P T t evolution of the Wilson Terrane metamorphic basement ...€¦ · P–T–t evolution of the Wilson Terrane metamorphic basement at Oates Coast, Antarctica Ulrich Schu¨ssler

Precambrian Research 93 (1999) 235–258

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

Ulrich Schussler a,*, Michael Brocker b, Friedhelm Henjes-Kunst c, Thomas Will aa Mineralogisches Institut, Universitat Wurzburg, Am Hubland, 97074 Wurzburg, Germanyb Institut fur Mineralogie, Universitat Munster, Corrensstraße 24, 48149 Munster, Germany

c Bundesanstalt fur Geowissenschaften und Rohstoffe, 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 distinguishedfrom high-grade zones to the east and west. In the central zone, P–T estimates of 8 kbar and 800°C derive from therelic assemblage: (1) Crd+Bt+Sil+Spl+Pl+Qtz for an earlier medium-pressure granulite-facies metamorphismwhich 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+Qtzand (3) Grt+Bt+Sil+Pl+Qtz±Spl. Decompression at still high temperatures and a clockwise directed P–T–t pathare 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 monaziteages 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 geochronologicaldata. Either they form relics of the Precambrian Antarctic Craton, or they represent an early metamorphic stage ofthe 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 and470±2 Ma for these biotites.

A late tectonic pegmatite yielded a concordant U–Pb monazite age of 489±3 Ma, while slightly discordant U–Pbdata of two zircon fractions are explained by recent minor lead loss of ca 490 Ma old zircons. Cooling to ca 500–350°Cis 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–Arin micas took place within ca 15–20 Ma. Cooling rates of 18–25°C Ma−1 can be derived, if continuous coolingis 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 leastpart of the migmatites derive from Early Proterozoic crustal protoliths.

Comparing the new P–T–t data 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 thePacific end of the Transantarctic Mountains, at least since the granulite-facies metamorphic event. © 1999 ElsevierScience 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: [email protected]

0301-9268/99/$ – see front matter © 1999 Elsevier Science B.V. All rights reserved.PII S0301-9268 ( 98 ) 00091-6

236 U. Schussler et al. / Precambrian Research 93 (1999) 235–258

1. Introduction performed for the first time during the GermanAntarctic North Victoria Land ExpeditionsGANOVEX 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 furGeowissenschaften und Rohstoffe (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 theWilson 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 tovery-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 withOpx±Cpx±Grt [Fig. 2; Schussler (1996)].in preliminary petrographic descriptions of some

outcrops ( Klimov and Soloviev, 1958; McLeod Interestingly, high-grade metamorphic rocks withsuch 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 NovaBay 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. ProbablePrecambrian and Cambro-Ordovician lithologies of the three tectonic terranes are undifferentiated except low-grade metasedimentsof McCain Bluff (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. Schussler 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 Terraneat Oates Coast. All mineral assemblages occur together with quartz, plagioclase, biotite and ±K-feldspar. Thrusts are labelled likein Fig. 2. The hatched area indicates the transition between eastern and central zone. A, Mt. Archer; BM, Berg Mountains; H, HaraldBay; LM, Lazarev Mountains; MC, McCain Bluff; RK, Ringgold Knoll; TP, Thompson Peak.

(Fig. 1). Several questions arise concerning the morphic rocks from the highest-grade core of theOates Coast crystalline basement. In an attempthigh-grade basement rocks of the Oates Coast.

What were the P–T conditions of different meta- to unravel the metamorphic history, we discussthe metamorphic P–T conditions and present newmorphic stages/events? To what extent can geo-

chronological data resolve the metamorphic and geochronological data. Compared with data fromthe southern Wilson Terrane at Terra Nova Bay,cooling history? How do P–T–t data from the

Oates Coast basement match those from other our results suggest a common tectonometamorphicevolution 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 NorthVictoria Land/Oates Coast sector of the Rossnological investigations were initiated. In this

paper, we concentrate on the evolution of meta- Orogenic belt.


2. Geological setting Terrane is well investigated in the Terra Nova Bayarea. Here, amphibolite-facies metamorphismreached 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 andRb–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 blockingtemperatures (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-bearingrelic 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 partreplaced 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 severaloutcrops 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 containrelic structures diverging from the common RossCraton during the Ross Orogeny. It is dominated

by medium- to high-grade metamorphic rocks Orogenic direction. P–T estimates yielded ca8 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 pointto Precambrian continental crust being involvedsuite of magmatic arc affinity (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; Schussler

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 EarlyPaleozoic 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 theTerra 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 (Schussler, 1996). Due tocompressive 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 ofthis 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 rocksof the Wilson Terrane [Figs. 1 and 2; Flottmannmetamorphic 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 zoneswith different 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 (Schussler, 1996). The WilsonThrust ( 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) formthe 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


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. Differences in

metamorphic grade indicate a tectonic contact re-equilibration and transformation of formerhigh-grade gneisses into muscovite–biotite gneissesbetween the central zone and an eastern zone

as well. with only few relics of cordierite and sillimanite(Schussler, 1996).The metamorphic basement consists of extensive

series of psammitic metasediments with local varia- The final stage of the high-grade metamorphicevolution was accompanied by the intrusion oftions to more quartzitic or more pelitic composi-

tions. Differences 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; Schussler metamorphic basement of the central zone atRinggold 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–Srmineral-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 pegmatiteemplacement. 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 ca470–475 Ma (Schussler and Henjes-Kunst, 1994).central zone.

All basement rocks were affected 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 partof 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 centralzone, 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 P–T conditions of the last metamorphic

Ms+Qtz=Sil+Kfs+V event which led to a widespread migmatizationand which is predominantly preserved in theBt+Sil+Qtz=Crd+Kfs+Vlarge majority of the basement rocks;

were recognized in pelitic bulk compositions [min- $ the P–T conditions of an earlier granulite-facieseral abbreviations after Kretz (1983)]. In the event which is evident from some relic mineralcentral zone, the higher-grade reactions assemblages preserved in part of the migmatites;

$ the time of the last metamorphic event; andBt+Sil+Qtz=Crd+Grt+Kfs+V$ the time of cooling subsequent to the last meta-

Bt+Sil+Qtz=Crd+Spl+Kfs+V morphic event.The new data regarding these points allow theare documented. In addition, a relic granulite-reconstruction of at least part of the P–T–t evolu-facies mineral assemblagetion of this basement sector and enable a compari-

Qtz+Pl+Bt+Opx±Grt±Cpx son with other basement segments of the WilsonTerrane. Additionally, new geochronological dataoccurs in migmatites of the central zone. A latefrom the Ringgold Knoll Pegmatite give moredecompression at still high temperatures for theprecise insight into the relations between meta-central zone is indicated by growth of Crd accord-morphism and magmatism of the region.ing to the reactionUnfortunately the relic granulite-facies mineralassemblages in the migmatites are rare and tinyGrt+Sil+Qtz+V=Crd.


and, in case of the Opx-bearing relics, sometimesheavily altered. Their preparation for isotopeanalysis to date the granulite-facies stage seems tobe impossible therefore.

