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1 Chemical Geology 261 (2009) Contents lists available at ScienceDirect Chemical Geology journal homepage: Metamorphic zircon, trace elements and Neoarchean metamorphism in the ca Ga Nuvvuagittuq supracrustal belt, Québec (Canada) N.L. Cates, S.J. Mojzsis Department of Geological Sciences, University of Colorado, Boulder, CO , USA article info abstract Article history: Received 1 February 2008 Received in revised form 27 January 2009 Accepted 29 January 2009 Editor: R.L. Rudnick Keywords: Eoarchean Nuvvuagittuq Metamorphism Zircon Geochronology Quebec The Nuvvuagittuq supracrustal belt (NSB) in northwestern Québec ranks as one of the oldest granitoid gneiss complexes thus far discovered. Emplacement ages for intrusive trondhjemitic dikes that transect amphibolites and quartz magnetite schists at the Porpoise Cove outcrops of the NSB cluster at 3.75 Ga. As with all other pre-3.7 Ga terranes thus far documented, the NSB has been metamorphosed and multiply deformed. In a combined approach to explore the thermal history of the NSB, conventional garnet biotite and plagioclase hornblende geothermometry was coupled with zircon U Th Pb depth profiles, REE partitioning and Ti-in-zircon thermometry to show that Neoarchean metamorphism in the belt reached upper amphibolite facies conditions (~640 C). This metamorphism resulted in some cases in thick zircon overgrowths on older cores and likely corresponds to the amalgamation of terranes in the Northeast Superior Province and the initiation of widespread igneous activity in the vicinity of the NSB. An earlier (resolvable) metamorphic episode from a zircon depth-profile, recorded at 3.62 Ga, probably corresponds to the intrusion of the granitoids which envelope the NSB Elsevier B.V. All rights reserved. 1. Introduction The discovery of pre-3.75 Ga rocks at the Porpoise Cove locality in the Nuvvuagittuq supracrustal belt (NSB), approximately 30 km south of Inukjuak in northern Québec (Canada), has greatly expanded the known inventory of Eoarchean volcano sedimentary rocks worldwide (Simard et al., 2003, Cates and Mojzsis 2007; Dauphas et al., 2007; O'Neil et al., 2007). In addition to the NSB, other known exposures of N3.6 Ga metasedimentary rocks now include the Itsaq gneiss complex in Greenland (Nutman et al., 1996), the Uivak gneisses in Newfoundland in Canada (Schiøtte et al., 1986), and the Anshan complexes in the North China Craton (Liu et al., 2008). Although the geology of the NSB bears some resemblance to the better known Ga supracrustal rocks of the Itsaq Gneiss Complex in West Greenland, there are several important differences (Cates and Mojzsis, 2006, 2007), which perhaps means that the two terranes are unrelated even if they are similar in age. If this is so, detailed analyses of this potentially separate ancient terrane to the West Greenland rocks becomes of fundamental importance. However, the NSB rocks are complicated. Some amphibolite components of the belt carry evidence for derivation from an ancient precursor (O'Neil et al., 2008) that carried deficits in the daughter Dedication: To Bob Pidgeon on the occasion of his Ga emplacement age. Corresponding author. Tel.: ; fax: address: nicole.cates@colorado.edu (N.L. Cates). product of 146 Sm decay ( 142 Nd; t 1/2 =103 Myr); these same rocks yield ages derived from 146 Sm 142 Nd systematics of up to 4.28 Ga, although conventional 147 Sm/ 143 Nd isochrons tend toward younger ages (ca. 3.8 Ga) in agreement with the zircon geochronology cited above. Because direct inferences of surface conditions on the early Earth rely heavily on isotopic and elemental ratios that can be disturbed by metamorphism, it makes sense to establish peak metamorphic conditions prior to further interpretation. Work on the NSB is still in its infancy. In addition to the paucity of geochronological data for these rocks, even less is known of their thermal histories. Percival et al. (2001) and Percival and Skulski (2000) noted that the Inukjuak and neighboring Tikkertuk domains host abundant cpx-bearing tonalitic to dioritic granitoid gneisses, Ga enderbite, and Ga granites that enclose older ( Ga) tonalitic units and yet older TTG gneiss enclaves ( 3.0 Ga; Leclair et al., 2006; Stevenson and Bizzarro, 2007). It would seem likely that the emplacement of these rocks thermally affected the NSB. To explore the metamorphic history of the NSB in greater detail, we undertook conventional garnet biotite and plagioclase hornblende mineral-pair geothermometry analyses by electron microprobe for previously described (Cates and Mojzsis, 2007) quartz biotite and amphibolitic schists collected from the Porpoise Cove outcrops of the NSB. We compared these results with a record of metamorphism from ion microprobe U Th Pb depth profiles in individual concordant zircons extracted from a thin (b1 m wide) intrusive homogeneous trondhjemitic gneiss sampled from within the mapped units of the /$ see front matter 2009 Elsevier B.V. All rights reserved. doi: /j.chemgeo

