Peak and post–peak development of UHT metamorphism at Mather Peninsula, Rauer Islands: Zircon and monazite U–Th–Pb and REE chemistry constraints

Abstract

Zircon and monazite in ultrahigh temperature (UHT) metamorphic rocks from the Rauer Islands of Prydz Bay in East Antarctica were investigated in terms of U–Th–Pb and rare earth elements (REE) chemistry along with textural context. All five analyzed samples, three from the Mather Paragneiss UHT unit and two from the host orthogneiss unit yield 522–517 Ma concordant zircon ages, with older protolith/inherited zircon ages of 3268 and 2800–2400 Ma along with highly discordant Mesoproterozoic to Neoproterozoic ages. Our data confirm the Archaean protolith age for the host orthogneiss surrounding the UHT Mather Paragneiss. The Archaean and Mesoproterzoic components of the Rauer Islands were not amalgamated in the Rauer Tectonic Event at 1030–990 Ma, and deposition of the Mather Paragneiss was considered at some time after the Rauer Tectonic Event. In contrast to the well–defined 520 Ma ages obtained from the zircons in the UHT rocks, monazite grains measured by electron microprobe show a distinct internal zonation, from 580–560 Ma dark–backscattered electron image (BSE) cores enriched in middle rare earth elements (MREE) and heavy rare earth elements (HREE) to 550–520 Ma mid–BSE mantles and 510–500 Ma bright–BSE rims. From the chemical and textural evidence we infer that the MREE–HREE–rich 580–560 Ma monazite cores may have formed through the decomposition of garnet during decompression just after the UHT event, whereas the MREE–HREE–depleted 550–500 Ma monazite grains/rims formed or recrystallized in reactions associated with subsequent extensive hydration during the upper–amphibolite to granulite–facies main Prydz Tectonic Event, which also caused marked recrystallization of zircon. The above data strongly support the interpretation that the UHT metamorphism occurred prior to 590–580 Ma.

Full text

Peak and post–peak development of UHT metamorphism
at Mather Peninsula, Rauer Islands: Zircon and monazite
U–Th–Pb and REE chemistry constraints
Tomokazu HOKADA*,**,***,†, Simon L. HARLEY***, Daniel J. DUNKLEY*,‡,
Nigel M. KELLY***,§ and Kazumi YOKOYAMA†
*National Institute of Polar Research, Tachikawa, Tokyo 190–8518, Japan
**Department of Polar Science, SOKENDAI (The Graduate University for Advanced Studies), Tachikawa, Tokyo 190–8518, Japan
***School of Geosciences, University of Edinburgh, Edinburgh EH9 3JW, Scotland, UK
†Department of Geology and Paleontology, National Museum of Nature and Science, Tsukuba 305–0005, Japan
‡Department of Applied Geology, Curtin University, Perth, WA 6845, Australia
§Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA
Zircon and monazite in ultrahigh temperature (UHT) metamorphic rocks from the Rauer Islands of Prydz Bay
in East Antarctica were investigated in terms of U–Th–Pb and rare earth elements (REE) chemistry along with
textural context. All five analyzed samples, three from the Mather Paragneiss UHT unit and two from the host
orthogneiss unit yield 522–517 Ma concordant zircon ages, with older protolith/inherited zircon ages of 3268
and 2800–2400 Ma along with highly discordant Mesoproterozoic to Neoproterozoic ages. Our data confirm the
Archaean protolith age for the host orthogneiss surrounding the UHT Mather Paragneiss. The Archaean and
Mesoproterzoic components of the Rauer Islands were not amalgamated in the Rauer Tectonic Event at 1030–
990 Ma, and deposition of the Mather Paragneiss was considered at some time after the Rauer Tectonic Event.
In contrast to the well–defined 520 Ma ages obtained from the zircons in the UHT rocks, monazite grains
measured by electron microprobe show a distinct internal zonation, from 580–560 Ma dark–backscattered elec-
tron image (BSE) cores enriched in middle rare earth elements (MREE) and heavy rare earth elements (HREE)
to 550–520 Ma mid–BSE mantles and 510–500 Ma bright–BSE rims. From the chemical and textural evidence
we infer that the MREE–HREE–rich 580–560 Ma monazite cores may have formed through the decomposition
of garnet during decompression just after the UHT event, whereas the MREE–HREE–depleted 550–500 Ma
monazite grains/rims formed or recrystallized in reactions associated with subsequent extensive hydration dur-
ing the upper–amphibolite to granulite–facies main Prydz Tectonic Event, which also caused marked recrystal-
lization of zircon. The above data strongly support the interpretation that the UHT metamorphism occurred
prior to 590–580 Ma.
Keywords: Monazite, Rare earth elements (REE), Ultrahigh temperature (UHT) metamorphism, U–Th–Pb geo-
chronology, Zircon
INTRODUCTION
Recent attempts at reconstructing the Neoproterozoic to
Cambrian formation of Gondwana from its pre–existing
continental fragments have inevitably led to a focus on
the amalgamation history of the East Antarctic Shield
(Fitzsimons, 2000; Harley, 2003; Harley and Kelly,
2007a; Boger, 2011). With this has come the realization
that the Prydz Bay region of East Antarctica is of central
importance in establishing where the final suturing of
Gondwana took place (e.g., Fitzsimons, 2003), the period
of time over which this occurred, and what impacts this
may have had on adjoining areas in former Gondwana.
The evidence preserved in high–grade metamorphic
gneisses and migmatites for the location of any possible
suture is inherently indirect, relying on discriminating ter-
ranes on the basis of at best unique or, more generally,
doi:10.2465/jmps.150829
T. Hokada, hokada@nipr.ac.jp Corresponding author
Journal of Mineralogical and Petrological Sciences, Volume 111, page 89–103, 2016

