Sonderdrucke aus der Albert-Ludwigs-Universität Freiburg

KURT BUCHER Mantle fragments in the Scandinavian Caledonides Originalbeitrag erschienen in: Tectonophysics 190 (1991), S. 173-192

Tectonophtsics, 190 (1991) 173-192

173

Elsevier Science Publishers B.V., Amsterdam

Mantle fragments in the Scandinavian Caledonides

Kurt Bucher-Nurminen *Department of Geology, University of Oslo, PB 1047, Blindern, N-0316 Oslo 3, Norway

(Received January 30, 1990; revised version accepted July 13, 1990)

ABSTRACT

Bucher-Nurminen, K.. 1991. Mantle fragments in the Scandinavian Caledonides. Tectonophysics, 190: 173-192.

Mantle fragments of ultramafic composition are widespread in the Scandinavian Caledonides (SC). Lenses and houdins ofAlpine-type peridotites in the Scandinavian Caledonides represent parts of dismembered ophiolite sequences and fragments ofsub-continental upper mantle. Metaperidotites of nappes in internal positions are generally isofacial with the metamorphicenvelope, usually Caledonian metasediments but in places also Precambrian metagranitoids forming the basement cores of thenappes. Caledonian metamorphism strongly modified the texture and mineralogy of the peridotites and resulted in asystematic metamorphic pattern which is consistent with the pattern observed in the envelope.

Metaperidotites of the external massifs display at least a two-stage metamorphic history: an early Caledonian high-pres-sure high-temperature phase related to early crustal stacking and a late Caledonian regional metamorphic overprint whichproduced a regular Barrovian-type metamorphic pattern of in-situ metamorphism.

Metaperidotites from nappes in intermediate positions (Iapetus Ocean ophiolites and ultramafic rocks from island arcenvironments) show strongly diverging histories. Metaperidotites from internal ophiolites (oceanic ophiolites. Köli) lack anyevidence of subduction metamorphism, are serpentinized to various degrees, show abundant primary mantle relic mineralogiesand the Caledonian metamorphic overprint is low. Metaperidotites from external (island arc) ophiolites and other associations(Seve) often show relic high-pressure metamorphism related to the Finnmarkian phase of the Caledonian orogeny. The Sevemetaperidotites are occasionally associated with eclogites and show a weak overprint of late Caledonian regional metamor-phism. Alpine-type peridotites are absent in the foreland of the Baltic Shield and in the innermost nappes (Lofoten).

The metamorphic characteristics and evolution recorded by the metaperidotites in the Scandinavian Caledonides allow ageneral reconstruction of the dynamics of collision belt formation.

Introduction

The Scandinavian Caledonides (SC) represent

an early Paleozoic collision belt of considerable

complexity with regard to the kinematics of the

orogenesis. The total exposed length of the belt on

the Scandinavian peninsula exceeds 2000 km which

corresponds to 1.5 times the total length of the

Alps. The Scandinavian Caledonides represent a

relatively deeply eroded mountain chain with the

typical erosion surface at mid-crustal levels of the

Caledonian structure. Much crucial information

* Present address: Mineralogisch-Petrographisches Institut,

Albert- Ludwigs- Universität, Alhertstr. 23h, D-7800 Freiburg

i. Br., F.R.G.

on the evolution of the belt which was contained

in the Caledonian low-grade sedimentary record

has been removed by erosion a long time ago. The

Caledonian orogenic belt was partly destroyed

and severely modified by the break of the Atlantic

Ocean in the Mesozoic and by continuous defor-

mation until the present (neotectonics).

In contrast to the various Mesozoic to Tertiary

belts, it appears impossible ever to reach an over-

all understanding of the evolution and large scale

kinematics of the Caledonian belt. However, the

present day deep erosion level in the Caledonian

mountain chain makes the belt suitable for study-

ing orogenic processes in the middle and lower

crust.

The Scandinavian Caledonides represent in

principle an Alpine-type orogenic belt which was

0040-1951/91/503.50 1991 – Elsevier Science Publishers B.V.

BERGENARCS

BALTIC SHIELD

CALEDONIAN FRONT

300 km

174 K. BU('HER-NURM►NEN

formed by a large cycle of ocean crust formation(Iapetus) with associated initial rifting and laterophiolite production, subsequent ocean crust con-sumption along a destructive plate margin, andfinally a continent–continent collision with stack-ing of the crust, crustal thickening and associatedmetamorphism and large lateral nappe displace-ments (e.g. Cuthbert et al., 1983; Dallmeyer, 1988;Stephens, 1988).

Significance of ultramafic rocks and purpose of re-view

A general feature of the Scandinavian Caledo-nides is the very widespread occurrence ofAlpine-type ultramafic rocks at all levels of thetectonostratigraphy (Qvale and Stigh, 1985). Al-pine-type ultramafic rocks (peridotites, serpen-tinites) are defined here as isolated solitary bodiesderived from the upper mantle (oceanic or con-tinental) which have crossed the mantle crustboundary by tectonic processes and which werecompositionally, mineralogically and texturallymodified in the crust during an orogenic cycle.This paper reviews some aspects of some Alpine-type ultramafic rocks in the ScandinavianCaledonides including their tectonic significanceand the general pattern of the Caledonian meta-morphism. The discussion is based on a compila-tion of new mineralogical data (assemblages, tex-tures, and mineral chemistry) from a large numberof occurrences of ultramafic rocks form the Centraland Southern Caledonides in addition to informa-tion retrieved from previously published data.

SCANDINAVIAN CALEDONIDES

Fig. 1. The Caledonian orogenic belt on the Scandinavianpeninsula is shown in grey together with some important place

names used in the text.

the tectonostratigraphy outlined below. Severalaspects of Fig. 4 will be discussed in later sectionsof the paper.

(a) Autochthonous foreland (autochthon)The foreland in the southeast of the Caledonian

front as it is exposed today is represented by theBaltic shield (which in turn consists of a Pre-cambrian basement and its thin Precambrian tolower Paleozoic cover). Ultramafic rocks are ex-tremely rare.

Caledonian tectonostratigraphy

For the purpose of the review of ultramaficrocks it is necessary to give a brief overview of thegeneral tectonostratigraphy of the ScandinavianCaledonides. Place names are given in Fig. 1 andnames of tectonostratigraphic units from Robertsand Gee (1985) are given in brackets. The regionalgeology of the central Scandinavian Caledonidesis summarized in Fig. 2 and of the southernScandinavian Caledonides in Fig. 3. A tentativeprofile across the Caledonian belt is presented inFig. 4. Figure 4 facilitates the understanding of

(h) External nappes and Western Gneiss Region(lower allochthon)The lowest of the transported units above the

Caledonian front are typical external nappes withlow grade Caledonian sediments, imbricate andduplex structures (Figs. 2, 3 and 4). The nappesrarely incorporate slices of the basement. A largeamount of cover shortening is recorded by thesecover units. The corresponding shortening in thebasement of the Baltic shield has occurred fartherto the northwest in particular in the WesternGneiss Region. The belt of external nappes locallyincludes Alpine-type ultramafics (Barkey, 1969).

SvartisenNappe

Complex

RüdingsfjelletNappe

Complex

AutochthonotiisBase Went

Baltic hield)HelgelandOphioliteComplex

!External nappes

HANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES

175

Fig. 2. Geological map of the central Scandinavian Caledonides based on the maps published by Sigmond et al. (1984), Magnusson

(1957) and Gee et al. (1985). Note, however, the distinction of three nappe complexes in the coastal area of Nordland, the

allochthonous nature of the gneiss nappes (dark grey shade), the uncertain status of the Lofoten area and the extensive ophiolite

complex along the Helgeland coast.

The erosion surface intersects the basement ofthe Baltic shield along the internal side of thehigher tectonic units of the Caledonian nappestack and the basement is exposed in the so calledWestern Gneiss Region (Fig. 3). The WesternGneiss Region thus represents a large window of alower tectonic level (several hundred km extensionalong the coast). Caledonian shortening, deforma-tion and thermal overprinting rapidly and con-tinuously increases towards the coastal area (Diet-ler, 1987).

