geochemical characterization and origin of granitoids from the south bohemian batholith in lower austria major and 31 minor elements have been determined in 39 large samples of variscan granitoids from 6 plutons or intrusions from the south bohemian batholith rastenberg weinsberg mauthausen schrems eisgarn and gebharts the granitoids are mainly granites but also diorites tonalites trondhjemites granodiorites average concentrations of ba th u la ce pb nd sr and k in the weinsberg mauthausen and schrems granites exceed those in average felsic i and s type granites by factors ranging between 2 1 and 1 3 the granites melts formed at waterundersaturated conditions and intruded at 10 to 15 km depth during late tectonic and post tectonic phases of the variscan orogeny about 330 to 300 ma ago hydrothermal or low temperature alteration is excluded for the majority of samples from a study of oxygen isotopes the thickness of the plutons is estimated at about 6 km from heat balance constraints by analogy with experimental partial melting three different sources of the granitoids can be identified and chemically characterized 1 the trondhjemites tonalites and diorites in the early rastenberg pluton are products of 15 to 40 melting respectively of a mafic partly amphibolitic lower crust redwitzites from the west bohemian massif which are comparable in age partly resemble the rastenberg rocks the mafic sources of the rastenberg granitoids and redwitzites are crustally contaminated as reflected in their sr nd isotopes 2 the very large syn tectonic weinsberg pluton was formed from about 30 partial melting of a tonalitic lower crust at 800 to 850 c its low proportion of ca 10 restite has a ferrodioritic composition the post tectonic fine grained mauthausen and schrems granites which tend to a granodioritic mode are very low in restite and are also products of melting of a tonalitic source 3 the youngest leuco granite the eisgarn pluton high in si p li rb cs u 87sr 86sr and low in ca sr ba reflects a pelitic source the change from mafic to tonalitic to pelitic source composition for the granitoid sequence may indicate that the depth of melt formation decreased with time the concentration of heavy rare earth elements decreased from weinsberg to eisgarn granites which indicates an increasing proportion of garnet in the source the orogenic heat conformable with a heat flow of about 100 mwm 2 was provided by mafic intrusions an alternative would be a drastic increase of the crustal thickness which cannot be recognized by barometry of the associated metamorphic rocks exposed metamorphic country rocks occur in higher amphibolite facies indicating about 5 kbar pressure mafic intrusions contain gabbros kleinzwettl or have formed quartz diorites gebharts the latter being contaminated by granitic melts from partial melting of the wall rocks mash process largescale contamination by crustal materials can be observed in 18o and in sr nd isotopes the major mafic activity was probably caused by depression of solidus temperatures in the mantle wedge above a subduction zone where water was available from dehydration of subducted ocean crust this water initiated partial melting of ultramafic rocks and metasomatism in the uppermost mantle above the level of melting the water also mobilized highly incompatible elements ba th u la ce pb nd sr and k from the uppermost mantle and transported them into the lower crust indicators of a nearby subduction or collision zone of late variscan age in addition to the specific association of granitoidal rocks are abundant upper mantle tectonites an alternate or additional source of metasomatic fluids may have been dehydration of lower crustal rocks during variscan high grade metamorphism

kinematics of crustal stacking and dispersion in the south eastern bohemian massif the south eastern bohemian massif consolidated during the late variscan orogeny by the oblique collision of two continental crustal blocks after closure of an oceanic realm one microcontinent comprises portions which are now distributed among moravian and moldanubian units and which are characterized by late proterozoic tectonothermal events especially by granitoid intrusions the other microcontinent includes the gfohl gneiss and granulites gfohl nappe of probable early palaeozoic protolith ages both continental blocks are separated by an ophiolite like assemblage which is preserved in portions of the raabs unit

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

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

mineral magnetic characteristics of podzolic soils developed on sand dunes in the lake gosciaz catchment central poland a number of samples from the a0 a2 b and c horizons have been collected from podzolic soils in the lake gosciaz catchment in central poland the catchment is very uniform consisting of 2 6 m high sand dunes with sparse stands of pine and a grass undervegetation radiocarbon dates of fossil soils covered by dune sand within the same type of environment as the lake gosciaz catchment indicate that the main dune forming phase is of older dryas age magnetic measurements have been carried out on bulk samples and on particle size fractions from the different soil horizons with respect to our mineral magnetic study it can be concluded that the soils are very uniform over the entire catchment relatively high concentrations of magnetic grains are found in the humus layer particularly in the 4 m size fraction as indicated by the highest x xfd sirm and hirm values the most likely mechanism for the magnetic enhancement in these podzolic top soils is continued burning processes low magnetic concentrations in the a2 horizon are due to eluviation and chelation local conditions can lead to a range of magnetic minerals precipitating in the b horizon

rock magnetic and paleomagnetic study on a 27 m long core from lake chapala western mexico paleoenvironmental implications for the last 10 ka abstract we report detailed rock magnetic and paleomagnetic results from a 27 m long sedimentary core collected at depocenter of lake chapala western trans mexican volcanic belt rock magnetic parameters point to psd grain size titanomagnetites as the dominant magnetic mineralogy observed variations in both concentration and grain size dependent magnetic parameters reflect changes in the sediment supply through the studied depth but mainly during the last half century stable characteristic remanent magnetization representative of secular variation of the geomagnetic field was retrieved after alternating field demagnetization of the natural remanent magnetization nrm the comparison of the obtained magnetic inclination and declination logs against the corresponding curves predicted by the global geomagnetic field model sha dif 14k enables to establish an alternative sedimentation rate independent age depth model for the whole core in reasonably good agreement with the one calculated using conventional radiocarbon dating this approach works not only for lake chapala but also for lake chalco the correlation established between rock magnetic parameters and paleoenvironments based on diatom identification of representative samples within specific zones can be considered as a novel proxy methodology this approach opens a new perspective for new more detailed rock magnetic paleomagnetic and paleoenvironmental investigations for better paleoclimatic reconstructions

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

glacial and nonglacial sediment contributions to wisconsin episode loess in the central united states the relative contributions of various glacial and nonglacial sediments to wisconsin episode loess units along the lower illinois and central mississippi valleys are estimated on the basis of a comparison of magnetic susceptibility and silt and clay mineralogy a mathematical method of source area calculation using four compositional parameters was guided by current knowledge of the regional glacial history on the basis of this technique the roxana silt along the illinois and mississippi river valleys is composed of significant superior lobe sediment 35 40 as well as wadena or des moines lobe sediment about 35 this can account for its high magnetic susceptibility feldspar content kaolinite content and pink hue lower peoria silt contains about 25 35 lake michigan lobe sediment with reduced contributions of the other sources after the mississippi river s diversion 20 4 ka the supply of superior des moines and wadena lobe sediment was cut off from the illinois valley in favor of lake michigan lobe sediment 75 80 contribution this major source area shift accounts for higher dolomite and illite contents and a more yellow hue in approximately the upper two thirds of peoria silt in the study area in loess south of st louis less pronounced compositional shifts occur because superior lobe sediment was not cut off and because des moines lobe wadena lobe and missouri river sediments having more intermediate composition compose 40 50 of the loess thereby diluting other source area changes nonglacial sediment from fluvial and periglacial sources is estimated to compose 10 40 of loess in both regions

transition from island arc to passive setting on the continental margin of gondwana u pb zircon dating of neoproterozoic metaconglomerates from the se margin of the tepla barrandian unit bohemian massif abstract sedimentological petrological and geochemical data combined with shrimp and la icp ms u pb ages of pebbles of granitic rocks and detrital zircons from neoproterozoic vletice metaconglomerates of the se margin of the tepla barrandian unit were used to characterize the regional geotectonic setting at the proterozoic paleozoic boundary the metaconglomerates form several metres thick intercalations in metasediments of the svrchnice formation roof pendants of central bohemian plutonic complex which is considered to have been deposited in neoproterozoic cambrian times a sedimentological study indicates that the metaconglomerates are products of debris flows deposited in submarine canyons or proximal parts of submarine fans geochemistry of metaconglomerate matrix and petrography of pebbles revealed an obvious difference from the western and central part of the tepla barrandian unit proterozoic where pebbles eroded from uplifted parts of the basin and island arc sources are most common the abundance of quartz quartzite granite and crystalline limestone in pebbles together with the more mature composition of host metagreywackes indicate the presence of relatively evolved continental crust in the source area for the vletice metaconglomerates and shallowing of the neoproterozoic basin to the se in the present geographic co ordinate system the neoproterozoic to lowermost cambrian age of the svrchnice formation suggested by its comparison with similar rocks in the central part of the tepla barrandian unit was confirmed by u pb dating ages of detrital zircons from the metaconglomerate matrix cluster around two values archaean to paleoproterozoic u pb ages between ca 2580 and 1780 ma and neoproterozoic to early cambrian ages between 660 and 520 ma with a significant peaks at 600 and 544 ma suggesting local volcanic activity extending shortly beyond the proterozoic paleozoic boundary zircons separated from two trondhjemite pebbles from the metaconglomerate yielded a mean u pb age of 610 17 ma arguing for active arc magmatism during the latest neoproterozoic the overall age distribution of detrital zircons and the absence of grenvillian zircon ages favour a location of the tepla barrandian unit close to the west african craton at the neoproterozoic paleozoic boundary the rapid onset of a transtensional geotectonic regime and maturing of sediments is evidenced by the overlying cambrian shallow water clastics alternating with predominantly felsic calc alkaline to alkaline volcanics during the cambrian oblique plate convergence the active continental margin was transformed into belts of horsts and narrow pull apart or small rift basins after the ordovician break off of armorican terrane assemblage blocks from gondwana the extensional regime in the tepla barrandian unit continued until the beginning of variscan orogeny in the middle devonian

