A Rb/Sr whole-rock isochron date from the lowermost gneiss complex of the area, west and its regional implications

FINN J. SKJERLIE & IAN R. PRINGLE

Skjerlie, F. J. & Pringle, I. R.: A Rb/Sr whole-rock isochron date from the lowermost gneiss complex of the Gaular area, west Norway and its regional implications. Norsk Geologisk Tidsskrift, Vol. 58, pp. 259-265. Oslo 1978. ISSN 0029-196X.

The Precambrian rocks within the Gaular area can be tectonically divided into two major rock units; a lower complex (Jostedal Complex), which largely consists of migmatites and gneiss-granites, and an overlying largely metasupracrustal sequence. The latter unit, which may be subdivided into the Holsen Gneiss (mainly quartz-rich paragneisses) and the Vevring Complex (mainly amphibolites, paragneisses, and eclogitic rocks), has been interpreted as a thurst sheet, the nappe. The Jostedal Complex yields a metamorphic Rb/Sr whole-rock isochron date of 1625 ±75 m.y. An initial 81Rb/'"Sr ratio of O. 701 ± 0.002 excludes an extended c rustal history prior to the major metamorphism. Two biotite/whole-rock ages of 392 ± 15 m.y. and 396 ± 15 m.y. provide evidence for a Caledonian influence in the area. It is assumed that the emplacement of the Sunnfjord nappe took place late in the Sveconorwegian (Grenvillian) orogeny.

F. J. Skjerlie, Geologisk institutt, avd. A, Allegt. 41, 50/4 Bergen-Univ., Norway. l. R. Pringle, Division de lnvestigaciones, Ministerio de Minas, Centro Simon Bo/ivar, Torre Norte P. 30, /9 Caracas, Venezuela.

