petrology geochemistry and environmental significance of silcrete calcrete intergrade duricrusts at kang pan and tswaane central kalahari botswana calcrete silcrete intergrade duricrusts are an important component of the kalahari group sediments of central southern africa and yet have neither been analysed systematically nor in any detail in this study the petrological and geochemical characteristics of suites of calcrete silcrete and intergrade duricrusts from two fresh relatively deep exposures at kang pan and tswaane adjacent to the okwa valley in the central kalahari botswana are described the duricrust profile at kang pan consists of a highly indurated crystalline non pedogenic calcrete which has been extensively silicified by chalcedony and rarely cryptocrystalline silica or microquartz silicification is most extensive in lower parts of the profile where replacement is related to groundwater fluctuations and in upper sections due to periodic flooding by ephemeral surface water the exposure at tswaane consists of a sequence of pale green glauconitic non pedogenic silcrete and cal silcrete overlain by non pedogenic calcrete all of which have formed within sediments situated upon granitoid gneiss bedrock the siliceous duricrusts are dominated by cryptocrystalline silica cements and appear to have developed through the replacement of a preexisting non pedogenic calcrete these siliceous duricrusts have also been calcified at a later date during the formation of the overlying calcrete to produce a complex range of silica carbonate cements at both sites the style and type of silicification present appears to be determined by the duration of wetting and the permeability of the precursor calcrete geochemical evidence indicates a lack of chemical weathering profile development within the granitoid gneiss bedrock and considerable differences between the chemical signature of the bedrock and combined duricrusts from kang and tswaane this suggests that bedrock made a minimal contribution in terms of silica and carbonate species to duricrust formation and that the majority of cementing agents were non local it would therefore appear likely that the geomorphological context of each site had a major influence upon the development of calcrete silcrete and intergrade duricrust cements

evolution of the eclogites in the rhodope zone of northern greece abstract the polymetamorphic complex of rhodope in the southern balkan peninsula has been affected by three successive metamorphic episodes these are 1 a high pressure 2 a barrow type and 3 a greenschist facies retrograde metamorphic episode based on the different metamorphic grades of the barrovian metamorphism the rhodope zone can be subdivided into two tectonic units a lower and an upper one commonly amphibolitised eclogites of probable mesozoic age occur in both units a in the shape of lenses b as concordant interbeds with other rock types such as gneisses and marbles and c as metagabbroic dykes the high pressure mineral assemblage is garnet omphacite quartz ca amphibole clinozoisite zoisite rutile kyanite in eclogites of gabbroic origin igneous relics such as igneous plagioclase augite and ilmenite are sometimes preserved pt conditions of 14 kbar and 550 600 c were determined for the high pressure metamorphism in the lower tectonic unit symplectites of clinopyroxene plagioclase ca amphibole plagioclase biotite plagioclase and spinel plagioclase are common and are attributed to post eclogitic reactions during uplift changes in mineral composition of high pressure phases such as garnet ca amphibole and na pyroxene reflect a re equilibration tendency during unloading

inferences on the mesozoic evolution of the north aegean from the isotopic record of the chalkidiki block the chalkidiki block is a major domain in the north aegean that contrary to other domains in the region largely escaped thermal perturbations during tertiary extension as a result the chalkidiki block is an ideal candidate to glean information related to the timing of mesozoic thermal events using appropriate geochronological techniques we have undertaken a laser ablation inductively coupled plasma mass spectrometry la icp ms study u th pb on monazites and u pb on zircons coupled with 40ar 39ar dating on nine samples from various structural levels within the thrust system of the chalkidiki block the eastern and structurally lower part of the system revealed a complete isotopic reset of carboniferous early triassic monazites coeval with partial monazite destruction ree mobilisation and formation of apatite allanite epidote coronas at 132 ma a reaction that is commonly observed in amphibolite facies rocks these coronas formed after crystallisation of garnet i e at t 580 c and in all probability either close to the peak temperature conditions 620 c on a prograde path or during retrogression between the peak temperature and the low temperature boundary of the amphibolite facies cooling of these rocks and arrival at mid crustal levels occurred at 95 100 ma by contrast the western and structurally uppermost part of the system went through the same event by 120 125 ma further structural considerations with respect to medium temperature geochronology data imply that syn metamorphic thrusting must have ceased by early late cretaceous we emphasize that with the sole exception of the chalkidiki block no pre 45 ma medium temperature geochronology data are preserved in other north aegean domains a feature that is clearly related to the extension induced thermal perturbation of the region during the tertiary

upper cretaceous exhumation of the western rhodope metamorphic province chalkidiki peninsula northern greece the vertiskos unit of northern greece is an elongated basement belt with a complex poly metamorphic history it extends from greece chalkidiki peninsula to the south up to serbia in the north and arguably represents the westernmost part of the rhodope metamorphic province northern greece to southern bulgaria the vertiskos unit experienced a medium pressure lower amphibolite facies metamorphic overprint during the alpine orogeny the available medium temperature geochronology implies that it remained at temperature of approximately 300 c or slightly higher during lower cretaceous in order to constrain its post lower cretaceous thermal history until near surface exposure we applied apatite fission track analysis the central ages obtained range from 68 5 3 8 to 46 6 3 6 ma uppermost cretaceous to middle eocene and mean track lengths between 13 and 13 5 m we applied two inverse thermal modeling approaches using either each sample independently high degree of freedomin the thermal history better data fit or all samples together interpreting them as a vertical profile simpler thermal history worse data fit irrespective of the modeling approach we conclude that the bulk thermal history of the vertiskos unit crosses the high temperature limit of the apatite partial annealing zone by the uppermost cretaceous and reaches near surface conditions as early as lower middle eocene these results contrast with the thermal history of the other domains of the rhodope metamorphic province further east namely the southern rhodope core complex and the northern rhodope complex and establish the vertiskos basement complex as the oldest exhumed coherent basement fragment of the rhodope metamorphic province and greece

u pb shrimp and nd isotopic data from the western bohemian massif bayerischer wald germany implications for upper vendian and lower ordovician magmatism u pb shrimp dating of zircons of metamagmatites from the bayerischer wald germany reveals a complex evolution of this section of the moldanubian zone exposed in the western bohemian massif of the central european variscan belt in the south western part of the bayerischer wald upper vendian magmatism is constrained by pooled 206pb 238u mean ages of 555 12 549 7 and 549 6 ma from metarhyolites and a metabasite inherited zircon cores were not observed zircon overgrowths yielding pooled 206pb 238u ages of 316 10 and 319 5 ma provide evidence for variscan metamorphic zircon growth cathodoluminescence imaging reveals a two stage metamorphic overprint

in situ cosmogenic radiocarbon production and 2 d ice flow line modeling for an antarctic blue ice area surface 14 c activity that differs by up to 25 from the activity calculated using an ablation only approximation which is used in all prior work and 3 that at high ablation margin sites solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10 as part of this effort we model twodimensional ice flow along the central flow line of taylor glacier we present two methods for parameterizing vertical strain rates and assess which method is more reliable for taylor glacier finally we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error the results presented here can inform ongoing and future 14 c and ice flow studies at ice margin

the mamonia complex sw cyprus revisited remnant of late triassic intra oceanic volcanism along the tethyan southwestern passive margin upper triassic volcanic and sedimentary rocks of the mamonia complex in southwestern cyprus are exposed in erosional windows through the post cretaceous cover where the mamonia complex is tectonically imbricated with the troodos and akamas ophiolitic suites most of these upper triassic volcanic rocks have been considered to represent remnants of triassic oceanic crust and its associated seamounts new nd and pb isotopic data show that the whole mamonia volcanic suite exhibits features of oceanic island basalts oib four rock types have been distinguished on the basis of the petrology and chemistry of the rocks volcanism began with the eruption of depleted olivine tholeiites type 1 and oceanic island tholeiites type 2 associated with deep basin siliceous and or calcareous sediments the tholeiites were followed by highly phyric alkali basalts type 3 interbedded with pelagic halobia bearing limestones or white reefal limestones strongly lree enriched trachytes type 4 were emplaced during the final stage of volcanic activity nd and pb isotopic ratios suggest that tholeiites and mildly alkali basalts derived from partial melting of heterogeneous enriched mantle sources fractional crystallization alone cannot account for the derivation of trachytes from alkaline basalts the trachytes could have been derived from the partial melting at depth of mafic material which shares with the alkali basalts similar trace element and isotopic compositions this is corroborated by the rather similar isotopic compositions of the alkali basalts and trachytes the correlations observed between incompatible elements nb th and varepsilon nd and pb isotopic initial ratios suggest that the mamonia suite was derived from the mixing of a depleted mantle dmm and an enriched component of high 238u 204pb himu type models using both nd and pb isotopic initial ratios suggest that the depleted tholeiites type 1 derived from a dmm source contaminated by an enriched mantle type 2 component em2 and that the oceanic tholeiites type 2 alkali basalts type 3 and trachytes type 4 were derived from the mixing of the enriched mantle source of the depleted tholeiites with a himu component none of the mamonia volcanic rocks show evidence of crustal contamination the upper triassic within plate volcanism likely erupted in a small southerly neotethyan basin located north of the eratosthenes seamount and likely floored by oceanic crust

structure and composition of calcified roots and their identification in calcareous soils abstract fine white branching structures isolated from calcareous soils in southern france were shown by microscopy and by chemical and mineralogical analysis to be calcified roots these structures occur mainly in rendzinas calcic xerosols and cambisols that are dense or rich in fines and where drainage is slow visible to the naked eye they can locally represent up to a quarter of the soil mass calcified roots consist of calcite elements whose mean size varies from 60 to 80 m and which retain the structure of the original cells of the root cortex each calcified cell has a central nucleus composed of two crystals on average and is coated with a thin mineral layer that seems to be poorly crystallized some retain organic remnants and the structure of cell walls calcified cells contain tubes less than 1 m in diameter which appear to have formed by weathering although they have generally not been recognized as calcified roots similar microstructures have been indicated in some calcareous soils and calcretes both present day and ancient ones that suggests that calcified roots are widespread and that their occurrence contributes to the genesis of some calcareous soils

