radon earthquake precursor studies in iceland discrete samples of geothermal water have been collected from a network of nine stations for analysis of radon 222 rn content the sampling network consisted of wells that range in wellhead temperature from 48 c to 100 c and the depths range from 38 m to 1338 m the sampling frequency at most stations was about once per week and twice per week at the station fludir the wells are either artesian or pumped more or less continuously the network covered two regions of transform faulting in iceland with seven stations in the southern iceland seismic zone sisz and two stations in northern iceland in the tjornes fracture zone tfz during 1978 and 1979 several anomalous changes in radon content were observed to precede some of the local earthquakes criteria based on tectonic regimes a magnitude distance relationship and time clustering were applied to select a set of 23 earthquakes that could be expected to be preceded by a radon anomaly the magnitude of these earthquakes ranged between 1 0 and 4 3 each of the 23 earthquakes was within the distance range of one or more stations such that altogether 57 potential observations of possible anomalies were available the method of analysis applied to the radon and earthquake data consisted of identifying radon anomalies in retrospect and resulted in nine precursory anomalies 48 cases of failure to observe an anomaly and seven false alarms the probability of observing radon anomalies before earthquakes with magnitudes between 2 0 and 4 3 the largest event observed was found to be approximately 65 based on a weekly sampling rate in the sisz five out of eight earthquakes m 2 were preceded by an anomaly in two cases anomalies were observed at two different stations prior to the same earthquake the anomalies appeared to occur farther away for larger earthquake magnitude an aquiclude that divides the sisz did not seem to affect the occurrence of radon anomalies but the amplitude of anomalies on the east side were larger than the ones on the west side the duration times of the anomalies ranged from 17 to 37 days the large number of failures to observe an anomaly indicated that the occurrence of an anomaly was more strongly dependent on local conditions and earthquake parameters than distance between epicenter and radon station some stations also appeared to be less sensitive than others and three stations never showed an anomaly during periods of bursts in local seismicity some stations reflected distinctively larger background fluctuations which hampered the correlation of the seismicity with the radon data most of the false alarms were related to disturbances caused by changes in well operation in the sisz the high rate of failure to observe an anomaly and the occurrence of false alarms was partly compensated for by operating a dense network of sampling stations wells that are utilized at moderate flow rates are less likely to show false alarms than wells utilized at high flow rates wells situated close to rock formations rich in radioactive minerals may be more likely to show anomalies than others

radon measurements for earthquake prediction along the north anatolian fault zone a progress report abstract radon 222rn concentration has been continuously measured since 1983 in groundwater at a spring and in subsurface soil gas at five sites along a 200 km segment of the north anatolian fault zone near bolu turkey the groundwater radon concentration showed a significant increase before the biga earthquake of magnitude 5 7 on 5 july 1983 at an epicentral distance of 350 km and a long term increase between march 1983 and april 1985 the soil gas radon concentration showed large changes in 1985 apparently not meteorologically induced the soil gas and groundwater data at bolu did not show any obvious correlation

origin of late variscan granitoids from ne bavaria germany exemplified by ree and nd isotope systematics sm nd isotopic results are presented for late and post tectonic granitoids comprising peraluminous granites and subordinate metaluminous dioritic rocks redwitzites from the fichtelgebirge fg and northern oberpfalz nop ne bavaria the data combined with a number of earlier geochromological studies on these granitoids place severe constraints on the crustal evolution of this region during the carboniferous and pre carboniferous epoch redwitzites range in nd t from 4 to 0 inclusive data from p k holl and coworkers the nd t values of the granites s s are restricted to overlapping ranges of 8 to 3 fg and 8 to 2 nop in both domains the older granites 330 325 ma are characterized by nd t values of 4 whilst the younger granites 315 305 and 290 ma have nd t values of 4 the diversity observed in nd isotopic characteristic is interpreted in terms of different source material the nd isotopic compositions of the redwitzites exhibit source heterogeneity suggesting mixing between mantle magma and crust the nd isotopic features of the older granites are consistent with the magmatic precursors having been generated by partial melting of pre existing mature crust variably contaminated by mantle material or alternatively by melting of chemically less evolved crust resembling paragneisses of teh zev structural unit the origin of the younger granites can be confidently linked to anatexis of common moldanubian and saxothuringian metasediments two stage nd model ages tdm of the granitoids are in the range 1 1 1 7 ga these ages provide further support of substantial involvement of pre phanerozoic crust in the generation of the granitoids the leuchtenberg granite and the g2 and g3 granites yield late variscan sm nd isochron ages which are concordant with previous rb sr data on these granites by contrast the nd isotopic data for barnau and flossenburg define pseudo isochrons suggestive of mixing between contrasting felsic melts

