Geochronological investigations in the area, central

STEINAR SOLHEIM

Solheim, S.: Geochronological investigations in the Oppdal area, central Norway. Norsk Geologisk Tidsskrift, Vol. 60, pp. 175-188. 1980. ISSN 0029-196X. The isochron ages obtained have been grouped in four geological time periods: 1700-1750 m.y. (dioritic gneisses, GråhØ/TronfjeU Complexes), 1450-1500 m.y. (augen gneisses and intrusives, GråhØ/TronfjeU Complexes), -1050 m.y. (Basement Complex and Flagstone Group), and -400 m.y. (minerals). The Caledonian influence in the area seems to be smaller than previously assumed since it has on! y caused a resetting of the isotopic system between minerals. The isochron ages of- 1050 m.y. are thought to reflect secondary events during the Sveconorwegian Cycle. The time of deposition/intrusion of the rocks of the Basement Complex and the Flagstone Group is thought to be earlier than 1450-1500 m.y. One extensive metamorphism has been recognized, and 1450-1500 m.y. is considered to be a minimum age for this event.

S. Solheim, Mineralogisk-geologisk museum, Sars gt. l, Oslo 5. Present address: Norsk Hydro, Oljedivisjonen, Kjørboko/len, 1300 Sandvika, Norway.

The Oppdal area is situated in southern ses, granitic gneisses and augen gneisses. Eggen TrØndelag (Fig. l). Geologically the area is (1977) interprets the banded gneisses as bordered by Caledonian rocks of the metavolcanics. In the southern part of the area Region in the east and Precambrian rocks of the the zone of outcrops of the banded gneisses Gneiss Region in the west. The aim of this work becomes narrower and it ends just south of the has been to date the various rock units in this area mapped by Eggen (1977), Evensen (1977) area and in this way to contribute to a hetter and Sjulsen (1977). understanding of the geological evolution of the The homogeneous gneisses. - These differ from area. the banded gneisses in having a uniform granitic composition. Eggen (1977) suggests that the rocks represent granites which have intruded the Geological setting volcanics and later been metamorphosed. Amphibolites appear as concordant layers and as The geological mapping of the Oppdal area has large massifs in the Basement Complex. The been carried out by Eggen (1977), Evensen latter are interpreted as metagabbros, while the (1977), and Sjulsen (1977). They suggest that concordant layers can be of intrusive or extru­ there are rocks here both in autochthonous and sive origin (Eggen 1977). allochthonous positions. The autochthonous rocks can be divided into two units, the Base­ ment Complex and the Flagstone Group. The Flagstone Group

The Flagstone Group is startigraphically above, The Basement Complex and has a concordant contact to the Basement Complex. A conglomerate with strongly de­ On the geological map (Fig. l) Eggen (1977), formed quartz pebbles occurs locally at this Evensen (1977), and Sjulsen (1977) have divided contact. At Morken the conglomerate has a the rocks in the Basement Complex into two thickness of about 50 metres (Rosenqvist 1941) groups: and in Åmotsdalen, just south of the map, the

The band ed gneisses. - The composition and equivalent conglomerate has a thickness of more structure of these rocks are very variable and than one hundred metres. This conglomerate give them a heterogeneous appearance. They indicates a geological time interval between the consist mainly of dioritic/quartz dioritic gneis- deposition/intrusion of the rocks of the Base- 176 S. Solheim NORSK GEOLOGISK TIDSSKRIFT 3 (1980)

LEGEND ------S implified geological map of TRONDHEIM the Oppdal Area D SUPERGROUP ------HORNET COMPLEX Hornblende- Mica Schist Group 1 f lagstone Gro up �

Rapakivi Group il -- - - g -- SLIPR AN COMP LEX � Hornblende- 2 Mica Schist Group :;;:

l. flagstone Group --- - GRÅHØ � TRONFjEll COMP LEX

' Gabbroic rocks

Augen gneisses

� fl>nstone Group � -8.8.:..8 ., ------il BASEMENT COMPLEX g .c:: :E

=:=:=3 Banded gneisses �

""""""" � Am phi boliles 1 :::..:::_:::_-StråiigråPhic�t bou �d�ry --- ectonT i c bo undary @ Granitic intrusives

Fig. I. Simplified geological map of the Oppdal area (after S. Eggen (1977), J. Evensen (1977), S. E. Sjulsen (1977)) showing sample localities. l - Bjørkeng, 2- Morken, 3- Slipran, 4- Vikasætra, 5- Storhaugfjellet, 6, 8- Engan, 7, 9, 10, Il- GråhØ, 12- Ol bu.

ment Complex and the Flagstone Group. The ses, gneisses, amphibolites, and leucogranites. composition of the rocks in the Flagstone Group The augen gneisses are the most typical rocks in is very uniform and there are obvious the area. The form of the augen varies; it can be mineralogical differences from the Basement rounded, oval or square and the longest axis can Complex. The content of quartz and white mica be up to 15 cm. The augen consist mainly of is higher in the Flagstone Group, while the K-feldspar and plagioclase with minor amounts content of biotite and plagioclase is lower. Eggen of quartz and white mica. The matrix is com­ (1977) defines the rocks of the Flagstone Group posed of quartz, feldspar, biotite, and white mica as meta-arkoses, and, because of the high con­ (Eggen 1977). The leucogranites occur within the tent of hematite, he proposes a continental to augen gneisses and vary from small lenses to shallow marine environment of deposition. large bodies up to hundreds of square metres in In spite of the metamorphism which has af­ area. The contact with the augen gneisses is fected all the rocks in this group, sedimentary disturbed by tectonic movements, and it was not features such as cross-bedding are well pre­ possible to decide from the field relations served. whether the granites intruded the augen gneiss The allochthonous rocks can be divided into complex before or after the formation of the three complexes: The GråhØ/Tronfjell Com­ augen (Eggen 1977). Evensen (1977) has studied plexes, the Slipran Complex, and the Hornet the rocks at Gråhø, in the western part of the Complex. complex. The metagabbros are the most typical rocks in this area. They intrude a complex of gneisses and augen gneisses. In addition to these The Gråhø/Tronj}ell Complexes rocks there are minor intrusions of leucogra­ nites. Both the augen gneisses and the leucogra­ The eastern part of these complexes, the area nites are equivalents to the corresponding rocks around Tronfjell, consists of coarse augen gneis- at Tronfjell. NORSK GEOLOGISK TIDSSKRIFT 3 (1980) Geochronology in the Oppdal area 177