3. Sample description: petrography and mineralchemistry

US-380 was taken from the eastern ridge ofThompson Peak (69°24∞30◊S, 157°44∞30◊E; Fig. 2)and forms part of monotonous migmatitic gneisses.The major minerals are Grt+Bt+Sil+Kfs+

Fig. 3. Relic granulite-facies mineral assemblage typicallyPl+Qtz, with accessory apatite, monazite and rareoccurring 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 Coastorientation 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) butmore 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 sillimanitewas 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 different 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 ca60 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 exsolutionlamellae 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 largegarnet 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% andpyrope 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 of66–67 mol% is identical to that of the cores. included in plagioclase.

Selected microprobe analyses of the mineralsUS-495 is a dark, gneissic migmatite and wascollected at the southwestern ridge of Mt. Archer which were taken for P–T calculations are given

in Table 1. The anorthite content of plagioclase(69°13∞30◊S, 157°36∞00◊E; Fig. 2). The outcrop isdominated 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.


tine and 16–20 mol% pyrope. The rims consist of68–72 mol% almandine, 12–16 mol% spessartineand 11–18 mol% pyrope.

US-501 was collected at the southwestern coast-line of Harald Bay, southeast of Mt. Archer(69°13∞20◊S, 157°44∞30◊E; Fig. 2). The outcrop isformed by rather homogeneous garnet-rich diatexi-tic migmatites which in part contain melanosomesand remnants of calc-silicate layers. The matrix ofUS-501 is formed by Grt+Bt+Kfs+Pl+Qtz+Gr. The structure of the medium- to coarse-grainedrock is more or less irregular. In part, biotite isclearly orientated. A weak differentiation into

Fig. 4. Relic granulite-facies mineral assemblage (1), occurring more melanocratic areas and more leucocratic onesin 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, xenomorphicand sillimanite (fibres) together with cordierite, plagioclase and

quartz ( light). grains within the leucocratic parts, accompaniedby 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 toseveral 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 minorcomponent 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. Accessoryminerals 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 perthiticexsolution 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 atthe 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 biotitesincluded 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 showeither 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 analmandine 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.


Table 1Compositions of phases taken for the calculation of P–T conditions 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 9bt 1 bt 2 crd 1 crd 2 pl spl bt crd pl spl bt1 bt2 bt 3 crd spl

Wt%SiO2 34.51 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 0.00TiO2 2.82 3.01 0.04 0.00 — 0.00 2.60 0.05 — 0.02 3.26 3.76 3.89 0.00 0.00Al2O3 19.04 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 57.80Cr2O3 0.11 0.03 — — — 0.21 0.03 — — 0.17 0.18 0.09 0.24 — 0.17Fe2O3 3.18 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 1.95MgO 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 3.17CaO 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 0.00MnO 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 0.46FeO 16.21 16.22 7.41 7.86 — 30.66 19.06 7.29 — 29.62 15.21 16.44 16.91 8.08 30.74ZnO — — — — — 4.83 — — — 6.24 — — — — 5.42Na2O 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

CationsSi 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 0.00Ti 0.16 0.17 0.00 0.00 — 0.00 0.15 0.00 — 0.00 0.18 0.22 0.22 0.00 0.00Al 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 1.94Cr 0.01 0.00 — — — 0.01 0.00 — — 0.00 0.01 0.01 0.01 — 0.00Fe3+ 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 0.04Mg 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 0.14Ca 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 0.00Mn 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 0.01Fe2+ 1.03 1.03 0.64 0.68 — 0.73 1.22 0.63 — 0.71 0.95 1.05 1.08 0.70 0.74Zn — — — — — 0.10 — — — 0.13 — — — — 0.12Na 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 oxygennumber 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 threesmall 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 3and 9, three calculations involving analyses ofUS-495 and US-501 employing the average P–T

method of Powell and Holland (1994) and using different 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 fiveindependent 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


Table 2P–T data with 2s errors and x2 values calculated for assemblage (1) Crd+Bt+Sil+Spl+Pl+Qtz which was found at the domains3, 7 and 9 of sample US-495

Assemblage XH20 P (kbar) 2s (P) T (°C ) 2s (T) x2

Loc 3crd1–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 7crd–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 9crd–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 andPowell (1990) and an updated version of the program THERMOCALC (Powell and Holland, 1988).

correlated and the best average P–T estimate is XH2O=1 range from 7.5 to 8 kbar and from 790to 840°C, indicating medium-pressure granulite-obtained by least-square techniques, allowing the

s uncertainties to be calculated. Finally, a x2 test facies conditions (Fig. 5). The calculated valuesdecrease by ca 50°C and 1 kbar when a fluidis applied to the average P–T result in order to

test for the reliability of the estimate. In the case composition of XH2O=0.6 is assumed. The P–Tof five independent reactions, as in assemblage(1), the x2 value should be <1.54. In all ourcalculations this value was well below 1.0 (Table 2)indicating the reliability of the results as well assupporting our initial assumption of localequilibrium.

All calculations were performed assuming ideal-mixing-on-sites activities, except for garnet andplagioclase. Garnet was described with the regularmodel 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. P–T data calculated for the medium-pressure granulite-out for various fluid compositions. The resultsfacies assemblages (1), #, $, and the low-pressure granulite-including the 2s uncertainties are given in Table 2facies to upper-amphibolite-facies assemblages (2), %, &; and

and are shown on the P–T diagrams in Figs. 5 (3),6, +. For one point of each metamorphic stage, the errorand 11. range is given. Filled symbols denote results from calculations

at XH2O=1, open symbols those at XH2O=0.6.For assemblage (1), all P–T estimates at


Tab

le3

U–P

ban

alyt

ical

resu

lts

for

zirc

ons

ofm

igm

atit

eU

S-50

1,fo

rm

onaz

ites

ofm

igm

atit

esU

S-38

0,U

S-49

5an

dU

S-50

1,an

dfo

rzi

rcon

san

dm

onaz

ite

ofpe

gmat

ite

RK

12

Sam

ple

Size

(mm

)C

once

ntra

tion

s(p

pm)

Mea

sure

dis

otop

era

tios

Cor

rect

edis

otop

era

tiosa

App

aren

tag

e(M

a)

UP

b206P

b208P

b/207P

b/206P

b/206P

b/±

2s207P

b/±

2s207P

b/C

orr.

206P

b/207P

b/207P

b/±

2sto

tal

rad.

206P

b206P

b204P

b238U

235U

206P

bco

eff.

238U

235U

206P

b

Zir

con

US-

501

>12

532

539

.532

.90.

1109

540.

