Late‐orogenic Svecofennian deformation in SW Finland constrained by pegmatite emplacement ages

Abstract

Late stages of the Svecofennian orogeny in SW Finland were related to a tranpressional stress field and dextral movements along crustal scale shear zones under decreasing temperature and pressure conditions. In the Kemiö area, a minimum estimate for the time span of movements along one of these shear zones is obtained from the ages of 1840 to 1830 Myr‐old microcline granite sheets, related to early ductile deformation, and by dating structurally late rare‐mineral granite pegmatites, related to brittle deformation. One pegmatite was emplaced when the rheological conditions in the gabbro changed from ductile to semi‐ductile and brittle. It has U‐Pb ferrotapiolite ages ranging between 1807.0 ± 2.9 Myr (2 s̀) and 1803.1 + 2.9/ ‐2.0 Myr (2 s̀). Another pegmatite emplaced under brittle conditions has an U‐Pb ferrotapiolite age of 1802.9 ± 1.3 Myr (2 s̀). These pegmatites were emplaced preferentially in gabbroic rocks, that showed brittle to semi‐ductile deformation at a time when their more felsic host‐rocks still showed ductile deformation. The age range bracketed by the microcline granite sheets and the structurally late pegmatites indicates that ductile deformation related to the transpressional Late Svecofennian tectonic regime in southwestern Finland, persisted for at least 30–40 Myr.

Structural and magmatic evolution in the Loimaa area, southwestern Finland

“…The granites were subsequently refolded in open folds with E-W trending fold axes. Lindroos et al (1996) dated a pegmatite in the Kemiö area and concluded that transpressional deformation lasted until 1803 Ma. As the overall structural pattern in the LSGM zone differs from that in the Loimaa area, where the E-W to NE-SW trending structures were developing already 1870 Ma ago, it is difficult to apply this model to the Loimaa area.…”

Apparent boudinage in dykes

“…Polyphase deformation, metamorphism, migmatisation and magmatic activity ranges in age from about 1890 to 1800 Ma (Ehlers et al, 1993;Lindroos et al, 1996). Magmatic activity included dyke-like intrusion of minor mafic magmas and abundant pegmatites into high metamorphic grade (up to anatexis) metasediments and granitoid complexes.…”

Secondary Fe–Mn-oxides in minerals heavily damaged by α-recoil: possible implications for palaeomagnetism

“…It contains about 1 wt% SnO 2 (i.e., 0.04 p.f.u.) and was dated at 1,802.9 ± 1.3 Ma (2r; Lindroos et al 1996). Material from the same, highly evolved pegmatite was taken by Pesonen (1999) and Pesonen et al (1999) for a palaeomagnetic study.…”

“…There is a tendency toward Mn-rich and Ta-rich compositions with pegmatite evolution, although there may be differences depending on tectonic setting and concomitantly melt source of the pegmatites (e.g., Cerny 1982aCerny , b, 1991. For this study, we selected columbite-tantalite minerals from the Svecofennian (Broddbo, Kolsva, Kimito) and Sveconorwegian (Tveit, Torsgruvan, Tangen, Vintergruvan, Karlshus) provinces of Finland, Sweden, and Norway, as well as from a Kibaran pegmatite (Ruhmebe) of Burundy (see Table 1), that had been previously dated by the U-Pb method (Romer and Lehmann 1995;Lindroos et al 1996;Romer and Smeds 1997). With the exception of samples Kimito and Broddbo, the investigated columbite-tantalite samples have low contents of Ti, W, and Sn, which may substitute for Nb and Ta, and low contents of Mg, Ca, and Sc, which may substitute for Mn and Fe (cf.…”

“…Searching for a carrier of magnetic remanence in highly evolved pegmatites, Pesonen (1999) and Pesonen et al (1999) noticed that ferrotapiolite or micro-inclusions in ferrotapiolite were carrying a magnetic remanence. Since ferrotapiolite allows the determination of precise and accurate U-Pb ages (Lindroos et al 1996), this mineral represents an example where remanence and age can be determined on the same crystal.…”