identification of inherited radiogenic pb in monazite and its implications for u pb systematics the simplest interpretation of uranium lead mineral ages in igneous rocks embodies the assumption that melting of pre existing rocks leads to homogenization of the lead isotopes thus resetting the radiometric clock zircons often violate this assumption by only partially dissolving and preserving a component of lead inherited from before the melting episode but until now such behaviour has not been observed in monazite here we report u pb data for zircons and monazites from a tertiary granite from the himalayas in which zircons and monazites contain inherited radio genic lead an analysis of a single entirely igneous zircon crystal plots on concordia at 19 5 0 4 myr eight monazite fractions lie on a line from an upper intercept of 471 10 myr to the youngest fraction which plots slightly above concordia at 21 myr the presence of excess 206pb in the youngest monazite fractions indi cates that the monazite data represent a mixing line between young magmatic monazite and an older inherited component rather than a resetting of 471 myr monazites at 20 myr these data suggest a closure temperature for pb in monazite significantly higher than previously published estimates

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

provenance of neoproterozoic and early paleozoic siliciclastic rocks of the tepla barrandian unit bohemian massif evidence from u pb detrital zircon ages abstract new la icp ms u pb detrital zircon ages from ediacaran and paleozoic siliciclastic rocks are used to constrain provenance and paleogeographic affinities of the tepla barrandian unit tbu in the centre of the bohemian massif central europe czech republic the samples taken span the period from 635 ma to 385 ma and permit recognition of provenance changes that reflect changes in geotectonic regime detrital zircon age spectra of two ediacaran one lower cambrian and three upper ordovician samples resemble the ages known from the nw african proportion of gondwana particularly the trans saharan belt while three rocks from higher lower cambrian to lowermost ordovician strata contain detritus that may have been derived exclusively from local sources the age spectrum of the devonian rock is a combination of the nw gondwanan and local features these new findings in combination with a wide range of published data are in agreement with a neoproterozoic subduction related setting at the margin of gondwana followed by a cambrian early ordovician rifting stage and an ordovician passive margin setting furthermore the data are in favour of a position of the tbu at the gondwanan margin throughout pre variscan times

cambro ordovician anatexis and magmatic recycling at the thinned gondwana margin new constraints from the kouřim unit bohemian massif the kou im unit represents one of the largest pre variscan meta igneous complexes in the bohemian massif and a geochronological whole rock geochemical and sr nd isotopic study was conducted to better understand the magmatic evolution of the early paleozoic gondwana margin five orthogneisses yield u pb zircon ages ranging from 492 4 to 484 2 ma two leucogranites give u pb zircon ages of 500 4 and 485 2 ma interpreted to be inherited from their orthogneiss host rock two samples from the metasedimentary host rock are dominated by neoproterozoic cambrian detrital zircons the abundance of zircon inheritance in the orthogneisses and whole rock sr nd isotopic composition imply an origin from relatively mature continental crustal material the subalkaline subaluminous slightly peraluminous and high k calc alkaline arc like geochemical signature of the orthogneisses is interpreted as inherited from the recycled cadomian metasedimentary source and both the magmatic and metasedimentary rocks are correlated with similar occurrences in the adjacent moldanubicum and tepla barrandian unit the late cambrian early ordovician magmatic activity is linked to crustal anatexis which was probably initiated by thermal and gravitational relaxation of the thickened cadomian arc type crust followed by lithospheric thinning assisted by far field forces the extensional event led to the formation of a passive margin associated with the opening of the rheic ocean supplementary material analytical methods petrographic descriptions with photographs of studied samples collections of cl images of representative zircon grains results of la icp ms u pb analyses of zircons and an additional geochemical diagram are available at https doi org 10 6084 m9 figshare c 4649255

correlation of allochthonous terranes and major tectonostratigraphic domains between nw iberia and the bohemian massif european variscan belt the nw iberian allochthon and the tepla barrandian and moldanubian zones represent the internal parts of the variscan belt in their respective domains a correlation based on the lithological association protolith ages metamorphic evolution detrital zircon age spectra and tectonic setting is attempted between the nw iberian massif and se bohemian massif in order to check whether they could have formed part of the same allochthonous stack the galicia tras os montes zone of the iberian massif and the internal zones of the bohemian massif include from bottom to top a parautochthon and lower allochthon representing the outer edge of the northern gondwana margin an oceanic middle allochthon with cambro ordovician and early devonian ophiolites and an upper allochthon interpreted as a peri gondwanan terrane early variscan subduction related high pressure metamorphism characterizes many of the allochthonous units with ages younging from the structurally upper to the lower units from 400 385 ma to 370 360 ma respectively high and ultrahigh pressure metamorphism occurred also in the saxothuringian autochthon at 360 340 ma but not in the nw iberian autochthon the different behavior of the autochthon in the iberian and bohemian massifs accounts for their distinct evolutions from 360 ma onward we conclude that the upper allochthon was a unique peri gondwanan terrane whereas the middle allochthon represents units of the same peri gondwanan ocean opened at the cambro ordovician boundary and having recorded localized renewed activity in the silurian early devonian no other oceans separated the lower allochthon parautochthon and autochthon

structure emplacement and tectonic setting of late devonian granitoid plutons in the tepla barrandian unit bohemian massif the st novice and cista granodiorite tonalite plutons are small 27 and 38 km2 respectively intrusions that are largely discordant to regional ductile structures in the center of the upper crustal tepla barrandian unit bohemian massif their whole rock and trace element compositions are consistent with medium k calc alkaline magma generated above a subducted slab in a continental margin arc setting the u pb zircon age of the st novice pluton newly determined at 375 2 ma using the laser ablation icp ms technique is within the error of the previously published pb pb age of 373 1 ma for the cista pluton the two plutons also share other characteristics that are typical of concentrically expanded plutons ceps such as elliptical cross section in plan view steep contacts inferred downward narrowing conical shape faint normal zoning and margin parallel magmatic foliation decoupled from the regional host rock structures we interpret the st novice and cista plutons as representing the initial late devonian stage of much more voluminous early carboniferous arc related plutonism represented most typically by the central bohemian plutonic complex in the upper crust of the central bohemian massif these two plutons are important tectonic elements in that they indicate an overall shift of the arc related plutonic activity from the nw to the se accompanied with a general compositional trend of the magmas from medium k calc alkaline to shoshonitic ultrapotassic such a pattern is compatible with se directed subduction of the saxothuringian ocean beneath the tepla barrandian overriding plate as a cause of arc related magmatism in this part of the bohemian massif

evidence of earthquake triggering by the solid earth tides article i nfo clearevidence forearthquake triggeringbythe earth tides hasremained elusive for morethan a century using the largest global earthquake catalog available the neic catalog with 442412 events we observe a clear correlation with 99 confidence between the phase of the solid earth tide and the timing of seismic events earthquakes occur slightly more often at the time of ground uplift by the earth tide i e when normal stresses are reduced within the lithosphere we observe that this phase distribution anomaly is larger for smaller and shallowerearthquakes although earthquakes inregionswithdominantlynormalandstrike slip faulting seem to exhibit more tidal triggering than regions dominated by thrust faulting there is no statistically significant evidence for a focal mechanism dependence on earthquake triggering finally we show here that it is highly probablethat the observedtriggeringiscaused bythe solid earth tide ratherthan byloading fromthe oceanor atmospheric tides although an additional impact due to loading from ocean tides is possible and probable we cannot detect it here because the earthquake database is not sufficiently complete and homogeneous more smallmagnitudeearthquakesinoceanicareasareneeded ourresultsareconsistentwiththeideaofadamped

patterns of earthquakes and the effect of solid earth and ocean load tides at mount st helens prior to the may 18 1980 eruption seismographs near mount st helens volcano recorded an earthquake swarm lasting nearly 2 months prior to the catastrophic may 18 1980 eruption the earthquakes are divided into four classes based on station cpw 116 km seismogram characteristics 1 events with sv p amplitude ratio 3 and dominant frequency 3 hz 2 events with sv p ratio between 1 and 3 and dominant frequency 2 hz 3 events similar to characteristic 2 but with a strong probably surface wave phase just after the s phase and 4 events with frequencies between 1 and 2 hz lacking a clear s phase the seismicity pattern for each of the four classes is unique and we assume each group of events with similar wave forms represents a common physical process depth source area or fault orientation maximum likelihood b values range between 0 97 0 05 and 1 81 0 09 for the four groups we calculate solid earth stress and strain tides at the average hypocentral depth of 4 km we also calculate stress and strain tides induced by ocean loading their amplitudes are typically 20 40 those of the solid earth tides at the location of mount st helens a weak but significant correlation exists between the latter two classes of events and the tides for a time interval of about 5 days preceding the first onset of volcanic tremor and about 5 days thereafter the polarity of the correlation is opposite for the two classes of events in each case the phase of the correlation changes systematically with time the changes coinciding with the onset of tremor on march 31 and with a pronounced decrease in earthquake energy release rate on april 3 there are no significant correlations between the tides and the number of events or energy release of these two classes of earthquakes during any other interval between march 20 and may 18 1980 the first two classes of events show no evidence of significant tidal correlation at any time during the study period

topographic inversion and changes in the sediment routing systems in the variscan orogenic belt as revealed by detrital zircon and monazite upb geochronology in post collisional continental basins abstract the basal alluvial and fluvial siliciclastic successions within the late carboniferous intracontinental basins of the west central bohemian massif provide an intriguing sedimentary record of changes in paleotopography and paleodrainage systems during and after orogenic collapse we analyze this record using the u pb laser ablation icp ms detrital zircon and monazite geochronology five distinct age groups have been identified across the examined stratigraphic succession i archean paleoproterozoic ii late neoproterozoic iii cambro ordovician iv late devonian and v mid to late carboniferous these age groups indicate mixing of both local and distal extra basinal sources and also point to a two stage post convergent geodynamic evolution of the variscan belt the first stage followed collapse of the central bohemian orogenic plateau at around 346 337 ma and involved rapid cooling at ca 340 320 ma and removal of ca 2 3 km of sedimentary overburden from the top of the plateau no sedimentary record of this erosional event suggests that the river systems transported detritus radially outwards from the inner part of the orogen to its forelands a paleorelief inversion and thus major reorganization of the sediment routing systems were then associated with uplift in the southerly and southwesterly peripheral domains of the orogen at ca 330 310 ma in turn these uplifted domains occupied by rapidly unroofed voluminous granite plutons shed detritus into the bohemian sink this was represented by the former plateau collapsed to relatively low elevations but still with significant internal paleorelief dominated by a system of tens of kilometers long nne ssw trending paleovalleys some of the material was further transported in extensive river systems crossing the former plateau to the northerly variscan foreland basin thus the radial sediment routing system changed into axial unidirectional finally we suggest that these major inversions of paleorelief and changes in the surface processes and paleodrainage systems may have been linked to distinct late orogenic lithospheric mantle delamination events first stage delamination occurred beneath the center of the bohemian massif to cause the plateau collapse denudation and radial sediment transport whereas the second stage delamination occurred along its periphery to cause uplift sourcing the intracontinental basins and unidirectional sediment transport end of deposition and magmatic shutdown at around 305 ma may then define the very cessation of late orogenic vertical movements and the transition to a post orogenic intra plate setting

