U–Pb Geochronology from Tonagh Island, East Antarctica: Implications for the Timing of Ultra-High Temperature Metamorphism of the Napier Complex

Precambrian Research 116 (2002) 237–263 www.elsevier.com/locate/precamres

U–Pb geochronology from Tonagh Island, East Antarctica: implications for the timing of ultra-high temperature metamorphism of the Napier Complex

C.J. Carson a,*, J.J. Ague a, C.D. Coath b

a Department of Geology and Geophysics, Yale Uni6ersity, P.O. Box 208109, New Ha6en, CT 06520-8109, USA b Department of Earth and Space Science, Uni6ersity of California, Los Angeles, CA 90095-1567, USA Accepted 13 February 2002

Abstract

Ion microprobe U–Pb zircon geochronology of an orthopyroxene-bearing felsic orthogneiss from central Tonagh Island, Enderby Land, East Antarctica provides insight into the chronological-metamorphic evolution of the Archaean Napier Complex, the details of which have been the source of debate for over two decades. The orthogneiss crystallised at 2626928 Ma, predating peak, ultra-high temperature (UHT) metamorphism and development of an / intense regional S1 gneissosity. Two subsequent episodes of zircon growth resetting can be identified. A minor period of zircon growth occurred at 2546913 Ma, the regional significance and geological nature of which is unclear. This was followed by an episode of abundant zircon growth, as mantles on 2626 Ma cores and as anhedral grains, partly characterised by high Th/U(\1.2), at 2450–2480 Ma. This age coincides with both lower and upper concordia intercept ages from other U–Pb zircon studies, and several Rb–Sr and Sm–Nd whole-rock isochron ages from the Napier Complex. We conclude that UHT metamorphism occurred at 2450–2480 Ma, and find no compelling evidence that UHT occurred much earlier as has been postulated. The zircon U–Pb data from this study also indicates a lower intercept age of 500 Ma, which coincides with the emplacement of Early Palaeozoic pegmatite swarms and synchronous infiltration of aqueous fluids into the southwestern regions of the Napier Complex. © 2002 Elsevier Science B.V. All rights reserved.

Keywords: Napier Complex; East Antarctica; U–Pb geochronology; Zircon; Ion probe

1. Introduction considerable attention over the past four decades (e.g. Sheraton et al., 1987; Harley and Black, The granulite-facies Archaean Napier Complex 1987, 1997), initially by Soviet and Australian of Enderby Land, East Antarctica has received geological expeditions, and, in recent years, by Japanese expeditions. The identiﬁcation of un- * Corresponding author. Present address: Geological Survey usual metamorphic mineral assemblages on a re- of Canada, 601 Booth Street, Ottawa, Ont., Canada K1A 0E8. gional scale, such as sapphirine+quartz E-mail address: [email protected] (C.J. Carson). (Dallwitz, 1968), osumilite9garnet, and orthopy-

0301-9268/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S0301-9268(02)00023-2 238 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 roxene+sillimanite+quartz, indicative of ultra- Mountains. Inland, to the north and east of high temperature (UHT) metamorphism, gener- Amundsen Bay, lie the scattered and sparsely ated great interest in the geological community. exposed nunataks of the Napier Mountains. Al- Subsequent detailed petrological studies confi- though the precise location of the boundary is rmed that the terrane experienced extreme tem- uncertain (Fig. 1), to the south and southeast the peratures approaching, even surpassing, 1000 °C Napier Complex is juxtaposed against the gran- at pressures of 8–11 kbars, temperatures quite ulite-facies early Neoproterozoic Rayner Complex unusual for mid-crustal metamorphism (Ellis, (1000 Ma; e.g. Black et al., 1987; Harley and 1980; Ellis et al., 1980; Grew, 1980, 1982; Harley, Hensen, 1990; Kelly et al., 2000). The south-west- 1985a, 1987, 1998; Sandiford and Powell, 1986a; ern Napier Complex is now thought to be bound Harley and Hensen, 1990; Harley and Motoyoshi, by Rayner Complex material, reworked by the 2000). These stimulating discoveries in the petro- eastward extension of Early Palaeozoic Lu¨tzow– logical arena were matched by chronological re- Holm Complex tectonism (520–550 Ma; Shirashi ports that suggested southwestern regions of the et al., 1997). terrane were composed of extremely ancient The geological evolution of the Napier Com- E crustal material (Pb–Pb ages 4000 Ma; plex presented here is condensed from Sheraton et Sobotovich et al., 1976; Lovering, 1979). Al- al. (1987) and Harley and Black (1987) and repre- though, subsequent isotopic work questioned the sents the generally accepted framework for the veracity of these very old protolith ages, an early terrane. The Complex is dominated by tonalitic to to mid Archaean origin for many of the or- granitic orthogneisses, with locally extensive lay- thogneiss units and paragneiss sequences across ered paragneiss sequences, which include quartz- the Napier Complex is now nevertheless well es- ites, ironstones, aluminous meta-pelites and tablished (Black and James, 1983; Black et al., -psammites. Mafic and ultramafic units are 1983b, 1986a,b; DePaolo et al., 1982; Owada et present, usually as narrow layers and boudinaged al., 1994; Harley and Black, 1997). However, the pods within felsic gneisses and layered paragneiss temporal framework of the structural-metamor- sequences. Calc-silicates and anorthosite bodies phic evolution of the Napier Complex, in particu- are rare. The first recognised deformation, D , lar the timing of UHT metamorphism, remains a 1 source of some debate amongst researchers (De- resulted in the development of a regional intense, Paolo et al., 1982; Harley and Black, 1997; Grew, flat-lying, strongly-lineated S1 gneissosity and the 1998). In this paper, we present new zircon U–Pb transposition and interleaving of lithological ion probe data from Tonagh Island, Amundsen units. The second deformation, D2, resulted in Bay, in which we offer additional evidence and refolding of S1 about F2 tight assymetrical folds constraints concerning the timing of UHT tec- with recumbent to reclined axial planes but only tonothermal events within the Napier Complex. localised S2 development. The third deformation, D3, resulted in upright broad (1–10 km) folding and produced regional dome and basin structures.

2. Geological setting In the Field Islands (Fig. 1), D3 was sufﬁciently intense to have developed a penetrative fabric, but

The Archaean Napier Complex is located over much of the Napier Complex S3 development within the East Antarctic Shield between latitudes was not pervasive and only locally developed in 66°–68°S and longitudes 48°–57°E and occupies F3 hinges. UHT metamorphic conditions are much of Enderby Land (Fig. 1). The south-west- thought to have evolved during D1 –D2 with up- ern regions of the Complex, well exposed as near- per-amphibolite conditions prevailing during D3, shore islands and coastal mountains in the vicinity after which the Complex was essentially cra- of Amundsen and Casey Bays, also include sev- tonised. Petrological studies have demonstrated eral broadly east–west trending ranges of nuna- that the P–T evolution of the Napier Complex taks and massifs of the Tula, Scott and Raggatt followed a near-isobaric cooling trajectory follow- C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 239 ing peak UHT conditions (Harley and Hensen, shear zones and pseudotachylites remain some- 1990). Tholeiitic dyke swarms were emplaced at what speculative. The final rock-forming events 2350 and 1190 Ma (Sheraton and Black, occurred with the emplacement of Early 1981), preceding the development of various lo- Palaeozoic planar pegmatites (500–522 Ma; Grew calised amphibolite-facies shear zones and pseu- and Manton, 1979; Black et al., 1983a; Carson et dotachylites (D4 –D5; Harley, 1985b; Sandiford, al., 2002) and rare mafic alkaline dykes (466–482 1985), movement along which is thought to have Ma; Black and James, 1983; Miyamato et al., resulted in partial exhumation of the terrane. 2000). − Features associated with D4 D5 have been generally attributed to the local manifestation of 2.1. Pre6ious geochronology granulite-facies tectonism which developed in the adjacent Neoproterozoic Rayner Complex ( A considerable amount of geochronological in- 1000 Ma), though this is largely based on indirect formation has been obtained from the Napier isotopic evidence, such as imprecise U–Pb lower Complex. Initial work focused of the identifica- concordia intercepts. The precise timing of these tion of very old crustal fragments (e.g. Sobotovich

Fig. 1. Location of Tonagh Island (indicated in box) within Archaean Napier Complex. Various localities discussed in the text are highlighted and labelled. Boundary between Napier and Rayner Complexes (dashed with question marks) after Harley and Black (1987) and Harley and Hensen (1990). Regional extent of sapphirine+quartz assemblages delineated by dashed line with barbs (after Harley and Motoyoshi, 2000); the area enclosed within this line also approximately delineates the areal extent of UHT metamorphism. Upper inset: location of Napier Complex within Antarctic continent. Lower inset: enlargement of Casey Bay region, with localities discussed in text (from Black et al., 1983a). 240 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 et al., 1976; Lovering, 1979) and was reﬁned and tak (Black and James, 1983), all of which indicate developed by subsequent work (e.g. DePaolo et evidence of an event at 2900 Ma. Harley and al., 1982; Black et al., 1983a,b, 1986a,b; Harley Black (1997) revised these earlier estimates on the and Black, 1997). Much of the research conducted timing of D2 based on their additional zircon data since the early 1980’s has focused on the construc- and stated that the entire D1 –D2 evolution may tion of a structural-metamorphic timeframe for have been achieved during the time interval of the evolution of the Napier Complex. 2850–2820 Ma, followed by an extended period The generally adopted chronological scheme of near-isobaric cooling. was largely developed by L.P. Black and cowork- Black et al. (1983a) presented conventional U– ers during the early to mid 1980’s, with subse- Pb analyses and centimetre-scale whole-rock Rb– quent reﬁnement by Harley and Black (1997), the Sr isochrons from felsic gneisses sampled from the results of which are summarised here. Initial con- Casey Bay region (Fig. 1) which record evidence straints on the timing of D1 were based on a for an event at 2450–2480 Ma. Black et al. Rb–Sr whole-rock isochron age of 3072934 Ma (1983a,b)) concluded that these data indicate re-

(Sheraton and Black, 1983) from a syn-S1 or- setting of Rb–Sr systems, zircon growth and ex- thogneiss from Proclaimation Island (Fig. 1), a tensive resetting of the U–Pb system in location somewhat distal to the region of extreme pre-existing zircon during D3, the effects of which metamorphism recorded in the Tula Mountains are pervasive in Casey Bay. A conventional zircon and Amundsen Bay regions to the SW. Based on U–Pb age from a syn-D3 pegmatite (Ayatollah these data, a number of workers maintained that Island; Fig. 1), yielded an upper intercept of 2456 UHT metamorphism occurred around this time +8/−5 Ma (Black et al., 1983a); a date that has (Black and James, 1983; Black et al., 1983a,b, become widely accepted as the best estimate for

