Precambrian Ages from the Geiranger-Tafjord-Grotli Area of the Basal Gneiss Region, West Norway

Precambrian ages from the Geiranger-Tafjord-Grotli area of the Basal Gneiss Region, west Norway

HANNES K.BRUECKNER

Brueckner, H. K.: Precambrian ages from the Geiranger-Tafjord-Grotli area of the Basal Gneiss Region, west Norway. Norsk Geologisk Tidsskrift, Vol. 59, pp. 141-153. Oslo 1979. ISSN 0029-196X.

An eclogite-bearing supracrustal complex (Vikvatn sequence) from the Geiranger-Grotli-Tafjord area, Basal Gneiss Region, west Norway givesan Rb-Sr whole-rock isochron age of 1775 ±57 m.y. A foliated granodiorite in more homogeneous gneisses (Fetvatn gneiss) gives an age of 960±lO m.y. Similar ages from other areas indicate that most of the Basal Gneiss Region originaled during the Svecofennian and Sveconorwegian orogenies. The role of the Caledonian orogeny in the evolution of the Basal Gneiss Region remains unsettled. The 960 m.y. intrusion possesses a pronounced schistosity indicating recrystallization in a stress field during the Caledonian orogeny or perhaps a late stage of the Sveconorwegian orogeny. The diminished role of the Caledonian orogeny in the formation of portions of the North Atlantic Caledonian System should modify theories for the evolution of this orogen.

H. K. Brueckner, Queens College of the City University ofNew York, Flushing, New York 11367, USA.

The eastem and southem margins of the Basal Gneiss Region contained late Precambrian Scandinavian Caledonides contain fossiliferous, and early Paleozoic geosynclinal rocks that had deformed geosynclinal sedimentary and volcanic been strongly metamorphosed and deformed rocks that indicate a late Proterozoic-early ductile into large recumbent nappes during the Paleozoic history for this orogen culminating in Caledonian orogeny. Muret (1960) and Bryhni the early Paleozoic Caledonian orogeny (Strand (1966) recognized that some relatively 1960). West and north of these marginal areas is homogeneous gneisses in the Basal Gneiss Re an extensive axial zone of highly metamor gion may have been strongly recrystallized phosed rocks that contain no fossils. Radiomet ('Caledonized') Precambrian basement. ric mineral ages from this belt range between 370 Sveconorwegian Rb-Sr whole-rock isochron and 590 m. y. indicating that the se rocks suffered ages from some of these gneisses (Brueckner et intense heating during the lower Paleozoic. The al. 1968, Brueckner 1972, Priem et al. 1973) are classical view of this zone is that it formed the consistent with this suggestion. Nevertheless, infrastructure of the Caledonian orogenic until the late 1960's, it was widely held that the system, possibly containing highly recrystallized supracrustal sequences within the Basal Gneiss Caledonian geosynclinal sediments and vol Region had a Caledonian origin. canics. Recent Rb-Sr whole-rock, U-Pb zircon and The Basal Gneiss Region of southem Norway Ar4°/Afl9 spectrum ages (Bryhni et al. 1971, (Fig. l) was believed to be a typical example of Brueckner 1972, Pidgeon & Råheim 1972, Mysen the Caledonian infrastructure (Holtedahl 1938). & Heier 1972) indicate that the supracrustal Predominantly quartzo-feldspathic gneisses are rocks within the Basal Gneiss Region may have associated with smaller quantities of schists, originated during the 1600 to 1800 m.y. quartzites, marbles, and calc-silicate rocks. Svecofennian cycle. These ages, if generally Meta-anorthosites, ultramafic lenses, and eclo applicable to the entire Basal Gneiss Region, gites are locally abundant. Regional metamor suggest that few, if any, of the meta-sedimentary phism was of medium grade, but high-grade and meta-volcanic rocks of the Basal Gneiss rocks and granuliteshave been reported from the Region originated during the Caledonian west (Lappin 1966, Bryhni et al. 1969, Carswell orogenic cycle. Furthermore, they raise the pos 1973, Råheim 1972). The region is structurally sibility that the Caledonian orogeny had a rather lower than the outer thrust sheets to the south trivial effect in what is generally considered the east and the Trondheim nappe to the east. Caledonian infrastructure. Similar results from Holtedahl (1938), Strand (1949), Muret (1960), the rest of the Scandinavian Caledonides (Sturt Gjelsvik (1951), and Hernes (1956) believed the et al. 1975, Heier & Compston 1969, Taylor 142 H. K. Brueckner NORSK GEOLOGISK TIDSSKRIFT 2 (1979)

l lq li �'l Sparagmite region

IOOkm

Fig. l. Location map of theBasal Gneiss Region and the Geiranger-Taford-Grotli area.