u pb and 40ar 39ar geochronology of the symvolon granodiorite implications for the thermal and structural evolution of the rhodope metamorphic core complex northeastern greece north aegean continental lithosphere was thickened by southwest vergent thrusting and continental subduction within the alpine collisional orogen but has subsequently been greatly extended on a northeast southwest axis in the back arc of the hellenic subduction zone crosscutting relationships with two granodiorite bodies emplaced at 31 33 ma the xanthi and eastern vrondou plutons constrain a pre mid oligocene origin of alpine convergent structures in northeastern greece post alpine thinning of the north aegean nappe pile began in earliest miocene time and has been accommodated by a succession of distinct structural systems the earliest of these the symvolon shear zone appears to represent a midcrustal coaxial rupture of the falakron marble series a carbonate platform 5000 m thick that was subducted northeastward beneath high grade rocks of the rhodope metamorphic province in late alpine time zircon and titanite u pb dates and hornblende 40ar 39ar dates obtained in this study constrain the intrusion and incipient mylonitization of the symvolon or kavala granodiorite within the symvolon shear zone at 21 22 ma following its emplacement the symvolon body resided at temperatures between 300 c and 500 c for 5 7 m y during which coaxial deformation may have continued within a widening symvolon rupture the strymon valley detachment a regionally south west dipping low angle normal fault succeeded the symvolon shear zone in middle miocene time southwestward displacement of the serbo macedonian gneiss complex by as much as 80 km in the hanging wall of this detachment facilitated the unroofing of the rhodope metamorphic core complex including the falakron marble series and several tertiary plutons in its footwall biotite and k feldspar 40ar 39ar dates from 11 1 0 2 ma to 15 5 0 3 ma yielded by symvolon granodiorite samples document the cooling of the rhodope core complex below 150 c 300 c during its southwestward progressive exhumation in the footwall of the strymon valley detachment

syn metamorphic nappe complex in the rhodope massif structure and kinematics mylonitic gneisses of the bulgarian and greek rhodope were deformed under medium pressure type metamorphism the kinematic information contained in these gneisses shows that shear deformation occurred during development of a nappe complex lithologies and metamorphic histories allow a lower footwall and an upper hanging wall terrane to be distinguished that define a crustal scale duplex as oceanic crust is involved collision between two continental units with subsequent crustal thickening is inferred the blocks would be moesia to the north and the lower rhodope promontory to the south which collided in the mesozoic to early cenozoic the nappe complex is characterized by south to southwestward foreland directed piling up and is associated with both coeval and subsequent extension the late extension is associated with the establishment of a high temperature low pressure metamorphic gradient and plutonism that predates but makes a transition to the lithospheric extension of the aegean arc

root calcretes and uranium bearing silcretes at sedimentary discontinuities in the miocene of the madrid basin toledo spain abstract this paper reports a detailed study of the calcrete and silcrete profiles in the miocene detrital deposits in the western area of madrid at the boundary of two main sedimentary units the aims of this work were to better understand the pedogenic and diagenetic environments in which these profiles formed and to determine the cause s of their enrichment in uranium calcrete and silcrete duricrusts are characteristic features of closed continental basins in semiarid climates this paper discusses the significance of duricrusts as indicators of important change in such basins the detailed macromorphological micromorphological and geochemical study of three duricrust profiles revealed the sequence of pedogenic vadose and groundwater processes responsible for their formation during the first stage of their development carbonate laminae formed a white grill like structure within the detrital parent materials the microstructure and macrostructure of the carbonate which includes alveolar septal structures and needle fiber calcite indicates the important role of roots and their associated microorganisms in calcrete formation early silicification occurred in the pedogenic vadose environment affecting the detrital parent material roots and calcretes forming an early silcrete defined by opaline glaebules and silica rhizoliths the detailed preservation of the cells in the silicified roots denotes the early replacement of root organic matter the green or green yellowish fluorescence of the silicified root structures under short wavelength uv shows their preferential enrichment in uranium calcitization and silicification coexisted in the pedogenic vadose environment leading to several reversible replacements of calcite and silica later the rise of the water table promoted silicification under phreatic conditions as indicated by the good preservation of the texture of the detrital host rocks and calcretes other silcrete textures such as ovoidal opaline accumulations intraclasts produced by autobrecchification and vadose silica cements indicate later vadose environments and consequently variations in the water table the geochemical features of the calcretes and silcretes major minor and rare earth elements were inherited from their parent materials the rare earth element patterns of some silcretes show them to have a positive ce anomaly suggesting that oxidizing conditions reigned during their formation the good correlation between silica and uranium suggests that the silica phases acquired uranium through the direct silicification of roots that had fixed uranium from organic matter this study shows that calcrete silcrete duricrusts provide detailed information regarding the processes occurring in semiarid continental basins in the studied basin roots played a key role in both the development of the duricrust profiles and their enrichment in uranium these duricrusts provide important information for understanding the overall stratigraphy of the studied basin and its large scale sequential evolution

tectonic development of the eastern mediterranean region the eastern mediterranean region is a classic area for the study of tectonic processes and settings related to the development of the tethyan orogenic belt the present set of research and synthesis papers by earth scientist from countries in this region and others provides an up to date interdisciplinary overview of the tectonic development of the eastern mediterrenean region from precambrian to recent key topics include continental rifting ophiolite genesis and emplacement continental collision extensional tectonics crustal exhumation and intraplate deformation e g active faulting alternative tectonic reconstructions of the tethyan orogen are presented and discussed with important implications for other regions of the world the book will be an essential source of information and interpretation for academic researchers geologists and geophysicists advanced undergraduates and also for industry professionals including those concerned with hydrocarbons minerals and geological hazards e g earthquakes

construction of spar calcite crystals around spores abstract growth of spar calcite crystals in rootcretes from the bluff formation of grand cayman was mediated by spores 1 m diameter through a three stage paragenetic sequence stage i involved calcification of the spore walls and in some cases precipitation of minute anhedral calcite crystals around the spores in stage ii spar calcite grew around the spores which acted as nuclei for precipitation stage iii was marked by the development of overgrowth zones around clusters of spores that occurred along the edges of smaller previously formed crystals precipitation of calcite inside the spores occurred at any stage spore identity was progressively lost as the crystals grew the resultant crystals appear identical to abiogenic crystals because they contain little or no evidence of he spores that mediated their formation

ophiolite genesis and global tectonics geochemical and tectonic fingerprinting of ancient oceanic lithosphere ophiolites and discussions on their origin and significance in earth s history have been instrumental in the formulation testing and establishment of hypotheses and theories in earth sciences the definition tectonic origin and emplacement mechanisms of ophiolites have been the subject of a dynamic and continually evolving concept since the nineteenth century here we present a review of these ideas as well as a new classification of ophiolites incorporating the diversity in their structural architecture and geochemical signatures that results from variations in petrological geochemical and tectonic processes during formation in different geodynamic settings we define ophiolites as suites of temporally and spatially associated ultramafic to felsic rocks related to separate melting episodes and processes of magmatic differentiation in particular tectonic environments their geochemical characteristics internal structure and thickness vary with spreading rate proximity to plumes or trenches mantle temperature mantle fertility and the availability of fluids subduction related ophiolites include suprasubduction zone and volcanic arc types the evolution of which is governed by slab dehydration and accompanying metasomatism of the mantle melting of the subducting sediments and repeated episodes of partial melting of metasomatized peridotites subduction unrelated ophiolites include continental margin mid ocean ridge plume proximal plume distal and trench distal and plume type plume proximal ridge and oceanic plateau ophiolites that generally have mid ocean ridge basalt morb compositions subduction related lithosphere and ophiolites develop during the closure of ocean basins whereas subduction unrelated types evolve during rift drift and seafloor spreading the peak times of ophiolite genesis and emplacement in earth history coincided with collisional events leading to the construction of supercontinents continental breakup and plume related supermagmatic events geochemical and tectonic fingerprinting of phanerozoic ophiolites within the framework of this new ophiolite classification is an effective tool for identification of the geodynamic settings of oceanic crust formation in earth history and it can be extended into precambrian greenstone belts in order to investigate the ways in which oceanic crust formed in the archean

therma volvi gomati complex of the serbo macedonian massif northern greece a middle triassic continental margin ophiolite of neotethyan origin we present field chemical and u pb age data for metamafic ultramafic bodies of the triassic rift related therma volvi gomati tvg complex of the serbo macedonian massif smm in the northern aegean region the tvg rocks record igneous textures overprinted by alpine metamorphism and deformation the high ti volvi body was transformed into amphibolites which also rim the low ti gomati and nea roda bodies the compositions are compatible with normal n and enriched e mid ocean ridge basalt morb and weak ocean island basalt oib signatures of the high ti rocks whereas the low ti rocks are depleted with respect to morb suggesting melt extraction from a mantle source u pb geochronology reveals triassic crystallization of the volvi body at 240 ma ordovician to carboniferous xenocrystic zircons were found in the stratoni body of the tvg complex which were sampled from the smm basement the triassic arnea rift related metagranite intrudes the volvi body comparison of the compositions of the volvi body and the arnea metagranite of the smm with triassic metagranite of the rhodope massif rm supports the presence of a regional bimodal rift suite the location of the tvg complex at the smm rm margin of eurasia confirms its origin as a middle triassic rift suite and its nature as a continental margin ophiolite representing an ocean continent transition of the propagating neotethys supplementary material whole rock chemical analyses and u pb geochronology analytical results data tables are available at https doi org 10 6084 m9 figshare c 4087022 thematic collection this article is part of the tethyan ophiolites and tethyan seaways collection9 available at https www lyellcollection org cc tethyan ophiolites and tethyan seaways

calcic paleosols of the plio pleistocene camp rice and palomas formations southern rio grande rift usa abstract seventy four separate paleosols are recognized in the plio pleistocene camp rice and palomas formations of the southern rio grande rift seven types of profiles are present a bw bk c a bt bk c bw bk c bt bk c bt k c bk c and ak calcic and petrocalcic horizons bk and k commonly overlain by argillic horizons bt dominate most paleosol profiles greater pore size and enhanced permeability typical of sandy and silty parent material favored the development of argillic horizons however micrite is the dominant form of calcium carbonate in bk and k horizons regardless of parent material grain size non calcified a horizons are typically lighter in color than adjacent subsurface pedogenic intervals due to their oxidized nature or lower clay content they are only rarely preserved probably due to erosion prior to final burial calcified root mats ak are not directly associated with other pedogenic horizons abundant horizontal roots suggest root mats formed under high water table conditions restriction of ak horizons to the distal piedmont lithofacies supports their development as spring or pond deposits near the toes of alluvial fans individual pedogenic horizons can often be traced hundreds of meters to kilometers normal and parallel to a basin s axis paleosol maturity and preservation potential are greatest for symmetrical basins especially where pedogenesis was far removed from active channels within the axial fluvial system or within distal piedmont settings of asymmetrical basins

paleosol classification problems and solutions abstract paleosols are soils that formed on landscapes of the geologic past three kinds exist buried exhumed and relict to help reconstruct paleoenvironments and for ease of comparison we suggest a property based classification system linked to genetic processes we use enduring properties because alteration of paleosols following burial is common morphological properties such as horizonation soil fabric root and worm casts and redoximorphic features are resistant to alteration and thus are valuable as criteria field observable and micromorphological properties degree of weathering and proportion of resistant minerals are also useful as criteria for paleosol orders total chemical analysis provides a proxy measure for base saturation and clay mineralogy we use proxy criteria to help classify paleosols that have changed markedly or have been lithified during or after burial to an earlier version of the system we add two new orders and include buried relict lithified and exhumed units at the suborder level our system clearly separates paleosol taxon names from those of all ground soils we use the prescript paleo at the order level and kryptic to designate the buried enduric to designate the relict lithic to designate the lithified and emergent to designate the exhumed paleosol suborders we use prescript modifiers to describe the physical characteristics of the paleosols and postscripts for parent material origin and the extensiveness of the paleosol landscape we present data and classify a number of paleosols as examples of the system