new analytical procedures to increase the resolution of zircon geochronology by the evaporation technique abstract in the evaporation technique kober b 1987 single zircon evaporation combined with pb emitter bedding for 207 pb 206 pb age investigations using thermal ion mass spectrometry and implications for zirconology contrib mineral petrol 96 63 71 of zircon dating individual zircon crystals are heated in a series of steps causing the thermal breakdown of the zircon along a sharply defined reaction front chapman h j roddick j c 1994 kinetics of pb release during the zircon evaporation technique earth planet sci lett 121 601 611 radiogenic 207 pb 206 pb ages for each step represent the average pb isotopic composition of the zircon consumed resulting in geologically meaningless mixed ages where the reaction front intersects multiple domains within a zircon recognition of whether 207 pb 206 pb ages are derived from single or heterogeneous components is central to age determinations by the evaporation technique replication of an age indicates that a single component is being dated as it is unlikely that several components will be repeatedly mixed in identical proportions thus increasing the number of heating steps taken for each zircon increases the resolution of information obtained presentation of data as age against order of acquisition allows clear visual interpretation of the degree of mixing behaviour within each zircon analysed ranking the heating steps in order of ascending age clarifies trends within the zircon population as a whole and also allows the use of an r2 test to delimit those steps which do or do not contain mixed components mixing behaviour may be further emphasised by superimposing the model initial th u ratio for each step on the same graph

erratum to metamictisation of natural zircon accumulation versus thermal annealing of radioactivity induced damage annealing of the radiation damage generated in decay events is a common phenomenon in natural zircons we have studied relationships between radiation damage age and radionuclide content of natural zircons on a micro scale the degree of metamictisation was estimated by means of confocal laser raman spectroscopic measurements raman band broadening in well ordered to moderately metamict zircons that have not experienced significant healing of the radiation damage since the time of their growth shows a nearly linear dependence on the fluence this dependence is proposed as a calibration line for examining unknown zircons in order to consider complete or incomplete damage storage which may contribute to the understanding of the low t thermal history of their host rocks seven examples for the application are presented radiation damage may anneal heterogeneously with preferred re ordering in more damaged micro areas results suggest that single point defects do not anneal continuously over geologic periods of time but are in contrast a comparably stable type of radiation damage in contrast to recrystallisation processes structural recovery due to simple thermal annealing is not necessarily connected with loss of radiogenic pb the most prominent example for this are concordant zircons from sri lanka although they are excellent standards for u pb dating sri lankan zircons should not be used as standards for structural radiation damage the experimental work is completed by monte carlo simulations of decay events in zircon

