Neoproterozoic metamorphic evolution of the Isbjørnhamna Group rocks from south-western Svalbardpor_186 250..264 Jaroslaw Majka,1 Jerzy Czerny,2 Stanislaw Mazur,3 Daniel K. Holm4 & Maciej Manecki2 1 Department of Earth Sciences, Uppsala University, Villavägen 16, SE-75236 Uppsala, Sweden 2 Department of Mineralogy, Petrography and Geochemistry, AGH–University of Science and Technology, 30 Mickiewicza Av., PL-30059 Kraków, Poland 3 GETECH, Kitson House, Elmete Hall, Elmete Lane, Leeds, LS8 2LJ, UK 4 Department of Geology, Kent State University, Kent, OH 44242, USA Keywords Abstract Caledonides; geothermobarometry; metapelites; Neoproterozoic; Spitsbergen. A metamorphosed volcano-sedimentary complex constitutes the Caledonian basement in the south-western part of Wedel Jarlsberg Land, Svalbard. Field, Correspondence textural and previous thermochronologic data indicate a weak, localized meta- Jaroslaw Majka, Department of Earth morphic Caledonian overprint (M2). Deformed M1 isograds and variation in Sciences, Uppsala University, Villavägen 16, pressure–temperature estimates indicate a pervasive Neoproterozoic SE-75236 Uppsala, Sweden. E-mail: amphibolite-facies metamorphism that pre-dates large-scale Caledonian age [email protected] folding. Garnet–biotite and garnet–Al silicate–plagioclase (GASP) geother- doi:10.1111/j.1751-8369.2010.00186.x mobarometry of the Isbjørnhamna Group mica schists, and their comparison with the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) petrogenetic grid, indi- cates a peak pressure of ca. 11 kbar, and a peak temperature of ca. 670°C during M1 metamorphism. A cooling rate of ca. 5°C My-1 is estimated on the basis of geothermobarometry and the available U–Th–total Pb and Ar–Ar data. Southern Wedel Jarlsberg Land (Fig. 1) uniquely pre- tectonic models proposed for this region. In this study we: serves a record of Neoproterozoic metamorphism on (1) quantify the peak pressure and temperature condi- Spitsbergen, Svalbard. In situ uranium–thorium–lead tions for Neoproterozoic metamorphism; (2) verify the electron microprobe (U–Th–Pb EMP) ages of metamor- correlation between the peak of metamorphism and the phic monazites from a Proterozoic volcano-sedimentary pervasive deformational fabric; (3) define zones of pro- complex date amphibolite-grade metamorphism at ca. gressive metamorphism and the relationship between 643 Mya (Majka et al. 2008). The northern extent of isograds and major tectonic structures; and (4) constrain this Neoproterozoic metamorphism is bounded by the the cooling history of this Neoproterozoic complex. Our Vimsodden–Kosibapasset zone (VKZ), which is a major study is based on extensive fieldwork supported by results strike–slip shear zone with sinistral kinematics (Fig. 2; of microstructural analysis and geothermobarometric Mazur et al. 2009). The VKZ separates the Neoproterozoic calculations. metamorphic complex from the remaining part of the south-western terranes of Svalbard, which were sub- Geological setting jected to younger, early Palaeozoic metamorphism (Gee & Tebenkov 2004; Czerny et al. 2010). The occurrence of The south-western part of Spitsbergen island exposes an a Neoproterozoic metamorphic domain in south-western elongate north–south trending metamorphic basement Spitsbergen has important bearing on terrane concepts exhumed within the axial zone of an Early Palaeogene for Svalbard construction in general, and specifically for fold-and-thrust belt (e.g., Lowell 1972; Dallmann et al. its proposed correlation with the Timanide metamorphic 1993; Braathen et al. 1995; Bergh et al. 1997). This belt of north-eastern Europe (Mazur et al. 2009). A better elevated basement high consists of Precambrian to understanding of the nature and extent of Neo- Early Palaeozoic rocks, the deformation and metamor- proterozoic metamorphism and associated pervasive phism of which were originally ascribed solely to the deformation, compared with earlier reports (Smulikowski Caledonian orogeny (e.g., Holtedahl 1926; Ohta 1982; 1965; Czerny et al. 1993; Majka et al. 2004), is an essen- Harland 1985; Harland et al. 1992). However, the occur- tial prerequisite for testing between, and refining, the rence of regionally extensive unconformities that are 250 Polar Research 29 2010 250–264 © 2010 the authors, journal compilation © 2010 Blackwell Publishing Ltd J. Majka et al. Neoproterozoic metamorphism in southern Svalbard Fig. 1 Geological sketch map of the Svalbard Archipelago (modified from Gee & Tebenkov 2004). associated with metamorphic and lithological contrasts western and south-western terranes (Fig. 1; Gee 1986; (e.g., Sandford 1956; Krasil’šcˇikov 1979; Birkenmajer Gee & Page 1994; Gee & Tebenkov 2004). 