0922

7111

039

0.11

7527

179

1.47

479

225

0.09

101

0.99

771

692

014

47±

1U

S-50

110

0–90

374

43.3

36.6

0.09

8822

0.08

4656

7150

0.11

3984

172

1.29

956

201

0.08

269

0.97

569

684

612

62±

2U

S-50

190

–80

386

43.3

36.8

0.09

3779

0.08

3283

7846

0.11

0909

167

1.24

612

189

0.08

149

0.99

167

882

212

33±

1U

S-50

112

5–10

037

143

.736

.90.

0988

220.

0864

9918

512

0.11

5567

174

1.36

625

206

0.08

574

0.99

870

587

513

32±

1U

S-50

180

–62

395

40.1

34.6

0.08

6616

0.07

7898

1112

40.

1018

8215

41.

0764

016

80.

0766

30.

969

625

742

1111

±3

RK

1250

0–25

063

9546

145

4b0.

0137

460.

0604

4642

450.

0777

5623

50.

6121

519

50.

0571

00.

948

483

485

495

±2

RK

1220

0–16

058

6742

241

4b0.

0163

040.

0614

0532

500.

0773

0323

80.

6076

020

40.

0570

10.

916

480

482

492

±3

Mon

azit

eU

S-50

112

5–10

019

4313

9613

2.8

9.33

3274

0.05

8691

7262

0.07

9579

243

0.62

196

193

0.05

668

0.98

349

449

147

9±

4U

S-50

110

0–80

1973

1420

135.

19.

3348

720.

0589

7573

720.

0796

8820

30.

6262

916

20.

0570

00.

988

494

494

492

±3

US-

495

125–

100

4499

1338

301.

53.

3408

970.

0579

1013

610

0.07

8007

178

0.61

135

141

0.05

684

0.99

048

448

448

5±

2U

S-49

5>

125

5291

1474

359.

83.

0064

960.

0576

7016

128

0.07

9144

178

0.61

947

140

0.05

677

0.99

549

149

048

3±

2U

S-38

0>

8071

4616

1248

1.9

2.26

5368

0.05

7922

1298

00.

0784

8612

10.

6147

197

0.05

680

0.99

348

748

748

4±

1U

S-38

080

–62

6150

1572

415.

22.

6966

690.

0584

2095

830.

0785

9311

90.

6166

295

0.05

690

0.98

948

848

848

8±

2R

K12

125–

8025

757

3118

3106b

0.71

7675

0.05

8633

8177

0.07

8838

237

0.61

890

189

0.05

694

0.98

748

948

948

9±

1

aCor

rect

ion

for

frac

tion

atio

n,sp

ike,

blan

k,in

itia

lco

mm

onle

ad.

bVal

ues

for

tota

lra

diog

enic

Pb.


data vary within a rather limited range of ca ±5°Cand ±0.1 kbar, if analyses of different cordieriteor biotite grains of the same domain are used forthe calculations, as shown for the domains 3 and9 in Tables 1 and 2.

Significantly lower P–T conditions of 640–650°Cand 4–5 kbar at XH2O=1.0 were found for assem-blage (2). If calculated with XH2O=0.6, the valuesdecrease by ca 30°C and 0.5 kbar, respectively.Assemblage (3) in US-501 gave pressures between4.1 and 5.7 kbar, however, in a wide temperaturerange from 720 to 870°C (the P–T range is definedwhen garnet core compositions are taken for thecalculations, but by using rim concentrations, thedata points plot into the same range). If calculated

Fig. 6. Concordia diagram with U–Pb dates between 484 andwith XH2O=0.6, the pressures range from 4.1 to494 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 morerealistic as the matrix of US-501 contains graphitewhich indicates lowered H2O activities (Fig. 5). below the closure temperature between 487 and

494 Ma. However, reverse discordance may alsoresult from analytical problems. For the presentstudy, most potential mechanisms [e.g. poor spike5. Isotope datacalibration, incomplete dissolution; Hawkins andBowring (1997) and references herein] can confi-5.1. Metamorphic rocksdently be ruled out, except mass spectrometricproblems 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 bythe 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 fractionsnot 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 fractionsindicate a more restricted time span between 484between these minerals. Two different 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 werealso 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 zirconfractions [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±8 Maand 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 threeeffects 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 fromtheir 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 Gacomponent.information, suggesting that the studied samples

experienced metamorphic crystallization or cooling Rb–Sr and K–Ar isotope analysis of biotite


two low-temperature increments. For the mediumto high-temperature increments, plateau ages of476.4±2.3, 472.2±3.3 and 470.1±2.3 Ma wereobtained (order of the data always with increasingsample numbers).

Sm–Nd isotope analysis of whole rock powdersfrom the same samples yielded model ages of 1.93,1.76 and 1.77 Ga (Table 6), calculated usingdepleted mantle parameters (DePaolo, 1981).

5.2. Pegmatite

The geochronological data of the garnet–tour-maline pegmatite RK12 from Ringgold Knoll[Fig. 2; Schussler and Henjes-Kunst (1994)] weresupplemented by U–Pb analyses on one monaziteand two zircon fractions (Table 3; Fig. 10). Ucontents of ca 6000 ppm for zircon and ca26 000 ppm for monazite which are up to 20 timeshigher than those of zircons and monazites fromthe metamorphic rocks support a magmatic originof these minerals in the pegmatite. RK12 zirconsare 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 zirconcrystallization. The monazite yielded a concordantU–Pb age 489±2 Ma. The zircon fractions plotslightly below the concordia, with U–Pb ages of

(a)

(b)

Fig. 7. (a) Concordia diagram with U–Pb data points of fivezircon fractions of the migmatite US-501. Three fractions (C–E)define a straight line which may be interpreted as a discordiawith a lower intercept at 469±8 Ma and an upper interceptwhich points to a protolith age of ca 1.9 Ga. (b) Fractions A–Cshow inherited cores in the zircons during cathodoluminescenceinvestigations ( 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 Maand 473, 469 and 470 (each ±5) Ma, respectively(Tables 4 and 5; Fig. 8), providing average ages of469±3 Ma for the Rb–Sr system and 471±3 Mafor the K–Ar system. 40Ar–39Ar total gas dates are474, 471 and 469 (±2) Ma. On all biotites,40Ar–39Ar incremental heating experiments wereperformed in order to verify the geological signifi-

Fig. 8. Comparison of Rb–Sr, K–Ar, 40Ar–39Ar total gas andcance of the conventional K–Ar dates. They 40Ar–39Ar plateau biotite ages of the investigated migmatitesyielded 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


Table 4Rb–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-380Whole rock 255.3 141.9 5.2321 0.759543 24 470±5Biotite 559.0 2.15 1489.2 10.697685 24



Table 5K–Ar analytical results for biotite fractions of samples US-380, US-495 and US-501

Sample K (wt%) Rad. Ar (nl g−1) Rad. Ar (%) Date (Ma±2s)

US-380 8.05 169.5 99.1 473.3±4.9US-495 7.72 160.5 98.7 468.9±4.8US-501 8.04 167.5 98.5 469.8±4.9

Table 6Sm–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 1.93US-495 42.94 8.376 0.1175 0.511975 −12.9 −8.0 1.76US-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. Discussionpoints can be explained by recent Pb loss of zirconswhich have crystallized ca 490 Ma ago. 6.1. Metamorphic historyInterestingly to note, the U–Pb data do not provideevidence for inherited zircon components within From our petrological data, two stages in the

metamorphic evolution of the central zone canpegmatite RK12.Schussler and Henjes-Kunst (1994) reported be distinguished. Within the migmatites, an earl-

ier, medium-pressure granulite-facies stage is40Ar–39Ar plateau ages on two muscovite fractionsof 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 reliablestandard 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.


