The age and regional correlation of the Svecofennian Geitfjell granite, Vestranden, Norway

LEIF JOHANSSON, HANS SCHOBERG & ZOLTAN SOLYOM

Johansson, L., Schoberg, H. & Solyom, Z.: The age and regional correlation of the Svecofennian Geitfjell granite, Vestranden, Norway. Norsk Geologisk Tidsskrift. Vol. 73, pp. 133-143. Oslo 1993. ISSN 0029- 196X.

U-Pb zin::on dating of granitoids from the northermost part of the Western Gneiss Region (Vestranden) has yielded the oldest ages so far recorded in central and southem Norway. The Geitfjell granite, a coane-grained granite near Grong, central Norway yielded a U-Pb zin::on age of 1828�!� Ma, whereas a Rb-Sr determination yielded an age of 1170±62 Ma. Its initial 86grrsr is 0.7094 and e..d 18( 30 Ma) value is -0.04. Petrographically sirnilar granites occur also in large areas in the northem half of the Vestranden region. The petrography, isotope chernistry, age and chemical composition of the Geitfjell granite closely resembles that of granites of the Revsund type, which intrude marine metasedimentary rocks in the Svecofennian Bothnian Basin in north-central Sweden. A correlation of these granites is thus suggested. The occurrence of this type of granite in the northemmost part of the Western Gneiss Region suggests that this region represents the westerly continuation of the Svecofennian Bothnian Basin rather than a part of the Southwest Scandinavian orogenic province or the Transscandinavian Granite Belt. Precambrian metasedimentary rocks of Bothnian Basin affinity may thus occur in Vestranden.

L. Johansson & Z. Solyom, Ins titute of Geology, Lund University, Solvegatan 13, S-22362 Lund, Sweden; H. Schoberg, Laboratory fo r /s otope Geo/ogy, Museum of Natura/ History, Box 50007, S-10405 Stockholm, Sweden.

The Baltic Shield is composed of successively younger crustal segments from the Archean region in the north­ ATLANTIC OCEAN east to the Southwest Scandinavian orogenic region in the southwest. In western and northem Scandinavia, all crustal segments, such as the Archean Domain (ca. 2900-2600 Ma), the Svecokarelian Domain (ca. 2000- 1750 Ma) with Svecofennian subprovinces, the Trans­ scandinavian Granite Belt (ca. 1800-1650 Ma) and the Southwest Scandinavian Region (ca. 1770-1550 Ma) are partly overlain by Caledonian nappes (Fig. 1). The west­ em parts of these regions have also been more or less reworked during Caledonian tectonothermal event(s). Studies of the lithotectonic evolution of the western 6()• continuations of these regions are important for under­ standing the successive growth of the Baltic Shield and for the plate tectonic reconstruction of the Precambrian and early Paleozoic continents (i.e. the relationships between the Baltic and Laurentian shields). These studies s8• must be based on work in the basement windows in the Ca1edonide�, in the Western Gneiss Region of Nor­ way, and on studies of far-travelled Caledonian nappes � 1 - 4 that contain tectonic slices of precambrian basement rocks. �2 115 The purpose of the present paper is to discuss the age 00km of the Geitfjell granite in the northemmost part of the � 3 116 3 Western Gneiss Region, the correlation of this granite Fig . l. Major tectonostratigraphic units of the western Bal tie Shield ( modified with the similar granites in the Bothnian Basin in the after Gaål & Gorbatschev 1981). l. North Svecofennian subprovince. 2. Central Caledonian foreland and, finally, to briefly discuss the Svecofennian subprovince, including Bothnian Basin. 3. South Svecofennian setting of the northem part of the Western Gneiss Re­ subprovince. 4. Transscandinavian Granite Belt. 5. Southwest Scandinavian orogenic province. 6. Caledonides. WGR =Western Gneiss Region, gion in the framework of orogenic segments in the Baltic VR = Vestranden, GOC = Grong-Olden Culmination, BW = BOrgefjell Window, Shield. TW = Tommerås Window, MZ = Mylonite zone, PZ = Protogine zone. 134 L. Johansson et al. NORSK GEOLOGISK TIDSSKRIFT 73 (1993)

