of the data points in the felsic- and intermediate-
igneous provenance fields may be correlated with the
isotope evidence in order to argue for a greater
contribution from magmatic rocks with less evolved
isotope compositions. Although the geochemical and
isotopic characteristics of all WT metasediments can
be explained by a binary mixing model using a
cratonic source of at least early Palaeoproterozoic
formation age on the one hand and a relatively young
and primitive source on the other hand, it is also
possible that more than two different sources were
involved. Another discrete source could have been
formed by a continental crust similar in average
composition to the old cratonic source envisaged
above but with on average late Palaeoproterozoic
mantle-separation age. In summary, the different
metasedimentary series of the northern Wilson Terrane
show almost identical provenances indicating a
common origin of their protoliths from predominantly
active-continental-margin rocks with on average late
Palaeoproterozoic mantle-separation ages. While there
is evidence for a stronger influence of an older
craton-type continental source in the western Wilson
Terrane close to its suspected boundary to the East
Antarctic Shield, series from the eastern “outboard”
terrane record a higher proportion of material derived
from a more juvenile magmatic-arc-type source.
The strong chemical heterogeneity of
metasediments from the Molar Formation (BT) can
readily be explained by variable amounts of
sedimentary input from two constrasting sources in
their protolith. One potential end member similar to
the old cratonic source already identified in
metasedimentary series from the western WT is well
represented by samples showing low εNd, t values of
-13 to -14 (Fig. 8b) and compositional affinities to an
active-margin-type continental crust. The second end
member is most likely the volcanogenic Sledgers
Group of the Bowers Terrane representing a Ross-
orogenic juvenile island-arc of Cambrian age (Weaver
et al. 1984; Wodzicki & Robert 1986). Sedimentary
input from primitive to locally evolved igneous rocks
of this island-arc source is identified by the
geochemical oceanic-island-arc affinity of some meta-
argillites (Fig. 6b), by elevated concentrations of
basaltophile elements (Ni, Cr, V; Fig. 4b) in most
samples and by a relatively primitive Nd isotope
composition in one sample (Fig. 8b). The
compositional variations of the Molar Formation (BT)
can thus be interpreted to indicate sedimentary
mixture of erosional material derived from an old
cratonic-type continental crust on the one hand and an
juvenile island-arc on the other hand. Similar to the
WT metasedimentary series, however, it is also
possible that additional sources as for instance
continental crust of late Palaeoproterozoic formation
age have contributed to the protolith of the bmo
metasediments.
The remarkable compositional homogeneity of
metapsammites and meta-argillites of the Robertson
Bay Group can be interpreted to indicate that the
sedimentary basin was constantly fed by one
homogenous source. In this model, the source can be
described as a continental crust built up by high
amounts of chemically evolved rocks having on
average late Proterozoic mantle-separation ages. This
F. Henjes-Kunst & U. Schüssler124
is very similar to what can be deduced for the
majority of samples from the WT metasedimentary
series. It is also possible, however, that repeated
transport and deposition in large turbiditic currents
may have led to almost perfect mixing of sedimentary
infill derived from two or more sources with different
chemical and isotopic characteristics as envisaged for
the BT and WT metasedimentary series. Geochemical
and Rb-Sr and Sm-Nd isotope investigations are not
capable of discriminating between these two models.
PLATE TECTONIC IMPLICATIONS
The constraints on the provenance of the
metasedimentary series of the three terranes in
northern Victoria Land and Oates Land can be used
to test plate tectonic models which were put forward
in the past for the geological evolution of the Ross
orogenic belt. Most models agree in that the outboard
Bowers Terrane, which formed as a primitive island-
arc, and the Robertson Bay Terrane were accreted by
plate convergence and subduction processes to the
active continental margin of the East Antarctic Shield
(Wilson Terrane) in late Cambrian to early Ordovician
times (Weaver et al., 1984; Gibson & Wright, 1985;
Kleinschmidt & Tessensohn, 1987; Flöttmann et al.,
1993; Matzer, 1995; Goodge & Dallmeyer, 1996;
Flöttmann et al., 1998). In this scenario, the
metasedimentary series in the Wilson Terrane can be
explained by early Ross-orogenic formation of
different basins which were situated at the margin of
the East Antarctic Shield and which were fed by
sedimentary material from the western Antarctic
craton and the eastern BT island arc (cf. Kleinschmidt
& Tessensohn, 1987). Cratonic continental crust
which fits the geochemical and isotopic specifications
of a potential sedimentary source is known from
Georg V Land and Terre Adélie some hundred
kilometers further west (Peucat et al., 1999) and from
other areas west of the Ross-Delamerian orogenic
Belt of East Gondwana (e.g. Fitzsimons, 2000).
