Geological Survey of Denmark and Greenland Bulletin 11, 115-123 115 New hornblende and muscovite 40Ar/39Ar cooling ages in the central Rinkian fold belt, West Greenland Ann-Sofie Sidgren, Laurence Page and Adam A. Garde The Palaeoproterozoic Rinkian fold belt in West Greenland consists of reworked Archaean basement, mainly orthogneiss, and the unconformably overlying Palaeoproterozoic Karrat Group. Both parts were intensely deformed and metamorphosed at around 1.87 Ga, at which time the crustal anatectic Prøven igneous complex was emplaced into the northern part of the belt. Seven new hornblende and muscovite 40Ar/39Ar cooling ages are presented from the central–northern parts of the Rinkian fold belt. Four 40Ar/39Ar hornblende ages ranging from 1795 ± 3 to 1782 ± 3 Ma were obtained from amphibolite and hornblendite enclaves in the Archaean orthogneiss, and two from relict dyke frag- ments in the latter that may be of Palaeoproterozoic age. Three 40Ar/39Ar muscovite ages of 1681 ± 6 Ma, 1686 ± 3 Ma and 1676 ± 3 Ma were obtained from samples of Karrat Group metagreywacke, andalusite schist and metasiltstone. The new 40Ar/39Ar ages, from hornblende and muscovite respec- tively, are very uniform and probably unrelated to local metamorphic grade and structural history, and are interpreted as regional late orogenic cooling ages. The new hornblende ages are significantly older than those previously obtained from the central and northern parts of the adjacent Nagssugto- qidian orogen to the south, and point to different uplift histories, which may suggest that the orogeny was not synchronous in the two regions. Keywords: Ar-Ar, geochronology, Rinkian, Palaeoproterozoic, West Greenland ____________________________________________________________________________________________________________________________________________________________________ A.-S.S. & L.P., Department of Geolog y, Geobiosphere Science Center, Lund University, Sölvegatan 12, S-223 62 Lund, Sweden. E-mail: Ann-Sofie.Sidgren@bd.lst.se A.A.G., Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. This paper presents seven new hornblende and muscovite 40Ar/39Ar cooling ages from the central part of the Palaeo- proterozoic Rinkian fold belt in West Greenland. The new data set provides insight into the cooling history of the Rinkian fold belt and can also be used to address its tem- poral relationship with the adjacent Nagssugtoqidian oro- gen to the south, from which other 40Ar/39Ar cooling ages have previously been published. Most of central and northern West Greenland consists of Archaean continental crust which was intensively rewor- ked during the Palaeoproterozoic. This reworking was first recognised in central West Greenland between 66° and 69°N by Ramberg (1949), who established the Nagssugtoqidian mobile belt (now called the Nagssugtoqidian orogen) in this area. Escher & Pulvertaft (1976) subsequently pro- posed that a separate Palaeoproterozoic mobile belt, the Rinkian fold belt, existed in central and northern West Greenland between 69° and 75°N (see inset map of Fig. 1). They noted that the latter region was dominated by an overall flat-lying tectonic foliation with superimposed large domes, and considered that these structures were of a dif- ferent nature from the generally steep foliations and tight folds that had previously been identified in the Nagssug- toqidian belt. In contrast to the collisional structures recog- nised within the Nagssugtoqidian orogen, it was thought that the Rinkian deformation had taken place without significant crustal shortening. Furthermore, whereas the collisional Nagssugtoqidian © GEUS, 2006. Geological Survey of Denmark and Greenland Bulletin 11, 115–123. Available at: www.geus.dk/publications/bull 116 orogen was originally believed only to comprise Archaean supracrustal and infracrustal rocks, the Rinkian fold belt contains a widespread, metamorphosed and deformed cover sequence, the c. 2 Ga old Karrat Group, which was unconformably deposited on the Archaean basement gneiss- es (Garde & Pulvertaft 1976; Henderson & Pulvertaft 1987; Kalsbeek et al. 1998). The lowest parts of the Kar- rat Group consist