M. Vaasjoki’s research while affiliated with Geological Survey of Finland and other places

Lead isotope analyses of 25 sulfide samples (galenas, iron sulfides, and sulfosalts) from five different mines of the Skellefte district, northern Sweden, demonstrate that the Pb-isotopic composition of galenas and other sulfides rich in lead varies between individual deposits within the district. This contrasts with many other base-metal districts, where ore lead is isotopically homogeneous on a regional basis. Although all of the Skellefte leads are depleted in 207Pb relative to average global lead evolution models, thus suggesting a large mantle-derived component in their sources, the Nasliden deposit lying at the contact of the host volcanic rocks and the overlying metasediments contains a significant component of crustal lead. It is concluded that while the Pb-isotope data are consistent with a volcanic exhalative origin of the ores of the Skellefte district, they also demonstrate that older crustal lead was incorporated into the sulfides during their emplacement and the subsequent period of magmatic and metamorphic activity which followed their deposition.

Reliable concordant U-Pb zircon data obtained for volcanic rocks in the Archaean greenstone belts in Finland indicate distinct age groups for each belt: Suomussalmi 2.94, 2.87 and 2.82 Ga; Kuhmo-Tipasjärvi 2.84-2.80 Ga; Ilomantsi-Kovero 2.88 and 2.75 Ga; and Oijärvi 2.82-2.80 Ga. The relative abundance of rocks within these age groups still remains unclear. Results from the Kuhmo belt indicate that the age of felsic and gabbroic rocks in the central part of the belt (Kellojärvi area) is 2798 ± 2 Ma, which is also the minimum age for the local mafic-ultramafic magmatism, including komatiites. Tholeiitic mafic rocks in the Kuhmo belt, as represented by the Moisiovaara gabbro, are 2823 ± 6 Ma in age, which is considered the maximum age for the komatiites. Both the Kuhmo and Tipasjärvi belts contain sedimentary rocks that were deposited after 2.75 Ga, and thus at least 50 Ma after the volcanism. Still younger sediments have been found in the Arola area of the Kuhmo belt, where a deformed quartzite contains detrital zircon as young as 2.70 Ga. The sediments in the paragneiss belts were deposited ca. 2.72 Ga ago. Sm-Nd isotopic results show that volcanic rocks in the Kuhmo and Tipasjärvi belts largely represent newly mantle-derived material. The bulk of the granitoids surrounding the belt postdate the volcanic rocks, and the isotope results as a whole suggest that the contribution of older crustal material was negligible and does not support the existence of continental basement during the formation of the supracrustal rocks within these belts. In contrast, in the Suomussalmi belt, Sm-Nd and Pb isotope results indicate a major involvement of significantly older crustal material (>3 Ga). A minor contribution of older crustal material is also evident in the Ilomantsi belt, where some igneous rocks contain xenocrystic zircon up to 3.3 Ga in age. Altogether, the isotope results suggest that the schist belts store a longlived (>200 Ma), fragmentary record of geological evolution, possibly in various geodynamic settings, including an oceanic plateau (Kuhmo, Tipasjärvi), island arc (Ilomantsi), back arc/intra-arc (paragneiss belts) and intra-continental rift (Suomussalmi). (.

The ∼500km long zone of late Svecofennian leucogranites in southern Finland is characterized by felsic, in places migmatitic magmatism that crosscuts older Paleoproterozoic crustal segments. SIMS and ID-TIMS U–Pb isotopic data on zircon and monazite from 15 granite and migmatite samples show that the emplacement of individual leucogranite plutons took place between ∼1.85Ga and ∼1.79Ga. This is a considerably longer period than what has traditionally been assigned to these granites. The youngest ages are found in the eastern part of the zone, which is the target area of this study. Significant age differences between granite plutons imply that the granites were emplaced in various depths and/or in various stages of the late Svecofennian, in support of a multi-stage accretionary history of the arc complexes of southern and western Finland. The late Svecofennian granites represent recycled crustal material and therefore commonly contain inherited zircons. The problem posed by the heterogeneity of zircon ages is overcome to some extent by the fact that the youngest zircons are invariably as old as the monazites, and thus monazite can be used as an emplacement age indicator. Zircon inheritance patterns vary somewhat between the different plutons within the granite zone. Archean inherited ages range from ∼2.8Ga to ∼2.5Ga and Paleoproterozoic 2.1–2.0Ga inheritance is also commonplace, despite the lack of known exposed source rocks of this age. The contrasting age distribution of inherited zircons suggests that both sedimentary and igneous rocks provided source material for these granites.

The first isotope datings from Finland were carried by Olavi Kouvo during his stay in the United States in the mid-1950s, and his doctoral thesis (1958) caused a fundamental change in the understanding of the Finnish Precambrian. It had been generally accepted that there were two great Precambrian orogenies in Finland: the older Svecofennian and the younger Karelian, but Kouvos results showed that the lithologic units associated with these orogenies were in fact coeval and that the granite-gneiss domain northeast of Karelides was much older than the southwestern part of the country. The existence of an ancient plate boundary along the Raahe-Ladoga zone became an accepted fact, not a mere working hypothesis, during the 1960's. The laboratory for isotope geology at the Geological Survey of Finland was established in 1964.

