a simple way to extract baddeleyite zro2 1 baddeleyite is a reliable geochronometer for dating the crystallization of mafic and other silica undersaturated intrusive rocks its high u and negligible initial pb content enable precise age determinations with statistical errors of a few million years or less the widespread use of baddeleyite for dating has however been limited by the low content of baddeleyite in many samples and by difficulties in isolating baddeleyite we have developed a new separation technique that utilizes differences in transport velocity among grains of varying size as they move across the deck of a water shaking table a small sample portion is loaded instantly on the table and only the finest and densest material remaining after similar to120 s is collected repeated recovery of sample portions yields a concentrate strongly enriched in baddeleyite and opaque minerals the latter is easily removed by a hand magnet the water based separation technique will improve yields of baddeleyite from samples processed on a water shaking table and is capable of recovering very fine grains as the entire separation is made in water problems arising from adhesive forces between minerals are minimized as is the risk of cross contamination no heavy liquids are necessary making separation quick uncomplicated and inexpensive

u th ages obtained by mass spectrometry in corals from barbados sea level during the past 130 000 years the study of the sea level record during the last glacial cycle has primarily proceeded indirectly by means of oxygen isotope measurements on foraminifera from deep sea sediments1 2 the direct approach of dating sea level indicators stagnated during the past decade mainly because the samples required to complete our knowledge of the past glaciations are below the present day sea level3 7 using the 14c ages of acropora palmata samples collected by drilling offshore the island of barbados fairbanks8 presented the first detailed chronology for the last deglaciation this radiocarbon chronology is limited to the past 30 kyr because of the short half life of 14c 5 730 yr we must therefore rely on other dating methods to obtain information for the whole last glacial cycle during the past four years 230th 234u dating of corals by thermal ionization mass spectrometry has been shown to be significantly more precise and accurate than the classical counting method9 14 we have used this technique to measure u th ages in coral samples from the barbados collection and from subaerially exposed outcrops see also ref 15 here we present results bearing on the sea level record for the past 130 000 years we conclude that the last deglaciation started 3 000 years earlier than previously thought and confirm that there were two surges in melt water8 at about 11 kyr and 14 kyr bp before present

the kivalliq igneous suite anorogenic bimodal magmatism at 1 75ga in the western churchill province canada abstract the kivalliq igneous suite 1 77 1 73 ga includes the nueltin granite intrusions gabbro and anorthosite intrusions three swarms of mafic dykes and basalt rhyolite and pyroclastic rocks of the pitz formation wharton group middle dubawnt supergroup it was emplaced in the reworked archaean hinterland of the trans hudson orogen in the central western churchill province wcp of the canadian shield the nueltin granites have field relations mineral assemblages and compositions typical of metaluminous to peraluminous proterozoic to recent rapakivi granites and the suite comprises a classic bimodal anorogenic province kivalliq rhyolites granites were erupted emplaced in a n s band the nueltin corridor about 700 km long and 300 km wide with volcanic rocks and dykes concentrated at the north end and coarse plutonic rocks to the south the mcrae lake dyke swarm originates among three prominent gabbro anorthosite intrusions inside the corridor and forms a narrow array extending a minimum of 600 km to the ne along the central axis of the wcp early mafic rocks intrusions and some lavas 1 77 ga had an alkali basalt parent with subalkaline basalts sills dykes and mafic enclaves present at the peak time of granite emplacement 1 75 ga average initial nd of basaltic rocks at 1 75 ga is 4 0 indicating a significant crustal component sm nd depleted mantle model ages of granites and rhyolites average initial nd 8 7 range from 2 2 to 3 ga consistent with melted archaean crust variably mixed with juvenile basalt metallogenic prospects of the kivalliq suite include u th zr y ree enriched granite and syenite epithermal ag au at silicic centres including mafic felsic ring complexes and pb cu sulphides associated with mafic intrusions topaz rhyolites are present but sn enriched rocks have not been observed the kivalliq suite is one of about 15 prominent anorogenic provinces and dyke swarms which erupted within the nuna supercontinent at ca 1 75 ga and the thermal effects of the kivalliq event are recorded in u pb and k ar data throughout most of the western churchill province

insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

precise u pb ages and geochemistry of palaeoproterozoic mafic dykes from southern west greenland linking the north atlantic and the dharwar cratons palaeoproterozoic magmatism throughout the north atlantic craton nac has produced dyke swarms of diverse orientations and emplacement ages in southern west greenland previously identified swarms include the ca 2500 ma kilarsaarfik dykes ca 2215 ma boninitic norite bn dykes and the abundant ca 2040 ma kangamiut and coeval 2050 2030 ma md3 dykes additional insight into the distribution of swarms is provided by new baddeleyite u pb id tims dates interpreted as emplacement ages for six dykes the geochemistry for a total of 42 samples on 40 dykes including the dated dykes provides some constraints on their petrogenesis two e w trending dykes yield ages of 2365 2 ma and 2374 4 ma representing a hitherto unknown magmatic event we propose the name graedefjord dykes for dykes of this age in southern west greenland one ne sw trending dyke is dated at 2209 5 ma close to a previously published 2214 10 ma age for a n s trending dyke member of bn dykes three dykes yield md3 ages of 2053 2 ma e w trending 2049 6 ma n s trending and 2042 2 ma wnw ese trending ca 2 21 ga old and predominantly ne se and nne sse trending dolerites some of which are high al basalts were derived from a relatively depleted mantle and injected during a boninite dolerite magmatic event together with nearby bn dykes derived from a harzburgitic yet highly lree and lile enriched subcontinental lithospheric mantle sclm ca 2 05 ga kangamiut md3 dolerites were derived from a similar ambient asthenospheric mantle and subsequently experienced crustal assimilation during fractional crystallisation the ca 2370 ma dykes in the nac are coeval with the dharwar giant dyke swarm of the dharwar craton dc and based on this unique age match it is proposed that the dharwar and nac were neighbours in the palaeoproterozoic furthermore both blocks share 222 2 21 ga dykes and matching the trends of 2 2 2 1 and 2 37 ga swarms between the two blocks allows several plausible reconstructions c 2012 published by elsevier b v

deglacial warming of the northeastern atlantic ocean correlation with the paleoclimatic evolution of the european continent abstract isotopic micropaleontologic and pollen analyses of deep sea cores from the bay of biscay and the northeastern atlantic ocean show that the deglacial warming of this oceanic area was closely correlated with the paleoclimatic evolution of the adjacent european continent temperatures were at least as warm as those of today in the bay of biscay between 13 300 and 11 000 b p coinciding with the combined bolling allerod warm continental events a major spread of polar water occurred between 11 000 and 10 000 b p during this event which coincides with the younger dryas continental cold event marine temperatures were almost as low as those of the last glacial maximum the final deglacial warming of the norteastern atlatntic ocean occurred during the following 3000 yr

the specific case of the mid proterozoic rapakivi granites and associated suite within the context of the columbia supercontinent abstract rapakivi granites and the associated suite anorthosite mangerite and charnockite that developed during the mid proterozoic escape the global rules that control usual granitic magma formation about 40 points that characterise the proterozoic magmatism confirm the originality of the magmatism rapakivi granites developed without a global orogenic context within the supercontinent columbia the intrusions extend from the east european shield to the western us through fennoscandia greenland and labrador other occurrences in the amazonian shield australia or south china are less well documented they show no definite trend in age or chemistry that would explain large scale mantle plume effects a mantle upwelling of material is contradictory with the re assembly of a supercontinent because it occurred before this magmatic episode 1 9 1 8 ga at 1 7 ga the supercontinent columbia amalgamated with the collision of the yapavai and mazatzal provinces a model is suggested that takes into account the supercontinent re assembly defining a downwelling flow in the mantle that anchors the continent above it in contrast to a material flow heat is still delivered to the base of the continental lithosphere and is focused toward the juvenile suture zone the base of the crust must reach 1200 1300 c before producing anorthosite magmas under such a high temperature gradient lasting over a long time magmas are transferred toward the upper crust giving the thin 5 km and square shape 100 km that the intrusions presently have heat delivery is essentially conductive leading to long time spans for intrusions the presence of the supercontinent immobile over a descending cell poloidal mode of convection developed a tangential force toroidal mode of convection that partly split the continent through strike slip deformation due to plate rotation it developed progressively between 1 57 and 1 3 ga starting from fennoscandia and then passing to amazonia western us and labrador in a clockwise sense the associated rotation induced sinistral shear manifested by small scale shear zones and the orientation of late magmatic facies topaz bearing granites in each province the proterozoic magmatism appears to be unique because it requires a supercontinent with a zone of juvenile crust surrounded by older cratons the present moho still shows remnants of this process having bumpy undulations that may reach 22 km in amplitude over a distance of 200 km

the origin of the palaeoproterozoic amcg complexes in the ukrainian shield new u pb ages and hf isotopes in zircon the ukrainian shield hosts two palaeoproterozoic anorthosite mangerite charnockite granite amcg complexes the korosten and korsun novomyrhorod complexes that intruded palaeoproterozoic continental crust in north western and central parts of the shield respectively we report results of u pb zircon and baddeleyite dating of 16 samples from the korosten plutonic complex kpc and 6 samples from the korsun novomyrhorod plutonic complex knpc fifteen zircon samples from both complexes were also analysed for hf isotopes these new together with previously published data indicate that the formation of the kpc started at c 1815 ma and continued until 1743 ma with two main phases of magma emplacement at 1800 1780 and 1770 1758 ma each of the main phases of magmatic activity included both basic and silicic members the emplacement history of the knpc is different from that of the kpc the vast majority of the knpc basic and silicic rocks were emplaced between c 1757 and 1750 ma the youngest stages of the complex are represented by monzonites and syenites that were formed between 1748 and 1744 ma both ukrainian amcg complexes are closely associated in space and time with mantle derived mafic and ultramafic dykes the hf isotope ratios in the zircons indicate a predominantly crustal source for the initial melts with some input of juvenile hf from mantle derived tholeiite melts the preferred model for the formation of the ukrainian amcg complexes involves the emplacement of large volumes of hot mantle derived tholeiitic magma into the lower crust this resulted in partial melting of mafic lower crustal material mixing of lower crustal and tholeiitic melts and formation of ferromonzodioritic magmas further fractional crystallization of the ferromonzodioritic melts produced the spectrum of basic rocks in the amcg complexes emplacement of the ferromonzodioritic and tholeiitic melts into the middle crust and their partial crystallization caused abundant melting of the ambient crust and formation of the large volumes of granitic rocks present in the complexes

