Mineral ages for multi isotope system in phlogopite-bearing pyroxenegranulite and felsic gneiss, the Howard Hills, Enderby Land,East Antarctica: Possible Proterozoic tectonothermalevents in the Napier Complex
Tomoharu Miyamoto+�, Yasutaka Yoshimura,, Daniel J. Dunkley-,Yoichi Motoyoshi- and Christopher J. Carson.
+Department of Earth and Planetary Sciences, Kyushu University, Hakozaki 0-chome, Fukuoka 2+,-2/2+
,Department of Geology, Kochi University, Akebono-cho ,-chome, Kochi 12*-2/,*
-National Institute of Polar Research, Kaga +-chome, Itabashi-ku, Tokyo +1--2/+/
.Northern Territory Geological Survey, GPO -***, Darwin NT, *2*+, Australia
�Corresponding author. E-mail: miyamoto@geo.kyushu-u.ac.jp
(Received February ,., ,**0; Accepted June 3, ,**0)
Abstract:Large maﬁc to ultramaﬁc blocks in felsic gneisses on the northern part ofthe Howard Hills, Napier Complex, East Antarctica are accompanied by phlogopite-bearing pyroxene granulite occurring in the margins of the block.In order tounderstand the crustal evolution of the Napier Complex, especially regarding thethermal history after peak metamorphism, Rb-Sr, Sm-Nd and U-Pb isotopic analyseshave been carried out on di#erent minerals from the phlogopite-bearing pyroxenegranulite and adjacent orthopyroxene felsic gneiss.Zircon grains from the orthopyroxene felsic gneiss yielded near concordant U-Pbisotopic ages of about ,./ Ga by conventional isotope dilution methods and deﬁned adiscordia with ,...�*.*, Ga lower intercept age.This age shows the waning stage ofUHT granulite facies metamorphism in the Howard Hills.Rutile fractions frompyroxene granulite yielded a near concordant U-Pb isotopic age of about +./ Ga.Thisage is interpreted as the ﬁnal thermal episode, excepting local igneous intrusions, in theHoward Hills region.Fluorophlogopite fractions from pyroxene granulite yielded Rb-Sr model ages ofabout +.2/ Ga, although evidence of retrograde metamorphism with ﬂuid activity ordeformation were poor in the pyroxene granulite.An internal Sm-Nd isochron ofwhole rock and orthopyroxene and feldspar separated from the same sample shows+.2/�*.+/ Ga.The Rb-Sr phlogopite model age, along with the Sm-Nd internalisochron age, records the time when the rocks of the Howard Hills underwent mediumto high grade metamorphism at temperatures well above the currently accepted closuretemperature of biotite (about -**�-/*�C).
key words: Napier Complex, UHT metamorphism, zircon, ﬂuorophlogopite, rutile
+.Introduction
The Napier Complex in Enderby Land, East Antarctica, consists of ultra-hightemperature (UHT) granulite-facies metamorphic rocks that are characterized by
Polar Geosci., +3, +/,�+1,, ,**0� ,**0 National Institute of Polar Research
osumilite-bearing, sapphirine-quartz and spinel-quartz assemblages (Sheraton et al.,+321).The temperature and pressure conditions of peak metamorphism have beenestimated at over +***�C and 1�++ kbar through analyses of phase relations and geo-thermobarometry (e.g., Harley and Hensen, +33*; Motoyoshi, +332).The complex issurrounded by the Proterozoic Rayner Complex, which is composed of upper-amphi-bolite to granulite-facies metamorphic rocks (Sheraton et al., +321).The Napier Complex is an important area for research into the structure andevolution of the Archean continental lithosphere.Many isotopic dating studies havebeen conducted on metamorphic rocks from the complex to reveal its geothermal history(Fig. +).Zircon grains with ion-microprobe (SHRIMP) U-Pb ages up to -.2 Ga havebeen found in orthogneiss from Mt. Sones (Williams et al., +32.; Black et al., +320).Harley and Black (+331) suggested that UHT granulite-facies metamorphism occurredat ,.2,�,.2. Ga deduced from their SHRIMP U-Pb geochronology.However, recentSHRIMP and CHIME dating of zircon and monazite in the granulite have yieldedabundant Late Archean�Early Proterozoic ages of ,.. to ,./ Ga (Hokada et al., ,**+,,**-, ,**.; Asami et al., ,**,; Carson et al., ,**,a, b; Suzuki et al., ,**0).Hokada etal. (,**+, ,**-, ,**.), Asami et al. (,**,) and Suzuki et al. (,**0) suggest that mostrocks in the Napier Complex underwent UHT granulite-facies metamorphism at about,..�,./ Ga.On the other hand, Harley et al. (,**+), Crowe et al. (,**,) and Kellyand Harley (,**/) infer it to be somewhat older, �,./, Ga, and up to � ,./3 Ga.Intheir research, Kelly and Harley (,**/) reported U-Pb ages of zircon from metamor-phic rocks on Gage Ridge and Dallwitz Nunatak where there are easterly ﬁelds in thewestern Napier Complex.A reason for the disagreement might be the local di#erenceof geothermal history in the Napier Complex.Further regional chronological researchis required to understand the thermal history of the Napier Complex.Many chronological studies have been performed to decide the timing of peakmetamorphism and the ages of older igneous events from detrital materials in the NapierComplex; however, chronological studies for post-peak thermal evolution are few (Fig.+).The Napier Complex has matured as a continental lithosphere after UHT granulitefacies metamorphism.Research on geothermal history after peak metamorphism isimportant for revealing the structure and evolution of the Archean continental crust.Only a few geochronological studies at Mount Riiser Larsen, Tonagh Islands, and someoutcrops around Casey Bay, however, have suggested the possibility of a geothermalevent after peak metamorphism (Grew et al., ,**+; Owada et al., ,**+, ,**-; Suzuki etal., ,**+, ,**0; Carson et al., ,**,a, Fig. +).The outcrops around Casey Bay are closeto the Rayner Complex, and are probably inﬂuenced by Proterozoic metamorphism atthe complex.Mount Riiser Larsen and the Tonagh Islands are far from the RaynerComplex; however, these crops are probably also inﬂuenced by the Proterozoictectonothermal event of the complex (Owada et al., ,**-).We can ﬁnd their trace inmetamorphic rock samples in the Napier Complex.Therefore, further systematicresearch on the thermal history after peak metamorphism is required to understand thehistory of transition from dynamic to static conditions during the maturation of thecontinental crust.In order to constrain the timing of later thermal events, especiallyﬁnding guideposts for the temperature-time relation after peak metamorphism, in thethermal history of the Napier Complex, Rb-Sr, Sm-Nd and U-Pb analyses were done on
Mineral chronology of metamorphic rocks from the Howard Hills153
Fig. +.