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 bythin layers and schlieren showing the relic assem-blage Qtz+Pl+Bt+Opx (±Grt±Cpx). Theserelics occur within the migmatites at several loca-tions of the central zone. In rare cases when garnetor clinopyroxene coexist with orthopyroxene, theorthopyroxene is in part or completely replacedby anthophyllite. Garnet profiles show rather uni-form compositions except some minor composi-tional variations along the rims; this lack ofzonation is interpreted as the result of a youngerre-equilibration of the former granulite-facies gar-nets. Therefore, garnet–orthopyroxene thermome-try on several samples gave variable temperaturesmuch below the lower stability of orthopyroxene,indicating disequilibrium conditions for these Opx-bearing relic assemblages (Schussler, 1996).

The formation of the assemblages (2) withFig. 9. 40Ar–39Ar spectra of biotite fractions from migmatitesUS-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 andUS-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 themetamorphic rocks which largely destroyed theare still preserved, as indicated by the narrow

range of P–T data 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. Thisis indicated by euhedral grain forms, by biotitenearly identical results from different domains

(Table 2). The medium-pressure granulite-facies inclusions showing the same orientation as thematrix 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


reactions in the KASH-system ( Xu et al., 1994).The same minimum temperature is given by thesolidus curve in the system Qtz–Or–An40–H2O[reaction A in Fig. 11; Johannes (1984)]. The highportions of former melt in the migmatite seriesshow that the melting temperature has beenexceeded considerably. The solidus curve andtherewith minimum temperature estimate for theassemblages (2) and (3) shift towards even highervalues if a H2O activity <1 is taken into account(Johannes and Holtz, 1990). Relic inclusions ofsillimanite in cordierite and in garnet provideevidence that reaction (3) Bt+Sil+Qtz=Crd+Grt+Kfs+V took place. Taking intoaccount the melt-in temperature of minimum700°C, pressures between 4 and 5.5 kbar arerequired for the reaction [ Xu et al. (1994)KFMASH-system; see also Holdaway and Lee

Fig. 11. Possible alternatives for the P–T–t evolution of the (1977)]. From all these arguments, a low-pressuremetamorphic basement in the northern Wilson Terrane: if the granulite-facies to upper-amphibolite-facies stagemedium-pressure granulite relics are of Precambrian age, then

of metamorphism at ca 4–5.5 kbar and minimumloop (I ) represents the Precambrian metamorphism and loop700°C, but most probably under significantly(II ) the metamorphic overprint during the early Paleozoic Rosshigher temperature conditions, can be postulatedOrogeny. If the relics are of early Ross age, then loop (I ) marks

the P–T–t evolution 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 ofGrt+Sil+Qtz+V=Crd ( Xu et al., 1994); (A) solidus curve incordierite around some garnets with sillimanitethe 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 caassemblage (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 andby different closing temperatures of the different Lee, 1977).chemical systems used for thermometry. However, All the migmatites that occur in the northernthey may also reflect disequilibrium for the mineral Wilson Terrane seem to have resulted from theassemblages 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 theresystem 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-pressureand the solidus for the system require a P–T range granulite-facies event. The position of P–T pointswhich is consistent with the calculated data. A for assemblage (1) at ca 8 kbar/800°C calculatedminimum temperature of 700°C derives from the at XH2O=1 clearly indicates the formation of meltupper 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 P–T field was experimen- and the calculated P–T values 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


migmatites of this early metamorphic stage in the disturbed later. Lower step ages for the firstincrements of the 40Ar–39Ar spectra demonstratefield may be explained as follows:

$ early migmatites exist, but appear quite similar that the K–Ar systems suffered from slight Ar lossin 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 areconcordant within the given errors. 40Ar–39Ar$ early migmatites have been completely

reworked during the last migmatization, except plateau ages for the biotite fractions suggest thatcooling 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 maypoint to some minor regional differences 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 fractionsC, D and E of sample US-501 define a discordia6.2. Age relationshipswith a lower intercept age of 469±8 Ma, suggest-ing that Pb loss or growth of new rims stopped atEstimates for the closure temperature of thethe same time when the Rb–Sr and K–Ar closingU–Pb system in monazite range between 730 andtemperatures of biotite were reached (nevertheless640°C (Copeland et al., 1988; Parrish, 1988;caution is required by interpreting the discordiaMezger, 1990). A value of 770°C is given by Dahlbecause of inherited cores in at least fraction C).(1997). These temperatures match in part theThis zircon intercept age corresponds well to atemperatures which were found for the low-pres-SHRIMP age of 469±4 Ma on zircon rims of asure granulite-facies to upper-amphibolite-faciesdiatexite from the Daniels Range [Fig. 1; Blackstage of the migmatites. The range in monaziteand Sheraton (1990)] which was interpreted toages between 484 and 494 Ma is therefore interpre-date a late thermal overprint. The upper interceptted to date this metamorphic stage in the centralof the discordia indicates a protolith age ofzone of the Oates Coast basement.1918±31, however, due to a complex history ofThe emplacement of the Ringgold Knoll pegma-the zircons in Ross time, combined with multistagetite is dated to ca 490 Ma by the new U–Pb agePb loss and/or zircon growth, no precise age infor-data of one monazite and two zircon fractions andmation can be inferred from this upper intercept.is further constrained by a Rb–Sr mineral-wholeThe larger size fractions A and B are interpretedrock isochron age of 492±8 Ma (Schussler andto be multiply discordant, and from their data,Henjes-Kunst, 1994). Metamorphism of the coun-even older protolith ages can be deduced. Thetry rocks and the subsequent intrusion of the late->1900 Ma protolith ages point to componentstectonic pegmatite took place within a very shortwithin the migmatites which derived from Earlytime span which cannot be resolved by U–PbProterozoic crustal rocks of probably Eastage dating.Antarctic provenance. This is corroborated by theFor biotite, closure temperatures of the Rb–SrSm–Nd whole rock model ages between 1.8 andand the K–Ar systems range from 350 to 300°C1.9 Ga from all three migmatite samples.(Purdy and Jager, 1976; Dodson, 1979; Harrison

et al., 1985), a temperature of 450°C for the K–Arsystem is given by Villa and Puxeddu (1994). The

7. Interpretationsmineral ages for biotites of US-380, US-495 andUS-501 are summarized in Fig. 8. From this, con-

7.1. P–T–t evolutioncordant Rb–Sr and K–Ar dates of, on average,469 and 471 Ma give a lower age limit for thecooling 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


a P–T–t path for the central zone of the Oates Gregory (1967) for several locations along thecoastline west of Oates Coast and by Stuwe andCoast crystalline basement (Fig. 11). The earliest

event documented was a medium-pressure granu- Oliver (1989) for King Georg V Land which isneighbouring 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 eventhas 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 P–T–tA 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 P–T

evolution. Between ca 490 and 473 Ma, the base- direction not verified, and loop (II ) of EarlyPaleozoic 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 metamorphismcould also have taken place early during the Rossclosing temperatures compiled by Villa (in press)].

