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

sedimentary recycling in arc magmas geochemical and u pb hf o constraints on the mesoproterozoic suldal arc sw norway the hardangervidda rogaland block within southwest norway is host to 1 52 to 1 48 ga continental building and variable reworking during the 1 1 to 0 9 ga sveconorwegian orogeny due to the lack of geochronological and geochemical data the timing and tectonic setting of early mesoproterozoic magmatism has long been ambiguous this paper presents zircon u pb hf o isotope data combined with whole rock geochemistry to address the age and petrogenesis of basement units within the suldal region located in the centre of the hardangervidda rogaland block the basement comprises variably deformed grey gneisses and granitoids that petrologically and geochemically resemble mature volcanic arc lithologies u pb ages confirm that magmatism occurred from 1 521 to 1 485 ma and conspicuously lack any xenocrystic inheritance of distinctly older crust hafnium isotope data range from ehf initial 1 to 11 suggesting a rather juvenile magmatic source but with possible involvement of late palaeoproterozoic crust oxygen isotope data range from mantle like 18o 5 to elevated 10 suggesting involvement of low temperature altered material e g supracrustal rocks in the magma source the hf o isotope array is compatible with mixing between mantle derived material with young low temperature altered material oceanic crust sediments and older low temperature altered material continent derived sediments this combined with a lack of xenoliths and xenocrysts exposed older crust afc trends and s type geochemistry all point to mixing within a deep crustal magma generation zone a proposed model comprises accretion of altered oceanic crust and the overlying sediments to a pre existing continental margin underthrusting to the magma generation zone and remobilisation during arc magmatism the geodynamic setting for this arc magmatism is comparable with that seen in the phanerozoic e g the sierra nevada and coast range batholiths with compositions in the suldal sector reaching those of average upper continental crust as within these younger examples factors that drive magmatism towards the composition of the average continental crust include the addition of sedimentary material to magma source regions and delamination of cumulate material underthrusting of sedimentary materials and their subsequent involvement in arc magmatism is perhaps a more widespread mechanism involved in continental growth than is currently recognised finally the suldal arc magmatism represents a significant juvenile crustal addition to sw fennoscandia

the bergslagen ore province sweden review and update of an accreted orocline 1 9 1 8 ga bp abstract we have revisited and re interpreted deformation structures in the classical ore province of west bergslagen part of the 1 9 1 8 ga old svecofennian orogen in south central sweden four successive phases and styles of deformation reflect 1 continental rifting associated with bimodal magmatism in continental back arc basins d1 2 closure of the rift s by upright tight to isoclinal folds on any scale d2 3 renewed rifting d3 with deposition of argillites conglomerates and submarine basalts in restricted rift basins and 4 intense buckling and folding of west bergslagen along variably but generally steeply plunging folds d4 strain localization during d4 initiated a network of conjugate retrogressive shear zones and closely related large scale vertical drag folds with the southwest northeast striking west bergslagen boundary zone system acting as the main tectonic boundary along which west bergslagen slipped relative to the central part of the province d4 transpression and variably oblique d4 shear may have caused the preservation of a low grade domain in west bergslagen as well as the exhumation of a granulite facies domain near the hinge of the orocline d4 structures include a regional scale oroclinal bend of 135 in southwest bergslagen connecting common southwest northeast strikes in west bergslagen to east west strikes to the south and east in central bergslagen the oroclinal bend tightens to 180 to the south it lines up with another orocline the two forming an s shaped pair of large scale buckle folds the orocline finds expression in the regional patterns of form lines and magnetic anomalies and in the spatial distribution of specific intrusive rock suites the northern structural boundary of the province is known as an east west striking zone of dip slip movement and local wrenching which we also interpret as a d4 effect the d4 structures imply the evolution of a separate ribbon like bergslagen micro continent at least 700 km long rifted from the former active continental margin during the d3 event and finally accreted slightly obliquely to pre existing fennoscandia to the present north by correlating d4 deformation structures with the areal distribution and previously determined radiometric ages of late orogenic anatectic granites we derive a provisional estimate 1 83 1 82 ga as the lower age limit to the initiation of terrane accretion d4 accretion is svecofennian orogeny proper in this part of the belt younger than the currently preferred estimate of 1 86 ga west bergslagen s synsedimentary hydrothermal ore deposits originating from 1 91 to 1 89 ga old volcanism and rifting have been remobilized to various extent during the late orogenic d4 phase and show pronounced concentration along d4 shear zones

the archaean proterozoic palaeoboundary in the lulea area northern sweden field and isotope geochemical evidence for a sharp terrane boundary abstract extensive sm nd isotope work has been carried out in northern sweden with the intention of studying crustal reactivation and assimilation processes relative to juvenile arc accretion at an archaean cratonic margin previous regional work has shown a gradual change from negative nd t values in northernmost sweden to positive values towards the skellefte sulphide ore district farther south isotopic variation and geophysical data show a break in the character of the crust along a wnw trending zone it is suggested that this zone which passes close to the towns of lulea and jokkmokk represents the transition from the archaean craton in the northeast to proterozoic juvenile crust in the southwest a profile across the eastern part of this transition zone has been studied in detail in the lulea area where archaean rocks recently have been discovered the distribution and character of the archaean rocks have been studied these occur as fragments ranging in size from several km 2 to dm in scale and consist of ca 2 7 ga orthogneisses of granodioritic to dioritic composition and porphyritic granitoids of quartz monzodioritic to granodioritic composition plutonic rocks of ca 1 9 ga age and archaean rocks were analysed for major and trace elements and for sm nd isotopes the calc alkaline ca 1 9 ga granitoids are dominated by granodiorites tonalites quartz diorites and quartz monzodiorites and there is a distinct boundary rather than a transition zone between intrusions with postive nd t values in the southwest and intrusions with negative nd t values in the northeast the locations of the outcropping archaean fragments coincides with this distinct boundary which is interpreted as a terrane boundary separating a ca 1 9 ga juvenile arc terrane in the southwest from the archaean continent in the northeast a possible explanation for this is that the proterozoic juvenile terrane was obducted onto the archaean craton during the final stages of the svecofennian orogeny

migratory tectonic switching western svecofennian orogen central sweden constraints from u pb zircon and titanite geochronology abstract the forsmark area in the western part of the svecofennian orogen central sweden is situated between two major palaeoproterozoic tectonic domains that show contrasting histories with respect to timing of igneous activity ductile deformation and metamorphism geological features common to both the adjacent domains are found at forsmark which consequently is a key area for understanding the tectonic evolution of the svecofennian orogen in this region new sims and tims geochronological data from several samples in a limited area at forsmark constrain the timing of igneous activity ductile deformation and metamorphism in this area u pb zircon dating in combination with field studies reveals the existence of two calk alkaline igneous suites at forsmark the older and most voluminous plutonic suite intruded at 1 89 1 87 ga and is affected by penetrative yet variably intense ductile deformation with the development of a ductile grain shape fabric under amphibolite facies metamorphic conditions and folding the younger and less voluminous hypabyssal suite intruded at 1 86 1 85 ga during the waning stages of this penetrative deformational phase furthermore u pb titanite dating shows that the area has been affected by one or more tectonothermal events after regional fabric development and possibly even after 1 83 ga the deformation during these events involved more restricted strain along discrete zones the forsmark data in combination with a compilation and evaluation of available geochronological data from the time interval 1 91 1 84 ga in central sweden points to the existence of at least two major tectonic cycles in this region each cycle is characterised by igneous activity associated at least partly with extension a short interval of compressional deformation and migration of the tectonic activity in this paper we discuss two highly contrasting tectonic models that may explain the cyclic tectonic evolution of the western part of the svecofennian orogen in central sweden the favoured model involves continuous subduction with a constant polarity beneath a single active continental margin combined with alternating subduction hinge retreat and advance the model involves migration of what has been described as tectonic switching in the younger accretionary orogenic systems of eastern australia palaeozoic lachlan orogen and new zealand

