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KURT BUCHER Mantle Fragments in the Scandinavian Caledonides

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KURT BUCHER Mantle Fragments in the Scandinavian Caledonides 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
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