Tectono-metamorphic evolution of metasedimentary host rocks of ultramafic rocks in the Heidal/Sel Group, Trondheim Nappe Complex, Folldal: mineralogical and micro-textural changes during the Scandian thrusting

Abstract

Abstract Several ultramafic outcrops are located along a specific tectono-stratigraphic unit in the Trondheim Nappe Complex, Central Scandinavian Caledonides. The relative timing and kinematics of these rocks are investigated by the use of structural, petrographical and mineralogical data from its host rocks. The metasedimentary host rocks are part of the Sel and Heidal Groups of the Røros and Remsklepp Nappe Complex, in the Upper and Middle Allochthon. The investigated area is located in Folldal and Dovre municipalities, 145 km south of Trondheim, in Hedmark County, in the southern Trondheim Region Caledonides. A geological map compiling data from petrographic and field observations show a wide range of mainly metasedimentary rocks hosting the ultramafic rocks. The most dominant host rocks are garnet-mica schists and quartz mylonites, but the various amount of mica and quartz generates several similar rock types, such as quartzites and metapsammites. After mica and quartz, garnets and chlorite are the most abundant minerals. The dominant fabric in the area is the main foliation, with a gentle, general dip toward southwest, referred to as S1 and a result of a D1 deformation event. Porphyroblasts of garnet and amphibole are interpreted to be from this phase. Mylonites, with amphibolite facies mineral assemblage are prominent at the contacts of the thrust nappes, indicating they formed during peak metamorphism, just before or during the D2 event. The metasedimentary rocks further away from the thrust nappes show greenschist facies assemblage without any obvious kinematic indicators, indicating less strain. Presence of chlorite indicates retrograde metamorphism from the D3 event. The ultramafic rocks are altered through serpentinization, with soapstone at some local sites. The serpentinites contain various amounts of chromite and magnetite and locally large amounts of talc. They have a sharp contact with the host rocks, and no sign of contact metamorphism or partial melting is observed, indicating the ultramafic rocks were tectonically placed on top of the Heidal Group in the late Precambrian or early Paleozoic, before sedimentation of the Sel Group.