experimental resetting of the u th pb systems in monazite abraded fragments 200 400 m of a large chemically homogeneous and non metamict brazilian monazite crystal characterised by a concordant u pb ages of 474 1 ma 208pb 206pb 19 5 were hydrothermally treated at varying temperatures with solutions of different compositions product monazites were analysed with scanning electron microscope sem electron microprobe emp secondary ion mass spectrometer sims and isotope dilution thermal ionisation mass spectrometer id tims experiments with pure water over a temperature range of 800 1200 c at 700 mpa and durations ranging from 5 to 60 days showed that even at 1200 c any dissolution and recrystallization of new monazite is confined to the outermost surface of the grain neither pb diffusion at the emp scale nor significant discordancy were observed we performed experiments at 800 and 1000 c for different durations using different fluid compositions at quartz saturation a 10 wt cacl2 fluid a 10 wt srcl2 fluid a 10 wt nacl fluid and a fluid containing nbs 982 pb standard 208pb 206pb 1 for all runs emp traverses revealed no pb diffusion profiles significant overgrowths of newly formed monazite are documented by sem analyses they occurred only in the 1000 c experiments when either cacl2 or pb bearing fluids were present in the cacl2 experiment two zones could be distinguished within the crystal a core possessing the initial monazite composition and a rim consisting of newly formed monazite produced by dissolution precipitation which was enriched in ca and pb free id tims dating of single grains treated with srcl2 and cacl2 solutions at 1000 c are significantly discordant experiments employing the nbs pb standard produced sub concordant monazite for which the 207pb 206pb apparent age has become older than prior to the experiment 544 ma at 800 c and 495 ma at 1000 c the newly grown monazite rim had obviously incorporated pb from the fluid none of our reaction products contained a detectable diffusion profile the only resetting mechanism we detected involved dissolution precipitation the extent of the dissolution precipitation process depends on fluid composition and is a more efficient mechanism than diffusion for controlling the resetting of monazite in natural rocks

monazite xenotime garnet equilibrium in metapelites and a new monazite garnet thermometer prograde suites of pelitic rocks were examined with electron microprobe introduction and laser ablation inductively coupled plasma mass spectrometry to monazite ce la th po4 plays an important role in determine the systematics of element partitioning between coexisting studies of igneous and metamorphic petrogenesis foremonazite xenotime and garnet monazite grains that grew in most monazite is used to date specific events in a equilibrium with xenotime are enriched in y and dy compared with petrogenetic sequence e g parrish 1990 harrison et al monazite that grew in xenotime absent assemblages y and heavy 1997 hawkins bowring 1999 foster et al 2000 rare earth element contents of monazite coexisting with xenotime monazite may contain a large percentage of the sample increase with rising temperature monazite xenotime y gd and rare earth element ree budget and thus exert an y dy partitioning is systematic within a metamorphic grade and important influence on the evolution of melt composition increases slightly with increasing metamorphic grade suggesting wark miller 1993 bea 1996 the thermobarometric that monazite xenotime pairs approached partitioning equilibrium potential of monazite coexisting with xenotime has been garnet and monazite in both xenotime bearing and xenotime absent recognized leading to calibration of monazite geoassemblages show a strong r 0 94 systematic relationship thermometers gratz heinrich 1997 1998 heinrich between inverse temperature and ln keq for the net transfer equiet al 1997 andrehs heinrich 1998 in addition the librium yag oh ap 25 4 qtz 5 4 grs 5 4 an recognition that nearly all lead in monazite is radiogenic 3ypo4 mnz 1 2h2o suggesting that garnet and monazite parrish 1990 montel et al 1994 has led to development crystallized in compositional equilibrium the following temand application of electron microprobe monazite dating perature keq relationship for the equilibrium above has been derived techniques suzuki adachi 1991 montel et al 1996 williams et al 1999 t c 1 45p bars 447772 32052 567 40 rln keq 273 15 full realization of monazite as a thermochronometer requires detailed understanding of the specific reactions responsible for its formation previous studies of monazite with a precision of some 30 c for temperature estimates our petrogenesis have focused on monazite compositional observations suggest that major and accessory phases interact in a zonation zhu o nions 1999a 1999b monazite coupled fashion during metamorphism and also approach a state growth kinetics ayers et al 1999 behavior of monazite of compositional equilibrium as reactions proceed during hydrothermal alteration poitrasson et al 1996 crowley ghent 1999 and melting events watt 1995 or to a limited extent the influence of major phase mineral assemblage in monazite reactivity zhu

the moldanubian zone in the french massif central vosges schwarzwald and bohemian massif revisited differences and similarities in order to portray the main differences and similarities between the northeastern variscan segments french massif central fmc vosges black forest and bohemian massif bm we review their crustal scale architectures the specific rock associations and lithotectonic sequences as well as the ages of the main magmatic and metamorphic events this review demonstrates significant differences between the moldanubian domains in thebmand the fmc on this basis we propose distinguishing between the eastern and western moldanubian zones while the vosges black forest mountains are an intermediate section between the bm and the fmc the observed differences are the result of first the presence in the french segment of an early large scale accretionary system prior to the main variscan collision and second the duration of saxothuringian armorican subduction which generated long lived magmatic arc and back arc systems in the bohemian segment while the magmatic activity in the fmc was comparably short lived

post granulite facies monazite growth and rejuvenation during permian to lower jurassic thermal and fluid events in the ivrea zone southern alps u pb analyses of single monazite grains from two granulite facies metapelites in the ivrea zone southern alps reveal the presence in both samples of at least three different ages and prove that earlier interpretations of supposedly concordant monazite data as cooling ages are unwarranted one group of monazite data defines a subconcordant discordia line with an upper intercept age of 293 4 5 8 ma and a lower intercept age of 210 14 ma the upper intercept is interpreted as the real cooling age of the monazites the lower intercept is interpreted as an episode of fluid driven pb loss indicated by the presence of internal and external corrosion structures not only of the monazites but also of the zircons in the same samples that are also rejuvenated at 210 12 ma another group of monazite data lies above the concordia the presence of excess 206pb indicates that these crystals have grown below the monazite blocking temperature thus after the granulite facies metamorphism the age of growth of the new monazite crystals is approached by their 207pb 235u ages that range between 273 and 244 ma the two groups of post cooling age post 293 4 5 8 ma monazite data correspond to two distinct late and post variscan geotectonic regimes that affected the southern alps 1 permian transtension with decompression and anatectic melting 2 upper triassic to lower jurassic rifting with geographically dispersed hydrothermal activity and alkaline magmatism

late cadomian crustal tilting and cambrian transtension in the tepla barrandian unit bohemian massif central european variscides the cadomian basement of the tepla barrandian unit is characterized by a classic barrovian type metamorphism the degree of which increases considerably towards the west reaching amphibolite facies conditions in the domaz lice crystalline complex dcc the number and volume of plutons also increases towards the west the emplacement ages of the te s ovice granite and the mrac nice trondhjemite have been determined at 521 7 2 ma and 523 4 5 ma respectively applying conventional u pb analyses of zircons pervasive high temperature prolate fabrics and north northwest dipping dextral oblique slip shear zones within the mrac nice trondhjemite suggest a synkinematic melt emplacement within a lower cambrian transtensional setting transtension is probably related to early stage rifting that introduced the separation of the tepla barrandian unit as part of armorica from gondwana structural and petrological data of the country rocks show that the barrovian type metamorphism and two deformation stages d1 with unknown kinematics and d2 top to the north shearing are older than the melt emplacement and thus can be attributed to the cadomian orogeny the intrusion depth of both plutons is nearly the same ca 7 km although the degree of barrovian type metamorphism differs significantly within the country rocks this suggests late cadomian eastward tilting of the metamorphic isograd planes the weak post plutonic lower greenschist to subgreenschist facies folding and thrusting result from variscan northwest southeast compression

conventional and in situ geochronology of the tepla crystalline unit bohemian massif implications for the processes involving monazite formation the tepla crystalline unit tcu western bohemian massif proves highly suitable for studying the effects of differential metamorphic reworking on the u th pb systematics in monazite as the overprint of variscan regional metamorphism onto high grade cadomian paragneisses intensifies progressively towards the northwest although variably hampered by scarcity small size and low uranium contents of monazite isotope dilution thermal ionisation mass spectrometry of monazite from paragneisses from the garnet staurolite and kyanite zones of the tcu gives a narrow 206pb 238u age range from 387 to 382 ma for variscan peak metamorphism these data are supported by 382 373 ma monazite ages derived from electron microprobe analyses inheritance of older components in grains from the central tcu imply major resetting of pre variscan monazite around 380 ma possibly due to widespread garnet growth during variscan metamorphism which led to the consumption of pre variscan high y monazite and subsequent growth of new low y monazite concordant 498 494 ma monazite ages in a migmatitic paragneiss close to the adjacent marianske lazn complex mlc grew in response to metagabbro emplacement in the mlc from 503 to 496 ma and not during either cadomian or variscan regional metamorphism backscatter imaging and electron microprobe analyses reveal that discordant monazite of the migmatite comprises a mix of various age domains that range from ca 540 to 380 ma combined evidence presented here suggests that instead of pb loss by volume diffusion the apparent resetting of the u th pb systematics in monazite rather involves new crystal growth or regrowth by recrystallisation and dissolution reprecipitation

timing styles and kinematics of cambro ordovician extension in the tepla barrandian unit bohemian massif and its bearing on the opening of the rheic ocean this paper describes late cambrian dikes and early ordovician volcano sedimentary successions of the prague basin bohemian massif to discuss the timing and kinematics of breakup of the northern margin of gondwana andesitic dikes indicate minor e w crustal extension in the late cambrian whereas the tremadocian to dapingian lithofacies distribution and linear array of depocenters suggest opening of this rheic ocean rift related basin during nw se pure shear dominated extension this kinematic change was associated with the onset of basic submarine volcanism presumably resulting from decompression mantle melting as the amount of extension increased we conclude from these inferences and from a comparison with other avalonian cadomian terranes that the rifting along the northern gondwana margin was a two stage process involving one major pulse of terrane detachment in the early cambrian and one in the early ordovician while the geodynamic cause for the former phase remains unclear but still may include effects of cadomian subduction roll back slab break off isostatic rebound or mantle plume the incipient stage of the latter phase may have been triggered by the onset of subduction of the iapetus ocean at around 510 ma followed by advanced extension broadly coeval tremadocian to darriwilian in large portions of the avalonian cadomian belt unequal amounts of extension resulted in the separation and drift of some terranes while other portions of the belt remained adjacent to gondwana

an electron microprobe study of the u th pb systematics of metamorphosed monazite the role of pb diffusion versus overgrowth and recrystallization abstract an electron microprobe emp was used to obtain u th pb chemical ages from monazite that underwent an amphibolite facies metamorphic overprint in the monashee complex southern canadian cordillera u pb isotope dilution dating of single grains yielded discordant ages that plot in linear arrays on a concordia diagram upper intercepts of the arrays indicate primary growth in the proterozoic and cretaceous and lower intercepts are coeval with a tertiary metamorphic event the microprobe s high spatial resolution 2 m and ability to quickly differentiate between domains with greatly contrasting u th and pb concentrations made it the ideal tool for investigating the cause of the u pb isotopic discordance the microprobe dating shows that proterozoic monazite lies directly adjacent to generally small

