multiple episodes of dike emplacement along the northwestern margin of the superior province manitoba three stable magnetization components a b and c have been isolated from a set of dikes that have been collectively referred to as the molson swarm positive baked contact tests and square shouldered thermal decay curves indicate that these components were acquired during three periods of dike emplacement dikes carrying a with a pole at 15 4 n and 96 5 w a95 4 1 n 16 sites have been dated by the u pb method at 1883 ma and are closely related to the paleoproterozoic trans hudson orogen ages of b with a pole at 27 1 n and 140 8 w a95 3 8 a95 3 0 n 30 and c with a pole at 40 7 n and 156 3 w a95 9 7 n 19 are estimated at 2170 2120 ma on the basis of paleomagnetic characteristics that are distinct from the a component the analysis of metamorphic history of the pikwitonei domain and comparison with well dated paleomagnetic poles from mafic igneous rocks in the superior province the associated b and c dikes were emplaced in normal and reversed geomagnetic fields when the northwestern margin of the superior province experienced initial rifting prior to the trans hudson orogen these earlier dikes may correlate with known 2094 ma mafic magmatism documented in the lower hurwitz group of the hearne province a stable magnetization component d with a pole at 20 9 n and 133 6 w a95 7 6 n 30 samples was also identified from unbaked granulites throughout the pikwitonei domain this component predates all dike intrusion in the region has an age range of 2200 2170 ma and may have been caused by earlier uplift of the northwestern superior province

microwave paleointensities from dyke chilled margins a way to obtain long term variations in geodynamo intensity for the last three billion years absolute paleointensity values of the geomagnetic field measured using microwaves are obtained from chilled margin samples of proterozoic dykes from the southern superior province of the canadian shield two age groups of dykes are studied the 2446 2473 ma matachewan swarm and 2076 2170 ma dykes from the biscotasing marathon and fort frances swarms all samples selected for paleointensity study carry a dominant primary magnetization component as determined from detailed paleomagnetic studies paleointensity results quoted below are based on individual sample determinations having a quality factor greater than 10 for the matachewan dykes the mean virtual dipole moment vdm is 2 53 0 93 a m 2 10 22 based on 12 samples from 12 dykes these results compare favourably with thellier data from the same swarm by who obtained a mean vdm value based on 25 samples from five dykes of 2 80 0 87 a m 2 10 22 for the biscotasing marathon and fort frances swarms the mean vdm based on 12 samples from 12 dykes is 4 13 1 82 a m 2 10 22 a value significantly greater than that obtained by macouin et al for the same swarms of 0 98 0 10 a m 2 10 22 based on 25 samples from nine dykes possible reasons for this difference are discussed the microwave method is shown to be an effective means for measuring paleointensity on dyke chilled margins and offers the opportunity that dyke swarms which occur frequently throughout the last three billion years of geological history can be used to track any long period variations in the geomagnetic field intensity that reflect stages in the evolution of the earth s core

integrated paleomagnetic and upb geochronologic studies of mafic intrusions in the southern canadian shield implications for the early proterozoic polar wander path abstract integrated paleomagnetic and u pb geochronologic studies are critical to establishing well dated apparent polar wander paths in the precambrian by collecting samples for both paleomagnetism and u pb geochronology at the same sampling sites and conducting rigorous paleomagnetic field tests to ensure that magnetizations are primary it is now possible to establish the age of paleomagnetic poles with uncertainties of no more than a few million years we have conducted such integrated studies on mafic intrusions of the southern superior province and adjacent portions of the southern province of the canadian shield these studies yield paleomagnetic tiepoints with high precision ages for use in polar wander path construction in the 2220 to 2110 ma period they indicate a direction of polar wander contrary to that of the most widely used paths for the superior province and for the canadian shield as a whole thus current interpretations of the movement of the superior province and the relative location and movement of the superior province and other structural elements of the canadian shield during this period are no longer valid

crustal shortening during the paleoproterozoic can it be accommodated by paleomagnetic data abstract in the last 130 million years perhaps as much as 2000 km of intra continental shortening has taken place within asia during 10 000 km of convergence between siberia and india with 8000 km being accommodated by oceanic subduction given that earth was hotter during the paleoproterozoic crustal shortening at this time may have been equally if not more significant compared to today warranting a search of the paleomagnetic data base for any supporting evidence paleomagnetic data from units on either side of the paleoproterozoic trans hudson orogen in canada have been chosen because this orogen bears close similarity both in size and history to the india siberia one the data do not prove but can accommodate continental crustal shortening between the archean slave and superior provinces of about 3000 1000 km the results allow crustal shortening to have been a significant component of continental collision in the paleoproterozoic an important issue in any re assembly of precambrian continents if true the present dimensions of precambrian shields may be significantly different compared to those existing in earlier times

anisotropy of magnetic susceptibility investigations of the st malo dyke swarm brittany france emplacement mechanism of doleritic intrusions a high quality aeromagnetic survey of northern brittany line spacing 250 m flight elevation 150 m has been used to delineate the lower carboniferous st malo dyke swarm in detail the dyke swarm fans at its northern and southern ends and is affected by n60 sinistral transcurrent faults after restoration of these offsets the full structure trends in a n s direction small dykes are not imaged and only one third of the swarm is evidenced by magnetism gravity and magnetic modelling shows that the swarm overlies a single n s elongated magma chamber the distribution of kmax inclinations of anisotropy of magnetic susceptibility ams suggests that the dykes display a fanning magma flow in section computed kmax inclinations are usually shallower than the measured geological dips probably because the flow becomes more disturbed as the dyke becomes shallower we observe that the mean susceptibility values increase when the magma flow is steeper than about 70 a detailed cross section of st briac dyke which is part of the st malo dyke swarm reveals that the main carrier of magnetization is magnetite in the centre of the dyke and magnetite maghaemite on its rims the emplacement of the st malo dyke swarm could have been contemporaneous with the n60 shearing which displaced the dyke swarm by about 20 km the dyke swarm is cut at its southern end by an e w orientated fault which probably acted during late carboniferous times

tectonic implications of paleomagnetic data from sills and dykes in the sverdrup basin canadian arctic mesozoic intrusives 71 sills and 12 dykes from different structures in the folded eastern sverdrup basin contain stable well grouped paleomagnetic directions of both normal and reverse polarity the mean direction is 274 79 k 173 95 3 7 with a corresponding pole position at 157 67 n k 61 a95 6 6 the paleomagnetic fold test shows that the major deformation involving tilting of strata around strike postdated the emplacement of sills and dykes minor preintrusion deformation may also have occurred but more importantly an offset of the paleopole from the cretaceous section of the polar wander path permits a postintrusion anticlockwise rotation of eastern sverdrup basin of about 15 20 with respect to stable north america if the intrusives are cretaceous the degree of rotation is close to that 11 estimated for greenland during the opening of the labrador sea no positive evidence has been found to support models of distributed shear with block rotations to accommodate the contentious northeast translation of greenland along the nares strait