overlapping of pedogenesis and meteoric diagenesis in distal alluvial and shallow lacustrine deposits in the madrid miocene basin spain the miocene distal alluvial fan and palustrine deposits of the madrid basin paracuellos de jarama area were examined to establish the sequence of its pedogenic diagenetic processes and the main factors controlling them in this area the diagenetic processes operated not only on carbonates as commonly studied but also in high magnesium clays and opaline cherts this paper provides a dynamic model for saline alkaline lake margins that complements the existing freshwater palustrine model three sections ber pel and upc containing limestones sepiolite dolostones and opaline cherts were examined these represent the entire transition from alluvial fan deposit to lake environment the alluvial fan deposits pel section show the most complex sequence of processes after a weak edaphic imprint the primary mudstone and siltstones are affected by silicification opaline levels formed mainly by selective replacement of sepiolites and later dolomitization several types of dolomite are present rounded crystals spherules and globular bodies being the most characteristic in the ponds situated at the foot of the alluvial fans or in interfan areas sepiolite precipitated within free water bodies ber section and an intense pedogenesis led to the formation of palustrine sepiolite deposits later the intense silicification of the sepiolite produced lenticular opaline levels that were locally transformed to quartz by aging in the shallow lakes upc section palustrine limestones and sepiolite were deposited the carbonates are partially affected by silicification the sepiolite becoming completely silicified the opaline levels clearly preserve the structure of the limestones and sepiolite host rocks all the opaline levels show local aging processes the silicification processes were different in the three settings due to the interplay of groundwater with sedimentary environments and facies in the distal alluvial fan sediments of the pel section the initial host rock affected by silicification is not preserved due to the later dolomitization that erased both it and its textures the silicification environment was therefore not easy to determine in the ber and upc section the main silicification took place in groundwater influenced environments but evidence was seen of cements and silicifications in vadose environments in all study settings the overlapping of pedogenic vadose and groundwater processes was controlled by changes in the position of the water table four stages were concluded to explain the transformation from the primary deposits to the later dolomitization process and the formation of vadose cements the majority of the silicifications seen occurred as responses to changes in the water table position linked to the aggradation of this closed basin in all the studied environments the sepiolite deposits were preferentially silicified their fibrous structure and high absorption capacity helping to retain interstitial fluids moreover sepiolite and opal are stable under relatively similar geochemical conditions in arid environments the dissolution of the sepiolite caused an increase in silica in the interstitial fluids favouring silicification and ruling out the need of a biogenic source the replacement of limestones by silica is usually driven by variations in ph and salinity which are very common in surface environments such as shallow lakes and soils

a classification of rhizogenic root formed calcretes with examples from the upper jurassic lower cretaceous of spain and upper cretaceous of southern france reply abstract studies of fluvial sequences from the upper jurassic and lower cretaceous of the cameros basin spain and the upper cretaceous of the aquitaine basin france have revealed large numbers of calcrete profiles which have formed wholly or largely through the action of plant root calcification those from spain have formed by the calcification of root mats whereas those from france resulted from the accumulation of calcified root cells microcodium these root related rhizogenic calcretes developed in stages allowing developmental time series chronosequences to be identified contrasting with existing models for calcrete profile growth they provide unique examples where vegetation has been the dominant control on profile development and probably indicate specific vegetation types and palaeoecological settings the role of root activity in forming some quaternary calcretes is also discussed

multiple generations of extensional detachments in the rhodope mountains northern greece evidence of episodic exhumation of high pressure rocks abstract an integrated structural and petrological study shows that exhumation of high rocks of the rhodope occurred in several pulses the structurally uppermost kimi complex recording an early cretaceous high p metamorphism was exhumed between about 65 and 42 ma the sidironero kardamos and kechros complexes that record early tertiary high metamorphism at least 19 kbar at 700 c were exhumed at 42 30 ma exhumation occurred with isothermal decompression strain episodes depict thrusting of medium high grade above lower grade high p rocks syn thrusting extension and post thrusting extension polyphase extension created several generations of detachment zones which in sum excise about 20 km of material within the crustal profile this reduction in crustal thickness is consistent with a reduction of the present crustal thickness from more than 40 km to less than 30 km in the eastern rhodope we mapped the post thrusting xanthi low angle detachment system over 100km from its break up zone above the sidironero complex central rhodope into the eastern rhodope this detachment shows an overall ese dip with a ramp and flat geometry cutting across the earlier thrust structures the kimi complex is the hanging wall of all synand post thrusting extensional systems on top of the kimi complex marine basins were formed from the lutetian c 48 43 ma through the oligocene during extension successively at c 26 to 8 ma the thasos pangeon metamorphic core complexes were exhumed in these times representing the early stages of aegean back arc extension the strymon and thasos detachment systems caused crustal thinning in the western rhodope renewed heating of the lithosphere associated with magmatism and exhumation of hot middle crust from beneath the sidironero complex occurred we focusoxn the geometry timing and kinematics of extension and contraction structures related to the 42 30 ma interval and how these exhumed high p rocks we interpret high rocks exhumed in this interval as a window of the apulian plate beneath the earlier in the cretaceous accreted kimi complex

late proterozoic and silurian basement units within the serbo macedonian massif northern greece the significance of terrane accretion in the hellenides

pedogenic and groundwater processes in a closed miocene basin northern spain abstract the sepulveda ayllon basin sab was an episodically closed miocene basin adjacent to the tertiary duero basin northern spain the sediments studied here represent a depositional system developed from the margin to the centre of the basin by alluvial fan mud flat and saline mud flat ephemeral lacustrine environments an assemblage of authigenic lithotypes was formed in the basin their distribution being influenced by that of the depositional environments within the basin authigenesis took place mainly before sedimentation of the next set of beds in the distal alluuial fan authigenesis of smectite outer part palygorskite inner part and calcrete predominates and the original muddy deposits are still recognizable in the mud flat a more intense transformation of the sediments took place with predominant formation of palygorskite outer part interstitial gypsum calcrete outer part dolocrete inner part and brown silcrete in the saline mud flat ephemeral lake original sediment was extensively disturbed and replaced by magnesian clays sepiolite and magnesian smectite interstitial gypsum dolocrete and cream and white silcrete authigenic alteration within each cycle varied in time and place and was both pedogenic and groundwater related the authigenic assemblages point to a closed basin under evaporation in a seasonally dry climate at the end of this period the sab became connected with the duero basin to the northeast thus losing the endoreic regime consequently the authigenic evaporitic assemblages were then unstable and were partially replaced by a late sparry calcrete

cerium anomalies in lateritic profiles the ree geochemistry and mineralogy have been studied in four lateritic profiles one derived from a syenite akongo sw cameroon the others being developed on a gneissic basement and located along a soil toposequence goyoum e cameroon there is a fractionation between lree and hree in the lateritic samples during weathering the weathered residual products being enriched in lree from la to eu and depleted in hree from gd to lu sampled waters are enriched in hree in relation to the syenite host rock a positive ce anomaly has been found systematically at the top of the saprolite beneath a zone of iron oxide accumulation in the laterite up to 2000 ppm ce may be present in the akongo profile cerianite ceo2 is present as very fine coatings in non ferruginous clayey domains primary ree bearing accessory minerals are weathered at the bottom of the profile specifically allanite and apatite are transformed into florencite and rhabdophane but these phases have no ce anomaly all the data are interpreted as the result of the following processes 1 1 ree leaching in a reducing environment 2 2 oxidation of ce3 to ce4 in an oxidizing environment 3 3 deposition of cerium as cerianite whereas the other ree remain in solution

lateral growth of the continental crust through deep level subduction accretion a re evaluation of central greek rhodope abstract the rhodope massif of northern greece and southern bulgaria has been variably interpreted as a fragment of pre alpine possibly hercynian or precambrian age continental crust that was actively involved in european margin convergent tectonics during the mesozoic and tertiary alternatively we propose that for central greek rhodope the lithological associations and tectonothermal history are consistent with progressive south directed mesozoic crustal growth through deep level subduction accretion a process which involved the near continuous transferral of material of oceanic and continental margin affinity from footwall to hanging wall within a north directed palaeotethyan subduction system central greek rhodope comprises a sequence of schists gneisses and migmatites of wide compositional variation that have undergone intense tectonic intermixing extensive occurrences of marble are associated with amphibolites variably retrogressed eclogites meta gabbros and ultramafic rocks quartzo feldspathic gneisses are abundant throughout the area and are associated with psammitic meta greywacke sequences pelites and quartzites the presence of intercalated marble slivers and thin manganiferous and iron rich siliceous bands is significant the various components exposed in the subduction accretion complex were first underthrust northwards to deep crustal levels subjected to high pressure eclogite facies metamorphism and underplated to the base of the overriding eurasian plate this latter accretion event was characterised by intense southwest directed ductile thrusting and pervasive amphibolite facies metamorphic overprinting it was accompanied by migmatisation and intracrustal melt generation and followed by subduction related magmatism deep level subduction accretion leads to southward growth of the continental margin which in turn leads to subduction retreat allowing the encroachment of the magmatic arc from the north subsequent extensional tectonic exhumation brought the complex up to its present position within the southern margin of eurasia this new interpretation may have implications for other high grade terrains within convergent and collisional systems that have been assumed to represent ancient or pre existing continental crust if so then the importance of deep level subduction accretion as a mechanism of continental crustal growth has hitherto been underestimated

needle fiber calcite a critical review and a proposed classification abstract from a review of the recent literature and our own observations the different morphologies of needle fiber calcite are classified they can be characterized as monocrystalline rods with or without serrated edges or as polycrystalline chains the simplest monocrystalline habit of needle fiber calcite is the individual micro rod m larger monocrystalline rods ma exist in multiples of two more complex forms are found as paired monocrystalline rods with serrated plates along their length mb this group includes three habits which can be distinguished by the number of plates and their orientation relative to the rod finally there is a monocrystalline form mab intermediate between the categories ma and mb which shows the transition from the simple coupled rods to rods with errated plates although polycrystalline needles p have rarely been reported in the literature three types of morphologies have been identified each type of crystal has a specific origin the m micro rods are physicochemical nuclei or calcified bacteria they do not seem to evolve towards larger or more complex morphologies the ma rods are related to biomineralization inside the fungal sheaths the mb rods postdate liberation of ma rods from mycelian bundles the serrated plates are secondary syntactic crystallizations of physicochemical origin whose growth can be inhibited for various reasons the origin of p chains is less well understood but they seem to be physicochemical crystallizations related to evaporation and desiccation therefore these three groups m ma mb and p have similar morphologies but do not seem to be related genetically the only known link is between the ma and mb rods there is no evolutionary s quence of needle fiber calcite habits but a convergence of forms between crystals whose origins are different and independent structures described in the literature as convoluted random cutane and nest like are the result of either evolution of the mycelian bundles or movement of material and organisms in the soil in the case of m micro rods the microfabric is related to the random precipitation around nuclei in pores filled with a highly supersaturated solution initially the p chains also precipitate without preferential organization and later on the growth seems to preferentially constitute chains although the mechanisms are still not understood moreover needle fiber calcites do not indicate a particular climatic environment

