metabasalts of the harbour main group eastern newfoundland a paleomagnetic study abstract the avalon peninsula of newfoundland is distinguished from the central part of that island by its large proportion of late precambrian rocks the study area is located in the colliers bay and bauline areas at longitude 53 w and latitude 47 30 n fifty two oriented samples 140 specimen were collected on ten different sites in the harbour main group volcanics which are older than 610 ma and younger than 770 ma remanent magnetization and magnetic susceptibility were measured both alternating field and thermal demagnetizations were used to characterize the stability of magnetizations four components of magnetization a b c d were isolated and their mean directions 317 22 175 34 190 27 266 37 are corresponding to paleopoles at 20 n 172 e 24 n 132 e 56 n 109 e and 18 s 205 e respectively component d is encountered on two sites only and is largely overprinted by the present earth s field component c is obtained in seven sites is characterized by a significantly negative fold test and is thus presumed to be secondary components a and b are found on most sites and are also characterized by negative fold tests but the test is more significant for b than a in addition the larger unblocking temperatures and median destructive fields of component a suggest that it is older than b a comparison of the paleopoles obtained with the apparent polar wander path for north america indicates that the age of acquisition of magnetization for c is carbo permian and for b it is upper devonian the age of d remains unresolved the pole of component a obtained in this study lies close to that of component a obtained in a previous study on the finn hill ignimbrites it is located within the devonian late hadrynian interval it is tentatively interpreted as the signature of the cadomian orogenic event that occurred at approximately 600 ma

model for the precambrian evolution of the avalon terrane in southern new brunswick canada the tectonostratigraphic record of the avalon terrane in southern new brunswick is interpreted as that of a wilson cycle that followed the grenvillian and preceded the opening of lapetus development of a late precambrian continental shelf is attributed to the cadomian ocean whose initial subduction at 800 ma led to platform collapse regional metamorphism and minor mafic to ultramafic plutonism in response to back arc extension and crustal separation oblique subduction of the resulting back arc basin at 600 630 ma produced calcalkaline plutonism and cogenetic volcanism that ended in dextral transtension and the emplacement of a bimodal dike complex initial rifting of lapetus which isolated this record from cratonic north america is recorded in eocambrian redbeds and bimodal volcanics and produced a cambro ordovician overlap sequence bearing acado baltic fauna

paleomagnetism of the sierra de las animas complex southern uruguay its implications in the assembly of western gondwana global paleogeographies in the neoproterozoic are poorly known in particular the assembly of gondwana at the end of the proterozoic is controversial in order to constrain the kinematic evolution of the rio de la plata craton a paleomagnetic study was carried out in the vendian to cambrian sierra de las animas magmatic complex exposed in southeastern uruguay 34 7 s 55 3 w one hundred and forty seven samples from 23 sites were studied these included two sites on the vendian glaciogenic sediments of the playa hermosa formation after stepwise af and thermal cleaning a characteristic remanence was isolated at several sites according to the available isotopic data for the complex and the correlation between remanence directions and lithologic type and age two paleomagnetic poles were computed for the sierra de las animas complex sa1 338 1 e 5 9 n dp 19 6 dm 26 7 n 7 sites mean declination 44 5 mean inclination 58 0 95 18 1 with a likely age around 520 ma shows a distribution of vgps that suggests a remanence acquisition that spanned a long time during a period of fast apparent polar wander for gondwana sa2 250 9 e 16 9 s dp 15 9 dm 21 5 n 6 sites mean declination 95 3 mean inclination 58 5 95 14 5 corresponds to a likely age around 550 ma and falls close to a pole of similar age from the congo craton this suggests that the rio de la plata craton was at that time part of gondwana and confirms previous postulates that a single apwp can be defined for the supercontinent since 550 ma a preliminary mean geomagnetic pole obtained from the playa hermosa formation ph 198 4 e 43 0 s dp 8 6 dm 16 0 n 6 samples mean declination 226 0 mean inclination 24 2 95 15 0 is consistent with a previous pole of 595 ma from the campo alegre ca lavas and may suggest if confirmed that the glaciogenic deposits were produced at low to intermediate latitudes despite high uncertainties a new speculative model on the process of assembly of gondwana is proposed