of quartzite, marble and minor amphi- bolite (the Qeqertarsuaq and Marmorilik Formations), which are overlain by a very uniform sequence of meta- greywacke, the Nukavsak Formation, which is several kil- ometres thick and occurs throughout most of the Rinki- an belt (Fig. 1; Henderson & Pulvertaft 1987). The geo- chemistry of the Karrat Group and studies of its detrital zircons indicate that the Karrat Group was derived from a mixed source including Palaeoproterozoic magmatic arc 483667 Hbl 1782 ± 2 Ma 483696 Hbl 1785 ± 3 Ma 483708 Hbl 1784 ± 3 Ma 483657 Hbl 1795 ± 3 Ma 483671 Mu 1676 ± 3 Ma 483654 Mu 1686 ± 3 Ma 483653 Mu 1681 ± 6 Ma 500 km Rinkian Nagssug- toqidian Disko Bugt 72°N 51°W 55°W 71°N Upernavik Kang illeq 50 km Ukkusissat Fjord Qeqertarsuaq Qinngusaaq Inland ice Surficial deposits Quaternary Basalt Paleocene Sandstone Karrat Group Palaeoproterozoic Prøven Igneous Complex Archaean, variably reworked Orthogneiss and minor supracrustal rocks Ar-Ar sample Hbl: hornblende Mu: muscovite Kang erlus suaq Uummannaq Appat Svartenhuk Nuussuaq Fig. 1. Map of the central Rinkian fold belt showing the locations of samples collected for 40Ar/39Ar age determination. The index map shows the position of Fig. 1 and the approximate extent of Nagssugtoqidian and Rinkian reworking in West Greenland. Modified from Garde et al. (2004). 117 rocks and Archaean basement rocks, and that it was de- posited at around 2.0–1.9 Ga ago (Kalsbeek et al. 1998; Thrane et al. 2003). The Rinkian fold belt also incorporates the Prøven igne- ous complex, a very large plutonic complex of granitic and microdioritic crustal melts that were emplaced under granulite facies conditions into the middle to upper crust in the Upernavik region of the Rinkian belt and is also found as the Cumberland batholith on adjacent Baffin Island, Canada. The pluton has previously yielded a Rb- Sr whole rock isochron age of 1860 ± 25 Ma with a high initial 87Sr/86Sr ratio (Kalsbeek 1981) and has recently al- so been studied by Thrane et al. (2005). The latter pro- duced a more precise zircon U-Pb ion probe age of 1869 ± 9 Ma and obtained negative εNd values from the plu- ton (calculated at 1870 Ma) ranging between –5.2 and –4.3. In agreement with the previous Rb/Sr data this shows that the plutonic complex contains a large Archaean con- tinental crustal component. It is therefore questionable whether the Prøven igneous complex – Cumberland batholith is subduction-related as has been proposed by Canadian workers. Thrane et al. (2005) suggest it repre- sents a crustal melt, induced by upwelling hot astheno- spheric mantle. Pulvertaft (1986), Henderson & Pulvertaft (1987), Grocott & Pulvertaft (1990) and Garde & Steenfelt (1999b) have described the structural evolution in vari- ous parts of the Rinkian fold belt, and recognised large- scale thrusts in its southern part. Following new field work in 2002–2003, the structural evolution in the Uumman- naq region is at present regarded as consisting of four main phases (briefly outlined by Garde et al. 2003, 2004). Defor- mation began with tight folding and possibly thrusting (D1), which developed prior to cleavage formation. This was followed by NE- to E-directed thrusting and ductile tectonic transport (D2) accompanied by formation of a penetrative schistosity, and then by NW- to W-directed tectonic transport (D3) and intensification of the pre-ex- isting schistosity. Lastly, very large, upright to overturned, dome-shaped anticlines and tight synclinal cusps were developed during continued shortening of the now strong- ly tectonically layered crust; these large structures are on- ly locally accompanied by a new tectonic fabric. The Prøven igneous complex was emplaced at a late stage of the main fabric-forming events and gave rise to a wide metamorphic aureole that was overprinted on rocks that were already regionally metamorphosed at high grade. 