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 , 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 of Alpine-type peridotites in the Scandinavian Caledonides represent parts of dismembered ophiolite sequences and fragments of sub-continental upper mantle. Metaperidotites of in internal positions are generally isofacial with the metamorphic envelope, usually Caledonian metasediments but in places also Precambrian metagranitoids forming the basement cores of the nappes. Caledonian metamorphism strongly modified the texture and mineralogy of the peridotites and resulted in a systematic 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 which produced a regular Barrovian-type metamorphic pattern of in-situ metamorphism. Metaperidotites from nappes in intermediate positions (Iapetus Ocean ophiolites and ultramafic rocks from island arc environments) show strongly diverging histories. Metaperidotites from internal ophiolites (oceanic ophiolites. Köli) lack any evidence of subduction metamorphism, are serpentinized to various degrees, show abundant primary mantle relic mineralogies and 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 . The Seve metaperidotites 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 a general reconstruction of the dynamics of collision belt formation.

Introduction on the evolution of the belt which was contained in the Caledonian low-grade sedimentary record The Scandinavian Caledonides (SC) represent has been removed by a long time ago. The an early Paleozoic collision belt of considerable Caledonian orogenic belt was partly destroyed complexity with regard to the kinematics of the and severely modified by the break of the Atlantic orogenesis. The total exposed length of the belt on Ocean in the Mesozoic and by continuous defor- the Scandinavian peninsula exceeds 2000 km which mation until the present (neotectonics). corresponds to 1.5 times the total length of the In contrast to the various Mesozoic to Tertiary . The Scandinavian Caledonides represent a belts, it appears impossible ever to reach an over- relatively deeply eroded mountain chain with the all understanding of the evolution and large scale typical erosion surface at mid-crustal levels of the kinematics of the Caledonian belt. However, the Caledonian structure. Much crucial information 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. * Present address: Mineralogisch-Petrographisches Institut, Albert- Ludwigs- Universität, Alhertstr. 23h, D-7800 Freiburg The Scandinavian Caledonides represent in i. Br., F.R.G. principle an Alpine-type orogenic belt which was

0040-1951/91/503.50 1991 – Elsevier Science Publishers B.V. 174 K. BU(HER-NURM►NEN formed by a large cycle of ocean crust formation SCANDINAVIAN CALEDONIDES (Iapetus) with associated initial rifting and later ophiolite production, subsequent ocean crust con- sumption along a destructive plate margin, and finally a continent–continent collision with stack- ing of the crust, crustal thickening and associated metamorphism and large lateral 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- BALTIC SHIELD nides is the very widespread occurrence of Alpine-type ultramafic rocks at all levels of the BERGEN CALEDONIAN FRONT ARCS tectonostratigraphy (Qvale and Stigh, 1985). Al- 300 km pine-type ultramafic rocks (peridotites, serpen- tinites) are defined here as isolated solitary bodies Fig. 1. The Caledonian orogenic belt on the Scandinavian derived from the upper mantle (oceanic or con- peninsula is shown in grey together with some important place tinental) which have crossed the mantle crust names used in the text. boundary by tectonic processes and which were compositionally, mineralogically and texturally modified in the crust during an orogenic cycle. the tectonostratigraphy outlined below. Several This paper reviews some aspects of some Alpine- aspects of Fig. 4 will be discussed in later sections type ultramafic rocks in the Scandinavian of the paper. Caledonides including their tectonic significance and the general pattern of the Caledonian meta- (a) Autochthonous foreland (autochthon) morphism. The discussion is based on a compila- The foreland in the southeast of the Caledonian tion of new mineralogical data (assemblages, tex- front as it is exposed today is represented by the tures, and mineral chemistry) from a large number Baltic shield (which in turn consists of a Pre- of occurrences of ultramafic rocks form the Central cambrian basement and its thin Precambrian to and Southern Caledonides in addition to informa- lower Paleozoic cover). Ultramafic rocks are ex- tion retrieved from previously published data. tremely rare.

Caledonian tectonostratigraphy (h) External nappes and Western Gneiss Region (lower allochthon) For the purpose of the review of ultramafic The lowest of the transported units above the rocks it is necessary to give a brief overview of the Caledonian front are typical external nappes with general tectonostratigraphy of the Scandinavian low grade Caledonian sediments, imbricate and Caledonides. Place names are given in Fig. 1 and duplex structures (Figs. 2, 3 and 4). The nappes names of tectonostratigraphic units from Roberts rarely incorporate slices of the basement. A large and Gee (1985) are given in brackets. The regional amount of cover shortening is recorded by these geology of the central Scandinavian Caledonides cover units. The corresponding shortening in the is summarized in Fig. 2 and of the southern basement of the Baltic shield has occurred farther Scandinavian Caledonides in Fig. 3. A tentative to the northwest in particular in the Western profile across the Caledonian belt is presented in Gneiss Region. The belt of external nappes locally Fig. 4. Figure 4 facilitates the understanding of includes Alpine-type ultramafics (Barkey, 1969). HANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES 175

Svartisen Nappe Complex Rüdingsfjellet Nappe Complex

Autochthonotiis Base Went Baltic hield) Helgeland Ophiolite Complex

!External nappes

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 of The Western Gneiss Region complex consists the Baltic shield along the internal side of the of a sequence of nappes separated by thin meta- higher tectonic units of the Caledonian nappe morphic cover sequences which have overprinted stack and the basement is exposed in the so called primary contacts towards the basement on one Western Gneiss Region (Fig. 3). The Western side and are bounded by thrust faults on the other Gneiss Region thus represents a large window of a side (Fig. 4). These thrust faults are crowded with lower tectonic level (several hundred km extension ultramafic lenses which are in turn very often along the coast). Caledonian shortening, deforma- associated with eclogites. The Caledonian meta- tion and thermal overprinting rapidly and con- morphic overprint gradually increases towards the tinuously increases towards the coastal area (Diet- coast (Medaris, 1984; Griffin et al., 1985). Condi- ler, 1987). tions reach upper amphiholite to the beginning of 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 Arcs). Deep seismic studies of the British (15-18 kbar in Western Gneiss Region, > 20 kbar Caledonides (Warner and McGeary, 1987) dis- in Bergen Arcs isofacial eclogites). The thrust faults covered a number of dipping seismic reflectors in also carry exotic (allofacial) ultramafic and mafic the deep crust and the upper mantle. These struc- fragments which were picked up by the faults tures have been interpreted as shear zones and at depth between 80 and 100 km (coesite- thrusts which, if the interpretation is correct, could kyanite-eclogites, some of the garnet-peridotites). be viewed as fossil equivalents of Caledonian shear This suggests that faulting and initial stacking has zones and thrusts. Ductile deformation (folding) occurred under fairly brittle conditions which per- and high-temperature metamorphism was prob- mitted very deep reaching faults (see also Bergen ably a consequence of subsequent thermal relaxa-

