geology and geochronology of the suomujarvi complex a new archaean gneiss region in the ne baltic shield finland abstract recent mapping in the eastern central lapland granitoid complex and dating of zircons at the nordsim facility was performed in order to highlight the existence of an 892 km2 area of archaean gneisses not previously identified in southeast lapland finland this archaean enclave is composed of northeast trending km scale bands of orthogneisses with varying mafic content and is named the suomujarvi complex along its southeastern boundary lies the aholanvaara volcano sedimentary package the evolution of the suomujarvi complex began at ca 2810 ma with intrusion of the homogeneous jumisko biotite tonalite granodiorite in the west this lithology becomes increasingly heterogeneous to the east where muscovite and an inherited ca 2870 ma old rock component represented by zircon cores are present these orthogneisses were later intruded by the 2755 18 ma 1 protolith of the miehinkavaara amphibole and biotite bearing diorite tonalite gneisses producing diffuse contacts characterized by gradual increases in mafic minerals all of the orthogneisses were then migmatized metamorphosed after ca 2730 ma the aholanvaara volcano sedimentary package was deposited its main quartzite unit was dominantly derived from an unknown 2731 17 ma 1 source but it also contains a ca 3 4 ga detrital component the absence of 2800 2820 ma detrital material in these sediments indicates they were not derived locally from the suomujarvi complex instead they share affinities with supracrustal packages further south in posio east in the belomorian province and north in the central lapland greenstone belt widespread voluminous intrusion of granites occurred in the area surrounding the suomujarvi gneisses and left a marked metamorphic overprint on the gneisses during the ca 1 8 extensional collapse of the svecofennian and lapland kola orogens multi scale folding produced subhorizontal southwest plunging tight to isoclinal upright folds and the dominant axial planer northeast trending fabric in the area this fabric is cut by a northwest trending fabric in the southwestern part of the area geological characteristics of the suomujarvi complex and ages of the orthogneisses within it indicate that it most likely represents the southwestern margin of the archaean belomorian block

the geochemical fingerprints of different tectonomagmatic environments using hygromagmatophile element abundances of tholeiitic basalts and basaltic andesites abstract using hygromagmatophile element abundances normalized to primordial mantle composition it is possible to discriminate between the tectonic settings of tholeiitic basalts and basaltic andesites 47 0 53 5 sio2 based on field and chemical evidence data from provinces or grouped localities have been compiled in order to establish chemical reference data sets for ocean island continental ocean ridge and floor subduction zone and back arc environments the disturbing effects of crustal contamination and hydrothermal alteration on the hygromagmatophile h element patterns are of minor significance in the recognition of plate tectonic settings on the basis of the data sets magmas from a number of ambiguous tectonic settings have been recognized using their h element patterns attention is directed to problems concerning rifting and drifting related basalts in an apparent continental setting and to the origin of ophiolite complexes using available data the pillow lavas of the troodos ophiolite complex cyprus are recognized as basalts erupted in a subduction zone environment

paleoproterozoic arc and ophiolitic rocks on the northwest margin of the trans hudson orogen saskatchewan canada their contribution to a revised tectonic framework for the orogen abstract a new lithotectonic framework for the northwestern reindeer zone of the trans hudson orogen divides rocks into five northwest to north dipping volcano sedimentary assemblages 1 at the structural base the 1 92 1 87 ga largely sedimentary levesque bay assemblage partly equivalent to former maclean lake gneisses which lies within the confines of the kisseynew domain and is tectonically imbricated with metasedimentary rocks of the the lawrence point assemblage is interpreted as a dismembered supra subduction zone ophiolite high mgo refractory harzburgite group 1 ultramafic rocks at the structural base of the assemblage is geochemically identical to the upper mantle section of selected supra subduction zone ophiolites and mantle tectonites chromite and olivine compositions of the group 1 ultramafic rocks are also comparable to those of ophiolitic harzburgite and mantle tectonite mafic metavolcanic rocks of the assemblage are classified as subalkaline tholeiitic basalts their trace element patterns and hf ta th y nb and la element ratios resemble those of modern back arc basin basalts the reed lake assemblage represents a subduction generated arc complex that was built on top of the lawrence point assemblage its mafic metavolcanic rocks are subalkaline basalts with calc alkaline trends and elevated th and ce concentrations and negative nb anomalies feldspar porphyry dykes intruding the lawrence point and duck lake assemblages constrain timing of lawrence point ophiolite emplacement onto the duck lake assemblage to 1 86 1 84 ga the trace element geochemistry of the dykes suggests continued arc volcanism after ophiolite emplacement mafic metavolcanic rocks of the levesque bay assemblage are geochemically similar to those of the lawrence point assemblage other ultramafic rocks peridotite to pyroxenite are abundant in the lawrence point assemblage but have similar geochemistry to small ultramafic bodies intruding the reed lake duck lake and levesque bay assemblages they represent a separate later phase 1 86 ga of ultramafic plutonism which post dates ophiolite emplacement timing of lawrence point ophiolite emplacement between 1 86 and 1 84 ga and geochemistry of later felsic and mafic ultramafic volcanism suggest that the lawrence point ophiolite and overlying reed lake arc assemblage were not accreted to the hearne craton prior to 1 86 ga but were first accreted to the flin flon glennie complex after 1 86 ga

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

extreme 13ccarb enrichment in ca 2 0 ga magnesite stromatolite dolomite red beds association in a global context a case for the world wide signal enhanced by a local environment abstract the palaeoproterozoic positive excursion of 13 c carb is now considered as three positive shifts of 13 c carb separated by returns to 0 which all occurred between 2 40 and 2 06 ma this isotopic event is unique in terms of both duration 300 ma and 13 c enrichment up to 18 the mechanism responsible for one of the most significant carbon isotopic shifts in earth history remains highly debatable to date 13 c of 10 to 15 cannot be balanced by organic carbon burial forg as there is no geological evidence for an enhanced corg accumulation prior to or synchronous with the excursion instead termination of these excursions is followed by formation of a vast reservoir of 13 c depleted organic material 45 at shunga and by one of the earliest known oil generation episodes at 2 0 ga none of the three positive excursions of 13 c carb is followed by a negative isotopic shift significantly below 0 as has always been observed in younger isotopic events reflecting an overturn of a major marine carbon reservoirs this may indicate that forg was constant implying that the mechanism involved in the production of corg was different onset of intensive methane cycling resulting in c change is another possibility the majority of sampled 13 c carb rich localities represents shallow water stromatolitic dolostones red beds and evaporites formed in restricted intracratonic basins and may not reflect global 13 c carb values closely spaced drill core samples n 73 of stromatolitic dolostones from the 1980 27 ma tulomozerskaya formation in the onega palaeobasin russian karelia have been analysed for 13 c carb and 18 o carb in order to demonstrate that different processes were involved in the formation of 13 c carb rich carbonates the 800 m thick magnesite stromatolite dolomite red beds succession formed in a complex combination of environments on the karelian craton peritidal shallow marine low energy protected bights barred basins evaporative ephemeral ponds coastal sabkhas and playa lakes the carbonate rocks exhibit extreme 13 c enrichment with 13 c values ranging from 5 7 to 17 2 vs v pdb mean 9 9 2 3 and 18 o from 18 6 to 26 0 vs v smow mean 22 0 1 6 the tulomozerskaya isotopic excursion is characteristic of the global 2 4 2 06 ga positive shifts of carbonate 13 c 12 c although it reveals the greatest enrichment in 13 c known from this interval an external basin s is considered to have provided an enhanced corg burial and global seawater enrichment in 13 c the global background value for the isotopic shift at tulomozero time ca 2 0 ga is roughly estimated at around 5 an explosion of stromatolite forming microbial communities in shallow water basins evaporative and partly restricted environments high bioproductivity enhanced uptake of 12 c and pene contemporaneous recycling of organic material in cyanobacterial mats with the production and consequent loss of co2 and ch4 are believed to be additional local factors which may have enhanced 13 c from 5 up to 17 such factors should be taken into account when interpreting carbon isotopic data and attempting to discriminate between the local enrichment in 13 c and globally enhanced 13 c values we propose that many previously reported 13 c values from other localities where environmental interpretations are not available or have not been taken into account may not represent the global 13 c values

geochemical evidence for the geotectonic setting of early proterozoic metavolcanic sequences in lapland systematic geochemical study of early proterozoic metavolcanic rocks from the northern part of the baltic shield supports the concept that a complex mosaic of continental masses volcanic arcs and marginal basins were welded into one cratonic massif during the svecokarelian orogeny 1900 1750 ma in the north in finnmark and northern norrbotten tholeiitic metabasalts together with thick shallow water clastic metasediments form the main part of the karelian supracrustal sequences which in turn overlie archaean basement the trace element signatures of these metabasalts are comparable to those of post palaeozoic continental tholeiites which erupted in zones of lithosphere attenuation and which suffered contamination by sialic crust it is not however possible to determine by geochemistry alone whether the continental rift basin in question lay in an intraplate or back arc setting metabasalts with a volcanic arc geochemical signature become an increasingly important component of supracrustal sequences as the diffuse southern margin of the karelian continent is approached in the skellefte field an ensialic volcanic arc developed and fed considerable quantities of immature clastic sediment into the deep water norrland geosyncline which may have been fully oceanic in character subduction magmatism continued during the svecokarelian orogeny giving rise to synorogenic metavolcanic suites such as the kiruna porphyries the evidence for marked spatial variation of magmatic activity in the early proterozoic is in contrast to the rather monotonous pattern of greenstone belt development reported from the archaean we believe that this spatial variability is a consequence of geotectonic processes similar to those of modern day plate tectonics and present a plate tectonic model for the early proterozoic development of the baltic shield

palaeoproterozoic rifting and formation of sulphide deposits along the southwestern margin of the svecofennian domain southern sweden abstract a mineralized palaeoproterozoic volcanic sequence has been identified southwest of vetlanda in southern sweden the sequence which is defined as the froderyd group is surrounded by 1 78 ga granitoids and may be considered as the southernmost segment of svecofennian crust in sweden the froderyd group has a bimodal geochemical character and is dominated by pillow basalts and associated gabbros but rhyolites thin intercalations of marble and stratiform sulphide ores do also occur the carbonate intercalations and the stratiform sulphide ores are interpreted as submarine exhalites formed syngenetically with the volcanic activity the mafic rocks have a typical mid ocean ridge basalt geochemistry with flat rare earth elements patterns the lead isotopic compositions of the mafic rocks and the associated stratiform sulphide ores support such a close affinity to juvenile sources with mantle character in contrast the rhyolites display more evolved geochemical signatures and isotopic patterns and are interpreted to represent remelted products of slightly older sialic crust a model for the formation of the froderyd group is presented suggesting rifting along a continental margin which may have occurred in the final stage of the svecofennian crust forming period

zircon and monazite geochronology of deformation in the pielavesi shear zone finland multistage evolution of the archaean proterozoic boundary in the fennoscandian shield the raahe ladoga shear complex is a major crustal structure representing the archaean palaeoproterozoic boundary in the fennoscandian shield the complex developed during the svecofennian orogeny c 1 9 1 8 ga beginning with regional thrust tectonic phases d 1 and d 2 followed by large scale shearing events d 3 and d 4 the pielavesi shear zone is a vertical north south trending shear zone within the raahe ladoga shear complex formed during regional d 3 shearing and later reactivated during the regional d 4 phase three north south trending elongate granitoid intrusions were selected as representative of silicic melts that intruded the transtensional pielavesi shear zone during the regional d 3 phase the oriented magmatic fabric of the granitoids indicates that they intruded coeval to the deformation event the zircon u th pb secondary ionization mass spectrometry sims ages of these intrusions 1888 4 1884 6 and 1883 5 ma overlap within error and provide a direct age for the regional d 3 deformation e hf t 1 1 to 3 4 and e nd t 1 2 to 0 4 values from these granitoids are both consistent with a predominantly juvenile source affected by a minor archaean component u th pb sims analyses of metamorphic monazite formed within a crosscutting blastomylonite provide an age for the regional d 4 phase and associated fluid activity of 1793 3 ma supplementary material analytical methods and tabulated analytical data are available at https doi org 10 6084 m9 figshare c 3498501