paleomagnetism of the latest precambrian cambrian unicoi basalts from the blue ridge northeast tennessee and southwest virginia evidence for taconic deformation abstract three components of magnetization have been observed in ninety six samples twelve sites of amygdaloidal basalts and sedimentary greenstones of the unicoi formation in the blue ridge province of northeast tennessee and southwest virginia these components could be isolated by alternating field as well as thermal demagnetization one component with a direction close to that of the present day geomagnetic field is ascribed to recent viscous remanent magnetizations another component with intermediate blocking temperatures and coercivities gives a mean direction of d 132 i 43 95 9 for n 10 sites before correction for tilt of the strata this direction and the corresponding pole position are close to ordovician silurian data from the north american craton and we infer this magnetization to be due to a thermal remagnetization during or after the taconic orogeny this magnetization is of post folding origin which indicates that the blue ridge in our area was structurally affected by the taconic deformation the third component with the highest blocking temperatures and coercivities appears to reside in hematite its mean direction d 276 i 17 95 13 8 for n 6 sites after tilt correction corresponds to a pole close to latest precambrian and cambrian poles for north america the fold test is inconclusive for this magnetization at the 95 confidence level because of the near coincidence of the strike and the declinations we infer this direction to be due to early high temperature oxidation of the basalts and argue that its magnetization may have survived the later thermal events because of its intrinsic high blocking temperatures a detailed examination of the paleomagnetic directions from this study reveals that the blue ridge in this area may have undergone a small counterclockwise rotation of about 15

ediacaran palaeomagnetism and apparent polar wander path for australia no large true polar wander summary we report new palaeomagnetic data for red beds from the ediacaran brachina and wonoka formations in the adelaide geosyncline south australia and discuss their place with previously determined poles in the ediacaran apparent polar wander path for australia both formations behave similarly on thermal demagnetization displaying high temperature components that decay to the origin at 680 c consistent with haematite being the only magnetic mineral present restoring the strata to the palaeohorizontal yielded positive fold tests for both units at 99 per cent confidence indicating that acquisition of magnetization occurred before the early palaeozoic delamerian orogeny for the brachina formation n 91 specimens the mean direction after unfolding is declination d 178 2 inclination i 22 6 95 4 4 indicating a palaeolatitude 11 8 2 5 and a pole position at latitude p 46 0 s longitude p 315 4 e with confidence semi axes dp 2 4 and dm 4 6 the mean direction for the wonoka formation after unfolding n 70 is d 255 9 i 23 7 95 6 4 indicating 12 3 3 8 3 4 and a pole position at latitude p 5 2 s longitude p 30 5 e dp 3 6 and dm 6 8 the mean directions for these units and other ediacaran units in the adelaide geosyncline are significantly different from each other which excludes blanket remagnetization of the units before delamerian folding and therefore gives strong preference to their magnetization dating from close to the time of deposition the late cryogenian ediacaran cambrian apparent polar wander path for south australia spans 150 myr from 635 to 490 ma and places australia in low palaeolatitudes throughout the interval studied the poles differ significantly from each other suggesting australia underwent continual drift during that time whereas the directional difference between the late cryogenian elatina formation and early ediacaran nuccaleena formation is mainly in inclination for most other contiguous stratigraphic units the differences are mainly in declination with minor inclination differences indicating australia was rotating about a nearby euler pole in low palaeolatitudes the large and perhaps rapid polar shifts at 615 590 and 575 565 ma in the laurentian apparent polar wander path are not evident in the ediacaran apparent polar wander path for australia because true polar wander should be recorded globally in the absence of evidence for any major stratigraphic break in the south australian succession we conclude that large true polar wander did not occur during the ediacaran

middle cambrian paleomagnetism of the avalon terrane in cape breton island nova scotia interbedded volcanics and sediments of the bourinot group mid cambrian in central cape breton island were studied as part of a paleomagnetic investigation to quantify the geologically plausible displacements of the avalon terrane with respect to the north american craton both volcanics and sediments reveal prefolding magnetizations with steep to intermediate inclinations dual polarity magnetizations in different volcanic flows are probably primary and they yield a mean direction of d 293 i 66 n 9 k 41 8 95 8 1 the sediments reveal upon detailed thermal demagnetization complex multivectorial remanences however a few samples show characteristic components and a large number of samples 38 have intersecting great circle remagnetization trajectories when combined this yields a mean direction of d 267 i 65 k 16 mean radius of error ellipse is 8 5 the mean directions for the bourinot rocks correspond to a paleolatitude of 49 s 11 which is different from the range of possible values for the predicted mid cambrian paleolatitudes for nova scotia 29 s to 18 n inferred from the north american apparent polar wander path this discrepancy between predicted paleolatitudes and the observed paleolatitude implies that nova scotia was not part of north america during the mid cambrian but rather they were separated by an ocean with a minimum width of 1000 km 20 10 this interpretation of the relative positions of nova scotia and north america based on paleomagnetism is consistent with the geological record in the northern appalachians and supports previous interpretations of the dissimilar early paleozoic faunas in avalon and north america it is paleomagnetically quite plausible that the avalon terrane formed part of a larger continent comprising africa gondwana and armorica during the cambrian

