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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 LREEenriched 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 Metamorphic age kernel density plots for the Blåhø (Seve) Nappe and Lower Allochthon rocks. A and D show eclogites, C and E show other metamorphic ages (e.g., pegmatites). Lower Allochthon ages are about 400 Ma, matching a minority of Seve Nappe metamorphic ages, most of which are pre-Scandian, probably when near the Laurentian margin. Also shown are Upper Alochthon igneous rock ages (C). Blåhø Nappe garnet ampibolite. Some have textural evidence that they were once eclogites, including plagioclase-diopsidehornblende symplectites around garnet, and plagioclase exsolution lamellae in diopside. One of the relatively rare Blåhø Nappe felsic gneisses. This one has deformed leucosomes, clearly having undergone partial melting at some point, probably pre-Scandian. Retrogressed eclogite boudin, surrounded by garnet amphibolite and kyanite schist. Kyanite-garnetmuscovite-biotite schist. The kyanitegarnet-quartz-feldspar leucosome is an indication of (probably) pre-Scandian partial melting. Regional map of central Norway, showing sample locations. There are 95 total samples for which whole rock data are shown in this poster. The data set includes amphibolite, eclogite, hornblendite, pyroxene granulite, and intermediate and felsic gneisses. Many eclogites and amphibolites have compositions that indicate that the protoliths were cumulate gabbros. BSE image of Blåhø amphibolite: Plagioclase-hornblende symplectite surrounds garnet, with larger hornblende and plagioclase crystals, and diopside with plagiolcase exsolution blobs. Cross-polarized light image of diopside in garnet amphibolite, with plagioclase exsolution lamellae and blobs, suggesting an omphacite precursor.

abstracts with programs - geological society of america

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