The age and status of the Gneiss Region of west standing of the chronology and the metamorphic Norway has been a matter of debate for many development of the Gneiss Region can only be years. Irgens & Hiortdahl (1864), Reusch (1881), obtained by applying a number of modem and Kolderup (1928) regarded the gneisses as radiometric dating techniques in combination forming a Precambrian basement. to the overly­ with known geological relationships. Several im­ ing Lower Palaeozoic sequence. Holtedahl portant contributions have already been made (1936, 1938, 1944) re-interpreted the gneisses as towards this end (Broch 1964, McDougall & having been formed by Caledonian high-grade Green 1964, Brueckner et al. 1968, Priem 1967, metamorphism, migmatization, and granitization Priem 1968, Priem et al. 1970, Bryhni et al. 1971, of both Lower Palaeozoic metasupracrustals and Brueckner 1972, Pidgeon & Råheim 1972, and Precambrian rocks. This interpretation was sup­ Sturt et al. 1975). ported by, amongst others, Kolderup (1952) and To date, however, the only Rb/Sr whole-rock Strand (1960). and mineral isochrons obtained from rocks in the Bryhni (1966), however, recognized the ex­ Gaular area are those from the Holsen Gneiss istence of a reworked pre-Eocambrian basement and associated pegmatites on the north shore of complex of relatively homogeneous gneisses - Holsavatn (Brueckner 1972). In this paper we the Jostedal Complex -overlain �y a heterogen­ add one new Rb/Sr whole-rock isochron age and ous complex -the Fjordane Complex -compris­ two Rb/Sr biotite/ whole-rock ages for the low­ ing Cambro-Silurian as well as older rocks cor­ ermost gneisses in the Gaular area. responding to the Jotun nappe. Also ·Skjerlie (1969) and Strand (1969) divided the gneisses in Askvoll-Gaular and Grotli areas respectively Geological setting into a Precambrian basement (identical with Bryhni's Jostedal Complex) and an overlying Skjerlie (1969) described the pre-Devonian rocks sequence of postulated Late Precambrian to in the Gaular area as Precambrian basal gne!sses Ordovician age. . overlain by a metasupracrustal sequence of pos­ It is quite obvious that a satisfactory under- tulated Late Precambrian to Ordovician age. In 260 F. J. Skjerlie & l. R. Pringle NORSK GEOLOGISK TIDSSKRIFT 4 (1978) the latter, the Holsen Group was identifiedas the the metasupracrustal sequence (Holsen Gneiss oldest unit, and this is succeeded by the Askvoll, and Vevring Complex) has been interpreted as a Stavenes and Lower Herland Groups. thrust sheet, the Sunnfjord nappe. Furnes et al. (1976), however, divided the The Gaular area has undergone at least three metasupracrustal rocks into two principal units phases of folding of presumed Caledonian age based on a pronounced change in metamorphic (S kjerlie 1969). Fold axes strike ES E-WNW grade and lithology across major faults between (oldest), NE-SW, and E-W (youngest). It ap­ the lower and upper parts of the former Askvoll pears (Fig. l) that the Vevring Complex occurs Group, suggesting quite different age relations. in separate eastern and western areas. Between The metamorphic grade of the upper unit does these two the Holsen Gneiss forms the core of a not exceed greenschist facies. This unit was recumbent anticline with an axial p1ane dipping assigned to the Askvoll Group and assumed to gently south-east. The formation of this anticlin­ be of Lower Palaeozoic age. The rocks of the al structure was produced by the second phase lower unit, however, preserve almandine­ of folding (NE-SW). amphibolite facies assemblages, and frequently contain inclusions of eclogitic rocks. This unit comprises the former Holsen Group and the Sampling higher grade metamorphic rocks of the former Askvoll Group. In the present account they are A number of samples were collected in fresh retermed the Holsen Gneiss and the Vevring roadcuts. Particular emphasis was laid on the Complex, respectively. Radiometric age-dates in Jostedal Complex and samples were taken from the Holsen area (Brueckner 1972), and in the gneissic rocks as well as aplites and palingenetic Nordfjord district farther towards the northwest granites. Samples were also collected from (Bryhni et al. 1971), have clearly proven a Pre­ granitic rocks which intrude the Vevring Com­ cambrian age for both the Holsen Gneiss and the plex. Unfortunately, however, both the aplites Vevring Complex. and the granites associated with the Jostedal The basal gneisses of Skjerlie (1969) un­ Complex as well as the granites intruding the doubtedly correspond to the Jostedal Complex Vevring Complex have very low and uniform as defined by Bryhni (1966), and the present Rb/Sr ratios. authors have, therefore, found it adequate to use A whole-rock Rb/Sr isochron date could, this term for the lowermost gneisses also in the however, be obtained on gneissic rocks from the Gaular area. area WNW of Hæstadfjorden (Fig. l). As pointed out by Bryhni (1966), the Jostedal Complex forms a large culmination and repre­ sents the deepest exposed level in the central Analytical techniques Caledonides. The complex consists of migma­ tites, granitic gneisses, and augen gneisses. The samples were analysed using standard Palingenetic granites, in places with originally isotope dilution techniques on an Atlas CH4 gneissic structures, also occur. The whole com­ mass spectrometer equipped with a digital out­ plex is frequently cut by pegmatites. put. 84S r spike was used for all strontium The rather monotonous Jostedal Complex is analyses and the data normalized to a 86Sr/88Sr overlain by the Holsen Gneiss which consists ratio of 0.1194. 87Rb spike was u sed for rubidium mainly of quartz-rich paragneisses, quartz analyses. The experimental uncertainties are schists, augen gneisses, and amphibolites. Peg­ estimated at 1.5% and O.l% for the 87Rb/86Sr and matites occur locally, and especially within the 87Sr/86Sr_ ratios respectively. The isochron ages Holsen district. were computed using York's model Il program

A gradual transition exists from the Holsen (York 1969) with >..87Rb = 1.39 X 10-11yr- 1 and Gneiss upwards into the Vevring Complex the errors given at the two sigma level. where the dominating rock types are amphibo­ lites, eclogitic rocks, paragneisses,- quartz schists, granitic rocks, and meta-anorthosites. Analytical results Ultrabasic and basic inclusions also occur. The Holsen Gneiss is separated from the In all, ten whole-rock samples from gnetsstc Jostedal Complex by a distinct thrust-zone and rocks from the Jostedal Complex were analysed, NORSK GEOLOGISK TIDSSKRIFT 4 (1978) Rb/Sr whole-rock isochron date from west Norway 261

f< t7

l:::::: l Vevring Complex

Fig. l. Simplified geological map of the Gaular area show ing sample localities and published whole-rock and mineral ages (in million of years). A and B: sample localities.