classification of paleosols despite increased interest in paleosols during the past decade no satisfactory classification is in current use presented here is a largely descriptive classification system that utilizes those pedogenic features that have the highest preservation potential in the rock record emphasized in the classification are morphological and mineralogical features that are easily recognizable in the field and through the petrographic microscope the classification is based on evaluation of the relative prominence in a paleosol of six pedogenic features or processes organic matter content horizonation redox conditions in situ mineral alteration illuviation of insoluble minerals compounds and accumulation of soluble minerals the most prominent of these six features processes provides the key to classifying a paleosol into one of nine orders four of the order names are borrowed from soil taxonomy histosol spodosol oxisol vertisol whereas the other five order names are presented here for the first time calcisol gypsisol gleysol argillisol protosol the orders may be preceded by one or more subordinate modifiers that describe other important features of the paleosol the classification is relatively easy to apply to the rock record and should enhance communication and aid in the standardization of terminology

the basement of the mount athos peninsula northern greece insights from geochemistry and zircon ages the mount athos peninsula is situated in the south easternmost part of the chalkidiki peninsula in northern greece it belongs to the serbo macedonian massif smm a large basement massif within the internal hellenides the south eastern part of the mount athos peninsula is built by fine grained banded biotite gneisses and migmatites forming a domal structure the southern tip of the peninsula which also comprises mount athos itself is built by limestone marble and low grade metamorphic rocks of the chortiatis unit the northern part and the majority of the western shore of the mount athos peninsula are composed of highly deformed rocks belonging to a tectonic melange termed the athos volvi suture zone avz which separates two major basement units the vertiskos terrane in the west and the kerdillion unit in the east the rock types in this melange range from metasediments marbles and gneisses to amphibolites eclogites and peridotites the gneisses are tectonic slivers of the adjacent basement complexes the melange zone and the gneisses were intruded by granites ierissos ouranoupolis and gregoriou the ouranoupolis intrusion obscures the contact between the melange and the gneisses the granites are only slightly deformed and therefore postdate the accretionary event that assembled the units and created the melange pb pb and u pb shrimp dating of igneous zircons of the gneisses and granites of the eastern athos peninsula in conjunction with geochemical and isotopic analyses are used to put athos into the context of a regional tectonic model the ages form three clusters the basement age is indicated by two samples that yielded permo carboniferous u pb ages of 292 6 2 9 ma and 299 4 3 5 ma the main magmatic event of the granitoids now forming the gneiss dome is dated by pb pb ages between 140 0 2 6 ma and 155 7 5 1 ma with a mean of 144 7 2 4 ma a within error identical age of 146 6 2 3 ma was obtained by the u pb shrimp method this late jurassic age is also known from the kerdillion unit and the rhodope terrane the rather undeformed granites are interpreted as piercing plutons the small granite stocks sampled have late cretaceous to early tertiary ages of 66 8 0 8 ma and 68 0 1 0 ma u pb shrimp 62 8 3 9 ma pb pb the main accretionary event was according to these data in the late jurassic since all younger rocks show little or no deformation the age distribution together with the geochemical and isotopic signature and the lithology indicates that the eastern part of the mount athos peninsula is part of a large scale gneiss dome also building the kerdillion unit of the eastern smm and the rhodope massif this finding extends the area of this dome significantly to the south and indicates that the tectonic boundary between the smm and the rhodope massif lies within the avz

tectonic development of the vardar ocean and its margins evidence from the republic of macedonia and greek macedonia abstract the tectonic development of the northeast margin of the korabi pelagonian continent and the adjacent vardar ocean is discussed based on study of nine traverses across the former northeastern margin of the korabi pelagonian continent represented by the west vardar subzone in the republic of macedonia a detailed correlation of units is also made across the international border with greece coupled with regional comparisons precambrian palaeozoic units were metamorphosed and intruded by granitic rocks followed by mid triassic rifting and the construction of a carbonate platform bordering a late triassic early jurassic ocean neritic deposition was followed by late jurassic syn or pre early tithonian subsidence with radiolarian and then terrigenous turbidite sedimentation overlying ophiolitic rocks are characterised by a relatively thin 200 km n s sheet of sheared blocky serpentinite mainly mantle harzburgite associated with ophiolite derived debris flows the emplaced ophiolitic rocks were subaerially weathered to form local fe ni accumulations following marine transgression a shallow water carbonate siliciclastic deposits including coralgal reefs accumulated during late tithonian early cretaceous after a hiatus mixed terrigenous carbonate gravity flows accumulated in deep water during the late cretaceous the vardar ocean opened during the late triassic early jurassic separating the korabi pelagonian and serbo macedonian continents northeastward subduction created a late jurassic magmatic arc along the southern margin of the serbo macedonian continent while future ophiolites formed by supra subduction zone spreading within the vardar ocean these ophiolites were emplaced in response to collision of the subduction trench with the korabi pelagonian continent the pre mesozoic basement detached and subducted undergoing high pressure low temperature metamorphism during the late jurassic tithonian followed by amphibolite greenschist facies metamorphism and exhumation prior to the late cretaceous a much reduced vardar ocean survived into the late cretaceous until this finally closed during the latest cretaceous early cenozoic triggering continental collision and thick skinned folding thrusting of the pelagonian vardar and serbo macedonian zones towards the w sw

the nature of triassic extension related magmatism in greece evidence from nd and pb isotope geochemistry the widespread triassic volcanic rocks of greece dismembered during the hellenide orogeny are used to interpret the nature of triassic rifting four assemblages of volcanic rocks are distinguished on geochemical criteria 1 a predominant subalkaline basalt andesite dacite series with a high proportion of pyroclastic rocks 2 minor shoshonites 3 alkali basalt and 4 morb the stratigraphic and palaeogeographic distribution of these rock types is synthesized new pb and nd isotopic data are used to discriminate between hypotheses suggesting that either subduction or extension was responsible for the triassic volcanism in the subalkaline basalt assemblage e nd is negative with depleted mantle model ages 1 5 ga pb isotopic compositions are mostly close to the very distinctive compositional field of cenozoic extensional rocks of the aegean area with very high 207 pb 204 pb for relatively low 206 pb 204 pb ratios these isotopic data confirm interpretations based on trace elements that subalkaline basalts were predominantly derived from melt depleted peridotite in the sub continental lithospheric mantle as a result of extension small areas of enriched hydrous mantle partially melted to yield shoshonitic magmas nd and pb isotopic compositions of the alkali basalts are quite different from those in other rock types and suggest a himu mantle source component derived from a small plume which also influenced morb compositions distribution of these various rock types is used to constrain palaeogeographic reconstruction of triassic micro continental blocks

characterisation of igneous terranes by zircon dating implications for uhp occurrences and suture identification in the central rhodope northern greece the occurrence of ultra high pressure uhp and high pressure hp relicts associated with oceanic material suggests the presence of a suture zone within the rhodope massif characterisation of the accreted igneous terranes and their relationship with the uhp eclogite occurrences provide new constraints on the location of this suture single zircon evaporation and sensitive high resolution ion microprobe dating of orthogneiss protoliths define two groups of intrusion ages permo carboniferous and late jurassic early cretaceous structurally the late jurassic gneissic complex overthrust a unit with permo carboniferous orthogneisses a melange zone marked by mylonites eclogites amphibolites and uhp micaschists separates these two units we interpret these observations in terms of two distinct igneous terranes the thracia permo carboniferous and rhodope late jurassic terranes separated by the nestos suture and assembled during the closure of an oceanic basin of the tethys geochemically the late jurassic rocks are akin to subduction magmatism possibly the same subduction that caused the uhp metamorphism of metasediments within the melange zone observed uhp hp relicts are restricted to the tectonic contact zone suggesting that a single subduction collison event can explain the occurrences of uhp relicts and eclogites in the central rhodope and that subducted rocks are exhumed only within the nestos suture

petrography and diagenesis of silcrete from kuwait arabian gulf abstract silcretic duricrust is found capping scattered isolated hills in the southern desert plain of kuwait it occurs in genetic association with surface and subsurface calcrete dolocrete and gypcrete deposits two main types of silcrete were found quartzitic silcrete and massive silcrete the first was developed by precipitation of microcrystalline drusy quartz cement in the intergranular pores of sandstones whereas the second type was formed by replacement of calcrete dolocrete and freshwater limestone by microcrystalline quartz silica precipitation took place in two stages in the first primary silicification caused the development of the main silcrete mass which is usually composed of microcrystalline quartz syntaxial quartz overgrowth and quartzine spherulites a second stag of secondary silicification was responsible for the precipitation of length fast chalcedony and megaquartz in cavities the field occurrence and the petrographic characteristics of the silcrete in kuwait provide support for the folk and pittman 1971 hypothesis on the replacement of evaporite and carbonate by quartzine and lutecite calcretization of kuwait group clastic deposits which resulted in complete or partial dissolution of siliciclastic grains and precipitation of carbonate minerals is likely to have provided soluble silica for groundwater a diagenetic model for the studied silcrete is proposed in which both vertical and lateral translocation of soluble silica in groundwater is considered silcrete has played an important role in the landscape development of the southern ar a of kuwait

sedimentation on the distal reaches of the okavango fan botswana and its bearing on calcrete and silcrete ganister formation the distal reaches or flood plain of the okavango alluvial fan of northern botswana are characterized by gently undulating topography with local relief generally less than two meters the entire area is blanketed by eolian sand although the area is semiarid with evapotranspiration exceeding precipitation the area is subject to seasonal flooding by annual influx of floodwater from subtropical angola to the north distributary channels on the flood plain are poorly defined and consist of sinuous depressions lacking normal fluvial characteristics such as levees bars or incision the flood water has very little suspended load higher ground on the flood plain forms islands during the seasonal flood elevated tracts arise by displacive subsurface crystallization of carbonate and silica which is induced by trees that grow on the higher ground sedimentation on lower lying areas occurs by a combination of 1 accumulation of fine clastic material derived from the flood water and phytolithic silica both of which are mixed into the sandy substrate by illuviation and bioturbation and 2 precipitation of fine grained amorphous silica from the groundwater induced by transpiration by aquatic grasses and sedges accumulation of silica in the soil profile produces a proto silcrete this sequence grades laterally into carbonate dominated island soils the ultimate cause of this association is an abundance of water with low suspended load in an environment with a high evapotranspiration rate ganisters in ancient rocks may have a similar origin

