Tectonostratigraphy in the Velfjord-Tosen Region, Southwestern Part of the Helgeland Nappe Complex, Central Norwegian Caledonides

Tectonostratigraphy in the Velfjord-Tosen region, southwestern part of the Helgeland Nappe Complex, Central Norwegian Caledonides

TERJE THORSNES & HELGE UZlSETH

Thorsnes, T. & Leseth , H. 1991: Tectonostrat igraphy in the Velfjord-Tos en region, southwestern part of the Heigeland Nappe Compiex, Central Norwegian Caledonides. Nor. geol. unders. Bull . 421, 1-18.

In the southwestern part s of the Helgeland Nappe Compl ex, dismembered ophiolites and their cover sequences are imbricated together with nappes consisting dominantly of continentally deri ved rock units. A local nappe sequence comprises three nappes, informally termed the Lower , Middle and Upper nappes . The Lower and Upper nappes con sist of partly migmatitic mica gneis ses and orthogn eisses associated with marbles and calc-silicate gneisses. Between these nap pes, the Middle nappe includes an igneous complex of mafic and ultramafi c rocks of cphiolitic affinity , unconformably overlain by a cover sequence with material derived partiy from the ophio lite substrate. Plutonic rocks , ranging from hornblende gabbros to diorites and granites, were emplaced at seve ral stages during the tectonometamorphic evolution of the region . Based on comparison with nearby areas, most of the regional metamorphism and deform ation would seem to have occurred in the Ordovician , with a later phase of thrusting during the Mid Silurian to Early Devonian Scandi an orogeny.

Terje Tttorsnes , Norges geologiske underseketse, Postboks 3006, 7002 Trondhe im, Norway. Helge t.esett; Institutt for Kontinentetsokketunderseketser, 7034 Trondheim , Norway.

Introduction been mapped between Grong and Bindal (Ro During the past decade, a number of detailed berts et al. 1983, Nordgulen & Bering 1987, studies of the southwestern parts of the Helge Husmo & Nordgulen 1988), where the Late land Nappe Complex (HNC) have been made, Ordovician /Early Silurian Heilhornet Pluton in order to elucidate the complex tectonic, (Nordgulen & Schouenborg 1990) intrudes metamorphic and intrusive history of this part both the underlying medium-grade supracrus of the Uppermost Allochthon. The present tal rocks (Kollung 1967, Schouenborg 1988) paper focuses on the tectonostratigraphical and the basal parts of the HNC (Fig. 1). North development of the Velfjord-Tosen region (Fig. west of the Heilhornet Pluton, the continuation 1), situated in the southwestern part of the of the base of the HNC has not been identifi HNC, based mainly on work forming parts of ed with certainty (Nordgulen & MitcheIl1988). cand.scient. theses (University of Bergen), Several authors (Prestvik 1972, Gustavson concentrating on the Velfjord (Leseth 1985) 1975, 1981, 1988, Nordgulen 1984, Bang 1985, and Sausvatn-Tosen areas (Thorsnes 1985). Leseth 1985, Sturt et al. 1985, Thorsnes 1985, 1987, Terudbakken & Mickelson 1986, Husmo & Nordgulen 1988) have shown that the consti Regional context tuent nappes of the HNC can be divided litholo The HNC forms the southern most part of the gically into two main groups: (1)nappes compo Uppermost Allochthon of the Scandinavian sed largely or entirely of high-grade gneisses, Caledonides in Central Norway (Gee et al. . partly migmatitic, associated with calcareous 1985). As the name suggests, it is composed metasedimentary rocks ; (2) nappes characteri of a series of nappes recording a complex sed mainly by mafic and ultramafic igneous tectonic evolution (Fig. 1). Towards the east rocks of ophiolite /island arc affinity, uncon and southeast , the HNC lies structurally above formably overlain by a sequence of polymict low grade rocks of the Kbli Nappes (Foslie & conglomerates, schists , psammites and mar Strand 1956, Ramberg 1967, Gustavson 1973, bles. The latter rocks are found in a belt along 1981, 1988, Lutro 1979, Dallmann 1986). In the the coast (Leka - Sauren - R0d0Y - Skalveer), southwestern part, the base of the HNC has in the Tosen - Velfjord area, and in the MOSj0- 2 Terje Thorsnes & Helge Lese tii NGU - BULl. 421. 1991

Fig. 1: Simplified geological map of the Helgeland area. Central Norwegian Caledonides, show ing the main Iithotecton ic units and intrusions (compiled from Gustavson 1981 . Norc -qulan ;: 1984. Bang 1985. Lesetn 1985. SKAL\ '/E R Sturt et al. 1985, Thorsnes 1985. Teruobakken & Mickelson 1986. and Nordgulen & Bering 1987). L - Lower nappe, M - Middle nappe, U • Upper nappe. H Helthornet, T - TerrAk.

1 1

4,0 HORTAVIER

~ I'ORI'H YRIC GRAMTE AND c::::::J ~II GM A TITI C G:-iEISS. GRANODIORITE :\IARBL E. SC HIST ~ MONZONITE. MO NZODIOR ITE ~ SC HIST. I'SAMI\ IIT E. :\IARBL E. CO:-iGLO:\IE RATE ~ GRANO DIO RITE . GRAl'o'lTE _ VLTRAM AFIC ROCKS. GABllRO. TON ALIT E, GRANODIORITE D AMI' HIIlOLITE. GREE:-iSTO:-iE ~ GABllRO. DIORITE t:zJ ROCKS BELOWT HE H:-iC en area (Fig. 1). The rocks found in the S0r metamorphic evolut ion (Kollung 1967, Nordgu fjord area possibly belong to the coastal belt. len 1984, Nordgulen & Schouenborg 1990). In On Leka, in the extreme southwestern part the southwestern parts of the HNC metamorp of the HNC, UlPb dating of zircon in quartz hic rocks of continental origin were intruded keratophyres of the Leka Ophiolite Comple x by tonal ites and granod iorites prior to c. 500 has yielded an age of 497 ± 2 Ma (Dunning Ma (Nissen 1986). These intrusions are consi & Pedersen 1988, Furnes et al. 1988) for an dered to have been emplaced in a foliation immature arc stage of the ophiolite. forming orogenic phase in Late Cambrian time A variety of Caledonian plutons collectively (Nissen 1986). Many of the large plutons collec termed the Bindal Batholith are found in the tively grouped as the Bindal Bathol ith appear HNC. The intrus ions range in composition from to be essentially of Late Ordov ician to Early gabbro to quartz diorite and granite , and were Silurian age (Nordgu len & Schouenborg 1990), emplaced at various stages during the tectono- and a Silurian mineral age has been recorded NGU - BULL. 421. 1991 Tecto nos tratigraphy in the Ve/fjord-Tosen region 3

VElFJ ORD

I"All---.lGNEOUS_ R Q C~ S m HORNBLENDE OIORITE . GRANI TE. GRA NOD IORlTE UPPER N APPE IT] MICAG"N EiSS

M!DOL E~A~E IT] ~~:~RL~~~TEM~ C H~;~ B l[ . GPEE N PSAMMITE ,

CD ~: ~g:~~ ~~s cg6~~~::J~:~;E POl YMICT CD ~~~~Al~~~iT. O~A~~~~~~~~R~~IST

I b I ~~~~ I ~~ c~~~~ ~~ ~G : A6~'N~~V:~;~T E IT] ~g~~;i.E~~,.~tttD S;SH~~~ I~:~C~~~O~SA R B l E co ~~~~ Lg~~~~~~EAATE . PQlYMICT MAFIC o:J ~t~:D~ : ~ I~S~M61~'R ~; ~~~~~O~~A :~I~~S ~ IGN£OUS BASEMENT UNIT , METAGABBRQ