Lithological differences in the metasedimentary series
of the WT may then be related to different positions
of the depositional environments: more variegated
series were formed as shallow-water continental-shelf
sediments while monotonous turbiditic series were
deposited at the continental slope.
For the Molar Formation of the Bowers Terrane,
palaeocurrent investigations (Laird et al., 1982;
Wodzicki & Robert, 1986) suggest that the clastic
material was derived from a landmass to the east to
northeast of the terrane (present-day position).
According to the data provided in this study the
landmass should have been built up by old “cratonic”
continental crust similar to what is envisaged for the
Wilson Terrane. Additionally, conglomerate horizons
in the Molar Formation containing fist-size cobbles of
granitic rocks strongly argue for a proximal position
of the landmass to the Bowers Terrane. Therefore, a
position east of the very wide Robertson Bay Group
depositional environment can be excluded. In none of
the geotectonic models cited above, however, a
landmass consisting of cratonic continental crust is
found located between the Bowers Terrane to the
west and the Robertson Bay Terrane to the east. If
there has originally been a continental landmass
between these two terranes plate tectonic processes
have erased any direct evidence of it. Similarities in
geochemical and isotopic characteristics of the
sources are compatible with a common origin of
“cratonic” materials in metasediments of the Bowers
and Wilson terranes from the continental crust of the
East Antarctic Shield. Results of recent
geochronological investigations on detrital components
in the Molar Formation (Bassett et al., 2002; Henjes-
Kunst, 2003) indicate that deposition of sediments in
the Bowers Terrane has occurred almost
contemporaneously with the onset of the Ross-
orogenic collision event by which the Bowers Terrane
was accreted onto the Wilson Terrane. Collision of
plates will result in thickening of crust and tectonic
exhumation which enhances erosion. It is therefore
likely that the “cratonic” component in the BT Molar
Formation was derived from the continental margin of
the East Antarctic Shield which was re-activated by
the Ross Orogeny.
The depositional environment of the Robertson
Bay Group has been modelled either in a very
proximal position to the east of the evolving Ross-
orogenic belt (Kleinschmidt & Tessensohn, 1987) or
in a very distal position close to a suspect continental
landmass in the east and separated from the
Palaeopacific margin of East Antarctica by oceanic
crust (Gibson & Wright, 1985; Matzer, 1995). In the
first model which is in accordance with the results of
sedimentological investigations by Wright (1981,
1985) and Wright et al. (1984), sedimentary material
is derived from the evolving Ross Orogen including
the already accreted Bowers Terrane island arc
(“dissected arc”). In the second model, the Robertson
Bay basin is mainly fed from the suspect landmass to
the east. Similarities in the geochemical and isotopic
source characteristics between metasediments of the
Robertson Bay Group and from the WT are in
accordance with the first model but cannot prove it.
Problems arise from the Bowers Terrane squeezed
between the Robertson Bay and Wilson terranes.
According to the available geochronological data, the
BT island arc was still active during turbiditic
sedimentation of the Robertson Bay Group. The rb
metasedimentary sequence, however, lacks indication
of an intervening juvenile island-arc activity.
Furthermore, the very remarkable compositional and
isotopic homogeniety of the rb metasediments across
the RBT including the area close to the boundary
against the BT contrasts with the heterogeneity of the
BT metasediments. The obvious hiatus in lithological
inventory and geochemical and isotopic characteristics
Metasedimentary Units of the Cambro-Ordovician Ross Orogen in NVL and Oates Land 125
of the metasedimentary sequences between the
Bowers and Robertson Bay terranes may be
interpreted in favour of the terrane model of Gibson
& Wright (1985) and Matzer (1995). Alternatively, it
is possible that a western part of the rb sedimentary
basin which evolved proximal to the BT island arc
was subducted beneath the two western terranes
during the Ross Orogeny. Origin of the sedimentary
material of the Robertson Bay Group from a suspect
landmass in the east cannot be evaluated since there
is only very little evidence for basement rocks in an
outboard position of the Ross-Delamerian Belt of SE
Australia or East Antarctica. The S-type metagranite
of Surgeon Island which is located at the northeastern
coast of northern Victoria Land was discussed by
Borg & DePaolo (1991) and Fioretti et al. (2002) as
evidence for a continent landmass east of the Ross
Orogenic Belt. The metagranite meets some of the
geochemical and isotopic requirements. Furthermore,
recent age determinations (Fioretti et al., 2001)
demonstrate a Cambrian formation age for the granite
which would also be in line with the required active-
continental-margin setting of the Robertson Bay
Group. Since Surgeon Island is almost completely
built up by a homogenous granite and because the
small island is in a very isolated position, a more
detailed discussion is not meaningful.
