Geochronology and isotopic data

Fig. 4. Sketch geological map in part after Mitrofanov, 1996 of the UGT showing sample localities. LKS, footwall boundary of the Lapland – Kola Suture. KB, Kolvitsa Belt. Box shows location of Fig. 2. Granulite facies paragneisses of the Umba Granulite Terrane UGT, Figs. 1 and 4 are gen- erally regarded as a southeastern correlative of the Lapland Granulite Terrane. These rocks struc- turally overly a highly deformed tectonic me´lange Balagansky et al., 1986, 1998a comprising UGT paragneisses and meta-igneous rocks of the under- lying Kolvitsa Belt. The Kolvitsa Belt and the overlying granulitic me´lange displays an inverted metamorphic gradient Priyatkina and Sharkov, 1979, similar to that documented within the Tanaelv Belt and between it and the overlying Lapland Granulite Terrane see above, Fig. 11. The high grade metamorphism within the UGT also took place c 1.90 – 1.92 Ga ago based on UPb zircon dates from sillimanite–garnet–bi- otite gneisses within the me´lange Bibikova et al., 1973 and from discordant leucosomes cutting granulite-facies mylonites Kislitsyn et al., 1999a. Metamorphic zircons in the underlying Kolvitsa gabbro-anorthosite massif also yielded similar ages Frisch et al., 1995; Kaulina, 1996. Follow- ing deformation, the UGT was intruded by the Umba Complex Fig. 4 of megacrystic granite, charnockite and enderbite at 1912 9 7 Ma Glebovitsky et al., 2000. The Umba Complex also intrudes the Tersk Terrane see above and thus is interpreted to stitch the Umba Granulite and Tersk terranes together.

2. Geochronology and isotopic data

2 . 1 . Strelna Domain and Tersk Terrane – Varzuga Ri6er section 2 . 1 . 1 . UPb ion-microprobe dating Sampling was carried out on a traverse along the Varzuga River Figs. 1 and 2. Samples were selected for dating on the basis of clear structural relationships as described below. Zircons were separated using standard electro- magnetic and heavy-liquid techniques at the Geo- logical Institute, Kola Science Centre, Apatity, Russia. Selected grains were hand-picked, mounted in epoxy resin and polished to reveal zircon interiors for scanning electron microscope SEM study Fig. 6 under cathodoluminescence CL and electron-backscattering BSE. UThPb analyses were performed at the Swedish Museum of Natural History, Stockholm, Sweden Nordsim facility on a high-mass resolution, high-sensitivity Cameca IMS 1270 ion-microprobe following routine methods previously described by Whitehouse et al. 1999, modified after White- J .S . Daly et al . Precambrian Research 105 2001 289 – 314 Table 1 UPb analytical data and calculated ages 1 [U] f 206 207 Pb 206 Pb 207 Pb 207 Pb 206 Pb 206 Pb 238 Pb Disc. [Pb] [Th] Samplegrain no. 206 Pb ppm 9s 238 U 9s 235 U 9s age Ma age Ma ppm ppm ThU 895-80 orthogneiss 0.196 0.7 0.511 2.0 13.78 229 2.1 228 2791 9 12 2659 9 43 5.7 322 5r 0.71 0.193 0.4 0.534 2.0 14.22 5c 2.0 194 2769 9 6 2758 9 44 0.5 147 167 0.86 0.191 0.5 0.556 2.0 14.62 2.0 2748 9 7 0.36 2850 9 45 126 − 4.6 0.57 6r 221 165 0.28 307 0.190 0.5 0.541 2.0 14.15 2.0 2739 9 8 2787 9 45 −2.2 237 297 0.97 6c 156 0.187 0.4 0.533 1.9 13.75 2.0 2718 9 7 2753 9 44 −1.6 113 100 0.64 3r 0.187 0.2 0.531 2.0 13.69 2.3 2717 9 19 2744 9 44 7C −1.2 0.98 24 19 25 6c2 0.187 463 1.1 0.526 1.9 13.55 1.9 2714 9 4 2725 9 43 −0.5 343 393 0.86 895-81 pegmatite 0.184 0.3 0.506 1.9 12.85 210 2.0 375 2690 9 5 2640 9 42 2.3 1.39 269 5c 1.51 0.181 0.3 0.492 1.2 12.25 1.2 2659 9 5 2578 9 25 3.7 328 3c 633 420 0.01 0.119 0.4 0.349 1.2 5.73 1.3 1941 9 6 1931 9 21 0.6 718 1r 10 1835 0.117 0.2 0.357 1.2 5.73 1.2 1903 9 4 1966 9 20 3r − 3.9 0.02 62 1527 3815 5161 0.110 0.3 0.261 1.2 3.94 1.2 1791 9 5 1494 9 16 18.6 1504 78 0.02 3r2 0.088 0.4 0.123 1.4 1.49 1.5 1377 9 8 749 9 10 48.3 0.01 75 726 5396 1r2 895-59 orthogneiss 0.190 0.6 0.522 1.2 13.66 1c 1.3 237 2742 9 9 2706 9 26 1.6 164 113 0.48 0.3 0.188 0.3 0.617 4.6 15.96 4.7 2722 9 6 3097 9 115 9r2 − 17.4 0.49 68 115 141 0.07 0.185 0.2 0.538 1.2 13.72 1.2 2699 9 4 2774 9 26 − 3.4 760 3r 85 1175 0.184 0.2 0.514 1.2 13.05 1.2 2689 9 3 2675 9 25 79 0.7 0.07 1r 1180 729 2.9 330 0.183 0.8 0.420 1.2 10.62 1.5 2683 9 14 2263 9 23 18.5 186 252 0.77 3c 9r3 0.182 153 0.7 0.483 4.6 12.14 4.6 2673 9 11 2541 9 97 6.0 101 80 0.52 0.176 0.6 0.541 4.6 13.09 4.6 2612 9 10 2787 9 105 −8.3 9r 160 114 82 0.51 895-62.1 pegmatite 0.15 0.117 0.3 0.354 1.2 