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The Paleoproterozoic Lomagundi-Jatuli positive [[delta].sup.13]C excursion in sedimentary carbonates represents an event whose magnitude and duration is unique in Earth history, although precise absolute chronology of this event remains poorly constrained. In northeastern Fennoscandia, an ~1300-m-thick sedimentary-volcanic succession of the Pechenga Greenstone Belt records decline of this isotopic excursion. Zircons from sedimentary rocks that occur within the decline have yielded [sup.207]Pb/[sup.206]Pb dates at 2058 [+ or -] 2 Ma ([+ or -]6 Ma including U decay constant uncertainties) and provide the first maximum age constraint on the termination of the Lomagundi-Jatuli event. Combined with existing constraints, these data indicate an ~140 m.y. interval characterized by [sup.13]C-rich carbonate accumulation. Keywords: Fennoscandia, Paleoproterozoic, dolostone, carbon, isotopes, age.

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 [sup.207]Pb/[sup.206] 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. Keywords: zircon, mantle origin, Archean, ophiolite, rifting, Fennoscandian Shield, Finland.

U-Pb age determinations on zircon from granitoids in the Archean of eastern Finland were conducted using the SIMS, LA-ICP-MS and TIMS techniques, with an emphasis on low-HREE granitoids. The oldest rocks in the Fennoscandian Shield are 3.4-3.5 Ga. Several samples were collected close to these rocks, but none of the samples were as old, indicating that the oldest rocks are only small, possibly allochthonous fragments in the Neoarchean bedrock. Some tonalite-trondhjemite-granodiorite (TTG) samples yielded homogeneous 2.72-2.73 Ga zircon populations, and in these samples, the initial εNd was also close to the depleted mantle (DM) values. However, several granitoid samples with a main zircon population of 2.7-2.8 Ga had 2.9-3.2 Ga grains or inherited cores, and in some samples, all grains were of 2.9-3.0 Ga. In these samples, the εNd value was also close to zero or slightly negative. These features suggest that apart from the juvenile Neoarchean magmas, the abundance of reworked 2.9 Ga material is considerable in the Archean crust, which developed during successive juvenile magmatic inputs that melted and assimilated the older sialic crust. The low-HREE geochemical character of granitoids has no correlation with their age, with the low-HREE granitoids yielding an age span of 2.72-2.98 Ga. [ABSTRACT FROM AUTHOR]

We have identified two contrasting styles of Paleoproterozoic metamorphism in the northern part of the Fennoscandian Shield. The Karelia and Lapland-Kola Provinces, comprising Archean and overlying Paleoproterozoic supracrustal rocks, show a typical medium pressure Barrovian-style metamorphism with commonly found kyanite-bearing mineral assemblages and ITD (isothermal decompression) PT paths. In the juxtaposed Svecofennia Province metamorphism represents low pressure-high temperature Buchan style with garnet-cordierite migmatites and intercalated andalusite-cordierite and andalusite-staurolite schists and sillimanite-muscovite gneisses. The retrograde PT paths show only a moderate uplift during cooling. U-Pb age determinations on monazite were made using the LA-ICP-MS from more than 80 samples from metasedimentary rocks. The sampling covered most parts of the Paleoproterozoic bedrock in Finland. The analyses reveal three peaks at c. 1.91 Ga, 1.86–1.88 Ga and at 1.79–1.81 Ga. The oldest, c. 1.91 Ga monazites are mostly found in the Lapland-Kola Province which is located in the northernmost Finland. In the Karelia Province where the Paleoproterozoic is underlain by Archean bedrock monazite yielded ages of 1.76‒1.81 Ga with only a few older exceptions in samples showing a spread of 207Pb/206Pb ages from c. 1.92–1.81 Ga. The Karelia Province underwent tectonic thickening, where monazite ages of around 1.80 Ga mostly represent exhumation near the temperature maximum. In the Svecofennia Province monazite ages vary from c. 1.89 to 1.78 Ga. In the Western Finland Subprovince the monazite ages in high-grade migmatites are mostly 1.86‒1.88 Ga but within the older migmatite areas there are lower grade zones where monazite yields ages of c. 1.80 Ga. Some samples also show a spread of 207Pb/206Pb ages from 1.89‒1.86 Ga to c. 1.78 Ga. In the Southern Finland Subprovince most ages are either 1.80‒1.78 Ga, especially in the andalusite grade schists, or the sample shows a spread of 207Pb/206Pb ages from c. 1.88 to 1.78 Ga. Only in the eastern part of the Southern Finland Subprovince there are rocks which yield merely 1.86‒1.89 Ga ages. Low pressure-high temperature metamorphism and lack of high or medium P/T rocks in the Svecofennia Province refers rather to accretionary than collisional processes. [ABSTRACT FROM AUTHOR]

