Zircon captures exhumation of an ultrahigh-pressure terrane, North-East Greenland Caledonides

Gondwana Research 25 (2014) 235–256

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Zircon captures exhumation of an ultrahigh-pressure terrane, North-East Greenland Caledonides J.A. Gilotti a,⁎, W.C. McClelland a, J.L. Wooden b a b

Department of Geoscience, University of Iowa, Iowa City, IA 52242, USA Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA

a r t i c l e

i n f o

Article history: Received 28 September 2012 Received in revised form 12 March 2013 Accepted 14 March 2013 Available online 13 April 2013 Handling Editor: A. Kröner Keywords: Ti-in-zircon thermometry Trace elements Ultrahigh-pressure metamorphism U–Pb geochronology Zircon

a b s t r a c t Zircon from the North-East Greenland ultrahigh-pressure (UHP) terrane formed over a 45 million year period from peak UHP conditions through the amphibolite facies. Our study utilizes sensitive high resolution ion microprobe-reverse geometry (SHRIMP-RG) mass spectrometry to assess the multiple ages and trace element patterns preserved in zircon from samples chosen to capture the exhumation history of these rocks. Peak UHP conditions from 365 to 350 Ma are derived from coesite-bearing samples, while a suite of progressively retrogressed quartzofeldspathic host gneisses and late-stage, leucocratic melts emplaced into the gneisses track exhumation. Melting occurred during all stages of exhumation, beginning with H2O-absent dehydration melting of phengite on the decompression path. A garnet-bearing leucosome in the neck of a kyanite-eclogite boudin that gives an age of 347 Ma is taken as the beginning of phengite melting. Leucosomes formed in HP granulite to amphibolite facies gneisses between 350 and 340 Ma, and fluid assisted melting continued until 320 Ma in the form of late, cross cutting pegmatites. Changes in the zircon trace element patterns are linked to decreasing temperature, and show that significant new zircon grew during melting on the exhumation path. Zircon cores recording protolith ages generally preserve magmatic temperatures (700 °C) and typical igneous REE patterns (Yb/Gd = 10). UHP/HP eclogite-facies zircon records higher T (900 °C) and flat HREE patterns (Yb/Gd = 1). Granulite to amphibolite facies zircon in quartzofeldspathic gneisses records both flat (Yb/Gd = 1) and steep (Yb/Gd = 100) HREE patterns at ca 700 °C suggesting the variable effects of garnet during decompression. Amphibolite facies pegmatites and leucosomes document a transition from moderate HREE (Yb/Gd = 10) at 700 °C to steep HREE (Yb/Gd = 100–1000) patterns at 600 °C. The pronounced steepening of the HREE patterns is attributed to garnet breakdown during amphibolite-facies metamorphism. The 30–50 million year spread of ages observed in individual samples records multiple periods of zircon growth and is interpreted as a characteristic signature of slowly exhumed UHP terranes. The data show that zircon ages combined with trace element and textural characterization of zircon from a broad suite of samples can successfully define the exhumation history of UHP terranes. © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction Subduction of continental crustal material to mantle depths is now a generally accepted phenomenon in collisional orogens (see Liou et al., 2009 for review). Major advancements in understanding the timing of continental subduction and exhumation have come from studies of zircon, which can grow at almost any stage on the pressure–temperature (P–T) path related to deep continental subduction depending on protolith lithology (Harley et al., 2007; Rubatto and Hermann, 2007). This is in part due to the fact that many ultrahigh-pressure (UHP) terranes experience partial melting during exhumation (see Zheng et al., 2011 for review), allowing for multiple zircon growth events (Hermann et al., 2001). Studies that utilize U–Pb ion microprobe geochronology on ⁎ Corresponding author at: Department of Geoscience, 121 Trowbridge Hall, University of Iowa, Iowa City, IA 52242, USA. Tel.: +1 319 335 1097; fax: +1 319 335 1821. E-mail address: [email protected] (J.A. Gilotti).

distinct cathodoluminescence (CL) domains that contain index mineral inclusion suites are particularly effective in linking age data to the P–T path (Gebauer et al., 1997; Hermann et al., 2001; Katayama et al., 2001). Examination of rare earth element (REE) concentrations in zircon has proven useful in relating age information to P–T conditions (Rubatto, 2002; Mattinson et al., 2006; Wu et al., 2008; Mattinson et al., 2009), as well as Ti-in-zircon thermometry (Watson and Harrison, 2005; Watson et al., 2006; Ferry and Watson, 2007). The application of these tools to a broad spectrum of lithologies can provide a chronology for exhumation of UHP terranes, despite the pitfalls in linking specific analytical spots in zircon to points on a P–T path (e.g., O'Brien, 2006). Estimates for the duration of formation and exhumation of UHP terranes vary considerably from relatively large provinces including the Western Gneiss region (Norway), Dabie-Sulu (China) and North Qaidam (China), which show exhumation rates of b10 mm/yr (Hacker et al., 2006; Mattinson et al., 2006; Song et al., 2006; Kylander-Clark et al., 2009; Mattinson et al., 2009; Zhang et al., 2009; Kylander-Clark et al.,

1342-937X/$ – see front matter © 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gr.2013.03.018

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a suite of progressively retrogressed quartzofeldspathic host gneisses and late-stage, leucocratic melts emplaced into the gneisses should track exhumation. Ti-in-zircon thermometry is correlated with trace element patterns to demonstrate that significant zircon grew in melts during decompression from UHP.

2012), to more quickly exhumed terranes (>10 mm/yr) including Kokchetav (Kazakhstan), Erzgebirge (Germany), Dora Maira (Italy), and Papua New Guinea (Hermann et al, 2001; Rubatto and Hermann, 2001; Massonne et al., 2007; Baldwin et al., 2008; Gordon et al., 2012). Evidence for melting during UHP exhumation is present in the more slowly exhumed Western Gneiss region (Labrousse et al., 2002) and Dabie-Sulu terrane (Zheng et al., 2011), as well as in some of the more quickly exhumed terranes including Kokchetav (Shatsky et al., 1999), Erzgebirge (Massonne et al., 2007), and Papua New Guinea (Baldwin et al., 2008; Gordon et al., 2012). Zircon from the North-East Greenland UHP terrane formed over a 45 million year (m.y.) period from peak UHP conditions through the amphibolite facies (Gilotti and McClelland, 2007), indicating that slow exhumation took place. Partial melting during exhumation is recorded by metapelitic rocks (Lang and Gilotti, 2007) and the abundant leucosomes in the UHP gneisses (Gilotti and McClelland, 2007; McClelland et al., 2009). In this study, we summarize results from high-resolution ion microprobe mass spectrometry to assess the multiple ages and trace element patterns preserved in zircon from twelve samples (Fig. 1) chosen to capture the exhumation history of the North-East Greenland UHP terrane. Previously published U–Pb and trace element data (Gilotti et al., 2004; McClelland et al., 2006; Gilotti and McClelland, 2007; McClelland et al., 2009; Gilotti and McClelland, 2011) is combined with new U–Pb data from 4 samples and trace element and Ti-in-zircon data from 10 of the 12 samples to generate a data set that covers the range of lithologies within the UHP terrane. The aims of this contribution are to (1) evaluate the significance of age variations observed within individual and among different samples using differences in zircon characteristics and chemistry as an aid in interpretation, (2) examine the change in trace element behavior of zircon during exhumation, and (3) assess the utility of Ti-in-zircon T estimates for establishing the exhumation history. The timing of UHP metamorphism should be derived from coesite-bearing samples, while

a)

20

2. Geology of the North-East Greenland UHP terrane A UHP terrane of unknown size is exposed on a small island at the eastern edge of the larger HP North-East Greenland eclogite province (Fig. 1; Gilotti et al., 2008 and references therein). Both the HP and UHP areas consist of the same Laurentian continental crust that was deformed and metamorphosed in the overriding plate of the Caledonian collision with Baltica (Gilotti and McClelland, 2007, 2011). HP metamorphism of the North-East Greenland eclogite province occurred from approximately 410 to 390 Ma (Gilotti et al., 2004) due to overthickening of the crust in response to continental collision, analogous to the great crustal thickness of the present-day Tibetan plateau (e.g. Wittlinger et al., 2004, 2009; Mechie et al., 2012). The southern Greenland Caledonides also contains regions with ≈400 Ma HP metamorphism, including Payer Land (McClelland and Gilotti, 2003) and Liverpool Land (Corfu and Hartz, 2011). UHP metamorphism took place later, at 365–350 Ma (McClelland et al., 2006), perhaps due to intracontinental subduction that broke the overriding plate near the end of plate convergence (Gilotti and McClelland, 2007). The North-East Greenland eclogite province is typical of continental eclogite terranes, where meter to kilometer scale mafic enclaves are encased in quartzofeldspathic ortho- and paragneiss. The protolith of this continental eclogite province is a 2.0–1.8 Ga calc-alkaline arc complex intruded by 1.75 Ga anorogenic granitoids (Kalsbeek et al., 2008). The majority of mafic rocks that became eclogites were originally layered gabbroic intrusive complexes, although mafic xenoliths in the calc-alkaline arc and later dikes also became eclogites (Gilotti et al., 2008).

o

18o 54'

b)

North-East Greenland HP eclogite province

coesite locality zircon samples (this study)

gneiss Jokelbugt

UHP terrane

pelitic paragneiss

Rabbit Ears Island

Storstrommen SZ

100 m contour

78 o

0

km

50

113 114 170 156

Qal

Greenland

limit of inland ice

154 155 110 111 123

Germania Land SZ

Caledonian orogen

Map area

184

water

143

78 8o 00 00'

120

201 77

193 194

130

o

145

Qal

147

0 Dove Bugt

1 km

Fig. 1. Geologic maps of (a) North-East Greenland Caledonides and (b) the ultrahigh-pressure terrane on “Rabbit Ears Island” showing sample locations referred to in the text. Modified from Gilotti and McClelland (2011).

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

The North-East Greenland UHP terrane belongs to a small group of hot UHP terranes (>800 °C) that include the Bohemian massif (Massonne and O'Brien, 2003; Kotková et al., 2011), the Kokchetav massif (Shatsky et al., 1995) and the Greek Rhodope mountains (Mposkos and Krohe, 2006). Peak P and T conditions determined from net transfer reactions among garnet + omphacite + kyanite + quartz/coesite + phengite from eclogites are 3.6 GPa and 950 °C (Gilotti and Ravna, 2002). Coesite has been identified in zircon from five localities (Fig. 1b; McClelland et al., 2006), including a new eclogite locality documented here. One of the coesite-bearing samples is a felsic gneiss, which supports the interpretation that the gneisses, and hence the entire crustal slice, experienced UHP conditions. In addition, many of the high-grade rocks on Rabbit Ears Island contain polycrystalline quartz inclusions with palisade texture in garnet that are clear pseudomorphs after coesite. Melting occurred during all stages of exhumation, and began just below the coesite stability field. Lang and Gilotti (2007) identified relicts of phengite decompression melting preserved in polyphase inclusions in restitic garnet within rare metapelites. Isochemical phase equilibrium modeling of the anatectic pelites shows that partial melting began at the coesite to quartz transition (Lang and Gilotti, 2011). Leucocratic bodies and dikes, ranging from undeformed to moderately deformed, are ubiquitous features of the UHP terrane. Leucosomes that are concordant with the foliation in the quartzofeldspathic gneisses, melts emplaced in the necks of eclogite boudins, and leucocratic pegmatites that cross-cut amphibolite facies deformation fabrics attest to fluid-enhanced melting events (Prince et al., 2001) that took place throughout the retrograde metamorphic history. U–Pb geochronology by ion microprobe has established a long history of zircon crystallization from UHP to retrograde amphibolite-facies conditions (Gilotti et al., 2004; McClelland et al., 2006; Gilotti and McClelland, 2007; McClelland et al., 2009; Gilotti and McClelland, 2011). Observed zircon is quite variable, ranging from well-preserved, Precambrian protolith zircon with little new growth, to inherited zircon overgrown by well developed rims, to populations with recrystallized cores and new rims that are chemically distinct but give similar ages. Gilotti and McClelland (2007) have proposed the following timing for UHP metamorphism and exhumation based on the existing data set: 1) UHP metamorphism between 365 and 350 Ma; 2) phengite decompression melting near the coesite to quartz transition (Lang and Gilotti, 2011) starting at ca 347 Ma; 3) granulite to amphibolite facies metamorphism accompanied by melting between 350 and 340 Ma; and 4) continued exhumation at amphibolite facies conditions with zircon growth in the youngest pegmatites at 320 Ma. Exhumation was at least a two stage process, with relatively rapid decompression through the eclogite and HP granulite facies (b10 m.y.) and slower, ≈ 20 m.y., in the amphibolite facies. Rapid decompression through the highest T part of the P–T path inhibits diffusion and resetting of entire mineral volumes. 3. Sample description Twelve samples from the North-East Greenland UHP terrane were chosen to represent the behavior of zircon during exhumation, including: two coesite-bearing rocks, a suite of quartzofeldspathic host gneisses and five small, post-UHP, leucocratic intrusive bodies. Sample locations are shown in Fig. 1b, and UTM coordinates are given in Table 1. Some petrologic, geochronologic and geochemical data relevant to this study are already published and here we include additional U–Pb geochronology and trace element analyses of zircon to augment the data set. McClelland et al. (2006) describe and present U–Pb data for coesite-bearing gneiss sample 03-184. Gilotti and McClelland (2007) describe and report U–Pb data for intrusion samples 03-113 and 03-201. McClelland et al. (2009) describe and provide U–Pb data for gneiss samples 03-130 and 03-154 and intrusion sample 03-155, as well as give trace element data for gneiss sample 03-184 and intrusion sample 03-113. Gilotti and McClelland (2011) describe geochemistry

237

and U–Pb data from gneiss samples 03-114, 03-170, and 03-156 in a discussion of protolith ages for the UHP terrane. This paper presents new descriptions, U–Pb and trace element data for a coesite-bearing eclogite (03-143) and two pegmatite samples (03-145 and 03-147). New trace element data is presented for 5 of the gneiss samples above (03-130, 03-114, 03-170, 03-154, and 03-156) and 2 intrusive samples (03-155 and 03-201). Additional U–Pb data is presented here for 03-154. Table 1 summarizes the mineral assemblages in the samples, and Fig. 2 illustrates the range of zircon morphologies and textures seen in CL images taken with the CL detector on the JEOL 5600 scanning electron microscope (SEM) at the School of Earth Sciences, Stanford University, Stanford, California. The CL image of coesite-bearing zircon from sample 03-143 in Fig. A1 of the online Appendix was captured with the ChromaCL detector on the Hitachi S-3400N SEM at the University of Iowa. 3.1. Coesite-bearing eclogite and gneiss Zircon from samples 03-143 and 03-184 contains coesite, and thus preserves direct evidence of the UHP metamorphism, as well as metamorphic rims formed on the retrograde PT path. McClelland et al. (2006) documented coesite inclusions in zircon separated from 03-184, and in three additional samples. Two coesite inclusions were recently identified in zircon from sample 03-143, a coarse-grained quartz eclogite from the east side of Rabbit Ears Island (Fig. 1) that has not been described previously. The laser Raman spectra and a CL image showing the coesite in grain 40 are given in the online supplement (Fig. A1a and b). The discovery of coesite in 03-143 on the eastern side of the island effectively doubles the documented thickness of the UHP terrane to 4 km, and bolsters our tacit assumption that all the rocks on the island have experienced UHP metamorphism (Gilotti and McClelland, 2007). Eclogite 03-143 (Fig. A2) consists of approximately 50% omphacite, 35% garnet, 12% quartz, and 2% retrograde biotite and plagioclase. Accessory minerals include rutile, zircon, ilmenite, and chlorite. Garnet is subhedral, 1–3 mm in diameter and occurs in aggregates. Quartz forms fairly large, isolated matrix grains with embayed boundaries shared with garnet and omphacite. Polycrystalline quartz inclusions with radial fractures and palisade texture are common in garnet, and represent pseudomorphs after coesite. Rutile is present in the matrix and as inclusions in garnet. The UHP assemblage in 03-143 was omphacite + garnet + coesite + rutile. The rock is somewhat retrogressed. Less than 15% of the omphacite has converted to fine-grained, lobate symplectites of diopside + plagioclase; plagioclase forms thin moats along some garnet and pyroxene grain boundaries; coesite has converted to quartz; and brown biotite is found in patches. Sample 03-184 is a garnet-rich, felsic gneiss that is interlayered on the centimeter to decimeter scale with more mafic, garnet + clinopyroxene rocks, and is representative of the best-preserved UHP features in the host gneisses. The sample is a weakly foliated gneiss composed of quartz, plagioclase, garnet, kyanite, clinopyroxene, amphibole with accessory rutile, zircon, phengite, epidote/clinozoisite, and titanite. Relict UHP minerals, garnet and kyanite, are fairly well preserved as porphyroblasts, but clinopyroxene forms skeletal remnants of former omphacite, some with lobate symplectites of diopside + plagioclase. Phengite, quartz, epidote/clinozoisite and rutile are the common inclusions in garnet. Rutile is also present in the matrix, with rare overgrowths of titanite. Blue-green amphibole, quartz and plagioclase are the dominant matrix phases; they also form very fine-grained kelyphites around garnet and clinopyroxene. 3.2. Quartzofeldspathic host gneisses Strongly deformed, quartzofeldspathic gneiss derived from a Paleoproterozoic calc-alkaline intrusive complex and anorogenic granites (Gilotti and McClelland, 2011) engulf the eclogite bodies on Rabbit Ears Island. The gneisses show progressive retrogression from rocks

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J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