Särv Jotun nappe External nappes Baltic shield

^+ ++++++++++++++++ f +++++++++++++++++ lower Köli F +++++++++++++ h +++++++++ + + + + + + + + + + + + + + + +++++++ ++++++ ^^^^^^•^^ .00 NW-Western 1++ ++ Svartisen Bergen arcs Gneiss Region ,

Fig. 4. Schematic geological cross section across the Scandinavian Caledonides showing the general positions of the major units. The profile 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- rocks include greenschists, amphibolites, mafic erate shortening. The Western Gneiss Region rep- granulites and eclogites. Ultramafic rocks of the resents (together with the Bergen arcs) the core Seve units include the whole spectrum from low area of Caledonian stacking and Caledonian ther- greenschist facies brucite + antigorite schists to mal overprinting during the Scandian continent- garnet peridotites. Paleogeographically the Seve continent collision (root zone) (Cuthbert, Harvey units may represent the transition zone, with all its and Carswell, 1983; Johansson and Möller, 1986; complexity, between the Baltic shield and the Möller, 1988) Iapetus ocean which is represented by the Köli units (see below). The ultramafics within the Seve (c) Säry realm (middle allochthon) complex partly belong to mafic/ ultramafic associ- The tectonic domain above the external nappe ations which resemble ophiolite sequences. Typi- province may be collectively termed the Säry re- cal for such transition zones are associations of alm (Figs. 2, 3 and 4). In the southern part of the within plate, continental and marginal basin char- Caledonides the tectonic province is mainly repre- acter (Stillman, 1988). The Seve units have been sented by the Jotun nappe complex and the Bergen involved in early Caledonian (Finnmarkian) arcs. The two nappe units comprise Precambrian tectonism and metamorphism (Dallmeyer and Gee, mafic-ultramafic igneous complexes in granulite 1986a, 1986b). The early Caledonian eclogites and facies of Precambrian age and its Caledonian garnet-olivine rocks may be related to a metamorphic cover. The main body of the far- Caledonian subduction process which consumed travelled Jotun nappe is formed by a large re- the the Iapetus ocean prior to the main Caledonian cumbent (e.g. Milnes and Koestler, 1983; (Scandian) continent-continent collision. Heim et al., 1977). Caledonian metamorphism gradually increases from the foreland towards the (e) Köli realm (upper allochthon) northwest. It reaches mid-greenschist facies condi- Includes most of the known true ophiolite se- tions at the internal boundary of the Jotun nappe. quences (Stillman, 1988) of the Scandinavian Most of the ultramafic rocks of the Jotun nappe Caledonides. The Köli nappes also include a very and the Bergen arcs are not Alpine-type ultra- large number of ultramafic bodies of all sizes and mafics but rather represent ultramafic cumulates varied sedimentary (partly fossiliferous) and in gabbro-anorthosite sequences. Parts of the volcanic/igneous record. The units represent the Bergen Arcs represents an internal equivalent of relics of the former Iapetus ocean and local margi- the Jotun nappe which have been subjected to nal basins. The main feature of the Köli ophiolites strong ductile deformation and high pressure is that they apparently have not been subducted metamorphism during the Caledonian cycle during the destruction of the Iapetus. There are no (Austrheim and Griffin, 1985). In the Central reported occurrences of eclogites and/or blue- Caledonides the Säry level of the tectonostratigra- schists in the Köli rocks. This is in marked con- phy consists mainly of Caledonian low grade trast to other Alpine-type orogenic belts. It may nappes which comprise both Precambrian gneisses be concluded from this lack of a regular subduc- and Caledonian sediments (particularly Vendian tion mechanism for the destruction of the Iapetus elastics). The local names of the nappes include ocean that this ocean consisted only very locally Särv, Offerdal, Risberget, Rondane, Valdres nappe of a strict oceanic crust-mantle sequence in the to name a few (Dyrelius et al., 1980). Metamor- modern (Mesozoic-Tertiary) sense. phic studies in these units are incomplete. (f) Helgeland nappe complex (uppermost alloch- (d) Seve realm (upper allochthon) thon) The Seve unit consists of a fairly heterogeneous The units which are collectively termed " upper- collection of different nappes. The lithologies most" allochthon (Gee et al., 1985) are very het- range from ultramafic and mafic rock sequences erogeneous and their tectonostratigraphic position to metapelitic and psamitic gneisses. The mafic is somewhat dubious. The various nappe corn- 178 K. BUCHER-NURMINEN

