Paleoproterozoic metamorphism in the northern Fennoscandian Shield: age constraints revealed by monazite

ABSTRACT

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.     

Erkki Mikkola und der heutige Stand der präkambrischen Geologie in Finnland

Ohne Zusammenfassung     

Internal Differentiation of the Archean Continental Crust: Fluid-Controlled Partial Melting of Granulites and TTG-Amphibolite Associations in Central Finland

Fault bound blocks of granulite and enderbite occur within upperamphibolite-facies migmatitic tonalitic–trondhjemitic–granodioritic(TTG) gneisses of the Iisalmi block of Central Finland. Theseunits record reworking and partial melting of different levelsof the Archean crust during a major tectonothermal event at2·6–2·7 Ga. Anhydrous mineral assemblagesand tonalitic melts in the granulites formed as a result ofhydrous phase breakdown melting reactions involving amphiboleat peak metamorphic conditions of 8–11 kbar and 750–900°C.A nominally fluid-absent melting regime in the granulites issupported by the presence of carbonic fluid inclusions. Thegeochemical signature of light rare earth element (LREE)-depletedmafic granulites can be modelled by 10–30 wt % partialmelting of an amphibolite source rock leaving a garnet-bearingresidue. The degree of melting in intermediate granulites isinferred to be less than 10 wt % and was restricted by the availabilityof quartz. Pressure–temperature estimates for the TTGgneisses are significantly lower than for the granulites at660–770°C and 5–6 kbar. Based on the P–Tconditions, melting of the TTG gneisses is inferred to haveoccurred at the wet solidus in the presence of an H₂O-rich fluid.A hydrous mineralogy, abundant aqueous fluid inclusions andthe absence of carbonic inclusions in the gneisses are in accordancewith a water-fluxed melting regime. Low REE contents and strongpositive Eu anomalies in most leucosomes irrespective of thehost rock composition suggest that the leucosomes are not meltcompositions, but represent plagioclase–quartz assemblagesthat crystallized early from felsic melts. Furthermore, similarplagioclase compositions in leucosomes and adjacent mesosomesare not a ‘migmatite paradox’, as both record equilibrationwith the same melt phase percolating along grain boundaries. KEY WORDS: Archean continental crust; fluid inclusion; granulite; migmatite; partial melting     

Trace element partitioning in the granulite facies

Analyses of trace elements in the mineral phases of granulites provide important information about the trace element distribution in the lower crust. Since granulites are often considered residues of partial melting processes, trace element characteristics of their mineral phases may record mineral/melt equilibria thus giving an opportunity to understand the nature and composition of melts in the lower continental crust. This study provides an extensive set of mineral trace element data obtained by LA-ICP-MS analyses of mafic and intermediate granulites from Central Finland. Mass balance calculations using the analytical data indicate a pronounced contribution of the accessory minerals apatite for the REE and ilmenite for the HFSE. Coherent mineral/mineral ratios between samples point to a close approach to equilibrium except for minerals intergrown with garnet porphyroblasts. Mineral trace element data were used for the formulation of a set of D ^mineral/melt partition coefficients that is applicable for trace element modelling under lower crustal conditions. D ^mineral/melt were derived by the application of predictive models and using observed constant mineral/mineral ratios. The comparison of the calculated D ^mineral/melt with experimental data as well as the relationship between mineral trace element contents and a leucosome with a composition close to an equilibrium melt provides additional constraints on mineral/melt partitioning. The D values derived in this study are broadly similar to magmatic partition coefficients for intermediate melt compositions. They provide a first coherent set of D values for Sc, V, Cr and Ni between clinopyroxene, amphibole, garnet, orthopyroxene, ilmenite and melt. In addition, they emphasize the strong impact that ilmenite exerts on the distribution of Nb and Ta.     

The age of mid-proterozoic phosphatic metasediments in Finland as indicated by radiometric UPb dates

Two uranium-bearing phosphatic metasedimentary occurrences at Lampinsaari, Vihanti and Temo, Nilsiä, in Finland have been dated by isotopic UPb analyses of whole rock samples to be 1876±2 Ma old. The dates reflect the time of the Svecokarelian regional metamorphism in high amphibolite facies. An analysis of a uranium thucolite from the deposit at Nuottijärvi, Paltamo, that was metamorphosed in intermediate greenschist facies gives a date of 1897±7 Ma, suggesting that the phosphatic sedimentation had occurred before that time. An upper time limit for the sedimentation is set by a previous date of 2080±45 Ma, which was obtained from a banded iron one formation. The absence of thorium in the deposits suggests a marine environment of deposition.     

