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P– T– t evolution of the Wilson Terrane metamorphic basement at Oates Coast, Antarctica

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Within the basement of the northern Wilson Terrane at Oates Coast, a very-high-grade central zone is distinguished from high-grade zones to the east and west. In the central zone, P–T estimates of 8 kbar and 800°C derive from the relic assemblage: (1) Crd+Bt+Sil+Spl+Pl+Qtz for an earlier medium-pressure granulite-facies metamorphism which is also documented by relic assemblages Qtz+Pl+Bt+Opx (±Grt±Cpx). A subsequent low-pressure granulite-facies to upper-amphibolite-facies stage with pervasive migmatization took place at 4–5.5 kbar and minimum 700°C, as derived from mineral reactions and thermodynamic calculations on the assemblages (2) Grt+Crd+Bt+Pl+Qtz and (3) Grt+Bt+Sil+Pl+Qtz±Spl. Decompression at still high temperatures and a clockwise directed P–T–t path are indicated by reactions Bt+Sil+Qtz=Crd+Grt+Kfs+V and Grt+Sil+Qtz+V=Crd.
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Precambrian Research 93 (1999) 235 –258
PTtevolution of the Wilson Terrane metamorphic basement
at Oates Coast, Antarctica
Ulrich Schu
¨ssler a,*, Michael Bro
¨cker b, Friedhelm Henjes-Kunst c, Thomas Will a
aMineralogisches Institut, Universita
¨tWu
¨rzburg, Am Hubland, 97074 Wu
¨rzburg, Germany
bInstitut fu
¨r Mineralogie, Universita
¨tMu
¨nster, Corrensstraße 24, 48149 Mu
¨nster, Germany
cBundesanstalt fu
¨r Geowissenschaften und Rohstoe, Postfach 510153, 30631 Hannover, Germany
Received 19 March 1998; received in revised form 24 August 1998; accepted 24 August 1998
Abstract
Within the basement of the northern Wilson Terrane at Oates Coast, a very-high-grade central zone is distinguished
from high-grade zones to the east and west. In the central zone, PTestimates of 8 kbar and 800°C derive from the
relic assemblage: (1 ) Crd+Bt+Sil+Spl+Pl+Qtz for an earlier medium-pressure granulite-facies metamorphism
which is also documented by relic assemblages Qtz+Pl+Bt+Opx (±Grt±Cpx). A subsequent low-pressure granulite-
facies to upper-amphibolite-facies stage with pervasive migmatization took place at 4–5.5 kbar and minimum 700°C,
as derived from mineral reactions and thermodynamic calculations on the assemblages ( 2) Grt+Crd+Bt+Pl+Qtz
and (3 ) Grt+Bt+Sil+Pl+Qtz±Spl. Decompression at still high temperatures and a clockwise directed PTtpath
are indicated by reactions Bt+Sil+Qtz=Crd+Grt+Kfs+V and Grt+Sil+Qtz+V=Crd.
The low-pressure granulite-facies to upper-amphibolite-facies stage is dated by six nearly concordant U–Pb monazite
ages of 484– 494 Ma from three migmatite samples and correlates to the late Pan-African Ross Orogeny in Cambro-
Ordovician times. The age of the medium-pressure granulite-facies assemblages is not constrained by geochronological
data. Either they form relics of the Precambrian Antarctic Craton, or they represent an early metamorphic stage of
the Ross Orogeny. Rb–Sr and K–Ar dating on biotites yielded 470, 468, 470 Ma and 473, 469, 470 Ma (±5 Ma each),
indicating the time of cooling to 450–300°C. This is confirmed by 40Ar–39Ar plateau ages of 476±2, 472±3 and
470±2 Ma for these biotites.
A late tectonic pegmatite yielded a concordant U–Pb monazite age of 489±3 Ma, while slightly discordant U–Pb
data of two zircon fractions are explained by recent minor lead loss of ca 490 Ma old zircons. Cooling to ca 500 –350°C
is dated to 472±2 Ma by concordant 40Ar–39Ar plateau ages of two muscovite fractions.
The cooling history of the basement from high-grade conditions to the blocking temperature of Rb–Sr and K–Ar
in micas took place within ca 15–20 Ma. Cooling rates of 18–25°CMa1can be derived, if continuous cooling
is assumed.
U–Pb data points of zircons as well as Sm–Nd whole rock model ages between 1.8 and 1.9 Ga indicate that at least
part of the migmatites derive from Early Proterozoic crustal protoliths.
Comparing the new PTtdata from the northern Wilson Terrane with those from the southern Wilson Terrane,
a common tectono-metamorphic history becomes evident for a 600 km long sector of the Ross Orogenic belt at the
Pacific end of the Transantarctic Mountains, at least since the granulite-facies metamorphic event. © 1999 Elsevier
Science B.V. All rights reserved.
Keywords: Age determination; Antarctica; Metamorphic evolution; Oates Coast; Ross Orogeny; Wilson Terrane
* Corresponding author. Fax: +49 931 888 4620; e-mail: uli.schuessler@mail.uni-wuerzburg.de
0301-9268/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.
PII S0301-9268( 98) 00091-6
236 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
1. Introduction performed for the first time during the German
Antarctic North Victoria Land Expeditions
GANOVEX V and VII in 1988/89 and 1992/The Oates Coast is located at the Pacific end of
the Transantarctic Mountains (155–160°E and 93, carried out by the Bundesanstalt fu
¨r
Geowissenschaften und Rohstoe (BGR,69–71°S, Fig. 1). The predominant high-grade to
very high-grade crystalline basement rocks of this Hannover).
Based on the regional distribution of criticalarea form the northern part of the Wilson Terrane,
which is the westernmost of three tectonometamor- mineral assemblages, the northern part of the
Wilson Terrane at the Oates Coast can be sub-phic terranes of the Early Paleozoic Ross Orogen.
Prior to 1988 only a few Soviet, Australian and divided into three metamorphic, high-grade to
very-high-grade units, one of them containingNew Zealand expeditions had touched parts of the
Oates Coast area between 1958 and 1967, resulting relics of granulite-facies mineral assemblages with
Opx±Cpx±Grt [Fig. 2; Schu
¨ssler (1996)].in preliminary petrographic descriptions of some
outcrops ( Klimov and Soloviev, 1958; McLeod Interestingly, high-grade metamorphic rocks with
such granulite-facies relics occur in a similar geolo-and Gregory, 1967; Sturm and Carryer, 1970).
Large-scale geological mapping and systematic gical and lithological setting in the Terra Nova
Bay region of the southern Wilson Terranesampling of the crystalline basement rocks were
Fig. 1. Geological sketch map of Oates Coast and North Victoria Land at the Pacific End of the Transantarctic Mountains. Probable
Precambrian and Cambro-Ordovician lithologies of the three tectonic terranes are undierentiated except low-grade metasediments
of McCain Blu(MC) and Berg Mountains ( B). Post-Ordovician lithologies are omitted. Black dots denote occurrences of granulite-
facies relics. WE, Western Exiles Thrust; EE, Eastern Exiles Thrust; W, Wilson Thrust.
237U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
Fig. 2. Sample locations and regional distribution of critical phases within the metamorphic complex of the northern Wilson Terrane
at Oates Coast. All mineral assemblages occur together with quartz, plagioclase, biotite and ±K-feldspar. Thrusts are labelled like
in Fig. 2. The hatched area indicates the transition between eastern and central zone. A, Mt. Archer; BM, Berg Mountains; H, Harald
Bay; LM, Lazarev Mountains; MC, McCain Blu; RK, Ringgold Knoll; TP, Thompson Peak.
(Fig. 1). Several questions arise concerning the morphic rocks from the highest-grade core of the
Oates Coast crystalline basement. In an attempthigh-grade basement rocks of the Oates Coast.
What were the PTconditions of dierent meta- to unravel the metamorphic history, we discuss
the metamorphic PTconditions and present newmorphic stages/events? To what extent can geo-
chronological data resolve the metamorphic and geochronological data. Compared with data from
the southern Wilson Terrane at Terra Nova Bay,cooling history? How do PTtdata from the
Oates Coast basement match those from other our results suggest a common tectonometamorphic
evolution for the high-grade metamorphic coreareas of the Wilson Terrane? In order to clarify
these questions, detailed petrological and geochro- complexes at both ends of the 600 km long North
Victoria Land/Oates Coast sector of the Rossnological investigations were initiated. In this
paper, we concentrate on the evolution of meta- Orogenic belt.
238 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
2. Geological setting Terrane is well investigated in the Terra Nova Bay
area. Here, amphibolite-facies metamorphism
reached maximum temperatures of 750°C at pres-The Ross Orogeny took place at the paleo-
Pacific margin of the Precambrian Antarctic conti- sures of 3.5–5 kbar (cf Schubert and Olesch, 1989;
Palmeri et al., 1991; Palmeri, 1997). U–Pb mona-nent in the Early Paleozoic as a result of accretion-
ary processes during an orthogonal plate zite, zircon and titanite ages dating the amphibo-
lite-facies stage range from 490 to 480 Ma ( Kleeconvergence (Stump, 1995; Tessensohn, 1997). The
Ross Orogen is roughly traced by the present et al., 1990; Klee, 1995 ). K–Ar, 40Ar–39Ar and
Rb–Sr mica data between 480 and 440 Ma indicategeomorphologic heights of the Transantarctic
Mountains. In the area of North Victoria Land cooling of the basement down to the mica blocking
temperatures ( Vita-Scaillet et al., 1994; Klee andand Oates Coast, plate convergence with a craton-
ward-directed subduction of oceanic crust led to Henjes-Kunst, in preparation). Opx–Grt-bearing
relic mineral assemblages of an earlier granulite-the accretion of the outboard Bowers and
Robertson Bay Terranes to the westernmost, facies metamorphism with Opx being in part
replaced by Ath during the subsequent amphibo-inboard Wilson Terrane [ Fig. 1; Kleinschmidt and
Tessensohn (1987); Kleinschmidt et al. ( 1992); lite-facies metamorphic stage were found in several
outcrops along the eastern edge of the Deep FreezeMatzer (1995)]. The Wilson Terrane is interpreted
as an active continental margin of the Antarctic Range (Fig. 1). Rocks from these outcrops contain
relic structures diverging from the common RossCraton during the Ross Orogeny. It is dominated
by medium- to high-grade metamorphic rocks Orogenic direction. PTestimates yielded ca
8 kbar and 800°C for the granulite-facies stage.which were intruded by syn- to post-tectonic calc-
alkaline igneous rocks of the Granite Harbour Sm–Nd model ages between 1.8 and 2.2 Ga point
to Precambrian continental crust being involvedsuite of magmatic arc anity (Gunn and Warren,
1962; Borg et al., 1987; Vetter and Tessensohn, in these high-grade metamorphic basement rocks
(cf Talarico and Castelli, 1995). From all this1987; Armienti et al., 1990; Fenn, 1993; Schu
¨ssler
et al., 1993). information, a multistage tectonometamorphic his-
tory involving a Proterozoic granulite-facies eventThe high-grade metamorphic rocks of the whole
Wilson Terrane are subdivided into an eastern, followed by an amphibolite-facies event in Early
Paleozoic time is deduced for the highest-grademedium- to high-pressure belt and a western, low-
pressure belt (Grew et al., 1984). Low-grade meta- basement rocks of the low-pressure belt in the
Terra Nova Bay area (cf Talarico and Castelli,sedimentary rocks accompany the low-pressure
belt along its western margin throughout almost 1995 and references given therein).
