Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite fragment, Trondheim, Norway: Petrogenesis and tectonic implications

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1 NORWEGIAN JOURNAL OF GEOLOGY Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite 167 Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite fragment, Trondheim, Norway: Petrogenesis and tectonic implications Trond Slagstad Slagstad, T.: Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite fragment, Trondheim, Norway: Petrogenesis and tectonic implications. Norwegian Journal of Geology, vol. 83, pp Trondheim 2003, ISSN X. The Early Ordovician Bymarka ophiolite fragment is situated in the Upper Allochthon of the central Norwegian Caledonides. Geochemical and geochronological investigations of this and other ophiolite fragments in the Norwegian Caledonides show that they formed between c. 500 and 470 Ma in broadly similar suprasubduction-zone settings. Geochemical investigations in the Bymarka ophiolite fragment, focusing on felsic, rhyodacitic and trondhjemitic rocks, demonstrate a geological evolution from the formation of new oceanic crust, through island arc development, to postobduction magmatism. The Klemetsaunet rhyodacite is interpreted to be analogous to plagiogranite, common in nearby ophiolite fragments, and probably formed by fractional crystallisation of mid-ocean ridge-like basaltic magmas in a back-arc basin. The slightly younger Fagervika trondhjemite is interpreted to represent construction of an island arc on recently formed oceanic crust, and may have formed by partial melting or fractional crystallisation of mafic island arc tholeiitic rocks or magmas. The Byneset trondhjemite typically occurs as dykes cutting the greenstones that make up the bulk of the Bymarka ophiolite fragment, and is interpreted to have formed by partial melting of mafic rocks at the base of a thick pile of ocean floor- and island arc-derived rocks stacked onto the outermost part of a continental or microcontinental margin. The geological evolution of the Bymarka ophiolite fragment and other ophiolite fragments in the central Norwegian Caledonides is similar to that of ophiolites elsewhere in the Caledonian-Appalachian orogen. The conclusions reached here thus support earlier ideas that the Caledonian-Appalachian ophiolites formed as part of the same, extensive island arc/back-arc system. Trond Slagstad, Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5. Present address: Geological Survey of Norway, 7491 Trondheim, Norway. Introduction Early Palaeozoic ophiolites and ophiolite fragments have been identified throughout the Upper Allochthon of the almost 2000 km-long Scandinavian Caledonides (Pedersen et al. 1988, Sturt & Roberts 1991). Furnes et al. (1985) divided these ophiolites into two age groups based on geological and biostratigraphic relationships, and this grouping has to some extent been confirmed by later U-Pb geochronology. The Late Cambrian-Early Ordovician group includes by far the largest number of fragmented ophiolites, whereas the younger group is represented mainly by the c. 440 Ma Solund-Stavfjord Ophiolite Complex (Pedersen et al. 1988). The older group of ophiolites probably formed in an oceanic suprasubduction-zone setting, whereas the Solund- Stavfjord Ophiolite Complex appears to have formed close to a mature island arc and/or a continental margin (Pedersen et al. 1988, Furnes et al. 1990). Ophiolite fragments in central Norway (e.g., Gale & Roberts 1974, Grenne et al. 1980, 1999, Prestvik 1980, Heim et al. 1987, Grenne 1989) belong to the older group of ophiolites, and recent work in Bymarka, on the outskirts of Trondheim (Fig. 1), has identified the partly preserved pseudostratigraphy (Slagstad 1998, this study) of a c. 480 Ma ophiolite (Roberts et al. 2002), referred to here as the Bymarka ophiolite fragment. In addition to the mafic rocks (dominantly greenstones representing pillow lavas and mafic dykes) that constitute the dominant portion of the Bymarka ophiolite fragment, three types of felsic rock have been identified based on field, geochemical, and petrographic investigations. The felsic rocks appear to represent different stages in the evolution of the ophiolite, from ocean-floor magmatism and subsequent construction of an island arc on the oceanic crust, to post-obduction magmatism. This paper presents field and geochemical data from the Bymarka ophiolite fragment, focusing on the felsic rocks, and discusses how the various rocks formed and their tectonic significance. Combined with recently published geochronological data, the geological evolution of this and nearby ophiolites is discussed in relation to that of ophiolites in other parts of the Caledonides.

