A Rb-Sr isotope study of granitoids and related mylonites in the Tännäs Augen Gneiss Nappe, southern Swedish Caledonides

Abstract

The Tännäs Augen Gneiss Nappe and parts of the related, subjacent Veman Nappe are composed of K-feldspar megacryst bearing granodiorites, which have been subject to low-grade metamorphism and varying degrees of deformation. They form parts of the Middle Allochthon — a long-transported nappe unit in the central Scandinavian Caledonides. The least-deformed granodiorite has a Rb-Sr age of 1610 ± 85 Ma and Sr₁ = 0.7029, while zircons from the same rock give a discordia line with an upper intercept at 1685 ± 20 Ma. These dates reflect the age of intrusion of the rock. The granodiorite is comparable in composition and age with post-Svecokarelian granitoids and related volcanites east of the mountain belt. The more typically deformed augen gneiss does not yield isochrons, but shows disturbed Rb-Sr patterns both for specimens sampled regionally and on smaller scales. At the thrust contacts towards over- and underlying units, where the deformation was extreme, the gneisses have been transformed to dense mylonites. One specimen from the mylonite which separates the Tännäs Augen Gneiss Nappe from the overlying Särv Nappe has been divided into thin slices; these give a Rb-Sr isochron with an early Ordovician age of 485 ± 50 Ma, while mylonites collected on a larger scale scatter about this isochron. The age most probably reflects the time of development of the mylonite. The Sr isotopic equilibration during the Caledonian orogeny has approached completion only in the mylonite and even there only on a decimetre-scale. It has probably taken place mainly by grain boundary diffusion combined with extensive recrystallization.

Key Words:

• Rb-Sr
• U-Pb
• absolute age
• dates
• whole rock
• mineral age
• zircons
• isochron
• Concordia diagram
• deformation
• thrust faults
• diffusion
• recrystallization
• granitoid
• mylonite
• augen gneiss
• Tännäs Augen Gneiss Nappe
• Veman Nappe
• Middle Allochthon
• Caledonides
• Lower Ordovician
• Proterozoic
• west central Härjedalen
• southwestern Jämtland
• west central Sweden
• N6220 N6240 E1345 E1205