application of thermochronology to hydrothermal ore deposits thermochronology finds many applications in economic geology utilizing temperature sensitive radiometric dating techniques to reveal low temperature upper crustal processes can elucidate many aspects of deposit genesis including timing and duration of mineralization processes rate of exhumation and erosion of intrusive ore deposits and comparative preservation potential the tools are utilized to best advantage when combined with other thermochronometry techniques that provide complementary information in addition when thermochronometers are combined with geochronometers e g zircon u pb over 800 c of thermal history is revealed from emplacement to erosion fig 1 with the advent of computational algorithms that provide more accurate and detailed models of thermochronology results the economic geologist has a powerful tool to use when assessing economic favorability of a region or prospect this chapter summarizes the various ways that low temperature thermochronometers have been utilized in studies of economic mineralization and primarily focuses on studies of porphyry ore deposits these deposits are well characterized and provide a good platform from which to demonstrate the extended understanding of ore formation processes that is provided by thermochronology studies in this section we review the fundamental application of u th he fission track and 40ar 39ar thermochronometry methods to mineralized systems we then present a synopsis of how these techniques can be used in thermal history analysis either alone or in combination with other chronometers in order to provide more information on deposit genesis the discussion is presented from low through to higher temperature thermochronometers corresponding to their presentation on figure 1 u th he thermochronology as documented earlier in the volume u th he thermochronology is based on measuring the accumulation of radiogenic 4he produced from u and th decay the daughter helium is retained until the mineral is heated to a temperature at which its structure and helium retentivity change this closure temperature varies from

correlation of triassic advanced rifting related neotethyan submarine basaltic volcanism of the darno unit ne hungary with some dinaridic and hellenidic occurrences on the basis of volcanological fluid rock interaction and geochemical characteristics comparative volcanological mineralogical petrological and geochemical studies of blocks of triassic submarine basalt occurrences hosted by the jurassic melange have been carried out the studied localities are located in displaced parts of the dinarides in ne hungary darno unit in the dinarides kalnik mts croatia and vares smreka bosnia and herzegovina and in the hellenides stragopetra greece the common characteristic of the studied occurrences is the well observable result of the lava water saturated sediment mingling i e the presence of the so called carbonate peperitic facies mixing of the basaltic lava with pelagic lime mud representing the unconsolidated stage of the red micritic limestone as well as fluid inclusion and chlorite thermometry data support that the carbonate peperite was formed above ccd and at the bosnian locality a shallower water about 1 4 km depth is proven the igneous rocks show mainly within plate basalt geochemical characteristics morb signatures are not common low temperature 200 c hydrothermal alteration is characteristic to the pillow basalt blocks with peperitic facies the similarities in the volcanological geochemical and textural characteristics observed at the different localities support a strong genetic connection among them the results of this study suggest to the advanced rifting stage origin of the triassic basaltic suits and their distinction from the true oceanic basalt pillow units of the dinarides can be based on the occurrences of the peperite facies

40ar 39ar dating of supergene mn oxides from the zunyi mn deposit guizhou plateau sw china implications for chemical weathering and paleoclimatic evolution since the late miocene 40ar 39ar laser incremental heating analyses of potassium bearing mn oxides from a weathering profile at the zunyi manganese deposit northern guizhou plateau sw china were applied to constrain the timing and rate of chemical weathering in critical zone and to provide insights into paleoclimatic conditions under which the weathering profile and associated mn oxides formed eighteen mn oxide grains from nine samples at three sites in saprolite zone of a single weathering profile yield well defined plateau or pseudo plateau ages ranging from 13 1 0 3 to 0 1 0 4 ma 2 the mn oxide ages are progressively younger toward lower elevations 13 1 0 3 to 11 0 1 8 ma at 951 m 3 69 0 07 to 2 16 0 16 ma at 922 m 1 3 0 2 ma at 914 m yielding an average weathering front propagation rate at 3 3 m ma for the past 13 1 ma the calculated weathering front propagation rate corresponds to a chemical denudation rate of 4 6 0 2 t km2 yr which is consistent with values deduced from mass balance calculation for suspended and dissolved loads of river discharge across the guizhou plateau the mn oxide 40ar 39ar ages suggest that humid climate conducive to intense weathering has prevailed over the guizhou plateau at least since the late miocene the climatic conditions inferred from the weathering geochronology are consistent with the continuous sedimentary and paleontological records from many cenozoic basins over the guizhou plateau confirming that supergene mn oxides can serve as a paleoclimate proxy for the past climatic condition the present 40ar 39ar dating results when combined with previous geochronological data of weathering profiles in the yunnan guizhou plateau and coastal areas of south china document progressively intensive erosion toward areas proximal to the eastern margin of the tibet plateau

preliminary model for an early transform history in central atlantic rifting dextral and sinistral shear along an arcuate intracontinental transform system parallel to the present coastlines of eastern north america and western africa is believed to have played a significant role in the decoupling of continental blocks during the extremely complex tectonic history of central atlantic rifting near pole small circle transform systems are developed as wide zones of shear during counterclockwise and clockwise rotations of africa and south america relative to north america about a pole of rotation in the sahara transform and rifting histories are combined in a model proposed for atlantic rifting that includes the following phases of rotation interacting with the linear belt of pre existing appalachian structural grain 1 late permian to middle triassic counterclockwise rotation of africa with dextral shear localizing an initial thermal arch and early sedimentation in southern appalachian grabens along a reactivated structural grain 2 dominant late triassic to early jurassic extensional reactivation in a northwest southeast direction with minor plate separation producing the main phase of graben sedimentation and a major 190 m y b p basalt sill and flow event 3 middle jurassic clockwise rotation of africa with sinistral shear resulting in the folding and faulting of graben sediments and the injection of basalt dikes 175 m y b p along a distinctive fan shaped distribution of extensional structures and 4 separation of the north american and african continents parallel to early atlantic fracture zone trends with the onset of plate accretion

tectonostratigraphic models of the alpine terranes and subduction history of the hellenides abstract the tectonostratigraphy of the hellenic terranes is related to their paleogeodynamic and paleogeographic evolution which comprises three major stages i continental rifting in the northern margin of gondwana characterised by volcanosedimentary complexes of late paleozoic triassic ii continental drifting and contemporaneous oceanic opening of tethyan basins in between the continental terranes during triassic paleogene with shallow water carbonate platforms on the continental terranes and ophiolite suites interlayered with pelagic sediments within the tethyan basins iii accretion of the tectonostratigraphic terranes with docking along the active european margin characterised by flysch melange sedimentation along the trenches from early jurassic to neogene two tectonostratigraphic models can be distinguished one for the continental terranes carbonate platforms and another for the oceanic basins the duration of each geodynamic stage for each terrane is obtained from the chronology of the tectonostratigraphic facies change the general trend is younger ages observed in the southern terranes and older ages towards the northern terranes the two alternative tectonostratigraphic models are applied in the two groups of terranes and the chronology of the geodynamic paleogeographic stages is estimated the hellenic subducted slab as shown in the seismic tomographic images is correlated to the terrane paleogeography using the estimated widths and the average subduction rates the upper 1700 km of the slab correspond to the three southern more external terranes whereas the remainingpart of the slab may represent the other more internal terranes a palinspastic model of the hellenides is presented taking into account the chronology of each stage for every terrane obduction of ophiolites over the southern platforms is observed in all four oceanic basins blueschist exhumation formation of core complexes and tectonic windows through extensional detachments occur after micro collision of continental terranes platforms and their isostatic uplift within the upper plate after their detachment from the subducting slab

some constraints on geochemical features in the triassic mantle of the easternmost austroalpine southalpine domain evidence from the karawanken pluton carinthia austria

late miocene drying of central australia abstract a back to the future approach to climate change refines neogene records of paleoclimatic cooling and drying as future climate states for a warming world near alcoota station in central australia is a late miocene fossil mammal site for alcoota 10 ma and ongeva 8 ma local faunas the alcoota fauna includes the largest known land bird dromornis stirtoni and early diprotodontid kolopsis torus but the geologically younger ongeva fauna has a larger diprotodontid zygomaturus gilli these faunas are evolutionary predecessors of australian pleistocene megafauna calcareous paleosols recorded a marked turn toward aridity compared with underlying thick lateritic duricrusts which lack fossils laterites of likely middle miocene age are evidence of wet 1100 mm mean annual precipitation tropical 17 c mean annual temperature paleoclimates but these were deeply eroded and redeposited by the time of the late miocene mammals a succession of 19 paleosols of 5 pedotypes were recognized in the 19 m thick section with shallow calcareous nodule bk horizons as evidence of mean annual precipitation ranges from 100 to 400 mm and mean annual range of precipitation of 50 mm very similar to the modern climate of alcoota 296 mm precipitation with 56 mm difference between wet and dry month this was neither a rainforest nor monsoonal paleoclimate and vegetation inferred from paleosols was open woodland and gallery woodland similar to that of today central australia was not covered by monsoon rainforest during the late miocene and its paleoclimatic history of neogene desertification matches well that of other parts of the world

on archeomagnetic secular variation curves and archeomagnetic dating abstract secular variation sv of the earth s magnetic field can be used for dating purposes by comparing archeomagnetic directions of unknown ages with a well dated reference curve in this study we propose a dating technique based on the statistics of mcfadden and mcelhinny geophys j int 103 1990 725 for testing the hypothesis that two fisherian distributions of individual directions share a common mean direction the statistics are adapted to test the degree of compatibility between one individual fisherian mean direction and a reference curve constructed using the bivariate extension of the fisher distribution furthermore as the density of the data which define the archeomagnetic reference curve varies in time we suggest that one computes the mean directions with moving windows of varying duration where both the window widths and the time shifts between successive mean directions are fixed when a minimum threshold density of data is reached within each time interval in our paper we apply this new procedure to the french archeomagnetic data set

fracture zones in the equatorial atlantic and the breakup of western pangea the early breakup of western pangea has been investigated by mapping the pattern of fracture zones and distribution of seismic reflectors within the sedimentary cover of the atlantic between the cape verde islands and the equator two distinct sets of transverse oceanic lineaments are present separated by the guinea fracture zone near lat 10 sup 0 n lineaments to the north are associated with the formation of the central atlantic in the late jurassic and early cretaceous those in the south relate to the cretaceous opening of the south atlantic the guinea fracture zone is thus the conjugate of the jurassic transform boundary under peninsular florida which linked the atlantic with the gulf of mexico the distribution of dated seismic reflectors suggests that deposition of deep water sediments was confined to the region north of the guinea transform until aptian time when the sierra leone basin began to open the latter started to widen at least 15 m y after the initiation of the cape basin off southwest africa an age difference that can be explained if a short lived plate boundary developed in either africa or south america during the early cretaceous neither the trends of the equatorial fracture zones nor the seismic stratigraphymore supports the existence of a predrift gap between west africa and brazil less

reply to d pană s discussion on the eastern carpathians ophiolites romania remnants of a triassic ocean lithos 108 2009 151 171

