magnetism and transmission electron microscopy of fe ti oxides and pyroxenes in a granulite from lofoten norway granulites from southwest lofoten norway often carry intense and stable components of natural remanent magnetization nrm coercivities in excess of 100 mt and blocking temperatures up to 580 645 c are common several phases in these rocks can carry stable components of nrm clinopyroxenes augite with exsolved pigeonite and orthopyroxene usually contain abundant fe ti oxides the majority of which are too small to be studied conclusively with visible light additionally these rocks typically contain coarse grained 100 1000 m exsolution intergrowths of rhombohedral fe ti oxides and coarse grained magnetite we have studied the magnetism and mineralogy of the titanohematite and the inclusion bearing augites in one granulite sample using transmission electron microscopy tem and magnetic experiments to understand better the remanence properties of naturally occurring fe ti oxides and their relationships to silicate minerals the coarse grained titanohematite has a coercivity of 350 mt and a laboratory saturation remanence that unblocks at 570 600 c the hysteresis characteristics of this titanohematite are typical of much smaller 10 m sized single domains which apparently reflects the presence of exsolved ilmenite lamellae as small as 10 nm in thickness these lamellae presumably inhibit 1 domain wall nucleation and 2 movement of walls that manage to nucleate the inclusions in the augites serve as carriers of lower coercivity remanence with blocking temperatures less than about 560 c magnetite hematite and ilmenite are present in the augite and intergrowth of the rhombohedral phases revealed by tem persists down to the 20 nm scale the distribution of the magnetite inclusions in the augite appears to be unrelated to any silicate microstructure while that of the rhombohedral fe ti oxides is strongly controlled by silicate microstructure with intergrowths of these oxides commonly decorating lamellae of pigeonite orthopyroxene and clinoamphibole within the augite the nrm of this sample possibly dates from the amphibolite facies metamorphism that the region experienced about 1000 m y ago paleomagnetic studies of these rocks could readily recover virtual geomagnetic poles dating from this event in addition to the included oxides the augites display a number of unusual features that probably result from the protracted polymetamorphic history of these granulites pigeonite lamellae are not parallel to the b crystallographic axis pigeonite lamellae cut across those of orthopyroxene and numerous included minerals occur rare earth elements are concentrated within inclusions that reside in the augite

magnetic fabric of granitoids from southern corsica and northern sardinia and implications for late hercynian tectonic setting anisotropy of magnetic susceptibility ams data 180 sites from hercynian granitoids from southern corsica france and northern sardinia italy provide a high density net of structural information for this part of the european hercynian belt two main groups of granitoids defined according to petrography and relative chronology provide contrasting ams results for both groups shallowly dipping magnetic foliations and lineations suggest that these granitoids were emplaced as large scale shallowly dipping sheets or open funnel shaped structures at a high structural level in non leucocratic granitoids 305 300 ma foliation and lineation trajectory maps reveal antiforms and synforms with a n130 axial trace intersection of the foliation planes indicates a n120 low plunging fold axis revealing a n120 maximum stretching direction slightly younger leucocratic granitoids 295 285 ma bounded by late hercynian faults display n50 lineations parallel to the elongated shapes of these intrusions this structural pattern is undisturbed by the faults and suggests an intrusion in decakilometre scale tensional fractures indicating a n140 instantaneous stretching direction compatible with the stretching direction evidenced in non leucocratic granitoids a younger c n20 oriented permian dyke swarm confirms the permanence of this stretching direction over at least 35 ma and indicates a coaxial strain regime to integrate these new data within a broader context in the hercynian belt corsica has to be back rotated clockwise by 70 to its permian position which indicates a north to n30 stretching direction for the 305 285 ma interval compatible with the post collisional strain and stress fields of the french massif central at about the same period

invariance and scaling properties in the distributions of contributing area and energy in drainage basins a the cumulative probability distributions for stream order stream length contributing area and energy dissipation per unit length of channel are derived for an ordered drainage system from horton s laws of network composition it is shown how these distributions can be related to the fractal nature of single rivers and river networks finally it is shown that the structure proposed here for these probability distributions is able to fit the observed frequency distri butions and their deviations from straight lines in a log log plot

microstructure and magnetic susceptibility applied to emplacement kinematics of granites the example of the foix pluton french pyrenees abstract the variscan foix granite 40 km 2 central pyrenees was subjected to detailed microstructural magnetic susceptibility and anisotropy studies magnetic susceptibility is shown to be of dominant paramagnetic origin its magnitudes clearly define a zoning which compares with the petrographic zoning and reveals that the pluton is the southern remnant of a normally zoned pluton truncated during alpine tectonics magnetic susceptibility anisotropy characterizes the planar and linear preferred orientations of the main iron bearing silicates namely biotite and accessory amphibole it is argued that these fabrics also reflect the flow pattern of the granite microstructural observations aid in distinguishing the magmatic and late magmatic fabric signatures main body from the post magmatic solid state signature leucogranite in order to account for the consistently subhorizontal and ne sw directed magmatic lineations in the northern sector of the foix granite an oblique intrusion is proposed it may be coeval with the early variscan regional sw trending shearing event steepening of the structures in at least part of the southern sector may be coeval with the late development of upright folds and vertical cleavage in the country rocks the utility of combined microstructural and magnetic susceptibility studies is demonstrated as an aid in the structural interpretation of granite intrusions which act as kinematic markers of their host terranes

orogen parallel tectonic transport in the variscan belt of northeastern sardinia italy implications for the exhumation of medium pressure metamorphic rocks a transpressive crustal scale dextral shear zone is documented in the variscan basement of northeastern sardinia it indicates the presence of a shear deformation parallel to the belt overprinting previous d1 structures related to nappe stacking and top to the s and sw thrusting the l2 stretch ing lineation points to an orogen parallel stretching and to a general change in the tectonic transport from d1 to d2 phase d1 developed during initial frontal collision whereas the d2 deformation was characterized by dextral shearing during the increasing curvature of the ibero armorican arc transpressional deformation developed in a regime of decreasing pressure it caused telescoping of the barrovian isograds and the exhumation of the low to medium grade metamorphic rocks in this sector of the variscan belt exhumation is due to continuing compression with an increasing compo nent of horizontal displacement the overall change of the shortening direction in a large sector of an orogenic belt with the occurrence of increasing orogen parallel displacement may be regarded as a general mechanism affecting the exhumation of rocks and preventing the thickened collisional crust from undergoing a generalized gravitational collapse

