Irmeli Mänttäri

Tectonic setting of post-collisional magmatism in the Palaeoproterozoic Svecofennian Orogen, SW Finland

Abstract Five bimodal post-collisional intrusions in southwestern Finland have been investigated. Geochemically, the mafic rocks are shoshonitic monzodiorites, which are highly enriched in Fe, P, Ti, F, LREE and in incompatible trace elements. The felsic rocks are garnet bearing peraluminous, S-type anatectic granites. New data on the mafic and the felsic intrusions yielded the same U–Pb zircon age of 1815 Ma. Therefore, the mafic and felsic intrusions are coeval but not cogenetic. Narrow contact metamorphic aureoles around the mafic intrusions contain garnet–orthopyroxene bearing assemblages, and thermobarometry indicates an intrusion depth of at least 15 km. Hence, there was little or no unroofing after peak regional metamorphism at 4–6 kbar. The geochemical characteristics of the mafic rocks suggest that they were derived from subcontinental lithospheric mantle that was previously enriched by fluids released during Svecofennian subduction. It is suggested here that hot upwelling asthenosphere convectively removed subcontinental lithospheric mantle and triggered partial melting of the enriched parts of the mantle. Uprising mafic melts increased the already high temperatures at mid-crustal levels and caused granulite facies metamorphism, crustal anatexis and production of granitic melts.

Svecofennian magmatic and metamorphic evolution in southwestern Finland as revealed by U-Pb zircon SIMS geochronology

Abstract Zircons from six samples collected from igneous and metamorphic rocks were dated using the NORDSIM ion microprobe, in order to investigate the tectonic evolution of the Palaeoproterozoic Svecofennian Orogen in southwestern Finland. These rocks represent pre-collisional, collisional and post-collisional stages of the orogeny. The ion microprobe results reveal two age groups of granodioritic–tonalitic rocks. The intrusions have different tectonic settings: the Orijarvi granodiorite represents pre-collisional 1.91–1.88 Ga island-arc-related magmatism and yielded an age of 1898±9 Ma, whereas the collision-related Masku tonalite was dated at 1854±18 Ma. The latter age accords with more accurate previous conventional zircon age data and constrains the emplacement age of collisional granitoids to ≈1.87 Ga. This is interpreted to reflect the collision between the Southern Svecofennian Arc Complex with the Central Svecofennian Arc complex and the formation of a suture zone between them during D2 deformation. Granulite facies metamorphism in the Turku area was dated at 1824±5 Ma using zircons from leucosome in the Lemu metapelite. This age constrains D3 folding related to post-collisional crustal shortening in this area. Crustal melting continued until ≈1.81 Ga, as indicated by the youngest leucosome zircons and metamorphic rims of enderbite zircons. New metamorphic zircon growth took place in older granitoids at granulite facies, but not at amphibolite facies. Detrital zircons with ages between 2.91 and 1.97 Ga were found in the mesosome of the Lemu metapelite and 2.64–1.93 Ga inherited cores were found in the 1.87 Ga Masku tonalite.

P–T–t development of Archaean granulites in Varpaisjärvi, Central Finland: II. Dating of high-grade metamorphism with the U–Pb and Sm–Nd methods