paleoproterozoic basin development and sedimentation in the lake superior region north america abstract the peneplaned archean craton in the lake superior region was the platform upon which a continental margin assemblage was deposited extension resulted in localized rifts that received thicker accumulations of sediments and volcanic rocks than did adjacent parts of the platform seas transgressed onto the continent several times and an ocean basin opened south of the present day lake superior island arcs that formed during subduction collided with the craton margin as the ocean basin closed oceanic crust is poorly preserved as a dismembered ophiolite sequence the arc volcanics are preserved as the wisconsin magmatic terranes the collision resulted in a fold and thrust belt known as the penokean orogen to the north of the fold and thrust belt a northward migrating foreland basin the animikie basin developed thick turbidite successions were deposited along the basin axis and terrigenous clastics and lake superior type iron formation were deposited on the shelf along the northern margin of the basin the primary paleoclimatic indicators are 1 glaciogenic rocks at the base of the paleoproterozoic succession in michigan indicating ice house conditions 2 remnants of a paleosol on the glaciogenic rocks indicative of deep weathering probably under subtropical conditions and therefore of greenhouse conditions and 3 carbonate minerals after gypsum halite and anhydrite in stromatolitic dolomite indicative of aridity three second order depositional sequences are bounded by major unconformities and can be correlated throughout the lake superior region

metasaprolite in the mcgrath gneiss minnesota usa viewing paleoproterozoic weathering through a veil of metamorphism and metasomatism abstract a 2100 ma 420 cm thick metasaprolite which is devoid of plagioclase but contains abundant microcline occurs in the archean mcgrath gneiss beneath the overlying paleoproterozoic denham formation despite being recrystallized under amphibolite facies conditions and chemically modified by potassium metasomatism mass fluxes related to weathering and metasomatism of the mcgrath metasaprolite can be estimated by judicious application of several geochemical parameters and the a c n k plot fedo et al 1995 the metasaprolite yields values of 67 94 for the plagioclase index of alteration 62 72 for the chemical index of alteration and 47 for the feldspar index of weathering and the total mass flux removed by weathering is a minimum of 2 4 moles cm 2 such values correspond to an intermediate degree or intensity of weathering compared to other more intensely weathered paleosols in the region such as those beneath the baraboo and sioux quartzites potassium metasomatism occurred at 1742 ma during the geon 17 yavapai tectonothermal event when a minimum of 0 33 moles cm 2 k 2 o was added to the metasaprolite although the absence of pedogenic textures precludes interpretation of climatic conditions during weathering of the mcgrath gneiss the level of atmospheric p co 2 can be estimated by application of sheldon s 2006 method which yields a minimum of 3 7 pal for a weathering duration of 100 000 years

restoring proterozoic deformation within the superior craton abstract geometrical patterns of paleoproterozoic dyke swarms in the superior craton north america and paleomagnetic studies of those dykes both indicate relative motion across the kapuskasing structural zone ksz that divides the craton into eastern and western sectors previous work has optimized the amount of vertical axis rotation necessary to bring the dyke trends and paleomagnetic remanence declinations into alignment yet such calculations are not kinematically viable in a plate tectonic framework here we subdivide the superior craton into two internally rigid subplates and calculate euler parameters that optimally group the paleomagnetic remanence data from six dyke swarms with ages between 2470 and 2070 ma our dataset includes 59 sites from the matachewan dykes for which directional results are reported for the first time our preferred restoration of the eastern superior subprovince relative to the western subprovince is around an euler pole at 51 n 85 w with a rotation angle of 14 ccw although we do not include data from the ksz in our rigid subplate calculations we can align its dyke strikes by applying a 23 ccw distributed shear that preserves line length of all dykes pinned to the western margin our model predicts approximately 90 km of dextral transpressional displacement at ca 1900 ma about half of which is accommodated by distributed strain within the ksz and the other half by oblique lateral thrusting with ne vergence across the ivanhoe lake shear zone we produce a combined apparent polar wander path for the early paleoproterozoic superior craton that incorporates data from both western and eastern subplates and that can be rotated to either of the subplates reference frames for the purposes of archean paleoproterozoic supercraton reconstructions

palaeomagnetism geochronology and geochemistry of the palaeoproterozoic rabbit creek and powder river dyke swarms implications for wyoming in supercraton superia it is likely that archaean cratons of laurentia had different palaeogeographic histories prior to their amalgamation new palaeomagnetic geochronological and geochemical evidence supports a reconstruction of the wyoming craton adjacent to the southern margin of the superior craton at 2 16 ga before rifting c 2 1 2 0 ga and eventual reamalgamation after the hudsonian orogeny c 1 8 ga u pb ages tims on baddeleyite from five dykes yield two groups of ages at c 2164 and 2155 ma the younger group of ages defines the rabbit creek swarm at 2161 2152 ma and precisely dates its palaeomagnetic pole two large and differentiated dykes 100 m in the bighorn and wind river uplifts are geographically related to the rabbit creek swarm but have slightly different orientations and yield slightly older ages at 2171 2157 ma these dykes may be parts of a single intrusion the great dyke of wyoming that spans over 200 km between uplifts possibly representing a different magmatic event this older event does not have enough distinct intrusions to provide a correctly averaged palaeomagnetic pole but correlates with magmatism in the superior craton and has a palaeomagnetic remanence comparable to the rabbit creek dykes with minor tilt corrections the palaeomagnetic data from the rabbit creek swarm and powder river south pass dykes support a reconstruction of the southeastern wyoming craton against the southern superior craton this fit juxtaposes the palaeoproterozoic huronian and snowy pass supergroups along two passive margins that experienced a prolonged period of mafic magmatism 100 myr and rift basin development although there are slight geochemical variations across the rabbit creek swarm all dykes fit into two distinct groups that are independently dated and internally consistent

paleomagnetism anisotropy of magnetic susceptibility and argon argon geochronology of the clearwater anorthosite saskatchewan canada abstract the clearwater anorthosite in saskatchewan is a major gabbro anorthosite layered intrusion lying within the archean rae province along the northwestern hinterland of the paleoproterozoic trans hudson orogen tho the intrusion which could be at least 2 4 ga old carries a well defined remanent magnetization with a direction of declination 112 and inclination 38 k 316 95 3 1 n 8 the corresponding paleomagnetic pole plat 6 5 n plong 48 2 w dp 2 2 dm 3 7 is the first one obtained from the archean western granulite domain of the rae province field observations supplemented by anisotropy of magnetic susceptibility data show that the magnetization post dates ductile deformation of the anorthosite argon argon plateau dates of 1917 7 ma for hornblende and 1839 5 ma for biotite are interpreted as cooling ages with the hornblende age approximating the time of remanence the pole position and others of comparable age from the northwestern margin of the tho form a loose cluster the mean of which is about 90 of arc away from that of a broadly contemporaneous group of poles from the superior province this relation supports previous interpretations of paleomagnetic data that prior to the tho at about 1 9 ga ago a major ocean the manikewan ocean at least 5000 km across separated the superior and rae hearne provinces