QI] ~~~X~T~ASSACl ~~~g~~ ~~~~T~.C ~I;~ B ~~ T H e;:12J;1 ~~~~~~~G ~TAES~ ~~ ~~T EU N IT

L9-'iLEA NA P P ~ ~ GRANIT IC DYK ES ~ M ICA GN EI SS . CALe -SI LI CATE GNEISS C!D MARBLE M AJ OR THRUST

....L---.... MI NOR THRUST __ FAU LT __ TECtONICAL LY MODI FIED UNCON FORMI TY

Fig. 2: Synoptic block diagram. showing the main lithOtectonic and structural units in the Velfjord-Nordfjellmark area .

from the Mosjeen Gabbro (Teru dbakken & while the final accretion of the HNC occurred Mickelson 1986). during the Late Silurian/Early Devonian Scandi Most of the deformation and thrusting of the an phase of the Caledonian Orogeny. HNC has traditionally been considered to have occurred during the Mid-Late Silurian to Early Devonian Scandian phase of the Caledonian Tectonostratigraphy: a summary Orogeny, though in several stages (Roberts In the Velfjord-Tosen region, the rocks can et al. 1983). In the Hattfjelldal area, along the be assigned to three nappes, informally named eastern margin of the HNC, Dallmann (1986, the Lower, Middle and Upper nappes (Fig. 2) 1987) considers that polyphase eastwards The Lower and Upper nappes compr ise litho thrusting of the HNC took place in Middle demic complexes of gneisses and calcareous Silurian times. This thrusting postdated thrust rocks . emplacement of the underlying Hattfjelldal The Middle nappe is the most extensively Nappe, forming part of the K61i Nappes (Dall studied nappe unit in the present investiga mann 1986). Radiometric datings by Nordgulen tion. It can be divided into two units forming & Schouenborg (1990), however, indicate that a basement-cover couplet. The basal unit con the juxtaposition of the HNC with the structu sists of variably altered ultramafic rocks and rally underlying medium-grade supracrustals metagabbros. In the Velfjord area, the Hegge in the Vestranden region along the south fjord ultramafite constitutes the major part of western margin of the HNC, and internal im the igneous basement unit. The cover unit brication of at least parts of the HNC, took consists of schists. metaconglomerates, psam place prior to or during the Late Ordovician, mites and marbles. 4 Terje Thorsnes & Helge Leseth GU · BULL. 421. 1991

The Heggefjord ultramafite shows evidence M ETAMORPH ICGRADE of several phases of deform ation which occur red prior to deposition of the overlying metase ' ~T IN G dimentary cover rock s. In the latter unit, 4 phases of deform ation have been established HIGH -- (0 1-04), with the peak of metamorph ism (medi um-grade) during 02. Structures assoc iated with 01 are few and usually obliterated by the M EDI UM - J~---- penetrative S2 foliation, formed as a result LOW -- / - __ of isoclinal folding and thru sting during 02 L------II~ T I ME (Fig. 3). Later deformation (03 and 04 ) refol D, ded the structures formed dur ing 0 1-02 and produced the presen t attitude of the rocks, Fig. 3: Schematic presentation of timing of metamorphism dipping 40-60° towards the ENE (for a more and thrusting for the cover sequence within the Middle nappe. detailed descr iption of the deform ation and metamorph ism, see page 14). metamorph ism in the area, intrude the contact (0. Nordgulen pers. comm. 1989). However, the present authors consider a thrust most Lower nappe likely, since the Lower nappe appare ntly struc The Lower nappe comprises a heterogeneous turally overlies the less deform ed and meta assemblage of gneisses, schists, calc-silicate morpho sed Sauren unit. The internal structure rocks and marbles (Fig. 1). It is intruded by a of the Lower nappe has never been studied variety of igneous rock s along its base (Fig. in detail, but from previous work it is clear 1). Tow ards the south , south of Tose n, the that the rocks are polyphasally deformed, local nappe becomes thinner as it is truncated by ly forming compl ex fold interference patterns intrusions belonging to the Bindal Batno lith. (Kollung 1967, Myrland 1972). Parts of the South of Terrak, the nappe is excised by mica gneisses are migmatitic ( ordg ulen these intrusions (Nordgulen et al. 1989). North 1984), indicating that these rock s have ex west of Tosen the structural thickness of the perienced metamorph ism at temperatures nappe increases to several kilometres. Here higher than the cover rock s of the Middle it is intruded by the Krakfjellet pluton along nappe. The gneisses of the Lower nappe may its southwestern margin (Nordgulen 1984). represent basement for the more evident supra In the north ern parts of the area, south of crustaI rocks of the nappe, but the generally Velfjord , the cont act relationships are less high strains have probably obliterated any clear. In the west, towards the Sauren-Torghat possible unconf orm able relationships. ten-area, beneath the Lower nappe (Fig. 1), a The gneisses of the Lower nappe are dark sequence of medium-gr ade schists , marbles grey, banded , coarse-grained, migmatitic bioti and metacong lomerates is seen to uncon te gneisses with abundant leucosome veins. form ably overlie a lower unit consisting of They show a well-developed penetrative folia variably altered ultram afic rock s and metagab tion, within which there are concordant leuco bros (Heldal 1987, Heldal & Hjelmeland 1988). somes compo sed mainly of quartz + oligocla Lithologically, the rock s bear several similariti se/andesine + biotite + muscovite + garnet + es to the Early Ordo vician Leka Ophiolite (fibrolitic) sillimanite ± microcline ::: horn Comp lex, positioned c. 50 km along strike blende ± chlorite. Accessory minerals are southwest of Sauren (Fig. 1). The rocks appe tourmaline, apatite, zircon, sphene, pyrite and ar to be less deformed and metamorphosed pyrrhotite.With increasing content of biotite than the rocks of the Lower nappe. It there and decreasing grain size, the gneisses grade fore seems appropriate to assign the rocks to into common ly rusty biotite-garnet schists , lo a separate tecton ic unit beneath the Lower cally with consp icuou s mats of fibrol itic sillima nappe, informally called the Sauren unit. nite. The precise nature of the contact between The calc-silicate rocks are generally dark, the Lower nappe and the Sauren unit is uncer banded and schistose consisting of hornblende tain, not least because a number of variably + plagioclase ± diops ide + biotite ± quartz sized plutons, producing widespread contact and minor amounts of calcite, apatite, allanite, NGU - BULL. 421,1 991 Tectonostratigraphy in the Velfjord-Tosen region 5

