Structure, geochemistry, and tectonic evolution of trench-distal backarc oceanic crust in the western Norwegian Caledonides, -Stavfjord ophiolite ()

Harald Furnes1,†, Yildirim Dilek 2,3, and Rolf Birger Pedersen1 1Department of Earth Science & Centre for Geobiology, University of , 5007 Bergen, Norway 2Department of Geology & Environmental Earth Science, Miami University, Oxford, Ohio 45056, USA 3School of Earth Science and Mineral Resources, China University of Geosciences, Beijing 100083, China

ABSTRACT fl uids . The evolution of the Solund-Stavfjord ultramafi c rocks in some ancient orogenic belts ophiolite complex oceanic crust occurred in as the remnants of former backarc basins. The The Late Ordovician (443 Ma) Solund- a short-lived (<20 m.y.), trench-distal, conti- Neotethyan realm in the eastern Mediterranean Stavfjord ophiolite complex in west Norway nent-proximal backarc basin, adjacent to region is a good example of this problem (Dilek represents the youngest phase of oceanic the eastern margin of Greenland-Laurentia, and Moores, 1990; Robertson, 2002; Dilek crust formation in the western Norwegian during the closure of Iapetus. This inferred and Thy, 2006), where several E-W–trend- Caledonides. It contains three structural tectonic setting is reminiscent of the modern ing ophiolite belts are separated by a series of domains with different crustal architecture Andaman at the eastern periphery of the Gondwana-derived continental fragments with that formed during two episodes of seafl oor Indian Ocean. no trace of a volcanic arc system (Tankut et al., spreading evolution of a Late Ordovician 1998; Dilek and Flower, 2003). Backarc basin marginal basin. The fossil oceanic crust of INTRODUCTION ophiolitic crust typically shows two important the younger episode contains pillow lavas, trace-element characteristics: (1) enhanced massive sheet fl ows, and hyaloclastites, NE- The majority of ophiolites around the world concentrations of subduction-mobile incom- trending sheeted dikes, and high-level isotro- represent fragments of oceanic lithosphere de- patible elements in comparison to normal mid- pic gabbros. The pillow lava versus massive veloped in suprasubduction-zone tectonic set- ocean-ridge basalt (N-MORB), and (2) reduced sheet fl ow distribution and the occurrence tings (Pearce et al., 1984), including incipient concentrations of subduction-immobile in- of an extensive sheeted dike complex in the arc, forearc, and backarc environments (Stern compatible elements (Pearce and Stern, 2006). Solund-Stavfjord ophiolite complex are and Bloomer, 1992; Hawkins, 2003; Robinson Hence, the extent and nature of subduction input typical of in situ oceanic crust developed at et al., 2008; Pearce and Robinson, 2010; Rea- into the melting regime and magma production modern intermediate-spreading mid-ocean gan et al., 2010; Dilek and Furnes, 2011). The systems are signifi cant parameters affecting ridges. The Solund-Stavfjord ophiolite com- incipient arc–forearc remnants that evolved in the geochemical fi ngerprint of backarc basin plex lavas and dikes are composed predomi- intra-oceanic arc-trench rollback systems (Dilek ophiolites. The distance from the mantle melt- nantly of normal mid-ocean-ridge basalt and Flower, 2003; Dilek and Thy, 2009; Dilek ing system to a trench and a subducting slab is (N-MORB) Fe-Ti basalts, and their trace- and Furnes, 2011) analogous to the modern highly important for the geochemical character element patterns indicate a weak subduction Izu-Bonin-Mariana system (Stern and Bloomer, of backarc basin ophiolites. Another signifi cant infl uence. The Nd isotope data of these rocks 1992; Bloomer et al., 1995; Stern et al., 2003, factor in the geochemical character of backarc suggest derivation of their magmas from an 2006; Takahashi et al., 2007) were incorporated basin ophiolites is whether their magmatic evo- isotopically homogeneous melt source with into ancient continental margins through colli- lution might have occurred near a continental no indication of continental crustal con- sional events during the closing stages of ocean mass where it was subjected to various degrees tamination. The Solund-Stavfjord ophiolite basins (e.g., Robertson, 2002; Wakabayashi and of crustal contamination. complex extrusive sequence contains phyl- Dilek, 2003; Dilek and Furnes, 2009). Backarc There are a few modern examples of backarc lite interlayers and is conformably over- basin ophiolites were emplaced onto the edges basins that have developed adjacent to large con- lain by a continentally derived, quartz-rich of the bounding island-arc complexes or micro- tinents. The best examples include the Sea of Ja- metasandstone that is intercalated with sills continents following basin collapse via intra- pan, Tasman Sea, Woodlark Basin, and Andaman of N-MORB basaltic lavas and shallow-level oceanic subduction (Dilek et al., 1999, 2008). Sea, all in the western Pacifi c, and the Tyrrhe- intrusions. The geochemical features of the These arc-forearc and backarc basin ophiolites nian Sea in the western Mediterranean. All these upper-crustal rocks of the Solund-Stavfjord subsequently became nested and amalgamated modern backarc basins evolved as a result of slab ophiolite complex indicate their forma- in some orogenic belts, where continental col- retreat and associated lithospheric extension in tion from magmas in which the melt evolu- lisions further shortened and deformed the re- the upper plates of subduction zones dipping to- tion involved only minor or no slab-derived cently accreted oceanic lithospheric fragments. ward continents. We have a limited knowledge Where volcanic arc units are missing, it of the structural and geochemical makeup of †E-mail: [email protected] may be diffi cult to recognize suites of mafi c- backarc oceanic crust from these modern basins

GSA Bulletin; July/August 2012; v. 124; no. 7/8; p. 1027–1047; doi: 10.1130/B30561.1; 14 fi gures; Data Repository item 2012218.

For permission to copy, contact [email protected] 1027 © 2012 Geological Society of America Furnes et al. because of limited deep drilling, dredging, and within the framework of the regional tecton- a trench-distal, fossil backarc oceanic crust, diving activities and related results. ics of the Caledonian–Appalachian orogenic and its evolutionary history provides an impor- In this paper, we document the internal struc- system. We also present a synoptic and sys- tant case study to examine in three dimensions ture and upper-crustal geochemistry of the Late tematic overview of the tectonostratigraphic the magmatic plumbing system and the geo- Ordovician Solund-Stavfjord ophiolite com- units that are spatially and temporally associ- chemical fi ngerprint of intermediate-spreading plex in the western Norwegian Caledonides ated with the Solund-Stavfjord ophiolite. The backarc oceanic crust. Our model suggests that (Fig. 1), and discuss its geodynamic evolution Solund-Stavfjord ophiolite complex represents this Late Ordovician backarc basin may have

′ WESTERN 4°45′ 5°00′ 5°15′ 5°00 ′ GNEISS TECTONOSTRATIGRAPHY REGION Devonian

Kalvåg Frøya CALEDONIDES 440 Ma mélange Hersvik Unit Smelvær Unit Thrust SSOC & ′ Kalvåg 61°15 ′ SM cover BASEMENT 61°15 Gåsøy 443 Ma mélange (SM)

SM Batalden ? Herland Group Eikefj. 10 km Skorpa ? Uncon- formity Høyvik Group

Askrova Svanøy Dalsfjord Suite 61°30′ 61°30 ′ Fault Group STAVFJORD Vågsøy A Tectonic contact E Western Gneiss Region Stave-

S Smelvær nes Moldvær LEGEND Heggøy Høyvik Devonian conglomerate Herland N SOLUND - STAVFJORD OPHIOLITE COMPLEX (SSOC) Tviberg Askvoll A AND ASSOCIATED SEDIMENTARY AND MAGMATIC ROCKS Alden I 443 Ma Granodiorite - granite Værlandet Atløy

G Gabbronorite - diorite (exposed) extrapolated E Kalvåg mélange ′ W 61°15 Smelvær Unit (pillow lava / metavolcaniclastics)

R Hersvik Unit (metagraywacke / volcaniclastics, lavas)

O Hers- Heggøy Fm (metagraywacke / phyllite, lava & SOLUND sill intrusions)

N SSOC (metagabbro / diorite, sheeted dikes, pillow and massive lavas, hyaloclastites) Lågøy 439 Ma Sunnfjord mélange & various schists Trygg- Oldra Sogneskollen Herland Group øy 434 Ma Høyvik Group N E Ø Dalsfjord Suite BALTICA J S Askvoll Group N E ′ G ′ 4°30 4˚45´ O 5°00′ 5°15 W Gneiss Region 61°00′ S

Figure 1. Simplifi ed geological map of the Solund-Bremanger area showing the stratigraphy and tectonostratigraphy of the various rock units represented in this region (modifi ed from Furnes et al., 1990). SSOC—Solund-Stavfjord ophiolite complex. The references to the radiometric ages are: Sogneskollen—434 Ma (Hacker et al., 2003); Hersvik—439 Ma (Hartz et al., 2002); Tviberg—443 Ma (Dunning and Pedersen, 1988); Gåsøy—443 Ma, and Bremanger—440 Ma (Hansen et al., 2002).

