Lithostratigraphy and UPb geochronology of the Telemark supracrustals in the Bandak-Sauland area, Telemark, South Norway

The Mesoproterowic Telemark supracrustals in southern Norway have been subdivided into four groups: (l) the ca. 1500 Ma volcanic Rjukan group overlain by (2) the quartzite-dominated Seljord group, itself overlain by (3) the ca. 1160 Ma volcanic-sedimentary Bandak group in the west and by (4) the undated Heddal group in the east. New mapping and U-Pb work provide considerable refinement to this stratigraphy. The Ljosdalsvatnet porphyry near the base of the Bandak gro up has an age of 1155 ± 3 Ma age, but it overlies that part of the Seljord gro up, which was folded and eroded before deposition of the Bandak gro up. The Brunkeberg porphyry, which has previously been correlated to the Rjukan gro up, yields an identical age of 1155 ± 2 Ma. It is unconformably overlain by a quartzite, which has also been included into the Seljord group. This group must, therefore, consist of two separate successions separated by a period of folding, here referred to as the Vindeggen gro up (older) and the Lifiell gro up (younger). The former represents a thick, fluvial, shallow sea sandstone - intertidal heterolithic sequence, whereas the latter is composed of a relatively thin and mature beach-shoreline sandstone sequence. The unconformity between the groups is displayed by in situ weathering breccias and quartzite-clast conglomerates at Heksfjellet. The gneissic and supracrustal rocks southwest of Brunkeberg, previously mapped as the Vemork formation, do not belong to the Rjukan gro up, as they overlie the 1155 Ma Brunkeberg formation. The traditional Bandak group is subdivided into the Oftefiell (older), and Høydalsmo (younger) separated by a major unconformity detected under the Røynstaul formation, and the Eidsborg formation ( topmost). The Høydalsmo gro up comprises the Dalaå porphyry that yields an age of 1150 ± 4 Ma. An unconformity has also been mapped under the Heddal group, which erodes both the Vindeggen and Lifjell groups. The Skogsåa porphyry, at the base of the Heddal gro up is dated at 1145 ± 4 Ma, bracketing the time of deposition of the Lifjell group between about 1155 Ma and 1145 Ma ago and making the Heddal group younger than the traditional Bandak rocks under the Dalaå formation. These data reflect a short-lived period of major tectonic activity accompanied by rifting and the generation of A-type intrusive bodies, probably related to an episode of mantle upwelling. There is also evidence for at !east two o ro geni c or major deformation stages: (l) the older p hase, which folded the Rjukan and Vindeggen groups prior to 1155 Ma, and (2) the younger p hase post -da ting 1145 Ma, which folded the Oftefjell, Høydalsmo, Lifjell, and Heddal groups and the Brunkeberg and Eidsborg formations for the first time, and the Rjukan and Vindeggen groups for the second time.