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

xenoliths in a mafic dyke at popes harbour nova scotia implications for the basement to the meguma group a recently recognized series of northwest trending mafic dykes cuts the cambro ordovician meguma group along the eastern shore of nova scotia some of the dykes contain gneissic and meta plutonic xenoliths interpreted here as including fragments of the basement to the meguma group most of the gneissic xenoliths in the dyke exposed at popes harbour are sillimanite and or kyanite bearing quartz poor metapelites and garnet orthopyroxene bearing tonalitic rock other enclaves include amphibolite quartzite gabbro granite granodiorite and granitic pegmatite textural features suggest that the earliest paragneissic assemblage included poikiloblastic garnet xfe 0 55 0 65 xmg 0 05 0 30 xca 0 02 0 16 xmn 0 02 0 16 biotite kyanite and or sillimanite oligoclase and quartz subsequent recrystallization at higher temperature and or lower pressure conditions is suggested by the replacement of kyanite by sillimanite mineral core composition data indicate early metamorphic conditions of 45

geological archive of the onset of plate tectonics plate tectonics involving a globally linked system of lateral motion of rigid surface plates is a characteristic feature of our planet but estimates of how long it has been the modus operandi of

granodiorites of the south mountain batholith nova scotia canada derived by partial melting of avalonia granulite rocks beneath the meguma terrane implications for the heat source of the late devonian granites of the northern appalachians abstract the late devonian south mountain batholith smb of nova scotia is the largest batholith of the northern appalachians the peraluminous granitic rocks range from biotite granodiorite to leucogranite samples collected from a drill core of the scrag lake granodioritic pluton of the western smb are chemically homogeneous from the surface to a depth of 1425 m the homogeneous composition implies that the granodiorite was derived from a relatively homogeneous source and that country rock assimilation was not an important source for the parental magma equilibrium partial melt modeling of underlying sub meguma granulite rocks indicates that they are the primary source rocks of the granodiorites we suggest that mantle derived magmas intruded the lower crust and induce large scale melting of the granulite basement rocks to produce the granodiorites fractional crystallization of the granodiorites plus assimilation of meguma supergroup metasediments likely produces the silica rich rocks of the smb the cause of mantle melting is uncertain however it may be related to the transitioning of the northern appalachians from a position above the deep mantle pacific large low shear velocity province llsvp to a higher shear velocity region of the mantle

geochemical and numerical constraints on neoarchean plate tectonics this paper discusses early neoarchean 2 8 2 7 ga plate tectonics by integrating knowledge from new geochemical observations and numerical models based on a geochemical dataset of 295 granitoid samples from the karelian and kola cratons of the fennoscandian shield we divide neoarchean juvenile extracted from oceanic crust or mantle granitoids into three groups 1 low hree heavy rare earth elements ttgs tonalite trondhjemite granodiorite high sio2 low mg low hree higher sr lower ybn and higher nb ta 2 high hree ttgs slightly lower range of sio2 larger range of mgo contents and higher cr and ni contents high hree lower sr higher ybn and lower nb ta and 3 high ba sr sanukitoids medium hree high mg and high k ba sr the main difference between the low and high hree groups lies in their pressure sensitive element contents which indicates high pressure melting conditions for the low hree group and low pressure conditions for the high hree group a possible tectonic scenario for the genesis of the two groups is an incipient hot subduction underneath a thick oceanic plateau protocrust melting in the lower part of thick basaltic oceanic crust could produce ttgs of the low hree type whereas low pressure melting of subducting slab and possible interactions with the mantle wedge at shallow depths would be capable of generating high hree ttgs the third group of archean high ba sr sanukitoids was formed after the ttgs their low sio2 and high mg k ba sr contents suggest an origin by melting in an enriched metasomatized mantle source probably as a result of a slab breakoff following a continental collision or attempted subduction of a thick oceanic plateau ttg protocontinent such hypothesis is supported by numerical modeling results that suggest an increased occurrence of slab breakoff in the archean which locally increased temperatures within the mantle wedge more frequent breakoff resulted because subducting plates were weaker due to rheologically thinner lithosphere and a thicker basaltic crust and tensile stresses within the subducting plate were larger due to a thick crust that transforms from buoyant basalt to dense eclogite

palaeomagnetism of flood basalts in the pilbara craton western australia late archaean continental drift and the oldest known reversal of the geomagnetic field 1 a late archaean circa 2775 2715 ma succession of terrestrial continental flood basalts mafic tuffs felsic volcanic rocks and clastic sedimentary rocks in the nullagine synclinorium and meentheena centrocline of the east pilbara basin western australia has been sampled for a palaeomagnetic study over 500 oriented mostly basalt drill cores were collected from the supracrustal succession and associated dykes thermal and alternating field demagnetization revealed two distinct components positive fold conglomerate and reversal tests confirm that the primary natural remanent magnetization nrm is still preserved the secondary component is interpreted as the record of remagnetization during a major thermal event possibly in the early proterozoic analysis of the primary nrm directions results in a magnetostratigraphy and an apparent polar wander path apwp for the 60 myr interval covered by the sampled succession assuming a geocentric axial dipole during this time interval the apwp shows that the pilbara craton was drifting during the late archaean and that drift rates probably varied significantly in particular a mean 27 2 shift in palaeolatitude is recorded across an unconformity that represents a relatively short time period and that marks a significant change in basalt geochemistry this study suggests that continents moved horizontally during the late archaean and that the rates of movement were significantly faster than in the phanerozoic in addition a reversed polarity interval with a positive reversal test is recorded we argue that it documents the oldest known geomagnetic reversals of the geomagnetic field

