Geology and zircon geochronology of the Acasta Gneiss Complex, northwestern Canada: New constraints on its tectonothermal history

Transcription

1 Precambrian Research 153 (2007) Geology and zircon geochronology of the Acasta Gneiss Complex, northwestern Canada: New constraints on its tectonothermal history Tsuyoshi Iizuka a,, Tsuyoshi Komiya a, Yuichiro Ueno a,1, Ikuo Katayama a,2, Yosuke Uehara a, Shigenori Maruyama a, Takafumi Hirata a, Simon P. Johnson b, Daniel J. Dunkley c a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo , Japan b Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth, WA 6004, Australia c National Institute of Polar Research, Kaga, Itabashi, Tokyo , Japan Received 27 June 2006; received in revised form 27 November 2006; accepted 28 November 2006 Abstract The Acasta Gneiss Complex of northwestern Canada contains the oldest known crustal rocks on Earth. Here we present a detailed geological map of the main area of the complex (around the sample locality of the oldest known rocks) and detailed sketch maps of critical geological outcrops. The geological map shows that the complex is divided, by a northeast-trending fault, into eastern and western domains. The eastern area is comprised from quartz dioritic gabbroic gneisses and multi-phase tonalitic granitic gneisses. The western area is comprised of layered quartz dioritic dioritic and tonalitic granitic gneisses and younger foliated granitic intrusions. The detail field observations reveal at least five tectonothermal events in the eastern area: (1 and 2) emplacement of mafic-intermediate magma (protolith of the quartz dioritic gneiss) and emplacement of felsic magma (protolith of the older felsic gneiss); (3) metamorphism to produce the gneissic structures of the felsic gneiss and quartz dioritic gneiss; (4) intrusion of felsic magma (protolith of the younger felsic gneiss), causing anatexis in some parts; (5) metamorphism and deformation to produce the gneissic structure of the younger felsic gneiss. In contrast, at least four tectonothermal events have been recognized in the western area: (1 and 2) emplacement of the protolith to the mafic-intermediate and felsic gneiss; (3) metamorphism and deformation to form the gneissic and layered structures; (4) intrusion of the granite sheet (the protolith of the foliated granite); (5) metamorphism and deformation of all lithologies. To constrain the timing of the tectonothermal events, we have carried out U Pb dating combined with cathodoluminescence imagery on zircon extracted from the gneisses and foliated granites. Our data reveal at least four tonalite granite emplacement events in the eastern area, at ca. 3.94, , 3.66 and 3.59 Ga, and tonalite emplacement at ca Ga and granite intrusion at 3.58 Ga in the western area. The field relationships between the felsic and quartz dioritic gneisses in the eastern area demonstrate that the two quartz dioritic gneiss protoliths were emplaced prior to 3.59 and 3.66 Ga, respectively. These results confirm findings of previous zircon geochronology that the protolith ages of Acasta gneisses are , and ca. 3.6 Ga. In addition, our comprehensive field observations, zircon internal structures and zircon U Pb dating clearly demonstrate that the early Archean Acasta rocks suffered anatexis/recrystallization, coincident with the emplacement of younger felsic intrusions, and that the 3.59 Ga Corresponding author. Present address: Earthquake Research Institute, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo , Japan. Fax: address: tiizuka@eri.u-tokyo.ac.jp (T. Iizuka). 1 Present address: Department of Environmental Science and Technology, Tokyo Institute of Technology, Midori-ku, Yokohama , Japan. 2 Present address: Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima , Japan /$ see front matter 2006 Elsevier B.V. All rights reserved. doi: /j.precamres