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

combined rock magnetic and geochemical characterization of angangueo mining district central mexico results of a combined rock magnetic and geochemical investigation carried out at angangueo mining district are presented the different thickness and color of layers observed in field are also sketched from the magnetic susceptibility vertical ground profile accomplished additionally an area with enhanced magnetic susceptibility values is clearly outlined from a radial survey on the top of one of the biggest tailing the internal differentiation of the tailing was also supported by a full set of laboratory rock magnetic and physical chemical parameters geochemical results of matched samples reveal a complex relationship along the profile between some heavy metal concentrations i e cu and pb and magnetic susceptibility never reported in mining wastes nor in environmental studies contrary to environmental investigations of topsoils magnetic susceptibility enhancements zones in a tailing do not correspond necessarily to more heavy metal contaminated zones these outcomes evidence the intricate nature of mining wastes and the need for a better and intensive investigation of these barely studied objects

comparison between petroleum hydrocarbon concentrations and magnetic properties in chennai coastal sediments bay of bengal india abstract petroleum hydrocarbon concentrations phc of surface sediments along the chennai coast india were measured by uv fluorescence uvf spectroscopy and the results are expressed in terms of chrysene equivalents the concentration of phc in sediment varies widely from 1 88 ppm to 39 76 ppm as compared to the baseline 1 88 ppm with higher values obtained in the northern part of the study area the highest magnetic susceptibility 96 8 10 8 m 3 kg 1 value was determined from the chennai harbour area the magnetic parameters show that the chennai coastal sediments are dominated by ferrimagnetic minerals the positive correlation r 2 0 86 p arm and sirm covary with the petroleum hydrocarbon concentration suggesting that large amounts of magnetic minerals originate from anthropogenic activities it is evident that using magnetic measurements may be considered a simple rapid cheap and non destructive method to determine petroleum hydrocarbon concentrations in coastal sediments furthermore this technique may be applied to petroleum exploration studies magnetic susceptibility measurements in sediments have been proposed as complementary or alternative means of exploration and assessment of hydrocarbon reservoirs

dating the onset of variscan crustal exhumation in the core of the bohemian massif new u pb single zircon ages from the high k calc alkaline granodiorites of the blatna suite central bohemian plutonic complex abstract the variscan central bohemian plutonic complex crops out between the upper crustal tepla barrandian and the high grade moldanubian units bohemian massif much of the complex is made up of calc alkaline plutonic rocks 1 the geochemically more primitive na rich 354 4 ma sazava suite which was emplaced syntectonically during regional shortening 2 the younger more evolved potassic blatna suite which records both the shortening along its nw contact and the onset of normal shearing related to exhumation of the moldanubian unit to the se new ion microprobe u pb zircon ages for the high k calc alkaline blatna suite pinpoint this major switch in the tectonic regime both blatna and kozarovice granodiorites 346 2 ma and 347 2 ma 2 were generated by melting of heterogeneous crust and were emplaced contemporaneously to form the blatna composite pluton from age spectra preserved in inherited zircons and whole rock sr nd isotope signatures the likely crustal sources for the magmas were immature greywackes rich in neoproterozoic 615 10 ma kozarovice or late cambrian early ordovician 492 4 ma blatna volcanogenic detritus an additional important petrogenetic process was variable mixing with enriched mantle derived monzonitic magmas which may also have supplied the extra heat for crustal anatexis supplementary material analytical techniques selected whole rock major and trace element geochemical analyses and u th pb data are available at http www geolsoc org uk sup18391

preserved pedogenic mineral magnetic signature pedogenesis and paleoclimate change pennsylvanian roca shale virgilian asselian central kansas usa abstract this study focuses on the field characteristics and mineral magnetism of four stacked paleosols in the upper pennsylvanian virgilian asselian roca shale exposed near manhattan kansas usa the roca shale paleosols pi at base p4 at top include a protosol p1 argillic calcisol p2 calcic argillisol p3 and calcic vertisol p4 soil climatic changes during the development of each soil profile are suggested by cross cutting relationships and superposition observations including 1 clasts of a lower vertic part are incorporated in an upper calcic horizon and 2 carbonate glaebules have argillans covered by hematitic ferrans decreasing carbonate content more strongly developed of vertic properties and argillans and more mature profiles in successive paleosols are consistent with an interpretation of a longer term soil climate shift from more arid to more seasonal or humid during development of the four paleosols several observations are consistent with a preserved pedogenic mineral magnetic signature in particular 1 each type of soil horizon b bt bk be and c within and among paleosols contains consistent magnetic susceptibility natural remanent magnetization nrm anhysteretic remanent magnetization arm and isothermal remanent magnetization irm signatures different from other horizons 2 similar redox trends are indicated by magnetic mineralogy and uranium concentrations 3 observed magnetic character and uranium concentrations are compatible with those expected in gleyed and oxidized intervals as defined by field observations 4 pedogenic carbonate glaebules and adjacent pedogenically altered siliciclastic strata preserve the same trends and 5 paleosols contain single domain sd magnetite and some maghemite similar to many modern soils comparison with other paleozoic vertisols indicates that the roca shale paleosols are characterized by more complex pedogenic and mineral magnetic attributes this variability in the roca shale paleosols is probably related to glacial interglacial climatic variability during pedogenesis the results of this study show that coupled field observation and mineral magnetic characterization of other pre quaternary paleosols potentially will yield significant paleoclimatic and pedogenic information

multiple zircon growth within early archaean tonalitic gneiss from the ancient gneiss complex swaziland abstract u pb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the ancient gneiss complex the oldest episode at3644 4ma 2 produced the dominant type of zircon characteristically purplish brown and massive in texture this probably constitutes unaltered zircon precipitated from the original magma recrystallization accompanied and obscured by early pb loss took place within the oldest grains at3504 6ma and3433 8ma whole new grains grew at these times also we interpret the post 3644 ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss in addition many grains are visibly overgrown by two layers of younger zircon of different colour and texture dated at2986 20ma and2867 30ma euhedral finely zoned whole grains having the 2986 ma age are present also evidently contributed by very thin felsic veins associated with the nearby lochiel granite the age of3644 4ma combined with precise zircon u pb dating of volcanics from the onverwacht group reported elsewhere demonstrates that at least part of the ancient gneiss complex is older than the barberton greenstone belt

magnetic imprints of pedogenesis in planosols and stagnic alisol from bulgaria iron in its diverse forms in soils is an important indicator of a number of past and or ongoing environmental processes iron oxide mineralogy pertinent to a particular soil type could be obtained with high resolution using magnetic methods planosols are a particular soil type characterized by surface water logging leading to development of great textural differentiation with depth this study presents magnetic investigation on three planosols and a stagnic alisol from bulgaria combined with conventional chemical extraction methods oxalate citrate dithionite and soil property characterization ph grain size fractions and organic carbon content extensive magnetic analyses on high resolution sampled depth profiles have been done for determination of iron oxyhydr oxide minerals their stability and variations of effective magnetic grain size with depth zones of intensive iron oxides dissolution the topmost soil horizons are magnetically depleted in the content of strongly magnetic minerals but the paramagnetic content detected through the ratio 77 293 is enhanced accordingly bleached eluvial horizons however contain stable crystalline magnetite maghemite fraction albeit in a trace amounts we ascribe this to the processes of lessivage and translocation of magnetite maghemite from the topmost humus horizon the main results on iron oxide mineralogy suggest that thin humus horizons are enriched with pedogenic maghemite thermal behavior of magnetic susceptibility suggests that ferrihydrite is present in humus and eluvial horizons of all profiles in illuvial horizons of planosols the common mineral is lepidocrocite while in alisol al substituted hematite planosols and stagnic alisol contain both poorly crystalline and well crystalline iron oxides increasing precipitation and weathering leads to higher proportion of well crystalline pedogenic phases in the alisol profile

exotic crustal components at the northern margin of the bohemian massif implications from uthpb and hf isotopes of zircon from the saxonian granulite massif abstract the saxonian granulite massif is located at the northern margin of the saxo thuringian zone of the peri gondwana bohemian massif eight felsic and mafic granulites were studied with respect to their geochemistry and u pb zircon geochronology the felsic granulites are interpreted to be derived from continental crust of possible granitoid composition an origin from depleted mantle sources with iat to morb composition can be assumed for the mafic granulites the peak of metamorphism is thought to be timed at about 340 ma while several earlier metamorphic events are supposed to have occurred at about 355 360 370 375 405 and 450 ma they reveal a complex and polyphased geologic evolution of the saxonian granulite massif protolith emplacement likely took place at c 450 and 494 ma hf isotopic data suggest mesoproterozoic crustal ages at least for parts of the massif as these crustal ages are exotic for the bohemian massif their origin has to be searched elsewhere potential source areas could be amazonia and baltica of which the latter is the one preferred furthermore a composite architecture with at least two components the felsic granulites with mesoproterozoic crustal model ages and the mafic granulites of potential island arc origin is hypothesised their amalgamation to the recent appearance of the saxonian granulite massif is likely bracketed between 375 and 340 ma

the late glacial pedogenesis interrupted by aeolian activity in central poland records from the lake gościąz catchment abstract an interdisciplinary study was undertaken to reconstruct the late glacial fluvioglacial and aeolian sediment transformations with a particular focus on soil forming processes the studies included complex pedological research physical and chemical properties x ray diffraction xrd analysis of clay minerals and optically stimulated luminescence osl dating on soil deposit sequences developed within the aeolian landscape of inland dunes in the lake go ci z catchment central poland late glacial buried bwb soil horizons are known as finow soils this paleosol was identified in the central part of inland dunes in vertical perspective developed from aeolian material deposited on fluvioglacial sediments the average content of silt and clay in this paleosols ranged from 7 3 to 41 1 and from 6 7 to 12 6 respectively in other soil horizons within profiles the average content of these granulometric fractions was about 3 and 1 5 respectively the dominance of dioctahedral vermiculite and the higher content of free iron oxides especially oxalate form and aluminium within bwb horizons indicates their pedogenic origin the osl dates suggested that finow soil fossilization began along with the development of aeolian processes in the late glacial cold periods the older dryas and the younger dryas while finow soil development took place in the warmer periods of the late glacial bolling allerod these results were consistent with data from germany other stands in northern poland and the tomsk priobye region se western siberia

characterisation of soils by mineral magnetic measurements processes of iron mineral authigenesis diagenesis and dissolution in interaction with primary inputs of iron minerals act to produce vertical differentiation of soil magnetic properties thus magnetic iron oxides in common with other iron forms may both respond to and reflect soil forming processes investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance in several areas of environmental study for example the observed magnetic variation can be used to discriminate between individual soils and soil horizons for the purposes of soil surveying sediment tagging and tracing and empirical modelling of source sediment linkages this paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance the effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters including magnetic susceptibility frequency dependent susceptibility and anhysteretic and saturation remanences contrary to current pedological thought but in accord with other magnetically based studies e g mullins the presence of magnetite maghemite typically of superparamagnetic single domain grain size within soils has been found to be an extremely widespread phenomenon its contemporary neoformation within the soil environment is inferred however the input of artificially generated ferrimagnetic material is indicated for one of the magnetically enhanced soils examined here and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic processes of magnetic enhancement are inferred from the data presented discrimination of industrially derived magnetic particles appears possible on the basis of their very low levels of frequency dependent susceptibility fd low anhysteretic remanence arm and dissociation from the finer clay sized not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons within gleyed waterlogged soils processes of magnetite dissolution appear to be grain size specific preferential reduction of those grains of ultrafine superparamagnetic and fine single domain magnetic grain size is indicated where soil forms a major contributor to catchment sediment loads this has significance in the area of source sediment modelling soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments further only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record