2 168 NORWEGIAN JOURNAL OF GEOLOGY Trond Slagstad Recently, Eide & Lardeaux (2002) identified a relict blueschist assemblage within the highly deformed basal part of the Bymarka ophiolite fragment. By analogy with interpretations of the Early Ordovician tectonic evolution and metamorphism elsewhere in the Appalachian-Caledonian orogen (e.g., van Staal et al. 1998), Eide & Lardeaux (2002) suggested that the blueschistfacies metamorphism represents Early Ordovician intra-oceanic subduction/obduction processes. Geology of the Bymarka ophiolite fragment Fig. 1. Simplified geological map showing the principal fragmented ophiolites in the Trondheim area (modified from Heim et al. 1987). Regional setting The Trondheim region ophiolites are situated in the Støren Nappe in the Upper Allochthon, equivalent to the exotic terranes of the Köli Nappes. The Støren ophiolite constitutes the base of the Støren Nappe and has been correlated with other ophiolites or ophiolite fragments to the northwest, e.g., the Vassfjellet (Grenne et al. 1980), Løkken (Grenne 1989), Resfjell (Heim et al. 1987), and Grefstadfjell (Ryan et al. 1980) ophiolites (Fig. 1). The Bymarka ophiolite fragment has also been correlated with this group of ophiolites (Grenne et al. 1980, Roberts et al. 2002). Geochemical investigations, focusing on the mafic lavas (greenstones) in these ophiolites, have revealed ocean-floor to arc affinities suggesting formation in a suprasubduction-zone setting (Gale & Roberts 1974, Roberts et al. 1984, Grenne 1989, Slagstad 1998), and radiometric dating, dominantly of zircon from plagiogranites, points to a Late Cambrian- Early Arenig age for many of the ophiolites (see Roberts et al for a review). The age of obduction is constrained by Late Arenig and younger fossils in the Hovin and Horg Groups (Vogt 1945, Bruton & Bockelie 1980), which unconformably overlie the deformed ophiolites. The Hovin and Horg groups were deformed and metamorphosed during the Late Silurian-Early Devonian Scandian orogeny, and dismemberment and fragmentation of the underlying ophiolites is probably in part related to this tectonic event. Ophiolite pseudostratigraphy The investigated area lies in the immediate vicinity of Trondheim (Figs. 1, 2). The most common rock types include greenschist-facies pillow lavas and hyaloclastites, diabase dykes, gabbros, agglomerates, and felsic trondhjemitic/tonalitic to granitic intrusions. Note that although the investigated rocks are deformed and metamorphosed, the prefix "meta" (e.g., "metagabbro" and "metatrondhjemite") is omitted for simplicity. The metamorphosed mafic lavas and dykes are referred to as 'greenstones', in accordance with historical terminology (e.g., Carstens 1920). Moderately SE-dipping foliations and NE-trending lineations, observed throughout most of the Bymarka ophiolite fragment, conform to regional structures and are interpreted to have resulted from Scandian orogenesis (Slagstad 1998). Despite moderate to strong Scandian deformation, a partly preserved ophiolite pseudostratigraphy has been identified (Fig. 2). Medium- to coarse-grained gabbros predominate in the northwestern part of the ophiolite, intruded by deformed mafic dykes typically <1 m thick. The gabbros give way southeastward to a unit dominated by mafic dykes overlain by a thick sequence of pillow lavas. Interlayered with the pillow lavas in the southeastern part of the study area is a supracrustal unit, several 10s of metres thick, comprising mafic and felsic agglomerates associated with cherts, thin felsic volcanic layers, and metre-thick, dark-blue and magnetite-rich quartz layers ('bluequartz'). Clasts from the bluequartz layers are common in the felsic agglomerate. The lateral extent of the supracrustal unit is probably of the order of km, indicated by several small exposures along strike. The supracrustal unit is interlayered with fine-grained greenstone, in places identifiable as deformed pillow lavas, suggesting alternate periods of mafic and felsic magmatism. In one outcrop, a c. 30 cm-wide transitional contact between greenstone and felsic agglomerate indicates a gradual change from mafic to felsic volcanism. The supracrustal unit appears to lack continental material and is interpreted here to represent ocean-floor sediments, i.e., the uppermost part of the ophiolite pseudostratigraphy. Ultramafic rocks corresponding to the mantle section of the ophiolite have not been identified. Although gabbros predominate in the northwestern part of the Bymarka ophiolite fragment, small gabbroic bodies, up to a few hundred metres across, are found associated with the supracrustal unit. The supracrustal

3 NORWEGIAN JOURNAL OF GEOLOGY Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite 169 Fig. 2. Geological map of the Bymarka ophiolite fragment (modified from Solli et al. 1999). The areal extent of individual ophiolite components (e.g., sheeted dyke-complex) could not be determined in detail due to poor exposure and relatively high strain, and is only roughly indicated on the map. unit dips moderately (20 30 ) to the southeast, and locally, over a distance of a few hundred metres of discontinuous outcrop, one moves in and out of agglomerate and gabbro suggesting an original cross-cutting relationship. The field relationships therefore suggest that the gabbros associated with the supracrustal unit formed comparatively late, and may be unrelated to formation of the ophiolite sensu stricto (i.e., oceanfloor magmatism). Here, gabbros in the western part of the area, constituting part of the ophiolite stratigraphy, are referred to as low-level gabbros, whereas gabbros associated with the supracrustal unit are referred to as high-level gabbros. Rhyodacites and trondhjemites Earlier mapping in the Trondheim region (Wolff 1979) indicated the presence of two types of felsic rock in the Bymarka ophiolite fragment, classified as quartz keratophyre 1 and albite granite. This investigation, however, has shown that there are three petrographically and geochemically distinct types of felsic rock in the area: the Klemetsaunet rhyodacite, Fagervika trondhjemite, and Byneset trondhjemite. The whitish-grey Klemetsaunet rhyodacite forms sheets and dykes ranging from a few dm up to several 10s of metres in thickness, in places continuous for several 100s of metres (Fig. 2). Apparently similar rhyodacites occur as thin sheets within the supracrustal unit, suggesting that they formed as ocean-floor extrusions and/or near-surface intrusions. Note, however, that none of the rhyodacites from the supracrustal unit have been analysed geochemically, thus the correlation with the Klemetsaunet rhyodacite is speculative. The Klemetsaunet rhyodacite is holocrystalline and granoblastic, composed mainly of microcrystalline, recrystallised albite and quartz. A weak to moderate foliation is defined by subparallel lensoid clusters of coarser (up to 0.5 mm), weakly to moderately strained quartz grains with irregular grain boundaries, or alternatively by small amounts of chloritized biotite or muscovite. In contrast to the Fagervika and Byneset trondhjemites, discussed below, no phenocrysts have been observed in these rocks. A characteristic feature of the Klemetsaunet rhyodacite is the presence of secondary euhedral to subhedral garnet and hornblende. The garnet is evenly distributed, and is locally seen transecting the foliation in the rocks; hornblende is evenly distributed or, in many 1 The term quartz keratophyre has traditionally been used in the Nordic countries to describe a metamorphosed, felsic extrusive rock, corresponding to rhyolite, dacite, or rhyodacite according to IUGS terminology.