distinctive event sequences and isotopic data recording
different geological ages for magmatism, sedimentation
and tectonism, and model ages for the ultimate sources
of sedimentary detritus in paragneisses or magmatic pre-
cursors of orthogneisses. Conversely, the timing of amal-
gamation, potentially via suturing, can be constrained by
identification of the earliest magmatism, sedimentation
or tectonothermal events shared by formerly distinct ter-
ranes (e.g., Fitzsimons, 2000, 2003; Harley, 2003).
The Prydz Belt of SE Prydz Bay is polymetamorphic
(Wang et al, 2008; Liu et al., 2009; Grew et al, 2012,
2013; Harley et al., 2013; Liu et al., 2013). It preserves
a complex record of latest Mesoproterozoic to early Neo-
proterozoic magmatism, sedimentation and granulite fa-
cies tectonism (Wang et al., 2008; Liu et al., 2009; Grew
et al., 2012) (Table 1) that is broadly comparable to tec-
tonothermal events in the Rayner Complex to the west
(Liu et al., 2013). This event sequence is strongly but var-
iably overprinted by a second granulite facies metamor-
phism in the early to middle Cambrian (545–515 Ma:
Hensen and Zhou, 1995; Zhao et al., 1995; Carson et al.
1996; Fitzsimons, 1996; Fitzsimons et al., 1997). Recent
structurally–controlled geochronology of the inferred
cover sequence, the Brattstrand Paragneiss (Fitzsimons,
1997), in the Larsemann Hills area of SE Prydz Bay
(Wang et al., 2008; Grew et al., 2012; Liu et al., 2013)
demonstrate a maximum deposition age close to 1000 Ma,
and minimum deposition age older than the 960–990 Ma
ages of metamorphosed felsic intrusive rocks (the Blun-
dell Orthogneiss of Grew et al., 2012). The basement
gneisses (Søstrene Orthogneiss: Fitzsimons, 1997), cover
metasediments (Brattstrand Paragneiss) and early intru-
sive charnockites and enderbites (Blundell Orthogneiss)
of the Prydz Belt all experienced a common high–grade
metamorphic event broadly constrained from complex zir-
con age populations to have progressed between 930 and
890 Ma, consistent with 900 Ma ages obtained from peg-
matites (Kelsey et al., 2008).
This chronology for SE Prydz Bay contrasts with
and supercedes previous interpretations of the detrital zir-
con and monazite isotopic record in the Brattstrand Para-
gneiss (Zhao et al., 1995; Carson et al., 2007; Kelsey
et al., 2008) that inferred mid–to late–Neoproterozoic
maximum deposition ages for their protoliths. These ear-
lier inferences led to the suggestion of a ‘Neoproterozoic
Basin’related to suturing (Kelsey et al., 2008), which
was generalized to include paragneisses from the margins
of the Prydz Belt in the Rauer Islands.
A complex and poly–metamorphic geologic record
has been documented from the Rauer Islands, where per-
vasive 540–510 Ma metamorphic ages have also been re-
ported (e.g., Kinny et al., 1993; Sims et al., 1994; Harley
et al., 1998; Kelsey et al., 2003a, 2007, 2008). Two para-
gneiss successions have been recognized, the Filla Para-
gneiss and the Mather Paragneiss (Harley and Fitzsimons,
1991; Harley et al., 1995; Harley, 2003). Zircon and mon-
azite age data on the Filla Paragneiss (Kinny et al., 1993;
Kelsey et al., 2007, 2008) indicate it to be polymetamor-
phic, with a 1030–970 Ma high–temperature metamorphic
event (Rauer Tectonic Event; Harley and Kelly, 2007b)
overprinted by the Cambrian Prydz event metamorphism
at 545–510 Ma (Prydz Tectonic Event; Harley and Kelly,
2007b). The Mather Paragneiss, outstanding for its pre-
servation of evidence for ultrahigh temperature (UHT)
metamorphism at 1.0–1.2 GPa and 990–1030 °C (Harley
and Fitzsimons, 1991; Harley, 1998; Kelsey et al., 2003b;
Harley and Kelly, 2007a), remains enigmatic as the age
and regional significance and extent of this UHT event
along with the relationship to the pervasive Prydz event
(545–510 Ma) is still not well constrained. Harley et al.
(2009) recently suggested, on the basis of U–Th–Pb chem-
ical ages of monazite associated with garnet + sapphi-
rine + quartz in a Mather UHT paragneiss from Torckler
Island, that UHT metamorphism occurred at >580–590
Ma and was associated with an event separate from the
major Prydz event at 545–510 Ma.
The revised chronology for Brattstrand Paragneiss
and SE Prydz Belt suggesting a correlation with the Ray-
Table 1. Geologic history of Rauer Islands and Prydz Bay regions
T. Hokada, S.L. Harley, D.J. Dunkley, N.M. Kelly and K. Yokoyama
90

ner Complex not only argues against any suturing and
terrane amalgamation between these areas but also raises
the question of whether the paragneisses of the Rauer
Islands to the NE are different in their depositional and
metamorphic ages, potentially indicating the Rauer Is-
lands as the focus for Cambrian terrane amalgamation
as suggested by Hensen and Zhou (1997). Given the un-
certainties surrounding its sedimentary provenance and
the age of its UHT metamorphism, this study presents
new zircon isotopic and monazite chemical age data on
the Mather Paragneiss and its host orthogneisses from the
type locality originally documented by Harley (1998) at
Mather Peninsula. These data are used to define the ages
and sources for the sedimentary precursors to the UHT
paragneisses, constrain their maximum ages of deposition
and infer the age of UHT metamorphism.
GEOLOGY OF RAUER ISLANDS
The Rauer Islands on the eastern coastline of Prydz Bay
are underlain by granulite facies orthogneisses and supra-
crustal sequences, denoted as the Rauer Terrane (e.g.,
Harley, 2003), located between the Neoproterozoic to
Cambrian Prydz Belt to the southwest and the Archaean
to earliest Palaeoproterozoic Vestfold Block to the north-
east (Fig. 1). The Rauer Terrane contains both Archaean
and Proterozoic crustal components (Kinny et al., 1993).
These tectonic blocks or crustal domains are now com-
plexly interleaved and infolded as a result of polyphase
high–grade deformation events that have cumulatively
produced isoclinal, sheath and interference folds with
steep SSE–plunging fold axes on all scales, and meter
to hundred meter wide arcuate to anastomosing high
strain zones (Harley, 1987; Sims et al., 1994).
The domains have been interleaved and co–deformed
in the Cambrian tectonic event at 530–510 Ma, but it is
still unclear as to whether they all share a prior history.
The Archaean domains are dominated by tonalitic to gran-
odioritic orthogneisses and composite layered mafic–in-
termediate orthogneisses with protolith and/or intrusive
ages from 3450 to 2550 Ma (Kinny et al., 1993; Harley
et al., 1998). These orthogneiss units were subjected to
high–grade metamorphism prior to 2550 Ma (Harley et
al., 1998; S. Harley, unpublished data) and subsequently
intruded by several generations of mafic dykes. These
mafic dykes are now metamorphosed into pyroxene gran-
ulites and variably deformed, being essentially parallel to
felsic and composite gneiss fabrics in high strain zones
(Harley, 1987; Harley and Fitzsimons, 1991; Sims et al.,
1994; Harley et al., 1995). The Archaean domain also
contains two groups of layered igneous complexes, the
2840 Ma ferrogabbroic–ferrodioritic Scherbinina Layered
Figure 1. Geological outline of the
Rauer Islands in Prydz Bay, East
Antarctica. (A) Map showing the
reconstruction of Gondwana su-
percontinent at 500 Ma and the lo-
cation of the Rauer Islands in East
Antarctica (modified after Harley
and Kelly, 2007a). VH, Vestford
Hills; PB, Prydz Belt. (B) Map of
the Rauer Group of Islands, show-
ing the key localities and the ap-
proximate distributions of pre-
dominantly Archaean rocks (after
Harley and Kelly, 2007a, 2007b).
(C) Geological map of Mather
Peninsula and Short Point (after
Harley, 1998). Sample localities
are shown as stars. Color version
is available online from http://
doi.org/10.2465/jmps.150829.
Age constraints of UHT metamorphism at Mather Peninsula, Rauer Islands 91