5.71 1.2 1909 9 5 1955 9 20 −2.8 856 2r2 305 2081 0.117 0.4 0.315 1.2 5.07 1.2 1908 9 7 1765 9 18 0.13 8.6 2r 1836 668 238 0.116 0.3 0.318 1.9 5.08 2.0 1891 9 5 6r3 1781 9 30 1022 6.7 370 90 0.09 0.22 0.116 0.2 0.317 1.9 5.05 2.0 1889 9 4 1774 9 30 3r2 7.0 0.10 126 445 1228 0.115 0.5 0.302 1.2 4.80 1.3 1887 9 10 6r 1700 9 18 1260 11.3 437 141 0.11 0.69 0.115 1.8 0.311 1.2 4.95 2.1 1887 9 31 1744 9 19 0.85 8.6 1565 560 189 0.12 3r 895-70 orthogneiss 0.45 0.07 0.125 0.5 0.341 2.0 5.89 2.0 2031 9 10 1892 9 32 7.9 84 17c 90 198 0.38 0.31 0.123 1.0 0.349 4.8 5.91 4.9 1994 9 17 1932 9 81 3.6 93 2r3 82 217 0.122 0.3 0.353 2.0 5.92 2.0 1979 9 5 1950 9 33 12c 1.7 0.40 189 204 468 481 0.15 0.121 0.4 0.350 2.5 5.83 2.5 1972 9 6 1932 9 42 2.3 206 187 27r 0.39 0.121 0.6 0.339 4.8 5.65 4.9 1970 9 11 0.19 1881 9 79 0.31 5.2 14r2 295 120 91 0.121 0.4 0.364 1.6 6.05 1.7 1964 9 8 14c 2001 9 28 462 −2.2 206 173 0.37 0.09 0.120 0.5 0.348 2.5 5.78 2.6 1960 9 9 0.16 1927 9 42 33r 2.0 285 0.36 103 121 J .S . Daly et al . Precambrian Research 105 2001 289 – 314 297 Table 1 Continued f 206 [U] 207 Pb 206 Pb 207 Pb 207 Pb 206 Pb 206 Pb 238 Pb Disc. [Pb] [Th] Samplegrain no. 206 Pb 9s 238 U 9s 235 U 9s age Ma age Ma ppm ppm ppm ThU 0.120 0.2 0.354 2.0 5.88 2.0 0.45 1960 9 4 12r 1955 9 33 0.3 0.02 292 284 645 501 0.03 0.120 0.4 0.358 4.8 5.91 4.8 1950 9 8 1973 9 83 −1.4 216 151 2c2 0.30 482 0.12 0.120 0.4 0.355 1.6 5.85 1.6 1950 9 7 1957 9 26 −0.4 207 149 14r2 0.31 0.120 0.6 0.352 2.1 5.81 2.2 1949 9 10 1946 9 35 47 0.2 0.32 17r 147 63 0.74 153 0.119 1.3 0.340 4.8 5.57 5.0 1940 9 23 1885 9 80 3.3 62 47 0.31 2r2 0.55 179 0.118 0.6 0.361 1.6 5.88 1.7 1930 9 11 1987 9 28 −3.4 77 52 0.29 2r 0.118 0.3 0.373 1.6 6.06 1.6 1925 9 6 0.04 2041 9 27 2c − 7.0 0.32 164 228 506 0.118 0.9 0.352 40r 2.5 362 5.71 2.6 1919 9 15 1945 9 42 −1.6 150 87 0.24 0.33 0.117 0.8 0.348 2.5 5.63 2.6 1916 9 15 1924 9 42 0.37 −0.5 148 61 40 0.27 34r 895-67 pegmatite 0.43 0.124 0.5 0.363 2.0 6.19 2.0 2007 9 8 1998 9 34 0.6 47 7c 45 104 0.04 0.122 0.3 0.345 4.6 5.78 4.6 1980 9 5 1910 9 77 4.1 495 17r2 55 1126 0.121 0.6 0.351 2.0 5.84 2.1 1964 9 11 1.34 1941 9 33 7r 1.4 0.04 30 291 726 0.120 0.6 0.358 4.6 5.94 4.6 1961 9 10 1972 9 79 −0.7 15r 842 341 33 0.04 0.118 0.3 0.357 4.6 5.80 4.6 1925 9 6 1967 9 78 56 −2.5 0.04 18r 1257 507 1223 0.117 0.3 0.358 2.0 5.79 2.0 1917 9 5 1972 9 34 −3.4 495 55 0.04 7r2 0.117 0.4 9r 0.345 1031 2.0 5.59 2.0 1917 9 7 1913 9 33 0.2 402 41 0.04 0.115 0.6 0.382 4.7 6.07 4.7 1883 9 10 2085 9 84 0.04 − 12.6 17r 1099 472 41 1 Note: Analyses for each sample are ordered by decreasing 207 Pb 206 Pb age; c, core; r, rim. Disc is the degree of discordance of 207 Pb 206 Pb and 206 Pb 238 U ages at the 2s level; negative values indicate reverse discordance; values in parentheses indicate that the analysis is concordant within 2s error. f206 is the amount of common 206 Pb, estimated from measured 204 Pb; blank values indicate that no common lead correction was applied due to statistically insignificant 204 Pb counts. ThU ratios are calculated from measured ThO and U assuming a relative sensitivity for these species which is derived from ThU in the 91500 reference zircon. These factors are calculated for each reference analysis using 208 Pb 206 Pb and the accepted 1065 Ma age of this zircon. Age errors are 1s. house et al. 1997. UPb ratio calibration was based on analyses of the Geostandards zircon 91500, which has an age of 1065.4 9 0.3 Ma and U and Pb concentrations of 80 and 15 ppm, respectively Wiedenbeck et al., 1995. For 206 Pb 238 U ratios, an error based upon the external reproducibility of multiple measurements of zir- con standard 91500 in a given analytical session in this study, from 1.2 to 4.5, 1 sigma has been propagated together with the observed analytical error from the unknowns. This external error generally dominates the error in this ratio. An assessment of the reproducibilty of 207 Pb 206 Pb ratios obtained with the ion-probe data is not so easy to make because the reference zircon has different age and Pb concentration from the un- knowns. In this study, we follow the practice described by Wiedenbeck and Watkins 1993 of taking the observed error in the ratio. This is generally larger than that resulting from counting statistics alone. Corrections for common Pb are based upon the measured 204 Pb signal, where statistically significant. The present day terrestrial average Pb-isotopic composition is used for this correction Stacey and Kramers, 1975 on the assumption that Pb is most likely introduced as a surface contaminant during sample preparation for detailed discussion of this rationale, see Zeck and Whitehouse, 1999. Data reduction employed Excel routines devel- oped by Whitehouse while age calculations were made using IsoplotEx v 2.05 Ludwig, 1999. UThPb data are presented in Table 1 and plot- ted as 2s error ellipses in Fig. 7. All age errors quoted in the text are 2s. 2 . 