4 170 NORWEGIAN JOURNAL OF GEOLOGY Trond Slagstad cases, concentrated along thin, bleached veins, suggesting that hornblende growth was related to fluid influx. The Fagervika trondhjemite is a large intrusive body of white to orange, medium-grained gneiss, extending from Trondheimsfjorden approximately 10 km southwest into Bymarka (Fig. 2). These rocks are holocrystalline and hypidiomorphic to allotriomorphic, with a trondhjemitic to granodioritic, and locally granitic, composition. Close to the contacts with the greenstones, the rocks are chilled with phenocrysts of plagioclase and less commonly quartz. Quartz occurs as irregular, recrystallised grains and larger phenocrysts, ranging from 0.1 to 2 mm, with moderate to strong undulose extinction and typically sutured grain boundaries. Plagioclase occurs as small (~0.1 mm) recrystallised grains, and as 1 2 mm, equigranular to tabular, subhedral to anhedral phenocrysts. The plagioclase phenocrysts commonly have moderately to highly altered cores with thin unaltered rims, possibly reflecting original zoning. The rocks carry a moderate to strong gneissic fabric defined by greenish bands of medium-grained muscovite, commonly interspersed with euhedral to anhedral epidote. In one outcrop, the foliation is overgrown by small idioblastic to hypidioblastic garnets. K-feldspar is absent from most of the body, but at Gråkallen, modal abundances are as high as 25%, and the rocks classify as granites. The K-feldspar is perthitic, mm across, subhedral to anhedral, and commonly mantles plagioclase. Other minerals present in varying amounts in these rocks include secondary biotite, chlorite, and calcite. The whitish-grey Byneset trondhjemite occurs as dykes and sheets ranging in thickness from < 1 m to several 10s of metres, dominantly in the western parts of the Bymarka ophiolite fragment (Fig. 2). Contacts with the surrounding greenstones are generally unexposed, but some of the thinner dykes have sharp, intrusive contacts. At Klemetsaunet, c.1 m thick dykes of Byneset trondhjemite cut the Klemetsaunet rhyodacite. The trondhjemites are fine- to medium-grained and typically granular in hand-specimen, with mm-size granules of white to greenish-white plagioclase. The thinner dykes are porphyritic, with phenocrysts of plagioclase. The rocks are holocrystalline, hypidiomorphic to allotriomorphic and composed essentially of strongly zoned, euhedral to subhedral plagioclase, commonly < 1 mm across, although grains as large as 2 mm are locally present. The plagioclase is moderately to strongly altered. Quartz occurs as < mm interstitial grains. Abundances of biotite, typically chloritised, are variable and the rocks have a trondhjemitic to tonalitic composition, defined as less and more than 10 modal% mafic minerals, respectively. Titanite occurs as irregular grains up to 2 mm across, although < 0.5 mm intergrown grains are more common. Other minerals include secondary epidote and muscovite, and strongly resorbed amphibole. Fig. 3. A. Zr-TiO 2 variation diagram. Zr and TiO 2 are well correlated suggesting limited mobility. The mobility of trace elements such as Th, Nb, Ta, and the REE cannot be tested in the same manner due to the small number of analyses, but are regarded as immobile. B. Zr-Sr variation diagram showing significant scatter. Similar scatter is observed when Zr is plotted against elements such as Ba, Rb, and K 2 O, suggesting that these elements were mobile. Recent U-Pb zircon dating of the Klemetsaunet rhyodacite and Fagervika trondhjemite by Roberts et al. (2002) yielded ages of 482±5 Ma and c. 481 Ma, respectively, interpreted by Roberts et al. to represent crystallisation ages. Geochemistry Analytical procedures Major and trace element compositions of 62 samples, including greenstones, gabbros, and felsic rocks, were determined on fused glass beads and pressed powder pellets, respectively, on a Philips PW 1480 x-ray spectrometer (XRF) at the Geological Survey of Norway