Natural bismuth vanadate from the Mutala pegmatite area,Mozambique

Bismuth vanadate (microprobe test) in varying shades of orange color and in well developed crystals (averaging 0.2 mm in size) occurs in bismutite in the Mutala granite pegmatite area, district of Zambezia, Mozambique. Two modifications of BiVO ₄were identified. An orthorhombic form is identical with pucherite and shows a₀ = 5.33₆ Å, b₀ = 5.05₃ Å, c ₀ = 12.02₁ Å. The crystal habit ranges from platy to stout prismatic. The X-ray powder pattern of the monoclinic form matches that of the synthetic monoclinic Bi-orthovanadate with a ₀ = 5.20₅ Å, b ₀ = 11.71₈ Å, c ₀ = 5.09₈ Å, = 90° 25. The crystal habit resembles that of a pyramidal scheelite crystal with the b-axis corresponding to the scheelite c-axis. Multiple twinning is seen on (101), in some instances with a composition plane (010).     

Spatial interpolation of monthly climate data for Finland: comparing the performance of kriging and generalized additive models

The Finnish Meteorological Institute has calculated statistics for the new reference period of 1981–2010. During this project, the grid size has been reduced from 10 to 1 km, the evaluation of the interpolation has been improved, and comparisons between different methods has been performed. The climate variables of interest were monthly mean temperature and mean precipitation, for which the spatial variability was explained using auxiliary information: mean elevation, sea percentage, and lake percentage. We compared three methods for spatial prediction: kriging with external drift (KED), generalized additive models (GAM), and GAM combined with residual kriging (GK). Every interpolation file now has attached statistical key figures describing the bias and the normality of the prediction error. According to the cross-validation results, GAM was the best method for predicting mean temperatures, with only very small differences relative to the other methods. For mean precipitation, KED produced the most accurate predictions, followed by GK. In both cases, there was notable seasonal variation in the statistical skill scores. For the new reference period and future interpolations, KED was chosen as the primary method due to its robustness and accuracy.     

Karibibite,a new FeAs mineral from South West Africa

Karibibite (ideally, Fe₂As₄O₉) occurs in vugs in massive loellingite of the Karibib pegmatite area, South West Africa. It is brownish yellow and finely fibrous. The thickness of the soft, single fibers is less than 1 micron, unsuitable for single-crystal X-ray study. Electron diffraction and X-ray powder pattern indicate that the mineral is orthorhombic, with $\text{a}{}_0\text{= 27.91}\text{A}\text{?}\text{, b}{}_0\text{= 6.53}\text{A}\text{?}\text{and c}{}_0\text{(}\text{fiber axis}\text{) = 7.20}\text{A}\text{?}$. The space group cannot be given. The mineral is paramagnetic with yellow fluorescence and is pleochroic with γ > 2.10, α = 1.96, 2V_α large, d = 4.07. It is soluble in acids and alkali hydroxide. Decomposition starts around 320 °C. The infra-red absorption spectrum indicates absence of AsO₄ groups. The mineral is classified tentatively as an oxide or arsenite.     

Dr. Erkki Mikael Mikkola

Ohne Zusammenfassung     

Archean of Greenland and Fennoscandia

The North Atlantic craton in southern West Greenland mainly consists of a tectonic collage of Mesoarchean continental crustal terranes, which were amalgamated at c. 2.7 Ga and are currently exposed at mid-crustal amphibolite to granulite facies levels. Tonalitic orthogneisses predominate, intercalated with slightly older tholeiitic to andesitic metavolcanic rocks and associated gabbro-anorthosite intrusive complexes. The North Atlantic craton also contains enclaves of Eoarchean, c. 3.86-3.6 Ga orthogneisses and supracrustal rocks including the Isua greenstone （or supracrustal） belt. This is the oldest known assemblage of rocks deposited at the surface of the Earth, comprising mafic pillow lavas, banded iron formations and metasedimentary schists with local disseminated graphite of possible biogenic origin. Eoarchean rocks have not been found in Kola and Karelia in Fennoscandia where most rocks are 2.9-2.7 Ga tonalitic-trondhjemitic-granodioritic orthogneisses with intercalated coeval greenstone belts and amphibolites. Mesoarchean 3.0-3.2 Ga rocks are found in the eastern and western parts of the Karelian province. Subduction-related rocks like the Iringora supra-subduction type ophiolite and basalt-andesite-dacite-rhyolite series volcanic rocks in many greenstone belts, as well as eclogites are found in the Archean of Fennoscandia. A clear distinction between Greenland and Fennoscandia is the abundance of 2.75-2.65 Ga igneous rocks in Fennoscandia which indicates that these two cratons had a separate evolution during the Neoarchean.