plexes of the " uppermost" allochthon typically (h) Rödingsfjellet nappe complex (uppermost al- include Precambrian continental rocks and Cale- lochthon) donian granitoids as well as Caledonian metasedi- This complex includes a series of nappes which ments. Therefore, the units either originate from are dominated by medium grade Caledonian cover the western continental margin of the Iapetus rocks particularly marbles. Precambrian gneiss ocean or alternatively they may represent units cores are, on the other hand, also the backbone of from a much lower position in the tectonostratig- these nappes. Ultramafic rocks from this nappe raphy on the Baltic Shield side. complex are distinctly different from those of the Ultramafic rocks occur at a number of locali- Svartisen nappe complex and suggest a different ties in the Helgeland nappe complex. Locally, source area in the mantle and a different geologi- thermal metamorphism associated with the intru- cal history after emplacement. sion of Caledonian gabbros and granitoids has strongly modified the ultramafic rocks which were (i) Lofoten present as serpentinites prior to the intrusions. A The tectonostratigraphic position of the Pre- number of ultramafic lenses occur in ophiolite cambrian rocks of the Lofoten area is unknown. associations along the Helgeland coast and their Several tectonic positions are possible (e.g. Andre- tectonostratigraphic relationship to the Helgeland sen and Rykkelid, 1989). The preferred solution nappe complex is uncertain. it is very likely that shown on Fig. 4 is consistent with the large scale all ophiolites (Stillman, 1988: Skäl y cr, Rodoy, metamorphic pattern of the area and with new Leka, Bronnoysund) belong to the same belt of geophysical observations available from the ophiolites along the southern parts of the Helge- Central Caledonides (Hurich et al., 1989). How- land coast and may collectively represent a Köli ever, it causes some problems with the widely used element. This Köli element may be connected with terminology of Caledonian tectonostratigraphy the equivalent elements in the East either above or (uppermost of uppermost allochthon?). The use of below the Helgeland nappe complex. The "above" generalized place names for the major units would solution for the western Köli elements brings the help to avoid such problems. The absence of pre- "uppermost" allochthon to the same tectonostrati- served Caledonian cover makes it difficult to de- grap hic level as the Western Gneiss Region. duce the effects of Caledonian metamorphism and deformation. however, regional Caledonian amphibolite facies metamorphism is indicated by (g) Svartisen nappe complex (uppermost alloch- Caledonian muscovite cooling ages (Griffin et al., thon) 1978). Deformation appears to be restricted to The nappes of this nappe complex are built up shear zones. Alkali amphibole bearing eclogites by large Precambrian gneiss cores and Caledonian developed along some of these shear zones in cover sequences separating the individual nappes. Precambrian gabbros which may represent the The tectonic style of the area has similarities to effects of early Caledonian stacking and transport the Penninic area of the Alps (see also Ruthland of the Lofoten nappes. and Nicholson, 1965). All units are meta- morphosed in at least upper amphibolite facies Ultramafic rocks and intense multi-phase ductile deformation is common. Metamorphism and deformation is re- General aspects lated to the Scandian continent–continent colli- sion. Ultramafic rocks are very abundant in the The typical occurrence of ultramafic rock bod- metasediments separating the gneiss cores of the ies is in the form of isolated lens shaped masses of nappes but several occurrences in the Precambrian various sizes ranging from a few centimeters to gneisses are known. All ultramafic rocks of this several kilometers in length. Very characteristic nappe complex are very similar as regards tex- for wide areas in the Caledonides is the presence tures, mineralogy and P–T–t-evolution. of serpentinite or peridotite humps and knolls of MANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES 179

