zircon hf o isotopes as melt indicator melt infiltration and abundant new zircon growth within melt rich layers of granulite facies lenses versus solid state recrystallization in hosting amphibolite facies gneisses central erzgebirge bohemian massif abstract in the central erzgebirge within the bohemian massif lenses of high pressure and ultrahigh pressure felsic granulites occur within meta sedimentary and meta igneous amphibolite facies felsic rocks in the felsic granulite melt rich parts and restite form alternating layers and were identified by petrology and bulk rock geochemistry mineral assemblages representing the peak p t conditions were best preserved in melanocratic restite layers in contrast in the melt rich leucocratic layers garnet and related hp minerals as kyanite are almost completely resorbed both layers display differences in accessory minerals melanosomes have frequent and large monazite and fe ti minerals but lack xenotime and apatite leucosomes have abundant apatite and xenotime while monazite is rare here we present a detailed petrographic study of zircon grains abundance size morphology inclusions in granulite facies and amphibolite facies felsic gneisses along with their oxygen and hafnium isotope compositions our data complement earlier u pb ages and trace element data ree y hf u on zircons from the same rocks tichomirowa et al 2005 our results show that the degree of melting determines the behaviour of zircon in different layers of the granulites and associated amphibolite facies rocks in restite layers of the granulite lenses small inherited and resorbed zircon grains are preserved and new zircon formation is very limited in contrast new zircons abundantly grew in the melt rich leucocratic layers in these layers the new zircons u pb age trace elements hf o isotopes best preserve the information on peak metamorphic conditions due to intense corrosion of other metamorphic minerals the new zircons often contain inherited cores compared to cores the new zircons and rims show similar or slightly lower hf isotope values slightly higher hf model ages and decreased oxygen isotope ratios the isotope compositions hf o of new zircons indicate partial hf isotope homogenization in the melt and melt infiltration from an external source new zircon was most likely formed by a peritectic reaction with melt above the wet solidus peritectic zircon conversely the amphibolite facies host gneisses lack indications of significant melt production pre metamorphic zircons experienced mainly solid state recrystallization and variable pb loss with only minor new zircon formation however subtle changes in cathodoluminescence pattern in the hf and o isotopes and in the lu hf yb hf ratios of zircons suggest that small volumes of melt were locally present in difference to granulites melt was internally produced the detection of low degree melts inferred from zircon geochemistry is extremely important for the rheology because these amphibolite facies rocks could act as large scale ductile shear zones the new zircon data support a different p t path for closely spaced amphibolite and granulite facies rocks

ultra high temperature metamorphism of mafic granulites from south altyn orogen west china a result from the rapid exhumation of deeply subducted continental crust

high temperature overprint in u hpm rocks exhumed from subduction zones a product of isothermal decompression or a consequence of slab break off slab rollback abstract this paper presents and discusses petrological observations from high to ultrahigh pressure u hp metamorphic terrains in relation to existing geophysical and numerical models for subduction and exhumation processes in orogenic belts the interpretations are mostly based on observations from gneiss terrains bearing abundant bodies mafic meta eclogite and ultramafic garnet peridotite and pyroxenite exposed in collisional orogens the inclusions and compositional zoning of minerals are considered to be first order information that is needed to constrain pt paths of hp uhp rocks and reconstruct the related geodynamic models for subduction and exhumation of crustal and mantle rocks the bohemian massif of the european variscides is used as the basis for a model example to explain these processes but u hp rocks from various other terrains are taken into consideration to discuss available pt paths in relation to proposed subduction and exhumation rates of u hp rocks based on geophysical and geochronological data primarily information used in this respect include textural relations and preserved prograde zoning in minerals from many u hp rocks which reveal that a relatively cool geothermal gradient typical of subduction zones tended to prevail during the prograde and peak pressure segments of pt paths prior to initiation of exhumation and may have continued even with cooling if exhumation rates were rapid the commonly applied interpretation of isothermal decompression during exhumation is critically appraised considering whether a simple thermal relaxation and radiogenic heating during exhumation is responsible for formation of post peak pressure retrograde mineral assemblages and textures observed in u hp rocks we go on to consider whether this can satisfactorily explain the often pervasive medium pressure high temperature metamorphic re equilibration of u hp rocks or whether an additional external source of heat is a better explanation we conclude that the commonly observed high temperature metamorphic overprint exhibited by u hp rocks occurs mostly after rocks have been exhumed from the subduction channel and have reached normal crustal positions when mantle upwelling resulting from slab breakoff delamination or slab rollback takes place at the onset of continent continent collision we also explore contrasting pt trajectories for mantle rocks that have been entrained into crustal material during their subduction or exhumation pt paths of mantle and subducted crustal rocks tend to converge as mantle rocks impinge upon the cooler subduction zone and once entrained share a common evolution that depends on the exhumation mechanism and rate considering all of the data presented in this work we conclude that the diverse polyphase metamorphic evolution exhibited by u hp terrains embodied in the pt paths of hp and uhp rocks has important consequences for reconstructing their changing thermal regimes and provides important constraints for geodynamic models involving subduction and the transition to collision

eclogite facies rocks of the monotonous unit clue to variscan suture in the moldanubian zone bohemian massif abstract the formation and exhumation of variscan high to ultrahigh pressure metamorphic rocks from the moldanubian zone in the bohemian massif are the subject of controversy regarding their unclear relationship to possible subduction zones in this paper we present the results of a petrological study of eclogites that occur together with serpentinites within amphibolite facies gneisses in the moldanubian zone east of the tepla barrandian block more than 100 bodies of retrogressed eclogite and serpentinite follow an approximately 250 km long sw ne trending zone in the central part of the bohemian massif together with surrounding gneisses the eclogites share a medium to low pressure amphibolite facies metamorphism p t conditions estimated for the eclogite facies stage indicate a relatively low temperature geothermal gradient similar to those observed in the saxothuringian zone which occurs north and northwest of the tepla barrandian block the presence and distribution of eclogites and comparison of their p t and age data with hp uhpm rocks in other units in the bohemian massif allow us to constrain the variscan suture which straddles the se of the tepla barrandian block the existence of this suture in relation to available geotectonic models and its possible continuation through other allochthonous units along the european variscan belt are discussed

individual flood events detected in the recent sediments of the petit lac d annecy eastern france a 90 cm long freeze core from the petit lac d annecy was sampled at 1 cm intervals and studied using magnetic measurements chemical element and pollen analysis the results reveal four distinctive horizons in each of which the different lines of evidence point to deposition during a single major flood event the magnetic evidence shows that the sediment deposited in each event included a high proportion of surface soil derived material and this inference is consistent with the geochemical and pollen records from these layers no firm dates can be ascribed to these inwash events at present but a tentative chronology derived for the core from 210pb measurements varve counting where possible and correlation with a nearby previously dated core suggest that these flood events date from the late seventeenth to early nineteenth centuries a tentative correlation is suggested between the single event deposits and the record of summer autumn floods during the period

metamorphic evolution during deep subduction and exhumation of continental crust insights from felsic granulites in south altyn tagh west china

topography of the variscan orogen in europe failed not collapsed the variscan orogenic collage consists of three subduction collision systems rheno hercynian saxo thuringian and massif central moldanubian devonian to early carboniferous marine strata are widespread not only in the individual foreland fold and thrust belts but also in post tectonic basins within these foreland belts and on the cadomian crust of peri gondwanan microcontinental fragments which represent the upper plates of the subduction collision zones these marine basins preclude high elevations in the respective areas and also in their neighbourhood widespread late carboniferous intra montane basins with their coal bearing sequences are likewise incompatible with high and dry plateaus while narrow belts with high elevations remain possible along active margins within the orogen comparison of the variscides with the himalaya tibetan plateau is unfounded plausible reasons for the scarcity of high variscan relief include subduction of oceanic and even continental crust subduction erosion orogen parallel extension and most important lithospheric thinning accompanied by high heat flow and magmatism in many areas timing and areal array of magmatism and ht metamorphism are not compatible with a model of tectonic thickening and subsequent gravitational collapse it is suggested instead that lithospheric thinning and heating are due to mantle activities caused by the tethys rift the lower and middle crust were thermally softened and rendered unfit for stacking and isostatic uplift in terms of topography the variscides represent a failed orogen the ht regime also explains the abundance of granitoids and ht lp metamorphic rocks typical of the variscides melting in the ht regime extracted mafic components from variscan and cadomian crust as well as from cadomian metasomatized lithospheric mantle thus mimicking subduction related magmatism the onset of the ht regime at c 340 ma may also have triggered the final ascent of hp uhp felsic metamorphic rocks

ultrahigh pressure minerals and metamorphic terranes the view from china abstract ultrahigh pressure uhp metamorphism refers to mineralogical modifications of continental and oceanic crustal protoliths associated mafic ultramafic rocks initially formed or emplaced in shallow levels of the lithosphere but which subsequently have experienced p t conditions within or above the coesite stability field 2 7 gpa 700 c typical products include eclogite garnet peridotite and uhp varieties of metapelite quartzite marble paragneiss and orthogneiss uhp metamorphic assemblages require relatively cold lithospheric subduction to mantle depths some recrystallization even occurs under forbidden p t conditions characterized by a geotherm of the current explosion of research on continental uhp terranes reflects their significance for mantle dynamics and the tectonics of continental subduction collision exhumation mantle slab interactions and geochemical recycling a further characterization of uhp phases and positive identification of uhp minerals requires new experimental studies coupled with state of the art analyses for example the very rare occurrence of microdiamond inclusions in zircons from dabie sulu uhp rocks may reflect high f o 2 attending recrystallization inasmuch as epidote is rather common rutile needles within garnets from sulu uhp eclogitic rocks may not be the result of exsolution so in such cases the apparent uhp pressure may have been over estimated hadean igneous microdiamond inclusions in jack hills detrital zircons could have originated from mantle xenoliths whereas abundant detrital phanerozoic diamonds containing inclusions of coesite and other eclogitic minerals from new south wales might have been derived from unexposed uhp metamorphic terranes micro mineral intergrowth and nano size minerals may hold important key to deciphering the actual p t paths of subduction and mantle return flow although most exhumed terranes have returned surfaceward relatively rapidly after short time of uhp condition the long duration of storage at great depth and slow exhumation for the largest uhp terranes remain as major problems