Table 1 Mineral assemblages and locations of studied samples. Sample

Grt

Cpx

Amp

Bt

Quartzofeldspathic gneisses 03-114 0548045, 8660006 03-123 0547885, 8659524 03-130 0549937, 8658933 03-154 0550877, 8660677 03-156 0548057, 8659855 03-170 0548018, 8659906

UTM coordinates (zone 27X WF)

p p p p p p

m/i i i

m

m/i

m m

m m/i m m/i m m

Coesite-bearing rocks 03-143 0491161, 8663588 03-184 0549935, 8662371

pb pb

m/s m/s

m, k

Late stage pegmatites 03-113 0548042, 03-145 0550158, 03-147 0550212, 03-155 0550872, 03-201 0548525,

8659980 8658375 8657142 8660556 8658834

m

Chl

Qtz

Fsp

m/i m

m/r/i m/r/i m/r/i m/i m m/i

m/r m/r m/r m m m/i

m m/i

s m/s/k

m m m m m

m m m m m

tr

pb

m/i

pb

m m m m

a

Ky

Ph/Ms

m/i

i i i

pb

Ep

Rt

Ttn

i i

p m a p m

m/i a/i m/i tr tr/i a/i

m m m m m m

i i

i

m/i m/i

m

Ilm

a a

a

a

a

pb

i

a

Mineral abbreviations follow those of Whitney and Evans (2010). Textual abbreviations are: p = porphyroclast, pb = porphyroblast, m = matrix, i = inclusion (primarily in garnet), r = ribbon, s = symplectite, k = kelyphite, a = accessory phase, and tr = trace amounts.

that still contain clinopyroxene and garnet with HP inclusions—such as kyanite, phengite and rutile—to gneisses where biotite is the major mafic phase, garnet is embayed and resorbed, and titanite replaces rutile (Table 1; Miller, 2008). The presence of HP inclusions in garnet, matrix clinopyroxene and kyanite in some samples, radial fractures around quartz inclusions in garnet, biotite + plagioclase symplectites after phengite (see photomicrographs in Gilotti and McClelland, 2011), as well as the proximity of the gneisses to coesite-bearing eclogites, strongly suggests that these gneisses also experienced UHP metamorphism. Samples 03-114, 03-130, 03-170 and 03-154 are intermediate in composition, and were originally part of the 2.0–1.8 Ga calc-alkaline intrusive complex, while 03-156 is derived from a 1.75 Ga granite (Gilotti and McClelland, 2011). The gneiss samples are variably retrogressed and mineral assemblages in individual samples are never in complete equilibrium; but for this reason, they capture different parts of the exhumation path. Samples 03-114, 03-130 and 03-170 are markedly less retrogressed than 03-154 and 03-156. All the samples contain quartz, feldspar, garnet and biotite. Garnet forms subhedral to anhedral porphyroblasts and preserves abundant inclusions in the least retrogressed samples, while it is fractured, resorbed and replaced by biotite and chlorite in the most retrogressed rocks. Both plagioclase and K-feldspar are present in all samples, but antiperthite with flame structure is found only in the least retrogressed samples. Quartz forms ribbons and matrix grains that display features of intracrystalline plasticity formed over a range of temperature (≈300–700 °C, Stipp et al., 2002), from embayed boundaries that signal high T grain boundary migration recyrstallization to undulose extinction at low T. Biotite becomes more abundant and changes from brown to green as retrogression proceeds. The main Ti-bearing phase changes from rutile to titanite on the retrograde P–T path. In 03-114, 03-130 and 03-170, garnet is rich in rutile inclusions, both rutile and titanite are common matrix phases, and titanite overgrowths on rutile are common. Trace amounts of rutile are preserved in 03-154 or 03-156, but titanite and ilmenite are abundant. Gneisses 03-170, 03-154 and 03-156 contain thin, wispy leucosomes parallel to gneissosity that are evidence of melting. A moderate to strong foliation, primarily defined by quartz ribbons and oriented biotite, is evidence of significant deformation on the exhumation path. In some cases, biotite-rich shear bands clearly deform the thin leucosomes. 3.3. Leucocratic layers and dikes The gneisses and eclogite bodies of the North-East Greenland eclogite terrane are cut by a variety of late-stage, commonly pegmatitic, leucocratic melt rocks that span the entire exhumation history from

approximately 350–320 Ma (Gilotti and McClelland, 2007; McClelland et al., 2009). Except for the leucosomes in the rare metapelites, which are clear partial melts (Lang and Gilotti, 2007) that did not grow zircon, most of the leucocratic dikes and layers probably formed by fluidassisted melting from externally derived fluids (e.g. Berger et al., 2008). Pegmatites are ubiquitous in the vicinity of the eclogite bodies, where they occur in the necks of boudins and in tension gashes. They range from undeformed to moderately deformed, and generally cut across the regional NNE-striking, sub-vertical foliation, but they can also occur in lenses parallel to gneissosity. Samples 03-113, 03-145, 03-147, 03-155 and 03-201 represent the broad spectra of post-UHP intrusive rocks. Garnet granitoid 03-113 is a medium-grained phase of a boudin neck pegmatite that lies adjacent to coesite-bearing, kyanite eclogites. It contains quartz, plagioclase, K-feldspar, garnet and biotite. Sample 03-201 is a distinctive, moderately deformed, very coarse grained, hornblende-bearing leucosome that cuts the foliation of garnet-hornblende gneisses. Poikiolitic hornblende crystals are blocky and up to 5 cm long. The rock matrix consists of quartz, feldspar and small garnet grains. Samples 03-145, 03-147 and 03-155 are from three separate E–W striking, cross-cutting pegmatites that represent the latest phase of intrusives seen on Rabbit Ears Island (Fig. A3). The pegmatites are 1–10 m thick, tabular bodies that contain xenoliths of the gneisses. They are coarse to very-coarse grained, relatively undeformed, and consist of feldspar and quartz, with a graphic granite texture, and large biotite books. 4. Analytical methods The elemental chemistry of magmatic and metamorphic zircon separated from the twelve samples described above was measured using the sensitive high resolution ion microprobe-reverse geometry (SHRIMP-RG) mass spectrometer at the U.S. Geological Survey—Stanford University ion probe facility, Stanford, California. U–Th–Pb isotopic data was collected separately from the trace element chemistry. SHRIMP-RG is well suited for studies of complexly zoned zircons because the spatial resolution attained by the 20–30 μm wide by 1–2 μm deep spot used for U–Pb geochronology allows for multiple analyses with negligible destruction. The spot size for trace element analysis is even smaller because a less energetic beam is applied. SHRIMP-RG was used in our previous studies of the Greenland samples, so our data sets were collected under a compatible set of conditions, following procedures outlined in Barth and Wooden (2006, 2010). Zircons were extracted from 0.5 to 3 kg samples by standard physical separation techniques and mounted in epoxy prior to CL imaging. Extra care was taken during polishing to expose the coesite in the

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

a) 03-143

239

g) 03-156

9.1t

5.1t

4.1

5.1

4.1t

9.1

5.2t 9.1

9.1t 14.1

4.2t

9.2t 9.2

5.2

b) 03-184

5.3t

h) 03-113

10.2t 9.1

13.2t

13.1t 13.1

10.3t

13.3t

10.1t 10.1

c) 03-130

i) 03-201

4.1t

4.1

21.1

23.2t

16.2t 16.1t 4.2t

d) 03-114 3.2

21.2t

23.1t 23.1

3.2t

3.1

21.3t 19.1t

9.1t

3.1t

23.3t

23.4t

9.1

21.1t

19.1

j) 03-145 9.2t

5.2

9.2

e) 03-170

5.1 10.1t

19.3t

17.2t

10.1

19.2 17.1t 17.1

19.2t

5.1

10.2t

5.1t

f) 03-154

100 microns

l) 03-147

9.1 9.1t

k) 03-155

4.1

2.1

9.2t 4.2t

4.1t

2.1t

Fig. 2. Cathodoluminescence (CL) images of zircon from (a) the coesite-bearing quartz eclogite 03-143; (b) coesite-bearing gneiss 03-184; (c–g) quartzofeldspathic host gneisses; (h) garnet-bearing, boudin neck granitoid 03-113; (i) hornblende-bearing leucosome 03-201; and (j–l) cross-cutting pegmatites. White circles are locations of trace element analyses and yellow circles are U–Pb analyses. Labels indicate grain number.spot number.

epoxy mount containing zircon grain 40 from quartz eclogite 03-143. The CL images (e.g. Fig. 2), together with transmitted light and reflected light images, were used to select spots for analysis. Concentrations and ratios of U, Th and Pb isotopes were measured using a 25–30 μm diameter, 5–10 nA O2− primary beam. Zircon standard CZ3 (550 ppm U; Pidgeon et al., 1994) was used to calibrate the U concentration. Isotopic ratios were calibrated against zircon standard R33 (420 Ma, Black et al., 2004; Mattinson, 2010), which was rerun after every fourth analysis. Calibration error for 206Pb/ 238U ratios of R33 for the analytical sessions was 0.61–0.88% (2σ). Ages were calculated as inverse-variance weighted mean 206Pb/238U ages with errors reported at the 95% confidence level or as concordia ages (Ludwig, 1998, 2001). Age calculations and Tera–Wasserburg diagrams were generated with the Isoplot/Ex program of Ludwig (2003). Discussion and interpretation of analyses from Paleozoic zircon are based on 206 Pb/ 238U ages calculated from ratios corrected for common Pb using the 207Pb method (Table 2).

Trace element compositions (Table 3) were collected from the same suite of zircons that were analyzed for geochronology. The grain mounts were lightly polished to remove the old coating and sputtered pits, recoated with gold and analyzed with a ≈15 μm spot and a 1–2 nA O2− primary beam. Analytical spots were placed within the same CL domain, adjacent to the initial U–Pb analysis, and extra spots were added to explore trace element variation among observed CL domains. The following peaks were measured: 7Li, 9Be, 11B, 19F, 23Na, 27Al, 30Si, 31P, 39K, 40 Ca, 45Sc, 48Ti, 49Ti, 56Fe, 89Y, 93Nb, 94Zr1H, 96Zr, 139La, 140Ce, 146Nd, 147 Sm, 153Eu, 165Ho, 157Gd16O, 159Tb16O, 163Dy16O, 166Er16O, 169Tm16O, 172 Yb16O, 175Lu16O, 90Zr216O, 180Hf16O, 206Pb, 232Th16O and 238U16O. Data reduction of elemental concentrations used zircon standards CZ3 and MAD (Mazdab and Wooden, 2006; Mazdab, 2009). Interference from inclusions or alteration was monitored through analysis of F, Al, P, Ca, Fe, Mg and K. The estimated errors based on repeated analysis of MAD are 3–10% for P, Y, Hf, Th, and U and the REE except for La (20%). We have previously published a short set of trace elements that were

240

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

Table 2 U–Pb geochronologic data and apparent ages. Spota

CL-age

Sample 03-143 15.1 r 37.1 c-s 32.1 r 22.1 m 38.1 c-s 25.1 c-s 36.1 c-s 11.1 r 39.1 r 33.1 r 8.1 r 5.1 r 10.1 r 1.1 m 27.1 c-s 2.1 c-s 40.1 c-s 6.1 c-s 7.1 c-s 20.1 c-s 23.1 r 14.1 r 21.1 r 18.1 r 19.1 c-s 30.1 c-s 3.1 c-s 24.1 m 28.1 m 35.1 m-epi 34.1 m-epi 17.1 m-epi 16.1 c-p 31.1 c-ep 9.1 c-ep 12.1 c2-ezi 13.1 c2-ezi 29.1 c3-ezi 26.1 c3-ezi 4.1 c3-ezi Sample 03-154 34.1 r 28.1 r 33.1 r 42.1 r 41.1 r 29.1 r 39.1 r 35.1 r 48.1 r 51.1 r 31.1 r 53.1 r 32.1 r 30.1 r 43.1 r 54.1 r 50.1 r 52.1 r 49.1 r 47.1 r 38.1 r 26.1 r 44.1 r 27.1 r 36.1 r 45.1 r 37.1 r 46.1 r 55.1 r 40.1 c

Th (ppm)

Th/U

206 Pb*b (ppm)

f206Pbcb

238

270 403 296 218 407 482 519 265 190 133 179 200 207 190 402 437 343 374 284 372 295 187 153 327 433 198 530 192 423 438 304 320 315 350 298 369 348 315 549 388

40 95 53 37 117 22 47 25 17 10 22 18 27 29 20 316 34 51 15 74 47 25 11 55 16 20 24 17 26 23 69 101 87 70 125 69 123 126 135 104

0.2 0.2 0.2 0.2 0.3 0.05 0.1 0.10 0.09 0.08 0.13 0.09 0.14 0.2 0.1 0.7 0.1 0.1 0.1 0.2 0.2 0.1 0.1 0.2 0.04 0.10 0.05 0.09 0.06 0.1 0.2 0.3 0.3 0.2 0.4 0.2 0.4 0.4 0.3 0.3

12 18 14 10 19 22 24 12 9 6 8 10 10 9 19 20 16 18 13 18 14 9 8 16 21 10 26 10 22 20 14 15 15 16 14 17 16 17 30 27

0.24 0.24 0.26 0.30 b0.01 0.07 0.06 b0.01 0.12 0.17 0.07 0.03 b0.01 0.17 0.09 0.05 0.14 0.31 0.11 b0.01 b0.01 b0.01 b0.01 b0.01 0.14 0.18 0.16 b0.01 b0.01 0.22 b0.01 0.03 0.12 0.22 b0.01 0.24 b0.01 1.47 1.16 2.32

19.409 19.307 18.813 18.604 18.811 18.766 18.748 18.520 18.462 18.355 18.326 18.053 18.096 18.352 18.429 18.419 18.384 18.273 18.252 18.118 17.562 17.514 17.339 17.299 17.653 17.503 17.228 16.623 16.529 18.626 18.471 17.921 18.390 18.948 18.154 18.605 18.379 15.883 15.679 12.443

(1.3) (1.2) (1.3) (1.3) (1.2) (1.2) (1.2) (1.3) (1.4) (1.5) (1.3) (1.3) (1.3) (1.3) (1.2) (1.2) (1.2) (1.3) (1.2) (1.2) (1.3) (1.3) (1.4) (1.2) (1.2) (1.4) (1.2) (1.4) (1.2) (1.2) (1.2) (1.2) (1.2) (1.2) (1.2) (1.2) (1.2) (1.3) (1.2) (1.3)

.05477 .05486 .05518 .05562 .05307 .05369 .05363 .05318 .05419 .05467 .05386 .05366 .04898 .05465 .05400 .05365 .05440 .05582 .05420 .05306 .05243 .05293 .05330 .05254 .05474 .05509 .05508 .05254 .05392 .05494 .05140 .05371 .05424 .05483 .05324 .05508 .05316 .06627 .06391 .07576

(2.1) (1.8) (2.1) (2.2) (1.9) (1.6) (1.5) (2.0) (2.5) (2.9) (2.5) (2.3) (2.3) (2.3) (1.7) (2.1) (1.8) (2.6) (1.9) (2.6) (2.0) (2.4) (4.7) (1.9) (1.6) (2.3) (2.5) (2.5) (1.5) (1.6) (2.0) (2.3) (1.9) (1.8) (2.1) (1.7) (1.8) (2.2) (1.3) (2.4)

113 64 345 154 141 59 70 85 108 126 70 68 128 80 338 95 79 83 95 86 116 111 45 92 67 94 12 73 72 141

2 2 20 7 7 1 2 4 6 6 4 3 6 4 8 5 3 4 5 10 3 10 2 7 5 2 1 1 4 37

0.02 0.03 0.06 0.05 0.05 0.02 0.03 0.05 0.06 0.05 0.05 0.04 0.05 0.05 0.02 0.06 0.04 0.06 0.06 0.12 0.03 0.09 0.04 0.08 0.08 0.02 0.07 0.01 0.06 0.27

5 3 15 7 6 3 3 4 5 6 3 3 6 4 16 5 4 4 5 4 6 5 2 5 3 5 1 4 4 31

0.7 0.8 0.3 0.5 b0.01 1.2 b0.01 b0.01 0.3 b0.01 0.3 b0.01 0.01 b0.01 0.1 0.2 0.3 b0.01 b0.01 b0.01 0.26 b0.01 b0.01 0.2 0.2 0.3 b0.01 0.2 b0.01 2.9

20.140 19.683 19.729 19.191 19.270 19.005 19.183 18.781 18.618 18.649 18.582 18.628 18.579 18.568 18.304 18.044 18.003 18.066 17.858 17.811 17.663 17.630 17.416 17.288 16.947 16.809 16.458 15.272 14.680 3.885

(1.8) (2.2) (1.3) (1.6) (1.7) (2.3) (2.2) (2.0) (1.8) (1.7) (2.1) (2.1) (1.7) (2.0) (1.4) (1.8) (2.1) (2.0) (1.9) (2.2) (1.8) (1.7) (2.5) (2.2) (2.0) (2.0) (4.3) (2.2) (2.2) (1.5)

.05828 .05909 .05521 .05671 .05291 .06224 .04976 .05130 .05597 .05284 .05568 .05244 .05331 .05141 .05418 .05482 .05582 .05025 .05258 .05213 .05572 .05226 .05335 .05568 .05523 .05638 .04836 .05629 .05529 .11557

(4.1) (5.7) (2.3) (3.5) (3.9) (6.6) (5.8) (6.7) (4.3) (3.9) (5.2) (5.6) (3.9) (5.2) (2.4) (4.3) (5.1) (5.0) (4.7) (5.8) (4.1) (4.1) (6.7) (4.9) (4.9) (4.8) (13.2) (6.1) (5.8) (1.2)