The Precambrian basement of the Scandinavian Cale­ Vestranden donides is exposed in many windows along the mountain Vestranden is, in this paper, defined as the broadly chain and in the Western Gneiss Region of Norway (Fig. triangular shaped area of the Western Gneiss Region 1). In the central Caledonides, the basement lithologies north of the Trondheim fjord, including also the coastal may be studied in the northem part of the Western areas just south of the Trondheim fjord (Fig. l). Gneiss Region (Vestranden), the Grong-Olden Culmi­ . Vestranden is dominated by granitic to granodioritic nation, The Borgefjell Window and the Tommerås Win­ gneisses. Tonalites, monzonites and mafic rocks occur dow (Figs. l and 2). In the Grong-Olden Culmination locally. Precambrian mafic dykes are common. These and in Vestranden, its western continuation, the Precam­ orthogneisses are metamorphosed and folded together brian basement crops out in near continuous exposure with supracrustal rock sequences. This folding, deforma­ over a distance of nearly 200 km across the strike of the tion and metamorphism is Late Silurian to Mid-Devo­ Caledonian orogen. With regard to lithologies, deforma­ nian in age. tion, metamorphism and age relationships this basement The age and origin of the supracrustal rocks is unclear area can be subdivided into an eastem part (Grong­ at present. Dating of the Vestranden orthogneisses north Olden Culmination) and a western part (Vestranden) of the Trondheim fjord has, until recently, been restricted (Johansson 1986). The eastem part is much less affected to a few Rb-Sr determinations (Priem et al. 1968; by the Caledonian orogeny than the western part. Råheim et al. 1979). More precise and reliable ages based on U-Pb dating of zircons were presented by Johansson ( 1986) and Schouenborg et al. ( 1991). The results of these suggest that the oldest crust in the northem part of Grong-Olden Culmination Vestranden is at !east ca. 1820 Ma old, whereas granitoid rocks in the southwestem Vestranden are significantly Radiometric dating suggests that the granites and rhyolites younger, and were formed mainly around 1650 Ma of the Grong-Olden Culmination were formed around (Tucker et al. 1987; Tucker & Krogh 1988; Johansson & 1650 Ma ago ( Stuckless et al. 1982; Wilson 1982; Stuckless Moller, in prep.). & Troeng 1984; Wilson et al. 1985). Fossen & Nissen Gorbatschev ( 1985) and Gaal & Gorbatschev ( 1987) (1991), however, presented a very well defined Rb-Sr correlated the Precambrian ciust of the entire Western whole-rock age of 1356 ± 29 Ma for the Blåfjellhatten Gneiss Region, including Vestramden, with that of the granite, a granite that is petrographically and chemically Southwest Scandinavian orogenic province (Fig. 1). The identical but isotopically distinctly different to the nearby correlation was based on geochronological and lithologi­ 1650 Ma old Olden granite. This suggests that there are cal similarities between the southem and central parts of differentgenerations of granites present within the Grong­ the Western Gneiss Region, southem Norway and south­ Olden Culmination. western Sweden. Reworking in the Grong-Olden Culmination is lim­ ited to the Caledonian Late Silurian to Early Devonian deformation and metamorphism, in contrast to the West­ em Gneiss Region, which is a polymetamorphic region Geitfjell granite with events in Gothian ( 1750- 1 550 Ma), Sveconorveg­ The Geitfjell granite occurs in the eastemmost part of ian-Grenvillian (1250-900 Ma) and Caledonian (650- Vestranden (Figs. 2 and 3). It fo rms a slice, less than 370 Ma) times (see Kullerud et al. 1986; Gaal & 500 m thick, of coarsely porphyritic, very inhomoge­ Gorbatschev 1987, and references therein). neously deformed, grey rock consisting of microcline, Based on the age, geochemistry and lithological simi­ quartz, oligoclase (An20_25) and biotite. Accessory miner­ larities, the granites and volcanic rocks of the Grong­ als are muscovite, hornblende, zircon, apatite, chlorite, Olden Culmination and the Tommerås Window are gamet, allanite, opaque minerals, and rarely diopside. generally considered to belong to the Transscandinavian Muscovite, chlorite, diopside, garnet and at !east some of Granite Belt of I-type granites and associated acidic the homblende and magnetite are secondary metamor­ volcanic rocks (Fig. 1), which extends from southem phic minerals. Flourite occurs rarely as thin coatings on Sweden to northem Norway (Gorbatschev 1985; Wilson fracture surfaces. Myrmekite is common and in some et al. 1985; Gaal & Gorbatschev 1987; Patchett et al.). Its samples string-perthitic feldspar is present. The K­ rocks were formed over a long period of time, between feldspar megacrysts reach up to 6 cm in length. Brown ca. 1800 Ma (Åberg & Persson 1984; Patchett et al. 1987; bio tite is the major Fe-Mg phase in the granite. Horn­ Mansfeld 1991) and 1650 Ma (Wilson et al. 1985; Patch­ blende, biotite and elongated quartz domains define the ett et al. 1987). Larson et al. ( 1991) subdivided the gneissic foliation. In many places, intense deformation granites of the Transscandinavian Granite Belt into two transformed the granite into augen gneiss. Sheared and age groups, a younger group comprising granites ca. almost isoclinally folded aplitic layers, thin mafic dykes 1680- 1650 Ma old and an older group of granites ca. and tonalitic xenoliths are common within the augen 1810- 1770 Ma. Granites of the younger group appear to gneiss. There are also large areas where the Geitfjell be more common along the western margin of the belt. granite is well preserved (Fig. 4). NORSK GEOLOGISK TIDSSKRIFT 73 (1993) The Svecofennian Geitf}e/1 granite, Vestranden, Norway 135

ATLANTIC OCE AN

BALTIC SEA

�z:

\, mm!ilCALEDONIAN NAPPES O 50km / I:;:::::::JWESTERN GNEISS REGION ) N ��!�!� �:�:�: TRANSSCANDINAVIAN BELT IC ROCKS f;�;;'�·:]REVSUND GRANITE ) t BOTHNIAN BASIN f:::=:j:::�=�SUPRACRUSTAL ROCKS

Fig. 2. Simplified geological map of northern central Sweden and Norway, compiled from Gavelin (1955), Greiling (1982), Troeng (1982), Lundegårdh et al. (1974), Sigmond et al. (1984) and Johansson (unpublished data). B =Borgefjell Window, K =Kolvereid, O=Olden granite, R =Riitan granite, S= Skellefteå, T= Tommerås Window, YR=Vestranden. l, RO and OS denote the approximate locations of the Ingdal granite, Roan granites and the Osen granitic dyke, respectively.

Fig. 3. Geological map of the Geitfjell area. The outcrop pattern of the western­ most part of the Geitfjell granite is based on the map by Sigmond et al. (1984). l. Granites and volcanics of the Grong-Olden Culmination. 2. Undifferentiated Precambrian granitoid gneisses. 3. Geitfjell granite. 4. Undifferentiated Caledo­ nian Nappes. 5. Thrusts. 6. Brittle faults. 7. Strike and dip of regional foliation. N= Namsen River, L= Lurudal River, R =Rognsmoklumpen, S = Sandiila River. Fig. 4. Typical coarse-grained grey Geitfjell granite. Length of the hammer is 55 cm.

Stratigraphically above the granite slice there is a fine-grained rock series. Similar, but somewhat more sequence of predominantly red and grey fine-grained reddish granitic gneisses occur beneath the Geitfjell gran­ gneisses interbanded by numerous amphibolite layers. ite slice. No tectonic or metamorphic break is found Mica schists and quartzites are found occasionally. between the Geitfjell granite and the surrounding Sheets of co arse- to medium-grained augen gneiss, metres gneisses. Close to the top of Geitfjell, the granite is partly or tens of metres thick, occur in the upper part of the migmatitic. The migmatite neosomes are medium- to 136 L. Johansson et al. NORSK GEOLOGISK TIDSSKR!Ff 73 (1993) coarse-grained with a patchy appearance. They consist granite type (Fig. 4) at Geitfjell. An extensive geochemi­ almost exclusively of feldspars and quartz, with only cal investigation of the Geitfjell granite is beyond the minor amounts of biotite, which define a weak Caledo­ scope of this paper. Details of the analytical procedures nian foliation. are described in Solyom et al. (1984). The small standard deviation seen in major and trace elements (Table l) shows that the samples were large Sampling enough to be representative of the coarse-grained gran­ ite. The molar oxide ratio of Al203fNa2O + K2 O + CaO Samples for chemical analysis and for isotopic dating (Table l) is slightly larger than 1.0, which indicates a were collected from fresh exposures along a road to the weak peraluminous character for the Geitfjell granite. top of Geitfjell. In order to satisfy representative sam­ This is also reflected by the presence of normative corun­ pling of the coarse-grained Geitfjell granite (Fig. 4), five dum (Table l). For comparison, analyses of the nearby samples of 6-7 kg each were collected for chemical anal­ Olden granite and the Revsund granite were carried out. ysis. For U-Ph zircon dating, approximately 70 kg of Granite of the Olden type together with rhyolitic por­ the coarse-grained granite were sampled. The samples for phyries occupy the major part of the Grong-Olden zircon dating and geochemical analysis were taken within Culmination east of Geitfjell (Figs. l and 2). The geo­ the same area and are petrographically identical. The chemistry and isotope geology of the Olden granite were whole-rock samples for Rb-Sr dating were collected discussed by Troeng ( 1982) and Stuckless et al. ( 1982) farther up along the same road. The maximum distance respectively. The Revsund granite occurs as large massifs between individual Rb-Sr smaple points is approxi­ east of the Caledonian Front and will be considered mately 20 m. Short petrographic descriptions of the Rb­ further below. Sr whole-rock samples are found in the Appendix. One of the samples collected for geochemical analysis was also used for the Sm-Nd whole-rock analysis. Results U-Pb dating Geochemistry Most zircons have subhedral outlines with well-devel­ It must be emphasized that the geochemistry presented oped prismatic shapes and less well-developed pyramidal here is representative only fo r the coarse-grained grey terminations (Fig. Sa). The length/width ratio is approx-