five (1005, 1006, 1009, 1010, 1011) of which were Table I. Rb-Sr data of the investigated samples plotted in the wrsr/86Sr versus 81Rb/86Sr diagram (Fig. 2). WR, whole-rock; collected at locality B (Fig. 1), the other five Bi, Biotite concentrate. (1014, 1017, 1018, 1020, 1021) at locality A. The Rb/Sr whole-rock isotopic data are reported in Sam p le Rb Sr "Rb/ 87Sr/ Table l, together with isotopi c analyses of biotite No. (ppm) (ppm) 86Sr 86Sr from two of the samples (1005, 1006). The analytical data are plotted on an isochron 1005 WR 78. 10 292.9 0.77 0.7206 diagram (Fig. 2). 1006 WR 127.7 125. 9 2.94 0.7656 The ten point isochron obtained indicates a 1009 WR 155.3 255.5 1.76 0.7375 IOIO WR 113.2 142.9 2.30 0.7539 metamorphic age of 1625 ±75 m.y. The low 1011 WR 101.2 119.5 2.46 0.7601 initial 87Sr/86Sr ratio (R1 =0.701±0.002) suggest 1014 WR 73. 14 94.3 2.25 0.7491 that the rocks could not have had a long crustal 1017 WR 57.92 124.8 1.34 0.7297 history prior to the major metamorphism. 1018 WR 71.80 98.44 2. 11 0.7495 The Rb/Sr biotite/ whole-rock isochrons 1020 WR 64.54 108.8 1.72 0.7455 obtained from two of the samples give ages of 1021 WR 56.89 50.42 3.28 0.7732 392 and 396 m.y. with initial 87Sr/86Sr ratios of 1005 Bi 377. 1 7.21 164.5 1. 6163 1006Bi 556.3 5.71 333.4 2.5916 0.7164 and 0.7494 respectively. These results are consistent with a number of Caledonian mineral

17 - GeologiskTidsskr. 4/78 262 F. J. Skjerlie & I. R. Pringle NORSK GEOLOGISK TIDSSKRIFT 4 (1978)

0.780

0.770

0.760

1010. 0.750 . 1018 fOll. . 1020 0.740

1- tl'l � � tl'l r- a:> o. 720

0.710

0.5 1.0 1.5 2..0 2.5 3.0 3.5 87R b/æsr

Fig. 2. Rb-Sr isochron plot of analytical results from whole-rock and biotite concentrates from gneissic rocks, the Jostedal Complex.