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

the weekend dykes a newly recognized mafic dyke swarm on the eastern shore of nova scotia canada a group of related mafic dykes is present along the eastern shore of nova scotia from halifax to country harbour the dykes are generally vertical strike about 150 and have an abundance of carbo

thermobarometric modelling of zircon and monazite growth in melt bearing systems examples using model metapelitic and metapsammitic granulites u pb age data collected from zircon and monazite are used to draw fundamental inferences about tectonic processes in the earth despite the emphasis placed on zircon and monazite ages the understanding of how to relate the timing of growth of zircon and monazite to an evolving rock system remains in its infancy in addition few studies have presented large datasets of geochronological data from zircon and monazite occurring in the same metamorphic rock sample such information is crucial for understanding the growth of zircon relative to monazite in a systematic and predictive manner as per this study the data that exist support the generally held conception that zircon ages tend to be older than monazite ages within the same rock here experimental data for zircon and monazite saturation in melt bearing rocks are integrated with phase diagram calculations the calculations constrain the dissolution and growth behaviour of zircon and monazite with respect to evolving pressure temperature and silicate mineral assemblages in high grade melt bearing metasedimentary rocks several key results emerge from this modelling first that in aluminous metapelitic rocks i e garnet cordierite sillimanite assemblages zircon ages are older than monazite ages in the same rock second that the growth rate of accessory minerals is nonlinear and much higher at and near saturation than at lower temperatures and third that the difference in zircon and monazite ages from the same rock may be ascribed to differences in the temperature s at which zircon and monazite grow rather than differences in closure temperature systematics using our methodology the cooling rate of granulites from the reynolds range central australia have been constrained at 4 c myr 1 this study serves as a first pass template on which further research in applying the technique to a field study can be based

origin of the late devonian weekend lamprophyre dykes meguma zone nova scotia the weekend dykes consist of 10 late devonian spessartite lamprophyres cropping out within the allochthonous meguma lithotectonic terrane of the northern appalachians the dykes have characteristic panidiomorphic textures with seriate phenocrysts of amphibole clinopyroxene and rare biotite set in a groundmass of intergrown plagioclase k feldspar and quartz with deuteric calcite and epidote all dykes intruded during one magmatic episode ca 370 ma following terrane accretion of the acadian orogeny the unaltered weekend dykes show restricted major element variation sio2 54 58 wt al2o3 14 16 wt mgo 7 11 wt and total alkalies 2 4 5 5 wt and have high mg 71 80 and moderate to high concentrations of ni 69 278 ppm and cr 390 992 ppm large ion lithophile element e g sr ba 294 1194 ppm and light rare earth element 13 67cn abundances are high relative to high field strength element e g nb ta y 0 45 26 ppm and heavy rare earth element 6 30cn abundances geochemical varia

co2 drawdown and cooling at the onset of the great oxidation event recorded in 2 45 ga paleoweathering crust abstract the archean atmosphere is thought to have been devoid of oxygen but instead containing high concentrations of greenhouse gases such as co2 and possibly ch4 that were required to keep the earth surface environments warm enough for maintenance of liquid water under faint young sun conditions earlier studies have suggested that the co2 concentrations must have been as high as over 1000 times present atmospheric levels pal to overcome the effects of weaker solar luminosity however more recent studies of precambrian paleosols imply existence of at least tenfold lower but still high pco2 levels the appearance of minute amounts of atmospheric o2 at the transition of archean and proterozoic is well documented however the bulk composition of the atmosphere and its dynamics at this crucial time is still widely debated different paleoclimate proxies of the 2 45 ga kuksha paleoweathering crust suggest its formation in a cool temperate climate paleoatmospheric pco2 estimates by geochemical and isotopic mass balance models suggest remarkably low paleoatmospheric co2 levels with the best guess estimates of atmospheric co2 between 1 and 10 pal we speculate that the kuksha paleoweathering crust either predates or partly overlaps with the onset of huronian glaciation and corresponds to a time interval of low co2 level during the first recorded large glaciation in earth history

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

the archaean sanukitoid series of the baltic shield geological setting geochemical characteristics and implications for their origin abstract archaean high mg granitoids sanukitoids occur in the karelian granite greenstone terrain in the baltic shield in two distinct zones in the west of the shield sanukitoid intrusions formed between 2700 and 2720 ma and consist of a single igneous phase that varies in composition from diorite to granite in the eastern part of the shield sanukitoid intrusions formed between 2730 and 2745 ma and are strongly differentiated varying in composition from ultramafic to felsic all the sanukitoids are enriched in light rare earth elements lree la 80 ppm ce 150 ppm la n yb n 30 40 sr 2000 ppm ba 2500 ppm p 2 o 5 1 5 alkalis na 2 o k 2 o 5 10 possess high mg values 0 50 0 65 and show a negative nb ta anomaly they are spatially and temporally related to syenite intrusions and lamprophyre dykes sanukitoid intrusions in the western and eastern zones differ in composition in the west they have higher sio 2 mainly 60 and lower alkalis sr ba lree than in the eastern zone intrusions the most differentiated intrusion the panozero intrusion in the eastern zone was formed in two magmatic cycles separated by ductile deformation in the first cycle ultramafic to monzonitic rocks formed whereas in the second cycle the magmas were monzodioritic to quartz monzonite ultramafic and mafic rocks make up about 10 of the outcrop and occur as enclaves in monzonites and monzodiorites and as dykes implies a number of discrete magmatic events all rocks of the panozero intrusion have high k 2 o and the composition of the initial melt calculated from the weighted average of the first cycle magmas is monzodiorite sio 2 52 mg 0 55 na 2 o k 2 o 6 the presence of magmatic hornblende and biotite a high carbonate content widely distributed explosive breccias and evidence of liquid immiscibility are consistent with a high h 2 o co 2 content in the sanukitoid melt the geodynamic model which most satisfactorily explains our geological and geochemical observations is the formation of the karelian sanukitoids in a cratonic environment adjacent to the belomorian karelian suture