5 NORWEGIAN JOURNAL OF GEOLOGY Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite 171 (NGU), using international standards (Govindaraju 1984, 1989). Rare earth elements (REE), Nb, Hf, Ta, and Th were determined on a subset of 9 felsic samples and 5 gabbros (2 low-level and 3 high-level), by inductively coupled plasma-mass spectrometry (ICP-MS) using a Na 2 O 2 sintering technique at Memorial University, Newfoundland. Longerich et al. (1990) discussed the analytical procedure, uncertainties, and precision of the ICP-MS analyses. Geochemical data from selected representative samples are presented in Tables 1A-D. A full dataset can be obtained from the author on request. Impact of hydrothermal alteration Field and petrographic observations from the Bymarka ophiolite fragment, in particular hornblende-rich, bleached veins in the Klemetsaunet rhyodacite, altered feldspars and biotite, and locally large abundances (10 20 modal%) of muscovite in the Fagervika trondhjemite, suggest that hydrothermal alteration may have been significant. Interpretation of tectonic setting and petrogenesis of metamorphosed rocks is based on the assumption that certain elements reflect the composition of the primary magma, i.e., that they are immobile or show only negligible mobility during secondary processes. Widely employed 'immobile' elements include the high-field strength elements (Ti, Zr, Y, Nb, Hf, and REE), whereas low-field strength elements such as Sr, Ba, Rb, Na, and K often are regarded as mobile (e.g., Higgins et al. 1985, Rollinson 1993). Similarly, Coish (1977), Coish & Church (1979), and Coish et al. (1982) argued that Al 2 O 3,TiO 2,P 2 O 5,Ni, Cr, Zr, Y, and the REE were relatively immobile during greenschist-facies hydrothermal alteration of the Betts Cove ophiolite, Newfoundland. Zirconium is considered to be a good indicator of fractionation, and is apparently immobile under most metamorphic conditions (Winchester & Floyd 1977). Plotting Zr against other immobile elements should, therefore, yield distinct correlations, whereas Zr plotted against a mobile element will result in significant scatter (see Fig. 3 and caption). Here, elements such as TiO 2,Zr, Y, Hf, and the REE are assumed to have been relatively immobile. In contrast, elements such as K 2 O, Ba, Rb, and Sr were probably mobilised during metamorphism. There are, however, significant and consistent differences between the investigated rock types with regard to these elements. These differences are interpreted to reflect mobilisation and redistribution of these elements on a sample- or outcropscale, but not on the scale of individual units. Greenstones The greenstones (Table 1A) range from c. 43 to 52 wt.% SiO 2,with Mg# (mole MgO/(MgO+FeO t ) between 0.40 and The greenstones are significantly enriched in Ti,Zr, and Y compared with island arc basalts, and have Fig. 4. Greenstones from the Bymarka ophiolite fragment plotted in the: A. Cr-Ti tectonic discrimination diagram of Pearce (1975), and B. Zr-Y*3-Ti/100 tectonic discrimination diagram of Pearce & Cann (1973). The fields in B. are: A-B=Low-K tholeiites, B=Ocean-floor basalts, C-B=calc-alkaline basalts, D=within-plate basalts. abundances of these elements comparable to midocean ridge basalt (MORB) (Woodhead et al. 1993, Smith & Humphris 1998). Their ocean-floor affinities are also indicated by a variety of tectonic discrimination diagrams (two of which are shown in Fig. 4), and Slagstad (1998) concluded that the greenstones most likely represent ocean-floor basalts, formed in a major oceanic basin or, alternatively, an oceanic back-arc basin. Grenne (1989) reached a similar conclusion for the Løkken ophiolite. Gabbros The varitextured low- and high-level gabbros (Table 1A) range from 46 to 50 wt.% SiO 2, and have indistinguishable major and trace element (excluding REE)

6 172 NORWEGIAN JOURNAL OF GEOLOGY Trond Slagstad Table 1A. Gabbros and greenstones. Geochemical data from selected representative samples. Low-level gabbro High-level gabbro Greenstones Sample SiO 2 (wt.%) TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O LOI Total Mg# Ba (ppm) b.d. 26 Rb b.d. b.d. b.d. 7 b.d. 6 b.d. b.d b.d. b.d. Sr Y Zr Nb n.a. n.a. n.a. n.a. n.a. n.a. n.a. Th n.a. n.a. n.a. n.a. n.a. n.a. n.a. Ni V Cr Hf n.a. n.a. n.a. n.a. n.a. n.a. n.a. Ta n.a. n.a. n.a. n.a. n.a. n.a. n.a. La n.a. n.a. n.a. n.a. n.a. n.a. n.a. Ce n.a. n.a. n.a. n.a. n.a. n.a. n.a. Pr n.a. n.a. n.a. n.a. n.a. n.a. n.a. Nd n.a. n.a. n.a. n.a. n.a. n.a. n.a. Sm n.a. n.a. n.a. n.a. n.a. n.a. n.a. Eu n.a. n.a. n.a. n.a. n.a. n.a. n.a. Gd n.a. n.a. n.a. n.a. n.a. n.a. n.a. Tb n.a. n.a. n.a. n.a. n.a. n.a. n.a. Dy n.a. n.a. n.a. n.a. n.a. n.a. n.a. Ho n.a. n.a. n.a. n.a. n.a. n.a. n.a. Er n.a. n.a. n.a. n.a. n.a. n.a. n.a. Tm n.a. n.a. n.a. n.a. n.a. n.a. n.a. Yb n.a. n.a. n.a. n.a. n.a. n.a. n.a. Lu n.a. n.a. n.a. n.a. n.a. n.a. n.a. b.d.=below detection limit of XRF (Ba-10ppm, Rb, Nb, Cr, Ni-5ppm). n.a.=not analysed.

