how does orogenic crust deform evidence of crustal scale competent behaviour within the partially molten middle crust during orogenic compression abstract granitic partial melts are generally thought to significantly weaken the orogenic crust and ultimately lead to the collapse of an orogen studies from different orogens have shown that the syn melt deformation behaviour at the orogenic scale is however more complex in addition once fully crystalline granitic material strengthens the crust linking the evolution from melt present to melt absent deformation at the scale of the orogen is a challenging but necessary task if one wishes to investigate the overall behaviour of the middle crust in this paper we make observations of orogen scale deformation developed over a period of 30 40 ma during crustal partial melting of the middle and lower crust of a palaeoproterozoic orogen the crust shows a globally common pattern where coeval partial melting compressional deformation and transtensional structures co exist we demonstrate this complex interaction through an integrated approach using multiple datasets the key observation is that of widespread regular orogen scale shortening by folding of sub horizontal anatectic granitic sheets and highly migmatized lithologies this shortening interacted with modest orogen parallel lateral stretch expressed as local extensional transtensional structures the pervasive and dominant deformation style is that of folding of the syn kinematic granite sheets and voluminous migmatites this demonstrates that the principal deformation style of deformation within the partially molten mid crust is competent rather than weak the observed evidence of weak behaviour such as strike slip or transtensional shear zones accommodating lateral escape are localized in comparison and not necessarily associated directly with the granite sheets or the highly migmatized volumes of the crust moreover the strain intensity fold wavelengths and amplitudes seems independent of the overall melt fraction the implications are that while widespread volumes of partial melt will weaken the crust overall compared to the brittle upper crust and while individual melt bodies can possibly persist for up to some millions of years in the middle crust i the relative strength between partially molten volumes is not primarily controlled by melt fraction and ii individual melt volumes may be too short lived at the time scale of orogenic deformation i e orogenic strain rates to significantly influence the overall deformation style

seismic and geoelectric evidence for collisional and extensional events in the fennoscandian shield implications for precambrian crustal evolution extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles normal incidence reflection profiles several hundred magnetotelluric soundings and several magnetovariational array observations results are compiled into four maps 1 moho depth 2 thickness of the lower crustal high velocity layer vp 7 km s 3 depth to the upper interface of the high velocity layer and 4 distribution of upper and middle crustal electrical conductors the moho depth is approximately 40 km in the archaean domain and varies between 40 and 65 km in the svecofennian domain most of the thickness variation 0 24 km is concentrated in the lower crustal high velocity layer electrically the crust is formed of several rather homogeneous either resistive or conductive blocks that are bounded by upper and middle crustal conductive zones the cratonized archaean crust experienced an extensional phase during which a thin mafic lower crustal high velocity layer was produced in association with mafic dykes 2 2 2 1 ga and well conducting volcanogenic and metasedimentary belts 2 5 2 0 ga the moho of the archaean domain was consequently reformed in the early proterozoic major thickening of the crust 50 65 km occurred when a svecofennian island arc and the archaean craton collided the svecofennian orogeny 1 76 1 9 ga resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc the internal terrane boundaries are seen as good reflectors and or inclined or vertical conductors that extend at least to the middle crust the thick high velocity 7 km s lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of late svecofennian subduction the collisional boundary is also preserved as a discontinuous conductive zone between the archaean and svecofennian domains the thickened crust was subsequently thinned to 42 km along e w striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths coeval mafic dykes and gabbro anorthosite bodies 1 6 1 5 ga the crust of the central part of the shield has since remained virtually intact