the tectonic significance of some basic dyke swarms in the canadian superior province with special reference to the geochemistry and paleomagnetism of the mistassini swarm quebec canada the mistassini dykes extend northwest from the mistassini embayment and comprise both tholeiitic and komatiitic suites they are probably 2000 ma old and yield two major paleomagnetic components one of these with a pole at 131 w 13 s is thought to be an overprint related to the elsonian disturbance 1400 1500 ma ago a very steeply down and reversed component may be primary and has a pole at 080 w 50 n these spacial chemical and age relationships between the mistassini molson marathon and payne river dyke swarms and the aphebian supracrustal fold belts on the perimeter of the superior province suggest a genetic relationship between the dyke swarms and the fold belts the supracrustal belts are evidence of the opening and closing of oceans and the dyke swarms are evidence of early stage failed arms related to these openings more rarely for example the payne river dykes early stage dyke swarms are developed and preserved parallel to the edges of newly developed spreading plate boundaries

the temporal evolution of sanukitoids in the karelian craton baltic shield an ion microprobe u th pb isotopic study of zircons abstract the u th pb isotopic systematics of zircons from six sanukitoid intrusions in the karelian craton has been investigated using the secondary ion mass spectrometer sims our results show that karelian sanukitoids formed between 2745 and 2705 ma ago and that there is an age difference of about 30 ma between sanukitoids in the eastern and western karelia results for intrusions made up of a number of magmatic phases show no internal age variation in the east of the study region zircons from a monzodiorite and two quartz monzonites from the panozero pluton were dated at 2737 11 2739 11 and 2741 8 ma in the hautavaara pluton a monzodiorite and a subalkaline granite were dated at 2743 8 and 2742 23 ma and a quartz diorite from the elmus pluton gave an age of 2741 7 ma in addition a lamprophyre dyke within the sjargozero pluton gave an age of 2742 14 ma in the west zircons from a quartz monzonite and a granodiorite in the njuk intrusion gave ages of 2709 10 and 2716 11 ma in the kurgelampi pluton zircons from a diorite and two granodiorites were dated at 2707 9 2719 6 and 2712 9 ma respectively zircons from the granodiorites and the alkaline granites contain older cores with 207 pb 206 pb ages greater than 2800 ma the absence of inherited zircons in the more mafic sanukitoids is attributed to their higher saturation levels for zirconium sanukitoid zircons have th u ratios above 0 5 and often higher whereas nearby tonalite tondhjemite granodiorites ttgs have th u ratios below 0 5 typically tonalites are older than sanukitoids by more than 100 ma and it is possible that tectonic processes associated with the collision of the belomorian mobile belt provided the heat necessary to initiate the generation of the sanukitoid melts

palaeomagnetism of the neoarchaean pongola and ventersdorp supergroups and an appraisal of the 3 0 1 9 ga apparent polar wander path of the kaapvaal craton southern africa the style of tectonic processes on the young earth is a topic of intense debate palaeomagnetism provides the only robust tests with which to settle theoretical disputes based on the rock record that infer large scale horizontal motions between palaeocontinents a possible apparent polar wander path apwp of the kaapvaal craton shows significant distances between adjacent palaeomagnetic poles that can be interpreted as evidence for such horizontal displacements during the archaean palaeoproterozoic there are however substantial uncertainties build into this apwp currently the published apwp for the kaapvaal craton comprises eight palaeopoles between 3 0 and 1 9 ga including several between 2 8 and 2 7 ma that indicate possible horizontal motions exceeding 6500 km the aim of this contribution is to increase the pole density within this interval and in doing so to test the robustness of the kaapvaal apwp within this time frame more than 250 samples of basalt diamictite and dolerite of the neoarchaean pongola ca 2 95 2 85 ga and ventersdorp ca 2 71 2 70 ga supergroups of the kaapvaal craton were sampled both at the surface and in an underground mine thermal demagnetisation revealed two magnetic components a high temperature component is present in three pongola formations and in two ventersdorp formations these may be recorders of the primary natural remanent magnetisation because of lack of precise geochronology and positive field tests this magnetisation can only be constrained to be older than ca 2 05 ga a medium temperature overprint in many samples probably records remagnetisation at ca 180 ma during the emplacement of basalts and dolerites of the mesozoic karoo large igneous province this remagnetisation appears to have affected a vast area of the kaapvaal craton our new palaeopoles together with those published previously demonstrate that the neoarchaean apwp of the kaapvaal craton remains poorly defined direct comparisons between the apwp of the kaapvaal craton with apwps of other archaean cratons to determine relative motions between archaean continents therefore need more reliable data

thermal history of the southwestern meguma zone nova scotia from an 40ar 39ar and fission track dating study of intrusive rocks two geologically distinct regions within the meguma terrane have been studied in detail and are shown to have contrasting thermal histories this conclusion is based on 31 new 40ar 39ar age spectra on micas k feldspars and hornblendes from several of the plutons in southern nova scotia which are satellites to the much larger south mountain batholith smb in addition we report four k feldspar age spectra from a locality in the northeastern part of the smb and seven fission track ages from both the southern and northern areas the complex age spectrum of hornblende from a mafic phase associated with the bamngton passage pluton suggests a minimum intrusive age of 385 ma a result consistent with geologic evidence that this pluton represents an early less evolved magmatic pulse later intrusive activity appears to have been coeval with the intrusion of the smb at ca 370 ma ago in the southern region argon clocks in feldspars were completely reset and mica clocks variably reset by a later thermal event the cumulative geochronologic and geologic evidence constrains this event to 300 320 ma ago in the smb this alleghanianl hercynian thermal disturbance is much less pronounced and appears to be localized to small areas that are often associated with economic mineralization k feldspars from the southern plutons record an episode of argon loss 220 230 ma ago which is less evident in the smb to the north upper triassic dike injection in the southern region associated with the initial rifting phase of the canadian atlantic margin accompanied this milder thermal pulse apatites in the two regions record a mean fission track age of ca 180 ma which we attribute to the final cooling of the terrane below about 100 c the timing of this event coincides with regional uplift recorded in sedimentary sequences along much of the maritime continental shelf deux kgions gtologiques distinctes inttgkes au terrane de meguma ont ctt ctudites en dctai1 et leurs histoires thermales

