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

pb pb baddeleyite ages of mafic dykes from the western dharwar craton southern india a window into 2 21 2 18 ga global mafic magmatism abstract paleoproterozoic mafic magmatism in the dharwar craton dc is evident by ubiquitous dykes of varied orientations and emplacement ages eight distinct mafic dyke events have been identified in the eastern dharwar craton edc while studies in the western dharwar craton wdc were restricted to a previously dated dandeli dyke precise baddeleyite te tims pb pb ages for five mafic dykes across the wdc reveal emplacement ages of 2212 3 0 8 ma 2212 9 0 4 ma 2208 1 0 8 ma 2205 2 0 8 ma for nw se to nnw sse trending dykes and 1957 3 0 7 ma for ene wsw dyke the ca 1 96 ga mundargi dyke establishes a new episode of mafic magmatism in the dc whereas ca 2 21 ga mafic dykes suggests widespread mafic magmatism across the dc the collective trend of ca 2 21 2 20 ga radiating dykes vary from n100 e in edc to n165 e in wdc converging towards the north western margin of dc fanning angle 65 where a mantle plume appears to be located it is postulated that ca 2 18 ga mahbubnagar dandeli dyke swarm of the dc also originated from a similar plume source but as a distinct pulse within a time span of 35 40 myr present study depicts barcode of the wdc and represents an epitome of correlations with the edc superior slave and greenland cratons widespread ca 2 21 2 18 ga lip event indicates that these cratons were located at proximity and suggests breakup of supercraton sclavia superia the single 1 96 ga mundargi dyke is coeval with the dykes and sills of karelia north china and north atlantic cratons

salinity history of the earth s early ocean it is commonly thought that the oceans are becoming saltier with time as sodium and chlorine are weathered out of continental rocks and transported to the sea here we argue that the salinity of the early ocean was 1 5 to 2 times the modern value and that it did not decline significantly until surprisingly late in the earth s history if correct this theory could help explain why the evolution of higher life took so long

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

dating of the ameralik dyke swarms of the nuuk district southern west greenland mafic intrusion events starting from c 3510 ma this paper presents shrimp u pb zircon dating on four meso to palaeoarchaean amphibolite facies ameralik dykes of differing compositions scattered throughout the palaeoarchaean 3850 3600 ma amphibolite granulite facies itsaq gneiss complex in the nuuk district southern west greenland in the north of the complex near the isua supracrustal belt an east west oriented 50 m thick metanoritic dyke with abundant relicts of original opx plag ol igneous mineralogy gave an age of 3512 7 ma interpreted as the intrusion age in general agreement with previous isotope dilution thermal ionization mass spectrometry data in the same area the best estimate for the intrusion age of a metadolerite dyke with the same orientation and similar degree of preservation is 3265 15 ma at the mouth of ameralik fjord 150 km to the sw a large noritic ameralik dyke the ugpik metadolerite was largely recrystallized to amphibolite in the later archaean relict magmatic zircon in this body implies an intrusion age of 3453 16 ma in itilleq fjord 60 km to the east of the ugpik metadolerite a dioritic component of a composite dioritic gabbroic intrusion cutting older more deformed ameralik dykes has yielded an age of 3257 14 ma for the time of intrusion these new sensitive high resolution ion microprobe dates on magmatic crystallization support previous field observations that the ameralik dykes consist of several unrelated sets of dykes different in age by up to 250 ma the magmatic zircons in three of the dykes have 2700 2500 ma overgrowths interpreted to have grown during late archaean amphibolite facies tectonothermal events in the region the original name ameralik dykes might propagate confusion that all such bodies can be regarded as the same age thus we recommend retaining the original definition of the ameralik dykes but using the term ameralik dyke swarms to emphasize that unrelated intrusions of more than one age are present

variation in u 238 and th 232 enrichment in u mineralized zone and geological controls on their spatial distribution singhbhum shear zone of india the determination of the natural radioactivity has been carried out by means of gamma ray spectrometry in exposed rock samples collected from various geological formations across the singhbhum shear zone of india the spatial variation in the concentration of primordial radionuclides reflects in general the geochemical variance among the analyzed rocks and affirms the earlier views on genesis of uranium mineralization this study further speculates new possibilities of thorium depletion during shearing and metamorphism all the obtained u values were more than the worldwide average u concentration which is about 40 bq kg interestingly u concentration in mica schists from singhbhum group of rocks and in slates and argillaceous quartzites from iron ore group of rocks was five to seven times higher than the global values in contrast the granites in study area showed comparatively lower u concentration the overall results suggest the control of complex metamorphism and shearing on u distribution in the terrane the contribution of 238u 232th and 40k to the total radiation dose in the study area was found to be significant ranging from 110 to 310 ngy h in comparison with the lower global as well as indian average values

uranium nickel cobalt molybdenum mineralization along the singhbhum copper belt india and the problem of ore genesis uranium mineralization is present at many places along the 200 km long singhbhum copper belt but the mineralization is relatively concentrated at the central part of it the belt is characterized by many shear zone features such as mylonites phyllonites and l s type of structures and of course copious metasomatism country rocks are basic schists metapelites quartzose rocks and albite schist gneiss soda granite orebodies are sheet like conformable with the pervasive planar structures in the host rocks no pronounced wall rock alteration accompanied the mineralization grade of the ore is low 0 1 u3o8 the principal uraniferous mineral uraninite occurs as dissemination other uranium bearing minerals include pitchblende allanite xenotime davidite clarkeite autunite metaautunite torbernite schoepite metaschoepite and uranophane uranium is also present in a number of refractory phases either as inclusion of uraninite or in the crystal structure additionally nickel cobalt and molybdenum are present at jaduguda bhatin in the form of millerite gersdorffite melonite nickel bearing pyrite molybdenite etc dominance of uraninite over pitchblende and the larger cell edge of uraninite development of hematite bearing quartz and na oligoclase at places in the ore zone association of uranium mineralization with ni co mo s as mineralization at jaduguda bhatin and continuation of the orebodies to considerable depths suggest that the uranium mineralization along the singhbhum belt belongs to moderate to high temperature vein type the age obtained by pb207 pb206 ratio and the concordia method suggest that the uranium mineralization in singhbhum took place 1500 1600 ma ago and this age is not far different from the age of formation of uranium vein deposits in many other precambrian shields of the world the following two mechanisms of the formation of the deposits are discussed 1 uranium precipitated in the dhanjori basal sediments was mobilized during deformation and metamorphism into ore deposits 2 the hydrodynamic system that leached out copper from the metabasic rocks to form the copper deposits at an earlier stage could in one of the oxidised pulses leach out uranium from the basal sediments and precipitate it in the favourable situations subsequent small scale redistribution is possible constituents of the ni co s as mineralization appear to have been contributed by the volcanic sedimentary pile