low temperature replacement of monazite in the ireteba granite southern nevada geochronological implications the ireteba pluton is a relatively homogeneous 64 ma zircon ion probe age two mica granite that was intruded by two 16 ma miocene plutons at depths ranging from 5 to 13 km deeper levels of the ireteba and miocene plutons were ductilely deformed at 15 16 ma at shallow levels remote from the miocene plutons the ireteba granite appears to have experienced little miocene heating and deformation monazites from different portions of the pluton reflect the different histories experienced by the host rock irregularly shaped patchy zones with high huttonite component thsio4 are widespread in monazite at deep levels adjacent to miocene plutons but less common in shallow level rock monazite grains with extensive replacement generally have irregular embayed surfaces in undeformed rocks distant from the miocene plutons monazites are less modified and more nearly euhedral though fine networks of replacement veins are common and irregular rims are evident in some grains secondary monazite from these samples is poorer in huttonite ion probe th pb dating yields 60 65 ma ages for magmatic and some replacement zones in monazite from the shallow samples and veins yield apparent ages as young as mid tertiary monazites from deep samples yield a few 55 65 ma ages for remnant magmatic zones and abundant miocene ages for replacement zones 14 18 ma these data demonstrate extensive miocene replacement of magmatic monazite especially at deep levels near miocene plutons and they suggest an early replacement episode as well both events were probably related to influxes of fluid the first may have been associated with initial solidification of the ireteba pluton and the second with the miocene plutons and or extensional deformation ambient temperatures at the time of replacement indicate that secondary monazite growth occurred at t as low as 400 c or less

an independent assessment of the load unload response ratio lurr proposed method of earthquake prediction the load unload response ratio lurr method is a proposed technique to predict earthquakes that was first put forward by yin 1987 lurr is based on the idea that when an area enters the damage regime the rate of seismic activity during loading of the tidal cycle increases relative to the rate of seismic activity during unloading in the months to one year preceding a large earthquake since earth tides generally contribute the largest temporal variations in crustal stress it seems plausible that earth tides would trigger earthquakes in areas that are close to failure e g vidale et al 1998 however the vast majority of studies have shown that earth tides do not trigger earthquakes e g vidale et al 1998 heaton 1982 rydelek et al 1992 in this study we conduct an independent test of the lurr method since there would be important scientific and social implications if it were proven to be a robust method of earthquake prediction smith and sammis 2004 undertook a similar study and found no evidence that there was predictive significance to the lurr method we have repeated calculations of lurr for the northridge earthquake in california following both the parameters of x c yin personal communication and the somewhat different ones of smith and sammis 2004 though we have followed both sets of parameters closely we have been unable to reproduce either set of results our examinations have shown that the lurr method is very sensitive to certain parameters thus it seems likely that the discrepancies between our results and those of previous studies are due to unaccounted for differences in the calculation parameters a general agreement was made at the 2004 aces workshop in china between research groups studying lurr to work cooperatively to resolve the differences in methods and results and thus permit more definitive conclusions on the potential usefulness of the lurr method in earthquake prediction

dating low grade metamorphic events by shrimp u pb analysis of monazite in shales although the u pb isotopic system is widely used to date magmatism and medium to high grade metamorphism dating low grade metamorphic events has been hampered by a scarcity of suitable minerals we show that monazite forms in shales during low grade metamorphism and can be dated by in situ u pb analysis providing precise ages for low temperature thermal events in proterozoic shales from the pine creek inlier of northern australia monazite and xenotime crystals that formed in a contact aureole yield ages synchronous with granite intrusion ca 1830 ma in the pilbara craton of northwestern australia monazite growth 2192 5 m y ago coincided with basaltic volcanism and rifting an event apparently responsible for hydrothermal iron oxide mineralization in a nearby giant iron ore deposit the mineralizing environment was highly oxic and bathed by meteoric fluids thus our results imply the existence of an oxygen rich atmosphere by 2200 ma given the abundance of shales in the stratigraphic record and the likely widespread distribution of monazite in such rocks in situ u pb dating of monazite may become an important chronometer for low grade metamorphic events

alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the oklo okelobondo and bangombe natural nuclear reaction zones franceville basin gabon abstract large scale light rare earth element lree uranium lead and phosphorus migration has been evidenced in the fa lower proterozoic sandstones of the franceville basin gabon hosting oklo natural nuclear reaction zones rz in relation with extensive accessory mineral alteration by highly saline diagenetic brines 28 7 wt nacl eq to 30 wt cacl2 eq at about 140 c and 1 kbar monazite is the most severely altered accessory mineral in the coarse grained sandstones of the basal fa formation detrital monazite crystals are altered to th oh silicate microcrystalline phase with very low concentrations of u and lree the th la ratio increase from non altered th la 0 27 to altered sandstones th la 1 14 shows that about 76 of the lree was leached this corresponds to the leaching of 2 01 109 metric tons at the scale of the fa formation in the franceville basin similarly the th u increase from monazite th u 18 6 to the th silicate phase th u 88 7 is interpreted as a result of an alteration by oxidizing brines with leaching of u together with lree and p it corresponds to the leaching of 9 06 106 metric tons of uranium this amount of uranium largely exceeds the known uranium reserves from the franceville basin in zircon crystals the cores are generally homogeneous weakly fractured and well preserved as attested by the archean ages 2867 24 and 2865 51 ma obtained by ionic microprobe analysis on zircon of the fa formation respectively from the marginal and central parts of the basin their composition corresponds to the pure end member zr hf sio4 poor in th and u th u 1 at the contrary their rims which present several growth zones with cracks fillings are enriched in ree p th and u with higher th u ratios 5 10 both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings for example in zircon the pb of galena crystals 3 23 wt largely exceeds the amount of pb maximum 0 1 wt that would have been produced in situ by radioactive decay in this mineral nearly all the lead were introduced into altered zones of accessories dissolution of accessory minerals occurred at 2000 ma producing a porous and distorted crystal structure which has allowed a later incorporation of pb galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non radiogenic pb isotopic compositions confirming the external origin of lead pb isotopic evolution models indicate a crystallization age sometime after 1000 ma both for galena located close to and far from u mineralizations and reactor zones which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms between 1000 and 750 ma at the scale of the franceville basin the present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride rich fluids during the diagenesis of a sedimentary basin

absence of earthquake correlation with earth tides an indication of high preseismic fault stress rate because the rate of stress change from the earth tides exceeds that from tectonic stress accumulation tidal triggering of earthquakes would be expected if the final hours of loading of the fault were at the tectonic rate and if rupture began soon after the achievement of a critical stress level we analyze the tidal stresses and stress rates on the fault planes and at the times of 13 042 earthquakes which are so close to the san andreas and calaveras faults in california that we may take the fault plane to be known we find that the stresses and stress rates from earth tides at the times of earthquakes are distributed in the same way as tidal stresses and stress rates at random times while the rate of earthquakes when the tidal stress promotes failure is 2 higher than when the stress does not this difference in rate is not statistically significant this lack of tidal triggering implies that preseismic stress rates in the nucleation zones of earthquakes are at least 0 15 bar h just preceding seismic failure much above the long term tectonic stress rate of 10 4 bar h

modeling and detecting interactions between earth tides and earthquakes with application to an aftershock sequence in the pyrenees abstract assuming that earthquakes are governed by solid friction and that the fault plane solutions are known a simplified model of correlation between the seismicity and a specified component of the tidal stress is proposed in the case of an earthquake sequence with a uniform time distribution and with a prevailing focal mechanism a test of statistical significance is derived from a binomial probability distribution this interaction model is checked over the aftershock sequence of a magnitude 5 earthquake which occurred within a seismic array in the pyrenees on 29 february 1980 the tidal stress field including the ocean tide effect is computed for this area the global time decrease of the seismicity is taken into account by a time transformation in order to deal with the time and space complexity of the stress field a moving window analysis is used it is shown that during one 7 day period out of a total of 30 days a positive correlation appears between earth tides and seismicity in agreement with the proposed model

tidal triggering of intermediate and deep focus earthquakes tidal stress changes within the earth due to the gravitational attraction of the sun and moon can exceed 20 mbar h whereas tectonic stress rates are of the order of 0 2 mbar h although the absolute magnitude of tidal stresses is less than 1 of the typical stress drops observed in earthquakes a correlation between the tidal stress history on a fault plane and the time of failure should exist if 1 tidal and tectonic stresses are combined in a simple manner 2 the tectonic stress buildup over time is smooth and 3 failure occurs immediately upon attainment of some critical stress to search for possible tidal triggering of intermediate and deep focus earthquakes 7359 events were selected from a global catalog and subdivided into 30 regions each event was assigned a tidal phase relative to both the semidiurnal and biweekly tidal stress curves at the time of occurrence all tidal phase distributions were checked for a preferred phase orientation using the rayleigh test and for nonrandomness using the chi square test results of this analysis for the semidiurnal earth tide show that three regions have a preferred phase direction and four regions display nonrandomness significant at the 90 confidence level which would be expected for this number of random data sets the biweekly tide when normalized for the unequal distribution of biweekly phase over time after correction for clustering of events in some of the data sets shows only three non random data sets at the 95 level as defined by the chi square test thus intermediate and deep focus earthquakes in most subduction zones occur independently of both the semidiurnal and biweekly tidal stress cycles and we may conclude that failure below 70 km does not occur at the instant a stress curve the simple sum of tectonic and tidal stresses reaches the yield strength of subcrustal rocks this implies that either tectonic stress rates prior to failure are not smooth and are unaffected by predictable sources of stress or that failure does not necessarily occur when a certain critical stress is reached these may in turn be due to some time delay characteristic of deep failure to the stress history of the material to the diffusion of fluids or to the mode of failure itself at present a unique and unexplained phenomenon at such high temperatures and pressures

two generations of variscan garnet implications from a petrochronological study of a high grade avalonia derived paragneiss from the drosendorf unit bohemian massif abstract garnets in metapelitic paragneisses from the southern drosendorf unit in the austrian part of the bohemian massif exhibit two episodes of growth during the variscan orogeny which can be distinguished on textural and chemical grounds the first garnet grt1 records evidence of high grade metamorphism in the late devonian frasnian famennian while the second garnet grt2 formed by a second high grade event in the early carboniferous visean both garnet generations contain abundant inclusions of which monazite rutile and crystallised melt droplets are particularly useful for reconstructing p t t conditions the late devonian age 373 9 ma for the first episode of garnet growth was obtained from chemical dating of monazite inclusions in grt1 metamorphic conditions during the first episode of garnet growth are estimated to have been between 0 7 and 0 8 gpa at 680 700 c and 0 95 1 10 gpa at 745 785 c there followed a phase of cooling and exhumation after which the second garnet grt2 were formed beginning under amphibolite facies conditions and continuing prograde to peak conditions of 0 95 1 10 gpa and 745 785 c which are similar to those of the first garnet forming event subsequently the rocks experienced near isothermal decompression to 0 5 0 8 gpa chemical dating of both monazite inclusions in grt2 and the matrix provide a visean age 343 3 ma a study of detrital zircons in these paragneisses revealed zircon forming events at around 1 2 1 5 and 1 8 ga suggesting an avalonian provenance the lack of zircons younger than 1 ga and the presence of cadomian metamorphic monazite relics 652 15 ma indicates an early neoproterozoic deposition age for the sedimentary protolith likely our documentation of a late devonian high grade metamorphic event in rocks derived from avalonian corroborates tectonic models which assume that frontal parts of the armorican terrane had already docked with avalonia by this time

