a 70 ma record of suprasolidus conditions in the large hot long duration grenville orogen petrochronology of magmatic monazite and apatite from a single paragneiss leucosome derived by insitu partial melting documents the thermal evolution of the allochthonous belt of the central grenville province monazite records suprasolidus metamorphism from ca 1080 to 1020 ma under high temperature up to 850 degrees c apatite from the same leucosome yields an age of 960 ma consistent with cooling of this crustal segment down to subsolidus conditions of ca 450 degrees c a pegmatitic granite dyke with lobate contacts previously dated at ca 1005 ma turlin et al 2017 is interpreted to intrude the paragneisses at a temperature of ca 650 degrees c close to the wet solidus these data document slow cooling at a rate of 2 6 degrees c ma for the middle crust of this part of the grenville hinterland marked by protracted suprasolidus conditions for at least 70 ma it supports the definition of the grenville orogen as a large hot long duration orogen

numerical simulation of metamorphic pressure temperature time paths and fluid production in subducting slabs in order to investigate metamorphic reactions and fluid production in subduction zones a 2 dimensional finite difference model has been constructed the model simulates heat transfer in a subduction zone with a 27 dip a spatial resolution of 1 km is used in the region of the subduction thrust in order to carefully monitor metamorphic reactions elsewhere the spatial resolution is 5 km in these calculations the oceanic crust is assumed to contain 2 wt bound volatiles distributed homogeneously within the 7 5 km thick crust fluids are released by continuous and discontinuous end members of three different dehydration models pressure sensitive temperature sensitive and amphibole negative dp dt dehydration subduction zone pressure temperature time p t t paths predicted by the model intersect the wet basaltic solidus only at the initiation of subduction in mature subduction zones and in the absence of significant frictional heating and induced mantle convection p t t paths encounter subsolidus conditions at depths of 100 150 km beneath magmatic arcs suggesting that slab dehydration reactions play an important role in arc magma genesis metamorphic dehydration reactions that consume 50 kj kg result in p t t paths that are only 5 k cooler than models that neglect such reactions in both the temperature and amphibole dehydration models the fluid production region moves to greater depths over time as the subduction zone cools within the oceanic crust fluid production starts at the top of the crust and migrates downward in the temperature and amphibole dehydration models this phenomenon coupled with the weakening effect of devolatilization reactions may result in the accretion of oceanic crust to the hanging wall of the subduction zone for a convergence rate of 3 cm yr average fluid fluxes out of the subducting slab range from 0 1 kg fluid m yr for the continuous reaction models to 1 kg fluid m yr for the discontinuous pressure and amphibole dehydration models vertical fluid fluxes on the order of 1 kg fluid m yr can substantially perturb the thermal structure of overlying rocks and can cause large scale hydration and metasomatism of the overlying mantle wedge in contrast the thermal effect of fluid flow parallel to the subduction thrust zone is insignificant compared to the thermal effect of subducting oceanic lithosphere

petrochronology and tims thermal ionization mass spectrometers or tims were developed by the pioneers of mass spectrometry in the mid 20th century and have since been workhorses for generating isotopic data for a wide range of elements later developed mass spectrometric techniques have many advantages over tims including higher spatial resolution with in situ techniques such as secondary ion mass spectrometry sims and laser ablation inductively coupled plasma mass spectrometry la icpms and greater versatility in terms the elements that can be easily and well measured the reason tims persists as an important method for geochronology is that for some key parent daughter systems e g u pb sm nd it can produce isotopic data and resultant dates with 10 100 times higher precision and more quantifiable accuracy than in situ techniques even when sample sizes are very small such as those that might result from single crystals or even small portions of zoned crystals for many questions in the geosciences the highest achievable precision and accuracy are required to resolve the timescales of processes and or correlate events globally as an example modern tims u pb geochronology is capable of producing dates with precision and accuracy better than 0 1 of the age for single crystals with only a few picograms pg of pb therefore it is possible to constrain the durations of single zircon crystal growth in magmatic systems over tens to hundreds of kyr in mesozoic and younger rocks if these dates and rates can be connected with other igneous processes such as magma transfer emplacement and crystallization then it becomes possible to calibrate thermal and mass budgets in magmatic systems and evaluate competing models for pluton assembly and subvolcanic magma storage as another example sm nd geochronology of garnet permits dates with precision better than 1 million years for garnets of any age including multiple concentric growth zones in single crystals such

past present and future of thermochronology in one form or another geochronologists have been practicing thermochronology 1 the use of radioisotopic dating to constrain thermal histories of rocks and minerals for over 40 years building from lessons learned over these four decades thermochronology continues to evolve due to technical developments increasingly sophisticated theoretical models and an expanding range of applications in geologic and planetary science most recently interest in earth surface processes and interactions between tectonics erosion and climate has drawn attention to techniques that can address the timing and rates of processes operating at temperatures below about 300 c the purpose of this rimg volume is to assess the current state of thermochronology as of circa 2005 which is coincidentally the 100th anniversary of the first radioisotopic date rutherford 1905 1906 excellent review papers and books on specific topics within this field have been published but no single volume has yet provided a comprehensive review of current practices basic theory and illustrative examples the motivation for this volume stems from these considerations knowing that in a fast developing field a book like this can quickly become dated we tried to include sufficient review of fundamentals and the literature to offer students and new users a useful introduction to thermochronology that may have some staying power in this chapter we first review the salient points of thermochronology s history before assessing our current capabilities and challenges and then taking the risk of suggesting where the field is headed we do not provide a comprehensive history of the method that does full justice to the work of the large and growing cohort of thermochronologists in this short space we instead opted to give our perspectives on where the intellectual and technological roots of the discipline lie which run deeper and go back farther than is sometimes appreciated

hydrological reconstruction of holocene paleofloods in baoji tianshui gorge upper weihe river basin china abstract a detailed field study was conducted on the palaeoflood deposit profile in the baoji tianshui gorge in the upper weihe river the sequence of palaeoflood events was established through field observation and grain size analysis with chronological supports from optical stimulated luminescence osl dates and from archaeologically constrained age frameworks the paleoflood peak water level and peak discharge were reconstructed using the pinch out elevation method and the thickness sediment content relationship the results show that three large scale paleoflood events occurred during a period of 3200 3000 yr bp in the baoji tianshui gorge upper weihe river the peak water levels varied between 660 48 and 661 40 m and the peak discharge varied between 23 200 and 25 200 m3 s among the three large scale paleoflood events the same methodology was used to calculate the modern flood peak discharges that yielded a deviation of only 3 6 between the reconstructed and measured peak discharges further verifying the acceptance of this method for the reconstruction of paleoflood peak discharges this studyexpanded the database of the palaeoflood hydrology research in the weihe river and the expanded database may be useful in providing important information for projects such as water conservancy hydropower engineering construction flood control and disaster mitigation

