u pb and sm nd isotopic constraints on the evolution of the paleoproterozoic perapohja belt northern finland ten localities representing metasedimentary and igneous rocks from the northern part of the paleoproterozoic perapohja belt fennoscandian shield were sampled for zircon u pb dating by la mc icp ms and sm nd analysis by tims the results revealed two phases of granitic magmatism that took place at 1 99 ga and 1 79 1 77 ga the former age 1989 6 ma was given by the kierovaara granite pluton representing the first example of felsic plutonism of this age in northern finland though a type felsic volcanic rocks of the same age have previously been recognized the strongly negative initial nd values obtained for granitoids ranging from 3 9 to 11 4 are compatible with the previous sm nd data indicating the presence of archean basement in northern finland at the time of granite emplacement a garnet whole rock sm nd age of 1752 14 ma was measured for a mica gneiss constraining the time of metamorphic cooling down to the closure temperature of garnet detrital zircon grains from quartzite samples belonging to the jatulian palokivalo formation and a stratigraphically unassigned lithodemic unit the mellajoki suite yielded solely archean zircon populations in contrast our sample from the martimo formation representing kalevian sedimentation revealed a 66 paleoproterozoic detrital zircon component with the youngest zircon grains showing an age of ca 1 91 ga the new age data allow us to constrain the depositional time of this sedimentary unit between ca 1 91 and 1 88 ga the latter being the time of the synorogenic plutonism our geochronological data coupled with recent data from other karelian belts suggest that the depositional time period for rocks assigned to the kaleva spans ca 150 ma and the simple division of these rocks into two non coeval units the lower and upper kaleva has become problematic

upb geochronology of layered mafic intrusions in the eastern baltic shield implications for the timing and duration of paleoproterozoic continental rifting abstract paleoproterozoic layered mafic ultramafic intrusions are widespread in the eastern baltic fennoscandian shield and mark the initial stages of continental rifting and outpouring of voluminous flood basalts u pb zircon baddeleyite ages have been determined for seven layered mafic intrusions and a subvolcanic felsic unit associated with two paleoproterozoic volcanic sedimentary belts the pechenga imandra varzuga belt in the kola peninsula and the sumi sariola belt in karelia the layered intrusions in the sumi sariola belt show an age progression from 2449 0 1 1 ma for the burakovka complex located at the southeastern edge of the belt to 2442 1 1 4 ma and 2441 3 1 2 ma for the lukkulaisvaara and tsipringa intrusions of the olanga complex in northern karelia northeastern part of the belt these age results indicate a total duration of mafic magmatism along the ca 800 km belt in karelia to be less than 10 m y the pechenga imandra varzuga belt kola peninsula contains two distinct age groups of layered mafic intrusions and related rocks the younger of these overlaps with the ages obtained for the olanga intrusions and includes the 2441 0 1 6 ma imandra lopolith and a spatially associated felsic subvolcanic unit dated at 2442 2 1 7 ma the older group of mafic layered intrusions in the kola peninsula includes the 2501 5 1 7 ma pana tundra intrusion the 2504 4 1 5 ma monche pluton and the 2505 1 1 6 ma general skaya mountain intrusion the temporal and spatial distribution of the 2 50 2 44 ga rift related igneous activity in the baltic shield is consistent both with the model of starting mantle plume and with the model of passive rifting controlled by pre existing lithospheric weaknesses

nitrogen isotopic composition of ammoniated phyllosilicates case studies from precambrian metamorphosed sedimentary rocks ammonium concentrations in phyllosilicates from archean and proterozoic schists and carbonates were evaluated by fourier transform infrared microspectrometry aftir the y 15 nair values of structural nitrogen in these phases were subsequently measured by laser mass spectrometry a rapid estimate of the concentration of nh4 in biotite by aftir allows the preselection of the samples to be analyzed for their isotopic composition micas from 1 90 2 09 ga metasediments from finland and from 1 025 1 500 ga biotite schists from the moine succession in scotland have high concentrations of structural ammonium between 176 and 1549 ppm suggestive of a biological origin for the nitrogen the heavy nitrogen isotopic signatures y 15 n 4 9x to 19 7x of structural nh4 in these micas are considered consistent with biological denitrification in operation at the time of sedimentation evidence for denitrification in the mesoproterozoic is in agreement with geological and geochemical data for abundant free oxygen in the atmosphere facilitating the stability of marine no3 sequences of early archean ca 3 8 ga garnet mica schists from the isua supracrustal belt isb in southern west greenland contain biotite rich in nh4 between 233 and 512 ppm and nitrogen isotopic signatures y 15 n 1 9x to 5 9x which possibly point to an early evolution of ammonium assimilation and or nitrogen fixation this result appears to be consistent with molecular phylogenetic studies in the bacterial and archaeal domains which suggest that nitrogen fixation and ammonium assimilation were metabolic features of the last common ancestor lca of all organisms d 2004 elsevier b v all rights reserved

neodymium isotope systematics of the mantle beneath the baltic shield evidence for depleted mantle evolution since the archaean abstract more than 300 published and unpublished nd isotopic analyses of mantle derived rocks from the baltic shield have been compiled the rocks range in age from archaean to phanerozoic within any given age interval the mantle derived rocks range in nd t from depleted mantle values at or above the growth curves of the global depleted mantle reservoirs of depaolo 1981 and depaolo et al 1991 to negative values initial neodymium isotopic compositions below the de paolo curve are best explained by interaction between depleted mantle derived magmas and local crustal contaminants the data now available lend no support to the existence of isolated less depleted or undepleted mantle domains beneath the baltic shield as was suggested by mearns et al 1986 and valbracht 1991a b

the os and nd isotopic systematics of c 2 44 ga akanvaara and koitelainen mafic layered intrusions in northern finland abstract we have determined os and nd isotopic compositions of various rock types in and adjacent to the c 2 44 ga akanvaara and koitelainen layered intrusions which are part of the extensive paleoproterozoic ultramafic to felsic mantle plume related magmatism in the fennoscandian shield the initial e nd in the layered sequences of both intrusions fall within a limited range around 2 0 being compatible with results from other coeval layered intrusion in the shield the initial os isotopic ratios in chromitites are near chondritic the total range of os t is from 1 to 6 with the highest values reported for low os chromites no systematic differences are found in the osmium isotopic compositions of the chromites between the intrusions or relative to the stratigraphic position of the sample the studied intrusions share similar isotopic and geochemical features with rocks from younger flood basalt provinces and also the same problem of the origin of the lithospheric source component viz crustal assimilation versus melt generation within lithospheric mantle the observed nd os isotopic systematics of the intrusions require that the magmas had negative e nd values already before emplacement and can best be accounted for by 15 25 deep crustal contamination of a komatiitic melt initially having a near chondritic os isotopic composition coupled with additional minor in situ assimilation of country rock material this hypothesis is supported by the existence of coeval crustally contaminated komatiitic volcanism on the fennoscandian shield

upwelling asthenosphere beneath western arabia and its regional implications two distinct phases of continental magmatism are evident in western arabia the first from about 30 to 20 ma produced tholeiitic to transitional lavas emplaced along nw trends the second from about 12 ma to recent produced transitional to strongly alkalic lavas emplaced along n s trends the older phase is attributed to passive mantle upwelling during extension of the red sea basin whereas the younger phase is attributed to active mantle upwelling but was facilitated by minor continental extension perpendicular to plate collision the younger magmatic phase is largely contemporaneous with a major period of crustal uplift to produce the west arabian swell after about 14 ma a variety of evidence suggests that the west arabian swell is thermally supported by hot upwelling asthenosphere in contrast to the distinct asymmetry of uplift and magmatism on opposing sides of the red sea basin these processes were symmetric across a n s line marking the central axis of the west arabian swell this axis coincides with two fundamental features the ha il rutbah arch in the north and the makkah madinah nafud mmn volcanic line in the south the symmetry of magmatism is demonstrated by petrochemical evidence that the mmn harrats were derived by greater degrees of partial melting at shallower depths than those harrats lying to the west and east of the mmn line the potential temperature of the asthenosphere is estimated to be about 1436 c beneath the mmn line and about 1354 c beneath the flanking more undersaturated harrats the source of upwelling is either a mantle plume centrally located beneath the west arabian swell or an elongated and extended lobe of hot asthenosphere emanating from the ethiopian mantle plume convective flow may have been channelled along a preexisting regional flexure in the continental lithosphere which concentrated hot asthenosphere beneath the central axis of the afroarabian dome this crest of mantle upwelling underlies the mmn line in the north and the danakil depression and ethiopian rift system in the south it also passes through the red sea basin at the midpoint of an unusual doubly propagating rift system where axial seafloor spreading began 4 5 m y ago the nw trend of the red sea basin was well established by crustal attenuation during the older magmatic phase the pliocene invasion of this basin by a n s zone of mantle upwelling has resulted in seafloor spreading parallel to the preexisting structure along a rift system that has continued to propagate away from its eccentric mantle source in two opposing directions

sm nd age of kambalda and kanowna greenstones and heterogeneity in the archaean mantle a precise sm nd age of 2 790 30 myr and an initial ratio corresponding to nd 3 4 0 6 has been determined for the kambalda sequence from the eastern yilgarn block of western australia the high initial 143nd 144nd implies a long lived light rare earth element depleted high sm nd source for the kambalda volcanics and therefore provides definitive evidence for ancient heterogeneities in the archaean mantle

the arabian continental alkali basalt province part i evolution of harrat rahat kingdom of saudi arabia the harrat rahat lava field forms a major component of an extensive but poorly known continental alkali basalt province extending from yemen in the south through saudi arabia and jordan to syria in the north the continental intraplate volcanism which produced the arabian lava fields harrats was contemporaneous with opening of the red sea collision of the arabian and eurasian plates and uplift of the afro arabian dome harrat rabat has evolved over the past 10 m y it contains two prominent lateritic disconformities which separate three stratigraphic units the shawahit 10 2 5 ma hammah 2 5 1 7 ma and madinah 1 7 recent basalts comprising 68 19 and 13 of the total harrat volume respectively the shawahit basalt is composed of coarse grained dictytaxitic olivine transitional basalt otb and minor 10 alkali olivine basalt aob which erupted through scoria cones and shield volcanoes in contrast the hammah basalt is dominated by equal volumes of fine grained intergranular to intersertal aob and hawaiite which erupted mainly through scoria cones and relatively few shield volcanoes sparse domes and flows of mugearite and benmoreite first appear in the hammah basalt the madinah basalt spans the full compositional range of harrat rahat 103 analyzed samples comprise otb 8 aob 47 hawaiite 32 mugearite 4 benmoreite 8 and trachyte 2 here scoria cones dominate whereas shield volcanoes are sparse and occur only in the lower madinah basalt domes and associated pyroclastic deposits are common the final events of harrat volcanism include 11 post neolithic eruptions 4 500 yr old and two historical eruptions in 641 and 1256 a d arabian harrat magmatism may have been initiated by partial melting of garnet perido tite in the asthenosphere at a depth 100 km accumulation of this partial melt at the crust mantle boundary 37 44 km may have resulted in miocene uplift of the afro arabian dome harrat rahat volcanism began shortly after significant initial uplift with the voluminous extrusion of shawahit otb lava chemical and petrographic data suggest that these otb magmas ascended rapidly with little crustal residence time the data support a model of open system fractionation for aob and hawaiite lavas of the younger hammah and madinah basalts in this model some of the potential shawahit otb was trapped in crustal chambers during magma ascent crystal fractionation in these chambers was accompanied by their periodic replenishment by rising otb magma this resulted in the mixing of magma types and the development of aob and hawaiite magmas which are anomalously enriched in highly incompatible elements the mugearite benmoreite and trachyte lavas are the products of advanced fractionation in these crustal chambers with little or no magma replenishment

