Geology of the Folldal Area, Southern Trondheirr Region Caledonides, Norway
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Geology of the Folldal area, southern Trondheirr Region Caledonides, Norway TERJE BJERKGA RD & ARNE BJ0RLYKK E Bjerkqard ,T. & Bjor lykke. A. 1994: Geology of the Folldal area, southern Trondheim Region Caledonides, Norway. Nor. geol. unders. Bull. 426, 53-75. The lowest stratigraphic unit in the Folldal area is the Gula Group, com prising turbidites and minor greenstones in the lower part (the Sinqsas Formation) and graphitic pelites with subordinate marble and conglome rate in the upper part (the Asli Formation). The overly ing Funds]o Group is bimodal with basalts and rhyodacit es, com agma tic gabbros and a large suovotcanlc trondhjem itic intrusion . The bimodality, high content of volcanlclastics, interca lations of continental sediments, geochemistry and lead isotopes of massive sulphides reveal that the Fundsje Gro up represents an island arc acc umulation formed close to a continent. The contact between the Fund sje and Gula Gro ups is interpreted to be primary, as shown by a grad ual lithological transition and no break in metamor phism and structural elements across the contact. The Folldal Trondhjemite in the Fundsjo Group has yielded a U Pb zircon age of 488 ± 2 Ma. Relationships between published ages from the Gula Group and rocks in the Seve Nappes suggest a possible tectonic and metamorphic link between the Seve and the lower part of the Gula Group . Either a primary unconformity or a thrus t marks the contac t between the Asli and Sinqsas Formations in the Gula Gro up. T. BjerkgaJd & A. Bjartykke I), Department of Geology, University of Oslo, P.o.Box 1047, Blindern, N-0316 Oslo, Norway. 1) Present address: Norges geol ogiske unoersoketse, Postboks 3006 - Lade, 7002 Trondheim, Norway. Introduction in relation to the volcanic and sedimentary Rocks of the Gula Group , which consist rocks of the Meraker and Steren Nappes, is mainly of metasediments, occupy the cen important to any tectonic model of the Cale tral part of the Caledonides in the Trond donides and has therefore been debated heim Region (Fig.1). The stratigraphic and since at least the beginning of this century tectono-stratigraphic position of this group, (see Gee et al. 1985 for a review). Tcctonostratigraphy of the Trondhcim Region West East Steren Nappc >r--oc: V/?/J r--o GulaN appc ~ Gula Nappe ~ ~ ~ •Meraker Nappe ~ Essandsje-Oyfjell Nappe Setra-Risbcrgct r·.....:··:·.·\·::··:l Remsklepp Nappe ~ ~ Nappe :.;'::.:.:-,::.:.:-,::.:.;.: R~ - ----- :I: t"!"l ~ D Rondane Nappe D ~:~;Rea PA RAUTOCHTHON· AUTOCHTHON • Major ore deposits Fig.1. Tectonostratigraphic map of the southern part of the Trondheim Region (modified from Nilsen 1988). The Folldal Area (fig.2) is indicated. 54 Terje Bjerkgard & Arne Bjorlykke GU• BULL 426. 1994 There is now a commo n opinion that the re of the contact between the Fundsjo and rocks of the Storen Group in the Storen Gula Groups and also the internal tectonic Nappe (west of the Gula Group) had a diffe structure of the Gula Group are critical for rent origin to that of the rocks of the Fundsjo the interpretation of the palaeotecton ic set Group in the Merake r Nappe. They probably ting of the Gula and Fundsjo Groups and for represent , respectively, an ocean floor and the existence of a separate Gula Nappe an island arc / marginal basin setting (Gale (Fig.1). & Roberts 1974, Grenne & Lagerblad 1985, Grenne 1988). The intervening sediments The Folldal area comprises both the Gula of the Gula Group have a more uncertain and the Fundsjo Groups, covering about origin. Based on lithology, sedimentology 150 km2 around the village of Folldal in the and geochemistry, Guezou (1978, p.14) county of Hedmark (Figs.1 & 2). Because of considered that the Gula metasediments in extensive mining activity, mapping has the Dornbas district are of epicontinental ori been carried out by several workers and gin and "are devoid of either turbidites or with different perspect ives (e.g. Bjorlykke rhythmic coarse clastic deposits ". Nilsen 1905, Marlow 1935, Page 1963, Heim 1968, (1978), however, reported that turbidites are 1972, Pedersen 1979 and by geologists at found in the central part of the group further Folldal Verk A/S). north, while graphitic phyllites with conglo merates and marbles occur both in the east In this paper, data on petrography, structu and in the west. Both back-arc and fore-arc res and tectonics, petrology and geoche positions have been inferred, either for the mistry and one U-Pb zircon age determinati whole group or part of the group (Gale & on will be presented from the Folldal area. Roberts 1974, Grenne 1988, Stephens & The intention is to throw light on the nature Gee 1989). of the contact between the Gula and Fund sjo Groups; and further, to compare petro Major differences in metamorphic grade graphy, geochemistry and the age of the and deformation (Wolff 1979) and age rela volcanic rocks with work done elsewhere in tionships (see