Geological Survey of Denmark and Greenland Bulletin 11, 101-114 101 207Pb-206Pb dating of magnetite, monazite and allanite in the central and northern Nagssugtoqidian orogen, West Greenland Henrik Stendal, Karsten Secher and Robert Frei Pb-isotopic data for magnetite from amphibolites in the Nagssugtoqidian orogen, central West Green- land, have been used to trace their source characteristics and the timing of metamorphism. Analyses of the magnetite define a Pb-Pb isochron age of 1726 ± 7 Ma. The magnetite is metamorphic in origin, and the 1726 Ma age is interpreted as a cooling age through the closing temperature of magne- tite at ~600°C. Some of the amphibolites in this study come from the Naternaq supracrustal rocks in the northern Nagssugtoqidian orogen, which host the Naternaq sulphide deposit and may be part of the Nordre Strømfjord supracrustal suite, which was deposited at around 1950 Ma ago. Pb-isotopic signatures of magnetite from the Arfersiorfik quartz diorite in the central Nagssugto- qidian orogen are compatible with published whole-rock Pb-isotopic data from this suite; previous work has shown that it is a product of subduction-related calc-alkaline magmatism between 1920 and 1870 Ma. Intrusion of pegmatites occurred at around 1800 Ma in both the central and the northern parts of the orogen. Pegmatite ages have been determined by Pb stepwise leaching analyses of allanite and monazite, and source characteristics of Pb point to an origin of the pegmatites by melting of the surrounding late Archaean and Palaeoproterozoic country rocks. Hydrothermal activity took place after pegmatite emplacement and continued below the closure temperature of magnetite at 1800– 1650 Ma. Because of the relatively inert and refractory nature of magnetite, Pb-isotopic measure- ments from this mineral may be of help to understand the metamorphic evolution of geologically complex terrains. Keywords: Pb isotopes, magnetite, Nagssugtoqidian orogen, Palaeoproterozoic, pegmatites, Pb stepwise leaching, supra- crustal rocks ______________________________________________________________________________________________________________________________________________________________________________ H.S. & K.S., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: hst@geus.dk R.F., Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. As part of the research programme 2000–2003 in the Nagssugtoqidian orogen of West Greenland by the Geo- logical Survey of Denmark and Greenland (GEUS), an assessment was made of the mineral resource potential of the region between Maniitsoq (Sukkertoppen; 66°N) and the southern part of Nuussuaq (70°15′N; Stendal et al. 2004). The present study comprises Pb-isotopic analyses of magnetite from amphibolites, hydrothermally altered amphibolites, the Arfersiorfik quartz diorite (see below), skarn, ultramafic rocks and pegmatites. Magnetite was chosen as a medium for analysis because of its abundance in amphibolites, even though the concentration of Pb in magnetite is generally low. In addition, an attempt was made to date monazite and allanite from pegmatites by the Pb stepwise leaching (PbSL) technique (Frei & Kam- ber 1995). The Pb-isotopic study of the amphibolites covers the Attu, Kangaatsiaq and Qasigiannguit regions (Fig. 1). The analysed pegmatites are from the Nordre © GEUS, 2006. Geological Survey of Denmark and Greenland Bulletin 11, 101–114. Available at: www.geus.dk/publications/bull 102 Strømfjord (Nassuttooq), Attu and Qasigiannguit areas. The aims of the study were (1) to use Pb-isotopic signa- tures of magnetite in an attempt to outline the metamor- phic history of the region; (2) to characterise the hydro- thermal overprinting in terms of its timing and Pb source; and (3) to place the results within the evolutionary frame of the Nagssuqtoqidian orogen. Regional geological setting The study region comprises the Palaeoproterozoic Nags- sugtoqidian orogen, a major collisional belt situated just north of the North Atlantic Craton (van Gool et al. 2002), as well as the southernmost part of the contemporaneous Rinkian fold belt (Garde & Steenfelt 1999; Connelly et al. 2006). Most of the region consists of Archaean ortho- gneisses, variably reworked during the Nagssugtoqidian and Rinkian tectonothermal events. Several thin belts of supracrustal rocks occur within the reworked Archaean gneiss terrain of the Nagssugtoqidian orogen (Fig. 1). Granitoid rocks and numerous pegmatites intrude the gneisses. Formations of Palaeoproterozoic age are limited to the Sisimiut igneous suite, Arfersiorfik quartz diorite, and minor supracrustal sequences including the Nater- naq supracrustal belt (Connelly et al. 2000; Thrane & Connelly 2006, this volume). The metamorphic grade is amphibolite facies, except for an area south of Ataneq in the south-western part