The Oates Coast area forms the northern partthe whole Wilson Terrane, but sporadically also
occur in a transition between the low-pressure and of the low-pressure belt ( Schu
¨ssler, 1996). Due to
compressive tectonics during Ross Orogenic ter-the medium- to high-pressure belt.
For the eastern, medium- to high-pressure belt, rane accretion, the high-grade basement rocks of
this area were detached and thrust to the east andwhich extends from the Lanterman Range to the
Dessent Ridge (Fig. 1), upper-amphibolite-facies to the west onto low-grade metasedimentary rocks
of the Wilson Terrane [ Figs. 1 and 2; Flo
¨ttmannmetamorphic conditions at 5–10 kbar were
deduced (cf Goodge and Dallmeyer, 1996 ). In the and Kleinschmidt (1991, 1993); Kleinschmidt
(1990)]. Within the high-grade basement, theseLanterman Range, high-pressure metamorphism
is documented by the occurrence of eclogites ( Di thrust tectonics also led to the occurrence of zones
with dierent metamorphic grades at the sameVincenzo et al., 1997). U–Pb and Sm–Nd ages
between 500 and 492 Ma from eclogites and level of exposure (Schu
¨ssler, 1996). The Wilson
Thrust ( W in Figs. 1 and 2) and the westerngneisses of the Lanterman Range were interpreted
to date the time of peak metamorphism (Goodge branch of the Exiles Thrust System ( WE ) form
the eastern and western borders of the high-gradeet al., 1995; Di Vincenzo et al., 1997).
The western, low-pressure belt of the Wilson basement rocks, respectively. The eastern branch
239U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
of the Exiles Thrust System ( EE ) separates a In parts of the basement rocks, a retrogressive,
but still syntectonic overprint caused far-reachingwestern zone from a central zone. Dierences in
metamorphic grade indicate a tectonic contact re-equilibration and transformation of former
high-grade gneisses into muscovite–biotite gneissesbetween the central zone and an eastern zone
as well. with only few relics of cordierite and sillimanite
(Schu
¨ssler, 1996 ).The metamorphic basement consists of extensive
series of psammitic metasediments with local varia- The final stage of the high-grade metamorphic
evolution was accompanied by the intrusion oftions to more quartzitic or more pelitic composi-
tions. Dierences in lithology and metamorphic late- to posttectonic pegmatites. A garnet–tourma-
line pegmatite (sample RK12) having intruded thegrade allow to distinguish the central zone from
the eastern and the western zones [Fig. 2; Schu
¨ssler metamorphic basement of the central zone at
Ringgold Knoll (Fig. 2) was dated by the Rb–Sr,( 1996)]. The metasediments in the latter two zones
are extremely monotonous, whereas abundant Sm–Nd, K–Ar and 40Ar–39Ar methods. A Rb–Sr
mineral-whole rock isochron yielded an age ofcalc-silicate interlayers, sometimes associated with
amphibolites, occur in the central zone. 492±8 Ma, interpreted to date the pegmatite
emplacement. Cooling to the blocking temperatureFurthermore, tectonic lenses of ultramafic rocks
within the metasediments are restricted to the of the K–Ar system in muscovite was dated to ca
470–475 Ma (Schu
¨ssler and Henjes-Kunst, 1994).central zone.
All basement rocks were aected by variable Except the critical mineral assemblages and reac-
tions observed in the migmatites and except somedegrees of migmatization. In the eastern and west-
ern zones, metatexites with in situ formation of geochronological data of the Ringgold Knoll peg-
matite, nothing is known about metamorphic evo-small leucosomes, or migmatites with up to deci-
metre-wide leucosomes and melanosomes are lution and age relations in the Oates Coast part
of the Wilson Terrane. To obtain more informa-common. In the central zone, additional diatexites
are widespread and show advanced magmatic tex- tion, three samples of migmatites of the central
zone, in part with granulite-facies relics, weretures with sporadic occurrence of nebulitic restite.
In the eastern and western zones, the prograde chosen for detailed petrological and geochronolog-
ical investigations to find out:metamorphic equilibria
$
the PTconditions of the last metamorphic
Ms+Qtz=Sil+Kfs+Vevent which led to a widespread migmatization
and which is predominantly preserved in the
Bt+Sil+Qtz=Crd+Kfs+V
large majority of the basement rocks;
were recognized in pelitic bulk compositions [min-
$
the PTconditions of an earlier granulite-facies
eral abbreviations after Kretz ( 1983)]. In the event which is evident from some relic mineral
central zone, the higher-grade reactions assemblages preserved in part of the migmatites;
$
the time of the last metamorphic event; and
Bt+Sil+Qtz=Crd+Grt+Kfs+V
$
the time of cooling subsequent to the last meta-
Bt+Sil+Qtz=Crd+Spl+Kfs+Vmorphic event.
The new data regarding these points allow the
are documented. In addition, a relic granulite- reconstruction of at least part of the PTtevolu-
facies mineral assemblage tion of this basement sector and enable a compari-
Qtz+Pl+Bt+Opx±Grt±Cpx son with other basement segments of the Wilson
Terrane. Additionally, new geochronological data
occurs in migmatites of the central zone. A late from the Ringgold Knoll Pegmatite give more
decompression at still high temperatures for the precise insight into the relations between meta-
central zone is indicated by growth of Crd accord- morphism and magmatism of the region.
ing to the reaction Unfortunately the relic granulite-facies mineral
assemblages in the migmatites are rare and tinyGrt+Sil+Qtz+V=Crd.
240 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
and, in case of the Opx-bearing relics, sometimes
heavily altered. Their preparation for isotope
analysis to date the granulite-facies stage seems to
be impossible therefore.
3. Sample description: petrography and mineral
chemistry
US-380 was taken from the eastern ridge of
Thompson Peak (69°2430S, 157°4430E; Fig. 2 )
and forms part of monotonous migmatitic gneisses.
The major minerals are Grt+Bt+Sil+Kfs+
Fig. 3. Relic granulite-facies mineral assemblage typically
Pl+Qtz, with accessory apatite, monazite and rare
occurring as layers and schlieren in migmatites of the Harald
opaque phases. Biotite shows a weak preferred
Bay and Mt. Archer area in the central zone of the Oates Coast
orientation as typical for migmatites. Garnet is
basement: orthopyroxene (medium grey) and biotite (dark)
together with plagioclase and quartz ( light ).
sometimes euhedral (up to 2 mm in grain size) but
more frequently is corroded along the edges.
Quartz, plagioclase and K-feldspar form gra- Layers or boudins of calc-silicate rocks are interca-
lated. At the northeastern flank of Mt. Archer,noblastic aggregates in interstices between biotite-
dominated parts. K-feldspar may have numerous decimetre to metre wide lenses of ultramafic rocks
are present. Opx-bearing granulite-facies relicssmall exsolutions of plagioclase. Rare sillimanite
was found as inclusion in plagioclase or in the have been found in the migmatites (Fig. 3). The
mineral content of US-495 is Grt+Bt+Crd+marginal parts of garnet.
Two dierent types of plagioclase compositions Sil+Spl+Kfs+Pl+Qtz. Leucocratic parts are
dominated by quartz and plagioclase, with subor-can be distinguished: anorthite contents of ca
60 mol%were found in rare grains with inclusions dinate biotite and K-feldspar. Myrmekitic
intergrowths of plagioclase and vermicular quartzof sillimanite, whereas plagioclase without silli-
manite inclusions has anorthite contents between can be observed. The melanocratic parts are
quartz-poor or even-free and are dominated by38 and 42 mol%. The latter also forms exsolution
lamellae in K-feldspar which is composed by biotite which defines the schistosity of the rock.
The biotite is accompanied by plagioclase±88–91 mol%orthoclase and 12 to 9 mol%albite.
Biotite has uniform XMg-values of 0.47–0.48. cordierite. Most garnets (0.5–4.0 mm in size) are
euhedral, some of them show corrosion texturesGarnet is almandine-dominated but contains con-
siderable amounts of pyrope and spessartine. A along the edges. Biotite inclusions may be strongly
orientated parallel to the matrix foliation.slight zonation was recognized across three large
garnet grains. Wide cores of 1.2–1.8 mm in size Cordierite, green spinel, plagioclase, quartz and
ilmenite form additional inclusions in garnet.have a composition of 65–67 mol%almandine,
21–22 mol%pyrope and 8–9 mol%spessartine. In Green spinel may also be part of the matrix
assemblage, but typically occurs in conspicuouscontrast, spessartine increases to 12–15 mol%and
pyrope decreases to 12–18 mol%in the 0.1–0.2 mm patches with the assemblage Crd+Bt+Sil+
Spl+Pl+Qtz ( Fig. 4 ). Sillimanite can also bewide rims, while the almandine content of
66–67 mol%is identical to that of the cores. included in plagioclase.
Selected microprobe analyses of the mineralsUS-495 is a dark, gneissic migmatite and was
collected at the southwestern ridge of Mt. Archer which were taken for PTcalculations are given
in Table 1. The anorthite content of plagioclase(69°1330S, 157°3600E; Fig. 2). The outcrop is
dominated by migmatites with clearly defined leu- varies between 27 and 42 mol%. In contrast to
sample US-380, plagioclases with and withoutcosomes and melanosomes and by diatexites.
241U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
tine and 16–20 mol%pyrope. The rims consist of
68–72 mol%almandine, 12–16 mol%spessartine
and 11–18 mol%pyrope.
US-501 was collected at the southwestern coast-
line of Harald Bay, southeast of Mt. Archer
(69°1320S, 157°4430E; Fig. 2). The outcrop is
formed by rather homogeneous garnet-rich diatexi-
tic migmatites which in part contain melanosomes
and remnants of calc-silicate layers. The matrix of
US-501 is formed by Grt+Bt+Kfs+Pl+Qtz+
Gr. The structure of the medium- to coarse-grained
rock is more or less irregular. In part, biotite is
clearly orientated. A weak dierentiation into
Fig. 4. Relic granulite-facies mineral assemblage ( 1), occurring
more melanocratic areas and more leucocratic ones
in several conspicuous patches of sample US-495 from Mt.
is evident from the hand specimen and the thin-
Archer in the central zone: spinel (dark), biotite (medium grey)
sections. K-feldspar forms large, xenomorphic
and sillimanite (fibres) together with cordierite, plagioclase and
quartz (light).
grains within the leucocratic parts, accompanied
by smaller, often platy plagioclase and quartz.
