U-PB ZIRCON AGES OF METATUFFITES AND OLDER GRANITE FROM THE TUNABERG AREA, SE BERGSLAGEN, SWEDEN

Two U-Pb zircon datings of metatuffites from the lower and upper unit of the metavolcanosedimentary sequence in the Tunaberg area, SE Bergsl agen, 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 upp er unit. An Older Granite in a folded, composite granite-basite sheet that is intrusive in the upper metavolcanosedimentary unit contains zi rcons 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 volca nosedimentary unit. The deposition of the southern Bergslagen metavolc anosedimentary 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 depo sition period of the 1.89-1.88 Ga western Bergslagen volcanosedimentar y sequence.