geochemistry u pb geochronology and sm nd data from the paleoproterozoic western finland supersuite a key component in the coupled bothnian oroclines abstract the western finland wf supersuite characterized by supracrustal rocks and intruding plutonic rocks forms the core component of the coupled bothnian oroclines in the central part of the svecofennian orogen we present here geochemical isotope data detrital zircon age data on sedimentary rocks and age data on granitoid plutons within the wf supersuite in the pirkanmaa pb and pohjanmaa pob belt representing the southernmost limb of the southern and the hinge zone of the northern bothnian orocline respectively the tampere belt tb in the north and hame belt hb in the south bound the pb basaltic to picritic volcanic rocks in the wf supersuite with a proposed eruption age at ca 1 91 ga 1 92 1 90 ga show locally pillow structures and are typically associated with black shales cherts carbonate bearing rocks based on geochemistry and sm nd data a variable morb wpb type origin is evident and an evolution scenario from earlier wpb type rifting to more morb affinity ocean sea opening is proposed detrital zircon age distributions from sedimentary rocks with 1 91 ga maximum depositional age in the tb pb and hb have been compared and typically the age distributions of

geochemistry geochronology and origin of the neoarchean planalto granite suite carajas amazonian craton a type or hydrated charnockitic granites abstract new whole rock geochemistry and la mc icpms and pb evaporation geochronological data were obtained on zircon from the neoarchean planalto suite granites and associated charnockitic rocks of the canaa area of the carajas province eastern amazonian craton brazil the pb evaporation ages of three samples from the planalto suite are around 2730 ma 2733 2 ma 2731 1 ma and 2736 4 ma whereas u pb la mc icpms concordia ages obtained for these samples are 2729 17 ma 2710 10 ma and 2706 5 ma respectively an orthopyroxene quartz gabbro associated with the pium complex and planalto suite yielded a u pb concordia age of 2735 5 ma interpreted as its crystallization age the planalto suite granites and the charnockitic rocks associated with the mesoarchean pium complex were probably crystallized at 2730 10 ma the planalto granites have ferroan character and are similar geochemically to reduced a type granites in previous studies they have been classified as such despite the fact that they are syntectonic the tectonic setting and the association between the planalto suite and charnockitic series led us to classify these biotite hornblende granites as hydrated granites of the charnockitic series the planalto suite and the neoarchean charnockitic magmas were more probably derived by partial melting of mafic to intermediate tholeiitic orthopyroxene bearing rocks similar to those of the pium complex at 2 76 ga upwelling of asthenospheric mantle in an extensional setting propitiated the formation of the carajas basin later on at ca 2 73 ga heat input associated with underplate of mafic magma induced partial melting of mafic to intermediate lower crustal rocks originating the planalto and charnockitic magmas the emplacement of these magmas occurred under active regional stress and resultant major shear zones found in the canaa dos carajas area the close association between the planalto suite and charnockitic rocks suggests that they are similar to the high temperature granite magmatism found near the borders of precambrian domains with different ages and tectonic evolution or in their zone of interaction

delamination of eclogitized lower crust control on the crust mantle boundary in the central fennoscandian shield abstract the crustal structure of the central part of the fennoscandian shield is dominated by a high velocity lower crust hvlc vp 7 3 7 5 km s at the depth of 40 63 km in an area of ca 400 km 600 km formed in the palaeoproterozoic svecofennian orogen at 1 91 1 87 ga in the area of the hvlc the moho is deeper than 50 km and attains depths of up to 63 km in eastern finland at the archaean proterozoic boundary zone we suggest that the present moho dates back to late svecofennian time and was determined by delamination of high density eclogitized lower crust after tectonic collisional thickening of crust densities deduced from seismic and gravity data suggest a very small or completely lacking density contrast between lower crust and uppermost mantle new geological evidence from kimberlite hosted garnet xenocrysts supports the presence of a garnet population with compositions in agreement with crustal eclogitic garnets in addition small clinopyroxene garnet mineral aggregates microxenoliths show omphacitic clinopyroxene compositions therefore the presence of eclogites in the high velocity lower crust is implied although the eclogite proportion as estimated from seismic velocity data is only a few percent in the present hvlc with results on near incidence and wide angle seismic studies and geological and geochemical data on heat producing elements we modeled the svecofennian collision and the thermal evolution of the crust to provide temperature constraints for the delamination model the constructed thermal model simulates the genesis of svecofennian synkinematic at 1 89 1 88 ga and postkinematic 1 88 1 87 ga granitoids as well as the general metamorphic grade of the surface rocks and lower crustal xenoliths moreover the narrow distribution of the ages of the svecofennian mafic ultramafic magmatism ca 1 89 1 87 ga dispersed around the hvlc area is explained by the homogeneous warming of the lithosphere the conditions of lower crustal delamination were estimated with a rayleigh taylor convective instability model constrained with lower crustal and upper mantle rheological data delamination of a lower crustal eclogite layer 5 30 km thick takes place in about 7 50 ma if the rock dynamic viscosity is 1 1021 pa s it requires temperatures of 1000 c in the lower crust and upper mantle in the svecofennian crust such temperatures were attained at about 1860 1850 ma i e after the orogenic peak we conclude that the moho of the hvlc area was controlled by delamination of a ca 5 10 km thick layer of high density lowermost crust into the mantle such a density controlled process may have controlled the moho depth also in other proterozoic shield areas