continental rifting and the implications for plate tectonic reconstructions previous plate tectonic reconstructions have tried to recreate the pre rifting pangea configuration of the continents by matching contours or lineaments that are thought to represent the continental boundaries such reconstructions have the inherent assumptions that no extension occurs within the continent during rifting that the continental boundaries are isochrons and that the continents rift without distortion this paper proposes a model for continental breakup with distortion where ocean basins are formed by propagating rifts this model challenges the assumptions made by previous reconstructions and presents a new method for representing the prerift geometry of the continents as rifts propagate through a continent the region in front of the rift extends by continental faulting and crustal thinning while the region behind the rift expands by seafloor spreading the propagating rift model implies that large amounts of continental extension up to 150 km occur and that the continental boundaries are not isochrons this extension due to rifting results in apparent overlap when the continents are returned to their original configuration because of the gradational nature of this extension the best representation of the pre rift configuration is obtained by matching the initial rifting point and having overlap increase in the direction of rift propagation the overlap is equivalent to the amount of extension that has taken place on both continental edges reconstructions based on this model are presented for the gulf of california the gulf of aden the norwegian greenland sea and the south atlantic ocean they are able to resolve the discrepancies that occur when rigid plate tectonics are applied this model has implications that can be used to predict the age and structure of the continental shelves and the location and orientation of the oldest magnetic isochrons

metallogeny of the northwestern and central dinarides and southern tisia abstract the dinaridic metallogenic province is a part of the alpine himalayan orogenic system developed as a result of opening and closure of the tethys ocean by convergence of the african and eurasian plates the northern boundary of the dinarides is related to the northern african margin adria apulia the tisia mega unit a small continental block positioned between the dinarides and the carpathians is genetically related to the south eurasian edge the geology of the dinarides is constrained by the alpine wilson cycle the major stages of the cycle are a permian early intra continental rifting b triassic advanced rifting c jurassic oceanization d cretaceous subduction e paleogene collision and f neogene post collision and extension followed by orogenic collapse each stage creates characteristic ore deposits related to the specific geological environments stage a bears hydrothermal siderite barite polysulphide deposits epigenetic sedimentary uranium deposits red bed type sabkha type copper and barite deposits and evaporites stage b favored sedex and hydrothermal iron polysulphide barite mercury and mvt deposits stage c developed chromites asbestos talc and magnesite deposits the spatial position of stage d remains poorly constrained the southern tisia unit might be a possible candidate for the tethyan active continental margin with the cretaceous subduction zone positioned beneath absence of voluminous subduction related magmatism and mineral deposits however favors subduction within the vardar zone the easternmost dinarides adjoined to the serbomacedonian ensialic terrain with its large cu porphyry deposits stage e was a prelude to the prolific phase f with its numerous hydrothermal pb zn and sb deposits that mostly occur in the western vardar zone the geology and metallogeny of southern tisia with medium high grade metamorphics i type s type granites resembles the middle austro alpine unit formed during the main carboniferous collisional stage this contribution provides a review of the metallogenic characteristics of the northwestern and central dinarides and southern tisia mega units based on recently gained knowledge on the regional geology petrology and genesis of mineral deposits establishment of the plate tectonic model several decades ago greatly contributed to an integrated interpretation of ore deposit genesis in turn basic research in the field of ore genesis generated new data that can be used to improve the plate tectonic model

ophiolites as indicators of the geodynamic evolution of the tethyan ocean abstract reviewing the ophiolites of the alpine chain from the western mediterranean to the indian ocean we subdivide them into ten groups of outcrops the geological environment through time the internal composition and history are given for each emphasis is put on the variegated and successive geodynamic processes which can be reconstructed for the different ophiolitic belts since their birth as an ocean like crust to their incorporation through accretion to a continent one can distinguish low versus high tholeiitic partial melting fairly or poorly established ridges simple ridge type crust versus complex ridge ensimatic arc and even more complex type long lived versus short lived oceanic crust stages abrupt large scale pure obduction versus progressive collision related obduction or simple progressive juxtaposition to continent the major events through time in the tethyan ophiolitic belts can be listed as follows spreading ridge activity during the jurassic liguria dinaro hellenides lesser caucasus and the cretaceous peri arabic nain sabzewar with a jump of ridge in between repeated change from ridge to ensimatic arc dinaro hellenic lesser caucasus peri arabic west indian during the latest jurassic and cretaceous upper cretaceous tertiary collision related obductions pontic lesser caucasus liguria squeezing out the elevated portions of these basins lack of oceanic magmatism during the cenozoic but progressive accretion along accretionary wedges of the elevated portions of the remaining mesozoic crust van nain sabzewar zahedan mekran

ensembles of low degree archeomagnetic field models for the past three millennia abstract we introduce ensembles of time varying archeomagnetic field models consisting of a reference model a mean model and a thousand individual models we present a set of three such ensembles built from archeomagnetic volcanic and sedimentary data sets that cover the past three millennia these ensembles can be used to describe the field at any location from the core surface to the magnetosphere and assess the way uncertainties due to the limited distribution and quality of the data affect any of its component or parameter such as individual gauss coefficients they provide alternative and we argue more complete descriptions of the archeomagnetic field to those provided by previously published archeomagnetic field models being better suited to existing and emerging needs such as those of geomagnetic data assimilation we present the data sets we rely on essentially the same as those used by other recent archeomagnetic field models and describe how errors affecting the data and errors due to non modelled small spatial scales of the field are taken into account we next explain our modeling strategy and motivation for building low degree spherical harmonic degree ensembles of models we carry on a number of end to end simulations to both illustrate the usefulness of such ensembles and point at the type of errors one should expect practical illustrations of what can be done with these three ensembles of models with examples of geomagnetic inferences are also described northern high latitude flux patches for instance appear to be the most robust features of all these patches tend to fluctuate but clearly have some favored locations resulting in the same clear signature with three tongues over northern america europe and asia in the time averaged field at the core mantle boundary similar to what had been found in earlier models inferences about the field behavior in the southern hemisphere are more difficult to draw still some suggestions that the well known present south atlantic reversed patch could have arisen as early as in 1500 a d are found in some of the ensembles we otherwise confirm that most of the current archeomagnetic field model limitations are related to a number of sediment cores identified as producing frequent outliers in the modeling process we provide evidence that such cores are likely affected by timing errors and timing delays between magnetization lock in and sediment deposition that future more advanced treatment should be able to handle all source files for the three ensembles of models together with appropriate matlab applications can be downloaded from http geomag ipgp fr download archeo fm zip

on the resolution of regional archaeomagnetism untangling directional geomagnetic oscillations and data uncertainties using the french archaeomagnetic database for between 1000 and 1500 ad as a guide a complexity is emphasised in the distribution of the french archaeomagnetic directions during the 13th and the 14th centuries ad data uncertainties and the smoothing introduced when estimating an average secular variation curve prevent scrutiny of the very nature of this complexity it might correspond to a directional yaw the nature of which would be compatible with the recent geomagnetic field evolution as traced by the gufm1 model in order to emphasise this indeterminacy a reference secular variation curve was constructed for between 1000 and 1500 ad including the yaw in question and synthetic databases that mimic the accuracy and density characteristics of the true french archaeomagnetic database were considered for some of these the synthetic curves hence obtained show that the dating accuracy of archaeomagnetic data is the crucial parameter for constructing a detailed secular variation path the significant impact of the experimental data accuracy is also illustrated even more crucial is the fact that precision of the data dating required to describe the directional variability over the century timescale largely exceeds the precision of the archaeological dates available for the structures generally studied this highlights the intrinsic limitation of archaeomagnetism for regional reconstruction of century scale geomagnetic field variations

paleomagnetism and the evolution of pangea we present new apparent polar wandering apw paths for the latest carboniferous to middle jurassic of europe northern asia north america and gondwana the paths have been constructed by ordering the paleopoles into a time sequence and calculating running averages globally this statistical analysis cannot be carried back before the latest carboniferous because the paleomagnetic record becomes inadequate the polar paths have all been rotated along with the land masses from which they have been observed into the classical configuration of wegener pangea a as quantified by later workers the mean paleopoles agree in the early jurassic but disagree in the latest carboniferous permian and early triassic this result is insensitive to reasonable choices made regarding the arrangement of the continents within gondwana it is true either for results from gondwana as a whole or from africa and south america alone this disagreement in apw paths suggests but does not prove that pangea a lasted for only a short interval of time in the early to middle jurassic 190 170 m y and perhaps the latest triassic 200 m y alternatively we may say either that the paleomagnetic record is biassed by sampling inadequacies or by systematically incorrect age assignments or that the earth s field has been non dipolar we regard these explanations as less likely although they cannot be discounted the polar paths can be reconciled by rotating gondwana anticlockwise by about 35 relative to laurasia placing africa beneath europe and south america beneath eastern north america this configuration is called pangea b pangea b could be a valid reconstruction for the latest carboniferous and early permian approximately 290 260 m y pangea b provides a statistically satisfactory resolution of the paleomagnetic problem it also provides a symmetrical arrangement of the continents bordering the appalachian hercynian fold belt for the late paleozoic which pangea a does not it provides a satisfactory framework for the early evolution of the gulf of mexico and it provides an explanation of the absence of geological evidence for a tethys ocean in the paleozoic the transition between b and a cannot be accurately reconstructed from the paleomagnetic data because the resolution of the record is as yet inadequate but it presumably occurred by a 3500 km minimum dextral megashear between laurasia and gondwana during the late permian and triassic approximately 250 200 m y without the formation of ocean between them and was accompanied by lesser motions between north america and europe the sense of megashear is the same as that invoked by arthaud and matte 1977 on geological grounds but the magnitude and timing are different our interpretation favors the idea that continental drift is a continuous process not confined to jurassic and later times but extending backwards into the triassic and permian we suggest that pangea was not an immobile configuration as envisaged by wegener and most subsequent workers but that it evolved more or less continuously