calcium in regolith carbonates of central and southern australia its source and implications for the global carbon cycle abstract the widespread regolith carbonates of the australian continent are a potential sink for co 2 we have used sr isotopes to investigate the source of the ca in regolith carbonates that cover approximately 1 6 10 6 km 2 of inland australia 87 sr 86 sr ratios for nearly all the carbonates were in the range 0 7094 to 0 7211 the results show that only about 10 of the ca in regolith carbonates is derived from weathered bedrock with the remaining component being derived from an external marine source the exceptions are the most northerly samples with a range of 0 7270 to 0 7374 in which bedrock content may be as high as 30 the likely ca source areas are sedimentary carbonates located on the continental shelf and calcareous aeolianites that surround much of the southern and western australian coastline winds passing over these carbonates continually rework and transport the material over the australian continent where it has settled and formed the regolith carbonates we see today the impact of this process is that despite the immense area covered australian regolith carbonates do not capture any additional co 2 instead the carbonate is simply being remobilised from one pool marine to another terrestrial

uranium rich opal from the nopal i uranium deposit pena blanca mexico evidence for the uptake and retardation of radionuclides abstract the nopal i uranium deposit of the sierra pena blanca mexico has been the focus of numerous studies because of its economic importance and its use as a natural analog for nuclear waste disposal in volcanic tuff secondary uranyl minerals such as uranophane ca uo2 sio3oh 2 h2o 5 and weeksite k na 2 uo2 2 si5o13 h2o 3 occur in the vadose zone of the deposit and are overgrown by silica glaze these glazes consist mainly of opal a which contains small particles of uraninite uo2 and weeksite close to a fault between brecciated volcanic rocks and welded tuff a greenish silica glaze coats the altered breccia yellow silica glazes from the center of the breccia pipe and from the high grade pile coat uranyl silicates predominantly uranophane and weeksite all silica glazes are strongly zoned with respect to u and ca and the distribution of these elements indicates curved features and spherical particles inside the coatings the concentrations of u and ca correlate in the different zones and both elements inversely correlate with the concentration of si zones within the silica glazes contain u and ca in a 1 1 ratio with maximum concentrations of 0 08 and 0 15 at for the greenish and yellow glazes respectively suggesting trapping of either ca1u1 aqueous species or particles in the colloidal silica x ray photoelectron spectroscopy xps fourier transform infra red spectroscopy ftir and oxygen isotope ratios measured by secondary ion mass spectrometry sims indicate higher u6 u4 ratios higher proportions of si oh groups and lower 18o values for the greenish silica glaze than for the yellow silica glaze these differences in composition reflect increasing brecciation porosity and permeability from the center of the breccia pipe yellow silica glaze toward the fault green silica glaze where the seepage of meteoric water and eh are higher

multi storey calcrete profiles developed during the initial stages of the configuration of the ebro basin s exorrheic fluvial network abstract multi storey calcrete profiles developed in the quaternary on strath terraces of the cinca and alcanadre rivers tributaries of the ebro river in ne spain two calcrete profiles tor 1 and tor 2 near the village of el tormillo show horizons with an arrangement that differs from that of commonly described calcrete profiles significant lateral changes occur in these profiles within a distance of less than 200 m reflecting their pedofacies relationship the tor 1 profile on terrace qt1 the highest and oldest consists of six horizons from bottom to top 1 coarse fluvial gravels 2 mudstones with carbonate nodules 3 a chalky horizon 4 laminar horizons including one peloidal horizon 5 a multi storey horizon formed of at least six minor sequences each of which includes a lower detrital layer a pisolithic horizon and a thin discontinuous laminar horizon these sequences indicate several cycles of brecciation and or reworking and 6 a topmost laminar and brecciated horizon also including reworked pisoliths some 200 m to the north of tor 1 horizon 5 undergoes a lateral change to channel fill deposits the infill of the channels shows a fining upwards sequence ranging from clasts of about 10 cm in diameter to red silts with sparse pebbles all the clasts come from the underlying calcrete horizons laminar horizons are interbedded with the clastic channel deposits the youngest calcrete profiles developed on terrace qt3 of the cinca river and on the qp4 and qp6 mantled pediment levels all show relatively simple profiles composed mostly of lower horizons of coated gravels with thin laminar horizons at the top most of the horizons especially the laminar ones show biogenic features such as alveolar septal structures calcified filaments biofilms spherulites micropores and needle like calcite crystals these features indicate the important role of vegetation in the formation of all the above profiles the interbedding of clastic sediments and pisolithic horizons within the tor 2 profile indicates several stages of stabilisation during profile formation these sequences are an indication of the sedimentation soil formation and reworking processes operating on the soil surface the alternation of these processes is interpreted as the result of climate vegetation changes the channel fills of tor 2 indicate erosion and reworking of the hard laminar calcrete horizon both tor 1 and tor 2 are multi storey profiles reflecting the complex sedimentation erosion pedogenesis relationships at the final stages of the development of its corresponding fluvial terrace the study of these calcretes shows that these supposedly abandoned terraces continue to be active even though the fluvial network is entrenched both the pedofacies relationships and the complexity shown by tor 1 and tor 2 reflect the complex and unstable geomorphic setting in which these profiles developed after the establishment of the exorrheic network less complex calcrete profiles were produced in the lower terraces

jurassic subduction zone tectonics of the rhodope massif in the thrace region ne greece as revealed by new u pb and 40ar 39ar geochronology of the evros ophiolite and high grade basement rocks abstract in the thrace region of ne greece the crystallization and post solidification cooling below 600 c of the intrusive suite of the circum rhodope belt arc related evros ophiolite span from 176 4 0 93 to 163 5 3 85 ma in the underlying rhodope high grade metamorphic basement ultra high pressure metamafic rocks are cross cut by orthogneisses with granitoid protoliths and crystallization ages between 160 0 69 and 154 1 5 ma this new u pb la icp ms zircon geochronological data provides evidence that the timing of the evolution of the evros ophiolite coincides within analytical error with granitoid magmatism in the underlying high grade basement one of the dated metagranitoid rocks cross cuts amphibolitized eclogite indicating that the high pressure conditions have a maximum age of 160 ma postdating one of the proposed ages of ultra high pressure conditions in the rhodope massif the currently earliest recognised rhodope high ultra high pressure metamorphic event is therefore related to the subduction setting which formed the evros ophiolite based on these new temporal constraints and the regional tectonic geochronologic framework we discuss and update the geodynamic context connected to the jurassic subduction collisional setting at the rhodope continental margin of eurasia

timing of tectonic emplacement of the ophiolites and terrane paleogeography in the hellenides the timing of tectonic emplacement of the ophiolites is analyzed in the four oceanic terranes of the hellenides h2 h4 h6 h8 the criteria for this analysis are based on a the post emplacement sedimentary cover or intrusive rocks b the syn emplacement tectonostratigraphic formations and c the youngest rocks involved in the structure of the autochthon and the allochthon unit in each case the timing becomes younger towards the more external tectonic units of the hellenides with i late eocene oligocene age in the external ophiolite belt of the pindos cyclades oceanic terrane h2 ii late jurassic early cretaceous age in the internal ophiolite belt of the vardar axios oceanic terrane h4 iii post liassic pre late jurassic age in the ophiolites of lesvos circum rhodope oceanic terrane h6 and iv pre late jurassic age in the ophiolites of volvi eastern rhodope terrane h8 an ophiolite obduction model can be applied with the ophiolitic nappes always emplaced on top of pre alpine continental terranes with mesozoic shallow water carbonate platforms the geometry of the continental terranes drifting during the mesozoic within the tethys ocean controls the number and dimensions of the tethyan oceanic basins where a continental terrane dies out the two adjacent oceanic basins merge into one larger basin this seems to be the case of the pelagonian terrane h3 which is terminated north of skopje where the pindos oceanic basin h2 merges with the vardar axios oceanic basin h4

multiple subduction cycles in the alpine orogeny as recorded in single zircon crystals rhodope zone greece abstract high and ultrahigh pressure metamorphic crustal rocks in orogenic belts provide evidence for subduction into the mantle and subsequent exhumation the timing of metamorphism s of complex high and ultrahigh pressure rocks can be registered in the robust mineral zircon able to preserve different growth generations here we present sensitive high resolution ion microprobe shrimp u pb age and ree compositional data from zircon in migmatitic gneisses from the ultrahigh pressure kimi unit of the alpine rhodope zone greece single zircon crystals preserve one to two magmatic one inherited and one syn magmatic and two to three metamorphic generations the 206pb 238u ages combined with ree zircon data mineral inclusions in metamorphic zircon petrological data from the matrix assemblages as well as pseudosection calculations are interpreted to reflect permian crystallization of the protolith of the studied gneisses and subsequent repeated subductions to high pressures over the course of the alpine orogeny at 158 ma suggested to be close to the time of ultrahigh pressure metamorphism at 74 ma and possibly also at 42 ma this inference stands in contrast to the general thinking about subduction exhumation cycles during alpine type orogenesis along a single path and is in line with recent simulation results about repeated subductions the recognition of two distinct high pressure subduction cycles within the alpine orogeny in a time frame of 80 ma and the indication of an additional subduction cycle 30 ma later in the same zircon crystal is remarkable it provides evidence for repeated deep subductions over longer 80 ma and possibly also shorter 30 ma frequencies and emphasizes the view that lithospheric plate collision subduction during the alpine orogeny in the rhodope zone does not have to be a continuous protracted process a regime of repeated subduction exhumation events involving micro continental fragments intervening between the european and adriatic margins may be rather responsible for the different u hp age clusters in the rhodope zone

origin of the early middle devonian magmatism in the sakarya zone nw turkey geochronology geochemistry and isotope systematics abstract a unique example of isotopically dated devonian metagranitoid the camlik metagranite okay et al 1996 crops out in the biga peninsula nw turkey although its contact relationships with the country rocks and geodynamic setting have remained to be enigmatic so far our field work however has shown that a number of metagranitoid bodies similar to the camlik metagranite intruded the country rocks and developed contact metamorphic zones consisting of andalusite and calcsilicate hornfelses garnet epidote and diopsite wollastonite skarns the country rocks of these metagranitoids are made up of regionally metamorphosed metaclastic successions with subordinate metacarbonate metachert metabasites the kalabak formation intercalated with tectonic slices of meta serpentinites the metagranitoids and the kalabak formation are collectively termed here the havran unit which forms a ne sw trending 20 km wide and 80 km long belt in the biga peninsula the havran unit is unconformably overlain by the late triassic shallow marine sediments and is in tectonic contact with the permo triassic karakaya complex interpreted as the palaeotethyan subduction accretion complex u pb shrimp ii and la icp ms dating of the zircons from four individual plutons yielded crystallization ages ranging from 389 1 2 6 to 401 5 4 8 ma i e early to middle devonian based on their geochemical characteristics the metagranitoids are divided into two groups named the camlik and yolindi metagranitoids both granitoids display distinct subduction signature and plot in the volcanic arc granite and post collisional granite fields on tectonic discrimination diagrams 87 sr 86 sr t values of these intrusions vary between 0 707367 and 0 715588 while their 143 nd 144 nd t values range from 0 51166 to 0 51187 very low values of end t from 5 3 to 9 1 imply that the lower to mid devonian metagranitoids in the havran unit might have been generated by partial melting of lower continental crust probably driven by a major magma underplating event delamination and or slab breakoff models are considered to be the most plausible mechanisms for the genesis of these metagranitoids the data obtained in this study has shown for the first time the widespread existence of devonian and older basement in nw turkey regional geological correlations imply that this basement may be an exotic terrane in the sakarya zone emplaced during the late triassic prior to its amalgamation with the palaeotethyan karakaya complex