zircon and opaque minerals . With increasing The contact relations betwee n the Lower contents of biotite and quartz + plagioclase, and Middle nappes are best seen along the they grade into hornblende-biotite gneiss and shore areas in Velfjord (Fig. 5, loc. A). The psammitic biotite gneiss. Locally , thin layers Heggefjord ultramafite is here thru sted over of marble are present (Nordgulen 1984). the marbles of the Lower nappe (Fig. 5). The Marbles are prominent in the Lower nappe marbles beneath the thrust consis t essentially (Kollung 1967), typically in the form of white of medium- to coarse-grained calcite with sub or grey, banded calcite marble. The marble is ordinate amounts of tremolite-actinolite, plagio usually coarse-grained with occasional specks clase, quartz and pyrite, and contain scatte red of pyrite oriented in rows parallel to the band boudins of rusty schists up to 5 m thick, and ing. The content of impur ities, such as eale granitic dykes and veins. Approx imately 50 m silicate minerals , is variable. away below the thrust, cross -cutting relation Various plutonic rocks , generally termed the ships are found between granitic dykes and Velfjord Massifs, which range from hornblende schist boudins, and between individual granitic gabbros to granites (Vogt 1897, Kollung 1967, dykes. Just below the contact , remains of the Myrland 1972), constitute an important part dykes are found only as boat-shaped bou of the Lower nappe (Fig. 1). On a regional dins , with a strong foliation giving rise to a scale, the large plutons are conformable with flaggy appearance. The cleavage is anastomo their metamorphic envelope , but on outcrop sing, and the dyke material bears a well develo scale the relationships may be discordant, ped stretching lineation parallel to the L2 with apophyses cutt ing the penetrative foliation lineation found in the metased imentary rocks in the surrounding metasedimentary rocks . of the Middle nappe : i.e. plunging 30-50° to Contact metamorphic aureoles are present wards the ENE-ESE. At this level, cross-cut around these massifs (Vogt 1897, Kollung ting relationships between dykes and schist 1967). The Velfjord Massifs were therefore boudins, or between individual dykes , can no considered by Vogt (1897) to postdate the longer be observed. Since no granitic dykes main phase of deformation and metamorphism have been found in the ultramafite, this is clear in the investigated area, whereas Kollung ly indicative of a ore-thrust age for the grani (1967) interpreted them as synorogenic. In the tic dykes, and a thrust boundary between the Sausvatn -Markafjell area (Fig. 4) a dioritic to marbles of the Lower nappe and the overlying monzodioritic pluton (Kollung 1967) belong ing ultramafic unit of the Middle nappe . to the Velfjord Massifs cuts the thrust contact A similar relationship, but on a larger scale, (02) between the Lower and Middle nappes is found along the northeastern side of the (Thorsnes 1985), but the pluton is deformed Tosen fjord . In a C. 400 m high cliff exposu re, by later deformation structures (03 and 04 ). marbles of the Lower nappe are superposed In our opinion, this shows that the Velfjord by Middle nappe rocks ; small lenses of serpen Mass ifs were emplaced at a compara tively tinite, mica schist, psamm ite and metacong lo late stage of the tectonometamorphic evolu merates (Fig. 6, UTM 925 317). Granitic dykes tion , postdating the main stage of deformation are nearly totally restr icted to marbles of the and thrusting. Lower nappe. In the lower part of the section , well below the contact between the two , the Contact relations between the Lower granitic dykes are found to transect the ban and Middle nappes ding in the marble. Towards the thrust con The Middle nappe is considered to have been tact with the Middle nappe, the granitic dykes thrusted over the Lower nappe on the basis gradua lly become sub-parallel to the banding. of evidence presented below. On a local scale, Since the dykes become highly deformed the foliation and lithological banding of the when approach ing the thrust plane betwee n rocks in the Lower nappe are concordant with the Lower and Middle nappes , this implies the thrust plane. However, on a regional scale, that the rocks of the Lower nappe were intru the base of the Middle nappe truncates lltholo ded by granitic dykes prior to the thrusting gical contacts in the footwall, as e.g. the con event. The few granitic dykes present in the tacts between gneiss and marble in the Lower Middle nappe are of unknown age, but are nappe (Fig. 5). probably not related in time or space to the dykes in the Lower nappe . 6 Terje Tnorsnes & Helge Lesetn GU -BULL.4 21. 1991

"> J I '" I . : '> J .. .'" . " • I '" '0 ~ J

Fig. 4: Simplified map over the Nordfjellmark area . Sausvatn, showing the main tectonos tratigraphic units and structures. For legend. see facing page.

Other intrusive rocks evidently postda te the forme r. On a larger scale, entire plutons can thrusting. At the Velfjord locality (Fig. 5, loc. be shown to cut the thrust. At Markafjell in A), one set of basic dykes showing chilled the Nordfjellmark area, C. 10 km south of margins cross-cuts both the thrust betwee n Velfjord (Fig. 4), a small (c. 4 km') dioritic to the Lower and Middle nappes , and the penetra monzonitic pluto n has been mapped by Kol tive foliation in the marble-schist unit of the lung (1967). This pluton cuts both the basal NGU- BULL. 421. 1991 Tectonostratigraphy in the Ve/fjord- Tosen region 7

LATE IGllE-'lUS ROCKS out in the northern and southweste rn parts ~ HORNBLENDE DIORITE IGRANITE, GRANODIORITE of the body, while the dunite occupies the