7 NORWEGIAN JOURNAL OF GEOLOGY Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite 173 Table 1B. Klemetsaunet rhyodacite. Geochemical data from selected representative samples. Sample A SiO 2 (wt.%) TiO Al 2 O Fe 2 O MnO b.d. MgO 0.44 b.d CaO Na 2 O K 2 O P 2 O b.d LOI Total A/CNK Ba (ppm) b.d Rb b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 8 Sr Y Zr Nb b.d b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 7.8 Th n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Ni b.d. b.d. b.d. 5 5 b.d. b.d. b.d. b.d. 5 b.d. b.d. V 6 b.d. b.d b.d. b.d. b.d b.d. Cr b.d. 5 b.d. b.d. b.d. b.d. b.d. b.d. b.d. Hf n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Ta n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a La n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Ce n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Pr n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Nd n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Sm n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Eu n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Gd n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Tb n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Dy n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Ho n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Er n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Tm n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Yb n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Lu n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a A/CNK= mole (Al 2 O 3 /CaO+Na 2 O+K 2 O)

8 174 NORWEGIAN JOURNAL OF GEOLOGY Trond Slagstad Table 1C. Fagervika trondhjemite. Geochemical data from selected representative samples. Sample G 23.3TK T1 T3 SiO 2 (wt.%) TiO Al 2 O Fe 2 O MnO b.d. MgO CaO b.d Na 2 O b.d K 2 O P 2 O LOI Total A/CNK Ba (ppm) Rb Sr b.d Y Zr Nb Th n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Ni b.d. b.d. b.d. 5 b.d. b.d. b.d. b.d. V Cr b.d. b.d b.d. b.d. b.d. 9 b.d. b.d. Hf n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Ta n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a La n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Ce n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Pr n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Nd n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Sm n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Eu n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Gd n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Tb n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Dy n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Ho n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Er n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Tm n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Yb n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a Lu n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. n.a. 0.57

9 NORWEGIAN JOURNAL OF GEOLOGY Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite 175 Table 1D. Byneset trondhjemite. Geochemical data. Sample B SiO 2 (wt.%) TiO Al 2 O Fe 2 O MnO MgO CaO Na 2 O K 2 O P 2 O LOI Total A/CNK Ba (ppm) b.d Rb Sr Y Zr Nb b.d. b.d. b.d. b.d. 5.3 b.d b.d. Th n.a. n.a. n.a. n.a n.a n.a. Ni b.d b.d. V Cr b.d b.d. b.d. Hf n.a. n.a. n.a. n.a n.a n.a. Ta n.a. n.a. n.a. n.a n.a n.a. La n.a. n.a. n.a. n.a n.a n.a. Ce n.a. n.a. n.a. n.a n.a n.a. Pr n.a. n.a. n.a. n.a n.a n.a. Nd n.a. n.a. n.a. n.a n.a n.a. Sm n.a. n.a. n.a. n.a n.a n.a. Eu n.a. n.a. n.a. n.a n.a n.a. Gd n.a. n.a. n.a. n.a n.a n.a. Tb n.a. n.a. n.a. n.a n.a n.a. Dy n.a. n.a. n.a. n.a n.a n.a. Ho n.a. n.a. n.a. n.a n.a n.a. Er n.a. n.a. n.a. n.a n.a n.a. Tm n.a. n.a. n.a. n.a n.a n.a. Yb n.a. n.a. n.a. n.a n.a n.a. Lu n.a. n.a. n.a. n.a n.a n.a. compositions, except for Ba and Sr which are higher in two of the high-level gabbros. The main difference between the low- and high-level gabbros is their REE patterns (Fig. 5). The field relationships suggest that the low-level gabbros are part of the ophiolite stratigraphy, and they have a MORB-like, slightly light REE (LREE)- depleted pattern ((La/Yb) N = ). In contrast, the high-level gabbros, situated higher in the ophiolite stratigraphy, have an island arc tholeiite (IAT)-like REE pattern characterised by slight LREE-enrichment ((La/Yb) N = ) (e.g., Pearce & Peate 1995). Rhyodacites and trondhjemites Major elements: Both the Klemetsaunet rhyodacite (Table 1B) and the Fagervika trondhjemite (Table 1C) are high in SiO 2,on average 76.5 and 74.6 wt.%, respectively, whereas the Byneset trondhjemite (Table 1D) ranges between 64.5 and 70.6 wt.% SiO 2.The rhyodacite is significantly enriched in iron relative to the trondhjemites, with an average FeO t /(FeO t +MgO) ratio of 0.87, typical of tholeiitic rocks. The Fagervika trondhjemite and Byneset trondhjemite, on the other hand, have lower values (average 0.74 and 0.68, respectively), typical of calc-alkaline rocks. The Klemetsaunet rhyodacite and Byneset trondhjemite straddle the boundary between metaluminous and peraluminous compositions with average A/CNK ratios of 1.02 and 1.01, respectively. The Fagervika trondhjemite is peraluminous with an average A/CNK ratio of 1.16.