lateglacial calendar year chronology based on annually laminated sediments from lake meerfelder maar germany abstract annually laminated sediments from lake meerfelder maar mfm in the eifel germany provide an independent calendar year timescale for the last glacial interglacial transition changes in varve thickness and varve facies reflect significant environment variations during this period the general micro facies pattern confirms that diatomaceous organic varves are favoured during warm climatic phases while cold periods generate higher allochthonous sediment fluxes an exception is the significant increase in diatom blooms at the beginning of the younger dryas six lateglacial biozones have been defined by litt and stebich quaternary international 1999 the sediment record comprises two tephra layers as chronomarkers the laacher see tephra 12 880 varve years bp is a continent wide correlation horizon while the ulmener maar tephra provides regional chronological links the beginning of the lateglacial warming is dated at 14 450 varve years vyrs bp which is regarded as the minimum age three abrupt cold spells interrupted the lateglacial interstadial at 13 800 vyrs bp oldest dryas 13 540 vyrs bp older dryas and 12 680 vyrs bp younger dryas the oldest dryas 130 years and the older dryas 190 years were rather short while the younger dryas lasted much longer 1090 years the mfm chronology was compared with ice core timescales from gisp2 and grip there is a good agreement between these timescales with the grip model age ss09 providing the best match to the mfm chronology

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

different origins of garnet in high pressure to ultrahigh pressure metamorphic rocks abstract garnet in high pressure hp to ultrahigh pressure uhp metamorphic rocks in subduction zone commonly shows considerable zonation in major and trace elements as well as mineral inclusions which bears information on its growth mechanism via metamorphic or peritectic reactions in coexistence with relic minerals and metamorphic fluids or anatectic melts at subduction zone conditions it provides an important target to retrieve physicochemical changes in subduction zone processes including those not only in pressure and temperature but also in the durations of metamorphism and anatexis garnet from different compositions of hp to uhp metamorphic rocks may show different types of major and trace element zonation as well as mineral inclusions discrimination between the different origins of garnet provides important constraints on pressure and temperature and the evolution history for the hp to uhp metamorphic rocks magmatic garnet may occur as relics in granitic gneisses despite metamorphic modification at subduction zone conditions with spessartine increasing or flat major element profiles from inner to outer core and exceptionally higher contents of trace elements than metamorphic mantle and rim metamorphic garnet can grow at different metamorphic stages during prograde subduction and retrograde exhumation with spessartine decreasing from core to rim if the intracrystalline diffusion is not too fast the compositional profiles of metamorphic garnet in the abundances of grossular almandine and pyrope are variable depending on the composition of host rocks and co existing minerals peritectic garnet grows through peritectic reactions during partial melting of hp to uhp rocks with the composition of major elements to be controlled by anatectic p t conditions and the compositions of parental rocks and anatectic melts trace element profiles in garnet with different origins are also variable depending on the coexisting mineral assemblages the garnet forming reactions and the property of metamorphic fluids or anatectic melts mineral inclusions not only present key clues to identify the different origins of garnet but also serve as sound candidates for the temporal constraint on garnet growth

holocene climate variability as reflected by mid european lake level fluctuations and its probable impact on prehistoric human settlements abstract a data set of 180 radiocarbon tree ring and archaeological dates obtained from sediment sequences of 26 lakes in the jura mountains the northern french pre alps and the swiss plateau was used to construct a holocene mid european lake level record the dates do not indicate a random distribution over the holocene but form clusters suggesting an alternation of lower and higher climatically driven lake level phases they provide evidence of a rather unstable holocene climate punctuated by 15 phases of higher lake level 11 250 11 050 10 300 10 000 9550 9150 8300 8050 7550 7250 6350 5900 5650 5200 4850 4800 4150 3950 3500 3100 2750 2350 1800 1700 1300 1100 750 650 cal bp and after 1394 ad a comparison of this mid european lake level record with the gisp2 polar circulation index pci record the north atlantic ice rafting debris ird events and the 14 c record suggests teleconnections in a complex cryosphere ocean atmosphere system correlations between the gisp2 pci the mid european lake level the north atlantic ird and the residual 14 c records suggest that changes in the solar activity played a major role in holocene climate oscillations over the north atlantic area

evolution of host inclusion systems a visco elastic model a visco elastic model is suggested to recover the p t t paths of spherically symmetrical host inclusion systems from residual pressure patterns for a coesite garnet aragonite garnet and diamond garnet system with partial transition of high pressure polymorphs to low pressure ones the model predicts that the exhumation history of ultra high pressure metamorphic rocks from depths below 90 km began with pressure decrease to 1 gpa 40 km depth while the temperature remained high 800 c then the system cooled and decompressed to ambient conditions non spherical inclusion shapes in the 3 d model lead to heterogeneities in residual pressure and shear stress distributions as well as to significant gradients of shear stress which may affect the kinetics and thermodynamics of phase transformation in the inclusions

precipitation of rutile and ilmenite needles in garnet implications for extreme metamorphic conditions in the acadian orogen u s a we report the discovery of oriented needles of rutile and ilmenite in garnet crystals from granulite facies metapelitic rocks of the merrimack synclinorium connecticut and present a precipitation model for their origin the rocks were strongly metamorphosed and deformed during the devonian acadian orogeny the needles are primarily elongated parallel to in garnet rutile has anomalous extinction angles as great as 35 cf griffin et al 1971 rutile and ilmenite needles are typically a few hundred nanometers to several micrometers in diameter and are several tens of micrometers to nearly a millimeter long other oxide inclusions that may be present include submicrometer to micrometer scale twinned rutile bicrystals as well as srilankite and a crichtonite group mineral some garnet cores have unusual box shaped quartz inclusions which coexist with ti fe oxide needles and commonly contain micrometer scale rods of f oh cl apatite negative garnet crystal pores are also widespread ti fe oxide needles are restricted to garnet core regions rims have a distinctly different inclusion population dominated by granulite facies minerals including sillimanite spinel cordierite and k feldspar consequently the garnet core regions represent an earlier distinct period of growth relative to the rims garnet cores contain 25 35 pyrope and a host of minor and trace constituents including tio2 0 07 0 6 wt cr2o3 0 01 0 10 wt na2o 0 01 0 03 wt p2o5 0 01 0 09 wt and zro2 up to 150 ppm na2o and zro2 correlate positively with tio2 titanium zoning is preserved in some garnets zoning profiles and two dimensional chemical mapping show that ti and to a lesser degree cr are depleted around ti fe oxide inclusions therefore we conclude that the needles are precipitates that formed from ti bearing garnet during exhumation and cooling garnet contained sufficient ti to form precipitates no ti source external to garnet was necessary titanium bearing garnets that contain oriented ti fe oxide needles are known primarily from ultrahigh pressure metamorphic rocks mantle peridotites and pyroxenites and high pressure granulites thus the presence of needle bearing garnets in connecticut strongly suggests that a previously unrecognized domain of extreme pressure and or temperature metamorphism exists in the acadian orogen

contrasting tectono metamorphic evolution of orogenic lower crust in the bohemian massif a numerical model abstract the bohemian massif located at the eastern margin of the european variscan belt is characterised by an exceptional accumulation of felsic high pressure granulites the petrological structural and geochronological studies of this region revealed systematic differences between the tectonometamorphic evolution of the southern moldanubian and northern west sudetes parts of the orogen two contrasting tectonic scenarios have been proposed gravity driven vertical mass exchanges followed by continental indentation in the moldanubian domain and crustal scale folding leading to gneiss dome formation in the west sudetes we present a numerical model in order to correlate the apparent differences between these two regions with the variations in the dynamics of the modelled system we model two colliding blocks an orogenic root where a felsic lower crust is overlain by a mafic layer and a middle crust and a continental indentor we examine the role of the rate of convergence of the two blocks radiogenic heat production within the felsic lower crust and efficiency of erosion the prograde part of the metamorphic evolution is controlled by the rate of convergence and the peak temperature depends on the heat production the retrograde evolution is controlled mostly by erosional processes in the models where the material is weakened due to the heating in the felsic lower crust the gravitational instability of the mafic and felsic layers causes their complete vertical exchange followed by a flow above the indentor in colder and or faster models the thickening is dominated by the buckling of the mafic layer these two styles of deformation i e gravity dominated and fold dominated models correspond to the structures observed in the moldanubian and the west sudetes moreover the calculated pressure temperature paths of the felsic lower crust are in agreement with available data

