paleomagnetism of archean granites and matachewan dikes in the wawa subprovince ontario reevaluation of the archean apparent polar wander path paleomagnetic measurements have been completed on 400 specimens from dated archean granites and matachewan dikes in the michipicoten and gamitagama greenstone belts in the western wawa subprovince

stratigraphy and petrology of the mulcahy lake layered gabbro an archean intrusion in the wabigoon subprovince ontario

preferred orientation and anisotropy of seismic and magnetic properties in gabbronorites from the bushveld layered intrusion abstract the preferred orientation of plagioclase and pyroxene in gabbronorites from the bushveld layered igneous intrusion south africa was investigated using electron backscatter diffraction in two localities 200 km apart belfast and rustenburg strong preferred orientation was observed with the principal fabric features aligned in the subhorizontal foliation plane defined by 010 planes of plagioclase and 100 planes of pyroxene the pattern appears to result from the orientation of tabular crystals during the dynamic intrusion events that formed the layered structure primary magmatic oxides are also influenced by the preferred orientation of plagioclase generating an oblate magnetic susceptibility fabric parallel to the foliation plane plagioclase preferred orientation gives rise to other directionally dependent geophysical properties including a large seismic anisotropy with p waves traveling 4 8 faster in the vertical than in the horizontal direction

magnetic anisotropy of serpentinized peridotites from the mark area implications for the orientation of mesoscopic structures and major fault zones 1 mantle derived serpentinized peridotites are exposed both along fracture zones and in areas of extreme tectonic extension at slow to intermediate spreading ridges and may constitute a significant volume of the shallow crust in these environments here we examine the potential of magnetic remanence data and structural features in serpentinized peridotites from ocean drilling program odp site 920 mid atlantic ridge south of kane mark to provide insights into the tectonic processes responsible for the exposure of these deep seated rocks at the seafloor paleomagnetic data from 214 samples from site 920 document a remarkably consistent inclination 36 1 0 8 1 4 that is shallower than either the expected geocentric axial dipole inclination 40 7 or present day inclination 41 9 at the site we show that the nearly univectorial remanence in these samples is likely to be a partial thermoremanence possibly augmented by viscous processes at moderate temperatures these properties were acquired during cooling from the relatively high temperatures 350 c at which serpentinization occurred the remanence directions therefore provide some information on the latest stages of uplift of the serpentinite massif however interpretation of this tectonic history is complicated by the presence of a pronounced magnetic fabric which presumably resulted in a deflection of the remanence we estimate the magnitude and direction of this deflection using a relationship between the anisotropy of magnetic susceptibility and remanence anisotropy the corrected remanent inclinations mean 39 5 more closely approximates the time averaged inclination at the site indicating that the massif experienced little or no resolvable tilt after serpentinization and cooling to 350 c accounting for the anisotropy related deflection of the remanence also allows us to more accurately restore various structural features within the core to their geographic orientation after this reorientation the dominant mesoscopic foliation in these rocks defined by the preferred orientation of orthopyroxene and subparallel serpentine veins has an average orientation that closely parallels the regional scale fault zones on the western median valley wall

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 red lake greenstone belt northwestern ontario possible evidence for the timing of gold mineralization we report paleomagnetic results from 1 largely unaltered batholiths and 2 sheared and altered units in the archean red lake greenstone belt a major gold producing region in the uchi subprovince of northwestern ontario group 1 includes the hammell lake killala baird trout lake and little vermilion lake batholiths which were intruded between 2730 and 2700 ma ago u pb zircon group 2 includes the howey diorite the dome stock and dikes and andesites from the dickenson mine these units from the mineralized deformation zones dzs between the batholiths were sheared and hydrothermally altered in the same event that caused gold mineralization in supracrustal units the batholiths carry a stable reversed remanence rlr withd 159 i 74 k 20 95 16 n 4 units 14 sites rlr has high coercivities unblocking temperatures and remanence intensities and appears to be a primary thermal remanence dating from 2700 ma the altered units from the gold bearing dzs carry a different remanence rlg withd 180 i 57 k 23 95 3 n 3 units 6 sites rlg also has relatively high coercivities and unblocking temperatures but its magnetization intensities are 1 2 orders of magnitude lower than those of rlr opaque minerals and silicates observed in thin section are highly altered rlg is therefore believed to be a chemical or thermochemical remanence acquired 2580 ma ago during the late stages of cooling and gold mineralization seeing as rlg characterizes the shear zone rocks but is absent from unaltered rocks outside the dzs it could be used as a rapid prospecting tool in delineating areas of alteration and possible mineralization the paleopoles for rlr 2700 ma and rlg 2580 ma link with those of the shelley lake granite 2580 ma 40ar 39ar hornblende and biotite and the matachewan hearst dikes 2450 ma u pb zircon and baddeleyite the sense of motion on the late archean early proterozoic apparent polar wander path for laurentia as defined by this sequence of paleopoles is opposite to that previously accepted