post tectonic magnetizations from the cordova gabbro ontario and palaeozoic reactivation in the grenville province summary the cordova gabbro of southern ontario intrudes 1300 myr old volcanic rocks of the hastings lowlands in the grenville structural province three distinct vector magnetizations a b and c have been isolated using a combination of stable endpoints subtracted vectors from orthogonal vector plots and converging remagnetization circles the a magnetization with mean direction d 294 i 55 5 k 42 ag5 5 5 n 18 sites is a high coercivity high blocking temperature remanence recorded by 49 samples the b magnetization was isolated in 33 samples and has a mean direction d 305 5 1 1 5 k 24 ag5 9 5 n 11 sites b has lower coercivities and blocking temperatures than a where the two are superimposed the a and b palaeopoles 151 e 10 5 s dp 6 dm 8 and 165 5 e 24 n dp so dm 9 5 fall on the grenville track around 900 and 820 ma respectively the a and b magnetizations thus date from uplift and cooling following the grenvillian orogeny the third magnetization the c component has been isolated in 23 samples its mean direction is d 180 i 27 5 k 18 ag5 10 5 n 12 sites the c is a low coercivity low blocking temperature overprint of a and b its palaeopole 102 e 31 n dp 6 5 dm 12 is unlike post 1300 precambrian poles for cratonic north america but matches silurian and late ordovician poles 40ar 39ar plateau ages of 446 and 447 ma determined by lopez martinez and york for plagioclases from one of the cordova samples confirm this age assignment the c magnetization therefore records a previously unrecognized mild thermal or hydrothermal event that occurred in palaeozoic time long after the grenvillian orogeny

effect of anisotropy on the paleomagnetic contact test for a grenville dike we report a detailed paleomagnetic contact test for a mafic dike and its tonalitic gneiss host rock in the precambrian grenville province of canada directions of natural remanent magnetization nrm in the country rock were corrected for assumed deflection by the strong magnetic anisotropy due to the rock fabric using the measured anisotropy of laboratory induced thermoremanent magnetization trm the trm ellipsoids have the same principal axes as the anisotropy of magnetic susceptibility ams ellipsoids but the anisotropy of remanence is more pronounced than the ams anisotropy correction moves nrm directions in the unbaked zone almost 20 bringing them close to typical grenvillian paleofield directions the anisotropy seems to be controlled by multidomain rather than single domain magnetic minerals nrm directions from the baked gneiss near the dike contact after anisotropy correction agree well with the nrm directions from the chilled margin of the early cambrian 570 3 ma dike this positive baked contact test is augmented by the presence of a hybrid zone in which country rock nrms are increasingly overprinted by the dike nrm as one moves from the unbaked toward the baked zone the dike nrm is therefore primary however the virtual geomagnetic pole vgp for the dike nrm 51 n 131 w dp 10 dm 12 n 24 samples or 41 n 153 w dp 13 dm 18 n 4 chilled margin samples does not fall near most established cambrian paleopoles post emplacement tectonic movement of the dike is unlikely it may be that the dike cooled and acquired trm during a geomagnetic field excursion because of its geologically short cooling blocking period of a few tens of years

paleomagnetism of the callander complex and the cambrian apparent polar wander path for north america the 7 km2 circular callander alkaline complex was emplaced into anorthositic and granitic gneisses of the grenville province in the canadian shield about 575 5 ma ago at the start of the cambrian the complex has not been subsequently metamorphosed or tilted detailed alternating field and thermal step demagnetization of 252 specimens from 29 sites led to the identification of a characteristic a magnetization component with a direction of d 82 2 i 82 7 95 3 1 k 83 n 26 sites in 5 sites of mesocratic to leucocratic syenite from the core of the complex in 5 sites of fenitized host rock from its aureole and in 16 sites of lamprophyre from radiating dikes isothermal remanent magnetization tests show that the a component is retained by both magnetite and hematite in a complete spectrum of domain sizes a reversals test suggests and a contact test shows the a component to be primary its pole position at 46 3 s 121 4 e dp 5 9 dm 6 1 does not fall on published but poorly defined

remagnetized cratonic cambrian strata from southern nevada stratigraphic sections of cambrian strata in southeastern nevada are akin to thin cratonic facies exposed in the grand canyon their structural setting is much more complicated however from mesozoic and cenozoic tectonism paleomagnetic samples from two sections through these strata appear to have been completely remagnetized coarse grained lightcolored sandstone from the tapeats sandstone yields scattered magnetizations residing in hematite that appear to reflect protracted diagenetic acquisition of remanence a hematitic sandstone in the bright angel shale yields relatively consistent paleozoic directions of magnetization but petrographic study shows that the hematite results from diagenetic oxidation and stratigraphic arguments suggest that the oxidation was not penecontemporaneous gray limestones of the jangle and muav limestones yield a magnetization residing in magnetite that may reflect late tertiary remagnetization being imposed during uplift related to the onset of basin and range deformation in any case this magnetization differs greatly in direction from a hematite magnetization reported from slightly younger muav limestone in the grand canyon both the sampling sites also appear to have been tectonically rotated but whether this rotation is true or a geometric artifact of the tilt correction cannot be determined from the present data these results suggest that paleomagnetic data from rocks as old as cambrian must be scrutinized very carefully before their magnetizations can be accepted as penecontemporaneous and such scrutiny must include geologic data