mid to late holocene flood frequency changes in the northeastern alps as recorded in varved sediments of lake mondsee upper austria abstract annually laminated varved lake sediments with intercalated detrital layers resulting from sedimentary input by runoff events are ideal archives to establish precisely dated records of past extreme runoff events in this study the mid to late holocene varved sediments of lake mondsee upper austria were analysed by combining sedimentological geophysical and geochemical methods this approach allows to distinguish two types of detrital layers related to different types of extreme runoff events floods and debris flows and to detect changes in flood activity during the last 7100 years in total 271 flood and 47 debris flow layers deposited during spring and summer were identified which cluster in 18 main flood episodes fe 1 18 with durations of 30 50 years each these main flood periods occurred during the neolithic 7100 7050 vyr bp and 6470 4450 vyr bp the late bronze age and the early iron age 3300 3250 and 2800 2750 vyr bp the late iron age 2050 2000 vyr bp throughout the dark ages cold period 1500 1200 vyr bp and at the end of the medieval warm period and the little ice age 810 430 vyr bp summer flood episodes in lake mondsee are generally more abundant during the last 1500 years often coinciding with major advances of alpine glaciers prior to 1500 vyr bp spring summer floods and debris flows are generally less frequent indicating a lower number of intense rainfall events that triggered erosion in comparison with the increase of late holocene flood activity in western and northwestern nw europe commencing already as early as 2800 yr bp the hydro meteorological shift in the lake mondsee region occurred much later these time lags in the onset of increased hydrological activity might be either due to regional differences in atmospheric circulation pattern or to the sensitivity of the individual flood archives the lake mondsee sediments represent the first precisely dated and several millennia long summer flood record for the northeastern ne alps a key region at the climatic boundary of atlantic mediterranean and east european air masses aiding a better understanding of regional and seasonal peculiarities of flood occurrence under changing climate conditions

mineralogy petrology u pb geochronology and geologic evolution of the dabie sulu classic ultrahigh pressure metamorphic terrane east central china the dabie sulu triassic collisional orogen in eastern asia was created by northward subduction of the yangtze continental crust capped plate beneath the sino korean craton eclogites garnet peridotites and surrounding country rock gneisses and marbles were all subjected to in situ uhp metamorphism as indicated by the presence of rare but widespread coesite inclusions in eclogitic minerals and in zircon crystals in the country rocks as well as by virtually identical metamorphic ages of various uhp rock types metamorphic p t estimates combined with investigations of mineral exsolution textures and high p polymorphs indicate that recovered depths of continental subduction may have exceeded 200 km parageneses of mineral inclusions in zoned zircon domains combined with u pb ages delineate a well constrained p t time path suggesting exhumation rates of 5 10 km myr a similar p t time trajectory has been established for the microdiamond bearing kokchetav massif thus far however diamond inclusions have not been confirmed from coesite bearing zircon domains of dabie sulu uhp rocks despite numerous detailed studies oxygen isotopes of minerals from many outcrop samples and the chinese continental scientific drilling ccsd project main hole cores indicate that delta 18 o depletion took place in a volume of proterozoic protoliths exceeding 100 000 km 3 along the northern edge of the yangtze craton evidently passive margin sediments and bimodal igneous rocks that had formed during rifting and breakup of the supercontinent rodinia were subjected to extensive meteoric water rock interactions attending terminal neoproterozoic snowball earth conditions such hydrothermal alteration volatilized and depleted c from the relatively oxidized protoliths accounting for the rare occurrences of graphite and apparent lack of microdiamond in dabie sulu uhp rocks

first finding of microdiamond coesite and other uhp phases in felsic granulites in the moldanubian zone implications for deep subduction and a revised geodynamic model for variscan orogeny in the bohemian massif abstract heavy mineral fractions and polished thin sections from felsic granulites from the moldanubian zone of the bohemian massif were thoroughly studied by means of raman microspectroscopy combined with optical microscopy and scanning electron microscopy the following phases were identified among others as inclusions in robust minerals such as garnet and zircon diamond characterized by an intense narrow peak at 1332 cm 1 was found in two inclusions in zircon they have a size of 5 m coesite identified by its very characteristic peak at 520 cm 1 was found in an inclusion in garnet together with quartz coesite has been almost completely transformed into quartz only minor coesite nano domains remain kumdykolite the orthorhombic polymorph of naalsi 3 o 8 characterized by strong peaks at 220 456 and 492 cm 1 occurs either as single crystals or as a part of multiphase inclusions in garnet and in zircon along with other mineral phases such as k feldspar phengite rutile moissanite sic exhibiting the characteristic raman bands at 767 788 and 969 cm 1 occurs as inclusions in garnet diamond and coesite are considered to have formed at the peak ultrahigh pressure metamorphic uhpm conditions kumdykolite has been proposed to be a metastable phase formed during rapid cooling from high temperature moissanite points to extremely reduced conditions during subduction to great depths the finding of uhp phases in felsic granulites in the moldanubian zone is clear evidence for subduction of crustal materials to mantle depths the garnet hosting the uhp phase inclusions usually preserves prograde compositional zoning this in combination with the uhpm mineral inclusions suggests that the felsic material should have passed uhp metamorphism at a low temperature gradient isothermal decompression the commonly accepted model at temperatures of 850 950 c would have substantially modified and homogenized the garnet composition eliminating any compositional zoning therefore the uhpm felsic rocks could have experienced only a short term granulite facies metamorphism after they were exhumed to crustal levels

regional metamorphism at extreme conditions implications for orogeny at convergent plate margins abstract regional metamorphism at extreme conditions refers either to alpine type metamorphism at low geothermal gradients of 30 c km extreme pressures refer to those above the polymorphic transition of quartz to coesite so that ultrahigh pressure uhp eclogite facies metamorphism occurs at mantle depths of 80 km extreme temperatures refer to those higher than 900 c at crustal depths of 80 km so that ultrahigh temperature uht granulite facies metamorphism occurs at medium to high pressures while crustal subduction at the low geothermal gradients results in blueschist eclogite facies series without arc volcanism heating of the thinned orogenic lithosphere brings about the high geothermal gradients for amphibolite granulite facies series with abundant magmatism therefore uhp metamorphic rocks result from cold lithospheric subduction to the mantle depths whereas uht metamorphic rocks are produced by hot underplating of the asthenospheric mantle at the crustal depths active continental rifting is developed on the thinned lithosphere in response to asthenospheric upwelling and this tectonism is suggested as a feasible mechanism for regional granulite facies metamorphism with the maximum temperature depending on the extent to which the mantle lithosphere is thinned prior to the rifting while lithospheric compression is associated with subduction metamorphism in accretionary and collisional orogens the thinned orogenic lithosphere undergoes extension due to the asthenospheric upwelling to result in orogen parallel rifting metamorphism and magmatism thus the rifting metamorphism provides a complement to the subduction metamorphism and its operation marks the asthenospheric heating of the orogenic lithosphere because of the partial melting and melt extraction of the lower continental crust contemporaneous granite migmatite granulite associations may serve as a petrological indicator of rifting orogeny that is superimposed on precedingly accretionary and collisional orogens the uht metamorphic rocks have occurred since the archean suggesting that the hot underplating has operated very early in the earth s history in contrast the uhp metamorphic rocks primarily occur in the phanerozoic indicating that the thermal regime of many subduction zones has changed since the neoproterozoic for the cold subduction

the kokchetav massif kazakhstan type locality of diamond bearing uhp metamorphic rocks abstract after the discovery of metamorphic coesite in crustal rocks from the western alps italy and the western gneiss region norway in the mid 1980s of the last century metamorphic diamond was observed only a few years later in situ in the kokchetav massif kazakhstan findings of such coesite and diamond bearing ultrahigh pressure metamorphic uhp rocks with protoliths formed or embedded in crustal levels and subsequently experienced pt conditions within or even higher than the coesite stability field have dramatically changed our geodynamic view of orogenetic processes these occurrences provide evidence that crustal rocks were subducted into mantle depths and exhumed to the surface recent studies even suggest continental subduction to depths exceeding 300 km these rocks have been extensively studied and many new and important observations have been made thus far more than 350 papers have been published on various aspects of kokchetav uhp rocks the kokchetav massif of northern kazakhstan is part of one of the largest suture zones in central asia and contains slices of hp and uhp metamorphic rocks classical uhp rocks mainly occur in the kumdy kol barchi kol and kulet areas and include a large variety of lithologies such as calcsilicate rocks eclogite gneisses schists marbles of various compositions garnet pyroxene quartz rocks and garnet peridotite most of them contain microdiamonds some of which reach a grain size of 200 m most diamond grains show cuboid shapes but in rare cases diamonds within clinozoisite gneiss from barchi kol occur as octahhedral form microdiamonds contain highly potassic fluid inclusions as well as solid inclusions like carbonates silicates and metal sulfides which favour the idea of diamond formation from a c o h bearing fluid nitrogen isotope data and negative 13c values of kokchetav diamonds indicate a metasedimentary origin pt estimates of kokchetav uhp rocks yield peak metamorphic conditions of at least 43 kbar at temperatures of about 950 1000 c some zircon separates show inherited proterozoic cores and 537 530 ma uhp metamorphic mantle zones several ar ar ages on micas scatter around 529 528 and 521 517 ma and reflect different stages of the exhumation history migmatization occurred during exhumation at about 526 520 ma isotopic studies on calcsilicate rocks confirm a metasedimentary origin 18o values of garnet and clinopyroxene of a layered calcsilicate rock rule out the possibility having a primitive mantle protolith similar studies on eclogites indicate their basaltic protolith having experienced water rock interaction prior to uhp metamorphism a number of unique mineralogical findings have been made on kokchetav uhp rocks k feldspar exsolutions in clinopyroxene demonstrate that potassium can be incorporated into the cpx structure under upper mantle pressures other significant observations are coesite exsolutions in titanite quartz rods in cpx the discovery of k tourmaline as well as new minerals like kokchetavite a hexagonal polymorph of k feldspar and kumdykolite an orthorhombic polymorph of albite the kokchetav uhp rocks represent a unique and challenging stomping ground for geoscientists of various disciplines from crystallography petrology and geochemistry to geophysics and geodynamics geotectonics it concerns all who are interested in the diverse metamorphic processes under upper mantle conditions