1028 Geological Society of America Bulletin, July/August 2012 The Solund-Stavfjord ophiolite, western Norwegian Caledonides opened up adjacent to the Laurentian margin the Herland and Høyvik Groups of the Middle meta-arkose (Fig. 3) in a Cr- and Ni-rich matrix of Eastern Greenland (in present coordinate Tectonic Unit, which in turn unconformably (Alsaker and Furnes, 1994). The high Cr and system), but its oceanic crust was subsequently rests on the mangeritic gneisses of the Dalsfjord Ni content of the matrix has been attributed to accreted onto the western continental margin of Suite (Figs. 1 and 3). Postcollisional Devonian the exposure of ophiolitic ultramafi c rocks on Baltica following closure of the Iapetus Ocean. conglomeratic rocks unconformably overlie the the seafl oor (Skjerlie and Furnes, 1990). The The tectonic evolution of the Solund-Stavfjord Solund-Stavfjord ophiolite complex, its sedi- Sunnfjord mélange in its type locality (Marka- ophiolite complex also provides us with an op- mentary cover (Heggøy Formation), and other vatn area; see Alsaker and Furnes, 1994) shows portunity to examine the diversity in the proc- tectonostratigraphic units in the region. many similarities to the mélanges occurring in esses of oceanic crust generation along strike in Of all these tectonostratigraphic assem- subduction-accretion complexes (Festa et al., the early Paleozoic Caledonides. blages constituting the Upper Tectonic Unit in 2010), such as the Franciscan complex, Califor- the western Norwegian Caledonides, only the nia (e.g., Wakabayashi, 2011), and the Ankara REGIONAL GEOLOGY AND Sunnfjord mélange and the Heggøy Forma- mélange, Turkey (Tankut et al., 1998; Dilek and TECTONOSTRATIGRAPHY tion have exposed contacts with the Solund- Thy, 2006; Danger fi eld et al., 2011). Stavfjord ophiolite complex. The stratigraphic The Sunnfjord mélange exposed between The development of oceanic crust and island- and/or tectonic position of the other units, (i.e., and Svanøy (Fig. 1) contains mainly arc systems associated with the closure of the the Hersvik and Smelvær Units and the Kalvåg turbiditic metagraywacke and metapelite, host- Iapetus Ocean has been well documented in the mélange) relative to the Solund-Stavfjord ophio- ing bodies of gabbro, pillow lavas, and dikes, geological record of the western Norwegian lite complex is uncertain, and can only be in- and volcaniclastic rocks. The turbiditic matrix Caledonides (e.g., Furnes et al., 1980; Stephens ferred on the basis of limited age constraints and here is known as the Høydalsfjorden com- et al., 1985; Rankin et al., 1988; Pedersen geochemical correlations. Thus, in this study and plex (Vetti, 2008). The bodies of gabbros, pil- et al., 1988; Pedersen and Furnes, 1991). The throughout this paper , we consider that: (1) the low lavas , and dikes either represent slivers of early Paleozoic ophiolite pulses as recorded in Sunnfjord mélange is structurally the lowest oceanic crust incorporated into the accretion- the history of the Norwegian Caledonides oc- tectonostratigraphic unit, (2) the Heggøy Forma- ary prism during subduction (e.g., Kimura and curred in two distinct episodes, an older phase tion rests with a primary stratigraphic contact on Ludden, 1995) or large olistoliths of ophiolitic in the Early Ordovician (ca. 500–470 Ma), the Solund-Stavfjord ophiolite complex, (3) the origin derived from the Solund-Stavfjord ophio- and a younger phase around the Ordovician- Hersvik Unit represents an island-arc complex lite complex in the upper plate. The strong my- Silurian boundary (ca. 440 Ma) (Dunning and coeval or younger than the Solund-Stavfjord lonitic fabric and the penetrative deformation of Pedersen, 1988; Pedersen and Dunning, 1997; ophiolite complex, (4) the Smelvær Unit consists the mélange make it diffi cult to test these two Peder sen et al., 1991). The remnants of the older of off-axis mafi c volcanic and intrusive rocks alternative hypotheses. In this study, we inter- episode of ophiolite formation are represented, that developed on and across the Solund-Stav- pret the Høydalsfjorden complex as part of the from south to north, by the Karmøy, Lykling, ophiolite complex oceanic crust, and (5) the Sunnfjord mélange. Gulfjellet , Løkken, and Leka ophiolite frag- Kalvåg mélange, containing material derived ments (e.g., Pedersen and Furnes, 1991). The from both the Hersvik and Smelvær Units, repre- Heggøy Formation 443 ± 3 Ma Solund-Stavfjord ophiolite complex sents the youngest tectonostratigraphic unit in the (Dunning and Pedersen, 1988), together with region. We present here a brief description of all The Heggøy Formation constitutes the sedi- the 437 ± 2 Ma Sulitjelma Gabbro in northern these tectonostratigraphic units, the geology of mentary cover of the Solund-Stavfjord ophio- Norway (Pedersen et al., 1991), is part of the which is closely related to the tectonic evolution lite complex and includes sedimentary and younger ophiolite occurrences in the western of the Solund-Stavfjord ophiolite complex. magmatic rock intercalations. On the island of Norwegian Caledonides (Fig. 2). Heggøy, an ~1000-m-thick-sequence of pre- The Solund-Stavfjord ophiolite complex is Sunnfjord Mélange dominantly calcareous metagraywacke rests spatially and temporally associated with several with a primary sedimentary contact directly on major tectonostratigraphic assemblages, which The Sunnfjord subophiolitic mélange (Fig. 1) the sheeted dike complex of the Solund-Stavf- collectively make up the Upper Tectonic Unit is a strongly tectonized sedimentary assemblage jord ophiolite complex (Furnes et al., 1990); in the western Norwegian Caledonides (Figs. that consists of a lower, fi ne-grained, quartz- in other areas, however, these metagraywacke 1 and 2). Graywacke and calc-alkaline basaltic bearing chlorite-muscovite schist, and upper rocks sit conformably on the ophiolitic pillow volcanic rocks of the Hersvik Unit, vol cani- metagraywacke rocks. Both the muscovite schist lavas. The metagraywacke sequence hosts nu- clastic rocks and alkaline pillow lavas of the and metagraywacke rocks contain blocks of merous sills and pillow lava horizons, and con- Smelvær Unit, and the Kalvåg mélange (Fig. 1) meta basalt, marble, and meta-arkose. Serpen- sists of dominantly fi ne- to medium-grained, are all inferred to be younger than the Solund- tinite blocks occur only in the metagraywacke thin- to thick-bedded, light to dark greenish- Stavfjord ophiolite complex and stratigraphi- rocks (Alsaker and Furnes, 1994) (Fig. 3). The gray sandstone-siltstone rocks (Fig. 3). The cally resting on it. The Sunnfjord mélange, type locality of the Sunnfjord mélange is in the sandstone unit includes quartz, albite, and mi- composed of a chaotic assemblage of blocks southern and eastern parts of the Stavenes Pen- nor rock fragments of greenstone and quartzite of sedimentary and magmatic rocks embedded insula, where the sheeted dike complex of the in a matrix composed of mica, chlorite, epidote, in a metagraywacke matrix, occurs tectonically Solund-Stavfjord ophiolite complex rests tecton- and variable amounts of calcite (Furnes, 1974; beneath the Solund-Stavfjord ophiolite complex ically on a 1350-m-thick deposit of the subophio- Furnes et al., 1990). These metagraywacke lay- and is inferred to have formed as a subophio- litic mélange (Figs. 1 and 3). Here, the mélange ers generally occur as thick, monotonous se- litic mélange during the emplacement of the consists predominantly of calcareous graywacke quences, commonly alternating with dark-green Solund-Stavfjord ophiolite complex (Alsaker with blocks of meta-arkose and meta basalt. The phyllite, quartzite, and minor limestone beds, and Furnes, 1994). This subophiolitic mélange upper part of the muscovite schist contains and locally with greenish-gray to dark-green tectonically overlies metasedimentary rocks of blocks of serpentinite, chert, meta basalt, and metavolcaniclastic rocks.

Geological Society of America Bulletin, July/August 2012 1029 Furnes et al.

′ ′ Tviberg A 5°00 E 5°10 60 STAVFJORDEN Do Stavestranda ~ 40 mai ~ n 2 Grimeli v v v Stave- v Smelvær v v ~ ~ Domain 3 Domain 1 neset v v ~ ~ v v ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ 5 km ~ ~ ~ Ultra- Heggøy ~ ~ mafic ~ Cross- Devonian conglomerates mélange ~ ~ cutting ~ ~ ~ dikes ~ Unconformity ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Metasediments and lavas of ~ ~ ~ ~ v 4°40′E ~ ~ ~ v the Hersvik & Smelvær Units & Tviberg ~ ~ ~ ~ the Heggøy Fm. / Later metagabbro ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Volcanic sequence ′ ~ ~ ~ ~ ~ 61°20 N Alden ~ ~ ~ ~ ~ ~ ~ ~ Transition zone of ~ ~ 61˚20´N volcanic and dike rocks Sheeted dike complex Værlandet (dike orientation indicated) ~ Metagabbro ophiolite complex Strongly deformed metabasalt Solund - Stavfjord ~ containing ultramafic bodies ~ Sunnfjord mélange ~ ~ ~ ~ Metasediments and -igneous rocks ~ of the Herland & Høyvik Groups, and the Dalsfjord Suite

Precambrian basement of the Western Gneiss Region Tectonic boundary Leknessund High-angle faults

Staveneset Stavestranda B W E 0 SOLUND Hersvik 61°10′N

Strand 100% dikes (m) Depth Langøy 800

S Ytrøy Langøy Strand Alden N Oldra 0 Sheet flows Pillow lava Volcaniclastic rocks Metagraywacke Steinsøy (m) Depth and phyllite 100% 100% Ytrøy 100% dikes dikes dikes Dike complex 4°40′E 4°50 ′ 800

Figure 2. (A) Detailed geological map of the Solund-Stavfjord ophiolite complex (SSOC) and the adjacent units. (B) Reconstruction of the vol- canic rocks of the ophiolite. Figure is modifi ed from Furnes et al. (2006). The inset map (upper-left corner) shows the distribution of the three structural domains, and the two dike generations (the old N60 °E, and the younger N20 °E; Dilek et al., 1997). The inset in the lower-right corner shows a reconstruction of the volcanic development of the Solund-Stavfjord ophiolite complex (modifi ed from Furnes et al., 2003).

U/Pb dating of detrital zircon and titanite Late Archean boundary (2495 Ma), Proterozoic to the main crustal construction periods of the grains from the metagraywacke rocks that are (1959–1024 Ma), and early-mid-Ordovician North Atlantic Shield (Pedersen and Dunning, intercalated with tholeiitic lava sills (location: (492–462 Ma). The Proterozoic zircons can fur- 1993), and they show that the Solund-Stavfjord between Grimeli and Stavestranda; see Fig. 2) ther be subdivided into three subgroups, i.e., ca. ophiolite complex was covered by clastic sedi- has revealed three distinct age groupings (Peder- 2000–1700 Ma, 1600–1400 Ma, and 1200–1000 ments derived from both the Precambrian and sen and Dunning, 1993): Early Proterozoic– Ma. These Precambrian age groups correspond Paleozoic terranes.

1030 Geological Society of America Bulletin, July/August 2012 The Solund-Stavfjord ophiolite, western Norwegian Caledonides

Tectonostratigraphy Kalvåg Smelvær Hersvik Heggøy SSOC Sunnfjord mélange Unit Unit Formation mélange Devonian cgl. SSOC unconf. Devonian b conglomerate

b ch Smelvær Unit (oceanic isl.) b ~ 350 m Upper Kalvåg b Mélange ~ 600 m 600 ~

Tectonic b Hersvik Unit b (island arc) ~ 2500 m s ~ 1350 m 1350 ~ ~ 1000 m a Unit Heggøy Fm (cover to SSOC) b

Solund- ~ 2000 m b Stavfjord ophiolite a complex (SSOC) ?

thrust

Sunnfjord Høyvik mélange Group Kalvåg mélange Hersvik & Smelvær Units Heggøy Fm & SSOC SSOC Sunnfjord mélange Limestone Olistoliths Herland Olistoliths shale b Metabasalt Phylite Group sandstone b Metabasalt Middle conglomerate Ignimbrite Green volcaniclastic rocks Chert s Serpentinite Tectonic Høyvik Sandstone Sandy turbidites Metasandstone ch Metachert Unit Group mica schist (deep marine) Conglomerate Siliciclastic sed. a Anorthosites (shallow marine) Calcareous Meta-arkose mangerites Sheeted dike complex metagreywacke/ Dalsfjord Suite gabbro Groundmass Basic intrusions chl.- musc.schist Marble granites Bituminous Granodiorite Lower Debris - flow Pillow lava Metagabbro Western Gneiss Undiff. deposits with schist intrusions Region Tectonic mylonites & channel - fill Massive lava Matrix-supported Main shear Unit gneisses conglomerates & intrusions polymict cgl. zone

Figure 3. Magmatic, volcanic, and sedimentary development of the Kalvåg mélange, Hersvik Unit, Smelvær Unit, Heggøy Formation, the Solund-Stavfjord ophiolite complex (SSOC), and the Sunnfjord mélange. Data sources: Furnes et al. (1990, 2000, 2003); Alsaker and Furnes (1994); and Ravnås and Furnes (1995).