a stable ediacaran earth recorded by single silicate crystals of the ca 565 ma sept iles intrusion the archetypical example of inertial interchange true polar wander iitpw the rapid rotation of the entire solid earth by 90 is based on two nearly orthogonal directions seen in paleomagnetic studies of the sept iles ca 565 ma intrusion quebec canada this motion has also been proposed as the driving force for the early cambrian explosion of life others have challenged these interpretations linking the data instead to flips between an axial and equatorial dipole field configuration we examine this enigma using single silicate crystal paleomagnetic analyses we conclude that only one of the previously reported magnetic directions is carried by single domain magnetic grains and can be considered primary thus the ediacaran diversification occurred on a rotationally stable earth driven by biotic and longer term abiotic forcing whether iitpw ever occurred in the past on earth remains uncertain

remagnetization and tectonic rotation of upper precambrian and lower paleozoic strata from the desert range southern nevada in the desert range of southern nevada miogeoclinal sedimentary rocks mostly shallow water limestones of latest precambrian through early ordovician age yield three components of magnetization having different blocking temperature spectra 1 a low blocking temperature component near the direction of the present axial dipole field 2 an intermediate blocking temperature component with northerly declination and inclination 60 and 3 a characteristic component with southeasterly declination and inclination 20 combinations of alternating field and thermal demagnetization indicate that the intermediate and characteristic components reside in magnetite the intermediate magnetization probably reflects a viscous partial thermoremanent magnetization vptrm imposed between the late cretaceous and mid cenozoic it was probably acquired when the strata were dipping slightly eastward the characteristic magnetization is probably a vptrm imposed during regional uplift in the late permian the degree of heating required to have destroyed any primary magnetization is consistent with the conodont color alteration index observed in the ordovician rocks additionally the characteristic magnetization in those rocks is younger than bedding disruption caused by major late stylolitization red purple mudstones from the middle member of the wood canyon formation in which a directionally similar characteristic magnetization resides in fine grained hematite also appear to have been remagnetized in these rocks the remagnetization probably reflects partial recrystallization as the blocking temperatures are too high to have been reset by burial heating the sampled sections have undergone relative tectonic rotation about a vertical axis consistent with late tertiary oroflexural bending that had been proposed on independent geologic evidence the characteristic magnetization probably provides a reliable estimate of the magnitudes of the vertical axis rotations as the regional geology suggests the desert range strata were essentially horizontal throughout the paleozoic the total observed relative rotation is 44 5 representing absolute counterclockwise rotation 27 4 of the northern part of the desert range and absolute clockwise rotation 17 5 of the southern part the only unit in the desert range sequence that may retain a primary magnetization is the late precambrian rainstorm member of the johnnie formation the characteristic magnetization of this unit exhibits two polarities and probably resides in specular hematite after correction for 18 of counterclockwise rotation the resulting pole 5 n 151 e is near other late hadrynian poles from north america

the sept iles anorthosite complex field relationships geochronology and petrology the sept iles anorthosite complex is located in the grenville structural province of the canadian shield geochronological studies of both the basic and acidic portions of the complex show that i

a paleomagnetic study of the layered mafic intrusion at sept iles quebec a paleomagnetic study of gabbroic and anorthositic members of the sept iles layered mafic intrusion has isolated a remanence with direction d 333 i 29 remanence a and a corresponding pole of 141 e 20 n the rocks are cut by numerous diabase dykes of unknown age the remanence carried by the dykes has direction d 188 i 85 remanence b with a corresponding pole of 116 e 59 s another remanence statistically identical to b d 186 i 85 is found at dyke contacts and in the local host rock its associated pole 115 e 61 s is identical to the one derived from the dykes this overprint is significantly different from remanence a and is obviously related to dyke emplacement the location of the pole derived from remanence a is in excellent agreement with many other cambrian poles from a variety of locations throughout cratonic north america the rocks at sept iles have been previously dated radiometrically at 540 ma thus all evidence indicates that remanence a is an original

paleomagnetism of the 550 ma skinner cove volcanics of western newfoundland and the opening of the iapetus ocean the skinner cove formation of western newfoundland is an alkali volcanic suite transported in the ordovician onto laurentia s iapetus ocean margin it has previously yielded a u pb zircon age of 550 5 3 2 ma has no penetrative deformation and has only been affected by zeolite facies metamorphism at 10 sites flows and dykes of the skinner cove formation exhibit a stable characteristic a remanence carried by magnetite this a remanence is shown to be primary by an intraformational conglomerate test using 20 trachybasalt clasts whereas 11 alkali basalt clasts were overprinted probably because they originally contained titanomagnetite from which rutile exsolved soon after burial by overlying flows after tilt correction the mean a remanence direction is southeast and down d 144 i 32 95 11 k 21 n 10 sites the paleolatitude calculated from the ten a site virtual geomagnetic poles is 19 s 9 which likely represents the paleolatitude of laurentia s iapetan margin at 550 ma evidence that the skinner cove formation originated at this margin includes its structural position within the humber arm allochthon its within plate trace element geochemistry and its similarity in age to other alkali magmatism that more certainly represents laurentia s iapetan margin comparison with other laurentian paleomagnetic data implies that laurentia drifted very rapidly northward from the south polar region to the equator between 570 and 550 ma we suggest that the start of this rapid northward drift at 570 ma marked the onset of iapetus sea floor spreading between laurentia and west gondwana further well constrained paleomagnetic data of 570 to 550 ma age are needed from laurentia to test its high rate of northward drift and from west gondwanan cratons to test that they remained relatively stationary