1 . 1 . 1 . Strelna Domain-Archaean orthogneisses. Orthogneisses from the Strelna Domain were in- vestigated to verify their assumed Neoarchaean age and to locate the boundary with the Tersk Terrane, which was known to contain Palaeoproterozoic elements Timmerman and Daly, 1995. Fig. 5. Fig. 5. a Field photograph showing the pegmatite, 895-81 for location, see Fig. 2 cutting the foliation in Archaean orthogneiss from the Strelna Domain, close to sample 895-80. The contact is arrowed. Camera faces south. Penknife is 10 cm long. b Field photograph showing the pegmatite, 895-67 for location, see Fig. 2 cutting the foliation and migmatitic leucosomes in Palaeoproterozoic orthogneiss from the Tersk Terrane, close to sample 895-70. The contact is arrowed. Camera faces south. Penknife is 10 cm long. c Field sketch showing the pegmatite 895-62.1 cutting bedding and foliation in the Sergozerskaya metasediments of the Tersk Terrane. Occasional migmatitic leucosomes not shown are also cut by the pegmatite vein. Sample 895-80 was collected along the Varzuga River just north of the confluence with the Pana River Figs. 2 and 5. This rock is a felsic orthogneiss with 74.6 SiO 2 , low CaO and K 2 ONa 2 O close to 1. The rock is foliated and the foliation is cut by a granitic pegmatite, 895-81 Fig. 5a and see below. Zircons from 895-80 Fig. 6a are rounded short prisms with aspect ratios of about 2.5. They show idiomorphic zon- ing under CL and several grains have conspicuous discordant cores. Five out of seven analyses of sample 895-80 3r, 6c, 6c2, 6r and 7c, Fig. 7a are concordant and have an average 207 Pb 206 Pb age of 2722 9 18 Ma. Analyses from both cores and rims Fig. 7a contribute to the c 2.72 Ga age. An older component of zircon may be present in grain 5, which has the highest 207 Pb 206 Pb ages of 2791 9 23 analysis 5r and 2769 9 12 Ma analy- sis 5c. It seems reasonable to conclude that sam- ple 895-80 has a Neoarchaean crystallisation age of c 2.72 Ga. Sample 895-59, collected further south Fig. 2 is similar in composition to 895-80 with slightly lower SiO 2 72.63. The zircons Fig. 6a are slightly rounded euhedral prisms with a rather uniform aspect ratio of c 2.5. They display id- iomorphic zoning under CL, usually with a non- luminescent dark-CL unzoned or complexly-zoned rim. A number of grains have discordant zoned cores. Seven analyses Fig. 7b have an average 207 Pb 206 Pb age of 2695 9 23 Ma indicating an Archaean age indistinguishable within the large error from that of 895-80. Five analyses 9r2, 3r, 1r, 9r3 and 3c define a discordia with intercepts at 2693 9 5 and 345 9 150 Ma MSWD = 1.5. However there is little justifica- tion for excluding analyses 9r and 1c. Further work is needed to define accurate ages from the Strelna Domain, but for the purposes of this study it is clear that both rocks formed in the late Archaean. A Palaeoproterozoic age is consid- ered highly unlikely. 2 . 1 . 1 . 2 . Tersk Terrane-Palaeoproterozoic or- thogneiss. Sample 895-70 Figs. 2 and 5b is a calc-alkaline felsic orthogneiss from the Tersk Terrane Sergozerskaya Unit with 68 SiO 2 , high Na 2 OK 2 O and high Ba. Zircons from this rock Fig. 6b comprise rounded, stubby, doubly- terminated prisms with aspect ratios between 1.5 and 2 as well as longer doubly-terminated prisms with aspect ratios close to 5. Most grains display strong idiomorphic zoning under CL and several show distinct cores with unconformable over- growths Fig. 6b. Fourteen out of sixteen analyses, from both cores and rims, overlap concordia within error Fig. 7c. One analysis 17c from the core of a grain that has a thin, zoned overgrowth Fig. 6b is discordant and has an older 207 Pb 206 Pb age of 2031 9 19 Ma. Although this result suggests an inherited component, it is unlikely to be of Ar- chaean age. Another discordant point 2c lies above concordia. Excluding these two points, the average 207 Pb 206 Pb age is 1961 9 9 Ma, which we interpret to date the magmatic age of this sample. 