scaling relationships of channel networks at large scales examples from two large magnitude watersheds in brittany france abstract we present a statistical analysis on two watersheds in french brittany whose drainage areas are about 10 000 and 2000 km 2 the channel system was analysed from the digitised blue lines of the 1 100 000 map and from a 250 m dem link lengths follow an exponential distribution consistent with the markovian model of channel branching proposed by smart 1968 the departure from the exponential distribution for small lengths that has been extensively discussed before results from a statistical effect due to the finite number of channels and junctions the strahler topology applied on channels defines a self similar organisation whose similarity dimension is about 1 7 that is clearly smaller than the value of 2 expected for a random organisation the similarity dimension is consistent with an independent measurement of the horton ratios of stream numbers and lengths the variables defined by an upstream integral drainage area mainstream length upstream length follow power law distributions limited at large scales by a finite size effect due to the finite area of the watersheds a special emphasis is given to the exponent of the drainage area a a that has been previously discussed in the context of different aggregation models relevant to channel network growth we show that a a is consistent with 4 3 a value that was obtained and analytically demonstrated from directed random walk aggregating models inspired by the model of scheidegger 1967 the drainage density and mainstream length present no simple scaling with area except at large areas where they tend to trivial values constant density and square root of drainage area respectively these asymptotic limits necessarily imply that the space dimension of channel networks is 2 equal to the embedding space the limits are reached for drainage areas larger than 100 km 2 for smaller areas the asymptotic limit represents either a lower bound drainage density or an upper bound mainstream length of the distributions because the fluctuations of the drainage density slowly converge to a finite limit the system could be adequately described as a fat fractal where the average drainage density is the sum of a constant plus a fluctuation decreasing as a power law with integrating area a fat fractal hypothesis could explain why the similarity dimension is not equal to the fractal capacity dimension as it is for thin fractals the physical consequences are not yet really understood but we draw an analogy with a directed aggregating system where the growth process involves both stochastic and deterministic growth these models are known to be fat fractals and the deterministic growth which constitutes a fundamental ingredient of these models could be attributed in river systems to the role of terrestrial gravity

influence of granitoid textural parameters on sediment composition implications for sediment generation the aim of this study is to determine and characterise the control exerted by parent rock texture on sand composition as a function of grain size the sands investigated were generated from granitoid parent rocks by the rhone damma and sidelen glaciers which drain the aar massif in the central alps switzerland and were deposited in glacial and fluvio glacial settings mechanical erosion comminution crystal breakdown and abrasion and hydraulic sorting are the most important processes controlling the generation of sediments in this environment whereas chemical and or biochemical weathering plays a negligible role by using a gis based microscopic information system mis five samples from the glacier drained portions of the aar basement have been analysed to determine textural parameters such as modal composition crystal size distribution and mineral interfaces types and lengths petrographic data of analysed sands include traditional point counts gazzi dickinson method minimum of 300 points as well as textural counts to determine interface types frequency and polycrystallinity in phaneritic rock fragments according to pettijohn s classification grain size dependent compositions vary from feldspathic litharenite 0 fraction via lithic arkose 1 and 2 to arkose 3 and 4 compositional differences among our data set were compared to modern plutoniclastic sands from the iberian massif spain and the st gabriel mts california usa which allowed us to assess the role exerted by glaciers in generating sediments by combining data from the mis with those from petrographic analysis we outlined the evolution of mineral interfaces from the parent rocks to the sediments

zircon coronas around fe ti oxides a physical reference frame for metamorphic and metasomatic reactions ilmenite in coronitic gabbros from the bamble and kongsberg sectors southern norway is surrounded by zircons ranging in diameters from a fraction of a micrometer to 10 m across the zircons are inert during subsequent metamorphism amphibolite to pumpellyite prehnite facies and metasomatism scapolitization and albitization and can be found as trails in silicates phlogopite talc chlorite amphibole albite and tourmaline in the altered rocks the trails link up to form polygons outlining the former oxide grain boundary this 3 dimensional framework of zircons is used to a recognize metasomatic origin of rocks b quantify the mobility of elements during mineral replacement c establish the growth direction of reaction fronts and to identify the reaction mechanism as dissolution reprecipitation zircon coronas on fe ti oxides have been described from a number of terrains and appear to be common in mafic rocks gabbros and granulites providing a tool for a better understanding of metasomatic and metamorphic reactions

gorongosa by the sea first miocene fossil sites from the urema rift central mozambique and their coastal paleoenvironmental and paleoecological contexts abstract the east african rift system ears has played a central role in our understanding of human origins and vertebrate evolution in the late cenozoic of africa however the distribution of fossil sites along the rift is highly biased towards its northern extent and the types of paleoenvironments are primarily restricted to fluvial and lacustrine settings here we report the discovery of the first fossil sites from the urema rift at gorongosa national park central mozambique at the southern end of the ears and reconstruct environmental contexts of the fossils in situ and surface fossils from the lower member of the mazamba formation estimated to be of miocene age comprise mammals reptiles fishes invertebrates palms and dicot trees fossil and geological evidence indicates a coastal plain paleoenvironmental mosaic of riverine forest woodland and estuarine habitats that represent the first coastal biomes identified in the neogene ears context receiving continental sediment from source terranes west of today s urema graben estuarine sequences accumulated prior to rifting as compound incised valley fills on a low gradient coastal plain following transgression modern environmental analogues are extremely productive habitats for marine and terrestrial fauna including primates thus our discoveries raise the possibility that the miocene coastal landscapes of gorongosa were ecologically favorable habitats for primates providing relatively stable maritime climate and ecosystem conditions year round freshwater availability and food both from terrestrial and marine sources the emerging fossil record from gorongosa is beginning to fill an important gap in the paleobiogeography of africa as no fossil sites of neogene age have previously been reported from the southernmost part of the ears furthermore this unique window into past continental margin ecosystems of central mozambique may allow us to test key paleobiogeographic hypotheses during critical periods of primate evolution