Exposed blocks of lower crustal Archaean granulites separated by Palaeoproterozoic faults occur near the western boundary of the Karelian craton in central Finland. The southwesternmost granulite block in the study area, the Jonsa block, differs from other granulites by having younger zircon U–Pb and Sm–Nd TDM model ages. The U–Pb ages on zircons and monazites from leucosomes and mesosomes of mafic and pelitic granulites in the Jonsa block are ca. 2.63 Ga, which is interpreted as the age of granulite metamorphism. Igneous enderbites, which make up a considerable part of the bedrock in granulite blocks, are ca. 50 Ma older. Outside the Jonsa block both conventional and ion probe zircon ages of mesosomes of migmatitic mafic granulites are 3.2–3.1 Ga, which are the protolith ages. Sm–Nd TDM model ages of the mafic granulites are 2.7–2.9 Ga in the Jonsa block but ca. 3.2 Ga in the other granulites. The age differences are interpreted to represent terrane accretion that juxtaposed 3.2 Ga rocks next to younger rocks. All granulites were metamorphosed in similar PT conditions at 2.63 Ga; granulite metamorphism affected the whole area but was not able to reset zircons in older rocks. In all blocks the Sm–Nd garnet-whole rock ages are younger, ranging from 2.48–2.59 Ga. This reflects relatively low closure temperature of the Sm–Nd system in garnet in these rocks. The U–Pb age of a Palaeoproterozoic dolerite which cuts the granulites is 2.3 Ga, close to the K–Ar age of amphibole in a retrogressed fracture.

Archean zircons from the mantle: The Jormua ophiolite revisited

The Jormua ophiolite represents seafloor from an Early Proterozoic ocean to continent transition zone that mainly consisted of Archean subcontinental lithospheric mantle. These mantle rocks were exhumed as a result of extreme crustal thinning and detachment faulting in association with the opening of the Svecofennian Sea. At the prerift stage of continental breakup, residual lithospheric peridotites became intruded by alkaline melts that formed a diverse suite of clinopyroxenite and hornblendite-garnetite dikes and veins. These Proterozoic dikes contain Archean zircon xenocrysts inherited from deeper sources of the continental mantle. The relatively large spread of 2 0 7 Pb/ 2 0 6 Pb ages between 3106 and 2718 Ma suggests that the zircons are derived from a variety of source rocks. Some xenocrysts have U and Th abundances comparable to zircon in common alkali basalts, whereas a population of Archean high-U and high-Th zircons is similar to those described from mica-amphibole-rutile-ilmenite-diopside-bearing mantle xenoliths. These are the oldest zircons found from upper-mantle rocks anywhere and imply that the Fennoscandian cratonic root was relatively cool and strongly metasomatized by ca. 3.1 Ga.

Petrology and geochemistry of mafic granulite xenoliths from the Lahtojoki kimberlite pipe, eastern Finland

Abstract The Lahtojoki kimberlite pipe in Kaavi, eastern Finland contains lower crustal mafic granulite xenoliths with a mineral assemblage cpx−amph−pl±grt±opx±bt. The TWEEQU thermobarometry indicates crystallization at ca. 800–900°C and 0.75–1.25 GPa, which corresponds with crustal depths of ca. 22–38 km. Reaction textures and results of thermobarometry in some xenoliths indicate post-peak decompression with cooling. Chemical composition of the xenoliths suggest that their protoliths crystallized from basaltic melts, with some K-enrichment. Their REE patterns are slightly LREE-enriched, some xenoliths having small negative Eu anomalies. The U-Pb ion probe dating of zircons from two samples yielded variable ages for zircons even in the same xenolith, between ca. 2.6 and 1.7 Ga. These ages correspond with main late Archaean and Palaeoproterozoic orogenic events in the Fennoscandian shield. The Sm–Nd garnet–clinopyroxene isochron age from one dated sample is 1.6 Ga, which represents either a cooling or reheating of the lower crust by rapakivi magmatism. Because the petrology, geochemistry and ages of xenoliths do not correlate well with the nearest exposed Archaean mafic granulites, it is evident that the present lower crust, underlying the Archaean rocks, has a considerable Palaeoproterozoic component.

