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

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

hydrodynamic influences on acoustical and optical backscatter in a fringing reef environment observations of hydrodynamics along with optical and acoustical water characteristics in a tropical fringing reef environment reveal a distinct signature associated with flow characteristics and tidal conditions flow conditions are dominated by tidal forcing with an offshore component from the reef flat during ebb measurements span variable wave conditions enabling identification of wave effects on optical and acoustical water properties high frequency acoustic backscatter 6 mhz is strongly correlated with tidal forcing increasing with offshore directed flow and modulated by wave height indicating dominant hydrodynamic influence backscatter at 300 and 1200 khz is predominantly diurnal suggesting a biological component optical backscatter is closely correlated with high frequency acoustic backscatter across the range of study conditions acoustic backscatter frequency dependence is used along with changes in optical properties to interpret particle size variations changes across wave heights suggest shifts in particle size distributions with increases in relative concentrations of smaller particles for larger wave conditions establishing a connection between the physical processes of a fringing tropical reef and the resulting acoustical and optical signals allows for interpretation and forecasting of the remote sensing response of these phenomena over larger scales

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

the use of age frequency distributions of micromammals in the determination of attritional and catastrophic mortality of fossil assemblages abstract many authors have used age frequency profiles of fossil populations of large mammals to determine whether an assemblage is the result of catastrophic or attritional mortality the smallest mammals previously used had calculated life spans of seven years or more the use of small mammals rabbit sized or smaller with much shorter life spans has never previously been done based on data from known fossil catastrophic assemblages and modern attritional assemblages of micromammals it is evident that age frequency profiles from these mammals are nearly identical to those of larger mammals and can be used in the same manner several fossil examples of both catastrophic and attritional mortality are cited one example of a micromammal fauna can best be interpreted as a combination of catastrophic and attritional mortality it is suggested that such a profile would not be recognizable in larger animals from the same horizon

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

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

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 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 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

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

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

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

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

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

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

the 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

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

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

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

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

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

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

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

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

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

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

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