1 8 ga svecofennian post collisional shoshonitic magmatism in the fennoscandian shield abstract at least 14 small 1 11 km across 1 8 ga svecofennian post collisional bimodal intrusions occur in southern finland and russian karelia in a 600 km long belt from the aland islands to the nw lake ladoga region the rocks range from ultramafic calc alkaline apatite rich potassium lamprophyres to peraluminous hibasr granites and form a shoshonitic series with k 2 o na 2 o 5 k 2 o na 2 o 0 5 al 2 o 3 9 over a wide spectrum of sio 2 32 78 although strongly enriched in all rocks the lile ba and sr and the lree generally define a decreasing trend with increasing sio 2 depletion is noted for hfse ti nb and ta available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded initial magmas mg 65 in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post collisional uplift some 30 ma after the regional svecofennian metamorphic peak however prior to the melting episode the lithospheric mantle was affected by carbonatite metasomatism more extensively in the east than in the west the melts generated in the more carbonate rich mantle are extremely enriched in p 2 o 5 4 f 12 000 ppm lile ba 9000 ppm sr 7000 ppm lree la 600 ppm and ce 1000 ppm the parental magma underwent 55 60 fractionation of biotite clinopyroxene apatite magnetite sphene whereupon intermediate varieties were produced after further fractionation 60 80 of k feldspar amphibole plagioclase minor magnetite sphene and apatite leucosyenites and quartz monzonites were formed in the west where the source was less affected by carbonatite metasomatism calc alkaline lamprophyres vogesites minettes and spessartites and equivalent plutonic rocks monzonites were formed removal of about 50 of biotite amphibole plagioclase magnetite apatite and sphene produced peraluminous hibasr granites the impact of crustal assimilation is considered to be low at about 1 8 ga the post collisional shoshonitic magmatism brought juvenile material particularly enriched in alkalis lile lree and f into the crust although areally restricted the regional distribution of the post collisional intrusions may indicate that larger volumes of 1 8 ga juvenile material resides in unexposed parts of the crust

aspects of the tectonic evolution of china the subject of this special publication is one of the most interesting in global geoscience the tectonic evolution of china the assemblage of terranes that underlie this part of the world provides outstanding opportunities to elucidate global processes and many of the factors that shape the earth s lithosphere are best exemplified by the geology of china and its immediately adjacent areas in addition there are geological features that are particular and unique to the region some have been the focus of recent attention and have attracted international interest because of their global importance this volume provides accounts of up to date research by chinese and international geological teams on key aspects of the tectonic evolution of china and its surrounding areas the papers describe the formation of the geological terranes that make up this part of east asia place constraints on plate tectonic models for their assembly and provide accounts of unique geological feature of the subcontinent

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

tectonic setting of post collisional magmatism in the palaeoproterozoic svecofennian orogen sw finland abstract five bimodal post collisional intrusions in southwestern finland have been investigated geochemically the mafic rocks are shoshonitic monzodiorites which are highly enriched in fe p ti f lree and in incompatible trace elements the felsic rocks are garnet bearing peraluminous s type anatectic granites new data on the mafic and the felsic intrusions yielded the same u pb zircon age of 1815 ma therefore the mafic and felsic intrusions are coeval but not cogenetic narrow contact metamorphic aureoles around the mafic intrusions contain garnet orthopyroxene bearing assemblages and thermobarometry indicates an intrusion depth of at least 15 km hence there was little or no unroofing after peak regional metamorphism at 4 6 kbar the geochemical characteristics of the mafic rocks suggest that they were derived from subcontinental lithospheric mantle that was previously enriched by fluids released during svecofennian subduction it is suggested here that hot upwelling asthenosphere convectively removed subcontinental lithospheric mantle and triggered partial melting of the enriched parts of the mantle uprising mafic melts increased the already high temperatures at mid crustal levels and caused granulite facies metamorphism crustal anatexis and production of granitic melts

palaeoproterozoic accretionary processes in fennoscandia abstract accretionary processes contributed to major continental growth in fennoscandia during the palaeoproterozoic mainly from 2 1 to 1 8 ga the composite svecofennian orogen covers c 1 10 6 km 2 and comprises the lapland savo fennia svecobaltic and nordic orogens it is a collage of 2 1 2 0 ga microcontinents and 2 02 1 82 ga island arcs attached to the archaean karelian craton between 1 92 and 1 79 ga andean type vertical magmatic additions especially at c 1 89 and c 1 8 ga were also important in the continental growth the palaeoproterozoic crust is the end product of accretionary growth continental collision and orogenic collapse preserved accretional sections are found in areas where docking of rigid blocks has prevented further shortening the pirkanmaa belt represents a composite accretionary prism and other preserved palaeosubduction zones are identified in the gulf of bothnia and the baltic sea areas in the southern segment of the lapland savo orogen collision between the archaean continent lower plate and the palaeoproterozoic arc microcontinent assembly upper plate produced a special type of lateral crustal growth the archaean continental edge decoupled from its mantle during initial collision and overrode the arc and its mantle during continued collision

the vaasa migmatitic complex svecofennian orogen finland buildup of a lp ht dome during nuna assembly tectonic evolution of the paleoproterozoic vaasa migmatitic complex vmc in the central part of the svecofennian accretionary orogen is deciphered using aeromagnetic and gravity maps deep seismic