modelling magnetization data using simplex two new approaches to modelling magnetization data are described the first approach models the magnetic properties of natural materials in terms of mixtures of possible source materials the second approach models the magnetic properties of natural materials in terms of mixtures of magnetite and haematite crystals of varying concentrations and grain sizes in the second approach the magnetic characteristics of magnetite and haematite and their variation with grain size are described by bicubic spline functions the methods are applied to remanence and susceptibility measurements on a suite of lake sediment particle size fractions and on a set of samples from a soil profile

assembly of the armorica microplate a strike slip terrane delivery evidence from u pb ages of detrital zircons abstract detrital zircons separated from the ordovician armorican quartzite in the foreland of the variscan belt of iberia were dated by laser ablation inductively coupled plasma mass spectrometry the age groups found correspond to proterozoic and archean crust formation events the proterozoic events reflected by zircon ages are avalonian cadomian pan african 550 800 ma grenvillean sunsas 900 1300 ma and eburnian transamazonian icartian 1800 2200 ma archean zircon ages are comprised between 2500 and 2800 ma central amazonian liberian events the presence of mesoproterozoic grenvillean zircons in the armorican quartzite which was deposited in a shallow platform bordering the nw african craton north gondwana margin suggests that the nw iberian terrane drifted from the south american margin where it was located during the neoproterozoic toward africa the drift of the iberian block likely achieved via transform faults was arrested by its docking and amalgamation with the peri nw africa

the plutonic rocks of the elbe valley zone germany evidence for the magmatic development from single zircon evaporation and kar age determinations abstract plutonic bodies of different geochemical characteristics and probably of different origin occur within the elbe valley zone e germany the complex of the so called meisen massif consists from rim to core of a nearly concentric sequence of diorite to monzodiorite monzonite and some granite intrusions field geological arguments point for an emplacement during the variscan orogenic cycle in contrast due to the lack of geochronological data the time of intrusion of the granodiorites from dohna and laas situated in the southeast and northwest of the massif is still under discussion according to 207 pb 206 pb ages of 355 12 ma produced by the single zircon evaporation technique all errors are given at the 1 level the pyroxene monzodiorite from groba represents an early stage of the variscan plutonic activity it is followed by different monzodiorites and monzonites of the meisen massif inherited zircon relics of 399 429 ma are comparable with similar zircon ages of some granulites and gneisses of the saxothuringian zone single grain determinations on zircons of the main granodioritic to monzogranitic intrusive rocks of the meisen massif gave only relic ages varying between 573 and 2222 ma these results are in agreement with the microscopic recognition of voluminous cores in the majority of the zircons based on k ar analyses the cooling of the main granitic intrusion below 350 50 c took place at 326 6 ma 207 pb 206 pb isotope ratios measured on 13 zircon grains yield implications for a cadomian emplacement of the granodiorites from dohna and laas 550 ma are interpreted as the most reliable minimum age of their intrusion in the case of laas a scattering of the data between 552 and 634 ma may be related to minor inherited components although no such indications were detected by microscopic investigations however a second group of zircons is characterized by the predominance of relictic domains showing strong crustal influences on these cadomian intrusions

chronology of late paleozoic tectonothermal activity in the southeastern bohemian massif austria moldanubian and moravo silesian zones 40ar 39ar mineral age controls abstract new 40ar 39ar hornblende and muscovite ages from the moldanubian and moravian zones of the southeastern bohemian massif indicate rapid cooling associated with continental underplating during early carboniferous plate collision and closure of the moravo silesian foredeep 40ar 39ar ages have been determined for hornblende concentrates from 1 amphibolite from the moldanubian nappe complex including the dobra gneiss rehberg amphibolite and loosdorf complex all part of the variegated unit 2 micaschist structurally overlying the bittesch gneiss and 3 amphibolite within the bittesch gneiss moravian zone the five hornblende concentrates display variably discordant 40ar 39ar age spectra because of contamination with extraneous argon components however 36ar 40ar vs 39ar 40ar isotope correlations are well defined for the five hornblende concentrates mswd the hornblende and muscovite ages are interpreted to date post variscan metamorphic cooling through ca 500 c and ca 400 c respectively no record of pre variscan thermal events is preserved in either the hornblende or muscovite argon systems however the widespread occurrence of extraneous argon components suggest a pre variscan stage of tectonothermal development the near concordancy of the hornblende and muscovite cooling ages suggest relatively rapid cooling through the contrasting argon closure temperatures and indicate that relatively high crustal levels had likely been attained during the early carboniferous

hercynian thrust related shear zones and deformation of the varied group on the contact of granulites southern moldanubian bohemian massif the three deformation phases inferred from the detailed structural analysis of the cesky krumlov varied group record the hercynian development of the southern moldanubian of the bohemian massif the deformation is related to the nw se thrusting of the large crustal units including granulites the formation of the ns and nw se trending shear zones is connected with the thrust movement the structural development begins with f1 isoclinal fold formation that could originate in unmetamorphosed sediments in the final stage they were strongly flattened and b1 boudinage developed in the rocks aplite dykes and migmatitization of paragneisses occurred at the same time the rocks were metamorphosed the d2 deformational phase was produced by the simple shear deformation of the unit and folds of various styles around rigid inclusions and the strike slip shear zone near the boundary of the granulite and the varied group were formed the f1 and f2 folds are parallel with the stretching and mineral lineation indicating a ns to nw se direction of the thrusting the youngest deformation is characterized by spectacular boudinage and by folding of the vertically oriented planes

long lasting cadomian magmatic activity along an active northern gondwana margin u pb zircon and sr nd isotopic evidence from the brunovistulian domain eastern bohemian massif manuscript published version available in international journal of earth sciences 2016 doi 10 1007 s00531 016 1416 y

petrogenesis of contrasting hercynian granitoids from the calabrian arc southern italy abstract the granitoids of the southern calabrian arc have been investigated for whole rock and phase chemistry zircon typology ree sr and nd isotopes two distinct granitoid associations which are related in time and partly in space are present a calc alkaline one and a peraluminous one the prevailing calc alkaline association is compositionally expanded sio2 48 70 and biotite dominated with tonalites and granodiorites as predominant rock types the peraluminous association is compositionally restricted sio2 67 76 and contains two mica al silicates distinct peraluminous typologies occur also as core facies within the calc alkaline types all granitoids are ilmenite bearing the cittanova cn villa s giovanni vsg and capo rasocolmo cr peraluminous granites display zircon typology ree patterns srt 51 to 113 and ndt values 8 5 to 4 6 at 290 ma suggesting a dominantly quartzofeldspathic metasedimentary source in more detail the cn types and the cr vsg types require heterogeneous and different sources the calc alkaline granitoids display very variable ree patterns cen 25 227 and ybn 3 5 18 5 in the tonalites granodiorites and variable age corrected ndt values 8 5 to 0 25 whereas sr values vary little 82 to 93 thus in terms of ndt srt covariation the data points define a vertical array which is inconsistent with a model involving crustal contamination by mantle derivatives a more viable mechanism seems to be the melting of hydrous and heterogeneous mafic lower crust and or basic underplate producing distinct magma batches evolving independently crustal contamination mingling and fractionation processes may all have contributed to the observed geochemical variations within the granitoids the peraluminous granodiorites occurring within the serre and capo vaticano multipulse calc alkaline plutons exhibit isotopic ratios ndt 6 11 to 0 33 and srt 93 to 97 which are similar to those of the calc alkaline host rocks suggesting a possible genetic link geologic and geochemical data indicate a continental collision setting for this plutonic activity the magmas were produced during late thickening to exhumation phases following collision and moderate crustal overthickening and were emplaced subsequently during a short time span a mantle contribution should have favoured melting which affected various sectors of the continental crust

zircon response to high grade metamorphism as revealed by u pb and cathodoluminescence studies correct interpretation of zircon ages from high grade metamorphic terrains poses a major challenge because of the differential response of the u pb system to metamorphism and many aspects like pressure temperature conditions metamorphic mineral transformations and textural properties of the zircon crystals have to be explored a large c 450 km2 coherent migmatite complex was recently discovered in the bohemian massif central european variscides rocks from this complex are characterized by granulite and amphibolite facies mineral assemblages and based on compositional and isotopic trends are identified as the remnants of a magma body derived from mixing between tonalite and supracrustal rocks zircon crystals from the migmatites are exclusively large 200 400 m and yield 207pb 206pb evaporation ages between 342 328 ma and single grain zircon fractions analysed by u pb id tims method plot along the concordia curve between 342 and 325 ma high resolution u pb shrimp analyses substantiate the existence of a resolvable age variability and yield older 206pb 238u ages 342 330 ma weighted mean age 333 6 3 1 ma for inner zone domains without relict cores and younger 206pb 238u ages 333 320 ma weighted mean age 326 0 2 8 ma for rim domains pre metamorphic cores were identified only in one sample 206pb 238u ages at 375 0 3 9 420 3 4 4 and 426 2 4 4 ma most zircon ages bracket the time span between granulite facies metamorphism in the bohemian massif 345 ma and the late variscan anatectic overprint bavarian phase 325 ma it is argued that pre existing zircon was variously affected by these metamorphic events and that primary magmatic growth zones were replaced by secondary textures as a result of diffusion reaction processes and replacement of zircon by dissolution and recrystallization followed by new zircon rim growth collectively the results show that the zircons equilibrated during high grade metamorphism and record partial loss of radiogenic pb during post peak granulite events and new growth under subsequent anatectic conditions

shear wave velocity tomography of the lithosphere asthenosphere system beneath the mediterranean area abstract the purpose of this study is deriving a 3d tomographic image of the shear wave velocity structure of the lithosphere asthenosphere system of the mediterranean region from rayleigh wave fundamental mode dispersion data the database consists of almost 200 wavetrains corresponding to regional events recorded at the mednet very broad band stations placed in the mediterranean area on the basis of the path averaged group velocities derived for each epicentre station trajectory local group velocities over the area covered by the seismic paths have been obtained for the 10 90 s period range by means of yanovskaya s formulation for laterally heterogeneous media according to the resolution of these local velocities a grid of 1 of latitude per 1 of longitude has been defined over the mediterranean area and a local group velocity dispersion curve has been then assigned to each gridpoint the stochastic inversion of almost 450 local dispersion curves permits deriving the respective 1d shear velocity structures from which the elaboration of a 3d tomographic image of the lithosphere asthenosphere system of the mediterranean region is straightforward as expected taking into account the complexity of this area the shear velocity model reveals significant lateral changes in the crust and uppermost mantle elastic structure as a main feature low shear velocities are deduced for most part of the central and eastern mediterranean basin at shallow depths down to 35 40 km whereas significantly higher velocities are obtained for the basins of the western mediterranean at this depth range this velocity pattern suggests a thicker crust in the centraleastern part with a greater thickness of sedimentary layers on account of the lower velocities deduced for the uppermost levels on the contrary for depths 80 km low shear velocities are obtained towards the west while the highest shear velocities are derived for the eastern mediterranean sea the aegean sea greece the centre and the south of the italian peninsula and the adriatic sea sicily and tunisia the velocity pattern at this depth range suggests a deeper beginning of the asthenosphere varying between 100 and 150 km under these areas while a shallower top around 70 km depth is found beneath the western mediterranean sea the areas where a thicker lithosphere has been deduced depict the main contact between the eurasian and african plates this feature being related to the collision and subduction processes linked to the convergence of both lithospheric plates