1986a,b). This estimate was later reassessed by the the timing of D3 (Black and James, 1983; Black et SHRIMP study of Harley and Black (1997) who al., 1983a,b, 1986a,b; Sheraton et al., 1987; concluded that UHT metamorphism and UHT- Harley and Black, 1997). Numerous studies, using related D1 could be no older than 2840 Ma, a variety of isotopic systems across the Napier based largely on the emplacement age of a syn-D1 Complex, report ages similar or identical to this orthogneiss at Dallwitz nunatuk, located within value and these have been interpreted as repre- the region of UHT metamorphism. They pro- senting isotopic disturbance during D3. posed a best estimate of 2837915 Ma for the However, the above chronological scheme for timing of ‘highest grade metamorphism’, evalu- the evolution of D1 –D3 has not been universally ated on zircon populations from three widely accepted. While there is little dispute concerning spaced samples. the antiquity of various protoliths across the

The timing of D2 has been subject to some Napier Complex, a number of authors have ex- uncertainty. Initially, the timing of D2 was largely pressed an alternative view for the timing of the based on a Rb–Sr whole rock isochron age of UHT metamorphism and structural evolution 2840 +220/−280 Ma (Black et al., 1986a) from (Grew and Manton, 1979; Grew et al., 1982; a granite body at Mt Bride (Fig. 1) for which a Grew, 1998; DePaolo et al., 1982). These authors syn-D2 emplacement age was inferred. An U–Pb maintain that the timing of UHT metamorphism zircon age of 2948 +38/−17 Ma from Mt Sones can be readily explained in terms of a short-lived is also considered to represent an event at this metamorphic event at 2450–2480 Ma (Grew, time (Black et al., 1986b; Black, 1988). Other, less 1998), in contrast to the much earlier D1 –D2 well-constrained, U–Pb ages have also been used tectonothermal evolution indicated above (Harley as evidence to constrain the timing of D2. These and Black, 1987, 1997). This alternative view include a concordant population of zircon ion evolved primarily over differing interpretations of probe U–Pb ages from a paragneiss from Mt syn-tectonic pegmatites in the Casey Bay region

Riiser-Larsen and a population of conventional (Fig. 1). The syn-D3 pegmatite dated by Black et U–Pb analyses from Mt Tod and Dallwitz Nuna- al. (1983a); 2456 +8/−5 Ma) is instead inter- C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 241

Fig. 2. Sample localities on Tonagh Island, including the sample site of Shirashi et al. (1997). Lithological subdivision boundaries (dashed and labelled) after Osanai et al. (1999). preted to date UHT metamorphism based on the subdivided the island into ﬁve lithological subdivi- preservation of UHT mineral assemblages in struc- sions (units I–V). These units represent coherent turally similar pegmatites (Grew, 1998). Sandiford blocks of rock associations, the boundaries of and Wilson (1984) also suggest that such peg- which are marked by steeply dipping to vertical matites were emplaced during D2 boudinage, dur- shear zones of unknown age. ing UHT conditions. In this contribution, we One sample (sample 27) for U–Pb zircon analy- document new zircon ion probe U–Pb data from sis was taken from an orthopyroxene-bearing a felsic orthogneiss exposed on central Tonagh quartzo-feldspathic orthogneiss (after the map unit Island and provide additional constraint on the of Osanai et al., 1999; hereafter called the Tonagh timing of UHT metamorphism and of subsequent Island orthogneiss in this contribution), the loca- events that affected the Napier Complex. tion of which is shown in Fig. 2. This unit is common on Tonagh Island and is light brown- to buff-coloured, homogeneous in appearance, con-

3. Tonagh Island orthogneiss taining an intense S1 gneissosity defined by 1–10 mm scale subtle discontinuous, quartzo-feldspathic Tonagh Island, the site of this study, is located rich segregations. Typically the unit contains an- in Amundsen Bay, south-western Napier Complex hedral quartz (35% mode), K-feldspar (14%) (Fig. 1). The geology of Tonagh Is. has been and plagioclase (22%) and large (2–3 mm described in detail by Osanai et al. (1999) and, like diameter) subhedral mesoperthite (10–20%), or- the majority of the Napier Complex, comprises a thopyroxene (3–5%) and minor clinopyroxene (B variety of felsic to intermediate orthogneisses, mi- 1%), with accessory ilmenite-magnetite, apatite, nor mafic and ultramafic granulites and subordi- zircon and rare monazite (typically as mantles on nate layered paragneiss sequences that include apatite). A typical chemical analysis is listed in quartzites and ironstones. Osanai et al. (1999) also Table 1 (sample 27) and is further discussed below. 242 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263

In addition to sample 27, multiple samples conditions (Carson et al., 2002). These samples were collected from an alteration selvedge or were collected originally in order to assess the zone immediately adjacent to an Early effect of Early Palaeozoic aqueous fluid influx Palaeozoic pegmatite. The location of samples on the host orthogneiss zircon population, the collected from the alteration selvedge (30, 28/6, results of which have been presented in Carson 28/1and28/2) are illustrated in Fig. 3. Within et al. (2002). However, additional data and fur- the alteration selvedge, the pyroxene-bearing or- ther assessment of the zircon U–Pb data of thogneiss had been extensively re-hydrated and Carson et al. (2002) is presented here within the recrystallised by aqueous fluids that accompa- context of the regional chronological evolution nied pegmatite emplacement, resulting in re- of the Napier Complex. placement of pyroxene with biotite–hornblende– epidote assemblages at upper amphibolite facies 4. Analytical procedures

Table 1 Zircons were extracted from bulk hand sam- Chemical composition of Tonagh Island orthogneiss ples (5 kgs) and were separated using stan- Oxides Sample 27 dard heavy liquid and magnetic techniques, mounted in epoxy, together with zircon standard 206 238 SiO2 71.08 AS3, (near concordant TIMS Pb/ U age of TiO 0.44 2 1099.190.5 Ma; Paces and Miller, 1993). The Al O 14.05 2 3 mounted grains were equatorially sectioned, pol- Fe2O3 3.19 MnO 0.04 ished and Au-coated. Backscattered electron MgO 0.90 (BSE) imaging was conducted to assess internal CaO 2.86 zircon structure and to facilitate selection of Na O 3.29 2 analysis sites (Fig. 4a–h) and were obtained us- K O 3.67 2 ing the JEOL JXA-8600 electron microprobe lo- P2O5 0.14 LOI 0.32 cated at Yale University. Zircon U–Pb ion Total 99.98 microprobe analyses were conducted on a Trace CAMECA IMS 1270, located at the Department Ba 1355 of Earth and Space Sciences, University of Cali- Nb 6 fornia, Los Angeles, USA. The analytical proto- Rb 69 cols for zircon analysis are summarized in Sr 257 Quidelleur et al. (1997). All isotopic ratios were Y 13 Zr 167 corrected, where necessary, for common lead us- 204 Ce (0.1) 90.0 ing measured Pb. Common Pb ratios used are U (0.1) 0.4 206Pb/204Pb=16.2, 207Pb/204Pb=15.35. Uranium Th (0.1) 5.8 and Th concentrations were estimated semi- Pb (2) 14 /94 /94 quantitatively using U Zr2O and Th Zr2Oin K/Rb 220.7 unknowns relative to that of the AS3 standard Ce /Y 15.5 N N and normalised against published U and Th Th/U 14.5 concentrations (Paces and Miller, 1993). Data Bulk rock geochemistry was conducted using XRF, conducted reduction and processing were performed using by X-ray assay laboratories (XRAL) in Don Mills, Ont., ZIPS v2.4.1 (C.D. Coath 2000, unpublished Canada. Major elements were determined on fused glass discs, software) and ISOPLOT v2.3 of Ludwig (1999). trace elements (Ba, Nb, Rb, Sr, Y and Zr) on pressed powder Zircon U–Pb isotope results have been pre- pellets and Ce, U, Th and Pb determined by ICPMS techniques (detection limits shown). Results shown are average of viously published in part (Carson et al., 2002; two samples for major and trace elements, from one sample alteration zone samples 28/6, 28/1 and 28/2; for ICPMS elements. unaltered protolith, sample 27; Fig. 3). C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 243

Fig. 3. Photograph and sketch map of sample site within the Tonagh Island orthogneiss adjacent to an Early Palaeozoic pegmatite. Note conspicuous ‘bleach’ zone adjacent to pegmatite (see also Carson et al., submitted for publication). The visible outer boundary of alteration zone indicates pyroxene-out isograd in the host orthogneiss. Samples 28/6, 28/1and28/2, 30 are 10, 160, 300 and 1100 mm respectively from left margin of the pegmatite; sample 27, 20 m to the left of pegmatite out of photograph. Photographer facing east, geologist for scale, sample location 67°05%37.3¦S, 050°17%09.8¦E.

We now evaluate these data presented by Carson and b). Typically this core region is overgrown et al. (2002) within the context of the regional by a volumetrically dominant mantle, which al- geochronological evolution of the Napier Com- though generally homogeneous (Fig. 4b and c), plex, and present additional isotopic data (Tables may exhibit oscillatory zoning at the micron 2–4). Errors ellipses shown in Figs. 5–9 are at scale (Fig. 4d). The mantle may crosscut the core the one sigma level, weighted mean and intercept regions (Fig. 4a) but more typically shows a con- ages quoted are at the two sigma level and in- formable relationship with them. Another major clude U decay constant uncertainties. population consists of anhedral spherical glassy ‘gem-like’ pinkish grains, up to 20–500 mmin diameter (Fig. 4g and h), which display minimal 5. Results structural development, and lack a clearly deﬁned euhedral core or micron scale oscillatory Zircons from the protolith and the alteration zoning, although some coarse (10–20 mm) zoning selvedge have similar morphologies. Most com- may be present (Fig. 4g). A narrow (B20 mm), mon are euhedral to subhedral, clear to honey low U outer rim is typically present on most coloured grains, 100–250 mm in length, that zircons (Fig. 4a–g) and may crosscut delicate have aspect ratios of 2–3. Under back scat- oscillatory zoning present in the mantle (Fig. 4c tered electron imaging, sectioned grains may ex- and d). In total, 133 U–Pb analyses were con- hibit fractured well deﬁned cores which may ducted on 70 zircon grains; Tables 2–4 sum- show micron-scale oscillatory zoning (Fig. 4a marise the isotopic data. 244 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263

