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Metamorphic and structural evolution of the Early Proterozoic Puolankajärvi Formation, Finland – II. The pressure–temperature–deformation–composition path
Author(s) -
TUISKU P.,
LAAJOKI K.
Publication year - 1990
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1990.tb00479.x
Subject(s) - geology , geochemistry , kyanite , andalusite , staurolite , sillimanite , proterozoic , biotite , cordierite , schist , metamorphic rock , partial melting , geothermobarometry , mineralogy , petrology , mantle (geology) , tectonics , paleontology , materials science , metallurgy , quartz , ceramic
The paragenetic relationships between sillimanite, andalusite, kyanite, chlorite, cordierite, biotite, garnet and staurolite in the Early Proterozoic Puolankajärvi Formation (PjF), together with mineral compositions, are used to construct a partial petrogenetic grid for metapelites with significant Mn content (MnO = 0.1–0.5%) by adding a six‐phase invariant point over the garnet‐absent invariant point for Mn‐free AMF‐phases. The grid and textural relations of the PjF are used to construct part of the P–T –deformation path for the PjF. Relatively short deformation pulses and associated flow of oxidizing fluid along shear zones were responsible for the paragenetic and compositional changes during cooling and decompression at 600–500°C and 6.0–2.5 kbar. Oxidation led to decreased Fe 2+ and further stressed the importance of Mn (increased Mn/divalent cations). A tectonothermal evolution of the Kainuu Schist Belt is presented which includes crustal thinning and steepening of a previously established thermal gradient. This was followed by thrusting and folding of the isotherms into a thermal antiform on the western side of the belt.