the saxonian granulite massif new aspects from geochronological studies geochronological and geochemical analyses were carried out to determine the age of protolith formation and the timing of eclogite and granulite metamorphism of the saxonian granulite massif sgm the age of the ultramafic protoliths of the garnet pyroxenites from the rim of the sgm has been determined to be late riphean 828 ma which together with geochemical and isotope data implies the extraction of the melt from an enriched mantle reservoir with an initial nd value of 3 4 the minimum emplacement age of the mafic to felsic granulites has been dated at 470 ma the source material of these granulites reflects an enriched mantle reservoir with an nd value of 2 6 470 ma

fission track dating of the south eastern bohemian massif waldviertel austria thermochronology and long term erosion apatite fission track thermochronology was applied to the outcropping variscan basement of the south eastern bohemian massif in order to resolve its post variscan cooling and denudation history nineteen samples could be dated by the grain population method the ages range from 235 to 90 ma middle triassic to middle cretaceous neglecting some local variations they show a decrease from the western edge of the moravian thaya window towards the west and the south fission track length histograms mainly exhibit broad distributions with arithmetic means between 12 3 and 13 4 m and standard deviations between 1 5 and 2 0 m thermochronological model calculations of the measured data demonstrate that the apatite fission track memory of the samples reaches back until the begin of the cretaceous and for some samples even until the permian the regional fission track cooling pattern is not related to the limitations and the internal structure of the moravian thaya window which appears to be an elder probably late carboniferous to permian tectonic feature also the vitis fault does not delimit areas of different thermochronological behaviour therefore significant vertical offset on that fault can be excluded for the cretaceous and cenozoic thermochronological model calculations of fission track ages and length data are consistent with the assumption of regional sedimentary reburial during the late cretaceous the amount of reburial could have attained 1 km the cenozoic bulk denudation was in the order of 1 to 3 km

deducing the ancestry of terranes shrimp evidence for south america derived gondwana fragments in central europe we present here an example of how the sensitive high resolution ion microprobe shrimp zircon dating method can provide a terrane specific geochronological fingerprint for a rock and thus help to reveal major tectonic boundaries within orogens this method applied to inherited zircons in a ca 580 ma metagranitoid rock from the eastern bohemian massif has provided for the first time in the central european variscan basement unequivocal evidence for mesoproterozoic and late paleoproterozoic geologic events ca 1 2 ga 1 5 ga and 1 65 1 8 ga the recognition of such zircon ages has important consequences because it implies that parts of the precambrian section of variscan central europe were originally derived from a grenvillian cratonic province as opposed to the common assumption of an african connection a comparison with previously published shrimp data suggests however that these mesoproterozoic and late paleoproterozoic zircon ages may be restricted to the moravo silesian unit in the eastern variscides whereas the saxothuringian and moldanubian zones appear to contain a typical north african i e neoproterozoic plus eburnian inherited zircon age spectrum this finding supports new tectonic concepts according to which variscan europe is composed of a number of completely unrelated terranes with extremely different paleogeographic origins the moravo silesian unit can be best interpreted as a peri gondwana terrane which was situated in the realm of the amazonian cratonic province by the late precambrian comparable to the avalonian terranes of north america and the united kingdom

magmatic stoping as an important emplacement mechanism of variscan plutons evidence from roof pendants in the central bohemian plutonic complex bohemian massif the presence of numerous roof pendants stoped blocks and discordant intrusive contacts suggests that magmatic stoping was a widespread large scale process during the final construction of the central bohemian plutonic complex bohemian massif the measured total length of the discordant contacts that cut off the regional cleavage and were presumably formed by stoping corresponds to about half of all contacts with the upper crustal host rocks in addition at least some of the straight cleavage parallel intrusive contacts may also have recorded complex intrusive histories ending with piecemeal stoping of thin cleavage bounded host rock blocks into the magma chamber based on the above we argue that the fast strain rates required for emplacement of large plutons of the central bohemian plutonic complex into brittle upper crustal host rocks over relatively short time span could not have been accommodated entirely by slow ductile flow or slip along faults instead the emplacement was largely accommodated by much faster thermal cracking and extensive stoping independent of regional tectonic deformation finally we emphasize that magmatic stoping may significantly modify the preserved structural patterns around plutons may operate as an important mechanism of final construction of upper crustal plutons and thus may contribute to vertical recycling and downward transport of crustal material within the magma plumbing systems in the crust

elevator tectonics and orogenic collapse of a tibetan style plateau in the european variscides the role of the bohemian shear zone variscan collision of peri gondwanan terranes led to a doubly vergent crustal wedge that was thicker than 55 km in the area of the bohemian massif this crustal thickness resulted in a highly elevated bohemian plateau with a topographic height 3 4 km the bohemian plateau was covered with unmetamorphic paleozoic strata all of which are today well preserved in the tepla barrandian unit because of crustal scale vertical slip along the bohemian shear zone bsz the bsz forms a subvertical ca 500 km long and up to 2 km wide belt of dip slip mylonites which show several 90 deflections in map view tepla barrandian down movements were active under retrograde metamorphic conditions starting with granulite and ceasing with greenschist facies conditions as slip along the bsz was largely vertical and led to a minimum throw of 10 km this type of crustal scale deformation is referred to as elevator tectonics the elevator style movements caused the juxtaposition of the supracrustal tepla barrandian lid the elevator against high grade rocks of the extruding orogenic root the bsz has further governed the foci of mantle derived plutonism new u pb zircon and monazite tims dating of six plutons suggest that emplacement of mantle derived melts along the bsz lasted for at least 20 m y starting with the emplacement of the klatovy granodiorite at 347 4 3 ma and ceasing with the emplacement of the drahotin pluton at 328 1 ma when taking into account the new ages of synkinematic plutons the simultaneous vertical slip along the individual segments of the bsz north west and central bohemian shear zone is bracketed to the period 343 337 ma elevator tectonics was probably controlled by delamination of thickened mantle lithosphere that caused a dramatic thermal turnover and heating up of the orogenic root the overheated lower crust was thermally softened by anatexis and diffusion creep resulting in channel flow vertical extrusion fast uplift and exhumation of the orogenic root

building up of a nested granite intrusion magnetic fabric gravity modelling and fluid inclusion planes studies in santa eulalia plutonic complex ossa morena zone portugal the santa eulalia plutonic complex sepc located in the ossa morena zone south portugal is composed of a medium to coarse grained pink granite g0 type and a central grey medium grained biotite granite g1 type available rb sr data indicates an age of 290 ma an emplacement model for the sepc is proposed taking into account magnetic fabric 2d gravity modelling and fluid inclusion planes studies the g0 and g1 types demonstrate different magnetic behaviour g0 is considered a magnetite type granite and g1 is an ilmenite type granite the formation of g0 required oxidized conditions related to the interaction of mafic rocks with a felsic magma the 2d gravity modelling and subvertical magnetic lineations show that the feeder zone of the sepc is located in the eastern part of the pluton confirming the role of the assumar and messejana variscan faults in the process of ascent and emplacement the magma emplacement was controlled by ene wsw planar anisotropies related to the final brittle stages of the variscan orogeny the emplacement of the two granites was almost synchronous as shown by their gradational contacts in the field the magnetic fabric however suggests emplacement of the g0 type first closely followed by emplacement of the g1 type pushing the g0 laterally which becomes more anisotropic towards the margin the g1 type became flattened acquiring a dome like structure the sepc is a nested pluton with g0 type granite assuming a tabular flat shape and g1 type forming a rooted dome like structure after emplacement sepc recorded increments of the late variscan stress field documented by fluid inclusion planes in quartz

using magnetic susceptibility to facilitate more rapid reproducible and precise delineation of hydric soils in the midwestern usa abstract standard field indicators currently used for hydric soil delineations usda nrcs 1998 field indicators of hydric soils in the united states version 4 0 in g w hurt et al ed united states department of agriculture nrcs fort worth tx are useful but in some cases they can be subjective difficult to recognize or time consuming to assess magnetic susceptibility ms measurements acquired rapidly in the field with a portable meter have great potential to help soil scientists delineate and map areas of hydric soils more precisely and objectively at five sites in illinois from 5 to 15 ha in area with contrasting soil types and glacial histories the ms values of surface soils were measured along transects and afterwards mapped and contoured the ms values were found to be consistently higher in well drained soils and lower in hydric soils reflecting anaerobic deterioration of both detrital magnetite and soil formed ferrimagnetics at each site volumetric ms values were statistically compared to field indicators to determine a critical ms value for hydric soil delineation such critical values range between 22 10 5 and 33 10 5 si in silty loessal or alluvial soils in illinois but are as high as 61 10 5 si at a site with fine sandy soil a higher magnetite content and slower dissolution rate in sandy soils may explain the difference among sites with silty parent material the lowest critical value 22 10 5 si occurs in soil with low ph 4 5 5 5 since acidic conditions are less favorable to ferrimagnetic mineral neoformation and enhance magnetite dissolution because of their sensitivity to parent material properties and soil ph critical ms values must be determined on a site specific basis the ms of studied soil samples 0 5 cm depth is mainly controlled by neoformed ultrafine ferrimagnetics and detrital magnetite concentrations with a minor contribution from anthropogenic fly ash neoformed ferrimagnetics are present in all samples but based on high fd 5 to 10 are most prevalent in high ph mollisols of northeastern illinois scanning electron microscope images display significantly more detrital magnetite alteration in hydric soils substantiating that reductive dissolution of magnetite aided by microorganisms is a primary cause for lower ms fly ash comprises 8 50 of the 5 m strongly magnetic particles and typically accounts for 5 15 of the total ms signal the proportion of fly ash in 5 m strongly magnetic fractions is greater in hydric soils because of lower natural magnetite contents possibly combined with historical topsoil accumulation in lower landscapes magnetic fly ash particles are also more altered in low ms soils implying that significant magnetite dissolution can occur in less than 150 years