U (ppm)

3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 2 2 2 2 2 2 2 1 1

3 3 3 3 3 3 3 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 1 1 1 1 1

U/206Pbc

207

Pb/206Pbc

206 Pb/238Ud (Ma)

323 325 333 337 334 334 335 339 340 341 342 347 349 341 340 341 341 342 344 346 358 358 362 363 355 358 363 377 379 336 341 350 341 331 346 337 342 388 394 487

207 Pb/206Pbd (Ma)

(4) (4) (4) (4) (4) (4) (4) (4) (5) (5) (5) (5) (5) (4) (4) (4) (4) (4) (4) (4) (4) (5) (5) (4) (4) (5) (4) (5) (4) (4) (4) (4) (4) (4) (4) (4) (4) (5) (4) (6)

310 (6) 317 (7) 318 (4) 326 (5) 326 (5) 327 (8) 329 (7) 335 (7) 336 (6) 337 (6) 337 (7) 337 (7) 338 (6) 339 (7) 343 (5) 347 (6) 347 (7) 349 (7) 352 (7) 353 (8) 354 (6) 356 (6) 360 (9) 362 (8) 369 (8) 371 (7) 383 (16) 408 (9) 425 (9) 1438 (20)

1879 (23)

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

241

Table 2 (continued) Spota

CL-age

Sample 03-145 1.1 r 12.1 r 17.1 r 18.2 r 18.1 r 15.1 rc 16.1 r 20.1 r 13.1 r 6.1 r 11.1 r 5.1 r 19.1 r 2.1 r 9.1 r 14.1 rc 10.1 m 22.1 r 7.1 r 19.2 m 8.1 r 21.1 m 3.1 r 5.2 rc 4.1 r Sample 03-147 12.1 c 8.1 c 14.1 c 3.1 c 4.1 c 11.1 c 13.1 c 7.1 c 6.1 c 10.1 c 5.1 c 1.1 c 2.1 c 9.1 c

U (ppm)

Th (ppm)

Th/U

206 Pb*b (ppm)

f206Pbcb

238

U/206Pbc

207

Pb/206Pbc

206 Pb/238Ud (Ma)

1

268 697 136 355 144 300 101 304 318 445 128 229 334 139 291 238 414 117 432 162 142 311 541 178 338

2 6 1 4 1 92 1 3 2 5 1 1 3 1 3 81 5 1 5 1 1 3 5 26 3

0.009 0.008 0.007 0.011 0.005 0.32 0.006 0.010 0.006 0.011 0.005 0.003 0.011 0.005 0.011 0.35 0.011 0.005 0.011 0.009 0.005 0.009 0.009 0.15 0.009

11 29 6 15 6 13 4 13 14 20 6 10 15 6 13 11 19 5 20 7 7 14 25 8 16

0.63 0.30 0.55 0.13 b0.01 3.62 0.45 0.15 0.28 0.22 0.93 b0.01 b0.01 0.03 b0.01 0.48 b0.01 0.39 b0.01 0.35 0.55 0.04 b0.01 0.21 0.17

20.527 20.483 20.106 20.121 20.084 19.232 19.811 19.821 19.707 19.602 19.377 19.421 19.380 19.291 19.290 19.116 19.201 18.958 18.753 18.661 18.594 18.669 18.498 18.388 18.336

(0.9) (0.5) (1.0) (0.6) (1.0) (0.8) (1.1) (0.7) (0.7) (0.6) (1.0) (0.9) (0.5) (1.2) (0.9) (0.8) (0.6) (1.1) (0.6) (0.9) (1.1) (0.7) (0.6) (0.9) (0.7)

0.05749 0.05487 0.05702 0.05369 0.05254 0.08179 0.05635 0.05391 0.05501 0.05457 0.06028 0.05150 0.05260 0.05318 0.05283 0.05684 0.05290 0.05620 0.05211 0.05594 0.05758 0.05350 0.05326 0.05501 0.05468

(2.7) (1.8) (3.3) (2.0) (3.2) (17.4) (3.4) (2.2) (2.2) (1.9) (3.2) (2.8) (1.8) (3.7) (2.5) (2.4) (2.0) (3.5) (1.9) (2.8) (3.4) (2.1) (1.8) (2.9) (2.1)

305 306 311 312 313 315 316 317 318 320 321 324 324 326 326 327 327 330 335 335 336 336 339 341 342

(3) (2) (3) (2) (3) (6) (3) (2) (2) (2) (3) (3) (2) (4) (3) (3) (2) (4) (2) (3) (4) (2) (2) (3) (2)

2 2 2 2 2 2 2 2 2 2 2 1 1 1

393 883 441 1598 939 229 1195 347 579 315 1123 704 403 980

1 9 2 23 12 1 9 1 3 2 8 3 3 10

0.004 0.011 0.004 0.015 0.013 0.003 0.008 0.004 0.006 0.005 0.008 0.005 0.007 0.010

17 38 19 73 41 10 52 15 26 14 50 32 18 45

0.40 0.20 0.14 5.28 0.09 0.41 0.06 0.34 b0.01 0.08 0.07 0.14 0.04 0.04

20.049 20.058 19.907 18.843 19.772 19.622 19.594 19.467 19.448 19.403 19.399 19.141 18.789 18.726

(0.8) (0.4) (0.7) (0.3) (0.5) (0.8) (0.4) (0.6) (0.5) (0.7) (0.4) (0.5) (0.6) (0.4)

0.05585 0.05424 0.05381 0.09514 0.05343 0.05607 0.05333 0.05560 0.05265 0.05352 0.05348 0.05413 0.05341 0.05347

(2.1) (1.4) (1.9) (5.3) (1.3) (2.5) (1.2) (2.0) (1.6) (2.2) (1.2) (1.5) (1.9) (1.2)

313 313 316 316 318 319 321 322 323 324 324 328 334 335

(2) (1) (2) (2) (2) (3) (1) (2) (2) (2) (1) (2) (2) (1)

2 2 2 2 2 2 2 2 2 2 2 2 2 2 1 1 1 1 1

207 Pb/206Pbd (Ma)

Note: All analyses were performed on the SHRIMP-RG at the United States Geological Survey-Stanford Microanalytical Center at Stanford University. Analytical procedure followed Williams (1998). Data reduction utilized the SQUID program of Ludwig (2001). a Abbreviations: 1.1 = grain number.spot number; c = core; rc = round core; m = mantle; r = rim; s = spherical; e = elongate; z = oscillatory zoned, p = patchy; i = igneous rare earth element signature. Numbers refer to age domains. See text for discussion. b Pb* denotes radiogenic Pb; Pbc denotes common Pb; f206Pbc = 100 ∗ (206Pbc/206Pbtotal). c Calibration concentrations and isotopic compositions were based on replicate analyses of CZ3 (550 ppm U) and R33 (420 Ma, Black et al., 2004; Mattinson, 2010). Reported ratios are not corrected for common Pb. Errors in parentheses are reported as percentage at the 1σ level. d Ages less than 1.0 Ga were calculated from 206Pb/238U ratios corrected for common Pb using the 207Pb method and ages greater than 1.0 Ga were calculated from 207Pb/206Pb ratios corrected for common Pb using the 204Pb method (see Williams, 1998). Initial common Pb isotopic composition approximated from Stacey and Kramers (1975). Uncertainties in millions of years reported as 1σ.

collected concurrently with U–Pb analysis, but only 03-113 and 03-184 (McClelland et al., 2009) had the complete set of elements listed above, which include Ti for accessory mineral thermometry. Additional REE data collected during U–Pb analysis of samples 03-143 and 03-154 are presented in Table A1 of the online Appendix. Both 48Ti and 49Ti were measured during the complete trace element routine on SHRIMP-RG; however, only the 49Ti data were used to determine the Ti content, as recommended by Watson and Harrison (2005), in order to avoid interference of 96Zr2+ with the 48Ti peak. The estimated error for 49Ti is 4% based on analysis of MAD which propagates to an analytical uncertainty less than that for the thermometer calibration (Watson et al., 2006) and uncertainties due to the effects of pressure above 1 GPa. 5. Zircon chemistry and geochronology Zircon from the UHP rocks in North-East Greenland is uniformly complex, with some preservation of inherited cores, metamorphic

recrystallization of those cores, and growth of new metamorphic rims. Use of the SHRIMP-RG to date zircon and collect trace element data often gives us the spatial resolution needed to determine whether or not individual CL domains (e.g. Fig. 2) correspond to actual age and chemical domains. Nevertheless, interpretation of the U/Pb age data from North-East Greenland remains challenging due to the common 50 m.y. spread in ages obtained for many samples. The spread may reflect analytical variation around multiple ages within a sample, i.e. distinct episodic events, or continuous growth of zircon throughout UHP metamorphism and exhumation. In addition, the spread in ages may reflect the presence of partially or wholly recrystallized inherited components or domains that have suffered Pb-loss. In the following discussion, we evaluate the age data in terms of episodic versus continuous age spectra using the age distribution (e.g., cumulative probability density plots, Fig. 4) and variation in zircon characteristics (e.g., CL images, Fig. 2), and trace element geochemistry (Fig. 3). Weighted mean 206Pb/238U ages calculated from distinct chemical or CL characteristics or age populations defined in cumulative probability density plots are used as a proxy for

242

Table 3 Zircon trace element data. Spota

CL

T (°C)

Th

U

Li

Be

B

F

Na

Al

P

K

Ca

Sc

49

Fe

Ti

Y

Nb

La

Ce

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf (×103)

35 355 20 12 37 29 49 118 46 35 28 22 74 8.4 377 40 41 38 64 145 177 155 173

220 563 200 167 236 194 513 290 238 237 204 314 439 202 624 406 309 505 197 548 516 428 370

0.3 0.2 0.2 0.1 0.1 0.1 0.2 0.1 0.2 0.3 0.1 0.3 0.5 0.1 0.2 0.4 0.2 2.8 0.1 0.1 0.3 0.1 0.1

0.003 0.020 0.010 b0.001 0.007 0.003 0.007 0.003 0.006 0.003 0.003 0.010 0.013 0.003 0.019 0.009 0.006 0.115 b0.001 0.021 0.016 b0.001 0.003

0.2 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.2 0.2 0.2 0.1 0.2 0.2 0.2 0.2 0.1 0.3 0.2 0.2 0.2

13 4 7 6 7 5 10 8 6 7 9 3 10 4 6 8 8 8 10 16 8 5 2

8.5 4.2 7.4 5.7 15.7 5.9 5.7 17.7 11.1 10.0 6.9 11.2 5.9 5.4 5.4 5.1 8.8 1956 5.6 6.2 4.9 3.7 4.1

27 19 24 26 35 21 29 24 24 24 21 21 25 20 23 27 28 2100 18 55 18 17 19

53 82 53 59 61 58 50 51 71 50 65 94 73 93 64 58 53 52 220 232 374 535 234

2.1 1.1 1.5 1.9 2.7 1.3 2.2 2.8 2.5 2.1 1.6 2.0 1.5 1.5 1.2 2.0 4.9 1.7 1.3 3.7 1.4 3.8 1.1

5.6 4.0 4.1 5.3 4.5 4.0 7.5 6.9 4.4 4.2 3.9 5.8 4.5 4.1 4.9 4.1 6.0 4486 4.4 28.6 4.1 3.4 4.2

34 54 36 38 42 41 28 28 51 36 42 82 53 64 40 39 40 39 48 42 125 97 74

34.3 45.6 35.3 38.3 35.9 36.6 36.5 33.0 38.6 34.7 38.7 19.4 49.0 9.2 46.3 51.9 29.6 62.4 16.1 21.6 46.3 46.8 49.8

5.2 11 4.7 4.2 5.6 4.0 12 7.5 4.3 4.5 4.1 4.0 6.5 2.3 12 6.2 4.3 1230 1.8 22 6.2 4.2 4.9

13 33 19 19 17 19 24 19 24 16 21 45 29 68 29 20 24 22 343 441 523 838 304

1.5 2.0 1.5 1.3 1.7 1.6 21 0.9 1.2 1.3 1.6 0.8 1.0 0.8 1.3 0.9 1.6 1.0 1.5 4.9 7.8 3.9 2.6

0.002 0.046 0.002 0.006 0.044 0.004 10.1 0.040 0.001 0.003 0.022 0.015 0.014 0.006 0.129 0.009 0.007 0.674 0.008 0.860 0.025 0.010 0.056

5.8 20 4.2 4.0 5.2 5.7 50 12 7.5 5.2 7.5 4.6 14 1.3 30 22 5.7 16 19 22 16 36 14

0.2 0.8 0.1 0.1 0.2 0.1 16.4 0.7 0.2 0.1 0.6 0.2 0.7 0.1 1.9 0.9 0.2 1.1 0.3 6.5 0.5 0.8 0.7

0.40 1.22 0.32 0.31 0.27 0.28 2.68 0.82 0.36 0.33 0.46 0.53 0.88 0.37 2.19 1.01 0.26 0.58 1.11 3.07 1.22 2.20 1.33

0.2 0.8 0.2 0.2 0.2 0.2 0.7 0.4 0.3 0.2 0.2 0.4 0.5 0.4 1.1 0.5 0.2 0.3 0.4 1.1 0.5 0.8 0.7

1.3 5.0 1.9 1.5 1.8 2.0 4.3 2.3 2.2 1.4 1.5 3.5 3.2 4.7 5.7 2.8 2.0 2.1 9.5 11 11 20 9.0

0.5 1.4 0.7 0.6 0.7 0.8 0.9 0.7 0.9 0.6 0.8 1.6 1.0 2.5 1.1 0.8 0.8 0.7 13 16 20 32 13

0.3 1.0 0.4 0.4 0.3 0.4 0.5 0.5 0.5 0.3 0.4 0.8 0.6 1.4 0.9 0.5 0.4 0.4 3.2 3.1 4.1 6.4 2.9

2.1 6.3 3.0 2.9 2.8 2.7 3.9 3.0 3.8 2.2 3.4 6.0 4.3 9.6 5.6 3.2 3.5 3.1 33 38 47 75 30

1.6 3.8 2.7 2.2 2.1 2.3 3.1 2.2 2.5 1.7 2.8 5.1 2.8 7.0 3.2 2.5 2.9 2.3 59 68 90 153 61

0.3 0.6 0.4 0.4 0.4 0.4 0.4 0.3 0.6 0.3 0.4 0.9 0.5 1.0 0.5 0.4 0.4 0.4 14 15 21 35 15

1.9 3.7 2.6 2.9 2.7 3.0 5.0 2.5 3.3 2.2 2.8 6.9 3.8 5.9 3.4 2.6 3.4 3.0 113 125 179 306 135

0.3 0.6 0.4 0.5 0.3 0.4 0.9 0.3 0.5 0.4 0.5 1.1 0.7 0.9 0.6 0.5 0.6 0.5 21 25 34 63 27

13.0 12.5 12.9 12.6 13.3 12.6 13.1 12.4 12.8 12.4 12.6 13.1 12.9 12.8 12.6 12.5 12.7 12.7 10.0 12.2 11.6 9.7 11.5

Sample GL03-130 2.1t r 870 4.1t r 877 8.1t r 878 9.1t r 887 10.1t r 887 15.1t r 853 16.1t r 958 2.2t c 948 4.2t c 928 9.2t c 906 16.2t c 900

28 23 29 24 31 6.6 29 84 57 6.3 65

228 195 236 173 257 225 227 156 178 60 159

0.20 0.28 0.14 0.13 0.29 0.15 0.08 0.00 0.01 0.01 0.02

b0.001 b0.001 0.003 b0.001 b0.001 b0.001 0.003 b0.001 b0.001 b0.001 b0.001

0.2 0.2 0.2 0.2 0.3 0.3 0.1 0.1 0.2 0.2 0.2

1 4 6 6 7 2 8 0 2 2 4

4.1 6.5 5.5 6.0 5.4 8.4 7.5 12.8 18.2 5.6 17.0

24 27 25 27 27 28 23 16 17 23 17

29 36 34 48 34 37 70 224 157 41 248

0.7 1.0 0.9 1.2 1.3 1.0 3.6 1.0 1.8 1.1 2.0

1.7 2.7 1.8 3.6 2.2 4.2 5.5 4.1 9.3 2.3 6.9

12 19 18 26 21 16 23 44 55 20 48

32.6 34.4 34.6 37.5 37.3 28.1 64.9 60.4 51.9 43.7 41.7

3.4 4.2 4.0 2.9 2.8 2.8 5.5 2.1 2.5 2.9 2.7

7 11 9 15 13 13 84 356 230 20 374

2.9 1.3 1.8 1.6 1.9 1.2 2.2 2.8 2.8 1.4 2.3

0.001 0.004 0.004 0.007 0.006 0.003 0.007 0.004 0.002 0.006 0.006

5.4 4.4 5.3 7.0 5.9 2.5 2.3 18 12 7.3 10

0.1 0.2 0.1 0.2 0.2 0.1 0.7 0.5 0.2 0.1 0.2

0.29 0.46 0.31 0.61 0.44 0.39 2.26 1.77 0.84 0.40 0.82

0.2 0.3 0.3 0.5 0.4 0.3 2.0 0.7 0.5 0.3 0.5

1.2 1.5 1.2 2.3 2.0 1.7 9.5 12 6.9 1.8 8.1

0.3 0.3 0.4 0.5 0.5 0.5 3.5 14 9.2 0.8 15

0.2 0.3 0.2 0.4 0.3 0.3 1.7 3.5 2.3 0.4 2.9

1.6 2.0 1.5 3.0 2.2 2.3 13 38 24 3.1 37

1.1 1.3 1.1 1.8 1.8 1.5 13 68 44 3.2 74

0.1 0.1 0.2 0.4 0.3 0.3 3 14 10 0.6 17

1.0 1.5 1.6 2.0 1.6 2.1 20 115 80 5.2 145

0.3 0.3 0.3 0.3 0.3 0.4 4.1 22 16 1.1 30

12.8 11.9 11.8 12.7 12.0 11.5 11.7 10.9 12.2 11.9 11.5

Sample GL03-114 2.1t r 685 2.2t r 830 3.1t r 832 4.1t r 786 5.2t r 817 7.2t r 823 8.1t r 792 9.1t r 785 9.2t r 829 2.3t c 725 3.2t c 742 5.1t c 793 7.1t c 732 8.2t c 766