The Slipran Complex is 550"C--600"C, but the pressure has not been determined. Using Winkler's (1974) subdivision The nappe complex is built up of rocks from the of metamorphic rocks into four grades, the rocks Homblende-mica schist Group and the underly­ in the area must be placed near the base of ing Flagstone Group. This means that rocks of 'medium grade'. There has not been any exten­ the Flagstone Group are both autochthonous and sive retrograde metamorphism in the area, but allochthonous, but no differences have been some chloritization of biotite is observed. found in petrology or chemistry of rocks in the two positions. The Homblende-mica schist Group consists of three different rock units (Eggen 1977, Evensen 1977). Structural geology

Amphibolites/garben schists. - These are the Sjulsen (1977) has carried out structural studies dominating rocks, and appear usually as layers and recognized several deformational events. In / of about 1 2 metre in thickness alternating with the first period of deformation (D1) there has the other units, but they are also observed with been isoclinal folding (F1) and all the rocks in the greater thickness. area have been folded during this deformation. During D2 the rocks were strongly deformed and Garnet-mica schists. - They have a banded the contacts of the Tronfjell/Gråhø and Slipran appearance with layers of quartz (thickness ab­ complexes were folded at this time (F2). These out l cm) alternating with layers consisting of nappes must, therefore, have been in their pre­ mica, gamet, plagioclase and quartz. sent position prior to the D2 deformational

Metapsammites. - They occur as layers varying period. D3 is a deformational period during in thickness from a few cm to about 50 cm. The which large recumbent folds were formed. The rock has a gneissic structure and is similar in Hornet nappe complex is a recumbent fold and appearance to a flagstone rich in biotite. No came into its present position during the D3-de­ sedimentary structures have, however, been formation. observed in this rock. During the last period of deformation (D4) a crenulation cleavage was locally developed in the rocks of the Basement Complex and the Flagstone Group. The Hornet Complex

In this complex also there are rocks of both the Homblende-mica schist Group and the Flag­ stone Group, and, as in the Slipran Complex, the flagstone is apparently identical to its au­ Previous interpretations tochthonous equivalent. The characteristic unit in this complex is a rock with a rapakivi texture. The Oppdal area has been studied by amongst In some zones the rapakivi is strongly deformed others: Tømebohm (1896), BjØrlykke (1905), and one can find transitions to augen gneisses Goldschmidt (1916), Carstens (1925), O. and mylonites. Holtedahl (1938, 1940, 1960), Barth (1938), Rosenqvist (1941, 1942), H. Holtedahl (1949), Gjelsvik (1952), Holmsen (1955), Strand (1960), Oftedahl (1964), and Hernes (1967). Radiometric Metamorphism age determinations on total rock systems have Eggen (1977) and Evensen (1977) have studied not previously been carried out. However, bio­ the metamorphism of the rocks in the area, and tites were analysed from two localities at LØnset have recognized only one extensive metamor­ and gave K-Ar ages of 370 m.y. and 388 m.y. phism. All the rock units discussed above appear (Broch 1964). to have been subjected to this metamorphic The rocks of the Basement Complex have event. This was tested by using the K�;.:'�� been correlated with the gneisses further west in geothermometer developed by Thomson (1976). the Gneiss Region. Hernes (1967) correlated No variation in metamorphic grade could be these rocks with the Lower Tingvoll Group, and detected between the different rock units tested. he suggested that they represented an The temperature indicated by this thermometer Eocambrian rock unit gneissified during the 178 S. Solheim NORSK GEOLOGISK TIDSSKRIFf 3 (1980)