Plagioclase and quartz sometimes occur in myr-sillimanite inclusions have similar anorthite
contents. K-feldspar shows a wide compositional mekitic intergrowth. Xenomorphic garnet (up to
several millimetre in diameter), biotite, plagioclaserange with orthoclase between 61 and 91 mol%
and 1–3 mol%celsiane, the rest being albite. XMg and small amounts of quartz are the main constitu-
ents of the more melanocratic parts. Garnet con-values of matrix biotite vary from 0.41 to 0.49.
Distinctly higher XMg values up to 0.65 were tains inclusions of biotite, cordierite, sillimanite,
green spinel, ilmenite, plagioclase and quartz.observed in biotites included in garnet, whereas
those included in cordierite have XMg values down Narrow, frayed strips of graphite as a minor
component are distributed in the whole matrix,to 0.42. Cordierite gave rather uniform XMg data
between 0.61 and 0.65 which may rise up to 0.68 but do not occur as inclusions in garnet. Accessory
minerals are monazite, zircon and rare apatite.when cordierite is included in garnet.
Green spinel is a hercynite-dominated solid solu- K-feldspar which usually shows thin perthitic
exsolution lamellae is composed of 84–92 mol%tion of 68–72 mol%hercynite, 11–15 mol%spinel
and 10–14 mol%gahnite, with 1–3 mol%magne- orthoclase, 7–15 mol%albite and 1–2 mol%celsi-
ane. Plagioclase has anorthite contents oftite and ca 1 mol%galaxite as minor endmembers.
Some grains show enhanced gahnite contents up 40–44 mol%.XMg for biotite in the matrix and at
the rims of garnet ranges from 0.54 to 0.57. Valuesto 28 mol%at the expense of hercynite. If included
in garnet, the contents of magnetite and spinel from 0.60 to 0.67 were observed for biotites
included in garnet. One green spinel included inmay increase up to 6 and 19 mol%, respectively,
whereas gahnite decreases to 7 mol%. garnet consists of 60 mol%hercynite, 28 mol%
spinel, 9 mol%gahnite and 1 mol%magnetite.Garnet is Ca-poor and has an almandine-domi-
nated pyralspite composition. Each of the seven Garnet is a nearly pure solid solution of alman-
dine and pyrope with subordinate spessartine andprofiles measured across garnet grains exhibits a
slight chemical zonation, usually with a homogen- andradite. Three profiles across garnet grains show
either a homogeneous composition of 66–67 mol%eous central part and a smaller rim of distinct
chemical composition. The rims are characterized almandine, 26–27 mol%pyrope, 3–5 mol%andra-
dite and 2–3 mol%spessartine, or a slight zonationby decreasing pyrope and increasing spessartine
contents. Almandine remains constant or increases with a homogeneous core composition and an
almandine increase and pyrope decrease to 70 andparallel to spessartine. Typical core compositions
are 67–69 mol%almandine, 9–12 mol%spessar- 23 mol%, respectively, at the rims.
242 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
Table 1
Compositions of phases taken for the calculation of PTconditions of the assemblage (1) in migmatite US-495
US-495 loc 3 loc 3 loc 3 loc 3 loc 3 loc 3 loc 7 loc 7 loc 7 loc 7 loc 9 loc 9 loc 9 loc 9 loc 9
bt 1 bt 2 crd 1 crd 2 pl spl bt crd pl spl bt1 bt2 bt 3 crd spl
Wt%
SiO234.51 33.84 47.75 48.41 61.22 0.02 35.00 48.40 59.99 0.00 35.65 34.34 33.79 48.33 0.00
TiO22.82 3.01 0.04 0.00 0.00 2.60 0.05 0.02 3.26 3.76 3.89 0.00 0.00
Al2O319.04 19.95 32.76 32.63 24.13 57.91 18.77 32.81 24.82 57.96 21.83 18.60 19.26 32.77 57.80
Cr2O30.11 0.03 0.21 0.03 0.17 0.18 0.09 0.24 0.17
Fe2O33.18 3.18 0.48 0.13 0.18 1.83 0.00 0.90 0.06 1.62 2.98 3.22 3.32 0.33 1.95
MgO 9.82 9.41 8.00 8.15 0.00 3.52 9.73 8.04 0.00 3.36 7.22 8.90 8.19 7.97 3.17
CaO 0.00 0.01 0.03 0.03 5.62 0.04 0.00 0.02 6.66 0.00 0.01 0.00 0.00 0.03 0.00
MnO 0.23 0.12 0.53 0.44 0.01 0.47 0.13 0.47 0.02 0.49 0.17 0.19 0.17 0.44 0.46
FeO 16.21 16.22 7.41 7.86 — 30.66 19.06 7.29 — 29.62 15.21 16.44 16.91 8.08 30.74
ZnO————— 4.83 — — — 6.24 — — — — 5.42
Na2O 0.30 0.33 0.36 0.28 8.21 0.21 0.35 7.84 0.23 0.31 0.29 0.25
K2O 9.05 8.88 0.00 0.01 0.10 8.85 0.04 0.14 8.05 9.09 9.13 0.00
Total 95.27 94.98 97.36 97.94 99.47 99.49 94.38 98.37 99.53 99.48 94.79 94.94 95.19 98.20 99.71
Cations
Si 2.62 2.57 4.97 5.01 2.73 0.00 2.68 4.99 2.68 0.00 2.66 2.62 2.58 4.99 0.00
Ti 0.16 0.17 0.00 0.00 0.00 0.15 0.00 0.00 0.18 0.22 0.22 0.00 0.00
Al 1.70 1.79 4.02 3.98 1.27 1.95 1.70 3.98 1.31 1.95 1.92 1.67 1.73 3.99 1.94
Cr 0.01 0.00 0.01 0.00 0.00 0.01 0.01 0.01 0.00
Fe3+0.18 0.18 0.04 0.01 0.01 0.04 0.00 0.07 0.00 0.04 0.17 0.19 0.19 0.03 0.04
Mg 1.11 1.07 1.24 1.26 0.00 0.15 1.11 1.23 0.00 0.14 0.80 1.01 0.93 1.23 0.14
Ca 0.00 0.00 0.00 0.00 0.27 0.00 0.00 0.00 0.32 0.00 0.00 0.00 0.00 0.00 0.00
Mn 0.01 0.01 0.05 0.04 0.00 0.01 0.01 0.04 0.00 0.01 0.01 0.01 0.01 0.04 0.01
Fe2+1.03 1.03 0.64 0.68 0.73 1.22 0.63 0.71 0.95 1.05 1.08 0.70 0.74
Zn ————— 0.10 — — — 0.13 — — — 0.12
Na 0.04 0.05 0.07 0.06 0.71 0.03 0.07 0.68 .03 0.05 0.04 0.05
K 8.88 0.86 0.00 0.00 0.01 0.87 0.01 0.01 0.77 0.88 0.89 0.00
Total 7.74 7.73 11.03 11.04 5.00 2.99 7.77 11.02 5.00 2.99 7.50 7.71 7.68 11.03 2.99
The indication of the analyses refers to that used in Table 2 for the individual calculations. Cations are normalized on an oxygen
number of 11 for biotite, 18 for cordierite, 8 for plagioclase and 4 for spinel. H2O contents are not considered.
4. Petrological results carried out on the assemblage (1) Crd+Bt+
Sil+Spl+Pl+Qtz which was analysed in three
small domains of the thin-section, signed as loca-Assuming local equilibria, conditions of forma-
tion were calculated for mineral assemblages of tions 3, 7 and 9 in Table 2. For the locations 3
and 9, three calculations involving analyses ofUS-495 and US-501 employing the average PT
method of Powell and Holland (1994) and using dierent biotite or cordierite grains were done.
Furthermore, assemblage (2) Grt+Crd+Bt+an updated version of the thermodynamic data set
of Holland and Powell ( 1990). Once the endmemb- Pl+Qtz was analysed and calculated, which typi-
cally forms most of the matrix of this sample. Iners of the minerals in an equilibrium assemblage
have been identified, it is possible to balance all US-501, the assemblage (3) Grt+Bt+Sil+
Pl+Qtz±Spl was used.the reactions among those endmembers. With the
thermodynamic data available, each reaction can For assemblage (1 ) it is possible to balance five
independent reactions between cordierite, biotite,be used for characterizing the pressure and temper-
ature of formation of the assemblage. sillimanite, spinel, plagioclase, quartz and H2O.
The calculated pressures and temperatures areRegarding sample US-495, calculations were
243U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
Table 2
PTdata with 2serrors and x2values calculated for assemblage (1) Crd+Bt+Sil+Spl+Pl+Qtz which was found at the domains
3, 7 and 9 of sample US-495
Assemblage XH20 P(kbar) 2s(P) T(°C) 2s(T) x2
Loc 3
crd1–bt1–sp1–pl –qtz–sil –fluid 1.0 7.6 1.2 827 124 0.58 (1.54)
0.6 7.1 1.1 779 110 0.49 (1.54)
crd1–bt2–spl–pl–qtz–sil–fluid 1.0 7.7 1.3 837 133 0.49 ( 1.54)
0.6 7.1 1.1 786 117 0.40 (1.54)
crd2–bt1–spl–pl–qtz–sil–fluid 1.0 7.8 1.2 833 125 0.63 ( 1.54)
0.6 7.5 1.1 788 112 0.53 (1.54)
Loc 7
crd–bt– spl–pl–qtz–sil–fluid 1.0 7.5 1.0 787 109 0.85 ( 1.54)
0.6 6.3 0.9 730 93 0.78 (1.54)
Loc 9
crd–bt1–spl–pl–qtz–sil–fluid 1.0 8.0 2.0 795 238 0.72 (1.54)
0.6 6.9 1.7 762 220 0.65 (1.54)
crd–bt2–spl–pl–qtz–sil–fluid 1.0 7.9 1.4 802 141 0.64 (1.54)
0.6 7.0 1.2 752 124 0.56 (1.54)
crd–bt3–spl–pl–qtz–sil–fluid 1.0 7.8 1.5 797 173 0.62 (1.54)
0.6 6.7 1.3 751 153 0.54 (1.54)
The calculations have been performed using an updated version of the internally consistent thermodynamic dataset of Holland and
Powell (1990 ) and an updated version of the program THERMOCALC ( Powell and Holland, 1988 ).
correlated and the best average PTestimate is XH2O=1 range from 7.5 to 8 kbar and from 790
to 840°C, indicating medium-pressure granulite-obtained by least-square techniques, allowing the
suncertainties to be calculated. Finally, a x2test facies conditions ( Fig. 5 ). The calculated values
decrease by ca 50°C and 1 kbar when a fluidis applied to the average PTresult in order to
test for the reliability of the estimate. In the case composition of XH2O=0.6 is assumed. The PT
of five independent reactions, as in assemblage
(1), the x2value should be <1.54. In all our
calculations this value was well below 1.0 ( Table 2)
indicating the reliability of the results as well as
supporting our initial assumption of local
equilibrium.
All calculations were performed assuming ideal-
mixing-on-sites activities, except for garnet and
plagioclase. Garnet was described with the regular
model of Newton and Haselton (1981), and plagio-
clase was modelled as a molecular solution employ-
ing the mixing parameters of Newton et al. ( 1980 ).