The Western Gneiss Region complex consistsof a sequence of nappes separated by thin meta-morphic cover sequences which have overprintedprimary contacts towards the basement on oneside and are bounded by thrust faults on the otherside (Fig. 4). These thrust faults are crowded withultramafic lenses which are in turn very oftenassociated with eclogites. The Caledonian meta-morphic overprint gradually increases towards thecoast (Medaris, 1984; Griffin et al., 1985). Condi-tions reach upper amphiholite to the beginning of

Baltic shieldSärvJotun nappe External nappes

lower Köli

^^^^^^•^^ .00 NW-Western 1++Bergen arcs Gneiss Region ,

^+ ++++++++++++++++f +++++++++++++++++

++++++++++++++++++++++

+ + + + + + + ++ + + + + + +

+++++++++++++

++Svartisen

Fh

176 K. BUCHER-NURMINEP

Fig. 3. Geological map of the southern Scandinavian Caledonides based on the maps published by Sigmond et al. (1984) and Gee et

al. (1985).

eclogite facies (resp. granulite facies) at the coast(15-18 kbar in Western Gneiss Region, > 20 kbarin Bergen Arcs isofacial eclogites). The thrust faultsalso carry exotic (allofacial) ultramafic and maficrock fragments which were picked up by the faultsat depth between 80 and 100 km (coesite-kyanite-eclogites, some of the garnet-peridotites).This suggests that faulting and initial stacking hasoccurred under fairly brittle conditions which per-mitted very deep reaching faults (see also Bergen

Arcs). Deep seismic studies of the BritishCaledonides (Warner and McGeary, 1987) dis-covered a number of dipping seismic reflectors inthe deep crust and the upper mantle. These struc-tures have been interpreted as shear zones andthrusts which, if the interpretation is correct, couldbe viewed as fossil equivalents of Caledonian shearzones and thrusts. Ductile deformation (folding)and high-temperature metamorphism was prob-ably a consequence of subsequent thermal relaxa-

Fig. 4. Schematic geological cross section across the Scandinavian Caledonides showing the general positions of the major units. Theprofile shows the structure of the belt after the main shortening, stacking and nappe transport but before modification by late folding

and differential uplift.

MANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES 177

tion of the thickened crust and continued mod-erate shortening. The Western Gneiss Region rep-resents (together with the Bergen arcs) the corearea of Caledonian stacking and Caledonian ther-mal overprinting during the Scandian continent-continent collision (root zone) (Cuthbert, Harveyand Carswell, 1983; Johansson and Möller, 1986;Möller, 1988)

(c) Säry realm (middle allochthon)The tectonic domain above the external nappe

province may be collectively termed the Säry re-alm (Figs. 2, 3 and 4). In the southern part of theCaledonides the tectonic province is mainly repre-sented by the Jotun nappe complex and the Bergenarcs. The two nappe units comprise Precambrianmafic-ultramafic igneous complexes in granulitefacies of Precambrian age and its Caledonianmetamorphic cover. The main body of the far-travelled Jotun nappe is formed by a large re-cumbent fold (e.g. Milnes and Koestler, 1983;Heim et al., 1977). Caledonian metamorphismgradually increases from the foreland towards thenorthwest. It reaches mid-greenschist facies condi-tions at the internal boundary of the Jotun nappe.Most of the ultramafic rocks of the Jotun nappeand the Bergen arcs are not Alpine-type ultra-mafics but rather represent ultramafic cumulatesin gabbro-anorthosite sequences. Parts of theBergen Arcs represents an internal equivalent ofthe Jotun nappe which have been subjected tostrong ductile deformation and high pressuremetamorphism during the Caledonian cycle(Austrheim and Griffin, 1985). In the CentralCaledonides the Säry level of the tectonostratigra-phy consists mainly of Caledonian low gradenappes which comprise both Precambrian gneissesand Caledonian sediments (particularly Vendianelastics). The local names of the nappes includeSärv, Offerdal, Risberget, Rondane, Valdres nappeto name a few (Dyrelius et al., 1980). Metamor-phic studies in these units are incomplete.

(d) Seve realm (upper allochthon)The Seve unit consists of a fairly heterogeneous

collection of different nappes. The lithologiesrange from ultramafic and mafic rock sequencesto metapelitic and psamitic gneisses. The mafic

rocks include greenschists, amphibolites, maficgranulites and eclogites. Ultramafic rocks of theSeve units include the whole spectrum from lowgreenschist facies brucite + antigorite schists togarnet peridotites. Paleogeographically the Seveunits may represent the transition zone, with all itscomplexity, between the Baltic shield and theIapetus ocean which is represented by the Köliunits (see below). The ultramafics within the Sevecomplex partly belong to mafic/ ultramafic associ-ations which resemble ophiolite sequences. Typi-cal for such transition zones are associations ofwithin plate, continental and marginal basin char-acter (Stillman, 1988). The Seve units have beeninvolved in early Caledonian (Finnmarkian)tectonism and metamorphism (Dallmeyer and Gee,1986a, 1986b). The early Caledonian eclogites andgarnet-olivine rocks may be related to aCaledonian subduction process which consumedthe the Iapetus ocean prior to the main Caledonian(Scandian) continent-continent collision.

(e) Köli realm (upper allochthon)Includes most of the known true ophiolite se-

quences (Stillman, 1988) of the ScandinavianCaledonides. The Köli nappes also include a verylarge number of ultramafic bodies of all sizes andvaried sedimentary (partly fossiliferous) andvolcanic/igneous record. The units represent therelics of the former Iapetus ocean and local margi-nal basins. The main feature of the Köli ophiolitesis that they apparently have not been subductedduring the destruction of the Iapetus. There are noreported occurrences of eclogites and/or blue-schists in the Köli rocks. This is in marked con-trast to other Alpine-type orogenic belts. It maybe concluded from this lack of a regular subduc-tion mechanism for the destruction of the Iapetusocean that this ocean consisted only very locallyof a strict oceanic crust-mantle sequence in themodern (Mesozoic-Tertiary) sense.

(f) Helgeland nappe complex (uppermost alloch-thon)The units which are collectively termed " upper-

most" allochthon (Gee et al., 1985) are very het-erogeneous and their tectonostratigraphic positionis somewhat dubious. The various nappe corn-

178 K. BUCHER-NURMINEN

plexes of the " uppermost" allochthon typicallyinclude Precambrian continental rocks and Cale-donian granitoids as well as Caledonian metasedi-ments. Therefore, the units either originate fromthe western continental margin of the Iapetusocean or alternatively they may represent unitsfrom a much lower position in the tectonostratig-raphy on the Baltic Shield side.

Ultramafic rocks occur at a number of locali-ties in the Helgeland nappe complex. Locally,thermal metamorphism associated with the intru-sion of Caledonian gabbros and granitoids hasstrongly modified the ultramafic rocks which werepresent as serpentinites prior to the intrusions. Anumber of ultramafic lenses occur in ophioliteassociations along the Helgeland coast and theirtectonostratigraphic relationship to the Helgelandnappe complex is uncertain. it is very likely thatall ophiolites (Stillman, 1988: Skäl y cr, Rodoy,Leka, Bronnoysund) belong to the same belt ofophiolites along the southern parts of the Helge-land coast and may collectively represent a Kölielement. This Köli element may be connected withthe equivalent elements in the East either above orbelow the Helgeland nappe complex. The "above"solution for the western Köli elements brings the"uppermost" allochthon to the same tectonostrati-grap hic level as the Western Gneiss Region.

(g) Svartisen nappe complex (uppermost alloch-thon)The nappes of this nappe complex are built up

by large Precambrian gneiss cores and Caledoniancover sequences separating the individual nappes.The tectonic style of the area has similarities tothe Penninic area of the Alps (see also Ruthlandand Nicholson, 1965). All units are meta-morphosed in at least upper amphibolite faciesand intense multi-phase ductile deformation iscommon. Metamorphism and deformation is re-lated to the Scandian continent–continent colli-sion. Ultramafic rocks are very abundant in themetasediments separating the gneiss cores of thenappes but several occurrences in the Precambriangneisses are known. All ultramafic rocks of thisnappe complex are very similar as regards tex-tures, mineralogy and P–T–t-evolution.

(h) Rödingsfjellet nappe complex (uppermost al-lochthon)This complex includes a series of nappes which

are dominated by medium grade Caledonian coverrocks particularly marbles. Precambrian gneisscores are, on the other hand, also the backbone ofthese nappes. Ultramafic rocks from this nappecomplex are distinctly different from those of theSvartisen nappe complex and suggest a differentsource area in the mantle and a different geologi-cal history after emplacement.

