u pb and sm nd isotopic constraints on the evolution of the paleoproterozoic perapohja belt northern finland ten localities representing metasedimentary and igneous rocks from the northern part of the paleoproterozoic perapohja belt fennoscandian shield were sampled for zircon u pb dating by la mc icp ms and sm nd analysis by tims the results revealed two phases of granitic magmatism that took place at 1 99 ga and 1 79 1 77 ga the former age 1989 6 ma was given by the kierovaara granite pluton representing the first example of felsic plutonism of this age in northern finland though a type felsic volcanic rocks of the same age have previously been recognized the strongly negative initial nd values obtained for granitoids ranging from 3 9 to 11 4 are compatible with the previous sm nd data indicating the presence of archean basement in northern finland at the time of granite emplacement a garnet whole rock sm nd age of 1752 14 ma was measured for a mica gneiss constraining the time of metamorphic cooling down to the closure temperature of garnet detrital zircon grains from quartzite samples belonging to the jatulian palokivalo formation and a stratigraphically unassigned lithodemic unit the mellajoki suite yielded solely archean zircon populations in contrast our sample from the martimo formation representing kalevian sedimentation revealed a 66 paleoproterozoic detrital zircon component with the youngest zircon grains showing an age of ca 1 91 ga the new age data allow us to constrain the depositional time of this sedimentary unit between ca 1 91 and 1 88 ga the latter being the time of the synorogenic plutonism our geochronological data coupled with recent data from other karelian belts suggest that the depositional time period for rocks assigned to the kaleva spans ca 150 ma and the simple division of these rocks into two non coeval units the lower and upper kaleva has become problematic

dating the termination of the palaeoproterozoic lomagundi jatuli carbon isotopic event in the north transfennoscandian greenstone belt abstract existing radio isotopic age constraints indicate that the global palaeoproterozoic lomagundi jatuli large positive carbonate carbon isotopic excursion with 13 c values 5 occurred between 2 2 and 2 06 ga in the north transfennoscandian greenstone belt of the kola peninsula nw russia northern norway and finland the lomagundi jatuli event is recorded in the carbonate rocks of the umba and kuetsjarvi sedimentary formations in the imandra varzuga and pechenga greenstone belts in both areas thick mafic volcanic units umba and kuetsjarvi volcanic formations overlie the carbon isotopic excursion bearing sedimentary units overlying and younger sedimentary units contain carbonate rocks with 13 c values typically ranging between c 1 and 3 signalling the termination of the lomagundi jatuli excursion two new u pb id tims isotope dilution thermal ionisation mass spectrometry zircon dates constrain this termination in both successions the lower unit of the il mozero sedimentary formation is a cross and parallel bedded volcaniclastic greywacke derived largely from erosion of the underlying umba volcanic formation it has yielded detrital zircons with 207 pb 206 pb dates as young as 2055 5 2 3 ma which is a maximum age for deposition and is inferred to date part of the underlying umba volcanics in pechenga the kolosjoki sedimentary formation was intersected by a drill hole obtained by the icdp international continental scientific drilling program supported far deep fennoscandian arctic russia drilling early earth project drilling programme zircons from a mafic fine tuff in this drill core have yielded a 207 pb 206 pb age of 2056 6 0 8 ma this age is interpreted as an eruption age contemporaneous with sedimentation the new age determinations overlap each other within uncertainty and is within error of previously published detrital zircon ages of 2058 2 ma from the kolosjoki sedimentary formation and 2049 28 ma from the kuetsjarvi volcanic formation combined these indicate that the lomagundi jatuli excursion terminated across fennoscandia by 2056 6 0 8 ma and may correlate with similar termination ages in fennoscandia and the transvaal south africa