For each assemblage, the calculations were carried
Fig. 5. PTdata calculated for the medium-pressure granulite-
out for various fluid compositions. The results
facies assemblages (1), #,$, and the low-pressure granulite-
including the 2suncertainties are given in Table 2
facies to upper-amphibolite-facies assemblages (2), %,&; and
and are shown on the PTdiagrams in Figs. 5
(3), 6,+. For one point of each metamorphic stage, the error
and 11.
range is given. Filled symbols denote results from calculations
at XH2O=1, open symbols those at XH2O=0.6.
For assemblage (1), all PTestimates at
244 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
Table 3
U–Pb analytical results for zircons of migmatite US-501, for monazites of migmatites US-380, US-495 and US-501, and for zircons and monazite of pegmatite RK12
Sample Size (mm) Concentrations (ppm) Measured isotope ratios Corrected isotope ratiosaApparent age ( Ma)
UPb206Pb 208Pb/207Pb/206Pb/206Pb/±2s207Pb/±2s207Pb/Corr. 206Pb/207Pb/207Pb/±2s
total rad. 206Pb 206Pb 204Pb 238U235U206Pb coe.238U235U206Pb
Zircon
US-501 >125 325 39.5 32.9 0.110954 0.092271 11039 0.117527 179 1.47479 225 0.09101 0.997 716 920 1447 ±1
US-501 100–90 374 43.3 36.6 0.098822 0.084656 7150 0.113984 172 1.29956 201 0.08269 0.975 696 846 1262 ±2
US-501 90–80 386 43.3 36.8 0.093779 0.083283 7846 0.110909 167 1.24612 189 0.08149 0.991 678 822 1233 ±1
US-501 125–100 371 43.7 36.9 0.098822 0.086499 18512 0.115567 174 1.36625 206 0.08574 0.998 705 875 1332 ±1
US-501 80–62 395 40.1 34.6 0.086616 0.077898 11124 0.101882 154 1.07640 168 0.07663 0.969 625 742 1111 ±3
RK12 500–250 6395 461 454b0.013746 0.060446 4245 0.077756 235 0.61215 195 0.05710 0.948 483 485 495 ±2
RK12 200–160 5867 422 414b0.016304 0.061405 3250 0.077303 238 0.60760 204 0.05701 0.916 480 482 492 ±3
Monazite
US-501 125–100 1943 1396 132.8 9.333274 0.058691 7262 0.079579 243 0.62196 193 0.05668 0.983 494 491 479 ±4
US-501 100–80 1973 1420 135.1 9.334872 0.058975 7372 0.079688 203 0.62629 162 0.05700 0.988 494 494 492 ±3
US-495 125–100 4499 1338 301.5 3.340897 0.057910 13610 0.078007 178 0.61135 141 0.05684 0.990 484 484 485 ±2
US-495 >125 5291 1474 359.8 3.006496 0.057670 16128 0.079144 178 0.61947 140 0.05677 0.995 491 490 483 ±2
US-380 >80 7146 1612 481.9 2.265368 0.057922 12980 0.078486 121 0.61471 97 0.05680 0.993 487 487 484 ±1
US-380 80–62 6150 1572 415.2 2.696669 0.058420 9583 0.078593 119 0.61662 95 0.05690 0.989 488 488 488 ±2
RK12 125–80 25757 3118 3106b0.717675 0.058633 8177 0.078838 237 0.61890 189 0.05694 0.987 489 489 489 ±1
aCorrection for fractionation, spike, blank, initial common lead.
bValues for total radiogenic Pb.
245U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
data vary within a rather limited range of ca ±5°C
and ±0.1 kbar, if analyses of dierent cordierite
or biotite grains of the same domain are used for
the calculations, as shown for the domains 3 and
9 in Tables 1 and 2.
Significantly lower PTconditions of 640–650°C
and 4–5 kbar at XH2O=1.0 were found for assem-
blage (2). If calculated with XH2O=0.6, the values
decrease by ca 30°C and 0.5 kbar, respectively.
Assemblage (3) in US-501 gave pressures between
4.1 and 5.7 kbar, however, in a wide temperature
range from 720 to 870°C (the PTrange is defined
when garnet core compositions are taken for the
calculations, but by using rim concentrations, the
data points plot into the same range). If calculated
Fig. 6. Concordia diagram with U–Pb dates between 484 and
with XH2O=0.6, the pressures range from 4.1 to
494 Ma for six monazite fractions of the investigated migmat-
5.5 kbar is identical to the range given for
ite samples.
XH2O=1. Nevertheless XH2O=0.6 seems to be more
realistic as the matrix of US-501 contains graphite
which indicates lowered H2O activities ( Fig. 5 ). below the closure temperature between 487 and
494 Ma. However, reverse discordance may also
result from analytical problems. For the present
study, most potential mechanisms [e.g. poor spike5. Isotope data
calibration, incomplete dissolution; Hawkins and
Bowring (1997) and references herein] can confi-5.1. Metamorphic rocks
dently be ruled out, except mass spectrometric
problems which may have occurred for a shortU–Pb isotope analysis was carried out on mona-
zite from US-380, US-495 and US-501. The mona- time. Even in this case, the age range indicated by
the 207Pb/206Pb ages ( 479–492 Ma) still appears tozites are commonly included in biotite, cordierite
and garnet, that is, the more melanocratic parts remain geologically significant. If the size fractions
not overlapping with the discordia are excluded,of the rocks, but they also occur as inclusions in
plagioclase and quartz or along grain boundaries the 207Pb/206Pb ages of the remaining fractions
indicate a more restricted time span between 484between these minerals. Two dierent size fractions
for each sample were analysed ( Table 3). Four and 492 Ma.
Five zircon size fractions (clear, short prismatic,fractions yielded concordant or nearly concordant
U–Pb ages which range between 484 and 494 Ma without visible inclusions) of sample US-501 were
also analysed by use of the conventional U–Pb(Fig. 6). In all samples, one grain size fraction
plots above the concordia. The highest degree of multigrain method (Fig. 7, Table 3). Three zircon
fractions [C, D, E in Fig. 7 (a)] define a ‘discordia’reverse discordance is observed for the largest
grain size fractions of samples US-501 and US-495. (MSWD 2.04) with a lower intercept at 469±8Ma
and an upper intercept at 1918±31 Ma.The reason for this result is poorly understood.
Reverse discordance as a product of excess 206Pb Cathodoluminescence studies document the pres-
ence of inherited cores in zircons in at least threeeects are found in magmatic monazites mainly
(Parrish, 1990), but can also be expected in mona- fractions [A, B, C in Fig. 7 (a); Fig. 7(b)] and from
their position in the concordia diagram, at leastzite newly grown during metamorphism. In this
case, the 207Pb/235U dates still provide reliable age fractions A and B appear to have a pre 1.9 Ga
component.information, suggesting that the studied samples
experienced metamorphic crystallization or cooling Rb–Sr and K–Ar isotope analysis of biotite
246 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
two low-temperature increments. For the medium
to high-temperature increments, plateau ages of
476.4±2.3, 472.2±3.3 and 470.1±2.3 Ma were
obtained (order of the data always with increasing
sample numbers).
Sm–Nd isotope analysis of whole rock powders
from the same samples yielded model ages of 1.93,
1.76 and 1.77 Ga ( Table 6), calculated using
depleted mantle parameters (DePaolo, 1981).
5.2. Pegmatite
The geochronological data of the garnet–tour-
maline pegmatite RK12 from Ringgold Knoll
[Fig. 2; Schu
¨ssler and Henjes-Kunst ( 1994)] were
supplemented by U–Pb analyses on one monazite
and two zircon fractions (Table 3; Fig. 10). U
contents of ca 6000 ppm for zircon and ca
26 000 ppm for monazite which are up to 20 times
higher than those of zircons and monazites from
the metamorphic rocks support a magmatic origin
of these minerals in the pegmatite. RK12 zircons
are free of inclusions and of brownish colour.
Some of the crystals show a length/width ratio
>10 which is also in favour of an igneous zircon
crystallization. The monazite yielded a concordant
U–Pb age 489±2 Ma. The zircon fractions plot
slightly below the concordia, with U–Pb ages of
Fig. 7. (a) Concordia diagram with U–Pb data points of five
zircon fractions of the migmatite US-501. Three fractions (C–E)
define a straight line which may be interpreted as a discordia
with a lower intercept at 469±8 Ma and an upper intercept
which points to a protolith age of ca 1.9 Ga. (b) Fractions A–C
show inherited cores in the zircons during cathodoluminescence
investigations ( left photo, prismatic zircon 180 mm in length;
right photo, prismatic zircons ca 200 mm in length).
from all three samples yielded 470, 468, 470 Ma
and 473, 469 and 470 (each ±5) Ma, respectively
(Tables 4 and 5; Fig. 8 ), providing average ages of
469±3 Ma for the Rb–Sr system and 471±3Ma
for the K–Ar system. 40Ar–39Ar total gas dates are
474, 471 and 469 (±2) Ma. On all biotites,
40Ar–39Ar incremental heating experiments were
performed in order to verify the geological signifi-
Fig. 8. Comparison of Rb –Sr, K–Ar, 40Ar–39Ar total gas and
cance of the conventional K–Ar dates. They
40Ar–39Ar plateau biotite ages of the investigated migmatites
yielded slightly disturbed age spectra ( Fig. 9 ) with
and of K–Ar and 40Ar–39Ar muscovite data of pegmatite RK12
found at Ringgold Knoll.
step ages of 300–380 and 400–460 Ma for the first
247U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
Table 4
Rb–Sr analytical results for whole rocks and biotite fractions of samples US-380, US-495 and US-501
Sample Concentration (ppm) Isotope ratios Age (Ma±2s)
Rb Sr 87Rb/86Sr 87Sr/86Sr 2s
US-380
Whole rock 255.3 141.9 5.2321 0.759543 24 470±5
Biotite 559.0 2.15 1489.2 10.697685 24
US-495
Whole rock 180.2 73.7 7.1153 0.775298 19 468±5
Biotite 324.5 1.71 855.65 6.431442 11
US-501
Whole rock 191.3 253.8 2.1855 0.732308 20 470±5
Biotite 642.6 3.61 776.55 5.915233 11
Table 5
K–Ar analytical results for biotite fractions of samples US-380, US-495 and US-501
Sample K (wt%) Rad. Ar (nl g1) Rad. Ar (%) Date (Ma±2s)
US-380 8.05 169.5 99.1 473.3±4.9
US-495 7.72 160.5 98.7 468.9±4.8
US-501 8.04 167.5 98.5 469.8±4.9
Table 6
Sm–Nd analytical data for whole rocks of the investigated migmatite samples from Oates Coast
Sample Nd (ppm) Sm ( ppm) 147Sm/144Nd 143Nd/144Nd e(Nd) t(Nd, DM)
pr.d. t
US-380 39.06 7.438 0.1147 0.511829 15.8 10.7 1.93
US-495 42.94 8.376 0.1175 0.511975 12.9 8.0 1.76
US-501 26.36 4.517 0.1032 0.511807 16.2 16.2 1.77
480–485 Ma. The discordant position of the data 6. Discussion
points can be explained by recent Pb loss of zircons
which have crystallized ca 490 Ma ago. 6.1. Metamorphic history
Interestingly to note, the U–Pb data do not provide
evidence for inherited zircon components within From our petrological data, two stages in the
metamorphic evolution of the central zone canpegmatite RK12.
