In situ Lu-Hf dating of garnets: reconstructing subduction zone histories in the Seve Nappe Complex, Scandinavian Caledonides

The Seve Nappe Complex (SNC) of the Scandinavian Caledonides preserves a record of HP-UHP metamorphism related to continental subduction along the Baltican margin. Geochronological studies traditionally identified a late Cambrian (c. 490–480 Ma) (U)HP event in the northern SNC, whereas the southern SNC was interpreted to record younger Ordovician metamorphism at c. 470–455 Ma (Gee et al. 2020, and references therein), leading to models of localized late Cambrian subduction restricted to the north.

In this contribution, we present a new approach using in situ Lu-Hf geochronology on polymetamorphic garnets to further constrain the subduction histories in the SNC. It is a powerful tool to directly date garnet growth associated with (U)HP conditions and allows recovery of early metamorphic histories in the Scandinavian Caledonides.

Preliminary results show ages of 495-480 Ma for a paragneiss in Marsfjället, a garnet schist in Avardo, two garnet schists in Lillfjället, two eclogites in Sjouten, and a schist in EASU. This extends the late Cambrian-early Ordovician subduction record to the central SNC. Furthermore, three Avardo eclogites yield ages of 460-450 Ma, indicating that the central SNC was affected by two metamorphic events, both possibly (U)HP.

In the ongoing project, in situ Lu-Hf dating will be applied on garnets farther south in the SNC to constrain the spatial extent of late Cambrian subduction of Baltica.

 

References:

Gee, D.G., Klonowska, I., Andréasson, P.G. and Stephens, M.B. 2020. Middle thrust sheets in the Caledonide orogen, Sweden: the outer margin of Baltica, the continent–ocean transition zone and late Cambrian–Ordovician subduction–accretion. Geological Society Memoir, 50, 517–548, https://doi.org/10. 1144/M50-2018-73