evolution of depleted mantle the lead perspective abstract isotopic data have established that compared to estimated bulk earth abundances the sources of oceanic basaltic lavas have been depleted in large ion lithophile elements for at least several billions of years various data on the tertiary mesozoic gorgona komatiite and cretaceous oka carbonatite show that those rocks also sample depleted mantle sources this information is used by analogy to compare pb isotopic data from 2 6 billion year old komatiite and carbonatite from the suomussalmi belt of eastern finland and munro township ontario that are with associated granitic rocks and ores that should contain marked crustal components within experimental error no differences are detected in the isotopic composition of initial pb in either of the rock suites these observations agree closely with sr and nd data from other laboratories showing that depleted mantle could not have originated in those areas more than a few tenths of billions of years before the rocks were emplaced on a world wide basis the pb isotope data are consistent with production of depleted mantle by continuous differentiation processes acting over approximately the past 3 billion years the data show that pb evolution is more complex than the simpler models derived from the rb sr and sm nd systems the nature of the complexity is still poorly understood

petrogenesis of silicic magmatism related to the 2 44 ga rifting of archean crust in koillismaa eastern finland abstract early paleoproterozoic extension in the archean craton of the fennoscandian shield led to the emplacement of several 2 44 ga layered gabbroic intrusive complexes in northern finland and adjacent russia closely associated with them are felsic rocks of similar age 1 the sirnio group volcanic rocks on top of the koillismaa layered complex 2 a quartz alkali feldspar syenite at kynsijarvi near the koillismaa complex and 3 an aluminous a type granite at nuorunen near the oulanka layered complex in the koillismaa area the ruptured archean crust consists of ortho and paragneisses that were intruded and migmatized by somewhat younger granites u pb zircon data indicate that the gneisses are at least 2 8 ga old and that the granites were crystallized at 2 7 ga both rock types show a common monazite age of 2695 ma that registers the peak of granulite facies metamorphism and possibly the intrusion of the granites the local neoarchean crust has nd at 2440 ma values between 5 and 8 5 the mafic rocks of the koillismaa complex show initial nd at 2440 ma values around 1 5 and those of the oulanka complex range from 2 1 to 0 the nd value 4 8 and t dm model age 2 9 3 0 ga of the kynsijarvi quartz alkali feldspar syenite are within the limits of the evolution path of the local archean crust the corresponding values for the nuorunen granite are 2 0 and 2 76 ga and are thus closer to those of the mafic rocks the volcanic rocks of the sirnio group show more scatter with initial nd at 2440 ma values of 1 1 to 5 3 the lowest nd values probably reflect later disturbance magmatic values cluster around 2 major and trace element modeling shows that fractional crystallization of the koillismaa complex parental magma or partial melting of the archean crust cannot account for the 2 44 ga silicic rocks of koillismaa the geochemical and nd isotope characteristics of the volcanic rocks and the kynsijarvi quartz alkali feldspar syenite are best explained by a combined assimilation fractional crystallization model involving a contaminated komatiitic parental magma and intermediate mafic lower crustal assimilant differences in the initial nd isotope composition of the 2 44 ga rocks reflect varying source components and evolutionary history for the archean lithosphere in eastern fennoscandia

neodymium strontium isotopic evidence for extreme contamination in a layered basic intrusion abstract neodymium and strontium isotopic analyses from the kalka layered basic intrusion central australia are arrayed parallel to the sr axis of a nd sr isotopic correlation diagram and are removed from the normal basaltic field initial 143 nd 144 nd ratios are almost constant and typical of continental basalts whereas initial 87 sr 86 sr ratios vary greatly and range to high values 0 7049 0 7088 acidic granulites from the country rock lie upon the continuation of the kalka trend at higher 87 sr 86 sr ratios outlining a mixing system between granulite and normal basaltic magma if average granulite acted as a contaminant isotope dilution calculations show that amounts assimilated reached as high as 25 bulk contamination is confirmed petrologically by the abundance of orthopyroxene particularly as basal orthopyroxenites and the restriction of olivine to less contaminated levels analogies between kalka and several other major layered intrusions such as the bushveld complex suggest that large scale contamination was a significant aspect of their genesis

conflicting sm nd whole rock and u pb zircon ages for archean lavas from newton township abitibi belt ontario abstract archean komatiitic and tholeiitic lavas from newton township ontario have a sufficient range of sm nd ratios to define a well constrained line on the normal 143 nd 144 nd vs 147 sm 144 nd isochron plot the data give an isochron age of 2826 64ma and an initial e nd of 2 65 0 26 however u pb analysis of zircons from a dacitic volcaniclastic that underlies the komatiite tholeiite suite give an age of 2697 1 1ma there is strong evident that the zircon age is the eruption age suggesting that the older sm nd age is incorrect and probably results from mixing between isotopically distinct mantle sources at the time of eruption the sources had e nd values from about 4 2 to 1 6 indicating that the archean mantle in this area was markedly heterogeneous and not uniformly depleted

the timing of uplift volcanism and rifting peripheral to the red sea a case for passive rifting prior to the formation of the red sea the northeastern afro arabian continent had low relief and was largely below sea level from the late cretaceous to the early oligocene the events leading to the formation of the red sea followed the sequence 1 alkaline volcanism and rifting beginning about 30 32 ma affecting a narrow linear zone in the continent 2 rotational block faulting and detachment faulting well underway by 25 ma 3 gabbro and diorite magmatism andesite to rhyolite volcanism and fine grained nonmarine sedimentation in the rift between 20 and 25 ma 4 fine grained marine sedimentation in the rift as the early shelves started to subside in the middle miocene and 5 uplift of the adjacent continents about 3 km and subsidence of the shelves about 4 km between 13 8 and 5 ma the youth of the uplift is suggested by 44 fission track dates on apatites from rocks of the proterozoic arabian shield that range in age from 13 8 to 568 ma the youngest of these ages coupled with the present high relief along the arabian escarpment and published heat flow measurements indicate that 2 5 4 km uplift has occurred in the last 13 8 m y the sequence volcanism rifting followed by uplift leads to our adoption of a passive mantle model for rift origin models that require uplift to create the rift are rejected because of the late uplift we advocate a model of lithospheric extension caused by two dimensional plate stress over those requiring tractional drag at the base of the lithosphere caused by vigorous flow in the asthenosphere it is acknowledged that traction models could explain the observed data but they imply a rigid static lithosphere and seem to require a link between the direction of flow in the asthenosphere and plate motions neither requirement is necessary in the extension model the rift starts with mechanical extension in a narrow zone of lithosphere between 25 32 ma in our model the thinned lithosphere is replaced by upwelling asthenosphere and by rocks from the adjacent deep continental lithosphere which flow into the rift ductile flow of the deep continental lithosphere is accelerated by partial melting as rocks flow upward toward the rift axis once partially melted rocks formerly part of the continental lithosphere join the upwelling asthenosphere resulting in a rapid erosion of the lithospheric mantle beneath the continent near the rift edge the resulting density decrease explains the uplift we think that the red sea began as a consequence of changing plate geometries resulting from the collision of india and eurasia after the collision the segment of the owens fracture zone north of the carlsberg ridge became locked forcing the northeast corner of afro arabia to rotate with the indian plate away from the rest of africa

sm nd and u pb dating of early proterozoic mafic felsic volcanism in northernmost sweden a sm nd mineral isochron of 1932 45 ma on a kiruna greenstone is defined by greenschist facies metamorphic minerals apparently older than the regional svecokarelian metamorphism 1880 ma however this age agrees within error with u pb zircon ages of 1909 1617 ma on quartz bearing prophyries which overlie but are in part intercalated with basic volcanics if these basic volcanics following previous studies are considered to be contemporary with the kiruna greenstones the present data suggest that eruption of both basic and acid volcanics and associated hydrothermal metamorphism took place over a short time interval close to 1 9 ga the greenstone sample is depleted in light rare earths and has high initial 145nd 144nd indicating a source in which such depletion has existed for a long time in contrast an albite diabase sample with a somewhat younger mineral age 1 80 0 08 ga is light rare earth enriched and has a correspondingly low initial nd isotopic composition

geochemistry and petrology of 2 40 2 45ga magmatic rocks in the north western belomorian belt fennoscandian shield russia abstract widespread mafic and ultramafic rocks drusites and minor granites of 2 40 2 45 ga age occur within the archaean gneisses of the belomorian fold belt several of these mafic ultramafic and acid intrusions in the north western part of that belt have been studied in detail the mafic and ultramafic rocks are rich in si and mg and also show lree enrichment they have nd 2 45 values varying between 0 2 and 1 8 assimilation fractional crystallization afc and mixing models in terms of nd t la sm ratios have been evaluated for the origin of the mafic rocks the results show that an afc model is preferable it involves 20 30 crustal rock assimilation by undepleted picrite magma with fractional crystallization the granites are similar in composition to a type within plate granites the tupaya bay granite pluton is characterized by geochemical layering which supports an anorogenic origin the granites and mafic ultramafic rocks drusites in the belomorian belt are similar in composition to coeval granites acid and mafic volcanic rocks mafic dykes and mafic layered intrusions within the kola and the karelian terrains and can be interpreted as part of the palaeoproterozoic igneous province connected with ascent of a mantle plume the small sizes corona textures and evidence for ductile deformation in the drusites are connected with their deeper level of emplacement p 8 12 kbar than that of the mafic layered intrusions 2 7 kbar