2 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) explore the differences between core zircon, grown in an igneous system, and rim zircon either precipitated from fluids expelled during metamorphism, resorbed, or recrystallized. We have explored how the new Ti-in-zircon thermometer (Watson and Harrison, 2005; Watson et al., 2006; Harrison et al., 2007; Hiess et al., 2008) can be used to compare the core-to-rim differences in apparent crystallization temperatures within a zircon. The result of this integrated analysis is that we can resolve different growth zones within a single grain to petrologic conditions that affected the rock which hosts the zircon, and use this information to begin to relate these to different metamorphic events that affected the belt at different times in its history. 2. Geological setting Fig. 1. Map of the geological domains within the northwestern Superior Province (NESP) in Québec, Canada. The Nuvvuagittuq supracrustal belt (NSB) is marked by a star. Map modified from Simard et al., supracrustal succession. The orthogneiss shares the deformational history of the surrounding supracrustal units and therefore should also share its metamorphic history which may be recorded as dissolution, recrystallization or precipitation of zircon. The depth-profile and mineral-pair thermometry were combined with the in situ ion microprobe analysis of rare earth elements (REEs) and Ti within specific domains of a single concordant zircon. We used REE partitioning to The Nuvvuagittuq rocks are located on the edge of the core Archean component of the Canadian Shield in the Northeast Superior Province (NESP; Fig. 1) of northern Québec. The NESP is the largest of the dominantly plutonic Archean terranes in the Superior Province, and is mainly composed of Archean plutonic tonalite trondhjemite granodiorite (TTG) and diorite gneisses which in turn enclose and in some case intrude older amphibolite- to granulite-facies supracrustal rafts and enclaves (Bédard, 2003). The rafts comprise long (up to ~150 km) narrow (1 10 km) linear belts of metamorphosed sedimentary and extrusive igneous rocks (reviewed in Percival, 2007), some of which preserve primary sedimentary and volcanic structures (Nadeau, 2003). The structure of the NESP is characterized by a NNW SSE structural trend and high temperature metamorphism and has been further subdivided into several domains based on regional mapping and aeromagnetic anomalies, the westernmost of which is the Inukjuak domain (Percival et al., 2001). While the style of tectonism Fig. 2. Simplified geologic map of the Inukjuak domain (after Simard et al, 2003). NSB is labeled. Note the other numerous unexplored supracrustals (units Ainn).

3 100 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) responsible for its final configuration remains unclear (vertical tectonics vs. Andean type subduction) the overall structure has been interpreted to be dominantly the result of a succession of crustal growth episodes and recycling over a period of ~300 Myr beginning at ca. 3.1 Ga (Simard et al., 2003; Bédard, 2003, Bédard et al., 2003; Percival et al, 2001; Lin et al., 1996, and references therein), finally culminating in the juxtaposition of the microcontinental blocks at about Ga when they were assembled through a series of Neoarchean orogens (Percival et al., 2006). A general overview of the petrogenetic history of the NESP has been accomplished in part through large-scale field studies that have concentrated on structural mapping and sample collection, from U Pb ID-TIMS geochronological surveys of several hundred zircons, and from Sm Nd studies commissioned by the Québec government (Simard et al., 2003; Leclair et al., 2006, Stevenson et al., 2006). In these studies, the westernmost Inukjuak domain was found to host rocks with westerly decrease in εnd model ages (T DM = Ga; Nd isotope data reported in Stern et al., 1994; Skulski et al., 1996; Boily et al., 2004, 2006, Stevenson et al., 2006), old inherited zircons, and scattered enclaves of supracrustal rocks and ultramafic sills. This is in marked contrast with the lithologically indistinguishable Tikkerutuk domain which records juvenile model ages (T DM =2.8 Ga; Stevenson et al., 2006), which means that the dominantly Ga granitoid rocks at Inukjuak tapped a significantly older crustal reservoir (Stevenson and Bizzarro, 2005; Stevenson et al., 2006). Whole-grain U Pb zircon TIMS data pointed to ages of some of the NSB gneisses of up to 3.8 Ga (Simard et al., 2003; reviewed in O'Neil et al., 2007; David et al., 2009). Ion microprobe studies of zircons extracted from thin trondhjemitic gneiss sheets collected during high-resolution (1:50) mapping of a section within the NSB at Porpoise Cove confirmed a minimum age of ca Ga for supracrustal enclaves in the westernmost Inukjuak domain (Cates and Mojzsis, 2007). We note that this belt is one of about a dozen or so supracrustal rafts captured in the gneisses that dominate the Inukjuak domain (Fig. 2), but remains the only one thus far studied in any detail. Structural analyses conducted in the NESP have identified multiple deformation events (Simard et al., 2003; Percival and Skulski, 2000; Lin et al., 1996). The oldest deformational events are only identifiable in the supracrustal rocks of the Innuksuac complex, of which the NSB is the largest raft thus far identified, and in small mafic enclaves in granitoids (Nadeau, 2003; Simard et al., 2003). The first phase of deformation (D 1 ) is recognized as a penetrative planar fabric and gneissosity in the