GEOCHEMICAL CONSTRAINTS ON PALEOZOIC ARC PETROGENESIS, SOUTH-CENTRAL NORWEGIAN CALEDONIDES

The Norwegian Caledonides contain numerous volcanic terranes of mostly oceanic origin. Recent debate has centered around the paleogeography of formation of the oceanic fragments, and the timing of their accretion to Baltica. In one interpretation, all the oceanic terranes formed outboard of Baltica and were accreted to the Baltic margin during the Ordovician. Justification of this model involves the interpretation of a terrane-linking unconformity that overstepped both Baltic margin strata and obducted oceanic fragments, prior to Late Ordovician time. An alternative view holds that many of the outboard terranes formed on the opposite side of Iapetus, possibly near the Laurentian(?) margin, and were not accreted to Baltica until the Siluro-Devonian Scandian orogeny. Most studies have focused on ophiolitic rocks in western Norway, where deformation and metamorphic effects are of relatively low intensity, and some ages are constrained by faunal and isotopic data. In the eastern Caledonides, such sequences are generally highly deformed and metamorphosed and fossil preservation is uncommon. In the study area, a domal structure exposes a complex sequence of metavolcanic and metasedimentary rocks overlying Neoproterozoic(?) continental-margin quartzites. In the past two decades, these rocks have been variously interpreted as 1) parts of several unrelated terranes separated by major thrusts; 2) as part of an internally deformed sheet of oceanic crustal rocks and associated sediments; and most recently, 3) as volcanic eruptions into a sedimentary basin developed on continental crust, above the major terrane-linking unconformity. This study reveals the presence of three geochemically distinct volcanic units: Unit A comprises island-arc tholeiites; Unit B contains highly depleted boninitic basalts; and Unit C may be characterized as primitive low-Ti, high-Mg tholeiites. These geochemical units are suggestive of the ‘stratigraphy’ recognized in some well-described modern and ancient supra-subduction zone arc sequences. Significantly, the available geochemical data do not indicate a continental crustal component in any of the units, thus making interpretation 3 (above) unlikely. Neodymium isotopic data from metabasalts and related metasedimentary rocks will be used to further evaluate these data.