Caledonian orogenic period. However, Råheim Analytical procedure (1977) has shown that rocks of the Tingvoll Group were formed much earlier in the Pre­ Most of the samples were collected from road cambrian at about 1620 m.y. (recalculated to cuts, where it was not difficult to get fresh A.= 1.42·10-11 yr-1). O. Holtedahl (1938) and samples. Between 5 and lO samples of the same Gjelsvik (1952) also postulated an Eocambrian rock type were collected at each locality within a age for the Basement Complex, while Rosen­ distance of about 10 metres. The weight of each qvist (1941, 1942) and H. Holtedahl (1949) were sample was between 3 and 10 kg. of the opinion that the conglomerate at the At GråhØ it was difficult to obtain fresh sam­ contact between the Basement Complex and the ples. In this case the average weight of each Flagstone Group was a basal-conglomerate sample is smaller, and fewer samples were col­ separating rocks of Precambrian and lected at each locality. Eocambrian age. The rocks were first crushed in a steel jaw­ Most geologists (e.g. O. Holtedahl l938, Barth crusher and then milled to powder in a steel 1938, Rosenqvist 1941, 1942, Gjelsvik 1952, swing mill. Rb and Sr were determined by X-ray Hernes 1967) have considered the Flagstone fluorescence spectrography. Mass spectrometry Group in Oppdal to be equivalent to the was performed on a V6 Micromass 30 mass Eocambrian light sparagmites in southern Nor­ spectrometer. Variable mass discrimination in way. Gjelsvik (1952) pointed out that if this was 87Sr/86 Sr was corrected by normalizing 86 Sr/ the case and the Basement Complex was a 88 Sr= 0.1194 (Faure & Hurley 1963). The 87Rb Precambrian rock unit, then the dark sparagmite decay constant used was 1.42 · 10-11 yr-1• would be absent in the Oppdal area. He con­ Regression lines were calculated using the cluded that the Basement Complex was not a technique of York (1969). As discussed by Precambrian rock unit, but an equivalent to the Brooks et al. (1968), 2.5 as a value of the quality dark part of the sparagmite sequence in southern of fit number (M SWD) has been used as a cutoff Norway and as such of Eocambrian age. level for a straight line where the scattering of Gee (1975a, 1975b, 1978) correlated the Flag­ the datapoints about the best fit line is only due stone Group with the rocks of the Ris back Group to experimental errors. in Jamtland, and he supposed that they rep­ The assigning error to the regression points, resented metasandstones deposited late in Pre­ the coefficient of variance for 87Rb/86 Sr, is taken cambrian time. The rocks of the Hornblende­ as l %, and the standard deviation for 87Sr/86 Sr mica schist Group have been proposed to be of as l · I0-4• All errors quoted in this pa per are two Camho-Silurian age (Rosenqvist 1941, 1942, H. sigma errors. Holtedahl 1949), and H. Holtedahl (1949) corre­ lated them with the Røros Group. Later Råheim (1977) carried out isotopic analysis of schists Results belonging to the so-called RØros Group in the syncline, and concluded that they were A summary of the ages obtained is given in Table metamorphosed in Precambrian time, probably l. The data which form the basis for this and about 1700 m.y. Figs. 2-8 are available from Mineralogisk­ Rosenqvist (1941, 1942) considered the geologisk museum, Oslo. metagabbros and the granites to be intruded and metamorphosed during the Caledonian orogenic The Basement Complex period. He also connected the formation of the augen gneisses in the Oppdal area with Caledo­ Samples from two localities (BjØrkeng (loe. l) nian events, while TØrnebohm (1896) and Cars­ and Morken (loe. 2)) were analysed, but only the tens (1925) suggested a Precambrian age for samples from BjØrkeng give an isochron (Fig. 2 these rocks. a). Here five of the six samples define an iso­ Several geologists (e.g. O. Holtedahl l938, H. chron age of 1055± 70 m.y. (M SWD= 1.05) and Holtedahl l949, Holmsen 1955) have recognized an initial 87Sr/86 Sr of 0.7078 ± 0.0005. Sample A that the rocks in the Oppdal area occur in a is excluded from the calculation. nappe system, and they have connected the The samples at Morken were collected close thrusting of the nappes with the Caledonian to the contact with the overlying conglomerate orogenic period. which separates the Basement Complex from the NORSK GEOLOGISK TIDSSKRIFT 3 (1980) Geochronology in the Oppdal area 179

b BASEMENT o. 78 8.0 BASEMENT c • Bjørkeng

• • Bjørkeng Morken o. A 120BA a SEMENT o o. 76 • Morken 6.0 Bjørkeng 86 .�· 87 Sri Sr o. 715 o. 74 87 B6 Sri Sr • 1055 ';..y. reference isochron . • 1055! 70 y. t m. o. 72 2.0 • o. 7078! o. 0005 o. 710 l 86 87 Rb1 Sr ..\.· 0· 707'------;;'-;=------.'- "----' 0· 7()L---'---;;"-;;,------'-----,J . o . 5 L o 2 . o 4 ';;-. o ---'---,,...6 . ;;---"o '--;;8-';. ,--�o 500 1000 1500 MO DEL AGE M. l Y. l Fig. 2 a-b) lsochron diagrams for total rock samples from the Basement Complex. In 2a, sample A is not included in the age calculation. c) 87Rb/'"Sr versus model age for total rock samples from the Basement Complex. Model ages use an assumed initial "Sr/88Sr of O. 7078, the value obtained from Joe. l (Fig. 2a).

Flagstone Group. In Fig. 2 b the data from The age of 396 ± 8 m. y. is in agreement with Morken and Bjørkeng are plotted together. The the ages reported by Broch (1964), who referred considerable scatter of the datapoints shows K/Ar-ages ranging from 370 m.y. to 388 m.y. on clearly that most of the samples analysed have a biotites from the Basement Complex. This is disturbed isotopic system. This is also demon­ also well within the range of Rb-Sr and K-Ar strated in Fig. 2 c, which illustrated that total ages of minerals reported from other parts of the rock ages for most of the samples are dependent Caledonian orogenic belt, the majority of which on the Rb/Sr ratio. It is clear that samples with Iie between 420 and 380 m.y. (Brueckner et al. higher Rb/Sr have more disturbed isotopic 1968, Strand 1969, Bryhni et al. 1971, Brueckner systems and give younger apparent ages. The 1972, Wilson 1972, Wilson & Nicholson 1973, same phenomenon has been pointed out by Page Råheim 1977). (1978) and Bell & Blenkinsop (1978) in their The data from the Basement Complex show studies of rocks in metamorphic terrains. clearly that this is a Precambrian rock unit Plagioclase, K-feldspar, and biotite were sepa­ influenced by Caledonian events. The influence rated from sample B. The minerals and the total has not been sufficient to reset the isotopes in rock together give an isochron age of 396 ± 8 the total rock samples completely, but the dis­ m.y. (Fig. 3). The value of the MSWD factor is turbance of the isotopic system in the samples 1.98. from Morken may be attributed to this.