1975, Brueckner 1973, Wilson & Nicholson (Brueckner et al. 1968) of this area postulated a 1973) and indeed for the Scottish Caledonides Caledonian supracrustal sequence resting on top (Van Breeman et al. 1974) and Greenland of a Caledonized 1000 m.y. old 'basement'. Caledonides(see Higgins 1976 for a review) have However, the results of this study suggest that far-reaching consequences for the entire North the supracrustal sequence originated at least Atlantic Caledonian system and would signifi 1700 m.y. ago and that the area was further cantly modify the theories for its evolution. modified by the addition of granitic intrusions This paper presents new Rb-Sr whole-rock about 1000 m.y. ago. The significance of the isochron data from the Geiranger-Tafjord-Grotli Caledonian orogeny to the evolution of this part area in the center of the Basal Gneiss Region of the Basal Gneiss Region remains an unsettled (Figs. l and 2). The previous interpretation question. NORSK GEOLOGISK TIDSSKRIFf 2 (1979) Precambrian agesfrom W. Norway 143

schists, and a few amphibolites to the west (the General geology Fetvatn gneiss of Brueckner 1977). The Vikvatn The Geiranger-Tafjord-Grotli area bad been sequence and the Fetvatn gneiss correlate mapped in reconnaissance by Strand (1949), broadly with portions of the Fjordane Complex Gjelsvik (1951, 1953) and O'Hara & Mercy and the Jostedal Complex, respectively, of (1963). More detailed investigations of parts of Bryhni (1966). The contact between these two this regionhave been conducted near Tafjord by units dips approximately 30" to the east between Brueckner (1969, 1977), near Grotli by Strand Tafjord and Grotli, but appears to be more (1969), and in the regions south of Grotli by complexly refolded at Tafjord and south of Grimstad (1973). Carswell (1968) published a Grotli. Both complexes contain at !east four detailed petrologic and chemical study of the generations of major and minor folds including gamet peridotite locality at Kallskaret, near Ta two sets of east-plunging, similar, isoclinal folds fjord. The following summary draws its data (F,A and F18), a set of concentric cross folds (F2), from all these sources. and a set of east-plunging chevron and open Fig. 2 is a simplified geologic map of the folds (F3). All rocks possess a strong regional Geiranger-Tafjord-Grotli area. The region is schistosity that generally strikes north-north transected by a north-northwest trending bound west and dips to the east at about 30". They also ary between a heterogeneous sequenc:e of gneis contain a strong east-plunging linear fabric de ses, augen gneisses, schists, amphibolitic rocks, fined by oriented minerals, quartz rods, sheared metasedimentary rocks including quartzites, out mineral grains, and boudin lines. These meta-anorthosites, eclogites, and peridotites to fabrics are related to both the F lA and F18 fold the east (the Vikvatn sequence of Brueckner systems, but it is difficult to distinguish which 1977) and a more homogeneous complex of schistosity or lineation belongs to which fold granodioritic gneisses, augen gneisses, some system because of their striking parallelism