2.6 11 19 3.5 33 30 10 22 11 88 42 247 48 126

96 220 417 244 378 539 288 846 211 211 118 476 262 319

0.07 0.16 0.52 0.22 0.20 0.89 0.40 0.98 0.15 0.07 0.10 0.05 0.18 0.08

b0.001 0.010 0.003 0.003 0.003 0.009 0.010 b0.001 0.007 0.019 0.003 0.010 0.013 0.014

0.2 0.3 0.4 0.4 0.2 0.4 0.4 0.3 0.3 0.2 0.2 0.2 0.4 0.3

5 2 6 2 4 7 7 3 4 4 10 5 7 6

14.3 7.0 5.3 8.0 5.8 7.3 15.1 6.0 5.9 49.8 4.6 14.3 20.6 10.0

16 22 23 22 20 25 25 22 22 18 20 21 22 19

40 53 66 91 77 53 72 101 64 649 279 947 237 634

2.5 1.3 1.1 1.3 1.0 1.1 1.2 1.1 1.0 6.4 1.3 4.6 2.0 2.9

3.6 2.6 2.7 3.4 2.8 3.3 4.0 4.3 2.6 10.4 3.0 6.0 9.4 17.7

17 27 37 46 38 28 37 36 31 31 31 44 23 27

5.0 23.0 23.3 15.1 20.3 21.4 16.0 14.9 22.6 8.0 9.6 16.2 8.6 12.3

1.1 2.8 3.5 2.7 4.2 4.6 2.7 6.6 3.2 2.3 1.6 5.4 2.8 3.8

39 39 47 77 65 36 53 118 46 1200 470 1801 410 1137

1.1 2.8 1.3 1.6 1.1 1.9 1.0 3.2 1.5 4.4 2.5 7.0 2.8 4.1

0.001 0.004 0.004 0.004 0.006 0.003 0.001 0.002 0.002 0.018 0.006 0.014 0.001 0.020

1.5 5.9 8.4 7.6 8.9 9.1 5.9 7.6 5.3 12 19 29 14 24

0.0 0.1 0.3 0.1 0.3 0.2 0.2 0.2 0.2 2.2 0.5 2.5 0.2 0.8

0.32 0.38 0.62 0.54 0.79 0.55 0.56 0.56 0.45 5.14 1.33 6.50 0.84 2.81

0.2 0.3 0.4 0.6 0.6 0.4 0.5 0.5 0.4 0.7 0.4 1.5 0.3 0.6

2.3 2.6 3.7 5.6 5.4 3.5 3.6 5.0 3.0 38 12 56 8.3 30

1.6 1.5 1.8 3.1 2.6 1.4 2.1 4.5 1.8 50 20 79 17 48

0.7 0.7 1.0 1.4 1.4 0.8 1.0 1.8 0.9 12.4 4.0 18.7 3.3 10.2

5.5 5.7 6.9 12 10 5.2 8.8 16 7.2 132 48 205 40 123

5.0 5.3 6.5 11 9.0 4.8 7.0 18 6.6 223 99 353 86 215

1.0 1.0 1.0 1.9 1.5 0.7 0.9 3.6 1.2 47 22 72 21 46

7.4 6.5 7.4 14 11 4.9 7.4 30 8.7 364 192 594 175 371

1.2 1.1 1.3 2.3 1.9 0.8 1.4 4.9 1.4 64 37 106 35 68

11.1 13.0 12.1 12.5 11.6 11.9 11.8 12.9 13.4 9.9 11.1 10.6 12.1 11.3

Sample GL03-170 1.1t r 872 1.2t r 842 2.1t r 837 4.2t r 763 7.1t r 817

16 19 24 62 7.4

257 251 326 587 1005

0.37 0.50 0.24 0.59 0.62

0.003 b0.001 b0.001 0.020 0.058

0.2 0.2 0.2 0.3 0.3

5 1 3 7 9

4.4 6.8 4.4 4.7 5.2

22 29 41 24 79

51 61 64 182 120

1.0 1.4 1.0 1.0 4.3

2.0 3.9 1.9 2.1 4.2

28 34 31 33 13

32.9 25.4 24.3 12.0 20.3

4.4 32 32 5.0 70

36 43 41 321 163

0.9 0.9 1.3 3.0 16

0.064 0.029 0.059 0.001 0.195

15 8.1 8.8 14 4.8

1.8 0.4 0.3 0.2 1.2

1.13 0.78 0.63 0.87 0.84

0.7 0.5 0.4 0.3 0.3

4.1 3.8 3.3 8.5 2.9

1.4 1.7 1.7 14 4.9

0.9 0.8 0.8 3.3 0.7

6.3 6.1 6.5 37 10

4.8 5.4 6.1 71 37

0.9 0.8 1.0 17 14

5.3 5.3 8.2 139 187

1.0 1.1 1.3 26 51

12.2 10.8 14.3 13.4 19.3

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

Sample GL03-143 1.3t r 876 2.2t r 912 4.2t r 880 5.1t r 890 7.3t r 882 10.1t r 884 26.2t r 884 29.2t r 872 38.2t r 891 40.2t r 878 1.1t m 891 7.1t m 812 7.2t m 921 1.2t c 739 2.1t c 914 38.1t c 928 40.1t c 859 10.2t c-p 953 4.1t c-ezi 792 5.2t c-ezi 824 26.1t c-ezi 914 29.1t c-ezi 915 9.1t c-ei 923

Spota T (°C)

Th

U

Li

8.1t 9.1t 10.2t 16.1t 17.1t 4.1t 10.1t 17.2t

r r r r r c c c

822 788 829 826 828 762 778 770

19 11 14 18 20 166 47 19

313 571 247 281 284 666 136 257

Sample GL03-154 13.1t r3 693 18.1t r3 705 1.1t r2 693 1.2t r2 706 4.1t r2 699 9.1t r2 712 4.2t c 735 9.2t c 746 18.2t c 739

3.1 6.5 8.4 6.3 2.9 6.2 39 176 174

Sample GL03-156 3.1t r2 720 5.2t r2 740 8.2t r2 740 9.2t r2 725 10.2t r2 704 20.2t r2 704 21.1t r2 709 5.3t r 760 7.1t r 707 20.1t r 705 5.1t c 672 8.1t c 671 9.1t c 703 10.1t c 677 Sample GL03-201 15.3t r3 657 18.1t r3 636 21.2t r3 655 22.1t r3 645 11.3t r2 631 15.1t r2 607 17.1t r2 616 19.1t r2 563 21.1t r2 597 22.3t r2 630 23.4t r2 615 11.1t r1 671 12.2t r1 650 16.1t r1 670 17.2t r1 666 19.2t r1 677 23.3t r1 662 11.2t m 879 12.1t m 860 15.2t m 837 16.2t m 848 18.2t m 705 19.3t m 855 20.1t m 698 21.3t m 853 22.2t m 848 20.2t c 821

Be

B

F

Na

Al

0.20 0.35 0.15 0.32 0.29 0.19 0.02 0.36

0.003 0.010 b0.001 b0.001 b0.001 b0.001 0.007 b0.001

0.2 0.2 0.2 0.2 0.2 0.4 0.2 0.4

3 6 2 6 4 6 2 7

3.7 6.0 4.1 7.4 4.4 44.6 5.4 4.6

65 140 169 96 87 102 149 416 332

b0.01 0.13 0.23 b0.01 b0.01 0.01 0.01 b0.01 0.01

b0.001 0.012 b0.001 0.010 0.003 b0.001 b0.001 0.003 0.007

0.1 0.3 0.3 0.2 0.1 0.2 0.2 0.1 0.2

1 4 6 6 2 3 5 2 12

1.9 4.0 32 1.4 1.2 1.7 1.2 3.1 2.2 7.8 56 20 34 30

134 137 279 96 89 113 86 156 143 272 107 37 75 58

b0.01 0.11 0.11 0.06 0.05 0.01 0.04 0.08 0.04 0.32 0.05 0.01 b0.01 0.03

b0.001 0.017 0.007 b0.001 0.007 0.003 0.003 0.003 b0.001 0.013 0.003 0.007 b0.001 0.003

0.1 0.2 0.1 0.2 0.1 0.2 0.4 0.3 0.2 0.2 0.3 0.2 0.1 0.1

b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 b0.01 0.014 0.283 0.202 0.055 0.152 0.077 19 14 23 22 2.0 12 1.5 29 21 82

13 7 16 12 0.38 0.23 0.34 2.55 0.30 2.47 0.24 20 72 31 17 31 15 303 219 327 315 254 260 238 385 321 687

b0.01 b0.01 0.01 0.01 b0.01 b0.01 0.02 b0.01 0.01 0.01 b0.01 b0.01 0.03 0.02 0.01 0.01 b0.01 0.40 0.12 0.36 0.41 0.32 0.21 0.33 0.36 0.31 0.17

0.002 b0.001 0.002 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 b0.001 0.003 0.002 b0.001 b0.001 0.002 b0.001 0.003 b0.001 0.002 b0.001 b0.001 0.000 0.001 b0.001 b0.001 0.006

0.1 0.02 0.1 0.1 0.1 0.1 0.4 0.1 0.05 0.03 0.1 0.1 0.3 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.2

Ca

Sc

49

P

K

Ti

Fe

Y

Nb

La

Ce

Nd

Sm

Eu

Gd

Tb

24 22 24 25 23 63 18 25

44 63 57 60 71 520 404 162

0.8 1.0 0.8 1.3 0.8 32.7 1.1 1.1

1.5 2.2 1.4 3.7 2.3 7.7 1.9 2.4

21 37 31 30 33 58 35 27

21.3 15.5 22.6 22.1 22.4 11.8 13.9 12.9

3.5 6.5 3.1 2.7 3.6 12 2.0 3.0

2.3 11.0 7.8 41.9 5.5 3.9 16.7 7.0 21.9

14 25 39 15 16 15 18 8 20

29 61 49 35 32 37 347 221 277

0.5 1.2 1.2 2.6 0.9 0.9 1.5 0.9 2.4

1.9 4.1 2.6 7.5 2.6 2.2 7.2 3.5 9.9

56 48 14 55 35 54 59 74 35

5.5 6.3 5.5 6.4 5.9 6.9 8.9 10.0 9.2

1 7 5 5 7 3 3 6 3 6 8 6 1 4

2.0 5.7 3.8 5.7 3.2 3.6 9.0 3.3 3.1 4.0 8.8 3.2 4.2 5.8

10 25 15 20 16 14 17 19 17 18 17 18 14 18

82 66 49 73 61 82 46 56 58 42 358 221 413 225

0.4 1.2 0.7 1.1 0.7 0.7 1.3 0.8 0.7 0.9 1.1 0.8 0.7 1.0

1.1 3.6 1.3 2.3 1.3 2.9 4.3 1.6 1.2 2.4 2.6 1.8 2.0 2.1

8 13 9 10 8 8 11 14 28 5 15 32 27 21

7 8 10 9 7 9 10 9 8 9 10 9 8 8 7 10 9 8 6 7 9 8 9 9 8 11 9

2.3 2.9 2.8 2.5 2.3 2.5 3.4 1.2 3.3 3.1 3.2 4.1 3.9 3.0 2.3 2.7 2.6 3.1 3.0 3.8 3.8 2.7 2.5 2.7 3.4 2.2 2.8

4 5 6 6 5 5 7 1 6 6 6 7 19 7 5 6 6 11 7 9 11 18 9 21 9 9 7

34 31 32 28 23 36 45 40 43 40 39 36 34 56 23 28 29 54 65 54 58 53 65 51 57 55 322

1.0 1.6 1.4 1.3 1.1 1.1 3.6 0.5 1.4 1.6 2.0 1.9 3.4 1.4 1.1 1.3 1.7 1.4 1.5 1.5 1.6 1.3 1.0 1.3 1.3 0.9 1.5

2.4 3.1 3.0 2.8 2.7 2.6 6.1 0.9 3.5 3.4 3.5 4.1 5.1 3.2 2.6 3.2 2.9 4.9 3.2 2.9 3.1 3.2 2.7 3.1 3.2 2.4 3.0

101 56 98 74 111 63 47 56 45 67 47 87 28 50 115 79 84 52 50 47 48 32 61 26 43 42 67

27 41 38 37 44 929 683 214

1.7 1.2 1.2 1.2 1.0 10 6.1 1.7

0.007 0.006 0.006 0.001 0.001 0.022 0.008 0.004

8.3 5.9 7.7 7.9 7.8 38 19 8.6

0.2 0.1 0.2 0.2 0.3 0.4 0.6 0.6

0.39 0.45 0.59 0.55 0.76 1.88 2.01 1.29

0.3 0.3 0.5 0.4 0.5 0.3 0.4 0.5

1.7 3.0 3.4 2.6 4.1 19 18 8.9

1.1 1.6 1.6 1.4 1.8 38 29 9.1

0.6 0.8 1.0 0.8 1.0 8.1 6.3 2.7

0.5 1.5 2.1 0.8 0.8 0.8 1.5 2.3 3.2

26 24 28 41 31 43 520 505 592

1.4 1.9 1.1 1.4 1.3 1.2 3.3 2.5 2.0

0.006 0.003 0.002 0.010 0.008 0.006 0.012 0.001 0.754

2.2 3.8 3.7 3.5 3.4 3.6 25 33 43

0.1 0.1 0.1 0.1 0.1 0.1 0.3 0.2 2.4

0.30 0.29 0.24 0.47 0.34 0.41 1.15 0.81 2.53

0.4 0.3 0.3 0.5 0.4 0.4 0.2 0.4 1.1

3.3 2.6 2.3 3.6 3.7 4.0 10 7.7 15

0.8 0.7 1.0 1.4 1.0 1.3 21 17 23

7.6 9.4 9.4 7.9 6.3 6.3 6.6 11.7 6.5 6.4 4.3 4.2 6.2 4.5

0.9 2.6 2.2 1.0 0.8 0.9 0.7 1.5 0.9 2.4 1.0 0.5 0.9 0.6

188 124 94 151 121 171 137 93 93 82 928 486 922 563

10 8.1 7.9 5.1 4.7 7.0 4.4 10 3.6 6.6 5.2 2.7 5.0 3.1

0.006 0.043 0.004 b0.001 0.006 0.002 0.002 0.001 0.004 0.004 b0.001 0.007 0.007 0.002

0.5 2.2 15 0.6 0.7 0.7 0.9 1.6 1.7 4.0 12 6.4 6.7 9.2

0.0 0.3 0.1 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.7 0.5 2.2 0.6

0.12 0.26 0.48 0.13 0.13 0.16 0.13 0.10 0.26 0.22 2.25 1.79 4.91 1.60

0.03 0.1 0.2 0.03 0.02 0.02 0.1 0.03 0.2 0.1 0.2 0.4 1.1 0.3

0.7 1.5 3.2 0.6 0.4 0.7 1.2 0.9 4.7 1.7 25 14 38 15

6.3 4.2 3.2 4.9 4.0 5.5 4.8 3.1 3.5 2.8 37 21 41 24

3.5 2.7 3.5 3.0 2.5 1.8 2.0 0.9 1.6 2.5 2.0 4.2 3.2 4.2 4.0 4.6 3.8 34.9 29.8 24.4 26.8 6.3 28.6 5.8 28.1 26.8 21.0

0.2 0.5 0.4 0.3 0.2 0.2 8.5 0.3 0.3 0.3 0.4 0.6 7.5 0.5 0.4 0.4 0.6 2.5 3.7 3.2 3.4 2.0 2.1 2.5 3.9 2.8 5.6

142 71 94 74 68 29 25 52 21 64 22 85 19 50 91 85 63 15 40 22 22 19 30 18 30 30 507

1.5 1.4 2.0 1.4 1.7 1.8 2.6 3.2 2.9 2.0 2.7 1.4 1.4 2.3 1.5 2.1 1.3 0.5 1.5 0.8 1.0 0.8 0.9 0.5 1.5 1.3 1.9

0.004 0.002 0.006 0.004 0.004 0.004 0.006 0.002 0.011 0.004 0.009 0.002 0.832 0.010 0.004 0.007 0.002 0.015 0.015 0.002 0.002 – 0.004 0.006 0.004 0.013 0.004

0.10 0.18 0.17 0.14 0.07 0.11 0.11 0.15 0.14 0.07 0.13 0.19 1.7 0.31 0.13 0.30 0.25 3.0 5.9 5.3 6.4 2.2 3.26 1.8 6.0 5.0 10.4

0.01 0.01 0.02 – 0.01 0.02 0.01 0.00 0.03 0.03 – 0.01 0.12 0.01 – 0.01 0.01 0.17 0.65 0.11 0.20 0.05 0.12 0.05 0.17 0.16 0.49

0.07 0.02 0.11 0.05 0.04 0.03 0.05 0.08 0.03 0.04 0.02 0.07 0.20 0.11 0.11 0.13 0.09 0.26 0.65 0.40 0.50 0.41 0.39 0.27 0.53 0.35 1.77

