Polarforschung 48 (1/2): 78-91, 1978

Caledanian Metamorphism in Svalbard,with Same Remarks an the Basement *

By Yoshihide Ohta ••

Abslract: Four zones of Caledonian rocks can be dislinguished in Svalbard. The geosynclinal sedimenlarypi les ar e 15-20 km thick i the lower part is eugeosynclinal and the middle and upper parts are

miogeosynclinal in character. The four zones represent two sets of paired metamorphie zones, each ofwhich is composed of the interrnediate T!P fades series and the high T fades series. The high T fadesseries resulted from the strang influence of migmatisation, while the blueschists of the W zone arethought to be a high P variety of th e intermediate T/P fades series. Migmatisation occurred later thanregional metamorphism. The former took pi ace in the Silurian-Devonian perfod. as shown by most of theKlAr ages, while the latter can be correlated with the Cambrian hiatus indicated by the preliminarystudies of the Rb/Sr isochrone ages. Chemical characteristics of the igneous rocks in the geosynclinalpiles do not show with certainty any ocenruc basement and a continental one is inferred from the pebblesof eonglomerates. The chemistry of syn - late orogenie gabbros of th e NE zone indicates an island-arccondition du ring the Caledonian orogeny in this area.

Zusammenfassung: Die Gesteine kaledonischen Alters werden in Svalbard in vier Zonen gegliedert. Diegeosynklinalen sedimentären Folgen sind 15 bis 20 km mächtig, wobei der liegende Teil eugeosynklinalen,die mittleren und oberen Bereiche miogeosynklinalen Charakter besitzen. Die vier Zonen bilden zweiReihen paariger metamorpher Bereiche, wovon jeder aus einer intermediären Druck-Temperatur-Faziesserieund einer Hochtemperatur-Faziesserie zusammengesetzt ist. Die Hochtemperatur-Faziesserie war das Er qeb­nis einer starken Uberprägung durch die Migmatisierung, während von der Glaukophansch ief'erfazfe sder Westzone angenommen wird, daß sie eine Hochdruckmodifikation der intermediären Druck-Temperatur­Fazies darstellt. Die Migmatisierung erfolgte später als die Regionalmetamorphose. Die erstere ereignetesich im Silur-Devon, wie die meisten der K/Ar-Altersbestimmungen zeigen, während die letztere mit demkambrisd1en Hiatus korreliert werden kann, wie die vorläufigen Ergebnisse der Rb/Sr-Isochrone ergebenhaben. Die chemischen Eigenschaften der magmatischen Gesteine in der geosynklinalen Folge zeigen mitSicherheit kein ozeanisches KrustenmateriaI. während ein kontinentaler Untergrund von den Geröllen derKonglomerate abgeleitet wird. Die Chemie der syn- bis spätorogenen Gabbros der Nordostzone zeigt Insel­bogen- Verhältnisse während der kaledonischen Orogenese in diesem Gebiet.

Caledonian rocks and structures are the fundamental framework of Svalbard. In thispaper a summary of Caledonian metamorphism will be given and some problems con­cerning the basement of the Caledonian geosyncline will be discussed.

THE CALEDONIAN GEOSYNCLINE

The geosynclinal sedimentary piles involved in the Caledonian orogeny in Svalbard arecalled the Hecla Hoek succession. Their total thickness is ab out 15-20 km and theyare of Precambrian to Lower Paleozoic age. These rocks are exposed along the westernand northern sides of the Svalbard archipelago, 650 km in the N-S direction along thegeneral trend of Caledonian structures and 500 km in the E-W direction aeross this trend.The rocks occur in four zones of a roughly N-S trend,and are separated by zones of youn­ger structures (Fig. 1): the W (subdivided into the WS and the WC areas), the NW, theNC and the NE zones, each zone forming an independent tectonic unit.

The lithologie characteristics of the geosynclinal sediments in the four zones aresimplified in Fag. 2. The lower Iimit of the succession is thought to be about 1,000m. y. b. p., which is estimated from the 800-900 m. y. Rb/Sr isochrone age of thevolcanic rocks of the Lower Hecla Hoek succession (RAHEIM, pers. corn., 19(8) and the1,275 m. y. Rb/Sr whole rock isochroneage of the granite-gneiss boulders of the UpperHecla Hoek succession (EDWARDS & TAYLOR, 19(4), both in the NE zone. The upperlimit is marked by the fossil evidence of the Ordovician-Silurian period in the WC area(SCRUTTON et al., 19(6). The lower parts of the Hecla Hoek succession in each zone are

• Paper presented at tbe "Conlerence on Geophysics, Geology. Geomorphology and Geodesy 01 Spits­bergen". held by the German Sociely 01 Polar Research in Hamburg, October 2-3, 1978.