D Vihatn soquonco

X 101111ple focafity

• ectoeite

� ultraMofic roe k / quartzite Ø' anorthosite

Fig. 2. Simplified geologic map o 2 3 of the Geinplger-Tafjord-Drotli l area showing sample localities. 144 H. K. Brueckner NORSK GEOLOGISK TIDSSKRIFr 2 (1979) throughout most of the area. A crenulation lieved. The present study was initiated to test cleavage linear fabric occurs in some of the more this possibility. schistose rocks and is probably related to the F3 fold system. The rocks are in the almandine amphibolite facies. Both sillimanite and kyanite have been recorded (Brueckner 1977, Strand Sample localities 1969). The folds and related fabrics and the Two large sets of suites (suites 'G' and suites metamorphism had been considered to have 'T') were collected from the Fetvatn gneiss and formed during the Caledonian orogeny (Brueck the Vikvatn sequence, respectively. The sam ner 1969, Strand 1969). Eclogites are common ples from the Fetvatn gneiss were collected near within the Vikvatn sequence, but are highly Geiranger, well away from the contact with the retrograded to amphibolites. A garnet-peridotite Vikvatn sequence. The specimens from the Vik lens occurs in the large dunite mass in vatn sequence were collected in Øyen and Øyste Kallskaret, near Tafjord. Rødal near Tafjord, where the stratigraphy of the rocks is well known (Brueckner 1977). The sam ple localities are shown in Fig. 2. The following description summarizes the more important Previous geochronology petrographic aspects of the analyzed samples. Portions of the Geiranger-Tafjord-Grotli area have been subjected to radiometric studies. The Fetvatn gneiss immediately below the contact Fetvatn gneiss, Geiranger with the Vikvatn sequence near Tafjord (locality Three suites were collected from road outcrops A in Fig. 2) gave a K-Ar biotite age of 382 m.y. along highway 58. Suite G-l was collected east (Brueckner et al. 1968). Strand (1969) reported of Geirangerat about 200 m a.s.l. The rocks are a K-Ar ages from biotites separated from rocks of somewhat heterogeneous suite of grey, banded the Vikvatn sequence near Grotli (localities B gneisses ranging from laminated, medium and C in Fig. 2) between 382 an

Øyen augen gneiss,

.760 Tafjord (T-5)

.740

.. . 720 / •

Rb87Jsr86

Fig. J. Rb-Sr isochron diagram of the Øyen augen gneiss (Suite T-5). Data plotted as triangles are not included in the regression shown.

Rb-Sr isochron diagrams in Figs. 3 and 4. The sequence (see Fig. 4) reveal that rocks could act results are mixed, but the data clearly indicate as open systems to the migration of rubidium, that the rocks of the Vikvatn sequence are not strontium, and/or radiogenic strontium without Caledonian, as previously interpreted (Brueck showing any obvious textural or mineralogical ner 1969), but rather are they Precambrian, and signs of this behavior in thin section and hand originated at !east as long ago as the Svecofen sample. Thus, the age and initial ratio obtained nian orogenic cycle. The clearest demonstration by deleting the three scattered samples (the age of this conclusion is from Øyen augen gneisses and ratio quoted in Fig. 3) are believed to be collected near the Tafjord Kraft dam (Suite T-5). most valid. Nine samples (Fig. 3) form a roughly co-linear The inclusion of sample T -6, an anorthosite array and define a Rb-Sr whole rock isochron intercalated between the Øyen augen gneisses, age of 1793 ± 110 m.y. (1u) and an initial Sr'I7/Sr86 in the fmal regression tightens the precision of ratio'of 0.7007±0.0018 (lu). As the scatter of the data to 1733±34 m.y. and 0.7025±0. 00036, the data is greater than can be attributed to respectively. These results neither prove nor analytical procedure, the results do not define, disprove that the anorthosite is related to the senso stricto, a true isochron. The arbitrary augen gneisses or that they were formed at the deletion of the three samples that cause most of same time. However, they do suggest that the the scatter (samples T-5-d, T-5-f- and T-5-j, initial Sr'I7/Sr86 ratios of the anorthosite and plotted as triangles on Fig. 3) changes the age to gneisses were close to the same 1700 m.y. ago, 1775±57 m.y. and raises the initial ratioslightly and thus raise the possibility that these two rock to 0.7017 ± 0.0009. Close examination of the types were genetically related. three deleted samples revealed no obvious signs Samples from suite T-l (not plotted on Fig. 2) that they acted as open systems. However, the do not defme an i\ochron, but plot as scattered results fromthe other rocks suites in the Vikvatn points below a 1700 m.y. reference isochron. NORSK GEOLOGISK TIDSSKRIFT 2 (1979) Precambrian agesfrom W. Norway 147