roundish or lensoid shape sticking prominently neous rock units of Precambrian age which form above the surrounding country rocks. The yellow- the basement for the Caledonian sediments. brownish weathering surface of the large mounds Caledonian sediments (and/or volcanics) are rocks usually measuring some hundreds of meters, con- of Precambrian to early Paleozoic age which expe- trasts with the standard grey gneisses of the rienced their first metamorphic overprint, usually Scandinavian mountain ranges. Ultramafic lenses in several phases, during the Caledonian orogenic occur locally in very large numbers although the cycle. The recognition of proven Caledonian rocks total surface covered by ultramafic rocks at the is difficult or ambiguous in many of the nappes. present erosion level does not exceed a few per- The ultramafic mantle fragments found along cent (referenced to a 16 km 2 grid). folded Caledonian thrust faults are interpreted The ultramafic rocks are always aligned along here as Caledonian rocks. They were picked up tectonic zones, e.g., shear zones, faults and nappe during Caledonian thrusting and consequently boundaries. They represent important markers of represent Caledonian mantle samples. The pres- otherwise often obscure nappe boundaries. Dozens ence of Alpine-type ultramafic rocks in the pre- of solitary Alpine-type peridotites often decorate a Caledonian basement prior to Caledonian thrust- major thrust at the present erosion surface in ing and stacking is less likely because of the a given local area. Therefore, the entire thrust scarcity of ultramafic rocks in the Baltic shield surface is likely to be plastered with peridotite outside the Caledonian belt (e.g. Modum fragments at depth. This surface geology suggests serpentinites). It is clear, however, that Caledonian that relatively large pieces of mantle peridotite ultramafic rocks could yield radiometric ages were picked up during the stacking of the Cale- ranging from the Archean to the Caledonian. The donian nappes. Furthermore, the stacking of the "age" is determined by the thermal structure of crust and its dissection was conducted by faults the mantle and the state of hydration at the time which cut across the crust mantle boundary. The of tectonic pick-up and by the subsequent thermal invasion of the crust by mantle ultramafics along (metamorphic) history of the fragment during its deep faults occurred initially in the form of large residence in the crust. mantle wedges which later became fragmented as a result of faulting, shearing, folding and Rock associations boudinage (Bucher-Nurminen, 1988). The geome- try of the thrust surface, together with the struc- The ultramafic rocks are normally associated tural features of the country rocks, indicates that with meta-supracrustal rocks. All typical meta- it underwent complex post-thrusting deformation supracrustals of a given nappe are found in the (see also fig. 5 in Cuthbert et al., 1983). envelope of the ultramafic lenses. There is no The field aspects of ultramafic rocks in the special predominance of associated mafic rocks Scandinavian Caledonides described above are (except in nappes where are they are generally general in the sense that the description is inde- abundant, e.g., in ophiolite associations). Rocks in pendent of the tectonostratigraphic level. Most of contact with the ultramafic rocks include marbles, the Alpine-type solitary ultramafic rock bodies amphibolites, micaschists, quartzites, greenschists, probably represent fragmented mantle material quartzo-feldspathic gneisses, eclogites and others. from the subcontinental mantle. An important In some of the nappes, ultramafic rocks with exception are ultramafic occurrences in ophiolite frequent mafic dykes are characteristic. The associations where parts of the ultramafic material mineral assemblages of the mafic dykes usually may represent fragments from the mantle beneath reflect the metamorphic grade of the country rocks. oceanic crust. Ultramafic lenses in the (ortho-) gneisses and migmatites of the basement cores of the nappes Caledonian and pre-Caledonian ultramafics are very rare. Some, however, do occur (e.g., Gro- In a discussion of the Caledonian orogeny one tli (Western Gneiss Region)–Barkey, 1969; Rodey has to distinguish between metamorphic and ig- (Salten)—Sorensen, 1955b) and their properties 180 K. BÜCHER-NURMINEN appear to be identical to those situated in the mafic bodies display microtextures and chemical nearby supracrustals. zoning patterns in refractory minerals which are Many of the large number of ultramafic rocks consistent with polystage or polyphase histories of in the nappe complex of the Western Gneiss re- recrystallization along complex P–T–t-paths. The gion are associated with quartzites and quartz-rich term "isofacial" (Evans, 1977) is therefore of micaschists. One would think that sedimentation limited significance. On the other hand, most of of probably late Precambrian (Vendian) sand- the ultramafic rocks also display complex multi- stones and arkoses on to the Baltic Shield is not stage microtextures. They often permit the distinc- the favorite geological environment for the em- tion of successive groups of mineral assemblages placement of solitary peridotites. Thus, it is sug- which may be related to a distinct P–T–t-path or gested that these ultramafics represent characteris- sequence of reactions relating them. Large por- tic examples of tectonically emplaced fragments of tions of the P–T–t-path are frequently shared by the subcontinental Caledonian mantle. The meta- the ultramafics and the envelope rocks. supracrustals (quartzites and quartzose mica- There are several categories of allofacial or schists) are found as continuous units in the exotic ultramafic rocks in the Scandinavian migmatitic gneisses (basement cores) and, together Caledonides. The most common is represented by with the Caledonian ultramafics, mark nappe the often poorly re-equilibrated meta-harzburgites boundaries within the Western Gneiss Region (or lherzolites) in ophiolite complexes (e.g. Leka; (Barkey, 1969; Krill, 1985; Bryhni, 1989). Prestvik, 1972; Fumes et al., 1988; Dunning and Pedersen, 1988). The assemblage olivine + Rock types and assemblages orthopyroxene + clinopyroxene + spinet repre- sents a nonequilibrium relict assemblage in a ter- There is essentially only one major ultramafic rain with chloritoid + chlorite in micaschists and rock type represented in all the continental associ- antigorite + brucite in partly equilibrated ultra- ations and that is an aluminous meta-harzburgite mafics. Garnet-bearing peridotites emplaced in (Qvale and Stigh, 1985). Subordinate are some amphibolite facies gneisses along shear zones and occurrences of meta-dunite. Aluminous meta- thrust faults together with high-pressure eclogites harzburgite dominates in nappes at all levels of (Smith and Lappin, 1989) (in contrast to low-P the tectonostratigraphy. This suggests that the crustal eclogites; e.g., Bryhni et al., 1977) repre- subcontinental Caledonian mantle was very uni- sent another type of allofacial ultramafic rocks. form in bulk composition. In ophiolite associa- Here, the envelope rocks did not share the very tions, ultramafic rocks occur as cumulates in mafic high pressure portion of the P–T–t-path followed sequences and as fragments from the sub-oceanic by the ultramafic rocks and parts of the associated mantle. The latter are also predominantly meta- mafic rocks. harzburgites with subordinate meta-lherzolite. Mineralogy of the ultramafic rocks Isofacial and exotic ultramafics The Al-rich metaharzburgite is usually isofacial Because of the simple total rock composition of with its envelope. This means that, for example, aluminous meta-harzburgites only a very limited an ultramafic lens in the Köli nappe complex number of different mineral assemblages was (upper allochthon) surrounded by greenschist formed in these rocks. All observed stable mineral facies micaschists is mineralogically composed of, assemblages are listed in 1. Anthophyllite e.g., antigorite + brucite rF chlorite, whereas an ul- and Mg-cummingtonite occur in hydrothermal re- tramafic rock of the same bulk composition in the action veins cutting across the ultramafic lenses. Svartisen nappe complex ("uppermost" alloc- Radial bundles of anthophyllite may measure 60 hthon) surrounded by upper amphibolite facies cm in diameter in some veins of the Svartisen micaschists contains forsterite + enstatite + complex. Anthophyllite rarely coexists with for- hercynite. Most of the envelope rocks of the ultra- 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 grade chrysotil + talc chlorite tremolite sub-greenschist Low grade antigorite + talc chlorite diopside low greenschist antigorite + brucite chlorite diopside low greenschist antigorite + forsterite chlorite diopside upper greenschist Medium grade antigorite + forsterite chlorite tremolite low amphibolite forsterite + talc chlorite tremolite mid amphibolite forsterite + enstatite chlorite tremolite upper amphibolite High grade forsterite + enstatite hercynite Al-amphibole low granulite forsterite + enstatite hercynite diopside high granulite

High P/low T forsterite + antigorite chlorite diopside/tremolite blueschist/low-T eclogites High P/mid T forsterite + talc (enstatite) chlorite tremolite eclogite High P/high T forsterite + enstatite chlorite a tremolite high- T eclogite hercynite diopside garnet a Stable Al-phase depends on precise P–T and on PH20. unknown from the Scandinavian Caledonides. CO2-rich fluid phase. At many localities the reac- Green hercynitic spinel is formed as a prograde tion went to completion and all olivine has been phase from the thermal decomposition of chlorite replaced by enstatite + magnesite. The source of (e.g., Svartisen). Green spinel occurs in many the fluid and location of the alteration remains spinel peridotites (e.g., Bergen Arcs, Western unknown. Magnesite (breunerite) is the dominant Gneiss Region, Seve) in equilibrium textures. carbonate mineral because CO 2-metasomatism of Magnetite is the common opaque phase in most meta-harzburgite is rarely associated with calcium ultramafic rocks. Its formation is usually related transfer. However, at some localities calcite and/or to oxidation associated with serpentinization. dolomite occur as reaction products of fluid/rock Chromite and brown chrome spinel are very com- interaction. These carbonates are often found in mon and occur as strongly resorbed and oxidized veins, shears and in black walls. The inferred relic phases (Bucher-Nurminen, 1988). Sulfides are calcium transfer which occasionally has accompa- generally rare. Nickel minerals occur as widespread nied CO2-metasomatism also led to the formation accessories. A comprehensive description of ore of abundant tremolite (often Cr-rich) in such al- minerals found in ultramafic rocks from the tered rocks. The observed calcium transfer sug- Trondheim nappe complex (Seve and Köli level) is gests infiltration by a mixed volatile fluid rather given by Nilsen (1978). than by pure CO 2 . Interaction with mixed H 20- Carbonates: The nearly universal presence of CO2 fluids resulted in the production of a number carbonate minerals in a large number of different of different rock types depending on the fluid assemblages appears to be special feature of the composition and prevailing P–T conditions dur- ultramafics of the Scandinavian Caledonides. Well ing interaction. Such rock types include various known examples are the enstatite–magnesite rocks ophicarbonate (antigorite + carbonate) rocks and from the Troms region (Schreyer et al., 1972; especially talc + magnesite rocks (soap stone). Oc- Ohnmacht, 1974) Very coarse enstatite + magne- casionally the formation of talc + magnesite from site rocks are also characteristic of all ultramafic enstatite + olivine occurs concurrently with growth lenses in the Svartisen nappe complex (Cribb, of amphiboles and other pyriboles (Jimthomp- 1982). Similar rocks have also been described from sonite, Chesterite). the Western Gneiss Region (Moore, 1977). The Titanium clinohumite (TICL) has not been rocks formed by the reaction of olivine with a found in serpentinites of the Scandinavian 182 K. BUGHER-NURMINETj