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

1 88 1 87 ga post kinematic intrusions of the central finland granitoid complex a shift from c type to a type magmatism during lithospheric convergence abstract the evolution of the svecofennian orogen spans from 1 95 ga to 1 80 ga with the main crust forming activity having taken place 1 90 1 87 ga ago subsequent deformation was largely concentrated to southern finland where transpressional structures indicate lithospheric convergence until 1 80 ga much of the svecofennian orogen in finland is occupied by the central finland granitoid complex cfgc that mainly consists of i type granodiorite and granite these 1 89 1 88 ga intrusive rocks are typically foliated and considered synkinematic a distinct suite of post kinematic undeformed or slightly foliated plutons crosscut the synkinematic rocks they are predominantly quartz monzonites and monzogranites and have been divided into three types according to their petrographic mineral and chemical characteristics compared to the calc alkaline synkinematic granitoids the post kinematic suite generally has an alkaline affinity with higher fe ti k ba zr and nb and lower mg ca and sr at a given sio 2 content the type 1 post kinematic plutons are peraluminous and contained a prominent sedimentary component in their source the types 2 and 3 plutons are marginally metaluminous to peraluminous the type 2 granites especially those in the western cfgc approach the classic 1 65 1 54 ga rapakivi granites of southern finland in their petrographic characteristics magmatic association tholeiitic mafic rocks and elevated contents of incompatible elements and hence have a type characteristics the type 3 plutons contain a pyroxene bearing margin type 3a or contain pyroxene throughout type 3b the type 3b rocks have c type charnockite magma type affinities and some plutons of this subgroup are alkaline the type 3a plutons appear to be transitional between the type 2 and type 3b plutons the synkinematic magmatic episode in the cfgc area probably involved partial melting of intermediate high k rocks in the lower crust with a magmatic addition from a mafic underplate this magmatic episode left a hornblende and biotite poor granulitic residue in the lower crust heat from the mafic underplate triggered partial melting of the granulite to produce the post kinematic magmatism and also introduced mafic material into the post kinematic magmas the post kinematic magmatism took place over quite a short time period ca 15 ma during overall lithospheric convergence it extended beyond the cfgc area and registers a temporal shift from the northeast toward the west and a general change in the character of magmatism from c type to a type the post kinematic magmatism probably resulted from extensional or transtensional events modifying the tectonically thickened crust partial melting of the mafic lower crust produced c type magmatism soon after the first melting episode the crust was already rather rigid and allowed the mafic magmas to follow a tholeiitic trend when the bimodal a type magmas were formed in the western cfgc

petrography geochemistry and geochronology of the sc enriched kiviniemi ferrodiorite intrusion eastern finland the kiviniemi mafic intrusion near the eastern margin of the paleoproterozoic central finland granitoid complex is both spatially and temporally associated with post kinematic fe ti p enriched svecofennian orogenic mafic magmatism the main rock types in this small 15 ha intrusion are garnet bearing fayalite ferrodiorite leucoferrodiorite ferromonzodiorite and pyroxene diorite the garnet bearing fayalite ferrodiorite and leucoferrodiorite contain 50 281 ppm sc 275 5600 ppm zr and 58 189 ppm y n 42 delineating a mineralized deposit some 2 5 ha in extent overall these rocks show an evolved iron enriched tholeiitic character low values of ni 20 40 ppm cr 20 ppm and cu 20 80 ppm and high contents of zn 213 700 ppm the rock forming minerals in the ferrodioritic rocks are ferro hedenbergite plagioclase an40 ferropargasite and ferroedenite almandine garnet and fayalite fo1 4 accessory minerals include zircon ilmenite fluorapatite biotite pyrite pyrrhotite potassium feldspar grunerite and clinoferrosilite some relict cumulate textures have been preserved but primary magmatic features have largely been overprinted by strong recrystallization and corona formation the main carriers of sc are amphibole clinopyroxene and apatite the remarkably strong enrichment of sc in ferromagnesian silicates and apatite rather than in specific sc minerals implies magmatic enrichment post kinematic mafic intrusions in central finland constitute a bimodal association with co existing granitoid counterparts the kiviniemi mafic intrusion is associated with a coarse megacrystic granite and the two rock type display mingled contacts indicative of contemporaneity of the two magmas this conclusion is in accord with the coincident u pb zircon ages for the ferrodiorite at 1857 2 ma multigrain id tims and the megacrystic granite at 1860 7 ma single crystal la mc icp ms the initial end value of the ferrodiorite and the granite are 0 1 and 2 5 respectively these nd isotope compositions probably reflect a chondritic mantle source for the ferrodiorite and suggest incorporation of some archaean crustal material into the granite in the course of magmatic evolution the resource estimation calculated for kiviniemi intrusion by using 40 g t sc cut off value is 13 4 mt of rock with an average grade of 162 7 g t scandium 1726 g t zirconium and 81 g t yttrium