Miyamoto et al.154
References 
( -I Mnz in reaction skam Beaver Island Asami et al. (2002) 
----•: • Zm in orthogneiss (rim) Dallwitz Nunatak Kelly and Harley (200S) -------- . -------------------------------------:!II Zm in orthogneiss (rim) Gage Ridge Kelly and Harley (200S) --------�----------------------------------
core & rirri Mt.Cronus 
I-core &\nm I Zrn in falsie gneiss Asami et al. (2002) 
j I core ---------r----------------------------------1-----t,--·  --- W.R(maficgneiss) Mt.Pardoe Tainosho etal. (1997) 
________________ "."':...R_:!f��-g�--_ __ __ _ _ _ _ _ __ _ _ _ _ Tainosho et al. (1994) 
Zm in falsie granu�te I Mt.Riiser Larsen Zrn in quartzose g�.  iss I Mnz in falsie granulite, core 
I &ri") 
Asami et al. (2002) 
... Zm in orthogneiss : + Minerals-WR. 
Zm in falsie gneiss (Spr-Qtz gneiss) 
: Mnz in falsie gneiss 
Zm in orthog� Zm in orthogneiss 
• Zrn in granite gneiss (core) 
Hokada et al. (200 I) Suzuki et al. (2001) Asami et al. (1998) 
Hokada et al. (2003) 
- Zm in granite gneiss (rim) Mnz in granite 
gneiss (core&mi) -t- Minerals-W.R.(granitic gneiss) Suzuki et al. (2006) 
; � Minerals-W.R.(psammitic gneiss) 
; Minerals-W.R.(mafic gneiss) 
-:::: _!'" i�felsicgneiss(L.I.) ___________________ Mt.Sones Williams etal. (1984) 
core & rim 
Zm in falsie gneiss 
\ Xtm in falsie gneiss 
cofe I Mnz in falsie gneiss 
Reference Peak 
Asami et al. (2002) 
----f Minerals-W.R.(Spr-Grtgneiss) � Tonaghlsl�d Osanaietal.(2001) 
W.R.(felsij: gneiss Grt-W.R.(falsic gneiss)+ Owada et al. (2001) & ultrar!lBfic rock) Minerals-W.R.(Grt-�neiss) Owada et al. (1994) =+ 
-+- Zrri in falsie orthogneiss 
+: :+ 
:+ :-1-
Zm in alteration selvage (range of Pb.f>b agoa) Carson et al. (2002a) 
Carson et al. (2002b) 
Zm in myfon�e ,f,i + 
Crowe et al. (2002) 
---------------------------------------------t-i W.R. (+sic gneiss - mafic, ultramafic rock) Howard Hills Miyamoto et al. (2004) 
: -+-Phl-Fld (Px granulite) 
Zm in fel�ic gneiss (l.l.) 
Field Island ri� 
Xtm in pegmatite -foore & W!!i-
This study 
Grew et al. (200 I) 
-------- .----------------------------------' : Fyfe Hills Black et al. (1983) 
W.R.(gabbro, arnfthosite) 
Zm in paragne-
i55:(rim) Zircon Point Harl-, and Black (1997) Zm in paragneiss (rim&sector-zoning) w:, 
2800 2600 
· Kelly and Harley (200S) 
2400 2200 2000 Age (Ma) 1800 1600 1400 
mineral fractions from metamorphic rocks in the Howard Hills, Enderby Land, EastAntarctica, and the relation between dating results and geothermal conditions consid-ered.The mineral fractions used are zircon, rutile, orthopyroxene, feldspar, andphlogopite which have di#erent closure temperatures (Tc).Their dating results prob-ably give us guideposts on the thermal history of the Napier Complex, even if textureand mineral assemblages in metamorphic rocks were poorly modiﬁed after peak meta-morphism.
,.General geology and sample description
,.+.General geology and occurrence of dated sampleThe Howard Hills lie on the eastern shore of Amundsen Bay in Enderby Land (Fig.,), and consist mainly of UHT granulite-facies metamorphic rocks (Yoshimura et al.,,***).The basement rocks include alternating layers of garnet-bearing felsic gneissand orthopyroxene-bearing felsic gneiss, with minor amounts of metamorphosed maﬁcto ultramaﬁc rocks occurring sporadically as thin lenticular layers (tens of centimetersto meters across).Peak metamorphic conditions of the gneisses in the Howard Hillshave reached ++/*�+,**�C (Yoshimura et al., ,***).Metamorphic rocks are intrudedby unmetamorphosed basaltic to doleritic Amundsen dykes at ca.+., Ga (Sheraton andBlack, +32+; Sheraton et al., +321).We investigated felsic gneisses cropping out in the northern part of the HowardHills.The felsic gneiss contains an over / m elongated block of metamorphosed maﬁcto ultramaﬁc rocks (Fig. -).The metamorphosed ultramaﬁc rock is composed ofolivine with subordinate orthopyroxene, spinel and phlogopite, and is locally penetratedby ﬁne serpentine veins (Miyamoto et al., ,**.).Pyroxene granulite divides theultramaﬁc core from the felsic gneiss and comprises orthopyroxene mainly with smallclinopyroxene and plagioclase, with granoblastic texture.A phlogopite-rich section ispresent in the boundary zone with the felsic gneiss (Fig. -b).Small patches oforthopyroxene felsic gneiss also occur in the pyroxene granulite.