paleomagnetism of ca 2 13 2 11 ga indin and ca 1 885 ga ghost dyke swarms of the slave craton implications for the slave craton apw path and relative drift of slave superior and siberian cratons in the paleoproterozoic of 35 archean cratons that have been identified around the globe only one the superior craton of the canadian shield has a reasonably well defined apparent polar wander apw path for much of paleoproterozoic time based on key i e well defined and precisely dated paleopoles as a result it has been difficult to compare the drift of these cratons or reliably test continental reconstructions of the archean cratons in the paleoproterozoic based on paleomagnetism in this study we report key paleopoles for the 2 13 2 11 ga northwest to north northwest trending indin dyke swarm 36 n 76 w a 95 7 and the 1 885 ga northeast to north northeast trending ghost swarm 2 n 106 w a 95 6 of the yellowknife region of the slave craton u pb baddeleyite ages have been determined at paleomagnetic sampling sites and baked contact tests establish that the remanences are primary combined with paleopoles from other precisely dated dyke swarms of the slave craton these data define a rudimentary apw path between ca 2 23 and 1 885 ga and permit a comparison of the drift of the slave and superior cratons over this interval both the indin and ghost poles are precisely matched in age with key poles on the superior apw path the slave and superior paths are not superimposed demonstrating that the two cratons were not in their present relative locations they have different overall shapes indicating relative drift during at least a portion of the period however the earlier 2 23 2 21 ga portions of the tracks appear broadly similar and could permit the two cratons to be on a single tectonic plate at that time although separated by a great distance a comparison of the ghost pole and a coeval key pole for siberia permits a 1 88 ga reconstruction with southern siberia facing the northern slave laurentia margin broadly similar to reconstructions that have been proposed at least as late as 1 38 ga although the distance between the cratons is still poorly constrained

differences in paleomagnetic interpretations due to the choice of statistical demagnetization and correction techniques kapuskasing structural zone northern ontario canada abstract the kapuskasing structural zone ksz reveals a section through the archean lower crustal granoblastic gneisses our new paleomagnetic data largely agree with previous work but we show that interpretations vary according to the choices of statistical demagnetization and field correction techniques first where the orientation distribution of characteristic remanence directions on the sphere is not symmetrically circular the commonly used statistical model is invalid fisher r a proc r soc a217 1953 295 any tendency to form an elliptical distribution indicates that the sample is drawn from a bingham type population bingham c 1964 distributions on the sphere and on the projective plane phd thesis yale university fisher and bingham statistics produce different confidence estimates from the same data and the traditionally defined mean vector may differ from the maximum eigenvector of an orthorhombic bingham distribution it seems prudent to apply both models wherever a non fisher population is suspected and that may be appropriate in any tectonized rocks non fisher populations require larger sample sizes so that focussing on individual sites may not be the most effective policy in tectonized rocks more dispersed sampling across tectonic structures may be more productive second from the same specimens mean vectors isolated by thermal and alternating field af demagnetization differ which treatment gives more meaningful results is difficult to decipher especially in metamorphic rocks where the history of the magnetic minerals is not easily related to the ages of tectonic and petrological events in this study thermal demagnetization gave lower inclinations for paleomagnetic vectors and thus more distant paleopoles third of more parochial significance tilt corrections may be unnecessary in the ksz because magnetic fabrics and thrust ramp are constant in orientation to the depth at which they level off at approximately 15 km depth with archean geothermal gradients primary remanences were blocked after the foliation was tilted to rise on the thrust ramp therefore the rocks were probably magnetized in their present orientation tilting largely or entirely predates magnetization

analysis of long term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics summary the 1167 published cooling unit cu palaeointensity estimates contained in the 400 10 ma portion of the pint global database were rigorously filtered according to accurate age determinations palaeodirectional reliability recognition of polarity and the method of palaeointensity acquisition the remaining 865 estimates group 1 were further filtered to ensure self consistency reducing the data set to 425 estimates group 2 group 1 and 2 data were clustered into temporally and or spatially distinct rock suites rs enabling each part of the record to be assessed for potential biasing by overrepresentation of palaeosecular variation psv the record was segmented according to the distribution of the data rather than using arbitrary time windows to ensure quasi consistent behaviour within each segment differences between these segments clearly indicate that a significant long timescale 10 7 and 10 8 yr variation of the mean geomagnetic poloidal field intensity gpfi occurred during the 400 10 ma period and hence that changing lowermost mantle conditions affect the capacity of the geodynamo to generate a poloidal field both the mean dipole moment and its standard deviation appear to be a function of the range of values each cu may adopt at one particular time this range is itself controlled by the variation of the maximum limit of dipole moment while the value of the minimum limit remains relatively constant tentative support is provided for the recent suggestion that psv may have been reduced during the cretaceous normal superchron cns though more data are needed in the range 120 60 ma to confirm this no conclusive evidence was found to support the suggestion that the gpfi record may be biased towards low or high values by palaeointensity determinations obtained using methods that do not adopt ptrm checks indeed offsets caused by unreliable data in well represented parts of the record are likely to be random and cancel one another out when gpfi variation is analysed at a sufficiently high resolution to allow comparisons with the geomagnetic polarity reversal frequency rf it is not possible to confirm whether the two parameters are anticorrelated decoupled or related in some more complex way however it is clear that gpfi and rf are definitely not positively correlated as has been previously suggested the present database documents sharp increases in gpfi around the onset times of the two recognized superchrons itself implying an anticorrelation the implications for geodynamo and mantle modelling of both an anticorrelation and a decoupling of the geomagnetic parameters are discussed briefly a generic geodynamic model is proposed to explain the relationship between observed longterm changes in gpfi and global geodynamic processes the model predicts that changes in gpfi result from a chain of geodynamic processes extending from crust to core beginning with plate reorganizations at the surface and culminating in increases in the vigour of outer core convection supercontinents are transient surface expressions of such geodynamic processes and provide the potential to test the generic model four time stages are proposed to describe the major long term changes in gpfi since the early devonian 400 350 350 250 250 175

an investigation of superior shoal central lake superior with a manned submersible a johnson sea link submersible was used to examine the geology of superior shoal in central lake superior here glacially scoured vertical cliffs some more than 100 m high are formed of 1 1 ga middle keweenawan basaltic lava flows displaying ophitic interiors and red amygdaloidal tops flat lying sandstones lithologically similar to the upper keweenawan bayfield jacobsville sequences occur to the north of the volcanic rocks these are inferred to have been downthrown along an eastward extension of the isle royale fault a major boundary fault of the midcontinent rift the volcanic rocks are normally magnetized supporting lithological evidence that they correlate with the middle keweenawan sequence on isle royale paleomagnetic data suggest that the volcanics have a complex structure possibly involving drag folding along the isle royale fault

paleomagnetism and u pb geochronology of diabase dyke swarms of minto block superior province quebec canada in the first collaborative study of paleomagnetism and precise u pb geochronology in the minto block of the superior province mafic dyke swarms with three widely divergent paleomagnetic signatures and isotopic ages have been identified the 2505 2 ma ptarmigan dykes trend north to northeast and have a virtual geomagnetic pole at 42 s 220 e similar to that of 2473 2446 ma matachewan dykes of the southern superior province the ca 2230 ma maguire dykes trend west to northwest and yield a paleopole at 9 s 267 e similar to those for 2216 8 4 ma senneterre dykes and 2217 2210 ma nipissing sills of the southern superior and southern provinces respectively the 2209 1 ma klotz dykes trend west northwest but do not carry a consistent magnetization direction finally 1998 2 ma minto dykes of west northwest to northwest trend identical in age to the 1998 ma 2 ma purtuniq ophiolite of the cape smith belt have a paleopole at 38 n 174 e the similarity of paleopoles for the ca 2 23 2 21 ga maguire

paleomagnetic pole from the yilgarn b yb dykes of western australia no longer relevant to rodinia reconstructions abstract the paleomagnetic pole from the yilgarn b yb dykes of western australia was for many years considered to be the only primary pole for australia in the interval 1050 700 ma and has been used to constrain reconstructions of the neoproterozoic supercontinent rodinia we conducted a detailed paleomagnetic study of one of the yb dykes resampling the same outcrops from which a positive baked contact test was reported previously our results yield a negative baked contact test and combined with petrographic and rock magnetic data indicate that the yb dyke magnetisation is a secondary overprint the yb dyke pole should no longer be used in testing rodinia reconstructions comparison with the apparent polar wander path for phanerozoic australia suggests the yb magnetisation is mesozoic in age although rocks along the western margin of the yilgarn craton preserve early paleozoic rb sr biotite ages