of the northern Nagssugtoqidian orogen (NNO; Fig. 1) and in most of the central Nagssugtoqidian orogen (CNO), where granulite facies rocks predominate. The gneisses are intensely folded and show a general E–W to NE–SW strike. Deformation of the Archaean gneisses in the NNO Greenland Inland Ice Greenland Canada 51° Us sui tNordre Strømfjord Arfersiorfik Aasiaat Qasigiannguit Kangaatsiaq Attu Sisimiut Kangerlussuaq Naternaq N ag ss u gt o q id ia n o ro ge n SN O C N O NSSZ ITZ N N O Inland Ice 0 50 km t t t t t t ttt Disko Bu gt 68° Jakobshavn Isfjord Kangersuneq Ataneq Quaternary surficial deposits Basalt Metasedimentary rocks Palaeoproterozoic Palaeogene Quaternary Sisimiut charnockite Arfersiorfik quartz diorite Archaean, variably reworked Granodioritic and granitic gneiss Orthogneiss Dioritic gneiss Metasedimentary rocks Amphibolite (including Proterozoic components) t t Thrust 457785457785 481049481049 481058481058 484883484883 484890484890 446633446633 446632446632 446626446626 481087481087 446603-04446603-04 44662344662320017362001736 485178485178 485179485179485193485193 223736223736 225348225348 223746223746 481070481070 Fig. 1. Geological map of central West Greenland, modified from van Gool et al. (2002). Red dots with numbers refer to samples analysed. CNO, Central Nagssugtoqidian orogen; NNO, Northern Nagssugtoqidian orogen; SNO, Southern Nagssugtoqidian orogen; ITZ, Ikertôq thrust zone; NSSZ, Nordre Strømfjord shear zone. 103 decreases gradually northwards, from high-strain to more open structures in the Archaean rocks. Steeply and shal- lowly dipping shear and fault zones are common in con- tact zones between different rock types. Major fault and shear zones generally strike NNE–NE. The gneisses of the NNO are late Archaean, with ages between 2870 and 2700 Ma (Kalsbeek & Nutman 1996; Connelly & Mengel 2000; Thrane & Connelly 2006, this volume). However, older rocks with ages ~3150 Ma appear to be present in the Attu area (Stendal et al. 2006, this volume). Only a few younger Palaeoproterozoic ages have been obtained from the NNO, including an undeformed pegmatite between Attu and Aasiaat with an intrusion age of about 1790 Ma (Connelly & Mengel 2000). The geological history of the study area can be sum- marised as follows (van Gool et al. 2002): • Deposition of supracrustal rocks: 2200–1950 Ma • Continental breakup – the Kangâmiut dyke swarm: 2040 Ma • Drifting – sediment deposition (supracrustal rocks) in the Nordre Strømfjord area: 2000–1920 Ma • Subduction – calc-alkaline magmatism, giving rise to the Sisimiut and Arfersiorfik igneous suites: 1920–1870 Ma • Peak metamorphism during collision (D1 and D2): 1860–1840 Ma • Large scale folding (D3): ~1825 Ma • Shearing in steep belts (D4): ~1775 Ma • Slow cooling following the shearing, with closing tem- perature of rutile (420°C) at around 1670 Ma (Con- nelly et al. 2000). Based on 40Ar-39Ar and U-Pb data for several minerals, Willigers et al. (2001) estimated cooling temperatures around 500°C at ~1700 Ma, 410°C at ~1640 Ma, and 200°C at ~1400 Ma. Previous investigations The Geological Survey, university research groups as well as exploration companies have been working in central West Greenland for decades and have collected signifi- cant amounts of data on the mineral potential of the re- gion (Stendal et al. 2002, 2004; Stendal & Schønwandt 2003; Schjøth & Steenfelt 2004; Steenfelt et al. 2004). Whole-rock Pb-Pb, Rb-Sr and Sm-Nd isotopic data from the study area have been presented by Kalsbeek et al. (1984, 1987, 1988), Taylor & Kalsbeek (1990) and Whitehouse et al. (1998), while e.g. Kalsbeek & Nutman (1996), Con- nelly & Mengel (2000), Connelly et al. (2000), Hollis et al. (2006, this volume) and Thrane & Connelly (2006, this volume) have published zircon U-Pb geochronologi- cal data. Pb-isotopic work has been carried out on sulphide sepa- rates, mainly pyrite, from a mineralisation in the Disko Bugt region north of the study area (Stendal 1998). In the latter study, two distinct mineralisation types in the Ar- chaean rocks were identified – a syngenetic, and at least one epigenetic type of ore formation. Pb-isotopic data of sul- phides from Proterozoic rocks yield a well-defined linear trend in a Pb-Pb isochron diagram, with a slope correspond- ing to an age of ~1900 Ma, and indicative of a primitive (i.e. low µ) source character of Pb in that mineralisation. Local geology and descriptions of the investigated rocks During this study Pb-isotopic analyses were carried out on magnetite from amphibolite (four samples), banded iron formation (one sample), hydrothermally altered amphibolite and calc-silicate skarn rock (four samples), the Arfersiorfik quartz diorite (three samples), magnetite skarn (one sample), ultramafic rock (one sample) and pegmatite (one sample). In addition, one amphibolite, one