Structural and metamorphic break between the Trondheim basin and the Surnadal synform ARNERÅHEIM Råheim, A.: Structural and metamorphic break between the Trondheim basin and the Surnadal synform. Norsk Geologisk Tidskrift, Vol. 59, pp. 195-198. Oslo 1979. ISSN 0029-196X. Recent field work shows that the rocks of the Caledonian Trondheim basin cannot be foUowed into the Surnadal synform as previously indicated on the 1960 Geo1ogical Map of Norway (Holtedahl & Dons 1960). The structurally complicated Surnadal synform is separated from the Støren Group (Støren nappe) by a thrust zone. The rocks of the Surna Group have suffered two metamorphic events, an earlier medium-grade event superimposed by a 1ow-grade event, white the Støren Group rocks have only been through the low-grade event. A. Råheim, Mineralogisk-geologisk museum, Sars gate l, Oslo 5, Norway. One of the larger extensions of crystallineschists Surnadal synform and the basement gneisses led from the Trondheim basin is the Surnadal Pidgeon & Råheim (1972) to suggest that at least synform (Ramberg 1977, Holtedahl & Dons the lower part of the Surnadal synform rocks, 1960). Strand (1953) and Hernes (1956, 1%7) the RØros (Hernes 1956, Råheim 1977) or Gula found no structural break between the rocks of Group (Gee 1975), may also belong to the this synform and the basal gneisses. The Svecofennian. Råheim (1972) queried a Caledo­ Surnadal synform is a complex nearly 'recum­ nian age for the development of the Surnadal bent' and isoclinal structure (Hernes 1956). Ac­ synform structure itself and suggested that the cording to Løset (1977) three phases of folding extension of the StØren Group from the have affected the rocks of the Surnadal synform Trondheim basin into the Surnadal syncline in the northern part of Trollheimen. These fold should be checked. phases have deformed all the rock groups, in­ Rb, Sr total rock results from the Røros or cluding the Svecofennian (1600-1800 m.y., Gula Group do not define a unique isochron age Priem et al. 1%7) gneiss groups belonging to the (Råheim 1977). The results, however, suggest basement. Løset also assigned the nearly recum­ that the rocks are older, possibly 1700 m.y., and bent and isoclinal development of the Surnadal that they were partially isotopically re­ synform to the first (F1) period of folding. It was equilibrated during the younger Sveconorwegian during this period of folding that the strongest and/or the Caledonian matamorphism. metamorphism took p1ace, where all the rocks of Råheim (1979) has recently shown that the the Surnadal synform as well as the basement main metamorphic and deformational episode of recrystallized at medium-grade metamorphism. the westerngneiss region, including the Surnadal Because the crystalline schists of the Surnadal synform and other stripes of crystalline schists synform were thought to be originally deposited (Fosen, Oppdal) with apparent direct extensions in the early Pa1aeozoic, LØset logically con­ from the Trondheim basin to the Gneiss Region, cluded that the recrystallization at medium­ very likely did occur in the Svecofennian (1500- grade metamorphism must be Caledonian. 1700 m.y. ago). However, a metamorphic or Recent geochronological studies at tectonic break between these rock units which Kristiansund (Pidgeon & Råheim 1972) and at differ in age by about 1200 m.y. had not been Surnadal (Råheim 1977) suggest, however, that demonstrated. all the rocks between the Surnadal syncline and Recent fieldwork to check on the apparent the coast may have been little affected by the extension of the Trondheim basin has now made Caledonian events. Instead, the results rather it clear that both a structural and a metamorphic indicate that the rocks may represent the 'root' break exist at the opening of the Surnadal to the Svecofennian mountain chain, some 1200 synform at Rindal (Fig. 1). A tectonized zone or million y�ars older. The lack of any structural or a thrust zone separates the structurally complex metamorphic breaks between the rocks of the sequence of medium grade rocks consisting of l% Note NORSK GEOLOGISK TIDSSKRIFT 2 (1979) garnet mica schists, amphibolites, garnet of the Trondheim Region is not correct with amphibolites, marbles, and associated calc sili­ regard to the Surnadal synform. During the first cate rocks from the much less structurally com­ and major metamorphic event of the Surna plicated low grade basic metavolcanics of the Group no significant metamorphic zonation ex­ Støren Group, or indeed the StØren nappe as isted within the Surnadal synform rocks. Instead proposed by Gale & Roberts (1974). Previous Goldschmidt's biotite zone marks the zone of correlations between the Røros or Gula parts of retrogression, where even the biotites are relicts the Surnadal synform and Caledonian Støren from the previous metamorphism. sequences are accordingly not appropriate. In The Surna Group rocks south of Rinna in the present study the rocks of the Surnadal Rindal have the same