Fig. 4. BSE images of representative zircon morphologies. All annotated ages are 207Pb/206Pb ages, which may differ from upper intercept ages quoted in the text depending on degree of discordance, scale bar in all images=100 mm: (a) sample number 28/2, grain 3, (disc 1), note zoned 2600 Ma core, overgrown by faintly zoned mantle and outer rim. Analysis point with 2526 Ma age rejected due to overlap with significantly older core; (b) sample number 28/2, grain 6 (disc 1), finely zoned 2600 Ma core, overgrown by wide homogenous mantle which grades into dark outer rim; (c) sample number 28/3 grain 14 (disc 1), unzoned 2600 Ma core, overgrown by zoned mantle and outer rim with 2550 Ma ages. Note the transgressive nature of the outer rim at the arrowed location; (d) sample 28/1, grain 5 (disc 7). Finely zoned core (reset to 2423 Ma?) overgrown by mantle and outer rim, outer rim appears to truncate mantle (e) sample 28/1, grain 7 (disc 7), coarsely zoned 2550 Ma euhedral core over grown by outer rim; (f) sample number 28/2, grain 1 (disc 7) (g) sample number 27, grain 14 (disc 1), auhedral grain with coarse concentric zoning. (h) sample 28/6, grain 15 (disc 7). Homogenous anhedral grain with negligible zoning. C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 245 location Defined Defined Defined Defined Defined Defined Defined Defined core c .spot c .grain ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ c sample ¯ .disc Date 2000-2Dec 281.d1.gr12.sp1 core c 281.d1.gr15.sp1 core 281.d1.gr16.sp1 core 281.d1.gr8.sp1 core 282.d1.gr3.sp1 core 282.d1.gr6.sp1 core 282.d1.gr8.sp1 283.d1.gr14.sp1 core O relative to that of measured values of AS3 2 Zr (ppm) 332 94 / (ppm) 838 399 2000-2Dec O and Th 2 Zr 94 / 101.7 U Disc.% U Th Spot concordant analysis. / = Th 0.3 97.9 765 221 2000-2Dec 0.5 89.7 410 237 2000-2Dec 100, 100% 9 0.001560 0.001270 + 100) Pb* × 206 1 / − Pb* 207 Pb age 206 / Pb 9 207 / U U age 238 / 238 / Pb* Pb 206 0.4870 0.00783 0.1758 0.4983 0.00593 0.1728 0.001080 0.4 100.9 748 0.4445 0.00833 0.1790 0.4688 0.01060 0.1759 0.002550 0.6 94.8 249 151 2000-2Dec 0.5198 0.00487 0.1799 0.001100 0.4 206 ( = 9 level. U and Th contents are semiquantitative estimates based on U | U 235 / Pb* 207 Pb* 206 % 14.7 10.5 15.9 11.8 10.2 Pb Pb; uncertainties are given at the 1 206 / 206 Pb 2585 207 Age (Ma) 2614 2609 2644 2652 9 11.6 17.8 15.5 2639 14.6 99.3 11.960 0.198 0.4859 0.00588 0.1785 0.001570 0.1 96.7 523 68 2000-2Dec 16.9 0.4931 0.00700 0.1753 0.001670 0.9 99.0 384 360 2000-2Dec 19.4 26.4 2634 12.6 99.6 12.100 0.340 0.4929 0.01370 0.1780 0.001350 0.8 98.1 568 479 2000-2Dec 10.2 25.4 U 235 / Pb Age (Ma) 2589 99.4 11.800 0.224 2594 99.7 11.870 0.146 207 2598 0.216 2521 99.4 10.970 0.229 2554 99.0 11.370 0.309 2672 99.6 12.900 0.139 33.9 25.5 99 25.5 30.2 99.5 37.2 59.1 46.5 2614 24.2 20.7 U 238 / Pb* denotes percentage of radiogenic Pb 206 standard and published U and Th concentrations. Disc.% is the individual discordance Age (Ma) 2558 2607 Table 2 U–Pb ages and isotopic compositions of selected zircon cores from206 Tonagh Island, Amundsen Bay 2553 2601 2584 11.920 2371 2583 2612 2478 % 2698 246 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 Defined Defined core Defined Mantle Defined Mantle Mantle Mantle Defined core Defined core c .spot c ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ .grain c sample ¯ .disc c 282.d7.gr11.sp3 282.d7.gr12.sp2 core 283.d7.gr1.sp1 282.d1.gr3.sp2 281.d7.gr7.sp1281.d1.gr3.sp3 283.d1.gr14.sp2 core 282.d7.gr16.sp2 282.d7.gr6.sp2 core 286.d1.gr10.sp1 O relative to that of measured values of AS3 2 (ppm) Th 439 2000-2Dec Zr 94 / 897 1121 617 2000-30Nov UDate (ppm) location O and Th 2 99.9 327 97 2000-1Dec Zr 104.7 103.7 654 700 2000-30Nov 102.2 500 57 2000-2Dec 94 / U Disc.% Spot / concordant analysis. Th 1.0 0.3 0.1 = 9 100, 100% + Pb* 100) 206 × / 1 − Pb* 207 Pb age 206 / Pb 9 207 / U 238 / U age 238 Pb* / 206 0.47030.4850 0.00763 0.1681 0.00552 0.1717 0.001580 0.1 0.000896 0.4 97.9 682 99.0 58 2000-2Dec 0.5029 0.00520 0.1682 0.000794 0.5 103.4 655 331 2000-30Nov Pb 206 ( = 9 U level. U and Th contents are semiquantitative estimates based on U | 235 / Pb* 207 Pb* 206 6.54 99.5 11.810 0.128 6.35 99.9 11.940 0.124 8.72 99.5 11.480 0.158 9 13.7 98.3 11.240 0.170 11.2 99.4 11.520 0.212 Pb 206 / Pb; uncertainties are given at the 1 Pb 206 207 2529 2548 0.5125 0.00542 0.1690 0.000640 0.5 2574 Age (Ma) 2548 2550 2545 2526 2542 9.34 9.72 9.05 2540 7.91 99.8 11.660 0.113 10.1 12.4 14.1 0.4833 0.00595 0.1687 0.001380 12.9 17.3 17.7 2539 15.8 99.4 10.900 0.207 17.2 0.4963 0.00764 0.1684 0.001130 9 Ua) 235 / Pb 2558 0.4953 0.00480 0.1671 0.000947 1.1 102.5 685 813 2000-30Nov Age (Ma) 207 2600 2590 0.5067 0.00475 0.1690 0.000660 2519 0.4695 0.00443 0.1692 0.001080 0.1 97.3 562 72 2000-30Nov 2543 2515 2563 2450 0.4420 0.00808 0.1668 0.001410 0.3 93.4 575 213 2000-2Dec 2566 2578 23.1 20.7 9.51 99.7 11.410 0.114 20.3 9 19.4 10.7 99.5 10.950 0.146 25.9 24.0 36.1 14.2 99.5 10.170 0.190 33.4 32.9 22.3 U) 238 / Pb* denotes percentage of radiogenic Pb 206 standard and published U and Th concentrations. Disc.% is the individual discordance 2667 2643 2593 Table 3 U–Pb ages and isotopic compositions of selectedAge zircons (Ma) from Tonagh206 Island, Amundsen Bay % 2481 2542 2360 % 2549 2485 2598 2626 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 247 Outer rim Outer Outer rim Outer rim Outer Outer rim UG Outer rim Outer Mantle Core of UG Outer rim UG Outer rim Outer rim Mantle c .spot c ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ .grain c sample ¯ .disc 283.d7.gr10.sp3 c 282.d7.gr16.sp1 282.d7.gr8.sp1 283.d7.gr1.sp2 282.d7.gr11.sp2 rim 282.d7.gr9.sp2 rim 282.d7.gr11.sp1 282.d7.gr1.sp3 rim 286.d7.gr10.sp1 283.d7.gr18.sp2 283.d7.gr3.sp2 283.d7.gr4.sp2 283.d7.gr16.sp1 283.d7.gr19.sp1 283.d7.gr7.sp2 283.d7.gr5.sp2 Th Date (ppm) location 352 2000-30Nov 49 94 267 2000-30Nov 65 196 2000-30Nov 47 206 2000-30Nov 76 267 2000-30Nov 78 193 2000-30Nov 111 114 2000-30Nov (ppm) 95.0 98.8 96.9 101.2 167 179 2000-30Nov 101.1 107.6 103.7 100 314 2000-30Nov 102.0 U Disc.% U Spot / Th 1.0 2.9 9 Pb* 206 / Pb* 207 9 U 238 / Pb* 206 0.5182 0.00689 0.1646 0.00253 1.5 107.5 163 271 2000-30Nov 0.4510 0.01090 0.1576 0.00531 1.0 0.4798 0.01420 0.1620 0.00353 2.3 9 U 235 / Pb* 207 Pb* 206 98.8 10.700 0.366 97.7 10.290 0.481 97.5 10.610 0.632 97.9 9.798 0.370 97.5 9.256 0.510 98.2 10.420 0.442 97.7 9.807 0.531 97.0 10.800 0.686 98.1 10.720 0.409 % 72.8 55.1 41.0 67.0 69.3 Pb 206 / Pb 24762457 46.9 97.8 25.32504 10.940 98.4 11.310 0.518 25.9 98.4 0.328 11.760 0.262 2430 57.1 207 2416 29.4 98.7 9.622 0.256 2474 0.4398 0.01570 0.1617 0.00528 2.6 2484 23.8 2449 2408 2398 Age (Ma) 2477 0.4834 0.01600 0.1621 0.00700 6.5 102.6 2477 36.7 2433 37.6 31.8 0.4769 0.01430 0.1627 0.00229 34.8 9 49.9 44.0 27.1 43.355.3 0.4683 0.01760 0.1594 0.4946 0.00386 2.8 0.02120 0.1555 0.00613 4.0 50.5 0.4340 0.01280 0.1547 0.00630 3.2 39.6 59.0 35.5 47.3 24.5 39.3 0.4790 0.01300 0.1578 0.00350 44.3 U 235 / Pb 2497 2586 25182549 0.4899 0.5122 0.01740 0.1620 0.01460 0.1601 0.00450 2.1 0.00240 103.8 1.2 60 108.5 136 154 2000-30Nov 209 2000-30Nov 2399 0.4465 0.00939 0.1563 0.00270 3.4 98.5 94 347 2000-30Nov 2461 2489 2416 24652364 0.4650 0.01860 0.1612 0.00233 3.2 99.8 78 277 2000-30Nov 207 Age (Ma) 2471 0.4622 0.01550 0.1631 0.00534 3.6 98.4 76 303 2000-30Nov 2473 2363 0.4195 0.00957 0.1600 0.00576 3.3 92.0 94 338 2000-30Nov 2499 29.2 20.9 62.6 75.2 62.1 48.5 81.7 24.4 99.0 10.340 0.442 69.5 2506 68.256.8 55.2 97.7 10.390 0.531 41.8 70.2 2417 43.5 60.8 97.6 9.254 0.447 61.9 99 U 238 / Pb Age (Ma) 24762570 77.2 2666 2590 91.6 2514 2692 2324 57.7 2400 2462 2468 Table 4 U–Pb ages and isotopic compositions of selected zircons from Tonagh206 Island, Amundsen Bay 2542 22582380 2350 2449 2455 2488 2523 2526 248 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 Outer Reset core Outer Outer rim Outer Spot Outer rim Outer Reset UG UG Outer Outer location Outer Outer rim Outer Mantle c .spot c ¯ ¯ ¯ ¯ ¯ ¯ ¯ .grain ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ c sample ¯ .disc 281.d7.gr20.sp2 rim 283.d7.gr5.sp3 rim 283.d7.gr10.sp4 rim 283.d7.gr7.sp1 281.d7.gr5.sp3 283.d7.gr10.sp1 core 281.d7.gr7.sp3 rim 282.d7.gr12.sp1 rim 286.d7.gr10.sp2286.d7.gr5.sp1 rim of UG 286.d7.gr15.sp1 282.d7.gr8.sp2 281.d1.gr15.sp2281.d1.gr8.sp3 rim 286.d1.gr10.sp2 rim c 281.d1.gr16.sp3 155 2000-1Dec Th 58 U (ppm) (ppm) 97.1 255 185 2000-2Dec 103.1 205 251 2000-30Nov 101.9 57 365 2000-1Dec Disc.% 104.6 1617 135 2000-30Nov 100.7 36 228 2000-30Nov 102.3 137 325104.1 2000-2Dec 56 294 2000-2Dec UDate / 2.2 105.2 175 415 2000-1Dec 1.1 5.9 Th 0.1 5.8 2.2 0.7 4.7 0.00326 9 Pb* 206 / Pb* 0.1568 0.00722 2.5 107.1 0.1579 0.00047 0.1 100.3 1611 242 2000-30Nov 0.1594 0.00184 1.4 101.4 168 254 2000-30Nov 0.1570 0.00208 0.1609 0.00299 2.0 100.8 115 246 2000-1Dec 0.1628 0.00208 1.3 96.0 170 246 2000-1Dec 0.1577 0.00056 0.1 104.2 2367 328 2000-30Nov 0.1587 0.00043 0.1573 0.00184 0.8 102.5 214 178 2000-30Nov 0.1624 0.00783 207 0.16120.1633 0.00421 1.0 0.00489 99.20.1570 188 202 0.00610 2000-2Dec 0.1552 0.00744 0.1609 0.00217 9 U 238 / Pb* 206 9 U 235 / Pb* 207 Pb* 206 % 4.6 99.9 10.650 0.094 0.4865 0.00420 78.2 96.3 10.700 0.735 0.4949 0.02290 33.9 98.5 11.150 0.336 0.4984 0.00906 0.1623 22.4 99.3 10.250 0.275 0.4734 0.01300 81.3 96.7 10.760 0.797 0.4804 0.02160 50.4 98.0 10.91065.9 0.533 96.2 0.4846 10.370 0.01300 0.712 0.4789 0.02350 9 81.4 96.1 9.752 0.613 0.4557 0.01700 22.8 99.0 9.973 0.267 0.4495 0.01090 Pb 206 / Pb 2421 2424 2465 2442 2481 2404 Age (Ma) 207 2468 2490 2465 2423 8.3 2433 5.0 99.9 10.010 0.090 0.4600 0.00381 8.2 7.1 2432 6.0 100.0 10.480 0.080 0.4818 0.00312 63.8 19.3 2450 19.5 99.0 10.340 0.215 0.4703 0.00759 24.8 23.0 2427 19.8 99.3 10.220 0.254 0.4710 0.01010 36.2 68.9 25.3 57.9 63.6 9 39.4 45.5 24.7 U 235 /