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

the early proterozoic pechenga varzuga belt a case of precambrian back arc spreading abstract the pechenga varzuga belt pvb in the kola peninsula russia comprises the western pechenga and the eastern imandra varzuga portions these are two major asymmetric synform structures infilled with proterozoic volcanogenic and sedimentary rocks dated between 2 45 and 1 9 ga two main stages of tectonic magmatic activity took place 1 a 2 45 2 3 ga basal siliceous high mg volcanic series typical for the sumian sariolian episodes and 2 the jutuli ludia and kalevian episodes which began 2 2 ga ago the lower level of the second sequence is made up of subaerial alkali basalts the upper 2 1 1 95 ga old portions of the sequence consist of tholeiitic pillow lavas hyaloclastites and ferropicrite basalt flows associated with deep water sediments such as phyllites silicites and turbidites the tholeiites exhibit ree distribution patterns similar to those of morb while the ferropicrite basalts resemble intraplate olivine basalts of continental and oceanic environments calc alkaline subaerial volcanism took place along the south western flank of the pechenga zone a final igneous stage is characterized by the emplacement of potassium granitic intrusions the main lapland thrust mlt occurs along the southern border of the paleoproterozoic lapland umba granulite belt and is a narrow zone of compression thrusting and folding accompanied by high pressure granulite metamorphism it is approximately coeval with the upper part of the pechenga varzuga belt and is interpreted as representing a complementary structure i e the zone of subduction of the karelian belomorian craton beneath the kola block thus the upper part of the pechenga varzuga belt is considered to have originated as a back arc basin in the rear of a zone of north directed subduction and continent collision it was eventually closed with the appearance of igneous suites typical of such events the pechenga varzuga belt main lapland thrust system thus provides an instance of an ancient 2 0 1 9 ga ago collision of lithospheric plates accompanied by back arc spreading and then closure of the back arc basin the geodynamic development is similar that which occurred more recently within the alpine himalayan belt

combined plate motion and density driven flow in the asthenosphere beneath saudi arabia evidence from shear wave splitting and seismic anisotropy a comprehensive study of mantle anisotropy along the red sea and across saudi arabia was performed by analyzing shear wave splitting recorded by stations from three different seismic networks the largest most widely distributed array of stations examined across saudi arabia to date stations near the gulf of aqaba display fast orientations that are aligned parallel to the dead sea transform fault most likely related to the strike slip motion between africa and arabia however most of our observations across saudi arabia are statistically the same showing a consistent pattern of north south oriented fast directions with delay times averaging about 1 4 s fossilized anisotropy related to the proterozoic assembly of the arabian shield may contribute to the pattern but is not sufficient to fully explain the observations we feel that the uniform anisotropic signature across saudi arabia is best explained by a combination of plate and density driven flow in the asthenosphere by combining the northeast oriented flow associated with absolute plate motion with the northwest oriented flow associated with the channelized afar plume along the red sea we obtain a north south oriented resultant that matches our splitting observations and supports models of active rifting processes this explains why the north south orientationmore of the fast polarization direction is so pervasive across the vast arabian plate less

association of gold with uraninite and pyrobitumen in the metavolcanic rock hosted hydrothermal au u mineralisation at rompas perapohja schist belt northern finland the perapohja schist belt comprises a supracrustal sequence of quartzites mafic volcanics and volcaniclastics carbonate rocks black shales mica schists and greywackes which were deposited from ca 2 44 to 1 91 ga during the rifting of the archaean basement in the eastern part of the fennoscandian shield metamorphism and multiple folding of the basin fill took place during the svecofennian orogeny 1 9 1 8 ga followed by intrusions of late orogenic 1 84 1 80 ga and post orogenic granitoids 1 79 1 76 ga the rompas au u mineralisation is hosted by deformed calcsilicate veins in mafic volcanic rocks and locally contains very high grade 10 000 g t au gold pockets with strict spatial association of gold minerals to uraninite and pyrobitumen chemical ages from the unaltered domains in the structure of uraninite indicate a 1 95 1 90 ga age for the deposition of the primary high temperature e g u th 100 in uraninite hydrothermal uranium mineralisation these data are in agreement with the results of previous u pb dating of uraninite by sims textural evidence suggests that metamorphic recrystallisation of the uraninite bearing quartz dolomite veins into calcsilicate mineral assemblages during the svecofennian orogeny 1 9 1 8 ga was followed by a hydrocarbon bearing fluid flow event and radiolytic polymerisation of hydrocarbons around grains of uraninite gold precipitated during a subsequent hydrothermal process in the fractures of uraninite as well as in the cracks and on the botryoidal surfaces of uraninite pyrobitumen nodules remobilisation and redeposition of uranium by these hydrothermal events produced secondary uraninite grains with chemical ages between 1 85 and 1 65 ga native gold is associated with galena altaite hunchunite nickeline and rare cobaltite pb bearing maldonite pyrite pyrrhotite chalcopyrite molybdenite and titanite raman spectra show disordered structure of undeformed pyrobitumen nodules in contrast with the well ordered graphite in calcsilicate veins mean random reflectance data for pyrobitumen indicate 270 340 c maximum temperature of thermal maturation this temperature range is also considered as the temperature of gold deposition results of multiple sulphur isotope analyses of organic material pyrite and acid volatile bound sulphur show distinct ranges of 34s values for sorg and scrs in uraninite pyrobitumen from 6 99 to 3 55 and from 10 02 to 4 41 respectively and uraninite pyrobitumen native gold mineral associations from 1 36 to 6 87 and from 0 42 to 9 7 respectively 33s data indicate local occurrence of nonmass dependent sulphur isotope fractionation owing to interaction of fluids with organic material concentration of lead in uraninite is depleted along the gold mineral filled fractures whereas the uranogenic lead isotope contents of galena altaite and hunchuite deposited in the same fractures are extremely high suggesting that the dominant source of lead for the crystallisation of these minerals was the radiogenic lead content of uraninite taking into account this source of radiogenic lead the calculated pb pb model ages for the lead minerals are between 1 75 and 1 70 ga sulphur and tellurium removal from the fluid by reaction with radiogenic lead released by uraninite appears to be an important mechanism in the strongly localised deposition of gold minerals scavenging of sulphur by pyrobitumen nodules from gold transporting fluids was an additional process triggering precipitation of gold carbon particles and organic functional groups in pyrobitumen probably acted as nucleation and adsorption centres for gold minerals

a sm nd isotopic study of the south harris igneous complex the outer hebrides sm nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods by coupling whole rock sm nd analyses of the principal members of the south harris igneous complex with sm nd mineral isochrons on two anothositic gabbros the age of granulite facios metamorphism has been defined whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2 18 0 06 gyr but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber whole rock data on the tonalite indicate that it is younger than 2 06 gyr its mean tchur age is 1 86 0 05 gyr garnet pyroxene amphibole plagioclase mineral isochrons on two anorthosite samples give identical 1 87 0 04 gyr ages which date cooling after the high pressure granulite facies metamorphism together with the tonalite whole rock data this defines the age of that metamorphism and confirms dearnley s original assignment of an early laxfordian age

early paleoproterozoic felsic and mafic magmatism in the karelian province of the fennoscandian shield abstract early paleoproterozoic 2 50 ga to 2 39 ga mafic and felsic intrusions are widespread in the fennoscandian shield the bimodal magmatic event reflects an extensional phase in the archean craton that had been tectonically quiescent for ca 200 ma after the amalgamation of the continent the early paleoproterozoic felsic intrusions are mostly granitic in composition only one syenitic pluton is known geochemically the felsic intrusions are a type the mafic layered intrusions were emplaced in the upper crust between the archean basement and overlying early paleoproterozoic volcanic rocks at least two different parental magma compositions were present in the mafic magmatic event partial melting of the archean lower crust due to heating effect of the hot mafic magmas most probably generated the felsic magmas which reflect the nd isotope composition of the source rocks the irregular distribution of coeval mafic and felsic intrusions within the karelian province is still an open question

8 precambrian geodynamics and ore formation the fennoscandian shield abstract compared with present day global plate tectonics archaean and palaeoproterozoic plate tectonics may have involved faster moving hotter plates that accumulated less sediment and contained a thinner section of lithospheric mantle this scenario also fits with the complex geodynamic evolution of the fennoscandian shield from 2 06 to 1 78 ga when rapid accretion of island arcs and several microcontinent continent collisions in a complex array of orogens was manifested in short lived but intense orogenies involving voluminous magmatism with a few exceptions all major ore deposits formed in specific tectonic settings between 2 06 and 1 78 ga and thus a strong geodynamic control on ore deposit formation is suggested all orogenic gold deposits formed syn to post peak metamorphism and their timing reflects the orogenic younging of the shield towards the sw and west most orogenic gold deposits formed during periods of crustal shortening with peaks at 2 72 to 2 67 1 90 to 1 86 and 1 85 to 1 79 ga the ca 2 5 to 2 4 ga ni cu pge deposits formed both as part of layered igneous complexes and associated with mafic volcanism in basins formed during rifting of the archaean craton at ca 2 5 to 2 4 ga svecokarelian ca 1 89 to 1 88 ga ni cu deposits are related to mafic ultramafic rocks intruded along linear belts at the accretionary margins of microcratons all major vms deposits in the fennoscandian shield formed between 1 97 and 1 88 ga in extensional settings prior to basin inversion and accretion the oldest cyprus type deposits were obducted onto the archaean continent during the onset of convergence the pyhasalmi vms deposits formed at 1 93 to 1 91 ga in primitive bimodal arc complexes during extension of the arc in contrast the skellefte vms deposits are 20 to 30 million years younger and formed in a strongly extensional intra arc region that developed on continental or mature arc crust deposits in the bergslagen uusimaa belt are similar in age to the skellefte deposits and formed in a microcraton that collided with the karelian craton at ca 1 88 to 1 87 ga the bergslagen uusimaa belt is interpreted as an intra continental or continental margin back arc extensional region developed on older continental crust iron oxide copper gold iocg deposits are diverse in style at least the oldest mineralizing stages at ca 1 88 ga are coeval with calc alkaline to monzonitic magmatism and coeval and possibly cogenetic subaerial volcanism more akin to continental arcs or to magmatic arcs inboard of the active subduction zone younger mineralization of similar style took place when s type magmatism occurred at ca 1 80 to 1 77 ga during cratonization distal to the active n s trending subduction zone in the west possibly interaction of magmatic fluids with evaporitic sequences in older rift sequences was important for ore formation finally the large volumes of anorthositic magmas that characterize the sveconorwegian orogeny formed a major concentration of ti in the sw part of the sveconorwegian orogenic belt under granulite facies conditions about 40 million years after the last regional deformation of the sveconorwegian orogeny between ca 930 and 920 ma

stratigraphy petrology and platinum group element mineralization of the early proterozoic penikat layered intrusion northern finland the penikat layered intrusion is part of an early proterozoic belt of layered intrusions 2 440 m y in age which extends across northern finland the intrusion is located between late archean granitoids and early proterozoic supracrustal rocks the marginal series with slightly contaminated chilled margin separates the penikat intrusion from the underlying basement complex the layered series is divided into five megacyclic units i to v from bottom to top each unit comprising an ultramafic lower part and a gabbroic upper part three major mineralized zones enriched in platinum group elements pge the sompujaervi sj ala penikka ap and paasivaara pv mineralizations have been delineated along almost 23 km of strike and occur in megacyclic unit iv the sompujaervi mineralization about 1 m thick is mostly associated with pyroxenitic rocks at the lower contact of the unit whereas the ala penikka zone about 0 35 m thick is associated with gabbronorite and poikilitic textured anorthosite in the middle part and the paasivaara zone about 1 m thick with leucogabbro and anorthosite very close to the upper contact the ratios of platinum group elements are relatively constant for each mineralization the paasivaara mineralization represents the most platinum rich variety and the ala penikka the most palladium rich one the ratio of pd to pt being typically about 2 for the sulfide free type and 1 5 3 5 and 0 7 for the sulfide bearing type for the sompujaervi ala penikka and paasivaara mineralizations respectively the sulfur free platinum group minerals pgm sperrylite and isomertieite are the dominant species encountered in the sompujaervi mineralization whereas sulfur bearing platinum group minerals together with sperrylite dominate in the ala penikka and paasivaara mineralizations these are represented by braggite and vysotskite in the ala penikka one and by braggite and cooperite in the more platinum rich paasivaara mineralization according to the new classification of platinum group element deposits proposed by cabri and naldrett 1984 the typically sulfide free sompujaervi mineralization may be included in the oxide silicate association as a possible new type whereas the ala penikka and paasivaara mineralizations may be classified as merensky reef type of the sulfide associations