40ar 39ar geochronological constraints on the evolution of lateritic iron deposits in the quadrilatero ferrifero minas gerais brazil weathering profiles overlying the sapecado pico and andaime iron ore deposits quadrilatero ferrifero qf minas gerais brazil reach depths of 150 400 m and host world class supergene iron orebodies in addition to hosting supergene ore bodies of global economic significance weathered banded iron formations at the quadrilatero ferrifero and elsewhere e g carajas hamersley are postulated to underlie some of the most ancient continuously exposed weathering profiles on earth laser incremental heating 40ar 39ar results for 69 grains of hollandite group manganese oxides extracted from 23 samples collected at depths ranging from 5 to 150 m at the sapecado pico and andaime deposits reveal ages ranging from ca 62 to 14 ma older mn oxides occur near the surface while younger mn oxides occur at depth however many samples collected at the weathering bedrock interface yield ages in the 51 41 ma range suggesting that the weathering profiles in the quadrilatero ferrifero had already reached their present depth in the paleogene the antiquity of the weathering profiles in the quadrilatero ferrifero is comparable to the antiquity of dated weathering profiles on banded iron formations in the carajas region brazil and the hamersley province western australia the age versus depth distributions obtained in this study but not available for other regions containing similar supergene iron deposits suggest that little further advance of the weathering front has occurred in the quadrilatero ferrifero lateritic profiles during the neogene the results suggest that weathering in some of these ancient landscapes is not controlled by the steady state advance of weathering fronts through time but may reflect climatic and geomorphological conditions prevailing in a remote past the geochronological results also confirm that the ancient landsurfaces in the quadrilatero ferrifero probably remained immune to erosion for tens of millions of years deep weathering mostly in the paleogene combined with low erosion rates account for the abundance and widespread distribution of supergene iron manganese and aluminum orebodies in this region

volcanic facies analysis of a subaqueous basalt lava flow complex at hruskovec nw croatia evidence of advanced rifting in the tethyan domain abstract the hruskovec quarry of basaltoid rocks is situated on the northwestern slopes of mt kalnik within the zagorje mid transdanubian zone a part of the north western dinarides the basaltoids are inter bedded with radiolarites of the middle and upper triassic age langobardian carnian norian spilites altered diabases and meta basalts form part of triassic volcanic sedimentary sequence made of sandstones shales micritic limestone altered vitric tuffs and radiolarian cherts incorporated tectonically into the jurassic cretaceous melange the architecture of the 2 km long and 100 m high pile of the extrusive basaltoid rocks is interpreted as a subaqueous basaltic lava flow the presented research deals with a variety of volcanic facies of the subaqueous basaltic lava flow which consists of several facial units 1 coherent pillow lavas with massive core the bending rims around the massive core 30 50 cm thick are dissected by polygonal columnar joints radiating from the pillow centres 2 closely packed pillows densely packed and contorted pillows due to emplacement accommodation clearly younging upward 3 pillow fragment breccia clast supported matrix poor monomict breccia formed proximal to the axis of the extrusion 4 isolated pillow breccia matrix supported clast poor breccia made of lava pipes and tubes within a matrix of fine grained sideromelan granules and shards 5 pyjama style pillows spherical decimetre to meter size pillow lava balls grown and chilled in isostatic state i e in a state of diminished density contrast within water soaked sediments named after peculiar alternating basaltic shelves inside the sphere which are encrusted with white secondary minerals 6 peperite and peperitic hyaloclastites blocky and globular peperites developed at the contact of soft wet sediment and hot intruding magma discovery of peperite and peperitic hyaloclastites within the triassic radiolarian cherts shales and micritic limestone with registration of conodonts index microfossils links their origin to magmatism during advanced rifting in a progressively subsiding basin during the triassic it precludes an affiliation of the subaqueous basaltic lava flow with the dismembered ophiolite formation a product of oceanization created in jurassic cretaceous times which is a commonly stated alternative explanation the basaltic rocks and their sedimentary hosts may be correlated with advanced triassic rifting and related volcanic sedimentary successions in the dinarides albanides helenides and further along the tethyan orogenic belt

chronology of exotic mineralization at el salvador chile by 40ar 39ar dating of copper wad and supergene alunite exotic copper mineralization is a complex hydrochemical process linking supergene enrichment lateral copper transport and precipitation of copper oxide minerals in the drainage network of a porphyry copper deposit at the el salvador porphyry copper deposit in northern chile the majority of the exotic ore comprised a mixture of copper bearing manganese oxyhydrates termed copper wad x ray diffraction scanning electron microscopy and electron probe microanalysis show that the copper wad is composed of copper bearing cryptomelane k1 2 mn3 mn4 8o16 x h2o and birnessite k0 33 batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document mn 7 3 9 end document o14 7h2o structures these natural occurrences within the exotic ore provide the opportunity to directly date the formation of these deposits using recent advances in 40ar 39ar geochronology of supergene k mn oxides formed by weathering a suite of copper bearing cryptomelane and birnessite samples from exotic deposits within the el salvador district were characterized and dated by 40ar 39ar laser step heating supergene alunite kal3 so4 2 oh 6 found in paleospring feeder systems leading from the source zones of copper outward to the exotic mineralization was dated to independently constrain the age of exotic ore formation although the ar retentivity of the layered birnessite structure has been questioned by others the ar retentivity of these samples is thought to be a function of their natural preservation and limited postcrystallization ground water interaction in the hyperarid atacama desert the 40ar 39ar analytical results show that ar and or k losses after crystallization excess 40ar and 39ar recoil do not pose significant problems therefore in the context of exotic copper deposits within hyperarid environments this dating method is applicable to both cryptomelane and birnessite within copper wad the 40ar 39ar dating of exotic mineralization at el salvador indicates that supergene and exotic mineralization processes were active at 35 ma about 5 m y after the emplacement of hydrothermal mineralization and continued until the middle miocene the majority of exotic mineralization extends from the oligocene miocene 24 ma boundary through the middle miocene 11 ma and relates to supergene fluid emanating in multiple directions from the source of copper in indio muerto which represents a topographic high above the exotic deposits one reconnaissance date of exotic mineralization at the chuquicamata porphyry copper deposit yielded an age of 17 03 0 03 ma coincident with the known supergene alunite dates for this deposit reconnaissance dating at exotica mina sur and el abra proved problematic due to the presence of contaminant silicate minerals within the copper wad ages derived here for exotic mineralization are similar to the known supergene ages throughout northern chile the dates indicate that supergene exotic mineralization began at the eocene oligocene boundary and continued through the oligocene eventually ceasing in the middle miocene due to desiccation of the atacama desert a series of 10 cm thick supergene alunite veins were microsampled across their widths and dated to address the kinetics of alunite vein growth apparent growth rates of the veins vary from 71 to 100 mm m y in the horizontal direction to 24 mm m y in the vertical presumably caused by differences in pressure regimes due to their orientation

new cocorp profiling in the southeastern united states part ii brunswick and east coast magnetic anomalies opening of the north central atlantic ocean new cocorp profiles on the coastal plain of georgia and northern florida support the hypothesis that the brunswick anomaly marks a late paleozoic suture they do not support the alternate view that this anomaly is caused by a mesozoic rift basin the trend of the brunswick anomaly relative to the appalachian gravity gradient indicates that in westernmost georgia and adjacent alabama african basement suwannee terrane is in proximity to autochthonous north american basement grenville farther east one or more paleozoic accreted terranes intervene between the north american and african sides of the orogen offshore the brunswick anomaly closely parallels the east coast magnetic anomaly this relationship implies that the east coast magnetic anomaly marks not only the present continental oceanic crustal transition but also the northward continuation of the late paleozoic suture between north america and africa transitional crust beneath the carolina trough baltimore canyon trough and correlative parts of the african atlantic margin is thus likely to have formed within a preexisting suture zone the dramatic change in character of the u s atlantic margin southward from the carolina trough to the blake plateau probably reflects the fact that two different types of prerift crust are juxtaposed in this region

triassic magmatism of the dinarides in yugoslavia abstract the triassic magmatism of the dinarides had a polyphase character it took place during a period of about 50 ma during the initial stages of the mesozoic wilson cycle their earliest phases might have been connected with rifting processes and the subsequent ones preceeded directly the opening of the dinaridic part of the tethys and the formation of a spreading ridge the triassic magmatism is represented by basic and more commonly by intermediate and acid plutonic and volcanic rocks with calc alkaline affinity the triassic magmatic rock association can be correlated on major element evidence with calc alkaline rocks of recent convergent plate margins however field relationships and the data on immobile trace elements indicate that lavas poured out over triassic sediments on a carbonate platform which was underlain by rocks of the continental crust the basic diversity of the triassic magmatism of the dinarides was influenced by partial melting other processes crystallization contamination and others must have played a secondary role

measuring lithospheric stretching on tethyan passive margins preliminary estimates of lithospheric stretching have been made by examining 63 stratigraphic sections from the northern and southern tethyan margins southeast france to bulgaria the study was centred on the italian southern alps the dolomites a portion of the southern margin within the external zones of the alpine orogen sections were decompacted and backstripped using the standard procedure allowing for errors in palaeowater depth estimates theoretical subsidence curves based on the finite duration stretching model were then fitted to the resultant water loaded subsidence curves in an attempt to clarify the pre compressional tectonic history of the region according to our results stretching started in the early to mid triassic and lasted 30 40 ma this conclusion is broadly in agreement with some previously published estimates the stretching factor is generally small 1 1 1 3 especially in southeast france and northern italy larger values of occur in the balkans c 1 5 to our knowledge the finite duration stretching model has not yet been applied to water loaded stratigraphic sections from the tethyan belt theory and observation appear to agree well

isotopic dating of supergene manganese oxides from the groote eylandt deposit northern territory australia a total of 51 conventional k ar dates on 22 manganese oxide samples from the groote eylandt manganese deposit is presented because most of the samples contained detrital mica from the bedrock k ar ages and their standard errors were corrected using material balance equations following determination of the contribution of radiogenic argon and potassium from the silicate residue in this way the times of formation of cryptomelane ba todorokite and romanechite and probably also vernadite were estimated in addition high resolution 40 ar 39 ar dating was used to date growth bands of pure void filling cryptomelane corrected k ar ages show that cryptomelane and ba todorokite the latter previously thought to be of diagenetic origin were precipitated between 18 and 6 ma early to late miocene as a result of chemical weathering through widespread dissolution and in situ replacement of pisoliths and ooliths romanechite in the lower part of the profile in the g south quarry must be older than 43 7 or 1 2 ma in the upper part of the g south quarry profile romanechite formed at about 30 ma these ages together with the cryptomelane and ba todorokite ages appear to define several episodes of intense chemical weathering in this deposit during the tertiary a pre late eocene episode older than 43 7 or 1 2 ma an oligocene episode around 30 ma and a miocene weathering megacycle 6 18 ma it is suggested that these weathering related processes had a much more significant effect on development of the deposit than previously thought the present mineralogy was produced by replacement processes during the tertiary associated with intense chemical weathering and not by sedimentary or diagenetic processes isotopic ages on manganese oxides from groote eylandt are similar to those determined on manganese oxides from northwestern australia and brazil indicating that periods of intense weathering in the tertiary recording humid climatic conditions were relatively widespread

geodynamic evolution of the central dinarides abstract the central part of the dinarides unlike the northwestern and southeastern dinarides shows a consistent pattern in the distribution of the large tectonostratigraphic units from southwest apulia to northeast the following five main units originating in different parts of the mesozoic tethys can be distinguished 1 adriatic dinaridic carbonate platform formations 2 carbonate clastic units flysch bosniaque of the passive continental margin 3 ophiolite bearing formations of the dinaridic ophiolite zone also containing radiolarites greywackes and shales including melange formations and overstepped late jurassic to cretaceous cover sequences 4 active continental margin units represented by upper cretaceous palaeogene trench sediments with blueschists tectonized ophiolite melange alpine medium pressure metamorphic rocks originated from upper cretaceous palaeogene sediments and alpine synkinematic granitoids 5 a nappe consisting of allochthonous palaeozoic triassic formations which is thrust onto the ophiolites and genetically related sedimentary formations the frontal parts of this nappe directly overlie the northeastern margin of the adriatic dinaridic carbonate platform i e the external dinarides the evolution of the central dinarides was controlled by a sequence of tectonic events which took place within the alpine cycle 1 rifting processes lasting some 40 50 ma which started in the late permian and ended in the norian 2 opening of the oceanic dinaridic tethys commenced in the late triassic and persisted for some 70 80 ma until the end of the jurassic 3 subduction processes and emplacement of ophiolites started in late jurassic early cretaceous times during the closing phase of the dinaridic ocean or dinaridic tethys the palaeozoic triassic nappe was emplaced on the ophiolites the thrusting was accompanied by alpine metamorphism 120 100 ma 4 the main alpine deformational event and medium pressure metamorphism with synkinematic granite plutonism took place in late eocene early oligocene times when the ophiolite and palaeozoic triassic nappes were emplaced on the adriatic dinaridic carbonate platform which at the same time was deformed resulting in the development of the external dinarides fold and thrust belt