constraints on the origin of zonation of the granite complexes in the fichtelgebirge germany and czech republic evidence from a gravity and geochemical study compositional zoning of granitic plutons is a common feature but the spatial variation of zoning is rarely studied in detail it is suggested that important additional information can emerge if the compositional zoning is regarded with relation to the root zones i e with the granite shape at depth a combined geochemical and gravity investigation was performed on the hercynian fichtelgebirge granites which are composed of an older intrusive complex oic and younger intrusive complex yic the oic and yic are distinct from each other with respect to magma origin differentiation pattern shape of the granite complexes at depth and the zonation pattern in relation to the root zone the oic shows normal compositional zoning whereas the yic both reverse yic 1 and normal zoning yic 2 the general zonation pattern of the oic and the yic is the result of a combination of multiple injections of single magma batches and in situ differentiation during magma emplacement in addition to the general zonation pattern local zonation in individual granite bodies of the yic was caused by sidewall crystallisation producing a less differentiated marginal facies g2 and a more differentiated core facies g3 and g4 it is suggested that the complex zonation patterns of the fichtelgebirge granite complexes reflect interactions between the rate of magma output which is partly governed by their rheological properties and the rate of deformation which provides the room for the magma to be emplaced

a theoretical model for change of shape of spring water radon anomalies with epicentral distance a model for radon anomalies before earthquakes with a relatively fast increase in radon concentration at the beginning of the anomaly is postulated a calculation based on this model shows that a parameter can be deduced from the form of the radon anomalies which depends on the epicentral distance of the measurement point after adjustment of some parameters the model calculation is in good agreement with observed earthquake precursor anomalies

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

zircon ages and nd isotopic and chemical compositions of orthogneisses from the black forest germany evidence for a cambrian magmatic arc single zircon u pb dating combined with 207pb 206pb ages obtained by the evaporation method constrains the emplacement of tonalitic trondhjemitic and granodioritic orthogneisses of the moldanubian zone in the black forest between 500 and 510 ma two detrital zircon populations of 1 9 and 1 6 ga indicate early middle proterozoic material in the former setting of the basement the initial end values range from 0 1 to 3 4 and mean crustal residence ages of 1 0 1 4 ga are consistent with involvement of early middle proterozoic crust and a subordinate juvenile component probably originating from subduction related melting of the mantle the orthogneisses have fractionated ree patterns and slightly higher k2o na2o ratios than typical low k tonalite trondhjemite granite suites the chemical data are interpreted as evidence for melting of amphibolite and contributions from evolved crust the emplacement of the orthogneisses was superceded by a high temperature metamorphic event at 480 ma which we interpret as a result of lithospheric thinning in a marginal basin behind a cambrian magmatic arc

granitoid magmatism of the nw bohemian massif revealed gravity data composition age relations and phase concept the late variscan granitoids of the nw bohemian massif northeast bavaria west bohemia constitute four partly contiguous granitoid complexes fichtelgebirge northern oberpfalz waidhaus rozvadov and bor incorporating more than 20 intrusive units based on gravity data the granites can be modeled as steeply inclined slab and wedge like bodies with thicknesses between 2 and 8 km a rough estimate of the total volume of the granites is approximately 18 000 km3 within the four areas named above composition ranges from less evolved dioritic rocks known as the redwitzite suite to highly evolved granites the redwitzites comprise metaluminous rocks with dominant i type features these rocks yield aberrantly old rb sr ages 545 415 ma low initial sr ratios 0 706 0 708 and high and variable nd t values 1 to 4 sr nd isotopes of the redwitzites show contamination trends towards the granites suggesting mixing between mantle magma and crustal granitic melts an older plutonic association granites of bor leuchtenberg weissenstadt marktleuthen zainhammer is mildly peraluminous displaying features of both i and s type granitoids these granites are characterized by lower carboniferous ages rb sr k ar u pb low to intermediate initial sr ratios 0 707 0 708 and high nd t values 2 to 4 which overlap with those of paragneisses from the zone of erbendorf vohenstrauss zev and from the western part of the tepla barrandian it is postulated that the older granites were formed either by partial melting of zev or tepla barrandian crust or alternatively of preexisting mature crust contaminated by mantle material the younger granites are strongly peraluminous and of s type they yield upper carboniferous rb sr and k ar ages and exhibit a range towards high initial sr ratios 0 710 0 720 and low nd t values 4 to 8 similar values are found in moldanubian paragneisses and in saxothuringian metasediments both of which provide potential source rock lithologies for these granites