paleomagnetism and u pb geochronology of the lac de gras diabase dyke swarm slave province canada implications for relative drift of slave and superior provinces in the paleoproterozoicgeological survey of canada contribution 20080350 lac de gras diabase dykes trend north to nne across the central slave province of the canadian shield u pb baddeleyite ages of 2023 2 and 2027 4 ma are interpreted as dyke emplacement ages th

geological and palaeomagnetic significance of the kulgera dyke swarm musgrave block nt australia summary the kulgera dyke swarm consists of olivine tholeiites which have intruded late proterozoic transitional granulite gneisses and granites of the eastern musgrave block in central australia preliminary rb sr results suggest that the dolerites were emplaced at 1054 f 14ma in addition a rb sr age of 1060 f 10ma on a biotite from a pegmatite indicates thermal resetting of the country rock minerals during dyke emplacement palaeomagnetic investigations of the dykes yield a primary thermoremanent magnetization direction corresponding to a palaeomagnetic pole at 17 s 266 e ag5 12 in addition to this primary magnetization an overprint component was present in many of the samples providing a palaeomagnetic pole at 30 s 138 e ags 24 which is similar to previous results from other central australian rocks affected by the alice springs orogeny the results extend the area of influence of the carboniferous alice springs orogeny southward into the musgrave block further the results provide no evidence for an earlier late proterozoic petermann orogeny affecting the musgrave block in the kulgera region however the possibility that a petermann orogeny thermal overprint has been erased by the alice springs orogeny cannot be dismissed

malley diabase dykes of the slave craton canadian shield u pb age paleomagnetism and implications for continental reconstructions in the early paleoproterozoic1geological survey of canada contribution 20110114 the ne trending malley dyke swarm dated herein at 2231 2 ma u pb baddeleyite extends from the central slave craton to the vicinity of the kilohigok basin and may continue farther to the nort

paleomagnetic and geochronological studies on paleoproterozoic diabase dykes of karelia east finland key for testing the superia supercraton paleomagnetic results are presented for two paleoproterozoic mafic dykes in the taivalkoski area in northern karelia province of the fennoscandian shield where based on k ar data the crust has seen minimal effects of the otherwise pervasive 1 8 1 9 ga svecofennian orogeny within this study a new u pb baddeleyite age of 2339 18 ma has been determined for one of the e w trending dykes dyke ad13 the paleomagnetic results show that a strong svecofennian overprinting is pervasive in the area upon thermal or af demagnetization four remanence directions were obtained most typical are the secondary svecofennian remanence direction a intermediate down to the nnw and remanence direction b intermediate down to the nne component d d 115 4 degrees 1 50 5 degrees alpha 95 2 6 degrees yielding a virtual geomagnetic pole vgp d plat 19 5 degrees n plon 263 3 degrees a95 3 1 degrees is obtained from baked rocks for dyke wd and based on a positive baked contact test is interpreted to represent the primary magnetization dating from about 2 4 ga dyke ad13 carries only secondary a and b components its unbaked host migmatites carry reversed a a r component and the baked host rock carries a component d d 134 5 degrees 1 7 3 degrees alpha 95 8 8 degrees which yields a vgp pole d plat 20 4 degrees n plon 257 3 degrees a 95 7 6 degrees possibly representing magnetization at 2 3 ga the new paleomagnetic data from the karelia province compared to similar aged paleomagnetic data from the superior province does not support the recently proposed superia configuration based upon dyke swarm trajectories c 2013 elsevier b v all rights reserved

paleomagnetism and u pb geochronology of the 2 17 ga biscotasing dyke swarm ontario canada evidence for vertical axis crustal rotation across the kapuskasing zone u pb dates on baddeleyite yield ages of 2167 8 2 2 and 2171 6 1 2 ma on two northeast trending dykes west of the kapuskasing zone in ontario canada these ages identify the dykes as belonging to the biscotasing dyke swarm east of the kapuskasing zone which was previously dated at 2166 7 1 4 ma by u pb on baddeleyite and zircon the new dates show that the biscotasing swarm was emplaced over an area of at least 300 000 km2 much larger than hitherto suspected and in a geologically short period of time of about 5 million years a comparison of paleomagnetic data from biscotasing and 2 45 ga matachewan dykes on either side of the kapuskasing zone suggests that the western half of the superior province has rotated about 10 20 counterclockwise relative to the eastern half across the kapuskasing zone this movement may have been accompanied by rifting farther north which ultimately led to the paleoproterozoic embayment underlying hudson bay that gives the superior province its characteristic butter

broad scale proterozoic deformation of the central superior province revealed by paleomagnetism of the 2 45 ga matachewan dyke swarm an extensive paleomagnetic study of the 2 45 ga matachewan dyke swarm of the north american superior province suggests that the interior of an archean shield can undergo broad scale distortion as a result of later proterozoic orogenic activity around the craton margins data collected from over 300 sites of which 137 are reported here for the first time reveal that the dykes contain a dual polarity primary remanence that varies across the swarm in both inclination and declination these regional variations are statistically significant at the 95 confidence level and cannot be attributed to remagnetization or to magnetic anisotropy inclination variation is probably due to real or apparent polar wander during the emplacement of the swarm and may in part explain the declination variation as well however for dykes within and northwest of the kapuskasing structural zone ksz a positive correlation is found between regionally averaged values of declination and dyke trend here the dykes appear to hav

a 2082 ma radiating dyke swarm in the eastern dharwar craton southern india and its implications to cuddapah basin formation abstract using consistent paleomagnetic data together with precise pb pb baddeleyite ages on a series of mafic dykes occurring over an area of at least 70 000 km2 a 2081 8 1 1 ma weighted mean of 4 dykes dyke swarm was identified intruding the archean basement rocks in the eastern dharwar craton skirting the cuddapah basin on its north northwest and western flanks the geometry of these dykes collectively due to their progressive variation in trend from n134 w to n28 e defines a fan angle of about 162 and forms a spectacular radiating swarm converging toward a focal point beneath the cuddapah basin anisotropy of magnetic susceptibility ams investigations on these dykes based on the orientation of principal eigenvectors indicate magma flow was vertically upward in them suggesting the magma source of these dykes to be proximal to the sampling sites these features together with reported geophysical evidence for high density material below the cuddapah basin suggest that this mafic volcanic province probably formed due to the impact of an asthenospheric mantle upwelling perhaps triggered by a plume head or other causal mechanisms like global warming of mantle or small scale instability like edge driven convection this may have resulted in the domal uplift of the continental lithosphere large scale crustal extension and thinning followed by thermal relaxation and subsidence that may have been responsible for the formation of the intra cratonic cuddapah basin shortly after 2082 ma dyke swarms of this age fort frances dykes 2076 5 4 ma or of slightly younger lac esprit dykes 2069 1 ma and older cauchon lake dykes 2091 1 1 8 2 1 ma ages are fairly wide spread in the superior province however a reconstruction of the paleopositions of dharwar cuddapah dykes 38 n 180 e a95 4 and superior at 2080 ma using paleomagnetic data fort frances dykes p 43 n lp 184 e does not suggest a close proximity for these provinces at that time their disparate locations could therefore suggest these were two distinct nodes of wide spread magmatism between 2080 ma and 2065 ma