U–Pb dating of zircons and monazites from Archean granulites in Varpaisjärvi, Central Finland:: Evidence for multiple metamorphism and Neoarchean terrane accretion

Abstract U–Pb age data from Archean lower crustal granulites in the Varpaisjarvi area, central Finland, indicate several metamorphic events that took place in Mesoarchean–Neoarchean and Paleoproterozoic time. Zircons separated from the granulites fall into two distinct age groups. In the older group, the protolith age is 3.2 Ga and many zircons indicate metamorphism at ca. 3.1 Ga. In contrast, the oldest zircons in the younger group are ca. 2.73 Ga. Both terranes were migmatized at ca. 2.70 Ga with the emplacement and crystallization of igneous enderbites. The enderbites and the leucosomes of migmatites in both granulite terranes have 2.7 Ga zircons. High grade metamorphic conditions prevailed from 2.70 to 2.63 Ga which is the age range of most zircons in the eastern terrane. Also the enderbites show metamorphic zircon growth at ca. 2.64 Ga. Two distinct age groups are interpreted to be the result of juxtaposition of two terranes of different ages. This indicates that collisional and accretionary processes operated during Neoarchean time. The area east from the eastern granulite terrane underwent almost pervasive deformation and amphibolite facies metamorphism during the Svecofennian orogeny at ca. 1.9 Ga. However, the U–Pb ages of zircons in the retrograde area are similar to those of the eastern terrane, thus indicating that these rocks were metamorphosed under granulite facies conditions (2.70–2.63 Ga). Although the ca. 1.9 Ga Svecofennian metamorphism is not evident in the ion microprobe results, some Paleoproterozoic Pb-loss is indicated by TIMS U–Pb analyses of zircons. In addition, monazite age of 1.89 Ga from the retrograde area reflects Svecofennian metamorphism.

The Kynsijärvi quartz alkali feldspar syenite, Koillismaa, eastern Finland—silicic magmatism associated with 2.44 Ga continental rifting

Abstract The Kynsijarvi quartz alkali feldspar syenite (KS) is a small A-type silicic pluton closely associated with the ∼2440 Ma Koillismaa complex, one of the Paleoproterozoic mafic layered intrusion complexes of central and northern Finland and Kola Peninsula. In the Koillismaa area, these mafic and felsic intrusions were emplaced into a late Archean (2900–2600 Ma) basement and stratigraphically underlie supracrustal rocks of the Paleoproterozoic Kuusamo schist belt. The KS is a medium-grained hypersolvus rock with mesoperthitic alkali feldspar, quartz, and ferro-edenite as the major rock forming minerals. Magnetite, titanite, zircon, fluorite, and secondary stilpnomelane are found in accessory amounts. The rock shows extensive subsolidus reactions, for example well-developed intergranular albite rims around alkali feldspar crystals. The KS is characterized by relatively high SiO2 (70.1–71.7 wt.%), total alkalies (∼10 wt.%), Ba (>700 ppm), Zr (>500 ppm), Ga (>30 ppm), and Nb (>40 ppm). Chondrite-normalized REE patterns are moderately enriched in the LREE [(La/Yb)N∼12.65] and show a negative Eu-anomaly (Eu/Eu*∼0.3). Four zircon fractions from the KS yield a U–Pb upper intercept age of 2442±3 Ma. The KS was probably emplaced in an extensional environment, the extension being caused by a mantle plume, which also generated the mafic layered complexes in the region. Overall, the 2.44 Ga mafic and felsic plutonic rocks mark a ∼400-km-long zone of early Paleoproterozoic rifting of the Fennoscandian Archean craton.

Geochronology and structural relationships of mesothermal gold mineralization in the Palaeoproterozoic Jokisivu prospect, southern Finland