nd and sr isotopic geochemistry of mafic layered intrusions in the eastern baltic shield implications for the evolution of paleoproterozoic continental mafic magmas nd and sr isotopic data are presented for the 2449 2441 ma olanga and burakovka layered mafic complexes in the eastern baltic shield these complexes have similar tectonic position but differ in two aspects the age of the enclosing crust and the post crystallization metamorphic history the sm nd isotopic results for the kivakka and lukkulaisvaara intrusions olanga complex are consistent with the model of closed system crystallization of a single magma without significant wallrock assimilation the rb sr systems of minerals were disturbed by late rb addition during 1 75 1 50 ga metamorphism the nd and sr isotopic systems in the burakovka complex show no metamorphic disturbance and indicate mixing of at least four isotopically distinct components isotopic variations in the burakovka complex can be explained by a 4 20 per cent contamination of a primary komatiitic or picritic magma with a mesoarchean crust similar to that exposed in the region a similar model applied to the olanga complex using a neoarchean crustal isotopic composition cannot reproduce the observed isotopic signature the nearly uniform initial nd values between 1 and 2 3 observed in the kivakka and lukkulaisvaara intrusions of the olanga complex as well as in the other 2 50 2 44 ga layered mafic intrusions throughout the eastern baltic shield are better explained by a mantle plume model with small amounts of crustal contamination and minor involvement of asthenospheric material this model is also consistent with the geological observations and the temporal distribution of the paleoproterozoic mafic magmatism in the eastern baltic shield as an alternative the enriched isotopic characteristics may be explained by melting of a metasomatically modified lithospheric mantle source

sedimentary and volcanic facies recording the neoarchaean continent breakup and decline of the positive δc excursion abstract the palaeoproterozoic kalix greenstone belt at the northern end of the bothnian bay in sweden comprises three groups the lower group a ca 3000 m thick unit of subaerially erupted tholeiitic intraplate basalts interbedded with fluviatile conglomerates represents the volcano sedimentary succession deposited in a rift setting these units are truncated by a break up unconformity above which is the middle group an 800 m thick succession of dolograinstones stromatolitic dolostones arenites volcanoclastic and mafic volcanic rocks stromatolite morphologies and the occurrence of distinctive sedimentary structures including planar and herringbone cross bedding hummocky cross stratification symmetrical and asymmetrical ripples desiccation cracks and tepees indicate markedly variable depositional environments associated with transition from a marine influenced rifted margin to a rimmed carbonate shelf sedimentation was accompanied by subaerial mafic volcanism and syndepositional tectonics the upper group a more than 2000 m thick unit of deep water shales was deposited on the drowned carbonate platform of the middle group in response to tectonically enhanced subsidence this study provides a detailed palaeoproterozoic depositional history of the fennoscandian shield from rifting to passive margin development associated with both dispersal of the neoarchaean supercontinent and decline of the proterozoic positive isotopic excursion of 13 c 12 c in sedimentary carbonates in contrast to many previously reported 13 c rich palaeoproterozoic carbonates which were deposited in non marine to restricted or partly restricted settings the carbonate facies reported herein were well connected to the oceanic realm and thereby represent a good opportunity to test restricted marine versus open marine models for how the carbon cycle functioned in deep time

sequence stratigraphy of the siliciclastic east puolanka group the palaeoproterozoic kainuu belt finland abstract the 2300 2600 m thick palaeoproterozoic east puolanka group within the central fennoscandian shield records four major transgressions on the cratonic margin within the approximate time period 2 25 2 10 ga stacking of siliciclastic facies in parasequences and parasequence sets provides data to evaluate oscillation of relative sea level and subsidence on different temporal scales the lowermost part of the passive margin prism is characterized by alluvial plain to shallow marine sediments deposited in incised valleys the succeeding highstand period is recorded by ca 250 m of progradational parasequence sets of predominantly rippled and horizontally laminated sandstones representing stacked wave dominated shoreline units in sequence 1 capped by a hiatus or in some places by a subaerial lava as relative sea level rose again sand rich barrier beach complexes developed with microtidal lagoons and inlets corresponding to a retrogradational parasequence set this was followed by a highstand period with aggradation and progradation of alluvial plain and coastal sediments grading up into wave tide influenced shoreline deposits in sequence 2 in sequence 3 the succeeding mudstones represent tidal flat deposits in a back barrier region with continued transgression the parasequences stacked retrogradationally each flooding episode being recorded by increasingly deeper water deposits above low angle cross bedded sandstones of the swash zones the succeeding highstand progradation is represented by alluvial plain deposits the next transgressive systems tract overlying an inferred erosional ravinement surface is recorded by a retrogradational parasequence set dominated by low angle cross stratified swash zone deposits in sequence 4 the large scale trough cross bed sets in these parasequences represent sand shoals and sheets of the inner shelf system the overall major transgression recorded in the lowermost part of the palaeoproterozoic cratonic margin succession was related to first to second order sea level changes probably due to increasing regional thermal subsidence of the lithosphere following partial continental breakup the stratigraphic evolution can be related to changes of relative sea level with a frequency of ca 25 million years probably propagated by episodic thermal subsidence the parasequences identified here are related to high frequency cycles of relative sea level change due to low magnitude eustatic oscillations

on the age of the kiruna greenstones northern sweden abstract three types of zircons from an albite diabase sill within the kiruna greenstone group have been investigated to determine their age and order of crystallization the youngest pink and transparent zircons probably crystallized in late fissures and in association with a thermal event the intermediate white zircon populations could not be readily dated because of significant lead loss the lead 207 206 ages of both the pink and white zircons agree roughly with that of the sm nd isochron about 1 93 ga on secondary minerals from a kiruna greenstone skiold and cliff the oldest brown zircons indicate an age of about 2 2 ga and probably reflect the initial crystallization of the rock the formation of the early proterozoic greenstones in the kiruna area thus commenced about 2 2 ga ago or earlier these results are consistent with the proposed chronostratigraphy of the jatulian formations in finland e g simonen merilainen a time interval of at least 0 3 ga is assumed for the volcano sedimentary period which pre dates the svecokarelian orogeny of the kiruna area

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

smnd pbpb and rbsr geochronology and petrogenesis of the mafic dyke swarm of mahbubnagar south india implications for paleoproterozoic crustal evolution of the eastern dharwar craton abstract the sm nd rb sr and pb pb isotopic systematics of whole rock and separated mineral fractions from four tholeiitic mafic dykes of the mahbubnagar swarm have been investigated this dyke swarm intrudes the granitic basement of the cuddapah basin in the eastern dharwar craton edc the sm nd data on whole rock samples of four gabbros and one dolerite and 15 mineral fractions yielded a linear array with an isochron age of 2173 64 ma and an initial nd value of 0 4 0 3 the sm nd data of a metapyroxenite and its three mineral fractions plot parallel to this array yielding an isochron age of 2189 123 ma with a negative initial nd value of 1 8 1 4 rb sr and pb pb internal isochron ages of three gabbros and a pb pb internal isochron age of a metapyroxenite dyke range between 2031 and 2284 ma and within the limits of experimental error broadly conform to the sm nd isochron age by contrast rb sr internal isochron ages of three other samples range between 1474 and 1952 ma and one sample yields a pb pb internal isochron age of 3446 ma showing the effects of partial isotopic resetting possibly due to hydrothermal alteration these data suggest that mafic dykes of the mahbubnagar swarm were emplaced at c 2170 ma marking an episode of the crustal extension and fracturing which ultimately resulted in the formation of the cuddapah basin different petrological and geochemical variants of mafic dykes seen in the area most probably represent generation and or crystallization of the progressively evolved batches of magma during a relatively short time interval the initial nd values calculated at 2170 ma range from 0 6 to 2 0 and can be explained as reflecting contamination of depleted mantle derived magma by crustal rocks of edc the initial 87 sr 86 sr ratios 0 7027 0 7064 and model 1 values 8 3 8 5 of apparently undisturbed samples are also consistent with this suggestion

sequence stratigraphy of the karelian formations 2 4 2 0 ga of the fennoscandian shield significance of major unconformities abstract established sequence stratigraphy for the palaeoproterozoic sedimentary sequences of the karelian formations of the fennoscandian shield illustrate first and second order base level cycles within a synrift to cratonic margin transition the stacking of siliciclastic facies in sequences and parasequence sets in the east puolanka group provide data to evaluate changes of relative sea level and basin subsidence in four third order sequences within a second order cycle in the kainuu belt the sedimentary sequences of the autochthonous part of the belt are up to 4700 m in total thickness the sedimentary sequences show the effects of very long duration first order cyclicity of sea level with a duration of ca 400 ma as part of the progressive development of a rifted neoarchaean continent attenuation of continental crust during rifting and breakup of the continent raised relative sea level from the autochthonous part of the kainuu belt three major basin evolution stages are recognized synrift transitional and initial passive margin each expressed by unconformity bounded second order cyclicity of duration more than 100 ma basal mafic volcanics and alluvial to glacial siliciclastics some 400 700 m thick of the kurkikyla and honkajarvi groups represent synrift deposits of a second order basin fill sequence related to a continental scale fragmentation of the crust ca 2 4 2 3 ga ago many of the shallow intra cratonic basins subsided below sea level some being affected by a major continental glaciation the subsequent peneplanation was dominated by an intensive period of subaerial chemical weathering apparently of the extent of an entire craton and identified as the next major unconformity the following second order sequence of deposits from quartz arenitic alluvial fans and broad braided alluvial plains of the korvuanjoki group up to 1400 m thick was deposited in half grabens still in a relatively stable cratonal setting and corresponding to the incipient stage in the development of a continental margin stacked highstand sequences were then encountered and extensive erosion and deep incised valleys were formed during the falling stage of relative sea level within the next unconformity bounded second order 2300 2600 m thick sequence of a passive margin prism of the east puolanka group ca 2 25 2 10 ga four third order cycles were identified defined by variable system tracts and sequence boundaries formed during a falling stage of relative sea level these sequences record lowstand and transgressive systems tracts followed by pronounced highstand systems tracts the sparse parasequence sets identified in these sequences are related to high frequency cycles of relative sea level change deposition was related to the increasing tectonic flexure and episodic regional subsidence of the cratonic margin followed later by distal shelf mudstones carbonates and deeper water deposition that rimmed much of the continental margin

smnd evidence for late archaean crust formation in the lapland kola mobile belt kola peninsula russia and norway abstract a sm nd model age traverse across the kola peninsula has been undertaken to gain insight into the major episodes of crustal growth and to test the possible presence of early archaean saamian crust in the region granitoid gneisses and subordinate metasediments have been sampled from the main archaean geological units i e from northeast to southwest the murmansk domain model ages of 2 68 2 96 ga central kola domain including samples from the superdeep borehole 2 74 2 92 ga belomorian domain including the eastern part of the tersk area 2 70 2 93 ga the narrow range of depleted mantle model ages from 2 68 to 2 96 ga for archaean granitoid orthogneisses close to the oldest crystallization ages from the region indicate a major episode of late archaean crustal growth and reveal no indication of ancient saamian material in keeping with other geochronological evidence similar results 2 86 to 2 93 ga for archaean metasediments support this conclusion proterozoic model ages for supracrustals from the western part of the tersk area 2 16 to 2 23 ga indicate that the crust in this area is much younger than previously recognized initial nd values for late archaean samples with reasonable age constraints fall within a range of 2 3 to 2 5 two well dated samples from the belomorian domain have the highest initial nd values nd 2 81 ga 2 0 and nd 2 78 ga 2 3 and suggest that a depleted mantle source was present in the kola region in late archaean times dated samples from the central kola domain have lower initial nd between 2 5 and 1 2 suggesting a possible regional difference in mantle character in contrast to the depleted signature of the archaean orthogneisses between 2 7 ga and 2 8 ga previous studies of earliest palaeoproterozoic rift related majic intrusives in the kola and karelia regions report negative nd values at 2 4 2 5 ga possibly these data reflect a change to more enriched mantle values

early proterozoic cu au and fe ore deposits associated with regional nacl metasomatism in northern fennoscandia scapolite is widely distributed in 1 9 2 5 ga volcano sedimentary rocks and 1 77 2 2 ga igneous rocks over several hundred square kilometres in northern fennoscandia comprising northern sweden no