hydrogeomorphic linkages of sediment transport in headwater streams maybeso experimental forest southeast alaska hydrogemorphic linkages related to sediment transport in headwater streams following basin wide clear cut logging on prince of wales island southeast alaska were investigated landslides and debris flows transported sediment and woody debris in headwater tributaries in 1961 1979 and 1993 widespread landsliding in 1961 and 1993 was triggered by rainstorms with recurrence intervals 24 h precipitation of 7 0 years and 4 2 years respectively occurrence distribution and downstream effects of these mass movements were controlled by landform characteristics such as channel gradient and valley configuration landslides and channelized debris flows created exposed bedrock reaches log jams fans and abandoned channels the terminus of the deposits did not enter main channels because debris flows spread and thinned on the unconfined bottom of the u shaped glaciated valley chronic sediment input to channels included surface erosion of exposed till rain splash sheet erosion and freeze thaw action and bank failures bedload sediment transport in a channel impacted by 1993 landslides and debris flows was two to ten times greater and relatively finer compared with bedload transport in a young alder riparian channel that had last experienced a landslide and debris flow in 1961 sediment transport and storage were influenced by regeneration of riparian vegetation storage behind recruited woody debris development of a streambed armour layer and the decoupling of hillslopes and channels both spatial and temporal variations of sediment movement and riparian condition are important factors in understanding material transport within headwaters and through channel networks copyright 2004 john wiley sons ltd

river response to rapid holocene environmental change evidence and explanation in british catchments macklin m g jones a f lewin j 2010 river response to rapid holocene environmental change evidence and explanation in british catchments quaternary science reviews 29 13 14 1555 1576 sponsorship welsh assembly government and the higher education funding council for wales for funding this study through its support of the centre for catchment and coastal research at aberystwyth university

raman spectroscopic and microscopic criteria for the distinction of microdiamonds in ultrahigh pressure metamorphic rocks from diamonds in sample preparation materials natural diamond from three ultrahigh pressure metamorphic uhpm terranes erzgebirge massif germany kokchetav massif northern kazakhstan rhodope metamorphic province greece and synthetic diamond from cutting and polishing materials paste spray saw blade were studied by means of optical microscopy and raman microspectroscopy to constitute a new petrographic and spectroscopic data set that might be a useful tool for identifying and characterizing metamorphic diamond several criteria are established for distinguishing natural microdiamond identified in a rock thin section from the externally introduced ones i e diamond as residual particles contaminants from the cutting and polishing material such as the diamond size the presence of inclusions coatings or coexistent phases and two diamond raman band parameters i e the raman shift and the full width at half maximum height fwhm

computational study of deformation mechanisms and grain size evolution in granulites implications for the rheology of the lower crust abstract most of granulite terrains worldwide are characterized by large mean grain sizes of 1 mm or more an important exception are the high pressure felsic granulites in the bohemian massif the european variscan belt there recrystallization of original coarse grained ternary feldspar led to formation of a fine grained 100 m mixed matrix dominated by plagioclase and k feldspar this change occurred at temperatures of 850 c and was probably caused by chemically induced decomposition related to slight cooling and enhanced by deformation during continental collision the resulting microstructure shows indications of diffusion creep assisted by melt enhanced grain boundary sliding further on minor coarsening occurred associated with deformation by dislocation creep and aggregation of mineral phases using a thermodynamics based model of grain size evolution we show that stability of the fine grained microstructure crucially depends on zener pinning in the two phase mineral matrix pinning efficiently hinders grain growth and the small grain size that resulted from the ternary feldspar decomposition can be stable even at high temperatures the late switch from the grain size sensitive creep to dislocation creep is rather difficult to explain by temperature and strain rate or stress changes only however a simple incorporation of melt solidification can successfully simulate this behavior alternatively the switch and the associated grain size growth can be related to mineral phase aggregation at lower pressure temperature conditions resulting into a decrease of pinning efficiency this study suggests that the fine grain size of the bohemian granulites in contrast to the common coarse grained type stems from abrupt recrystallization during the high pressure high temperature conditions and pinning in the fine grained matrix such a process may in some cases significantly and suddenly reduce the strength of the lower continental crust and allow for its efficient redistribution

pre mesozoic alpine basements their place in the european paleozoic framework prior to their alpine overprinting most of the pre mesozoic basement areas in alpine orogenic structures shared a complex evolution starting with neoproterozoic sediments that are thought to have received detrital input from both west and east gondwanan cratonic sources a subsequent neoproterozoic cambrian active margin setting at the gondwana margin was followed by a cambrian ordovician rifting period including an ordovician cordillera like active margin setting during the late ordovician and silurian periods the future alpine domains recorded crustal extension along the gondwana margin announcing the future opening of the paleotethys oceanic domain most areas then underwent variscan orogenic events including continental subduction and collisions with avalonian type basement areas along laurussia and the juxtaposition and the duplication of terrane assemblages during strike slip accompanied by contemporaneous crustal shortening and the subduction of paleotethys under laurussia thereafter the final pangea assemblage underwent triassic and jurassic extension followed by tertiary shortening and leading to the buildup of the alpine mountain chain recent plate tectonic reconstructions place the alpine domains in their supposed initial cambrian ordovician positions in the eastern part of the gondwana margin where a stronger interference with the chinese blocks is proposed at least from the ordovician onward for the visean time of the variscan continental collision the distinction of the former tectonic lower plate situation is traceable but becomes blurred through the subsequent oblique subduction of paleotethys under laurussia accompanied by large scale strike slip since the pennsylvanian this global collisional scenario has been replaced by subsequent and ongoing shortening and strike slip under rising geothermal conditions and all of this occurred before all these puzzle elements underwent the complex alpine reorganization

orbital and solar forcing of shifts in mid to late holocene flood intensity from varved sediments of pre alpine lake ammersee southern germany abstract microfacies analyses and x ray fluorescence scanning xrf at sub mm resolution were conducted on the varved mid to late holocene interval of two sediment profiles from pre alpine lake ammersee southern germany the coring sites are located in a proximal as10prox and distal as10dist position towards the main tributary river ammer in 1 8 km distance from each other to shed light on sediment distribution within the lake particular emphasis was 1 the detection of intercalated detrital layers and their micro sedimentological features and 2 intra basin correlation of these event deposits detrital layers were dated by microscopic varve counting verified by accelerator mass spectrometry 14c dating of terrestrial plant macrofossils since 5500 varve years vyr bp in total 1573 detrital layers were detected in either one or both of the investigated sediment profiles based on their microfacies geochemistry and proximal distal deposition pattern detrital layers were interpreted as river ammer flood deposits earlier studies on flood layer seasonality have proven that flood layer deposition occurs predominantly during spring and summer the flood season at lake ammersee most prominent features of the record are the onset of regular flood layer deposition at 5500 vyr bp in as10prox and 2800 vyr bp in as10dist as well as three major increases in mean flood layer thickness at 5500 2800 and 400 vyr bp integrating information from both sediment profiles allowed to interpret these changes in terms of shifts towards higher mean flood intensity proposed triggering mechanisms are gradual reduction in northern hemisphere orbital summer forcing and superimposed centennial scale solar activity minima likely responses to this forcing are enhanced equator to pole temperature gradients and changes in synoptic scale atmospheric circulation the consequences for the ammersee region are more intense cyclones leading to extremer rainfall and flood events in spring and summer

coincidence of gabbro and granulite formation and their implication for variscan ht metamorphism in the moldanubian zone bohemian massif example from the kutna hora complex abstract leucocratic metagabbro and amphibolite from a mafic ultramafic body within migmatite and granulite in the kutna hora complex were investigated the mafic ultramafic rocks show amphibolite facies metamorphism but in the central part of the body some metagabbro preserves cumulus and intercumulus plagioclase clinopyroxene and spinel spinel forms inclusions in both clinopyroxene and plagioclase and shows various degree of embayment structure that was probably a result of reaction with melt during magmatic crystallization in the metagabbro garnet forms coronae around clinopyroxene at the contacts with plagioclase amphibolite contains garnet with prograde zoning and plagioclase phase relations of igneous and metamorphic minerals indicate that magmatic crystallization and subsequent metamorphism occurred as a result of isobaric cooling at a depth of 30 35 km u pb dating on zircon from leucogabbro yielded a variscan age 337 7 2 ma that is similar or close to the age of granulite facies metamorphism ca 340 ma in the moldanubian zone based on the calculated pt conditions and age data both the mafic ultramafic body and surrounding granulite shared the same exhumation path from their middle lower crustal position at the end of variscan orogeny the coincidence of mafic ultramafic intrusives and granulite amphibolite facies metamorphism is explained by lithospheric upwelling beneath the moldanubian zone that occurred due to slab break off during the final stages of subduction of the moldanubian plate beneath the tepla barrandian block the model also addresses questions about the preservation of minerals and or their compositions from the early metamorphic history of the rocks subjected to ultradeep subduction and subsequent granulite facies metamorphism