Caledonides. This is remarkable because this Roermund, 1989). The ultramafic rocks in the mineral is widespread in serpentinites of the Alps Seve nappes are often associated with mafic rocks and other mountain belts. The absence of TICL in eclogite facies and with metapelites showing from serpentinites of the Scandinavian Caledo- high pressure assemblages. The Seve high grade nides is not a consequence of a low Ti content but rocks represent the only convincing example of rather could be the result of different conditions transported Caledonian metamorphism in the during serpentinization. It is also remarkable in Caledonides (there is much transported Pre- this context that only one single occurrence of cambrian metamorphism of course). The main rodingite (meta-rodingite) has been reported from Caledonian metamorphism in the Seve units re- the Scandinavian Caledonides (Brie, 1985). The sulted in local retrogression of the early assem- absence of TICL and the scarcity of rodingite has blages where fluids became available (e.g. Calon. probably the same but unknown reasons. 1979). The assemblages produced indicate a meta- morphic grade comparable to the Köli unit to- Metamorphism of ultramafic rocks wards the west. Regional Caledonian metamor- phism post-dating nappe transport has not ex- All ultramafic fragments from the mantle are ceeded greenschist facies. metamorphic rocks. Mantle fragments in the All other localities shown in Fig. 5 display Caledonian belt may have a complex polymeta- assemblages which fit into a regular large scale morphic history. Any sample may contain relict metamorphic pattern with the metamorphic grade textures and minerals from the mantle period of increasing from southeast to northwest. The pat- its history as well as assemblages which resulted tern has been produced by the main (Scandian) from modifications during its residence in the Caledonian metamorphism. The lowest grade is crust. Depending on the detailed history of the indicated by the assemblages from Köli ultra- ultramafic occurrence it may have been meta- mafics in the southern part of the map and from morphosed during several Precambrian and both the Leka ophiolite (Fig. 5). All assemblages from Caledonian thermal events (Finnmarkian and the Helgeland nappe complex ("uppermost" al- Scandian). In addition the early (Finnmarkian) lochthon) and from the Mo i Rana district are and main (Scandian) Caledonian metamorphism diagnostic for conditions near the greenschist- are regionally diachronous (Dallmeyer, 1988). amphibolite facies boundary which is consistent with chlorite + staurolite grade metapelites and Central Caledonides tremolite + calcite + dolomite + quartz in mar- bles. However, the intrusion of Caledonian However, the regional distribution of observed granitoid plutons and smaller gabbro stocks and textures and mineral assemblages shows a rela- sheets in the Helgeland nappes (or Köli ophiolites; tively clear and simple picture. Figure 5 shows the see above) resulted in local contact aureoles with maximum temperature assemblages of Caledonian prograde contact metamorphic sequences in ultra- metamorphism in some ultramafic occurrences mafic (and other) rocks. Typical for these occur- from the Nordland, Västerbotten and Norbotten rences is the presence of well developed pseudo- areas respectively (geology in Fig. 2). The occur- spinifex (Jack-straw) textures with extremely rences in the Seve units generally display high elongated olivine (cf. also Bakke and Kornelius- grade assemblages which do not fit into the gen- sen, 1986). Assemblages in ultramafic rocks from eral large scale picture of low grade metamor- both sides of the nappe boundary between the phism in the proximity of the Caledonian front Köli and Helgeland nappe complex are very simi- (Calon, 1979). The maximum temperature and lar (cf. also Lutro, 1989) and are consistent with a pressure assemblages have been formed during post-transport metamorphism. The Köli occur- early Caledonian prograde metamorphism. Some rences on the west side of the Saltfjellet window allofacial garnet–olivine rocks have been em- (more than ten large lenses) display mid- placed in the Seve units during stacking (Van amphibolite facies assemblages and very similar 154 K fit ( III K NI. KMINE-\

16 C I + 14 S p1 metamorphic 12 geotherm

1(1

h

6

4

4()) S0() 600 7(x1

Temperature (T)