2 . 1 . 1 . 3 . Pegmatites. Three granitoid pegmatite samples from one locality within the Strelna Do- main and from two localities within the Tersk Terrane were sampled in an attempt to constrain the time of deformation. Sample 895-81 Fig. 2 is from a thin c 10 cm vein that cuts the foliation in orthogneiss 895-80 Fig. 5a. The pegmatite is itself folded about a steep axial plane. Zircons from 895-81 Fig. 6c have a bimodal distribution comprising cloudy, fractured elongate prisms with aspect ratios of 4 – 5 and strongly rounded prisms with aspect ratios of 1.4 – 2.3, similar to those in the host gneiss, 895-80. Both types have a similar appear- ance under CL. They show narrow CL-dark or mottled rims that define a crude idiomorphic zon- ing. The rims unconformably overgrow inner cores showing strong, fine-scale idiomorphic zon- ing Fig. 6c that make up most of the grain. Two near-concordant analyses from the rims 1r and 3r have Palaeoproterozoic 207 Pb 206 Pb ages Fig. 7d, the more concordant of which being 1941 9 13 Ma. Two core analyses 3c and 5c have 207 Pb 206 Pb ages of 2659 9 9 and 2690 9 11 Ma, respectively, indicating that the cores are inher- ited. The cores have a much higher ThU value of c 1.5 compared with typical values of less than 0.02 for the rims. The cores 3c and 5c also have lower U contents of 420 and 269 ppm while the rims have high U contents ranging from 1835 to Fig. 6. a SEM CL images of zircons from Archaean orthogneisses from the Strelna Domain: sample 895-59 left, grains 1, 3 and 9 and sample 895-80 right, grains 3, 5, 6 and 7. Nordsim ion microprobe analytical spots, numbered as in Fig. 7 and Table 1. c, core; r, rim. Scale bar = 100 mm. b SEM CL images of zircons from Palaeoproterozoic orthogneiss from the Tersk Terrane: sample 895-70 grains 2, 12, 14, 17, 27, 33, 34 and 40. Nordsim ion microprobe analytical spots, numbered as in Fig. 7 and Table 1. c, core; r, rim. Scale bar = 100 mm. c. SEM CL images of zircons from pegmatites: sample 895-81, which cuts Archaean orthogneiss, 895-80, from the Strelna Domain top left, grains 1, 3 and 5; sample 895-67, which cuts Palaeoproterozoic orthogneiss, 895-70, from the Tersk Terrane right, grains 7, 9, 15, 17 and18 and sample 895-62.1, which cuts the Sergozerskaya metasediments in the Tersk Terrane bottom left, grains 2, 3 and 6. Nordsim ion microprobe analytical spots, numbered as in Fig. 7 and Table 1. c, core; r, rim. Scale bar = 100 mm. Fig. 7. a UPb discordia diagram for sample 895-80 showing the average 207 Pb 206 Pb age calculated from five concordant data points, outlined in black. Data Table 1 are plotted as 2s error ellipses. Selected points on this and other parts of Fig. 7 are labelled with the analytical spot as numbered in Table 1 and Fig. 6. b UPb discordia diagram for sample 895-59 showing the average 207 Pb 206 Pb age calculated from all six data points outlined in black. Data Table 1 are plotted as 2s error ellipses. c UPb discordia diagram for sample 895-70 showing the average 207 Pb 206 Pb age calculated from fourteen concordant data points outlined in black. Grey data points are excluded as discussed in the text. All data Table 1 are plotted as 2s error ellipses. d UPb discordia diagram for sample 895-81 showing a discordia line that excludes two analyses 3c and 5c interpreted as inherited cores. Data Table 1 are plotted as 2s error ellipses. e UPb discordia diagram for sample 895-62.1 showing a discordia line fitted to all six data points. Data Table 1 are plotted as 2s error ellipses. f UPb discordia diagram for sample 895-67 showing the average 207 Pb 206 Pb age for all samples. Data points Table 1 are plotted as 2s error ellipses. Labelled data points and the three outlined in bold are discussed in the text. 5396 ppm. Excluding grains 3c and 5c, the rim analyses define a poorly fitted discordia with in- tercepts at 1910 9 93 and 472 9 290 Ma MSWD = 23. While no precise estimate is possi- ble from these data, it seems clear that the peg- matite has a Palaeoproterozoic c 1.9 Ga rather than Archaean age. The significance of the non- zero intercept is unknown but this is similar to that found in the late Archaean orthogneisses. It