correction for deviations due to magnetic anisotropy of palaeomagnetic directions from the mulcahy gabbro north west ontario summary the mulcahy gabbro is an archean layered intrusion in the wabigoon subprovince of north western ontario alternating field and thermal demagnetization of specimens from it have provided consistent within site characteristic remanence chrm directions in eight of 19 sites antipodal polarities of remanence are recovered and the overall in situ formation mean is d 240 i 51 k 21 this direction is unlike any reported archean direction from the superior province of north america the degree of anisotropy of low field magnetic susceptibility is high with k k3 values up to 1 3 cooling selected specimens in weak fields indicate that trm is deflected towards the magnetic fabric plane accordingly the in situ chrm was corrected for the effect of magnetic anisotropy two methods of correction give similar results however the resultant formation mean remains essentially unchanged as a consequence of sufficient variability in the attitudes of k k planes although uncertainty in the initial dip renders the tilt correction problematical partial tilt correction 36 per cent results in a formational mean with better precision than the in situ formation mean furthermore partial untilting of remanence directions results in a positive reversal test with a better classification than that obtained using in situ directions this partially tilt corrected result d 202 i 49 k 35 yields a pole latitude 62 n longitude 42 e which is archean in aspect

anisotropy of magnetic remanence a brief review of mineralogical sources physical origins and geological applications and comparison with susceptibility anisotropy the magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low field magnetic susceptibility ams however alternative methods based on remanent magnetization measured in the absence of a magnetic field rather than induced magnetization measured in the applied field have distinct advantages for certain geological applications this is particularly true for 1 adjunct studies in paleomagnetism in order to assess the fidelity with which a natural remanence records the paleofield orientation 2 studies of weakly magnetic or weakly deformed rocks for which susceptibility anisotropy is very difficult to measure precisely and 3 quantitative applications such as strain estimation the fundamental differences between susceptibility and remanence and their respective anisotropies are due to several factors 1 susceptibility arises from all of the minerals present in a sample whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals 2 ferromagnetic minerals are generally more anisotropic than para and diamagnetic minerals 3 for ferromagnetic minerals remanence is inevitably more anisotropic than susceptibility and 4 a number of common minerals including single domain magnetites possess an inverse anisotropy of susceptibility i e they tend to have minimum susceptibility parallel to the long axis of an individual particle remanence is immune to this phenomenon as a consequence of all these factors remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample

late cretaceous cenozoic development of outer continental margin southwestern nova scotia the growth pattern for the outer continental margin of nova scotia during the late cretaceous and cenozoic was studied using seismic stratigraphy and well data sediment accumulation was broadly controlled by temporal changes in relative sea level but significant spatial and temporal changes in accumulation patterns were caused by changes in sediment supply rate morphology erosion by abyssal currents and salt tectonics a jurassic early cretaceous carbonate platform remained exposed until the late cretaceous and controlled the location and steepness of the paleoslope until the late miocene local erosion of the outer shelf and slope in the late paleocene early eocene produced chalky fans on the upper rise the relationship between erosion of the shelf in the late eoce e and early oligocene and abyssal current erosion of the upper rise in the oligocene is unclear seaward extensions of tertiary shelf edge canyons are poorly defined except for the eocene fans in the miocene abyssal currents eroded a bench on the upper continental rise subsequently sediments lapped onto and buried the paleoslope the lower rise above horizon au oligocene is composed of fans and olistostromes shed from halo kinetic uplift of the upper rise current eroded unconformities are common in the rise sequence but the only current deposit is a pliocene interval 300 m restricted to the lowermost rise pleistocene turbidity currents eroded the present canyon morphology