1891 1883 ma southern bastar cuddapah mafic igneous events india a newly recognized large igneous province abstract a newly recognized remnant of a paleoproterozoic large igneous province has been identified in the southern bastar craton and nearby cuddapah basin from the adjacent dharwar craton india high precision u pb dates of 1891 1 0 9 ma baddeleyite and 1883 0 1 4 ma baddeleyite and zircon for two se trending mafic dykes from the bd2 dyke swarm southern bastar craton and 1885 4 3 1 ma baddeleyite for a mafic sill from the cuddapah basin indicate the existence of 1891 1883 ma mafic magmatism that spans an area of at least 90 000 km2 in the south indian shield this record of 1 9 ga mafic ultramafic magmatism associated with concomitant intracontinental rifting and basin development preserved along much of the south eastern margin of the south indian shield is a widespread geologic phenomenon on earth similar periods of intraplate mafic ultramafic magmatism occur along the margin of the superior craton in north america 1 88 ga molson large igneous province and in southern africa along the northern margin of the kaapvaal craton 1 88 1 87 ga dolerite sills intruding the waterberg group existing paleomagnetic data for the molson and waterberg 1 88 ga large igneous provinces indicate that the superior and kalahari cratons were at similar paleolatitudes at 1 88 ga but a paleocontinental reconstruction at this time involving these cratons is impeded by the lack of a robust geological pin such as a limpopo like 2 0 ga deformation zone in the superior province the widespread occurrence of 1 88 ga intraplate and plate margin mafic magmatism and basin development in numerous archean cratons worldwide likely reflects a period of global scale mantle upwelling or enhanced mantle plume activity at this time

mafic dike swarms as absolute time markers in high grade terranes 40ar 39ar geochronological constraints on the kangâmiut dikes west greenland the paleoproterozoic kang miut dikes are valuable time markers in the complex high grade terranes of west greenland they are used to separate the archean basement from areas affected by the proterozoic nagssugtoqidian orogen it is shown here that both unmetamorphosed and metamorphosed kang miut dikes exposed in the foreland of the orogen preserve pre nagssugtoqidian 40ar 39ar ages two suites of kang miut dikes have been identified ca 2530 ma and ca 2040 ma metamorphism of the dikes in the foreland occurred shortly after their emplacement within the resolution of the argon system we suggest that crustal heating during dike emplacement or heat derived from the individual dikes caused the development of metamorphic minerals and textures internal shearing of the dikes in the foreland occurred as late as 1740 ma contemporaneous with closure of the argon system in hornblende inside the orogen following peak nagssugtoqidian metamorphism ca 1850 ma inside the orogen reworked equivalents of the kang miut dikes record parts of the post nagssugtoqidian cooling path thermotectonic activity during the nagssugtoqidian orogeny is bracketed between intrusion of the youngest kang miut dikes ca 2040 ma and post peak metamorphic cooling of the orogen to 500 c at 1740 ma

geochemistry and petrogenesis of amphibolites kolar schist belt south india evidence for komatiitic magma derived by low percentages of melting of the mantle the kolar schist belt of the dharwar craton of south india is an archean greenstone belt dominated by metavolcanic rocks the mafic metavolcanic rocks occur as komatiitic and tholeiitic amphibolites the komatiitic amphibolites occur along the margins of the n s trending synformal belt they are much less abundant than the tholeiitic amphibolites and have 14 to 21 3 wt per cent mgo the komatiitic amphibolites from the west central part of the belt have two distinctive ree patterns 1 those enriched in the middle to light ree but depleted in ce relative to nd and 2 those with patterns that are convex up i e depleted in both light and heavy ree although more depleted in the light ree associated tholeiites have light ree depleted to flat ree patterns komatiitic and tholeiitic amphibolites from the eastern part of the belt have enriched light ree patterns the tholeiitic amphibolites from the kolar schist belt are similar to the th i and th ii types of archean tholeiites of condie 1981 the komatiitic amphibolites are similar to komatiites and komatiitic basalts of barberton mountainland but have higher feo and tio 2 abundances and lower yb gd ratios the petrogenetic interpretations for these rocks are based primarily on a modification of the mgo feo diagram of hanson langmuir 1978 and modelling of zr ni and ree all of the rocks have undergone some fractionation while the modelling does not give accurate temperatures pressures compositions and extents of melting of the mantle sources for the various amphibolites it does present an approach which can be used for estimating these parameters for example the komatiitic amphibolites appear to be derived from melts generated by 10 to 25 per cent melting of the mantle over a range of depths and temperatures greater than 80 km and 1575 c the variation in the p t conditions of magma generation is possibly due to adiabatic melting in mantle diapirs with a range of feo mgo ratios if the tholeiitic amphibolites are derived from similar mantle sources it is not clear that they are their parent melts may have been generated by similar extents of melting but at depths of less than 80 km the komatiitic amphibolites from the west central part of the belt were generated from light ree depleted mantle whereas those from the eastern part of the belt appear to have been generated from light ree enriched mantle the sources for the komatiitic amphibolites in both areas were significantly enriched in feo relative to pyrolite thus a light ree depleted and a light ree enriched source appear to have provided mafic volcanics with similar major element chemistry to this belt during its evolution

emplacement ages of paleoproterozoic mafic dyke swarms in eastern dharwar craton india implications for paleoreconstructions and support for a 30 change in dyke trends from south to north abstract large igneous provinces lips and especially their dyke swarms are pivotal to reconstruction of ancient supercontinents the dharwar craton of southern peninsular india represents a substantial portion of archean crust and has been considered to be a principal constituent of superia sclavia nuna columbia and rodinia supercontinents the craton is intruded by numerous regional scale mafic dyke swarms of which only a few have robustly constrained emplacement ages through this study the lip record of the dharwar craton has been improved by u pb geochronology of 18 dykes which together comprise seven generations of paleoproterozoic dyke swarms with emplacement ages within the 2 37 1 79 ga age interval from oldest to youngest the new ages integrated with u pb ages previously reported for the hampi swarm define the following eight swarms with their currently recommended names ne sw to ese wnw trending ca 2 37 ga bangalore karimnagar swarm n s to nne ssw trending ca 2 25 ga ippaguda dhiburahalli swarm n s to nnw sse trending ca 2 22 ga kandlamadugu swarm nw se to wnw ese trending ca 2 21 ga anantapur kunigal swarm nw se to wnw ese trending ca 2 18 ga mahbubnagar dandeli swarm n s nw se and ene wsw trending ca 2 08 ga devarabanda swarm e w trending 1 88 1 89 ga hampi swarm nw se ca 1 79 ga pebbair swarm comparison of the arcuate trends of some swarms along with an apparent oroclinal bend of ancient geological features such as regional dharwar greenstone belts and the late archean ca 2 5 ga closepet granite batholith have led to the hypothesis that the northern dharwar block has rotated relative to the southern block by restoring a 30 counter clockwise rotation of the northern dharwar block relative to the southern block we show that pre 2 08 ga arcuate and fanning dyke swarms consistently become approximately linear two possible tectonic models for this apparent bending and concomitant dyke rotations are discussed regardless of which deformation mechanisms applies these findings reinforce previous suggestions that the radial patterns of the giant ca 2 37 ga bangalore karimnagar dyke swarm and probably also the ca 2 21 ga anantapur kunigal swarm may not be primary features

evidence from the rare earth element record of mantle melting for cooling of the tertiary iceland plume widespread flood basalt volcanism and continental rifting in the northeast atlantic in the early tertiary period 55 myr ago have been linked to the mantle plume now residing beneath iceland1 2 3 4 5 although much is known about the present day iceland plume6 7 8 9 its thermal structure composition and position in the early tertiary period remain unresolved estimates of its temperature for example range from 1 600 c in some plume models3 to 1 500 c based on the volume and composition of basaltic crust10 11 12 several recent studies4 have emphasized similarities in the thermal and chemical structure of the tertiary and present day plumes to argue for stability of the mantle anomaly whereas others12 13 relate variations in basalt volumes and compositions to changes in plume flux moreover some authors1 2 13 have assumed that the plume was rift centred for its entire history whereas others argue that it became ridge centred only after plate separation14 15 here we report compositional data for 6 000 metres of flood basalts erupted in east greenland close to the inferred plume axis that we use to constrain the tertiary plume structure rare earth element systematics place limits on the pressures and extents of mantle melting and show that the mantle was initially moderately hot 1 500 c but that its temperature declined during flood volcanism these observations are difficult to reconcile with current plume head models and call for important lithospheric control5 10 16 17 18 on actively upwelling mantle along the rifted margin