1900 1700 0 4020 60 80 100 1750 1850 1800 1795 ± 3 Ma Integrated age = 1797 ± 3 Ma Hornblende483657 1800 1600 1650 1750 1700 1681 ± 6 Ma* Integrated age = 1703 ± 3 Ma Muscovite483653 1600 1650 1750 1700 1686 ± 3 Ma* Integrated age = 1716 ± 2 Ma Muscovite483654 1600 1650 1750 1700 1676 ± 3 Ma Integrated age = 1684 ± 3 Ma Muscovite483671 1700 0 4020 60 80 100 1750 1850 1800 1782 ± 2 Ma Integrated age = 1801 ± 2 Ma Hornblende483667 1900 1700 1750 1850 1800 1785 ± 3 Ma Integrated age = 1864 ± 2 Ma Hornblende483696 1700 0 4020 60 80 100 1750 1850 1800 1784 ± 3 Ma Integrated age = 1798 ± 3 Ma Hornblende483708 Cumulative % 39Ar released A pp ar en t ag e (M a) Ukkusissat area North-eastern Uummannaq area South-eastern Uummannaq area Fig. 2. 40Ar/39Ar plateau age spectra from the Rinkian fold belt. Ages with an asterisk (*): 40Ar/39Ar plateau age representing less than 50% of total 39Ar release. Ages without an asterisk: 40Ar/39Ar plateau age. 118 483653 Muscovite (J = 0.01071 ± 0.00001): C 1.4 0.00001 0.001 0.00011 0 147.485 0.093 10.9 10.9 100 1709.6 4.4 D 1.4 0.1461 0.004 0.00008 26.5 147.544 0.306 35.6 46.4 100 1710 1.7 •E 1.5 1.9453 0.021 0.00097 27.6 143.552 0.011 1.2 47.7 99.9 1679.9 13.4 •F 1.6 2.875 0.004 0.00041 96.1 143.488 0.009 1 48.7 100 1679.4 17.2 •G 1.9 0.4853 0.003 0.00010 68.7 143.739 0.187 21.7 70.3 100 1681.3 2.7 H 4 0.3402 0.004 0.00019 24.8 147.664 0.256 29.7 100 100 1710.9 1.6 Integrated (total fusion) age: 1703 3 (•) Plateau age: 23.9 1681 6 483654 Muscovite (J = 0.01071 ± 0.00001): A 1.4 0.005 0.003 0.00106 0.1 150.444 0.767 26.9 26.9 99.8 1731.6 1.0 B 1.5 0.006 0.005 0.00033 0.2 149.670 0.692 24.3 51.2 99.9 1725.9 1.2 •C 1.6 0.0499 0.009 0.00063 1.1 144.382 0.265 9.3 60.5 99.9 1686.2 1.5 •D 1.7 0.9006 0.020 0.00077 16.1 143.337 0.075 2.6 63.1 99.9 1678.2 3.3 •E 1.8 0.1521 0.007 0.00147 1.4 144.323 0.153 5.4 68.5 99.7 1685.7 2.2 •F 1.9 0.357 0.009 0.00015 32.9 144.706 0.251 8.8 77.3 100 1688.6 3.1 G 2.3 0.0741 0.003 0.00003 35.7 148.939 0.648 22.7 100 100 1720.4 2.1 Integrated (total fusion) age: 1716 2 (•) Plateau age: 26.1 1686 3 483657 Hornblende (J = 0.01071 ± 0.00001): A 1.8 11.847 0.123 0.04105 4 184.220 0.019 1 1 94 1965.7 11.4 B 1.9 11.642 0.027 0.00559 28.7 163.894 0.059 3.1 4.1 99.3 1828.5 3.3 •C 2 10.312 0.013 0.00193 73.5 159.127 1.198 63.2 67.3 99.9 1794.7 1.1 •D 2 10.03 0.014 0.00155 89 159.084 0.463 24.4 91.7 100 1794.4 1.3 •E 2.1 11.862 0.043 0.00182 90 158.317 0.028 1.5 93.2 100 1788.9 8.0 •F 2.6 11.62 0.018 0.00223 71.9 159.178 0.130 6.8 100 99.9 1795.1 2.3 Integrated (total fusion) age: 1797 3 (•) Plateau age: 95.9 1795 3 483667 Hornblende (J = 0.01071 ± 0.00001): A 1.8 14 0.277 0.08890 2.2 501.205 0.011 0.5 0.5 95.1 3339.8 25.5 B 1.9 8.88 -0.010 0.01236 9.9 166.723 0.006 0.2 0.7 98.1 1848.2 27.0 C 2 8.133 0.066 0.00845 13.3 196.061 0.068 3 3.7 98.9 2041 3.3 D 2 8.823 0.082 0.01454 8.4 148.220 0.042 1.8 5.5 97.4 1715.1 6.7 E 2.1 5.161 0.073 0.01482 4.8 146.685 0.009 0.4 5.9 97.2 1703.6 17.3 •F 2.2 8.525 0.054 0.00482 24.4 157.760 0.118 5.2 11 99.3 1784.9 2.3 •G 2.2 9.17 0.048 0.00241 52.4 157.782 0.188 8.2 19.2 99.8 1785.1 1.7 •H 2.3 8.683 0.050 0.00316 37.8 157.288 0.206 9 28.2 99.6 1781.6 1.6 •I 2.3 8.979 0.056 0.00234 53 157.382 0.328 14.3 42.5 99.8 1782.2 1.4 •J 2.4 8.623 0.055 0.00105 113.4 157.274 0.432 18.8 61.3 100 1781.5 1.9 •K 2.6 8.346 0.056 0.00150 77 157.395 0.561 24.4 85.8 99.9 1782.3 1.1 •L 2.8 9.598 0.067 0.00374 35.4 156.861 0.081 3.5 89.3 99.5 1778.5 3.0 •M 4 10.4 0.062 0.00273 52.5 157.231 0.246 10.7 100 99.8 1781.1 1.9 Integrated (total fusion) age: 1801 2 (•) Plateau age: 94.1 1782 2 483671 Muscovite (J = 0.01071 ± 0.00001): A 1.4 0.2482 0.005 0.00153 2.2 145.263 0.449 27.4 27.4 99.7 1692.9 1.2 B 1.5 0.7642 0.007 0.00189 5.6 145.703 0.168 10.3 37.6 99.6 1696.2 1.9 •C 1.5 0.3775 0.007 0.00197 2.6 143.327 0.168 10.2 47.8 99.6 1678.2 1.6 •D 1.6 0.4299 0.017 0.00377 1.6 142.856 0.089 5.4 53.3 99.2 1674.6 2.8 •E 1.6 1.5215 0.010 0.00180 11.7 142.234 0.057 3.5 56.7 99.7 1669.8 3.7 •F 1.7 0.9017 0.017 0.00144 8.7 142.377 0.093 5.7 62.4 99.7 1670.9 3.0 •G 1.7 0.2113 0.005 0.00004 79.6 143.047 0.230 14 76.4 100 1676 1.7 •H 1.8 1.123 0.015 0.00144 10.7 142.970 0.065 4 80.4 99.7 1675.4 23.0 •I 2 0.4492 0.005 0.00121 5.1 143.638 0.152 9.3 89.7 99.8 1680.5 4.2 J 2.3 0.2898 0.004 0.00172 2.3 144.357 0.169 10.3 100 99.7 1686 3.2 Integrated (total fusion) age: 1684 3 (•) Plateau age: 52.1 1676 3 Table 1. 