,.,.Petrography and mineral remarks of dated samplesPhlogopite-bearing pyroxene granulite (TM32+,,3-*-E) and orthopyroxene felsicgneiss (TM32+,,3-*-Ef) were taken from the edge of the maﬁc block (Fig. -b).Sample TM32+,,3-*-E is composed mainly of orthopyroxene, with interstitial phlo-
Fig. + (opposite).Compiled age data between ,.2 and +.. Ga for metamorphic rocks from the NapierComplex.A vertical dotted line shows the Archean-Proterozoic boundary.Red: Sm-Nd isochronage, Orange: Rb-Sr isochron age, Green: U-Th-total Pb CHIME age, Blue: U-Pb SHRIMP age, andBlack: U-Pb age deﬁned by conventional method.Thin colored lines represent the data values ofone analysis with error ranges.Thick colored lines show the dated ranges involving a series ofanalyses.“U.I.” and “L.I.” represent the upper and lower intercept ages for discordant U-Pb age.Fld: feldspar, Grt: garnet, Mnz: monazite, Phl: phlogopite, Px: pyroxene, Qtz: quartz, Rtl: rutile, Spr:sapphirine, Xtm: xenotime, Zrn: zircon, and W.R.: whole rock sample.It is clear that datingresults for SHRIMP U-Pb dating and U-Th-Pb CHIME dating methods of zircon and monazite areconcentrated between the Late Archean and the Early Proterozoic.Other methods mostly showProterozoic.
Mineral chronology of metamorphic rocks from the Howard Hills155
gopite, alkali feldspar, plagioclase, sapphirine and spinel.Sapphirine occurs as por-phyroblasts with core parts of greenish spinel and as thin ﬁlms along the grainboundaries of granoblastic orthopyroxene.This spinel-sapphirine corona was syn-thesized artiﬁcially for this granulite sample at high-pressure (3 kbar) and high-tem-perature (++**�C) during an annealing experiment involving heating to +,**�C (Sato etal., ,**.a).The sapphirine ﬁlms locally contact quartz (Miyamoto et al., ,**.).Alkali feldspar is sanidine and/or perthite.Rutile is present as a minor phase, andoccursalonggrainboundariesandasacicularinclusionsinorthopyroxene.Orthopyroxene shows aluminous composition, and the content decreases from the core(3.3 wt� of Al,O-, *.., per 0-oxygen formula unit) to the rim (/.- wt�, *.,, per 0-oxygen formula unit) although their Mg/Fe ratios have small di#erences (Mg/(Mg�Fetotal)�*.10�*.13) (Miyamoto et al., ,**.).Large orthopyroxene grains include ru-tile, phlogopite and unidentiﬁed ﬁne minerals.Phlogopite is rich in ﬂuorine (nearly .wt�; Sato et al., ,**.b) and poor in H,O (H,O(�)�*.*0 wt�, H,O(�)�*./. wt�,as determined by the Karl-Fischer method (Muroi, +313)).Sample TM32+,,3-*-Ef consists of quartz, plagioclase, alkali feldspar and ortho-pyroxene.Alkali feldspar represents perthite to mesoperthite.Rutile and zirconoccur as accessory minerals in the matrix and as inclusions in orthopyroxene.Somezircon grains show oscillatory zoning without an inherited-core (Fig. .a), and othershave an inherited core with mosaic texture (Fig. .b).Quartz, feldspar, pyroxene andbiotite were found in some zircon grains as inclusions (Figs. .a, b and c).Chemical
Fig. ,.Location map of the Howard Hills in western Enderby Land, East Antarctica.Inset, �: Schir-macher Hills, �: S�r Rondane Mountains, �: Yamato-Belgica Complex, and �: Prince CharlesMountains.
Miyamoto et al.156
Amundsen Bay 
Casey Bay 
'Mt. Cronus � �G, 
� Reference Pk. 
� � � 
\))T \\ufffd '� 
� � 49°30' E 50'00' E 3 ' 51'00' E 
67'30' S 
51'30' E 
compositions of pyroxene and biotite included in zircon are listed in Table +.Evidenceof hydrothermal activity at a later stage after the UHT metamorphism was not found,except for occurrence of serpentine in the associated metamorphosed ultramaﬁc rock.
,.-.Compositional remarks of the rock samplesMajor and minor element compositions of phlogopite-bearing pyroxene granulite(TM32+,,3-*-E), orthopyroxene felsic gneiss (TM32+,,3-*-Ef) and associated rocksare reported by Miyamoto et al. (,**.).Estimated Nd model ages of the orthopy-roxene felsic gneiss (TM32+,,3-*-Ef; Miyamoto et al., ,**.) are tNd,CHUR�-.-1�*.+0Ga and tNd,DM�-./*�*.+1 Ga (based on +.-Nd/+..Nd (present)�*./+,0-2 and +.1Sm/+..Nd(present)�*.+300forchondriticuniform reservoir (CHUR) parameters
Fig. -.Occurrence of metamorphosed ultramaﬁc block and granulite in felsic gneiss (a; photograph, b;sketch, with sampling points of TM32+,,3-*-E and TM32+,,3-*-Ef).Their petrographies andcompositions are described in Miyamoto et al. (,**.).
Mineral chronology of metamorphic rocks from the Howard Hills157
(Hamilton et al., +32-), and +.-Nd/+..Nd (present)�*./+-+/ and +.1Sm/+..Nd (present)�*.,+-1 for depleted mantle (DM) parameter (Peucat et al., +322)).In that study,Rb-Sr and Sm-Nd isotope data of the metamorphic rocks were arranged as lines on theisochron diagrams with contemporary apparent ages of about ,.0/ Ga within analyti-cal errors with ordinate intercepts with 21Sr/20Sr�*.1-/1�*.**,. and +.-Nd/+..Nd�*./*2/0�*.****. (eNd,CHURisochron,t ��+,.-�*.3).Miyamoto et al. (,**.) interpreted thesewhole rock isochron ages as the time of UHT granulite facies metamorphism with
Fig. ..Back scattered electron (BSE) images of zircon grain prepared for U-Pb isotopic analyses fromorthopyroxene felsic gneiss (TM32+,,3-*-Ef).The images were pictured by a scanning electronmicroscope (SEM: JEOL JSM-/2**LV) with an energy dispersive X-ray analytical system (EDS:OXFORD Link ISIS) at Kyushu University.For chemical zoning and mosaic core in the images,light and dark areas are Hf rich and poor, respectively.Zoning (a) and mosaic core (b) are faintlyvisible.(c) Zircon with many kind of inclusions in the same grain.Afs: alkali-feldspar, Bt: biotite,Px: pyroxene, Qtz: quartz.Chemical compositions of biotite and pyroxene inclusions are listed inTable +.
Miyamoto et al.158
partial melting of LILE-enriched protoliths, after denying the possibility of pseu-doisochrons due to mechanical mixing among the dated samples in terms of petrochem-ical evidence.