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

two dimensional modeling of the p t t paths of regional metamorphism in simple overthrust terrains quantitative modeling of metamorphic p t t paths in thrust belts has so far been limited to one dimensional simplification we have developed a two dimensional time dependent finite element procedure capable of tracing and updating fault and strata boundaries during faulting for modeling thermal evolution of overthrust terrains we show that the two dimensionality of an overthrust and the speed and duration of thrusting have significant effects on the p t t paths of rocks in orogenic belts the results suggest that petrological studies of p t paths of rocks from different lateral positions in an orogenic belt along with two dimensional modeling procedure can put strong constraints on models of regional tectonic processes

hp uhp exhumation during slow continental subduction self consistent thermodynamically and thermomechanically coupled model with application to the western alps abstract the exhumation of ultra high pressure uhp crustal material often occurs at the rear of sedimentary accretion wedges e g alps himalaya norway however the mechanisms of deep 100 km burial in slow 10 mm yr 1 and slow at the later stage

experimental evidence for diamond facies metamorphism in the dora maira massif the peak metamorphic conditions of subducted continental crust in the dora maira massif western alps have been revised by combining experimental results in the kcmash system with petrologic information from whiteschists textural observations in whiteschists suggest that the peak metamorphic assemblage garnet phengite kyanite coesite talc originates from the reaction kyanite talc garnet coesite liquid in the experimentally determined petrogenetic grid this reaction occurs above 45 kbar at 730 c at lower pressures talc reacts either to orthopyroxene and coesite or together with phengite to biotite coesite and kyanite the liberated liquid contains probably similar amounts of h2o and dissolved granitic components the composition of the liquid in the whiteschists at peak metamorphic conditions a major unknown in earlier studies was probably very similar to the liquid composition produced in the experiments therefore the experimentally determined petrogenetic grid represents a good model for the estimation of the peak metamorphic conditions in whiteschists experimentally determined si isopleths for phengite further constrain peak pressures to 43 kbar for the measured si 3 60 of phengite in the natural whiteschists all these data provide evidence that the whiteschists reached diamond facies conditions the fluid absent equilibrium 4 kyanite 3 celadonite 4 coesite 3 muscovite pyrope has been calibrated on the basis of garnet and phengite compositions in the experiments and serves as a geothermobarometer for ultra high pressure uhp metapelites for graphite bearing metapelites and kyanite phengite eclogites forming the country rocks of the whiteschists peak metamorphic pressures of about 44 3 kbar were calculated from this barometer for temperatures of 750 c estimated from garnet phengite thermometry therefore the whole ultra high pressure unit of the dora maira massif most likely experienced peak metamorphic conditions in the diamond stability field while graphite is common in the metapelites diamond has not been found so far the absence of metamorphic microdiamonds might be explained by the low temperature of metamorphism the absence of a free fluid phase in the metapelites and a short residence time in diamond facies conditions resulting in kinetic problems in the conversion of graphite to diamond

sedimentary records of large holocene floods from the middle reaches of the yellow river china slackwater deposits have been used as evidence for the magnitude and frequency of palaeofloods which occurred prior to gauged and historical data sets e g holocene extreme flood reconstruction in north america and australia the slackwater deposits from xiaolangdi reach of the yellow river have been identified and related to floods according to both eyewitness accounts and the analysis of sediments the technique successfully delineated the peak flood level and estimated a discharge of the 1994 ad flood in the yellow river the discrepancy of the estimated discharge was less than 5 compared with the instrumental records when we applied the technique to palaeoflood analysis the highest slackwater deposits and flood indicators from the tributary mouths the low river terraces and the loess platforms and loess alcoves together with radiocarbon dating provided evidence of at least six floods with peak discharges greater than 27 300 m3 s 1 at xiaolangdi section over the last 8500 yr the largest flood in ca 7362 yr b p had an estimated discharge of 42 900 m3 s 1 which is the maximum flood event known in the yellow river the results provide a basis for a determining the floods which were comparable to the most cataclysmic flood during the last 2360 yr in 1843 ad and b estimating the flood magnitude for a 10 000 yr time scale relevant for building xiaolangdi dam one of the largest dams under construction in the world the reconstruction of palaeofloods also provides evidence for a phase between ca 6000 8000 yr b p which was much wetter than today as a result of increased summer precipitation and enhanced pacific monsoon conditions in eastern china

petrology and geochronology of rutile rutile tio2 is an important accessory mineral that when present offers a rich source of information about the rock units in which it is incorporated it occurs in a variety of specific microstructural settings contains significant amounts of several trace elements and is one of the classical minerals used for u pb age determination here we focus on information obtainable from rutile in its original textural context we do not present an exhaustive review on detrital rutile in clastic sediments but note that an understanding of the petrochronology of rutile in its source rocks will aid interpretation of data obtained from detrital rutile for further information on the important role of rutile in provenance studies the reader is referred to previous reviews e g zack et al 2004b meinhold 2010 triebold et al 2012 coarse rutile is the only stable tio2 polymorph under all crustal and upper mantle conditions with the exception of certain hydrothermal environments smith et al 2009 as such we will focus on rutile rather than the polymorphs brookite anatase and ultrahigh pressure modifications in this chapter we first review rutile occurrences trace element geochemistry and u pb geochronology individually to illustrate the insights that can be gained from microstructures chemistry and ages then in the spirit of petrochronology we show the interpretational power of combining these approaches using the ivrea zone italy as a case study finally we suggest some areas of future research that would improve petrochronologic research using rutile rutile is a characteristic mineral in moderate to high pressure metapelitic rocks in high pressure metamorphosed mafic rocks and in sedimentary rocks e g force 1980 frost 1991 zack et al 2004b triebold et al 2012 rutile also occurs rarely in magmatic rocks e g anorthosites as well as in some hydrothermal systems coarse grained