40Ar-39Ar analytical data for step heating experiments on amphiboles and muscovites from the Rinkian fold belt Step Pwr/T°C Ca/K Cl/K 36Ar/39Ar %36Ar(Ca) 40*Ar/39Ar Mol 39Ar % Step %39Ar %40*Ar Age (Ma) ± Age Cumulated (2σ) J: irradiation parameter. 40*Ar: radiogenic 40Ar. Steps marked with dot (•) are included in the plateau age for each sample. 119 Whereas the tectonic model of Grocott & Pulvertaft (1990) operated with four contractional and three exten- sional events in an epicontinental marginal basin, four main phases of deformation that developed during pro- gressive crustal shortening are now recognised. It has been debated in recent years whether the previous distinction between the Rinkian and Nagssugtoqidian belts in West Greenland is meaningful in tectonic terms (e.g. Garde & Steenfelt 1999a; van Gool et al. 2002), and it has now been proposed that the two belts represent the northern and southern parts of a common, more than 1100 km wide collisional orogen, separated by a suture located in the Disko Bugt region (Fig. 1; Connelly et al. 2005). The continuous crustal shortening in the Rinkian fold belt throughout its tectonic evolution is in agreement with a setting within the northern of two colliding plates at some distance from the suture, and thus in accordance with the proposed tectonic linkage to the Nagssugtoqidian orogen. However, the 40Ar/39Ar data presented in the following section may be interpreted to indicate that the tectono- metamorphic events in the Rinkian and Nagssugtoqidian belts were not contemporaneous. Descriptions of samples and results of 40Ar/39Ar age determinations Four hornblende samples and three muscovite samples were collected at the head of Ukkusissat Fjord close to the Prøven igneous complex, between Svartenhuk and Uum- mannaq, and close to the north coast of Nuussuaq (Fig. 1). Sample numbers refer to the data base of the Geological Survey of Denmark and Greenland. Ukkusissat Fjord near the Prøven igneous complex A sample with hornblende was collected south of the Prøven igneous complex, within the high-grade contact metamorphic aureole where extensive partial melting has been observed, particularly within the Karrat Group (Gro- cott & Pulvertaft 1990). The sample (483657, Fig. 1) comes from a homogeneous, medium-grained, amphib- olitic relict dyke within the regional flat-lying tonalitic orthogneiss basement. The amphibolite dyke is approxi- mately one metre thick, a few metres long, and has been isoclinally folded. The sample is mostly composed of light to dark green hornblende between 0.5 and 1 mm in dia- meter, together with some plagioclase and minor phases Step Pwr/T°C Ca/K Cl/K 36Ar/39Ar %36Ar(Ca) 40*Ar/39Ar Mol 39Ar % Step %39Ar %40*Ar Age (Ma) ± Age Cumulated (2σ) 483696 Hornblende (J = 0.01071 ± 0.00001): A 1.9 4.9568 0.167 0.02251 3 557.080 0.091 1.9 1.9 98.9 3501.9 4.4 B 2 5.6395 0.198 0.00248 31.3 176.447 0.447 9.5 11.5 99.7 1914.4 1.3 C 2.1 5.7159 0.202 0.00177 44.4 164.542 0.370 7.9 19.4 99.8 1833 1.7 D 2.2 5.9588 0.197 0.00468 17.5 161.233 0.042 0.9 20.3 99.3 1809.7 6.6 E 2.2 5.7368 0.198 0.00147 53.9 161.903 0.098 2.1 22.3 99.9 1814.5 2.5 F 2.3 5.3872 0.190 0.00107 69.3 162.278 1.212 26.4 48.8 99.9 1817.1 1.0 •G 2.3 5.3337 0.184 0.00097 75.5 157.503 1.620 34.6 83.4 100 1783.1 1.3 •H 2.4 5.3052 0.209 0.00097 75.2 157.732 0.242 5.2 88.5 100 1784.7 1.6 •I 2.6 6.2547 0.255 0.00436 19.8 161.354 0.017 0.4 88.9 99.4 1810.6 13.4 •J 2.9 7.1602 0.246 0.00815 12.1 156.396 0.009 0.2 89.1 98.7 1775.1 13.0 •K 4 5.4765 0.189 0.00181 41.7 158.216 0.511 10.9 100 99.8 1788.2 1.6 Integrated (total fusion) age: 1864 2 (•) Plateau age: 51.2 1785 3 483708 Hornblende (J = 0.01071 ± 0.00001): A 1.9 7.672 0.051 0.00834 12.7 181.053 0.134 8.2 8.2 98.8 1945 2.2 •B 2 9.5527 0.062 0.00197 66.7 157.766 0.855 52.6 60.8 99.9 1785 1.0 •C 2.1 9.6506 0.062 0.00317 42 157.637 0.376 23.2 84 99.7 1784.1 1.5 •D 2.2 8.7213 0.062 0.00050 241.2 157.711 0.054 3.3 87.3 100.1 1784.6 3.8 •E 2.3 10.597 0.109 0.00836 17.5 157.274 0.024 1.5 88.7 98.7 1781.5 8.4 •F 2.4 13.824 0.144 0.00502 38 156.401 0.012 0.7 89.5 99.4 1775.2 12.1 •G 2.7 10.368 0.067 0.00298 47.9 157.265 0.171 10.5 100 99.7 1781.4 3.3 Integrated (total