Complex (Snape et al., 1997; Harley et al., 1998; Harley
and Kelly, 2007b) and the Mg–rich gabbro–anorthositic
Torckler–Tango Complex (Harley et al., 1995).
The Mather Paragneiss, as originally defined (Harley,
1998), includes a varied suite of protoliths hosted or form-
ing boudinaged lenses and stringers within or adjacent
to the Archaean tonalitic gneisses described above. The
Mather Paragneiss is composed of magnesian and alu-
minous garnet–orthopyroxene–sillimanite–bearing meta-
pelite with secondary sapphirine and cordierite, orthopy-
roxene–sillimanite metaquartzite, magnesian garnet–silli-
manite metapelite, orthopyroxene–bearing leucogranite,
and garnet–bearing mafic granulite (Harley and Fitzsi-
mons, 1991; Harley, 1998). It is also considered to include
forsterite–diopside marble and related metasomatic diop-
sidite rinds, andraditic and Fe–Mn skarn rocks, and gar-
net–bearing metabasic granulites (Harley and Fitzsimons,
1991; Buick et al., 1994; Harley et al., 1995). This suite
can be traced as thin and laterally discontinuous horizons
from Mather Peninsula itself through to Short Point in the
eastern Rauer islands (Fig. 1c).
The Archaean basement rocks of the Rauer Islands
are interleaved with a Mesoproterozoic to earliest Neo-
proterozoic domain characterized by the migmatised Fe–
pelites, semipelites and quartzites of the Filla Paragneiss,
leucogneisses and leucogranites that occur as sheets, sills
and stocks within the Filla Paragneiss, granitic to dioritic
orthogneisses and gneissic granitoids, and a varied suite
of metabasites with associated metacarbonate horizons
that are interpreted to be metavolcanic rocks associated
with the Filla Paragneiss (Harley and Fitzsimons, 1991;
Harley et al. 1995). Zircon U–Pb age dating of the meta-
granitoids, leucogneiss and cross–cutting aplite indicate
that these rocks, and the Filla Paragneiss, underwent
granulite facies metamorphism and deformation in the
interval 1030–990 Ma (Kinny et al., 1993). Metamorphic
monazite in these paragneisses usually records two U–
Th–Pb age peaks, a younger one at 540–510 Ma and an
older one, mainly recorded from monazite cores, that
ranges from 1000 to 900 Ma (S. Harley, unpublished da-
ta; Kelsey et al. 2007). Hence, the isotopic record of the
Filla Paragneiss is consistent with it being polymetamor-
phic, recording deformation and metamorphism in both
the end–Mesoproterozoic to earliest Neoproterozoic Ra-
uer Tectonic Event and the Cambrian Prydz Tectonic
Event. This also demonstrates that the Filla Paragneiss
was deposited in the Mesoproterozoic, prior to 1030
Ma. A possible minimum age of deposition is provided
by a 1058 Ma zircon age obtained by Kinny et al. (1993)
from a small–scale partial melt patch in the Filla Meta-
basite, which occurs in close association with the Filla
Paragneiss throughout the Rauer Islands.
SAMPLE DESCRIPTIONS
Five samples, three from the main Mg–Al–rich Mather
Paragneiss horizon, one from an adjacent tonalitic ortho-
gneiss, and one from another pelitic gneiss horizon within
the orthogneiss (Figs. 1c and 2), have been investigated.
The Mg–Al–rich gneiss (SH/88/218 and TH/06/30C;
both collected from precisely the same sample location) in
Mather Peninsula. They preserve mineral assemblages in-
cluding garnet, high–Al orthopyroxene and/or sillimanite
that are constrained to have equilibrated at UHT condi-
tions, from minimum conditions of 0.9–1.0 GPa and 980
°C (Kelsey et al. 2005) up to 1.1–1.2 GPa and 1030 °C
Figure 2. Field photographs taken in
the Mather Peninsula of the Rauer
Islands, showing the modes of oc-
currence of Mather UHT gneisses
and the host orthogneiss. (A) Math-
er Paragneiss layer including Mg–
Al–rich UHT gneiss is intercalated
within felsic orthogneiss layers.
(B)–(D) Close up of Mg–Al–rich
UHT gneiss in Mather Paragneiss
layer. Color version is available
online from http://doi.org/10.2465/
jmps.150829.
T. Hokada, S.L. Harley, D.J. Dunkley, N.M. Kelly and K. Yokoyama
92

(Harley, 1998). The peak garnet + orthopyroxene + silli-
manite assemblages are locally to extensively replaced by
fine–grained symplectites composed of combinations of
sapphirine, cordierite, orthopyroxene, spinel or plagio-
clase (Fig. 3a). For example, former orthopyroxene +
sillimanite is generally replaced by fine–grained symplec-
tites of sapphirine + cordierite, and garnet by sapphirine +
orthopyroxene with sillimanite or cordierite (Figs. 3b and
3c), and also extensively hydrated to form biotite (Figs. 3d
and 3e). As described and analyzed in detail by Harley
and Fitzsimons (1991), Harley (1998) and Kelsey et al.
(2003a, 2005), the range of textures formed in different
micro–scale bulk compositions are consistent with an ini-
tial post–peak evolution involving appreciable decom-
pression of up to 0.4 GPa under granulite–facies condi-
tions. Whilst the precise dP/dT of this post–peak decom-
pressional path is subject to some debate (Kelsey et al.,
2005, 2007; cf. Harley, 1998, 2003) it is generally agreed
that the Mg–Al granulite post–peak P–Tpath traversed
through 0.7–0.8 GPa and 850–900 °C. The UHT gneiss
was affected by extensive hydration to form biotite–bear-
ing reaction coronas and biotite–rich reaction zones, some
of which are then overprinted by low–Al orthopyroxene +
cordierite symplectites. Symplectite coarsening, produc-
ing blocky orthopyroxene and sapphirine, is associated
with this biotite overprint. Late–stage garnet breakdown
to cordierite and biotite, and replacement of earlier sap-
phirine + orthopyroxene by these phases, is also observed.
Biotites in these textural settings preserves low fluorine
(<0.9 wt%) and chlorine (<0.3 wt%) with variable TiO2
(1–3 wt%) contents consistent with their formation on
cooling below 800–900 °C.
Sample TH/06/30J was collected from an Opx–
bearing leucosome occurring with Mg–Al–rich gneisses.
Patchy or vein leucosomes similar to that sampled occur
on centimeter to decimeter width scales and are commonly
deformed, consistent with the host gneisses. The main
leucosome constituents are quartz and orthopyroxene;
plagioclase and biotite are minor phases and K–feldspar is
generally absent. Orthopyroxene is porphyroblastic with
grains up to 2–5 mm across. Biotite is usually anhedral and
occurs interstitially around quartz and orthopyroxene crys-
tals. Zircon, rutile and late muscovite are minor.
The tonalitic orthogneiss sample (TH/06/33A) is from
the orthogneiss units that encloses Mather UHT gneiss lay-
er (SH/88/218, TH/06/30C and TH/06/30J). It is represen-
tative of the typical tonalitic orthogneisses of the eastern
Rauer Islands and Mather Peninsula area, and consists of
quartz, plagioclase, perthite, hornblende, and relatively
minor amount of orthopyroxene and clinopyroxene. Zir-
con, monazite and opaque minerals are also present.
The pelitic gneiss sample (TH/06/33B) consists of
coarse–grained garnet crystals up to a few centimeters in
diameter, which enclose fibrous sillimanite inclusions and
are surrounded by coarse–grained (1–2 mm across) pris-
matic sillimanite that forms the main foliation. Quartz,
plagioclase and biotite are minor phases. Biotite is com-
monly anhedral and interstitial, indicating formation at
post–peak. Cordierite occurs interstitially among garnet
crystals, and is apparently secondary. Zircon and opaque
minerals are present as accessory phases.
ION MICROPROBE ZIRCON CHRONOLOGY
Analytical method
U–Pb and REE data from zircon were obtained using the
ion microprobe SHRIMP II at the National Institute of
Polar Research, Tokyo. Zircons in the Mg–Al–rich Mather
Figure 3. Photomicrographs of the Mather UHT gneiss sample
SH/88/218. (A) The distributions of zircon grains analyzed in
situ in polished thin section by SHRIMP. (B) Zircon (#6) in-
cluded in Opx. (C) Zircon grains (#2 and #7) occur in the sym-
plectite zone composed of sapphirine, orthopyroxene, cordierite,
spinel and plagioclase. (D) Zircon grains (#5 and #12) associ-
ated with secondary cordierite and biotite. (E) Zircon intersti-
tially occurs with secondary biotite. Color version is available
online from http://doi.org/10.2465/jmps.150829.
Age constraints of UHT metamorphism at Mather Peninsula, Rauer Islands 93