grenville age a type and related magmatism in southern laurentia texas and new mexico u s a abstract large grenville age plutons are known to be present in the 1340 1370 ma basement in texas and eastern new mexico but their distribution and compositional ranges are poorly known new u pb zircon dates shrimp rg stanford university on subsurface samples have identified widespread presence of compositionally diverse grenville age 1070 1110 ma plutonic rocks in the texas panhandle core from a 175 m thick sill like gabbroic intrusion yielded a date of 1081 8 3 ma in situ differentiation of this tholeiitic magma led to a 7 fold increase in incompatible element concentrations as ree patterns remained essentially flat trace element data suggest an e morb like source to the south alkali feldspar granite with a type affinities was recovered from drill core from the abilene gravity minimum near albany texas it yielded a date of 1078 23 ma similar to undeformed post orogenic granites in the llano uplift further west an anorthosite xenolith from the eocene three sisters intrusion in el paso texas yielded a date of 1068 30 ma which is within error of the 1110 19 ma age determined for the main stage of the nearby a type red bluff granitic suite xenoliths from potrillo maar volcano in the center of the southern rio grande rift include monzonitic xenoliths with granulitic texture which yielded dates of 1072 ma in contrast potrillo maar xenoliths with igneous textures yielded u pb dates of 27 ma development of the granulitic texture is interpreted to reflect granulite facies metamorphism during tertiary time 26 27 ma on the basis of age zoning and u concentrations in the zircon crystals our results show that grenville age magmatism in the texas and new mexico subsurface was widespread and was coeval with syn and post deformation granites in the llano uplift of central texas the compositions of dated samples suggest that mafic magmas were broadly tholeiitic in character and that granitic magmas were a type these magmatic affinities are thought to be associated with crustal extension rather than with subduction moreover the presence of anorthosite xenoliths in the el paso kilbourne hole region the broadly tholeiitic character of mafic rocks and the a type nature of the granitic rocks are broadly similar to amcg anorthosite mangerite charnockite granite suites present in the grenville province of eastern laurentia

petrogenesis of the post kinematic magmatism of the central finland granitoid complex i radiogenic isotope constraints and implications for crustal evolution post kinematic granitoids of the central finland granitoid complex introduction cfgc were emplaced into the palaeoproterozoic crust of central in many precambrian shield areas extensive palaeofinland shortly after the 1 89 ga peak of the svecofennian proterozoic granitoid magmatism marks the mature stage orogeny they are found as discordant generally non foliated plutons of crustal accretion that led to rapid growth of juvenile with aand c type granite characteristics and a bimodal mafic terranes around pre existing archaean cratons patchett felsic magmatic association conventional and ion microprobe et al 1981 van schmus bickford 1981 patchett u pb data show that the post kinematic magmatism commenced bridgwater 1984 huhma 1986 sadowski bettenin the northeastern cfgc at 1 885 ga and gradually moved court 1996 on a global scale these processes led to the to the west where the plutons are dated at 1 870 ga radiogenic establishment of the palaeoproterozoic supercontinent isotope nd sr pb composition of the post kinematic plutons nena northern europe north america gower et al indicates homogeneous initial ratios nd at 1 875 ga 0 sr 1990 rogers 1996 the scattered remnants of which sr 0 703 and stacey kramers type pb with low longcurrently constitute a substantial fraction of the conterm th u 2 these help to constrain the protolith history of tinental crust of the northern hemisphere the post kinematic magmatism nd and pb isotopic data for in the finnish part of the fennoscandian or baltic palaeoproterozoic granitoids pb isotopic data for associated sulphshield fig 1 the evolution of the 1 9 ga svecoides and nd and pb isotopic composition of the classic 1 6 ga fennian orogen involved several magmatic events e g finnish rapakivi granites reveal several distinct crustal domains in korsman et al 1999 emplacement of early orogenic the finnish svecofennian orogen and indicate that the central part 1 93 1 91 ga tonalites was followed by predominant of the svecofennian orogen consists of several terranes microsynorogenic 1 90 1 87 ga magmatism characterized by continents that were amalgamated in palaeoproterozoic times the i type tonalite granodiorite and granite and arc type cfgc region is the oldest 2 0 ga among these terranes volcanism at 1 84 1 81 ga late orogenic mainly stype granites were emplaced as a broad belt across southernmost finland these and associated mafic rocks have been ascribed to crustal shortening vaisanen

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 bothnian coupled oroclines of the svecofennian orogen a palaeoproterozoic terrane wreck the accretion of magmatic arcs gives rise to elongate linear orogens and is a key process in forming new continental crust many precambrian continents are however presently equidimensional or have large areas without any clear linearity such as the central part of the palaeoproterozoic svecofennian orogen 1 92 1 77 ga one way of forming an equidimensional continental domain is by buckling of a linear orogen about vertical axes of rotation into one or more coupled oroclines here we reinterpret existing data and demonstrate the occurrence of coupled bothnian oroclines in the svecofennian orogen palinspastic restoration of the southern and northern bothnian oroclines brings a 1000 km long segment of the svecofennian orogen into an originally linear nw striking geometry that restores the lithological belts metamorphic zones and structural vergences to a common direction and which indicates that the orogen consists of a sw facing arc which has been shortened along ne verging folds and thrust faults

highly varying radiogenic heat production in finland fennoscandian shield abstract radiogenic heat production in finland has been previously studied using airborne gamma ray surveys and glacial till measurements alike for the first time this paper presents a detailed survey on the spatial variation in radiogenic heat production determined using outcrop samples obtained from all important lithologies of the country the dataset of 6465 samples represents mostly mesoarchean about 2 7 ga paleoproterozoic ca 2 2 1 8 ga and mesoproterozoic ca 1 6 1 3 ga rocks nearly all data are from precambrian fennoscandian shield area but heat production appears to be highly variable and above global archean and proterozoic averages spot readings show an arithmetic average of 1 34 1 19 wm 3 and a range from 0 02 to 19 4 wm 3 the interpolated areal average of the whole area is 1 42 1 41 wm 3 the high standard deviation of data is related to the geochemical characteristics of uranium u thorium th and potassium k resulting in a skewed distribution of heat production mesoproterozoic anorogenic rapakivi granites and late paleoproterozoic svecofennian granitoids show the highest heat production values in the range of 3 5 wm 3 the results show no distinct dependencies of heat production with geological age metamorphic grade nor seismic p wave velocity but an increasing trend of heat production with sio2 content and decreasing trends of heat production with fe2o3 content and with rock density are evident surface heat flow 44 borehole data values correlates weakly with heat production r 0 35 the general heterogeneity of heat production calls for supporting information from other geophysical methods for better understanding of the thermal state of the lithosphere in finland and beyond