Schu
¨ssler and Henjes-Kunst (1994) reported be distinguished. Within the migmatites, an earl-
ier, medium-pressure granulite-facies stage is40Ar39Ar plateau ages on two muscovite fractions
of the pegmatite of 471.0±4.2 and 471.1±4.2 Ma. clearly documented by the relic assemblage (1):
Crd+Bt+Sil+Spl+Pl+Qtz. This assemblageRecalculation of these data using a more reliable
standard led to slightly higher plateau ages of which only occurs in very small domains within
the rock matrix indicates temperatures of472.0±2.2 and 472.3±2.2 Ma, respectively.
248 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
Fig. 10. Concordia diagram with U–Pb data points of one mon-
azite fraction and two zircon fractions of pegmatite RK12.
on the calculations are taken into account ( Fig. 5;
Table 2). This metamorphic stage is confirmed by
thin layers and schlieren showing the relic assem-
blage Qtz+Pl+Bt+Opx (±Grt±Cpx). These
relics occur within the migmatites at several loca-
tions of the central zone. In rare cases when garnet
or clinopyroxene coexist with orthopyroxene, the
orthopyroxene is in part or completely replaced
by anthophyllite. Garnet profiles show rather uni-
form compositions except some minor composi-
tional variations along the rims; this lack of
zonation is interpreted as the result of a younger
re-equilibration of the former granulite-facies gar-
nets. Therefore, garnet–orthopyroxene thermome-
try on several samples gave variable temperatures
much below the lower stability of orthopyroxene,
indicating disequilibrium conditions for these Opx-
bearing relic assemblages (Schu
¨ssler, 1996).
The formation of the assemblages ( 2) with
Fig. 9. 40Ar–39Ar spectra of biotite fractions from migmatites
US-380, US-495 and US-501.
Grt+Crd+Bt+Pl+Qtz and (3) Grt+Bt+Sil+
Pl+Qtz±Spl in the matrix of samples US-495 and
US-501, respectively, can be related to the wide-790–840°C at nearly 8 kbar. Despite of the relic
nature of assemblage ( 1 ), equilibrium conditions spread migmatization and recrystallization of the
metamorphic rocks which largely destroyed theare still preserved, as indicated by the narrow
range of PTdata obtained when various mineral former medium-pressure granulite-facies assem-
blages. Most of the garnets in the migmatites eitheranalyses from the assemblage at the same location
were used for the calculation. This is supported by grew during this stage or were re-equilibrated. This
is indicated by euhedral grain forms, by biotitenearly identical results from dierent domains
(Table 2). The medium-pressure granulite-facies inclusions showing the same orientation as the
matrix biotite and by the flat zonation profilesconditions significantly stand out from those of
the later low-pressure granulite-facies to high mentioned above.
The strongly varying temperatures calculatedamphibolite-facies stage, even if the uncertainties
249U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
reactions in the KASH-system ( Xu et al., 1994).
The same minimum temperature is given by the
solidus curve in the system Qtz–Or–An40–H2O
[reaction A in Fig. 11; Johannes (1984)]. The high
portions of former melt in the migmatite series
show that the melting temperature has been
exceeded considerably. The solidus curve and
therewith minimum temperature estimate for the
assemblages (2) and (3) shift towards even higher
values if a H2O activity <1 is taken into account
(Johannes and Holtz, 1990 ). Relic inclusions of
sillimanite in cordierite and in garnet provide
evidence that reaction (3) Bt+Sil+Qtz=
Crd+Grt+Kfs+V took place. Taking into
account the melt-in temperature of minimum
700°C, pressures between 4 and 5.5 kbar are
required for the reaction [ Xu et al. ( 1994)
KFMASH-system; see also Holdaway and Lee
Fig. 11. Possible alternatives for the PTtevolution of the
(1977)]. From all these arguments, a low-pressure
metamorphic basement in the northern Wilson Terrane: if the
granulite-facies to upper-amphibolite-facies stage
medium-pressure granulite relics are of Precambrian age, then
of metamorphism at ca 4–5.5 kbar and minimum
loop (I ) represents the Precambrian metamorphism and loop
700°C, but most probably under significantly
(II ) the metamorphic overprint during the early Paleozoic Ross
higher temperature conditions, can be postulated
Orogeny. If the relics are of early Ross age, then loop ( I ) marks
the PTtevolution during the Ross Orogeny [loop (II ) may
for the investigated rocks.
be neglected in this case]. ( 1 ) Ms+Qtz=Sil+Kfs+V
Decompression at still elevated temperatures is
(Chatterjee and Johannes, 1974; Xu et al., 1994 ); (3 )
evident from reaction (5) Grt+Sil+Qtz+V=
Bt+Sil+Qtz=Crd+Grt+Kfs+V ( Xu et al., 1994) ; ( 5)
Crd, which led to the formation of a rim of
Grt+Sil+Qtz+V=Crd ( Xu et al., 1994 ); (A ) solidus curve in
cordierite around some garnets with sillimanite
the system Qtz–Or–An40–H2O (Johannes, 1984 ); (B) aluminum
silicate triplepoint (Holdaway and Mukhopadhyay, 1993). $,
inclusions in samples from the central zone. The
&and +, Data points for the medium-pressure granulite-facies
reaction indicates minimum pressures of ca
assemblage (1 ) and the low-pressure granulite-facies to upper
3–4 kbar, as calculated for the pure Fe endmemb-
amphibolite-facies assemblages ( 2) and (3) , taken from Fig. 5.
ers in the KFASH-System [Fig. 11; Xu et al.
(1994)]. Introduction of MgO will shift the reac-
from the assemblages (2) and ( 3) may be explained tion to even higher pressures ( Holdaway and
by dierent closing temperatures of the dierent Lee, 1977).
chemical systems used for thermometry. However, All the migmatites that occur in the northern
they may also reflect disequilibrium for the mineral Wilson Terrane seem to have resulted from the
assemblages used. This is the more likely explana- younger low-pressure granulite-facies to upper-
tion, as the influence of a melting phase on the amphibolite-facies metamorphism. But there
system is not considered in the calculations. On remains one question concerning a possible mig-
the other hand, discontinuous mineral reactions matization during the earlier medium-pressure
and the solidus for the system require a PTrange granulite-facies event. The position of PTpoints
which is consistent with the calculated data. A for assemblage ( 1 ) at ca 8 kbar/800°C calculated
minimum temperature of 700°C derives from the at XH2O=1 clearly indicates the formation of melt
upper stability of muscovite according to reaction (Fig. 11). If XH2O was 0.5, the solidus curve shifts
(1) Ms+Qtz=Sil+Kfs+V ( Fig. 11 ). The posi- to ca 770°C at 7 kbar (Johannes and Holtz, 1990)
tion of this reaction in the PTfield was experimen- and the calculated PTvalues for assemblage ( 1 )
tally defined by Chatterjee and Johannes (1974) decrease below 780°C ( Fig. 5), this is close to or
even below the solidus temperature. The lack ofand is corroborated by calculation of mineral
250 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
migmatites of this early metamorphic stage in the disturbed later. Lower step ages for the first
increments of the 40Ar–39Ar spectra demonstratefield may be explained as follows:
$
early migmatites exist, but appear quite similar that the K–Ar systems suered from slight Ar loss
in post-Ordovician time, although K–Ar dates andto the younger ones and can not be distin-
guished from these; 40Ar–39Ar plateau ages for the same samples are
concordant within the given errors. 40Ar39Ar
$
early migmatites have been completely
reworked during the last migmatization, except plateau ages for the biotite fractions suggest that
cooling temperatures were already reacheda few small relics like the Opx or the very tiny
patches with assemblage (1); between 470 and 476 Ma ago. This variation may
point to some minor regional dierences in the
$
the medium-pressure granulite-facies metamor-
phism was ‘dry’ and without formation of melt; late cooling history of the whole migmatite com-
plex. The 40Ar–39Ar plateau ages of 472 Ma of thein this case, the rehydration of the central zone
during the low-pressure granulite-facies to high- pegmatitic muscovites indicate the common cool-
ing history of migmatites and pegmatites.amphibolite-facies event has to be explained.
U–Pb zircon data of the smallest size fractions
C, D and E of sample US-501 define a discordia
6.2. Age relationships
with a lower intercept age of 469±8 Ma, suggest-
ing that Pb loss or growth of new rims stopped at
Estimates for the closure temperature of the
the same time when the Rb–Sr and K–Ar closing
U–Pb system in monazite range between 730 and
temperatures of biotite were reached (nevertheless
640°C (Copeland et al., 1988; Parrish, 1988;
caution is required by interpreting the discordia
Mezger, 1990). A value of 770°C is given by Dahl
because of inherited cores in at least fraction C ).
(1997). These temperatures match in part the
This zircon intercept age corresponds well to a
temperatures which were found for the low-pres-
SHRIMP age of 469±4 Ma on zircon rims of a
sure granulite-facies to upper-amphibolite-facies
diatexite from the Daniels Range [ Fig. 1; Black
stage of the migmatites. The range in monazite
and Sheraton (1990)] which was interpreted to
ages between 484 and 494 Ma is therefore interpre-
date a late thermal overprint. The upper intercept
ted to date this metamorphic stage in the central
of the discordia indicates a protolith age of
zone of the Oates Coast basement.
1918±31, however, due to a complex history of
The emplacement of the Ringgold Knoll pegma-
the zircons in Ross time, combined with multistage
tite is dated to ca 490 Ma by the new U–Pb age
Pb loss and/or zircon growth, no precise age infor-
data of one monazite and two zircon fractions and
mation can be inferred from this upper intercept.
is further constrained by a Rb–Sr mineral-whole
The larger size fractions A and B are interpreted
rock isochron age of 492±8 Ma (Schu
¨ssler and
to be multiply discordant, and from their data,
Henjes-Kunst, 1994). Metamorphism of the coun-
even older protolith ages can be deduced. The
try rocks and the subsequent intrusion of the late-
>1900 Ma protolith ages point to components
tectonic pegmatite took place within a very short
within the migmatites which derived from Early
time span which cannot be resolved by U–Pb
Proterozoic crustal rocks of probably East
age dating.