) Pb 2466 2436 2536 28.1 2479 2497 2457 2474 2440 2493 2455 2478 2502 2516 2468 2459 Age (Ma) 207 2412 2432 33.3 16.8 39.0 98.9 56.7 42.0 31.4 98.2 10.430 0.285 0.4701 0.00958 70.2 2485 21.6 98.8 10.050 0.394 0.4478 0.01580 18.2 44.1 13.6 93.9 56.6 52.7 98.1 10.270 0.437 0.4620 0.01200 75.1 48.3 9 102.0 continued ( U 238 / Pb 2592 2440 2485 2499 2607 2535 2488 2385 2529 2484 Age (Ma) 2555 2448 44.1 2421 2547 2393 2523 Table 4 206 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 249 UG UG UG location Outer Mantle Mantle UG UG Mantle Outer Core Mantle Outer Outer rim Outer Outer rim c .spot c .grain ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ c sample Spot ¯ .disc 286.d1.gr4.sp2 30.d1.gr15.sp1 30.d1.gr18.sp1 30.d1.gr18.sp2r c 286.d1.gr13.sp3 rim 286.d1.gr14.sp2 30.d1.gr19.sp1r 30.d1.gr19.sp2r 30.d1.gr2.sp1 30.d1.gr2.sp3 rim 30.d1.gr4.sp1 of UG 30.d1.gr6.sp1 30.d1.gr4.sp2 rim of UG 30.d1.gr6.sp2r 30.d1.gr6.sp3r rim 281.d1.gr12.sp3 78 311 2000-2Dec 132 278 2000-2Dec 376 236 2000-2Dec U (ppm) (ppm) 99.0 64 497 2000-2Dec 97.9 94.9 196 338 2000-2Dec 94.7 39 309 2000-2Dec 89.7 136 322 2000-2Dec 99.0 104.9 Disc.% 103.2 531 326 2000-2Dec UThDate / Th 7.1 1.6 0.6 7.1 2.2 9 0.00245 Pb* 206 / Pb* 0.1649 0.00256 0.6 101.8 393 261 2000-2Dec 0.1607 0.00363 2.5 100.6 101 277 2000-2Dec 207 0.1594 0.00668 1.9 0.1609 0.1517 0.00393 0.1628 0.00647 3.6 0.1578 0.00252 4.0 98.8 149 661 2000-2Dec 0.1602 0.00331 0.1616 0.00622 1.6 92.3 181 323 2000-2Dec 0.1620 0.00101 0.4 98.5 616 239 2000-2Dec 0.1646 0.00182 0.1621 0.00477 1.7 100.9 146 269 2000-2Dec 0.1595 0.00883 0.1610 0.00490 9 0.01020 2.1 96.1 144 325 2000-2Dec U 238 / Pb* 206 9 U 235 / Pb* 207 Pb* 206 % 25.8 98.6 9.847 0.280 0.4439 70.9 97.2 10.760 0.610 0.4894 0.01370 44.2 97.6 9.163 0.358 0.4380 0.01410 67.0 96.9 10.300 0.633 0.4588 0.01760 34.9 98.4 9.633 0.304 0.4361 0.00973 18.6 99.4 11.200 0.255 0.4934 0.00848 93.7 94.2 9.526 0.877 0.4333 0.02650 51.4 97.3 9.091 0.454 0.4095 0.01540 27.9 99.0 10.310 0.340 0.4624 0.011 0.1617 0.00267 0.6 Pb 206 / Pb 2449 2507 2463 207 2365 2485 2458 2504 2450 2466 2474 52.7 25.4 33.8 9 35.8 56.9 32.7 2487 30.4 98.6 10.030 0.355 0.4465 0.01310 0.1630 0.00294 1.5 95.7 195 315 2000-2Dec 27.0 29.1 51.2 21.3 13.6 39.7 2477 49.7 98.2 10.580 0.453 0.4736 0.01200 84.6 45.7 30.5 U 235 /

) Pb 2527 2470 Age (Ma) 2502 207 2421 26.2 2465 2354 2462 2420 2400 2385 2460 2540 2487 2390 2347 2463 45.7 26.1 99.0 11.040 0.302 0.4855 0.01050 53.2 38.1 97.9 10.390 0.379 0.4688 0.01210 58.4 59.3 45.5 77.7 63.3 54.7 2433 27.0 98.8 9.844 0.288 0.4523 0.01230 43.7 58.5 2473 65.0 96.9 9.470 0.528 0.4249 0.01290 27.9 2477 10.6 99.5 10.280 0.151 0.4602 0.00633 36.6 52.4 70.7 48.4 99 119.0 continued ( U 238 / Pb 2568 2551 2478 Age (Ma) Age (Ma) 2368 2380 2438 Table 4 206 2434 2342 2405 2333 2283 2440 2499 2585 2321 2213 2450 250 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 O relative to 2 UG UG UG Outer rim location Outer Mantle UG Outer rim UG Mantle Zr 94 / c .spot c O and Th 2 Zr 94 .grain / ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ c sample Spot concordant analysis. ¯ = .disc 27.d1.gr2.sp1 27.d1.gr9.sp1 27.d1.gr14.sp1 27.d1.gr14.sp2 2000-2Dec c 281.d1.gr3.sp227.d1.gr1.sp1 27.d1.gr1.sp2 rim 286.d1.gr4.sp3 27.d1.gr3.sp1 27.d1.gr3.sp2 100, 100% 394 2000-2Dec 359 + 100) 64 420 2000-2Dec 459 350 2000-2Dec 361 554 2000-2Dec 384 × U (ppm) (ppm) 1 − 95.8 96.7 95.5 Disc.% Th Date Pb age U 206 / / Th Pb 207 / U age 9 238 / Pb Pb* 206 ( 206 / = Pb* 207 9 U 238 / Pb* 206 9 level; UG, anhedral unstructured grain. U and Th contents are semiquantitative estimates based on U U | 235 / Pb* 207 Pb* 206 99.7 10.470 0.221 0.4748 0.0101 0.1599 0.00109 0.9 102.1 99.3 9.829 0.194 0.4428 0.00667 0.1610 0.00196 0.7 99.3 10.490 0.196 0.4586 0.00743 0.1658 0.00132 1.4 98.2 10.340 0.441 0.4519 0.0166 0.1659 0.00445 6.0 99.2 10.330 0.280 0.4593 0.0115 0.1631 0.00197 1.2 97.9 273 % 11.5 Pb 206 / Pb; uncertainties are given at the 1 Pb 206 2454 207 2517 45.1 2389 60.0 97.0 9.752 0.499 0.4597 0.0163 0.1539 0.00542 5.7 102.1 60 374 2000-2Dec 2466 2516 2442 19.2 99.3 9.701 0.167 0.4432 0.00523 0.1587 0.00180 0.9 96.8 378 370 2000-2Dec 16.9 2442 14.3 25.1 2488 20.3 19.6 17.3 9 39.5 21.3 18.2 35.9 U 235 /