2 180 T. Iizuka et al. / Precambrian Research 153 (2007) granitic gneiss protolith in the eastern area contains zircon xenocrysts with ages up to 3.9 Ga. In this context, we also discuss the tectonothermal evolution of the Acasta Gneiss Complex on the basis of these results and those from previous studies, and its implications for radiogenic isotopic studies Elsevier B.V. All rights reserved. Keywords: Acasta gneiss; Ancient zircon; LA-ICPMS; Hadean; Crustal reworking; U Pb dating 1. Introduction The geological, petrological and geochemical signatures of early Archean rocks provide the most direct evidence for the early crust formation processes. The Acasta Gneiss Complex is an early Archean gneiss complex exposed along the western margin of the Slave Province, northwestern Canada (Fig. 1). The rocks was first dated by Bowring and Van Schmus (1984) using thermal ionization mass spectrometry (TIMS) zircon geochronology, to test a hypothesis that an early Proterozoic terrane had overthrust the western edge of the Slave Province, and they obtained a zircon U Pb age of 3.48 Ga. They carried out further geological mapping, sampling and dating by TIMS in the region, and recognized that the rocks have Nd CHUR model ages up to 4.1 Ga and contained zircon cores older than 3.84 Ga (Bowring and Van Schmus, 1984; Bowring et al., 1989a). Subsequent in situ sensitive high-resolution ion microprobe (SHRIMP) dating combined with imaging studies (optical microscopy with HF etching, cathodoluminescence (CL) imagery and/or back-scattered electron (BSE) imagery) revealed protolith ages of , and 3.6 Ga for the tonalitic granitic gneisses, and of 4.0 and 3.6 Ga for amphibolitic gneisses (Bowring et al., 1989b; Bowring and Housh, 1995; Bleeker and Stern, 1997; Stern and Bleeker, 1998; Bowring and Williams, 1999; Sano et al., 1999). The Ga protoliths are recognized as the oldest known terrestrial rocks. More recently, Iizuka et al. (2006) reported the occurrence of a 4.2 Ga zircon xenocryst within a ca Ga tonalitic gneiss protolith based on in situ zircon dating by SHRIMP and LA-ICPMS combined with CL and BSE imaging studies. Despite extensive geochronological studies of Acasta gneisses, the lithology and geology of them remain poorly understood. The detailed geological and lithological descriptions are critical for adequate interpretation of the geochronological, geochemical and isotopic data from the gneisses and for better understanding the nature of very early continental crust formation and its subsequent tectonothermal history. In this paper, we show a 1:5000 geological map of the main area of the complex (around the sample locality of the Acasta gneisses reported by Bowring et al., 1989a), 1:100 to 1:10 sketch maps of critical geological outcrops, and zircon U Pb data on 13 Acasta rock samples. The geological, lithological and zircon geochronological data combined with CL imaging of the zircon reveal that the early Archean Acasta gneisses experienced a long and complex geological history, including deformation, recrystallization and anatexis. Along with the results obtained in this and previous studies, we discuss the tectonothermal history of the Acasta Gneiss Complex, and its implications for whole-rock Sm Nd and zircon Lu Hf isotopic studies. 2. Geology 2.1. Slave Province The Slave Province is a well-exposed Archean terrane located in the northwestern part of the Canadian Shield (Fig. 1), and covers an area of approximately 190,000 km 2 (Hoffman, 1989). It is bounded to the east and southeast by the Ga Thelon-Taltson orogen and to the west by the Ga Wopmay orogen. Rocks of the Slave Province can be classified into three main lithotectonic assemblages: (1) pre-2.8 Ga basement; (2) Ga supracrustal rocks of the Yellowknife Supergroup, with syn-volcanic plutons; (3) Ga plutons (Henderson, 1970; van Breemen et al., 1992; Villeneuve and van Breemen, 1994). The pre-2.8 Ga basement consists of amphibolitegrade granitic gneisses, granitoids and supracrustal rocks comprising quartzite, banded iron formation, conglomerate and volcanic rocks that pre-date the deposition of Yellowknife Supergroup supracrustal rocks (Easton, 1985; Henderson, 1985; Frith et al., 1986; Isachsen and Bowring, 1994, 1997; Villeneuve and van Breemen, 1994; Bleeker et al., 1999a,b; Yamashita and Creaser, 1999; Yamashita et al., 2000a,b; Ketchum et al., 2004). These pre-2.8 Ga rocks have only been documented to the west of a north-trending Nd Pb isotopic boundary at approximately 111 W in the central Slave Province (Davis and Hegner, 1992; Thorpe et al.,

3 T. Iizuka et al. / Precambrian Research 153 (2007) Fig. 1. (a) Simplified geological map of the Slave Province in northwestern Canada. Modified from Hoffman (1989) and Bleeker et al. (1999a,b). The box shows location of (b). (b) Lithological map along the Acasta River after St-Onge et al. (1988) and Bowring et al. (1990). Barbed and solid lines are Proterozoic thrust faults and transcurrent faults, respectively. Arrows and dashed line represent Proterozoic fold axes. The box shows location of Fig ; Davis et al., 1996; Bleeker and Davis, 1999). The Acasta Gneiss Complex is located within this basement complex. The Ga Yellowknife Supergroup supracrustal sequence is exposed throughout the Slave Province and covers 30% of the province (Padgham and Fyson, 1992). It mainly consists of mafic to felsic volcanic rocks and greywacke-mudstone turbidites. The volcanic rocks are accompanied by synchronous plutonic rocks of intermediate composition. The turbidites are largely derived from the Ga volcanic and plutonic rocks (Jenner et al., 1981; Sircombe et al., 2001) and make up 80% of the supracrustal sequence. Throughout the Slave Province, plutons intruded the supracrustal sequence between 2.62 and 2.58 Ga (van Breemen et al., 1992; Villeneuve and van Breemen, 1994). The syn- to post-deformational plutons are generally felsic, ranging from tonalite to granite (Davis et al., 1994) and comprise 50% of the exposed province. These plutons caused regional deformation and low to intermediate P/T metamorphism, ranging from greenschist to granulite facies (Thompson, 1978; Henderson and Schaan, 1993; Pehrsson and Chacko, 1997) Geological framework of the Acasta Gneiss Complex The Acasta Gneiss Complex is exposed along the Acasta River in the westernmost Slave Province (Fig. 1). So far, the most detailed geological descriptions of the Acasta Gneiss Complex have been given by