may reflect a real thermal or hydrothermal event related to Devonian magmatism in the region Kramm et al., 1993. Deformation of the Archaean orthogneiss oc- curred before c 1.9 Ga. The pegmatites probably belong to the same suite as those cutting the metasediments and orthogneisses arc rocks of the Tersk Terrane to the south. Given the similar structural grain, we tentatively correlate the defor- mation event preceding pegmatite emplacement in the Archaean rocks with that affecting the Tersk Terrane to the south. Pegmatite 895-62.1 was collected south of the confluence of the Krivets and Varzuga rivers Fig. 2. This vein cuts the early foliation and leuco- somes in metasediments of the Sergozerskaya Unit Fig. 5c. Sample 62.1 is also folded by later folds and locally foliated parallel to their axial plane. Zircons from 895-62.1 Fig. 6c comprise bipyramidal elongate needles with aspect ratios of 5 – 9, as well as squat bipyramidal prisms with aspect ratios of c 3. Most grains have clearly- defined, generally CL-light cores with discordant, idiomorphically zoned, overgrowths, sometimes mottled and generally CL-darker towards the edge of the grains. Six analyses, all from rims Fig. 6c and with high U concentrations and uniform ThU ratios of c 0.13, have an average 207 Pb 206 Pb age of 1896 9 10 Ma. Only one of these 2r2 overlaps concordia and has a 207 Pb 206 Pb age of 1909 9 11 Ma. All define a discordia MSWD = 1.3 with an upper intercept age of 1906 9 9 Ma Fig. 7e and a non-zero lower inter- cept 260 9 170 Ma within error of those of other discordia lower intercepts from the area cf. 895- 59 and 895-81. Pegmatite 895-67 Figs. 2 and 5b cuts the lithological layering, migmatitic leucosomes, folia- tion and lineation in the orthogneiss, 895-70. Zircons from 895-67 Fig. 6c are haematite- stained, doubly terminated squat prisms with as- pect ratios between 2 and 3. Most have CL-dark rims which overgrow cores with finer-scale id- iomorphic zoning under CL. Eight analyses, of which seven were aimed to date the rims, all plot on or close to concordia Fig. 7f and have an average 207 Pb 206 Pb age of 1944 9 31 Ma. The core analysis is distinctive in having a higher ThU ratio 0.43 than the rims c 0.04. Three analyses from outer rims 7r2, 9r and 18r, plotted in bold on Fig. 7f are concordant and have an average 207 Pb 206 Pb age of 1920 9 7 Ma, poten- tially the best estimate of the age of the pegmatite. One core analysis 7c is concordant with a 207 Pb 206 Pb age of 2007 9 16 Ma. The remaining ‘rim’ analyses 7r, 15r and 17r2; Fig. 6c all plot be- tween these two and yield an average 207 Pb 206 Pb age of 1962 9 60 Ma, possibly because the analyt- ical spot has sampled both core and rim. Further data are required to resolve these complexities. 2 . 1 . 2 . Age of metamorphism 40 Ar 39 Ar analyses were performed at Leeds University using a modified AEI MS10 mass spec- trometer followed experimental procedures de- scribed in detail by de Jong et al. 2000. Hand-picked hornblende aliquots 0.06 and 0.1 g were irradiated in high-purity Al foil for 10 h at the Risø facility Roskilde, Denmark with a fast neutron dose of approximately 9 × 10 17 neutron cm 2 . Flux variation over the length of the canister was of the order of 5 – 6, as monitored by co-ir- radiated aliquots of mineral standards Tinto: Rex and Guise, 1986; HB3gr: Turner et al., 1971. Flux variation over the length of the canister was of the order of 5 – 6. The irradiation parameter, J, was obtained from the 40 Ar 39 Ar K of the mon- itors using a polynomial fit Dodson et al., 1996. All errors are quoted at the 1s level unless other- wise stated. Additional analytical details are given in the footnote of Table 3. 40 Ar 39 Ar ages have been obtained from three hornblende samples Figs. 2 and 10. Sample 8 95-86 Fig. 9a, from a concordant amphibolite band within the Archaean TTG gneisses of the Strelna Domain close to the contact with the Peschanoozerskaya metasediments Fig. 2, yielded a total gas age of 1900 9 3 Ma. It has a plateau age of 1904 9 3 Ma 67 of the 39 Ar, 2s that is interpreted as dating cooling following Palaeoproterozoic reworking. Sample 895-60 Fig. 10b is from the same locality as sample 895-62.1 see above within the Sergozerskaya metasediments of the Tersk Ter- rane. 895-60 yielded an age spectrum with progres- sively decreasing apparent ages