The palaeoproterozoic Svecofennian orogen in southern Finland contains a number of orogenic gold occurrences. The Jokisivu gold deposit, comprising auriferous quartz veins, is hosted by syn-tectonic quartz diorites to gabbros. Mineralization occurs in approximately WNW–ESE- and WSW–ENE-trending shear zones, which probably branch from regional-scale NW–SE-trending shears. Ore zone fabrics post-date regional-scale folding and the metamorphic peak, and can be correlated with late Svecofennian regional shear tectonics (D 6 ; 1.83–1.78 Ga), indicating that mineralization formed during the late stages of orogenic evolution. SIMS and TIMS U–Pb dating of three samples place tight constraints on the age of gold mineralization. Zircons from both unaltered and altered quartz diorites have ages of 1884±4 Ma and 1881±3 Ma, respectively. These are interpreted as the crystallization age of the rock and as providing the maximum age for mineralization. Zircon rims from an altered quartz diorite from the ore zone give ages of c . 1802±15 Ma, which overlap with the 1801±18 Ma titanite (mean Pb–Pb) age from the ore zone. The ages are similar to the age of the pegmatite dyke that cuts the ore zone and whose zircon age of 1807±3 Ma is approximately the same as the 1791±2 Ma monazite age (TIMS) giving the minimum age of the gold mineralization. The mineralization and its structural framework can be correlated with coeval late Svecofennian shear tectonics related to WNW–ESE-oriented shortening in southern Finland. Extensive c . 1.8 Ga granite magmatism, shear zone development and associated gold mineralization are of regional importance also in the northern and western Fennoscandian Shield (Finnish Lapand and Sweden). A Cordilleran-type setting can explain the widespread distribution of magmatism and gold mineralization associated with shortening, as well as the required heat source triggering hydrothermal fluid flow along shear zones.

A priori evidence for zircon antecryst entrainment in megacrystic Proterozoic granites

Entrainment of antecrystic zircon in early saturated main silicate phases has been recognized as a complicating factor in high-precision U-Pb geochronology of Phanerozoic granitic systems, but has not been demonstrated for Precambrian rocks. We report U-Pb ages (secondary ion mass spectrometry) and trace element (laser ablation–inductively coupled plasma–mass spectrometry) compositions of zircon from three main ferroan granite types of the late Paleoproterozoic Wiborg rapakivi granite batholith of southeastern Finland and vicinity. Zircon was extracted from the interior parts of megacrystic (diameters as large as 15 cm) rapakivi-textured alkali feldspar ovoids using a microdrilling technique. Compared to zircon in ovoid groundmass, zircon in the ovoids has higher incompatible element values (e.g., uranium and rare earth elements) and, for two of the three samples examined, slightly older (by ∼1 m.y.) average U-Pb ages. These observations suggest that the ovoid material of the rapakivi granites entrained zircon from earlier magmas that were compositionally different than the final magmatic host of the ovoids. Groundmass zircon implies similar average U-Pb ages for all the studied samples, ca. 1628 Ma; this is regarded as the final crystallization age of the granites. These observations show that it is viable to measure statistically valid age differences between antecrysts and groundmass of Proterozoic granites. Overall, our sampling technique provides enhanced textural control of antecrystic zircon in a wide range of sample materials.

Palaeomagnetism and U–Pb geochronology of c. 1570 Ma intrusives from Åland archipelago, SW Finland – implications for Nuna

Abstract We report new palaeomagnetic and isotope age data of Early Mesoproterozoic (i.e. Subjotnian) intrusions from the Åland archipelago, SW Finland. The palaeomagnetic results reveal dual-polarity magnetizations with a pronounced reversal asymmetry occurring in dykes. We explain the asymmetry by an unremoved secondary component, which is affecting more N-polarity dykes. Other explanations, such as the age difference of magnetization between normal and reversed polarity dykes, are discussed. The primary nature of magnetization in dykes for both normal (N) and reversed (R) groups is verified by positive baked contact tests. A dyke showing reversed polarity from Korsö is dated 1575.9±3.0 Ma (U–Pb) in this study. This and previous U–Pb data tighten the magmatic activity in Åland to 1580–1570 Ma. We combined new palaeomagnetic data with those from earlier studies to provide a new key-palaeomagnetic pole for Baltica. Our data positions Baltica on equatorial latitudes, supporting the NENA (North Europe–North America) connection between Baltica and Laurentia at 1.59–1.58 Ga. Palaeomagnetic data support that NENA was valid at 1.75, 1.58, 1.46, and 1.26 Ga, forming the core of Mesoproterozoic Nuna (a.k.a. Columbia) supercontinent.