GEOCHEMICAL AND TECTONIC EVOLUTION OF A TRENCH-DISTAL BACKARC OPHIOLITE, SOLUND-STAVFJORD COMPLEX (443 MA), IN THE WESTERN NORWEGIAN CALEDONIDES

The late Ordovician (443 Ma) Solund–Stavfjord ophiolite complex (SSOC) and associated rocks exposed in the southwest Norwegian Caledonides display characteristic features of an oceanic crust developed in a trench-distal backarc basin adjacent to a continental margin, analogous to the modern Andaman Sea. The structural architecture of the SSOC complex, particularly the orientation of its sheeted dike complex, suggests two different stages of ocean-floor spreading during its evolution. The oceanic crust in the younger domain contains pillow lavas, massive sheet flows and hyaloclastites, NE-trending sheeted dikes, and high-level isotropic gabbros. The proportions of pillow lava to massive sheet flows, the thick (~1.2 km) sheeted dike complex, and the hydrothermal alteration patterns as defined by δ¹⁸O data are reminiscent of the modern intermediate-spreading oceanic crust (e.g. the 5.9 Ma Costa Rica Rift). The metabasaltic rocks in this domain are predominantly Fe-Ti basalts of N-MORB composition and show a large range in their incompatible (e.g. Zr) and compatible (e.g. Cr) element contents as a result of magma mixing and fractional crystallization. Insignificant variations in the Nd-isotopic character of these rocks suggest derivation of their magmas from an isotopically homogenous melt source. Abrupt changes in their chemostratigraphy within short distances along-strike of the ophiolite indicate, however, magma delivery from small, separate melt lenses that evolved independently of each other beneath the spreading axis. The oceanic crust in this domain developed along a rift system that propagated northeastward (in the present coordinate system) in a late Ordovician backarc basin, similar to the tectonics of the modern Lau Basin. The pre-existing oceanic crust in this basin is represented by the older domain, consisting of NW-trending sheeted dikes and massive to layered gabbros.

The extrusive sequence in the SSOC locally contains phyllite intercalations, and is conformably overlain by continentally derived quartz-rich meta-sandstone that contains N-MORB basaltic lavas and shallow-level intrusions. These late-stage igneous rocks in the sedimentary cover show weak subduction geochemical signatures, indicating their formation in a basin that was in close-proximity to continental crust on-land. The occurrence of calc-alkaline lavas and sedimentary, serpentinite-bearing mélanges and olistrostromes, tectonostratigraphically beneath the SSOC attests to the presence of an island arc and associated accretionary prism complex at the time of the evolution of the backarc basin. During the emplacement of the SSOC onto the Baltica continental margin in the middle Silurian, a different mélange unit that contains mafic-ultramafic components from different structural levels of the ophiolite, developed beneath the advancing ophiolite nappe. The evolutionary history of the SSOC thus involves oceanic crust formation in a short-lived (< 20 m.y.), trench-distal, continent-proximal backarc basin during the late closing stage of the Iapetus Ocean.