10 176 NORWEGIAN JOURNAL OF GEOLOGY Trond Slagstad Trace and rare earth elements: The Klemetsaunet rhyodacite has a flat primitive mantle-normalized pattern, with small negative Ta and Nb anomalies, and large negative Sr, P, and Ti anomalies (Fig. 6A). The rhyodacites are enriched in Zr, Y, and heavy REE (HREE) compared with the Fagervika and Byneset trondhjemites. The rhyodacites have slightly LREE-depleted ((La/Yb) N = ) REE patterns, similar to the lowlevel gabbros, but with significantly more negative Eu anomalies ((Eu/Eu*) N = ) (Fig. 7). In tectonic discrimination diagrams, the Klemetsaunet rhyodacite plots in the field of ocean ridge granite (Fig. 8) (Pearce et al. 1984). The Fagervika trondhjemite is enriched in Ba, Rb, and Th, with negative Ta, Nb, Sr, P, and Ti anomalies in the primitive mantle-normalized diagram (Fig. 6B). This rock type is enriched in LREE ((La/Yb) N = ) with negative Eu anomalies ((Eu/Eu*) N = ) (Fig. 7), and plots in the volcanic arc field in tectonic discrimination diagrams (Fig. 8). The Byneset trondhjemite has a negatively sloping primitive mantle-normalized pattern, with small negative Ta,Nb, and Nd anomalies and a positive Sr anomaly (Fig. 6C). The rocks are low in HREE with (La/Yb) N ranging from 9.6 to 15.3, and have small, positive Eu anomalies ((Eu/Eu*) N = ) (Fig. 7). Like the Fagervika trondhjemite, the Byneset trondhjemite plots in the volcanic arc field in tectonic discrimination diagrams (Fig. 8). Petrogenesis of the rhyodacite and trondhjemites Rare earth element modelling Rare earth element modelling, used below to discuss the petrogenesis of the investigated rocks, faces a number of pitfalls including i) the effects of accessory phases, ii) variation in published partition coefficients, and iii) the assumption that the investigated rocks represent liquid compositions. It is well known that accessory minerals such as apatite, zircon, allanite, and monazite may be extremely rich in certain trace elements (e.g., LREE in allanite and monazite, HREE in zircon), and in many cases can obscure the effects produced by the major phases during partial melting and fractional crystallisation (e.g., Bea 1996 and references therein). This problem is, however, likely to be small in the cases discussed here that involve fractional crystallisation and partial melting of mafic rocks that contain a small proportion of accessory phases. I have, therefore, not included accessory phases in the models presented below. Fig. 5. Chondrite-normalised REE pattern for gabbros in the Bymarka ophiolite fragment. All chondrite- and primitive mantlenormalised diagrams are normalised to the values of Sun & McDonough (1989). Partition coefficients vary according to the structure and composition of the melt (e.g., Mahood & Hildreth 1983, Nash & Crecraft 1985), mineral composition (e.g., Blundy & Wood 1991, Icenhower & London 1996), temperature (Icenhower & London 1995, Chappell 1996), and water content (Mahood & Hildreth 1983). The partition coefficients used here are largely based on consideration of melt composition. Fractional crystallisation of basaltic magmas is modelled using partition coefficients compiled by Smith & Humphris (1998) and used by them to model fractional crystallisation of MORB from the mid-atlantic ridge. Modelling of fractional crystallisation of tonalitic/trondhjemitic melts and partial melting of mafic rocks producing such melts, uses partition coefficients similar to those used by Borg & Clynne (1998) for partial melting of mafic rocks at lower crustal levels (P < ca. 800 MPa) producing felsic melts. Modelling of partial melting of mafic rocks at higher pressures (in the stability field of garnet, i.e., P > 800 MPa) uses partition coefficients from Martin (1987) and Barth et al. (2002). The assumption that immobile elements from the investigated rocks represent liquid compositions is difficult to test. In particular, the low- and high-level gabbros may contain a significant proportion of cumulate phases. Data from the Løkken ophiolite (Grenne 1989), however, show that the gabbros and greenstones there (the greenstones more likely to represent liquid compositions) have similar REE patterns. Thus, the assumption that the investigated rocks represent, or at least approximate, liquid compositions appears to be valid. Klemetsaunet rhyodacite The Klemetsaunet rhyodacite is geochemically similar to plagiogranites from the Løkken ophiolite (Fig. 6A).

11 NORWEGIAN JOURNAL OF GEOLOGY Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite 177 Fig. 7. Chondrite-normalised REE pattern for the Klemetsaunet rhyodacite, Fagervika trondhjemite, and Byneset trondhjemite. At Løkken, the plagiogranites are associated with varitextured gabbros (Grenne 1989) and this common transition between plagiogranites and gabbroic cumulates observed in many ophiolites is often cited as evidence for fractional crystallisation (Coleman & Donato 1979, Lippard et al. 1986, Grenne 1989). In the Bymarka ophiolite fragment, a few, small occurrences of rhyodacite are spatially associated with the low-level gabbros. However, the larger plagiogranite bodies, such as the one at Klemetsaunet and several smaller plagiogranite dykes or sheets, are found at higher levels within the ophiolite, associated with mafic dykes and pillow lavas. In addition, similar rocks appear to be associated with the agglomerates, suggesting that they formed as high-level intrusions or extrusions. Although plagiogranites are typically associated with the gabbroic cumulates in an ophiolite, stratigraphically higher intrusive plagiogranites and their extrusive counterparts have been described from other ophiolites (e.g., Brown et al. 1979, Floyd et al. 1998), and are by no means uncommon. Although most workers favour fractional crystallisation of gabbroic magmas as the most likely petrogenetic model for plagiogranites, some workers have proposed that they can form by partial melting of a mafic source (e.g., Gerlach et al. 1981, Flagler & Spray 1991, Selbekk et al. 1998). Here, REE modelling is used to test whether partial melting of a low-level gabbro or fractional crystallisation of a low-level gabbroic magma can account for the composition of the Klemetsaunet rhyodacite. Fig. 6. Primitive mantle-normalised trace element contents of A. Klemetsaunet rhyodacite, B. Fagervika trondhjemite, C. Byneset trondhjemite. The Løkken plagiogranite in A. is from Grenne (1989). Partial melting: Phases such as plagioclase, hornblende, pyroxenes, and Fe-Ti oxides are likely to be the dominant residual minerals after partial melting of a mafic source at the pressure-temperature conditions typical of plagiogranite formation. Taking the average lowlevel gabbro composition to represent the source, and assuming a plagioclase-pyroxene-dominated residue (wt.% Pl 44 Opx 40 Cpx 16 ), the model melts produced by 10 20% equilibrium batch melting (Shaw 1970) are