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

genesis of a variscan batholith field petrological and mineralogical evidence from the corsica sardinia batholith abstract the genesis of the corsica sardinia batholith csb can be explained in a continental collision setting during the time of construction of the batholith which is dated at about 70 ma three magmatic associations were emplaced mg k calc alkaline u1 composite mafic silicic calc alkaline u2 and post collisional alkaline u3 in addition the level of intrusion became progressively shallower from 5 6 kbar for u1 to less than ikbar for u3 in a large scale uplift associations u1 and u2 exhibit non cogenetic mafic and felsic components the processes and products of interaction between mantle and crustal derived melts are however different and may be related to the thickness of the crust which controls both the parameters t pco2 ph2o andptot and the time of mingling of the products of anatexis and basaltic melts both u1 and u2 granitoids were formed by crystallization of a magma derived by fusion of a source of greywacke composition the melting occurred successively under granulitic u1 and amphibolite facies conditions u2 in association u1 basaltic melts displaying potassic lamprophyre like composition up to 8 k2o are of uncertain affinity furthermore crustal interaction with the basic magma precludes precise definition of the source in the u2 composite association the primary tholeiitic character of the mantle derived basic rocks is constrained by both mineralogical and geochemical data dry and hydrated subunits are distinguished among the basic rocks associated with the granitoids no hybridization processes have been identified in the core of the mafic complexes of the dry subunits but they are recognized at the level of emplacement at the margins at the contact with the granitoids contamination in the mafic hydrated subunit is due to the increase in water content of crustal origin in the magma during the ascent and emplacement of the basic complexes leading to the development of abundant green amphibole extensive hybridization is restricted to the base of the crust where it led to incomplete mixing yielding granodiorites and enclaves the genesis and emplacement of the composite calc alkaline u2 association took place in an extensional setting in an uplifting basement postdating a collisional event during the devonian an eastern austro alpine block was thrust onto the western ebro balearic continent the u1 mg k granitoids resulted from anatexis under conditions of pco2 ph2o this took place at the base of the austro alpine crust which was undergoing fusion due to its adiabatic ascent the u2 calc alkaline granitoid rocks were derived in a hydrated environment by fusion of the same austro alpine crust in corsica and of the ebro balearic crust on sardinia and in the pyrenees the conditions of interaction between mafic and felsic magmas may serve as a geodynamic marker providing information on the thickness of the crust in time and space and on the genesis of continental crust the persistence during the palaeozoic of dominantly tholeiitic mafic magmatism suggests that hot spot activity occurred in this area of southwestern europe during pre variscan and variscan times up to the time of opening of the liguro piedmontese ocean birthplace of the alpine orogen

ca distribution between almandine rich garnet and plagioclase in pelitic and psammitic schists from the metamorphic basement of north eastern sardinia garnet and plagioclase pairs from fourteen selected samples from garnet to sillimanite zones collected along a ns traverse throughout the metamorphic basement of ne sardinia have been analyzed by microprobe

on the description of the basin effective drainage structure abstract the relationships between the local properties of the morphological elements of a drainage basin and its drainage structure are analysed two theoretical approaches are used for which hydro dynamics and basin morphology are respectively assumed to represent the lowest and the highest starting scale level the results show a convergence toward the idea of slope scaling with contributing area in river networks this concept is then used as an indicator of channel initiation and maintenance for the identification of the effective drainage structure of the basin on the basis of existing digital elevation models dems integral properties of the single dem pixel are finally introduced to reduce the noise owing to a possible dem low accuracy e g in the estimation of the local slope value from the elevation data set the resulting network is characterized by a non uniform drainage density as expected for natural river networks

transformation of cumulate mafic rocks to granulite and re equilibration in amphibolite and greenschist facies in ne sardinia italy abstract the ultramafic amphibolites hosted in the hercynian migmatite of ne sardinia consist of three main compositional layers a b c from a few metres to a few decametres thick layer a is made up of coarse grained olivine chlorite amphibole spinel minor pyroxene garnet and rarely plagioclase layer b is made up of coarse grained plagioclase olivine pyroxene spinel garnet and amphibole layer c consists mainly of porphyroblastic garnet pyroxene large amphibole grains up to 5 cm and minor plagioclase on the basis of mineral assemblages and microstructures three stages of mineralogical re equilibration can be recognised granulite amphibolite and greenschist primary igneous olivine and anorthite reacted under granulite conditions to produce coronas consisting of orthopyroxene clinopyroxene green spinel and garnet the amphibolite stage is characterised by the formation of brown and green clinoamphiboles between pyroxene and corona garnet anthophyllite talc mg rich chlorite plagioclase and cr bearing spinel greenschist stage minerals mostly replacing the mafic minerals consist of tremolite fayalite epidote albite calcite dolomite and serpentine the history of the ultramafic amphibolites started with igneous crystallisation and continued through granulite t 700 740 c p 8 10 kbar amphibolite t 580 640 c p 4 6 kbar and greenschist facies t 330 400 c p evaluation of p t conditions indicated a p t path from the protolith to granulite stage characterised by an increase in pressure and temperature and then from the granulite facies through amphibolite to the greenschist stage characterised by a decrease in pressure and temperature the petrological evolution of the ultramafic amphibolites and the p t time path is discussed in the context of the hercynian orogeny in sardinia

magnetic susceptibility of the mont louis andorra ilmenite type granite pyrenees a new tool for the petrographic characterization and regional mapping of zoned granite plutons we have observed remarkably consistent patterns of concentric zonation in the values of low field magnetic susceptibility measurements over the variscan age mont louis andorra granite pluton of the eastern pyrenees of andorra spain and france this zonation is a function of the rock petrology since a close correlation is shown between the petrographic nature as defined by chemical analysis and the magnetic susceptibility magnitude of the rocks it reflects the dominantly paramagnetic nature of the granite carried by fe bearing silicates and this is demonstrated by the fact that low field susceptibilities of representative specimens are almost equal to 1 their high field susceptibilities and 2 their calculated susceptibilities using fe contents assuming a null ferromagnetic contribution we conclude that this technique accurately reflects the modal abundances of ferromagnesian phases in rocks of the ilmenite series and that it represents a powerful and efficient tool for the reconnaissance surveying of petrological variations in granitoid plutons

