CONSTRAINTS ON SYN-INTRUSIVE CA. 1.8 GA CONGLOMERATE DEPOSITS ASSOCIATED WITH THE SMALAND-VARMLAND BELT, SE SWEDEN - PB-PB ZIRCON EVAPORATION DATING OF THE MALMBACK CONGLOMERATE

Single-zircon evaporation datings of three porphyritic clasts from the Malmback conglomerate yield ages of 1796+/-8, 1807+/-7, and 1811+/-11 Ma. The precursors of the reworked volcanics are interpreted as Smala nd-Varmland igneous rocks of the Transscandinavian Igneous Belt (TIB). Two granites, located east and west of the conglomerate, give ages of ca. 1700 and greater than or equal to 1780 Ma, respectively. The coex istence of the ca. 1800 Ma old granite and reworked volcanics (the con glomerate clasts) of similar age is suggested to reflect an extensiona l tectonic setting during the earliest TIB magmatism, although the ove rall tectonic regime may have been compressional.