UHT gneiss samples (SH/88/218 and TH/06/30C) were
analyzed in–situ in polished thin section in addition to
analyses obtained from mineral separates. Detail of the an-
alytical method is described in Supplementary Documen-
tation (Supplementary Documentation is available on-
line from http://doi.org/10.2465/jmps.150829). The proce-
dures for the Pb and U isotopic analyses of zircon followed
those of Compston et al. (1984) and Williams (1998) with
U–Pb measurements were calibrated against 204Pb–correct-
ed (Pb/U)/(UO/U)2values for standard zircon FC1 (1099
Ma, Paces and Miller, 1993). Data reduction and process-
ing were performed using the Excel add–in program
SQUID (Ludwig, 2001) and plots were generated using
ISOPLOT (Ludwig, 2003). Common Pb contents were
corrected using the measured 204Pb and a Stacey and
Kramers (1975) model for ages approximating those of
standard and unknown zircon ages (see Ludwig, 2001
for details). Errors on single spot ratios and ages are quoted
at 1–sigma, whereas pooled ages and concordia intercept
ages are quoted at 95% confidence levels. Zircon REE
contents for standard reference material 91500 (Wieden-
beck et al., 2004) were within 10% of published values.
Results
The Mg–Al–rich gneiss (TH/06/30C = SH/88/218, the
original UHT rock described by Harley and Fitzsimons
(1991) and Harley (1998)) includes subhedral to rounded
zircon grains, some of which preserve a marked discon-
tinuous internal zonal structure (Fig. 4). U–Pb analyses
were made on a total of 129 analytical spots on 107 zir-
con grains from the separates mounted in the epoxy resin
disc (TH/06/30C) and 27 analytical spots on 17 zircon
grains from the polished thin section (SH/88/218) (Sup-
plementary Table S1: Supplementary Table S1 is avail-
able online from http://doi.org/10.2465/jmps.150829).
Both groups of zircon analyses yield consistent results,
with a large spread in zircon analyses lying between a
main upper intercept at 2600 Ma and lower intercept near
500 Ma. A few zircon cores yielded Archaean ages older
than 2600 Ma (>2700–3200 Ma: Figs. 5a and 5b). In de-
tail, the spread of data on Tera–Wasserburg Concordia
diagrams (Figs. 5a and 5b) defines a broadly triangular
field from near 2600 Ma to lower intercepts ranging from
<1000 to 500 Ma, with concordant younger age domains
being either near 500 Ma or between 900–710 Ma (207Pb/
206Pb ages). The pooled ages of the younger group of in–
situ analysis and grain mount analysis yielded concordia
ages of 522 ± 13 Ma (decay–constant errors included,
MSWD of concordance = 1.7, probability of concor-
dance = 0.19) and 519.8 ± 4.7 Ma (decay–constant errors
included, MSWD of concordance = 1.8, probability of
concordance = 0.17), respectively. We carefully investi-
gated the modes of occurrence of zircon grains along
with their internal structure using CL imaging. Despite
their varied textural settings (Figs. 3 and 4) there appears
to be no systematic age–texture relationships.
REE contents were determined for some of the zir-
con grains analyzed in–situ in the thin section of sample
SH/88/218 along with garnet from the same sample (Sup-
plementary Table S2: Supplementary Table S2 is available
online from http://doi.org/10.2465/jmps.150829). All zir-
cons displayed fractionated REE patterns typified by en-
richments in HREE compared with MREE and LREE,
and strong negative Eu anomalies (Eu/Eu* = 0.3) (Fig.
6a). The most concordant older zircons (>2 Ga) have
YbN/GdNof ~ 19 at GdNof 100, whereas the near–con-
cordant Cambrian zircons (530 Ma) have YbN/GdNof
19 at GdNof 40. Three highly discordant zircon rims
on Archaean grains have elevated YbN/GdN(up to 150)
caused by depletions in MREE compared with the cores.
Garnet in SH/88/218 preserves a flat HREE pattern with
YbN/GdNof 1.2 at GdNof 16 (Fig. 6a). All zircon, irre-
spective of age and textural type, is highly enriched in the
MREE–HREE relative to the garnet. DREE(Zrn/Grt) val-
ues calculated from Archaean and discordant ‘old’zircon
increase from 1.5 at Eu and 2–5.5 at Gd up to 19–38 at Er
and 50–110 at Yb. DREE(Zrn/Grt) values calculated using
the Cambrian zircons increase from 1.5 at Eu and 2 at Gd
up to 12 at Er and 17 at Yb (Fig. 6b). One zircon grain
(Zrn–#1, Fig. 4) enclosed within garnet shows younger
(440–390 Ma) rim around the discordant 2060 Ma core.
REE pattern of this younger zircon (spot 1.4 in Fig. 6a)
indicate some enrichment of LREE which typical of rel-
atively lower–grade hydrothermal alteration. So, we con-
sider that the younger 400 Ma rim of this particular zir-
con may reflect the late stage alteration rather than a
high–grade metamorphic event.
Figure 4. Cathodoluminescence (CL) images of selected zircon
grains from Figure 3 with 206Pb/238U ages.
T. Hokada, S.L. Harley, D.J. Dunkley, N.M. Kelly and K. Yokoyama
94

The UHT orthopyroxene–bearing leucosome (TH/
06/30J) also yields scattered and discordant zircon data
with an upper intercept of 2460 Ma and lower intercept of
540 Ma based on 36 analytical spots on 30 zircon grains
(Fig. 7a). Notably, this analytical population does not
contain any zircon, concordant or discordant, that falls
in the age range 1000 ± 100 Ma. Three concordant analy-
ses and one slightly discordant point plot at 540–520 Ma,
whilst one analysis occurs near 635 Ma.
Both Mather UHT gneiss (SH/88/218 and TH/06/
30C) and Opx–bearing leucosome (TH/06/30J) preserve
a similar age spectrum (Figs. 5c, 5d, and 7d). Th/U ratios
in zircons are in the range of 0.4–1.0 for 2400–2600 Ma
concordant (and near upper intercept) zircon domains and
below 0.4 for 520–500 Ma concordant (and near lower
intercept) domains.
Thirteen analyses on 12 zircon grains from the tona-
litic orthogneiss (TH/06/33A) defined a discordant array
with several near–concordant analyses bracketing an up-
per intercept age of 3268 ± 4 Ma (Fig. 7b). This age is
coincident with the 3269 ± 9Ma age obtained by Sheraton
et al. (1984) for an orthogneiss from Short Point, about 1
km south of the Mather Peninsula UHT site (Fig. 1).
In the case of the metapelitic garnet–sillimanite gneiss
(TH/06/33B), 49 analytical spots were obtained on 39 zir-
con grains. The zircons have no inherited magmatic grain
cores or textural domains and so are all interpreted to be
recrystallized metamorphic grains. The analytical popula-
tion (n = 49) yielded a precise concordia age of 516.6 ± 2.0
Ma (decay–constant errors included, MSWD of concor-
Figure 5. Tera–Wasserburg concor-
dia diagrams showing the results
of U–Pb SHRIMP zircon dating
of the Mather UHT gneiss (SH/
88/218 and TH/06/30C). Uncer-
tainties in the calculated ages are
reported at the 2–sigma level. Also
shown are Th/U ratios versus
U–Pb ages analyzed by SHRIMP.
Open circles are <10% discord-
ance. Solid circles are discordant
analyses (more than 10% discord-
ance). (A) Results of U–Pb dating
of zircon in–situ of thin section
(SH/88/218). (B) Results of U–Pb
dating of zircon grains mounted
on epoxy resin (TH/06/30C). (C)
Th/U ratios versus U–Pb ages for
SH/88/218. (D) Th/U ratios versus
U–Pb ages for TH/06/30C.
Figure 6. (A) Chondrite normalized REE data for monazite, zir-
con and garnet from SH/88/218 (chondrite values after Anders
and Grevasse, 1989). Zircon and garnet data were obtained by
SHRIMP and monazite data were analyzed by electron microp-
robe. Y data of monazite were plotted as proxy for Ho and Er.
(B) DREE(Zrn/Grt) values for sample SH/88/218. Color version
is available online from http://doi.org/10.2465/jmps.150829.
Age constraints of UHT metamorphism at Mather Peninsula, Rauer Islands 95

