U-Pb ages and Nd isotope characteristics of the lateorogenic, migmatizing microcline granites in southwestern Finland

Abstract

U-Pb ages and whole-rock Nd isotope data have been obtained from the Paleoproterozoic lateorogenic migmatizing microcline granites of southwestern Finland. Isotope dilution and ion microprobe U-Pb data on zircons and monazites show that the age spectrum of these granites is at least 1.85-1.82 Ga. Commonly, zircons and monazites record the same ages. The age variation in the Veikkola granite area is of the order of 25 Ma and indicates that this seemingly homogeneous granite consists of two separate intrusions. The zircons of the lateorogenic granites are pervasively altered and conventional U-Pb results are commonly discordant. The ion microprobe studies reveal that the granites contain very few inherited zircons with preserved original U-Pb isotope ratios, with the exception of the Oripää granite. Initial ε Nd values, mostly in the range of -0.5 to -1.0, imply a moderate input of older crustal material into most of the lateorogenic granites. A shift from more juvenile to less radiogenic Nd isotope composition is observed from north to south, and the variation pattern of ε Nd values of the lateorogenic granites is thus similar to that of the surrounding synorogenic granitoid rocks.

Microstructures associated with the Sottunga-Jurmo shear zone and their implications for the 1.83-1.79 Ga tectonic development of SW Finland

“…D3 corresponds best to the ~1.84-1.80 Ga Svecobaltic orogeny of Lahtinen et al (2005) and is approximately simultaneous with the late phase of LSGM anatectic granite magmatism and migmatisation at ~1.85-1.82 (e.g. Kurhila et al 2005). However, magmatism and extensive migmatisation of that age have not been observed within and to the south of the SJSZ.…”

“…The LSGM rocks are extensively migmatised and intruded by subhorizontal sheets of anatectic granites and tonalites at 1.85-1.82 Ga (e.g. Johannes et al, 2003;Kurhila et al, 2005;Stålfors & Ehlers, 2005). In contrast to the LSGM, the SJSZ and the area to the SW consist mainly of amphibolite facies orthogneisses dominated by steep foliations and fold axial surfaces (Fig.…”

“…Väisänen & Hölttä, 1999;Kurhila et al, 2005). From c. 1.85-1.84 Ga onwards, much of the regional deformation partitioned into regional folds and ductile crustal-scale shear zones throughout the Fennoscandian shield (e.g.…”

U-Th-Pb zircon geochronology on igneous rocks in the Toija and Salittu Formations, Orijärvi area, southwestern Finland: Constraints on the age of volcanism and metamorphism

“…From southeastern Finland ages on metamorphism and crustally derived granites ranges from c. 1840 Ma to 1795 Ma (Korsman et al 1984;Nykänen, 1988;Vaasjoki & Sakko, 1988). Theses data, with apparent clusters at c. 1850 − 1840, 1830 − 1820, 1815 and 1795 Ma, suggest that high heat flow events may have been episodic rather than continuous, in accordance with the view of Kurhila et al (2005) who stated that the heat flow took place in separate pulses. Successive intrusion of mafic magmas into mid-crustal levels could account for the episodic heat pulses (Schreurs & Westra, 1986).…”

“…In southwestern Finland there is zircon data on 1.85 Ga and 1.83 Ga metamorphism (Torvela et al, 2008), 1824 ± 5 Ma and 1804 ± 14 Ma leucosomes and c. 1830 Ma and younger monazite data from granites (Suominen, 1991;Nironen, 1999;Ehlers et al, 2004). From the West Uusimaa area there are from c. 1.85 Ga to 1813 ± 4 Ma granites (Kurhila et al, 2005;Skyttä & Mänttäri, 2008) and from 1832 ± 2 Ma to 1792 ± 5 Ma metamorphic ages (Mouri et al, 2005 and this study). From southeastern Finland ages on metamorphism and crustally derived granites ranges from c. 1840 Ma to 1795 Ma (Korsman et al 1984;Nykänen, 1988;Vaasjoki & Sakko, 1988).…”