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

paleomagnetism of 2 44 ga mafic dykes in russian karelia eastern fennoscandian shield implications for continental reconstructions abstract results of a paleomagnetic study of paleoproterozoic dykes and archean host rocks in the eastern fennoscandian shield are reported here most of the dykes are related to ca 2440 ma layered intrusions and have isotopic ages between 2349 and 2476 ma the dykes form an extensive nw se and e w trending swarm the majority are unaltered gabbronorites which carry stable remanent magnetizations with multicomponent nature the high temperature remanence component d which is thought to represent the primary 2 44 ga magnetization of the dykes has a paleomagnetic pole at 20 s 278 8 e a95 6 0 n 11 sites another d like component d with shallower inclination vgp 9 6 n 256 2 e eight samples at one site from baked archean host rock may also represent the primary remanence of the dykes the commonest remanence component a records an extensive svecofennian partial overprint and is interpreted to be ca 1 84 ga old it yields a paleomagnetic pole at 52 6 n 226 8 e a95 2 7 n 23 the fourth component b has a paleopole at 53 9 n 169 6 e a95 6 3 n 16 and is inferred to be ca 1 75 ga on the basis of isotopic dating although ages of 2 1 1 3 and 0 3 ga are also possible baked contact tests for the 2 44 ga dykes are negative as shown by the occurrence of component d in archean host rocks beyond one dyke width it is proposed that the negative baked contact test indicates regional reheating and remagnetization of the archean basement at ca 2 44 ga ago due to the spatially vast occurrence of coeval 2 44 ga mafic dykes layered intrusions volcanics and granites in the area based on this hypothesis it is suggested that component d in the dykes may be primary and acquired at ca 2 44 ga ago the similarity of pole d with poles from coeval dykes and layered intrusions elsewhere in the karelian province and kola peninsula suggest that within the archean domain of the fennoscandian shield no large scale block movements have taken place since 2 44 ga at 2 44 ga the fennoscandian shield was located at the latitude of ca 30 s which is different to that at 1 84 and 1 75 ga when fennoscandia was located at the latitudes of ca 30 n a continental reconstruction based on pole d and the pole from the 2 45 ga matachewan dykes of the superior craton in laurentia places the kola peninsula adjacent to southern greenland reconstructions based on recent geological interpretations and a reconstruction yielding pole d are also discussed

multiple post svecofennian 1750 1560 ma pegmatite dykes in archaean palaeoproterozoic rocks of the west troms basement complex north norway geological significance and regional implications abstract the west troms basement complex north norway consist of meosarchaean neoarchaean and palaeoproterozoic crust which likely represent the continuation of the karelian craton of the fennoscandian shield beneath and across the caledonian nappes the complex is transected by several meta sedimentary volcanic belts with ages from ca 2 7 to 1 9 ga and linear ductile shear zones that record polyphase deformation after emplacement of several late svecofennian plutonic complexes 1 80 1 79 ga the astridal supracrustal belt of the senja shear zone was intruded by various generations of granite pegmatite dykes which yield zircon and titanite u pb ages ranging between 1758 and 1562 ma with evidence of a weak caledonian overprint at about 400 ma that formed or reset some but not all titanites the age range indicates that formation of the pegmatites occurred after termination of the main svecofennian orogeny 1 92 1 79 ga of the karelian craton comparison within the larger scale context of the fennoscandian shield suggests that emplacement of these pegmatites may have been caused by late stage reactivation of intracratonic shear zones possibly reflecting distal effects of deeper magmatic processes such as intrusion of coeval aged rapakivi granite suites ca 1 65 1 5 ga in finland and or the mesoproterozoic gothian orogeny and magmatism affecting the southwestern parts of fennoscandia the mesoproterozoic ages however also correspond to late laxfordian ca 1 7 1 6 ga tectonic events in the lewisian basement on the laurentian margin of scotland suggesting a potential early link between these basement complexes

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

u pb dating of zircons and monazites from archean granulites in varpaisjarvi central finland abstract u pb age data from archean lower crustal granulites in the varpaisjarvi area central finland indicate several metamorphic events that took place in mesoarchean neoarchean and paleoproterozoic time zircons separated from the granulites fall into two distinct age groups in the older group the protolith age is 3 2 ga and many zircons indicate metamorphism at ca 3 1 ga in contrast the oldest zircons in the younger group are ca 2 73 ga both terranes were migmatized at ca 2 70 ga with the emplacement and crystallization of igneous enderbites the enderbites and the leucosomes of migmatites in both granulite terranes have 2 7 ga zircons high grade metamorphic conditions prevailed from 2 70 to 2 63 ga which is the age range of most zircons in the eastern terrane also the enderbites show metamorphic zircon growth at ca 2 64 ga two distinct age groups are interpreted to be the result of juxtaposition of two terranes of different ages this indicates that collisional and accretionary processes operated during neoarchean time the area east from the eastern granulite terrane underwent almost pervasive deformation and amphibolite facies metamorphism during the svecofennian orogeny at ca 1 9 ga however the u pb ages of zircons in the retrograde area are similar to those of the eastern terrane thus indicating that these rocks were metamorphosed under granulite facies conditions 2 70 2 63 ga although the ca 1 9 ga svecofennian metamorphism is not evident in the ion microprobe results some paleoproterozoic pb loss is indicated by tims u pb analyses of zircons in addition monazite age of 1 89 ga from the retrograde area reflects svecofennian metamorphism