.. Dr , Yoshihide Ohta , Norsk Poiarinstilutt. P. O. Box 158, N-1330 Os10 Lufthavn (Norway).

78

eugeosynclinal to some extent, while the middle and upper parts are miogeosynclinaland are dominated by quartzite, limestone-doiomite and areno-argiliaceous rocks.Recently, two horizons of basic rocks have been recognized in the Middle and UpperHecla Hoek in the W zone, and the intermediate volcanfc rocks (the Kapp HansteenFo rrna t io n] of the NE zone ha ve shown its st ra t iq raph i c posit io n to be younger than that

Kvit0ya/)

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76'

77'

79"

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o Ep-Amphibolite facies

~/I'.!1 Amphibolite fac!es"~I (mostly minmattuc l

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D Post-Caledonian rccks

E: ~:] Poat-tectonlc Caledoman granite

Lower Patecz ITIIill upper}IQ.Cil!!1.Qd.'!..n_ f7777J Mu:ldle Hec.la Hoek qeosvnclinal~ sediments

Precambr1an~LI;;;;;;,d 0wer

21'

21'

-f ltE zone

19'

Bjornoya

~

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Ne

12'

50

15"

Wzone

o,

12'

Fig. I: D'is t r ihu ti on of t he Hc cl a Ho e k r ock s an d t h ei r me t ö mo rp hic grade in Svetbard •

Abb. 1: Verteilung der Hek la-Hoek-Ges telne und ihre Metamorphosegrade in Svalbard.

of the argillo-arenaceous sediments (the Brennevinsfjorden and the Austfonna Forma­tions), The amount of volcanigenic rocks does not exceed 10 "/0 of the Lower Hecla Hoeksuccession and a typical ophiolitic assemblage does not occur in any zone. There is nodistinct difference in the amount of volcanigenic rocks in the four zones and thecontrast between the eu- and mio-geosynclinal character can always be observedbetween the lower and the middle-upper successions.

CALEDONIAN METAMORPHISM

The metamorphic grade increases roughly parallel to the stratigraphie depth in all zones,

19

as shown by the general positions of iso grades in Fig, 2; areverse relationship canrarely be observed. This evidence gives the impression that metamorphism influencedupon the geosynclinal piles is a single prograde series in general, although local corn­plexities do occur.

W zone

WS area WC areaNW zone NC zone NE zone

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- - ky- -

~-=-1 Argillac,ous rocks ~ Limestone Dolomit, - -- -

~~gö~ Conqlornerate Ii3illlgn,ous rocks- - sil- -- -- --

Fig. 2: Simplified lithostratigraphic columns of the Hecla Hoek rocks from the four z on e s in Svalbard.A, Ir and B along t he Ieft side of the columns: acidic, intermediate, and basic igneous activitiesrespectively, bi, g, cord, st, ky, si l, and hy along the right side 01 the colums: rough position 01 theisogrades of the minerals.

Abb. 2: Vereinfachte lithostratigraphische Profilsäulen der Hekla-Hoek-Polqen von den vier Zonen inSvalbard. A, I und B rechts der Profilsäulen bedeuten A = saurer, I = intermediärer und B = basischerMagmatismus, Bi (~ Biotit). g (~ Granat), cord (~ Cordierit), st (~ Staurolith). ky (= Disthen), s il(= Sillimanit) und hy (Hypersten) auf der rechten Seite geben ungefähr die Lage der Isograden derMinerale an.

80

Metamorphism of the WC area arid the NW zone has been studied in detail by thepresent author, while the WS area and the NC zone have been investigated mainly byPolish and British expeditions respectively. The NE zone is now being studied in detailby the staff of the Norsk Polarinstitutt. Some metamorphic characteristics will be outlinedbelow, ranging through the W to the NE zone.

The Wzone

Two phases of deformation are evident from the analyses of mesoscopic structuralelements in Hornsund, Bellsund, St. Jonsfjorden and Prins Karls Forland: the older oneis mainly in the E-W trend, with considerable variations and the younger one in theNNW-SSE trend, both having been disturbed by Tertiary movements. BIRKENMAJER(1975) has suggested a few possrb le Precarnbrian deformations, his theory being basedon the presence of unconformities and the granite pebbles of conglomerates in theHecla Hoek successions of the WS area. However, linear structures representiriq theolder deformation phase were found in the Middle and Upper Hecla Hoek rocks of theWC area, thus indicating their Caledonian origin. The younger trend is superimposedby the Tertiary deformation, roughly in the same trend, and is therefore difficult todistinguish from the latter. However, the specific style of the Io lds and the associatedmineralisation, neither of which are found in the post-Middle Paleozoic rocks, giveevidence of their Caledonian origin. These two deformation phases can be used astime markers for Caledonian metamorphism in the W zone.