(i) LofotenThe tectonostratigraphic position of the Pre-

cambrian rocks of the Lofoten area is unknown.Several tectonic positions are possible (e.g. Andre-sen and Rykkelid, 1989). The preferred solutionshown on Fig. 4 is consistent with the large scalemetamorphic pattern of the area and with newgeophysical observations available from theCentral Caledonides (Hurich et al., 1989). How-ever, it causes some problems with the widely usedterminology of Caledonian tectonostratigraphy(uppermost of uppermost allochthon?). The use ofgeneralized place names for the major units wouldhelp to avoid such problems. The absence of pre-served Caledonian cover makes it difficult to de-duce the effects of Caledonian metamorphism anddeformation. however, regional Caledonianamphibolite facies metamorphism is indicated byCaledonian muscovite cooling ages (Griffin et al.,1978). Deformation appears to be restricted toshear zones. Alkali amphibole bearing eclogitesdeveloped along some of these shear zones inPrecambrian gabbros which may represent theeffects of early Caledonian stacking and transportof the Lofoten nappes.

Ultramafic rocks

General aspects

The typical occurrence of ultramafic rock bod-ies is in the form of isolated lens shaped masses ofvarious sizes ranging from a few centimeters toseveral kilometers in length. Very characteristicfor wide areas in the Caledonides is the presenceof serpentinite or peridotite humps and knolls of

MANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES 179

roundish or lensoid shape sticking prominentlyabove the surrounding country rocks. The yellow-brownish weathering surface of the large moundsusually measuring some hundreds of meters, con-trasts with the standard grey gneisses of theScandinavian mountain ranges. Ultramafic lensesoccur locally in very large numbers although thetotal surface covered by ultramafic rocks at thepresent erosion level does not exceed a few per-cent (referenced to a 16 km 2 grid).

The ultramafic rocks are always aligned alongtectonic zones, e.g., shear zones, faults and nappeboundaries. They represent important markers ofotherwise often obscure nappe boundaries. Dozensof solitary Alpine-type peridotites often decorate amajor thrust fault at the present erosion surface ina given local area. Therefore, the entire thrustsurface is likely to be plastered with peridotitefragments at depth. This surface geology suggeststhat relatively large pieces of mantle peridotitewere picked up during the stacking of the Cale-donian nappes. Furthermore, the stacking of thecrust and its dissection was conducted by faultswhich cut across the crust mantle boundary. Theinvasion of the crust by mantle ultramafics alongdeep faults occurred initially in the form of largemantle wedges which later became fragmented asa result of faulting, shearing, folding andboudinage (Bucher-Nurminen, 1988). The geome-try of the thrust surface, together with the struc-tural features of the country rocks, indicates thatit underwent complex post-thrusting deformation(see also fig. 5 in Cuthbert et al., 1983).

The field aspects of ultramafic rocks in theScandinavian Caledonides described above aregeneral in the sense that the description is inde-pendent of the tectonostratigraphic level. Most ofthe Alpine-type solitary ultramafic rock bodiesprobably represent fragmented mantle materialfrom the subcontinental mantle. An importantexception are ultramafic occurrences in ophioliteassociations where parts of the ultramafic materialmay represent fragments from the mantle beneathoceanic crust.

Caledonian and pre-Caledonian ultramaficsIn a discussion of the Caledonian orogeny one

has to distinguish between metamorphic and ig-

neous rock units of Precambrian age which formthe basement for the Caledonian sediments.Caledonian sediments (and/or volcanics) are rocksof Precambrian to early Paleozoic age which expe-rienced their first metamorphic overprint, usuallyin several phases, during the Caledonian orogeniccycle. The recognition of proven Caledonian rocksis difficult or ambiguous in many of the nappes.

The ultramafic mantle fragments found alongfolded Caledonian thrust faults are interpretedhere as Caledonian rocks. They were picked upduring Caledonian thrusting and consequentlyrepresent Caledonian mantle samples. The pres-ence of Alpine-type ultramafic rocks in the pre-Caledonian basement prior to Caledonian thrust-ing and stacking is less likely because of thescarcity of ultramafic rocks in the Baltic shieldoutside the Caledonian belt (e.g. Modumserpentinites). It is clear, however, that Caledonianultramafic rocks could yield radiometric agesranging from the Archean to the Caledonian. The"age" is determined by the thermal structure ofthe mantle and the state of hydration at the timeof tectonic pick-up and by the subsequent thermal(metamorphic) history of the fragment during itsresidence in the crust.

Rock associations

The ultramafic rocks are normally associatedwith meta-supracrustal rocks. All typical meta-supracrustals of a given nappe are found in theenvelope of the ultramafic lenses. There is nospecial predominance of associated mafic rocks(except in nappes where are they are generallyabundant, e.g., in ophiolite associations). Rocks incontact with the ultramafic rocks include marbles,amphibolites, micaschists, quartzites, greenschists,quartzo-feldspathic gneisses, eclogites and others.In some of the nappes, ultramafic rocks withfrequent mafic dykes are characteristic. Themineral assemblages of the mafic dykes usuallyreflect the metamorphic grade of the country rocks.

Ultramafic lenses in the (ortho-) gneisses andmigmatites of the basement cores of the nappesare very rare. Some, however, do occur (e.g., Gro-tli (Western Gneiss Region)–Barkey, 1969; Rodey(Salten)—Sorensen, 1955b) and their properties

180 K. BÜCHER-NURMINEN

appear to be identical to those situated in thenearby supracrustals.

Many of the large number of ultramafic rocksin the nappe complex of the Western Gneiss re-gion are associated with quartzites and quartz-richmicaschists. One would think that sedimentationof probably late Precambrian (Vendian) sand-stones and arkoses on to the Baltic Shield is notthe favorite geological environment for the em-placement of solitary peridotites. Thus, it is sug-gested that these ultramafics represent characteris-tic examples of tectonically emplaced fragments ofthe subcontinental Caledonian mantle. The meta-supracrustals (quartzites and quartzose mica-schists) are found as continuous units in themigmatitic gneisses (basement cores) and, togetherwith the Caledonian ultramafics, mark nappeboundaries within the Western Gneiss Region(Barkey, 1969; Krill, 1985; Bryhni, 1989).

Rock types and assemblages

There is essentially only one major ultramaficrock type represented in all the continental associ-ations and that is an aluminous meta-harzburgite(Qvale and Stigh, 1985). Subordinate are someoccurrences of meta-dunite. Aluminous meta-harzburgite dominates in nappes at all levels ofthe tectonostratigraphy. This suggests that thesubcontinental Caledonian mantle was very uni-form in bulk composition. In ophiolite associa-tions, ultramafic rocks occur as cumulates in maficsequences and as fragments from the sub-oceanicmantle. The latter are also predominantly meta-harzburgites with subordinate meta-lherzolite.

Isofacial and exotic ultramaficsThe Al-rich metaharzburgite is usually isofacial

with its envelope. This means that, for example,an ultramafic lens in the Köli nappe complex(upper allochthon) surrounded by greenschistfacies micaschists is mineralogically composed of,e.g., antigorite + brucite rF chlorite, whereas an ul-tramafic rock of the same bulk composition in theSvartisen nappe complex ("uppermost" alloc-hthon) surrounded by upper amphibolite faciesmicaschists contains forsterite + enstatite +hercynite. Most of the envelope rocks of the ultra-

mafic bodies display microtextures and chemicalzoning patterns in refractory minerals which areconsistent with polystage or polyphase histories ofrecrystallization along complex P–T–t-paths. Theterm "isofacial" (Evans, 1977) is therefore oflimited significance. On the other hand, most ofthe ultramafic rocks also display complex multi-stage microtextures. They often permit the distinc-tion of successive groups of mineral assemblageswhich may be related to a distinct P–T–t-path orsequence of reactions relating them. Large por-tions of the P–T–t-path are frequently shared bythe ultramafics and the envelope rocks.

There are several categories of allofacial orexotic ultramafic rocks in the ScandinavianCaledonides. The most common is represented bythe often poorly re-equilibrated meta-harzburgites(or lherzolites) in ophiolite complexes (e.g. Leka;Prestvik, 1972; Fumes et al., 1988; Dunning andPedersen, 1988). The assemblage olivine +orthopyroxene + clinopyroxene + spinet repre-sents a nonequilibrium relict assemblage in a ter-rain with chloritoid + chlorite in micaschists andantigorite + brucite in partly equilibrated ultra-mafics. Garnet-bearing peridotites emplaced inamphibolite facies gneisses along shear zones andthrust faults together with high-pressure eclogites(Smith and Lappin, 1989) (in contrast to low-Pcrustal eclogites; e.g., Bryhni et al., 1977) repre-sent another type of allofacial ultramafic rocks.Here, the envelope rocks did not share the veryhigh pressure portion of the P–T–t-path followedby the ultramafic rocks and parts of the associatedmafic rocks.