fusion) age: 1798 3 (•) Plateau age: 91.8 1784 3 Table 1 (continued) 120 such as biotite, titanite and zoizite. The biotite is inter- grown with hornblende, and the plagioclase is partly al- tered to sericite. The obtained plateau age is 1795 ± 3 Ma (Fig 2; Table 1). North-eastern Uummannaq Four samples were collected in the Kangilleq–Kangerlus- suaq area, 75–100 km north of Uummannaq (Fig. 1). This area was less intensely affected by Rinkian metamor- phism than other areas investigated in this study, with chlorite schist locally preserved on the north coast of Qeqertarsuaq. Samples 483653 and 483654, both from the Nukav- sak Formation, were collected at two localities close to each other near the southern end of Qeqertarsuaq (Fig. 1). Sample 483653 (Fig. 1) is a greywacke consisting of biotite, sillimanite, quartz, muscovite and small amounts of tourmaline and zircon. It is a fine-grained rock, where biotite and muscovite together define the main tectonic foliation. Fibrolitic sillimanite occurs in broom-shaped clusters close to muscovite. It was difficult to obtain a good separate from this sample because the muscovite is very fine grained, intergrown with biotite, and sometimes has altered rims. This sample yielded a u-shaped spectrum, with a minimum which yields an age of 1681 ± 6 Ma and represents 24% of the total 39Ar-release (Fig. 2; Table 1). Sample 483654 (Fig. 1) is a fine-grained andalusite schist with centimetre-sized andalusite poikiloblasts in a matrix of biotite and quartz, minor tourmaline and mus- covite. Partial recrystallisation of andalusite to fibrolite was observed. The biotite shows two different orientations implying growth both during D2 and D3 deformation. The muscovite crystals are very small and often occur close to biotite, but sometimes also in small separate clusters. This sample gave a u-shaped spectrum, with a minimum representing 26% of the total 39Ar-release and yielding an age of 1686 ± 3 Ma (Fig. 2; Table 1). Sample 483667 (Fig. 1) was collected from a decime- tre-thick, boudinaged, homogeneous amphibolite band in tonalitic reworked orthogneiss on Qinngusaaq (Fig. 1). Folds, lineations, δ- and σ-shaped porphyroclasts and folia- tions representing both D2–D3 and D4 occur at the sam- pling locality. The light to dark brownish-green hornblende forms well-crystallised medium-grained aggregates with interstitial plagioclase partly altered to sericite. Small grains of pale green pyroxene, probably diopside, occur together with the hornblende. A hornblende plateau age of 1782 ± 2 Ma was obtained (Fig. 2; Table 1). Sample 483671 (Fig. 1) consists of fine-grained meta- sandstone to metasiltstone from the Nukavsak Formation with quartz, biotite, muscovite and sillimanite as main minerals. Biotite, muscovite and sillimanite define the main tectonic foliation, where sillimanite often occurs in clusters containing small muscovite and biotite grains. At this locality, quartz pods display distinct asymmetries in two different directions. The asymmetric pods on rock faces with SW–NE orientations suggest top-to-NE tec- tonic transport (during D2), whereas rock faces with SE– NW orientations suggest transport to the NW (during D3) and contain biotite lineations with that trend. Mus- covite from this rock, presumably grown during D3, gave a plateau age of 1676 ± 3 Ma (Fig. 2; Table 1). South-east of Uummannaq Two samples with hornblende were collected from the Archaean basement south-east of the Uummannaq area, close to the north coast of Nuussuaq (Fig. 1). Sample 483696 (Fig. 1) comes from a hornblenditic layer in a leucogabbro that occurs as enclaves in quartzo-feldspath- ic orthogneiss. The sample consists almost exclusively of light to dark green, medium- to coarse-grained horn- blende. The hornblende plateau age is 1785 ± 3 Ma (Fig. 2; Table 1). Sample 483708 (Fig. 2) comes from an amphibolite dyke that cuts the fabric of the surrounding augen gneiss and is probably Palaeoproterozoic in age. Both the amphi- bolite and the host gneiss are intensely deformed. This sample has biotite and hornblende growing together, feld- spars partly altered to sericite, and minor amounts of quartz. The