Hersvik Unit ner (2–5 m). The remaining sedimentary and of which (in particular, the high Zr and light rare volcaniclastic rocks are dominated by calcare- earth element [LREE] contents) are compati- The Hersvik Unit is best observed in the ous metagraywacke, displaying cross-bedding, ble with trachytes and comendites/pantellerites Solund region (Figs. 1 and 2), where it is un- graded-bedding, and fl ame structures. These (Furnes and Lippard, 1983). U-Pb dating of the conformably overlain by the Devonian con- rocks have been interpreted as turbiditic se- felsic lava rocks yielded an Early Silurian age of glomerate (Nilsen, 1968; Osmundsen and quences (Furnes, 1974). In the higher parts of 439 ± 1 Ma, and these magmatic rocks are inter- Andersen, 2001) of the Solund basin (Fig. 3). the Hersvik Unit, the lava units are intercalated preted as landslide deposits of the substrate to the Its basal part is nowhere exposed, and hence with thin conglomerate beds with quartzite as Devonian rocks (Hartz et al., 2002). The Early its exact stratigraphic position is uncertain. The the main pebble type, and in the uppermost part Silurian age of the felsic lava rocks suggests that Hersvik Unit consists of mafi c lavas, green vol- of the Hersvik Unit, the sequence is dominated these landslide deposits may have been spatially caniclastic rocks, metagraywacke, conglomer- by fi ne-grained, chlorite-rich schists, represent- and temporally associated with plagioclase- ate, and minor gabbroic intrusions in the upper ing mafi c volcaniclastic rocks (Fig. 3). phyric lavas of the Hersvik Unit. We interpret the part of the sequence (Furnes, 1974). About 50% In the southernmost part of the Hersvik area mafi c lavas and the associated metasedimentary- of the Hersvik Unit is composed of mafi c lavas, (Fig. 2), fl ow-banded and brecciated felsic lava, volcaniclastic rocks of the Hersvik Unit as sub- which are made of massive and plagioclase- granite, and gabbro rocks are mixed with a De- marine deposits, and the trachyte, comendite, and phyric basalt intercalated with sedimentary and vonian conglomerate (Steel, 1976). The felsic pantellerite as probable subaerial lavas, repre- volcaniclastic rocks. Stratigraphically higher lava occurs in two sheets (up to 200 m long and senting the lower and higher (and mature) parts, in the sequence, the lava units become thin- 10–20 m thick), the geochemical compositions respectively, of an island-arc complex.

Geological Society of America Bulletin, July/August 2012 1031 Furnes et al.

Smelvær Unit A B C The lithological units of the Smelvær Unit are only seen on a series of islands (Fig. 1) with no lower and upper contacts exposed, and hence its exact stratigraphic position is also uncertain. In its type locality on the island of Smelvær, the rock sequence consists of 350 m of mainly nonvesicular pillow lavas, minor mas- sive lavas, and green volcaniclastic rocks, which are interbedded with thin beds (up to 30 cm) of 1 m 0.5 m 0.5 m black chert and graphite-bearing black schists Doomed rift (Fig. 3). On the island of Vågsøy farther to the D NE (Fig. 1), the Smelvær Unit is composed pre- dominantly of green volcaniclastic rocks and (Old) minor fl aser-gabbro. Tviberg Fracture zone B & C Kalvåg Mélange A The Kalvåg mélange (Fig. 1) represents a >2500-m-thick, olistostromal mélange (Ravnås and Furnes, 1995). Olistolith blocks are variable Alden D in size, up to ~3 km long and 0.5 km thick, and are 1 m composed mainly of shallow-marine rocks, deep- E marine turbidites, bedded chert, pebbly mud- stone, calcareous rocks, basaltic to andesitic lava E fl ows, rhyolites, and rhyolitic ignimbrites. These Domain I blocks are embedded in a sheared mudstone (new) matrix, with subordinate black shale and inter- Oceanic crust bedded sandstone-conglomerate beds (Fig. 3). Domain I generated by The geochemical character of the magmatic Oldra & propagating rift

Propagating rift fragments defi nes a large compositional range of adjacent Oceanic crust islands Domain II generated by mafi c volcanic rocks of tholeiitic, boni nitic, and doomed rift calc-alkaline to alkaline affi nities (Ravnås and Furnes, 1995), demonstrating that the material Domain III Sinistral shear zone 100 m for this mélange originated from sources having F & G different tectonomagmatic settings. The Kalvåg Sheeted dikes mélange is intruded by the Gåsøy gabbronorite/ F diorite and Bremanger granodiorite plutons G (Fig. 1), dated at 443 ± 4 Ma and 440 ± 5 Ma, respectively (Hansen et al., 2002).

Structure of the Solund-Stavfjord Ophiolite Complex

The Solund-Stavfjord ophiolite complex in- cludes three structural domains (Fig. 2, upper left inset; Fig. 4) that display different types of 2 m crustal architecture developed during its multi- stage seafl oor spreading evolution (Dilek et al., Figure 4. Sketch map showing the inferred distribution of the three structural domains of the 1997; Furnes et al., 1998). Domain 1, best ex- Solund-Stavfjord ophiolite complex (SSOC), and representative fi eld pictures. (A) Basaltic dike posed on the island of Oldra, has a NNE-trend- complex intruded by light-gray to white quartz diorite (location: western part of Tviberg; ing structural grain defi ned by the consistent Fig. 2). (B) Strongly sheared plagiogranite (light gray), crosscut by a basalt dike (N60 °E), orientation of dike swarms in the sheeted dike which is in turn intruded by an undeformed N20°E basalt dike (lower left). (C) Strongly complex (Fig. 2). Its exposed units include an sheared serpentinite (central part), cut by N20°E basalt dikes. (D) Dike (N20 °E) crosscut- extrusive sequence (Figs. 4E and 4F), a well- ting gabbro (location: Tviberg, Fig. 2). (E) Massive sheet fl ows (location: Alden; Fig. 2). developed sheeted dike complex (Fig. 4G), and (F) Three pillow bodies defi ning a triangular space, fi lled with pillow breccia and a micro- underlying isotropic gabbros (Fig. 4D). The ex- pillow in the center (location: little island [Kattøy] south of Steinsøy, Solund). (G) Sheeted trusive rocks, up to 800 m in thickness, include dike complex (location: Oldra, Solund; Fig. 2). pillow lava, pillow breccias, and hyaloclastite

1032 Geological Society of America Bulletin, July/August 2012 The Solund-Stavfjord ophiolite, western Norwegian Caledonides with local occurrences of jasper and chert, 80 SSZ (FA) 4 massive sheet fl ows, and fossil lava lakes. An SSZ (BA-FA) 70 extensive network of normal faults that are asso- SSZ (CBA) 3 SSZ (OBA) - SSOC ciated with hydrothermal alteration zones and 60 2 2 epidosite formations occurs in the hyaloclastite 2 TiO and pillow breccia horizons. An ~100-m-thick SiO 50 transition zone composed of dike swarms and 1 40 pillow breccias separates the lowermost section of the extrusive sequence from the underlying 30 0 sheeted dikes. The NE-trending and steeply to 0 5 10 15 20 25 0 5 10 15 20 25 moderately dipping sheeted dikes are mostly 20 20 basaltic in composition and display one- or two- sided chilled margins. Epidote vein networks 15 are common along dike margins. 15 3 t

Sheeted dikes are crosscut by two different O

2 10 FeO sets of oceanic faults. One set includes dike- Al 10 parallel, moderately to gently dipping faults 5 accompanied by epidote-chlorite veins and hydro thermal breccia zones. The other fault set 5 0 consists of steeply dipping dike-perpendicular 0 5 10 15 20 25 0 5 10 15 20 25 oblique-slip faults, which are also associated with epidote-quartz and epidote-chlorite vein 600 3000 networks. The plutonic sequence below the sheeted dikes consists mainly of fi ne- to coarse- 400 2000 grained gabbros that are locally intruded by V V 10- to 20-cm-thick, aphanitic, irregular basaltic Cr dikes. Apophyses of microgabbro also intrude 200 1000 into the dike rocks, indicating mutually cross- cutting relations between the sheeted dikes and 0 0 the gabbros. 0 5 10 15 20 25 0 5 10 15 20 25 Domain 2, best exposed on the island of Tvi- berg, represents the oldest preserved oceanic 500 120 crust in the ophiolite and consists of pillow 400 lavas , W-NW–trending sheeted dikes and under- 90 lying isotropic gabbros (Fig. 2). The contacts 300

Y Y 60 between these lithologies are commonly faulted Zr and sheared, although primary mutual intrusive 200 relationships between dikes and gabbros are lo- 30 100 cally well preserved. Dike rocks are composed mainly of aphyric to strongly porphyritic dia- 0 0 base and are locally net-veined by diorite and 0 5 10 15 20 25 0 5 10 15 20 25 quartz diorite intrusions (Fig. 4A). The plutonic MgO MgO rocks beneath the dike complex include gabbro, (t) Figure 5. Bowen diagrams for MgO versus SiO 2, TiO 2, Al 2O3, FeO , V, Cr, Zr, and Y. Data microgabbro, and quartz diorite with varying sources: Forearc ophiolite–suprasubduction-zone (forearc [FA]): Betts Cove ophiolite, New- grain sizes. foundland (Bedard, 1999); backarc to forearc ophiolites–suprasubduction zone (backarc- Domain 3 occurs in the central part of the forearc [BA-FA]): Mirdita ophiolite, Albania (Dilek et al., 2008); Kizildag ophiolite, Turkey Island of Tviberg, where domains 1 and 2 in- (Dilek and Thy, 1998, 2009); Oman ophiolite (Lippard et al., 1986; Einaudi et al., 2003; tersect each other along an ~1-km-wide, NE- Godard et al., 2003); Troodos ophiolite, Cyprus (Auclair and Ludden, 1987; Rautenschlein trending shear zone (Fig. 2). The sheeted dikes et al., 1985; Taylor, 1990); continental backarc–suprasubduction-zone (CBA): South Chilean of domain 2 change their orientation from NW ophiolites: (Elthon, 1979; Saunders et al., 1979; Stern, 1979, 1980; Stern and Elthon, 1979); to ENE within this shear zone, indicating a and the Solund-Stavfjord ophiolite complex (SSOC)–suprasubduction-zone (OBA, oceanic signifi cant shift in the paleostress regime. Do- backarc): Furnes et al. (2006, and references therein), this work. main 3, represented by this shear zone, consists of dike swarms, plutonic rocks, serpentinite breccias, and fault-bounded serpentinite sliv- meter-scale, discrete sinistral shear zones. Some saltic dikes. We structurally and geochemically ers (Figs. 4B and 4C) in deformed isotropic to of the ENE-oriented sinistral shear zones are correlate these N20°E-trending, undeformed fl aser gabbros. Pegmatitic gabbros are locally spatially associated with serpentinite intrusions dikes with the dike swarms of the NE-oriented crosscut by N60°E-striking subvertical dikes, in the gabbro-quartz diorite, and these serpenti- sheeted dike complex of domain 1 farther south and both the dikes and their gabbroic host rocks nite intrusions and the sinistral shear zones are on the island of Oldra (see Fig. 2). Zircon grains are plastically deformed along millimeter- to crosscut by N20°E-trending, undeformed ba- separated from an undeformed quartz diorite in-