pointing to excess argon incorporation. The total gas age of 1899 9 3 Ma and the 1902 9 3 Ma integrated age, excluding the strongly discordant age steps, are thus probably elevated to some degree. Variation of CaK ratio and atmospheric contamination of the two sharply discordant steps Table 3 suggest sample inhomo- geneity. However, the result provides a minimum estimate of the cooling age following amphibolite- facies metamorphism of the metasediments. Sample 895-97 Fig. 10c, from the Sergozer- skaya orthogneiss Tersk Terrane, was collected south of sample 895-70 and occurs as a concordant band, probably within the same orthogneiss unit. Hornblende from this sample yielded a total gas age of 1869 9 3 Ma and defines a plateau age six steps with 86.5 of the 39 Ar released of 1875 9 3 Ma. This result is consistent with the time constraint provided by the post-D1 pegmatite and is inter- preted as a cooling age following amphibolite facies metamorphism. 2 . 1 . 3 . Crustal residence ages Five new SmNd analyses are presented for the Varzuga River samples Fig. 2, Table 2 and two are available from Timmerman and Daly 1995. The two Archaean orthogneisses from the Strelna domain 895-80 and 895-59, which have been dated by ion microprobe, have depleted mantle model ages t DM , DePaolo, 1981 of 2948 and 3035 Ma, respectively. One psammite 895-90 from the Strelna Domain Peschanoozerskaya Suite, col- lected close to the confluence of the Falaley and Varzuga rivers Fig. 2, has a much younger t DM age of 2686 Ma suggesting a mixture of Palaeoproterozoic and Archaean source material and implying a Palaeoproterozoic depositional age for these metasediments. Thus both Archaean and Palaeoproterozoic materials are represented within the Strelna Domain. The juvenile character of the Tersk Terrane, suggested on the basis of SmNd data alone samples 2021 and 2066; Timmerman and Daly, 1995, is confirmed by the analysis of the c 1960 Ma old orthogneiss 895-70, which has a t DM age of 2221 Ma and an initial o Nd value of 0.9. Metased- iments from the Tersk Terrane Table 2, Fig. 2 and Timmerman and Daly, 1995 have similarly young t DM ages suggesting a mainly Palaeoproterozoic source. 2 . 2 . Lapland Granulite Terrane and Umba Granulite Terrane 2 . 2 . 1 . Crustal residence ages Eleven whole rock samples, eight metasediments and three calc-alkaline orthogneisses, from both the Finnish and Russian parts of the Lapland Granulite Terrane Table 3 have been analysed for SmNd isotopes in order to calculate their depleted mantle model or crustal residence ages and o Nd values DePaolo, 1981. The results are shown in Table 2 and in Fig. 3 and Fig. 8. SmNd depleted mantle model ages Table 2, Fig. 3 range from 2005 to 2355 Ma for or- thogneisses, including data recalculated from Bernard-Griffiths et al. 1984. The paragneisses yielded t DM ages in the range 2185 – 2557 Ma Table 2. This range narrows significantly to 2185 – 2355 Ma when the sample with the highest model age which also has the highest SmNd ratio is ex- cluded. The results suggest a predominantly Palaeoproterozoic provenance for the metasedi- ments and a similarly young source for the or- thogneisses. This is in marked contrast to the late Archaean signatures Fig. 7 from the surrounding terranes and clearly demonstrates that the pro- toliths of the Lapland Granulite Belt must be younger than late Archaean. SmNd analyses Table 2, Fig. 8 are available for six metasediments from the UGT and for nine samples from the intrusive Umba Complex. Sample locations are shown in Fig. 4. Metasediments from the UGT have t DM ages in the range 2123 – 2454 Ma. The intrusive Umba Complex has similar model ages suggesting that similar material makes up its source or that the complex is heavily contam- inated by the metasediments. However, some sam- Table 2 SmNd whole-rock and mineral data Sm Nd Sample 147 Sm 144 Nd Description 143 Nd 144 Nd t DM 1 Lapland Granulite Terrane 8.44 50.76 G14A 0.1005 Paragneiss 0.511415 9 7 3 2185 0.511406 9 10 3 2.39 18.31 0.0789 G21A 0.511036 9 8 3 Paragneiss L 0.511026 9 8 3 Garnet 7.05 9.17 0.4652 0.515789 9 8 3 0.515771 9 7 3 0.85 9.43 0.0545 Feldspar 0.510745 9 8 3 Paragneiss M G21R 3.87 20.12 0.1164 0.511507 9 7 3 2403 0.511495 9 7 3 6.07 7.06 0.5203 Garnet 0.516478 9 8 3 0.516478 9 8 3 3.55 20.98 0.1023 Ya49 0.511383 9 8 3 Paragneiss 2265 4.01 31.49 0.0771 Paragneiss 