0.03 0.01 0.06 0.04 0.004 0.001 0.01 0.002 0.001 0.004 0.004 0.05 0.08 0.05 0.12 0.12 0.07 0.18 0.34 0.28 0.24 0.24 0.27 0.19 0.22 0.24 1.09

0.7 0.3 0.6 1.0 0.3 0.03 0.05 0.1 0.03 0.2 0.03 1.1 0.7 0.9 1.4 1.6 1.0 1.2 1.9 1.8 1.7 1.9 2.3 1.7 2.1 1.9 9.9

0.52 0.19 0.45 0.34 0.11 0.04 0.05 0.07 0.04 0.07 0.03 0.46 0.12 0.33 0.62 0.59 0.41 0.22 0.68 0.47 0.36 0.37 0.50 0.34 0.49 0.44 2.71

Dy

Ho

Er

Tm

Yb

3.8 6.4 6.3 5.7 7.7 97 73 27

3.5 5.6 5.1 4.9 6.3 183 138 43

0.5 0.8 0.8 0.7 0.9 41 29 9

4.3 7.9 5.6 6.1 6.8 355 237 77

0.8 1.1 0.9 1.2 1.2 67 45 15

13.3 14.5 11.1 13.3 10.9 12.8 9.6 10.9

0.6 0.5 0.6 1.0 0.9 1.0 4.0 2.9 5.1

4.7 4.0 4.9 6.9 6.4 7.0 50 37 59

2.0 2.2 2.9 3.7 2.1 3.9 110 96 118

0.3 0.2 0.5 0.5 0.2 0.5 27 27 28

1.4 1.7 2.7 3.2 1.7 3.0 250 280 257

0.2 0.3 0.5 0.5 0.1 0.5 47 67 52

12.0 12.2 12.0 11.7 12.0 12.0 11.7 12.8 11.7

0.5 0.6 0.8 0.4 0.5 0.5 0.7 0.4 1.5 0.6 8.6 5.1 11.1 5.4

10 8.7 9.2 8.5 8.3 9.0 10 7.0 16 8.4 98 55 115 58

47 25 16 35 25 41 23 18 10 13 172 100 174 109

16 6.5 3.9 10 6.6 13 5.0 5.7 1.2 2.9 35 20 34 23

182 67 37 118 65 140 44 55 7.4 27 280 166 267 187

42 16 8.3 28 14 31 8.6 13 1.2 5.3 52 31 48 34

12.7 12.6 13.7 12.8 13.0 13.0 11.7 13.9 13.1 12.7 11.9 10.0 9.9 10.8

10 4.7 7.3 6.6 3.4 1.8 2.1 3.4 2.1 3.7 2.3 7.1 1.9 4.8 7.5 8.1 6.6 3.5 6.1 4.1 3.7 3.3 4.8 3.5 5.1 5.2 38

5.2 2.3 2.9 2.7 2.2 0.9 0.8 1.7 0.7 2.0 0.6 2.9 0.7 1.8 3.3 2.8 2.4 0.5 1.5 0.8 0.8 0.8 1.2 0.7 1.1 1.1 19

6.4 4.1 4.1 3.2 8.3 4.6 3.0 6.0 3.2 5.7 2.7 3.6 1.4 1.8 3.1 2.8 2.3 0.4 1.0 0.5 0.6 0.4 0.6 0.4 0.7 0.7 23

57 39 35 29 108 70 45 77 46 69 44 34 22 15 25 22 20 2.8 7.8 4.0 4.6 2.7 5.6 2.2 4.9 4.8 205

11 8.5 7.7 6.0 27 21 12 21 13 16 12 7.0 5.9 2.7 5.7 5.1 3.9 0.5 1.3 0.5 0.9 0.5 1.0 0.4 1.0 1.0 44

13.4 14.2 13.0 13.3 14.8 17.4 15.8 25.4 17.9 13.5 15.3 13.7 16.1 13.0 13.7 12.4 12.6 16.1 14.9 13.8 14.1 11.3 15.0 11.0 13.7 15.1 12.5

26 16 17 14 20 10 7.3 14 6.3 17 6.3 15 3.9 8.2 14 12 10 1.8 5.5 2.7 3.1 2.2 3.8 2.0 4.2 3.5 99

Lu

Hf (×103)

243

(continued on next page)

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

CL

244

Table 3 (continued) Spota

CL

T (°C)

Th

U

Li

Be

B

F

Na

Al

P

K

Ca

Sc

49

Ti

Fe

Y

Nb

La

Ce

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Hf (×103)

61 24

526 209

99 41

12.3 13.7

Sample GL03-201 23.1t 23.2t

c c

812 861

623 648

0.10 0.26

0.001 0.001

0.2 0.02

14 12

5.5 2.8

14 8

738 251

3.5 1.8

8.9 2.9

57 38

19.4 30.1

6.4 5.7

1346 463

5.3 2.5

0.020 0.017

10.1 6.3

0.48 0.24

1.73 0.83

0.21 0.19

23 8.8

9.60 4.05

117 54

56 21

279 113

Sample GL03-145 11.3t r 653 12.2t r 703 13.1t r 684 14.2t r 689 15.2t r 703 16.2t r 681 17.2t r 689 18.2t r 654 19.1t r 676 19.3t r 689 20.2t r 700 21.1t r 649 22.1t r 632 11.2t m 676 12.1t m 693 17.1t m 700 18.1t m 717 21.2t m 687 11.1t rc 911 14.1t rc 952 15.1t rc 934 16.1t rc 892 19.2t rc 946 20.1t rc 896

0.8 3.6 3.9 5.9 4.4 2.1 3.8 1.2 1.7 3.9 4.2 1.5 0.6 3.8 4.7 4.0 5.0 2.8 12 111 80 11 64 10

150 360 349 573 403 197 341 160 178 381 399 220 130 338 370 388 570 284 147 237 725 131 291 111

b0.01 0.05 0.03 0.06 0.05 0.03 0.06 0.03 0.02 0.05 0.06 0.05 0.05 0.06 0.03 0.05 0.07 0.03 0.08 0.05 0.23 0.06 0.06 0.06

0.006 0.034 0.053 0.021 0.023 0.022 0.005 0.017 0.019 0.007 0.007 0.003 0.003 0.022 0.005 0.010 0.076 0.033 0.008 0.019 0.075 0.001 0.007 0.001

0.03 0.05 0.1 0.04 0.1 0.1 0.05 0.04 0.1 0.1 0.02 0.02 0.04 0.1 0.1 0.05 0.1 0.1 0.2 0.2 0.4 0.1 0.1 0.1

10 14 14 14 13 15 13 11 12 11 10 14 17 10 12 12 11 21 89 16 20 22 12 11

2.6 3.2 4.2 3.0 2.9 3.2 3.0 2.6 2.8 3.0 2.9 2.6 3.7 2.8 2.7 2.7 3.3 2.6 4.5 10.8 9.1 5.3 3.4 3.0

5 6 9 6 7 8 6 5 6 7 5 5 9 6 6 5 7 5 16 32 25 16 8 10

40 77 48 76 54 44 53 46 51 54 64 57 51 46 56 53 74 76 65 460 301 72 155 61

1.6 1.8 2.5 1.6 1.8 1.9 1.8 1.4 1.4 1.6 1.1 1.8 2.1 1.4 1.3 1.2 1.5 1.5 2.8 22.7 3.0 3.3 1.4 1.2

2.7 3.7 4.6 3.0 3.3 5.7 2.8 2.7 3.5 3.3 2.9 3.6 4.1 3.3 3.1 2.5 2.8 3.1 7.4 3.1 18.4 5.6 3.4 3.6

121 216 120 104 145 124 127 129 159 135 169 70 36 126 150 139 254 204 51 243 70 45 68 45

3.4 6.2 5.0 5.3 6.2 4.8 5.3 3.4 4.5 5.2 6.0 3.2 2.6 4.5 5.5 6.0 7.3 5.1 45.2 62.0 54.2 39.0 59.3 40.1

1.0 1.2 1.2 1.6 1.4 2.8 1.3 0.8 0.8 1.1 1.3 1.4 1.0 1.2 1.1 1.1 1.6 1.2 6.0 3.1 46 4.2 3.9 4.7

204 342 206 189 244 165 210 281 255 238 304 112 53 217 254 235 399 301 44 522 459 40 177 29

3.4 5.0 3.0 3.7 3.7 2.8 2.9 3.4 4.6 3.2 3.8 3.3 2.2 2.7 3.3 3.5 6.3 4.0 1.4 4.1 3.0 1.4 1.8 1.4

0.007 0.010 0.009 b0.001 0.005 0.140 0.015 0.005 0.005 0.007 0.005 0.027 0.003 0.004 b0.001 0.005 0.019 b0.001 0.291 0.018 0.439 0.017 0.002 0.002

0.8 2.4 2.7 3.2 3.0 2.3 2.4 0.8 1.4 2.5 2.5 1.4 1.0 2.4 2.9 2.9 3.3 1.8 4.5 18.5 10.7 3.9 8.8 3.1

0.05 0.1 0.1 0.05 0.1 0.6 0.1 0.04 0.04 0.04 0.1 0.1 0.01 0.1 0.1 0.1 0.1 0.1 1.3 0.9 1.2 0.1 0.5 0.1

0.21 0.61 0.44 0.39 0.42 0.54 0.40 0.29 0.23 0.55 0.66 0.15 0.08 0.54 0.57 0.54 0.86 0.37 1.02 2.57 1.14 0.56 1.05 0.37

0.2 0.5 0.4 0.4 0.4 0.5 0.5 0.3 0.3 0.5 0.5 0.1 0.1 0.5 0.5 0.5 0.7 0.4 1.0 1.4 1.1 0.4 0.8 0.4

2.7 6.9 5.2 4.6 6.2 4.2 5.4 3.9 4.4 6.1 7.1 1.6 0.7 5.3 6.5 6.5 8.8 4.6 3.9 19 6.7 2.7 5.8 2.2

1.4 2.9 1.8 1.7 2.3 1.3 2.2 1.9 1.9 2.3 2.6 0.84 0.32 1.8 2.4 2.1 3.4 2.0 0.82 5.3 2.4 0.66 1.6 0.55

21 35 23 20 23 16 25 26 24 26 31 11 5 24 28 25 39 27 6 54 33 6 18 5

8 13 8 7 10 6 8 11 10 9 12 4 2 9 10 9 16 11 2 19 17 2 7 1

37 56 34 31 41 28 36 50 41 41 53 20 10 35 40 39 72 51 5 83 94 5 33 3

9 12 7 7 9 6 8 11 9 8 11 4 2 7 8 8 16 10 0.9 16 23 1.0 7 0.7

71 103 61 49 74 49 62 87 68 68 91 36 20 60 72 69 136 91 7.2 131 218 8.1 70 4.7

14 20 12 10 13 9 13 16 14 13 18 7 3 11 13 14 27 17 1.2 26 46 1.2 15 0.8

13.0 10.3 12.2 10.5 10.9 12.0 11.2 12.4 11.8 11.3 11.4 12.8 13.0 11.0 10.4 11.1 10.9 10.7 12.7 11.3 15.1 12.7 9.9 11.9

Sample GL03-155 1.1t r 638 3.1t r 766 4.1t r 714 5.1t r 636 7.1t r 813 2.1t c 746 6.1t c 714 7.2t c 714

0.8 20 14 1.2 34 15 7.7 7.0

220 1768 1345 192 3113 1229 801 760

0.04 0.09 0.06 0.02 0.20 0.04 0.02 0.02

0.003 0.595 0.091 0.026 0.129 0.268 0.085 0.045

0.2 1.8 0.5 0.3 1.0 0.3 0.2 0.1

8 36 4 7 10 12 8 4

4.6 12.8 4.7 6.9 19.2 10.4 4.2 7.6

16 62 26 18 48 20 19 11

55 113 89 42 192 208 154 142

0.8 2.9 1.2 1.0 3.2 2.1 0.9 0.9

1.8 23.9 8.1 4.1 15.9 2.8 3.5 4.8

45 229 131 72 373 477 322 301

2.8 12.3 7.1 2.7 19.5 10.1 7.1 7.1

1.4 25 3.8 1.7 15 4.0 2.5 1.9

55 739 391 107 1098 1128 584 540

1.7 15 10 1.9 22 8.1 4.8 4.3

0.001 1.256 0.216 0.007 0.072 0.047 0.003 0.008

1.0 15 6.1 1.2 10 6.8 3.8 3.1

0.0 3.3 1.3 0.0 0.5 0.3 0.1 0.1

0.11 2.99 1.41 0.17 2.60 1.56 0.93 0.81

0.0 2.2 0.9 0.1 2.6 1.5 0.9 0.8

0.7 21 11 2.0 31 22 11 11

2.2 30 16 4.7 50 46 25 22

4.7 83 41 11 139 112 62 58

13 133 74 23 229 208 119 108

3.5 27 16 4.8 44 42 25 24

36 216 122 45 340 339 217 198

9.4 39 23 8.4 63 63 40 38

11.2 12.3 12.8 13.9 21.5 11.6 12.0 12.1

Sample GL03-147 1.1t c 650 2.1t c 680 3.1t c 791 4.1t c 686 5.1t c 698 6.1t c 699 7.1t c 673 8.1t c 756 9.1t c 713 10.1t c 668 11.1t c 643 13.1t c 666 13.1t c 704 14.1t c 666

5.9 2.7 19 11 8.6 3.3 2.2 9.3 10 1.3 0.7 1.6 8.3 1.5

745 381 1404 857 1085 544 424 846 922 295 234 401 1050 309

0.08 0.04 0.01 0.05 0.07 0.03 0.02 0.05 0.08 0.03 0.03 0.03 0.10 0.03

b0.001 0.016 0.205 0.039 0.087 0.051 0.049 0.075 0.109 0.025 0.018 0.013 0.039 0.028

0.1 0.1 0.1 0.1 0.03 0.1 0.1 0.1 0.1 0.1 0.03 0.1 0.1 0.1

45 16 19 12 17 17 16 16 17 17 20 24 19 22

11.9 4.3 3.9 3.8 3.5 2.9 3.4 5.1 4.6 3.8 4.5 3.5 4.5 4.0

10 10 6 7 6 5 5 8 8 6 8 8 8 8

70 122 249 88 152 156 151 191 172 84 50 87 163 80

6.1 2.0 1.8 2.0 1.7 1.4 1.5 2.4 2.3 1.7 2.4 1.7 2.4 2.1

7.8 4.3 2.9 3.2 2.5 2.6 3.1 4.3 4.2 3.1 4.0 5.3 4.3 4.2

113 294 695 236 302 342 230 548 379 217 129 161 328 209

3.3 4.7 15.8 5.1 5.8 5.9 4.4 11.2 7.0 4.1 3.0 4.0 6.3 3.9

1.3 1.4 2.6 1.5 1.9 1.4 1.3 2.3 2.2 0.8 0.8 1.6 2.1 1.0

562 490 2244 943 795 628 620 1280 818 389 251 411 777 375

4.9 5.0 17 10 7.7 7.6 6.4 9.3 8.8 4.5 4.3 6.4 6.5 4.5

0.042 0.008 0.003 0.005 b0.001 0.005 0.003 0.003 0.005 0.005 0.003 0.280 0.003 0.003

1.2 1.2 6.4 3.2 2.7 1.9 1.3 4.3 3.1 1.4 0.7 2.2 2.5 1.0

0.2 0.1 0.2 0.5 0.2 0.1 0.05 0.1 0.1 0.02 0.1 0.3 0.1 0.03

0.72 0.38 1.65 1.69 0.78 0.49 0.29 0.98 0.71 0.27 0.19 0.37 0.63 0.25

0.6 0.4 1.9 1.5 0.7 0.4 0.3 1.2 0.6 0.3 0.1 0.2 0.6 0.3

23 19 86 38 31 24 23 49 31 15 9 15 29 13

4.4 3.1 16.0 8.4 5.3 3.8 3.0 8.0 5.1 2.2 1.4 1.9 4.5 2.0

11 6.7 34 22 12 8.7 5.9 18 11 4.7 2.2 3.9 10 3.8

100 95 405 165 146 116 120 222 146 73 51 77 141 73

22 21 83 33 32 26 28 46 32 16 12 19 31 17

174 182 644 261 265 210 258 382 263 140 115 172 263 155

0.3 7.1 3.9 0.9 12.2 9.4 5.0 4.4

51 45 203 99 72 58 50 113 70 29 20 32 71 33

35 36 121 48 51 43 51 73 52 30 23 36 50 31

20.6 12.5 10.9 16.7 17.4 13.2 13.8 10.2 17.1 12.9 14.7 15.8 17.5 14.1

Note: All analyses were performed on the SHRIMP-RG at the United States Geological Survey-Stanford University Microanalytical Center, Stanford, CA following the procedure outlined in Mazdab (2009) and Mazdab and Wooden (2006). All abundances are expressed in ppm. Analyses in italics reflect high contaminant levels and are therefore removed from the discussion. a 1.1 = grain number.spot number; c = core; rc = round core; m = mantle; r = rim; s = spherical; e = elongate; z = oscillatory zoned, p = patchy; i = igneous rare earth element signature.

