The Säm fold structure : characterization of folding and metamorphism in a part of the eclogite–granulite region, Sveconorwegian orogen

Abstract: Structural and petrological studies on complexly folded mafic and felsic layered sequences have been mapped in the parautochthonous basement of the Sveconorwegian Province, Sweden. Three deformation phases (D1, D2, D3)involving at least two fold phases (F2, F3) and two lineation phases (L2, L3) characterize a polyphase fold, which is primarily defined by a migmatized and highly competent garnet amphibolite. This unit was folded and boudinaged into map-scale tectonic lenses during upper amphibolite to granulite facies metamorphism. Penetrative deformation during D3 and D2 created strong, composite fabrics. The oldest, D1 event is interpreted as simply a deformation phase forming a gneissic S1. D2 folded the S1 into similar-shaped, upright to inclined, closed to isoclinal folds (F2). L2 lineations have a bimodal distribution with two distinct clusters trending 37/318 and 26/160. The F2 folds were subsequently refolded into a NE-plunging, open, Class 1C, Type 3 interference pattern folds (F3) during D3. The majority of L3 lineations align parallel with the F3 fold axis. L3 has shallow to moderate plunges all trending NE (mean vectors: 19/052, 29/039 & 08/060). Syn-kinematic shear-sense indicators are consistently top-to-the-east. Upper amphibolite to granulite facies was achieved in the garnet amphibolite gneiss. The earliest assemblages preserved plagioclase inclusions in clinopyroxene, and orthopyroxene forming coronas around clinopyroxene and Fe-Ti oxides, indicative of high temperatures ~800-850 °C at ~10 kbar. The texturally equilibrated mineral assemblage consists of garnet + clinopyroxene + plagioclase + quartz + hornblende + Fe-Ti oxide + biotite, and is found stable at P-T conditions of ~700-800 °C and ~9-14 kbar. Mineral assemblages studied in the sillimanite-bearing quartz-feldspar gneiss showed a clockwise P-T-t evolution. A single grain of kyanite is believed to have been found. Tentatively, the paragneiss travelled through the relatively higher pressure, kyanite stability field; possibly during D2. A stable mineral assemblage containing quartz + K-feldspar + plagioclase + biotite + sillimanite + garnet + Fe-Ti oxide, suggests P-T conditions of ~650-850 °C and ~3-10 kbar. Cordierite was found reacting with garnet in the presence of K-feldspar, which suggested D3 decompression of ≤5 kbar occurred at high temperatures (≥650 °C). Tectonic implications relating this structure with deformation observed elsewhere in the Eastern Segment is discussed.