2 45 ga break up of the archaean continent in northern fennoscandia rifting dynamics and the role of inherited structures within the archaean basement abstract constraints from restoration of minor rift related faults preserved from subsequent palaeoproterozoic overprint along the southern margin of the perapohja belt northern fennoscandia indicate local ne sw extension directions prevailed at the time of 2 44 ga rifting of the archaean continent this palaeostress field is attributed to pull apart setting of the perapohja belt at a left overstepping zone of major sinistral n s trending deformation zones consequently the variably overprinted nw se trending structures of the perapohja belt including the nw se central graben were originally generated as normal faults the proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step over zone between strike slip faults by contrast the faults bounding e w to ene wsw trending basement horsts are the result of reactivation of older basement structures underlying the pull apart basin based on the results we infer that the final break up of the archaean continent in northern fennoscandia utilized the 2 44 ga rift related nw se trending crustal anisotropy parallel with dyke swarms with pre existing archaean n s trending structures reactivated as second order shear zones in between the voluminous syn rift 2 5 2 4 ga and later pulses of mafic magmatism 2 32 1 98 ga are considered indicative of active type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the superior hearne and karelian kola cratons the presented model supports the continuity of the archaean craton beyond the caledonian orogen towards north west and explains its segmentation suggesting a model for the localization of the palaeoproterozoic supracrustal belts of northern fennoscandia moreover this paper provides an approach to understand the basement cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits

age and isotope geochemistry of the archaean pongola and usushwana suites in swaziland southern africa a case for crustal contamination of mantle derived magma the igneous rocks of the pongola supergroup ps and usushwana intrusive suite uis represent a case of late archaean continental magmatism in the southeastern part of the kaapvaal craton of south africa and swaziland u pb dating on zircons from felsic volcanic rocks of the ps yields a concordia intercept age of 2940 22ma that is consistent with a sm nd whole rock age of 2934 114ma determined on the ps basalt rhyolite suite the initial end of 2 6 0 9 is the lowest value so far reported for archaean mantle derived rocks rb sr whole rock dating of the ps yields a younger isochron age of 2883 69ma which is not significantly different form the accepted u pb zircon age an internal cpx opx plag whole rock isochron for a pyroxenite from the younger uis yields an age of 2871 30 ma and initial 143nd 144nd that lies off the chur growth curve by end 2 9 0 2 however sm nd whole rock data for the uis yield an excessively high age of 3 1 ga that conflicts with firm geological evidence showing the uis to be intrusive into the ps the negative deviations of initialend from the chondritic nd evolution curve suggest significant contamination of the ps and uis melts by older continental crust a mixing process with continental crust after magma segregation is supported by a high initial 87sr 86sr ratio of0 703024 24 for a clinopyroxene sample from a uis pyroxenite compared with an expected value of 0 701 for the 2 9 ga mantle we therefore interpret the linear array of data points for the uis gabbros as a mixing line between 2 87 ga old magma and older continental crust parallel lree enriched ree patterns negative nb ti anomalies a distinctive and uniform ratio of ti zr 46 and a narrow span of initial nd indicate a common source for both the ps and uis suites which is different from primitive mantle

early proterozoic continental tholeiites from western bergslagen central sweden ii nd and sr isotopic variations and implications from smnd systematics for the svecofennian sub continental mantle abstract a subset of nine least altered least fractionated within plate continental metatholeiites from western bergslagen yields a sm nd isochron age of 1886 48 ma with an nd t of 1 1 this age is in agreement with previous u pb zircon dating of a coeval felsic intrusion variation of nd t in other subsets is shown to be caused by crustal contamination of the basic magma to explain the remarkable isotopic homogeneity of the least altered least fractionated metatholeiites and their combined lree enrichment and isotopic depletion three petrogenetic models are discussed the first model involves incipient melting of a depleted mantle in the garnet stability field the second model presumes complete homogenization of a depleted mantle derived melt contaminated by older continental crust and the third model proposes melting of an enriched mantle source in light of the depleted nature of the baltic asthenosphere absence of enriched archaean crust in the bergslagen area negative hfse zr hf ti anomalies in the metatholeiites and enriched isotopic characteristics of lamprophyres occurring 50 km to the west model three is preferred in a tentative scenario three time scales are inferred from sm nd isotope systematics to generate the metatholeiites 1 long term 1 ga lree depletion of the asthenosphere by extraction of continental crust 2 shorter term pre 1 88 ga bp lree enrichment of this depleted mantle by supra subduction zone modification of the mantle wedge during protocrust formation and 3 short term around 1 88 ga bp generation of basic magma by melting of this enriched mantle as a result of continental rifting of the early proterozoic protocrust

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

a nd and sr isotopic study of the trinity peridotite implications for mantle evolution field evidence indicates that the trinity peridotite was partially melted during its rise as a part of the upwelling convecting mantle at a spreading center a sm nd mineral isochron for a plagioclase lherzolite yields an age t 427 32 ma and initial e nd 10 4 0 4 which is distinctly higher than that expected for typical depleted mantle at this time this age is interpreted as the time of crystallization of trapped melt in the plagioclase lherzolite p t field this time of crystallization probably represents the time when the massif was incorporated as a part of the oceanic lithosphere the sm nd model age of the plagioclase lherzolite total rock is t chur nd 3 4 ae this suggests that the trinity peridotite was derived from a mantle that was depleted rather early in earth history the peridotite contains many generations of pyroxenite dikes and some microgabbro dikes we report data for two dikes that clearly crosscut the main metamorphic fabric of the peridotite a microgabbro dike yields a sm nd mineral isochron age of t 435 21 ma and e nd 6 7 0 3 a pyroxenite dike yields an initial e nd 7 3 0 4 the initial e nd values for the pyroxenite and gabbro dikes are fairly similar to those for the depleted mantle at this time and are distinct from the lherzolite demonstrating that they are not genetically related rb sr data do not give any coherent pattern however some bounds can be put on initial sr values of e sr 21 for the plagioclase lherzolite and e sr 8 7 for the microgabbro dike it is plausible that the dikes represent cumulates left behind from island arc magmas that rose through the the oceanic lithosphere within the vicinity of a subduction zone major and trace elements and sm nd isotopic data indicate a multiple stage history for the trinity peridotite a small melt fraction was extracted from an undepleted source 3 4 ae or more ago to produce the proto lherzolite a large fraction of melt 12 to 23 was extracted from the proto lherzolite to produce the present rock the lherzolite was then crosscut by dikes from average depleted mantle 0 44 ae ago the data are compatible with the depleted mantle source being formed very early in earth history although most available data indicate that the depleted upper mantle has been relatively well stirred through time the trinity data suggest that very ancient nd isotopic values are preserved and thus chemical and physical heteorgeneities are sometimes preserved in the depleted source of mid ocean ridge basalts as well as the oceanic lithosphere which they intrude

on the lomagundi jatuli carbon isotopic event the evidence from the kalix greenstone belt sweden we report a significant new database on the carbon and oxygen isotopic analyses 233 whole rock and 43 microcored samples obtained from sedimentary dolostones in the kalix greenstone belt from the fennoscandian shield of sweden this characterises the second half of the prominent palaeoproterozoic lomagundi jatuli positive carbon isotope excursion within a 600 m thick succession of alternating volcanic volcaniclastic siliciclastic and dolomitic rocks from greenschist epidote amphibolite metamorphic facies the least altered dolostone samples show a gentle oscillation between 2 and 4 throughout stratigraphy with a second order positive excursion from 4 through 8 and gradually back again to 4 in the c 150 m thick unit in the middle and upper parts of the succession this second order excursion is superimposed on the general relatively rapid 13c decline at the second half of the lomagundi jatuli isotopic event and coincides with the transition from a marine influenced rift to a passive margin setting if the excursion is global it would reveal internal fine structure of the lomagundi jatuli isotopic excursion if not basinal local factors are the main players lithostratigraphic and c isotope correlation between kalix and neighbouring perapohja schist belt successions 40 km apart suggest an incomplete geological record in both successions despite the absence of obvious non depositional unconformities

nd isotopic systematics and chemistry of central australian sapphirine granulites an example of rare earth element mobility abstract lower proterozoic sapphirine bearing and associated granulites from central australia exhibit the greatest range of present day 143 nd 144 nd ratios nd o 26 5 to 112 3 yet reported for rocks believed to be cogenetic the nd isotopic data and ree abundances of these rocks demonstrate extreme fractionation of the rare earths during the formation of stratiform cu pb zn sulfide deposits with which they are closely associated field relationships petrography and chemistry of the sapphirine granulites suggest that their protoliths comprised chlorite rich rocks which were generated by hydrothermal alteration of a range of rock types prior to metamorphism calculations employing ree abundances of the sapphirine granulites and associated rocks combined with bulk solid fluid distribution coefficient data yield high fluid rock ratios consistent with a pre metamorphic hydrothermal origin for the unusual ree patterns the sm nd data for these rocks define an age of 1760 75ma which is significantly younger than the crust formation age of the terrain 2070 125ma but indistinguishable from the rb sr whole rock age for granulite facies metamorphism 1790 35ma these data are interpreted in terms of major hydrothermal fractionation of the rare earths shortly perhaps tens of millions of years before granulite facies metamorphism followed by redistribution of nd isotopes or small fractionations of the sm nd ratio during the granulite facies event and possibly also during intense retrogression which reset rb sr whole rock and u pb zircon and monazite systematics at about 1700 ma

