A Proterozoic mylonite zone in the Series north of , south central

IAN C. STARMER

Starmer, l. C.: A Proterozoic mylonite zone in the Kongsberg Series north of Hokksund, south central Norway. Norsk Geologisk Tidsskrift, Vol. 60, 189-193. 1980. ISSN 0029-196X. A north-south zone of mylonite and ultramylonite, in the middle of the Kongsberg Series, dissipates in a wide zone to the north, but separates two areas of very different geology. It seems to reflect an upthrusting of the eastem (' area') block. Late faulting in the opposite sense may represent a Precambrian initiation of the Oslo Graben.

I. C. Starmer, Department ofGeological Sciences, Queen Mary College, University of London, London E. I. 4NS, England.

Within the Kongsberg Series, a zone of Pro­ and more variable lithologies are usually terozoic mylonite and ultramylonite traces from grouped as part of the Kongsberg Series (e.g. Hokksund (20 km NE of Kongsberg) for some 15 Barth & Dons 1960) but sometimes distinguished km north to Kopland (Fig. l). Further north, it as a 'Modum area' subdivision ('Modumforma­ dissipates in a broad zone of protomylonites and sjonen' of A. Bugge 1928, 'Modumfeltet' of thin mylonites, with its western edge along the Josang 1966 and 'Modum-området' of Dons & shores of (for about 8 km) thence curving Jorde 1978). This Modum area complex has been north of the map area through Medrutjern to likened to the Bamble Series of the Skagerrak Krøderen (some 40 km north of Hokksund): the coast (A. Bugge 1928, 1937, Josang 1966). eastern parts are partially lost in beits of albitised In the east of the map (Fig. 1.) the Pre­ breccia. The cataclased zone separates two dif­ cambrian is covered by Cambro-Ordovician ferent areas of north-south striking, upper sediments of the Oslo Graben, from whence a amphibolite facies lithologies (with late ret­ variety of Permian intrusives injected rocks rogressions). It is relatively concordant to the throughout the area. Only the larger bodies have structure of the western area, but, in places, been shown on Fig. l. cross-cuts the lithobanding of the eastern area. Adjacent rocks have been partially deformed into structural conformity with the cataclased zone. The Kongsberg Series west of the The whole area forms part of that described by mylonite zone A. Bugge (1937) in the memoir ' og ' and is included on the 1:250,000 map These rocks are supracrustals (metasediments sheet 'Skien' (Dons & Jorde 1978). Cobalt and metavolcanics) with original upper amphibo­ mineralisations and enclosing rocks in the lite facies assemblages. West of the map area Skuterud area (2 km north of Overntjern, Fig. l) (Fig. l) granulite facies rocks have been reported have been discussed by Rosenqvist (1949) and by Jacobsen & Heier (1978) from Holtefjell Gammon (1966). Josang (1966) has described the (about 8 km west of ). geology north of the present area and, on Fig. l., The supracrustals are variable quartz­ the triangular area between Øverbykollen, plagioclase-biotite gneisses and protomylonites HØgåsen, Vestre Spone and Snarumselva is with occasional hornblende and almandine. taken largely from his mapping, to show the Quartz (reaching 0.8 mm in protomylonites and 2 correlation. mm in gneisses) has often recrystallised and late West of the mylonite zone and Simoa, rather fabrics of biotite-muscovite-clinozoisite ( -pista­ monotonous Kongsberg Series supracrustals ex­ cite) overprint and may be partially mimetic. tend for some distance west of the map area (Fig. Thin concordant and discordant amphibolites l) being punctuated only by the Vatnås granite occur sporadically. Sillimanitic rocks were not (10 km west of Kopland). To the east, different observed in t�e present area, but were found 190 /. C. Starmer NORSK GEOLOGISK TIDSSKRIFT 3 (1980) down strike at Eiker kobberverk (6 km WSW of East from Simoa to Øverbykollen and HØgå­ Hokksund). sen, the supracrustals are dominated by quartz­ Some coarser quartz-plagioclase-biotite- biotite/phlogopite schists with layers of quartz­ muscovite gneisses occur and sporadic layers biotite gneiss. They carry minor hematite and contain plagioclase augen and porphyroclasts up are equivalent to the quartz-phlogopite-hematite