REGIONAL COMPARISON
Turner et al. (1993) performed a detailed
geochemical and Rb-Sr and Sm-Nd isotope study on
Neoproterozoic to Ordovician sedimentary sequences
of the Delamerian orogenic Belt and on Archaean to
Mid-Proterozoic basement as their potential source
rocks in SE Australia. On the basis of the Nd isotope
compositions of two samples from the Wilson Terrane
which were investigated for comparison, they argued
that the WT metasediments correlate with passive-
margin sedimentary strata of the Neoproterozoic
Adelaidean sequence but not with the Cambrian
Normanville and Kanmantoo sequences of SE
Australia. This is not supported by the results of the
present study. The Nd isotope composition of the WT
metasediments is quite variable. Their range of εNd, t
values (-14 to -5; n = 36) overlaps with the values
obtained for sediments of Neoproterozoic (-10.6 to -
4.4; n = 15) and of Cambrian ages (-13.4 to -8.5; n =
11) of the Delamerian orogenic Belt (Turner et al.,
1993). Lithological and age constraints clearly support
a correlation of the WT metasedimentary units in East
Antarctica with the Cambrian Normanville and
Kanmantoo groups of SE Australia. Age spectra of
detrital zircons additionally argue in favour of
common provenances for metasediments from
northern Victoria Land and Oates Land and Cambrian
sediments from SE Australia. Zircons from WT
samples and from the turbiditic Kanmantoo Group of
SE Australia show a very prominent late-Pan-African
age component for which there is only minor
evidence in the Neoproterozoic Adelaidean and the
Cambrian Normanville sediments (Ireland et al., 1995,
1998, 1999; Berry et al., 2001; Henjes-Kunst et al.,
2003, in prep.). The greater variability in Nd isotope
composition of metasediments from East Antarctica as
compared to Cambrian strata in SE Australia then
indicates more variable provenances and a greater
influence from primitive sources in Antarctica.
Acknowledgements - We are grateful to Schorse
Kleinschmidt, Thomas Flöttmann, Michael Schmidt-Thomé
and Heinz Jordan for companionship during field trips and
for providing additional samples. Don Henry, Julian
Lodziak, Detlef Requard, Monika Bockrath, Ilse Deneke,
Beate Eichmann and Peter Macaj are thanked for laboratory
assistance at BGR Hannover, Peter Späthe and Rosemarie
Baur for thin section preparation and laboratory assistance
at the Institut für Mineralogie, Würzburg, and Axel
Höhndorf for discussions of the analytical results. Two
constructive reviews by Anna M. Fioretti and Sergio Rocchi
helped to improve the manuscript. US gratefully
acknowledges financial support by the Deutsche
Forschungsgemeinschaft (Schu 243/9-1; Schu 873/1-1).
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... Here, I will refer to this assemblage generally as Wilson Group. The Wilson assemblage consists of micaceous paragneiss, mica schist, psammitic schist, quartzite, marble and local migmatite (Ravich et al., 1965;Gair et al., 1969;Sturm and Carryer, 1970;Stump, 1995;Henjes-Kunst and Schüssler, 2003;Estrada et al., 2016). The other units are mainly metamorphosed siliciclastic rocks (including greywacke, psammite, and argillite protoliths) with minor calcareous units. ...