(1. [ham! relationships in ultramafic rocks slightl y modified from Bucher-Nurminen ( 1985). I he metamorphic conditions during the 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- Caledonides is shown in Fig. 7 (geology in Fig. 3). plexes (each with its series of nappes) have been The pattern shows a gradual increase of transported along the same major . metamor-phic grade normal to the long axis of the Some localities also show additional complexity as belt from the Caledonian front in the southeast to a result of local thermal effects from igneous the coastal area in the northwest. All of the very intrusions. Finally. Köli ultramafics from south of man y occurrences in the Seve units south of the Narvik also show mid-amphiholite facies assem- "Trondheim nappes (Fig. 7) display low grade as- blages (Crowle y and Spear. 1987). Crowle y and semblages of the middle greenschist facies. The Spear (1987) have also shown that individual uniform assemblages indicate that the strike of the nappes followed separate earl y metamorphic P units parallels the metamorphic isograds. The low T- -t -path but the paths eventuall y converge. grade character of the Seve rocks contrasts sharply The general metamorphic P T-conditions for with the granulite to eclogite facies assemblages in the main Caledonian phase May he placed on a the equivalent Seve units north of the (;rang Olden regional metamorphic gradient (Fig. 6). The meta- culmination. The si g nificance of this contrasting morphic gradient is constrained b y the "peak" metamorphic evolution has not vet been investi- assemblages in ultramafic rocks and metapelites in gated. the envelopes of the ultramafic lenses. The ultra- Note in Fig. 7 that the ultramafic rocks of the mafic rocks have equilibrated or were partly "re- .lotun nappe and the external nappes north of the set" along this t ypical " Barrovian" type metamor- .10t Lin nappe are of the same metamorphic grade phic geotherm after the large scale nappe trans- as the Seve ultramafics mentioned above. To- port. This observation is consistent with data from gether these occurrences define a belt of middle to metapelites from the Ofoten nappe stack (Hodges upper greenschist facies grade which parallels the and Rovden. 1984: Steltenpohl and I3artley. 1987: main axis of the Caledonides and cuts across all Barker. 1989). nappe boundaries. The metamorphic grade recorded b y the ultra- Southern C(tic(1(u (i(Ic°s mafic rocks of the Seve units rapidl y increases towards the north along the west side of the The distribution pattern of Caledonian mineral Trondheim nappes and reaches mid-amphiholite assemblages in ultramafic rocks for the Southern facies in the Oppdal area ( Fig. 7). The same MANTLE FRAGMENTS IN THF. SCANDINAVIAN CALEDONIDES 183

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 is depicted 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 up textures in all ultramafic rocks from the Salten to 30 cm in size) in a talc matrix. These occur- coast Sorensen, 1955a,b; Bucher-Nurminen, 1988; rences are very distinct and must mark a major Svartisen nappe, Högtuva nappe, Sjona nappe, thrust fault. A progressive increase in metamor- nappes of the Glomfjorden area). In addition, phic grade is indicated by the assemblages of the magnesite + enstatite rocks can be found at all ultramafic rocks from the Saltfjellet towards the localities in the area. Upper amphibolite facies coastal nappe complex (Svartisen area). The conditions are characteristic for the coastal area of highest metamorphic grade is characterized by the the Svartisen nappe complex. The general char- assemblage En + Fo + Prg + Spl (abbreviations of acteristics of the ultramafic occurrences in the mineral names after Kretz, 1983) which is uni- Troms district are very similar to the ones in the formly found in very coarse post deformation Svartisen nappe complex (Ohnmacht, 1974; Qvale \1 \ \ I I I FRA(i\1F\ls IN TM . SCANDINAVIAN CAI 1-UOVII)FS 185

NIJosa

Precambrian granulite Caledonian facies 10 Front ultramafics 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 shown randomly oriented in a finer grained forsterite by the ultramafics of the Western Gneiss Region matrix. The fabric is a dramatic contrast to the along a profile from Lom to Alesund (Fig. 7). The fabric of the surrounding gneisses. A close-up of picture is also manifest in mafic rocks (Griffin et the microtexture of the En + Fo rock is also shown al.. 1985) and metapelites (Krill, 1985). The situa- in Fig. 8. Locally a ghost texture, marked by fine tion in the Western Gneiss Region is complex in parallel hands of very fine grained magnetite, is detail for the reasons explained above and because overgrown by coarse forsterite crystals and the of the presence of a variety of genetically dis- enstatite megacrystals. The texture (and the com- tinctly different ultramafic rocks. position of the minerals) suggests that the as- A particularly interesting series of ultramafic semblage is a prograde metamorphic assemblage rocks occur as lenses in a metasedimentary se- which progressively replaced a low grade pre- quence running SE–NW from the Lom area to the cursor rock, most likely an antigorite + magnetite coast Markey, 1969). Some textural details of one schist. The fabric suggests that the En + Fo grade typical lens is shown in Fig. 8. The envelope is metamorphism occurred after the emplacement of made up of finehanded supracrustal gneisses with the lens in its present gneissic environment. Small a very strong foliation and strong houdinage of relict fragments of meta-eclogite are also associ- amphibolite hands. The ultramafic lens consists of ated with the ultramafic lens. These rocks also two parts: (a) an enstatite + forsterite rock which show a strong tectonic fabric. The structure of the is the product of Caledonian regional metamor- occurrence suggests that fragments of eclogite and phism and (h) a magnesite + talc rock which re- serpentinite have been picked up by faults from a sulted from interaction of the en + fo rock with source area in the mantle. This implies that eclo- externall y derived CO,-rich fluids. Strongly elon- gite formation occurred in the stability field of gated enstatite crystals of up to 12 cm length are antigorite (serpentine), e.g., below about 650 °C at 186 K. BU(HF:R-NtIRMINI:N