paleomagnetic results from the cambrian and ordovician sediments of bornholm denmark and southern sweden and paleogeographical implications for baltica 1 if apparent polar wander paths apwp cross the question arises how to prove the older magnetization to be primary and not just a younger overprint this problem is typically met in areas affected by percolating mineralizing fluids and or heating due to a younger regional igneous activity the permian magnetic overprint is the classical example earlier paleomagnetic studies over the lowermost cambrian nekso sandstone fm nsf of bornholm denmark yielded a characteristic remanent magnetization chrm similar to the permian directions for baltica since a possible reason could be a chemical overprint we checked whether this phenomenon did take place on a regional scale some samples therefore were collected from other lower cambrian clastics of bornholm and southern scandinavia in result we show that the well grouped and stable chrm of the nsf contrasts with fairly chaotic soft and badly preserved magnetizations of the balka hardeberga mickwitzia and lingulid sandstones of bornholm and southern sweden thus not indicating widespread paleomagnetic overprint we demonstrate that the chrm of the nsf is most probably of syndepositional early diagenetic origin and its similarity to the permian direction for baltica is only casual we propose a normal polarity and a near equatorial position on the southern hemisphere for baltica in the early cambrian time as well as a more complicated trend of the apwp for this paleocontinent than envisaged by other authors paleomagnetic results from the arenigian limestones of the laesaa formation bornholm that yield excellently defined but most probably only secondary components are also presented

gravity field expansion in ellipsoidal harmonic and polyhedral internal representations applied to vesta abstract a 20th degree ellipsoidal harmonic gravity field of vesta is determined by processing radiometric doppler and range data from the dawn mission the gravity field shows sensitivity up to degree 18 and the coefficients are globally determined on average to degree 15 gravity anomalies are mapped to the brillouin ellipsoid 304 289 247 km which is a substantially closer fit to the surface than the reference ellipsoid 290 290 265 km used to map the conventional spherical harmonic series especially near the poles two models of internal structure are subsequently explored in which density variations are permitted in the uppermost layer i e crust in order to explain vesta s local gravitational signature these models include the case of a two layer model with an average crustal thickness of 55 5 km and a three layer model with an average crustal thickness of 22 4 km for both two layer and three layer scenarios the bouguer gravity anomaly is minimized for a crustal density of 2970 kg m 3 the remaining bouguer anomalies can be explained by lateral crustal density variation of 2310 3440 kg m 3 and 2660 3240 kg m 3 for the 22 4 km and 55 5 km crustal thickness models respectively the general trend of the estimated lateral crustal densities for the two cases is very similar with a wider range for the 22 4 km case due to a thinner crust this indicates that a thick crust e g 55 5 km would be more favorable for minimizing the range of lateral crustal density variations consideration of independent geochemical and petrological constraints suggests that a three layer model is a more appropriate representation of vesta s internal structure despite the fact that two layer models provide a satisfactory fit to gravity data alone in detail it is found that densities derived from gravity data assuming three layer models and those derived from the howardite eucrite diogenite meteorites and estimates of plausible bulk vesta composition show an excellent level of mutual consistency

geophysics of the quebec appalachians abstract this paper deals with a synthesis of the geophysics of the quebec appalachians an emphasis is given to the regional geophysical surveys for the final geological interpretation regional residual and downward continued aeromagnetic am data yield information on the tectonic history of the region the main features are pattern recognition of folded and particularly faulted structures and the boundaries of the tectonic domains three main fault systems have been identified and classified using am information in addition depths to the crystallise basement and to the cambro ordovician magnetic layers were obtained for sectors of the gulf of st lawrence gaspe region nine main geological features are extracted from the processed am data detailed geological modelling of some of the identified structures e g nappes are presented five main geological features are recognized from the gravity data these features are correlated with the ones obtained from am data whenever possible geological models based on information from surface geology rock density contrasts and occasional contraints from regional seismic reflection data are presented the boomerang shape of the sutton bennett anticlinorium and the planar or curved slab like shape of the obducted ophiolite bodies upper mantle proto oceanic floor may be considered as the significant features regional reflection seismic data give a clear geological picture of three out of four distinguishable zones of the quebec appalachians that is autochthonous parautochthonous and transition zone parautochthonous allochthonous corresponding approximately to the external domain the pattern recognition of the allochthonous approximate internal domain is somewhat more difficult to decipher from the reflection seismic data intraplate seismicity and seismotectonics indicate the presence of four zones of high seismicity in southern quebec in almost all zones the seismotectonic history goes back to late precambrian hadrynian times the scarce and scattered paleomagnetic data of geological formations located on the platform and the external domain of the quebec appalachians and in the internal domain are compared with those belonging to the stable north american plate the paleomagnetic results indicate that the angular difference between the poles from the platform and external domain is large at cambrian times 40 and small at devonian times 5 10 this suggests a fast rate of motion and accretion of the internal domain relative to the north american plate in the cambrian lower devonian time span 550 385 ma

