Late-orogenic, ductile shear zones and protolith ages in the Svecofennian domain, central Sweden

Abstract: The timing of deformation along ductile shear zones in the Svecofennian Domain in central Sweden has generally been constrained by their relationship to magmatic and metamorphic events of known age. A better resolution for the timing of the deformation can be obtained by direct, radiometric dating of minerals which are constituents of the shear fabric.In this study, several, high-T shear zones related to two large-scale, shear zone systems in central Sweden, the Hassela Shear Zone (HSZ) and the Storsjön-Edsbyn Deformation Zone (SEDZ) have been dated using the U-Pb isotopic system in titanite. The microstructures in these zones indicate that the titanite is part of the deformational fabric. The U-Pb titanite results define a narrow time interval between c. 1815 Ma and 1800 Ma as a period of ductile shearing. In combination with previous and new kinematic data - this dominated by dextral horizontal components - indicates that ductile shearing was related to approximately N-S contraction during the late stage of the Svecokarelian orogeny. Temporally and kinematically these results are in accordance with shear zone activity in southeastern Sweden and southern Finland.The character of the shear zones is dependant on the nature of the protolith rocks and the temperature during deformation. In the northwest, where the HSZ and the SEDZ partly join, coarse C/S-mylonites are typical for deformation zones developed in 1.86-1.85 Ga K-feldspar megacryst bearing granites. Deformation zones in older rocks are usually plane-parallel and commonly consist of juxtaposed lithologies. The kinematic characteristics of these, essentially coeval, zones vary. Strain partitioning during transpressive conditions is indicated by the high proportion of orthogonal pure shear recorded in the most prominent zone (the Forsaån zone) and the oblique dextral, and dextral strike-slip kinematics of neighbouring shear zones.Some of the shear zones in this area appear to have been reactivated, or affected by a hydrothermal or magmatic event at c. 1.75 Ga, as indicated by U-Pb titanite and apatite data. One of these zones has further localised warm, saline fluids in Phanerozoic time. These fluids have affected the outer parts of titanite crystals, formed composite zircon grains and almost completely reset the U-Pb system in metamict zircon. These minerals yield a lower intercept age of c. 0.4 Ga in a U-Pb diagram, suggesting that the fluid flow was driven by the Caledonian orogeny at temperatures as low as 150-200oC.The shear zones are mainly separated by generally undeformed K-feldspar megacryst bearing granites. These granites were previously interpreted to be part of the c. 1.80 Ga Revsund suite. However, the unexpectedly high emplacement ages of 1.86-1.85 Ga obtained here for this type of granites requires a reassessment of that assumption. Their, to a large extent, undeformed character may also indicate that the regional tectonometamorphism occurred earlier in this area than further to the south.