pressure temperature time paths from two dimensional thermal models prograde retrograde and inverted metamorphism two dimensional thermal models of time transitive crustal thickening and subsequent unroofing in large scale overthrust terrains generate pressure temperature time ptt paths that generally resemble those produced by earlier one dimensional instantaneous models but that differ in detail these differences in ptt path morphology are most pronounced proximal to major fault zones where postthrusting geotherms are characterized by large temperature inversions in one dimensional models but generally lack inversions in two dimensional models tests using the two dimensional fault model developed here indicate that 1 burial rate proportional to dp dt not thrust fault geometry dip angle controls the topology of synthrusting pt paths but plays only a minor role in determining the maximum temperature that rocks attain during the later unroofing stage in their thermal histories 2 normal ranges of thrusting and erosion rates and fault parameters result in pt paths with the usual sense clockwise on conventional pt diagrams counterclockwise in our diagrams 3 the amount of heating and duration of heating following the end of thrusting are a function of the rate of unroofing dp dt during this period 4 fast unroofing rates lead to the attainment of lower maximum temperatures after greater amounts of unroofing 5 the initial thermal state of the lithosphere prior to thrusting has a profound effect on pt path morphologies and on the peak metamorphic conditions attained by samples 6 for excess heat distributed across the entire lithosphere e g due to increased background thermal gradients a plot of peak temperatures experienced by metamorphic rocks versus structural depth tmax plot closely represents the initial geotherm 7 excess heat in the crust only e g increased radioactive heating in a layer yields a different result with the tmax plot corresponding to the initial geotherm only near the top of the hanging wall tmax plots for both 6 and 7 show no temperature inversion which exceeds the nominal uncertainties 50 k for geothermometric data 8 shear heating can lead to significant temperature inversions at the fault zone if the frictional coefficient is 0 6 or greater and 9 simultaneous thrusting and erosion produce pt loops significantly narrower than those resulting from sequential thrusting and erosion suggesting that any formulation which fails to account for some degree of simultaneity between thrusting and erosion represents a far endmember model forward models like those presented here provide important guidelines for understanding the sensitivity of metamorphic ptt paths to various thermal mechanical and geometric factors related to tectonism but they are generally inappropriate for reconstructing metamorphic thermal histories from actual petrologic and geochronologic data analytical inversion techniques that use a postthrusting thermal regime consistent with two dimensional forward models and that integrate values of dp dt dt dt and radioactive heating rates extracted from suites of metamorphic rocks provide the best hope for furthering our understanding of the thermal evolution of metamorphic terrains

extraordinary hydro climatic events during 1800 1600 yr bp in the jin shaan gorges along the middle yellow river china abstract paleoflood slackwater deposits swds of the holocene were found at many sites along the jin shaan gorges in the middle yellow river basin a set of four paleoflood swd beds was identified at the pingduguan pdg sites and studied by field observations and laboratory analysis including particle size distribution and optically stimulated luminescence osl dating hydrological reconstruction using the hec ras model shows that the paleoflood peak discharges were between 39 000 and 50 220 m 3 s 1 they are about double the largest gauged flood 24 000 m 3 s 1 that has occurred since 1976 along the jin shaan gorges these extraordinary flood events were osl dated to between 1800 and 1600 yr bp during which climatic deterioration and disasters were documented over the yellow river basin various evidences show that severe flood and drought disasters during the episode resulted in frequent harvest failures famines social upheavals and population reduction invasions by nomads and even the fall and replacement of the dynasties climate decline documented by proxy records such as tree rings stalagmites ice cores and lake sediments from over the world is in agreement with the paleoflood events identified along the jin shaan gorges these mean that the extraordinary paleoflood events are closely related to increased climatic variability and instability this result provides solid evidence for understanding the response of hydroclimatic system to global climate change in the semi arid and sub humid regions of the world

palaeoflood record of the tagus river central spain during the late pleistocene and holocene abstract palaeoflood hydrology of the tagus river central spain was reconstructed from slackwater deposits and palaeostage indicators in two canyon reaches located at el puente del arzobispo in the central part of the catchment 35 000 km 2 in drainage area and in alcantara at the lower part of the catchment 52 000 km 2 in drainage area near the portuguese border the palaeoflood record with more than 80 flood events shows clusters of floods at specific periods from 9440 to 9210 14 c yr bp 8540 8110 bc 8500 to 8000 14 c yr bp 7500 7000 bc 6750 14 c yr bp 5000 bc 1200 to 950 14 c yr bp ad 785 1205 410 14 c yr bp ad 1450 1500 170 to 80 14 c yr bp ad 1670 1950 the largest flood s occurred during the periods 9440 to 9210 14 c yr bp 6750 14 c yr bp and 1200 to 950 14 c yr bp reaching minimum discharge estimates of between 4000 and 4100 m 3 s 1 in the el puente del arzobispo reach and 13 700 15 000 m 3 s 1 in the alcantara reach these periods with increased flood magnitude and or frequency in the tagus river are strongly related to increased moisture influx and winter precipitation in the iberian peninsula especially in the western zone proxy records sensitive to winter precipitation such as lake levels and vegetation changes indicated by pollen records are in good agreement with the clusters of floods found in the tagus river this flood variability seems to correspond to changes in the prevailing atmospheric circulation pattern affecting the iberian peninsula

depositional development of an isolated mound and adjacent area in the southern yellow sea during the last postglacial sea level rise abstract a detailed analysis of the chirp 2 7 khz profiles core sediments and 14c ages from an isolated mound and adjacent area in the southern yellow sea off the jiangsu coastal area reveals a highly complex development of transgressive and highstand deposits during the last postglacial sea level rise in water depths of 50 60 m the terrace consists of thin transgressive sediments directly above transgressive surface the isolated mound 40 7 km long and 29 6 km wide occurs above 40 50 m in water depth and is detached from the jiangsu and east china sea tidal sand ridges it comprises the sse ward prograding sandy unit pu and the overlying muddy flank unit fu at the outer margin the terrace with thin transgressive sediments was most likely formed when sea level rose rapidly from 58 to 45 m around about 11 6 ka as rate of sea level rise was attenuated from 45 to 36 m between ca 11 6 and 9 6 ka the nearly flat lower boundary of the pu was produced at water depths of 37 42 m by a ravinement process subsequent rapid sea level rise from 36 to 16 m between ca 9 6 and 9 1 ka created accommodation for deposition of the pu sea level rise stagnated from 16 to 10 m between ca 9 1 and 7 5 ka during which the huanghe river apparently shifted south of the shandong peninsula and discharged directly into the western yellow sea the combination of slow sea level rise and sediment supply from the early holocene huanghe delta by the southward flowing coastal currents most likely favoured the formation of the sse ward prograding transgressive sandy pu the highstand muddy fu overlying the pu was accumulated after ca 7 5 when sea level approached its present position and the huanghe river shifted back to the north development of transgressive and highstand deposits in the study area during the last postglacial sea level rise was highly variable in time and space in response to changes in rate of sea level rise together with variations in sediment supply caused by the shifting of the huanghe river

an evaluation of the simulated water balance of eurasia and northern africa at 6000 y bp using lake status data changes in the water balance of eurasia and northern africa in response to insolation forcing at 6000 y bp simulated by five atmospheric general circulation models have been compared with observations of changes in lake status all of the simulations show enhancement of the asian summer monsoon and of the high pressure cells over the pacific and central asia and the middle east causing wetter conditions in northern india and southern china and drier conditions along the chinese coast and west of the monsoon core all of the models show enhancement of the african monsoon causing wetter conditions in the zone between ca 10 20 n four of the models show conditions wetter than present in southern europe and drier than present in northern europe three of the models show conditions similar to present in the mid latitude continental interior while the remaining models show conditions somewhat drier than present the extent and location of each of the simulated changes varies between the models as does the mechanism producing these changes the lake data confirm some features of the simulations but indicate discrepancies between observed and simulated climates for example the data show 1 conditions wetter than present in central asia from india to northern china and mongolia indicating that the simulated asian monsoon expansion is too small 2 conditions wetter than present between ca 10 30 n in africa indicating that the simulated african monsoon expansion is too small 3 that northern europe was drier but the area of significantly drier conditions was more localized around the baltic than shown in the simulations 4 that southern europe was wetter than present apparently consistent with the simulations but pollen data suggest that this reflects an increase in summer rainfall whereas the models show winter precipitation and 5 that the mid latitude continental interior was generally wetter than present

