TY - JOUR
T1 - A regional 1.92 Ga tectonothermal episode in Ostrobothnia, Finland
T2 - Implications for models of Svecofennian accretion
AU - Williams, Ian S.
AU - Rutland, R. W.Roye
AU - Kousa, Jukka
PY - 2008/8/20
Y1 - 2008/8/20
N2 - The mostly metasedimentary Svecofennian Western Pohjanmaa belt in Ostrobothnia, Finland, can be divided into two stratigraphic groups separated by a major unconformity that reflects deformation following regional metamorphism. The western Lappfors group, interpreted as a Svionian basement complex, has strong W-trending folding and aeromagnetic signatures that contrast with the overlying eastern Evijärvi group, interpreted as lower Bothnian, which has more open N-trending folding and magnetic patterns. Several lines of evidence date the unconformity at ∼1.92 Ga. Detrital zircons from two samples of Lappfors group metasediment, and a sample of the basal Nivala gneisses in the Eastern Pohjanmaa belt, have 1.92-1.91 Ga post-depositional low-Th/U metamorphic overgrowths. The maximum deposition age of the Lappfors sedimentary protoliths, based on detrital zircon ages, is between ∼1.99 and ∼1.95 Ga. Three samples of Bothnian sediments lack pervasive ∼1.91 Ga overgrowths, instead having a variety of detrital zircons as young as ∼1.95-1.91 Ga, reflecting recycling of the underlying basement complex. The maximum deposition age of the Bothnian sedimentary protoliths is inferred to be ∼1.91 Ga. The Niska granitoid, which intrudes the Evijärvi group and is deformed only by the younger tectonic episode affecting that sequence, has a zircon age of 1896 ± 6 Ma. That episode, which established the present relationships between basement and cover, is dated by ∼1.88 Ga metamorphic zircon overgrowths in both the Svionian and Bothnian samples, and by 1878 ± 4 Ma metamorphic monazite from a metasediment from the Savo belt, east of the Nivala district. The post-1.91 Ga volcanic sequences of the Svecofennian Province are unlikely to represent arc accretion. The Svionian metamorphic sequences are probably the remnants of a widespread marginal basin that formed between ∼1.97 and 1.92 Ga, then was accreted to the craton during an Early Svecofennian (∼1.92-1.91 Ga) orogenic phase, forming the basement on which the Bothnian volcano-sedimentary sequences were subsequently deposited.
AB - The mostly metasedimentary Svecofennian Western Pohjanmaa belt in Ostrobothnia, Finland, can be divided into two stratigraphic groups separated by a major unconformity that reflects deformation following regional metamorphism. The western Lappfors group, interpreted as a Svionian basement complex, has strong W-trending folding and aeromagnetic signatures that contrast with the overlying eastern Evijärvi group, interpreted as lower Bothnian, which has more open N-trending folding and magnetic patterns. Several lines of evidence date the unconformity at ∼1.92 Ga. Detrital zircons from two samples of Lappfors group metasediment, and a sample of the basal Nivala gneisses in the Eastern Pohjanmaa belt, have 1.92-1.91 Ga post-depositional low-Th/U metamorphic overgrowths. The maximum deposition age of the Lappfors sedimentary protoliths, based on detrital zircon ages, is between ∼1.99 and ∼1.95 Ga. Three samples of Bothnian sediments lack pervasive ∼1.91 Ga overgrowths, instead having a variety of detrital zircons as young as ∼1.95-1.91 Ga, reflecting recycling of the underlying basement complex. The maximum deposition age of the Bothnian sedimentary protoliths is inferred to be ∼1.91 Ga. The Niska granitoid, which intrudes the Evijärvi group and is deformed only by the younger tectonic episode affecting that sequence, has a zircon age of 1896 ± 6 Ma. That episode, which established the present relationships between basement and cover, is dated by ∼1.88 Ga metamorphic zircon overgrowths in both the Svionian and Bothnian samples, and by 1878 ± 4 Ma metamorphic monazite from a metasediment from the Savo belt, east of the Nivala district. The post-1.91 Ga volcanic sequences of the Svecofennian Province are unlikely to represent arc accretion. The Svionian metamorphic sequences are probably the remnants of a widespread marginal basin that formed between ∼1.97 and 1.92 Ga, then was accreted to the craton during an Early Svecofennian (∼1.92-1.91 Ga) orogenic phase, forming the basement on which the Bothnian volcano-sedimentary sequences were subsequently deposited.
KW - Finland
KW - Metasediments
KW - Ostrobothnia
KW - Svecofennian
KW - Zircon U-Pb ages
UR - http://www.scopus.com/inward/record.url?scp=48349121150&partnerID=8YFLogxK
U2 - 10.1016/j.precamres.2008.05.004
DO - 10.1016/j.precamres.2008.05.004
M3 - Article
SN - 0301-9268
VL - 165
SP - 15
EP - 36
JO - Precambrian Research
JF - Precambrian Research
IS - 1-2
ER -