Structure, Geochemistry, and Tectonic Evolution of Trench-Distal Backarc Oceanic Crust in the Western Norwegian Caledonides, Solund-Stavfjord Ophiolite (Norway)
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Structure, geochemistry, and tectonic evolution of trench-distal backarc oceanic crust in the western Norwegian Caledonides, Solund-Stavfjord ophiolite (Norway) Harald Furnes1,†, Yildirim Dilek 2,3, and Rolf Birger Pedersen1 1Department of Earth Science & Centre for Geobiology, University of Bergen, 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 Sea 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 Bremanger CALEDONIDES 440 Ma mélange Hersvik Unit Smelvær Unit Thrust SSOC & ′ Kalvåg 61°15 ′ SM cover BASEMENT 61°15 Gåsøy 443 Ma Sunnfjord mélange (SM) SM Batalden ? Herland Group Eikefj. 10 km Skorpa ? Uncon- formity Høyvik Group Kinn Askrova Svanøy Dalsfjord Suite 61°30′ 61°30 ′ Fault Askvoll 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 vik 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