ages within the range 372-427 m.y. which have Pidgeon & Råheim (1972) obtained Rb/Sr been recorded from other parts of the Gneiss whole-rock isochron ages of 1708±60 m.y. for Region (Broch 1964, McDougall & Green 1964, gneissic rocks belonging to the Kristiansund Brueckner et al. 1968, Strand 1969, Priem et al. (Raudsand) and Frei Groups in the Kristiansund 1970, Bryhni et al. 1971, Brueckner 1972, area. These authors interpreted the results as Pidgeon & Råheim 1972, Sturt et al. 1975). reflecting the age of the amphibolite facies metamorphism; the initial 87 Sr/86Sr ratio of O. 7038± 0.0018 was taken as an indication that Conclusion and discussion the rocks of the Kristiansund Group did not have a long crustal history prior to 1700 m.y. Neither The age of the major metamorphism of the was any isotopic event detected between 1700 Jostedal Complex in the Gaular area, definedby m.y. and 400 m.y. a ten point Rb/Sr whole-rock isochron is Skjerlie (1969) correlated the Frei and 1625±75 m.y. This isochron age fits into the age Raudsand (Kristiansund) Groups with the range of the Svecofennian orogen (Kratz et al. Jostedal Complex. This correlation was con­ 1968) and also corresponds to the second Lax­ firmed from the Tafjord-Grotli area where, ac­ fordian metamorphism (LM2) of the Scottish cording to Hernes (1970), the contact between Lewisian (Moorbath & Park 1971). The low the Jostedal Complex and the metasupracrustal initial 87Sr/86Sr ratio (R1 =O. 701 ± 0.002) suggests sequence coincides with the contact between the that these gneissic rocks from the Jostedal Com­ Raudsand (Kristiansund) Group and the overly­ plex did not have a long crustal history prior to ing Tingvoll Group of the Møre district. the major metamorphism. Brueckner (1972) obtained a Rb/Sr whole-rock NORSK GEOLOGISK TIDSSKRIFT 4 (1978) Rb/ Sr who/e-rock isochron date from west Norway 263 isochron age of 1253 ± 100 m.y. and an initial most geologists which have studied these rocks 87Sr/86Sr ratio of O. 7088 ± 0.0047 on gneissic interpret them as in situ crustal metamorphic rocks from the north shore of Holsavatn, and he rocks (Gjelsvik 1952, Hernes 1954, Kolderup drew the conclusion that this age represented the 1%0, Schmidt 1%0, Bryhni et al. 1%9, Skjerlie Sveconorwegian orogenic cycle. Brueckner be­ 1%9, Bryhni et al. 1970). The results of lieved that the analysed gneisses belonged to the McDougall & Green (1964) could, therefore, Jostedal Complex. According to Skjerlie (l%9), suggest a minimum age of about 1800 m.y. for however, these are part of the Holsen Gneiss the eclogite bearing metasupracrustals. (Fig. 1). The Rb/Sr whole-rock isochron obtained (Fig. The possibility that the Holsen Gneiss is con­ 2) is not ideal. The dispersion of the analytical siderably older than the Rb/Sr whole-rock iso­ results reflects a later metamorphic event of chron age obtained by Brueckner (1972) must probableSveconorwegian age during which both not, however, be ruled out. Considering the ini­ the Rb/Sr and 87Sr/86Sr ratios were slightly dis­ tial 87Sr/86 Sr ratio (R1 =0.7088 ±0.0047) of the turbed. The Jostedal Complex seems not, how­ analysed samples, the age of 1253± 100 m.y. ever, to have been so strongly influenced by the could reflect a metamorphic overprint. Such an Sveconorwegian orogeny as the overlying overprint has been reported from the Vevring Holsen Gneiss and Vevring Complex. This Complex farther towards the northwest, where circumstance could be explained by assuming McDougall & Green (1964) obtained K-Ar ages on that the Precambrian metasupracrustal rocks un­ eclogites in the 1740-1850 m.y. and the 950-1170 derwent Sveconorwegian reworking somewhere m.y. range. These results give a minimum age of outside the Gaular area, and were later brought about 1800 m.y. for the ecologites and a into their present position by thrust tectonics. metamorphic overprint due to the Sveconor­ The emplacement of the tectonic unit most prob­ wegian orogeny. Bryhni et al. (1971) suggest an ably took place late in the Sveconorwegian important overprint in the Fjordane Complex orogeny since the Vevring Complex already con­ (Vevring Complex of Skjerlie & Pringle) at stituted considerable portions of the continental around 1022±18 m.y. ago on the basis of 40Ar/ crust by the opening stage of the lapetus ocean in 39 Ar age-spectrum analysis of pyroxene con­ latest Precambrian-earliest Cambrian time centrate. (Furnes et al. 1976). Rb/Sr whole-rock analyses of schists and It is difficult to forward proof of tectonic gneisses from Almklovdalen, which undoubtedly emplacement in the Precambrian due to the fact correspond with the Vevring Complex in the that younger Caledonian events have led to Gaular area, yield an age around 1150 m.y. and disturbances and small-scale thrustings along the an initial 87Sr/86Sr ratio of 0.710±0.01 (Brueck­ supposed older thrust-plane. ner 1972). Brueckner suggested that there is Brueckner et al. (1968) obtained a Rb/Sr some evidence that this apparent age is not a whole-rock isochron age of 1060± 160 m.y. simple in situ crystallization age or homogeniza­ (1.39 x1o-11y-1 87Rb decay constant) for gneis­ tion age, and that the rocks may have undergone ses from the Tafjordarea which were believed to some isotopic redistribution at some stage in belong to the Jostedal Complex. The sample their history. In the present authors' opinion, the localities were, however, situated very close to relatively high initial 87 Sr/86 Sr ratio suggests that the supposed contact between the Jostedal Com­ the obtained age of 1150 m.y. represents a plex and the overlying metasupracrustal se­ metamorphic overprint. quence. The analysed rocks are monotonous On the basis of the different analytical results strongly banded gneisses containing no obvious from different parts of the · Precambrian metasedimentary or metavolcanic features and metasupracrustal rocks (Holsen Gneiss and have a tectonic fabric parallel to that in the Vevring Complex), it seems reasonable to con­ overlying metasupracrustal rocks. In the present clude that these rocks have been affected by a authors' view these rocks probably correspond penetrative reworking during the Sveconor­ to mylonitic rocks in the Gaular area which wegian (Grenvillian) orogeny. occur in a broad zone along the contact between Even if some authors argue that country rock the Jostedal Complex and the Holsen Gneiss. eclogites in are tectonically The age obtained by Brueckner et al. (1%8), emplaced lower crustal or upper mantle environ­ therefore, probably represents an overprint ments (O'Hara & Mercy 1963, Lappin 1%6) caused by movements associated with the 264 F. J. Skjerlie & l. R. Pringle NORSK GEOLOGISK TIDSSKRIFT 4 (1978) mylonitization. References Pegmatitic rocks are usually rather scarce Broch, O. A. 1964: Age determinations of Norwegian minerals within the Precambrian metasupracrustals. At up to March 1964. Nor. Geo/. Unders. 