early and late paleozoic remagnetization of the cambrian peerless formation colorado abstract the upper cambrian peerless formation in central colorado contains two secondary components of magnetization the dominant component was removed during initial af demagnetization of specimens whereas a weaker component was removed during subsequent thermal demagnetization irm acquisition experiments suggest that the dominant component has low coercivity whereas the weaker component has high coercivity the latter component has southeasterly and shallow directions and the corresponding pole position is located at 40 0 n 134 5 e the high coercivity blocking temperatures over 600 c and petrographic evidence suggest this component resides in diagenetic hematite the magnetization is interpreted as a crm acquired in the late paleozoic the low coercivity component directions fall on a great circle that passes through the modern field direction and two modes at 107 e 10 and 287 e 10 this component is interpreted as a vector sum of two antipodal crms and a modern vrm the pole position 10 n 150 e suggests acquisition in the early paleozoic and the remanence resides either in maghemite or in diagenetic magnetite

paleomagnetism of the catoctin volcanic province a new vendian cambrian apparent polar wander path for north america the vendian cambrian segment of the lauretian apparent polar wander path apwp has been poorly constrained and the subject of some controversy the catoctin volcanic province in central virginia is well dated at 570 35 ma rb sr and 597 18 ma u pb and therefore presented an excellent paleomagnetic target for resolving the laurentian vendian cambrian apwp a total of 206 samples from 32 sites were collected from the catoctin basalts feeder dikes and sills the study revealed three ancient directions of magnetization the youngest c component fails the fold test and yielded a characteristic in situ direction of d 147 i 44 k 21 95 9 the corresponding paleopole falls along the middle ordovician segment of the laurentian apwp and we consider this component to be the result of a taconic remagnetization the second component the b component is carried by hematite exhibits dual polarities and passes a fold test the tilt corrected b component characteristic direction is d 92 i 17 k 16 95 13 the corresponding paleopole at 4 s 193 e falls near a well established late cambrian 505 ma pole for laurentia and we consider this component to be a remagnetization during a late cambrian tectonic event in the central appalachians the third component isolated in the catoctin basalts the a component yields a tilt corrected mean of d 68 i 84 k 59 95 9 this component passes a fold and reversal test a suite of samples was collected from two catoctin feeder dikes and surrounding country rocks that yield a positive baked contact test the a pole at 43 s 128 e falls significantly away from previously proposed vendian poles for laurentia a reevaluation of previous paleomagnetic studies from coeval rock units reveals similarly steep directions and leads us to propose a new apwp this new apw track indicates that laurentia was located near the pole during the interval 615 580 ma and drifted rapidly 16 cm yr 1 toward its late cambrian equatorial position

the sept iles layered mafic intrusion geophysical expression eighty eight percent of the cambrian sept iles layered mafic intrusion possibly the second largest on earth lies under the estuary of the st lawrence river a marine geophysical cruise in 1986 t

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

geochemistry and petrogenesis of the tibbit hill metavolcanic suite of the appalachian fold belt quebec vermont a plume related and fractionated assemblage the late neoproterozoic metavolcanic tibbit hill formation thf covers a large crustal segment exposed along a belt about 250 km long in the appalachian fold belt quebec vermont it is predominantly basaltic in composition but contains a minor component of felsic and intermediate alkaline volcanic lithologies geochemically the tibbit hill formation forms a continuum in composition and exhibits a wide range of sio2 44 76 wt percent covering the entire spectrum from alkali basalt to trachyte and comendite this mildly alkaline suite is relatively enriched in incompatible elements and exhibits a wide range of zr 138 1493 ppm nb 15 139 ppm and y 18 185 ppm concentrations among other elements the concentrations of the hfse and the ree gradually increase toward the more evolved lithologies the chondrite normalized ree patterns are fractionated lree enriched over hree parallel to subparallel and generally uniform but with negative eu anomalies developed in the more felsic varieties these geochemical features underline the comagmatic nature of the entire suite and are consistent with a fractionated basalt to comendite suites the incompat ible element profiles suggest that most of these elements including nb zr ti y and the ree have not been affected by metamorphism as they remained largely intact within the thf rocks chemical features of the mafic rocks are typical of within plate basalts and suggest that their melts were derived from a fertile or plume related mantle source chemical features of the felsic and intermediate rocks are typical of anorogenic a1 type suites related to hotspots mantle plumes or continental rift zones this is consistent with the regional geological context with the volcanism associated with an iapetan rrr triple junction occurring shortly before the onset of seafloor spreading geochemical modelling shows that the thf basaltic magma was produced by a very small degree of batch partial melting f 2 5 percent of a garnet bearing primitive mantle source garnet lherzolite a final basaltic melt segrega tion depth is estimated at 80 to 100 km melting probably occurred within the thermal anomaly of a rising mantle plume beneath the sutton mountains triple junction near the quebec maine border fractionation of thf basaltic magma produced minor trachytic and comenditic magmas the volcanic assemblage of the afar rift for example boina centre appears to represent a modern analogue to the thf volcanic suite