a paleomagnetic and u pb geochronology study of the western end of the grenville dyke swarm rapid changes in paleomagnetic field direction at ca 585 ma related to polarity reversals a paleomagnetic study of the western end of the similar to 585 ma grenville dyke swarm shows that individual dykes are characterized by high coercivity and unblocking temperature magnetizations that can differ in direction by as much as 90 degrees field tests including baked contact studies and the continuity of paleomagnetic direction along dyke strike suggest that the magnetizations are primary precise u pb baddeleyite dating on these dykes indicates that changes in magnetization direction of similar to 90 degrees occur in less than 4 million years and their close temporal association with reversals of the axial dipole field suggest that certain dykes are recording an equatorial dipole field as a transitional field between opposite polarity states the documented instability in the earth s field occurs less than 10 myr before the first recorded appearance of macroscopic multi cellular organisms on earth inviting speculation that an extended period of frequent magnetic reversals may play a part in faunal evolution c 2014 elsevier b v all rights reserved less

relationship between co2 dominated fluids hydrothermal alterations and gold mineralization in the red lake greenstone belt canada abstract the red lake greenstone belt is one of the foremost au mining camps in canada and hosts the world class campbell red lake au deposit belt scale hydrothermal alteration is characterized by proximal ferroan dolomite zones associated with au mineralization surrounded by distal calcite zones both being accompanied by potassic alterations sericite muscovite and biotite at the campbell red lake and cochenour deposits au mineralization in particular high grade ore is associated with silica and sulfides especially arsenopyrite that replace carbonate quartz veins and the host rocks the prevalence of carbonic fluid inclusions and rare occurrence of aqueous bearing inclusions in carbonate quartz au veins in the campbell red lake deposit and the consistency of homogenization temperatures of carbonic inclusions within individual fluid inclusion assemblages fia have been interpreted to indicate that h2o poor co2 dominated fluids were responsible for the carbonate veining and au mineralization further studies of fluid inclusions in carbonate quartz veins within and outside the deformation zone hosting the campbell red lake deposit the red lake mine trend including the cochenour au deposit the redcon au prospect and outcrops in the distal calcite zone also reveal the dominance of carbonic fluid inclusions in vein minerals these studies indicate that co2 dominated fluids were flowing through fractures during carbonate vein formation and au mineralization both within and outside major structures the carbonic fluid may have been initially undersaturated with water or it may have resulted from phase separation of an h2o co2 nacl fluid in the latter case phase separation modeling indicates that the initial fluid likely had x co 2 values larger than 0 8 calculations based on hydrothermal mineral assemblages indicate x co 2 values in the host rocks from 0 025 to 0 85 reflecting a change from co2 dominated fluids in the fractures veins to h2o dominated fluids in the host rocks away from the fractures the co2 dominated fluids were likely advected from granulite facies in the deeper crust whereas the h2o dominated fluids were derived from the ambient host rocks of amphibole to greenschist facies calculations based on co2 requirements and source constraints indicate that the mineralizing fluids were likely two orders of magnitude more enriched in au than the commonly assumed values of a few g l which may explain why the campbell red lake deposit has a very high grade of au average 21 g t for the whole deposit and 81 g t for the goldcorp high grade zone fluid inclusion data suggest that the carbonate veining and au mineralization likely took place at depths from 7 to 14 km the development of crustiform colloform structures in the carbonate quartz veins which was previously interpreted to indicate relatively shallow environments may alternatively have been related to extremely high fluid pressures and the co2 dominated nature of the fluids which could have enhanced the brittle properties of the rocks due to their high wetting angles

age of the richmond gulf group and implications for rifting in the trans hudson orogen canada abstract the aphebian richmond gulf group of the east coast of hudson bay is a fluvial sandstone sequence containing basalt flows it was shed eastward from a rifting related thermal dome situated in the present central part of hudson bay sandstone from the base of the group contains a uraniferous diagenetic apatite cement u pb and pb pb isochron analysis from this material show the age of the apatite haematite cement to be 2025 25 ma old these data provide an older age bracket for the deposition of the richmond gulf nastapoka and belcher groups they also date the formation of the richmond gulf graben an aulacogen related to the initial rifting of the trans hudson orogen

a precursor to the matuyama brunhes reversal in chinese loess and its palaeomagnetic and stratigraphic significance summary we report high resolution palaeomagnetic results across the lower part of s8 in the luochuan area northwest of china using parallel subsets samples a palaeomagnetic anomaly with low palaeointensity and disordered magnetic direction was identified 21 ka prior to the matuyama brunhes mb polarity reversal and is consistent with the mb precursor observed from marine sediments and lava flows this is the first convincing report of the mb precursor in terrestrial material chinese loess which attests the global feature of the mb precursor constrained by the stratigraphic position of both the mb polarity reversal and the mb precursor the loess unit l8 and palaeosol unit s8 of the chinese loess sequence are undoubtedly tied to the marine isotope stages 18 and 19 respectively this correlation is critical for an accurate understanding of the correspondences of palaeomagnetic records between the chinese loess and marine sediments

the matachewan dyke swarm canada an early proterozoic magnetic field reversal abstract consistent age relationships between oppositely magnetized dykes of the 2 45 ga matachewan dyke swarm suggest that only a single magnetic field reversal occurred during the period of igneous activity the magnetic field throughout most of this time was characterized by a ssw declination and shallow negative inclination but reversed toward the waning stages of magmatism the new paleomagnetic data provide the oldest known magnetic reversal for which the relative reversal sense is known

a 100 km long paleomagnetic traverse radial to the sudbury structure canada and its bearing on proterozoic deformation and metamorphism of the surrounding basement abstract the 1850 ma sudbury igneous complex sic considered to be a composite melt sheet of a major meteorite impact has been deformed into an oval shaped basin known as the sudbury structure this paper explores to what extent this deformation has been communicated to the surrounding archean basement around the northern margin of the sic paleomagnetism of 2450 ma matachewan dykes and 1850 ma impact breccia along a traverse about 100 km long and normal to strike of the contact between the sic and the basement suggests that the basement beneath the nw corner of the sudbury structure has been tilted to the se within about 10 km of the contact at this distance a possible fault separates the tilted region from one that shows no evidence of tilting petrographically the dykes out to a distance of about 50 km distant from the sic are altered to upper greenschist facies of metamorphism with a fibrous amphibole replacing pyroxene and with loss of primary texture that characterizes less altered matachewan dykes at distances greater than 50 km the direction of magnetization found in the altered matachewan dykes is an overprint which is probably associated with regional metamorphism related to orogenesis or possibly with thermo chemical alteration associated with sic emplacement the direction of the component is compatible with an age of about 1 8 to 1 9 ga suggesting that the penokean orogen is the most likely cause if not the impact event the paleomagnetism of the breccias together with shatter cone orientation data suggests that within 10 km of the sic basement contact basement tilting to the southeast increases towards the sic

petrogenesis of the early proterozoic matachewan dyke swarm canada and implications for magma emplacement and subsequent deformation major minor and rare earth element analyses of fine grained marginal material from 249 dykes of the matachewan giant radiating swarm show that the range in compositions cannot be produced by any simple fractionation model instead a two stage process is required whereby melts were modified by both fractionation and assimilation of lower crustal material before rising and ponding at shallower crustal levels 15 20 km maximum where a combination of fractional crystallization plagioclase phenocryst formation and periodic melt replenishment took place these shallow crustal chambers acted as feeders for the numerous dykes one hypothesis compatible with these observations involves an extensive rising plume that would provide heterogeneous melts over a large area accompanied by stresses that could produce a radial pattern of dykes in a search for indications of lateral magma flow a comparison of longitudinal variations in composition along four sample traverses across the western subswarm showed no t