Model ages for these samples, assuming an in other Sveconorwegian ages from the Basal itial ratio of 0.702, range from 450 to 840 m.y. Gneiss Region. However, the scatter of the data These numbers suggest that samples weighing is much greater than can be attributed to analyti one to two kilograms acted as partially opened cal techniques. Furthermore, the fine structure systems during the Sveconorwegian and/or the of the data points on the isochron plot suggests Caledonian orogenies. Some investigators be other possible 'ages'. For example, four rocks Heve that augen gneisses mark shear zones, from the T-3 suite with the weakest tectonic sometimes shear zones of great displacement. If fabrics (samples T-3-a, T-3-b, T-3-c and T-3-d) so, the open system behavior of the T-1 samples are remarkably co-linear and define a Caledo should not be surprising. However, it is im nian 'age' of 412 ±42 m.y. and an initial Sr87/S� portant to rembember that the T-5 suite, which ratio of 0.7090 ±0.0001. Most of the rest of the gives a good isochron age, is also from a zone of samples, including all the moderately to weakly augen gneisses. It is a puzzling dilemma that foliate rocks, but excluding the pegmatitic sam some augen gneisses show evidence of open ple T-4, scatter about an isochron 'age' of system behavior and others do not. approximately 1500 m.y. with a rough initial The data from suites T-2 and T-3 and from Sr87/S� ratio of 0.706. Finally, if just the sample T -4 are plotted in Fig. 4 as triangles, strongly foliate samples from suite T-3 are dots, and a square, respectively. A best fit line considered (i.e. samples T-3-e, T-3-f and T-3-g), through all the points defines an apparent 'age' then an 'age' of about 1700 m.y. and an initial of 1144 ±100 m.y. and an apparent 'initial ratio' ratio of about 0.705 will result. These 'ages', of 0.7074 ±0.0004. This 'age' coincides with based on different groupings of samples, are not

Table l. Rb-Sr analytical data for rocks of the Vikvatn sequence, Tafjord.

Sample Rock Type Rb(ppm) Sr (ppm) Rb87/Sr"" Sr"/Sr""

Rødal augen gneiss T-1-a Augen gneiss 209 379 1.60 0.7180 T-1-b Augen gneiss 310 448 2.01 0.7147 T-1-c Augen gneiss 206 389 1.54 0.7202

Rødal 'granu/ite' T-2-a Moderately foliate gneiss 46.1 418 0.320 0.7125 T-2-b Moderately foliate gneiss 45.9 418 0.319 0.7126 T-2-c Moderately foliate gneiss 46.9 434 0.313 0.7126 T-2-d Moderately foliate gneiss 44.2 429 0.298 0.7120 T-2-e Moderately foliate gneiss 41.5 429 0.280 0.7122 T-2-f Moderately foliate gneiss 43.4 449 0.280 0.7119

T-3-a 'Granu1ated' gneiss 39.9 558 0.207 0.7102 T-3-b 'Granulated' gneiss 26.5 653 0.653 0.7097 T-3-c 'Granulated' gneiss 35.7 602 0.172 0.7100 T-3-d 'Granulated' gneiss 20.5 590 0.101 .0.7096 T-3-e F olia te schist 47.3 408 0.336 0.7133 T-3-f Foliate schist 30.1 472 0.184 0.7113 T-3-g Foliate schist 31.1 480 0.189 0.7098

T4 Pegmatite 139 644 0.624 0.7171

Øyen augen gneiss T-5-a Augen gneiss 143 644 2.12 0.7553 T-5-b Se hist 70.1 254 0.800 0.7219 T-5-c Coarse augen gneiss 139 203 1.98 0.7510 T-5-d Augen gneiss 180 222 2.35 0.7642 T-5-e Schist 71.4 248 0.833 0.7214 T-5-f Schist 88.3 250 1.02 0.7232 T-5-g Sheared augen gneiss 154 245 1.81 0.7477 T-5-h Se hist 74.9 363 0.598 0.7174 T-5-i Schist, some small augen 161 223 2.10 0.7481

T-6 Anorthosite 6.80 858 0.023 0.7032 148 H. K. Brueckner NORSK GEOLOGISK TIDSSKRIFT 2 (1979)

Rød al 'gran u lite', Tafjord (T-2, T-3)

.700 0.1 0.2 0.3 Rb87Jsr86

Fig. 4. Rb-Sr isochron diagram of weakly to strongly granulated rocks from RØdal, near Tafjord. Suite T-2 plotted as triangles. Suite T-3 plotted as dots. Sample T-5 plotted as a square. 'Ages' are for reference only.