Tab/e l. Chemical composition (wt%) of the Geitfjell, Revsund and Olden granites. PERAL=the molar ratio of Al203 to (Na20 + K20 + CaO); D.l.=Sum of nonnative quartz, albite and orthoclase (differentiation index of Thomton & Tuttle 1960). x =average value: SD = standard deviation.

Geitfjell Revsund I Revsund II Olden (5 samples) ( 26 sarnples) (99 samples) (35 sarnples)

X SD X SD X SD X SD

Si02 71.48 0.22 70.75 1.02 69.89 3.36 75.2 2.20 Ti02 0.33 0.01 0.46 0.22 0.52 0.25 0.16 0.09 At, o, 14.10 0.06 14.08 0.72 14.08 0.88 13.0 0.09 Fe203 0.31 0.03 0.41 0.26 0.43 0.31 0.40 0.30 FeO 2.27 0.08 2.80 0.68 3.24 1.26 1.10 0.40 MnO 0.03 0.07 0.11 0.06 0.05 0.04 0.02 MgO 0.44 0.02 0.59 0.23 0.73 0.45 0.18 0.18 CaO 1.45 0.03 1.64 0.46 1.82 0.69 0.60 0.40 Na20 2.77 0.02 2.76 0.42 2.78 0.54 3.70 0.40 K20 5.72 0.10 4.88 0.45 4.61 0.67 5.10 0.40

Rb (ppm) 248 5 Sr (ppm) 133 2 Zr (ppm) 194 6

CIPW NORM Qz 28.2 29.7 28.8 31.9 Qrt 33.7 28.8 27.2 30.1 Alb 23.5 23.4 23.5 31.3 An 7.2 8.1 9.0 3.0 Cor 0.7 1.3 1.2 0.3 Hy 3.4 4.2 4.9 1.5 En I.l 1.5 1.8 0.5 Ilm 0.6 0.9 1.0 0.3 Mt 0.5 0.6 0.6 0.6

PERAL 1.05 1.10 1.09 1.02 D.I. 85.4 81.9 79.5 93.3 NORSK GEOLOGISK TIDSSKRIFT 73 (1993) The Svecofennian Geitf]ell granite, Vestranden, Norway 137

grain. The discordant outer shell has an idiomorphic outline and possibly represents a recrystallized part or a metamorphic growth zone of the zircon. The analytical data are shown in Table 2. The results from regression of different size-fraction combinations are summarized in Table 3. The oldest age is obtained by regression of all abraded zircon fractions. The age is 1828=�� Ma (MSWD = 2.1) with a lower intercept of

Table 3. Summary of different alternatives. Upper Lower intercept intercept Alternative n age (Ma) age (Ma) MSWD

All abraded zircon fractions 4 1828:�� 499::� 2.1 All zircon fractions except 74-106jml 8 1823:�� 487:� 1.2 All zircon fractions 9 1795:!� 421:::: 4.1 All non-abraded zircon fractions except 74-106 I1ID 4 tm:U 362:m O.l 2 All non-abraded zircon fractions 5 1758:�� m:��� 4.1

00:::> Fig. 5. a. Zircon from the Geitfjell granite. Length of scale is 100lliD· b. � - Backscatter electron image of a sectioned zircon. Note the concordance between GEITFJELL GRANITE crystal growth planes and the compositional zoning. Length of scale is l 00jml. if � 0.30 +88 Ma imately 2.5. The colour is light brown to light grey. U i =1828 -6S Backscatter electron images of polished sections of zir­ cons reveal a compositional zoning as dark and light areas in Fig. 5b. This variation is caused largely by 0.20 differences in the average atomic weight of the elements present, with light areas being relatively enriched in heavier elements, probably hafnium. A majority of the zircons imaged are compositionally zoned and in most O .lO cases zoning is concordant to crystal growth planes, probably the result of successive growth of zircon. The isotopic data do not suggest the presence of any inherited 3.06 3.12 3.18 0.00 L:-:----1----'-----JL----1----'------l-...J components. No rounded cores indicating the presence o.oo 2.00 4.00 2o1pb 1z3sv of older detritial zircons were found. In some zircons Fig. 6. U-Pb concordia plot of zircons from the Geitfjell granite. Open eir­ there is a thin outer zone of zircon that is discordant to eies= non-abraded zircon fractions; filled circles= abraded fractions.The zircon the crystallographic planes of the internal part of the fractions are numbered in accordance with Table 2. MSWD value is 2.1.

Table 2. U-Pb analytical data. Concentration lsotopic ratios Age (Ma) Fraction Sample u Pb,..d 206pb• 207pbb 208pbb 206pbb 207pbb 206pb 207pb 207pb 2 206 206 2 2 2 s 2 206 no. (lliD) (ppm) (ppm) 04pb pb pb 3•u 3>u 3 u 3>u U

l <45 1181 247 41800 0.1025 0.0342 0.21410 ±58 3.0260 ±124 1251 1414 1670 2 45-74 1163 247 62200 0.1028 0.0347 0.21729 ±50 3.0786 ±83 1268 1427 1675 2 Abraded 1134 242 32400 0.1029 0.0347 0.21802 ± 33 3.0925 ±68 1271 1431 1677 3 74-106 1102 233 7675 0.1030 0.0364 0.21608 ± 37 3.0682 ±68 1261 1425 1679 3 Abraded 1178 259 33300 0.1035 0.0364 0.22434 ± 47 3.2012 ±77 1305 1458 1688 4 106-150 1133 247 >100000 0.1033 0.0346 0.22320 ±Sl 3.1782 ±153 1299 1452 1684 4 Abraded 1115 245 64100 0.1036 0.0359 0.22406 ±50 3.2009± 78 1303 1457 1690 5 >ISO 1164 251 39000 0.1029 0.0352 0.22014 ± 77 3.1240 ±35 1283 1439 1678 5 Abraded 1369 288 37200 0.1025 O.o313 0.21586 ± 108 3.0511 ±153 1260 1420 1670