nature of the early proterozoic outokumpu assemblage eastern finland abstract the 1970 ma old outokumpu assemblage consists of serpentinites gabbro and mafic metavolcanites with pelagic and chemogenic sediments lying in the early proterozoic 2200 1950 ma karelides of eastern finland its formation occurred after the development of the jatulian 2200 2000 ma quartzite dominated shelf sediments and prior to the deposition of the kalevian flysch the outokumpu assemblage has been compared to an ophiolite and though the lithologies are all present sheeted dykes excepted the internal stratigraphy suggests otherwise the mafic metavolcanites have a more depleted chemistry than n morb and were erupted into shallow water along with the deposition of pelagic shales carbonates and chemogenic sediments including metalliferous exhalites ultramafic sills intruded these rocks at a later stage prior to rapid subsidence and flysch deposition the recently discovered jormua ophiolite complex may constitute part of the depositional basement to the outokumpu assemblage and the kalevian flysch which formed over stretched crust in an ensialic basic

geochemistry and origin of saline groundwaters in the fennoscandian shield chemical and isotopic analyses of water from drill holes and mines throughout the fennoscandian shield show that distinct layers of groundwater are present an upper layer of fresh groundwater is underlain by several sharply differentiated saline layers which may differ in salinity relative abundance of solutes and o h sr and s isotope signature saline groundwater can be classified into four major groups based on geochemistry and presumed origin brackish and saline waters from 50 200 m depth in coastal areas around the baltic sea exhibit distinct marine chemical and isotopic fingerprints modified by reactions with host rocks these waters represent relict holocene seawater inland three types of saline groundwater are observed an uppermost layer of brackish and saline water from 300 900 m depth saline water and brines from 1000 2000 m depth and superdeep brines which have been observed to a depth of at least 11 km in the drill hole on the kola peninsula u s s r electrical and seismic studies in shield areas suggest that such brines are commonly present at even greater depths the salinity of all inland groundwaters is attributed predominantly to water rock interaction the main solutes are cl ca na and mg in varying proportions depending on the host rock lithology the abundance of dissolved gases increases with depth but varies from site to site the main gas components are n2 ch4 up to 87 vol and locally h2 the 13c value for methane is highly variable 25 to 46 and it is suggested that hydrothermal or metamorphic gases trapped within the surrounding rocks are the most obvious source of ch4 the uppermost saline water has meteoric oxygen hydrogen isotopic compositions whereas values from deeper water plot above the meteoric water line indicating considerably longer mean residence time and effective low temperature equilibration with host rocks geochemical and isotopic results from some localities demonstrate that the upper saline water cannot have been formed through simple mixing between fresh water and deep brines but rather is of independent origin the source of water itself has not been satisfactorily verified although superdeep brines at least may contain a significant proportion of relict precambrian hydrothermal or metamorphic fluids

early proterozoic deposition and deformation at the karelian craton margin in southeastern finland abstract the margin of the karelian craton underwent complex though not chaotic deformation during the svecokarelian orogeny the resultant structural sequence can be subdivided into an early phase that involved translation of thrust nappes and recumbent folds verging towards the craton and a younger phase having more zonal and upright structures these latter include nw trending ductile shear zones associated with widespread pluton emplacement and later n trending faults and shear zones having substantially vertical displacements these features are interpreted in terms of a relative increase in vertical crustal stresses resulting from gravitational and thermal disequilibrium in the overthrust archaean basement despite this structural complexity two distinct depositional provinces are recognized in north karelia separated by a major thrust zone the more easterly hoytiainen province is a narrow essentially intracratonic feature containing locally derived archaean and proterozoic detritus an early phase of chemogenic and pelitic deposition with sporadic tholeiitic volcanism was followed by an influx of coarse clastic detritus largely deposited from turbidity currents within a prograding submarine fan the province apparently records deposition in a transtensional en echelon system of basins initiated near the craton margin at about 2 1 ga in contrast the savo province is largely allochthonous and includes the distinctive outokumpu assemblage commonly interpreted as a remnant of lower proterozoic oceanic crust provenance for the bulk of the overlying metasediments remains problematical although local evidence for deposition on archaean basement exists if an accretionary margin context is considered several potential niches for savo province deposition can be found that appear to satisfy constraints imposed by both isotopic data relating to provenance and the sequence and geometry of svecokarelian deformation

precambrian ophiolite belts of southern siberia russia and their metallogeny abstract greenstone and ophiolite belts of different ages in southern siberia are compared the archaean olondo greenstone belt 3 0 2 9 ga within the aldan shield is different from younger ophiolite belts the early proterozoic baikal muya ophiolite belt 1 9 ga containing both komatiites and a complete ophiolitic sequence of the tholeiite series and the late proterozoic early cambrian 0 6 0 52 ga kurtushibin and ilchir ophiolite belts in western and eastern sayan are fairly comparable with young ophiolites and modern crust of oceanic type in the olondo greenstone belt there are komatiite and tholeiite series that are represented by both volcanic rocks and differentiated sills but no dyke complex is present in the baikal muya ophiolite belt there are metavolcanic rocks including komatiites and komatiitic basalts a dyke complex upper cumulate gabbros and lower metaperidotites ophiolites of the kurtushibin belt in the western sayan and ilchir belt in eastern sayan are examples of complete ophiolitic sequences with long and complicated evolution histories including development of a marianite boninite series of primitive island arc type asbestos chromite fe ti oxide polymetallic and gold deposits occurring precambrian ophiolites are briefly described when ophiolites appeared about 2 0 1 9 ga ago cr ni cu and asbestos were concentrated within them the composition of ophiolites and associated ores changed significantly with time a change in upper mantle convection may be the key to the magmatic and metallogenic evolution

precambrian blueschist terranes of the world blueschists are most common in post paleozoic orogenic belts higher geothermal gradients and a thinner lithosphere in the precambrian are widely thought to have disfavored blueschist formation however several precambrian blueschist belts have been reported from eurasia on the basis of established geologic and geochronologic constraints these belts are grouped as suspect possible and definite precambrian blueschists suspect ones include those in alaska in anglesey u k in the scandinavian caledonides and in yunnan china possible precambrian blueschist belts are those in delhi india in central china in heilongjiang china and in the urals and altai sayan tien shan u s s r definite precambrian blueschist terranes are those in western africa and in aksu china these blueschist belts are described with emphasis on the age constraints the recently investigated aksu blueschist terrane is presented as an example this high p t coherent blueschist greenschist terrane at aksu is unconformably overlain by a thick unmetamorphosed sedimentary sequence ranging in age from late sinian ca 600 800 ma through eocambrian to paleozoic the basal sinian conglomerates contain subrounded to rounded clasts of the underlying schist clearly demonstrating the pre sinian age of blueschist facies metamorphism characteristic features of the above mentioned blueschist belts include 1 1 the blueschist terranes are coherent and primary compositional lithological layers are well preserved melange belts are not common 2 2 protoliths include pelite sandstone bimodal volcanics limestone and small quantities of pillow basalt and pelagic sediment ultramafics are rare 3 3 the protoliths were subjected to multiple deformation and transitional blueschist greenschist facies metamorphism typical precambrian blueschist contains the assemblage crossitic amphibole epidote chlorite albite quartz actinolite rare lawsonite jadeitic pyroxene and aragonite have been described in few belts 4 4 the metamorphic facies series belongs to the high pressure intermediate type of miyashiro 1961 the precambrian blueschist and other pre mesozoic intracontinental blueschist of eurasia formed before rather than during final closure of intervening ocean basins between pairs of continents they do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction underplating and imbrication of accretionary wedges including both oceanic and continent derived elastics the rare preservation of precambrian blueschist is due to greenschist overprinting when the heat flow and geothermal gradient was high rather than reflecting lack of production their existence in china and africa demonstrates that requisite conditions for blueschist facies metamorphism existed at least 700 ma ago

high pressure eo alpine parageneses developed in magnesian metagabbros gruppo di voltri western liguria italy aluminous magnesian metagabbroic layers and pods occur in the beigua serpentinite of western liguria where they are associated with more voluminous fe ti rich lithologies both types were subjected to localized early rodingitization all rocks show the effects of a pervasive essentially isochemical high pressure early alpine metamorphism for which a sequence of mineral assemblages is partially preserved eclogite glaucophanic eclogite na al actinolitic amphibolite greenschist the nature of the eo alpine high pressure recrystallization and the origin of the protoliths are the subjects of this paper

geochemistry and tectonic setting of lewisian clastic metasediments from the early proterozoic loch maree group of gairloch nw scotland abstract the early proterozoic 2 0 ga loch maree group lmg of the lewisian complex consists of a supracrustal sequence of metasedimentary schists and associated tholeiitic metavolcanic amphibolites within the archaean basement the metasediments comprise three main mapped units the flowerdale schists kerrysdale schists and charlestown schists together with numerous narrower bands the dominant clastic metasediments within the schist units semipelites and psammites are chemically distinguished as a series of greywackes and lithic sandstones with variable lil and transition element abundances fractionated ree patterns and overall compositions similar to paas three chemical groups are recognized whose range of composition largely reflects the mixing between two main source components lewisian gneissose basement with an intermediate acid continental crust composition and local rift related amphibolites with a primitive tholeiitic composition which also incorporates components not preserved within the lmg the geochemistry of the clastic metasediments confirms a previous tectonic model for the lmg as an intracontinental extensional rift receiving clastic detritus from the adjacent uplifted basement as well as from internal volcanogenic sources

petrology and geochemistry of metabasalts from the 1 95 ga jormua ophiolite northeastern finland the jormua ophiolite exposes a unique fragment of red seatype oceanic crust formed in a setting related to continental break up 1950 ma ago two distinct types of basalt are represented the early dykes and the main basalt suite early dykes have fractionated h ree patterns oib like trace element patterns low 2j nb 6 and ejwfyss ga 0 6 indicative of their derivation from an enriched source the remaining dykes and all lavas belong to the second e morb like main basalt suite which is characterized by high mg number and cr contents flat ree patterns zr b 6 17 chondritic th ta and only moderately depleted isotopic signatures zjufi gs a 4 1 9j most main suite samples cannot be related solely by fractional crystallization to a common parental magma rather they represent distinct melt fractions that underwent variable amounts of chromite olivine plagioclase fractionation during ascent a significant part of the compositional diversity of the main basalt suite can be explained by mixing a depleted source with a relatively uniform proportion of an enriched component similar to that represented by the oib like early dykes it is probable that during the latest stages of continental rifting the oib type melts metasomatized the upper part of the depleted asthenospheric mantle which became the source of the main basalt suite soon after the old continental lithosphere was ruptured the complete absence of any evidence for a subduction related component in the basalts implies that jormua is not a back arc ophiolite

rapid growth of some major segments of continental crust some major segments of continental crust display a narrow range of sm nd crustal formation ages the sizes of the canadian shield the svecokarelian province of northern europe the west central united states and the arabian nubian shield suggest rapid crustal growth island arc accretion models rank among the most favored tectonic models for the formation of these areas a quantitative comparison of the growth rates of these crustal segments to mesozoic cenozoic arc addition rates shows however that island arc accretion alone seems insufficient to account for the amount of crust that was produced in each of these terrains other additional mechanisms such as hot spot volcanism and underplating may have been active in addition to arc accretion alternatively large amounts of preexisting basement have gone so far undetected

metallogeny and tectonic evolution of the early proterozoic skellefte district northern sweden abstract the various mineralizations in the early proterozoic skellefte district are discussed and placed in a palaeotectonic context the metallogeny together with volcanic stratigraphy geochemistry isotope studies and structural geology favour a former destructive plate margin at 1 89 ga the area resembled a modern island arc environment where volcanogenic massive sulphide and gold rich porphyry type deposits formed mafic to ultramafic sills and dykes hosting ni mineralizations intruded a greywacke sequence immediately south of the skellefte district as a result of large scale rifting at the end of the volcanic period gold was emplaced 20 60 ma later during peak metamorphism at the end of the svecofennian orogeny finally at 1 80 ga after accretion of the island arc to the archaean continent in the northeast large granitoids intruded some of these have associated w u mo and li mineralizations an evidence for a crustal or cratonic influence