The Flagstone Group

• 1.00 This group has been sampled at several iotite 0.90 BA SEMENT / L localities. Six of the sev en samples from a quarry Bjørkeng 10 40 0.80 O. 710 at Kleivan about 5 km south of Engan define an

• K-leldspar isochron age of 1035± 94 m.y. (MSWD= 0.89) • t 396±8 m. y. with an initial ratio of 0.7110± 0.0019 (Fig. 4 a). This locality is situated outside the area mapped Total rock by Eggen (1977), Evensen (1977) and Sjulsen O. 7JS (1977), (Fig. l), but there is no doubt that it is a

• Plagioclase part of the Hornet Complex (Eggen pers. comm. 1979). About 1.5 km west of Slipran (loe. 3) ll samples were collected and analysed from the authochthonous part of the Flagstone Group. Jf

Fig. 3. Isochron diagram for minerals and total rock (B in Fig. they are regressed together they do not define 2a) from loe. l. any isochron age. However, five of the sam- 180 S. Solheim NORSK GEOLOGISK TIDSSKRIFT 3 (1980)

l a FLAGSTONE O. 735 · Kleivan

; O. 76 b / o c 78 FLAGSTONE FLAGSTONE • o. ' ..Storhaugfjellet Slipran o. 75 / � Vikasætra o o .. • l· O. 76 .. oD .. O. 730 O. 74 .... o .. • /. � l Vl l .... �-.:: Vl ... 73 74 t • 1035±94 m. y. o. ,_Vl o. ;· 00 �-.:: /"' • 7110±0.0019 • 1052±43 m. y. l o. t ,_Vl 1035 00 m. y. reference • 7104±0. 0011 o. o. 72 l o. isochron o. 72 7 7 8 Rb/86Sr mEl 8 Rb/86Sr o. 710 �..L..---1---'----'--'------' o. 71 o .l 1.5 1.0 2.0 . 3.0 4.0 2.0 4.0 6.0

Fig. 4. lsoehron diagrams for rocks from the Flagstone Group a) Kleivan. The loeality is situated km south of loe. 8 (Fig. 1). 6 Sample plotted as open circle is not included in the age calculation. b) Loe. 3. Samples plotted as open squares are not included in the age calculation. c) Loe. 4 and loe. 5. The isochron obtained from Kleivan is shown as a reference isoehron. ples Iie on a straight line with a best fit slope fjell Complex), GråhØ (loe. 7, the GråhØ Com­ corresponding to an age of l 052 ± 43 plex) and Stekern. The locality at Stekern is m.y. (MSWD= 0.95) and initial 87Sr/86Sr of about 2 km west of the area mapped by Eggen 0.7104 ± 0.0011 (Fig. 4 b). There is no significant (1977), Evensen (1977), and Sjulsen (1977), but difference between these data and the data the augen gneisses here are equivalent to those obtained from the samples at Kleivan. When at Tronfjell and GråhØ (Eggen pers. comm. data from the two isochrons are regressed to­ 1979). Isotopic studies of nine samples from gether they define a linear array; a best fit line Engan define an approximate isochron age of gives t= 1037± 31 m.y. (MSWD= 0.87) and I= 0.7109± 0.0007. / a 0 o The samples collected at Vikasætra (loe. 4) 5 . 6. and Storhaugfjellet (loe. 5) south of GråhØ (the FLAGSTONE authochthonous part of the Flagstone Group) O. 77 • Kleivan • Sl ipran show too much scatter to define an isochron • Vikasætra • (Fig. 4 c). In Fig. 5 a all the datapoints from the O. 76 Storhaugfjellet Flagstone Group are plotted together, and, as in • the case of the Basement Complex, few of the o. 5 samples fall on the isochron. The disturbance of 7 ;.· ',-,---,--,--: � : . b. the isotopic system is also demonstrated in Fig. 5 FLAGSTONE 74 l. 0 b which illustrates that samples with high Rb/Sr 7 6� O. 8 Sr/86Sr •• show younger apparent ages than samples with r 0 low Rb/Sr. 4. o • . . 73 V: . m, .. ference . . O · 72 �ochron The Gråhø/Tronj]ell Complexes .." . 87 Rb/86Sr MO DEL AGE l M. Y. l O. 7 00 The GråhØ and Tronfjell Complexes have similar ! <------'1 -0 --2-'- . 0---'-0- '-�5 ;;:--'-.-'----..____-'-100-0 -'---' . 3 . tectonic positions (Eggen 1977, Sjulsen 1977) Fig. 5. a) All the analyzed samples from the Flagstone Group and the results of analyses from the two com­ are plotted. The isoehron obtained from Kleivan is shown as a plexes will therefore be discussed together. reference isochron. b) ''Rb/86Sr versus model age for total rock samples from the Augen gneisses. - Samples of augen gneisses Flagstone Group. Model ages use an assumed initial 87Sr/86Sr have been collected at Engan (loe. 6, The Tron- of 0.7110, the value obtained from Kleivan. NORSK GEOLOGISK TIDSSKRIFT 3 (1980) Geochronology in the Oppdal area 181