Mineralogy of the ultramafic rocks

Because of the simple total rock composition ofaluminous meta-harzburgites only a very limitednumber of different mineral assemblages wasformed in these rocks. All observed stable mineralassemblages are listed in Table 1. Anthophylliteand Mg-cummingtonite occur in hydrothermal re-action veins cutting across the ultramafic lenses.Radial bundles of anthophyllite may measure 60cm in diameter in some veins of the Svartisencomplex. Anthophyllite rarely coexists with for-sterite and prograde anthophyllite + forsterite is

MANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES 181

TABLE 1

Observed mineral assemblages in ultramafic rocks of the Scandinavian Caledonides

Metamorphic grade MSH +Al

+ Ca Metamorphic facies

Very low gradeLow grade

Medium grade

High grade

High P/low THigh P/mid THigh P/high T

chrysotil + talcantigorite + talcantigorite + brucite

antigorite + forsterite

antigorite + forsterite

forsterite + talcforsterite + enstatite

forsterite + enstatite

forsterite + enstatite

forsterite + antigorite

forsterite + talc (enstatite)

forsterite + enstatite

chlorite tremolitechlorite diopsidechlorite diopsidechlorite diopsidechlorite tremolitechlorite tremolitechlorite tremolitehercynite Al-amphibolehercynite diopside

chlorite diopside/tremolitechlorite tremolitechlorite a tremolitehercynite diopsidegarnet

sub-greenschistlow greenschistlow greenschistupper greenschistlow amphibolitemid amphiboliteupper amphibolitelow granulitehigh granulite

blueschist/low-T eclogiteseclogitehigh- T eclogite

a Stable Al-phase depends on precise P–T and on PH20.

unknown from the Scandinavian Caledonides.Green hercynitic spinel is formed as a progradephase from the thermal decomposition of chlorite(e.g., Svartisen). Green spinel occurs in manyspinel peridotites (e.g., Bergen Arcs, WesternGneiss Region, Seve) in equilibrium textures.Magnetite is the common opaque phase in mostultramafic rocks. Its formation is usually relatedto oxidation associated with serpentinization.Chromite and brown chrome spinel are very com-mon and occur as strongly resorbed and oxidizedrelic phases (Bucher-Nurminen, 1988). Sulfides aregenerally rare. Nickel minerals occur as widespreadaccessories. A comprehensive description of oreminerals found in ultramafic rocks from theTrondheim nappe complex (Seve and Köli level) isgiven by Nilsen (1978).

Carbonates: The nearly universal presence ofcarbonate minerals in a large number of differentassemblages appears to be special feature of theultramafics of the Scandinavian Caledonides. Wellknown examples are the enstatite–magnesite rocksfrom the Troms region (Schreyer et al., 1972;Ohnmacht, 1974) Very coarse enstatite + magne-site rocks are also characteristic of all ultramaficlenses in the Svartisen nappe complex (Cribb,1982). Similar rocks have also been described fromthe Western Gneiss Region (Moore, 1977). Therocks formed by the reaction of olivine with a

CO2-rich fluid phase. At many localities the reac-tion went to completion and all olivine has beenreplaced by enstatite + magnesite. The source ofthe fluid and location of the alteration remainsunknown. Magnesite (breunerite) is the dominantcarbonate mineral because CO 2-metasomatism ofmeta-harzburgite is rarely associated with calciumtransfer. However, at some localities calcite and/ordolomite occur as reaction products of fluid/rockinteraction. These carbonates are often found inveins, shears and in black walls. The inferredcalcium transfer which occasionally has accompa-nied CO2-metasomatism also led to the formationof abundant tremolite (often Cr-rich) in such al-tered rocks. The observed calcium transfer sug-gests infiltration by a mixed volatile fluid ratherthan by pure CO 2 . Interaction with mixed H 20-CO2 fluids resulted in the production of a numberof different rock types depending on the fluidcomposition and prevailing P–T conditions dur-ing interaction. Such rock types include variousophicarbonate (antigorite + carbonate) rocks andespecially talc + magnesite rocks (soap stone). Oc-casionally the formation of talc + magnesite fromenstatite + olivine occurs concurrently with growthof amphiboles and other pyriboles (Jimthomp-sonite, Chesterite).

Titanium clinohumite (TICL) has not beenfound in serpentinites of the Scandinavian

182 K. BUGHER-NURMINETj

Caledonides. This is remarkable because thismineral is widespread in serpentinites of the Alpsand other mountain belts. The absence of TICLfrom serpentinites of the Scandinavian Caledo-nides is not a consequence of a low Ti content butrather could be the result of different conditionsduring serpentinization. It is also remarkable inthis context that only one single occurrence ofrodingite (meta-rodingite) has been reported fromthe Scandinavian Caledonides (Brie, 1985). Theabsence of TICL and the scarcity of rodingite hasprobably the same but unknown reasons.

Metamorphism of ultramafic rocks

All ultramafic fragments from the mantle aremetamorphic rocks. Mantle fragments in theCaledonian belt may have a complex polymeta-morphic history. Any sample may contain relicttextures and minerals from the mantle period ofits history as well as assemblages which resultedfrom modifications during its residence in thecrust. Depending on the detailed history of theultramafic occurrence it may have been meta-morphosed during several Precambrian and bothCaledonian thermal events (Finnmarkian andScandian). In addition the early (Finnmarkian)and main (Scandian) Caledonian metamorphismare regionally diachronous (Dallmeyer, 1988).

Central Caledonides

However, the regional distribution of observedtextures and mineral assemblages shows a rela-tively clear and simple picture. Figure 5 shows themaximum temperature assemblages of Caledonianmetamorphism in some ultramafic occurrencesfrom the Nordland, Västerbotten and Norbottenareas respectively (geology in Fig. 2). The occur-rences in the Seve units generally display highgrade assemblages which do not fit into the gen-eral large scale picture of low grade metamor-phism in the proximity of the Caledonian front(Calon, 1979). The maximum temperature andpressure assemblages have been formed duringearly Caledonian prograde metamorphism. Someallofacial garnet–olivine rocks have been em-placed in the Seve units during stacking (Van

Roermund, 1989). The ultramafic rocks in theSeve nappes are often associated with mafic rocksin eclogite facies and with metapelites showinghigh pressure assemblages. The Seve high graderocks represent the only convincing example oftransported Caledonian metamorphism in theCaledonides (there is much transported Pre-cambrian metamorphism of course). The mainCaledonian metamorphism in the Seve units re-sulted in local retrogression of the early assem-blages where fluids became available (e.g. Calon.1979). The assemblages produced indicate a meta-morphic grade comparable to the Köli unit to-wards the west. Regional Caledonian metamor-phism post-dating nappe transport has not ex-ceeded greenschist facies.

All other localities shown in Fig. 5 displayassemblages which fit into a regular large scalemetamorphic pattern with the metamorphic gradeincreasing from southeast to northwest. The pat-tern has been produced by the main (Scandian)Caledonian metamorphism. The lowest grade isindicated by the assemblages from Köli ultra-mafics in the southern part of the map and fromthe Leka ophiolite (Fig. 5). All assemblages fromthe Helgeland nappe complex ("uppermost" al-lochthon) and from the Mo i Rana district arediagnostic for conditions near the greenschist-amphibolite facies boundary which is consistentwith chlorite + staurolite grade metapelites andtremolite + calcite + dolomite + quartz in mar-bles. However, the intrusion of Caledoniangranitoid plutons and smaller gabbro stocks andsheets in the Helgeland nappes (or Köli ophiolites;see above) resulted in local contact aureoles withprograde contact metamorphic sequences in ultra-mafic (and other) rocks. Typical for these occur-rences is the presence of well developed pseudo-spinifex (Jack-straw) textures with extremelyelongated olivine (cf. also Bakke and Kornelius-sen, 1986). Assemblages in ultramafic rocks fromboth sides of the nappe boundary between theKöli and Helgeland nappe complex are very simi-lar (cf. also Lutro, 1989) and are consistent with apost-transport metamorphism. The Köli occur-rences on the west side of the Saltfjellet window(more than ten large lenses) display mid-amphibolite facies assemblages and very similar

C I+

S p1

metamorphicgeotherm

16

14

12

1(1

h

6

4

154 K fit ( III K NI. KMINE-\

4()) S0()

600

7(x1

Temperature (T)

(1. ['ham! relationships in ultramafic rocks slightl y modified from Bucher-Nurminen ( 1985). I he metamorphic conditions duringthe Scandian main phase for the localities in the central Scandinavian (aledonides have peen deduced from the ultrainafics and their

I sotacial envelope. The defined smooth metamorphic geotherm correspond to the regular metamorphic pattern slim\n in Fig. 12.

and Stigh. 1985) which suggests that both com-plexes (each with its series of nappes) have beentransported along the same major thrust fault.Some localities also show additional complexity asa result of local thermal effects from igneousintrusions. Finally. Köli ultramafics from south ofNarvik also show mid-amphiholite facies assem-blages (Crowle y and Spear. 1987). Crowle y andSpear (1987) have also shown that individualnappes followed separate earl y metamorphic PT- -t -path but the paths eventuall y converge.