-.Analytical procedure
Phlogopite, orthopyroxene, feldspar and rutile were separated from phlogopite-bearing pyroxene granulite (TM32+,,3-*-E).Zircon was also taken from the ortho-pyroxene felsic gneiss (TM32+,,3-*-Ef).The minerals were concentrated by magnet-ic separation with a Frantz isodynamic separator and by gravity separation with tetra-bromoethane and methylene iodide.All mineral fractions were puriﬁed by hand-picking under a binocular-microscope.For Rb-Sr and Sm-Nd isotopic analysis, +**�,** mg of each mineral fraction wasused.They were cleaned with acetone and distilled water before decomposition.Conventional isotope dilution methods were applied to determine Rb, Sr, Sm and Ndconcentrations of the samples.A decomposed sample fraction was resolved in a
Table +.Chemical compositions of pyroxene and biotite includedin a zircon from TM32+,,3-*-Ef (Fig. .c).
pyroxenebiotite
wt�SiO,TiO,Al,O-FeO�
MnOMgOCaONa,OK,OTotal
CationsSiTiAlFeMnMgCaNaKTotal Cations
.1.+-*.*0+..0++,./+*.+,,+.,3,..0+.*,
(O�0)+.031*.**,*.0,**.-11*.**.+.+.-*.*3/*.*1+
.+.*+..31+-./10.3-*.+*+3.0*
+.*+3.-030.//
(O�,,)/.2*+*./,3,.,0,*.2,**.*+,..+--
*.,10+.03*+/./,-
Chemical compositions were determined using a scanning electronmicroscope (SEM: JEOL JSM-/2**LV) with an energy dispersiveX-ray analytical system (EDS: OXFORD Link ISIS) at KyushuUniversity.� Total Fe as FeO.
Mineral chronology of metamorphic rocks from the Howard Hills159
mixture of hydrochloric acid (HCl) and oxalic acid ((COOH),), and passed throughDOWEX /*W-X2, ,** mesh cation exchange resin to separate Rb, Sr and REE.Samarium and neodymium were dissolved by *.,N ,-methyllactic acid, and separatedfrom the REE by passing through DOWEX /*W-X2, ,** mesh cation exchange resinadjusted with ammonia, following the method of Kubota (+33,) modiﬁed from Notsuet al. (+31-).Rubidium concentrations were determined with a HITACHI RMU/Gmass spectrometer with a single collector.Strontium and neodymium isotope compo-sitions and total Sr, Sm and Nd concentrations were determined with a JEOL JMS*/RBmass spectrometer with a single collector.The strontium standard of Eimer andAmend gave 21Sr/20Sr�*.1*2*�*.***+ (+s) and standard NBS32- gave 21Sr/20Sr�*.1+*-�*.***+ (+s).The neodymium standard of JNdi-+ (GSJ (Geological Surveyof Japan) standard with recommended value of+.-Nd/+..Nd�*./+,++/�*.*****1;Tanaka et al., ,***) gave +.-Nd/+..Nd�*./+,+,�*.****. (+s).Relative analyticalerrors in Rb, Sr, Sm and Nd concentrations are ,�, +�, *./� and +�, respectively.Contamination levels of Rb, Sr, Sm and Nd are under +�+*�+*g, -�+*�+*g, +�+*�+*gand +�+*�+*g per sample.For U-Pb isotopic analysis, +*�-* dark reddish-brown grains of rutile were pickedfor each run (Nos. /3-, /33, and 0*, in Table ,).For zircon analysis, one (Nos. 0-2,0.,, 0.. and 0./), two (Nos. 0-1 and 0.+), and four (No. 0-0) grains were used assample fractions.All zircon grains chosen are dark-colored and translucent withsmooth surfaces.All rutile fractions and one zircon fraction (No. 0-0) were cleanedwith warmed dilute nitric acid to remove common lead contamination on grain surfaces.Other zircon fractions were cleaned by warming in distilled water for an hour forsurface cleaning before decomposition.Conventional isotope dilution methods wereapplied to determine U and Pb compositions of the samples.Chemical treatment andanalytical method for U-Pb isotope determination in zircon and rutile followed princi-pally the conventional method of Krogh (+31-), and is described in Miyamoto andYanagi (+330).The decay constants used for age calculations are 0./.�+*�+, year�+ for +.1Sm(Lugmair and Marti, +312), +..,�+*�++ year�+ for 21Rb, 3.2.2/�+*�+* year�+ for ,-/Uand +.//+,/�+*�+* year�+ for ,-2U, and the present ,-2U/,-/U ratio is +-1.22 (Steiger andJa¨ger, +311).Samarium-neodymium isochrons were determined by the least squaresregression method of York (+300).Error estimation with correlations for U-Pb iso-tope ratios follows Ludwig (+32*).A discordia was deﬁned from the U-Pb isotoperatios of zircon by the least squares regression method of York (+303) for data withcorrelated errors.
..Results
Analytical U-Pb isotope data for rutile fractions from phlogopite-bearing pyroxenegranulite (TM32+,,3-*-E) and zircon fractions from orthopyroxene felsic gneiss (TM32+,,3-*-Ef) are listed in Table ,.A concordia diagram for the U-Pb compositions isshown in Fig. /.Some of the zircon fractions have almost concordant U-Pb composi-tions that correspond to ages of around ,../ Ga (Fig. /a).Forced extrapolation ofU-Pb compositions of zircon fractions except No. 0-0 make us estimate a discordia with
Miyamoto et al.160
lower intercept age of ,...�*.*, Ga and upper intercept age of -.3�*./ Ga.Rutilefractions have near concordant U-Pb compositions (Fig. /b) and correspond to ages of+./�+.0 Ga, which is the youngest age estimation among U-Pb, Sm-Nd, and Rb-Sr resultsin this study.Analytical Rb-Sr isotopic compositions for feldspar and phlogopite fractions fromphlogopite-bearing pyroxene granulite are shown in Table -.Phlogopite fractionscontain abundant rubidium, and show extremely high Sr isotope ratios.Model ages ofphlogopite (tmodel) were determined as ,-points isochron ages for all fractions byassuming an initial ratio equal to the Rb-Sr isotopic ratio of the feldspar fraction (Fig.0).The tmodel values are +.13�+.3* Ga with analytical errors of about *.*0 Ga.Analytical Sm-Nd isotopic data for whole-rock, feldspar and orthopyroxene frac-tions from phlogopite-bearing pyroxene granulite are shown in Table ..Isotope ratiosapproximate a regression line with a slope corresponding to an age of +.2/�*.+/ Gawith an initial ratio of (+.-Nd/+..Nd)I.R.�*./*3*1�*.***++ on an isochron diagram(Fig. 1).