4 182 T. Iizuka et al. / Precambrian Research 153 (2007) Fig. 2. Four main lithofacies in the Acasta Gneiss Complex: (a) mafic-intermediate gneiss series; (b) felsic gneiss series; (c) layered gneiss series with rhythmical layering of leucocratic and melanocratic layers; (d) foliated granite preserving original igneous textures. Bowring et al. (1989b, 1990) and Bowring and Williams (1999). They describe the complex as a heterogeneous assemblage of biotite hornblende tonalitic to granitic orthogneiss commonly inter-layered on a centimeter- to meter-scale with amphibolitic and chloritic schlieren, boudins and layers. Large areas of amphibolite also occur, together with less abundant calc-silicate gneiss, quartzite, biotite schist and ultramafic schist. All the rocks are intruded by weakly to strongly foliated and mylonitic granite. Bleeker and Stern (1997), Stern and Bleeker (1998) and Bleeker and Davis (1999) have also carried out comprehensive geological mapping of the Acasta region and adjacent regions in the Slave Province. We have made a 1:5000 geological map of 6km 6 km of the main area and 1:100 to 1:10 sketch maps of critical areas in the Acasta Gneiss Complex, and collected about 1000 rocks during field seasons conducted in 2000 and We have classified the major assemblage of foliated to gneissic rocks into four lithofacies by the composition and texture of the rocks on the basis of field observation and mineralogy (Fig. 2): (1) a mafic-intermediate gneiss series (quartz dioritic, dioritic and gabbroic gneisses) (Fig. 2a); (2) a felsic gneiss series (tonalitic, trondhjemitic, granodioritic and granitic gneisses) (Fig. 2b); (3) a layered gneiss series (layered mafic-intermediate and felsic gneiss; Fig. 2c); (4) a foliated granite (preserving original igneous textures; Fig. 2d). The identification of these major lithofacies is consistent with the previous work (Bowring et al., 1990; Bowring and Williams, 1999), but our geological map (Fig. 3) shows detailed field relationships amongst them which were not previously described. The mapped main area is subdivided into two main domains by a northeast-trending fault, which juxtaposes contrasting lithologies and which is marked by many quartz veins from sub-millimeters to meters thick. At various localities, the orientation of gneissic structures and fabrics also change abruptly across the fault. The felsic gneiss series occurs predominantly in the eastern area and as minor intrusions in the western area, whereas the mafic-intermediate gneiss series occurs mainly as rounded to elliptical enclaves and inclusions within the felsic gneiss (Fig. 4a). In the eastern part of the eastern area the felsic gneiss series have northwest-trending foli-

5 T. Iizuka et al. / Precambrian Research 153 (2007) Fig. 3. Geological map of the Acasta Gneiss Complex. It comprises mafic-intermediate, felsic and layered gneiss series, and foliated granite with minor felsic and mafic dikes. Northeast-trending fault runs at the center. Data sources of ages (protolith ages of gneisses) in the legend are as follows: (1) this study; (2) Bowring and Williams (1999); (3) Bowring and Housh (1995); (4) Stern and Bleeker (1998); (5) Bowring et al. (1989b); (6) Iizuka et al. (2006); (7) Bleeker and Stern (1997).

6 184 T. Iizuka et al. / Precambrian Research 153 (2007) Fig. 4. (a) Enclaves of quartz dioritic gneiss in granitic gneiss. (b) Hornblendite inclusion in quartz dioritic gneiss. (c) Hornblendite along the boundary between quartz dioritic and granodiorite gneisses. (d) Co-occurrence of granitic and tonalitic gneisses, suggesting the multi-felsic intrusions. (e) Porphyroblasts of feldspar with asymmetrical structures in the layered gneiss. (f) Folded layered gneiss and intrusion of foliated granite. ations that dip westward, but in the western part they trend north and dip eastward. The layered gneiss series is present mainly in the western area where the gneissic foliation generally trends north south and dips to the west. These structures are often oblique to the boundary of the foliated granite. The foliated granite predominantly occurs as intrusions up to 200 m wide that generally trend north south, whereas much thinner intrusions of granite and aplite are present throughout the complex. The granitic intrusions in the western region are cut by the main central fault. Mafic dikes are widespread throughout the area, are generally northwest-trending, and cut the main central fault.

7 T. Iizuka et al. / Precambrian Research 153 (2007) Lithology and petrography The mafic-intermediate gneiss series (Fig. 2a) predominantly occurs as 3 km 1 km to 10 cm 10 cm enclaves within the felsic gneiss, and forms blocks, boudins and bands in the eastern area (Fig. 4a). The mafic-intermediate gneiss series contains both mesocratic and melanocratic portions, and includes gabbroic, dioritic and quartz dioritic gneisses. Quartz dioritic gneiss is the predominant phase, which is comprised by the mineral assemblage hornblende, plagioclase, quartz and biotite (main constituent minerals are put in descending order of abundance) and accessory alkali-feldspar, zircon, titanite, apatite, garnet and opaque; however, in the northern part of the eastern area gabbroic gneiss is more abundant. Some gneisses, especially in the northern part of the eastern area, have abundant garnet porphyroblasts. Occasionally, massive hornblendite inclusions are present within the mafic-intermediate gneisses (Fig. 4b), and frequently occur along the boundary between the mafic-intermediate and felsic gneisses (Fig. 4c). The felsic gneiss series (Fig. 2b) is widely distributed in the eastern area of the Acasta Gneiss Complex and occurs as massively or banded leucocratic gneiss including tonalitic, trondhjemitic, granodioritic and granitic gneiss. The mineral assemblage ranges from plagioclase, quartz, hornblende and biotite with accessory alkalifeldspar, zircon, titanite, apatite and opaque to quartz, alkali-feldspar, plagioclase and biotite with accessory zircon, titanite, apatite and opaque. At some localities, the different type compositions occur together providing evidence for multiple phases of felsic magamtism (Fig. 4d). The layered gneiss series (Fig. 2c) is characterized by both continuous layering of felsic and mafic-intermediate lithological suites (gneisses) on a centimeter- to meter-scale and a prominent preferred orientation of platy and prismatic minerals. The layered gneiss series occurs predominantly in the western area together with the foliated granite (Fig. 3); although the banding of felsic and mafic-intermediate gneisses outcrop at some localities in the eastern area, the continuity of the banding structure is poor. The mineralogy and bulk compositions of the felsic and mafic-intermediate lithological suites of the layered gneiss series are equivalent to the felsic and mafic-intermediate gneiss series in the eastern area, respectively. There are many large porphyroblasts of quartz and feldspar (Fig. 4e). In addition, some mafic-intermediate suites contain abundant garnet porphyroblasts. Thin boudins and layers of coarsegrained hornblendite are also present sporadically along the layering. The foliated granite (Fig. 2d) predominantly occurs in the western area as intrusions up to 200 m wide. Original igneous textures are preserved and the unit is comprised of plagioclase, alkali-feldspar, quartz, hornblende and biotite (the abundances of plagioclase, alkali-feldspar and quartz are nearly equal) with accessory zircon, titanite, apatite and opaque. Some of the granites are inter-folded with the layered gneiss series (Fig. 4f). Mafic dikes post-date the formation of the central fault and generally trend northwest southeast. The intrusions are fine-grained and have a typical mineral assemblage of actinolite hornblende, plagioclase, quartz, epidote and chlorite and accessory biotite, titanite, apatite and opaque, indicating metamorphism under the epidote amphibolite to amphibolite facies conditions. In addition, some of the gneisses contain calcite, epidote and secondary biotite, indicating that they suffered post-magmatic metasomatic alteration and infiltration of mobile elements such as Ca and K Field relationships amongst the gneisses and foliated granite The Acasta gneisses have been modified during several metamorphic ductile deformation events. In addition, some of them have been strongly deformed or migmatized, adding to the difficulty of understanding their ancient tectonic history. However, detailed field observations provide critical insights into the relationships amongst the gneisses and granites. Below we show sketches and photos of critical field outcrops (Figs. 5 8). Fig. 5 shows the field relationships between the intermediate and felsic gneisses. The outcrop consists of quartz dioritic gneiss, coarse-grained granodioritic gneiss, and granitic gneiss with pegmatites and hornblendite pods. The pegmatites mainly occur on the fringe of the granitic gneiss. The hornblendite pods are present along the boundary between the quartz dioritic and the granitic gneisses accompanied by a relatively quartzrich quartz dioritic gneiss. The boundary between the quartz-rich quartz dioritic gneiss and the quartz dioritic gneiss is vague. The granitic and quartz-rich quartz dioritic gneisses exhibit subparallel gneissosity to their outer form, whilst the gneissic structure within the quartz dioritic gneiss is partly obscured due to the paucity of quartzofeldspathic minerals giving gneissosity (compositional banding). In contrast, the gneissosity of the coarse-grained granodioritic gneiss is obliquely cut by the granitic gneiss. These observations indicate that the protolith of the granitic gneiss intruded into the quartz dioritic and coarse-grained granodioritic gneisses, and that during the crystallization of the granite intrusion,