revisiting the paleomagnetism of the neoarchean uaua mafic dyke swarm brazil implications for archean supercratons abstract the original connections of archean cratons are becoming traceable due to an increasing amount of paleomagnetic data and refined magmatic barcodes the uaua block of the northern sao francisco craton may represent a fragment of a major archean craton here we report new paleomagnetic data from the 2 62 ga uaua tholeiitic mafic dyke swarm of the uaua block in the northern sao francisco craton eastern brazil our paleomagnetic results confirm the earlier results for these units but our interpretation differs we suggest that the obtained characteristic remanent magnetization for the 2 62 ga swarm is of primary origin supported by a provisionally positive baked contact test the corresponding paleomagnetic pole 25 2 n 330 5 e a95 8 1 n 20 takes the present northern part of the sao francisco craton to moderate latitudes based on the comparison of the paleolatitudes of cratons with high quality paleomagnetic data and magmatic barcodes we suggest that the northern part of the sao francisco craton could have been part of the proposed supervaalbara supercraton during the archean supervaalbara is proposed as including but not limited to the part of the sao francisco craton as well as the superior wyoming kola karelia zimbabwe kaapvaal tanzania yilgarn and pilbara cratons

plate kinematics of the rocas verdes basin and patagonian orocline the processes of orocline formation are a topic of debate in geosciences the patagonian orocline has been a case in point for over a century large anomalous paleomagnetic pole rotations show that the orocline started to form at the same time as mid cretaceous closure of the rocas verdes basin today known from ophiolitic and basin fill remnants in the patagonian and fuegian andes some studies therefore present bending of the andes and closure of the basin as shared consequences of rotation of a small plate that was driven by subduction related forces at the pacific margin of gondwana an alternative view of the orocline is as a product of cretaceous to paleogene aged sinistral oblique convergence at the plate boundary scale geological data from tierra del fuego have been interpreted in support of both views here i test these suggestions by comparing the rocas verdes basin s tectonostratigraphy to predictions of a plate kinematic model for fragmentation of the western interior of gondwana the model is sufficient to explain the known history of basin opening to a width of 100 300 km during the period 152 141 ma and later closure in oblique plate convergence as this convergence occurred by motion around a distant euler pole it could not have produced the patagonian orocline by rotation of a lithospheric plate on its pacific flank the large anomalous paleomagnetic rotations of tierra del fuego instead are likely to have occurred within the crust by rotation and deformation of regional strike slip faults and the intervening rocks to accommodate oblique convergence of the south american and antarctic plates between albian and paleocene times

paleomagnetic constraints on an archean paleoproterozoic superior karelia connection new evidence from archean karelia abstract charno enderbitic granitoids in the karelia craton of the fennoscandian shield have been studied paleomagnetically the characteristic remanence component has a steep negative inclination and is interpreted to record magnetization at a maximum age of 2684 2 ma consistently stable results were obtained from 12 sites in the koitere area corresponding to regions with high positive magnetic anomalies and high remanence intensities petrographic studies coupled with rock magnetic investigations indicate that the remanence resides in fine sd psd magnetite grains formed during neoarchean clinopyroxene alteration cross cutting vertical subvertical paleoproterozoic dolerite dykes suggest that the koitere granitoids are in their original orientations and were not affected by svecofennian deformation at ca 1 9 1 8 ga the koitere granitoids have an opposite polarity compared to the steep positive inclination remanence direction of the previously studied ca 2 63 ga varpaisjarvi enderbites and granulites the data from koitere and varpaisjarvi imply that at ca 2 7 2 6 ga the karelia craton was located at high latitudes of 80 60 whereas previous paleomagnetic data from ca 2 5 ga formations in the vodlozero terrane in nw russia indicate a near equatorial position comparison of paleomagnetic data from the koitere and varpaisjarvi granulite grade rocks with rocks of similar age in the superior craton shows that at ca 2 7 2 6 ga the superior and karelia cratons were located at high latitudes and in close proximity although the present data cannot demonstrate that the cratons were amalgamated however during the archean paleoproterozoic transition at ca 2 50 ga both cratons experienced significant rotation and drifting to near equatorial paleolatitudes suggesting that the superior and karelia cratons may have been attached at that time

constraints on the u pb systematics of metamorphic rutile from in situ la icp ms analysis abstract in situ laser ablation icp ms u pb dating of metamorphic rutile from granulite facies metapelitic rocks of the archaean pikwitonei granulite domain manitoba canada provides constraints on pb diffusion and characterizes the closure behavior of rutile the analysis of transects of 35 m spots across 15 rutile grains having a size of 120 to 280 m yielded concordant ages with core ages of ca 2450 ma and core to rim younging towards 2280 ma age profiles indicate that volume diffusion of pb occurs in rutile implying that the ages represent cooling ages to investigate the closure behavior of pb in rutile closure temperature profiles t c x were constructed based on different models combined with experimentally determined diffusion parameters the classical t c x model of dodson 1986 mat sci forum 7 145 154 indicates a rapid decrease of t c in the rims of grains providing unrealistic estimates for the cooling rate when combined with u pb ages a new t c x model was constructed based on the analyzed age profiles that are described by an error function this model shows a more steady decrease in t c throughout the grain from ca 640 c in the core depending on grain size to a rim intercept t c rim of 490 c 7 2 which is interpreted to be the extrapolated theoretical absolute temperature of insignificant pb diffusion in rutile the new model provides a better description of the relation between age and t c for the analyzed grains however both t c x models demonstrate that even in small grains the variations of t c can be significant making it impossible to derive one representative t c for pb in rutile the error function based t c x model allows the determination of cooling rates which show a decrease over time from ca 2 2 to 0 4 c ma agreeing well with previous estimates for the pikwitonei granulite domain this consistency supports the validity of our model and indicates that cooling rates can be estimated from single grains by la icp ms u pb dating of rutile providing constraints on the cooling history of a metamorphic terrane the slow cooling rates imply that exhumation was slow