in situ u pb geochronology and geochemistry of a 1 13 ga mafic dyke suite at bunger hills east antarctica the end of the albany fraser orogeny abstract antarctica contains continental fragments of australian indian and african affinities and is one of the key elements in the reconstruction of nuna rodinia and gondwana the bunger hills region in east antarctica is widely interpreted as a remnant of the mesoproterozoic albany fraser orogen which formed during collision between the west australian and mawson cratons and is linked with the assembly of rodinia previous studies have suggested that several generations of mafic dyke suites are present at bunger hills but an understanding of their origin and tectonic context is limited by the lack of precise age constraints new in situ shrimp u pb zircon and baddeleyite dates of respectively 1134 9 ma and 1131 16 ma confirm an earlier rb sr whole rock age estimate of ca 1140 ma for emplacement of a major mafic dyke suite in the area existing and new geochemical data suggest that the source of the dyke involved an emorb like source reservoir that was contaminated by a lower crust like component the new age constraint indicates that the dykes post date the last known phase of plutonism at bunger hills by ca 20 million years and were emplaced at the end of stage 2 of the albany fraser orogeny in current models post orogenic uplift and progressive tectonic thinning of the lithosphere were associated with melting and reworking of lower and middle crust that produced abundant plutonic rocks at bunger hills a major episode of mafic dyke emplacement following uplift cooling and plutonic activity with increasing mantle input suggests that the dykes mark the end of a prolonged interval of thermal weakening of the lithosphere that may have been associated with continued mafic underplating during orogenic collapse if the undated olivine gabbro dykes with similar trend geochemistry and petrology at windmill islands are coeval with the ca 1134 ma dyke at bunger hills this would suggest the presence of a major dyke swarm at least 400 km in extent in such case the dykes could have been emplaced laterally from a much more distant mantle source possibly a plume and interacted with the locally heterogeneous and variably metasomatised lithosphere

paleomagnetism and upb geochronology of volcanic rocks from michipicoten island lake superior canada precise calibration of the keweenawan polar wander track abstract paleomagnetic poles have been determined for normally magnetized samples of late proterozoic volcanic rocks from 23 sites on michipicoten island lake superior canada the results are in general accord with an earlier less precise study by one of the authors and indicate a small polar shift between rocks of the mamainse point formation 186 e 38 n and the michipicoten island formation 175 e 26 n a quartz feldspar porphyry intruding the lower mamainse point formation but unconformably overlain by the michipicoten island formation was sampled for u pb zircon dating the zircons define an age of emplacement of 1086 5 1 3 3 0 ma this age and the pole positions are consistent with and supplement ages previously determined for the keweenawan paleomagnetic polar wander curve these data define an apparent polar wander rate of 14 cm a 1 this age combined with previously published u pb ages indicates a time span of 22 ma for most keweenawan igneous activity in addition the time of the keweenawan paleomagnetic reversal is constrained to be between 1086 5 1 3 3 0 and 1097 6 3 6 ma the age of the upper part of the reversely magnetized osler group the zircon population contained an inherited component from a source 1403 50 ma old this suggests that the porphyry magma was at least partly derived by melting of an anorogenic pluton emplaced during the mid proterozoic

co 2 rich fluid inclusions with chalcopyrite daughter mineral from the fenghuangshan cu fe au deposit china implications for metal transport in vapor co2 rich fluid inclusions containing opaque mineral crystals were found in the fenghuangshan skarn porphyry cu fe au deposit in tongling anhui china these inclusions show variable co2 contents and are accompanied by aqueous inclusions both occurring as secondary inclusions in quartz and being locally associated with chalcopyrite mineralization laser raman microspectroscopic analyses confirm the predominance of co2 in the vapor and the presence of h2s as high as 8 mol and identify the opaque mineral with yellow reflectance color in the inclusions as chalcopyrite more than half of the co2 bearing inclusions contains chalcopyrite whereas few of the associated aqueous inclusions do so the chalcopyrite occupying less than 1 volume of the inclusions is interpreted to be a daughter mineral and calculated cu concentrations in the inclusions range from 0 1 to 3 4 wt copper is inferred to have been transported in co2 dominated fluids as hs complexes the occurrence of chalcopyrite daughter crystals in co2 rich fluid inclusions indicates that co2 rich vapor has the capacity of transporting large amounts of cu and possibly au this finding has significant implications for metal transport and mineralization in hydrothermal systems enriched in co2 such as orogenic type and granitic intrusion related gold deposits

paleomagnetism of mesoproterozoic margins of the anabar shield a hypothesized billion year partnership of siberia and northern laurentia abstract siberia and laurentia have been suggested as near neighbors in proterozoic supercontinents nuna and rodinia but paleomagnetic evidence has been sparse and ambiguous here we present four new paleomagnetic poles from undeformed paleo mesoproterozoic lower riphean sedimentary rocks and mafic intrusions of the northwestern anabar uplift in northern siberia combining these results with other proterozoic data from siberia and laurentia we propose a tight juxtaposition of those two blocks euler parameters 77 098 137 for anabar to north america spanning the interval 1 7 0 7 ga constituting a long lived connection that outlasted both the nuna and rodinia supercontinental assemblages

palaeomagnetic and rock magnetic investigations of the sipoo subjotnian quartz porphyry and diabase dykes southern fennoscandia abstract the sipoo quartz porphyry dyke swarm u pb zircon age 1 63 ga in southern finland carries an intermediate to high coercivity remanent magnetization a n d 22 2 i 1 8 k 34 2 95 13 3 n 5 dykes which is regarded as primary the sipoo diabase dykes emplaced slightly before the granite porphyries yield a highly scattered antiparallel direction a r d 198 0 i 6 5 k 6 2 95 40 1 n 4 dykes isolated in intermediate to high coercivities based on positive contact test results remanence a r is primary in the diabases the best preserved diabase dyke as determined by petrographic and bulk magnetization properties carries both the a r magnetization and the a n magnetization as a secondary component in some samples the most altered diabase dyke is almost completely overprinted by a n the overprinting has taken place as a result of hydrothermal alteration and formation of new magnetite during the emplacement of quartz porphyry dykes and rapakivi granite the quartz porphyries and diabases carry mixed sd and md magnetic grain sizes the large md grains being responsible for a relatively large present earth s field pef magnetization in addition a high coercivity magnetization b d 23 9 i 52 2 k 215 8 95 6 3 n 4 dykes presumably a chemical overprint has been isolated in the quartz porphyries the b magnetization is interpreted to be either an intermediate direction between a n and pef with no geological significance or to represent a separate reactivation of the crust at about 1 32 ga ago

paleomagnetism and orientation of precambrian dykes eastern lake superior region and their use in estimates of crustal tilting archean rocks form the eastern margin of the 1 1 ga old central north american rift along the eastern shore of lake superior and have been tilted westwards in response to rifting paleomagnetic and structural data from 2 6 ga old matachewan dykes suggest a westward crustal tilt of about 60 which agrees well with dips recorded in nearby keweenawan volcanics that rest directly on basement rocks the matachewan dyke swarm occurs throughout the east shore region of lake superior whereas keweenawan supracrustal sequences which give a more precise estimate of tilt are restricted to a few isolated shoreline patches estimates of crustal tilt can be obtained from the dykes on a regional basis thus generating a more complete picture of basement deformation adjacent to a major intracratonic rift