transient hot channels perpetrating and regurgitating ultrahigh pressure high temperature crust mantle associations in collision belts abstract two dimensional numerical modeling of early continental collision associated with subduction of the lithospheric mantle shows that the formation and exhumation of coesite and diamond bearing rocks metamorphosed at 700 to 900 c in the presence of dense supercritical silicate fluids and melts may be explained by a transient hot channel effect anomalously high temperature is caused by intense viscous and radiogenic heating in the channel composed of deeply subducted radiogenic upper crustal rocks especially sediments of passive margin origin and mantle rocks heating is associated with partial melting of crustal rocks caused by pervasive flow of aqueous fluids relieved by rapid dehydration deserpentinization of the overriding mantle lithosphere that has been hydrated during earlier subduction stages the channel penetrates the plate interface to the bottom of the lithosphere of the overriding plate 150 200 km and is characterized by metamorphic temperatures reaching 700 to 900 c low effective viscosity of rocks subsequent to increased temperature partial melting and fluid infiltration permits profound mixing of hydrated mantle and crustal rocks the hot channel exists only during early collision but rapidly produces large amounts of ultrahigh pressure high temperature rocks further collision closes the channel by squeezing rheologically weak partially molten buoyant rocks between the strong lithospheric mantles of the two colliding plates assemblages of complicated p t paths with repetitive loops characterize exhumation of ultrahigh pressure rocks in the convoluted flow pattern of the hot channel

mineral accumulations induced by biological activity on granitic rocks in qinghai plateau china fragments of weathered granitic rocks from the kunlun shan qinghai plateau china were investigated to elucidate the influence of biotic crusts on the breakdown of granitic rocks in an alpine environment scanning electron microscopy with energy dispersive system and x ray diffractometry were used to describe the nature and properties of mineral accumulations on the rock surface results showed that organic salts such as calcium oxalate and calcium formate are associated with aspicilia caesiocinera nyl ex malbr arnold caloplaca sp xanthoria elegans link th fr and lecidea plana lahm nyl secondary accumulations of 2 i clays minerals are found in a caesiocinera while oxides of manganese are associated with x elegans coatings of goethite iron oxides are believed to form from biological activity associated with the presence of hyphae and rodlet structures on the flakes calcium oxalate crystallizes into several morphologies such as druse hexagonal plates and lenticular containing between 20 and 48 per cent calcium by weight calcium formate and iron oxide goethite occur together in the form of red desert varnish observed black coatings contain as much as 37 per cent manganese and 22 per cent iron clay accumulations have plate like morphology and contain c 2 1 silicon to aluminium contents we argue that organic acids from the activities of biotic crusts contribute to the breakdown of granitic rocks fungi accelerate the breakdown of granitic rocks through the growth of fungal hyphae along the 001 cleavage planes in primary chloritic minerals copyright c 2003 john wiley sons ltd

the alps 2 controls on crustal subduction and ultra high pressure rock exhumation in alpine type orogens building on our previous results we use 2 d upper mantle scale thermomechanical numerical models to explore key controls on the evolution of alpine type orogens and the alps per se focusing on ultra high pressure u hp metamorphic rocks the models show that uhp rocks form and exhume by burial and subsequent buoyant ascent of continental crust in the subduction conduit here we test the sensitivity of the models to surface erosion rate crustal heat production plate convergence divergence rates geometry of the subducting continental margin and strength of the retrocontinent surface erosion affects crustal exhumation but not early buoyant exhumation metamorphic temperatures increase with crustal radioactive heat production maximum burial depth prior to exhumation increases with plate convergence rates but exhumation rates are only weakly dependent on subduction rates onset of absolute plate divergence does not trigger exhumation in these models we conclude that contrasting peak pressures exhumation rates and volumes of u hp crust exhumed in the alps orogen primarily reflect along strike contrasts in the geometry thermal structure and or strength of the subducting microcontinent brianconnais and continental european margin crust the experiments also support the interpretation that the western alps u hp internal crystalline massifs exhumed as composite stacked plumes and that these plumes drove local crustal extension during orogen scale shortening for weak upper plate retrocrusts postexhumation retrothrusting forms a retrowedge overall these results are consistent with predictions using the exhumation number ratio of buoyancy to side traction forces in the conduit which expresses the combined parameter control of the depth volume of crustal subduction and the transition to buoyant exhumation

electron microprobe petrochronology the term petrochronology has increasingly appeared in publications and presentations over the past decade the term has been defined in a somewhat narrow sense as the interpretation of isotopic dates in the light of complementary elemental or isotopic information from the same mineral s kylander clark et al 2013 although complementary isotopic and elementary information are certainly a central and critical part of most if not all petrochronology studies the range of recent studies that might use the term covers a much broader scope the term petrochronology might alternatively be defined as the detailed incorporation of chronometer phases into the petrologic and tectonic evolution of their host rocks in order to place direct age constraints on petrologic and structural processes as noted by kylander clark et al 2013 the linkage between geochronology and petrology can involve a variety of data including mineral textures and fabrics the distribution of mineral modes or volume proportions compositional zoning mineral inclusion relationships and certainly major element trace element and isotopic composition of the chronometer and all other phases electron probe micro analysis epma has a central and critical role to play in establishing the linkage between chronometer phases and their host assemblage the basic instrument is an electron microscope which can be used in either scanning or fixed beam modes with integrated wavelength dispersive spectrometers wds energy dispersive spectrometers eds electron detectors to image secondary and backscattered signals a light optical system and optionally cathodoluminescence cl detection the electron microprobe is used to investigate the distribution composition and compositional zonation of all mineral phases the data that underpin thermobarometric analysis and modeling of p t histories the microprobe with m scale spatial resolution can also characterize compositional zonation in very small accessory phases including monazite xenotime zircon allanite titanite apatite and others this as discussed below can be a