sm nd isotope systematics of 1 9 1 8 ga granites from western bergslagen sweden inferences on a 2 1 2 0 ga crustal precursor the bergslagen ore province forms part of the svecofennian domain of the baltic shield the initial nd values of least altered and albitized granite samples of the 1 88 ga bergslagen older granite suite vary between 1 2 and 2 5 the granite samples of the 1 78 ga fellingsbro malingsbo suite vary between 0 3 and 7 7 the rb sr system of the older granite suite yields an errorchron of 1 70 0 01 ga with an initial 87sr86sr ratio of 0 7053 indicating tectonothermal disturbance perhaps due to the intrusion of younger granites of the transscandinavian igneous belt the initial nd isotopic composition of the 1 88 ga older granite suite can be interpreted in two extreme end member models a interaction between depleted mantle derived melts and archaean crustal material at 1 88 ga and b remelting of early proterozoic pre 1 88 ga crust previous studies in the baltic shield have advocated the former model and discarded the latter because of absence of 2 0 2 4 ga crust likewise in the case of western bergslagen no archaean crust is present it is hence argued that the nd isotopic data may also be interpreted in terms of generation of the older granite suite through remelting of a crustal precursor in this case the 2 1 2 0 ga depleted mantle model ages of the older granite suite could give an indication of the time that such a precursor was extracted from the depleted mantle

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

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

origins and ages of proterozoic granitoids in the bothnian basin central sweden isotopic and geochemical constraints abstract the paleoproterozoic bothnian basin in central sweden and south central finland is dominated by a 10 km thick sequence of metasediments mainly metaturbidites these have been intruded by granitoids which have traditionally been classified as early late and postorogenic relative to the c 1 9 1 8 ga s vecofennian orogeny the early orogenic granitoids are part of a calcalkaline suite ranging in composition from ultramafic to granitic while the late orogenic granitoids mainly are purely granitic and associated with abundant pegmatites the postorogenic group forms large massifs of mainly granitic composition and associated pegmatites are rare an early orogenic tonalite yields a u pb zircon age of 1877 6 ma which is within the age range for s vecofennian granitoids of this tectonic position elsewhere in sweden and finland a late orogenic granite belonging to the harno granites from the southern part of the bothnian basin gives a u pb monazite age of 1822 5 ma this provides a minimum age for the main phases of svecofennian deformation in the region and a correlation with the 1 84 1 83 ga old granite migmatite zone of southern finland is suggested a postorogenic granite belonging to the revsund granitoids gives a u pb zircon age of 1798 8 ma together with previously published age data this indicates that the revsund granitoids were emplaced successively during an age interval of at least between 1 80 and 1 77 ga ree patterns and trace element model calculations indicate that late orogenic granites could have been produced by partial melting of metasediments similar to those exposed at the surface a contribution from the early orogenic granitoids is possible but not discerned by these calculations the postorogenic granites on the other hand have too high concentrations of sr ba y and hree to be derived from such a metasedimentary source the initial e nd values for the early orogenic granitoids range between 2 and 1 corresponding values for both late and postorogenic granites range between 1 and 3 which is on average slightly higher than those for surrounding metasediments the initial sr isotope ratios for the harno granites are intermediate between those for the metasediments and the early orogenic granitoids while the initial sr isotope ratios for the revsund granites are distinctly lower the geochemical and isotopic data support a model in which the harno granites were derived mainly from bothnian basin metasediments with a contribution from early orogenic granitoids the composition of the revsund granitoids is however inconsistent with such a derivation for these rocks a deeper as yet unknown crustal source with a significant pre 1 9 ga crustal component is indicated

full scale crustal interpretation of kokkola kymi kokky seismic profile fennoscandian shield the kokkola kymi deep seismic sounding profile crosses the fennoscandian shield in northwest southeast nw se direction from bothnian belt to wiborg rapakivi batholith through central finland granitoid complex cfgc the 490 km refraction seismic line is perpendicular to the orogenic strike in central finland and entirely based on data from quarry blasts and road construction sites in years 2012 and 2013 the campaign resulted in 63 usable seismic record sections the average perpendicular distance between these and the profile was 14 km tomographic velocity models were computed with jive3d program the velocity fields of the tomographic models were used as starting points in the ray tracing modelling based on collected seismic sections a layer cake model was prepared with the ray tracing package seis83 along the profile upper crust has an average thickness of 22 km average and p wave velocities vp of 5 9 6 2 km s near the surface increasing downward to 6 25 6 40 km s the thickness of middle crust is 14 km below cfgc 20 km in se and 25 km in nw but vp ranges from 6 6 to 6 9 km s in all parts lower crust has vp values of 7 35 7 4 km s and lithospheric mantle 8 2 8 25 km s moho depth is 54 km in nw part 63 km in the middle and 43 km in sw yet a 55 km long section in the middle does not reveal an obvious moho reflection s wave velocities vary from 3 4 km s near the surface to 4 85 km s in upper mantle consistently with p wave velocity variations results confirm the previously assumed high velocity lower crust and depression of moho in central finland

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

evidence for 1 82 ga transpressive shearing in a 1 85 ga granitoid in central sweden implications for the regional evolution abstract two crustal scale shear zone systems the storsjon edsbyn deformation zone and the hassela shear zone join in the western part of central sweden and form an anastomosing pattern of ductile shear zones several of these up to 1 km wide zones truncate an intrusion previously defined as a component of the major ca 1 80 1 77 ga magmatic suite the revsund granitoids in north central sweden however within the southern part of the intrusion titanites that are components of the fabric in a pervasively deformed shear zone forsaan have a u pb tims age of 1816 2 ma zircon porphyroclasts in the same fabric yield a u pb sims age of 1849 14 ma overlapping with the u pb tims age of 1851 2 ma for titanites from an undeformed granitoid these results are interpreted to define the ages of ductile shearing and magma emplacement respectively this interpretation is supported by microstructures as well as the regional structural pattern characterised by shear zones enveloping virtually undeformed pods of the granitoid previously reported and locally existing magmatic flow structures indicate some syn emplacement deformation the kinematic results characterised by a high proportion of pure shear within the granitoid and oblique dextral shearing along its margin reflect strain partitioning under transpressive conditions the kinematic and geochronological results fit into the regional framework of late orogenic localised deformation along crustal scale shear zones recent absolute datings of shear fabrics defines ca 1 82 1 80 ga as a period of shear zone activity in central sweden temporally this period overlaps with the emplacement of s type granites and granitoids derived from deeper crustal levels a similar situation exists in southern finland structural geochronological and geophysical data suggest that the crustal scale shear zones in central sweden and southern finland may be related temporally and spatially the age and the post tectonic nature on a regional scale of the dated 1 85 ga granitoid have important regional implications as both the age of the intrusion and the ductile shearing are older than the established age range 1 80 1 77 ga of the revsund granitoids either an extended intrusive history for the suite is indicated or the granitoids in this part of the type areas should be excluded from the revsund suite their post tectonic nature implies that the pervasive svecokarelian deformation in the area occurred earlier than generally assumed 1 85 1 80 ga on the other hand approximately coeval calc alkaline granitoids to the south east of the investigated area are folded and foliated pervasively indicating a later svecokarelian evolution in that area i e that the granitoids are located in domains with separate orogenic evolutions