The older phase of deformation created the cleavages associating with the kyanite-,staurolite- and garnet-bearing mineral assemblages which represent the intermediateT!P metamorphic facies series. The Tertiary movements made only retrogressive changesof the Caledonian assemblages. A stage of static re-crystallization of micas has beenreported elsewhere in the WS area and is definitely older than the Tertiary deformation(SMULIKOWSKI, 1965, 1968).

Similarly, two phases of re-crystallization have been abserved in the high-grade meta­morphic rocks which occur along the eastern and western si des of the ForlandsundetGraben in the WC area (HJELLE et al., 1979). The older assemblages here, includingsillimanite, garnet arid micas, were strongly modified by mechanical crushing with anintroduction of quartz, and large unoriented biotite f lakes grew at a later date, regard­less of the schistosities; these flakes, however, were folded by chevron folds of theyounger deformation phase. Thus static thermal re-crystallization of the W zonedefinitely dates from the Caledonian period.

~

0 Amphibo-""cc lite f.'"uiE Epidota-arn-a:0 phibolite f.:;;

""f-~ Greensch-:;;

ist I.

DEFORMATION F1PHASE

NATURES OF THE Trqht-visoclinal.mainly E-W. but

DEFORMATION variable trends

Localmylo- Chevron-crenul.nitisatinn NNW-SSE.

Open folds

Open foldsNNW-SSE

thrust to NE

Fig. 3: Sequence of Caledonianmetamorphic events in the Wzone of Svalbard.

PER IOD Caledonlan period Tert iaryAbb. 3: Abfolge kaledonlsdlermetamorpher Ereignisse in derWestzone von Svalbard.

81

From the above description, it may be concluded that there were two phases ofre-crystallization in the W zone, occurring within the time range between the twodeformation phases of the Caledonian period: an intermediate T!P facies series ofregional metamorphism associated with the formation of cleavages during the olderphase of deformation, and static thermal metamorphism which occurred later than theformation of cleavages, but which is older than the younger phase of deformation (Fig. 3).The albite-granite pebbles of the conglomerate of the Middle Hecla Hoek succession inthe WS area were considered to be derived from the underlying middle Lower HeclaHoek rocks, and Precambrian metamorphism has been suggested by BIRKENMAJER(1975). However, an apparent similarity of these pebble-rocks with the underlying onesis not enough to correlate them with certainty, and more definite data, such as theirage and sedimentological support etc., are required.

The blueschists and eclogite of the WC area make a particular rock association, withiso la ted occurrences as thrust schuppen in the Upper Hecla Hoek rocks (OHTA,1979).The eclogite occurs as small tee tonic blocks in the calcareous schists, and is convertedinto glaucophane-bearing rocks along the cracks and margins. Chloritoid-phengite-quartzschist also has glaucophane idioblasts and shows a partly feather amphibolite texture.Typical blueschist is composed of almost pure glaucophane groundmass with variousamounts of large size gamet. Basic dykes of a few meters in thickness cut the dolomitebed sharply, making a gamet-bearing skarn zone in the laller, and the dyke rocks were

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40 Lirnc s tono various mrx ture s 01limestone -pelite

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0.1 0.2 0.3 004 0.5mg 0.6 0.7 0~8 0.9

Fig. 4: Ori qi n a l r ock s of the blue s chis t s from Mot al afj ell a. the ",,fe e r e a (the Ni qqli-val u e diagram).broken curve . differentiation trend of tholeiitic rock s , chained curve: differentiation trend of alkalinerocks, dot: epidote-amphibolite without alkatine-arnphibole, open cirele: glaucophane·bearing schist, cross:mica-quartz schist without alkaline-amphibole.

Abb. 4: Ursprungsgesteine der Glaukophanschiefer von Motalafjellet, des Westzentral-Gebietes (Niqqli­Werte-Diagramm). Gerissene Linie: Differenzierungstendenz tholeiitisd1er Gesteine, strid1punktierte Linie:Differenzierungstendenz alkalischer Gesteine, Punkt: Epidot-Amphibolit ohne Alkali-Hornblende, offenerPunkt: Glaukophan-führende Schiefer, Kreuz: Glimmer-Quarz-Schiefer ohne Alkali-Hornblenden.

converted into pure glaucophane schist. The bulk chemical analyses of blueschists showthat the original rocks are mixtures of pelitic sediments and the early differentiates ofbasic magma (Fig. 4). No transitional relation has been observed from the blueschist tothe associated epidote amphibolites. The oriqin of the eclogite is unknown, while theassociated glaucophane rocks are evidently retrogressive products from the former.

