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The Rjukan Proterozoic rift basin, its basement and cover, volcanic and sedimentary infill, and associated intrusions

ELLENM.O.SIGMOND, SVEIN GJELLE&ARNE SOLLl

Geologicol Survey ofNorway,P.O.Box 3006-Lade, N-7002 Trondheim, .

Stratigraphy. The classic stratigraphy of the Group, a few granites probably also t he Bandak Supracrustals wo rked out by Don s (1960) disting uished Group. The plu tonic rocks range from gabbros to granit es three groups of metamorphosed rocks separated from one (Fig. 2). A special suite of int rusive rocks, the Grotte Suite, another by major unconformities:the Rjukan Group (oldest), consisting of dio rites, quartz diorit es, tonalites and granodi­ the Seljord Group, and the Bandak Group (youngest). In ad­ orites, have U-Pb ages of around 1509 Ma (Ragnhildstveit et dition, Dahlgren (pers.comm. 1996) has established a fourth al. 1994). Their age and field relation s indicate that they group, the Heddal Group, concordantly overlying the Seljord could be conn ected wit h th e magm a- and riftform ing event qua rtzites, and olde r than the Bandak Group. that led to t he erupti on of t he acidic Rjukan volcanites.The Areal extent. Our mapping northwards from the Rju kan U-Pb age of a Rjukan rhyolite has been determined to be area confirms the stratigraphy establ ished by Dons and around 1514 Ma (Dahlgren, pers. comm.) and 1511 Ma Dahlgren.All four groups have been followed no rt hwards to (Sigmond & Tucker in prep .) the Caledon ian front at Reineskarvet (Figs.1a.b,c), TheSeljord and Heddal Groups . After this period with ac­ Basement. The boundary relationship between the tive rifti ng and vol canism t here followed a long quiet period Telemark supracrusta ls and the underlying and surrounding with almost complete levelli ng of the old landscape.The de­ gneisses and granites has been a matter of discussion in position of the pure quartzites of th e Seljo rd Group took pla­ Norwegian geology for over 100 years.We have now found ce mainly in an epicontinent al shelf environment. The the basement to the Rjukan group, the Goyst Complex, in Hedd al Group was deposit ed in an environment w here the Uvdal area and just east of the Mandal-Ustaoset Fault some volcanic activity took place. The sediments of these Zone. The Goyst Complex consists of dark-coloured supra­ two Groups cover a larger area than the Rjukan volcanites crustal gneisses typically with rusty weat heri ng, and in many and have an eastward areal exten sion independen t of th e places developed as schlieric migmatites. The gneisses are earlier rift basin (Fig. 1b), a fact support ing the drastic diffe­ intruded by fine-grained grey dior ites, and all are folded and rences in deposit iona l and tectonic environment during the deformed in a complex matter. There is a mark ed difference depos ition of the rocks of the Rju kan and Seljo rd Grou ps. in the degree of deformation and metamorphism between The Bandak Group. The rocks of th e Band ak Group have the slightly deformed Rjukan rhyolites and the strongly de­ been deposited wi th an angular unconformity on the older formed and metamorphosed, migmatitic rocks of the Goyst folded and erod ed groups The rocks are metamorphosed Complex. This Complex represents a true basement upon immatu re sandston es, breccias and con glomerates w ith lay­ which the Rjukan rhyo lites have been deposited. ers of basic and acid ic volcanic rocks. The rocks alon g the The Rjukan volcanic rocks. A thick sequence of meta­ western boundary were deposited alon g an active fau lt mo rphosed acidic lavas and tuffs is overla in by metamor­ marg in (the rej uvenated Mandal-Ustaoset Fault Zone),w hile phosed basic lavas and tuffs. They were depos ited in a rift the eastern bou nda ry from Numedal to Tinnsjoen is a post­ basin limi ted by the Mandal-Ustaoset Fault Zone in the west depositional fau lt. Thus, the Bandak Group could also have (Sigmond, in Tobi & Touret 1985),and a possible faul t also in been deposited in an intraconti nental rift basin. th e east. The eastern boundary is drawn along the eastern­ The Rjukan Rift Basin. The depositional environment of most outcrops of the Rjukan volcanites (Fig. 1a). the Telemark sup racrustals has been discussed by many aut­ The Uvdal plutonic belt. On the 1:1 M geo log ical map of hors.The field relati ons revealed by our new mapping, wi th Norway (Sigmond et al. 1984) th e Uvdal-Geilo area has been th e very large volumes especially of acidic volcani c rocks presented as a belt of gneisses wi t h a zone of Telemark su­ within fault bou ndaries, clearly indi cate that th e Rjukan vol­ pracrustals on each side, a picture w hich could easily be canic rocks were depos ited in a rift basin. thought to represent a folded orogenic belt. Our mapping This basin is of much the same size and has t he same N-S has shown that th is is not correct. There isjust one broad belt extent as the Perm ian Oslo Rift (Fig.2).The Rjukan rocks were with Telemark supracrustals, mainly Rjukan rhyolites, with a intruded by numerous igneous rocks (Fig. 2). Some of them, central area from Uvdal to Geilo consisting of younger plu­ e.g. the Grotte suite, were pro bably connected w ith t he rift­ tons int ruding th e rocks of t he Rjukan Group, partly also the forming event, whil e others, especially t he large gran ites, are NGU - BULL 4 ~ 3_,_ 19 9 ? - PAGE 7 I