major trace and rare earth element ree geochemistry of different colored fluorites in the bobrynets region ukraine abstract the bobrynets fluorite mineralization in the exo contact zone of krivograd novoukrainian granite intrusion is found in the southern part of the kirovograd cherkassy fault zone between the novoukrainian anticlinoria and the ingulo ingulets synclinoria the fluorite mineralization appears as vein and disseminat as well as in the form of a cement in the tectonic breccia among the metasomatically altered granites fluorite mineralization is observed for 5 km along this fault zone down to a depth of 300 m with the thickness of the individual mineralized zones ranging from a few meters to 75 m with average fluorite contents ranging from 1 2 to 28 the estimated reserve of bobrynets fluorites is approximately 1 56 million tonnes fluid inclusion data in the studied fluorites indicate that the study area has two types of fluorites high temperature fluorites 240 280 c and low temperature fluorites 100 150 c the total rare earth element ree contents of the fluorites range from 62 4 ppm to 527 9 ppm with a mean of 218 ppm the mean ree concentrations of different colored massive fluorites range from 80 2 ppm for yellow fluorites to 258 ppm for green fluorites 97 4 ppm for violet fluorites and 526 5 ppm for granite samples the mean y concentrations of the studied fluorites are 86 2 ppm for violet fluorites 104 5 ppm for yellow fluorites 242 5 ppm for green fluorites and 39 9 ppm for granite samples the chondrite normalized ree patterns of the bobrynets fluorites exhibit trends similar to each other and show a decrease from light rees lree to heavy rees hrees all fluorites in the study area have positive ce anomalies between 1 06 and 1 16 with mean of 1 12 negative eu anomalies with a range of 0 35 to 0 58 mean 0 46 and positive y anomalies between 1 38 and 2 67 with mean of 2 12 the y ho values change from 48 3 to 92 2 with mean of 71 1 the tb ca and tb la ratios of the bobrynets fluorites are between 0 00000249 and 0 0000404 and between 0 098 and 0 30 respectively and these ratios also indicate a formation from fractionated ore bearing fluids at an early stage of mineralization mainly a hydrothermal and less pegmatitic stage the tb yb n la yb n ratios are low because these fluorites are not lree rich when compared to other fluorite deposits the la yb n eu eu n values also indicate low negative eu anomalies and low lree enrichments for the bobrynets fluorites the sc eu vs sr diagram and sc vs ree diagram show that the bobrynets fluorites have low sr contents and high sc eu values and that they cover the same areas as the rift deposits of new mexico and the skarn fluorite deposits of akdagmadeni the 208pb 204pb isotope values of fluorites vary between 53 5 and 62 6 and the 206pb 204pb isotope values of fluorites vary between 23 3 and 26 8 the 208pb 204pb vs 206pb 204pb ratios indicate that fluorites originated from upper crust materials all these findings indicate that ree contents of the bobrynets fluorites have different physicochemical parameters such as decrease in temperature or increases in o2 and ph high oxygen fugacity in the source of the hydrothermal fluids and low sr and high sc values similar to rift and skarn deposits

rainy periods and bottom water stagnation initiating brine accumulation and metal concentrations 1 the late quaternary a working hypothesis is proposed to account for the present accumulation of brines in isolated pockets of the ocean floor and for the formation of the underlying organic and metal rich sediments these are the tyro and bannock basins in the east mediterranean the red sea deeps and the orca basin in the northern gulf of mexico initiation of brine derived deposition in the red sea deeps and orca basin occurred between 12 000 and 8000 years b p this time bracket also encompasses the formation of the latest east mediterranean sapropel and the wettest global climate since the last glacial maximum this wet period first appeared in the tropics around 12 000 years b p then in the subtropical and middle latitudes during the same period the 23 000 year precession cycle brought the summer insolation of the northern hemisphere to its peak at 11 000 years b p with retreating northern hemisphere ice sheets the red sea deeps and the orca basin became anoxic during this humid period and metal rich sapropel deposition then began in contrast the tyro and bannock basins began accumulating a brine long before and persisted beyond this climatic stage the hypothesis involves two propositions 1 as in the eastern mediterranean sea marine anoxia was mainly the consequence of the large influx of continental runoff and local precipitation longer residence time of bottom waters so called stagnation in silled rimmed basins would have resulted from lower salinity at the sea surface in areas of deep water formation in the eastern mediterranean the red sea and the gulf of mexico and 2 miocene or older evaporites underlie these basins or outcrop on their flanks leaching from these evaporites was an ongoing process before the quasi stagnation phase but the initial leachate much less saline than the present brines was continuously flushed by bottom circulation the climate induced quiescence of bottom waters in these basins enabled the leachate to accumulate the feedback of stagnation by increased density progressively raised the salinity of entrapped bottom waters to the present brine concentration the high density has resisted brine removal by bottom circulation until present time long after cessation of the initiating wet period the brines therefore are stagnant fossil waters

sveconorwegian massif type anorthosites and related granitoids result from post collisional melting of a continental arc root article i nfo two magmatic suites were emplaced during the post collisional evolution of the sveconorwegian orogeny an anorthosite mangerite charnockite suite amc suite and an hornblende and biotite bearing granitoids suite hbg suite the amc suite is exclusively located in the westernmost and warmest part of the orogen in granulite facies gneisses whereas the hbg suite intruded in the rest of the orogen but not in the granulite domain new u pb zircon geochronological data confirm previous age determinations 970 932 ma hbg suite and 933 916 ma amc suite the mafic facies of the two post collisional magmatic suites have similar geochemical compositions but the hbg differentiation trend displays higher cao sr u and th as well as lower k2o and feot mgo than the amc differentiation trend the hbg suite is hydrous and has a broadly higher fo2 whereas the amc suite is anhydrous the inferred parent magmas of both suites have overlapping initial sr nd and pb isotopic compositions with increasing differentiation the two trends point towards two different crustal contaminants together with the recent recognition of a major crustal shear zone located just east of the amc suite this difference suggests that the suites were emplaced in two different lithotectonic units using published experimental constraints and geochemical modeling we suggest that the mafic facies of both suites were produced by partial melting of lower crustal sources which were previously underplated probably during the evolution of a long lasting convergent margin later these lower crustal sources were modified by the regional granulite facies metamorphism 1 035 to 0 97 ga prevailing in the westernmost part of the orogen thus producing an anhydrous lower crustal source for the amc suite accordingly we conclude that the sveconorwegian massif type anorthosites result from partial melting of the continental arc root this process if accepted for other amcg anorthosite mangerite charnockite granite complexes was possible in the proterozoic because of a sufficiently high temperature but not in the archean because in subduction zones the main transfer to the crust was then felsic tonalites granodiorites and not basaltic we thus further suggest that the onset of massif type anorthosites at the beginning of the proterozoic may mark the time when plate tectonics began to operate in a similar way as today

palaeomagnetic studies of proterozoic rocks from the lake onega region southeast fennoscandian shield summary in a recent compilation elming et al 1993 presented a new apparent polar wander path for the proterozoic of the fennoscandian shield however only a minor portion of the data used for this and for the earlier compilations has been obtained from the eastern part of the fennoscandian shield thus prompting the present study in this paper we present new palaeomagnetic data for the lake onega region the sections studied comprise quartzites as well as extrusive and intrusive rocks of vepsian 1900 1650 ma and jatulian 2300 2100 ma ages which represent svecofennian and jatulian tectonomagmatic events respectively thermal and alternating field demagnetization experiments reveal the presence of several components of the natural remanent magnetization of normal and reversed polarity component an has been identified in a sill of vepsian gabbro dolerites 1770 ma exposed at rybreka locality and yields a palaeopole at 37 8dn 210 6de dp 4 0d dm 7 7d this magnetization is interpreted to be of thermoremanent origin in the vepsian shoksha quartzite ca 1800 ma cut by the sill and in the underlying basalts the palaeopoles indicated by the presumably primary components an and ar are at 45 1 dn 206 4de dp 3 6d dm 6 5d and 44 2dn 241 5de dp 6 1d dm 9 6d respectively component an was isolated in the jatulian basaltic lavas ca 2200 2300 ma at girvas locality also however in this case it appears as secondary and yields nearly the same palaeopole 43 1dn 211 5de dp 4 9d dm 8 8d component br was isolated in vepsian gabbro dolerites and quartzites sampled from zones altered by monzonite veins representing the final phase of sill emplacement this component yields a palaeopole at 13 9dn 171 3de dp 2 4d dm 4 8d for gabbro dolerites and at 10 5dn 201 8de dp 8 4d dm 14 9d for quartzites the vepsian and jatulian rocks also show the presence of presumably younger 1500 ma magnetization components c d and e the presumably primary stable dual polarity components determined in the jatulian lavas and interlayers of the red siliceous rocks yield a palaeopole at 0 8ds 245 4de dp 7 2d dm 11 6d for the upper part of the section component f and a palaeopole at 36 0ds 307 4de dp 9 9d dm 14 9d for the lower part of the section component g these palaeomagnetic poles are compared with the apparent polar wander path of fennoscandia for the interval 2400 1200 ma after elming et al 1993 and with some recent results from vepsian and jatulian rocks

volcanic arc emplacement onto the southernmost appalachian laurentian shelf characteristics and constraints in the southernmost appalachians the hillabee greenstone an ordovician volcanic arc fragment lies directly atop the outermost laurentian devonian earliest mississip pian shelf sequence at the structural top of the greenschist facies talladega belt the frontal metamorphic allochthon along this orogenic segment the hillabee greenstone was emplaced between latest devonian and middle mississippian time it and the upper most laurentian section were later repeated together within a series of map scale imbri cate slices of a postmetamorphic dextral transpressional alleghanian thrust duplex system that placed the high grade eastern blue ridge allochthon atop the talladega belt geochemical and geochronologic u pb zircon studies indicate that the hillabee greenstone s interstratifi ed tholeiitic meta basalt and calc alkaline metadacite rhyolite formed within an extensional setting on con tinental crust ca 460 470 ma palinspastic reconstructions of the southern appalachian ordovician margin place the hillabee green stone outboard of the present position of the pine mountain terrane and suggest links to ordovician plutonism in the overlying east ern blue ridge and possibly to widespread k bentonite deposits within ordovician plat form units the tectonic evolution of the hil labee greenstone exhibits many unusual and intriguing features including 1 premeta morphic emplacement along a basal cryptic thrust which is remarkably concordant to both hanging wall and footwall sequences across its entire extent 230 km 2 forma tion transport and emplacement of the arc fragment accompanied by minimal deforma tion of the hillabee greenstone and under lying outer margin shelf rocks 3 emplace ment temporally coincident with the adjacent collision of the younger tectonically indepen dent ouachita volcanic arc with southeastern laurentia these features highlight strong contrasts in the ordovician taconian evolu tion of the southern and northern parts of the appalachian orogen