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

103 46

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

zircon/chondrite

103

a) 03-143

245

i) 03-201

e) 03-170

102 101

m

100

r1 r2 r3

10-1 10-2

103

zircon/chondrite

c

b) 03-184

j) 03-145

f) 03-154 r1

102 101

r2-3

100 10-1 10-2

zircon/chondrite

103

c) 03-130

g) 03-156

k) 03-155

d) 03-114

h) 03-113

l) 03-147

102 101 100 10-1 10-2

zircon/chondrite

103 102 101 100 10-1 10-2

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

cores - steep HREE, magmatic signature

steep HREE, magmatic signature

flat HREE, high-T with garnet signature

very steep HREE, metamorphic without garnet signature

flat HREE, low-T with garnet signature Fig. 3. Chondrite-normalized REE patterns for the different CL domains of zircons from (a) coesite-bearing eclogite, (b) coesite-bearing gneiss; (c–g) quartzofeldspathic host gneisses; (h) garnet-bearing granitoid; (i) hornblende-bearing leucosome; and (j–l) cross-cutting pegmatites. Pm (gray label) is interpolated. Normalization uses the chondrite REE abundances of Anders and Grevesse (1989) multiplied by 1.36 (Korotev, 1996). Symbols depict five different REE patterns.

periods of major zircon growth. The new U–Pb data presented in Table 2 and U/Pb data from zircon rims in gneiss samples 03-114, 03-170, 03-156 that was reported but not discussed in Gilotti and McClelland (2011) are plotted on Tera–Wasserburg diagrams and age distribution diagrams (Fig. 4). Previously published U/Pb data is presented only on age distribution diagrams (Fig. 4). New trace element data including

Ti is presented for 10 of the 12 samples in Table 3. Trace element and Ti-in-zircon data for other samples (03-184, 03-113) can be found in McClelland et al. (2009). Characterizing zircons in terms of domains defined by U–Pb age, CL response, Th/U chemistry, and rare earth element (REE) patterns has proven useful in discriminating zircon production in different environments. Igneous zircon is characterized by relatively

246

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

steep heavy rare earth element (HREE) patterns, a pronounced negative Eu anomaly and a positive Ce anomaly (Hoskin and Schaltegger, 2003). Eclogite facies metamorphic zircon generally varies from this pattern with a flattening of the HREE pattern, decrease in ∑REE, loss of the Eu anomaly, and decrease in Th/U (Rubatto, 2002). Zircons from partial melts (e.g., leucosomes) have a range of signatures from flat to very steep HREE, variable presence of a negative Eu anomaly, and different ∑REE (e.g., McClelland et al., 2009; Rubatto et al, 2009). 5.1. 03-143 coesite-bearing quartz eclogite Zircon in quartz eclogite 03-143 is spherical to ellipsoidal (100–300 μm in the longest dimension), with complex cores and rims—a common characteristic of high-grade, metamorphic zircon (Corfu et al., 2003). Some oscillatory zoned, elongate cores are preserved (e.g. grain 4 in Fig. 2a); but in most cases, CL-dark cores are mottled in appearance and surrounded by multiple, nearly concentric, homogeneous rims that are moderately luminescent in CL (Fig. 2a). Six oscillatory zoned cores (c2-ezi and c3-ezi, Table 2; c-ezi, Table 3) have Th/U = 0.3–0.5, steep HREE (Yb/Gd = 11–16), ∑ REE = 286–729, and a well preserved, negative Eu anomaly. Three homogeneous mantles (m-epi, Table 2) on elongate, zoned cores have steep but lower abundance HREE patterns as well. Core and mantle domains with steep HREE patterns are interpreted to represent zircon from the mafic protolith, consistent with similar cores in previously dated coesite-bearing zircons in eclogites that retain relicts of their Paleoproterozoic protoliths (McClelland et al., 2006). Three of the cores give older, strongly discordant ages (c3, Table 2, Fig. 4a) indicating partial resetting of the protolith U/Pb systematics. The other domains that retain the original magmatic REE signature (c2-ezi, m-epi, Table 2) were entirely reset through extensive Pb-loss and give U/Pb ages from 336 to 350 Ma. The remaining cores are mottled or patchy (c-p, -ep, Tables 2 and 3) to homogeneous in CL (c, Tables 2 and 3). Analyses 9.1t (Table 3) and 9.1 (Table 2) show steep and flat HREE patterns, respectively, from the patchy core of grain 9 (Fig. 2a) indicating that the patchy cores are likely a heterogeneous mix of extensively recrystallized, protolith zircon (steep HREE) and newly formed zircon (flat HREE). The homogeneous cores of spherical grains (c-s, Tables 2 and 3) are compositionally and texturally similar to mantles (m, Tables 2 and 3) that overgrow the zoned or patchy protolith cores. Both CL domains have Th/U = 0.04–0.6, flat HREE (Yb/Gd = 0.6–2.0), reduced ∑REE (15–37), and no negative Eu anomaly (Fig. 3a). The CL domain containing coesite (grain 40, Fig. A1) falls in the homogeneous core group. The mantles are typically overgrown by multiple, faintly zoned metamorphic rims (r, Tables 2 and 3) that are generally brighter in CL, but the ranges of Th/U (0.07–0.6), Yb/Gd (0.7–2.0), and ∑REE (15–99) and the age range of 323–363 Ma overlap with that of the cores. The homogeneous core, mantle and rim domains are interpreted as newly grown metamorphic zircon on the basis of the presence of UHP inclusions and uniform trace element signatures that are distinct from the original magmatic signature. The U/Pb ages for the metamorphic zircon vary from 379 to 320 Ma with a small cluster at ~360 Ma, and most ages falling around 340 Ma (Fig. 4a). The lack of correlation between core versus rim CL domains and U/Pb age makes it difficult to distinguish between continuous and episodic crystallization. Two older concordant analyses at 377–379 Ma (Group 1, Table 2; Fig. 4a) are consistent with the age of HP metamorphism in the NEGEP (Gilotti et al., 2004), indicating that zircon crystallization began when the crust thickened to form the HP terrane. All remaining analyses give Carboniferous metamorphic 206Pb/238U ages

that can be interpreted in two fundamentally different ways. Assuming that zircon growth occurs during distinct events, the data can be divided into two groups (Groups 2 and 3, Table 2; Fig. 4a) of mixed core, mantle and rim analyses with mean ages of 359 ± 3 Ma (n = 7) and 340 ± 2 Ma (n = 18). Alternatively, the ages can be interpreted to record continuous growth from 365 to 330 Ma. The 7 analyses of Group 2 that cluster around 360 Ma are clear outliers with respect to the younger analyses (Fig. 4a), and are therefore interpreted to record zircon growth at 359 ± 3 Ma during UHP metamorphism, consistent with ages of coesite-bearing zircon domains in other samples (McClelland et al., 2006). The uniform spread in the younger ages is best interpreted to record semi-continuous or continuous zircon growth during multiple closely spaced intervals from 350 to 330 Ma (Fig. 4a), since there is no clear CL and trace element variation as a function of age. The one analysis of zircon surrounding the coesite inclusion in grain 40 (Fig. A1) is from a dark gray core that gives a 206Pb/238U age of 341 Ma, which is considerably younger than the 365–350 Ma ages previously determined for UHP metamorphism (McClelland et al., 2006). We infer that the coesite is armored by a thin domain of older, ~360 Ma zircon which is overgrown by more voluminous 350–330 Ma zircon with similar trace element but slightly distinct CL characteristics (Fig. A1). Thus obtaining a direct age on UHP metamorphism for this sample was not successful due to the small domain size. Most of the zircon growth in this sample is interpreted to have occurred during exhumation after 345 Ma while preserving flat HREE patterns and slight negative Eu anomalies. The significance of the 2 youngest analyses is uncertain. 5.2. 03-184 coesite-bearing felsic gneiss Zircon morphology, inclusions, trace element chemistry and U–Pb ages for coesite-bearing gneiss sample 03-184 have been previously described (McClelland et al., 2006, 2009), but we include this sample because it provides a starting point for trace element behavior in felsic gneiss at UHP conditions. CL-dark cores with faint oscillatory zoning and variably developed patchy zones of recrystallization are overgrown by coesite-bearing, CL-bright mantles and CL-moderate rims (Fig. 2b). Most protolith cores have steep HREE (Yb/Gd = 20–64) and show depletion in abundance (∑REE = 64–701) and lowering of Th/U (0.02–0.35) with progressive recrystallization (Fig. 3b). The CL-bright, low-U coesite-bearing domain and CL-moderate rims have relatively flat HREE (Yb/Gd = 4–154), very depleted ∑REE (3–6; Fig. 3b) and low Th/U (0.01–0.14), and persistent negative Eu anomaly. Uncertainties on the individual measurements are high (±10 m.y. for most, 1σ) due to the generally low U concentrations (b15 ppm). Ages from both domains vary smoothly from 380 to 340 Ma (Fig. 4b) and give a weighted mean 206Pb/238U age of 358 ± 4 Ma (McClelland et al., 2006) that we have interpreted to represent the time of UHP metamorphism at 365–350 Ma (McClelland et al., 2009). An alternative interpretation of continuous metamorphic zircon growth and/or recrystallization from 380 to 340 Ma is permissible on the basis of textural evidence suggestive of a complex history of zircon growth and recrystallization (McClelland et al., 2009). In this case, the older ages (>350 Ma) are interpreted as dating UHP metamorphism because of the coesite inclusions, and the younger ages (b350 Ma) reflect growth or modification during exhumation. 5.3. 03-130 intermediate gneiss Zircon in 03-130 is described in McClelland et al. (2009) as ovoid, and more similar to the coesite-bearing grains in 03-143 and 03-184 above,

Fig. 4. Plots of U/Pb data for (a) coesite-bearing eclogite, (b) coesite-bearing gneiss; (c–g) quartzofeldspathic host gneisses; (h) garnet-bearing granitoid; (i) hornblende-bearing leucosome; and (j–l) cross-cutting pegmatites. Terra–Wasserburg plots show data published herein or presented but not discussed in Gilotti and McClelland (2011). Data are 1σ error ellipses uncorrected for common Pb. Filled ellipses are used in calculating weighted mean 206Pb/238U ages discussed in the text. Errors are reported at the 95% confidence level. Age distribution and cumulative probability density plots are provided for all samples discussed in the text. Ages are divided into 10 m.y. increments to help visually track the variation through the sample suite.

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

a) 03-143

247

e) 03-170

0.062

(MSWD = 0.5; n=7)

(MSWD = 1.2; n=18)

206

206

Pb

370

350

330

reset protolith cores

207

Age

380 370 360 350 340 330 320

20

400 238

12

U/

206

340

20

Pb

238

17

320

360 60

359 + 3 Ma

320

370 360 350 340 330 320

U/

206

Pb

19

343 + 4 Ma

Number 1 350-330 Ma

f) 03-154 0.07

Number 3

338 + 3 Ma (MSWD = 0.2; n=13)

Pb

1

r2

206

b) 03-184

Age

360

0.05

3

2

1

380

Age

Pb/

c3

0.05

310

207

390

440

(MSWD = 0.7; n=4)

Pb/

0.08

480

343 + 4 Ma

0.06

Pb

359 + 3 Ma

340 + 2 Ma

207

Pb/

400 390 380 370 360 350 340 330 320

358 + 4 Ma

380 420

r1

r3

356 + 5 Ma

340

(MSWD = 1.3; n=11)

1

Number 5

Age

400 390 380 370 360 350 340 330 320

Age

c) 03-130

347 + 3 Ma

cores

400 390 380 370 360 350 340 330 320

238

321 + 3 Ma

(MSWD = 1.0; n=11)

0.04 16

U/

206

356 + 5 Ma

360-320 Ma

1 Number 5 Number 4

d) 03-114

g) 03-156

0.064

0.065

Pb

342 + 3 Ma (MSWD = 2.3; n=11)

core

core

207

390

400

360

207

Pb/

Pb/

206

206

Pb

22

Pb

rims 1

300

370

350

330

0.050

0.055

(MSWD = 2.1; n=8)

362 + 4 Ma (MSWD = 0.5; n=3)

370 360 350 340 330 320

238

U/

16 206

Pb

19

362 + 4 Ma Age

Age

16

350-330 Ma 13

370-350 Ma

380 370 360 350 340 330 320

238

U/

206

Pb

20

335 + 4 Ma

Number 2

Number

350-340 Ma

320

335 + 4 Ma

340-330 Ma

330-320 Ma

248

J.A. Gilotti et al. / Gondwana Research 25 (2014) 235–256

h) 03-113 Age

390 380 370 360 350 340 330 320

347 + 2 Ma 329 + 5 Ma mantles rims

350-320 Ma

Number 2

i) 03-201

Age

380 370 360 350 340 330 320

322 + 8 Ma (MSWD = 1.3) grey rims

mantles

5.4. 03-114 intermediate gneiss

329 + 5 Ma (MSWD = 2.8) 1Number 3

j) 03-145 0.064

338 + 3 Ma

207

Pb/

206

Pb

(MSWD = 1.3; n=6)

350

0.050

310

330

320 + 3 Ma (MSWD = 5.2; n=16)

238

Age

18 350 340 330 320

U/

206

Pb

21

338 + 3 Ma 320 + 3 Ma

Number 2

Age

k) 03-155 350 340 330 320

321 + 2 Ma

330 + 2 Ma 1Number 3

l) 03-147 0.059

320 + 3 Ma

207

Pb/

206

Pb

(MSWD = 6.1; n=9)

340

330

320

310

0.051

Age

18.6 340 330 320 310

than those in the other gneiss samples. Mottled, moderately luminescent cores with faint oscillatory zoning cut by veins and healed fractures are partially surrounded by homogeneous, gray rims that are overgrown by very narrow, very CL-bright rims (Fig. 2c). The cores preserve a protolith trace element signature with steep HREE (Yb/Gd = 10–18), high ∑REE (207–309), Th/U = 0.2–0.5, and a negative Eu anomaly (Fig. 3c). The gray rims show flat HREE (Yb/Gd = 0.8–3), low ∑REE (12–75) and Th/U (0.03–0.14), and no Eu anomaly. In stark contrast to the geochemistry, the U–Pb geochronology shows that the Precambrian protolith cores are reset and give the same Carboniferous metamorphic age as the rims. Assuming that the oldest core age and the youngest core and rim ages are outliers, the remaining cores and rims give a well defined age of 347 ± 3 Ma (McClelland et al., 2009). The two youngest analyses may reflect growth or modification at ~330 Ma, as more robustly seen in other samples.

238

U/

206

Pb

20.6

Elongate to subequant zircon in 03-114 has complex, oscillatoryzoned cores with patchy bright areas indicating modification by recrystallization (Fig. 2d). Up to three different rims, each homogeneous in CL, overgrow the cores. The cores show magmatic REE patterns (Yb/Gd = 10–21; ∑ REE = 401–1524, Th/U = 0.2–0.5; Fig. 3d) and give discordant U/Pb analyses that define a protolith age of 2005 ± 33 Ma (Gilotti and McClelland, 2011). The rim domains have flat HREE patterns (Yb/Gd = 10–21), low ∑REE (27–92), low Th/U (0.01–0.1), and U/Pb ages ranging from 324 to 365 Ma. The U/Pb analyses define an older cluster with a weighted mean age of 362 ± 4 Ma, and a second group that can be interpreted to record continuous or semi-continuous zircon growth from 350 to 330 Ma or a single event at 343 ± 3 Ma based on the weighted mean 206Pb/238U ages of 11 rim analyses (Fig. 4d). The multiple rims observed in CL images do not match simple age variations, and thus cannot discriminate between the alternative interpretations of episodic versus continuous growth. The single young analysis, clearly an outlier, records additional growth or Pb-loss. 5.5. 03-170 intermediate gneiss Zircons in 03-170 are elongate to subequant ellipsoids containing distinct cores and thick rims (Fig. 2e). The cores are generally CL-bright with complex oscillatory and parallel zoning that is crosscut and overprinted by mottled and patchy areas with embayed boundaries. The cores give discordant U/Pb ages that define a protolith age of 1983 ± 34 Ma (Gilotti and McClelland, 2011) and steep HREE (Yb/Gd = 9–64) and high ∑ REE (203–849) characteristic of igneous zircon (Fig. 3e). The cores are overgrown by thick CL-dark rims with faint zoning that have flat HREE (Yb/Gd = 1–3), low ∑ REE (26–43), low Th/U (0.02–0.08), and no negative Eu anomaly. Very thin, CL-bright outer rims too small to analyze are present on most grains. Only 7 nearly concordant U/Pb rim ages representing metamorphic growth are available for this sample. The 207Pb-corrected 206Pb/238U ages range from 329 to 361 Ma; 4 analyses give a weighted mean age of 343 ± 4 Ma (Fig. 4e), which is taken as the dominant time of zircon growth for this sample. The ~360 Ma analysis and two ~330 Ma analyses are consistent with the ages observed in the other gneiss samples. Additional data might show that zircon growth was continuous from 350 to 330 Ma.

320 + 3 Ma 5.6. 03-154 intermediate gneiss

Number 2 Fig. 4 (continued).

Zircon in the most retrogressed gneiss (03-154) contains beautiful, oscillatory-zoned, protolith cores overgrown by light gray metamorphic rims with faint zoning (Fig. 2f). The cores define steep HREE patterns (Yb/Gd = 17–36), high ∑REE (546–606) and Th/U (0.3–0.5), and give a protolith age of 1982 ± 11 Ma (McClelland et al., 2009). Additional

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Fig. 5. Summary of U/Pb ages from the North-East Greenland UHP terrane for eclogites, quartzofeldspathic gneisses and leucocratic intrusions. Additional data is from (a) Gilotti et al. (2004); (b) McClelland et al. (2006); (c) McClelland et al. (2009); (d) Gilotti and McClelland (2011); (e) Gilotti and McClelland (2007). Symbols are the same as those used in Figs. 3 and 6; c indicates coesite. Samples in italics are not discussed in this paper.