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

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

isotopic and geochemical constraints on the evolution of the 1 93 1 79 ga svecofennian crust and mantle in finland the central part of the svecofennian domain in finland is characterized by thick lithosphere with crustal thickness up to 60 65 km major collisional events at 1 91 1 90 ga and 1 89 ga and a thrusting event at 1 86 1 84 ga the latter due either to continuation of shortening or to a separate intracrustal collision are considered the major causes of crustal thickening the origin of the svecofennian domain has been formerly attributed to mixing of depleted mantle melts with variable amounts of archaean crustal component via subduction but the new data and reinterpretation of older data imply that besides juvenile crust the svecofennian domain also contains older palaeoproterozoic crustal components no archaean component is found in the 1 93 1 91 ga gneissic tonalites and related felsic volcanics end t 1 4 which occur in the savo belt sb adjacent to the archaean craton the depleted mantle model ages vary from 1 94 ga to 2 33 ga with most values 2 0 ga the analysed syntectonic post thickening granitoids 1 89 ga in the sb exhibit positive end t values ranging from 2 8 to 3 0 pointing to a juvenile source the hypersthene granitoids are similar in age but show epsilon values around 1 which are only slightly lower than found in em derived mafic rocks the isotopic and geochemical data for the central finland granitoid complex and tampere schist belt indicate the occurrence of evolved thick crust 2 0 ga and associated lithospheric mantle already at 1 91 ga the southernmost part of the southern svecofennian in finland also shows evolved type crust 2 0 ga a tentative model for the development of crust and mantle is presented it includes the occurrence of continental nucleus surrounded by more juvenile island arc accretions two successive collisional events thickened both the crust and the subcontinental lithospheric mantle with the greatest relative amount of thickening occurring in the mobile zones convective removal of the lowermost parts of thickened lithosphere caused an upwelling of hot asthenosphere increase in the geotherm and melting of both the enriched subcontinental lithospheric mantle and the convective depleted asthenosphere mafic magmatism during the post thickening stage 1 89 ga thus varied from dominantly enriched to dominantly depleted mantle melts and was associated with variable crustal contamination the thickening of the lithosphere in the central part of the svecofennian domain was not followed by large extension suggesting that the thickness was already great 1 88 1 80 ga ago this together with the thick high velocity lower crust could have provided the isostatic balance needed to stabilize the thick crust

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

granites and early proterozoic crustal evolution in sweden evidence from smnd upb and o isotope systematics abstract u pb zircon and whole rock rb sr sm nd and o isotope results are reported for granitoids from three proterozoic provinces in sweden that represent different crustal environments the radiometric ages span the period 1 89 1 65 ga and record events ranging from original igneous crystallisation to late metamorphic resetting these events occurred at different times in the crustal segments the o isotope ratios identify different crustal source materials for the granitoids the sm nd results indicate that the major crustal source components common to most of the granitoids formed shortly before 1 9 ga close in time to the formation of the earliest granitoids contributions from recycled archaean basement are minimal the geographically widespread occurrence of such crustal sources indicates that the period around 2 ga was of great importance for the growth of new continental crust in sweden

mineral chemistry constraints on the evolution of the 1 88 1 87 ga post kinematic granite plutons in the central finland granitoid complex abstract a suite of post kinematic 1 88 1 87 ga silicic plutons crosscut 1 89 1 88 ga synkinematic granitoids in the central finland granitoid complex cfgc in south central finland the plutons range from biotite hornblende quartz monzonite to syenogranite and include pyroxene and olivine bearing varieties mineral chemical data on feldspars biotite amphibole pyroxenes olivine and oxides of the post kinematic plutons are presented the data are interpreted to show that these plutons register 1 a considerable range in pressure from 2 4 kbar amphibole barometry to 5 7 kbar olivine pyroxene barometry 2 temperatures mostly reflecting resetting during cooling 450 800 c quilf thermometry and 3 low fo2 log fo2 fmq 0 3 to 1 5 quilf equilibria in particular plutons with olivine and pyroxene bearing margins and amphibole dominated central parts record progressive oxidation and hydration upon cooling shifting from the quilf equilibrium toward kuilb the post kinematic granites can be considered post collisional in regard to compressional events in the cfgc and display many of the characteristics of the anorogenic 1 6 ga rapakivi granites further south they were presumably derived from a deep and dry crustal source like the rapakivi granites

origin of continental crust of 1 9 1 7 ga age nd isotopes in the svecofennian orogenic terrains of sweden initial nd isotopic ratios are reported for 57 samples representing all crustal components in the central and southern parts of the 1 9 1 7 ga old svecofennian orogenic terrains of sweden and the 1 8 1 7 ga old transscandinavian granite porphyry belt transscandinavian igneous belt u pb zircon ages are determined where the magmatic ages were not sufficiently constrained the nd isotopic results show that there are no archaean crustal blocks within the studied 1 9 1 7 ga old crustal segment this is consistent with results from finland the main input of recycled archaean crust to the svecofennian was in the form of sediments which are concentrated in the central svecofennian subprovince of north central sweden this is consistent with the vicinity of this subprovince to the archaean craton of the baltic shield these turbidite sediments contain up to 40 archaean material but s type granitoids associated with them have a smaller archaean component around 20 most of the quantitatively predominant i type granitoids and volcanics as well as all the basaltic rocks studied have nd from 1 to 3 and consist of 90 material newly derived from the mantle 1 9 1 7 ga ago we argue that the archaean material was introduced in the form of sediments but we cannot distinguish between pelagic sediments delivered on subducting oceanic crust and turbidite accumulations like the ones seen in north central sweden in either case an overall 15 archaean contribution for the swedish svecofennian was reasonably well homogenized with mantle derived magmatic products for this to occur an active near surface tectonic environment such as a complex of subduction zones must have been necessary