82

Three mineral assemblages indicating different physical conditions of formation are dis tin­guished in these basic rocks (Fig. 5). The physical conditions of the formation of the eclo­

gite were calculated from the partitioning of Fe and Mg in the garnet-pyroxene parage­nesis to be of minimum P = 9.7 kb and T= 540-570° C, and of the blueschist, using theSi +4 values of phengite referring to the oxygen isotope temperature for the Californiantype IV blueschists, P = 8 kb, when T = 400-550° C. These data are summarizedtoqetlier with those of other rocks in Fig. 7. Characteristic is the lack of the low-gradeside of the high P metamorphie facies series of the eclogite and blueschists. The conver­sion from eclogite to blueschist resulted from an isothermal retrogressive process ofpressure, while the epidote amphibolites and the chloritoid-quartz schists make aseriesof intermediate T/P type metamorphism ofabout 25° C/km temperature gradient. Althoughthe origin of the eclogite is unknown, the formation of the blueschists can b e consideredto be a high pressure deviation of the jntermediate T/P metamorphie facies series andis classified as the high temperature glaucophane schist facies of WINKLER (1967) and

A A

Fig. 5: ACF diagram of the basic rocks from the WC area, indicating three as s emb le qes . Broken tieli ne : epidote actinolite greenschists facies, solid tie line: glaucophane schist fades, chained tie line :eclogite. Dot: epidote amphibolite, open circle : glaucophane schist, cross : mica-quartz schist.

Abb. 5: ACF-Diagramm der basischen Gesteine aus dem Weslzentral-Gebiet mit drei Mineralvergesell­schaftungen. Gerissene Linie: Epidot-Aktinolith-Crünschiefer-Fazies, ausgezogene Linie: Glaukophan­Schiefer-Fazies. strichpunktierte Linie: Eklogite, Punkt: Epidot-AmphiboJit, offener Punkt: Glaukophan­Schiefer, Kreuz: Gli mrn er-Ouarz-Schiefer .

TAYLOR & COLEMAN (1968). A local tee tonic overpressure is enough to produce thistype of blueschist and it is not necessary to introduce a subduction zone in th'e presentcase as HORSFIELD (1972) did.

TheNW zone

This zone is characterized by an intensive development of migmatites, which form twozones of a toughly N-S trend. The major anticlinorium in the western part has its axisslightly plunging to the south; thus t.he deeper rocks are exposed towards the north.The stratigraphie position of the original rocks is difficult to determine because of strongmetamorphie modifications, but the rocks are roughly correlated to the lower Middle andLower Hecla Hoek successions.

83

The metamorphic events are summarized in Fig. 6. Migmatisation of the F2-F3 stagecovers most of the area and the low-grade rocks remain in the northeastern and south­western parts and in a N-S zone between two migmatite zones. In the latter twolocalities, the effects of migmatisation gave all the intermediate grade rocks high T typemetamorphic mineral assemblages, and it is difficult to classify the type of metamorphismin the low grade phyllites. The conversion of cordierite into almandine garnet can oftenbe observed in the pelitic gneisses. This indicates that the thermal gradient of meta­morphism roughly coincides with the andalusite-kyanite inversion line (30° Ckm) in theamphibolite facies range (HIRSCHBERG & WINKLER, 1968).

However, in the northeastern part, GEE (1966) reported kyanite-staurolite assemblagesin the pelitic gneiss associating with eclogite. Sm all remnants of kyanite were also foundin the cordierite-sillimanite-garnet-two-mica gneiss paleosomes of migmatite in Danskeyain the northwestern part of this zone. The corundum-spinel crots found in the

Granu Ilte

~

faciesca - - - ---

~--- ;4f --- -- -- - - -

<r spcc Amphibolite sil'"

~u facies~

cordI -- - _s~u_ ---- - --- - - -c,cc

-/alm.e:> Epldote-amph-

:2 x.;~~<t ibollte laciesf-~ --- --- ---- -- --:2 qz.rntro- hi.

Greenschistfacies dun ion

TECTONIC PHASE pre-Fl F 1 Fl-F2 F2 F2-F3 F 3 post- E3 E4interval interval

N-Slolds NNW-SSE NNW-SSE Mechanlcaltlgbt- isncl. uqht lolds shearing

TECTONIC EVENTS loldsWeak re- Cleavages. Feldspathi- Retrogres- Intrusion of Intrusion of

ANO crystaill- regronalrlle- sauen, Ior- sive myloni- grey granl- post-tec-sation tamorphism mation 01 Migmatisation tisation te. aqmatrt- toruc

METAMORPHISM laverinq Granit ic metatects isation. ther qrarutsmal meta-morphism

Fig. 6: Celedonien tectonic and me tarnorphi c events in the NW zone of Svalb a rd.