hornblende plateau age is 1784 ± 3 Ma (Fig. 2; Table 1). Discussion and conclusions The results from this study provide the first published constraints on cooling ages of hornblende and muscovite in the central Rinkian belt. Hornblende 40Ar/39Ar plateau age spectra from samples 483657, 483667 and 483708 yield ages between 1795 and 1782 Ma. These ages all form well-defined plateaus, and the plateaus represent more than 90% of total 39Ar release. Sample 483696 yielded a pla- teau age of 1785 Ma for 51% of the total 39Ar release, and is consistent with the other hornblende ages. Muscovite samples 483653 and 483654 both yield u-shaped age spec- tra with minima representing less than 50% of the total 39Ar release, at 1681 and 1686 Ma respectively. Sample 483671 provides a plateau age of 1676 Ma defined by 121 52% of the total 39Ar release. The muscovite plateau age spectrum for sample 483671 is consistent with the mini- ma provided by samples 483653 and 483654. These tak- en together suggest a relatively consistent muscovite cool- ing age below 350°C of c. 1680 Ma in the central Rinkian belt. The obtained hornblende and muscovite ages at 1795– 1782 and 1686–1676 Ma, respectively, are remarkably uniform, although they cover a distance of c. 200 km in chlorite to sillimanite grade amphibolite facies terrain across the entire central part of the Rinkian fold belt. This 40Ar/39Ar age study shows that the temperatures reached during the Palaeoproterozoic tectonothermal reworking were everywhere sufficiently high to reset the 40Ar/39Ar hornblende and muscovite systems in the rocks examined. The ages date the cooling below the closure temperature of Ar diffusion in hornblende and muscovite after the Palaeoproterozoic metamorphic event, and the data sug- gest a slow cooling rate of c. 1.5°C/Ma between c. 1780 and 1680 Ma, using closure temperatures of 500°C for hornblende and 350°C for muscovite (McDougall & Harrison 1999). Due to recent recalculation of the primary and second- ary standards used in 40Ar/39Ar geochronological experi- ments (Renne et al. 1998), the previously published 40Ar/ 39Ar ages from the Nagssugtoqidian belt (Rasmussen & Holm 1999; Willigers et al. 2001, 2002), which use the older standard age, have to be multiplied by 1.009 in or- der to compare directly with the new 40Ar/39Ar ages pre- sented here from the Rinkian belt. In the northern part of the Nagssugtoqidian orogen, Willigers et al. (2001, 2002) obtained 40Ar/39Ar hornblende ages of 1756–1733 Ma (re- calculated from 1740–1717 Ma) and muscovite ages of c. 1715 Ma (recalculated from 1700 Ma). In the central part of the orogen still farther south, their hornblende ages range between c. 1750–1700 Ma and muscovite ages between c. 1765–1715 Ma (recalculated from 1750–1700 Ma). In the Disko Bugt area (Fig. 1), where Connelly et al. (2005) proposed a suture between the two belts, a set of 40Ar/39Ar and K-Ar hornblende age data reported by Rasmussen & Holm (1999) scatter between Archaean ages and a K-Ar age of c. 1765 Ma, revealing that tempera- tures during the Palaeoproterozoic thermal event were not sufficiently high in all parts of this area to reset the K-Ar isotope system (Rasmussen & Holm 1999). The uniformity of the new 40Ar/39Ar ages from the cen- tral and northern Rinkian fold belt suggests that the ages are largely unrelated to the metamorphic grade and to the structural history of the geographical locations of the sam- ples, with the possible exception of sample 483657 from Ukkusissat Fjord (see below). Accordingly, the 40Ar/39Ar data are interpreted as regional, late orogenic cooling ages which are not directly related to the tectono-metamor- phic history of the individual samples. This conclusion is supported by (in part unpublished) U-Pb zircon ages of syn- to late-kinematic Palaeoproterozoic pegmatites from the same region, which are older than 1800 Ma (Thrane et al. 2003; K. Thrane, personal communication 2004). Willigers et al. (2002) reached the same conclusion from their 40Ar/39Ar studies of the central and northern Nags- sugtoqidian orogen reported above, pointing out that their study area represents a section of middle to