the accretionary svecofennian orogen insight from the babel profiles abstract the babel profiles b c 1 7 form a 1200 km long nearly continuous cross section through the svecofennides the near vertical marine reflection profiles display a wide range of crustal structures that can be associated with both the accretionary svecofennian orogeny 1 96 1 75 ga and the following subjotnian and jotnian rift stages 1 65 1 11 ga the svecofennian accretionary orogeny took place when a number of micro plates with island arc affinities and surface expression of a large archipelago accreted to the continental karelian plate some of the accreting terranes seem to have had older cores that have acted as crustal indentors during the collision babel profiles 3 and 4 image a series of collisional terrane boundaries between karelian continental margin savo arc sa and central finland arc cfa in the north the karelian margin has been both over and underthrust by the savo arc cfa comprises a folded thrust package on top of a continental nucleus and the savo arc the associated subduction zone and accretionary prism are interpreted to lie to the south underneath the bothnian basin area where prominent ne dipping lower to middle crustal reflections are found along babel profiles 1 and 4 an oblique collision of the central finland arc and the continent resulted in the development of the strike slip fault on the young hot savo arc babel profiles 1 6 7 and c image the internal architecture of the southern and central finland arcs the unusually thick crust 55 60 km hosts unreflective high density mafic intrusions acting as a crustal indentor a highly reflective antiform structure developed on the southern side of it southern finland arc complex sfa is an imbrication structure comprising stacked slices of arc related crust on an older continental nucleus bergslagen nucleus prior to the collision the sfa suffered from gravitational collapse during which the crust was thinned profile b images the architecture of the central and southern swedish svecofennides the sormland terrane sob is interpreted as the accretionary prism of the southern finland arc squeezed between the svecofennian collage in the north and the advancing continent to the south the southern continent island arc is characterised by ne dipping crustal reflections and moho offsets as well as step wise increasing thickness of the crust after the final collision large volumes of mantle derived material intruded the crust as large mushroom shaped plutons they are interpreted as the heat source for the tib magmatism in southern and western scandinavia

archean proterozoic collision boundary in central fennoscandia revisited abstract archean proterozoic ap collision boundary in central fennoscandia is a complex collage of archean basement rocks its paleoproterozoic supracrustal cover rocks and svecofennian 1 93 1 78 ga supracrustal and plutonic rocks the apparent steep nature and complexity of the ap boundary has led to contradictory tectonic models varying from arc continent collision via back arc closure to accretion of arc terranes along a strike slip shear zone we present new geochronology data from detrital zircons combined with whole rock geochemical and sm nd isotopic data on paleoproterozoic sedimentary and volcanic rocks across the boundary to revise its nature the 2 06 ga siilinjarvi volcanic complex and associated turbidites with an inferred deposition age at 2 1 2 06 ga are part of the cover sequence the siilinjarvi volcanic rocks derive from both plume oib and sub continental lithospheric mantle sclm magma sources and are often contaminated by the archean crust the sahara suite sedimentary cover rocks are derived from local 1 95 1 92 ga bimodal volcano plutonic source which source also explains the common age population found in younger sedimentary rocks the nalantojarvi suite psammites with maximum depositional ages of 1 97 1 93 ga are continental slope to deep water sediments deposited at the continental margin contemporaneous mafic magmatism seen as lavas and dykes show a change from sclm dominated present east to asthenospheric depleted mantle dm dominated present west melts the interpretation of the nalantojarvi suite as cover rocks of the lower plate indicates that the archean crust continues at least 50 km more to the west than thought earlier parautochthonous sedimentary rocks from salahmi eastern kansanneva and loutemaki with maximum depositional ages at 1 92 1 91 ga are inferred as collision related foredeep deposits from local sources both the lampaanjarvi suite and pyhajarvi suite sedimentary rocks have deposited at 1 89 1 88 ga or later whereas the lampaanjarvi suite rocks are dominated by a local source and the pyhajarvi suite rocks derive from a mixture of older sediments and 1 89 1 88 ga granitoids at least two tectono metamorphic stages separated with a major unconformity can be differentiated in the study area the early deformation and metamorphic stage in the nalantojarvi suite has occurred before the intrusion of the late to post collisional pyroxene granitoids at 1 88 ga the pyhajarvi suite sedimentary rocks have been deposited after the intrusion of these granitoids between 1880 and 1875 ma and then subsequently deformed and metamorphosed at 1875 ma we propose a tectonic model where the continental breakup occurred at 2 1 2 06 ga followed by a new spreading within the pre existing passive margin at 1 98 1 93 ga during initial continent arc collision the subducted slab steepened strongly at high angle subduction ceased rapidly and finally detached and shallow slab breakoff occurred at 1 92 1 91 ga followed subsequently by ophiolite obduction and a formation of a foreland fold and thrust belt mantle crust decoupling of the archean lithosphere occurred and a crocodile jaw formed where the arc rocks are partly buried beneath the bounding archean continent similar features have been recorded in other ap collision zones which could indicate that the buoyant nature of the archean lithosphere combined with denser paleoproterozoic lithosphere may be a general cause for decoupling of archean crust and mantle during arc continent collision in paleoproterozoic

the annieopsquotch ophiolite belt of southwest newfoundland geology and tectonic significance the annieopsquotch ophiolite belt consists of ophiolite fragments exposed from buchans in central newfoundland to the southwest tip of the island the largest remnants belong to the annieopsquotch complex in the northeast and the long range mafic ultramafic complex in the southwest the arenigian annieopsquotch complex exposes an ophiolite stratigraphy from layered cumulate gabbro moho sequence through high level gabbro sheeted dikes pillow lava and chert the long range mafic ultramafic complex consists largely of cumulate and high level metagabbro with subordinate ultramafic rock and metabasalt all engulfed as massif to xenolith size fragments in ordovician tonalitic and younger mafic to felsic granitoid intrusions the latter rocks are intersected by a wide locally gently inclined high temperature ductile thrust zone which also encompasses amphibolite semipelitic schist and marble between buchans and the southwest tip of newfoundland the tonalite terrane encloses many smaller ophiolitic fragments the ophiolite fragments of the northern half of the belt have similar dike trends have n type morb chemistry and likely formed one major allochthon of iapetus oceanic crust this was disrupted by the intrusion of tonalite and other magmas in an arc or arc trench environment and by subsequent faulting an attempt is made to assess the significance of the annieopsquotch belt in relation to ether newfoundland ophiolites

the petrochemistry of ophiolite gabbroic complexes a key for the classification of ophiolites into low ti and high ti types ophiolites have been divided into two groups high ti and low ti types these can be discriminated by studying the fractionation trends of both gabbroic complexes this work and lavas and dykes 16 particularly in the tio2 m i diagram the first type typically shows morb like magmas whereas in the second the magma types have a spectrum of composition from mid ocean ridge basalts to island arc tholeiites and boninite like magmas often occur high ti ophiolites are petrologically and geochemically similar to major oceanic and ensialic back arc basin crusts as well as oceanic crust generated during the intermediate and late stage opening of intraoceanic back arc basins parental magmas and fractionation processes of low ti ophiolites fit with an hypothesis of their formation in the early stage of opening of intraoceanic back arc basins

tectonic evolution in the archaean and proterozoic kre high heat flow probably favoured global hotspot activity with crustal growth of iceland type through vertical magmatic accretion the tectonic environment for the generation of about 3 9 2 5 ga granite gneiss greenstone terrains with volurnlnous production of tonalite trondhjemite granodlorite magmas is still uncertain and both piate ma and intrapiate scenarios arc possible deformanon atylea in greenstones and adjacent high grade orthognersses with intercalated shallow water supracrustal assemblages suggest extensive horizontal shortening and crustal interstacking this probably resulted from collisional and or rotational motion and produced significant intracrustal melting as early as 3 9 ga by the end of the archaean and locally much earlier crustal thicknesses reached 35 50 km or more and the cxtl aordinary hhlding to erogenic deformation may have been due to delamination and crustal subduction towards the end of the proterozoic evidence for modern style plate tectonics becomes abundant through the recognition of laland a16 accretion ophiolite obducdon foreland thrust and fold belts and exotic terranes one of the best documented example for this tectonic environment is the arabian nubian shield 4lthough it is doubtful whether modern plate tectonic settings can serve as analogues for all precambrian tectonic styles it i clear that global crustal evolution since the early archaean was governed by the motion of plates which repeatedly came togethel tv fotm supercontinents and then dlspersetl again we shall not understand the detailed mechanism of precambrian crustal itlton cs before we learn horn lo irltejpret recent large scale crustal deformation and its causes

metallogeny of gold in the precambrian of northern europe fennoscandian gold deposits have been successfully explored in a wide range of precambrian geological environments in sweden and finland during the last two decades under explored areas still exist at other sites in the fennoscandian shield particularly in russia and norway and there is a high potential for future new discoveries of economic gold in northern europe careful geological documentation of a number of previously productive gold deposits such as haveri and saattopora finland boliden adelfors and enasen sweden and eidsvoll and bidjovagge norway is now available in the literature kutemajarvi pahtavaaraa and suurikuusikko finland and bjorkdal akerberg harnas and pahtohavare sweden represent a new generation of gold mines and recent research on these enables an improved base for formulating regional and local metallogenetic models younging trends from northeast to southwest characterize both the crust and ore forming regional patterns in the fennoscandian shield orogenic or mesothermal gold deposits are found in a wide range of host rocks and are closely linked to either the paleoproterozoic svecokarelian or the neoproterozoic sveconorwegian orogeny unlike these shear zone related deposits significant amounts of gold were concentrated in volcanogenic massive sulfide vms deposits by magmatic hydrothermal processes in conjunction with formation of paleoproterozoic svecofennian juvenile crust hundreds of gold deposits of a variety of ages and genetic styles are therefore now known in the precambrian of northern europe of which some are economic others subeconomic and still others only of scientific interest one hundred representative deposits in sweden norway finland and russia are discussed in this review literature references are provided for each of them and detailed maps show the location of each deposit or prospect listed data on tonnage and grades for the most recent exploration targets as well as the economically and historically most significant deposits show that they were formed during the entire precambrian yield of gold in the fennoscandian shield is so far confined to proterozoic deposits but the potential to find economically viable gold deposits in the russian parts of the archean greenstone belts should not be underestimated

metamorphic and structural evolution of the early proterozoic puolankajarvi formation finland i structural and textural relations the amphibolite facies puolankajarvi formation pjf occupies the western margin of the early proterozoic kainuu schist belt ksb of northern finland the lower and middle parts of the pjf consist of turbiditic psammites and pelites and tempestitic semipelites this report concentrates on the pelitic lithologies which include quartz two mica plagioclase schists with variable amounts of garnet staurolite andalusite and biotite porphyroblasts as well as sillimanite and cordierite segregations the ksb forms a major north south trending synclinorium between two archaean blocks it contains both autochthonous and allochthonous units and is cut by faults and shear zones the pjf lies on the western side of the ksb and is probably allochthonous the formation has undergone six major deformation phases d1 d2 d3a d3b d4 and d5 during d3a d5 the maximum principal stress 1 changed in a clockwise direction from south west to north east between d2 and d3 the intermediate principal stress 2 changed from horizontal to vertical and the interval between d2 and d3 marks a transition from thrust to strike slip tectonics relict structures in the porphyroblasts indicate the following mineral growth deformation evolution in the pjf 1 throughout the pjf there was a successive crystallization of garnet syn d1 poryphyroblastic biotite inter d3 4 and staurolite inter d3 4 during the pre d4 stage 2 a syn d4 inter d4 5 crystallization of kyanite sillimanite fibrolite porphyroblastic tourmaline magnetite rutile cordierite and muscovite biotite plagioclase pseudomorphs after staurolite was most localized at and near d4 shear zones 3 a syn to post d5 generation of andalusite ilmenohematite and sheet silicates after staurolite and after cordierite occurred near d5 faults the evolution outlined here permits the relative dating of the pjf parageneses which is used in the second part of the study tuisku laajoki 1990 and together with the knowledge of the pressure temperature conditions during various growth events makes it possible to compile pressure temperature deformation paths for the pjf

the svecofennian orogen a tectonic model abstract recent geological studies have provided new concepts concerning the timing of the final break up of the archaean craton age data on detrital zircons from sedimentary rocks in sweden and finland show marked similarities and indicate pervasive homogenization of accumulated material geochemical and isotopic studies in sweden and finland suggest the existence of two volcanic belts separated by an area dominated by sedimentary rocks however the ages of metamorphism and plutonism differ in particular the late post 1 85 ga granitoid magmatism was more voluminous in sweden than in finland the svecofennian crust in finland is anomalously thick up to 65 km and is characterized by a high velocity lower crust of variable thickness the evolution of the svecofennian orogen is modelled by opening of the pre svecofennian ocean at 1 95 ga and progressive accretion of two arc complexes to the archaean craton between 1 91 1 87 ga initial accretion at 1 91 ga led to thrusting of the slices of an island arc complex and proterozoic marine strata onto the thickening craton margin subduction reversal with simultaneous anti clockwise rotation of the converging plate or convergence of another plate resulted in subduction below the newly accreted arc complex the tectonic style at the craton margin changed from thrusting to lateral displacement in the east in finland in the west in sweden the subduction zone retreated along a transform fault a large sedimentary basin closed in the east 1 89 ga ago when another arc complex collided against the accreted one collision resulted in lithospheric thickening subsequent delamination of the thickened lithosphere and compensation by a hot mantle underplate and large scale granitoid magmatism as a result of mixing between mantle magma and remelted arc complex material the thick dense layer that formed in the lower crust as the result of underplating kept the thickened crust in isostatic balance thus the exhumation rate in this area was similar to the rate in areas of thinner crust subduction and sediment accumulation continued in the west until at least 1 87 ga when large areas in the east were already stabilized an intracratonic transpressional zone developed as the result of continued convergence subsequent extensional collapse along the transpressional zone caused melting of svecofennian sedimentary rocks migmatization and generation of the 1 84 1 82 ga anatectic granites of southern finland and central sweden the voluminous post 1 85 ga granitoid magmatism in sweden is tentatively associated with an east west extension that is not related to the extensional collapse