•) Corrected for blank and mass spectrometer fractionation. b) Corrected for common lead, blank and mass spectrometer fractionation. Errors are ±211in the last digits. Mass spectrometer fractionation: U, 0.10% perAMU; Pb, 0.12% perAMU. Common lead: 206Pb/'04Pb= 15:8, 207Pb/'07Pb = 15.3, 2 2 08Pbf 04Pb= 35.4 (Stacey & Kramers (1975) growth curve). 138 L. Johansson et al. NORSK GEOLOGISK TIDSSKRIFT 73 (1993)

499�l �� Ma (Fig. 6), and is considered as the best age estimate of crysta11ization of the Geitfjell granite. When all nine zircon fractions are regressed, the discordia obtained has an upper intercept age of 1795��� Ma and a 1ower intercept age of 42 1�ll� Ma. The MSWD value 0.9000 for this line is 4. 1. This s1ightly high MSWD is caused 0.8500 1arge1y by the poor fit of the non-abraded 74-106 J.lm fra ction. When the zircons of this fraction were abraded 0.8000 1770±62Ma and ana1ysed, the data obtained fitted the discordia very Init. Sr ratio= 0.7094 well. If the non-abraded 74-106 J.lm fraction is excluded 0.7500 MSWD=4.0 from the age calculation the resu1ting upper intercept age is 1823��� Ma, the 1ower intercept age is 487�:� Ma and 3.00 4.00 5.00 6.00 the MSWD va1ue is 1.2. When all the non-abraded zircon Fig. 7. Rb-Sr diagram for the Geitfjell whole-rock samples. Migmatite neosome fractions are regressed together the resu1ting upper inter­ samples are shown with filled error boxes. cept age is 1758�W Ma, and if the fraction 74- 106 J.lm is excluded, the age obtained is 1771� �� Ma, with a lower intercept age of 362: m Ma. The U and Pb contents in the Table 5. Sm-Nd analytical data for the Geitfjell granite. different zircon fra ctions are quite uniform except fo r the Sm (ppm) 9.64 abraded 150 J.lm fraction, which has a considerably Nd (ppm) 47.98 > 7 higher U and Pb content. All abraded zircon fractions, t4 Smf'"Nd 0.122296 143Ndf144Nd (measured value) 0.511740 except the > 150 J.lm fra ction are less discordant than 143Ndf144Nd (recalculated to 1830 Ma) 0.510268 non-abraded zircons of the same size fraction. eNd (1830 Ma) -0.04 There are very small differences in the U and Pb T0M (Ma)• 2092 (Ma)b 1834 contents of the non-abraded zircons and the internat TcHUR parts of the zircons (Table 2). However, the abrasion of ") Model age based on a depleted mantle source (DePaolo 1981). b) Model age the zircons results in a slightly higher upper intercept based on a chondritic mantle source. age, suggesting that there is a small age difference be­ tween the internat and outer parts of the zircons. The between the migmatitic and the non-migmatitic samples regression of the four abraded zircon fractions, yielding is seen. The seven non-migmatitic rocks yield an age of an age 1828��� Ma, is therefore the best estimate of the 1770 ± 62 Ma with an initial 87Sr/86Sr ratio of 0.7094 ± age of the Geitfjell granite. The abrasion also reduces 0.0024, MSWD = 4.0. The data from the three possible inftuence on the age calculation by recent lead migmatitic samples fail to yield a well-defined regression loss in the outer parts of the zircons. line but scatter around a line corresponding to an age of 1670 Ma (Fig. 7). The Geitfjell granite has not been dated using the Rb -Sr dating and Sm-Nd analysis Sm-Nd method, but a Sm-Nd whole-rock analysis was Among the 10 ana1ysed who1e-rock samples, three were made in order to obtain some genetic information on the collected from migmatite neosomes, the other samples source material of the granite. The Sm-Nd analytical represent different types of granite or rocks clearly re- results are shown in Table 5. 1ated to the formation of the Geitfjell granite. The Rb -Sr data are 1isted in Tab1e 4 and plotted on an isochron diagram (Fig. 7), where a clear difference Discussion Geitfje/1 granite Rb-Sr analytical data. Table 4. The 1828��� Ma upper intercept age is interpreted as the Rb" 87Rbb 87SRc intrusion age of the Geitfjell granite. Zircon dating of Sample rock type Rb" Sr" rocks from northern Vestranden has given the lower no. (ppm) Sr 86Sr 86Sr intercept ages 434 Ma, 393 Ma and 366 Ma (Schouen­ l Pegmatite 333 122 2.72 8.04 0.9183 borg et al. 1991) and in the Roan area 352 Ma, 372 Ma 2 Augen gneiss 258 106 2.43 7.17 0.8904 and 104 Ma (Johansson & Moller, in prep.). All these 3 Granite (neosome) 153 178 0.86 2.50 0.8015 4 Granite (neosome) 227 lll 2.05 6.03 0.8838 ages except the last one are within a time interval charac­ 5 Hybride rock 210 223 0.94 2.74 0.7799 terized by high grade metamorphic conditions or subse­ 6 Granite (neosome) 159 175 0.91 2.65 0.8002 quent cooling (Johansson et al. 1987; Moller 1988; 7 Coarse augen gneiss 263 104 2.53 7.45 0.9032 8 Coarse augen gneiss 245 146 1.69 4.94 0.8324 Dallmeyer et al. 1992). The 499�l �� Ma lower intercept 9 Coarse augen gneiss 261 103 2.54 7.46 0.9018 age (Table 3) most likely reftects the Caledonian distur­ l O Coarse augen gneiss 263 109 2.41 7.13 0.8876 bance of the U-Pb system. •) XRF, approximate content. b) XRF, and MS, estimated precision ±l% The age of the Geitfjell granite gneiss is so far the (2<1). C) MS, typical precision ±6.10-'. oldest documented age recorded in the Western Gneiss NORSK GEOLOGISK TIDSSKRIFT 73 (1993) The Svecofennian Geitj}ell granite, Vestranden, Norway 139