holocene paleofloods related to climatic events in the upper reaches of the hanjiang river valley middle yangtze river basin china abstract holocene slackwater deposits along the river channels were used to study the magnitude and frequency of the paleofloods that occurred prior to gaged and historical data sets all over the world paleoflood hydrological investigations were carried out in the upper reaches of the hanjiang river china four bedsets of paleoflood slackwater deposits were found interbedded in the holocene loess soil sequence within the cliff riverbanks they were studied by field observations and laboratory analysis including magnetic susceptibility particle size distribution and optically stimulated luminescence osl dating these provide a complete catalog of holocene flood events at a watershed scale that is very important in understanding the regional hydrological response to monsoonal climatic change four episodes of holocene overbank flooding have been recorded over the basin these hydroclimatic events were dated to 8500 8400 4200 4000 3200 2800 and 1800 1700 ybp by using the osl method in combination with pedostratigraphic correlations with the previously studied holocene pedostratigraphy in the hanjiang river basin the peak discharges of these flood episodes vary in a range from 26 610 to 47 900 m 3 s 1 as estimated by using the slope area method of streamflow measurement these flood events are therefore considered a regional expression of known climatic events and demonstrate holocene climatic instability the study provides a solid reference that magnitudes and recurrence frequencies of floods are highly sensitive to climate change in the monsoonal zone in other parts of the world

zircon the metamorphic mineral a mineral that forms under conditions as variable as diagenesis to deep subduction melt crystallization to low temperature alteration and that retains information on time temperature trace element and isotopic signatures is bound to be a useful petrogenetic tool the variety of conditions under which zircon forms and reacts during metamorphism is a great asset but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure temperature deformation fluid context under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information in the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology with the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high grade metamorphic rocks since then there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures its capacity to protect mineral inclusions zircon thermometry trace element patterns and their relation to main mineral assemblages solubility of zircon in melt and fluids and isotopic systematics in single domains that go beyond u pb age determinations metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks but has become a truly indispensable mineral in reconstructing pressure temperature time fluid paths over a wide range of settings an obvious consequence of its wide use is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial in this chapter reference to published works are intended as examples and not as a compilation this chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture deformation structure and mineral chemistry including trace element and isotopic systematics linking this information together highlights the potential of zircon as a key mineral in petrochronology

river patterns and their meaning abstract rivers are largely responsible for shaping the earth s continental landscapes river patterns the spatial arrangements of channels in the landscape are determined by slope and structure at site and sector scale channel morphology varies spatially and in time but river patterns and drainage texture or the frequency of stream lines per unit area together determine the intricacy or otherwise of topography most river patterns evolve through natural selection slope induces the formation of such patterns as parallel radial and distributary while structure produces straight angular trellis and annular arrangements once established patterns tend to persist nevertheless at many sites the usual patterns have been disturbed and patterns that are anomalous in terms of slope and structure have been produced by diversion tectonism volcanism glaciation mass movements and human activities by antecedence superimposition inheritance or underprinting by the persistence of deeply eroding rivers which encounter alien structures and by climatic change river patterns provide clues to underlying structure and to the chronology of events they have also proved significant in the search for minerals

extraordinary floods of 4100 4000 a bp recorded at the late neolithic ruins in the jinghe river gorges middle reach of the yellow river china abstract holocene palaeohydrological investigations were carried out in the middle reach of the jinghe river that drains the central part of the loess plateau a set of palaeoflood slackwater deposits was found interbedded in the holocene loess soil sequence within the cliff riverbanks both the sedimentary criteria and the analytical results show that this sediment was sourced from a suspended sediment load of floodwater the major cultural layer of a late neolithic settlement 4300 4000 a bp occurring at the same site is blanketed by this flood deposit directly slopewash with pottery shards charcoal and burnt earth occurs in between the palaeoflood slackwater deposit beds it indicates that each of the slackwater deposit beds has recorded one individual flood event and that these extraordinary floods occurred when the late neolithic people occupied the riverbank terrace land this pre historical settlement was abandoned after the floods eventually due presumably to repeated inundation by overbank floodwater the flood events were osl dated to between 4100 and 4000 a bp and checked by archaeological dating of the anthropogenic remains retrieved from the sequences peak discharges of the floods were estimated to between 19 500 and 22 000 m3 s 1 which is several times gauged maximum floods during the 4200 4000 a bp climatic event severe droughts were documented in china s monsoonal regions and great floods were known from an ancient legend this study provides a well dated sediment record of pre historical floods which occurred at the transition from the neolithic to the bronze age in the yellow river basin it shows that extraordinary floods were part of the abrupt climatic variability during the 4200 4000 a bp climatic event the highly variable unstable and catastrophic climate typified by both droughts and floods resulted in settlement abandonment and possibly the decline of the highly developed late neolithic civilizations in china s monsoonal regions

investigating extreme flood response to holocene palaeoclimate in the chinese monsoonal zone a palaeoflood case study from the hanjiang river abstract palaeoflood events recorded by slackwater deposits swds were investigated extensively by sedimentological criteria of palaeohydrology along the upper hanjiang river valley modern flood swds were collected for comparison with palaeoflood swd in the same reaches three typical palaeoflood swds were observed within holocene loess soil blanket on the first river terrace land the grain size distributions of palaeoflood swds are similar to modern flood swds whereas they are different from eolian loess and soil palaeoflood swd lies in three major pedo stratigraphic boundaries ts l 0 l 0 s 0 and s 0 l t in the holocene loess soil profiles the chronology of three palaeoflood episodes was established by osl dating and pedo stratigraphic correlation with the well dated holocene loess soil profiles in the upper hanjiang river basin holocene palaeoflood events were dated to 9500 8500 3200 2800 and 1800 1700 a b p respectively palaeoflood discharges were estimated by the palaeoflood model i e slope area method and step backwater method the highest discharges are 51 680 53 950 m 3 s 1 at the 11 500 time scale in the xunyang reach of the upper hanjiang river valley holocene extraordinary hydroclimatic events in the hanjiang river often result from abnormal atmospheric circulations from southwest monsoons in the chinese monsoonal zone these results provide a regional expression of extreme flood response to holocene palaeoclimate to understand the effects of global climatic variations on the river system dynamics

petrochronology of zircon and baddeleyite in igneous rocks reconstructing magmatic processes at high temporal resolution zircon zrsio4 and baddeleyite zro2 are common accessory minerals in igneous rocks of felsic to mafic composition both minerals host trace elements substituting for zr among them hf th u y rees and many more the excellent chemical and physical resistivity of zircon makes this mineral a perfect archive of chemical and temporal information to trace geological processes in the past utilizing the outstanding power and temporal resolution of the u pb decay schemes baddeleyite is a chemically and physically much more fragile mineral it preserves similar information only where it is shielded from dissolution and physical fragmentation as an inclusion in other minerals or in a fine grained or non reactive rock matrix it offers the potential for dating the solidification of mafic rocks with high precision through its crystallization in small pockets of zr enriched melt after extensive olivine and pyroxene fractionation zircon and baddelelyite u pb dates are for an overwhelming majority of cases and where we can assume a closed system considered to reflect the time of crystallization the development of the u pb dating tool ca id tims chemical abrasion isotope dilution thermal ionization mass spectrometry since 2005 has led to unprecedented precision of better than 0 1 in 206pb 238u dates bowring et al 2005 increased sensitivity of mass spectrometers and low laboratory blanks due to reduction of acid volumes allow routine u pb age determinations of micrograms of material at sufficiently high radiogenic common lead ratios see schoene and baxter 2017 this volume in situ u pb age analysis using laser ablation or primary ion beam sputtering allows analysis of sub microgram quantities of zircon material from polished internal sections or zircon surfaces with spot diameters ranging from 30 m for laser ablation inductively coupled plasma mass spectrometry la icp ms to 10 m for secondary ion mass spectrometry sims lateral resolutions of 2 5 m for nanosims