progressive development of structures in a ductile shear zone in the singhbhum shear zone of eastern india successive generations of folds grew in response to a progressive ductile shearing during this deformation a mylonitic foliation was initiated and was repeatedly transposed the majority of fold hinges were formed in an arcuate manner at low angles to the y axis in an e w trending subhorizontal position and major segments of the fold hinges were then rotated towards the down dip northerly plunging x axis the striping and intersection lineations were rotated in the same manner the down dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations the consistent asymmetry of the folds the angular relations between c and s surfaces and the evidence of two dimensional boudinage indicate that the deformation was non coaxial but with a flattening type of strain with 1 2 the degree of non coaxiality varied both in space and time from the progressive development of mesoscopic structures it is concluded that the 2 3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale when a new set of shear lenses was superimposed on already sheared rocks the preexisting foliation generally lay at a low angle to the lenses no new folds developed where the acute angle was sympathetic to the sense of shear displacements where the acute angle was counter to the sense of shear the pre existing foliation lying in the instantaneous shortening field was deformed into a set of asymmetric folds

ideas about the surface runoff features on mars in their report evidence of recent groundwater seepage and surface runoff on mars 30 june p 2330 1 malin and edgett propose that the seepage and flow of water may have occurred on mars in the recent geologic past on the basis of the observation of a large number of channel features on

the enigma of prokaryotic life in deep hypersaline anoxic basins deep hypersaline anoxic basins in the mediterranean sea are a legacy of dissolution of ancient subterranean salt deposits from the miocene period our study revealed that these hypersaline basins are not biogeochemical dead ends but support in situ sulfate reduction methanogenesis and heterotrophic activity a wide diversity of prokaryotes was observed including a new abundant deeply branching order within the euryarchaeota furthermore we demonstrated the presence of a unique metabolically active microbial community in the discovery basin which is one of the most extreme terrestrial saline environments known as it is almost saturated with mgcl2 5 m

new pieces to the archaean terrane jigsaw puzzle in the nuuk region southern west greenland steps in transforming a simple insight into a complex regional tectonothermal model in the south of the nuuk region of west greenland our 1980s mapping recognized four archaean gneiss terranes f ringehavn tre brodre tasiusarsuaq and akia terranes with different protolith ages and separate early tectonothermal histories later in the archaean these were juxtaposed and then experienced the same 2700 2500 ma tectonothermal events here we abandon extrapolation of only these four terranes across the whole region and distinguish two new terranes in the ne the northernmost isukasia terrane previously regarded as the northernmost exposure of the f ringehavn terrane consists of palaeoarchaean rocks 3600 ma tectonically bounded on its south by the 3075 2960 ma kapisilik terrane these were juxtaposed and metamorphosed together by 2950 ma the previously recognized f ringehavn terrane to the sw is another separate entity of palaeoarchaean rocks that was juxtaposed with adjacent terranes only after c 2800 ma hence in an increasingly complex regional model there were several mid to neoarchaean terrane assembly events with superimposed orogenies from c 2950 ma until after 2700 ma although the f ringehavn and isukasia terranes were incorporated into the later archaean terrane collage at different times they might be fragments from a larger palaeoarchaean complex rifted apart from c 3500 ma onwards

early proterozoic metallogeny tectonics and geochronology of the singhbhum cuu belt eastern india abstract the singhbhum thrust belt hosts three types of mineralisation sulphides of copper and other metals uranium oxides and apatite magnetite regional geological investigations reveal that the thrust belt and the mineralisations evolved through several distinct geological episodes 1 the oldest is the deposition of the protoliths of the older metamorphic group their subsequent deformation and metamorphism and the post kinematic intrusion of the singhbhum granite the older metamorphic orogeny 3 15 3 0 ga the chotanagpur gneiss to the north is also thought to have originally belonged to this period 2 the chotanagpur gneiss and the singhbhum granite together formed an extensive sialic basement of the archaean period which subsequently underwent extension producing a continental basin filled with platform sediments of the iron ore group and deep sea basins on sialic basements filled with flysch like sediments of the singhbhum group this was accompanied by extensive mafic volcanism the dalma volcanics the dhanjori volcanics and the dolerite dyke the sulphide ores were formed as strata bound layers associated with the volcanism of the dhanjori volcanics uranium minerals and phosphates were deposited as sediments or related to later felsic plutonic activity volcanism sedimentation and mineralisation were controlled by rift fractures forming the basins these were later subjected to deformation and metamorphism of the singhbhum orogeny 2 05 2 0 ga the rift fractures developed into thrust belts felsic magmatic activity took place hydrothermal activity along the thrust belt modified the mineralised belt 3 a later thermal event 1 6 ga was imprinted on the rocks of the thrust belt

ediacaran seawater temperature evidence from inclusions of sinian halite abstract seawater temperatures throughout earth s history have been suggested to illustrate a long term cooling trend from nearly 70 c at 3500 ma to around 20 c at 800 ma the terminal neoproterozoic prior to the cambrian explosion is a key interval in evolutionary history as complex multicellularity appeared with the advent of the ediacara fauna these organisms were likely the first that required higher levels of atmospheric and dissolved marine oxygen for their sustainability it is known that most modern macroinvertebrates are intolerant of temperatures in excess of 45 c perhaps more importantly these high seawater temperatures limit the potential of dissolved oxygen and therefore become an integral part of this evolutionary story previously our understanding of seawater temperature during the terminal neoproterozoic comes only from 18 o 16 o and 30 si 28 si ratios ascertained from a limited number of cherts isotopic ratio methods for assessing seawater temperatures are inherently indirect and have a wide range of oscillation however maximum homogenization temperatures th max of primary fluid inclusions in halite provide a direct means of assessing brine temperature and have been shown to correlate well with average maximum air temperatures the oldest halites date to the neoproterozoic lower paleozoic 700 500 ma and ediacaran representatives can be found in sichuan province china which do preserve primary fluid inclusions for analysis via cooling nucleation methods we utilized halite samples from the changning 2 well correlative to the dengying formation 551 542 ma to provide a direct assessment of terminal neoproterozoic seawater temperature our measurements indicate that seawater temperatures where these halites formed are highly similar to tropical phanerozoic seawater temperature estimates from compiled paleotemperature data the decline in seawater temperatures over the course of the proterozoic accompanied by the reduction of seawater salinity with the sequestration of salt in massive halite deposits in the neoproterozoic allowed the ocean system to accumulate more dissolved oxygen and potentially paved the way for the evolutionary innovation of complex multicellularity

paleoproterozoic mafic dike swarms of northeast laurentia products of plumes or ambient mantle abstract the occurrence of mafic dike swarms in archean and paleoproterozoic terranes is often cited as evidence for mantle plume activity associated with abnormally high mantle potential temperatures t p quantitative modeling of rare earth element ree abundances for the 2 04 ga kang miut dike swarm in west greenland and comparisons among other paleoproterozoic dike swarms in northeast laurentia indicates that the conditions for mantle melt generation were similar beneath much of laurentia and only ca 120 c hotter than present day ambient mantle t p ca 1300 c these thermal conditions correspond closely to estimates of ambient mantle potential temperatures during the paleoproterozoic era based on secular cooling models and continental freeboard constraints thus the geochemistry of most paleoproterozoic dikes of northeast laurentia can be accounted for by partial melting of upwelling mantle accompanying passive rifting and do not necessitate plume activity ambient mantle conditions during the paleoproterozoic t p ca 1420 c are similar to those inferred for the paleogene volcanic rift margins of the north atlantic igneous province where the rate of crustal production associated with early oceanic spreading was two to three times higher than at the modern ridge likewise rifting under ambient mantle conditions in the precambrian could have resulted in similar rates of crustal productivity and thus contributed significantly to continental growth

