the development of the central european continental crust since the early archaean based on conventional and ion microprobe dating of up to 3 84 b y old detrital zircons abstract archaean components up to 3 84 b y old have been detected by ion microprobe dating of zircon from various sediments and metasediments of the european hercynides moldanubian of northeastern bavaria and the massif central the early archaean age of 3 84 b y was obtained within the euhedral core of a detrital grain from a moldanubian paragneiss the same detrital zircon grain reveals at least three metamorphic overprints at 2 59 b y b p 1 94 b y b p and 460 m y b p and at least two cycles of mechanical abrasion during surface recycling in the early proterozoic and after the pan african cycle it is thus the crystal with the most complex geological history ever detected further archaean ages for the crystallization of primary magmatic zircons probably in the course of continental crust formation in the first cycle provenances of the two studied areas were found at 3 15 2 9 2 76 and 2 65 b y b p and in a few cases between 2 6 and 2 5 b y b p early proterozoic crust forming events between about 1 75 and 2 15 b y b p play an important role especially for the sediments and metasediments of the montagne noire at the southern rim of the massif central middle and late proterozoic ages cluster around 1 0 b y b p and 600 m y b p for all analysed samples therefore the deposition of the metasedimentary precursors took place after the pan african cycle which is in line with the rb sr whole rock systematics of central european metasediments the ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains the following sequence coinciding with times of magmatic events has been so far established 2 59 1 94 and 1 0 b y b p and 600 m y b p thus ion probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles the ubiquitous presence of pan african detrital zircons in european metasediments is a strong indication that the detrital material was derived from gondwana however from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m y and 330 m y ago it is probable that various gondwana derived microcontinental plates contributed to the different basin fillings the gondwana derived detritus is also supported by the observation of two periods of relative quiescence or non activity around 2 3 b y ago and especially around 1 5 b y ago which is a typical feature observed in gondwana cahen et al 1984 but not in laurasia the other possible source of the detritus the lack of zircons crystallized between 3 84 and 3 15 b y ago might have different causes thus the continental crust of the european hercynides probably developed since the early archaean and grew via many crustal accretion events to its present state

ion microprobe ages of zircons from early archaean granite pebbles and greywacke barberton greenstone belt southern africa abstract we report single grain and grain domain u pb zircon ages for three granitoid clasts from the moodies conglomerate and for detrital zircons from a fig tree greywacke from clastic sediments in the early archaean barberton greenstone belt of southern africa the crystallization ages obtained for the moodies clasts vary between 3570 6 ma and 3518 11 ma while the fig tree zircons show a surprisingly uniform source terrain age of 3453 9 ma that may date the stratigraphically underlying onverwacht group felsic metavolcanics the discordancy patterns are ascribed to a combination of variable pb loss at unspecified times in the past and new zircon growth and or recrystallization the shrimp data record details of isotopic variation even within single zircons that result in precise definition of upper concordia intercept ages our results suggest an age range between 3460 and 3570 ma for the source region of the barberton belt clastic metasediments presumably the ancient gneiss complex of swaziland and felsic volcanics of the overwacht group this source included geochemically evolved granitoids that predate the oldest greenstones by some 100 ma

u pb zircon and sm nd model ages of high grade moldanubian metasediments bohemian massif czechoslovakia we report single grain u pb ion microprobe as well as conventional bulk size fraction ages for zircons from 3 metasediment samples of the moldanubian complex bohemian massif one of the largest crystalline complexes of the hercynian foldbelt in europe these are complemented by whole rock sm nd model ages the metasediments are of upper amphibolite to granulite grade and come from the bory massif in moravia nw of brno sample aa 1 and from the varied group aa 2 and monotonous group aa 3 in the ceske budejovice region of sw bohemia

