Ophiolitic Rocks
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J. geol. Soc. London, Vol. 141, 1984, pp. 1015-1032, 10 figs, 1 table. Printed in Northern Ireland, Lower Palaeozoic convergent plate margin volcanism on B0ml0, SW Norway, and its bearing on the tectonic environments of the Norwegian Caledonides Harald Brekke, Harald Furnes, Jan NordQs & Jan Hertogen SUMMARY:The island of B0mlopresents a geological section throughaCaledonian convergent plate margin. The Lykling Ophiolite and the overlying ensimatic arc sequence of the Geitung Unit were emplaced onto the Baltic Shield, probably in Arenig times. After a period of deep erosion, the Basin and Range type volcanics of the Siggjo Complex were deposited followed by a hiatus in the magmatism and a weak but widespread phase of vertical tectonism, possibly dueto the successful subduction of aspreading ridge. Atthe top of the mainly sedimentary AshgilVLlandovery Vikafjord Group, the Basin and Rangetype volcanism resumed. This was probably preceded immediately Sy the calc-alkaline, subaerial volcanism at the base of the Langevsg Group, which passes stratigraphicallyupwards into basin-fill lithologies such as submarine volcaniclastics, cherts, turbidites and submarine greenstones of chemistrycompatible with the earlystages of ensialic back-arc basin development. The stratigraphy-_. and volcanism are discussed in context with recent views onthe rest of the Norwegian Caledonides. The island of B0mlo is situated in the Sunnhordland correlatives.Extrusive quartz-keratophyres from the region of SW Norway (Fig. 1). B0mlo is underlain by overlying Geitung Unit have yielded a Rb-Sr whole- Caledonian rocks ranging in age from the Cambrian rock isochron age (A = 1.42 X 10-l1 yr-’) of 535 f 46 into the Silurian and belonging to the ‘upper alloch- Ma. (Furnes et al. 1983). Rhyolites from the uncon- thon’ of the Caledonian nappe pile of Norway formably overlying Siggjo Complex aredated at (Roberts et al. 1981). This constitutes the uppermost 464 f 16 Ma. (Rb-Srisochron) (Furnes et al. 1983) allochthon in the Hardangervidda-Ryfylke area (Solli which is the Llanvirn-Llandeilo boundary according to et al. 1978). the time scale of McKerrow et al. (1984). Similar rocks The earliest description of the geology of B0mlo is on the island of Stord, the Kattnakkenvolcanics, have that of Reusch (1888a). Renewed geological investiga- yielded Rb-Sr whole rock dates at 445 f 5 Ma. (Priem tions started only recently (Songstad 1971; Suthren & & Torske 1973; Lippard 1976) indicating Uppermost Furnes 1980, Nordiis et al. 1984; Amaliksen 1983; Caradoc. These dates are in good agreement with the Brekke 1983). The recent work allows a subdivision of biostratigraphy since the lower part of the unconform- the rocks of Bomlo into five lithostratigraphic units: ably succeeding Vikafjord Group contains limestones the Lykling Ophiolite,the Geitung Unit, the Siggjo with a fossil fauna of Ashgill to possibly lowermost Complex, theVikafjord Group and the Langevdg Llandovery age. In a similar group of rocks on Stord, Group (Fig. 2). Theterm ‘Lykling OphioliticCom- the Dyvikvdgen Group, a Lower Llandovery graptolil plex’ used by Nordds et al. (1984) includes both the tic shale occurs on top of the limestones (Kiaer 1929; Lykling Ophiolite and Geitung Unit, but the authors Ryan & Skevington 1976; Thon et al. 1980). Thick now find it useful to distinguish these. Thestratigraphy piles of coarse clastics, the Sagvatn Formation, overlie presents a very complete succession of old ocean crust, the BergesvatnFormation with a slight angular island arc and marginal basin sequences. discordance. On Stord the base of a similar unit, the The present paper briefly describes and analyses the Utslettefjell Conglomerate, has been demonstrated to relationship betweenthe magmatic activity andthe be erosive down into the fossiliferous strata (Fxrseth stratigraphy of the area, with special emphasis on the & Ryan 1975; Frerseth & Steel 1978, Thon et al. 1980). post-Arenig period. A more detailed lithological On Bomlo the SagvatnFormation is capped by the description is given by Nordds et al. (1984). The subaerial volcanics of the Eriksvatn Formation. magmatic and geological history is put in context with The Langevdg Group is probably the youngest current views on the rest of the Norwegian Caledo- group of supracrustalrocks in thearea. This is in nides and models forthe overalltectonomagmatic contrast to the view of Faerseth (1982) who included settings are discussed. these rocks in the very heterogeneous ‘Hardangerfjord Group’ which he holds to be the oldest Caledonian Stratigraphy rocks of the area. On Born10 the volcanics in the lower part of the Langevdg Group are seen to makeprimary The stratigraphy of Bomlo is summarized in