believed to date any true events. They do sug uted to analytical uncertainties. All of this gest, however, that the rocks cou1d have origi scatter is from the coarse-grained samples that nated as long ago as the Svecofennian orogenic occur as scattered clots within the generally fine cycle, and that their present scattered distribu to medium-grained granodiorite. These samples tion on a Rb-Sr isochron diagram was the result are plotted as triangles in Fig. 5. Deleting these of incomplete open system behavior during both samples increases the precision of the age and Sveconorwegian and Caledonian orogenies. It is initial Sfl7/Srss ratio to 960±10 m.y. and noteworthy that, in general, the more foliate 0.7062 ±0.0003, respectively. The scatter of samples tend to define older 'ages' whereas the these points can be attributed to experimental rocks with the granulated textures tend to define error and so the results define an isochron age in younger 'ages'. the strict sense. This age confirms the similar ages obtained from the Fetvatn gneiss in Tafjord (Brueckner et al. 1968) and in other parts of the Fetvatn gneiss, Geiranger presumably contiguous Jostedal Complex Analyses of samples from the Fetvatn gneiss (Brueckner 1972, Priem et al. 1973). The age is near Geiranger are listed in Table 2 and plotted believed to date the intrusion of the granodiorite in Figs. 5 and 6. As with the data from Tafjord, body during the Sveconorwegian orogenic cycle. many of the data points plot in a scattered The alternative explanation, that the isochron fashion. The striking exceptions to this scatter represents a re-homogenization age of rocks are from the samples of granodiorite of the G-3 older than Sveconorwegian, is not considered as suite (Fig. 5) which define an age of 953 ±41 like ly because of the striking linearity of the data m.y. (lo-) and an initial Sfl7/Srss ratio of and the relatively low initial Sfl7/Srss ratio. 0.7067 ±0.0013 (lu). The scatter of the data However, the granodiorite has suffered events points is only slightly greater than can be attrib- subsequent to its intrusion as shown by its NORSK GEOLOGISK TIDSSKRIFf 2(1979) Precambrian agesfrom W. Norway 149

Table 2. Rb-Sr analytical data for rocks of the Fetvatn gneiss, Geiranger

Sample Rock Type Rb(ppm) Sr(ppm) Rb"/Sr"" Sr"/Sr""

G-1-a Foliate granite 176 555 0.917 0.7269 G-1-b Foliate granite 151 639 0.685 0.7305 G-1-c Laminated schist 129 386 0.967 0.7268

G-2-a Granite 208 274 2.20 0.7457 G-2-b Mixed granite & schist 108 245 1.28 0.7335 G-2-c Coarse foliate granite 163 315 1.50 0.7320 G-2-d Granite 154 305 1.46 0.7351 G-2-e Microcline-rich gneiss 196 341 1.67 0.7378 G-2-f Plagioclase-rich gneiss 83.7 280 0.866 0.7296 G-2-g Schist 102 181 1.64 0.7340

G-3-a Fine foliate granodiorite 199 224 2.59 0.7406 G-3-b Fine foliate granodiorite 218 174 3.64 0.7555 G-3-c Fine foliate granodiorite 211 184 3.33 0.7516 G-3-d Fine foliate granodiorite 220 175 3.64 0.7553 G-3-e Fine foliate granodiorite 165 402 1.19 0.7223 G-3-f Fine foliate granodiorite 194 214 2.63 0.7420 G-3-g Coarse, weakly foliate 217 222 2.84 0.7441 G-3-h Mixed coarse & fine 193 229 2.45 0.7389 G-3-i Fine foliate granodiorite 196 229 2.47 0.7394 G-3-j Fine foliate granodiorite 195 229 2.47 0.7394 G-3-k Mixed coarse & fine 196 231 2.46 0.7396 G-3-1 Coarse, weakly foliate 255 374 1.98 0.7365

. 760

Fol iate granodiorite,

Geiranger (G-3)

.740

• . 720

o. 70 62 ±o.o oo3 Rbs115,ss .700 L------L----�L------L----��----�------L-----�---- 1.0 2.0 3.0

Fig. 5. Rb-Sr isochron diagramof Geiranger granodiorite.Samples plotted as triangles contain coarse grained materialand are not included in the age calculation. 150 H. K. Brueckner NORSK GEOLOGISK TIDSSKRIFT 2 (1979)

.740

• .730

.72 0

Mixed gneiss, Geiranger (G-1, G-2)

.710

.700 �------�------4------L------L------� 2.0

Fig. 6. Rb-Sr isochron diagram of mixed gneisses from near Geiranger. Suite G-1 plotted as triangles. Suite G-2 plotted as dots. The 'age' is for reference only.