provenance of the upper triassic siliciclastics of the mecsek mountains and villany hills pannonian basin hungary constraints to the early mesozoic paleogeography of the tisza megaunit the tisza megaunit in the southern pannonian basin formed part of the southern margin of the european plate in the early mesozoic era its exact paleo position and relation to other structural blocks is disputed for a long time detrital zircon u pb dating heavy mineral analysis and petrographical examination of carnian to pliensbachian sandstone members lead to better understanding of the provenance of clastic deposits after the ladinian carnian carbonate to siliciclastic facies shift in the southwestern tisza megaunit investigations allow for constraining its paleogeographic relation to adjacent units the carnian and pliensbachian siliciclastics of the villany hills derive from inside the southwestern tisza megaunit i e the medium grade polymetamorphic rocks of the adjacent slavonian mountains or similar basement fragments the upper triassic clastic deposits of the mecsek mountains most likely derive from variscan felsic plutonic rocks of the local basement or partially from the southern southwestern bohemian massif about 200 ma zircon u pb ages are tentatively interpreted as traces of synsedimentary distal volcanism in the central atlantic magmatic province

constriction due to subduction evidence for slab pull in the marianske lazně complex central european variscides pervasive l and l s tectonites of the marianske lazn eclogites include pencil fabrics constrictional folds and mullions that result from the first deformation increment observed petrological and microstructural data suggest that these structures formed by constrictional flow at deep subduction levels after or even during the eclogite facies stage and before the subsequent high pressure granulite facies overprint constriction at mantle depths is explained by taking into account the geometry and seismic properties of recent subduction zones here constrictional strain and associated l s tectonites should be expected below the rheologycontrolled slab bend at about 40 km depth where dip parallel extension and a reduced viscosity of the downgoing slab are compatible with a coaxial flow regime

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

pulse of the seafloor tidal triggering of microearthquakes at 9 50 n east pacific rise 1 unequivocal evidence of tidal triggering is observed for microearthquakes 0 4 to 2 0 ml recorded between october 2003 to april 2004 near 9 50 n on the east pacific rise epr although semidiurnal tidal stress changes are small 99 9 nonrandom temporal distribution with events occurring preferentially near times of peak extension due to the proximity of this site to an ocean tidal node where changes in sea surface height are minimal periodic stress changes are dominated by the solid earth tide in contrast previous studies on the juan de fuca ridge have shown microearthquake triggering to be a response to seafloor unloading during times of low ocean tide the modulation of 9 50 n microearthquakes by small amplitude periodic stresses is consistent with earthquake nucleation within a high stressing rate environment that is maintained near a critical state of failure by on axis magmatic and hydrothermal processes

tidal triggering of earthquakes in the ning er area of yunnan province china abstract to investigate the potential effect of tidal modulation on the seismicity in the ning er area a seismically and geothermally active zone in yunnan province china we studied the correlation between earth tides and the occurrence of m 6 0 earthquakes dating back to 1970 as well as their aftershock sequences using theoretically calculated tidal stresses and a statistical test the results show a significant correlation between earth tides and the occurrence of earthquakes six of seven main events occurred when the earth tide increased the coulomb failure stress on the source fault four main events occurred in a narrow range of phase angle corresponding to the maximum loading rate of tidal stress furthermore the histories of the aftershock sequence as a function of the tidal phases demonstrate clear tidal modulation with a high significance thus we conclude that earth tides have a clear role in triggering or modulating the rupture of the fault systems in the ning er area

different age response of zircon and monazite during the tectono metamorphic evolution of a high grade paragneiss from the ruhla crystalline complex central germany results of tims sims and sem analyses show that zircon and monazite in a high grade paragneiss of the ruhla crystalline complex central germany were formed and or altered during different stages of a tectono metamorphic history between early devonian and permian times detrital zircon cores of 460 ma place an older limit on the age of anatexis and show that the paragneiss sequence contains rocks at least as young as early cambrian metamorphic zircon growth commenced at 365 ma peaking at 360 355 ma at the same time that granite dykes were emplaced in contrast monazite in the paragneiss preserves little record of the metamorphic peak most monazite grains grew or were recrystallised in the lower carboniferous at 339 ma contemporaneous with the emplacement of voluminous diorite and granite bodies these intrusions and related tectonics caused some of the high u zircon overgrowths to undergo moderate to severe pb loss a second pb loss event between 300 and 280 ma can be related to late carboniferous early permian large scale block faulting

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

resolving cambrian carboniferous permian and alpine monazite generations in the polymetamorphic basement of eastern crete greece by means of the electron microprobe the electron microprobe based investigation of accessory monazites in polished thin sections is a helpful tool in resolving the geochronology of a polymetamorphic basement the method was applied to variably altered gneisses and micaschists from the retrogressed originally amphibolite facies basement in eastern crete greece the presented data indicate that most monazite formed or recrystallized in response to high fluid activity during alpine low temperature metamorphism this low temperature monazite is characterized by distinctly low yttrium uranium and thorium contents however older grains were able to survive in less retrogressed samples and have been traced with the electron microprobe using microstructural and compositional criteria in situ chemical th u pb dating of these pre alpine monazites provides evidence for an igneous event in the cambrian and two different metamorphic events in the carboniferous and permian

significance of in situ sims chronometry of zoned monazite from the lewisian granulites northwest scotland abstract in situ studies of lewisian metamorphic monazites show them to be strongly zones in th content th u ratio and 207 pb 206 pb age traverses across compositional zones of individual monazites by sims and empa show that there are no resolvable shifts between 232 th 238 u and 208 pb 206 pb ratios and between th and pb contents in any of the profiles obtained furthermore the difference in 208 pb 232 th ages between two zones of monazites in the same thin section is indistinguishable from the difference in 207 pb 206 pb ages determined by sims these observations together with simple models suggest that transport of pb relative to th and u did not occur significantly during younger e g the laxfordian events in the lewisian and the ages preserved in these zoned monazites are close to concordant moreover monazites having different compositions yield indistinguishable 207 pb 206 pb ages which suggests that the closure temperature of the u pb system in these monazites is not significantly dependent upon their composition consequently the pattern of age distribution observed in the monazites is taken to reflect the metamorphic history of the lewisian the results are consistent with the recent interpretations of the lewisian thermal history which suggest two high grade metamorphic events separated by 200 ma

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

the east variscan shear zone geochronological constraints from the capo ferro area ne sardinia italy abstract the exhumation of the variscan high grade metamorphic complex of sardinia was driven by large scale asymmetric shear zones which reached middle crustal levels at 325 1 3 u pb monazite ma exhumation was accompanied by heating and partial melting the migmatization process was followed by the emplacement of syntectonic granites dated with u pb on zircon at 318 3 ma and 317 2 ma which produced contact metamorphism at 315 1 3 ma u pb monazite in mylonitic silurian orthogneisses the nw se dextral shear zone of the capo ferro area ne sardinia is affected by partial melting by granite emplacement and shows remarkable petrotectonic analogies to and can thus be related to the east variscan shear zone a major intra continental shear zone which was active in the southern european variscides from 325 ma to 315 ma and that has been precisely dated with the u pb monazite system the emplacement of migmatitic gneisses along large shear zones within this time interval is not restricted to sardinia but has also been observed in southern corsica in the french massif central in the western alps and in central spain this pattern of shear zones is related to a complex geodynamic scenario linked to the oblique convergence of the gondwana and laurussia plates during the middle upper carboniferous

tidal triggering of earthquakes analysis of the tidal stress tensor at the time of moderate to large earthquakes strongly suggests that shallow 30 km larger magnitude oblique slip and dip slip earthquakes are triggered by tidal stresses no corresponding triggering effect is seen for shallow strike slip earthquakes or for any type of intermediate or deep focus earthquakes which have been studied tidal triggering is also discussed from the viewpoint of the dilatancy diffusion model specifically the model as usually stated excludes the possibility of small earthquakes being tidally triggered

seismic tremor at the 9 50 n east pacific rise eruption site ocean bottom seismic observations within the 9 50 n region of the east pacific rise indicate persistent low amplitude tremor activity throughout the october 2003 through february 2007 period of monitoring these signals exhibit either monochromatic or polychromatic spectral characteristics with a 6 hz fundamental frequency and up to two harmonics individual events cannot be correlated between nearby 1 km stations implying the presence of multiple small amplitude sources positioned within the shallow crust tremor exhibits a semidiurnal periodicity with some stations recording activity during times of increasing tidal extension and others detecting tremor signals during times of increasing compression the amplitude duration and rate of activity also correlate positively with fortnightly changes in the amplitude of the tides these spatiotemporal patterns are consistent with tremor generation in response to tidally modulated fluid flow within a network of shallow cracks tremor energy flux is spatially and temporally heterogeneous however there are extended periods of greater and lesser activity that can be tracked across portions of the array despite their shallow crustal origin changes in tremor amplitude and spectral character occur in the months prior to a major microearthquake swarm and inferred seafloor spreading event on 22 january 2006 with an increase in the degree of correlation between tremor activity and tidal strain in the weeks leading up to this event after the spreading event two eruption surviving stations near the axis continue to show high rates of tremor activity whereas these signals are suppressed at the single station recovered from the near axis flanks this off axis quiescence may result from the dike induced closing of cracks or perhaps from the emplacement of impermeable flows near the station

chronology of the saxothuringian subduction in the west sudetes bohemian massif czech republic and poland isotopic dating of monazite and garnet from high pressure metamorphic rocks exposed in the northern part of the saxothuringian palaeo suture in the bohemian massif revealed a diachronous metamorphism of various rock types that are now closely associated within allochthonous units representing the devonian carboniferous subduction accretionary complex mafic blueschists of the middle unit yielded a lu hf garnet age of 363 9 1 3 ma the blueschists occur within high pressure garnet free phyllites monazite extracted from this rock type yielded a u pb isotope dilution thermal ionization mass spectrometry age of 336 5 0 5 ma garnet bearing micaschist of the lower unit contains monazite with a u pb secondary ion mass spectrometry age of 341 3 ma consistent with lu hf garnet whole rock ages of 344 5 1 3 and 342 7 ma obtained from the same rock type existing tectonic models of the bohemian massif and particularly of its northern part assume that the period of oceanic subduction was terminated at c 380 375 ma by the arrival of an attenuated saxothuringian continental crust which was partly subducted and partly relaminated underneath the overriding tepla barrandian domain however our data as well as data from mafic high pressure rocks in the southern part of the saxothuringian domain suggest that the initial collision was probably caused by the arrival of a smaller crustal block present within the saxothuringian oceanic domain after its subduction and relamination the oceanic subduction was re established and terminated by continental subduction and later collision of the saxothuringian passive margin at c 345 335 ma