from static to dynamic provenance analysis sedimentary petrology upgraded abstract the classical approach to sandstone petrology established in the golden years of plate tectonics and based on the axiom that detrital modes of sandstone suites primarily reflect the different tectonic settings of provenance terranes has represented a benchmark for decades the composition of sand and sandstone however simply provides us with a distorted image of the lithological structure of source terranes and gives us little clue whether they are allochthonous or autochthonous orogenic or anorogenic young or old what we may able to see reflected in detrital modes is the nature of source terranes continental arc oceanic and the tectonostratigraphic level reached by erosion in space and time the proposed new approach to the petrology of sand and sandstone 1 starts with a simple classification scheme circulated since the 1960s which is purely descriptive objective and free of ill defined ambiguous terms and 2 focuses on the nature and tectonostratigraphic level of source terranes further steps are essential to upgrade provenance analysis acquiring knowledge from modern settings is needed to properly identify and wherever possible correct for physical and chemical processes introducing environmental and diagenetic bias and thus address nature s complexities with adequate conceptual tools equally important is the integration of multiple techniques ideally including bulk sediment multi mineral and single mineral methods bulk sediment petrography remains the fundamental approach that allows us to capture the most precious source of direct provenance information represented by the mineralogy and texture of rock fragments bulk sediment geochemistry applicable also to silt and clay carried in suspension is a superior method to check for hydraulic sorting chemical weathering and fertility of detrital minerals in different sediment sources detrital geochronology thermochronology and isotope geochemistry reveal the diverse time structures of source rocks and have become necessary complementary techniques in modern provenance analysis inferences on geodynamic processes need independent geological information and come last but if tackled properly they can lead us much farther than the standard label obtained by using triangular diagrams uncritically as if they were infallible oracles

conceptualization and scale in hydrology abstract it is argued that the levels of scale at which a meaningful conceptualization of physical processes is possible are not arbitrary and their range is not continuous formulations appropriate at a given level usually are not applicable at the immediately adjoining levels this is seen as one of the important reasons for the slow progress of hydrological science on basin scale since it is dominated by different effects than those observed at the immediately lower level of channel reaches and hillslopes which represent the focus of current hydrological research the search for an appropriate level of conceptualization can proceed either upward or downward along the hierarchy of scales both possibilities are discussed and an example of the downward route is given

stable isotope dietary reconstructions of herbivore enamel reveal heterogeneous savanna ecosystems in the plio pleistocene malawi rift abstract the plio pleistocene expansion of eastern africa savanna ecosystems was a major driver for morphological and behavioral innovations in hominin evolution most evidence for hominin ecosystem reconstructions originates from the eastern rift in today s somali masai endemic zone we provide stable carbon 13 c and oxygen 18 o isotope data from 22 bovid equid suid hippopotamid and elephant teeth from the southern part of the east african rift and supplement these by 18 o values of present day meteoric water this region karonga basin malawi hosts remains of homo rudolfensis and paranthropus boisei it is situated between hominin bearing sites in eastern and southern africa and therefore fills an important geographical gap for hominin evolution tooth enamel 13 c and 18 o data of 14 large bodied mammalian herbivore species from units 2 and 3 ca 4 3 to 0 6 ma in the chiwondo beds provide information about foraging strategies and vegetation patterns our stable isotope data reflect c 3 dominated ecosystems that permitted access to open c 4 environments for migratory mammals many analyzed taxa show evidence for mixed diets with high abundances of c 3 biomass consumption only selected bovids were specialized grazers 18 o of present day meteoric water reflects rainfall amount and seasonality elevation and lake evaporation 18 o values of karonga basin herbivore enamel are generally low and cover values similar to present day drinking water indicating both evaporated and freshwater sources comparison of 13 c values from the karonga basin with other eastern african herbivore communities reveals significant differences in dietary patterns with much more generalized mixed c 3 c 4 and pure c 3 feeding behaviors in the malawi rift similarly enamel 18 o values are lower than in most plio pleistocene eastern african herbivores suggesting more humid conditions collectively our data support models in which early hominin foraging activity included access to c 3 dominated biomes complicating traditional interpretations linking early human evolutionary innovations to a shift towards savanna habitats

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

the nature of calcareous deposits along pan margins in eastern central namibia in a region along the western margin of the kalahari in eastern namibia and western botswana many pan basins have mainly calcareous deposits along part of their margins these are typically lined by low vertical scarps in namibia these pans are mainly located in dry river beds the petrographical study of these deposits demonstrates that they consist of lacustrine sediments that have to a varying extent been affected by early diagenetic processes and by the formation of late diagenetic features the original composition of the deposits ranges from highly calcareous sediments typically with ostracod diatom and charophyte remains to entirely non calcareous diatomites the deposits generally show an upward increase in total carbonate content which is mainly a synsedimentary feature the early diagenetic processes that affected the deposits include the formation of orthic siliceous nodules at a later stage secondary calcite enrichment occurred leaving only the silica impregnated sections unaffected this enrichment partly accounts for the upward increase in total carbonate content in some profiles and often resulted in the development of a highly calcareous surface horizon sepiolite and amorphous silica that are part of the groundmass of the deposits may also partly have formed at this stage late diagenetic features include various forms of secondary calcite and silica this study of pan basins in eastern central namibia indicates that a lacustrine rather than purely pedogenic origin should also be considered for calcareous deposits that commonly occur along pan margins in other parts of southern africa copyright 2002 john wiley sons ltd