8 186 T. Iizuka et al. / Precambrian Research 153 (2007) Fig. 5. A sketch map and photo of outcrop of quartz dioritic, coarse-grained granodioritic and granitic gneisses with minor pegmatite and hornblendite layers. The outcrop displays that granitic gneiss occurs as intrusions into quartz dioritic and coarse-grained granodioritic gneisses. In addition, the quartz dioritic gneiss differentiated into relatively quartz-rich quartz dioritic gneiss leucosome and hornblenditic residual layers along boundary between the quartz dioritic and granitic gneisses, indicating anatexis of the quartz dioritic blocks by intrusion of granitic magmas. fluids (most likely hydrous melts) were released to form the pegmatites on the fringe of the granite intrusion. The occurrence of hornblendite pods accompanied by quartz-rich quartz dioritic gneiss between the quartz dioritic and granitic gneisses suggests that fluid infiltration and thermal metamorphism during granite intrusion also caused partial melting (anatexis) of the host gneiss to form a hornblendite restite with a quartz-rich quartz dioritic gneiss leucosome. Hence, we have recognized at least five tectonothermal events in the eastern area from these fabrics: (1 and 2) emplacement of quartz dioritic magma (protolith of the quartz dioritic gneiss) and emplacement of granodioritic magma (protolith of the coarse-grained granodioritic gneiss); (3) metamorphism to produce the gneissic structures of the coarse-grained granodioritic gneiss (and quartz dioritic gneiss); (4) intrusion of granitic magma (protolith of the granitic gneiss), causing anatexis and formation of hornblendites and quartz-rich quartz dioritic gneiss; (5) metamorphism and deformation to produce the gneissic structures of granitic and quartz-rich quartz dioritic gneisses. The relationship between the quartz dioritic and granitic gneisses is also shown in Fig. 6. The outcrop comprises of quartz dioritic and granitic gneisses with hornblendite pods and pegmatites. The gneissic structure of the granitic gneiss is subparallel to the direction of their outer form. The gneissic structures of the quartz dioritic gneiss are more complicated than those of the granitic gneiss, and oblique to them at some points. Pegmatites occur along the margin of the granitic gneiss as well as within the quartz dioritic gneiss, suggesting its derivation from fluids released during the crystallization of the granite intrusion. Fourteen hornblendite pods are sporadically distributed within the quartz dioritic gneiss body and, unlike those illustrated in Fig. 5, most of them are not associated with quartzrich dioritic gneiss layer. In addition, the deformation structures imprinted on them are consistent with those within the quartz dioritic gneiss. These observations suggest that the hornblendite pods are xenoliths entrained by the quartz dioritic magma. Therefore, we have recognized five tectonomagmatic events from this outcrop: (1) formation of the hornblendite xenolith protoliths; (2)