1468± 103 m. y. (MSWD= 19. 7). The initial ratio There is no geological reason to suggest that is 0. 7078±0. 0056. the augen gneisses at GråhØ are younger than Ten samples from Stekern indicate an age of those at Engan and Stekern and the most Iikely 1347± 225 m.y. (MSWD= 29. 7) and initial 87Sr/ interpretation is that the 'age' of 1129 m. y. is a 86Sr of 0. 7098 ±0. 0082. If samples A and B are mixed one and as such has no geological mean­ excluded from the regression analysis, the age is ing. increased to 1443 ±120 m.y. , the initial ratio is 0. 7080±0. 0040, and the isochron fit improves Augen from the augen gneisses. - Seven augen (MSWD= 26.4). There is, however, no obvious from the augen gneis ses have been separated and reason for rejecting any of the data as the analysed. Two samples were collected at freshness and degree of the alteration of the Stekern and five samples at Engan (loe. 6). The samples are uniform. Brueckner (1979) points scatter of the datapoints exceeds experimental out, however, that rocks can act as open systems uncertainties (MSWD = 20. 9) and results in a to the migration of rubidium and strontium with­ best fit solution with an age of 1372± 145 m.y. out showing any obvious textural or mineralogi­ and an initial 87Sr/86Sr of 0. 7119± 0. 0060. If ca! signs of this behaviour either in thin section samples A and B are excluded from the regres­ or in hand sample. sion analysis, the age is 145 9±84 m. y., and the When the samples from Engan and Stekem value of the MSWD factor decreases signifi­ (excluding A and B) are regressed together, the cantly to 4.2. The initial ratio is 0. 7095±0. 0032 age is 1463±68 m. y. (MSWD=24. 8) and the (Fig. 7 b). initiai87Sr/86Sr is 0. 7076± 0. 0028 (Fig. 6 a). Samples A and B weighed only 100-150 grams The high value of the MSWD factor (ll> 2. 5) and were smaller than the other samples (ca. obtained from the analyses of the augen gneisses 200-300 gram). They would, therefore, be more at Engan and Stekern implies that the scatter of sensitive to minor movements of Rb and/or Sr the datapoints about the isochrons cannot be during the superimposed events. This can be the solely due to experimental error, but must also reason for the poor fit to the isochron. reflect geological disturbance. The most likely Taking the uncertainty of the data into consid­ explanation is that the total rock Rb-Sr isotopic eration, there is no significant difference be­ systems have been disturbed during later tween the age of the augen and of the augen metamorphic/deformational periods, the Sve­ gneisses. The implication of this for the general conorwegian (-1000-1100 m.y. ) and/or the history of the rock will be discussed later. Caledonian (-400 m. y. ). Granitic intrusion at Engan. - Ten samples of Biotite, K-feldspar, and plagioclase have been the granitic intrusion at Engan (loe. 8) give an separated from sample C. Together with the total approximate isochron age of 1489± 116 m.y. rock sample they give an approximate isoehron (MSWD=28. 9). The initial ratio is 0. 7036± age of 392± 21 m.y. (MSWD=6.2 ) (Fig. 6 b). 0.01 16 (Fig. 7 c). This result is consistent with the mineral age The high value of the MSWD factor indicates a obtained from the Basement Complex. disturbance of the isotopic systems as in the case At GråhØ there are augen gneisses of the same of the augen gneis ses. type as at Stekern and Engan. The samples at Two samples are excluded from the regression Stekem and Engan were taken from new analysis. They both have a relatively high Rb/Sr, roadcuts and were therefore fresh. At GråhØ it and, as discussed earlier, such samples tend to was not possible to obtain samples of equivalent have more disturbed isotopic systems than those quality. Nevertheless, ten samples from loeality with low Rb/Sr. 7 have been analysed and eight of them give an The low value of the initial 87Sr/86Sr may apparent age of 1129± 302 m. y. (MSWD= 5.9), suggest a mantle origin of the magma. However, and an initial ratio of 0. 7177± 0.0084 (Fig. 7 a). the data are so uncertain that this cannot be The isochron age is not well defined. This can confrrmed. be due to the sampling problems at GråhØ. After the cleaning (cut by saw) the final sample size The intrusions at Gråhø. - Ten samples have here was very small. Such samples place even been collected and analysed from one of the stronger limitations on allowable minor move­ small granitic intrusions at GråhØ (loe. 9). The ments of Rb and Sr during the superimposed scatter of the data exceeds experimental uncer­ deformational and/or metamorphic events. tainties (MSWD = 14. 5), and results in a best-fit 182 S. Solheim NORSK GEOLOGISK TIDSSKRIFT 3 (1980)

1.00 a AUGEN GNEISS • Engan / 1.30 • b • Stekern c MINERALISOCHRON 0.90 1.20 AUGEN GNE l S S Engan 87 Srt86sr t • 1463�68 m.y. 1.10 • l • o. 7076� o. 0028 87Srt86sr • 392±21 m.y. 1.00 t

0.80 o •• 0.90 �B ". 0.80

87Rbi86Sr 87Rb/86Sr o. 70 o. 70 10 10 20 30 40 50 60 70

Fig. 6. Isochron diagrams for total rocks and minerals from the GråhØ/Tronfjell Comp1exes. a) Augen gneisses from Engan (loe. 6) and Stekem (3 km west of loc. 3). Samples A and B are not included in the age calculation. b) Minerals (K-feldspar, biotite and plagioclase) and total rock (sample C in Fig. 6a) from loe. 6.