supracrustals and the second phase of deformation (D 2 ) resulted in isoclinal folding of the planar fabric (Simard et al., 2003). The third phase of deformation (D 3 ) recognized in the Inukjuak domain corresponds to the main regional deformation and metamorphic event that affects virtually all lithologies in the domain and resulted in the NNW SSE regional structural trend observed throughout the NESP (Fig. 2; Simard et al., 2003). This event is largely preserved in the ca Ma granitoids as magmatic to submagmatic deformation and partial assimilation of smaller mafic enclaves (Simard et al., 2003). The fourth phase of deformation (D 4 ) can be recognized as tight to open folds that are superimposed on the main deformational fabric. A later, localized, shear fabric (D 5 ) and a series of large, Proterozoic E W brittle faults (D 6 ) are the final deformational events recognized in the Inukjuak domain, though they do not appear to have affected the NSB (Simard et al., 2003). The NSB is an 8 km 2 volcano sedimentary sequence arranged in a south-plunging synform (Fig. 3) surrounded, and intruded in some cases, by 3.6 Ga granitoid gneisses (Stevenson and Bizzarro, 2007; David et al., 2009). A small coastal outcrop at Porpoise Cove (Fig. 4) is dominated by amphibolites, but also contains metasedimentary rocks comprising sequences of both chemical sedimentary (finely laminated banded iron-formations, BIFs) and detrital origin (quartz biotite schists, possible conglomerate; Fig. 5). Subsequent to emplacement, this package of supracrustal rocks was intruded by several generations of granitoids, the oldest of which is 3.75±0.01 Ga (Cates and Mojzsis, Fig. 3. Generalized geologic map of the Nuvvuagittuq supracrustal belt. From O'Neil et al. (2007). 2007). The oldest generation of granitoids shares the entire deformational history of the supracrustal belt and therefore constrains the minimum age of deposition and maximum age of D 1. Though the granitoids have for the most part been brought into parallelism, rare cross-cutting relationships have been identified (Cates and Mojzsis, 2007). Recent work by O'Neil et al. (2008) shows that some of the amphibolite units yield a 142 Nd/ 144 Nd vs. 147 Sm/ 144 Nd isochron age of 4.28 Ga, and are the first terrestrial rocks to record depletion of the extinct isotope 146 Sm from resolvable deficits in 142 Nd. Unfortunately, the apparent absence of igneous zircon in these units (David et al., 2009) precludes a definitive answer to whether parts of the belt are indeed 4.28 Ga or are instead derived from a source that was isolated since that time. In spite of deformation and metamorphism, some of the units are indisputable BIFs that preserve finely laminated quartz and Fe-oxide and Fe-silicate textures and some ultramafic rocks locally show features reminiscent of pillow structures. It was also found that these rocks preserve elevated 56 Fe/ 54 Fe compared to surrounding units of igneous origin, which not only provides independent confirmation of their sedimentary protolith, but also suggests that Fe-isotopic diffusional exchange between the BIFs and amphibolites due to metamorphism has not been an important process (Dauphas et al., 2007). Younger pegmatitic granitoids (Lg, Fig. 4) which cross-cut the supracrustal units have not been affected by the first three phases of deformation, but have undergone gentle open folding that has been ascribed to D 4 (Simard et al., 2003). These pegmatites have been dated

4 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) Fig. 4. Geologic map of the Porpoise Cove locality within the NSB. Labels correspond to sample numbers discussed in text. From Cates and Mojzsis (2007). by TIMS analysis of monazite to 2686±12 Ma (David et al., 2009) and constrain both the maximum age for D 3 and the minimum age for D Methods 3.1. Mineral-pair thermometry Prior to analysis all doubly-polished thin-sections were mapped (Fig. 6) using back-scattered electron imaging prior to analysis on the JEOL JXA-733 electron microprobe in the Department of Geological Sciences, University of Colorado. Electron probe mineral analyses were preformed on adjacent grains, as well as across grains to check for zoning, and results from these analyses are presented in Table 1. Metamorphic temperature conditions were determined only using mineral pair measurements from grains which were in physical contact in thin-section. The plagioclase hornblende thermometers of Holland and Blundy (1994) were applied to two amphibolite samples, and garnet biotite thermometry was used on two garnet bearing quartz biotite schists employing the program WINTWQ (version 2.32; Berman, 1991; Berman et al., 2007). Activity composition relations for garnet and biotite were computed with the activity models of Berman et al. (2007) and Berman and Aranovich (1996) respectively U Th Pb zircon depth profiles U Pb zircon depth profiles were performed on the UCLA Cameca ims 1270 high-resolution ion microprobe under routine conditions (e.g. Mojzsis and Harrison, 2002; Trail et al., 2007); a short summary is Fig. 5. Annotated field photographs of lithotypes used for thermometry. A. Quartz biotite schist (meta-conglomerate?). Brunton compass for scale. B. Amphibolite. Hammer for scale (~40 cm long). See Fig. 4 for precise sample locations.