U–Pb and trace element data was collected from zircon rims for this study (Tables 2 and A1) to more clearly define the spread in rim ages, particularly the younger ages ~320 Ma. U/Pb analyses from the rims are divided into 3 populations (r1, r2, and r3 in Tables 2 and 3) on the basis of chemistry and age. The older population, r1, is chemically distinct with HREE patterns similar to other gneiss samples with Yb/ Gd = 4–24 (Fig. 3f) and a weighted mean age of 356 ± 5 Ma. The HREE pattern flattens more in the younger r2 and r3 populations with Yb/Gd = 0.7–1 largely due to depletion of HREE relative to MREE, ∑REE = 16–26 and Th/U = 0.03–0.07. Rims r2 and r3 give weighted mean ages of 338 ± 3 Ma and 321 ± 3 Ma (Fig. 4f). The alternative interpretation that zircon growth was continuous from 360 to 320 Ma and accompanied by changing trace element compositions at ~350 Ma is permissible. By comparison with other gneiss samples, we infer that the compositionally distinct ~356 Ma domain reflects UHP growth and rims r2 and r3 record growth from 350–320 Ma during exhumation. 5.7. 03-156 granitic gneiss Sample 03-156 is dominated by elongate, subhedral grains with prominent cores and rims (Fig. 2g). The cores variably preserve faint oscillatory zoning and are commonly affected by recrystallization along fractures, veins and in irregular patches. Homogeneous metamorphic rims are developed on most of the grains, particularly near the tips. There is typically a CL-dark inner rim, a lighter CL-gray outer rim, and a very thin CL-white rim that is too small to analyze. The cores with steep HREE patterns, ∑REE = 422–743 and Th/U ≈ 0.5 yield a protolith age of 1756 ± 36 Ma (Gilotti and McClelland, 2011). A few inner rims lack a Eu anomaly and display flat HREE (Yb/Gd = 2) and lower ∑REE (48–97). However, most metamorphic rims have Th/U = 0.01–0.03 and very steep HREE (Yb/Gd = 15–266) with abundances approaching levels of the protolith magmatic signature (∑REE = 99–305; Fig. 3g). The U–Pb rim ages range from 315 to 380 Ma with no defined variation between the CL rim domains. A weighted mean age of 335 ± 4 Ma defined by 8 of 13 rim analyses is interpreted as the time of major zircon rim growth (Fig. 4g). A longer period of metamorphic growth or modification is suggested by the scattered rim ages of ~380, ~355 and ~320 Ma that are outliers in the data set (Fig. 4g). Zircon with similar

characteristics (Yb/Gd = 15–266; ∑ REE = 113–256; Th/U = 0.02–0.03) from another garnet-bearing metagranitoid sample (03-123) gives rim ages that cluster around 330 Ma (330 ± 3 Ma, MSWD = 0.4, n = 6) and an additional 4 analyses that cluster at ~347 Ma (Gilotti and McClelland, 2011).

5.8. 03-113 garnet-bearing, boudin neck granitoid U/Pb ages from the garnet-granitoid (03-113) collected from the neck of an eclogite boudin are critical in establishing the timing of exhumation of the UHP terrane (Gilotti and McClelland, 2007). Zircon typically forms elongate, euhedral to subhedral prisms of up to 500 μm long. The grains have oscillatory zoned, xenocrystic cores overgrown by CL-dark mantles and lighter CL-gray rims that are best developed at grain tips (Fig. 2h). The cores have steep HREE and high ∑REE (367–1605) and Th/U (0.4–0.9) typical of magmatic zircon and give discordant U/Pb ages (McClelland et al., 2009). Grouping the analyses on the basis of CL domains, 5 older CL-dark mantle ages range from 360 to 350 Ma, 7 additional CL-dark mantles give an age of 347 ± 2 Ma and the CL-gray rims give an age of 329 ± 5 Ma (Gilotti and McClelland, 2007). The intermediate CL-dark domain analyses between 340 and 330 Ma record overlap between the clear mantle and rim domains; they may represent analytical mixing of the two domains or continuous growth or modification between the mantles and rims. Approximately half of the trace element analyses for granitoid 03-113 had elevated Fe, Al and Ca concentrations, suggesting significant alteration of the zircon population. The 360–350 Ma analyses have flat HREE patterns similar to the UHP metamorphic zircon in the adjacent coesite-bearing eclogite (sample 03-110, McClelland et al., 2006). The younger zircon is characterized by steep HREE patterns (Yb/Gd = 7–45), variable ∑REE (62–627), and low Th/U = 0.01–0.1. There is a slight compositional difference observed with a steepening of the HREE from the inner mantles and outer rims (McClelland et al., 2009). The older 360–350 Ma domains are interpreted as inherited metamorphic grains from the adjacent gneiss and eclogite. The mantles and rims are interpreted to record emplacement and crystallization at ~347 Ma (Gilotti and McClelland, 2007) followed by continuous or

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additional periods of zircon growth with compositional changes affecting the zircon between 347 and 320 Ma. 5.9. 03-201 hornblende-bearing leucosome Zircon from 03-201 has distinctive, well developed, fairly homogeneous, CL-gray and CL-white rims that surround complex, dark cores with low aspect ratios that are not always located in the center of the grains (Fig. 2i). The cores include xenocrystic grains that give discordant protolith ages (23.1, Fig. 2i) and a homogeneous CL-dark mantle that gives U–Pb ages ranging from 323 to 380 Ma (dc in table 1 of Gilotti and McClelland, 2007). The xenocrystic cores have steep HREE, ∑REE = 483–1183, and Th/U = 0.07–0.17 (Fig. 3i). The CL-dark mantles have flat HREE (Yb/Gd = 1–3) with no negative Eu anomaly and lower ∑REE (15–27), but define two populations on the basis of low Th/U (0.05–0.07) and very low Th/U (0.006–0.008). The inner CL-gray rims have steeper HREE (Yb/Gd = 14–141) with a slight negative Eu anomaly, higher ∑REE (36–117), but very low Th/U (0.001–0.006). Excluding the 4 oldest CL-dark mantles between 380 and 340 Ma as clear outliers, the gray rims and remaining dark mantles yield an age of 328 ± 3 Ma; excluding the 3 youngest CL-gray rims gives an age of 329 ± 3 Ma (Gilotti and McClelland, 2007). Separating the dark mantles from gray rims gives weighted mean 206Pb/238U ages of 329 ± 5 and 322 ± 8 Ma, respectively (Fig. 4i), consistent with the observation that the gray rims overgrow the dark mantles but are not distinguishable within uncertainty of the calculated ages. The outer CL-white rims have steeper HREE (385–2223) due to MREE depletion, a well developed, negative Eu anomaly, increased ∑REE (67–170) and higher Th/U (0.005–0.05) due to U depletion. U–Pb analysis of the CL-white rims was unsuccessful due to the very low U concentrations (0.2–2 ppm). Zircon from 03-201 clearly records a complex history. The CL-dark mantles are interpreted to record initial leucosome crystallization at ~330 Ma. An abrupt compositional change occurs between the CL-gray rims and CL-white rims. The U–Pb data is consistent with interpretation as two distinct events or as a continuum that records distinct compositional changes between 330 and 320 Ma. 5.10. 03-145 cross-cutting pegmatite Sample 03-145 yielded large, euhedral grains often with round cores overgrown by oscillatory zoned CL-dark to CL-light zircon (Fig. 2j). The round cores are distinguished from the oscillatory zoned zircon by high Th (10–111 ppm) and Th/U (0.1–0.5) ratios. Five of 6 cores analyzed for trace elements had elevated Fe, Al and Ca concentrations suggesting significant alteration of the zircon (Table 3); 3 of the cores have flat HREE (Yb/Gd = 2–3; Fig. 3j) whereas the other 3 are steep (Yb/Gd = 7–33). The cores are interpreted as inherited components from the adjacent gneisses. U/Pb ages of the cores vary from 341 to 315 Ma suggesting that the U/Pb systematics has been reset. Oscillatory zoned zircon is generally CL-dark in the grain interiors or mantles and becomes alternating CL-dark and CL-light toward the rims. All of the zoned zircon is relatively homogeneous with steep HREE (Yb/Gd = 11–28), a slight negative Eu anomaly, elevated ∑REE (44–322) and low but variable Th/U (0.005–0.013). U/Pb ages for the oscillatory zoned domains range from 305 to 342 Ma. A mix of 6 mantle and rim analyses define an older age cluster with a weighted mean U/Pb age of 338 ± 3 Ma; the remaining rim ages define a weighted mean U/Pb age of 320 ± 3 Ma, assuming that the 2 youngest outliers reflect Pb-loss (Fig. 4j). The large age range for this sample is surprising considering its apparently simple, oscillatory zoned grains and consistent trace element signature. We assume that the main zircon growth occurred at ~325 Ma during emplacement and crystallization of the cross-cutting pegmatite. The 340–330 Ma ages may record zircon crystallization associated with early melting in the gneisses, but are distinctly younger than the 347 Ma event recorded in granitoid 03-113.

5.11. 03-155 cross-cutting pegmatite Zircons from 03-155 are elongate, euhedral grains with oscillatory and lesser sector zoned CL-dark cores overgrown by thin CL-bright rims (Fig. 2k). Taking the oldest analysis as an outlier, the U–Pb ages span a shorter period than observed in 03-145, ranging from 332 to 314 Ma. The analyses plotted in the cumulative probability density plot define two age populations with peaks at ~330 and ~320 Ma with weighted mean 206Pb/238U ages of 330 ± 2 Ma and 321 ± 2 Ma (Fig. 4k), but no clear correlation between core versus rim domain as a function of age (McClelland et al., 2009). Both cores and rims have steep HREE (Yb/Gd = 10–20), ∑ REE = 317–925, and low Th/U (0.01). Two rims with young ages (≈ 318 Ma) have steeper HREE (22–52) and lower ∑ REE (71–101). 5.12. 03-147 cross-cutting pegmatite Sample 03-147 contains simple prismatic to elongate zircons with well developed oscillatory and sector zoning (Fig. 2l), steep HREE (Yb/Gd = 12–52), a slight negative Eu anomaly, high ∑ REE (234–1603) and Th/U = 0.003–0.01 (Fig. 3l). U/Pb analyses range from 335 to 313 Ma with the three oldest ages as clear outliers and the remaining 11 analyses giving a weighted mean 206Pb/238U age of 320 ± 3 Ma (Fig. 4l). As with sample 03-145, the large spread in ages is at odds with the apparent simplicity of the grains. We interpret that the bulk of the zircon growth at ~320 Ma accompanied emplacement and crystallization of the cross-cutting pegmatite. 6. Ti-in-zircon thermometry Titanium content was measured in 221 different spots from 121 zircons in the twelve samples described above (Table 2). Ti ranges from 0.9 to 65 ppm. Multiple analyses of individual zircons were necessary to sample the different age and chemical domains. Ti-in-zircon temperatures were calculated using equation 15 in Ferry and Watson (2007). Because all twelve samples contain free quartz, the activity of SiO2 is set equal to one (aSiO2 = 1). Rutile is present in the coesite-bearing samples and the quartzofeldspathic gneisses, allowing aTiO2 = 1. Rutile is absent in the leucocratic layers and dikes, so temperature was calculated with aTiO2 = 0.7, based on Hayden and Watson's (2007) conclusion that hydrous, siliceous melts rarely have aTiO2 b 0.6, and some of the Greenland samples contain ilmenite. Temperature is plotted as a function of Yb/Gd (Fig. 6) in order to distinguish among individual chemical domains in zircon, particularly the MREE in relation to the HREE. Fig. 6a shows representative zircon data from the literature, with T calculated from Ferry and Watson (2007), which delineates a field of zircon growth in the presence of magmatic and metamorphic melts. Trace element concentrations and ratios in magmatic zircon vary as a function of melt composition during fractionation and crystallization upon cooling, but typically exhibit Yb/Gd ratios between ≈8 and 50 as indicated by the data sets of Claiborne et al. (2006, 2010) and Barth and Wooden (2010). Trace element composition of zircon in metamorphic environments is more variable, but appears dominated by the presence or absence of externally derived fluids (Rubatto et al., 2009) and the impact of metamorphic reactions involving garnet during zircon growth (Rubatto, 2002; Whitehouse and Platt, 2003). Metamorphic zircon from UHP eclogite in the Sulu orogenic belt, eastern China, defines a field of Yb/Gd = 1–50 over a T range of 650–750 °C (Gao et al., 2010). Zircon from UHT granulites of the Anápolis–Itauçu complex, Brazil, formed at sub-peak T of 750–950 °C with Yb/Gd = 0.1–40 (Baldwin and Brown, 2008). The trend toward lower Yb/Gd values reflects the influence of garnet in sequestering HREE in lower T and higher T settings, respectively (Rubatto, 2002; Whitehouse and Platt, 2003). Zircon grown during cooling of granulite facies rocks in southern India formed with Yb/Gd = 1 at 675–725 °C (Clark et al., 2009). Zircon rims with Yb/Gd = 2 precipitated in the

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a) Data from literature 950

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Yb/Gd Fig. 6. Temperatures derived from Ti-in-zircon thermometry vs. Yb/Gd exhibit decreasing temperature on the exhumation path from UHP conditions. The region of Yb/Gd = 8–60 delineates magmatic and metamorphic melts. (a) Compilation of magmatic and metamorphic zircon compositions and T estimates from the literature. (b–e) Greenland samples: (b) inherited Precambrian cores from all samples; (c) metamorphic zircon in coesite bearing samples; (d) metamorphic zircon from quartzofeldspathic gneisses; (e) Caledonian magmatic zircons from leucocratic layers and dikes. Symbols are the same as those used in Figs. 3 and 5.

presence of leucogranitic melt at ≈840 °C during cooling and exhumation of granulite migmatites in the Bohemian Massif (Kotková and Harley, 2010). Finally, zircon produced during fluid-assisted melting in amphibolite facies migmatites in the Central Alps defines a general trend of increasing Yb/Gd from ≈1 to 675 with decreasing temperature from ≈725 to 600 °C (Rubatto et al., 2009). The data presented in Fig. 6a provides a comparative template for interpretation of the REE data observed in the Greenland samples. Both intact and disturbed Paleoproterozoic zircon cores from the Greenland sample suite have temperatures in the range of 675–950 °C (Fig. 6b), with the least altered cores preserving lower values (675–800 °C) similar to the magmatic T of the original igneous protoliths. The Yb/Gd ratios from the inherited cores fall in the range of magmatic zircon, which is consistent with their formation in layered, mafic intrusive complexes, calc-alkaline batholiths and anorogenic granites—i.e. the igneous protoliths that comprise the bulk of the North-East Greenland eclogite province (Gilotti and McClelland, 2011). Elevated T estimates of 850–950 °C are recorded by protolith cores in the coesite-bearing samples (03-143, 03-184) and an intermediate gneiss (03-130). This increase reflects either resetting during UHP

metamorphism reaching >950 °C (Gilotti and Ravna, 2002) despite the expected slow rate of Ti diffusion in zircon (Cherniak and Watson, 2007), or may preserve a higher magmatic T for more mafic igneous protoliths. Carboniferous metamorphic zircon in coesite-bearing samples returns high temperatures ranging from 775 to 950 °C and typically show decreased Yb/Gd (Fig. 6c). The temperature range 800–950 °C of zircon in quartz eclogite 03-143 is associated with a 10-fold lowering of Yb/Gd. Coesite-gneiss sample 03-184 shows a much less pronounced shift in Yb/Gd at the T range of 800–900 °C, but has a remarkable drop in ∑ REE. As discussed below, interpretation of the Ti data from these samples is problematic due to potential pressure effects on T estimates for zircon formed above 2 GPa (Ferriss et al., 2008). Metamorphic zircon from the quartzofeldspathic gneisses displays a range of temperature and trace element behavior (Fig. 6d). The least retrogressed gneisses based on petrology (03-114, 03-130, 03-170) have zircon similar to that in the eclogite sample (03-143). Metamorphic zircon shifts from the magmatic protolith signature toward a higher range of Ti-in-zircon temperatures (775–900 °C with a few outliers) and lower Yb/Gd (≈1–5), suggesting growth in the presence

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of garnet. In contrast, zircon from granitic gneiss 03-156 and the most retrogressed gneiss sample (03-154) has temperature ranges of 690–715 °C and 700–750 °C, respectively. Lower T zircon growth in the retrogressed gneiss takes place in the presence of garnet as suggested by low Yb/Gd. In contrast, the granitic gneiss has markedly high Yb/Gd ratios suggesting growth during garnet breakdown. The persistent negative Eu anomaly in this sample indicates zircon crystallization in the presence of feldspar (e.g., Heaman et al., 1990; Schaltegger et al., 1999). Zircon derived from garnet granitoid 03-113 shows a large variation in temperature and composition defined by roughly half of the analyses, which have elevated Fe, Ca, Al, and LREE concentrations that are interpreted to record extensive alteration of the zircon population. On the basis of grains least affected by alteration, T estimates range from 650 to 750 °C (Fig. 6e). Three analyses are obvious xenocrysts of protolith gneiss. Two low Yb/Gd analyses are from zircon grown in the presence of garnet. The remaining analyses overlap the magmatic signature of the protolith cores and loosely define a core to rim increase in Yb/Gd with decreasing T between 347 and 330 Ma. The hornblende leucosome 03-201 has the largest range in temperature and composition (Fig. 6e). Analyses define very systematic temperature groups that correspond to the dark cores (812–879 °C), gray rims (636–705 °C) and outermost white rims (587–655 °C). The high-T cores are interpreted as xenocrysts of protolith zircon. Highand low-T cores with low Yb/Gd and an age of ≈330 Ma may either be inherited material or record a protracted history of zircon growth. The gray rims grew between 330 and 320 Ma during cooling and garnet breakdown, defining a trend of increasing Yb/Gd with decreasing T and decreasing ∑REE. The white rims continue the trend to below 600 °C after 320 Ma. The dramatic increase in Yb/Gd with decreasing T in the white rims is accompanied by an increase in ∑REE but with a decrease in MREE, which is attributed to co-crystallization with titanite (Mazdab et al., 2007). Zircon from cross-cutting pegmatites (03-145, 03-147, and 03-155) gives T estimates ranging from 625 to 820 °C, except for one outlier. Sample 03-147 has the simplest set of 320 Ma magmatic zircon with a temperature range between 665 and 790 °C. Ti-in-zircon temperatures for 03-155 have a spread of 813–636 °C that corresponds to a decrease in age from 330 to 320 Ma. Older zircon in 03-145 clusters around 680 °C at 338 Ma while younger rim domain analyses range down to 625 °C. The outlier in this sample from a round core falls within the high T, low Yb/Gd field defined by zircon in the gneisses, supporting interpretation of these cores as xenocrysts of older metamorphic zircon. Data from the three pegmatites define a general trend of increasing Yb/Gd with decreasing T. The trend is accompanied by decreasing ∑ REE and increasing Hf, similar to melt fractionation trends in magmatic zircon (cf. Barth and Wooden, 2010).