) Pb Age (Ma) 2422 2477 2419 2464 207 2465 2432 2479 2428 31.9 9.866 0.180 0.4509 0.00718 0.1587 0.00134 0.8 98.2 349 310 2000-2Dec 29.8 71.9 50.7 23.4 240773.8 15.8 42.1 16.0 99.3 9.970 0.231 0.4413 0.00941 0.1638 0.00156 1.5 94.4 270 438 2000-2Dec 32.8 13.4 58.6 43.0 98.4 9.927 0.386 0.4524 0.0132 0.1591 0.00404 1.5 98.3 116 186 2000-2Dec 99 continued ( U 238 / Pb* denotes percentage of radiogenic Pb 206 that of measured values of AS3 standard and published U and Th concentrations. Disc.% is the individual discordance % 2505 44.0 2433 2399 99.4 2438 2412 47.2 Age (Ma) Age (Ma) Table 4 206 2357 2496 2365 2404 2363 20.5 2436 2406 2447 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 251

A subgroup of analyses (eight analyses from from 0.1–1.1. This population displays a range eight grains) have 207Pb/206Pb ages between from reversely to normally discordant with a 2600–2650 Ma (Fig. 5; Table 3). These analyses maximum of 104–93%, respectively. An error are all from texturally well-defined zircon cores weighted regression (MSWD=2.1) gives an up- (e.g. Fig. 4a–c), and have U and Th contents per concordia intercept of 2546913 Ma, with between 250–840 ppm and 70–480 ppm re- an imprecise lower intercept of 459640 Ma. spectively, with Th/U of between 0.1–0.9. Analy- Most analyses form a population with 207Pb/ ses from this population range from slightly 206Pb ages around 2450–2470 Ma (Fig. 7; reversely discordant (102%, one analysis), to nor- Table 4 and including previously published data mally discordant (90%). The data approximates from Carson et al., 2002). Many of these analy- a linear trend (MSWD=4.7) with an upper con- ses are from mantles and low-U outer rim sites, cordia intercept of 2626928 Ma and a very although several are from defined cores and imprecise lower intercept of 16491200 Ma. may represent isotopically reset magmatic cores Another small subgroup of analyses (10 analy- (Fig. 4d). No concordant age could be calculated ses from 10 grains) has 207Pb/206Pb ages between for these data (Ludwig, 1998), but they form 2529–2575 Ma (Fig. 6; Table 3). Analysis sites a near linear trend, with some excess scatter, of this population are well defined variably- for which an error weighted regression (n=115, zoned cores (Fig. 4e) or mantles that overgrow MSWD=4.0) gives an upper concordia intercept cores of 2600 Ma age (Fig. 4c). Uranium and of 2476.999 Ma with a lower intercept Th contents are variable, 320–1100 ppm and of 5319200 Ma. In further considering these 60–810 ppm respectively, with Th/U ranging data, we subdivided these analyses into ei-

Fig. 5. Conventional concordia diagram showing zircon U–Pb isotope data from a population of old well-deﬁned cores (Fig. 4a–c). Line is an error-weighted regression with an upper intercept of 2626928 Ma and a poorly deﬁned lower intercept of 16491200 Ma. 252 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263

Fig. 6. Conventional concordia diagram showing zircon U–Pb isotope data from a selected population of analyses that form an error-weighted regression with an upper intercept of 2546913 Ma and a poorly defined lower intercept of 459640 Ma. ther high Th/U types or outer rims (which may 6. Discussion include analyses with high Th/U). Analyses with high Th/U(\1.2) define a linear trend with an Prior to discussing the zircon isotopic data error weighted regression with no excess scatter below, it is appropriate to deliberate on the chem- (n=52, MSWD=1.03) and an upper concordia ical characteristics of the Tonagh Island or- intercept of 2483.099 Ma (Fig. 8). Common Pb thogneiss (Table 1). Sheraton and Black (1983) / is typically higher in analyses with elevated Th U, made two broad distinctions of felsic to interme- / and with lower U contents (Table 2). High Th U diate orthogneiss units within the Napier Com- analysis sites are commonly broad mantles, but plex, ‘undepleted’ and ‘depleted’ varieties, the may also include occasional narrow outer rims latter defined by strong depletion of a variety of and, less commonly, indistinct unzoned cores. All elements and also on the basis of various elemen- low U outer rims (with ‘dark’ BSE response; Fig. tal ratios, including Th/U and chondrite nor- 4b–g), regardless of Th/U, were regressed to- malised Ce /Y . Sheraton and Black (1983) gether and show a linear trend with no excess N N scatter (n=24, MSWD=0.67) and an upper in- concluded that depleted varieties acquired their tercept of 2455916 Ma (Fig. 9). It is tempting to particular geochemical signature primarily as a suggest that the narrow outer rims may represent result of partial melting of a garnet-bearing mafic the later stage of a progression of zircon growth source, rather than resulting from UHT metamor- (2455 Ma) which commenced with the growth phism, though this effect was also considered. of the high Th/U group at 2483 Ma. Treatment This geochemical discrimination was used by as separate populations is supported by a MSWD Sheraton and Black (1983) to characterise intru- of B1 for each group (high Th/U and the outer sive suites that pre-dated UHT metamorphism. rim groups) when regressed separately. However, it should be stressed that chemical sig- C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 253 natures alone cannot provide a robust criteria for conclude that the Tonagh Island orthogneiss be- assessing orthogneiss emplacement relative to longs to the ‘depleted’ class of felsic orthogneisses UHT metamorphism. The purpose of the follow- defined by Sheraton and Black (1983). ing geochemical information is to further describe The small population of clearly defined zircon the nature of the orthogneiss under investigation. cores (n=8) with an upper intercept age of In presenting this data, we do not imply any timing 2626928 Ma (Fig. 5) has Th/U values typically relationships of orthogneiss emplacement relative around 0.3–0.5 (with three analyses at 0.1, 0.8 and to UHT conditions. 0.9), values typical of magmatic zircon (Maas et The Tonagh Island orthogneiss illustrates many al., 1992). It is also significant to note that there of the defining characteristics of ‘depleted’ or- are no ages older than 2650 Ma in 133 analyses thogneisses. This unit has low values of Y relative of 70 zircon grains from this orthogneiss, preclud- to TiO2 (cf. Fig. 10 of Sheraton and Black (1983)), ing any significant prior geological history. The low Th (5.8 ppm) and U (0.4 ppm) and very high magmatic values of Th/U, together with the well- Th/U (14.5; Fig. 13 of Sheraton and Black (1983)). defined, micron-scale oscillatory zoned nature of

Relative to SiO2, Ce, Nb, Ti, Zr, Rb and Y all are the cores suggests an igneous origin. We conclude comparable with the depleted orthogneiss ﬁelds that the emplacement of this voluminous ‘de- shown in Fig. 5 of Sheraton and Black (1983). pleted’ felsic intrusive on Tonagh Island occurred Based on these geochemical characteristics, we at 2626928 Ma.

Fig. 7. Conventional concordia diagram showing all remaining zircon U–Pb isotope data. These data approximate a linear tread (n=115; MSWD=4.0) and an error-weighted regression yield an upper intercept of 2476.999 Ma and a lower intercept of 5319200 Ma. The diagram is visually dominated by 59 analyses with large error ellipses; such analyses typically include high Th/U and low U outer rims, that have elevated common Pb levels and have correspondingly large Pb/U errors. The high Th/U and low U outer rim analyses are separately treated in Figs. 8 and 9 respectively. The inset at lower right shows a selected subset (n=56) of U–Pb analyses with Pb* contents \99% and correspondingly smaller errors. 254 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263

Fig. 8. Conventional concordia diagram showing zircon U–Pb isotope data from high Th/U(\1.2) analyses, a subdivision of the data presented in Fig. 7. These analyses form an error-weighted regression, with no excess scatter (n=52; MSWD=1.03) with an upper intercept of 2483.099 Ma and a poorly deﬁned lower intercept of 3709350 Ma.

The population of ages with an upper concor- tain that the geological significance of this period dia intercept of 2546913 Ma (Fig. 6) is some- of zircon growth is currently unclear given the what problematic. Shirashi et al. (1997) also noted limited geological and isotopic information on a small population of zircons with 207Pb/206Pb this event. ages 2550 Ma in their analysis of a quartz- feldspar gneiss from layered paragneiss sequence on northern Tonagh Island (Fig. 2), but were 6.1. Significance of the 2450–2470 Ma ages and unclear on the significance of these. They specu- the timing of UHT metamorphism lated that the population may represent a episode The majority of zircon U–Pb analyses from of new zircon growth. Recent SHRIMP analysis this study form a population that have an upper on zircons extracted from a leucosome from concordia intercept 2450–2470 Ma, statistically McIntyre Is (Fig. 1) also yield U–Pb ages of identical with many isotopic ages from across the 2560–2590 Ma (S.L. Harley; unpublished data). Napier Complex. Partial resetting of old zircon The growth of new zircon is also suggested at this U–Pb systems from orthogneiss protoliths results time (S.L. Harley personal communication). in (generally imprecise) lower intercepts at These interpretations are consistent with our ob- 2400–2500 Ma (Black and James, 1983; Williams servations in that analysis sites that yield 2550 et al., 1984; Black et al., 1986a,b; Harley and Ma represent either cores (Fig. 4e) or overgrowths Black, 1997), interpreted to indicate a period of (Fig. 4c) on older 2600 Ma cores. We acknowl- major isotopic disruption. Upper intercepts of edge that there was a period of new zircon growth other zircon populations also commonly indicate at 2546913 Ma, with formation of new grains ages of 2450–2470 Ma, which has been inter- and overgrowths on 2600 Ma cores, but main- preted as a period of zircon growth or of the C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 255 complete resetting of pre-existing zircon (e.g. that UHT metamorphism occurred 400 Ma Black et al., 1983a, 1986a,b). Rb–Sr and Sm–Nd earlier. Grew (1998) maintains that the timing of whole-rock isochrons ages also indicate a period UHT metamorphism can be readily explained in of extensive isotopic re-equilibration at this time terms of a short-lived metamorphic event at (e.g. Black and James, 1983; Black et al., 1983a; 2450–2480 Ma in contrast to the much earlier

Black and McCulloch, 1987; Owada et al., 1994; D1 –D2 tectonothermal evolution preferred by Tainosho et al., 1994, 1997). The interpretation of Harley and Black (1987, 1997). This alternative this strong isotopic signature at 2450–2480 interpretation is largely based on the preservation

Ma, recorded from a wide variety of rock types of UHT assemblages within ‘syn-D3’ pegmatites and isotopic systems is critical to models of the (Grew, 1998) and syn-D2 UHT pegmatites (Grew geochronological-metamorphic framework of the et al., 1982; Sandiford and Wilson, 1984; Sandi- Napier Complex. ford and Powell, 1986b) both of which yield up- Black et al. (1983a), for example, maintained per intercept ages of 2500 Ma (Grew et al., that the widespread and strong isotopic signature 1982). These contrasting interpretations must be at 2460 Ma is due to the pervasive effects of D3 addressed within the context of the presently and recrystallisation associated with the develop- available data and take into account the following ment of S3, and that the D1 –D2 and UHT evolu- facts. tion occurred 400 Ma earlier, during the mid to (1) In the Field Islands (Fig. 1) the effects of D3 late Archaean. In contrast, Grew (1998) acknowl- are reported to be locally intense (Black et al., edges that while all studies seem to agree that 1983a) and proceeded at upper-amphibolite facies isotopic systems were highly disturbed and reset conditions (650–720 °C and 5–8 kbar; Harley at 2450–2480 Ma, he questions the assertion and Black, 1987). This D3 event is thought to