to 5 mm size. Rarely, coarser varieties are schists ('kvarts-flogopit-hematitskifer') of developed, with augen reaching l cm or more in Josang (1966): these rocks were reported to size, particularly along the western margin of the contain sporadic cordierite and some highly ultramylonite between Nykirke and Skotselv, altered orthopyroxene. where they alternate with more equigranular Some of the sillimanitic mica schists and gneisses. Late fabrics of biotite-muscovite­ quartzites have quartz-sillimanite lensoids. clinozoisite cut the rocks, but, for the most part, Many of those observed (e.g. SE of Overntjern) wrap around remnant porphyroclast 'islands' or are identical to lithologies referred to as 'nodular augen of quartz and plagioclase. In places, repla­ rocks' in the Bamble Series. They are not of cive microcline has converted them to granitic isolated occurrence and trace 15 km north of the augen gneisses with granitisation of adjacent map area (Fig. l) to Krøderen. ultramylonite. The quartzites usually carry some mica Although weak granitisation effects are ubi­ (muscovite, biotite, or phlogopite) and may con­ quitous west of the mylonite zone, they are tain oligoclase, sillimanite, and/or cordierite. particularly concentrated in certain beits and the The large bodies in the north are represented by granitic augen gneisses developed on the eastern thinner bands further south along the strike, margin of one of these. Locally, the granitised between Åmot and Hokksund, where they beits contain granitic gneiss layers and zones formed the mylonite and ultramylonite. with thin veins of granitic pegmatite. Although Gabbros and amphibolites show some north­ granitic gneisses rarely show weak shearing, the south elongation, but partially cross-cut. No­ replacive microclines grew after the main tably, mylonitised quartzites are sometimes cut cataclasis, but at elevated metamorphic condi­ discordantly by thin amphibolites. Around tions, with thin granitic veins in the northwest Dramselva, in Åmot, altered, actinolitic meta­ corner of Fig. l. carrying almandines. gabbro has layers of actinolitic mica gneiss rep­ resenting protomylonite enclaves. Albitisation was a late process and cut some The 'Modum area' rocks east of the scapolite veins. In this area, it normally followed mylonite zone late breccias; e.g. on the south shore of Simoa, just south of Kopland (Fig. l) albitised breccias The supracrustals here are dominant!y, if not traverse a granite band, which cuts (and contains exclusively, metasediments ( with ubiquitous xenoliths of) amphibolite intruded into ultra­ concordant and discordant intrusions of thin mylonite. amphibolites). They are variable quartz-mica A number of small lenses of magnesite­ (-plagioclase) gneisses, schists, and protomylo­ serpentine rocks occur throughout the Modum nites (with both cordierite and almandine) fre­ area from Snarumselva to south of Åmot, and quently grading into numerous thin and im­ have been described in detail by Josang (1966). persistent quartzitic layers and sillimanite-gneiss They are concordant units in the supracrustals. bands (granitised to sillimanitic granites). In Sporadic cataclasis is observed east to Åmot addition they have olivine gabbros (with corona and Vestre Spone. Quartz grains in the gneisses growths, amphibolitisation, and scapolitisation), and quartzites are often less than l mm size but ortho amphibole-cordierite rocks, large granitic are reduced to less than 0.05 mm in some pro­ and quartz-plagioclase pegmatites, and albitites. tomylonites and mylonites, with subsequent All of these lithologies, including the more un­ coarser recrystallisations and late muscovite­ usual varieties, are typical of the Bamble Series biotite-clinozoisite ( -chlorite) fabrics. and contrast with those in the Kongsberg Series west of the mylonite zone. They were originally upper amphibolite facies assemblages with Fig. l. Geology of the area north of Hokksund. The district traces of granulite (or hornblende-granulite) between Øverbykollen, Høg11sen, Vestre Spone and facies, in places. Snarumselva is taken largely from Jiisang (1966). NORSK GEOLOGISK TIDSSKRIFT 3 (1980) Proterozoic mylonite zone 191

