Evidence for 1.82 Ga transpressive shearing in a 1.85 Ga granitoid in central Sweden: implications for the regional evolution

Abstract

Two crustal-scale shear zone systems, the Storsjön–Edsbyn deformation zone and the Hassela shear zone join in the western part of central Sweden and form an anastomosing pattern of ductile shear zones. Several of these, up to 1 km wide, zones truncate an intrusion previously defined as a component of the major, ca. 1.80–1.77 Ga magmatic suite (the Revsund granitoids) in north central Sweden. However, within the southern part of the intrusion, titanites that are components of the fabric in a pervasively deformed shear zone (Forsaån) have a U–Pb TIMS age of 1816±2 Ma. Zircon porphyroclasts in the same fabric yield a U–Pb SIMS age of 1849±14 Ma, overlapping with the U–Pb TIMS age of 1851±2 Ma for titanites from an undeformed granitoid. These results are interpreted to define the ages of ductile shearing and magma emplacement, respectively. This interpretation is supported by microstructures as well as the regional structural pattern characterised by shear zones enveloping virtually undeformed pods of the granitoid. Previously reported and locally existing magmatic flow structures indicate some syn-emplacement deformation. The kinematic results, characterised by a high proportion of pure shear within the granitoid and oblique dextral shearing along its margin, reflect strain partitioning under transpressive conditions. The kinematic and geochronological results fit into the regional framework of late-orogenic localised deformation along crustal scale shear zones. Recent, absolute datings of shear fabrics defines ca. 1.82–1.80 Ga as a period of shear zone activity in central Sweden. Temporally, this period overlaps with the emplacement of S-type granites and granitoids derived from deeper crustal levels. A similar situation exists in southern Finland. Structural, geochronological and geophysical data suggest that the crustal scale shear zones in central Sweden and southern Finland may be related temporally and spatially. The age and the post-tectonic nature (on a regional scale) of the dated 1.85 Ga granitoid have important regional implications. As both the age of the intrusion and the ductile shearing are older than the established age range (1.80–1.77 Ga) of the Revsund granitoids, either an extended intrusive history for the suite is indicated, or the granitoids in this part of the type areas should be excluded from the Revsund suite. Their post-tectonic nature implies that the pervasive Svecokarelian deformation in the area occurred earlier than generally assumed (1.85–1.80 Ga). On the other hand, approximately coeval, calc-alkaline granitoids to the south-east of the investigated area are folded and foliated pervasively, indicating a later Svecokarelian evolution in that area, i.e. that the granitoids are located in domains with separate orogenic evolutions.