Evolution of a gneiss in the Seve nappe complex of central Sweden – Hints at an early Caledonian, medium-pressure metamorphism

Abstract

Ultrahigh-pressure (UHP) metamorphism has recently been reported from various crustal rocks in the Seve nappe complex (SNC) in which microdiamonds were found. We studied a gneiss from the Åreskutan Nappe in central Sweden. This nappe of the SNC was recently interpreted to have experienced UHP metamorphism. The studied medium-grained and well foliated gneiss contains quartz, biotite, garnet, K-feldspar, plagioclase, and potassic white mica with accessory kyanite, sillimanite, rutile, ilmenite, and monazite. Pressure (P) – temperature (T) pseudosections were mainly used to decipher the metamorphic evolution of the gneiss. According to polyphase inclusions such as biotite + chlorite + potassic white mica in the garnet core, the prograde P-T conditions were approximately 600 °C and 8–8.5 kbar. An isobaric heating to 700 °C followed, constrained by homogenous garnet with XCa = 0.03 and XMg = 0.295, the highest Si content of 3.15 per formula unit in potassic white mica, and Zr-in-rutile thermometry. A late migmatitic stage occurred at about 725 °C and 6.5 kbar. U-Th-Pb dating of monazite, included in garnet, with the electron microprobe yielded two populations at 463 ± 12(2σ) Ma (Y2O3 > 2.0 wt%) and 458 ± 7(2σ) Ma (Y2O3 < 0.5 wt%) that were assigned to the prograde evolution before and during/after garnet growth. A matrix population gave 448 ± 12(2σ) Ma (Y2O3 < 0.5 wt%) which we relate to the late migmatitic stage. We hypothesize that the studied rock experienced prograde metamorphism in the uppermost portion of a downgoing continental plate (Baltoscandia) by thrusting under an island arc/microcontinent. Afterwards, the rock was exhumed by mass flow in an exhumation channel during the early Caledonian orogeny.