The general metamorphic P T-conditions forthe main Caledonian phase May he placed on aregional metamorphic gradient (Fig. 6). The meta-morphic gradient is constrained by the "peak"assemblages in ultramafic rocks and metapelites inthe envelopes of the ultramafic lenses. The ultra-mafic rocks have equilibrated or were partly "re-set" along this t ypical " Barrovian" type metamor-phic geotherm after the large scale nappe trans-port. This observation is consistent with data frommetapelites from the Ofoten nappe stack (Hodgesand Rovden. 1984: Steltenpohl and I3artley. 1987:Barker. 1989).

Southern C(tic(1(u (i(Ic°s

The distribution pattern of Caledonian mineralassemblages in ultramafic rocks for the Southern

C'aledonides is shown in Fig. 7 (geology in Fig. 3).The pattern shows a gradual increase ofmetamor-phic grade normal to the long axis of thebelt from the Caledonian front in the southeast tothe coastal area in the northwest. All of the verymany occurrences in the Seve units south of the"Trondheim nappes (Fig. 7) display low grade as-semblages of the middle greenschist facies. Theuniform assemblages indicate that the strike of theunits parallels the metamorphic isograds. The lowgrade character of the Seve rocks contrasts sharplywith the granulite to eclogite facies assemblages inthe equivalent Seve units north of the (;rang Oldenculmination. The si g nificance of this contrastingmetamorphic evolution has not vet been investi-gated.

Note in Fig. 7 that the ultramafic rocks of the.lotun nappe and the external nappes north of the.10t Lin nappe are of the same metamorphic gradeas the Seve ultramafics mentioned above. To-gether these occurrences define a belt of' middle toupper greenschist facies grade which parallels themain axis of the Caledonides and cuts across allnappe boundaries.

The metamorphic grade recorded b y the ultra-mafic rocks of the Seve units rapidl y increasestowards the north along the west side of theTrondheim nappes and reaches mid-amphiholitefacies in the Oppdal area ( Fig. 7). The same

183MANTLE FRAGMENTS IN THF. SCANDINAVIAN CALEDONIDES

Fig. 5. Distribution of metamorphic mineral assemblages in ultramafic rocks from the central Scandinavian Caledonides. Circles:En + Fo, squares: Tlc + Fo, diamonds: Atg + Fo, filled diamonds: Atg + Brc. The metamorphic significance of the assemblages isdepicted on Fig. 6 and expressed on Table 1. All mineralogical information on this figure is based on samples collected by the author

(all Norwegian localities) and on Calon (1979), and Crowley and Spear, (1987) for some of the localities in Sweden.

textures with large roundish olivine aggregates upto 30 cm in size) in a talc matrix. These occur-rences are very distinct and must mark a majorthrust fault. A progressive increase in metamor-phic grade is indicated by the assemblages of theultramafic rocks from the Saltfjellet towards thecoastal nappe complex (Svartisen area). Thehighest metamorphic grade is characterized by theassemblage En + Fo + Prg + Spl (abbreviations ofmineral names after Kretz, 1983) which is uni-formly found in very coarse post deformation

textures in all ultramafic rocks from the Saltencoast Sorensen, 1955a,b; Bucher-Nurminen, 1988;Svartisen nappe, Högtuva nappe, Sjona nappe,nappes of the Glomfjorden area). In addition,magnesite + enstatite rocks can be found at alllocalities in the area. Upper amphibolite faciesconditions are characteristic for the coastal area ofthe Svartisen nappe complex. The general char-acteristics of the ultramafic occurrences in theTroms district are very similar to the ones in theSvartisen nappe complex (Ohnmacht, 1974; Qvale

Precambriangranulite

faciesultramafics

NIJosa

Caledonian10 Front

185\1 \ \ I I I FRA(i\1F\'ls IN TM . SCANDINAVIAN CAI 1-UOVII)FS

Fig. 7. Distribution of metamorphic mineral assemblages in ultramafic rocks form the southern Scandinavian Caledonides. Circles:I n — Fo. squares: Tlc + Fo. diamonds: Atg + Fo + Di, filled diamonds: Atg + Fo + Tr, pointing down triangles: Atg + Tlc,triangles: Atg + ßrc. the metamorphic significance of the assemblages is shown on Fig. 7 and explained in Table 1. All mineralogical

information in this figure is based on samples collected b y the author.

general increase in metamorphic grade is shownby the ultramafics of the Western Gneiss Regionalong a profile from Lom to Alesund (Fig. 7). Thepicture is also manifest in mafic rocks (Griffin etal.. 1985) and metapelites (Krill, 1985). The situa-tion in the Western Gneiss Region is complex indetail for the reasons explained above and becauseof the presence of a variety of genetically dis-tinctly different ultramafic rocks.

A particularly interesting series of ultramaficrocks occur as lenses in a metasedimentary se-quence running SE–NW from the Lom area to thecoast Markey, 1969). Some textural details of onetypical lens is shown in Fig. 8. The envelope ismade up of finehanded supracrustal gneisses witha very strong foliation and strong houdinage ofamphibolite hands. The ultramafic lens consists oftwo parts: (a) an enstatite + forsterite rock whichis the product of Caledonian regional metamor-phism and (h) a magnesite + talc rock which re-sulted from interaction of the en + fo rock withexternall y derived CO,-rich fluids. Strongly elon-gated enstatite crystals of up to 12 cm length are

randomly oriented in a finer grained forsteritematrix. The fabric is a dramatic contrast to thefabric of the surrounding gneisses. A close-up ofthe microtexture of the En + Fo rock is also shownin Fig. 8. Locally a ghost texture, marked by fineparallel hands of very fine grained magnetite, isovergrown by coarse forsterite crystals and theenstatite megacrystals. The texture (and the com-position of the minerals) suggests that the as-semblage is a prograde metamorphic assemblagewhich progressively replaced a low grade pre-cursor rock, most likely an antigorite + magnetiteschist. The fabric suggests that the En + Fo grademetamorphism occurred after the emplacement ofthe lens in its present gneissic environment. Smallrelict fragments of meta-eclogite are also associ-ated with the ultramafic lens. These rocks alsoshow a strong tectonic fabric. The structure of theoccurrence suggests that fragments of eclogite andserpentinite have been picked up by faults from asource area in the mantle. This implies that eclo-gite formation occurred in the stability field ofantigorite (serpentine), e.g., below about 650 °C at

talc -+magnesite.

roots/.•

meta=eclogite

thrustfault

strongly fdrietedgneisses

and boudinägedampliibolites

Schematic map of an ultramafic boudin in the WesternGneiss Region near Grotli (Ottadalen)

186 K. BU('HF:R-NtIRMINI:N

Fig. 8. General mesoscopic texture of an ultramafic rock lens

in the Western Gneiss Region near Grotli, Ottadalen. The inset

shows a general microtextural detail found in many medium to

high grade ultramafic rocks of the Scandinavian Caledonides.

16 kbar. The temperature limit given here is con-sistent with the isotherms mapped by Griffin et al.(1985) for the eclogite stage of the Scandian mainphase. The subsequent formation of coarse en-statite + forsterite rock can then be explained as

the result of thermal relaxation, rapid uplift anddecompression of the nappe stack of the WesternGneiss Region.