lithospheric structure of the arabian shield and platform from complete regional waveform modelling and surface wave group velocities summary regional seismic waveforms reveal significant differences in the structure of the arabian shield and the arabian platform we estimate lithospheric velocity structure by modelling regional waveforms recorded by the 1995 1997 saudi arabian temporary broadband deployment using a grid search scheme we employ a new method whereby we narrow the waveform modelling grid search by first fitting the fundamental mode love and rayleigh wave group velocities the group velocities constrain the average crustal thickness and velocities as well as the crustal velocity gradients because the group velocity fitting is computationally much faster than the synthetic seismogram calculation this method allows us to determine good average starting models quickly waveform fits of the jon and sn body wave arrivals constrain the mantle velocities the resulting lithospheric structures indicate that the arabian platform has an average crustal thickness of 40 km with relatively low crustal velocities average crustal p and s wave velocities of 6 07 and 3 50 km s 1 respectively without a strong velocity gradient the moho is shallower 36 km and crustal velocities are 6 per cent higher with a velocity increase with depth for the arabian shield fast crustal velocities of the arabian shield result from a predominantly mafic composition in the lower crust lower velocities in the arabian platform crust indicate a bulk felsic composition consistent with orogenesis of this former active margin p and s wave velocities immediately below the moho are slower in the arabian shield than in the arabian platform 7 9 and 4 30 km s 1 and 8 10 and 4 55 km s respectively this indicates that the poisson s ratios for the uppermost mantle of the arabian shield and platform are 0 29 and 0 27 respectively the lower mantle velocities and higher poisson s ratio beneath the arabian shield probably arise from a partially molten mantle associated with red sea spreading and continental volcanism although we cannot constrain the lateral extent of a zone of partially molten mantle

delineation and character of the archaean proterozoic boundary in northern sweden abstract before the deposition of a proterozoic cover and the repeated proterozoic reworking of the older rocks the presently exposed archaean areas in northern sweden formed part of a coherent craton in the present study we have used sm nd isotopic analyses of proterozoic granitoids and metavolcanics to delineate the archaean palaeoboundary in a regional context the transition from strongly negative nd t values in the northeast to positive values in the southwest is distinct and approximately defines the border of the old craton the archaean palaeoboundary extends in a wnw direction and is subparallel to the longitudinal axis of the skellefte sulphide ore district but it is situated 100 km farther to the north the 1 9 ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments but those situated to the north have higher contents of lile and lree at similar contents of si this indicates that they were generated in an area with thicker crust and supports the location of the archaean proterozoic palaeoboundary there is no simple correlation between the archaean palaeoboundary as defined by the isotopic results and any of the major fracture systems as interpreted from regional geophysical measurements reflection seismic work indicates that juvenile volcanic arc terrains to the south have been thrust onto the archaean craton possible thrust faults have been identified from aeromagnetic measurements rifting of the archaean craton created a passive margin 2 0 ga ago spreading shifted to convergence with subduction beneath the archaean continent 1 9 ga ago subsequently the resulting juvenile volcanic arc collided with the old continent and the archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting the boundary then was disturbed by later deformation predominantly along nne trending fracture systems

cenozoic epeirogeny of the arabian peninsula from drainage modeling it is generally accepted that the arabian peninsula has been uplifted by subcrustal processes positive residual depth anomalies from oceanic crust in the red sea and in the gulf of aden suggest that a region surrounding this peninsula is dynamically supported admittance calculations surface wave tomography studies and receiver function analyses all imply that regional topography is generated and maintained by some combination of mantle convective circulation and lithospheric thickness changes despite these significant advances the spatial and temporal uplift rate history of the arabian peninsula is not well known here we show that a regional uplift rate history can be obtained by jointly inverting 225 longitudinal river profiles that drain this peninsula our strategy assumes that shapes of individual river profiles are controlled by uplift rate history and moderated by erosional processes we used local measurements of incision rate to calibrate the relevant erosional parameters in our inverse algorithm uplift rate is permitted to vary smoothly as a function of space and time but upstream drainage area remains invariant we also assume that knickzone migration is not lithologically controlled implications of these important assumptions have been investigated our results suggest that the arabian peninsula underwent two phases of asymmetric uplift during the last 20 30 ma at rates of 0 05 0 1 mm a 1 the southwestern flank of the peninsula has been uplifted by 1 5 2 5 km regional stratigraphic constraints the age and composition of volcanism paleosol formation incised peneplains emergent marine terraces and thermochronometric measurements corroborate our calculated patterns of uplift progressive development of three domal swells along the western margin of the peninsula is consistent with localized upwelling of hot asthenospheric mantle

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

40ar 39ar geochronology of the neogene quaternary harrat al madinah intercontinental volcanic field saudi arabia implications for duration and migration of volcanic activity abstract new 40 ar 39 ar ages based on incremental heating techniques for groundmass separates of 25 samples are presented for the harrat al madinah volcanic field part of harrat rahat in the north western part of the arabian plate this area is an active volcanic field characterized by the occurrence of two historical eruptions approximately in 641 and 1256 ad field investigations of the main volcanic landforms indicate dominantly monogenetic strombolian eruptions in addition to local more explosive eruptions the lavas consist mainly of olivine basalt and hawaiite flows with minor evolved rocks of mugearite benmoreite and trachyte that occur mainly as domes tuff cones and occasionally as lava flows previous k ar dating shows that the harrat al madinah lava flows and associated domes comprise seven units spanning an age range of ca 1 7 ma recent the new 40 ar 39 ar age determinations confirm to a great extent the previously obtained k ar ages in the sense that no major systematic biases were found in the general stratigraphy of the different flow units however the 40 ar 39 ar plateau ages show that volcanism in this area began in the neogene 10 ma and continued to recent with the most voluminous eruptions occurring in the quaternary neogene volcanism occurred in at least three pulses around 10 5 and 2 ma whereas quaternary volcanism produced at least seven units reflecting lava flow emplacement in the time period of 1 90 ma recent thus the whole duration of volcanic activity in the harrat al madinah 10 ma recent appears much longer than that previously identified the longevity of volcanism in the same part of the moving arabian plate and absence of evidence for uni directional migration of volcanic activity indicate that there is no fixed plume beneath this region the nnw trending distribution of the volcanic vents is parallel to the red sea and suggests their origin is related to periodic extensional episodes along the reactivated red sea fault system

pb and nd isotopic study of two archean komatiitic flows from alexo ontario isotopic analysis of two archean komatiitic flows from alexo ontario gives a pb pb isochron age of 2690 15 ma and a sm nd isochron age of 2752 87 ma these ages agree well with u pb zircon ages from underlying and overlying volcanics the variations in element ratios that define the isochrons were not produced during crystallization of the lavas the spread in u pb was caused by submarine alteration soon after eruption and the spread in sm nd resulted from a differences in the composition of the residue of melting and b contamination of the upper komatiite flow through thermal erosion of the lower flow the 147sm 144nd ratio of uncontaminated komatiite is 0 25 which reflects the depleted nature of its mantle source the th u ratio of about 3 4 is probably also representative of depleted mantle the initial nd of 2 44 0 51 indicates that the mantle depletion took place long before magma formation

growth of archaean crust in the kuhmo district eastern finland u pb and sm nd isotope constraints on plutonic rocks abstract neoarchaean plutonism in the kuhmo district in eastern finland karelia includes tonalites sanukitoids leucocratic granodiorites and leucogranites a u pb sims and tims study of zircons and titanites places age constraints on this plutonic activity and whole rock sm nd isotopes are used to characterise source variation u pb results indicate that the tonalites were emplaced during at least three separate periods 2 81 2 78 and 2 76 2 74 ga the initial nd values vary from 0 3 to 2 2 and suggest no major involvement of significantly older crust in the genesis of tonalites the tonalites were followed by 2 74 2 70 ga possibly also 2 68 ga sanukitoids with initial nd values of 0 7 to 1 2 close temporal relations between the 2 74 ga tonalites and sanukitoids indicate that their genesis may be linked subsequent leucocratic granodiorites and leucogranites were emplaced at 2 70 2 68 ga and commonly contain inherited zircons the initial nd values of these granodiorites and granites vary from 1 1 to 3 4 indicative of multiple magma sources the general decrease in initial nd values with decreasing age demonstrate progressive recycling of pre existing crust thus stressing the importance of crustal recycling as a process in forming the leucocratic granitoid rocks overall the archaean plutonic rocks of the kuhmo district are temporally similar to those in the adjacent russian karelia suggesting that 2 83 2 68 ga was one of the most extensive periods of crustal growth throughout the karelia craton

crustal structure of the northern harrat rahat volcanic field saudi arabia from gravity and aeromagnetic data abstract new gravity data reveal a prominent negative anomaly along the main vent axis of the northern harrat rahat volcanic field in saudi arabia the low continues north of the volcanic field onto exposures of proterozoic rocks indicating that the low is caused not only by the volcanic field and possibly also underlying cenozoic sediments but also the underlying proterozoic basement an inversion of the gravity field guided by analysis of aeromagnetic data indicates 1 a broad depression of the basement surface that is deeper along the main vent axis in the eastern part of the harrat and in the southwest part of the volcanic field and 2 less dense basement beneath the vent axis low densities within the basement most likely arise from lithologic variations in the basement predating cenozoic volcanism although our analysis does not rule out small volumes of partial melt and higher temperatures or extensive fracturing at depth

sm nd and pb isotopic study of mafic rocks associated with early proterozoic continental rifting the peropohja schist belt in northern finland the perapohja schist belt in northern finland rests unconformably on archaean granitoids and marks the early stages of proterozoic crustal evolution in the fennoscandian baltic shield 2440 ma old layered mafic intrusions predate the supracrustal and ca 2200 ma old sills of the gabbro wehrlite association intrude the lowest quartzites and volcanics runkaus of the sequence the sm nd mineral isochron of the penikat layered intrusion gives an age of 2410 64 ma the initial nd values of the penikat intrusion nd 2440 1 6 and the runkausvaara sill nd 2200 0 suggest that these mafic magmas were contaminated by older crustal material the sm nd and pb isotopic results on the 2 44 2 2 ga old runkaus volcanics indicate mobility of pb fractionation of sm nd during late greenschist facies metamorphism and crustal contamination the pb pb data provide an age of 1972 80 ma with a high initial 207pb 204pb ratio 1 8 49 while scattered sm nd data result in an imprecise age of 2330 180 ma with an initial nd value of about zero secondary titanite gives an u pb age of ca 2250 ma the jouttiaapa basalts in contrast ascended from the mantle without interaction with older crust these lree depleted tholeiites mark a break in continental sedimentation and yield a sm nd age of 2090 70 ma their initial nd 4 2 0 5 implies that the subcontinental early proterozoic mantle had been depleted in lree for a long period of time the first lava flows are strongly depleted in lree suggesting that their source was significantly more depleted than the source of mid ocean ridge basalts today

geochemical and sm nd isotopic study of amphibolites in the cathaysia block southeastern china evidence for an extremely depleted mantle in the paleoproterozoic abstract geochemical and sm nd isotopic results are reported for late paleoproterozoic mafic amphibolites from sw zhejiang and nw fujian parts of the cathaysia block of se china two suites of contemporaneous amphibolites are distinct in their geochemical characteristics group 1 samples from nw fujian have chemical compositions of transitional and alkali basalts show lree enriched patterns and plot mainly in the field of within plate basalt on a number of trace element discrimination diagrams group 2 rocks from sw zhejiang have tholeiitic compositions and are characterized by flat to lree depleted patterns and fall into the morb field all the amphibolite samples have high end t values of 5 6 to 8 5 t 1766 19 ma a positive correlation between end t and nb th suggests possible mixing of a mantle derived magma and a crustal component with the least contaminated samples having very high end t values 8 8 5 and nb th ratios of 13 the geochemical and isotopic characters and close temporal relationship of these two suites of amphibolites suggest that their magmatic precursors were likely formed in an environment similar to an ensialic rift developing into a proto oceanic basin e g the gulf of tadjoura the exceptionally high end t values of up to 8 5 for some of the amphibolites suggest the presence of a time integrated extremely depleted mantle source beneath cathaysia during the paleoproterozic