Fig. 9. Conventional concordia diagram showing zircon U–Pb isotope data from low U outer rim analyses only, a subdivision of the data presented in Fig. 7. These analyses form an error-weighted regression, with no excess scatter (n=24; MSWD=0.67) with an upper intercept of 2455916 Ma and a poorly deﬁned lower intercept of 6289580 Ma. 256 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263

Fig. 10. Summary concordia diagram of the U–Pb isotopic results from two pre-UHT orthogneiss units (Tonalitic Gneiss, Mt Sones, Black and James, 1983; Williams et al., 1984; Harley and Black, 1997; and a charnockitic leucogneiss from Fyfe Hills, Black and James, 1983). Of importance here is the lack of disturbance at the timing of UHT metamorphism (2820–2850 Ma) favoured by Harley and Black (1997); indicated by the solid star). Although the lower intercept indicated by the study of Williams et al. (1984) is somewhat younger than 2500 Ma (20309170 Ma), that study concluded the lower intercept was the likely result of disturbance during an event at 2400 Ma, but did not elaborate on the nature of that event. have been responsible for extensive resetting of observations suggest that the orthogneiss was em- Rb–Sr whole-rock and U–Pb zircon systematics placed into the crust prior to UHT conditions, at 2450–2480 Ma. However, intensity of D3 and subsequently endured the entire D1 –D2 struc- across the Napier Complex is quite varied, yet this tural evolution, including UHT metamorphism. strong isotopic imprint at 2450–2480 Ma is Since orthogneiss emplacement, based on the data almost always present. At the sample location for presented here, occurred at 2626928 Ma, UHT this study (Fig. 3), for example, there is no local metamorphism and D1 cannot be older than this. penetrative fabric development or any deforma- (3) A more general observation from the nu- tion that can be attributed to D3, yet the upper merous isotopic studies is that a number of zir- intercept age for the majority of analyses pre- con U–Pb systems, which date the emplacement / sented here ranges between 2455–2483 Ma. of pre-D1 UHT orthogneiss units, show little or (2) The sampled orthogneiss on Tonagh Island no isotopic disturbance at the timing of UHT contains a pervasive, generally flat-lying gneissos- conditions favoured by either Black et al. (1983a) ity, S1, that parallels a S1 gneissosity that is or Harley and Black (1997) (Black and James, defined by UHT assemblages in adjacent parag- 1983, Fig. 3; Black et al., 1983b, 1986b, Fig. 6; neiss sequences. The orthogneiss also contains Williams et al., 1984, Fig. 3; Fig. 4; Harley abundant (10–20% modal) mesoperthite. These and Black, 1997, Fig. 3a and b). These units C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 257 instead show discordia with (albeit imprecise) sen Bay are not time equivalents, although this lower intercepts consistent with disturbance at presents no particular geological difficulty. 2400–2500 Ma (Fig. 10). Indeed, the possibility of high-grade polymeta- Based on our data and observations, UHT morphism in the Napier Complex has been previ- metamorphism must have post-dated emplace- ously explored by Hensen and Motoyoshi (1992) ment of the Tonagh Island orthogneiss at 26269 and Harley and Black (1997). In one proposal, 28 Ma. There are two periods of zircon growth Hensen and Motoyoshi (1992) suggested that sep- subsequent to 2626928 Ma identified in this arate overprinting high temperature events may study, one at 2546913 Ma and again at 2476.99 offer a better explanation for petrological features

9 Ma (taking the weighted mean of all analyses of associated with D1 and D2 at Mt Riiser-Larsen 2450–2480 Ma as shown in Fig. 7); UHT meta- (Fig. 1), considering that the time span originally morphism must have occurred during one of these proposed for the D1 –D2 evolution ( 3072–2850 episodes. Given the intense isotopic disturbance in Ma) is considered too long to sustain tempera- a number of isotopic systems across the entire tures in excess of 900 °C (Harley and Hensen, Napier Complex at 2450–2480 Ma and the 1990). Harley and Black (1997) also address the absence of any signiﬁcant D3 effects at the sample possibility of multiple tectonothermal events. locality (point 1), we propose that UHT metamor- Those authors propose that a high-grade fabric 9 phism and D1 –D2, at least in the Tonagh Island forming event at 2980 9 Ma, synchronous region, occurred at 2450–2480 Ma, supporting with their revised timing of emplacement of the the assertion of Grew (1998). We acknowledge Proclaimation Island orthogneiss, occurred in that the growth of zircon at 2546913 Ma may northern regions of the terrane. This fabric form- indicate a metamorphic episode at this time. How- ing event was superseded by D1 –D2 and UHT ever, we discount the possibility that UHT meta- metamorphism in the Tula-Scott mountains re- morphism occurred at this time, given the paucity gion, at 2850–2820 Ma, the time of UHT fa- of data of 2550 Ma from presently available voured by Harley and Black (1997). While these chronological studies of the Napier Complex. discussions do not directly relate to the suggestion Our conclusion contrasts with the structurally- offered here, in that UHT metamorphism in the constrained chronological framework presented Amundsen Bay region at 2470 Ma post-dated by, for example, Harley and Black (1997) which an earlier episode UHT metamorphism in the argues for UHT metamorphism at 2850–2820 Tula-Scott mountains region at 2850–2820 Ma,