below) indicate that an the 'greenstone belt', especially in the unconfo rmity or thrust may exist internally Roros-Meraker district. These data also within the Gula Group. Therefore, the natu- form a basis for a discussion of the origin of Tec tono stra tigrap hy and Tec tonic Co mplex Terrane (Stcphens & Gce 1989) Lithological Units (Mod ified (Gee et al. 1985) from Nilscn 1988) Metamorphic Gula , appc; Exotic tcrranes complexes Gula Group derived from outboard of Baltic, Meraker Nappc: Oceanic sequences UPPE R Fundsj o Group. SIIMmo Group ALLOCHTHON Suspect terranes: probably outer Essandsjo-Oyfjell Nappc; Seve Nappcs tcct. shonened Aursund Group margin of Baltica Remsklepp Nappc: Platformal and Augen Gneiss Unit. Tynset Gro up Tectonically MIDDLE miogcoclinal shortened margin ALLOCHTHO sediments and Rondanc Nappe of Baltica Precambrian crystalline rocks LOWER Osen-Roa l appc ALLOCHTHON Platformal sedimcnts PARAUTOCHTHON/AUTOCHTHON Table 1. The divisions in tectonostra Cratonic Baltic. tigraphy. terranes and litholoqical Prec, cryst. basement PRECAMBRIAN CRYST, BAS EM E1-·/T units in the Southern Trondheim Region. NGU - BULL 426. 1994 Terje Bjerkgard & Arne Bjarlykke 55 LEGElI:D TYNSET & AURSUND GRO UPS r::'::' ::' :'~ Mica schists and :';::'.:::'.:::'.::. quartzites. subordinate :.;::.;.,'..:::.;:. ultrarnafic rocks and greenstones SULAMo GROUP Micaschist s o 0 0 0 Conglomerate ~0000 RJNDSJ0 GROUP Tuffite Metasediments, partly graphitic. Metarhyodacite Metaba salt Metagabbro Trondhjemite wl mali c xenoliths GULAGROUP <=0_"_" Ii-:, ,". Asli Formation, pelites Singsas Forma tion. banded quartzites and mica schists d - Sheeled dyke complex p - Pillow lavas Strike and dip Major fault , Sulphide mine Sample locality * for dating I km I------l Fig.2. Simplified geolog ical map of the Folldal Area. 56 Terje Bjerkgard & Ame Bjorlykke GU - BULL 426. 994 and connection between the Gula Group two sedimentary units, the Asli and Sinqsas and the Fundsjo Group and finally for the Formations, (Nilsen 1978) in the eastern interpretations and understa nding of the Trondheim Region. The Gula Greenstone, a setting of the ore deposits in Folldal, work metabasalt unit, is common ly found betwe which is currently in progress. en the two and also within the Sinqsas For mation (Nilsen & Mukherjee 1972). All units are represented in the Folldal area and Geological setting were described by Heim (1972) under the name Storho i Formation. The Gula Group The Caledonides of the southern Trond constitutes the Gula Nappe according to heim Region comprise tectonic and litho Roberts (1978) or Gula Nappe Complex logical units derived from several d i ff e r~ n t (Gee et al. 1985). Because of possible con environ ments, thrust upon a Precambnan fusion with the lithological term Gula Group , basement with a thin, mostly Cambrian, the terms Gula Nappe and Nappe Complex sedimentary cover. According to Gee et al. should really be avoided as names for this (1985), the tectonostratigraphy of this regi tectonic unit. They will, however, be used in on can be divided into three major alloch this paper because of their common usage thonous complexes; namely the Lower, in the literature. Middle and Upper Allochthons (Table 1, b e ~ n Fig.1). A new division of units has pr? The Sinqsas Formation consists of psam posed for the Scandinavian Caledonides In mitic to semipelitic quartz-plagioclase-m ica the last five years based on the terrane con schists. They show typically a rhythmical cept (e.g. Roberts 1988, Stephens 1988). variation in quartz and mica content as well Terranes are defined on the basis of their as grain size, on 0.5 - 2 m scale, from near overall geological features and histories, ly pure quartzites to mica schists. Porphyro which are distinctive from those of neigh blasts of garnet and c1 ino-amph ibole are bouring terranes. In the Scandinavian Cale common . No primary sedimentary structu donides, outside of the tectonically shorte res have been found in the Folldal area, but ned shelf and miogeoclinal sequences rela in other places, e.g. the Roros area, graded ted to Baltica there are several suspect and bedding, cross bedding and load casts are exotic terranes of uncertain palaeogeo commonly observed in this unit and the graphic origin (Table 1). rocks have been interpreted as turbidites (Rui 1972, Nilsen 1978). The Folldal area (Fig.2) comprises units occurring above the arkosic rocks of the The Asli Formation is dominated by semi Rondane Nappe (Bockelie & Nystuen 1985) pelitic garnet-chlorite-bearing mica sc ~ i .sts . in the Middle Allochthon . The structurally Based on a varying content of phyllosilica lower units in the area are dominated by tes and quartz the formation might be divi argillaceous sediments occurring both in the ded in a lower unit of quartz-rich schists and Remsklepp Nappe (Wolff 1979) of the Midd a higher unit of partly graphitic phyl.losilica le Allochthon and in the Essandsjo-0yfjell te-rich schists, a division also noticed by Nappe (Nilsen 1988) of the Upper Alloch Heim (1972).