Palaeoproterozoic prograde metasomatic-metamorphic overprint zones in Archean tonalitic gneisses, eastern Finland

• Published in: Bulletin of the Geological Society of Finland
• Main Authors: M. Pajunen, M. Poutiainen
• Format: Article in Journal/Newspaper
• Language: English
• Published: Geological Society of Finland 1999
• Subjects: Archean; Paleoproterozoic; tectonics; reactivation; overprint; metamorphism; metasomatism; P-T path; mineral reaction; fluids; fluid inclusions; kyanite; cordierite; xenotime; U/Pb age; eastern Finland; Geology; QE1-996.5; Kainuu; karelia*
• Online Access: https://doi.org/10.17741/bgsf/71.1.005; https://doaj.org/article/ca41137158b3426f902a3dd45525cf77

Several occurrences of coarse-grained kyanite rocks are exposed in the Archaean area of eastern Finland in zones trending predominantly northwest-southeast that crosscut all the Archaean structures and, locally, the Palaeoproterozoic metadiabase dykes, too. Their metamorphic history illustrates vividly Palaeoproterozoic reactivation of the Archaean craton. The early-stage kyanite rocks were formed within the framework of ductile shearing or by penetrative metasomatism in zones of mobile brecciation. Static-state coarse-grained mineral growth during the ongoing fluid activity covered the early foliated fabrics, and metasomatic zoning developed. The early-stage metasomatism was characterized by Si, Ca and alkali leaching. The late-stage structures are dilatational semi-brittle faults and fractures with unstrained, coarse-grained fabrics often formed by metasomatic reactions displaying Mg enrichment along grain boundaries. Metamorphism proceeded from the low-T early-stage Chl-Ms-Qtz, Ky/And-St, eventually leading to the high-T late-stage Crd-Sil assemblages. The thermal peak, at 600-620°C/4-5 kbar, of the process is dated to 1852+2 Ma (U-Pb) on xenotime. Al-silicate growth successions in different locations record small variations in the Palaeoproterozoic clockwise P-T paths. Pressure decreased by c. 1 kbar between the early and late stage, i.e. some exhumation had occurred. Fluid composition also changed during the progression, from saline H2O to CO2, rich. Weak retrograde features of high-T phases indicate a rapid cooling stage and termination of fluid activity. The early-stage Ky-St assemblages resemble those described from nearby Palaeoproterozoic metasediments in the Kainuu and North Karelia Schist Belts, where the metamorphic peak was achieved late with respect to Palaeoproterozoic structures. The static Ky-St metamorphism in kyanite rocks was generated by fluid-induced leaching processes at elevated T during the post-orogenic stage after collision of the Palaeoproterozoic island arc complex with the Archaean ...