Summary
Zircon fractions extracted from a metatrondhjemite sheet interbanded with
mafic volcanites on the island of Ytterøy in the Støren Nappe, Trondheims-
fjorden, have yielded a U-Pb age of 495.3 +- 2.5 Ma. This is interpreted as
the crystallisation age of this subvolcanic felsic intrusion. Field
relationships indicate that the trondhjemite and greenstone volcanites are
more or less coeval; both rock-types have participated in all the Scandian
deformation phases recorded on the island. A nearby metalimestone with
gradational contacts into mafic volcanites is also considered to be roughly
the same age as the trondhjemite.
Based on comparisons with other U-Pb dated trondhjemitic sheets associated
with metabasalts in this part of Norway, the Ytterøy greenstone pile appears
to be one of the oldest volcanite units in the Trondheim Nappe Complex. Both
the metabasalts and the metalimestone are probably of latest Cambrian to
very earliest Ordovician age.