time of formation and peak of variscan hp ht metamorphism of quartz feldspar rocks in the central erzgebirge saxony germany the variscan erzgebirge represents an antiform with a core of gneisses and mica schists surrounded by a phyllitic mantle the gneiss eclogite unit geu in the central part is a composite tectonometamorphic assemblage characterized by a hp ht imprint and comprises migmatitic para and orthogneisses ht mylonites hp granulites eclogites and garnet peridotites it is tectonically sandwiched between two major units with distinctly lower pt histories the geu experienced a characteristic kinked retrograde pt path after hp ht equilibration with 1 strong near isothermal decompression at high temperatures 2 extensive re equilibration at medium pressures followed 3 by rapid cooling during continued uplift we dated zircons pb pb evaporation from granitoid orthogneisses and metapelites of the geu the orthogneisses contain euhedral long prismatic zircons of igneous origin that provided protolith ages between 470 and 524 ma metapelites retain well preserved granulite facies mineral assemblages and contain spherical multifaceted metamorphic zircons that grew near the peak of hp ht metamorphism inclusions of prograde hp phengite 15 kbar and rutile are included in one such zircon metamorphic zircons of three samples from different localities yielded identical 207pb 206pb ages of 340 5 0 7 ma 341 2 0 5 ma and 341 6 0 5 ma respectively consideration of these zircon ages with published 39ar 40ar white mica ages suggests fast cooling and uplift rates in excess of 50 c ma and 4 km ma this is typical for large scale extensional tectonic unroofing of the ultra deep part of a fossil thickened variscan continental crust 60 km during continuing continental collision and orogenic collapse

mineral magnetic and textural interpretations of sedimentation in the skagerrak eastern north sea abstract the mineral magnetic properties of surface sediments in the northern skagerrak show systematic trends which allow the identification of sediment provinces and the interpretation of transport pathways this application is especially valuable considering the importance and difficulty of environmental analyses involving fine grained deposits magnetic susceptibility and saturation isothermal remanent magnetisation sirm have different geographic patterns but their individual trends are consistent in the skagerrak muds clay content 25 60 over the entire studied area the values correlate positively with general trends in particle size whereas sirm does not show particle size dependency the present study confirms that the majority of sediments accumulating in the northern skagerrak are supplied from the south the mineral magnetic parameters facilitate detailed tracking of the counterclockwise transport of sediments around and into the norwegian trench

thick skinned versus thin skinned thrusting rheology controlled thrust propagation in the variscan collisional belt the southeastern bohemian massif czech republic austria the variscan nappe assembly within the southeastern bohemian massif includes 1 crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels 2 nappe assembly of footwall units under very low to low grade metamorphic conditions and 3 frontal thin skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate thick skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin skinned tectonic character of foreland units 40ar 39ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during late variscan thrusting between 340 ma and 325 ma the 40ar 39ar mineral ages together with previously published 40ar 39ar and sm nd data record systematically decreasing variscan mineral ages structurally downward into footwall units this is explained by rapid exhumation and cooling synchronously with thrusting along crustal scale ramps during foreland directed nappe propagation the location of decollement zones within specific structural levels was apparently controlled by crustal scale rheological con trasts late variscan cooling ages contrast with cadomian 40ar 39ar hornblende and muscovite ages recorded within the structural basement immediately below the major variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during variscan thrusting these record a sequence of cadomian magmatic and metamorphic events in foreland units including 1 pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions 610 ma 2 greisen formation during a cadomian collisional event 600 ma and 3 intrusion of calcalkaline granitoid bodies with i type affinities 590 ma

anhydrite bearing rocks from the rožna district moldanubian zone czech republic high grade metamorphosed exhalites several types of anhydrite bearing rocks have been found in the amphibolite facies metamorphosed rocks at the north eastern margin of the moldanubian zone anhydrite either forms monomineralic bands up to 40 cm thick or occurs in the form of disseminated grains in surrounding calc silicate gneiss together with feldspar scapolite amphibole pyroxene epidote and pyrite the isotopic composition of sulphur 34s 30 6 to 32 3 and strontium 87sr 86sr 0 70797 to 0 70781 in anhydrite may indicate a marine source of sulphate the isotopic ratio of strontium is in the same range as that of metamorphosed strata bound barite sulphide ores which have been previously described in the same area the 34s values of coexisting pyrite range from 21 4 to 22 5 the 34sanhydrite pyrite corresponding to the metamorphic temperature of 600 to 660 c in contrast to many submarine exhalative deposits the oxygen isotopic compositions of anhydrite 18o 9 3 to 10 2 are lighter than that of barite 18o 10 4 to 13 8 this indicates that the both minerals are not in isotopic equilibrium therefore it is probable that anhydrite and barite from the ro na district were deposited from fluids that contained different proportions of seawater and hydrothermal fluids or from hydrothermal fluids that underwent variable extent of oxygen isotope exchange with seafloor rocks the 13c values in calcite 13c 17 2 to 18 7 from anhydrite bearing rock are lower than those in distant marbles as graphite is absent in anhydrite and calcite bearing rocks impoverishment in the 13c isotope cannot be attributed to the graphite carbonate isotopic exchange during metamorphism it is proposed that low 13c values in carbonates are caused by pre metamorphic oxidation of organic matter in course of hydrothermal processes anhydrite and anhydrite bearing calc silicate gneiss from the north eastern part of the moldanubian zone are interpreted to be the high grade metamorphosed analogue of anhydrite rich exhalites commonly found in submarine exhalative hydrothermal deposits

mineral magnetic properties of holocene lake sediments and soils from the karsa valley lappland sweden and their relevance to palaeoenvironmental reconstruction a mineral magnetic study of lacustrine sediments and soils from the karsa valley in lappland sweden was undertaken to estimate the potential of mineral magnetic techniques in elucidating the variability of climate recorded by glacier fluctuations and soil development sediment cores from two lake basins receiving sediment input from a glacial outflow stream and their respective catchments show a mineral magnetic record that can be interpreted in terms of glacial activity and soil development dissolution of magnetite is indicated during periods of climatic amelioration and causes a high s ratio during periods of glacial advance and colder wetter climate the minerogenic sediment supply is increased and magnetite is preserved in the sediment resulting in a low s ratio variations in the s ratio can then be used as a proxy climate indicator

late quaternary stage 2 and 3 meltwater and heinrich events northwest labrador sea two major meltwater events are documented in cores from the nw labrador sea one occurred ca 20 000 14c yr b p in association with deposition of a major detrital carbonate unit both prior to and after this event 18o values of near surface planktonic foraminifera were 4 5 indicating fully enriched glacial values a younger event ca 14 000 14 c yr b p is characterized by a dramatic change in 18o from 4 5 to 2 0 and coincided with the retreat of ice from the outer se baffin shelf possibly into hudson strait these meltwater events coincide with heinrich h layers 1 and 2 from north atlantic sediments the 14 000 14c yr b p meltwater event indicates that the eastern margin of the laurentide ice sheet also underwent rapid retreat at approximately the same time as other ice sheet margins around the ne north atlantic a third major detrital carbonate event at the base of hu87 033 009 possibly correlative with heinrich layer 3 occurred ca 33 960 675 14 c yr b p however this is older than the accepted date for h 3 of 27 000 14c yr b p and may be h 4

shear wave attenuation tomography of the lithosphere asthenosphere system beneath the mediterranean region abstract 3 d tomographic images of the shear wave attenuation structure of the lithosphere asthenosphere system beneath the mediterranean region are derived from the inversion of anelastic attenuation coefficients obtained for the fundamental mode of nearly 3100 rayleigh wavetrains recorded at very broad stations placed in the mediterranean region the two station method is used to compute 124 interstation path averaged attenuation coefficients within the 15 120 s period range an adaptation to attenuation coefficients of the yanovskaya ditmar formulation specifically devoted to path averaged surface wave dispersion data and body waves permits to obtain the local attenuation coefficients and their uncertainties for all the period range at every node of a grid covering the studied region maps of stretching azimuths and mean radii of the resolution ellipses are also obtained for every period all of these parameters are strongly related to the spatial path coverage data quality and data coherency a local attenuation dispersion curve is defined then in each grid node and the stochastic inversion of these curves yields 1 d models of inverse shear wave intrinsic quality factor q 1 the shear wave velocity models obtained for every grid node in an elastic tomography developed in a previous work for the same domain are taken as reference models for the inversion process the kriging contouring algorithm is used to represent the resulting 3 d q 1 structure by means of horizontal and vertical cross sections down to 160 km depth the main patterns of this anelastic structure are mostly in agreement with the structural and tectonic features obtained in the reference shear velocity tomography the local attenuation maps manifest notable lateral changes for periods of 20 and 40 s whereas more moderate spatial variation is observed for longer periods the 3 d anelastic models show that at shallow levels the attenuation in the western mediterranean is usually slightly higher than in the central eastern part the area from corsica sardinia islands to italy presents high q 1 values of 40 0 70 0 10 3 down to 75 km depth whereas q 1 does not exceed 20 0 10 3 in the rest of the mediterranean basin except for the south eastern part where slightly higher values are obtained 20 0 30 0 10 3 those high q 1 values suggest the presence of asthenospheric material at shallow levels which would be related to distensive tectonic processes responsible for the recent opening of extensional basins as the tyrrhenian sea and the ligurian provencal basin in the eastern mediterranean the vertical cross sections at different latitudes and longitudes reveal at deep levels some structural patterns consistent with signs of subduction which are linked to the convergence between the african and eurasian plates

ultrapotassic dykes in the moldanubian zone and their significance for understanding of the post collisional mantle dynamics during variscan orogeny in the bohemian massif abstract we report mineral textures geochemistry and age relations of two ultrapotassic dykes from a dyke swarm in the western part of the moldanubian zone at contact to the tepla barrandian block the dykes have orientation almost perpendicular to the nne ssw trending central bohemian plutonic complex and cross cut metamorphic foliation in basement gneisses and migmatites based on mineral compositions and geochemistry the dykes show close relations to mg k syenite plutons in the moldanubian zone the two dykes are vaugnerite and syenite in compositions and contain talc pseudomorphs after olivine within a fine grained matrix that consists of k feldspar phlogopite with small amounts of clinopyroxene and accessory quartz apatite titanite and sulphides of fe cu ni the syenite porphyry dyke cross cuts the vaugnerite it contains quartz phenocrysts and comparing to vaugnerite has lower modal content of talc pseudomorphs the vaugnerite and syenite porphyry have high k 2 o 6 to 7 wt and mg number 0 66 0 74 but low cao and na 2 o contents the vaugnerite is markedly rich in p 2 o 5 2 wt and comparing to syenite porphyry has higher amount of mantle incompatible elements e g rb cs ba pb th u v and cr id tims analyses on titanite in vaugnerite and on zircon in syenite porphyry yielded 338 59 0 68 and 337 87 0 21 ma respectively mineral and bulk rock chemistry of the dykes indicates that the source magma was formed by a low degree of partial melting of a highly anomalous domain in the upper mantle which subsequently fractionated and was contaminated with crustal material during its ascent formation of ultrapotassic magma is explained by transcurrent shear zones in the mantle lithosphere that occurred due to block rotation and post collisional mantle dynamics initiated by slab break off and asthenosphere upwelling into the moldanubian accretionary complex during the variscan orogeny