hydrodynamic description for advective sediment transport processes and rill initiation a physically based model for advective sediment transport during rill initiation is developed based on 1 a short wavelength rescaling of the momentum balance equations for the surface water flux 2 a deforming coordinate system which characterizes local morphological changes of the surface during erosion and 3 a generalized transport function for the surface sediment flux which distinguishes the contributions of diffusive and advective transport a linear stability analysis of the coupled system indicates that an intrinsic minimum transverse length scale for rill initiation is forced by the cross slope pressure gradient in the surface water the wavelength of this initial rill spacing is dependent upon the relative significance of the advective component of sediment transport and the hydraulic efficiency of the surface water flux and is independent of the total length of the hillslope the hydrodynamic model presented can also be used to evaluate the nonlinear development of rills and rill networks subsequent to initiation

aeromagnetic anomalies magnetic petrology and rock magnetism of hemo ilmenite and magnetite rich cumulate rocks from the sokndal region south rogaland norway aeromagnetic maps of the egersund mid proterozoic igneous province show a spectacular range of positive and negative magnetic anomalies with a contrast up to 15 600 nt the positive magnetic anomalies are over magnetite norites and overlying mangerites and quartz mangerites of the bjerkreim sokndal layered intrusion these rocks are dominated by multi domain md magnetite the negative magnetic anomalies are over ilmenite rich norites of the bjerkreim sokndal layered intrusion the tellnes ilmenite norite ore deposit and massif anorthosites these rocks are dominated by hemo ilmenite and or by silicates containing fine grained oxide exsolution lamellae electron microprobe analyzes of coexisting fe ti oxides in the layered intrusion confirm earlier observations that oxides in early magmatic rocks are dominated by hemo ilmenite with minor end member magnetite followed by more reduced oxides dominated by titanomagnetite with minor near end member ilmenite what is not fully understood is the property of ilmenite with hematite exsolution lamellae or even more striking hematite with ilmenite lamellae to produce strong remanent magnetization of high coercivity and with a neel temperature equal to or above the curie temperature of magnetite this property makes the rhombohedral oxides an important candidate to explain some high amplitude deep crustal anomalies on earth or strong remanent magnetization on other planets a remarkable feature in the egersund province is that primitive magmas produced rocks rich in hemo ilmenite causing negative magnetic anomalies related to magnetic remanence and more evolved magmas produced rocks rich in magnetite related to positive induced magnetic anomalies all in the course of crystallization differentiation

stochastic forcing of sediment supply to channel networks from landsliding and debris flow sediment influx to channel networks is stochastically driven by rainstorms and other perturbations which are discrete in time and space and which occur on a landscape with its own spatial variability in topography colluvium properties and state of recovery from previous disturbances the resulting stochastic field of sediment supply interacts with the topology of the channel network and with transport processes to generate spatial and temporal patterns of flux and storage that characterize the sedimentation regime of a drainage basin the regime varies systematically with basin area we describe how the stochastic sediment supply is generated by climatic topographic geotechnical and biotic controls that vary between regions the general principle is illustrated through application to a landscape where sediment is supplied by mass wasting and the forcing variables are deterministic thickening of colluvium random sequences of root destroying wildfires and random sequences of rainstorms that trigger failure in a population of landslide source areas with spatial variance in topography and colluvium strength landslides stop in channels or convert to scouring debris flows depending on the nature of the low order channel network sediment accumulates within these channels for centuries before being transferred downstream by debris flows time series of sediment supply transport and storage vary with basin scale for any combination of climatic topographic and geotechnical controls in a companion paper benda and dunne this issue we use simulations of timing volumes and locations of mass wasting to study the interaction between a stochastically forced sediment supply and systematic changes of storage and flux through channel networks

magnetic fabric vs magnetite and biotite shape fabrics of the magnetite bearing granite pluton of gameleiras northeast brazil abstract a well defined orientation pattern has been evidenced by the magnetic fabric lineation and foliation of the granite pluton of gameleiras northeast brazil this magnetic fabric is roughly parallel to the magmatic fabric defined by the elongate mafic enclaves and by the planar arrangement of tabular k feldspar phenocrysts low field anisotropy of magnetic susceptibility properties as well as magnetic mineralogy studies and optical microscopy observations indicate that multidomian iron rich magnetite is the main carrier of the magnetic susceptibility and its anisotropy to examine in more detail the relationships between the magnetic and magmatic fabrics measurements of two dimensional shape fabrics of grain populations of biotite and magnetite have been carried out on a total of 500 grains using an automatic shape fabric analysis procedure on carefully selected and oriented thin sections angular departure has been calculated between the maximum elongation of the mineral shape fabric of the given grain population and the maximum susceptibility axis of the corresponding macroscopic specimen we found that the fabric of biotite correlates strongly with the magnetic fabric 7 8 the fabric of magnetite although not as well defined mainly because of its weak shape anisotropy still correlates with the magnetic fabric 12 18 origin of low field magnetic fabric in the pluton of gameleiras is concluded to be controlled principally by the statistical alignment of the long axes of inequant magnetite grains magnetic interactions between the grains of magnetite where they form clusters may account for the observed scattering or abnormal magnitudes of the magnetic anisotropy concerning the orientation of the magnetic fabric as the long dimensions of the magnetites preferentially align with the boundaries of the other minerals particularly biotite and titanite the magnetic fabric as defined by the grains of magnetite is concluded to approximate closely the magmatic fabric defined by the shape fabric of biotite however as no correlation exists between shapes or clustering of magnetites and strain undergone by the deforming magma the strength of the magnetic fabric in ferromagnetic granites cannot be used in a simple way as a measure of strain intensity

magnetic petrology of variably retrogressed eclogites and amphibolites a case study from the hercynian basement of northern sardinia italy 1 this study constitutes the first attempt to integrate rock magnetism and metamorphic petrology in the hercynian basement of northern sardinia the investigation focused on the magnetic petrology of variably retrogressed eclogites and amphibolites from two suites of basic meta igneous rocks which occur along a major tectonic line posada asinara line within a medium grade mg and a high grade hg metamorphic complex consistent with petromagnetic results hg metabasites contain variable amounts of monoclinic pyrrhotite intergrown with rutile and titanomagnetite occurring as inclusions in garnet abundant ilmenite associated to secondary hornblende and with sphene low ti magnetite rims and rutile both as inclusions in ilmenite and as discrete grains in mg metabasites pyrrhotite is restricted to amphibolites of the posada valley area where it occurs as rare inclusions in garnet all samples are characterized by variable amounts of ilmenite rutile and sphene which show the same microstructural features described in hg rocks microstructural evidence and geothermobarometric data indicate that 1 pyrrhotite and titanomagnetite likely formed prior to and remained stable during the eclogite facies metamorphic peak and 2 the growth of ilmenite and sphene can be attributed to the amphibolite facies retrogression mainly due to model reactions such as garnet omphacite rutile h2o hornblende plagioclase ilmenite and amphibole ilmenite o2 sphene magnetite quartz h2o the results from our combined petrological and petromagnetic study corroborate the hypothesis that significant volumes of mafic ultramafic rocks similar to some of the investigated outcrops may account for the magnetic anomalies flanking the northeastern part of the posada asinara line