the role of fungal biomineralization in the formation of early carboniferous soil fabrics paleosols in the lower carboniferous limestones of south wales commonly contain needle fibre calcite which is an unusual form of calcite recently shown to form by the calcification of fungal hyphae in present day soils the needle fibre calcite occurs in two associations in the paleosols as coatings on sediment grains and as rhizocretions the former can be compared with the microbial grain coatings of quaternary calcretes the latter represent the sites of fungal coats on roots and are interpreted as probable ectomycorrhizae a symbiotic fungal sheath root association these findings suggest that biomineralization was important in the formation of soil fabrics during the carboniferous as it is in present day soils

penetrative calcretes and their stratigraphic implications multiple calcrete horizons previously considered representative of true subaerial exposures may in fact be false penetrative calcretes related to only one subaerial exposure event individual subaerial exposure periods can produce a true surficial calcrete plus a series of penetrative calcretes although penetrative calcrete horizons are not laterally traceable over long distances they are very similar to surficial calcretes are commonly found at sequence and or lithologic boundaries permeability anomalies and can be easily misinterpreted in core borings as representing individual exposure events recent penetrative calcretes are common on elevated limestone ridges of the caicos islands where roots penetrate downward seeking ground water moisture in glacial times penetrative calcretes may have been present throughout carbonate platforms as ground water levels were much lower penetrative and surficial calcretes can be differentiated by the lower concentrations of insoluble al and fe in penetrative calcretes

mineralogical forms of silica and their sequence of formation in silcretes the paragenesis and the sequence of formation of silica polymorphs have been studied in three examples of silicification in the regolith of the paris basin and the central massif of france the initial silica deposits are most frequently opal with varying degrees of structural organization subsequently recrystallization produced varieties of progressively better organized quartz namely fibrous quartz chalcedony and lutecite which developed from deposits of microlaminated opal and microcrystalline quartz which formed from nodular opal however even during the formation of microcrystalline quartz facies dissolution and recrystallization processes continued resulting in progressively bigger quartz crystals in the final stages of silcrete development hyaline quartz crystals grew in voids these mineral sequences are determined by the solubility of each of the component parts solubility is controlled by the purity and the degree of order of the crystalline structure the most disordered minerals are the most soluble the degree of order is controlled in part by foreign ions incorporated into the lattice but also by the degree of supersaturation of silica in solution in the system which in turn controls the rate of formation of nuclei dissolution and recrystallization processes are dependent on the percolation of dilute solutions through the silcretes the concentrations of silica and other cations in these solutions is a function of the solubility of the silica phases in the host material and of the rate of percolation thus the porosity of the host material is important rapid percolation favors dissolution but in zones with slow percolation equilibrium is attained and no transformation of existing silica phases occurs it is in the zones of intermediate porosity that progressive recrystallization of quartz occurs the effect of rate of percolation on dissolution and recrystallization explains the vertical distribution of different silica polymorphs observed in numerous silcrete duricrusts

microcodium revisited root calcification products of terrestrial plants on carbonate rich substrates abstract microcodium is a problematic calcitic microfeature of many calcretes and calcareous paleosols in the cretaceous and tertiary continental and marine successions of the peri tethyan realm the main controversy about the microcodium structures is their origin and possible relation with calcified plant roots microcodium and rhizogenic root formed microfabrics were studied in calcrete profiles within the paleocene shallow marine carbonate succession in southwestern slovenia the prominent laminar calcrete horizons contain abundant calcite aggregates 150 m to 1 cm in size with perfectly preserved structural details of plant root tissues morphology and structure of these aggregates indicate that they formed through biologically controlled precipitation of calcium carbonate within the root cortical cells the morphologies intermediate between the typical microcodium aggregates composed of a single layer of individual elongate pyramidal or prismatic crystals of calcite measuring 100 500 m in length and 20 70 m in width and calcified roots with multilayer arrangement of isodiametric cells were observed and this supports previous rhizogenic interpretations of microcodium structures intermediate forms show that the typical microcodium aggregates formed through morphological transformation of the root tissue by growth of the calcite within the cortical cells which distorted the cell shape calcification of roots and the creation of microcodium structures can be explained as an effective nutrient acquiring mechanism used by certain types of terrestrial plants inhabiting nutrient poor calcareous substrates the widespread occurrence of microcodium in almost unaltered shallow marine limestones indicates that its formation took place during early stages of paleosol development probably reflecting specific types of vascular plants of a pioneer community that were able to colonize carbonate substrates during the early phases of subaerial exposure

unravelling calcrete environmental controls in volcanic islands gran canaria island spain abstract volcanic islands are unique contexts to analyze the controls on calcrete development in settings with varied relief climate and vegetation and lacking carbonate host rocks this paper discusses the formation of quaternary calcrete profiles in gran canaria volcanic island under direct influence of the sal saharian air layer calcrete profiles are multi storey and composed of prismatic massive laminar pisolithic ooid bearing sandy mudstones with rhizoliths and brecciated horizons host rocks include basalts siliceous mudstones volcanic sands and gravels and bioclastic rich sands carbonate is mostly calcite but some dolomite occurs in massive horizons the main microfabrics include laminated micrite fine crystalline dolomite desiccated micrite micrite clay groundmass oriented mg rich clays peloids spherulites and coated grains 13c varies from 3 26 to 9 18 vpdb and 18o from 0 86 to 3 24 vpdb the 87sr 86sr ratios are between 0 707504 and 0 708860 indicating that calcium was mostly supplied from the caco3 rich aeolian dust coming from the sahara desert calcrete profiles are mostly pedogenic with roots and microorganisms enabling carbonate precipitation within the soils the correlation trend of 13c and 18o points to a climate control on precipitation with the heavier isotope values in the horizons containing dolomite sedimentation rates interplayed with calcrete formation processes and erosion to outline the profiles horizons amalgamated laminar horizons formed when the rate of calcrete formation was higher than sedimentation and erosion whereas areas or periods with relatively higher sedimentation allowed the separation of calcrete horizons the direction of the winds supplying aeolian dust and the varied relief vegetation and climate favored better development of calcretes in the relatively arid and lowland areas of the eastern side of the island our study shows that calcretes contain the record of the paleoenvironmental controls that operated in volcanic islands with no previous carbonate substrates

lower miocene gypsum palaeokarst in the madrid basin central spain dissolution diagenesis morphological relics and karst end products the miocene sedimentary record of the madrid basin displays several examples of palaeokarstic surfaces sculpted within evaporite formations one of these palaeokarstic surfaces represents the boundary between two main lithostratigraphic units the miocene lower and intermediate units of the madrid basin the palaeokarst formed in lacustrine gypsum deposits of aragonian age and corresponds to a surface palaeokarst epikarst further buried by terrigenous deposits of the overlying unit karst features are recognized up to 5ae5 m beneath the gypsum surface exokarst and endokarst zones are distinguished by the spatial distribution of solution features i e karren dolines pits conduits and caves and collapse breccias sedimentary fills and alteration of the original gypsum across the karst profiles the development of the gypsum palaeokarst began after drying out of a saline lake basin as supported by recognition of root tubes later converted to cylindrical and funnel shaped pits at the top of the karstic profiles the existence of a shallow water table along with low hydraulic gradients was the main factor controlling the karst evolution and explains the limited depth reached by both exokarst and endokarst features synsedimentary fill of the karst system by roughly laminated to massive clay mudstone with subordinate carbonate and clastic gypsum reflects a punctuated sedimentation regime probably related to episodic heavy rainfalls typical of arid to semi arid climates duration of karstification is of the order of several thousands of years which is consistent with previous statements that gypsum karstification can develop rapidly over geologically short time

initial stages of laminar calcrete formation by roots examples from the neogene of central spain thin carbonate laminae formed by root activity are common within miocene detrital deposits of the duero and madrid basins the laminae are about 3 cm thick extending several metres laterally and displace the original detrital sediment which ranges in grain size from fine gravel to sandy clay the thickness morphology microstructure and stable isotope compositions of the laminae indicate that they formed by the activities of roots and associated microorganisms within the soil the laminae are similar to those commonly recognised in thick laminar calcrete profiles three types of laminae are recognised differences in the micromorphology of the laminae are explained as reflecting the different organisms involved and whether calcification occurred when the root was alive or decaying the first type occurs in a profile at paracuellos the jarama where the laminae consist of a mosaic of 20 mm calcite crystals whose arrangement indicates that calcification took place in the medulla of the root and probably occurred while the plant was alive in a second type in the villacadima profile laminae comprise calcified root mats whose formation indicates the interplay of roots and fungi calcification of the cell walls and intracellular spaces took place in the cortices of the roots and not in the medullas as revealed by the presence of central pores in every calcified root a third type of lamina is recognised in profiles at vin egra de moran a and consists of mucilaginous sheets coated by needle fibre calcite crystals the occurrence of associated small root casts indicates that formation of the laminae occurred while the root was decaying and was also influenced by fungal activity these laminae occur within poorly developed soils and their formation was controlled by the relationship between sedimentation erosion and soil formation processes thus the occurrence of these laminae interbedded with detrital sediments reflects environments where sedimentation was relatively low and episodic so after the detrital sediment input surfaces were stable and root mats were able to develop renewed sedimentation accounted for the death of the root mats and the development of new ones on the new surfaces where the sedimentation rate is lower the laminae tend to amalgamate and thicker laminar calcrete profiles form with little or no interbedded detrital sediment