dance = 4.4, probability of concordance = 0.036) (Fig. 7c).
ELECTRON MICROPROBE MONAZITE
CHRONOLOGY
Analytical method
Chemical analyses were made on monazite grains in a
normal polished thin section from sample SH/88/218 us-
ing electron microprobe with a wavelength–dispersive X–
ray analytical system (JEOL JXA–8800M) at the Nation-
al Museum of Nature and Science, Tokyo, Japan. The
theoretical basis of electron microprobe dating follows
that of the chemical Th–U–total Pb isochron method
(CHIME) described by Suzuki et al. (1991). Detail of
the analytical method is described in Supplementary
Documentation, which essentially follow that described
in Hokada et al. (2004). Internal monazite standards with
their ages ranging from 3460 to 1 Ma have been routinely
monitored to check the reliability of the obtained ages
(Santosh et al., 2006).
Results
Despite their varied textural settings (i.e., inclusions in
UHT orthopyroxene, grains in symplectites, grains grown
with late biotite) almost all monazite grains yield U–Th–
Pb electron microprobe chemical ages in the range 580–
450 Ma, with weak evidence for 650 Ma inheritance
(Figs. 8 and 9) (Supplementary Table S3: Supplementary
Tables S3 is available online from http://doi.org/10.2465/
jmps.150829). The full group of analyses (total 100 anal-
yses on 69 grains) treated without consideration of texture
or chemistry yielded a weighted average age of 521 ± 28
Ma. However, the monazites occasionally showed distinct
internal structures, typified by dark–BSE cores, mid–BSE
mantles, and bright–BSE rims. These variations in BSE
correlate with grain chemistry (see Fig. 6; please note that
Y is commonly used as proxy for Ho and Er): the dark–
BSE monazite cores (e.g., Mnz–#15, #27, and #14 in Fig.
8) are relatively enriched in M–HREE and Y (depleted in
ThO2), whereas the bright–BSE mantles or structureless
grains (e.g., Mnz–#15, #9, and #10 in Fig. 8) have lower
Figure 7. Tera–Wasserburg concor-
dia diagrams showing the results
of U–Pb SHRIMP zircon dating
of felsic orthogneiss, Opx–bearing
leucosome and Grt–Sil gneiss as-
sociated with the Mather UHT
gneiss. Uncertainties in the calcu-
lated ages are reported at the 2–
sigma level. Also shown are Th/
U ratios versus U–Pb ages ana-
lyzed by SHRIMP. Open circles
are <10% discordance. Solid cir-
cles are discordant analyses (10%
discordance or more). (A) Results
of U–Pb zircon dating of Opx–
bearing leucosome sample (TH/
06/30J) from the Mather UHT
gneiss horizon. (B) Results of U–
Pb zircon dating of felsic ortho-
gneiss sample (TH/06/33A) near
the Mather UHT gneiss horizon.
(C) Results of U–Pb zircon dating
of Grt–Sil gneiss (TH/06/33B) as-
sociated with the felsic orthog-
neiss. (D) Th/U ratios versus U–
Pb ages for TH/06/30J. (E) Th/U
ratios versus U–Pb ages for TH/
06/33A. (F) Th/U ratios versus
U–Pb ages for TH/06/33B.
T. Hokada, S.L. Harley, D.J. Dunkley, N.M. Kelly and K. Yokoyama
96

M–HREE and Y (higher ThO2) and the outermost bright
BSE–rims the lowest M–HREE and Y (highest ThO2)
concentrations. These chemical variations may reflect
changes in the monazite forming reactions, coexisting
phases, or fluid / melt composition. Sixteen analytical
spots from the dark–BSE monazite core domains give
ages in the range 580–560 Ma and a weighted average
age of 556 ± 21 Ma, and 15 analytical spots from the
distinctive low–REE bright–BSE–rims yield a weighted
average of 512 ± 18 Ma. The mid–BSE mantles record
intermediate ages in the range 550–520 Ma.
DISCUSSION
Relationships between Archaean and Proterozoic
crustal domains in the Rauer Islands
The Rauer Islands contain both Archaean and Proterozoic
crustal components (e.g., Kinny et al., 1993; Harley and
Kelly, 2007a, 2007b), with the Archaean domain domi-
nant at in the Scherbinina Island –Mather Peninsula and
Torckler Island areas (Fig. 1). The Archaean/Proterozoic
boundary is inferred to run through Mather Peninsula (Fig.
1; Harley and Kelly, 2007a, 2007b) some 500 metres to
the west of the UHT locality. The Mather Paragneiss de-
trital zircon age spectra (dominantly >2800–2400 Ma) and
3267 Ma age for the protolith of the host tonalitic ortho-
gniess obtained in this study are consistent with the pre-
viously reported zircon dates from Archaean orthogneisses
of the Rauer Islands (2800 Ma: Kinny et al., 1993; 3279
Ma: Sheraton et al., 1984; 2840 and >3300 Ma: Harley et
al., 1998) and confirm that the host orthogneiss surround-
ing the UHT paragneisses at Mather Peninsula is Archaean
in protolith age. The zircon age spectrum for the Mather
Paragneiss is also consistent with its highly evolved Nd
isotope signature and whole rock model age (TDM = 2900–
3700 Ma: Hensen and Zhou, 1995, 1997).
As noted above, it is now generally accepted that
the Mesoproterozoic lithotectonic units are polymetamor-
phic, and experienced granulite–grade tectonism at both
1000 Ma (Rauer Tectonic Event) and 530 Ma (Prydz Tec-
tonic Event). However, it remains a matter of debate as
to whether the Prydz Tectonic Event was (1) responsible
for the high–grade amalgamation of formerly separate
Archaean and Mesoproterozoic lithotectonic units, or (2)
only responsible for reworking a mixed basement that
included Archaean and Mesoproterozoic gneisses. In the
first model, proposed initially by Hensen and Zhou
(1997), the Archaean and Mesoproterozoic lithotectonic
units would have had distinct and unrelated crustal histo-
ries up until amalgamation in the Cambrian. In the second
model, the Archaean and Proterozoic gneisses may have
been assembled in and shared the 1030–990 Ma Rauer
Tectonic Event (Kinny et al., 1993; Harley et al., 1998).
This ambiguity arises for two main reasons. Firstly,
as recognized by Hensen and Zhou (1997), the Archaean
orthogneisses do not record in their zircon U–Pb isotopic
signatures the effects of the 1000 Ma Rauer Tectonic
Event, but instead preserve a spectrum of Archaean ages
disturbed by resetting at 540–505 Ma and new zircon
growth at 530 Ma (e.g., Kinny et al., 1993; Harley et
al., 1998; Harley and Kelly, 2007b). Secondly, the dom-
inant Mesoproterozoic sedimentary suite, the Filla Para-
gneiss, is found to lack (Kinny et al., 1993) or have very
few (Kelsey et al., 2008) detrital zircons derived from
Figure 8. Backscattered electron images (BSE) of monazite dated
by electron microprobe. Numbers indicate apparent U–Th–Pb
chemical age (Ma).
Figure 9. Histograms showing the monazite U–Th–Pb ages. See
text for detail.
Age constraints of UHT metamorphism at Mather Peninsula, Rauer Islands 97