source of boron in the palokas gold deposit northern finland evidence from boron isotopes and major element composition of tourmaline the recently discovered palokas gold deposit is part of the larger rompas rajapalot gold mineralized system located in the paleoproterozoic perapohja belt northern finland tourmaline is an important gangue mineral in the palokas gold mineralization it occurs as tourmalinite veins and as tourmaline crystals in sulfide rich metasomatized gold bearing rocks in order to understand the origin of tourmaline in the gold mineralized rocks we have investigated the major element chemistry and boron isotope composition of tourmaline from three areas 1 the palokas gold mineralization 2 a pegmatitic tourmaline granite and 3 the evaporitic petajaskoski formation based on textural evidence tourmaline in gold mineralization is divided into two different types type 1 is located within the host rock and is cut by rock forming anthophyllite crystals type 2 occurs in late veins and or breccia zones consisting of approximately 80 tourmaline and 20 sulfides commonly adjacent to quartz veins all the studied tourmaline samples belong to the alkali group tourmaline and can be classified as dravite and schorl the 11b values of the three localities lie in the same range from 0 to 4 tourmaline from the au mineralization and from the petajaskoski formation has similar compositional trends mg is the major substituent for al inferred low fe3 fe2 ratios and na values 0 8 atoms per formula unit apfu of all tourmaline samples suggest that they precipitated from reduced low salinity fluids based on the similar chemical and boron isotope composition and the re os age of molybdenite related to the tourmaline sulfide quartz veins we propose that the tourmaline forming process is a result of a single magmatic hydrothermal event related to the extensive granite magmatism at around 1 79 1 77 ga tourmaline was crystallized throughout the hydrothermal process which resulted in the paragenetic variation between type 1 and type 2 the close association of tourmaline and gold suggests that the gold precipitated from the same boron rich source as tourmaline

association of gold with uraninite and pyrobitumen in the metavolcanic rock hosted hydrothermal au u mineralisation at rompas perapohja schist belt northern finland the perapohja schist belt comprises a supracrustal sequence of quartzites mafic volcanics and volcaniclastics carbonate rocks black shales mica schists and greywackes which were deposited from ca 2 44 to 1 91 ga during the rifting of the archaean basement in the eastern part of the fennoscandian shield metamorphism and multiple folding of the basin fill took place during the svecofennian orogeny 1 9 1 8 ga followed by intrusions of late orogenic 1 84 1 80 ga and post orogenic granitoids 1 79 1 76 ga the rompas au u mineralisation is hosted by deformed calcsilicate veins in mafic volcanic rocks and locally contains very high grade 10 000 g t au gold pockets with strict spatial association of gold minerals to uraninite and pyrobitumen chemical ages from the unaltered domains in the structure of uraninite indicate a 1 95 1 90 ga age for the deposition of the primary high temperature e g u th 100 in uraninite hydrothermal uranium mineralisation these data are in agreement with the results of previous u pb dating of uraninite by sims textural evidence suggests that metamorphic recrystallisation of the uraninite bearing quartz dolomite veins into calcsilicate mineral assemblages during the svecofennian orogeny 1 9 1 8 ga was followed by a hydrocarbon bearing fluid flow event and radiolytic polymerisation of hydrocarbons around grains of uraninite gold precipitated during a subsequent hydrothermal process in the fractures of uraninite as well as in the cracks and on the botryoidal surfaces of uraninite pyrobitumen nodules remobilisation and redeposition of uranium by these hydrothermal events produced secondary uraninite grains with chemical ages between 1 85 and 1 65 ga native gold is associated with galena altaite hunchunite nickeline and rare cobaltite pb bearing maldonite pyrite pyrrhotite chalcopyrite molybdenite and titanite raman spectra show disordered structure of undeformed pyrobitumen nodules in contrast with the well ordered graphite in calcsilicate veins mean random reflectance data for pyrobitumen indicate 270 340 c maximum temperature of thermal maturation this temperature range is also considered as the temperature of gold deposition results of multiple sulphur isotope analyses of organic material pyrite and acid volatile bound sulphur show distinct ranges of 34s values for sorg and scrs in uraninite pyrobitumen from 6 99 to 3 55 and from 10 02 to 4 41 respectively and uraninite pyrobitumen native gold mineral associations from 1 36 to 6 87 and from 0 42 to 9 7 respectively 33s data indicate local occurrence of nonmass dependent sulphur isotope fractionation owing to interaction of fluids with organic material concentration of lead in uraninite is depleted along the gold mineral filled fractures whereas the uranogenic lead isotope contents of galena altaite and hunchuite deposited in the same fractures are extremely high suggesting that the dominant source of lead for the crystallisation of these minerals was the radiogenic lead content of uraninite taking into account this source of radiogenic lead the calculated pb pb model ages for the lead minerals are between 1 75 and 1 70 ga sulphur and tellurium removal from the fluid by reaction with radiogenic lead released by uraninite appears to be an important mechanism in the strongly localised deposition of gold minerals scavenging of sulphur by pyrobitumen nodules from gold transporting fluids was an additional process triggering precipitation of gold carbon particles and organic functional groups in pyrobitumen probably acted as nucleation and adsorption centres for gold minerals

