Olav Eklund

1.8 Ga Svecofennian post-collisional shoshonitic magmatism in the Fennoscandian shield

Abstract At least 14 small (1–11 km across) 1.8 Ga Svecofennian post-collisional bimodal intrusions occur in southern Finland and Russian Karelia in a 600-km-long belt from the Aland Islands to the NW Lake Ladoga region. The rocks range from ultramafic, calc-alkaline, apatite-rich potassium lamprophyres to peraluminous HiBaSr granites, and form a shoshonitic series with K 2 O+Na 2 O>5%, K 2 O/Na 2 O>0.5, Al 2 O 3 >9% over a wide spectrum of SiO 2 (32–78%). Although strongly enriched in all rocks, the LILE Ba and Sr and the LREE generally define a decreasing trend with increasing SiO 2 . Depletion is noted for HFSE Ti, Nb and Ta. Available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded. Initial magmas (Mg#>65) in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post-collisional uplift some 30 Ma after the regional Svecofennian metamorphic peak. However, prior to the melting episode, the lithospheric mantle was affected by carbonatite metasomatism; more extensively in the east than in the west. The melts generated in the more carbonate-rich mantle are extremely enriched in P 2 O 5 ∼4%, F∼12,000 ppm, LILE: Ba∼9000 ppm, Sr∼7000 ppm, LREE: La∼600 ppm and Ce∼1000 ppm. The parental magma underwent 55–60% fractionation of biotite+clinopyroxene+apatite+magnetite+sphene whereupon intermediate varieties were produced. After further fractionation, 60–80%, of K-feldspar+amphibole+plagioclase±(minor magnetite, sphene and apatite), leucosyenites and quartz-monzonites were formed. In the west, where the source was less affected by carbonatite metasomatism, calc-alkaline lamprophyres (vogesites, minettes and spessartites) and equivalent plutonic rocks (monzonites) were formed. Removal of about 50% of biotite, amphibole, plagioclase, magnetite, apatite and sphene produced peraluminous HiBaSr granites. The impact of crustal assimilation is considered to be low. At about 1.8 Ga, the post-collisional shoshonitic magmatism brought juvenile material, particularly enriched in alkalis, LILE, LREE and F, into the crust. Although areally restricted, the regional distribution of the post-collisional intrusions may indicate that larger volumes of 1.8 Ga juvenile material resides in unexposed parts of the crust.

1.8 Ga magmatism in southern Finland: strongly enriched mantle and juvenile crustal sources in a post-collisional setting

Whole-rock and isotope geochemistry of six ∼1.8 Ga post-kinematic intrusions, emplaced along the ∼1.9 Ga Southern Svecofennian Arc Complex (SSAC) and in the SW part of the Karelian Domain in Finland, was studied. The intrusive age [U–Pb secondary ion mass spectrometer (SIMS)] of one of these, the Petravaara Pluton, was determined as 1811 ± 6 Ma. Basic-intermediate rocks are alkali-rich (K2O + Na2O > 4 wt.%) and typically shoshonitic, strongly enriched in large ion lithophile elements and light rare earth elements, but relatively depleted in high field strength elements and heavy rare earth elements. The enrichment is much higher than can be accounted for by crustal contamination and requires previously melt-depleted mantle sources, subjected to variable metasomatism by carbonate-rich fluids and sediment-derived melts. These sources are inferred to consist of phlogopite ± amphibole-bearing peridotites from depths below the spinel–garnet transition, as shown by the high Ce/Yb ratios. 87Sr/86Sr(1.8 Ga) ratios in the range 0.7027–0.7031 and ‘mildly depleted’ ϵNd(1.8 Ga) values (+0.1 to +1.4), with T DM values <2.1 Ga, suggest that mantle enrichment was associated with the previous Svecofennian subduction–accretion process, when enriched sub-Svecofennian mantle sections developed, dominantly characterized by 147Sm/144Nd ratios of 0.14–0.17. The associated granitoids are diversified. One group is marginally peraluminous, transitional between I (volcanic-arc) and S (syn-collisional) types, and was derived from mixed igneous and sedimentary, but juvenile Svecofennian source rocks, as supported by near-chondritic ϵNd(1.8 Ga) and somewhat elevated 87Sr/86Sr(1.8 Ga). The other group is transitional between I and A (within-plate) types in character and had dominantly igneous protoliths. The whole-rock geochemistry and isotopes suggest that the compositional variation between ∼50 and 70 wt.% SiO2 may be explained by hybridization between strongly enriched mantle-derived magmas and anatectic granitic magmas from the juvenile Svecofennian crust. One intrusion in the east contains a significant portion of Archaean, mostly igneous protolithic material (ϵNd(1.8 Ga) = –2.8 and ϵHf(t) for zircons between +2.8 and −11.9, with an average of −4.9). The ∼1.8 Ga post-kinematic intrusions were emplaced within the SSAC subsequent to the continental collision with the Volgo-Sarmatia craton from the SE, during a shift from contraction to extension, that is, in a post-collisional setting.