5 102 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) Fig. 6. Annotated backscattered electron images of thin sections used for thermometry. Numbers correspond to geochemical analyses performed by electron microprobe (Table 1). Bt biotite; Grt garnet; Hbl hornblende; Il ilmenite; Ms muscovite; Qtz quartz; Rt rutile.

6 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) Table 1 Mineral analyses determined by electron microprobe. Garnet analyses Spot # SiO 2 TiO 2 Al 2 O 3 Cr 2 O 3 FeO MnO MgO CaO Total IN05042 A_ A_ A_ A_ B_ B_ B_ C_ C_ C_ C_ C_ C_ IN05037 A_ A_ B_ B_ B_ B_ B_ B_ B_ Biotite analyses SiO 2 TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2 O Cl Total IN05042 A_ A_ A_ A_ A_ B_ B_ B_ B_ B_ C_ C_ C_ C_ C_ C_ IN05037 A_ B_ B_ B_ B_ B_ B_ B_ B_ Plagioclase analyses SiO 2 Al 2 O 3 CaO Na 2 O K 2 O FeO Total IN (continued on next page) (continued on next page)

7 104 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) Table 1 (continued) IN IN05024 A_ A_ A_ A_ A_ A_ B_ Amphibole analyses SiO 2 TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2 O Cl Total IN IN05024 A_ A_ A_ A_ A_ B_ provided here. Two previously characterized b2% discordant zircons were selected for depth-profiling from two related trondhjemitic gneiss samples (IN and -022 in Fig. 7; Cates and Mojzsis, 2007). The grains were individually plucked from their original ion microprobe geochronology mount (NUV_01) with a fine needle under a binocular microscope, and each placed onto separate new mounts with standard zircon AS3 (Paces and Miller, 1993; Black et al., 2003; Schmitz et al., 2003). The zircons were positioned such that original unpolished zircon prism faces were flush with the surface, and afterwards each sample mold was cast in epoxy. Following a light hand polish with paper impregnated with 1 μm diamond paste, the epoxy surface was cleaned in successive 6 N HCl and ultra-distilled water baths under brief (30 s) sonication. A sputter coater was used to plate a 100 Å layer of Au on the sample to facilitate conductivity and the mounts were photographed in reflected light. Operating conditions for the U Pb depth-profiles reported here were that a ~10 na O 2 primary beam was focused to a 25 μm spot at a mass resolving power of ~6000 to resolve isobaric interferences. We used oxygen flooding to a pressure of Torr to increase Pb+ yields (Schuhmacher et al., 1994). Multiple analyses of standard AS3, which has a concordant 206 Pb/ 238 U and 207 Pb/ 235 U age of ±0.5 Ma (Schmitz et al., 2003), were made prior to and after each depth profile run in order to determine a working curve to which the data are fit. However, instead of running for cycles as is normally the case for conventional ion microprobe geochronology, 50 cycles were programmed for each Fig. 7. Backscattered electron images of zircons selected for depth profiling. Labeled ages of spots are in Ma and were previously determined by standard ion microprobe techniques. The data are reported in Cates and Mojzsis (2007). Scale bars are 100 μm.