Geological Society of America Bulletin, July/August 2012 1033 Furnes et al. trusion, which is spatially associated with these used for analysis of Sc, Nb, Cs, the rare earth Solund-Stavfjord ophiolite complex meta- dikes (domain 1), yielded a U-Pb age of 443 ± elements (REEs), Pb, and Th. The solution ICP- basalts predominantly plot in the MORB fi eld, 3 Ma (Dunning and Pedersen, 1988). We inter- MS analyses were done on an HP4500 ICP-MS although we also see some samples plotting in pret domain 3 as a segment of a fossil oceanic at Memorial University of Newfoundland fol- the within-plate basalt (WPB) and island-arc fracture zone (Skjerlie et al., 1989; Skjerlie and lowing acid digestion. The full details of the tholei ite (IAT) fi elds. All upper-crustal rocks Furnes, 1990; Dilek et al., 1997). procedure are given in Longerich et al. (1990). of the Solund-Stavfjord ophiolite complex plot Sheeted dikes in the Solund-Stavfjord ophio- within or very close to the mantle array in the lite complex form parallel to subparallel dike Solund-Stavfjord Ophiolite Complex Th/Yb–Nb/Yb diagram (Fig. 8C). The lavas swarms, with their thicknesses ranging from Dikes and Lavas and dikes of the backarc-forearc ophiolites, on extremely thin (1.5 cm dikelets) up to ~2 m, the other hand, defi ne a much wider geochemi- with an average thickness of ~70 cm (Ryttvad Figure 5 shows a selection of Bowen dia- cal range, and those of the forearc ophiolites (t) et al., 2000). The grain size of the tabular bod- grams (SiO 2, TiO 2, Al 2O3, FeO , V, Cr, Zr, and Y generally plot in the boninite fi eld. ies varies from fi ne grained to medium grained. vs. MgO) for the main types of suprasubduction- We show the fracture zone–related Tviberg Dark, dense, fi ner-grained aphanitic basalt dike zone ophiolites. The suprasubduction-zone–type dikes and gabbros (Fig. 2) (Skjerlie et al., 1989; swarms intrude into and between the coarser- ophiolites that developed in an extended back- Skjerlie and Furnes, 1990), representing domain grained diabase dikes. In places, these basaltic arc-forearc region (e.g., Mirdita, Albania; Dilek 3 in the Solund-Stavfjord ophiolite complex dikes form dike swarms with one-sided chilled et al., 2008) show the largest geochemical range, (Dilek et al., 1997), separately in Figure 9. The margins. Up to six successive dike injections whereas those formed strictly in a forearc setting REE patterns of these dikes and gabbros defi ne have been observed within the dike complex (e.g., Betts Cove, Newfoundland; Bedard, 1999) a large compositional range from fl at to LREE- (Dilek et al., 1997). include the most MgO-rich lavas and dikes enriched patterns, with heavy (H) REEs (17–58 × Gabbro occurrences vary from fine- to (boni nites). Compared to the backarc-forearc chondrite) and LREEs (28–360 × chondrite), medium-grained, vari-textured gabbros. Fine- and forearc suprasubduction-zone ophiolites, the and the most fractionated samples demonstrate grained gabbro occurs as irregular bodies, locally composition of the Solund-Stavfjord ophiolite weak to moderate negative Eu anomalies (Fig. intruded by regular to irregular dikes, defi ning a complex lavas and dikes defi nes a rather narrow 9A). Thus, the REE patterns of Tviberg dike and root zone of the sheeted dike complex (Peder- MgO range (mostly between 5 and 9). The con- gabbro samples differ signifi cantly from those sen, 1986; Nicolas and Boudier, 1991; Dilek centration ranges of TiO 2, V, Cr, Zr, and Y show of the other Solund-Stavfjord ophiolite complex and Eddy, 1992). Primary plagioclase, clino- large variations as a result of different degrees of dikes and lavas (as shown in Fig. 6A). Figure pyroxene, and magnetite are partly preserved in partial melting, mixing, and fractional crystalli- 9B shows the MORB-normalized multi-element these isotropic gabbros, but the dominant min- zation of magmas (Furnes et al., 2006). patterns of the Tviberg dikes and gabbros. In erals include the lower-greenschist-facies asso- Figure 6 displays REE patterns of the Solund- general, there is a steady increase in the normal- ciation, consisting mainly of actinolite, epidote, Stavfjord ophiolite complex lavas and dikes ized ratios from the least (Lu) to the most (Cs) albite, chlorite, and leucoxene. compared with those of backarc-forearc and incompatible elements. Most of the samples dis- forearc ophiolites. The Solund-Stavfjord ophio- play negative Nb and Sr anomalies, and all sam- GEOCHEMISTRY lite complex metabasaltic rocks defi ne rather ples defi ne positive Pb anomalies. Chromium fl at to slightly upward-convex patterns between defi nes a large range from MORB-normalized We present here new geochemical data from Lu and Pr, whereas La and Ce are slightly de- values varying from 1.1 to 0.015 (Fig. 9B), the Solund-Stavfjord ophiolite complex (Table pleted (La: 6–35, Gd: 15–50, Lu: 12–40 times suggesting that the majority of the samples are DR1 1) and the associated tectonostratigraphic chondrite). The forearc types invariably show strongly fractionated, as also demonstrated by units (Table DR2 [see footnote 1]), and com- characteristic, saddle-shaped REE patterns, the negative Sr and Ti anomalies. Some samples pare the geochemical fi ngerprint of the Solund- whereas the backarc-forearc ophiolites dis- also have very high values of Cs and Th. Stavfjord ophiolite complex with the different play a much broader range. Figure 7 shows types of suprasubduction-zone ophiolites that MORB-normalized multi-element diagrams Associated Tectonostratigraphic Units are discussed in Dilek and Furnes (2011). for the Solund-Stavfjord ophiolite complex, Analytical Procedures and for backarc-forearc and forearc ophiolites, Figure 10 shows chondrite-normalized REE with their average values also depicted (Fig. diagrams of mafi c rocks in different tec tono- Major- and trace-element (V, Cr, Co, Ni, Cu, 7D). Figures 7A through 7C display the same stratigraphic units that are spatially and tem- Zn, Rb, Sr, Y, Zr) analyses were performed on patterns for the REEs, i.e., pronounced enrich- porally associated with the Solund-Stavfjord an X-ray fluorescence spectrometer (XRF). ment of Pb, Cs, and Th, and depletion of Nb. ophiolite complex, as discussed earlier. Intru- The glass-bead technique of Padfi eld and Gray The forearc-type ophiolites demonstrate the sive rocks and lavas from the Heggøy Forma- (1971) was used for the major elements and most prominent enrichments of Pb, Cs, and Th, tion (Fig. 3) show fl at REE patterns and are pressed-powder pellets for the trace elements, whereas the Solund-Stavfjord ophiolite com- slightly depleted in LREEs, making them indis- using international basalt standards with recom- plex exhibits the least. tinguishable from those of the Solund-Stavfjord mended or certifi ed values from Govin daraju In Figure 8, the geochemical data from vari- ophiolite complex rocks (Figs. 10A and 10B). (1994) for calibration. Inductively coupled ous suprasubduction-zone type ophiolites are However, the REE patterns of the lavas from the plasma–mass spectrometer (ICP-MS) was plotted in selected discrimination diagrams. Hersvik and Smelvær Units are distinctly dif- In the V-Ti diagram (Fig. 8A), the Solund- ferent, showing a continuous increase from Lu Stavfjord ophiolite complex lavas and dikes through La, with La contents up to ~200 × chon- 1GSA Data Repository item 2012218, Tables DR1 and DR2, is available at http://www.geosociety.org fall within the fi eld defi ned by Ti/V ratios be- drite (Figs. 10C and 10D). Multi-element dia- /pubs/ft2012.htm or by request to editing@geosociety tween 20 and 50, typical of MORB (Shervais, grams for the Heggøy Formation and Hersvik .org. 1982). In the Zr/Y–Zr diagram (Fig. 8B), the and Smelvær Units are shown in Figure 11. The

1034 Geological Society of America Bulletin, July/August 2012 The Solund-Stavfjord ophiolite, western Norwegian Caledonides greenstone samples from the Heggøy Formation 100 show a fl at pattern with near-MORB concentra- tions, and one of the samples shows a minor negative Nb anomaly (Fig. 11A). The green- stone samples from the Hersvik and Smelvær 10 Units all display increasing rock/MORB ratios toward the more compatible elements (Figs. Rock/Chondrite 11B and 11C), but those from the Hersvik Unit A. SSOC (dikes and lava) defi ne pronounced negative Nb anomalies. The 1 major geochemical differences among the vol- La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu canic rock samples from various units are well demonstrated in the Zr-Nb diagram (Fig. 12). 100 The volcanic rocks of the Smelvær Unit are dis- tinctly different from those of the Hersvik Unit by their much higher Nb at a given Zr content, although their REE patterns are indistinguish- able (Figs. 10A and 10B). The bimodal Zr-Nb 10 character of the olistolithic blocks in the Kalvåg mélange (Fig. 12) suggests that these magmas Rock/Chondrite may have been generated in different tectonic B. SSZ ophiolites - backarc to forearc environments. 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu TECTONIC EVOLUTION OF THE SOLUND-STAVFJORD 10 OPHIOLITE COMPLEX BACKARC OCEANIC CRUST