palaeomagnetism of precambrian basic intrusion and dykes of northern karelia eastern fennoscandian shield abstract palaeomagnetic results for mafic and ultramafic intrusions and their country rocks in russian karelia are presented the oldest rock type is pyroxene hornblende gabbro regarded as late archaean or palaeoproterozoic in age later at the early stage of formation of a volcanic plutonic association including layered intrusions and metavolcanites gabbro norite dykes were emplaced sm nd isotopic analyses of this association yield mineral and whole rock ages of 2 47 2 35 ga with nd 0 4 to 1 2 for gabbro norite dykes based on field relations the youngest rock types are pyroxene plagioclase porphyry dykes and gabbro dolerite dykes three distinct magnetization components were isolated by alternating field and thermal demagnetization the most common remanence component b with a mean direction of d 30 i 58 k 52 95 10 n 5 geological units is an overprint interpreted to have been acquired at the last stage of the svecofennian orogeny 1 90 1 70 ga component a with a mean direction of d 350 i 49 k 69 95 9 n 5 geological units is also an overprint probably acquired at the onset of the svecofennian orogeny both components are found in all rocks studied component c with a mean direction of d 126 i 73 k 167 95 10 n 3 geological units is isolated as a primary remanence in the pyroxene hornblende gabbro intrusion and gabbro norite dykes the results are compared with other contemporaneous palaeomagnetic poles from finland and russian karelia

single grain zircon u pb age constraints of the western and eastern sanukitoid zones in the finnish part of the karelian province abstract a single grain zircon u pb secondary ion mass spectrometer sims study of nine granitoid intrusions with sanukitoid geochemistry from the finnish part of the karelian province shows their time limited emplacement the age results indicate emplacement at 2715 and 2745 ma we observed also a significant number of relict cores in the zircons with 207 pb 206 pb ages up to 3 2 ga emphasizing the importance of inheritance processes in the petrogenesis of the sanukitoids compiled single grain age data of the finnish and russian bibikova et al 2005a parts of the karelian province confirm the occurrence of two temporally diverging sanukitoid zones throughout the province the western zone shows a younger average age 2718 ma and the eastern zone an older average age 2740 ma with an apparent age difference of 20 ma between the zones most of the sanukitoid intrusions in finland belong to the western sanukitoid zone but the southernmost intrusions can be regarded as a part of the eastern sanukitoid zone the boundary between the sanukitoid zones specifies the boundary of the western and central karelian terranes worldwide sanukitoid magmatism at 2 95 2 55 ga as reported from several archean blocks followed sharply after prolonged tonalite trondhjemite granodiorite ttg magmatism indicating a short lived tectonomagmatic phenomenon at the end of convergent accretion

the shelburne dike an early mesozoic diabase dike in nova scotia mineralogy chemistry and regional significance a diabase dike about 140 km long the shelburne dike cuts in a northeasterly direction across the southwestern part of nova scotia the dike recently dated at 201 ma forms part of a major appala

discrimination and origin of the sanukitoid series geochemical constraints from the neoarchean western karelian province finland abstract neoarchean late to post tectonic sanukitoid intrusions are found in the western part of the karelian province in finland variable sized even grained to k feldspar porphyritic intrusions form a series of diorites tonalites and granodiorites that are calc alkaline to alkali calcic magnesian and mostly peraluminous the major and trace element geochemistry of the intrusions show typical sanukitoid affinities a mantle signature high content of mg ni cr and high mg and enrichment in lile especially k ba and sr the intrusions form a distinctive series that can be distinguished from the ttg series of the karelian province by their high ba sr signature low na 2 o k 2 o ratio and uniform hree pattern the sanukitoid series can be compositionally constrained as follows sio 2 55 70 wt na 2 o k 2 o 0 5 3 mgo 1 5 9 wt mg 45 65 k 2 o 1 5 5 0 wt ba sr 1400 ppm and gd er n 2 6 the probable source was enriched subcontinental lithosphere mantle with high lile concentrations in phlogopite and or sanidine the accumulation of k ba and sr in the mantle source may have occurred as a consequence of two separate metasomatic events 1 by subduction related fluids melts and 2 by metasomatism related to asthenospheric mantle upwelling caused by slab breakoff the slab breakoff at the end stage of subduction is also a viable trigger for the sanukitoid magmatism

late devonian lamprophyric dykes in the meguma zone of nova scotia hornblende rich lamprophyric dykes from southeastern nova scotia are contemporaneous with the large peraluminous granitic batholiths ca 370 ma of the meguma zone and may be closely associated with their origin

magma injection directions inferred from a fabric study of the popes harbour dike eastern shore nova scotia canada a 15 m wide dike at popes harbour on the eastern shore of nova scotia shows an increase in both the size up to 1 m and density up to 50 of predominantly pelitic xenoliths toward the dike center these features reflect flowage differentiation processes during dike emplacement flow direction is indicated by sillimanite xenocrysts that represent the unassimilated products of xenolith disaggregation the xenocrysts are highly aligned parallel to the dike margins northwest southeast with a mean plunge of 20 se suggesting that the magma last moved and may have been injected in a subhorizontal northwestward direction the available information indicates that lateral injection of magma in mafic dikes is more common than predominantly vertical movement this study shows that an analysis of fabrics in mafic dikes should be useful in further evaluating the role of lateral magma injection in dike formation

relative age of otto stock and matachewan dykes from paleomagnetism and implications for the precambrian polar wander path north trending diabase dykes within and near the otto syenite stock of the kirkland lake area of the superior province carry a stable paleomagnetic remanence identical to that of 2454 2 ma matachewan dykes far from the stock a positive baked contact test for the remanence of a dyke within the stock demonstrates that the remanence is primary thus 1 these dykes belong to the matachewan swarm 2 the otto stock has not magnetically overprinted matachewan dykes in its contact aureole as previously reported and 3 the stock must be older than 2454 ma the magnetic remanence associated with the otto stock previously thought to be primary must now be considered of uncertain age nevertheless the positive baked contact test for the remanence of the matachewan dyke within the stock demonstrates that the remanence associated with the stock predates intrusion of the matachewan swarm the conventional polar wander path for precambrian north america decreases in age from the matachewan paleopole to the otto