solution of 1455 ± 97 m.y. and an initial 87 Sr/87 Sr crustal origin. This is in good agreement with the of 0.7171 ± 0.0124 (Fig. 7 d). conclusions reached by Evensen (1977). Metagabbro is the dominant rock type in the Dioritic gneiss, Stekern. - As discussed earlier GråhØ Complex. It forms intrusions of various the rocks at Stekem are thought to be a part of sizes, and samples were collected from two of the GråhØ/ Tronfjell Complexes. these (loe. 10, 11). Eight samples were analysed. A dioritic gneissic rock rich in biotite has been Seven of them give an approximate age of dated. This rock unit can probably be correlated 1271 ± 163 m.y. (M SWD= 243.0). The initial with the gneiss horizons in the GråhØ/ Tronfjell ratio is 0.7036± 0.0019 (Fig. 7 e). The very high Complexes described by Eggen (1977) and value of the MSWD factor implies considerable Evensen (1977). Because of the complex folding disturbance of the isotopic systems. of the rocks at Stekem it was not possible to If samples A and B are excluded from the determine the relation between the gneiss and regression analysis, the value of MSWD de­ the surrounding granites and augen gneisses. creases to 104.3, while the age increases to Eight samples were analysed and six of them 1646± 323 m.y. The initial 87 Sr/86 Sr will then be form a roughly co-linear array and define an 0.7024 ± 0.0015. isochron age of 1734± 106 m.y. (M SWD= 3.3) The data leave considerable uncertainty as to and an initial 87 Sr/86 Sr of 0.7017 ± 0.0012 (Fig. the age of the metagabbro at GråhØ. However, 70. the field relations in the area do not indicate any This is the highest 'age' obtained from the time difference between the formation of the rocks in the Oppdal area, and the very low initial gabbro and granites. Therefore, the ages of 1271 ratio indicates that the rock cannot have had a m.y. or 1646 m.y. do not seem to be geologically long crustal history prior to this time. significant. From the other results from the area, an age between 1450 m.y. and 1500 m.y. seems The Slipran Complex more.likely. It is interesting to note that the two samples with the highest Rb/ Sr (A, B) are those Six samples of a gamet-mica schist (belonging to which deviate most strongly from a linear arra y. the Homblende - mica schist Group) were col­ The low value of the initial 87 Sr/86 Sr of the lected near Olbu (loe. 12), but the datapoints do metagabbros indicates a mantle origin of the not fall on an isochron (Fig. 8 a). The reason for magma whereas the adjacent granites have a the scatter probably Iies in the fact that the considerably higher initial ratio indicating a samples were penetrated by narrow veins of NORSK GEOLOGISK TIDSSKRIFT 3 (1980) Geochronology in the Oppdal area 183

a 0.80 b c A GEN GNEISS • 1.10 o O. 750 U AUGEN Gr�hø GRANITE l • Engan Engan • Stekern o - •• �B o. 745 1.00 o ,. 87 86 o. 75 Sri Sr o

t • 1459+84 m.y.

l • o. 7095 to. 0032 t • 1489!116 m.y. l • o. 7036!0.0116

87 86 Rb/ Sr 0.80 • I. O 2.0 3.0 4.0

d e AO MET AG ABBRO GR ANITE O. 70 o. 1.00 73 Grå hø lO 15 20 J so o . 73 t • 323 m.y . o 16116! DIORITIC GNEISS . 90 l . l • o. 7024!0. 0015 Stekern t • 55 m.y. / / 14 !97 • o o. o ;· l . 7171:1: 0124 . 72 . ./ • o 8 0. 0 t • 1734±106 m.y. o O. 71 l. . 7017!0.0012

8 87Rb/86Sr 7Rb/86Sr 87Rb/86Sr ______- - o o. IOL_ _L ..I. -,'::--' O. 70'----'..,- ..___J'- -'-----:L --'--.....I . 70.._..__.__.__.__ '---'----'-----'-----'--'---L....J l O 3 O I 0 15 . 2. 0 . 0. 5 1. 0

Fig. 7. Isochron diagramsfor total rock from the Grllhø/Tronfjell Complexes. Samples plotted as open circles are not included in the age calculation. a) Augen gneisses from loe. 7. d) Granitie intrusive from loe. 9. e) Metagabbro from loe. 10 and loe. Il. b) Augen from Engan (loe. 6) and Stekern. " c) Granitic intrusive from loe. 8. O Dioritie gneiss from stekern (3 km west c,Cloe. 3).

quartz which may have disturbed the isotopic and Sjulsen (1977). It is obvious, however, that systems. this locality is a part of the Hornet Complex. The rocks of the Hornblende - mica schist The datapoints do not define an isochron (Fig. Group are stratigraphically above the Flagstone 8 b). It was impossible to get fresh samples and Group, but there is probably no great time this is probably the reason for the scatter of the difference between the deposition of these two datapoints. units since there is no metamorphic or structural

break between them. o. 74 b RAPAKIVI a HORNBLENDE-MIC A • Tøttan • SCHIST o. 73 O. 7 l O l bu es: . The Hornet Complex ... o. 72 o. 72 "' ... The analytical results from the flagstone in the � • :;o 0.71 "' O. 71 Hornet Complex have been discussed together 87 86 :;o Rbi Sr 87Rbi86Sr with the autochthonous part of the Flagstone o. 70 o. 70 0.5 1.0 1.5 2.0 2. 5 0.5 1.0 1.5 2.0 Group. 8. a) Homblende-miea sehist from Olbu (loe. 12). No iso­ Samples of the rapakivi granite were collected Fig. chron can be defined. about 6 km south of Engan which is outside the b) Rapakivi from Tøftan. The loeality is situated about 8 km area mapped by Eggen (1977), Evensen (1977) south of loe. 8. No isoehron can be defined. 184 S. Solheim NORSK GEOLOGISK TIDSSKRIFT 3 (1980)

Table 1. Rb-Sr isochron regression results for total rock and minerala + total rock from the Oppdal Area. For sample locality, see Fig. 1.