The maximum assemblages shown in Fig. 7 canbe placed onto the P–T-plane of Fig. 9. Thevarious localities define a similar metamorphicgradient as shown in Fig. 6 for the CentralCaledonides. The metamorphic gradient of Fig. 9is related to the main Caledonian (Scandian)metamorphism of the area and is probably youngerthan the major lateral movements of the nappesbut still older than some late extension tectonismwhich has modified parts of the Caledonian geom-etry of the nappes (Norton, 1986). The variousunits have, however, reached their position alongthe metamorphic geotherm along different P–T-t-paths which are characteristic of the individualunits. Early Caledonian metamorphism may be

represented by some Seve eclogites of the Oppdalarea which have not been studied in detail so far.The position of Bergen arcs shown in Fig. 9 alongthe main trend is given by the late amphibolitefacies overprinting of the rocks of the area includ-ing the ultramafic rocks. Both the Bergen arcs andthe Western Gneiss Region have reached theirposition along the regional trend from an eclogitestage which is also related to the main Caledonian

300 4(X) 500 6(X) 700 8(X) 9(X)

Temperature (°C)

Fig. 9. Phase relationships in ultramafic rocks slightly modified from Bucher-Nurminen (1988). The metamorphic conditions during

the Scandian main phase for the localities in the central Scandinavian Caledonides have been deduced from the the ultramafics and

their isofacial envelope. The defined smooth metamorphic geotherm corresponds to the regular metamorphic pattern shown in Fig.

12.

metamorphicgeotherm —

^ B shield

geother

=— Ht

MANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES 187

stacking and thickening of the crust during conti-nent-continent collision (e.g., Cuthbert et al.,1983).

Discussion

The maximum metamorphic conditions for theCentral and Southern Caledonides are sum-marized in Figs. 10 and 11 respectively and areshown together with some geotherms and P-T-t-path for some of the regions and units. In theCentral Caledonides (Fig. 10) several features mustbe emphasized. The coastal strip from about theRana Fjorden (Svartisen nappe complex, Saitencoast) extending probably to the Troms region,represents the highest grade area in the CentralCaledonides. It is the metamorphic core area ofthe Caledonian main phase. Some hornblendemineral ages are available from the Precambriangneiss cores of the Svartisen nappe complex (thegneiss cores are erroneously designated as"windows" in the a large part of the literature(e.g., Stephens, 1988)). The 401 to 418 Ma40Ar/39Ar plateau ages (Dallmeyer, 1988) are interpre-ted as cooling ages dating the post-Scandian cool-ing through about 500 ° C. The maximum temper-ature of the rocks was about 700-750 ° C and itrequired about 25 Ma to cool the rocks from themaximum temperature to the closing temperatureof the hornblendes assuming a moderate, time

dependent cooling rate of about 5-20 ° C/Ma. Letus assume that the Caledonian orogeny followed asimilar large scale orogenic cycle of lithosphericthickening, heating, isothermal decompression andcooling as many other orogenic belts. The onset ofcooling from the 700-750 ° C maximum tempera-ture was at about 420-440 Ma and the tectonicthickening of the crust by stacking and thrustingended at about 440-460 Ma (assuming a typicaltime lag of about 20 Ma between the crustalthickening and the onset of uplift and cooling—e.g., England and Thompson, 1984; Thompsonand Ridley, 1987). According to this crude esti-mate, the 440-460 Ma time range marks the endof the major Scandian deformation (formation ofthe nappe stacks, thickening of the crust). Thecoastal units of the Saiten, Ofoten and Troms areawere about 40 to 50 km deep at about 440-460Ma before the onset of uplift and cooling. It istherefore unlikely that Scandian high-grade meta-morphic nappes incorporate Caledonian sedi-ments younger than about 440-460 Ma. Sedimen-tation of flysch-type sediments associated withsynorogenic volcanics will be expected to accom-pany the main Scandian compression and stackingphase of Caradoc-Ashgill age. The youngestmarine sediments in West Norway are of lateLlandovery age (Bruton and Harper, 1988) whichagrees with the proposed 420-440 Ma for theonset of the major uplift and cooling phase.

2 Leka 1 1 I I I I I I I I I

300 400

Temperature (°C)

Fig. 10. Tentative P—T-path for localities from the central Scandinavian Caledonides.

500 600 700 800 900

16

14

12

10

TØverdalen Dovre)Otta

OppdalFolldalTynsetRoros

Sognefjell

metamorphic,geotherm

shieldgeotherm

MQHO►

T̂r unndalenollheimen

188 K. nU('nER-Nt%RMINE"

2300 400 500 600 7(X) 8(X) 9(X)

Temperature (°C)

Fig. 11. Tentative P–T-path for localities from the southern Scandinavian C'alcdonides.

It is also important to recognize the large meta-morphic differences within the very different het-erogeneous units of the so called "uppermost"allochthon. The maximum temperatures reachedby the Helgeland and Svartisen ultramafics differby as much as 250-300 °C. The rocks of thesouthern Helgeland coast are typical mid-crustalrocks (about 5 kbar), whereas the Svartisen rocksrepresent rock volumes from the thickened con-tinental root of the Caledonian belt.

A similar dramatic difference of the tectono-thermal evolution can be deduced for the Kölielements (Fig. 10). The difference in maximumtemperature attained during the Caledonian mainphase is of the order of 200 °C. The Köli units ofthe southern regions in Fig. 2 were only mod-erately heated (< 400 ° C) and may therefore stillpreserve parts of the early pre-ScandianCaledonian tectonothermal evolution. The Seveunits were not heated above about 450 °C (butprobably less than 400 ° C) during the Caledonianmain phase. As a consequence, biotite andmuscovite ( 4(Ar/ 39Ar) cooling ages may indicatethe Caledonian main phase, whereas hornblendemay (partly) preserve the early Caledonian evolu-tion (Dallmeyer, 1988). However, the position ofthe Seve units in Fig. 10 is strictly for the units inthe Västerbotten and Jämtland districts (Fig. 2),Farther north (Sarek) and in internal Seve windows

the Scandian metamorphism may have locally re-set all minerals.

The general pressure-temperature path for theSeve units is shown as "Seve loop" in Fig. 10. Thehigh-pressure high-temperature part of the loop isrelated to the early tectonothermal evolution ofthe Caledonides (destruction of the Iapetus ocean).The units subsequently returned to shallow levelsof the crust during the main Caledonian nappetransport and cooled to the regional main Cale-donian metamorphic geotherm.

The prograde metamorphic path for ultramaficfragments which have been incorporated in theSvartisen nappe stack during the collision andstacking related to the Caledonian main phasemay have followed section A (Fig. 10). As sug-gested by the textures and the mineral composi-tions, most of the ultramafics were picked up inserpentinized form by the thrusting. The steadystate geotherm of the continental areas prior tcthe onset of the Caledonian compression is poorlyknown but it was probably close to a normalshield geotherm since the cratonization of theBaltic shield was accomplished about 1 billionyears prior to the onset of the Caledonian cycle.The Baltic crust was not involved in theCaledonian cycle until the Scandic main phase.Initial stacking, thickening and faulting of theBaltic crust therefore occurred above a cold and