+ vertical .,... 75 -as· ". 60 -70" "., 45-55" .... 30 -40" T 15-25" Mi nor folds with Larger Oslo Region intrusives 15....- plunge - with horizontal axis

--- Late fault 192 /. C. Starmer NORSK GEOLOGISK TIDSSKRIFf 3 (1980)

for some 15 km north of the map area, to The mylonite zone KrØderen, with only isolated, thin mylonites in From Hokksund to Kopland, the main zone of protomylonites and no major movement zone. ultramylonite and mylonite is predominantly These small-scale displacements on many planes white or cream in colour and developed in suggest that, despite some movement (and minor quartzitic (or quartz-rich) rocks. Adjacent lith­ transpositions during metamorphism and defor­ ologies, richer in biotite, underwent much less mation) there may be more of an original severe cataclasis: some thin layers of this type stratigraphic sequence in the north, with the also occur within the ultramylonite and mylonite western Kongsberg Series supracrustals passing shown on the map (Fig. 1). (Similarly, the zones more gradually east into those of the Modum marked as protomylonite usually have a uniform area. weak cataclasis, but occasionally denote bands In the present area, weak cataclasis extends of gneiss with thin mylonite layers.) eastwards to Vestre Spone and some more in­ The mylonite and ultramylonite have a fine­ tense movement planes may have been obscured grained fabric, with quartz grain sizes< O.l mm by the north-south, albitised breccias. This can (and often < 0.05 mm) but showing some coarser be compared with the late, brittle movements of recrystallisations, in part contemporaneous with the 'friction breccia' which followed an earlier development of late muscovite-biotite-clinozois­ ultramylonite - mylonite zone along the western ite fabrics. In some cases, these blastomylonitic margin of the Kongsberg Series. The major recrystallisations have produced coarse growths albitised breccia runs north from Kopland to the of muscovite or biotite and, in well-foliated east of Øverbykollen (Fig. l) and was reported examples, later tale may coat foliation planes. by Jøsang (1%6) to trace for another 8 km The cataclased rocks (including the ultramy­ further north. lonite) are granitised, cut by metagabbros, Strong blastomylonitic recrystallisations may amphibolites, granitic veins, and granitic pegma­ have helped to disguise cataclased zones in the tites, and are brecciated. Rarely the amphi­ north. These have already been mentioned, but bolitised intrusions broke up the ultramylonite are especially strong near Simoa. They were (e.g.·near the church Nykirke), but brecciation possibly enhanced by intrusion of gabbros and was usually due to subsequent movements re­ later pegmatites together with the pneumatolytic lated to both the albitised breccias and the Oslo and hydrothermal activity. Graben faulting. Mylonite, 4-5 km north of Because of all the se factors, it is impossible to Hokksund, is broken up by late concentric fold­ determine whether there was any original de­ ing. crease, northwards, in the intensity of the move­ From the few localities where lineations and ments. rolling can be observed, the cataclasis seems to have reflected an upward movement of the east­ Late folding em block. At Hassel gruver (Fig. l) gabbros and thin Late concentric folding developed within, and concordant amphibolites have caused a heavy on either side of, the mylonite zone. magnetite - hematite impregnation of ultramylo­ Wavelengths range from 20 m to less than I m nite and mylonite. Quartz recrystallisation oc­ and the folds have a variety of axial directions curred in places and there were sporadic over­ and plunges, with no consistent pattern. The growths of muscovite and biotite. structures deform some thin amphibolites, but The northerly dissipation of the ultramylonite are occasionally cut by small veins of granitic in a broad area(containing some protomylonites pegmatite. Particular concentrations of these and sporadic mylonites) from Simoa east to minor folds are developed in mylonite on the Vestre Spone, occurred largely because the main east of Dramselva, where the y cause some brec­ movement zone coincided here with more mica­ ciation. In the adjacent ultramylonite, about 3 rich rocks. The wide belt of quartz-mica schists km north of Hokksund, some thin protomylo­ between Simoa and Øverbykollen contains a few nite-mylonite layers have developed small(< 0.5 thin mylonites, representing quartzitic layers. m wavelength) monoclinal folds: their axes are in The total movement was taken up on a large the strike of the north-south cataclastic foliation number of planes over a wide area and micace­ and they reflect a late upthrust of the western ous rocks could glide. The same effects are seen side, possibly consistent with the late faulting. NORSK GEOLOGISK TIDSSKRIFT 3 (1980) Proterozoic mylonite zone 193

