U-Pb dating of metatrondhjemite from the island of Ytterøy. Trondheimsfjorden, Central Norway.

Abstract

Zircon fractions extracted from a metatrondhjemite sheet interbanded withmafic 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 asthe crystallisation age of this subvolcanic felsic intrusion. Fieldrelationships indicate that the trondhjemite and greenstone volcanites aremore or less coeval; both rock-types have participated in all the Scandiandeformation phases recorded on the island. A nearby metalimestone withgradational contacts into mafic volcanites is also considered to be roughlythe same age as the trondhjemite.Based on comparisons with other U-Pb dated trondhjemitic sheets associatedwith metabasalts in this part of Norway, the Ytterøy greenstone pile appearsto be one of the oldest volcanite units in the Trondheim Nappe Complex. Boththe metabasalts and the metalimestone are probably of latest Cambrian tovery earliest Ordovician age.