high temperature memory in calcic amphiboles and constraints on compositional control of their 40ar 39ar ages well characterized calcic amphiboles from an incompletely mixed magmatic rock association of mainly intermediate dioritic composition were studied by incremental heating experiments of 40 ar 39 ar the rocks in question were thermally overprinted by late hercynian granites microprobe and scanning electron microscopy analyses show that the amphibole grains consist of two different calcic hornblende varieties 1 a si and mg rich variety and 2 a variety enriched in al ti fe k and na the mg si rich hornblende can be further characterized by its higher cao k 2 o ratios 30 50 compared to the al ti fe k na rich hornblende 12 15 three hornblende separates that previously had yielded conventional k ar ages of 320 317 and 302 ma have been dated by the 40 ar 39 ar step heating technique the low and medium temperature parts of the 40 ar 39 ar age spectrum show discordant ar systematics with single step ages generally younger than 325 ma the gas released at the highest temperature steps preserves an exceptional record of old and remarkably similar integrated ages 342 346 and 344 ma the high temperature ages correspond to phases with higher apparent cao k 2 o ratios 30 45 and are interpreted to result from ar release from mg si hornblende domains that are assumed to have been very resistent to ar loss during the late hercynian thermal events the results of this study suggest that ar retentivity differs among calcic amphiboles and mg si rich hornblendes are more ar retentive than their al ti fe k na rich counterparts

late variscan magmatism revisited new implications from pb evaporation zircon ages on the emplacement of redwitzites and granites in ne bavaria pb evaporation zircon analyses coupled with a detailed cathodoluminescence cl study on the complete series of granitoids from the northern oberpfalz ne bavaria provide new evidence for the commencement and timing of late variscan magmatism all granitoids analysed in this study were dated before by rb sr and or k ar methods investigated samples comprise medium grained i type dioritic rocks redwitzites i s type granites leuchtenberg marktredwitz g1 zainhammer and s type granites falkenberg liebenstein mitterteich friedenfels steinwald flossenburg barnau the zircon evaporation technique reveals three groups of 207pb 206pb ages which are interpreted to represent magmatic crystallisation 1 ages of 324 321 ma are found in all analysed redwitzites and in almost all i s type granites 2 the granites of falkenberg and liebenstein yield ages of 315 ma 3 ages between 312 and 310 ma are recorded in the mitterteich friedenfels steinwald and flossenburg granites titanite crystals from different redwitzite bodies yield conventional u pb ages of 325 322 ma identical to the pb evaporation zircon data of these rocks the s type granites of groups 2 and 3 contain zircons with relict cores but only a small number of them yield older ages indicating that some of the cores must have lost their radiogenic pb from the geochronological data we infer that metamorphic conditions of the variscan crust produced different granite types at different times the data support a model involving an early period of mantle related magmatism which postdates the final convergence stage of the variscan orogen this magmatic activity was at the same time as the thermal peak of regional metamorphism and is followed by a late period of crustal anatexis which is probably related to post collisional extension of the thickened variscan crust

radon content of groundwater as an earthquake precursor evaluation of worldwide data and physical basis the properties of a worldwide data set of 91 radon 222 rn anomalies the frequency of occurrence the precursor time interval and the distribution of peak amplitudes are correlated with earthquake data such as the respective magnitude and epicentral distance these anomalies were reported as precursors to earthquakes in the united states ussr china japan and iceland although the data set is incomplete and limited by experimental deficiencies several consistent patterns emerge radon anomalies from different tectonic regions show similar patterns the radon anomalies occur at greater epicentral distances for earthquakes of the larger magnitude anomalies preceding large earthquakes m 6 are frequently observed at a distance of 100 to 500 km these distances are larger than several times the rupture dimensions of the future earthquakes the time from the onset of an anomaly to the time of the earthquake the precursor time increases with magnitude but decreases with distance between epicenter and radon station in addition radon anomalies are observed more frequently prior to large earthquakes than prior to small ones indicating that the preparation zone increases in size as magnitude increases the peak amplitude does not scale with magnitude but forms a consistent pattern with epicentral distance in that the larger the earthquake magnitude the farther away the largest amplitudes tend to occur the preparation zone of the earthquake where the anomalies occur forms an almost continuous annulus that expands with time away from the future rupture zone the outer radius of this annulus scales with the earthquake magnitude model calculations indicate that strain fields of at most 10 6 to 10 8 strain caused the radon anomalies if these strains are divided by the appropriate precursor time minimum strain rates from 10 7 day 1 to 10 10 day 1 are obtained such small strains and strain rates suggest that in most cases neither mechanical crack growth induced by dilatancy nor mechanical coupling between pore pressure and the rock matrix caused the anomalies large changes in the orientation of the local strain field however could occur and affect the local stress intensity factor since changes in the stress intensity factor can result in stress corrosion the occurrence of radon anomalies is attributed to slow crack growth controlled by stress corrosion in a rock matrix saturated by groundwater