Abb. 6: Kal ed ontschs, tektonische und metamorphe Ereignisse in der NW-Zone von Sv alb ar d.

neighbouring area are considered to have been converted from former staurolite orchloritoid porphyroblasts (HJELLE & OHTA, 1974). This evidence indicates that the Flmetamorphism in the NW zone is of an intermediate TiP facies series.

The Ne zone

This zone has been studied over a long period, mostly by Cambridge expeditions, andthe best preserved succession of Hecla Hoek rocks of about 20 km in thickness wasestablished (WILSON, 1958; HARLAND, 1959; GAYER & WALLIS, 1969. HARLANDet al., 1966; GAYER, 1969). The present author has been working in this area since 1978.

Dominant structures with a N-S strike determine the rock distribution, and the mainanticline, whose co re is composed of the highest grade of metamorphic rocks, occursalong the western part of the zone in the southwestern part of Ny Friesland.The western limb of the anticline is cut by the Wijdefjorden fault, while the easternlimb shows aseries of prograde metamorphism. The biotite isograde roughly coincideswith the top of the Lower Hecla Hoek rocks in the northern part of this zone, while it

84

passes obliquely through the Middle Hecla Hoek rocks in the southern part of Ny Fries­land. The Upper Hecla Hoek rocks along the Hinlopenstretet in the east have been leftalmost entirely unmetamorphosed and contain well-preserved Lower Paleozoic fossils(FORTEY & BRUTON, 1973).

The main anticline shows an axial culmination in south-western Ny Friesland, wherekyanite-, staurolite- and sillimanite-bearing assemblages have been reported. Theseminerals show a strong preferred orientation controlled by cleavages and layeredstructures developed along the axial plane of tight isoclinal folds. This is the main

rnelt

NW zoneNE zone

800

II

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600

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Fig. 7: Met am or ph i c facies s erie s of Ce ledoni an rocks in thc four zones of Svalbard.

Abb. 7: Metamorphe Faziesserien kaledonischer Gesteine in den vier Zonen Svalbards.

85

regional Caledonian event and the metamorphism is of the intermediate T!P facies series.The cleavages and layered structures were involved in the main anticline structure,and the axial culmination indicates the upheaval of the highly metamorphosed core,probably composed of migmatites. Since the migmatites did not reach the level of thepresent surface, the thermal effect was weak even in the crestal part 01 the anticline,and the character of the regional metamorphism has been weil preserved.

Owing to the similar structural trends of the earlier regional metamorphism and latermigmatite upheaval, it is difficult to recognize the tectonic phases simply from thestudies of mesoscopic structural elements; however, detailed petrographic analyses willsolve this problem.

TheNEzone

Intensive migmatites and intrusive granite developed in the northern and eastern partsof Nordaustlandet, weakly metamorphosed Hecla Hoek rocks occur in the western andmiddle parts of this area, and the southern part is covered by Upper Paleozoic strata.The migmatites and granites show sharp intrusive contact with the Hecla Hoek rocks.The highest grade of metamorphism achieved in the lat te r is lower lhan the biotiteisograde even in the middle Lower Hecla Hoek rocks, and the type of metamorphismis unknown. All biotite-bearing rocks are related to the high temperature contact meta­morphism of the migmatites and granites, and almandine, andalusite and staurolite wereformed locally. The migmatites include numerous paleosomes of schistose and gneis so serocks, most of them have sillimanite, cordierite and alma·ndine garnet with a largeproportion of micas. Rhombic pvroxene-bearinq. garnet-mica-amphibole gneis ses havebeen found in the north-eastern and eastern parts of Nordaustlandet (HJELLE et al.,1978), indicating that the highest grade was the granulite facies befo re the invasion ofthe migmatitic metatects. It is notable that a large amount of syn- to late-tectonicgabbros occurs in the north-eastern part of Nordaustlandet and on adjacent islands(OHTA,1978).

I PAIR II PAIR

W NW NC NE

Dominating metamer- Intermediate T/P High T Intermediate T/P High Tphic facies series

Post-tectonic Hornemantoppen Newtontoppen Rijpljorden

'" granite granite (410-385) granite (402-385) granite (380-340)

"'" High T series High T series> Migmatisation (And. Cord. Alm. weak invasion (Bi. Alm. And, SI.'"" phase No migmatite Sill. Co. Sp.) 01 apophyses Cord. Sill.)'" (m.y.) (375) Iarqe amount of (436-365) large amount 01'a0 migmatites (450-379) migmatites (430-355)'0

'" regional Intermediate T/P Intermediate T/P Intermediate T/P (lower than<iU metamorphism (ChI. Alm. St. (SI. Ky. 'Sill.) (St. Ky. Sill.) Bi-isograd)

phase Ky. Sill.) (541) (1) (530±)(584-402)

Granite pebbles Syn- & late-in tillile. tectonic gabbros.