9 T. Iizuka et al. / Precambrian Research 153 (2007) Fig. 8. A photo of outcrop of layered gneiss and foliated granite. The gneissic structure of the layered gneiss is cut obliquely by the foliated granite, indicating the formation of gneissic structure prior to the granite intrusion. Fig. 6. A sketch map of the locality A showing the relationships between quartz dioritic and granitic gneisses with minor pegmatite and hornblendite layers. emplacement of quartz dioritic magma entraining those hornblendite xenoliths; (3) metamorphism and deformation to produce a gneissosity of the quartz dioritic gneiss; (4) intrusion of granitic magma (granitic gneiss protolith); (5) metamorphism and deformation to produce gneissosity within the granitic gneiss. Fig. 7 shows the relationship between the coarsegrained granodioritic gneiss and the granitic gneiss. In this outcrop, which is some 400 m southwest of locality of Fig. 5, the coarse-grained granodioritic gneiss occurs as enclaves or boudins within the granitic gneiss. The gneissic structure of the granitic gneiss is concordant with the shape of the granodioritic gneiss enclaves and boudins. These observations also indicate that the granodioritic gneiss was intruded by the granitic magma. Fig. 8 displays the field relationship between the layered gneiss and foliated granite. The layered gneiss series, which mainly consists layered mafic gneiss with thin layers of felsic gneiss, is intruded by 1 m thick foliated granite sheet. The foliated granite has a moderate to weak foliation but no obvious gneissic structures. Importantly, the layering and gneissic structures within the layered gneiss series are obliquely cut by the foliated granite intrusion, indicating that the formation of layering structures preceded intrusion of the foliated granite. These observations demonstrate at least five tectonomagmatic events in the western area: (1 and 2) emplacement of the protoliths to the mafic felsic gneisses; (3) metamorphism and deformation forming the gneissic and layered structures of the layered gneiss; (4) intrusion of the granite sheet (the protolith of the foliated granite); (5) metamorphism and deformation of all lithologies. 3. Zircon geochronology 3.1. Analytical methods Fig. 7. A photo showing coarse-grained granodioritic enclaves within granitic gneiss, indicating that the former was intruded by the latter. Zircon grains were separated from rock samples using standard crushing, magnetic separation and heavy-liquid techniques. The grains were mounted in epoxy and were polished. Before U Pb isotopic analyses, we rigorously checked the external and internal structures of the zircons using transmitted/reflected light microscopy and

10 188 T. Iizuka et al. / Precambrian Research 153 (2007) CL imaging. CL images were obtained using a JEOL JSM-5310 scanning electron microprobe combined with an Oxford CL system at Tokyo Institute of Technology. U Pb dating was performed on the laser ablationinductively coupled plasma mass spectrometer (LA- ICPMS) at Tokyo Institute of Technology. The ICPMS instrument used in this study was a ThermoElemental VG PlasmaQuad 2 quadrupole based ICPMS equipped with S-option interface (Hirata and Nesbitt, 1995) and chicane ion lens. The laser ablation system used in this study was a MicroLas production (Gottingen, Germany) GeoLas 200CQ laser ablation system. This system utilizes Lambda Physik (Gottingen, Germany) COMPex 102 ArF excimer laser as a 193 nm DUV (deep ultraviolet) light source. The details of these systems are described elsewhere (Iizuka and Hirata, 2004). Helium gas was flushed into the ablation cell, minimizing aerosol deposition around the ablation pit and improving transport efficiency (Eggins et al., 1998). In order to improve the stability of the signals, a gas expansion chamber was inserted between the ablation cell and the ICP ion source (Tunheng and Hirata, 2004). The U Pb data were obtained from two different sizes of ablation craters (16 and 32 m) with the integration time of 20 s, laser repetition rate of 4 6 Hz, and emission power of 4 5 mj. The instrumental bias for the 206 Pb/ 238 U ratio was corrected by normalizing against SL13 (572 Ma; Roddick and van Breemen, 1994) and Nancy standard zircons ( Ma; Wiedenbeck et al., 1995). Common Pb was corrected using 204 Pb. The isobaric interference of 204 Hg on 204 Pb was corrected by monitoring 202 Hg. In order to reduce the isobaric interference of 204 Hg, a Hg-trap device using an activated charcoal filter was applied to the Ar make-up gas before mixing with He carrier gas (Hirata et al., 2005). No common Pb correction has been applied to analyses where the corrected ratio is within analytical uncertainty of uncorrected ratio. Analytical uncertainties combine the counting statistics and the reproducibility of the standard analyses (NIST SRM 610 for 207 Pb/ 206 Pb and the standard zircons for 206 Pb/ 238 U, respectively), added in quadrature. 207 Pb concentrations of NIST SRM 610 are higher than those of the analyzed zircon samples. The precision and accuracy of our zircon U Pb dating technique was assessed by analyzing the zircon standard QGNG and in-house zircon standard PMA7. QGNG is one of the oldest zircon standards with a 207 Pb/ 206 Pb age of ± 0.6 Ma (Black et al., 2004). The LA- ICPMS U Pb isotopic data were obtained from 20 separate spots (Appendix A in supplementary material), and yielded a mean age of 1813 ± 52 Ma (2S.D.), consistent with the literature data. In addition, there is a variation in the obtained LA-ICPMS 207 Pb/ 206 Pb ages beyond analytical uncertainty (Appendix A in supplementary material), indicating that the analytical uncertainty for each LA-ICPMS analysis is an underestimate. The underestimation results from significant differences between 207 Pb concentrations of NIST SRM 610 and natural zircon. In this study, therefore, a further 0.75% uncertainty is assigned to the errors of the 207 Pb/ 206 Pb isotope ratios for NIST SRM 610 and propagated through the error calculation. This results in LA-ICPMS 207 Pb/ 206 Pb ages for QGNG that are within analytical uncertainty of the standard age. Zircon PMA7 was previously analyzed by Menot et al. (1993) using SHRIMP technique (Menot et al., 1993). We also measured the U Pb isotopes of zircon PMA7 by SHRIMP and LA-ICPMS (Appendix B in supplementary material). The SHRIMP results of previous and present studies revealed a variation in 207 Pb/ 206 Pb ages beyond analytical uncertainty, indicating that some grains have suffered significant ancient Pb loss. After culling the youngest 207 Pb/ 206 Pb age that is not identical to the other 207 Pb/ 206 Pb ages within analytical uncertainty, a weighted mean 207 Pb/ 206 Pb age of 2438 ± 4Ma (2σ) was obtained and interpreted as the crystallization age of PMA7. Nineteen LA-ICPMS U Pb isotopic data from 10 grains also showed a variation in 207 Pb/ 206 Pb ages, and gave a 207 Pb/ 206 Pb mean age of 2417 ± 39 Ma (2S.D.; from the 17 highest 207 Pb/ 206 Pb ratios) (Appendix B in supplementary material), consistent with the SHRIMP results. This demonstrates that our method is capable of producing accurate 207 Pb/ 206 Pb dates with precision of ca. 2% from early Proterozoic grains with U concentrations of ppm Samples and zircon U Pb data In this study, we have analyzed zircons from 13 samples (6 felsic gneisses, 2 intermediate gneisses, 2 layered gneiss felsic suites, 2 foliated granites, and 1 pegmatite) from 9 localities (Fig. 3). We interpret the age of the samples by a combination of U Pb zircon data, field observations, and CL imagery of zircon zonation patterns (Fig. 9). All analytical data is summarized in Table 1 and Fig Locality A (AC458, AC460 and AC461) Locality A (Fig. 3) consists of quartz dioritic gneiss and granitic gneiss with pegmatites (Fig. 6 and see also Section 2.4). We analyzed zircon grains extracted from three rock samples at this locality: AC458 (granitic gneiss), AC460 (pegmatite) and AC461 (quartz dioritic gneiss) (Fig. 6).