altered amphibolite and one sample of banded iron for- mation were subjected to PbSL procedures (Frei & Kam- ber 1995), and allanite (two samples) and monazite (three samples) from pegmatites were analysed by PbSL in an attempt to date their emplacement. Brief descriptions of the investigated rocks are given below. Amphibolitic rocks Amphibolites occur together with garnet-mica schists/ gneisses in supracrustal sequences, interlayered with ortho- gneiss. Some amphibolite layers in the gneiss terrain can be followed continuously along strike for up to tens of kilometres. They are heterogeneous in composition. They are found in three associations: (1) rusty weathering, me- dium-grained garnet amphibolite layers (c. 0.5 m thick) folded together with the orthogneisses, (2) dark, fine-grai- ned amphibolite, occurring as layers up to 10 m thick, and (3) medium-grained, commonly garnetiferous, lay- ered amphibolite. Layered amphibolites are the most com- mon, and occur as units up to 200 m thick, although layers only 10–20 m thick are more common. The three different types of amphibolite form separate outcrops and do not occur together. The Pb-isotopic analyses reported in this paper refer to magnetite from the layered amphib- olites (type 3). 104 The supracrustal sequences consist of garnet-mica schist/gneiss, together with amphibolite (Fig. 2a) and rusty weathering layers c. 1 m thick of quartz-garnet rich gneiss with some iron sulphides (1 vol.%). Within the layered amphibolite sequences, magnetite-bearing horizons 1–10 m thick occur. The magnetite occurs in laminae 1–10 mm thick, alternating with quartz-feldspar laminae of the same thickness. Alteration is common within the layered am- phibolites. Altered amphibolite Some amphibolites have been hydrothermally altered and sulphide mineralised and may contain calc-silicates. This type of amphibolite is dominated by layered garnet-rich amphibolite, interlayered with magnetite-bearing and rusty weathering layers, with disseminated pyrite (Fig. 2b). The layers are generally 0.5–2 m thick; in some cases layered amphibolite is intercalated with rusty weathering layers 10–30 cm thick, consisting of quartz-bearing mica schist with iron sulphides and staining of malachite. Within the altered amphibolite calc-silicate minerals are found in zones 1–2 m thick or as smaller lenses, comprising hornblende, diopside, garnet and magnetite. A B Fig. 2. Amphibolite (A) and hydrothermally altered amphibolite (B) from the Attu area. 105 Banded iron formation at Naternaq The supracrustal belt at Naternaq (Fig. 1) consists of meta- volcanic rocks interlayered with pelitic and psammitic schists and gneisses, marble units, exhalites and chert-rich layers with minor quartzite and banded iron formation. In total, these units define a supracrustal sequence up to 3 km thick, which is folded into a major shallowly dip- ping WSW-trending antiform. The supracrustal sequence can be traced for approximately 30 km along strike and is intruded by granite sheets and pegmatite veins. Østergaard et al. (2002) and Stendal et al. (2002) give detailed de- scriptions of the stratigraphy of the supracrustal rocks. The banded iron formation (Fig. 3) occurs locally associ- ated with the amphibolite in zones composed of centime- tre-thick layers of magnetite and siderite quartz and calc- silicates. The depositional environment is of a sedimentary type comprising true sediments, submarine volcanic rocks and exhalites. A range of variably altered conformable horizons of very fine-grained siliceous and sulphide rich lithologies associated with either amphibolite or marble are interpreted as volcanogenic-exhalitic rocks (Østergaard et al. 2002; Stendal et al. 2002). Arfersiorfik quartz diorite The Arfersiorfik quartz diorite (Kalsbeek et al. 1987) is located in the eastern part of the fjord Arfersiorfik (Fig. 1) and covers several hundreds of square kilometres. Within the quartz diorite body, magnetite occurs in hornblende- rich rocks (hornblende, quartz, feldspar, and chlorite) and often shows paragenetic relation with iron sulphides (pre- dominantly pyrrhotite). The Arfersiorfik quartz diorite was emplaced in the period 1920–1870 Ma (Kalsbeek et al. 1987; Connelly et. al. 2000). Ultramafic rocks near Qasigiannguit An ultramafic body 300 × 300 m large is located on the north side of Kangersuneq, forming rusty weathered hills. On its eastern and western sides the ultramafic body is bounded by fault zones, invaded by pegmatites. On its northern and southern sides it is bordered by amphibolite and garnet amphibolite, respectively. Because of the pen- etrative weathering it is difficult to sample fresh material from the ultramafic body. In its centre, an intensely rusty weathered and eroded ‘joint’ zone cuts the ultramafic rocks. This contains 1–10 vol.