MULTIPHASE TRONDHJEMITES FROM THE TRONDHEIM REGION, CENTRAL NORWEGIAN CALEDONIDES: AGES, GEOCHEMISTRY AND PETROGENESIS

Trondhjemite, as a rock name, has existed since the time of Goldschmidt's (1916) pioneering studies in central and southern Norway, and his work in and around the type locality at Follstad, south of the city of Trondheim. In the Norwegian Caledonides, trondhjemites occur widely in the outboard, oceanic and arc terranes of the Köli nappes, which accreted to the Baltoscandian margin of Baltica during the Scandian orogeny, in Siluro-Devonian time. Two principal types of trondhjemite are known: (1) plagiogranitic bodies, associated with ophiolites, that have yielded U-Pb zircon ages generally in the range 495-480 Ma; and (2) dykes and major plutons with continental-margin geochemical signatures, which range in age from Early/Mid Ordovician to Late Silurian (468 to 419 Ma). In Central Norway, our work has been concentrated on the trondhjemites in the Støren and Gula Nappes. In the Støren Nappe, plagiogranite-type trondhjemites occur as diverse intrusive bodies associated with ophiolitic gabbros, and their extrusive equivalents are present in the overlying volcanic sequences. These rocks also commonly occur as clast material in unconformably overlying conglomerates. The continental-margin type, high-Al/low-Yb trondhjemites are found in both the Støren and the Gula Nappes, but are most common in the latter which also hosts the type-locality, Follstad body (7 km long, 0.4 km wide). Both the Follstad trondhjemite (U-Pb dated to 432 + 3 Ma) and other variants of trondhjemite in the Gula (either plutons or dykes) transect an early (inferred Early Ordovician) foliation in the host rocks, but they are themselves variably folded, weakly foliate, and carry a Scandian, upper greenschist-facies mineral assemblage. The continental-margin trondhjemites are considered to have formed as melting products of garnet-bearing amphibolite, with peak emplacement relating to a high-T, extension-related scenario in Late Ordovician/Early Silurian time. By way of comparison, trondhjemites in the eastern Blue Ridge terrane of the Southern Appalachians in Alabama, indicate a petrogenesis more related to remobilized continental margin lithosphere during the Late Devonian, Acadian Orogeny.