11 T. Iizuka et al. / Precambrian Research 153 (2007) Fig. 9. Cathodoluminescence images of zircons from (a) a pegmatite AC460, (b) a granitic gneiss AC458, (c) a quartz dioritic gneiss AC461, (d) a granitic gneiss AY066, intruding quartz dioritic gneiss, (e) a tonalitic gneiss AC580, (f) a granitic gneiss AC584, (g) a quartz dioritic AC579, (h) a granitic gneiss AY120, (i) a coarse-grained granodioritic gneiss AY199, (j) a foliated granite AC476, (k) a tonalitic suite of layered gneiss AC478, (l) a foliated granite AC020 and (m) a tonalitic suite of layered gneiss AC023. Spot numbers correspond to those in Table 1. Values record 207 Pb/ 206 Pb age of each spot. t and Hf represent analytical spots of trace elements and Lu Hf isotope, respectively.

12 190 T. Iizuka et al. / Precambrian Research 153 (2007) Fig. 9. (Continued ). Sample AC460 (pegmatite) yielded euhedral to subhedral zircons with low aspect ratios ( 2). The zircons are typically m long and relatively clear (i.e. not metamict). These textures are different from those of zircon extracted from the granitic gneiss (AC458) and quartz dioritic gneiss (AC461), suggesting that the zircons from AC460 are magmatic rather than being inherited from the adjacent gneisses. When viewed under CL, weak oscillatory zoning is common (Fig. 9a). The oscillatory zoning is a typical internal structure of zircons grown in equilibrium with a melt or a fluid (Rubatto and Gebauer, 1998; Corfu et al., 2003). Locally along fractures, the oscillatory zoning structure is disrupted by dull (under CL) discordant areas. These observations suggest that the zircons were altered along the cracks by fluids (Corfu et al., 2003). The CL images show no evidence of any older cores within the grains. Six U Pb analyses were carried out on four oscillatory-zoned grains, and all analyses but one yielded close to concordant ages around 3.6 Ga (Fig. 10a). The discordant data extends to the right side of concordia indicating recent Pb loss. All obtained 207 Pb/ 206 Pb ages

13 Table 1 LA-ICPMS U Pb isotopic analytical data for zircons from the Acasta Gneiss Complex Grain spot Internal structure a U (ppm) Th (ppm) Th/U 204 Pb/ 206 Pb 206 Pb c / 238 U(2σ) 207 Pb c / 206 Pb c (2σ) Age (Ma) Disc. (%) 206 Pb/ 238 U(2σ) 207 Pb/ 206 Pb (2σ) AC460 (pegmatite) 01-l b osc < ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc ± ± ± ± 26 1 AC458 (granitic gneiss) 01-1 osc < ± ± ± ± Altered core < ± ± ± ± w-osc overgrowth ± ± ± ± Altered core ± ± ± ± osc overgrowth < ± ± ± ± osc overgrowth ± ± ± ± Altered core ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± w-osc overgrowth < ± ± ± ± Altered core ± ± ± ± osc overgrowth < ± ± ± ± Altered core < ± ± ± ± osc < ± ± ± ± Altered core < ± ± ± ± Altered core < ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc < ± ± ± ± 31 3 AC461 (quartz dioritic gneiss) 01-1 Altered < ± ± ± ± Altered ± ± ± ± Altered ± ± ± ± Altered < ± ± ± ± Altered < ± ± ± ± Altered ± ± ± ± T. Iizuka et al. / Precambrian Research 153 (2007)