serbo macedonian revisited a silurian basement terrane from northern gondwana in the internal hellenides greece abstract new geochronological and geochemical data on basement orthogneisses from the vertiskos unit of the serbo macedonian massif smm internal hellenides northern greece are used in order to constrain the pre alpine tectonic history of the basement units in the metamorphic hinterland of the hellenides the prevailing rock types in the vertiskos crystalline basement are coarse grained biotite augengneisses with subordinate leucocratic muscovite gneisses and two mica gneisses zircon pb pb and u pb ages on 20 samples range from 425 9 4 2 ma to 443 4 5 5 ma with a mean of 432 2 3 2 ma and are interpreted as primary crystallisation ages of the basement granites on the basis of the magmatic internal structure of the zircon grains trace element and isotope geochemistry of the gneisses show that they originated in a magmatic arc setting but also contain material from pre existing continental crust the rocks are slightly peraluminous and some leucocratic gneisses are strongly depleted in incompatible trace elements the zircon ages document an early silurian magmatic phase in the internal hellenides the association of lithologies and ages is distinctly different from that of the adjacent basement massifs this difference in basement provenance and the fact that the vertiskos unit is bordered by ophiolitic material both to the west and to the east leads to the conclusion that this part of the smm is an exotic terrane of northern gondwanan origin which was finally accreted to its present position during the alpine orogeny the vertiskos terrane may have been part of the hun superterrane which evolved at the northern active continental margin of gondwana in the early palaeozoic and rifted away from it during the opening of the rheic ocean in the cambrian to silurian parts of this superterrane such as the vertiskos terrane are present throughout the variscan and alpine orogens

sources of strontium and calcium in desert soil and calcrete the carbon cycle significance of soil carbonate fluxes is subject to large uncertainties because it is not clear precisely how much calcium is derived from atmospheric sources compared with that from the chemical weathering of silicate minerals in the petrocalcic horizon calcrete of a pleistocene soil from the usda scs desert project area near las cruces nm approximately 1 5 g ca cm 3 has been added with an associated expansion of the profile of200 strontium isotope values for the labile cations and carbonate from the a b and k soil horizons have 87 sr 86 sr values that range from 0 7087 to 0 7093 similar to the values for easily soluble local dust and rain the parent material non calcareous camp rice alluvial sediment has a 87 sr 86 sr ratio of0 7165 mixing calculations indicate a minimum atmospheric contribution to soil carbonate calcium of 94 the more likely scenarios indicate at least 98 of the ca originated from atmospheric input the variations in 87 sr 86 sr ratios of soil silicate 0 7131 to 0 7173 are consistent with weathering of volcanogenic sediments and neoformation of clay minerals in the petrocalcic horizon moreover the sr isotope data suggest that 50 70 of silicate in the uppermost 25 cm of the profile could be atmosphere derived the isotopic composition of labile strontium in the a horizon and the mass distribution of silicon and calcium indicate that the uppermost portion of the profile is the present zone for the release of cations due to silicate weathering steady state models of the whole profile yield a sr weathering flux ranging from200 to 400 m gc m 2 ma 1 the results indicate that both the present day and long term contribution of calcium from silicate weathering is less than 2 of that supplied from the atmosphere and confirm that desert soil formation is not a significant sink for atmospheric carbon 1999 elsevier science b v all rights reserved

late cenozoic extension of the alpine collisional orogen northeastern greece origin of the north aegean basin the rhodope metamorphic core complex exposed beneath the strymon valley detachment in northeastern greece comprises a platform carbonate sequence 5000 m thick intruded by tertiary calc alkaline plutons the final thickening of the north aegean alpine collisional orogen in paleogene time and its neogene quaternary extensional dismemberment in the backarc of the hellenic subduction zone produced structures within and above the rhodope core complex that are related here to four successive deformations in early middle eocene time d1 the falakron marble series was subducted northeastward beneath the serbo macedonian west thracian gneiss complex a heterogeneous ophiolite bearing high grade metamorphic terrane that was accreted to southeastern europe in cretaceous time the orogen began to extend on a northeast southwest axis in earliest miocene time evidenced in the rhodope core complex by the emplacement of the symvolon granodiorite ca 21 ma within a northwest trending midcrustal coaxial rupture of the falakron slab d2 the strymon valley detachment system succeeded the d2 symvolon rupture and related structures facilitating unroofing of the core complex and a transition from ductile to brittle deformation ca 16 3 5 ma d3 the serbo macedonian gneiss complex the island of thasos and a supradetachment basin were translated relatively southwestward as much as 80 km in the d3 hanging wall balanced reconstructions of d2 and d3 displacements predicate two new hypotheses concerning the cenozoic tectonic evolution of northeastern greece first the southwest vergent thasos detachment and northeast vergent low angle normal faults in the olympos region may both have rooted in the coaxial symvolon rupture zone to form a bivergent early miocene extensional system second the olympos and falakron carbonate platforms may be correlative remnants of the subducted eastern margin of the apulian microcontinent implying that the vardar zone the putative alpine suture is a rootless ophiolite belt and that segments of the alpine suture are actually exposed over a zone as wide as 200 km including the rhodope province cumulative d2 and d3 stretching of greater than or equal to 100 created the north aegean basin the north anatolian fault which accommodates the westward escape of anatolia from the pontide suture propagated into the north aegean region in late pliocene time its offshore continuation the north aegean trough transfers dextral strike slip displacement into extension to the north principally expressed in northern greece by the thermaikos strymon and drama half grabens d4

blueschist facies overprint of late triassic tethyan oceanic crust in a subduction accretion complex in north central anatolia turkey the late cretaceous accretionary complex along the izmir ankara erzincan suture zone northern turkey includes various types of metamorphic rock together with radiolarite sandstone mudstone serpentinite basalt and limestone meta plagiogranite blocks up to 5 m in diameter and a meta gabbro slice 800 500 m cross cut by meta plagiogranite are observed in a matrix of serpentinite mudstone and radiolarian chert these meta gabbros and meta plagiogranites show subduction related geochemical characteristics laser ablation inductively coupled plasma mass spectrometry u pb dating of zircons from two meta plagiogranites yielded 222 3 1 5 and 227 2 1 6 ma crystallization ages this study shows that the meta gabbro and meta plagiogranite from the accretionary complex are remnants of the metamorphosed equivalents of the late triassic lower crustal rocks of the subducted tethyan oceanic crust they are the oldest lower crustal rocks observed in the izmir ankara erzincan suture zone both the meta gabbros and meta plagiogranites have na ca amphiboles indicating blueschist facies metamorphism however phengites from a blueschist facies rock yielded an 40 ar 39 ar age of 104 3 0 7 ma indicating that blueschist metamorphism occurred during the albian all these data suggest that the oceanic crust formed during the norian metamorphosed during the jurassic to early cretaceous and then dismembered during accretionary complex formation in the late cretaceous supplementary material a complete description of the analytical methods electron microprobe la icp ms zircon u pb and 40 ar 39 ar age analyses pseudo section modelling and thermocalc are available at https doi org 10 6084 m9 figshare 6509984

triassic rift related meta granites in the internal hellenides greece the serbo macedonian massif is a basement complex in the internal hellenides of northern greece situated between the vardar zone to the west and the rhodope massif to the east the serbo macedonian massif comprises several distinct basement units interpreted as terranes the largest of which is the gondwana derived vertiskos terrane in the northwestern and central parts of the massif a series of leucocratic meta granites intrude the silurian orthogneiss basement of the vertiskos terrane no similar granites are found in any of the other units of the internal hellenides the meta granites have a pronounced crustal within plate signature which is visible in lithology major and trace element geochemistry and the sr isotopic compositions these intrusions were dated using the pb pb single zircon evaporation method and yielded a triassic age of between 240 7 2 6 ma and 221 7 1 9 ma on 17 samples with a mean age of 228 3 5 6 ma the zircons are purely magmatic indicating that ages are primary crystallization ages a rb sr errorchron of the whole rock samples of the arnea granite yielded an age of 231 6 9 9 ma mswd 82 and a mean 87 sr 86 sr initial ratio is 0 7142 indicating a crust dominated source and suggesting an a type origin for the granites the a type meta granites together with mafic intrusive bodies amphibolites in the vertiskos terrane may be evidence of triassic rifting that led to the formation of a branch of neotethys vardar meliata ocean similar rock associations are also exposed in the cyclades and in massifs of the wider eastern mediterranean realm related to the gondwana derived hun terrane indicating that the arnea type granites are representatives of a major regional rifting event in triassic times

ground water silicification and leaching in sands example of the fontainebleau sand oligocene in the paris basin in the fontainebleau sand oligocene superposed lenses of quartzite are restricted to outcrops and structurally low zones of the formation the quartzite lenses result from a silicification process controlled by water table levels during the recent geomorphologic evolution of the landscape the quartzite lenses pinch out rapidly under the limestone capped plateaus and are related to the development of a bleached sand profile above the water table the sand is bleached and the former quartzite lenses are corroded along their margins moreover flint pebbles are dissolved and primary smectite and illite are weathered to kaolinite dissolved silica can thus reach supersaturated levels allowing quartz precipitation quartz grains are overgrown at the base of the profile in zones of ground water outflow related to spring lines the adjustment of the water table during the landscape incision led to formation of quartzite lenses at deeper levels this silicification process has no climatic significance but is dependent on the stability of the landscape and on its progressive dissection the rate of quartz cementation indicated by geomorphologic considerations is consistent with hydraulic and geochemical calculations it appears to be very rapid approximately 30 000 yr for a quartzite lens

tethyan ophiolites and tethyan seaways tethyan ophiolites have played a major role in the development of the ophiolite concept during the past 200 years and contributed significantly to the formulations of the suprasubduction zone ssz ophiolite paradigm and the plate tectonics theory their melt evolution and magmatic construction witnessed the dispersal of the supercontinent pangaea via continental rifting and seafloor spreading opening and closure of multiple seaways in an eastward widening latitudinal ocean basin neotethys and deep mantle recycling processes through subduction zone tectonics and plume activities neotethyan ophiolites are highly diverse in their crustal mantle structures and compositions pointing to major differences in their melt sources and tectonic settings of magmatic construction the papers in this thematic set demonstrate these differences in the neotethyan ophiolite record through new structural geochemical isotopic and geochronological data and interpretations the jurassic western alpine ophiolites formed in a narrow basin alpine tethys that developed between europe and north africa adria iberia their peridotites represent exhumed continental lithospheric mantle the triassic jurassic and cretaceous ophiolites east of adria formed in different neotethyan seaways and their ssz magmatic construction involved multiple episodes of melting depletion and refertilization of subduction modified oceanic mantle the late cretaceous peri arabian and peri indian ophiolites which are discontinuously exposed along a c 9000 km long belt from sw anatolia to se tibet formed above a trans tethyan subduction accretion system within southern neotethys diachronous collisions of arabia and india with this intraoceanic subduction accretion system in the latest mesozoic and the paleogene respectively resulted in diachronous ophiolite emplacement in the mediterranean and tibetan himalayan orogenic belts thematic collection this article is part of the tethyan ophiolites and tethyan seaways collection available at https www lyellcollection org cc tethyan ophiolites and tethyan seaways