8 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) analysis session that lasted approximately 1 h. The cumulative result is a prolonged and continuous collection time from the same spot as the ion beam slowly sputters into the zircon. To accurately determine the depth of the zircon pit excavated by the O 2 ion beam after each 50 cycle run, the pit depth was measured using an ADE Phase Shift MicroXAM Interferometric Surface Profiler which allowed us to accurately compute the rate at which the primary ion beam eroded the sample; the rate of sputtering was then used to estimate the depth of each datum. During acquisition of one of our depth profiles (grain IN05003_18), the instrument experienced minor primary beam instabilities which affected the 207 Pb/ 206 Pb ratios. Those cycles were easily identifiable by dips in the counting statistics and were excluded from the final data analysis Rare earth elements in zircon Rare earth elements were measured in zircon grain IN05003_18 with the Cameca ims 3f ion microprobe at the CRPG in Nancy, France. Instrument conditions follow those described elsewhere (Martin et al., 2008), and a short summary is provided here. An O 2 primary beam was accelerated at 10 kv with a beam current of ~20 na, and secondary ion intensities were measured by counting at low mass resolution (~300) and with an energy filtering of 80±20 ev. Intensities of 96 Zr, REE and Hf isotopes between masses 134 and 180 were measured by peak switching. Counting times were 10 s for each mass, and for each analysis 15 cycles were accumulated. The mass intensities were converted to elemental concentrations in ppm from the abundance matrix of the REE isotopes over the measured masses and normalized to 96 Zr intensities using the zircon standard (Wiedenbeck et al., 1995, 2004; Sano et al., 2002). In each analytical session, instrument stability was monitored by periodic REE measurements on standard zircon Titanium in zircon Titanium concentrations in zircon were also measured using the Cameca ims 3f at the CRPG-Nancy, using a protocol similar to Watson et al. (2006); a brief summary is provided. Measurements on zircon grain IN05003_18 were made at different grain locations which correspond to previously obtained REE analyses. The grain was not re-polished between REE and Ti analyses. A primary 10 na O 2 beam was focused to a 10 μm spot on synthetic glass standards (NSB 610, 612 and 614) and a zircon standard (91500) of known Ti concentrations. Minimum detection limits for Ti by this method are ~0.1 ppm and the analytical uncertainty for zircons that crystallize in the C range relevant to this study is ~5 C (1σ). The measured concentrations applied to the equation of Watson et al. (2006) define a log linear dependence of Ti with temperature. Cracks or the grain/epoxy interface were avoided because it has been found that measured Ti concentrations are often elevated when analyses occur on cracks in zircon (Harrison and Schimtt, 2007; Hiess et al., 2008). To test for this, one of our analyses was directed over the interface between the zircon and the mount medium. The result was an exceptionally high count rate for Ti that resulted in an untenable temperature estimate that was not considered further. 4. Sample descriptions 4.1. Zircons chosen for depth profiling Two zircons that were previously analyzed by conventional ion microprobe U Th Pb geochronology in spot mode, and concordant to within 2%, were selected for depth profiling (zircons IN05022_26, IN05003_18; Cates and Mojzsis, 2007). One of these was sub-sampled for REE and Ti measurements (IN05003_18). Both grains were collected from the same 3751±10 Ma trondhjemitic orthogneiss but were sampled in two locations, and have slightly different morphologies (Fig. 7). Zircon 022_26 is a ~150 μm long and stubby grain with slightly rounded facets. During back-scattered electron imaging carried out prior to conventional ion microprobe U Pb geochronology, the entire zircon was found to preserve strong oscillatory zoning with no evidence of a significant rim overgrowth or resorption/recrystallization. In contrast, although zircon 003_18 is similar in size and overall morphology to 022_26, it has a large (b30 µm wide) unzoned overgrowth over an igneous core with fine oscillatory zoning. This made it an appealing target for comparison of core and rim chemistry Sample mineralogy for thermometry Quartz biotite schists Samples IN05037 and IN Within the mapped portion of the belt, we collected from a minor unit sandwiched between amphibolites that was earlier described as a polymict quartz biotite conglomeratic schist with stretched polycrystalline quartz clasts up to cm (Cates and Mojzsis, 2007). This rock also contains small mafic clasts suspended in a matrix of biotite+disseminated quartz+ clinozoisite±muscovite±garnet ±abundant Fe-sulfides (Fig. 6). Grains of the minor minerals magnetite, ilmenite, apatite, and zircon are also present throughout. In addition to the present mineralogy, previously published major and trace element geochemistry are also consistent with a detrital sedimentary protolith for this unit and therefore was deemed suitable for garnet biotite thermometry. Two garnet-bearing samples from this unit were chosen for thermometry. The garnets are anhedral, range from several mm to several cm in diameter, tend to occur in clusters and often have inclusions of biotite±muscovite±quartz. The garnets are unzoned, with very restricted variation in composition with average endmember compositions of py 59.8 alm 25.9 gro 8.2 and 3.2 uv 0.7 sp 0.7. Biotites occur as small laths (b1 cm long) often with small oxide inclusions and appear to be interstitial to quartz and garnet. Compositionally, they vary little with low Al and Ti contents which are consistent with a metamorphic origin (Deer et al., 1992) Amphibolites Samples IN05021 and IN Amphibolites dominate the mapped area at Porpoise Cove. Most of these have been strongly affected by chemical weathering, and therefore clean plagioclases for thermometry were restricted to two samples (Fig. 6). The mineralogy comprises amphibolite+plagioclase+quartz±k-feldspar. Trace minerals are dominantly Fe-oxides, apatite and Fe-sulfides. The amphiboles are mostly hornblende, with minor tschermakite and cummingtonite. Plagioclases are all labradorite and are compositionally unzoned. 5. Results and discussion 5.1. Mineral-pair thermometry Garnet biotite thermometry We carried out 74 coupled analyses on 15 garnet biotite mineral pairs for thermometry: seven from sample IN05037, and eight from sample IN Results from the thermometry are summarized in Table 2. While the individual mineral pair analyses may differ by as much as 70 C (621±14 C 554±12 C at 5 kbars), weighted mean data for both IN05037 and -042 yield similar temperatures (581±20 and 584±17 at 5 kbars, respectively) consistent with amphibolite facies metamorphic conditions for pressures equivalent to regional metamorphism (3 7 kbars) Plagioclase hornblende thermometry The results of our analyses of 47 mineral pairs from sample IN05021, and 16 from sample IN05024, are summarized in Table 2.