timing and duration of supergene mineralization at the xinrong manganese deposit western guangdong province south china cryptomelane 40 ar 39 ar dating supergene mn oxide deposits are widely distributed in guangxi guangdong yunnan and hunan provinces south china accounting for 18 of the total mn reserves in the country direct dating of supergene mn enrichment however is lacking in this paper we present high resolution 40ar 39ar ages of mn oxides from the xinrong mn deposit western guangdong to place numerical constraints on the timing and duration of supergene mn enrichment a total of ten cryptomelane samples spanning a vertical extent of 67 m were dated using the 40ar 39ar laser incremental heating technique with seven samples yielding well defined plateau or pseudo plateau ages ranging from 23 48 0 91 to 2 06 0 05 ma 2 one sample yields a staircase spectrum that does not reach a plateau the spectrum however indicates the presence of two or more generations of mn oxides in the sample whose ages are best estimated at 22 34 0 31 and 10 2 0 86 ma respectively the remaining two samples gave meaningless or uninterpretable results due to significant 39ar recoil and contamination by old phases the 40ar 39ar data thus reveal a protracted history of weathering and supergene mn enrichment that started at least in the end of the oligocene or beginning of miocene and extending into the latest pliocene staircase apparent age spectra resulting from banded or botryoidal samples yield an average growth rate of mn oxides at 0 6 0 7 10 3 mm kyr 1 the values indicate that a 1 mm grain of mn oxides may host minerals precipitated during a time span of ca 1 5 m y and accumulation of mn oxides to form economic deposits under weathering environments may take millions of years the distribution of weathering ages shows that the oldest mn oxides occur on the top of the profile whereas the youngest minerals are found at the bottom suggesting downward propagation of weathering fronts however two samples located at the intermediate depths of the profile yield ages comparable with those occurring at the highest elevations such a complexity of age distribution is interpreted in terms of preferential penetration of mn rich weathering solutions along more permeable fault zones or as a result of multi stages of dissolution and re precipitation of mn oxides a synthesis of geochronological and geological data suggests that formation of the xinrong deposit was a consequence of a combination of favorable lithological climatic and structural conditions because the climatic and structural conditions are similar among the provinces of south china during the cenozoic the geochronological results obtained at xinrong may also have implications for the timing of supergene mn enrichment throughout south china

geomagnetic field variations in western europe from 1500bc to 200ad part i directional secular variation curve abstract to improve the geomagnetic field secular variation curve svc of western europe during protohistoric times archaeomagnetic directions of 39 archaeological kilns or hearths from france were investigated the dating of each archaeological structure was established with archaeological or chronometric methods thirty seven of these structures are dated from the first millennium bc one from the end of the second millennium bc and the last one from the fourth millennium bc thermomagnetic curves unblocking temperatures and coercivities suggest that the main carrier of the remanent magnetization is a ti poor titanomagnetite archaeodirections were obtained by alternating field and thermal demagnetizations on almost 900 specimens the anisotropy tensor of thermoremanent magnetization was determined for 35 structures and 22 mean archaeodirections were corrected for anisotropy the new archaeodirections are very consistent with previously published data a new directional svc was built using bivariate statistics with selected western europe data located within 1000 km of paris selection criteria include the number of samples the dating reliability and the accuracy of the mean archaeodirection resulting secular variation between 1500bc and 0ad mainly shows large changes in declination while inclinations are bracketed between 65 and 75 the declinations show a strong maximum with values 30 35 around 800 750bc followed by a sharp decrease to values around 0 at 500bc and close to 7 around 250bc the main features of the secular variation from 1500bc to 0ad appear to be a dominant westward drift and two major changes around 800 and 250bc compared to the global and regional geomagnetic models the new reference data are better fitted by arch3k cst 1 and scha dif 3k than by arch3k 1 and cals3k 4 models the strong variation of the archaeodirection between 1000 and 500bc makes archaeomagnetism very useful for dating purposes

timing of supergene enrichment of low grade sedimentary manganese ores in the kalahari manganese field south africa abstract low grade carbonate rich manganese ore of sedimentary origin in the giant kalahari manganese field south africa is upgraded to high grade todorokite manganomelane manganese ore by supergene alteration below the unconformity at the base of the cenozoic kalahari formation incremental laser heating 40ar 39ar dating of samples from the supergene altered manganese ore suggest that chemical weathering processes below the kalahari unconformity peaked at around 27 8 ma 10 1 ma and 5 2 ma ago older ages are dominant in the upper part of the weathering profile while younger ages are characteristic of the deeper part of the profile younger ages partially overprint older ages in the upper part of the weathering profile and demonstrate the downward progression of the weathering front by as little as 10 cm per million years the oldest age obtained in the weathering profile namely 42 ma is considered a minimum estimate for the onset of the post african i cycle of weathering and erosion that followed the break up of gondwanaland and formation of the cretaceous to early cenozoic african land surface the youngest ages recorded at around 5 ma in turn correspond well to the pliocene transition from humid to arid climatic conditions in southern africa

advances in archaeomagnetic dating in britain new data new approaches and a new calibration curve abstract archaeomagnetic dating offers a valuable chronological tool for archaeological investigations particularly for dating fired material the method depends on the establishment of a dated record of secular variation of the earth s magnetic field and this paper presents new and updated archaeomagnetic directional data from the uk and geomagnetic secular variation curves arising from them the data are taken from publications from the 1950 s to the present day 422 dated entries derived from existing archaeo and geomagnetic databases are re evaluated and 487 new directions added resulting in 909 entries with corresponding dates the largest collection of dated archaeomagnetic directions from a single country an approach to improving the largest source of uncertainty the independent dating is proposed and applied to the british iron age resulting in 145 directions from currently available databases being updated with revised ages and or uncertainties and a large scale reassessment of age assignments prior to inclusion into the magnetic moments of the past and geomagia50 databases from the significantly improved dataset a new archaeomagnetic dating curve for the uk is derived through the development of a temporally continuous geomagnetic field model and is compared with previous uk archaeomagnetic dating curves and global field models the new model arch uk 1 allows model predictions for any location in the uk with associated uncertainties it is shown to improve precision and accuracy in archaeomagnetic dating and to provide new insight into past geomagnetic field changes

weathering geochronology by u th he dating of goethite nine samples of supergene goethite feooh from brazil and australia were selected to test the suitability of this mineral for u th he dating measured he ages ranged from 61 to 8 ma and were reproducible to better than a few percent despite very large variations in u and th in all samples with internal stratigraphy or independent age constraints the he ages corroborated the expected relationships these data demonstrate that internally consistent he ages can be obtained on goethite but do not prove quantitative 4he retention to assess possible diffusive he loss stepped heating experiments were performed on two goethite samples that were subjected to proton irradiation to produce a homogeneous distribution of spallogenic 3he the 3he release pattern indicates the presence of at least two diffusion domains one with high helium retentivity and the other with very low retentivity at earth surface conditions the low retentivity domain which accounts for 5 of 3he contains no natural 4he and may represent poorly crystalline or intergranular material which has lost all radiogenic 4he by diffusion in nature diffusive loss of 3he from the high retentivity domain is independent of the macroscopic dimensions of the analyzed polycrystalline aggregate so probably represents diffusion from individual micrometer size goethite crystals the 4he 3he evolution during the incremental heating experiments shows that the high retentivity domain has retained 90 95 of its radiogenic helium this degree of retentivity is in excellent agreement with that independently predicted from the helium diffusion coefficients extrapolated to earth surface temperature and held for the appropriate duration considering both the high and low retentivity domains these data indicate that one of the samples retained 90 of its radiogenic 4he over 47 5 ma and the other retained 86 over 12 3 ma thus while diffusive loss corrections to supergene goethite he ages are required these initial results indicate that the corrections are not extremely large and can be rigorously quantified using the proton irradiation 4he 3he method

the evolution of the meliata hallstatt ocean and its significance for the early evolution of the eastern alps and western carpathians abstract the evolution of the meliata hallstatt ocean and of its southern and northern margin in the inner western carpathians are described the main rifting began during the pelsonian and the sea floor spreading ended at the beginning of the middle carnian contemporaneously with the raibl event the subduction began in the latest triassic and the final closing of the ocean accompanied by uplift of the adjacent marginal areas was dated as basal oxfordian remnants of the oceanic suboceanic sequence occur in two tectonic positions 1 obducted nappes that contain mostly as tectonic melanges the whole middle triassic to middle jurassic sequence including large bodies of dismembered ophiolites of ladinian to early carnian age 2 evaporite melanges at the base of nappes that are derived from the northern margin of the ocean these evaporite melanges consist of late permian evaporitic matrix and blocks of a dismembered ladinian to lower carnian ophiolitic sequence in the inner western carpathians both the northern and southern marginal sequences of the meliata hallstatt ocean and the transitions into the carbonate platforms are present in different nappes the southern marginal development is characterized by middle carnian distal clastic raibl beds and by middle jurassic rhyolithic volcanism both missing in the northern marginal development because the subduction related middle jurassic volcanism contemporaneous with the turbidites in the southern meliaticum is restricted to the marginal area south of the meliata hallstatt ocean southward directed subduction is indicated to the south on the adjacent carbonate platform a jurassic basin with aalenian to bajocian ophiolites opened this may be a back arc basin in the eastern alps only the northern marginal zone and the adjacent carbonate platform are preserved but parts of the dismembered ophiolites in the haselgebirge may be of ladinian to late carnian age like in evaporitic melanges in the same tectonic position in the inner western carpathians the final closing of the meliata hallstatt ocean is indicated by the abrupt end of the turbiditic sedimentntation in the oceanic suboceanic domain and by uplift in the marginal areas where lower oxfordian radiolarites are overlain by shallow water upper oxfordian to tithonian or neocomian limestones silica nappe hallstatt nappes with the evidence of the final closing of the meliata hallstatt ocean near the middle late jurassic boundary the cimmerian orogenesis in the genetic sense of engor 1984 1985 is now also proven in the inner western carpathians and in the eastern alps a continuation of the meliata hallstatt ocean into the pontide paleotethys cimmerian ocean sensu kozur 1990 through the transylbanian oceanic domain and the strandzha unit is assumed a connecction with the vardar ocean is impossible because in the vardar zone no triassic ophiolites or basic volcanics are present and the ophiolites have jurassic age