Evidences Rhyolile pebbles Basic rocks Acidic & basic Granite pebblessuggesling in upper L. H. H. in L. H. H. rocks in L. H. H. in tillile.

the basement .Basic rocks in Eclogite Acid-intermediateL., M., & U. H. H. volcanics in 1. H. H.

Eclogite Hypersthene-hearing gneisses.

Tab. 1: Summaries 01 metamorphism in the lour zones 01 Svalbard and the evidence to suggest thebasement.

Tab. 1: Zusammenfassung der Metamorphose-Ereignisse in den vier Zonen von Svalbard und Hinweiseauf die Deutung des Untergrundes.

86

SUMMARY OF CALEDQNIAN METAMORPHISM

Four zones of Caledonian metamorphie roeks in Svalbard represent two sets of ap­parently paired metamorphie zones (MIY ASHIRO, 1973). as shown in Tab. 1 and Fig. 7.However, the high T facies series of the NW and NE zones is eaused by the develop­ment of intensive migmatites, and regional metamorphism whieh preeeded miqrnatisationis of the intermediate TiP facies series, although that of the NE zone is uneertain.Considering the teetonie history (Tab. 1), the two sets of apparently paired metamorphiezones are results of the different extents of migmatisation in a later period of Caledonianorogeny and the preeeding regional metamorphism is probably the result of the inter­mediate T!P facies series in all four zones.

It is generally said that the zones of different metamorphie facies series are eloselyrelated to the different nature of geosynclinal sedimentary piles: the high P type faciesseries to the more eugeosynclinal one. The Hecla Hoek sueeessions of the four zones inSvalbard do not show any lithologieal eontrast with eaeh other . instead, the eontrast isfound between the Lower and the Middle-Upper sueeessions. This evidenee is eonfirmedby the same metamorphie facies series in all four zones.

Aeeording to the radiomctric ages, the end of the migmatisation period was about350 m. y. ago, and it was followed by the post-teetonie granite. The regional metamor­phism had its elimax about 530 m. y. ago. The former age eorresponds to the so-ealledmain Caledonian event elsewhere, while the latter ean be eorrelated with the earlyCaledonian phase as established in north-western Finmark, northern Norway (PRINGLE,1971) and Seotland (pANKHURST & PIDGEON, 1973). whieh is represented 'by the Middleand Upper Cambrian hiatus of the Upper Hecla Hoek sueeessions. This is eonfirmed bythe faet the Ordovician-Silurian Bulltinden Formation of the WC area includes pebblesof phyllites and sehists whieh are probably derived from the underlying Middle andLower Hecla Hoek sueeessions.

Although the origin is eomplex, the two sets of paired metamorphie zones are exhibitedon the present-day surface: The eontrast between the paired zones is not as strong asbetween the younger pairs whieh eonsist of the high P type and the high T typemetamorphie facies series. This weak eontrast between the paired zones and the laek ofdistinet lithologie differenees in the geosynclinal piles are eharaeteristie for the SvalbardCaledonides.

THE BASEMENT OF THE HECLA HOEK GEOSYNCL1NE

There may be so me diastrophisms in the Upper Proterozoie to Eoeambrian: period, andthe Hecla Hoek geosynclinal sediments might have been subjeeted to metamorphismand deformation, as BIRKENMAJER (1975) has suggested for the WS area. Even so,there is good reason to believe that the development of the Heela Hoek geosynclineforms a large unit in the geoteetonie history of Svalbard.

A possible Arehean basement of the geosyncline has been suggested for the NE zone,based on the faets that the grade of metamorphism deereases towards the east in thesediments arid that a wide migmatite eomplex with gentle undulating struetures oeeurs(SANDFORD, 1926, 1954; SOKOLOV et al., 1968). Gneissie paleosomes of the migmatitesin the NW zone have been eonsidered by KRASILSCIKOV (1973) and RAVICH (1979)as being derivatives from the basement.

The faets in favour of a eontinental basement nature are the pebbles and boulders ofgranite in the eonglomerates of the Heela Hoek sueeessions: elsewhere in the tillite

87

of the Upper Hecla Hoek and in the Slyngfjella Conglomerate of Middle Hecla Hoekin the WS area. The granite-gneiss boulders from the tillite of the NE zone were datedat 1,257±45 m. y. by the Rb/Sr whole rock isochrone method and the high initialSr 87/ 86 ratio. 0.72172 ± 0.00056 obtained, implies that these rocks are from even oldercrustal rocks (EDWARDS & TAYLOR, 1974).