late gauss to early matuyama invasions of neogloboquadrina atlantica in the mediterranean and associated record of climatic change a well dated high resolution record of climatic change is presented for the late pliocene mediterranean principal component analysis on abundances of 14 planktonic foraminiferal species reveals a series of late gauss to early matuyama surface water cooling events which can be correlated with north atlantic glacial isotope stages 108 to 94 the abundance record of the benthic foraminifer trifarina angulosa suggests lowered bottom water temperatures at times of surface water cooling the record of surface water oxygen and carbon isotopes shows an inverse pattern with 18o maxima and 13c minima at times of cool sea surface temperature conditions concomitant decrease in p b value and increased admixtures of clastic material suggest that late gauss to early matuyama surface water cooling events in the mediterranean are accompanied with glacioeustatic sea level low stands major surface water cooling events are associated with invasions of neogloboquadrina atlantica this north atlantic mid to high latitude species is at 2 55 ma close to the gibraltar portal whereafter it could invade the mediterranean repeatedly during periods of climatic cooling until it disappears from all over the north atlantic and mediterranean at the end of glacial stage 96

crustal scale pop up structure in cratonic lithosphere dobre deep seismic reflection study of the donbas fold belt ukraine the dobre project investigated the interplay of geologic and geodynamic processes that controlled the evolution of the donbas fold belt ukraine as an example of an inverted intracratonic rift basin a deep seismic reflection profile provides an excellent image of the structure of the donbas fold belt which is the uplifted and compressionally deformed part of the late paleozoic pripyat dniepr donets basin both the effects of rifting and those of later structural inversion are recognized in the seismic and geologic data the interpretation of the reflection data shows that the inversion of the donbas fold belt occurred at the crustal scale as a mega pop up which involved a major detachment fault through the entire crust and an associated back thrust the dobreflection image provides a simple concept of intracratortic basin inversion the crustal pop up being uplifted and internally deformed the association of such a structure with inverted intracratonic basins such as the donbas fold belt implies brittle deformation of relatively cold crust

geochronological constraints on the emplacement history of an anorthosite rapakivi granite suite u pb zircon and baddeleyite study of the korosten complex ukraine u pb zircon baddeleyite ages obtained for the korosten anorthosite rapakivi granite complex ukrainian shield suggest that different magmatic phases were emplaced during a period of ca 30 million years as a series of distinct igneous episodes the earliest 1789 1 2 0 ma anorthosites were followed by 1781 3 3 2 ma dykes of plagiogranite porphyries the emplacement of a major rapakivi granite phase took place at 1767 4 2 2 ma and was followed by emplacement of layered intrusions of anorthosites gabbronorites diabases and ultrabasic rocks between 1761 and 1758 ma the minimum duration of magmatism of about 30 million years the 6 15 million years interval between igneous pulses and alternation of discrete episodes of basic and felsie magmatism are common features of major anorthositemangerite charnockite rapakivi granite complexes temporal distribution of igneous activity in the korosten complex shows that the gabbro anorthosites and the granites are not comagmatic although they are possibly cogenetic and that at least four portions of granitic and basic magmas were generated during a relatively long period of at least 30 million years the time gap of about 20 25 million years between early basic and later and more voluminous granitic magmatism characteristic of the korosten pluton wiborg and salmi batholiths probably reflects the duration of extensional processes before the generation of large volumes of magma in the lower crust

the evolution of pleistocene climate a time series approach abstract in the later part of the pleistocene variations in global ice volume have been dominated by an approximate 100 000 year cycle analysis of 2 myr long oxygen isotope record from an equatorial pacific core indicates that this is true only for the last 900 000 years prior to this time the amplitude of the 100 000 year cycle is much reduced as is the variance of all oscillations with periods greater than 60 000 years based on results of time series analysis of this 2 myr long record the pleistocene glacial cycles can be divided into three sections 1 the late pleistocene 0 900 kyr b p where the variations in the isotope record are dominated by the 100 000 year cycle 2 the middle pleistocene 900 1450 kyr b p in which low frequency components are not as important as in the later period of the pleistocene and 3 the early pleistocene late pliocene 1450 2000 kyr b p where general reductions of importance at all frequencies is seen as compared to the later intervals recent modeling efforts which describe variations in global ice volume show that the dominant low frequency component observed in the late pleistocene can result from different time constants for the rate of glacial growth and decay in response to variations in the earth s orbital parameters it is hypothesized that during the early pleistocene the rate of growth and decay of glaciers were more similar and that continental erosion by successive glacial advances lowered the land surface in areas of ice cap formation to below sea level when the ice caps became marine based more rapid decay of the ice became possible

late pliocene to early pleistocene astronomically forced sea surface productivity and temperature variations in the mediterranean abstract principal component analysis of late pliocene to early pleistocene planktonic foraminiferal abundances from the mediterranean yielded two significant components which represent first order approximations for sea surface productivity and temperature this interpretation is confirmed by the good fit between the score plots and sea surface productivity and sea surface temperature proxy records based on selected eutrophic and temperature sensitive species using present day habitat characteristics spectral analysis revealed significant peaks in the orbital frequency bands of the spectrum suggesting that variations in sea surface productivity and temperature are controlled by the quasi periodic variations in the earth s orbit precession forced variations are reflected primarily in sea surface productivity and to a lesser degree in sea surface temperature maxima in sea surface productivity and temperature in this frequency band occurred at times of sapropel formation the sea surface productivity maxima can be explained by an increased input of river borne nutrients and by the shoaling of the pycnocline and the associated intensification of the deep chlorophyll maximum dcm layer precession forced variations in sea surface temperature are not related to glacial cycles since this frequency component is almost completely absent in ice volume proxy records for this time interval sapropel bound sea surface temperature maxima presumably reflect periods of maximum insolation during the northern hemisphere summer the contrasting winter signal corresponds to minimum insolation values which may be reflected by peak abundances ofgloborotalia inflata obliquity forced variations are reflected primarily in sea surface temperature and not in sea surface productivity these temperature fluctuations correspond with ice volume changes registered in the oxygen isotope record at the north atlantic dsdp site 607 this shows that obliquity forced sea surface temperature variations in the mediterranean are primarily driven by glacial cycles

the magnetostratigraphy and a 1780 ma palaeomagnetic pole from the red sandstones of the vazhinka river section karelia russia summary a palaeomagnetic study of the vepsian sandstones of the upper part of the shoksha formation has revealed a stable remanence rock magnetic and mineralogical studies suggest diagenetic haematite as the main carrier of this remanence two magnetic polarities with a regular stratigraphic zonation have been found the reversal test is positive the tilt corrected palaeomagnetic direction is n 36 d 354 3u i 21 6u k 22 3 a95 5 2u the corresponding palaeomagnetic pole of 39 7un 221 1ue dp 2 9u dm 5 5u is proposed as a key pole for 1790 1770 ma for fennoscandia

glacial holocene salinity changes in the mediterranean sea hydrographic and depositional effects oxygen isotope changes as recorded by pleistocene foraminifera from open ocean sediments are primarily a function of changes in the isotopic composition of the global seawater reservoir caused by ice volume changes with a secondary temperature component1 oxygen isotope records from marginal basins such as the gulf of mexico2 the mediterranean sea3 5 and the red sea6 also exhibit the global signal but are far more complicated owing to the strong overprint of local climate conditions in particular glacial interglacial changes in continental aridity and river runoff produce significant salinity changes in these semi enclosed basins using available oxygen isotope records we estimate that glacial 18 000 yr bp salinities in the eastern and western mediterranean sea were 2 7 and 1 2 higher respectively than at present these elevated glacial salinities played an important part in preconditioning the eastern mediterranean for the eventual accumulation of organic carbon rich sediments sapropels at the end of deglaciation

integrated magnetostratigraphy and biostratigraphy of the upper pliocene lower pleistocene from the monte singa and crotone areas in calabria italy the results of a detailed magnetostratigraphic and biostratigraphic study of late pliocene to early pleistocene marine marl sequences from the monte singa and crotone areas in calabria italy are presented the magnetostratigraphy from the monte singa sequence ranges from below the gauss matuyama boundary up to and including the lower olduvai boundary normal polarities at a level corresponding to isotope stage 81 most probably represent the r union subchron from the lower olduvai boundary upward a reliable magnetostratigraphy could not be established due to increased weathering of the marls resulting in mainly secondary magnetizations the magnetostratigraphy from the composite sequence of the crotone area belongs to a large part of the matuyama chron and includes the olduvai subchron the position of the lower and upper boundaries of the olduvai subzone could be established more precisely than from earlier results moreover the upper boundary of the olduvai subzone poses an ambiguity a relatively long normal polarity interval representing the main olduvai subchron and corresponding to a duration of 115 ka is followed by a short 30 ka reversed subchron and the short 15 ka normal vrica subchron another option and more in accordance with the duration of the olduvai subchron in literature would be to consider the complete n r n polarity succession with a total duration of 160 ka as representing the olduvai subchron implying that this olduvai subchron has a short reversed interval in its upper part linear interpolation and extrapolation yield ages for the most important late pliocene early pleistocene biostratigraphic datum levels an age of 1 69 ma is found for the pliocene pleistocene boundary using the conventional polarity time scale dated with radiometric results however hilgen 1 in correlating the sapropel groups and patterns to the precession curve of the earth s orbit obtained significantly different ages for the polarity transitions of the present study according to this astronomically calibrated polarity time scale the age of the pliocene pleistocene boundary is 1 81 ma

the crustal tongue melting model and the origin of massive anorthosites recent detailed field studies in several anorthosite complexes have shown that anorthosites are frequently associated with weakness zones in the crust which may have favoured their emplacement at mid crust levels recent experimental data have shown that the parent magma compositions of various anorthosite massifs lie on thermal highs in the relevant phase diagrams at 10 13 kbar indicating that these magmas cannot be derived by fractionation of peridotitic mantle melts but by melting of gabbronoritic sources in the lower crust at 40 50 km depths in the sveconorwegian province terne boundaries have been traced in deep seismic profiles to moho offsets or to tongues of lower crustal material underthrust to depths higher than 40 km in southern norway we suggest that a lithospheric scale weakness zone the feda transition zone has channelled the rogaland anorthosites through linear delamination asthenospheric uprise and melting of a mafic lower crustal tongue