pb diffusion in zircon abstract chemical diffusion of ti under anhydrous conditions at 1 atm and under fluid present elevated pressure 1 1 1 2 gpa conditions has been measured in natural zircon the source of diffusant for 1 atm experiments was a zro2 tio2 zircon mixture with experiments run in crimped pt capsules diffusion experiments done in the presence of h2o co2 fluid were run in a piston cylinder apparatus using a source of ground tio2 zrsio4 and sio2 with oxalic acid added to produce h2o co2 vapor and partially melt the solid source material yielding an assemblage of rutile zircon melt vapor nuclear reaction analysis nra with the resonant nuclear reaction 48ti p 49v was used to measure diffusion profiles for both sets of experiments the following arrhenius parameters were obtained for ti diffusion normal to cover the temperature range 1350 1550 c at 1 atm d ti 3 33 10 2 exp 754 56 kj mol 1 r t m 2 s 1 ti diffusivities were found to be similar for experiments run under fluid present conditions a fit to all of the data yields the arrhenius relation d 1 34 102 exp 741 46 kj mol 1 rt m2 s 1 these data suggest that zircon should be extremely retentive of ti chemical signatures indicating that the recently developed ti in zircon crystallization geothermometer watson e b harrison t m 2005 zircon thermometer reveals minimum melting conditions on earliest earth science 308 841 844 will be quite robust in preserving temperatures of zircon crystallization titanium diffuses somewhat faster in zircon than larger tetravalent cations u th and hf but considerably more slowly than pb the ree and oxygen hence ti crystallization temperatures may be retained under circumstances when radiometric ages or other types of geochemical information are lost

anticorrelated rb sr and k ar age discordances leuchtenberg granite ne bavaria germany the leuchtenberg granite oberpfalz ne bavaria displays a continuous differentiation trend ranging from mildy peraluminous coarse grained porphyritic biotite granites bg to strongly peraluminous medium to fine grained garnet bearing muscovite granites gmg the rb sr and k ar age determinations of whole rock and mineral samples from the granite and associated intermediate rocks redwitzites have revealed two divergent age gradients rb sr wholerock dates decrease and initial 87sr 86sr ratios increase for successively more evolved subsets of the granite all bg samples 87rb 86sr 2 16 yield a date of 326 2 ma with a low initial 87sr 86sr ratio of 0 70778 0 00013 1 while all gmg samples 87rb 86sr 70 to 1000 yield a younger date of 317 2 ma with an enhanced initial 87sr 86sr ratio of 0 7146 0 0039 the k ar measurements on biotites and muscovites give closely concordant dates for the gmg 326 323 ma and the southern lobe of the bg 324 320 ma the northern lobe of the bg including the redwitzites shows a well defined trend of decreasing k ar dates from 320 ma to 300 ma towards the northwest critical consideration of both isotope systems leads to the conclusion that the rb sr system of the gmg was disturbed by a later hydrothermal event the ca 326 ma whole rock rb sr date for the bg is not in conflict with any of the k ar mineral dates and is taken as approaching the crystallization age of the leuchtenberg granite the k ar age progression within the northern lobe of the bg indicates that this part either cooled down over a protracted period of some 20 ma or experienced reheating at ca 300 ma the study highlights the potential of combined rb sr and k ar dating in deciphering detailed chronology on the scale of a single igneous intrusion