the eastern carpathians ophiolites romania remnants of a triassic ocean abstract mesozoic ophiolitic and related rocks in the eastern carpathians occur in three areas from north to south rar u h ghimas and persani mts they are found as blocks ranging from few metres to a few kilometers in size and as centimetre sized in breccias most likely embedded in the late barremian early albian wildflysch formation compositionally they range from lherzolites and harzburgites to mafics such as feti gabbros dolerites basalts and to andesites the volcanics comprise highly depleted basalts andesites to enriched type mid ocean ridge basalts additionally they include ocean island basalts and calc alkaline basalts andesites and trachytes based on paleontological evidence their age is middle to late triassic they can be clearly compared with remnants of the meliata hallstatt ocean in the western carpathians but do not match the jurassic ophiolites and island arc volcanics in the mures zone of the southern apuseni mts we propose a triassic ocean connected with the meliata hallstatt ocean between a the bucovinian sub bucovinian continental crust b the infrabucovinian and finally c the northern apuseni microcontinents this ocean closed in the late triassic to early jurassic causing close juxtaposition of all three microcontinents an ophiolite complex together with ocean island basalts and calc alkaline basalts andesites remained from this ocean and was subsequently eroded and transported as blocks of different size into the lower cretaceous wildflysch basin together with blocks and clasts of limestones similar to the mesozoic sedimentary sequences in the northern apuseni realm the wildflysch formation was thrust as an independent unit during the albian over the bucovinian nappe in the eastern carpathians and the northern apuseni continental crust respectively

dating of chemical weathering processes by in situ measurement of u series disequilibria in supergene fe oxy hydroxides using la mc icpms abstract constraints on the timing of weathering processes in the northern australian regolith have been obtained by in situ measurement of u series disequilibria in u rich supergene fe oxy hydroxides using a laser ablation mc icpms technique this approach has permitted the measurement of 234 u 238 u and 230 th 238 u activity ratios in finely crystalline fe oxy hydroxides from the ranger uranium deposit with sufficient precision and spatial resolution to constrain the age of formation of these phases and to develop a geochronological framework for weathering processes 230 th 238 u systematics in the finely crystalline fe oxy hydroxides yield 230 th ages ranging from 60 to 350 ka the most reliable 230 th ages cluster between 122 and 216 ka suggesting fe oxy hydroxides formation and associated weathering peaked during the previous 2 interglacial periods this is supported by the 230 th ages and isotope composition of pisolith fe oxy hydroxide pedogenic nodules cores which are demonstrated to behave as closed systems the u isotopic composition of the fe oxy hydroxides is consistent with an origin from groundwater in equilibrium with dissolved uraninite secondary overprinting is evident in some samples as a large range in 234 u 238 u our results suggest that weathering intensity varies with global climate cycles and that together with weathering events dated by 40 ar 39 ar of mn oxides elsewhere in northern australia feng y x and vasconcelos p 2001 quaternary continental weathering qeochronology by laser heating 40 ar 39 ar analysis of supergene cryptomelane geology 29 7 635 638 weathering rates in this region are orbitally forced

the tectonic evolution of a critical segment of the dinarides alps connection kinematic and geochronological inferences from the medvednica mountains ne croatia the transition zone between the alps and dinarides is a key area to investigate kinematic interactions of neighboring orogens with different subduction polarities a study combining field kinematic and sedimentary data microstructural observations thermochronological data rb sr and fission track and regional structures in the area of medvednica mountains has revealed a complex polyphase tectonic evolution we document two novel stages of extensional exhumation the first stage of extension took place along a late cretaceous detachment following the late early cretaceous nappe stacking burial and greenschist facies metamorphism two other shortening events that occurred during the latest cretaceous oligocene were followed by a second event of extensional exhumation characterized by asymmetric top ne extension during the miocene top nw thrusting took place subsequently during the pliocene inversion of the pannonian basin the cretaceous nappe burial late cretaceous extension and the oligocene earliest miocene contraction are events driven by the alps evolution in contrast the latest cretaceous eocene deformation reflects phases of dinaridic contraction furthermore the miocene extension and subsequent inversion display kinematics similar with observations elsewhere in the dinarides and eastern alps all these processes demonstrate that the medvednica mountains were affected by alpine phases of deformations to a much higher degree than previously thought similarly with what has been observed in other areas of contractional polarity changes such as the mediterranean black sea or new guinea systems the respective tectonic events are triggered by rheological weak zones which are critical for localizing the deformation created by both orogens

triassic shoshonites from the dolomites northern italy alkaline arc rocks in a strike slip setting triassic igneous rocks of the dolomites are typical shoshonites with high k2o sio2 and k2o na2o ratios and low fe contents they have mid ocean ridge basalt like abundances of high field strength elements but are highly enriched in large ion lithophile lil elements rb sr ba k the effects of crustal contamination and fractional crystallization have been superimposed on initial lil element enrichment lavas of similar composition occur in papua new guinea fiji the absaroka mountains and the eolian islands these suites are all associated with destructive plate margins but were not erupted until after subduction had ceased examination of mediterranean paleogeography indicates that no subduction event which could have given rise to the shoshonites of the dolomites occurred during the triassic in fact the tectonic setting at the time of eruption of the lavas was probably one of strike slip faulting the suite may have inherited its arc signature from permo carboniferous subduction of the ocean between laurasia and gondawanaland prior to the hercynian orogeny

quaternary continental weathering geochronology by laser heating 40ar 39ar analysis of supergene cryptomelane incremental laser heating analyses of supergene cryptomelane clusters extracted from three distinct weathering profiles from the mary valley region southeast queensland australia yield reproducible and well defined plateau ages ranging from 346 15 to 291 14 ka 2 a precipitation of supergene cryptomelane in this period implies that relative humid climate prevailed in southeast queensland from 340 to 290 ha a result consistent with oxygen isotope analyses of marine sediments from ocean drilling program site 820 and with regional pollen and spore records these results the first report on the precise ar 40 ar 39 dating of quaternary supergene cryptomelane indicate that ar 40 ar 39 analysis of pedogenic minerals provides a reliable geochronometer for the study of quaternary surficial processes useful in the study of soil formation rates continental paleoclimates and archaeological sites devoid of datable volcanic minerals

mesozoic plate tectonic reconstruction of the carpathian region abstract palaeomagnetic palaeobiogeographic and structural comparisons of different parts of the alpine carpathian region suggest that four terranes comprise this area the alcapa tisza dacia and adria terranes these terranes are composed of different mesozoic continental and oceanic fragments that were each assembled during a complex late jurassic cretaceous palaeogene history palaeomagnetic and tectonic data suggest that the carpathians are built up by two major oroclinal bends the alcapa bend has the meliata oceanic unit correlated with the dinaric vardar ophiolite in its core it is composed of the western carpathians eastern alps and southern alcapa units transdanubian range bukk this terrane finds its continuation in the high karst margin of the dinarides further elements of the alcapa terrane are thought to be derived from collided microcontinents czorsztyn in the n and a carbonate unit tisza in the se the tisza dacia bend has the vardar oceanic unit in its core it is composed of the bihor and getic microcontinents this terrane finds its continuation in the serbo macedonian massif of the balkans the bihor getic microcontinent originally laid east of the western carpathians and filled the present carpathian embayment in the late palaeozoic early mesozoic the vardar ocean occupied an intermediate position between the western carpathian austroalpine transdanubian high karst margin and the bihor getic serbo macedonian microcontinent the vardar and pindos oceans were opened in the heart of the mediterranean adriatic microcontinent in the late permian middle triassic vardar subducted by the end of jurassic causing the bihor getic serbo macedonian microcontinent to collide with the internal dinaric western carpathian margin an external penninic vahic ocean tract began opening in the early jurassic separating the austroalpine western carpathian microcontinent and its fauna from the european shelf further east the severin ceahlau magura also began opening in the early jurassic but final separation of the bihor getic ribbon and its fauna from the european shelf did not take place until the late middle jurassic the alcapa and the tisza dacia were bending during the albian maastrichtian the two oroclinal bends were finally opposed and pushed into the gates of the carpathian embayment during the palaeogene and neogene at that time the main n s shortening in distant alpine and hellenic sectors was linked by a broader right lateral shear zone along the former vardar suture

geochronological evidence for pervasive miocene weathering minas gerais brazil ar 40 ar 39 laser incremental heating analyses of 22 individual grains of supergene cryptomelane from three weathering profiles up to 400 km apart in the rio doce valley and barbacena regions at minas gerais brazil show that the formation of weathering profiles in these regions is contemporaneous suggesting a strong weathering event in the middle to late miocene 10 8 ma the preservation of these miocene samples at or near the present surface suggests that either erosion rates have been very low in the region since the miocene or that a much thicker weathering mantle was present in the region originally assuming a constant thickness of weathering profiles in the region throughout the tertiary we may calculate weathering front propagation rates of 4 8 m myr 1 during the past 10 ma copyright c 2004 john wiley sons ltd

kinematics of foreland vergent crustal accretion inferences from the dinarides evolution one of the most common observation in mediterranean areas is the migration of contractional deformation and associated slabs through time towards external orogenic areas associated with lower plate crustal accretion the dinarides orogen of central europe is an optimal place to study such a sequence of contractional deformation compared with other areas contraction in the dinarides was less overprinted by subsequent extension while a remnant of the subducted slab is observed in a far external orogenic position understanding the deformational evolution of the dinarides is hampered by the reduced availability of kinematic studies therefore we have performed a surface kinematic study in the external parts of the dinarides by correlating with available geophysical and evolutionary constraints we constructed two large scale kinematically controlled regional transects the results demonstrate a long lived evolution of shortening that affected the dinarides lower orogenic plate while the late jurassic earliest cretaceous deformation was associated with an earlier obduction moment the latest cretaceous onset of continental collision has gradually focused deformation at inherited rheological weakness zones we show that shortening was interrupted by a period of miocene extension that affected all orogenic areas and created the dinarides lake system the extension was followed by renewed shortening that started during the latest miocene and remains presently active whose kinematics in the central and se part of the dinarides is revealed for the first time by our study these results indicate a lower plate crustal accretion mechanism that was spatially and temporally connected with gradual slab retreat in the dinarides

from nappe stacking to extensional detachments at the contact between the carpathians and dinarides the jastrebac mountains of central serbia reactivation of inherited nappe contacts is a common process in orogenic areas affected by back arc extension the amount of back arc extension is often variable along the orogenic strike owing to the evolution of arcuated mountain chains during stages of rapid slab retreat this evolution creates low rates of extension near rotation poles where kinematics and interplay with the pre existing orogenic structure are less understood the amount of miocene extension recorded by the pannonian basin of central europe decreases se wards along the inherited cretaceous paleogene contact between the dinarides and carpathian mountains our study combines kinematic data obtained from field and micro structural observations assisted with fission track thermochronological analysis and u pb zircon dating to demonstrate a complex poly phase evolution in the key area of the jastrebac mountains of serbia a first event of late cretaceous exhumation was followed by latest cretaceous eocene thrusting and magmatism related to a continental collision that sutured the accretionary wedge containing contractional trench turbidites the suture zone was subsequently reactivated and exhumed by a newly observed miocene extensional detachment that lasted longer in the jastrebac mountains when compared with similar structures situated elsewhere in the same structural position such extensional zones situated near the pole of extensional driven rotation favour late stage truncations and migration of extension in a hanging wall direction while directions of tectonic transport show significant differences in short distances across the strike of major structures