conventional and ion microprobe u pb dating of detrital zircons of the tentudia group serie negra sw spain implications for zircon systematics stratigraphy tectonics and the precambrian cambrian boundary conventional multi grain and ion microprobe dating of detrital zircons from a very low grade metagraywacke of the tentudia group upper part of the serie negra ossa morena zone sw spain reveals an uppermost vendian age for the deposition of the metagraywacke the youngest detrital zircon grain provides a maximum depositional age of about 565 ma thus these data contradict earlier middle to upper riphean ca 1350 850 ma estimates on the age of the tentudia group and favour a precambrian cambrian boundary falling into the range of 540 to 530 ma the presence of about 20 of pan african detrital zircons ranging from about 700 to 550 ma indicates the derivation from gondwana from the upper intercept ages of the fan shaped data field defined by conventionally determined zircon fractions it can be deduced that 2 1 ga old zircons as well as archean zircons existed in the provenance s of the serie negra sediments this mixing of crustal components of different ages is in line with the nd crustal residence age of 1 9 ga the latter value as well as other model ages of the iberian massif indicates unusually high amounts of ancient crust to be present in the strata this is different to other meta sediments of the european hercynides and suggests that the iberian strata of uppermost precambrian age may contain the detritus of more internal older parts of gondwana than other european strata of comparable ages geochemical data on the analysed sample and further metagraywackes of the tentudia group argue for a deposition in an arc environment such a scenario would conform with the syn to post orogenic shallow marine deposition of the studied sediments furthermore an upper time limit for the pre lower cambrian deformational history including two phases of regional deformation is given by the maximum age of deposition implying a very short time interval for deposition and deformation of the tentudia group concerning the u pb systematics of detrital zircon fractions it is probable that numerous previously published conventional multi grain zircon data on meta sedimentary rocks of the european hercynides readily can be explained by the presence of up to 20 of pan african detrital zircons and later phanerozoic lead loss during metamorphic transformation of the sedimentary protoliths moreover this implies that such metasediments originated from post pan african sedimentary precursors

evidence for an early archean component in the middle to late archean gneisses of the wind river range west central wyoming conventional and ion microprobe u pb data gneissic rocks that are basement to the late archean granites comprising much of the wind river range west central wyoming have been dated by the zircon u pb method using both conventional and ion microprobe techniques a foliated hornblende granite gneiss member from the southern border of the bridger batholith is 2670 13 ma zircons from a granulite just north of the bridger batholith are equant and faceted a typical morphology for zircon grown under high grade metamorphic conditions this granulite which may be related to a second phase of migmatization in the area is 2698 8 ma south of the bridger batholith zircons from a granulite charnockite which is related to an earlier phase of migmatization in the range yield a discordia with intercept ages of about 2 3 and 3 3 ga however ion microprobe analyses of single zircon grains indicate that this rock contains several populations of zircon ranging in age from 2 67 to about 3 8 ga based on zircon morphology and regional geologic relationships we interpret the data as indicating an age of 3 2 ga for the first granulite metamorphism and migmatization older possibly xenocrystic zircons give ages of 3 35 3 65 and 3 8 ga younger zircons grew at 2 7 and 2 85 ga in response to events including the second granulite metamorphism at 2 7 ga that culminated in the intrusion of the bridger batholith and migmatization at 2 67 ga these data support the field and petrographic evidence for two granulite events and provide some temporal constraints for the formation of continental crust in the early and middle archean in the wyoming province

obtaining diagenetic ages from metamorphosed sedimentary rocks u pb dating of unusually coarse xenotime cement in phosphatic sandstone the depositional age of nonfossiliferous metamorphosed sedimentary rocks is commonly bracketed between the age of the youngest detrital mineral and the age of the oldest metamorphic mineral the technique of dating diagenetic xenotime by ion microprobe can provide robust minimum ages for sediment deposition however in most cases xenotime is only a few microns m in size and rarely exceeds 10 m the minimum size for in situ ion microprobe analysis phosphatic sandstone in the greenschist facies mount barren group in southwestern australia contains unusually abundant xenotime occurring as exceptionally coarse 200 m pore filling cement that nucleated on detrital zircon grains the optimum environmental site for the formation of the cement was sand beds within a black shale condensed section analysis of xenotime by sensitive high resolution ion microprobe yields two age populations 1696 7 ma for cement adjacent to detrital zircon grains and 1646 8 ma for outer zones preserved textures show that initial xenotime growth was early diagenetic establishing the ca 1700 ma age as a proxy for the depositional age of the mount barren group the younger age ca 1650 ma is regarded as burial related the xenotime data reduce considerably the previous limits on the age of the succession i e between ca 1850 ma youngest zircon population and ca 1200 ma peak metamorphism the significant achievement of our results is establishing that early diagenetic xenotime retains its physical form and u pb isotopic age despite greenschist facies metamorphism and penetrative deformation