development of the mozambique and ruvuma sedimentary basins offshore mozambique two major sedimentary basins have been identified on the continental margin of mozambique mozambique basin and ruvuma basin the formation of the basins is related to the break up of gondwana and opening of the western part of the indian ocean the basins are relatively young having developed discordantly to the structural plan of gondwana sedimentary basins the history of the formation of the east african continental margin sedimentary basins within mozambique has been studied on the basis of the present day concept of gondwana break up and madagascar s drift with respect to africa two stages in the history of the east african basins can be recognized late gondwana and post gondwana the late gondwana stage 303 157 ma is typified by sedimentary and volcanogenic rocks of the karoo group the post gondwana stage 157 0 ma corresponds to the period of active gondwana break up and the formation of indian ocean marginal basins the mozambique basin occupies both central and southern parts of the coastal plain of mozambique extending onto the continental shelf and slope the sedimentary fill is composed of upper jurassic cretaceous and cenozoic rocks which discordantly overly the karoo basalts the upper jurassic occurs as continental red beds mostly distributed within buried grabens cretaceous rocks occur as terrigenous sediments of continental and marine genesis cenozoic deposits are of predominantly marine and deltaic origin the ruvuma basin situated in the north of mozambique is part of an extended east african marginal basin which includes parts of the coastal plains and continental margins of tanzania and kenya the basement of the basin is composed of crystalline and metamorphic rocks of pre cambrian age the sedimentary fill is represented by continental terrigenous karoo sediments marine and lagoonal jurassic deposits and marine and deltaic rocks of cretaceous and cenozoic age the main difference between the sedimentary fill of the mozambique and ruvuma basins is related to the changing sedimentary environment in the period of gondwana break up jurassic and early cretaceous when the drift of madagascar and sea floor spreading took place and the marine basin gradually extended southwards between the east african margin and madagascar from the late cretaceous on the sedimentary environment within both basins was similar the mozambique and ruvuma basins forming a single system on the east african continental margin the main stages of the basins development are illustrated by a series of palaeogeographic reconstruction maps

micromorphological fabrics and developmental stages of carbonate nodular forms related to soil characteristics abstract carbonate nodules often constitute the first stage of calcrete formation micromorphological analysis of carbonate nodules and nodular fabrics in different soil materials shows that their development is a function of several factors nature of the host matrix texture porosity carbonate and non carbonate clay distribution bulk density and the interactions among them different micromorphological stages therefore exist in different soil materials in calcareous medium textured soil materials the stages of nodule formation are related to the increase of the density of the nodule caused by the accumulation of microcalcites in such a soil material the following stages occur 1 microcalcites within the low to moderate density matrix 2 microcalcites of moderate to high density and moderately dense diffuse nodules parallel calcans may occur 3 microcalcites of high density with dense microcalcitic nodules during these stages the amount of non carbonate clay decreases but is homogeneously dispersed and disseminated with the microcalcites in general the size of microcalcites 1 8 m is inversely related to the clay content in non calcareous medium to fine textured soil materials secondary carbonate precipitates in intrapedal and or interpedal voids as large sparry crystals due to pedoturbation processes the nodules thus formed are subjected to recrystallization resulting in a microsparry pattern the development stages are 1 few calcans and crystal chambers 2 crystic nodules mainly of sparry crystals and an increase of calcans and crystic chambers 3 recrystallized nodules composed mainly of microspars in coarse grained materials secondary carbonates precipitate directly in voids as sparry crystals and the resulting nodules are of the crystic type the large crystals are often subjected to recrystallization in partly calcareous coarse grained materials with additions of calcareous dust particular micromorphological carbonate forms occur in the c horizon cutanic calciasepic fabric forms besides zones of crystic fabric which gradually become calciasepic in the b horizon and include the typical nodular forms of the calcareous medium textured soil material together with appreciable amounts of glaebular halos the processes leading to the formation of these stages occur essentially without a significant biogenic influence where carbonate segregation occurs as rhizomorphs surrounding active roots the fabric is always micritic even in coarse textural soil material

application of lattice gas techniques to the study of sediment erosion and transport caused by laminar sheetflow in order to gain a deeper understanding of the dynamics of erosion and sediment transport on hillslopes it seems important to clarify the role of some basic mechanisms involved in these processes while there is evidence that this cannot be done using the theoretical framework of river hydraulics the use of numerical analysis could be of considerable help the nature of the problem requires a technique capable of solving navier stokes equations at low reynolds number with geometrically complex boundaries and solid particles moving inside the flow field all these requirements make a novel method known as lattice gas automaton lga a natural candidate for the study of the hydrodynamics of sheetflows however due to the recent introduction of this technique there is a lack of a clear definition of its operational limits considering the case of a viscous sheetflow on an erodible rough boundary we argue that by using lga the stream reynolds number can be increased only at the expense of a reduction of the boundary shearing stress accordingly lga cannot profitably be used to study the beginning of sediment motion and transport on the other hand a further evolution of lga known as the lattice boltzmann method seems highly promising for the numerical study of the erosion processes that eventually lead to drainage network evolution along hillslopes 1997 by john wiley sons ltd