a matlab tool for archaeomagnetic dating a matlab tool for archaeomagnetic dating has been developed in this work well dated palaeosecular variation curves psvcs can be used to date archaeological artefacts with unknown ages the archaeomagnetic direction declination and or inclination and the archaeointensity obtained from the archaeological artefact are compared with a master psvc in addition historical lava flows with controversial ages can be dated using this methodology the dating process follows the descriptions given by lanos 2004 which is based on the combination of temporal probability density functions of the three geomagnetic field elements here we develop an interactive tool in matlab code to carry out archaeomagnetic dating by comparing the undated archaeomagnetic or lava flow data with a master psvc the master psvcs included with the matlab tool are the different european bayesian curves and those generated using both regional and global geomagnetic field models a case study using all the psvcs available in europe and some undated archaeomagnetic data has been carried out to analyze how the different psvcs affect the dating process in addition the dating uncertainty and the relocation error have been analyzed in the european region results show that some regional bayesian psvcs and the regional scha dif 3k archaeomagnetic model are the best choices to obtain an accurate date in europe moreover when it is available the full geomagnetic field vector must be used for archaeomagnetic dating

the middle triassic magmatic tectonic arc development in the southern alps abstract the geodynamic meaning of the middle triassic magmatic and tectonic events in the southern alps is still uncertain the main problem is the significance of the compressional tectonic structures recognized in the central dolomites in fact these structures are incompatible with the extensional geodynamic interpretation crustal rifting assumed by several authors therefore the compressional tectonic elements of the dolomites have been mostly ignored or not seriously taken into account or considered as local phenomena due to the volcanic tectonic interplay other more modern interpretations are founded on regional transcurrent and or transpressive tectonics recent discovery of a middle triassic thrust system located in the piave river area some tens of kilometres east of the central dolomites lends more credence to the compressional middle triassic event the magmatic series of volcanic and plutonic middle triassic rocks display an orogenic character with typical calc alkaline trends recognized across the whole southern alps as well as in the dinaric and hellenic domains such a character is rather uncommon in the crustal evolution by rifting but is more in accord with a compressional geodynamic setting furthermore the time space distribution of the south alpine magmatic rocks coupled with their petrochemical affinity are close to the modern volcanic arcs nevertheless the general geodynamic context of the southern alps owing to their pronounced ensialic character does not fit the oceanic model

three millennia of directional variation of the earth s magnetic field in western europe as revealed by archeological artefacts directional secular variation of the geomagnetic field over the last 2000 years has been defined in western europe from numerous archeomagnetic studies however the number of archeomagnetic results for older periods is much more limited for this reason we present new data obtained from fired archeological structures found in two french sites loupiac and aspiran dated within the first millennium b c latest bronze earliest iron age transition 850 700 b c and iron age 525 475 b c respectively from a compilation of archeomagnetic results from western europe great britain italy and france and northern africa tunisia we propose a directional secular variation curve for western europe that covers the entire first millennium b c this curve exhibits a large clockwise motion with rapid changes during the first half of the millennium while the last four centuries b c are characterized by weak variations

overview of ophiolites and related units in the late palaeozoic early cenozoic magmatic and tectonic development of tethys in the northern part of the balkan region abstract the northern balkan peninsula including serbia montenegro bosnia croatia and the former yugoslavian republic of macedonia represents an excellent region for the study of tectonic processes related to mesozoic tethyan ophiolite genesis and emplacement we first summarise the main tectonic units of the northern balkan peninsula and then use this information to discuss tectonic processes including rifting sea floor spreading ophiolite genesis and emplacement melange accretion ocean basin closure and collision we then discuss alternative models of ophiolite genesis and emplacement for the region and suggest that multi ocean basin interpretations fit the data better than single ocean basin interpretations rifting of adria gondwana during the triassic created a rift in the south budva zone and opened a triassic oceanic basin further north dinaride ocean occurrences of inferred sub continental mantle lithosphere in the dinaride ophiolite belt e g zlatibor may record extensional exhumation within an ocean continent transition zone bordering the adria dinaride continent this was followed by emplacement together with ophiolites and melange during upper jurassic early cretaceous time upper triassic radiolarites and mid ocean ridge type basalts formed at a spreading ridge after continental break up the oceanic lithosphere of the dinaride ophiolite belt was partly generated above a subduction zone the metamorphic soles of the dinaride ophiolites formed during mid late jurassic mainly based on k ar dating widespread melange that is associated with the ophiolites represents a subduction complex controlled by tectonic accretion and sedimentary reworking in trench and fore arc basin settings a possible cause of jurassic dinaride ophiolite emplacement was collision of a subduction trench with a continental margin further north mesozoic oceanic lithosphere subducted northeastwards present coordinates opening a late jurassic marginal basin in the main vardar zone the dinaride ocean in the south closed during late jurassic early cretaceous time tithonian berriasian deformed oceanic crust melange and magmatic arc rocks further north main vardar zone were transgressed by mainly clastic sediments during the early cretaceous however part of the vardar ocean vardar zone western belt or sava zone remained partially open until latest cretaceous time generally northward subduction within this remnant ocean triggered further supra subduction zone ophiolite genesis during the late cretaceous the ocean closed by the maastrichtian followed by early cenozoic regional scale southward thrusting that locally intercalated older and younger mesozoic ophiolites and melanges future progress particularly depends on determining the crystallisation ages of the ophiolites obtaining better structural data on the direction of initial ophiolite emplacement and unravelling the palaeozoic tectonic development of the eurasian continental margin

synthesis of the tectonic sedimentary evolution of the mesozoic early cenozoic pindos ocean evidence from the nw peloponnese greece

late palaeozoic to early mesozoic evolution of pangaea several possible configurations of the pangaea supercontinent have been suggested for the interval from late carboniferous to early triassic here we re examine the palaeomagnetic basis for these models emphasizing the trends of the paths of apparent polar wander for the individual components of the supercontinent rather than simply averaging poles of presumed similar age two of the alternatives pangaea b and c may result from averaging poles of dissimilar age along common polar wander paths giving rise to spurious tectonic displacements the most likely model appears to be the formation in late devonian time of a modified pangaea a2 followed by evolution to the traditional configuration a during the triassic

chronology of pleistocene weathering processes southeast queensland australia 40ar 39ar dating of mn oxides from the upper kandanga uka mn deposit southeast queensland australia shows that the weathering profile overlying the deposit is younger than 1 ma seventy plateau ages obtained for 81 grains from 17 samples fall into three groups i 1000 800 ka n 2 ii 630 510 ka n 2 and iii 400 50 ka n 66 the last age group can be further resolved using the mixture modelling solution by sambridge and compston sambridge m s compston w 1994 mixture modeling of multi component data sets with application to ion probe zircon ages earth planet sci lett 128 373 390 into two major age clusters sub groups iiia and iiib with peak values at 313 4 ka n 43 with an outlier and 213 7 ka n 22 respectively the results suggest that a supergene blanket in a mn deposit may develop relatively fast if weathering and erosional conditions are appropriate the age peaks measured in this study correspond to warm periods identified in specmap oxygen isotope records of deep sea sediments and local paleoclimatic data the correlation of weathering age peaks with oceanic climatic indicators suggests a climatic control on continental chemical weathering where warm and humid conditions are conducive to increasing weathering rates the clustering of supergene mn oxide precipitation ages suggests that weathering rates vary with time supporting an episodic weathering history for the profiles investigated

triassic volcanogenic sediments from the southern alps italy austria yugoslavia a contribution to the pietra verde problem abstract during the middle triassic anisian and ladinian volcanogenic sediments of silicic explosive volcanism have been deposited throughout the southern alps these sediments show an insignificant variation in mineralogical composition and contain various amounts of shards and pumice fragments individual deposits are 1 cm to 150 m thick and are interbedded between basinal or rare shallow water limestones sedimentary structures often clearly exhibit redeposition and primary settings neither in palaeogeography including basins and reefs nor in field evidence can we find an indication where eruption sites had been located scanning electron microscope sem studies of shards and pumice fragments elucidate primary textures and alteration processes alteration and or diagenesis caused the mobility of main elements so primary chemical composition is camouflaged sizes and shapes of shards and pumice fragments infer an alkali trachytic source for deposits of the eastern part of the southern alps

first 40ar 39ar dating of intense late palaeogene lateritic weathering in peninsular india lateritic surface processes have shaped large platform and cratons of the tropical belt constraining the timing of such processes is crucial to decipher their role in cratonic morphogenesis and their response to long term climatic change and lithospheric deformation weathering histories have been documented for south america africa and australia but precise time constraints of the lateritic weathering processes in south india are still lacking we present 40ar 39ar ages of supergene cryptomelane k mn oxide formed in the sandur mn ore deposits exposed on the highest lateritic paleolandsurface that once covered the mysore plateau and the adjacent deccan traps significant 40ar 39ar ages are estimated between 36 and 26 ma from well defined plateaus in step heating 39ar release spectra and from best fitted inverse isochrones these ages constitute firm time constraints that document intense late eocene oligocene lateritic weathering over peninsular india under the influence of warm and wet climate comparable to that prevailing in tropical humid forests these results imply that southern india was weathered between 36 and 26 ma and may have been dissected mostly in the neogene

middle late jurassic sedimentary melange formation related to ophiolite obduction in the alpine carpathian dinaridic mountain range abstract the middle late jurassic mountain building process in the western tethyan realm was triggered by west to northwestward directed ophiolite obduction onto the wider adriatic shelf this southeastern to eastern adriatic shelf was the former passive continental margin of the neo tethys which started to open in the middle triassic its western parts closed from around the early middle jurassic boundary with the onset of east dipping intra oceanic subduction ongoing contraction led to ophiolite obduction onto the former continental margin since the bajocian trench like basins formed concomitantly within the evolving thin skinned orogen in a lower plate situation deep water basins formed in sequence with the northwest westward propagating nappe fronts which served as source areas of the basin fills basin deposition was characterized by coarsening upward cycles i e sedimentary melanges as synorogenic sediments the basin fills became sheared successively by ongoing contractional tectonics with features of typical melanges analyses of ancient neo tethys melanges along the eastern mediterranean mountain ranges allow both a facies reconstruction of the outer western passive margin of the neo tethys and conclusions on the processes and timing of jurassic orogenesis comparison of melanges identical in age and component spectrum in different mountain belts figured out one neo tethys ocean in the western tethyan realm instead of multi ocean and multi continent scenarios