The Otanmäki area in central Finland hosts two occurrences of Nb-Zr-REE mineralization, Katajakangas and Kontioaho, within a suite of ca. 2.04-2.06 Ga (A1-type) gneissic granites, syenite and monzonite-monzodiorite. They exhibit trace element characteristics and whole-rock eNd(2050 Ma) values (from +2.6 to -1.3) consistent with derivation by differentiation of mantle-derived mafic magmas with variable interaction with crustal material. The mineralization is localized in 0.1- to 1.4-m-thick dikes (Katajakangas) and a 30- to 50-m-thick sheet-like body (Kontioaho), containing allanite-(Ce), zircon, titanite, and Nb-REE-Th-U oxides. Their wall rocks are composed of ca. 2.06 Ga peraluminous monzogranite, which is genetically unrelated to the mineralized rock units, as evidenced by whole-rock chemical and Sm-Nd isotope data and zircon U-Pb geochronology. Instead, the mineralization is linked to the nearby peralkaline to metaluminous alkali feldspar granite magmatism dated at ca. 2.04-2.05 Ga. The development of REE-HFSE enrichment in the mineralized rock units required extensive crystallization of a peralkaline granite magma, producing residual metaluminous, highsilica melts enriched in REE-HFSE, Ca, and Fe relative to Na, K, and Al. The REE-HFSE and Ca enrichment was further promoted by volatile complexing with dissolved F-, CO3 2- and SO4 2-. These highly evolved melts were parental to the mineralized dikes and the sheet-like intrusive body, which were emplaced into the monzogranite capping the intrusions of peralkaline granite. [ABSTRACT FROM AUTHOR]

Over the past tens of years, glacial sandstone erratics variably enriched In Zn, Pb and Cu have been collected from the coast south of Raahe, but their source has remained ambiguous. In these non-metamorphosed and non-deformed boulders, detrltal grains of quartz and minor feldspar are cemented by calcium carbonate, which is partly or wholly replaced by ore minerals, Including sphalerite, galena, pyrlte, marcaslte, and chalcopyrite. Analyzed mineralized boulders have yielded total base metals contents between 1 and 19 wt.%. The FeS content in sphalerite Is low and variable, ranging commonly between 0.5 and 15 mol.%, which is in harmony with its coexistence with pyrlte. Galena shows very radiogenic Pb isotope compositions, with 206Pb/204Pb and 207Pb/204Pb falling In the ranges of 20.55-21.06 and 15.90-15.94, respectively. These compositions are similar to those measured for the Lalsvall MVT Pb-Zn deposit in the Swedish Caledonides, being consistent with a similar Ordovician age of ore formation. However, S isotope analyses yielded heterogeneous compositions, with pyrlte showing mostly negative δ34S values from -15.6 to -7.6%o, Indicating partly a strong bacteriogenic signature, which are distinctly different from the generally heavy sulfur isotope compositions (avg. δ34S +24%O) reported from the Lalsvall-type deposits, pointing to a different source of sulfur. These results together with some mlneralogical differences (relatively abundant chalcopyrite, scarcity of fluorite and barite) suggest that the boulders were not derived from the eastern front of the Caledonian orogen but their provenance occurs much closer, potentially In a so-far-undiscovered occurrence of stratabound base metal mineralization in Cambrian sediments under the Bothnia Bay. The direction of transportation of the sandstone boulders can be estimated using associated carbonate and mafic metavolcanite boulders whose carbon isotope and major and trace element compositions, respectively, Indicate their derivations from the Kalix area, northern Sweden. [ABSTRACT FROM AUTHOR]