% magnetite. Magnetite-rich skarn at Qasigiannguit Near Qasigiannguit a skarn rock is found in the contact zone striking 66° and dipping 77°SE between mica schist and quartzite and a marble-calc-silicate sequence. It com- prises magnetite skarn (0.5 m thick) in close contact with the mica schist and quartzite. Towards the south-east the magnetite skarn is followed by alternating layers of calc- silicate rocks and marble (including a quartzitic, sulphide- rich layer), followed by a pegmatite body. Fig. 3. Banded iron formation sequence from the Naternaq area. 106 A B Fig. 4. Pegmatites and minerals analysed. A: Pink discordant pegmatite and allanite (inset) from the Attu area. B: White pegmatite and monazite (inset) from the Nordre Strømfjord (Nassuttooq) area. 107 Pegmatites Pink pegmatites. Throughout the study area, especially in the outer fjord zone from south of Attu northward to Kangaatsiaq, the country rocks are intruded by granite and by pink pegmatites with alkali feldspar crystals com- monly more than 10 cm in size. The pegmatites occur mostly as discordant decimetre- to metre-thick bodies within the gneisses, at contacts between major lithologi- cal units, and within supracrustal rocks where they are clearly cross-cutting. The dominant minerals in the pink pegmatite are alkali feldspar, quartz, biotite and subordi- nate allanite, titanite, apatite, magnetite and Fe-sulphides (Fig. 4a). Zonation is occasionally seen with quartz-rich centres bounded by alkali feldspar-rich parts. White pegmatites. White pegmatites are generally concor- dant (but locally discordant) to the foliation of the adja- cent country rocks, typically grey gneiss and supracrustal rocks. The pegmatites are 5–20 m wide and 50–200 m long with a general trend of NW–SE all over the Nordre Strømfjord and Ussuit areas. Gradational contacts to the host rocks are common. Quartz and feldspar dominate the white pegmatites, with garnet, biotite, monazite, magnetite and zircon as characteristic minor constituents. Monazite is found as 0.5–5 mm orange crystals that mainly occur in plagioclase- and biotite-rich pegmatites (Fig. 4b). Monazite crystals are euhedral and occur in lens-shaped layers accompanied by biotite, set in a granoblastic ma- trix of primarily plagioclase (Secher 1980). Analytical methods Pb isotope analyses for this study were carried out at the Danish Centre for Isotope Geology, Geological Institute, University of Copenhagen. Mineral fractions were sepa- rated from dry split aliquots of crushed and sieved (100– 200 µm) rock powders using a hand magnet, a Frantz isodynamic separator and heavy liquid techniques. No further purification was carried out, and the mineral frac- tions may contain minor proportions of foreign minerals. Pb was separated conventionally on 0.5 ml glass columns charged with anion exchange resin, followed by a clean up on 200 µl Teflon columns. A standard HBr-HCl solu- tion recipe was applied in both column steps. Total proce- dural blanks for Pb amounted to < 120 pg which is consi- dered insignificant for the measured Pb-isotopic results, relative to the amount of sample Pb estimated from the mass spectrometer signal intensities. Isotope analyses were Magnetite from amphibolite and banded iron formation 446626 Ataneq 68°.061 53°.510 Amphibolite 18.815 0.026 15.549 0.022 44.971 0.068 0.973 0.953 446632 Ataneq 68°.047 53°.179 Amphibolite 28.204 0.042 16.518 0.026 38.467 0.066 0.971 0.906 446623 Attu 67°.837 53°.408 Amphibolite 17.522 0.025 15.404 0.024 38.344 0.066 0.961 0.880 481070 Attu 67°.915 53°.231 Amphibolite 17.388 0.019 15.376 0.018 41.241 0.053 0.978 0.936 484883 Naternaq 68°.398 51°.941 BIF in amphibolite 54.587 0.028 19.316 0.012 36.871 0.030 0.942 0.886 Magnetite from altered amphibolite and calc-silicate rock 446633 Niaqornaarsuk 68°.217 53°.028 Altered amphibolite 26.387 0.505 16.465 0.316 47.423 0.908 0.998 0.999 446603 Attu 67°.927 53°.622 Altered amphibolite 18.398 0.030 15.620 0.026 41.710 0.074 0.978 0.956 446604 Attu 67°.927 53°.622 Altered amphibolite 18.256 0.046 15.638 0.042 40.443 0.122 0.930 0.845 484890 Naternaq 68°.408 51°.935 Calc-silicates (skarn) 16.534 0.009 15.372 0.010 35.562 0.028 0.957 0.913 Magnetite from the Arfersiorfik quartz diorite 485178 Arfersiorfik 67°.970 50°.430 Quartz diorite 17.790 0.016 15.469 0.016 36.703 0.049 0.919 0.782 485179 Arfersiorfik 67°.967 50°.412 Quartz diorite 18.071 0.018 15.498 0.016 36.343 0.041 0.975 0.954 485193 Arfersiorfik 67°.956 50°.594 Hornblenditic rock 17.007 0.013 15.456 0.013 37.124 0.037 0.959 0.902 Magnetite from pegmatite 481087 Attu 67°.890 53°.517 Pegmatite 28.393 0.043 16.602 0.026 189.024 0.311 0.982 0.971 Magnetite from ultramafic rock and skarn 481049 Qasigiannguit 68°.801 50°.973 Ultramafic rock 25.827 0.039 16.103 0.025 51.334 0.084 0.979 0.949 481058 Qasigiannguit 68°.800 51°.169 