Oxygen isotope ratios of igneous zircon from magmatic rocks in Finland provide insights into the evolution and growth of the Precambrian crust during the Svecofennian orogeny. These data preserve magmatic δ18O values and correlate with major discontinuities in the lower crust. Oxygen isotope ratios of zircon across the 1.88–1.87 Ga Central Finland granitoid complex (CFGC) range from 5.50‰ to 6.84‰, except for three plutons in contact with the adjacent greenstone and metasedimentary belts (δ18O(Zrc) = 7.60‰–7.78‰). There is a systematic variation in δ18O(Zrc) with respect to geographic location in the CFGC, ranging from 6.60±0.23‰ (σ) in the northeast to 5.90±0.40‰ in the west-southwest. These values correlate with a change in crustal thickness and shift in geochemical composition. The oxygen isotope composition of the 1.65–1.54 Ga rapakivi granites and related rocks in southern Finland show a decreasing trend from north to south, independent of their emplacement age. The southern anorogenic granite group has an average δ18O in zircon of 6.14±0.07‰ and the northern anorogenic group has an average δ18O in zircon of 8.14±0.59‰. This difference reflects the boundary between island arc terrains accreted during the Paleoproterozoic.

The central part of the Fennoscandian shield in southwestern Finland and central Sweden hosts the remnants of a 1265-Ma tholeiitic magmatic system associated with fault-bounded basins and continental red-beds. The magmatic rocks are represented by transitional basalt dykes and sills - possibly feeders to once-extensive lavas - that delineate an area of circa 500 km in diameter. Farther to the east, a suite of 1460-Ma dolerites is found in the Lake Ladoga basin of Russian Karelia. These are more alkaline and form at least two major sills that can be followed circa 150 km along strike. In the Lake Ladoga region, an extrusive lave succession is also present. Epsilon-Nd (at 1265 Ma) values of the Finnish (Rämö, 1990 and unpublished data) and Swedish (Patchett et al., 1994) tholeiites range from +0.4 to +3.7 and average at +2.2 ± 0.9 (1 S.D., n = 25). The geographic position of the analysed samples reveals regionally different initial Nd isotope compositions that are not correlative with the Nd isotope composition of the enclosing Palaeoproterozoic crustal country rocks. The alkaline dolerites of the Lake Ladoga basin are distictly less radiogenic (lower long-term Sm/Nd) than the Finnish and Swedish transitional and tholeiitic basalts, having epsilon-Nd (at 1460 Ma) values between -8.6 and -9.2 and a mean value of -8.8 ± 0.2 (1 S.D., n = 7). Overall, the initial Nd isotope compositions probably reflect variation in the mantle source composition of the basalts rather than crustal contamination. The 1265-Ma magmatism in southwestern Finland and central Sweden may reflect the onset of the Sveconorwegian-Grenvillian Wilson cycle, while the 1460-Ma magmatism in Russian Karelia probably reflects thermal contraction of ruptured, relatively thin crust in the aftermath of the thermal perturbations that created the classic Finnish rapakivi granite suites.

The presence of 1.91–1.93 Ga old granitoids at the Archean–Proterozoic boundary along the Raahe–Ladoga zone in Finland has been demonstrated on various occasions. These rocks have been considered to represent juvenile crustal material, as their εNd values are markedly positive. However, as Svecofennian metasediments contain detrital zircons derived from a ca. 2 Ga old source, the possibility has existed that the 1.92 Ga age may have been a mixture between 2 and 1.89 Ga old zircon populations, as such mixing would not markedly affect their neodymium isotopic properties. Also, some syntectonic 1.89 Ga old Svecofennian granitoids contain heterogeneous zircon populations, but it has been impossible to determine the age and origin of the older zircons by conventional methods.NORDSIM ion probe results on three samples from the 1.92 Ga age group confirm the earlier conclusions. Especially important is that no zircons older than 1.95 Ga were detected in the 1.92 Ga group samples. Thus, the 1.92 Ga event was the beginning of the formation of new continental crust in the primitive Svecofennian island arc and these granitoids formed by partial melting of basaltic magmas derived from a depleted mantle source. One sample also contains a younger zircon population formed during the orogenic culmination at 1.89 Ga. In contrast, one grain from a sample representing the 1.89 Ga age group contains an Archean core, which is considered to represent sedimentary detritus assimilated during either magma formation or intrusion.While the results prove the true igneous nature of the 1.92 Ga event, they also rule out these rocks as a possible provenance for the ca. 2 Ga old zircons encountered in the Svecofennian metaturbidites. Thus, there is still no direct evidence from granitoid rocks for an extensive Svecofennian protocrust, the existence of which has been postulated on the basis of geochemical and SmNd isotopic data.