... Sr-and Nd-isotope compositions of S-type Granite Harbour intrusions within the Wilson assemblage indicate derivation by melting of pre-existing continental crust (Borg et al., 1987), which likely reflects a cratonic sedimentary source of detritus. The geochemical and isotopic compositions of Preistley Formation samples likewise indicate that Wilson material was derived from a crustal source with an average early Paleoproterozoic formation age (Henjes-Kunst and Schüssler, 2003). Other units within the Wilson metamorphics show Ross Orogen sources and have maximum depositional ages of 525-560 Ma, indicating a Cambrian or younger stratigraphic age. ...
... The geology of northern Victoria Land is dominated by three tectonic assemblages that developed largely during the Ross Orogeny -the Wilson, Bowers, and Robertson Bay terranes -consisting mostly of sedimentary and volcanic material of early Paleozoic age and volumetrically significant arc-type intrusions. As discussed above, many of the rocks assigned to the Wilson terrane are actually Cambrian in age (detrital zircon and muscovite ages indicating deposition b550-525 Ma; Henjes-Kunst and Schüssler, 2003;Henjes-Kunst et al., 2004;Di Vincenzo et al., 2014;Paulsen et al., 2016) and have a combined cratonic and early Ross Orogen provenance. The detrital zircon provenance data illustrate that, rather than representing reworked Precambrian crust, much of the Wilson terrane consists of Cambrian syn-orogenic clastic deposits sourced from, and metamorphosed during, the Ross Orogeny. ...
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The Transantarctic Mountains (TAM) are one of Earth's great mountain belts and are a fundamental physiographic feature of Antarctica. They are continental-scale, traverse a wide range of latitudes, have high relief, contain a significant proportion of exposed rock on the continent, and represent a major arc of environmental and geological transition. Although the modern physiography is largely of Cenozoic origin, this major feature has persisted for hundreds of millions of years since the Neoproterozoic to the modern. Its mere existence as the planet's longest intraplate mountain belt at the transition between a thick stable craton in East Antarctica and a large extensional province in West Antarctica is a continuing enigma. The early and more cryptic tectonic evolution of the TAM includes Mesoarchean and Paleoproterozoic crust formation as part of the Columbia supercontinent, followed by Neoproterozoic rift separation from Laurentia during breakup of Rodinia. Development of an Andean-style Gondwana convergent margin resulted in a long-lived Ross orogenic cycle from the late Neoproterozoic to the early Paleozoic, succeeded by crustal stabilization and widespread denudation during early Gondwana time, and intra-cratonic and foreland-basin sedimentation during late Paleozoic and early Mesozoic development of Pangea. Voluminous mafic volcanism, sill emplacement, and layered igneous intrusion are a primary signature of hotspot-influenced Jurassic extension during Gondwana breakup. The most recent phase of TAM evolution involved tectonic uplift and exhumation related to Cenozoic extension at the inboard edge of the West Antarctic Rift System, accompanied by Neogene to modern glaciation and volcanism related to the McMurdo alkaline volcanic province. Despite the remote location and relative inaccessibility of the TAM, its underlying varied and diachronous geology provides important clues for reconstructing past supercontinents and influences the modern flow patterns of both ice and atmospheric circulation, signifying that the TAM have both continental and global importance through time.
... The WT mostly comprises of poly-deformed medium-to high-grade metamorphic rocks such as schists, gneisses and migmatites (GANOVEX Team, 1987;Henjes-Kunst and Schussler, 2003;Talarico et al., 2004). These rocks were intruded by the Granite Harbour Intrusives (GHI), a calc-alkaline plutonic suite including both S-and I-type granitoids with magmatic arc affinity, of Cambrian-Ordovician age Black and Sheraton, 1990;Di Vincenzo and Rocchi, 1999;Giacomini et al., 2007;Renna et al., 2011). ...
... The RBT consists of a thick succession of very low-grade metaturbidites of the Cambro-Ordovician period (Wright et al., 1984;Stump, 1995;Henjes-Kunst and Schussler, 2003;Rossetti et al., 2006). The Robertson Bay Group (Late Cambrian to Early Ordovician) with very thick, folded successions of monotonous turbiditic greywackes alternating with silty mudstones represents a roughly 200 km wide metasedimentary unit east of the BT. ...
... The Robertson Bay Group (Late Cambrian to Early Ordovician) with very thick, folded successions of monotonous turbiditic greywackes alternating with silty mudstones represents a roughly 200 km wide metasedimentary unit east of the BT. Their base is unexposed and a minimum thickness of 3000 m has been estimated (GANOVEX Team, 1987;Henjes-Kunst and Schussler, 2003). This succession represents a mainly very low-grade to low-grade metamorphosed flysch sequence originally deposited in a deep-sea fan to basin plain environment. ...