Antarctic provenance. This is corroborated by the
For biotite, closure temperatures of the Rb–Sr
Sm–Nd whole rock model ages between 1.8 and
and the K–Ar systems range from 350 to 300°C
1.9 Ga from all three migmatite samples.
(Purdy and Ja
¨ger, 1976; Dodson, 1979; Harrison
et al., 1985), a temperature of 450°C for the K–Ar
system is given by Villa and Puxeddu ( 1994 ). The
7. Interpretations
mineral ages for biotites of US-380, US-495 and
US-501 are summarized in Fig. 8. From this, con-
7.1. PTtevolution
cordant Rb–Sr and K–Ar dates of, on average,
469 and 471 Ma give a lower age limit for the
cooling of the migmatites from regional-metamor- The new petrological and geochronological data
presented here can be used to define a segment ofphic temperatures, if the isotope systems were not
251U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
aPTtpath for the central zone of the Oates Gregory (1967) for several locations along the
coastline west of Oates Coast and by Stu
¨we andCoast crystalline basement ( Fig. 11). The earliest
event documented was a medium-pressure granu- Oliver ( 1989 ) for King Georg V Land which is
neighbouring Oates Coast at its western side.lite-facies stage at ca 790–840°C/8 kbar. This was
followed by a low-pressure granulite-facies to Given that a Precambrian granulite-facies event
has to be expected for the precursors of the Wilsonupper-amphibolite-facies stage at minimum 700°C
and 4–5.5 kbar, dated to ca 490 Ma ago. This Terrane rocks. The medium-pressure granulite-
facies assemblages observed in the rocks investi-second metamorphic stage clearly took place in
the course of the Early Paleozoic Ross Orogeny. gated could be interpreted as relics of the remobil-
ized Precambrian basement. In this case, a PTtA decompression at high temperatures as indicated
by reactions (3) and (5) is taken as evidence for a path with at least two separate loops would result
( Fig. 11 ), loop ( I ) of Proterozoic age, clockwiseclockwise direction of the Ross Orogenic PT
evolution. Between ca 490 and 473 Ma, the base- direction not verified, and loop (II ) of Early
Paleozoic age in clockwise direction. Alternatively,ment cooled down from 730–700°C to 350–300°C
[or from 770 to 450°C, taking into account the a medium-pressure granulite-facies metamorphism
could also have taken place early during the Rossclosing temperatures compiled by Villa (in press)].
Therewith a cooling rate of 18–25°CMa1is Orogeny and may have immediately preceded the
low-pressure granulite-facies to upper-amphibo-indicated if continuous cooling is assumed.
The cooling history between 490 and 473 Ma is lite-facies stage during one single PTtloop ( I )
in clockwise direction. In this case, the centralcorroborated by age dates from a pegmatite
located at Ringgold Knoll ( Fig. 2). The intrusion zone can be interpreted as a deeper crustal level
of the Ross metamorphic crystalline complex atinto the hot basement took place ca 490 Ma ago,
as established by new U–Pb data on monazite and Oates Coast, compared to the eastern and western
zones (Schu
¨ssler, 1996 ). At the moment, no realzircon and by Rb–Sr and Sm–Nd whole rock–min-
eral isochrons (Schu
¨ssler and Henjes-Kunst, 1994). evidence for either of these two possibilities was
recognized.Final cooling to ca 350°C [500°C, Hammerschmidt
and Frank (1991 )] was dated at 472±2Ma by
means of two 40Ar–39Ar spectra. In addition, these 7.2. Regional aspects
data demonstrate that intrusive activities took
place more or less contemporaneously with the 7.2.1. The western low-pressure metamorphic belt
of the Wilson Terranelow-pressure granulite-facies to upper-amphibo-
lite-facies event. The low pressure belt extends from the Pacific
end of the Wilson Terrane at the Oates Coast forStill unknown is the age of the earlier medium-
pressure granulite-facies event at the Oates Coast. ca 600 km in southeastern direction to the Terra
Nova Bay region ( Deep Freeze Range, see Fig. 1 )The Wilson Terrane is interpreted as an active
continental margin of the East Antarctic craton at the Ross Sea. By comparing the geological
situations, striking similarities become obvious. Induring Ross Orogeny (e.g. Kleinschmidt and
Tessensohn, 1987), and at least parts of the base- both regions, high-grade metamorphic complexes
are in tectonic contact to low-grade metasedimen-ment should therefore derive from Precambrian
continental crust. This is substantiated by U–Pb tary series to the west. At the Oates Coast, these
metasediments are represented by the Berg Groupdata points of zircons from sample US-501, but
also by the whole rock Sm–Nd model ages. Sm–Nd (Skinner et al., 1996 ), at Terra Nova Bay by the
Priestley Formation (Skinner, 1983, 1989; Faddamodel ages of 1.8 to 2.2 Ga (Talarico et al., 1995)
and a U–Pb zircon upper intercept age of et al., 1994). Evidence for a west-southwest
directed thrusting of the high-grade basement2028+30/38 Ma ( Klee et al., 1990 ) from the
Terra Nova Bay in the southern Wilson Terrane towards the craton over the Berg Group metasedi-
ments along the western branch of the Exilessupport this interpretation. Granulite-facies rocks
from the craton were described by McLeod and Thrust System at the Oates Coast was found by
252 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
Flo
¨ttmann and Kleinschmidt (1991, 1993). A sim- 1991; Talarico et al., 1995; Palmeri, 1997 ). Both
contain migmatites which were formed during PTilar thrusting in the Terra Nova Bay region is
assumed, but has not yet been proved conditions cited above. For the polymetamorphic
complex, the migmatite stage is part of a decom-( Kleinschmidt and Matzer, 1990 ). A tectonic con-
tact (Boomerang Thrust) between the high-grade pressional PTpath, whereas the migmatites of
the metasedimentary sequence represent the high-basement and the Priestley Formation is described
by Skinner (1991), but is not regarded as an est metamorphic stage of a counterclockwise heat-
ing path. These contrasting PTpaths areequivalent of the Exiles Thrust at Oates Coast
( Kleinschmidt, 1990 ). interpreted as PTtrajectories of dierent crustal
levels in the same tectonic setting that developedThe high-grade metamorphic basement at Terra
Nova Bay (Deep Freeze Range) is formed by during the Cambro-Ordovician Ross Orogeny
(Palmeri, 1997). Dierent crustal levels in the samemonotonous metamorphic, in part migmatitic
series which are quite similar in lithology to the tectonic setting were also recognized for the vari-
ous zones of the Oates Coast basement (Schu
¨ssler,Oates Coast metamorphic basement. Also,
dierent metamorphic zones can be distinguished 1996 ), with a decompressional PTevolution for
the rocks of the central zone (PTtpath I in(Palmeri et al., 1991). In the highest-grade parts,
the basement contains assemblages with Fig. 11). The PTevolution of the eastern and
western zones has not yet been investigated inGrt+Crd+Bt+Sil+Spl+Kfs+Qtz (Schubert
and Olesch, 1989) and also granulite-facies relics detail.
Monazite, zircon and titanite from several gneisswith Opx–Grt and Opx–Grt–Crd bearing assem-
blages (Talarico et al., 1989, 1995; Talarico, 1990; samples of the Terra Nova Bay basement (with
and without granulite-facies relics) yielded concor-Castelli et al., 1991; Talarico and Castelli, 1995).
For garnet and orthopyroxene bearing granulite- dant to nearly concordant U–Pb ages of
480–490 Ma. A six point zircon discordia definesfacies relics at Terra Nova Bay, Castelli et al.
(1991) and Talarico and Castelli (1995) estimated a lower intercept age of 488±9 Ma for one sample
( Klee et al., 1990; Klee, 1995 ). These dates closelyPTconditions of 800–825°C and 7–8 kbar which
correspond well to the PTdata for the medium- resemble our U–Pb ages on monazites from the
Oates Coast which are interpreted to date the low-pressure granulite-facies stage of the Oates Coast
basement. A second, low-pressure granulite-facies pressure granulite-facies to upper-amphibolite-
facies metamorphism of the Ross Orogeny. Similarstage is proved for the Terra Nova Bay basement
at ca 750–875°C and 4–5 kbar (Castelli et al., to the Oates Coast migmatites, the upper intercept
age of the zircon discordia for a Terra Nova Bay1991; Talarico and Castelli, 1994). PTconditions
for a late amphibolite-facies stage in the migmatites migmatite provides evidence for crustal compo-
nents of Early Proterozoic age ( Klee et al., 1990 ).at Terra Nova Bay are estimated by Schubert and
Olesch (1989) to ca 650°C at 5 kbar. Palmeri et al. This is corroborated by Sm–Nd ages of 1.8–2.2 Ga
(Talarico et al., 1995 ). The cooling history appears(1991 ) and Palmeri ( 1997) calculated 750°Cat
5 kbar as maximum conditions for this stage. to be more complex in the Terra Nova Bay region,
as indicated by K–Ar and 40Ar–39Ar mineral data650°C at 3–4 kbar are given by Talarico and
Castelli (1994) for a retrograde stage within the scattering between ca 480 and 440 Ma (Vita-
Scaillet et al., 1994; Klee and Henjes-Kunst, ingranulite relics. These data generally match those
for the low-pressure granulite-facies to upper- preparation).
From the strong similarities in petrology andamphibolite-facies stage of the Oates Coast
basement. geochronology between Oates Coast and Terra
Nova Bay it is likely that both regions underwentThe metamorphic basement at Terra Nova Bay
can be subdivided into a polymetamorphic com- a common tectonometamorphic history, at least
since the medium-pressure granulite-facies event.plex with granulite relics and into a monometamor-
phic metasedimentary sequence ( Palmeri et al., This assumption is supported by results from the
253U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
Daniels Range which is located in striking direction Monazites and titanites from the Lanterman
between Oates Coast and Terra Nova Bay (Fig. 1). metamorphic complex yielded U–Pb ages of ca
Ulitzka (1987) estimated PTconditions of ca 498 Ma, dating the time when metamorphic tem-
730°C and 4 kbar for the formation of migmatites peratures were ca 650–700°C (Goodge et al.,
in the Daniels Range and reported a steep meta- 1995 ). Ages of 500±5 and 492±3 Ma derive from
morphic gradient to low-grade metasediments Sm–Nd mineral-whole rock isochrons of the eclog-
towards the west. Within the high-grade series, the ite lenses, U–Pb two point discordias (rutile, whole
occurrence of a granulite-facies assemblage with rock) result in ages of 495±6 and 503±6 Ma for
Qtz+Pl+Opx+Grt+Crd+Sil+Spl was noted the same samples (Di Vincenzo et al., 1997),
by Plummer et al. ( 1983) from a breccia pipe. interpreted to approximately constrain the time of
Most of the K–Ar muscovite and biotite dates the high-pressure event. Subsequent cooling
reported by Kreuzer et al. ( 1987) from the Daniels to temperatures <400°C is indicated by 40Ar–
Range vary between 470 and 475 Ma. These dates 39Ar plateau ages of ca 482 Ma on muscovites
are corroborated by new 40Ar–39Ar plateau ages (Goodge and Dallmeyer, 1996; Henjes-Kunst, in
of 475±2 and 476±2 Ma (Henjes-Kunst, unpub- preparation).
lished) and thus correspond to the cooling ages The U–Pb monazite and titanite data suggest
obtained for the Oates Coast basement. In conclu- that the high-grade metamorphism took place ca
sion, the available data imply that during the Late 10 Ma earlier in the Lanterman Range than in the
Precambrian/Early Paleozoic, a rather homogen- Oates Coast area. 40Ar39Ar plateau ages clearly
eous style of orogenic evolution has taken place indicate that during the cooling history the
over a strike length of at least 600 km along the Lanterman Range reached the muscovite closure
Pacific margin of the old Antarctic craton. temperature again ca 10 Ma earlier than the Oates
Coast basement, most likely due to an earlier
uplift.