0.510969 9 16 Ya63 2345 Paragneiss LN124 5.15 22.73 0.1369 0.511924 9 12 2215 3.46 15.96 0.1310 0.511813 9 16 LN126 2262 Paragneiss 4.14 17.43 0.1436 Paragneiss 0.511870 9 10 S-57 2557 Granodiorite Ya42 5.21 24.18 0.1302 0.511823 9 12 2220 4.36 24.93 0.1057 0.511371 9 8 3 G81 2355 Diorite 0.511365 9 9 3 L162.4 7.02 Diorite 32.02 0.1325 0.511877 9 12 2182 Umba Granulite Terrane 1.79 13.98 0.0772 0.511034 9 14 2236 DB95-16 Garnet quartzite 4.70 28.52 0.0995 Garnet quartzite 0.511256 9 12 101068 2380 Psammite 992-30 2.67 15.84 0.1018 0.511329 9 10 2329 4.75 27.14 0.1059 0.511305 9 16 992-32 2454 Psammite 4.34 23.37 0.1123 Psammite 0.511428 9 16 992-36 2425 993-62 3.89 Psammite 19.17 0.1225 0.511768 9 18 2123 Umba Complex 6.81 993-63 35.27 Porph. granite 0.1167 0.511679 9 16 2136 5.77 33.16 0.1051 Enderbite 0.511652 9 14 7767 1943 Charnockite 8067 10.20 58.37 0.1056 0.511675 9 14 1920 3.15 18.36 0.1036 101246 0.511491 9 10 Enderbite 2141 6.61 34.69 0.1152 Charnockite 0.511625 9 12 107171 2187 Charnockite 107172 6.72 32.67 0.1243 0.511742 9 14 2212 5.69 26.78 0.1284 0.511725 9 12 895-99 2352 Granite 12.28 68.13 0.1089 Granite 0.511491 9 18 895-100 2251 895-101 8.62 Megacrystic granite 36.27 0.1436 0.511896 9 10 2497 Strelna Domain 1.81 17.03 0.0641 895-59 0.510152 9 18 felsic gneiss 2948 1.86 12.61 0.0891 felsic gneiss 0.510571 9 10 895-80 3035 895-90 2.48 Psammite 12.94 0.1157 0.511320 9 14 2686 Tersk Terrane Biotite schist 895-65 5.13 26.97 0.1149 0.511595 9 12 2229 5.59 25.41 0.1330 Felsic gneiss 0.511864 9 12 895-70 2221 Metagreywacke 2021 2 4.78 25.60 0.1130 0.511565 9 18 2231 5.37 23.21 2066 2 0.1399 Metadacite 0.511993 9 12 2162 1 SmNd depleted mantle model age Ma after DePaolo 1981. 2 From Timmerman and Daly 1995. 3 Analysed at Department of Geological Sciences, University of Michigan, Ann Arbor following methods described by Mezger et al. 1992; other samples analysed at University College Dublin following methods described by Menuge 1988 as modified by Menuge and Daly 1990. All 143 Nd 144 Nd ratios have been corrected to a value of 0.511847 9 5 for the La Jolla standard. Age calculations were made using 2s errors of 0.1 UCD data or 0.15 Michigan analyses in 147 Sm 144 Nd and 0.002 in 143 Nd 144 Nd. L, leucosome; M, mesosome in migmatite, G21; porph., porphyritic. Table 3 40 Ar 39 Ar analytical data of hornblende separates 1 Temperature 39 Ar K 37 Ar Ca 38 Ar Cl CaK 40 Ar 39 Ar K 40 Ar Atm 39 Ar K Age error 1s Vol 10 − 9 cm 3 STP °C Ma J-value: 0.004910 9 0.2 KAr age: 1808954 Ma 2 Weight g: 0.09899 K: 0.230 2 895-60 0.70 0.04 24.0 755 545.3 0.06 43.9 0.9 2349 79 1.36 0.01 28.1 435.8 0.10 9.3 915 1.4 2064 28 1.38 965 25.70 0.11 37.0 390.4 1.0 20.3 1931 2 1.23 980 23.01 0.10 37.2 380.3 0.8 18.1 1900 2 16.31 0.07 37.1 379.4 0.88 1.1 1000 12.9 1898 2 4.90 0.02 36.2 369.8 1020 2.0 0.27 4.0 1868 12 3.74 0.02 36.0 363.3 0.21 2.4 1040 3.0 1847 11 1.18 1080 22.23 0.10 37.4 387.1 0.7 17.4 1921 2 23.45 0.10 1200 35.0 1.33 372.6 0.8 19.6 1876 2 1.97 0.01 22.5 255.6 4.8 2.6 1467 0.18 13 1320 Plateau age: no K = 0.20 wt Total gas age: 1899 9 3 Ma Weight g: 0.07401 895-86 J-value: 0.004950 9 0.5 KAr age: 1910956 Ma 2 K: 0.487 2 1.21 0.04 35.4 433.0 0.07 37.9 700 0.5 2066 56 0.27 875 4.07 0.13 29.6 311.4 4.4 2.2 1683 11 1.13 935 16.33 1.89 28.8 371.6 0.5 9.0 1883 3 24.15 4.00 23.0 378.2 2.09 0.3 960 16.7 1903 1 22.24 3.85 22.1 975 377.9 2.00 0.3 16.1 1903 1 29.09 5.40 20.8 379.3 2.79 0.1 990 22.3 1907 1 1.49 1020 18.04 2.90 24.1 378.6 0.2 11.9 1905 2 1.15 1060 15.61 2.23 27.1 379.9 0.5 9.2 1909 3 21.33 1.96 42.3 380.6 1.00 0.1 1145 8.0 1911 2 145.91 0.90 590.0 383.6 0.5 1320 3.9 0.49 1920 6 Plateau age: 1904 9 3 Ma 2s K = 0.49 wt Total gas age: 1900 9 3 Ma Weight g: 0.06290 895-97 J-value: 0.004830 9 0.2 KAr age: 1870956 Ma 2 K: 0.982 2 785 0.09 0.38 0.03 8.4 203.1 67.3 0.4 1233 72 950 0.44 2.18 0.05 9.9 351.1 3.6 2.2 1789 6 5.36 0.14 8.4 376.5 1.26 0.7 970 6.3 1870 1 12.12 0.31 8.5 990 377.5 2.84 0.2 14.1 1873 1 16.65 0.43 8.5 378.3 3.89 0.1 1010 19.3 1875 1 4.00 1025 17.20 0.45 8.6 378.2 0.2 19.8 1875 1 1.88 1045 8.13 0.22 8.6 379.0 0.2 9.3 1877 1 8.31 0.21 8.7 377.9 1.91 0.1 1090 9.5 1874 1 13.01 0.33 8.9 378.8 1150 0.1 2.93 14.5 1876 1 2.10 0.05 9.5 372.0 0.44 0.5 1250 2.2 1855 5 0.48 1315 2.17 0.05 9.0 366.2 0.6 2.4 1837 5 Total gas age: 1869 9 3 Ma Plateau age: 1875 9 3 Ma 2s K = 0.95 wt 1 The temperature