lower crust that was only slowly exhumed by erosion. In preserved upper crustal levels of younger orogens it is commonly possible to date specific tectonic events us- ing the 40Ar/39Ar method, because the dated units were either transported rapidly to these crustal levels and are not yet eroded away, or the minerals grew at temperatures near or below their closing temperature and thus con- strain the age of the prograde tectonothermal event itself. The 1795 ± 3 Ma age of the hornblende from Ukkusissat Fjord (sample 483657, about 12 Ma older than the other hornblende ages) may point to early uplift of this particu- lar area, which is a domain of early NE-directed D2 thrust- ing that was not affected by the subsequent NW-directed tectonic transport during D3. The cooling rate of c. 1.5°C/Ma documented by this study (using hornblende and muscovite closure tempera- tures of 500°C and 350°C) is only slightly slower than the 2–3°C/Ma reported by Willigers et al. (2001, 2002) from the central Nagssugtoqidian orogen, but considera- bly slower than rates between 5° and 7°C/Ma reported by the latter authors from the northern Nagssugtoqidian oro- gen. Willigers et al. (2001, 2002) used less accepted clo- sure temperatures of 580°C and 410°C for hornblende and muscovite, respectively, implying a difference of 170°C between hornblende and muscovite closure temperatures. The latter temperature gap is larger than the 150°C used in this study, but this makes little difference to the calcu- lation of cooling rates. The uniform 40Ar/39Ar hornblende ages resulting from the present investigation are significantly older than those in both the northern and central parts of the Nagssugto- qidian orogen (Willigers et al. 2001, 2002). As regards muscovite, the Rinkian muscovite ages are younger than muscovite ages in the northern Nagssugtoqidian belt, but older than those in the central Nagssugtoqidian orogen (Willigers et al. 2001, 2002). The fact that Rinkian horn- blende ages are older than those in the Nagssugtoqidian orogen shows that cooling below 500°C took place earlier in the Rinkian fold belt than in both the central and north- ern parts of the Nagssugtoqidian orogen. It is therefore 122 plausible that uplift began significantly earlier in the Rin- kian belt but was slower than in the Nagssugtoqidian oro- gen, which may in turn suggest that the main phases of compression and peak metamorphism in the two belts were not synchronous. These interpretations are consistent with the observa- tion by Taylor & Kalsbeek (1990) that Pb-Pb whole-rock isochron ages of marbles in the two belts (interpreted as representing recrystallisation of the marbles during peak metamorphism) differ significantly from each other. Mar- bles collected on Appat island in the central Rinkian belt (Fig. 1) yielded a Pb-Pb isochron of 1881 ± 20 Ma, whereas an age of 1845 ± 23 Ma was obtained from marbles in the central part of the Nagssugtoqidian orogen. Our inter- pretations are also consistent with the fact that the 40Ar/ 39Ar data reported here show no signs of having been af- fected by a contact metamorphic aureole around the Prøven igneous complex. The intrusion age of the latter at 1869 ± 9 Ma is coeval with the youngest members of the Arfersiorfik complex and Sisimiut charnockite in the central Nagssugtoqidian orogen (Connelly et al. 2000; van Gool et al. 2002). The Prøven igneous complex has in- truded rocks belonging to the Karrat Group that were already intensely deformed and metamorphosed prior to the intrusion, but before the last major deformation and peak metamorphism (Thrane et al. 2005); the Prøven ig- neous complex represents a crustal melt that was appar- ently not related to subduction processes. In contrast, the Arfersiorfik complex and Sisimiut charnockite in the south represent I-type magmas that were related to precollision subduction. Notwithstanding the overall structural and geochrono- logical evidence for a direct linkage between the Rinkian fold belt and the Nagssugtoqidian orogen, the age rela- tionships outlined above may imply that collision-related deformation, metamorphism and magmatic activity took place in the northern