40ar 39ar dating of the brunhes matuyama geomagnetic field reversal magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale gpts to date events in the range 0 to 5 million years ago a critical tie point on the gpts is the potassium argon age of the most recent brunhes matuyama geomagnetic field reversal astronomical values for the forcing frequencies observed in the oxygen isotope record in ocean drilling project site 677 suggest that the age of this last reversal is 780 ka thousand years ago whereas the potassium argon based estimate is 730 ka results from 4039 ar incremental heating studies on a series of lavas from maui that straddle the brunhes matuyama reversal give an age of 783 11 ka in agreement with the astronomically derived value the astronomically based technique appears to be a viable tool for dating young sedimentary sequences

petrogenesis of a mid proterozoic anorthosite mangerite charnockite granite amcg complex isotopic and chemical evidence from the nain plutonic suite the nain plutonic suite nps about 1 30 ga underlies some 19 000 km intruding the boundary between middle to late archean rocks in the east nain province and late archean and early proterozoic rocks in the west churchill province nd isotopic compositions in most igneous units of the nps reflect their geographic position relative to the inferred boundary between the nain and churchill provinces rare exceptions are local intrusions of olivine bearing basic rocks with highest epsilon nd 1 3 ga near 3 0 present in both eastern and western sectors of the nps anorthositic rocks have variable and significant contributions of crustal nd but notably lower i sr than granitoid rocks ferrodiorites are similar isotopically to the anorthositic group and have chemical and mineral compositions more akin to the anorthositic group than the granitoids aluminous orthopyroxene megacrysts in nps anorthositic rocks indicate crystallization pressures of 6 2 to 11 0 kbar consistent with lower cru

early proterozoic palaeomagnetism new results from the intrusives and related rocks of the karelian belomorian and kola provinces eastern fennoscandian shield the palaeomagnetic data that are available for the eastern fennoscandian shield are too sparse to allow any palaeomagnetic constraints to be determined for intracratonic horizontal movements in this part of the shield to address this issue we have obtained new palaeomagnetic data from mafic and ultramafic plutons sills dykes and basement country rocks in the kola belomorian and karelian provinces of the fennoscandian shield a number of natural remanent magnetization nrm components were isolated and we associate these with various events of precambrian tectonomagmatic activity within the eastern fennoscandian shield 36 palaeomagnetic poles were obtained 12 poles with ages ranging from 1700 ma to 2450 ma satisfy the accepted reliability criteria corresponding to the b category of briden duff 1981 of these six might be classified as reference poles for the kola and belomorian provinces there are poles from the main range gabbro dolerites 16 7 s 304 8 e a95 8 3 u pb age of 2436 25 ma the voche lambina ultramafics and amphibolites 12 9 n 314 1 e a95 8 6 rb sr age of 2198 172 ma and the kolvitsa peridotites and schists 44 2 n 232 6 e a95 5 4 assigned magnetization age of 1900 ma for the karelian province reference poles are presented for the burakovka gabbro norites 19 5 s 262 6 e a95 7 2 sm nd age of 2439 22 ma the matozero doleritic dykes 20 6 s 308 5 e a95 12 3 u pb age of 2150 60 ma and the ropruchey sill gabbro dolerites 40 5 n 229 8 e a95 8 1 u pb age of 1770 12 ma although some nrm components of other rocks are not as precisely dated a number of palaeomagnetic poles are in good internal agreement forming two tracks of the lower proterozoic apparent polar wander path apwp a preliminary version of the pre svecofennian apwp tracks for the karelian and kola belomorian provinces is proposed our results confirm the general drift pattern of fennoscandia in the proterozoic and support the idea of the absence of large scale block movements within the fennoscandian shield since jatulian times 2300 2100 ma to help constrain the relative positions of the various tectonic blocks of the eastern fennoscandian shield during pre jatulian times more new palaeomagnetic and radiometric data need to be added to the available palaeomagnetic database

the fedorivka layered intrusion korosten pluton ukraine an example of highly differentiated ferrobasaltic evolution abstract this study documents the petrography and whole rock major and trace element geochemistry of 38 samples mainly from a drill core through the entire fedorivka layered intrusion korosten pluton as well as mineral compositions microprobe analyses and separated mineral fraction analyses of plagioclase ilmenite magnetite and apatite of 10 samples the fedorivka layered intrusion can be divided into 4 lithostratigraphic units a lower zone lz 72 m thick a main zone mz 160 m thick and an upper border zone itself subdivided into 2 sub zones ubz2 40 m thick ubz1 50 m thick igneous lamination defines the cumulate texture but primary cumulus minerals have been affected by trapped liquid crystallization and subsolidus recrystallization the dominant cumulus assemblage in mz and ubz2 is andesine an39 42 iron rich olivine fo32 42 augite en29 35fs24 29wo42 44 ilmenite hem1 6 ti magnetite usp52 78 and apatite the data reveal a continuous evolution from the floor of the intrusion lz to the top of mz due to fractional crystallization and an inverse evolution in ubz resulting from crystallization downwards from the roof the whole rock fe mg ratio and incompatible element contents e g rb nb zr ree increase in the fractionating magma whereas compatible elements e g v cr steadily decrease the intercumulus melt remained trapped in the ubz cumulates due to rapid cooling and lack of compaction and cumulus mineral compositions re equilibrated e g olivine fe ti oxides in lz the intercumulus melt was able to partially or totally escape the major element composition of the mz cumulates can be approximated by a mixing linear relationship between a plagioclase pole and a mafic pole the latter being made up of all mafic minerals in nearly constant relative proportions by analogy with the ferrobasaltic jotunitic liquid line of descent defined in rogaland s norway and its conjugated cumulates occurring in the transition zone of the bjerkreim sokndal intrusion rogaland a monzonitic 57 sio2 melt is inferred to be in equilibrium with the mz cumulates the conjugated cumulate composition falls within error on the locus of cotectic compositions fixed by the 2 pole linear relationship ulvospinel is the only ti phase in some magnetites that have been protected from oxidation quilf equilibria in these samples show that magnetite and olivine in mz have retained their liquidus compositions during subsolidus cooling this permits calculation of liquidus fo2 conditions which vary during fractionation from fmq 0 7 to 1 4 log units low fo2 values are also evidenced by the late appearance of cumulus magnetite fo42 and the high v3 content of the melt reflected in the high v content of the first liquidus magnetite up to 1 85 v

evaluation of the astronomically calibrated time scale for the late pliocene and earliest pleistocene an astronomically calibrated time scale was recently established hilgen 1991a b for the pliocene and earliest pleistocene based on the correlation of dominantly precession controlled sedimentary cycles sapropels and caco3 cycles in mediterranean deep sea sections to astronomical solution ber90 berger and loutre 1991 this time scale which differed substantially from the conventional time scales then widely used is essentially confirmed by a new generation of radiometric dates using the ar 40 ar 39 laser fusion technique to evaluate this time scale we here extract both precession and obliquity related components from late pliocene earliest pleistocene climatic proxy records in the mediterranean and determine coherencies and phase time lags between these components and the respective orbital variations this work is found to support the new time scale because significant coherencies are found between the astronomically related components in our proxy records and the orbital variations not only at the main period of precession but also at the main period of obliquity and because of the eccentricity modulation of precession in the low frequency eccentricity bands of the spectrum but the calculated time lag between obliquity and 41 kyr components in the climatic proxy records shows a significant increase of 5 6 1 kyr with respect to the late pleistocene if the phase relation with precession is kept constant this increase can be explained by a change in the climate response time to obliquity or to precession by a small uncertainty in the astronomical solution or by a combination of these factors

in situ u pb and lu hf isotopic studies of zircons from the sancheong hadong amcg suite yeongnam massif korea implications for the petrogenesis of 1 86 ga massif type anorthosite abstract isotopic and geochemical characteristics of proterozoic anorthosite mangerite charnockite granite amcg suite have long been used for tracing the mantle crustal source and magmatic evolution we analyzed lu hf isotopic compositions of zircon from the sancheong hadong amcg complex yeongnam massif korea in order to understand tectonomagmatic evolution of the paleoproterozoic amcg suite occurring at the southeastern margin of the north china craton ncc the anorthositic rocks in this complex associated with charnockitic and granitic gneisses were recrystallized to eradicate magmatic features in situ shrimp sensitive high resolution ion microprobe u pb analyses of zircon from a leuconorite and an oxide bearing gabbroic dyke yielded weighted mean 207 pb 206 pb ages of 1870 2 ma and 1861 6 ma respectively charnockitic granitic and porphyroblastic gneisses yielded weighted mean 207 pb 206 pb zircon ages of 1861 6 ma 1872 6 ma and 1873 4 ma respectively these crystallization ages together with our previous geochronological data for anorthosites 1862 2 ma are indicative of episodic amcg magmatism over an 10 ma interval initial e hf t values of zircon analyzed from five anorthositic rocks and four felsic gneisses range from 2 1 to 6 1 and 0 3 to 5 4 respectively zircon hf isotopic data in combination with available whole rock sr nd isotopic data suggest that anorthositic parental magma was most likely derived from a mantle source and variably affected by crustal contamination this crustal component is also reflected in charnockitic granitic magmas produced primarily by the melting of lower crust taken together the amcg magmatism at 1 87 1 86 ga in the yeongnam massif is most likely a late orogenic product of paleoproterozoic ncc amalgamation tectonically linked to assembly of the columbia supercontinent

the sarmatian crustal segment precambrian correlation between the voronezh massif and the ukrainian shield across the dniepr donets aulacogen abstract sarmatia is the southernmost crustal segment of the east european precambrian craton the phanerozoic dniepr donets aulacogen dda subdivides it into unequal halves the ukrainian shield in the south and the voronezh massif in the north correlation between the ukrainian shield and the voronezh massif is reviewed the overall structure and age province subdivision of sarmatia are assessed and constraints are imposed on displacement along the dda it is shown that differences in the trends of the principal basement structures highlighted by magnetic anomalies outlining palaeoproterozoic banded iron formations are a consequence of the heterogeneous structure of the archaean palaeoproterozoic basement rather than the kinematics of the opening of the dda only minor right lateral displacement along the dda has occurred the main crustal units of the ukrainian shield i e the azov middle dniepr and ingul ingulets blocks correlate excellently with the oskol sumy and sevsk blocks of the voronezh massif which allows the definition of three coherent oskol azov sumy dniepr and sevsk ingulets crustal domains presently available radiometric datings show that these three domains all have archaean crustal ages of 3 65 3 0 3 2 3 1 and 3 1 2 8 ga respectively the inter domain boundaries appear largely to have existed already in the archaean but were subsequently masked by palaeoproterozoic deformation and overlain by proterozoic cover and nappe piles amongst the three crustal domains of central and eastern sarmatia the oskol azov and sevsk ingulets ones have been reworked substantially in the palaeoproterozoic while the sumy dniepr domain acted as a stable cratonic unit although apparently rotating anti clockwise between 2 3 and 2 1 ga ago the palaeoproterozoic banded iron belts of sarmatia can be subdivided into the cratonic margin krivoy rog type and the interior intra domain oskol type the former outline the cratonic sumy dniepr domain while the latter associate with nw se continental rifting that followed archaean zones of weakness inside the oskol azov domain the bryansk block in the northwestern part of the voronezh massif is a part of the large bryansk bragin granulite domain which was formed between ca 2 2 and 2 1 ga ago and extends into the northwestern ukrainian shield in the east the lipetsk losev volcanic belt and the east voronezh turbidite schist province follow the boundary between the sarmatian and volgo uralian crustal segments together these two crustal units appear to represent a palaeoproterozoic orogenic environment ca 2 1 2 0 ga in age