late orogenic svecofennian deformation in sw finland constrained by pegmatite emplacement ages late stages of the svecofennian orogeny in sw finland were related to a tranpressional stress field and dextral movements along crustal scale shear zones under decreasing temperature and pressure conditions in the kemio area a minimum estimate for the time span of movements along one of these shear zones is obtained from the ages of 1840 to 1830 myr old microcline granite sheets related to early ductile deformation and by dating structurally late rare mineral granite pegmatites related to brittle deformation one pegmatite was emplaced when the rheological conditions in the gabbro changed from ductile to semi ductile and brittle it has u pb ferrotapiolite ages ranging between 1807 0 2 9 myr 2 s and 1803 1 2 9 2 0 myr 2 s another pegmatite emplaced under brittle conditions has an u pb ferrotapiolite age of 1802 9 1 3 myr 2 s these pegmatites were emplaced preferentially in gabbroic rocks that showed brittle to semi ductile deformation at a time when their more felsic host rocks still showed ductile deformation the age range bracketed by the microcline granite sheets and the structurally late pegmatites indicates that ductile deformation related to the transpressional late svecofennian tectonic regime in southwestern finland persisted for at least 30 40 myr

the early proterozoic orijarvi triangle southwest finland a key area on the tectonic evolution of the svecofennides abstract a consistent pattern of three successive deformation phases is present in the west uusimaa area in the south svecofennian schist belt of finland in which the orijarvi triangle bordered by shear zones forms a low strain area it is shown that the well preserved early folds and foliation in the orijarvi triangle correlate with the regionally oldest and completely transposed structures a model is given for the tectonic development of the west uusimaa area involving 1 an early phase of subhorizontal probably thrust related folding 2 a second phase of crustal shortening and 3 a late phase of folding and formation of vertical shear zones the first two phases took place during prograde metamorphism whereas the last one records retrogression shear zones were probably active during all phases subhorizontally thrusting during d 1 transversely wrenching during d 2 and vertically uplift during d 3 it is suggested that the d 1 d 2 sequence represents a progressive deformation in a plate collision system whereas d 3 is related to a 1533 23 ma old post svecofennian possibly gothian event a model is presented in which the development and preservation of the orijarvi triangle is explained by the local development of transversal shear zones along anatectic plutons during d 2 these shear zones were then reactivated during d 3 the model presented implies that the sigmoidal pattern present in southern finnish geology was caused during a late stage of the d 2 crustal shortening event along three broad parallel sinistral shear zones it is suggested that these shear zones represent deep seated parts of wrench faults in a complex plate tectonic system involving several island arcs and that they were caused by differential movement of the south svecofennian schist belt with respect to the tampere schist belt

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

application of paleomagnetism to accretionary tectonics and structural geology the use of paleomagnetism to determine translations and rotations of geologic terranes has contributed to a better understanding of deformation within accretionary complexes shear zones and rifts in this report to the i u g g we review the progress resulting from paleomagnetic studies during the last four years in the cordillera of north america the appalachian orogen and the circum pacific region

138la β decay constant estimated from geochronological studies abstract la ce ages are reported for two sets of finnish pegmatites lovbole and mustikkamaki and for an ami tsoq gneiss greenland when 138 la value 2 29 10 12 yr 1 obtained by radioactivity measurement 1 is used for the chronological calculation the la ce ages 2129 2325 3271 myr evaluated for these rocks are 18 35 older than the sm nd ages for the same samples to make the la ce age fit to the sm nd age for the same sample a new value of 2 77 0 21 10 12 yr 1 is evaluated for 138 la in this calculation the la ce and sm nd ages reported for a bushveld gabbro 2 have been also taken into account together with those for the lovbole pegmatite and the mustikkamaki pegmatite while the ami tsoq gneiss ggu110999 has been omitted because of the complicated thermal history of this sample