Caledonian MetamorphismScandian main phase

Seve nappe complexwith early Caledonianmetamorphism

very low grade

greenschist facies

low amphibolite facies

amphibolite facies

high grade (incl. eclogites etc)

unknown (greenschist or higher)

early Caledonian eclogites

no Caledonian metamorphism300 km

MANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES

189

brittle mantle. The mantle on a shield geotherm(Fig. 10) is in the stability field of antigorite(+ Fo, or + Brc; Fig. 6). If H 2O is available insuch a mantle, serpentinites will readily form fromharzburgites (Bucher-Nurminen, 1990). The accessof the required H 2O was certainly facilitated bythe inferred deep faulting and stacking. The sec-tion B of the P—T-path (Fig. 10) was followed bythe crustal segment (mid-crustal Precambriangranitoids and its Caledonian sedimentary cover)which is today exposed in the Svartisen nappestack prior to the pick-up of the ultramafic frag-ments. Section A is the path jointly followed bythe crustal and mantle rocks after ultramafic rockswere picked up. The meta-harzburgitic and partlyserpentinized ultramafics on the shield geothermunderwent further serpentinization as water be-came available in the tectonic faults and shearzones. The possibility remains that the lithospheremight have been heated and/or thinned by anevent related to the Iapetus formation leading tothe dehydration of ultramafic rocks previouslypresent in a hydrated form. However, such a ther-mal event was unlikely to affect the lithosphericmantle beneath the shield away from the axis ofrifting. It cannot be ruled out that some of theprograde ultramafic rocks (dehydrated serpen-tinites) found in the nappes with continental affin-ity, may be partially related to the rifting stageduring Iapetus formation rather to the Caledonianmain phase. However, the large scale consistencyof prograde textures suggests that most of theobserved ultramafic rocks were dehydrated duringthe Caledonian cycle. Figure 11 shows similartentative models for the tectonothermal history forthe Southern Caledonides. The minimum hydratedstate of the ultramafic (and other) rocks wasreached during the Scandian main phase and de-fines the regional metamorphic trend (also previ-ously shown in Fig. 9). The Seve equivalent unitsof the area (Bläho, Surnadal, etc; locality namesSunndalen, Trollheimen, Dovre, Folldal, Tynset,Roros in Fig. 9) again span a wide range ofmetamorphic conditions depending on the posi-tion of the Seve localities in relation to the mainCaledonian isograd pattern and the temperaturerange is from 350 ° to 600 ° C. Here, the lowestgrade Scandian Seve localities which may have

preserved a pre-Scandian history are the ones alongthe South-rim of the Trondheim nappe complex.The Bergen arcs have arrived at their position onthe metamorphic geotherm from a Scandian eclo-gite stage (Austrheim and Griffin, 1985). WesternGneiss Region localities differ by about 100-150 ° C with regard to maximum temperature re-ached during the Scandian main phase. It is sug-gested that slicing and stacking of Baltic continen-tal crust has resulted in the piercing of the im-bricates by mantle wedges. The mantle materialincluded serpentinites, meta-harzburgites, meta-lherzolites (with garnet or spinel or chlorite; de-pending on the PH,() and the degree of equilibra-tion) and was further serpentinized during theinitial stages of tectonic transport. Finally, theultramafics of the Western Gneiss Region traveledalong general P–T paths as shown in Fig. 11,which involved both high pressure and high tem-perature modifications of previously inhomoge-neously equilibrated and hydrated mantle rocks.

Bryhni and Brastad (1980) and Bryhni (1983)presented overview maps of the Scandinavian

Fig. 12. Metamorphic map of the Scandinavian Caledonidesshowing the general pattern of the Scandian main phase of

Caledonian metamorphism.

190 K. BUGHER-NURMINEN

Caledonides showing the maximum metamorphicgrade attained by all rocks or the maximum gradeof Caledonian metamorphism attained by Cale-donian rocks (metasediments of assumed or provenpost-Vendian age) excluding the Caledonian meta-morphism in pre-Vendian rocks (Caledonian base-ment). A new compilation of metamorphic infor-mation based on the data presented above is givenin Fig. 12 The regular general pattern of theregional "in situ" metamorphism is related to theScandian main phase.

Acknowledgements

This is publication number 104 of theNorwegian contribution to the InternationalLithosphere Project. Financial support from theNorwegian Science Foundation (NAVF; grantD.440.90/009) is gratefully acknowledged. 1 grate-fully acknowledge the critical and constructivereviews by Jacoby and an anonymous reviewer.

References

Andresen, A. and Rykkelid, E., 1989. Basement shorteningacross the Caledonides in the Torneträsk-Ofoten area.Geol Fören. Förh., 111:381-383.

Austrheim, H. and Griffin. W.L., 1985. Shear deformation andeclogite formation within granulite-facies anorthosites ofthe Bergen Arcs, Western Norway. Chem. Geol., 50: 267-281.

Bakke, S. and Korneliussen, A., 1986. Jack-straw-texturedolivines in some Norwegian metaperidotites. Nor. Geol.Tidsskr., 66: 271-276.

Barker, A.J., 1989. Metamorphic evolution of the Caledoniannappes of north central Scandinavia. In: R.A. Gayer (Edi-tor), The Caledonide Geology of Scandinavia. Graham andTrotman, London, pp. 193-204.

Barkey, H., 1969. Structural map of Western Jotunheimen,Norway. Geological Survey of Norway, Trondheim, 4sheets.

Bee, R., 1985. Rodingite from Lindas, western Norway. Nor.Geol. Tidsskr., 65: 301-320

Bruton, D.L. and Harper, D.A.T., 1988. Arenig-Llandoverystratigraphy and faunas across the Scandinavian Caledo-nides. In: A.L. Harris and D.J. Fettes (Editors), The Cale-donian-Appalachian Orogen. Geol. Soc. London, Spec.Publ., 38: 247-268.

Bryhni, I., 1983. Regional overview of metamorphism in theScandinavian Caledonides. In: P.E. Schenk (Editor), Re-gional Trends in the Geology of the Appalachian-Cale-

donian- Hercynian- Mauritanide Orogen. Reidel, Dor-drecht, pp. 193-204.

Bryhni, I., 1989. Status of the supracrustal rocks in the West-ern Gneiss Region, S Norway. In: R.A. Gayer (Editor), TheCaledonide Geology of Scandinavia. Graham and Trotman,London, pp. 221-228.

Bryhni, I. and Brastad, K., 1980. Caledonian regional meta-morphism in Norway. J. Geol. Soc., 137: 251-259.

Bryhni, I., Krogh, E.J. and Griffin, W.L., 1977. Crustal deriva-tion of Norwegian eclogites: a review. Neues Jahrb.Mineral., Ahh., 130: 59-68.

Bucher-Nurminen. K., 1988. Metamorphism of ultramafic rocksin The Central Scandinavian Caledonides. Nor. Geol. Un-ders., Spec. Publ., 3: 86-95.

Bucher-Nurminen, K., 1990. Transfer of mantle fluids to thelower continental crust; constraints from mantle mineral-ogy and MOHO temperature. In: B.K. Nelson and P. Vidal(Editors), Development of the Continental Crust throughGeological Time. Chem. Geol., 83: 249-261.

Calon, T.J., 1979. A study of the alpine-type peridotites in theSeve-Koli Nappe Complex Central Swedish Caledonideswith special reference to the Kittelfjäll peridotite. Doct.Thesis, Univ. of Leiden, 236 pp.

Cribb, St. J., 1982. The Torsvik sagvandite body, North Nor-way . Nor. Geol. Tidsskr., 62: 161-168.

Crowley, P.D. and Spear, F.S., 1987. The P-T evolution of themiddle Koli Nappe Complex, Scandinavian Caledonides(68° N) and its tectonic implications. Contrib. Mineral.Petrol., 95: 512-522.

Cuthbert, S.J, Harve y, M.A. and Carswell, D.A., 1983. Atectonic model for the metamorphic evolution of the BasalGneiss Comples, Western South Norway. J. Metamorph.Geol., 1: 63-90.

Dallmeyer, R.D., 1988. Polyphase tectonothermal evolution ofthe Scandinavian Caledonides. In: A.L. Harris and D.J.Fettes (Editors), The Caledonian-Appalachian Orogen.Geol. Soc. London, Spec. Publ., 38: 365-379.

Dallmeyer, R.D. and Gee, D.G., 1986a. 4()Ar/ 3°Ar mineraldates from retrogressed eclogites within the Baltoscandianmiogeocline: implications for a polyphase Caledonian oro-genic evolution. Geol. Surv. Am. Bull., 97: 26-34.

Dallmeyer R.D. and Gee, D.G., 1986b. Polyorogenic 4` Ar/ 39Armineral age record in the Seve and Koli nappes of theGäddede area, northwestern Jämtland, central Scan-dinavian Caledonides. J. Geol., 96: 181-198.

Dallmeyer, R.D., Gee, D.G. and Beckholmen, M., 1985.4` Ar/ 39Ar mineral age record of early Caledoniantectonothermal activity in the Baltoscandian miogeocline,central Scandinavia. Am. J. Sci., 285: 532-658.

Dietler, T.N., 1987. Strukturgeologische and tektonische En-twicklung des Western Gneiss Complexes im Sognefjord-Querschnitt, Westliches Norwegen. Ph.D. Thesis, SwissFederal Technical University (ETH), Zurich, 8194: 234 pp.

Dunning, G.R. and Pedersen, R.B., 1988. U/Ph ages of ophio-lites and arc-related plutons of the Norwegian Caledonides:

IANTLE FRAGMENTS IN THE. SCANDINAVIAN CALEDONIDES 191

implications for the development of Iapetus. Contrib.Mineral. Petrol., 98: 13-23.