Magnetite skarn 35.124 0.024 17.028 0.013 38.847 0.035 0.968 0.929 Table 1. Pb isotope ratios of magnetite from different rock types BIF: Banded iron formation. * Errors are 2σ absolute (Ludwig 1990). ** r1 = 206Pb/204Pb versus 207Pb/204Pb error correlation (Ludwig 1990). † r2 = 206Pb/204Pb versus 208Pb/204Pb error correlation (Ludwig 1990). Sample Locality Latitude Longitude Rock 206Pb ± 2σ* 207Pb ± 2σ 208Pb ± 2σ r1 ** r2† N W 204Pb 204Pb 204Pb 108 Magnetite, banded iron formation within amphibolite 484883, locality 68°.398 N, 51°.941 W [1] 1 N HBr 30' 48.234 0.030 18.615 0.014 37.418 0.032 0.968 0.919 [2] 1 N HBr 1 h 120.002 0.091 26.407 0.022 3.862 0.005 0.978 0.760 [3] 4 N HBr 3 h 94.120 0.363 23.247 0.090 37.841 0.149 0.995 0.987 [4] 8 N HBr 6 h 30.176 0.239 16.809 0.134 35.387 0.282 0.994 0.994 [5] 8 N HBr 12 h 18.799 0.062 15.506 0.052 34.534 0.116 0.991 0.987 [6] HF 12 h 18.799 0.124 15.539 0.103 35.013 0.233 0.995 0.993 Magnetite, amphibolite 446632, locality 68°.047 N, 53°.179 W [1] 1 N HBr 30' 19.932 0.026 15.621 0.021 37.462 0.055 0.970 0.920 [2] 1 N HBr 1 h 38.765 0.032 17.639 0.016 39.167 0.040 0.978 0.946 [3] 4 N HBr 3 h 35.439 0.060 17.098 0.030 42.830 0.079 0.982 0.948 [4] 8 N HBr 6 h 37.099 0.150 17.595 0.072 36.972 0.154 0.993 0.979 [5] 8 N HBr 12 h 40.431 0.219 18.100 0.099 34.544 0.189 0.994 0.994 [6] HF 12 h 28.265 0.083 16.910 0.051 34.598 0.106 0.987 0.977 Magnetite, altered amphibolite 446633, locality 68°.217 N, 53°.027 W [1] 1 N HBr 30' 25.206 0.021 16.298 0.015 45.932 0.047 0.971 0.914 [2] 1 N HBr 1 h 27.276 0.170 16.492 0.104 47.555 0.300 0.993 0.990 [3] 4 N HBr 3 h 25.359 0.022 16.261 0.016 45.944 0.049 0.975 0.948 [4] 8 N HBr 6 h 26.018 0.024 16.333 0.016 46.646 0.051 0.968 0.936 [5] 8 N HBr 12 h 27.705 0.073 16.539 0.046 48.236 0.138 0.965 0.937 Allanite, pegmatite 2001-736, locality 67°.883 N, 53°.523 W [1] 1 N HBr 30' 22.579 0.029 15.860 0.021 137.787 0.193 0.982 0.960 [2] 1 N HBr 1 h 21.528 0.013 15.728 0.011 123.978 0.101 0.971 0.939 [3] 4 N HBr 3 h 79.398 1.929 22.265 0.542 1360.842 33.070 0.999 1.000 [4] 8 N HBr 6 h 2527.779 37.253 295.424 4.391 52501.990 774.512 0.992 0.999 [5] 8 N HBr 12 h 7804.112 133.916 882.216 15.164 161163.864 2767.112 0.999 1.000 [6] HF 12 h 551.839 2.322 74.260 0.322 11002.171 46.823 0.973 0.994 [7] HF 2 d 111.336 0.543 25.175 0.136 1948.881 9.628 0.901 0.991 Table 2. Pb-Pb step leaching data for magnetite, allanite, and monazite in banded iron formation, amphibolite and pegmatites in the Nagssugtoqidian orogen Code Acid Time 206Pb/204Pb ± 2σ * 207Pb/204Pb ± 2σ * 208Pb/204Pb ± 2σ * r1 r2 * Errors are 2σ absolute (Ludwig 1990). For explanations of r1 and r2, see Table 1. Allanite, pegmatite 457785, locality 68°.834 N, 51°.226 W [1] 1 N HBr 30' 36.910 1.617 17.616 0.772 538.548 23.595 1.000 1.000 [2] 1 N HBr 1 h 44.614 0.389 18.318 0.160 714.245 6.237 0.998 0.999 [3] 4 N HBr 3 h 26.321 0.210 16.341 0.130 270.923 2.164 0.998 0.999 [4] 8 N HBr 6 h 21966.303 527.491 2410.934 57.939 328786.908 7897.797 0.999 1.000 [5] 8 N HBr 12 h 26.193 0.574 17.165 0.390 161.467 3.639 0.964 0.972 [6] HF 12 h 15.058 1.448 15.006 1.443 39.562 3.805 1.000 1.000 [7] HF 2 d 27.000 0.230 15.806 0.207 186.650 2.183 0.651 0.729 Monazite, pegmatite 223736, locality 67°.680 N, 52°.565 W [1] 1 N HBr 30' 136.776 2.665 28.301 0.574 6619.883 131.103 0.960 0.984 [2] 1 N HBr 1 h 222.475 3.796 37.330 0.643 12548.515 214.343 0.991 0.999 [3] 4 N HBr 3h 219.227 2.039 36.918 0.346 12871.792 120.474 0.992 0.995 [4] 8 N HBr 6 h 184.897 2.055 33.259 0.373 10581.910 118.035 0.993 0.997 [5] 8 N HBr 12 h 89.680 0.811 23.046 0.213 3554.756 32.275 0.979 0.998 [6] HF 12 h 39.412 0.038 17.328 0.018 88.489 0.099 0.980 0.957 [7] HF 2 d 261.196 1.366 41.737 0.220 175.006 0.931 0.994 0.987 Monazite, pegmatite 223746, locality 67°.674 N, 52°.456 W [1] 1 N HBr 30' 20.955 0.097 15.264 0.071 156.248 0.733 0.995 0.994 [2] 1 N HBr 1 h 26.768 0.169 16.158 0.103 315.778 2.010 0.997 0.997 [3] 4 N HBr 3h 215.079 2.560 37.781 0.451 5263.963 62.733 0.997 0.999 [4] 8 N HBr 6 h 2978.834 188.060 343.540 21.694 75533.892 4768.946 1.000 1.000 [5] 8 N HBr 12 h 6392.913 99.502 721.057 11.259 157988.821 2460.811 0.997 1.000 [6] HF 12 h 468.603 5.974 66.525 0.919 10731.171 137.428 0.923 0.996 [7] HF 2 d 2161.547 97.250 254.541 11.458 51062.309 2297.561 1.000 1.000 Monazite, pegmatite 225348, locality 67°.833 N, 52°.323 W [1] 1 N HBr 30' 139.662 2.160 29.676 0.459 1656.012 25.630 1.000 1.000 [2] 1 N HBr 1 h 140.039 1.416 29.481 0.299 1544.411 15.642 0.996 0.999 [3] 4 N HBr 3h 877.104 4.121 108.678 0.517 12440.807 58.889 0.990 0.997 [4] 8 N HBr 6 h 25.459 0.418 16.750 0.456 225.146 4.515 0.604 0.820 [5] 8 N HBr 12 h 40549.054 1970.494 4458.008 222.550 602889.998 29313.071 0.973 1.000 [6] HF 12 h 8037.949 261.731 893.317 29.098 122611.734 3993.108 1.000 1.000 [7] HF 2 d 4389.260 