new evidence for block and thrust sheet rotations in the central northern calcareous alps deduced from two pervasive remagnetization events 1 we present 81 paleomagnetic sites early triassic to early cretaceous from the central sector of the northern calcareous alps nca eastern alps austria and germany stepwise thermal demagnetization defines three magnetic directions mostly carried by low unblocking temperature and low coercivity minerals j3 350 j2 500 j1 575 and 680 j3 and j2 show positive inclinations whereas j1 very seldom is of dual polarity the fold tests show that a j3 can be interpreted as a postfolding and posttilting remagnetization and j2 as a postfolding and pre syn tilting j1 can be considered as primary because of the occurrence of two polarities and evidence presented by other authors in the area all three components show a systematic and significant clockwise rotation after comparing with the expected european references j2 or j1 are marked by higher rotation values than j3 j2 shows different inclinations depending on the structural position north or southward dips considering the structural evolution and the observed inclinations the first postfolding and pretilting remagnetization event j2 could have taken place between late cretaceous and eocene times but certainly before the thrusting of the nca over the rhenodanubian flysch and northward tilting caused by the stacking of the lower austroalpine nappes the second postfolding and posttilting remagnetization j3 would have been acquired after the final thrusting of the nca over penninic units from then the nca behaved as a set of rigid blocks recording the main stage of vertical axis clockwise rotation 65 in average associated with the continental collision the variable degree of rotation in the different positions from 40 to 134 can be explained by individual vertical axis rotation in a block system trying to adjust to space problems the constant declination differences between j2 and j3 25 in average would reflect the rotation related with the lateral differences of shortening during the oblique thrust of the austroalpine units over the penninic units

prograde metamorphic zoning in amphibolite facies pelitic gneisses from kemio southwest finland the metamorphism of the svecofennides in southwestern finland has been studied in pelitic gneisses from the kemio region three metamorphic zones have been distinguished which are characterized in order of increasing grade by the parageneses muscovite quartz biotite sillimanite k feldspar and cordierite k feldspar temperature and load pressure estimates of metamorphism range from 560 25 c 3 0 5 kb for the upper boundary of the muscovite quartz zone to 670 25 c 4 0 5 kb for the cordierite k feldspar zone at a partial water pressure of 0 4 0 1 ptotal

magnetic fabrics and paleomagnetism of the storsjon edsbyn deformation zone central sweden abstract a regional deformation zone separating tectonomagmatic blocks in the central part of the fennoscandian shield has been studied using anisotropy of magnetic susceptibility and paleomagnetism in order to define the tectonic character and age of the zone and to put the deformation into a plate tectonic context the deformation zone the storsjon edsbyn deformation zone sedz separates the ratan granite of the trans scandinavian igneous belt tib and the southern svecofennian subprovince in the southwest from older svecofennian granitoids of the central svecofennian subprovince in the northeast the sedz is the most prominent deformation zone in a regional pattern of nw se trending shear zones on basis of ams data the width of the zone which cuts through 1 7 1 9 ga old rocks is estimated at 10 12 km in its central southern part and 20 25 km in its northern part the orientations of the susceptibility axes vary systematically across the zone in a way that suggests that the sedz is a pure shear dominated transpression zone caused by a compressive stress from the present ssw paleomagnetic data from a dolerite sill plat 10 plon 166 and from a dyke plat 25 plon 190 that cut the sedz indicate that no plastic deformation has occurred along the zone during the past 1 25 ga and probably not for the past 1 5 1 6 ga ams data do not point to any deformation of the 1 70 ga old ratan granite which further limits the time span of deformation the main phase of deformation most likely took place between 1 80 and 1 70 ga ago the geological evolution of this part of the fennoscandian shield at the time between 1 80 and 1 70 ga ago is very complex and in part still enigmatic this time period involves magmatism of the tib which is related to eastward subduction it also comprises the beginning of the gothian orogeny and the development of a regional system of transpressive deformation zones the compressive stress field from the present ssw indicated in this study is interpreted to be related to an initial stage of plate collision between the southwestern scandinavian subprovince and older parts of the fennoscandian shield this collision resulted in a system of regional nw se trending intra plate deformation zones one of which is the sedz

basal drainage system response to increasing surface melt on the greenland ice sheet surface meltwater reaching the bed of the greenland ice sheet imparts a fundamental control on basal motion sliding speed depends on ice bed coupling dictated by the configuration and pressure of the hydrologic drainage system in situ observations in a four site transect containing 23 boreholes drilled to greenland s bed reveal basal water pressures unfavorable to water draining conduit development extending inland beneath deep ice this finding is supported by numerical analysis based on realistic ice sheet geometry slow meltback of ice walls limits conduit growth inhibiting their capacity to transport increased discharge key aspects of current conceptual models for greenland basal hydrology derived primarily from the study of mountain glaciers appear to be limited to a portion of the ablation zone near the ice sheet margin