a 4500 year record of large floods on the colorado river in the grand canyon arizona a sequence of flood deposits left by the colorado river in the grand canyon arizona provides evidence of at least 15 floods with peak discharges greater than 5500 m 3 sec 1 over the last 4500 yr ten floods during the last 2000 2300 yr had discharges greater than 6800 m 3 sec 1 one flood 1600 1200 yr ago had a discharge exceeding 14 000 m 3 sec 1 a flow rate more than twice the largest gaged flood this record of flooding is one of the longest for a major u s river and combined with the gaged record of twentieth century floods allows determination of the frequency and history of large floods that have affected key aspects of colorado river geomorphology

paleoflood hydrology and extraordinary flood events paleoflood hydrology is the study of past or ancient flood events the most accurate technique involves the analysis of slackwater deposits and paleostage indicators swd psi slackwater deposits are sand and silt emplaced from suspension in exceptionally deep high velocity floods that characterize narrow deep canyons in resistant geologic materials these and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events favorable sites for slackwater deposition include channel margin areas where stresses and velocities are reduced below critical values necessary to maintain fine grained bedload material sand and silt in suspension computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections the flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers correlation of multiple swd psi sites along a river reach is used to identify the maximum paleostage achieved by a given flood advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years the major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits an important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14c tiny blebs of charcoal seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance since swd psi studies yield very accurate determinations of paleoflood ages and magnitudes there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood frequency analysis nevertheless swd psi paleoflood hydrology has moved beyond the research phase its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites

40ar 39ar dating of the thanet gabbro ontario looking through the grenvillian metamorphic veil and implications for paleomagnetism in this first dating study of the thanet gabbro complex from the hastings basin area we have obtained k ar evidence to support the geological contention that this pluton and by inference others nearby has escaped the severity of the grenvillian metamorphic overprint specifically a reliable date of 1200 ma for a hornblende demonstrates that this body was emplaced at least 100 ma earlier than the peak of metamorphism in the adjacent haliburton highlands from considerations of possible cooling histories and a proposed grenville apparent polar wander apw curve we deduce that only the thanet magnetization component with the lowest laboratory blocking temperatures th3 was likely acquired as a thermoremanent magnetization whereas the other two components were probably acquired as low temperature chemical remanent magnetizations furthermore the th3 component was likely set before 1 0 ga consistent with our earlier proposed grenville loop

petrochronology based on ree minerals monazite allanite xenotime apatite ree minerals monazite xenotime and allanite are ree1 minerals sensu stricto because lanthanides la lu and yttrium are critical constituents in them apatite does not require ree but because it contains substantial ree in many rocks it is included in this review all four minerals also host unusually high radionuclide concentrations notably th and u forming the basis of their utility as geochronometers this quartet of accessory minerals is playing an increasingly important role in petrochronology because they provide ways to link robust spot ages to petrogenetic p t conditions so can lend petrogenetic context to chronology based on other minerals part i of this review assembles the basic requisites prior to integrative petrochronologic analysis individual characteristics of the four ree minerals are addressed first i e their crystal chemistry and stability relations thermobarometers and trace element geochemistry used for tracing petrogenesis are discussed next and finally their chronology is summarized part ii presents case studies to highlight the specific strengths of ree minerals used to resolve the dynamics of a broad range of processes from diagenetic to magmatic conditions finally a brief section at the end outlines a few of the current challenges and promising perspectives for future work to introduce the four ree minerals in style let us recall the origins of their names the three phosphates have well respected greek grandparents and allanite has solid scottish roots yet of all four of them show idiosyncracies in etymology or type material apatite had long puzzled naturalists as it shows great chemical and physical variability and can resemble other minerals once properly identified abraham gottlieb werner named it apatite his reasoning referred to the greek root and giving the precise latin translation decipio taken literally both mean i deceive or i mislead which sounds like an apt confession from this mineral for having fooled

zirconium abundance in granulite facies minerals with implications for zircon geochronology in high grade rocks zircon growth in high grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium zr we have measured the concentration of zr in the major minerals in granulite facies rocks of differing bulk composition both garnet and hornblende contain tens of parts per million zr and no other major phase contains significant zr simple calculations show that reaction of either garnet or hornblende to form non zr bearing phases will release sufficient zr to account for at least some new zircon growth u pb ages from new zircon grown as a result of either hornblende or garnet breakdown are not expected to record the time of peak metamorphism but rather will record the time of particular metamorphic reactions allowing direct correlation of zircon ages with petrologically derived pressure temperature time paths this approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible

cosmogenic 3he and 21ne age of the big lost river flood snake river plain idaho the big lost river flood in southeastern idaho occurred 20 500 calibrated yr b p on the basis of dates derived from cosmogenic 3 he and 21 ne measurements of samples from flood deposited boulders and from scour features this date corresponds to a date of 16 900 14 c yr b p and is close in age to several other cataclysmic flood events in western north america it may mark evidence for widespread warming at the end of the pleistocene in western north america the big lost river flood was smaller than some other late pleistocene floods such as the bonnevi1le flood and the missoula floods thus some samples exposed after the flood had significant amounts of cosmogenic 3 he and 21 ne that was acquired before the flood occurred

lake evolution during the past 30 000 years in china and its implications for environmental change abstract taking china as a whole lake expansion occurred 500 b c 0 a d 650 950 a d and 1250 1650 a d over the past 3000 yr the expansions were mainly due to climatic fluctuations and correspond to frequent flood reports three regional characteristics of lake development have been recognized 1 fluctuations of near coastal lakes and those in other regions as well were out of phase during 500 b c 500 a d which possibly resulted from out of phase fluctuations of climate and sea level on 10 to 100 yr time scale 2 as the lower yellow river channel migrated southward the lakes in the south of the north china plain formed and reached their largest sizes much later than those in the north 3 for the lakes in the middle and lower yangtze river valley the higher a lake basin lies above a neighboring river and the further the lake is from the river s mouth in general the younger the lake is through land reclamation the ancient chinese have changed most of the lakes greatly however human activities were also constrained in certain ways by the natural changes of the lakes reclamation usually was banned and farmland was abandoned to repair reservoirs while lakes were expanding