new u pb age constraints for the laxford shear zone nw scotland evidence for tectono magmatic processes associated with the formation of a paleoproterozoic supercontinent the lewisian gneiss complex in north west scotland is a part of the extensive network of archaean cratonic areas around the margins of the north atlantic it is considered to be made up of a number of terranes with differing protolith ages which have been affected by a range of different metamorphic events a major shear zone the laxford shear zone forms the boundary between two of these terranes new dates presented here allow us to constrain the timing of terrane assembly related to the formation of palaeoproterozoic supercontinents early deformation along the laxford shear zone and primary accretion of the two terranes occurred during the inverian event at c 2480 ma this was followed by extension and the intrusion of the mafic scourie dykes subsequently renewed silicic magmatic activity occurred at c 1880 ma producing major granite sheets considered to have formed as part of a continental arc a further collisional event began at c 1790 ma and was followed by slow exhumation and cooling this laxfordian event caused widespread crustal melting metamorphism and deformation and is considered to represent the final assembly of the lewisian gneiss complex within the major supercontinent of columbia or nuna

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

timing of palaeoproterozoic intra orogenic sedimentation in the central fennoscandian shield evidence from detrital zircon in metasandstone detrital zircon u pb sims data on quartz rich metasandstone units presumed to belong to the upper part of the svecofennian stratigraphy in southeastern finland and east central sweden suggest the existence of clastic sedimentary basins between the two main orogenic phases at 1 89 1 86 ga and 1 83 1 79 ga during a period referred to as the intra orogenic phase 1 86 1 83 ga stratigraphically below the metasandstone at hamrange east central sweden is a metadacite with an u pb zircon tims age of 1888 6 ma which indicates the maximum age of sedimentation it also indicates that an earlier proposed correlation of hamrange metavolcanic rocks and 1 86ga equivalents at los to the northwest must be rejected instead there is a temporal affinity to the metavolcanic rocks in the bergslagen province to the south or southern finland to the east quartz rich metasandstone samples from four localities luukkola pyhantaka and tiirismaa in finland and hamrange in sweden yield multimodal detrital zircon age distributions with main populations at 2 95 2 60 ga 2 10 1 95 ga and 1 92 1 85 ga the groups are similar in all four samples and they are comparable to previously reported detrital ages in this part of the fennoscandian shield the oldest zircon analysed gave an age of 3 32 ga tiirismaa the maximum ages of sedimentation and of subsequent deformation and metamorphism indicated by the youngest detrital zircon from the four localities are 1842 10 ma luukkola 1865 11 ma pyhantaka 1848 13 ma tiirismaa and 1855 10 ma hamrange respectively possible source rocks for these zircon grains are found within and around the vast ljusdal batholith in sweden and in the arc complexes of western and southern finland it is concluded that several intra orogenic sedimentary basins existed during the time interval 1 86 1 83 ga ago between two major orogenic events in the fermoscandian shield less

precambrian domains in lithuania evidence of terrane tectonics abstract the west lithuanian granulite wlg and east lithuanian domains eld form the proterozoic basement of lithuania and can be distinguished on the basis of differing structural patterns lithologies and evolutionary histories they are juxtaposed along the mid lithuanian suture zone mlsz in the wlg the main lithotectonic complexes comprise felsic and intermediate mostly metasedimentary granulites in the south west and mafic metaigneous granulites in the north east the former are interpreted as marine metapelites while most of the mafic ones have been derived from island arc tholeiites these rock complexes trend nw se and are marked by contrasting gravity and magnetic anomalies ne and e w striking faults and shear zones complicate the potential field patterns sets of nw trending anomalies also extend from lithuania across the baltic sea to south central sweden and indicate that the wlg complexes continue into the baltic fennoscandian shield voluminous anatectic granites alternate with the metapelites whereas the mafic granulites occur together with enderbites and charnockites in the eld the main structures produce strong nne ssw oriented gravity and magnetic anomalies which trend parallel to the belarus baltic granulite belt bbg and other terranes situated still farther east the eld is composed of metasedimentary rocks interpreted as one time graywackes shales and dolomites accumulated in continental margin arc and shallow water basinal environments amphibolites and gabbros with morb and iat characteristics and voluminous granitoids are also present the coexistence of juvenile mafic rocks with continental margin and shelf sediments suggests an oceanic back arc setting the two lithuanian basement domains display contrasting metamorphic histories that suggest separate developments before the eventual amalgamation in the wlg the metapelites indicate peak metamorphism at high temperatures up to 850 900 c and moderate pressures 8 10 kbar this was followed by cooling and reheating and then an uplift event repeated magmatic underplating accompanied the metamorphism in the eld in contrast the rocks have been subjected to comprehensive metamorphism under moderate amphibolite facies conditions that metamorphism however was not uniform throughout the metasediments in the east have recorded pressures similar to those in the neighbouring bbg 7 8 kbar but lower temperatures 650 680 c while in the central and western parts of the eld metamorphism occurred at ca 480 580 c with pressures increasing from 3 4 kbar in the centre to 6 kbar close to the western boundary reheating to 700 c due to a ca 1 5 ga magmatic event is characteristic the mlsz which separates the two lithuanian basement domains from each other is a n s oriented ca 30 50 km wide westward plunging crustal discontinuity marked by magnetic and gravity highs mafic and felsic intrusions and sheared rocks crustal thicknesses change from 42 44 km in the west to 50 km in the eastern side of the zone which also truncates a crustal low velocity layer characteristic of the wlg the amalgamation of the wlg and eld along the mlsz occurred at ca 1 71 1 66 ga after which time both domains were affected by the same post kinematic anorogenic magmatism ca 1 58 1 45 ga ago that event and related shearing were responsible for some ultimate refragmentation of the lithuanian basement terranes

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

stratigraphy and geochemistry in the proterozoic mafic volcanic rocks of the nagu korpo area sw finland the volcanic rocks of the nagu korpo area in sw finland form a thin 1000 m formation that is folded into narrow synforms separated by migmatite filled antiforms in nagu the volcanic formation comprises three units a lower unit of subvolcanic banded gabbro sills a middle unit of amphibolitic volcanic rocks and an upper unit consisting of a thin ultramafic volcanic rock layer the volcanic formation is interlayered within a thicker sequence of metasedimentary gneisses geochemically the metavolcanic rocks are tholeiitic and plots based on ti y zr and other trace elements show that they resemble recent within plate lavas morb and mantle normalised trace element plots show patterns strongly differing from those of recent arc lavas indicating a tectonic environment of plate boundary related initial rifting the upper ultramafic lavas are enriched in lree relative to the lavas of the much thicker amphibolitic middle unit below a model for the initial formation of the volcanic rocks of the svecofennian fold belt must acknowledge both the episode of initial rifting and the later evolution of an island arc like environment