Early Ordovician Ophiolite and 1 Island-Arc Development Rock/Chondrite In this section, we present a brief overview of C. SSZ ophiolites - forearc the historical geology of the older, Early Ordovi- 0.1 cian ophiolites and island-arc occurrences in the La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu western Norwegian Caledonides in order to pro- vide a regional geological background for our Figure 6. Chondrite-normalized rare earth element (REE) patterns of: (A) dikes and lavas of interpretation of the development of the Solund- the Solund-Stavfjord ophiolite complex (SSOC); (B) Mirdita (Albania) and Kizildag (Turkey) Stavfjord ophiolite complex backarc oceanic ophiolites (suprasubduction zone [SSZ]–backarc to forearc), where the light-gray lines repre- crust and associated tectonostratigraphic units. sent light (L) REE–depleted samples, and the dark-gray lines represent LREE (La, Ce, Nd)–en- Prior to the development of the backarc basin in riched samples; and (C) Betts Cove (Newfoundland) ophiolite (suprasubduction zone–forearc). which the Solund-Stavfjord ophiolite complex For data sources: See Figure 5. Chondrite data (in ppm): La = 0.2347, Ce = 0.6032, Pr = 0.0891, and these other units formed, the Early Ordo- Nd = 0.4524, Sm = 0.1471, Eu = 0.056, Gd = 0.1966, Tb = 0.0363, Dy = 0.2427, Ho = 0.0556, Er = vician oceanic crust and island-arc complex of 0.1589, Tm = 0.0242. Yb = 0.1625, Lu = 0.0243 (Anders and Grevasse, 1989). the Iapetus Ocean, as represented by widespread remnants in (see Pedersen and Furnes, 1991), were already accreted into a con- West Karmøy igneous complex contain inher- placed as stitching plutons after the obduction of tinental margin (Pedersen et al., 1992). Based on ited zircons as old as 2491 Ma, which together the Early Ordovician ophiolites into the Lauren- the faunal characteristics (Toquima–Table Head with other geochemical evidence suggests that tian margin so that the granitoid magmas were faunas) of the sedimentary rocks overlying the they were generated from melting of sediments able to acquire the Archean–Early Proterozoic Early Ordovician (Arenig-Llanvirn) ophiolites partly derived from an Archean–Early Protero- zircons from a Laurentian provenance. (Bruton and Bockelie, 1980; Stephens and Gee, zoic province (Pedersen and Dunning, 1997; The timing of the emplacement of the Early 1985), Pedersen et al. (1988, 1992) proposed that Hamnes, 1998). The nearest Archean province Ordovician ophiolite and island-arc complex to these ophiolites were accreted into the margin of for these zircons is ~1200 km farther to the the Laurentian margin is not well constrained. the Laurentian craton in the Middle Ordovician. north on the Baltic Shield, whereas the Archean An ~1000-m-thick sequence of subaerial, 473 ± Further evidence for the Laurentian affi nity of terrane of northwest (Pedersen et al., 2 Ma, calc-alkaline volcanic rocks (basalts, ba- the Early Ordovician ophiolite–island-arc ter- 1992) and the Archean–Proterozoic terrane of saltic andesites, andesites, and rhyolites) on the rane of western Norway comes from a granitoid East Greenland (e.g., Higgins et al., 2004), both island of Bømlo in western Norway (Nordås intrusion in the West Karmøy igneous complex of which were part of the Laurentian craton, are et al., 1985) rests unconformably on 494 ± 2 Ma that has yielded a crystallization age of 474 ~300 km to the west. Therefore, we infer that submarine island-arc tholeiitic lavas (Peders en +3/–2 Ma (U/Pb zircon dating; Pedersen and the Middle Ordovician granitoid intrusions of and Dunning, 1997). This subaerial vol canic Dunning, 1997). The granitic intrusions of the the West Karmøy igneous complex were em- unit may mark the onset of continental-arc

Geological Society of America Bulletin, July/August 2012 1035 Furnes et al.

100 Late Ordovician Rift-Drift and A. SSOC (dikes & lava) Seafl oor Spreading Evolution of the 10 Solund-Stavfjord Ophiolite Complex Backarc Basin 1

Rock/MORB The Late Ordovician marginal basin devel- 0.1 oped initially by rifting via trench-slab rollback of the active continental margin of Laurentia. 0.01 By Ashgill time, the Early Ordovician ophio- Cs Th Nb La Ce Pb Pr Nd Zr Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Sc Cr lites were deeply eroded and covered by thick 100 sedimentary deposits, related to the formation B. SSZ ophiolites – backarc to forearc (BA-FA) of active, fault-bounded basins (Thon, 1985). 10 Magmatic rocks of this age (around the Ordo- vician-Silurian boundary) occur throughout the 1 Upper Allochthon of the Norwegian Caledo- nides (see Pedersen et al., 1992). However, Rock/MORB 0.1 only the S olund-Sunnfjord marginal basin de- veloped into an oceanic backarc basin, with its 0.01 well-developed seafl oor spreading system, and Cs Th Nb La Ce Pb Pr Nd Zr Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Sc Cr island-arc and off-axis volcanism. The internal structure and geochemical signa- 100 C. SSZ ophiolites – forearc (FA) tures of various crustal subunits in the Solund- Stavfjord ophiolite complex indicate that the 10 Late Ordovician ophiolite developed during at least two phases of oceanic spreading (Skjerlie 1 and Furnes, 1990; Dilek et al., 1997; Furnes

Rock/MORB et al., 1998). The NE-trending domain 1 rep- 0.1 resents a remnant of an intermediate-spreading mid-ocean ridge that propagated northeastward 0.01 Cs Th Nb La Ce Pb Pr Nd Zr Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Sc Cr (in present coordinate system) into a preexisting oceanic crust (domain 2), which had developed 100 along a NW-trending spreading system earlier D. Comparison SSOC with BA-FA & FA in the same basin. This crosscutting relationship

10 between the two domains, representing differ- ent seafl oor spreading episodes, is best exposed on the island of Tviberg (Fig. 2), where younger 1 sheeted dikes of domain 1 intrude into a rem-

Rock/MORB nant of the older oceanic crust (sheeted dike 0.1 complex and gabbro of domain 2) (Fig. 2). The Average SSZ ophiolites – forearc NE-oriented domain 3 represents a paleoceanic Average SSZ ophiolites – backarc to forearc Average SSOC (dikes & lava) fracture zone associated with the fi rst episode of 0.01 Cs Th Nb La Ce Pb Pr Nd Zr Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Sc Cr seafl oor spreading within the basin. Domains 1 and 2 of the Solund-Stavfjord Figure 7. Mid-ocean-ridge basalt (MORB)–normalized multi-element diagrams of: (A) dikes ophio lite complex consist of well-preserved pil- and lavas of the Solund-Stavfjord ophiolite complex (SSOC); (B) Mirdita (Albania) and lowed and massive lava fl ows, hyaloclastites, Kizildag (Turkey) ophiolites (suprasubduction zone [SSZ]–backarc to forearc); (C) Betts sheeted dikes, and isotropic gabbros (Figs. 2 Cove (Newfoundland) ophiolite (suprasubduction zone–forearc); and (D) average composi- and 4). The extrusive sequence ranges in thick- tions of the three aforementioned types. MORB data (in ppm): Cs = 0.007, Th = 0.12, Nb = ness from ~470 m to 800 m. Pillow lava sequences 2.3, La = 2.5, Ce = 7.5, Pb = 0.3, Pr = 1.32, Nd = 7.3, Zr = 74, Sm = 2.63, Gd = 3.68, Ti = 7614, locally show development of megacycles char- Tb = 0.67, Dy = 4.55, Y = 28, Ho = 1.01, Er = 2.97, Tm = 0.456, Yb = 3.05, Lu = 0.455, V = 300, acterized by extrusion of large (mega) pillows at Sc = 40, Cr = 275 (Pearce and Parkinson, 1993). the base, overlain by lavas with decreasing pil- low sizes that are in turn capped on top by pillow breccias or hyaloclastites. We interpret each of magmatism prior to, or synchronous with the margin during the Middle to Late Ordovician, these pillow lava units as the product of a dis- emplacement of the Early Ordovician ophiolite predating the Taconic orogeny by 5–15 m.y. tinct eruptive event (Furnes et al., 2001, 2003). and island-arc complex to the Laurentian conti- (van Staal et al., 2007, 2009; Zagorevski et al., Massive lavas occur mainly in Stavestranda and nental margin, and may indicate timing of a sub- 2009). Underthrusting of ophiolites following Alden (Fig. 2, lower right inset; Fig. 4E) and duction polarity fl ip. This timing corresponds to this polarity fl ip and the initiation of a west- defi ne volcanic eruptive centers (Furnes et al., the obduction of the Early Ordovician arc and dipping subduction took place during Middle 2003). Farther to the south in the Solund area backarc terranes to the Laurentian continental Ordovician (Zagorevski et al., 2007, 2009). (between Langøy and Ytrøy), pillow lavas are

1036 Geological Society of America Bulletin, July/August 2012 The Solund-Stavfjord ophiolite, western Norwegian Caledonides

the dominant extrusive rock type (Fig. 2) and 1000 are most commonly aphyric; plagioclase-phyric A Boninite SSZ (FA) pillow lavas (up to10%) are relatively rare. The 800 SSZ (BA-FA) pillow and massive lava rocks are sparsely vesic- SSZ (OBA) - SSOC ular to nonvesicular, suggesting eruption in a deep-water environment. Tholeiitic pillow lavas 10 20 30 600 on the Pacifi c Ocean fl oor near Hawaii (Moore,

V 1965) have nearly zero vesicularity at an erup- 400 tion depth of ~4 km, indicating that lavas extrud- ing in deep water do not commonly acquire a 50 vesicular texture. The Solund-Stavfjord ophiolite 200 complex pillow lavas are geochemically similar to those reported by Moore (1965) from the Pa- 0 cifi c, and hence we infer that they likely erupted 0 5000 10000 15000 20000 25000 at great depths, approaching more than 3 km be- Ti low sea-level.

10 The depth of active spreading ridges varies B as a function of spreading rate (Kasting et al., WPB 2006). Fast-spreading ridges may reach depths SSZ (FA) of 2.5–2.9 km below sea level (Bischoff, 1980), Boninite whereas slow-spreading ridges commonly occur SSZ (BA-FA) at depths of 3.5 km or more (van Damm, 1990). Spreading rates in modern backarc basins vary SSZ (CBA)

Zr/Y Zr/Y from slow (<50 mm/yr) to fast (>100 mm/yr), SSZ (OBA) - and their water depths change considerably SSOC MORB (Taylor and Martinez, 2003) from ~1500 m to >5000 m (Stüben et al., 1998; Leat et al., 2000; Chung-Hwa et al., 1990; Keller et al., 2008). IAT Nearly 300 km off the coast of Burma and 1 ~100 km SE of its spreading axis, the depth of 1 10 100 1000 Zr the continent-proximal Andaman Sea backarc basin reaches 2360 km (core MD77–169, see 10 table 1 in Colin et al., 1999). We propose that the C Andaman Sea backarc basin is a viable modern analogue for the Late Ordovician Solund-Stav- OIB fjord ophiolite complex backarc basin. Using the existing information on the occur- 1 Volcanic arc array rence and distribution of pillow versus mas- sive lava fl ows at modern spreading centers, MORB - OIB array Bonatti and Harrison (1988) established a re- Th/Yb Th/Yb lationship between the spreading rate and the 0.1 E-MORB proportion of pillow to massive lavas. They concluded that pillow lavas are predominant SSZ (BA-FA) at slow-spreading mid-ocean ridges, whereas SSZ (OBA) - SSOC N-MORB SSZ (FA) massive lavas are generally more widespread at fast-spreading mid-ocean ridges. Applying this 0.01 0.1 1 10 100 empirical relationship to the Solund-Stavfjord Nb/Yb ophiolite complex extrusive sequence and con- sidering the internal structure of the ophiolite, Figure 8. Lava and dike rocks from suprasubduction-zone (SSZ) ophiolite types (backarc- we infer an intermediate spreading rate for the forearc [BA-FA], forearc [FA], continental backarc [CBA], and oceanic backarc [OBA] magmatic evolution of the Solund-Stavfjord represented by the Solund-Stavfjord ophiolite complex [SSOC]), plotted in various dis- ophiolite complex (Dilek et al., 1997; Furnes crimination diagrams. (A) Ti-V plot (after Shervais, 1982). The Ti/V ratios are charac- et al., 2003). Combining the vesicularity-depth teristic of the following: 10–20 = island arc; 20–50 = mid-ocean-ridge basalt (MORB); relationships outlined herein and the spreading 20–30 = mixed MORB and island arc; 10–50 = backarc basins; boninite fi eld. (B) Zr–Zr/Y rate estimates for the Solund-Stavfjord ophio- plot (after Pearce, 1980; modifi ed by Furnes et al., 2007). WPB—within-plate basalt; lite complex, we think a realistic water depth IAT—island arc tholeiite. (C) Nb/Yb–Th/Yb plot (after Pearce, 2008). Suprasubduction for the Solund-Stavfjord ophiolite complex zone data: See Furnes et al. (2006, and references therein) and Dilek and Furnes (2011, backarc basin is around 2500–3000 m, which and references therein). is in the middle of the depth range for modern backarc basins.