extension of the astronomically calibrated polarity time scale to the miocene pliocene boundary the early pleistocene to late pliocene astronomically calibrated time scale of shackleton et al 1 and hilgen 2 is extended to the miocene pliocene boundary this is done by correlating the detailed record of caco3 cycles in the trubi and the lower part of the overlying narbone formation rossello composite section sicily to the astronomical record using 1 inferred phase relations between these caco3 cycles and the corresponding orbital cycles of precession and eccentricity and 2 calibration points provided by the previously established astronomical calibration of sapropelitic layers which occur in the topmost part of the caco3 record 2 this correlation allows all small scale caco3 cycles in the rossello composite section to be coded after the correlative precession peak and to be dated astronomically with an accuracy of 1 ky in combination with the available high resolution magnetostratigraphy it further allows the construction of an astronomically calibrated geomagnetic polarity time scale for the major part of the gilbert and gauss chrons this new time scale gives ages for the reversal boundaries of the kaena 3 04 3 11 ma and mammoth 3 22 3 33 ma the gilbert gauss chron boundary 3 58 ma and the reversal boundaries of the cochiti 4 18 4 29 ma nunivak 4 48 4 62 ma sidufjall 4 80 4 89 ma and thvera 4 98 5 23 ma these ages are significantly older than the ages given by conventional time scales age discrepancies gradually increase with increasing age the age of 5 32 ma for the miocene pliocene boundary as defined by the base of the trubi on sicily is almost 0 5 m y 9 5 older than the presently accepted age of 4 86 ma 3 these considerable discrepancies with the conventional time scale can either be explained by an consistent error in the decay constants used in k ar radiometric dating or more likely by diffusional loss of radiogenic argon

the origin of basic rocks of the korosten amcg complex ukrainian shield implication of nd and sr isotope data abstract the korosten complex is a paleoproterozoic gabbro anorthosite rapakivi granite intrusion which was emplaced over a protracted time interval 1800 1737 ma the complex occupies an area of about 12 000 km2 in the north western region of the ukrainian shield about 18 of this area is occupied by various mafic rocks gabbro leucogabbro anorthosite that comprise five rock suites early anorthositic a1 1800 1780 ma main anorthositic a2 1760 ma early gabbroic g3 between 1760 and 1758 ma late gabbroic g4 1758 ma and a suite of dykes d5 before 1737 ma in order to examine the relationships between the various intrusions and to assess possible magmatic sources nd and sr isotopic composition in mafic whole rock samples were measured new sr and nd isotope measurements combined with literature data for the mafic rocks of the korosten complex are consistent and enable construction of rb sr and sm nd isochronous regressions that yield the following ages 1870 310 ma rb sr and 1721 90 ma sm nd these ages are in agreement with those obtained by the u pb method on zircons and indicate that both rb sr and sm nd systems have remained closed since the time of crystallisation in detail however measurable differences in isotopic composition of the korosten mafic rock depending on their suite affiliation were revealed the oldest a1 rocks have lower sr 87sr 86sr 1760 0 70233 0 70288 and higher nd end 1760 1 6 0 9 isotopic composition the most widespread a2 anorthosite and leucogabbro display higher sr and lower nd isotopic composition 87sr 86sr 1760 0 70362 end 1760 varies from 0 2 to 0 7 the g3 gabbro norite has slightly lower end 1760 varying from 0 7 to 0 9 finally g4 gabbroic rocks show relatively high initial 87sr 86sr 0 70334 0 70336 and the lowest nd isotopic composition end 1760 varies from 0 8 to 1 4 of any of the mafic rocks of the korosten complex studied to date on the basis of sr and nd isotopic composition we conclude that korosten initial melts may have inherited their nd and sr isotopic characteristics from the lower crust created during the 2 05 1 95 ga osnitsk orogeny and 2 0 ga continental flood basalt event indeed end 1760 values in osnitsk rocks vary from 0 0 to 1 9 and from 0 2 to 3 4 in flood basalts we suggest that these rocks being drawn into the upper mantle might melt and give rise to the korosten initial melts 87sr 86sr 1760 values also support this interpretation we suggest that the sr and nd isotopic data currently available on mafic rocks of the korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by paleoproterozoic amalgamation of sarmatia and fennoscandia

palaeoceanographic record of the last deglaciation in the strait of sicily abstract the palaeoceanographic evolution of the levantine waters during the last deglacial time is investigated using the sedimentary record of a deep sea core cs 70 5 from the linosa basin 35 44 4 n 13 11 0 e 1486 m water depth radiocarbon dating and oxygen isotope stratigraphy based on 18 o changes in globigerina bulloides allow us to recognize and to date the different steps of the deglaciation these steps are synchronous with those reported in the north atlantic but correspond to a 18 o decrease of higher amplitude than in the alboran sea or in the north atlantic ocean major faunal events permit the establishment of a local biozonation which differs from those reported either for the alboran sea or the eastern mediterranean basin major breaks in the faunal assemblages occurred during termination i a within the first step of the deglaciation around 14 kyr b p and near 10 6 kyr b p within the younger dryas the onset of the last deglaciation induced important changes in the characteristics of the levantine and atlantic water masses which occupied the strait of sicily the 13 c records of globigerina bulloides and cibicidoides pachyderma indicate that the modifications observed in the assemblages of deep faunas are controlled by the oxygenation of the water column 13 c records of globigerina bulloides are similar throughout the west mediterranean as well as in sediments located below the present mediterranean outflow the distributional pattern as well as its 13 c record suggest that this species could be a good recorder of the upper levantine waters and more precisely of the mixing layer of the overlying atlantic waters with the levantine ones major influxes of atlantic waters during terminations i a and i b could have slowed down the vertical mixing of the different water layers present in the strait of sicily and caused a decrease in the oxygenation of the water column during termination i b the effect of atlantic influxes was reinforced by the occurrence of a low salinity layer in the eastern basin which led to the stagnation of the deep waters the two episodes of decreased oxygenation in the levantine waters also favored the precipitation of inorganic magnesium calcite

astronomical solutions for paleoclimate studies over the last 3 million years three solutions numerical qtd90 semi analytical la90 and analytical ber90 for the planetary and precessional elements of the earth have recently become available they allow the computation of the fundamental astroclimatic parameters eccentricity climatic precession and obliquity the agreement between these different solutions is remarkably good reflecting their accuracy in such a way that they can be considered reliable enough for paleoclimate studies over at least 3 myr the difference between these solutions in eccentricity is less than 0 005 over this time interval while the differences in obliquity and climatic precession are larger but mainly due to phase lag between the solutions at 3 myr bp ber90 leads la90 by 6 x 10 3 yrs and qtd90 by 9 x 10 3 yrs for the obliquity and it leads la90 by 2 x 10 3 yrs and qtd90 by 4 x 10 3 yrs for the climatic precession

geochemistry petrogenesis and tectonic setting of amphibolites from the southernmost exposure of the appalachian piedmont the uchee belt of the southern appalachian piedmont consists of a sequence of amphibolite amphibolitic gneiss and gneissic calc silicate surrounded by highly strained mylonitic rocks associated with motion along the bartletts ferry goat rock fault system amphibolites from three major localities within the belt the hudson mill rapids davis mill and lindsey creek localities have been analyzed for their major and trace element chemistry the mafic amphibolites are concluded to have been derived from intrinsically basaltic protoliths as determined by the presence of patchy zoning in relict igneous plagioclase normative mineralogy niggli trends and a variety of diagnostic geochemical criteria davis mill amphibolites are characterized by high ree abundances and flat ree pattern la yb sub n 1 3 to 2 3 similar to volcanic rocks from pacific type island arcs geochemical parameters such as high la nb and v ni ratios low ti v and ti zr ratios and discriminant trace element diagrams based on elements generally considered immobile during metamorphism indicate an island arc and back arc basin affinity for the amphibolites hudson mill rapids amphibolites characterized by a wide range of ree concentrations and strongly negative ce anomaly are interpreted as having been derived from parental basalts produced in a back arc basin by melting of a descending slabmore plus mantle wedge the chemical diversity of the amphibolites and the presence of thrusts strike slip faults and highly strained rocks are consistent with the premise that the western uchee belt is a deformed composite part of an extensive arc system less

african monsoons an immediate climate response to orbital insolation the croll milankovitch astronomical theory of climate1 3 has received strong support from the evidence of a linear climatic forcing by obliquity and precession although nonlinearity had to be assumed for eccentricity4 5 moreover interglacials have appeared to be controlled by the orbital insolation6 although a phase shift of 6 000 5 000 yr is seen between an astronomical climate index and terrestrial climate indicators dominated by the northern ice sheet dynamics climate proxy data of low latitudes are less directly dependent on the ice sheets here it is shown that during the past 464 000 yr african monsoons signalled by the east mediterranean sapropels were heaviest always and only when a northern summer monsoon index computed from the orbital variation of insolation reached maximum values this occurred during all the interglacials but also twice during glacial periods thus tropical aridity is not the single climatic pattern of glacial periods an immediate terrestrial climate response to orbital variations of insolation is established

calibration of the c 14 timescale over the past 30 000 years using mass spectrometric u th ages from barbados corals uranium thorium ages obtained by mass spectrometry from corals raised off the island of barbados confirm the high precision of this technique over at least the past 30 000 years comparison of the u th ages with 14c ages obtained on the holocene samples shows that the u th ages are accurate because they accord with the dendrochronological calibration before 9 000 yr bp the 14c ages are systematically younger than the u th ages with a maximum difference of 3 500 yr at 20 000 yrbp the u th technique thus provides a way of calibrating the radiocarbon timescale beyond the range of dendrochronological calibration

precise timing of the last interglacial period from mass spectrometric determination of thorium 230 in corals the development of mass spectrometric techniques for determination of 230 th abundance has made it possible to reduce analytical errors in 238 u 234 u 230 th dating of corals even with very small samples samples of 6 x 10 8 atoms of 230 th can be measured to an accuracy of 3 percent 2 and 3 x 10 10 atoms of 230 th can be measured to an accuracy of 0 2 percent the time range over which useful age data on corals can be obtained now ranges from about 50 to about 500 000 years for young corals this approach may be preferable to 14 c dating the precision with which the age of a coral can now be determined should make it possible to critically test the milankovitch hypothesis concerning pleistocene climate fluctuations analyses of a number of corals that grew during the last interglacial period yield ages of 122 000 to 130 000 years the ages coincide with or slightly postdate the summer solar insolation high at 65 n latitude which occurred 128 000 years ago this supports the idea that changes in pleistocene climate can be the result of variations in the distribution of solar insolation caused by changes in the geometry of the earth s orbit and rotation axis