early paleoproterozoic magmatism in the yangtze block evidence from zircon u pb ages sr nd hf isotopes and geochemistry of ca 2 3 ga and 2 1 ga granitic rocks in the phan si pan complex north vietnam abstract our understanding of the early evolution of the yangtze block is limited by the sparsely dispersed nature of pre neoproterozoic exposures new integrated petrographic zircon u pb age and hf nd isotope analyses and whole rock geochemical data for early paleoproterozoic granites in the phan si pan complex provides new insights into the evolution of the yangtze block as well as its role in the pre nuna supercontinent la icp ms zircon u pb dating of magmatic zircons from quartz monzonite and gneissic granite yielded 207pb 206pb ages of 2306 12 ma and 2096 15 ma respectively zircons from the quartz monzonite have ehf t values ranging from 4 1 to 2 1 corresponding to tdm2 model ages of 3002 2890 ma whereas zircons in the gneissic granite have ehf t values between 0 95 and 1 72 and corresponding tdm2 model ages of 2660 2516 ma which are consistent with their whole rock nd isotope values geochemically the quartz monzonites are i type granites combined with their relatively high sr y ratios and low y concentrations as well as fractionated ree patterns with relatively high lree but low hree concentrations they were probably generated by partial melting of the thickened middle lower crust under elevated temperature geochemical and isotopic signatures suggest that the ca 2 1 ga gneissic granites are high k calc alkaline ferroan a type granites formed by partial melting of juvenile crustal source at high temperature and low pressure with little involvement of ancient crustal material the phan si pan complex has a distinct early paleoproterozoic crustal evolution history compared with the other crustal provinces of the yangtze block suggesting independent histories that were not unified until the late paleoproterozoic during the assembly of nuna moreover the magmatism and tectonic evolution of the north vietnam region is broadly similar to that of the arrowsmith orogen of the rae craton in laurentia suggesting a potential spatial linkage the geologic record of the yangtze block does not support an early paleoproterozoic shutdown of plate tectonics

eutectic brines on mars origin and possible relation to young seepage features abstract if the recently observed young gullies on mars represent seeps or outflows of aqueous fluid that fluid must be highly concentrated brine to be even moderately stable under current martian surface conditions such brines are to be expected in the megaregolith if an initial saline hydrosphere underwent evapoconcentration via water escape from the atmosphere evolved into subsurface cacl 2 enriched brines by chemical interaction with mafic rocks and then froze until eutectic compositions were reached although most such brines would tend to sink deep into the megaregolith they could be preserved locally at relatively high elevations as aquifers or frozen paleoaquifers perched between ground ice and crystalline salts

comment to paper timing of magmatism and metamorphism in the gruinard bay area of the lewisian gneiss complex comparison with the assynt terrane and implications for terrane accretion by g j love p d kinny and c r l friend contr mineral petrol 2004 146 620 636

metallogeny of south greenland a review of geological evolution mineral occurrences and geochemical exploration data abstract south greenland has been the site of historic mining of cryolite copper graphite and gold hosts mineral deposits with gold uranium zinc niobium tantalum zirconium hafnium ree iron titanium vanadium fluorite and graphite and has additional potential for lithium beryllium phosphorus gallium and thorium data from stream sediment geochemical surveys document that south greenland is enriched in a range of these elements relative to the rest of greenland and to estimates of the upper crust composition distribution patterns for individual elements within south greenland exhibit enriched regions that are spatially related to lithological units crustal structure and known mineralisation the northern domain of south greenland includes the southernmost part of the orthogneiss dominated north atlantic craton orogenic gold mineralisation is hosted by quartz veins and hydrothermally altered rocks associated with shear zones intersecting the mesoarchaean tartoq group of mafic metavolcanic rocks geochemical exploration indicates that additional potential for gold mineralisation exists within palaeoproterozoic supracrustal rocks overlying the archaean basement rocks formed during the palaeoproterozoic ketilidian orogeny occupy a major part of south greenland and has been divided into two domains the central domain is underlain by the julianehab igneous complex forming a 100 km wide ene wsw zone centrally across south greenland intrusive and extrusive mostly felsic magmatic rocks were emplaced in two main stages 1850 1830 and 1800 1780 ma in a continental arc setting positive anomalies in aeromagnetic data indicate that mafic plutons are common in the late igneous complex intra arc mafic metavolcanic rocks contain syngenetic stratabound copper sulphide and epigenetic shear zone hosted copper silver gold mineralisation at kobberminebugt and kangerluluk whereas metasedimentary and metapyroclastic rocks contain stratabound uraninite mineralisation orthomagmatic iron titanium vanadium mineralisation is hosted by a gabbro a potential for porphyry type mineralisation related to the late intrusive stages of the julianehab igneous complex is suggested by showings with copper molybdenum and gold together with stream sediment anomalies for these elements vein type uranium mineralisation occurs in fault zones in the julianehab igneous complex related to mesoproterozoic rifting the southern domain contains an assemblage of palaeoproterozoic metasedimentary and metavolcanic rocks that underwent moderate to strong deformation peak ht lp metamorphism and partial melting with subsequent retrograde exhumation at 1790 1765 ma the supracrustal rocks contain syngenetic au as sb u and zn mineralisation in volcanic or graphite and sulphide rich sedimentary environments graphite was mined historically at two sites many stream sediment gold anomalies are located in a ne trending belt along the boundary between the early julianehab complex and the supracrustal rocks to the south they reflect a number of auriferous quartz vein occurrences including the nalunaq gold deposit hosted in a system of shear zones and probably generated as orogenic gold during ketilidian accretion the 1755 1730 ma a type ilua plutonic suite is the latest magmatic event in the ketilidian orogen the 1300 1140 ma gardar period involved continental rifting sedimentation and alkaline magmatism numerous dykes and 10 ring shaped intrusion complexes were formed across south greenland an orthomagmatic iron titanium vanadium deposit is hosted by troctolitic gabbro residual magmas and fluids resulting from extreme magmatic differentiation possibly combined with assimilation of older crust created mineral deposits including cryolite that was mined at ivigtut large low grade deposits of uranium rare earth elements zinc at kvanefjeld and tantalum niobium rare earth element zirconium at kringlerne in the ilimaussaq complex as well as tantalum niobium rare earth elements at motzfeldt so in the igaliko complex the south greenland crustal evolution records effects of mantle processes such as lithospheric extension subduction and underplating which resulted in recurrent magma emplacement in tectonically active environments as such the geology of south greenland reflects events and circumstances that are favourable to the generation and preservation of hydrothermal ore forming fluid systems during the ketilidian orogeny as well as to the development of extreme rock compositions within the gardar alkaline igneous province