Region. For the reconsiruction of the westerly continua­ zircons and may therefore also include younger compo­ tions of different orogenic regions in the Baltic Shield an nents of the zircon rims. The regression of non-abraded important question is whether rocks of similar age also zircons from Geitfjellet yielded and age of 17 58 ::� �2 Ma occur farther south in the Western Gneiss Region or (Table 3). If the age of the Geitfjell granite only is whether they are restricted to the northernmost part of considered, it can be classified as a Late Svecofennian the region. O'Nions ( 1973) and Brueckner ( 1972) re­ granite corresponding in time, for instance, to the ported Rb-Sr ages of 1900 Ma and 1840 Ma respectively 1825 ± 5 Ma old Harno granite ( Claesson & Lundqvist for gneisses in the central part of the Western Gneiss 1990) of the Bothnian basin. The Harno granite is, Region. These age deterrninations are, however, of however, finer grained and equigranular, the eNd value is doubtful significance as they have large uncertainties and generally lower and the zircons are different in morphol­ are poorly docurnented. Kullerud et al. (1986) considers ogy compared to zircons of the Revsund granite. ages around 1750 Ma to be the maximum for gneisses The 87Srf86Sr initial ratio of the Revsund granite from the southern and central part of the Western Gneiss (0.7074 ± 0.0006) (Welin et al. 1971) is close to the initial Region. 87Sr/86Sr ratio of the Geitfjell granite (0.7094 ± 0.0024). The 1828=�� Ma age of the Geitfjell granite shows that The eNd values and depleted mantle model ages of Sve­ it is not related to the nearby granites of the Grong­ cofennian granites intruding the Bothnian Basin and of Olden Culmination or the Tommerås Window. Further­ the Geitfjell granite are summarized in Table 6. The eNd

more, the Olden granite is chernically different from the value of the Geitfjell granite is - 0.04, which is close to ·coarse-grained Geitfjell granite. the highest eNd values obtained for the Revsund granites, In contrast, the Geitfjell granite is very similar to the by Wilson et al. ( 1985), and somewhat higher than values Revsund granite, which intrudes metasedimentary and obtained by Patchett et al. ( 1987) and Claesson & volcanic rocks of the Bothnian Basin in north-central Lundqvist (1987). A rninor change (a few per cent) in the Sweden (Fig. 2). The Revsund granite massifs are very proportions of a mantle component and an old crustal large and consist of many smaller plutons, but they are component (particularly if the crust is Archean) in the petrographically and geochernically rather homogeneous source of the granite may lead to a significant change in over large areas. The type Revsund granite is a grey, the eNd value of the granite. Results from several investi­ coarse-grained porphyritic rock with K-felspar mega­ gations suggest that northern Scandinavia during Mid­ crysts up to 6-7 cm in size. Equigranular and reddish Proterozoic time was underlain by a depleted mantle varieties occur locally. The granite is locally enriched in (Wilson et al. 1985; Huhma 1986; Claesson 1987; Patch­ Å U, W, Sn and Mo (Gavelin 1955; Wilson & kerblom ett et al. 1987). The negative eNd value suggests that the 1982; Wilson et al. 1985). Claesson & Lundqvist ( 1987, Geitfjell magma was derived from a source that bad a 1990) subdivided the granites within the Bothnian Basin long residence time within the crust. This is also sup­ into early, late, and post-orogenic granites. They consider ported by the comparatively high initial Sr ratio. the Revsund granite to be post-orogenic in relation to the Occurrence of coarse-grained grey granite of the Geit­ Svecofennian orogeny. Geochernistry and oxygen isotope fjell type also has been reported farther west and north­ data indicate a major pelitic component in the source of west in the Vestranden area. These granite bodies are the granite (Wilson et al. 1985). Claesson & Lundqvist probab1y much 1arger than the Geitfjell granite (A. Solli, ( 1990) stated that, based on geochernical and isotopic pers. cornrn., Namsos map sheet; L. Johansson, unpub­ data, it is not very likely that the Revsund granite fo rrned lished results). by melting of the surrounding greywackes - an opinion Farther north, in the Kolvereid area (Fig. 2), zircons proposed by Gavelin (1955) and Svensson (1970). from tonalitic gneiss have yielded an age of 1819 ± 6 Ma, The geochemistry of different Revsund plutons has with a lower intercept age of 393 ± 35 Ma (Schouenborg been studied by Svensson ( 1970, 1979), Einarsson ( 1978) and Persson (1978). The Revsund I group (Table l) Table 6. �d values and depleted mantle model ages of the Geitfjell granite and includes 26 selected analyses of grey coarse-grained por­ granitesin the Bothnian Basin. phyritic granite, with Si02 contents close to the average Sam ple a ToM (Ma)b Reference of the Geitfjell granite. The average composition of the 'Nd Geitfjell granite is within the range of the Revsund Geitljell -0.04 2170 This paper average, plus or minus the standard deviation. The only Storuman 1336 -0.40 2090 Wilson et al. 1985 exception is K 0, which is somewhat higher in the Storuman 1347 +0.29 2030 Wilson et al. 1985 2 Storuman 1368 +0.27 2080 Wilson et 1985 Geitfjell samples. The Revsund Il group comprises 99 al. Joran 1395 -0.50 2090 Wilson et al. 1985 analyses of grey Revsund granites from the entire out­ Grundfors 1138 -0.20 2150 Wilson et al. 1985 crop area in the Bothnian Basin. SOrbygden SW 49 -2.02 2300 Patchett et al. 1987 Gallo SW 50 -2.42 2280 Patchett et al. 1987 The ages of some Revsund granite massifs have been Pilgrimstad SW 51 -1.43 2180 Patchett et al. 1987 deterrnined by the U-Pb zircon method. The granites Hammerdal SW 54 -2.78 2310 Patchett et al. 1987 fo rrned in the interval 1770-1800 Ma ago (Wilson et al. Stromsund SW 55 -1.10 2230 Patchett et al. 1987 1985; Patchett et al. 1987; Claesson & Lundqvist 1990). ")At the age of intrusion ( see original papers). b)T DM= depleted mantle model These datings are in most cases based on non-abraded age ( DePaolo 1981 ). 140 L. Johansson et al. NORSK GEOLOGISK TIDSSKRIFT 73 (1993)

