Tectonometamorphic history of the Østfold Gneiss Complex, Idefjorden lithotectonic unit, southeastern Norway

The Sveconorwegian Orogen comprises diverse lithotectonic units formed and shaped by magmatic, sedimentary and tectonic processes at different times in the Meso- and Neoproterozoic. Evaluations of the development of each lithotectonic unit are necessary to verify the still controversial mechanisms proposed for the evolution of the orogen. The present study is focused on a particular occurrence within the Østfold Gneiss Complex of the Idefjorden lithotectonic unit. The studied locality, near Halden, south Norway, offers a 250 m-wide section that was saw-cut with a helical diamond-wire. The country rock is a high-strain shear zone in migmatitic biotite gneiss, which contains dismembered (boudinaged, sheared and flattened) fragments of garnet amphibolite and several generations of pegmatite pods and veins. Field mapping and U–Pb dating of zircon, monazite, titanite and rutile reveal the following:(1) The protolith of the host gneiss was likely greywacke, with a Mesoproterozoic detrital zircon population derived from the Gothian Stora Le–Marstrand volcanic assemblage. (2) A Gothian event of metamorphism and gneissification occurred at 1549 ± 4 Ma (2 sigma uncertainty). (3) Major Sveconorwegian metamorphism and deformation caused by SE-directed contraction of the Idefjorden unit spanned several stages between c. 1041 and 1033 Ma. (4) Local retrogression, with formation of secondary titanite, followed at around 920 Ma, likely related to emplacement of the Bohus–Idefjorden granite intrusion, and possibly linked to uplift and rotation of the shear zone and normal faulting, the last of which occurred in the brittle regime.