Geochronological investigations in the Oppdal area, central Norway STEINAR SOLHEIM Solheim, S.: Geochronological investigations in the Oppdal area, central Norway. Norsk Geologisk Tidsskrift, Vol. 60, pp. 175-188. Oslo 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 Trondheim 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 COM PLEX ' 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).