paleomagnetism u pb geochronology and geochemistry of lac esprit and other dyke swarms james bay area quebec and implications for paleoproterozoic deformation of the superior province an extensive set of north to northwest trending diabase dykes termed the lac esprit swarm is identified in the superior province east of james bay based on geological mapping and a distinctive paleomagnetic pole 61 7 n 169 1 e dm 7 7 dp 5 5 the lac esprit swarm yields a u pb baddeleyite age of 2069 1 ma similar to that of the 2076 5 4 ma fort frances swarm of the western superior province their paleomagnetic declinations differ by 23 12 after correction to a common reference locality the difference is likely due mainly to counterclockwise rotation about a vertical axis of the fort frances area relative to the lac esprit area differential rotation of 10 20 has been proposed more locally across the kapuskasing structural zone separating the eastern and western superior province in earlier paleomagnetic studies of ca 2450 ma matachewan and 2170 ma biscotasing dyke swarms thus relative rotation may have involved the entire eastern and western superior province perhaps in response

paleomagnetism of ordovician alkalic intrusives and host rocks from the pedernal hills new mexico positive contact test in remagnetized rocks jackson m van der voo r and geissman j w 1988 paleoma etism of ordovician alkalic intrusives and host rocks from the pedemal hills new mexico positive contact test in remagnetized rocks tectonophysics 147 313 323 a set of thin dikes from central new mexico dated at 469 7 ma rb sr loring and armstrong 1980 have yielded a virtual geomagnetic pole which lies on the late paleozoic segment of the north american apparent polar wander path the remanence of the dikes appears to be a product of late paleozoic hydrothermal alteration paradoxically however the magnetization of the host rocks is most simply explained in terms of a positive contact test samples collected between 0 2 and 0 5 dike widths from the contact contain a component of remanence parallel to the magnetization in the dikes with unblocking temperatures which decrease with distance from the dikes host rocks from a distance of more than 1 dike width show no evidence of the characteristic dike magnetization there are two possible resolutions of this paradox 1 the magnetization of the host rocks is secondary despite the apparent positive contact test and is a product of hydrothe al fluid migration through the dikes or along the contact zones or 2 the magnetization of the dikes is primary but not representative of the ordovician paleofield for north america possible reasons for inaccurate representation include a incomplete averaging of secular variation b tectonic rotation with respect to the stable craton or c erroneous age determination for the rocks we argue that explanation 1 is the most likely

palaeoproterozoic indian shield in the global continental assembly evidence from the palaeomagnetism of mafic dyke swarms abstract palaeomagnetic study of palaeoproterozoic mafic dykes in the basement along the margins of the cuddapah basin the largest precambrian sedimentary basin in south india is presented in detail for a general discussion of palaeoproterozoic igneous activity in india the results are compared with all earlier published data on mafic dykes in india and are integrated with recently published high precision u pb baddeleyite ages to provide a comprehensive account of palaeoproterozoic igneous activity in india the analysis consolidates palaeomagnetic poles for six age divisions between 2 45 and 1 85 ga with robust statistical criteria our best estimates of overall mean poles from 241 dykes are situated at 1 17 7 n 106 0 e a95 9 0 7n 24 at c 2 45 ga 2 7 1 n 57 2 e a95 4 5 n 69 at c 2 37 ga 3 41 6 s 5 5 e a95 5 1 n 34 at c 2 22 ga 4 4 7 n 343 0 e a95 4 4 n 31 at 2 18 ga 5 49 2 n 332 9 e a95 4 8 n 24 at 1 99 1 89 ga and 6 73 7 n 282 6 e a95 2 9 n 39 at 1 86 ga the data permit us to construct an apparent polar wander path for the indian shield for an 600 ma interval of the palaeoproterozoic eon 2 45 1 85 ga testing and evaluation of continental reconstructions for this interval which are mostly based on geological correlations reveal many inconsistencies between 2 45 and 2 37 ga the indian shield was situated at higher latitudes similar to the yilgarn craton of australia it was subsequently located near the equator at 2 22 2 18 1 99 and 1 86 ga thus an india australia connection is supported during these times but a proposed australia kaapvaal link in zimvaalbara and a dharwar india slave connection in sclavia or a superior zimbabwe india connection in superia are inconsistent with indian data in addition the close palaeomagnetic comparison between the palaeoproterozoic dykes of dharwar bastar bundelkhand cratons in india indicates an age of 2 45 ga for orogenic activity along the central indian tectonic zone hence matching this zone with the 2 0 1 8 ga trans north china orogenic belt or positioning north china adjacent to india or juxtaposition of the indian shield along the western margin of laurentia in the columbia reconstructions is not supported the indian data appear to be in accord with the essential features of the refined protopangaea model and the original ur configuration finally the results are interpreted in terms of four dyke emplacement events in the age range 2 45 2 18 ga linked to short lived 5 10 ma lips developed over mantle plumes the dykes of 1 99 1 89 ga age probably relate to continued long lived igneous activity while the 1 86 ga dykes are relatively fewer in number and may represent waning stage of a large igneous event related to a major mantle plume

low paleointensities recorded in 1 to 2 4 ga proterozoic dykes superior province canada abstract paleointensity studies have been conducted on six mafic dyke swarms from the superior province in canada with u pb ages between 1141 ma and 2473 ma buchan et al tectonophysics 319 2000 167 198 the mean direction of the characteristic magnetization for each dyke swarm coincides with results of earlier studies in which the primary origin of the magnetization was established on the basis of contact and secular variation tests this primary component was isolated over a very narrow range of high unblocking temperatures usually 550 580 c indicating that it is carried by magnetite or low titanium titanomagnetite magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo single domain or single domain grains taken together the characteristics of this component suggest that it is thermal in origin paleointensity experiments were conducted with a specially designed oven using a revised approach of the thellier coe method eighty five successful determinations of paleointensity were obtained for 20 sites from six dyke swarms doubling the number of existing well constrained precambrian data the paleofield estimates for the dyke swarms vary between 5 and 11 2 t yielding virtual dipole moments between 0 85 0 1 and 2 8 0 87 1022 am2 the overall convergence between these new results and the rest of the database strongly reinforces the existence of a weaker geomagnetic field during the 1000 2400 ma period than has been recorded between 0 3 and 400 ma however additional determinations are required particularly for the 400 2000 ma interval in order to establish whether a long term evolution in the time averaged field intensity can be linked to the onset of the growth of the earth s inner core

rock magnetic studies and a comparison between microwave and thellier palaeointensities for canadian precambrian dykes abstract a selection of precambrian dolerite dykes from the canadian shield has been studied to ascertain the magnetic field strength at the time of formation chilled margins samples from dyke swarms with u pb ages between 2450 and 2104 ma were analysed using the microwave and thellier palaeointensity techniques rock magnetic analyses have been carried out in order to evaluate magnetic mineralogy and grain size thermomagnetic analyses show single distinct curie temperatures at 585 c indicating pure magnetite hysteresis and backfield irm measurements suggest grains within the pseudo single domain psd grain size range lowrie fuller tests suggest that single domain grains carry the remanence high quality palaeointensity estimates were obtained showing markedly low palaeointensities throughout the proterozoic corresponding virtual dipole moments vdms were calculated typically being less than 4 10 22 am 2 less than half the value of the present field these data are similar to recently published palaeointensities from the same dyke swarms determined using the conventional thellier procedure