The occurrence of K20 rich rhyolite pebbles (K20 wt Ofo = 9.40 and 10.21; SMULIKOWSKI,1968) in the conglomerate of the Vimsodden Formation in the upper Lower Hecla Hoekof the WS area, suggests a source area for the pebb les with a continental crust. Thefeldspar porphyroblastic gneisses of the Planetfjella Group of upper Lower Hecla Hoekin the NC zone are thought to be derived from acidic volcanic rocks (W ALUS, 1969).Some of them include a significant amount of potash feldspar, suggesting a potashalkaline acidic volcanic rock Origin. These rocks have a significance similar to therhyolite pebbles of the Vimsodden conglomerate.

The paleosomes of granulite facies rocks in the migmatites of the NE zone are anotherpossible indication of a crystalline basement. If the correlation of the Kapp HansteenFormation of this zone to the Planetfjella Group of the NC zone is correct, then theareno-argillaceous rocks of the Brennevinsfjorden, Kapp Platen and Austfonna For­mations are comparable to the middle and lower Lower Hecla Hoek successions. Thebiotite iso grade of Caledonian regional metamorphism had not been achieved in thelowest part of the metasediment successions in the NE zone, while the paleosomes hadthe mineral parageneses of the granulite facies. The granulitic paleosomes could nothave been produced.by the migmatisation which occurred under the conditions of theamphibolite facies. Thus, a large gap of metamorphic grade is estimated between thelower part of the Lower Hecla Hoek successions and the granulitic paleosomes, andthis may indicate the difference between the geosynclinal sediments and the basement.On the other hand, an ancient oceanic basin, i. e. Paleo-Atlantic or Iapetus, has beeninferred from the existence of abundant basic volcanic rocks in the Lower Hecla Hoeksuccession of the NC zone (HARLAND & GAYER, 1972), and a mid-oceanic spreadingcentre was induced from the gabbro-amphibolites of the WS area during the LowerHecla Hoek period (BIRKENMAJER, 1975). HORSFIELD (1972) concluded that the

Alkaline basalt Subalkaline basalt Transitional basalt

Q)Q)

~~;;; ;;; ;;;

"~~ .S ....... ;.::: ....... "";:: <~ ~"~ .g§~ .g"~1: '(ti '(tim ro- ~ro;;;

0_

.~~~ Total-",ro o ro ' ro ~:-2 ~........,;: ro

0>'" -"'''' (iji2 j'§;a 0'" .0",I;<'V) ::2._ ro _ro -ro ,"ro 0'" ro ro",ro

:r;.o ':'.0 , .0 ~.o ~.o :r:.o ,..,,,.0 ~".o Z~.cro ~ g g~ Z

WC area 0 1 0 1 3 0 2 0 3 10WS area 0 2 4 1 1 1 2 1 5 11

Total 0 3 4 2 4 1 4 1 8 21

IAlkaline and transition al basalt Subalkaline and

transitional basalt

IPotash series Sodic series

High-KSodic

Flood High-Alseries Trachybasalt Potassic Hawaiite basalt basalt

basalt basalt i

WC area 0 (1)

I

(5) 8 10 1 2 3

WS area 0 (3) (11)6 52 1 9 2

Total 0 2 2 [ 11 5 14 6-

Tab. 2: Classification of the selected b as ic rocks from the W zone of Svalbard.

Tab. 2: Einteilung ausgewählter basischer Gesteine der Westzone Svalbards.

88

blueschists of the WC area wereformed from the oceanic basic rocks involved in asubduction zone during the Caledonian orogeny.

However, the mere existence of abundant basic rocks is not enough tosuggest anoceanic basement, but detailed examination of the nature of the basic rocks is necessary.For this purpose, uncontaminated basic rocks were selected from available bulk chemiealanalyses sampled from the W zone and classified according to MIDDLEMOST (1975),as shown in Tab. 2. The ratio of alkaline and subalkaline rocks is 2:1 in the WSarea and 1:4 in the WC area. This dissimilarity indicates the difference in the strati­graphie position of the basic rocks of the two areas; however, the existence of potash­alkaline rocks, although present in small amounts, is notable. In the classification whichincludes the transition al rocks, Na-rich hawaiite and flood basalt are dominant in bothareas. Comparing the Ti02 and P205 with the avcraqe oceanic rocks (Tab. 3). the dominantrocks of the present areas are not similar to the oceanic ones: the present hawaiiteshave a distinctly lower Ti02 and P205 than the oceanic ones, and the present floodbasalts are similar to the continental tholeiites. Low potash subalkaline rocks are saidto be typical of oceanic floor basalt. This type of rock does exist in the present areas,however, occurring as lamprophyre and porphyrite dykes which cut shallow sea sedi­ments; thus they are not likely to be of the oceanic floor type. The Ti02 and P205 alsoconfirm a non-oceanic origin.On the whole, as far as we know up until now, the basicrock assemblages of the W zone are not of an oceanic type at all , and more systematicstudies are required for any significant conclusion to be drawn on the tectonic settingof the area.

