Metamorphism in the Tømmerås area, western Scandinavian Caledonides

Abstract

Structural and metamorphic discontinuities in the Tømmerås basement-cover sequence confirm the allochthonous character of the Lower Snåsa Group, the Leksdalsvann Group, and the upper basement of the Tømmerås Window. Three major deformational events are recognized. During an early event the upper unit (the Snåsa Group amphibolites and gneisses) was metamorphosed under amphibolite facies conditions and thereafter became emplaced above the unmetamorphosed Leksdalsvann sediments. The interkinematic (D1-D2) parageneses of these sediments indicate a high P _H2O and T ca. 450°C. D2 recrystallization, corresponding to the transitional greenschist-amphibolite facies (500–550°C, P > 600 MPa), is largely controlled by the distribution of shear strain. Calcium-rich mantles of late D2 garnets in the same sediments indicate a pressure peak before the main thrusting (late D2). D3 folded the thrusts and the isograd pattern established during D2 around the NE-SW trending axis of the Tømmerås Antiform. Various features indicate the subordinate role of Caledonization in the Tømmerås ‘autochthon’: the decrease of grade with increasing tectono-stratigraphic depth, the open megastructures and the mylonites and local PT-peaks. This is noteworthy because Tømmerås is located on the very border of the coastal gneiss region, an area previously supposed to have been subject to Caledonian mobilization.

Key Words:

• Metamorphism
• deformation
• allochtons
• nappes
• basement-cover relations
• geologic thermometry
• paragenesis
• texture
• greenschist-amphibolite
• facies
• phengite
• biotite
• garnet
• chemical composition
• garnet zoning
• metasedimentary rocks
• metamorphic rocks
• amphibolite
• Tømmerås Antiform
• Tømmerås Window
• Leksdalsvann Group
• Lower Snåsa Group
• Bjørntjern schist
• Caledonides
• Ordovician
• Silurian
• Trondheimsfjord
• central Norway
• N6346 N6414 E1239 E1119