Pre-1.94 to post-1.88 Ga sediment depositional environment and c. 1.94 Ga felsic magmatism in the Knaften area, northern Sweden

Abstract

ABSTRACTThe Knaften area is situated in the northern part of the Bothnian Basin in the central Svecofennian province of Sweden. Sedimentary rocks are here conformably intercalated with mafic to intermediate volcanic rocks. The depositional environment in the south-eastern part of the Knaften area was syn-orogenic deep water where tectonic activity caused volcanism and faulting. Mass flow processes transported material down slope and formed turbidites and debris flow deposits. Later, two phases of deformation affected the supracrustal rocks. Detrital zircons in a turbiditic sedimentary rock gave ages between 2.07 and 1.88 Ga, and between 2.93 and 2.62 Ga, indicating a substantial Archean component. The 1.88 Ga maximum depositional age is coeval with those of other supracrustal successions in the region, e.g., the mainly sedimentary Härnö and Vargfors groups, as well as the mainly volcanic Skellefte and Arvidsjaur groups. Zircons in the Knaften granite and the Knaften porphyry dyke have U-Pb ages of 1939 ± 3 Ma and 1939 ± 4 Ma, respectively. They confirm previously poorly constrained 1.95–1.94 Ga ages for these rocks. The intrusions cross-cut the mafic to intermediate volcanic rocks that, by consequence, must be older than 1.94 Ga. The new zircon data thus point to an age difference of about 50 to 60 million years between the felsic magmatism and the deposition of the dated turbiditic sediments. The oldest volcanic rocks (pre-1.94 Ga) in the Knaften area are evidence of at least one additional depositional stage in the Svecofennian tectonic evolution.