Petrology, tectonics, and hydrothermal alteration of a fossil backarc oceanic crust: Solund-Stavfjord ophiolite complex of the western Norwegian Caledonides- A review

Abstract

The Late Ordovician Solund-Stavfjord ophiolite complex in southwest Norway developed in a backarc basin through at least two episodes of seafloor spreading. The youngest phase is a structural domain (domain 1) in oceaniccrust that formed along a backarc rift system that opened at an intermediate spreading rate and propagated north-northeastward into preexisting oceanic crust (domain 2). These two domains are separated by a 1-km-wide zone of anomalous oceanic crust, with fracture-zone affinity (domain 3), that is composed of strongly sheared gabbros and metabasalt dikes hosting abundant serpentinite bodies. Domains 1 and 2 contain high-level gabbro, sheeted dikes, a transition zone of dikes and volcanic rocks, and a volcanic suite consisting of various proportions of pillow lavas, massive lava flows, and hyaloclastite breccias. The metabasalts of both domains have MORB (mid-oceanic-ridge basalt) composition, with minor evidence of an island-arc component, and those of domain 1 are dominantly Fe-Ti basalts. Their contents of FeO t (total Fe as FeO), MgO, Al 2 O 3 , Cr, Ni, and Zr indicate that differences in depth of melting (30 to 12 kbar), crystal fractionation of olivine, plagioclase, and clinopyroxene at different levels in the mantle and crust (<8 kbar), and magma mixing controlled the composition of the metabasalts. The high proportion of massive lava flows, the lack of pervasive seafloor extensional tectonic deformation, and the hydrothermal history of the fossil oceanic crust in domain 1 are best comparable to the features of young oceanic crust of the Costa Rica Rift. The Solund-Stavfjord ophiolite complex is conformably overlain by (1) a heterogeneous suite (Stavenes Group) of quartz-dominated metasediments hosting pillow lavas, volcaniclastic rocks, and minor intrusions of island-arc tholeiite-like composition with a clear subduction signature (the Heggoy Formation) and (2) the supposedly later calc-alkalic and alkalic rocks of the Hersvik and Smelyaer units, respectively. This progressive change in chemical composition of the igneous rocks stratigraphically upward within the sedimentary cover indicates the development, through time, of a mature arc in the subduction-zone environment represented by the Solund-Stavfjord ophiolite complex. The mode and nature of magmatic, tectonic, and hydrothermal processes that operated during the evolution of the complex in a backarc basin are similar to those observed from modern mid-ocean ridge settings, particularly at intermediate-spreading ridge axes. The geologic features of the sedimentary cover of the Solund-Stavfjord ophiolite complex and the underlying Sunnfjord melange suggest that the inferred Caledonian backarc basin evolved in proximity to a continental landmass, reminiscent of the tectonics of the modern Andaman Sea region.