IGNEOUS GEOCHEMISTRY OF THE BLÅHØ NAPPE OF THE MIDDLE ALLOCHTHON, NORWEGIAN CALEDONIDES: A DEEPLY SUBDUCTED SEGMENT OF ROCKS GEOCHEMICALLY SIMILAR TO THOSE EXPOSED IN THE STØREN GROUP OF THE UPPER ALLOCHTHON

The Scandinavian Caledonides are a complex orogenic belt that was assembled in the middle Paleozoic from several tectonic segments from within and on both margins of Iapetus. Some parts of the Caledonide belt developed in the Iapetan Ocean, including volcanic arcs, that were first accreted onto the Laurentian side of the ocean. These were transferred, with parts of the Laurentian margin, to the Baltican side during the culminating Siluro-Devonian Scandian collision between Laurentia and Baltica. Caledonide architecture now includes these assembled pieces as the Lower Allochthon (Baltican margin), Middle Allochthon (Baltican rifted margin, possibly with outboard volcanic arcs), Upper Allochthon (volcanic arcs), and Uppermost Allochthon (pieces of Laurentian margin), all above autochthonous Baltican crust.

The Blåhø Nappe of Norway is an assemblage of metamorphosed sedimentary and igneous rocks, with eclogites in some places, that is interpreted to be the remnants of an outboard arc, similar to upper parts of the Middle Allochthon (Seve Nappe) in Sweden. Here, we present 95 new whole-rock chemical analyses of Blåhø igneous rocks that include metamorphosed gabbros, basalts, and sparse intermediate, dacitic, and rhyolitic rocks. Mafic rocks are generally tholeiitic, felsic rocks generally calc-alkaline. REE diagrams for mafic rocks range from somewhat LREE-enriched to LREE-depleted. Multi-element and other diagrams indicate variable arc influence. Comparisons indicate derivation from an arc-back-arc system that is geochemically indistinguishable from ophiolitic and related volcanic and plutonic rocks in the Støren Group of the Upper Allochthon. Sparse Blåhø Nappe igneous ages are also similar to those of the Støren Group.

Eclogites within the Blåhø Nappe give metamorphic ages that are Scandian (based on literature), as do eclogites in the Lower Allochthon of the Western Gneiss Region. Middle Allochthon eclogites and gneisses in Jämtland, Sweden, record Ordovician 445-490 Ma metamorphic ages, from earlier subduction probably on the Laurentian side of Iapetus. Based on eclogite ages and new geochemistry, we present a model in which the Blåhø Nappe resulted from deep Scandian subduction of part of the same volcanic-sedimentary assemblage that also became the Upper Allochthon Støren Group.