Therewith a cooling rate of 18–25°C Ma−1 is Orogeny and may have immediately preceded thelow-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 P–T–t loop (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 levelof 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 westernzones (Schussler, 1996). At the moment, no realzircon and by Rb–Sr and Sm–Nd whole rock–min-

eral isochrons (Schussler and Henjes-Kunst, 1994). evidence for either of these two possibilities wasrecognized.Final cooling to ca 350°C [500°C, Hammerschmidt

and Frank (1991)] was dated at 472±2 Ma bymeans of two 40Ar–39Ar spectra. In addition, these 7.2. Regional aspectsdata demonstrate that intrusive activities tookplace 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 activecontinental 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 andTessensohn, 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 Precambriancontinental 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, butalso 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 theTerra 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 rocksfrom the craton were described by McLeod and Thrust System at the Oates Coast was found by


Flottmann and Kleinschmidt (1991, 1993). A sim- 1991; Talarico et al., 1995; Palmeri, 1997). Bothcontain migmatites which were formed during P–Tilar thrusting in the Terra Nova Bay region is

assumed, but has not yet been proved conditions cited above. For the polymetamorphiccomplex, the migmatite stage is part of a decom-( Kleinschmidt and Matzer, 1990). A tectonic con-

tact (Boomerang Thrust) between the high-grade pressional P–T path, whereas the migmatites ofthe 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 P–T paths areequivalent of the Exiles Thrust at Oates Coast

( Kleinschmidt, 1990). interpreted as P–T trajectories of different crustallevels 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). Different 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 (Schussler,Oates Coast metamorphic basement. Also,

different metamorphic zones can be distinguished 1996), with a decompressional P–T evolution forthe rocks of the central zone (P–T–t path I in(Palmeri et al., 1991). In the highest-grade parts,

the basement contains assemblages with Fig. 11). The P–T evolution of the eastern andwestern 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 (withand 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 of480–490 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 closelyP–T conditions of 800–825°C and 7–8 kbar which

correspond well to the P–T data for the medium- resemble our U–Pb ages on monazites from theOates 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 interceptage of the zircon discordia for a Terra Nova Bay1991; Talarico and Castelli, 1994). P–T conditions

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°C at

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 TerraNova 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 leastsince 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


Daniels Range which is located in striking direction Monazites and titanites from the Lantermanbetween Oates Coast and Terra Nova Bay (Fig. 1). metamorphic complex yielded U–Pb ages of caUlitzka (1987) estimated P–T conditions 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 frommorphic 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, wholeoccurrence of a granulite-facies assemblage with rock) result in ages of 495±6 and 503±6 Ma forQtz+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 ofMost of the K–Ar muscovite and biotite dates the high-pressure event. Subsequent coolingreported 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 muscovitesare corroborated by new 40Ar–39Ar plateau ages (Goodge and Dallmeyer, 1996; Henjes-Kunst, inof 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 suggestobtained for the Oates Coast basement. In conclu- that the high-grade metamorphism took place casion, the available data imply that during the Late 10 Ma earlier in the Lanterman Range than in thePrecambrian/Early Paleozoic, a rather homogen- Oates Coast area. 40Ar–39Ar plateau ages clearlyeous style of orogenic evolution has taken place indicate that during the cooling history theover a strike length of at least 600 km along the Lanterman Range reached the muscovite closurePacific margin of the old Antarctic craton.

temperature again ca 10 Ma earlier than the OatesCoast basement, most likely due to an earlieruplift.7.2.2. The eastern medium- to high-pressure

metamorphic belt of the Wilson TerraneOne of the best investigated areas of this belt is

the Lanterman Range in the northwestern WilsonTerrane (Fig. 1). For the metamorphic rocks of

Acknowledgmentthis area, Grew and Sandiford (1984) calculatedP–T conditions of ca 700°C at 8 kbar for an early

The Bundesanstalt fur Geowissenschaften undstage of metamorphism, followed by an intermedi-Rohstoffe in Hannover is thanked for the invitationate overprint at 650–700°C and 5.5–6.4 kbar andof U. Schussler to participate in the GANOVEXa final greenschist-facies stage at 300–370°C andProgram. Many thanks are due to all who contrib-3–5 kbar. Conditions of 700°C and up to 8 kbaruted to the success of the field work with theirare also suggested by Roland et al. (1984) whologistic or professional help. H. Klappert, M. Metz,mention that a subsequent, second metamorphicM. Bockrath and P. Macaj are thanked for labora-stage should have taken place at somewhat lowertory assistance at the BGR. Thanks are due to H.P–T conditions. For metamorphism in the DessentBaier (Munster) for laboratory assistance and S.Formation in the southeastern Wilson Terrane,Rochnowski (Munster) for support on the massKleinschmidt et al. (1984) estimated 600°C atspectrometer. K.-P. Kelber kindly prepared the6–7 kbar. Recently, eclogite lenses were foundfigures, P. Spathe the thin sections. U. Schusslerwithin the Lanterman Range, documenting meta-wants to thank W. Schubert for involving him inmorphic conditions of maximum 850°C at mini-geological research in North Victoria Land and formum 15 kbar for a high-pressure stage, followedhelpful advice and discussion. J.D. Kramers, M.by medium- and low-pressure stages underOkrusch and F. Talarico are thanked for con-amphibolite-facies conditions (Di Vincenzo et al.,

1997). structively reviewing the manuscript. Financial sup-


port from the Deutsche Forschungsgemeinschaft is US-495 and US-501 was carried out atthe Zentrallaboratorium fur Geochronologie,gratefully acknowledged.Munster. 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-Appendixtop 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-typeMineral compositions were determined in pol- Teledyne mass spectrometer with a single Faraday

ished thin-sections using a CAMECA SX50 collector. Sr was loaded with H3PO4 on single Taelectron microprobe with wavelength-dispersive filaments and analysed on a VG Sector 54 multicol-spectrometers at the Mineralogisches Institut, lector mass spectrometer in dynamic mode. DuringUniversitat Wurzburg. The operating conditions the period of this study, the 87Sr/86Sr ratio of thewere: 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/86Srbackgrounds 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 reportedwere used for reference standards. Matrix correc- at the 2sm level, 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 magnificationfor major elements was verified by repeated meas- based on the spike/sample ratio and the blankurements on respective standards. For low concen- correction. Rb–Sr ages were calculated using thetrations, 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 1.42×10−11 a−1