Timing and geochemistry of potassic magmatism in the eastern part of the Svecofennian domain, NW Ladoga Lake Region, Russian Karelia

Abstract The Puutsaari intrusion is a potassium-rich magmatic complex in the eastern part of the Svecofennian domain close to the Archaean border. The intrusion is generally undeformed in contrast to 1880–1875 Ma-old country rock tonalitic migmatites and diatectites. The main rock types are: (1) mafic rocks of a gabbro–norite–diorite–quartz monzodiorite series; (2) quartz diorite–tonalite–granodiorite; and (3) coarse-grained microcline granite. The three rock-types intruded coevally forming a peculiar three-component mingling system. The mafic rocks, enriched in K, P, Ba, Sr and LREE, have marked shoshonitic affinities (K2O=1.97–5.40, K2O/Na2O=0.6–2.37). On a regional scale they demonstrate transitional geochemistry between less enriched syn-orogenic 1880 Ma-old gabbro–tonalite complexes and strongly enriched 1800 Ma post-collisional shoshonitic intrusions. The microcline granite as well as the tonalite–granodiorite rocks are geochemically similar to crustal anatectic granitoids of the NW Ladoga Lake area. The three rock groups do not form a single trend on Harker-type diagrams and are unlikely to be related by fractional crystallisation or mixing. Zircons from the Puutsaari microcline granite and from the mafic rock series have been dated by ion-microprobe (NORDSIM) at 1868.2±5.9 and 1869±7.7 Ma, respectively. Most zircons recovered from a granite sample had zoned or homogeneous cores and unzoned fractured rims. No statistically significant variation of zircon core and rim ages from the granite was established in the course of this study. Zircons from the mafic rock are unzoned. It is suggested that the mafic rocks at Puutsaari were derived from an enriched mantle shortly after the main Svecofennian collisional event and the roughly 1.88 Ga regional metamorphic culmination. The emplacement of the mafic melt caused anatectic melting of various crustal protoliths and produced coeval granitic and tonalitic compositions.

Generation of A-type granitic melts during the late Svecofennian metamorphism in southern Finland

Abstract Across southern Finland the Late Svecofennian Granite Migmatite zone contains large amounts of migmatites and S-type granites formed during the high temperature and low pressure metamorphism between 1.84 and 1.80 Ga. Within this zone, the Karjaa granite intrudes the surrounding migmatites. The granite is more fine-grained and darker than the surrounding anatectic S-type granites, which are associated with the migmatites. The Karjaa granite cuts the migmatites suggesting that it is coeval or younger than the migmatites. It is a two-feldspar biotite granite containing apatite and zircon as accessory minerals. The granite displays elevated TiO2, P2O5 and F contents and is characterized by high Ba, Zr, Nb, and Ga contents. The REE patterns indicate strong enrichment in LREEs and a pronounced europium minimum. The crystallization temperature of the granite is estimated to about 900°C using the P2O5 and Zr-saturation methods. Cathodoluminescence images on zircons indicate core domains and overgrowth structures. SIMS dating of the zircon cores and rims yielded concordia ages of 1880±16 Ma and 1826±11 Ma, respectively. On the basis of these data, it seems that c. 1880 Ma old igneous rocks at deeper crustal levels partially melted during at c. 1825 Ma metamorphism and generated hot melts having a composition close to A-type granites.

Proterozoic Processes in the Fennoscandian Shield

1 Department of Geology, Turku University, Fi-20014, Turku, Finland; olav.eklund@utu.fi 2 Department of Earth Sciences, Uppsala University, Villavägen 16 SE-752 36 Uppsala, Sweden; ulf.andersson@geo.uu.se 3 Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm 4 Deptartment of Geology, Lund University, Sölvegatan 12, SE-22362 Lund, Sweden; karin.hogdahl@geol.lu.se

CHARACTERIZATION OF DIFFERENT ORIGIN LIMESTONES BY ESCA AND SEM/EDX IN ORDER TO DETERMINE THEIR SUITABILITY FOR DESULPHURIZATION

CARETECH is a multidisciplinary project supported by the Academy of Finland. The aim of this project is to determine the best natural materials that can be used in desulphurization and carbonation processes using methods from materials science, mineralogy and geology. In this paper, we discuss various properties of limestones, such as the chemical composition, surface structure and define their influence on desulphurization.