structural elements as synform are therefore considered together and reported by Løset (1977). However, the general termed the Surna Group. trend of the main foliation has been pushed or During the metamorphism and thrusting of the bent subparallel to the foliation of the Støren StØren nappe the medium-grade rocks of the nappe rocks. The binge for this 'hending' is a Surna Group suffered pronounced superimposed marked tectonized zone (Fig. l) at which both low-grade effects, particularly at the thrust con­ basement rocks and Surna Group rocks show an tact. Considerable amounts of chlorite formed in abrupt change in the strike direction (Fig. l). the Surna Group rocks during this event; Several shear zones with strong retrogression amphibolites were transformed partly or com­ parallel to the general strike occur between this pletely to green schists or green stones, and even marked tectonized zone and the 'thrust zone' metapelites took on a very similar 'green appear­ between the Surna Group rocks and the Støren ance'. In the Surna Group rocks, however, nappe. The best place to oQserve the thrust zone pockets of the earlier medium-grade amphibo­ and the marked change in metamorphism is at lites, garnet amphibolites, and garnet mica­ the farm Heggem, where garnet amphibolite and schists are still preserved, showing that these garnet micaschists are found only about 100 m rocks have been through two separate metamor­ from the rocks of the Støren nappe with low phic events. The Støren Group rocks have on the grade mineral assemblages. other hand, suffered only one; the latter low­ A structural discordance between the main grade metamorphic event. The similar 'green structures of the Surna Group rocks and the appearance' on both the retrogressed Surna foliation of the Støren Group rocks apparently Group rocks and the Støren Group rocks made it exist at Trønsdal. Unfortunately the actual difficult to distinguish them in the field, and thrust zone between the two groups is covered, prevented recognition of the break by earlier but at the closest outcrop of the Støren Group at workers. The rather few outcrops in the critical Lyngset (Fig. l) the foliation strikes 100" (Fig. l) areas also obscure the thrust contact, which is and at Trønsdal the general trend of the much therefore marked with a 'thrust zone' in Fig. l. more complicated structures in the Surna Group Another zone of strong retrogression of the is 60" (Fig. l). At Trønsdal the Surna Group Surna Group rocks also occurs nearly parallel to rocks are strongly retrogressed but with 'pock­ the structural trend of the Surnadal synform ets' where rocks with medium-grade mineral (Fig. l). It is this retrogressed zone in the assemblages are still preserved. A sulfide de­ Surnadal syncline which has wrongly been taken posit occurs in the amphibolites of the Surna for the Støren Group equivalent, because of the Group rocks at Trønsdal, and sulfides associated general green appearance of the rocks. How­ with amphibolites (or retrogressed amphibolites) ever, 'pockets' of medium-grade rocks, can be followed westwards along the Surnadal amphibolites, and gamet micaschists are pre­ synform. These sulfides have been correlated served in this zone together with the strongly with the Caledonian sulfide deposits which are retrogressed rocks with low-grade mineral as­ mined in the StØren Group at LØkken. However, semblages (chlorite, albite, serrisite, actinolite, a correlation with the sulfide deposits in the and quartz). The so-called Støren-Bymark Gneiss Region, for example south on Averøya, is Group (Hernes 1956) of the Surnadal synform thought to be much more favorable. Here, has thus been through at least two metamorphic sulfide deposits occur in amphibolites associated events, while this is not the case with the Støren with garnet micaschists, garnet amphibolite Group proper. The map of Goldschmidt (19 15) eclogite rocks, marbles, and calcsilicate gneis­ showing metamorphic zones of the southem part ses, which not only have the same structural NORSK GEOLOGISK TIDSSKRIFT 2 (1979) Note 197 Fig. l. Geological map showing the main rock units of the upper Surnadal-Rindal area. The map has been compiled from LØset (1977) and own observations. l. Basement gneisses, with or without augen. 2. Quartzites and quartzschists. 3. The Surna Group. 4. Thrustzone between the Surna Group and the Støren Group (nappe) and marked zones of retrogression. 5. The Støren Group (or Støren nappe). 6. Metagabbro (low grade). The dark lense in VindØladalen marks the location of a medium grade metagabbro, which is transformed to eclogite at the margins. The dotted line marks a tectonized zone, which acts as the binge for an abrupt change in the strike direction. Strike and dip are given for the main foliation. elements as the Surna Group rocks, but also situation, as well as the available radiometric age have the same rock or chemical ingredients. The data. only difference between the rocks of the two Acknowledgements.