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

u pb isotopic dating of columbite tantalite minerals development of reference materials and in situ applications by ion microprobe columbite tantalite group minerals are the most common nb ta minerals columbite tantalite is particularly suitable for u pb dating due to its high u and low common pb contents in situ isotopic dating of columbite tantalite by la icp ms or sims requires certified reference material to properly account for potential matrix effects linked to substitutions between nb and ta and between mn and fe our study has two objectives i establish a database of reference materials for in situ u pb isotopic dating of columbite tantalite minerals and ii test the capability of sims to in situ u pb date columbite tantalite minerals of different chemical composition tests of in situ u pb dating demonstrate that sims can easily be used to date columbite tantalite minerals with errors and precisions overlapping the reference id tims age there are however significant matrix effects for non matching nb ta fe mn compositions of sample and reference material matrix effects are highly correlated with the ta ta nb ratio of columbite tantalite due to the significant difference in the atomic mass of nb and ta the mn mn fe ratio does not significantly contribute to the observed matrix effect as the two elements have similar atomic masses the linear correlation between ta nb ta and 206pb 238u sims 206pb 238u id tims obtained for columbite tantalite minerals of known id tims age demonstrates that the sims matrix effect can be properly accounted for by using the chemical composition as determined by empa the ability to measure 204pb by sims also allows the use of reference materials with a small common lead contribution and to calculate accurate and precise ages for columbite tantalite minerals with contributions of common lead

biogenic magnetite as a primary remanence carrier in limestone deposits abstract studies on the microbial communities and magnetic phases of samples collected from carbonate oozes at sugarloaf key fl u s a and calcareous laminated sediments from laguna figueroa baja california mexico have revealed the existence of magnetotactic bacteria and ultrafine grained single domain magnetite in both environments magnetotactic bacteria were identified by light and electron microscopy the single domain magnetite was detected by coercivity spectra analysis with a squid magnetometer and examined under the transmission electron microscope the similarity in terms of size and shape between the single domain magnetite found in these sediments and the magnetite observed in the bacterial magnetosome from enriched cultures indicates the ultrafine grained magnetite in these two marine environments was biologically formed these results combined with the common occurrences of ultrafine grained magnetite in limestone deposits detected rock magnetically suggest biogenic magnetite may be present and contribute to the magnetic remanence in these rocks several cambrian limestone samples separately collected from siberia china and kazakhstan were examined for the presence of bacterial magnetite samples from the lower cambrian sinskian formation at siberia platform were found to contain both a large amount of apparently bacterial magnetite particles and a very stable primary magnetic component post cambrian diagenesis does not seem to affect the microgranulometry of these apparently bacterial magnetite crystals or the magnetic remanence carried by them assessing the potential role of biogenic magnetite as a primary remanence carrier in other phanerozoic limestone deposits ought to be further pursued

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

strain ellipsoids from incompetent dykes application to volume loss during mylonitization in the singo gneiss zone central sweden we describe how to constrain bulk strain ellipsoids by distinguishing the attitudes of mullioned from planar contacts in incompetent dykes our method is demonstrated by using metabasaltic dykes to

svecofennian crustal deformation of the baltic shield and upb age of late kinematic tonalitic intrusions in the burtrask shear zone northern sweden post collisional crustal deformation of the svecofennian region involved 1 lateral escape movements along crustal scale dominantly dextral shear zones that roughly correspond to boundaries between volcanic and sedimentary terranes and 2 crustal thickening eventually resulting in regional migmatite formation and granitic magmatism u pb dating of titanite from skarnified limestone in the contact metamorphic aureole of late kinematic tonalites in the burtrask shear zone one of these major dextral shear zones in northern sweden yields 1825 4 3 ma 2 this age corresponds to the end of ductile deformation along the burtrask shear zone but it also coincides with important migmatite formation and granite emplacement in the southern part of the svecofennian region the available geochronologic data from the svecofennian region illustrate that as migmatites and granites become regionally important in the interior of sediment dominated terranes movements along shear zones at the margin of terranes become subordinate and compressional deformation is concentrated into the interior of the terranes