1975; Bjørnerud 1990) and geochronological studies North of Isfjorden, the south-western terranes of both provide evidence that the Caledonian basement Svalbard include a blueschist–eclogite allochthon known includes Proterozoic crustal domains juxtaposed by Early as the Vestgötabreen complex (e.g. Ohta 1979) that Palaeozoic tectonic events (e.g., Peucat et al. 1989; yielded argon-40 (40Ar)–argon-39 (39Ar) mineral cooling Balashov et al. 1995; Gee et al. 1995; Hellman et al. 1997; ages of ca. 460 My (Dallmeyer et al. 1990). The complex Johansson et al. 2004; Pettersson et al. 2009). These is non-conformably overlain by Middle Ordovician domains, together with their sedimentary cover, are conglomerates and limestones (Armstrong et al. 1986), interpreted to be individual tectonostratigraphic terranes succeeded by Late Ordovician to Early Silurian turbidites assembled during the Caledonian orogeny (e.g., Harland (Scrutton et al. 1976). The metamorphic basement, 1972; Harland & Gayer 1972; Gee & Page 1994). Based on together with its Early Palaeozoic cover, is thrust over a important contrasts in stratigraphy, structural develop- succession of diamictites of inferred Vendian age (Gee & ment and tectonothermal evolution, the Caledonian Tebenkov 2004). To the south of Isfjorden, the south- rocks of Svalbard have been divided into eastern, north- western terranes consist of a Meso- to Neoproterozoic Polar Research 29 2010 250–264 © 2010 the authors, journal compilation © 2010 Blackwell Publishing Ltd 251 Neoproterozoic metamorphism in southern Svalbard J. Majka et al. Fig. 2 Geological sketch map of the south- western part of Wedel Jarlsberg Land (modified from Czerny et al. 1993). metamorphic complex, which is divided into three and a metavolcanic succession of the Eimfjellet Group units: Isbjørnhamna, Eimfjellet and Deilegga groups (Birkenmajer 1958; Czerny et al. 1993). The Isbjørn- (Birkenmajer 1981, 1991). The latter is non-conformably hamna Group is composed of garnet–mica schists, overlain by the low-grade Neoproterozoic Sofiebogen paragneisses, calc-silicate rocks and marbles subjected Group, comprising a succession of conglomerates, lime- to amphibolite-grade Barrovian metamorphism, which stones and dolomites passing up into fine-grained was followed by greenschist facies metamorphism siliciclastic rocks (Bjørnerud 1990; Czerny et al. 1993; (Smulikowski 1965; Majka et al. 2004). The Eimfjellet Gee & Tebenkov 2004). The Sofiebogen Group is capped Group consists of quartzites, amphibolites and green- by Vendian tillites of the Kapp Lyell Group, and comprises schists, accompanied by minor metarhyolites and a thick series of diamictites (Bjørnerud 1990; Dallmann muscovite schists. These rocks were subjected to green- et al. 1990; Harland et al. 1993). The Cambro–Ordovician schist to lower amphibolite facies (Czerny et al. 1993). succession is tectonically emplaced atop the Proterozoic The contact between the two crustal blocks exposed in complexes (Dallmann et al. 1993; Mazur et al. 2009). the south-western part of Wedel Jarlsberg Land between Wedel Jarlsberg Land, located in the southern part of the Hornsund fjord and Torellbreen glacier is represented the south-western terranes (Fig. 2), consists of two Prot- by the ca. 2 km wide VKZ. The deformation zone is char- erozoic basement blocks with contrasting metamorphic acterized by a heterogeneous array of structures formed histories. The northern block comprises rocks of the during ductile shear deformation. Structural features of metasedimentary Deilegga and Sofiebogen groups, which the VKZ indicate bulk sinistral strike-slip to oblique-slip are separated by a Late Proterozoic angular unconformity shear strain formed under greenschist facies conditions. (Torellian unconformity; Birkenmajer 1975). The south- Thermochronologic data obtained from the Isbjørn- ern block corresponds to a stratified volcano-sedimentary hamna and Eimfjellet groups indicate a Neoproterozoic polymetamorphic complex that consists of a metasedi- age of metamorphism. 40Ar–39Ar step heating of mineral mentary sequence known as the Isbjørnhamna Group separates yielded a 616 My age for hornblende from the 252 Polar Research 29 2010 250–264 © 2010 the authors, journal compilation © 2010 Blackwell Publishing Ltd J. Majka et al. Neoproterozoic metamorphism in southern Svalbard Eimfjellet Group, and 585–575 My ages for muscovite consists of carbonate–mica schists, with irregular bands from the Isbjørnhamna Group (Manecki et al. 1998). of mica schists, paragneisses of the Skoddefjellet type Manecki et al. (1998) interpreted these ages as represent- and minor marble intercalations. A mineral assemblage ing a previously unrecognized episode of Neoproterozoic characteristic of carbonate-mica schists comprises Cal + reheating, probably connected with the opening of the Qtz + Bt + Grt +