Fig. 3. unconformablecontacts only with the ophiolitic The oldest rock unit is the Lykling Ophiolite and its rocks-all contacts with the Siggjo Complex and Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/6/1015/4888215/gsjgs.141.6.1015.pdf by guest on 30 September 2021 1016 H. Brekke, H. Fumes, J. Nordds & J. Hertogen 0- I 0- Kx) 200 kr FIG. 1. Simplified map of the NorwegianCaledonides showing thedistribution of ophiolitesand volcanic arc sequences. Vikafjord Group are faulted. However, on the island The Lykling Ophiolite of Huglo, E of Stord, greywackes andcherts corre- lated with theLangevig Group overlie fossiliferous The Lykling Ophiolite crops out in the area N of limestones of possible Ashgill age (Monsen 1937; C. Lykling (Fig. 2). This rock unitpresents an almost Magnus pers. comm.). Hence, if the unconformity at complete ophiolite pseudo-stratigraphy (see Nordis et the base of the Sagvatn Formation in the Vikafjord al. 1984) and is invaded by voluminous plagiogranites Group is of regional significance, it may be coeval with which are geochemically similar tothe extrusive the unconformity at the base of the Langev5g Group, quartz-keratophyres of the overlying Geitung Unit. implying that the upper part of the Vikafjord Group Rocks correlated with the Lykling Ophiolite crop out and the Langevgg Groupare penecontemporaneous in areasfurther south on B0mloand on southern (see Fig. 3). Moster (Fig. 2). Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/6/1015/4888215/gsjgs.141.6.1015.pdf by guest on 30 September 2021 Lower Palaeozoic convergent plate margin volcanism 1017 ,:L!;,,=[ ,:L!;,,=[ --___,-,.-.l, Langev6g Group _____ _________ Vi kafjordGroup - Unconforrnity SiggjoComplex - Unconformitv S.Lyklingholmenunit Geitungunit '1 Early xxxxx island X xxxXxx Plagiogranitearc IQ :.,-,*x>a; ,<?<:*:>*?:. L ++.~.~~;$ yklingophiolite m.. Ophiolitic Rocks . -' (Correlatedwith Lykling G - Geitung M - Moster L - Langev6g N - NordCyane Lg - Lyngs~y S - Siggjo Ly - Lykling SL - Sere Lykling, holmen L6 - L6ten9y V - Vikafjord I 5km i FIG.2. Simplified geological map of Bemlo. The Geitung Unit with thin, laterally persistentchert-like horizons and minorconglomerates and sandstones(Amaliksen The Geitung Unit is a mixed extrusiveisedimentary 1983; Nordis et al. 1984). rock unitunconformably overlying the Lykling The greenstones represent pillow lavas, micro- Ophiolite. The unitcomprises greenstones, quartz- pillow lavas, pillow breccias and hyaloclastites, all keratophyresand volcaniclastic breccias interbedded highly vesicular. Theseare interbedded with more Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/6/1015/4888215/gsjgs.141.6.1015.pdf by guest on 30 September 2021 1018 H. Brekke, H. Furnes, J. Nordds & J. Hertogen SIGGJO COMPLEX /U I 3z ??- LANGEVAG GROUP _I- Ashgill ESPESTAO FM. 4 TAVA N E SE T FM.' VIKAFJORD ESTHAUGEN FM. GROUP ORLAND FM. Fault YRKJETUFT FM. 46W6M.a 468t23M.a. 014 CONGLOMERATESAND LIMESTONES COARSESANDSTONES ...... ....... SANDSTONES AND . BEDDEDCHERTS ACIDVOLCANICS U. PHYLLITES p - PILLOW LAVAS Tb- TUFF-BRECCIAS - UNCONFOIU.IITY 6.U.- GEITCNGUNIT L.0.- LYKLINGOPHIOLITE FIG.3. The stratigraphy of the post-Arenig rocks of B0mlo. massive andautobrecciated, slightly vesicular, sub- upper range of those reported fromintra-oceanic marine lavas. island arc volcanics (Furnes et al. 1983). Quartz-and feldspar-phyricquartz-keratophyres Unconformably overlying the GeitungUnit and the occur as massive and flow-banded flows, as tuffaceous, Lykling Ophiolite are coarse, unsortedsedimentary apparently welded beds and as hyaloclastites. One of breccias (the Sore Lyklingholmen Unit) derived from the flow-banded units, dated at 535 f 46 Ma, has an the local substrate (Fig. 2). These are thought to be initial 87Sr/86Srratio of 0.7048 which is within the associated with faultscarps along an ancient, oceanic Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/141/6/1015/4888215/gsjgs.141.6.1015.pdf by guest on 30 September 2021 LowerPalaeozoic convergent marginplate volcanism 1019 fracture zone (Nordiset al. 1984; Amaliksen in prep.). 1980), andrepresent arapid transgression (possibly related to the Upper Ordovician ice-age). The basal deposits are overlain by conglomerates and coarse sandstones interpreted as submarine, resedimented The Siggjo Complex conglomerates of the typedescribed by Davies & The Siggjo Complexunconformably overlies the Walker (1974) andWalker (1975). Theyprobably Lykling Ophioliteand Geitung Unit (Fig. 2). The represent an ancientfan-delta that built out froma complex consists of volcanics, largely subaerial, with tectonically active coastline in the W. minor intercalations of sedimentary rocks (Nordis et On top of the fan-delta deposits there is a thick pile al. 1984). A reverse fault runs