palaeomagnetic and geochemical evaluation of a terrane linking ignimbrite evidence for the relative position of the grampian and midland valley terranes in late silurian time the proposed correlation of lower old red sandstone dacitic ignimbrites between the northern midland valley and grampian terranes of central scotland has been investigated in a combined palaeomagnetic and geochemical study statistically indistinguishable late silurian palaeolatitudes grampian terrane 27 0 s 3 5 northern midland valley terrane 22 9 s 1 8 comparable remagnetization histories a shared normal polarity for the primary magnetization and equivalent gyroremanence acquisition favour correlation across the highland boundary fault similar chemistry for biotite phenocrysts further corroborates correlation which was initially made on the basis of a similar field appearance and petrography this correlation together with evidence from provenance studies suggests that the separation of the northern midland valley and grampian terranes inferred for the ordovician period was largely removed by late silurian times with scope for relative displacements of the order of only tens of kilometres after lower old red sandstone deposition

ductile shear zones related to crustal shortening and domain boundary evolution in the central fennoscandian shield the paleoproterozoic part of the fennoscandian shield is composed of crustal components formed in different tectonic settings and generally separated by well defined shear

implications of upb ages of columbite tantalites from granitic pegmatites for the palaeoproterozoic accretion of 1 90 1 85 ga magmatic arcs to the baltic shield abstract the palaeoproterozoic growth of the baltic shield involved the accretion of several 1 90 1 85 ga old magmatic arcs to the southwest of the archaean craton and the deformation and migmatization of sedimentary basins located between the arcs during crustal thickening after the collision of the arcs the sedimentary basin fill became intruded by peraluminous two mica granites locally columbite bearing pegmatites are genetically associated with these granites columbites from lithium cesium tantalum type lct type pegmatites from the stockholm area sormland gneisses yield u pb ages at 1815 1820 ma while niobium yttrium fluorine type nyf type pegmatites from the same area are younger 1795 2 ma 2 farther to the north lct type pegmatites from the central bothnian basin that correspond geochemically and mineralogically to those of the stockholm area yield u pb columbite ages at 1795 1800 ma while lct type pegmatites in the sedimentary basin between skelleftea and lulea yield less well constrained u pb columbite ages at 1765 1775 ma lct type pegmatites are mainly associated with crustal melts that form during postcollisional thickening of continental crust they represent markers for the time when the palaeoproterozoic baltic shield suffered sufficient thickening to yield voluminous anatectic melts the u pb columbite ages from the lct type pegmatites indicate that comparable phases of post collisional crustal thickening of the svecofennian area of the baltic shield occurred later to the north

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

the thermotectonic evolution of a proterozoic low pressure granulite dome west uusimaa sw finland three fold generations have been recognized in svecofennian rocks 1 800 ma from west uusimaa sw finland the first one f1 might be related to thrusting and imbrication tectonics at plate collision contacts the main generation f2 is due to a n s horizontal crustal shortening which created at first e w trending upright folds in the whole region and later tightened these f2 folds in the western part of the belt whereas conjugate shear zones and tectonic lenses of competent rock bodies developed in the eastern part simultaneously the metamorphic conditions rose from amphibolite to granulite facies in this eastern part which is known as the west uusimaa complex the amphibolite to granulite facies transition zone along the western boundary of the granulite facies complex is studied in detail a number of prograde mineral reactions are telescoped in this transition zone the breakdown of biotite and amphibole to ortho clino pyroxene in metaigneous rocks the appearance of garnet in cordierite bearing metapelites and the appearance of scapolite in calcareous rocks distinct mineralogical changes also occur in this zone which cross cuts all major structures and rock units and are only affected by late f3 folding open disharmonic folds with approximately n s trending axial planes and young shear zones associated with pseudotachylite generation the absence of any evidence of block faulting and tilting of the crust that could be associated with the granulite complex suggests that the whole region represents one crustal level

