The Early Ordovician Bymarka ophiolite fragment is situated in the Upper Allochthon of the central Norwegian Caledonides. Geochemical and geochronological investigations of this and other ophiolite fragments in the Norwegian Caledonides show that they formed between c. 500 and 470 Ma in broadly similar suprasubduction-zone settings. Geochemical investigations in the Bymarka ophiolite fragment, focusing on felsic, rhyodacitic and trondhjemitic rocks, demonstrate a geological evolution from the formation of new oceanic crust, through island arc development, to postobduction magmatism. The Klemetsaunet rhyodacite is interpreted to be analogous to plagiogranite, common in nearby ophiolite fragments, and probably formed by fractional crystallisation of mid-ocean ridge-like basaltic magmas in a back-arc basin. The slightly younger Fagervika trondhjemite is interpreted to represent construction of an island arc on recently formed oceanic crust, and may have formed by partial melting or fractional crystallisation of mafic island arc tholeiitic rocks or magmas. The Byneset trondhjemite typically occurs as dykes cutting the greenstones that make up the bulk of the Bymarka ophiolite fragment, and is interpreted to have formed by partial melting of mafic rocks at the base of a thick pile of ocean floor- and island arc-derived rocks stacked onto the outermost part of a continental or microcontinental margin. The geological evolution of the Bymarka ophiolite fragment and other ophiolite fragments in the central Norwegian Caledonides is similar to that of ophiolites elsewhere in the Caledonian-Appalachian orogen. The conclusions reached here thus support earlier ideas that the Caledonian-Appalachian ophiolites formed as part of the same, extensive island arc/back-arc system.
Geochemistry of trondhjemites and mafic rocks in the Bymarka ophiolite fragment, Trondheim, Norway: Petrogenesis and tectonic implications