paleomagnetism and u pb geochronology of the clarence head dykes arctic canada orthogonal emplacement of mafic dykes in a large igneous province the north south trending clarence head dyke swarm located on devon and ellesmere islands in the canadian high arctic has a trend orthogonal to that of the neoproterozoic franklin swarm that surrounds it the clarence head dykes are dated by the u pb method on baddeleyite to between 716 1 and 713 1 ma ages apparently younger than but within the published age range of the franklin dykes alpha recoil in baddeleyite is considered as a possible explanation for the difference in ages but a comparison of the u pb ages of grains of equal size from both swarms suggests that recoil distances in baddeleyite are lower than those in zircon and that the clarence head dykes are indeed a distinctly younger event within the period of franklin magmatism the clarence head dykes represent a large swarm tangential to and cogenetic with a giant radiating dyke swarm 800 km from the indicated source the preferred mechanism for the emplacement of the clarence head dykes is the exploitation of concentric zones of e

paleomagnetism u pb geochronology and geochemistry of marathon dykes superior province and comparison with the fort frances swarm we report the first detailed study of the paleomagnetism u pb geochronology and major element geochemistry of paleoproterozoic north trending marathon dykes north of lake superior the paleomagnetic and geochemical results demonstrate that marathon dykes can be divided into two subsets one of normal magnetic polarity the other of reverse polarity normal and reverse marathon paleomagnetic poles at 43 n 196 e dm 9 dp 7 number of dykes n 16 and 51 n 175 e dm 9 dp 6 n 12 respectively are statistically distinct and may indicate different ages of normal and reverse dyke emplacement a u pb baddeleyite age of ma has been obtained at a normally magnetized marathon paleomagnetic site the reversely magnetized marathon dykes are undated but have a paleopole rather close to that of the reversely magnetized ma fort frances dykes and major element geochemical signatures as portrayed on jensen plots that are identical to those of the fort frances swarm therefore reverse marathon an

paleomagnetism structure and longitudinal correlation of middle precambrian dykes from northwestern ontario and minnesota a primary magnetization about 2 1 ga old is well preserved in the kenora kabetogama dyke swarm in minnesota and ontario its direction the mean for 12 dykes incorporating results from 30 paleomagnetic sites is 115 9 55 6 yielding a paleomagnetic pole position of latitude 42 7 s longitude 4 2 e dm 7 3 dp 5 2 the primary nature of the magnetization can be demonstrated using geochemistry to correlate individual dykes over distances that can exceed 300 km whereas the remanence direction varies between dykes it remains constant along a dyke sufficient proof that it was formed during initial cooling of the intrusion a weak longitudinal increase in hydrous alteration mgo content and degree of magnetic overprinting is observed as the swarm is followed to the south and may reflect increasing proximity to a precambrian continental margin that underwent rifting sedimentation and deformation during the time interval 2 2 1 6 ga bp although the cause of the longitudinal changes in terms of ma

the paleoproterozoic marathon large igneous province new evidence for a 2 1 ga long lived mantle plume event along the southern margin of the north american superior province abstract three paleoproterozoic diabase dyke swarms the marathon kapuskasing and fort frances which outcrop around the northern margin of lake superior radiate from a focal region south of the lake u pb dating shows that the marathon and kapuskasing swarms range in age between 2126 and 2101 ma and the fort frances between 2076 and 2067 ma the convergence of the dyke swarms to a common focal area and the lack of apparent polar wander during the time interval 2125 2067 ma suggest that a single periodically active plume was responsible this 60 m y period of magmatism manifest over an area of at least 400 000 km2 is designated as the marathon large igneous province only a single magnetic field reversal has been recognized throughout the 60 m y of plume activity this observation together with u pb geochronology has helped to place dykes within each swarm in a chronological order based on their magnetic polarity for the dyke population as a whole older dykes tend to show more enriched and more fractionated incompatible trace element patterns compared to younger ones a result which suggests that the marathon kapuskasing and fort frances swarms are genetically related and underwent a secular reduction both in the degree of crustal contamination and or depth of mantle melting over the course of plume activity a similar geochemical relationship has also been found for the 2171 2166 ma biscotasing swarm but determining whether it is a common feature of plume development must await further study

the paleomagnetic significance of new u pb age data from the molson dyke swarm cauchon lake area manitoba u pb geochronology paleomagnetism and petrography indicate that the molson dyke swarm along the western margin of the superior craton is a composite of at least two ages of intrusion the more extensive younger dyke set the molson swarm sensu stricto generally has a 030 trend is mainly pyroxenitic to noritic with subordinate diabase and has been related to rifting in a back arc environment during closure of the manikewan ocean at about 1920 1800 ma which culminated in the trans hudson orogen a u pb zircon age from one of these dykes located at cauchon lake indicates emplacement at 1877 7 150 4 similar to two previous u pb age determinations on molson dykes another dyke from cauchon lake yields a baddeleyite zircon u pb date of 2091 2 ma and appears to be part of an older mainly diabasic suite of east northeast trending dykes that may represent a continental rifting episode that preceded the opening of the manikewan ocean the new u pb age data require a revision to the interpretation of

the gold carbonate association source of co2 and co2 fixation reactions in archaean lode deposits abstract abundant carbonate is a ubiquitous feature of archaean lode gold deposits both in domains of alteration enveloping veins and in rocks enclosing auriferous sediments detailed studies of chemical mass balance in metabasalts progressively altered towards gold bearing quartz carbonate veins disposed within shear zones at yellowknife reveal massive additions of co2 k si and fe accompanying mineralisation with concomitant depletions of na coherent behaviour of al sc zr also v and nb provides a reference for constraining the volume relations during hydrothermal alteration in the peripheral regions of alteration depletions of ca and mg result in overall volume reduction but these elements are added to veins and their immediate alteration envelopes where there is a large positive volume change precious metals together with cr ni cu zn pb rb cd and ba have been added to the veins and altered wall rocks whereas quartz noble metals and other trace elements have been precipitated from hydrothermal fluids solution of ca mg and some fe indigenous to peripheral alteration regions combining with co2 from the mineralising reservoir appears to be the process for forming the abundant ferri dolomite gangue observations of many archaean au deposits reveals that carbonate chemistry reflects the nature of wall rocks with wall rocks donating the bivalent metal cations and hydrothermal fluids the co2 alteration reactions at yellowknife involved hydrolysis of albite accompanied by fixation of kaqueous to produce muscovite with na loss and hydrolysis of chlorite epidote with co2 fixation to form ferri dolomite studies of oxygen isotopes fe 2 fe 2 fe 3 and structure reveal that au quartz and carbonate were precipitated in the presence of fluids of probable metamorphic origin 18 o 8 9 at low redox potential and at ambient temperatures of 400 450 c during episodes of hydrofracturing the abundant co2 and k required for extensive carbonate muscovite replacement alteration could be supplied by fluids released during prograde metamorphism under greenschist or greenschist amphibolite facies conditions where the relative proportion of co 2 h 2 o is in the order of 0 2 to 0 5 and k na 1 given high co2 co in the hydrothermal reservoir these molecules may act as complexing agents for transport of au and other rare elements e g w pd ni cr