boundaries of mantle lithosphere domains in the bohemian massif as extinct exhumation channels for high pressure rocks abstract five domains microplates have been recognized by seismic anisotropy in the mantle lithosphere of the bohemian massif the mantle domains correspond to major crustal units and each of the domains bears a consistent fossil olivine fabric formed before their variscan assembly the present day mantle fabric indicates that this process consisted of at least three oceanic subductions each followed by an underthrusting of the continental lithosphere the seismic anisotropy does not detect remnants of the oceanic subductions but it can trace boundaries of the preserved continental domains subsequently underthrust along the paths of previous oceanic subductions the most robust continent continent collision was followed by westward underthrusting of the brunovistulian mantle lithosphere still detectable by seismic anisotropy more than 100 km beneath the moldanubian mantle lithosphere major occurrences of the high pressure ultra high pressure hp uhp rocks follow the ene and nne oriented sutures and boundaries of the mantle lithosphere domains mapped from three dimensional modeling of body wave anisotropy the hp uhp rocks are products of oceanic subductions and the following underthrusting of the continental crust and mantle lithosphere exhumed along the mantle boundaries the close relation of the mantle sutures and occurrences of the hp uhp rocks near the paleosubductions testifies for models interpreting the granulite garnet peridotite association by oceanic continental subduction underthrusting followed by the exhumation of deep seated rocks our findings support the bivergent subduction model of tectonic development of the central part of the bohemian massif the inferences from seismic anisotropy image the bohemian massif as a mosaic of microplates with a rigid mantle lithosphere preserving a fossil olivine fabric the collisional mantle boundaries blurred by tectonometamorphic processes in easily deformed overlying crust served as major exhumation channels of the hp uhp rocks

delamination in collisional orogens thermomechanical modeling 1 modes of mantle delamination in collision zones are discussed in the light of 2d numerical modeling freely evolving thermomechanical models take into account phase changes in the mantle and melting in the crust distinct modes are identified in which orogens undergo mantle delamination concurrently or after collision delamination propagates along the moho of the subducted plate together with the retreating trench provided that slab pull is sufficient and that the meta stability of the crust lithospheric mantle interface is overcome topography is an instantaneous response to delamination and migrates with the focused and localized separation between crust and lithospheric mantle delamination front early exhumation of high pressure rocks is followed by exhumation of high temperature partially molten rocks convective stabilization of delamination outlasts slab break offs and impedes renewed build up of mechanically strong lithospheric mantle by static cooling parameter studies investigate the sensitivity of delamination to the age of the subducted oceanic plate the collision rate the mantle rheology and explore the effects of melting and sedimentation mantle flow patterns that form around delamination and subduction are sensitive to and often limited by the semi permeable 670 km spinel perovskite transition demonstrating that phase changes are essential factors delamination may account for the formation of complex boundary zones between continents such as between africa and europe in the mediterranean where transient compression and extension the lack of mantle lithosphere and the exhumation of high grade metamorphic core complexes are features also obtained in our modeling results

from sedimentary records to sediment budgets multiple approaches to catchment sediment flux this paper reviews the traditional approach to sediment budget studies in geomorphology new approaches and more specifically the potential impact of new methodological advances each component of the budget is discussed including the spatial and volumetric estimations of erosion deposition and storage and the precision and accuracy of flux rate estimation examples are used from recent work in germany the netherlands and the uk and include the pedogenic approach to erosion estimation remote sensing lidar geophysics and sediment based dating techniques for flux rate estimation the precision and accuracy of catchment sediment flux estimates based upon sediment storage is not only dependant upon volumetric accuracy but also on the precision and accuracy of sediment dating in this area there has been a revolution with direct sediment dating techniques tl osl esr freeing budget studies from the constraints and biases of radiocarbon of particular importance is the use of cosmogenic nuclides for dating but which can also be used to derive long term erosion rates but only using a steady state assumption finally a tentative initial application of the sediment budget approach to pleistocene terrace staircase in unglaciated basins is discussed it is argued that only now do we have the techniques available to be able to produce accurate sediment budget estimations at spatial scales greater than that of zero order basins and over time periods greater than that covered by direct observations

new evidence of blueschist facies rocks and their geotectonic implication for variscan suture s in the bohemian massif blueschist facies rocks exposed within consolidated continental blocks provide some of the best evidence for the existence of previous suture s they usually occur as lenses or layers embedded within greenschist or amphibolite facies rocks and indicate reequilibration at medium to low pressure conditions in the bohemian massif a few occurrences of blueschists have been reported and here new evidence of high pressure hp metamorphism in various lithologies is presented that suggests a larger extent of blueschist facies rocks along the northern border of this massif an earlier blueschist facies metamorphism is documented by inclusions of glaucophane in garnet epidote and titanite from metabasites along with zoned white mica having a phengitic core and a muscovite rim in metapelites and orthogneisses the estimated p t conditions obtained using pseudosections and mineral isopleths correspond to blueschist and low temperature eclogite facies conditions 1 1 2 0 gpa at 350 550 c together with medium temperature eclogites from different units in the bohemian massif they indicate a geothermal gradient of 8 10 c km 1 which is typical for cool subduction radiometric dating on phengite from metapelites confirms an early palaeozoic cooling age of c 360 ma for this hp metamorphic event the presence of blueschist facies rocks their p t relations and age constraint together with those from eclogite facies rocks allows us to locate the variscan suture which straddles the se margin of saxothuringian zone from erzgebirge to sudetes and its possible continuation to the moldanubian zone where eclogite facies and uhpm rocks are abundant

kumdykolite from the ultrahigh pressure granulite of the bohemian massif we report the first occurrence of kumdykolite a high temperature analog of albite in the european variscan belt it was discovered in an ultrahigh pressure diamond bearing quartzofeldspathic granulite from the northern bohemian massif it is associated with phlogopite and quartz in a multiphase solid inclusion within garnet considered to represent a trapped fluid or melt phase micro raman analysis and mapping along with bse revealed the presence of a sub equant elongated grain of kumdykolite reaching 20 mu m in length wdx analysis has shown that kumdykolite contains 2 wt cao probably indicating significant miscibility with the ca end member svyatoslavite similar to the case of microdiamond inclusions the kumdykolite bearing multiphase inclusion is located in the ca and mg rich central part of the garnet and thus must have been trapped at p 4 gpa the inclusion minerals however crystallized upon decompression and cooling during the exhumation kumdykolite preservation thus provides independent evidence for high temperature of the original trapped fluid or melt crystallization and rapid cooling of the rocks our results imply that kumdykolite and other feldspar modifications stable at elevated pressures and temperatures may be common phases in quartzofeldspathic granulites and need to be searched for

composition and origin of volcanic ash zones in late quaternary sediments from the reykjanes ridge evidence for ash fallout and ice rafting abstract seven quaternary volcanic ash zones including the well known ash zones az i and az ii have been investigated in five well dated sediment cores from the reykjanes ridge between 59 and 60 n the zones were defined on their geochemical composition morphological variation of the particles and stratigraphic position the ash zones az i and az ii show a mixture of colorless bubble wall shards and brown blocky glasses whereas ash zones vz 1 to 5 are composed of dark to light brown volcanic glasses ranging in shape and texture from blocky nonvesicular shards to highly vesicular pumiceous types the dark brown glass shards of ash zones vz 1 to 5 represent subalkalic and low k subalkalic basaltic series geochemical correlation of the ash zones with possible source areas indicates that they all derived from iceland the colorless shards in az i and az ii are of rhyolitic composition the eastern volcanic zone on iceland is the most important source with abundant volcaniclastics being produced by subaerial subglacial eruptions and secondary erosion of volcanic rocks explosive volcanism on iceland is manifested by marine ash layers az i and az ii most of the volcaniclastic zones vz 1 to 5 coincide chronologically with peak abundances of coarse ice rafted material ird indicating that most of this volcanic glass was incorporated into the icelandic ice sheet and released to the ocean by iceberg calving and subsequent melting however two layers in vz 1 and vz 5 have no correlatives in the ird record suggesting that they were probably transported by local sediment gravity flows from this we must infer that most of the ice rafting episodes bearing volcanic particles as observed in sediments from reykjanes ridge monitor glacier fluctuations on iceland

past hydrological events reflected in holocene history of polish rivers abstract a database of 335 radiocarbon dates from holocene fluvial sediments in poland has been compiled each sample is characterized by 25 parameters all samples were grouped into one of 7 regions characterized by hydrological regime and one of 8 depositional environments statistical analysis was used to enhance the palaeoenvironmental data and 17 phases may be distinguished 10 of which have a distinct sharp peak abandoned paleochannels 149 dates clearly reflect the major wetter phases of the holocene an opposite picture is presented by overbank aggradation 86 dates connected mainly with expansion of agriculture preliminary correlations are made on peat bog records changes in vegetation lake water level rates calcareous tufa and speleothems growth landslides phases and debris flow phases these show many convergences explained by climatic fluctuations during the holocene

discovery of metamorphic microdiamonds from the parautochthonous units of the variscan french massif central the variscan french massif central fmc is classically described as a stack of nappes with increasing metamorphism from the bottom parautochthonous unit to the top lower and upper gneiss units ultra high pressure uhp metamorphism was already recognized in the uppermost units with notably coesite bearing rocks we report the first finding of metamorphic microdiamonds in the parautochthonous unit revealing that the uhp event affected the whole stack of nappes and also that the pressures reached are above what was previously expected since the presence of diamond according to the peak temperature estimates for this unit indicates pressures of ca 3 gpa i e 100 km depth at the scale of the fmc this finding adds complexity to the established models on a broader scale this adds evidence of similarity between this part of the belt and other uhp variscan terranes such as the erzgebirge and the bohemian massif