MIDDLE NAPPE central part. Wehrlite and pyroxenite occur in E -I NORDLl AN MA. MARBLE, GREEN PSAMM ITE, a small fault-bounded block in the south - - STAUROLlTESCH IST =01 LAN GFJORD CONGLO MERATE, POLYM ICT western parts. ~ CALCAREOUS CONGLOMERATE The ultramafic rocks are interpreted to be r.-=l BUOYA SCHIST, CALCAREO US SCHIST l=...LJ WITH LEN SES OF CONGLOMERATE of mantle affinity, and the structures and textu r.-ol SIORVIKA CONGLOMERATE. POLYM ICT res described below are considered to be rela L....!...... J MAFIC TO CALCAREO US CONGLOMERATE r::=1NORDFJ ELLMARK SCHI ST. CALCAREOUS ted to early, non-orogenic deformation under -.:---= SCH IST, BANDED PSAMM ITES AND MARBLE upper mantle or lower crustal conditions (for ~ ANES CONGLOMERATE, POLYMICT MA FIC ~ CONGLOMERA TE a detailed descr iption and discuss ion, see = GLOMM EN PSAMMIT E . CALCAREOUS, HORN ~ BLENDE-RICH OR QUARTZ -RICH PSAMM ITES Lesetn (1985)). The harzburg ites show an ~ IGNEOUS BASEMENT UNI T. METAGABBRO imperfect foliation defined by zones or bands enriched and/or depleted in flattened grains e -d AUNVATN SC. CALCAREOUS SCHI ST WITH --- LOCAL BASAL CONGLOMERATE, MARBL E of enstat ite partly replaced by bastite. This ~ IGNEOUS BASEM ENT UN IT harzburgite 'foliation' is parallel to pyroxenite ~ HARZB URGITE I DUNITE veins and tabular bodies of dunite (partly veins, LOWER NAPp E partly bodies of uncertain origin). In dunitic ~ GRAN ITIC DYKES bodies, chromite bands form a distinct ban 1-_- 1M ICA GNE ISS . CALC - SILI CATE GNEISS ding, and flattened chromite grains may form ~ MARB LE an axial planar cleavage in the fold hinges -"--"- MA JQR THRUST -'---'- MINOR THRUST of open to isoclinally folded chrom ite layers. - - FAULT Cross-cutting relationsh ips may be observed ~- TECTON ICALLY MOD IF IED UNCONFORM ITY between individual pyroxe nite veins, and bet ~ FOLIATION INMETASEDIM ENTARY ROCKS ~ TRANSPOSED BANDING IN ULTRAMA FIC ROCKS ween pyroxen ite veins and sheet-like dunite -+- ANTIFORM TRACE bodies (Fig. 7). These planar structures app _ SYNFORM TRACE ear to have formed during the same stage of early, non-orogenic deformation, and are normally parallel or subparallel. Microst ructures and text ures indicative of upper mantle/lower crust deformation of the thrust and the internal imbrication of the Midd ultramafic rocks are also found. In the north le nappe (Figs. 2 & 4), and also postdates the ern harzburgite, two types of relict coarse penetrative S2 foliation in its metasedimentary grained olivine occur. The first type is that of cover unit. 0.5-2 cm equant olivine cryst als which have straight grain boundaries meeting in triple junc 0 Middle nappe tions at 120 • The second type is usually over The lower part of the Middle nappe is compo 2 cm in size, with quite irregular grain bounda sed of a unit of igneous rocks .It comprises ries and common kink bands, giving a porphy variably altered ultramafic rocks and lenses roclastic appearance. These textures corr e of metagabbro, cropp ing out discontinuously spond well with text ures termed 'coarse equ along the base of the Middle nappe. ant' and 'coarse porphyroclastic' by Nicolas The largest and best preserved igneous et al. (1 980), who concluded that they develo complex is found at Nevernes, where there is ped during plastic deformation simultaneous an approx imately 5 km long and up to 1 km with crystal slip and recrystallization at tempe wide body, the Heggefjord ultramaf ite (Fig. 5). ratures between 10000 and 13000 C, formed The main lithologies in the ultramafite are during asthenosphere flow at seafloor sprea harzburg ite (dominant), dunite and minor ding sites. amounts of clinopyroxen ite and wehrlite. The A third type of olivine is represented by fine mineralogy of these rocks has been extens ive grained, elongated and recrystallized olivine ly modified by both regional and contact meta crystals or aggregates defining an LS fabric morphic processe s. Some relict primary mine in the southwestern parts of the ultramafite. rals and textures are, however, preserved The fine-grained textures are associated with particularly in the northern and central parts a strong and partly mylonitic foliation. This can of the Heggefjord body. The harzburgite crops to some extent be compared to textures ter- 8 Terje Thorsnes & Helge Lose th NGU · BULL. 421, 1991

o 2 k m N '. V E LFJ ORD

Fig. 5: Simplified map over the Nevernes area, Velfjord, showing the main tectonostratigraphic units and structures. For legend, see previous page. A- locality with well exposed relations between the Lower and Middle nappes. B - locality of fig. 7. NGU - BULL. 421,1 991 Tectonostratigraphy in the Velfjord-Tosen region 9

Fig. 6: Cliff expo sure of the contact between the Lower and Middle nappes (the contact runs between the two white marker s, middle left to top right). Note the abundance of granitic dykes in truding marbles and eale-silicate rocks in the Lower nappe. In the lower part, discordant relat i onships are found, whil e close to the contact the dykes are concordant to the base of the Middle nappe. The Middle nap pe is virtually free of granite dykes at this locality. The height of the cliff expos ure is c. 400 m. The cliff is located on the south eastern side of Tosen (UTM 92531 7).

med fine-grained porphyroclastic to mylonitic A number of minor faults and fractures hav by Nicolas et al. (1980). Such textures are ing the same trend may also be related to most common in the basal part of an ophiolite this brittle deformation, During regional defor pseudostratigraphy, but can also be found in mation and metamorphism, reactivation of the the transitional zone between mantle tectonites se faults and fractures probably accommod a and cumulates in an ophiolite , where the majo ted the regional strains and served as path rity of the structures relate to deformation ways for fluid migration. Extensive retrogres under upper mantle or lower crustal condi sion took place in these zones, and large tions. Early, non-orogenic deformation has porphyroblasts of serpentine and magnesite also been reported from the harzburgite tecto (up to 10 cm) are present. Spectacu lar ring nite of the Leka Ophiolite Complex (Furnes growth structures also occur where faults inter et al. 1988), sect, leaving less altered peridotite fragments The early, non-orogenic deformation of the in a talc-antigorite matrix (Fig. 9). Locally, the ultramaf ic rocks was followed by faulting un retrogression zones are strongly deformed , der brittle conditions; Le. at high levels in the produc ing talc-antigorite schists where the crust , possibly related to a post-obduction schistosity is parallel to the margins of the event predating the deposition of the overlying alteration zones, while porphyroclasts of mag metasedimentary sequence, The best example nesite and antigorite can be found in the cen of these structures is the fault contact bet tral parts. Apparently, the alteration zones ween the northern harzburgite and the central have had a great ability for accommodating dunite which causes a discordant relationship between the chromite layering in the dunite L~ c=J PYRQXENITE VEIN ~ -:; .;:j DUNlTE HARZB URGtTE and the pyroxenite veins in the harzburgite ...... These planar fabrics are normally parallel, belonging to the early tectonic structures (Fig, 8). This fault ends at the base of the overlying metasedimentary rocks , thus indicating that brittle deformation of the ultramafite occurred prior to deposition of the cover sequence. This is a similar situation to Leka, where the b : :::: :::::: >~:·: ·: : : ::::· major movements on some of the faults in the LaC clearly predate the deposition of the Fig. 7: Drawing of observed field relationships showing unconformably overlying Skei Group (Fumes tabular dunite bodies in harzburgite cross -cut by a folded et al. 1988), pyroxe nite vein. For location , see fig. 5. 10 Terje Thorsnes & Helge t.esetn GU· BULl. 421. 1991

M IDDLE NAP PE 1=-1 ~~ 6~~~~ E :~A~~ D E - ENSTATITE FOLIATI ON Kj, ~ EOUS SASE WEHT. U N IT C - CH ROMI TE LAYER ING } NOR MA LLY PARA LLE L P - PYROXENITE VEINS o HARZBURGITE ...... : :: :: : ::: : :~ OU" "TE ...... LOWER NAP PE ...... ~ ~ ~tAB LlN E 1 S ...... 1000""

N ::\\:\\ \:\\\ \\\\:ki s :\:\::[:\\\::: :\:\\\\\\[:\\\:\\\\\lL\[::::::::' ~ ::: : :: :: :: : : : : : : :: : : :: : : : : :: :::: :: : : : : : :: : : : :: : : : : :: : : : : : :: :: :: : :~. " : \ i)W: ......

:::::....:: :: ..::: \':: :::: :::: :::: X:32 :::· ·:::· :::>'· :-:-: -:.:-:- :-:.:-:- :-:-:- :-:-.r,::-:-:-

::..:::::. :::::. . .:\\. :::. .::.:::::. . :.::.::::.. ::::::., ......

