The Palaeoproterozoic Grythyttan Field in the Svecofennian Orogen, West Bergslagen, Central Sweden: structure, stratigraphy and age

Abstract

A number of discrete slate belts of limited size occur in the Palaeoproterozoic volcano-sedimentary Bergslagen Group of western Bergslagen in the Fennoscandian Shield of south-central Sweden. The Grythyttan Slate Belt (GSB), studied for more than a century, forms a single basin with the nearby Saxån Slate Belt (SSB). We use the lithostratigraphy of the Grythyttan Slate Formation of the Grythyttan belt, based on basin-scale sedimentary facies associations of the volcaniclastic sediments in time and space, as a method to interpret overall tectonic structure. Contrary to traditional views, we reconstruct the GSB as a single overturned limb of a km-scale anticline with horizontal axis in the hanging wall of an east-vergent thrust fault, reactivating a listric extensional fault. The fold connects the GSB to the neighboring SSB. Folding and thrusting were related to tectonic closure of a volcanic back-arc or intra-arc basin. The early folds were subsequently affected by strike-slip shearing and folding around vertical fold axes, which partitioned preferentially into the least competent lithologies (slates and marbles), significantly modifying the map appearance of the slate belt. The late shearing and folding resulted from accretion of Bergslagen onto the Fennoscandian continental margin during the late, Svecobaltic phase of the Svecofennian orogeny. The GSB forms a thin, intraformational wedge in the Bergslagen Group and represents a relatively short interlude with a conglomeratic alluvial fan and turbiditic volcaniclastics followed by more felsic volcanic rocks resembling those of the Bergslagen Group. We suggest that the GSB, and by inference the other slate belts, stem from calderas, as either terrestrial volcanic lakes or shallow submarine eruption centres. The conglomerates are pre-orogenic, not a post-orogenic molasse as traditionally conceived, and their clast fabric has resulted from sedimentary processes only. Newly determined SIMS U-Pb zircon ages constrain the age of the GSB at c. 1895 Ma.

A 1.9 Ga glacial sedimentary-facies association at low palaeolatitude in the Bergslagen Group, Grythyttan Field, Central Sweden

“…The suggestion of a glacial palaeoclimate at low latitude and elevation stimulated us to scrutinise the Bergslagen Group (BG) for additional evidence of a (peri-)glacial sedimentary environment. Herein, we will describe an ice-front sedimentary facies association in the southern Grythyttan Field (GF), previously dated at c. 1895 Ma (Kuipers et al, 2018).…”

“…The Group has been dated from different, widespread locations to approximately 1.91-1.89 Ga (Stephens et al, 2009). Shallow submarine deposition prevailed, as witnessed by a variety of sedimentological, mineralogical and geochemical details, but occasionally subaerial conditions prevailed (for references see Kuipers et al, 2018). Throughout Bergslagen the BG is accompanied by a synvolcanic intrusive suite of 'older' granites, diorites and gabbros (GDG suite; Stephens et al, 2009; Fig.…”

“…Coordinates: Swedish RT90 grid (km units). Legend: ochre -'older', synvolcanic Svecofennian (GDG) granites; pink -'younger' Svecofennian (GSDG) (Kuipers et al, 2018). Note location and image of ice-wedge cast (Kuipers et al, 2013).…”

“…2) includes a low greenschist-facies metamorphic subdomain of the BG also known as the Grythyttan Field (GF), described in theses by Sundius (1923) and Kuipers (1987); it is intruded on its western side by granites of the post-Svecofennian, <1.8 Ga Transscandinavian Igneous Belt (TIB). The stratigraphy, structure and age of the GF have recently been re-assessed by Kuipers et al (2018). The subparallel Saxån Slate Belt (SSB) and the Grythyttan Slate Belt (GSB) mark the western and eastern limbs of a N-S-trending, east-vergent, map-scale anticline, resulting from upright folding (D2) of the previously extended back-arc or intra-arc basin (D1).…”

Evidence of permafrost in the Paleoproterozoic (c. 1.9 Ga) of Central Sweden

“…The Group has been dated in different, widespread locations to approximately 1.91–1.89 Ga 32 . Shallow marine deposition prevailed, as witnessed by a variety of sedimentological, mineralogical and geochemical details, such as the occurrence of wave‐base sediments and (stromatolitic) limestone, 28 but occasionally subaerial conditions prevailed (for references see 28,33 ). Throughout Bergslagen the BG is accompanied by a synvolcanic intrusive suite of “older” granites, diorites, and gabbros (GDG suite 32 ).…”