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Satellite remote sensing imagery is especially useful for geological investigations in Antarctica because of its remoteness and extreme environmental conditions that constrain direct geological survey. The highest percentage of exposed rocks and soils in Antarctica occurs in Northern Victoria Land (NVL). Exposed Rocks in NVL were part of the paleo-Pacific margin of East Gondwana during the Paleozoic time. This investigation provides a satellite-based remote sensing approach for regional geological mapping in the NVL, Antarctica. Landsat-8 and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) datasets were used to extract lithological-structural and mineralogical information. Several spectral-band ratio indices were developed using Landsat-8 and ASTER bands and proposed for Antarctic environments to map spectral signatures of snow/ice, iron oxide/hydroxide minerals, Al-OH-bearing and Fe, Mg-OH and CO 3 mineral zones, and quartz-rich felsic and mafic-to-ultramafic lithological units. The spectral-band ratio indices were tested and implemented to Level 1 terrain-corrected (L1T) products of Landsat-8 and ASTER datasets covering the NVL. The surface distribution of the mineral assemblages was mapped using the spectral-band ratio indices and verified by geological expeditions and laboratory analysis. Resultant image maps derived from spectral-band ratio indices that developed in this study are fairly accurate and correspond well with existing geological maps of the NVL. The spectral-band ratio indices developed in this study are especially useful for geological investigations in inaccessible locations and poorly exposed lithological units in Antarctica environments.
... In addition, the c. 500 Ma thermal event indicates post-or pre-D 2 migmatization. The GHI (granite and granodiorite) related to the Ross Orogeny has been reported at Mt. Murchison and its western region [46]. Several types of granites crosscut the gneissosity of the Mt. ...
... In addition, the c. 500 Ma thermal event indicates post-or pre-D2 migmatization. The GHI (granite and granodiorite) related to the Ross Orogeny has been reported at Mt. Murchison and its western region [46]. Several types of granites crosscut the gneissosity of the Mt. ...
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The Ross(–Delamerian) Orogeny significantly impacted the formation of the tectonic structure of the Pacific Gondwana margin during the early Paleozoic era. Northern Victoria Land (NVL) in Antarctica preserves the aspect of the Ross Orogeny that led to the union of the Wilson (WT)–Bowers (BT)–Robertson Bay Terrane. The aspect of the Ross Orogeny in the NVL is characterized by subduction of oceanic domains toward the continental margin (continental arc) and the accretion of the associated marine–continental substances from 530–480 Ma. In the Mountaineer Range in NVL, the Ross Orogeny strain zone is identified at the WT/BT boundary regions. In these areas, fold and thrust shear zones are observed and aspects of them can be seen at Mt. Murchison, the Descent Unit and the Black Spider Greenschist zone. The Dessent Unit corresponds to a tectonic slice sheared between the WT and BT. The metamorphic evolution phase of the Dessent Unit is summarized in the peak pressure (M1), peak temperature (M2) and retrograde (M3). The sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb ages of 514.6 ± 2.0 Ma and 499.2 ± 3.4 Ma obtained from the Dessent Unit amphibolite are comparable to the M1 and M2 stages, respectively. The Dessent Unit underwent intermediate pressure (P)/temperature (T)-type metamorphism characterized by 10.0–10.5 kbar/~600 ˚C (M1) and ~7 kbar/~700 ˚C (M2) followed by 4.0–4.5 kbar/~450 ˚C (M3). Mafic to intermediate magmatism (497–501 Ma) within the WT/BT boundary region may have given rise to the M2 stage of the Dessent Unit, and this magmatism is synchronous with the migmatization period of Mt. Murchison (498.3 ± 3.4 Ma). This indicates that a continuous process of fold-shearing–magmatic intrusion–partial melting, which is typically associated with a continental arc orogeny, occurred before and after c. 500 Ma in the Mountaineer Range. During the Ross Orogeny, the Dessent unit was initially subducted underneath the WT at depth (10.0–10.5 kbar, ~35 km) and then thrust into the shallow (~7 kbar, ~23 km), hot (≥700 ˚C) magmatic arc docking with the Mt. Murchison terrain, where migmatization prevailed.