/.Discussion
The pyroxene granulite and felsic gneiss on the Howard Hills were residue and/orinﬂuenced by melt generated during partial melting of LILE-enriched protoliths withhigh Sr isotope ratio (Yoshimura et al., ,***; Miyamoto et al., ,**.).Partial meltingunder granulite-facies conditions can cause growth of zircon grains in metamorphicrocks (Rubatto et al., ,**+).It is signiﬁcant that zircon includes biotite and pyroxenein addition to quartz and feldspar (Fig. .c).The biotite inclusion might be evidence ofa protolith of metamorphic rocks since biotite was not found in the host felsic gneissmatrix (TM32+,,3-*-Ef).The pyroxene has aluminous composition (+..0+ wt� ofAl,O-, corresponding to *.0, Al atoms per 0 oxygens) with Mg/(Mg�Fetotal)�*.1/.
Table ,.Analytical data for rutile fractions in phlogopite-bearing pyroxene granulite (TM32+,,3-*-E)and zircon fractions in the orthopyroxene-bearing felsic gneiss (TM32+,,3-*-Ef) from theHoward Hills, Enderby Land, East Antarctica.
Mineral chronology of metamorphic rocks from the Howard Hills161
Sample U (ppm) Pb (ppm) 2°'Pb/206Pb* 201Pb/206Pb* 208Pb/206Pb* li06 (Ga)* lio1 (Ga)* 1207.2()6 (Ga)* 
Rutile fractions in phlogopite-bearing pyroxene granulite (1M981229-03E) No.593 63.4 16.1 0.0003 ± 0.0003 0.1022 ± 0.0087 0.0104 ± 0.0050 1.51 ± 0.49 1.54 ± 0.32 1.58 ± 0.25 No.599 63.9 14.9 0.0001 ± 0.0001 0.0949 ± 0.0026 0.0058 ± 0.0010 1.42 ± 0.17 1.46 ± 0.11 1.50 ± 0.09 No.602 66.6 15.0 0.0003 ± 0.0003 0.1026 ± 0.0026 0.0095 ± 0.0007 1.35 ± 0.08 1.45 ± 0.05 1.58 ± 0.09 
Zircon fractions in Opx-bearing felsic gneiss (1M981229-03Ef) No.636 1240 572 0.0001 ± 0.0001 0.1711 ± 0.0002 0.0488 ± 0.0002 2.36 ± 0.04 2.47 ± 0.02 2.56 ± O.ot No.637 672 322 0.0002 ± 0.0002 0.1631 ± 0.0002 0.0582 ± 0.0005 2.43 ± 0.03 2.45 ± 0.02 2.46± 0.01 No.638 819 427 0.0010 ± 0.0001 0.1772 ± 0.0003 0.1001 ± 0.0001 2.48 ± 0.02 2.49 ± 0.01 2.51 ± 0.01 No.641 452 220 0.0002 ± 0.0001 0.1671 ± 0.0001 0.0555 ± 0.0001 2.47 ± 0.01 2.49 ± 0.01 2.51 ± 0.01 No.642 245 118 0.0001 ± 0.0001 0.1620 ± 0.0002 0.0623 ± 0.0003 2.44 ± 0.02 2.45 ± 0.01 2.46 ± O.ot No.644 1081 529 0.0001 ± 0.0001 0.1746 ± 0.0001 0.0409 ± 0.0002 2.50 ± 0.02 2.56 ± 0.01 2.60 ± 0.01 No.645 500 244 0.0003 ± 0.0003 0.1724 ± 0.0003 0.0368 ± 0.0002 2.49 ± 0.02 2.53 ± 0.02 2.56 ± 0.01 
*Errors given are ±la. For error calculation, see Ludwig (1980). 
These characteristics are consistent with composition of porphyroblastic orthopyroxenein the gneiss matrix.Therefore, the zircon potentially grew before or at the time ofreaction with biotite by melting and/or after production of pyroxene.The composition of No. 0-0 fraction lies between those of the others and the originon the concordia diagram (Fig. /).Since this zircon fraction was rinsed with warmednitric acid diluent, the discordant composition is probably attributed to Pb loss from
Fig. /.U-Pb concordia diagram for zircon grains from orthopyroxene felsic gneiss (TM32+,,3-*-Ef) andrutile fractions from phlogopite-bearing pyroxene granulite (TM32+,,3-*-E).Figures /a and /bare expanded diagrams for the zircon and rutile fractions.Ellipses express data points for eachfraction with correlated errors.Rutile fractions which are dark reddish-brown under the microscopecomprise +*�-* grains (Nos. /3-, /33, and 0*,).For zircon fractions (Nos. 0-0�0.0), Nos. 0-2,0.,, 0.. and 0./ fractions are single grain, Nos. 0-1 and 0.+ fractions comprise two grains, and theNo. 0-0 fraction comprises four grains.Forced extrapolation of U-Pb compositions of zirconfractions except No. 0-0 estimates a discordia with lower intercept age of ,...�*.*, Ga and upperintercept age of -.3�*./ Ga.