geochemical characterisation of a polyphase deformed altered and high grade metamorphosed volcanic terrane implications for the tectonic setting of the svecofennides south central finland abstract the virtasalmi metavolcanic belt forms part of the early proterozoic svecofennide terrane in central finland the preserved supracrustal lithologies 40 of the assemblage have undergone extensive synvolcanic hydrothermal alteration polyphase regional ductile deformation low pressure regional polyphase metamorphism to granulite facies with subsequent amphibolitisation and extensive emplacement of a synkinematic i type plutonic suite despite these overprinting events three chemically distinct suites of metatholeiites and one suite of coeval coarser grained intrusive lithologies are recognized and their petrogenesis resolved primary igneous geochemistry is preserved mainly by the hfses and cr and ni variable major and lil element mobility is demonstrated significant differences in geochemistry are reflected in different metamorphic assemblages between suites preservation of vesicular pillow lavas indicate subaqueous extrusion of the three metatholeiitic suites trace element geochemistry indicates that suite i compositions are controlled by low pressure fractional crystallization in contrast suite iii amphibolite compositions are controlled by batch partial melting with limited modification by open system fractional crystallization these differences in fractionation history point to the evolution of these two main suites within distinct upper crustal magmatic plumbing systems also differences in more hyg less hyg element and other incompatible element ratios point to derivation of suites i and iii by different degrees of partial melting from different heterogeneous mantle source regions coeval eruption of these suites indicates that variations in source geochemistry and melting processes reflect spatial rather than temporal differences the volumetrically smaller suite ii comprises lava flows and pillowed units that are interlayered with but have a more depleted chemistry than the other suites marked variation in cr and ni contents within a narrow range of zr contents in individual flows in subgroups iia 1 and iia 2 and marked tio 2 enrichment in subgroup iib suggests that the more depleted character of this suite is due to fractional crystallisation and crystal settling in high level magma chambers suite iv amphibolites comprise a number of small intrusive bodies that are emplaced within the volcanic pile they are coarser grained than the enveloping amphibolites and are interpreted as tectonised relics of high level magma chambers and or sills emplaced into the coeval metavolcanic assemblage they have a depleted geochemistry and there is textural and geochemical evidence to suggest a partial cumulate origin similarities in fractionation histories suggest that suite iv intrusions are cogenetic with suites i and ii enrichment in incompatible elements particularly nb demonstrates that the virtasalmi metatholeiites are not related to supra subduction zone magmatism all the amphibolites are within plate basalts or e type morbs with similar compositions known today from a range of tectonic settings however the interlayering of e morb metavolcanic rocks with marbles and voluminous fine grained clastic metasediments is more likely to indicate an intracratonic rift passive margin geotectonic setting subsequent deformation and metamorphism of the volcanics and syn kinematic calc alkaline intrusives are interpreted as accretion at a destructive margin the latter was most probably a continental calc alkaline arc various crustal levels of which are now exposed in the study area

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

effects of frictional viscous oscillations and fluid flow events on the structural evolution and re os pyrite chalcopyrite systematics of cu rich carbonate veins in northern norway abstract mesothermal chalcopyrite pyrite magnetite bearing calcite dominated vein deposits in the repparfjord tectonic window northern norway have been studied to constrain the mechanics of their emplacement and the timing of initial vein formation and reactivation the veins cut across paleoproterozoic tholeiitic metabasalts and present textural contrast between their hydrofractured coarse grained margins and the fine grained mylonitic cores they formed under overall viscous conditions although cyclically increased fluid pressures caused transient embrittlement as mineral precipitation sealed the fractures deformation was accommodated again viscously leading to mylonitization of the vein s core local brecciation of the calc mylonite demonstrates the cyclicity of this process re os chalcopyrite pyrite and k ar fault gouge dates define an almost continuous age range from 2540 ma to 460 ma regression of three re os analyses yields a 2069 14 ma age 187os 188os 0 18 0 04 interpreted as the age of vein emplacement sulfide precipitation and initial frictional viscous deformation k ar ages are mixed ages that constrain a maximum age of faulting in association with the veins at approximately 460 ma hence indicating structural reactivation connected with silurian caledonian orogenic compression the spread in re os model ages reflects this reactivation wherein renewed strain accommodation and circulating oxidizing fluids caused fracturing dynamic recrystallization and isotopic disturbance of the sulfides the study provides evidence for fluid flow during viscous deformation and demonstrates that strain and flow of oxidizing fluids can have a significant yet localized control on the integrity of the re os systematic in pyrite and chalcopyrite

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

multiple post svecofennian 1750 1560 ma pegmatite dykes in archaean palaeoproterozoic rocks of the west troms basement complex north norway geological significance and regional implications abstract the west troms basement complex north norway consist of meosarchaean neoarchaean and palaeoproterozoic crust which likely represent the continuation of the karelian craton of the fennoscandian shield beneath and across the caledonian nappes the complex is transected by several meta sedimentary volcanic belts with ages from ca 2 7 to 1 9 ga and linear ductile shear zones that record polyphase deformation after emplacement of several late svecofennian plutonic complexes 1 80 1 79 ga the astridal supracrustal belt of the senja shear zone was intruded by various generations of granite pegmatite dykes which yield zircon and titanite u pb ages ranging between 1758 and 1562 ma with evidence of a weak caledonian overprint at about 400 ma that formed or reset some but not all titanites the age range indicates that formation of the pegmatites occurred after termination of the main svecofennian orogeny 1 92 1 79 ga of the karelian craton comparison within the larger scale context of the fennoscandian shield suggests that emplacement of these pegmatites may have been caused by late stage reactivation of intracratonic shear zones possibly reflecting distal effects of deeper magmatic processes such as intrusion of coeval aged rapakivi granite suites ca 1 65 1 5 ga in finland and or the mesoproterozoic gothian orogeny and magmatism affecting the southwestern parts of fennoscandia the mesoproterozoic ages however also correspond to late laxfordian ca 1 7 1 6 ga tectonic events in the lewisian basement on the laurentian margin of scotland suggesting a potential early link between these basement complexes

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

continental margin magmatism and migmatisation in the west central fennoscandian shield the ljusdal batholith ljb is a major component of the central svecofennian domain in sweden it is separated from the bothnian basin to the north by the 1 82 1 80 cia crustal scale hassela shear zone hsz the ljb has emplacement ages of 1 86 1 84 cia is mainly alkali calcic metaluminous has epsilon nd values between 0 3 and 1 2 and was formed in a magmatic arc setting during the svecokarelian orogeny the ljb was affected by at least three fold episodes large scale folded screens of migmatised metasedimentary rocks occur in the eastern part of the batholith and to the north of the hsz there is a 50 km wide diatexite belt the transition belt trb consisting of 1 88 1 85 ga granitoids is located at the northwestern extension of this belt a calc alkaline and peraluminous composition combined with negative epsilon nd values 1 7 to 0 8 indicates a large proportion of metasediments in the source for these granitoids u pb sims data on zircon rims from migmatites and leucogranites to the north and east of ljb yield ages of 1 87 1 86 ga i e coeval with the granitoids of the ljb and the trb there is thus a close relationship between the ljb the trb and the migmatites in both space and time syn migmatitic shearing along the hsz indicates that a proto hsz was initiated already at c 1 86 ga and the location of the proto hsz is inferred to be controlled by two older nuclei present in the lower parts of the crust as crustal scale shear zone systems are known to act as ascent pathways for sheet like flow in active orogenies the trb may represents accumulations of melts that were attracted and extracted by the proto hsz and intruded in a block that was not pervasively affected by subsequent shear along the hsz an active continental margin setting for the ljb implies subduction at c 1 86 ga and provides a heat source for both the migmatites and the trb a later migmatisation at 1 82 ga has been recorded to the south of the hsz within the ljb the 1 82 ga stromatic migmatites are folded by f 2 folds and the fabric is truncated by 1 80 cia pegmatites