p t t development of archaean granulites in varpaisjarvi central finland ii dating of high grade metamorphism with the u pb and sm nd methods exposed blocks of lower crustal archaean granulites separated by palaeoproterozoic faults occur near the western boundary of the karelian craton in central finland the southwesternmost granulite block in the study area the jonsa block differs from other granulites by having younger zircon u pb and sm nd tdm model ages the u pb ages on zircons and monazites from leucosomes and mesosomes of mafic and pelitic granulites in the jonsa block are ca 2 63 ga which is interpreted as the age of granulite metamorphism igneous enderbites which make up a considerable part of the bedrock in granulite blocks are ca 50 ma older outside the jonsa block both conventional and ion probe zircon ages of mesosomes of migmatitic mafic granulites are 3 2 3 1 ga which are the protolith ages sm nd tdm model ages of the mafic granulites are 2 7 2 9 ga in the jonsa block but ca 3 2 ga in the other granulites the age differences are interpreted to represent terrane accretion that juxtaposed 3 2 ga rocks next to younger rocks all granulites were metamorphosed in similar pt conditions at 2 63 ga granulite metamorphism affected the whole area but was not able to reset zircons in older rocks in all blocks the sm nd garnet whole rock ages are younger ranging from 2 48 2 59 ga this reflects relatively low closure temperature of the sm nd system in garnet in these rocks the u pb age of a palaeoproterozoic dolerite which cuts the granulites is 2 3 ga close to the k ar age of amphibole in a retrogressed fracture

late archean high mg granitoids sanukitoids in the southern karelian domain eastern finland pb and nd isotopic constraints on crust mantle interactions late archean ca 2 73 ga porphyritic high k and high mg granitoids in the nilsia and lieksa areas eastern finland have geochemical signatures similar to those of high mg sanukitoid series from other parts of the karelian domain of the fennoscandian baltic shield and the archean sanukitoid suites from canada the nilsia and lieksa granitoids have low sio2 contents 62 7 67 0 and high mg numbers 0 45 0 52 they are enriched in k2o 2 40 4 73 wt p2o5 0 25 0 42 wt sr 610 850 ppm ba 1200 2300 ppm cr 40 80 ppm and lree la yb n 19 65 they are characterized by a negative eu anomaly eu eu 0 53 0 81 and show strong depletion in u high th u ratio low u pb ratio high 2 values for the source 9 positive nd 2 7 ga values of 0 3 1 4 and nd depleted mantle model ages 2 75 2 86 ga the high 2 values pointing to a significantly older crustal source apparently contradicts the positive nd 2 7 ga values supporting a mantle origin the geochemical and isotopic data suggest that the nilsia and lieksa granitoids originated at ca 2 73 ga from a mantle wedge source enriched in lile u th and pb by recycling of continental material in subduction related slab dehydration processes shortly up to 200 ma before melting a considerably large amount of crustal lead was contributed from subducting sediments into the overlying mantle wedge consequently the isotopic composition of mantle wedge lead was overprinted by crustal lead isotope signatures it is suggested that crustal recycling through subduction zone processes played an important role for the enrichment of the mantle wedge and generation of mantle derived high granitoids in the archean

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

the late archean record a puzzle in ca 35 pieces abstract the global archean record preserves ca 35 large cratonic fragments and a less well defined number of smaller slivers most archean cratons display rifted margins of proterozoic age and therefore are mere fragments of supercratons which are defined herein as large ancestral landmasses of archean age with a stabilized core that on break up spawned several independently drifting cratons the tectonic evolution of individual archean cratons such as the slave craton of north america or the kaapvaal craton of southern africa should therefore always be considered in the context of their ancestral supercratons this is particularly true for many of the smaller cratons which are too limited in size to preserve the complete tectonic systems that led to their formation these limitations not only apply to the crustal geology of archean cratons but also to their underlying lithospheric mantle keels if these keels are archean in age as their broad correlation with ancient surface rocks suggests they also are rifted and drifted remains of larger keels that initially formed below ancestral supercratons the study of archean cratons and their lithospheric keels should thus be global in scope in the search for late archean supercratons a craton like the slave with three to four rifted margins has a ca 10 maximum probability of correlating with any of the ca 35 remaining cratons around the globe assuming wholesale recycling of archean cratons has been limited since ca 2 0 ga alternatively if the original number of independently drifting cratons was significantly larger the probability of successful correlations is much less due to repeated cycles of break up and plate tectonic dispersal since ca 2 0 ga the probability of correlation is probably independent of present day proximity unless there is independent evidence that two neighbouring cratons were only separated by a narrow ocean nevertheless most previously proposed craton correlations rely erroneously on present day proximity implicitly extrapolating relatively recent paleogeography back to the archean considering the fundamental differences between some of the better known cratons such as the slave superior and kaapvaal it seems likely that these cratons originated from independent supercratons sclavia superia vaalbara with distinct amalgamation and break up histories this is contrary to widely held opinion based largely on an idealized view of the supercontinent cycle that all cratons shared a common history in a single late archean supercontinent in contrast to the superior and kaapvaal cratons which are radically different from the slave craton and each other the dharwar craton of peninsular india the zimbabwe craton of southern africa and the wyoming craton of north america all show significant similarities to the slave craton it thus seems likely that at least some of these were nearest neighbours to the slave craton in the ca 2 6 2 2 ga sclavia supercraton

