The role of felsic and mafic igneous rocks in deciphering the evolution of thrust-stacked terranes: An example from the north Norwegian Caledonides

Abstract

Excellent exposure of appropriate structural levels across the trace of the regional Ofoten synform, north Norway, make this an exceptional area in which to investigate the role of felsic and mafic igneous rocks in the amalgamation and final emplacement of a stack of far-traveled terranes. The nappe sequence in Ofoten is composed, from bottom to top, of the Narvik, Ofoten, and Niingen nappe complexes. Each of these nappe complexes contains multiple, internal thrust sheets, or nappes, that were emplaced onto Precambrian-Cambrian rocks of the Baltic shield along east-directed thrusts during the Caledonian orogeny. Distinct contrasts in rock types, structural arguments, and regional correlations indicate that the nappes are suspect or exotic terranes. Whole-rock chemistry, petrography, and field relations of felsic and mafic igneous rocks within each allochthon are used to explore the tectonic setting of each terrane, the crustal levels of their amalgamation, and the relative and absolute timing of their amalgamation. Three different types of felsic intrusions were distinguished in each lithologic package, with the exception of the Evenes Group (Ofoten nappe complex) which has none. Caledonian plutons also are lacking in the underlying Baltic basement complex. The intrusions are separated temporally into early-phase, nappe-specific plutons and late-phase intrusions, some of which cut across nappe boundaries. Most are S- and I-type collisional granitoidal rocks. Early-phase granitoids in the Bogen Group (Ofoten nappe complex), however, have an A-type chemistry and mineralogy, probably indicating either second-cycle melting of a granulite terrane or melting of calc-alkaline rocks. The lack of Caledonian intrusives in both the Evenes Group and the basement requires that allochthons overlying these units rooted to the west. Discrimination of the various tectonic settings based on the geochemistry of the felsic intrusions dovetails closely with that determined from a sampling of mafic rocks. Each nappe originated as a laterally-distinct entity that rode piggyback with the Niingen nappe complex being emplaced first and the composite nappe stack thrust lastly upon the basement complex. Previously reported lithologic correlations with fossiliferous units and U-Pb and 40Ar/39Ar mineral cooling dates combine with new Rb/Sr and U-Pb dates to place the relative nappe-stacking chronology into an absolute timeframe. Metamorphism of the Narvik nappe complex was first at ∼432 Ma, followed by metamorphism of the Niingen around ∼430 Ma, which later metamorphosed the Ofoten by down heating between ∼430 and 425 Ma. The stacking process took only ∼7 my and resulted from the Silurian-Devonian Scandian orogeny. Pre-Scandian relics occur within each of the nappe complexes and were strongly reworked and emplaced during the Scandian event.