u pb columbite tantalite chronology of rare element pegmatites using tims and laser ablation multi collector icp ms u pb isotopic analyses using tims and laser ablation multi collector icp ms were carried out on columbite tantalite minerals from three suites of rare element li cs ta pegmatites in the superior province of canada conventional tims analyses of these columbite tantalite crystals produce scattered data and reverse discordance even after hf leaching of the grains prior to dissolution possibly reflecting the incomplete removal of the open system metamict segments during sample preparation la mc icp ms analyses of unleached primary columbite free from inclusions and alteration give consistent and precise 0 5 pb pb ages demonstrating the utility of this approach however normal and reverse discordance is also observed in u pb data from la mc icp ms analyses this discordance represents either u pb mobilisation during recent weathering sample preparation and or an analytical artefact originating from variable elemental fractionation between u and pb during ablation and ionisation that itself may have its origin in the contrasting metamictization of the dated columbite and the monazite standard used best age estimates of columbite from pegmatites in the superior province are 2670 5 ma for the pakeagama lake pegmatite 2644 7 ma for the separation rapids group and 2665 8 ma for the mavis lake group the ages broadly show that the rare element pegmatites are temporally synchronous with adjacent peraluminous granites

melt extraction and accumulation from partially molten rocks current models for melt segregation and ascent are not adequate to accurately describe transport and accumulation in combination we propose that transport is discontinuous and in batches and that accumulation occurs by stepwise merging of batches a simple numerical model of jostling spheres that merge when they touch was used to represent stepwise accumulation and transport of batches by propagation of hydrofractures results of the numerical model indicate that such a system may quickly develop into a self organised critical soc state in this state the distribution of melt batch volumes can be described by a power law with an exponent m that lies between 2 3 and 1 once a self organised critical state is established the system is capable of discharging any additional melt without further change to itself deformation aids melt extraction efficiency as it increases the mobility of hydrofractures enhances accumulation and hence lowers the exponent m full connectivity of melt needs never to be reached in the system and melt transport and extraction can occur at very low melt fractions the chemical evolution of melt from source to emplacement level will be governed by the discontinuous mixing and mingling of batches each with different histories and possibly different sources if no subsequent homogenisation occurs in a magma chamber or the final emplacement structure the process can be identified by chemical heterogeneity of plutons and volcanic rocks

petrogenesis of the post kinematic magmatism of the central finland granitoid complex ii sources and magmatic evolution the 188 187ga post kinematic granitoids of the central finland granitoid complex cfgc provide a key geochemical link to understanding granite formation in paleoproterozoic orogenic and post orogenic terrains thickness of the crust and intra crustal differentiation processes played an important role in the formation of three granitoid types that shortly followed the peak of the svecofennian orogeny in the eastern cfgc pyroxene bearing plutons with c type geochemical affinities predominate these were formed from a mixture of low to moderate degree partial melts 30 of mafic mantlederived basaltic 49 sio2 source rocks and partial melts of pre existing mafic granulite lower crust at depth in the western cfgc high silica iron rich fluorite bearing plutons with a type granite characteristics predominate a higher thermal gradient thinner upper and lower crust and significantly more shallow moho depth resulted in higher proportions of crustal melts 03 0 4v s 0 1 02 in the eastern cfgc incorporated into the partial melts of a mafic mantle derived source a geochemical model focusing on the j and honkajoki plutons of the post kinematic suite is presented constraining the nature of the source rock s the degree of partial melting proportions of partial melts and fractionation processes involved