the growth of early proterozoic crust new evidence from svecofennian detrital zircons ion microprobe u pb analyses of zircons from the 1 9 gyr old svecofennian metasediments demonstrate the presence of both archaean and early proterozoic grains whereas all conventional multi grain analyses give apparent u pb ages of about 2 3 gyr most zircons are 1 9 2 0 gyr old suggesting that a major still unknown early proterozoic terrain older than 1 9 ga supplied detritus to the early orogenic svecofennian turbidites these results imply that the formation of early proterozoic felsic crust started c 100 myr earlier than has previously been inferred from isotopic data models in which nd model ages of 2 1 2 4 gyr have been interpreted as real crustal formation ages are not supported by our results

temporal constraints on the paleoproterozoic lomagundi jatuli carbon isotopic event the paleoproterozoic lomagundi jatuli positive 13c 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 207pb 206pb dates at 2058 2 ma 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 13c rich carbonate accumulation

the role of inclusions in u pb and sm nd garnet geochronology stepwise dissolution experiments and trace uranium mapping by fission track analysis the u pb and sm nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes however the budgets of uranium lead samarium and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon a combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium lead samarium and neodymium in garnets used for u pb and sm nd chronology variations of th u u nd and sm nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions the stepwise dissolution procedure does not induce any apparent artifacts on 207pb 206pb and sm nd ages the technique has been applied to garnets from three metamorphic terranes almandine garnet samples from the high grade pikwitonei granulite domain manitoba canada and the wind river range wyoming usa have the majority of their uranium samarium neodymium and radiogenic lead pb hosted by micrometer scale inclusions of monazite fission track densities reveal that uranium is 108 times more abundant in inclusions than in garnets stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite in contrast grossularandradite garnets from cascade slide adirondack highlands ny usa show little variation in fission track density and similar isotopic ratios between dissolution steps indicating that the budgets of uranium samarium neodymium and pb are not significantly influenced by inclusions the demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in u pb and sm nd garnet geochronology of metasedimentary rocks

2 45 ga break up of the archaean continent in northern fennoscandia rifting dynamics and the role of inherited structures within the archaean basement abstract constraints from restoration of minor rift related faults preserved from subsequent palaeoproterozoic overprint along the southern margin of the perapohja belt northern fennoscandia indicate local ne sw extension directions prevailed at the time of 2 44 ga rifting of the archaean continent this palaeostress field is attributed to pull apart setting of the perapohja belt at a left overstepping zone of major sinistral n s trending deformation zones consequently the variably overprinted nw se trending structures of the perapohja belt including the nw se central graben were originally generated as normal faults the proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step over zone between strike slip faults by contrast the faults bounding e w to ene wsw trending basement horsts are the result of reactivation of older basement structures underlying the pull apart basin based on the results we infer that the final break up of the archaean continent in northern fennoscandia utilized the 2 44 ga rift related nw se trending crustal anisotropy parallel with dyke swarms with pre existing archaean n s trending structures reactivated as second order shear zones in between the voluminous syn rift 2 5 2 4 ga and later pulses of mafic magmatism 2 32 1 98 ga are considered indicative of active type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the superior hearne and karelian kola cratons the presented model supports the continuity of the archaean craton beyond the caledonian orogen towards north west and explains its segmentation suggesting a model for the localization of the palaeoproterozoic supracrustal belts of northern fennoscandia moreover this paper provides an approach to understand the basement cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits

on the kriging of water table elevations using collateral information from a digital elevation model in unconfined aquifers flowing under topographic gradients the water table is a subdued replica of the ground surface above this principle is the basis for using detailed collateral or secondary information from digital elevation models to supplement sparse observations from water wells in the mapping of phreatic surfaces data from dem derived secondary variables are incorporated into the estimation of water table elevations using the geostatistical method known as kriging with an external drift ked two different ked models are proposed based on the choice of secondary variable in the first water table elevation is expressed as the sum of a deterministic trend given by topographic elevation and a residual random component representing depth to water table in the second depth to water table is expressed as a linear function of a deterministic trend given by the topmodel topographic index and a residual random error the relationship between water table depth and topographic index is derived from simplified groundwater dynamics and forms the basis of topmodel type rainfall runoff models the two ked models are applied to the mapping of water table elevations in the oak ridges moraine orm an unconfined aquifer near toronto canada results show that ked with topographic elevation as external drift is the more robust of the two models despite its strong theoretical basis the second model yields kriged water table elevations that are not always physically plausible in part this is because field observations of water table depth do not verify the predicated relationship with topographic index in large parts of the study area however this relationship may be valid in other cases and at other spatial scales in such cases the second model would provide a very powerful approach for mapping water table elevations and for calibrating distributed parameters of the topmodel equations on water well observations

long lived lree mobility in the cratonic rift and foredeep to foreland sedimentary cover at the western margin of the karelia province abstract the western margin of the karelia province of the fennoscandian shield consists of archean crust covered by remnants of paleoproterozoic c 2 5 1 9 ga metasedimentary rocks the study area in the eastern part of the margin shows 2 3 2 1 ga cratonic sedimentation followed by 2 1 2 05 ga rift sedimentation and c 1 91 1 92 ga foredeep to foreland sedimentation related to continent arc continent collision extreme lree mobility loss up 95 of la is recorded by many samples from these units monazite bastnasite and in some cases allanite have been the prealteration lree host phases in the lree depleted samples monazite was altered to th silicates with variable loss of the lree the remaining part of which was incorporated in th poor monazite the depositional age of the sedimentary units was studied by dating la mc icpms detrital zircon grains and the age of lree mobility by dating monazite and xenotime and by whole rock sm nd modeling the maximum deposition age of the foredeep to foreland basin is 1 91 1 92 ga based on detrital zircon data and the minimum age of the main foliation in the study area is 1 87 1 86 ga based on the younger age limit of late to post tectonic xenotime and the ages of few probable metamorphic zircons coeval bastnasite with xenotime in a lree enriched sample suggests that this was also one important stage of lree mobility monazite crystallizations have occurred 1 76 1 78 ga ago while the rb sr whole rock data suggest that the closure of this isotope system took place 1732 21 ma ago the sm nd modeling gives ages at 0 4 1 0 ga for lree depletion the sm nd age of 412 27 ma can be considered as the age of the latest major lree depletion event we propose that alkali bearing oxidizing fluids started to form in the cratonic sequences due to diagenetic reactions maybe as early as 2 3 2 1 ga alkalinity of fluids was later increased by interaction with alkaline 2 05 ga source materials before and during the c 1 91 1 92 ga foredeep to foreland basin stage basin inversion and metamorphic crystallization occurred at 1 91 1 87 ga episodes of fluid migration at 1 78 1 73 ga and afterwards were focused along fracture zones and in microfracture networks the 0 4 ga stage is the most prominent of the