the brabant massif as part of armorica gondwana upb isotopic evidence from detrital zircons u pb isotopic ratios of zircons are used as time dependent geochemical tracers for the formation of the zircons and for the sum of processes and their intensities effective for the disturbance of the u pb systems during the history of the zircons by comparison detrital zircons can thus serve as indicators for the provenance of clastic sediments the u pb systematics of 57 carefully selected fractions of detrital zircons from four cambrian sandstones of the ardennes and the brabant massif have been investigated according to the u pb data features of color and shape of these zircons are suited for distinguishing to major groups 1 1 colourless mainly rounded subordinately subhedral or euhedral zircons which are characterized by u contents below 300 ppm and by apparent 207pb 235u ages between 0 53 ga and 1 65 ga 2 2 reddish generally subrounded to very well rounded zircons with u contents from 240 ppm to more than 800 ppm and apparent 207pb 235u ages between 1 75 ga and 2 4 ga from the group of colourless zircons only one fraction yields concordant u pb data and documents a cadomian panafrican crystal growth at 545 ma u pb data of the other colourless zircons arrange in a fan shaped area in the concordia diagram below concordia and from 530 ma to 600 ma zircons with such u pb characteristics are well known from central and southern europe they suggest a strong cadomian pan african influence the reddish zircons are dominantly derived from archean to early proterozoic crystalline rocks based on these results and on comparison with u pb data of other detrital zircons the ardennes and the brabant massif must be regarded to be coherent to the gondwana continental plate before the caledonian orogeny

two orogenies in the grenville belt the age of the grenvillian event is commonly taken to be 1 000 myr but depending on the isotopic systems used and the rocks examined dates vary from 1 100 myr or more1 to 700 myr ref 2 k ar mineral dates and 40ar 39ar dates reflect blocking temperatures that correspond to regional cooling they should therefore be younger than the true age of a thermal event that would be more accurately dated by rb sr whole rock isochrons or by u pb dating of zircons however because blocking temperature for argon is estimated to be 550 600 c in hornblende3 hornblende dates should at least correspond to the time at which medium to high grade metamorphism was last reached in the rock if systems have remained closed ever since in the grenville province the discrepancy between 1 100 and 950 myr dates recorded by different radiometric clocks has commonly been attributed to a progressive and slow cooling of the region4 i suggest here that the presence of at least two distinct thermal events in that time interval is a plausible explanation

origin and evolution of avalonia evidence from u pb and lu hf isotopes in zircon from the mira terrane canada and the stavelot venn massif belgium laser ablation inductively coupled plasma mass spectrometry u pb and lu hf isotope analyses of detrital zircon from neoproterozoic cambrian clastic sedimentary rocks in the mira terrane cape breton island nova scotia canada west avalonia and the stavelot venn massif east belgium east avalonia support deposition on an originally coherent microcontinent crustal evolution trends defined by hf t values varying with age reflect juvenile magma production in the source continent at 1 2 2 2 and 2 4 3 1 ga mixing of juvenile and recycled crust in continental magmatic arcs is recognized at 0 5 0 72 1 4 1 7 1 8 2 2 and 2 4 2 7 ga these results concur with the crustal evolution in amazonia the likely parent craton crustal evolution in avalonia is recorded in detrital and magmatic zircon from neoproterozoic arcs 680 550 ma positive hf t values suggest juvenile input and mixing with recycled crust most negative hf t values represent recycling of predominantly mesoproterozoic underlying crust avalonian arc magmatism was followed by late neoproterozoic early cambrian sedimentation in various belts in west avalonia these belts were juxtaposed by strike slip during late early cambrian deposition in a rift basin the youngest detrital zircon population c 517 ma probably represents synrift magmatism before break up of avalonia migmatization at 406 2 ma in a xenolith from the east avalonian crust reflects post collisional heating supplementary materials details of sample locations and analytical results are available at www geolsoc org uk sup18641