evolution of the adriatic carbonate platform palaeogeography main events and depositional dynamics abstract the adriatic carbonate platform adcp is one of the largest mesozoic carbonate platforms of the perimediterranean region its deposits comprise a major part of the entire carbonate succession of the croatian karst external or outer dinarides which is very thick in places more than 8000 m and ranges in age from the middle permian or even upper carboniferous to the eocene however only deposits ranging from the top of the lower jurassic toarcian to the top of the cretaceous can be attributed to the adcp defined as an isolated palaeogeographical entity although the entire carbonate succession of the karst dinarides was deposited within carbonate platform environments there were different types of carbonate platforms located in different palaeogeographical settings carboniferous to middle triassic mixed siliciclastic carbonate deposits were accumulated along the gondwanian margin on a spacious epeiric carbonate platform after tectonic activity culminating by regional middle triassic volcanism recorded throughout adria the african promontory a huge isolated carbonate southern tethyan megaplatform abbreviated as stm was formed with the area of the future adcp located in its inner part tectonic disintegration of the megaplatform during the middle to late early jurassic resulted in the establishment of several carbonate platforms including the adriatic apenninic and apulian separated by newly drowned deeper marine areas including the adriatic basin as a connection between the ionian and belluno basins lagonero basin and the area of the slovenian and bosnian troughs the adcp was characterised by predominantly shallow marine deposition although short or long periods of emergence were numerous as a consequence of the interaction of synsedimentary tectonics and eustatic changes also several events of temporary platform drowning were recorded especially in the late cretaceous when synsedimentary tectonics became stronger leading up to the final disintegration of the adcp the thickness of deposits formed during the 125 my of the adcp s existence is variable between 3500 and 5000 m the end of adcp deposition was marked by regional emergence between the cretaceous and the palaeogene deposition during the palaeogene was mainly controlled by intense synsedimentary tectonic deformation of the former platform area some carbonates mostly eocene in age were deposited on irregular ramp type carbonate platforms surrounding newly formed flysch basins and the final uplift of the dinarides reached its maximum in the oligocene miocene the adriatic carbonate platform represents a part although a relatively large and well preserved one of the broader shallow water carbonate platform that extended from ne italy to turkey although its continuity is somewhat debatable in the area near albanian greece boundary this large carbonate body which was deformed mostly in the cenozoic including a significant reduction of its width needs a specific name and the central mediterranean carbonate platform is proposed abbreviated to cmcp although the local names such as adcp for its nw part should be kept to enable easier communication and to facilitate description of local differences in platform evolution

the secular variation of earth s magnetic field models of the magnetic field at the core mantle boundary at selected epochs from 1715 0 to 1980 0 reveal novel features in the field at the core these suggest that the flow of core fluid is coupled to the mantle and that magnetic diffusion is significant

late archean manganese mineralization and younger supergene manganese ores in the anmod bisgod region western dharwar craton southern india geological characterization palaeoenvironmental history and geomorphological setting abstract the field setting mineralogical geochemical genetic and depositional features of late archean metasedimentary manganese as well as cenozoic supergene manganese and ferromanganese ores along the 75 km long linear tract of the anmod bisgod region in the western dharwar craton southern india are described the 2700 ma metasedimentary manganiferous formation is underlain by stromatolitic limestone dolomite and quartzite arenite and also occurs within the former the older quartzite arenite which contains redox sensitive detrital pyrite and uraninite as well as thucolite indicates sedimentation under anoxic to oxic transitional palaeoenvironmental condition whereas the younger stromatolitic limestone dolomite indicates development of oxygen in the depositional environment the manganese ore types have been classified into three categories one of which is a mn rich duricrust once capping the late archean manganiferous formation it formed during a cenozoic lateritic weathering event and was eroded in response to crustal uplift perhaps uniquely clastic deposits of the eroded mn duricrust occur in mineable quantities in the form of fluvial palaeochannel fills these cryptomelane rich supergene ores are also suitable for radiometric dating knowing their age would help to constrain the timing of mn duricrust formation and indirectly the regional weathering event responsible for the extensive laterite occurrences of southern india

the role of subduction accretion processes in the tectonic evolution of the mesozoic tethys in serbia abstract ophiolitic rocks and melange occur in two belts in serbia to the northeast the vardar zone and to the southwest the dinaride ophiolite belt of the drina ivanjica belt which is interpreted as being a microcontinent rifted from apulia the tethys in the vardar zone has a long and complex palaeozoic early tertiary history that is still poorly understood ophiolitic rocks are known to have formed at least in the early late jurassic some of the extrusives apparently formed in a supra subduction zone setting based on immobile element geochemical evidence the mesozoic tethys in the vardar zone closed at least partly by the latest jurassic evidence from the apulian margin in the southwest and from the drina ivanjica belt further to the northeast shows that a small red sea type oceanic basin rifted in the late permian early triassic followed by spreading in the mid late triassic to early jurassic evidence from immobile trace elements shows that these extrusives are of mid ocean ridge and within plate type during the early late jurassic this southerly oceanic strand began to close creating melange as a classic subduction accretion complex morb and seamount type lavas channelised micaceous turbidites rare calciturbidites and large limestone masses capped by condensed successions were incorporated in the melange through tectonic processes many of these units originated at or near the southwestern margin of the drina ivanjica inferred microcontinent less deformed debris flows olistostromes reflect gravity reworking and or diapirism with accumulation mainly in perched basins above the subduction zone large overriding harzburgitic ophiolite thrust sheets may have developed in response to incipient spreading above a subduction zone dipping to the southwest these ophiolites were emplaced northeastwards complete with metamorphic soles in response to collision of the trench with the drina ivanjica belt southwest directed sliding of sediments into the foredeep took place prior to suturing in the tithonian and later transgression by clastic sediments

tectonic units of the alpine collision zone between eastern alps and western turkey abstract we present a map that correlates tectonic units between alps and western turkey accompanied by a text providing access to literature data explaining the concepts used for defining the mapped tectonic units and first order paleogeographic inferences along strike similarities and differences of the alpine eastern mediterranean orogenic system are discussed the map allows 1 for superimposing additional information such as e g post tectonic sedimentary basins manifestations of magmatic activity onto a coherent tectonic framework and 2 for outlining the major features of the alpine eastern mediterranean orogen dinarides hellenides anatolides and taurides are orogens of opposite subduction polarity and direction of major transport with respect to alps and carpathians and polarity switches across the mid hungarian fault zone the dinarides hellenides taurides and apennines consist of nappes detached from the greater adriatic continental margin during cretaceous and cenozoic orogeny internal units form composite nappes that passively carry ophiolites obducted in the latest jurassic earliest cretaceous or during the late cretaceous on top of the greater adriatic margin successions the ophiolites on top of composite nappes do not represent oceanic sutures zones but root in the suture zones of neotethys that formed after obduction suturing between greater adria and the northern and eastern neotethys margin occupied by the tisza and dacia mega units and the pontides occurred in the latest cretaceous along the sava izmir ankara erzincan suture zones the rhodopian orogen is interpreted as a deep crustal nappe stack formed in tandem with the carpatho balkanides fold thrust belt now exposed in a giant core complex exhumed in late eocene to miocene times from below the carpatho balkan orogen and the circum rhodope unit its tectonic position is similar to that of the sakarya unit of the pontides we infer that the rhodope nappe stack formed due to north directed thrusting both rhodopes and pontides are suspected to preserve the westernmost relics of the suture zone of paleotethys

an updated archeomagnetic directional variation curve for france over the past two millennia following 25 years of additional data acquisition abstract nearly 40 years ago emile thellier published an article summarizing the archeomagnetic data he had obtained during his career which had allowed him to recover the main features of the directional variations of the geomagnetic field in france for the last two millennia this database went on to be significantly completed 25 years ago by ileana bucur who had taken over thellier s work on archeomagnetism this forms the current basis of our knowledge of the directional evolution of the geomagnetic field in france since then archeomagnetic studies have been continued at thellier s historical laboratory in saint maur this article presents the directional archeomagnetic data obtained in france over the past 25 years in this same laboratory a total of 528 new data are presented which together with the 170 results obtained on the french territory previously listed in bucur 1994 constitute the french directional database 698 data in all all but two of these data were obtained using the experimental protocol developed by e thellier based on the analysis of large samples and on a magnetic viscosity test that has proved its reliability on numerous occasions the directions from the entire french database have been precisely defined with 95 of the 95 values being less than 1 9 and 50 being less than 0 8 the selection of 286 data with dating uncertainties of 50 years allowed us to compute a new reference directional variation curve for france since the first century bc using sliding windows the variable durations and shifts of which were adapted to the time distribution of the available data and the bivariate extension of the fisher statistics this shows ample and smooth variations fairly similar to those previously determined from a much smaller database the resulting secular variation curve is particularly well suited for use in archeomagnetic dating

discrimination criteria for assigning ore deposits located in the dinaridic palaeozoic triassic formations to variscan or alpidic metallogeny among the ore deposits located in the palaeozoic triassic formations of the dinarides siderite ankerite and barite mineralization hosted by carboniferous and lower permian sediments as well as polimetalic deposits in the mid bosnian schists mts mb coused a long standing dipute concerning there metallogenic affiliations

structure and provenance of late cretaceous miocene sediments located near the ne dinarides margin inferences from kinematics of orogenic building and subsequent extensional collapse the ne part of the dinarides mountain chain located near their junction with the carpatho balkanides is an area where sedimentary basins associated with the neotethys subduction and collision are still exposed we performed a provenance study based on detrital fission track thermochronology combined with zircon upb magmatic geochronology and existing studies of kinematics and exhumation our study shows rapid sedimentation in the trench and forearc basin overlying the upper european tectonic plate a number of latest cretaceous early paleocene igneous provenance ages show a dominant magmatic source area derived from a late cretaceous subduction related arc this arc shed short time lag sediments in the forearc and the trench system possibly associated with focused exhumation in the serbo macedonian margin this was followed by burial of the trench sediments and a novel stage of middle late eocene exhumation driven by continued continental collision that had larger effects than previously thought the collision was followed by late oligocene miocene exhumation of the former lower adriatic plate along extensional detachments that reactivated the inherited collisional contact along the entire dinarides margin this event re distributed sediments at short distances in the neighboring miocene basins our study demonstrates that the dinarides orogenic system is characterized by short lag times between exhumation and re deposition whereas the upper tectonic plate is significantly exhumed only during the final stages of collision such an exhumation pattern is not directly obvious from observing the overall geometry of the orogen

a revised fit of south america and south central africa abstract a new set of parameters for the total plate tectonic reconstruction of south america and south central africa is presented euler pole 46 75 n 32 65 w rotation angle 56 40 this fit is constrained by at least three pre drift tectonic features crossing from one continent to the other 1 the geophysically defined eastern and western boundaries of the submarine jurassic outeniqua basin south africa and the falkland plateau basin 2 the late precambrian transcurrent fault and mylonite belts of pernambuco brazil and foumban west africa and 3 the triassic northern tectonic front of the cape fold belt and the major morphological feature on the falkland plateau with which it is closely lined up isotopic ages of falkland plateau gneisses correspond to cape pluton and cape fold belt ages suggesting their palaeoposition was within the realm of the cape fold belt in addition the bathymetrically and geophysically defined northeastern apex of the falkland plateau fits into the re entrant angle defined on the south african margin by the steep southeast facing sheared agulhas margin and the southern face of the tugela cone simultaneously known precambrian outcrops in northeastern brazil and in the gulf of benin area of west africa are juxtaposed rather than overlapped reconstructions producing a closer fit of these cratonic areas are considered untenable