svecofennian detrital zircon ages implications for the precambrian evolution of the baltic shield metasediments intruded by 1 90 1 87 ga old plutonic rocks form the oldest major proterozoic crustal component in the svecofennian domain of the baltic fennoscandian shield their ndtdm model ages and conventional multigrain zircon u pb ages between 2 4 and 2 1 ga have previously been interpreted either as mixing ages between 1 9 ga old juvenile materials and a minor archaean component or as actual rock and protolith ages to resolve the ensuing controversy 120 individual detrital zircons from svecofennian metasediments in sweden and finland were analysed using the shrimp ion microprobe the oldest materials in this array are a 3 44 ga old zircon from the tampere schist belt in finland and a 3 32 ga old crystal from southeastern sweden about 30 of the analysed crystals are 2 97 2 60 ga old while 65 have ages between 2 12 and 1 88 ga thus there is no evidence of 2 6 2 1 ga old protoliths but the age range of the proterozoic zircons indicates that a major area of 2 1 1 9 ga old crust was in erosional position 1 9 ga ago this implies that the formation of palaeoproterozoic crust in the baltic shield or its one time close neighbourhood must have commenced 100 200 ma earlier than hitherto assumed in conjunction with previously obtained isotopic data the youngest detritus ages of the present study constrain the age of svecofennian sedimentation it can also be concluded that the archaean zircons found in quartzites from southern sweden may have been derived from source areas to the southwest of the central svecofennian marine depositional basin the so called bothnian basin separating southern sweden from the archaean craton in the northeastern part of the shield

new geochronological and sm nd constraints across the pajala shear zone of northern fennoscandia reactivation of a paleoproterozoic suture abstract altogether 1130 u pb analyses on zircons from 25 samples across the pajala shear zone in northern fennoscandia were performed by la icpms methodology to study the origin of the zone the samples were also analyzed for their sm nd isotope systematics and geochemically the new data is tested with existing geological information to elaborate the provenance and maximum depositional ages the stratigraphic position of studied samples and finally to assess the tectonic evolution in the pajala shear zone area chromium enriched metasedimentary rocks are found within and to the east and west of the n s trending pajala shear zone no material originating from proterozoic felsic igneous zircons or mafic sm nd rocks was found in these rocks and thus only neoarchean or very early paleoproterozoic ages of deposition are possible our new results indicate that the bimodal 1 99 1 97 ga supracrustal rocks of the rovaniemi supersuite cover a large area east of the pajala shear zone based on the absence of archean zircons typical of the metasedimentary rocks covering the archean basement it seems that the basement and its sedimentary cover were not exposed at the time of deposition of the rovaniemi supersuite the metasedimentary rocks of the uusivirka supersuite located within the pajala shear zone vary from hornblende bearing metasedimentary rocks metapelite and meta arkose to orthoquartzite their characteristic features include a dominant proterozoic zircon population 65 87 with a high proportion of analyzed grains yielding ages between 1 96 ga and 1 91 ga and maximum depositional ages of 1 92 1 91 ga we record at least three metamorphic events at ca 1 92 1 90 ga ca 1 85 ga and at 1 82 1 78 ga of which the youngest can possibly be separated into two events at 1 83 1 82 ga and 1 79 1 77 ga respectively we propose a tectonic model in which continental breakup occurred at 2 1 2 05 ga followed by the development of a magmatic arc kittila arc at ca 2 0 ga above a subduction zone two archean continental units norrbotten and karelia collided at ca 1 92 1 91 ga the kittila arc was obducted upon the karelia continent as a foreland fold and thrust belt and the metasedimentary rocks of the uusivirka supersuite were emplaced as foreland deposits the proposed tectonic model is based on the absence of 2 44 ga layered intrusions and especially 2 06 ga rocks and 1 99 1 97 ga rocks of the rovaniemi supersuite that are found only east of the pajala shear zone in our model the pajala shear zone originated as a divergent plate boundary and was multiply reactivated after continental collision with both lateral and vertical movements