9 106 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) Table 2 Thermometry results in C. Pressure (kbar) Garnet biotite thermometry Plagioclase hornblende thermometry Therm A Therm B IN05037 IN05042 All IN05021 IN05024 All IN05021 IN05024 All N N N N N N W W W W W W ±20 567±17 567±11 645±35 592±26 631±43 636±25 565±29 629± ± ± ±11 647±35 608±21 637±36 665±22 609±26 650± ±20 584±17 583± ±34 626±19 642±32 684±21 638±25 672± ±21 592±17 590± ±35 644±18 647±31 703±19 667±23 694±26 Therm A silica saturated thermometer. Therm B silica unsaturated thermometer. Holland and Blundy (1994) present two thermometers, one calibrated for an assemblage including quartz and one for silica undersaturated assemblages. Both thermometers should yield similar results if the rock is silica saturated, which is the assumed state for both IN05021 and -042 as they contain quartz. As anticipated, we find that the results for both thermometers are similar (IN05021: 648 ±34 C and 684±21 C at 5 kbars; IN05024: 626±19 C and 638±25 C at 5 kbars) and results from both thermometers are presented for completeness. have yet to be identified in our samples from the NSB. However, the results from the thermometers presented above, are relatively insensitive to pressure, varying only by 15 C for garnet biotite thermometry and by 30 C for the silica-saturated plagioclase hornblende thermometer at pressures reasonable for regional metamorphism. Additionally, certain qualitative constraints can be inferred. If regional metamorphic conditions and a steep geotherm (70 C/km) at 3.75 Ga Metamorphic grade Assessment of the degree of metamorphism in a volcano sedimentary sequence is the first step in determining the likely preservation of isotopic and elemental ratios in protoliths. In pelitic schists the mineral assemblage of garnet + biotite + muscovite + quartz combined with the absence of sillimanite can be used to place an upper limit to the metamorphism of ~650 C at 3 kbars, although muscovite can persist above 700 C at higher pressures (Spear, 1993). Garnet biotite thermometry in the matrix of the conglomeratic schists in the NSB is consistent with the observations noted above, with temperatures of ~ C at regional metamorphic conditions (3 7 kbars; Table 2, Fig. 8). Results from the silica-saturated plagioclase hornblende thermometry are slightly elevated when compared to those from the garnet biotite thermometry ( C; Fig. 9), but nearly within error for both thermometers (Table 2). This may be the result of post-peak metamorphism re-equilibration in the pelitic schist or may be simply an artifact of using two separate thermometers that do not use the same thermodynamic data sets. Efforts to place constraints on the pressure are more problematic as appropriate mineral assemblages for barometry or thermobarometry Fig. 8. Results of garnet biotite thermometry. Fine dashed lines correspond to 1σ errors. Fig. 9. Results of plagioclase hornblende thermometry using the Holland and Blundy (1994) thermometers. A. Mean temperatures (1σ indicated by dashed lines) yielded using the silica-saturated thermometer. B. Mean temperatures (1σ indicated by dashed lines) yielded using the silica-unsaturated thermometer.