a c

Volcanic rocks of the Rjukan Group , Supracrustal rocks Supracrustal rocks CJ and p lutonic rocks with xenolit hs of CJ Hedda l Group Banda k Group Rjuka n volcanites .

Quartzites Basement gneisses, part ly migmatitic. CJ CJ Seljord Group Fig. 1. Present exposed Mandal - Ustaoset Fault Zone areas of t he Rjukan, 5eljord, Hedd al and Postulated eastern lault margin of Rjukan rift basi n Sigmond 1997 Band ak Groups.

OSLO RIFT, PALAEOZOIC _ Pluton ic rocks, Permian

_ Volcanic rocks, Permian

Sedimentary rocks, Cambro - Silurian

RJUKAN RIFT, PROTEROZOIC ~ Granite/gabbro. diabase

~ ~ Dior ite to gra nodio rite (Grotte SUite)

~ Augen granite / granitic gneiss

Postulat ed extent of Rjukan volca nites , c::=J including areas cove red by you nge r rocks

c::=J Basement gne isses (Geyst Com plex) ROCKS MAINLY OUTSIDE THE RIFT BASINS, PROTEROZOIC Gneisses and granites , both olde r and young er than the Rjukan Group .

Fig.2.The Rj ukan Rift com pared to th e Oslo Rift. Sigmond 1997 most probably too young to be directly connected with thi s References event. Furthermore, the new age determin ations have Don s JA 1960:Telemark supracrust als and associated rocks. In Holtedahl, shown that th ere is a gap of 360 million years between the O.(ed.) Geology of Norw ay.Norgesgeologisk e undersekelse208,49-58. Ragnhild stveit, J., 5igm ond, E.M.O &Tucker, AD. 1994: Early Proterozoic depos ition of the Rjukan rhyolites and th e Bandak supra­ supracrustal roc ks west of the Manda l-Ustaoset Fault Zone, crustals.This indicates that th e different groups in Telemark , South Norway . Terra abstracts.Abstrac t supplement should preferably be treat ed and discussed separately, and No 2 to Terra Nova 6, 1S-16. th e term Telemark Supergroup should be dropp ed. In dis­ Sig mo nd, E.M.O., Gust avson, M. & Roberst, D. 1984: Bergg runn skart over Norge M 1:1 m illion Norges geologiske undersekelse. cussing all fou r groups it is more appropriate to use the in­ Tobi, A.C& Tour et, J.L.R. (eds.) 1985: The Deep Proteroz oic crust in the form al term Telemark supracrustals. North At lantic Provinces. D. Reid el Publishing Company, Dord recht, Holl and, 603 pp .