of the double-vacuum, resistance-heated furnace was monitored with a MinoltaLand™ Cyclops 52 infra-red optical pyrometer and is estimated to be accurate to 9 25°C with reproducibility of 9 5°C. 40 Ar atm , atmospheric 40 Ar; 40 Ar, radiogenic 40 Ar; 39 Ar K , 38 Ar Cl and 37 Ar Ca formed from K, Cl and Ca during neutron irradiation of the sample. All errors are quoted at the 1s level, unless otherwise stated. J-value uncertainty is included in the errors quoted on the total gas and plateau ages but the individual step ages have analytical errors only. Ages of individual steps are corrected for irradiation-induced contaminant Ar-isotopes derived from Ca and K in the sample. Correction factors used were: 36 Ar 37 Ar Ca 0.255×10 − 3 , 39 Ar 37 Ar Ca 0.67×10 − 3 and 40 Ar 39 Ar K 0.48×10 − 1 . Ages were calculated using the decay constants given by Steiger and Ja¨ger 1977. 2 Data obtained by Dave Rex and Rodney Green, Leeds University. Fig. 8. SmNd evolution diagram showing data Table 2 from the Lapland Granulite Terrane, Tersk Terrane, UGT and surrounding Archaean areas Timmerman and Daly, 1995 and this paper. Orthogneiss and granitoid samples are plotted as large circles, paragneisses as small squares. DM, depleted mantle DePaolo, 1981. Fig. 10. Ar-Ar step-heating age spectra. ples have t DM ages as young as 1920 Ma, indicating the presence of a mantle component in addition. In common with the Lapland Granulite Terrane, it appears from these data that any Archaean component in the UGT is minor. 2 . 2 . 2 . Age of metamorphism SmNd dating of garnet was attempted on sev- eral samples from the Lapland Granulite Terrane that display several petrographic varieties of gar- net. Unfortunately, some of these did not yield useful ages because the garnets did not have suffi- ciently high SmNd ratios, probably due to the presence of submicroscopic inclusions such as ap- atite and monazite. In the absence of such con- Fig. 9. SmNd isochron diagram for sample 21, located south- east of Ivalo Fig. 3 L = leucosome, R = mesosome. taminants, garnet is one of the most important target minerals for metamorphic geochronology. Firstly, it occurs as a major modal component of common rock types and thus may be texturally constrained. Secondly, in combination with other phases it can yield PT and PTt information as well as geochronological data. Recent discussion on the interpretation of isotopic ages from garnet have focussed on the SmNd system with esti- mates of the closure temperature ranging from 600 Mezger et al., 1992 to 750°C Zhou and Hensen, 1995. Daly et al. in review report SmNd ages for different petrographic varieties of garnet whose PT history has been inferred from reaction textures and determined independently using conventional thermobarometry. In one rock, the garnet SmNd age is identical to that for UPb in metamorphic zircon and 20 Ma older than a concordant UPb monazite age, indicating a high closure temperature, above about 650 – 700°C for the garnet SmNd system. In this case the SmNd system seems to be dating metamor- phic events that correlate with the petrography. This is consistent with some studies, such as those of Vance and O’Nions 1990 and Hensen and Zhou 1995, which have concluded that the SmNd system is capable of dating garnet crys- tallisation during high grade metamorphism at temperatures up to 700°C and of surviving net transfer reactions in the same rock at tempera- tures as high as 500°C. However these studies disagree with the conclusion of Mezger et al. 1992 that the garnet SmNd system was limited by closure temperature, which they argued must be as low as 600°C to be consistent with their data from the Adirondacks. SmNd mineral isotopic data Table 2 are presented for both the leucosome and mesosome from one migmatite sample 21, Fig. 3. Garnet in this rock exhibits only one petrographic variety. The leucosome sample 21A yields a garnet- whole-rock SmNd age of 187096.5 Ma. Gar- nets from the mesosome sample 21R yield an identical SmNd age garnet–WR of 187096.4 Ma. Combining these data, and including analy- ses of a feldspar separate, yields a combined SmNd isochron age of 187097 Ma MSWD= 2.0. These data provided a minimum age for garnet growth during melting and M2 metamor- phism see above. The M1 metamorphism has not been dated in this study and remains to be evaluated.

3. Discussion