a modeling scheme for the study of drainage density an analytical approach is taken for the modeling of the drainage density of a basin a rainfall model a runoff model and an erosion model are coupled together to estimate from climatic and geomorphologic characteristics the expected length of no erosion in a basin and through this parameter the drainage density of the watershed the convergence of overland flow as an important cause of the growth upslope of the channels is explicitly taken into account in the model as well as a simple approximation to the evolution in time of the contributing area at the source the approach is applied to two basins in puerto rico with encouraging results

estimating peak currents at ground lightning impacts using remanent magnetization 1 the impact of lightning is permanently recorded in struck material by a secondary magnetization called lightning induced remanent magnetization lirm we introduce an original rock magnetic method based on alternating field demagnetization to define both the geometry and the intensity of the peak lightning magnetic field it also provides positive evidence than a material has effectively been struck on a schoolyard tree struck by lightning we were able to locate the impact to within a few centimeters to prove that it was a vertical negative current and to estimate a peak current intensity of 99 7 ka this value is 14 ka higher than the one provided by meteorage lightning detection network and offers an independent calibration

erosion processes at the hillslope scale the results are presented of an intensive monitoring programme to determine eroded volumes due to observed rainfall events on a hillslope surface the surface investigated has been reproduced through a digital elevation model and analysed in terms of drainage network contributing area and slope by performing planimetric and altimetric analysis erosion maps were derived from a comparison of consecutive digital elevation models relative to time these maps reveal the spatial and temporal evolution of the erosion process at the hillslope scale the erosion was uniform across the surface supporting the assumption of randomness in the erosion process commonly used in surface drainage development models the observed value of erosion has been estimated at approximately 0 11 m m 2 per year with almost 500 mm of total annual rainfall

strain patterns within the late variscan granitic dome of velay french massif central the velay granites are intruded as a late variscan dome in the french massif central strain patterns within the velay granites reflect the geometry and kinematics of late carboniferous deformation at the site of final emplacement the internal deformation developed during granitic magmatism is not homogeneously distributed from inner to outer zones increasing deformation is marked by changes from high temperature foliations to s c orthogneisses strain concentration at the margins reflects a strength contrast due to cooling and subsequent magma hardening according to the internal strain field two zones can be distinguished in the velay dome i a central and southern zone where concentric patterns indicate a southward expansion of granites and ii a northern zone where the lateral expansion is limited and combined with top to the north extensional shearing along the mont pilat detachment zone and high temperature wrenching these asymmetric strain patterns relate to the expansion and deformation of granites emplaced below a detachment fault during the uplift of a thickened area post thickening combination of extensional and wrench tectonics is inferred

the na ca amphibole albite chlorite epidote quartz geothermobarometer in the system s a f m c n h2o 1 an empirical calibration the proposed geothermobarometer is based on an empirical calibration which takes account of two equilibria involving the tremolite edenite pargasite and hastingsite components in amphiboles it has applications to assemblages found in metabasic rocks of widely different chemical compositions magnesian to fe rich metabasalts and for metamorphism ranging from lower greenschist to highest amphibolite facies knowing the si t1 aliv alvi fe3 fe2 mg ca nam4 naa and a vacancy in an amphibole and the al3 and xmg in coexisting epidote and chlorite it is possible to calculate two values of in kd for this assemblage these equilibria involve edenite tremolite and pargasite hastingsite tremolite end members in amphibole the calculation program is given for these equilibria the isopleths iso values of kd have been calculated for 0 27 xmg 0 75 and 0 xfe3 fe3 fe3 alvi 0 8 it is then possible to determine pressure and temperature directly when xmg xfe3 in kd for tremoliteedenite and in kd for pargasite hastingsite tremolite are known application of this geothermobarometer is limited to ca free plagioclase assemblages and complete p t paths can be drawn only if all the minerals are considered together phase relations at successive stages of crystallization can be constrained by studying the relationships between the coexisting minerals their zoning and the metamorphic fabrics

magnetic petrology of deep crustal rocks ivrea zone italy magnetic petrology is an extension of petrology integrating magnetic property studies with conventional petrology for the purpose of understanding the development and modification of the magnetization in rocks the magnetic properties of a suite of samples from the ivrea zone northern italy are described with the aim of developing a magnetic petrology for deep crustal rocks samples studied include a variety of metamorphosed mafic granulites granofelses plagioklasfelse amphibolite pyriclasite and pelitic quartzo feldspathic rocks acid granulites stronalite and kinzigite gneisses schists and representative ultramafic rocks phlogopite peridotite pyroxenite hornblendite ilmenite is the chief oxide mineral in a majority of the samples but in a few mafic rocks magnetite is the principal oxide constituent cr al spinel is common in ultramafic samples and rutile is present in some metapelites sulfides are generally subordinate in abundance to oxide minerals the mafic rocks in the ivrea section have the broadest range of magnetic properties and include the most strongly magnetic samples the ivrea ultramafic rocks are moderately magnetic the magnetite present appears to be secondary and is associated with crustal alterations while pelitic and quartzo feldspathic lithologies are dominantly paramagnetic or only weakly magnetic the main ferromagnetic mineral in all strongly magnetic samples is magnetite that is nearly pure fe3o4 in composition consequently curie temperatures are 565 580 c for these rocks pyrrhotite contributes to the magnetism in several ultramafic and mafic rocks and it is the sole ferromagnetic mineral in the pelitic and quartzo feldspathic samples the results of our study indicate that certain mafic rock types amphibolites mafic granulites pyriclasites are the most likely source of the ivrea zone s regional scale magnetic anomalies if the ivrea zone represents a tectonically exposed cross section of continental crust the geographic distribution of these rock types suggests the presence of multiple and variably thick strongly magnetic layers with magnetite curie temperatures in the deep crust petrographic evidence suggests however that the magnetic mineralogy of the sampled ivrea rocks might have been changed subsequent to peak metamorphic conditions therefore considerable care must be taken in evaluating the magnetic record in these crustal cross sections particularly if they are to be used to model the magnetic characteristics of the lower crust

