U-Pb zircon ages of metatuffites and Older Granite from the Tunaberg area, SE Bergslagen, Sweden

Abstract

Two U-Pb zircon datings of metatuffites from the lower and upper unit of the metavolcanosedimentary sequence in the Tunaberg area, SE Bergslagen, Sweden, yield an age of 1871 ± 7 Ma for the metatuffite in the lower unit and an age of 1836 ± 9 Ma for the metatuffite in the upper unit. An Older Granite in a folded, composite granite-basite sheet that is intrusive in the upper metavolcanosedimentary unit contains zircons with cores that are apparently inherited from older crust; the U-Pb zircon age of 1912 ± 29 Ma obtained for this Older Granite must be discarded as the intrusion age of this granite. The maximum age of intrusion of the pre-main folding composite sheet of Older Granite and metabasite is constrained by the 1836 ± 9 Ma age of the upper volcanosedimentary unit. The deposition of the southern Bergslagen metavolcanosedimentary sequence in the Tunaberg area occurred in the period 1.88-1.83 Ga, i.e. a somewhat later and much longer period than the deposition period of the 1.89-1.88 Ga western Bergslagen volcanosedimentary sequence.