Schematic map of an ultramafic boudin in the Western the result of thermal relaxation, rapid uplift and Gneiss Region near Grotli (Ottadalen) decompression of the nappe stack of the Western talc -+ thrust magnesite. fault Gneiss Region. roots/. The maximum assemblages shown in Fig. 7 can • be placed onto the P–T-plane of Fig. 9. The various localities define a similar metamorphic gradient as shown in Fig. 6 for the Central meta=eclogite Caledonides. The metamorphic gradient of Fig. 9 is related to the main Caledonian (Scandian) metamorphism of the area and is probably younger than the major lateral movements of the nappes but still older than some late extension tectonism which has modified parts of the Caledonian geom- strongly fdrieted etry of the nappes (Norton, 1986). The various gneisses and boudinäged units have, however, reached their position along ampliibolites the metamorphic geotherm along different P–T- t-paths which are characteristic of the individual units. Early Caledonian metamorphism may be Fig. 8. General mesoscopic texture of an ultramafic rock lens in the Western Gneiss Region near Grotli, Ottadalen. The inset represented by some Seve eclogites of the Oppdal shows a general microtextural detail found in many medium to area which have not been studied in detail so far. high grade ultramafic rocks of the Scandinavian Caledonides. The position of Bergen arcs shown in Fig. 9 along the main trend is given by the late amphibolite 16 kbar. The temperature limit given here is con- facies overprinting of the rocks of the area includ- sistent with the isotherms mapped by Griffin et al. ing the ultramafic rocks. Both the Bergen arcs and (1985) for the eclogite stage of the Scandian main the Western Gneiss Region have reached their phase. The subsequent formation of coarse en- position along the regional trend from an eclogite statite + forsterite rock can then be explained as stage 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. MANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES 187 stacking and thickening of the crust during conti- dependent cooling rate of about 5-20 ° C/Ma. Let nent-continent collision (e.g., Cuthbert et al., us assume that the Caledonian orogeny followed a 1983). similar large scale orogenic cycle of lithospheric thickening, heating, isothermal decompression and Discussion cooling as many other orogenic belts. The onset of cooling from the 700-750 ° C maximum tempera- The maximum metamorphic conditions for the ture was at about 420-440 Ma and the tectonic Central and Southern Caledonides are sum- thickening of the crust by stacking and thrusting marized in Figs. 10 and 11 respectively and are ended at about 440-460 Ma (assuming a typical shown together with some geotherms and P-T-t- time lag of about 20 Ma between the crustal path for some of the regions and units. In the thickening and the onset of uplift and cooling— Central Caledonides (Fig. 10) several features must e.g., England and Thompson, 1984; Thompson be emphasized. The coastal strip from about the and Ridley, 1987). According to this crude esti- Rana Fjorden (Svartisen nappe complex, Saiten mate, the 440-460 Ma time range marks the end coast) extending probably to the Troms region, of the major Scandian deformation (formation of represents the highest grade area in the Central the nappe stacks, thickening of the crust). The Caledonides. It is the metamorphic core area of coastal units of the Saiten, Ofoten and Troms area the Caledonian main phase. Some hornblende were about 40 to 50 km deep at about 440-460 mineral ages are available from the Precambrian Ma before the onset of uplift and cooling. It is gneiss cores of the Svartisen nappe complex (the therefore unlikely that Scandian high-grade meta- gneiss cores are erroneously designated as morphic nappes incorporate Caledonian sedi- "windows" in the a large part of the literature ments younger than about 440-460 Ma. Sedimen- (e.g., Stephens, 1988)). The 401 to 418 Ma40Ar/ tation of flysch-type sediments associated with 39Ar plateau ages (Dallmeyer, 1988) are interpre- synorogenic volcanics will be expected to accom- ted as cooling ages dating the post-Scandian cool- pany the main Scandian compression and stacking ing through about 500 ° C. The maximum temper- phase of Caradoc-Ashgill age. The youngest ature of the rocks was about 700-750 ° C and it marine sediments in West Norway are of late required about 25 Ma to cool the rocks from the Llandovery age (Bruton and Harper, 1988) which maximum temperature to the closing temperature agrees with the proposed 420-440 Ma for the of the hornblendes assuming a moderate, time onset of the major uplift and cooling phase.

metamorphic geotherm —

B ^ shield geother

=— Ht

2 Leka 1 1 I I I I I I I I I 300 400 500 600 700 800 900

Temperature (°C) Fig. 10. Tentative P—T-path for localities from the central Scandinavian Caledonides. 188 K. nU(nER-Nt%RMINE"

16

14

12 shield metamorphic, geotherm geotherm

10 MQHO

^ unndalen Tr ollheimen

Otta Folldal Oppdal Tynset T Øverdalen Dovre) Roros Sognefjell 2 300 400 500 600 7(X) 8(X) 9(X)

Temperature (°C) Fig. 11. Tentative P–T-path for localities from the southern Scandinavian Calcdonides.