(a)(b)
Miyamoto et al.162
::> 0.55 
CX) C') � 6: 0.5 
(0 0 C\l 0.45 
::::, CD Cf) C\J ..Q a.. CD 0 C\J 
:2.0 Ga 0.3 
I J I I l Rutile 1 (No.593: 0.2 1.0 Ga __ -���] 
11 13 
207Pb/235LJ 
207pb/235LJ 
15 20 
0.4 .----------------, 
0.35 ::> � 0.3 � 
a..o.25 (0 0 C\l 0.2 
0.151 
1.0 Ga 
2 3 4 
207pb/235LJ 
metamict zircon during this cleaning process.An extrapolation of U-Pb compositions of zircon fractions except No. 0-0 shows-.3�*./ Ga as the upper intercept age (Fig. /a).Unfortunately, this upper interceptage has large uncertainty although it is similar to the Nd model age of the host rock(tNd,CHUR�-.-1�*.+0 Ga and tNd,DM�-./*�*.+1 Ga for TM32+,,3-*-Ef: Miyamoto etal., ,**.); therefore, it does not require detailed examination.One thing, however, iscertain: this discordance is consistent with the presence of an inherited domain in somezircons (Fig. .b).The discordant U-Pb composition is probably followed by theexistence of a surviving old domain in zircon as an inherited core.The extrapolation shows ,...�*.*, Ga as the lower intercept age (Fig. /a).Thisage probably shows an age of UHT granulite facies metamorphism.However, Miya-moto et al. (,**.) reported simultaneous ,.0/ Ga by Rb-Sr and Sm-Nd whole rockisochron ages using metamorphic rocks with ultramaﬁc to felsic compositions on the
Table -.Analytical data for feldspar and phlogopite fractions in phlogopite-bearing pyroxene granulite(TM32+,,3-*-E) from the Howard Hills, East Antarctica.
Rb (ppm)Sr (ppm)21Rb/20Sr21Sr/20Sr�tmodel (Ga)�
FeldsparPhlogopite*./*�*.1+ mm*.-/�*./* mm*.,/�*.-/ mm
+-2,+2*,+1**
,..-,..*+3.2
,31.201*.
*.133/2�*.****2
2.231+ �*.***.+-.,30�*.**-+3..31�*.**,
+.3*�*.*0+.13�*.*0+.2/�*.*0
� Error given are �+s.Model ages (tmodel) of phlogopite fractions were estimated on from the pairs offeldspar and phlogopite fractions.
Fig. 0.Rb-Sr isotope ratios of phlogopite and feldspar from pyroxene granulite (TM32+,,3-*-E) on anisochron diagram.Three model isochron ages (tmodel) connecting with the feldspar fraction andthree phlogopite fractions are also given in Table -.
Mineral chronology of metamorphic rocks from the Howard Hills163
20 ,-----���������--,-�����--,....,....,�-, Bt,0.25-0.35mm 
,,__ (f) co 
� 10 (f) r--co 
Bt,0.50-0. 71 mm 
tmodel :c: 1.85 ± 0.06 Ga// / /. /.<··' .....: ... · Bt,0.35-0.50mm 
tmodel" 1.79 ± 0.06 Ga 
--0--: Bt,0.50-0.71mm 
D : Bt,0.35-0.50mm 
-[}-: Bt,0.25-0.35mm Feldspar o���-��-���-��-���-��-�� 0 200 600 800 
Howard Hills.He concluded that the ,.0/ Ga whole rock age indicates the time ofUHT granulite facies metamorphism with partial melting of LILE-enriched protolithswith high Sr isotope ratio, after eliminating the possibility of pseudoisochrons bymechanical mixing of the analyzed samples.Ellis and Green (+32/) and Harley (+32/,+332) proposed that isobaric cooling took place in the UHT metamorphic rocks afterpeak metamorphic condition in the Napier Complex.In addition, Yoshimura et al.(,***) also suggested the same possibility for metamorphic rocks from the HowardHills.Cherniak and Watson (,***) suggested that zircon with +** mm radius wouldlose over 3*� of Pb if residing at +***�C for +*1�+*2 years, at ++**�C for +*0 years orat +,**�C for +*/ years.Occurrence of ﬂuid activity produces much Pb-loss from zir-con crystal during metamorphism at even lower temperature condition (Pidgeon et al.,
Fig. 1.Sm-Nd isochron diagram for mineral fractions and whole rock samples.Errors given are �+s. -�errors were estimated for Sm/Nd ratios of each sample as a convention for isochron determination.MSWD for the isochron was estimated below +.*. Letters identifying points refer to the sample.Small deviation of whole rock sample (W.R.) from regression may be caused by occurrence of spinelwhich is regarded as a container of REE in a rock sample.
Table ..Analytical data for whole rock and mineral fractions of phlogopite-bearing pyroxenegranulite (TM32+,,3-*-E) from the Howard Hills, East Antarctica.
Sm (ppm)Nd (ppm)+.1Sm/+..Nd+.-Nd/+..Nd�
Whole rock (W.R.)Orthopyroxene (Opx)*./*�*.1+ mm*.-/�*./* mmFeldspar
*..23*../0,.,,
+.02,.+-,,.0
*.+10*.+,0*.*/3/
*./+*/2�*.****/
*./+++0�*.****0*./+*0-�*.****2*./*310�*.****,
� Error given are �+s.