geodynamic model for the palaeozoic crustal consolidation of western and central europe abstract the crystalline basement of western and central europe consists of a mosaic of crustal elements which were consolidated during pre grenvillian the grenvillian dalslandian morarian cadomian caledonian and hercynian orogenic cycles contemporaneous with the caledonian suturing of the precambrian laurentia greenland and fennosarmatian shields a number of gondwana derived cadomian micro cratons were accreted to the southern margin of laurasia following the late caledonian paroxysm the devonian and early carboniferous evolution of europe was dominated by continued northward subduction of the proto tethys plate at an arc trench system parallelling the southern margin of laurasia the accretion of additional gondwana derived continental fragments back arc rifting and a sinistral translation of major proportions between the laurentia greenland and the fennosarmatian sub plates the acadian and bretonian orogenies were of the pacific type the visean collision of gondwana with laurasia marked the onset of the himalayan type variscan orogeny during which collision related compressive stresses overpowered the devonian early carboniferous back arc rift systems and caused the development reactivation of a subduction zones the central armorican and saxothuringian successor basins became folded and destroyed during the latest visean whilst the variscan foredeep became scooped out in part by nappes involving the basement during the late westphalian major crustal shortening during the variscan diastrophism was accompanied by the anatectic remobilisation of subducted crustal and upper mantle material and a widespread syn and late orogenic magmatism the latest carboniferous early permian reorientation of the convergence direction between gondwana and laurasia induced the development of a complex wrench fault system transecting the variscan fold belt and an extensive post orogenic volcanism the hypotheses summarised here require confirmation by further palaeomagnetic and radiometric data and faunal analyses

deformation and related radiochronology in a late pan african mylonitic shear zone adrar des iforas mali the influence of a deformation on the zircon u pb system was studied using the example of the western margin of the iforas granulitic unit mali where undeformed sub alkaline leptynites have evolved to strongly foliated mylonites and then to ultramylonites the deformation episod was late pan african and occurred at low temperature zircons show evidences of a brittle behavior with microcracks development then fracturation of the crystals and dispersion of the fragments in the ductile matrix an equilibrium grain size was reached which probably depended of the stress but was independant of the initial size of the zircons and of the strain

monazite and xenotime petrogenesis in the contact aureole of the makhavinekh lake pluton northern labrador high temperature 700 900 c metamorphism in the contact aureole of the makhavinekh lake pluton mlp northern labrador led to the growth of monazite and xenotime during progressive replacement of regional garnet bearing assemblages m1 by lower pressure symplectitic coronas of orthopyroxene cordierite m2 in the inner aureole 500 m from the contact where m1 garnet is strongly resorbed high y hree monazite xy hree 0 14 0 18 occurs as small isolated grains and as discontinuous rims on partially resorbed pre m2 monazites that were liberated from garnet xenotime also occurs as small isolated grains within m2 coronas ion microprobe dating of thin high y rims indicates that new monazite growth occurred during m2 monazite xenotime miscibility gap temperatures are consistent with al solubility in orthopyroxene thermometry estimates indicating that peak temperatures in the inner aureole are accurately recorded and preserved by monazite m2 monazite records therefore the temperature and timing of m2 metamorphism two net transfer reactions modelled using singular value decomposition in the system p y hree lree are proposed to account for the growth of m2 phosphates 1 38 grt1 1 mnz1 1 13 mnz2 and 2 737 grt1 1 ap 1 mnz2 3 4 xno2 reaction 1 conserves p and gave rise to locally coronitic high y overgrowths on partially resorbed pre m2 monazite whereas reaction 2 accounts for the growth of small new monazite and xenotime grains both reactions were highly localized within individual m2 coronas due to slow intergranular diffusion accompanying fluid undersaturated metamorphism in the mlp aureole similar monazite forming reactions are expected in other polymetamorphosed granulites

arsenian monazite ce and xenotime y ree arsenates and carbonates from the tisovec rejkovo rhyolite western carpathians slovakia composition and substitutions in the ree y xo4 system x p as si nb s abstract a unique ree y th p as si nb s accessory assemblage was identified in a small body of lower triassic a type rhyolite of the silicic superunit near tisovec rejkovo central slovakia arsenian monazite ce phosphatian gasparite ce and arsenian xenotime y phosphatian chernovite y solid solutions in association with ree carbonates bastnasite parisite rontgenite synchysite and rare cerianite ce form anhedral to subhedral grains and aggregates 0 3 mm scattered in the groundass or as intergrowths with zircon and fe ti oxides compositions show a wide asp 1 substitution in monazite gasparite and xenotime chernovite solid solutions atom as as p 0 00 to 0 73 and 0 10 to 0 94 respectively the presence of s in monazite gasparite s s 0 14 apfu together with ca enrichment indicates clinoanhydrite substitution cas ree y 1 p as 1 rather than the incorporation of brabantite along the cath ree y 2 exchange vector moreover the th and nb rich members reveal thorite and fergusonite substitutions thsi y ree 1 p as 1 and nb p as 1 respectively the as rich ree phases originated probably during the post magmatic alteration of primary monazite ce and xenotime y by as rich high fo2 fluids whereas cerianite ce and the ree carbonates are products of a later hydrothermal overprint of the rhyolite by co2 rich solutions

