Isotope geochronology in the high-grade metamorphic Precambrian of southwestern Norway: new data and reinterpretation.

Abstract

A new series of isotopic age data is reported from the high-grade metamorphic Precambrian in S.W. Norway, mainly from the Sirdal-Ørsdal area. U-Pb investigations on suites of zircons from three augen gneisses and a pyroxene syenite produce discordias with upper intercepts between 1070 and 1030 Ma; these ages are assigned to the highest-grade phase of Sveconorwegian metamorphism, M2, induced by the intrusion of the older part of the Bjerkreim-Sokndal lopolith. Some zirkon fractions from antoher pyroxene syenite and a garnetiferous migmatite contain an inheritance of old radiogenic lead, but no conclusions are possible regarding the age of the older component. The lower intercepts of the discordia trajectories are interpreted as reflecting episodic loss of radiogenic lead in the Palaezoic, probably in relation to nearby belt of Caledonian orogenesis and \/or post-orogenic uplift; the lead-loss event is tentatively correlated with M4 phase of incipient very-low-grade metamorphism.