7.2.2. The eastern medium- to high-pressure
metamorphic belt of the Wilson Terrane
One of the best investigated areas of this belt is
the Lanterman Range in the northwestern Wilson
Terrane (Fig. 1). For the metamorphic rocks of Acknowledgment
this area, Grew and Sandiford (1984) calculated
PTconditions of ca 700°C at 8 kbar for an early The Bundesanstalt fu
¨r Geowissenschaften und
stage of metamorphism, followed by an intermedi- Rohstoe in Hannover is thanked for the invitation
ate overprint at 650–700°C and 5.5–6.4 kbar and of U. Schu
¨ssler to participate in the GANOVEX
a final greenschist-facies stage at 300–370°C and Program. Many thanks are due to all who contrib-
3–5 kbar. Conditions of 700°C and up to 8 kbar uted to the success of the field work with their
are also suggested by Roland et al. ( 1984 ) who logistic or professional help. H. Klappert, M. Metz,
mention that a subsequent, second metamorphic M. Bockrath and P. Macaj are thanked for labora-
stage should have taken place at somewhat lower tory assistance at the BGR. Thanks are due to H.
PTconditions. For metamorphism in the Dessent
Baier (Mu
¨nster) for laboratory assistance and S.
Formation in the southeastern Wilson Terrane,
Rochnowski (Mu
¨nster) for support on the mass
Kleinschmidt et al. (1984) estimated 600°Cat
spectrometer. K.-P. Kelber kindly prepared the
6–7 kbar. Recently, eclogite lenses were found
figures, P. Spa
¨the the thin sections. U. Schu
¨ssler
within the Lanterman Range, documenting meta-
wants to thank W. Schubert for involving him in
morphic conditions of maximum 850°C at mini-
geological research in North Victoria Land and for
mum 15 kbar for a high-pressure stage, followed
helpful advice and discussion. J.D. Kramers, M.
by medium- and low-pressure stages under
Okrusch and F. Talarico are thanked for con-
amphibolite-facies conditions (Di Vincenzo et al.,
1997). structively reviewing the manuscript. Financial sup-
254 U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
port from the Deutsche Forschungsgemeinschaft is US-495 and US-501 was carried out at
the Zentrallaboratorium fu
¨r Geochronologie,gratefully acknowledged.
Mu
¨nster. For Rb–Sr analyses, mineral separates
(40–50 mg) and whole rock powders (ca 100 mg)
were mixed with a 87Rb–84Sr spike in Teflon screw-
Appendix top vials and dissolved in a hot HF–HNO3(5:1)
mixture. Rb and Sr were separated by standard
A1.1. Analytical methods ion-exchange procedures using 2.5 N HCl as elu-
tant. Rb was loaded as chloride on a double
A1.1.1. Mineral compositions Ta-filament assembly and analysed on a NBS-type
Mineral compositions were determined in pol- Teledyne mass spectrometer with a single Faraday
ished thin-sections using a CAMECA SX50 collector. Sr was loaded with H3PO4on single Ta
electron microprobe with wavelength-dispersive filaments and analysed on a VG Sector 54 multicol-
spectrometers at the Mineralogisches Institut, lector mass spectrometer in dynamic mode. During
Universita
¨tWu
¨rzburg. The operating conditions the period of this study, the 87Sr/86Sr ratio of the
were: 15 kV accelerating voltage; 15 nA sample NBS-987 standard was 0.71025±0.00003 ( 2s).
current; and 1–2 mm beam size. Element peaks and Based on repeated measurements, the 87Rb/86Sr
backgrounds were each measured over 20 s, except ratios were assigned an uncertainty of 1%(2s).
for Fe ( 30 s). Synthetic silicate and oxide minerals For other isotope ratios uncertainties are reported
were used for reference standards. Matrix correc- at the 2smlevel, taking into account the within-
tion was calculated by the PAP-program supplied run precision, an estimated uncertainty of 0.1%
by CAMECA. An analytical error of 1%relative for the 87Sr/86Sr spike ratio, the error magnification
for major elements was verified by repeated meas- based on the spike/sample ratio and the blank
urements on respective standards. For low concen- correction. Rb–Sr ages were calculated using the
trations, higher errors must be taken into account. least squares regression technique of York (1969 ).
The detection limit is at concentrations of Errors are reported on the 95%confidence level.
0.05–0.1 wt%for the operating conditions used. A decay constant of lRb 1.42×1011 a1
(Neumann and Huster, 1974) was used for age
calculations. The analytical results and apparentA1.1.2. Isotope analysis
For mineral separation, ca 1.5–2.5 kg of sample ages are given in Table 4.
Decomposition of monazite or zircon and chem-material were crushed in a steel mortar and succes-
sively ground in a tungsten carbid mill for a few ical separation of U and Pb largely followed the
procedures suggested by Krogh (1973) for zircon,seconds. After sieving and washing, monazite and
zircon were extracted from the size fraction but using 6 N HCl instead of HF for dissolution
of monazite. A 235U/208Pb mixed spike was used<350 mm by standard magnetic and heavy liquid
techniques. Biotite was enriched from the size for isotope dilution. U and Pb were analysed on
a VG Sector 54 multicollector mass spectrometerfraction 250–180 mm with a magnetic separator.
All mineral fractions for U–Pb and Rb–Sr analysis in static and dynamic mode, respectively. Pb was
loaded on rhenium single filaments with silica gelwere carefully handpicked. Biotite for Rb–Sr and
K–Ar analyses was also ground under ethanol to and H3PO4. U was loaded with graphite and
H2O on rhenium and analysed using a tripleremove interlayer inclusions. Monazite and zircon
were washed in high purity HCl, H2O and aceton filament configuration. Pb isotope ratios were cor-
rected for 0.012%fractionation per atomic massto remove surface contaminations. Mica concen-
trates were washed in ethanol (p.a) in an ultrasonic unit as determined from measured values of NBS
981. Total analytical Pb blanks were <0.2 ngbath. For Rb–Sr, K–Ar and 40Ar–39Ar analyses,
dry aliquots of the same biotite separates were during the period of this study. An assumed uncer-
tainty in the blank amounts of 50%was used inused.
U–Pb and Rb–Sr isotope analysis of US-380, the error propagation. The calculation of the error
255U. Schu
¨ssler et al. /Precambrian Research 93 ( 1999) 235–258
ellipses in Fig. 6 follows Ludwig ( 1980 ) and con- the isotope abundances, the mass intensities were
corrected for eects of mass discrimination, step-siders the internal precision (2s) of the mass
spectrometric measurements, an estimated uncer- temperature dependent total-system blank, decay
and interfering isotopes produced during irradia-tainty of 0.15%of the U/Pb ratio in the spike,
the error magnification from the spike/sample tion. The error in the 40Ar39Ar age was calculated
by statistical propagation of in-run uncertainties,ratio and the estimated uncertainty (±1%)
in the isotopic composition of the blank the errors of the correction factors and the blank
determinations. Total-gas ages were computed byPb (208Pb/204Pb=37.5, 207Pb/204Pb=15.5, 206Pb/
204Pb=17.72). For initial lead correction, isotopic appropriate weighting of the age, percentage 39Ar
released and calculated uncertainty of the indivi-compositions according to the model of Stacey
and Kramers (1975) were employed. Zircons and dual temperature step. A 40Ar–39Ar age plateau’
is defined by ages recorded by two or more contigu-monazite of the RK12 pegmatite were treated in
a similar way during U–Pb analysis at the ous gas fractions, each representing >5%(and
together >50%) of the total 39Ar released andBundesanstalt fu
¨r Geowissenschaften und
Rohstoe (BGR). being mutually identical within their calculated
uncertainties. Both, the errors of the total-gas andK–Ar and 40Ar–39Ar analyses of biotite sepa-
rates were carried out at the BGR, Hannover. For the plateau ages additionally take into account the
uncertainties derived for the flux-calibration factor.conventional K–Ar dating, K was determined by
flame photometry using Li as an internal standard, All ages and element concentrations were calcu-
lated using the IUGS recommended constantsAr by total-fusion isotope-dilution static analysis
on a MAT CH4 mass spectrometer (cf Seidel et al., (Steiger and Ja
¨ger, 1977). All errors quoted are
calculated on the 95%confidence level (2s).1982). Mean intralaboratory uncertainties ( 2s)in
radiogenic Ar and K are ±0.3 and 1%, respec-
tively. Note that our K–Ar date for the standard
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... Metamorphic rocks in the Lanterman and Salamander ranges are also probably correlative with the Wilson Group (Talarico et al., 1998). Regional metamorphism is variable from greenschist to granulite facies, and in many areas these rocks are intimately mixed with granitoids and migmatites of the Granite Harbour batholith (Babcock et al., 1986;Schüssler et al., 1999Schüssler et al., , 2004. Despite variable and locally high-grade metamorphism, available geochronologic evidence shows that the Wilson terrane does not represent primary Archean or Paleoproterozoic crust of the East Antarctic shield (westward, the first such rocks occur in the Terre Adélie craton of Wilkes Land), but rather is a composite metasedimentary assemblage of latest Neoproterozoic to Cambrian age. ...
... Northern Victoria Land is underlain by three major late Neoproterozoic and early Paleozoic tectonic elements, as noted earlier, which are variably deformed and metamorphosed. The presence of map-scale contractional structures (mainly upright folds and thrusts), discontinuities between different metamorphic P/T types, and early to middle Paleozoic I-type plutonic suites are all indicative of a convergent-margin tectonic setting in the latest Precambrian to early Paleozoic Bradshaw et al., 1985;Gibson and Wright, 1985;Borg et al., 1987;Kleinschmidt and Tessensohn, 1987;Kleinschmidt, 1991a, 1991b;Dallmeyer and Wright, 1992;Goodge and Dallmeyer, 1996;Ricci et al., 1997;Schüssler et al., 1999). Others have stressed the role of strike-slip tectonics (Weaver et al., 1984) or proposed the accretion of allochthonous terranes (Bradshaw et al., 1985), yet numerous connections are apparent between the various tectonic elements and reflect a coherent convergent-margin architecture (see, for example, Tessensohn and Henjes-Kunst, 2005;Federico et al., 2006;Rocchi et al., 2011). ...
... Wilson Group gneisses are separated from the Bowers terrane by steeply-dipping faults and shear zones Gibson, 1984;Roland et al., 1984;Sandiford, 1985;Capponi et al., 2002), but the age and tectonic significance of these zones are controversial. In northern Victoria Land the Ross Orogeny resulted in pronounced folding of Lower Ordovician rocks in the Bowers and Robertson Bay terranes (Bradshaw et al., 1985;Gibson and Wright, 1985), in diachronous cleavage development within the Robertson Bay Group (Dallmeyer and Wright, 1992), and ductile thrusting of Wilson Group metamorphic rocks (Kleinschmidt and Tessensohn, 1987;Flöttmann and Kleinschmidt, 1991a;Schüssler et al., 1999). ...