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

statistical significance of the fold test in palaeomagnetism summary the significance of the change in direction dispersion on applying the fold test in palaeomagnetism is used as the criterion for deciding whether or not the fold test has statistical significance a set of tables is compiled which can be used for quick application of the test

role of dynamic grain boundary wetting in fluid circulation beneath volcanic arcs 1 we report the influence of shear deformation on the microstructure of water saturated olivine and clinopyroxene aggregates prior to deformation the aqueous fluid was isolated in pockets along grain corners in the olivine water aggregates while it was interconnected by a network of grain edge tubules in the clinopyroxene water aggregate during deformation of both types of aggregates the aqueous fluid phase segregates into grain boundaries at an angle of 16 to the shear direction inclined in a sense antithetic to the applied shear fluid rich planes formed by such dynamic wetting of grain boundaries lead to the formation of high permeability paths in the matrix in a deforming mantle such high permeability paths will enhance the efficiency of aqueous fluid extraction from subducting slabs

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

zircon u pb ages from the wabigoon manitou lakes region wabigoon subprovince northwest ontario three distinct stratigraphic units have been recognized in the crow lake savant lake belt these are a basal unit consisting of high magnesium tholeiitic flows a middle unit consisting mostly of tholeiitic to calc alkaline pyroclastic rocks and flows and an upper unit of high iron tholeiitic flows a zircon u pb study has been carried out on seven rocks from the wabigoon manitou lakes region of this belt in the manitou lakes area a minimum age of 2755 ma on a porphyry pluton intruded into the wapageisi volcanics gives a minimum age for the high magnesium tholeiitic sequence a trondhjemite phase of the atikwa batholith near wabigoon lake that is dated at 2732 2 2 9 ma has the same age within experimental error as a rhyolite flow from the lower wabigoon volcanics in the same area that is dated at however a trondhjemite phase of the atikwa batholith at eagle lake that is dated at is distinctly younger than a dacite flow collected nearby from the lower wabigoon volcanics that is dated at both thes

the effect of magnetic anisotropy on paleomagnetic directions in high grade metamorphic rocks from the juiz de fora complex se brazil the juiz de fora complex is mainly composed of granulites and granodioritic migmatite gneisses and is a cratonic basement of the ribeira fold belt which was formed during the last stage of brasiliano orogeny samples widely distributed over the studied region se brazil rio de janeiro state were collected for paleomagnetic and magnetic anisotropy determinations after measurement of anisotropy of low field magnetic susceptibility ams the same specimens were submitted to both alternating field for anisotropy of remanent magnetization arm determinations and thermal demagnetization for paleomagnetic analyses demagnetization processes allowed isolating an eastern steep positive inclination characteristic remanent magnetization chrm direction from the rocks the studied specimens are strongly heterogeneous at sample scale regarding the amount of magnetic minerals even for specimens from the same core rock magnetism indicates that there is a complex mixture of magnetic minerals with both low and high coercivity which are associated with coarse grained titano magnetite and either fine grained titano magnetite or titanohematite grains respectively these magnetic minerals are responsible for both remanent direction and magnetic anisotropies even though for some specimens paramagnetic minerals are the carriers of ams fabric the chrm direction found for the rocks should represent the cooling phase of the brasiliano event in the area and it was acquired after the magnetic fabric was formed the ams and arm fabrics are coaxial and tectonic in origin and compare favorably with the mesoscopic scale fabrics in the adjacent areas however arm measurements have reached only low coercivity grains and it could be determined in few specimens then only an estimate of arm effect on chrms correction is shown the rocks are strongly magnetically anisotropic 50 and foliated the paleomagnetic directions from each specimen within a site tend to approach its ams or arm foliation kmax kint or armmax armmin plane these directions were corrected specimen by specimen from the sites taking into account the pole of ams or arm foliation kmin or armmin and the value of the degree of anisotropy p results show that even for specimens with high p the angular difference between corrected and uncorrected magnetization direction may be insignificant if the geomagnetic field at the time of acquired magnetization was approximately parallel to the magnetic foliation of the specimens our data set suggests that ams is a powerful tool to correct the chrm deviation from paleofields in rocks with complex magnetic mineralogy such as those from the juiz de fora complex

