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 of Alpine-type peridotites in the Scandinavian Caledonides represent parts of dismembered ophiolite sequences and fragments of sub-continental upper mantle. Metaperidotites of nappes 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 orogeny. 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 erosion 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 Alps. 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 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- 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- rock 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 fold (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 fault 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 Table 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