monazite xenotime miscibility gap thermometry i an empirical calibration rare earth element ree and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the variscan fold belt in ne bavaria germany the metapelites include a continuous prograde mainly low p 3 5 kbar metamorphic profile from greenschist c 400 c to lower granulite facies conditions c 700 c the lree la sm are incorporated preferentially in monoclinic monazite reo9 polyhedron whereas the hree plus y are concentrated in tetragonal xenotime reo8 polyhedron the major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade monazite consists mainly of la ce and nd la0 20 0 23 ce0 41 0 45 nd0 15 0 18 po4 all other elements are below 6 mol likewise xenotime consists mainly of ypo4 with some dy and gd solid solutions y0 76 0 80 dy0 05 0 07 gd0 04 0 06 in contrast the minor hree concentrations in monazite increase strongly with increasing metamorphic grade y dy and gd increase by a factor of 3 5 from greenschist to granulite facies rocks monazite crystals often show zonation with cores low in hree and rims high in hree that is interpreted as growth zonation attained during prograde metamorphism similarly sm and nd in xenotimes increase by a factor of 3 4 with increasing metamorphic grade prograde zonation in single crystals of xenotime was not observed the xhree y in monazite and xlree in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c 400 c to 700 c the empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry due to its contents of u and th monazite has often been used for u pb age determination the combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature time paths

accessory mineral behavior during differentiation of a granite suite monazite xenotime and zircon in the sweetwater wash pluton southeastern california u s a abstract compositional and textural characteristics of the accessory minerals monazite xenotime and zircon in the sweetwater wash granites and related aplites indicate that these phases not only controlled much of the trace element geochemistry of the suite but that they also record the melt compositional changes that occurred during magmatic differentiation fractionation of monazite due to decreasing saturation levels of its essential structural constituents with falling temperature was likely responsible for an ongoing trend of light rare earth element ree depletion this was accompanied by increasing heavy ree concentrations until xenotime joined the crystallizing assemblage subsequently combined monazite xenotime fractionation resulted in lowering of the entire ree budget zircon which contains inherited cores that were apparently resorbed and rounded during initial anatexis was saturated throughout the differentiation history of the sweetwater wash suite accessory phases in granite and aplite exhibit strong compositional differences at a variety of scales the differences are best displayed by monazite on average crystals in more differentiated rocks aplites are relatively depleted in light ree s and contain higher concentrations of the substituting elements u and th zircon displays complimentary increases in hf and y while xenotime shows a slight increase in th and in gd ho ratios both phases also exhibit higher u concentrations in aplites than in granites these differences in accessory mineral compositions are observed not only between granites and the more differentiated aplites but also within individual thin sections due to in situ fractionation on an even smaller scale strong compositional variations are present within single crystals possibly due to diffusion controlled melt compositional gradients in the regions a adjacent to growing major phases and b adjacent to the growing accessory crystal itself our observations indicate that compositional variations among accessory minerals are potentially useful for tracking of magmatic processes but that the scale of observed variations must be carefully considered

differential response of u pb systems in coexisting accessory minerals winnipeg river subprovince canadian shield implications for archean crustal growth and stabilization the u pb isotopic systems of zircon monazite titanite and some apatite and the pb isotopic composition of k feldspar have been investigated in three areas of the winnipeg river subprovince wrs of the superior province canada in order to define the timing of magmatic and metamorphic processes in this archean gneissic granitoid terrain