two neoarchean supercontinents revisited the case for a rae family of cratons abstract a variety of early paleoproterozoic supercontinents or supercratons have been proposed including kenorland superia and vaalbara improved geologic knowledge of the churchill province canadian shield suggests that the classical kenorland configuration with linked rae and hearne cratons is invalid and represents a younger ca 1 9 ga nuna configuration we review the early paleoproterozoic tectonic history of the rae craton and propose that the ca 2 55 2 50 ga macquoid and 2 50 2 28 ga arrowsmith orogens represent critical piercing points for supercontinent reconstruction utilizing the stratdb and dateview databases we have identified nearly two dozen regions worldwide that show effects of these orogenic events including the rae gawler mawson peninsular india west africa amazonia north china and sask crustal blocks we propose that these cratonic blocks form a distinct rae family lineage utilizing new time space plots for the rae family and components of the superia and vaalbara supercratons we demonstrate that the rae family lacks the distinctive early paleoproterozoic depositional and glaciogenic record characteristic of superia and vaalbara but has an important contemporaneous record of juvenile crust formation and contractional orogenesis we propose the collective history of the rae family is best reconciled if it represents the now dispersed blocks of a third earliest paleoproterozoic supercraton that we term nunavutia in this model the magmatic and tectonometamorphic record of these cratons includes an early 2 55 2 45 ga period of interior collisional orogenesis in which the supercraton amalgamated and a later phase of long lived peripheral orogenesis 2 45 2 28 ga prior to its partial break up at ca 2 05 ga forming within an otherwise globally quiescent period the history of the rae family is important for assessment of constrasting models of early paleoproterozoic geodynamic activity

the silurian passamaquoddy bay mafic dyke swarm new brunswick petrogenesis and tectonic implications the volcanic and sedimentary rocks of the passamaquoddy bay pb area of southeastern new brunswick are part of the silurian devonian coastal volcanic belt cvb an extensive belt of bimodal volca

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

early paleoproterozoic archean dykes and gneisses in russian karelia of the fennoscandian shield new paleomagnetic isotope age and geochemical investigations abstract we present here new palaeomagnetic isotopic age and geochemical data from archean and early palaeoproterozoic rocks in the eastern fennoscandian shield we have studied ne sw trending gabbronorite dyke sets and their host archean basement rocks in the vodlozero block near the 2449 ma burakovka layered intrusion in southern russian karelia both dyke sets are genetically related to the burakovka intrusion the other ca 25 km long avdeev dyke locating a few kilometers south from the burakovka intrusion yields a stable single component remanence direction that is in agreement with the direction previously obtained from the burakovka intrusion another ne sw trending dyke 0 8 m wide shalskiy diabase dyke about 30 km south of the burakovka intrusion yields a similar remanence direction as the avdeev dyke the overall mean remanence direction has a palaeopole at plat 12 3 n plong 243 5 e a 95 15 4 4 sites 28 samples the thin shalskiy diabase dyke transects a similarly ne sw trending 500 m wide coarse grained gabbronorite dyke which has now been dated by sm nd method as 2608 56 ma geochemically all the dykes are quite similar showing slight calc alkaline affinity and low tio 2 and high sio 2 with moderate mgo and low cr and ni furthermore the dykes are geochemically identical to the 2 45 ga dyke swarm in the northern karelian province the remanence direction of the thin shalskiy diabase dyke differs significantly from the high temperature and high coercivity remanence component of the unbaked archean gabbronorite dyke which yields a palaeopole at plat 22 7 n plong 222 1 e dp 8 2 dm 16 2 five samples on the basis of different remanence directions of the diabase dyke and the unbaked archean gabbronorite dyke the baked contact test for the diabase dyke is positive in addition to the high temperature and high coercivity component of the baked and unbaked archean gabbronorite dyke in low temperatures and coercivities we isolated a similar component as in the diabase dyke a comparable remanence component was also obtained from the archean basement at ca 8 km from the dykes we propose that in the studied area the archean basement and the archaean dyke were partly remagnetized due to emplacement and subsequent uplift and cooling of the large burakovka layered intrusion and related dykes at about 2 40 ga ago this interpretation lends support from a new 40 ar 39 ar dating of hornblende from another area lake paajarvi area in northern karelia there a negative baked contact test was previously obtained for the remanence of the dated ca 2 45 ga dyke rocks related to the ca 2 45 ga oulanka layered intrusion the 40 ar 39 ar dating of the unbaked archean basement which yields the same remanence component as the dykes shows a plateau age of ca 2 6 ga but in addition it also shows resetting of the basement at ca 2 4 ga ago the dating thus supports reactivation and partial remagnetization of the archean basement at ca 2 4 ga ago our new palaeomagnetic results from the burakovka dykes and the new 40 ar 39 ar dating from the lake paajarvi area give support to our previous interpretation that at lake paajarvi area the remanence component suggested to be 2 4 ga despite to negative baked contact test is indeed of this age therefore it is implied that the results can be used for continental reconstructions

u pb zircon geochronology in the southwestern abitibi greenstone belt superior province the abitibi belt is the largest continuous greenstone belt in the superior province of the canadian shield it comprises several composite komatiitic tholeiitic calc alkalic and sedimentary sequences that are folded transected by major faults and intruded by various generations of plutonic rocks precise u pb geochronology has been carried out in the belt for the past decade to solve chronostratigraphic and metallogenetic problems this paper presents new zircon ages and reassesses previously published ones now refined by the addition of abraded and concordant zircon analyses volcanic and subvolcanic units of the timmins area yield the following ages 2727 1 5 ma for a tuff of the upper deloro formation 2703 1 5 ma for a tuff of the upper tisdale formation and 2698 4 ma for the krist fragmental assigned to the top of the tisdale formation the age of a dunite intrusive into the upper deloro formation is revised at 2707 3 ma whereas the aquarius diorite east of timmins yields a poorly define