87 86 locality Rock type No. of samples MSWD Age (m.y.) Initial sr; sr Total rock/ minerala + total rock

Basement Complex Bjørkeng Gneiss 1.05 1055+70 o. 7078+0.0005 total rock Bjørkeng Gneiss 4 1. 98 396+8 0.7134+0.0001 minerals + total rock

Flagstone Group

Kle ivan Flagstone 0.89 1035+94 0.7110+0.0019 total rock Slipran Flagstone 0.95 1052+43 0.7104+0.0011 total rock Kleivan + Flagstone 11 0.87 1037+31 0.7109+0.0007 total rock Slip ran Gråhe/Tronfjell Complexes

Engan Augen gnei ss 19.7 1468+103 0.7078+0.0056 total rock Stekern Augen gneiss 26.4 1443+120 0.7080+0.0040 total rock Engan + Augen gneiss 17 24.8 1463+68 0.7076+0.0028 total rock Stekern

Engan Augen gneiss 4 6.2 392+21 0.8707+0.0033 minerals + total rock o. Grå he Augen gneiss 5.9 1129:302 7177+0.0084 total rock Engan Augen 4.2 1459+84 0.7095+0.0032 total rock

Engan Granite 10 28. 9 1489+116 0.7036:0.0116 total rock Gråhe Granite 10 14.5 1455+97 o. 7171+0.0124 total rock Gråhe Gabbro 104.3 1646+323 0.7024+0.0015 total rock

Stekern Gneiss 3.3 1734+106 o. 7017+0.0012 total rock

Discussion The isochron of the augen gneisses in the The age of the augen gneisses Oppdal area could represent the age of the It is important to know how the augen gneisses 'parent rock'. This could mean that when the formed when interpreting the age obtained. Sev­ augen were formed, say during the Caledonian eral theories about this have been put forward. Cycle, the migration of ions was on such a small An anatectic origin of augen has been pro­ scale that it did not disturb the isotopic systems posed by Mehnert (1968). This cannot, however, of the total rock samples. The data obtained on be the case in the Oppdal area, because the separated augen give the same age as the total metamorphic grade has not been high enough to augen gneisses. This implies that if the augen form a melt (Eggen 1977). Furthermore, augen gneisses were formed by the processes sug­ formed by anatectic melting should have a gested by StrØmberg (1962) and Eggen (1977), granitic composition (Mehnert 1968), which the this must have taken place at least 1450-1500 augen in the Oppdal area do not have. m.y. ago. It is most reasonable to correlate these Rosenqvist (1941) suggested that the augen events with the strongest deformational period gneisses in the Oppdal area were formed by (D2) in the area. potassium metasomatism which took place on a regional scale during the Caledonian Orogeny. The thrusting of the Hornet Complex StrØmberg (1962) proposed that augen can be developed during strong deformation of rocks. The Hornet Complex is tectonically higher than First a migration of ions takes place during the Slipran Complex. It is a large recumbent fold deformation and then during post deformation with the fold axis in the N-S direction. It has crystalloblastesis the augen can be formed. Eg­ been thrust after the D2 deformational period gen (1977) suggested that the augen gneisses in since the nappe borders are not folded by F2• the Oppdal area had been formed in this way, but The rapakivi forms the central parts of the fold, unlike Rosenqvist he did not think that the and zones of the rapakivi and the surrounding migration of ions had taken place on a regional flagstone have been strongly deformed and scale. mylonitized during the folding and thrusting of NORSK GEOLOGISK TIDSSKRIFT3 ( 1980) Geochronology in the Oppdal area 185 the complex. A recrystallization has occurred in while the superimposed rocks in the area give the mylonitic parts of the rapakivi. higher ages. An obvious question is whether this The mylonite is dated by A. Krill (pers. comm. age represents the time of the deposition/intru­ 1979) to 1500-1600 m.y. This age can be in­ sion of the rocks or reflects a secondary event. terpreted in two ways; either it reflects the time The latter interpretation seems to be the most of the intrusion of the rapakivi (primary age) or it probable one. reflects the time of mylonitization (secondary The fact that 1450-1500 m.y. is a minimum age age). for the main metamorphic event in the area That re setting of isotopic systems occurs dur­ excludes the possibility that the ages obtained ing mylonitization has been demonstrated re­ from the Basement Complex and the Flagstone peatedly in other parts of the world (e.g. Odom Group reflect the time of intrusion/ deposition of & Fullagar 1970, Abbot 1972, Russen 1976, these rocks. Furthermore, the thrusting of the Gabrielsen et al. 1979). Hornet Complex is determined to - 1450-1500 The most reasonable interpretation of the age m.y., and since it is thrusted above the Base­ of the mylonite is therefore that it reflects the ment Complex and the Flagstone Group, these time of mylonitization. rocks have to be older. Taking the uncertainty of the data into consid­ If it is postulated that the age of - 1050 m.y. eration there is no significant difference between reflects secondary events it is necessary to ex­ the data obtained on the mylonite (1500-1600 plain re setting of Rb and Sr in these rocks only m.y.) and the augen gneisses of the Tronfjell/ and not in the adjacent granites and augen gneis­ GråhØ Complexes (1463± 68 m.y.). The age of ses. The cause of the resetting is probably to be the augen gneisses is correlated with the strong found in the structural conditions in the area. F2 folding and this implies that the thrusting of Råheim & Compston (1977) and Black et al. the Hornet Complex (Da) must have taken place (1978) have demonstrated that development of a just after this folding. It is therefore most crenulation cleavage can lead to a reequilibration reasonable to look at D2 and Da as two events in of Rb and Sr in rocks because it opens the rock the same orogenic period. to fluids and to a migration of Rb and Sr. A crenulation cleavage occurs at some localities in the Oppdal area. It is best developed in the The time of metamorphism flagstone, but it also occurs in the basement Only one extensive metamorphism is observed (Sjulsen 1977). It is developed at BjØrkeng where in the area, and all rocks show the same the 1055 m.y. isochron of the basement is de­ metamorphic grade (Eggen 1977, Evensen 1977). fined. The cleavage does not appear to be de­ An opinion common to many geologists work­ veloped in the metagabbro, granites, and usgen ing the Oppdal area (e.g. Rosenqvist 1941, 1942, gneisses (Sjulsen 1977). It is the youngest O. Holtedahl 1938, H. Holtedahl 1949, Oftedahl structural element in the area and formed during 1964) is that the rocks were metamorphosed the last deformational period (D4). during the Caledonian Orogenic Cycle. The new results, however, indicate that the metamor­ Comparisons with other results from the phism occurred in Precambrian time. A correla­ Gneiss Region tion in time between the extensive metamorph­ ism and the considerable deformation and fold­ The rocks and minerals analysed are grouped in ing during D2 and Da seems probable. This four geological time periods: 1700-1750 m.y., means that the main metamorphic event took 1450-1500 m.y., -1050 m.y., -400 m.y. Only place at about 1450-1500 m.y. the augen gneisses and the metagabbro of the The chloritization of biotite may be connected GråhØ Complex fall outside one of these periods. with events during the Sveconorwegian (- l 050 However, the uncertainty in the data is rather m.y.) or Caledonian (- 400 m.y.) Cycles. high, and as discussed previously there is no reason to suggest that these rocks are younger than the granites at GråhØ and the augen gneis­ The age of the Basement Complex and the ses at Engan and Stekern (1450 m.y. - 1500 Flagstone Group m.y.). The rocks of the Basement Complex and the Ages between 1650 m.y. and 1750 m.y. have Flagstone Group show ages of about 1050 m.y. been reported from several parts of the Gneiss

