From crystal to crust: the Proterozoic crustal evolution of southwest Norway

The geology of the Suldal Sector, southwest Norway, comprises exposures from three orogenic periods; the Telemarkian, Sveconorwegian and Caledonian. Telemarkian (~1500 Ma) basement rocks are interpreted to be the oldest crust in the region; these are intruded by Sveconorwegian granitoid intrusions (~1070-930 Ma). Crystalline nappe units overlie the Mesoproterozoic basement, and from reconnaissance U-Pb dating and zircon hafnium isotopes, are believed to comprise slices of the Mesoproterozoic Norwegian continental margin. The Telemarkian basement comprises meta-plutonic/volcanic lithologies that represent the deformed upper crustal section of a continental arc - the Suldal Arc; U-Pb dating suggests this arc was active from ~1520 to 1475 Ma. Whole-rock geochemistry and hafnium and oxygen isotopes measured in zircon, suggest that arc magmatism recycled older continental crust (20-50% contribution) that had been mixed with mantle-derived material in the lower crust; the older crustal component comprised late-Palaeoproterozoic sedimentary material derived from the Fennoscandian continent. During the arc’s evolution, dehydration of mafic source magma induced by heat from magmatic underplating, and subsequent melting of dehydrated crust enhanced by asthenospheric upwelling, allowed for the intrusion of iron-enriched tholeiitic magmas. The Suldal arc and by extension, the Telemarkia terrane, represent the last stages of continental crust formation within a retreating accretionary orogen that was active since ~1.8 Ga. Based on whole-rock geochemistry, U-Pb, hafnium and oxygen isotopes in zircon, Sveconorwegian granite suites formed between 1.07 and 0.92 Ga, and are largely derived from ~1.5 Ga mafic lower crust with a limited contribution of juvenile mantle-derived material. The geodynamic setting of granitic magmatism evolved from supra-subduction, to overthickened crust, to thinned crust with possible lithospheric delamination. The varying geochemistry of the granite suites (I- to A-type) is controlled not by geodynamic setting, but dominantly by water content in the magma source. Sveconorwegian deformation in the Suldal Sector is bracketed between ~1069 and ~1047 Ma by intrusions of the Storlivatnet plutonic complex.