Archaean sources and records Nd and Sr isotope signa-
tures consistent with derivation from dominantly Palaeo–
Mesoproterozoic basement (Kinny et al., 1993; Hensen
and Zhou, 1995).
Given this uncertainty, the presence of Archaean
and Mesoproterozoic material in the zircon age spectra
from the Mather Paragneiss is of critical importance. This
study and two previous zircon isotopic studies of samples
of the Mather Paragneiss (Wang et al., 2007; Kelsey et
al., 2008) indicate that near–concordant zircons of late–
Mesoproterozoic to Neoproterozoic age occur in these
rocks. In the present study we have identified 4 high–U
(3000–1100 ppm), low Th/U (0.01), near–concordant zir-
con grains with 207Pb/206Pb ages in the range 900–710
Ma in the Mg–Al pelite, and none in the Opx–leucosome.
Notably, no concordant zircon grains or grain rims have
been found with ages in the range corresponding to the
1030–990 Ma Rauer Tectonic Event.
Wang et al. (2007) reported SHRIMP U–Pb data for
zircons separated from a UHT Mather Paragneiss sample.
Their data were also dominated by highly discordant to
concordant Archaean core populations, with weighted
mean 207Pb/206Pb ages of 2657 ± 17 Ma and 2532 ± 12
Ma, in most cases overgrown by rims with ages consistent
the Prydz Tectonic Event (532 ± 7 Ma). That study also
identified a texturally distinctive set of near–concordant
high–U, low Th/U (<0.02) overgrowths or mantles on
some Archaean zircon cores, which yielded a weighted
mean 207Pb/206Pb age of 995 ± 15 Ma. On the basis of
these near–concordant zircons, and a discordia defined
by analyes of some of Archaean zircon cores that projects
down to a lower intercept near 1000 Ma, Wang et al.
(2007) suggested that the Mather Paragneiss is polymeta-
morphic, affected not only by the Prydz Tectonic Event
but also the Rauer Tectonic Event at 1000 Ma. If this was
the case, it would uniquely imply that the Archaean and
Mesoproterozoic rocks of the Rauer Islands shared their
geological evolution from at least 1000 Ma. In the inter-
pretation of Wang et al. (2007) the sedimentary precursors
of the Mather Paragneiss would be older than 1000 Ma,
like the Filla Supracrustals, but sourced from a basement
dominated by the local Archaean gneisses rather than per-
haps a more distal Proterozoic source region.
Kelsey et al. (2008) documented the LA–ICP–MS U–
Pb age spectra of zircons separated from two Mather Para-
gneiss samples. Their results were consistent with those
of Wang et al. (2008) in defining broad triangular fields
of mostly discordant analyses on the Wetherill concordia
diagrams, dominated by detrital zircon cores of Archaean
to early Palaeoproterozoic age (2800–2300 Ma). Near–
concordant zircon grains or rims ranging in apparent
age from 980–580 Ma, as well as 550–500 Ma rims,
formed the younger age components of their zircon spec-
tra. In contrast to Wang et al. (2007), Kelsey et al. (2008)
interpreted their 980–580 Ma analytical group, which also
were mainly characterised by low Th/U (<0.1), as detrital
grains of metamorphic provenance. Based on this inter-
pretation Kelsey et al. (2008) concluded that the Mather
Paragneiss precursors were deposited in the late Neopro-
terozoic with varied detrital sources that ranged in age to
as young as 580 Ma.
Like the Mather Paragneiss zircon data reported here
and by Wang et al. (2007), the discordant zircon data of
Kelsey et al. (2008) indicate extensive Pb loss from the
Archaean zircons at ages older than 800 Ma. The zircon
data presented in Kelsey et al. (2008) could therefore be
interpreted as reflecting the dominance of originally Arch-
aean detrital zircon grains that have been variably affected
and disturbed by the Rauer Tectonic Event at 1000 Ma, as
well as by the later Prydz Tectonic Event. Examination of
the CL images of Kelsey et al. (2008) suggests that many,
if not all, of the Neoproterozoic zircon grain domains have
experienced textural changes, such as oscillatory zone
‘blurring’and ‘bleaching’and invasion by lobate chemi-
cal fronts. These internal textural features are typical of
metamorphic modification (e.g., Harley et al., 2007) facili-
tated by coupled dissolution/reprecipitation (Geisler et al.,
2007). The non–uniform spread in the near–concordant
Neoproterozoic zircon U–Pb data in Kelsey et al. (2008)
is re–interpreted here as reflecting the variable effects of
the Prydz Tectonic Event at 530 Ma on pre–existing Meso-
proterozoic metamorphic zircon formed during the Rauer
Tectonic Event. In this interpretation no exotic sources of
mid–Neoproterozoic zircons are required.
The evidence from Wang et al. (2007) and in–situ
analyses here which show that some >1000 Ma zircon
may have nucleated on 2600–2400 Ma Archaean detrital
cores, isotopically disturbed during the Rauer Tectonic
Event, and could be interpreted to reflect the juxtaposition
or amalgamation of the Archaean and Mesoproterozoic
components of the Rauer Islands prior to or during the
Rauer Tectonic Event at 1030–990 Ma. However, this is
not a necessary conclusion. The detrital zircon spectrum
obtained by Kelsey et al. (2008) on a sample of undisputed
Proterozoic Filla Paragneiss (11104B, west Mather Penin-
sula) includes 3 Archaean grains with 207Pb/206Pb inter-
cept ages of 2950 to 2600 Ma, and a sample from Lunnyy
Island contains one 2670 Ma detrital grain. Hence, it
is possible that zircons with Archaean cores rimmed by
1000 Ma overgrowths could be derived from the erosion
of the Filla Paragneiss or similar rocks, whether the Arch-
aean component of the Rauer Islands was present or not.
This observation, coupled with the lack of any 1000 Ma
zircon record in the Archaean orthogneisses reported in
T. Hokada, S.L. Harley, D.J. Dunkley, N.M. Kelly and K. Yokoyama
98