14 Table 1 (Continued) Grain spot Internal structure a U (ppm) Th (ppm) Th/U 204 Pb/ 206 Pb 206 Pb c / 238 U(2σ) 207 Pb c / 206 Pb c (2σ) Age (Ma) Disc. (%) 206 Pb/ 238 U(2σ) 207 Pb/ 206 Pb (2σ) 07-1 Altered < ± ± ± ± Altered < ± ± ± ± 56 1 AY066 (granitic gneiss) 01-1 osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc ± ± ± ± 26 0 AY066 (granitic gneiss) 10-1 osc ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± 27 1 AC580 (tonalitic gneiss) 01-1 osc < ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± w-osc ± ± ± ± w-osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc ± ± ± ± Altered < ± ± ± ± AC584 (granitic gneiss) 01-1 w-osc ± ± ± ± w-osc ± ± ± ± osc core < ± ± ± ± osc overgrowth ± ± ± ± osc core ± ± ± ± osc core < ± ± ± ± osc overgrowth ± ± ± ± w-osc core < ± ± ± ± osc overgrowth ± ± ± ± T. Iizuka et al. / Precambrian Research 153 (2007)

15 05-1 osc core ± ± ± ± osc core < ± ± ± ± osc core < ± ± ± ± osc overgrowth < ± ± ± ± osc core < ± ± ± ± osc core < ± ± ± ± osc overgrowth < ± ± ± ± osc overgrowth < ± ± ± ± Altered core ± ± ± ± Altered core ± ± ± ± osc overgrowth ± ± ± ± Altered core < ± ± ± ± osc overgrowth < ± ± ± ± osc overgrowth < ± ± ± ± Altered core < ± ± ± ± Altered core ± ± ± ± osc overgrowth < ± ± ± ± osc core < ± ± ± ± osc core < ± ± ± ± osc overgrowth < ± ± ± ± osc core < ± ± ± ± osc overgrowth < ± ± ± ± osc core < ± ± ± ± Altered core ± ± ± ± osc core < ± ± ± ± osc core < ± ± ± ± osc overgrowth ± ± ± ± 49 8 AC579 (quartz dioritic gneiss) 01-1 osc ± ± ± ± osc < ± ± ± ± w-osc ± ± ± ± w-osc < ± ± ± ± Altered core < ± ± ± ± Altered core < ± ± ± ± osc ± ± ± ± osc overgrowth ± ± ± ± Altered core ± ± ± ± Altered core ± ± ± ± osc overgrowth ± ± ± ± Patchy zoning core ± ± ± ± Patchy zoning core < ± ± ± ± Patchy zoning core < ± ± ± ± Patchy zoning core < ± ± ± ± osc overgrowth < ± ± ± ± Altered core ± ± ± ± w-osc overgrowth ± ± ± ± 34 3 T. Iizuka et al. / Precambrian Research 153 (2007)

16 Table 1 (Continued) Grain spot Internal structure a U (ppm) Th (ppm) Th/U 204 Pb/ 206 Pb 206 Pb c / 238 U(2σ) 207 Pb c / 206 Pb c (2σ) Age (Ma) Disc. (%) 206 Pb/ 238 U(2σ) 207 Pb/ 206 Pb (2σ) 11-2 Altered core ± ± ± ± w-osc ± ± ± ± Altered core < ± ± ± ± osc overgrowth ± ± ± ± Altered core < ± ± ± ± osc ± ± ± ± w-osc ± ± ± ± osc overgrowth < ± ± ± ± Altered core ± ± ± ± AY120 (granitic gneiss) 01-1 osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± AY199 (coarse-grained granodioritic gneiss) 01-1 osc < ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± Altered ± ± ± ± Altered ± ± ± ± osc ± ± ± ± osc < ± ± ± ± AY199 (coarse-grained granodioritic gneiss) 12-1 osc ± ± ± ± 45 1 AC476 (foliated granite) 01-1 osc ± ± ± ± T. Iizuka et al. / Precambrian Research 153 (2007)

17 01-2 osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± AC478 (tonalitic suite of layered gneiss) 01-1 osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± Recryst ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc ± ± ± ± Recryst ± ± ± ± osc ± ± ± ± osc ± ± ± ± 22 1 AC020 (foliated granite) 01-1 osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc < ± ± ± ± osc ± ± ± ± osc ± ± ± ± T. Iizuka et al. / Precambrian Research 153 (2007)

18 Table 1 (Continued) Grain spot Internal structure a U (ppm) Th (ppm) Th/U 204 Pb/ 206 Pb 206 Pb c / 238 U(2σ) 207 Pb c / 206 Pb c (2σ) Age (Ma) Disc. (%) 206 Pb/ 238 U(2σ) 207 Pb/ 206 Pb (2σ) 03-3 osc ± ± ± ± Altered ± ± ± ± Altered ± ± ± ± Altered ± ± ± ± 56 2 AC020 (foliated granite) 05-1 osc < ± ± ± ± osc ± ± ± ± osc ± ± ± ± osc ± ± ± ± Altered ± ± ± ± AC023 (tonalitic suite of layered gneiss) 01-1 osc core ± ± ± ± osc overgrowth ± ± ± ± osc core ± ± ± ± osc core ± ± ± ± osc core < ± ± ± ± osc overgrowth ± ± ± ± osc core ± ± ± ± osc overgrowth < ± ± ± ± osc core ± ± ± ± osc core ± ± ± ± osc overgrowth ± ± ± ± osc core ± ± ± ± osc core ± ± ± ± osc core < ± ± ± ± osc overgrowth < ± ± ± ± osc core ± ± ± ± osc overgrowth ± ± ± ± osc core ± ± ± ± osc overgrowth < ± ± ± ± osc core ± ± ± ± osc overgrowth < ± ± ± ± osc core < ± ± ± ± osc overgrowth ± ± ± ± osc core < ± ± ± ± osc core < ± ± ± ± osc overgrowth < ± ± ± ± T. Iizuka et al. / Precambrian Research 153 (2007) Note: All errors are quoted at 2σ level. a Internal structures in cathodoluminescence images osc: oscillatory zoning; w-osc: weakly oscillatory zoning; Recryst: recrystallized. b Bold used in the calculation of the crystallization and/or recrystallization ages of the protolith of Acasta gneisses. c Common Pb corrected using 204 Pb. No common Pb correction has been applied to analyses for which the corrected ratio is within 2σ of the uncorrected ratio.