12 178 NORWEGIAN JOURNAL OF GEOLOGY Trond Slagstad HREE-depleted relative to the rhyodacites (Fig. 9A). Mineral abbreviations are after Kretz (1983). Changing the proportion of plagioclase and pyroxenes or adding hornblende and/or Fe-Ti oxides to the residue does not change this result. It is, therefore, concluded that partial melting of low-level gabbro or equivalent rocks is unlikely to have formed the Klemetsaunet rhyodacite. Fig. 8. Y+Nb-Rb tectonic discrimination diagram (Pearce et al. 1984). Samples from the Klemetsaunet rhyodacite with Rb and Nb contents below the detection limit were assumed to have Rb and Nb contents equal to the detection limits for these elements (5 ppm), thus representing maximum values for the Rb and Nb contents of these samples. The Nb contents have little influence on the Y+Nb-total and moving the samples to lower Rb contents does not alter the conclusions reached in the text. Abbreviations: Syn-COLG=syn-collision granites, VAG=volcanic arc granites, WPG=within-plate granites, ORG=ocean ridge granites. Fractional crystallisation: The fractional crystallisation modelling, using the equation for Rayleigh fractional crystallisation (Allègre & Minster 1978), was performed in two stages to accommodate changing partition coefficient and changes in the crystallising assemblage. The low-level gabbro was taken to represent the composition of the starting melt. The fractionating assemblage was (wt.% Ol 53 Pl 26 Cpx 21 ) during stage 1 and (wt.% Pl 66 Cpx 44 ) during stage 2. The model melt obtained after 50% fractional crystallisation during stage 1 was used as the starting composition for stage 2. This value was arbitrarily selected, but anywhere from 40 to 60% fractional crystallisation during stage 1 yields essentially similar results for stage 2. The results of the modelling are presented in Fig. 9B, and show that the Klemetsaunet rhyodacite could have formed by 75 85% fractional crystallisation (total% stage 1 and 2) of a low-level gabbroic magma. Fagervika trondhjemite The geochemical composition of the Fagervika trondhjemite suggests an island arc origin for this rock type, and two possible petrogenetic models can be envisaged. One is partial melting of basaltic rocks at the base of the arc, the second extensive fractional crystallisation of a mantle-derived basaltic magma. A third model involving fractional crystallisation of a basaltic magma followed by partial melting of the fractionated rock is also possible. Partial melting: The presently available dataset suggests two possible sources for the Fagervika trondhjemite, represented by the high- and low-level gabbros. Although the high-level gabbros constitute a volumetrically minor portion of the total mafic rocks in the Bymarka ophiolite fragment, it is likely that the base of the proposed island arc (in which the Fagervika trondhjemite is suggested to have formed) was dominantly made up of such rocks. Fig. 10A shows the results of Fig. 9. A. Modelled REE patterns produced by 10 20% equilibrium batch melting of average low-level gabbro. The grey field shows the range of REE compositions for the Klemetsaunet rhyodacite. Partition coefficients are from a compilation by Borg & Clynne (1998). B. Modelled REE patterns produced by Rayleigh fractional crystallization of an average low-level gabbro. Partition coefficients for stage 1 from Smith & Humphris (1998), and for stage 2 from Borg & Clynne (1998).

13 NORWEGIAN JOURNAL OF GEOLOGY Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite 179 partial melting REE modelling, taking the slightly LREE-enriched high-level gabbros to represent the source. The results show that partial melting of a highlevel gabbro can account for the HREE contents and Eu anomalies of the Fagervika trondhjemite, leaving a plagioclase-pyroxene dominated residue with small amounts of magnetite and hornblende (wt.% Pl 53 Opx 23 Cpx 13 Mag 7 Hbl 4 ). The observed LREEenrichment could, however, not be reproduced and appears to require a source that is more strongly enriched in LREE. The results of the modelling also mean that the LREE-depleted low-level gabbros are an unlikely source. Fractional crystallisation: Fig. 10B shows the results of fractional crystallisation REE modelling of a high-level gabbroic magma. As for the Klemetsaunet rhyodacite, the modelling was performed in two stages, and the starting composition for stage 2 was taken to be the model melt produced by 50% fractional crystallisation in stage 1. The fractionating assemblage was (wt.% Pl 40 Ol 21 Cpx 21 Hbl 10 Mt 8 ) during stage 1, and (wt.% Pl 54 Cpx 33 Mt 9 Hbl 4 ) during stage 2. Model melts representing a total of 75 and 85% fractional crystallisation are somewhat higher in LREE than those produced by partial melting in Fig. 10A, but the fractional crystallisation model nevertheless fails to account for the LREE enrichment observed in the Fagervika trondhjemite. Fractional crystallisation followed by partial melting of fractionated rock: The third petrogenetic model for the Fagervika trondhjemite tested here involves fractional crystallisation of a high-level gabbroic magma, followed by partial melting of the fractionated rock. Fig. 10C shows the results of the modelling. The composition of the fractionated rock is taken to be similar to the model melt produced by 50% fractional crystallisation of olivine, plagioclase, clinopyroxene, and magnetite in similar proportions to that of stage 1 in Fig. 10B. Model melts produced by 10 and 20% partial melting of the fractionated rock, leaving a plagioclase-pyroxene-dominated residue (wt.% Pl 52 Cpx 22 Opx 17 Hbl 4 Mt 5 ), are slightly depleted in LREE and have less negative Eu anomalies relative to the Fagervika trondhjemite. Byneset trondhjemite The Byneset trondhjemite has major and trace element compositions similar to rocks formed by partial melting of a mafic source leaving a garnet-bearing amphibolitic or eclogitic residue (Martin 1987, Rapp et al. 1991, Winther & Newton 1991, Sen & Dunn 1994, Rapp & Watson 1995). This is illustrated in the Y vs. Sr/Y diagram (Drummond & Defant 1990) in Fig. 11. Also plotted in this diagram is the Fagervika trondhjemite, which, in contrast to the Byneset trondhjemite, plots in the field of typical island arc tonalites and trondhjem- Fig. 10. A. Modelled REE patterns produced by equilibrium batch melting of an average high-level gabbro. The grey field shows the range of REE compositions for the Fagervika trondhjemite. Partition coefficients from a compilation by Borg & Clynne (1998). B. Modelled REE patterns produced by Rayleigh fractional crystallization of an average high-level gabbro. Partition coefficients for stage 1 from Smith & Humphris (1998), and for stage 2 from Borg & Clynne (1998). C. Modelled REE patterns produced by 50% Rayleigh fractional crystallization of an average high-level gabbro, followed by equilibrium batch melting of the fractionated rock (i.e., model melt from stage 1). Partition coefficients for stage 1 from Smith & Humphris (1998), and for stage 2 from Borg & Clynne (1998).