late pleistocene variability of the carbon isotopic composition of organic matter in the eastern mediterranean monitor of changes in carbon sources and atmospheric co2 concentrations the organic carbon isotopic record of the sapropels s1 and s3 s10 and intercalated marl oozes has been determined in a 12 m piston core from the eastern mediterranean the 13corganic values are systematically lighter mean 21 0 0 82 in all sapropels and heavier mean 18 8 1 07 in the marl oozes these differences are not due to variable marine and terrestrial organic matter mixtures because all values are heavier than modern plankton in the mediterranean there is no relationship between the corganic n ratios and the isotopic values and published information on the abundance and distribution of organic biomarkers shows that terrestrial material constitutes a minor fraction of the total organic matter temperature effects on isotope fractionation are also discounted because the change in 13corganic values between glacial and interglacial horizons is in the opposite sense diagenesis which can produce relatively small changes in the carbon isotopic composition of sedimentary organic matter under certain circumstances is unlikely to have caused the observed differences because this mechanism would cause an enrichment in 12c implying that all values were even heavier originally and there is no secular trend in the 13corganic record the observed differences in 13corganic between the two lithologies are probably produced by changes in the isotopic composition and the concentration of dissolved co2 first freshwater flooding during the formation of the sapropels caused the isotopic composition of the dissolved inorganic carbon in the surface waters of the mediterranean to become lighter because of the 13c deficiency in fresh waters hence photosynthesis would have produced isotopically lighter organic material second changes in atmospheric pco2 between glacial and interglacial periods as shown by the vostok ice core caused marked changes in the concentration of free dissolved co2 in the mixed layer lower values during glacial maxima caused a smaller fractionation of the carbon isotopes by phytoplankton whereas levels were less limiting during the interglacials concentrations of dissolved co2 could also have been much higher during the deposition of the sapropels because of the supply of regenerated co2 to the mixed layer by upwelling and this could have further lightened the 13corganic values in the sapropels themselves carbon isotope records may provide an alternative method for estimating atmospheric pco2 levels over longer time periods than can be obtained from ice cores

anorthosites granulites and the supercontinent cycle abstract the pattern of occurrence of the massif type anorthosites of the proterozoic on a rodinia reconstruction suggests that the geneses of the anorthosites the associated granulites and their chief repository the grenville age mobile belt may be interrelated this relationship has been examined within the broad geodynamic and spatial temporal framework of the proterozoic supercontinent all over the grenville age mobile belt approximately within 1500 1000 ma two successive back to back episodes can be recognized a continent continent collisional episode with massive sedimentation great crustal shortening and thickening with large folds and nappes metamorphism and calcalkaline magmatism and accretion of juvenile crust was followed within little over a hundred million years or so by an extensional episode beginning with large scale mantle derived basic magma invasion ponding and differentiation at the base of the thickened orogen anorthosite diapirism and pervasive thermal overprinting of the lower and middle crust producing granulite belts cooling unroofing and erosion of the orogen coincided with the later stages of the extension which ended with splitting of the supercontinent we have argued that the first episode marks the phase of closing in and amalgamation of the converging continental lithospheric plates and the second episode represents the phase of reversal rifting plate separation and drifting away of the post split continental blocks we believe the supercontinental closing and opening cycles provide a realistic force fields and appropriate spatial temporal and thermal tectonic framework for the origin of the proterozoic massif type anorthosites and the associated granulites and b an explanation for the spatially overlapping but apparently conflicting evidence of both collisional and extensional tectonic signatures in the grenville age orogenic belts

proterozoic basic dykes in the ukrainian shield a palaeomagnetic geochronologic and geochemical study the accretion of the ukrainian shield to fennoscandia a palaeomagnetic geochronologic and geochemical study of basic dykes in the ukrainian shield has been undertaken with the purpose of testing a hypothesis for timing of accretion of the ukrainian shield with fennoscandia on the basis of geological data it has been suggested that the two shields amalgamated at ca 1900 1800 ma with this study of 11 basic dykes the palaeomagnetic database for the ukrainian shield is extended the palaeomagnetic data indicate three generations of dykes this is supported by the age data and the difference in geochemical compositions ages of two ca 25 m wide dykes susly in the north western block of the shield have been determined one for which a positive palaeomagnetic field test is demonstrated the lower of the two ar 40 ar 39 hornblende ages of this dyke 2061 20 ma is similar to the u pb zircon age of the granitic host rock ca 2067 ma u pb zircon which suggests that the dyke intruded shortly after crystallization of the granite combined with the magnetization of the baked contact and that of another dyke a new ca 2060 ma pole is presented plat 15 7 degrees plon 182 9 degrees a 95 13 7 degrees which may be regarded as a vgp an u pb age of 1722 12 ma of baddeleyite from the khmelnik dyke which yields a well defined vgp is similar to the age of nearby anorthosite complexes a new 1740 1770 ma key pole for the ukranian shield is also presented plat 26 5 degrees plon 169 1 degrees a 95 3 9 degrees on the basis of palaeomagnetic and geochronological data a 2060 1720 ma apparent polar wander for the ukrainian shield is defined this apparent polar wander is significantly different from that of fennoscandia during the same time interval and indicates that the two shields were not in their present relative positions at ca 1770 ma a scenario is here proposed where the ukrainian shield and sarmatia collided with fennoscandia at ca 1900 1800 ma and after that event at ca 1720 1660 ma it rotated approximately 43 degrees into its present position c 2010 elsevier b v all rights reserved less

a reappraisal of the geomagnetic polarity time scale to 4 ma using data from the turkana basin east africa recalibration of the pliocene and early pleistocene geomagnetic time scale using the k ar dated fluvial sequence of the turkana basin in east africa agrees with calibrations based on astronomical calculations ages estimated here are olduvai subchron 1 78 1 96 ma reunion subchrons 2 11 2 15 ma and 2 19 2 27 ma matuyama gauss boundary 2 60 ma kaena subchron 3 02 3 09 ma mammoth subchron 3 21 3 29 ma gauss gilbert boundary 3 57 ma

precambrian mafic dyke swarms in the singhbhum craton eastern india and their links with dyke swarms of the eastern dharwar craton southern india abstract based on trend cross cutting relationships and u pb dating precambrian mafic dykes in the singhbhum craton earlier collectively identified as newer dolerite swarm have been separated into seven distinct swarms which are thought to be the plumbing systems for large igneous provinces lips these singhbhum swarms range in age from 2 80 ga to 1 76 ga and include the 2 80 ga ne sw trending keshargaria swarm 2 75 2 76 ga nne ssw to ne sw trending ghatgaon swarm the 2 26 ga ne sw to ene wsw trending kaptipada swarm based on a new u pb id tims age 2256 6 ma the 1 77 ga wnw ese trending pipilia swarm the early paleoproterozoic e w to ene wsw trending keonjhar swarm the middle paleoproterozoic nw se to nnw sse trending bhagamunda swarm and the late paleoproterozoic n s to nne ssw trending barigaon swarm two of the singhbhum swarms the 2 26 ga kaptipada and 1 77 ga pipilia are closely matched with the 2 26 2 25 ga ippaguda dhiburahalli and 1 79 ga pebbair swarms respectively of the eastern dharwar craton the correlations suggest that the singhbhum and dharwar cratons were close enough at these times to share two reconstructed lips a 2 26 2 25 ga kaptipada ippaguda dhiburahalli lip and a 1 79 1 77 ga pipilia pebbair lip and if so both swarms must be present in the intervening bastar craton candidates are proposed also the 2 76 2 75 ga ghatgaon swarm of the singhbhum craton can be provisionally correlated with 2 7 ga keshkal swarm of the bastar craton the 2 26 2 25 ga kaptipada ippaguda dhiburahalli lip of the singhbhum bastar dharwar reconstruction has age matches in the vestfold hills of antarctica 2 24 ga dykes the kaapvaal craton the 2 25 2 23 ga hekpoort lavas and perhaps the zimbabwe craton 2 26 ga chimbadzi troctolite intrusions the 1 76 1 79 ga pipilia pebbair lip of the singhbhum bastar dharwar reconstruction has age matches in the north china australian shield amazonian rio de plata and sarmatia cratons the relevance of these matches for reconstructions will require future testing using paleomagnetic studies while there are 2 7 2 8 ga lip type greenstone belts in many crustal blocks there are no precise matches with the 2 76 2 75 ga ghatgaon swarm of the singhbhum craton however the 2 80 ga keshargaria swarm can be potentially linked with units in the pilbara and yilgarn cratons

the lower crust of se belarus petrological geophysical and geochemical constraints from xenoliths abstract highly altered alkaline lamprophyre tuffs of devonian age from se belarus contain lower crustal xenoliths that vary from feldspar rich mafic granulites to rarer feldspar poor eclogitic granulites together with coarse grained hornblendites textural evidence mineral compositions and t p data 700 800 c 0 9 1 1 gpa of the garnet granulite xenoliths indicate that equilibration occurred in the lower crust and therefore we consider that these xenoliths represent samples of the in situ lower crust they are highly altered but the main primary minerals are almandine garnet diopside and andesine plagioclase k feldspar scapolite biotite apatite ilmenite ti magnetite and rutile are accessory minerals a subsequent metasomatic event post dating the granulite facies metamorphism has introduced widespread pargasitic amphibole the granulites are basic in composition are enriched in the most incompatible trace elements and have large negative nb anomalies la n nb n range from 3 to 12 inferring a subduction related origin ree patterns show variable lree enrichment la yb 6 27 and some positive eu anomalies indicating plagioclase accumulation trace element and ree patterns for one sample suggest a melt composition the granulites may represent a gabbroic underplate of unknown age and may be petrogenetically related to the osnitsk mikashevichi plutonic belt hornblendite xenoliths are relatively unaltered and consist of pargasite or pargasite plus magnetite and apatite amphibole compositions are very homogeneous and resemble those in the granulite xenoliths the 40 ar 39 ar plateau age is 381 2 ma lree enriched patterns low 87 sr 86 sr ratios and textural constraints suggest that these hornblendites precipitated from a magma formed from a small degree partial melt of a garnet bearing mantle they may be petrogenetically related to the host lamprophyres possibly as a cumulate from an earlier flux of magma and related to the metasomatic event that affected the granulites whole rock vp values calculated from modal mineralogies of the granulite xenoliths range from 6 7 to 7 5 km s assuming equilibration at 800 c 1 0 gpa they are compatible with seismic velocity measurements of the lower crust beneath se belarus which reach 7 2 km s xenoliths with the least modal plagioclase and to some extent the most modal clinopyroxene and garnet give the highest whole rock vp values calculated vp values for hornblendite xenoliths 6 75 km s at 800 c 1 0 gpa suggest that identification of hornblendite in the lower crust from vp values alone would be difficult