228, 84-113. some localities, however, the Holsen Gneiss Brueckner, H. K. , Wheeler, R. L. & Armstrong, R. L. 1%8: contains abundant pegmatite bodies which as a Rb-Sr isochron for older gneisses of the Tafjord area, basal rule show deformation phenomena. Four sam­ gneiss region, south-western Norway. Nor. Geo/. Tidsskr. 48, 127-131. ples from a single pegmatite body from the north Brueckner, H. K. 1972: Interpretation of Rb-Sr ages from the shore of Holsavatn defined a whole-rock iso­ Precambrian and Paleozoic rocks of southern Norway. Am. chron age of 676 ± 110 m.y. and an unusually J. Sei. 272, 334-358. Bryhni, l. 1%6: Reconnaissance studies of gneisses, ultraba­ high initial 87 Sr/ 86Sr ratio of O. 7659 ± 0.0055 sites, eclogites and anorthosites in outer Nordfjord, Western (Brueckner 1972). If this apparent age is ac­ Norway. Nor. Geo/. Unders. 241, 68 p. cepted as meaningful, according to Brueckner it Bryhni, 1., Bollingberg, H. J. & Graff, P. R. 1969: Eclogites in would suggest that the host rocks suffered a quarzofeldspathic gneisses of Nordfjord, West Norway. period of pegmatite emplacement late in the Nor. Geo/. Tidsskr. 49, 193-225. Bryhni, 1., Green, D. H., Heier, K. S. & Fryfe, W. S. 1970: On Sveconorwegian cycle or early in the Caledonian the occurrence of eclogite in western Norway. Contrib. cycle. Samples from three separate pegmatite Mineral. Petro/. 26, 12-19. bodies were not co-linear and did not define a Bryhni, 1., Fitch, F. J. & Miller, J. A. 1971: 40 Arf39Ar dates meaningful isochron age. This could, according from recycled Precambrian rocks in the Gneiss region of the Norwegian Caledonides. Nor. Geo/. Tidsskr. 51, 391-406. to Brueckner, be explained either by local Furnes, H., Skjerlie, F. J. & Tysseland, M. 1976: Plate anatexis of the immediately adjacent country tectonic model based on greenstone geochemistry in the rocks or by intrusion from a common parental Late Precambrian-Lower Paleozoic sequence in the magma. In the latter case the pegmatites could Solund-Stavfjorden areas, west Norway. Nor. Geo/. Tidsskr. 56, 161-186. have aquired different initial 87Sr/86Sr ratios Gjelsvik, T. 1952: Metamorphosed dolerites in the gneiss area through differential contamination during intru­ of SunnmØre. Nor. Geo/. Tidsskr. 30, 33-134. sion. In either case, the host rocks would have Hernes, I. 1954: Eclogite-amphibolite on the Molde been partially open to the migration of Rb and peninsula, southern Norway. Nor. Geo/. Tidsskr. 33, 163- 184. Sr. In the present authors' opinion the pegmati­ Hernes, l. 1970: Tafjord-Grotli-området og grensen mellom tic bodies were most probably formed by local det eldre prekambriske kompleks og den senprekambrisk­ anatexis of the immediately adjacent host rocks eokambriske lagrekke. Nor. Geo/. Tidsskr. 50, 261-268. as a result of deformation connected to thrust Holtedahl, O. 1936: Trekk av det skandinaviske fjell­ kjedestrøks historie. Nordiska naturforskarmøtet i Helsing­ tectonics late in the Sveconorwegian orogeny. fors 1936, 129-145. The two biotite/ who1e-rock ages of 392 m.y. Holtedahl, O. 1938: Geological observations in the Opdai­ and 396 m.y. for the Jostedal Complex WNW of Sunndal-Trollheimen district. Nor. Geo/. Tidsskr. 18, 29-53. Hæstadfjorden suggest a Caledonian influence in Holtedahl, O. 1944: On the Caledonides of Norway with some scattered local observations. Skr. Nor. Vidensk. Akad. i the area. These results are consistent with a Oslo, Mat.-Naturvidensk. Kl., 1944, No. 4, 31 p. biotite age of 408 m.y. obtained by Brueckner Irgens, M. & Hiortdahl, Th. 1864: Om de geologiske forhold (1972) for the Holsen Gneiss from the north paa kyststrækningen af Nordre Bergenhus Amt. Uni­ shore of Holsavatn. Other data reported from versitetsprogram, 2. Halvaar, Christiania 1864, 14 p. Kolderup, N.-H. 1928: Fjellbygningen i kyststrØket mellem the Basal Gneiss Region (Broch 1964, Nordfjord og Sognefjord. Bergens Mus. Årb. 1928. McDougall & Green 1964, Brueckner et al. 1968, Naturvidensk. r.7, 222 p. Strand 1969, Priem et al. 1970, Bryhni et al. Kolderup, N .-H. 1952: The age of gneisses and migmatites in 1971, Brueckner 1972, Pidgeon & Råheim 1972, the "North-West Block" of southern Norway. Trans. Edin. Geo/. Soc. 15, 234-240. Sturt et al. 1975) also confirm that an important Kolderup, N.-H. 1%0: Origin of Norwegian eclogites in gneis­ Caledonian event took place around 400 m.y. ses. Nor. Geo/. Tidsskr. 40, 73-76. ago. Kratz, K. 0., Gerling, E. K. & Lobach-Zhuchenko, S. B. 1%8: The isotope geology of the Precambrian of the Baltic Acknowledgements. - The authors are much indebted to Dr. Shield. Can. J. Earth Sei. 5, 657-660. A. Råheim for stimulating discussions on problems of the Lappin, M. A. 1%6: The field relationships of basic and Precambrian geology of west Norway. Thanks are als o due to ultrabasic masses in the Basal Gneiss Complex of Stadlandet Dr. B. Robins for criticism and comments on the manuscript. and Almklovdalen, Nordfjord, southwestern Norway. Nor. The illustrations were kindly prepared by E. Irgens. Geo/. Tidsskr. 46, 439-496. McDougall, l. & Green, D. H. 1964: Excess radiogenic argon and isotopic ages on minerals from Norwegian eclogites. Nor. Geo/. Tidsskr. 44, 183-1%. Moorbath, S. & Park, R. G. 1971: The Lewisian chronology of the southern region of the Scottish mainland. Scott. J. Geo/. 8, 51-74. NORSK GEOLOGISK TIDSSKRIFT 4 (1978) Rb/Sr whole-rock isochron date from west Norway 265