Geological Society of America Bulletin, July/August 2012 1037 Furnes et al.

1000 A. Tviberg (dikes & gabbro)

100

Rock/Chondrite 10

Figure 9. (A) Rare earth element (REE) and (B) mid-ocean-ridge basalt (MORB)–normalized 1 multi-element patterns of dikes La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu and gabbro from Tviberg (So- 1000 lund-Stavfjord ophiolite com- B. Tviberg (dikes & intrusions) plex). Location: See Figures 1 100 and 2.

10

1 Rock/MORB

0.1

0.01 Cs Th Nb La Ce Pb Pr Sr P Nd Zr Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Sc Cr

ε The Solund-Stavfjord ophiolite complex samples) shows that the Nd values display mi- mélange at this time signals an episode of sig- shows subduction infl uence in its geochemi- nor variation (7.77–8.42), with an average value nifi cant shortening across the basin as a result ε cal fi ngerprint, indicated by weak to moderate of 8.14 ± 0.2 (Furnes et al., 2006). This Nd value of its ongoing closure, similar to other supra- enrichment of Cs, Pb, and Th, and depletion of plots within the array of the depleted mantle ophiolitic mélange occurrences in the Tethyan Nb in its dike and lava rocks. However, com- growth curve (at 443 Ma) and hence suggests realm (Dilek et al., 2007). The presence in this pared to the trench-proximal forearc ophiolites that the Solund-Stavfjord ophiolite complex mélange of blocks of boninite cannot easily be (e.g., Mirdita, Albania; see Dilek et al., 2008), magmas were not contaminated by continental explained by their derivation from the Solund- the Solund-Stavfjord ophiolite complex meta- crust even though the Solund-Stavfjord ophio- Stavfjord ophiolite complex, as crustal units basalts display typical MORB to WPB charac- lite complex marginal basin evolved adjacent to with boninitic affinities are lacking in the ter in discriminant diagrams (Figs. 7–9). The a continent. Solund-Stavfjord ophiolite complex. However, geochemistry of the dikes and gabbros at Tvi- An island-arc system was constructed in boninitic rocks together with island-arc tholei- berg is in general more enriched in the incom- front of the Solund-Stavfjord ophiolite com- itic basalts are abundant in the older Early patible elements compared with the rest of the plex marginal basin and above the retreating Ordo vician ophiolites and island-arc units. We, ophiolite (Fig. 9). This is a feature attributed to trench-slab by the Late Ordovician–Early Si- therefore, infer that the Kalvåg mélange was re- the fracture zone tectonic setting in which this lurian. The Hersvik Unit in the western Nor- ceiving material from these older ophiolites and part of the ophiolite was formed (Skjerlie and wegian Caledo nides represents the remnants island-arc units exposed on the seafl oor, consis- Furnes, 1990; Furnes et al., 1998). However, of this arc complex (Fig. 13). Alkaline lavas tent with our interpretation that they were once although the incompatible trace elements are of the Smelvær Unit formed as part of an off- making up part of the substratum of the Late more enriched due to smaller degrees of partial axis magmatic phase that may have eventually Ordo vician island-arc complex (Fig. 13A). melting of a less-depleted source (Furnes et al., developed into ocean islands within this back- The presence of continentally derived sedimen- 1990), the weak subduction signal is still visible arc basin. The nonvesicular pillow lavas of tary rocks (phyllite and graywacke), inter calated in the multi-element diagram, as shown by the the Smelvær Unit (on the island of Smelvær; with the uppermost volcanic rocks of the Solund- enrichment of Pb, Th, and Cs, and depletion in Fig. 1) are inferred to represent submarine vol- Stavfjord ophiolite complex, and also conform- Nb (Fig. 9). Therefore, we infer that the Solund- canic units erupted in deep water. The Kalvåg ably covering the complex (Heggøy Formation), Stavfjord ophiolite complex metabasalts formed mélange developed as a supra-ophio litic mé- suggests that the Solund-Stavfjord ophiolite from magmas that were produced in a trench- lange overlying the Solund-Stavfjord ophiolite complex basin evolved adjacent to a continent distal suprasubduction-zone setting, which was complex with olisto lith blocks of alkaline and (Fig. 13A). The detrital zircon and titanite ages suffi ciently far from the subducting slab to have calc-alkaline rocks that were derived from both of 2495–462 Ma from the clastic sedimentary only minor infl uence of slab-derived fl uids in the Hersvik and Smelvær Units (Fig. 13). Thus, rocks of the Heggøy Formation indicate that the their melt evolution (Fig. 13). An extensive Nd- the Kalvåg mélange is the only linkage between detrital material was from a Precambrian shield isotope study of the metabasalts of the Solund- the Solund-Stavfjord ophiolite complex and the as well as from an Ordovician Caledonian terrane Stavfjord ophiolite complex (51 lava and 4 dike Smelvær Unit. The development of the Kalvåg (Peder sen and Dunning, 1993).

1038 Geological Society of America Bulletin, July/August 2012 The Solund-Stavfjord ophiolite, western Norwegian Caledonides

1000 There is no direct radiometric age data for the A. SSOC (dikes and lava) timing of the closure of the Solund-Stavfjord Staveneset, Lågøy, Oldra 100 ophiolite complex backarc basin. However, on the island of Atløy (Fig. 1), the Sunnfjord sub- ophiolitic mélange locally rests along a strati- 10 graphic (unconformable) contact on the rocks Rock/Chondrite of the Middle Silurian Herland Group, which 1 represents the uppermost sedimentary cover of La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu the crystalline basement of Baltica (Andersen et al., 1990) (Fig. 1). Hence, we deduce that the 1000 B. Heggøy Formation emplacement of the Solund-Stavfjord ophio lite complex onto the sedimentary cover of the 100 Western Gneiss Region (Fig. 13) might have occurred during Wenlock (428–423 Ma) time, when the sediments of the Herland Group were 10 being deposited (Andersen et al., 1990, 1998; Rock/Chondrite Pedersen et al., 1992). In addition, geochemi- 1 cal modeling of the 434 Ma Sogneskollen La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu granite complex along the northern edge of the 1000 Sognesjøen (Hacker et al., 2003) (Fig. 1) sug- C. Hersvik Unit gests that its genesis is compatible with partial melting of the granitoid rocks of the Dalsfjord 100 Suite (Fig. 1), graywackes, and interlayered volcaniclastic rocks of the Sunnfjord sub- 10 ophiolitic mélange at pressures of ≤10 kbar

Rock/Chondrite and temperatures close to or below 800 °C (Skjerlie et al., 2000). We, therefore, favor the 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu interpretation that the tectonic emplacement of the Solund-Stavfjord ophiolite complex onto 1000 the Baltica continental margin was already in D. Smelvær Unit progress by 430 Ma.

100 These temporal relationships suggest that the life span of the Solund-Stavfjord ophiolite com- plex backarc basin from the development of its 10 ocean fl oor to the tectonic emplacement of its Rock/Chondrite oceanic crust onto the Baltica continental mar-

1 gin (marking the fi nal stages of basin closure) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu was in the order of 10–20 m.y. (Fig. 13B). This time span is largely consistent with the forma- Figure 10. Rare earth element (REE) patterns of lavas and dikes from (A) Solund-Stav- tion and the subsequent tectonic accretion of fjord ophiolite complex (SSOC), (B) Heggøy Formation, (C) the Hersvik Unit, and (D) the ophiolites and arc-backarc complexes (5–10 Smelvær Unit. m.y.) in the Canadian Appalachians during the Taconic orogeny (Lissenberg et al., 2005; Malo et al., 2008), as well as with the inferred time Time Constraints for the Development of landslide deposits in the southern part of the brackets of the Tethyan marginal basins (Dilek the Solund-Stavfjord Ophiolite Complex Hersvik area (Fig. 2), likely marks an advanced et al., 1999; Dilek and Moores, 1990; Dilek and Associated Units stage of the formation of the Hersvik Unit as et al., 2007, 2008). the upper crust of the island arc (Fig. 13A). The The available time constraints for the devel- 443 Ma calc-alkaline Gåsøy intrusion (Fig. 1) Paleogeography of the Solund-Stavfjord opment of the oceanic crust of the Solund-Stav- may mark an early stage of the island-arc devel- Ophiolite Complex Backarc Basin fjord backarc basin, the island-arc construction opment. These limited age data suggest that the (Hersvik Unit), and the off-axis volcanism island-arc construction and the backarc spread- The magmatic events along the eastern mar- (Smelvær Unit) are limited. A diorite rock from ing of the Solund-Stavfjord ophiolite complex gin of the Iapetus Ocean were highly different domain 1 in the Solund-Stavfjord ophiolite may have occurred simultaneously. We can and diachronous as recorded in the Caledonian– complex on Tviberg (Fig. 2) yielded an age of deduce the age of the Smelvær Unit indirectly, Appalachian orogenic belt (Fig. 14, upper left). 443 Ma for the youngest crust of the ophiolite, based on stratigraphic relations. The Kalvåg The Caledonides in Eastern Greenland display but there is no age constraint from the oldest do- supra-ophiolitic mélange contains material de- a sedimentary record of a long-lived (Neo- main representing the earliest seafl oor-spread- rived from the Smelvær Unit and is intruded by protero zoic to early Paleozoic) passive-margin ing product of the Solund-Stavfjord ophiolite the 443 Ma Gåsøy gabbronorite (Fig. 1). Hence, evolution (mainly carbonates) on this part of the complex. The 439 Ma rhyolite, occurring in the the Smelvær Unit should be 443 Ma or older. Laurentian Shield (Henriksen, 1985; Andresen

Geological Society of America Bulletin, July/August 2012 1039 Furnes et al.