60.969 493.058 6.880 66031.253 918.275 0.996 0.999 109 carried out on a VG Sector 54-IT instrument. Fractiona- tion for Pb was controlled by repetitive analysis of the NBS 981 standard (values of Todt et al. 1993) and amoun- ted to 0.103 ± 0.007% / amu (2 σ; n = 11). Stepwise Pb leaching (PbSL) experiments followed methods described in Frei & Kamber (1995). The programmes and parameters of Ludwig (1990) were used for the isochron calculations. Model first-stage µ 1 values were calculated using 4.55 Ga for the age of the earth. All age and isotope data in this paper are given with 2 σ precisions. Results The Pb-isotopic results are given in Tables 1–3. The ura- nogenic Pb-isotopic composition of magnetite from the amphibolites (four samples; squares in Fig. 5) together with the banded iron formation (Naternaq; one sample outside the range of Fig. 5) define an isochron with an age of 1726 ± 7 Ma (2 σ; MSWD = 1.4; model µ 1 = 7.89 ± 0.02), which corresponds to a late stage in the metamor- phic evolution of the Nagssugtoqidian orogen (cf. Willigers et al. 2002). This isochron intercepts the Stacey & Kram- ers (1975) Pb-isotopic growth curve at ~2140 Ma. Four mineral separates from altered amphibolite, rep- resented by calc-silicate rich phases and by hydrothermal- ly altered and mineralised samples, have Pb-isotopic com- positions that plot above the 1726 Ma isochron (diamonds in Fig. 5). This more radiogenic Pb-isotopic composition indicates admixture of a more evolved Pb component in- to the alteration fluids. The Pb-isotopic compositions of magnetite from an ultramafic rock and a magnetite skarn from the Qasigiannguit area plot below the isochron (out- 5 samples Amphibolite/BIF Magnetite Bulk 1726 6.5 1.38 2140 7.89 0.02 484883 BIF Magnetite PbSL 1756 36 8.70 2401 7.7 0.11 223736 Pegmatite Monazite PbSL 1797 13 4.44 2925 7 0.05 223746 Pegmatite Monazite PbSL 1816 16 41.7 2784 7.24 0.12 225348 Pegmatite Monazite PbSL 1787 11 76.9 2271 7.81 1.00 447783 Pegmatite Allanite PbSL 1785 9.2 12.9 2335 7.76 0.00 2001-736 Pegmatite Allanite PbSL 1818 12 53.8 2453 7.66 0.02 BIF: Banded iron formation; PbSL: Pb step leaching. Table 3. Pb isotope ages, µ1-values, and intercepts with the Stacey & Kramers (1975) Pb-isotopic growth curve for magnetite, allanite, and monazite from amphibolite, BIF and pegmatites Sample Rock Mineral Method Age (Ma) ± 2σ MSWD Intercept (Ma) with µ1 ± 2σ Stacey & Kramers (1975) 1915 16 17 18 20 7 P b/ 20 4 P b 206Pb/204Pb 15.2 15.3 15.4 15.5 15.6 2000 1600 1200 800 400 0Magnetite Amphibolite Altered amphibolite Arfersiorfik - this study Arfersiorfik - Kalsbeek et al. (1987) Arfersiorfik - Whitehouse et al. (1998) Age = 1726 ± 7 Ma MSWD = 1.4 Fig. 5. 206Pb/204Pb-207Pb/204Pb diagram. Squares, Pb isotope ratios of magnetite from amphibolites; diamonds, magnetite from altered amphibolites (data from sample 446632 outside the range of Fig. 5, see Table 1). Arfersiorfik quartz diorite: circles, this study; crosses, data from Kalsbeek et al. 1987; filled triangles, data from Whitehouse et al. 1998. The isochron intercepts the Stacey & Kramers (1975) Pb- isotopic growth curve (blue) at ~2140 Ma. 110 side the range of Fig. 5; see Table 1), suggesting a slightly more primitive Pb source. The uranogenic vs. thorogenic isotopic patterns (not shown in a figure) are complex and do not add to a better understanding of the uranogenic Pb-isotopic data. As ex- pected, they reflect differences in U/Th ratios among the different samples analysed. The Arfersiorfik quartz diorite has been dated at ~1920 Ma (Kalsbeek et al. 1987). Three magnetite samples from this igneous suite have been included in the present study. The uranogenic Pb-isotopic compositions of these mag- netites (circles in Fig. 5) are similar to the whole-rock Pb- isotopic signatures (crosses in Fig. 5; data from Kalsbeek et al. 1987). Four additional whole-rock analyses (filled triangles in Fig. 5; data of Whitehouse et al. 1998) show wider scatter than the data of Kalsbeek et al. (1987) and the results of this study. The PbSL data obtained on magnetite from three of these samples are shown in Fig. 6. A regression for the steps defined by the sample of banded iron formation, 484883 (excluding step 3; Table 1) yields a best-fit line with a slope corresponding to an age of 1756 ± 36 Ma (MSWD = 8.70; model µ 1 = 7.70 ± 0.11; lower intercept with the Stacey & Kramers Pb-isotopic growth curve at ~2400 Ma), similar to the age obtained from the amphi- bolites. PbSL analyses of two other samples (446632, amphibolite and 446633, altered amphibolite) are closely scattered around the 1756 correlation line. PbSL data obtained on allanite from a pink pegmatite (sample 2001-736) resulted in a well-defined errorchron with an age of 1818 ± 12 Ma (MSWD = 53.8; model µ 1 = 7.66 ± 0.02; lower intercept with the Stacey & Kramers Pb-isotopic growth curve at ~2450 Ma; Fig. 7A). The thorogenic vs. uranogenic isotopic pattern (Fig. 7B) re- veals that essentially only one phase has dominantly con- tributed Pb to the leaching acids, as a nearly perfect linear relationship is indicated by the data points. This points to a more or less constant Th/U in the recovered Pb frac- tions. For this reason, the age of 1818 ± 12 Ma can be interpreted with great confidence to represent the emplace- ment age of the pegmatite. The Pb-isotopic composition of magnetite (sample 481087, Table 1) from this pegma- tite plots on the allanite isochron (Fig. 7A), indicating preservation of isotopic equilibrium between these two phases. PbSL data on monazite from a white pegmatite (sam- ple 223736) also yield an errorchron, the slope of which corresponds to an age of 1797 ± 13 Ma (MSWD = 4.44; model µ 1 = 7.00 ± 0.05; lower intercept with the Stacey & Kramers Pb-isotopic growth curve at ~2925 Ma; Fig. 8A). The thorogenic vs. uranogenic isotopic pattern (Fig. 8B) again indicates a predominantly single phase that con- tributed Pb to the leaching acids, as the data points define a near perfect linear relationship. Consequently, with great confidence, the age of 1797 ± 13 Ma is interpreted as the intrusion age of this pegmatite. 12 16 20 24 28 484883 486632 486633 Intercept ~ 2401 Ma Banded iron formation 484883 Altered amphibolite 486633 Amphibolite 486632 Age = 1756 ± 36 Ma MSWD = 8.70 Magnetite step leaching 1200 20 40 1401008060 20 7 P b/ 20 4 P b 206Pb/204Pb Fig. 6. 206Pb/204Pb-207Pb/204Pb diagram of step leaching results of magnetite for three samples. The errorchron intercepts the Stacey & Kramers (1975) Pb-isotopic growth curve (blue) at ~2400 Ma. 111 Three more step-leaching experiments were performed on allanite (1) and monazite (2) separates from other peg- matites (Fig. 9). The ages defined by the respective er- rorchrons are similar to the ones presented above, and are close to 1800 Ma. Results of the isochron calculations are listed in Table 3. Discussion The age defined by the Pb-isotopic compositions of magne- tite from the amphibolites (1726 ± 7 Ma) is younger than the latest major tectonometamorphic event in the region (D4, strike-slip shearing and granite intrusion at 1780– 1770 Ma; see Connelly et al. 2000 and van Gool et al. 2002), and may be interpreted as a cooling age after the D4 event. Metamorphic conditions in the CNO reached temperatures above 650°C at 1800 Ma and approximate- ly 540°C by c. 1740 Ma (Connelly & Mengel 2000; Con- nelly et al. 2000; Willigers et al. 2001). Slow cooling fol- lowed with closing temperatures of rutile (420°C) around 1670 Ma (Connelly et al. 2000). Based on 40Ar-39Ar and U-Pb data of several minerals, Willigers et al. (2001) esti- mated cooling temperatures around 500°C at ~1700 Ma, 410°C at ~1640 Ma and 200°C at ~1400 Ma. A continu- ous magnetite-ulvöspinel solid solution series exists, with exsolution taking place below 600°C (Deer et al. 1966; Ramdohr 1969). Thus, the ages of the magnetite may date 20 7 P b/ 20 4 P b 10 30 50 70 Allanite 2001-736 Magnetite Age = 1812 ± 12 Ma MSWD = 54 20 8 P b/ 20 4 P b 206Pb/204Pb 0 4000 2000 8000 10000 0 100 200 300 400 500 600 Magnetite A B Fig. 7. Step leaching 206Pb/204Pb-207Pb/204Pb and 208Pb/204Pb-206Pb/ 204Pb diagrams of allanite from red pegmatite (sample 2000736). The errorchron intercepts the Stacey & Kramers (1975) Pb-isotopic growth curve (blue) at ~2925 Ma. 20 7 P b/ 20 4 P b 10 20 30 40 50 Monazite 223736 Age = 1797 ± 13 Ma MSWD = 4.4 20 8 P b/ 20 4 P b 206Pb/204Pb 0 4000 12000 16000 0 100 200 300 A B Fig. 8. Step leaching 206Pb/204Pb-207Pb/204Pb and 208Pb/204Pb-206Pb/ 204Pb diagrams of monazite from white pegmatite (sample 223736). The errorchron intercepts the Stacey & Kramers (1975) Pb-isotopic growth curve (blue) at ~2453 Ma. 112 the timing where exsolution in magnetite ceased (< 1800 Ma), that is, after peak metamorphic conditions. Model first-stage µ 1 values associated with Pb-Pb iso- chrons have been used elsewhere in Greenland to judge the influence of Pb from Archaean sources on the Pb- isotopic characteristics of Palaeoproterozoic igneous rocks (e.g. Kalsbeek & Taylor 1985). Rocks derived from Prot- erozoic sources commonly have model µ 1 values around 8, while contamination with Pb from Archaean sources tends to lower the µ 1 values. The high µ 1 value (7.89; Ta- ble 3) obtained for the amphibolite isochron and the lower intercept with Stacey & Kramers (1975) Pb-isotopic growth curve at 2140 Ma suggest a mainly Palaeoproter- ozoic Pb source for the amphibolites. This source is pro- bably also related to the origin of the supracrustal rocks. Detrital zircon U-Pb ages of metasedimentary rocks of the Nordre Strømfjord suite (2200–1950 Ma; Nutman et al. 1999) and the Naternaq supracrustal belt (c. 1950– 