a 450 year record of spring summer flood layers in annually laminated sediments from lake ammersee southern germany 1 a 450 year spring summer flood layer time series at seasonal resolution has been established from the varved sediment record of lake ammersee southern germany applying a novel methodological approach the main results are 1 the attainment of a precise chronology by microscopic varve counting 2 the identification of detrital layers representing flood triggered fluxes of catchment material into the lake and 3 the recognition of the seasonality of these flood layers from their microstratigraphic position within a varve tracing flood layers in a proximal and a distal core and correlating them by application of the precise chronology provided information on the depositional processes comparing the seasonal flood layer record with daily runoff data of the inflowing river ammer for the period from 1926 to 1999 allowed the definition of an approximate threshold in flood magnitude above which the formation of flood layers becomes very likely moreover it was possible for the first time to estimate the completeness of the flood layer time series and to recognize that mainly floods in spring and summer representing the main flood seasons in this region are well preserved in the sediment archive their frequency distribution over the entire 450 year time series is not stationary but reveals maxima for colder periods of the little ice age when solar activity was reduced the observed spring summer flood layer frequency further shows trends similar to those of the occurrence of flood prone weather regimes since a d 1881 probably suggesting a causal link between solar variability and changes in midlatitude atmospheric circulation patterns

climatic variability in the northwestern alps france as evidenced by 600 years of terrigenous sedimentation in lake le bourget cores recovered from periglacial lake le bourget deep basin northwestern alps were investigated to examine the ine uence of the little ice age lia on terrigenous lacustrine sedimentation growing glaciers in the regional watershed induced catastrophic rhone river e oods and major undere ow deposits in the deep basin during the early e fteenth the sixteenth and the mid eighteenth centuries the lia is characterized by a decrease in deposition from intere ows from ad 1550 to 1740 and an increase in deposition from undere ows from ad 1550 to 1800 on one hand spectral analyses of the laminations in intere ow deposits reveal 4 5 years cyclicities from ad 1440 to 1550 as well as 7 8 and 13 14 years cyclicities from ad 1740 to 1870 on the other hand spectral analyses of a clay mineral ratio ree ecting undere ow deposits highlight 45 50 years cyclicities from ad 1550 to 1800 these pluriannual decadal and pluridecadal periods are typical of the north atlantic oscillation nao a nao like period in our data would be a consequence of periodical variations in rainfall and snow accumulation during late autumn and winter over lake le bourget s watershed

significance of zr in rutile thermometry for deducing the decompression p t path of a garnet clinopyroxene granulite in the moldanubian zone of the bohemian massif

10 000 yr record of extreme hydrologic events well dated lacustrine sediments provide a hydrologic record indicating that the frequency and magnitude of runoff events and by inference storms have varied over the past 10 k y in northern new england we used five sediment cores and radiocarbon dating to develop a chronology of holocene hydrologic events for the ritterbush pond basin northern vermont chemical and physical analyses allow us to identify 52 distinct layers of predominately inorganic sediment that represent terrestrially derived material delivered to the pond by runoff events the thickness of some layers suggests hydrologic events at least equal in size to and probably much larger than any storm or flood recorded during nearly 300 yr of written regional history layer thickness and frequency and by analogy storm size and recurrence change through the holocene the largest events occurred 2620 6840 and 9440 calibrated 14c years before present cal 14c yr b p the most frequent hydrologic events occurred in three periods 1750 to 2620 6330 to 6840 and 8600 cal yr b p the recurrence interval of layer deposition during stormy periods averages 130 100 cal yr whereas the recurrence interval during less stormy periods is longer 270 170 cal yr the ritterbush pond event record illustrates the potential of inorganic lacustrine sediment to serve as a proxy record for estimating paleoflood frequency and deciphering climate change

transformation weakening diffusion creep in eclogites as a result of interaction of mineral reactions and deformation abstract the deformation of eclogites and the driving forces for their fabric development are an important topic potentially allowing to determine deformation rates and stresses in subduction zones where the greatest number of large earthquakes occurs here fabric studies of grain size and shape texture and chemical composition from two locations of variscan and alpine eclogites are presented all samples show a well developed crystallographic preferred orientation cpo of omphacite with a strong maximum of 001 in the lineation direction and a weaker maximum of poles to 010 normal to foliation garnet shows no systematic cpo anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde in terms of pressure change p t path the individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations chemical microstructural and cpo data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures the dominant deformation is inferred to be diffusion creep where dissolution of material took place in reacting mafic phases plagioclase pyroxene and precipitation took place in the form of new eclogite facies minerals omphacite garnet zoisite this type of diffusion creep deformation represents a transformation process involving both deformation and metamorphic reactions it is emphasized that the weakening is directly connected to the transformation and therefore transient the weakening facilitates diffusion creep deformation of otherwise strong minerals pyroxene garnet zoisite at far lower stresses than dislocation creep the results imply low stresses during the deformation of eclogite blocks in subduction zones these results can be applied to other rock types too

preserved near ultrahigh pressure melt from continental crust subducted to mantle depths remnants of hydrous melt formed at mantle depths have been identified and characterized within high pressure leucogranulites of the orlica snieznik dome bohemian massif central europe they occur as nanogranites in garnet formed via partial melting of granitoids during the variscan orogeny melt composition and h2o content have been investigated in situ after experimental re homogenization of the nanogranites and are consistent with melts produced experimentally from crustal lithologies at mantle depths this is the first geochemical study of melt inclusions from natural crustal rocks equilibrated close to the stability field of coesite shedding light on how continental crust melts during deep subduction whereas decompressional melting is commonly invoked for deeply subducted crustal lithologies melting occurred near or at the metamorphic peak pressure in the orlica snieznik granulites melting of deeply subducted crustal rocks significantly modifies the rheology and thus promotes fast exhumation this process has a critical influence on the geodynamic evolution of subduction collision orogens as well as crustal differentiation at depth

recycling of crustal materials through study of ultrahigh pressure minerals in collisional orogens ophiolites and mantle xenoliths a review abstract newly recognized occurrences of ultrahigh pressure uh p minerals including diamonds in ultrahigh temperature uh t felsic granulites of orogenic belts in chromitites associated with ophiolitic complexes and in mantle xenoliths suggest the recycling of crustal materials through deep subduction mantle upwelling and return to the earth s surface this circulation process is supported by crust derived mineral inclusions in deep seated zircons chromites and diamonds from collision type orogens from eclogitic xenoliths in kimberlites and from chromitities of several alpine himalayan and polar ural ophiolites some of these minerals contain low atomic number elements typified by crustal isotopic signatures ophiolite type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone in addition to ringwoodite and inferred stishovite a number of nanometric minerals have been identified as inclusions employing state of the art analytical tools reconstitution of now exsolved precursor uh p phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier uh p conditions for example tibetan chromites containing exsolution lamellae of coesite diopside suggest that the original chromitites formed at p 9 10 gpa at depths of 250 300 km the precursor phase most likely had a ca ferrite or a ca titanite structure both are polymorphs of chromite and at 2000 c would have formed at minimum pressures of p 12 5 or 20 gpa respectively some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin including zircon feldspars garnet kyanite andalusite quartz and rutile the zircons possess much older u pb ages than the time of ophiolite formation these uh p mineral bearing chromitite hosts evidently had a deep seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes these new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere recycling of surface organic carbon into the lower mantle and ascent to the earth s surface through mantle upwelling intensified study of uh p granulite facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling

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

deciphering high pressure metamorphism in collisional context using microprobe mapping methods application to the stak eclogitic massif northwest himalaya the stak massif northern pakistan is a newly recognized occurrence of eclogite formed by the subduction of the northern margin of the indian continent in the northwest himalaya although this unit was extensively retrogressed during the himalayan collision records of the high pressure hp event as well as a continuous pressure temperature p t path were assessed from a single thin section using a new multiequilibrium method this method uses microprobe x ray compositional maps of garnet and omphacitic pyroxene followed by calculations of 200 000 p t estimates using appropriate thermobarometers the stak eclogite underwent prograde metamorphism increasing from 650 c and 2 4 gpa to the peak conditions of 750 c and 2 5 gpa then retrogressed to 700 650 c and 1 6 0 9 gpa under amphibolite facies conditions the estimated peak metamorphic conditions and p t path are similar to those of the kaghan and tso morari high to ultrahigh pressure hp uhp massifs we propose that these three massifs define a large hp to uhp province in the northwest himalaya comparable to the dabie sulu province in china and the western gneiss region in norway

diamond and coesite discovered in saxony type granulite solution to the variscan garnet peridotite enigma diamond and coesite were discovered in situ as inclusions in garnet kyanite and zircon in high pressure granulites from northern bohemian massif these continental crustal rocks were therefore subducted to depths of c 140 km which also explains their common association with mantle garnet bearing peridotites models involving crustal thickening for these high pressure granulites need to be significantly modified whole variscan belt with numerous hp granulite occurrences can represent a large ultrahigh pressure terrain