Miyamoto et al.164
"'O z 'SI" 'SI" ' "'O z 
0.512 ,----------------------, 
0.511 
0.509 
Age= 1.85 ±0. 15 Ga ( 143Nd/144Nd)LR. = 0.50907 ±0.00011 0.508 ��������������������� 0 0.05 0.1 0.15 0.2 
147Sm/144Nd (Errors given are ±1 a) 
+300; Sinha et al., +33,).Consequently, the radiogenic-Pb clock of zircon can be resetto a high temperature (�+***�C) if such high temperature is continued over +* millionyears.The younger U-Pb zircon age than whole rock ages for metamorphic rocks fromthe Howard Hills can be explained by continuation of the UHT granulite faciesmetamorphism from ,.0/ Ga to ,... Ga.The period of UHT metamorphism for the Napier Complex is estimated as *.+billion years by Kelly and Harley (,**/) and as *.*0 billion years or shorter by Suzukiet al. (,**0).In this study, the time-interval between the U-Pb zircon age and theRb-Sr and Sm-Nd whole rock ages is *.,+ billion years, and corresponds to twice theperiod of UHT metamorphism estimated by Kelly and Harley (,**/) and over threetimes that estimated by Suzuki et al. (,**0).However, previously reported ages ofUHT metamorphism range from ,./3 to ,..* Ga (Fig. +).The length of this period isequivalent to the time-interval between the U-Pb zircon age and the whole rock ages forthe Howard Hills.The length of this time-interval is not unreasonable for the period ofUHT metamorphism in the Napier Complex.We should consider the continuation ofthe UHT granulite facies metamorphism after obtaining more regional and systematicdating results from the Napier Complex.Rutile fractions have near concordant U-Pb ages of ca.+./ Ga. Mezger et al. (+323)estimated the closure temperature (Tc) of rutile as -/*�./*�C from ﬁeld evidence.ThisTc is lower than those in other minerals used for U-Pb dating, such as garnet, zircon,monazite, xenotime or titanite.Cherniak (,***) has also suggested from the di#usionparameter that Pb can be moved easily from rutile crystals under low-temperaturegeothermal activity.Therefore, we interpret the ca.+./ Ga rutile age as representingthe last stage of the regional thermal condition with low-temperature in the NapierComplex, except for intrusion of the Amundsen dykes or pegmatite (Sheraton andBlack, +32+; Osanai et al., ,**+).The ﬁne serpentine vein penetrated in metamor-phosed ultramaﬁc rocks in the maﬁc to ultramaﬁc block (Miyamoto et al., ,**.) isevidence of minor geothermal activity at low temperature in the Howard Hills.Noother obvious trace of retrograde metamorphism with reaction nor deformation texturewas found in the samples and associated rocks.This minor hydrothermal activitymight have occurred at around +./�+.0 Ga.Phlogopite occurs with feldspar (perthite, sanidine, and plagioclase) in the inter-stices of granoblastic orthopyroxene in pyroxene granulite.Feldspar is the mostsuitable mineral for di#usive exchange of Sr with phlogopite since other minerals cancontain negligible strontium.A Rb-Sr model age for phlogopite, therefore, showstiming of equilibration between phlogopite and feldspar in the sample.Phlogopite fractions from pyroxene granulite yielded Rb-Sr model ages (tmodel) ofaround +.2/ Ga (Fig. 0), although the phlogopite also shows di#erent Rb compositionsin di#ering grain sizes (Table -).Biotite grains can locally preserve heterogeneouscompositions corresponding to di#erent ages of their crystallization (Hart, +30.; Yama-shita and Yanagi, +33.).The compositional variation is probably originated from theirheterochronous crystallization.Biotite and phlogopite are susceptible to homogeniza-tion of Sr isotopic ratios under relatively low-temperature conditions.Similarity oftmodel between phlogopite fractions of di#ering grain sizes reﬂects an inﬂuence of re-equilibration of Sr isotope ratio between phlogopite and feldspar.The phlogopite,
Mineral chronology of metamorphic rocks from the Howard Hills165
however, contains much ﬂuorine.Enhancing ﬂuorine content in phlogopite can raisethe closure temperature (Tc) depending on the e#ect of Sr di#usion (Hammouda andCherniak, ,***; Ku¨hn et al., ,***), as well as expanding the stability ﬁeld toward highertemperature (Munoz, +32.; Motoyoshi and Hensen, ,**+).The presence of ﬂuoro-phlogopite with ca.+.2/ Ga gave us the highest limit of temperature from its Tc, whichmetamorphic rocks su#ered at that time.There is a di#erence in Sm/Nd ratio between large and small orthopyroxene frac-tions (Table .; Fig. 1).The orthopyroxene includes ﬁne rutile, phlogopite, and un-identiﬁed minerals.Suzuki et al. (,**0) suggests that the existence of inclusions suchas monazite modiﬁes Sm/Nd ratios of host mineral fractions.The compositionaldi#erence among orthopyroxene fractions probably depends on quantitative variation ofthe inclusions as monazite, apatite, or zircon, although their inﬂuences were notanalyzed either quantitatively or qualitatively for this sample.Neodymium composi-tion of the mineral is generally believed to be resistant to disturbance by metamorphismand hydrothermal alteration.Neodymium isotopic compositions of orthopyroxene ingranulite or peridotite, however, sometimes deviate from precisely determined internalisochrons (“cryptic retrogression”, Burton and O’Nions, +33*; Pan et al., +333; Zhanget al., ,**+).Re-equilibration of Nd isotope ratios in orthopyroxene is believed to takeplace at amphibolite to granulite-facies conditions.The ca.+.2/ Ga age from Rb-Sr and Sm-Nd mineral dating signiﬁcantly postdatesthe UHT granulite facies metamorphism.Previous geochronological research atMount Riiser Larsen, Tonagh Islands, and some outcrops around Casey Bay havesuggested a possibility of a geothermal event following peak metamorphism regionally inthe Napier Complex (Grew et al., ,**+; Osanai et al., ,**+; Owada et al., ,**+, ,**-;Suzuki et al., ,**+, ,**0, Fig. +).Especially, Osanai et al. (,**+) and Owada et al.(,**+, ,**-) reported Sm-Nd internal isochron ages with ca.+.3 Ga and +.0 Ga.Geochronological results in this study are consistent with the previous works.Suzukiet al. (,***), and Owada et al. (,**+, ,**-) have suggested the possibility of atectonothermal event involving tholeiitic magmatism at ca.+.3 Ga that a#ected gneiss atMount Riiser Larsen and on Tonagh Island, the Napier Complex.Unmetamorphosedbasaltic to doleritic dykes that occur in the Howard Hills are considered to be muchyounger (ca.+., Ga, Sheraton and Black, +32+).Although no ﬁeld evidence of igneousactivity at ca.+.3 Ga has been found in the Howard Hills, Rb-Sr and Sm-Nd mineral agessupport a possibility of regional tectonothermal event that inﬂuenced the NapierComplex after UHT granulite-facies metamorphism.Grew et al. (,**+) reported a CHIME U-Th-Pb apparent age of +2+3�.2 Ma forxenotimes in a beryllium pegmatite and another age of ,+/.�/- Ma for a xenotime inanother pegmatite from Christmas point on the Napier Complex (Fig. +).The mineralage of +.2/ Ga agrees with the xenotime CHIME ages.A closure temperature for theU-Pb system of the xenotime is estimated at �0/*�C (Heaman and Parrish, +33+) andup to about 2**�C (Kamber et al., +332).Suzuki et al. (,**+, ,**0) reported Sm-Ndinternal isochron ages of ,.,�,.. Ga and regard the age as a record of cooling below0**�1**�C.These ages are older than the tectonothermal event involving tholeiiticmagmatism at ca.+.3 Ga in Mount Riiser Larsen (Suzuki et al., ,***).The Sm-Ndinternal isochron age dated in this study is +.2/ Ga for the Howard Hills.The
Miyamoto et al.166
Fig. 2.Apparent temperature-time (T-t) relation for metamorphic rocks from the Howard Hills.Age dataare taken from the result of this study and Miyamoto et al. (,**.).The Sm-Nd whole-rockisochron age is regarded as the time of peak metamorphism for the dated sample (Miyamoto et al.,,**.).The metamorphic temperature (+***��+,**�C) is quoted from Yoshimura et al. (,***).Estimations of closure temperatures for each mineral are as follows: +*/*��++**�C for closuretemperature of zircon age (Cli#, +32/): 0/*��1**�C for ﬂuorophlogopite under dry condition (Ku¨hnet al., ,***): -/*��./*�C for rutile age (Mezger et al., +323), and based on di#usion parameters(thick and broken lines): +***��++**�C for zircon (Cherniak and Watson, ,***): ./*��//*�C forrutile (Cherniak, ,***).The Th-U-Pb CHIME ages for monazite and xenotime are obtained fromGrew et al. (,**+), Asami et al. (,**,), and Suzuki et al. (,**0).Their temperature estimationsare based on the di#usion parameter: 1**��1/*�C for monazite (Parrish, +33*; low Tc estimation inthe ﬁgure) and 3**��+***�C for monazite (Cherniak et al., ,**., high Tc estimation in the ﬁgure):0/*��2**�C for xenotime (Heaman and Parrish, +33+; Kamber et al., +332).The shaded square isan area of T-t points for monazite.