gas geochemistry applied to earthquake prediction an overview terrestrial gases in groundwater and soil air have been extensively studied in recent years in seismically active areas especially in ussr china japan and united states in search of premonitory changes that might be useful for earthquake prediction concentrations of radon helium hydrogen mercury carbon dioxide and several other volatiles have been found generally to be anomalously high along active faults suggesting that the faults may be paths of least resistance for the terrestrial gases generated or stored in the earth to escape to the atmosphere temporal gas concentration changes with durations of a few hours to many months have been observed before many large earthquakes at a relatively small number of favorably situated stations at epicentral distances of up to several hundreds of kilometers these sensitive stations are generally located along active faults especially at intersections or bends of faults or some other structurally weak zones possibly because of tectonic strain concentration at such places however gas concentrations especially those measured at or near the ground surface may also be significantly affected by meteorological hydrological and other nontectonic changes in the environment the nontectonically induced variations must be carefully eliminated or recognized in the search of true earthquake related anomalies

mapping of subsurface contamination zone using 3d electrical resistivity imaging in hilla city iraq electrical resistivity imaging eri method is very useful in mapping subsurface contamination zones in this study 3d eri is used to identify the distribution and depth of subsurface soil contamination zone near al furat state company for chemical and pesticides industries in hilla city iraq a 3d pure image of electrical resistivity distribution is obtained from a perpendicular square grid 80 80 m which consists of three parallel and three perpendicular lines the electrical resistivity values range from 1 to 21 o m and the total root mean square rms after four iterations is about 7 the image gives eight different depth slices for the estimated model with a depth interval of about 1 m the maximum depth of investigation is 13 7 m the results mapped the contamination zone where high electrical resistivity values about 21 o m are observed possibly due to accumulation of alkaline waste disposed from the company the eri results show that the subsurface layers up to a depth of 4 99 m are of moderate to high electrical resistivity values the study reveals that 3d eri is an effective tool for subsurface contamination zone mapping in various depths which extends in this study from the near surface to a depth of about 5 m below the ground level

relationship of the 1979 southern california radon anomaly to a possible regional strain event during the second half of 1979 anomalously high emanation of radon was recorded at two stations of the automated radon thoron monitoring network operated by the w k kellogg radiation laboratory of the california institute of technology the two stations exhibiting major anomalies kresge and dalton canyon are located approximately 30 km apart on the frontal fault system of the transverse ranges of southern california at kresge the anomaly began on june 21 1979 and continued through december 1979 at dalton canyon the anomaly started about 3 weeks later and also continued through december 1979 at both sites the anomalous levels of radon decreased but did not return entirely to normal values shortly before october 15 1979 during the week of october 15 1979 a 6 6 m earthquake occurred about 290 km to the southeast of the two stations and later in that week earthquakes of magnitude 4 2 and 4 1 occurred at malibu and lytle creek the latter two events were within 60 km of the monitors a radon thoron monitor at lytle creek recorded no long term anomaly but did record a sharp spikelike decrease in the radon level on october 13 1979 coincident with our observations of anomalous radon levels other investigators have reported anomalies or suspected anomalies in several other geodetic geophysical and geochemical signals from the same general region the rapid temporal development of several of the anomalies together with the large area over which they were observed suggests that a large scale strain event took place which may have been responsible both for the widespread anomalies and for the seismicity that occurred in the region subsequent to the onset of the anomalies