on the formation of sand ramps a case study from the mojave desert sand ramps are dune scale sedimentary accumulations found at mountain fronts and consist of a combination of aeolian sands and the deposits of other geomorphological processes associated with hillslope and fluvial activity their complexity and their construction by wind water and mass movement means that sand ramps potentially hold a very rich store of palaeoenvironmental information however before this potential can be realised a full understanding of their formation is necessary this paper aims to provide a better understanding of the principal factors influencing the development of sand ramps it reviews the stratigraphic chronometric and sedimentological evidence relating to the past development of sand ramps focussing particularly on soldier mountain sand ramp in the mojave desert as well as using observations of the modern movement of slope material to elucidate the formation of stone horizons within sand ramps findings show that sand ramps cannot easily be interpreted in terms of a simple model of fluctuating palaeoenvironmental phases from aeolian dominated to soil fluvial dominated episodes they accumulate quickly perhaps in 5 ka probably in a single phase before becoming relict based on the evidence from soldier mountain they appear strongly controlled by a window of opportunity when sediment supply is plentiful and cease to develop when this sediment supply diminishes and or the accommodation space is filled up contemporary observations of stone movement both on rock and sandy sloping surfaces in the mojave region indicate movement rates in the order of 0 6 and 11 mm yr 1 which is insufficiently fast to explain how stone horizons could have been moved across and been incorporated into sand ramps on multiple occasions stone horizons found within the aeolian sediments lack evidence for soil development and are interpreted as very short term events in which small streams moved and splayed discontinuous stone horizons across the sand ramp surface before aeolian deposition resumed surface stone horizons may form by creep from mountain slope sources across sand ramps but require enhanced speed compared to measured rates of runoff creep we propose the mechanism of fluvio aeolian creep our study suggests that current models of alternating aeolian and colluvial deposition within sand ramps their palaeoenvironmental significance and indeed how sand ramps are distinguished from other dune forms require amendment

kinematics of the variscan thrusting in the eastern moldanubicum bohemian massif czechoslovakia evidence from the naměst granulite massif abstract the tectonic history of the nam st granulite massif eastern part of the bohemian massif moldanubian zone czechoslovakia is interpreted in terms of polyphased generally north directed variscan nappe stacking original acid granulites formed during a presumed phase of lithospheric subduction 750 c 11 14 kbar 370 350 ma were converted into ht mylonites during the dominant deformation phase d 1 as a consequence of complex thrust wrench processes related to intra moldanubian deep crustal stacking 700 c 9 6 kbar 350 330 ma bringing granulites and the surrounding gfohl gneiss on top of the moldanubian nappe sequence this phase was followed by a northeastward superposition of the already stacked moldanubian nappe over the moravian zone phases d 2 d 3 650 400 c 5 3 kbar 335 325 ma which also marginally affected the cadomian basement of the brunovistulicum the variscan tectonic history was terminated by a namurian to lower permian phase of sinistral strike slip movements along nne ssw trending faults

evidence from zircon dating for existence of approximately 2 1 ga old crystalline basement in southern bohemia czech republic zircon ages are reported for three moldanubian amphibolite grade orthogneisses from the southern bohemian massif obtained by conventional u pb analyses for two of these orthogneisses conventional u pb data are supported by ion microprobe single zircon ages or single grain evaporation data the amphibolite grade orthogneisses occurring in three small tectonic lenses within the varied group close to the south bohemian main thrust are of tonalitic granodioritic or quartz dioritic composition

chronological constraints on the pre orogenic history burial and exhumation of deep seated rocks along the eastern margin of the variscan orogen bohemian massif czech republic key lithological units of the high grade eastern margin of the bohemian massif were dated using the u pb and pb pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the variscan orogenic root the protolith ages for metagranitoids orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from cambro ordovician to early devonian times probably related to discontinuous intracontinental rifting of neoproterozoic crust our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted neoproterozoic crust preserved farther to the east as the brunia microcontinent zircon ages for felsic granulites high grade gneisses of the lower crust and of a syn convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 ma exhumation of the lower crust to mid crustal levels was a localized process that occurred at 340 ma and was associated with crustal scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin both types of exhumation mechanisms were driven by deep level wedging indentation of the easterly brunia continent followed by deposition of heavy minerals and pebbles derived from high pressure rocks in the adjacent foreland basin final orogenic development was characterized by ne sw dextral transpressive shearing parallel to the brunia margin as well as dextral trans tension associated with activity along the elbe lithospheric fault these processes affected the marginal parts of the orogenic root and were accompanied by 330 to 325 ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries rotation of the assembled orogenic belt accompanied by lithospheric faulting driven by westerly subduction roll back may be the most plausible model to explain late deformation of the orogenic root

u pb zircon and sm nd dating of moldanubian hp ht granulites from south ohemia czech republic conventional multigrain u pb systematics of zircon and single grain evaporation data of zircons in combination with nd model ages are presented for pyroxene free felsic garnetiferous granulites and orthopyroxene bearing granulites of felsic mafic and intermediate composition from the blansky les mountains granulite complex s bohemia zircons from mafic and intermediate granulites yield concordia intercept ages between 346 5 and 351 6 ma as well as a single grain evaporation age of 346 12 ma these ages define the time of variscan high grade metamorphism in s bohemia and are supported by an internal sm nd garnet zircon plagioclase whole rock isochron age of 343 21 ma euhedral zircons from felsic granulites of probable granitic derivation yield u pb ages of 366 5 ma and 373 12 ma interpreted as intrusion ages of the granitic precursor an inherited proterozoic crustal component is documented by poorly defined concordia upper intercept ages around 1700 ma and nd model ages between 1 3 and 2 1 ga

re os geochemistry and geochronology of the ransko gabbro peridotite massif bohemian massif the ransko gabbro peridotite massif in eastern bohemia is a strongly differentiated intrusive complex which hosts low grade ni cu ores mainly developed close to the contact of olivine rich rocks with gabbros in troctolites and to a much lesser extent in both pyroxene and olivine gabbros and plagioclase rich peridotites gabbro troctolite peridotite and ni cu ores from the jezirka ni cu pge deposit considered to be a typical example of the liquid segregation style of mineralization were analyzed for re os concentrations and isotopic ratios seven barren and mineralized samples from the jezirka deposit yielded a re os regression of 341 5 7 9 ma mswd 69 strongly mineralized peridotite with mantle like initial 187os 188os ratio of 0 125 suggests that os as well as other pge present in the ni cu mineralization are predominantly of mantle origin on the other hand barren and low mineralized samples have radiogenic initial 187os 188os ratios of 0 14 0 16 suggesting some import of re and or radiogenic 187os most likely through contamination by continental crust during magma emplacement the re os age of the ransko massif is significantly younger than the previously suggested lower cambrian age but it is similar to and or younger than the age of metamorphism of the adjacent kutna hora crystalline complex and the moldanubian unit therefore it is likely that the emplacement of the ransko massif and its ni cu mineralization was closely connected with the late stage evolution of the kutna hora crystalline complex

origin of migmatites by deformation enhanced melt infiltration of orthogneiss a new model based on quantitative microstructural analysis a detailed field study reveals a gradual transition from high grade solid state banded orthogneiss via stromatic migmatite and schlieren migmatite to irregular foliation parallel bodies of nebulitic migmatite within the eastern part of the gfohl unit moldanubian domain bohemian massif the orthogneiss to nebulitic migmatite sequence is characterized by progressive destruction of well equilibrated banded microstructure by crystallization of new interstitial phases kfs pl and qtz along feldspar boundaries and by resorption of relict feldspar and biotite the grain size of all felsic phases decreases continuously whereas the population density of new phases increases the new phases preferentially nucleate along high energy like like boundaries causing the development of a regular distribution of individual phases this evolutionary trend is accompanied by a decrease in grain shape preferred orientation of all felsic phases to explain these data a new petrogenetic model is proposed for the origin of felsic migmatites by melt infiltration from an external source into banded orthogneiss during deformation in this model infiltrating melt passes pervasively along grain boundaries through the whole rock volume and changes completely its macro and microscopic appearance it is suggested that the individual migmatite types represent different degrees of equilibration between the host rock and migrating melt during exhumation the melt topology mimicked by feldspar in banded orthogneiss forms elongate pockets oriented at a high angle to the compositional banding indicating that the melt distribution was controlled by the deformation of the solid framework the microstructure exhibits features compatible with a combination of dislocation creep and grain boundary sliding deformation mechanisms the migmatite microstructures developed by granular flow accompanied by melt enhanced diffusion and or melt flow however an ams study and quartz microfabrics suggest that the amount of melt present did not exceed a critical threshold during the deformation to allow free movements of grains

early archaean zircon ages from orthogneisses and anorthosites at mount narryer western australia abstract u th pb isotopic analyses have been made with an ion microprobe of 20 30 m areas within zircons from gneisses adjacent to the mount narryer quartzite western australia in which detrital zircons up to 4200 ma old have been found the cores of the zircons yield protolith crystallization ages of 3678 6 ma 95 confidence limits for the meeberrie banded monzogranite gneiss and 3381 22 ma for the dugel syenogranite leucogneiss in agreement with observed intrusive relationships zircons from inclusions of deformed leucogabbro and meta anorthosite within the dugel gneiss formerly part of the manfred layered igneous complex are 3730 6 ma old the remnants of the manfred complex are therefore the oldest rocks currently known on the australian continent analyses of discrete post magmatic grains and of overgrowths on older grains indicate an episode of subsolidus zircon growth in the region at 3296 4 ma which is interpreted as a time of local metamorphism of the gneisses subsequently the zircons have been rounded and embayed and have lost radiogenic pb in both ancient and recent times all the zircon ages determined in this study are within the age range for detrital zircons in the mount narryer quartzite it is therefore possible that these felsic gneisses and anorthositic rocks were a local source for some of the sedimentary material however the lateral extent of this early archaean terrain and the source rocks for the rare 4100 4200 ma old zircons in the metasediments are yet to be determined