foliate texture and the fact that the coarser suffered a partial redistribution of rubidium, grained material does not always plot on the strontium and strontium isotopes. isochron. The data from suites G-1 and G-2 are plotted on Fig. 6 as triangles and dots, respectively. The Discussion data scatter widely about an isochron that defines an 'age' of about 800 m.y. and an initial Only two of the five suites analyzed for this Sr87/Sfl6 ratio of approximately 0.719. These study defined meaningful isochron ages, il numbers are not believed to be meaningful. lustrating the difficultyof extracting useful infor Different groupings of the data based on com mation from metamorphic rocks. Best fit iso position and/or texture do not reveal any dis chron 'ages' fitted to scattered data points for cemible pattems. The fact that the initial Sr87/ the other three suites should be discarded as Sfl6 ratio is rather high might suggest that these meaningless, despite the fact that some of these rocks originated prior to the 800 m.y. 'age', 'ages' coincide with known thermal events. possibly during the Svecofennian orogeny. How ever, the rocks could equally well have formed Time of origin of the rocks of the during a Sveconorwegian event with a high, Geiranger-Tafjord-Grotli area 'crustal' initial Sr87/Sfl6 ratio. In any case, the scattered distribution of the data points on the Few, if any, of the rocks from the Geiranger isochrondiagram indicates that the ro�k systems Tafjord-Grotli area originated during the NORSK GEOLOGISK TIDSSKRIFr 2 (1979) Precambrian agesfrom W. Norway 151

Caledonian orogenic cycle. Carswell (1973) pro tween the Jostedal Complex and the Fjordane poses that Caledonian supracrustalsequences do Complex does not appear to be valid in the exist in the Basal Gneiss Region and are distin northern Basal Gneiss Region near guished from Precambrian supracrustal rocks by Kristiansund. Pidgeon & Råheim (1972) and lacking eclogites, meta-anorthosites, and Råheim (1977) report Rb-Sr whole-rock isochron garnetiferous ultramafic rocks. Unfortunately, ages of about 1700 m.y. from the Raudsand there is as yet no geochronological evidence to (Kristiansund), Frei, and Tingvoll Groups. The sustain this view. The dated rocks of the Vikvatn Frei Group contains abundant supracrustal sequence certainly originated during the rocks as well as eclogites (Råheim 1972). Thus Svecofennian cycle. The low SrB7/Sr86 ratio for the Frei Group and perhaps the quartzite-bearing these rocks (0.7017) does not indicate a long portions of the Tingvoll Group could be correla history prior to the Svecofennian. These conclu tive with the Fjordane Complex. But the sions are similar to those reached by Pidgeon & Raudsand Group and parts of the Tingvoll Group Råheim (1972), Mysen & Heier (1972), Råheim are relatively homogeneous granitic gneisses (1977) and others in other portions of the Basal (Råheim 1972) and resemble the Jostedal Com Gneiss Region. Further chronological work plex. If these units are correlative, as suggested should be aimed at sampling and analyzing pos by Hernes (1970), then it becomes necessary to sible Caledonian supracrustal sequences. explain the disparity in their ages. One possi bility is that the central and western Basal Gneiss Region was more severely affected by Sveconorwegian recrystallization and intrusion The Fjordane Complex versus the Jostedal than the northern Basal Gneiss Region.' Complex An alternate explanation is that the Jostedal Bryhni (1966) introduced the terms Fjordane Complex and the Raudsand Group are not the Complex and Jostedal Complex to distinguish same. Mapping between the Geiranger-Tafjord between a sequence of supracrustal rocks, some Grotli area and the Kristiansund area will be of which contain eclogites, anorthosites, man necessary to test this possibility. A major bound gerites, and garnetiferous ultramaficrocks, and a ary between these regions may mark either the more homogeneous sequence of gneisses, re northern limit of Sveconorwegian intrusions or a spectively. The distinction between these units major thrust zone subsequently healed by later has been used by some investigators as a useful (Caledonian?) recrystallization. This latter pos mapping convenience. The question remains sibility gains some support from the work of whether there is any chronological distinction Andresen et al. (1974) on the Hardanger-Ryfylke between these two complexes. Evidence from Nappe in southern Norway. They document the the central and western Basal Gneiss Region existence of a major thrust separating an upper suggests that there is a difference. All four Rb-Sr sequence of 1640 m.y. old metamorphic rocks whole rock isochron ages from the Jostedal (Kvitenut Comp1ex) from a lower sequence of Complex, which includes the Fetvatn gneiss of 1290 m.y. old metamorphic rocks (Dyrskard this study, range between 960 and 1200 m.y. Group). A similar situation may exist in the (Brueckner et al. 1968, Brueckner 1972, Priem et central and western Basal Gneiss Region, but al. 1973). On the other hand, most of the 1550 to not in the northern Basal Gneiss Region. 1900m.y. Svecofennian ages have been obtained Neither of the two explanations rule out the from rocks that can be correlated with the possibility that relicts of Svecofennian rocks Fjordane Complex, including the Vikvatn se occur within the Jostedal Complex, though these quence of this study (Bryhni et al. 1971, Brueck relicts have not yet been found. In any case, the ner 1972, Mysen & Heier 1972). Sveconorwegian abundance of ages around 1000 m.y. testifies to ages that have been determined from this com the importance of the Sve<',onorwegian event in plex (see, e.g., Bryhni et al. 1971 and Brueckner the central and western Basal Gneiss Region. 1972) can generally be considered re-equilibra The severity of this event presumably increased tion ages subsequent to the Svecofennian cycle. southward, toward the Sveconorwegian pro These results confirm the validity of dividing the vince of the Precambrian shield of southern central and western Basal Gneiss Region into Norway. two major units. However, the chronological distinction be- 152 H. K. Brueckner NORSK GEOLOGISK TIDSSKRIFf 2 (1979)