saudi arabian seismic refraction profile a traveltime interpretation of crustal and upper mantle structure abstract the crustal and upper mantle compressional wave velocity structure across the southwestern arabian shield has been investigated by a 1000 km long seismic refraction profile the profile begins in mesozoic cover rocks near riyadh on the arabian platform trends southwesterly across three major precambrian tectonic provinces traverses cenozoic rocks of the coastal plain near jizan and terminates at the outer edge of the farasan bank in the southern red sea more than 500 surveyed recording sites were occupied and six shot points were used including one in the red sea two dimensional ray tracing techniques used to analyze amplitude normalized record sections indicate that the arabian shield is composed to first order of two layers each about 20 km thick with average velocities of about 6 3 km s and 7 0 km s respectively west of the shield red sea margin the crust thins to a total thickness of less than 20 km beyond which the red sea shelf and coastal plain are interpreted to be underlain by oceanic crust a major crustal inhomogeneity at the northeast end of the profile probably represents the suture zone between two crustal blocks of different composition elsewhere along the profile several high velocity anomalies in the upper crust correlate with mapped gneiss domes the most prominent of which is the khamis mushayt gneiss based on their velocities these domes may constitute areas where lower crustal rocks have been raised some 20 km two intracrustal reflectors in the center of the shield at 13 km depth probably represent the tops of mafic intrusives the mohorovicic discontinuity beneath the shield varies from a depth of 43 km and mantle velocity of 8 2 km s in the northeast to a depth of 38 km and mantle velocity of 8 0 km s depth in the southwest near the shield red sea transition two velocity discontinuities occur in the upper mantle at 59 and 70 km depth the crustal and upper mantle velocity structure of the arabian shield is interpreted as revealing a complex crust derived from the suturing of island arcs in the precarnbrian the shield is currently flanked by the active spreading boundary in the red sea

crustal architecture of the inverted central lapland rift along the hukka 2007 profile we have studied the lateral velocity variations along a partly buried inverted paleo rift in central lapland northern europe with a 2d wide angle reflection and refraction experiment hukka 2007 the experiment was designed to use seven chemical explosions from commercial and military sites as sources of seismic energy the shots were recorded by 102 stations with an average spacing of 3 45 km two dimensional crustal models of variations in p wave velocity and vp vs ratio were calculated using the ray tracing forward modeling technique the hukka 2007 experiment comprises a 455 km long profile that runs nnw sse parallel to the kittila shear zone a major deformation zone hosting gold deposits in the area the profile crosses paleoproterozoic and reactivated archean terranes of central lapland the velocity model shows a significant difference in crustal velocity structure between the northern distances 0 120 km and southern parts of the profile the difference in p wave velocities and vp vs ratio can be followed through the whole crust down to the moho boundary indicating major tectonic boundaries upper crustal velocities seem to vary with the terranes compositional differences mapped at the surface the lower layer of the upper crust displays velocities of 6 0 6 1 km s both paleoproterozoic and archean terranes are associated with high velocity bodies 6 30 6 35 km s at 100 and 200 350 km distances the central lapland greenstone belt and central lapland granitoid complex are associated with a 4 km thick zone of unusually low velocities 6 0 km s at distances between 120 and 220 km we interpret the hukka 2007 profile to image an old partly buried inverted continental rift zone that has been closed and modified by younger tectonic events it has structural features typical of rifts inward dipping rift shoulders undulating thickness of the middle crust high velocity lower crust and a rather uniform crustal thickness of 48 km

sm nd isotope evidence of a collisional event in the precambrian of northern sweden abstract the sm nd isotope characteristics of c 1 9 and c 1 8 ga granitoids occurring close to the edge of the archaean craton in northern sweden have been studied strongly negative e nd t values occur farther southwest in the potassic 1 8 ga granitoids than in the calc alkaline 1 9 ga granitoids intrusions of the younger granitoids with distinctly negative e nd t values are surrounded by granitoids of the older group with positive values northeast of the 1 8 ga granitoids discussed here the c 1 9 ga calc alkaline intrusions formed within the archaean continent have the same sm nd isotope characteristics as the 1 8 ga granitoids to the southwest our interpretation is that a juvenile volcanic arc terrane to the south collided with and was thrust onto the archaean continent after the formation of the 1 9 ga granitoids the outcome was that these juvenile rock masses were obducted onto the reworked archaean continent the 1 8 ga granitoids were formed by remobilisation of continental crust partial melting at 1 8 ga resulted in the intrusion of granitoids carrying the sm nd isotopic signature of the archaean continent into juvenile rocks it is probable that the collision discussed here was part of a major accretion and amalgamation to the archaean craton after the formation of the calc alkaline 1 9 ga granitoids but before the formation of the 1 8 ga granitoids

enriched komatiitic basalts from newton township ontario their genesis by crustal contamination of depleted komatiite magma lree enriched komatiitic basalts are commonly found in association with lree depleted komatiites this association is found in a sequence of late archaean lavas from newton township ontario the komatiitic lavas at newton township differ from most late archaean examples in that both the komatiites and the komatiitic basalts are depleted in al and in the hree the close association and distinctive al and hree depletions of the two lava types strongly suggest a common origin despite their contrasting lree patterns a model is proposed whereby the lree depleted komatiites represent the parental magma to the lree enriched komatiitic basalts the two being linked by a combination of crystal fractionation and crustal assimilation the composition of the contaminant is estimated by comparing the lree enriched komatiitic basalts with the evolved part of a thick layered komatiite flow that has similar major element chemistry the contaminant composition coincides closely with taylor mclennan s estimate of the composition of the archaean upper crust it is concluded that lree enriched komatiitic basalts can be produced from lree depleted komatiite parent magmas by combined assimilation and fractionation and that such a process best explains the geochemistry and nd isotopic features of most komatiitic basalts

approximating edges of source bodies from magnetic or gravity anomalies cordell and grauch 1982 1985 discussed a technique to estimate the location of abrupt lateral changes in magnetization or mass density of upper crustal rocks the final step of their procedure is to identify maxima on a contoured map of horizontal gradient magnitudes we attempt to automate their final step our method begins with gridded magnetic or gravity anomaly data and produces a plan view of inferred boundaries of magnetic or gravity sources the method applies to both local surveys and to continent wide compilations of magnetic and gravity data e g zietz 1982 simpson et al 1983a kane et al 1982

timing and duration of palaeoproterozoic events producing ore bearing layered intrusions of the baltic shield metallogenic petrological and geodynamic implications there are two 300 500 km long belts of palaeoproterozoic layered intrusions the northern kola belt and the southern fenno karelian belt in the baltic fennoscandian shield new u pb tims ages and radiogenic isotopic nd sr he data have been determined for mafic ultramafic cu ni ti cr and pgebearing layered intrusions of the kola belt u pb ages on zircon and baddeleyite for gabbronorite and anorthosite from the fedorovo pansky monchepluton and main ridge monchetundra and chunatundra mt generalskaya intrusions and gabbronorite and dykes from the imandra lopolith of the kola belt define a time interval of more than 130 million years from ca 2 52 ga to 2 39 ga at least four intrusive phases have been distinguished three pge bearing and one barren this spread of ages is wider than that for intrusions of the fenno karelian belt which clusters at 2 44 ga nd isotopic values for the northern belt range from 1 1 to 2 4 implying an enriched mantle em 1 type reservoir for these layered intrusions initial sr isotopic data for the rocks of the intrusions are radiogenic relative to bulk mantle with isr values from 0 703 to 0 704 geochemical data and 4he 3he isotopic ratios of the minerals reflect a significant contribution from a mantle source rather than the influence of crustal processes during emplacement the geological and geochronological data indicate that in the eastern part of the baltic shield mafic ultramafic intrusive magmatism was active over a protracted period and was related to plume magmatism associated with continental breakup that also involved the superior and wyoming provinces

geochronologic significance of lead lines from old cratons abstract lead isotope data from geologically mixed suites of archaean gneisses often yield secondary isochrons that have statistically better fits than might be expected from the geological constraints on the range of samples used accepted as true isochrons these pb pb lines are commonly used both to determine the age of the rock suite and as evidence that all the rocks that yield data points within error of the isochron were formed at the same time from the same source using within geologically reasonable limits random data for initial lead and secondary lead growth we demonstrate that heterogeneous sample sets may yield statistically valid secondary isochrons this implies that the fit to a common secondary isochron does not necessarily mean that the samples used have the same age or an isotopically similar source numerical modelling of artificial data sets suggests that high correlation between 1 and 2 values is the single most important factor that enhances the statistical fit of the samples about a lead line and smoothes the initial scatter in contrast initial scatter will be preserved in data sets with a random relation between 1 and 2 values the 1 value estimated from the lead line will be biased towards the sample with the lowest 2 value

sr isotopic composition of paleoproterozoic 13c rich carbonate rocks the tulomozero formation se fennoscandian shield abstract rb sr systematics has been studied in 13c rich carbonate rocks of the paleoproterozoic 2 09 0 07 ga tulomozero formation in the northern onega lake area the se fennoscandian shield the formation is divided into eight members a f consisting of greenschist facies grade variegated sandstones siltstones mudstones stromatolitic dolostones and subordinate crystalline limestones samples of carbonate rocks were obtained from two overlapping drillholes intersecting the entire thickness of the tulomozero formation prior to isotope analysis the rocks powders were treated with 1n ammonium acetate for partial removal of the late epigenetic carbonate phases major resetting of the rb sr systems in the tulomozero carbonate rocks appears to take place during the svecofennian regional metamorphic event and it was screened by using mn sr fe sr mg ca and 18o 16o ratios high sr content up to 2080 g g in limestones and 530 g g in dolostones coupled with low fe mn 0 40 in dolostones of the upper part of member b and of members g and h is indicative of seawater influxes sea transgression into the tulomozero basin the 87sr 86sr values in the least altered mn sr