geochemistry and petrology of recycled orogen derived sediments a case study from upper precambrian siliciclastic rocks of the central iberian zone iberian massif spain abstract two areas of upper precambrian siliciclastic rocks separated from each other by 150 km were selected in the central iberian zone ciz for a petrological and geochemical study the petrology of the coarser grained rocks indicates the absence or scarcity of igneous clasts the presence of extrabasinal and intrabasinal clasts and homogeneous features of sandstones ranging from very fine to fine grained the modal data of these rocks plot close to the qt l and qm lt sides of qtfl and qmflt triangular diagrams suggesting provenance from a recycled orogen the geochemical data on the shales and sandstones from the two areas studied are very uniform both groups of rock show similar ranges of relevant major element ratios such as al 2 o 3 tio 2 17 46 21 51 17 46 21 34 for shale and sandstone respectively al 2 o 3 feo t 2 40 3 79 2 18 3 33 and feo t feo t mgo 0 71 0 77 0 68 0 76 together with parallel average rare earth element ree patterns and similar values for some key trace element ratios la th th yb th u zr hf sm nd and cr sc other trace element ratios la sc th sc cr th and cr zr are uniform in the shales group and also in the sandstones group although minor differences become apparent when both groups are compared these differences are probably related to the quartz dilution effect affecting the relative abundance of phyllosilicates in these rocks and indirectly the sc and cr contents given their covariations with al2o3 the zr yb zr y and zr th ratios of the shales are uniform the sandstones show higher and more variable values and there is no correlation between zr abundance and any of these ratios in the shales or sandstones together with the parallel ree patterns shown by both groups of rock these results suggest the absence of heavy mineral fractionation during the transport of detrital material and or the relative importance of other minerals probably phyllosilicates in the control of yb y and th abundances apparently all the groups of rocks show a remarkable geochemical homogeneity despite the distance between the sampling areas thus the petrological and chemical data are consistent and point to a very extensive homogeneous and stable source region as the most plausible contributor to the upper precambrian siliciclastic rocks in the ciz the possible relationship between this zone and n gondwana is also discussed

evolution of moldanubian rocks in austria review and synthesis the moldanubian zone in austria comprises three major lithological units despite general agreement that nappe tectonics contributed to its current structure the number and position of tectonic boundaries or continental pieces that were involved in its evolution as well as the age extent and position of oceanic sutures are disputed recent models ascribe the moldanubian tectonostratigraphic structure to its oblique n to ne directed collision with moravia only the rocks of the moldanubian bunte series and gfohl unit experienced a common intensive overprint in the range 700 800 c and 8 11 kbar textural evidence suggests that this overprint was attained during nearly isothermal decompression so the rocks experienced higher pressures prior to this overprint these conditions constrain a continent continent collision environment that contributed to the formation of the moldanubian granulites the estimated metamorphic temperatures are close to tmax during this hercynian high t overprint the minerals underwent extensive diffusion controlled homogenization of elements the early stages of retrogression of these units were characterized by isobaric cooling at c 6 kbar in the range 650 500 c that is related to the oblique collision of the moldanubian and moravian zones cooling to c 400 c is demonstrated by unstrained diasporized corundum inclusions in garnet of common moldanubian granulites the available age data including cooling ages from metamorphic rocks show a very wide variation between 490 and 280 ma that depends on sample characteristics and the dating method used they demonstrate clearly however that the metamorphic overprint is hercynian the possibility that the large variation in ages reflects homogenization resetting and closure of the isotopic systems attained at different sample and method specific times is discussed age data varying between c 370 and c 346 ma tentatively date different stages during the hercynian high t decompression the majority of zircon and monazite u pb ages as well as the hornblende and muscovite ar ar cooling ages cluster between c 345 and c 326 ma and date the effective closure conditions and the onset of rapid nearly isobaric cooling the continent continent collision that formed the granulites pre dates c 370 ma the intra moldanubian nappe stacking pre dates thrusting of the moldanubian zone over the moravian zone the range c 340 335 ma is the lower limit for completion of tectonic activity in the moldanubian zone the moldanubian series are post tectonically intruded by granitoids of the southern bohemian pluton recent age determinations and geochemical evidence suggest that the formation of the early granitoid types took place in the lower crust in connection with the hercynian high grade overprint the moldanubian monotone series in austria is separated from the other moldanubian units by a conspicuous tectonic horizon it also differs from them by its characteristic high t low p overprint which is best demonstrated by a widespread cordierite gneiss

thermobarometry diffusion modelling and cooling rates of crustal garnet peridotites two examples from the moldanubian zone of the bohemian massif abstract different origins for crustal garnet peridotites are illustrated by two variscan bodies in adjacent tectonically juxtaposed terranes in the moldanubicum of the bohemian massif eastern moravia the nove dvory garnet peridotite contains lenses of eclogite and is allochthonous and allofacial with respect to surrounding gfohl gneiss the mohelno spinel garnet peridotite is devoid of eclogite and has concordant contacts with surrounding felsic granulite the bulk of the mohelno body is spinel peridotite and garnet occurs only along its margins where it has formed at the expense of spinel thermobarometry of mineral cores from garnet bearing assemblages yields values for nove dvory from 1195 c 54 6 kbar to 970 c 41 8 kbar and for mohelno from 1265 c 27 9 kbar to 1070 c 23 4 kbar the compositional zoning profiles in garnets at both localities are matched closely by diffusion modelling in which bilinear cooling rates are utilized at nove dvory initial cooling rates of 50 200 c ma are followed by slower rates of 10 50 c ma in contrast at mohelno an extremely rapid initial cooling rate of 1 c year was followed by one of 500 c ma an idealized tectonic model is proposed for the emplacement of the mohelno peridotite to account for its cooling history the predicted garnet zoning profiles that result from conductive cooling agree closely with the observed zoning profiles both peridotites were derived from the upper mantle the nove dvory body was emplaced from a deep fault zone into the crust as garnet peridotite and the mohelno body was emplaced as a hot slab of spinel peridotite in which garnet formed only along contacts as the body cooled into the garnet stability field

early palaeozoic tectonothermal events in basement complexes of the eastern greywacke zone eastern alps evidence from u pb zircon data field observations and u pb zircon data yield evidence that the kaintaleck complex represents a distinct crystalline basement unit of the eastern greywacke zone and contrasts partly with other pre alpine basement complexes of the eastern alps the age data indicate possible late archean rock formation several magmatic and metamorphic events in the early palaeozoic ca 520 490 and ca 400 360 ma and low grade metamorphic overprint in the cretaceous zircons from a garnet gneiss layer in a plagioclase gneiss and amphibolite sequence yield an upper intercept age of ca 2 55 ga which is interpreted as a possible protolith age the lower intercept age at 514 ma represents either 1 a magmatic event or 2 a metamorphic overprint within amphibolite facies conditions magmatic zircons derived from granitic orthogneiss boulders of the kalwang conglomerate which covers the crystalline basement crystallised at ca 500 ma the lower discordia intercepts of the orthogneiss boulders mark the cretaceous low grade metamorphic overprint the lower intercept age of paragneiss zircons from another slice of the kaintaleck complex suggests a likely devonian metamorphic event at ca 390 400 ma the new data demonstrate that the kaintaleck complex experienced several stages of tectonothermal evolutions which are in contrast to the evolution of the main mass of the austroalpine basement the data constrain linkages of this part of the austroalpine basement to the cordillere ligerienne and cadomian block within west european variscides

zircon dating of north bohemian granulites czech republic further evidence for the lower carboniferous high pressure event in the bohemian massif u pb zircon and rutile multigrain ages and 207pb 206pb zircon evaporation ages are reported from high pressure felsic and metapelitic granulites from northern bohemia czech republic the granulites in contrast to those from other occurrences in the bohemian massif do not show evidence of successive ht mplp overprints multigrain size fractions of nearly spherical multifaceted metamorphic zircons from three samples are slightly discordant and yield a u pb concordia intercept age of 348 10 ma whereas single zircon evaporation of two samples resulted in 207pb 206pb ages of 339 1 5 and 339 1 4 ma respectively a rutile fraction from one sample has a u pb concordia intercept age of 346 14 ma all ages are identical within error and a mean age of 342 5 ma was adopted to reflect the peak of hp metamorphism because rutile has a lower closing temperature for the u pb isotopic system than zircon the results and the p t data imply rapid uplift and cooling after peak metamorphism the above age is identical to ages for high grade metamorphism reported from the southern bohemian massif and the granulite massif in saxony it can be speculated that all these granulites were part of the same lower crustal unit in early carboniferous being separated later due to crustal stacking and subsequent late variscan orogenic collapse

a single devonian subduction event for the hp ht metamorphism of the cabo ortegal complex within the iberian massif abstract ion microprobe data are reported for zircons from ultramafic mafic and metasedimentary high pressure hp rocks of the cabo ortegal allochthonous complex galicia nw spain the new data suggest that the protoliths of eclogites morb type and mg and al rich and mafic granulites have formed in early ordovician times 495 11 469 9 and 478 12 ma respectively involving oceanic and or continental mantle sources we have found no evidence for a hp metamorphic event at 480 ma and for the previously proposed second hp event in the silurian instead a single subduction induced pt t loop with a climax around 390 ma early devonian can be deduced from the relevant domains of zircons from the following rocks two different types of eclogites two hp paragneisses one garnet pyroxenite and one spinel harzburgite rapid cooling and exhumation minimum rates of ca 90 c ma and ca 5 8 km ma are derived for the time immediately after the peak conditions after cooling to ca 500 c slower cooling and exhumation rates ca 15 18 c ma and ca 0 18 km ma are probably artefacts due to an episode of reheating at ca 350 ma this interpretation is supported by the scatter of various mineral ages and the presence of synkinematic granitoids a subduction related acadian early devonian hp metamorphism followed by an early carboniferous collision can thus be demonstrated for the first time also for the iberian part of the european variscides

crystal chemistry and origin of grandidierite ominelite boralsilite and werdingite from the bory granulite massif czech republic a veinlet croscutting the host granulite foliation in the bory granulite massif contains mineral assemblage involving grandidierite ominelite boralsilite fe analogue of werdingite dumortierite locally sb ti rich tourmaline and corundum the assemblage formed at medium pressure and moderate temperature during the exhumation of the granulite body

review of nd isotopic data and xenocrystic and detrital zircon ages from the pre variscan basement in the eastern bohemian massif speculations on palinspastic reconstructions

behaviour of zircon in high grade metamorphic rocks evidence from hf isotopes trace elements and textural studies hf isotopic data of minerals in a mafic pyroxene granulite from the southern bohemian massif together with their major and trace element composition and petrological observations were used to decipher the metamorphic history and behaviour of zircon in the granulite the hf isotopic composition in the minerals was used to estimate whether the decompression reaction namely the consumption of garnet and rutile could have provided zr for the formation of newly grown metamorphic zircon the age of the decompression reaction indicated by the evolution of hf isotopes in garnet and orthopyroxene is between 333 and 331 ma i e ca 7 ma younger than the available u pb zircon ages from the moldanubian granulites and than the newly obtained 343 2 ma laser ablation icp ms u pb age of zircons the combination of bulk and in situ hf isotopic data major and trace element composition and petrological modeling of p t evolution revealed that the formation of zircons can not be related to the decompression phase of the evolution of the mafic granulites