an early proterozoic ophiolite the jormua mafic ultramafic complex northeastern finland abstract the 1 96 ga old jormua complex comprises a fault bounded unit 2 5 20 km in the centre of the kainuu schist belt at the eastern margin of the early proterozoic svecokarelides the complex was dismembered and metamorphosed to lower amphibolite facies during the svecokarelian orogeny in the intact complex the basal unit was formed by serpentinites the middle unit of mafic dykes and the uppermost extrusive unit of basaltic pillow lavas gabbroic rocks are present as intrusive bodies in the serpentinites and narrow interdyke screens within the dyke unit trondhjemitic rocks occur as minor segregations in the gabbroic rocks or as cross cutting narrow dykes in the mafic dyke unit the dyke unit is locally composed exclusively of subparallel metabasalt and metadolerite dykes in places interdyke metagabbro and serpentinite screens are abundant well preserved chilled margins are common and many dykes are split and intruded by younger dykes the chemical composition of the serpentinites indicates derivation from harzburgitic dunitic mantle peridotites the gabbroic rocks which represent obvious tholeiitic cumulates show extreme differentiation similar to gabbros of the high ti ophiolites and gabbros dredged from present midocean ridges in chemistry the trondhjemitic rocks are close to granitic rocks from mid ocean ridges and high ti ophiolites the similar chemical composition of the dykes and lavas of the complex indicates that they are cogenetic both being tholeiitic metabasalts with chondrite normalized ree patterns ranging from slightly lree depleted to slightly lree enriched ones compared with modern rocks the jormua metabasalts most closely resemble transitional midocean ridge basalts it is suggested that the jormua complex and related mafic ultramafic bodies in eastern finland originated as divergent margin ophiolites concomitantly with the break up of the archaean craton in the extensional stage of the svecokarelian orogeny which culminated in the opening of a major ocean after 1 97 ga the tectonic emplacement of these ophiolites into their present locations occurred in connection with thrusting during the compressional stage of the orogeny about 1 9 ga ago

the temporal record of magmatism at cerro uturuncu bolivian altiplano twenty six new 40ar 39ar plateau ages for 23 lavas and domes from the uturuncu volcano in the altiplano of sw bolivia reveal a protracted eruptive history from 1050 5 to 250 5 ka eruptions have been exclusively effusive producing some 50 km3 of high k dacites and silicic andesites bimodal mineral compositions complex mineral textures the presence of andesitic magmatic enclaves within dacites and linear chemical trends on binary element plots all indicate that magma mixing is an important petrogenetic process at uturuncu post 458 ka distinct high and low mgo cr magmas are resolved these magmas erupt during similar times suggesting that eruptions are tapping different parts of the magma system albeit from the same vent system volcanic and petrological features are consistent with the existence of a vertically extensive magma mush column beneath uturuncu and calculated buoyancy forces are sufficient to drive effusive eruptions eruptive activity is episodic with six eruptive periods separated by hiatuses of 50 kyr cumulative volume curves demonstrate that the majority of the edifice formed between 595 and 505 ka the episodicity of eruptions is most likely to be related to fluctuations in the magma supply to the underlying altiplano puno magma body

new constraints for the source characteristics deposition and age of the 2 1 1 9 ga metasedimentary cover at the western margin of the karelian province abstract the paleoproterozoic cover sequence at the 100 150 km wide western margin of the archean karelian province is dominated by deep water lower and upper kaleva metasediments we present here an interpretation of sm nd isotope and geochemical data on 36 samples tims multi grain u pb zircon analyses on nine samples and ca 100 sims analysis of detrital zircon grains from four upper kaleva and one lower kaleva samples the lower kaleva is characterized by autochthonous parautochthonous lithologically heterogeneous metaturbidites showing common enrichment in quartz all the analysed detrital zircons are of a local neoarchean source but tdm variation up to 2 4 ga combined with geochemical data indicate abundant mixing of paleoproterozoic mafic material presumably from 2 1 ga plateau lavas and dykes in most of the lower kaleva samples the upper kaleva is dominantly allochthonous with tectonically enclosed fragments of ophiolite bodies and it is characterized by lithological and geochemical isotopic homogeneity geochemical isotopic and detrital zircon data favour material derived from an orogenic domain comprising both archean and proterozoic units followed by effective mixing during the transport the archean zircon grains 25 are mostly neoarchean the paleoproterozoic grains lack zircons at 2 5 2 2 ga and plot dominantly 92 between 1 92 and 2 05 ga the indicated maximum deposition ages vary from 1 95 1 94 ga to 1 92 ga the main source area proposed is the himalaya type lapland kola orogen now in the northeast which experienced mountain building and erosion at 1 95 1 91 ga the western margin of the karelian province shows evidence of rifting and lithosphere thinning from 2 1 to 1 95 ga but it is still under debate whether the craton breakup occurred at 2 06 ga in a volcanic or later at 1 95 ga in a non volcanic margin setting one hypothesis is that the onset of collision in the northeast changed plate motion and lead to a new spreading within the pre existing passive margin at 1 97 1 95 ga thus both a volcanic margin at 2 06 ga and a non volcanic margin at ca 1 95 ga could have been operated at the western margin of the karelian province