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

multiple sulfur isotopes of sulfides from sediments in the aftermath of paleoproterozoic glaciations geochemical evidence reported from paleoproterozoic sediments has long been used to evaluate the transition from the anoxic archean atmosphere to an oxygenated atmosphere sulfur isotopes 32s 33s 34s and 36s in sedimentary sulfides and sulfates are an especially sensitive means to monitor this transition such that the timing of the paleoproterozoic great oxidation event can be investigated using mass independently fractionated mif sulfur isotope systematics expressed as 33s here we report data from 83 individual analyses of pyrite pyrrhotite and chalcopyrite on a new suite of 30 different samples from finland south africa wyoming and ontario that span 600 my and follow one or several snowball earth events in the paleoproterozoic the samples were measured using a high resolution secondary ion mass spectrometry technique in multicollection mode that investigates multiple sulfur isotopes in microdomains 30 m within individual sulfide grains while preserving petrographic context we focused on sediments deposited in the aftermath of the paleoproterozoic glaciations between 1 9 and 2 2 ga to trace fluctuations in atmospheric o2 concentrations that were likely affected by an interplay of o2 sinks in the atmosphere and the upper ocean and continental crust and by the emergence and diversification of aerobic organisms our results demonstrate that mif sulfur isotopes are absent in sediments deposited after the period of protracted global cooling in the paleoproterozoic and independently confirm observations that mif ceased during this time we interpret our results by integrating 33s and 34s data in sulfides 13c data in carbonates and the estimated timing of glaciation events in the paleoproterozoic data strongly hint at the presence of microbial sulfate reduction and fluctuations in the concentration of dissolved seawater sulfate and or in 34ssulfate in the aftermath of glaciations and likely were affected by changing erosion rates and nutrient delivery to the oceans these changes modulated the population of primary producers especially oxygenic photosynthesizers and led to fluctuations in the abundance of atmospheric o2 co2 and ch4 our results support the interpretation that the world wide 13ccarb excursion observed between 2 25 and 2 05 ga karhu and holland 1996 was a period of significant accumulation of o2 in the atmosphere

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

bizarre geochemistry of komatiites from the crixas greenstone belt brazil komatiite lava flows in the crixas greenstone belt goias brazil have textures and volcanic structures typical of archean komatiites but are geochemically most unusual the flows are porphyritic and massive or layered with spinifex upper parts and olivine cumulate lower parts mgo contents range from 18 to 40 in such lavas only olivine and minor chromite can have crystallized but neither major nor trace elements fall on olivine control lines in mgo variation diagrams cao and sr fall on lines with slopes steeper than olivine control lines sio2 feo na2o k2o and y show little systematic variation zr shows a large variation that does not correlate with mgo and al2o3 decreases markedly with decreasing mgo the aberrant behaviour is highlighted by the ree rare earth elements in spinifex and olivine cumulate layers from three flows in the spinifex layers chondrite normalized ree patterns are hump shaped with maxima at nd or sm la sm n 0 6 gd yb n 1 6 2 3 whereas cumulate zones in the same flows have steadily sloping patterns with lree enriched relative to hree la sm n 1 3 gd yb n 1 4 neither normal magmatic processes acting during emplacement of the komatiites nor thermal erosion and wall rock assimilation can explain these effects and we speculate that elements commonly thought of as immobile e g al zr ree migrated during hydrothermal alteration or metamorphism a pb pb whole rock isochron gave an age of 2 728 140 ma and selected sm nd analyses an apparent isochron age of 2 825 98 ma nd 0 the pb pb age is believed to be the approximate time of emplacement interpretation of the sm nd data is complicated by the evidence of mobility of ree

isotope data on the age and genesis of layered basic ultrabasic intrusions in the kola peninsula and northern karelia northeastern baltic shield abstract new u pb zircon ages and sm nd data are presented for layered basic ultrabasic intrusions of the kola peninsula and northern karelia in the northeastern part of the baltic shield all ages are in the range between 2 5 and 2 4 ga but there appear to exist three phases of intrusion thus the monchegorsk intrusion and the fedorovy pansky massif have ages between 2493 and 2470 ma the kivakka and lukkulaisvaara massifs of the olanga group of intrusions between 2445 and 2435 ma while the imandra lopolith is 2396 7 ma old a narrow range of negative nd t values between 2 3 and 1 2 suggests formation by partial melting of a semi homogeneous enriched mantle source

the western border of the archaean province of the baltic shield evidence from northern sweden restricted areas of acid to intermediate gneisses in northernmost sweden are known to be archaean and several other gneissic granodioritic to tonalitic massifs have been suggested to be of this age to delimit the western border of the archaean province of the baltic shield and to characterize the swedish archaean we have studied the 2 83 ga old soppero gneiss and 8 of the proposed archaean massifs in northern sweden by sm nd isotopic analyses u pb zircon dating in two cases and geochemistry

the madinah eruption saudi arabia magma mixing and simultaneous extrusion of three basaltic chemical types during a 52 day eruption in 1256 a d 0 5 km3 of alkali olivine basalt was extruded from a 2 25 km long fissure at the north end of the harrat rahat lava field saudi arabia the eruption produced 6 scoria cones and a lava flow 23 km long that approached the ancient and holy city of madinah to within 8 km three chemical types of basalt are defined by data point clusters on variation diagrams i e the low k high k and hybrid types all three erupted simultaneously their distribution is delineated in both scoria cones and lava flow units from detailed mapping and a petrochemical study of 135 samples six flow units defined by distinct flow fronts represent extrusive pulses the high k type erupted during all six pulses the low k type during the first three and the hybrid type during the first two

crustal reflectivity near the archaean proterozoic boundary in northern sweden and implications for the tectonic evolution of the area summary sm nd isotope ratios of 1 9 1 8 ga granitoids delineate the archaean proterozoic boundary in northern sweden an important feature in the fennoscandian shield the boundary strikes approximately wnw ese and is defined as a c 20 km wide zone with juvenile palaeopro terozoic rocks to the ssw and archaean and proterozoic rocks derived to a large extent from archaean sources to the nne it therefore constitutes the strongly reworked margin of the old archaean craton extrapolation of the boundary offshore into the bothnian bay and correlation with the marine reflection seismic babel lines 2 and 3 4 indicates that the boundary dips to the south southwest consistent with interpretation of the sm nd data in order to tie the babel results with onshore surface geology and obtain detailed images of the uppermost crust a short 30 km of subsurface coverage pilot profile was acquired in the lulea area of northern sweden during august 1999 the profile consisted of a high resolution shallow com ponent 1 kg shots and a lower resolution deep component 12 kg shots both components image most of the reflective crust with the deep component providing a better image below 10 s comparison of signal penetration curves with data acquired over the trans scandinavian igneous belt a large batholith indicate the transparent nature of the crust there to be caused by geological factors not acquisition parameters lower crustal reflectivity patterns on the lulea test profile are similar to those observed on the babel lines suggesting the same lower crust onshore as offshore interpreted archaean reflective upper crust in the ne ex tends below more transparent proterozoic crust in the sw this transparent crust contains a number of high amplitude reflectors that may represent shear zones and or mafic rock within granite intrusions a marked boundary in the magnetic field in the sw has been interpreted as being the result of a gently west dipping contact zone between meta sediments and felsic volcanic rocks however the seismic data indicate a near vertical structure in this area by correlating the onshore and offshore seismic data we have better defined the location of the archaean proterozoic boundary on the babel profiles our new interpretation of the crustal structure along the northern part of the babel line 2 shows a more bi vergent geometry than previous interpretations comparison of the re interpreted crustal structure in northern sweden with that found in the middle urals shows several similarities in particular the ac cretion of a series of arcs to a stable craton based on this similarity and geological data we deduce that a continental arc accreted to the southwestern margin of the archaean craton at c 1 87 ga shortly thereafter the skellefte island arc underthrust the continental arc owing to a collision further to the southwest resulting in the bi vergent crustal structure observed today

applications of attenuation tomography to imperial valley and coso indian wells region southern california spatial variations in seismic wave attenuation were tomographically imaged from observed s to p wave amplitude ratios in the imperial valley and coso indian wells regions of southern california in the coso indian wells region a highly attenuating body s wave quality factor q 30 coincides with a slow p wave anomaly mapped by walck and clayton 1987 this coincidence suggests the presence of a 3 to 5 km depth magmatic or hydrothermal body in the indian wells region in the imperial alley slow p wave travel time anomalies and highly attenuating s wave anomalies were found in the brawicy seismic zone at a depth of 8 12 kin the effective s wave quality factor is very low q 20 and the p wave velocity is 10 slower than the surrounding area these results suggest either magmatic or hydrothermal intrusions or fractures at depth possibly related to active shear in the brawley seismic zone this attenuation tomographic technique is shown to be useful in delineating the spatial variations in seismic wave attenuation and in estimating the effective seismic quality factor of attenuation anomalies

linkage of crustal deformation between the archaean basement and the proterozoic cover in the perapohja area northern fennoscandia abstract reconstructed variations in stratigraphic thicknesses combined with mapped variations in fold and thrust belt structural styles and patterns of the entire palaeoproterozoic perapohja belt in northern finland show that both the deposition of the 2 4 1 9 ga supracrustal rocks and the structural evolution during the subsequent svecofennian orogeny 1 9 1 8 ga have been tightly controlled by i the palaeo topography of the archaean 2 5 ga basement and ii the reactivation of archaean basement faults during the multiple stages of rifting and deposition of the supracrustals as well as during subsequent basin inversion and fold and thrust belt development folds and thrusts in the supracrustal sequence are found to be largely detached from the underlying basement along the petajaskoski detachment zone pdz that formed within a mechanically weak stratigraphic unit in the basal part of the perapohja belt stratigraphy the pattern of contractional structures is complex but developed during one phase of simple progressive south vergent thrusting two styles of basement cover linkage are recognised the hard linked style is best illustrated by the reverse sihtuuna fault currently defining the northern margin of the e w to ene wsw trending basement horsts and the overlying anticlines and juxtaposing the stratigraphically lowermost perapohja belt rocks with higher up stratigraphy indicating the fault is inherited after a pre existing basement structure based on the distribution and thickness variation of the palaeoproterozoic supracrustal units the faults bounding the basement horst were active as extensional faults during deposition the soft linked style relationship is evident as abrupt terminations of thrust related elongate domes and localisation of the compressional overprint into the nw se trending central graben understanding the basement architecture and basement cover linkages is highly significant because the majority of the known mineralizations of the belt are located along or close to the basement faults

the lithospheric shear wave velocity structure of saudi arabia young volcanism in an old shield abstract we investigate the lithospheric shear wave velocity structure of saudi arabia by conducting h stacking analysis and jointly inverting teleseismic p receiver functions and fundamental mode rayleigh wave group velocities at 56 broadband stations deployed by the saudi geological survey sgs the study region the arabian plate is traditionally divided into the western arabian shield and the eastern arabian platform the arabian shield itself is a complicated melange of crustal material composed of several proterozoic terrains separated by ophiolite bearing suture zones and dotted by outcropping cenozoic volcanic rocks locally known as harrats the arabian platform is primarily covered by 8 to 10 km of paleozoic mesozoic and cenozoic sedimentary rocks our results reveal high vp vs ratios in the region of harrat lunayyir which are interpreted as solidified magma intrusions from old magmatic episodes in the shield our results also indicate slow velocities and large upper mantle lid temperatures below the southern and northern tips of the arabian shield when compared with the values obtained for the central shield we argue that our inferred patterns of lid velocity and temperature are due to heating by thermal conduction from the afar plume and possibly the jordan plume and that volcanism in western arabia may result from small scale adiabatic ascent of magma diapirs

high grade metamorphism of 2 45 2 4 ga age in mafic intrusions of the belomorian belt in the northeastern baltic shield

broad accommodation of rift related extension recorded by dyke intrusion in saudi arabia volcanic fields at the eastern margin of the red sea rift were regarded as seismically quiet until a swarm of 30 000 earthquakes struck in 2009 geological analyses reveal the intrusion of a volcanic dyke and indicate that rift related extension is spread over a broad region far from the rift axis