et al. 1991), whereas three Rb-Sr datings of basement 1500-1400 Ma, 1250 -900 rna (Sveconorvegian-Grenvil­ gneisses (Priem et al. 1968) gave ages in the interval lian orogeny) and 600-370 Ma (Caledonian orogeny) 1763-1864 Ma (recalculated for aRb decay constant of (Gaål & Gorbatschev 1987). There is no evidence that 1.42 w-ll year-1). the northern Vestranden region has been influenced by The correlation between Geitfjell and Revsund gran­ any orogenic event between 1600 Ma and 700 Ma. A ites is also supported by the similarity between the large part of the Vestranden region north of the Trond­ granites in the Borgefjall Window and the Revsund heim fjord is made up of granitoids > 1800 Ma old, granites (Kautsky 1948; Zachrisson 1969; Gustavsson similar to those of the central Svecofennian subprovince, 1973; Greiling 1974, 1982). The Borgefjell Window (Fig. intruded by y ounger granites ca. 1650 Ma old. Available 2) is situated approximately halfway between the Rev­ age data on the younger granites suggest a very short sund province in Vasterbotten and Vestranden. The win­ and intense period of granite formation at around dow is dominated by coarse porphyritic granite of the 1650 Ma. Revsund type ( Greiling 1982). There are also Precam­ Some of the ca. 1650-Ma-old intrusions in Vestranden brian rocks of supracrustal origin, which rnay correspond include rocks of monzonitic composition (Roan area), to the metasedimentary and metavolcanic rocks of the which also are typical of granitoid suites of the Trans­ Bothnian Basin farther east. Granodiorites and syenites scandinavian Granite Belt. These granites in Vestranden occur in subordinate amounts. could either betong to the younger group, defined by The Rb-Sr isotope system in the Geitfjell migmatite Larson et al. ( 1991), of granites of the Transscandina­ samples is clearly disturbed. The scatter around the vian Granite Belt, or be easterly located ca. 1650-Ma-old 1670-Ma reference line (Fig. 7) may be explained if the granites of the Southwest Scandinavian orogenic prov­ entire Vestranden-Grong-Olden region is considered. ince. The choice between these two alternatives is not Numerous granites intruded the area in the time interval easy because the relationships between these two regions, 1650 ± 30 Ma. The Olden granite (ca. 1650 Ma; Stuck­ especi'ally with respect to the age of crust formation, are less et al. 1982; Stuckless & Troeng 1984), the lngdal a matter of debate and far from clear. granite ( 1664 Ma; Tucker & Krogh 1988), the Roan Rocks that have been derived from areas west of granitoids (ca. 1650 Ma; Johansson & Molter, in prep.) Vestranden and which now occur in the far-travelled and granite dykes in the Osen region (Fig. 2) (ca. 1630; Seve Nappes of the Caledonian Upper Allochthon origi­ Schouenborg et al. 1991) were all intruded around nated from a source area dominated by rocks with ages 1650 Ma in the Grong-Olden and Vestranden areas. in the range 1730-1400 Ma, with a minor component at . Also, within the allochthonous nappes in the central !east as young as 1000 Ma (Williams & Claesson 1987). Caledonides, there occur granites broadly coeval with These data indicate the presence of a terrane typical of these ca. 1650-Ma-old granites (ca. 1685 Ma: Tannas the Southwest Scandinavian orogenic province along the Augen gneiss, Claesson 1980). The formation of mig­ Atlantic margin of the Baltic Shield. This a1so fits well matites at Geitfjell may be caused by increased temper­ with the evidence of Sveconorvegian metamorphism and atures in the crust during the intrusion of the granites. sedimentation on the Lofoten islands (Griffin et al. 1978) In conclusion, the Geitfjell granite is a slightly older and magmatism and metamorphism in the Bergen Are counterpart to the late to post-Svecofennian Revsund (Cohen et al. 1988). granites intruding the Bothnian Basin in north-central In terms of the traditional subdivision of the Precam­ Sweden. brian of the Baltic Shield the Vestranden area north of the Trondheim fjord is best described as a composite region built up of Svecofennian granitoids > 1800 Ma old and intruded by granitoids ca. 1650 Ma old of prob­ Vestranden able Transscandinavian Granite Belt affinity, which The recognition of rocks of Svecofennian affinity in finally were subject to partial reworking during the Cale­ Vestranden supports the common opinion that Ves­ donian orogenic event. This scenario is different from tranden crystalline rocks belong to the Baltic Shield and earlier models in that it considers parts of the northem­ not to an 'exotic' crustal block accreted to the shield most Vestranden area as a westerly continuation of the during the Caledonian continent-continent collision as Svecofennian orogenic domain rather than as the indicated in tectonic models by Mykkeltveit et al. ( 1980) northermost part of the Southwest Scandinavian oro­ and Gilotti & Hull ( 1993). genic province (Gorbatschev 1985, Gaål & Gorbatschev The Vestranden area has generally been considered as 1987) or a continuation of the post-Svecofennian Transs­ a part of the Southwest Scandinavian orogenic province candinavian Granite Belt (Tucker et al. 1991). (Fig. l) (Gaål & Gorbatschev 1987) or as a continuation There is a possibility that not only the granites of the of the Transscandinavian Granite Belt (Tucker et al. Bothnian Basin but also greywackes, pelites, pillow Javas, 1991) The Southwest Scandinavian orogenic province is metavolcanites and other Svecofennian supracrustal characterized by major crustal formation in the interval rocks may occur in Vestranden. Rocks of Bothnian 1750-1550 Ma, and subsequent repeated deformation, affinity may also have been tectonically incorporated into metamorphism and igneous activity in the time intervals the Caledonian nappes that were derived from the outer NORSK GEOLOGISK TIDSSKRIIT 73 (1993) The Svecofennian Geitf}el/ granite, Vestranden, Norway 141