1900 Ma, Thrane & Connelly 2006, this volume) indi- cate erosion of a predominantly Palaeoproterozoic hinter- land. It implies that the stratabound, semi-massive sul- phide deposits associated with banded iron formation at Naternaq (Stendal et al. 2004) were also deposited during Palaeoproterozoic time. The results of allanite and monazite PbSL experiments indicate pegmatite formation around 1800 Ma in both the CNO (at Nordre Strømfjord) and NNO (at Attu and Qasigiannguit). This is in agreement with ages reported by Kalsbeek & Nutman (1996) and Connelly et al. (2000), which are slightly younger (1780–1770 Ma) or within error overlapping those reported here. The pegmatites were emplaced after post-collisional deformation, large scale folding, and shear zone formation (D3) which ended around 1825 Ma (van Gool et al. 2002). The wide range in model µ 1 values (7.00–7.81) and lower intercepts with the Stacey & Kramers (1975) Pb- isotopic growth curve (2271–2925 Ma) indicate variable contributions of Archaean and Palaeoproterozoic coun- try rocks to the petrogenesis of the pegmatites: Pegmatite sample 223736 (µ 1 = 7.00; lower intercept at 2925 Ma) may largely consist of remelted Archaean country rock, whereas sample 225348 (µ 1 = 7.81; lower intercept at 2271 Ma) appears to be mainly derived from Palaeoproterozoic sources. Hydrothermal activity in the region probably contin- ued after the time of pegmatite emplacement and after the magnetite had cooled through its closing temperature (~600°C), which means that the temperatures of the hy- drothermal fluids ranged from 650°C to 400°C in the period 1800–1650 Ma. The Pb-isotopic signatures of the ultramafic rock and 20 7 P b/ 20 4 P b 206Pb/204Pb 0 100 150 300 200 250 50 0 C Monazite 223746 Age = 1816 ± 16 Ma MSWD = 41.7 400 800 1200 1600 2000 2400 2800 20 7 P b/ 20 4 P b 15 16 17 18 19 Allanite 457785 Age = 1785 ± 9.2 Ma MSWD = 12.9 206Pb/204Pb 0 Ma 800 1600 A 15 25 35 45 20 8 P b/ 20 4 P b 206Pb/204Pb 0 200 400 600 800 1000 B 0 2000 4000 6000 8000 Monazite 225348 Age = 1787 ± 11 Ma MSWD = 77 Fig. 9. Step leaching 206Pb/204Pb-207Pb/204Pb diagrams of monazite from white pegmatite (samples 225348 and 223736) and allanite from pink pegmatite (sample 447783). The errorchron intercepts of the Stacey & Kramers (1975) Pb-isotopic growth curve (blue) are given. 113 the magnetite skarn from the Qasigiannguit area do not lend themselves to deduce whether these formations were formed during the Palaeoproterozoic or represent rem- nants of Archaean origin. It has been suggested that many of the epigenetic gold and copper occurrences in the Ataa area north-east of Disko Bugt, about 75 km north of Jakobshavn Isfjord (Fig. 1), are contemporaneous with the peak metamorphism at ~1900 Ma in that area (Stendal 1998). This 1900 Ma metamorphic-hydrothermal event is not reflected in the magnetite Pb-isotopic data of the present study area. Conclusions Pb-isotopic data of magnetite can be related to the gener- al geological evolution of the Nagssugtoqidian orogen and are thus a useful tool for studying the metamorphic his- tory of Palaeoproterozoic events in West Greenland. A drawback of magnetite Pb-isotopic analysis, however, is the generally low Pb concentration in this mineral, which makes analysis difficult. Magnetite in the amphibolites was formed during sev- eral stages of metamorphism. The isochron age of ~1726 Ma probably represents a cooling age after a prominent late tectonometamorphic event in the region dated at ~1775 Ma. The isotopic data suggest a Palaeoproterozoic (mantle?) source for the Pb in the amphibolites. The Nor- dre Strømfjord supracrustal suite, formed by erosion of a similar juvenile Palaeoproterozoic hinterland, was depos- ited between 2000 and 1920 Ma. It is suggested that the Naternaq sulphide deposit is part of this supracrustal suite. Calc-alkaline magmatism related to subduction (1920– 1870 Ma; Connelly et al. 2000) gave rise to the formation of the Arfersiorfik quartz diorite. The Pb-isotopic signa- ture of magnetite from these rocks is comparable with that of whole-rock samples. Allanite and monazite PbSL analyses yield pegmatite formation ages of ~1800 Ma for both the Nordre Strøm- fjord, Attu and Qasigiannguit regions. The formation of pegmatites is therefore post-collisional. The pegmatites were formed by melting of the local country rocks; Pb- isotopic data indicate that variable proportions of Late Archaean and Palaeoproterozoic age contributed to their petrogenesis. 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Journal of Petrology 42, 1729– 1749. __________________________________________________________________________________________________________________________________________________________________________________ Manuscript received 28 October 2004; revision accepted 19 December 2005