the evolution of the polymetamorphic basement in the central alps unravelled by precise u pb zircon dating the u pb age determinations of zircon and rutile from the aar massif reveal a complex evolution of the central alpine basement the oldest components are found in zircons of metasediments which bear cores of archean age the u pb age of discordant prismatic zircons of the same rocks ranges between 580 and 680 ma an age that is typical for pan african metamorphism the zircons are interpreted as pan african detritus with archean inheritance the provenance region of the pan african zircons is assumed to be a terrane of gondwana affinity i e the w african craton or the pentevrian microplate the caledonian metamorphism left a pervasive structural imprint in amphibolite facies on the rocks of the aar massif it is dated at 456 2 and 445 ma by zircons of a layered migmatitic gneiss and a migmatitic leucosome respectively both occurring in the northernmost zones of the massif hercynian metamorphism never exceeded greenschist facies conditions and is recorded by zircon in a garnet amphibolite and by rutile in a meta psammite that yield an age of 330 ma both zircon and rutile are considered to be products of retrograde mineral reactions and therefore do not date the peak conditions of hercynian metamorphism the gastern granite at the western end of the aar massif is a contaminated granite that intruded at 303 4 ma contemporaneously with the wide spread late hercynian post collisional i type magmatism the study demonstrates the potential of isotope dilution u pb dating of single grains and microfractions in deciphering complex evolutionary histories of polymetamorphic terrains

northward sediment transport from the late carboniferous variscan mountains zircon evidence from the oslo rift norway proto rift late carboniferous fluvial and deltaic sandstones from the asker group are locally preserved in the upper palaeozoic oslo rift high precision u pb ages of single detrital zircon grains from a moscovian late westphalian to early stephanian sandstone and overlying tuffaceous sediments that predated initial basaltic volcanism have been used to study the provenance of the detritus and to evaluate the tectonic significance of the deposits we find late neoproterozoic of avalonian affinity cambro ordovician and early carboniferous detrital zircons requiring that one of the source regions was located along or south of the axis between southern england and northeastern germany and implying sediment transport from the south probably as a consequence of uplift during the asturic compressional phase of the variscan orogeny other age groups reflect typical baltic sources possibly from the ringkobing fyn high detrital zircons and monazite from underlying silurian sandstone wenlock ludlow yield svecofennian to sveconorwegian as well as late ordovician ages but lack late neopoterozoic avalonian ages it is thus unlikely that the bulk of the carboniferous detritus could have been derived by recycling of preexisting silurian sandstones these data imply that the rivers feeding the asker group originated in the variscan mountains and entered the oslo proto rift from the south possibly via the horn graben or along the sorgenfrei tornquist zone

paleozoic evolution of the armorica plate on the basis of paleomagnetic data new paleomagnetic data support the contention that armorica and gondwana formed a coherent block until ordovician time however by early devonian time armorica collided with the north american northern european assembly to form the old red continent and the collision itself was responsible for the acadian orogeny not for the earlier taconic orogeny as postulated previously the carboniferous collision of gondwana with the old red continent subsequently formed pangaea and produced the appalachian hercynian orogeny

detrital composition of ordovician sandstones from the rugen boreholes implications for the evolution of the tornquist ocean the lower palaeozoic sequences of the rugen boreholes are composed of pelitic clastic sediments which range in age from the cambro ordovician boundary to the late ordovician provenance studies have been carried out on cambro ordovician sandstones from the loissin borehole and on middle upper ordovician greywackes of the rugen 5 borehole

age and tectonic setting of late archean greenstone gneiss terrain in henan province china as revealed by single grain zircon dating the authors report precise u pb zircon ages for single grains of a metarhyodacite from the late archean dengfeng greenstone belt in henan province china near the southern margin of the north china craton most grains belong to an igneous population whose u pb isotopic systematics define a straight line intersecting concordia at 2512 12 ma and this is interpreted as the time of crystallization of the original greenstone volcanics several grains are distinctly older between 2576 9 and 2945 44 ma and the authors interpret the older grains as xenocrysts of pre greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava the xenocrysts suggest evolution of the dengfeng greenstone belt in a continental environment that may be represented by the taihua high grade gneisses bordering the dengfeng greenstones and for which the authors obtained ages of 2806 7 and 2841 6 ma the data add evidence to the now widely held concept that most archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination in such cases conventional zircon dating may not always record the precise age of rock formation