parts of the Baltoscandian margin. These rocks would DePaolo, D. J. 1981: Neodymium isotopes in the Colorado Front Range and then be 'hidden' in the numerous 'cover' sequences that crust-mantle evolution in the Proterozoic. Nature 291, 193-196. Einarsson, O. 1978: Stratigrafisk och petrografisk undersåkning av den prekam­ occur in many places in the Vestranden area, their true briska berggruden i Doubblonområdet Vasterbottens llin. Sveriges Geo/ogiska nature being obscured by Caledonian metamorphism and Undersokning C748, 123 pp. deformation. Since the Bothnian Basin may represent an Fossen, H. & Nissen, A. L. 1991: Rb-Sr age of the Blåfjellhatten granite in the Olden Window, Central Norway. Norges geologiske undersøkelse Bulletin 42 0, outer are accretionary prism environment (Wilson et al. 51-56. 1985), the rocks may, even if they are completely petro­ Gavelin, S. 1955: Beskrivning till berggrundskarta over Vlisterbottens !lin. genetically unrelated, share many petrological features Urberget inom Vasterbottens llin. Sveriges Geo/ogiska Undersokning Ca 37, 1-99. with the rocks of the much younger Caledonian out­ Gorbatschev, R. 1985: Precambrian basement of the Scandinavian Caledonides. board terranes. It would then be very difficult to distin­ In Gee, D. G. & B. A. Sturt (eds.): The Ca/edonide Orogen Scandinavia and guish between rocks from the two different sources, Related Areas, 197-212. Wiley, Chichester. Gaål, G. & Gorbatschcv, R. 1987: An outline of the Precambrian evolution of the particularly in their present deformed and metamor­ Baltic Shield. Precambrian Research 35, 15-52. phosed state. Gilotti, J. A. & Hull, J. M. 1993: Kinematic stratification in the hinterland of the central Scandinavian Caledonides. Journal of Structura/ Geo/ogy 15, 629-646. In conclusion, there are petrographical, geochemical, Greiling, R. 1974: Das kristalline, prakambrische Grundgebirge im ostlichen Teil isotopic and geochronological similarities between the des BOrgefjeli-Fensters (Zentrale Kaledoniden Skandinaviens). Geo/ogiska Geitfjell and Revsund granites. Based on these facts, on Foreningens i Stockholm Forhand/ingar 96. 247-251. the occurrence of other Svecofennian granitoids in Ves­ Greiling, R. 1982: Precambrain basement complcxes in the north-central Scandi­ navian Caledonides and their pre-Caledonian tectonic evolution. Geologische tranden and the absence of ages and orogenic events Rundschau 71, 85-93. typical for the Southwest Scandinavian orogenic prov­ Griffin, W. L., Taylor, P. N., Hakkien, J. W., Heier, K. S., Iden, l. K., Krogh, E. J., Malm, 0., Olsen, K. 1., Ormaasen, D. E. & Tveten, E. 1978: Archean and ince, it is suggested that the northern and eastern part of Prterozoic evolution in Lofoten-Vesterålen, N. Norway. Journal of Geological Vestranden, north of the Trondheim fjord, is a western Society, London 135, 629-647. continuation of the Svecofennian subprovince of central Gustavsson, M. 1973: Description of the geological map Borgefjell l: l 00 000. Norges geologiske undersøkelse 298, 1-43. Sweden, intruded by younger granites probably belong­ Huhma, H. 1986: Sm-Nd, U-Pb and Pb-Pb isotopic evidence for the origin of ing to the youngest phase of granite formation within the the Early Proterozoic Svecokarclian crust in Finland. Geological Survey of Transscandinavian Granite Belt. Finland Bulletin 337, 52 pp. Johansson, L. 1986: The relationship between the Grong-Olden and the Ves­ tranden basement regions, central Norway. In Basement and cover re/ationships Acknowledgements. - This paperis a reworked and extendedversion of part of a in the Vestranden -Grong-O/den region, central Scandinavian Caledonides: PhD thesis ( 1986; U). Professors Ro land Gobatschev (Lund) and Allan Krill petro/ogy, age-relationships, structures and regional correlation. Unpublished (Trondheim), and Drs. Oystein Nordgulen (Trondheim) and Robert D. Tucker PhD thesis, University of Lund, 142 pp. (Toronto) made many valuable suggestions which improved the manuscript Johansson, L., Lindqvist, J.-E. & Moller, C. 1987: High thermal gradients during considerably. The use of the Sm-Nd analytical facilities at the Mineralogicai­ retrograde mctamorphism in the central Scandinavian Caledonidcs. Geo/ogiska Geological museum in Oslo is gratefully acknowledged. Foreningens i Stockholm Forhand/ingar 109, 347-350. Kautsky, G. 1948: Diskussionsbidrag. Geo/ogiska Foreningens i Stockholm Manuscript received October 1991 Forhand/ingar 70, 501. 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Persson, L. 1978: The Revsund-Siirvikgranites in the Westernparts of the province Precambrian rocks of Vasterbotten county Sweden. Sveriges Geo/ogiska Un­ of Ångennanland centralSweden. Sveriges Geo/ogiska UndersokningSerie C 741, dersokning C 764, 19 pp. 59 pp. Thornton, C. P. & Tuttle, O. F. 1960: Chemistry of igneous rocks: pt. I, Priem, H. N. A., Boelrijk. N. A. l. M., Verschure, R. H., Hebeda, E. H. differentiation index. American Journal of Science 258, 664-684. & Verdunnen, E. A. Tb. 1968: Second Progress Report on the lsotopic Dating Troeng, B. 1982: Uranium-rich granites in the Olden Window, Sweden. Miner­ Project in Norway. Z. W. O. Laboratory for Isotope Geology,Amsterdam, alogica/ Magazine 46, 217-226. 42 pp. T.ucker, R. D. & Krogh, T. E. 1988: Geochemical investigation of the Ingdal Provost, A. 1990: An improved diagram for isochron data. Chemical Geo/ogy Granite Gneiss and discordant pegrnatites Western Gneiss Region, Norway. (lsotope Geoscience section) 80, 85-99. Norsk Geologisk Tidsskrift 68, 201-210. Råheim, A., Gale, G. H. & Roberts, D. 1979: Rb-Sr ages of basement gneisses Tucker, R. D., Råheim, A., Krogh, T. E. & Corfu, F. 1987: Uranium-Lead and supracrustal rocks of the Grong area, Nord-Trondelag, Norway. Norges zircon and titanite age from the northern portion of the Western Gneiss geologiske undersøkelse 354, 131-142. Region, south-central Norway. Earth and P/anetary Science Letters 81, 203- Schouenborg, B. E., Johansson, L. & Gorbatschev, R. 1991: U-Pb zircon ages of 211. basement gneisses and discordant felsic dykes from Vestranden, westemmost Tucker, R. D., Krogh, T. E. & Råheim, A. 1991: Proterozoic evolution and Baltic Shield and central Norwegian Caledonides. Geo/ogische Rundschau 80, agc-province boundaries in the central part of the Western Gneiss Region, 121-134. Norway: results of U-Pb da ting of accessory tninerals from the Trondheims­ Sigmond, E. M. 0., Gustavsson, M. & Roberts, D., 1984: Berggrunnskart over fjord to Geiranger. In Gower, C. F., Rivers, T. & Ryan, A. B. (eds.): Norge - M. l: l million. Norges geologiske undersøkelse 13-84. Mid-Proterozoic Laurentia-Baltica, 175-196. Geo/ogica/ Association of Canada Solyom, Z., Andreasson, P-G., Johansson, l. & Hedvall, R. 1984: Petrochemistry Specia/ Paper 38. of late Proterozoic rift volcanism in Scandinavia 1: the Blekinge-Dalarna Wclin, E., Christiansson, K. & Nilsson, O. 1971: Rb-Sr radiometric ages of Dolerites (BDD) volcanism in a failed arm of Iapetus. Lund Publications in cxtrusive and intrusive rocks in northcrn Sweden. l. Sveriges Geo/ogiska Geo/ogy 23, 56 pp. Undersokning C 666, 38 pp. Stacey, J. S. & Kramers, J. D. 1975: Approximation of terrestrial lead isotope Wilson, M. R. 1982: Magma types and the tectonic evolution of the Swedish evolution by a two-stage model. Earth and Planetary Science Letters 26, Proterozoic. Geo/ogische Rundschau 71, 120-129. 207-221. Wilson, M. R. & Åkerblom, G. V. 1982: Geological setting and geochetnistry of Steiger, R. H. & Jager, E. 1977: Subcommision on Geochronology: convention on uranium-rich granites in the Proterozoic of Sweden. Mineralogical Magazine the use of decay constants in geo- and cosmochronology. Earth and Planetary 46, 233-246. Science Letters 36, 359-362. Wilson, M. R., Hamilton, P. J., Fallick A. E., Aftalion, M. & Michard, A. 1985: Stuckless, J. S. & Troeng, B. 1984: Uranium tnineralizationin response to regional Granites and early Proterozoic crustal cvolution in Sweden: cvidcnce from metamorphism at Lilljuthatten, Sweden. Econmic Geology 79, 509-528. Sm-Nd, U-Pb and O isotope systematics. Earth and Planetary Science fetters Stuckless, J. S., Troeng, B., Hedge, C. E., Nkomo, l. T. & Simmons, K. R. 1982: 72, 376-388. Age of uranium tnineralization at Lilljuthatten in Sweden, and constraints on Williams, l. S. & Clacsson, S. 1987: Isotopic evidcnce for the Precambrian ore genesis. Sveriges Geo/ogiska Undersokning C 798, 49 pp. provenance and Caledonian metamorphism of high grade paragneisses from Svensson, U. 1970: Geochemical investigation of the principal Precambrian rocks the Seve Nappes,Scandinavian Caledonides. Il. Ion tnicroprobezircon U-Th­ of Vasterbotten county Sweden. Sveriges Geologiska Undersokning C 652, 73 Pb. Contributions to Mineralogy and Petro/ogy 97, 205-217. pp. Zachrisson, E. 1969: Caledonian geology of northern Jamtland-southern Svensson, U. 1979: Geochemical investigation of minor elements of the principal Vasterbotten. Sveriges Geo/ogiska Undersokning C 644, 1-33. NORSK GEOLOGISK TIDSSKRIIT 73 (1993) The Svecofennian Geitjje/1 granite, Vestranden, Norway 143