possibility. The youngest meaningful age report The role of the Caledonian orogeny in the ed is the 960 m.y. age of the granodiorite body Basal Gneiss Region near Geiranger (suite G-3). This body possesses Most of the Basal Gneiss Region as well as much a pronounced schistosity caused by the parallel of the other high-grade metamorphic terrains of alignment of biotite flakes. Since this schistosity central and northem Norway appear to have was imprinted on the rock after its solidification, formed well before the inception of the Caledo it seems likely that it developed during Caledo nian geosyncline. This fact should cause consid nian metamorphism, although it is also possible erable revisions of models, including plate that it developed during a very late phase of the tectoJiic models (see, e.g., Dewey 1969), of the Sveconorwegian orogeny. The Hestbrepiggen evolution of the Caledonian system. However, granite, which gives about the same age as the the importance of Caledonian metamorphism Geiranger granodiorite (Priem et al. 1973), also and deformation in the development of these contains a pronounced 'foliation'. The regions is still open to question. schistosity of the Geiranger granodiorite paral Strongly deformed metamorphic rocks of un lels the regional schistosity which is, in turn, disputed Caledonian origin occur to the west, parallel to the axial surface of similar, isoclinal, east, and south of the Basal Gneiss Region and it recumbent F18 folds (Brueckner 1977). These therefore seems likely that the Basal Gneiss considerations might suggest that the F18 fabric Region bad to suffer some effects as well. Fur and subsequent fabrics formed during the thermore, most Rb-Sr and K-Ar mineral ages Caledonian orogeny, testifying to the possible from the Basal Gneiss Region give Caledonian severity of this orogeny in the Basal Gneiss numbers (see Brueckner 1972 for a review). Region. Under this model, the pre-existing F1A Finally, some late muscovite-bearing pegmatites fold system could be a 'homogenized' fabric that in the northem Basal Gneiss Region give Rb-Sr was previously a rather heterogeneous assort whole rock ages of around 385 m.y. (Pidgeon & ment of older (Svecofennian and Sveconor Råheim 1972). The high initial SJ:'I7/SJ:'I6 ratios of wegian) structures, but that were severely com these intrusions (0.715) suggest formation by pressed and pervasively rotated to their present Caledonian partial fusion of deep crustal rocks. configuration during intense F18 deformation. Although these considerations argue for some Priem et al. (1973) indicate that the 975 m.y. sort of 'Caledonization' of the Basal Gneiss Hestbrepiggen granite intruded during the for Region, they do not provide unique information mation of the F2 folds of the area. If so, the F1 on its severity. Two extreme views are pre folds must have developed earlier than 975 m.y. sented below that are believed to encompass all (i.e. during the Svecofennian or early Sveconor the likely possibilities. wegian orogeny) whereas the F3 folds of the area Limit ed 'Caledonization'. - Caledonian ef must have developed later (late Sveconorwegian fects under this model were limited to a slight or Caledonian). re-heating event (ca. 300"C to reset mineral The presently available data do not appear to ages). Any deformation was limited to the de favor one over the other of the above models. velopment of open, east-west folds, some The view tentatively adopted in this paper is that boudinage (Sturt et al. 1975), and perhaps a all three of the major orogenic events in southem fracture cleavage in some rocks. Pidgeon & Norway (Svecofennian, Sveconorwegian, and Råheim (1972) and Råheim (1977) argue that the Caledonian) played a role in the origin and major metamorphism of the Basal Gneiss Region development of the rocks of· the Basal Gneiss occurred around 1700 m.y. ago. If so, a subse Region. quent weaker metamorphism would be expected to develop retrograde mineral assemblages, a feature not commonly observed. If, however, the major metamorphism occurred during the Conclusions Caledonian orogeny, retrograde assemblages Most of the rocks of a heterogeneous sequence would not be expected. of supracrustal rocks in the central part of the Severe 'Caledonization'. - This model calls Basal Gneiss Region of Norway (the Vikvatn for severe recrystallization during the Caledo sequence) originated about 1775 m.y. ago during nian Orogeny, probably accompanied by ductile the Svecofennian orogenic cycle. At least some deformation. Results from this study support the of the rocks of the underlying, more homogene- · NORSK GEOLOGISK TIDSSKRIFT 2 (1979) Precambrian agesfrom W. Norway 153