14 180 NORWEGIAN JOURNAL OF GEOLOGY Trond Slagstad ites. The low HREE and Y contents of the Byneset trondhjemite indicate that garnet was stable in the source, suggesting pressures in excess of 800 MPa (Rapp et al. 1991, Rapp & Watson 1995). As is the case with the Fagervika trondhjemite, the two most likely sources for the Byneset trondhjemite are greenstones and low- and high-level gabbros. REE modelling is used to further constrain the source of the Byneset trondhjemite. The results of partial melting REE modelling of a LREE-depleted source (low-level gabbro) are shown in Fig. 12A. The modelling shows that a LREE-depleted source, such as the low-level gabbros, can produce model melts with similar middle REE (MREE) and HREE contents as the Byneset trondhjemite at reasonable degrees of partial melting (10 40%) leaving a garnet-bearing amphibolitic residue (wt.% Hbl 70 Pl 17 Grt 13 ). However, the LREE-depleted pattern of the source is inherited by the model melts and the latter do not resemble the composition of the Byneset trondhjemite. In contrast, a LREE-enriched source, such as the highlevel gabbros, produces model melts with similar REE compositions as the Byneset trondhjemite for 10 40% melting, leaving a garnet-bearing amphibolitic residue (wt.% Hbl 62 Cpx 15 Grt 12 Pl 11 ) (Fig. 12B). Thus, the highlevel gabbros or equivalent rocks are considered a potential source of the Byneset trondhjemite. Fig.11. Y-Sr/Y diagram showing that the Byneset trondhjemite is similar to Archean TTGs (tonalite-trondhjemite-granodiorite) and adakites interpreted to have formed by partial melting of a mafic source in the stability field of garnet (Drummond & Defant 1990). The Fagervika trondhjemite is compositionally similar to tonalitic and granodioritic rocks formed in island arcs, either by fractional crystallisation of basaltic magmas or partial melting of mafic sources. The Klemetsaunet rhyodacite plots at higher Y values than covered by the diagram, indicated by the arrow. Compositional fields after Hansen et al. (2002). negative Ta-Nb anomalies (Fig. 6A) (Grenne 1989), but a more detailed study, including a larger number of samples, is needed to determine whether or not this difference is real. Tectonic significance of the rhyodacite and trondhjemites Klemetsaunet rhyodacite The Klemetsaunet rhyodacite appears to have formed in an ocean-floor setting, coevally with the low-level gabbros and greenstones making up the bulk of the Bymarka ophiolite fragment. This interpretation is strengthened by the occurrence of fine-grained mafic dykes, compositionally identical to the greenstones, cutting the rhyodacites (Slagstad 1998). The most likely petrogenetic model is fractional crystallisation of a basaltic magma, similar in composition to the low-level gabbros. Regional geological considerations, discussed above, suggest that the rocks may have formed in a suprasubduction-zone setting. No clear evidence of this is seen in either the Klemetsaunet rhyodacite or the surrounding greenstones, although a suprasubductionzone setting may be supported by the negative Ta-Nb anomalies in the primitive mantle-normalized diagram (Elthon 1991); alternatively, the anomalies could result from fractional crystallisation of Fe-Ti oxides. The plagiogranites from Løkken appear to lack significant Fagervika trondhjemite Although the geochemical data point to an island arc setting for the Fagervika trondhjemite, the REE modelling does not yield conclusive results regarding the source and petrogenesis of this rock type. The modelling indicates a source with a stronger LREE-enrichment than observed in the high-level gabbros, resulting, for example, from a stronger subduction influence. Although such a source has not been identified, this should not come as a surprise considering that the ocean floor/island arc assemblages observed in the area are strongly dismembered and fragmented. For the same reason, a model involving fractional crystallisation of basaltic magmas cannot be discounted despite the lack of intermediate rocks in the study area. Byneset trondhjemite Drummond & Defant (1990) argued that adakites, which have several geochemical characteristics in common with the Byneset trondhjemite (e.g., Fig. 11), form by partial melting of subducted oceanic crust. These melts must pass through the overlying mantle wedge