timing of magmatism and migmatization in the 2 0 1 8 ga accretionary svecokarelian orogen south central sweden the palaeoproterozoic 2 0 1 8 ga svecokarelian orogen in central sweden consists of a low pressure predominantly medium grade metamorphic domain central part of bergslagen lithotectonic unit enclosed to the north and south by low pressure migmatite belts two periods of metamorphism 1 87 1 85 and 1 83 1 79 ga are known in the migmatite belts in this study new u th pb ion microprobe data on zircon and monazite from twelve samples of locally migmatized gneisses and felsic intrusive bodies determine both protolith and metamorphic ages in four sample areas north of stockholm inside or immediately adjacent to the medium grade metamorphic domain two orthogneiss samples from the rimbo area yield unusually old protolith ages of 1909 4 and 1908 4 ma while three orthogneisses from the skutskar and forsmark areas yield more typical protolith ages between 1901 3 and 1888 3 ma migmatized paragneiss samples from this and two earlier studies contain a significant detrital component sourced from this 1 9 ga magmatic suite they are interpreted to be deposited contemporaneously with or shortly after this magmatism migmatization of the paragneiss at rimbo was followed by intrusion of leucogranite at 1846 3 ma even in the other sample areas to the north hedesunda tierp skutskar and forsmark metamorphism including migmatization is constrained to the 1 87 1 85 ga interval and penetrative ductile deformation is limited by earlier studies in the forsmark area to 1 87 1 86 ga however apart from a metamorphic monazite age of 1863 1 ma precise ages were not possible to obtain due to the presence of only partially reset recrystallized domains in zircon or highly discordant u rich metamict and altered metamorphic rims migmatization was contemporaneous with magmatic activity at 1 87 1 84 ga in the bergslagen lithotectonic unit involving a mantle derived component and there is a spatial connection between migmatization and this magmatic phase in the hedesunda tierp sample area the close spatial and temporal interplay between ductile deformation magmatism and migmatization the p t metamorphic conditions and the continuation of similar magmatic activity around and after 1 8 ga support solely accretionary rather than combined accretionary and collisional orogenic processes as an explanation for the metamorphism the generally lower metamorphic grade and restricted influence of the younger metamorphic episode at least at the ground surface level distinguishes the central part of the bergslagen lithotectonic unit from the migmatite belts further north and south

contrasting source components of the paleoproterozoic svecofennian metasediments detrital zircon u pb sm nd and geochemical data abstract the paleoproterozoic svecofennian orogen in the fennoscandian shield can be regarded as one entity or as a collage of several accretionary units to test for the possible occurrence of a suture zone dividing the svecofennian orogen into central and southern parts we report new ion microprobe data on clastic zircons of nine samples of metasediments whole rock sm nd and geochemical data were acquired to characterize the source components of the metasediments the central svecofennian metasediments in finland can be divided to lower and upper types with inferred deposition ages at 1 92 1 91 and 1 90 1 88 ga respectively both types are immature and show a derivation from an unweathered orogenic source the lower central svecofennian metasediments show a dominant proterozoic age population at 1 93 2 02 ga and a variation in end 1 9 values from about 3 to 0 5 which is mainly explained by variable amounts of archean detritus 30 45 the upper central svecofennian metasediments have end 1 9 at about 0 similar to values obtained from adjacent volcanic and plutonic arc rocks one meta arkose is tentatively classified as a molasse type sediment with a source dominated by earlier svecofennian sediments southern svecofennian metasediments can be divided into mature and less mature types with dominant deposition ages at 1 90 1 88 ga and end 1 9 values from 3 to 1 mature quartzites seem to occur at least in two stratigraphic levels where the upper quartzites have maximum deposition ages at 1 87 1 86 ga a characteristic feature of some southern svecofennian metasediments is a dominant proterozoic age population at 2 0 2 1 ga coincident deposition of mature and immature sediments and volcaniclastic rocks indicate that different sources weathered and non weathered basement and syn depositional volcanic centers produced simultaneously detritus to a large subsiding basin mature pelites and quartzites suggest a long residence time and stable passive margin or cratonic environment the tectonic setting and possible provenance areas for the central and southern svecofennian metasediments are also discussed as a conclusion the metasediments from the central and southern parts of the svecofennian orogen show different lithological characteristics and have contrasting source components the presented isotope data indicate a different evolution of these parts prior to 1 89 ga and the existence of a suture zone between them as proposed in earlier studies

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

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

preservation of palaeoproterozoic early svecofennian structures in the orijarvi area sw finland evidence for polyphase strain partitioning abstract the predominantly migmatitic palaeoproterozoic uusimaa belt preserves early lower grade svecofennian structures in the orijarvi area in sw finland this study aims at explaining the deformational history responsible for its preservation and also at defining the age of the early svecofennian deformation detailed structural analysis reveals that the preservation was enabled by polyphase strain partitioning which initiated during the early svecofennian d2 deformation 1875 ma ago as revealed by ion microprobe u pb data on zircons from granodioritic and intermediate syn d2 intrusive dykes the d2 structures were low strain upright folds at high crustal levels and sub horizontal high strain folds at deeper crustal levels the sub horizontal d2 structures were refolded into upright folds during the subsequent late svecofennian d3 deformation whereas the upright d2 structures behaved as almost rigid blocks that caused strain partitioning into high strain zones along the block margins this accounts for the low cumulative strain in specific parts of the orijarvi area further strain partitioning during d4 caused reverse dip slip movements along regional scale shear zones crustal depth controlled the metamorphic grade during d2 when local migmatisation took place at deep crustal levels later metamorphic overprint during d3 deformation is evident from post d2 growth of sillimanite and a second generation of andalusite similarities in the structural patterns between the orijarvi area and the tampere vammala area 100 km to the north suggest that irrespective of the age of the later overprint subsequent deformation was localised along the margins of the early formed upright domains while the low grade rocks within the domains were preserved

integrated geophysical modelling of a lateral transition zone in the lithospheric mantle under norway and sweden summary the scandinavian mountain chain the scandes exhibits characteristics that are unusual for an old palaeozoic mountain belt these include renewed neogene uplift in a passive margin setting and the lack of a pronounced crustal root we investigate the influence of present day thermal compositional and geometric structures in the crust and mantle on the topography of the southern scandes and the resulting implications for the tectonic history of fennoscandia a self consistent 3 d subsurface model of southern norway and sweden is constructed from recent geophysical data sets that constrain the crustal architecture lithosphere geometry density distributions and thermal properties recent seismological studies show evidence for a relatively abrupt transition from seismically slower lithospheric mantle underneath southern norway to seismically faster lithospheric mantle underneath southern sweden we find that a transition from thin subcontinental lithospheric mantle sclm underneath southern norway to a significantly thicker sclm beneath southern sweden can explain this velocitytrend adifferenceincompositionbetweenthetwosclmdomains owingtodifferent depletion refertilization is required to satisfy the gravity field and isostatically compensated topography but contributes only slightly to the observed velocity contrast such a lateral transition zone constituting a major change in the lithospheric structure in southwestern fennoscandia can be related to several major proterozoic and phanerozoic tectonic events such as the sveconorwegian orogeny the permian oslo rift and perhaps even the opening of the atlantic

evidence for early proterozoic plate tectonics from seismic reflection profiles in the baltic shield plate tectonics provides the linking framework for all tectonic and magmatic activity seen today but it is not known when plate tectonics first developed on earth new deep seismic reflection and