100 A. Heggøy Formation

10

1 Rock/MORB

Figure 11. Mid-ocean-ridge ba- 0.1 Th Nb La Ce Pr Nd Zr Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Co Cr salt (MORB)–normalized multi- element patterns of metabasalts 100 from (A) the Heggøy Formation, B. Hersvik Unit (B) the Hersvik Unit, and (C) the

Smelvær Unit. All the Solund- 10 Stavfjord ophiolite complex data are in Table DR1 (see text foot- note 1); the data from the Heg-

Rock/MORB 1 gøy Formation, the Smelvær and Hersvik Units are from Table DR2 (see text footnote 1) and un- 0.1 published data (H. Furnes, 1988, Th Nb La Ce Pr Nd Zr Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Co Cr personal obs.), and all the data from the Kalvåg mélange are 100 from Ravnås and Furnes (1995). C. Smelvær Unit

10

Rock/MORB 1

0.1 Th Nb La Ce Pr Nd Zr Sm Eu Gd Ti Tb Dy Y Ho Er Tm Yb Lu V Co Cr and Hartz, 1998; Andresen et al., 1998; Hig- and quartz diorites that are interpreted as part of Subduction Polarity during Formation gins and Leslie, 2000; Higgins et al., 2004). magmatic arc development during the subduc- of the Solund-Stavfjord Ophiolite The Canadian Appalachians farther south, on tion of the Iapetus oceanic lithosphere beneath Complex Backarc Basin the other hand, demonstrate a more complicated Laurentia-Greenland (Higgins et al., 2004). The magmatic and tectonic evolutionary path by 435–425 Ma granites and migmatites in Eastern We infer a westward-dipping (in present co- comparison along the same western side of the Greenland (Fig. 14, panel A2) indicate a much ordinate system) subduction zone during the closing Iapetus Ocean. more robust and intrusive magmatic activity rift-drift stage of the Solund-Stavfjord ophiolite By the end of the Ordovician, the Iapetus than in the Early to Middle Ordovician (Watt complex marginal basin in our tectonic model Ocean had experienced a protracted period of et al., 2000; Kalsbeek et al., 2001; Higgins (Fig. 13A). This inferred tectonic setting of the contraction, and the width of the ocean basin (at et al., 2004). This is in strong contrast to the Solund-Stavfjord ophiolite complex basin is the latitude of the Solund-Stavfjord ophiolite Middle-Late Cambrian and Ordovician mag- analogous to the geodynamics of the modern complex) was in the order of 500–600 km, as matic/tectonic history of the North American Andaman Sea (Furnes et al., 1990, 2000). inferred from the paleogeographic reconstruc- Laurentian margin (Figs. 14C and 14D). While The complex tectonic and magmatic history tion of Blakey (2011) (Fig. 14, upper right). At the northern part of the Laurentian margin in of the Canadian Appalachians during the Late this stage in the closure of the Iapetus Ocean, Greenland was undergoing limited magmatism Cambrian through Silurian has been discussed the magmatic and tectonic development of the and tectonic quiescence, the central part of the in detail by Zagorevski and van Staal (2011). orogen, from Newfoundland to central-east Laurentian margin at the latitude of the Cana- These authors proposed a westward-directed Greenland (a lateral distance in the order of dian Appalachians was experiencing continen- subduction at the late stages of the closure of ~3000 km), was profoundly different and dia- tal extension and multiple episodes of backarc the Iapetus Ocean (ca. 455–440 Ma). The mag- chronous (Figs. 14A–14D). There is only scant basin formation, leading to the generation of matic rocks of this age span in the Canadian ε evidence of magmatic activity in the southern microcontinents and subsequent collisional Appa lachians show a wide range in their Nd part of the East Greenland Caledonides in the events (arc-continent, continent-continent) values (Whalen et al., 1996,1997, 2006; Kurth time period of 466–360 Ma. The earliest mag- (e.g., Ryan and Dewey, 2004; Zagorevski et al., et al., 1998; Kurth-Velz et al., 2004; Zagorevski matic activity in this region is represented by 2009; van Staal et al., 2009; Zagorevski and et al., 2006; van Staal et al., 2007), which are all ca. 466 Ma, I-type calc-alkaline granodiorites van Staal, 2011). lower than that of the time-appropriate depleted-

1040 Geological Society of America Bulletin, July/August 2012 The Solund-Stavfjord ophiolite, western Norwegian Caledonides

120 mantle growth curve (Fig. 14). This isotopic fea- Kalvåg Melange - olistoliths ture implies variable but signifi cant amounts of Kalvåg Melange - debris old, recycled crustal material involved in their Hersvik Unit 100 melt production. The most likely crustal source Smelvær Unit for this feature is the Grenvillian Laurentian Heggøy Fm continental margin (e.g., van Staal et al., 2007). SSOC 80 Similarly, many of the Cambrian–Early Ordovi- cian ophiolite and island-arc rocks in the western Norwegian Caledo nides also show relatively low ε Nd values (Fig. 14). The West Karmøy igneous 60 Nb complex, which developed after the ophiolite emplacement, displays even lower values than any of the magmatic rocks in the Canadian 40 Appalachians (Pedersen and Hertogen, 1990; Pedersen and Dunning, 1997), indicating the in- tegration of older crustal material into the mantle 20 source. This inference is compatible with our in- terpretation, suggesting that the Cambrian–Early Ordovician ophiolites and island-arc units were originally emplaced onto the southeastern mar- 0 0 100 200 300 400 500 600 gin of Greenland (Fig. 14). On the contrary, the ε Zr Nd values of the metabasalts from the Solund- Stavfjord ophiolite complex defi ne only minor Figure 12. Diagram showing Zr-Nb plots for magmatic rocks of the variations, and they generally plot within the ar- Solund-Stavfjord ophiolite complex (SSOC), the Heggøy Formation, ray defi ned as the depleted-mantle growth curve ε the Smelvær and Hersvik Units, and the Kalvåg mélange. (Fig. 14). However, the Nd values of the younger

A Solund - Stavfjord backarc basin

?

ry prism

Tviberg cretiona Figure 13. (A). Tectonic model Ac Smelvær Kalvåg Hersvik for the development of the So- Unit mélange Unit Older ophiolitesHeggøy Fm lund-Stavfjord ophiolite com- + isl. arcs Older ophiolites Western Gneiss plex (SSOC) and the associated SSOC SSOC Region (Baltica) magmatic and sedimentary Laurentia (undifferentiated) units in a backarc basin, adja- Fractional crystallization in magma chamber cent to the Laurentian continen- ubducting Mixing of melts S tal margin of East Greenland ~ 440 Ma Primary melt Counter-flow Lithosphere (ca. 440 Ma). (B) During the of enriched Mafic to felsic intrusions mantle time span of 430–420 M, the Subduction component large / small Solund-Stavfjord ophiolite com- Sla plex and associated magmatic b re treat and sedimentary units were Kalvåg tectonically emplaced upon the mélange B Upper Smelvær Baltic margin, with concomitant Accreted oceanic terranes of the Caledonides Unit (SU) LAURENTIA BALTICA Tectonic formation of the subophiolitic Island Hersvik (SE Greenland) Unit (HU) Sunnfjord mélange. SM—Sunn- Kalvåg SU Arc SSOC & HF Unit E. Ordovician Oph. mélange (HU) HG Heggøy Fm fjord mélange; HG—Herland SM (HF)(HF) Group; DS—Dalsfjord Suite; SSOC Sunnfjord WGR—Western Gneiss Region. mélange DS HG R WG Middle Tectonic Høyvik Unit Group Mantle lithosphere DS ~425 Ma of Baltica Lower Mantle lithosphere Tectonic WGR of Laurentia Stitching granitic plutons Unit

Geological Society of America Bulletin, July/August 2012 1041 Furnes et al.

Figure 14. (Upper left) Map of the Svalbard A Paleozoic fold belts (Caledonides, BALTICA d C n A n a A G I a a Appalachians, and Variscan) at l l e e C reenland T n I d e c o N T their relative position ca. 300 Ma n e B o L E r i d G A e R (after Henriksen et al., 2008). (Upper B s U s A u right) Paleogeographic map recon- L t e struction at around 450 Ma (after p a ariscan I Blakey, 2011). The section lines A, V C B, C, and D depict the profi le lines LAURENTIA across East Greenland, west Nor- s n

a D i Avalon way, Scotland, and Newfoundland, h c er la nd respectively. (A1–D1) Reconstruc- a a pp AFRICA G tions of the sedimentary, magmatic, A and tectonic evolution in the time A1: ~ 460 Ma E. Greenland A2 : ~ 425 Ma interval ca. 460–440 Ma of the Scandian Laurentia Granitoids Laurentian margin of East Green- Central E. Greenland Cambrian - Ordovician nappes sediments land, Scotland, and Newfound- Meso- /Neoprote Laurentia rozoic ted land. (A2–C2) Reconstructions of Archean to Baltica fferentia Paleoproterozoic Gneiss Compl. undi ltica the collision between Laurentia Iapetus Ba and Baltica. (D2) Reconstruction ocean crust of the collision between Lauren- B1: ~ 450–440 Ma W. Norway B2: ~ 425 Ma Isl. arc (HU) Undiff. tia and Gander. The eight panels Early Ord. SSOC sediment ~500 - 450 Ma SSOC oph. + isl.arc SM covers are modifi cations from: A1—Hig- Oph. + isl.arc basin & isl.arc KM gins et al. (2004); A2—Henriksen SU + Laurentia tica Baltica of Bal et al. (2008); B1—Pedersen et al. Laurentia (SE Greenland) gion s Re neis (1992); Yoshinobu et al. (2002), this W. G work; B2—Hacker et al. (2003, Granitoids 2010), this work; C1 and C2— C1: ~ 460–450 Ma Scotland C2: ~ 425 Ma Leslie et al. (2008); D1—Zagorev- Scandian ski and van Staal (2011); D2—van HBF SUF nappes Acc. prism Laurentia Staal et al. (2008). Abbreviations: MVT Compl.—complex; oph.—ophiolite; Laurentia MVA Baltica Baltica isl.arc—island arc; SM—Sunn fjord BV & TV mélange; SU—Smelvær Unit; KM— Granite Kalvåg Mélange; MVA—Mid- D1: ~ 455–440 Ma Newfoundland D2: ~ 425 Ma land Valley arc; BV and TV—Ben Brunswick LBOT & Vuirich Granite and Tayvallich Ganderia Complex BOVT AAT volcanics; Acc.prism.—accretion- SF T - E Ba Laurentia Gander P ary prism; MVT—Midland Valley margin T Laurentian Crust margin Gander terrane; HBF—Highland Bound- PVA DM ary fault; SUF—Southern Upland Dashwood fault; LBOT—Lushs Bight oceanic 10 tract; BOVT—Baie Verte oceanic Depleted mantle curve tract; AAT—Annieopsquotch ac- 5 cretionary tract; DM—Dunnage 0 mélange; PVA—Popelogan–Victoria –5 Caledonides Norway (CN) arc; SF—Salinic forearc; T-E BA— CN: SSOC-metabasalts CN: SSOC-Isl. Arc rocks Tetagouche-Exploits backarc; P— Epsilon Nd –10 CN: SSOC-Late granites Popelogan arc; T—Tetagouche Caledonides Greenland block. The lowermost diagram –15 Caledonides Scotland Increasing crustal influence Caledonides Ireland shows the relationship between ε Age (m.y.) Appalachian Newfoundland Nd –20 and age for magmatic rocks of the 420 430 440 450 460 470 480 490 500 510 Caledonides (East Greenland, west Norway, and Scotland) and the Appalachians (Newfoundland). The data are from the following sources: Norway (except Solund-Stavfjord ophiolite complex and associated rocks): Barnes et al. (2002, 2005); Pedersen and Hertogen (1990); Hamnes (1998). Solund-Stavfjord ophiolite complex-metabasalts: Furnes et al. (2006); Solund-Stavfjord ophiolite complex–island-arc rocks: Skjerlie (1992); Hansen et al. (2002). Solund- Stavfjord ophiolite complex–late granites: Skjerlie (1992); Skjerlie et al. (2000). E. Greenland: Kalsbeek et al. (2008). Scotland: Kneller and Aftalion (1987); Chew et al. (2007); Clemens et al. (2009). Ireland: Draut et al. (2004). Newfoundland: Whalen et al. (1996, 1997, 2006); Kurth et al. (1998); Kurth-Velz et al. (2004); Zagorevski et al. (2006); van Staal (2007). Depleted mantle growth curve is from DePaolo (1988).