Fig. 8: Fault con tact betw een harzburgite and dun ite, truncated by the overlying metasedimen tary rocks (le of arrow . map to the left. The black rect ang le shows the outline of the map to the right ). Cnrorrute layero ng and pyroxenite vems are norm al ly para llel. but show discordant relationships across the fau lt (map to the right).

most of the strain during regional deforma Contact-metamorphic microtextures within tion, leaving the less altered and more rigid the ultramafic rocks have been described by parts of the ultramafite virtually undeformed. Bakke & Korneliussen (1986). They found that Structures related to the translation of nap while the northern part of the dunite contains pes and deformation of the entire sequence primary olivine crystals with deformation lamel are found mainly in the marginal parts, or in lae, the olivines in the southern part were shear zones within the ultramafite. In the recrystallized into a microscopic jack-straw marginal zones, particularly along the thrust text ure. This textur e can be ascribed to con betwee n the ultramafite and the Lower nappe, tact metamorp hism caused by the nearby diori but also locally along the contact with the tic to gabbroic plutons of the Velfjord Massif. overlying metasedimentary rocks, retrogres Another part of the basal igneous unit is sion of the ultramafite has produced talc found south of Sausvatn, at the western part antigorite schists with a strong foliation, How of Markafjell (UTM 907 448). A lens of metagab ever, these zones seldom exceed one metre bro (c. 1 x 3 km) lies between two of the litho in thickness . In the northernmo st part of the logical units within the lower part of the meta ultramafite, shear zone structures are com sedimentary rocks. This is considered to repre mon. These are observed either as eyes or sent a dismembered part of the basal igneous lenses of relatively fresh harzburgite set in a unit, whose present structural position is rela serpentine schist matrix, or as mega-augen ted to imbrication of the lower parts of the where relatively fresh peridotite is separated Middle nappe. by thinner zones of altered rock s. The structu In the eastern, lithostratigraphically upper res range in size from a few centimetres up most parts , the metagabbro is coarse- to very to 18 m in thickness and several hundred coarse-grained. It is weakly to moderately folia metres in length. A strongly developed serpen ted, with the foliation expressed by elongated tine foliation is usually associated with the aggregates of hornblende, parallel to a ban zones. The trend of the foliation varies, but ding defined by bands of dark hornblende, is normally parallel to the surfaces of unalte alternating with discontinuous, irregular lami red harzburgite. nae of greyish plagioclase. The hornblende NGU · BULL. 421.1991 Tectonostratigraphy in the Velfjord- Tosen region 11

