Bulletin of the Geological Society of Finland (Jun 1999)
Palaeoproterozoic prograde metasomatic-metamorphic overprint zones in Archean tonalitic gneisses, eastern Finland
Abstract
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 craton in Palaeoproterozoic and/or reactivated Neoarchaean zones of weakness. The distribution of late-stage Crd-Sil metasomatism-metamorphism indicates that the corresponding thermal event was widespread in the Nurmes-Sotkamo area. Isotopic studies on Archaean granitoids and greenstone belts also indicate such late heating. According to pressure determinations, the Archaean±Palaeoproterozoic crust has been uplifted and exhumed about 15-20 km since 1850 Ma. Contemporaneous magmatic activation occurred in the North Karelia Schist Belt, too. The seismic deep structure of the crust, particularly the thick high-velocity layer in the lower crust, postulates some kind of disturbance in the lower crust and lithospheric mantle. The dilatational late-stage heat flow, CO2-rich fluidization and Mg metasomatism, and exhumation are connected with this disturbance. It is suggested that they were related to magmatic underplating into the lower crust after the Svecofennian-Archaean collision.
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