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

Two crustal-scale shear zone systems, the Storsjon-Edsbyn deformation zone and the Hassela shear zone join in the western part of central Sweden and f orm an anastomosing pattern of ductile shear zones. Several of these, up to 1 km wide, zones truncate an intrusion previously defined as a component o f the major, ca. 1.80-1.77 Ga magmatic suite (the Revsund granitoids) in no rth central Sweden. However, within the southern part of the intrusion, tit anites that are components of the fabric in a pervasively deformed shear zo ne (Forsaan) have a U-Pb TIMS age of 1816 +/- 2 Ma. Zircon porphyroclasts i n 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 granit oid. These results are interpreted to define the ages of ductile shearing a nd magma emplacement, respectively. This interpretation is supported by mic rostructures as well as the regional structural pattern characterised by sh ear zones enveloping virtually undeformed pods of the granitoid. Previously reported and locally existing magmatic flow structures indicate some syn-e mplacement deformation. The kinematic results, characterised by a high prop ortion of pure shear within the granitoid and oblique dextral shearing alon g its margin, reflect strain partitioning under transpressive conditions. T he kinematic and geochronological results fit into the regional framework o f late-orogenic localised deformation along crustal scale shear zones. Rece nt, 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 t he dated 1.85 Ga granitoid have important regional implications. As both th e age of the intrusion and the ductile shearing are older than the establis hed 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 par t of the type areas should be excluded from the Revsund suite. Their post-t ectonic nature implies that the pervasive Svecokarelian deformation in the area occurred earlier than generally assumed (1.85-1.80 Ga). On the other h and, approximately coeval, calc-alkaline granitoids to the south-east of th e 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. (C) 2001 Elsevier Science B. V. All rights reserved.