timing of magmatism and metamorphism in the gruinard bay area of the lewisian gneiss complex comparison with the assynt terrane and implications for terrane accretion reply u pb zircon data from selected gneiss samples provide estimates of the timing of igneous protolith emplacement and granulite facies metamorphism in the gruinard bay badcall bay and lochinver areas of the mainland lewisian gneiss complex of north west scotland zircons from the gneisses contain complex zoning patterns making them amenable to shrimp ion microprobe analysis with inherited zircon distinguishable from magmatic growth and metamorphic recrystallisation from gruinard bay a tonalitic granulite yielded a protolith age of 2 825 8 ma for oscillatory zoned zircon cores with recrystallised low u rims preserving an age of 2 733 12 ma an inherited 2 905 15 ma age component was recognised in a sample of trondhjemitic gneiss the protolith of which is estimated to have been emplaced at 2 858 11 ma low u rims yielding an age of 2 729 20 ma are also present which together with the rim age from the granulite are considered to date granulite facies metamorphism in the gruinard region for comparison with the gruinard bay gneisses a trondhjemitic partial melt and trondhjemite sheet were analysed from the granulite facies rocks at badcall bay in the assynt terrane age data do not show definitive relationships with zoning patterns in the zircons and show a spread along concordia typical of previous ion microprobe age data from the scourie gneisses both samples do however show a significant isotopic disturbance at 2 480 ma the most recent estimate for the badcallian metamorphism protolith ages for components of the assynt terrane range between ca 2 900 and 3 030 ma and appear to be older than the 2 860 2 825 ma gneiss protoliths from the gruinard bay area the timing of granulite facies metamorphism also appears to be different between the two areas suggesting that the two granulite regions of the mainland may not be related as previously thought instead they are interpreted in terms of a terrane accretion model whereby the two areas evolved as separate blocks prior to their juxtaposition during the palaeoproterozoic the boundary between the two terranes is thought to lie to the south of lochinver corresponding with the strathan line because of the observed 2 480 ma age signature to the north and 2 730 ma signature to the south by defining a new mainland terrane the gruinard terrane the number of mainland blocks which includes the assynt central rhiconich northern and southern terranes has been expanded

pixe and microthermometric analyses of fluid inclusions in hydrothermal quartz from the 2 2ga ongeluk formation south africa implications for ancient seawater salinity abstract the ongeluk formation mainly consists of submarine volcanics pillow lavas and sheet flows composed of basaltic andesites that are between underlying and overlying glaciogenic deposits i e the makganyene diamictite and a dropstone layer at the base of the hotazel formation respectively the stratigraphic position of the ongeluk formation indicates that the ongeluk volcanism occurred during a period of global glaciation the ongeluk volcanic rocks are host to subseafloor hydrothermal quartz deposits as drainage cavities and interpillow voids the hydrothermal quartz contains many primary type 1 and secondary type 2 liquid vapor fluid inclusions as well as inclusions that are randomly distributed without a trace of secondary healed cracks type 3 all these fluid inclusions types were individually analyzed with microthermometry and particle induced x ray emission pixe methods the results show that type 1 fluid inclusions are highly saline whereas type 2 fluid inclusions are relatively less saline type 3 fluid inclusions have bimodal peaks of salinity corresponding to those of type 1 and type 2 fluid inclusions and thus appear to represent mixtures between latter two inclusion types among the various fluid inclusion salinities of fluid inclusions a wide range in na ca values was identified in the high salinity fluid inclusions i e type 1 and a subset of type 3 which are thought to represent subseafloor fluids circulated by the ongeluk submarine volcanism the wide range in na ca values can be explained by a mixing process between na rich and ca rich fluids associated with albitization of the host basaltic andesites in the albitization process na is removed from the fluids and fixed by the host rocks by contrast ca is released from the host rocks into the fluids pixe analysis also showed two distinct trends i e vertical and horizontal on variation diagram of ca cl versus cu cl and positive correlations between mn cl cu cl zn cl and pb cl these data are best explained by the presence of other mixing processes between 1 a na rich ca and cu poor fluid 2 a ca rich na and cu poor fluid and 3 a na and cu rich ca poor fluid the cu mn zn and pb rich fluid likely represents a high temperature hydrothermal fluid from a deep reaction zone in the ongeluk subseafloor hydrothermal system by contrast the 1 na and k rich ca and cu poor endmember for the high salinity primary inclusions is considered to represent the composition of 2 2 ga ongeluk seawater we propose the estimation of 2 2 ga ongeluk seawater composition is 2237 mmol kg of na 200 1000 mmol kg of k

clues to martian brines based on halogens in salts from nakhlites and mer samples 1 chlorine and br abundances in fracture filling secondary salts in nakhla veins determined in this study by synchrotron x ray microprobe br and electron microprobe cl techniques compare well to the halogens determined recently by apxs instruments in soils and rock rinds at the gusev and meridiani sites the salts in these mars rocks arise from martian brines that have undergone evaporative or freezing concentration the halogen abundances in these salts are corrected for dilution effect because of mixing with halogen poor phases to obtain their true abundances which seem to be close to the halogen abundances in terrestrial sea brines consistent with the petrographic evidence for evaporative salt deposition sequence of carbonate sulfate halite in nakhlite meteorites nakhla veins yield high br 250 ppm and low cl br ratios 10 50 suggesting possible salt deposition from concentrated brines whereas lafayette iddingsite with low br 11 ppm and high cl br ratios 250 300 indicates salt deposition from dilute brines on mars the low cl br ratios 20 to 80 in salts from gusev and meridiani rock rinds indicate that they presumably originate from concentrated subsurface martian brines the high cl br ratio 270 in salts deposited by dilute solutions in lafayette and adirondack is similar to the chondritic cl br ratio which seems to characterize the dilute brines on mars prior to halite precipitation including early mars waters noachian or earlier our results seem to be consistent with the burt knauth model involving density stratification during evaporation freezing of brine solutions in martian regolith

high precision u pb dating of complex zircon from the lewisian gneiss complex of scotland using an incremental ca id tims approach abstract a novel approach of thermally annealing and sequentially partially dissolving single zircon grains prior to high precision isotope dilution thermal ionization mass spectrometry id tims is presented this technique is applied to complex zircon from the precambrian lewisian gneiss complex of scotland up to six partial dissolutions were conducted at incrementally higher temperatures and analysed at each successive step id tims analyses reveal the portions of zircon affected by the lowest temperature partial dissolution step have suffered pb loss successively higher temperature partial dissolution steps yield a series of analyses from the younger domains followed by mixing trajectories with older components presumably from the inner domains specifically for a partially retrogressed granulite tonalite gneiss from the central block assynt high grade metamorphic zircon ages of c 2500 ma and c 2700 ma are resolved with a protolith age of c 2860 ma also recognised this unequivocally demonstrates two separate episodes of high grade metamorphism affected rocks from this region the c 2700 ma age provides a minimum age constraint on the highest pressure event known from archean crustal rocks using this technique of pseudo spatial resolution coupled with high precision analysis it is possible to recognise discrete pb loss and multiple stages of zircon growth or isotopic resetting within single grains to within 0 1 0 2 error 2 on individual 207pb 206pb ages this method has relevance to u pb zircon geochronology where conventional micro beam techniques are unable to resolve between separate ages within single grains