a long term flood discharge record derived from slackwater flood deposits of the llobregat river ne spain abstract slackwater palaeoflood deposits were identified along two bedrock gorge study reaches of the llobregat river at pont de vilomara and monistrol de montserrat the compiled palaeoflood record consists of two principal flood series a a relatively complete record of low to high magnitude flood events from the last ca 100 years and b evidence of the largest palaeoflood events that have occurred over the last ca 2700 years the longer term extreme palaeoflood record indicates that the discharge of the 1971 flood the largest on record was exceeded on at least eight occasions with two periods of high magnitude flooding identified a the late bronze age 2500 2700 years ago and b the little ice age ad 1500 1700 at pont de vilomara palaeodischarge estimates of 3700 4300 m 3 s compare to a discharge of 2300 m 3 s for the 1971 event downstream at monistrol an estimate of 4680 m 3 s for flood deposits dated as ad 1516 1642 and believed to be those of the ad 1617 flood compared to 2500 m 3 s for the 1971 flood

reconciling plate tectonic reconstructions of alpine tethys with the geological geophysical record of spreading and subduction in the alps article a new reconstruction of alpine tethys combines plate kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution from sea floor spreading through subduction to collision in the alps unlike previous models which relate the fate of alpine tethys solely to relative motions of africa iberia and europe during opening of the atlantic our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record the motions of these microplates adria iberia alcapia alkapecia and tiszia relative to both africa and europe during late cretaceous to cenozoic time involved the subduction of remnant tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces 1 131 84 ma intra oceanic subduction of the ligurian part of alpine tethys attached to iberia coincided with eo alpine orogenesis in the alcapia microplate north of africa these events were triggered primarily by foundering of the older 170 131 ma neotethyan subduction slab along the ne margin of the composite african adriatic plate subduction was linked by a sinistral transform system to e w opening of the valais part of alpine tethys 2 84 35 ma subduction of primarily the piemont and valais parts of alpine tethys which were then attached to the european plate beneath the overriding african and later adriatic plates nw translation of adria with respect to africa was accommodated primarily by slow widening of the ionian sea 3 35 ma recent rollback subduction of the ligurian part of alpine tethys coincided with western alpine orogenesis and involved the formation of the gibraltar and calabrian arcs rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable retreating adriatic and african slabs during slow convergence of africa and europe the upper european iberian plate stretched to accommodate this slab retreat in a very mobile fashion while the continental core of the adriatic microplate acted as a rigid indenter within the alpine collisional zone the subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the alps and apennines as well as beneath parts of the pannonian basin the adriatic sea the ligurian sea and the western mediterranean the predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10 30 indicating that parts of slabs may be superposed within the mantle transition zone and or that some of this subducted lithosphere became seismically transparent

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

response of extreme floods in the southwestern united states to climatic variations in the late holocene a regional synthesis of paleoflood chronologies on rivers in arizona and southern utah reveals that the largest floods over the last 5000 years cluster into distinct time periods that are related to regional and global climatic fluctuations the flood chronologies were constructed using fine grained slackwater deposits that accumulate in protected areas along the margins of bedrock canyons and selectively preserve evidence of the largest events high magnitude floods were frequent on rivers throughout the region from 5000 to 360014c yrs bp dendrocalibrated age 3800 2200 bc and increased again after 2200 bp 400 bc with particularly prominent peaks in magnitude and frequency around 1100 900 bp ad 900 1100 and after 500 yrs bp ad 1400 in contrast the periods 3600 2200 bp 2200 400 bc and 800 600 yrs bp 1200 1400 ad are marked by sharp decreases in the occurrence of large floods on these rivers in the modern record storms that generate large floods 10 year in the region fall into three categories 1 winter north pacific frontal storms 2 late summer and fall storms that draw in moisture from recurved pacific tropical cyclones and 3 summer storms mainly convective thunderstorms winter storms and tropical cyclones are associated with the most severe floods on the rivers in this study and are the most probable causes of the paleofloods over the last 5000 years floods from both winter storms and tropical cyclones occur when deep mid latitude troughs steer storm systems into the region composite anomaly maps of daily 700 mbar heights indicate that these floods are associated with a low pressure anomaly off the california coast and a high pressure anomaly over the aleutians or gulf of alaska a strong connection exists between the negative phase of the southern oscillation index often associated with el nin o conditions and the large floods associated with winter storms and tropical cyclones the paleoflood records confirm the existence of centennial scale variations in the conditions conducive to the occurrence of extreme floods and flood generating storms in this region the episodes with an increased frequency of high magnitude floods coincide with periods of cool wet climate in the western u s whereas warm intervals such as the medieval warm period are times of dramatic decreases in the number of large floods a positive relationship between the paleofloods and long term variations in the frequency of el nin o events is evident over the last 1000 years this relationship continues over at least the last 3000 years with warm coastal sea surface temperatures indicative of el nin o like conditions

chronometry and speedometry of magmatic processes using chemical diffusion in olivine plagioclase and pyroxenes the magmatic processes that fuel volcanism crustal growth ore formation and discharge of volcanic gases and aerosols to the atmosphere occur across a range of timescales from millions of years to just a few seconds for example the production of new oceanic crust at mid ocean ridges is a near continuous process that can operate in any one ocean basin on timescales of more than 100 m y however the driving force for such processes is the spreading of the ocean plates that happens on a cm yr timescale at the other end of the spectrum explosive volcanic eruptions involve the ascent and fragmentation of magma at velocities of the order of 100 m s such that the journey from a magma chamber to an ash cloud may take place in a matter of minutes in this case the driving force is the rapid expansion of magmatic gas in response to changes in pressure at intermediate timescales magmatic processes may give rise to hydrothermal ore deposits on timescales of less than a million years for an individual deposit while growth of giant granite batholiths may require piecemeal assembly of magma batches on timescales of a few million years although each of these processes has a characteristic time averaged timescale on which it operates this is typically the end result of one or more natural processes that operate on much shorter timescales for example mid ocean ridges do not extrude magma continuously onto the ocean floor mineralising fluids do not discharge continuously through the shallow crust and granitic magmas do not dribble continuously into evolving batholithic chambers in some cases it is the long term timescales that are important for example the spreading rate of ocean basins in others it is the short term timescales that are important for example the episodic growth of lava domes at active volcanoes although