middle precambrian provenance of jurassic sandstone in the mino terrane central japan th u total pb evidence from an electron microprobe monazite study precise electron microprobe analyses have been made on detrital monazites in jurassic sandstones of the mino terrane central japan and their th u pb relations are examined in terms of pbo and tho2 tho2 tho2 plus the equivalent of uo2 the pbo tho2 ratios of most detrital monazites 403 of 581 grains cluster in the range from 0 0763 to 0 0618 other grains show the ratio of about 0 0541 46 grains 0 0364 29 grains 0 0170 24 grains and 0 0116 79 grains the pbo tho2 ratio of 0 0763 0 0618 is nearly identical with that of monazites in a middle precambrian 1740 ma gneiss clast from the kamiaso conglomerate of the mino terrane this strongly suggests that the detrital monazites were derived from gneisses and or granitoids of middle precambrian age the ratio of 0 0116 is nearly the same as that of monazites in a permian 274 ma paragneiss of the hida terrane north to the mino terrane the pbo tho2 monazite data coupled with palaeocurrent analyses and petrological characteristics of sandstones show that middle precambrian high grade metamorphic rocks and granitoids were widespread both in the northern and southern source regions of the e w trending jurassic basin of the mino terrane the pbo tho2 ratio of detrial monazites is a useful new measure for the provenance analysis of clastic rocks

evidence for tidal triggering of earthquakes as revealed from statistical analysis of global data 1 we observe tidal triggering of earthquakes by measuring the correlation between the earth tide and earthquake occurrence we used the times locations and focal mechanisms of the 9350 globally distributed earthquakes with magnitude 5 5 or larger from the harvard centroid moment tensor catalog the tidal stress was theoretically computed by using the preliminary reference earth model and a recent ocean tide model nao 99b we considered the shear stress on the fault plane and the trace of stress tensor j1 defining the tidal phase angle at the occurrence time for each earthquake we statistically tested the phase selectivity using the schuster s method for all the earthquakes no significant correlation is found between the earth tide and earthquake occurrence both for the shear stress and for j1 by classifying the data set according to fault types however we find a high correlation with the shear stress for reverse fault type the correlation is particularly clear for shallow and smaller earthquakes of this type significant correlation with j1 also appears for larger earthquakes of reverse fault type and for shallow and larger ones of normal fault type we find no correlation for strike slip type for all the cases of high correlation earthquakes tend to occur when the tidal stress accelerates the fault slip indicating that high correlation is not coincidental but is physically justified this result strongly suggests that a small stress change due to the earth tide encourages earthquake occurrence when the stress in the future focal area is near a critical condition

chemical differences between the island and submarine pumice layers of thera abstract investigations on tephra layers of deep sea cores from near the island of santorini showed that only the pumice clasts from the west of the island can be attributed to the minoan eruption the tephra layers within the caldera are of uncertain origin

pb diffusion in monazite a combined rbs sims study abstract we report measurements of pb diffusion in both synthetic cepo4 and natural monazites run under dry 1 atm conditions powdered mixtures of prereacted cepo4 and pbzro3 were used as the source of pb diffusant for in diffusion experiments conducted in sealed pt capsules for durations ranging from a few hours to several weeks following the diffusion anneals pb concentration profiles were measured with rutherford backscattering spectroscopy rbs and supplemented by measurements with secondary ion mass spectrometry sims in order to evaluate potential compositional effects upon pb diffusivity and simulate diffusional pb loss that might occur in natural systems we also conducted out diffusion experiments on pb bearing natural monazites in these experiments monazite grains were surrounded by a synthetic zircon powder to act as a sink monazites from these experiments were analyzed with sims over the temperature range 1100 to 1350 c the arrhenius relation determined for in diffusion experiments on synthetic monazite is given by d 0 94 exp 592 39 kj mol 1 rt m 2 s 1 diffusivities for synthetic and natural monazites are similar as are results of measurements made on the same samples using both rbs and sims the activation energy for pb diffusion we determined is more than three times that for natural monazite reported previously with pb diffusivities at

a lifetime of the variscan orogenic plateau from uplift to collapse as recorded by the prague basin bohemian massif the ordovician to middle devonian prague basin bohemian massif represents the shallowest crust of the variscan orogen corresponding to c 1 4 km palaeodepth the basin was inverted and multiply deformed during the late devonian to early carboniferous variscan orogeny and its structural inventory provides an intriguing record of complex geodynamic processes that led to growth and collapse of a tibetan type orogenic plateau the northeastern part of the prague basin is a simple syncline cross cut by reverse thrust faults and represents a doubly vergent compressional fan accommodating c 10 19 nw se shortening only minor syncline axis parallel extension and significant crustal thickening the compressional structures were locally overprinted by vertical shortening kinematically compatible with ductile normal shear zones that exhumed deep crust in the orogen s interior at c 346 337 ma on a larger scale the deformation history of the prague syncline is consistent with building significant palaeoelevation during variscan plate convergence based on a synthesis of finite deformation parameters observed across the upper crust in the centre of the bohemian massif we argue for a differentiated within plateau palaeotopography consisting of domains of local thickening alternating with topographic depressions over lateral extrusion zones the plateau growth involving such complex three dimensional internal deformations was terminated by its collapse driven by multiple interlinked processes including gravity voluminous magma emplacement and thermal softening in the hinterland and far field plate boundary forces resulting from the relative dextral motion of gondwana and laurussia

electron microprobe observations of pb diffusion in metamorphosed detrital monazites electron microprobe analyses have been made on monazite grains from paragneiss samples in the andalusite sillimanite transition 620 15 c and sillimanite orthoclase 680 15 c zones of the cretaceous ryoke metamorphic belt southwest japan monazites from pelitic gneisses are of metamorphic origin euhedral to subhedral and chronologically homogeneous giving chemical th u total pb isochron chime ages of 98 8 3 3 98 0 3 2 ma two psammitic gneisses of individual metamorphic grade contain both metamorphic monazite grains and detrital ones as old as ca 1700 ma most detrital monazite grains are heterogeneous in the tho2 and uo2 concentrations and have multiple or single rims as young as ca 100 ma several detrital monazite grains are well rounded in form exhibit homogeneous th and u distributions and show a pb diffusion profile in the margin the width of the diffusion zones is approximately constant throughout grains from each psammitic gneiss 18 22 m for 620 c and 48 58 m for 680 c assuming the isothermal diffusion of pb from homogeneous monazite spheres during a 5 ma duration of peak metamorphism we obtain diffusion coefficients of 1 9 0 3 10 21 and 1 5 0 3 10 20 cm2 s at 620 c and 680 c respectively these data derive an activation energy of 2 44 2 85 1 26 105 j mol and a frequency factor of 3 4 10 7 8 5 10 12 2 2 107 cm2 s taking account of uncertainties of 15 c in the temperatures and of 20 in the diffusion coefficients the diffusion parameters obtained from natural samples in this study provide a reliable insight into the closure temperature for pb in monazite that has been poorly understood so far

influence of fortnightly tides on earthquake triggering at the east pacific rise at 9 50 n the study of tidal forcing provides an opportunity to view the response of the solid earth to known changes in stress earthquakes triggered by diurnal and semidiurnal tidal forces have been well documented for the deep ocean and some terrestrial environments however few studies examine the influence of fortnightly tides on earthquake triggering and only in terrestrial environments we assess triggering of microearthquakes related to true fortnightly tides and fortnightly tidal modulations along the east pacific rise at 9 50 n where strong diurnal and semidiurnal triggering of earthquakes has been observed an ocean bottom seismograph experiment collected microearthquake data between october 2003 and january 2007 a time period that included a well documented submarine eruption culminating in january 2006 we examine how triggering changed through the build up to and aftermath of this seafloor spreading event results show that earthquakes occur preferentially during times of increasing peak volumetric stress leading up to the eruption with strong statistical significance schuster test p values up to 2 22e 31 however the correlation loses its strength immediately prior to and during the eruptive period p value as high as 0 2837 when competing tectonic and magmatic stresses may occur post eruption triggering reappears p value 1 45e 26 but with opposite phase statistical significance is also explored using monte carlo method highlighting limitations of swarm dominated time series for the schuster test nevertheless the observations are consistent with a shift in the dominating stress triggering the earthquakes from extensional tectonics to magmatic processes followed by a post eruption period of cooling relaxation and possibly magma chamber refilling

domainal variations of u pb monazite ages and rb sr whole rock dates in polymetamorphic paragneisses ktb drill core germany influence of strain and deformation mechanisms on isotope systems polyphase metamorphic paragneisses from the drill core of the continental deep drilling project ktb nw bohemian massif are characterized by peak pressures of about 8 kbar medium p metamorphism followed by strain accumulation at t 650 c initially by dislocation creep and subsequently by diffusion creep u pb monazite ages and rb sr whole rock data vary in the dm scale indicating ordovician and mid devonian metamorphic events such age variations are closely interconnected with dm scale domainal variations of microfabrics that indicate different predominant deformation mechanisms u pb monazite age variations dependent on microfabric domains exceed grain size dependent age variations in mylonitic domains recording high magnitudes of plastic strain dislocation creep and minor static annealing monazite yields concordant and near concordant lower ordovician u pb ages and the rb sr whole rock system shows isotopic disequilibrium at an mm scale in mineral growth mobilisate domains in which diffusive mass transfer was a major strain producing mechanism promoting diffusion creep of quartz and feldspar and in which static recrystallization annealing reduced the internal free energy of the strained mineral aggregates concordant u pb ages are mid devonian locally in such domains rb sr dates among mm3 sized whole rock slabs reflect post ordovician resetting in transitional domains the u pb ages are discordant we conclude that medium p metamorphism occurred at 484 2 ma and a second metamorphic event at 380 370 ma mid devonian caused progressive strain in the rocks dislocation creep at high rates even at high temperatures does not reset the rb sr whole rock system while diffusion creep at low rates and stresses i e low e deff ratios static annealing and the presence of intergranular fluids locally assist resetting at temperatures above 650 c diffusive pb loss did not reset ordovician u pb monazite ages and in domains of overall high imposed strain rates and stresses resetting was not assisted by dynamic recrystallization crystal plasticity however during diffusion creep at low rates pb loss by dissolution and precipitation recrystallization of monazite produces discordance and devonian concordant u pb monazite ages hence resetting of these isotope systems reflects neither changes of temperature nor directly the presence or absence of strain

geochronology and geochemistry of eclogites from the marianske lazně complex czech republic implications for variscan orogenesis the marianske lazn complex mlc is located in the bohemian massif along the north western margin of the tepla barrandian microplate and consists of metagabbro amphibolite and eclogite with subordinate amounts of serpentinite felsic gneiss and calcsilicate rocks the mlc is interpreted as a metaophiolite complex that marks the suture zone between the saxothuringian rocks to the north west and the tepla barrandian microplate to the south east sm nd geochronology of garnet omphacite pairs from two eclogite samples yields ages of 377 7 and 367 4 ma samples of eclogite and amphibolite do not define a whole rock sm nd isochron even though there is a large range in sm nd ratio implying that the suite of samples may not be cogenetic eclogites do not have correlated nd values and initial 87sr 86sr ratios five of the eight eclogite samples have high nd values 10 2 to 7 1 consistent with derivation from a morb like source but variable 87sr 86sr ratios 0 7033 to 0 7059 which probably reflect hydrothermal seawater alteration three other eclogite samples have lower nd values 5 4 to 0 8 and widely variable 87sr 86sr ratios 0 7033 to 0 7096 such low nd values are inconsistent with derivation from a morb source and may reflect a subduction or oceanic island basalt component in their source the mlc is an important petrotectonic element in the bohemian massif providing evidence for cambro ordovician formation of oceanic crust and interaction with seawater late devonian frasnian famennian high and medium pressure metamorphism related to closure of a saxothuringian ocean basin early carboniferous visean thrusting of the tepla terrane over saxothuringian rocks and late visean extension

