Metamorphic Evolution of the Tjeliken Garnet-Phengite Gneiss, Northern Jämtland, Swedish Caledonides

University essay from Uppsala universitet/Institutionen för geovetenskaper


The Tjeliken Mountain in northern Jämtland, central Scandinavian Caledonides is by most authors considered to belong to the Lower Seve Nappe Complex (SNC). However, recently P-T conditions similar to the Middle Seve have been constrained for the eclogite at the top of the mountain, revitalizing the tectonic debate about Tjeliken. Also the timing of high-pressure metamorphism is debated. Two earlier studies of the eclogite yield ages between 464 Ma and 446 Ma. This study focuses on the garnet-phengite gneiss hosting the eclogite. By construction of P-T conditions and dating the two discrepancies above are investigated.

U/Pb zircon dating by secondary ion mass spectrometry technique (SIMS) targeted on metamorphic rims yield a concordia age of 460.2 ± 2.7 Ma corresponding well to earlier c. 463.7 ± 8.9 Ma Sm/Nd dating of the eclogite.

The inferred peak mineral assemblage of the gneiss is garnet + phengite + quartz + K-feldspar + titanite ± H2O. Thermodynamic modelling reveal that garnet cores equilibrated within 1.9 - 2.6 GPa and 600 - 700 oC. Fe2+-Mg garnet-phengite thermometry involving garnet rims yields temperatures of c. 650 - 715 oC revealing relatively similar temperatures during growth of garnet core and rim, respectively.

Garnet chemistry is characterised by oscillatory zoning with an antithetic pattern of Ca and Fe. The former decreases from core to rim, whereas the latter increases. The opposite trend is observed in epidote-group minerals suggesting exchange between the two minerals during garnet growth. Skeletal textures and atoll textures together with observed chemical pattern may indicate multiple garnet growth episodes.

The results of the study points toward similar P-T history of the Tjeliken eclogite and gneiss in favour of the interpretation of considering the whole Tjeliken to belong to the Lower Seve. The obtained U/Pb age support other age constraints in the area suggesting high-pressure metamorphism at c. 460 Ma related to a subduction event affecting the central Scandinavian Caledonides at c. 460 - 450 Ma.


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