Northeastern Section–41st Annual Meeting (20–22 March 2006)

Paper No. 3
Presentation Time: 9:05 AM


HOLLOCHER, Kurt T., Geology Department, Union College, Nott St, Schenectady, NY 12308, ROBINSON, Peter, Geol Survey of Norway, Trondheim, N-7491, Norway, WALSH, Emily, Geology Department, Cornell College, Mount Vernon, IA 52314 and TERRY, Mike, Bayerisches Geoinstitut, Universität Bayreuth, D-95440, Bayreuth, Germany,

The Sætra Nappe (=Särv) is one of many thrust sheets transported eastward from the Baltica margin during the Silurian – Early Devonian Scandian Orogeny. The Nappe is mostly metamorphosed Neoproterozoic sandstone up to 2 km thick that was intruded by diabase dikes during rifting of Rodinia from Baltica to produce the Iapetus Ocean. Sampling from 15 localities was done to extend correlation of the Sætra Nappe from lower-grade rocks in west-central Sweden and eastern Norway to highly deformed amphibolite- and eclogite-facies rocks in the Western Gneiss Region, W and SW of Trondheim, where the Nappe is as thin as 1 m. Mafic rocks were also collected from two units underlying the Sætra: augen gneisses of the Risberget (=Tännäs) Nappe, and nearby dike-rich basement gneisses ("sausage rock"). The dikes at most suspected Sætra sites are indistinguishable from those at the classic Oppdal locality, though there are subtle variations. They are somewhat LREE-enriched, with most having Lan/Smn ratios 1.0-1.8 (Oppdal Group), but with more alkaline varieties having ratios up to 2.4 (Ystland Group). All have Nb/La ratios of 0.8-1.4. Negative Eu anomalies are small to none, except large for obviously evolved compositions. Spider patterns show a consistent lack of Nb-Ta anomalies, with common slight negative Zr-Hf anomalies, and common small positive P and U anomalies. These characteristics indicate a continental-rift or flood-basalt eruptive setting. Many dikes cutting Risberget augen gneiss are compositionally indistinguishable from those in the Sætra, as are dikes in basement "sausage rock", and are interpreted to represent the same dike swarms. This suggests that the basement "sausage rock" areas are part of the Middle Allochthon with Sætra and Risberget. Other, generally coarser mafic rocks in the Risberget are different from Sætra and probably Mid-Proterozoic, having large negative Nb-Ta anomalies and other characteristics indicative of arcs or continental crust contamination. Data from this and other studies from the Sætra Nappe show geographic compositional variations, with MORB-like compositions to the north and south and more alkaline compositions at about the latitude of northern Trondheimsfjord. We interpret the latter to have been influenced by enriched mantle of a nearby hot-spot during opening of Iapetus.