O'Hara, M. J. & Mercy, E. L. P. 1963: Petro1ogy and fjord, Søndfjord og Sogn. Nyt. Mag. Naturvitensk. 26, petrogenesis of some garnetiferous peridotites. Trans. Roy. 93-170. Soc. Edinburgh 65, 251-314. Schmitt, H. H. 1964: Metamorphic eclogites of the Eiksund Pidgeon, R. T. & Råheim, A. 1972: Geochronological in­ area, Sunnmøre, Norway. Am. Geophys. Union Trans. 43, vestigations of the gneisses and minor intrusive rocks from 128. Kristiansund, west Norway. Nor. Geo/. Tidsskr. 52, 241- Skjerlie, F. J. 1969: The pre-Devonian Rocks in the Askvoll­ 256. Gaular Area and adjacent Districts, Western Norway. Nor. Priem, H. N. A. 1967: Progress Report on the lsotopic Dating Geo/. Unders. 258, 325-359. Project in Norway. Z. W. O. Laboratory for isotope Strand, T. 1960: The pre-Devonian rocks and structures in the geology. Amsterdam. region of Caledonian deformation. In Holtedahl, O. (ed.) Priem, H. N. A. 1968: Second Progress Report on thelsotopic Geology of Norway. Nor. Geo/. Unders. 208, 170-284. Dating Project in Norway. Z. W. O. Laboratory for isotope Strand, T. 1969: Geology of the Grotli area. Nor. Geo/. geology. Amsterdam. Tidsskr. 49, 341-360. Priem, H. N. A., Boelrijk, N. A. J. M., Hebeda, E. H., Sturt, B. A., Skarpenes, 0., Ohanian, A. T. & Pringle, I. R. Verdurmen, E. A. Th. & Verschuse, R. H. 1973: A note on 1975: Reconnaissance Rb/Sr isochron study in the Bergen the Hestbrepiggan Granite in West Jontunheimen. Nor. Are System and regional implications. Nature 253, 595-599. Geo/. Unders. 289, 31-35. York, D. 1969: Least-squares fitting of a straight line with Reusch, H. H. 1881: Konglomerat-Sandstenfelteme i Nord- correlated errors. Earth Planet. Sei. Lett. 5, 320-324.