nd isotopic composition of cratonic rocks in the southern death valley region evidence for a substantial archean source component in mojavia thirty early proterozoic intermediate to silicic metasedimentary and metaigneous rocks in the southern death valley region and vicinity show nd values of 1 6 to 6 3 at 1 7 ga and nd model ages of 2 1 to 2 6 ga these cratonic rocks thus reveal an older signature than so far reported for nd province 1 of the western united states as much as 30 40 of their mass may be archean crustal material the archean component was introduced in the form of sedimentary detritus that was probably subducted and mixed with juvenile material at a convergent margin three younger precambrian rocks associated with the cratonic rocks also have a nd isotopic composition of province 1 type

fingerprinting gondwana versus baltica provenance nd and sr isotopes in lower paleozoic clastic rocks of the malopolska and łysogory terranes southern poland abstract neodymium and strontium isotope compositions of clastic material in lower paleozoic rocks in addition to detrital zircon and mica geochronology and faunal data are used to constrain the geotectonic provenance of the malopolska and ysogory terranes in southern poland being characterized by markedly different geological histories during early paleozoic times malopolska and ysogory were most often interpreted as exotic terranes but their identity provenance and accretionary history are still being discussed this study presents the first systematic nd and sr isotope survey of the lower paleozoic siliciclastic rocks cambrian clastic rocks of malopolska differ in their nd and sr isotope compositions from those of ysogory the former are characterized by a wide range of the end 505 ma values from 11 7 to 5 7 sr isotope ratios from 0 712 to 0 767 and model ages t2dm predominantly younger than 1 9 ga whereas the end t values of the latter vary within a fairly narrow range of the end 505 ma from 11 0 to 8 3 sr isotope ratios from 0 722 to 0 754 and model ages are older than 1 9 ga new nd data combined with detrital zircon ages provide evidence that malopolska must have been supplied with clastic material from amazonian sources during cambrian times being also progressively sourced from the baltic svecofennian crust in the middle cambrian detritus recognized in ysogory has mostly been derived from the svecofennian basement of fennoscandia but it has also a small addition from cadomian sources all provenance and biogeographic data suggest that malopolska was situated close to the amazonian craton near the western end of the cadomian belt during the early cambrian and it was not until the late middle cambrian that malopolska collided with the baltica margin during the late cambrian ysogory occupied a position between baltica and the cadomian belt but it was probably not yet accreted to the baltic crust at that time

a late mesoproterozoic 40ar 39ar age for a melt breccia from the keurusselka impact structure finland field investigations in the eroded central uplift of the 30 km keurusselka impact structure finland revealed a thin dark melt vein that intersects the autochthonous shatter cone bearing target rocks near the homestead of kirkkoranta close to the center of the impact structure the petrographic analysis of quartz in this melt breccia and the wall rock granite indicate weak shock metamorphic overprint not exceeding 8 10 gpa the mode of occurrence and composition of the melt breccia suggest its formation as some kind of pseudotachylitic breccia 40ar 39ar dating of dark and clast poor whole rock chips yielded five concordant late mesoproterozoic miniplateau ages and one plateau age of 1151 10 ma 11 ma 2 mswd 0 11 p 0 98 considered here as the statistically most robust age for the rock the new 40ar 39ar age is incompatible with 1 88 ga svecofennian tectonism and magmatism in south central finland and probably reflects the keurusselka impact followed by impact induced hydrothermal chloritization of the crater basement in keeping with the crosscutting relationships in the outcrop and the possible influence of postimpact alteration the late mesoproterozoic 40ar 39ar age of 1150 ma should be treated as a minimum age for the impact the new 40ar 39ar results are consistent with paleomagnetic results that suggested a similar age for keurusselka which is shown to be one of the oldest impact structures currently known in europe and worldwide

fulgurites in the southern central sahara republic of niger and their palaeoenvironmental significance the mineralogical and chemical characteristics of fulgurites natural glasses formed by lightning strikes to the ground from the southern central sahara niger are presented the fulgurites are indicators of thunderstorms the northernmost important fulgurite formation in the study area reached up to about 18 n with decreasing fulgurite concentration from south to north their distribution pattern and the relative dating of their formation in relation to landscape history from the late pleistocene onwards e g palaeolakes palaeosols and to neolithic settlement reveals their value as palaeoenvironmental indicators they indicate 1 local palaeoenvironmental conditions depending on the topographical situation in a complex dune relief 2 climatic change during the mid holocene from northerly rains to southerly rains and 3 the northernmost limit of important thunderstorms and rainfall activity since this time in the southern central sahara