geochemical characteristics of granulite facies rocks in the archean varpaisjarvi area central fennoscandian shield abstract archean lower crustal granulites are exposed as fault bounded blocks in the varpaisjarvi area central finland the prevailing rock types in these blocks are igneous enderbites tonalitic trondhjemitic granodioritic migmatites and mafic granulites although metamorphosed at moderate pressures 8 10 kbar the varpaisjarvi granulites are not in a geochemical sense good representatives of depleted restitic lower crust having invariably negative or negligible eu anomalies only the enderbites have k rb ratios showing depleted granulite trends the enderbites are orthopyroxene bearing igneous rocks ranging in composition from diorite to tonalite they have distinctive flat lree patterns with la sm from 2 03 2 86 which may be due to the removal of a lree rich phase during fractionation if not the result of infiltration by a co 2 rich fluid mafic granulites occur as intercalations within the tonalitic trondhjemitic granodioritic gneisses and while all are tholeiitic in composition they fall into two groups on the basis of major and trace element abundances the group 1 tholeiites are mostly basaltic with steeply increasing alkali contents over a short range in sio 2 ti zr ratios are close to the chondritic value of 110 the group 2 tholeiites vary from picrobasaltic to andesitic in composition with the latter being dominant ti zr ratios lie below the chondritic value the group 1 rocks have higher ni and cr and lower zr y ratios than their group 2 counterparts the varpaisjarvi area represents at least two accreted blocks or terranes that exhibit marked lithological and geochemical differences with the jonsa block being the most distinctive the main differences are the two contrasting groups of tholeiitic mafic granulites the jonsa block having only the group 2 metatholeiites and the presence of quartz cordierite and cordierite orthoamphibole orthopyroxene rocks only in the jonsa block where they are found as interlayers amongst the group 2 tholeiites the ti zr and zr y ratios of these latter lithologies are closely comparable with those of the group 2 tholeiites the jonsa block cordierite orthoamphibole orthopyroxene rocks have high mgo mgo feo tot ratios compared to most cordierite orthoamphibole rocks produced by seawater basalt interaction their major and trace element geochemistry nevertheless indicates that they were derived from a mafic igneous precursor

u pb zircon ages and rb sr whole rock isotope studies of early proterozoic volcanic and plutonic rocks near tampere southern finland abstract the tampere schist belt southern finland is a well preserved volcanic sedimentary belt in the early proterozoic svecofennian terrain the metasedimentary rocks of the belt are dominated by turbiditic greywackes siltstones and mudstones the metavolcanics range from basalt to rhyolite in composition intermediate rocks being the most common near tampere the belt comprises an east west trending major syncline the u pb zircon ages of the 1 9 ga old metavolcanics cover an age interval of 15 ma the oldest age 1904 4 ma is from the intermediate unit at orivesi which is dominated by calc alkaline high k dacites and andesites this unit is the lowest in the northern limb of the syncline at orivesi at ylojarvi a sample from the lower volcanic unit has an age of 1898 4 ma while a sample from the upper volcanic unit has the youngest crystallisation age of 1889 5 ma these ages agree with the structure and stratigraphy of the belt the implications of the 1891 16 and 1889 19 ma ages of two feldspar porphyries remain obscure here the error limits are wide and it is not known with certainty whether the rocks are dykes sills or volcanics an age of 1901 28 ma was obtained for the varmala granitoid which intrudes the schists although the error is large it is possible that some of the zircon has been inherited from older rocks the rb sr whole rock ages of the volcanics agree within error limits with the u pb zircon ages the rb sr age of the hameenkyro batholith is however about 100 ma lower than the equivalent u pb age the i sr values of the metavolcanics and the 1 882 ga old hameenkyro batholith range between 0 7020 and 0 7033 these low figures suggest that their protoliths separated from upper mantle sources within a short time interval the sm nd and lu hf isotopic data available in the literature support the conclusion that the period around 1 9 ga represents an important episode of continental growth which is particularly evident in the svecofennian ketilidian trans hudsonian terrains

paleoproterozoic sediment recycling during multiphase orogenic evolution in fennoscandia the tampere and pirkanmaa belts finland abstract geochronology of detrital zircons and their overgrowths combined with whole rock geochemical and sm nd isotopic data can be used to distinguish different stages of sediment recycling and metamorphism during multiphase orogenic evolution this approach is applied to the paleoproterozoic sedimentary rocks of the tampere and pirkanmaa belts southern finland in the center of the composite svecofennian orogen the lower part of the tampere belt succession and bulk of the pirkanmaa belt are characterized by turbidites whereas the upper part of the tampere belt succession is dominated by 1 90 1 89 ga mature arc type volcanic rocks detrital zircon geochronology indicates that the tampere and pirkanmaa belts have a coeval 1 92 1 89 ga depositional and tectonic history ages of pre depositional zircon overgrowths vary from 1 91 ga to 2 0 ga with clusters at 1 92 ga and 1 98 ga within the pirkanmaa belt post depositional zircon overgrowths indicate metamorphic culmination at c 1885 ma in the vammala ni zone and at c 1875 ma in the northern part the lower conglomerates and graywackes in the tampere belt and their equivalents in the pirkanmaa belt are rich both in neoarchean and paleoproterozoic grains the latter ranging in age from 1 9 to 2 1 ga compared to these a sample from the vammala ni zone has an exotic provenance with at least c 1 90 ga 2 04 2 15 ga 2 38 2 48 ga and 2 57 2 63 ga components a sedimentary recycling and tectonic model for the central fennoscandia is proposed in which the paleoproterozoic keitele bergslagen continent was formed during an unnamed orogeny at 1 98 1 97 ga the archean norrbotten microcontinent was attached to the continent at 1 97 1 93 ga upper kaleva turbidites derived from the lapland kola orogen in the north were deposited before 1 92 ga on a passive margin of the archean karelia craton the karelia craton collided with the keitele bergslagen norrbotten continent at c 1 92 ga forming the lapland savo orogen subsequent evolution led to rifting and break up of the latter continent into two microcontinents in the hinterland at 1 92 1 91 ga the rift was developed into a subsiding passive margin of the keitele microcontinent with voluminous turbidite deposition now seen as graywackes in the tampere pirkanmaa and pohjanmaa western finland belts the turbidite material was derived from the rising lapland savo orogen and included recycled upper kaleva recycled sandstones and first cycle 2 03 1 97 ga detritus from keitele 1 93 1 92 ga savo arc material as well as detritus from the archean craton and its cover deposits the collision between karelia and keitele caused a subduction reversal and the onset of tampere arc volcanism at 1 90 ga arc derived materials started to deposit and were mixed with older sedimentary rocks and trench parallel distal turbidites from exotic source were being deposited in the accretionary wedge

the precambrian supracrustal rocks of finland and their tectono exogenic evolution abstract the precambrian supracrustal rocks of finland were produced during four main evolutionary stages 1 late archaean pre svecokarelidic greenstone schist evolution on an unknown basement 2 at least four cycles of early proterozoic karelian continental epi pericontinental sedimentation and volcanism on or next to the cratonized late archaean basement 3 early proterozoic svecofennian eugeosynclinal evolution west of or in part on the late archaean basement and its cratonic cover and 4 middle to late proterozoic jotnia continental sedimentation on the stabilized early proterozoic post svecokarelidic basement

chromites from the early proterozoic outokumpu jormua ophiolite belt a comparison with chromites from mesozoic ophiolites abstract chromites from the ultramafic rocks of the early proterozoic outokumpu jormua ophiolite belt were found to possess textural and cr cr al and mg mg fe 2 ratios that resemble those described in mesozoic ophiolite complexes chromites of cumulus texture were found at all locations along the belt whereas residual and podiform chromites were found only in the outokumpu complex the chromites in the outokumpu complex originated in a supra subduction zone environment whereas those in the jormua ophiolite complex originated in a mid ocean ridge environment the main difference between the early proterozoic ophiolitic chromites and those of mesozoic age is the relatively high zn content in the former which is a metasomatic feature related to the presence of large amounts of cu co zn au ore the data and observations presented provide evidence contributing to the concept of proterozoic plate tectonics

plumes and hotspots as sources of greenstone belts abstract greenstone belts that are older than 0 18 ga formed under circumstances that generate a buoyant garnet poor root because they do not generate enough depletion in the lithospheric mantle juvenile arcs involving average temperature early precambrian mantle were not a significant source of early precambrian greenstone belts instead early precambrian greenstone belts originated from mantle plumes and hotspots we focus on the origin of highly deformed morb like greenstone belts that are underlain by tonalitic to trondhjemitic plutons and surrounded by metamorphosed accretionary wedge like sediments these greenstone belts could form as the consequence of subducting a ridge segment within 500 km of a hotspot located beneath the continent as has happened on the taitao peninsula in chile the tonalitic to trondhjemitic plutons result from partial melting of oceanic crust thickened by extra melt derived from the hotspot the morb like rocks result from decompressive partial melting within the slab window produced by the subducting ridge the stratigraphy and tectonic setting of the recently emplaced taitao ophiolite are a possible phanerozoic analog of some morb like archean and early proterozoic greenstone belts

structure of the outokumpu ore district and ophiolite hosted cu co zn ni ag au sulfide deposits revealed from 3d modeling and 2d high resolution seismic reflection data abstract the outokumpu district within the north karelia schist belt in eastern finland hosts a number of cu co zn ni ag au sulfide deposits that are associated with palaeoproterozoic ophiolitic metaserpentinites derived from depleted mantle peridotites that were subsequently tectonically interleaved with allochthonous metaturbidites the metaperidotites have been extensively metasomatized to quartz carbonate calc silicate rocks of the outokumpu assemblage the outokumpu area has been affected by a multiple phase tectonic history comprising various phases of folding and shearing followed by several faulting events future exploration has to expand the search into deeper areas and requires knowledge of the subsurface geology in order to unravel the complex structure 3d geologic models of different scales have been built using a variety of information including geological aeromagnetic and gravity maps digital terrain models and mine cross sections as well as data like drill core logs combined with observations from underground mine galleries structural measurements aeromagnetic data and seismic surveys for crustal structures data from seismic surveys lines have been reprocessed for our purpose both deposit scale and regional scale models allow the reconstruction of a sequence of structural events the mined ore has formed during remobilization of a proto ore and is closely related to shear zones thrusts that truncate the outokumpu assemblage later faults dismembered the ore explaining the variable depth of the different ore bodies along the outokumpu ore zone on larger scale at least four km scale thrust sheets separated by major listric shear zones can be identified in the ore belt which are internally further imbricated by subordinate shear zones these thrusts separate a number of lens shape metaperidotite bodies that are probably surrounded by outokumpu assemblage rocks thrust stacking was followed by at least three stages of faulting that divided the ore belt into fault bounded blocks with heterogeneous displacements i faulting along nw dipping faults with unresolved kinematics ii reverse faulting along c 50 60 se dipping faults and iii sw ne to ssw nne striking faults which may have formed at an earlier stage and have been reactivated the specific outokumpu alteration assemblage around metaperidotite bodies combined with shear zones acting as path ways for fluids are the main vectors to mineralization seismic reflection data do not provide a simple tool to directly detect the sites of outokumpu assemblage bodies at depth but they identify strong reflector zones which are characteristic for though not exclusive to the assemblage shear zones can be recognized as curved dislocations in the seismic lines our study shows that 3d modeling when used in combination with surface geology and other geophysical data and good knowledge about the structural evolution clearly improves the interpretation of reflectors and enables the identification of strong reflector packages as outokumpu assemblage that due to absent geological control have first been mapped as unknown reflector it thus enhances the chances for locating potentially economic horizons at depth and to delineate target areas for detailed exploration