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

structural evolution of the accretional continental margin of the paleoproterozoic svecofennian orogen in southern sweden abstract volcanic and sedimentary rocks of the vastervik and valdemarsvik groups along the southern margin of the 1 90 1 85 ga south svecofennian crustal province in sweden have been migmatised and intruded by granitoids and gabbros during extension deformation phase d1 at a depth of at least 10 km peak metamorphic conditions were reached around 1825 ma extension switched to compression before cooling the partially molten middle crust facilitated shortening and thickening by isoclinal and sheath folds on scales from a decimetre to 10 km d2 a pervasive mylonitic s2 foliation developed these structures were refolded into eastward plunging crustal scale upright folds with e se w nw trending axial planes d3 d2 d3 deformation structures are colinear and developed during top to the wnw slip along s2 locally zones of high ductile strain in garnet bearing migmatite assemblages record considerable vertical displacement from 850 to 300 mpa possibly in nappe roots some major d2 sheath folds are embraced by d3 like but nnw trending megafolds d4 apparently resulting from shortening around the former d2 d4 deformations are interpreted to be broadly coeval and related to transpressive crustal shortening against the northern svecofennian continental back stop after cooling to high greenschist facies conditions early structures underwent renewed transpression in north south direction expressed in localized retrogressive shear zones phase d5 much d5 strain was accommodated by the 10 km wide nw se striking right lateral loftahammar linkoping deformation zone lldz operative around 1800 1780 ma regional strain partitioning caused transverse shortening in 15 20 km wide border zones of the lldz later overprints d6 involve narrow subparallel nw se striking and steeply ne dipping low greenschist facies mylonite zones some of which merge into the regional polyphase asbro norrkoping deformation zone d6 was transpressive oblique left lateral ne block up slip suggests a w sw e ne orientation of 1 peak metamorphism and partial melting in the supracrustal rocks was approximately coeval with 1800 1835 ma old subduction related magmatism in the oskarshamn jonkoping belt ojb 100 km to the south west seismic reflection profiling off the baltic coast babel line b suggests northward polarity of subduction beneath the ojb and a back arc environment of the vastervik and valdemarsvik groups between the ojb arc and the margin of the 1850 ma old svecofennian orogen we suggest that d1 d5 deformation was caused by intracontinental back arc extension and subsequent closure of the back arc basin by oblique accretion of the ojb marginal arc onto the northern continent accretion involved northward indentation by mid crustal wedges one of which coincides with and possibly caused the nucleation of the d5 stage lldz 40ar 39ar data from hornblende and white mica identify a prolonged cooling history of the area 1810 1490 ma influenced by the nearby late and postorogenic transscandinavian igneous belt and possibly by largely hidden c 1530 ma old anorogenic rapakivi type intrusions

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

high resolution body wave tomography beneath the svekalapko array i a priori three dimensional crustal model and associated traveltime effects on teleseismic wave fronts summary assessment of contributions from shallow lithosphere to teleseismic wave front distortion is a prerequisite for high resolution regional teleseismic tomography several methods have been proposed in the past for the correction of these effects e g application of station correction terms we propose an approach that is independent of the subsequent inversion and uses the available a priori knowledge of the crustal structure to calculate crustal traveltime effects of teleseismic wave fronts our approach involves the construction of a 3 d crustal model based on controlled source seismology data and calculation of the associated traveltime anomalies for incoming teleseismic wave fronts the model for central fennoscandia shows a maximum crustal thickness of 64 km and includes a high velocity lower crust as derived for parts of the study area by previous authors traveltimes calculated using finite differences for teleseismic waves travelling through this crustal model are compared with those from the standard reference model iasp91 and the residuals are used to correct observed teleseismic arrival times at the svekalapko array to test the performance of this approach in a second part of the study a synthetic traveltime data set is obtained by tracing wave fronts through a mantle structure with known velocity anomalies and the 3 d crustal model this data set is inverted with and without correction for crustal effects the 3 d crustal effects alone with a homogeneous mantle are also inverted and the results show that the crustal effects propagate down to 450 km the comparison of the inversion results demonstrates the need to apply appropriate 3 d crustal corrections in high resolution regional tomography for upper mantle structure beneath the baltic shield

the effect of mineralogy and effective normal stress on frictional strength of sheet silicates abstract phyllosilicates are common constituents of upper crustal faults and subduction forearcs we studied the effect of mineralogy and controlled effective normal stress between 5 mpa and 100 mpa on frictional strength although the effect of mineralogy on frictional strength of single phase phyllosilicate gouges has been previously studied the influence of effective normal stress has not we conducted water saturated and vacuum dry frictional tests on single phase phyllosilicate gouges using a triaxial apparatus minerals included talc pyrophyllite kaolinite lizardite illite montmorillonite chlorite muscovite phlogopite and biotite particle size 20 mpa dry friction coefficients are constant and wet friction coefficients show a small increase this is attributed to loss of water and increased contact area the results indicate that frictional strength of clay rich faults increases at depths less than 1 km under hydrostatic pore fluid pressures

geological geochemical and geochronological evidence for a new palaeoproterozoic terrane in southeastern sweden the geological and geochronological relationships between a rhyolite belonging to the transscandinavian igneous belt tib and an intrusive suite the backaby intrusion southeast sweden have been investigated by means of bedrock mapping geochemistry and radiometric age determinations u pb on zircons field relations suggest that the rhyolite rests unconformably on the backaby intrusion which previously has been correlated with 1 9 ga granitoids within the svecofennian domain the rhyolite yielded an age of 1800 8 ma and a tonalite belonging to the backaby intrusion basement yielded an age of 1834 3 ma the age of the rhyolite is slightly older 10 15 ma than adjacent granitoids belonging to the transscandinavian igneous belt and is considered as the earliest magmatic expression of tib in the area the age and the geochemical trends for the backaby intrusion is significantly different from that of the surrounding tib granitoids and indicate a rock forming event at about 1830 1840 ma in southeastern sweden represented by the oskarshamn jonkoping belt this event is significantly younger than the older svecofennian rocks within the svecofennian domain proper indicating a hitherto unidentified geological terrane in southeastern sweden

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

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

shrimp data on zircons from the wanzi series constraints on the rock formation time and implications of migmatization at 2 1 2 0 ga in the fuping complex north china craton in most of the rocks of the fuping complex in the trans north china orogen a large quantity of leucocratic veins dykes and granitic intrusions are present as migmatites the incongruent partial melting of biotite to hornblende suggests the water fluxed anatexis of the migmatization of which the melt is manifested by the euhedral crystallization of some quartzofeldspathic minerals and presence of albite rim on earlier plagioclase or k feldspar importantly some melt may be segregated in the sillimanite gneisses or extracted and injected as faint dispersed melt into the competent metasedimentary quartzite of the wanzi series rocks the quartzite has plenty of zircon ages cluster at c 2 5 ga and some at c 2 1 2 0 ga the former may be detrital zircon age while the latter represents the age of migmatization or crystallization of the injected melt not detrital zircon ages as conventionally believed the data have constrained that the sedimentation time of the protolith of the quartzite of the wanzi series was between c 2 5 ga and c 2 1 2 0 ga not younger than c 2 1 2 0 ga as once proposed therefore it must be cautious in applying detrital zircon ages in constraining the time of sedimentation if the rocks have undergone middle high grade metamorphism especially when migmatization was involved in the metamorphism process