analog modeling of one way gravitational spreading of hot orogens a case study from the svecofennian orogen fennoscandian shield abstract geodynamic models offer insight into deformation styles of syn and post collisional extensional processes such models often ignore the effect of lateral viscosity contrast between accreted terranes and inherited tectonic boundaries this paper presents a set of crustal scale analog experiments of one way post collisional gravitational spreading testing the effect of crustal heterogeneity by using two mechanically contrasting blocks pre existing cuts with varying dips and three layers with contrasting rheologies two blocks represent strong mature and weak juvenile crustal parts both blocks have three horizontal crustal layers brittle upper weak partially molten middle and dry strong lower layer cuts represent pre existing crustal scale shear zones and tectonic boundaries the experiments indicated that tectonic boundaries may facilitate exhumation and increase extensional strain when the boundaries dip opposite to the flow direction the tectonic boundaries may also split and divide the crust into separate blocks with independent deformation signatures shortening elongation or rotation the tectonic boundaries rotated along vertical axes toward the flow direction the modeling results suggest that in areas with inherited tectonic boundaries and compositional differences the amount of extension is unevenly distributed between the different parts during the widespread unilateral gravitational spreading and that the flow has different effects on the contrasting crustal units in both lateral and vertical directions to validate the modeling the results are compared to geophysical and geological data from the paleoproterozoic svecofennian orogen in the fennoscandian shield the comparison suggests that the orogen underwent 50 extension and was thinned by maximum of 20 via westward gravitational spreading this spreading rotated the crustal scale blocks shear zones and tectonic contacts it also brought exhumed high grade middle crustal blocks into contact with subsided low grade upper crustal blocks we suggest that results of the analog models can be used as proxies in both ancient and modern hot accreted orogens which have undergone post convergence continental mid crustal weakening and have comprised of juxtaposed terranes with varying rheological compositions

geochemistry zircon ages and whole rock nd isotopic systematics for palaeoproterozoic a type granitoids in the northern part of the delhi belt rajasthan nw india implications for late palaeoproterozoic crustal evolution of the aravalli craton determination of zircon ages as well as geochemical and sm nd isotope systematics of granitoids in the khetri copper belt of the aravalli mountains nw india constrain the late palaeoproterozoic crustal evolution of the aravalli craton the plutons are typical a type within plate granites derived from melts generated in an extensional tectonic environment they display ree and multi element patterns characterized by steep lree enriched and almost flat hree profiles and distinct negative anomalies for sr p and ti initial e nd values range from 1 3 to 6 2 and correspond to crustal sources with mean crustal residence ages of 2 5 to 2 1 ga a lower mafic crustal anatectic origin is envisaged for these granitoids and the heterogeneous e nd t values are inferred to have been acquired from the magma source region zircon pb pb evaporation and u pb ages indicate widespread rift related a type magmatism at 1711 1660 ma in the northern delhi belt and also suggest a discrete older magmatic event at around 1800 ma the emplacement ages of the compositionally distinct a type granitoid plutons and virtually coeval granulite metamorphism and exhumation in another segment of the aravalli mountains further signify that part of the aravalli crust evolved during a widespread extensional event in late palaeoproterozoic time

three stages to form a large batholith after terrane accretion an example from the svecofennian orogen abstract the magmatic history of batholiths formed as the result of tectonic accretion mirrors the evolution of the underlying crust from island arcs to accretion and collapse chemical composition age and structure give information about the magma source and emplacement depths the target of this study is the central finland batholith cfgc that provides a magmatic record after terrane accretion during the svecofennian orogeny followed by orogenic collapse the crustal source of partial melting has changed during the crustal evolution and differentiation after the terrane accretion and when the stress regime has changed from contraction to extension here we present a three stage magmatic evolution model for accretional batholith in the svecofennian orogen in the first stage the lower crust of the accreted crust melts partially and produces calcic tonalitic melts that are emplaced above the upper middle crustal detachment zone group 1 1887 3 ma in the second stage the middle crust begins to melt producing calc alkalic granodioritic melts that are emplaced close to the surface group 2 1890 1886 ma and at both sides of the upper middle crustal detachment zone group 2 1884 1883 ma in the third stage the lower crust remelts and produces bimodal calc alkalic gabbroic and alkali calcic to alkalic granitic melts group 3 1881 1880 ma which are emplaced above the upper middle crustal detachment zone this kind of crustal differentiation produces a mafic lower an intermediate middle and a felsic upper crust the third stage represents the last magmatic event of the svecofennian accretional orogeny where the high velocity lowermost crust is formed during this stage crustal thickening and widespread mid crustal partial melting have inititated lateral flow after or concurrently with the emplacement of the group 2 granodiorite the lateral flow has reactivated large scale shear zones allowing the transportation of the group 3 rocks from the lower crust to the upper middle crustal detachment zone the change in the degree of deformation at the exposed parts from undeformed through local to pervasive is attributed to differences in the exhumation level

