Sveconorwegian vs. Caledonian orogenesis in the eastern Oygarden Complex, SW Norway - Geochronology, structural constraints and tectonic implications

The Oygarden Complex is the westernmost basement window in the Norwegian Caledonides, yet, the age and evolution of this part of the Baltic Shield is largely unknown. We examined the eastern part of the window by detailed field mapping and SIMS U-Pb zircon geochronology, to disentangle the record of Caledonian and Sveconorwegian orogenesis and to constrain the long-term crustal evolution. The eastern Oygarden Complex comprises mainly Sveconorwegian metaigneous rocks, which intrude Telemarkian granitic basement, dated at 1506 +/- 5 Ma. Sveconorwegian magmatism occurred in two distinct phases: Contemporaneous hornblende biotite granite and gabbro intrusions revealed crystallization ages of 1042 +/- 3 Ma and 1041 +/- 3 Ma, respectively. We dated younger leucogranitic intrusions at 1027 +/- 4 Ma, 1024 +/- 6 Ma and ca. 1022 Ma. The new ages clearly identify the Oygarden Complex as a part of Telemarkia and correlate it with the Sirdal Magmatic Belt. Furthermore, they show that the Precambrian evolution of the Oygarden Complex is distinctly different from the Western Gneiss Region. Bimodal magmatism at 1041 Ma and the absence of Sveconorwegian high-grade metamorphism in the eastern Oygarden Complex support the idea of an accretionary Sveconorwegian orogen. Following long-term residence at low temperatures, a temperature increase caused resetting of high-U metamict zircons at ca. 482 Ma. This early Ordovician thermal event might reflect extension of the Baltican margin or early Caledonian convergence. Caledonian ductile reworking involved top-to-E shearing and recumbent lineation-parallel folding followed by the formation of ductile-to-brittle normal-sense shear zones. We discuss this structural evolution in the light of existing and new tectonic models, including early Devonian core-complex exhumation of the Oygarden Complex.