may form individual crystals up to 1 cm long, and in places shows relict 'sub-ophitic' textu res. Along strike, the metagabbro shows appa rently primary textura l and modal variation. The metagabbro is cut by three types of dykes ; amphibolitic, trondhjemitic and granitic. The amphibolitic dykes are dark, fine-grained and massive, with the margins transposed by the penetrative 'S2 foliation'. They contain dominantly dark hornblende and plagioclase, with locally abundant opaque minerals and minor epidote and quartz. The light-coloured trondh jemitic dykes are up to 1 m thick, and show a well developed foliation with elongated Fig. 9: Ring growth structure within the Heggefjord ultrama quartz grains set in a fine-grained matrix of fite. produced by alteration of peridotite to talc and antigor i quartz, plagioclase, hornblende and minor bio te along intersecting brittle faults. Pencil (14 cm) for scale. tite and epidote. The amphibolitic and trend hjemitic dykes are found only in the metagab bro, and carry a similar imprint of deformation part of the metagabbro are probab ly the res and metamorphism. We therefore consider ult of thrusting and imbrication of the lower them therefore to be genetically related to the parts of the Middle nappe during regional D2 metagabbro. deform ation. The granitic dykes in the metagabbro are medium- to coarse-grained, weakly foliated, Metasedimentary rocks in the Middle nappe and, composed of quartz, K-feldspar, plagio The metasedimentary rocks, which structurally clase and biotite, with minor hornblende , sphe overlie the basal igneous unit, compr ise a ne and epidote. Their width varies from 1 to series of psammites, schists, conglomerates, 25 metres. The dykes are also found in the marbles and possible felsic volcanites. Despite metasedimentary units surrounding the meta high strains, various lines of evidence indicate gabbro , and they cross-cut the penetrative that the contact between the igneous unit and foliation. They thus postdate the penetrative the metasedimentary rocks is a primary uncon foliation and the early deformation of the formity, though modified during later regional metagabbro . They are, however, affected by deformation and metamorph ism. the later folding of the metagabbro. The contact between the basement unit and Moving towards the tectonostratigraph ically the metasedimentary rocks is sharp. Lithologi lower part of the metagabbro, there is a pro cal banding in the metasediments, regarded gressive mylonitization demonstrated by dec as transposed bedding, is concord ant with the reasing grain size and development of planar contact on both outcrop and regional scale. and linear fabrics. In the middle part of the Near the contact, however , the metasediment metagabbro, anastomosing shear zones have ary rocks exhibit comparatively low strains. isolated lenses of little deformed coarse This is shown by the presence of pebbles in grained metagabbro in a matrix of fine-grained a conglomerate and phenocrysts in crystal amphibolite. In thin-section, porphyroclasts of tuffs immediately overlying the Heggefjord ut hornblende with recrystallized margins are tramafite. Furthermore, in the same area, bou commonly observed. dins of amphibole-rich layers in psammite for In the western, lowermost part, the original med during D1 are preserved within 02 fold metagabbro is transformed into a fine-grained cores. Such structures have not been obser hornblende schist showing a well-developed ved away from the contact. In the Heggefjord LS fabric and is considered to be a mafic ultramafite, no increase in strain is evident mylonite. Sporadic, light-coloured, concordant when approaching its upper contact, but a and isoclinally folded, quartzo-feldspathic thin (generally c. 1 m thick) talc-antigorite bands exhtbitinq features indicative of large schist is locally developed along the contact , ductile strains are considered to be myloniti possibly due to the difference in competence zed trondhjemitic dykes . between the rocks. In the metagabbro in the The large strains record ed in the lowermost western part of Markafjell, there is a distinct 12 Terje Thorsnes & Helge Lesetti NGU · BULL. 421. 1991 decrease in strain towards the contact with and tectonostratigraphic units is shown in overlying metasedimentary rocks. Fig.2. The lithology of the overlying metasedimen In the Velfjord area, the lowest unit is term tary rocks yields evidence for erosion of an ed the Aunvatn schist (Fig. 5). It comprises a assemblage of rocks partly comparable to the basal conglomerate, partly calcareous psammi igneous basement unit. In the psammites im tes, hornblende-bearing psammites, garnet mediately above the Heggefjord ultramafite, biotite-sch ists, thin marble horizons and subor aggregates of tremolite-actinolite associated dinate amounts of fine-grained acidic igneous with chromite, together with pebbles of meta rocks (possibly extrusions or shallow intrusi gabbro , constitute part of the pebble popula ons). The structura l thickness of this unit in tion within pockets of a calcareous conglo creases from c. 100 m to c. 200 m, from north merate. The tremolite-actinolite aggregates are to south. At the base of the unit, there is a considered to represent metamorphosed ultra calcareous banded psammite with a locally mafic c1asts carrying chromite. developed basal polymict conglomerate , up to In the Nordfjellmark area (Fig. 4, UTM 910 10 m thick, directly above the basal igneous 435), similar psammites and schists overlie unit (Figs. 2 & 4). The conglomerate is matrix serpentinized ultramafic rocks . In the ultra supported, with up to 70% calcite in the ma mafic rocks , pyrrhot ite grains with pentlandite trix. Locally, the matrix is green due to a relati lamellae are present. Immediately overlying vely high content of tremolite-actinolite. Pebb these rocks , there are biotite-quartz-feldspar les and clastic grains occur scattered in the schists with recrysta llized pyrrhotite with pent matrix. The clast population is made up of landite lamellae (L.P. Nilsson , pers.comm . gabbros, fine-grained quartz-plagioclase-cal 1991). cite rocks , and aggregates of tremo lite-actino Further up in the sedimentary sequence, the lite associated with chromite grains. The clas pebble populat ion of polymict conglomerates tic grains are of chromite and pyrrhotite to includes a high proportion of metagabbro, gether with thin bands rich in chromium probab greenstones, felsic volcanites and scattered ly representing altered, smeared-out chromite tremolite-actinolite aggregates . The combined grains, and plagioclase partly recrystallized to evidence for low strain near the basement a fine-grained granular texture. The Aunvatn cover contact , together with the lithology of schist appears to be separated from overlying the overlying metasedimentary rocks , the ear mica schists and polymict conglomerates by ly ductile deformation in the basal igneous a tectonic break, most probably a minor thrust unit, and the brittle fault ending at the cover (see p. 15). sequence, provide sufficien t evidence to inter This mica schist and polymict conglomerate pret the contact as a primary depositional unit can be traced for c. 15 km southwards unconform ity. During regional deformation, to Nordfjellmark, c. 2 km south of Sausvatn, some modification of the contact probably where similar lithologies overlie lenses of ser occurred , as shown by the talc-antigorite pentinized ultramafic rocks included in the schist zones locally developed along the mar basal igneous unit. A lens of metagabbro struc gin of the igneous complex . Similar primary turally overlies the Aunvatn schist in the Nord stratigraphic unconformities have been descri fjell mark area. This lens is considered as a bed between cover sequences and ophiolitic dismembered part of the basal igneous unit, rocks at a number of localities in the region which most probably was imbricated during (Bang 1985, Sturt et al. 1985, Heldal 1987, thrusting of the Middle nappe. This is overlain Husmo & Nordgulen 1988). by a thick metasedimenta ry sequence, start ing Due to D2 imbrication in the lower parts of with psammites. the Middle nappe, the pattern of lithostratigra The psammites have a structural thickness phy changes from north to south. In the Vel of c. 100 m, and are termed the Glernrnen fjord area, contact relations between the igne psammite. Three varieties occur; grey quartz ous basement unit and the overlying metasedi rich psammites, green calc-silicate-rich psam mentary rocks are best preserved , while the mites, and dark hornblende -rich psammites. most complete tectono- and lithostratigraphy The mineralogical immaturity and locally high is found in the Nordfjellmark area, midway content of mafic minerals such as hornblend e between Velfjord and Tosen. A synopt ic pro indicate erosion from a mafic source. file summarising the main structural features The psammites pass upwards into the Anes NGU · BULL. 421, 1991 Tectonostratigraphy in the Velfjord- Tosen region 13 conglomerate, which is an up to 200 m thick polymict 'mafic' conglomerate, with horn blende-rich matrix and a number of clasts reflecting erosion from a mafic to ultramafic source (Fig. 10). The matrix is dominated by hornblende , plagioclase and epidote, with pos sible clastic grains of chromite and plagiocla se. The clast size normally ranges from 0.5 X 2 to 5 X 20 cm, though some clasts may re ach up to 20 X 80 cm (in X-Z sections). In places, hornblende-rich psammites are found within the conglomerate. The clast populat ion is dominated by med ium- to coarse-grained metagabbros, fine- to Fig. 10: Polymict mal ic conglomerate (Anes conglomerate). very fine-grained greenstones and green with ctast population dominated by gabbro and greens tone (UTM 948 355). Pencil (14 cm) for scale. schists, trondh jemite and altered ultramafic clasts (tremolite-actinolite aggregates).In addi tion, epidote nodules , quartzite, vein quartz , granite and possible chert or jasper is pre sent,·especially in the upper parts of the con glomerate. Many of the metagabbro pebbles are comparable to the metagabbro at the western part of Markafjel l. The conglome rates are followed by c. 150 m of schists, marbles, and banded psammites , assigned to the Nord fjellmark schist. In the Velfjord area, clastic grains of pyrrhotite with pentlandite exsolution lamellae are found in the psammites. These are similar to those found in sediments imme diately overlying the Heggefjord ultramafite. At the top of the unit, a laterally persistent Fig. 11: Isoclinally folded psammite with in the BU0ya schist , marble horizon can be traced for at least 30 Nordfjellmark area. Bands rich in quartz and plagioclase alternate with bands rich in biotite and hornblende. Pencil km, probably indicating stable depositional (14 cm) lo r scale. conditions. A new period of tecton ic instability is ex pressed by two conglomerate horizons, sepa rated by a calcareo us schist. The lowermost glomerates in being dominantly calcareous . of these conglomerates , the Storvika conglo The matrix may conta in up to 80% calcite, and merate, is dark , polymict and matrix- supported large marble clasts are commo n. The propo r with rather small clasts, seldom exceeding 2 x tion of granitoid and quartzitic clasts is high 5 cm. The clast population is basically similar in compar ison with the Anes conglomerate, to that of the Anes conglomerate, but locally while that of gabbro, greenstone and trend marble clasts form an important part. The hjemite is correspondingly lower. Scattered matrix is generally rich in hornblende and pla clasts up to 10 x 25 cm (in X-Z section), of gioclase, though locally there is a higher qu medium- to coarse-grained quartzo-feldspath ic artz content , together with calcite and calc rocks exhibit a pre-pebble foliation, and were silicate minerals. Clastic chromite grains occur probably derived from acidic orthogneisses. sporad ically in the matrix. Also quartzitic to psammitic pebbles may have The Storvika conglomerate is overlain by the a pre-pebble foliation. This indicates that meta BU0ya schist (Fig. 5), compr ising mainly cal morphic sialic crust must have been available careous schists, with local development of for erosion, at the time of deposition. In the quartz-rich psammites (fig. 11), conglomerate upper parts of the unit, psammites and schists lenses and thin layers of quartz-keratophyres. with sporadic conglomerate lenses become The upper conglomerate horizon, the Lang increasingly important. The schists are either fjord conglomerate, differs from the other con- calc-silicate or biotite schists. Staurolite and 14 Terje Tnorsnes & Helge l.ese tn GU · BULl. 421.1991