multiple episodes of gold mineralization in the east kunlun orogen western central orogenic belt china constraints from re os sulfide geochronology abstract the gouli goldfield 110 t au located in the east kunlun orogen western central orogenic belt of china is one of the most important goldfields in this area in the last decade a number of orogenic gold deposits e g guoluolongwa and annage have been shown to be hosted by rock units of different lithology and ages rhenium osmium re os geochronology of sulfides from gold bearing veins was performed to define the chronologic relationships between gold mineralization present in the metamorphic rocks proterozoic and silurian of the east kunlun orogen sulfides pyrite and chalcopyrite from pyrite quartz vein and polymetallic sulfides quartz vein in the guoluolongwa gold deposit yield re os isochron dates of 374 15 ma mswd 4 6 initial 187os 188os ratio osi 0 06 0 22 and 354 7 ma mswd 0 18 osi 0 13 0 01 respectively similar ages are also revealed by the pyrite mineral separates from the annage gold deposit 383 8 ma and 349 6 ma these ages are interpreted to record the timings of the formation of the two vein types in these deposits which are nominally separated by 20 ma the new re os ages presented here identify the first two late paleozoic devonian and early carboniferous gold mineralizing events in the east kunlun orogen and thus indicate at least two mineralization epochs in this area given ages late triassic of other gold systems and field observations considering the geological background and temporal distribution of gold deposits in adjacent areas western qinling and qaidam qilian we suggest that gold deposits in the western central orogenic belt were formed in collisional post collisional settings being controlled by common tectonic magmatic activities related to the evolution of both the prototethys ocean proterozoic paleozoic and paleotethys ocean paleozoic early cenozoic further the initial os osi obtained from the re os isochron suggest that for the two vein types in the guoluolongwa gold deposit the os and by inference the ore metal au were derived from a mantle like source osi values 0 12 0 13 which should be related to the contemporaneous mantle like magmatism in contrast the pyrite quartz vein in the annage gold deposit possesses a significantly radiogenic osi value 3 65 0 51 given the similar timing of mineralization between the guoluolongwa and annage deposits it is considered that the ore metal likely has a similar origin i e a mantle like source however at annage the hydrothermal fluid interacted with the proterozoic metamorphic host rocks and leached radiogenic os that masks any evidence of a mantle like source

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

nd model age mapping of the southeast margin of the archean foreland in the grenville province of ontario nd model ages are used to map the southeast margin of superior southern province basement within the grenville province of ontario this boundary located 60 km southeast of the grenville front is tentatively identified as a collisional suture between an archean foreland to the northwest and a proterozoic island are of penokean age to the southeast

baltica during the ediacaran and cambrian a paleomagnetic study of hailuoto sediments in finland abstract we present a new late neoproterozoic paleomagnetic pole for baltica from an inclined 272 m deep oriented sedimentary drill core in hailuoto western finland the depositional age of the hailuoto sediments is poorly constrained at 570 600 ma three components of magnetization were isolated with thermal and alternating field af demagnetization treatments the chrm characteristic remanence magnetization component is a high coercivity unblocking temperature dual polarity component that passes a reversal test the combined observed chrm component of the hailuoto sediments d 334 2 i 44 4 95 7 2 k 16 5 yields a paleomagnetic pole of plat 48 7 n and plon 241 1 e with a95 8 1 the inclination corrected direction f 0 6 of d 334 4 i 57 7 95 5 8 k 25 2 yields a paleomagnetic pole of plat 60 5 n and plon 247 9 e with a95 7 6 as it is a dual polarity chrm carried by both magnetite and hematite with no resemblance to younger events we interpret it as a primary component a paleolatitude for hailuoto of 38 3 was calculated from the chrm two secondary components were identified the first is a low coercivity blocking temperature component with a remanent magnetization of d 239 0 i 67 3 95 8 7 n 13 samples which we interpret as drilling induced remanent magnetization dirm the second secondary component has a remanent magnetization of d 49 4 i 34 9 95 8 6 n 5 samples and is commonly seen in fennoscandian formations the chrm hailuoto pole adds to the scattered ediacaran paleomagnetic data of baltica and indicate large distances between other late neoproterozoic and early cambrian paleomagnetic poles we present reconstructions of baltica and laurentia between 616 and 550 ma which move baltica from high latitudes 615 ma over the polar region to low latitudes 550 ma and laurentia from low latitudes 615 ma to a polar position 570 ma and back to an equatorial position 550 ma a low to mid latitude position of baltica determined by the hailuoto paleomagnetic pole and the lack of glaciogenic sediments determined in an earlier study of hailuoto sediments indicate a warm deposition environment