previous studies or in the tonalitic orthogneiss document-
ed here from Mather Peninsula, leads us to conclude that
the Archaean and Mesoproterzoic components of the Ra-
uer Islands were not amalgamated in the Rauer Tectonic
Event. The near–concordant early Neoproterozoic (mainly
1000–800 Ma) zircons recorded in this study and reported
in Kelsey et al. (2008) and Wang et al. (2007) are inter-
preted similarly to Kelsey et al. (2008), as detrital grains
signifying deposition of the Mather Paragneiss at some
time after the Rauer Tectonic Event. However, in contrast
to Kelsey et al. (2008) the range in Neoproterozoic ages
from 1000 to 710 or even 580 Ma is considered to reflect
the variable effect of the Prydz Tectonic Event at 530 Ma
on metamorphic zircon formed during the Rauer Tectonic
Event by 970 Ma, rather than defining a younger maxi-
mum age of deposition for the Mather Paragneiss. Similar
zircon age spectrum was reported also from the nearby
Larsemann Hills in the Prydz Belt (Grew et al., 2012),
and they also concluded that the wide apparent age range
(from 905 to 510 Ma) is explained as variable resetting of
earlier metamorphic zircon during the later stage meta-
morphism. Although there is no clear evidence of the mu-
tual relationships between the Mather Paragneiss and the
surrounding Archaean tonalitic gneiss, the modes of oc-
currence of the Mather Paragneiss layers localized in the
Archaean tonalitic gneiss unit indicate that the UHT meta-
morphism was restricted to a specific slice containing the
Mather Paragneiss assemblage, which was interleaved
with the other units in the Rauer Islands only later, in a
separate episode related to the Prydz event at 545–510 Ma.
Timing of UHT metamorphism and tectonic implica-
tions
Placing a strong time constraint on the UHT event in the
Mather Paragneiss is critical for models of the develop-
ment of the Rauer Islands, the Prydz Belt and the amal-
gamation of East Antarctica (Harley et al., 2013). The
first indication that these paragneisses might record an
event distinct in age from the Prydz Tectonic Event
was reported by Hensen and Zhou (1995, 1997), who
obtained a 600 Ma Sm–Nd garnet–whole rock isochron
for Mather Paragneiss SH/88/218. This result is unique
within their garnet–whole rock Sm–Nd dataset for the
eastern Prydz Bay region, as all other paragneiss samples
yielded isochron ages near 510–500 Ma. This would
be consistent with the retention of a pre–Prydz Tectonic
Event garnet formation episode in the Mg–Al UHT gran-
ulites of the Mather Paragneiss.
As noted in the previous section, Wang et al. (2007)
ascribed the formation of 1000 Ma age high–U, low Th/U
rims or mantles on Archaean zircons in the Mather Para-
gneiss to the UHT metamorphic event. They further sug-
gested that the anomalously high temperatures were
caused by syn–metamorphic enderbitic magmatism, re-
quiring the host orthogneiss to be of that age. The age
of 3268 ± 4 Ma obtained in the present study for the host
tonalitic orthogneiss means that this cannot be a heat
source for UHT metamorphism if that occurred at 1000
Ma, and hence argues strongly against a key facet of the
interpretation of Wang et al. (2007).
Based on their interpretation that 980–580 Ma zircons
in their Mather Paragneiss sample were detrital grains of
varied metamorphic provenance, Kelsey et al. (2008) con-
cluded that the Mather Paragneiss was not older than 580
Ma, citing this as proof that the UHT metamorphism oc-
curred in the Prydz Tectonic Event from 575 through to
510 Ma, in accord with evidence from monazite chemical
dating (Kelsey et al. 2003b, 2007). As argued above, the
broad range in Neoproterozoic ages could plausibly be the
result of variable resetting of early Neoproterozoic detrital
grains by the Prydz Tectonic Event, so that the Mather
Paragneiss could have been deposited as early as 1000
Ma but after the Rauer Tectonic Event. Hence, the previ-
ous zircon data and our new data also allow UHT meta-
morphism to be as old as 1000 Ma, but do not require it.
All five analyzed samples in this study, three from
the Mather UHT unit (TH/06/30C and SH/88/218: Mg–
Al–rich UHT gneiss, TH/06/30J: orthopyroxene–bearing
leucosome) and two from the host orthogneiss unit (TH/
06/33A: felsic orthogneiss, TH/06/33B: Garnet–silliman-
ite–bearing metapelitic gneiss lens in orthogneiss) yield
concordant to near–concordant 530–510 Ma zircons, con-
sistent with previous age data on zircons from the Mather
Paragneiss (Wang et al., 2007; Kelsey et al., 2008) and
nearby orthogneisses (Kinny et al., 1993; Harley et al.,
1998). These clearly only place a lower age limit on UHT
metamorphism. The Opx–bearing leucosome TH/06/30J,
which is likely to have formed through melting and melt–
wall rock interaction syn–or late during the UHT event,
preserves one near–concordant zircon with a 206Pb/238U
age of 635 Ma, but otherwise there is no clear evidence
in the zircon data from the Mather Paragneiss samples
to precisely constrain an age for the onset and duration
of UHT metamorphism.
The calculated apparent REE distribution coeffi-
cients [DREE(Zrn/Grt)] can be compared with empirical
or experimentally derived D values inferred to represent
equilibrium (e.g., Kelly and Harley, 2005). Although gar-
net generally has much less (1/100) REE concentration
than zircon as can be seen from Figure 6, the modal pro-
portion of garnet is much greater than that of zircon —
probably 1000 times or more especially for the case of
the current sample which has >10% modal proportion of
garnet (see Figs. 2c and 3a) in the rock. Therefore, even
Age constraints of UHT metamorphism at Mather Peninsula, Rauer Islands 99

though garnet has much less REE contents than zircon,
garnet still contributes to control the REE pattern of met-
amorphic zircon if zircon growth/recrystallization took
place at the presence of garnet. Although, there still re-
mains several discrepancies among recent experimental
results (Rubatto and Hermann, 2007; Taylor et al.,
2015), comparison of the DREE(Zrn/Grt) values calculat-
ed for all zircon types in SH/88/218 with experimentally–
constrained values for DMREE–HREE(Zrn/Grt) under UHT
conditions of >950 °C (Rubatto and Hermann, 2007;
Taylor et al., 2015) shows that none of these zircons have
equilibrated with the garnet present in the UHT gneiss.
As the garnet in the studied UHT sample shows relatively
low–Ca and metamorphosed under T> 900 °C, the ex-
perimental conditions of Taylor et al. (2015) are to be
more appropriate. However, the DREE(Zrn/Grt) values
of the studied sample suggests HREE–enriched D pattern
that is far different from the experimental results of Tay-
lor et al. (2015) suggesting flat M–HREE D pattern to be
equilibrium partitioning (Fig. 6). Hence, none of the zir-
con ages obtained in our study provide an age of the peak
UHT event in which garnet was stable. These observation
implies that the zircon grains, even if they are included in
the orthopyroxene or garnet porphyroblasts, were more or
less affected by lead loss, partial resorption–reprecipita-
tion and/or recrystallization promoted by pervasive hy-
dration during the upper–amphibolite to granulite–facies
main Prydz Tectonic Event at 520–510 Ma.
Monazite chemical dating offers an alternative means
of addressing the age of the UHT event in the Mather Para-
gneiss, especially when considered using in–situ analysis
and monazite mineral chemistry. Kelsey et al. (2003b)
reported monazite electron microprobe chemical ages
for the Mather Paragneiss that ranged from 521 ± 8 to
500 ± 6 Ma. These data were complemented by in–situ
results from two further samples that yielded ages of up
to 544 ± 11 Ma for monazites hosted in or exhumed from
garnet porphyroblasts (Kelsey et al., 2007). Kelsey et al.
(2007) also recognized a 574 ± 16 Ma monazite age pop-
ulation, hosted in orthopyroxene, which they ascribed to a
prograde, pre–garnet, stage of UHT metamorphism that
later peaked at 545 Ma as part of the regionally docu-
mented Prydz Tectonic Event.
Monazite grains analyzed in–situ in the present study
by electron microprobe show distinct internal zonation:
580–560 Ma dark–BSE cores, 550–520 Ma mid–BSE
mantles and 510–500 Ma bright–BSE rims, with minor
inheritance of an earlier but poorly defined population
of 650 Ma monazite. The 580–560 Ma monazite cores
have relatively high M(–H)REE whereas the 550–520
Ma mantle domains and structureless grains preserve low-
er M(–H)REE contents, suggesting their growth or mod-
ification under different conditions (Fig. 6a). The 510–500
Ma outermost rims have the lowest M(–H)REE concentra-
tions, again suggesting an evolution in the REE chemistry
of their growth environment that may be related to the
formation or consumption of other REE–bearing minerals.
A simple interpretation may be the 580–560 Ma monazite
to be formed prograde metamorphism pre–dating the per-
vasive 520 Ma ages obtained from both zircon and mon-
azite. It cannot be ruled out the possibility that the older
monazite dates just inherited from the 1000 Ma Rauer
Tectonic Event. In order to assess this we need more data
and information of the nature of Rauer Tectonic Event,
and at moment we could not discuss more on this.
The Mg–Al–rich gneisses in Mather Peninsula pre-
serve UHT mineral assemblages including garnet, ortho-
pyroxene and/or sillimanite that are locally replaced by
fine–grained symplectite composed of sapphirine, cordier-
ite, orthopyroxene, spinel or plagioclase along the post–
peak decompressional P–Tpath (Harley, 1998). These
gneisses have also experienced extensive hydration, man-
ifested in the formation of biotite–bearing reaction coronas
and localized biotite–rich zones and reaction selvedges.
Almost all monazite grains are distributed in the symplec-
titic reaction zones. From the chemical and textural evi-
dence we infer that the M–HREE–rich 580–560 Ma mon-
azite cores may have formed not prograde but through the
decomposition of garnet, for example to orthopyroxene +
sapphirine, during decompression just after the UHT
event, whereas the M–HREE–depleted 550–500 Ma mon-
azite grains/rims formed or recrystallized in reactions as-
sociated with the subsequent extensive hydration, which
also caused the marked recrystallization of zircon.
The monazite data support the interpretation pro-
posed by Harley et al. (2009) that the UHT metamor-
phism occurred prior to 590 Ma, and are consistent with
the 600 Ma Sm–Nd garnet isochron obtained for the same
rock by Hensen and Zhou (1995, 1997). The observation
that this age (600–580 Ma) is only recorded in the UHT
Mather Paragneiss and is not present in adjacent or near-
by Archaean or Meso–Neoproterozoic units within the
Rauer Islands, all of which record the 530 Ma Prydz Tec-
tonic Event, supports the speculation of Harley et al.
(2009) that the Mather Paragneiss experienced its UHT
metamorphism prior to final accretion to and during in-
terleaving with those other units in the Cambrian.
SUMMARY AND CONCLUSIONS
Given its present position the evolution of the Rauer
Terrane is critical to understanding the amalgamation of
East Antarctica, and particularly the relative importance
of 1000 and 530–510 Ma tectonothermal events in weld-
T. Hokada, S.L. Harley, D.J. Dunkley, N.M. Kelly and K. Yokoyama
100