significance of crustal scale shear zones and synkinematic mafic dykes in the nagssugtoqidian orogen sw greenland a re examination the published accounts of the structural geology and tectonic evolution of the early proterozoic nagssugtoqidian orogen sw greenland require modification in several fundamental respects the geometry of the orogen has been attributed to important displacements on major conjugate shear zones and thrust zones from new field observations we are unable to confirm that crustal scale shear zones have played an important role in nagssugtoqidian tectonics the dextral component of conjugate shear zones is demonstrably sinistral and transpressive strike slip shear zones are incipient and a major thrust zone within the orogen cannot be confirmed at its projected inland location the sinistral strike slip nordre stromfjord shear zone is an order of magnitude smaller than previously thought it comprises an array of non linked segments of annealed mylonite and cannot have accommodated large displacements the dextral itivdleq shear zone is characterised by heterogeneous sinistral noncoaxial flow but compared with deformation elsewhere in the orogen it is not a large scale zone of strain localisation ikertoq thrust zone is not an orogen scale shear zone but appears to be part of a large scale rheological boundary application of an en relais fracture array model to what is classically identified as the kanga miut mafic dyke swarm suggests that the sinistral vorticity determined throughout much of the nagssugtoqidian orogen is detectable in the archean foreland up to 150 km south of the orogenic front

u pb ages of syndeformational dykes associated with the mesoproterozoic nain plutonic suite labrador field and u pb zircon geochronological studies in a suite of crosscutting deformed and metamorphosed basic dykes show that they were emplaced in a syndeformational environment between 1328 2 and 1316 5 1 6 ma earlier dykes in the suite are granulitic commonly folded along north south axes or strongly rotated into north south trending shear zones later dykes are metamorphosed to amphibolite grade these sometimes retain igneous porphyritic textures and occasionally crosscut the granulite dykes the amphibolite dykes were emplaced in a less ductile crustal environment and a structural environment less dominated by shear deformation both granulitic and to a lesser extent amphibolite dykes show evidence of synshear emplacement the episode of emplacement is broadly coeval with the injection of the mid proterozoic nain plutonic suite nps and suggests that the dykes were injected as a series of leaks from nps feeder chambers if so the granulite dykes may represent leaks intruded near the culmination of nps activity whereas the amphibolite dykes were probably injected during the waning phase of tectono thermal conditions

filling in the juvenile magmatic gap evidence for uninterrupted paleoproterozoic plate tectonics abstract despite several decades of research on growth of the continental crust it remains unclear whether the production of juvenile continental crust has been continuous or episodic throughout the precambrian models for episodic crustal growth have gained traction recently through compilations of global u pb zircon age frequency distributions interpreted to delineate peaks and lulls in crustal growth through geologic time one such apparent trough in zircon age frequency distributions between 2 45 and 2 22 ga is thought to represent a pause in crustal addition resulting from a global shutdown of magmatic and tectonic processes the 2 45 2 22 ga magmatic shutdown model envisions a causal relationship between the cessation of plate tectonics and accumulation of atmospheric oxygen over the same period here we present new coupled u pb hf and o isotope data for detrital and magmatic zircon from the western churchill province and trans hudson orogen of canada covering an area of approximately 1 3 million km2 that demonstrate significant juvenile crustal production during the 2 45 2 22 ga time interval and thereby argue against the magmatic shutdown hypothesis our data is corroborated by literature data showing an extensive 2 22 2 45 ga record in both detrital and magmatic rocks on every continent and suggests that the operation of plate tectonics continued throughout the early paleoproterozoic while atmospheric oxygen rose over the same time interval we argue that uninterrupted plate tectonics between 2 45 and 2 22 ga would have contributed to efficient burial of organic matter and sedimentary pyrite and the consequent rise in atmospheric oxygen documented for this time interval

meso and neoarchaean geological history of the bjornesund and ravns storo supracrustal belts southern west greenland settings for gold enrichment and corundum formation to understand the mesa and neoarchaean geological history of the bjornesund region southern west greenland ca 62 63 degrees n we have undertaken an integrated study combining field observations with u pb zircon geochronology by la icp ms metamorphic petrology and microstructural analyses the bjornesund region links the southern part of the fiskenaesset anorthosite complex to the well preserved metavolcanic rocks of the ravn story supracrustal belt and the bjornesund supracrustal belt so the development of this region is of key interest to unravelling the archaean geology of the entire region in this study we couple the observed phases of gold enrichment and corundum formation to the geological history the leucogabbroic rocks of the bjornesund region which are part of the fiskenaesset complex intruded at ca 2 95 ga into the tholeiitic and calc alkaline metavolcanic rocks of the two studied belts following isoclinal folding the rocks were intruded by diorite 2 92 ga and granodiorite 2 91 and 2 88 ga which are now texturally orthogneisses the area was affected by three major phases of folding f1 undated f2 at 2 85 2 83 ga and f3 at 2 71 2 70 ga the former deformation event is associated with the gold enrichment in alteration zones in shear folds in metavolcanic amphibolite near the contact with the orthogneiss intrusion of granitic sheets during the f3 deformation triggered the formation of corundum which is locally of gem quality at the contact between leucogabbroic and ultramafic rocks peak metamorphic conditions were calculated as 580 630 degrees c and 6 4 kbar at 2 85 2 83 ga later retrogression in the southern part of the area at ca 500 450 degrees c and 4 5 3 kbar and extensional fault reactivation overprinted some of the older structures c 2014 elsevier b v all rights reserved less

john joly 1857 1933 and his determinations of the age of the earth abstract john joly 1857 1933 was one of ireland s most eminent scientists of the late nineteenth and early twentieth centuries who made important discoveries in physics geology and photography he was also a respected and influential diplomat for trinity college dublin and various irish organizations including the royal dublin society measuring the age of the earth occupied his mind for some considerable time a problem he was to address using a diverse range of methods his sodium method of 1899 for which he is best known was hailed by many as revolutionary but it was later superseded by other techniques including the utilization of radiometric dating methodologies although joly himself carried out much research in this area he never fully accepted the large age estimates that radioactivity yielded nevertheless joly s work in geochronology was innovative and important for it challenged earlier methods of arriving at the earth s age particularly those of lord kelvin although his findings and conclusions were later discredited he should be remembered for his valuable contribution to this important and fundamental debate in the geological sciences

age and origin of early proterozoic dolerite dykes in south west greenland samples of three dolerite dykes from south west greenland give a pooled weighted mean rb sr whole rock isochron age of 2 130 65 ma initial 87sr 86sr ratios are 0 70155 0 00018 for one of the dykes naujat and 0 70277 0 00012 for the other two torssut and akinaq a fourth dyke qaqarssuaq did not yield an isochron torssut samples have significantly lower initial 207pb 204pb ratios than samples from the naujat dyke indicating that the torssut dyke has been affected by crustal contamination

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

oxygen and stable carbon isotopes from a nautiloid from the middle pennsylvanian late carboniferous impregnation lagerstatte buckhorn asphalt quarry primary paleo environmental signals versus diagenesis abstract two shell fragments one of an undetermined cephalopod and one of an orthoconic nautiloid pseudorthoceratidae gen et sp indet from the pennsylvanian buckhorn asphalt lagerstatte oklahoma were studied to reconstruct paleotemperatures and to explore possible diagenetic alterations for the first time not only parts of the shell of an orthoconic nautiloid but also its cameral deposits were sampled in high resolution the mineralogy geochemistry and microstructure of both shell remains the pseudorthoceratid and the undetermined cephalopod were investigated in detail before high resolution transects for oxygen and stable carbon isotopes were sampled the shell fragment of the undetermined cephalopod showed no or very little diagenetic alteration therefore the measured 18o variation 18o range of 0 68 and the inferred temperature range of 28 to 31 c are interpreted to represent an original signal the temperatures indicate that the specimen lived in shallow warm tropical waters the 13c signal with a 13c range of 3 43 is interpreted to be the consequence of a strong metabolic effect a septum of the phragmocone of the pseudorthoceratid nautiloid was only slightly altered by diagenesis and a largely original 18o signal could be obtained the 18o values exhibit an inferred temperature range of 14 to 15 c indicating that this species lived in deeper and cooler waters the cameral deposits were investigated in detail by energy dispersive x ray edx and by xrd gadds x ray diffraction general area detector diffraction system analyses part of the cameral deposits in the orthoconic nautiloid turned out to originally consist of high mg calcite hmc instead of the estimated aragonite anyhow the massive cameral deposits show signs of strong small scale alteration resulting in a distinct spreading of oxygen values the oxygen and carbon isotope plots show a linear trend for the oxygen carbon correlation this is interpreted as the mixing of two end members with the lightest end members suggesting a meteoric diagenesis with a diagenetic fractionation of the aragonite to low mg calcite lmc respectively the hmc to lmc and dolomite the calculated temperatures of the pseudorthoceratid specimen and the undetermined cephalopod indicate two different paleoenvironments additionally the pseudorthoceratid nautiloid displays the narrow range of temperatures in the deeper parts of the sea and the undetermined cephalopod the more fluctuating temperatures in the shallow areas it is also shown how important a detailed investigation of microstructures and mineralogy of samples with an according cautious interpretation of the achieved dataset is the importance of an intensive examination of samples is pointed out to avoid small scale alterations to be overlooked since this would lead to a misinterpretation of the results