10 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) are assumed, with subduction of hot, young crust (or sinking as a dense cold-finger in the vertical tectonics model), a temperature of 650 C would be reached at a pressure of b3 kbars (assuming an average continental crust density of 2.7 g/cm 3 ). If the geotherm was even more pronounced, the temperature may be reached at an even shallower depth. We view it as unlikely that the geotherm was substantially less than what is observed at modern non-orogenic regions (25 C). Under those conditions, 650 C would be reached at a pressure of 7.6 kbars U Th Pb zircon depth profiles The resilience of zircons to geologic processing that would otherwise consume a mineral back into the rock cycle means they can provide a unique window into the metamorphic history of an assemblage of related rocks. A single zircon can preserve a complex record of metamorphic recrystallization, partial dissolution and growth over the entire history of the grain (e.g. Pidgeon et al., 2000). This refractory characteristic is what makes the study of zircon so appealing as it stands in contrast to all other minerals historically used for geochronology such as feldspar for common-pb; mineral or whole rock isochrons from 87 Sr/ 86 Sr, 147 Sm/ 143 Nd, etc. This latter category will typically preserve information only for the last moderately to highly elevated thermal event, depending on the closure temperature of the system (usually below 650 C) and other effects. Furthermore, the 2-D spatial resolution of zircon analysis by ion microprobe is particularly well suited to zircon geochronology and trace element studies because of the in situ nature of the analyses. Compared with techniques that require dissolution of whole grains or grain aggregates, the ion microprobe can analyze specific growth zones within a zircon with a ~10 20 μm diameterby b1 μm depth spot (e.g. Pidgeon et al., 1998). Despite this obvious analytical power, a limitation to traditional ion microprobe techniques has been the size of the spot that can be analyzed; growth zones smaller than ~5 µm are usually impossible to investigate via U Pb isotope geochronology, even by the new generation of high resolution, high transmission secondary ion mass spectrometers (Cameca ims 1270 and 1280; SHRIMP-II and -RG). This Table 3 Results of depth-profiling study of zircon IN05022_26. Depth (μm) 206 Pb / 238 U ratio 207 Pb / 235 U ratio 207 Pb / 206 Pb ratio 207 Pb/ 206 Pb age Pb (%) Th/U Th/U

11 108 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) down to facilitate analysis from an unpolished younger exterior through to any preserved overgrowths or recrystallizations recorded by the grain, and ultimately to the oldest core region. For zircons, this technique is effective because it takes advantage of the sub-micron resolution of the depth of the analysis point into a mineral with a Pb payload that is often N99% radiogenic (Mojzsis and Harrison, 2002; Carson et al., 2002; Manning et al., 2006; Trail et al., 2007). In this way, the chemistry of multiple zircon growth events as narrow as 0.1 μm can be documented within a single grain. Comparison of trace elements such as Th/U (and potentially other tracers in development for depthprofile studies such as REEs, Ti and P) are now possible for each growth zone as a function of the depth into the grain, and can be used to aid in the interpretation of (for example) metamorphic vs. igneous growths and rate of growth, magma chamber histories, compositions of fluids, and the thermal history of a rock. Fig. 10. U/Pb age vs. depth-profile of zircon IN05022_26 and corresponding Th/U. Boxed areas contain analyses used to determine age and Th/U. The shaded region is the predicted Th/U for a zircon grown in a melt of the composition of the host rock (based on a whole rock Th/U=5.2, reported in Cates and Mojzsis, 2007). apparent drawback can be transcended by 3-D techniques uniquely suited to ion microprobe mass spectrometry: Depth-profile compositional analysis of minerals (Grove and Harrison, 1999). In this mode the primary ion beam is used to erode through a previously documented mineral that was removed from its original mount and recast upside Zircon IN05022_26 The core age from this zircon, recorded from conventional ion microprobe geochronology was 3755±7 Ma (Cates and Mojzsis, 2007). Depth-profile analysis revealed the outermost surface of the grain preserved a young age (~2800 Ma), but this quickly increased to the core age of 3802±12 Ma by a depth of ~2.5 μm. The Th/U results track the different age domains, with very low outermost values (b0.02 at ~2800 Ma) that quickly increase to a value of 0.31 for the oldest ages (Table 3, Fig. 10). This is just slightly lower than the field of predicted values for a zircon derived from a rock of this composition as determined by the whole-rock Th/U and a rock/zircon partition coefficient of 0.2 (Mahood and Hildreth 1983; Mojzsis and Harrison, 2002). This small Table 4 Results of depth-profiling study of zircon IN05003_18, IN Depth (μm) 206 Pb / 238 U ratio 207 Pb / 235 U ratio 207 Pb / 206 Pb ratio 207 Pb/ 206 Pb age Pb (%) Th/U Th/U