low K sub-a!kaline basalt sub-alkaline basalt

ocean foor b as al t flood basalt Svalbard bigh Al Svalbard

Ocean sea Mt. & Svalbard conti- oceanic TH WC WS basalt WC WSIloor acean WC area nente l TH (Hawaii) area area (Japan & area area

floor- (Karroo) Korea)

rio, 1.5 1.3 3.3 1.1 2.5 1.6 1.7 0.8 2,3 1.5P20S 0.13 0.13 0.22 0.13 0,30 0.12 0.15 0'.14 0,30 0.33

No. 01data 44 19 I 21 200 4 7 11' 2 4

alkaline basalt

potash series Svalbard sodic series Svalbard

Ocean ridges deep WC WS Canary Hawaii WC WS(Gough is1.) subducted area area isl. is1. area area

zone(Japan)

Ti02 3.3 2.2 2.5 1.8 2.6 3.4 2.0 1.8PzOs 0:33 0.70 0.25 0.61 0.84 0.70 0,24 0.25

No. 01data 10 27 3 3 22 62 4 10

Tab. 3: Comparison of average TiOz and PzOs between the basic rocks of the W. zone of Svalbard andvarious rock types from different tectonic settings.

Tab. 3: Vergleich des durchschnittlichen Gehalts an. TiO z und PZ0 5 zwischen den basischen Gesteinen derWestzone Spitzbergens und verschiedenen Gesteinstypen unterschiedlicher tektonischer Position.

Basic rocks exist in relatively small amounts in the NW zone and have been stronglymodified by migmatisation. It is almost impossible, therefore, to work out their originalnature. In the Ne zone, the Harkerbreen Group of the middle Lower Hecla Hoeksuccession includes a large amount of amphibolite and gabbroic rocks, but no chemicalstudy has yet been undertaken.

Intermediate and acidic volcanic rocks predominate in the upper Lower Hecla Hoek

89

successions of the NE zone. The Kapp Hansteen Formation, in particular, has a successionof more than a few km in thickness, composed ofandesite and porphyrite. This volcanismis certainly not of an oceanic type. Syn- to late-tectonic Caledonian gabbros arewidespread in the north-eastern part ofNordaustiandet. One of them in Storoya showsdistinct layered structure and cumulative textures, indicating astratiform basic complex.The later differentiates of this complex are calc-alkaline quartz dioritic rocks whichoccupy ab out half of the exposures. The basic rocks, both of Storoya and Kvitoya, showa relatively high concentration of iron in the middle stage of the differentiation andare of a tholeiitic nature. Some high Al basalts are also accompanied (OHTA, 1978).This association of rock types is typical of the recently formed island-arc areas: thewell-developed island-arc with a continental crust of about 20-30 km in thickness

F'lg. 8: Trace element ratios of the Caledoniangabbros fr orn the NE zone of Svalbard (afterPEARS & CAN, 1973). A: ocean ic floor basalts,B: low potash basalts of tsl and-arcs , C: calc-alkalicbasalts. Dot: Kvitaya gabbros, open circle : Storayagabbros.

Abb. 8: Spurenelement-Gehalte der kaledonischenGabbros aus der NE-Zone von Svalbard (nachPEARS & CAN 1973). A: Basalte des Oz eanbodens ,B: Kaliarme Basalte der Inselbögen, C: Kalk­Alkali-Basalte. Punkt: Kvitey a-Gabbros , offenerPunkt Storeya-Gabbros .

(MIYASHIRO, 1975). Prorn theseresults and the trace .elernent ratios shown in Fig. 8,it may be concluded that the eastern part of the NE zone had a continental crustof moderate thickness during the period of Caledonian oroqeny.

To sum up: according to our present knowledge of the basic rocks, an oceanic crustbasement to the Hecla Hoek geosyncline is not likely. A continental basement is morelikely.

ACKNOWLEDGEMENTS

The author is grateful for constant discussions with and encouragement by thegeological staff of the Norsk Polarinstitutt; thanks are due to Mrs. A. Brekke forimprovements to the language.

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