a reappraisal of the lewisian gneiss complex geochronological evidence for its tectonic assembly from disparate terranes in the proterozoic new u pb single zircon geochronology undertaken on tonalitic gneisses granite sheets migmatites and metasediments from the lewisian gneiss complex on the mainland and the northern part of the outer hebrides nw scotland have been used to test the correlation of so called laxfordian events across the complex from the outer hebrides to the mainland and the current model for the evolution of the complex as a whole the study has revealed that the granite sheets originated in two quite different melting events those on the mainland at loch laxford are ca 1 855 ma old whereas those on harris and lewis with which they are presently correlated are ca 1 675 ma old grey gneisses associated with granites on the south side of loch laxford are confirmed to belong to the northern region a migmatitic grey gneiss on harris has given a protolith age of ca 3 125 ma the currently oldest recognised in the complex detrital zircons in the leverburgh and langavat belts range in age from 2 780 to 1 880 ma and unequivocally demonstrate deposition in the palaeoproterozoic the granulite facies metamorphism in this block is dated from zircon overgrowths at ca 1 880 ma the laxford shear zone which separates the northern and central regions is interpreted to have evolved post 1 860 ma during amphibolite facies metamorphism accompanying deformation which took place at ca 1 740 ma in both regions on harris the langavat finsbay shear zone developed after 1 675 ma when a ca 1 880 ma granulite facies proterozoic arc was juxtaposed against amphibolite facies archaean rocks to the north therefore the shear zones which bound tectonic blocks in the lewisian complex evolved at different times and can be interpreted as terrane boundaries the new data confirm that the lewisian complex was not constructed from one contiguous piece of archaean crust reworked in the proterozoic but was progressively assembled from several discrete terranes during the proterozoic accordingly the former regional divisions of the lewisian complex are here renamed as follows on the mainland the northern region is called the rhiconich terrane and the central region the assynt terrane on the outer hebrides the archaean gneisses of lewis and the northern part of harris comprise the tarbert terrane whereas the newly accreted proterozoic blocks are called the roineabhal terrane in harris and the niss terrane in the north on lewis wider correlations show that the geology of the outer hebrides has more in common with east greenland than mainland scotland on the eastern side of the minch fault

a mohr circle construction for the opening of a pre existing fracture abstract the opening of a fracture is dependent on the fluid pressure and the local stress regime around the fracture for a fracture to open the fluid pressure must exceed the normal stress acting on the fracture the mohr circle provides a useful tool allowing the visualization of these relationships this construction determines the range of fracture orientations that are able to open as well as determining the opening direction of these fractures this is then used to produce ratios of the magma pressure and the principal stresses for some dykes from the lizard ophiolite u k

early proterozoic mafic dykes in the north atlantic and baltic cratons field setting and chemistry of distinctive dyke swarms

geochemical evolution and unusual pyroxene chemistry of the md tholeiite dyke swarm from the archaean craton of southern west greenland

u pb baddeleyite ages and geochemistry of dolerite dykes in the bas draa inlier of the anti atlas of morocco newly identified 1380 ma event in the west african craton in the bas draa inlier anti atlas morocco the paleoproterozoic basement which is cut by the ediacaran taourgha granite is also crosscut by numerous dykes of a variety of trends mostly of uncertain age two doleritic dykes are dated by the id tims u pb method on baddeleyite and yield emplacement ages of 1381 8 ma mswd 0 84 and 1384 6 ma mswd 1 4 determined for a n135 degrees e and a n40 degrees e trending dyke respectively these dates represent the first geochronological evidence of a mesoproterozoic magmatic event in the anti atlas this magmatic event falls in the previously considered ca 1 7 1 0 ga mesoproterozoic gap in geological activity in the anti atlas the poorly dated taghdout and taarotihate sequences could represent remnants of the ca 1380 ma magmatism and rift related sedimentation the mesoproterozoic sedimentary succession of the atar group in the taoudeni basin mauritania could also represent a good candidate for rift related sedimentation but it postdates the 1380 ma magmatic event by 270 ma the dated 1380 ma dykes are transitional to mildly alkaline basalts not unlike some hawaiian lavas however these dykes have a distinct negative nb anomaly a common features in many large igneous provinces lips and this requires interaction with the lithosphere this interaction may have occurred at the level of the lithospheric mantle or the crust these newly dated 1380 ma dykes may converge to the north speculatively suggesting a magmatic center associated with a 1380 ma mantle plume along the northern margin and possibly linked to rifting and possible breakup on that margin and also to a regional uplift that largely removed the evidence of a 1380 ma cover sequence contemporaneous 1380 1390 ma magmatism is reported elsewhere on other crustal blocks and that in northeastern laurentia northern greenland northern siberia anabar shield and baltica southern urals can be reconstructed with that of the bas draa inlier anti atlas region of the west african craton wac into a single large igneous province lip extending over an area of 1 million km 2 and associated with the final fragmentation of the columbia nuna supercontinent c 2012 elsevier b v all rights reserved

1 99 ga mafic magmatism in the rona terrane of the lewisian gneiss complex in scotland abstract the scourie dyke swarm has long been important to unravelling the geological history of the lewisian gneiss complex lgc and the north atlantic craton recent dating has documented that the majority of those dykes were emplaced between c 2418 2375 ma here we show that a quartz dolerite dyke in the rona terrane of the lgc has a u pb zircon age of 1989 08 4 3 0 99 ma this is the first mafic dyke to be dated from the rona terrane the southernmost of those proposed for the terrane model of the lgc our new age also overlaps with the c 1992 ma age of a previously dated olivine gabbro dyke in the northern region of the lgc and shows that the lgc contains at least three and possibly four temporally discrete episodes of palaeoproterozoic mafic magmatism the scourie dyke swarm sensu stricto at c 2 4 ga a suite of younger dykes at c 1 99 ga referred to here as the strathan dyke swarm and the c 1 99 1 90 ga mafic rocks that are part of the palaeoproterozoic loch maree group a c 2 04 ga dyke in the assynt terrain may be part of the younger suite of dykes or perhaps records a temporally separate event crucially our new age data demonstrate that suites of mafic dykes emplaced across the mainland lgc are similar in age and supports the correlation of structural and metamorphic features across that complex