(Neumann and Huster, 1974) was used for agecalculations. 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 theprocedures 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 dissolutionof 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 ona 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 wasloaded 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 andH2O 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 NBS981. 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


ellipses in Fig. 6 follows Ludwig (1980) and con- the isotope abundances, the mass intensities werecorrected for effects of mass discrimination, step-siders the internal precision (2s) of the mass

spectrometric measurements, an estimated uncer- temperature dependent total-system blank, decayand 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 40Ar–39Ar age was calculatedby 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 blankdeterminations. 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 39Arreleased 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% (andtogether >50%) of the total 39Ar released andBundesanstalt fur Geowissenschaften und

Rohstoffe (BGR). being mutually identical within their calculateduncertainties. 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 theuncertainties 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 Jager, 1977). All errors quoted arecalculated 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 standardglauconite GL-O is ca 1% younger than the mean Referencesvalue of the compilation of Odin (1982). For40Ar–39Ar analysis, sample aliquots of ca 15 mg Armienti, P., Ghezzo, C., Innocenti, F., Manetti, P., Rocchi,

S., Tonarini, S., 1990. Isotope geochemistry and petrology ofwere wrapped into aluminium foil and tightlygranitoid suites from Granite Harbour Intrusives of thestacked into a quartz ampoule. An intralaboratoryWilson Terrane, North Victoria Land, Antarctica. Eur.standard biotite (SN: 313.7±1.8 Ma) which wasJ. Mineral. 2, 103–123.

used as a flux monitor was placed in double Black, L.P., Sheraton, J.W., 1990. The Influence of Precambrianaliquots on top of and below each stack and in Source Components on the U–Pb Zircon Age of a Palaeozoic

Granite from Northern Victoria Land, Antarctica.single aliquots between every two mica samples.Precambrian Research 46, 275–293.The ampoule was evacuated, pre-heated at 150°C

Borg, S.G., Stump, E., Chappell, B.W., McCulloch, M.T.,and sealed. Irradiation with fast neutrons wasWyborn, D., Armstrong, R.L., Halloway, J.R., 1987.

performed for 96 h in position I/44 of the 5 MW Granitoids of northern Victoria Land, Antarctica: implica-scientific reactor FRG I, Forschungszentrum tions of chemical and isotopic variations to regional crustal

structure and tectonics. Am. J. Sci. 287, 127–169.GKSS, Geesthacht on ampoules shielded fromCastelli, D., Lombardo, B., Oggiano, G., Rossetti, P., Talarico,thermal neutrons by a 1 mm thick Cd liner and

F., 1991. Granulite Facies Rocks of the Wilson Terranesealed in an aluminium container. Gas was(northern Victoria Land): Campbell Glacier. Mem. Soc.

extracted in 17 heating steps up to a maximum Geol. It. 46, 197–203.temperature of 1550°C in order to ensure total Chatterjee, N.D., Johannes, W., 1974. Thermal stability and

standard thermodynamic properties of syntheticdegassing, each step maintaining a constant tem-2M1-muscovite, KAl2[AlSi3O10(OH)2]. Contrib. Mineral.perature for 30 min. Gas purification was per-Petrol. 48, 89–114.formed by SAES Zr/Al getters heated to 800°C

Copeland, P., Parrish, R.R., Harrison, T.M., 1988.for 3 min. The intensities at masses 40, 39, 37 and Identification of inherited radiogenic Pb in monazite and its36 were measured by peak jumping on a VG implication for the U–Pb system. Nature 333, 700–763.

Dahl, P.S., 1997. A crystal-chemical basis for Pb retention andMM1200 mass spectrometer. For calculation of


fission-track annealing systematics in U-bearing minerals, and natural observations. Contrib. Mineral. Petrol. 63,175–198.with implications for geochronology. Earth Planet. Sci. Lett.

Holdaway, M.J., Mukhopadhyay, B., 1993. A reevaluation of150, 277–290.the stability relations of andalusite: thermochemical data andDePaolo, D.J., 1981. Neodymium isotopes in the Coloradophase diagram for the aluminum silicates. Am. Mineral. 78,Front Range and crust-mantle evolution in the Proterozoic.298–315.Nature 291, 193–196.

Holland, T.J.B., Powell, R., An enlarged and updated internallyDi Vincenzo, G., Palmeri, R., Talarico, F., Andriessen, P.A.M.,consistent thermodynamic dataset with uncertainties and cor-Ricci, C.A., 1997. Petrology and Geochronology of Eclogitesrelations: the system K2O–Na2O–CaO–MgO–MnO–FeOfrom the Lanterman Range, Antarctica. J. Petrol. 38,–Fe2O3–Al2O3–TiO2–SiO2–C–H2–O2. 1990. J. metamorphic1391–1417.Geol. 8, 89–124.Dodson, M.H., 1979. Theory of cooling ages. In: Jager, E.,

Johannes, W., 1984. Beginning of melting in the granite systemHunziger, J.C. (Eds.), Lectures in Isotope Geology. Springer,Qz–Or–Ab–An–H2O. Contrib. Mineral. Petrol. 86, 264–273.Berlin, pp. 194–202.

Johannes, W., Holtz, F., 1990. Formation and composition ofFadda, S., Franceschelli, M., Giorgetti, G., 1994. MineralogyH2O-undersaturated granitic melts. In: Ashworth, J.R.,and metamorphic zonation in low-grade metasediments fromBrown, M. (Eds.), High-temperature Metamorphism andthe Priestley Glacier, Northern Victoria Land, Antarctica.Crustal Anatexis. Unwin Hyman, pp. 87–104.Terra Antartica 1, 33–36.

Klee, S., 1995. Altersbestimmung hochmetamorpher GesteineFenn, G., 1993. Petrogenese der Granite Harbour Intrusives indes sudlichen Wilson Terranes, Nord-Victorialand,Nord Victoria Land und der Prince Albert MountainsAntarktis. Berichte zur Polarforschung 170, 50–56.(Antarktis). Dr.-Thesis, University of Bremen, Bremen,

Klee, S., Baumann, A., Thiedig, F., 1990. Age Relations of thepp. 1–174.High Grade Metamorphic Rocks in the Terra Nova BayFlottmann, T., Kleinschmidt, G., 1991. Opposite thrust systemsArea, North Victoria Land, Antarctica: a preliminary report.in northern Victoria Land, Antarctica: imprints ofPolarforschungen 60 (1992), 101–106.Gondwana’s Paleozoic accretion. Geology 19, 45–47.