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

age of deformation episodes in the palaeoproterozoic domain of northern sweden and evidence for a pre 1 9 ga crustal layer it is proposed in a companion paper that the volcanic rocks of the skellefte group in northern sweden were deposited on a pre 1 9 ga basement rather than formed as a juvenile volcanic arc and subsequently accreted to the continent it is suggested that the proposed basement the robertsfors group to the south of skelleftea was already deformed during an early episode of deformation d1 prior to the deposition of the skellefte stratigraphic sequence during an extensional tectonic episode a younger episode of deformation d2 then affected both areas this hypothesis has important implications for the evolution of the svecofennian province to test it by constraining the ages of the principal deformation episodes we have undertaken both isotope dilution and ion microprobe studies of zircon and monazite from three localities in the robertsfors group a phase of migmatisation directly related to d2 shear zones allows the dating of d2 at 1860 ma other planar granitoid bodies which cut the d1 structures confirm the age of this event in the context of earlier published data we conclude that the correlation of d2 in the robertsfors group with the main folding in the skellefte group is valid another granitoid body folded by d2 but emplaced after d1 contains a suite of zircons of apparent magmatic aspect but with a spread of ages from 1870 to 1900 ma because of the high mswd the suite was divided into two groups with ages of 1896 and 1874 ma recognising the possibilities of inheritance and of updating by younger events we have preferred to interpret the younger age as a minimum age for magmatic intrusion this is consistent with the basement hypothesis but is our only definite constraint on the age of d1 however we infer using other evidence that d1 is probably older than 1900 ma xenocrysts interpreted as detrital zircons derived from the associated metasediments give ages between 2716 and 1941 20 ma the latter age provides a maximum age of deposition for part of the robertsfors group and a maximum age for d1 other sequences to the south and west considered broadly equivalent to the robertsfors group are intruded by granitoids dated between 1920 and 1960 ma providing local minimum ages of deposition for the sequences involved regional correlation with respect to age and deformation style can be made with the pre 1 9 ga kalevian rocks to the ne deposited on archaean basement on the sw margin of the karelian province we suggest that pre 1 9 ga complexes equivalent to the robertsfors group are widespread in the svecofennian province and were deposited in a marginal basin which may have been initiated as early as 2 2 ga ago this hypothesis of an extensive basement crustal layer evidently requires that a substantial volume of crustal growth occurred prior to 1 9 ga in contrast to previous hypotheses erosion of the pre 1 9 ga basin rocks after d1 is also the likely source of the 2 1 1 9 ga detrital zircons in younger sediments associated with the overlying post 1 9 ga volcanics the source of these detrital zircons has previously been problematical

troodos extrusive series of cyprus a comparison with oceanic crust the extrusive section on the northern flank of the troodos ophiolite is 1 000 to 1 500 m thick and consists of about 12 lithologic units of pillowed and massive lava and breccia some pillowed units which average 100 to 200 m in thickness vary systematically from lava tubes up to 6 m in diameter at the base through common pillows less than 1 m in diameter to small pillows and extrusive breccias at the top these assemblages appear to be individual volcanoes suggesting episodic volcanic accretion the upper 100 to 200 m of the section is pervasively altered and oxidized but fresh volcanic glass is preserved throughout most of the extrusive section the lavas are subdivided on geochemical evidence into a lower differentiated andesite to dacitic andesite series resembling evolved island arc tholeiites and an upper series of picrite mafic basalt and basaltic andesite very low in tio2 k2o p2o5 nb rb and ba the highly depleted basalt and basaltic andesite resemble lavas from ocean crust near island arcs lateral continuity of lithologic units interpreted as individual volcanoes and the relatively constant dips of the extrusive rocks suggest that tilting and rotation have not played major roles in the accretion process the occurrence of fractionated lavas at the base and primitive lavas at the top suggests a broad zone of accretion located above a subduction zone

general geology and evolutionary history of the early proterozoic polmak pasvik pechenga imandra varzuga ust ponoy greenstone belt in the northeastern baltic shield abstract the early proterozoic polmak pasvik pechenga imandra varzuga ust ponoy greenstone belt is situated in the northeastern part of the baltic shield and is discontinuously developed over a length of ca 1000 km it comprises five main zones which are separated by longitudinal and a number of transverse syndepositional faults each zone contains a number of lithostratigraphic units at the rank of groups that are subdivided into several sedimentary and volcanic formations with a total thickness over 16 000 m the sequence comprises a series of sedimentary volcanic rhythms usually separated by stratigraphic breaks marked by palaeo weathering the groups are separated by major disconformities which may mark record rift inversion or orogenic episodes at least seven distinctive lithostratigraphic markers occur in the belt including two komatiitic volcanic series separated by immature conglomerates two red coloured proto evaporitic formations separated by the alkaline volcanics and two different black shale formations perhaps the main characteristic of the belt is the apparently long history of sedimentary volcanic development at the same site spanning a time period of 2500 to 1800 ma a three stage rift development model is considered 1 an intracontinental rift stage 2500 2100 ma separated by the major inversion 2400 2330 ma and comparable with present day afar triangle and east african rifts 2 a transitional from intracontinental to to intercontinental rift stage 2100 1970 ma with possible short lived spreading about 1990 1970 ma 3 a collision related intercontinental rift stage 1970 1800 ma followed by the svecofenian orogeny 1800 1700 ma the observed distribution of platinum group elements pge mineralization is related to layered gabbro norite complexes fedorova and panskie tundry gora general skaya intrusions dated at 2470 ma which are apparently co magmatic and coeval with the first komatiitic volcanic series seidorechka formation found in the eastern part of the belt only the major chromite occurences are connected with a layered gabbro norite intrusion imandra lopoloth which is co magmatic and coeval between 2440 2330 ma with the second komatiitic volcanic series rusinga polisarka akhmalahti and malbekk formations the ni cu deposits of the belt are connected with a differentiated gabbro werhlite complex which is co magmatic and coeval ca 1970 ma with the specific fe and ti rich ferropicrites with komatiitic affinities