petrological and geochemical characterization of proterozoic ophiolitic melange nellore khammam schist belt se india abstract the ophiolitic melange occurs close to a major fault within the metavolcanics of kanigiri area in the western margin of the khammam nellore schist belt andhra pradesh india the unique feature of kanigiri melange is the occurrence of ophiolitic bodies of various sizes most of which display petrological and geochemical characteristics typical of supra subduction zone oceanic crust the ophiolitic melange is composed mainly of blocks of variably deformed and metamorphosed pillow basalts serpentinized ultramafic rocks meta gabbros cherts and pelagic sediments in a pervasive scaly argillite matrix the melange has a tectonic contact with a granite intrusion and numerous thrust faults cut across the melange exposures primary magmatic features such as drainage cavities in pillow basalts magmatic layering in gabbros and clinopyroxene cumulates in ultramafic rocks are well preserved in low strain domains mafic components enclosed in the melange are characterized by low pressure secondary mineral assemblages retrograded from amphibolite down to greenschist facies as commonly observed in subducted and exhumed orogens all amphiboles in the studied fragments are calcic and cluster in the range from winchite actinolite to hornblende amphibole compositions are characterized by al 2 o 3 2 4 13 8 feo 5 13 k 2 o 0 04 0 7 and x mg 0 67 0 77 the metabasalt and wherlite samples exhibit light rare earth element lree enriched patterns and negative high field strength element hfse zr and ti consistent with a suprasubduction zone geodynamic setting a notable feature of both metabasalt and wherlite samples is their pronounced negative ce anomaly and a positive eu anomaly as observed in a number of modern island arc suites as the petrological features and the geochemical characteristics of the remnant fragments we analyzed in this study are reminiscent of their phanerozoic ophiolite counterparts we suggest that these rocks were derived from a proterozoic oceanic plate belonging to a suprasubduction ophiolite assemblage subduction zone geochemical signatures and ophiolite like crustal lithologies in the study region are consistent with the accretion of juvenile oceanic fragments to the pre existing continental blocks through a forearc continent collision

origin and emplacement of ophiolites ever since their recognition ophiolites have been a source of controversy although all workers have agreed upon their importance a number of conflicting schools of thought have become unified in the interpretation of ophiolite complexes as fragments of oceanic crust and mantle formed at spreading centers an expanded version of the penrose ophiolite definition is proposed herein called the ophiolite association taking into account rocks which bear upon the origin and significance of ophiolite complexes an ideal ophiolite association includes the following units from bottom to top 1 a crystalline basement and shallow water sedimentary sequence 2 a tectonic unit of thrust slices of continental margin and rise and abyssal sediments and or melange 3 a metamorphic unit as much as a few hundred meters thick generally with higher grade rocks over lower grade ones 4 ultramafic tectonite unit composed dominantly of multiply deformed peridotite dunite and minor chromitite 5 a cumulate complex ultramafic at the base grading to mafic or intermediate at the top 6 a noncumulus unit of varitextured gabbro and minor trondhjemite 7 a sheeted dike complex 8 an extrusive section of massive and pillowed flows and intercalated sediments 9 an abyssal or bathyal sediment sequence which may include radiolarian chert red pelagic limestone metalliferous sediments breccias and or pyroclastic deposits and 10 postemplacement deposits of laterite reef limestone or shallow marine or subaerial sediments this expanded ophiolite association allows greater precision in inferring original origin and emplacement some ophiolite complexes particularly in the tethys herein designated tethyan type exhibit units 1 2 3 and 4 whereas others particularly in the cordilleran belt herein termed cordilleran type lack units 1 2 3 and in places 4 the emplacement of the former probably occurred during collision of a passive margin with a subduction zone or incipient subduction zone emplacement of the cordilleran type generally is not clear cut the nature of the spreading center which produced a given ophiolite is indicated more clearly by the presence or absence of units 5 9 than by chemistry or mineralogy some ophiolite complexes lack complete sections and may have formed along transform faults others contain volcanic pyroclastic deposits instead of pelagic sedimentary sequence and probably formed within or near an arc several ophiolite complexes may have originated by conversion of a transform fault into a subduction zone such conversion may result from normal plate evolution or from major plate reorganization major times of ophiolite emplacements appear to correspond with global plate reorganizations

preservation of biosignatures in metaglassy volcanic rocks from the jormua ophiolite complex finland evidence of microbially mediated alteration of basaltic glass is preserved in originally glassy basalts rims of pillow lavas hyaloclastite breccias and chilled margins of dykes from the well preserved 1 95ga jormua ophiolite complex joc of northeastern finland although textural evidence of microbial alteration is commonly observed in relic glass from recent oceanic crust and some ophiolites these textures have been destroyed during greenschist to lower amphibolite facies regional metamorphism and deformation of the joc however another robust biosignature is found in the generally depleted d 13 c values of disseminated carbonate extracted from originally glassy basalts relative to crystalline samples the same distribution of d 13 c values is well documented in samples from recent oceanic crust as well as ophiolites of phanerozoic age this characteristic contrastinthe d 13 cvaluesofdisseminatedcarbonateisinterpretedtoresultfrommicrobe inducedfractionationduringoxidation of organic matter x ray mapping of initial alteration zones has identified residual carbon associated with highly concentrated s that is unrelated to carbonate we attribute these biosignatures to microbially mediated alteration of originally glassy material prior to ophiolite emplacement 2004 elsevier b v all rights reserved

geology geochemistry and tectonic significance of mafic ultramafic rocks of mesoproterozoic phulad ophiolite suite of south delhi fold belt nw indian shield abstract a thick sequence of mafic ultramafic rocks occurs along a major shear zone phulad lineament running across the length of aravalli mountain range for about 300 kms it has been suggested that this sequence may represent a fragment of ophiolite or a rift related metavolcanic suite made up of basalts and fractionated ultramafics the geological and tectonic significance of the complex is assessed using field relationships petrography and geochemistry structurally the lowest part of the complex comprises a discontinuous band of plastically deformed harzburgite mantle component followed by layered cumulus gabbroic rocks crustal component a complex of non cumulus rocks comprising hornblende schists gabbros sheeted dykes and pillowed basalts structurally overlies layered gabbros huge bodies of diorite intrude volcanics geochemical classification suggests that all non cumulus mafic rocks are sub alkaline basalts except one variety of dykes which shows mildly alkaline character the sub alkaline rocks are tholeiite to calc alkaline with boninite affinity tectono magmatic variation diagrams and morb normalised patterns suggest a fore arc tectonic regime for the eruption of these rocks the mafic rocks of phulad ophiolite suite are zoned across the strike in terms of their distribution from west to east the hornblende schists and basalts are exposed at the westernmost margin followed by gabbros and dykes the alkaline dyke occurs at the easternmost part the rocks of phulad suite are juxtaposed with shallow water sediments in the east followed by platformal sediments and then continental slope sediments in the further east indicating gradual thickening of the crust from west to east and an eastward subduction the geochemical interpretation presented in this study together with discussion of lithological association is used to decipher the tectonic evolution of the mesoproterozoics of nw indian shield

a teleseismic study of the lithospheric structure across the ladoga bothnian bay zone in finland abstract in order to study teleseismic arrivals times across the ladoga bothnian bay zone a proposed suture zone that separates the karelian region 2 7 ga from the svecofennian region 1 8 ga a seismic array of 22 event recorders was deployed along the sveka profile in central finland the p travel time delays are shown to vary by less than 0 2 s along the profile and consistent delay patterns are observed for the various earthquakes the travel time delays delineate zones that appear to correlate with different geological terrains a 2 d inversion of the data yields a tomographic image of the crust and lithosphere across the ladoga bothnian bay zone where 1 velocity variations are resolved the southwestern proterozoic crust is slower 0 15 s than the northeastern archean crust in agreement with a relatively thinner about 6 km archean crust a subcrustal lower velocity 0 8 dome like structure is present beneath the kuhmo block northern part of the profile and is bordered by a southwest dipping higher velocity 0 8 body that seems to connect to the surface trace of the kuhmo suomussalmi greenstone belt

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

spatial and temporal diversity of early proterozoic volcanic sequences comparisons between the baltic and laurentian shields abstract early proterozoic volcanic supracrustal sequences in the baltic shield exhibit considerable diversity in their geochemical composition sedimentary and metallogenetic associations the earliest volcanic sequences are calc alkaline associations whose age and mode of origin is poorly understood at present tholeiitic metabasalts erupted at about 2 2 2 0 ga form a significant component of lapponian and jatulian supracrustal sequences in the northern part of the shield these lavas were erupted in marine basins and rifts within archaean cratonic lithosphere ultramafic lavas are present but appear to be spatially restricted to certain supracrustal belts marginal to possible crustal suture zones e g the karasjok belt tholeiitic metabasalts of slightly younger age 1 96 ga form part of an ophiolitic association at jormua in north central finland other kalevian suites of similar age at outokumpu and in northern finland may have formed in marginal basins adjacent to a proterozoic ocean the svecofennian domain of central sweden and southern finland locally contains primitive oceanic arc like terranes representing a juvenile addition to the proterozoic crust most of the svecofennian complex is however composed of turbiditic sediments felsic calc alkaline volcanics dated at 1 92 1 87 ga and plutonic suites which reflect sedimentary and igneous reprocessing of the juvenile arc terranes the porphyry volcanics of norrbotten were subaerially erupted between 1 9 and 1 87 ga they reflect maturation of the svecofennian arc system to an andinotype continental margin terrestrial calc alkaline volcanism continued until 1 6 ga in the transscandinavian granite porphyry belt all the volcanic components of the classic wilson cycle have now been recognised in the baltic shield the early proterozoic history of which is thus readily explicable in terms of plate tectonic concepts in the laurentian shield tholeiitic metabasalts are associated with ultramafic lavas in the cape smith belt and elsewhere along the northern margin of the superior craton the ophiolitic association has recently been described from the cape smith belt the trans hudson and svecofennian belts are complex accretional orogens with comparable magmatic and tectonic histories pre grenville reconstructions suggest that the continuation of the transscandinavian granite porphyry belt is to be found in labrador and possibly in the southern part of the concealed mid continent region in the usa the numerous similarities between the two shields suggest they underwent a parallel and possible juxtaposed evolution in early proterozoic time

short lived 1 9 ga continental margin and its destruction wopmay orogen northwest canada a 1 9 ga continental margin in the northwest of the canadian shield evolved in less than 15 m y from a zone of crustal stretching through a phase of passive margin subsidence to attempted subduction beneath an exotic microcontinent reversal of subduction polarity and generation of a major episutural magmatic arc occurred within 15 m y following microcontinental accretion terminal collision occurred 90 m y at most after initial rifting certain differences in detail between this collided margin and those of phanerozoic age may be related to subduction of newly rifted lithosphere rapid evolution of the orogen the contracted passive margin phase in particular is consistent with but does not prove more rapid recycling of oceanic plates at that time

the origin of oceanic plagiogranites from the karmoy ophiolite western norway both field relationships and geochemical characteristics indicate two suites of plagiogranitic and related rocks coexisting in the higher parts of the karmoy ophiolite of western norway the plutonic zone of this ophiolite can be subdivided into three complexes the east karmoy igneous complex the visnes high level complex and the veavagen igneous complex and plagiogranitic rocks are well developed in the first two of these within the east karmoy igneous complex plagiogranites are associated with high temperature pre basic dyke shear zones rare earth element modelling indicates that these plagiogranites were derived by anatexis of amphibolite hydrated diabase assuming a starting material consisting of 40 hornblende and 60 plagioclase and that batch melting occurred within the stability field of hornblende in comparison plagiogranite occurs in a number of bodies in the upper part of the visnes high level complex and forms a sandwich horizon together with biotite diorites and epidosites between a roof assemblage of dykes microgabbros and magnetite gabbros and a floor assemblage of layered and non layered gabbros the r e e modelling of the petrogenesis of this series of plagiogranites indicates that they were derived by filter pressing of a differentiated interstitial liquid to the vari textured gabbros although the distribution of highly hygromagmatophile elements such as k rb ba etc cannot be explained satisfactorily by this model alone depletion in these elements appears to be an autometasomatic effect