deep subduction of felsic rocks hosting uhp lenses in the central saxonian erzgebirge implications for uhp terrane exhumation abstract contrasting metamorphic conditions determined by chemical geothermobarometric investigations of ultrahigh pressure uhp lenses surrounded by high pressure hp and medium pressure mp felsic country rocks are an enigmatic feature of uhp terranes one of the major questions arising is whether the uhp lenses and the country rocks are a product of different peak metamorphic conditions corresponding to different maximum depth or whether country rocks also experienced uhp conditions but equilibrated and or re equilibrated at a different metamorphic stage here we address this question to the central saxonian erzgebirge in the northwestern bohemian massif germany in order to screen the variety of garnet from lithologies occurring in the study area we analyzed the detrital garnet record from seven modern stream sands in addition to 700 inclusion bearing garnet grains previously studied from the 125 250 m grain size fraction we analyzed the 63 125 and 250 500 m fractions and extended the dataset to overall 2100 inclusion bearing grains the new findings of coesite and diamond inclusions in several garnet grains which are in compositional contrast to garnet of the known uhp lenses but match with those of the felsic country rocks show that considerable parts of the country rocks underwent uhp metamorphism melt inclusions containing cristobalite kokchetavite and kumdykolite in garnet derived from the country rocks point to partial melting and re equilibration during exhumation at hp ht conditions although an amalgamation of rocks which reached different maximum depth may be responsible for some of the contrasting peak metamorphic conditions the mineralogical evidence for uhp conditions in the felsic country rocks surrounding the uhp lenses proves a largely coherent slab subducted to uhp conditions furthermore the presence of coesite in the subducting voluminous felsic crust and its transformation to quartz during exhumation have great implications for buoyancy development during the metamorphic cycle which may explain the high exhumation rates of uhp terranes

collision vs subduction related magmatism two contrasting ways of granite formation and implications for crustal growth earth s continental crust is dominantly made of buoyant felsic igneous material granitoids that were ultimately extracted from the mantle as a result of earth s differentiation since felsic melts are not in chemical equilibrium with the mantle they can originate either from melting of older crustal lithologies or from differentiation of a primitive mantle melt only the latter case will contribute to crustal growth to understand the mechanisms of continental crust growth and differentiation through time it is therefore necessary to unravel the respective contribution of these two different mechanisms in the genesis of granitoid suites in modern earth granitoids are chiefly generated in convergent plate boundaries subduction and collision this paper examines the granitic suites in a late collision environment the variscan french massif central fmc and compares them with the suites found in an oceanic arc we therefore describe and compare two end members sites of granite generation in the fmc several main types of granites are described muscovite and cordierite bearing peraluminous granites resp mpg and cpg contain large amounts of inherited zircons and their chemistry demonstrates that their sources were older crustal material resp metasediments and metaigneous on the other hand potassic calc alkaline granites kcg associated to potassic diorites vaugnerites do not contain inherited zircons and ultimately derive from the vaugnerites the vaugnerites in turns form by partial melting of a mantle contaminated by the regional crust therefore although they are isotopically similar to the crust the kcg are net contributors to crustal growth thus we conclude that although late orogenic settings are dominated by crustal melting and recycling they may be sites of net crustal growth even though this is not visible from isotopes only in contrast arc granitoids are purely or almost purely mantle derived however the preservation potential of arcs is much smaller than the preservation of late orogenic domains such that at the scale of a whole orogenic belt late orogenic magmatism is probably as important as arc magmatism finally we speculate that the situation may have been similar in the archaean or even more skewed towards late orogenic sites or similar environments dominated by melting of a altered mafic protocrust owing to the hotter mantle and less stable subductions during that period

composition of barian mica in multiphase solid inclusions from orogenic garnet peridotites as evidence of mantle metasomatism in a subduction zone setting multiphase solid inclusions in minerals formed at ultra high pressure uhp provide evidence for the presence of fluids during deep subduction this study focuses on barian mica which is a common phase in multiphase solid inclusions enclosed in garnet from mantle derived uhp garnet peridotites in the saxothuringian basement of the northern bohemian massif the documented compositional variability and substitution trends provide constraints on crystallization medium of the barian mica and allow making inferences on its source barian mica in the multiphase solid inclusions belongs to trioctahedral micas and represents a solid solution of phlogopite kmg3 si3al o10 oh 2 kinoshitalite bamg3 al2si2 o10 oh 2 and ferrokinoshitalite bafe3 al2si2 o10 oh 2 in addition to ba 0 24 0 67 apfu mica is significantly enriched in mg xmg 0 85 to 0 95 cr 0 03 0 43 apfu and cl 0 04 0 34 apfu the substitution vector involving ba in the i site which describes the observed chemical variability can be expressed as bafeivalclk 1mg 1si 1 oh 1 a minor amount of cr and vial enters octahedral sites following a substitution vector vi cr al 2 vi mg fe 3 towards chromphyllite and muscovite as demonstrated by variable ba and cl contents positively correlating with fe barian mica composition is partly controlled by its crystal structure textural evidence shows that barian mica together with other minerals in multiphase solid inclusions crystallized from fluids trapped during garnet growth the unusual chemical composition of mica reflects the mixing of two distinct sources 1 an internal source i e the host peridotite and its garnet providing mg fe al cr and 2 an external source represented by crustal derived subduction zone fluids supplying ba k and cl at uhp uht conditions recorded by the associated diamond bearing metasediments c 1100 c and 4 5 gpa located above the second critical point in the pelitic system the produced subduction zone fluids transporting the elements into the overlying mantle wedge had a solute rich composition with properties of a hydrous melt the occurrence of barian mica with a specific chemistry in barium poor mantle rocks demonstrates the importance of its thorough chemical characterization

synchronous holocene climatic oscillations recorded on the swiss plateau and at timberline in the alps eight synchronous pre roman cold phases were found at 9600 9200 8600 8150 7550 6900 6600 6200 5350 4900 4600 4400 3500 3200 and 2600 2350 radiocarbon years bp by reconstructing past climate at two sites on the swiss plateau and at timberline in the alps the cooling events during the early and mid holocene represent temperature values similar to today and apparently the onset of cooling events represents a deviation from today s mean annual temperature of about 1 c and is triggered at a 1000 year periodicity at wallisellen langachermoos 440 m a former oligotrophic lake near zurich the correlation between sum mertime lake levels and the seed production of the amphi atlantic aquatic plantnajas flexilis was used to reconstruct lake levels over a 3000 year period during the first part of the holocene at lake seedorf on the western swiss plateau 609 m the sedimentological palynological and macrofossil record revealed fluctuations of lake levels for the complete holocene from lago basso in the

reply to j berger s comment on the article discovery of metamorphic microdiamonds from the parautochthonous units of the variscan french massif by thiery v et al 2015 gondwana research 28 954 960

oriented multiphase needles in garnet from ultrahigh temperature granulites connecticut u s a this study investigates distinctive oriented needle shaped multiphase inclusions found within cores of garnets from felsic granulites of the upper member of the bigelow brook formation and the brimfield schist northeastern connecticut u s a the rocks crop out in the southern end of the central maine terrane and thus are a part of the acadian neoacadian orogen the typical mineral assemblage is garnet sillimanite k feldspar plagioclase quartz cordierite biotite spinel the sillimanite is commonly pseudomorphous after kyanite the multiphase needle inclusions and associated exsolved needles of rutile and ilmenite are mostly oriented parallel to of garnet the multiphase needles contain various combinations of quartz micas chlorite rutile graphite a siliceous compositionally variable phase phase cv zn spinel apatite zircon and rare ilmenite we hypothesize that they represent inclusions of fluid that adhered to exsolving ti fe oxide needles mostly rutile or extended along zones of weakness in garnet this requires that multiphase needle formation occurred in response to cooling and or decompression the needles ultimately decrepitated during retrogression we note that micaceous needle shaped multiphase inclusions are rarely described the closest analogs of which we are aware are found in uhp garnets of the erzgebirge perchuk 2008 the brimfield schist in this area underwent ultrahigh temperature metamorphism uhtm of 1000 c at a minimum pressure of 1 gpa ague et al 2013 here we provide new temperature estimates for the adjacent upper member of the bigelow brook formation ternary feldspar reintegration using the activity model of benisek et al 2004 and zr in rutile thermometry tomkins et al 2007 give average temperatures of 990 and 1010 c respectively at 1 gpa for this unit therefore the recently discovered uht zone in the brimfield schist of northeastern connecticut extends to the east to include the upper member of the bigelow brook formation consequently the first confirmed regional uht locality in the united states is larger than initially recognized and is at least 25 km long and 5 10 km wide the oriented elongate multiphase inclusions and petrographically obvious oriented ti fe oxide needles may be useful indicators of extreme temperature and or pressure rocks in other field areas

anatomy of a diffuse cryptic suture zone an example from the bohemian massif european variscides the fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late paleozoic variscan belt in continental europe in particular the bohemian massif at its eastern extremity preserves well the evolution of an andean type orogen involved in continental collision this process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate this led to gravity driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials in the studied example this zone of interaction repeatedly reappears within the orogen forming a so called diffuse cryptic suture zone

failed silurian continental rifting at the nw margin of gondwana evidence from basaltic volcanism of the prague basin tepla barrandian unit bohemian massif the silurian volcanic rocks of the prague basin represent within plate transitional alkali to tholeiitic basalts which erupted in a continental rift setting through the thick cadomian crust of the tepla barrandian unit bohemian massif despite the variable often intense alteration resulting in post magmatic replacement of the basalt mass due to carbonatization the geochemical signatures of silurian basalts are still sufficiently preserved to constrain primary magmatic processes and geotectonic setting the studied interval of silurian volcanic activity ranges from wenlock homerian 431 ma to late ludlow gorstian 425 ma with a distinct peak at the wenlock ludlow boundary 428 ma trace element characteristics unambiguously indicate partial melting of a garnet peridotite mantle source wenlock basalts are similar to alkaline oib with depleted radiogenic nd signature compared to ludlow basalts which are rather tholeiitic emorb like with enriched radiogenic nd signature the correlation of petrogenetically significant trace element ratios with nd isotopic compositions points to a mixing of partial melts of an isotopically heterogeneous possibly two component mantle source during the wenlock ludlow melting lava eruptions were accompanied by intrusions of doleritic basalt and meimechite sills the latter represent olivine rich cumulates of basaltic magmas of probably predominantly ludlow age meimechites with dolerites and to a lesser extent some lavas were subject to alteration due to wall rock fluid interaction the trigger for the wenlock to ludlow 431 425 ma extension and related volcanism in the prague basin is related to far field forces namely slab pull regime due to progressive closure of the iapetus ocean the main stage of the baltica laurentia collision then caused the prague basin rift failure at ca 425 ma that has never reached an oceanic stage