faunal evidence for oceanic separations in the palaeozoic of britain the faunal criteria for the recognition of the relative separation of old continents are 1 the contrast between shallow not deep water faunas off opposite shorelines 2 the recognition of deeper water facies surrounding the edges of continents and 3 the recognition of the disposition of ancient climatic belts these criteria are defined and used to assess the relative position of the various parts of what is now britain in the palaeozoic the iapetus ocean is well characterized by faunal differences from the cambrian until the lower ordovician but from caradoc times onwards these differences dwindle until by the latest silurian only the distribution of the ostracods separates the two sides of the ocean on faunal grounds in the early ordovician s british faunas are comparable with those from bohemia france and elsewhere to the s indicating connection with gondwanaland and these faunas differ from those in the baltic area a distribution which we attribute to a true ocean tornquist9s sea between n and s europe tornquist9s sea probably closed in the upper ordovician and from ashgill times onwards the s british and baltic faunas remained similar during the middle and late silurian there is evidence of increasing faunal distinction between britain the baltic area and north america on the one hand and southern europe and gondwanaland on the other indicating an opening rheic ocean which appears to have persisted until late carboniferous times

the geological and tectonic evolution of the precambrian of northern sweden a case for basement reactivation abstract this study is the result of a regional mapping programme of the precambrian to the extreme north of sweden by the geological survey of sweden in the years 1976 1981 it takes into account most of the geological geochemical and geophysical data concerning the region a brief description and revision of the stratigraphy is given in the light of recent mapping results and radiometric age determinations an attempt is made to trace the main petrogenetic trends of the volcanic and the plutonic rock series in order to determine which geotectonic model can best be inferred for the region following the archaean consolidation phase two major tectonic episodes have been recognized 1 a tensional episode characterized by incipient rifting which resulted in the extrusion of basic magma and in the deposition of predominantly quartzitic sediments and 2 a complex episode of reactivation of older structures and formations corresponding to the peak of the svecokarelian orogeny and to later consolidation this tectonic evolution is interpreted here as a phase of basement reactivation and shows much resemblance to ensialic mobile belts described in other parts of the world a characteristic feature of the aeromagnetic gravimetric and landsat maps is the profusion of circular and arcuate structures of variable magnitude and distinctiveness the possibility that some of these are due to the process of sequential inheritance of early impact scars is discussed

investigation of the archaean crust by single grain dating of detrital zircon a greywacke of the slave province canada u pb dating of single zircon grains from a graywacke of the point lake area of the archaean slave structural province in canada indicates that this sediment is derived from two sources 1 the ancient shield parts of which are at least 3100 ma old which is the basement of the slave province supracrustal rocks and 2 younger rocks about 2700 ma old this implies the creation of large amounts of acidic to intermediate rocks during the 2700 ma event it also indicates that magma generation and intracontinental sedimentary basin formation and deepening are strongly related

u pb systemstics in single zircons from the pontiac sediments abitibi greenstone belt twenty six detrital zircon grains from the late archaean pontiac sediments of the abitibi greenstone belt were dated individually by the u pb method in order to identify possible elements of an ancient pre 2 7 ga shield the results show that at least two components were present at the source areas of these sediments 1 materials of about 2700 2725 ma which were most likely derived from contemporaneous acid igneous rocks and 2 significantly older materials parts of which are at least 2940 ma old which were either part of an uplifted sedimentary basin or a sialic basement complex these results can be used to suggest a close link between igneous activity and basin subsidence and that the development of the abitibi volcanic belt was closely associated with the presence of some older sialic blocks

understanding geologic processes with xenotime composition chronology and a protocol for electron probe microanalysis abstract xenotime ypo 4 is iso structural with zircon group minerals and lies at one end of a compositional solid solution with monazite as with monazite and zircon naturally occurring xenotime may accommodate significant concentrations of rare earth elements as well as th and u making it an important repository for these valuable geochemical marker elements and a potential geochronometer xenotime grows in sediments during diagenesis in low to granulite grade metamorphic rocks in migmatites and is common in peraluminous igneous rocks xenotime can be a complement or indeed an alternative to monazite and or zircon in geochemical and geochronological studies to maximise the petrographic potential of xenotime a complete understanding of its composition and relationship to rock forming assemblages is essential electron beam techniques provide micron scale resolution of compositional and textural variation and yield precise dates for individual micro volumes in multi domain grains methods for collecting trace element data by electron probe microanalysis differ from those routinely employed to collect major element concentrations careful characterisation of background curvature and background interferences around the th u and pb peaks is required to avoid large overlap correction for y l 2 3 on pb m analysis of pb is made on the m peak a broad array of geochemical and geochronological applications can take advantage of the links between xenotime composition and texture and the evolution of silicate mineral assemblages in a diversity of environments one example of metasomatised xenotime from a pegmatite in sw norway demonstrates a practical application to dating geologic processes as well as the potential limitations that arise in compositionally heterogeneous xenotime