the correlation between the palaeoproterozoic orogens and granulite belts in the baltic shield and north america craton a suggested model of palaeoproterozoic plate tectonics abstract the earth s archaean history was completed with the formation of the first supercontinent pangaea 0 at ca 2 5 ga ago the main tectonic units of the younger palaeoproterozoic crust in the north atlantic realm include the collision and accretional orogens and in some cases the granulite thrust nappe belts three cycles of geodynamic activities are known in palaeoproterozoic at 2 5 2 33 ga 2 25 1 9 ga and 1 9 1 82 ga not only the starting and final events of the palaeoproterozoic megacycle with the corresponding extension and compression stages but also each cycle of evolution of the collision orogens including both pechenga imandra varzuga trans hudson cape smith new quebec and other suture belts and lapland snowbird queen maud granulite thrust nappe belts and of accretional orogens svecofennian penokean wopmay are strongly correlatable the palaeoproterozoic plate tectonic evolution appears unique in the absence of 1 general dispersal of the archaean supercontinent and 2 manifestation of subduction related magmatism in the accretional orogens along the craton margins of the north atlantic realm during 2 45 to 1 93 ga i e for about 600 ma an exception from the above generalization is the reported intrusion of calc alkaline granitoid complex cutting the lower part of the huron supergroup along the passive margin of the superior craton during 2 39 2 33 ga it is presumed that during palaeoproterozoic the oceanic crust in the oceanic hemisphere underwent rapid recycling that was accompanied by unusually slow wilson type evolution within the supercontinent that was probably nowhere disrupted in full rapid recycling of oceanic crust appears possible if the mid ocean ridges mors are presumed to lie above the sea level this might have prevented hydration of oceanic lithosphere which inhibited evolution of calc alkaline magmas this particular model of global tectonics during the palaeoproterozoic may be styled as microocean tectonics to distinguish it from the archaean microplate tectonics and the phanerozoic plate tectonics

geochemistry and tectonomagmatic affinities of the metavolcanic rocks of the early proterozoic tampere schist belt southern finland abstract the c 1 9 ga old tampere schist belt is part of the svecofennian province of finland and sweden which was formed by extensive mantle derived crustal growth during the early proterozoic the volcanic rocks of the belt range widely in chemical composition part of the variation is due to alteration and reworking but the main control are primary compositional characteristics the rocks range from almost low k tholeiitic units to shoshonitic and trachytic units intermediate rocks of calc alkaline affinities being most common by and large the volcanic rocks resemble those of present day volcanic arcs calc alkaline intermediate rocks dominate and the contents of tio2 in mafic rocks are often low some units are exceptional by exhibiting non arc affinities they are fairly high in ti and may thus mark extensional tectonic events the percentage of andesites dacites rhyolites or percent adr mostly exceeds 50 which suggests that the crust was more than 20 km thick during the eruption of most of the studied rocks the tampere schist belt thus resembles mature island arcs or volcanic arcs at or close to active continental margins rather than immature intra oceanic island arcs archaean crust is absent and it can be inferred that the crust was segregated from the mantle during the proterozoic the svecofennian province as a whole appears to differ from present day convergent plate margins by the rapidity of its crustal evolution

geochemical evidence for the hydrothermal origin of sulphur base metals and gold in proterozoic metamorphosed black shales kainuu and outokumpu areas finland metamorphosed black shale formations 120 150 m thick with median concentrations of 7 non carbonate carbon and 6 8 sulphur are abundantly met in drill cores in the kainuu and outokumpu areas carbon isotope 3c values are comparable to the average isotopic composition of non carbonate carbon in sedimentary rocks the rare earth element patterns show cerium depletion indicating a marine origin base metal concentrations in black shales are high in the talvivaara mineralization ore estimate 300 mt with 0 26 ni 0 14 cu and 0 53 zn and in the vicinity of ophiolite complexes in the kainuu schist belt the median value for gold is 35 ppb max 170 ppb in the western part and 16 ppb max 180 ppb in the east highest platinum and palladium concentrations encountered are 60 70 ppb it is shown that in the kainuu black shales concentrations of sulphur base metals and precious metals have been increased by hydrothermal processes as indicated by positive europium anomalies elevated mercury concentrations max 7 5 ppm high sulphur isotope 34s values and the enrichment of sulphur and base metals relative to carbon the processes operative during the deposition of these shales find an analogy in recent processes in ocean ridge spreading axes

detachment faulting and the evolution of passive continental margins major detachment faults play a key role in the lithospheric extension process in the basin and range province and may also be important in other continental extension terranes such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift subsidence patterns detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins we therefore suggest the existence of upper plate and lower plate passive margins these give rise to a complementary asymmetry of opposing margins after continental breakup transfer faults offset marginal features and allow margins to switch from upper plate to lower plate characteristics along strike

the cheyenne belt analysis of a proterozoic suture in southern wyoming abstract the cheyenne belt of southeastern wyoming is a major shear zone which separates archean rocks of the wyoming province to the north from 1800 1600 ma old eugeoclinal gneisses to the south miogeoclinal rocks 2500 2000 ma old unconformably overlie archean basement immediately north of the shear zone and were deposited under transgressive conditions along a rift formed continental margin intrusive tholeiitic sills and dikes are interpreted as rift related intrusions and a date of 2000 ma on a felsic differentiate of these intrusions gives the approximate age of rifting there are no known post 2000 ma felsic intrusions north of the cheyenne belt volcanogenic gneisses and abundant syntectonic calc alkaline plutons of the southern terrane are interpreted as island are volcanic and plutonic rocks the volcanics are a bimodal basalt rhyolite assemblage plutons include large gabbroic complexes and quartz diorite 1780 ma syntectonic granitoids 1730 1630 ma and post tectonic anorthosite and granite 1400 ma there is no evidence for archean crust south of the cheyenne belt structural data thrusts in the miogeoclinal rocks vertical stretching lineations and the same fold geometries north and south of the shear zone suggest that juxtaposition of the two terranes took place by thrusting of the southern terrane island arc over the northern terrane craton and miogeocline probably as a continuation of the south dipping subduction which generated calc alkaline plutons of the southern terrane a metamorphic discontinuity across the shear zone with greenschist facies rocks to the north and upper amphibolite facies rocks and migmatites to the south also suggests thrusting of the southern terrane deeper crustal levels over the northern terrane shallower levels the cheyenne belt may be a deeply eroded master decollement perhaps analogous to a ramp in the master decollement in the southern appalachians this interpretation of the cheyenne belt as a proterozoic suture zone provides an explanation for the geologic geochronologic geophysical metallogenic and metamorphic discontinuities across the shear zone

transport of nordic seas overflow water into and within the irminger sea an eddy resolving simulation and observations 1 results from a climatologically forced eddy resolving 1 12 atlantic simulation using the hybrid coordinate ocean model help clarify some presently unresolved connections between volume transports of nordic seas overflow water at key locations in the northernmost north atlantic ocean the model results demonstrate that in addition to the known westward flow through the charlie gibbs fracture zone cgfz some iceland scotland overflow water isow flows westward through gaps in the reykjanes ridge north of the cgfz into the irminger sea and some flows southward along the eastern flank of the mid atlantic ridge into the west european basin these results provide insights into the well known inconsistency between observed westward transport of isow through the cgfz 2 4 sv and the transports upstream at southeast of iceland section 3 2 sv and downstream in the western irminger sea 4 5 sv although the portion of the simulated isow that flows through cgfz is about 500 m deeper than observed the model results also show two isow pathways of this flow into the irminger sea one northward along the western flank of the reykjanes ridge and the other westward before turning north eastward on the western side of the irminger basin comparisons with the long term moored instrument database in the irminger sea show that the model based mean circulation is in reasonable agreement with observed volume transports of overflow water and that it gives approximately correct temperature and salinity characteristics

1 92ga kimberlitic rocks from kimozero nw russia their geochemistry tectonic setting and unusual field occurrence abstract the kimozero body is inferred to represent the oldest known diamond bearing kimberlite it was emplaced into a basaltic sequence as a gently dipping sheet like unit comprising several rock varieties with textures indicating a supracrustal origin minor subvolcanic bodies are represented by several narrow dykes and diatremes which formed during at least two intrusive phases the kimberlitic rocks have been exposed to extensive interactions with hydrous and co 2 rich fluids which led to extreme changes in the chemical compositions of the rocks making it difficult to identify their origin geochemical data indicate a within plate anorogenic tectonic setting of the kimozero magmatic body trace element compositions show that the rocks have a clear geochemical affinity to orangeite micaceous group ii kimberlite and olivine lamproite nd and sr isotope compositions of the kimozero rocks show that they were derived from a mantle source resembling bulk silicate earth new zircon u pb isotope data indicate emplacement age of the kimberlite at 1 92 ga suggesting that the rocks were associated with a late stage of paleoproterozoic rifting of the karelian craton related to the formation and growth of the svecofennian ocean

reactions of hydrothermal solutions with organic matter in paleoproterozoic black shales at talvivaara finland evidence from multiple sulfur isotopes abstract stable isotopic studies of archean paleoproterozoic sedimentary rocks older than 2 4 ga have documented large mass anomalous fractionations of sulfur isotopes 33 s 0 0 2 while younger rocks record little to no anomalous fractionation of sulfur isotopes this change from large anomalous fractionations to sulfur isotope values that fall on the terrestrial fractionation line has been proposed to represent the transition from an essentially anoxic archean atmosphere to an oxygenated atmosphere here we present 34 s and 33 s data for 28 core samples from paleoproterozoic 2 1 1 9 ga metamorphosed black shales and associated sulfide ores in eastern finland previous 34 s of studies of the talvivaara and outokumpu deposits of eastern finland focused on single grain analyses of sulfides while this study presents data from sequentially extracted sulfur fractions e g acid soluble sulfides chrome reducible sulfides elemental sulfur the sulfur isotope results range between 16 6 and 11 6 for 34 s 8 6 and 6 0 for 33 s and 1 25 and 0 55 for 33 s the anomalously fractionated samples are not consistent with previous findings that large anomalous fractionations of sulfur isotopes are absent in sedimentary metasedimentary rocks younger than 2 4 ga it is unlikely that talvivaara sulfides are the products of ultraviolet photolysis of volcanic so 2 in an oxygen poor atmosphere alternatively these sulfides could in part be the products of diagenetic reactions between sediments enriched in organic matter c org and hydrothermal solutions rich in sulfate i e thermochemical sulfate reduction laboratory experiments on thermochemical sulfate reduction have shown that mass anomalous fractionations of sulfur isotopes can be recorded in reduced sulfur products from reactions between simple amino acids and sulfate at 150 300 c a thermochemical sulfate reduction pathway for the mass anomalous signatures in talvivaara samples is consistent with previous genetic models proposed for the origin of the sulfur and organic carbon rich black shales now schists associated with the talvivaara and outokumpu ore deposits at talvivaara metalliferous black shales apparently were deposited under an anoxic to sulfidic marine water column the reported multiple sulfur isotopic data are key geological evidence for mass anomalous fractionations of sulfur isotopes during hydrothermal alteration of fine grained organic rich sediments approximately 20 of samples in this study show large offsets in 33 s values between different extracted sulfur fractions from the same sample which can be explained by classical isotope effects associated with the formation of hydrothermal alteration products that predominantly are retained within the precursor fine grained organic rich units

a paleoproterozoic ultramafic mafic assemblage and associated volcanic rocks of the boromo greenstone belt fractionates originating from island arc volcanic activity in the west african craton abstract the loraboue birimian ultramafic mafic assemblage located in the boromo greenstone belt burkina faso is interpreted as the remains of a magma chamber that crystallized at the base of an island arc the ultramafic rocks exhibit an heteradcumulate texture being generally made up of wehrlites and more rarely dunites the crystallization sequence inferred from the cumulates is olivine chromite followed by clinopyroxene amphibole orthopyroxene biotite the gabbroic rocks are mainly layered and grade into more differentiated facies with sub pegmatitic texture containing up to 70 modal plagioclase including zircon and more commonly apatite crystals textural relationships and mineral phase chemistry are indicative of crystallization at elevated pressures 8 kbar the parental magma being generated by a moderate to high degree of partial melting of a mantle source affected by previous metasomatic events above a subducted oceanic slab the loraboue volcanic formations exhibit a range of geochemical features they consist dominantly of calc alkaline basalts pyroclastites and rhyolite and more rarely of basalt dolerite and gabbro of tholeiitic affinity these different types of basalt as well as the dolerite and the isolated massive gabbro show the classic features of arc magmatic suites namely lile and pb enrichment depleted hfse patterns and high ce nb and th nb ratios thus the calc alkaline plutonic and volcanic assemblages of loraboue could represent the roots of an island arc and the associated coeval volcanic rocks the paleoproterozoic crust of the west african craton was heterogeneous and was not the consequence of a single process of genesis as some modern igneous province the birimian crust was generated by both volcanic arc accretion and oceanic plateau accretion