It is also important to recognize the large meta- the Scandian metamorphism may have locally re- morphic differences within the very different het- set all minerals. erogeneous units of the so called "uppermost" The general pressure-temperature path for the allochthon. The maximum temperatures reached Seve units is shown as "Seve loop" in Fig. 10. The by the Helgeland and Svartisen ultramafics differ high-pressure high-temperature part of the loop is by as much as 250-300 °C. The rocks of the related to the early tectonothermal evolution of southern Helgeland coast are typical mid-crustal the Caledonides (destruction of the Iapetus ocean). rocks (about 5 kbar), whereas the Svartisen rocks The units subsequently returned to shallow levels represent rock volumes from the thickened con- of the crust during the main Caledonian nappe tinental root of the Caledonian belt. transport and cooled to the regional main Cale- A similar dramatic difference of the tectono- donian metamorphic geotherm. thermal evolution can be deduced for the Köli The prograde metamorphic path for ultramafic elements (Fig. 10). The difference in maximum fragments which have been incorporated in the temperature attained during the Caledonian main Svartisen nappe stack during the collision and phase is of the order of 200 °C. The Köli units of stacking related to the Caledonian main phase the southern regions in Fig. 2 were only mod- may have followed section A (Fig. 10). As sug- erately heated (< 400 ° C) and may therefore still gested by the textures and the mineral composi- preserve parts of the early pre-Scandian tions, most of the ultramafics were picked up in Caledonian tectonothermal evolution. The Seve serpentinized form by the thrusting. The steady units were not heated above about 450 °C (but state geotherm of the continental areas prior tc probably less than 400 ° C) during the Caledonian the onset of the Caledonian compression is poorly main phase. As a consequence, biotite and known but it was probably close to a normal muscovite (4(Ar/ 39Ar) cooling ages may indicate shield geotherm since the cratonization of the the Caledonian main phase, whereas hornblende Baltic shield was accomplished about 1 billion may (partly) preserve the early Caledonian evolu- years prior to the onset of the Caledonian cycle. tion (Dallmeyer, 1988). However, the position of The Baltic crust was not involved in the the Seve units in Fig. 10 is strictly for the units in Caledonian cycle until the Scandic main phase. the Västerbotten and Jämtland districts (Fig. 2), Initial stacking, thickening and faulting of the Farther north (Sarek) and in internal Seve windows Baltic crust therefore occurred above a cold and MANTLE FRAGMENTS IN THE SCANDINAVIAN CALEDONIDES 189 brittle mantle. The mantle on a shield geotherm preserved a pre-Scandian history are the ones along (Fig. 10) is in the stability field of antigorite the South-rim of the Trondheim nappe complex. (+ Fo, or + Brc; Fig. 6). If H 2O is available in The Bergen arcs have arrived at their position on such a mantle, serpentinites will readily form from the metamorphic geotherm from a Scandian eclo- harzburgites (Bucher-Nurminen, 1990). The access gite stage (Austrheim and Griffin, 1985). Western of the required H 2O was certainly facilitated by Gneiss Region localities differ by about 100- the inferred deep faulting and stacking. The sec- 150 ° C with regard to maximum temperature re- tion B of the P—T-path (Fig. 10) was followed by ached during the Scandian main phase. It is sug- the crustal segment (mid-crustal Precambrian gested that slicing and stacking of Baltic continen- granitoids and its Caledonian sedimentary cover) tal crust has resulted in the piercing of the im- which is today exposed in the Svartisen nappe bricates by mantle wedges. The mantle material stack prior to the pick-up of the ultramafic frag- included serpentinites, meta-harzburgites, meta- ments. Section A is the path jointly followed by lherzolites (with garnet or spinel or chlorite; de- the crustal and mantle rocks after ultramafic rocks pending on the PH,() and the degree of equilibra- were picked up. The meta-harzburgitic and partly tion) and was further serpentinized during the serpentinized ultramafics on the shield geotherm initial stages of tectonic transport. Finally, the underwent further serpentinization as water be- ultramafics of the Western Gneiss Region traveled came available in the tectonic faults and shear along general P–T paths as shown in Fig. 11, zones. The possibility remains that the lithosphere which involved both high pressure and high tem- might have been heated and/or thinned by an perature modifications of previously inhomoge- event related to the Iapetus formation leading to neously equilibrated and hydrated mantle rocks. the dehydration of ultramafic rocks previously Bryhni and Brastad (1980) and Bryhni (1983) present in a hydrated form. However, such a ther- presented overview maps of the Scandinavian mal event was unlikely to affect the lithospheric mantle beneath the shield away from the axis of rifting. It cannot be ruled out that some of the Caledonian Metamorphism prograde ultramafic rocks (dehydrated serpen- Scandian main phase tinites) found in the nappes with continental affin- ity, may be partially related to the rifting stage during Iapetus formation rather to the Caledonian main phase. However, the large scale consistency of prograde textures suggests that most of the observed ultramafic rocks were dehydrated during Seve nappe complex the Caledonian cycle. Figure 11 shows similar with early Caledonian metamorphism tentative models for the tectonothermal history for the Southern Caledonides. The minimum hydrated state of the ultramafic (and other) rocks was reached during the Scandian main phase and de- very low grade fines the regional metamorphic trend (also previ- greenschist facies ously shown in Fig. 9). The Seve equivalent units low amphibolite facies of the area (Bläho, Surnadal, etc; locality names amphibolite facies Sunndalen, Trollheimen, Dovre, Folldal, Tynset, high grade (incl. eclogites etc) Roros in Fig. 9) again span a wide range of unknown (greenschist or higher) metamorphic conditions depending on the posi- early Caledonian eclogites tion of the Seve localities in relation to the main 300 km no Caledonian metamorphism Caledonian isograd pattern and the temperature Fig. 12. Metamorphic map of the Scandinavian Caledonides range is from 350 ° to 600 ° C. Here, the lowest showing the general pattern of the Scandian main phase of grade Scandian Seve localities which may have Caledonian metamorphism. 190 K. BUGHER-NURMINEN

Caledonides showing the maximum metamorphic donian- Hercynian- Mauritanide Orogen. Reidel, Dor- grade attained by all rocks or the maximum grade drecht, pp. 193-204. Bryhni, I., 1989. Status of the supracrustal rocks in the West- of Caledonian metamorphism attained by Cale- ern Gneiss Region, S Norway. In: R.A. Gayer (Editor), The donian rocks (metasediments of assumed or proven Caledonide Geology of Scandinavia. Graham and Trotman, post-Vendian age) excluding the Caledonian meta- London, pp. 221-228. morphism in pre-Vendian rocks (Caledonian base- Bryhni, I. and Brastad, K., 1980. Caledonian regional meta- ment). A new compilation of metamorphic infor- morphism in Norway. J. Geol. Soc., 137: 251-259. Bryhni, I., Krogh, E.J. and Griffin, W.L., 1977. Crustal deriva- mation based on the data presented above is given tion of Norwegian eclogites: a review. Neues Jahrb. in Fig. 12 The regular general pattern of the Mineral., Ahh., 130: 59-68. regional "in situ" metamorphism is related to the Bucher-Nurminen. K., 1988. Metamorphism of ultramafic rocks Scandian main phase. in The Central Scandinavian Caledonides. Nor. Geol. Un- ders., Spec. Publ., 3: 86-95. Acknowledgements Bucher-Nurminen, K., 1990. Transfer of mantle fluids to the lower continental crust; constraints from mantle mineral- ogy and MOHO temperature. In: B.K. Nelson and P. Vidal This is publication number 104 of the (Editors), Development of the Continental Crust through Norwegian contribution to the International Geological Time. Chem. Geol., 83: 249-261. Lithosphere Project. Financial support from the Calon, T.J., 1979. A study of the alpine-type peridotites in the Norwegian Science Foundation (NAVF; grant Seve-Koli Nappe Complex Central Swedish Caledonides D.440.90/009) is gratefully acknowledged. 1 grate- with special reference to the Kittelfjäll peridotite. Doct. Thesis, Univ. of Leiden, 236 pp. fully acknowledge the critical and constructive Cribb, St. J., 1982. The Torsvik sagvandite body, North Nor- reviews by Jacoby and an anonymous reviewer. way. Nor. Geol. Tidsskr., 62: 161-168. Crowley, P.D. and Spear, F.S., 1987. The P-T evolution of the References middle Koli Nappe Complex, Scandinavian Caledonides (68° N) and its tectonic implications. Contrib. Mineral. Andresen, A. and Rykkelid, E., 1989. Basement shortening Petrol., 95: 512-522. across the Caledonides in the Torneträsk-Ofoten area. Cuthbert, S.J, Harve y, M.A. and Carswell, D.A., 1983. A Geol Fören. Förh., 111:381-383. tectonic model for the metamorphic evolution of the Basal Austrheim, H. and Griffin. W.L., 1985. Shear deformation and Gneiss Comples, Western South Norway. J. Metamorph. eclogite formation within granulite-facies anorthosites of Geol., 1: 63-90. the Bergen Arcs, Western Norway. Chem. Geol., 50: 267- Dallmeyer, R.D., 1988. 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