the fate of zircon during uht uhp metamorphism isotopic u pb δ18o hf and trace element constraints garnet clinopyroxene ultra high pressure uhp rocks from the northern bohemian massif contain zircon with micro diamond inclusions trace element concentrations oxygen and hafnium isotopic composition and u pb age of distinct textural domains in zircon characterize their growth conditions and temporal evolution diamond bearing zircon mantle domains with relicts of oscillatory zoning have uniform th u ratios 0 1 0 2 high ti contents 110 190 ppm corresponding to temperatures of at least 1100 c and some two of 17 mantle analyses preserve steep heavy rare earth element hree patterns with ybn gdn 10 11 with a weak negative eu anomaly these signatures are consistent with crystallization from a melt under uhp ultra high temperature uht conditions some of the bright cathodoluminscence cl rims preserve th u and ti values characteristic of the zircon mantles but others show elevated th u ratios of 0 3 0 4 and lower ti contents 20 40 ppm only 13 ppm in a rare low cl outer rim as they feature flat hree patterns and negative eu anomalies and commonly make embayments and truncate the mantle zoning we suggest that they have formed through recrystallization in the solid state during exhumation of the rock when both garnet and plagioclase were stable the three zircon domains that is cores mantles and rims yield u pb concordia ages of 340 9 1 5 340 3 1 5 and 341 2 3 4 ma respectively when linked to the previously reconstructed p t path of the rock the error limits of the zircon mantle and rim ages constrain the exhumation of the rocks from depth of 140 km uhp to 80 km hp to a minimum rate of 1 5 cm yr 1 the zircon cores are heterogeneous in terms of th u ratio below 0 1 but also above 0 2 and ree characteristics and their ehf values scatter between 15 7 and 4 8 with similar values for individual domains within a single zircon grain suggesting a very localized control on hafnium isotope composition on a grain scale the non equilibrated ehf values as well as a large range of the hf depleted mantle model ages possibly reflect the presence of a heterogeneous population of old zircon consequently the uniform and young 238u 206pb ages may represent near complete resetting of the u pb geochronometer during the uhp uht event at c 340 ma through dissolution reprecipitation process in contrast to hf the oxygen isotope composition of zircon is homogeneous ranging between 7 8 and 9 6 vsmow reflecting a source containing upper crustal material and homogenization at uhp uht conditions our study documents that continental crust was subducted to mantle depths at c 340 ma during the variscan orogeny and was subsequently very rapidly exhumed implying that the sequence of events was faster than can be resolved by the secondary ion mass spectrometry technique

environmental responses to lateglacial climatic fluctuations recorded in the sediments of pre alpine lake mondsee northeastern alps investigation of the sedimentary record of pre alpine lake mondsee upper austria focused on the environmental reaction to rapid lateglacial climatic changes results of this study reveal complex proxy responses that are variable in time and influenced by the long term evolution of the lake and its catchment a new field sampling approach facilitated continuous and precisely controlled parallel sampling at decadal to sub annual resolution for mu xrf element scanning carbon geochemistry stable isotope measurements on ostracods pollen analyses and large scale thin sections for microfacies analysis the holocene chronology is established through microscopic varve counting and supported by accelerator mass spectrometric 14 c dating of terrestrial plant macrofossils whereas the lateglacial age model is based on delta 18 o wiggle matching with the greenland ngrip record using the gicc05 chronology microfacies analysis enables the detection of subtle salimentological changes proving that depositional processes even in rather large lake systems are highly sensitive to climate forcing comparing periods of major warming at the onset of the lateglacial and holocene and of major cooling at the onset of the younger dryas reveals differences in proxy responses reflecting threshold effects and ecosystem inertia temperature increase vegetation recovery decrease of detrital flux and intensification of biochemical calcite precipitation at the onset of the holocene took place with only decadal leads and lags over a ca 100 a period whereas the spread of woodlands and the reduction of detrital flux lagged the warming at the onset of the lateglacial interstadial by ca 500 750a cooling at the onset of the younger dryas is reflected by the simultaneous reaction of delta 18 o and vegetation but sedimentological changes reduction of endogenic calcite content increase in detrital flux were delayed by about 150 300 a three short term lateglacial cold intervals corresponding to greenland isotope substages gl 1d gl 1c2 and gl 1b also show complex proxy responses that vary in time copyright c 2011 john wiley sons ltd

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

raman study of diamond based abrasives and possible artefacts in detecting uhp microdiamond abstract raman spectral characteristics of a range of diamond based abrasives powders and sprays and drilling and cutting tools originating from preparation laboratories worldwide are presented some abrasives show strong broadening of the main diamond band fwhm full width at half band maximum 5 cm 1 accompanied by strong band downshift 1316 1330 cm 1 others are characterised by moderate band broadening fwhm 1 8 5 cm 1 at rather regular band position 1331 1333 cm 1 in addition we found that a fresh abrasive and its used analogue may in some cases show vast differences in their raman spectra the raman parameters of diamond based abrasives overlap widely with raman parameters of uhp ultra high pressure microdiamond it is hence impossible to assign diamond detected in a geological specimen to either an introduced artefact or a genuine uhp relict from the raman spectrum alone raman is an excellent technique for the detection of minute amounts of diamond however it does not provide conclusive evidence for the identification of uhp microdiamond the latter requires thorough verification for instance by optical microscopy or if doubts cannot be dispelled transmission electron microscopy

microdiamonds frontier of ultrahigh pressure metamorphism a review abstract this is a comprehensive review paper devoted to microdiamonds from ultrahigh pressure metamorphic uhpm terranes incorporated in orogenic belts formed at convergent plate boundaries in paleozoic mesozoic alpine time when in 1980 the first small diamonds were discovered within amphibolite granulate facies metamorphic rocks it came as a great surprise that buoyant continental crust could be subducted to depths of hundreds of kilometers and then subsequently exhumed since then much progress has been made in understanding the mechanism of these diamonds formation and the number of new diamond bearing uhpm terranes was significantly increased especially within european orogenes moreover new variations in tectonic settings in which uhp rocks can be formed and exhumed came to the attention of geologists simply due to the finding of diamonds in places previously forbidden for their formation e g oceanic islands ophiolites and forearc environments over the past decade the rapidly moving technological advancement has made it possible to examine microdiamonds in detail and to learn that part of them has a polycrystalline nature that they contain nanometric multiphase inclusions of crystalline and fluid phases and that they keep a crustal signature of carbon isotopes scanning and transmission electron microscopy focused ion beam techniques synchrotron infrared spectroscopy micro x ray diffraction and nano secondary ion mass spectrometry studies of these diamonds provide evidence that they keep traces of fluid originated from both crustal and mantle reservoirs and that they probably interacted with deep mantle plumes hypotheses proposed for diamond formation in subduction zones founded on both analytical and experimental studies are discussed the paper also emphasizes that the discovery of these microdiamonds as well as coesite triggered a major revision in the understanding of deep subduction processes leading to a clear realization of how continental materials can be recycled into the earth s mantle and geochemically rejuvenate it

late holocene environmental change in the howgill fells northwest england abstract new data from chapel beck in the southwest howgill fells combined with existing data support the identification of three phases of extensive hillslope gullying after 2200 between 1250 700 and after 500 cal bp examination of cumulative probability density function cpdf plots of the distribution of calibrated radiocarbon dates confirms the identification of these phases there are differences in the spatial distribution of geomorphic activity between these phases with the c 2200 1800 cal bp episode limited to the western howgill fells in comparison the 1250 700 and post 500 cal bp episodes are more widespread cross region phenomena and affected the more remote central fells pollen and geochemical data reveal several phases of increased human activity in these uplands with small scale temporary clearances during the late bronze age early iron age and more substantial clearances during the late iron age and romano british times 2300 1500 cal bp and these were followed by woodland expansion and decline in human activity during both the mid to late iron age and the period after the roman withdrawal from britain after 1500 cal bp the final substantial woodland clearances occurred from c 1100 900 cal bp after which there has been little woodland recovery a range of pollen sites across the howgill fells indicate that there is spatial variability in the level of human activity and impact iron age romano british woodland clearances appear to have affected the west of the region more than the remote central valleys whereas the later 1100 900 cal bp woodland reduction affected the entire region this pattern is similar to that identified from the geomorphology and demonstrates the importance of pollen sites that record local palaeo vegetation information and the significance of using a series of sites to explore intra regional variability in the vegetation history we suggest that human activity is a critical factor in mediating the late holocene geomorphic record but climate and particularly individual storms are the trigger mechanism responsible for erosion and incision on hillslopes and within the wider fluvial system palaeoclimate data humification and plant macrofossil from peat deposits at archer moss identify shifts to wetter conditions c 3100 2700 2400 1700 1300 1100 and after 650 cal bp and probably reflect increases in summer wetness of these the 2400 1700 1300 1100 and post 650 cal bp wetter phases coincide with increased geomorphic activity and perhaps encouraged incision on hillslopes rendered more susceptible to erosion by the activities of people