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

trm deviations in anisotropic assemblages of multidomain magnetite anisotropic assemblages of multidomain magnetite particles develop an anisotropy of magnetic susceptibility ams which in turn induces deviations of thermo remanent magnetization trm from the field direction from the theories of multidomain trm acquisition it is shown that the trm anisotropy tensor has its eigenvalue ratios ti related to the principal weak field susceptibility ratios pi by the order of magnitude ti p2i this relation has been experimentally verified on two sets of highly anisotropic rock samples the exponent has been determined to be 1 94 in the samples from a peruvian gabbro and 1 81 in those from the granite of flamanville nw france accounting for experimental difficulties in determining the trm anisotropy tensors these exponents are judged to agree well with the expected one it is therefore stressed that ams measurements provide a good means of evaluating the magnetic field direction from deviated trm directions providing magnetic carriers are mainly multidomain magnetites

late pleistocene sedimentation on the continental slope and rise off western nova scotia reflectivity patterns in echograms recorded from relatively smooth continental slope and rise off nova scotia can be grouped into four depositional associations the shallowest association corresponds to gravelly sand on the outer shelf that spills over the shelf edge and mixes with mud down to 500 m water depth a sharp boundary near 62 30 w extending from 500 m down to the abyssal plain separates associations farther seaward sediments transported downslope by near bottom gravity processes accumulated west of the boundary hemipelagic sediments accumulated east of the boundary from 500 to 4 300 m below 4 300 m sediment waves are common in a contonrite fan association these associations indicate that during the late pleistocene radically different sedimentation processes were juxtaposed across a sharp boundary down the slope and rise west of 62 30 w small scale slope failure occurred over a wide area due to relatively rapid accumulation of fine grained glacial debris the slope progiraded seaward the lower rise received distal turbidity current and debris flows deposits east of 62 30 w relatively less failure occurred in hemipelagic sediment lower on the rise seaward of the hemipelagics turbidity currents and contour currents carried sediment originating in deep canyons to the east of 61 30 w along the lower rise toward the south and west in the west the primary depocenter was the slope and upper rise in the east the depocenter was on the lower rise a glacial sediment source on land and relict shelf morphology probably controlled sedimentation processes and thus the location of depocenters seaward of the shelf edge

age of the mulcahy lake intrusion northwest ontario and implications for the evolution of greenstone granite terrains abstract zircon data from a 63 km 2 layered mafic intrusion in the wabigoon subprovince the mulcahy lake gabbro show that the gabbro crystallized at 2733 2 0 9 1 0 ma the largely unaltered and unmetamorphosed gabbro intrudes tholeiites of the crow lake savant lake greenstone belt which were generated over at least a 10 ma time span rb sr and sm nd data were obtained for pristine whole rock and mineral separates from the intrusion the rb sr data show disturbances in the system particularly in the case of pyroxenes samples with low rb sr ratios indicate an initial sr ratio of 0 7007 for an age of 2733 ma the sm nd age is 2744 55ma and an epsilon nd value of 2 6 1 2 indicates a light rare earth element depleted source the sm nd data show less disturbance than the rb sr data granitic and mafic intrusives with similar initial ratios occur 80 km to the south in the quetico subprovince primary hornblende was analyzed for 40 ar 39 ar the age obtained is 2703 20ma clearly younger than the zircon age indicating some argon loss the mulcahy lake gabbro is one of four layered mafic plutons of similar petrographic character and stratigraphic age which are arrayed in an arc in the crow lake savant lake greenstone belt the plutons appear to represent subvolcanic chambers which developed simultaneously and which crystallized at the end of the volcanic cycle all intrude basalts and are intruded in turn by the atikwa batholith a massive granitic intrusion of over 1000 km 2 in area previously published zircon ages for several sample sites within the batholith indicate crystallization at 2731 8 1 3 1 6 ma to 2732 2 2 9ma these data show that the batholith overlaps the age of the gabbros or is at most 5 ma younger this virtual contemporaneity is hidden in the rb sr and sm nd data the cessation of mafic magma production and the immediate if not parallel voluminous production of calc alkaline liquid may represent a process important in the development of greenstone granite belts a process not apparent when ages are not so precisely determined