Rinkian belt while subduction was still going on south of the recently proposed suture in the Disko Bugt region. It may be speculated that such dia- chronism is also reflected in the dissimilar 40Ar/39Ar cool- ing ages from the Rinkian and Nagssugtoqidian parts of the entire Palaeoproterozoic orogenic complex in West Greenland. Alternatively, the different Rinkian and Nags- sugtoqidian cooling ages might relate to different depths of burial. However, this is not supported by the uniform hornblende 40Ar/39Ar cooling ages found within the Rin- kian belt itself, regardless of geographical distance and metamorphic facies; further discussion of large-scale plate- tectonic implications is beyond the scope of the present paper. Analytical procedure Four hornblende and three muscovite separates from the central Rinkian belt have been dated with the 40Ar/39Ar- method. The hornblende separates were obtained from amphibolite and diorite, and muscovite from metasedi- mentary rocks, by crushing, sieving and handpicking. The hornblende and muscovite samples selected for 40Ar/39Ar geochronology were irradiated together with the DRA-2 sanidine standard (25.26 Ma; Wijbrans et al. 1995, recal- culated following Renne et al. 1998), for 35 hours at the NRG-Petten HFR RODEO facility in Petten, The Neth- erlands. J-values (the irradiation parameter) were calcu- lated with a precision of 0.5%. The 40Ar/39Ar geochronology laboratory at the Univer- sity of Lund employs a Micromass 5400 mass spectrome- ter with a Faraday cup and an electron multiplier. A met- al extraction line, which contains two SAES C50-ST101 Zr-Al getters and a cold finger cooled to c. –155°C by a Polycold P100 cryogenic refrigeration unit, is also present. One or two grains of hornblende or muscovite were load- ed into a copper planchette that consists of several 3 mm holes. Samples were step-heated using a defocused 50W CO2 laser. Sample clean-up time was 5 minutes, using the two hot Zr-Al SAES getters and the cold finger. The laser was rastered over the samples to provide even heat- ing of all grains. The entire analytical process is automat- ed and runs on a Macintosh computer with software de- veloped at the Berkeley Geochronology Center by Al Deino and modified for the laboratory at the University of Lund. Time zero regressions were fitted to data collected from 10 scans over the mass range of 40 to 36. Peak heights and backgrounds were corrected for mass discrimination, isotopic decay and interfering nucleogenic Ca-, K-, and Cl-derived isotopes. Isotopic production values for the cadmium lined position in the Petten reactor are 36Ar/ 37Ar(Ca) = 0.000270, 39Ar/37Ar(Ca) = 0.000699, and 40Ar/ 39Ar(K) = 0.00183. 40Ar blanks were calculated before eve- ry new sample and after every three sample steps. 40Ar blanks were between 5.0 and 3 × 10–16. Blank values for masses 39 to 36 were all less than 7 × 10–18. Blank values were subtracted for all incremental steps from the sample signal. The laboratory was able to produce very good in- cremental gas splits, using a combination of increasing time at the same laser output, followed by increasing laser output. Age plateaus were determined using the criteria of Dalrymple & Lanphere (1971), which specify the pres- ence of at least three contiguous incremental heating steps with statistically indistinguishable ages and constituting greater than 50% of the total 39Ar released during the ex- periment. Inverse isochrons yield ages statistically indis- 123 tinguishable from those given by the plateaus and are not presented here. 40Ar/39Ar plateau age spectra are presented in Fig. 2 and the analytical data in Table 1. Acknowledgements The authors thank J.N. Connelly, J. Grocott, M. Hand, K.J.W. McCaffrey and K. Thrane for discussions leading to the preparation of this manuscript, which also draws on their collective field observations in 2002–2003. We are grateful to J. Grocott and Å. Johansson for critical reviews. References Connelly, J.N., van Gool, J.A.M. & Mengel, F.C. 2000: Temporal evo- lution of a deeply eroded orogen: the Nagssugtoqidian orogen, West Greenland. 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