secondary fe mn oxides in minerals heavily damaged by α recoil possible implications for palaeomagnetism sub micron fe mn oxides in columbite tantalite minerals are bound to metamict domains in the host these nano oxides are secondary minerals as the metamict zones formed through accumulation of damages from recoil each of which in a small volume destroys the crystal lattice of the u and th bearing columbite tantalite host transmission electron microscope investigations demonstrate that the oxides fall in the compositional range of magnetite jacobsite iwakiite fe mn 3o4 and of hematite type minerals along the fe2o3 mn2o3 join i e minerals that are known to be major carriers of magnetic remanence measured magnetic properties of isolated columbite tantalite crystals demonstrate in some samples magnetic remanence which is bound to sub micron pseudosingle domain crystals rather than to the host thus the magnetic remanence is not related to the formation of columbite tantalite but to the crystallization of the nano oxides which occur in metamict domains of the host and therefore are secondary depending on the content and distribution of u and th the nano oxides may form diachronously within an individual and among several host crystals magnetic secondary nano oxides are not restricted to columbite tantalite minerals but may occur in other minerals where high contents of fe and mn are associated with high contents of u and th rocks that show the same spatial distribution for u and fe as for instance some red sandstones may display magnetic properties that are controlled by recoil induced growth of secondary fe oxides

u pb dating of columbites a geochronologic tool to date magmatism and ore deposits we have developed techniques for the u pb analysis of the mineral columbite fe mn ta nb 2o6 and report u pb ages obtained for three early proterozoic columbites from the baltic shield of northern sweden the u pb ages of these columbites agree with other available geochronological data u pb dating of columbite is therefore a potentially powerful tool in establishing reliable ages of pegmatites alkaline and carbonatitic intrusions and ore deposits of sn w and rees all of which often contain columbite furthermore columbite can be used to date peraluminous granites as it often occurs within li p ree pegmatites associated with such granites

evidence for early tertiary remagnetization of devonian rocks from the orcadian basin northern scotland and associated transcurrent fault motion the moderately inclined reversed field vectors exhibited by many of the devonian rocks of the orcadian basin are reinterpreted as resulting from a regional early tertiary remagnetization event in northern scotland relating to the emplacement of the british tertiary igneous province the nature of the remagnetization appears to be primarily chemical and may relate to the circulation of magma derived hydrothermal fluids in the porous old red sandstone formations fine grained matrix hematite appears to be an important carrier of this remagnetization component but other secondary opaque minerals also contribute to the magnetizations of certain sites although orcadian basin rocks are currently at some distance from the nearest tertiary igneous center skye geologic and tectonic evidence may allow for southward translation of the skye igneous center in earliest tertiary time

apparent boudinage in dykes abstract intrusive rocks may be arranged in the form of strings of lenses or beads as found on the cap de creus peninsula ne spain and in the south finland migmatite granite belt these structures first appear to be the result of stretching and boudinage of intrusive sheets or dykes however closer examination reveals that they are not boudins but are instead primary intrusive structures a detailed study was performed on a swarm of pegmatite intrusions at cap de creus layering is often continuous between beads and in some cases individual beads exhibit a very irregular shape these observations are shown to be incompatible with an origin by boudinage analogue experiments were used to test the effect of the two models boudinage and emplacement on the structures around beads and show that an emplacement model of local expansion and collapse of magmatic sheets is the most compatible with field observations such structures can only form when dykes intrude hot rocks when magma solidifies slowly enough to allow enough ductile flow of the wall rock to accommodate the formation of the beads the fact that the pegmatite bead strings are not boudins has repercussions for the interpretation of the deformation at cap de creus and necessitates caution in the interpretation of deformation based on apparent boudins in intrusive rocks in other areas