whole grain evaporation for 207 pb 206 pb age investigations on single zircons using a double filament thermal ion source a technique has been developed and tested to analyse 207pb 206pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains 0 3 mm the evaporation analyses are performed in the double filament arrangement of a thermal ion mass spectrometer thims the method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons the evaporation of test samples results in 207pb 206pb apparent ages in good agreement with u pb ages known from literature single zircons from a granite of the marble mountains california yield an age of crystallization of 1 410 30 ma ceylon zircons from heavy mineral bearing gravels yield 560 40 ma as age of crystallization of the pegmatitic gravel sources individuals from a heterogeneous zircon population of a diatexite from the southern schwarzwald sw germany indicate metamorphic zircon formation around 500 ma and the existence of middle proterozoic relics 1 95 0 05 ga

k ar geochronology of different tectonic units at the northwestern margin of the bohemian massif abstract the polymetamorphic moldanubian mo of the northeasthern margin of the bohemian massif has been thrust to the north onto the mainly paleozoic sedimentary saxothuringian of the fichtelgebirge fg these two units have undergone polyphase deformation and the last regional event to affect both units was a low pressure metamorphism in which temperatures decreased towards the north in contrast the nappe units of the erbendorf vohenstrauss zone zev and the erbendorf greenschist zone egz which partly cover the border of the moldanubian and the saxothuringian and the munchberg nappe pile mm which lies on the saxothuringian were in parts subjected to a late medium pressure metamorphic event the zev the egz the mo and the fg are intruded by late carboniferous granites conventional k ar analyses mainly of hornblendes and muscovites from the autochthonous fg and mo the units beneath the nappes have yielded exclusively carboniferous dates the oldest dates point to a regional cooling of the rocks which outcrop at the present day surface at about 330 320 ma i e at the early late carboniferous boundary the late carboniferous cooling history was largely governed by the thermal influence of the post kinematic granites 320 295 ma especially in the fg and the northern mo the high grade metamorphic rocks in the western part of the zev and in the upper three nappes of the mm mostly yield dates around 380 ma i e early devonian the results show a relatively wide scatter moreover biotites frequently appear to be older than the coexisting muscovites both observations indicate that the rocks underwent a later thermal influence whether some groups of older dates e g 400 ma are due to excess argon or to inherited argon is still open to discussion slightly scattered muscovite dates around 366 ma were obtained for the prasinite phyllite series one of the lower nappes of the mm a single hornblende from the egz gave the same age these two nappes have therefore probably been affected by a late devonian thermal and or tectonic event the muscovite dates obtained from the paleozoic bavarian lithofacies the lowermost nappe of the mm and the hornblende dates from the eastern part of the zev are indistinguishable from those of the autochthonous units fg and mo

secondary ca al silicates in plutonic rocks implications for their cooling history secondary ca al silicates are used to constrain the p t x conditions of the very early post magmatic stage of the intermediate to basic hercynian plutonic complexes of charroux civray nw massif central france and fichtelgebirge ne bavaria germany the secondary ca al silicates hydrogarnet prehnite pumpellyite epidote and laumontite form lenses within unaltered or only slightly chloritized biotite hydrogarnet as the first occurring ca al silicate phase crystallizes at temperatures above 340 c the common paragenesis prehnite pumpellyite post dates hydrogarnet and indicates rather narrow ranges of temperature 200 280 c and pressure 2 3 kbar laumontite is formed at the end of ca al silicate crystallization 180 260 c 1 3 kbar mostly in small fractures in association with prehnite and adularia the observed crystallization sequence of the ca al silicates and their stabilities define a retrograde alteration path for the plutonic rocks the ca al silicate assemblage results from an early pervasive alteration of the plutonic rocks by low xco2 fluids during post magmatic cooling subsolidus cooling starts at about 4 kbar at solidus temperatures as indicated by magmatic epidote stability hornblende barometry and fluid inclusion data and continues under slightly decreasing pressure uplift down to 2 3 kbar at 200 280 c prehnite pumpellyite paragenesis this shows that ca al silicate assemblages may be a unique tool to constrain the p t conditions of the subsolidus cooling of intermediate to basic plutonic bodies