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

structure of the crust and uppermost mantle beneath southern finland revealed by analysis of local events registered by the svekalapko seismic array abstract in 1998 1999 a large scale seismic array was deployed in finland as a part of the europrobe svekalapko subproject involving 14 european universities and research institutes the objective of the project was to map the deep lithosphere structure and thickness beneath the fennoscandian shield by means of teleseismic events in addition about 580 local seismic events were registered during the data acquisition period among them only eight local earthquakes were recorded the rest being quarry blasts from mining sites in russia finland estonia and sweden in this study we present the analysis of the seismic wave field from the strongest local events registered by the majority of the stations of the svecofennian karelian lapland kola transect svekalapko array with the aim of mapping the structure of the upper mantle beneath the array for this purpose we selected the events corresponding to a single source type and compared these recordings with those from wide angle reflection and refraction experiments in the area to identify the regional phases the record sections of selected events demonstrate strong reflections pmp from the moho boundary the refracted pn phases can be seen as first arrivals at distances of about 200 400 km from the source at offsets of about 400 800 km phases reflected from inhomogeneities in the uppermost mantle p1 and double reflections from the moho boundary pmppmp were recorded results from 2d forward ray trace modeling of reflected and refracted p waves along four profile swathes from svekalapko stations demonstrate that the mantle reflections originate from two different groups of boundaries beneath the array one group of phases arrive from subhorizontal and gently dipping reflectors below the moho boundary at a depth of 70 90 km while the other group are phases originating from a depth of 100 to 130 km based on the irregular character of the first group of reflections their different spatial orientation and correlation with the moho offsets we interpret the boundaries of this group as relicts of ancient subduction and collision processes the second group of reflections can be explained by a transition from mechanically strong to mechanically weak lithosphere

reactive monazite and robust zircon growth in diatexites and leucogranites from a hot slowly cooled orogen implications for the palaeoproterozoic tectonic evolution of the central fennoscandian shield sweden monazite in melt producing poly metamorphic terranes can grow dissolve or reprecipitate at different stages during orogenic evolution particularly in hot slowly cooling orogens such as the svecofennian owing to the high heat flow in such orogens small variations in pressure temperature or deformation intensity may promote a mineral reaction monazite in diatexites and leucogranites from two svecofennian domains yields older coeval and younger u pb sims and emp ages than zircon from the same rock as zircon precipitated during the melt bearing stage its u pb ages reflect the timing of peak metamorphism which is associated with partial melting and leucogranite formation in one of the domains the granite and diatexite belt zircon ages range between 1 87 and 1 86 ga whereas monazite yields two distinct double peaks at 1 87 1 86 and 1 82 1 80 ga the younger double peak is related to monazite growth or reprecipitation during subsolidus conditions associated with deformation along late orogenic shear zones magmatic monazite in leucogranite records systematic variations in composition and age during growth that can be directly linked to th u ratios and preferential growth sites of zircon reflecting the transition from melt to melt crystallisation of the magma in the adjacent ljusdal domain peak metamorphism in amphibolite facies occurred at 1 83 1 82 ga as given by both zircon and monazite chronology pre partial melting 1 85 ga contact metamorphic monazite is preserved in spite of the high grade overprint by combining structural analysis petrography and monazite and zircon geochronology a metamorphic terrane boundary has been identified it is concluded that the boundary formed by crustal shortening accommodated by major thrusting

facies analysis of a 1 9 ga continental margin back arc felsic caldera province with diverse zn pb ag cu au sulfide and fe oxide deposits bergslagen region sweden the bergslagen mining district in south central sweden has been a major metal producer for more than 1 000 years in geologic terms bergslagen is the intensely mineralized part of a large early proterozoic mainly 1 90 1 87 ga felsic magmatic region of mainly medium to high metamorphic grade in the baltic shield the district contains a diverse range of ore deposit types including banded iron formation magnetitecalc silicate skarn manganiferous skarn and carbonate hosted iron ore apatite bearing iron ore stratiform and strata bound zn pb ag cu au sulfide ores and w skarn most ore deposits occur in hydrothermally altered metavolcanic rocks and associated metalimestones and skarns this study is a field oriented regional facies analysis based on recognition of primary volcanic and sedimentary features through the strong overprints of hydrothermal alteration deformation and metamorphism the region is interpreted as an extensional probably back arc active continental margin magmatic region it evolved through stages of intense magmatism thermal doming and crustal extension followed by waning extension waning volcanism thermal subsidence reversal from extension to compressional deformation regional metamorphism and structural inversion the volcanic successions comprise interfingering proximal near vent medial volcano flanks and distal volcano margin facies associations of rhyolitic pyroclastic caldera volcanoes and subordinate dacite rhyolite complexes nonwelded to poorly welded pyroclastic flow and fallout units and their rapidly resedimented subaerial and subaqueous equivalents are the most abundant volcanic facies subvolcanic porphyritic intrusions and cryptodomes are also abundant the pyroclastic debris was shed from mainly shallow water and subaerial proximal areas to shallow and moderately deep water above and below storm wave base distal areas most ore deposits formed during the regional waning volcanic stage and occur in medial to distal facies associations that comprise rhyolitic ash siltstone limestone and vitric crystal sandstone and breccia these mineralized facies associations were deposited in subaqueous environments mainly below the fair weather wave base but locally above the storm wave base water depths are interpreted to have been mainly 10 to 500 m with local deeper areas although the ore deposits occur in medial to distal facies associations many occur above or laterally adjacent to subsided volcanic vent complexes this suggests a spatial temporal and possible genetic relationship between several ore deposit types and the late magmatic stage of these volcanic centers except for the apatite bearing iron deposits it is uncertain if the ores are genetically related to specific magmatic hydrothermal events from specific volcanoes this is possible especially for the base metal sulfide deposits which have intense localized footwall alteration in felsic volcanic rocks however an important role of the volcanic centers may have been to provide strong positive pertubations or magmatic hydrothermal input to the regional convective geothermal system the main zn pb ag cu au ore deposits span a range between two end member types stratiform ash siltstone hosted sea floor deposits that have similarities to some stratiform sediment hosted zn pb ag deposits especially the broken hill type deposits and strata bound volcanic associated limestone skarn subsea floor replacement deposits that more closely resemble felsic volcanic associated massive sulfide deposits facies models are presented for both the banded iron formations are interpreted as chemical suspension deposits rhythmically interbedded with rhyolitic ash siltstone the skarn iron ores are strata bound in limestones and include metamorphosed sedimentary ores and metasomatic replacements apatite bearing iron ores differ in setting from the other ores in being formed within a dacitic intrusive complex that may be the root zone of a dacite rhyolite volcanic center

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

proterozoic sutures and terranes in the southeastern baltic shield interpreted from babel deep seismic data abstract a hitherto unknown terrane and its bounding sutures have been revealed by a combined study of normal incidence and wide angle seismic data along the babel profile in the baltic sea this intermediate terrane is situated between a northern terrane of svecofennian age and a southwestern terrane of gothian age it is delimited upwards by two low angle and oppositely dipping sutures and occupies mainly middle and lower crustal levels with a width of 300 km at moho level the 1 86 ga suture against the northern terrane is imaged by a prominent almost continuous ne dipping crustal reflection from 3 5 to 14 s twt over 175 km where it downlaps on the moho sub moho velocities change from 8 2 to 7 8 km s 0 2 over less than 25 km a relatively strong ne dipping normal incidence and wide angle reflection at 19 23 s twt indicates that the suture extends into the upper mantle the pervasive ne dipping reflection fabric of the intermediate terrane is interpreted as shear zones that developed during collision and possibly were reactivated by later events high poisson s ratios suggest a mafic composition or high fluid content the 1 86 ga collision was probably succeeded by continental break up and removal of an unknown continent except for the intermediate terrane subsequent formation of an east dipping subduction zone further to the west led to the emplacement of 1 81 1 77 ga old granitoids in the southern part of the transscandinavian igneous belt the 1 65 1 60 ga suture against the southwestern terrane is defined by a semi continuous band of strong sw dipping reflections between 3 and 8 s twt over 65 km which are interpreted as a low angle thrust zone along which gothian crust overrode the intermediate terrane the identification of three individual seismic terranes in the southeastern part of the baltic shield provides new evidence for palaeoproterozoic plate tectonic processes