Mineral chronology of metamorphic rocks from the Howard Hills167
T(°C) 
Whole Rock isochron age (UHT metamorphism) t j& Zircon 
I 1 Zircon lfl·'l'i Mooazj,e 
: ·•· ·· •··· ·•· •.· ··· ·•··. : (high T estimation) : .·· •.. • ·. ·• .· ·. •·: C 
: .·.····.· .. ·. · . ··· . .  · ·.· ···: 
XenotimeT I , I ' ,.r .•....... h Monazite �I /> [ : : . . ··· II ..• · Ori (low T est1mat1on)::� ·•·• ·•· :: •.• ·. : C ···'·· '··· I I .!.xenotime .!. I ir Fluorophlogopite 
Cooling rate 
/Myr) 
'r - l .Rutile r, "" I 
250 ����������������� 3 2.5 2 Age (Ga) 1.5 1 
deviation between these Sm-Nd internal isochron ages is probably caused by the stronginﬂuence of a tectonothermal event at ca.+.3 Ga, which is unidentiﬁed on the HowardHills; otherwise, it might be caused by local di#erence of the cooling process, that is tosay, faster cooling of metamorphic rocks in Mount Riiser Larsen than other outcrops.Apparent temperature-time (T-t) relations for zircon, ﬂuorophlogopite and rutilefrom metamorphic rocks on the Howard Hills are shown in Fig. 2.T-t relations ofmonazite and xenotime from other localities in the Napier Complex (Asami et al., +332,,**,; Grew et al., ,**+; Suzuki et al., ,**0) are also shown in the same ﬁgure.Temperature constraints were given by adoption of UHT granulite-facies condition(Yoshimura et al., ,***) for whole rock age, and closure temperatures evaluated fromﬁeld evidences or di#usion parameters (Cli#, +32/; Mezger et al., +323; Parrish, +33*;Heaman and Parrish, +33+; Kamber et al., +332; Ku¨hn et al., ,***; Cherniak, ,***;Cherniak and Watson, ,***; Carson et al., ,**,a; Cherniak et al., ,**.) for zircon,monazite, xenotime, ﬂuorophlogopite, and rutile ages.The apparent T-t relations forthe Howard Hills are linear within the uncertainties of age and estimated temperature.We suggest two plausible explanations for the T-t relation: one suggests slowcooling by passing each T-t point in turn after UHT granulite facies metamorphism.Alternatively, dating results might detect individual heated episodes on the jaggedcooling history of the Howard Hills.Osanai et al. (,**+) and Owada et al. (,**+,,**-) reported two individual geothermal events at ca.+.3 Ga and +.0 Ga, and con-cluded that these two events reﬂect the regional scale of tectonothermal events amongthe East Gondwana fragments.Geochronological results in this study might also showthe Proterozoic episodic thermal events in the Napier Complex.
0.Conclusions
Analytical U-Pb data for zircon fractions from orthopyroxene felsic gneiss in theHoward Hills, Enderby Land, deﬁne a discordia with lower intercept age of ,...�*.*,Ga.This age shows the time of termination of the UHT granulite facies metamor-phism that has occurred several times since ,.0/ Ga at the Napier Complex.AnalyticalU-Pb data for rutile fractions in phlogopite-bearing pyroxene granulite from the sameoutcrop show nearly a concordant +./ Ga age.This age represents the last stage ofthermal activity, with the exception of the later intrusion of tholeiitic dykes or peg-matites.The rubidium-strontium model age is about +.2/ Ga for phlogopite in pyroxenegranulite.Samarium and neodymium compositions of whole-rock, feldspar and ortho-pyroxene fractions of the same pyroxene granulite were regressed on an isochron with anage of +.2/�*.+/ Ga.Although ﬁeld evidence of igneous activity at +.3 Ga has notbeen found in the Howard Hills, the correspondence of Rb-Sr and Sm-Nd ages probablyrecords a regional tectonothermal event which inﬂuenced the Napier Complex after thepeak metamorphism at Archean.
Acknowledgments
We express our sincere thanks to the members of JARE-.* and crew of the ice-
Miyamoto et al.168
breaker “Shirase”, especially Messrs. Y. Ohashi, K. Maki, S. Harigai, T. Takei and Drs.H. Yamauchi and K. Shiraishi who assisted in the helicopter operation.We would liketo thank Profs. Edward S. Grew, S. L. Harley, M. Arima, H. Ishizuka, T. Kawasaki, Y.Osanai and T. Yanagi; Drs M. Owada, T. Tsunogae, T. Ikeda, T. Hokada, N. Nakanoand K. Sato; and an anonymous reviewer for their constructive comments and discus-sions during the ﬁeld survey and preparation of this manuscript.Constructive criti-cisms of the anonymous reviewer and Dr. T. Hokada, and the support of editors aremuch appreciated.Thanks also go to Dr. K. Yamashita for advice in chemicalanalyses.Miyamoto’s research was supported ﬁnancially by a Grant-in-Aid for En-couragement of Young Scientists (No. *31.*.*1) from the Ministry of Education,Science, Sports and Culture, Japan.
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