7. Discussion The Carboniferous history of UHP metamorphism and exhumation in North-East Greenland is the youngest event known in the Caledonian collision between Baltica and Laurentia. UHP metamorphism in the overriding plate of Laurentia requires that plate convergence continued for at least 50 m.y. beyond the culmination of the Scandian collision at ≈400 Ma (Gee et al., 2008). Plate divergence is first possible at the beginning of exhumation of the North-East Greenland UHP terrane— hence the importance of this data set. The variation in zircon behavior by sample across a range of lithologies, combined with differences in U/Pb ages (Fig. 5), trace element chemistry (Fig. 3) and Ti-in-zircon thermometry (Fig. 6), facilitates understanding of the nature and timing of zircon growth during the UHP cycle. We discuss aspects important to interpreting zircon behavior below.

7.1. Exhumation history defined by U/Pb ages from all lithologies U/Pb ages from gneisses and intrusions in the UHP terrane (Fig. 5) suggest ongoing UHP metamorphism from 365 to 350 Ma, onset of exhumation and dehydration melting at ≈347 Ma, and retrograde amphibolite-facies metamorphism between 330 and 320 Ma, prior to emplacement of cross-cutting pegmatite dikes (Gilotti and McClelland, 2007). Most eclogite and gneiss samples experienced some zircon growth at UHP conditions, starting at ≈365 Ma and continuing to 350 Ma (Fig. 5). The age range observed between coesite-bearing samples implies a minimum of 15 m.y. residence time at UHP conditions for the eclogite and host gneiss units. The few older ages between 400 and 365 Ma from zircon with flat HREE patterns typical of eclogite facies zircon may record HP metamorphism of the broader North-East Greenland eclogite province, which was ongoing at 415–390 Ma (Gilotti et al., 2004). Melting accompanied by zircon rim growth in the gneisses was abundant and widespread across the UHP terrane beginning at ≈347 Ma, the age of the oldest granitoid emplaced during exhumation (Fig. 5). Most of the gneiss samples contain wispy leucosomes as evidence of melting. Phengite dehydration melting during decompression occurred just below the coesite to quartz transition in metapelites (Lang and Gilotti, 2007, 2011), and this is probably true of the host gneisses, which contain rare phengite inclusions in garnet. Zircon growth at ≈330 Ma is better recorded by the leucocratic intrusions and leucosomes (Fig. 5) than the gneisses, which show very little evidence for zircon growth except in the most retrogressed samples (03-154 and 03-156). The widespread occurrence of amphibolitefacies leucosomes in the UHP terrane suggests that exhumation to at least lower crustal levels occurred by 330 Ma. Pegmatites that crosscut amphibolite-facies structural fabrics in the gneisses record the end of melting and ductile deformation at ≈320 Ma (Fig. 5). The pegmatites are interpreted to be the last hydrous melt emplaced after leucosome crystallization upon approaching the wet granite solidus during exhumation to shallow crustal levels (cf. Zong et al., 2010). The data shows that timing of exhumation is best established by multiple lithologies within the UHP terrane, particularly those involved in or resulting from melting. 7.2. Trace element and Ti-in-zircon variation in eclogites and gneisses during exhumation The REE patterns and Ti-in-zircon T estimates described for the North-East Greenland UHP terrane broadly define 5 groups. Garnet is known to sequester HREE (Rubatto, 2002; Whitehouse and Platt, 2003), so zircon grown in the presence of garnet commonly has flat to low HREE; whereas, zircon will incorporate the HREE if garnet is not present. This, together with the behavior of Eu, helps to interpret the source of the zircon. The characteristics and origin of the zircon groups are: (1) steep HREE, a negative Eu anomaly and variable T that preserve the igneous signature of the protolith; (2) flat HREE, no negative Eu anomaly, and high T typical of UHP eclogite-facies zircon; (3) flat HREE, no negative Eu anomaly, lower T associated with metamorphic zircon formed in the presence of garnet; (4) steep to very steep HREE, lower T metamorphic signature related to garnet breakdown; and (5) steep HREE, intermediate to lower T signature of zircon grown in presence of a metamorphic melt (Figs. 3 and 6). New zircon formed at UHP conditions in most samples displays trace element characteristics of eclogite facies formation—flat HREE and no or limited negative Eu anomaly—indicating zircon growth in the presence of garnet but not feldspar (Rubatto, 2002). The UHP zircon records T lower than the peak T of 950 °C determined from thermobarometry (Gilotti and Ravna, 2002). The significance of the Ti-in-zircon estimates is difficult to evaluate given the uncertainty in pressure corrections for P > 2 GPa (Ferriss et al., 2008). Nevertheless, the high T estimates indicate growth of metamorphic zircon with a Ti signature similar to zircon in granulite terranes (Fig. 6a). Despite

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this high metamorphic T, cores retain clear evidence of their lower T protolith and—with the exception of 03-130—Paleoproterozoic age (Fig. 6b,c). These observations reflect slow Ti and Pb diffusion in zircon (Cherniak and Watson, 2007), and support the clear distinction between inherited igneous zircon and new metamorphic growth. Metagranitoid samples 03-156 (Fig. 3g) and 03-123 maintain a negative Eu anomaly and steep HREE over most of their zircon growth history from 347 to 330 Ma. The total REE increases with increasing Yb/Gd, reflecting garnet-breakdown during zircon growth. These rocks experienced the same P–T history as the calc-alkaline gneisses, thus the difference in zircon chemistry is due to the protolith composition. Rims on UHP zircon formed during exhumation preserve the same REE signature and high T estimates as the UHP domains, despite the younger U/Pb ages observed in some samples (Fig. 5). Eclogite sample 03-143 retains a single high T estimate through youngest rim growth at ≈335 Ma (Figs. 5, 6c). Data from gneiss samples 03-114, 03-170, and 03-130 collectively define an increase in Yb/Gd with decreasing T (Fig. 6d). The individual samples do not define this trend and U/Pb ages from 03-114 do not show a correlation of decreasing T with younger ages. Metagranitoid sample 03-156 and gneiss sample 03-154, which record growth at lower T (700–750 °C) conditions at ≈340 Ma, also show unchanging REE signatures and T in younger rims. These observations suggest that the initial rims often adequately record T, but generally fail to record changes in Ti or REE through time. The most extreme example is 03-154, which records a shift to lower T, but displays flat HREE and no Eu anomaly under amphibolite facies conditions at 320 Ma. A similar lack of trace element adjustment is demonstrated in Dabie Shan, China, where zircon rims contain plagioclase and amphibole inclusions but still display a HP trace element signature (Liu and Liou, 2011). Although zircon from the North-East Greenland host gneisses fail to record the transition to lower T, amphibolite facies conditions, leucosomes and pegmatites capture this portion of the exhumation history. The lack of consistent behavior observed for metamorphic zircon with regards to Ti-in-zircon T estimates is consistent with complexities outlined for igneous zircon by Fu et al. (2008). 7.3. History of melting in the UHP terrane zircon Zircon from the leucocratic intrusions records complex Ti and REE chemistry (Figs. 3 and 6e) that can readily be interpreted in the context of inheritance and multiple melting events. High U, low Th/U zircon formed at 347 Ma in the garnet-bearing granitoid (03-113) has a REE signature similar to that of zircon formed from hydrous melts (Fig. 6e). Cores with igneous protolith signatures indicate inheritance of zircon from the host gneiss. Zircon with low Yb/Gd can be similarly interpreted as metamorphic zircon derived from the host gneiss. The remaining analyses indicate crystallization at 725 °C at ≈347 Ma with rim overgrowths recording T = 675 °C at ≈330 Ma. Older zircon in the pegmatites, particularly 03-145 and 03-155 (Fig. 5), reflects involvement of older leucosome in pegmatite generation. The similarity in trace element chemistry, general decrease in T with time, and large spread in observed U/Pb ages suggest that the cross cutting pegmatites culminate a protracted history of melting and amphibolite-facies retrograde metamorphism of the gneisses from ca 340 to 320 Ma. For example, the older domain in 03-145 records T = 700 °C at ≈340 Ma, while the younger zircon in the cross-cutting pegmatites gives T = 650 °C at ≈320 Ma. Distinct cores engulfed by multiple growth domains of zircon from leucosome sample 03-201 record a complex melting history as well. Xenocrysts of protolith zircon from the gneisses are easily identified by discordant U/Pb ages and trace element signatures (Fig. 3). Cores with eclogite facies REE patterns formed at high T or low T are similar to older metamorphic zircon (Fig. 6d,e). These cores are interpreted as recycled UHP (high-T) and decompression melting (low-T) metamorphic zircon. The ages of the metamorphic cores are reset to 330 Ma. Rims formed at 320 Ma and 650 °C have the same age as the cross cutting

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pegmatites. The younger rims show decreasing T at b625 °C, recording cooling during final zircon growth in the leucosome. 7.4. Interpretation of multiple age domains in zircon Many of the North-East Greenland samples record multiple or extended periods of zircon growth (Fig. 5) that punctuate the exhumation history of the UHP terrane. Distinguishing between continuous growth or recrystallization and episodic growth or recrystallization remains problematic for individual samples in this suite. Data in some samples can successfully be interpreted in the context of specific CL domains or variation in trace element geochemistry, suggesting episodic growth and justifying grouping of analyses (e.g., samples 03-154 and 03-201). Many however cannot and, as discussed above, most younger zircon domains in particular do not record changes in REE chemistry. In other cases, the U/Pb signature is entirely reset, regardless of CL or compositional domain (e.g., sample 03-130). Nevertheless, the large age range for individual samples is interpreted to record progressive zircon growth or modification during exhumation, and be a characteristic signature of a slowly exhumed UHP terrane. Evidence for zircon growth at UHP conditions between 365 and 350 Ma is present in nearly all of the eclogite and gneiss samples. The protolith cores are easily distinguished from metamorphic overgrowths because they retain their igneous trace element signature even though the U/Pb systematics is variably reset through a combination of recrystallization and veining. The presence of coesite and other HP/UHP inclusions of kyanite and omphacite in zircon with flat HREE patterns indicates growth of metamorphic zircon rather than recrystallization of existing protolith zircon. There is no direct evidence for the presence of melt at UHP conditions. Zircon thus likely formed at 365–350 Ma from sub-solidus modification of UHP phases and protolith zircon (e.g., Fraser et al., 1997; Bingen et al., 2001; Degeling et al., 2001) in the presence of aqueous to supercritical fluids (Liati and Gebauer, 1999; Rubatto and Hermann, 2003; Zheng, 2009). Zircon within pegmatites and leucosomes precipitated from hydrous melts, forming euhedral grains with well developed oscillatory zoning. Younger rims on zircon in both leucosomes and the host gneiss that truncate earlier zoning (Fig. 2) are suggestive of a decoupled dissolution/reprecipitation mechanism for zircon growth, in which zircon is dissolved during partial or fluid assisted melting and reprecipitated on cooling (Ayers et al., 2003; Rubatto et al., 2009). Some samples show textural evidence of coupled dissolution/reprecipitation (Geisler et al., 2007) on fractures within the core domains, but in some cases the fractures cut younger rims (McClelland et al., 2009). The age range observed for individual samples across the sample suite is interpreted to reflect continuous to episodic availability of fluids controlled by melting and retrograde reactions during exhumation. Defining precise ages for particular growth events is generally not possible in the absence of clear CL or geochemical variation. However, the age distributions for individual samples clearly define periods of more abundant growth (e.g., 350 and 330 Ma, Fig. 5) that help establish the timing of specific events (e.g., UHP metamorphism and dehydration melting) in the metamorphic evolution of the UHP terrane. Weighted mean ages of the major age populations approximate the timing of the specific events although the calculated uncertainty will not incorporate uncertainty due to prolonged or continuous growth. The large age range defined by small volume sampling afforded by the SIMS technique is characteristic of slowly exhumed UHP terranes. Interpretation of ages defined by individual samples is greatly improved by having large multi-element data sets from numerous samples. The North East Greenland UHP terrane clearly provides a challenging data set with regard to how U–Pb data from complex zircon suites is interpreted, but it is not unique. SIMS data sets that show a significant range in ages have been generated for other UHP terranes, such as the North Qaidam (e.g., Yang et al., 2005; Liu et al., 2006; Mattinson et al., 2006; Song et al., 2006; Mattinson et al., 2007; Zhang et al.,

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2008; Mattinson et al., 2009; Zhang et al., 2010) and Dabie-Sulu (e.g., Hacker et al., 2006; Liu and Liou, 2011) regions of China, the Western Gneiss region of Norway (Root et al., 2004), and in Papua New Guinea (Monteleone et al., 2007). Interpretation of most SIMS data typically involves grouping data based on similarity in trace element geochemistry, CL characteristics, inclusion suites, or age and calculation of a weighted mean age. Outliers to specific age groups are dismissed assuming inheritance of older material, recrystallization or Pb-loss. In some samples the groups are well behaved, while the rationale is less well justified in others. Calculating a weighted mean age for metamorphic zircon domains assumes that all analyses record a distinct temporal event. This assumption is rarely justified since—as discussed above for the Greenland examples—data sets that record a large age range can either reflect: (1) continuous zircon growth, recrystallization or Pb-loss during metamorphism, such that the scale of growth domains or zones of recrystallization is smaller than the volume of sample analyzed, or (2) a series of closely spaced events that cannot be resolved within analytical uncertainty of the dating technique. The calculated weighted mean age and associated uncertainty likely do not adequately reflect the complexity of the age variation in the zircon population for any of these scenarios. Nevertheless, the distribution of ages within individual samples appears to define periods of peak activity. Evaluating the timing of these peaks and their possible geologic significance is more informative than simply interpreting the peaks as statistically indistinguishable within a continuum. Our view is that for most UHP terranes, including North-East Greenland, grouping ages in context of peak ages and compositional variation provides useful information regarding the timing of UHP metamorphism and subsequent exhumation. Refinements to analytical techniques will advance our ability to pull apart complex zircon populations (i.e. reduce uncertainty). Application of chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) is promising, but has failed to resolve the problem of continuous versus episodic zircon growth in UHP zircon (e.g., Root et al., 2004; Gordon et al., 2012). The lack of spatial resolution with CA-TIMS produces a high-precision average age from multiple domains; these age estimates are no more satisfactory than weighted mean ages produced by grouping subsets of SIMS data. Analytical advancements that couple increased precision with smaller sample volume will ultimately help refine the complex metamorphic histories recorded in zircon from slowly exhumed UHP terranes, as well as other complex metamorphic and igneous settings. 7.5. Implications for exhumation of UHP terranes The North-East Greenland UHP terrane records a 45 m.y. history of UHP metamorphism and exhumation to amphibolite facies conditions. The zircon data breaks this long-lived history into UHP metamorphism at 365–350 Ma, isothermal decompression at 347 Ma and reaching crustal levels at least by 330 Ma. Assuming exhumation from 120 km to lower crustal levels of approximately 30–40 km gives an exhumation rate of 4–5 mm/yr, a rate comparable to other large UHP terranes (cf. Liou et al., 2009). The thermal history is equally sluggish. Zircons record cooling from ≈950 °C at UHP conditions to ≈725 °C over 20 m.y. and from 725 °C to 650 °C over 25 m.y. yielding cooling rates of 11 and 3 °C/m.y., respectively. Cooling between UHP conditions and granite emplacement at 347 Ma occurred during or shortly after exhumation. The very slow cooling between 347 and 320 Ma is consistent with stalling of the UHP terrane at the base of the crust (e.g. Walsh and Hacker, 2004). The exhumation and cooling rates observed for the North-East UHP terrane are interpreted as characteristic of UHP terranes formed in the overriding plate of a collisional orogen by intracontinental subduction. 8. Conclusions The North-East Greenland UHP terrane formed over 15 million years, and experienced slow exhumation and cooling to amphibolite facies

over a 30 m.y. period during the Carboniferous. The tempo of the exhumation and cooling history is successfully provided by U/Pb ages, REE chemistry and Ti-in-zircon temperature estimates from a spectrum of lithologies. Zircon in UHP gneisses records the timing of decompression to the lower crustal levels but often does not track changes in T very well. Some zircon growth during melting is seen in the gneiss samples, but zircon from leucosomes and pegmatites best records melting during exhumation—including timing and changes in T during the decompression and retrograde history. Most samples contain complex zircon populations, with up to three different trace element signatures and what we interpret to be multiple age domains. Inclusion suites and trace element patterns in zircon from a significant number of samples and a variety of lithologies are, therefore, required to capture the timing of formation and exhumation of hot UHP terranes. 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