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

significant ages an introduction to petrochronology question why petrochronology why add another term to an already cluttered scientific lexicon answer because petrologists and geochronologists need a term that describes the unique distinctive way in which they apply geochronology to the study of igneous and metamorphic processes other terms just won t do such evolution of language is natural and well established for instance geochronology was originally coined during the waning stages of the great age of the earth debate as a means of distinguishing timescales relevant to earth processes from timescales relevant to humans williams 1893 eighty eight years later berger and york 1981 coined the term thermochronology which has evolved as a branch of geochronology aimed at constraining thermal histories of rocks where typically the thermally activated diffusive loss of a radiogenic daughter governs the ages we measure thermochronology may now be distinguished from plain vanilla geochronology whose limited purpose in the words of reiners et al 2005 is exclusively to determine a singular absolute stratigraphic or magmatic or metamorphic formation age with little concern for durations or rates of processes that give rise to these rocks neither of these terms describes what petrologists do with chronologic data a single date is virtually useless in understanding the protracted history of magma crystallization or metamorphic pressure temperature evolution and we are not simply interested in thermal histories but in chemical and baric evolution as well rather we petrologists and geochronologists strive to understand rock forming processes and the rates at which they occur by integrating numerous ages into the petrologic evolution of a rock it is within this context that a new discipline termed petrochronology has emerged1 in some sense petrochronology may be considered the sister of thermochronology petrochronology typically focuses on the processes leading up to the formation of igneous and metamorphic rocks the minerals and textures we observe

paleoflooding reconstruction from holocene levee deposits in the lower meuse valley the netherlands this study investigates the holocene levee deposits and paleoflooding history of the lower meuse in the netherlands based on archeological investigations and sedimentary analyses grain size end member modelling magnetic susceptibility and thermogravimetric analyses the levee on the left bank in the study area near ooijen contains a continuous sedimentary record in the nw downstream part the archeological evidence and sedimentary results including a grain size based flood energy index lfei indicate that the lower meuse experienced a quiescent flooding period and low sedimentation rates during the mid late mesolithic when the levee was low and human influence was minor deposition during the mesolithic shows a fining upward trend and a highly developed soil containing abundant artefacts during the neolithic and bronze age the flooding intensity was low but gradually increased to a moderate level probably because of deforestation and increased runoff limited bronze age findings may reflect decreased human activity because of the increased flooding starting from the iron age the meuse experienced a generally enhanced flooding regime which is reflected by erosion and coarsened sedimentation in the iron age and roman period the high peak discharges during the iron age may have re opened a low lying paleochannel near to the levee during the intensified floods this general increasing trend is interrupted by the lower flooding phase in the early middle ages dark ages from the middle to the late middle ages the floods intensified again the coarsening deposition and higher sedimentation rates since the iron age resulted from increased floods and higher sediment supply by deforestation and soil erosion the findings of this study agree with a recent paleoflood reconstruction for the lower meuse by using a floodplain archive implying that levee sediment records have potential in paleohydrological studies if the completeness of the record and chronological information is guaranteed 2019 elsevier b v

diffusion obstacles and opportunities in petrochronology many of the approaches in petrochronology are rooted in the assumption of equilibrium diffusion is an expression of disequilibrium the movement of mass in response to chemical potential gradients and isotopes in response to isotopic gradients it is extremely important that we be aware of how the effects of diffusion can place obstacles across our path towards petrochronologic enlightenment conversely the effects of diffusion also provide opportunities for understanding rates processes and conditions experienced by rocks the enormity of the field does not permit us to provide a comprehensive review of either the mathematics of diffusion or quantitative data that have been obtained relevant to the interpretation of diffusive processes in rocks and minerals many resources cover these topics including rimg volume 72 diffusion in minerals and melts zhang and cherniak 2010 and several textbooks crank 1975 glicksman 2000 particularly relevant to the discussion of petrochronology are summaries of the theory and controls on diffusion brady and cherniak 2010 zhang 2010 as well as diffusion rates in feldspar cherniak 2010a accessory minerals cherniak 2010b garnet ganguly 2010 mica pyroxene and amphibole cherniak and dimanov 2010 and melts zhang and ni 2010 zhang et al 2010 rather than duplicate that material our goal is to explore the obstacles and opportunities presented by the effects of diffusion as they inform the rates of petrologic processes to achieve this goal we emphasize key principles and illustrative examples quantitative interpretation of the effects of diffusion assumes predictability of numerous factors that may affect chemical or isotopic transport including temperature initial and boundary conditions water and oxygen fugacities activities of other components multiple mechanisms of diffusion and crystal chemistry coupling of the substitution of elements into different crystallographic sites additionally the extraction of meaningful ages durations of events and temperatures requires

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

geochemistry of sediments from the huaibei plain east china implications for provenance weathering and invasion of the yellow river into the huaihe river abstract the sediments of the huaibei plain in semi humid mid eastern china represent valuable geological records with respect to eolian fluvial interactions depositional environments and climate change in this region provenance and weathering conditions are often reconstructed using sedimentary geochemistry methods in this study an 8 m core from huainan and a set of loess samples from northern and southern china were analyzed for major trace and rare earth elements rees results were compared to determine the samples provenance the major trace rare earth elements contents and grain size distribution were found to fluctuate widely in the 2 8 m section of the huainan core and more narrowly closer to the surface 0 2 m this suggests a provenance shift at a depth of 2 m the tio 2 al 2 o 3 sio 2 al 2 o 3 th nb la nb values and ree patterns in the upper core 0 2 m are similar to those found in samples from the chinese loess plateau clp these results suggest that the clp in northern china is likely to be the primary origin of the upper part 0 2 m of the huainan core compared with clp samples the upper part of the huainan core exhibits lower k 2 o al 2 o 3 values and higher chemical alteration indices this is indicative of the material s substantial weathering during transportation and re deposition and implies that these sediments could reasonably be classified as typical recycled loess the sediments may have been transported from the clp to huainan as yellow river flood events probably during the last deglaciation 13 2 ka as a result of increased precipitation along with glacier and snow melt in the upper reaches of the yellow river catchment during this period this suggests that the yellow river may have migrated into the huaihe river catchment much earlier than the earliest historical records 361 bce suggest the implications of this would be profound with respect to chinese history

dating recurrent shear zone activity and the transition from ductile to brittle deformation white mica geochronology applied to the neoproterozoic dom feliciano belt in south brazil abstract argon geochronology of white mica has a good potential of recording recurrent dynamic recrystallization in shear zones in the transition from ductile to brittle deformation as temperature ranges for mineral crystallization and isotopic closure of different grain size fractions overlap between ca 275 425 c this study presents over 40 k ar ages of coarse grained muscovite and multiple clay sized white mica fractions extracted from mylonites and fault gouges in southernmost brazil complemented by detailed structural and xrd characterization the widespread obtained ages are best explained in the context of recurrent deformation under retrograde conditions during progressive strain localization and transition from low grade ductile deformation into brittle faulting the onset of the ibare shear zone is constrained in the tonian followed by reactivation during the ediacaran evolution of the adjacent dom feliciano belt which records a remarkable long term history of recurrent deformation for over 100 myr new data extends the geochronological constraints of the dorsal do cangucu shear zone into low temperature conditions estimating the onset of brittle activity at 525 470 ma by dating early fault gouges formed shortly after final syn kinematic white mica crystallization in the hosting mylonites dating of recurrent faulting events during the paleo mesozoic complement the regional sedimentary and thermochronological record