Geochemical characteristics of the Feragen ultramafic body, central Scandinavian Caledonides: Implications for petrogenesis and tectonic significance

We present new mineral chemistry, whole-rock major and trace element data for the mantle rocks of the Feragen ultramafic body (FUB), one of the largest ultramafic fragments within the Scandinavian Caledonides. Solitary peridotite bodies are important components of the orogenic belts worldwide as they represent tectonically sampled fragments of the mantle. Compositions of these peridotites provide information that contributes to comprehension of regional geology and evolution of orogens. The FUB comprises three different types of mantle lithologies: (1) peridotites with moderately depleted mineral compositions (Mg-number of clinopyroxene 92–94, Al2O3 of clinopyroxene >3 wt.% and spinel Cr-numbers in the range between 0.1 and 0.6); (2) dunites with highly depleted compositions (Mg-number of rare clinopyroxene grains 94–96, Al2O3 of clinopyroxene <2 wt.%, Cr-number of spinel >0.6); (3) lithologies transitional between the two that mark contacts between dunites and host peridotite and show clinopyroxene compositions similar to those in dunites and a wide range of spinel Cr-number between 0.4 and 0.8. The dunites of the FUB are interpreted as end-products of the reaction between high-Mg high-Si melts and the ambient mantle peridotite. Mineral chemistry data, coupled with extremely low REE content of the rocks, suggest that the FUB represents a fragment of highly depleted, possibly fore-arc, mantle that has  experienced high degrees of partial melting. Two alternative scenarios for the formation of the FUB are proposed, where: (1) the FUB represents a mantle root of an arc that existed within the Iapetus Ocean and was later thrusted onto the Baltican continent during the Scandian phase of the Caledonian orogeny; (2) the FUB was exhumed along the hyperextended Baltican continental margin during the incipient stage of the Iapetus opening and could represent a remnant of a “fossil” oceancontinent transition zone, now incorporated into the Caledonide Orogen, with the refractory nature of the FUB being inherited from an older, pre-Caledonian, tectonothermal event.