Appendix achieved by long and gentle polish­ dating at UM 662 460. These coor­ ing in an air abrasion device, con­ dinates refer to map sheet 1823 IV Analytical procedures for structed and operated according to Grong l:50,000. radiometric dating Krogh ( 1982), with the exception U-Pb and Rb-Sr isotopic analyses that no pyrite was used. Samp/e l. Pegmatite, forming pods were performed in 1984 at the Mu­ and patches in the granite. The peg­ seum of Natural History in Stock­ Rb -Sr. - The Rb/Sr ratios and the matite pod is not intruding the holm and the sample for Sm-Md contents of these elements were de­ granite but represents a late mag­ was analysed in 1988 at Mineralo­ termined by XRF spectrometry. Af­ matic phase in the crystallization of gisk-Geologisk Museum in Oslo. ter HF-HCL0 dissolution, foll­ the granite. It is slightly foliated. 4 owed by cation-exchange purifica­ tion of Sr ( Christiansson 1982), the U-Pb. - Zircons were separated Samp/e 2. Reddish, well-foliated 87Srf86Sr ratios were measured on a from the granite sample using stan­ augen gneiss. The augens consist of MAT 261 mass spectrometer, which dard mineral separation techniques. numerous small microcline crystals. for measurements of NBS 987 gave The least magnetic fraction was fur­ Many augens are strongly elongated a mean value of 0.71025 ± 3 during ther split into size fractions from and ftattened. The average diameter the time of data collection. The data which impurities, inclusion-rich or of the least deformed augens are were corrected fo r fractionation by irregularly shaped crystals were re­ approximately 20 mm. Plagioclase, normalizing the 86Sr/88Sr ratio to moved by hand-picking. U and Pb quartz and biotite occur as inclu­ 0.1194. From these measurements were extracted from the zircons and sions in the augens. The matrix is 87Rbf86Sr ratios were calculated separated for mass spectrometric dominated by quartz, K-feldspar, with a precision of 0.6%. The Rb­ analysis fo llowing the method of plagioclase and biotite. Sr whole-rock ages and initial 87Sr/ Krogh ( 1973), with some modifica­ 86Sr ratios were calculated following tions which included electrolytic de­ Sample 3. (Neosome) White, the method of Provost ( 1990) using position of Pb ( described in Chris­ medium-grained and even-grained a decay constant of 1.42 10-11 tiansson 1982). Uranium was mea­ x granite from a migmatite neosome. year- 1 for 87Rb (Steiger & Jager sured on a MAT 261 mass spec­ The granite is only slightly foliated. 1977). Errors in Rb-Sr ages are trometer in a multicollector config­ Biotite is very subordinate. Small­ given as 2u expanded errors Clifford uration using single Re filaments garnets ( <0.5 mm) are common. (1973). and the silica gel technique. Data reduction of isotopic compositions Sample 4. (Neosome) Reddish, followed the procedure of Ludwig Sm -Nd. - A Sm-Nd whole-rock medium-grained granite with well­ ( 1980), whose method also gives analysis was made on one of the developed granitic texture and al­ calculated ages with uncertainties samples collected for geochemical most no foliation visible. The due to the analytical errors at the 2u analysis. The analytical procedure sample comes from a migmatite levet. The decay constants recom­ essentially fo llowed that described neosome. mended by the IUGS Subcommis­ by Meams ( 1986). Isotopic ratios sion on Geochronology (Steiger & for Nd were normalized to 146Nd/ Samp/e 5. Hybrid rock occurring 144Nd = 0.7219. The decay con­ Jager 1977) were used. Mass frac­ along the margin of a xenolith, stant used fo r 147Sm = 6.54 10-12 tionation corrections of 0.10%/ x partly assimilated in the granite. year- 1 . The values are calcu­ AMU for U and 0. 12%/AMU for eNd The contact zone is 10-15 cm wide. Pb Pb lated relative to CHUR, with were made. The total con­ The rock is light grey, fine-grained present-day 147Sm/144Nd = 0.1967 tamination as determined by parat­ and rich in biotite and plagioclase. lei blanks was about 2.4 ng and to and 143Nd/144Nd = 0.512638. compensate for common Pb the fol­ Iowing values were calculated based Sample 6. (Neosome) White to on a Stacey & Kramers (1975) light grey granite from a migmatite Brief descriptions of the Rb -Sr growth curve: 206Pb/204Pb = 15.8, neosome. The rock has a diffuse whole-rock samp/es zo7Pb/z04pb = 15.3, zospbj204Pb = banding with centimetre-wide hands 35.4. In order to study the homo­ All Rb-Sr samples consist of somewhat richer in biotite. Small geneity of the zircons and the possi­ quartz, K-feldspar, plagioclase and garnets ( <0.5 mm) are common. ble presence of older inherited biotite. Accessory minerals are mus­ cores, all size fractions except covite, apati te, zircon, ± allanite, Samp/es 7- 10. Coarse augen gneiss < 45 llm were abraded until, on av­ ±gamet, ± chlorite. The samples with K-feldspar megacrysts up to erage, 60% of the initial mass re­ for U-Pb dating and geochemical 6 cm across. These four samples are mained, after which a new analysis analysis were taken at UTM coordi­ almost identical to the rock sampled was performed. The abrasion was nate UM 685 478 and for Rb-Sr fo r the zircon dating.