13- Geologisk Tidsskr.3/80 186 S. Solheim NORSK GEOLOGISK TIDSSKRIFT 3 (1980)

Region. Råheim (1977) dated gneisses at dated gneisses in the Tafjord area at l 058 ± 150 Surnadalen at 1671± 63 m.y. and Pidgeon & m.y. (A= 1.42· w-•• yr-•). Pyroxene from gneis­ Råheim (1972) obtained an age of 1672± 60 m.y. ses from Gloppenfjorden was dated by Bryhni et from gneisses in the area. Gneisses al. (1971) using Ar isotope analysis to 1022± 18 from Hareidland are dated by Mysen & Heier m.y. Priem et al. (1973) dated the Hestbrepiggan (1972) at 1646± 70 m.y. Brueckner (1979) has Granite at 1009 m.y. (A= 1.42· 10-u yr-•). carried out isotopic analy sis of augen gneisses at These results indicate that large parts of the Tafjord and obtained an age of 1738± 114 m.y. Gneiss Region have been affected by the All of these data are recalculated to the new Sveconorwegian events. decay constant for 87Rb (1.42 · 10-u yr-•). The augen gneisses and granites in the Oppdal area give ages between 1450 and 1500 m.y. Ages in this interval have previously been obtained in Conclusion the Gloppen-Eikefjord area where Abdel­ Monem & Bryhni (1978) have dated mangeritic The time of deposition/intrusion of the rocks in rocks to 1479± 64 m.y. (recalculated to the Basement Complex and the Flagstone Group 1 1450-1500 A= 1.42 · 10-u yr- ). They conclude that the age has not been determined, but ca. m.y. must be taken as a minimum age. is considered to be a minimum age. The same may be the situation in the Oppdal No age has been obtained from the Horn­ area, i.e. that the ages between 1450 and 1500 blende - mica schist Group but, as discussed m.y. reflect a secondary event which has not previously, there can be no great time difference affected the isotopic systems in the rocks dated between the deposition/intrusion of this rock by Pidgeon & Råheim (1972), Mysen & Heier unit and the underlying Flagstone Group since (1972), Råheim (1977), and Brueckner (1979) no structural or metamorphic breaks have been either because these rocks have been situated observed. outside the area of influence or because the After the deposition/intrusion of these rocks, influences have not been sufficiently strong to the GråhØ/Tronfjell Complexes and the Slipran cause resetting of Rb and Sr. A problem in this Complex were thrust into their present position connection is to explain why no re-equilibration (D1 deformational period) and folded (F 1). of the isotopes has occurred in the dioritic gneiss The formation of the augen gneisses is corre­ at Stekern (t= 1734± 106 m.y.). A more detailed lated with the strong F2-folding (minimum age of study of the rock unit may give an answer. 1450-1500 m.y.). An alternative explanation is that the ages The Hornet Complex Iies tectonically above between 1450 and 1500 m.y. do not represent the GråhØ/Tronfjell Complexes and the Slipran any specific geological event but are mixed ages. Complex. The contact of this complex is not As discussed previously in this paper, rocks with folded by F2 as the others, and it must therefore high Rb/Sr often show more disturbed isotopic have been thrust into position after the D2 defor­ systems and give younger apparent ages than mational period. The age obtained for the samples with low Rb/Sr. The dioritic gneiss at mylonitic parts of the complex (- 1500-1600 Stekern which gives an age of 1734± 106 m.y. m.y.) probably indicates the time of thrusting has a considerably lower Rb/Sr than the rocks (Da). It is most reasonable to look at D., D2 and giving ages in the interval 1450-1500 m.y. This Da as events in the same orogenic period, and the may be why the age of 1734 m.y. has been main metamorphic event should be correlated preserved. with this (minimum age of 1450-1500 m.y.).

In any case the ages between 1450 and 1500 The ages of - 1050 m. y. probably reflect the m.y. must be considered as minimum ages for development of a crenulation cleavage during the formation and metamorphism of the augen the Sveconorwegian Orogenic Period (D4). gneisses, granites and metagabbro in the Oppdal The rocks in the Oppdal area have been af­ area. fected by the Caledonian Orogenic Cycle, but The ages of ca. 1050 m.y. obtained in the area the new data indicate that the influence is smal­ are probably secondary ages reflecting ler than previously thought. A resetting of Rb Sveconorwegian events. Ages of this magnitude and Sr has taken place only between minerals. have also been reported from other parts of the This is consistent with results from other parts of Gneiss Region. Brueckner et al. (1968) have the Gneiss Region. NORSK GEOLOGISK TIDSSKRIFT 3 (1980) Geochronology in the Oppdal area 187

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