hydrodynamic description of the erosional development of drainage patterns the erosional development of drainage networks is analyzed by modeling two dimensional flow of water and sediment over a bare slope deterministic equations of mass and momentum conservation are coupled with random fluctuations of sediment pickup and transportation processes acting at the hydrodynamic scale these fluctuations are then shown to be able to provide an active source of channelization at the hillslope scale controlled experiments are also carried out by simulation runs aimed at investigating model performance 1 to describe formation of natural drainage patterns 2 to account for different factors i e soil characteristics hillslope geometry and rainfall intensity which control network formation and development and 3 to reproduce differentiation of drainage patterns due to above factors

magnetism at depth a view from an ancient continental subduction and collision zone recent sophisticated global data compilations and magnetic surveys have been used to investigate the nature of magnetization in the lower crust and upper mantle two approaches to constraining magnetizations are developed providing minimum 0 01 si and maximum 0 04 si susceptibility estimates given some assumed thickness 15 km here these values are higher than are found in many continental rocks are there rocks deeper in the crust or upper mantle that are more magnetic than expected or are the model assumptions incomplete what is the magnetic behavior of deep crustal and upper mantle rocks when slightly cooler than the curie or neel temperatures of their magnetic minerals after being exhumed from locations of high grade metamorphism at greater depth different sets of equilibrium metamorphic minerals can be considered that would form under different conditions results on 1501 samples from the western gneiss region wgr norway mainly from mafic and ultramafic bodies subducted to depths of 60 200 km and temperatures of 750 up to 950 c at the very highest pressures show that rocks did not fully equilibrate to the dominant metamorphic facies conditions there is a large variation in petrophysical properties oxide minerals and mineral assemblages in wgr samples though they cannot explain the broad high amplitude deep seated anomalies measured in this region the presence of magnetite and exsolved titanohematite and hemo ilmenite in samples shows those magnetic phases are preserved even at eclogite facies conditions in part because complete eclogite facies equilibrium was rarely achieved

inception of channelization and drainage basin formation upstream driven theory the ubiquitous presence of river drainage basins in the terrestrial environment suggests that distributed overland flow generated by rainfall tends to spontaneously organize itself into dendritic systems of discrete channels several recent numerical models describe the evolution of complete drainage basins from the initial condition of rainfall on a flat tilted plateau the surface of which has been provided with random elevation perturbations these analyses model overland flow via the assumption of a perfect balance between gravitational and frictional terms i e in terms of normal flow linear stability analysis applied to the normal flow model has been shown however to fail to select a wavelength corresponding to a finite distance of separation between incipient basins this suggests that the normal flow model may not be a sufficient basis for studying drainage basin development especially at the finest scales of morphologic significance here the concept of a threshold condition for bed erosion is combined with an analysis of the full equations of shallow overland flow in order to study wavelength selection classical linear stability analysis is shown to be inadequate to analyse the problem at the level of inception an alternative linear analysis of bed perturbations based on the threshold condition is developed and shown to lead to the selection of finite wavelength of the correct order of magnitude the analysis here is driven from the upstream direction in that bed erosion is first caused only when sufficient flow has gathered from upstream due to rainfall a downstream driven theory of incipient channelization that is not necessarily dependent upon rainfall is presented in izumi 1993 and is presently in preparation for publication

magnetic petrology of high fe ti eclogites from the ccsd main hole implications for subduction zone magnetism 1 the high fe ti eclogites with exsolved lamellar in 530 600 m depths from the chinese continental scientific drilling ccsd main hole in the sulu ultra high pressure uhp metamorphic belt eastern china record an anomalously high susceptibility natural remanent magnetization nrm and koenigsberger ratio q nrm ji ji is induced magnetization this provides us with a good opportunity to study the effects of magnetic minerals and exsolution lamellae on the magnetic properties of deep rocks in this paper we have measured systematically magnetic properties and mineral assemblage and structure for three special samples no 83 no 86 and no 89 results show that these fe ti rich eclogites are the result of the fine grained titano magnetite exsolution in pyroxene and lamellar structure consisting of finely interlayered ilmenite and hematite in exsolved hemo ilmenite we found that the dominant fe bearing oxide minerals in samples studied are ilmenite hematite hem ilm up to 25 little titano magnetite and pyrite the ferromagnetic susceptibility is mainly controlled by fine grained titano magnetite and nrm is closely related to the exsolved lamellar structure we hence propose that the anomalous magnetism of these eclogites observed in our samples is the result of exsolution from homogenous pyroxene and ilmeno hematite during cooling and decompression processes these fe ti rich eclogites might be one of the sources of high magnetic anomalies observed in the sulu subduction zone eastern china

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

petrophysical data and opaque mineralogy of high grade and retrogressed lithologies implications for the interpretation of aeromagnetic anomalies in northern vestranden central norway abstract a regional aeromagnetic anomaly centred around roan northern vestranden western gneiss region central norway is caused by the presence of high grade orthogneisses and metabasites with 1 2 vol magnetite granulites are surrounded and partly covered by amphibolite facies rocks which consist of reworked retrogressed granulite facies orthogneisses and bands of metasediments prograde metamorphism from amphibolite to granulite facies led to production of magnetite as a result of the breakdown of hydrous fe mg al silicates the magnetite occurs as 0 5 1 mm amoeboid grains surrounded by plagioclase and k feldspar this magnetite is nearly pure with only a negligible content of ulvospinel fetio4 in solid solution early retrogression at low ph2o conditions is characterized by breakdown of garnet producing fine grained magnetite in almandine rich varieties this secondary magnetite is common in basic rocks where it can be the dominant type grains smaller than 10 m are associated with high coercivities and this magnetite may be the carrier of remanence in such lithologies a probably contemporaneous development of magnetite can be seen in clinopyroxenes where it occurs as 0 1 mm long needles or rods within crystallographic planes clinopyroxene that crystallized at high pressure can hold an excess of fe in the lattice but upon decompression will release this fe producing fe ti oxides late retrogression at high ph2o leads to reduction of the magnetite content due to replacement of magnetite by silicates mainly hornblende and sphene also leucoxene magnetite is still stable into the upper amphibolite facies the close correlation between petrophysical data aeromagnetic anomalies and geological units allows mapping of these units in the offshore areas metasedimentary rocks that are downfolded in the orthogneisses show low magnetic susceptibility values and can be traced as zones of lowest magnetization level