Ma. The timing of syn-D1 felsic intrusives, in they do indicate willingness of researchers to ad- particular the Dallwitz Nunatak orthogneiss ( dress the possibility of polymetamorphic events in 2850 Ma), is central to the calibration of the order to account for inconsistencies in the petro- revised chronological framework of Harley and graphic, chronological and structural framework Black (1997). In order to reconcile these differ- for the Napier Complex. ences, one might argue for multiple episodes of In spite of the abundance of isotopic data col- UHT metamorphism across the Napier Complex. lected in the Napier Complex, there remains in- An episode of UHT metamorphism that peaked in sufficient data to fully test and assess the the eastern periphery of the Tula Mountains at possibility of two UHT events, one at 2850–2820 2850 Ma (Harley and Black, 1997) may have been Ma and another at 2450–2480 Ma in the post-dated by a younger episode of UHT meta- Amundsen Bay area. We believe that another morphism at 2470 Ma in the Amundsen Bay difficulty with this suggestion is that it fails to region, and which also exerted a strong isotopic explain point 3 above, in that some lithologies overprint across the terrane and cumulated in the located in the eastern Tula Mountains, such as the regional development of upright D3 fold struc- Tonalitic Gneiss at Mt Sones and the Gage Ridge tures at upper amphibolite grade. This suggestion orthogneiss, show no clear evidence of isotopic would imply that the D1 –D2 structural features disturbance at the timing of UHT metamorphism between the eastern Tula Mountains and Amund- favoured by Harley and Black (1997). It would 258 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 seem unusual that UHT metamorphism at However, in a zircon U–Pb ion probe study 2850 Ma (Harley and Black, 1997), an event of of the Scandinavian Caledonides, Williams and extreme metamorphic conditions (\1100 °C) Claesson (1987) concluded that the growth of and intense pervasive deformation, across a con- zircon, with characteristically low Th/U, oc- siderable region, left an comparatively minor curred during the transitional amphibolite–gran- isotopic imprint and only in some rock types. In ulite Caledonian orogeny, at which time Th and addition, one might argue that it seems equally U were mobilised. They discounted the alterna- unusual that D3, a relatively lower grade event tive possibility that loss of Th and U from the 0 ( 700 °C), with marked heterogenous struc- host rock occurred prior to the growth of the tural distribution, resulted in a widespread, re- low Th/U zircon. They based this on the obser- markably consistent, strong isotopic imprint at vation that a spectrum of pre-existing Precam- 2450–2480 Ma in virtually all rock types and brian zircon grains, with ‘‘normal’’ Th/U isotopic systems (e.g. Sandiford and Wilson, (typically 0.1–0.5), have Caledonian age lower 1984; Sheraton et al., 1987). intercepts, a period of metamorphism during which new zircon growth occurred, characterised 6.2. High Th/U zircons by low Th/U(B0.1). An intriguing feature of the zircon data of Similarly, following Williams and Claesson Black et al. (1986b), and of this study, is the (1987), we propose that the zircon population / presence of a near concordant zircon population with high Th U described here characterises zir- with an unusually high Th/U(\1.2). Many zircon growth during UHT metamorphism. Al- con analyses of the volumetrically dominant though the example of Williams and Claesson mantle and outer rims in this study have high (1987) specifically addresses the growth of low Th/U (Table 4; Carson et al., 2002) which, if Th/U zircon during Palaeozoic metamorphism regressed as a separate population, yield an up- in the Scandinavian Caledonides, the logic of per intercept age of 2483.099Ma(n=52, Fig. their conclusion is relevant here. We suggest 8). This age is identical to that reported by that the actinides were mobilised during UHT Black et al. (1986b) for a near concordant pop- metamorphism, with preferential extraction of U ulation of high Th/U(\1.2) ‘structureless crys- over Th from the terrane, providing a chemical tals’ which yielded a ‘preferred’ 207Pb/206Pb environment during peak conditions that facili- mean age of 2479923 Ma. Although exhaustive tated the growth of high Th/U zircon. Discor- studies of the processes and conditions con- dance trends in much older, \2470 Ma, / trolling zircon Th U remain to be conducted, in pre-existing zircon populations reported in other general, zircons grown under metamorphic con- U–Pb studies from the Napier Complex fre- / B ditions frequently have low Th U, typically quently have (albeit imprecise) lower intercepts 0.05–0.1 (Williams and Claesson, 1987; Maas et that coincide with the growth of the high Th/U al., 1992). Indeed, reported Th/U values greater zircon population at 2470 Ma (see point 3 than unity are quite uncommon, and the high above). We reaffirm, therefore, that the upper Th/U zircon population presented here, and in intercept age of 2483.099 Ma for the high Th/ Black et al. (1986b), clearly indicates the forma- U analyses (Fig. 8) may indicate the best esti- tion of zircon in an unusual chemical environment. Black et al. (1986b) argued that the mate for the timing of UHT metamorphism. growth of zircon with such unusual chemistry is The low U outer rims (Fig. 9) yield an upper 9 due to crystallisation in an environment depleted intercept of 2455 16 Ma, which is significantly / in U relative to Th. Those authors also argue distinct from the high Th U zircon population that depletion must have occurred prior to crys- (2483.099 Ma). Zircon from the low U popula- tallisation of this distinctive zircon population, tion are restricted to the outer margins of zircon during UHT metamorphism at 3070 Ma. grains (e.g. Fig. 4b–g) and are generally charac- C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 259 terised by low U (and relatively low Th) con- 6.4. Significance of post-2460 Ma lower intercept tents. The low U outer rims may result from ages zircon growth during continued U depletion of the terrane resulting from UHT metamorphism. The present study also permits some discussion Based on the textural context of the low U zircon of the significance of post-2460 Ma lower inter- (as rim material), we suggest that the low U cepts reported in many U–Pb studies of the outer rim zircon represents the final period of Napier Complex. A number of imprecise U–Pb zircon growth that occurred during waning UHT lower intercepts at 1000 Ma have been docu- metamorphism. mented from the Field Islands, near the boundary with the NeoProterozoic Rayner Com- 6.3. Re6erse discordance plex (Fig. 1). These lower intercepts have been attributed to isotopic disturbance in response to It should be noted that a number of the U–Pb 1000 Ma granulite-facies tectonothermal activ- analyses shown in Fig. 7 are reversely discordant. ity in the adjacent Rayner Complex (Grew et al., Although discussion of the significance of the 1982; Black et al., 1983a, 1986b; Sandiford and observed reverse discordance for this dataset Wilson, 1984). However, Shirashi et al. (1997) is presented elsewhere (Carson et al., 2002), a re-examined the chronological evolution of the brief summary is offered here. Reverse discor- western-most regions of the Rayner Complex and dance is considered to be either genuine, resulting showed that, immediately adjacent to the Napier from intragrain net excess of Pb* due to redistri- Complex, the Rayner Complex experienced high- bution during a geological disturbance (e.g. grade metamorphism during the Early Palaeozoic Williams et al., 1984; Harley and Black, 1997) (500–550 Ma). Shirashi et al. (1997) proposed or an analytical artefact due to differential ioni- that the western extremities of the Rayner Com- sation efficiencies between standard and un- plex instead represent the eastward extension of known resulting in enhanced Pb ion yields the Early Palaeozoic Lu¨tzow–Holm Complex (e.g. McLaren et al., 1994; Wiedenbeck, 1995). and that the western Rayner Complex may in Carson et al. (submitted for publication) con- fact represent a collage of NeoProterozoic and cluded that reverse discordance exhibited by a Early Palaeozoic terranes, including 770 Ma subset of the zircon population illustrated in Fig. felsic intrusive bodies. This scenario clearly intro- 7 is genuine, based on a non-zero lower intercept, duces some complexity in the interpretation and as defined by the spread of both normal significance of the 1000 Ma lower intercept and reversely discordant data, that coincides with ages reported from within the Napier Complex. a known episode of geological disturbance Although, imprecise due to extended extrapola- (Section 6.4), and evidence of sub-micron scale tions to concordia, the U–Pb zircon analyses Pb and U heterogeneities. An analytical artefact presented here (Fig. 7) indicate an Early rationale for the observed reverse discordance Palaeozoic lower intercept (see also Grew and was discounted, primarily on the absence of Manton, 1979). Carson et al. (2002) concluded a zero lower intercept; as a zero intercept is that, for the orthogneiss studied here, zircon dis- a characteristic feature of differing ionisation cordance was directly related to disturbance due efficiencies between the standard and a set of to aqueous fluid infiltration, at upper-amphibolite unknowns (e.g. McLaren et al., 1995; Wieden- grade (8kband670 °C; Carson et al., sub- beck, 1995). It should be noted, however, mitted for publication), accompanying emplace- that regardless of the nature of reverse discor- ment of an Early Palaeozoic pegmatite. This dance, real or analytical artefact, the upper inter- raises the possibility that post-2450 Ma concor- cept age is still considered robust (Wiedenbeck, dia lower intercepts may result, at least in part, 1995), and the geochronological information from isotopic disturbance due to aqueous fluid derived from upper intercepts in this study re- infiltration into the Napier Complex during the main valid. Early Palaeozoic, particularly for lithologies 260 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 where there is demonstrable hydration due to followed by a major episode of growth com- proximity to Early Palaeozoic pegmatites. For mencing with the high Th/U mantles at example, based on the description of the biotite- 2483.099 Ma and terminating with outer bearing ‘grey gneiss’ by Black et al. (1983a) from rim development at 2455916 Ma. Ayatollah Island, Casey Bay, that unit is clearly (c) UHT metamorphism occurred, at least in the related to the proximity of a large pegmatite body Tonagh Island region, synchronous with the of Early Palaeozoic age (Rb–Sr whole-rock age of growth of high Th/U zircon at 2483.099 522910 Ma; Black et al., 1983a). Almost cer- Ma, with zircon growth continuing at least tainly, the formation of this ‘grey gneiss’ resulted until 2455916 Ma as represented by the from hydration and recrystallisation of the anhy- growth of low-U outer rims. We find no drous ‘pink gneiss’ by influx of aqueous fluids compelling evidence that supports UHT accompanying pegmatite emplacement (Grew and metamorphism at either 2840 or 3070 Manton, 1979; Carson et al., 2002). This conclu- Ma. sion is supported by a Rb–Sr whole-rock (d) Post 2460 Ma zircon lower intercepts re- isochron from this ‘grey gneiss’ which yields a ported from the Napier Complex are com- somewhat imprecise age of 6509145 Ma (Black monly considered to result from isotopic et al., 1983a). Although the lower intercept based disturbance during adjacent Rayner Complex on conventional U–Pb zircon analyses from this tectonothermal activity (1000 Ma). We unit is 1183 +433/−446 Ma (Black et al., stress that such lower intercepts may also be 1983a), a likely cause of zircon discordance is the complicated by isotopic disturbance due to influx of magmatic fluids due to the emplacement aqueous fluid influx, at elevated conditions, of the nearby Early Palaeozoic pegmatite. Al- into the Napier Complex which accompanied though, some well-defined lower intercepts are Early Palaeozoic pegmatite emplacement at clearly related to the effects of tectonothermal 500 Ma. activity of Rayner age (Black et al., 1984), the interpretation of other less precise lower intercepts and ‘geologically meaningless’ lower inter- Acknowledgements cepts (Black et al., 1986a, Fig. 7b and c) across the Napier Complex are problematic in this light, CJC acknowledges participation in the 40th and should address the possibility of superim- Japanese Antarctic Research Expeditions and the posed effects of fluid influx into the Napier Com- SEAL (Structure and Evolution of East Antarctic plex, during the Early Palaeozoic. Lithosphere) program, at the kind invitation of K. Shiraishi and Y. Motoyoshi of the National Insti- tute of Polar Research, Toyko. CJC also wishes to 7. Conclusions thank the expeditioners of JARE-39 and -40, particularly the Tonagh Island field party, and the The zircon U–Pb data presented from Tonagh officers and crew of the icebreaker Shirase, for the Island provide additional constraints on the tim- hospitality extended to him during JARE-40. As- ing of UHT metamorphism within the Napier sistance by, and discussion with, T.M Harrison, Complex, East Antarctica, which has been a topic M. Grove, K. McKeegan and C.M. Breeding of debate for two decades. We conclude that: during ion probe sessions at the Department of (a) the emplacement of a major felsic orthogneiss Earth and Space Sciences, UCLA, and with J. body on Tonagh Island, occurred at 26269 Eckert during electron microprobe sessions at 28 Ma, prior to the onset of UHT metamor- Yale University is appreciated. E.S. Grew pro-

phism and intense D1 –D2 deformation. vided the photograph in Fig. 3 and comments on (b) Two episodes of zircon growth occurred sub- early version of the manuscript. The manuscript sequent to orthogneiss emplacement. A minor beneﬁted from valuable and constructive reviews episode of growth occurred at 2546913 Ma, from Simon Harley and an anonymous reviewer. C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 261