sm nd and pb isotopic study of mafic rocks associated with early proterozoic continental rifting the peropohja schist belt in northern finland the perapohja schist belt in northern finland rests unconformably on archaean granitoids and marks the early stages of proterozoic crustal evolution in the fennoscandian baltic shield 2440 ma old layered mafic intrusions predate the supracrustal and ca 2200 ma old sills of the gabbro wehrlite association intrude the lowest quartzites and volcanics runkaus of the sequence the sm nd mineral isochron of the penikat layered intrusion gives an age of 2410 64 ma the initial nd values of the penikat intrusion nd 2440 1 6 and the runkausvaara sill nd 2200 0 suggest that these mafic magmas were contaminated by older crustal material the sm nd and pb isotopic results on the 2 44 2 2 ga old runkaus volcanics indicate mobility of pb fractionation of sm nd during late greenschist facies metamorphism and crustal contamination the pb pb data provide an age of 1972 80 ma with a high initial 207pb 204pb ratio 1 8 49 while scattered sm nd data result in an imprecise age of 2330 180 ma with an initial nd value of about zero secondary titanite gives an u pb age of ca 2250 ma the jouttiaapa basalts in contrast ascended from the mantle without interaction with older crust these lree depleted tholeiites mark a break in continental sedimentation and yield a sm nd age of 2090 70 ma their initial nd 4 2 0 5 implies that the subcontinental early proterozoic mantle had been depleted in lree for a long period of time the first lava flows are strongly depleted in lree suggesting that their source was significantly more depleted than the source of mid ocean ridge basalts today

near orthogonal deformation successions in the poly deformed paleoproterozoic martimo belt implications for the tectonic evolution of northern fennoscandia abstract structural mapping integrated with aeromagnetic and anisotropy of magnetic susceptibility ams data form complimentary and robust tools for analysis of regional structures and fabric overprinting relationships this multi scale approach is used to constrain the geometry and tectonic evolution of sparsely exposed and flat lying outcrops of the martimo belt mb of northern fennoscandia the mb records a history of near orthogonal deformations upper greenschist to lower amphibolite facies metamorphism between ca 1 92 1 79 ga the earliest deformation d1 involved east directed thin skin thrusting and the development of recumbent f1 folds and a n s trending s1 fabric d2 deformation is prominent in the central part of the mb and is characterized by n s shortening with the development of e w trending f2 folds an associated pervasive steeply dipping s2 foliation and type 2 fold interference pattern the mb and mellajoki suite exhibit regional scale d2 synformal and antiformal refolded structures respectively d3 is more heterogeneously developed and manifests bimodal orientations with nnw sse and wnw ese structural trends it is pronounced in the central and eastern parts of the mb but weakly developed in the western part the steep dip of s3 in the region suggests that it formed in response to ne sw bulk shortening d4 structures have a conspicuous nne ssw trend that is visible as lineaments on the aeromagnetic image and locally in the field the last deformation event d5 affected almost the entire mb and was characterized by broadly wsw ene oriented crustal shortening the n s oval shape geometry of the eastern part of the mb is attributed to the d5 event contrasting with well developed overprinting relationships between d1 and d3 structures the relative timing of d3 d5 could not be established in the study area but can be inferred from structures with similar trends in the central lapland granitoid complex which is located north of the mb the d1 d5 deformation scheme can be readily reconciled with the tectonic evolution of the lapland savo fennian svecobaltic and nordic orogens of the composite svecofennian orogen the age of d1 in the northern part of the lapland savo orogen is 1 91 ga based on crosscutting granitoids and maximum deposition ages of sedimentary rocks d2 1 90 1 89 ga has been linked to orogen parallel shortening during the late stage of southern part of the lapland savo orogen several orthogonal tectonic events in northern fennoscandia indicate amalgamation of crustal blocks around the core of the archean karelia continent at 1 92 1 77 ga and that fennoscandia was a key component of the columbia hudsonia nuna supercontinent