restoration of early variscan structures exposed along the tepla shear zone in the bohemian massif constraints from kinematic modelling perpendicular or high angle relationship of lineations is common in shear zones where synchronous fabric transposition and shearing occur this paper documents a polyphase early variscan structural record exposing the transition from orogenic superstructure to infrastructure along a major detachment shear zone at the western margin of the tepla barrandian domain bohemian massif on the scale of tens of kilometers this region shows major but continuous reorientation of s2 s3 intersection lineation from its nne ssw trends in the superstructure to the ese wnw trends in the infrastructure a simple kinematic numerical model which allows deformation parameters such as fabric attractor deformation symmetry and proportion of simple shear to be varied was formulated to explain the rotation of fabrics observed in the field in line with the results of our kinematic modelling these rotations are interpreted as a result of progressive transposition of the originally subvertical s2 foliation into the gently se dipping s3 marked by stretching at high angle to the incipient s2 s3 intersection the mineral fabric relations suggest that the s2 was associated with crustal thickening and is well preserved in the superstructure while s3 was associated with exhumation of deeper parts of the infrastructure the kinematic model confirmed the operation of major detachment shear zone and allowed a set of the most probable deformation parameters leading to successful simulation of the fabric geometries to be identified in addition the successful model parameters were used to restore the vertical structure of the studied region prior to exhumation on the basis of available pressure temperature data we believe that unlike the commonly used balanced cross section restoration our restoration technique can be successfully applied in orogen interiors if the mineral fabric relations are well constrained and high quality pressure temperature estimates exist

ediacaran to cambrian oceanic rocks of the gondwana margin and their tectonic interpretation in tectonic maps of variscan europe allochthonous pieces of cadomian basement clearly stand out with their predominant metabasic to ultrabasic elements the so called exotic terranes with ophiolites most of these domains are observed in basements of the central iberian allochthone the south armorican domain the nappe structures of the french massif central the saxothuringian zone and the bohemian massif similar relics can be recognized in many alpine basement areas and correlations with supposedly more autochthonous basements such as the ossa morena zone and the central iberian basement can be envisaged all of these relics are thought to represent the interrupted trace of a former continuous or discontinuous structure characterized by the presence of ocean derived proto rheic rock suites these can be interpreted as pieces of former magmatic arcs of ediacaran to cambrian age accreted to the gondwana margin which later were scattered as allochthonous units during the variscan plate tectonic processes the presence of similar rock suites of ordovician age in the alpine realm is explained by the accretion of exotic china derived basements and their collision with the gondwana margin during the opening of the rheic ocean

structural style deformation mechanisms and paleodifferential stress along an exposed crustal section constraints on the rheology of quartzofeldspathic rocks at supra and infrastructural levels bohemian massif abstract investigations of variscan shortening of quartzofeldspathic rocks have been carried out along an exposed crustal section located in the western part of the tepla barrandian unit bohemian massif the study focuses on determining differential stress deformation mechanisms and geometry of cadomian metagreywackes and cambrian granitoids which show different grade of variscan barrovian type metamorphism ranging from the greenschist subgreenschist facies boundary semibrittle deformation regime to the high grade amphibolite facies the semibrittle deformation regime t ca 300 c is characterized by large scale thrusting within a folding environment and by similar intensities of cataclasis diffusive mass transfer and crystal plasticity of quartz the latter including low temperature migration recrystallization within cadomian quartz veins of the greenschist facies part dislocation creep of quartz operated at differential stresses ranging from ca 70 mpa in the lowermost greenschist facies to ca 20 mpa in the uppermost greenschist facies these values have been derived by paleopiezometry using the grain size of recrystallized quartz they are interpreted as upper bounds for the bulk stress because of intense diffusive mass transfer within the adjacent metagreywackes a marking increase in recrystallized grain size of quartz occurs within the lower greenschist facies where low temperature grain boundary migration is replaced by subgrain rotation thus causion is needed when applying the recrystallized grain size paleopiezometer to this crustal level pressure solution of quartz and development of discrete crenulation cleavage was common in fine grained metagreywackes of the lower greenschist facies whereas the upper greenschist facies t ca 500 570 c is characterized by increasing intensity of metamorphic reactions grain coarsening and development of a schistosity the entire greenschist facies level forms a suprastructure where viscosity contrasts between mica and quartz rich layers were sufficiently high for buckle folding strain protected intercalated metagranitoids on the other hand do not show pervasive deformation fabrics the amphibolite facies part forms an infrastructure characterized by pervasive mylonitic shearing of both cambrian granitoids and metagreywackes it is suggested that the combination of the following processes has contributed to weaken the amphibolite facies level making nonlocalized flow possible 1 increased recrystallization of feldspar 2 crystal plastic softening of quartz due to high temperature grain boundary migration and activity of prism slip and 3 enhanced diffusive mass transfer in the form of metamorphic reactions the structural breaks at the semibrittle crustal level and at the greenschist amphibolite facies boundary supports the view that rheological boundaries are prone to form major detachment horizons in the continental crust

neoproterozoic to early cambrian history of an active plate margin in the tepla barrandian unit a correlation of u pb isotopic dilution tims ages bohemia czech republic abstract the tepla barrandian unit tbu of the bohemian massif shared a common geological history throughout the neoproterozoic and cambrian with the avalonian cadomian terranes the neoproterozoic evolution of an active plate margin in the tepla barrandian is similar to avalonian rocks in newfoundland whereas the cambrian transtension and related calc alkaline plutons are reminiscent of the cadomian ossa morena zone and the armorican massif in western europe the neoproterozoic evolution of the tepla barrandian unit fits well with that of the lausitz area saxothuringian unit but is significantly distinct from the history of the moravo silesian unit the oldest volcanic activity in the bohemian massif is dated at 609 17 19 ma u pb upper intercept subduction related volcanic rocks have been dated from 585 7 to 568 3 ma lower intercept rhyolite boulders which pre dates the age of sedimentation of the cadomian flysch st chovice group accretion uplift and erosion of the volcanic arc is documented by the neoproterozoic dob is conglomerate of the upper part of the flysch the intrusion age of 541 7 8 ma from the zgorzelec granodiorite is interpreted as a minimum age of the neoproterozoic sequence the neoproterozoic crust was tilted and subsequently early cambrian intrusions dated at 522 2 ma t sovice granite 524 3 ma vsepadly granodiorite 523 3 ma smr ovice tonalite 523 1 ma smr ovice gabbro and 524 0 8 ma orlovice gabbro were emplaced into transtensive shear zones

seismic triggering effect of tidal stress abstract the tidal stress field of a layered spherically symmetric earth due to the sun and moon is used to investigate its relationship to the occurrence of earthquakes for 70 severe earthquakes which occurred in or near china during the last 24 years we computed the tidal stress in spherical polar coordinates at the focus at the time of occurrence the normal and tangential stress components along the slip direction on the earthquake fault plane were then found by a coordinate transformation coulomb s criterion of shear fracture is used to determine whether these stress components will trigger the earthquakes or not the results show that of the 70 earthquakes 43 have been subjected to a triggering effect and for the 18 events that occurred in north china 14 have been triggered it may be concluded that for shallow strike slip earthquakes there is a significant triggering effect while for oblique slip and dip slip earthquakes the tidal stress has only a small triggering effect from 72 events which occurred in other parts of the world similar conclusions are obtained finally for the purpose of predicting earthquakes the time interval during which the tidal stress may trigger an earthquake on faults in north china is investigated

u pb zircon ages and structural development of metagranitoids of the tepla crystalline complex evidence for pervasive cambrian plutonism within the bohemian massif czech republic u pb zircon dating of three metagranitoids situated within a tilted crustal section at the northwestern border of the tepla barrandian unit tepla crystalline complex tcc yields similar cambrian ages the u pb data of zircons of the tepla orthogneiss define an upper intercept age of 513 7 6 ma the 207pb 206pb ages of 516 10 and 511 10 ma of nearly concordant zircons of the hanov orthogneiss and the lestkov granite are interpreted to be close to the formation age of the granitoid protolith similar to the cambrian granitoids of the southwestern part of the tepla barrandian unit doma lice crystalline complex dcc the middle cambrian emplacement of the tcc granitoids postdates cadomian deformation and metamorphism of the upper proterozoic country rocks but predates variscan tectonometamorphic imprints structural data as well as sedimentological criteria suggest a dextral transtensional setting during the cambrian plutonism related to the early paleozoic break up of northern gondwana due to strong variscan crustal tilting the degree of variscan tectonometamorphic overprint is strikingly different in the dated granitoids it is lowest in the weakly or undeformed lestkov granite located in the greenschist facies domain the tepla orthogneiss in the north underwent pervasive top to nw mylonitic shearing under amphibolite facies conditions there is no indication for a resetting of the u pb isotopic system of the tepla orthogneiss zircons that could be attributed to this imprint radiation damages accumulated until recent have probably caused lead loss

10 km minimum throw along the west bohemian shear zone evidence for dramatic crustal thickening and high topography in the bohemian massif european variscides the west bohemian shear zone wbsz forms a steep collapse structure along which east side down normal movements led to the juxtaposition of the relatively cold cadomian basement of the tepla barrandian unit against high grade moldanubian rocks synkinematic plutons straddle the wbsz the mut nin pluton intruded into moldanubian crust at a depth of 23 4 km as derived by using al in hornblende barometry the tepla barrandian babylon pluton intruded at 12 km depth as indicated by phengite barometry and petrogenetic considerations both emplacement depths together with mineral cooling ages result in a minimum vertical displacement of 10 km between 340 and 320 ma this large throw could be explained by over thickened crust that was weakened from below the alkaline signature of the mut nin diorite indicates that mantle melting was important to thermally weaken the crust at 340 ma the cold tepla barrandian upper crust sank into its weak partly molten moldanubian substratum resulting in elevator style movements not only along the wbsz but also along the hoher bogen and central bohemian shear zone all these ductile normal shear zones were active simultaneously during the lower carboniferous and dip steeply towards the tepla barrandian unit that probably formed a highly elevated plateau at this time

second order moment of central california seismicity 1969 1982 the second order moment cross correlation function of earthquakes in the u s geological survey central california catalog between 1969 and 1982 was calculated with respect to a magnitude threshold m 4 0 over interevent distances up to 80 km and interevent times up to 320 days the statistical procedure results in a representation of the spatial temporal structure of the catalog associated with m 4 0 earthquakes and is capable of revealing patterns too weak to be detected in the space time distribution of seismicity for individual earthquake sequences a method is introduced for identifying aftershocks based on a physical two parameter model of the earthquake interaction process the results show that the aftershock process dominates the second order moment and may even obscure the statistical expression of a precursory process a concentration of foreshocks within 15 km and 3 days of m 4 0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2 6 5 3 km d this concentration may be related to an observed tendency for m 4 0 events to cluster auto correlate over this interevent range with the identified aftershocks removed the residual catalog is poissonion in space and time when two m 4 0 earthquakes occur within 80 km and 40 days of each other aftershock productivity appears to be relatively enhanced in the earlier sequence this suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes

the birth of the rheic ocean early palaeozoic subsidence patterns and subsequent tectonic plate scenarios abstract new plate tectonic reconstructions of the gondwana margin suggest that the location of gondwana derived terranes should not only be guided by the models but should also consider the possible detrital input from some asian blocks hunia supposed to have been located along the cambrian gondwana margin and accreted in the silurian to the north chinese block consequently the gondwana margin has to be subdivided into a more western domain where the future avalonian blocks will be separated from gondwana by the opening rheic ocean whereas in its eastern continuation hosting the future basement areas of central europe different periods of crustal extension should be distinguished instead of applying a rather cylindrical model it is supposed that crustal extension follows a much more complex pattern where local back arcs or intra continental rifts are involved guided by the age data of magmatic rocks and the pattern of subsidence curves the following extensional events can be distinguished during the early to middle cambrian a back arc setting guided the evolution at the gondwana margin contemporaneous intra continental rift basins developed at other places related to a general post pan african extensional phase affecting africa upper cambrian formation of oceanic crust is manifested in the chamrousse area and may have lateral cryptic relics preserved in other places this is regarded as the oceanisation of some marginal basins in a context of back arc rifting these basins were closed in a mid ordovician tectonic phase related to the subduction of buoyant material mid ocean ridge since the early ordovician a new phase of extension is observed accompanied by a large scale volcanic activity erosion of the rift shoulders generated detritus armorican quartzite and the rift basins collected detrital zircons from a wide hinterland this phase heralded the opening of palaeotethys but it failed due to the silurian collision eo variscan phase of an intra oceanic arc with the gondwana margin during this time period at the eastern wing of the gondwana margin begins the drift of the future hunia microcontinents through the opening of an eastern prolongation of the already existing rheic ocean the passive margin of the remaining gondwana was composed of the galatian superterranes constituents of the future variscan basement areas remaining under the influence of crustal extension they will start their drift to laurussia since the earliest devonian during the opening of the palaeotethys ocean

geochronology of polygenetic monazites constrained by in situ electron microprobe th u total lead determination implications for lead behaviour in monazite abstract monazite grains are generally concordant and with an electron microprobe give th u total pb isochron ages in agreement with conventional u pb data monazites from four samples of post archean migmatite from the ivory coast record two main ages an archean age at 2 80 ga partly overprinted by a late migmatization event at 2 03 ga the in situ electron microprobe determination has also shown a third event that systematically appears at 2 72 ga for all the monazites from two of the four studied rocks the three events at 2 80 2 72 and 2 03 ga recorded by the th u total pb system in monazite were each obtained on independent homogeneous parts of a single grain with no sign of pb diffusion thus the oldest monazite remained closed during a high grade resetting migmatization at a temperature estimated at around 700 c and also remained closed during the growth of secondary monazite conventional u pb data isotope dilution on monazite from the same rocks did not allow precise age determinations because the representative points were highly up to 83 discordant and did not fit well along a chord in the concordia diagram the u pb isotope dilution method gave a lower intercept age of 2029 25 ma in agreement with the electron microprobe age although with an unusually high uncertainty due to a poor fit of the data the pb evaporation method gave an old archean age at 2830 7 ma and proterozoic ages ranging from 2417 10 to 2074 7 ma thus although the oldest archean age obtained with the pb evaporation and electron microprobe methods is similar within the analytical error the proterozoic ages obtained by the pb evaporation method are significantly older than the th u total pb proterozoic age obtained with the electron microprobe this discrepancy is presumed to be due to the influence in the pb evaporation method of radiogenic lead contained within archean zones of the monazite grains in other words it means that migmatization took place later than 2074 7 ma and probably around 2030 ma the second archean event at 2 72 ga revealed by high spatial resolution of the electron microprobe could not be determined by other methods the th u total pb method appears to be an inexpensive alternative method for dating simple monazites with a precision close to 20 ma even if the u pb isotopic dilution technique remains the reference method moreover owing to the very high spatial resolution 1 m of the electron microprobe it appears to have no competitor for dating complex polygenetic monazites especially when more than two events are recorded homogeneous zones of different ages coexist in a single grain with no major indication of lead diffusion a procedure for age and error calculations is defined

combined lu hf and sm nd geochronology of the marianske lazně complex new constraints on the timing of eclogite and granulite facies metamorphism abstract lu hf and sm nd garnet whole rock geochronology combined with petrographic observations minero chemical variations thermodynamic modelling and structural data was used to constrain the p t t d evolution of eclogites from the marianske lazn complex bohemian massif boudins of mostly isotropic eclogite with relict steep eclogite facies fabric are affected by steep migmatitic foliation which is followed on a regional scale by the development of almost pervasive predominantly se dipping extensional foliation the structural succession shows continuous transition from eclogite to garnetiferous migmatitic amphibolite and to amphibolite migmatite a least retrogressed sample of eclogite shows clusters of fine grained inclusion poor garnet omphacite relicts surrounded by a fine grained clinopyroxene plagioclase symplectite with minor amphibole biotite plagioclase intergrowths after white mica kyanite with plagioclase spinel coronas and accessory rutile rare potassic white mica occurs as inclusions in omphacite a more retrogressed eclogite with no omphacite or kyanite relicts contains inclusion poor garnet surrounded by amphibole plagioclase corona in a matrix dominated by plagioclase amphibole symplectite with minor clinopyroxene in places the symplectite is overgrown by coarse grained amphibole peak p t conditions inferred from combined conventional thermobarometry and phase equilibria modelling and based on inclusions of white mica up to 3 33 si p f u matrix omphacite jd33 36 and garnet core alm33 38prp38 42grs22 25sps1 compositions are 25 kbar at 650 750 c a ht overprint occurred at 14 18 kbar and 800 c based on coexisting clinopyroxene jd18 24 plagioclase an18 35 and amphibole na b lu hf and sm nd garnet geochronology has been applied to both samples an older age c 390 ma obtained by the lu hf method is interpreted as the timing of hp metamorphism while c 15 ma younger ages were obtained by the sm nd method as temperatures for the ht overprint exceed the empirically and experimentally determined closure temperature of the sm nd system the sm nd ages are interpreted to date cooling following the ht overprint combined together contrasting eclogite and amphibolite facies migmatite fabrics the mineral textures calculated p t conditions and distinct lu hf and sm nd ages provide a complete p t t d path characterised by rapid 15 ma transition from hp subduction crustal thickening to extensional ht shearing this unconventional exhumation path does not fit to models of monocyclic exhumation in a subduction channel proposed for the munchberg and zev devonian hp units to the west in contrast the post peak extensional low angle shearing shortly after subduction and collision resembles more the geodynamic model typical for the iberian subduction system both in time scales and sequence of tectonic events

partial resetting of the upb isotope system in monazite through hydrothermal experiments an sem and upb isotope study abstract natural monazites of different grain sizes were treated experimentally under hydrothermal conditions and their u pb isotope systematics measured before and after the runs runs were carried out with monazite grain size fractions ranging from 40 to 125 m in diameter and on 207 pb 206 pb ages close to 380 ma u concentrations remained always unchanged all recrystallized monazites from the powdered starting material define a discordia with intercept ages of 379 3 2 3 and 7 18 ma the latter indicating the hydrothermally induced pb loss at the time of the experiment sem observations of monazite grains from hydrothermally treated grain size fractions exhibited only minor dissolution features confined to the grain surfaces run products from monazite powders showed recrystallization and grain growth induced by a dissolution precipitation mechanism increasing temperature resulted in a distinct enlargement of grain sizes at 650 and 750 c a dissolution precipitation process in presence of a fluid phase rather than pb volume diffusion acts as a very efficient mechanism for the resetting of the u pb isotope system of monazites even at low temperatures this process is most important in shear zones where channelized fluid flow may occur

the problem of dating high pressure metamorphism a u pb isotope and geochemical study on eclogites and related rocks of the marianske lazně complex czech republic this study presents new geochemical major and trace element nd sr isotope and u pb zircon monazite titanite and rutile data for various rock types eclogite high pressure granulite amphibolite orthogneiss leucosome of the high grade metamorphic marianske lazn complex in the western bohemian massif concordant u pb zircon analyses from the mafic to intermediate samples disclose the marianske lazn complex as a mixture of c 540 ma oceanic rocks juxtaposed at depth with lower crustal rocks of the structurally overlying tepla barrandian unit this interpretation refutes earlier models in which the marianske lazn complex was interpreted as a dismembered cambro ordovician ophiolite complex affected by variscan subduction metamorphic zircon in mafic rocks of the marianske lazn complex as well as monazite from orthogneiss and titanite in leucosome yield ages around 380 ma on the basis of detailed petrography and cathodoluminescence imaging we conclude that the c 380 ma age reflects the timing of variscan exhumation and associated decompression melting under upper amphibolite facies to granulite facies conditions thereby the data derived from metamorphic zircon of eclogites and high pressure granulite unexpectedly do not date the timing of eclogitization which could have happened just before variscan exhumation or even shortly after late cadomian protolith formation

the central sudetic ophiolites sw poland petrogenetic issues geochronology and palaeotectonic implications abstract the central sudetic ophiolites cso including the largest and best studied leza ophiolite are relatively well preserved complete palaeozoic ophiolitic sequences at the ne margin of the bohemian massif near the eastern edge of the variscan belt in central europe despite a detailed study and a well documented morb type affinity the geochemical complexity of these ophiolites has only recently been appreciated allowing construction of petrogenetic models we review here petrological and geochemical data indicating a contrast between the plutonic and volcanic members of the leza ophiolite this contrast was previously interpreted in terms of a model of two stage incremental melting of a depleted asthenospheric source producing basaltic magmas and subsequent melts crystallizing as gabbros however we argue that the geochemical differences between the plutonic and volcanic members can be explained by differentiation and cumulation processes of broadly similar parental magmas preliminary interpretation of new shrimp zircon age data from plutonic and volcanic members are consistent with earlier u pb zircon ages and provide evidence for magmatic crystallization of the leza ophiolite at c 400 ma the cso interpreted as traces of the rheic ocean are key for reconstructing the early to mid palaeozoic rifting processes distinct similarities exist with time equivalent ophiolitic units in the western variscan segment of the nw iberian massif the cso represent c 400 ma oceanic crust fragments and their structural position delineates likely major variscan tectonic sutures and tectonic mosaic domains within the variscan accretionary prism

a metamorphic history from micron scale 207pb 206pb chronometry of archean monazite ion microprobe measurements of pb isotope ratios in monazites have been obtained in situ from thin sections using the cambridge isolab 120 molecular interferences are sufficiently resolved at an rp of 6500 to allow 2 7pb z 6pb dating of monazite with precisions as low as 4 5 ma 2 r the results presented here provide important information on the chronological history of the late archean metamorphism of the wind river range wyoming usa matrix monazites and monazite inclusions in garnets from a metapelite from the northern wind river range have been analysed by sims in a previous study peak metamorphic conditions t 800 c p 8 1 kb were estimated using inclusion assemblages in garnets from this same sample isolated monazite inclusions in garnet yield 2 7pb z 6pb age estimates of 2781 6 to 2809 10 ma those along fractures yield lower ages 2603 2687 ma which are similar to tims and sims ages of matrix monazites a single large 500 xm monazite grain locally preserves growth zoning but has a recrystallised core and a resorbed recrystallised rim age estimates for these three regions are 2788 9 ma 2663 4 and 2523 6 ma respectively thus the inclusion assemblages of sharp and essene may record peak metamorphic conditions at ca 2 8 ga and indicate a phase of metamorphism that predates by over 100 ma the emplacement of the bridget batholith the major lithologic component of the northern wind river range the analysed monazite grains appear to preserve ca 300 ma history even within a single grain monazite inclusions in garnet that are fully armoured may provide estimates for the time of garnet growth even in high grade terranes where most chronometers are reset the age pattern preserved by the large monazite grain cannot be simply related to diffusion controlled closure instead a chronology is preserved which can be related to the petrographic setting of individual grains through in situ analysis