volcanic rocks of the 1985 tibet geotraverse lhasa to golmud volcanic rocks encountered during the tibet geotraverse have been studied in the field in thin section and by major and trace element geochemistry in order to determine their most probable original eruptive environment rocks from a total of eleven distinct volcanic provinces were studied in this way they provide evidence for an active continental margin or post collision province of probable devonian early carboniferous age in the northern kunlun mountains an active continental margin of late carboniferous age in the southern lhasa terrane permian continental rifts in the central qiangtang and central kunlun terranes triassic volcanic arcs in the southern lhasa and northern qiangtang terranes a triassic active continental margin dyke swarm in the northern kunlun mountains a jurassic post collision or back arc rifting province in the southern qiangtang terrane a jurassic island arc in the northern lhasa terrane a cretaceous post collision province in the northern lhasa terrane possibly extending into the southern qiangtang terrane and a palaeogene active continental margin in the southern lhasa terrane an oligocene trachyte plug in the northern qiangtang terrane was the only evidence encountered during the geotraverse of volcanism post dating the palaeogene india eurasia collision however the composition of this plug coupled with new and published analyses from miocene volcanics in the southern lhasa terrane and from the pliocene recent volcanic province of northwest tibet places important constraints on models for post collision underplating of tibet by continental lithosphere any underplating is likely to have been a much later than the start of collision b directed beneath tibet from the north as well as the south and c limited in extent

geochronology of the baye mn oxide deposit southern yunnan plateau implications for the late miocene to pleistocene paleoclimatic conditions and topographic evolution 40ar 39ar laser incremental heating analyses of supergene k mn oxides from weathering profiles at the baye mn deposit southern yunnan plateau sw china were carried out to place constraints on the timing of weathering and derive insights into local paleoclimatic and landscape evolution weathering profiles in the baye mn mine are dominated by 20 30 m thick saprolites which are locally covered by unconsolidated alluvial deposits we analyzed 70 grains from 35 hand samples collected from four sites located at distinct elevations in most cases different grains from the same hand sample and different samples from the same site have reproducible 40ar 39ar plateau ages the plateau ages of individual grains are also consistent with the respective isochron integrated and ideogram ages this age consistency attests to the reliability of the geochronological results as numerical constraints on the formation and evolution of these weathering profiles sixty four grains from 32 hand samples collected from weathering profiles at four sites a to d yield well defined plateau or pseudoplateau ages ranging from 2 98 0 07 to 0 16 ma 2 the age results show that mn oxides become progressively younger towards lower elevations samples collected from the highest elevation site 1855 m a s l have the oldest ages at 2 98 0 07 to 2 83 0 13 ma samples from intermediate elevations 1821 to 1815 m a s l yield ages ranging from 2 91 0 10 to 2 08 0 11 ma and those from the lowest sites 1768 to 1753 m a s l are younger than 1 25 0 08 ma the age versus elevation relationship indicates fast downward propagation of weathering front since the late pliocene and permits estimating the weathering rates at 3 3 3 8 6 6 1 2 and 11 1 1 9 m myr 1 during the time periods of 2 98 2 83 2 91 2 08 and 1 25 to 0 16 ma respectively the average river incision rates estimated from the surface exposed ages range from 235 10 to 416 19 m myr 1 during the past 3 million years the incision rate calculated from minimum exposed ages at sites b d is 33 2 m myr 1 for the time period of 2 91 1 25 ma which is consistent with the slow incision of streams on the ridges over the yunnan plateau since the early pleistocene three mn oxide pebbles contained in the alluvial deposits overlying the saprolite dominated weathering profiles at the two highest sites provided additional information on the timing of weathering four grains from two pebbles yield plateau ages of 6 32 0 19 to 5 27 0 10 ma whereas the other two grains from the third pebble indicate the minimum formation ages of 8 2 0 4 and 9 3 0 3 ma these ages confirm the existence of older weathering profiles now dismantled in the region manganese oxide 40ar 39ar ages of the baye deposit when combined with results from other localities indicate that lateritic weathering and supergene mn enrichment and by inference warm and humid climates conducive to intense weathering have prevailed over the plateau since the middle to late miocene the climatic conditions inferred from the weathering geochronology are consistent with multiple independent marine and terrestrial sedimentary and paleontological records confirming that supergene mn oxides can be used as a useful proxy for past climate age clusters of mn oxides at 2 9 2 4 1 2 0 8 and 0 6 0 4 ma are broadly coincident with and thus likely reflect intensification events of the indian summer monsoon that brings moisture and abundant precipitation to the yunnan plateau these clusters also coincide with the periods of significant surface uplift in the yunnan plateau demonstrating a causal link between topographic evolution plateau uplift and intensification of the monsoonal climate

pangea in permian to jurassic time the apparent polar wander paths apwps for the northern and southern continents have been redetermined using revised time scales and data when plotted on the conventional pangea pangea a the paths are distinct but essentially linear and parallel from late carboniferous to late permian time both paths then undergo a sharp change in direction and converge so as to overlap in early to middle jurassic time if the permo triassic magnetic field was an axial dipole then paleomagnetic data show that pangea was mobile during most and possibly all of the mid triassic to mid jurassic interval because of the uncertainties in pole ages and positions and errors in measurements the new apwps are consistent with a range of pangeas in particular the data are compatible with two paleomagnetic pangeas a2 and a new class of pangea here named the d pangeas these resemble morel and irving s 1981 pangea b and the c pangeas of smith et al 1981 unlike the a2 pangeas the b c and d pangeas all require large 2000 km relative motion between the northern and southern continents geological data require the transition from an earlier pangea to pangea a to take place along a transform zone here named the pangea transform zone or ptz the ptz is considered to be technically analogous to the great glen fault of scotland four types of possible ptz are examined passing through either the western or eastern end of the brunswick magnetic anomaly of eastern north america or north and south of iberia the geological data suggest that those pangeas that require large displacements on ptzs that pass east of the brunswick magnetic anomaly are improbable b and c pangeas are snapshots giving precise paleomagnetic reconstructions d pangeas fit long stretches of the apwps as well as possible a2 pangeas fit the apwps moderately well but also require the geological displacement to be smaller the differences in displacement between the d and a2 pangeas reflect geometrical relationships between euler poles ptz curvature and the apwps the general geological evidence suggests that d pangeas are less probable than a2 pangeas but that they cannot be rejected transition from a2 to a is most likely to have occurred via a sequence of rotations about different poles rather than a single rotation in detail the conventional pangea requires modification africa must be broken into at least two fragments along the benue trough and its northern continuation northwest africa then fits more tightly into northern south america which modifies the apwps and reduces the gap between them by including the effects of continental extension and using detailed geological data the gap can be reduced to one in which the a63 circles touch or overlap non dipole fields or an expanding earth need not be invoked to explain the mismatch of the apwps on the conventional pangea the finite rotation required for the older pangea to pangea a is very close to van der voo and french s 1974 single rotation for a2 to a but is actually the sum of three smaller rotations that reflect the modification to pangea a continental extension and the postulated ptz displacement of the possible ptzs a ptz parallel to the west african continental magnetic anomaly with a displacement of about 170 km gives a better fit of this anomaly to the east coast magnetic anomaly of north america further refinements to pangea a2 will depend on a better knowledge of stretching factors on the passive continental margins of the modified pangea a and incorporation of rotations due to major faults such as the south atlas fault in morocco

metalliferous manganese oxide mineralization associated with the boleo cu co zn district mexico apart from its large reserves of cu co zn mineralization contained within stratiform ore beds or mantos the boleo district of central baja mexico also contains significant concentrations of mn oxides typically 2 3 wt mn interspersed with hydrothermal sulfides and both hydrothermal and weathering related metal oxide minerals within the mantos mn oxides occur as concentric nodules disk shaped concretions laminae granules or aggregates veins dendrites and various replacements the oxides are composed of varying amounts of hollandite cryptomelane todorkite romanechite pyrolusite and x ray amorphous mn oxide chemically the manto oxides are unique owing to their extraordinarily high abundances of co cu pb and zn typically 2 5 wt but up to 23 wt existing mineralogical and geochemical classifications are ambiguous regarding the genetic classification of mn oxides within the mantos a new mno 2 fe 2 o 3 t mgo diagram is proposed in order to discriminate among highly metalliferous mn oxides based on the new classification scheme manto mn oxides record a complex interaction between hydrothermal and diagenetic processes to better understand the mechanism of oxide deposition radiogenic isotopes sr and pb and ree geochemistry were used to delineate fluid and metal sources isotopic and ree constraints indicate that the boleo mn oxides are largely hydrothermal in origin however they show varying degrees of modification resulting from diagenetic processes hydrothermal mn oxides are syngenetic synsedimentary to syndiagenetic in origin and were precipitated in association with sulfides reflecting localized variations in redox conditions within the manto sequence due to the downward infiltration of metal bearing brines apart from morphological and stratigraphic evidence e g occurrence of nodular crusts along the upper contact of the manto a syngenetic origin is supported by a single k ar date of 7 0 0 2 ma which is within the estimated age range for deposition of boleo formation clastic sediments interaction between brine and seawater or brine and pore fluid as it infiltrates the sedimentary pile produced hydrothermal precipitates with certain hydrogenetic attributes i e positive ce anomaly lree enrichment on the other hand oxides with hydrogenetic and with mixed hydrothermal hydrogenetic affinities represent diagenetic alteration and remobilization of earlier formed hydrothermal oxides by meteoric fluids by far the greatest degree of remobilization occurred in response to compactional dewatering of the manto where the dewatering pore waters enriched the uppermost manto strata in mn fe zn co and to lesser degrees cu later diagenetic modifications coincided with the development of a supergene overprint which typifies boleo formation strata at elevations above the present day water table

balancing cross sections with kinematic constraints the dolomites northern italy balancing of structural cross sections is often hindered by a limited database or tectonic complications in such cases kinematic constraints may help to better constrain the construction of a cross section and its extrapolation to depth the kinematic implications of a known structure generally at the surface have to be considered for the unknown structure and for the internal consistency of a cross section a key point is the quantification of shortening of the known structure the extrapolation of this parameter allows balancing in an approximate way emphasizing major structures while minor structures are either neglected or treated in a simplified way with this approach a quick overview can be achieved on how much leeway for interpretation exists critical points for testing various working hypotheses become more evident and the trial and error effort during the construction of a balanced section is considerably shortened still the implications the observed structures have on the deeper crustal levels can be assessed this concept is applied to a section across the italian dolomites a brittle fold and thrust belt lacking reflection seismic data so far and thus with a poorly known deep structure still active south vergent alpine thrusting is oriented approximately perpendicular to previous dinaric thrusting the ramp flat style is dominant due to rheological constrasts within the stratigraphic sequence thanks to kinematic balancing based on surface data three alpine thrust systems could be identified each defining a thrust sheet with its particular basement unit and its sedimentary part superposition of the two more external basement units led to the presently observed high elevation of the triassic sequence in the dolomites shortening since late miocene times amounts to more than 50 km retrodeformation of alpine thrusting is a key for better understanding the previous dinaric structures and kinematics