1042 Geological Society of America Bulletin, July/August 2012 The Solund-Stavfjord ophiolite, western Norwegian Caledonides

ε arc-related rocks (the 443 Ma Gåsøy intrusion associated trench system retreated eastward, The Nd values of the Sogneskollen granite com- and the 440 Ma Bremanger granitoid complex), the upper-plate extension led to rift drift and plex vary between –7 and –10, demonstrating its with ages comparable to that of the Solund- then seafl oor spreading tectonics in the open- derivation from melting of the supracrustal rocks Stavfjord ophiolite complex, are signifi cantly ing Solund-Stavfjord ophiolite complex basin, of the Baltic Shield (Skjerlie, 1992; Skjerlie lower (~+1.5 to –2.2; see Skjerlie, 1992; Hansen reminiscent of the geodynamic evolution of the et al., 2000; Hansen et al., 2002). et al., 2002) than those of the meta basalts of the Lau Basin in the western Pacifi c and the Anda- Solund-Stavfjord ophiolite complex (average man Sea in the eastern Indian Ocean. Isolated CONCLUDING REMARKS +8.14). The youngest stitching plutons (the ca. sheets of the Early Ordovician ophiolites and 434 Ma Sogneskollen intrusion and minor gran- island-arc units that were previously emplaced The Late Ordovician (ca. 443 Ma) Solund- ε ite intrusions) defi ne the lowest Nd values of –7 onto the eastern Greenland margin were rifted Stavfjord ophiolite complex and its conform- to –10 (Skjerlie et al., 2000) (Fig. 14). off during the opening of the Solund-Stavfjord able cover sequence (Heggøy Formation) are Continentally derived, old sediments gener- ophiolite complex basin and were preserved part of the Upper Tectonic Unit of the alloch- ε ally have highly negative Nd values (the mag- in the forearc basement of the east-facing Late thonous rock sequence in the western Norwe- nitude depending on the age) and high content Ordovician arc-trench system. These older gen- gian Caledo nides. The calc-alkaline and alkaline of Th (Plank and Langmuir, 1998). When sedi- erations of ophiolitic and arc units were sub- magmatic, volcaniclastic, and sedimentary rocks ments on oceanic lithosphere are subducted to sequently telescoped into the Baltica margin of the Hersvik and Smelvær Units (respec- suffi cient depths, they may partially melt, and during the fi nal closure of the Solund-Stavfjord tively) and the olistostromal Kalvåg mélange the produced melt would percolate into the ophiolite complex basin. The Kalvag and Sunn- are all tectonically associated with the Solund- upper mantle above the subduction zone. The fjord mélanges were developed as supra- and Stavfjord ophiolite complex, and they are also melt product of this hybrid mantle would then subophiolitic mélanges, respectively, during the part of this Upper Tectonic Unit. Collectively, carry the chemical and isotopic signatures from ophiolite emplacement and basin closure stages these allochthonous rock units are thrust over the the subducted sediments (e.g., Pearce et al., in the Middle Ordovician. subophiolitic Sunnfjord mélange and are uncon- 2005). Our data from the Solund-Stavfjord ophi- formably overlain by a Devonian conglomerate. olite complex show no evidence of Th enrich- Emplacement of the Solund-Stavfjord The Solund-Stavfjord ophiolite complex vol- ment in the multi-element diagrams (Fig. 7), and Ophiolite Complex and Associated canic rocks are locally intercalated with phyllites the Nd-isotope data from the Solund-Stavfjord Rocks onto Baltica and are conformably overlain by quartz-rich, ophiolite complex metabasalts give no indica- continentally derived sediments indicating that tion of mixing of the mantle with melts derived The subophiolitic Sunnfjord mélange rests the formation of the Solund-Stavfjord ophiolite from old sediments. These data do not support tectonically on the Wenlock or older continental complex oceanic crust was proximal to a conti- an eastward subduction during the generation of margin sedimentary deposits of Baltica (Alsaker nental margin. the Solund-Stavfjord ophiolite complex backarc and Furnes, 1994). The protoliths of various The Solund-Stavfjord ophiolite complex basin (Andréasson et al., 2003), in which case schist types in the Sunnfjord mélange consist of oceanic crust, with its well-preserved deep- ε one would expect much lower Nd values as well graywackes, volcaniclastic rocks with slivers of water volcanic rocks, sheeted dike swarms, and as higher Th contents for the Solund-Stavfjord massive greenstones (pillow lavas and dikes), and gabbros, formed during two episodes of diach- ε ophiolite complex metabasalts. The lower Nd gabbros. The provenance of these rocks in the ronous seafl oor spreading within a marginal values of the coeval arc-related rocks (Fig. 14) mélange was likely the Late Ordovician island basin. The tholeiitic Solund-Stavfjord ophio- are likewise inconsistent with derivation of their arc (Hersvik Unit). Blocks of greenstone and lite complex lavas and dikes are predominantly melts during eastward subduction. gabbroic rocks are either scraped-off fragments Fe-Ti basalts showing an N-MORB geochemi- The cover sediments of the Solund-Stavfjord of the downgoing slab that were attached to the cal affi nity. Their minor to moderate enrichment ophiolite complex (the Heggøy Formation) Baltic Shield, or slices of the older oceanic base- in Cs, Pb, and Th relative to Nb suggests sub- show a wide range of zircon ages from ca. ment on which the Hersvik Unit was constructed duction infl uence in their melt evolution, and 500 Ma to 460 Ma, distinct Proterozoic age (Fig. 13). As the Iapetus Ocean closed, metasedi- the Nd-isotope values show derivation of their groupings, and the Proterozoic-Archean bound- mentary rocks and gneisses of the Middle (Her- melt from a homogeneous mantle source with ary (Pedersen and Dunning, 1993). This age land and Høyvik Groups and Dalsfjord Suite) no crustal contamination. These features, com- spectrum of detrital zircons is consistent with and the Lower (Western Gneiss Region) Tec- bined with the fi eld observations and regional their derivation from the Early to Middle Ordo- tonic Units of the downgoing Baltica continental tectonics, indicate that the basin in which the vician oceanic crust and associated island-arc margin, as well as the Solund-Stav fjord ophiolite Solund-Stavfjord ophiolite complex formed was units that were accreted to the Laurentian mar- complex and other associated rock assemblages a short-lived (~10–20 m.y.), trench-distal back- gin earlier, and from a large distribution of Pre- of the Upper Tectonic Unit (Figs. 1–3), provided arc basin (the Solund-Stavfjord backarc basin). cambrian rocks with variable ages. The most material to the subduction-accretion complex The lava sequences of the calc-alkaline Hers- obvious continental source would be southern beneath the ophiolite, now represented by the vik Unit and alkaline Smelvær Unit represent Greenland, where the Precambrian sequences Sunnfjord subophiolitic mélange (Fig. 13). Syn- island-arc and backarc oceanic-island basalt are represented by Proterozoic and Archean to post emplace ment granitic plutons were em- (OIB) developments, respectively. Sedimentary rocks (e.g., Higgins et al., 2004). placed into the recently accreted oceanic material material derived from these two different mag- We envision the initial tectonic setting of the and their tectonic basement of Baltica affi nity as matic environments contributed to the formation Solund-Stavfjord ophiolite complex backarc ba- stitching intrusions. These stitching plutons (i.e., of the supra-ophiolitic Kalvåg mélange, as evi- sin as a continent-proximal marginal basin that Sogneskollen granite complex, 434 Ma; Hacker denced from the geochemistry of the sedimen- evolved above a subduction zone dipping west- et al., 2003) provide important spatial and tem- tary debris and the olistoliths. We infer that the ward beneath today’s Greenland (Fig. 14B). poral constraints for the basin closure and the Solund-Stavfjord backarc basin opened adjacent As the subducting oceanic lithosphere and the syncollisional tec tonics of the Baltica margin. to the eastern margin of Greenland–Laurentia

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Dilek, Y., and Thy, P., 1998, Structure, petrology, and sea- This study was carried out with fi nancial support Bischoff, J.L., 1980, Geothermal system at 21°N, East Pa- fl oor spreading tectonics of the Kizildag ophiolite, cifi c Rise: Physical limits on geothermal fl uid and role Turkey, in Mills, R.A., and Harrison, K., eds., Modern from the Norwegian Research Council, the Univer- of adiabatic expansion: Science, v. 207, p. 1465–1469, Ocean Floor Processes and the Geological Record: sity of Bergen, and Meltzers Høyskolefond. In ad- doi:10.1126/science.207.4438.1465. Geological Society of London Special Publication 148, dition to the fi eld work carried out by the authors Blakey, R., 2011, Northern Arizona University Geology p. 43–69. in the Solund-Stavfjord region, fi eld work was also Web Page: http://www2.nau.edu/rcb/ (accessed Octo- Dilek, Y., and Thy, P., 2006, Age and petrogenesis of plagio- conducted by a large number of former graduate ber 2011). granite intrusions in the Ankara mélange, Central Tur- students in the Solund-Stavfjord region from 1985 Bloomer, S.H., Taylor, B., MacLeod, C.J., Stern, R.J., Fryer, key: The Island Arc, v. 15, no. 1, p. 44–57, doi:10.1111 through 2001, including K.P. Skjerlie, R.J. Johansen , P., Hawkins, J.W., and Johnson, L., 1995, Early arc /j.1440-1738.2006.00522.x. T. Carlsen, E. Alsaker, R. Ravnås, Ø. 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Dilek acknowledges of basalt in oceanic spreading ridges and seamounts: rift tectonics of a Caledonian marginal basin: Multi- the Miami University Distinguished Professorship Effects of magma temperature and viscosity: Jour- stage seafl oor spreading history of the Solund-Stavfjord funds for his research. Associate Editor Cees van nal of Geophysical Research, v. 93, p. 2967–2980, ophiolite in western Norway: Tectonophysics, v. 280, Staal and reviewers Alexandre Zagorevski and Paul doi:10.1029/JB093iB04p02967. p. 213–238, doi:10.1016/S0040-1951(97)00036-X. Bruton, D.L., and Bockelie, J.F., 1980, Geology and Dilek, Y., Thy, P., Hacker, B., and Grundvig, S., 1999, Struc- D. Ryan provided objective and insightful reviews paleon tology of the Hølonda area, western central ture and petrology of Tauride ophiolites and mafi c dike of the manuscript that helped us improve the text Norway—A fragment of North America?, in Wones, intrusions (Turkey): Implications for the Neo-Tethyan and illus trations, and for which we are most grateful. D.R., ed., The Caledonides in the USA: Proceedings ocean: Geological Society of America Bulletin, Jane Ellingsen kindly helped with the illustrations. of the International Geological Correlation Programme v. 111, no. 8, p. 1192–1216, doi:10.1130/0016-7606 Caledonide Orogen Project 27: Blacksburg, Virginia, (1999)111<1192:SAPOTO>2.3.CO;2. 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