40Ar/' 39Ar mineral ages within the Western Gneiss Terrane, Troms, Norway: evidence for polyphase Proterozoic tectonothermal activity (svecokarilian and sveconorwegian)

Abstract

Hornblende concentrates from units of the Western Gneiss Terrane exposed on Kvaløy and northeastern Senja, Troms, Norway, display variably discordant 40Ar/ 39Ar age spectra. They suggest a complex Precambrian tectonothermal evolution involving: (1) middle Proterozoic (∼ 1700–1800 Ma) partial rejuvenation of older (> 2000 Ma) intracrystalline argon systems; and (2) variable rejuvenation of argon systems by a late Proterozoic tectonothermal event. Samples completely rejuvenated during the lastest event record plateau isotope correlation ages of 941 ± 6 Ma and 916 ± 5 Ma. There is no record of any Caledonian rejuvenation in the 40Ar/ 39Ar results. This contrasts markedly with southwestern Senja where previously reported 40Ar/ 39Ar mineral ages indicate a penetrative, polyphase Caledonian history. This indicates that the intervening Svanfiellet shear zone may have profound tectonic significance. The record of late Proterozoic tectonothermal history provided by the 40Ar/ 39Ar results suggests that at least local sectors of the Western Gneiss Terrane must be comprised of relatively high-level Caledonian structural units, which include elements of Baltic continental crust derived west (present reference) of the Sveconorwegian-Grenville ‘front’ extrapolated from southern Scandinavia.