... Mudstones + thin beds of sandstone ± conglomerates of the Molar Formation were deposited from dominant northwest-to-southeast paleocurrents in a southwest-sloping basin (Bradshaw et al., 1985;Wodzicki and Robert Jr., 1986) that was possibly fault-bounded. Clasts in the conglomerates were derived from Antarctica and comprise volcanic detritus, granitoids, quartzites, metamorphic quartz, and rare limestone (Weaver et al., 1984;Bradshaw et al., 1985;Cooper et al., 1996;Henjes-Kunst and Schüssler, 2003). ...
Article
New insights into the Late Precambrian-latest Devonian evolution of the Pacific margin of Gondwana are obtained by treating the margin in terms of three key tectonic elements: i) the in situ part of the Ross Orogen of Eastern Antarctica (Wilson Terrane) built on, and fringing, older crust; ii) the largely in situ southern Tasmanides of eastern Australia; and iii) offshore basement and island arc terranes now accreted either to the Ross Orogen, the Tuhua Orogen of southwestern New Zealand or, in one case, to the Australian Tasmanides. Detailed correlations between these elements suggest that the onset of convergence was essentially simultaneous along the margin over an original distance of ~1000 km. The first appearance of subduction-related igneous rocks occurred at ~540–530 Ma in the Tasmanides; ~535–530 Ma in the Tiger Arc of northern Victoria Land; and 550 Ma in southern Victoria Land of the Ross Orogen. New correlations of this paper suggest possible but unconstrained trajectories of offshore terranes. The Bowers Terrane was accreted to the East Gondwana margin at ~491 Ma, producing the main Ross Orogeny. The adjoining Takaka Terrane had docked briefly with that margin at ~497–494 Ma (Haupiri Disturbance in New Zealand) before crustal extension rifted it oceanward to drift away in the latest Cambrian to become subsequently amalgamated with the sedimentary Buller Terrane at ~390 Ma. The West Tasmania Terrane was accreted to the East Gondwana margin beginning at~500–499 Ma (generating phase 3 of the Tyennan Orogeny) and the connected Selwyn Block to the Tasmanides at ~500 Ma (main Delamerian Orogeny). Our new interpretations suggest that previous lithological correlations of subduction-related volcanics between the Ross Orogen and southern Tasmanides did not take into account major rollback in the Tasmanides from ~514 to ~503 Ma. Similarly, they suggest that the ~550–480 Ma Granite Harbour Intrusive roots of the continental margin Ross Arc are not correlatives of 514, 505 and ~ 495–470 Ma granites intruding the Kanmantoo Group in the Delamerian Orogen of South Australia, either in time or in tectonic setting. We also recognize an early (~520–516 Ma) boninitic infant arc event in the outboard West Tasmania, Bowers, and Takaka terranes that predated ~500 Ma more mature arcs in the last two. Arc-related magmatism in the Ross Orogen reflects the interplay between two main subduction systems — that which generated the Ross Arc and an outboard one that generated intraoceanic arcs. Three major turbidite fan systems developed along the East Gondwana margin as responses to major deformations. Early Cambrian fan system 1 postdates the Beardmore Orogeny and includes the Kanmantoo Group in the Delamerian Orogen and the Berg and upper Priestley formations in the Wilson Terrane. Cambrian-Ordovician fan system 2 (the Robertson Bay Group, the Swanson Group in Marie Byrd Land, the Greenland Group in the Buller Terrane and the St Arnaud Group in the Delamerian Orogen) and Lower-Middle Ordovician fan system 3 (turbidites of the Eastern Lachlan Orogen, the Buller Terrane (New Zealand) and East Tasmania Terrane) both postdate different parts of the Ross Orogeny. Cessation of fan system at ~458 Ma correlates with ‘accretion’ of the Robertson Bay Terrane in northern Victoria Land.
... Also selected is sample SAX20 (Table 2), which is overall the most incompatible element enriched sample (Fig. 6) and has been equated to a primary (metasomatic) melt by Coltorti et al. (2004) and Perinelli et al. (2006). Crustal rocks from the NVL consist of gabbro, diorite and granodiorite (Di Vincenzo & Rocchi, 1999;Dallai et al., 2003), metasediment (Henjes-Kunst & Schussler, 2003;Di Vincenzo et al., 2014) and eclogite (Di Vincenzo et al., 1997;Ghiribelli, 2000). Gray fields encompass AFC model curves [shown in detail in (c) and (d)] using D Rb ¼ 0Á01, D Sr ¼ 0Á1, D Nd ¼ 0Á4 and proportion of assimilation to crystallization (r) ¼ 0Á8. ...
