THE EARLY EVOLUTION OF THE SOUTHWEST SWEDISH GNEISS PROVINCE - GEOCHRONOLOGICAL AND ISOTOPIC EVIDENCE FROM SOUTHERNMOST SWEDEN

The continental crust in southwestern Sweden has a complicated history of formation, apparently commencing similar to 1.75 Ga ago and continuing until the end of the Sveconorwegian orogeny similar to 0.9 Ga ago. Most of the rocks are polymetamorphic gneisses, but massifs of intrusive rocks can commonly be recognized. This study reports new U-Pb zircon ages from the province of Skane in the southernmost part of the region. Two gneiss samples and four samples of granitoid rocks were dated by the U-Pb zircon method. In the two gneiss samples, selected single zircon crystals were analyzed in addition to the standard multi-grain size fractions. The multigrain zircon analyses yielded an upper intercept age of 1613 +/- 6 Ma for a grey gneiss from the central part of the province of Skane, and 1557(-27)(+32) Ma for a red aplitic gneiss from southern Skane. Individual zircon crystals from these samples gave Pb-207/Pb-206-ages between 1335 and 1669 Ma. For the grey gneiss, the individual zircons define a separate discordia with an upper-intercept age of 1640 +/- 16 Ma; in the red gneiss sample they scatter around a line with an upper intercept at 1685 +/- 25 Ma. Core-bearing zircons from a red gneissic granite in central Skane yielded an age of 1575(-61)(+77) Ma, while similar zircons from a strongly foliated red granite from the Kullaberg horst in northwestern Skane gave an age of 1497(-34)(+47) Ma. Core-free zircons from the Beden granodiorite in southern Skane have an age of 1449(-11)(+23) Ma, and core-bearing zircons from a gneissic sic charnockite near (Orkelljunga in northern Skane yielded an age of 1452(-50)(+350) Ma. Although the latter age is poorly defined, it allows for similar intrusion ages of the Orkelljunga charnockite and the Beden granodiorite. Sm-Nd isotopic analyses yield T-DM ages of 1.84 to 1.99 Ga, and T-CHUR ages of 1.48 to 1.60 Ga for these rocks, while Rb-Sr analyses give T-UR ages between 1.46 and 1.71 Ga. The combined evidence suggests that the main crust-forming episode in this part of southwestern Sweden occurred 1.6 to 1.7 Ga ago, with presumably subduction-related magmatism forming the protoliths of the red and grey gneisses. Around 1.5 to 1.6 Ga ago red anatectic granites were generated. The Beden granodiorite and the Orkelljunga charnockite, as well as the Varberg charnockite farther north, were all formed similar to 1.45 Ga ago, possibly by mantle-generated anorogenic magmatism. Some 0.9 Ga ago, after the Sveconorwegian crustal thickening and high-pressure metamorphism, uplift within the Southwest Swedish Gneiss Province exposed lower- to mid-crustal granulitic and charnockitic rocks in its southern part.