u pb zircon and sr nd pb whole rock investigations from the continental deep drilling ktb among the large number of gabbroic intrusions within the profile of the continental deep drilling ktb several types can be distinguished in this study petrographic geochemical investigations conventional u pb zircon data and results of sr nd pb isotope characteristics of meta gabbros amphibolites and felsic rocks are reported the main results are u pb zircon age data reveal a magmatic cambro ordovician evolution of the erbendorf vohenstrauss zone zev the range of the ages of magmatic intrusions varies from 496 to 476 ma probably as a result of overlap of magmatic and metamorphic processes cambro ordovician magmatic ages were observed within different mafic b unit as well as in variegated units v unit of the ktb profile the silurian devonian metamorphism is dated at around 398 ma the silurian as well as the variscan metamorphic events apparently did not influence the u pb system the cambro ordovician mafic rocks from the ktb scatter on a nd evolution diagram several meta gabbros cluster around the ndt 500 value of 5 but others vary from 8 6 to 0 65 and suggest variable degrees of contamination with continental crust material a contamination of only 5 10 is required to reduce nd from 8 6 to approximately 5 according to the ree patterns and the sr nd pb data the mafic rocks could either derive from an enriched mantle source or else their protolith magma originated from a depleted mantle and was contaminated by continental crust the data for the mafic rocks of the zev are in accord with observation by other authors regarding the evolution of the late proterozoic to early palaeozoic magmatism in the bohemian massif

coronitic metagabbros of the marianske lazně complex and tepla crystalline unit inferences for the tectonometamorphic evolution of the western margin of the tepla barrandian unit bohemian massif

subduction driven shortening and differential exhumation in a cadomian accretionary wedge the tepla barrandian unit bohemian massif abstract the tepla barrandian unit tbu of central europe s bohemian massif exposes perhaps the best preserved fragment of an accretionary wedge in the avalonian cadomian belt which developed along the northern active margin of gondwana during late neoproterozoic in the central tbu three ne sw trending lithotectonic units domains 1 3 separated by antithetic brittle faults differ in lithology style and intensity of deformation magnetic fabric ams and degree of cadomian regional metamorphism the flysch like domain 1 to the nw is the most outboard trenchward unit which has never been significantly buried and experienced only weak deformation and folding the central melange like domain 2 is characterized by heterogenous intense deformation developed under lower greenschist facies conditions and was thrust nw over domain 1 along a se dipping fault to the se the most inboard arcward domain 3 is lithologically monotonous dominated by graywackes and slates was buried to depths corresponding up to the lower greenschist facies conditions where it was overprinted by a pervasive se dipping cleavage and then was exhumed along a major nw dipping normal fault we interpret these domains to represent allochtonous tectonic slices that were differentially buried and then exhumed from various depths within the accretionary wedge during cadomian subduction the nw directed thrusting of domain 2 over domain 1 may have been caused by accretion at the wedge front whereas the se dipping cleavage and se side up exhumation of domain 3 may record inclined pervasive shortening during tectonic underplating and subsequent horizontal extension of the rear of the wedge the boundary faults were later reactivated during cambro ordovician extension and variscan compression compared to related terranes of the cadomian belt the tbu lacks exposed continental basement evidence for regional strike slip shearing and extensive backarc magmatism and lp ht metamorphism which could be interpreted to reflect flat slab cadomian subduction this in turn suggests that cadomian accretionary wedges developed in a manner identical to those of modern settings elevating the tbu to a key position for understanding the style kinematics and timing of accretionary processes along the avalonian cadomian belt

emplacement depths and radiometric ages of paleozoic plutons of the neukirchen kdyně massif differential uplift and exhumation of cadomian basement due to carboniferous orogenic collapse bohemian massif abstract the igneous complex of neukirchen kdyn is located in the southwestern part of the tepla barrandian unit tbu in the bohemian massif the tbu forms the most extensive surface exposure of cadomian basement in central europe cambrian plutons show significant changes in composition emplacement depth isotopic cooling ages and tectonometamorphic overprint from ne to sw in the ne the vsepadly granodiorite and the smr ovice diorite intruded at shallow crustal levels t 20 km the teufelsberg cert v kamen diorite on the other hand forms an unusual intrusion dated at 359 2 ma concordant u pb zircon age k ar dating of biotite of the teufelsberg diorite yields 342 4 ma these ages together with published cooling ages of hornblende and mica in adjacent plutons are compatible with widespread medium to high grade metamorphism and strong deformation fabrics suggesting a strong variscan impact under elevated temperatures at deeper structural levels the plutons of the neukirchen area are cut by the steeply ne dipping hoher bogen shear zone hbsz which forms the boundary with the adjacent moldanubian unit the hbsz is characterized by top to the ne normal movements which were particularly active during the lower carboniferous a geodynamic model is presented that explains the lateral gradients in cambrian pluton composition and emplacement depth by differential uplift and exhumation the latter being probably related to long lasting movements along the hbsz as a consequence of lower carboniferous orogenic collapse

the fortnightly tide and the tidal triggering of earthquakes in this paper we test the southern california network earthquake catalog and the world wide earthquake catalog for a fortnightly tidal periodicity and find none in studies that test the hypothesis of tidal triggering of earthquakes it is usually necessary to resolve the tidal shear stress onto the plane of the fault and in the direction of the slip vector to determine if the stress is compatible with the fault motion heaton 1975 1982 this analysis requires accurate focal mechanisms and knowledge of the focal plane on which slip occurred for as many earthquakes as possible to improve the statistical sampling of the data set however if we consider the fortnightly tide which is a simple amplitude modulation focal mechanism information is not required our analysis assumes that we have a large data base of earthquakes with varying mechanisms the alignment of the slip vector with the tidal shear stress will vary from earthquake to earthquake however if tidal triggering of earthquakes does occur more earthquakes should be triggered out of the total population when the tidal stress is large than when it is small if the largest semi diurnal tide is normalized to an amplitude of 1 0 the fortnightly tide would have an amplitude of 0 17 on this scale and a period of about 2 weeks munk and macdonald 1960 therefore the effect on the occurrence of earthquakes by the tidal shear stress will be modulated with a 2 week period considering the fortnightly tide allows us to utilize entire catalogs of data to test the tidal triggering hypothesis without knowledge of the focal mechanisms

textural chemical and isotopic insights into the nature and behaviour of metamorphic monazite monazite is a mineral of choice for dating metamorphism in amphibolite and granulite grade metapelites however there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems the two main obstacles addressed in this contribution are firstly the minor but significant e g 1 30 ma dispersal in duplicate isotope dilution thermal ionisation mass spectrometry id tims u pb age data commonly recorded from a single rock and secondly the difficulty of attaching monazite age data to pressure and temperature information through a multidisciplinary approach utilising tims and laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms isotope data quantitative and qualitative emp chemical analyses of monazite and textural studies we assess the significance of pb loss older components and continuous and episodic monazite growth in the generation of dispersed age data three samples from the canadian cordillera and one sample from the himalaya of pakistan are examined each sample exhibits an age dispersion of between 1 and 12 ma for single crystal and multi grain tims u pb monazite age determinations consideration of the closure temperature for pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive pb loss did not play a significant part in generating this age dispersal the la mc icpms study indicates that an older component 100 ma older than the tims ages contributed to the age dispersal in three of the four samples in all the samples however chemical analyses identified that the majority of monazites examined exhibited significant intra crystalline zoning in y content the la mc icpms analysis of one sample that was constrained to zones of distinct y content indicates that these zones are of distinct age we suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in y whereas monazite grown after garnet is relatively poor in y a combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event this behaviour contributes to and in one of the samples controls the observed age dispersal this recognition allows the generation of pressure temperature time points by combining textural and chemical information of monazite with in situ age determinations and pressure temperature information from garnet thus the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool

deciphering the variscan tectonothermal overprint and deformation partitioning in the cadomian basement of the tepla barrandian unit bohemian massif the tepla barrandian unit tbu has long been considered as a simply bivergent supracrustal median massif above the saxothuringian subduction zone in the variscan orogenic belt this contribution reveals a much more complex style of the variscan tectonometamorphic overprint and resulting architecture of the neoproterozoic basement of the tbu for the first time we describe the crustal scale ne sw trending dextral transpressional krakovec shear zone ksz that intersects the tbu and thrusts its higher grade northwestern portion severely reworked by variscan deformation over a southeastern very low grade portion with well preserved cadomian structures and only brittle variscan deformation the age of movements along the ksz is inferred as late devonian 380 370 ma on the basis of structural microstructural and anisotropy of magnetic susceptibility data from the ksz we propose a new synthetic model for the deformation partitioning in the tepla barrandian upper crust in response to the late devonian to early carboniferous subduction and underthrusting of the saxothuringan lithosphere we conclude that the saxothuringian tepla barrandian convergence was nearly frontal during 380 346 ma and was partitioned into pure shear dominated domains that accommodated orogen perpendicular shortening alternating with orogen parallel high strain domains that accommodated dextral transpression or bilateral extrusion the synconvergent shortening of the tbu was terminated by a rapid gravity driven collapse of the thickened lithosphere at 346 337 ma followed by or partly simultaneous with dextral strike slip along the baltica margin parallel zones driven by the westward movement of gondwana from approximately 345 ma onwards

provenance of upper devonian clastic meta sediments of the bollstein odenwald mid german crystalline zone variscides detrital zircons separated from paragneisses of the northern bollstein odenwald mid german crystalline zone variscides yielded cambrian to upper devonian u pb ages the age of the youngest detrital zircon population 371 3 ma and the lower carboniferous metamorphic overprint indicate an upper devonian depositional age of the paragneiss protolith the age spectra of detrital zircons suggest the latter to be derived from 1 ordovician igneous rocks of the saxothuringian zone 2 a late cadomian magmatic arc tepla barrandian unit and 3 a silurian devonian magmatic arc cadomian igneous activity is also documented by cambrian zircon cores in lower to middle devonian detrital zircons meso and paleoproterozoic detrital zircons which are typical for the old red continent and the rhenohercynian zone are entirely lacking the restricted palaeozoic detrital zircon age spectrum is attributed to a silurian devonian intra arc or trench setting both the lack of mesoproterozoic detrital zircons and the striking similarity of the u pb ages of the detrital zircon obtained from the bollstein odenwald with u pb ages from crystalline rocks of the saxothuringian basement rules out that the bollstein odenwald is forming a tectonic window rhenohercynian lower plate inside the mid german crystalline zone saxothuringian upper plate