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Alkaline magmatism associated with the West Antarctic rift system in the northwest Ross Sea (NWRS) includes a north-south chain of shield volcano complexes extending 260 km along the coast of Northern Victoria Land (NVL) as well as numerous small volcanic seamounts located on the continental shelf and hundreds more within a ~35,000 km2 area of the oceanic Adare Basin. New 40Ar/39Ar age dating and geochemistry confirm that the seamounts are Pliocene‒Pleistocene in age and petrogenetically akin to the mostly middle to late Miocene volcanism on the continent as well as to a much broader region of diffuse alkaline volcanism that altogether encompasses areas of West Antarctica, Zealandia and eastern Australia. All of these continental regions were contiguous prior to the late‒stage breakup of Gondwana at ~100 Ma, suggesting that the magmatism is interrelated, yet mantle source and cause of melting remain controversial. The NWRS provides a rare opportunity to study cogenetic volcanism across the transition from continent to ocean and consequently offers a unique perspective from which to evaluate mantle processes and roles of lithospheric and sub-lithospheric sources for mafic alkaline magmas. Mafic alkaline magmas with > 6 wt.% MgO (alkali basalt, basanite, hawaiite, and tephrite) erupted across the transition from continent to ocean in the NWRS show a remarkable systematic increase in silica-undersaturation, P2O5, Sr, Zr, Nb and light rare earth element (LREE) concentrations as well as LREE/HREE and Nb/Y ratios. Radiogenic isotopes also vary with Nd and Pb ratios increasing and Sr ratios decreasing ocean-ward. These variations cannot be explained by shallow-level crustal contamination or by changes in degree of mantle partial melting but are considered to be a function of the thickness and age of the mantle lithosphere. We propose that the isotopic signature of the most silica-undersaturated and incompatible element enriched basalts best represent the composition of the sub-lithospheric source with low 87Sr/86Sr (≤ 0.7030) and δ18Oolivine (≤ 5.0 ‰), high 143Nd/144Nd (~ 0.5130) and 206Pb/204Pb (≥ 20) ratios. The isotopic ‘endmember’ signature of the sub-lithospheric source is derived from recycled subducted materials and was transferred to the lithospheric mantle by small degree melts (carbonate-rich silicate liquids) to form amphibole-rich metasomes. Later melting of the metasomes produced silica-undersaturated liquids that reacted with the surrounding peridotite. This reaction occurred to a greater extent as the melt traversed through thicker and older lithosphere continent-ward. Ancient and/or more recent (~550‒100 Ma) subduction along the Pan-Pacific margin of Gondwana supplied the recycled subduction-related residue to the asthenosphere. Melting and carbonate metasomatism were triggered by major episodes of extension beginning in the Late Cretaceous but alkaline magmatism was very limited in its extent. Significant delay of ~30 to 20 Myr between extension and magmatism was likely controlled by conductive heating and the rate of thermal migration at the base of the lithosphere. Heating was facilitated by regional mantle upwelling, possibly driven by slab detachment and sinking into the lower mantle, and/or by edge‒driven mantle flow established at the boundary between the thinned lithosphere of the West Antarctic rift and the thick East Antarctic craton.
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New field data from the Lanterman Range - Mountaineer Range region suggest that the easternmost portion of the Wilson Terrane is composite, including four major metamorphic complexes, each of them characterized by distinct lithological assemblages and metamorphic patterns. These data, in conjuction with available geochronological constraints, support the subdivision of the Ross Orogen into an inner, broad, low P - high T belt hosting the magmatic arc, and an outer narrow belt including three distinct tectonic units composed by medium- to high-pressure rocks, some of them of mafic composition and retaining a MORB-like geochemical affinity. An orogenic model involving subduction-accretion at a continent/ocean plate boundary consistently integrates the Late Precambrian-Ordovician petrological, geochronological and structural records of the northern Victoria Land segment of the Antarctic paleo-Pacific margin of eastern Gondwana.
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