)yrelius, D., Gee, D.G., Gorbatschev, R., Ramberg, H. andZachrisson, E., 1980. A profile through the central

Scandinavian Caledonides. Tectonophysics, 69: 247-284..ngland, P.C. and Thompson, A.B., 1984. Pressure-tempera-

ture-time paths of regional metamorphism, I. Heat transfer

during the evolution of regions of thickened continentalcrust. J. Petrol., 25: 894-928.

:vans, B.W., 1977. Metamorphism of Alpine peridotite andserpentine. Ann. Rev. Earth Planet. Sei.. 5: 397-447.

:urnes, H., Pedersen, R.B. and Stillman, C.J., 1988. The Lekaophiolite complex, central Norwegian Caledonides: fieldcharacteristics and geotectonic significance. J. Geol. Soc.London, 145: 401-412.

see. D.G., Kumpulainen, R., Roberts, D., Stephens, M.B.,Thon, A. and Zachrisson, E., 1985. Scandinavian

Caledonides: Tectonostratigraphic Map. In: D.G. Gee andB.A. Sturt (Editors), The Caledonide Orogen-Scandinaviaand related areas. Wiley, Chichester.

griffin, W.L., Taylor, P.N., Hakkinene, J.W., Heier, K.S.,Iden, LK., Krogh, E.J., Malm, 0., Olsen, K.I., Ormaasen,D.E. and Tveten, E., 1978. Archaean and Proterozoic crustal

evolution in Lofoten-Vesterälen, N Norway. J. Geol. Soc.London, 135: 629-647.

Griffin, W.L., Austrheim, H.. Brastad, K., Bryhni, I., Krill, A.,Mork, M.B.E., Qvale, H. and Torudbakken, B., 1985.High-pressure metamorphism in the Scandinavian Caledo-nides. In: D.G. Gee and B.A. Sturt (Editors), The

Caledonide Orogen-Scandinavia and Related Areas. Wi-ley, Chichester, pp. 783-802.

Heim, M., Schärer, U. and Milnes, G., 1977. The nappe

complex in the Tyin-Bygdin-Vang region, central southernNorway. Nor Geol. Tidsskr., 57: 171-178.

Hodges, K.V. and Royden, L., 1984. Geologic thermobarome-try of retrograded metamorphic rocks: an indication of theuplift trajectory of a portion of the northern ScandinavianCaledonides. J. Geophys. Res., 89: 7077-7090.

Hunch, C.A., Palm, H., Dyrelius, D. and Kristoffersen, Y.,1989. Deformation of the Baltic continental crust duringCaledonide intracontinental subduction: views from seismicreflection data. Geology, 17: 423-425.

Johansson, L. and Möller, C., 1986. Formation of sapphirineduring retrogression of a basic high-pressure granulite,Roan, Western Gneiss Region, Norway. Contrib. Mineral.Petrol., 94: 29-41.

Kretz, R., 1983. Symbols for rock-forming minerals. Am.Mineral., 68: 277-279.

Krill, A.G., 1985. Relationships between the Western GneissRegion and the Trondheim Region: stockwerk-tectonicsreconsidered. In: D.G. Gee and B.A. Sturt (Editors), TheCaledonide Orogen-Scandinavia and Related Areas. Wi-ley, Chichester, pp. 475-483.

Lindqvist, J-E., 1987. Metamorphism in basement rocks andthe implications for the tectonic evolution, Nasafjäll

Window, Scandinavian Caledonides. Geol. Rundsch., 76(3):837-850.

Lutro. 0., 1979. The geology of the Gjersvik area, NordTrondelag, Central Norway. Nor. Geol. Unders., 354: 53-100.

Magnusson, N.H., 1957. Pre-Quaternary rocks of Sweden-Scale 1 :1 million, Sver. Geol. Unders., Ser. Ba 16.

Medaris, L.G., 1984. A geothermobarometric investigation ofgarnet peridotites in the Western Gneiss Region of Nor-way. Contrib. Mineral. Petrol., 87: 72-86.

Milnes, A.G. and Koestler, A.G., 1983. Geological structure ofJotunheimen, southern N orway (Sognefjell-Valdrescross-section). In: D.G. Gee and B.A. Stunt (Editors), TheCaledonide Orogen-Scandinavia and Related Areas. Wiley,Chichester, pp. 457-474.

Möller, C., 1988. Geology and metamorphic evolution of theRoan area, Vestranden, Western Gneiss Region, CentralNorwegian Caledonides. Nor. Geol. Unders., 413: 1-31.

Moore, A.C., 1977. The petrography and possible regional

significance of the Hjelmkrona ultramafic body (sagvan-dite), Nordmore, Norway. Nor. Geol. Tidsskr., 57: 55-64.

Nilsen, 0., 1978. Caledonian sulphide deposits and minor

iron-formations from the Southern Trondheim Region,Norway. Nor. Geol. Unders., 340: 35-85.

Norton, M.G., 1986. Late Caledonide extension in WesternNorway: a response to extreme crustal thickening. Tecton-ics, 5: 195-204.

Ohnmacht, W., 1974. Petrogenesis of carbonate-orthopyroxen-ites (sagvandites) and related rocks from Troms, northernNorway. J. Petrol., 15: 303-323.

Prestvik, T., 1972. Alpine-type mafic and ultramafic rocks ofLeka, Nord-Trondelag. Nor. Geol. Unders., 273: 23-34.

Roberts, D. and Gee, D.G., 1985. An introduction to thestructure of the Scandinavian Caledonides: In: D.G. Gee

and B.A. Sturt (Editors), The Caledonide Orogen:Scandinavia and Related Areas. Wiley, Chichester, pp. 55-68.

Qvale, H. and Stigh, J., 1985. Ultramafic rocks in theScandinavian Caledonides: In: D.G. Gee and B.A. Stunt(Editors), The Caledonide Orogen-Scandinavia and Re-lated Areas. Wiley, Chichester, pp. 693-716.

Rutland, R.W.R. and Nicholson, R., 1965. Tectonics of theCaledonides of part of Nordland, Norway. Q. J. Geol. Soc.London, 121: 73-109.

Schreyer, W., Ohnmacht, W. and Mannchen, J., 1972.Carbonate-orthopyroxenites (sagvandites) from Troms,northern Norway. Lithos, 5: 345-363.

Sigmond, E.M.O., Gustayson, M. and Roberts, D., 1984. Be-drock map of Norway-Scale 1:1 million. Norges Geol.Unders., Trondheim.

Smith, D.C. and Lappin, M.A., 1989. Coesite in the Straumenkyanite-eclogite pod, Norway. Terra Nova, 1: 47-56.

Sorensen, H., 1955a. A petrographical and structural study of

the rocks around the peridotite at Engenbrx, Holandsfjord,Northern Norway. Nor. Geol. Unders., Ärb., 191: 71-102

192 K. BUCHER-NURMINEN

Sorensen, H., 1955b. A preliminary note on some peridotitesfrom Northern Norway. Nor. Geol. Tidsskr., 35; 93-104.

Steltenpohl M.G. and Bartley, J.M., 1987. Thermobarometricprofile through the Caledonian nappe stack of WesternOfoten, North Norway. Contrib. Mineral. Petrol., 96: 93-103.

Stephens, M.B., 1988. The Scandinavian Caledonides: a com-plexity of collisions. Geol. Today., 20-26.

Stillman, C.J., 1988. Ordovician to Silurian volcanism in theAppalachian-Caledonian orogen. In: A.L. Harris and D.J.Fettes (Editors), The Caledonian-Appalachian Orogen.Geol. Soc. London, Spec. Publ., 38: 275-290.

Thompson, A.B. and Ridley, J.R., 1987. Pressure-tempera-ture-time (P -T - t) histories of orogenic belts. Philos.Trans. R. Soc. London, Ser. A, 321: 27-45.

Van Roermund, H.L.M., 1989. High-pressure ultramafic rocksfrom the allochthonous nappes of the Swedish Caledonides.In: R.A. Gayer (Editor), The Caledonide Geology ofScandinavia. Graham and Trotman, London, pp. 205-219.

Warner, M. and McGeary, S., 1987. Seismic reflection coeffi-cients from mantle fault zones. Geophys. J. R. Astron. Soc.,89: 223-230.