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... 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-highgrade central zone from high-grade eastern and western zones (Schüssler 1996). ...
... Initial 87 Sr/ 86 Sr 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. ...
... 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 ε Nd 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 ε Nd, t values from -1 to -14. ...
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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.
... In addition to speculations about the position of the boundary between the East Antarctic Craton and the Ross-orogenic belt, there has been some debate about the presence and nature of basement rocks of cratonic origin in the Ross-orogenic WT. Based on the finding of relict granulite-facies minerals in high-grade metamorphic rocks of the westernmost WT it has been argued that these rocks represent East Antarctic cratonic crust reworked in the course of the late Precambrian to early Palaeozoic Ross Orogeny (Talarico & Castelli, 1995;; see also Schüssler et al., 1999). In addition, the results of a Rb-Sr whole-rock dating study on high-grade metamorphics and related magmatites from the coastal area of Oates Land were interpreted to indicate a pre-Ross-orogenic age of parts of the basement in the northwestern WT (Adams & Roland, 2002). ...
... This was in line with the interpretation of earlier conventional U-Pb zircon ages for anatectic rocks of the Daniels Range (northwestern WT) (Sheraton et al., 1987). However, in the course later conventional and in part SHRIMP U-Pb investigations on zircon, monazite and sphene from high-grade basement rocks of the western "granulitic" belt of the WT, these results could not be verified (Black & Sheraton, 1990;Klee et al., 1990;Klee, 1995;Schüssler et al., 1999). In order to clarify the discrepancies in geochronological results for basement rocks of the western WT, a SHRIMP dating study on high-grade metamorphic rocks and undeformed intrusives from the coastal area of Oates Land was initiated. ...
... The present study was performed on high-grade metamorphic and related igneous rocks from the Wilson Hills located in the northwestern coastal area of Oates Land ("Oates Coast") ( Fig. 1) and forming part of the inboard high temperature-dominated metamorphic belt of the WT. In this area, major thrusts activated during final tectonometamorphism of the Ross Orogeny separate units with contrasting metamorphic degrees (Flöttmann & Kleinschmidt, 1991;Schüssler et al., 1999;Schüssler et al., 2004). These structures were interpreted to indicate that blocks from 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). ...
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High- to very-high-grade migmatitic basement rocks of the Wilson Hills area in northwestern Oates Land (Antarctica) form part of a low-pressure high-temperature belt located at the western inboard side of the Ross-orogenic Wilson Terrane. Zircon, and in part monazite, from four very-high grade migmatites (migmatitic gneisses to diatexites) and zircon from two undeformed granitic dykes from a central granulite-facies zone of the basement complex were dated by the SHRIMP U-Pb method in order to constrain the timing of metamorphic and related igneous processes and to identify possible age inheritance. Monazite from two migmatites yielded within error identical ages of 499 ± 10 Ma and 493 ± 9 Ma. Coexisting zircon gave ages of 500 ± 4 Ma and 484 ± 5 Ma for a metatexite (two age populations) and 475 ± 4 Ma for a diatexite. Zircon populations from a migmatitic gneiss and a posttectonic granitic dyke yielded well-defined ages of 488 ± 6 Ma and 482 ± 4 Ma, respectively. There is only minor evidence of age inheritance in zircons of these four samples. Zircon from two other samples (metatexite, posttectonic granitic dyke) gave scattered 206Pb-238U ages. While there is a component similar in age and in low Th/U ratio to those of the other samples, inherited components with ages up to c. 3 Ga predominate. In the metatexite, a major detrital contribution from 545 - 680 Ma old source rocks can be identified. The new age data support the model that granulite- to high- amphibolite-facies metamorphism and related igneous processes in basement rocks of northwestern Oates Land were confined to a relatively short period of time of Late Cambrian to early Ordovican age. An age of approximately 500 Ma is estimated for the Ross-orogenic granulite-facies metamorphism from consistent ages of monazite from two migmatites and of the older zircon age population in one metatexite. The variably younger zircon ages are interpreted to reflect mineral formation in the course of the post-granulite-facies metamorphic evolution, which led to a widespread high-amphibolite-facies retrogression and in part late-stage formation of ms+bi assemblages in the basement rocks and which lasted until about 465 Ma. The presence of inherited zircon components of latest Neoproterozoic to Cambrian age indicates that the high- to very-grade migmatitic basement in northwestern Oates Land originated from clastic series of Cambrian age and, therefore, may well represent the deeper-crustal equivalent of lower-grade metasedimentary series of the Wilson Terrane.
... The northernmost part of the Wilson Terrane was basically investigated for the first time during the German Antarctic North Victoria Land Expeditions GANOVEX V and VII in 1988/89 and 1992/93. There, a subdivision of the crystalline basement into three different NNW–SSE trending zones was recognised: a central zone with granulite-facies gneisses and migmatites is flanked by one eastern and one western zone with gneisses which were formed under lower amphibolite-to lowest granulite-facies conditions (Schüssler, 1996; Schüssler et al., 1999 Schüssler et al., , 2004 ). The zones are confined by the prominent Wilson, Exiles, and Lazarev thrust systems ( Kleinschmidt, 1991, 1993; Läufer et al., 2006 ) except the boundary between the eastern and the central zone which is still of unclear character. ...
... Previously, monazite U–Pb isotopic data have been published from four samples collected in the central, granulite-facies zone of the Wilson Terrane. Two of them were analysed by SHRIMP (Henjes-Kunst et al., 2004), the other two by TIMS (Schüssler et al., 1999Schüssler et al., , 2004). The SHRIMP analysed samples are G8-57.4 ...
... Monazite in medium-to high-grade metapsammitic to metapelitic gneisses and migmatites from the eastern, central and western zonesSchüssler et al., 1999) and are listed inFig. 8. ...
... A still earlier age of 544 ± 4 Ma (SHRIMP U-Pb zircon data) was interpreted as the crystallization age of an S-type granite in the Daniels Range, north-western Wilson Terrane (Black and Sheraton, 1990). But the rock was identified as diatexite by Schüssler et al. (1999) and the 544 Ma zircon population is probably inherited from the sedimentary source rock . Synorogenic to post-orogenic Granite Harbour Intrusives in the Deep Freeze Range have U-Pb zircon ages of 508 to 493 Ma for the oldest, strongly foliated synkinematic granites and tonalites, and 489 to 481 Ma for the younger calc-alkaline granites and mafic dykes (Bomparola et al., 2007). ...
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The Ross-Delamerian orogenic belt was formed along the eastern side of the Australian-East Antarctic continent during west-directed subduction of the Palaeo-Pacific Ocean in the early Palaeozoic. Northern Victoria Land (NVL) in Antarctica was located at a central position of the Ross-Delamerian system. Its metamorphic basement is formed by three lithotectonic units formerly interpreted as terranes: the Wilson, Bowers and Robertson Bay terranes (from west to east). Dating of detrital zircons from 14 meta¬sedimentary samples of these terranes combined with petrographical and whole-rock geochemical studies give new insights into the stratigraphic and tectonic evolution of NVL. All samples show very similar zircon age spectra with two main intervals, a Ross/Pan-African-age interval (470–700 Ma) and a Grenville-age interval (900–1300 Ma), as well as subordinate craton-related ages dispersed over the range of ca. 1600–3500 Ma. The Ross/Pan-African-age zircon population tends to get more dominant from the Priestley Formation of the Wilson Terrane to the Molar Formation of the Bowers Terrane, and finally to the Robertson Bay Group, whereas the number of craton-related ages diminishes in this direction. A common East Antarctic source area is indicated for all analyzed samples. The Priestley Formation was deposited on the Palaeo-Pacific passive continental margin of East Gondwana in the late Neoproterozoic after Rodinia breakup. The sequence was subsequently metamorphosed and intruded by the Granite Harbour Intrusives during the Ross Orogeny. The Molar Formation of the Bowers Terrane is interpreted as a turbiditic sequence deposited in an accretionary setting on the active continental margin in the Late Cambrian during and after accretion of the Glasgow island arc allochthon. The thick, homogeneous sequence of the Robert¬son Bay Group resulted from continuous turbiditic sedimentation in an accretionary wedge in front of the Ross Orogen after docking and imbrication of the Glasgow island arc in the Early Ordovician.
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The history of the East Antarctic ice sheet provides important understanding of its potential future behaviour in a warming world. The provenance of glaciomarine sediments can provide insights into this history, if the underlying continent eroded by the ice sheet is made of distinct geological terranes that can be distinguished by the mineralogy, petrology and/or geochemistry of the eroded sediment. We here present a multi-proxy provenance investigation on Pliocene sediments from Integrated Ocean Drilling Program (IODP) Site U1361, located offshore of the Wilkes Subglacial Basin, East Antarctica. We compare Nd and Sr isotopic compositions of < 63 μm detrital fractions, clay mineralogy of < 2 μm fractions, ⁴⁰Ar/³⁹Ar ages of > 150 μm ice-rafted hornblende grains, and petrography of > 2 mm ice-rafted clasts and > 150 μm mineral grains. Pliocene fine-grained marine sediments have Nd and Sr isotopic compositions, clay mineralogy, and clast characteristics that can be explained by mixing of sediments eroded from predominantly proximal crystalline terranes with material derived from inland sources from within the currently glaciated Wilkes Subglacial Basin. Conversely, evidence for such an inland source is absent from ice-rafted hornblende ages. We render a lithological bias against hornblende grains in the doleritic and sedimentary units within the basin the most likely explanation for this observation. ⁴⁰Ar/³⁹Ar hornblende ages however record additional provenance from the distal margins of the Ross Sea, and possibly even the West Antarctic area of Marie Byrd Land. The latter lies > 2000 km to the east and hints at significant iceberg release from the West Antarctic ice sheet during warm intervals of the Pliocene. Together our results make a strong case for combining geochemical and mineralogical signatures of coarse- and fine-grained glaciomarine sediment fractions in order to derive robust provenance interpretations in ice covered areas.
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It is widely accepted that radiometric ages determined on metamorphic minerals from orogenic belts generally reflect their cooling history rather than their primary crystallisation. Armstrong (1966) first developed this concept in detail, while Harper (1967) pioneered its application to the British Caledonides. Its general acceptance, however, stems largely from the investigations of the Central Alps by Professor Jäger and her colleagues at Bern, reviewed elsewhere in this volume. In a young mountain belt the precision of some dating systems, notably Rb-Sr on biotite, K-Ar on biotites and muscovites, and U-fission tracks on apatite, is sufficient to build up a consistent picture of rocks cooling through a succession of different “blocking temperatures”, that is, temperatures at which the various systems start to retain their radiogenic daughter products. Above its blocking temperature each system appears to lose its daughter product completely, and below it the daughter product is quantitatively retained.