absolute dating of dedolomitization and the origin of magnetization in the cambrian morgan creek limestone central texas goethite associated with dedolomite in the morgan creek limestone in central texas carries a chemical remanent magnetization crm the pole position 88 n 96 w k 67 95 2 7 which is close to that of the modern pole and the fact that only normal directions are present suggest that the component is a post brunhes 700 000 yr crm dedolomite is common in the morgan creek and consists of calcite rhombohedra with relict dolomite goethite occurs surrounding the rhombs and in rhombic zones within the crystals the goethite precipitated when iron was released from ferroan dolomite during the dedolomitization process the paleomagnetic data therefore date the dedolomitization process as modern dedolomitization is probably related to surface weathering of the morgan creek in addition to goethite associated with dedolomite the morgan creek contains hematite replacing fossils and glauconite and detrital hematite the morgan creek also contains three other magnetic components one of these components is a viscous remanent magnetization aligned with the present magnetic field another component is a triassic jurassic crm residing in hematite a fourth component in the morgan creek is an early paleozoic magnetization residing in hematite and minor magnetite and represents the vector addition of a detrital remanent magnetization and one or more crms the results of this study demonstrate that paleomagnetic and petrographic techniques can be used to date a diagenetic event and also illustrate the complex nature of magnetization in a sedimentary unit

circum iapetus paleogeography of the precambrian cambrian transition with a new paleomagnetic constraint from laurentia abstract the paleogeography of the precambrian cambrian transition is still poorly known but is fundamental for understanding late neoproterozoic climate extremes and the spatial associations of the rapidly evolving ediacaran and cambrian fauna this period has also been proposed to host unusual geodynamic events such as rapid plate motions and or the bulk tumbling of the earth with respect to its spin axis called true polar wander tpw new paleomagnetic and 40ar 39ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western quebec the mont rigaud and the chatham grenville stocks are related to a failed rift arm of the iapetus ocean both intrusions bear hornblendes which provide overlapping 40ar 39ar plateau ages of 533 2 1 1 ma mont rigaud and 531 4 3 4 ma chatham grenville corresponding to early cambrian nemakit daldynian time characteristic paleomagnetic reversed directions from both intrusions are easterly and shallow d 89 3 i 30 1 95 6 2 k 38 8 n 15 sites and the mont rigaud intrusion also carries antipodal normal polarity directions the combined result yields a north paleopole at location 11 9 s 184 6 e dp 3 8 dm 6 9 placing the montreal region at 16 4 s paleolatitude in the early cambrian in good agreement with late ediacaran and middle to late cambrian paleomagnetic results from laurentia laurentia likely resided at low southerly paleolatitudes throughout the cambrian ruling out a major inertial interchange tpw event for early to middle cambrian time a comparison of late ediacaran to middle cambrian paleomagnetic results from laurentia baltica siberia and elements of west gondwana implies that by early cambrian time a large paleolatitudinal gap existed between laurentia and its presumed conjugate rift margins thus placing a wide iapetus ocean between them the position of baltica within the circum iapetus paleogeography is contentious due to differing interpretations of the few paleomagnetic results during ediacaran time laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 ma to the south polar region at 590 575 ma and then rapidly back to low latitude by 565 550 ma the large and perhaps rapid apparent polar wander of laurentia and other continents in the ediacaran remains enigmatic and controversial

paleomagnetism of the cambro ordovician mcclure mountain alkalic complex colorado the middle to late cambrian loop in the north american apparent polar wander path apwp has been variously attributed to tectonic rotations remagnetizations and primary magnetizations although no primary thermal remanent magnetizations or primary detrital remanent magnetizations have as yet been demonstrated the temporally self consistent nature of the loop has been used as an argument for primary magnetizations we have studied 535 5 ma nepheline syenites and syenites of the mcclure mountain alkalic complex as well as 495 10 ma red trachyte dikes which intruded the complex in an effort to find a primary trm because zijderveld analysis yielded consistent results for only one trachyte dike remagnetization great circle analysis was employed giving a pole for the trachyte dikes at the tip of the loop 43 n l14 e while the syenites and nepheline synenites gave a pole at the base of the loop 18 n 142 e the magnetic carrier in the trachytes is hematite which apparently formed during a pervasive hydrothermal alteration k ar whole rock dating of the trachytes suggests a pennsylvanian age for the alteration and thus a late paleozoic remagnetization of the trachytes thus the low latitude cambrian pole is confirmed but we find no evidence in this study to support the primary nature of the cambrian apwp