archaean chronology in the eastern part of the baltic shield a synthesis abstract twenty seven gneisses and granodiorites of the archaean basement of eastern finland have been chosen for rb sr geochronological studies the results were used together with other radiometric measurements to discuss the time space relationships between the various rock units this review can be summarized as follows from 3 0 to 2 8 ga a first generation of grey orthogneisses kivijarvi type was emplaced subsequently to an important tectono metamorphic event a second generation of grey orthogneisses naavala type intruded the kivijarvi gneisses 2 69 2 65 ga the two generations of grey gneisses form the basement on which were deposited the mafic and ultramafic magmas of the lower volcanic sequence of the kuhmo suomussalmi greenstone belt at 2 65 ga both greenstone and gneiss basement were intruded by calc alkaline intermediate to felsic magmas they were emplaced as volcanites in the upper part of the greenstone belt 2 5 ga or as granodioritic augen gneisses into the basement 2 50 2 55 ga the end of the archaean magmatic activity is marked in this part of the baltic shield by the intrusion of pink leucogranites 2 41 ga these age data are consistent with the field relative chronology and show that 1 the finnish archaean terrains developed over a period of at least 0 45 ga followed by a 0 40 ga period of geologic quiescence 2 there is a gneissic basement to the kuhmo suomussalmi greenstone belt the kivijarvi grey gneisses were emplaced at least 0 25 ga prior to the greenstones 3 the zircons extracted from recycled magmatic rocks give c 2 7 ga ages and are interpreted as retaining a memory of their ancient protolith and consequently do not indicate the emplacement age of their present host rock this interpretation is subject to great controversy and discussion among geologists 4 the rb sr whole rock system is not modified by the svecokarelian metamorphism but the strongly heterogeneous rocks as well as the minerals were affected and recorded the thermal influence of that orogeny

magmatic to hydrothermal crystallization in the w sn mineralized mole granite nsw australia part i crystallization of zircon and ree phosphates over three million years a geochemical and u pb geochronological study abstract zircon monazite and xenotime crystallized over a temperature interval of several hundred degrees at the magmatic to hydrothermal transition of the sn and w mineralized mole granite magmatic zircon and monazite thought to have crystallized from hydrous silicate melt were dated by conventional u pb techniques at an age of 247 6 0 4 and 247 7 0 5 ma respectively xenotime occurring in hydrothermal quartz is found to be significantly younger at 246 2 0 5 ma and is interpreted to represent hydrothermal growth from associated fluid inclusions it is concluded that it precipitated from a hydrothermal brine 600 c which is below the accepted closure temperature for u pb in this mineral these data are compatible with a two stage crystallization process precipitation of zircon and monazite as magmatic liquidus phases in deep crustal magma followed by complete crystallization and intimately associated sn w mineralization after intrusion of the shallow sill like body of the mole granite later hydrothermal formation of monazite in a biotite fluorite topaz reaction rim around a mineralized vein was dated at 244 4 1 4 ma which distinctly postdates the mole granite and is possibly related to a younger hidden intrusion and its hydrothermal fluid system obtaining precise age data for magmatic and hydrothermal minerals of the mole granite is hampered by uncertainties introduced by different corrections required for multiple highly radiogenic minerals crystallising from evolved hydrous granites including 230 th disequilibrium due to th u fractionation during monazite and possibly xenotime crystallization variable th u ratios of the fluids from which xenotime was precipitating elevated contents of common lead and post crystallization lead loss in zircon enhanced by the fluid saturated environment the data imply that monazite can also survive as a liquidus phase in protracted magmatic systems over periods of 10 6 years the outlined model is in agreement with prominent chemical core rim variation of the zircon