Proterozoic Evolution and Provenance of the High-Grade Jotun Nappe Complex, SW Norway: U–Pb Geochronology A.M
Total Page：16
File Type：pdf, Size：1020Kb
Available online at www.sciencedirect.com Precambrian Research 159 (2007) 133–154 Proterozoic evolution and provenance of the high-grade Jotun Nappe Complex, SW Norway: U–Pb geochronology A.M. Lundmark a,∗, F. Corfu a,S.Spurgin¨ b, R.S. Selbekk b,c a Institute of Geosciences, Sem Sælands vei 1, 0371 Oslo University, Norway b Mineralogisch-Geochemisches Institut, Albert-Ludwigs-Universit¨at, Albertstraße 23b, 79104 Freiburg, Germany c Natural History Museum (Geology), Oslo University, PB 1172 Blindern, 0318 Oslo, Norway Received 14 June 2006; received in revised form 21 December 2006; accepted 25 December 2006 Abstract New U–Pb geochronological data are used to explore the Proterozoic history and provenance of the Jotun Nappe Complex in the Scandinavian Caledonides, SW Norway. Orthogneisses in the upper part of the complex, the high-grade Upper Jotun Nappe, yield protolith ages of 1.66, 1.63 and 1.26 Ga. During the Sveconorwegian orogeny, the rocks underwent penetrative deformation and metamorphism up to granulite facies conditions. Later shearing, hydration and retrogression under amphibolite-facies conditions is dated to 954 ± 3 Ma by local, syntectonic melting of granitic gneiss and 950 ± 1 Ma emplacement of syntectonic, syn-retrogression pegmatites, suggested to reflect decompression and partial exhumation of the lower crustal rocks. A second cycle of granulite- facies metamorphism is dated to 934 ± 1 Ma, coeval with 934 ± 3 Ma anatexis of granitic gneiss. We propose that the second cycle represents post-collisional elevated heat flow, correlated to granulite-facies metamorphism in the related Lindas˚ Nappe, and to anorthosite and granite plutonism at higher crustal levels in the western Baltic Shield, possibly in response to late-orogenic underplating or delamination. The tectonometamorphic record of far-travelled, high-grade crystalline rocks in the Jotun Nappe Complex, the Lindas˚ Nappe and the Dalsfjord Nappe confirms their origin in the Baltic Shield, and suggests that they provide a glimpse of a deeper segment of the Sveconorwegian orogen than presently exposed in the basement of SW Norway. © 2007 Elsevier B.V. All rights reserved. Keywords: Caledonides; Southwest scandinavian domain; U–Pb geochronology; Zircon; High-grade metamorphism; Proterozoic; Sveconorwegian 1. Introduction ern Norway along with the allochthonous sedimentary cover of the Baltic Shield and exotic fragments of Iapetan As Baltica collided with Laurentia during the Cale- affinities (Fig. 1). The crystalline nappes have tradition- donian orogeny, crustal shortening was partly accommo- ally been thought to represent the imbricated margin dated by nappe stacking, leading to the emplacement of the Baltic Shield (Roberts and Gee, 1985), but their of a series of thrust sheets on top of the Baltic Shield. enigmatic positions in the nappe sequence have also Far-travelled crystalline crustal rocks in the Jotun Nappe led to proposals that they may constitute exotic ter- Complex, the Lindas˚ Nappe and the Dalsfjord Nappe ranes (Andersen and Andresen, 1994; see also Emmett, are present in the Caledonian thrust sheets of southwest- 1996), and the validity of correlations between the crys- talline nappes has been questioned (Milnes et al., 1997; Wennberg et al., 1998). ∗ Corresponding author. Fax: +47 228 54215. Comparisons of the tectonometamorphic histories E-mail address: [email protected] (A.M. Lundmark). of the nappe complexes and the Baltic Shield have 0301-9268/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2006.12.015 134 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 Fig. 1. Geology of southwestern Norway (modified after Gee et al., 1985; Lutro and Tveten, 1996; Fossen and Holst, 1995; Fossen and Dallmeyer, 1998; Sigmond, 1998; Tucker et al., 1990). The Valdres Sparagmite is interpreted to represent the cover sequence of the Lower Jotun Nappe in the area of the Jotun Nappe Complex (see text), the units are not shown separately on the map. Localities are: (I) Hurrungane; (II) Lake Galgebergstjernet; (III) Fannaraken;˚ (IV) Rambera. so far been hampered by a lack of age data from age, are present only in the Upper Jotun Nappe, thus the high-grade rocks of the Jotun Nappe Complex, separating the complex into two parts, juxtaposed dur- the largest of the crystalline units in the Caledonian ing the Caledonian orogeny (Lundmark and Corfu, allochthon of southwestern Norway. The matter has 2007). been further confused by the report of an erroneous In this study, new U–Pb geochronological data are Precambrian age of granitic dykes in the Jotun Nappe reported and used to examine the Proterozoic history of Complex. The granites, now known to be of Silurian the Upper Jotun Nappe, and to attempt to constrain its A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 135 provenance and relation to the Lower Jotun Nappe, the 2.2. Caledonian nappes Lindas˚ Nappe and the Dalsfjord Nappe. The collisional phase of the Caledonian orogeny was 2. Geological setting