ous, Fetvatn gneiss formed later, about 960 m.y. CarsweU, D. A. 1968: Picritic magma-residual dunite relation ago during the Sveconorwegian orogenic cycle. ships in gamet peridotite at Kalskaret near Tafjord, south Norway. Contrib. Mineral. Petro/. /9, 97-124. The distinction between these two units in the CarsweU, D. A. 1973: The age and status of the Basal Gneiss Geiranger-Tafjord-Grotli area is based on solid Complex of north-west southem Norway. Nor. Geo/. chronological grounds bot may not be valid in Tidsskr. 53, 65-78. the northem Basal Gneiss Region. The Caledo Dewey, J. 1969: Evolution of the Appalachian/Caledonian Orogen. Nature 222, 124-129. nian orogenic cycle did not play a major role in Gjelsvik, T. l 95 l: Oversikt over bergartene i Sunnmøre og the formation of these rocks, bot it is still possi tilgrensende deler av Nordfjord. Nor. Geo/. Unders. 179, 45 ble that Caledonian metamorphism and deforma p. tion gave the rocks some of their present Gjelsvik, T. 1953: Det nordvestlige gneis-område i det sydlige Norge, aldersforhold og tektonisk-stratigrafisk stilling. Nor. structural and metamorphic character. Geo/. Unders. /84, 71-94. Gjelsvik, T. 1957: Geochemical and mineralogical investiga Acknowledgements. - This work was carried out at tions of titaniferous iron ores, west coast of Norway. Econ. Mineralogisk-geologisk museum in Oslo, Norway, during my Geology 52, 482-498. sabbatical leave from Queens College of the City University of Grimstad, E. 1973: En petrografisk og strukturgeologisk un New York. I wish to thank the staff of the museum for the dersøkelse av området sør for Grotli. Unpublished cand. opportunity to work here and, in particular, I wish to thank real. thesis, Universitetet i Oslo, 147 p. Toril Enger, who did much of the chemical work and X-ray Hansen, B. T. , Oberli, F. & Steiger, R. H. 1973: The geo fluorescence work, Bjørn Sundvoll, who designed and chronology of the Scoursby Sund Area. Program report 4. maintained the mass spectrometer, Ame Råheim, who helped Rb/Sr whole rock and mineral ages. Rapp. Grønn/and Geo/. collect the samples in the field, and Fredrikke Dons, who Unders. 58, 55-58. typed the manuscript. Ame Råheim, Inge Bryhni, and William Heier, K. S. & Compston, W. 1969: Interpretation of Rb-Sr L. Griffin critically read the manuscript and their suggestions pattems in high-grade metamorphic rocks, northern Nor are gratefully acknowledged. This study was partially sup way. Nor. Geo/. Tidsskr. 49, 257-283. ported by a National Science Foundation Grant (EAR-73- Hernes, I. 1956: The Surnadal Syncline, Central Norway. Nor. 00601). Geo/. Tidsskr. 36, 25-40. Hernes, l. 1970: Tafjord-Grotli området og grensen mellom det eldre Prekambriske kompleks og den Senprekambriske Eokambriske lagrekke. Nor. Geo/. Tidsskr. 50, 261-268. · References Higgens, A. K. 1976: Pre-Caledonian metamorphic complexes within the southem part of the East Greenland Caledonides. Andresen, A., Heier, K. S., Jorde, K. & Naterstad, J. 1974: A J. Geo/. Soc. London 132, 289-305. preliminary Rb/Sr geochronological study of the Hardan Holtedahl, O. 1938: Geological observations in the Opdal gervidda-Ryfylke Nappe system in the Røldal area, south Sunndal-Trollheimen district. Nor. Geo/. Tidsskr. /8, 29-53.

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