the gothara plagiogranite evidence for oceanic magmatism in a non ophiolitic association north khetri copper belt rajasthan india abstract the gothara granitoid a small pluton in the vicinity of khetri town is one of several granitoid bodies that intrude the palaeo mesoproterozoic delhi supergroup of the khetri copper belt kcb in north rajasthan india detailed petrological and geochemical investigations characterise the gothara granitoid as a plagiogranite it exhibits profuse granophyric intergrowth between quartz and chessboard albite besides having the low al2o3 extremely low k2o rb ba and high na2o that are characteristic of plagiogranites and matches some typical oceanic plagiogranites of the world in totality however unlike the other known plagiogranites it is mineralogically and chemically exceptionally homogeneous almost free from alteration and occurs in a non ophiolitic setting the field relationships of the gothara plagiogranite particularly its dual and overlapping relationship with the mafic magmatic rocks its accessory mineral assemblage i type characteristics and the major trace and ree geochemistry all point towards its fractionation related petrogenesis from a mafic magma of mantle origin in an oceanic ridge tectonic regime this discovery is of importance not only for the petrotectonic evolution of the khetri copper belt but also for the petrogenesis of plagiogranites elsewhere in the world

multiply deformed paleoproterozoic foreland fold and thrust belt in northern fennoscandia the peripheral kuusamo belt as a key example abstract the central lapland belt clb perapohja belt pb and kuusamo belt kb form an important part of northern fennoscandia and record ca 700 ma 2 5 1 8 ga of precambrian tectonic processes the 2 5 2 1 ga sedimentary volcanic successions in these belts form a basin wide surfaces with a wide lateral continuum tidal influenced shallow epeiric sea setting dominated at 2 4 2 1 ga followed by a continental breakup and subsequent formation of a passive margin at 2 1 ga the collisional evolution 1 93 1 76 ga started with an e w locally nw se and ne sw compressional phase d1 1 93 1 90 ga that resulted in at least 600 km long and in part over 300 km wide e vergent lapland foreland fold and thrust belt ftb the kb is the best preserved example of this ftb and shows e vergent imbricated structures in the east and e vergent out of sequence thrusting in the west the southern part of the kb has been affected by n s shortening d2 seen as upright to slightly inclined open to tight f2 folds and as oval shaped dome and basin structures the d3 overprinting causes folding with nw se trending spaced foliation in less competent rocks and parallel folds and faults in orthoquartzites the d4 ne sw trending overprinting deformation is characteristic for the high grade areas of the kb and is seen in outcrop to map scale tight folding in general the easternmost peripheral part of the e vergent lapland foreland ftb formed upon non stretched archean basement is only moderately affected by the later deformation events in the stretched part of the archean basement the earlier rift structures have facilitated the n s shortening and the d2 structures dominate the map pattern the d3 due to a sw ne shortening shows strong strain partitioning and localized deformation after prolonged heating the archean basement started to act ductile and long wavelength crustal scale ne sw trending d4 folds formed the metamorphic zonation in the kb from greenschist facies in the south to medium pressure amphibolite facies in the north is explained by a major d4 anticlinorium wavelength 70 100 km

accretion tectonism in the proterozoic svecokarelides of the baltic shield the duality of early middle proterozoic terranes in the baltic shield reflects fundamental differences in history between coeval mobile belts and the manner of their interaction the karelides formed on a continent ocean transcurrent margin in which offsets developed as arcs and back arc basins on stretched continental crust at least one back arc basin was obducted onto the continental margin preserved as an allochthonous terrane in the karelian nappe pile remnant arc fragments form accreted exotic terrane in pohjanmaa the svecofennides developed as a succession of island arcs while the site of subduction moved south and successive extinct arcs were accreted onto the archean craton arc terrane impingement affected deformation style in the karelides which changed from one of major horizontal translation to one dominated by major dextral wrench faults analogies can be drawn with the phanerozoic evolution of the northeast pacific margin

pre critical wide angle reflections from the baltic shield evidence for a 1 8 ga subduction complex abstract during the collaborative babel experiment in 1989 over 30 000 airgun shots were fired along reflection profiles in the gulf of bothnia and baltic sea and recorded at 62 onshore seismic stations giving densely sampled wide angle data of many forms at one such station in line and three dimensional wide angle arrivals from 14 40 km offset were recorded on a linear geophone array placed off end to marine reflection line 2 the multifold data from this station have been cdp sorted and stacked and also displayed as a single fold high resolution time distance record section with an average trace spacing of 4 m prominent wide angle reflections observed on both wide angle stacks and receiver gathers can be correlated with steep northward dipping reflectors observed in the lower crust on coincident near normal incidence reflection marine profiles high resolution ray tracing through a three dimensional model shows that the observed reflections likely occur in a reflective zone 8 km thick composed of individual reflectors 100 300 m thick the necessary impedance contrasts can be produced by alternating high and low velocities or densities these reflectors parallel the trend of the skelleftea mineral district and its associated conductivity anomaly and are here interpreted as metamorphosed remnants of subducted oceanic type crust and its overlying sediments

the josephine ophiolite northwestern california the 157 m y old josephine ophiolite is a complete ophiolite consisting of the following units harzburgite tectonite cumulate ultramafics cumulate gabbro interlayered with cumulate ultramafics cumulate gabbro high level gabbro and diorite a 1 5 km thick sheeted dike complex and 0 4 km of pillow lava and breccia the ophiolite is conformably overlain by the galice formation consisting of a thin siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental margin sources the ophiolite and overlying galice formation were intruded by numerous 150 to 151 m y old calc alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite pumpellyite to lower greenschist facies the josephine peridotite makes up the base of the ophiolite and covers 800 km2 it is an alpine type peridotite that represents upper mantle material and the residue of partial melting ultramafic rocks overlying the josephine peridotite are olivine spinel clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene olivine plagioclase cumulates and finally to clinopyroxene plagioclase cumulates high level gabbros and diorites are characterized by highly variable textures complex intrusive relationships strongly zoned plagioclase and abundant mafic dikes plagiogranites occur at this level as dikes small intrusions and screens between mafic dikes but they are rare the sheeted dike complex consists of mafic dikes averaging 0 6 m in thickness chilled margins are locally well preserved and many of the dikes have been split and intruded by younger dikes resulting in half and marginless dikes the dike complex is composed of 100 sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts the dikes are inclined at 40 to bedding after correcting for folding and are inferred to have been rotated shortly after formation the extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas some of the breccias formed as talus deposits whereas others are graded and matrix supported indicating transport and redeposition possibly in an area of rugged submarine topography the dikes and lavas have geochemical affinities to island arc tholeiites but are in some aspects transitional to mid ocean ridge basalts the dikes and lavas are cogenetic with the underlying plutonic sequence as indicated by similarity in crystallization sequence contradictory intrusive relationships between sheeted dikes and high level plutonic rocks and lack of diabase dikes in the lowermost cumulates and harzburgite the upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center resulting in downward changes in mineralogy texture and 18o in addition metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs a geothermal gradient of 250 350 c km is inferred from mineral assemblages on the basis primarily of its regional setting the ophiolite is inferred to have formed in a back arc basin in particular it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex remnant arc this interpretation is consistent with abundant volcanic arc detritus in graywackes overlying the ophiolite the island arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back arc spreading

comparing petrographic signatures of bioalteration in recent to mesoarchean pillow lavas tracing subsurface life in oceanic igneous rocks abstract bioalteration of basaltic glass in pillow lava rims and glassy volcanic breccias hyaloclastites produces several distinctive traces including conspicuous petrographic textures these biologically generated textures include granular and tubular morphologies that form during glass dissolution by microbes and subsequent precipitation of amorphous material such bioalteration textures have been described from upper in situ oceanic crust spanning the youngest to the oldest oceanic basins 0 170 ma the granular type consists of individual and or coalescing spherical bodies with diameters typically around 0 4 m these are by far the most abundant having been traced up to 550 m depths in the oceanic crust the tubular type is defined by distinct straight to irregular tubes with diameters most commonly around 1 2 m and lengths exceeding 100 m the tubes are most abundant between 50 m and 250 m into the volcanic basement we advance a model for the production of these bioalteration textures and propose criteria for testing the biogenicity and antiquity of ancient examples similar bioalteration textures have also been found in hyaloclastites and well preserved pillow lava margins of phanerozoic to proterozoic ophiolites and archean greenstone belts the latter include pillow lavas and hyaloclastites from the mesoarchean barberton greenstone belt of south africa and the east pilbara terrane of the pilbara craton western australia where conspicuous titanite mineralized tubes have been found petrographic relationships and age data confirm that these structures developed in the archean thus these biologically generated textures may provide an important tool for mapping the deep oceanic biosphere and for tracing some of the earliest biological processes on earth and perhaps other planetary surfaces

provenance of early proterozoic and archaean metasediments in finland a smnd isotopic study abstract sm nd isotopic analyses of early proterozoic and archaean clastic metasediments from finland are presented they have been used to calculate crustal residence ages and to constrain the sedimentary provenance the mean crustal residence age t dm for the svecofennian metagreywackes in the tampere schist belt is 2 22 ga which is about 0 3 ga in excess of the assumed stratigraphic age this indicates that the bulk of the sediments were eroded from newly formed crust their major source was most likely svecofennian orogenic material mixed with a small amount of archaean derived detritus the precursors of many granitoids in the svecofennian terrain also had similar short crustal residence times the kalevian metasediments which were deposited 2 05 1 9 ga ago on the edge of the archaean craton in eastern and northern finland have a mean t dm of 2 41 ga which is 0 4 0 5 ga more than the inferred stratigraphic age hence their source was on average older than that of the svecofennian metasediments investigated but definitely not purely archaean neither could crust similar to the bulk of the 1 9 ga old svecofennian terrain have been the only source various sources contributed at least locally but the bulk of the sedimentary material probably had a common provenance major sources consistent with existing isotopic data could have been 2 1 ga old jatulian rocks the c 2 0 ga old granulite belt in northern finland and or the svecokarelian early orogenic arc terrain mixed with archaean derived nd the method and data do not allow a distinction to be made between the possible alternatives the t dm model ages for two analysed archaean metasediments are slightly over 3 ga which is about 0 3 ga in excess of their assumed stratigraphic age thus providing no evidence for crust much older than 3 ga in this area

the tampere schist belt structural style within an early proterozoic volcanic arc system in southern finland abstract the central part of the narrow e w trending tampere schist belt comprises volcanic and sedimentary rocks of low metamorphic grade surrounded by intrusive rocks and migmatites the first folding event with subvertical axial surfaces and originally subhorizontal fold axes dominates the structure of the belt well preserved primary structures indicate a major e w trending syncline in the northern part of the belt with minor upright open to tight synclines and anticlines in the limb areas f 1 fold axes in the limbs have been rotated towards the steeply to vertically plunging stretching lineation dextral f 2 folds with subvertical e w to ne sw striking s 2 crenulation cleavage and steeply plunging fold axes overprint penetrative s 1 schistosity subhorizontal dextral shear is inferred from the d 2 structures d 3 kinking and fracturing is restricted to narrow nw se to n s trending zones the shear zone between the schists and the migmatites to the south is a south dipping thrust or reverse fault with uplift of the higher grade rocks in the northern part of the tampere schist belt another major fault is postulated both shear zones were formed at a late stage of progressive d 1 deformation on the basis of these structures and published sedimentological geochemical and isotopic data the tampere schist belt is interpreted as an upper part of a thrust sheet within a forearc of an ancient volcanic arc system the vergence of structures suggests subduction toward the present south