composition and evolution of the early oceans evidence from the tagish lake meteorite abstract laboratory leaching studies of the tagish lake meteorite demonstrate that the most readily leached cations of carbonaceous chondrite meteorites are mg ca na and k with the most readily leached anions being so4 cl and po4 in decreasing order of abundances soluble organics were not analyzed due to limited sample sizes magnesium and sulfate the most abundant solutes leached from tagish lake meteorite are also the most abundant ions of salts lining fractures of aqueously altered meteorites these same ions probably were the most abundant solutes of the first permanent oceans which probably formed during or soon after the late heavy bombardment stage 4 2 to 3 7 ga considering the residence time of cl in modern seawater and the composition of the exposed continental crust to at least 3 3 ga evolution of seawater from mg so4 dominated to nacl dominated would have required about 3 105 yr and could not have been complete until about 3 0 to 3 3 ga the calculations also suggest that the first permanent oceans were more saline than modern seawater and as argued by others these aspects have significant implications for evolution of life many solar system bodies besides earth probably derived a significant proportion of their volatile and soluble constituents from carbonaceous chondrite material therefore primordial solutions similar to those presented here were likely important in the early evolution of aqueous systems throughout the solar system including those present on carbonaceous chondrite parent asteroids and icy outer solar system satellites

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 and u pb geochronology of easterly trending dykes in the dharwar craton india feldspar clouding radiating dyke swarms and the position of india at 2 37 ga abstract a u pb baddeleyite age of 2367 1 ma from a diabase dyke together with previously published age data suggests that a major early proterozoic dyke swarm cuts across the structural grain of the archean dharwar craton in india paleomagnetic data suggest the swarm is at least 300 km wide and 300 km long and has a fan angle of at least 30 with convergence to the west it was originally emplaced at high latitudes and together with the widgiemooltha dykes of the yilgarn block of australia may have been a segment of a larger radiating swarm related to a long lived plume event that was active for about 50 my from 2418 to 2367 ma a regional change in the intensity of brown feldspar clouding in the dykes suggests that the dharwar craton was tilted northwards in harmony with previous observations on the structure and metamorphism of the archean rocks towards the south the brown feldspar clouding becomes more intense and locally assumes a blacker more sooty appearance the black clouding whose precise origins remain unknown is accompanied by a remagnetization and appears to coincide closely with a region of carbonatite magmatism at 800 ma and with a shear zone and change in structural trend related to pan african deformation at 550 ma paleomagnetic studies suggest that i the high coercivity part of the remanent magnetization is carried by magnetite exsolved within either brown or black clouded feldspars and ii as a more general observation diabase with brown clouded feldspar can carry a primary magnetization but not if the clouding is black

low δ18o rocks in the belomorian belt nw russia and scourie dikes nw scotland a record of ancient meteoric water captured by the early paleoproterozoic global mafic magmatism abstract the 2 45 2 38 ga intrusions and their host rocks in the belomorian belt of nw russia and the scourie dikes of the lewisian complex in nw scotland are marked by extremely low 18o values their isotope signatures reflect high temperature exchange between rocks and low 18o meteoric waters representing a record of active hydrologic cycle early in the earth s history here we explore triple oxygen and hydrogen isotope systematics d 18o and 17o of these rocks to evaluate the isotope composition of meteoric water the new occurrences from belomorian belt range in 18o from 14 to 2 combined with previously reported data ranging between 27 and 3 the low 18o rocks are traced across 500 km of the belomorian belt based on spatial distribution of the values hydrothermal alteration driven by the emplacement of the mafic intrusions is the most consistent explanation for the low 18o values recorded in the belomorian belt the 17o systematics reveals contribution of distinct meteoric waters with 18o of 38 and 9 recorded at different localities across the belt we also present new data for the 2 41 2 38 ga scourie dikes with the lowest 18o value of 2 5 recorded in the amphiboles of loch na h dike unlike the belomorian belt the low 18o signature of scourie dikes requires igneous incorporation of low 18o hydrothermally altered rocks through contamination of mantle derived melts the 17o data analyzed at the loch na h dike reveals that the incorporated material experienced water rock interaction with meteoric water that had 18o of 35 10 the exact mechanism for incorporation of such low 18o rocks in the mafic scourie dike melts is equivocal and could involve subduction or assimilation of low 18o hydrothermally altered mafic rocks these reconstructed isotope compositions of meteoric waters are diverse and comparable to the modern day precipitation of high latitude regions e g greenland 18o 35 and precipitation of mid latitude regions 9 possibly reflecting spatial 18o gradients resulted from an active hydrologic cycle during the cold climate of the early paleoproterozoic as depicted by previous isotope enabled global circulation models of snowball earth state

precise u pb baddeleyite ages of mafic dykes and intrusions in southern west greenland and implications for a possible reconstruction with the superior craton the archaean block of southern greenland constitutes the core of the north atlantic craton nac and is host to a large number of precambrian mafic intrusions and dyke swarms many of which are regionally extensive but poorly dated for southern west greenland we present a u pb zircon age of 2990 13 ma for the amikoq mafic ultramafic layered intrusion fiskefjord area and four baddeleyite u pb ages of precambrian dolerite dykes specifically a dyke located se of ameralik fjord is dated at 2499 2 ma similar to a previously reported ar 40 ar 39 age of a dyke in the kangamiut area for these and related intrusions of ca 2 5 ga age in southern west greenland we propose the name kilaarsarfik dykes three wnw trending dykes of the md3 swarm yield ages of 2050 2 ma 2041 3 ma and 2029 3 ma a similar u pb baddeleyite age of 2045 2 ma is also presented for a se trending dolerite iglusuataliksuak dyke in the nain province the rifted western block of the nac in labrador we speculate that the md3 dykes and age equivalent nne trending kangamiut dykes of southern west greenland together with the iglusuataliksuak dyke after closure of the labrador sea represent components of a single areally extensive radiating swarm that signaled the arrival of a mantle plume centred on what is presently the western margin of the north atlantic craton comparison of the magmatic barcodes from the nain and greenland portions of the north atlantic craton with the established record from the north eastern superior craton shows matches at 2500 ma 2214 ma 2050 2030 ma and 1960 1950 ma we use these new age constraints together with orientations of the dyke swarms to offer a preliminary reconstruction of the north atlantic craton near the north eastern margin of the superior craton during the latest archaean and early palaeoproterozoic possibly with the core zone craton of eastern canada intervening c 2010 elsevier b v all rights reserved less

noritic dykes of southern west greenland early proterozoic boninitic magmatism two major swarms of early proterozoic ca 2 1 ga basic dykes occur within the archaean craton of southern west greenland one swarm comprises ophitic and sub ophitic tholeiitic dolerites while the other the bn dyke swarm constitutes mainly norites in which pyroxenes and olivine are enclosed by plagioclase oikocrysts the close geochemical similarity between a quenched norite and the coarser grained varieties indicates that the composition of the latter type has not been significantly modified by crystal accumulation the bn dykes are geochemically distinctive most having high mgo ca 16 cr and ni contents in conjunction with relatively high sio2 light rare earth ree and large ion lithophile lil element concentrations the texture mineral chemistry and petrochemistry of the quenched noritic dyke all bear strong resemblances to those features in modern boninitic lavas the bn dykes also correspond to proposed parental liquids of the bushveld complex and other major layered basic igneous intrusions the two dyke swarms are petrogenetically distinct the tholeiitic dolerites were derived from a relatively undepleted primordial mantle while the noritic dykes originated from a metasomatized harzburgitic source the wide spread distribution of similar proterozoic intrusions suggests crustal underplating by harzburgitic mantle on a world wide scale at this time