linkage of crustal deformation between the archaean basement and the proterozoic cover in the perapohja area northern fennoscandia abstract reconstructed variations in stratigraphic thicknesses combined with mapped variations in fold and thrust belt structural styles and patterns of the entire palaeoproterozoic perapohja belt in northern finland show that both the deposition of the 2 4 1 9 ga supracrustal rocks and the structural evolution during the subsequent svecofennian orogeny 1 9 1 8 ga have been tightly controlled by i the palaeo topography of the archaean 2 5 ga basement and ii the reactivation of archaean basement faults during the multiple stages of rifting and deposition of the supracrustals as well as during subsequent basin inversion and fold and thrust belt development folds and thrusts in the supracrustal sequence are found to be largely detached from the underlying basement along the petajaskoski detachment zone pdz that formed within a mechanically weak stratigraphic unit in the basal part of the perapohja belt stratigraphy the pattern of contractional structures is complex but developed during one phase of simple progressive south vergent thrusting two styles of basement cover linkage are recognised the hard linked style is best illustrated by the reverse sihtuuna fault currently defining the northern margin of the e w to ene wsw trending basement horsts and the overlying anticlines and juxtaposing the stratigraphically lowermost perapohja belt rocks with higher up stratigraphy indicating the fault is inherited after a pre existing basement structure based on the distribution and thickness variation of the palaeoproterozoic supracrustal units the faults bounding the basement horst were active as extensional faults during deposition the soft linked style relationship is evident as abrupt terminations of thrust related elongate domes and localisation of the compressional overprint into the nw se trending central graben understanding the basement architecture and basement cover linkages is highly significant because the majority of the known mineralizations of the belt are located along or close to the basement faults

paleocene orthopteroids from south central alberta canada new paleocene orthopteroid taxa from the paskapoo formation are described all are represented by adult wings albertoilus cervirufi n gen n sp superfamily hagloidea family prophalangopsidae

age and structural constraints on the tectonic evolution of the paleoproterozoic central lapland granitoid complex in the fennoscandian shield abstract the paleoproterozoic central lapland granitoid complex clgc forms the northern core of the fennoscandian shield we present here new u pb age data and review the existing ones from plutonic and metasedimentary rocks of the clgc and integrate the results with the structural field observations to decipher the tectonic history of this region granitoids vary in age from 2 43 ga to 1 78 ga whereas metamorphic zircon overgrowths both in granites and metasedimentary rocks vary between 1 90 ga and 1 76 ga with several age peaks indicating prominent metamorphic pulses the almost continuous growth of metamorphic zircon domains favors that the central part of the clgc was near solidus for 100 120 ma a succession of five deformations and associated foliation producing events which were near orthogonal to each other have been identified east vergent thrusting and stacking d1 commenced at ca 1 92 ga and led to the onset of barrovian type metamorphism followed by a radical tectonic shift that yielded e w trending d2 structures exhumation at 1 88 1 87 ga caused decompression melting of thermally evolved thickened crust and led to the emplacement of large volumes of granites and triggering of buchan type metamorphic conditions ne sw collision d3 at the southwestern margin of fennoscandia heterogeneously overprinted the s3 fabric on d2 structures orogen parallel nnw sse buckling at 1 87 1 85 ga formed oroclines in the hot linear arc segments renewed nnw sse shortening d4 at 1 84 1 81 ga produced long wavelength folds and caused the formation of domal structure in the clgc the final 1 78 1 76 ga far field collision d5 in the west overprinted all the pre existing structures this complex multi directional tectonic scenario provoked the intermittent exhumation phases between different contraction events of the clgc