groundwater silicification in paris basin limestones fabrics mechanisms and modeling abstract paris basin tertiary formations contain large deposits of lacustrine limestones the limestones have a low clastic content and many secondary dewatering and pedogenic paludine fabrics indicating deposition in shallow environments these lacustrine limestones commonly contain cherts that crosscut sedimentary structures the silicified zones may be pervasive and retain the structure and dull aspect of the limestone or form irregularly shaped translucent nodules quartz is almost the only silica phase present in the cherts two main types of silicification occur together 1 voids partly or entirely filled with quartz and 2 limestone matrix that has been replaced by microcrystalline quartz with preservation of most of the primary limestone fabric there is a systematic relationship between silicification and high porosity zones the replacement of the limestone matrix by quartz is directly connected to voids infilled with quartz because the limestones are pure without clayey layers the silica must have come from other formations overlying sands and soils and been introduced by groundwater flow in view of the weak solubility of silica in surficial waters substantial groundwater flow is needed to supply the silica precipitated from the solution this explains the observed relationships between voids and silicification a coupled mathematical model reaction transport of this type of silicification was used to characterize the physicochemical conditions and to attempt a quantitative treatment of the phenomenon kinetics seem to be the limiting factor of quartz precipitation in the voids however the modeling shows that the kinetics of quartz precipitation limit the development of the silica replacement whereas the diffusion of the dissolved species from the replacement front towards the voids seems to limit calcite dissolution limestone is replaced by silica without any increase in the porosity if the groundwater is close to equilibrium with calcite the precipitation rate of the quartz depends on the number of quartz nuclei the model predicts that silica deposition and calcite replacement can be completed in about 10 000 to 100 000 years

kinematics of the southern rhodope core complex north greece the southern rhodope core complex is a wide metamorphic dome exhumed in the northern aegean as a result of large scale extension from mid eocene to mid miocene times its roughly triangular shape is bordered on the sw by the jurassic and cretaceous metamorphic units of the serbo macedonian in the chalkidiki peninsula and on the n by the eclogite bearing gneisses of the sideroneron massif the main foliation of metamorphic rocks is flat lying up to 100 km core complex width most rocks display a stretching lineation trending ne sw the kerdylion detachment zone located at the sw controlled the exhumation of the core complex from middle eocene to mid oligocene from late oligocene to mid miocene exhumation is located inside the dome and is accompanied by the emplacement of the synkinematic plutons of vrondou and symvolon since late miocene times extensional basin sediments are deposited on top of the exhumed metamorphic and plutonic rocks and controlled by steep normal faults and flat ramp type structures evidence from thassos island is used to illustrate the sequence of deformation from stacking by thrusting of the metamorphic pile to ductile extension and finally to development of extensional plio pleistocene sedimentary basin paleomagnetic data indicate that the core complex exhumation is controlled by a 30 dextral rotation of the chalkidiki block extensional displacements are restored using a pole of rotation deduced from the curvature of stretching lineation trends at core complex scale it is argued that the rhodope core complex has recorded at least 120 km of extension in the north aegean since the last 40 my

mineralogy and origin of rhizoliths on the margins of saline alkaline lake bogoria kenya rift valley abstract a wide range of rhizoliths occurs around the margins of lake bogoria kenya these include root casts moulds tubules rhizocretions and permineralised root systems these rhizoliths are variably composed of opaline silica calcite zeolites mainly analcime fluorite and possibly fluorapatite either alone or in combinations some rhizoliths are infilled moulds with detrital silicate grains most rhizoliths are in situ showing both vertical and horizontal orientations reworked rhizoliths have been concentrated locally to form dense rhizolites hot spring fluids concentrated by evapotranspiration and capillary evaporation have provided most of the silica for the permineralisation of the plant tissues precipitation involved the growth of silica nanospheres and microspheres that coalesced into homogeneous masses calcite rhizoliths formed following evaporative concentration evapotranspiration and or co2 degassing of ca bearing runoff water that infiltrated the sediment or by mixing of runoff with saline alkaline groundwater fluorite precipitated in areas where mixing of hot spring and meteoric waters occurred or possibly where hot spring fluids came into contact with pre existing calcite zeolitic rhizoliths formed during a prolonged period of aridity when capillary rise and evaporative pumping brought saline alkaline waters into contact with detrital silicate minerals around roots

nodular chert formation in carbonate rocks mapping of carbonate dissolution in nodular chert bearing strata indicates that nodular chert formation in a diverse suite of phanerozoic limestones occurred in bulk pore waters that were at calcite saturation opal ct and quartz supersaturation was achieved in most chert bearing strata by the intraformational dissolution of amorphous silica skeletal material chertification occurred by a combination of force of crystallization controlled replacement of the host carbonate whereby non hydrostatic stresses resulting from opal ct and quartz crystal growth caused calcite dissolution and the isomineralic heterogenous nucleation of new opal ct and quartz crystal at nodule peripheries the restriction of the solution phase at silica carbonate contacts to thin films permitted the preservation of ghosts of micron sized features in the chert heterogeneities in sediment organic matter content porosity and biogenic silica concentrations promote the nucleation of chert nodules in some formations

the maliac ocean the origin of the tethyan hellenic ophiolites the hellenides part of the alpine orogeny in greece are rich in ophiolitic units these ophiolites and associated units emplaced during jurassic obduction testify for the existence of one or several tethyan oceanic realms the paleogeography of these oceanic areas has not been precisely described however all the authors now agree on the presence of a main triassic jurassic ocean on the eastern side of the pelagonian zone vardar domain we consider that this maliac ocean is the most important ocean in greece and albania here we limit the detailed description of the maliac ocean to the pre convergence period of approximately 70 ma between the middle triassic rifting to the middle jurassic convergence period a quick overview on the destiny of the different parts of the maliac ocean during the convergence period is also proposed the studied exposures allow to reconstruct 1 the middle to late triassic maliac oceanic lithosphere corresponding to the early spreading activity at a mid oceanic ridge 2 the western maliac margin widely exposed in the othris and argolis areas 3 the eastern maliac margin in the eastern vardar domain peonias and paikon zones we established the following main characteristics of the maliac ocean 1 the middle triassic rifting marked by a rapid subsidence and volcanism seems to be short lived few my 2 the maliac lithosphere is only represented by middle to late triassic units especially the fourka unit composed of wpb oib and morb pillow lavas locally covered by a pelagic middle triassic to middle jurassic sedimentary cover 3 the western margin is the most complete and our data allow to distinguish a proximal and a deeper distal margin 4 the evolution of the eastern margin peonias and paikon series is similar to that of the w margin except for its jurassic terrigenous sediments while the proximal w margin was dominated by calcarenites 5 we show that the w and e margins are not volcanic passive margins and 6 during the middle jurassic convergence period the eastern margin became an active margin and both margins were affected by obduction processes

tectonic stages along a traverse cross cutting the rhodopian zone greece in a cross section along the rhodopian zone from the greece bulgarian border through the postalpine basins in the xanthi area to thasos island detailed structural mapping and collection of field data enable us to determine a sequence of tectonic stages that affected this area this allows us to propose a model of continental collision during the early eocene ne sw directed movements produced a southwest facing pile of crystalline nappes with internal deformation comprising nw and ne trending isoclinal folds and a ne sw trending stretching lineation after a short time interval nappe movements started again under brittle deformation conditions and produced composite thrust sheets wnw and nne trending chevron folds and thick cataclasite late erogenic uplift and extension caused the formation of nw and ne trending normal faults these faults bounded the oligocene flysch and molasse basins and provided pathways for voluminous magmatic suites this tectonic process was interrupted at the oligocene miocene boundary by a late orogenic compression reflected in nw open folds and dissolution cleavages subsequently differential uplift and subsidence associated with nw and ne trending normal faults has continued up to the present time from the early pliocene new fragmentation and thinning of the crust reflected in wnw listric faults and nne transfer faults has affected the southern parts of the rhodopian zone nappe forming movements are the result of a progressive collisional process involving a kimmerian microcontinent lying between the eurasia and apulia continents in the final stages of this collision uderplating processes led to regional uplift and extension additional extensional movements were related to the formation of the aegean back arc basin caused by the late cenozoic hellenic subduction zone

study of a modern calcrete forming in guadalajara central spain an analogue for ancient root calcretes abstract the pajares calcrete in central spain a modern calcrete consists of an accumulation of powder carbonate around the roots of living trees and bushes which penetrates miocene deposits by more than 3 m calcrete development is mostly vertical and oblique but thinner horizontal calcified root mats also occur carbonate accumulation up to 15 cm thick is made up of micrite with some etched clasts alveolar septal structures calcified rootlets coated grains and micrite grains in detail many of these features are composed of needle fibre calcite and micro rods accumulation of carbonate was produced both by roots through direct calcification of their cells and also by microbial activity within the rhizosphere this case study provides a good example of how carbonate is accumulating around roots at present and may allow better understanding of ancient calcretes especially those which formed directly around roots in direct contact with the host rock in addition the calcrete s setting and its occurrence around roots belonging to various types of plants suggest that carbonate accumulation in modern landscapes may be a relatively rapid common process even in erosive regimes and unrelated to specific vegetation types

evolution of a conjugate passive margin pair in mesozoic southern turkey in the western tauride belt of southern turkey a north pointing cusp isparta angle occurs along the mesozoic carbonate axis and encompasses passive continental margin sequences rift assemblages mantle peridotites an ophiolite complex and platform fragments the contacts between these tectonostratigraphic units are commonly tectonic with the carbonate platforms forming the autochthonous basement to the west and east directed thrust sheets both in the west and the east rift assemblages consist of chert pelagic limestone mudstone sandstone and alkaline lavas ranging in age from upper triassic to upper cretaceous and locally include serpentinite lenses mantle peridotites are commonly harzburgitic cut by numerous doleritic dikes and underlain by a syntectonic melange the ophiolite complex occurs in the south adjacent to and overlying the western platform bey daglari and consits mainly of peridotites and cumulate gabbros with a poorly developed dike complex and rare extrusive rocks platform fragments are represented by north south trending slivers of limestones that have affinity with the western platform they are technically overlain by the rift assemblages and or mantle rocks within the isparta angle rock types and associated structures within the isparta angle indicate a progressive evolution from continental rifting and passive margin development to ocean basin formation in the tauride belt of the neotethyan realm during triassic to late cretaceous time the pronounced asymmetry in the crustal architecture of the conjugate margins suggests that initial stages of rifting and subsequent extension were probably facilitated by crustal failure along an east dipping in present coordinates low angle fault continued rifting and associated crustal thinning resulted in upwelling and emplacement of the mantle beneath the lower plate and in partial melting of this elevated mantle generating alkaline lavas further extension in the south established a spreading system along which cretaceous oceanic crust now represented by the tekirova ophiolite was created contractional deformation in late cretaceous to late tertiary time caused telescoping of rock units within the isparta angle and emplacement of thrust sheets onto the platform carbonates both in the east and the west the opening and closure stages of the ocean basin within the isparta angle were diachronous while the bounding carbonate platforms were probably revolved around a pivoting pole located at or near the apex of this sharp bend in the tauride belt