earl y (Precambrian) expressions of the Oslo Gra­ Faulting and brecciation ben movements. A num ber of minor faults are developed, but the Prior to the mylonitisation, supracrustals on most notable features are the north-south brec­ either side generally had upper amphibolite cia zones, which usually dip east, with latest facies assemblages. After the cataclasis, gabbros movements giving a normal or oblique down­ and dolerites were intruded, subsequently un­ throw of the eastern side. Some small, late faults dergoing upper amphibolite facies metamor­ show the same sense of movement. One larger phism and amphibolitisation and being cut by fault (shown in the west of Fig. l) becomes more granite veins, pegniatites and albitised breccias. significant north of Simoa (i.e. north of the A late metamorphism, at epidote-amphibolite present area) where it forms the western bound­ facies grade, produced muscovite-biotite­ ary of the 'Modum area' complex. clinozoisite fabrics in gneisses and mylonites. These were partially mimetic but also cross-cut­ In the south of the area (Fig. l) breccias may be ting, indicating some directed stress. associated with the Oslo Graben. About 5 km The gabbros and amphibolites therefore have north of Hokksund (just west of the Oslo Graben the same relative age as the similar 'Vinor' sediments) mylonite is brecciated and included intrusions of the Kongsberg Series and the 'hy­ in a dolerite dyke which has been cut by a later perites' of the Bamble Series (Starmer 1977). fault. Further north, about 5 km south of Åmot, Acknowledgements. -Thanks are due to Prof. Audley-charles a mylonitised quartzite layer, within protomy­ for perniission to use the facilities of the Department of lonites, is intruded by small metagabbros, Geological Sciences, Queen Mary College. amphibolites, and pegmatites, and cut by a later, December 1979 north-south breccia, itself traversed by Permian injections. In the north of the area, albitised breccias References trend north-south. They may reflect early ex­ pressions of the Oslo Graben movements or may Bugge, A. 1928: En Forkastning i det Syd-Norske Grunnfjell. Nor. Geo/. Unders. 130, 124 pp. be equivalent to displacements further west, on Bugge, A. 1937: Flesberg og Eiker. Nor. Geo/. Unders. 143, the margin of the Kongsberg Series, developing 118 pp. the 'friction breccias' (which could also repres­ Barth, T. F. W. & Dons, J. A. 1960: Pre-Cambrian of Southem ent initiation of the Oslo Graben). Norway, 6-67. In Holtedahl, O. (ed.) Geology of Norway. Nor. Geo/. Unders. 208. Gammon, J. B. 1%6: Fahlbands in the Precambrian of South­ em Norway. Econ. Geo/. 61, 174-188. Jacobsen, S. B. & Heier, K. 1978: Rb-Sr isotope systematics Summary in metamorphic rocks, Kongsberg sector, South Norway. Lithos Il, 257-276. The zone of my1onite and u1tramy1onite dissi­ Josang, O. 1%6: Geologiske og petrografiske undersøkelser i pates in a wide zone to the north, but separates Modumfeltet. Nor. Geo/. Unders. 235, 148 pp. two different geological complexes. It seems to Rosenqvist, I. Th. 1949: Noen observasjoner og refleksjoner omkring Modum koboltgruver. Nor. Geo/. Tidsskr. 27, 187. have resulted from an upthrusting of the eastern Starmer, I. C. 1977: The geo1ogy and evolution of the south­ block, but later faults and breccias show an western part of the Kongsberg Series. Nor. Geo/. Tidsskr. opposite sense of movement and may re present 57, 1-22.