CJC acknowledges receipt of the Damon Wells facies rocks from Fyfe Hills, Enderby Land, Antarctica. fellowship at Yale University; analytical costs Nature 298, 614–618. Ellis, D.J., 1980. Osumilite–sapphirine–quartz granulites from were supported by National Science Foundation Enderby Land, Antarctica: P–T conditions of metamor- grant EAR-0001084 and -9810089. phism, implications for garnet–cordierite equilibria and the evolution of the deep crust. Contrib. Mineral Petrol. 74, 201–210. References Ellis, D.J., Sheraton, J.W., England, R.N., Dallwitz, W.B., 1980. Osumilite–sapphirine–quartz granulites from En- derby Land, Antarctica—mineral assemblages and reac- Black, L.P., James, P.R., 1983. Geological history of the tions. Contrib. Mineral Petrol. 72, 123–143. Archaean Napier Complex in Enderby Land. In: Oliver, Grew, E.S., 1980. Sapphirine+quartz association from Ar- R.L., James, P.R., Jago, J.B. (Eds.), Antarctic Earth Sci- chaean rocks in Enderby Land, Antarctica. Am. Mineralo- ence. Cambridge University Press, Cambridge, pp. 11–15. gist 65, 821–836. Black, L.P., James, P.R., Harley, S.L., 1983a. Geochronology Grew, E.S., 1982. Osumilite in the sapphirine–quartz terrane and geological evolution of metamorphic rocks in the Field of Enderby Land, Antarctica: implications for osumilite Islands area, East Antarctica. J. Metamorphic Geol. 1, petrogenesis in the granulite-facies. Am. Mineralogist 67, 277–303. 762–787. Black, L.P., James, P.R., Harley, S.L., 1983b. The geochronol- Grew, E.S., 1998. Boron and beryllium minerals in granulite- ogy, structure and metamorphism of early Archaean rocks facies pegmatites and implications of beryllium pegmatites at Fyfe Hills, Enderby Lands, Antarctica. Precambrian for the origin and evolution of the Archaean Napier Com- Res. 21, 197–222. plex of East Antarctica. Memoirs of National Institute of Black, L.P., Fitzgerald, J.D., Harley, S.L., 1984. Pb isotopic Polar Research, Special Issue, 53, 74–92. composition, colour, and microstructure of monazites from Grew, E.S., Manton, W.I., 1979. Archaean rocks in Antarc- a polymetamorphic rock in Antarctica. Contrib. Mineral tica: 2.5-billion-year uranium–lead ages of pegmatites in Petrol. 85, 141–148. Enderby Land. Science 206, 443–445. Black, L.P., Sheraton, J.W., James, P.R., 1986a. Late Ar- Grew, E.S., Manton, W.I., Sandiford, M., 1982. Geochrono- chaean granites of the Napier Complex, Enderby Land, logical studies in East Antarctica: age of pegmatites in Antarctica: a comparison of Rb–Sr, Sm–Nd and U–Pb Casey Bay, Enderby Land. Antarct. J. US 17, 1–2. isotopic systematics in a complex terrain. Precambrian Harley, S.L., 1985a. Garnet–orthopyroxene bearing granulites Res. 32, 343–368. from Enderby Land, Antarctica: metamorphic pressure– Black, L.P., Williams, I.S., Compston, W., 1986b. Four zircon temperature–time evolution of the Archaean Napier Com- ages from one rock: the history of a 3930 Ma old granulite plex. J. Petrol. 26, 819–856. from Mt Sones, Enderby Land, Antarctica. Contrib. Min- Harley, S.L., 1985b. Paragenetic and mineral-chemical rela- eral Petrol. 94, 427–437. tionships in orthoamphibole-bearing gneisses from En- Black, L.P., McCulloch, M.T., 1987. Evidence for isotopic derby Land, east Antarctica: a record of Proterozoic uplift. re-equilibration of Sm–Nd whole-rock systems in Early J. Metamorphic Geol. 3, 179–200. Archaean crust of Enderby Land, Antarctica. Earth Planet Harley, S.L., 1987. Pyroxene-bearing granulites from Tonagh Sci. Lett. 82, 15–24. Island, Enderby Land, Antarctica: further evidence for Black, L.P., Harley, S.L., Sun, S.S., McCulloch, M.T., 1987. very high temperatures (\950 °C) Archaean regional The Rayner Complex of east Antarctica, Complex isotopic metamorphism in the Napier Complex. J. Metamorphic systematics within a Proterozoic mobile belt. J. Metamor- Geol. 5, 241–356. phic Geol. 5, 1–26. Harley, S.L., 1998. An appraisal of peak temperatures and Black, L.P., 1988. Isotopic resetting of U–Pb zircon and thermal histories in ultra high-temperature (UHT) crustal Rb–Sr and Sm–Nd whole-rock systems in Enderby Land, metamorphism: the signiﬁcance of aluminous orthopyrox- Antarctica: implications for the interpretation of isotopic ene. Memoirs of National Institute of Polar Research, data from polymetamorphic and multiply deformed ter- Special Issue, 53, 49–73. rains. Precambrian Res. 38, 355–365. Harley, S.L., Black, L.P. 1987. The Archaean geological evolu- Carson, C.J., Ague, J.J., Grove, M., Coath, C.D., Harrison, tion of Enderby Land, Antarctica. In: Park, R.G., Tarney, T.M., 2002. U-Pb isotopic behaviour of zircon during J. (Eds.), Evolution of the Lewisian and Comparable Pre- upper-amphibolite facies fluid inflitration in the Napier cambrian High-grade Terrains. Geological Society Special Complex, east Antarctica. Earth and Plantary Science Let- Publication No. 27, pp. 285–296. ters, in press. Harley, S.L., Hensen, B.J., 1990. Archaean and Proterozoic Dallwitz, W.B., 1968. Coexisting sapphirine and quartz in high-grade terranes of East Antarctica (40°E–60°E): a case granulites from Enderby Land, Antarctica. Nature 219, study of the diversity in granulite facies metamorphism. In: 476–477. Ashworth, J.R., Brown, M. (Eds.), High Temperature DePaolo, D.J., Manton, W.I., Grew, E.S., Halpern, M., 1982. Metamorphism and Crustal Anatexis. Unwin Hyman, Sm–Nd, Rb–Sr and U–Th–Pb systematics of granulite London, pp. 320–370. 262 C.J. Carson et al. / Precambrian Research 116 (2002) 237–263

Harley, S.L., Black, L.P., 1997. A revised Archaean chronol- Geophys. Res. 98, 13997–14013. ogy for the Napier Complex, Enderby Land, from Quidelleur, X., Grove, M., Lovera, O.M., Harrison, T.M., SHRIMP ion-microprobe studies. Antarct. Sci. 9, 74–91. Yin, A., Ryerson, F.J., 1997. The thermal evolution and Harley, S.L., Motoyoshi, Y., 2000. Al zoning in orthopyrox- slip history of the Renbu Zedong Thrust, southeastern ene in a sapphirine quartzite: evidence for \1120 °C Tibet. J. Geophys. Res. 102, 2659–2679. UHT metamorphism in the Napier Complex, Antarctica, Sandiford, M., 1985. The origin of retrograde shear zones in and implications for the entropy of sapphirine. Contrib. the Napier Complex: implications for the tectonic evolu- Mineral Petrol. 138, 293–307. tion of Enderby Land, Antarctica. J. Struct. Geol. 7, Hensen, B.J., Motoyoshi, Y., 1992. Osumilite-producing re- 477–488. actions in high temperature granulites from the Napier Sandiford, M., Wilson, C.J.L., 1984. The structural evolu- Complex, East Antarctica: tectonic implications. In: tion of the Fyfe Hills-Khmara Bay region, Enderby Yoshida, Y., Kaminuma, K., Shirashi, K. (Eds.), Recent Land, East Antarctica. Aust. J. Earth Sci. 31, 403–426. Progress in Antarctica Earth Science. Terra Sciencific Sandiford, M., Powell, R., 1986a. Pyroxene exsolution in Publishing Company, Tokyo, pp. 87–92. granulites from Fyfe Hills, Enderby Land, Antarctica: Kelly, N.M., Clarke, G.L., Carson, C.J., White, R.W., 2000. evidence for 1000 °C metamorphic temperatures in Ar- Thrusting in the lower crust: evidence from the Oygarden chaean continental crust. Am. Mineralogist 71, 946–954. Islands, Kemp Land, East Antarctica. Geol. Mag. 137, Sandiford, M., Powell, R., 1986b. The origin of Archaean 219–234. Lovering, J.F., 1979. The evidence for (4000 m.y. crustal gneisses in the Fyfe Hills region, Enderby Land; field material in Archaean times. J. Geol. Soc. Aust. 26, 268 occurrence, petrology and geochemistry. Precambrian abstract. Res. 31, 37–68. Ludwig, K.R., 1998. On the treatment of concordant ura- Sheraton, J.W., Black, L.P., 1981. Geochemistry and nium-lead ages. Geochim. Cosmochim. Acta 62, 315– geochronology of Proterozoic tholeiite dykes of East 318. Antarctica: evidence for mantle metasomatism. Contrib. Ludwig, K.R., 1999. User’s manual for Isoplot/Ex, v2.3, a Mineral Petrol. 78, 305–317. geochronological toolkit for Microsoft Excel. Berkeley Sheraton, J.W., Black, L.P., 1983. Geochemistry of Precam- Geochronological Centre Special Publication No. 1a, pp. brian gneisses: relevance for the evolution of the East 54. Antarctic Shield. Lithos 16, 273–296. McLaren, A.C., Fitzgerald, J.D., Williams, I.S., 1994. The Sheraton, J.W., Tingey, R.J., Black, L.P., Offe, L.A., Ellis, microstructure of zircon and its influence on the age de- D.J., 1987. Geology of Enderby Land and western Kemp termination from Pb/U isotopic ratios measured by ion Land, Antarctica. Bureau of Mineral Resources, Aus- microprobe. Geochim. Cosmochim. Acta 58, 993–1005. tralia, Bulletin, no. 223, pp. 51. Maas, R., Kinny, P.D., Williams, I.S., Froude, D.O., Comp- Shirashi, K., Ellis, D.J., Fanning, C.M., Hiroi, Y., Kagama, ston, W., 1992. The earth’s oldest known crust: a H., Motoyoshi, Y., 1997. Re-examination of the meta- geochronological and geochemical study of 3900–4200 morphic and protolith ages of the Rayner Complex, Ma old detrital zircons from Mt. Narryer and Jack Hills, Antarctic: evidence for the Cambrian (Pan-African) re- Western Australia. Geochim. Cosmochim. Acta 56, gional metamorphic event. In: Ricci, C.A. (Ed.), The 1281–1300. Antarctic Region: Geological Evolution and Processes. Miyamato, T., Grew, E.S., Sheraton, J.W., Yates, M.G., Terra Antarctica Publication, Siena, pp. 79–88. Dunkley, D.J., Carson, C.J., Yoshimura, Y., Motoyoshi, Sobotovich, E.V., Kamenev, Y.N., Komaristyy, A.A., Rud- Y., 2000. Lamproite dykes in the Napier Complex at nik, V.A., 1976. The oldest rocks of Antarctic (Enderby Tonagh Island, Enderby Land, East Antarctica. Polar Land). Int. Geol. Rev. 18, 371–388. Geosci. 13, 41–59. Tainosho, Y., Kagami, H., Takahashi, Y., Iizumi, S., Os- Osanai, Y., Toyoshima, T., Owada, M., Tsunogae, T., anai, Y., Tsuchiya, N., 1994. Preliminary result for the Hokada, T., Crowe, W.A., 1999. Geology of Ultrahigh- Sm–Nd whole-rock age of the metamorphic rocks from Temperature metamorphic rocks from Tonagh Island in the Napier Complex, East Antarctica. Polar Geosci. 12, Mount Pardoe in the Napier Complex, East Antarctica. 1–28. Proc. NIPR Symp. Antarct. Geosci. 7, 115–121. Owada, M., Osanai, Y., Kagami, H., 1994. Isotopic equili- Tainosho, Y., Kagami, H., Hamamoto, T., Takahashi, Y., bration age of Sm–Nd whole-rock system in the Napier 19947. Preliminary result for the Nd and Sr isotope char- Complex (Tonagh Island), East Antarctica. Proc. NIPR acteristics of the Archaean gneisses from Mount Pardoe, Symp. Antarct. Geosci. 7, 122–132. Napier Complex, East Antarctica. Proc. NIPR Symp. Paces, J.B., Miller, J.D., 1993. Precise U–Pb ages of Duluth Antarct. Geosci. 10, 92–101. Complex and related mafic intrusions, Northeastern Min- Wiedenbeck, M., 1995. An example of reverse discordance nesota: geochronological insights to physical, petroge- during ion microprobe zircon dating—an artifact of en- netic, paleomagnetic, and tectonomagnetic processes hanced ion yields from a radiogenic labile Pb. Chem. associated with the 1.1 Ga midcontinental rift system. J. Geol. 125, 197–218. C.J. Carson et al. / Precambrian Research 116 (2002) 237–263 263

Williams, I.S., Compston, W., Black, L.P., Ireland, T.R., Williams, I.S., Claesson, S., 1987. Isotopic evidence for the Foster, J.J., 1984. Unsupported radiogenic Pb in zircon: a Precambrian provenance and Caledonian metamorphism cause of anomalously high Pb–Pb, U–Pb and Th–Pb of high grade paragneisses from the Seve Nappes, Scandi- ages. Contrib. Mineral Petrol. 88, 322–327. navian Caledonides. Contrib. Mineral Petrol. 97,205–217.