Kleinschmidt, G., 1990. The Southern Continuation of theFlottmann, T., Kleinschmidt, G., 1993. The Structure of OatesWilson Thrust. Polarforschungen 60 (1992), 124–127.Land and Implications for the Structural Style of Northern

Kleinschmidt, G., Matzer, S., 1990. Structural field observa-Victoria Land, Antarctica. Geol. Jb. E 47, 419–436.

tions in the basement between Fry and Reeves Glacier,Goodge, J.W., Dallmeyer, R.D., 1996. Contrasting Thermal

Victoria Land, Antarctica. Polarforschungen 60 (1992),Evolution within the Ross Orogen, Antarctica: Evidence

107–109.from Mineral 40Ar/39Ar Ages. J. Geol. 104, 435–458.

Kleinschmidt, G., Tessensohn, F., 1987. Early Paleozoic west-Goodge, J.W., Walker, N.W., Dallmeyer, R.D., 1995. Thermal ward directed subduction at the Pacific margin of Antarctica.

and kinematic history of the Ross orogen, Antarctica. Geol. In: McKenzie, G.D. (Ed.), Gondwana six: structure, tecton-Soc. Am. Abs. Prog. 27, 126 ics and geophysics. Am. Geophys. Union, Geophys. Monogr.

Grew, E.S., Sandiford, M., 1984. A staurolite–talc assemblage Series 40, 89–105.in tourmaline–phlogopite–chlorite schist from northern Kleinschmidt, G., Roland, N.W., Schubert, W., 1984. TheVictoria Land, Antarctica, and its petrogenetic significance. Metamorphic Basement Complex in the Mountaineer Range,Contrib. Mineral. Petrol. 87, 337–350. North Victoria Land, Antarctica. Geol. Jb. B 60, 213–251.

Grew, E.S., Kleinschmidt, G., Schubert, W., 1984. Contrasting Kleinschmidt, G., Buggisch, W., Flottmann, T., 1992.Metamorphic Belts in North Victoria Land, Antarctica. Geol. Compressional causes for the Early Paleozoic Ross Orogen—Jb. E 60, 253–263. evidence from Victoria Land and the Shackleton Range. In:

Gunn, B.M., Warren, G., 1962. Geology of Victoria Land Yoshida, Y. (Ed.), Recent Progress in Antarctic Earthbetween the Mawson and Mulock Glaciers, Antarctica. Bull. Sciences. Terra Scientific, Tokyo, pp. 227–233.NZ. Geol. Surv. 71, 1–157. Klimov, L.V., Soloviev, D.S., 1958. A preliminary communica-

Hammerschmidt, K., Frank, E., 1991. Relics of high pressure tion about geological observations in eastern Antarctic.metamorphism in the Lepontine Alps (Switzerland)– Inform. Bull. Sov. Antarct. Exped. 16, 7–15.40Ar/39Ar and microprobe analyses on white micas. Schweiz. Kretz, R., 1983. Symbols for rock-forming minerals. Am.Mineral. Petrogr. Mitt. 71, 261–274. Mineral. 68, 277–279.

Harrison, T.M., Duncan, I., McDougall, I., 1985. Diffusion of Kreuzer, H., Hohndorf, A., Lenz, H., Muller, P., Vetter, U.,40Ar in biotite: temperature, pressure and compositional 1987. Radiometric ages of pre-Mesozoic rocks from Northerneffects. Geochim. Cosmochim. Acta 49, 2461–2468. Victoria Land, Antarctica. In: McKenzie, G.D. (Ed.),

Hawkins, D.P., Bowring, S.A., 1997. U–Pb systematics of mon- Gondwana six: structure, tectonics and geophysics. Am.azite and xenotime: case studies from the Paleoproterozoic Geophys. Union, Geophys. Monogr. Series 40, 31–47.of the Grand Canyon, Arizona. Contrib. Mineral. Petrol. Krogh, T.E., 1973. A low contamination method for hydrother-127, 87–103. mal decomposition of zircon and extraction of U and Pb for

Holdaway, M.J., Lee, S.M., 1977. Fe–Mg cordierite stability in isotopic age determinations. Geochim. Cosmochim. Acta37, 485–494.high-grade pelitic rocks based on experimental, theoretical,


Ludwig, K.R., 1980. Calculations of uncertainties of U–Pb iso- Lanterman metamorphics, North Victoria Land, Antarctica.Geol. Jb. B 60, 319–361.topic data. Earth Planet. Sci. Lett. 46, 212–220.

Matzer, S., 1995. Palaozoische Akkretion am palaopazifischen Schubert, W., Olesch, M., 1989. The petrological evolution ofthe crystalline basement of Terra Nova Bay, North VictoriaKontinentalrand der Antarktis in Nordvictorialand–

P–T–D–Geschichte und Deformationsmechanismen im Land, Antarctica. Geol. Jb. E 38, 277–298.Schussler, U., 1996. Metamorphic rocks in the Northern WilsonBowers Terrane. Ber. Polarforsch. 173, 1–234.

McLeod, I.R., Gregory, M.C., 1967. Geological investigations Terrane, Oates Coast, Antarctica. Geol. Jb. B 89, 247–269.Schussler, U., Henjes-Kunst, F., 1994. Petrographical and geo-along the Antarctic Coast between longitudes 108°E and

166°E. Rep. Bur. Miner. Recourc. Geol. Geophys. Canberra chronological investigations on a garnet–tourmaline pegma-tite from Ringgold Knoll, Oates Coast, Antarctica. Chem.78, 1–53.

Mezger, K., 1990. Geochronology in Granulites. In: Vielzeuf, Erde 54, 297–318.Schussler, U., Skinner, D.N.B., Roland, N., 1993. Subduction-D., Vidal, P. (Eds.), Granulites and Crustal Evolution.

Kluwer Academic, Dodrecht, pp. 451–470. related Mafic to Intermediate Plutonism in the NorthwesternWilson Terrane, North Victoria Land, Antarctica. Geol. Jb.Neumann, W., Huster, H., 1974. The half-life of 87Rb measures

as a difference between the isotopes 87Rb and 85Rb. Z. Phys. E 47, 389–418.Seidel, E., Kreuzer, H., Harre, W., 1982. A late Oligocene/early270, 121–127.

Newton, R.C., Haselton, H.T., 1981. Thermodynamics of the Miocene high pressure belt in the external Hellenides. Geol.Jb. E 23, 165–206.garnet–plagioclase–Al2SiO5–quartz geobarometer. In:

Newton, R.C., Navrotsky, A., Wood, B.J. (Eds.), Advances Skinner, D.N.B., 1983. The geology of Terra Nova Bay. In:Oliver, R.L., James, P.R., Jago, J.B. (Eds.), Antarctic Earthin Physical Geochemistry, vol. 1. Springer, Berlin,

pp. 131–147. Science. Cambridge University Press, Cambridge,pp. 150–155.Newton, R.C., Charlu, T.V., Kleppa, O.J., 1980.