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

palaeoceanography of the last deglaciation in the alboran sea western mediterranean stable isotopes and planktonic foraminiferal records abstract oxygen isotope changes of the foraminiferal species globigerina bulloides and uvigerina peregrina eleven radiocarbon dates on the sediment fraction above 50 m and 13 biostratigraphical events allow us to correlate and to date the sediment record of the last climatic cycle in four cores from the alboran sea nine ecozones are recognized which differ from those previously established for atlantic sediments cored immediately west of gibraltar these ecozones can be grouped into two broad phases from the last glacial maximum up to near 9 500 yr b p cold north atlantic microfaunas are the major component of the assemblages during this interval dominant species replace each other according to the varying temperature and density stratification of the water masses from 9 500 yr b p up to the present time subtropical forms from the central atlantic dominate the northern atlantic forms an abrupt increase in the percentages of g inflata occurred near 7 000 b p this event was diachronous within the mediterranean during the last glacial g inflata was endemic inside the mediterranean and its abrupt increase near the end of the deglaciation did not result from replenishment from the atlantic between 4 000 and 5 000 yr b p percentages of g bulloides also increased abruptly warming conditions associated with the onset of a seasonal thermal stratification and a contrasting winter vertical convection are strongly suggested

age of the plio pleistocene boundary in the vrica section southern italy new palaeomagnetic data from the proposed plio pleistocene boundary stratotype section vrica section southern italy are presented here in order to improve the correlation of these sediments to the magnetic polarity time scale if the interpretation preferred here is correct the boundary falls above the olduvai normal subchron at approximately 1 6 myr

the 1 80 1 74 ga gabbro anorthosite rapakivi korosten pluton in the ukrainian shield a 3 d geophysical reconstruction of deep structure the deep structure of the gabbro anorthosite rapakivi granite amcg type korosten pluton kp in the northwestern ukrainian shield was studied by 3 d modelling of the gravity and magnetic fields together with previous seismic data the kp occupies an area of ca 12 500 km 2 and comprises several layered gabbro anorthositic intrusions enveloped by large volumes of rapakivi type granitoids between 1 80 and 1 74 ga the emplacement of mafic and associated granitoid melts took place in several pulses the 3 d geophysical reconstruction included a modelling of the density distribution in the crust using the observed bouguer anomaly field constrained by seismic data on moho depth and b modelling of the magnetic anomaly field in order to outline rock domains of various magnetisation size and shape in the upper and lower crust the density modelling was referred to three depth levels of 0 to 5 5 to 18 and 18 kin to moho respectively the 3 d reconstruction demonstrates close links between the subsurface geology of the kp and the structure of the lower crust the existence of a non magnetic body with anomalously high seismic velocity and density is documented most plausibly it represents a gabbroic stock a parent magma chamber with a vertical extent of ca 20 km penetrating the entire lower crust this stock has a half cylindrical shape and a diameter of ca 90 kin it appears to be connected with a crust mantle transitional lens previously discovered by eurobridge seismic profiling the position of the stock relative to the subsurface outlines of the kp is somewhat asymmetric this may be due to a connection between the magmatism and sets of opposite dipping faults initially developed during late palaeoproterozoic collisional deformation in the sarmatian crustal segment continuing movements and disturbances of the upper mantle and the lower crust during post collisional tectonic events between 1 80 and 1 74 ga may account for the long lived recurrent amcg magmatism less

polybaric fractional crystallization of high alumina basalt parental magmas in the egersund ogna massif type anorthosite rogaland sw norway constrained by plagioclase and high alumina orthopyroxene megacrysts bulk analyses of plagioclase and high alumina orthopyroxene megacrysts decimetreto metre sized crystals with plagioclase lamellae from the late proterozoic egersund ogna massif type anorthosite rogaland anorthosite province swnorway are used to constrain the parental magma compositions and differentiation processes within this 30 km diameter diapir spatial compositional variations show that two types of anorthosite occur high sr 720 1090 ppm andesine anorthosite an48 4 in the centre of the intrusion and low sr 320 620 ppm labradorite anorthosite an59 6 in the margin two populations of orthopyroxene megacrysts are clearly discriminated by their mn and cr contents but display similar ranges of mg number 55 79 interpretation of trace element data and comparison with inversion models suggest that the two types of anorthosite result from crystallization of different parental magma compositions with high alumina basalt affinities and similar mg numbers but different wollastonite sr mn and cr contents a rayleigh fractional crystallization model of the trace element concentrations vs mgo content in orthopyroxene constrains the cotectic orthopyroxene plagioclase proportions to 0 33 0 67 in both magma types before the appearance of fe ti oxide minerals on the liquidus at f 0 7 where f is melt fraction polybaric crystallization is recorded by variable alumina contents in the orthopyroxene megacrysts 2 3 8 5 wt corresponding to crystallization pressures in the 3 13 kbar range as shown by experimental data this implies that the high alumina orthopyroxene megacrysts mainly crystallized en route during diapiric rise of the anorthositic mush modelling the plagioclase compositions with experimentally determined partition coefficients for ca k sr and ba confirms that pressure variation during polybaric crystallization is the main controlling factor on compositional variations in the anorthosite pluton

pinning down the brunhes matuyama and upper jaramillo boundaries a reconciliation of orbital and isotopic time scales until recently the temporal calibration of the plio pleistocene portion of the geomagnetic reversal time scale grts was based exclusively on kar isotopic ages of volcanic rocks the so called chronogram technique estimates the age that minimizes discrepancies between a given age assignment for a reversal boundary and the population of dates of known polarity the most widely quoted date 0 73 ma for the brunhes matuyama boundary was derived in such a manner 1 radioactive decay is not the only source of temporal information in the geologic record however variations in several of the orbital parameters of the earth e g precession obliquity eccentricity can be calculated for the last several million years and the climatic response approximated a recent analysis of the so called astronomical technique on oxygen isotopic data from deep sea sediments suggested an age of 0 78 ma for the brunhes matuyama boundary 2 some 50 000 years older than the chronogram estimate we reconcile the two estimates for the age of the last reversal by first presenting new magnetostratigraphic data tied to high quality40ar39ar dates our date for the upper jaramillo boundary is0 992 0 039 ma considerably older than the estimate of 0 91 ma in the standard chronogram based time scale 3 furthermore an age of0 746 0 009 ma was obtained for sediments immediately overlying the brunhes matuyama boundary these data are consistent with the astronomical estimates of 0 78 ma and 0 99 ma for the brunhes matuyama and upper jaramillo boundaries respectively finally we present a new method for estimating the uncertainties of ages calculated using the chronogram technique that employs a simple bootstrap resampling scheme the 95 confidence region for the chronogram estimate of the brunhes matuyama boundary using an updated compilation of date polarity pairs spans from 0 73 to 0 78 ma we find therefore that the discrepancies between the isotopic data set and the astronomical calibration of the brunhes matuyama boundary disappear when appropriate uncertainties are applied for the most recent compilation of isotopic data

geochemistry and tectonic significance of the ongarbira metavolcanic rocks singhbhum district india abstract the volcanic sedimentary sequence of ongarbira occurs to the south of the singhbhum shear zone and rests uncomformably on a basement topographic low between the chakradharpur granite and the singhbhum granite structural analysis reveals that the ongarbira synform and other adjoining structures are kilometre scale first generation folds on e w striking axial planes these structures are in marked contrast with the corresponding kilometre scale first generation folds typical of the iron ore group synclinorium towards the south of the ongarbira belt the ongarbira metavolcanic rocks exhibit a limited range of tholeitic basalt compositions the basalts are grouped according to their mg and the details of their ree patterns groups i and ii are relatively unevolved and very similar to lree and lil depleted ocean ridge tholeiites the most evolved group iii with lree enrichment is most similar to lil enriched ocean floor basalts but may be contaminated with crustal materials in general the ongarbira volcanics evolve upward stratigraphically petrogenetic major element modelling of the ongarbira basalts suggests that the primary melt was generated by batch melting 30 of lherzolite at pressures of at least 1 5 gpa sequential fractionation of ol o1 cpx pl and cpx pl followed comparison of incompatible and compatible element distributions indicates a very similar batch m melting fractional crystallization sequence for ongarbira groups i and ii basalts group iii basalts may have been derived from a batch melt initially more enriched in incompatible elements trace element discrimination of the ongarbira basalts suggests that they were generated in an extensional environment primordial mantle normalized element distributions indicate that the basalts are similar to continental rift basalts but with oceanic affinities the chemical data along with the associated sediments indicate that the ongarbira basalts were probably emplaced in a mature continental rift although a back arc origin cannot be completely disregarded it is concluded that the ongarbira volcanics were emplaced as part of the same rifting event that resulted in outpourings of the proterozoic dalma and dhanjori volcanics implying that the ongarbira suite is proterozoic and not part of the archaean iron ore group the structural geometry of the ongarbira rocks is more conformable to that of the singhbhum mobile belt than to the iron ore group synclinorium in the singhbhum craton it is likely that the southern limit of the singhbhum mobile belt is defined by the interface of its e w structures with the nne structures of the iron ore group rocks thus the singhbhum shear zone does not mark the interface between the archaean singhbhum craton and the proterozoic singhbhum mobile belt in the eastern indian shield

late palaeoproterozoic mafic dyking in the ukrainian shield of volgo sarmatia caused by rotation during the assembly of supercontinent columbia nuna the ukrainian shield comprises the exposed crust of the large palaeoproterozoic protocraton volgo sarmatia which together with the fennoscandian crustal segment constitutes the east european craton baltica geological and geophysical data indicate that 1 80 to 1 75 ga mafic dykes related to anorthosite mangerite charnockite granite amcg plutons are widespread within the ukrainian shield we examined their ages distribution patterns orientations and compositions in three different crustal blocks volyn ingul and azov and found close spatial relationships with major strike slip fault systems developed during two phases of extension the early 1 80 1 77 ga generation of mafic dykes mostly follows nw 330 20 degrees and more rarely n s or e w trending faults corresponding to major ne sw extension the submoshorino phase these dykes contain olivine dolerites picrites camptonites lamprophyres kimberlites and other rocks belonging to tholeiitic and subalkaline jotunitic series the compositions of these dykes differ between the host blocks but all feature upper mantle geochemical signatures such as high contents of ni and cr and positive values of epsilon nd 1800 up to 2 8 high degrees of ree fractionation indicate deep levels of mantle melting which is particularly characteristic of the ingul block as marked by the most extensive and dense mafic dyke swarms the later 1 76 1 75 ga dyke swarms occur close to the most voluminous amcg suites of similar age and were emplaced during the second korsun phase of faulting when all the older strike slip fault zones were reactivated and partly transformed to tensional faults by e w extension these dyke swarms mainly trend 030 20 degrees they are jotunitic and their isotopic signatures indicate a greater participation of crustal sources in the parent melts the overall transtensional tectonic setting of the mafic dyking associated with the amcg magmatism in volgo sarmatia was created by convergent tectonics and postcollisional collapse of the thickened lithosphere as well as by mantle delamination coupled with the rotation of volgo sarmatia between 1 80 and 1 75 ga this agrees with palaeomagnetic reconstruction suggesting rotation s of volgo sarmatia during its protracted oblique docking with fennoscandian terranes and laurentia as supercontinent columbia nuna was assembled c 2012 elsevier b v all rights reserved less