garnet may be important accessory minerals logy, typically being grey, coarse-grained and in the mica schists. intercalated with calc-silicate bands. In addi The uppermos t lithological unit, the Nordlian tion, there are variably abundant garnet-fibro schist, comprises marbles interbanded with lite-biotite schists and amphibolites. green psammites in the lower part, and stauro The Upper nappe is cut by a number of lite schists in the upper part. The marbles are acidic to intermediate intrusions. A large elon green-grey and have occas ional pebbly beds. gated pluton, termed granite by Kollung (1967) dominated by rounded quartz ite fragments. has intruded approximately along the contact The staurolite schists are green-coloured. with of the Middle and Upper nappes east of Lang staurolite porphyrob lasts up to 1.5 cm in diame fjorden (Figs. 1 & 4). Recent mapping and ter. Bands of greenschist may represent meta geochemical investigations (0 . ordgulen pers. morphosed and retrogressed basic tavas, sills comm. 1990) show that this is a light-colour or transposed dykes. ed. medium- to fine-grained, strongly foliated tonalitic rock. In most areas the tonalite is cut Contact relations between the Middle and by various pegmatites and granitic dykes . Upper nappes Similar metamorphic and plutonic rock s On the basis of regional considerations, the outcrop extensively in the HNC outside the contact between the supracrustal rocks of the investigated area (Gustavson 1981, 1988). but Middle nappe and the overlying, partly mig it is not known whether these rocks belong matitic gneisses and metased imentary rocks , to the same or a different tectonic unit. is interpreted to be a thrust. The contact, howev er, is concordant on the outcrop scale. No mylonites have been obse rved at the con Deformation and metamorphism tact , only a well developed foliation. Consi dering that the thrust separates metasedimen in the metasedimentary rocks of tary rocks rich in phyllosilicates. and that the Middle nappe movement most probably occurred under The predominant stage in the tectono-rneta medium-grade cond itions. this is perhaps to morphic evolution of the metasedimentary be expected. On a regional scale, there are rock s of the Middle nappe was 02 . with the indications that some of the marble units with formation of a penetrative LS fabric nearly in the Upper nappe become excised or at le obliterating earlier structures. The following ast attenuated along the base of the nappe pre-02 structures are collectively termed 0 1, (at Fuglvatn, east of Tosen (Nordgulen et al. even though they may possibly belong to diffe 1989)). rent events. South of Tosen , the contact between the Porphyroblasts of staurolite and garnet have Middle and Upper nappes is cut by granites overgrown a foliation defined by straight or and granodiorites of the Bindal Batholith (0 . weakly curved trails of very fine-grained to Nordgulen pers. comm. 1986). fine-grained quartz and opaque minerals, re garded as a S1 foliation. These porphyro blasts are deform ed by 02 structures, and Upper nappe must have grow n interkinematically 0 1-02 , or The Upper nappe comprises a heterogeneous in the initial stages of 02. assemblage of meta-sedimentary rocks and In hinges of asymm etrical F2 folds, a folia gneisses (Kollung 1967, Gustavson 1981. tion defined by tiny biotite flakes forms part of 1988). They are generally rather coar se-grain a composite SO/S1 foliation which is overgrown ed and partly migmatitic, with a penetrative by somewhat larger biotite grains parallel to folia tion. T he gne isses range from lig ht- co lo ur the a x ial surface of the F2 fo ld (SO is a comp o ed, quartzo-feldspat hic acidic gneisses, to sitional banding, assumed to be represent dark varieties rich in biotite and cafe-silicate bedding). minerals. Close to the contact with the ultramafic rock s The metased imentary rocks are dominated in Velfjord (Fig. 5), boudins of amphibole-rich by calcareous psammites and schists, in which layers in psammite are found, oriented perpen the conte nt of calc-silicate minerals such as dicular to the S2 foliation, in a F2 fold hinge. epidote , actinolite and diopside may be signifi These must have formed by 0 1 boudinage cant. Marbles form a locally prom inent litho- prior to the development of S2 foliation. NGU · BULL.4 21,1991 Tectonostratigraphy in the Velfjord-Tosen region 15

The structures ascribed to 02 include isocli basis of refolding of F2 folds, and/or fold ing nal folds with assoc iated penetrative axial pla of the L2 lineation, and/or the formation of a nar schistos ity (S 2, normally transpos ing bed S3 crenulation cleavage from the S2 foliation . ding and earlier cleavage), strong flattening The regional extent of this deformation is of pebbles, well developed mineral lineations uncertain, but the folds have the same orienta and stretching of pebbles (L2), and boudinage tion as the major , open , map-sca le folds which parallel to S2. Most of the small-scale F2 fold can be seen in the Velfjord-Tosen area (Fig. axes and the L2 lineations plunge c. 20°-40° 1). A locally developed stretching lineation can towards the E-ESE. However, due to later be seen in the Markafjell pluton (Fig. 4), defi refold ing (03, 04 ), the direction and dip of ned by the preferred orientation of hornblende these structures may vary considerably. The and plagioclase laths. This lineation has broad attitude of the S2 foliation is similarly gover ly the same trend as the L2 lineation, but can ned by the later folding. In the Nordfjellmark be separated from it because the pluton clear area, a large isoclinal S-fold (Fig. 4) with a ly transects the 02 fabrics in its envelope , and fold axis plunging c. 45° towards the ENE is thus predates them. However , this coincidence a prominent structure. The amplitude and of trends may indicate that 02 and 03 are wavelength of this fold is estimated to 300 m coaxial. and 700 m, respectively. Another large isocli The last deformation event observed in the nal fold is outlined by a marble horizon found invest igated area is termed 04 , and includes within the Aunvatn schist in the Nevernes area gentle to close folds , in many places of chev (Fig. 5). This fold is somewhat strange since ron-type, with fold axes plunging shallowly no complementary fold has been found. Most towards the N-NE or the S-SW, and with axi probably, the stratigraphy has been excised al planes dipping towards the W-NW. A local due to movements along the northeastern limb ly prominent crenulation cleavage , S4, can be of this structure. This proposed tectonic break found . The folds are found only as local, is placed at the top of the Aunvatn schist. small-sca le structures , and probably have little However, it should be noted that the sedi regional importance. ments above this tectonic break do not differ The metamorphism accompanying the defor significantly in terms of metamorphism or struc mation is characterised by prograde meta tural style. This tectonic break, however, corre morphism which reached medium-grade cond i sponds well with the minor thrust found at the tions during 02 , followed by post-02 retrog res base of the metagabbro in the Nordfjellmark sion. Metamo rphism prior to 02 is poor ly cons area. trained, but growth of biotite and plastic defo r 02 was characterised by large ductile stra mation of quartz during 01 tend to indicate ins as recorded by the isoclinal folding, pene low-grade cond itions during this early event. trative schistosity and a prominent stretch ing Porphyroblasts of staurol ite and garnet, gro lineation; and the S2 foliation is parallel or wing interkinematically (01-02) or in the initial sub-para llel to the thrust plane and the im stage of 02 , indicate that medium-grade cond i brication zone in the lower part of the Middle tions were reached prior to or in the initial nappe . Consequently, the most likely interpre stage of 02 . These conditions persisted tation is that the thrusting and imbrication of throughout 02 , as shown by the growth and the nappes took place during 02. Provided recrystallization of minerals such as biotite, this is correct, then the trend of the L2 linea garnet, fibrol itic sillimanite and hornblende tion may be taken to indicate the direction of along the S2 foliat ion. Retrogression of the nappe transport. medium-grade parageneses occurred during The third deform ation event, 03 , is character 03 , as shown by the growth of chlor ite, chlor it ised by a number of open to isoclinal, small oid and white mica. No metamorphic reac to medium-scale folds (wavelength s in the tions have been observed in assoc iation with order 0.1 to 20 m), with associated L3 lineati 04 structures. on and a S3 crenulation cleavage . The trend of the fold axes is difficult to distinguish from the F2 folds , while the axial planes and the Summary and discussion S3 crenulation cleavage have more of an E-W Regional and local studies (Kollung 1967, orientation. The 03 deformation can, in most Myrland 1972, Nordgulen 1984, Thorsnes 1985, places, easily be separated from 0 2 on the Nissen 1986) show that the lithologies of both 16 Terje Thorsnes & Helge l.eseth NGU - BULL. 421, 1991