a preliminary magnetic study of soil samples from west central minnesota abstract various rock magnetic techniques were applied to characterize magnetically the samples of a soil profile taken from west central minnesota there is a marked change in magnetic properties as a function of depth in the core x ray analysis and curie temperature measurements carried out on the magnetic fractions indicate that magnetite is the dominant iron oxide in both the top soil and the subsoil the intensity of anhysteretic remanent magnetization arm decreases sharply as the depth increases in contrast the stability of arm was found to be higher for the subsoil the surface soil sample was capable of acquiring a significant amount of viscous remanent magnetization vrm the vrm acquisition coefficient s a of the subsoil s a 3 18 10 6 emu g 1 3 18 10 6 a m 2 kg 1 was about ten times weaker than that of the top soil sample s a 3 868 10 7emu g 1 3 868 10 7 a m 2 kg 1 the magnetic domain state indicator the ratio of coercivity of remanence to coercive force h cr h c was 1 5 and 3 85 for the top soil and subsoil respectively it appears that the observed variations in magnetic properties down the present soil core is due only to a difference in grain size we conclude that the magnetic grains in surface soil samples were more single domain sd like whereas the magnetite grains in the subsoil samples were more likely in pseudo single domain psd or small multidomain md range the observed lower stability for the surface soil samples is attributed to the presence of superparamagnetic grains whose presence was confirmed by transmission electron micrographs

magmatism and early variscan continental subduction in the northern gondwana margin recorded in zircons from the basal units of galicia nw spain in situ uranium lead dating la sf icpms and sims and lu hf isotope analyses la mc icp ms of zircon from eclogite facies rocks from the basal units of the variscan belt in galicia constrain their magmatic and metamorphic evolution and give some clues about the nature and origin of the involved basement the samples studied are two felsic gneisses two eclogites and one eclogitic gneiss of intermediate composition metatonalite oscillatory zoned zircon cores from the felsic samples gave a main clustering of u pb ages at 493 2 and 494 2 ma and some older ages that represent inherited cores zircon grains from the intermediate and one of the mafi c rocks show no inherited cores and yielded ages of 494 3 and 498 6 ma respectively interpreted as time of protolith crystallization variably developed homogeneous zircon rims in one felsic gneiss yielded an age of 372 3 ma and very tiny zircons of one eclogite gave 350 2 ma both of which we interpret as metamorphic ages the new age data demonstrate that the calc alkaline magmatic suite described in the basal unit is ca 20 ma older than the alkaline to peralkaline plutonic suite of the same unit dated at 472 2 ma rodriguez et al 2007 and thus probably represents a distinct geologic event overgrowth rims are interpreted as metamorphic on the basis of their lu hf and th u ratios the 372 3 age is considered as dating the high pressure high p metamorphism and is essentially in agreement with previous ar ar and rb sr data this high p metamorphism marks the initial early variscan subduction of the gondwana margin the inherited zircon ages and hf isotopic composition of zircons point to a considerable input of crustal material with west african craton provenance to the felsic magma

meteorites from the united arab emirates description weathering and terrestrial ages we report on the first meteorite search campaign in the united arab emirates uae the geology and proximity of our search region suggest that it is the north western extension of the oman meteorite fields we found 26 ordinary chondrites bringing the total number of official meteorites from the uae to 28 the campaign was organized and conducted in close cooperation with the uae government and the main masses of the meteorites remained in the country where they will become part of an exhibition the bulk composition of five meteorite and three soil samples indicates an uptake of u mo sr ba li and pb from the soil into the meteorites during terrestrial weathering terrestrial ages determined from 14c decay of 21 meteorites range from recent falls to 24 4 ka with two meteorites having 37 ka and approximately 39 ka respectively weak correlations between weathering degree meteorite bulk chemical composition and terrestrial age suggest highly localized weathering conditions possibly related to abundant occurrences of sabkhas in the search region

unsupported radiogenic pb in zircon a cause of anomalously high pb pb u pb and th pb ages ion microprobe u th pb isotopic analyses of zircons from a granodioritic orthogneiss from the napier complex mount sones enderby land antarctica have identified an unambiguous example of unsupported radiogenic pb in a 3 950 ma old crystal at one 40 m spot on the crystal an unusually large heterogeneity in pb content was found the concentration of radiogenic pb ranging from 5 to 50 percent higher than could have been generated in 3 950 ma by radioactive decay of the co existing u and th this relative excess of radiogenic pb is attributed to pb gain rather than to u and th loss because first the pb content varied by more than the u or th contents and secondly changes in the pb u pb th and pb isotopic composition correlated directly with changes in the pb concentration the individual 207pb 206pb apparent ages ranged from 4 000 ma to 4 145 ma all greater than the inferred age of the crystal a correlation between 207pb 206pb and pb u shows that the pb excess has not resulted from recent pb movement the spot apparently gained radiogenic pb about 2 500 ma ago at the same time as the majority of the other zircons in the rock suffered substantial pb loss the pb movement occurred in response to a discrete geologic event reverse discordance is a phenomenon that must be considered when interpreting zircon u pb ages especially 207pb 206pb ages of single crystals or portions of crystals

tectonometamorphic evolution of the bohemian massif evidence from high pressure metamorphic rocks the variscan orogenic belt of which the bohemian massif is a part is typically recognized for its characteristic low pressure high temperature metamorphism and a large volume of granites however there are also bodies of high pressure rocks eclogites garnet peridotites and high pressure granulites which are small in size but widely distributed throughtout the massif initially the high pressure rocks were considered to be relicts of a much older orogenic event but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have palaeozoic protoliths metamorphic ages from the high pressure rocks define no single event instead a number of discrete clusters of ages are found between about 430 ma and the time of the dominant low pressure event at around 320 330 ma

exhumation of high pressure rocks of the kokchetav massif facts and models abstract the exhumation of ultrahigh pressure uhp metamorphic units from depths more than 100 120 km is one of the most intriguing questions in modern petrology and geodynamics we use the diamondiferous kumdy kol domain in the kokchetav massif to show that exhumation models should take into consideration initially high uplift velocities from 20 down to 6 cm year and the absence of the deformation of uhp assemblages the high rate of exhumation are indicated by ion microprobe shrimp dating of zircons from diamondiferous rocks and supported by the low degree of nitrogen aggregation in metamorphic diamonds diamondiferous rocks in the kumdy kol domain occur as steeply dipping 60 80 thin slices few hundred metres within granite gneiss using geological petrological and isotopic geochemical data we show that partial melting of diamondiferous metamorphic rocks occurred a very important factor which has not been taken into account in previous models deformation of diamondiferous rocks at kumdy kol is insignificant diamond inclusions in garnet are often intergrown with mica crystals carrying no traces of deformation all these facts could be explained by partial melting of metapelites and granitic rocks in the kumdy kol domain the presence of melt is responsible for an essential reduction of viscosity and a density difference between crustal rocks and mantle material and reduced friction between the upwelling crustal block the subducting and overriding plates besides the exhumation rate seems to depend on internal pressure in the subducting continental crustal block which can be regarded as a viscous layer between subducting continental lithosphere and surrounding mantle we construct different models for the three stages of exhumation a model similar to corner flow for the first superfast exhumation stage an intermediate stage of extension most important from structural point of view and a very low rate of exhumation in final diapir erosional uplift

model of syn convergent extrusion of orogenic lower crust in the core of the variscan belt implications for exhumation of high pressure rocks in large hot orogens reflection seismic section field structural analysis and gravimetric modelling of orogenic lower crust in the core of a carboniferous orogenic root reveal details of the polyphase process of exhumation subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the nw the fabrics are developed in both mid crustal and lower crustal high pressure granulite units as a result of intensive nw se intraroot horizontal shortening driven probably by the west directed collision in granulites the steep fabrics originated as a result of extrusion of orogenic lower crust in a 20 km wide vertical ascent channel from lower crustal depths at 350 340 ma the large granulite bodies preserve older granulite facies fabrics documenting a two stage evolution during the exhumation process surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth suggesting preservation of the 20 km wide subvertical tabular structure reaching moho depths horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units this structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid crustal levels 765 c and 0 76 gpa at 342 337 ma large massifs of extruded and progressively dismembered felsic granulites disturbed mid crustal fabrics in the surrounding horizontally flowing partially molten crust the horizontal mid crustal flow resulted in collapse of the supra crustal tepla barrandian unit interpreted as the orogenic lid along a large scale crustal detachment above the extruded lower crustal dome the presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens

u pb zircon age and nd isotopic composition of granitoids charnockites and supracrustal rocks from heimefrontfjella antarctica heimefrontfjella and mannefallknausane in dronning maud land antarctica comprise an amphibolite facies terrain and a granulite terrain separated by a major mylonite zone the amphibolite terrain is made up of mafic to felsic metavolcanics and metasediments intruded by granitoid plutons the granulite terrain has supracrustal rocks with similar lithologies intruded by felsic plutonic rocks that crystallized as charnockites

age of detrital zircon and titanite in the meguma group southern nova scotia canada clues to the origin of the meguma terrane abstract precise u pb isotopic analysis of seventeen selected high quality laboratory abraded zircon single grains seven multigrain fractions of similar material and four paramagnetic titanite fractions record the ages of the various detrital grain types from two turbidite samples collected from the top and about 5 km below the top of the goldenville formation of the meguma group the results show that the source of both sediments contained 600 ma and 2000 ma old volcanic plutonic components whereas a third 3000 ma old component occurs exclusively in the upper sample and the youngest zircons and titanite are found in the lower sample lead lead ages of bulk fractions at 2300 ma and two nearly concordant single grains at 2040 ma and 2140 ma upper sample together with 2157 ma bulk fraction and 2060 ma and 2080 ma single grain fractions lower sample indicate that the 2000 ma component dominates the pink purple euhedral grain types detrital titanite gives ages of 605 ma upper sample and 552 ma lower sample with the latter sample containing a minor 1975 ma component colourless zircons record this same age difference the lower sample contained the youngest concordant single zircon grain 566 ma and 580 605 ma bulk grain fraction even though two concordant single grains have ages of 632 and 643 ma estimated bulk fraction ages for certain zircon grain types from the upper sample fall in the range of 605 to 675 ma these data suggest that while the youngest rocks in the source region were removed by erosion during the early stages of deposition of the goldenville formation the oldest source rocks were not unroofed until later in the depositional history the 566 8 ma age of the youngest concordant zircon from the stratigraphically lower sample provides a maximum age for the deposition of this grain with the base of the cambrian placed between 545 and 530 ma the goldenville formation may straddle the precambrian cambrian boundary graptolites from the immediately overlying halifax formation are of early ordovician tremadocian age the lower sample contains spherical zircons that may have been derived from mafic granulite with an estimated age of 2000 2200 ma with this one possible exception all of the zircons analyzed appear to be igneous in origin and all are relatively euhedral indicating that rocks of the ages referred to above probably outcrop in the source area anomalous discordance patterns imply that part of the 2960 ma area was metamorphosed by the 2000 ma event and that both the 2960 and 2000 ma regions were metamorphosed during the 600 ma event the complete lack of detritus with ages between 680 and 2040 ma virtually eliminates a northern north american or baltic shield provenance however the available data are not sufficient to distinguish between a west african or guyana shield source