fertile granites of precambrian rare element pegmatite fields is geochemistry controlled by tectonic setting or source lithologies abstract a total of twenty fertile granites and their rare element pegmatite aureoles was examined to correlate their tectonic position relative to the preceding orogenic event with their geochemical signature namely li rb cs be ga sn nb ta y ree sc ti zr be th u f nyf the range of t time difference between metamorphism and or deformation and granite intrusion is 20 to 465 ma for the examined cases there is some tendency toward syn to late orogenic origin of lct s as opposed to post to anorogenic timing of nyf s but the correlation is poor compared to the previously accepted stereotype similarly both metaluminous and peraluminous fertile granites are about equally spread among syn late and anorogenic environments lct systems are largely associated with peraluminous granites whereas all metaluminous to peralkaline plutons bear the nyf signature tectonic control on geochemistry of granite pegmatite suites is poor and secondary to the fundamental influence of the source lithologies yielding granitic melts the lct type suites are derived from undepleted upper crustal lithologies suffering their first anatectic event which mobilizes the most volatile components into low temperature low percentage melts supracrustal sequences as well as ortho and para lithologies of their basement serve as protoliths with variable degrees of involvement the nyf type suites whose plutons have the geochemical signature of a granites may be derived by melting of depleted lower crustal sources residual after preceding anatectic events or possibly by melting of juvenile lithologies with short crustal residence differentiation from mantle derived magmas is highly improbable volatile extraction of nyf elements from the mantle and their pre enrichment in protoliths or melts seems to be required the few mixed systems with variable expressions of a nyf lct signature are derived basically as nyf suites but contaminated by the lct elements from undepleted upper crust via several possible mechanisms

beryllium mineral evolution beryllium is a quintessential upper crustal element being enriched in the upper crust by a factor of 30 relative to primitive mantle 2 1 vs 0 07 ppm most of the 112 minerals with be as an essential element are found in granitic pegmatites and alkalic rocks or in hydrothermal deposits associated with volcanic and shallow level plutonic rocks and skarns because of the extensive differentiation needed to enrich rocks sufficiently in beryllium for beryllium minerals to form these minerals are relative latecomers in the geologic record the oldest known is beryl in pegmatites associated with the sinceni pluton swaziland 3000 ma in general beryllium mineral diversity reflects the diversity in the chemical elements available for incorporation in the minerals and increases with the passage of geologic time furthermore the increase is episodic that is steep increases at specific times are separated by longer time intervals with little or no increase in diversity nonetheless a closer examination of the record suggests that at about 1700 ma the rate of increase in diversity decreases and eventually levels off at 35 species formed in a given 50 ma time interval between 1125 and 475 ma then increases to 39 species at 125 ma except for four spikes before dropping off to 30 species for the last 100 ma these features appear to reflect several trends at work 1 diversifications at 2475 1775 and 525 ma which are associated with highly fractionated rare element granitic pegmatites and with skarns at langban and similar deposits in the bergslagen ore region of central sweden and which are inferred to correspond to the collisional phases of the supercontinents kenorland nuna and gondwana respectively 2 diversification at 1175 ma due to the rich assemblage of beryllium minerals in the ilimaussaq peralkaline complex gardar province west greenland in an extensional environment 3 diversification at 275 ma which is largely attributable to granitic pegmatites appalachian mountains u s a and urals russia and the larvik alkalic complex norway but nonetheless related to continental collision and 4 limited exhumation of environments where beryllium minerals could have formed in the last 100 ma that the maximum diversity of be minerals in any one geologic environment could be finite is suggested by the marked slowing of the increase in the number of species formed in a given 50 ma time interval whereas the drop off at 100 ma could be due to 100 myr being too short a time interval to exhume the deep seated occurrences where many be minerals had formed the relative roles of chance vs necessity in complex evolving systems has been a matter of considerable debate one equally applicable to what extent the temporal distribution of beryllium minerals is a matter of contingency on the one hand the appearance of the most abundant be minerals such as beryl and phenakite early in the history of be mineralization appears to be a deterministic aspect since these minerals only require the abundant cations al and si and crystallize at relatively low concentrations of be in aqueous solution or granitic magmas on the other hand it could be argued that the very existence of most other be minerals as well as the temporal sequence of their appearance is a matter of chance since 55 of the 112 approved be minerals are known from a single locality and many of these phases require an unusual combination of relatively rare elements consequently we cannot exclude the possibility that other equally rare and thus contingent potential be minerals await discovery in as yet unexposed subsurface deposits on earth and we suggest that details of be mineral evolution on other earth like planets could differ significantly from those on earth