the Lower and the Upper nappes are part of ver sequence unconformably overlying the a metamorphic complex comprising both para Lyngen Gabbro - considered to be part of and orthogneisses. which togeth er with huge an ophiolite comp lex. In the lower parts of volumes of pluton ic rocks constitute the bulk this sedimentary sequence, rock s derived from of the HNC. In the southern part of the HNC, the ophiolite dominate. while in the upper fine-gra ined granodioritic dykes (526 ± 10 parts there is a significant contr ibution from Ma) and dark . medium-grained tonalitic dykes metamorphic continental rock s. This influx of (503 ± 26 Ma) intruding paragne iss and mar continental pebbles is interpreted by Sturt & ble, have been dated by whole -rock Rb-Sr Robert s (1991) to be associated with the ob methods (Nissen 1986). Based on field relati duction of the Lyngen ophiolite onto the west ons, Nissen (1 986) conclude d that the dykes ward extension of the continent Baltica, follow were syn-tectonic, and that the tonalite is ol ed by uplift, erosion and subsequent depos i der than the granodiorite. Conseque ntly. the tion of a sedimentary sequence reflecting ero sequence was Cambrian or older. and presu sion from both continental and ophiolitic sour mably deformed in Late Cambrian to Early ces. Such far-reaching conclusions cannot be Ordov ician times. The dykes are considered drawn from the present study , but the occur to have intruded during a foliation-forming rence of probab le continental rocks in the orogenic phase in Late Cambrian time (Nissen upper metasedimentary units of the Middle 1986). nappe indicates that the ophiolite was empla These findings have important implications ced on to some sort of metamorphic, sialic for the understanding of the tectono-meta crust of pre-obd uction age. prior to the depos i morp hic history of the HNC. Based on lithologi tion of the metasedimentary rocks. cal similarities. the ophiolitic basement unit The poorly sorted nature and mineralogical of the Middle nappe seems to corre late with immaturity of the matrix of the conglomerates the Leka Ophiolite Complex (LOC) (Prestvik indicate rapid depos ition. possibly due to syn 1972, Sturt et al. 1984) in the western parts sedimentary faulting. The recognition of the of the HNC. Radiometric dating (U/Pb zircon) basement-cover contact as a modified deposi by Dunning & Pedersen (1988) of a quartz tional unconformity implies that the sequence keratophyre dyke in the LOC has yielded 497 in the lower parts youngs towards the east. ± 2 Ma, and this implies an Early Ordovician Except for a large F2 fold in the Nordfjellmark age for the form ation of the primitive arc sta area. and the invoked tecton ic discontinuity in ge of the LOC. Hence. if the corre lation is the Velfjord area, the sequence seems general valid. regional deformation (0 1-04) and meta ly to have a younging toward s the east, parti morph ism in the igneous basement unit and cularly when the change of clast populations the metasedimentary rocks of the Middle nap in the conglomerates is taken into considera pe must be younge r than the Early Ordovician. tion. However, the generally high strains, gene In summary. the metasedimentary rocks of ral lack of reliable way-up criteria and lack the Middle nappe are dominated by schist s of faunal evidence precludes any definite state and conglomerates, with minor amounts of ments regard ing the true stratigraphy of these marbles. The bulk of the clasts in the conglome rocks . Conseque ntly. no formal Iithostratigra rates . particularly in the lower parts . reflects phic division has been applied. eros ion of an assemblage of rocks correspon Timing of the thrusting of the Middle nappe ding to an ophiolite pseudostratigraphy. In the can be constrained by a comparison with the upper conglomerates, a relatively high propor tectonostratigraphic pattern in the Serfjord tion of clasts such as quartzite, granite, tonali area, in the southern part of the HNC (Fig. 1). te. marble and possible acidic orthogneiss An imbricated sequence of metagabbro and reflects derivation from a different assemblage 9reenstones with island arc affinity uncon of rocks. of probab le continental affinity. The formab ly overlain by polymict conglomerates. lithodemic comp lexes in the Lower and Upper psammites, marbles. schists and minor felsic nappes have lithologies in part comparable to volcanites (Husmo & Nordgu len 1988) is cut these pebbles, but the significance of this is by the large Heilhornet pluton. Granodiorites uncerta in. The sequenc e of metasedimentary from the latter also intrude the medium-grade units in the Middle nappe is broadly equiva supracrustals structurally overlying the Vest lent to the stratigraphical pattern in Lyngen, randen orthogne isses. and have been dated where Minsaas & Sturt (1985) describe a eo- by U/Pb in zircons, yielding a Late Ordovician- NGU - BULL. 421, 1991 Tectonostratigraphy in the Velfjord- Tosen region 17

Early Silurian age of 444 ± 11 Ma (Nordgulen structural relations indicate that the nappes & Schouenborg 1990). Based on combined with dominantly continental material have ex field and isotope data, Nordgulen & Schouen perienced a more protracted tectono-meta borg (1990) conclude that an important part morphic history than the nappes comprising of the tectonometamorphic development, inclu ophiolite detritus. ding nappe imbrication and emplacement of the HNC over the Vestra nden orthogne issl Acknowledgements supracrustal complex , took place in the Ordovi The Velfjord-Tosen area was suggested for study by BA Sturt, NGU, Trondh eim. He also supervised the work at cian. Even though the imbricated sequence in all stages, for which we are most grateful. During the field Sertjorden belongs to a lower tectonostrati work and preparation of the theses, we benefilt ed from inspiring discussions with fellow students in Bergen, particu graph ic level within the HNC, the overall simila larly N. Bang, H. Brekke, T. Heldal, H. Hjelmeland and 0. rities in terms of the 'ophiolitic' basement and Nordgulen. M. Gustavson, 0 . Nordgu len, D. Roberts, B.A. Sturt, F.C. Wolff (all Trondheim) and an anonymous referee cover sequence indicates that this dating is are thanked for their detailed and critical comments, which relevant for the Middle nappe, too . Hence, it greatly improved the manuscript. During the wet summers is inferred that regional deformation, initial of fieldwork , invaluable practical and moral support was given by the Anes family in Nordfjellmark to TT, and by thrusting of the Middle nappe, and its imbrica the Lien family at Nevernes to H.L. Finally, we wish to tion with the Lower and Upper nappes took acknowledge financial support from Sulfidmalm AIS and the University of Bergen. place during the Ordovician. The final accre tion of the HNC and parts of the underlying supracrustal rocks of the Vestranden sequen ce onto the Baltoscandian continent probably occurred during the Middle Silurian to Early References Devonian Scandian phase of the Caledonian oroge ny (Nordgulen & Schouenborg 1990). Bakke, S. 1979: Ultrabasiske bergarter i Velfjord , Nordtand, Unpubl. NGU-report 1625/4. One should, however, note that there are indi Bakke, S. & Korneliussen, A. 1986: Jack-straw textured cations for Middle Silurian eastward thru sting olivines in some Norwegian metaperidotites. Nor. Ge of the HNC from the Hattfjelldal area,along 01. Tidsskr. 66, 271-276. the eastern boundary of the HNC (Dallmann Bang, N. 1985: The strat igraphy and structura l develop ment of the R0d0y-Halt0Y area, outer Vefsnfjord. 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Manuscript received January 199 1; revised m/s April 1991; accepted May 1991.