accompanied by southeast-directed thrusting in south- western Norway and emplacement of nappes of various 2.1. Western Gneiss Region origins on top of the Baltic Shield. The nappes have traditionally been divided into four levels according to The Proterozoic basement of the Western Gneiss their inferred origin; the Lower and Middle Allochthon Region (WGR), southwestern Norway, is overlain by represent parts of Baltica and its margin, the Upper a set of Caledonian thrust sheets and is itself inter- Allochthon consists of the outermost border of Baltica nally imbricated (e.g., Tucker et al., 2004)(Fig. 1). and exotic fragments of Iapetan affinities, and the Upper- The parautochthonous WGR is generally regarded as most Allochthon is comprised of rocks of Laurentian the continuation of the Baltic Shield, although its rela- affinity (Roberts and Gee, 1985; Gee et al., 1985). tion to the autochthonous basement is obscured by the The Jotun Nappe Complex, dominating the Cale- overlaying thrust sheets (e.g., Gaal´ and Gorbatschev, donides of southwestern Norway, and the smaller Lindas˚ 1987; Milnes et al., 1997). The WGR consists mainly Nappe and Dalsfjord Nappe, are allochthonous slices of juvenile crust formed at 1.7–1.55 Ga (Kullerud et al., of Proterozoic crystalline crust (Fig. 1). The high-grade 1986; Tucker et al., 1990). During the ca. 1.25–0.9 Ga gneisses in the nappes have been correlated by field geol- Sveconorwegian orogeny (Starmer, 1990), part of the ogists since they were first described by Goldschmidt in assembling of the supercontinent Rodinia, the southwest 1916 as the “Bergen-Jotun Stamm”. The origin of the margin of the Baltoscandian craton and the southwest- nappes is uncertain, but they are generally assigned to ern parts of the WGR (the Southwest Scandinavian the Middle Allochthon, implying an origin in the pre- Domain; Gaal´ and Gorbatschev, 1987) were extensively imbricated Baltoscandian margin. reworked (Tucker et al., 1990; Skar˚ and Pedersen, 2003). The heterogeneous Dalsfjord Nappe is dominated by In contrast, the northeastern parts of the WGR under- syeno-monzonites and minor gabbro and anorthosite. went no corresponding isotopic and tectonic disturbance Zircon U–Pb protolith ages of 1634 ± 3 and 1464 ± 6Ma at this time. This led to the proposal that the Pro- for monzonite (partially retrogressed mangerite) and togine Zone/Sveconorwegian Frontal Deformation Zone gabbro, respectively, were reported by Corfu and (PZ/SFDZ), which marks the boundary of Sveconor- Andersen (2002), while an imprecise lower intercept wegian influence in Sweden, resurfaces west of the age of 882 ± 29 Ma along with concordant titanite ages Caledonian allochthon in the WGR (Tucker et al., 1990). interpreted to reflect ≥960 and ≤920 Ma titanite growth, In the WGR southwest of the SFDZ, high-grade testify to a strong Sveconorwegian influence. The nappe metamorphism has been dated to 987 ± 10 Ma (Røhr displays a meta-sedimentary cover sequence, the Høyvik et al., 2004). Extensive, ca. 970–960 Ma migmatiza- Group, possibly a correlative of late Precambrian arkoses tion under amphibolite facies conditions was triggered and conglomerates that make up the Valdres Sparagmite by widespread emplacement of metaluminous gran- (Andersen et al., 1998), which in turn is interpreted as ites, dated at one locality to 966 ± 3Ma (Skar˚ and the allochthonous cover of the Baltic craton (Roberts Pedersen, 2003). Post-tectonic dykes were emplaced at and Gee, 1985). Phengite 40Ar/39Ar ages date metamor- ca. 950–940 Ma (Tucker et al., 1990), corresponding to phism of the sediments to ca. 447 Ma (Andersen et al., a monazite age of 949 ± 3 Ma interpreted to date the 1998). In the Silurian, the Høyvik Group was once more waning stages of metamorphism (Røhr et al., 2004). exposed at the surface and was unconformably over- The Sveconorwegian event in the WGR ended with the lain by the sediments of the Herland Group (Brekke and intrusion of granites dated to 932 ± 8Ma(Corfu, 1980). Solberg, 1987). Following the break-up of Rodinia and the formation The Lindas˚ Nappe, part of the Bergen Arc Complex, is of the Iapetus Ocean, orogenic activity resumed during mainly composed of Proterozoic anorthosites, charnock- the Ordovician-Silurian Caledonian orogeny. The colli- ites to mangerites and banded gneisses (Kolderup and sional phase involved subduction of the leading edge of Kolderup, 1940) that, in the field, are indistinguishable the Baltoscandian shield, during which the northwest- from rocks exposed in the Jotun Nappe Complex. The ern parts of the WGR were extensively deformed (Rice, only exception is the presence of Caledonian eclog- 2005) and exposed to high pressure/ultra high-pressure ites in the Lindas˚ Nappe, absent in the Jotun Nappe conditions (e.g., Cuthbert et al., 2000; Carswell et al., Complex. Discordant zircon data from a charnockite 2003). in the Lindas˚ Nappe yield a protolith (upper inter- 136 A.M. Lundmark et al. / Precambrian Research 159 (2007) 133–154 cept) age of 1237(+43/−35) Ma and date high-grade commonly exhibiting primary compositional layering, metamorphism (lower intercept) at 932(+28/−36) Ma.