Thermochemistry of the high structural state plagioclase. Skinner, D.N.B., 1989. Terra Nova Bay and the Deep FreezeRange—the southern allochthonous border of NorthGeochim. Cosmochim. Acta 44, 933–941.

Odin, G.S., 1982. Interlaboratorial standards for dating Victoria Land, Antarctica. Mem. Soc. Geol. It. 33 (1987),41–58.purposes. In: Odin, G.S. (Ed.), Numerical Dating in

Stratigraphy. Wiley, Chichester, pp. 123–150. Skinner, D.N.B., 1991. Metamorphic Basement ContactRelationships in the Southern Wilson Terrane, Terra NovaPalmeri, R., 1997. P–T paths and migmatite formation: an

example from Deep Freeze Range, northern Victoria Land, Bay, Antarctica—the Boomerang Thrust. Mem. Soc. Geol.It. 46, 163–178.Antarctica. Lithos 42, 47–66.

Palmeri, R., Talarico, F., Meccheri, M., Oggiano, G., Pertusati, Skinner, D.N.B., Jordan, H., Schmidt-Thome, M., 1996. TheBerg Group of Oates Land, East Antarctica. Geol. Jb. BP.C., Rastelli, N., Ricci, C.A., 1991. Progressive deformation

and low pressure/high temperature metamorphism in the 89, 271–293.Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrialDeep Freeze Range, Wilson Terrane, northern Victoria Land,

Antarctica. Mem. Soc. Geol. It. 46, 179–195. lead isotope evolution by a two stage model. Earth Planet.Sci. Lett. 26, 207–221.Parrish, R.R., 1988. U–Pb systematics of monazite and its clo-

sure temperature based on natural examples. Geol. Assoc. Steiger, R.H., Jager, E., 1977. Subcommision on geochronol-ogy: convention to use of decay constants in geo- and cos-Can. Prog. w. Abstr. 13, A94

Parrish, R.R., 1990. U–Pb dating of monazite and its applica- mochronology. Earth Planet. Sci. Lett. 36, 359–362.Stump, E., 1995. The Ross Orogen of the Transantarctiction to geological problems. Can. J. Earth Sci. 27, 1431–1450.

Plummer, C.C., Babcock, R.S., Sheraton, J.W., Adams, C.J.D., Mountains. Cambridge University Press, Cambridge.Sturm, A., Carryer, S.J., 1970. Geology of the Region betweenOliver, R.L., 1983. Geology of the Daniels Range, Northern

Victoria Land, Antarctica: a preliminary report. In: Oliver, the Matusevich and Tucker Glaciers, North Victoria Land,Antarctica. NZ. J. Geol. Geophys. 13, 408–435.R.L., James, P.R., Jago, J.B. (Eds.), Antarctic Earth Sience.

Cambridge University Press, Cambridge, pp. 113–117. Stuwe, K., Oliver, R., 1989. Geological history of Adelie Landand King George V Land, Antarctica: evidence for a polycy-Powell, R., Holland, T.J.B., 1988. An internally consistent

thermodynamic data set with uncertainties and correlations. clic metamorphic evolution. Precambrian Res. 43, 317–334.Talarico, F., 1990. Reaction textures and metamorphic evolu-III. Application methods, worked examples and a computer

program. J. metamorphic Geol. 6, 173–204. tion of aluminous garnet granulites from the Wilson Terrane(Deep Freeze Range, North Victoria Land Antarctica). Mem.Powell, R., Holland, T.J.B., 1994. Optimal geothermometry and

geobarometry. Am. Mineral. 79, 120–133. Soc. Geol. It. 43 (1988), 49–58.Talarico, F., Castelli, D., Migmatitic metasedimentary granu-Purdy, J.W., Jager, E., 1976. K–Ar ages on rock forming miner-

als from the Central Alps. Mem. Inst. Geol. Petrol. Univ. lites from Mills Peak and Mt. Emison (Wilson Terrane,Northern Victoria Land, Antarctica): a case history of pro-Padova 30, 1–321.

Roland, N.W., Gibson, G.M., Kleinschmidt, G., Schubert, W., cesses involved in the formation of garnet±orthopyroxeneleucocratic segregations. 1994. Terra Antartica 1, 19–22.1984. Metamorphism and structural relations of the


Talarico, F., Castelli, D., 1995. Relict granulites in the Ross Vetter, U., Tessensohn, F., 1987. S- and I-type granitoids ofNorth Victoria Land, Antarctica, and their inferred geotec-orogen of northern Victoria Land (Antarctica), I. Field

occurrence, petrography and metamorphic evolution. tonic setting. Geol. Rundsch. 76, 233–243.Villa, I.M., in press. Isotopic closure. Terra Nova.Precambrian Res. 75, 141–156.

Talarico, F., Memmi, I., Lombardo, B., Ricci, C.A., 1989. Villa, I.M., Puxeddu, M., 1994. Geochronology of theLarderello geothermal field: new data and the ‘‘closure tem-Thermo-barometry of granulite rocks from the Deep Freeze

Range, North Victoria Land, Antarctica. Mem. Soc. Geol. perature’’ issue. Contrib. Mineral. Petrol. 115, 415–426.Vita-Scaillet, G., Feraud, G., Ruffet, G., Lombardo, B., 1994.It. 33, 131–142.

Talarico, F., Borsi, L., Lombardo, B., 1995. Relict granulites K/Ar and 40Ar/39Ar laser-probe ages of metamorphic micasand amphibole of the Wilson Terrane and Dessent Unit,in the Ross orogen of northern Victoria Land (Antarctica)

II. Geochemistry and paleo-tectonic implications. Northern Victoria Land (Antarctica): their bearing on theregional post-metamorphic cooling history. Terra AntarticaPrecambrian Res. 75, 157–174.

Tessensohn, F., 1997. Shackleton Range, Ross Orogen and 1, 59–62.Xu, Guowei, Will, T.M., Powell, R., 1994. A calculated petroge-SWEAT Hypothesis. In: Ricci, C.A. (Ed.), The Antarctic

Region: Geological Evolution and Processes. Terra netic grid for the system K2O–FeO–MgO–Al2O3–SiO2–H2O, with particular reference to contact-metamor-Antartica, Siena, pp. 5–12.

Ulitzka, S., 1987. Petrology and geochemistry of the migmatites phosed pelites. J. Metamorphic Geol. 12, 99–119.York, D., 1969. Least squares fitting of a straight line withfrom Thompson Spur, Daniels Range, North Victoria Land,

Antarctica. Geol. Jb. B 66, 81–130. correlated errors. Earth Planet. Sci. Lett. 5, 320–324.

Documents

P T t evolution of the Wilson Terrane metamorphic basement ...€¦ · P–T–t evolution of the Wilson Terrane metamorphic basement at Oates Coast, Antarctica Ulrich Schu¨ssler