ing together the various Archaean and Proterozoic crustal
blocks (Zhao et al., 1995; Hensen and Zhou, 1995; Car-
son et al., 1996; Hensen and Zhou, 1997; Fitzsimons,
2000; Harley, 2003; Carson et al., 2007; Wang et al.,
2007; Kelsey et al., 2007, 2008; Boger, 2011; Harley et
al., 2013). The Mather Paragneiss detrital zircon age
spectra (upper intercepts of 2800–2400 Ma) and the pro-
tolith zircon age of the host tonalitic orthogniess (3268 ±
4 Ma) obtained in this study are consistent with the pre-
viously reported zircon dates from Archaean orthogneiss-
es of the Rauer Islands, and confirm that the host orthog-
neiss surrounding the UHT paragneisses are Archaean in
protolith age. Although we have identified a few near–
concordant 900–700 Ma zircon grains from the Mg–Al
UHT gneiss, no concordant zircon grains or grain rims
have been found with ages in the range corresponding to
the 1030–990 Ma Rauer Tectonic Event. This observation
leads us to conclude that the Archaean and Mesoproter-
zoic components of the Rauer Islands were not amalga-
mated in the Rauer Tectonic Event, and that deposition of
the Mather Paragneiss was at some time after the Rauer
Tectonic Event. The near–concordant early Neoprotero-
zoic (700 Ma) zircons are considered to reflect the vari-
able effects of the Prydz Tectonic Event at 530 Ma on the
pre–existing metamorphic zircon, rather than defining a
younger maximum age of deposition for the Mather Para-
gneiss.
Two alternative tectonic scenarios have been pro-
posed previously for the context and development of
the UHT metamorphism in the Mather Paragneiss. The
first scenario, that of a single tectonic event, proposed
that UHT, ITD and subsequent biotite formation all oc-
curred during the age interval >575–510 Ma and hence
reflect the Prydz Belt tectonism seen further SW in Prydz
Bay (Kelsey et al., 2007, 2008). The second, two tectonic
event scenario of Wang et al. (2007), proposed that an
older possibly 1000 Ma UHT metamorphism was over-
printed by the later high–Thydration event at 580–510
Ma. Neither of these scenarios fully accounts for the data
presented in this study, in particular the monazite age–
chemistry evidence which suggest that 580–560 HREE–
enriched monazite formed in the Mather Paragneiss ac-
companying the decomposition of high–Mg garnet at or
following UHT conditions. The implications of the mon-
azite data are that UHT metamorphism occurred at or just
prior to 580 Ma and was followed by extensive decom-
pression and cooling, whereas the development of M–
HREE depleted 550–500 Ma monazite grains/rims re-
flects a distinct overprinting episode associated with the
subsequent extensive hydration related to the upper–am-
phibolite to granulite–facies main Prydz Tectonic Event
in the Rauer Islands.
The zircon and monazite data present in this study
imply the contrasting behavior of these minerals in re-
sponse to crystallization/modification during high–T/
UHT metamorphism. Zircon is commonly a robust min-
eral that can retain older protolith/inherited isotopic com-
positions, but conversely is not always a reliable record-
er of HT and UHT metamorphic events (e.g., Kelly and
Harley, 2005) as zircon may not grow in the absence of
suitable melts, and furthermore may be extensively modi-
fied by coupled dissolution–precipitation under relatively
lower–Tbut fluid–rich conditions (e.g., Geisler et al.,
2007). In contrast, whilst monazite has less ability to re-
tain older protolith/inherited isotopic compositions, it may
have greater potential to record the HT–UHT metamor-
phic events. The combined or integrated use of isotopic
and chemical analysis of zircon and monazite, preferably
in–situ and in concert with host silicate and other minerals
that may compete for REE, offers greater potential to pro-
vide insights on the development of complex and poly–
metamorphic terranes than the use of zircon alone.
ACKNOWLEDGMENTS
Samples used in the study were collected during the
1988/89 and 2006/07 austral summer seasons, and the
Australian Government Antarctic Division (AGAD) and
the expedition members of Australian National Antarctic
Research Expedition (ANARE) are thanked for their sup-
port while in the field and at Davis Station. Constructive
reviews by Ian Buick and Ed Grew, and the editorial as-
sistance by N. Nakano improved the manuscript consid-
erably. Thanks are also due to them. This work was sup-
ported by JSPS KAKENHI Grant Number 25287132
and NIPR Project Research KP–7 to TH, and by UK
Natural Environment Research Council (NERC) grant
NE/B504157/1 and Antarctic Science Advisory Council
(ASAC) field support award 2690 (2006–2007) to SLH.
The production of this paper was supported by an NIPR
publication subsidy.
SUPPLEMENTARY MATERIALS
Supplementary Documentation, Tables S1–S3, and color
version of Figures 1–3, and 6 are available online from
http://doi.org/10.2465/jmps.150829.
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Manuscript received August 29, 2015
Manuscript accepted February 12, 2016
Manuscript handled by Nobuhiko Nakano
Age constraints of UHT metamorphism at Mather Peninsula, Rauer Islands 103