19 T. Iizuka et al. / Precambrian Research 153 (2007) are equal within analytical uncertainty, and yield a mean age of 3589 ± 30 (2S.D.) that we interpret to be the age of crystallization of the pegmatite. The granitic gneiss (AC458) yielded generally euhedral to subhedral zircons m in length. The zircons have typical aspect ratios of 2 4 and under CL illumination, the grains generally preserve oscillatory zoning structures (Fig. 9b). Locally the zoning has been altered producing discordant, dull and occasionally mosaic or feathery textures under CL (Corfu et al., 2003). In addition 5% of zircons contained non-zoned to dull cores that are overgrown by oscillatory-zoned mantles (Fig. 9b), suggesting that AC458 contains xenocrystic zircon cores. Twenty-five U Pb age determinations were undertaken on 16 grains. Analyses of oscillatory-zoned sites yielded variably discordant 207 Pb/ 206 Pb ages from ca. 3.6 to ca. 3.3 Ga producing an average 207 Pb/ 206 Pb age of around 3.6 Ga (Fig. 10b). In contrast, the non-zoned cores range in 207 Pb/ 206 Pb age from ca. 3.0 to 3.9 Ga. The data demonstrate that a granitic magma, containing xenocrystic zircons, was emplaced at ca. 3.6 Ga, which subsequently suffered metamorphism. Determination of the precise crystallization age is difficult because of the variation in 207 Pb/ 206 Pb ages, as well as the presence of xenocrystic cores. We estimate the crystallization age of the protolith of AC458 from the oldest 207 Pb/ 206 Pb ages of oscillatory-zoned zircons on the assumption that their age variation was caused by both ancient and recent (downward and rightward distribution in the concordia diagram, respectively) Pb-loss events from one generation of zircon, rather than by mixing of multigenerations of zircons (see discussion in Nutman et al., 1997; Bowring and Williams, 1999). The 11 highest 207 Pb/ 206 Pb measurements on oscillatory-zoned zircons are equal within analytical uncertainty, and yield a mean 207 Pb/ 206 Pb age of 3585 ± 70 Ma (2S.D.). This crystallization age is identical to the crystallization age of the pegmatite AC460, showing consistency with the field observation. The zircon grains separated from quartz dioritic gneiss (AC461) are subhedral to auhedral and small ( 50 m long). In CL images, the zircons are dull and have partly mosaic textures and do not have any oscillatory zoning (Fig. 9c). We determined eight U Pb ages on eight grains. The U Pb data are variably discordant with a range of 207 Pb/ 206 Pb ages from ca. 3.4 to ca. 3.0 Ga (Fig. 10c). In addition, some of the zircons have low Th/U ratios ( 0.1; Table 1), suggesting that they are metamorphic origin rather than magmatic origin (Rubatto, 2002). These results, as well as the zircons subhedral to euhedral form and lack of oscillatory zoning, suggest that the U Pb data reflect Pb-loss from igneous and/or metamorphic zircon grains during later metamorphism, and do not represent the crystallization age of the protolith. Therefore, we could not determine the precise age of AC461. However, the field relationships and the U Pb data from AC458 and AC460 indicate that it is older than 3.59 Ga Locality B (AY066) Locality B is located at westernmost part of the eastern area (Fig. 3) and consists of granitic gneiss and quartz dioritic gneiss. Fig. 11 shows the field relationships between the two rock units. The quartz dioritic gneiss occurs as enclaves and boudins within the granitic gneiss whose gneissic structure is concordant with the boudins of quartz dioritic gneiss. These fabrics indicate that the protolith of granitic gneiss intruded the quartz dioritic gneiss. We analyzed zircon grains from the granitic gneiss (AY066). Sample AY066 yielded euhedral to subhedral, coarse ( m long), and low aspect ratio ( 2) zircon. Oscillatory zoning is common (Fig. 9d), and there is no evidence of any older xenocrystic cores. Twelve U Pb analyses were carried out on 12 oscillatory-zoned grains, and all analyses lie close to concordia at ca. 3.6 Ga (Fig. 10d). All 207 Pb/ 206 Pb ages are within analytical uncertainty, and yield a mean age of 3586 ± 26 (2S.D.), the crystallization age of the granite (protolith of AY066) Locality C (AC579, AC580 and AC584) Locality C is located at the easternmost region of the mapped area (Fig. 3) and consists of quartz dioritic gneiss and two kinds of felsic gneiss (tonalitic and granitic) with late felsic dikes. Fig. 12 shows a sketch of the locality. The gneisses are highly deformed. The quartz dioritic gneiss occurs as enclaves within the tonalitic and granitic gneisses. The gneissic structure of the quartz dioritic gneiss blocks is locally oblique to the shape of the blocks and the gneissic structure of the felsic gneisses. The gneissic fabrics within the felsic gneisses are generally concordant with their margins, but at some localities they are oblique to each other. An aplite dike intruded through all of the gneisses, and was itself deformed by the north south-trending strike strip fault. Late stage, thin, north south-trending felsic dikes, parallel to the deformation fabrics, intruded all of the rocks at this locality. Because of the strong deformation fabrics imparted on the gneisses, it is difficult to determine the relative intrusion history for the gneisses. However, the oblique relationships amongst the gneissic structures indicate

20 198 T. Iizuka et al. / Precambrian Research 153 (2007)