age and tectonic framework of structurally controlled palaeoproterozoic gold mineralization in the hame belt of southern finland abstract a number of gold occurrences in the palaeoproterozoic svecofennian domain of southern finland show a clear structural control on mineralization and are classified as orogenic gold samples of pre syn and late to post mineralization rocks taken from two gold prospects in the somero tammela gold zone of the hame belt have been studied to gain absolute ages of mineralization by using sims u pb of zircon and titanite and ar ar single grain step heating of hornblende and biotite gold mineralization in the somero tammela gold zone is closely related to quartz veins and controlled by wsw ene to sw ne trending shear zones and faults formed during prolonged dextral oblique contraction in the late stages of svecofennian orogenic evolution zircons from a felsic volcanic host rock give an age of 1881 3 ma interpreted as the crystallization age of the rock and providing a maximum age for mineralization inherited zircons in a gold bearing quartz vein with the youngest zircon rim of 1832 15 ma give a maximum age of quartz vein emplacement a 1786 7 ma mean 207pb 206pb age of a pegmatitic quartz feldspar dyke providing a minimum age for mineralization is interpreted as reflecting lead loss whereas an 1815 4 ma zircon age is probably closer to the crystallization age of the dyke this age overlaps with 1805 4 5 4 ma 39ar 40ar age of hornblende from a quartz vein 1 78 and 1 76 ga 39ar 40ar biotite cooling ages mark the minimum age for mineralization in summary the age of mineralization can be estimated at ca 1 82 1 79 ga which is in good agreement with the age of dextral shearing in southern finland the controlling structures wsw ene to sw ne dextral shear zones and nw se oriented faults formed during approximately wnw ese oriented sub horizontal shortening during crossing ductile brittle boundary conditions the steep shear zones and faults and the intersection lines of the two sets impart a directional permeability allowing channelling of the mineralizing fluids in these structures this was supported by prolonged dextral oblique contraction enabling continuous or episodic slip along the faults and shear zones the late svecofennian tectonic episode could be described as an extension contraction cycle in a convergent plate margin setting above a retreating subduction zone oblique extension 1 86 1 83 ga caused by slab rollback was followed by a rapid switch to oblique contraction at ca 1 82 ga induced by arrival of positively buoyant crustal fragments e g oceanic ridge oceanic plateau or continental sliver at the subduction zone the contractional period characterized by dextral transpression provided a favourable setting for orogenic gold mineralization the termination of the svecofennian orogenic evolution in finland after 1 79 ga might be caused by lock up of the subduction zone and slab break off the striking nw se alignment of orogenic type gold occurrences in the svecofennian domain of southern finland suggests that the late svecofennian 1 82 1 79 ga gold mineralizing event may be regionally more widespread and of greater importance than hitherto expected

sources of svecofennian granitoids in the light of ion probe upb measurements on their zircons abstract the presence of 1 91 1 93 ga old granitoids at the archean proterozoic boundary along the raahe ladoga zone in finland has been demonstrated on various occasions these rocks have been considered to represent juvenile crustal material as their e nd values are markedly positive however as svecofennian metasediments contain detrital zircons derived from a ca 2 ga old source the possibility has existed that the 1 92 ga age may have been a mixture between 2 and 1 89 ga old zircon populations as such mixing would not markedly affect their neodymium isotopic properties also some syntectonic 1 89 ga old svecofennian granitoids contain heterogeneous zircon populations but it has been impossible to determine the age and origin of the older zircons by conventional methods nordsim ion probe results on three samples from the 1 92 ga age group confirm the earlier conclusions especially important is that no zircons older than 1 95 ga were detected in the 1 92 ga group samples thus the 1 92 ga event was the beginning of the formation of new continental crust in the primitive svecofennian island arc and these granitoids formed by partial melting of basaltic magmas derived from a depleted mantle source one sample also contains a younger zircon population formed during the orogenic culmination at 1 89 ga in contrast one grain from a sample representing the 1 89 ga age group contains an archean core which is considered to represent sedimentary detritus assimilated during either magma formation or intrusion while the results prove the true igneous nature of the 1 92 ga event they also rule out these rocks as a possible provenance for the ca 2 ga old zircons encountered in the svecofennian metaturbidites thus there is still no direct evidence from granitoid rocks for an extensive svecofennian protocrust the existence of which has been postulated on the basis of geochemical and sm nd isotopic data

paleoproterozoic lateritic paleosol ultra mature mature quartzite meta arkose successions in southern fennoscandia intra orogenic stage during the svecofennian orogeny abstract a paleoproterozoic lateritic paleosol overlain by ultra mature mature quartzite is reported for the first time from three localities in the svecofennian province in southern finland these are important indicators for warm paleoclimate and continental paleoenvironment ultra mature quartzites above the lateritic paleosols are overlain by matrix supported conglomerates followed by mature quartzites these latter rocks are related to incipient rifting followed by main rift stage starting with increasingly more immature meta arkose we present a tectonic model in which exhumation after 1 88 1 87 ga contraction unroofed plutonic rocks volcanism and exhumation related erosion produced deposition of both volcanic and sedimentary material weathering of these rocks and deposition of ultra mature mature quartzites took place between 1 87 and 1 85 1 84 ga followed by rifting at 1 84 ga tectonic slicing during basin inversion thrust the pre rift quartzites and rift related volcanic and sedimentary rocks over older mid crustal rocks and produced a low angle stacking system to accommodate voluminous magmatism at deeper crustal levels and widespread sedimentation at the surface we propose that exhumation due to faulting followed by stabilization of the upper crust 0 15 20 km occurred simultaneously as the detached hot middle and lower crust experienced vertical shortening via ductile flow and extrusion the ca 1 85 ga paleosol quartzite successions in southern fennoscandia have many similarities with the widespread ca 1 7 ga paleosol quartzite successions in the united states both are intra orogenic formed on hot orogens and have simultaneous magmatism at deeper crustal levels and sedimentation at the surface accretion type models presented for southern fennoscandia at 1 87 1 80 ga are not compatible with the available data and either far field setting during outboard subduction or a newly formed continental interior setting are proposed anyhow tectonic models proposed for southern fennoscandia between 1 87 and 1 80 ga should take into account the occurrence of lateritic paleosols and ultra mature quartzites

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km