Geochemistry and petrogenesis of trondhjemites and granodiorite from Gauldalen, Central Norwegian Caledonides

Abstract

In the Gauldalen district of Central Norway, three varieties of trondhjemite cutting rocks of the Gula Complex have been distinguished on the basis of nuances of colour and field relationships. The high Al2O3, Rb and Sr, and low Yb contents of all three types of trondhjemite signify their generation in a continental margin palaeotectonic setting. They are considered to have formed as melting products of a garnet-bearing amphibolite. Less common, spessartine-garnet granodiorites in this same area are calc-alkaline and peraluminous, and may have derived from an Al-rich source. Field relations denote that greenish-white trondhjemite dykes and sheets are the oldest intrusions. One of these dykes has provided a Rb-Sr whole-rock isochron (minimum) age of 465+11 Ma. The garnetiferous granodiorites transect these early dykes and are themselves cut by dykes and larger bodies of white trondhjemite, including the type-locality Follstad trondhjemite U-Pb-dated (zircon and titanite) to 432+3 Ma.\rAll the trondhjemite and the granodiorite postdate an existing foliation in the host Gula rocks, but are themselves weakly foliated, folded and in places cut by a later cleavage. The overall, local and regional isotopic data and biostratigraphic constraints suggest that an early Caledonian tectonothermal event affected the Gula rocks before 465 Ma, and probably in Early to Mid Arenig time. The foliation, folds and cleavage in the trondhjemites and granodiorite, on the other hand, must postdate 432 Ma and are considered to have formed during the main Scandian tectonometamorphic event dated isotopically to around 425 Ma.