paleomagnetism of 1 79 ga para de minas mafic dykes testing a sao francisco congo north china rio de la plata connection in columbia abstract paleogeographic connections between sao francisco congo north china and rio de la plata cratons through the paleoproterozoic have been proposed on the basis of geological and paleomagnetic data we conducted new paleomagnetic analyses for mafic dykes of the 1790 ma para de minas swarm occurring in the southern sao francisco craton the data revealed south southwestern low inclination magnetic directions after alternating field af and thermal demagnetization providing a paleomagnetic pole at 39 8 s 196 8 e a95 17 0 the characteristic remanent magnetization is interpreted as a thermo remanent magnetization carried by stable pseudo single domain magnetite grains being acquired during the cooling of the dykes at 1790 ma as attested by a positive baked contact test performed on one of the dykes the new pole in conjunction with 1790 1750 ma paleomagnetic poles available for other blocks allowed an improved reconstruction of columbia nuna supercontinent at this time frame laurentia baltica siberia proto amazonia west africa and proto australia were positioned as in previously proposed models the main difference from our reconstruction concerns the link of the sao francisco congo north china and rio de la plata cratons implying that much of the columbia supercontinent had already agglutinated around 1850 1800 ma ago this configuration is consistent with the onset of eventual taphrogenic episodes over most blocks accompanied by the emplacement of mafic dyke swarms and granitic intrusions at 1790 1750 ma

quest for palladium application of the groves hypothesis to precambrian terranes of northeast north america a compilation of all types of magmatic deposits in precambrian terranes of northeast north america revealed a continental scale metallic zonation nickel is homogeneously distributed palladium and chromite are centered on the archean craton as predicted by the groves hypothesis whereas low ti cu u rare earth element iron oxide deposits occur just south and east of the archean and paleoproterozoic margin within the grenville province ti is concentrated along this margin in association with anorthosite nickel is still abundant there but palladium and chromite are remarkably absent this grenvillian ni pd cr paradox suggests that the fractionation process invoked in the classical plume model is inadequate to explain the geochemical characteristics of the anorthosite parental magmas the crustal tongue melting model seems to better explain the metallic zonation observed at the margin of the archean and proterozoic terranes

retreat velocity of the north atlantic polar front during the last deglaciation determined by 14 c accelerator mass spectrometry the glacial oscillations which dominate the quaternary global climate are closely related to changes in incoming solar radiation but the details of palaeoclimatic spectra suggest that other parameters usually referred to as feedback have significantly increased the climatic response to extraterrestrial forcing1 the last deglaciation appears to be one of the more characteristic examples of nonlinearity because its abruptness strongly contrasts with the smooth shape of the associated solar radiation maximum dated at 11 000 yr bp in addition the presence of the younger dryas cold event concomitant with the solar radiation maximum could be interpreted as a transitory phenomenon caused by the rapid meltwater influx linked to the first phase of the ice sheet s disintegration2 3 here using accelerator mass spectrometry we date precisely the movements of the polar front during the whole period of glacial retreat 15 000 8 000 yr bp our results show that the polar front has moved between 35 n and 55 n in 1 000 yr for the earliest retreat 12 500 12 000 bp and moved almost instantaneously that is 400 yr during the two abrupt climatic changes associated with the younger dry as cold event

the jotunite of the korosten amcg complex ukrainian shield crust or mantle derived abstract the korosten plutonic complex kpc of palaeoproterozoic age is made up of an amcg suite of rocks jotunitic ferromonzodioritic magmas have been emplaced at the margins or within massive anorthosites and in hypabyssal dykes coeval with the amcg plutonism major and trace element geochemistry shows a calc alkalic to alkali calcic trend with large variations of composition with sio2 ranging from 44 to 62 and mg from 19 to 36 high concentrations of fe ti and p up to 20 fe2o3t 4 tio2 and 2 p2o5 the korosten jotunitic sequence of rocks matches the jotunite liquid line of descent that has been defined in the rogaland anorthosite province both trends are compared to experimental data in dry conditions and medium to high pressure on the evolution of olivine tholeiite melt it is shown that the initial step of the jotunite evolution characterized by a decrease in sio2 correlated with increases in fe and p is not reproduced in the experiments at medium pressure at high pressure such evolution is possible but cannot yield sio2 rich melts with further fractionation because of the existence of a thermal barrier derivation of jotunite melts from mantle derived olivine tholeiite would be possible in a polybaric evolution of the melt however the polybaric evolution of the anorthosite mush with extraction of the interstitial jotunite melt to form the mafic marginal intrusion is not supported by field evidence of filter press mechanism derivation of the jotunite from melting of gabbroic rocks in the crust and their evolution at medium pressure appears as a more convincing mechanism the ages of formation of the korosten and rogaland anorthosites are separated by 800 myr it is speculated that the secular cooling of the earth has modified the generation process of the anorthosite in the kpc a hot zone developed in the crust due to an asthenosphere uprise concomitant with the development of ni rich tholeiitic dykes and in rogaland the cooler and more rigid crust was able to subduct and form crustal tongues that were potential source of melts

u pb id tims baddeleyite ages and paleomagnetism of 1 79 and 1 59 ga tholeiitic dyke swarms and position of the rio de la plata craton within the columbia supercontinent abstract the tandilia terrane southernmost fringe of the rio de la plata craton is an igneous and metamorphic complex produced by an accretionary orogeny 2 25 2 02 ga calc alkaline acidic dykes with e w strike and a major shear zone with similar orientation are related with the late orogeny stage as supported by field relations in a previous study the acid dykes gave 40 ar 39 ar ages of 2007 24 ma to 2020 24 ma a n and nw trending tholeiitic dyke swarm tandil swarm is also present in the tandilia terrane one sample from the nw trending subset previously gave a u pb id tims baddeleyite age of 1588 11 ma new precise u pb id tims baddeleyite dating of both n and nw trending tandil dykes yielded crystallization ages of 1589 3 ma 1588 3 ma and 1588 3 ma significantly older tholeiitic dykes known as the florida swarm occur in the northern rio de la plata craton for which a u pb id tims baddeleyite age of 1790 5 ma was previously reported consequently intermittent rifting 1 79 1 59 ga took place after tectonic stabilization of the late paleoproterozoic lithosphere proto rio de la plata craton the available geochemical data for the 1 59 ga tandil dykes define low and high tio 2 trends although only the low tio 2 subgroup is firmly dated both the tandil and florida dykes have geochemical and nd sr characteristics consistent with derivation from heterogeneous mantle sources that underwent metasomatic effects the tandil dykes may be linked with the 1 57 0 02 ga capivarita anorthosite which occurs to the east of the northern part of the craton correlatives on other crustal blocks may include those in baltica such as bimodal rock association including the breven hallefors and aland aboland diabase dykes and in the reconstructed gawler craton nw laurentia dolerites bimodal magmatism and iocg deposits contemporary within plate bimodal associations are also present in the sw amazonian craton paleomagnetic data for the 1790 ma florida dykes allows three possible reconstruction scenarios for the position of rio de la plata craton i in the southern hemisphere linked to the pampia terrane and the amazonia rio apa landmass as close as they are in present day ii in the northern hemisphere as a nearest neighbor to the reconstructed nw laurentia and gawler blocks proto australia or iii in the northern hemisphere near the boundary of amazonia proto amazonian craton and baltica positions 2 and 3 are consistent with all three criteria paleomagnetic poles and proximity to matching ca 1790 ma and 1590 ma mafic magmatism our data are consistent with the idea that the columbia supercontinent started major break up at 1 59 ga

geology and geochronology of the paleoproterozoic hart dolerite western australia abstract the hart dolerite is the intrusive component of the hart carson large igneous province which extends over an area of 160 000 km2 in the speewah and kimberley basins in the western part of the north australian craton in the core of the speewah dome a ca 500 m thick composite sill of hart dolerite intruded metasedimentary rocks of the speewah group which have a maximum depositional age of 1814 10 ma referred to here as the speewah sill it comprises four discrete gabbroic units and two later stage dyke and sill units of potassic granophyre the gabbros locally contain sulfide poor stella type platinum group element enrichment and vanadiferous titanomagnetite mineralisation new id tims u pb geochronology of baddeleyite from gabbro units in the speewah sill yield upper intercept ages all 2 of 1792 7 1 3 ma and 1791 7 4 1 ma these results are complemented by a shrimp age of 1792 1 5 9 ma 2 for baddeleyite in a late stage pigeonite vein in hart dolerite 120 km south of the speewah dome these new data are consistent with previous regional geochronology of late stage quartz feldspar granophyres and with a model of rapid emplacement of the hart dolerite at 1792 6 1 2 ma in an intracratonic extensional setting globally the period around ca 1793 ma includes several large igneous provinces suggesting that this interval was characterised by high mantle heat flow continental extension and voluminous mafic magmatism over large areas

u pb age and hf isotope compositions of zircons from the north western region of the ukrainian shield mantle melting in response to post collision extension terra nova 24 373 379 2012 abstract the north western region of the ukrainian shield represents palaeoproterozoic continental crust formed essentially during two orogenic events at ca 2 1 2 05 and 2 0 1 95 ga at 1 8 1 74 this area was intruded by huge korosten anorthosite mangerite charnockite granite complex at least three generations of mafic dykes can be distinguished in the region we report new u pb data obtained for a suite of ni enriched dolerite dykes and layered gabbroic intrusions our results indicate that the tomashgorod dyke was formed at 1787 4 6 4 ma the prutivka intrusion at 1777 0 4 7 ma and the kamenka massif at ca 1 79 ga this time coincides with the collision of the fennoscandian and sarmatian segments of the east european craton and it is contemporaneous with the initial stages of evolution of the korosten complex hf isotopic compositions of zircons indicate that these rocks originated due to melting of moderately depleted mantle material