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

摘要

The Late Ordovician (443 Ma) Solund-Stavfjord ophiolite complex in west Norway represents the youngest phase of oceanic crust formation in the western Norwegian Caledonides. It contains three structural domains with different crustal architecture that formed during two episodes of seafloor spreading evolution of a Late Ordovician marginal basin. The fossil oceanic crust of the younger episode contains pillow lavas, massive sheet flows, and hyaloclastites, NE-trending sheeted dikes, and high-level isotropic gabbros. The pillow lava versus massive sheet flow distribution and the occurrence of an extensive sheeted dike complex in the Solund-Stavfjord ophiolite complex are typical of in situ oceanic crust developed at modern intermediate-spreading mid-ocean ridges. The Solund-Stavfjord ophiolite complex lavas and dikes are composed predominantly of normal mid-ocean-ridge basalt (N-MORB) Fe-Ti basalts, and their trace-element patterns indicate a weak subduction influence. The Nd isotope data of these rocks suggest derivation of their magmas from an isotopically homogeneous melt source with no indication of continental crustal contamination. The Solund-Stavfjord ophiolite complex extrusive sequence contains phyl-lite interlayers and is conformably overlain by a continentally derived, quartz-rich metasandstone that is intercalated with sills of N-MORB basaltic lavas and shallow-level intrusions. The geochemical features of the upper-crustal rocks of the Solund-Stavfjord ophiolite complex indicate their formation from magmas in which the melt evolution involved only minor or no slab-derivedfluids. The evolution of the Solund-Stavfjord ophiolite complex oceanic crust occurred in a short-lived (<20 m.y.), trench-distal, continent-proximal backarc basin, adjacent to the eastern margin of Greenland-Laurentia, during the closure of lapetus. This inferred tectonic setting is reminiscent of the modern Andaman Sea at the eastern periphery of the Indian Ocean.