links between palaeoproterozoic palaeogeography and rise and decline of stromatolites fennoscandian shield through a review of literature and new data we have documented two major events in the palaeoproterozoic history of stromatolites as indicated by palaeontological and palaeoenvironmental studies with a time resolution of between 40 and 200 ma we confirm semikhatov and raaben s and awramik s albeit approximately maximum in diversity and abundance of stromatolites between 2330 and 2060 ma ago jatulian diversification we suggest that this taxonomic diversity was driven by a major phase of cratonisation formation of the karelian carbonate platform and numerous rift related shallow water carbonate basins supersaturated with ca 2 mg 2 and co2 the jatulian stromatolite explosion is synchronised with a positive 13ccarb shift of jatulian age carbonates we also document stromatolite decline which occurred on the fennoscandian shield somewhere between 2060 and 1900 ma ago this decline both in abundance and in taxonomic diversity is interpreted as having been caused by the first phase of oceanisation the oceanisation led to the considerable reduction in ecological niches that could be utilised by cyanobacteria the post jatulian decline of stromatolites coincides with an abrupt downward 13corg shift from 19 to 38 and is roughly coeval with the appearance of the first eukaryotic algae documented elsewhere the systematics of the fennoscandian diversity of palaeoproterozoic stromatolites is identical to that reported from india and china and reveals a dissimilarity with abundance and diversity patterns in australia and northern america

metallogeny of gold in the fennoscandian shield gold occurs in a number of different ore types in the fennoscandian shield ranging in age from late archean to late proterozoic until recently the metal was exploited primarily as a byproduct in volcanogenic massive sulphide deposits but during the 1980s more gold mines have been opened than during any other episode in the mining history of northern europe the occurrence of gold in the fennoscandian shield is reviewed in the context of the major tectonostratigraphic units 1 in the karelian province gold is hosted by greenstone belts of the archean basement complex e g at ilomantsi eastern finland greenstone belts of the nordkalott province which are interpreted as part of an early proterozoic cover sequence contain gold deposits associated with copper bidjovagge saattopora and pahtohavare gold is also associated with cobalt in the metasomatically altered early proterozoic cover in north eastern finland meurastuksenaho and juomasuo 2 in the svecofennian domain the major gold deposits were generated during the emplacement of 1 92 1 87 ga old accretional magmatism these deposits occur in the northeastern part of the svecofennian domain close to the archean proterozoic boundary they comprise two major types a the porphyry type and shear zone gold hosted by tonalite at tallberg laivakangas kopsa and osikonmaki b as a component of volcanogenic massive sulphide deposits e g holmtjarn boliden and pyhasalmi other types are c gold bearing quartz alumina alteration zones formed during the 1 92 1 87 ga magmatic period enasen d gold in massive sulphide and iron ore deposits in bergslagen 3 gold associated with 1 84 1 54 ga granites has been reported from several sites in the shield including quartz veins and contact metasomatic deposits in addition shear zone related gold deposits post dating these granites have been identified in southeastern sweden adelfors 4 in the sveconorwegian domain the gold deposits at bleka eidsvoll glava and hamas are associated with shear zones which developed penecontemporaneously with the intrusion of late 1 0 0 9 ga granites

evidence for the emplacement of ca 3 0 ga mantle derived mafic ultramafic bodies in the ukrainian shield abstract the archean bedrock of the ukrainian shield is divided into western and central blocks by a n w trending tectonic zone known as the golovanevsk suture zone within this zone there are numerous mafic ultramafic bodies including the chromitite bearing lipovenki and kapitanov massifs the chromitites of these massifs possess mineralogical and textural features and platinum group element systematics similar to those typically observed in ophiolitic podiform chromitites although there is no field evidence for an ophiolitic structure the geochemical evidence strongly suggests an origin in the oceanic lithosphere the re os ratios of the chromites and whole rock chromitites are extremely low and consequently their 187 os 188 os ratios have increased little since crystallization osmium isotope model ages indicate formation at approximately 3 0 ga suggesting that the massifs represent portions of the archean oceanic lithosphere

drift and aspect sensitivity of scattering irregularities in the upper equatorial e region recent observations at jicamarca peru of vhf radio signals coherently scattered from the upper equatorial e region during midday hours show that the corresponding irregularities in the electron density distribution are aligned along the earth s magnetic field vertical electron drift velocities in the altitude region between 120 km and 170 km show spatial and temporal variations of as much as 30 these variations are attributed to small scale varying electric fields which are generated by the dynamo action of gravity waves in the neutral atmosphere

volcanic and geochemical stratigraphy of the nussir group of arctic norway an early proterozoic greenstone suite the nussir group of west finnmark is a suite of metabasaltic lavas and tuffs about 1 8 km thick associated with continental and shallow marine sediments this assemblage is typical of karelian early proterozoic supracrustal sequences throughout northern norway and northernmost sweden primary textures are well preserved and permit the establishment of a volcanic stratigraphy for the nussir group the krokvatn formation is dominated by mafic aquagene tuffs while the overlying svartfjell formation is composed mainly of submarine pillow basalt petrographic evidence for the petrogenesis of the metabasalts is poor because of greenschist facies metamorphism during the svecokarelian orogeny c 1840 ma the earliest lavas have the most primitive compositions but subsequent flows exhibit strong enrichment in fe ti and incompatible elements as a consequence of extensive crystal fractionation the tholeiitic nature of the suite is confirmed by the trace element data the later lavas are more uniform and less evolved and are considered to represent steady state compositions produced by a periodically replenished tapped open system magma chamber the field relationships lithological and geochemical evidence suggest that the nussir group lavas were erupted in a submarine rift basin within continental crust they form part of an extensive tholeiitic magmatic province which occupied much of northern norway sweden and finland in the early proterozoic the tectonic environment of this province is poorly constrained by the geochemical data continental plateau basalt provinces such as the columbia river and karoo provide the closest phanerozoic geochemical analogues

metamorphic and structural evolution of the early proterozoic puolankajrvi formation finland ii the pressure temperature deformation composition path the paragenetic relationships between sillimanite andalusite kyanite chlorite cordierite biotite garnet and staurolite in the early proterozoic puolankajarvi formation pjf together with mineral compositions are used to construct a partial petrogenetic grid for metapelites with significant mn content mno 0 1 0 5 by adding a six phase invariant point over the garnet absent invariant point for mn free amf phases the grid and textural relations of the pjf are used to construct part of the p t deformation path for the pjf relatively short deformation pulses and associated flow of oxidizing fluid along shear zones were responsible for the paragenetic and compositional changes during cooling and decompression at 600 500 c and 6 0 2 5 kbar oxidation led to decreased fe2 and further stressed the importance of mn increased mn divalent cations a tectonothermal evolution of the kainuu schist belt is presented which includes crustal thinning and steepening of a previously established thermal gradient this was followed by thrusting and folding of the isotherms into a thermal antiform on the western side of the belt

crustal conductors along the sveka profile in the fennoscandian baltic shield finland summary electrical properties of the crust in the central fennoscandian baltic shield are examined along the 435 km magnetotelluric profile sveka a 2 d geoelectric model shows large variations in the crustal resistivity with the model resistivities ranging from 0 1 to 35 000 m the main geoelectrical features of the crust and upper mantle are 1 a resistive archaean lower crust conductance 30 s 2 a conductive palaeoproterozoic lower crust beneath the central finland granitoid complex 170 900s free h2o rich fluids trapped into the lower crust seem less plausible causes for enhanced conductivity as long retention times are required and no possible sources seem to be available to produce fluids into the dry lower crust most likely causes for permanent enhancement of lower crustal conductivity include the conductive fill of ductile shear zones conductive graphite bearing lamellae conduction in mafic rocks perhaps via conduction in magnetite or conductive solid remnants of an ancient shear zone between two crustal blocks thin graphitic grain boundary films precipitated from co2 rich fluids also seem less plausible 3 a very resistive 10 000 35 000 m upper crust of the palaeoproterozoic central finland granitoid complex and the archaean domain 4 narrow and elongated conductors 0 1 100 m beneath the palaeoproterozoic kainuu ksb and tampere tsb schist belts are caused by graphite bearing metasedimentary rocks buried deep into the crust in the collisions of crustal terranes during the formation of the palaeoproterozoic svecofennian crust the 500 km long tsb conductor represents an internal svecofennian terrane boundary whereas the ksb conductor is a part of a discontinuous archaean proterozoic boundary conductor 5 resistive upper mantle there are no indications of an upper mantle conducting layer within the uppermost 300 km in the central part of the fennoscandian shield

characterization of sulfide deposits by airborne magnetic and gamma ray responses in eastern finland abstract integrated interpretation of aerogeophysical data permits geophysical characterization of sulfide deposits in eastern finland the structural control and the hydrothermal alteration of the rocks can be discerned from airborne magnetic and radiometric data the cu co zn deposits associated with serpentinite dolomite calc silicate rock siliceous rock black shale settings of the outokumpu type are locally characterized by reduced intensity of magnetization and electrical conductivity and reduced eth and k radiation enhanced eu eth anomalies are commonly spatially related to major structural lineaments such as the boundaries of structurally more coherent bedrock blocks or intersections of faults and shear zones electromagnetic surveys can be used to distinguish black shales and serpentinites in the region the sulfide deposits in eastern finland can be divided into two groups on the basis of airborne geophysical characteristics 1 intense magnetization and high electrical conductivity and 2 moderate magnetization and electrical conductivity in the sulfide deposits of group 1 ore forming constituents mainly precipitated during primary sedimentation for example the black shale hosted ni cu zn ore at talvivaara group 2 is related to sulfide deposits where major ore forming processes have affected the associated black shale formation after the sedimentation phase outokumpu type cu co zn deposits in cases where the magnetization and electrical conductivity of black shales are especially weak ore formation processes probably continued during late metamorphic or post metamorphic deformation the theoretical basis for the observed differences in magnetic response between different deposits has to do with the mineralogy of the black shales pyrite changes into ferrimagnetic monoclinic pyrrhotite during metamorphism above greenschist facies especially along tectonic structures if no further alteration occurs pyrrhotite bearing black shale formations appear as highly magnetic and electrically conductive zones on geophysical maps wherever intense tectonic deformation emplacement of quartz sulfide veins hydrothermal alteration or contact metamorphism has affected the black shale formation after sedimentation both magnetic and electromagnetic responses may be weakened because of diminished amounts of the pyrrhotite bearing and graphite rich material in addition the presence of antiferromagnetic pyrrhotite of hexagonal crystal structure may decrease the magnetic response the classification presented here applies specifically to eastern finland but may be widely applicable to terranes with abundant black shale formations that have undergone regional metamorphism of upper greenschist facies or higher association of sulfide ores with black shales is a common feature in the geological record our integrated geophysical method offers a quick way of examining large areas and it should prove useful in the search for sulfide deposits in other parts of the world

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

ni cu pge deposits in finland geology and exploration potential abstract several types of nickel sulphide deposits occur in finland some of them are also important sources of cu co and pge the main deposit types are related to archaean and palaeoproterozoic mafic to ultramafic rocks palaeoproterozoic metamorphosed black shales of the talvivaara type or the ultramafic and metasomatic calc silicate rocks of the ophiolitic outokumpu association several komatiite hosted ni cu pge deposits have been discovered in the archaean and paleoproterozoic greenstone belts in northern and eastern finland but only one has been briefly mined in 1989 many 2 44 ga palaeoproterozoic mafic ultramafic layered intrusions in finland host contact type pge ni cu deposits and significant reef type pge deposits typically the ni resources are of low grade but large tonnage the mafic ultramafic magmatism at ca 2 06 ga produced significant ni cu pge resources in the central lapland greenstone belt including the kevitsa and sakatti deposits the svecofennian domain in central and southern finland contains hundreds of small ca 1 88 ga mafic ultramafic orogenic intrusions many of these host nickel mineralization nickel exploration potential was evaluated in general based on identification of critical factors within a mineral system framework and a quantitative assessment of the potential occurrence of undiscovered nickel resources the mgo content of the parental magma was estimated for 28 deposits 6 komatiite hosted occurrences and 22 svecofennian orogenic intrusions the estimated mgo content of the parental magmas for the selected deposits varies from around 7 to 15 in basaltic to komatiitic basalt systems and up to 30 in more magnesian komatiitic systems the ni cu value varies mostly between 0 7 and 12 correlating in general positively with the mgo content of the magma exceptionally high or low ni cu values in some deposits may result from for example copper loss during post magmatic processes serpentinization and or metamorphism or part of the sulphide ore zoned by the ni cu value still being unknown recent advances in understanding of komatiites in finland indicates high exploration potential for nickel within the paleoproterozoic komatiitic formations which typically are also significantly enriched in cu and pge however the quantitative assessment of undiscovered nickel resources in finland suggests that the contact type ni cu pge deposits hosted by 2 44 ga mafic ultramafic layered intrusions in northern finland have the greatest potential for hosting undiscovered resources among the assessed nickel deposit types these occur within restricted areas resulting in a high deposit density finally considerable nickel resources might exist in undiscovered talvivaara kevitsa or sakatti type deposits these deposit types should be assessed as soon as more geological information becomes available