Ordovician–Silurian deformation of the Neoproterozoic upper gneiss unit in the northern Seve Nappe Complex: implications for subduction of the Baltican margin

Abstract

The upper gneiss unit is exposed in the northernmost Seve Nappe Complex (SNC) in the Scandinavian Caledonides. To investigate the Caledonian tectonic history of the unit, in situ white mica and biotite 40 Ar/ 39 Ar geochronology was applied to a leucogranite and two paragneisses. The leucogranite exhibits low-strain traits. Biotite porphyroblasts yielded a cooling age of 459 ± 2 Ma. White mica that replace biotite and plagioclase provided a crystallization age of 436 ± 5 Ma. White mica in both paragneisses exhibit high-strain characteristics associated with top-to-the-east sense of shear. One paragneiss provided dispersed late Cambrian to Late Ordovician dates with the youngest approximating deformation at 459 ± 2 Ma. The older dates are interpreted to reflect 40 Ar inherited from a previous metamorphic event. The second paragneiss yielded a deformation age of 434 ± 2 Ma. The collective dataset is interpreted to record exhumation of the upper gneiss unit at c. 459 Ma and deformation and fluid infiltration at c. 434 Ma during continental collision. The events closely resemble the deformation histories of other northern SNC terranes. Synthesizing these results with those for other northern SNC terranes suggests that the Baltican margin underwent oblique, south-to-north subduction during late Cambrian time. Supplementary material: Tables providing mineral chemistry of white mica and biotite and 40 Ar/ 39 Ar geochemistry results are available at https://doi.org/10.6084/m9.figshare.c.6639993 Thematic collection: This article is part of the Caledonian Wilson cycle collection available at: https://www.lyellcollection.org/topic/collections/the-caledonian-wilson-cycle