12 N.L. Cates, S.J. Mojzsis / Chemical Geology 261 (2009) the Boizard suite (David et al., 2009). The core of the belt is also dominated by the same lithology, although, to the best of our knowledge no geochronology of that particular unit has yet been performed. The ca Ma gneiss does not share the initial deformational events recorded by the supracrustals of the NSB, suggesting that D 1 and D 2 must predate 3650 Ma Ti-in-zircon thermometry Fig. 11. U/Pb age vs. depth-profile of zircon IN05003_18 and corresponding Th/U. Boxed areas contain analyses used to determine age and Th/U. The shaded region is the predicted Th/U for a zircon grown in a melt of the composition of the host rock (based on a whole rock Th/U=3.7, reported in Cates and Mojzsis, 2007). discrepancy may be the result of a small error in the partition coefficient used as a change of 0.02 in the partition coefficient results in a ±0.05 change in the predicted Th/U for the zircon. Alternatively, a small error in the rock chemical analysis due, for example, to incomplete zircon digestion or an inhomogeneous sample, could also lead to a slightly elevated Th/U for the rock, which has less than 2 ppm U Zircon IN05003_18 The original overgrowth age of 2713±9 Ma recorded during conventional ion microprobe geochronology is also recorded in depth profiling with an age of 2738±25 Ma and Th/Ub0.01 (Table 4, Fig.11). However, a second, older zone with an age of 3622±46 Ma and Th/U plateau of 0.25 was also resolved. The core age of 3752±10 Ma recorded during conventional geochronology was reached at a depth of 5 μm. The core age and Th/U ratios measured during depth profiling (0.34) are both equivalent to those obtained via conventional ion microprobe analysis (Cates and Mojzsis, 2007) Regional geology and timing of metamorphism Both the ca Ma and the ca Ma ages recorded by zircons correspond to known regional geologic events. The western-most portion of the Innukjuak domain is dominated by the tonalitic Ma Qilalugalik and the 2750 Ma Boizard suites (Simard et al., 2003). The ~250 km 2 Boizard suite, which is composed of heterogeneous biotitebearing tonalitic gneisses and rare, small, partly resorbed mafic enclaves, surrounds and encloses the entire NSB (Fig. 2), and we speculate that its emplacement is the likely source for the 2750 Ma metamorphic event recorded by the zircon rims and coincides with the regional D 3 event. The ca Ma recrystallization/overgrowth zone in zircon IN05003_18 corresponds to the age (3658.5±2.5 Ma) of a thin (b20 m thick at its widest point), variably mylonitized tonalitic to granodioritic gneiss that surrounds the northwestern margin of the belt and is itself intruded by Titanium concentration in zircon ([Ti] zircon ) is a geochemical tracer that has been developed to constrain zircon crystallization temperature (Watson and Harrison, 2005). The thermometer presumes coexistence of rutile (essentially pure TiO 2 ) with zircon at time of crystallization, which is a valid assumption for most felsic igneous rocks. On the basis of this assumption, an equilibrium constant can be calculated from a(tio 2 ) in zircon by setting the activity of rutile equal to ~1 (Watson and Harrison, 2005). The thermometer has been calibrated experimentally for high ( C) temperatures, and natural zircons as well as glass standards have been used for crystallization temperatures of ~580 C 1170 C (Watson et al., 2006). The diffusion of Ti in zircon is slow (Cherniak and Watson, 2007), and retention of tetravalent Ti in zircon is apparently aided by the fact that it substitutes without charge compensation most favorably into the Si 4+ site (Harrison et al., 2007; Ferry and Watson, 2007). This substitution has recently been shown to be pressure sensitive at PN1 GPa(Ferriss et al., 2008). The utility of the [Ti] zircon thermometer remains in the developmental stage. The Ti thermometer is best applied to systems that contain apuretio 2 phase (e.g., rutile), but is suitable for melts with a Tisaturated phase such as ilmenite. When ilmenite is present in the source melts, a(tio 2 )isabout0.6(watson et al., 2006). The effect of sub-unity Ti activity (b0.6) in the thermometer is to cause a small shift from the original calibration of Watson and Harrison (2005) of about +50 C. Results show that a siliceous melt, independent of water content, will often saturate in a Ti bearing phase before zircon, and Ti-activity in melts has been characterized for magmas of diverse compositions (Ryerson and Watson, 1987). This has also been experimentally shown for siliceous melts such as trondhjemite, S-type granite, and metaluminous granite (Hayden et al., 2005). Arguments have been raised that the Ti thermometer is ambiguous for some thermometry studies (Kamber et al., 2005; Nutman, 2006; Glikson, 2006; Valley et al., 2006; Coogan and Hinton, 2006; Hiess et al., 2008; Fu et al., 2008). A common thread to these reports is that Zr and Ti activities in zircon source melts can be highly variable, and that a late stage mafic melt may be able to saturate the residuum at low temperatures in Zr (and Si) enough to subsequently crystallize zircon. It was argued that zircon crystallization temperatures may not be used to uniquely separate this process (or an analogous process) from granitic, water-saturated minimum melt conditions (Valley et al., 2006; Coogan and Hinton, 2006). Nutman (2006) and Hiess et al., (2008) suggested that tonalites could crystallize zircons with Ti temperatures indistinguishable from minimum melt conditions implicated in granite genesis. Current [Ti] zircon data, however, suggests that the majority of zircons that crystallize from mafic sources (including late-stage residual melts) are demonstrably different in peak and distribution from felsic sources (Harrison and Schmitt, 2007; Harrison et al., 2006, 2007; Watson and Harrison, 2006). Table 5 Ti-in-zircon thermometer results for zircon IN05003_18. Spot # 29 Si/ 30 Si Error 47 Ti/ 30 Si Error 48 Ti/ 30 Si Error 49 Ti/ 30 Si Error 50 Ti/ 30 Si Error 49 Ti/ 50 Ti Error Ti (ppm) Temperature ( C) IN53_18r E E E E E E E E IN53_18c E E E E E E E E IN53_18c E E E E E E E E IN53_18c E E E E E E E E IN53_18r E E E E E E E E IN53_18r E E E E E E E E